CRITICAL LITERATUREREVIEW OFRELATIONSHIPSBETWEENPROCESSINGPARAMETERSAND PHYSICALPROPERTIESOFPARTICLEBOARD

USDA FOREST SERVICEFOREST PRODUCTS LABORATORYGENERAL TECHNICAL REPORTFPL-101977

U.S. DEPARTMENT OF AGRICULTUREFOREST SERVICEFOREST PRODUCTS LABORATORYMADISON, WIS.

CRITICAL LITERATURE REVIEWOF RELATIONSHIPS

BETWEEN PROCESSING PARAMETERSAND PHYSICAL PROPERTIES

OF PARTICLEBOARDby

MYRON W. KELLY

GENERAL TECHNICAL REPORT FPL-10May 1977

Prepared forForest Products Laboratory

Forest ServiceU.S. Department of Agriculture

Kelly is a member of the faculty of theDepartment of Wood and Paper Science

School of Forest ResourcesNorth Carolina State University, Raleigh

ABSTRACT

The pertinent l iterature has been re-viewed, and the apparent effects of selectedprocessing parameters on the resultant parti-cleboard properties, as generally reported inthe literature, have been determined. Resinefficiency, type and level, furnish, and pressingconditions are reviewed for their reportedeffects on physical, strength, and moistureand dimensional properties.

A serious deficiency in the researchappears to be lack of consideration of thewithin-panel density gradient and its effectson physical properties.

ACKNOWLEDGMENTS

The author wishes to acknowledge thefinancial assistance provided for this work bythe Forest Service of the United States Depart-ment of Agriculture and the National Particle-board Association, Silver Spring, Maryland.The assistance provided by the personnel atthe Forest Products Laboratory in Madison,Wisconsin is also appreciated.

Forest Products Laboratory is maintainedby the U.S. Department of Agriculture, ForestService, in cooperation with the University ofWisconsin, Madison.

TABLE OF CONTENTS

LIST OF FIGURES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

PHYSICAL PROPERTIES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Vertical Density Gradient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Formaldehyde Release. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Springback. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Surface Characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

PROCESSING PARAMETERS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Resin Efficiency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Adhesive Type and Level. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Furnish . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Pressing Conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Mat Moisture Content. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Press Closing Rate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Press Time, Temperature, and Pressure

STRENGTH PROPERTIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Bending Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Modulus of Rupture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Particle configuration and orientation

Modulus of Elasticity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Particle configuration and orientation

Internal Bond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Other Strength Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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MOISTURE AND DIMENSIONAL PROPERTIESEquilibrium Moisture Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Dimensional Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Water Absorption and Thickness SwellingDensity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Particle configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Resin and wax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Linear Expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Particle configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Resin and wax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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LIST OF FIGURES

Figures

1. Vertical density gradient of a three-layer particleboard . . . . . . . . . . . . . . . . . . . . . . . . . .

2. Internal bond variation with thickness for a three-layer particleboard . . . . . . . . . . . . . .

3. Influence of initial moisture content and press time on the core temperaturewith the granular-type particle mat (SG 0.65). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4. Influences of initial moisture content and press time on the core temperaturewith the flake-type particle mat (SG 0.75) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5. Moisture distribution in particle mats as influenced by press time . . . . . . . . . . . . . . . .

6. Relationship between initial moisture content and the press time at whichthe core temperature rises above 100° C for the granular-type particle . . . . . . . . . . . .

7. Effect of various moisture and temperature conditions on vertical densitygradients in 0.5-inch particleboard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8. Influence of heating temperature on residual formaldehyde . . . . . . . . . . . . . . . . . . . . . .

9. Influence of particleboard moisture content on residual formaldehyde . . . . . . . . . . . . .

10. Thickness springback as influenced by flake dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . .

11. Influence of resin droplet size on tensile strength perpendicular and parallelto particleboard surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

12. Relationship between wood density and modulus of rupture forfour particleboard densities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

13. Relationship between specific gravity and modulus of elasticity ofparticleboard face layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

14. Absorption isotherms for particleboards compared to wood, at 70° F . . . . . . . . . . . . . .

15. Relationship between thickness swelling and moisture content increase . . . . . . . . . .

16. Influence of particleboard density on rate of thickness swelling . . . . . . . . . . . . . . . . .

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IV

CRITICAL LITERATURE REVIEWOF RELATIONSHIPSBETWEEN PROCESSING PARAMETERSAND PHYSICAL PROPERTIESOF PARTICLEBOARDS

byMYRON W. KELLY

INTRODUCTION

The development of the particleboardindustry has been characterized by large anddramatic changes in equipment, resins, andlevels of automation since its inception in theearly forties. The large number of processescurrently used by the industry to produce flat-pressed particleboard all have similar manu-facturing steps. Subtle equipment and pro-cessing modifications to circumvent patents,as well as improvements based on experiencewith earlier plants, have practically guaranteedthat no two plants are exactly the same. Whilespecific methods may vary from plant to plant,they all have particle preparation, drying, andscreening steps, as well as resin application,mat formation, hot pressing and edge trim-ming stations. A detailed development ofmanufacturing processes can be found inKollmann et al. (1975).

The particleboard literature containsresults of thousands of research projects con-ducted by many researchers over the past fortyyears. A large portion of this literature is inGerman or Russian publications, many ofwhich are unavailable. Consequently, thisreview is based primarily on research pub-lished in the English literature with only a fewreferences to untranslated foreign papers.

The total number of particleboard-relatedpublications is overwhelming, as is evident byquickly scanning the “Particleboard andRelated Structural Panel Products” section ofthe Abstract Bulletin of the Institute of PaperChemistry and other abstracting services.Restriction of this review to the effects ofvarious processing parameters on panel pro-perties reduced the number of applicablepublications to a more manageable number.However, this is not to imply all pertinent

publications were reviewed, although nonethat were applicable and useful were inten-tionally excluded.

The difficulty encountered in a review ofthis nature is attempting to develop a relation-ship between a processing parameter andspecific particleboard properties from thelarge number of reports and the wide range ofexperimental conditions represented by thesereports. Initially, efforts were made to quantifythe effect of processing parameters on theproperties of the resultant particleboard--e.g.,the influence of flake length on the bendingstrength and dimensional stability of theboard. This proved to be a futile exercise be-cause of the interaction of other processingvariables and the wide range of experimentalconditions used by the various researchers.Consequently, this report qualitatively pre-sents the reported relationships between pro-cessing variables and the resultant particle-board properties.

The pertinent literature has been reviewedand the apparent effects of selected process-ing parameters on the resultant particleboardproperties, as generally reported in the litera-ture, have been determined. Emphasis hasbeen placed on the literature in which strengthand dimensional properties, as influenced byprocessing parameters, have been studied.However, the initial portion of the report isdevoted to a review of general particleboardproperties and processing parameters. Anumber of reports emphasize these generalproperties and contain much useful infor-

1 This is Paper No. 5065 of the Journal Series of theNorth Carolina Agricultural Experiment Station,Raleigh.

mation. The individual reports are reviewed insome detail within the respective sections ofthis review. Not all investigators agree as to theeffect of a given processing variable on theresultant panel properties; however, in manyinstances these disputes can be explained bydifferences in experimental conditions. Inmany other instances, further research isnecessary to obtain more information beforethe contradictory findings can be resolved.

A few general comments on the particle-board literature are possible from a review ofthis nature. The most obvious deficiencythroughout the literature, with a few notableexceptions, is the lack of detailed experimentalprocedures in the published reports. In someextreme cases the experimental details con-sisted only of a statement that particleboardwas made in the laboratory, with no details asto particle type, adhesive content, or pressingconditions. Fortunately, this example is notrepresentative; more information is normallygiven. However, lack of detail on pressingconditions, such as mat moisture content andpress closing rate, is widespread in the litera-ture. It is hoped that this deficiency will beremedied in the future.

Another unfortunate characteristic is thepractice of reporting board density figureswith no details of the moisture content at whichit was determined. The nonuniformity of testmethods for water absorption and dimensionalstability also effectively limits comparisonsbetween reported values of individual re-searchers.

By far, the most serious deficiency in theparticleboard literature is the lack of emphasison the vertical density gradient. Innumerablearticles appear in which results reportedlyshow the effect of processing parameterchanges on the board physical properties.Unfortunately, these parameter changes mayinfluence the vertical density gradient also, butthis remains undetected because the gradient,too often, is not determined. Much more validrelationships between processing parametersand board properties could be obtained if thevertical density gradients were determined. Itis hoped that both the vertical density gradientand the moisture content gradient will eventu-ally be routinely determined by all researchers.

The above criticisms are not meant as ablanket indictment of the particleboard litera-ture. A tremendous volume of useful infor-mation pertaining to particleboard manufac-ture and properties is available, and more isadded daily. The rapid growth of this industrywas possible only with the information andknowledge acquired through the excellentresearch conducted by many investigators.Further advances, in both processing andapplications, will also be realized with con-tinued research efforts of many individuals.

Comments and criticisms which are appli-cable to more than one section in the reporthave been included in all pertinent sections.This results in duplication in the respectiveareas but the usefulness of the report to pro-spective readers may be enhanced by thismethod of presentation.

PHYSICAL PROPERTIES

Various particleboard physical propertiesare not readily classified as either strength orhygroscopic characteristics, but are importantfor most particleboard applications. Many ofthese properties, such as density, verticaldensity gradient, and springback, have a signi-ficant influence on the resultant strength andhygroscopic characteristics of the particle-board panel. However, these, as well as formal-dehyde release and surface characteristics,are also dependent upon processing para-meters such as pressing conditions, resincontent, and particle geometry. It will be help-ful to develop these physical properties in thissection and various processing parameters inthe next section before discussing the inter-dependent effects of physical properties andprocessing variables on the strength andhygroscopic characteristics of particleboard.

Density

The two most important factors control-ling the average final density of a particleboardare the raw material density and the compac-tion of the mat in the hot press. Any change inone of these factors requires an adjustment ofthe other if the average board density is toremain constant. Likewise, either of thesefactors can be changed to increase or de-crease the average particleboard density.However, a higher density panel produced byincreasing the compaction level will not haveproperties equal to the same-density panelproduced with higher density wood furnish.This is only one example of the interdepen-dence of processing parameters on the pro-perties of the resultant particleboard. Isolationof the effect of individual processing para-

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meters on particleboard properties is ex-tremely difficult and can, at best, be onlyapproximated.

The pressing operation consolidates theparticle mat to the desired thickness and poly-merizes the binder system between individualparticles. The first function eliminates many ofthe voids in the mat and compresses the woodstructures; the latter function ensures reten-tion of the consolidated mat upon release ofthe pressure. The amount and condition of thematerial in the mat, together with the pressingtechniques, determine the average resultantboard density. Thickness control of mostparticleboard production is attained by using“stops”: Incompressible material of the appro-priate thickness is placed on two sides of eachpress opening and the platens are closedagainst these stops, resulting in a gap betweenthe two platens equal to the desired particle-board thickness.

The maximum pressure employed forparticleboard in a press equipped with stopscontrols the rate of press closing, assumingthe pressure exceeds the minimum required tocompact the mat to the desired thickness. Therate of press closing on a given weight of woodfurnish does not affect the average final parti-cleboard density. That density is independentof the pressing operation when the presstemperature and pressing time are sufficient topolymerize the adhesive system and allowevaporation of the excess water. The averageparticleboard density is dependent upon thequantity and density of the wood furnish usedto make a particleboard of a given thickness,but the maximum pressure--and hence the rateof closing to stops--has an extremely criticaleffect on the vertical density gradient, asshown in the next section.

The final average density of particleboardis dependent not only upon the amount ofwoody material in the mat but also upon pro-cessing conditions prior to the pressing oper-ation--specifically, furnish species, prepara-tion and drying, adhesive content, and otheradditives.

The literature contains many examples inwhich the specific gravity or density is statedambiguously; a reported panel density of45 Ib/ft3 (pounds per cubic foot) is inadequate.For example, this could mean 45 pounds ofovendry particleboard per one cubic foot ofdry particleboard, or 45 pounds of ovendryparticleboard per one cubic foot of particle-board at any moisture content, or it could evenmean 45 pounds of particleboard and waterper one cubic foot of particleboard at anymoisture content. Consequently, the equili-

brium moisture content (EMC) at which thevolume and weight are determined for thedensity calculation must be specified. Specificgravity is always calculated with the ovendryweight in the numerator but the EMC at whichthe volume is determined for the denominatormust also be specified. Therefore, a density of45 Ib/ft3 (ovendry weight, volume at 12 percentmoisture content) is the same as a specificgravity of 0.721 (ovendry weight, volume at 12percent moisture content) and also the sameas 50.5 Ib/ft3 (weight and volume at 12 percentmoisture content). Clearly, more care shouldbe exercised in the reporting of board densityand specific gravity.

Unless specifically stated in this report, thedensities quoted from the literature sourcesare at unspecified moisture contents, as therequired information is not contained in theoriginal report.

The effect of furnish species on particle-board physical properties is primarily relatedto the wood density and the necessity of com-pressing the wood to obtain satisfactory parti-cleboard. In general, a conventional particle-board with a density lower than the density ofthe wood furnish will be unsatisfactory (Lar-more 1959; Lynam 1959; Suchsland 1967;Hse 1975). The compaction of the mat to anaverage density higher than the density of thefurnish will allow better surface contact be-tween the component particles of the mat. Thisresults in better adhesive utilization becausemore adhesive-coated particles will be inintimate contact with other wood particlesinstead of with voids.

Suchsland (1959) developed a statisticalmodel for a particleboard mat relating thedegree of densification to particle geometry,relative air volume, and component wooddensity. The distribution of total particle thick-ness varies randomly in a mat; regions of hightotal particle thickness will obviously be com-pressed to a greater extent than will corres-ponding regions of low total particle thicknesswhen the entire mat is compressed to a givenuniform thickness. Suchsland further deter-mined that relative compression area--not themore commonly accepted board density--isthe significant factor in developing bendingstrength in a flakeboard.

However, board density and area of com-paction are directly related for mats containingdifferent amounts of a given species of thesame particle configuration and pressed toconstant thickness. Larmore (1959) has shownthat the modulus of rupture for particleboardof specific gravity 0.73 (ovendry weight, vol-ume at 75°F and 65 percent relative humidity)

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is higher for aspen furnish (specific gravity0.37, ovendry weight, green volume) than foryellow birch (specific gravity 0.65, ovendryweight, green volume). Larmore also reportedextremely low modulus of rupture values forparticleboard made with yellow birch furnishto a specific gravity of 0.56 (ovendry weight,volume at 75° F and 65 percent relative humi-dity), lower than its component species. Thisfurther illustrates the necessity of compressingthe wood to some extent to obtain satisfactoryboard properties.

Obviously, at some high specific gravitythe effect of the furnish specific gravity on theresultant board strength properties shoulddisappear, since the board specific gravity hasa theoretical maximum equal to the specificgravity of the wood substance. However, formedium- to high-density particleboard, woodsof specific gravity below that of the finishedboard will result in a more satisfactory product.

Density of the raw material furnish is themost critical parameter when determining thepotential of a given species for particleboardmanufacture. The literature contains refer-ences to this interaction by noting an improve-ment in resin efficiency when either usingwoods of lower specific gravities to produceparticleboard of constant final specific gravityor using a constant wood specific gravity andproducing boards with higher final specificgravities (Burrows 1969; Hann et al. 1962;Lehmann 1970; Shuler 1974; Hse 1975). Theseall indicate the necessity of compressing thewood particles to a higher specific gravity thanthe original wood to attain sufficient inter-particle contact and allow the use of a loweradhesive level than would otherwise berequired.

The literature appears to be devoid ofother species effects in relation to particle-board specific gravity. This is not surprising;extractive content, acidity, grain deviation, andother species characteristics should have anegligible influence on the final board specificgravity.

The ovendry specific gravity of a particle-board can be readily controlled and accuratelycalculated from the dry weight of wood andother additives used to produce a mat of speci-fied volume. However, higher density particle-board will have a significant effect upon thepress cycle--more wood has to be heated,more water removed, and more adhesivereacted (Heebink et al. 1972).

The particle configuration should, likesome species effects, have a negligible effecton the final specific gravity, assuming theapplied pressure is sufficient to compress the

mat to the desired thickness. Suchsland (1959)reported a higher pressure required to reacha desired specific gravity for narrower andthicker flakes as opposed to wider and thinnerflakes.

The particle moisture content entering thepress also fails to influence the average oven-dry specific gravity of the final particleboard,but the moisture content effect on the verticaldensity distribution is highly significant, as willbe shown in the next section. Because thecompressive strength of wood is inverselyrelated to the moisture content, the pressurerequired to compress the mat to a given thick-ness will be lower as the moisture contentincreases. However, an elevated moisturecontent significantly extends the total presstime: excess moisture has to be removed fromthe mat before the adhesive will cure and be-fore the danger of delamination due to steamrelease at the time of press opening is reduced.

Vertical Density Gradient

The density of a mat-formed hot-pressedparticleboard is not uniform in the thickness(vertical) direction, but varies through thethickness. The density profile of a board ishighly dependent upon the particle configu-ration, moisture distribution in the mat enter-ing the press, rate of press closing, tempera-ture of the hot press, reactivity of the resin, andthe compressive strength of the componentwood particles. The effect of these variables onthe vertical density gradient will be examinedin this section.

The vertical density gradient in particle-board substantially influences many strengthproperties, as adequately demonstrated bynumerous studies found in the literature.However, much more numerous in the litera-ture are reports in which the vertical densitygradient and its influence on particleboardproperties are completely ignored. Certainproperties, most notably bending strength, aresignificantly enhanced by the presence of thisvertical density gradient while other proper-ties, such as tensile strength perpendicular tothe panel surface and interlaminar shear, areadversely affected by this density distribution.Therefore, depending upon which propertiesare most critical in the ultimate application ofthe panel, modifications of the pressing pro-cedure may be justified either to enhance or torestrict the formation of this gradient.

Shen and Carroll (1970) studied the verti-cal density gradient and vertical layer strengthas determined by the torsion shear technique(Shen and Carroll 1969). The torsion shearstrength was linearly correlated for 12 com-

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mercial boards, indicating a potential appli-cation of the torsion shear test as a method fordetermining the vertical density gradient.

Plath and Schnitzler (1974) have recentlyshown the high correlation between verticaldensity gradient and tensile strength perpen-dicular to the panel surface. The standard testspecimen for tensile strength perpendicularto the surface consists of the entire boardthickness (ASTM 1975); consequently, rupturewill occur at the weakest point in the thicknessdirection. Plath and Schnitzler determined thetensile strength perpendicular to the surfaceat various points throughout the panel thick-ness (tensile strength profiles) and correlatedthe observed values with the actual density atthat point. The plots of vertical density and

tensile strength versus specimen thicknesspossess remarkably similar shapes (figs. 1 and2). Plath and Schnitzler (1974) also presentvertical density gradient diagrams for threeparticleboards--an air-felted board, a three-layer board, and a three-layer board subjectedto prepress--and a medium-density fiber-board. Dramatic differences were noted be-tween the air-felted particleboard and thethree-layer particleboard not subjected toprepressing. The air-felted particleboardexhibited a symmetrical “U”-shaped gradient,but the three-layer board had a very thin regionof high density approximately 2 mm beloweach surface. The three-layer board subjectedto precompression possessed a vertical den-sity distribution very similar to the air-felted

Figure 1.--Vertical density gradient of a three-layer particleboard.(From Plath and Schnitzler 1974)

(M 144 401)

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board except the bottom surface had a lowerdensity. The medium-density fiberboard had avertical density gradient similar to the air-felted particleboard, except the upper surfacewas lower in density. Consequently, the verti-cal density gradient of the medium-densityfiberboard and three-layer precompressedparticleboard would be expected to exhibitunbalanced dimensional changes with relativehumidity changes because of the unsymmetri-

cal nature of the respective vertical densitygradients.

The influence of mat moisture content anddistribution, average board density, faceweight of three-layer boards, press closurerate, and press cycle on the vertical densitygradients in particleboard have been welldocumented by Strickler (1959), Heebink et al.(1972), and Geimer et al. (1975). Unfortunately,neither Strickler nor Geimer et al. shows the

Figure 2.--Internal bond variation with thickness for a three-layerparticleboard.(From Plath and Schnitzler 1974)

(M 144 402)

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vertical density throughout the entire thick-ness so the presence of unsymmetrical gradi-ents remains undetected. However, the exis-tence and influence of this vertical densitygradient have been recognized by theseauthors.

The normal method of determining verti-cal density gradients is to accurately deter-mine the weight and thickness of a givenspecimen, remove a thin layer from one sur-face, and accurately determine the weight andthickness of the resultant specimen. The den-sity of the removed layer can then be deter-mined from the weight and volume of materialremoved. This is normally referred to as thegravimetric method. This is repeated on bothsurfaces until the complete density profile isdetermined or on only one surface if only halfof the density profile is desired (Strickler 1959;Heebink et al. 1972; Geimer et al. 1975). Un-fortunately, there appears to be a general lackof concern as to the moisture distribution inthe panels used for the vertical density gradi-ent determinations. Because the gravimetricmethod used to measure the density gradientis not independent of moisture, variations inthe moisture distribution in the thicknessdirection will be reflected in the density gradi-ent. Consequently, the moisture gradientshould also be determined on matched speci-mens when the vertical density gradient isdetermined by the gravimetric method. Nomeasurements of moisture distribution werereported by Strickler (1959), Heebink et al.(1972), Plath and Schnitzler (1974), or Geimeret al. (1975).

An x-ray absorption method for measur-ing the vertical density gradient appears inthe literature (Henkel 1969; Polge and Lutz1969). The influence of a moisture gradient onthis method is not discussed in either paper.

The vertical density gradient in mat-formed hot-pressed particleboard arises fromthe influence of temperature and moisture onthe compressive stress of wood perpendicularto the grain (Suchsland 1962). The elevatedpress temperatures used to quickly polymerizethe adhesives in the particleboard drasticallyreduce the compressive strength of wood. Inaddition, the mat normally enters the press at8 to 15 percent moisture content. The com-bined effect of moisture and heat severelyreduces the compressive strength of wood.However, the vertical density gradient does notresult from high temperature and moisturecontent, but is due to the unequal distributionof moisture and heat as a result of the pressingoperation. If the moisture and heat are equallydistributed throughout the mat and remain

equally distributed throughout the entirepressing cycle, the stress-strain relationshipfor all particles of a homogeneous particle-board would be equal and no vertical densitygradient would result (Suchsland 1962). This isobviously not the case with heated platen hotpresses used in particleboard manufacture;consequently, hot-pressed particleboardpossesses a vertical density gradient.

A closer look at the pressing operation isin order to more fully understand the under-lying causes of the vertical density gradient.Strickler (1959) and Suchsland (1962) haveeach published excellent explanations of thephenomena present in a particleboard matduring consolidation in a heated platen press.An uncompressed mat placed in a hot press iscompressed to the desired thickness over afinite period of time, depending upon thespeed of press closure, final thickness, and thecompressive strength of the component woodpart ic les. Homogeneous part ic leboards,pressed at various closure speeds to a con-stant final thickness determined by the stops,will possess different vertical density gradientsbecause of the time-dependent nature of theheat and moisture transfer through the matand their resultant effect on the compressivestrength of the various particle layers (Suchs-land 1962). The time required to close thepress to stops is directly related to the initialpressure; hence, a high initial pressure willcause the mat to reach final thickness beforesufficient heat is transferred from the platensto the mat interior. Because the surface layersare hot, and therefore have lower compressivestrength than the interior layers, compressionof the wood structure in the surface layers willoccur before compressive failure occurs in theinterior. If the press closes to stops beforethe compressive strength of the interior layersis exceeded, all of the compressive failure willhave occurred near the surfaces resulting inhigh-density face regions and a much lowerdensity core, hence a sharp vertical densitygradient. With a lower initial pressure, pressclosure speed will be slower, thereby allowingthe interior to attain a higher temperature anda lower compressive strength before the stopsare reached. The surfaces and interior will bemore equal in density and the slower the pressclosure speed, the lower will be the verticaldensity gradient (Suchsland 1962).

Using the press closure time to control thevertical density gradient has some very obvi-ous limitations. The adhesives used in particle-board production are temperature sensitive--i.e., polymerization to a cross-linked polymeroccurs rapidly at elevated temperatures. Con-

7

sequently, long press closure times will causethe adhesive on the particles in contact withthe bottom hot platen to polymerize beforesufficient interparticle contact has occurred.As a result, the bonding between surface parti-cles will be drastically reduced and a very poorsurface will be formed. This condition is refer-red to as a “precured” surface; the bendingstrength of “precured” particleboard is severe-ly reduced. Another difficulty encountered inattempting to use press closure rate to limitthe vertical density gradient is the lengthypress cycle which results. The capacity ofproperly designed particleboard plants islimited by the press cycle; increasing the pressclosure time will have an immediate and ad-verse effect upon the production level. Con-sequently, most processing changes whichincrease the press cycle are to be avoided.

In addition to closure rate, another meth-od, which has been frequently studied and iswidely used by the industry to control thevertical density gradient, is the “steam shock”treatment (Fahrni 1956; Strickler 1959; Deppeand Ernst 1965; Heebink et al. 1972; Geimeret al. 1975). This technique uses steam toquickly heat the mat interior to more nearlyequalize the rate of reduction in compressivestrength through the mat thickness. Also,because the compressive strength of wood isinversely related to the moisture content, theadditional moisture will further reduce thecompressive strength, allowing mat compres-sion to occur at a lower pressure.

It would be helpful to review the work ofMaku et al. (1959) and Strickler (1959) to deter-mine the rate of heat transfer from, the hotplatens to the mat as well as to determine theeffect of this unsteady state heating processon moisture distribution and movement acrossthe mat thickness. It is the rate at which mois-ture and heat migrate into the mat during thepress closing period that determines wherecompressive failure will occur to allow consoli-dation of the mat to the desired thickness.Once the stops have been reached, very littleadditional compressive failure occurs; con-sequently, the density gradient is establishedduring press closing.

Maku et al. (1959) studied the moistureand temperature distribution in homogeneousparticleboard mats composed of two differentparticle configurations, with moisture con-tents of 1, 11, 14, 22 and 30 percent, when sub-jected to pressure in a hot press. Thermo-couples located at various points through themat thickness allowed them to monitor thetemperature at each location as it varied withpress time, at a press temperature of 135° C.

Because no adhesive was used, moisture con-tent samples at various thickness locationswere also obtained to determine the moisturecontent distribution as a function of time. Thepress times used by Maku et al. for 3/4-inch-thick particleboard were unrealistically long(up to 60 minutes). However, this allowed themto monitor the mat temperature and moisturecontent for extended time periods.

The core temperature as a function ofpress time was distinctly different for the twoparticle types (figs. 3 and 4). The particleconfiguration referred to as granular was 0.2 to0.4 inch long, 0.04 to 0.2 inch wide, and 0.004 to0.08 inch thick and consisted of an unspecifiedmixture of birch and beech species. The flakeparticles were practically square thin particlesent i rely of birch; the range in part ic ledimensions was reported as 0.3 to 0.6 inchlong, 0.1 to 0.8 inch wide, and 0.004 to 0.02 inchthick. For all mats with granular-type particles,with the exception of the mat at 1 percentmoisture content, the core temperatureincreased rapidly to 100° C and remainedconstant at this temperature for various timeperiods depending on the initial mat moisturecontent. However, in the mats with the flake-type particles and high initial mat moisturecontents, the core temperature reached amaximum above 100° C in the early stages ofpressing, decreased slowly to 100° C,remained at this temperature for some periodof time, and then increased smoothly towardthe platen temperature at extended presstimes. The maximum temperature reached bythe mat core in the early stages of pressing wasa function of furnish moisture content; furnishat a 30 percent moisture content resulted in ahigher core temperature than furnish at 20percent moisture content.

Strickler (1959) reported the maximuminitial temperature rise in the core of mats withuniform moisture contents of 6, 9, and 12 per-cent to be inversely proportional to moisturecontent. The moisture content range studiedby Strickler includes the range in which Makuet al. (1959) did not observe a core temperaturemaximum. Strickler (1959) states: “...1) themaximum temperature achieved (core) by theinitial rise was governed by moisture contentand was controlled thereafter by moisture con-tent, and; 2) the temperature tended to dip afterthe initial-rise maximum.” Because the particlesize used by Strickler (0.5 inch long, 0.375 inchwide, and 0.008 inch thick) was very similar tothe flake-type particles of Maku et al. (1959),and because the press temperatures and spe-cific gravities were comparable, the reporteddifferences in the core temperature at similar

8

Figure 3.--Influence of initial moisture content and press time on the coretemperature with the granular-type particle mat (SG 0.65).(From Maku et al. 1959)

(M 144 403)

moisture contents must be the result of some maximum core temperatures as the surfaceother unrecognized and undetermined varia- moisture contents increased from 12 to 30ble. Strickler (1959) did report higher initial percent.

9

Figure 4. --Influence of initial moisture content and press time on the coretemperature with the flake-type particle mat (SG 0.75).(From Maku et al. 1959)

(M 144 404)

Maku et al. (1959) also presented dataillustrating the effect of mat moisture distri-bution on the core temperature with three matsof granular-type particles. All mats had the

same average moisture content of 23 percentbut different initial distributions -- i.e.: a uni-form distribution of 23 percent moisture con-tent; inner three-fourths at 10 percent and the

10

surfaces at 58 percent moisture content; andthe inner half at 10 percent and the surfaces at35 percent moisture content. No initial maxi-mum in the core temperature was reported; allmats exhibited a plateau at 100° C. The coretemperature of the three-layered constructionwith 58 percent surface moisture content in-creased above 100° C in the shortest time; thecore temperature of the mat with a uniformmoisture content of 23 percent remained at100° for a substantially longer time,

Maku et al. (1959) found the maximuminitial core temperature to be a function of theboard specific gravity for the flake-type, butnot the granular-type, particles at a moisturecontent of 25 percent. Strickler (1959) foundno relationship between the board specificgravity and the maximum initial core tempera-ture at moisture contents of 6, 9, and 12 per-cent. However, both Strickler and Maku et al.found it took longer to reach the maximumcore temperature as the specific gravity in-creased, although the increased time was notproportional to the additional mass.

Maku et al. (1959) determined the coretemperature rise for three-layer particle-board with different ratios of surface tocore particles consisting of flake- and granu-lar-type, respectively. The moisture contentwas constant (25 percent) for all mats. Theresults are consistent with homogeneousboards: As the relative amount of flake-typematerial increased, the initial core temperatureincrease above 100° C was higher, the subse-quent decrease to 100° C was more obvious,and the time period until the core temperaturefinally increased above 100° C was longer.

Strickler (1959) did not measure moisturedistribution in the mat as a function of presstime,asdid Maku et al. (1959)(fig.5). Maku et al.found the moisture distribution to be symmet-rical around the board centerline and, for ahomogeneous board with the granular-typeparticle at 21 percent initial moisture content, areverse moisture gradient evident at a 5-minutepressing period indicated moisture had mi-grated from the surface towards the core butthe core was still at the initial moisture content.The moisture distribution in a homogeneousboard with the flake-type particle did notpossess this reverse moisture gradient. Un-fortunately, the data do not allow a directdetermination of the relationship betweenparticle geometry and moisture movement inthe mat since the particleboards producedwith each particle were of different specificgravities--flake-type, 0.65 and granular-type,0.8 (conditions at which specific gravity deter-mined unknown). However, the conclusion

drawn by Maku et al. (1959) that verticalmoisture movement is very slight with theflake-type particle (as compared with sub-stantial vertical moisture movement in thegranular-type particle) appears to be valid. Forboth of the above homogeneous particle-boards, a moisture content gradient still ex-isted after 45 minutes in the hot press at 135° C.

The three-layer board, with the flake-typeparticle surface representing 2/7 of the totalthickness and an initial moisture content of 43percent, and the core with the granular-typeparticle at an initial moisture content of 7 per-cent, had the expected reverse moisture gradi-ent at 3 minutes pressing time. The moisturecontent at 1/4 thickness was approximately 18percent, while the surfaces and core both wereat approximately 10 percent. At a pressing timeof 10 minutes no reverse gradient was evident;the core moisture content was 10 percent.After 35 minutes in the press the gradient wasentirely lacking.

Unfortunately, Maku et al. (1959) did notproduce bonded particleboard; their objectivewas to follow the temperature and moisturebehavior in a mat with no interference fromthe adhesive. Consequently, they could notdetermine vertical density gradients. The re-sults of portions of their study are in goodagreement with many of Strickler’s (1959)results and, because Strickler did determinevertical density gradients, it is possible to usethese two papers to review the effect of mois-ture and temperature on the vertical densitygradients in hot-pressed mat-formed particleboard. However, there are also areas in whichdifferences exist in the results of the respectivepapers and the lack of experimental detail inMaku et al. prevents resolution of thesedifferences.

Strickler (1959) showed that high initialpressure (rapid press closing) resulted in ahigh surface density and a low center density.This results from the short time period for heatand moisture transfer into the mat; conse-quently, compressive failure occurs only in thewood of the surface layers, not in the corelayer.

Heat transfer to the center of a particle matin a hot press is strongly influenced by mois-ture content (Maku et al. 1959; Strickler 1959).Strickler pointed out that two moisture gradi-ents exist in a particle mat: one of increasingmoisture content from the hot platens to themat center and another of decreasing moisturecontent from the middle area of the mat to theedges, along the centerline plane. Moisture atthe mat surface vaporizes when the presscloses and the resultant steam flows to a cooler

11

Figure 5.--Moisture distribution in particle mats as influenced by presstime. (A) Granular-type particle, SG 0.8, initial mat moisture content21 percent. (B) Flake-type particle, SG 0.65, initial mat moisturecontent 21 percent. (C) SG 0.65, initial mat moisture content, core 7percent (granular type), face 43 percent (flake type).(From Maku et al. 1959) (M 144 405)

region (toward the mat center) where it con- ever, the steam resulting from vaporization ofdenses. Consequently, the moisture content of the surface moisture will transfer heat to thethe intermediate and core layers should in- mat core much more quickly than if transfercrease in the early portion of the press cycle. were entirely by conduction (Strickler 1959).This was reported by Maku et al. (1959). How- Consequently, increasing the surface mois-

12

ture content will speed the temperature in-crease of the core.

Strickler (1959) and Maku et al. (1959)both reported data indicating that the initialtemperature rise in the core for mats of flake-type particles produced a temperature exceed-ing the boiling point of water. The core temper-ature after this initial maximum decreased tonear the boiling point of water as the presstime increased. Strickler also showed that formoisture contents below 12 percent the maxi-mum temperature rise in the core decreases asthe moisture content increases. He attributedthis to higher vapor pressures which force themoisture to flow along the centerline to the matedges and escape more rapidly. However,vapor pressures are a function of temperature,not water content; more water may be evapora-ting from the mat edges to the atmosphere ina given time period at the higher moisturecontent, and evaporative cooling may keep thecore temperature lower. Maku et al. (1959) incontrast to Strickler did not find a maximumcore temperature for the flake-type particles at12 percent moisture content. Particle geome-try cannot account for the different resultsobtained by Maku et al. (1959) and by Strickler(1959) as particle configuration was similar inboth studies. The contrasting results mustarise from other unrecognizable sources.

Particle configuration and its relationshipto moisture migration from the mat centralareas to the edges along the core may accountfor the core temperature differences betweenmats of flake-type and mats of granular-typeparticles noted by Maku et al. (1959). The initialcore temperature of mats with granular-typeparticles did not rise above 100° C for anymoisture content conditions tested: 1, 11, 14,22, and 30 percent. If the porosity of the matwith this particle was sufficiently high to allowrapid venting of the center moisture directly tothe edges, the core temperature should notexceed the boiling point of water until the freewater in the core has evaporated. Also, the timeperiod during which the core will remain atconstant temperature, assuring constant heatflow to the center, should be directly propor-tional to the quantity of water evaporated to theatmosphere. The data (Maku et al. 1959) showthe core temperature to increase above 100° Cat a press time of 14, 23, 35, and 51 minutes fora moisture content of 11, 14, 22, and 30 per-cent respectively. Figure 6 (from Maku et al.1959) shows the linear relationship betweenthe initial mat moisture content and the time inthe press before the core temperature risesabove 100° C. This appears to illustrate thatfree water is present in the core of the mat withthe granular-type particle and its rate of re-

Figure 6.--Relationship between initial moisture content and the presstime at which the core temperature rises above 100° C for the granu-lar-type particle. (Adapted from Maku et al. 1959)

13

(M 144 406)

moval is controlled by heat flow to the center.The flake-type mats in which the core tempera-ture rose above 100° C do not have this linearrelationship between initial mat moisturecontent and the time of final core temperaturerise above 100° C.

From the above it appears either that thegranular-type particles allow faster watermovement along the center to the mat edgesthan do the flake-type particles, or that theflake-type particles allow more rapid heatmovement from the mat surfaces to the core.However, because the rate of the initial tem-perature rise of the core appears to be approx-imately the same for the two particle typesat equal moisture contents, the increasedporosity of the granular-type particle matappears to be indicated. This particle type isprobably stiffer than the flake-type and doesnot compress and mat together as well as theflakes.

The mats with flake-type particles at high(20 and 50 percent) moisture contents exhib-ited a peak in the core temperature above 100°C (Maku et al. 1959). This indicates heat trans-fer to the core from the faces is faster than heattransfer to the mat edges for this particle type.However, ‘because most of the heat is trans-ferred by steam, data indicate that, in the earlystages of pressing, moisture movement fromthe faces to the core is quicker than from thecore to the edges, resulting in a superheatedinterior. Unfortunately, the moisture contentgradients presented by Maku et al. cannot beused to compare the moisture distribution asa function of press time for the two particleconfigurations as mats of unequal specificgravity are shown.

The dip in the core temperature after theinitial temperature rise, observed by bothMaku et al. (1959) and Strickler (1959), indi-cates a net loss of heat in this region. This ismost likely due to a net loss of moisture in thecore; more heat is being consumed to evap-orate water to the atmosphere through thecenterline-than is being released by condensa-tion of water migrating to the core from themat surfaces. The moisture gradients of Makuet al. do show a decreasing core moisturecontent at a press time approximately equal tothe temperature dip. Strickler also pressed amat without adhesive to verify that the exo-thermic heat of condensation of the resin wasnot responsible for the core temperature riseand subsequent dip.

The effect of moisture content on verticaldensity gradient was shown by Strickler (1959)to result in more severe gradients as the mois-ture content increased. Because stops were

14

not used for thickness control, it is impossibleto correlate the results directly to a processusing stops; however, the data do illustratethat moisture content, initial pressures, andspecific gravity are all positively correlated tothe magnitude of the vertical density gradient.Strickler also showed that a high surfacemoisture content increased the density of thesurface and intermediate layers while de-creasing the center density. Therefore, mois-ture distribution as well as moisture contenthas a substantial effect on the vertical densitygradient. Thus, the “steam shock” surfacetreatment introduced by Fahrni (1959) servesto magnify density variations in the thicknessdirection of particleboard.

Heebink et al. (1972), as part of a largerstudy, examined the effect of various proces-sing factors on density and vertical densitygradients in particleboard. Variables studiedwere species, particle geometry, board den-sity, board thickness, moisture content anddistribution, and press time, temperature, andclosing speed. Moisture content and distribu-tion within the mat were the most importantvariables affecting the vertical density gradi-ent. Press closing speed and press tempera-ture also contributed to the vertical densitygradient; all other variables had secondaryeffects. Strength properties were also highlydependent upon the vertical density gradient.

Only at extemely rapid closure speeds (15seconds) did the maximum density result atthe surface. With longer closing times, thevertical density distribution was lower at thesurface, rising rapidly to a maximum within3/32 inch from the surface and then decreasingto a lower density core (Heebink et al. 1972). Avery sharp vertical density gradient was ob-tained with a high surface moisture content (15percent) and low core moisture content (5 per-cent); however, with the core at 15 percent andthe surface at 5 percent, a relatively flat gra-dient with the maximum density at the corewas obtained (fig. 7). Unfortunately, moisturecontent gradients in the particleboard as itcame from the press were not determined, onlythe average moisture content of the board. Theresults obtained by Heebink et al. (1972) on theeffect of initial mat moisture distribution on thevertical density gradient agree with those ofStrickler (1959). As the closing time increased(Heebink et al. 1972), the vertical density gra-dient was reduced--i.e., the density of themaximum-density region decreased and thecore density increased.

Geimer et al. (1975) determined the verti-cal density gradients on three-layer Douglas-fir structural particleboards with two surface-

to-core ratios and three press closing speeds.Surface flakes were 2 by 0.5 by 0.02 inch inlength, width, and thickness, respectively. Thelarge surface flakes retard moisture movementto the core so a larger portion of the compres-sive failure occurs in the surface resulting inhigher surface densities. Therefore, not onlypress c losure speed, but a lso di f ferentamounts of surface material for a given thick-ness will contribute to the vertical densityprofile. Geimer et al. also showed that, de-pending upon the vertical density gradient ob-tained with a given press closure, the averagesurface density can be increased or decreasedwith an increase in the surface material.

Included also in Geimer’s report (1975)was the effect of particleboard thickness, at aconstant density, on the vertical density gra-dient. At a constant average density, increasedthickness resulted in an increase in the facespecific gravity at both face-core levelsstudied. Therefore, thicker boards will havesteeper vertical density gradients at constantaverage density and closure speeds because

heat and moisture will not reduce the compres-sive strength in the interior quickly enough toallow the compressive failure to be moreequally distributed across a larger portion ofthe total thickness.

Maloney (1970) included density profilemeasurements. as part of a larger study onresin distribution in three-layered and multi-layered particleboards. The particleboardswith high resin faces had surface specificgravities approximately 11 percent higher thanboards produced under the same experimentalconditions but with lower surface resin con-tents. The average initial mat moisture contentwas given as approximately 10 percent in bothcases, but the higher resin content surface alsomust have had a higher moisture content sincethe solids content of the adhesive was notadjusted. Therefore, the density profile differ-ences reported as due to resin content couldinstead have been the result of moisturedifferences.

The development of the vertical densitygradient within the hot press appears to be

Figure 7.--Effect of various moisture and temperature conditions onvertical density gradients in 0.5-inch particleboard.(From Heebink et al. 1972)

(M 144 407)

15

fairly well documented although not allparameters contributing to its developmenthave been fully studied. However, the lack ofinformation relative to its influence in themajor portion of the particleboard literature isdistressing and it is hoped that determining thevertical density gradients will become muchmore widely practiced by researchers in thenear future. Many observed particleboardproperties attributed to various processingparameters may be the result of unmeasureddifferences in the vertical density gradients.

Formaldehyde Release

The release of formaldehyde fumes fromparticleboard bonded with urea-formaldehyderesins is a problem which has been frequentlystudied. The human nose is extremely sensi-tive to formaldehyde; formaldehyde detectionis possible at 0.8 ppm (Walker 1964). The rateof formaldehyde release from particleboarddecreases with time from the press; tempera-ture and humidity during storage, shipping,and usage also influence the rate of release.Processing conditions, such as time in thepress, moisture content, adhesive content andreactivity, and press temperature all influencethe rate of formaldehyde release. The difficultyencountered in attempting to study factorsaffecting formaldehyde release is the lack of aquantitative analytical procedure for the lowconcentrations of formaldehyde normallyencountered and correlation of the test resultsto formaldehyde released from the particle-board in use. The procedures used to studyformaldehyde release from particleboardare many and varied. This discussion will be onthe results obtained by investigators usingvarious procedures in studying the effect ofprocessing variables on formaldehyde releasefrom urea-formaldehyde bonded particle-board. The release of formaldehyde fromphenol-formaldehyde-bonded boards is not aproblem, due to the irreversibility of thispolymer.

Neusser and Zentner (1968) have pre-sented a general description of the problemsencountered with urea-formaldehyde-bondedparticleboard. They believe it is necessary toapply an impermeable coating to particleboardif large amounts of the board are to be used inan enclosed environment.

Wittmann (1962) has pointed out thenecessity of free, or unreacted, formaldehydein urea-formaldehyde resins to allow the con-densation reaction to occur. Most of this for-maldehyde is consumed in the curing reactionin particleboard manufacture, but a portionremains in the voids and will gradually escape.

Even heating the board under vacuum to re-move the free formaldehyde in the voids willnot produce an odor-free particle board(Wittmann 1962). Formaldehyde is also re-leased at the press during manufacture(Petersen et al. 1972), but this formaldehyde isof less concern to the end-user than is theformaldehyde released from the cured resin.Residual formaldehyde is normally consideredto be the formaldehyde loosely bound in thefinished particleboard, which tends to split offthe polymer structure after the manufacturingprocess is completed.

Wittmann (1962) studied the residualformaldehyde released from particleboardsproduced with various resin contents, presstime, and catalyst contents, stored at variousconditions. Increasing the resin content from6 to 12 percent (resin solids on ovendry weightof wood) did not significantly increase theresidual formaldehyde. Particleboards pro-duced with increased press times, at constantammonium chloride levels, had decreasedresidual formaldehyde. Increasing the ammo-nium chloride content at constant press timesalso decreased the residual formaldehyde.Wittmann also found reduced levels of residualformaldehyde in particleboards containingincreasing amounts of ammonium hydroxideas well as increasing amounts of urea.Petersen et al. (1974) found formaldehydelevels reduced with ammonium salts but notwith the heavy metal salts such as magnesiumchloride or aluminum sulfate. Deppe and Ernst(1965) reported that formaldehyde release waslowered with increasing levels of ammoniumchloride. Ginzel (1973) also found a lowerresidual formaldehyde level in particleboardswith resins containing ammonium salts andurea. Plath (1968) attributed the reducedresidual formaldehyde found with ammoniumsalts to the reaction between the ammoniumgroup and formaldehyde which results inhexamethylenetetramine. Wittmann (1962)pointed out that addition of ammonia and urea,while helping to alleviate the residual formal-dehyde release, would result in slow resin cureand detrimental strength and hygroscopicproperties, respectively.

Plath (1966; 1967a; 1967b; 1967c; 1968)using a calorimetric method specifically forformaldehyde, studied the effect of press timeand temperature, ammonia level, and matmoisture content on residual formaldehydereleased from laboratory-produced particle-board. He found a decrease in residual formal-dehyde as the press temperature increased(125, 135, 145, and 155° C) and as the presstime increased (3, 6, 9, and 12 minutes). In-

16

creased press temperature was more effectivethan increased press time in reducing theresidual formaldehyde (Plath 1967a). Dataalso indicated that the rate of resin cure wasinversely related to the residual formaldehyde.At higher press temperatures the adhesive inthe surface regions should cure more quicklythan at lower temperatures, but the curing rateof the core will be much less increased by thehigher temperatures because of the watermigrating to the core. Hence, increasing presstemperature will increase the curing rate of theresin at the surfaces more than the resin in thecore and, since the residual formaldehydefrom the surface was reduced more than thatfrom the core with increasing press tempera-ture, it was concluded that rate of cure was asubstantial influence on residual formal-dehyde. The quantity of formaldehyde re-leased from the core was always greater thanfrom the surface. The degree of cure, as well asthe rate of cure, was also inversely related tothe residual formaldehyde release as indicatedby less residual formaldehyde at longer presstimes.

Addition of ammonium-containing hard-eners to the urea-formaldehyde resin alsoreduced the residual formaldehyde (Plath1967b). Hardeners with 10 to 30 percent am-monium reduced the formaldehyde to a levelcomparable to that obtained by increasing thepress time from 3 to 12 minutes. However, thestrength and hygroscopic properties were alsoreduced at the high ammonium levels. In-creasing mat moisture content increased theresidual formaldehyde (Plath 1967c); the in-crease was greater at the surface than at thecore.

Plath (1968) also found an increasedresidual formaldehyde content when thepanels were hot-stacked out of the press. Hot-stacking also resulted in a higher residualformaldehyde release from the surface thanpresent immediately out of the press. Plath(1968) attributed this to the migration of themoisture with dissolved formaldehyde fromthe core to the surface during equalization,although an equally valid reason would be mi-gration of formaldehyde from the highly con-centrated core region to the surface which ini-tially has a lower formaldehyde concentration.

Petersen et al. (1972) measured the for-maldehyde released during hot pressing of amat at 16 percent moisture content and foundit to be 50 percent greater than from a mat at10 percent moisture content. The formalde-hyde/urea molar (F/U) ratio also had a signifi-cant effect on formaldehyde released duringpressing. A resin with an F/U ratio of 1.8/1.0,

17

with a particle mat at 10 percent moisture con-tent, released 2.5 times more formaldehydethan did a resin with an F/U ratio of 1.4/1.0.Petersen et al. showed that increasing thesolid resin content did not proportionally in-crease the formaldehyde liberated duringpressing. Also, increasing the mat moisturecontent increased the formaldehyde releasedduring pressing more at the 5 percent resinsolids level than occurred at 12 percent resinsolids. More total formaldehyde was releasedduring pressing as press time and temperatureincreased. The board which released the mostformaldahyde in the press did not releaseas much residual fomaldehyde after storage,but the decrease in residual formaldehyde wasnot as great as the increase in formaldehydereleased during pressing. For example, with aF/U molar ratio of 1.8/1.0 and a press temper-ature of 160° C, an 8-minute press cycle re-leased, during pressing, approximately twicethe formaldehyde of the 5-minute press cycle.However, after storage, the residual formal-dehyde determined on the 8-minute board wasonly slightly less than the residual formalde-hyde in the board pressed for 5 minutes.Therefore, the total formaldehyde releasedwas greater for the board with the longer presscycle.

Formaldehyde liberated during the press-ing cycle for three particle species was alsodetermined by Petersen et al. (1973). Oakliberated the most formaldehyde, spruce theleast, and beech was midway between. This isthe only information found in which formalde-hyde was released by mats of different specieshas been determined.

Deppe and Ernst (1965) indicated theformaldehyde released at the press was re-duced as the amount of moisture evaporatedper unit surface area of the mat increased--i.e.,as the mat moisture content increased, theformaldehyde at the press decreased. Plath(1967c) found that residual formaldehydereleased from finished particleboards in-creased as the mat moisture content enteringthe press increased. The lower formaldehydeat the press reported by Deppe and Ernst(1965) could have been the result of a lowerdegree of reaction; hence the formaldehydewould weakly bond through ether linkages inthe press and would subsequently be releasedas residual formaldehyde (Plath 1967c). If thisis in fact the case, a compromise in mat mois-ture content is then required to insure that theformaldehyde fumes in the press area and theresidual formaldehyde released from theboard are both at acceptable levels.

Christensen (1972) found moisture con-

tent, temperature, and resin composition tohave the greatest effect on formaldehyde liber-ation. He studied the residual formaldehyde inparticleboards by heating the panel undervacuum and adsorbing the evolved formalde-hyde in water. The results were comparable tothose of earlier investigators in that formalde-hyde emission increased with board moisturecontent and decreased with increasing presstime. With the two resin content levels usedby Christensen (1972), the formaldehydeemission increased approximately linearlywith the resin content increase. This is contra-

dictory to the results of Petersen eta/. (1973) inwhich formaldehyde release did not increasein proportion to the increased resin content.

Christensen (1972) also presented a plotin which the logarithm of formaldehyde re-lease was linearly related to the particleboardtemperature (fig. 8). A plot of formaldehydeemission versus board moisture content waslinear at moisture contents below 9 percent;above 9 percent the formaldehyde emissioncontinued to increase, but at a decreasing rate(fig. 9). The concentration of formaldehydereleased at 70° C was the same as that released

Figure 8.--Influence of heating temperature on residual formaldehyde(From Christensen 1972)

(M 144 408)

18

at 3 percent moisture content. Therefore, innormal interior applications for particleboardit would appear that the moisture content ismore important than the temperature in con-trolling formaldehyde release.

Many factors appear to influence formal-dehyde release from particleboard; no single

processing parameter appears to be control-ling this phenomenon. Resin changes whichwill potentially reduce formaldehyde liberationwill also lengthen the press cycle, result inresin precure, or have adverse effects upon theproduct’s strength properties. Manufacturingcompromises are required and are normally

Figure 9.--Influence of particleboard moisture content on residualformaldehyde.(From Christensen 1972)

(M 144 409)

19

made on subjective instead of objective factorsdue to lack of a reproducible, meaningful,quick analytical procedure for measuringformaldehyde release from both a particle-board panel and a mat in the press.

Springback

Springback is the nonrecoverable thick-ness swelling that occurs when a finishedparticleboard, in equilibrium with a giventemperature and humidity, is exposed to anelevated humidity, or liquid water, and sub-sequently returned to the original humidityand temperature condition. Particleboard ismuch more dimensionally unstable in thethickness direction than would be expectedfrom the normal shrinking and swelling of thecomponent wood particles.

The additional thickness swelling in parti-cleboard exposed to moisture, greater thanthat normally expected for wood material, isdue to the release of residual compressivestresses imparted to the board during thepressing of the mat in the hot press. As shownearlier, compressive failure of at least a portionof the wood particles is required to produceparticleboard. The moisture content reductionwhile the mat is restrained in the hot pressreduces the plasticity of the wood and resultsin a semipermanent “set” of these compressivestresses. However, at any future date when themoisture content increases, the additionalmoisture will plasticize the wood and allowthese stresses to be relieved, allowing expan-sion in the thickness direction. The particle-board will also swell due to adsorption of waterby the hygroscopic particles of wood. The totalthickness swelling for particleboard exposedto moisture is the sum of these two compo-nents. Subsequent redrying will result in thick-ness shrinkage equal only to the shrinkage ofthe particles; none of the compressive stressrelease will be recovered (Neusser et al. 1965;Beech 1975). Additional wetting and dryingcycles will also result in further irrecoverablethickness swelling, but at a decreasing rate.

Because most of the literature reviewed inthe moisture and dimensional properties sec-tion does not differentiate between hygro-scopic swelling and compressive stress relief,the literature pertaining to springback studieswill be reviewed here.

Childs (1965) studied the effect of chipwidth, resin content, and board density onthe springback of urea-formaldehyde-bondedparticleboards made from sweetgum chips.As expected, springback increased with in-creasing board density and decreased withincreasing resin content. As the board density

increases, more compressive failure is im-parted to the particles which will springbackon rewetting and, as the resin content increases, the component particles are stabi-lized so springback will be reduced. Spring-back characteristics were independent of chipwidth.

Saito (1972) studied the springback ofboth commercial and laboratory particle-boards soaked in water to constant thicknessand subsequently oven-dried. Phenolic bond-ed boards were more stable than boards withurea-formaldehyde resins; melamine-formal-dehyde bonded particleboards were close tothe phenolics. Water temperature duringsoaking had no effect on springback, but thetemperature used for preliminary drying wasdirectly related to the amount of springback,because higher temperatures plasticized thewood to a greater degree.

Beech (1975), using a phenol-formalde-hyde resin, reported a significant lowering ofspringback when the phenolic adhesive levelwas increased from 9 to 12 percent. This wasattributed to increased bonding between theparticles. The portion of the irreversible thick-ness swell associated with glue failure wasfound to be slight as compared to thicknessswell as a result of stress release. Beech alsoreported that post-treatment with saturatesteam at 160° C was effective in reducing bothirreversible and reversible thickness swelling.Apparently the high temperature steamchanged the hygroscopicity of the wood andallowed stress release to occur.

Turner (1954) reported springback inphenolic-bonded particleboards to be gov-erned by resin content and panel density.However, his definition of springback includesthickness swelling as well as true springbacksince the reported values are the percent thickness increase after exposure to two watersoak-drying cycles. This study examined theproperties of homogeneous panels producedwith various particle configurations; all parti-cle types with 4 percent resin yielded boardswith less thickness swelling after two exposurecycles than seven-ply Sitka spruce plywood.This is one of the rate instances where particle-board has been reported to have lower thick-ness swelling than plywood; the high specificgravity (0.80, at unspecified moisture contentof the particleboard could account for thisresult.

Post (1961) reported that springbackincreased as flake thickness increased in urea-formaldehyde-bonded particleboard. Thiseffect was evident with all flake lengths al-though an interrelationship between flake

20

length and thickness was apparent (fig. 10).Four-inch-long flakes resulted in significantlylower springback at greater thicknesses.

Springback is a characteristic property offlat-pressed particleboard and is present in allboards. Adjustments in processing parameterscan be used to reduce the springback ten-dency, but due to the nature of the product itcannot be eliminated. Whenever particleboardis subjected to conditions which increase the

plasticity of the wood (high moisture, or bothhigh temperature and high moisture) the thick-ness swelling will exceed that predicted bymoisture absorption alone.

There have been various attempts to re-duce dimensional changes in the thicknessdirection of particleboard by reducing thehygroscopicity of the particles or reducing thecompressive set in the final panel. Theseattempts have been reviewed by Halligan

Figure 10.-- Thickness springback as influenced by flake dimensions.(From Post 1961)

(M 144 410)21

(1970) and do not presently appear to be com-mercially feasible. Steam and/or heat treat-ment of the cured particleboard appears toeffectively reduce springback in phenolic-bonded material (Heebink and Hefty 1969;Halligan and Schniewind 1972; Shen 1973; andBeech 1975). Heebink and Hefty reported thata 10-minute treatment with saturated steam at360° F was the most effective post-treatment toreduce springback. Post-treatment withoutthickness restraint under the above conditionshad no effect on the linear stability and only aslight reduction in bending strength. Becausespringback is the result of compressive failuresin the hot press, pretreatment methods to beonly one-half as effective in reducing spring-back as the most post-treatment on the finalparticleboard.

Halligan and Schniewind (1972) found adirect linear relationship between the spring-back and thickness swelling for urea-formal-dehyde-bonded particleboards exposed tovarious relative humidities. The close relation-ship between spr ingback and thicknessswelling led Halligan and Schniewind to con-clude there was no need to consider the twoproperties separately. However, because theirdata are based on only the first adsorptioncycle after pressing, it would be expected thatsubsequent sorption cycles would result inmuch less springback and hence a differentrelationship between the two properties.

Halligan and Schniewind (1972) also post-treated phenolic-bonded particleboard withsteam and found springback eliminated below90 percent relative humidity and reduced toless than one-third of untreated boards atrelative humidities above 90 percent. Post-treatment of urea-formaldehyde particle-boards with steam is not feasible due to thedegradation of the resin under heat and mois-ture conditions.

Shen (1973), in an attempt to producephenolic-bonded particleboard with lowerspringback and without steam post-treatment,adapted a hot press to allow direct injection ofsteam from the press platens into the mat. Thesteamcured particleboards had improveddimensional stability but came out of the pressat 10 to 15 percent moisture content instead ofthe normal 2 to 5 percent. The potentialadvantage of this method is that the resin iscured in a steam atmosphere which allowsrelaxation of a large portion of the compressivestresses induced in the particles duringpressing. However, this method does notappear feasible due to the high moisturecontent of the produced board and thedifficulty inherent in attempting to force the

steam through the mat.Suchsland and Enlow (1968) used a 2-

hour heat treatment at 425° F to allow relax-ation of the compressive stresses in phenolic-bonded particleboard. Boards subjectedto this treatment exhibited better surfacesmoothness, less thickness swelling and morestrength retention at high humidities than diduntreated boards. This severe treatment re-sulted in a slight reduction in the modulus ofelasticity, which was attributed to thermaldeterioration of the wood components.

Mikhailov and Ostapenko (1972) attri-buted the springback of particleboard to anonuniform moisture content distribution inthe thickness direction. They recommendedlengthening the press time and reducing themat moisture content to alleviate this tenden-cy. This practice should reduce the moisturegradient from surface to core in the final panel,but the moisture gradient normally would beexpected to contribute in only a minor way toparticleboard springback.

Neusser (1967) and Deppe and Ernst(1965) have used the term springback to char-acterize the thickness increase of particle-board immediately out of the press. This is notthe same phenomenon referred to here asspringback; this expansion occurs for mostparticleboard upon pressure release in the hotpress. Neusser (1967) used this initial expan-sion as a measure of the curing rate of variousadhesives; the more completely the resincured in the press the less was this immediateexpansion. Deppe and Ernst (1965) found thisinitial expansion to be a problem with veryshort press times at moisture contents above10 percent. This initial or immediate expansionis no problem in properly produced particle-board where the resin is sufficiently cured.

Springback resulting from residual com-pressive stress release appears to be mosteffectively controlled by a steam post-treat-ment. As a result of the production process forparticleboard, compressive stresses are incor-porated which, upon subsequent exposure tohigher moisture, result in excessive thicknessswell and subsequent application difficulties.Allowing the release of these residual com-pressive stresses by steaming results in a moredimensionally stable product for all applica-tions. Most studies concerned with thicknessswelling of particleboard, as will be shown inthat section, combine the thickness resultingfrom moisture adsorption with the springbackfrom stress release. Therefore, the thicknessswelling reported is only accurate for theinitial moisture content increase; all subse-quent moisture content cycles should have a

22

lower percentage thickness swelling, but thetotal thickness will continue to increase (Liiri1961).

Surface Characteristics

The particleboard literature is surprisinglydeficient in studies concerned with processingparameters and their effect upon the surfacesmoothness. Obviously, roughness in the plat-ens or cauls used to press the particleboardwill be reflected on the board surfaces, but thiscause of roughness is easily eliminated. Othersurface smoothness influences--moisturecontent, particle type, and other controllableprocess parameters--are touched on in theliterature reviewed here.

Plath (1971a) has reviewed the require-ments for particleboards laminated with plas-tic overlays and has concluded it is impossibleto develop criteria applicable for all laminatingapplications. In addition to a smooth and uni-form surface, moisture and density distribu-tions, chip characteristics, and bendingstrength are also important.

The lack of a simple and objective testprocedure to quantify surface properties re-mains a problem (Hunt 1965; Gatchell et al.1966; Kehr 1966; Neusser, Krames, Haidinger,and Serentschy 1969; Neusser, Karmes, andZentner 1969; Kehr and Jensen 1970; Neusserand Krames 1971). Optical methods with re-flected light are normally used. Gatchell et al.used the shadow projected by two parallelwires as a semiquantitative measure for sur-face roughness; the sharpness of the shadowsand the variability of the distance betweenthe two shadows were related to surfacesmoothness.

Kehr (1966), Kehr and Jensen (1969), andNeusser, Krames, and Zentner (1969) all re-ported improved surface characteristics as theparticle size decreased. Kehr and Jensen pointout that strength properties are reduced anddimensional changes increased with decreas-ing particle size. Neusser, Krames, and Zent-ner also mention this and further point out thedifficulty in blending and mat forming withfibrous surface particles, although this con-figuration does not adversely affect thestrength. They further report that soft to semi-soft, diffuse-porous hardwoods are more suit-

able for surfaces than pine which splinters.

Kehr (1966) also reported improved sur-face characteristics obtained with addition ofwax. This is possibly due to the inhibitingeffect of the wax on absorption of liquid waterwhich reduces the ensuing swelling. Hunt(1965) studied the effect of moisture on show-through of veneered particleboard and foundthe show-through reversible with moisturecontent. Show-through at high moisture con-tents disappeared at lower moisture contents.Also, compressed softwood particles moved3 to 8 times as much as uncompressed wood ofthe same species upon exposure to relativehumidity changes. Undoubtedly, this addi-tional movement was due to the springbackmentioned earlier.

Neusser, Krames, Haidinger, and Seren-tschy (1969) have shown the importance ofchip configuration on various particleboardproperties. Surface stability is a function ofthickness, planarity, and swelling character-istics of the chip, and surface porosity is afunction of the chip thickness, density, andcompressibility.

Shen (1974) reported an improvement insurface smoothness by repressing particle-board at 500 to 650° F under 250 to 600 poundsper square inch (Ib/in.2) for a few seconds.This treatment plasticized the surface andresulted in a smooth, nonabsorbent, andhardened face. The treated surface was im-proved with no detrimental effects, and report-edly bonded satisfactorily with commercialurea-formaldehyde laminating resins. Longertimes at high temperature and pressure re-sulted in some deterioration of the surface.

The increased application of particle-board in direct surface finishing will result inincreased emphasis on surface quality. Quan-titative methods of assessing surface qualityhave to be developed and more emphasisplaced on techniques for producing a uniformsurface during manufacture. The variousmethods used in the above literature illustratethe need for standard test methods; eachinvestigator has developed his own test andcomparisons between his criteria for accept-able surfaces and those of another investigatorare impossible.

PROCESSING PARAMETERSResin efficiency, adhesive type and level,

furnish, and pressing conditions are the pro-cessing parameters examined here for theireffects on particleboard properties,

Resin Efficiency

Resin costs are a major portion of the totalmanufacturing expense of particleboard.Consequently, optimum board properties with

23

minimum adhesive consumption is the goal ofall manufacturers; this concept is referred toas resin efficiency. Continuous, cured glue-lines are not obtained between individualparticles because the resin is normally sprayedon the particles in discrete individual droplets.Not all particles are in intimate contact withneighboring particles over their entire surfaceafter mat consolidation in the press. Adhesivedeposited on surfaces adjacent to interparticlevoids will not contribute at all to bonding.Therefore, minimizing the interparticle voids--increasing board density--will normally im-prove the board properties at constant resincontent.

Resin efficiency in particleboard is diffi-cult to study directly due to the problemsencountered in quantitatively measuring theamount of liquid adhesive sprayed on the sur-face of irregularly shaped particles. Theamount of resin solids normally used in parti-cleboard manufacture is only a fraction of thatused for plywood and other solid wood gluing.Meinecke and Klauditz (1962) have discussedthe theoretical glueline thickness obtained ifthe entire particle surface is coated with a resinsolution of 50 percent total solids. Even with achip 0.03 inch thick sprayed with 50 percentresin solution to 8 percent solids on a dry woodbasis, the resulting film of water and resinwould be only 25 µm thick (Meinecke andKlauditz 1962).

The adhesive is normally applied to thecomponent particles by spraying fine dropletsof the resin solution on continuously agitatedchips. This agitation allows some rubbingaction between adjacent chips to further dis-perse the adhesive. Resin efficiency, as mea-sured by improved physical and strengthproperties, increases as the droplet size fromthe spray nozzle decreases and the degree ofdispersion of the adhesive solution increases(Carroll and McVey 1962; Meinecke and Klau-ditz 1962; Kehr et al. 1964; Lehmann 1970;Christensen and Robitschek 1974). Therefore,the finer the adhesive spray, the better is thepossibility of maximizing the fraction of parti-cle surface covered.

In addition to a well-dispersed spray,increasing the rubbing action between indi-vidual particles as they are agitated in theblender enhances the possibility for resintransfer from one particle with excessive adhe-sive to another particle deficient in adhesive.Christensen and Robitschek (1974) reportedhigher internal bond values for particleboardsproduced from furnish exposed to extendedpost-blending agi tat ion as compared toboards from furnish without the post-blending.

They also showed, by mixing two screenedfractions of chips with and without urea-formaldehyde adhesive, that interparticle resintransfer occurred during agitation. Subse-quent screening and nitrogen determinationon the initially resin-free fraction showed thatresin transfer to this fraction from the otherfraction had occurred in the blender. Kehr et al.(1968) also showed that resin transfer fromneighboring particles in the blender contri-buted to improved particleboard properties.

Carroll and McVey (1962) in a similarstudy found only slightly improved propertieswith extended mixing time. They sprayed therequired adhesive on only 50 percent of thefurnish, added the remaining furnish, andmixed the total in a rotating drum blender for30 minutes. The internal bond and modulus ofrupture properties for the resultant particleboard were less than 5 percent higher thanthose in which the post-blending of the twofractions was only 5 minutes. This illustratesthe relatively limited value of trying to improvethe resin distribution by prolonged post-mixing; much better results can be obtained byoptimizing the original blending.

Meinecke and Klauditz (1962) reported adrop in tensile strength both perpendicularand parallel to the surface as the resin dropletsize increased from 35 to 100 µm (fig. 11).Droplets smaller than 35 µm did not improvethe tensile strengths. Because sections exa-mined under the microscope showed onlyrelatively few areas without adhesive, improve-ments in strength by further decreasing thedroplet size would be expected to be small.Meinecke and Klauditz also stated that indus-trial blenders normally operate with a dropletdiameter around 80 µm and, consequently,improved board properties could be obtainedcommercially by reducing droplet size. Carrolland McVey (1962) also reported lower internalbond and modulus of rupture values for indus-trial boards as compared to identical labora-tory boards. This was attributed to poorerindustrial blending; when laboratory-blendedmaterial was pressed in the industrial press,the particleboard was equal in strength to thelaboratory board. The industrial blender didnot blend the furnish and resin as well as thelaboratory blender, possible due to the largerdroplet size in the industrial blender (Meineckeand Klauditz 1962).

Carroll and McVey (1962) do not agreewith Meinecke and Klauditz (1962) concerningthe value of a continuous film on the particlesurface. Carroll and McVey found extremelylow internal bonds (30 to 40 lb/in.2), even at 7percent resin content (resin solids on ODwood), for a phenolic adhesive that flowed into

24

a continuous film. However, when they used aparticleboard phenolic resin that did not havethe high flow properties, excellent internalbonds were obtained (180 lb/in.2). They be-lieved the thin film which resulted when the lowresin content spread ino a continuous filmformed a starved glue joint. Carroll and McVeybelieved a large number of spot welds uni-formly distributed throughout the mat wouldbe superior to a continuous film.

Lehmann (1965) obtained optimum boardproperties at a given resin content when auniform distribution of resin was applied byfine atomization. Modulus of rupture, modulusof elasticity, internal bond, and dimensionalstabilization all improved when the dropletsize of the urea-formaldehyde adhesive de-creased. Burrows (1961), using a phenolicbinder, found modulus of rupture to be inde-pendent of the atomization level. The atomi-zation which produced the best internal bondwas related to the adhesive content; fine atomi-zation at 2 percent and coarse atomization at6 percent adhesive content produced thehighest internal bonds (Burrows 1961).

Spraying with adhesives of lower totalsolids has been shown to increase the resinefficiency in particleboard (Carroll and McVey1962; Carroll 1963). This will result in less resinsolids in each droplet on the particle surfacebut more droplets per unit of particle area.

This would appear to coincide with the theoryof Meinecke and Klauditz (1962) and Marian(1958) that a continuous resin film is the opti-mum condition for particleboard production. Alow level of water dilution significantly lowersresin viscosity but also increases the amountof water added to the furnish. The additionaladhesive volume may lengthen the blendingtime or force the blenders to operate with aneven larger droplet size (Carroll and McVey1962; Meinecke and Klauditz 1962). Kehr et al.(1964) have shown the detrimental effect onproperties with increasing “throughput pernozzle.” As the “throughput per nozzle” in-creased, the blending time, dimensional stabi-lity, and strength properties of the particle-board all decreased. Lower “throughput pernozzle” resulted in a more uniform distribu-tion, due to the smaller droplet size, and allproperties subsequently improved.

Pecina (1963) measured the dimensionand distribution of resin droplets on particlesurfaces by adding fluorescent dye to theresin, photographing the particle under UVradiation, and determining the fraction of sur-face area covered with resin. His paper has noinformation comparing different blendingtechniques, only development of the testprocedure.

Lehmann (1970) added a dye to a urea-formaldehyde adhesive and determined the

Figure 11.--Influence of resin droplet size on tensile strength perpen-dicular and parallel to particleboard surface.

(M 144 411)25

relative area of resin coverage on particleswith three different resin contents (2, 4, and 8percent) and two atomization levels (fine andcoarse) by measuring the reflection of thedyed resin. Fine atomization was found toresult in better particle coverage at all resincontents and particle sizes. Resin efficiencyalso increased with board density but this wasattributed to better bonding efficiency be-tween particles, not to resin distribution.

Lehmann (1968) also added a fluorescentdye to urea-formaldehyde and phenol-formal-dehyde adhesives to study the interparticlebonding in flakeboard. Two atomization levelswere used and the location and distribution ofthe adhesive in the finished particleboard wasdetermined by photomicrographs under ultra-violet (UV) light and correlated to the result-ant physical properties. Resin applied as afine spray always produced better propertiesthan the same resin applied as a coarse spray.Photomicrographs revealed that continuousgluelines were produced with 6 percent urea-formaldehyde (resin solids on ovendry wood--OD--wood) and almost continuous gluelineswith only 4 percent of the phenol-formalde-hyde (resin solids on OD wood) when bothwere applied as a fine spray. Lehmann (1968)also photographed boards made with diluted,low-viscosity urea-formaldehyde resin appliedin a fine spray. No penetration of the resin intothe particles was observed, and with the ex-ception of the internal bond, the physicalproperties were not decreased. Lehmann(1968) also used a diluted, low-viscosityphenol-formaldehyde resin with an airlessspray; results were the same as those obtainedwith the diluted urea-formaldehyde resin. Thisis contradictory to the results of Carroll andMcVey (1962) with a high flow phenol-formal-dehyde resin. Carroll (1963) also reported thatresin efficiency for urea-formaldehyde adhe-sives is lost as a result of apparent migrationof low-molecular-weight components into thewood substance.

Lehmann (1965) reported that airlessspraying did not distribute the resin as wellas the air sprayer; agitation of the particles wasless without the excess air. There appeared tobe no large significant differences in the boardproperties attributable to spraying method.Fine atomizat ion of the resin improvedstrength and dimensional stability to a greaterextent than did spraying method, sprayingtemperature, or solids content of the resin(Lehmann 1965). Best results were obtained byspraying at room temperature with a high resinsolids content. This is contradictory to thefindings of Carroll and McVey (1962) and

26

Carroll (1963), in which better resin efficiencywas obtained by spraying diluted resins. How-ever, the decrease in properties with decrea-sing solids content found by Lehmann (1965)was, in all cases, relatively small.

One additional problem encounteredwhen applying resin to wood particles is theparticle size distribution normally found inall samples of wood particles: Resin treatmentlevels in particleboard are normally based onresin solids as a percent of the ovendry woodweight. However, the resin function in particle-board is to bond individual particles into aconsolidated mat by establishing an adhesivebond between the surfaces of adjacent parti-cles. As the particle size at a constant weight ofovendry wood decreases, the total surfacearea increases. Therefore, to obtain the sameresin solids level per unit surface area, the solidresin content, expressed as a percentage of theovendry wood weight, has to increase. Theexperimental difficulty of quantitatively deter-mining the surface area of a normal particlefurnish, consisting of various configurationsand particle volumes, is enormous; conse-quently, most researchers report the resin levelon a weight basis (Turner 1954). Duncan(1974) has recently shown that larger particlesreceive proportionately more resin than thesurface area they contribute to the furnish. Hehas developed a mathematical relationshipbetween screen analysis data and relative sur-face area to show that the resin weight per unitof surface area is higher for larger particlesthan for small particles.

Post (1958, 1961), using surface areas ofaccurately cut flakes as the basis for resintreatment levels, found only moderate in-creases in bending strength when the resintreatment increased from 3.27 to 13.07 gramsof resin solids per square meter of surfacearea. Post (1958) stated that using particlesurface area rather than ovendry wood weightas the basis for resin application appears to bea reasonable method of separating effects ofparticle geometry. This method was satisfac-tory for his controlled particle geometry, butin a normal furnish where there is a distributionof particle configurations, enormous difficul-ties are encountered in accurately determiningthe true surface area.

Maloney (1970) studied the resin distri-bution in layered particleboard by varying theresin level in the component layers while main-taining a constant resin content in the board.All of his data is on an ovendry wood weightbasis and, consequently, it is not surprisingthat the data show that fine particles receiveda disproportionate share of the resin. The sur-

face fines attained a 12 percent resin treat-ment as opposed to a 0.9 percent treatment forthe core when the multi-layer board furnishwas blended together. When 80 percent of thetotal resin was applied to only the core (larger)particles, followed by the addition of surfacefines and the remaining resin, the surfaceparticles had only a 4.0 percent resin treat-ment while the core resin content was 7.7 per-cent. Maloney (1970) also determined the sur-face area of 25 representative particles of eachlayer used for the multi-layer particleboard.Resin contents were then calculated on aparticle surface area basis. These resultsshowed that the resin content of the surfacelayer was slightly less than double that of thecore when all layers were blended together.For the case where 70 percent of the resin wasinitially applied to only the coarser particles,the core resin content was approximately 3times greater than the surface on a weight perarea basis. Maloney concluded that about 8percent resin solids by weight on the surfacefines is sufficient for adequate bonding.

Maloney (1970) further concluded that aneven distribution of resin by surface area is notthe optimum condition for the production ofmulti-layer boards with high density surfacesand low density cores. He believed optimumconditions existed when a uniform distributionof resin by weight of particles was approached,because the high-density surfaces resulted inefficient usage of the applied adhesive; moreadhesive per unit area was required in the lowdensity core where the adhesive was not usedefficiently. This is in direct contrast to Dun-can’s results (1974).

Lehmann (1970) reported data, obtainedby screen analysis and reflectance measure-ments with a urea-formaldehyde adhesivecontaining a fluorescent dye, that show thesmaller particles always have a larger percent-age surface area coverage than the coarserparticles when the two are blended together,regardless of whether a fine or coarse spray isused. Therefore, the fines had a higher resincontent on an ovendry wood weight basis butno data were presented as to the resin contenton a surface area basis.

lstrate et al. (1964) have used a slender-ness coefficient to describe the particle shape.Particles with high slenderness coefficients(length/thickness) could be satisfactorilyglued into particleboards without requiring acontinuous layer of adhesive, as was neces-sary for the particles with low slendernesscoefficients. Thicker particles had less surfacearea per unit of weight but required more adhe-sive than did thinner particles. lstrate et al.

attributed this to the lack of contribution fromthe particle edges to the bonding mechanism.They reported a slenderness coefficient be-tween 60 and 120 to be the optimum, depend-ing upon the location of the chip in the particlemat. This agrees with the findings of Post(1961) in which the rate of increase in boardstrength decreased above a length/thicknessratio of approximately 100.

There appears to be no definitive literatureavailable which conclusively proves exactlythe location and distribution of adhesive inparticleboard. On a weight basis, smaller parti-cles consume more than their share of theadhesive, but opinions differ as to whether theconsumption of adhesive on an area basis isgreater or less for the smaller particles. Differ-ences also exist as to whether it is desirable tohave equal resin treatments, either on a weightor area basis, for different particle sizes. Fur-ther research on this problem is required.

Adhesive Type and Level

Adhesives currently used in particleboardmanufacture consist of urea-formaldehydeand phenol-formaldehyde polymers. Phenol-formaldehyde usage is limited in this countrysince most particleboard is used in interiorapplications and urea-formaldehyde adhe-sives, which enjoy a large cost advantage, areentirely adequate for this use. The increasedemphasis on structural-grade particleboardshould provide impetus for the further devel-opment and utilization of phenol-formalde-hyde or other durable adhesives in particle-board.

All investigators are unanimous in theirfindings that as the adhesive content increasesall strength properties of the resultant particle-board increase. Consequently, this review willbriefly cover adhesive variables such as reacti-vity, synthesis conditions, and type, as theyhave been reported to affect particleboardproperties.

Deppe and Ernst (1971) produced parti-cleboards with diisocyanate binders andobtained modulus of rupture (MOR) values at2.5 percent diisocyanate comparable to thoseobtained at 3 percent phenol-formaldehyde(adhesive solids on OD wood). Press times forthe diisocyanate are much shorter than therelatively long times for phenol-formaldehydebut the diisocyanate-bonded material stuck tocauls and other metal surfaces during manu-facture. Schorning et al. (1972) attempted todevelop new particleboard adhesives, bothunder alkaline and acidic curing conditions.Roffael and Rauch (1971a, 1971b, 1972b,

27

1973a, 1973b, 1973c, 1974) have studied thepotential of sulfite liquor from the pulpingprocess as an adhesive system for particle-boards. They have also incorporated bothphenol-formaldehyde resoles and novolacswith the liquor and various heating post-treat-ments to improve properties of the resultantboards. High temperature treatment of diiso-cyanate-bonded boards improved dimen-sional stabi l i ty but lowered mechanicalstrength (1973c).

Anderson et al. (1974a, 1974b, 1974c) havestudied the bark extracts of various species forpotential particleboard adhesives. Bark extractas a possible adhesive source is an extremelyactive research area and a review of this fieldwill not be attempted here. It does appear thatbark extract offers a very good potential butfurther research is necessary to fully exploit.this material.

Hse (1974a, 1974b, 1974c) has recentlyreported on a large study of urea-formalde-hyde adhesives synthesized under variousacidic and alkaline conditions with a number offormaldehyde/urea molar ratios at a series ofdifferent reaction concentrations and temper-atures. Adhesives with a high methylol contentproduced boards with higher internal bond,modulus of rupture values. Resin shrinkagein curing was negatively correlated with theabove properties but free formaldehyde con-tent was linearly and positively correlated withthese board properties. Hse (1974b) also madea series of urea-formaldehyde adhesives inwhich the methylolation pH was varied be-tween 7 and 10 and the polymerization pH wasvaried between 3.8 and 5.8. Methylolationunder neutral or slightly alkaline conditionsproduced adhesives which resulted in particle-boards with higher internal bond and modulusof rupture values. Variations in the acidity hada smaller effect on the resins produced but theoptimum was obtained when polymerizationwas accomplished at pH 5.8 following methyl-olation under neutral conditions. Hse (1974c)also found sodium hydroxide to be muchbetter for catalyzing the methylolation reac-tion than either hexamethylenetetramine ortriethanolamine. With sodium hydroxide asthe methylolation catalyst, polymerizationwas possible with acetic acid, hydrochloricacid, ammonium chloride, and phosphoricacid, but with other methylolation catalysts theacid catalysts which produced satisfactoryresins were limited.

Temperature is an important parametercontrolling resin reactivity in particleboardproduction. Because heat has to migrate fromthe platens to the mat center in heated platen

28

presses, the longer the press cycle the higherthe core temperature and more completelycured is the adhesive, as indicated by internalbond and thickness swell tests. Moisturemigration to the core is also confounded withheat migration, as pointed out in the section“Vertical Density Gradient.” Consequently, thetemperature of the core does not rise signifi-cantly above 100° C until the liquid water hasevaporated from the mat edges. Press temper-ature must be limited to prevent board surfacesin contact with the platens from being de-graded; therefore, increasing the core temper-ature is normally accomplished by length-ening the press cycle. Because phenol-formal-dehyde adhesives are not as reactive as theurea-formaldehyde adhesives, higher boardtemperatures are required, and this is reflectedin longer press; cycles for phenolic-bondedparticleboard (Carroll 1963; Lehmann et al.1973).

Roffael et al. (1972) found that raising thecore temperature to 220° C, either by highfrequency heating or extended press cycles,resulted in improved dimensional stabilityand internal bonds for phenol ic-bondedScotch pine particleboard.

Turner (1954) used a suspension of water-insoluble phenol-formaldehyde adhesive inhis study of particle size and shape, and gen-erally found that modulus of rupture in-creased, but at a decreasing rate as the resincontent increased from 2 to 4 to 8 percent.Lehmann (1974) found an adhesive level of atleast 1 pound of solids per 1,000 square feet ofparticle surface was required to produceoptimum board properties with phenolic-bonded flakes 2 inches in length. This wasapproximately 5 percent adhesive (resin solidson OD wood) for this flake configuration.Lehmann et al. (1973) produced adequate 0.5-inch thick (40 Ib/ft3) phenol-formaldehyde-bonded particleboard with a 2-minute presscycle at 375° F by adding moisture to the matand catalyzing the adhesive. With these con-ditions and a 15-second press closing time, thecore temperature was above 250° F in less thanone minute. This they attributed primarily tomoisture distribution and not press temper-ature or catalyst.

Kimoto et al. (1964) using urea-formal-dehyde adhesive levels of 8, 10, and 15 percent(resin solids on OD wood), generally foundonly slight improvement in both strength andswelling properties with 15 percent as com-pared to 10 percent adhesive. The most no-table exception to this was a greater than 50percent reduction in thickness swelling forlauan particleboard when exposed to satu-

rated water vapor. The average particle dimen-sions used in this board were 2.172, 0.179, and0.088 cm for the length, width, and thickness,respectivley. None of the properties decreasedas the resin content increased, but manyproperties were equal at either two or all threeof the studied resin levels. Press times used inthis study were exceptionally long (18 min.),the press temperature was only 130° C, and theparticleboards were approximately 0.5 inchthick.

Post (1958) found that resin content hasonly a slight effect on the modulus of rupture(MOR) values with urea-formaldehyde adhe-sive spreads of 3.27, 6.54, and 13.07 grams persquare meter (gm/m2) on flakes with a numberof different configurations. Particle geometrywas much more significant than adhesive con-tent. Lehmann (1970) using a urea-formal-dehyde adhesive at 2, 4, and 8 percent solids(resin solids on OD wood) found only smallincreases in MOR, modulus of elasticity(MOE), and thickness swelling when adhesivecontent increased from 4 to 8 percent, but theinternal bond continued to increase substan-tially in this region.

Hann et al. (1963) reported improvedparticleboard durability when the adhesivelevel was increased from 3 to 6 percent and 4 to8 percent for phenol-formaldehyde and urea-formaldehyde adhesives, respectively. Theyattributed this to the increased resistance ofthe board to springback with the additionaladhesive content, Lehmann (1974) also re-ported much lower springback after acceler-ated aging when the phenol-formaldehydeadhesive content was increased to 3 from 9percent (resin solids on OD wood).

Schuler (1974) made particleboard fromchipping residues with seven levels of urea-formaldehyde adhesive ranging from 2 to 12percent (resin solids on OD wood). No im-provements were evident in MOR and MOEwhen the adhesive level was increased above 5percent. For both properties, the 12 percentadhesive content was below the 10 percentlevel at all particleboard densities. The percentthickness swell, after both 2 and 24 hours watersoak attained a minimum at 10 percent andwere both higher at 12 percent adhesivecontent.

Clad (1967) found that high moisturecontent in the particle mat retarded curing ofphenol-formaldehyde adhesives, whereas alow moisture content reduced the flow of theresin which reduced board quality. He alsoreported no improvements in board qualitywhen the adhesive content increased above 12percent.

Addition of a phenolic impregnating resinto the particles has been studied as a possiblemethod for improving particleboard properties(Browne et al. 1966; Haygreen and Gertje-jansen 1971). Haygreen and Gertjejansen(1972) made wafer-type particleboard with3 percent phenol-formaldehyde adhesiveand added 7 percent of an impregnatingphenol-formaldehyde resin. Much better prop-erties were obtained when the impregnatingresin was added to green flakes instead ofto the dry flakes. Thickness swelling andspringback was significantly improved as wasthe strength and surface smoothness after onevacuum-pressure-soak cycle. Browne et al.(1966) used phenol-formaldehyde impregna-ting resin in both urea-formaldehyde- andphenol-formaldehyde-bonded particleboards.Impregnation reduced the irreversible par-ticleboard swelling by either reducing thehygoscopicity or bulking the structure or,more likely, a combination of both. Obviously,the cost of producing this impregnatingmaterial severely limits the applicability.

There appear to be no consistent dataavailable which indicate a particular adhesivelevel is optimum. Much of this inconsistencyis due to the lack of uniformity in stating theadhesive content levels. Adhesive contentsbased on the ovendry wood weight are ex-tremely dependent upon particle configura-tion; however, the experimental difficultiesinherent in determining the particle surfacearea per unit wood weight limits the usefulnessof calculating spreads per unit of particle sur-face area. Consequently, each particle config-uration will have an optimum adhesive contentdependent upon the desired panel productsand the economics of production. Multi-layerparticleboard with different adhesive levels inthe core and surface layers is one widely-usedtechnique to obtain satisfactory board proper-ties with the most economic use of adhesive.

Furnish

Most particleboard consists of over 90percent ligno-cellulosic material on a dryweight basis; consequently, the properties ofthis raw material have a significant effect onboth the manufacture and the physical proper-ties of the final product. This section will brieflyreview the available literature pertaining to theeffect of wood species and other ligno-cellulo-sic-based materials on properties of resultantparticleboards. Specific effects on strengthand dimensional properties will be more fullypresented in these respective chapters.

The quality of wood furnish in particle-board manufacture IS controlled by the species

29

properties such as density, acidity, extractivecontent, and machinability. The effect of wooddensity has been extensively studied and thegeneral conclusion reached that species oflower density than the final particleboarddensity are required to economically producehigh quality particle board (Larmore 1959;Vital et al. 1974; Hse 1975). This is believed tobe related to the compressive failure pre-viously mentioned.

Hse (1975) made particleboards of threedensities with nine hardwood species usinglarge flakes (3- by 3/8- by 0.015 inch) and aphenol-formaldehyde adhesive. He reported acompaction ratio (board density/wood den-sity) of 1.25 required to obtain bendingstrength necessary for exterior particleboardapplications. Thus, low-density species arerequired for low- to medium-density particle-board and high-density woods can only behigh-density particleboard. However, thick-ness stability is inversely related to boarddensity, severely limiting the potential ofhigh-density particleboard.

Anthony and Moslemi (1969) reportedsatisfactory strength and dimensional proper-ties for hickory flakeboard at 44 to 48 Ib/ft3

(unspecified moisture content). Unfortu-nately, they do not state the density of theoriginal hickory but most hickory species usedin particleboard of the density would result incompaction ratios significantly less than 1.25.

Vital et al. (1974) produced three-layerparticleboards from four exotic hardwoodspecies of widely different densities. Boardswith all combinations of one, two, three, andfour species were made at each of two com-paction ratios, 1.2 and 1.6. The properties ofthe particleboards with mixed species wereprimarily dependent upon the average densityof the species mixture used and the bendingstrength could be predicted from the speciesmix. They also found that, with boards at equaldensities, MOR and MOE were directly relatedto compaction ratio. This was attributed to thegreater volume of wood compacted and theincreased interparticle contact, which, com-bined, produced better bonding.

Other species variables are of only minorimportance in determining species suitabilityfor particleboard. Interior particleboard isproduced with urea-formaldehyde adhesiveswhich cure under acidic conditions. Theacidity of the furnish should be monitored toallow proper adjustment of the catalyst level toprevent unduly long press cycles or precure ofthe mat before reaching the proper consolida-tion. Frequent species changes in the furnishshould be avoided.

Extractives can also initiate or inhibit resincure, but this effect can be corrected by cat-alyst adjustment. Non-hygroscopic extrac-tives can also serve as a dimensional stabiliz-ing agent, by either bulking the wood struc-ture or inhibiting the adsorption of moisture.

There have been a number of studies todetermine the potential of incorporating otherligno-cellulosic materials into particleboard.Rootwood (Howard 1974), sycamore saplings(Rice 1973), bark (Dost 1971; Gertjejansen andHaygreen 1973; Anderson et al. 1974a, 1974b,1974c), and forest residues (Heebink 1974;Lehmann and Geimer 1974) have all beenstudied as potential particleboard furnishes.

Slash pine rootwood particleboards madewith large flakes and a phenol-formaldehydeadhesive had slightly better bending strengththan, and twice the internal bond of, boardsproduced from stemwood (Howard 1974). Thelower specific gravity of the rootwood resultedin a greater compaction at the same particle-board density, hence the better properties.However, grain deviation in the rootwood, aswell as the greater compaction, resulted inpoorer dimensional stability for these boardsthan was found in the stemwood boards.

Three-layer redwood particleboards,incorporating 0, 10, 20, and 30 percent red-wood bark, were studied at three urea-formal-dehyde resin levels (Dost 1971). Results showsignificant decrease in MOR, MOE, internalbond, and dimensional stabi l i ty with in-creasing bark content at all resin levels. The24-hour water absorption decreased as barkcontent increased, reflecting the lower hygro-scopicity of the bark. Experimental difficultieswere also encountered in blending the mater-ial; consequently, a part of the decreasedproperties may be due to a lack of adequateresin distribution.

Anderson et al. (1974a; 1974b; 1974c), in aseries of studies on bark extract-bondedparticleboard and particleboard with a barkcore, have shown the potential of incorpor-ating limited quantities of bark in extract-bonded exterior particleboard. Particleboardsmade with 100 percent bark had very low MORand high linear expansion values, but particle-boards with a bark core and wood surfaceswere satisfactory for Ponderosa pine and whitefir furnish. It remains to be seen if this barkcore particleboard is suitable for general exter-ior applications.

Gertjejansen and Haygreen (1973) studiedthe effect on the resultant physical propertiesof incorporating aspen bark in the furnish ofwafer- and flake-type particleboards. A phe-nol-formaldehyde adhesive at 3 and 8 percent

30

was used for the wafer and flake boards,respectively. They concluded that the entiretree bole could be utilized for either particle-board type, provided. the bark was removedfrom the butt log. The linear stability was dras-tically reduced when the butt log bark wasincluded but bark from upper logs had littleeffect.

Rice (1973) showed the possibility ofusing young, fast-grown sycamore saplingsfor a flake-type particleboard. The only diffi-culty noted was the filtering of the bark finesto the lower surface of the mat. Rice (1973)believes this layering would be considerablyreduced in mechanized mat formers and thatwhole-tree conversion of fast-grown sycamoresaplings into flake-type particleboards of avariety of densities and resin contents ispossible.

Chen et al. (1972) made particleboardfrom spruce with 100 percent total tree mater-ial (called biological surface mass) as well asusing this material in combination with con-ventional debarked spruce furnish. Boardswere of 8 percent urea-formaldehyde resin(resin solids on OD wood) and 0.5 percent wax(was solids on OD wood). Particleboards madewith 100 percent total tree material did notmeet standard bending specifications, butmixing this material with debarked spruceparticles on an equal weight basis producedacceptable particleboards.

Stayton et al. (1971) reported that accept-able particleboards with 0.5-inch-long flakescould be produced from mixtures of high-den-sity paper birch and low-density aspen with 8percent urea-formaldehyde and 6 percentphenol-formaldehyde adhesives (resin solidson OD wood) at average densities of 45 and 46Ib/ft3, respectively. All-aspen boards weresuperior to boards containing birch but an all-birch board retained 80 percent of the strengthof the all-aspen board. Stayton et al. (1971)reported the surface quality decreased withincreasing birch but they believed acceptableparticleboard could be produced with varyingmixtures of birch and aspen flakes. Unfortun-ately, the density of the two species was notincluded in the above report.

Heebink (1974) and Lehmann and Geimer(1974) have produced structural, phenolic-bonded particleboard from forest residues oflodgepole pine and Douglas-fir, respectively.Heebink reported that structural particleboardcould be produced from lodgepole pine forestresidue greater than 3 inches in diameter. Healso showed that a mixture of live and deadlodgepole pine forest residue would produceacceptable part ic leboard for over laying

31

applications.Lehmann and Geimer (1974) reported

significantly lower panel properties whenmore than 12 percent bark was included in thefurnish for particleboard produced fromDouglas-fir forest residues. Panels composedentirely of tops and branches or less than 75percent sound wood had even lower proper-ties. The percent retention of the originalproperties was also determined after ASTMaccelerated aging exposure. Excessivestrength loss as a result of this exposureoccurred in panels with 25 percent bark, 100percent branchwood, and 100 percent de-cayed wood; panels with lower percentages ofthe above residues did not exhibit strengthlosses signi f icant ly higher than controlboards.

Heebink et al. (1984) compared the qualityof par t ic leboard produced f rom planershavings obtained with various planer settings.Planer shavings normally result in lowerquality particleboard than that obtained fromflakes. Consequently, this research was anattempt to improve the applicability of planershaving for particleboard furnish by planerdesign modifications. Many of the changeseffective in producing higher quality flakesreduced the quality of the planed surface orrequired substantial planer changes. However,the single most important variable whichimproved the part ic leboard qual i ty wasplaning green stock; evidently the green par-ticles did not crack as easily across the grain.

Lutz et al. (1969), using an experimentalblanker to surface softwood lumber across thegrain, showed the improved quality of theresultant flake, as opposed to planer shavings,for particleboard furnish. The flakes producedby this cross-grain cutting method resulted inparticleboard properties equal to flakes from adisc flaker. These crossgrained flakes were notbroken and curled along the grain as is com-mon for planer shavings (Heebink et al. 1964).

Stewart (1970) reported on the highquality surfaces and flakes produced by cross-grain planing of hard maple and the widevariation of chip type available with a helicaledge tool. Stewart and Lehmann (1973) pro-duced particleboards with cross-grain, knife-planed flakes of four hardwood species; theboards showed exceptional linear stability.Stewart and Lehmann (1974) also showed theexcellent strength and dimensional stabilityattainable in urea-formaldehyde-bonded par-ticleboards from flakes produced by cross-grain cutting with helical cutters.

The material from which high-qualityparticleboard can be economically produced

is limited to those ligno-cellulosic materialsof adequate inherent strength which can bereadily broken down and glued together with-out unduly destroying the native strength.Cost, as influenced by resin content, is thecontrolling factor. Many ligno-cellulosicscould theoretically be used for particleboard,but the higher adhesive contents required toobtain a satisfactory product are presentlyprohibitively high.

Pressing Conditions

The pressing operation is obviously anextremely critical step in particleboard pro-duction. It is during this step that many of thephysical properties are determined, especiallythose properties influenced by the verticaldensity gradient. Mat moisture content, pressclosing speed, and press time and temperatureare the most significant pressing conditionsaffecting the properties of particleboard.

Mat Moisture Content

The mat moisture content is an extremelycritical factor not only for the total press time,but also in development of the vertical densitygradient. As discussed earlier, the surfacemoisture evaporates and migrates to the matcenter when the hot platens contact the mat.This speeds the transfer of heat to the corewhich allows the adhesive to react more quick-ly than if the heat moved by conductionthrough the wood and air spaces. However,excessive moisture migration to the coreimposes the requirement of excessively longpress cycles to allow removal of moisturethrough the edges to prevent delamination ofthe board upon pressure release. Excessivemoisture also interferes with the chemicalreaction of condensation polymerization, bywhich urea-formaldehyde and phenol-formal-dehyde adhesives harden. Moisture at the matsurface also reduces the compressive strengthof the wood and results in surface densifi-cation from compressive failure as the press isclosed. Excessive surface densification resultsin low-density cores for a given average boarddensity. The bending strength and tensilestrength parallel to the board surface willincrease as this densification occurs but theinternal bond, interlaminar shear, and screwwithdrawal will be lower as a result of the low-density core.

The method commonly used to circum-vent the adverse moisture effects, but stillretain the advantage of rapid heat transfer tothe core, is a nonuniform moisture distributionthrough the mat thickness. With higher levelsof moisture at the surfaces than at the core,

32

rapid heat transfer occurs without undulylengthening the press cycle. This is readilyaccomplished in layered and homogeneousconstructions in which the core material isdried and blended separately from the surfacematerial. A homogeneous board with all ma-terial dried and blended in the same systemcan be given a nonuniform moisture distri-bution across the thickness by spraying themat surface or cauls with water. Normally, anonuniform moisture distribution, with thesurfaces at a higher moisture content than thecore, increases the vertical density gradient.

Maku and Hamada (1955) found a de-creased thickness swelling and water absorp-tion as the moisture content of the chipsincreased. However, it is unclear from thereport whether the mats were pressed at highmoisture contents or whether they were con-ditioned to 12 to 13 percent after blending athigher moisture contents.

Heebink et al. (1972) reported that a mois-ture distribution of 5 percent in the mat plus a5 percent surface spray allowed the core toreach 220° F in a shorter time period thanrequired for mats with a uniform moisturedistribution of 10 percent for two differentpress temperatures and two different closingrates. However, the vertical density gradientdata show a lower gradient for the boards withthe uniform moisture content of 10 percent,although this comparison of density gradientsis confounded with adhesive and press tem-perature differences. Heebink et al. (1972)reported 10 to 12 percent to be the optimummoisture content for mats with uniform distri-butions. Lower moisture contents requiredhigher pressures to consolidate the mat andwere characterized by poor interparticlebonding. Higher moisture contents necessi-tated longer press cycles to allow sufficientmoisture to escape.

Heebink (1974) found that higher MORMOE values for three-layer flakeboards thanfor those of an earlier study (unpublished)were partially due to a nonuniform moisturedistribution. As opposed to a uniform distri-bution of 12 percent moisture content, a faceand core moisture content of 15 and 5 percent,respectively, contributed to improved boardstrength.

Strickler (1959) showed that the timerequired for heating the mat core decreasedwith increasing surface moisture content. Athigher surface moisture contents (above 15percent) the MOR and MOE decreased whenthe core was at 9 percent moisture content.The internal bond also decreased but thethickness swelling improved as the surface

moisture content increased.Strickler (1959) showed that the time

required for heating the mat core decreasedwith increasing surface moisture content. Athigher surface moisture contents (above 15percent) the MOR and MOE decreased whenthe core was at 9 percent moisture content,The internal bond also decreased but thethickness swelling improved as the surfacemoisture content increased.

Kehr and Schoelzel (1965) also found asharp decrease in the resistance of the matto compression as increased quantities ofwater were sprayed on the mat surface. This,again, was the result of the reduced compres-sive strength of wood at higher moisture levels.

Lehmann (1960) found MOR values in-creased with increasing mat moisture to 13.4percent moisture content, and then decreasedat moisture contents of 16.5 and 20.0 percent.Internal bond increased as the moisture con-tent decreased. Lehmann (1960) also showedan increased dimensional stability and a re-duced water absorption as the mat moisturecontent increased. Rice (1960) used mat mois-ture contents of 9, 12, and 15 percent to studythe effect of mat moisture on board properties.The MOR and MOE values were increased 18and 13 percent respectively, by increasing themat moisture content from 9 to 15 percent.Likewise, the dimensional stability of the panelimproved substantially. The increased matmoisture content also reduced the closing timeto stops with a constant pressure but the prob-lem of thickness shrinkage in the press wasgreater with high moisture content mats.Gatchell et al. (1966) reported an optimumin static bending strength and internal bond ata moisture content of 12 percent.

The mat moisture content and moisturecontent distribution can be varied only withina rather limited range, hence the effect of thisvariable on particleboard properties is limited.With a uniform mat moisture content, theoptimum range appears to be 11 to 14 percent.Much higher surface moisture contents arepossible if the core region is at a correspond-ingly lower moisture content. Basically, itappears that the vertical density gradient ischanged with changing moisture contents anddistributions but the lack of vertical densitygradient data prevents positive proof of thiseffect.Press Closing Rate

Generally, the closing rate of most hotpresses is established by adjustment of theinitial pressure; higher initial pressures resultin the platens closing to stops, or desired

thickness, more quickly than closing speedsattained at lower initial pressures (Kehr andSchoelzel 1967). Most authors, if they mentiona closing speed, will normally give the infor-mation as the time to stops, which is the mini-mum time for exposure of the lower mat sur-face to the bottom platen before the mat hasreached complete compaction. Time to stopsis not the same as time of compaction: duringthe time from start of closing to loss of daylightbetween the mat and the upper platen no matcompaction occurs. The time of compaction aswell as the total press closing time should bespecified by researchers more frequently.

The closing rate of the press and themoisture content of the mat are both importantfactors in the formation of the vertical densitygradient. Quickly closing the press will subjectthe mat surfaces to the platen heat and allowcompressive failure at the mat surface beforethe interior has warmed sufficiently to allowdistribution of the compressive failure througha greater portion of the mat thickness. Conse-quently, faster press closings would be ex-pected to increase the vertical density gradientand improve the bending strength but at theexpense of the internal bond and screwholding strength.

Rice (1960) reported improved MOR andMOE values as the closing rate increased buthe found internal bond, springback, andswelling properties to be independent ofclosing times. Bismarck (1974) reported areduction in internal bonds as the pressclosing speed increased above 40 in/min(inches per minute), but the bending strengthcontinued to increase. He also reported thatpanels produced at the highest rate of com-paction and with the shortest press cycle hadthe lowest formaldehyde emission. Bismarckdeveloped a pressing technique in which theclosing speed was related to the ratio of panelcompression to the compressive resistance ofthe panel until desired thickness was attained.This technique prevented rapid compaction ofa mat with low compressive strength whichnormally occurs with conventional presscontrols. Tests of his technique against theconventional procedure resulted in 5 and 9percent improvements for bending and inter-nal bond strength, respectively, a 10 percentreduction in thickness swelling, and a 16percent reduction in pressing time. Thistechnique most likely reduces the verticaldensity gradient although further studies arenecessary to verify this.

Liiri (1969) monitored the pressure on aseries of mats pressed with increasing closingtimes and found the maximum pressure re-

33

quired for mat consolidation decreased withincreased closing times. The longer the matwas exposed to elevated temperature, thehigher was the degree of wood plasticizationand the lower the pressure required to com-pact it to the desired thickness.

Rice et al. (1967) reported decreasinginternal bond values for flakeboard as theclosing time of the press increased. Rapidpress closure increased MOR, as expected,and substantially increased thickness swellingfor uncatalyzed adhesives, but only slightlyincreased the thickness swelling for the cata-lyzed resin. The internal bond of flakeboardswith both catalyzed and uncatalyzed adhe-sives dropped approximately 50 percent whenthe time to press closing was increased from1.5 to 3.4 minutes.

Heebink et al. (1972) found the reverserelationship between internal bond and pressclosure time; i.e., as the closing time increasedthe core density increased and the internalbond increased. The lower face density withlonger closure times also reduced the bendingstrength.

Maku and Hamada (1955) graphed thick-ness swelling versus pressure at various mois-ture contents and two chip thicknesses. Amaximum thickness swelling occurred for allmoisture contents and chip thicknesses atapproximately 20 kg/cm2 (kilograms persquare centimeter) (284 Ib/in.2). Unfortu-nately, experimental details are not given; itappears that the mats were not pressed toconstant thickness but instead the variouspressures resulted in boards of various speci-fic gravities. Consequently, their data may beconfounded with density effects.

Geimer et al. (1975) have recently pub-lished vertical density gradients obtained from0.85-inch-thick three-layer boards at threedifferent closing speeds. Extremely fast clo-sure (20 seconds) resulted in a continuouslydecreasing density from the surface to thecore. One-minute closure time produced amaximum density approximately 1/16 inchbelow the surface while the 3-minute closuretime resulted in a vertical density gradient witha board peak from approximately 0.1 to 0.2inch below the surface. The maximum of the3-minute gradient was at a substantially lowerdensity than the maximum for the l-minuteclosure time.

The effect of press closure speed on theproperties of particleboard appears to be fairlywell established. The press closure rate influ-ences the properties related to the verticaldensity gradient since this is highly dependenton closing speeds. Press closing rates can be

adjusted to optimize the desired properties,but adjustments have to be within fairly narrowboundaries as dictated by the resin reactivityand mechanical limitations of the press.

Press Time, Temperature, and Pressure

The function of the hot press in particle-board production is to consolidate the chipmat to the desired thickness and densityfollowed by polymerization of the adhesivebetween adjacent chips into a cross-linkedsolid polymer to hold the mat in this consoli-dated state when removed from the press. Tofacilitate the chemical reaction and allowreasonable press times, all production pressesin the particleboard industry employ somemethod of heating the mat. Most commonly,this is done by heated platens which contactthe mat surfaces. Heat then flows from theplatens through the mat surfaces into theinterior. Because the entire board is not uni-formly heated throughout the thickness, thecuring of the adhesive does not occur uni-formly; the adhesive at the mat surface is thefirst to cure and that at the central region thelast. Because the mat center is always at thelowest temperature, the pressing time andtemperature should be such to ensure that thecore reaches a sufficiently high temperatureto allow the resin to cure. This can be accomp-lished by increasing the press time at a con-stant temperature or by increasing the presstemperature at a constant pressing time.

Particleboard produced in presses equip-ped with a high frequency generator areheated more uniformly because heat is gener-ated within the mat and does not migrate fromthe hot platens. The adhesive cures muchmore uniformly through the thickness and themat center is not any cooler than the remainderof the mat.

Superimposed upon the requirement ofreaching a given core temperature is the effectof moisture migration to the core. The presstime has to be sufficiently long to allow excesswater to migrate to the board edges beforetermination of pressing. A board with a proper-ly cured center will delaminate upon pressurerelease if the steam pressure is higher than theinternal bond of the particleboard (Lehmann etal. 1973).

Lehmann et al. (1973) found that, with acatalyzed urea-formaldehyde adhesive, acenterline temperature above 200° F for 0.25minute was sufficient to cure the adhesive. Therate of heat transfer to the center controlledthe total pressing time for a given board; there-fore, shorter press times were obtained withhigher press temperatures, “steam shock”

34

treatments, and thinner boards.In the same study, phenol-formaldehyde

resins were found to have a slower curing ratethan the urea-formaldehyde binders. In thiscase, 1.75 minutes above 220° F was requiredto satisfactorily cure the adhesive in the core.The catalysts used in this study (methylethylketone hydroperoxide, potassium chromate,and phenol-resorcinol resin) were not effectivein reducing press times of the phenol-formal-dehyde adhesives. The data appeared to indi-cate that phenol-formaldehyde adhesives aremuch more adversely affected by high mois-ture levels in the curing zone than are the urea-formaldehyde resins.

Otlev (1971) reported the moisture con-tent of particleboard core and surfaces afterpressing to be 11 to 13 percent and 3 to 5 per-cent, respectively. Moisture sprayed on thesurface did not produce a higher center mois-ture content but did speed the rate of resincuring.

Heebink et al. (1972) reported satisfactoryurea-formaldehyde-bonded particleboardscould be produced when the center tempera-ture was above 220° F for 0.5 to 0.7 minute.They also reported a decrease in total pressingtimes of 1 to 2 minutes when the press tem-perature was increased from 375° to 475° F.This higher temperature allowed faster heattransfer and’ shifted the maximum densityregion toward the interior, so the internal bondstrength increased and the bending strengthdecreased.

Liiri (1969) reported a decrease in themaximum pressure required to compact themat as the press temperature increased. Thisis the result of increased plasticity of the woodas the temperature increases.

Lynam (1959) stated that longer presscycles at lower temperatures were desirablebecause less water was removed from theboard and the subsequent moisture adsorp-tion from the atmosphere was reduced. Longpress cycles also allowed more water to es-cape but Lynam (1959) believed long cycles atlower temperatures were better than shortcycles at higher temperatures, because lesswater was evaporated and the remaining waterwas more uniformly distributed in the finishedboard. He has no experimental data in thisreport on which to base his conclusions.

The effect of platen temperature on com-pression time for three-layer particleboardshas been studied by Kehr and Schoelzel (1965)at three moisture contents. Increasing theplaten temperature from 120° to 180° C rapidlyreduced the compression time required for amat moisture content of 11 percent. However,

very little change in compression time withtemperature was evident at 18 percent mois-ture content, although the compression timewas lower than for that of the 11 percent mat.This indicates that wood at 18 percent mois-ture content is sufficiently plasticized forcompression and that additional plasticizationimparted by the heat is not required.

Lehmann (1960) used three press tem-peratures (227, 307, and 344° F) to study theeffect on dimensional and strength propertiesof yellow poplar flakeboard. The extremelylong press times employed (20 to 45 minutes)probably allowed complete cure of the adhe-sive in all cases and may account for the lack ofsignificance of press temperature on mostproperties. Thickness of the board out of thepress was significantly decreased as the presstemperature increased, probably due toincreased drying and subsequently highershrinkage.

Pressing time and temperature are ex-tremely important parameters in particleboardmanufacturing and have to be carefully con-trolled to ensure that the core temperatureattains the level required to cure the adhesivewithout subjecting the board surface to a high,degradative temperature. Improvements inparticleboard adhesive technology have signi-ficantly lowered the time period required tocure the adhesives and have allowed theindustry to attain relatively short press cycles.Therefore, the press temperature and pressingtime need to be closely controlled to ensureadequate temperature levels are reached in thecore at the most economical combination ofthese two factors.

Press pressure is of minor importance inparticleboard produced with press stops, oncethe initial mat consolidation is reached. Kehrand Schoelzel (1965) produced particleboardswith reduced pressure at various time intervalsafter the desired thickness was reached. Nosignificant property deterioration occurredwhen the pressure was released after 2 min-utes on stops for the boards at 0.65 gm/cm3.The higher density boards (0.70 gm/cm3)exceeded the desired thickness and hadslightly lower internal bond when the pressurewas reduced after 2 minutes.

Liiri (1969) measured the pressure withinthe board as a function of press timeand founda sharp maximum occurred at the point atwhich target thickness was attained. This wasfollowed by an immediate drop to a very lowpressure at the end of the press cycle. Thispattern was obtained at all board densities andall closing times although the maximumpressure was higher for the higher density

35

board and occurred at later times as theclosing time increased. Therefore, the pres-sure in the board increases quickly during matcompaction and decreases once the thicknessis obtained, at which time the press pressurecan also be reduced.

Lehmann et al. (1973) obtained similarresults of board pressure versus press timewith catalyzed urea-formaldehyde Douglas-firflakeboards. The pressure diagrams for threethicknesses of 40 Ib/ft3 boards were nearlyidentical; a maximum pressure of approxi-mately 600 lb/in.2 was obtained at a press timeof 1 minute. The 45 and 50 Ib/ft3 boards at-tained a pressure of approximately 850 and950 Ib/in.2, respectively, at a press time of 1minute. Heebink et a/. (1972) found similarsharp decreases in the board pressure andconcluded that the minimum press time couldbe determined as the point at which the re-quired pressure fell below the internal bondstrength of the board.

Unt i l fur ther improvements in presstechnology are realized, the practice of con-trolling particleboard thickness with stops inmulti-opening presses will continue. Hence,

the function of the pressure is to bring thecomponent particles into intimate contact byconsolidating the mat to the thickness of thestops. Once stops have been reached, furtheradditional pressure is useless and, in fact, ithas been adequately shown that the pressurecan be reduced as the pressing operationcontinues. The only influence press pressurehas on the resultant panel properties is relatedto the speed of closure and its effect on thevertical density gradient, assuming platencontact on the stops continues throughoutthe press cycle.

One further pressing technique, whichhas appeared in the literature, is the injectionof high pressure steam into the consolidatedmat in the hot press (Shen 1973). The steamreportedly cured the phenol-formaldehydeadhesive very rapidly and the resultant parti-cleboard possessed superior dimensionalstability and strength properties compared toa conventionally platen-heated board. Furtherresearch and development are required to fullystudy this method before it is widely appliedin industry.

STRENGTH PROPERTIESThe various processing parameters have

various effects on the resultant panel strengthproperties. A number of difficulties are en-countered when reviewing the literature forthis information.

Foremost of these difficulties are thereports which do not furnish sufficient infor-mation as to experimental procedures andconditions. Many instances were found inwhich important processing parameters, suchas mat moisture content and distribution, rateof press closing, and particle configuration,were not given. As pointed out in the verticaldensity gradient section, the above variablesare extremely important in controlling thisgradient, and bending and internal bondproperties are strongly influenced by thisgradient.

The effects of interaction between pro-cessing variables on the resultant propertiesare normally quite large and in many instancesseparation of these interactions proved to beimpossible. The moisture content of the boardat test has been shown by Halligan andSchniewind (1974) to have a significant effectupon the modulus of rupture, modulus ofelasticity, and internal bond. Some reports,however, do not include the moisture contentat which the various tests were conducted.

One further difficulty is the applicabilityof laboratory research results to the particle-board production line. Optimum, well-con-trolled production conditions in the laboratoryoften are not duplicated on the production linedue to the desire to keep production at thehighest possible level or due to lack of properequipment maintenance. Consequently, mostlaboratory-produced particleboards will havehigher strength values than those obtainedfrom a production line with the same furnishand adhesive content. An example of this is thecommercial blender, which operates at ratesabove those shown to produce optimumblending (Carroll and McVey 1962). Anotherexample is moisture migration to the boardedges in the hot press; in small laboratory-produced boards, migration will occur in lesstime than in large production-size boards.Therefore, commercial press times are nor-mally longer than would be predicted fromlaboratory results. Mat felting is normallymore uniformly accomplished in commercialproduction than is possible with hand feltingin a laboratory. Consequently, density varia-tions across the board are normally less pro-nounced in commercial products.

The information reviewed here was ob-tained from tests conducted on laboratory-

36

produced boards, but the trends and conclu-sions should also apply to commercial prod-ucts. The literature, on the effects of boarddensity and particle configuration and orien-tation is reviewed for each major strengthproperty. Variables which have been reportedto affect a number of properties will be dis-cussed separately in the respective strengthproperty section. Literature pertaining to otherprocessing parameters, such as adhesivecontent, moisture content, and pressing con-ditions, which affect the panel strength pro-perties can be found in the section, “Process-ing Parameters.”

Bending Strength

The bending strength of particleboard ismeasured on the ASTM Standard MethodD 1037 for static bending. The four strengthproperties obtainable from this test (ASTM1975) are the (a) modulus of rupture; (b)modulus of elasticity; (c) stress at proportionallimit; and (d) work at maximum load. Themodulus of rupture is the most widely deter-mined property from static bending tests ofparticleboard. The modulus of elasticity, orstiffness, is much less often determined andstress at proportional limit and work to maxi-mum load are rarely encountered in theliterature.

Modulus of Rupture

The modulus of rupture (MOR) is animportant property determining the applica-bility of particleboard for structural compo-nents. Many processing parameters and theireffects on the MOR have been studied; themost widely reported parameters are boarddens i ty and par t i c le con f igura t ion andorientation.

Density.--The density of the board di-vided by the density of the wood equal thecompaction ratio (Hse 1975). Hse illustrated ahigh correlation between compaction ratioand MOR for particleboards at three differentdensities produced from nine hardwoodspecies from low to high specific gravity.Unfortunately, the equation presented to fitthe plot of MOR versus compaction ratio doesnot fit the data (Hse 1975). Figure 1 of thatpaper, erroneously entitled MOE instead ofMOR, has a linear equation which gives anexcellent fit to the data but a minus sign beforethe intercept has been omitted. The plot of theexperimental points does appear to illustratean excellent linear relationship betweenmodulus of rupture and compaction ratio forall species used in this study.

The compaction ratio has been practically

37

ignored by most researchers in particleboardtechnology; many make reference to thenecessity of compacting the material to adensity greater than the original wood, but fewhave actually calculated a compaction ratio.The board density used in the calculation isthe average board density; therefore, com-paring the relationship of MOR to compactionratio between researchers is useless unlesssimilar vertical density gradients are present inthe respective boards.

Howard (1974) determined the densifica-tion (compaction) ratio for flakeboards madefrom slash pine stems and root material. TheMOR for boards made from the root materialwas higher than the same-density board fromstemwood and, since the density of the rootmaterial was less than the stemwood, the com-paction ratio was higher for the root material.

Vital et al. (1974) found that particle-boards from four exotic hardwoods of widelyvarying specific gravity, made to constantboard density, had higher MOR values as thecompaction ratio increased from 1.2 to 1.6.However, boards made to the same compac-tion ratio from different furnish specific gra-vities did not result in constant MOR values,as reported by Hse (1975), but increasedlinearly with increasing board density.

Stewart and Lehmann (1973) found theMOR to increase linearly with increasing paneldensity for four hardwood species ranging inspecific gravity from 0.37 to 0.67 (OD weight,volume at 8 percent moisture content). How-ever, the modulus of rupture decreased as thespecies density increased--i.e., as the compac-tion ratio decreased--for all board densities.

Stegmann and Durst (1965) have illus-trated (fig. 12) the relationship between theMOR values for particleboards of differentdensities produced from furnishes of variouswood species with different densities. As thefurnish density increases, the MOR valuesdecrease at constant particleboards densities,although a sharper drop in the MOR at lowboard densities and high wood densities wouldbe expected than shown by this curve.

Obviously, compaction ratio is directlyproportional to the particleboard density forfurnish with constant specific gravity. Allstudies in which board density versus MORhas been determined report an increase inmodulus of rupture with increasing boarddensity--that is, as the compaction ratio in-creases the modulus of rupture increases.Consequently, compaction ratio may be anexcellent method of quantitatively determiningthe relationship between board density andmodulus of rupture. Determining the furnish

specific gravity is necessary in order to calcu-late the compaction ratio; this practice will alsoresult in additional benefits to the particle-board literature, which is currently lacking infurnishing specific gravity data.

Further research is required to determinethe feasibility of using compaction ratio as apredictor for the MOR of particleboard. Com-paction ratio alone is not the only factor in-fluencing the MOR values; pressing condi-tions, adhesive content, and many otherprocessing factors are extremely important,but the compaction ratio warrants furtherstudy.

It is unanimously accepted, and adequateevidence is available, that MOR increases withboard density when all other factors (particleconfiguration and orientation, species, adhe-sive content, pressing conditions, etc.) are

constant. The reviewed literature does nothave any reported decrease in modulus ofrupture when average board density increases.

The surface density strongly affects MORbecause bending stresses are higher at thesurfaces. Consequently, MOR values arehighly dependent upon the vertical densitygradient; widely different values can be ob-tained for equal average board densitiessimply by changing the processing variablesaffecting the vertical density distribution.Rice (1960) increased the MOR 18 percent byincreasing the mat moisture content into thepress from 9 to 15 percent for sweetgum flake-board. Lehmann (1960) reported similar re-sults with yellow poplar furnish but noted adecrease in MOR when the moisture contentwas increased from 16.5 to 20 percent. Highermoisture contents increase the compressibility

Figure 12.--Relationship between wood density and modulus of rupturefor four particleboard densities.(From Stegman and Durst 1965)It is recommended that these curves be used with care, as a boarddensity of 0.5 is not current/y possible with wood of 0.6 density.

(M 144 412)

38

of the mat surface during press closing, result-ing in increased surface densification andhigher MOR values (Strickler 1959; Heebink eta/. (1972). The adverse effect of high uniformmat moisture content on press time and adhe-sive hardening is reduced by adding the mois-ture to the surfaces and not to the core.

Kehr and Schoelzel (1967) reported noclear effect of press closing speed on thevertical density gradient or MOR values. How-ever, Heebink et al. (1972) found a definitesurface densification increase as the closingspeed increased and a direct relationshipbetween MOR and surface density. Therefore,it appears that useful information could beobtained by determining the vertical densitygradient, as well as the average board density,and relating it to MOR values, as recentlyproposed by Plath and Schnitzler (1974).

Particle configuration and orientation.--Many reports appear in the literature onstudies concerned with the effect of particlegeometry and alinement on the resultant parti-cleboard strength properties. The difficulty inreviewing these and finding a consensus of theoptimum particle size for maximum MOR, orany other property, is the nonuniformity ofthe other variables used in the studies. Species,average board density, adhesive type andamount, as well as pressing conditions affect-ing the vertical density gradient, are not con-stant. However, the many reports available doallow useful comparisons. It should be remem-bered that the particle size for optimum MORis not necessarily the optimum for dimensionalstability or internal bond; compromises inparticle geometry have to be made to produceboards which have satisfactory properties tofulfill desired end-use functions.

Turner (1954) published results of one ofthe first investigations on particle size andshape and the influence of these on thestrength properties of particleboard. Fourpart ic le conf igurat ions-- f lat f lakes, th instrands, helical ribbons, and cubes--wereglued into panels of 0.5 to 1.05 specific gravitywith 2, 4, and 8 percent phenol-formaldehydeadhesive. Equal weights of blended materialwere pressed in a mold at 250, 500, and 1,000lb/in.2 to obtain the various densities; conse-quently, panel thickness was not constant. At aconstant panel density, the flakes 3 incheslong and 0.015 to 0.020 inch thick had theoptimum MOR values.

Post (1958) found a continuous increasein MOR for oak particleboard with increasingflake length over the studied range of 0.5 to 4inches, but the rate of increase decreased withlengths greater than 2 inches. However, as the

flake thickness increased above 0.010 inch,MOR decreased for all flake lengths. The flakelength/thickness ratio was found by Post(1958) to be closely related to MOR at all flakelengths and thicknesses. MOR continued toincrease but only slowly even at the highestratio (300) used in the study. In a related study,Post (1961) stated that the length/thicknessratio is a better indicator of the effect of particleconfiguration on MOR than either dimensionindividually.

Brumbaugh (1960) studied the effect offlake size on Douglas-fir particleboard ofthree densities. Modulus of rupture valuesincreased with increasing flake length withinthe studied range of 0.5 to 4 inches. Flakethicknesses of 0.009 to 0.018 inch resulted inno significant effect on MOR. Brumbaughindicated optimum overall board propertiesrequired a length/thickness ratio of at least400, but his reported MOR values increased asthe ratio increased to 250 and remained con-stant at higher ratios.

Heebink and Hann (1959) studied theeffect of particle shape on homogeneousparticleboard properties of northern red oak;their results also showed l-inch flakes to havesignificantly higher MOR values than 0.25-inchflakes, at an equal thickness of 0.015 inch.Planer shavings, sawdust, slivers, and fines allresulted in lower MOR values than did the 0.25-inch-long flake.

Kimoto et al. (1964) made low-density(specific gravity 0.3 to 0.4) particleboard fromthree configurations of lauan flakes and splin-ters with length/thickness ratios of from 10 to100. The highest MOR values were obtained atthe highest ratio for both particles, but theresults with the splinters were not statisticallysignificant. The flake length/thickness ratiowas highly significant; MOR increased withincreasing ratios even up to the highest ratio(100). Rackwitz (1963) also reported optimumstrength values achieved at ratios of from 100to 130 with particles up to 0.75 inch long.Heebink (1974), with a constant length/thick-ness ratio (100), found higher MOR values forthe boards produced with flakes 2 inches longthan with flakes 3 inches long.

Lehmann (1974) found increasing flakethicknesses always reduced MOR, when otherfactors were constant for phenol-formalde-hyde bonded flakeboard for structural applica-tions. Gatchell et al. (1966) found an increasein MOR as flake thickness decreased withphenolic-bonded flakes. Flake length (1 and 2inches) did not greatly affect the MOR. Planershavings resulted in reduced MOR values.Stewart and Lehmann (1973) reported very

39

little effect of flake thickness on MOR in thethickness range of 0.006 to 0.018 inch forparticleboard from four hardwood speciesTalbot and Maloney (1957) studied particle-boards made from coarse and fine planershavings at three board densities with threelevels of wax and powdered phenol-formalde-hyde adhesive. The coarse shavings (retainedon 4 mesh screen) resulted in higher MORvalues than the fine shavings (passed 4 mesh,retained on 16 mesh screen) for all resin andwax levels at all board densities used.

The available literature seems to indicatethat particle thickness has more influence onMOR values than does particle length, at leastat lengths greater than 1 to 2 inches. At thoselengths, significantly lower MOR values havebeen found as the particle thickness increasesabove 0.020 inch. The above studies in whichthe length/thickness ratio influence on theMOR values have been reported were normallyconducted by holding one particle dimensionconstant. This allowed comparisons of MORfor particleboards with component particlesof di f ferent length/ thickness rat ios. Thevarious investigators differ as to the optimumratio, but all appear to agree that MOR in-creases with an increasing length/thicknessratio. Heebink (1974), holding the ratio con-stant by changing both dimensions, con-cluded that increasing the particle thicknessfrom 0.02 to 0.03 inch had a more detrimentaleffect on the MOR than decreasing the particlelength from 3 to 2 inches. Hence, it appearsthat much of the literature indicating a rela-tionship between particle length and thicknessis, in fact, i l lustrating the possibly moreimportant effect of particle thickness.

Very few studies have been conductedrelating the MOR to the particle width. Kusian(1968) found MOR increased as the widthincreased, but as the width approached par-ticle length the MOR decreases. Suchsland(1959) stated that the ideal particle configura-tion for a three-layer particleboard was nar-row, thick particles in the core and thin, short,square particles at the surfaces. This, ac-cording to Kusian (1968) will not optimize theMOR property, but the square particles shouldresult in a smoother surface.

Hse et al. (1975) have recently publishedresults of a study with layered particleboardmade from flakes 3 inches long, consisting ofequal portions of hickory, white oak, southernred oak, sweetgum, and southern pine. (Thispaper has the same graph of MOR versuscompaction ratio Hse had published earlier(1975) with the minus sign missing from theintercept term of the regression equation.)

Unfortunately. Hse et al. presented no dataon face-core proportions so a 20 percentreduction in MOR resulting from random faceorientation when compared to alined faceorientation (both on random cores) is difficultto interpret. Geimer et al. (1975) have pre-sented data which indicate face orientationperpendicular to test direction results in MORvalues only half those obtained when the faceflakes are alined parallel to the long dimensionof the test specimen. This result is based onl/Z-inch board with approximately 40 percentof the total board weight in the faces.

Klar and Stofko (1965) studied differentsurface thicknesses of oriented and unori-ented particles with sawdust cores, at constantboard thicknesses. They found increasingMOR values with decreasing core thickness(increasing surface proportion). Klauditz et al.(1960), using oriented particles approximately2 inches long throughout the entire thickness,found MOR values in both grain directions tobe at least as strong as wood, up to a densityof 0.6. The inherent weakness of wood inbending perpendicular to the grain was greatlyexceeded by the alined particleboard whentested perpendicular to the particle alinement,indicating completely alined particleboarddoes not have as large differences in bendingstrength between the two directions as iscommon for wood.

Larger particles can also be satisfactorilybonded at lower resin contents, because theparticle surface area is drastically reduced asthe particle size increases. Gertjejansen andHaygreen (1973) indirectly show the advan-tage of 1.5-inch-long wafers over 0.5-inchflakes on phenol-formaldehyde adhesive con-sumption when producing homogeneousparticleboards with the respective particles.The MOR was reduced approximately 9 per-cent when wafers were used instead of flakesfrom the same wood source. However, theadhesive content of the waferboard was only 3percent as compared to 8 percent (resin solidson OD wood) for the flakeboard; conse-quently, tremendous adhesive savings arepossible with the larger wafer, providing otherproperties are maintained.

The effect of particle geometry on theresultant panel MOR values appears to befairly well established. Particles of high length/thickness ratios, in which structural damage isminimal, normally produce particleboardswith superior MOR values. Planer shavings areusually damaged and consequently produce aboard with inferior MOR values (Hart and Rice1963; Heebink et al. 1964). Large particles atthe board surface normally do not felt as well

40

or produce as smooth a surface as smallerparticles. Consequently, a balance has to beestablished between modulus of rupture andother board properties; one widely used com-promise is a three-layer construction in whichsmall particles, for smoothness, are used forparticleboard surfaces and larger particles,with less adhesive, are used in the core.

Modulus of Elasticity

The modulus of elasticity (MOE) is animportant property because it is a measure ofthe stiffness, or resistance to bending, whena material is stressed. The effective MOE ofparticleboard is measured by mid-spanloading as described in ASTM D 1037 (1975).The nonuniformity of platen-pressed particle-board in the thickness direction prevents thedetermination of a true MOE. Effective MOEused here is board stiffness divided by momentof inertia -- i.e., El/l.

In general, modulus of elasticity andmodulus of rupture are affected similarly byvarious processing parameters. Increasingboard density increases both properties;increasing surface density and surface particlealinement increases both properties; andhigher adhesive contents normally increaseMOR and MOE. Howard (1974) and Vital et al.(1974) have calculated regression equationsfor MOE versus MOR values for the data oftheir respective studies. Howard (1974) pre-sents regression equations for boards fromboth stemwood and rootwood of slash pinemade with 3-inch-long flakes. The scatter forthese equations is large, especially for thestemwood material (r2 = 0.22) whereas Vitalet al. (1974) obtained highly significant correl-ations (r2 > 0.96) for regression equations atthe two compaction ratios studied with 3/4-inch-long flakes. The regression equations ofeach paper predict widely different MOEvalues, so comparisons are useless. This is notunexpected as the flake length strongly affectsthe bending strength; consequently, therespective regression equations are not appli-cable to particleboards produced with pro-cessing variables at levels other than those atwhich the equations were calculated.

The effective MOE is not determined asfrequently as MOR by various researchers;consequently, there are many reports in theliterature in which no information is availableon MOE. As was done in the section, “Modulusof Rupture,” the parameters affecting MOE willbe reviewed individually in the followingdensity and particle configuration and orienta-tion subsections.

Density.--Particleboards of constant aver-

age density possess higher MOE values asthe wood density decreases -- i.e., as thecompaction ratio increases. Hse (1975) plottedMOE versus compaction ratio for phenol-formaldehyde-bonded particleboards fromflakes of eight different hardwoods with widelydifferent wood densities. The rapid increase inMOE with increasing compaction ratio washighly significant. The regression equationfitting this data predicted an MOE of 670,000lb/in.2 at a compaction ratio of 1.0, and 800,000lb/in.2 at a compaction ratio of 1.24. Theexcellent correlation between MOE and com-paction ratio found in this data should not beextrapolated to other studies unless the pro-cessing parameters are the same as in Hse’sstudy.

In contrast to Hse, Vital et al. (1974) usedtwo compaction ratios with various mixtures offour exotic hardwood species to produceparticleboards of various densities. Within acompaction ratio, a highly significant linearrelationship was obtained between effectiveMOE and board density. Hse’s (1975) graphindicates that any given value of MOE has onlyone compaction ratio, independent of boarddensity, while Vital et al. (1974) indicate thatfor a given MOE, a compaction ratio of either1.2 or 1.6 is possible, depending upon theboard density. However, no direct compari-sons should be attempted between thesestudies: Hse (1975) used large flakes (3 incheslong) and 4 percent phenol-formaldehydeadhesive while Vital et al. (1974) used smallerflakes (3/4 inch long) and 7 and 9 percent urea-formaldehyde adhesive in core and surfacelayers, respectively (resin solids on OD wood).This lack of uniformity illustrates the difficultyin comparing individual reports.

The vertical density gradient, as influ-enced by face weight/total board weight ratioand press closing time, has been shown byGeimer et al. (1975) to have a tremendousinfluence on the effective MOE, even at a con-stant board density. Consequently, unless thevertical density gradient is determined and itseffect recognized, erroneous conclusions canbe drawn about the effect of other variables,such as particle configuration and resin con-tent, on the effective MOE. Most researchersattempt to circumvent the vertical densitygradient effect by using a constant pressclosing rate, press temperature, and mat mois-ture content. The effectiveness of this practice,when various levels of a particular processparameter are studied, has not been reportedin the available literature. Until the practice ofdetermining vertical density gradients is morewidespread, many properties reportedly due to

41

processing changes may be, in reality, due tothe vertical density gradient effect.

Lehmann (1970) reported the effectiveMOE to be significantly affected by boarddensity for particleboard produced fromDouglas-fir flakes with 2, 4, and 8 percentadhesive (resin solids on OD wood). There wasalso a significant resin content-density inter-action indicating more efficient resin usageat the higher densities. Shuler (1974) andLehmann (1974) found a linear relationshipbetween MOE and density at all resin contentsstudied.

Heebink et al. (1972) measured the verticaldensity gradient and MOE for particleboardpressed with 5 and 15 percent moisture con-tent in the core and surfaces, respectively. Thehigh density regions within 1/16 inch of thesurfaces resulted in high effective MOE values.Plath (1971b) showed mathematically therelationship between MOE and density ofvarious layers through the thickness direction.He presented an equation from which the MOEcan be calculated if the vertical density gradi-ent is known and also pointed out the optimumgradient for maximum MOE is a relatively high-density surface. Bismarck (1974) reported anincrease in MOE as the speed of press closurewas increased from 15 to 30 mm/min. This alsodemonstrates the influence of the verticaldensity gradient and the improvement possi-ble in the effective MOE with higher densitysurfaces.

The literature contains many studieswhich report a direct relationship betweenboard density and the effective MOE; all unani-mously agree that an increase in density willincrease MOE. No general quantitative rela-tionship is obvious between studies, as veryfew reports are complete enough in the pro-cessing details to allow comparisons. Forexample, very few reports contain mat mois-ture content into the press, press closingspeed, and all other pressing conditions toallow comparisons with other reports. Twoinvestigators can report widely different MOEvalues for the same average board density orresin content, but no comparisons can bemade because the experimental details arem i s s i n g .

Particle configuration and orientation.--The modulus of elasticity is strongly depen-dent upon flake length; longer flakes produceparticleboards with substantially higher effec-tive MOE (Heebink and Hann 1959; Heebink etal, 1964; Lehmann 1974).

The effect of particle thickness on MOEdoes not appear to be as well defined as theeffect of length. Stewart and Lehmann (1973)

did not find a significant effect of flake thick-ness on the effective MOE for particleboardsproduced from cross-grain flakes in the thick-ness range 0.006 to 0.018 inch. Gatchell et al.(1966) found an increase in MOE when flakethickness decreased from 0.030 to 0.015 to0.007 inch. Rackwitz (1963) found the optimumMOE with a length/thickness ratio range of100 to 130; the MOE increased as the ratioincreased to this range and remained constantat higher ratios. Particle width was reportedto have only a slight influence on MOE. Leh-mann (1974) also found a decrease in theeffective MOE when the flake thickness in-creased from 0.030 to 0.045 inch, at a constantflake length of 2 inches and at all phenol-formaldehyde adhesive contents studied.

The effect of surface particle alinement onMOE is similar to the effect on MOR, as dis-cussed in that section. Geimer et al. (1975)have shown that, for a constant average boarddensity of 40 Ib/ft3, the effective surface MOEincreases at a faster rate above a face specificgravity of 0.75. Below a face specific gravity of0.75, the rate of increase in surface MOE withincreasing face specific gravity is approxi-mately constant (fig. 13). The increasing rateabove 0.75 face specific gravity occurs withrandom as well as with alined surface flakes,although the effective MOE values are sub-stantially higher for boards of parallel aline-ment. This again illustrates the influence of thevertical density gradient on particleboard MOEvalues as well as the benefit of surface particlealinement. At a surface specific gravity of 0.7,the particleboard with parallel alined surfaceflakes had approximately 2.5 times the effec-tive MOE of the random surface and approxi-mately 14 times the effective MOE when deter-mined in the direction perpendicular to thesurface flake alinement. This data illustratesthe large differences possible in MOE valuesfor a constant average board density simply bychanging the vertical density gradient. Thisfurther strengthens the argument for deter-mining the vertical density gradient whenmeasuring various strength properties.

Maloney (1975), in a study to determinethe effect of short-retention-time blenders onlarge flake furnishes, found an increase inMOE as the board specific gravity increasedfor all resin levels studied (2, 4, and 6 percentresin solids on OD wood). Negligible improve-ments in the MOE at all board specific gravitieswere obtained on increasing the resin contentfrom 4 to 6 percent. Hse et al. (1975) alsoobtained significantly higher MOE values at allboard densities with alined surfaces on ran-dom cores as opposed to random surfaces on

42

Figure 13.--Relationship between specific gravity and modulus of elasti-city of particleboard face layers.(From Geimer et al 1975)

(M 144 413)

43

random cores.The particleboard literature appears to

lack high-quality information pertaining toprocessing variables and their effect on MOE.This is not surprising in that applications ofparticleboard have, in the past, been moreconcerned with the ultimate strength and notthe board‘s stiffness. However, with increasinginterest in structural applications, additionalwork is necessary to consistently attain thehigh MOE required in such applications. Thework of Geimer et al. (1975) has shown theinfluence of surface density on the effectiveMOE and future researchers should not simplydetermine MOE values as a function of averageboard density, but should also measure thevertical density gradient.

Internal Bond

The internal bond (IB), or tensile strengthperpendicular to the board surface, is a widelydetermined property in all particleboard re-search. A particleboard with a well-curedadhesive will normally fail when stressed intension perpendicular to the board surface,near the midpoint of the thickness. This is thelowest density region in a hot-pressed panelparticleboard and consolidation of the mat toobtain intimate particle-particle contact islowest in this region. Most researchers havefound higher IB values with increasing boarddensity, increasing resin content, and in-creasing press time and temperature. Con-sequently, this portion of this review will berestricted to studies concerned with the effecton internal bond of particle geometry andrelated parameters.

The normal vertical density gradient inflat-pressed particleboard adversely affectsthe IB. Highly densified surfaces increase thebending strength of particleboard, as shownby Geimer et al. (1975), but the resultant lowerdensity core region normally reduces the IB(Strickler 1959; Plath and Schnitzler 1974).However, Strickler (1959) and Geimer et al.(1975) did not find a strong correlation be-tween core density and internal bond; Stricklerattributed this to moisture and press cycleeffects.

Kehr et al. (1964) found a sharp decreasein the IB as the resin output per spray nozzlewas increased above 100 gm/min. The largeadhesive droplets did not result in as goodblending as obtained with small adhesivesdroplets (see “Physical Properties”).

Excessive moisture in the particleboardmat will migrate to the core during hot pressingand interfere with the chemical reaction neces-

sary to cure the resin (Lehmann 1960; Hart andRice 1963; Gatchell et al. 1966; Heebink et al.1972). Roffael et al. (1972) increased the coretemperature to 220° C and obtained improvedinternal bonds, since at this temperature mostof the water is removed and does not interferewith the adhesive curing.

Vital et a/. did not find a close relation-ship between IB and board density for particle-boards made from mixtures of four exotichardwoods. They reported a decrease in IB atconstant board density as the compactionratio increased from 1.2 to 1.6. The IB decreasewas attributed to the increased amount of flakedamage at the high compaction. In contrast,Hse (1975) found increasing internal bondswith increasing compact ion rat io at al lcompaction ratios studied between 0.9 and 1.5with 3-inch-long flakes from nine hardwoodspecies. The larger f lakes (Hse 1975)responded to the higher compaction byappearing to better util ize the 4 percentphenolic adhesive, while the shorter andgenerally lower density flakes (Vital et a/. 1974)did not require the high pressure for goodbonding and appeared to be weakened at thehigher compaction ratio.

Maloney (1975), in his study of shortretention-time blenders, reported decreasedIB values at board specific gravities above 0.75for a 6 percent level of phenol-formaldehydeadhesive (resin solids on OD wood) with flakesblended in a laboratory rotary blender.Generally, Maloney’s (1975) results show adirect linear relationship between IB and boardspecific gravity with a phenol-formaldehydeadhesive content of 2 percent, but even at thehigh specific gravity, IB was below 100 lb/in.2

At 4 and 6 percent resin, the IB increased withboard specific gravity but at a decreasing rate,except for the instance mentioned abovewhere the IB decreased.

The internal bond strength improves asthe core particle configuration changes from along wide flake to planer shavings or slivers(Childs 1956; Talbott and Maloney 1957;Suchsland 1959; Brumbaugh 1960; Rackwitz1963; Kimoto et al. 1964; Gatchell et al. 1966;Stewart and Lehmann 1973; Lehmann 1974) asshown in the “Bending Strength” in this report,the reverse is true for modulus of rupture andmodulus of elasticity--i.e., large flakes improvethese properties. Consequently, in making ahomogeneous particleboard, the manufac-turer must reach a compromise on whichparticle configuration to use to meet the pro-jected application demands. A three-layerparticleboard construction, with coarse parti-cles in the core and flakes at the surface, is

44

often used to optimize both properties in thesame board (Keylwerth 1958; Suchsland 1960).

Talbott and Maloney (1957) reported asignificant improvement in internal bond forall board specific gravities, particle sizes, andphenol-formaldehyde resin contents when0.75 percent was was added to the furnish.Hann et al. (1962) also reported increased IBvalues when 1 percent wax was added to parti-cleboard bonded with 4 percent urea-formal-dehyde adhesive. Heebink and Hann (1959)found no improvement in strength with addi-tion of 1 percent wax to a board with 8 percenturea-formaldehyde adhesive. Gatchell et al.(1966) found a wax content above 2 percentinterferred with inter-particle bonding. Steg-mann and Durst (1965) also found reducedstrength with increasing wax content. Waxaddition to particleboard improves the liquidwater resistance but it is extremely doubtfulthat it would also improve the internal bond, ashas been reported, since there is no strongchemical attraction between wax and woodparticles sufficient to improve the interparticlebonding. Excessive wax addition will hinderbond formation.

Other Strength Properties

The lack of additional mechanical tests inthe particleboard literature is surprising,especially when the standards for conductingthese tests appear in the ASTM StandardMethod D 1037. Very few reports appear inwhich shear strength in the plane of the boardand tensile and compressive strengths parallelto the surface have been determined, and thenumber of reports concerned with screw with-drawal is not much greater. The addition ofinterlaminar shear and edgewise shear onlyrecently to the ASTM Standard Method ac-counts for the lack of this data. The otherproperties have appeared in the Methods for anumber of years.

McNatt (1973) has evaluated nine com-mercially produced particleboards for basicengineering properties in a attempt to eventu-ally establish design stresses for particle-board. The boards selected represented arange of thicknesses, resin types, particleconfigurations and wood species, as well aslayered and homogeneous structures. Inter-laminar shear strength and internal bond werepositively and linearly correlated for all parti-cleboards. This is not surprising, as both are ameasure of the interparticle bonding in thecore. A positive and linear correlation betweenmodulus of elasticity in bending and plateshear modulus was also obtained for allboards. Tensile strength parallel to the surfacewas linearly and positively correlated to themodulus of rupture and the logarithm of tensile

strength parallel to the surface was correlatedto the edgewise shear strength. The latterregression was significantly improved byremoving the data for a board with large aspenflakes which had a phenol-formaldehydeadhesive and was intended for structural appli-cations. The large component flakes of thismaterial resulted in greater variability in theoverall properties as compared to boardsmade with smaller particles. Another signi-ficant finding (McNatt 1973) was a lack of anylarge consistent differences in properties dueto orientation of test specimens either parallelor perpendicular to the mat forming direction.This report by McNatt (1973) begins to fill alarge void in the literature and it is hoped otherresearchers will follow his example.

In a related study, McNatt (1974) deter-mined the effect of various relative humidityconditions on the same engineering propertiesdetermined earlier (McNatt 1973). Five com-mercial particleboards were tested at 65 per-cent relative humidity to establish the controllevel followed by repeated testing after furtherrelative humidity exposures of various timeperiods. No significant differences were notedin the response to relative humidity changesfor the different boards, they all behavedsubstantially the same regardless of resin type,species, or density. All properties increasedabove 80 percent, whereas the bending, ten-sion, and shear properties decreased, and thecompressive properties increased at ovendryconditions. At 30 percent relative humidity allproperties, except bending strength, werehigher than the controls were at 65 percentrelative humidity.

Superfesky (1974), in a study to comparetwo different sheet metal screws for the screwwithdrawl resistance test in particleboard,found edge withdrawal lower than face with-drawal for the eight particleboards tested. Norelationship was found between density andscrew withdrawal resistance of these com-mercial boards because board structure, parti-cle size, resin type, species, and processingvaried between boards. Post (1961) reportedonly a weak relationship between screw with-drawal resistance and resin content. Flakesize, unless excessively thick, also had only asmall effect.

Didriksson et al. (1974) have reported onthe edge-splitting tendency of wood-basedmaterials, including particleboard. Their tech-nique consisted of measuring the internalbond both before and after placing a screwinto the sample edge. In general, the IB wasapproximately 10 percent lower with the screwinserted at the thickness midpoint in the boardedge. Carroll (1970) has shown the importance

45

of not overdriving a screw in particleboard;overdriving substantially reduces screw with-drawal strength. Carroll suggested this drasticreduction in screw withdrawal resistance as aresult of overdriving may contribute to parti-cleboard’s poor reputation for fastening withscrews.

Johnson and Haygreen (1974) have meas-ured the impact strength, with a sandbag droptest, of four particleboard types and comparedthem to plywood. The impact strength ofparticleboard was well-correlated to thetoughness, but only moderately and negativelycorrelated to board density and torsionalinternal bond. No correlation existed betweenimpact strength and static bending properties.

Relatively little literature is available on therheological behavior of particleboard. Thecreep of particleboard subjected to long termstatic loads under cyclic relative humidityconditions will become extremely important asthe structural use of particleboard increases,Norimoto and Yamada (1966) found particle-board possessed significantly more creep thandid solid wood. Bryan (1960) published one ofthe first articles which appeared in this countryon the long term loading behavior of particle-board, He measured the time to failure forboards loaded to various percentages of theirmaximum load and reported that adhesive typeand density were not significant factors. Thecorrelation coefficients for a linear relation-ship between the flexural stress ratio and thelogarithm of time were variable and not highlysignificant.

Bryan and Schniewind (1965) studied theeffect of cyclic moisture conditions on thecreep properties of both urea-formaldehydeand phenol-formaldehyde bonded particleboards and reported increases in creep whenthe moisture content of the specimen changed.Adsorption (from 6 to 8 percent MC) resultedin higher creep levels in all cases but desorp-tion for high initial moisture contents (from 18to 6 percent MC) did not increase creep; de-sorption from intermediate moisture contents(from 12 to 6 percent MC) exhibited creepvalues similar to adsorption. Bryan andSchniewind (1965) also presented datashowing creep in particleboard to be moresevere under cyclic relative humidity condi-tions and to show only very little recoverywhen the relative humidity decreases from 50to 30 percent. Here also, no significant differ-ences between urea-formaldehyde and phe-nol-formaldehyde bonded boards were noted.

Many further studies are presently beingconducted on the creep and impact strength ofparticleboard. Not only are these propertiesnot adequately known for particleboard butthe effects of processing factors on theseproperties are also unknown. However, noinformation is available specifying impactstrength and creep resistance required for amaterial to be applicable for constructionpurposes. When this is determined, work canproceed on designing a particleboard productto meet these requirements, if presently avail-able particleboard is unsatisfactory.

MOISTURE ANDDIMENSIONAL PROPERTIES

Particleboard is hygroscopic and dimen-sionally unstable when exposed to water vaporor liquid water. Because the material pos-sesses hygroscopic properties similiar to solidwood, it will adsorb moisture from high humid-ity atmosphere and increase in volume; how-ever, subsequent drying does not result in areturn to the original volume. Excessive dimen-sional changes in the material after installationcan be disastrous; proper installation andefforts to eliminate large moisture fluctuationsare mandatory for satisfactory utilization ofparticleboard. This irreversible, dimensionalinstability of particleboard is believed to be theresult of compressive stresses incorporatedinto the panel in the pressing operation, and ofpossible hydrolysis of the urea-formaldehydeadhesives at elevated moisture conditions. Therelease of compressive stresses, called spring-

back, has been reviewed earlier; this sectionwill review moisture content changes anddimensional properties as affected by boarddensity, particle configuration, adhesives, andwax content.

Equilibrium Moisture Content

Relatively little literature is available inwhich the effect of particleboard processingon the equilibrium moisture content (EMC) forvarious relative humidities has been studied.Halligan and Schniewind (1972) presentedsorption isotherms for two particleboards ofdifferent densities and different urea-formal-dehyde adhesive levels. A 50 Ib/ft3 board with10 percent adhesive content (resin solids onOD wood) had lower equilibrium moisturecontents at all relative humidities than didboards of 37 Ib/ft3 with a 4 and 6 percent adhe-

46

sive content in the core and surface layers,respectively (fig. 14). Below 30 percent relativehumidity, the isotherm for the 50 Ib/ft3 boardwas slightly below the isotherm for solid wood.Above 30 percent re lat ive humidi ty, the in-crease in moisture content wi th increasingrelative humidity was slower for both particle-boards when compared to solid wood. At theh igher re la t i ve humid i t i es , the so l id woodattained an EMC at least 5 percent above thatof either particleboard.

From the tabulated data presented (Halli-gan and Schniewind 1972), board density wasthe most significant factor in reducing EMC; asdensity increased within each resin level, EMCdecreased. Only at relative humidities above90 percent was the EMC consistently reducedas the resin content increased. However, theincreased bonding efficiency believed to occuras the board dens i t y inc reased may havespread the adhesive sufficiently to block addi-

tional sorption sites. The resulting reducedsorption would appear to have been due tohigher density but may, in fact, have been dueto the adhesive. No mention of this possibilitywas given by Halligan and Schniewind nor canthis hypothesis be tested on their data. Higheramounts of wood material should not reducethe EMC as the hygroscopicity of the addi-tional wood is the same, but the additionalpress t ime required for the higher densi tyboard may have reduced the hygroscopicity ofthe wood.

No good comparison between the equili-brium moisture content of urea-formaldehydeand phenol- formaldehyde-bonded boards ispossible from the work of Halligan and Schnie-wind (1972) because the adhesive and waxc o n t e n t s w e r e n o t e q u a l . S t e a m - t r e a t e dphenol-bonded boards of 37 Ib/ft3 appeared tohave lower EMC values, at relative humiditiesabove 50 percent, than the untreated boards.

Figure 14.--Absorption isotherms for particleboards compared to wood,’at 70° F.(From Halligan and Schniewind 1972)

(M 144 414)47

However, the treated 44 Ib/ft3 board had lowerequilibrium moisture contents at only 5 of the12 relative humidities studied. The results donot entirely confirm the author’s statement thatsteam treatment lowered the EMC values.

Suchsland (1972) determined adsorptionand desorption isotherms for ten commercialparticleboards of both interior and exteriortypes. All boards exhibited large sorptionhysteresis, and isotherms were lower thanthose of solid wood, but the interior boards(assumed to be urea-formaldehyde bonded)always possessed a higher moisture content ata given relative humidity than did the exteriorparticleboards (assumed to be phenol-formal-dehyde bonded). Suchsland theorized thatexposure to high temperatures commonlyencountered in drying was responsible for thereduced hygroscopicity of particleboard ascompared to solid wood. He also pointed outthat press temperatures for the exterior parti-cleboards are higher than those used forinterior particleboard, possibly accounting forthe lower hygroscopici ty of the exter iorboards.

Jorgensen and Odell (1961) found thatparticleboards with intermediate adhesivespread levels (6.54 gm resin solids/m2 of parti-cle surface) reached a lower EMC than did theparticleboard with either a lower or higheradhesive spread level (3.27 and 13.07 gm resinsolids/m2 of particle surface). Jorgensen andOdell (1961) could not explain this behaviorand the lack of further research in this areadoes not allow confirmation of these findings.

Schneider (1973) reported extremely highEMC values for phenol-formaldehyde-bondedparticleboard at relative humidities above 90percent. At relative humidities below 90 percent, the EMC values were not significantlyhigher than solid wood. Urea-formaldehyde-bonded particleboard had isotherms similar tothose of wood below 60 percent relative hu-midity but above this level the EMC was belowthat of wood.

Thermal treatments to which particle-boards are subjected in the hot press arebelieved to reduce the sorption ability of thewood (Seifert 1972; Wittmann 1973). Seifertfound heat treatment to be much more effec-tive in reducing adsorption at low relativehumidities; he reported that capillary conden-sation at high relative humidities is less affec-ted by thermal treatments. Seifert also foundthat reduction of sorption caused by non-hygroscopic adhesives blocking the sorptionsites does not occur.

There appears to be no consistent patternin the literature for the effect of various pro-

cessing parameters on the equilibrium mois-ture content of particleboard. Very little infor-mation has been published in this area. Thelack of this fundamental knowledge has notrestricted the use of particleboard and, asthere are many other problems which needmore urgent attention, research in this areashould not be given top priority.

It is obvious that particleboard--directlyderived from the hygroscopic material, wood--will also adsorb and release moisture whensubjected to high and low relative humidities.The exact equilibrium moisture content at-tained by a particular particleboard at a givenrelative humidity is of much less importancethan are the dimensional changes which occuras the moisture content fluctuates. Thesedimensional changes have been widely studiedand are reviewed in the following section.

Dimensional Properties

Solid wood shrinks and swells when sub-jected to environmental conditions causingdesorption or adsorption of water. Particle-board also shrinks and swells under the sameconditions, but the magnitude of this dimen-sional change is much greater in the thicknessdirection of conventionally flat-pressed parti-cleboard than would be expected from thenormal shrinking and swelling of wood mater-ial. The linear dimensional changes in particle-board are normally slightly greater than thelongitudinal changes found in solid wood.

Kollman et al. (1975) graphically com-pared the dimensional stability of particle-board to that of solid pine wood. The averagelinear expansion compares favorably with thelongitudinal swelling of pine, but the thicknessswelling is much greater than the tangentialswelling and continues to increase at an in-creasing rate with moisture content. Thisreflects the large amount of springback asso-ciated with flat-pressed particleboard as aresult of the compressive set during manu-facture.

The abnormal thickness swelling ob-served in flat-pressed board subjected to highhumidities is not entirely reversible when theboard is subsequently redried. This irrevers-ible thickness swelling is the springbackdiscussed earlier and results from the com-pressive stresses which are “set” in the boardduring hot pressing. Plasticization by moisturereleases this set and, in some instances, thespringback can be sufficiently large to destroya major portion of the interparticle bonding.

The dimensional changes of particleboardsubjected to various relative humidities havebeen widely studied, but direct comparison

48

between reports is often difficult due to theenormous number of processing variableswhich directly and indirectly affect dimen-sional changes. In addition, there are basicallytwo methods which are used to study thesechanges: ASTM Standard Method D 1037(ASTM 1975) and the vacuum-pressure-soak(VPS) method first reported in the particle-board literature by Heebink (1967).

The ASTM Method D 1037 for thicknessswell requires moisture conditioning to aconstant weight, then submersion of the speci-men in water with no pressure. Thicknessmeasurements are made after 2 and 24 hourssubmersion and dimension changes are ex-pressed as a percent of the original thickness.Linear dimensional changes are determinedfrom another specimen conditioned to 90percent relative humidity at 20° C after thesample has been initially conditioned to 50percent relative humidity. Linear expansion isthen expressed as a percent increase per inchof the original dimension. Initial humiditylevels other than 50 percent are often used;30 to 90 percent is a frequently used relativehumidity exposure test.

The VPS method uses matched samplesof particleboard--one sample ovendried, theother subjected to 30 minutes of vacuumfollowed by 22 hours submerged in waterunder static pressure. The linear dimensionalchanges for the two specimens are added to-gether and expressed as a percent change perinch of the original dimension. The thicknesschange is calculated from the soaked speci-men as a percent of the ovendry thickness.

The advantage of the vacuum-pressure-soak method is that prior conditioning of thespecimens to a constant weight is not neces-sary as all dimensional changes are basedupon the ovendry dimensions. This reducesthe time required to obtain the information, butthe ASTM method, with the conditioning stepprior to testing, should allow the moisturegradients induced in the boards during hotpressing to be substantially eliminated. Theeffect, if any, of this moisture content gradienton the dimensional properties of particleboardis not known.

Water Absorption and Thickness Swelling

Water absorption and thickness swellingnormally are determined on the same speci-men in both the ASTM and VPS methods. TheATSM method specifies horizontal submer-sion of a 12- X 12-inch specimen and notesvertical submersion will result in absorptionof more water; therefore, comparisons be-tween water absorption and thickness swelling

results obtained by the two techniques is notpossible. The specimen size used in the ASTMtest by various researchers is variable, limitingdirect comparisons.

Johnson (1964) studied 24-hour watersoaking in both vertical and horizontal posi-tions of 26 commercial particleboard speci-mens, 3 X 15 inches. In general, more waterwas absorbed by the vertically soaked speci-mens, although five of the 26 particleboardsabsorbed more water in the horizontal posi-tion. The vertically soaked samples absorbedthe moisture more quickly than did the hori-zontally soaked samples, but at an equal mois-ture content increase there was very littledifference in the thickness swelling. However,because the vertically soaked specimen ab-sorbed more water and absorbed it faster, italso had more thickness swelling at any givensoaking time.

Johnson (1964) also measured the mois-ture content and thickness swelling of boardsconditioned at 65 percent relative humidityand 70° F for 44 days (fig. 15). These sampleswere initially conditioned for one month at 30percent relative humidity at 90° F. The thick-ness swelling at 65 percent relative humiditywas less than 2 percent and the moisture con-tent increase was approximately 2.5 percent.The data indicate that samples attained aconstant EMC and thickness swelling duringthe 154 days at this humidity. However, thedata for the 90 percent humidity conditiondefinitely showed thickness swelling stillincreasing rapidly at the end of the 44-dayperiod. Thickness swelling versus moisturecontent was approximately linear between 2and 14 days at this condition. After 14 daysthe moisture content increased less than onepercent for the remaining 30 days at this rela-tive humidity but the thickness swelling in-creased more than 3 percent and was stillincreasing when the test was terminated. Thisindicates that a relaxation or other time-dependent phenomenon such as springback,independent of moisture content, is control-ling the thickness swelling. The author madeno comment on this behavior.

The processing parameters and theireffect on water absorption and thicknessswelling cannot be studied from Johnson’spaper; no identification or description wasgiven other than the particleboard thick-nesses. Johnson’s paper does illustrate thelong time period required to attain equilibriumthickness swelling data at various relativehumidities. The literature appears to containmuch swelling data which has been obtainedunder non-equilibrium conditions as a result

49

of short conditioning periods (Neusser et al.1965).

Density.--The effect of board density onthe thickness swelling of particleboard isconfounded with the springback behavior.Higher density boards will possess more com-pressive set than lower density boards whenboth are made with the same wood furnish.

Therefore, it is not surprising that thicknessswelling is normally reported to increase withincreasing board density (for example, Gat-chell et al. 1966; Halligan and Schniewind1972; Hse 1975). However, this finding has notbeen unanimous; other authors have foundeither an increase or no change in thicknessstability with increasing particleboard density

Figure 15.--Relationship between th ickness swel l ing and mois turecontent increase.(From Johnson 1964)

(M 144 415)

50

(Lehmann and Hefty 1973; Vital et al. 1974)Vital et al. (1974), with particleboards

from exotic hardwoods of four different wooddensities, studied the water absorption andthickness swelling properties. Two compac-tion ratios were used and boards were madewith all possible species mixtures of one, two,three, and four species. Water absorption andthickness swelling values were determined forall boards over the relative humidity incre-ments of 30 to 90 percent, 50 to 90 percent, and30 percent to water soaking for 24 hours.Unfortunately, the specimen sizes used are notreported, nor is the conditioning time at eachrelative humidity given in the paper. Since thehighest density board was 0.93 gm/cm3 (gramsper cubic centimeter) (OD weight, volume at65 percent relative humidity) the time to equi-librium moisture content can be assumed tohave been quite long.

Not surprisingly, the water absorption inthe 24-hour water soak test was highly cor-related with the board density (Vital et al.1974). For all species combinations, the highercompaction ratio (1.6) always absorbed alower amount of water than the lower compac-tion ratio (1.2). Water entry into the higherdensity boards occurred at a slower rate due tothe decreased porosity and the increasedwood material. Total water absorbed from theatmosphere (30 to 90 and 50 to 90 percentrelative humidity increments) was also higherin the lower compaction boards in all cases,but the difference between the two ratios wasmuch less significant (approximately 1 percentmoisture content). The longer times at theserelative humidities obviously allowed themoisture to equalize much more uniformlythrough the board than was possible in the24-hour water soak.

With only a few exceptions, an increase inboard density resulted in a decrease in thick-ness swelling for all three exposure conditions.The lower moisture absorbed by the higherdensity boards was one possible explanationfor this behavior. Additional reasons givenwere the increased interparticle bonding at thehigher density or the higher moisture contentduring pressing, as reduced porosity of thehigh-density mat limits the excape of moistureand allows increased compressive set in thefinal panel.

Lehmann and Hefty (1973) found norelationship between particleboard densityand thickness swelling except at the 2 percentadhesive level. At this level of urea-formalde-hyde resin, the lower density board, 0.65gm/cm3 (OD weight, volume at 65 percentrelative humidity), had lower thickness swell-

ing than the board with a density of 0.75 gm/cm3 (OD weight volume at 65 percent relativehumidity).

Gatchell et al (1966) determined the thick-ness swelling of phenolic bonded particle-board specimens. 22 inches long. 5/8 inchwide, and of various densities. The exposureconditions for determining the thicknessswelling were from ovendry to 30, 65, 80, and90 percent relative humidity. as well as a 30-day water soak period. Thickness swelling wasindependent of density up to 80 percent rela-tive humidity for densities of 30, 40, and 50Ib/ft3. Above 80 percent the high-densityparticleboard had higher thickness swelling.attributed to springback from the higher com-pressive set at this density. Lehmann (1970)also found no significant effect of density onthe thickness swelling of Douglas-fir particle-board between 30 and 90 percent relativehumidity.

Roffael and Rauch (1972a) reported on thethickness swelling of particleboards with awide range of densities (0.51 to 0.94 gm/cm3)when subjected to water soaking at 20° C.They found a decrease in absorption but anincrease in thickness swelling as the densityincreased. The ratio of thickness swelling towater absorption at low board densities wasconstant for practically the entire 10-daysoaking period. For the high-density boards,this ratio increased to a maximum with in-creasing soaking time followed by a decreasein the longer soaking times. This behaviorreflects the slow rate of thickness swellingfound for the boards with densities above 0.7gm/cm3 (fig.16) and is attributed to low diffusionrates in the high-density boards. However, thereduced porosity in the high-density boards,as compared to those of lower density, pre-vented rapid liquid water penetration through-out the board; consequently, the diffusion pathof the water into individual component parti-cles was much longer and the subsequent rateof thickness swelling was reduced in the high-density material.

Halligan and Schniewind (1972), for aseries of particleboards with three resin con-tents at each of three densities, found a higherthickness swelling as the board density in-creased for moisture contents above 10 per-cent. Below 10 percent moisture content,board density appeared to have little influenceon thickness swelling. The boards reached 10percent moisture content at approximately 80percent relative humidity; therefore, the dataagree well with that of Gatchell et al. (1966).

Stewart and Lehmann (1973) did not find aconsistent relationship between thickness

51

swelling and board density for particleboardsproduced from cross-grain, knife-planedflakes from four different hardwoods. Bass-wood particles produced particleboard inwhich thickness swelling decreased signifi-cantly as board density increased. Yellowpoplar, red oak, and hickory all had approxi-mately the same thickness swelling regardlessof particleboard density.

Hann et al. (1992) reported increasedthickness swelling in 24-hour water soakingwhen the density of Douglas-fir particleboardwas increased from 34 to 43 Ib/ft3. This wastrue for both urea-formaldehyde- and phenol-formaldehyde-bonded boards.

Suchsland (1973) determined the thick-ness swelling of ten commercial particle-boards under cyclic relative humidity andwater soak exposure and found no correlationbetween board density and thickness swelling.The rate of thickness swelling was initially lessthan the rate of water volume absorptionwhich, according to Suchsland, indicatesinternal swelling. As the moisture contentincreased, the rate of swelling was higher thanthe rate of water volume absorption and wasattributed to internal failures in the lowerdensity core. Suchsland stated that efforts toreduce thickness swelling should concentrateon reducing the horizontal density distribution

Figure 16.--Influence of particleboard density on rate of thicknessswelling.(From Roffael and Rauch 1972a)

(M 144 416)52

so that high- and low-density regions areeliminated. This ideal can never by obtainedwith wood particles but it can be approachedas the particle size decreases.

Lehmann (1974) found a relatively minoreffect of density on thickness swelling with l-and 30-day water soaking. The results wereobtained with Douglas-fir particleboard madeat two densities (37.5 and 42.5 Ib/ft3, OD weightand volume at 65 percent relative humidity)with various flake configurations and threelevels of phenol-formaldehyde adhesive con-tent. The higher density board exhibited lessswelling and water absorption with the 24-hoursoak but more swelling in the 30-day soak andin the OD-VPS test than the lower densityboard. This is to be expected because theslower diffusion rate in the higher densitymaterial prevents attainment of moisture equi-librium in the 24-hour soak. However, no sta-tistically significant effect was found relatingparticleboard density to the thickness swellingin this study.

Hse (1975), in contrast to Lehmann (1974),found increased thickness swelling with in-creased board density in the 5-hour boil andthe VPS tests for phenol-formaldehyde-bond-ed particleboard. However, in the 50 to 90 per-cent relative humidity tests no density effectwas obtained. Gertjejansen et al. (1973) foundincreased thickness swelling with increasedboard density for phenolic-bonded wafer-type particleboard made with various mixturesof aspen, tamarack, and paper birch. Theyused the VPS test for determining thicknessswelling.

A consistent relationship between boarddensity and thickness swelling is not obviousin the present literature. Reports of increasedthickness swelling with increasing boarddensity are balanced by other reports in whichno relationship is evident. In most instances inwhich thickness swelling was reported to berelated to board density, the test method em-ployed was the VPS test or a boiling water test.These two methods and--to a slightly lesserdegree--the 24-hour water soak will allowfaster compressive stress release than willmethods in which the boards are conditionedat various relative humidities. Also, moistureequilibrium at a given relative humidity doesnot imply thickness swelling equilibrium,especially at higher relative humidities. Com-pressive stress release, resulting in additionalthickness swelling, can probably continue tooccur long after moisture equilibrium is at-tained. Higher density boards are subjected tohigher compressive failure during production;therefore, more springback would be expectedfrom these boards when re-wetted. In appli-

cations such as furniture corestock, in whichexposure to liquid water is minimal, the thick-ness swelling determined with various relativehumidity exposures appears to be more appli-cable. The long exposure times required forthis test severely limit its use by most re-searchers.

Particle configuration.--The effect ofparticle size and shape on the thicknessswelling of particleboard has been widelyreported in the literature. Turner (1954) wasone of the first investigators to systematicallystudy this variable. His test method consistedof determining the thickness after two water-soaking-ovendrying cycles and expressingthis thickness as a percent of the originalthickness. This procedure is more severe thanthe presently used 24-hour water soak. Turner(1954) showed that flake length has no signi-ficant effect upon thickness swelling. Long,thin, narrow strands with 2 percent phenolicresin resulted in a poorer thickness stabilitythan the flat flakes, but, with increased resincontents of 4 and 8 percent, the stability of theboard produced with the strands was betterthan the flakes at equal resin content (resinsolids on OD wood).

Lehmann (1974), in his study of Douglas-fir flakes 0.5, 1.0, and 2.0 inches long and 0.030and 0.045 inch thick, found no significanteffect of flake length on particleboard thick-ness swelling with either the VPS or relativehumidity exposure tests. However, the thinnerflakes resulted in slightly less thicknessswelling, especially in the OD-VPS test. Brum-baugh (1960) also reported improved thick-ness stability with thin (0.009 inch) Douglas-firflakes and a decrease in thickness stabilitywith a flake length of 0.5 inch. The waterabsorption of the OS-inch-long flake wassignificantly higher, reflecting the increasedpercentage of end-grain on the flake surface.Longer flakes (1, 2, and 4 inches) produced amore stable board as determined by the 24-hour water soak test. Jorgensen and Odell(1961) and Post (1956) also found improvedthickness stability in particleboard producedwith oak flakes 0.006 inch thick as comparedto the other flake thicknesses studied (0.012,0.025, and 0.050 inch). The flake length used inthis study (0.5, 1, 2, and 4 inches) did notsignificantly affect the thickness stability.

Stewart and Lehmann (1973), using cross-grain, knife-planed hardwood flakes of fourspecies and three thicknesses (0.006, 0.012,and 0.016 inch), found no relationship betweenthe flake thickness and thickness swelling inthe resulting particleboard. Kimoto et al.(1964) reported a slight decrease in thicknessswelling, determined by the water soak test,

53

as the particle dimensions increased for low-density lauan particleboard. No effect ofparticle configuration on thickness stabilitywas evident in the relative humidity exposuretest.

Heebink and Hann (1959) found northernred oak flakes 1 inch long produced a morestable particleboard than did flakes 0.25 inchlong. Post (1961) reported that flake length hadno relationship to thickness stability when theflake thickness was below 0.012 inch. Withflakes thicker than that, stability was improvedwith increasing flake length.

Beech (1975) studied the thicknessswelling properties of phenol-formaldehyde-bonded particleboard made with angle-cutparticles of Scots pine. The angle-cut flakeswere produced in such a manner that theorientation of the grain was approximately20° out of the flake plane. This allowed a por-tion of the large dimensional movement acrossthe grain to be shifted to the linear dimensionalmovement in the produced particleboard andfurther allowed the wood longitudinal direc-tion to be slightly oriented in the board thick-ness. This reduced the thickness swelling forparticleboards produced for these angle-cutparticles below the level obtained in particle-board produced with standard particles. How-ever, as would be expected, this particle alsoresulted in boards with substantially reducedstrength. Beech (1975) did not determinelinear expansion but this, too, would be ex-pected to increase with the angle-cut particles.The use of angle-cut particles as a method forreducing the thickness swelling in particle-board appears to have limited potential.

The available literature appears to bepractically unanimous in that better thicknessstability is obtained with boards producedfrom thin particles than from thick particles.The lower wood mass in each particle and theincreased number of particle-particle inter-faces possibly allows better dispersion of thehygroscopic swelling into the interparticlevoids. Consequently, this swelling into themacroscopic board voids, not internal swellingwithin the wood particles, results in less thick-ness swelling.

Much less agreement is found regardingthe influence of particle length on thicknessswelling. In general, it appears that increasingparticle length improves the thickness stabil-ity. A possible reason for this may be as theparticle length increases and the particle isbonded to a larger region of the board, it ismore effective in dispersing localized swellingstresses over a larger region--i.e., it serves asa moderating influence and reduces the effect

of high density regions on thickness swelling.Further research is required on both themacrostructure and microstructure of particle-board before the true influence of particlelength on the thickness swelling of particle-board is known.

Resin and wax.--Increasing the resincontent improves the thickness stability ofparticleboard (Hann et al. 1963; Kimoto et al.1964; Gatchell et al. 1966; Lehmann and Hefty1973). Lehmann (1970) found optimum thick-ness swelling for urea-formaldehyde-bondedDouglas-fir particleboard occurred below 8percent adhesive (resin solids on OD wood).Lehmann (1974) also found improved thick-ness stability in Douglas-fir flakeboard withincreasing levels of phenol-formaldehydeadhesive. The three adhesive levels studied(3, 6, and 9 percent resin solids on OD wood)did not indicate an optimum level, but theimprovement obtained between 6 and 9 per-cent was lower than the improvement between3 and 6 percent. Beech (1975) reported re-duced irreversible swelling or springbackwhen the phenol-formaldehyde adhesivecontent was increased from 9 to 12 percent.He also found less reversible and irreversibleswelling for Scots pine and beech particle-board subjected to a 24-hour water soak whenbonded with phenol-formaldehyde instead ofurea-formaldehyde adhesive. Oak particle-board bonded with urea-formaldehyde wasmore stable than when phenol-formaldehydewas the binder. This was attributed to the inter-ference of the acidic extracts on the alkalinecatalyst in the phenol-formaldehyde adhesive.

Increasing the resin content in a givenparticleboard will result in improved inter-particle bonding which should also improvethe thickness stability. However, in all proba-bility there is a level at which additional resinwill no longer enhance this bonding and servesonly to impregnate or coat the componentparticles. This is the basis upon which impreg-nating resins have been used to limit thehygroscopicity of the material, but the econo-mics do not presently justify this treatment.

The use of a paraffin wax is widespread inthe particleboard industry for both urea-formaldehyde- and phenol-formaldehyde-bonded boards. Contradictory claims appearin the literature as to the value of the wax inreducing the dimensional changes of particle-board. Gatchell et al. (1966) and Heebink andHann (1959) found no reduction in either mois-ture content or dimensional changes for wax-treated particleboard under long-term expo-sure to water vapor. Stegmann and Durst(1965) reported a large reduction in both water

54

absorption and thickness swelling in the 24-hour water soak test by addition of wax. How-ever, other researchers (Maku et al. 1956,1957; Heebink and Hann 1959) indicate waxaddition moderates dimensional changesduring fluctuations in relative humidity andshort-term exposures to liquid water. Exces-sive amounts of wax, greater than 1 percentwax solids on ovendry wood, interferes withadhesive bonding and reduces the strengthproperties.

Other proposals which have been made toreduce the hygroscopicity and consequentlyimprove the dimensional stability of particle-board are heating the unbonded particles orthe finished particleboards with or withoutsteam (Suchsland and Enlow 1968; Heebinkand Hefty 1969; Halligan and Schniewind1972; Burmester 1973; Hujanen 1973). Post-steaming appears to be the most effectivemethod for improving dimensional stabilitybut the additional costs of this treatment haveprevented its acceptance by the industry.Linear Expansion

The linear expansion of particleboardwhen exposed to moisture is much less thanthe radial swelling, but greater than the longi-tudinal swelling, of solid wood. Consideringthe orientation of the component particles inthe mat, this response is not surprising. Mostparticles are oriented with grain parallel to theplane of the particleboard. In a randomlyformed board, particles are desposited at allpossible angles to each other but still with thegrain direction basically in the plane of theboard. Consequently, the linear expansion inthe x and y directions will be equal to eachother but greater than the longitudinal swellingand less than the transverse swelling of solidwood. In oriented-flake particleboards, inwhich the individual particles are alined par-allel to one another in the panel, the linearexpansion in the two directions of the panelwill not be equal (Geimer et al. 1975). Thelinear expansion in the direction of orientationwill decrease and approach the longitudinalswelling of the solid wood while the linearexpansion perpendicular to the alinement willincrease and approach the transverse swellingof solid wood. With increased usage of alinedparticleboard in the construction industry,the isotropic swelling behavior of this materialwill have to be recognized and installationadjustments made to insure satisfactoryservice.

The ASTM Standard Method D 1037(1975) for determining linear expansion re-quires equalizing a 3- by 12-inch specimen to

50 percent relative humidity, conditioning thespecimen at 90 percent relative humidity, anddetermining the percent expansion of thespecimen between the respective relativehumidities. Many investigators have used30 to 90 percent relative humidity incrementsand specimens not of the standard sizes.These adaptations of the standard methodshould not severely affect the final resultsprovided the specimens have attained equili-brium at the relative humidity conditions used.Board density and particle configuration arediscussed separately here regarding theireffects on linear expansion.

Density.--Vital et al. (1974), in their studyof particleboard pressed to two compactionratios, found a slightly greater linear expan-sion with the higher compaction ratio--i.e.,higher board density. They used both 30 to 90and 50 to 90 percent relative humidity incre-ments and, although the effect of compactionratio on linear expansion was small and some-what variable, a trend toward greater linearexpansion with the higher compaction ratiowas ev ident fo r bo th re la t i ve humid i tyexposures.

Stewart and Lehmann (1973), in theirstudy of particleboard made with cross-grain,knife-planed flakes of four hardwood species,found an increase in linear expansion withincreasing board density for a 30 to 90 percentrelative humidity exposure with all speciesexcept the low-density basswood. This sameeffect was found for particleboard made withthe two high-density species, red oak andhickory, after a 30-day water soak exposure.However, the linear expansion for the 30-daywater soak test decreased with density for theb o a r d s m a d e w i t h y e l l o w - p o p l a r a n dbasswood.

Stewart and Lehmann (1973) also re-ported swelling data for basswood particle-boards which show the linear expansion fromboth 30 to 90 percent relative humidity expo-sure and the 30-day water soak test to bepractically equal. Basswood particleboardsof 40 and 45 Ib/ft3 nominal density had 0.10and 0.08 percent linear expansion, respec-tively, for the 30 to 90 percent relative humidityexposure. However, the same boards, sub-jected to a 30-day water soak test, had 0.12 and0.09 percent linear expansion. All other parti-cleboards made in this study had linear expan-sions in the 30-day water soak test twice thoseobtained from the 30 to 90 percent relativehumidity exposures. This unique swellingbehavior of the basswood particleboards wasnot discussed by the authors.

55

Suchsland (1972) found no clear relation-ship between board density and linear expan-sion for ten commercia l part ic leboardsexposed to a 40 to 93 percent relative humidityincrement. However, the two boards with thehighest density also had the highest linearexpansion; the remaining eight boards hadapproximately the same density but the linearexpansions were widely different. Gertje-jansen et al. (1973) found no effect of boarddensity on linear expansion with phenolic-bonded waferboard composed of tamarack,paper birch, and aspen. Lehmann (1974) alsofound no effect of board density on the linearexpansion of phenol-formaldehyde-bondedparticleboard made with Douglas-fir flakesof various lengths and thicknesses.

Hse (1975) determined the linear expan-sion of particleboards produced from ninesouthern hardwoods subjected to three differ-ent exposure tests. In all three tests (VPS, 50to 90 percent relative humidity, and 5-hourboil), the linear expansion of sweetbay andred maple boards of 44.5 Ib/ft3 was less thanthat of the boards of 39.5 Ib/ft3 which was lessthan that of the boards of 49.5 Ib/ft3. Particle-boards from the other species did not consist-ently exhibit this behavior. The author did notcomment upon this minimum linear expansionat the 44.5 Ib/ft3 density for sweetbay and redmaple, and no obvious explanation is evidentfrom the data.

In general, there appears to be no consist-ent and reproducible relationship betweenparticleboard density and linear expansion.There are indications that increasing densitymay tend to increase linear expansion but theevidence is not overwhelming. No investi-gators have found a statistically valid relation-ship between linear expansion and boarddensity. Linear expansion is much moredependent upon part ic le geometry andalinement.

Particle configuration.--Gatchell et al.(1966) found increased linear expansion inDouglas-fir particleboard at all relative humid-ities when the flake thickness increased above0.015 inch. Very little difference in linearstability was noted when the flake thicknesswas reduced to 0.007 inch. Linear expansionwas also independent of flake length whenthe flake length was increased from 1 to 2inches. However, with flakes 0.5 inch long,the resulting particleboard had significantlyhigher linear expansion. When the particleconfiguration was changed from flakes toslivers to hammermilled planer shavings, thelinear stability also decreased. The authorsattributed the linear expansion of the panels

with slivers to the cylindrical shape of theslivers in which the restraint of the longitudinalgrain on the transverse grain was reduced fromthat of the flakes. The hammermilled planershavings were somewhat cubical and did notfelt with the longitudinal grain parallel to thepanel surface as is typical with flakes. Conse-quently, the random orientation of the grainwithin the particleboard increased the linearexpansion but at the same time reduced thethickness swell

Turner (1954) also showed the tremen-dous improvement possible in linear stabilityby decreasing the flake thickness from 0.030 to0.009 inch. Particleboard with flakes 1.5 incheslong had slightly better linear stability thanwith flakes 3 inches long for flake thicknessof 0.030 inch. However, when the thicknesswas reduced to 0.018 inch, the linear expan-sion of the particleboard with the 1.5-inch flakewas reduced to much more than for the 3-inchflake, and was comparable to that of the 3-inchflake 0.009 inch thick. Heebink and Hann(1959) also found that red oak flakes 1 inchlong produced more linearly stable particle-board than did 0.25-inch-long flakes, shavings,sawdust or slivers. Post (1961) also found thatlinear stability was not greatly affected bychanges in flake length or thickness below athickness of 0.012 inch; above this thickness,decreasing length reduced linear stability.Brumbaugh (1960) also found increased linearexpansion in particleboard from Douglas-firflakes as the flake length decreased and theflake thickness increased.

Lehmann (1974) subjected particleboardsmade with Douglas-fir flakes of three lengths(0.5, I, and 2 inches) and two thicknesses(0.030 and 0.045 inch) to five exposure con-ditions. These exposure tests were 30 to 90and 50 to 90 percent relative humidity steps,1- and 30-day water soak, and ovendry-vacuum pressure soak (OD-VPS). In all teststhe panels with the 2-inch flakes had the bestlinear stability. Decreased flake thickness andincreased flake length produced better linearstability in the water soak tests but flake thick-ness had no effect in the relative humiditytests. These results indicate that the exposuretest used to study linear stability influencesthe results obtained.

Heebink (1974) found reduced linearstability with flakes 3 inches long by 0.030 inchthick as compared to flakes 2 inches long by0.020 inch thick. This indicates flake thicknessis a more important factor than either flakelength or length/thickness ratio in controllinglinear expansion. Heebink used the OD-VPStest.

56

Suchsland (1972) also concluded thatparticle size was the most important factorcontrolling linear expansion in his study of tencommercial particleboards. The exposure testhe used was 40 to 93 percent relative humidity.

Gertjejansen and Haygreen (1973) re-ported somewhat better dimensional stabilitywith flakes (0.5 inch long by 0.015 inch thick)than with wafers (1.5 inches long by 0.025 inchthick). The resin content was 8 and 3 percentrespectively (resin solids on OD wood) for theflakes and wafers, and, assuming equal parti-cle widths, the flakeboard had somewhathigher resin content per unit of surface area.However, both board types had excellentlinear stability and the absolute differenceswere small.

Howard (1974) found that linear expan-sion of particleboard made from slash pinerootwood was greater than that made from thestemwood when both were subjected to theVPS test. One possible explanation for thisbehavior was the higher grain deviation pre-sent in the flakes from the rootwood.

Stewart and Lehmann (1973) found parti-cleboard produced with cross-grain planerflakes were exceptionally stable in both rela-tive humidity exposure and 1- and 30-daywater soak. This is attributed to small intra-particle separations between fibers producedby the cutting action of the planer, preventingswelling from accumulating over the entireflake.

The various researchers appear to agreethat particleboards composed of flakes havethe highest linear stability and that thin flakes(less than 0.012 to 0.015 inch) further improvethe stability. The effect of flake length doesnot enjoy a consensus but it does appear that aflake length below 1.0 inch has a detrimentaleffect on linear expansion.

Resin and wax.--The linear expansion ofparticleboard subjected to various exposureconditions is only slightly reduced by in-creasing the resin content (Gatchell et al. 1966;Lehmann 1974). An exception to this appearsto be that, at extremely low resin contents,linear expansion is substantially increased, butabove a resin content high enough to ade-quately bond the particles, further resinaddition is of little benefit.

Urea-formaldehyde resins usually resultin less dimensional stability than the phenol-formaldehyde class because of resin hydroly-sis when boards are exposed to moisture.

The effect of wax addition on linear expan-sion has not been widely studied becauselinear expansion is relatively low in mostparticleboards. However, wax addition wouldbe expected to have a similar influence onlinear expansion as has been reported for thethickness swelling--i.e., wax will reducedimensional changes under cyclic relativehumidity conditions and short-term liquidwater exposure.

LITERATURE CITED

1. American Society for Testing and Materials1975. Standard methods of evaluating theproperties of woodbase fiber and panelmaterials. ASTM D 1037-72. Philadelphia,Pa.

2. Anderson, A. B., A. Wong, and K.-T. Wu.1974a. Utilization of white fir bark in parti-cleboard. For. Prod. J. 24(1):51-54.

3. Anderson, A. B., A. Wong, and K.-T. Wu.1974b. Utilization of white fir bark and itsextract in particleboard. For. Prod. J.24(7):40-45.

4. Anderson, A. B., A. Wong, and K.-T. Wu.1974c. Utilization of Ponderosa pine barkand its extract in particleboard. For. Prod.J. 24(8):48-53.

5. Anthony, C. P. and A. A. Moslemi1969. Strength and dimensional proper-ties of hickory flakeboards. For. Prod. J.19(7):54-55.

6. Beech, J. C.1975. The thickness swelling of woodparticleboard. Holzforschung 29(1):11-18.

7. Bismarck, C.1974. (Optimizing the pressing of particle-boards. The manufacture of particle-boards with urea-formaldehyde bindersusing special automated regulation sys-tems for the pressing process.) Holz-Zentralbl. 100(80):1247-1249.

8. Browne, F. L., D. L. Kenaga, and R. M.Gooch1966. Impregnation to control dimen-sional stability of particleboard and fiber-board. For. Prod. J. 16(11):45-53.

9. Brumbaugh, J. I.1960. Effect of flake dimensions on prop-erties of particleboard. For. Prod. J. 10(5):243-246.

57

10. Bryan, E. L.1960. Bending strength of particleboardunder long-term load. For. Prod. J. 10(4):200-204.

11. Bryan, E. L. and A. P. Schniewind1965. Strength and rheological propertiesof particleboard as affected by moisturecontent and sorption. For. Prod. J. 15(4):143-148.

12. Burmester, A.1973. Effect of heat-pressure treatmentsof semi-dry wood on its dimensionalstability. Holz Roh- Werkst. 31(6):237-243.

13. Burrows, C. H.1961. Some factors affecting resin effi-ciency in flakeboard. For. Prod. J. 11(1):27-33.

14. Carroll, M. N.1963. Efficiency of urea- and phenol-formaldehyde in particleboard. For. Prod.J. 13(3):113-120.

15. Carroll, M. N.1970. Relationship between driving torqueand screw-holding strength in particle-board and plywood. For. Prod. J. 20(3):24-29.

16. Carroll, M. N. and D. McVey1962. An analysis of resin efficiency inparticleboard. For. Prod. J. 12(7):305-310.

17. Chen, T. Y., M. Paulitsch, and G. Soto1972. On the suitability of the biologicalsurface mass from spruce thinnings as rawmaterial for particleboard. Holz Roh-Werkst. 30(1):15-18.

18. Childs, M. R.1956. The effect of density, resin content,and chip width on springback and certainother properties of dry formed flat pressedparticleboard. M.S. Thesis, Dept. of Woodand Pap. Sci., North Carolina State Univ.,Raleigh.

19. Christensen, R. L.1972. Test for measuring formaldehydeemission from formaldehyde bondedparticleboards and plywood. For. Prod. J.22(4):17-20.

20. Christensen, R. L. and P. Robitschek1974. Efficiency of resin-wood particlebonding. For. Prod. J. 24(7):22-25.

21. Clad, W.1967. Phenolic-formaldehyde conden-sates as adhesives for particleboardmanufacture. Holz Roh- Werkst. 25(4):137-147.

58

22. Deppe, H. J. and K. Ernst1965. Problems of pressing-time reduc-tion in particleboard manufacture. HolzRoh- Werkst. 23(11):441-445.

23. Deppe, H. J. and K. Ernst1971. lsocyanates as adhesives for parti-cleboard. Holz Roh- Werkst. 29(2):45-50.

24. Didriksson, E. I. E., J. O. Nyren, and E. L.Back1974. The splitting of wood-base buildingboards due to edge screwing. For. Prod. J.24(7):35-39.

25. Dost, W. A.1971. Redwood bark fiber in particle-board. For. Prod. J. 21(10):38-43.

26. Duncan, T. F.1974. Normal resin distribution in particle-board manufacture. For. Prod. J. 24(6):36-44.

27. Fahrni, F.1956. (The pressing of particleboards withparticles of high moisture content in thesurface layers.) Holz Roh- Werkst. 14(1):8-10.

28. Gatchell, C. J., B. G. Heebink, and F. V.Hefty1966. Influence of component variableson properties of particleboard for exterioruse. For. Prod. J. 16(4):46-59.

29. Geimer, R. L., H. M. Montrey, and W. F.Lehmann1975. Effects of layer characteristics onthe properties of three-layer particle-boards. For. Prod. J. 25(3):19-29.

30. Gertjejansen, R. and J. G. Haygreen1973. Effect of aspen bark from butt andupper logs on the physical properties ofwafer-type and flake-type particleboards.For. Prod. J. 23(9):66-71.

31. Gertjejansen, R., M. Hyvarinen, J.Haygreen, and D. French1973. Physical properties of phenolicbonded wafer-type particleboard frommixtures of aspen, paper birch, and tama-rack. For. Prod. J. 23(6):24-28.

32. Ginzel, W.1973. Contribution to the hydrolysis inurea-resin-bonded particleboard. HolzRoh- Werkst. 31(1):18-24.

33. Halligan, A. F.1970. A review of thickness swelling inparticleboard. Wood Sci. Technol. 4(4):301-312.

34.

35.

36.

37.

38.

39.

Halligan, A. F. and A. P. Schniewind1972. Effect of moisture on physical andcreep properties of particleboard. For.Prod. J. 22(4):41-48.

Halligan, A. F. and A. P. Schniewind1974. Prediction of particleboard mechan-ical properties at various moisture con-tents. Wood Sci. Technol. 8(1):68-78.

Hann, R. A., J. M. Black, and R. F.Blomquist1962. How durable is particleboard? For.Prod. J. 12(12):577-584.

Hann, R. A., J. M. Black, and R. F.Blomquist1963. How durable is particleboard? II.The effect of temperature and humidity.For. Prod. J. 13(5):169-174.

Hart, C. A. and J. T. Rice1963. Some observations on the develop-ment of a laboratory flakeboard process.For. Prod. J. 13(11):483-488.

Haygreen, J. G. and R. O. Gertjejansen1971. Improving the properties of particle-board by treating the particles with phe-nolic impregnating resin. Wood & Fiber3(2):95-105.

40. Haygreen, J. G. and R. O. Gertjejansen1972. Influence of the amount and type ofphenolic resin on the properties of awafer-type particleboard. For. Prod. J.22(12):30-34.

41 Heebink, B. G.

42.

43.

44.

45.

1967. A procedure for quickly evaluatingdimensional stability of particleboard.For. Prod. J. 17(9):77-80.

Heebink, B. G.1972. Irreversible dimensional changes inpanel materials. For. Prod. J. 22(5):44-48.

Heebink, B. G.1974. Particleboards from lodgepole pineforest residue. USDA For. Serv. Res. Pap.FPL 221. For. Prod. Lab., Madison, Wis.

Heebink, B. G. and R. A. Hann1959. How wax and particle shape affectstability and strength of oak particle-boards. For. Prod. J. 9(7):197-203.

Heebink, B. G., R. A. Hann, and H. H.Haskell1964. Particleboard quality as affected byplaner shaving geometry. For. Prod. J.14(10):486-494.

46. Heebink, B. G. and F. V. Hefty1969. Treatments to reduce thicknessswelling of phenolic-bonded particle-board. For. Prod. J. 19(11):17-26.

47. Heebink, B. G., W. F. Lehmann, andF. V. Hefty1972. Reducing particleboard pressingtime: Exploratory study. USDA For. Serv.Res. Pap. FPL 180. For. Prod. Lab., Madi-son, Wis.

48. Henkel, M.1969. Measurement of density profiles inparticleboards and fibreboards by X-rays.Holztechnologie 10(2):93-96.

49. Howards, E. T.1974. Slash pine rootwood in flakeboard.For. Prod. J. 24(6):29-35.

50. Hse, C.-Y.1974a. Characteristics of urea-formalde-hyde resins as related to glue bond qualityof southern pine particleboard. J. Jap.Wood Res. Soc. 20(10):483-490.

51. Hse, C.-Y.1974b. Reaction pH of urea-formaldehyderesins as related to strength properties ofsouthern pine particleboard. J. Jap. WoodRes. Soc. 20(10):491-493.

52. Hse, C.-Y.1974c. Reaction catalysts of urea-formal-dehyde resins as related to strengthproperties of southern pine particleboard.J. Jap. Wood Res. Soc. 20(11):538-540.

53. Hse, C.-Y.1975. Properties of flakeboards fromhardwoods growing on southern pinesites. For. Prod. J. 25(3):48-53.

54. Hse, C.-Y., P. Koch, C. W. McMillin andE. W. Price1975. Laboratory scale development of astructural exterior flakeboard from hard-woods growing on southern pine sites.For. Prod. J. 25(4):42-50.

55. Hujanen, D. R.1973. Comparison of three methods fordimensional ly stabi l iz ing wafer-typeparticleboard. For. Prod. J. 23(6):29-30.

56. Hunt, D. G.1965. The show-through of veneeredparticleboard. J. Inst. Wood Sci. 8(14):47-72.

59

57. Istrate, V., G. Filipescu, and C. Stefu1964. (The influence of chip dimensionon the consumption of glue and on thequality of particleboard.) Industr. Lemn.17(4):133-137.

58. Johnson, J. W.1964. Effect of exposure cycles on stabil-ity of commercial particleboard. For. Prod.J. 14(7):277-282.

59. Johnson, A. and J. G. Haygreen1974. Impact behavior of particleboardas related to some other physical proper-ties. For. Prod. J. 24(11):22-27.

60. Jorgensen, R. M. and R. L. Odell1961. Dimensional stability of oak flake-board as affected by particle geometryand resin spread. For. Prod. J. 11(10):463-466.

61. Kehr, E.1966. On the improvements of particle-board surfaces. Holz Roh- Werkst. 24(7):295-305.

62. Kehr, E. and U. Jensen1969. On thickness variations of cali-brated particleboards. Holztechnologie10(2):97-104.

63. Kehr, E. and U. Jensen1970. Manufacture and properties ofparticleboard with fine particle surfacelayers. Holz Roh- Werkst. 28(10):385-391.

64. Kehr, E., K. H. Macht, and G. Riehl1964. Contributions to the gluing andbonding process with chips in particle-board manufacture. I. About the influenceof the throughput of binders when whirl-ing nozzles are used. Holztechnologie5(1):17-26.

65. Kehr, E., K. H. Macht, and G. Riehl1968. Contributions on glue-spreadingand gluing of chips in manufacturingparticleboards. V. Investigations on thecirculating of chips in the mixer, arrange-ment of nozzles, and the stripping effectwith the spray and circulating glue spread-ing methods. Holztechnologie 9(3):169-175.

66. Kehr, E. and S. Schoelzel1965. The investigation on pressing con-ditions in the manufacture of particle-boards. Drev. Vysk. 1965(3):133-147.

6 0

67. Kehr, E. and S. Schoelzel1967. Investigations of the pressing dia-gram. II. Influence of chip moisture, mouldclosing time, and moulding pressure onthe compression characteristics duringhot-pressing particleboards. Holztechno-logie 8(3):177-181.

68. Keylwerth, R.1958. On the mechanics of the multi-layerparticleboard. Holz Roh- Werkst. 16(11):419-430.

69. Kimoto, R., E. Ishimoir, H. Sasaki, andT. Maku1964. Studies on particleboards. VI.Effects of resin content and particledimension on the physical and mechani-cal properties of low-density particle-boards. Wood Res. Kyoto U. 32:1-14

70. Klar, G. V. and J. Stofko1965. (“Combined” wood particleboardswith higher strength.) Dievo 20(1):5-7.

71. Klauditz, W., H. J. Ulbricht, W. Kratz, andA. Buro1960. The production and properties ofwoodchip material with oriented strength.Holz Roh- Werkst. 18(10):377-385.

72. Kollmann, F. P., E. W. Kuenzi, and A. J.Stamm1975. Principles of wood science andtechnology. Vol. II. Wood-based materials.Chap. 5. Springer-Verlag, New York,703 p.

73. Kusian, R.1968. Model-investigations about theinfluence of particle size on structuraland strength properties of particle mate-rials. II. Experimental investigations.Holztechnologie 9(4):241-248.

74. Larmore, F. D.1959. Influence of specific gravity andresin content on properties of particle-board. For. Prod. J. 9(4):131-134.

75. Lehmann, W. F.1960. The effects of moisture content,board density, and press temperature onthe dimensional and strength propertiesof flat-pressed particleboard. M.S. Thesis,Dep. of Wood and Pap. Sci., North Caro-lina State Univ., Raleigh.

76. Lehmann, W. F.1965. Improved particleboard throughbetter resin efficiency. For. Prod. J. 15(4):155-161.

77. Lehmann, W. F.1968. Resin distribution in flakeboardshown by ultralight photography. For.Prod. J. 18(10) :32-34.

78. Lehmann, W. F.1970. Resin efficiency in particleboard asinfluenced by density, atomization, andresin content. For. Prod. J. 20(11):48-54.

79. Lehmann, W. F.1974. Properties of structural particle-board. For. Prod. J. 24(1):19-26.

80. Lehmann, W. F. and R. L. Geimer1974. Properties of structural particle-boards from Douglas-fir forest residues.For. Prod. J. 24(10):17-25.

81. Lehmann, W. F., R. L. Geimer, and F. V.Hefty1973. Factors affecting particleboardpressing time: Interaction with catalystsystems. USDA For. Serv. Res. Pap. FPL208. For. Prod. Lab., Madison, Wis.

82. Lehmann, W. F. and F. V. Hefty1973. Resin efficiency and dimensionalstability of flakeboards. USDA For. Serv.Res. Pap. FPL 207. For. Prod. Lab., Madi-son, Wis.

83. Liiri, O.1961. Investigations into long-term moist-ening of wood particle board and theeffect of changes in moisture on its quali-ties. Pap. Puu 43(12):737-748.

84. Liiri, O.1969. The pressure in the particleboardproduction. Holz Roh- Werkst. 27(10):371-378.

85. Lutz, J. F., B. G. Heebink, H. R. Panzer,F. V. Hefty, and A. F. Mergen1969. Surfacing softwood dimensionlumber to produce good surfaces and highvalue flakes. For. Prod. J. 19(2):45-50.

86. Lynam, F. C.1959. Factors influencing the propertiesof chipboard. J. Inst. Wood Sci. 2(4):14-27.

87. Maku, T. and R. Hamada1955. Studies on chipboard. II. Swellingproperties of chipboard. Wood Res. KyotoU. 15:53-59.

88. Maku, T., R. Hamada, and H. Sasaki1957. Studies on the chipboard. Ill.Some experiments on the improvementsof dimensional stabilities of chipboard.Wood Res. Kyoto U. 17:16-24.

89. Maku, T., R. Hamada, and H. Sasaki1959. Studies on the particleboard (chip-board). IV. Temperature and moisturedistribution in particleboard during hot-pressing. Wood Res. Kyoto U. 21:34-46.

90. Maku, T., H. Sasaki, and R. Hamada1956. The effect of paraffin emulsion onthe hygroscopic, swelling, and mechani-cal properties of chipboard. J. Jap. WoodRes. 2(3):130-132.

91. Maloney, T. M.1970. Resin distribution in layered parti-cleboard. For. Prod. J. 20(1):43-52.

92. Maloney, T. M.1975. Use of short-retention-time blenderswith large-flake furnishes. For. Prod. J.25(5):21-29.

93. Marian, J. E.1958. Adhesive and adhesion problems inparticleboard production. For. Prod. J.8(6):172-176.

94. McNatt, J. D.1973. Basic engineering properties ofparticleboard. USDA For. Serv. Res. Pap.FPL 206. For. Prod. Lab., Madison, Wis.

95. McNatt, J. D.1974. Properties of particleboards atvarious humidity conditions. USDA For.Serv. Res. Pap. FPL 225. For. Prod. Lab.,Madison, Wis.

96. Meinecke, E. and W. Klauditz1962. (Physics and technology of glueapplication and mutual bonding of woodparticles in the manufacture of particle-boards.) Inst. Wood Res., Forschungsber.Landes Nordheim-Westfalen Rept. 1053,Köln and Opladen, W. German. Transl.available from U.S. Dept. of Commerce.

97. Mikhailov, N. A. and N. I. Ostapenko1972. (Internal stresses in particleboard.)Derevoobrab. Prom. 21(10):9-10.

98. Neusser, H.1967. Practical testing of urea resin gluesfor particleboard, with regard to short-ening of pressing time. Holzforsch. Holz-verwert. 19(3) :37-40.

99. Neusser, H. and V. Krames1971. Surface texture of particleboard, itsassessment and aesthetic effect. HolzRoh- Werkst. 29(3):103-118.

61

100. Neusser, H., V. Krames, and K. Haidinger1965. The effect of moisture on particle-boards with special consideration ofswelling. Holzforsch. Holzverwert. Pt. I.17(3):43-53; Pt. II. 17(4):57-69.

101. Neusser, H., V. Krames, K. Haidinger, andW. Serentschy1969. The character of particles and hisinfluence on quality of surface layersof particleboards. Holzforsch. Holzver-wert. 21(4):81-94.

102. Neusser, H.,V. Krames, and M. Zentner1969. Particleboards with different sur-face layers as basic material for overlays.Holzforsch. Holzverwert. 21(3):56-61.

103. Neusser, H. and M. Zentner1968. On the causes and control of form-aldehyde odour with wood-based ma-terial, especially particleboards. Holz-forsch. Holzverwert. 20(5):101-112.

104. Norimoto, M. and T. Yamada1966. On the creep of particle boards.Wood Res. Kyoto U. 39:22-28.

105. Otlev, I. A.1971. (Change in the moisture content ofthe chip mat during hot pressing.) Dere-voobrab. Prom. 20(10):3-4.

106. Pecina, H.1963. Determination of the dimensions ofparticle surface wetted by the adhesive.Holztechnologie 4(1):68-70.

107. Petersen, H., W. Reuther, W. Eisele, andO. Wittmann1972. Investigations on the formalde-hyde liberation during the manufactureof particleboard with urea-formaldehydeadhesives. Holz Roh- Werkst. 30(11):429-436.

108. Petersen, H., W. Reuther, W. Eisele, andO. Wittmann1973. Further investigations on the form-aldehyde liberation during particleboardproduct ion wi th urea-formaldehydeadhesives. II. Effect of resin quantity,pressing time, and temperature. HolzRoh- Werkst. 31(12) :463-469.

109. Petersen, H., W. Reuther, W. Eisele, andO. Wittmann1974. Further investigations on the form-aldehyde liberation during particleboardproduct ion wi th urea-formaldehydeadhesives. III. Influence of hardener typeand amount of formaldehyde-bindingagents. Holz Roh- Werkst. 32(10):402-410.

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110. Plath, L.1966. The determination of formaldehydeliberation from particleboards by themicrodiffusion method. Holz Roh-Werkst. 24(7) :312-318.

111. Plath, L.1967a. Tests on formaldehyde Iiberationfrom particleboard. II. The influence ofpressing time and temperature on formal-dehyde liberation. Holz Roh- Werkst.25(2) :63-68.

112. Plath, L.1967b. Tests on formaldehyde liberationfrom particleboard. III. Influence ofhardener compounds on formaldehydeliberation. Holz Roh- Werkst. 25(5):169-173.

113. Plath, L.1967c. Tests on formaldehyde liberationfrom particleboard. IV. Effect of moisturecontent in chip mat on formaldehydeliberation. Holz Roh- Werkst. 25(6):231-238.

114. Plath, L.1968. Tests on formaldehyde liberationfrom particleboard. V. The influence ofcuring acceleration and aging period onformaldehyde liberation. Holz Roh-Werkst. 26(4):125-128.

115. Plath, L.1971a. Requirements for particleboardswith plastic layers. Holz Roh- Werkst.29(10) :369-376.

116. Plath, L.1971b. A contribution on particleboardmechanics. Holz Roh- Werkst. 29(10):377-382.

117. Plath, L. and E. Schnitzler1974. The density profile, a criterion forevaluating particleboard. Holz Roh-Werkst. 32(11):443-449.

118. Polge, H. and P. Lutz1969. Possibilities of the density meas-urement of particleboards perpendicularto the plane by X-rays. Holztechnologie10(2):75-79.

119. Post, P. W.1958. The effect of particle geometry andresin content on bending strength of oakparticleboard. For. Prod. J. 8(10):317-322.

20. Post, P. W.1961. Relationship of flake size and resincontent to mechanical and dimensionalproperties of flakeboard. For. Prod. J.11 (1) :34-37.

121. Rackwitz, G.1963. Influence of chip dimensions onsome properties of wood particleboard.Holz Roh- Werkst. 21(6):200-209.

122. Rice, J. T.1960. The effects of selected variables onthe properties of flatpressed flakeboards.M.S.Thesis, Dep. of Wood and Pap. Sci.,North Carolina State Univ., Raleigh.

123. Rice, J. T.1973. Particleboard from “silage” syca-more--laboratory production and testing.For. Prod. J. 23(2):28-34.

124. Rice, J. T., J. L. Snyder, and C. A. Hart1967. Influence of selected resin andbonding factors on flakeboard proper-ties. For. Prod. J. 17(8):49-57.

125. Roffael, E. and W. Rauch1971a. Production of particleboardsusing sulfite black liquor as binder. I.Literature review and own investigations.Holzforschung 25(4):112-116.

126. Roffael, E. and W. Rauch1971b. Production of particleboardsusing sulfite black liquor as binder. II.A new and fast method for the productionof particleboards. Holzforschung 25(5):149-155.

127. Roffael, E. and W. Rauch1972a. Inf luence of densi ty on theswelling behavior of phenolic-resin-bonded par t i c leboards . Ho lz Roh-Werkst. 30(5):178-181.

128. Roffael, E. and W. Rauch1972b. Production of particleboardsusing sulfite black liquor as binder. III.Possibilities of shortening the post-thermal t reatment for sul f i te l iquorbonded particleboards of 9 mm thick-ness. Holzforschung 26(6): 197-202.

129. Roffael, E. and W. Rauch1973a. (Use of sulfite black liquors incombination with alkaline phenolic re-s ins as adhesives in particleboardgluing.) Holzforschung 27(5):178-179.

130. Roffael, E. and W. Rauch1973b. Production of particleboard usingblack liquor as binder. IV. Use of sulfitel iquor in combinat ion with alkal inephenolic resins. Holzforschung 27(6):214-217.

131. Roffael, E. and W. Rauch1973c. Influence of temperature andthermal post-treatment on some physicalproperties of diisocyanate bonded parti-cleboard. Holz Roh- Werkst. 31(10):402-405.

132. Roffael, E. and W. Rauch1974. (Comparing certain properties ofchipboards made with phenolic resinsand sulfite liquor.) Holz-Zentralbl. 100(43/44):702-704.

133. Roffael, E., K. Schaller and W. Rauch1972. (Manufacture of phenol-resin-bonded particleboards. I. The effect ofpressing temperature on some physicaland technological properties.) Holz-Zentralbl. 98(136):2003-2004.

134. Saito, F.1972. Spring back of particle boards.Wood Ind. (Jap) 27(1):14-18.

135. Schneider, A.1973. The sorption-behavior of phenolic-bonded and urea-bonded particleboards.Holz Roh- Werkst. 31(11):425-429.

136. Schorning, P., E. Roffael, and G.Stegmann1972. New wood-to-wood bonds in chipmaterials by chemical polyfunctionalsystems. I. Hot pressing with amine-alkaline bond systems. Holz Roh- Werkst.30(7):253-258.

137. Schorning, P. and G. Stegmann1972. New wood-to-wood bonds in chipmaterials by chemical polyfunctionalsystems. II. Hot pressing with woodenboards and particles in a weak acid bondsystem. Holz Roh- Werkst. 30(9):329-332.

138. Seifert, J.1972. On the sorption and swelling ofwood and wood base materials. I. Influ-ences on the sorption of wood-basematerials. Holz Roh- Werkst. 30(3):99-111.

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139. Shen, K. C.1973. Steam-press process for curingphenolic-bonded particleboard. For.Prod. J. 23(3):21-29.

140. Shen, K. C.1974. Improving the surface quality ofpart ic leboard by high-temperaturepressing. For. Prod. J. 24(10):36-39.

141. Shen, K. C. and M. N. Carroll1969. A new method for evaluation ofinternal strength of particleboard. For.Prod. J. 19(8):17-22.

142. Shen, K. C. and M. N. Carroll1970. Measurement of layer strength dis-tribution in particleboard. For. Prod. J.20( 6) :53-55.

143. Shuler, C. E.1974. Pilot study of the use of pulpwoodchipping residue for producing particle-board in Maine. Tech. Bull. No. 67, LifeSci. and Agr. Exp. Sta., Univ. of Maine,Orono.

144. Stayton, C. L. and M. J. Hyvarinen, R. O.Gertjejansen, and J. G. Haygreen1971. Aspen and paper birch mixtures asraw material for particleboard. For. Prod.J. 21(12):29-30.

145. Stegmann, G. and J. Durst1964. Particleboard from beech wood.Holz-Zentralbl. 90(153):313-318.

146. Stewart, H. A.1970. Cross-grain knife planing hardmaple produces high-quality surfacesand flakes. For. Prod. J. 20(10):39-42.

147. Stewart, H. A. and W. F. Lehmann1973. High-quality particleboard fromcross-grain, knife-planed hardwoodflakes. For. Prod. J. 23(8):52-60.

148. Stewart, H. A. and W. F. Lehmann1974. Cross-grain cutting with seg-mented helical cutters produces goodsurfaces and flakes. For. Prod. J. 24(9):104-106.

149. Strickler, M. D.1959. Effect of press cycle and moisturecontent on properties of Douglar-firflakeboard. For. Prod. J. 9(7):203-205.

150. Suchsland, O.1959. An analysis of the particleboardprocess. Q. Bull., Mich. Agr. Exp. Sta.,Mich. State Univ. 42(2):350-372.

151. Suchsland, O.1960. An analysis of a two-species three-layer wood flakeboard. Q. Bull., Mich.Agr. Exp. Sta., Mich. State Univ. 43(2):375-393.

152. Suchsland, O.1962. The density distribution in flake-board. Q. Bull., Mich. Agr. Exp. Sta.,Mich. State Univ. 45(1):104-121.

153. Suchsland, O.1967. Behavior of a particleboard matduring the pressing cycle. For. Prod. J.17(2):51-57.

154. Suchsland, O.1972. Linear hygroscopic expansion ofselected commercial particleboards. For.Prod. J. 22(11):28-32.

155. Suchsland, O.1973. Hygroscopic thickness swellingand related properties of selected com-mercial particleboards. For. Prod. J.23(7):26-30.

156. Suchsland, O. and R. C. Enlow1968. Heat treatment of exterior particle-board. For. Prod. J. 18(8):24-28.

157. Superfesky, M. J.1974. Screw withdrawal resistance oftype A and AB sheet metal screws inparticleboard and medium density hardboard. USDA For. Serv. Res. Pap. FPL239. For. Prod. Lab., Madison, Wis.

158. Talbott, J. W. and T. M. Maloney1957. Effect of several production vari-ables on the modulus of rupture andinternal bond strength of boards madeof green Douglas-fir planer shavings. For.Prod. J. 7(10):395-398.

159. Turner, H. D.1954. Effect of particle size and shape onstrength and dimensional stability ofresin bonded wood-particle panels. For.Prod. J. 4(5):210-222.

160. Vital, B. R., W. F. Lehmann, and R. S.Boone1974. How species and board densitiesaffect properties of exotic hardwoodparticleboards. For. Prod. J. 24(12):37-45.

64 5.0 - 66 - 5 - 77

U.S. GOVERNMENT PRlNTlNG OFFlCE 1977 750-027 13

161. Walker, J. F.

162.

1964. Formaldehyde. 3rd ed. ReinholdPubl. Corp., New York. 701 p.

Wittmann, O.1962. The subsequent dissociation offormaldehyde from particleboard. HolzRoh- Werkst. 20(6):221-224.

Wittmann, O.1973. Alkali in phenolic resin bondedparticleboard. Holz Roh- Werkst. 31(11):419-425.

163.

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