resin hydrolysis and mechanisms of formaldehyde release from … · resin hydrolysis and mechanisms...

RESIN HYDROLYSIS AND MECHANISMS OF FORMALDEHYDE RELEASE FROM BONDED WOOD PRODUCTS

George E. Myers Research Chemist

Forest Products Laboratory One Gifford Pinchot Drive Madison, WI 53705-2398

Abstract

In this paper I examine the available evidence for and against the influence of urea-formaldehyde (UF) resin hydrolysis on formaldehyde emission from UF-bonded wood products. That evidence includes literature reports from 1950 through 1984 as well as unpublished results from my own recent studies. I have considered three related aspects: (a) the chemistry and hydrolytic stability of UF and phenol-formaldehyde (PF) materials and rates of formaldehyde liberation from these materials, (b) the chemistry and hydrolytic stability of formaldehyde-wood and UF- wood reaction products and rates of formaldehyde liberation from these products, and (c) the formaldehyde liberation behavior of UF and PF particleboards. The primary conclusions are: (a) In an acid-catalyzed UF board, formaldehyde can exist in a wide variety of states, including dissolved methylene glycol monomer and oligomers, parafonn, hexa, chemically bonded UF resin states, chemically bonded UF-wood states, cellulose hemiformals and formals. Each of those states is a potential source of formaldehyde emission by evaporation (methylene glycol) or initial hydrolysis. At present, we cannot quantify the relative contributions of these states over time. (b) In a base- catalyzed PF board, formaldehyde states may include methylene glycol monomer and oligomer, chemically bonded PF resin states, chemically bonded PF-wood states, and cellulose hemiformals. Emission sources apparently include methylene glycol, cellulose hemiformals, and possibly phenolic methylols. (c) Diffusion processes very likely exert a major influence on panel emission rates and may involve movement of methylene glycol in the wood's moisture or of gaseous formaldehyde within the board or within the board-air interface.

In: Christiansen, Alfred W.; Gillespie, Robert; Myers, George E.; River, Bryan H., eds. Wood adhesives in 1985: Status and needs. Proceedings 47344 of a conference; 1985

Introduction

General Background

Formaldehyde emission from some urea-formaldehyde (UF) bonded wood products has been recognized for a number of years as a potential source of indoor air pollution, leading to inhabitant discomfort and possibly to health problems. In response to a need expressed by industry representa- tives for an independent evaluation and summation of data from diverse sources, I have prepared a series of six critical reviews of the literature on different technical and technologi- cal aspects of this problem. This paper is the last of those critiques and concerns the extent to which resin hydrolysis in the cured board contributes to formaldehyde emission from the board. As with the other critiques (75-79), this one is based on a bibliography (80)1 derived from several sources; it covers the period from 1950 through 1984. In addition, this critique also includes considerable unpublished data from my own work. Note that a useful earlier review of some related aspects of this hydrolysis question is available (64).

Over the past decade or so, great progress has been made (21,63,106) in improving the formaldehyde emission from wood products such as particleboard, hardwood plywood paneling, and medium-density fiberboard. Beneficial steps have included reducing the Formaldehyde-to-urea (F/U) mole ratio

impregnating the wood furnish (substrate) with a formaldehyde scavenger having hindered access to the UF adhesive (78), and treating boards with formaldehyde scavengers and/or barrier coatings after manu- facture (79). For example, many plants in Europe now produce particleboard, that meets the German E-l standard recommending large test chamber formaldehyde levels of

1Copies of the bibliography may be requested from the Formaldehyde Institute, 1075 Central Park Avenue, Scarsdale, NY 10583.

May 14-16; Madison, WI. Madison, WI: Forest 119 Products Research Society; 1986: 119-156.

<0.1 ppm (29). The United States wood products industry is now producing particleboard and hardwood plywood paneling that meet the recently imposed Housing and Urban Development product standards (20) aimed at main- taining formaldehyde levels in new mobile homes < 0.4 ppm.

Despite this practical progress, great uncertainty still exists as to the precise mechanism by which formaldehyde is held within a board and slowly released as a gas to the atmos- phere. Historically, the emission potential of a board has been thought to be governed, particularly in a board's early life, by the board's so-called "free" formaldehyde content (82). "Free" formaldehyde is presumed to derive from the excess formaldehyde present in the UF resin. It exists in some ill-defined, relatively loosely bound states within the board, states whose stabilities are sensitive to temperature and humidity. At high resin F/U ratios, the "free" formaldehyde content and board emission rate fall rapidly after pressing and later decrease more slowly. The "free" formaldehyde content and board emission rate are lower after pressing when using resins with F/U ratios approaching 1.0, and they decrease more slowly with time. What has never been clear, however, is whether actual UF resin hydrolysis, with attendant formaldehyde production, causes a significant amount of the board's emission, and if so, at what point in the board's life that occurs.

The question of the contribution of UF resin hydrolysis to board emission is not a trivial one. If resin hydrolysis contributes significantly to emission, then, in principle, the board would retain the potential to emit during its useful life, in contrast to the situation in which all the emission results from "free" formaldehyde. In the former case, efforts to minimize emission must be directed toward resin stabilization and/or to ensuring that incorporated formaldehyde scavengers retain their effectiveness at low formaldehyde activities for the board's entire useful life. Another consequence of continued resin hydrolysis is possible limits on the durability of UF bonded products; in this case improvement may be expected from more stable resins.

Objective and Approach of Paper

The overall objective of this paper is to define the extent to which

board formaldehyde emission is controlled by resin hydrolysis or other processes. To that end, I present the results of an examination of the relevant literature plus the results of recent experiments carried out at the Forest Products Laboratory (FPL). The literature review was not intended to be an encyclopedic presentation of literature results, but rather a brief summary of primarily those findings that I consider most relevant to the issue. Experimental details of the recent FPL testing are in the Appendix along with explanations of calculation procedures.

In addition to UF systems, this paper includes a more abbreviated treatment of phenol-formaldehyde (PF) systems. There are two reasons for this inclusion. First, PF-bonded boards have been considered as one possible lower-emitting substitute for UF boards, and they thus provide a useful baseline system for comparison. Second, the PF chemistry is very different from that of the UF system and comparisons between the two may yield insights into their emission mechanisms.

I have divided the overall presentation into three related aspects, each addressing specific questions as outlined in the following:

Part 1. Formaldehyde-urea and formaldehyde-phenol states.

a. Do UF systems hydrolyze to produce formaldehyde?

b. Are hydrolysis rates sufficient to account for formaldehyde emission rates from UF-bonded boards under reasonable exposure conditions (e.g., 20-40°C, 30-80%, relative humidity (RH), pH 2-4)?

c. What are the answers to the above when applied to phenol-formaldehyde systems in alkali?

Part 2. Formaldehyde-cellulose and resin-cellulose states.

a. What chemical products (chemical states) are formed when wood is exposed to formaldehyde and/or resin molecules under the temperature and acidity conditions of adhesive bond formation?

b. Are those states susceptible to hydrolysis, thereby liberating formaldehyde?

c. Are hydrolysis rates of those states sufficient to account for board emission rates?

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Part 3. Formaldehyde emission from boards.

a. Can we clearly distinguish formaldehyde emitted from boards via cured resin hydrolysis mechanisms from that emitted via other likely mechanisms?

Part 1. Formaldehyde-Urea and Formaldehyde-Phenol States

Chemical Background

Formaldehyde-urea states. That UF materials hydrolyze to produce formaldehyde is incontrovertible. Over the past 30 years, investigators have extensively examined the structure of low molecular weight UF compounds and the physical chemistry of their formation and degradation in aqueous systems. Earlier work by de Jong and de Jonge (17-19), Landquist (54,55) and others (16,25, 100) used classical solution techniques to study the kinetics and equilibria of formation and hydrolysis. Subsequently, chromatographic (24,53,56) and nuclear magnetic resonance (NMR) (27,49,81,90,98,107) techniques were applied. In effect, many of the reactions that form UF products may be formally regarded as reversible condensation processes that eliminate water in the forward direction and, therefore, involve hydrolysis in the reverse direction. For some of the lowest molecular weight products, for example, the reversible reactions are:

(1)

(2)

(3)

(4)

(5)

(6)

Elevated temperature and neutral or acid pH lead to polymer formation primarily via forward reactions (3) through (6), the last being responsible for branching and the possibility of chain crosslinking. Resin cure is normally conducted at temperatures above 120°C and pH below 5 and is presumed to leave the following moieties (formaldehyde states) that are subject to hydrolysis, with direct or eventual liberation of formaldehyde (R = H or -CH2):

Because most, if not all, of the reactions leading to these formaldehyde states are reversible, the use of acid catalysts to hasten adhesive bond cure unfortunately increases the rates of hydrolysis and formaldehyde liberation. Some evidence also exists for acid-catalyzed hydrolysis of the amide C-N bond (2,110), in which case the product is not formaldehyde but an amine and CO,. However, the extent of this amide hydrolysis has never been made clear.

As partial justification for the above statements and to illustrate the intensive investigation of the UF system, I will briefly summarize two studies. Slonim and coworkers (98) employed C-13 NMR to measure the equilibrium constants (K) at pH 7 and ambient temperature for several formation/hydrolysis reactions, including five reactions to produce the mono, di, and tri methylolureas. Corrected for differences in the number of reactive sites per molecule, the K values in L/mole are: 350 for formation of monomethylolurea (UF)

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φ

φ

from urea (U) and formaldehyde (F); 300 for N,N’-dimethylolurea (FUF) from UF and F; 60 for trimethylolurea (FUF2) from UF and F; and 40 for FUF, from FUF and F. At equilibrium, therefore, species concentrations decrease as the amide hydrogens on urea are replaced by formaldehyde, and the effect is particularly strong for replacing the second hydrogen on a given nitrogen. Similarly, the K for Reaction 4 is approximately 2 and that for formation/hydrolysis of the hemiformal of monomethylolurea is approximately 13, indicating even less tendency to form those species.

In contrast to Slonim et al’s work, Braun and coworkers (8) studied the acid-catalyzed hydrolysis of care- fully characterized methylene-diurea (UFU). In this case the initial hydrolysis products (Reaction 3) are urea and monomethylolurea, the latter then degrading to urea and CH2O (Reaction 1). At 80°C and pH 5, about 20 percent degradation to CH2O is observed after 3 hours, whereas at pH 3 an equilibrium state is reached at about 40 percent degradation in only 30 minutes. Applying de Jong’s activation energy of 19.5 Kcal/mole-K (15) to the pH 3 results at 80°C yields a degradation rate of approximately 0.5 percent per hour at 25°C and pH 3. Thus, even the methylene link, customarily presumed to be quite stable in cured resins, will degrade at a significant rate when dissolved in aqueous acid at 25°C.

Formaldehyde-phenol states . Whether these states undergo sisnifi- cant hydrolysis at relevant exposure conditions to liberate formaldehyde is not clear. In an alkaline resole system cured between 130 and 200°C, the major formaldehyde states are thought (48) to be residual methylols and methylene bridges between phenolic rings, e.g.,

(ML85 5504)

At temperatures not far above 130°C and for short cure times, significant amounts of methylene ether bridges may also remain; e.g.,

(ML85 5505)

Formalin-phenol solutions contain hemiformals of the type φ - OCH2O(CH20)nH with n = 0 to 3 (49). Similarly, formalin benzyl alcohol solutions contain the hemiformals HO - φ - CH2O(CH2O)nH, with n = 0 to 3. While such states probably will not withstand typical alkaline cure conditions, they may re-form after cure of a high formaldehyde PF resin.

Finally, recent solid-state C-13 NMR studies (59) have confirmed the likelihood of other states, e.g.,

(ML85 5503)

Except for the hemiformals, none of the above states is expected to be highly, or even moderately, unstable. For example, the durability of PF-bonded wood joints is limited by the endurance of the wood, not the resin (68). This recognized stability and the generally much lower formaldehyde emission from PF-bonded products (28,74,94) have exempted the PF system from any intensive study of its inherent tendency for formaldehyde liberation under near ambient exposure conditions. However, 1.6 and 13.0 mole percent formaldehyde was liberated from o- and p-methylolphenols, respectively, after 6 hours in boiling water (8), indicating the possibility of reversion of the formaldehyde ring substitution reaction. For cured PF resins 3.6 percent of the resin weight was lost as formaldehyde after about

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5 hours in boiling water, and corresponding figures for 0.1N NH4OH and (29) H2SO4 were 2.3 and 0.7 percent

Rosenberg studied the rate of formaldehyde release from a cured PF resin in 1N H2SO4 at 78°C and observed a rapid initial release, followed by a slower first-order kinetics region (95); the first-order rate constant decreased with increasing resin cure. Summers found that cured novolak resins can be solubilized by digestion in 3 percent alkali at 280 to 320°C for several minutes (105). My own experiments on cured PF resin at more realistic conditions (Fig. 1) showed measurable amounts of liberated formaldehyde and a strong increase with humidity.

Thus, the limited available data indicate that formaldehyde liberation from cured PF resins can indeed occur. However, little is known about the underlying chemical mechanisms.

Other formaldehyde states . In addition to the above UF and PF formaldehyde states, two others must be considered. Formaldehyde can undergo reaction with itself to produce polymer such as paraform (Reaction 7), and it may react with the ammonia that is often present from UF cure catalysts to yield hexamethylenetetramine (hexa, Reaction 8). Both materials will liberate formaldehyde by reversion (hydrolysis).

(7)

(8)

Both the rate and extent of paraform dissociation (hydrolysis) are strongly pH dependent, being minimal between pH 2.5 to 4.5 and increasing exponen- tially below and above that range (114). Formation of paraform, therefore, seems to be much less likely in an alkaline PF system than in a moderately acidic UF system. Although the potential for hexa formation exists only for UF systems, hexa is relatively unstable in acid; if formed during UF cure, hexa may be relatively shortlived in some UF systems.

Formaldehyde Liberation Rates

I conclude that formaldehyde can indeed be produced by hydrolysis of UF and other (paraform, hexa) states and by some unknown mechanism from PF systems. The question next arises

whether known hydrolysis rates of UF and PF systems are sufficient to account for a significant portion of observed or permissible formaldehyde emission rates from particleboards. For example, if formaldehyde were known to be liberated by hydrolysis of all UF states at rates far below permissible board emission rates, then we could presumably ignore UF resin hydrolysis as a potential source of board emissions.

To attempt an answer to this question I transformed existing data on hydrolysis rates of model compounds and cured resins into formaldehyde emission rates from a hypothetical particleboard by the following steps:

1. Converting measured liberation rates for model chemicals and resins into units of mg liberated formaldehyde per g dry board per hour.2 The calculation assumes a board of 16 mm thickness and 0.65 g/cc density, with 7 percent resin solids. It also assumes that the influence of the board matrix on formaldehyde liberation is limited to the board's composition and geometry.

2. Selecting a standard state for hypothetical UF boards of (a) 25°C; (b) 50 percent RH (water activity 0.5); and (c) pH 3.0 or the actual acidity of bondline, solid resin, or solid model compound. For hypothetical PF boards the same standard state was used except for a pH of 10 or actual basicity of bondline, etc.

3. Emphasizing data close to the standard conditions and making approximate corrections to those conditions (parenthetic rate values in Table 1) using factors from the actual reference whenever possible.

4. Defining maximum allowable

board emission rates of 9 x 10-5 mg/g board-hr for the HUD large chamber standard of 0.3 ppm (20) and 2 x

10-5 mg/g board-hr for the German E-1 (29) of 0.1 ppm (see Appendix 3C for method of calculation).

Table 1 summarizes some transformed hydrolysis and formaldehyde liberation rate information from the literature and from my own recent experiments (Figs. 1,3,5,7).2,3

Values corrected to standard conditions are in parentheses. Before examining those transformed data,

2See Appendix 3 for methods of calculation.

3See Appendix 1a for methods of measurement.

1 2 3

however, some limitations should be recognized.

1. Many of the corrections to the specified standard conditions are quite approximate. In addition, the selection of pH 3 as standard for UF's and pH 10 as standard for PF's is a somewhat arbitrary compromise because of variations in wood acidity and catalyst levels among boards, plus uncertainty about correlations between measured board or resin pH's and actual bondline PH.4 Thus it would be best to regard most of the corrected rate values (parenthetic values) as significant within perhaps one order of magnitude.

2. In many cases, reported first-order kinetic rate constants were determined from measurements during early stages of hydrolysis. Because my calculated liberation rates assume the same rate constant throughout the material's degradation course, some rates calculated over a longer time may be less correct. Similarly, most, if not all, of the experiments were conducted under conditions in which the formaldehyde vapor pressure or concentration was maintained at very low levels. Consequently formaldehyde liberation rates were not affected by back-reactions and were maximized. In a board, however, it is conceivable that these maximum hydrolysis rates are not achieved due to locally high formaldehyde concentrations. Moreover, actual board emission rates may be greatly lessened by formaldehyde-wood interactions and diffusion effects. By the same token, the degrading material will then act as a formaldehyde source for longer times.

3. As will be noted later, hydrolysis rates for dissolved species are very likely to be higher than for analogous species present in an insoluble, crosslinked resin.

With these constraints in mind, examination of Table 1 and the relevant figures leads to the following conclusions for the various systems.

4For example, suppose a pH of 4.0 is measured for a ground UF particleboard as a 1:10 slurry in H2O (see Appendix 1b). The same board condi- tioned at 50 percent RH would be in contact with nearly 100-fold less H2O. For an acid such as HCl, this might mean the actual bondline pH at 50 percent RH is closer to 2.0. For acetic acid, however, the corresponding change might be more like pH 4 to 3.

UF Models. Conclusions are as follows:

1. Initial rates for pure compounds from solution data can be very high relative to the board standards

of 9 x 10-5 or 2 x 10-5 mg CH2O/g-hr. Even if the particular formaldehyde state represented by a model compound were present as only a fraction of the resin, initial emission rates could be well above the allowable levels. This is illustrated in Figure 2 for -NCH2OH assumed present to the extent of 10 percent of the resin's formaldehyde, for -NCH2N- present at 80 per

cent, and for -NCH2OCH2N- present at 10 percent.

2. At such high early rates, however, formaldehyde sources are not long lived and may not constitute an important direct emission source after a few weeks. On the other hand, short-lived primary sources might still result in longer term board emission because their liberated formaldehyde could react with wood substance to yield secondary emission sources. This necessitates, of course, that liberation rates from secondary sources are lower than from primary sources due to inherent stability or to diffusional delays within the board.

3. In contrast to the situation where initial rates are very high, the liberation rate from a moderately stable material might be maintained at rates above allowable standards for a much longer time. Thus, intermediate stability might be less desirable than low or high stability.

4. The durability of UF-bonded wood products may be limited by both the inherent hydrolytic sensitivity of the resin and bond rupture caused by swelling and shrinkage stresses (see discussion in Part 3). Both events, however, may eventuate in formaldehyde liberation. Consideration of durability should therefore allow us to estimate some formaldehyde liberation rates for comparison with model compound and resin data. Let us assume that board failure occurs due to rupture of one chemical bond type which liberates one molecule of formaldehyde. Consider two cases: (a) a rather conservative case in which only 5 percent of the bonds rupture in 50 years, i.e., probable board durability greater than 50 years; and (b) a much less conservative case in which 30 percent of the bonds rupture in 20 years, i.e., probable failure in 20 years or less. Case (a) leads to a

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first-order rate constant of 3.3 x -1 l0-11 s and a liberation rate, given

by the "high durability" curve in Figure 2, below the E-1 allowable line. Case (b) leads to a first-order

-1 rate constant of 5.7 x 10-10 s and a liberation rate, given by the "low durability" curve in Figure 2, above the HUD allowable line. Note that both rate constants are several orders of magnitude smaller than those in Table 1 for UF model compound degradation.

5. The available data on formaldehyde liberation from paraform (entry A7) and hexa (entry A8; see also Ref. 114) are not directly relevant to the possible contribution of these states to liberation from cured resin samples. At pH 8, paraform dis- sociates and vaporizes completely within 2 days (entry A7 and Fig. 3) but at pH 2 to 3, its dissociation rate may be one or two orders of magnitude less (114). Hexa, on the other hand, will hydrolyze approximately an order of magnitude faster at pH 2 to 3 (114) than at pH 8 (entry A8 and Fig. 3). Data are needed on the stability of hexa and paraform at pH 2 to 3.

6. Formaldehyde liberation rates calculated from model compound degra- dations in aqueous solutions obviously are more than enough to account for short-term (weeks) board emission. But the dropoff with time is much too rapid to account for longer term board emission and is inconsistent with durability considerations; moreover, the rates are well above those observed for cured resins. I therefore conclude that such homogeneous solution data are not quantitatively applicable to the heterogeneous solid state/aqueous liquid/air system that exists with actual cured resins or bonded wood. This conclusion perhaps especially applies to methylene links (NCH2N) which should constitute a major part of the cross-linked network backbone and whose formaldehyde liberation rates (Table 1 and Fig. 2) are much too high. In an actual cured resin, many of these links may exist within crystallites (104) or tightly crosslinked non-polar regions and are thus relatively inaccessible to water and acid. Moreover, methylene links with tertiary nitrogens are presumed to be less hydrolytically sensitive than those with secondary nitrogens (64,73).

Therefore, and not unexpectedly, this re-examination of UF model compound solution hydrolysis data

confirms the qualitative possibility of significant formaldehyde liberation from cured resins and, thereby, the necessity for further quantitative consideration of liberation rates by cured resins under realistic conditions.

Cured UF Resins . The following comments are primarily based on entries B7 through B8 of Table 1 because those liberation experiments most nearly approximated conditions in actual bondlines (see column 2 and Appendix 1 for experimental details) and because the liberation rates were directly obtained from slopes of experimental data plots, not from rate constants that are assumed to apply throughout the degradation process.

1. Liberation rates cover a wide range, corrected rate values at one day differing by a factor of twenty.

Liberation rate is decreased by lowering F/U ratio (entry B7a versus B8a and B7e versus B8c), by lowering humidity (entry B7a versus B7b, and Fig. 4) and by extending cure (entry B7a versus B7c, B8a versus B8b).

3. It is possible that a major portion of the liberated formaldehyde might be due not to hydrolysis of chemical states but to removal of physically sorbed formaldehyde at rates controlled perhaps by diffusion within the resin particles. However, in my view several findings argue in favor of hydrolysis as the major source:

(a) Experiments B4 through B8 all employed -80 mesh (<l80 µm) pow- ders, and other experiments demonstrated little or no influence of particle size on liberation rates by fractions below that size (73).

(b) Very strong increases in liberation rate occurred with increasing humidity (entry B7a versus B7b, and Fig. 4).

(c) Only a small decrease in rate resulted from vacuum pumping at ambient temperature for 17 days (entry B7a versus B7d). The observed decrease could be due to removal of some physically sorbed formaldehyde or to dissociation of some chemical formaldehyde states, e.g., methylols or paraform.

(d) Significant liberation rates remained after lengthy post-cure under vacuum (entry B7a versus B7c and B8a versus B8b).

(e) Lengthy toluene elution (Appendix 1e) of vacuum-pumped UF and PF resins at a ambient

1 2 5

temperature (Fig. 5) removed amounts of formaldehyde that were significant fractions of the respective Perforator (Appendix 1c) values (about 40% for the UF and 20% for the PF) but were much smaller fractions of the amounts liberated by exposure to 80 percent RH (about 7% for both resins). Elution from the PF resin hardly changed between 10 and 50 days while that from the UF resin continued to increase at a slow rate after 50 days. In either case the elution may be removing physically sorbed formaldehyde, dissolving a formaldehyde compound (paraform ?) or causing, particularly in the UF, dissociation of chemical states by imposing a very low concentration of formaldehyde.

4. Cure in the presence of only 25 percent wood flour leads to significantly greater liberation rates compared to neat resin (entry B7e versus B7f, B8c versus B8d, and Fig. 6). Moreover, the resin/wood composites maintain higher rates for longer times, with the consequence that estimated times to reach the allowable rates are greater than for neat resin. It is not clear whether the wood interferes with resin cure or acts as a formaldehyde sink during cure and as a secondary source thereafter. However, Perforator values (in mg CH2O/g resin) are 3.5 and 3.3 for F/U 1.6 resin and resin/wood samples (entries B7e and B7f) respectively, and 1.5 and 1.2 for F/U 1.2 resin and resin/wood samples (entries B8c and B8), respectively. The lower Perfora- tor values-- and nearly identical pH's--for the resin/wood composites may indicate that cure interference did not occur; if so, the increased liberation in the presence of wood must be due to hydrolysis of formaldehyde-wood states that are resistant to the Perforator conditions (boiling toluene, 2 hours).

5. By integrating the rates, one can estimate the fractional loss of formaldehyde that a given sample has suffered by the time its liberation rate reaches the allowable levels. The F/U 1.6 resin (entry B7a), for example, would lose about 18 percent of its original formaldehyde before it reached the allowable level, whereas the F/U 1.2 resin (entry B8a) would lose about 6 percent. Such levels may or may not be significant for the integrity of a bonded product,

depending upon the relative contributions to the loss of formaldehyde by nonstructural moieties (hexa, paraform, -N(CH2O)nOH) and by structural moieties (-NCH2N- and -NCH2OCH2N- in effective network chains). Moreover, because of these varied sources, it seems unlikely that current spectro- scopic techniques can adequately define the structural changes that may occur in similarly aged resins (12,43,57).

6 Liberation rates transformed into hypothetical board emission rates (Fig. 7) are significantly above the board allowable levels and fall rather rapidly, the F/U 1.2 sample reaching the HUD standard in approximately 1 month. These high liberation rates contradict the general experience that boards made with low F/U resins (< 1.2) can and do meet the HUD standard (32,75). This discrepancy possibly results from greater hydrolysis rates of neat resins due to absence of a wood sink for excess acid and/or slower formaldehyde emission from real boards due to diffusional restrictions. Whatever the details, cured UF resins clearly have the potential for significant contribution to board emission.

To summarize this section on UF resins, the salient points to be noted are:

1. Hydrolysis of chemically bonded formaldehyde states in neat cured UF resins is, in my view, responsible for the major portion of liberated formaldehyde from those materials.

2. Resin hydrolysis is potentially a significant contributing mechanism towards board emission.

3. Data on neat resins do not permit us to quantify resin contributions towards board emission.

Phenol-Formaldehyde (PF) Models and Cured Resins. In contrast to UF resins, phenol-formaldehyde resins are normally-cured under alkaline conditions for somewhat longer times and at higher temperatures. The more rigorous cure conditions plus the inherently greater hydrolytic stability of the linkages present in PFs result in a cured product whose potential formaldehyde liberation rate should be much less than that of a UF system. In fact, the general experience with PF particleboard shows that formaldehyde emission levels are as little as one-tenth of the levels of

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UF boards made from medium F/U ratio resins (28,74,94).

Whether the limited model compound data (Table 1, entries C1 to C3) agree with this conclusion is unclear because the completeness of degradation during the rather short exposures was not described in the original references and because the conditions were far removed from our standard condition for PF resins in boards (25C/50% RH/inherent alkalinity or pH 10). However, the data do indicate a significant potential for formaldehyde liberation. For example, if this degree of degradation were spread over 50 to 500 days at standard conditions with a cured resin that still contained 10 percent of its original methylol groups, the corresponding formaldehyde liberation rate would be

approximately 2 x 10 to

2 x 10-5 mg/g bd-hr, i.e., close to the allowable values. (The assumption of 10 percent residual methylols may be conservative in view of Fyfe et al's [35] finding that after 160 minutes at 180°C, the residual CH2/CH2OH ratio in a PF resin was only 2.9.) The PF resin data of Freeman and Kreibich (33, entries Dla, Dlb) were also obtained at rather extreme conditions but indicate a finite amount of liberated formaldehyde. Here, spreading that amount over 50 to 500 days yields liberation

rates in the range of 10-4 to 10-3 mg/g bd-hr.

My own experiments on a cured PF resin (entries D2a, b, c, and Fig. 1) were much closer to standard conditions and warrant the following comments:

1. A small, but finite, formaldehyde liberation occurs. At 80 percent RH, the rate for the PF (entry D2a) after 1 day was approximately a seventh of that for the F/U 1.6 UF resin (entry B7d) but nearly equalled that for the F/U 1.2 UF resin (entry B8a). The initial corrected rate was well above allowable levels but dropped below them within 2 to 6 weeks (Fig. 7).

Liberation rate is strongly decreased by lower humidity and extended cure (Fig. 1 and entries D2).

3. Similar to studies on the UF resins, several findings argue against the likelihood that all of the liberated formaldehyde derives from physically adsorbed material. These findings include retention of finite liberation rates after 20 days of

vacuum pumping (entry D2a) or after exposure to 200°C for 30 minutes in vacuum (entry D2c); the very strong influence of humidity (entry D2a versus entry D2b); and the elution by toluene of only a small fraction of the formaldehyde liberated at high humidity (Fig.5).

4. Hexa is not a formaldehyde source here because no ammonia was present. Paraform seems an unlikely source because the alkaline pH should greatly increase its hydrolysis rate above that in entry A6 where paraform was totally vaporized within 2 days. Solid state carbon-13 NMR spectra on sample D2a showed no detectable formaldehyde bands (58), although Fyfe et al. (35) reported trace amounts still present in similar spectra after curing 160 minutes at 180°C.

5. In view of point 4 and the reported formaldehyde liberation from methylolphenols (entries C1, C2), it is possible that residual phenol methylols account for observed formaldehyde from this PF resin. However, we cannot rule out the possibility of liberation due to degradation of residual methylene ether linkages between phenols.

Conclusions--Part 1

On the basis of information in the literature plus the results of my own experiments, I offer the following conclusions regarding resin contributions to board emission.

1. There is little doubt that most, perhaps all, formaldehyde states in a cured UF resin are susceptible to hydrolytic degradation.

2. Most of the hydrolysis reactions in UF resins are reversible but can ultimately lead to formaldehyde liberation if allowed to proceed.

3. Other sources of formaldehyde liberation may coexist with a UF resin, e.g., paraform and hexamethylenetetramine. Present data are insuf- ficient to define the extent to which these materials may be responsible for observed formaldehyde liberation rates from cured UF resins and boards, and this reservation should be borne in mind when considering the succeeding remarks.

4. Initial formaldehyde liberation rates observed for neat UF resins that are cured at conditions comparable to particleboard press conditions can be significantly above levels that are allowable for particleboard under current emission standards (HUD, 0.3 ppm; German E-1, 0.1 ppm).

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5. The fact that boards manu- factured with similar UF resins (F/U 1.2) can meet the HUD standard indicates that the wood system strongly mitigates the influence of resin hydrolysis on board emission. As a consequence, quantitative statements about the actual contribution of UF resin hydrolysis to board emission cannot be made, although it seems likely that contribution can be significant.

6. One cured PF resin exhibits an initial formaldehyde liberation rate approximately equal to that from a UF resin with F/U 1.2. Phenol methylol and/or methylene ether links may be the sources of liberated formaldehyde. Statements similar to those in point 5 may also be relevant here.

Part 2. Formaldehyde-Cellulose and Resin-Cellulose States

During hot pressing of an acid- catalyzed UF particleboard, some resin species, formaldehyde, and acid leave the bondline and penetrate into and interact with the wood substrate to yield formaldehyde states in addition to those in the cured resin bondline. Similarly, in an alkaline-catalyzed PF board, additional states can result from penetration of resin, formaldehyde, and base into the wood. These states within the wood constitute, or contribute to, what has been called "free" formaldehyde. What we must examine is the nature of these states, their hydrolytic stability, and their likely contribution to both short and long term formaldehyde emissions compared to those from the resins themselves.

To that end, I present abbreviated reviews of the chemistry of formaldehyde-wood and resin-wood reactions and of selected literature and FPL data to illustrate possible formaldehyde rates from the presumed formaldehyde states. As in Part 1, my inten- tion is not to offer a comprehensive review but merely to provide evidence for some likely interactions in a bonded wood board and for the possible emission consequences of these interactions.

Chemical Background

Acid-Catalyzed Formaldehyde- Cellulose Reactions . The acid- catalyzed reactions of cellulose with formaldehyde and also with a wide variety of N-methylolureas have been intensively investigated, primarily as a consequence of their potential for

imparting creaseproof properties to cotton textiles (13,93,114). Less intensive studies have also been carried out into the acid-catalyzed reactions between formaldehyde and wood or paper because of interest in the resultant dimensional stabilization (70,103,114,115).

Within the wood substance the formaldehyde may conceivably exist as a solution of monomer (presumably as HOCH2OH) or oligomer, or even as insoluble paraform, with the monomer and oligomer being highly H-bonded to water and wood substance.5 That strong interactions occur in the formaldehyde-water-wood system is evidenced by slow release of formaldehyde from impregnated veneer (51) and by wood density changes (44). In addition, however, it is known that formaldehyde will react with cellulosic hydroxyls at elevated temperatures in acid to yield a hemiformal (4,5,34, 83,99,110,111,114):

(9)

The hemiformal in turn can react with another cellulose hydroxyl to give a formal crosslink:

(10)

More generally, a polyformal crosslink may form via direct reaction of a

5A 5 percent loss of a UF resin's formaldehyde into the wood corresponds approximately to 100 mg CH2O per 100 g dry wood. At 50 percent RH this would lead to a nominal formaldehyde concentration in water of about 1 percent. Since the water by definition has thermodynamic activity of only 0.5, however, this nominal 1 percent formaldehyde concentration may have an effectively higher activity. Note also that even at 5 percent concentration only about 20 percent of the formaldehyde exists as oligomer or polymer at equilibrium (114), and the equilibrium probably would be achieved very slowly in a board.

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formaldehyde oligomer with cellulose or by stepwise build-up from the hemiformal; the overall effect is:

(11)

(12)

Reactions (9) to (12) apparently take place primarily with secondary cellulose hydroxyls (45,113). Andrews has reviewed the evidence for cellulose- formaldehyde reactions (4).

On the basis of reported amounts of formaldehyde that can be bound (not removed by neutral water rinsing) by cotton, it seems likely that formaldehyde could be bound by the wood substance during pressing of a UF board in amounts that are potentially significant for emission. For example, 1.4 percent of bound formaldehyde resulted from treating cotton with 6.5 percent formaldehyde in water at pH 2.2 and subsequently curing for about 4 minutes at 105oC (67). Bound quantities increase with formaldehyde concentration, with greater acidity, and with higher cure temperature (67,114). At 1.4 percent, the bound formaldehyde would tie up only a small fraction (<5 mole %) of the secondary hydroxyls present in the cellulose of a particleboard. Moreover, that amount of cellulose binding capacity perhaps would not be needed since it corresponds to 10 to 20 percent of the UF resin's total formaldehyde content; i.e., probably more than a low F/U resin could afford to lose without undesirable consequences for cure and for board properties.

The combined formaldehyde-acid permeation into the wood during pressing could also be expected to produce methylolation of the lignin (96). To what extent this reaction will compete with the above formaldehyde-cellulose reaction is not apparent. In any event, qualitative or quantitative information on possible rates of formaldehyde liberation from aromatic methylols is not available.

Base-Catalyzed Formaldehyde- Cellulose Reactions . We are concerned here with base-catalyzed PF systems and the situation is less clear than it is for the acid-catalyzed UF system. Treatment of wood or paper with formaldehyde under either neutral or alkaline conditions reportedly causes little or no change in chemical or physical properties, in distinct contrast to treatment under acid conditions (103,114). This is taken as evidence that only cellulose hemiformals (Reactions 9 or 11) are produced without further reaction between the hemiformal and cellulose to provide formal crosslinks (103, 114). Studies of reactions between formaldehyde and simple alcohols verify the absence of formals in base (14,114). With regard to reaction with lignin, the situation is similar to that in acid.

"Uncatalyzed, Ambient" Formaldehyde-Cellulose Reactions . At their natural acidities, wood and other cellulosics sorb formaldehyde and can then act as formaldehyde emitters. Almost certainly, the formaldehyde state resulting from such sorption is not cellulose formal because the conditions are too mild and because the formaldehyde is easily removed by water (114 and Fig. 8). Other alternatives include paraform, cellulose hemiformal, and/or water solutions of methylene glycol and oligomers, but available data do not allow an unequivocal choice among these. Recent NMR experiments, for example, indicate little hemiformal formation between formaldehyde and cellobiose or ethylene glycol in water (65), but Kamath et al. found the heat of sorption of formaldehyde on cellulose at 25°C to 100°C to equal the heat of formation of simple hemiformals (46), and we know that lower alcohols readily form hemiformals with formaldehyde (114). Within a post- production particleboard, some degree of dynamic cellulose hemlformal formation/hydrolysis may take place due to the presence of acid (UF) or base (PF) and the cellulosic degradation products resulting from hot pressing (89).

Acid-catalyzed N-methylolurea- Cellulose Reactions. That reactions of the type shown in equation (13) do occur between cellulose and N-methylolureas has been amply demonstrated and, in fact, studied in detail by those interested in textile crease- proofing (4,6,13,66,84,85,91,102,114).

1 2 9

(13)

Here, R1, R2, and R3 may be H- or -CH2-. The reactions are reversible and acid-catalyzed, and their rates and equilibrium positions are affected by the structures of R1, R2, and R3. If any of the R's possesses an N- methylol group, then additional reactions with other cellulose hydroxyls may occur, leading to cellulose crosslinking. Because R1, R2, and R3 will possess N-methylols and/or active N-H, bridging between substrate and the resin matrix is even more likely.

One illustration will suffice to demonstrate the possible extent of bonding to cellulose under conditions that approximate those during UF particleboard hot pressing (100 to 160°C, 3 to 6 minutes, acid catalysis). Cotton was treated with an aqueous solution of N, N'-dimethylolurea at NH4Cl concentrations from 0 to 2 per cent, subsequently dried at 115°C, and heated at 150°C for 10 minutes (89). The amount of nonwashable material bound to the cotton varied from 2 to 6 percent as the NH4Cl concentration increased. Whether such chemical reaction does indeed occur between UF adhesive and wood substrate during bond formation has not been definitely established. However, there is every reason to expect numerous such resin- cellulose reactions within the resin- wood interphase region, and we need to consider their possible contributions to board formaldehyde emission.

Base-Catalyzed PF Resin-Cellulose Reactions. As with the UF resins, evidence for chemical bonding between PF resins and wood components during board pressing is not definitive. However, Allan and Neogi showed reaction between lignin and a PF model compound under alkaline conditions and elevated temperature (1). Thus, there is good reason to expect the formation of methylene and/or methylene ether links between PF and lignin molecules; such states should possess comparable stability to the analogous ones in the resin itself, however, and do not re- quire explicit consideration. I have also chosen not to consider possible formals produced by reaction between phenolic methylols and cellulose hydroxyls because those are relatively unlikely in strong base and because I am not aware of any relevant data.

Conclusions. In view of the discussion thus far in Part 2, I conclude that the following events are sufficiently probable and important to warrant examining their consequences for formaldehyde emission from boards:

1. Some undefined portion of a UF resin's formaldehyde (as hydrated monomer or oligomer) will permeate into the wood during hot pressing and, catalyzed by acid from the resin and/or from the wood itself, will produce bound hemiformal and formal states by reaction with cellulose.

2. UF resin species will penetrate the wood to a lesser extent but can also undergo acid-catalyzed reaction with cellulose to form N-methylolethers.

3. During hot pressing of a PF board,formaldehyde and alkali will penetrate into the wood and react with cellulose hydroxyls to yield bound hemiformal states but probably not formals.

4. Some of the formaldehyde that penetrates into the wood during hot pressing of a UF board may homopoly- merize, but this is unlikely in an alkaline PF board.

5. After board production, some cellulose hemiformal may be forming and hydrolyzing within the board.

Formaldehyde Liberation Rates

Many of the reactions discussed in Part 2 (reactions 9-13) are reversible condensations that produce water as well as the particular formaldehyde states. Therefore, each of those formaldehyde states is hydrolyzable, with the potential for formaldehyde liberation. Moreover, as with most of the reactions in UF systems, the rates of these forward and reverse reactions are strongly influenced by acidity (84,85,110).

Table 2 and Figure 2 present selected data illustrating the stability and formaldehyde liberating tendencies of several cellulose- formaldehyde compounds or states. As with the data in Part 1, the cellulose-related values have also been transformed to liberation rates from a hypothetical particleboard and, in some cases, have been corrected to standard conditions. They are subject, therefore, to the same limitations discussed in Part 1.

Within these constraints, the Table 2 values plus other observations in the literature lead to the following comments.

1 3 0

Cellulose - CH2O Models and Compounds . Conclusions are as follows:

1. The corrected acid-catalyzed formaldehyde liberation rates from cellulose formal models are within an order of magnitude of those for actual cellulose-formals (entries Al to A3 versus B2b and B3b). The latter behavior may approach that of similar states in a board since both situa- tions involve nonhomogeneous systems,

solid phase in contact with liquid.

2. Inter-cellulose polyformal bridges hydrolyze in acid 3 to 4 times faster than bridges composed of single formal links (entry A2 versus Al and B3b versus B2b).

3. The corrected cellulose formal liberation rates are an order of magnitude less than initial rates from realistically-cured UF resins (c.f. entries B7a, c and B8a, c in Table 1). Cellulose formals are, therefore, relatively stable states.

4. Assuming that during board pressing 2 percent of the resin's formaldehyde yields cellulose formals, subsequent hypothetical board emission rates at 25°C/50 percent RH/pH3 are approximated by curve (d) in Figure 2. The rate is just below the E-l allowable for at least a year, indicating that cellulose formals might constitute a significant source of formaldehyde emission from UF boards.

5. Although quantitative information is lacking on hydrolysis rates of proven cellulose hemiformals, their rates are generally assumed to be much faster than hydrolysis rates of cellulose formals. For example, investigators of textile cross-linking almost universally assume that hot water or alkaline washes after cure will remove cellulose hemiformals but leave cellulose formals intact (entries B1, 2) (45,113). Experience with the acetals and hemiacetals formed from simple alcohols also lends credence to greater hydrolysis rates of hemiformals relative to formals (14). Thus, cellulose hemiformals formed during pressing of a UF board should hydrolyze rapidly and they may account for the early high emission of UF boards, particularly where high F/U resins will have contributed greater amounts of formaldehyde to the wood substance.

6. Under the alkaline conditions of interest for a PF board, we could expect cellulose formals to hydrolyze at rates comparable to those at pH 2 to 4 (compare entries B3c). But, as noted earlier, such groups apparently do not form to any great extent under

alkaline conditions (103,114). There- fore, we should expect their contribution to PF board emission to be slight. On the other hand, cellulose hemiformals apparently can be produced under alkaline conditions and are more stable to hydrolysis in base than in acid (114). Quite possibly, then, cellulose hemiformals account for some of the small but finite formaldehyde emission by PF boards.

Cellulose-UF Resin States . The cellulose-UF reaction products of con- cern are N-methylolurea ethers whose hydrolysis yields first the corresponding N-methylolurea (reverse of Reaction 13), which then can decompose into the substituted urea and CH2O (reverse of modified Reaction 1). It has been clearly established that the rate constants for the first step are generally much greater than the rate constants for the second step (3,85, 110). (This, of course, is just the reverse of the situation with cellulose formals and hemiformals.) There- fore, formaldehyde liberation rates from the cellulose-UF reaction products are controlled not only by those products (ethers) but also by their N-methylol daughters. On a relative basis over time, liberation rates will initially be zero, will increase as the daughter concentration builds up, and will eventually decrease as both parent and daughter become depleted. This is illustrated in entry D and Figure 2 (curve e) for the ether resulting from the reaction between cellulose and dimethgloldihy- droxyethyleneurea (DMoDHEU).6 Al- though this particular material is not a realistic component of a conven- tional UF wood adhesive, it is one of the few N-methylol compounds for which relevant, self-consistent data are available. Assuming that only 1 percent of the resin's formaldehyde is involved in such Cell-OCH-N bonds, the hypothetical formaldehyde liberation rate at 25°C/50 percent RH/pH 3 exceeds the HUD allowable level for most of the first 100 days (curve e, Fig. 2). Although the relevance of this particular calculation can be questioned, significant formaldehyde liberation emission by this mechanism is at least conceivable.

6This discussion and the rate calculations assume no methylols existed prior to the hydrolysis experiment.

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Conclusions--Parts 1 and 2

It is well to emphasize once again the semiquantitative nature and even dubious applicability of some of the rate calculations presented in Parts 1 and 2. Nevertheless, I feel that the following overall conclusions are warranted from these guidelines.

Formaldehyde States in Cured UF Resins . Conclusions are as follows:

1. Most of the formaldehyde emitted by cured UF resins results from hydrolysis of chemically bonded formaldehyde states. All such states are susceptible to hydrolysis and most of the hydrolysis reactions produce formaidehyde.

Initial formaldehyde liberation rates from LJF resins (F/U 1.2 to 1.6) that have been cured under realistic conditions but with little or no wood can be several-fold greater than allowable rates based on the HUD particleboard emission standard. This implies that wood alters the resin cure state or its pH or that wood mitigates the rate of formaldehyde loss.

3. Paraform and hexa hydrolysis may be significant contributors to emission but data are not yet sufficient to prove or disprove this point.

Formaldehyde States in Cured PF Resins . Conclusions are as follows:

1. One cured PF resin exhibits an initial formaldehyde rate approximately equal to that from a UF resin with F/U 1.2. Degradation of phenol methylol groups and/or of methylene ether links presently appears the most likely mechanism for this formaldehyde production.

Formaldehyde States With Cellulose. Conclusions are as follows:

1. Small amounts of reaction between UF resin methylols and cellulose to yield N-methylolethers are likely. These states are very susceptible to hydrolysis and may well contribute to board emission in early stages and somewhat lengthen the time for emission rate to fall below the standard levels.

2. Cellulose formals are also likely hydrolyzable products in an acid-catalyzed UF board. These appear to be relatively, stable states, however, and probably need to involve several percent of a resin's formaldehyde before making a substantial contribution to overall board emission. But their stability also

implies that their contribution to emission may be long-lasting. Cellu- lose formals are unlikely in an alkaline-cured PF board.

3. Cellulose hemiformals will probably also be present in both an acid-catalyzed UF board and an alkaline-cured PF board, particularly with high formaldehyde resins. Such hemiformals, as well as N-methylol hemiformals, are quite unstable in acid, and these states may account for much of the initial, rapidly falling emission from boards made with high F/U resins. Hemiformals hydrolyze more slowly in base but may contribute significantly to the normally low emission from PF boards.

Part 3. Studies on Wood Systems

Here we need to examine the evidence available from studies on actual wood systems that relates to the mechanisms controlling formaldehyde emission from bonded wood products. Factors to be considered include: (a) the degree to which formaldehyde emission rate from wood systems is controlled by diffusion processes, (b) the contribution of resin hydrolysis to emission rate, and (c) the contribution of formaldehyde-wood states to emission rate.

In the following section, I first briefly summarize the reported evidence regarding diffusion control and resin hydrolysis in actual bonded products. I then present and discuss some of my own recent experiments on wood systems that attempted to shed additional light on the questions of resin hydrolysis and the mechanism of formaldehyde emission.

Literature Evidence for Diffusion Control

Although published evidence is sparse, there is little doubt that diffusion processes can play an important role in board emission. Some of the more critical findings are as follows:

1. Particleboard emits two to three times less formaldehyde after conditioning than do exposed core surfaces (38).

2. Emissions are higher from board edges than from board faces (several studies, including 11).

3. Emission levels are decreased at higher board density (11,60) and at lower board porosity (11).

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4. Ventilation rate and board loading effects on emission levels in chambers can be quantitatively described by equations that are based upon the assumption that diffusion across a board-air interface layer governs the emission rate (76). At sufficiently high ventilation rates, the dependence on ventilation rate disappears and formaldehyde loss is governed by within-board processes (47).

It therefore appears that formaldehyde emission rate from a given large panel may be controlled by diffusion either in the board-air interface or within the board or by chemical processes within the board. Which of these predominates depends upon the board's age, composition, physical structure, and exposure conditions.

Literature Evidence for Resin Hydrolysis in Actual Boards

Despite the rather massive literature on formaldehyde emission from UF-bonded wood products, evidence for a direct causal relationship between resin hydrolysis and formaldehyde emission from bonded products is almost nonexistent. Indeed, evidence in the literature that UF resin hydrolysis actually does occur in a board arises primarily from studies of whether the limited durability of UF-bonded wood products is caused by resin hydrolysis or by a particular susceptibility of UF resin-wood bonds to rupture from swelling/shrinkage stresses.

That UF resin hydrolysis does occur in boards is strongly indicated by the following:

1. higher rates of strength loss for UF boards and joints compared to those made with other adhesives (phenolics, isocyanates, melamines) during aging at constant temperature/humidity, particularly at high temperature/ humidity (7,83,92);

2. decrease in board modulus of rupture (MOR) but not in internal bond after spraying only the surface mat with water prior to pressing (92);

3. increase in solubility of cured resin in both UF-bonded particleboard and UF-bonded Perlite (nonswelling volcanic glass) board during aging (37);

4. decrease in loss of strength during constant temperature/humidity aging of plywood after soaking in

NaHCO3 to neutralize the acid cure catalyst, which would otherwise catalyze resin hydrolysis (40).

Evidence that the lower durability of UF-bonded products can also be brought about by swelling/shrinkage stresses in a board includes the following:

1. faster strength losses for UF boards than for others (phenolic, iso- cyanate, melamine) during cyclic humidity/temperature aging, where swelling/shrinkage stresses can be high (23,37,62,71,88,97,116);

2. greater internal bond (IB) loss and thickness swelling increase with UF particleboard than with a UF Perlite board (36,37);

3. no change in modulus or strength of cured neat UF resin films during humidity cycling, i.e., when no swelling/shrinking substrate is present (22);

4. increase in thickness swelling of boards with low F/U resins both before and after cyclic weathering (61), accompanied by the postulate (22) that low F/U resins are more brittle than high F/U resins;

5. decreased strength loss on boiling plywood bonded with UF resins containing polyfunctional ureas which are postulated to produce more flexible binder networks (41);

6. accelerated aging under stress of UF joints relative to PF joints (50).

Finally, several studies have yielded results whose interpretation is less clear-cut:

1. far greater cumulative amounts of formaldehyde emitted by boards than can be accounted for by their Perfora- tor (see Appendix 1c) values (74), which have often been presumed to measure primarily non-resin formaldehyde. Unfortunately, it will be shown later that the Perforator value does not necessarily measure all formaldehyde-wood states or only non-resin formaldehyde.

2. reduced rate of cured resin film cracking by incorporating acid-reactive filler. Such materials will decrease the acidity within the resin, thereby decreasing hydrolysis; however they also reduce the extent of resin

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cure, thereby decreasing brittleness and tendency to crack (31).

3. decreased strength loss of UF particleboards by using less acidic cure catalyst (92) or by incorporating acid scavengers (42). (Arguments here are identical to those described above.)

4. greater mat moisture content (MC) yielded greater formaldehyde emission during particleboard pressing (86) and after pressing (87). Plausible alternatives to resin hydrolysis stipulate that greater mat MC facilitates formaldehyde movement to the board surface and/or that it enhances hydrolysis of cellulose formals and hemiformals.

Overall, the available literature supports the prevalent view that the durability of UF-bonded wood products is governed by the susceptibility of cured UF resin bonds to scission by both hydrolysis and swell/shrink stresses. Moreover, in either case formaldehyde is a probable ultimate product. Furthermore, mechanical stress enhances the rates of many chemical reactions (9). In fact, simplistic calculations based on formaldehyde liberated from bond ruptures that limit product service life led to the emission curves (f) and (g) in Figure 2; those curves at least indicate the possibility that formaldehyde from swell/shrink stress rupture could contribute significantly to total emission.

Thus, qualitatively we might expect UF resin bond scission to be one source of board formaldehyde emission. However, the available studies do not permit quantitative statements about the relative magni- tudes of that source compared to other sources, such as formaldehyde-wood states, during board lifetime.

Recent FPL Studies

To shed additional light on the emission mechanism and the contribution of resin hydrolysis to formaldehyde emission, my recent experiments have examined the liberation or extraction of formaldehyde from particleboards, from wood containing sorbed formaldehyde, and from cured resins. Some of the resin data were discussed in Part 1; here, I present results from particleboard and formaldehyde-sorbed wood experiments in which rates of formaldehyde removal were measured by three different procedures (see Appendix 1 for experimental details).

Formaldehyde Removal By Gas Elution. These experiments involved the continuous collection of formaldehyde removed by a controlled flow of gas over the wood samples. Variables included time, gas flow rate, sample comminution, gas type, humidity, and adhesive type.

1. Comminution and flow rate effects. Elution rates were measured from UF particleboard at two geometries--e.g., shredded (85 pct < 1 mm) and 25x25x16 mm pieces. Shredding was conducted in a sealed system so that no formaldehyde was lost during that operation. The eluting gas was nitrogen at zero and 20 percent RH and at flow rates corresponding to 0.4 to 4.5 changes in gas volume per minute (NCM).

Small effects of flow rate were found with dry nitrogen between 0.5 and 1.0 NCM but none with 20 percent RH nitrogen between 0.8 and 4.5 NCM. Figure 8 compares results for pieces and shredded particleboards at two levels of Perforator (see Appendix 1c) values. Several points should be noted:

(a) Elution from shredded UF board was only slightly faster than from the 25x25 mm pieces, and the increase was consistent with observed effects of the flow rate difference (1.0 NCM for shredded versus 0.5 for pieces). This similarity in elution rates indicates that the rate-limiting step in formaldehyde release in these experiments is not "macro- diffusion" within voids but either "micro-diffusion" within the wood or an actual bond rupture step.

(b) No burst of liberated formaldehyde was observed during shredding of the 25x25x16 mm pieces in any of the tests on shredded UF board. Apparently, no significant amount of formaldehyde existed as gas within voids; i.e., all formaldehyde in the board pieces was present in a physically dissolved or sorbed state or in a chemically reacted state. This is consistent with point (a) and with the discussion in earlier sections on the high reactivity of formaldehyde with water, urea, and wood components.

(c) The elution process was quite slow and had not reached any obvious endpoint after 10 days, although the evolved formaldehyde totalled only about 20 to 30 percent of that removed by the

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2-hour toluene boiling in the Perforator test. Obviously, therefore, dry nitrogen is not an acceptable eluant for readily removing formaldehyde because of the nonpolar nature of nitrogen and the removal of water from the board. 2. Other eluant gases . The

effectiveness of dry N2, CO and CO2, as eluants (Fig. 9) was compared in brief tests. The three gases provided no differentiation between formaldehyde states in UF board.

3. Gas moisture effects. As expected, the influence of moisture in the eluting nitrogen was very strong (Fig. 10). Points to be noted here are as follows:

(a) The observed absence of an endpoint to the dry gas elution from UF board after 10 days (Fig. 8) was extended to 40 days (Fig. 11).

(b) During about 15 days of elution at 80 percent RH (Fig. 10), the UF board sample lost an amount of formaldehyde equal to approximately 80 percent of the original Perforator value and the rate showed no indication of slowing. Similarly, at 20 percent RH a UF particleboard lost formaldehyde to the extent of about 50 percent of the Perforator value in 40 days.7 Clearly, moisture in the eluting gas removed formaldehyde from states within the board that were not affected by the Perforator conditions (toluene reflux, 2 hours). Whether those states include formaldehyde bonded to resin, i.e., whether resin hydrolysis occurs under the elution conditions, cannot be firmly stated. However, the rapid liberation rate noted earlier for cured resin at high humidity (Fig. 4) provides strong, indirect evidence for resin hydrolysis contributions to the observed board losses at high humidities.

4. Resin effects. Elution experiments were also performed on PF-bonded particleboard and on Southern pine chips (furnish without resin) that had sorbed formaldehyde via room temperature vapor phase

7Perforator values for one UF board were not increased by extending the toluene reflux time beyond the standard 2 hours.

equilibration (see Appendix 1d and 2). Points to be noted are as follows:

(a) The elution patterns from zero to 20 percent RH for the PF board (Fig. 12) were very similar to those for the UF board. How- ever, the formaldehyde losses for the PF board were approximately ten-fold less than for the UF, and the PF losses at 20 percent RH are likely to exceed the Perforator value sooner than in the case of the UF board.

(b) The elution patterns from zero to 20 percent RH for the formaldehyde-sorbed furnish (Fig. 13) were again similar to those for the two board types, although elution rates were faster, relative to the respective Perforator values, for the furnish than for the boards.8 Obviously, the Perforator test does not measure the total of all possible formaldehyde non-resin states, even where those states are formed in the absence of heat or resin cure catalysts (furnish pH = 3.9).

Formaldehyde Liberated in Weighing Bottle Test. Tests equiva- lent to those on ground resins (Part 1) were conducted on ground UF and PF particleboards and on ground Southern pine that had first been impregnated with tartaric acid solutions at pH 2 or 3, then vapor-sorbed with formaldehyde, and finally either aged at room temperature for 2 weeks or heated 4 minutes at 160°C to model board pressing conditions. Liberation tests were run at 27°C and at both 33 percent and 80 percent RH on -80 mesh (< 180 µm) materials and on several particle sizes between 180 µm and 62 µm. Points to be noted are as follows:

(a) At 33 percent RH (Fig. 14) the formaldehyde-sorbed wood vir- tually completed its loss of formaldehyde after about 15 to 20 days, whereas the UF particleboard appeared to continue to slowly liberate formaldehyde. (The PF particleboard liberation is an order of magnitude below that of the UF particleboard and possesses poor accuracy.) Heating the formaldehyde-sorbed wood has caused either a loss of

8Negligible amounts of formaldehyde were eluted from the same furnish unexposed to formaldehyde.

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(b)

(c)

formaldehyde or a stronger bonding to the wood (perhaps formals). Liberated amounts for the formaldehyde-sorbed wood equalled or slightly exceeded the Perforator values, while the UF board Perforator amount was exceeded quite early.

At 80 percent RH (Fig. 15) the above differences between UF particleboard and formaldehyde-sorbed wood were magnified. Liberation from the UF particleboard continued rapidly at 30 days while that from formaldehyde-sorbed wood became nearly constant in about 5 days. wood samples also liberated total amounts that were close to their Perforator values measured at high moisture. Most of the formaldehyde in the formaldehyde- sorbed wood was, therefore, very weakly bonded (perhaps hemiformal and methylene glycol) although there may have been small quantities that were liberated with greater diffi- culty, particularly at pH 3 relative to pH 2. The UF board, in contrast, apparently contained little of the very loosely bound formaldehyde but contained greater amounts of more strongly bound formaldehyde, as would be expected. The PF board liberation was again well below that of the UF board and behaved similarly to the formaldehyde-sorbed wood samples except for greatly exceeding its Perforator value.

At 80 percent RH the UF and PF boards exhibited no significant particle size effects on liberation rates between particle sizes of approximately 60 and 180 µm. In that size range, therefore, within-particle diffusion did not influence liberation rate from the boards.

Formaldehyde extracted in water . Formaldehyde liberated during continuous exposure to water at pH 3 was also measured on the same materials used in the weighing bottle test. Very dilute slurries of -80 mesh material were held at 25°C in the presence of sodium azide as bacterial inhibitor. In 1 or 2 hours almost all removable formaldehyde was extracted from the formaldehyde-sorbed wood samples (Fig. 16); the total amounts were nearly identical to those at 80 percent RH and to the Perforator values.

However, liberation from the UF board continued rapidly after 6 days and at 30 days far exceeded the amounts at 80 percent RH and the amounts from the wood samples in water. Interestingly, liberation from the PF board in water also exceeded that at 80 percent RH and may have occurred in two or more stages; even the apparent initial stage, however, was an order of magnitude greater than the Perforator value.

Interpretation and Extrapolation to Boards in Service

In this section, I offer an analysis of the Part 3 results in terms of the findings from Parts 1 and 2, and on that basis, speculate about their implications for large panel formaldehyde emission.

Interpretation for Comminuted Systems. The similarities and differences noted in Part 3 for the kinetics of formaldehyde removal from UF and PF particleboards and from formaldehyde- sorbed wood are brought out more clearly by plotting relative formaldehyde losses versus time. Loss ratios, i.e., formaldehyde loss by any material divided by the UF board loss at the same time, are shown in Figures 17 and 18; included in Figure 17 are analogous ratios for resin data from formaldehyde liberation (weighing bottle test, Appendix la) and formaldehyde elution (Appendix 1d) experiments. Examination of the data leads to the following additional comments:

1. Southern pine containing formaldehyde that was sorbed at the wood's natural pH or at pH 2 to 3 held the formaldehyde in a state that was strongly retained at low humidity but relatively labile at moderate to high humidities. The formaldehyde was nearly completely released, for example, in 12 days at 33 percent RH (Fig. 14) in 5 days at 80 percent RH (Fig. 15) and in 0.2 days in pH 3 water (Fig. 16). The available information (Part 2) indicates this formaldehyde is present as monomer (methylene glycol) or oligomer dissolved in the wood's moisture or possibly as cellulose hemiformals.

2. With PF board, the amount of formaldehyde released within the time scale of these experiments varied more with humidity, and did not obviously exist in only one state (cf. Figs. 11, 14-16). While a portion of the removable formaldehyde very likely exists in the same state(s) as in the

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formaldehyde-sorbed wood, a major portion is more strongly held but still sensitive to moisture. The latter state perhaps is phenolic methylols.

3. The UF board undoubtedly contained some of the same moisture- labile states that were present in the formaldehyde-sorbed wood, and these account for some of the initially rapid loss observed at 80 percent RH (Fig. 15) and in water (Fig. 16). Up to 20 percent RH the release pattern from the UF board by nitrogen elution was very similar to that from the PF board and formaldehyde-sorbed wood, indicating similar release mechanisms from all three comminuted wood systems under those conditions (Fig. 17). In the other types of experiments at higher humidities, however, the release pattern from the comminuted UF board clearly differed from the patterns observed in the other two wood systems (Fig. 18). The continued evolution of formaldehyde from the UF board beyond the very early portion and at rates increasing with humidity strongly indicates extensive hydrolytic sources other than those present in the PF and formaldehyde-sorbed wood. Obviously, those additional sources are most likely UF resin and UF-wood states, with some possibility of cellulose formals, as discussed in Parts 1 and 2.

4. Point 3 suggests a similar release mechanism for the shredded boards and furnish particles during nitrogen elution at 20 percent RH and below (Fig. 17). This implies identical rate-limiting steps, which might involve a chemical bond rupture or a monomeric formaldehyde diffusion process. The nature of the three systems dictates that a chemical process most probably involves hydrolysis of cellulose hemiformals. However, the evidence for significant amounts of that formaldehyde state to be present is not clear-cut (Part 2). Since small nitrogen flow rate effects were observed (Fig. 8) in the range employed in these experiments (0.5 NCM), some control of elution rate by gaseous formaldehyde diffusion through the shredded or furnish particle-gas interface (vaporization) must have existed. Intraparticle diffusion limitations also seem likely at these particle sizes (~100 to 1,000 µm), although particle size effects were not observed in the high humidity weighing-bottle tests with particle sizes below 180 µm. Intra- particle diffusion presumably involves methylene glycol, whose effective diffusion rate in the wood's water may

well be decreased by strong interactions with cellulosics during its passage to the particle surface.

Implications For Formaldehyde Emission From Large Panels . Much of the above discussion should be directly relevant to large panel emission. If intraparticle diffusion of methylene glycol is hindered under some conditions with comminuted materials, it will obviously continue to be hindered in an actual board. More- over, gaseous diffusion through particle-gas interfaces will be greatly slowed in a large panel because no eluting gas is present to reduce the concentration gradient and the interface layer thickness. In addition, the diffusion path to the panel surface will be tortuous, and panel surface-air layer gaseous diffusion limitations may exist. Diffusion effects are therefore very important in panel emission rates.

For a given board composition and structure, the presence of diffusion limitations leads to lower emission rates and somewhat higher internal concentrations of dissolved methylene glycol. The concentration increase may be sufficient to slow the net production of formaldehyde via reversible hydrolyses, thereby lowering and pro- longing the emission contributions from hydrolytic processes. Unfortu- nately, given the currently available knowledge, we can only speculate about which formaldehyde states in the board may be responsible for emission at various points in the board's life. However, the water extraction data (Fig. 16) suggest the possibility of distinguishing between "loosely held" formaldehyde (perhaps methylene glycol monomer and oligomers and cellulose hemiformal) and more firmly bonded formaldehyde, the latter presumably including hydrolytic sources (perhaps UF, UF-wood, and cellulose formal). The shape of the UF board curve in Figure 16 indicates that from 20 to 40 mg of formaldehyde per 100 g of board belongs in the "loosely held" category. In addition, the Perforator value for this board (11 mg/lOO g) indicates that it should meet the HUD and possibly the E-l standards, leading to emission rates at standard

conditions between 2 x 10-5 and

9 x 10-5 mg per g board per hour (Appendix 3c). If "loosely held" formaldehyde is primarily responsible for those emission rates, the time required to dissipate those formaldehyde states is 3 to 6 months at the HUD level and 1 to 2 years at the E-1

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level. Continuing with the argument, subsequently emitted formaldehyde should derive from hydrolytic processes. Obviously, additional water extractions plus measurements of actual emission rates on identical boards would be needed to confirm this approach towards distinguishing formaldehyde sources within boards.

Summary and Conclusions

This paper addresses the general question of the extent to which resin hydrolysis is responsible for formaldehyde emission from bonded wood products. I have both reviewed briefly the relevant literature and presented original FPL data in three areas: the chemistry and hydrolytic stability of formaldehyde resins and model compounds (Part 1); the reactions of formaldehyde and UF compounds with wood components and the hydrolytic stability of their products (Part 2); and formaldehyde emissions from bonded wood products (Part 3).

Major Findings. The major findings are as follows:

1. In an acid-catalyzed UF- bonded board, formaldehyde can exist in a wide variety of states. These states may include dissolved methylene glycol monomer and oligomers, paraform, hexa, chemically bonded UF resin states, chemically bonded UF-wood states (amidomethylene ethers with cellulose), cellulose hemiformals, and cellulose formals.

2. Each of these states is a potential source of formaldehyde emission by evaporation (methylene glycol) or by initial hydrolysis (all others). Unfortunately, we cannot now provide a complete listing of states in the order of their potential importance as emission sources. Clearly, however, some of the most weakly held states would be methylene glycol, cellulose hemiformal, amido- methylols, and cellulose amidomethylene ethers.

3. In a base-catalyzed PF-bonded board, formaldehyde states may include: methylene glycol monomer and oligomers, chemically bonded PF resin states, chemically bonded PF-wood states, and cellulose hemiformals. Emission sources apparently include methylene glycol, cellulose hemiformals, and a PF resin state-- possibly phenolic methylols.

4. In Southern pine containing formaldehyde that was sorbed at room temperature and at the wood's natural pH or at pH 2 or 3, formaldehyde

states may include methylene glycol monomer and oligomers and possibly cellulose hemiformals. These are all apparently readily removed from the comminuted wood at 80 percent RH (5 days) or in pH 3 water (0.2.day).

5. Diffusion processes can very likely exert a major influence on emission rates from large panels. Depending upon board structure, composition, age, and exposure condition, emission-limiting diffusion steps may involve methylene glycol within the board's water or gaseous formaldehyde within the board or within the board- air interface.

Subsidiary Findings. The subsidiary findings were as follows:

1. Formaldehyde liberation from cured neat resins (PF and UF) is much greater than expected for those same resins cured in a particleboard, indicating that the wood alters the resin cure and/or the bondline pH or that diffusion effects predominate in the board.

2. A cured PF resin liberates formaldehyde at significant rates that increase with humidity.

3. The Perforator test measures formaldehyde in states that are present in cured neat PF and UF resins, in boards made with both resins, and in formaldehyde-sorbed wood. In all but the last, the Perforator values are much less than the amounts removable by simple exposure to high humidity.

4. The limited durability of UF-bonded wood products probably results from the susceptibility of UF resin and UF-wood bonds to chain scission from both hydrolysis and swell/shrink stresses. In either case, formaldehyde is a possible product.

Acknowledgment

This work was partially funded by the Formaldehyde Institute. Vital assistance was provided by two groups at the Forest Products Laboratory--the staff of the Library in literature search and retrieval, and the Systems and Automatic Data Processing group in establishing and using a computerized literature file. I am also greatly indebted to members of the Technical Committee of the Formaldehyde Institute for advice and for supplying materials. Professor James Koutsky, University of Wisconsin-Madison Chemical Engineering Department, and several of his students aided this effort with both advice and laboratory

1 3 8

aid. Much of the FPL data were obtained by Ralph Schaeffer and Jill Wennesheimer.

Appendix 1. Experimental Procedures

a. Formaldehyde liberation by weighing bottle technique

Ground, sieved (-80 mesh or smaller) powder was weighed (10 to 150 mg) into a glass weighing bottle (40 mm dia. x 40 mm high), a small glass cross placed on the bottom of the container, a glass beaker (22 mm dia. x 25 mm high) containing 5 ml of sulfuric acid or salt solution placed on the cross, and the bottle sealed with a greased cap. The assembly was then stored in a temperature chamber (usually at 27°C) for a specified period. The beaker was then removed and replaced with a fresh solution. For a given weighed sample, the solution was replaced no more than twice. At each removal the solution was analyzed for formaldehyde, usually by the chromotropic acid procedure.

Resin samples were vacuum-dried before weighing into the bottles and thereafter evacuated for various times to minimize physically sorbed formaldehyde. Humidity in the sealed bottles was controlled by the concentration of sulfuric acid or salt.

b. pH of cured resins and model compounds

The ground sample (usually -80 mesh) was shaken with distilled water at a ratio of 1/10 in a capped vial at least overnight. The pH of the supernatant was measured using a combination electrode.

c. Perforator test

With particleboard the standard procedure (27) was followed. About 100 g of 25 x 25 mm specimens were refluxed in toluene for 2 hours with continuous extraction of formaldehyde into water. The water was then analyzed for formaldehyde concentration by the acetylacetone fluorometric method (72). For ground resins and other materials, sample amounts were adjusted to produce comparable formaldehyde concentrations.

d. Nitrogen elution of particleboard, furnish, and cured resin

25 x 25 x 16 mm specimens were placed on a wire screen inside a hori- zontal glass tube (30 mm diam. x

750 mm long). Smaller particle size material was placed either in a similar vertical tube, with bottom gas feed, or in a continuously shaken Erlenmeyer flask, with gas feed via a tube leading to the flask's bottom. Entering gas was preconditioned by passage through or over saturated salt solutions at room temperature (23 ± 1oc). Exiting gas was continuously scrubbed of formaldehyde by passage through a series of impingers containing water and held in ice water. The number of impingers in series varied with gas flow rate and scrubbing time, based on prior experiments that had established conditions pro- viding greater than 95 percent scrubbing efficiency. The gas flow was interrupted at intervals to allow changing to a series of fresh impinger solutions; the removed impinger solutions were analyzed separately or in combination, usually with the acetylacetone fluorometric method (72). A variety of tests confirmed that no significant formaldehyde losses were caused by adsorption on the polyethylene tubing or by leaks. A number of analyses by both the acetylacetone and chromotropic acid methods showed no significant differences.

Ground resin was eluted by nitrogen in a similar manner; the primary exception was the use of only a few grams held in a glass tube that contained sintered glass frits at both ends.

e. Toluene elution of cured resin

About 1 g of vacuum-dried resin (-80 mesh) was weighed into a glass tube (16 mm diam. x 30 mm long) containing a medium porosity glass frit and evacuated for a specified period before being placed in the elution column. Under a dry nitrogen atmos- phere, toluene was allowed to drip slowly (0.05-0.15 ml/min) through the resin to the bottom of a water column (-300 mm long), where each drop of toluene bubbled back up through the water and was then collected in a separate flask. The toluene flow was interrupted at intervals to change the toluene collection flask and the water in the extraction column. The toluene in the flask was re-extracted with water and that water and the column water were analyzed for formaldehyde by the acetylacetone method. All but a few percent of the formaldehyde was in the column water.

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f. Water extraction of ground wood or board

Approximately 0.4 g of ground (-80 mesh) sample were placed in a stoppered flask with 75 mL of water made to pH 3 with HCl and containing 100 mg/L sodium azide as bacterial inhibitor. The flasks were shaken at 25°C and 10 mL aliquots were removed periodically by sucking through a sintered glass filter. At each removal, 10 mL of fresh liquid were added to the flask through the filter; each flask was sampled no more than three times. Aliquots were analyzed by the fluorometric acetylacetone procedure (72).

Appendix 2. Materials

The two UF resins (F/U 1.2 and 1.6) were prepared using a hybrid recipe agreed upon by three adhesive manufacturers as representative of current commercial procedures. The synthesis began with UF concentrate at F/U 4.8 to which urea was added in three separate steps: the first at pH 6.8 to 7.2 at 95°C, the second at pH 6.2 to 6.4 at 85°C, and the third at pH 7.0 to 7.5 at 60°C. Final pH was adjusted to 7.5 to 7.8 and solids content about 60 percent. The standard cure condition was 4 minutes at 160°C with 0.7 percent NH4Cl. The cured material was vacuum-dried, ground, sieved, further vacuum dried, and stored cold over desiccant.

For the resin-wood composites, the wood furnish was ground and sieved to produce -80 mesh material which was mixed with resin in a 1:4 wood:resin solids ratio. NH4Cl catalyst was added at 0.7 percent (resin solids basis) and the mixture cured as a thin film under nitrogen for 30 minutes at 100°c. Neat resin was treated similarly. The cured products were treated as above.

The PF resin was a commercial product with solids content of 51 percent, viscosity F by Gardner-Holt, and pH 9.8. The standard cure condition used was 3.5 minutes at 205C.

The UF particleboard was a commercial low emission product made with a resin having an F/U ratio below 1.2. The PF board was an experimental industrial product, and the furnish was standard industrial Southern pine material.

Formaldehyde-sorbed Southern pine furnish was prepared by allowing furnish to equilibrate for several days at room temperature over water solutions of salts and formaldehyde,

with the salt serving to control humidity. Formaldehyde-sorbed ground Southern pine was similarly prepared except for prior soaking in tartaric acid solution (with sodium azide) at pH 2 or 3.

Appendix 3. Methods of Calculation

Formaldehyde liberation rates on hypothetical board basis

Liberation rate = R in units of mg CH20 per g dry board per hour Assumptions:

- 0.07 g resin solids per 100 g dry board

- 16-mm thick board of density 0.64 g/cc

- emission from both sides of large panel

- mean F/U = 1.4 - decompositions are all apparent

first order with rate constant

k in hour-1 except where rates are measured directly from slopes of experimental data

(1) Rate constant k measured on molar basis--e.g., for a model compound.

(A1)

where

M = molecular weight of starting compound

(2) Rate constant k measured on weight basis--e.g., for cured resin.

(A2)

(3) Rate measured from slope of data plot.

(A3)

where

S = slope of plot of cumulative mg liberated CH2O per g starting material versus time in hours

(4) Rate R' from component of resin present as resin weight fraction wi.

(A4)

140

b. Calculation of rate variation with c. Rates for particleboard emission time, using rate constants . standards

(In all cases F = formaldehyde Assuming a steady state condition and zero subscripts indicate initial for the concentration CS in ppm of amounts.) formaldehyde in air and an emission

rate ER from board in units of mg CH2O per g dry board per hour:

(A10)

(A5) where

K = constant for conversion of units

N = ventilation rate in hours

L = board loading in m2 exposed

board area per m3 of air space

(A6)

Literature Cited

(A7)

with

(A8)

(A9)

141

TABL

E 1.-

-FOR

MAL

DEHY

DE L

IBER

ATIO

N FR

OM M

ODEL

COM

POUN

DS A

ND R

ESIN

S EX

PRES

SED

AS H

YPOT

HETI

CAL

BOAR

D EM

ISSI

ON R

ATES

TABL

E 1.-

-FOR

MAL

DEHY

DE L

IBER

ATIO

N FR

OM M

ODEL

COM

POUN

DS A

ND R

ESIN

S EX

PRES

SED

AS H

YPOT

HETI

CAL

BOAR

D EM

ISSI

ON P

ATES

--con

t.

TABL

E 1.-

-FOR

MAL

DEHY

DE L

IB&R

ATIO

N FR

OM M

ODEL

COM

POUN

DS A

ND R

ESIN

S EX

PRES

SED

AS H

YPOT

HETI

CAL

BOAR

D EM

SSIO

N RA

TES-

-CON

T.

TABL

E 2.-

-FOR

MAL

DEHY

DE L

IBER

ATIO

N FR

OM C

H 2O-

CELL

ULOS

E ST

ATES

EXP

RESS

ED A

S HY

POTH

ETIC

AL B

OARD

EM

ISSI

ON R

ATES

TABL

E 2.-

-FOR

MAL

DEHY

DE L

IBER

ATIO

N FR

OM C

H 2O-

CELL

ULOS

E ST

ATES

EXP

RESS

ED A

S HY

POTH

ETIC

AL B

OARD

EM

ISSI

ON R

ATES

--con

t.

Figure l.-- Formaldehyde liberation from cured PF resin. Weighing bottle test at 27C. Cured 3.5 min. 205C. -80 mesh. Evacuated 17 days.

Figure 2. --Calculated CH2O liberation rates for hypothetical board from data on models. Corrected to 25C/pH 3.O/RH 50 pct. HUD and E-l lines are allowable rates. (a) -NCH2N- at w = 0.8; (b) -NCH2OCH2N- at w = 0.1; (c) -NCH2OH at w = 0.1; (d) Ce110(CH20) Cell at w = 0.02; (e) Ce110CH2N- at w = 0.1; (f) "High" durability case; 5 pct loss in 50 years; (g) "Low" durability case; 30 pct loss in 20 years.

Figure 3.-- Formaldehyde liberation rates for model compounds. Weighing bottle test at 27C, 80 pct relative humidity.

Figure 4.-- Formaldehyde liberation from cured UF resin; F/U 1.6 Weighing bottle test at 27C. Cured 4 min. 160C, 0.7 pct NH4Cl. Evacuated 17 days.

152

! ! "

Figure 5. --Toluene elution of cured resins. 0.05-0.15 ml/min. 25C. -80 mesh. Evacuated 3-4 weeks. Cured as in Figs. 1 and 4.

Figure 7. --Calculated liberation rates for hypothetical board from data on cured resins. PF as in Fig. 1. UF samples as in Fig. 6.

Figure 6. --Formaldehyde liberation from resin versus resin/wood. 80 pct relative humidity/27C. Cured 30 min. 130°C. -80 mesh. Evacu- ated 2 weeks. Resin/plywood at 3/1. P = Perforator value in mg/g resin.

Figure 8.-- Particleboard elution by dry nitrogen; sample geometry effects (o ! shredded; 1.0 NCM;! " 25x25x16 mm 0.5 NCM; duplicate runs. P = Perfor-ator value in mg/l00 g dry board, measured on starting material at ~6 pct moisture content.

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Figure 9. --Particleboard elution by different dry gases. Differences in the two curves due to different flow rates and experimental configur- ations. P as in Fig. 8.

Figure 10. --Urea-formaldehyde particleboard elution by nitrogen; relative humidity (RH) effects. (0.4 NCM. P as in Fig.8.)

Figure 11.--Urea-formaldehyde particleboard elution by nitrogen at different relative humidities (RH). (0.5 NCM. P as in Fig. 8.)

Figure 12. --Phenol-formaldehyde particleboard elution by nitrogen at different relative humidities (RH). (0.5 NCM. P as in Fig. 8.)

1 5 4

∩ !

#

!

Figure 14. --Formaldehyde liberation from particleboards and CH2O-sorbed wood at 27°C and 33 percent relative humidity (RH); weighing bottle

Figure 13. --Elution of test with -80 mesh materials. o Southern pineformaldehyde-sorbed fur- impregnated with pH 2 tartaric acid and vapor-nish by nitrogen at dif- equilibrated with CH2O/salt solution at ~50ferent relative humidi- pct RH; ∩ as before except heated 4 min. atties (RH). (0.5 NCM. P 160°C after CH2O sorption; ! urea-formaldehydeas in Fig. 8.) particleboard; phenol-formaldehyde particle-

board, values approximate; P = Perforator value at indicated moisture content [MC).

Figure 15. --Formaldehyde liberation from particleboards and CH2O-sorbed wood at 27°C and 80 percent relative humidity (RH). Weighing bottle test with -80 mesh materials. o Southern pine impregnated with pH 2 tartaric acid and vapor-equilibrated with CH2O/salt solution at ~75 pct RH; # Figure 16. --Formaldehyde liberation as before except pH 3 tartaric acid; in water at 25°C and pH 3 from parti- ! urea-formaldehyde particleboard; cleboard and CH2O-sorbed wood; all

phenol-formaldehyde particle- materials -80 mesh. Sodium azide in board, parentheses indicating ap- water at 100 mg/L as preservative;

Fig. 14. 14. proximate values; P and MC as in symbols and abbreviations as in Fig.

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Figure 17.--Formaldehyde loss ratios at 20 percent relative humidity for various materials. Formaldehyde removed from a material divided by that removed from urea-formaldehyde (UF) particleboard. Board elution by nitrogen. Resin liberation by weighing bottle test. PF = phenol-formaldehyde.

Figure 18.--Formaldehyde loss ratios at 80 percent relative humidity (RH) and in water. Loss ratio = CH2O liberated relative to that from urea-formaldehyde (UF) particleboard in same test. WB = weighing bottle test; PF = phenol-formaldehyde; aq = water extraction test at pH 3. All materials -80 mesh. Southern (So.) pine impregnated with pH 2 tartaric acid and CH2O vapor-sorbed.

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