pvc handbook

PVC Handbook

Charles E. Wilkes, Charles A. Daniels, James W. Summers

ISBN 3-446-22714-8

Vorwort

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Preface

In this single handbook the editors aim to give a diverse audience of readers a complete accountof all aspects of PVC – from monomer manufacture to polymerization; the gamut of suchadditives as stabilizers, lubricants, plasticizers, impact modifiers, fillers and reinforcing agents;blends and alloys; compounding and processing; characterization; combustion resistance andweatherability; product engineering design; applications; environmental and safety; and finallythe PVC industry dynamics. Jim Summers’ Introduction presents a good historical backgroundon PVC and several of the individual chapters give a historical perspective to the technologiestherein. The handbook contains both practical formulation information as well as a mechanisticview of why PVC behaves as it does. The authors are from both industry and academia. Notsurprisingly, many of the industry authors are from the former BF Goodrich laboratories,where much of the industry’s technology was developed. Overall, however, about ten PVCand chemical- supplier companies are represented by the authors.

When I joined the BF Goodrich laboratories from graduate school in the mid-sixties, PVCresearch was one of the many challenges there. I had the great privilege of many conversationswith Dr. Waldo Semon, who was still roaming our halls at that time. One of his quotes thatwill always stick with me was: “Chuck, you’ll find that PVC is perhaps the most inert chlorinecompound in existence.” His words preceded by decades the health and safety concerns withchlorinated materials in general. But they are true today. PVC is a very safe material whenused and disposed of properly.

I probably wouldn’t have agreed to take on this daunting task if it weren’t for tremendouscontribution of my co-editors, Jim Summers and Chuck Daniels. With the addition of theirbroad network of PVC experts, we were able to organize this sterling group of authors. And,they made many detailed technical enhancements to the diverse chapters. Thanks also toChristine Strohm and coworkers at Carl Hanser Publishers for soliciting outside recommen-dations regarding the make-up of the handbook and for many enhancements in the chapters’readability.

Many of the fundamental technology discoveries related to PVC were made in the 1930’sthrough the 1950’s. Since those times, continuous improvement, broadening of applications,and process improvements for cost reduction and safety have been the mainstay of PVCresearch. The billions of pounds of PVC made today still use the free radical catalysts discoveredin the ‘30’s. On the innovation front, some readers will be familiar with the PVC TechnologyConsortium organized by the Edison Polymer Innovation Corporation and carried out overthe period 1998 to 2005. I have been privileged to direct this consortium of twenty one sponsorcompanies from fourteen countries who funded 12 research projects in six universities. I’dlike to point out a few highlights of results from that consortium. Rich Jordan University ofChicago), Bill Brittain (University of Akron) and Tony Rappe (University of Colorado) andcoworkers gained great understanding toward the metallocene polymerization of vinyl chloride,but in the end were unsuccessful. Their very excellent work has been published and willhopefully stimulate future success in some laboratory. On the other hand, Virgil Percec(University of Pennsylvania) and coworkers have succeeded in living radical polymerization

XII

of vinyl chloride. This technology has produced narrow molecular weight distributionhomopolymers as well as a range of block copolymers (both high Tg and low Tg). Severalpublications and patents have resulted from this landmark work. Bill Starnes (College ofWilliam & Mary) and coworkers have discovered a family of new non-metal PVC stabilizers.Chapter 4 herein and several publications and patents describe the results. Jim White andcoworkers (University of Akron) have published and patented new high performance alloytechnology, including new block-copolymer compatibilizers. Joe Kennedy and coworkers(University of Akron) have published and patented some novel modifications of PVC viaanionic polymerization. Kyonsuku Min and coworkers (University of Akron) published onthe preparation of PVC-polyurethane alloys by their reactive formation in a twin screw extruder.Eric Baer and Anne Hiltner (Case Western Reserve University) published excellent fundamentalstudies of toughness, creep and fatigue resistance in PVCs. Jerry Lando and Morty Litt (CaseWestern Reserve University) attempted to modify PVC stereostructure by polymerizationadditives. And, Miko Cakmak (University of Akron) published on the characterization ofchemical and morphological changes in PVC compounds during extrusion processing by on-line measurements. I personally hope that this body of published and patented knowledgewill result in a renaissance for PVC – resulting in many new applications and continued goodgrowth.

Charles E. Wilkes

Akron, OhioApril 2005

Charles Daniels Charles Wilkes James Summers

Preface

PVC Handbook


ISBN 3-446-22714-8

Leseprobe 1




5 Plasticizers

LEONARD G. KRAUSKOPF, ALLEN GODWIN

5.1 Introduction

In 1951, the International Union of Pure and Applied Chemistry (IUPAC) developed auniversally accepted definition for a plasticizer as a substance or material incorporated in amaterial (usually a plastic or an elastomer) to increase its flexibility, workability, or distensibility.A plasticizer may reduce the melt viscosity, lower the temperature of a second-order transition,or lower the elastic modulus of the product.

In 2003, the worldwide market for plasticizers was more than 4.6 million metric tonnes(10 billion pounds), with approximately 90% applied as plasticizers for PVC. In North America,plasticizer consumption was about one million metric tonnes (2.2 billion pounds), withExxonMobil Chemical, BASF, Sunoco, and Eastman Chemical Company as the majorproducers. The plasticizer market in Europe is about 1.3 million metric tonnes (2.8 billionpounds), with the three largest producers being ExxonMobil Chemical, Oxeno, and BASF.The region of the world with the largest plasticizer production is the Far East, with approxi-mately 2.2 million metric tonnes (5 billion pounds) produced annually. There are numerousplasticizer producers in that region, the major producers being Nan Ya Plastics, UnionPetrochemical Corp., Dahin Co., Aekyung Industrial Co, and LG Chemical.

Throughout the period from 1970 to 1995, the worldwide plasticizer markets grew at ratesabove the various GNPs; however this trend has started to decrease in North America and inEurope. In recent years, the average growth rate in those regions has ranged between 2 and3%, with projected growth rates of only 1–2%. The Far East is not only the largest market forplasticizers but continues to show the highest growth rates, with the Chinese plasticizer marketreported to have grown in excess of 12% in 2002. This rapid growth in China has alsocontributed to the decline in growth rates in many other parts of the world, as Chinese importshave displaced locally produced materials.

5.2 Historical Developments

Several authors have documented the historical developments of plasticizers and their use inPVC. Sears and Darby [1] provide an extensive review, including citations of the use of waterand other liquids as “quasi-plasticizers” in non-polymeric materials. The use of plasticizers inPVC and other polymers originated as extensions from low volatility solvents. Weinberg [2]

174 5 Plasticizers [References on Page 198]

points out that Waldo Semon, of B. F. Goodrich, originated the use of plasticized PVC usingplastisols (dispersion of PVC particles in plasticizers). Semon’s objective was to apply corrosion-resistant linings to metal storage tanks, which he accomplished via fused plastisol coatings onwire mesh secured to tank interiors. Krauskopf [3] reviewed plasticizers used in polymers,beginning with the use of camphor in nitrocellulose (1868) by the Hyatt brothers and up toGresham’s patented use of DOP (di-2-ethylhexyl phthalate) in PVC [4] in the early 1940s.The use of DOP prevailed as the preferred general-purpose plasticizer for PVC until the late1970s. In 1968, more than 550 different materials were listed as commercial plasticizers, availablefrom over 75 suppliers in the USA [5].

Changes in costs and availability of raw materials that serve as plasticizer feedstock have causeda significant reduction in the number of plasticizer suppliers and plasticizer products in use.Although there are still approximately 70 different plasticizers available, about 80% of theworldwide consumption is comprised of three plasticizers, di-2-ethylhexyl phthalate (DOP),diisononyl phthalate (DINP), and diisodecyl phthalate (DIDP).

5.3 Mechanisms of Plasticization

For a plasticizer to be effective, it must be thoroughly mixed and incorporated into the PVCpolymer matrix. This is typically obtained by heating and mixing until either the resin dissolvesin the plasticizer or the plasticizer dissolves in the resin. The plasticized material is then moldedor shaped into the useful product and cooled. Different plasticizers will exhibit differentcharacteristics in both the ease with which they form the plasticized material and in the resultingmechanical and physical properties of the flexible product.

Several theories have been developed to account for the observed characteristics of theplasticization process. A significant review of the theoretical treatment of plasticization isdescribed by Sears and Darby [6]. In this treatment, plasticization is described by three primarytheories, with some modifications.

According to the Lubricating Theory of plasticization, as the system is heated, the plasticizermolecules diffuse into the polymer and weaken the polymer-polymer interactions (van derWaals’ forces). Here, the plasticizer molecules act as shields to reduce polymer-polymerinteractive forces and prevent the formation of a rigid network. This lowers the PVC Tg andallows the polymer chains to move rapidly, resulting in increased flexibility, softness, andelongation.

The Gel Theory considers the plasticized polymer to be neither solid nor liquid but anintermediate state, loosely held together by a three-dimensional network of weak secondarybonding forces. These bonding forces acting between plasticizer and polymer are easilyovercome by applied external stresses allowing the plasticized polymer to flex, elongate, orcompress.

Free Volume is a measure of the internal space available within a polymer. As free volume isincreased, more space or free volume is provided for molecular or polymer chain movement.A polymer in the glassy state has its molecules packed closely but is not perfectly packed.

1755.3 Mechanisms of Plasticization

The free volume is low and the molecules cannot move past each other very easily. This makesthe polymer rigid and hard. When the polymer is heated to above the glass transitiontemperature, Tg, the thermal energy and molecular vibrations create additional free volumewhich allows the polymer molecules to move past each other rapidly. This has the effect ofmaking the polymer system more flexible and rubbery. Free volume can be increased throughmodifying the polymer backbone, such as by adding more side chains or end groups. Whensmall molecules such as plasticizers are added, this also lowers the Tg by separating the PVCmolecules, adding free volume and making the PVC soft and rubbery. Molecules of PVC canthen rapidly move past each other. If the plasticizer uniformly went into the PVC, it wouldbehave similarly to an uncured rubber, with lots of creep and high compression set. For example,uncured tires do not hold their shape; they require a crosslinking cure to give them dimensionalstability. Likewise, a thermoplastic elastomer such as PVC requires physical crosslinks whichare meltable to make them thermoplastic. These meltable crosslinks are the PVC crystalliteswhich give PVC a physical cure. Therefore, the plasticizer must not be a powerful solvent forall the PVC parts, but must be selective in enterring the amorphous PVC part and must notenter and destroy the crystalline part of PVC.

The mechanistic explanation of plasticization considers the interactions of the plasticizer withthe PVC resin macromolecules. It assumes that the plasticizer molecules are not permanentlybound to the PVC resin molecules but are free to self-associate and to associate with thepolymer molecules at certain sites such as amorphous sites. As these interactions are weak,there is a dynamic exchange process whereby, as one plasticizer molecule becomes attached ata site or center, it is readily dislodged and replaced by another. Different plasticizers yielddifferent plasticization effects because of the differences in the strengths of the plasticizer-polymer and plasticizer-plasticizer interactions. At low plasticizer levels, the plasticizer-PVCinteractions are the dominant interactions, while at high plasticizer concentrations plasticizer-plasticizer interactions can become more significant. This can explain the observation of “antiplasticization”, wherein low plasticizer levels (< 15 phr) increase rigidity in PVC, as measuredby modulus, tensile strength, elongation and low temperature properties.

For a plasticizer to be effective and useful in PVC, it must contain two types of structuralcomponents, polar and apolar. The polar portion of the molecule must be able to bind reversiblywith the PVC polymer, thus softening the PVC, while the non-polar portion of the moleculeallows the PVC interaction to be controlled so it is not so powerful a solvator as to destroy thePVC crystallinity. It also adds free volume, contributes shielding effects, and provides lubricity.Examples of polar components would be the carbonyl group of carboxylic ester functionalityor, to a lesser extent, an aromatic ring; the non-polar portion could be the aliphatic side chainof an ester. The balance between the polar and non-polar portions of the molecule is criticalto control its solubilizing effect; if a plasticizer is too polar, it can destroy PVC crystallites; if itis too non-polar, compatibility problems can arise. Useful tools in estimating plasticizercompatibility are the Apolar/Polar Ratio method developed by Van Veersen and Meulenberg[7] and the solubility parameter methods [8–11].


5.4 Types of Plasticizers

Plasticization is achieved by incorporating a plasticizer into the PVC matrix through mixingand heat. Plasticizers may be classified as either monomeric or polymeric plasticizers, dependingon their synthesis steps, which relates in part to their molecular weight. It is preferred tocategorize plasticizers on the basis of their chemical structure and associated performancewhen employed in PVC.

The IUPAC definition of a plasticizer is entirely focused on performance characteristics whencombined with a polymer; there is no implication of chemical structure or physical propertiesof the plasticizer per se. Early technical publications, therefore, presented rather vaguecategorizations based on observed performance properties. Attempts to correlate neatplasticizer properties with performance characteristics were unsuccessful; generalizationsbecame possible only after development of large, coherent databases of properties measuredon flexible PVC as a function of a broad range of plasticizer levels (i.e., 20–90 phr) for manycommercial and experimental plasticizers [12]. The key performance properties are influencedby plasticizer level (phr) as well as the chemical type. In addition, variations in isomeric structureand homologues within any given chemical family contribute performance variations thathave been measured in flexible PVC compositions. Table 5.1 shows the major chemical familiesof PVC plasticizers vs. key performance criteria.

An orderly comparison of plasticzers is facilitated by separating all plasticizers types intothree subgroups relating to their performance characteristics in PVC:

� General Purpose (GP): plasticizers providing the desired flexibility to PVC along with anoverall balance of optimum properties at the lowest cost. These are dialkyl phthalatesranging from diisoheptyl (DIHP) to diisodecyl (DIDP), along with low cost oils called“extenders”.

� Performance Plasticizers (PP): contribute secondary performance properties desired inflexible PVC beyond the GP type, while imposing somewhat higher costs. Table 5.1identifies these key performance criteria as “Strong solvaters”, “Low temperature” and“Low volatility”. These include specific phthalates and other types of plasticizers. Strongsolvaters have higher polarity and/or aromaticity. Conversely, low temperature types, suchas aliphatic dibasic esters, are less solvating and have higher diffusivity. Low volatilityrequires high molecular weight plasticizers, such as trimellitates and polyesters (poly-meric).

� Specialty Plasticizers (SP): provide properties beyond those typically associated with flexiblePVC designed for general purpose or specialty characteristics. These exceptional charac-teristics are typically a function of specific chemical plasticizer families and may vary as afunction of isomeric structure and/or homologues. Such properties are shown in Table 5.1as “Low diffusivity”, “Stability”, and “Flame resistance”. Few phthalates meet these specialrequirements. Polyester plasticizers provide low volatility and low diffusivity, along withlow smoke (in the absence of aromaticity) under fire conditions. Epoxy plasticizers provideadjuvant thermal stability to PVC; phosphates and halogenated plasticizers provide fireretardant properties. Specialty plasticizers impose even higher costs than PP gradeplasticizers.

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Table 5.1 indicates the primary performance characteristics associated with each chemicalfamily by “X”, while “√” denotes secondary functions associated with products in that class ofplasticizers. Formulating refinements in plasticizer performance and cost constitute theselection of preferred isomers and homologues of any given chemical family, or combinationsthereof. Phthalates are the most widely used class of plasticizers in PVC. As shown, theycontribute the most complete array of required performance properties in flexible PVC. Inaddition, their cost and availability supports their preference. While historically DOP –di(2-ethylhexyl) – phthalate has been the product of choice, the current market for GP plasti-cizers includes dialkyl phthalates that are slightly different homologues of DOP, such as di-isoheptyl (C7), diisooctyl (C8), diisononyl (C9) and diisodecyl (C10) phthalates; their combinedusage totals more than 80% of the worldwide plasticizer market. Note that the family ofphthalate plasticizers show an offering in all of the performance categories, as indicated by the“√”. Performance comparisons of these materials are reviewed in Sections 5.5 through 5.9.

� Phthalate esters: prepared by the esterification of two moles of a monohydric alcohol withone mole of phthalic anhydride. Although phthalate esters can be prepared from manydifferent alcohols, the range of alcohols used to make plasticizers for PVC applications isgenerally limited from C4 to C13 alcohols. Phthalate esters prepared from alcohols below C4are too volatile, while phthalate esters prepared from alcohols greater than C13 have limitedcompatibility. Many commercial grade phthalates are prepared using a mixture of mono-meric alcohols, such as butanol with 2-ethylhexanol, or blends of linear heptanol, nonanol,and undecanol, and so forth. Di-2-ethylhexyl phthalate (DOP), which is prepared from2-ethyl hexanol, establishes the standard against which other plasticizers may be compared.

� Extenders: shown in the general purpose plasticizer category because they are mostcommonly employed with phthalates to reduce costs in general purpose flexible PVC.

Table 5.1 Plasticizer Family/Performance Grid

Performance plasticizers Specialty plasticizers Family Generalpurpose

Strongsolvent

Lowtemp

Lowvolatility

Lowdiffusion

Stability Flame resistance

Phthalates X √ √ √ √ √

Trimellitates √ X √

Aliphatic dibasic esters X

Polyesters X X

Epoxides √ √ X

Phosphates √ √ X

Extenders X

Miscellaneous X X X

X = Primary performance function√ = Secondary performance function



These low cost oils have limited compatibility in PVC; for example, naphthenic hydro-carbons may be used up to 35 weight% in dialkyl phthalate plasticizers, while aliphatichydrocarbons are limited to less than 10%. Higher molecular weight phthalates are lesstolerant of extender levels due to their reduced compatibility in PVC. Chlorinated paraffinextenders are not widely used in the U.S., but are commonly employed as secondaryplasticizers worldwide. Chlorinated paraffins are produced by chlorination of hydrocarbonsup to a chlorine content in the range of 30–70%. These secondary plasticizers are used toreduce cost and to improve fire resistance. The plasticizers with lower chlorine contenthave lower specific gravity, viscosity, and color, while higher chlorine content impartsincreased fire resistance.

“Performance Plasticizers” have three subgroups:

� “PP-SS”, strong solvator;

� “PP-LT”, low temperature, and

� “PP-LV”, low volatility.

In addition to selected phthalate candidates, other chemical structures contribute desiredperformance attributes.

“PP-SS”s, strong solvaters, are a result of increased polarity and/or aromaticity. Thus, lowermolecular weight phthalates such as dihexyl (DHP) and butyl, octyl (BOP), as well as butyl-benzyl (BBP) phthalate fall into this category; these plasticizers also contribute to volatilefuming during processing and volatilization in end use applications. In addition, there arenon-phthalate plasticizers of high aromaticity that serve as strong solvators. Such materialsare benzoate esters and tri(cresyl) phosphate.

“PP-LT”s are low temperature phthalates made with normal or “linear” alcohols. These less-branched alkyl groups contribute improved low temperature properties in all the chemicalfamilies of plasticizers. The entire family of aliphatic dibasic esters contributes exceptional lowtemperature properties. They are prepared by the esterification of one mole of dibasiccarboxylic acid, such as adipic or azelaic acid, with two moles of monohydric alcohols. Lowermolecular weight alcohols are used with higher molecular weight acids, and vice versa, suchthat the total carbon content per molecule ranges between C18 and C26. This maintains theapolar/polar ratio required to provide PVC compatibility along with low temperatureproperties. Di-2-ethylhexyl adipate (DOA) is the standard and most widely used plasticizer inthis class. Di-2-ethylhexyl azelate (DOZ), di-2-ethylhexyl sebacate (DOS), and diisononyladipate (DINA) are used for low temperature applications requiring lower plasticizer volatility.

Plasticizer structural relationships with low temperature performance will be reviewed inSections 5.5 through 5.9.

“PP-LV”s are low volatility plasticizers primarily because of their high molecular weight, whichis also reflected in low vapor pressure. High molecular weight phthalates that serve as “LV”plasticizers include those having molecular weights greater than DIDP (446). Increasing themolecular weight of phthalates increases the ratio of apolar/polar functionality until loss ofPVC compatibility occurs at molecular weights greater than that of DTDP (530). Highmolecular weight phthalates having low volatility and compatibility with PVC include DIUP,UDP, DTDP, 911P, and DUP, all shown in Table 5.2. Two chemical families are noted for theiruse as low volatility plasticizers – trimellitates and polyesters (also referred to as polymerics).

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Table 5.2 Plasticizer Acronyms, Chemical Compositions, and Substitution Factors

Acronym Chemical structure Molecular weight

Subst.* factor

Phthalates

BBP butyl, benzyl ca. 312 0.94

BOP butyl, 2-ethylhexyl ca. 365 0.94

DHP di(isohexyl) 334 0.96

DIHP di(isoheptyl) 362 0.97

DOP di(2-ethylhexyl) 390 1.00

DIOP di(isooctyl) 390 1.01

DCP di(2-normal-octyl) (aka capryl) 390 NA

DINP di(isononyl) 418 1.06

DIDP di(isodecyl) 446 1.10

DIUP di(isoundecyl) 474 1.16

UDP di(iso C11

, C12

, C13

) ca. 502 1.21

DTDP di(isotridecyl) 530 1.27

Linear phthalates

DBP di(n-butyl) 278 0.86

79P di(linear C7, C

9) ca. 390 1.00

NHDP(610P) di(n-C6, C

8, C

10) ca. 418 0.99

DNNP di(n-nonyl) 418 0.94

L9P di(linear nonyl) 418 0.99

7911P di(linear C7, C

9, C

11) ca. 418 1.00

911P di(linear C9, C

11) ca. 446 1.05

DUP di(linear C11

) 474 1.14

Trimellitates

NODTM tri(n-C8, C

10) ca. 592 1.12

TOTM tri(2-ethylhexyl) 546 1.17

TIOTM tri(isooctyl) 546 1.19

TINTM tri(isononyl) 588 1.27

Adipates

79A di(linear C7, C

9) ca. 370 0.90

DOA di(2-ethylhexyl) 370 0.93

DIOA di(isooctyl) 370 0.94

DINA di(isononyl) 398 0.98



Trimellitates are the product of three moles of monohydric alcohols and trimellitic anhydride(TMA). The third alkyl group, compared to phthalates, contributes higher molecular weight;the third ester group contributes sufficient polarity to maintain PVC compatibility.

“Specialty Plasticizers” are also divided into three subgroups:

� “SP-LD” for low diffusion;

� “SP-Stab” for stabilizing function, and

� “SP-FR” for fire resistance in PVC.

Low diffusivity is contributed by high molecular weight and highly branched isomericstructures. Diisodecyl phthalate (DIDP) and diisotridecyl phthalate (DTDP) impart improvedresistance to diffusion-controlled plasticizer losses, and are sometimes used in combinationwith more costly diffusion-resistant plasticizers. But the polyester family is noted for itsoutstanding performance in this category.

Polymeric plasticizers are typically polyesters, with a molecular weight range from 1,000 to8,000. Polyethylene copolymers (EVA’s,VAE’s, etc.) and terpolymers can range up to > 500,000.Polyesters are prepared by the esterification of propylene glycol or butylene glycol with aliphaticdibasic acids. The greater the plasticizer viscosity, or molecular weight, the greater itspermanence. Polymeric plasticizers composed of branched structures are more resistant todiffusivity losses than those based on linear isomeric structures; on the other hand they aremore susceptible to oxidative attack. The polarity, or the oxygen-to-carbon ratio, also impactsextraction resistance of the polymerics. Lower polarity materials exhibit better extractionresistance towards polar extraction fluids such as soapy water. Glutarate polymerics reportedlyhave a proven history of providing good weathering resistance [13].

Table 5.2 (continued)

Acronym Chemical structure Molecular weight

Subst.* factor

Phosphates

DDP isodecyl, diphenyl 390 0.96

TOF tri(2-ethylhexyl) 435 1.00

TCP tricresyl 368 1.31

Epoxides

OET 2-ethylhexyl epoxy tallate ca. 410 0.96

ESO epoxidized soybean oil ca. 1,000 1.10

Others

DOTP di(2-ethylhexyl)terephthalate 390 1.03

DINCH di(isononyl) cyclohexane-1,2-dicarboxylate 422 NA

* Substitution factor = PHR required for 80A Durometer hardness at room temperature vs.required DOP level (52.9 phr).

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Interestingly, the trimellitate plasticizers demonstrate improved resistance to diffusivity-controlled losses only under certain conditions. Trimellitates in combination with polyesterplasticizers control migration from PVC refrigerator gaskets, which can cause crazing of theABS door liner. However, trimellitates fail to provide reduction of plasticizer diffusivity underoil immersion tests.

Pentaerythritol esters are a type of “miscellaneous” plasticizers that impart both low volatilityand diffusivity. Pentaerythritol and dipentaerythritol are tetra and hexa alcohols, respectively;they are esterified with a stream of straight chain fatty acids to make plasticizers. Hercoflex®600 is the pentaerythritol tetraester and 707 is a mixture of tetra and hexa esters, using amixture of pentaerythritol and dipentaerythritol. Their molecular weights are approximately600 and 750, respectively, which contributes to both low volatility and diffusivity.

Epoxy plasticizers enhance thermal and UV stability of PVC. They are the only class ofplasticizers that undergo a chemical grafting onto the PVC polymer at the site of labile chloridesin the presence of mixed metal stabilizers [14]. This chemical family is composed of essentiallytwo types of epoxidized natural products. Epoxidized oils, such as soybean oil (ESO) andlinseed oil (ELSO) are prepared by the use of peracetic acid, which adds the oxirane structureat unsaturated (double bond) sites. These oils have molecular weights of approx. 1,000, causingthem to perform as low volatility plasticizers. The other group of epoxy plasticizers isrepresented by octyl epoxy tallate (OET). This product results from the epoxidation of tall oilesters, which are the esterified product of tall oil acids. The OET has a molecular weight ofapprox. 410, and is a monoester. This causes it to have more limited compatibility in PVC, andto contribute toward lower plastisol viscosity and low temperature properties. The primaryperformance attributes of epoxy plasticizers are their role in PVC stabilization, which isaccomplished at less than 10 phr levels. Therefore, while they contribute to the plasticizationin PVC, the secondary plasticizer effects are minimized. A commercial curiosity of the“epoxidized phthalate-type” structure was found to contribute beneficially to thermal stability,while otherwise imparting the expected properties of the dialkyl phthalate counterpart [15].

Flame resistant plasticizers include halogenated (preferably brominated) phthalates and thephosphate family. Brominated phthalate esters are produced by the esterification of tetra-bromophthalic anhydride with various alcohols, most typically 2-ethylhexanol. Phosphateplasticizers which may be considered as “inorganic esters” are prepared by the slow additionof phosphorous oxychloride to alcohol or phenol. The highly aromatic tricresyl phosphate(TCP) is the most effective fire retardant, but generates high smoke under fire conditions.Trialkyl phosphates (like TOF) are less efficient in fire resistant properties. Commercialphosphate plasticizers use combinations of aryl and C8 and C10 alkyl groups to offer a balanceof fire reduction, volatility, and efficiency. A combination of phosphate plasticizers, antimonytrioxide, and zinc borate yields a superior flame retardant grade of PVC for demandingapplications such as plenum cable jacketing and electrical insulation [16]. Phosphate plasticizersmay be combined with phthalates to reduce formulating costs.

Miscellaneous plasticizers include “phthalate-like” esters, benzoates, sulfonates, pentaerythritolesters, citrates, and similar materials.

“Phthalate-like” esters include DOIP [di (2-ethylhexyl) meta (called “iso”) phthalate], andDOTP [di (2-ethylhexyl) para (called “tere”) phthalate], which are isomeric structures of DOP.DOTP is commercially available at similar costs to DOP; Section 5.8 reviews the performance



characteristics of DOIP and DOTP. Hexamoll® DINCH is di (isononyl) cyclohexane-1,2-di-carboxylate [17], the hydrogenated product of the corresponding di C9 phthalate ester(DINP).As indicated [15], its performance characteristics in PVC are expected to be similar to thephthalate counterpart, except for having less solvency for PVC. DINCH has recently beenintroduced by BASF as a candidate for applications with sensitivity for peculiar health andenvironmental concerns. These sensitivities are also addressed with the recent introductionof a novel triester plasticizer completely devoid of carbon ring configurations; it is claimed foruse in medical applications and “low smoke” grade PVC electrical insulations [18].

Benzoates are the esterification products of benzoic acid and selected glycols, usually diols.Preferred glycols are dipropylene glycol and butane diols. One commonly used benzoate isdipropylene glycol dibenzoate (DPGDP, commercially Benzoflex® 9-88). Its preferred use isin PVC flooring products, owing to its strong solvating strength, and it reportedly controlsplasticizer bleeding into asphalt adhesives. Benzoflex® 1046 is a mixed ester of benzoate esterof Texanol®. Texanol is an “ester-ol” resulting from the Aldol and Tischenko condensationreactions of three moles of isobutyraldehyde. Esterification with benzoic acid yields the mixedester. Its preferred use is in vinyl sheet flooring, where the benzoate enhances processing,while the low molecular weight contributes a hardened, stain resistant surface, due tovolatilization.

Sulfonates also exhibit strong solvency for PVC. Mesamoll® is a product of Bayer; it is describedas the phenyl cresyl esters of pentadecyl sulfonic acid. It is reportedly resistant to hydrolysesand diffusion controlled plasticizer losses.

Citrates are promoted for plasticized PVC applications facing exceptional toxicological and/orenvironmental constraints. However, neat citric acid does not meet PVC compatibilityrequirements. Therefore, citrate plasticizers are tetraesters, resulting from the reaction of onemole of an organic acid (with the single alcohol group) and three moles of alcohol, whichesterify the three acid groups.

5.5 Plasticizer Performance

The previous section summarized key plasticizer families and associated performancecharacteristics. Within each family, there are a variety of isomeric structures and homologuesthat contribute measurable variations in performance. The phthalate family includes a greatvariety of isomers and homologues that are useful as plasticizers for PVC. This, then, is anappropriate plasticizer family for the evaluation of the effects of chemical structures in PVC.Generalizations derived from phthalate structures appear to translate into other chemicalfamilies as well. Performance plasticizers (PP) contributing low volatility include those havingmolecular weights greater than that of DIDP (446) and those of less branching (oxidativeresistance). Linear alkyl structures contribute low temperature properties as well as lowervolatility. All aliphatic dibasic esters, such as the adipates, contribute exceptional lowtemperature properties. The chemical structures that contribute improved low temperatureproperties typically impart lower plastisol viscosities, due to their own lower viscosity; likewise,their reduced tendency for solvation of PVC resin contributes to improved viscosity stability

183

under storage conditions. On the negative side, the low temperature type plasticizers impaircompatibility in PVC and diffusion-controlled plasticizer loss in end use applications. Octylepoxy tallate (OET) and tri(2-ethylhexyl) phosphate demonstrate performance characteristicssimilar to those of low temperature plasticizers.

Performance plasticizers having strong solvating characteristics include phthalates havinghigher polarity and aromaticity. These structural features also contribute increased volatilitydue to smaller non-polar tails. Non-phthalate plasticizers having strong solvency are highlyaromatic (benzoates, TCP) or other more polar structures such as sulfonates.

Epoxy plasticizers contribute the unique feature of enhancing thermal stability, and alsocontribute plasticization properties consistent with their molecular structure. That is,epoxidized oils (about 1,000 molecular weight) enhance low volatility, while monoester OETenhances low temperature performance.

Disciplined studies of carefully controlled model formulations, sample preparation, andconditioning have shown little correlation between neat plasticizer properties and theirperformance in PVC [19–22].

Key physical properties appear to correlate with plasticizer performance in PVC only when thecomparisons are restricted to homologues of a given chemical family. Further correlation ofphysical properties of the neat plasticizer with performance in PVC is confounding; a summaryof key physical and performance properties of plasticizers for PVC verifies the point [15].

The only mild chemical interaction between plasticizers and PVC polymers allows for thecalculation of predicted specific gravity of the plasticized PVC compositions. Yet this chemicalassociation precludes quantitative prediction of plasticizer losses due to volatilization (i.e.,using vapor pressure), or diffusion-controlled migration [19, 23–25]. Oxidative or hydrolyticdegradation, on the other hand, have the opposite effect when attempting to predict plasticizertransience from PVC.

The color of plasticized PVC compositions is typically not altered by the plasticizer. This isbecause most commercial grade plasticizers are near “water-white” in color. Highly colored(amber–brown) plasticizers would, of course, impart undesired color to flexible PVCcompositions.

The effects of a wide variation in plasticizer level on the mechanical properties of PVCcompounds are listed in Table 5.3.

This example shows typical properties for general purpose PVC containing DINP levels rangingfrom zero (rigid vinyl) to about 600 phr (parts per hundred resin, by weight). The averageconsumption of plasticizers in flexible PVC stands at 50 phr. Useful commercial productstypically range from about 20 to 100 phr; fishing lures, at about 600 phr, are an exceptionalproduct.

Reliable generalizations of plasticizer structure/performance relationships require extensiveevaluations using disciplined model formulations, raw materials, sample preparation, andconditioning in the measurement and cataloging of the data. Such procedures have beendescribed [12]. Other publications have utilized these data to provide analyses and comparisonsof various commercial and experimental plasticizers [15, 23, 26–28]. The translation of thisbasic information to specific property requirements of flexible PVC products is enhanced byan awareness of some generalizations relevant to properties influenced by plasticizer chemicalstructure as well as level (phr) in the PVC, as follows:



� Hardness (softness) is significantly influenced by plasticizer level, as well as type of plasticizer,which controls plasticizer “efficiency”.

� Tensile strength and ultimate elongation (% extension at failure) are influenced by plasticizerlevel, but these properties are not significantly altered as function of plasticizer type withPVC formulated to specified room temperature hardness.

� Modulus (stiffness, flexibility) may be measured under tensile stress (ASTM D 882) at aspecified strain level, or under flexural stress (ASTM D 747), or under torsional stress(ASTM D 1043). Modulus values vary significantly as a function of plasticizer level, andare somewhat influenced by plasticizer type (efficiency) when measured at room tem-perature and formulated to specified hardness. The three different techniques fordetermination of modulus at room temperature result in significantly different absolutevalues, due to variations in test methodology forces of extension, compression, and shear.

Table 5.3 Typical Properties of General Purpose Vinyl Plastic Products

Rigid Semirigid Flexible Very flexible

Extremelyflexible

DINP, phr 0 34 50 80 600

Wt% of composition 0 25 33 44 86

Typical properties

Specific gravity, 20/20 °C 1.40 1.26 1.22 1.17 1.02

Hardness Durometer A, 15 s – 94 84 66 < 10

Flexural stiffnessa at 23 °CMPa psi

> 900 > 130,000

6910,000

121,700

3.4500

– –

Tensile strengthb

MPa psi

> 41> 6,000

31 4,500

21 3,100

14 2,000

– –

Elongation (%)b < 15 225 295 400 –

Brittlenessc

°C°F

> 23> 73

–16+3

–32–26

–47–53

– –

Examples Bottles,pipe, siding,records

Shades, shoe heels, thin films,produce wrap

Wall-cover-ing, book-binders, upholstery,garden hose

Boots, gloves, water beds

Fishinglures

a ASTM D 747b ASTM D 882c ASTM D 746

Source: Krauskopf, L. G., in Encyclopedia of PVC, 2nd ed., Nass, N. L. and Heiberger, C. A. (Eds.), Marcel Dekker (1988), p. 149(reprinted with permission)

185

� Low temperature properties, both low temperature modulus and brittleness, are significantlyinfluenced by plasticizer level (phr) and type. Low temperature modulus values aredetermined by ASTM D 1043 (Clash-Berg, Tf), while brittleness temperature is determinedby ASTM D 746 (TB).

NOTE: Reliable measurements of failure properties require that test specimens be completelyfree of surface imperfections (nicks/cuts), completely fused and conditioned at roomtemperature (ASTM conditions preferred) for at least 24 hours following preparation ofspecimens.

Variations in isomeric structure are primarily imparted by the nature of the alkyl moiety.These configurations are a function of the starting materials and processes used to producethe plasticizer-grade alcohols. Plasticizers made with more linear (less branched) molecularstructures are more efficient, and impart improved low temperature properties and volati-lization and oxidative resistance [29, 30]. Krauskopf [31] differentiated plasticizer performanceeffects as a function of five different degrees of branching using commercial grade di(C8),di(C9),and di(C10) phthalates; these were grouped as:

� Normal: 100% unbranched; primarily an academic product, except for a limited amountof commercial products based on mixtures of normal C6, C8, and C10 alcohols.

� Linear: a mixture of normal and monomethyl branched alcohols. These “linear” alcoholsare produced by the hydroformylation (Oxo process) of normal alpha olefins. The resultantalcohol is approximately a 70/30 molar ratio of normal/2-methyl branched isomers.

� Slightly branched (SLBR): primarily a mixture of monomethyl and dimethyl branchedalcohols. Commercially, the “slightly branched” alcohols are the hydroformylation productsof octenes that are dimerized normal butenes. The resultant alcohol (nonanol) is a randommixture of monomethyl octanols and dimethyl heptanols.

� Moderately branched (MODBR): primarily dimethyl or monoethyl (i.e., 2-ethylhexyl)branched; these serve as the major type in “General Purpose” plasticizer category.“Moderately branched” nonanol is the hydroformylation product of mixed olefinsgenerated by the dimerization of a mixture of propylene and normal butene feeds. Theresultant olefin is a mixture of hexenes, heptenes, and octenes, which are separated bydistillation. The octenes are hydroformylated to give nonanol mixtures primarily composedof dimethyl substituted C7 backbones.

� Highly branched (HIBR): triple methyl branched; a specific product available in Europe.The “highly branched” nonanols are the hydroformylation product of trimethyl branchedpentenes which are a product of dimerized isobutene; the resultant alcohol is primarily3,5,5-trimethyl hexanol. These are notably susceptible to oxidative attack, while showingincreased resistance to diffusivity.

These variations in degrees of branching demonstrate measurable effects on PVC propertieswith respect to plasticizing efficiency, low temperature properties, diffusivity, volatility, andstability to thermal and oxidative degradation. The normal C9 phthalate has a substitutionfactor (S.F.) of 0.94, while the plasticizing efficiency of the “linear” nonyl phthalate is equivalentto that of DOP (S.F. = 1.00). When compared at equivalent hardness in PVC, the “linear”phthalate is only slightly deficient to the normal nonyl phthalate with respect to low temperature



properties and volatility. The “moderately branched” (MODBR) DINP shows less plasticizingefficiency (S.F. = 1.06) and a deficiency in low temperature properties and volatility vs. thenormal nonyl phthalate. Compared to DOP performance at specified room temperaturehardness, the DINP (MODBR) provides equivalent low temperature properties with signifi-cantly lower volatility. The performance of the “slightly branched” (SLBR) DINP is essentiallyequivalent to a 50/50 mixture of di (normal nonyl) phthalate and moderately branched DINP(MODBR).

Wadey studied a series of designed isomeric variations of DINP and the effects on plasticizerperformance in PVC [30]. The conclusions are consistent with the generalizations cited above.

5.6 Plasticizer Efficiency

Plasticizer “efficiency” may be quantified as a function of PVC Durometer hardness. Similarcomparisons may be made for other mechanical properties, but hardness test reliability andthe common practice of a designated room temperature hardness value supports its use toquantify plasticizing efficiency. Figure 5.1 graphically portrays quantitative determination ofplasticizer efficiency, expressed as “Substitution Factor” (SF); in this example, the hardnessvalues are compared for DINP (MOD-BR) to DOP [di-2-ethylhexyl phthalate] plasticizedPVC.

DINP

50

55

60

65

70

75

80

85

90

95

20 30 40 50 60 70 80 90 100

Plasticizer phr

Sh

ore

A

D

uro

me

te

r h

ard

ne

ss

(1

5 s)

52.9 56.2

DOP

Figure 5.1 Durometer A hardness of DINP vs. DOP(Source: Krauskopf, L. G., in Handbook of PVC Formulating, Wickson, E. J. (Ed.) (1993) Wiley, New York, p. 171,courtesy John Wiley, reprinted with permission)

187

It is shown that 80 Durometer A hardness is provided by 52.9 phr DOP, while 56.2 phr DINPis required to provide the same hardness. Thus, the substitution factor (SF) for DINP vs. DOPis 1.06, as shown in Equation 5.1:

Substitution Factor (SF) = ⎛ ⎞ ⎛ ⎞

=⎜ ⎟ ⎜ ⎟

⎝ ⎠⎝ ⎠

phr plasticizer at Durometer 80 56.2 phr DINP

phr DOP at Durometer 80 52.9(5.1)

The “SF” indicates that DINP-MODBR is 6% less efficient than the plasticizing efficiency ofDOP. In other words, DINP needs to be added at a level 6% higher than the DOP level, inorder to achieve the same hardness or softness. It has been found that this ratio (substitutionfactor) is consistent over plasticizer levels ranging from about 20 to 90 phr. The question ofacceptability, then, rests on comparative formulating costs and other critical properties providedat the specified room temperature hardness. In general, it is found that when compared atequivalent hardness, the DINP-MODBR plasticized PVC will have preferred low temperatureproperties as well as significantly less plasticizer loss due to volatilization and diffusivity. Mostcommercial grades, and many experimental plasticizers, have been evaluated in PVC over awide range of levels (phr). The disciplined cataloguing of the performance properties allowsfor easy comparisons of cost effective formulating options at specified hardness values as wellas at specified plasticizer levels. A computer program includes optimizations as a function offiller content as well as plasticizer selection and predicted properties in PVC [22]. Table 5.2lists relative plasticizing efficiency values determined for commercial grade plasticizers, alongwith acronyms, chemical compositions, and molecular weights.

5.7 Low Temperature

Tables 5.4 and 5.5 show low temperature flex (Tf) and brittleness (TB) for PVC using differentplasticizers. Table 5.4 compares low temperature properties when formulated to equivalentroom temperature hardness (80A Durometer); Table 5.5 shows low temperature propertiesat equivalent plasticizer levels (50 phr, by weight), in a recipe commonly used for “screening”plasticizer performance.

It is shown that for given alkyl structures, the trimellitate family provides similar or onlyslightly improved low temperature properties vs. the phthalate counterparts, because of theneed for higher plasticizer levels to provide the target room temperature hardness, due to thelower plasticizing efficiency of trimellitates versus phthalates. Adipate plasticizers, on the otherhand, impart significantly improved low temperature properties (by about –25 to –35 °C)versus their phthalate counterparts, in spite of the fact that they are more efficient (substitutionfactors of < 1.00) in providing target room temperature hardness. The more linear alkylstructures in the plasticizer contribute improved low temperature properties (by about –5 to–7 °C) vs. the branched isomers. They also have lower substitution factors (higher plasticizingefficiency) to meet room temperature hardness. It should be noted that end-use productsrequire low temperature tests to be conducted on whatever form, or shape, the product has.

5.7 Low Temperature


Table 5.4 Low Temperature Properties of Unfilled, General Purpose PVC, Formulated to Meet 80 ADurometer Hardness at Room Temperature

Tf (°C)a T

B (°C)b

Phthalates

BBP –10.7 –12.1

BOP –25.8 –31.1

DIHP –24.6 –32.9

DOP –27.7 –34.9

DIOP –27.5 –32.8

DOTP –31.9 –36.5

DINP –29.2 –35.8

DIDP –31.6 –37.8

DIUP –32.2 –37.8

UDP –32.8 –41.8

DTDP –39.3 –42.9

610P –36.1 –45.5

79P –35.7 –40.2

7911P –34.9 –42.2

L9P –37.4 –44.4

911P –39.8 –47.6

DUP –43.0 –53.7

Trimellitates

TOTM –29.4 –39.0

TIOTM –27.5 –37.8

TINTM –31.4 –38.8

Adipates

DOA –50.9 –61.7

DIOA –49.3 –63.2

DINA –51.6 –64.4

79A –52.5 –66.1

PVC formula by weight:PVC-100, plasticizer (concentration adjusted to yield a shore A of 80),Liquid Ba/Cd/Zn stabilizer – 2.0, stearic acid 0.25.a ASTM D1043b ASTM D746

189

Table 5.5 Low Temperature Properties of Unfilled, General Purpose PVC at 50 PHR Plasticizer

Tf (°C)a T

B (°C)b

Phthalates

BBP –11.0 –12.3

BOP –26.9 –31.9

DIHP –23.6 –32.3

DOP –24.9 –32.9

DIOP –23.8 –30.0

DOTP –27.7 –33.4

DINP –23.6 –31.8

DIDP –23.6 –31.8

DIUP –22.4 –30.5

UDP –20.9 –32.9

DTDP –24.4 –31.5

610P –33.5 –43.5

79P –32.7 –38.0

7911P –32.0 –40.2

L9P –34.3 –42.1

911P –34.3 –43.4

DUP –33.3 –45.9

Trimellitates

TOTM –19.2 –31.8

TIOTM –15.9 –29.1

TINTM –17.3 –27.9

Adipates

DOA –52.8 –62.7

DIOA –49.8 –63.5

DINA –50.5 –63.8

79A –55.3 –67.6

PVC formula by weight:PVC-100, plasticizer – 50Liquid Ba/Cd/Zn stabilizer – 2.0, stearic acid 0.25.a ASTM D1043b ASTM D746

5.7 Low Temperature


Thus, performance test results measured on finished commercial PVC plastics may beinfluenced by factors other than the formulating predictions. For example, incompletely fusedor improperly conditioned, or otherwise damaged, specimens may experience undue failurein mechanical property testing.

Likewise, the translation of catalogued values may require experience. For example, properlyprepared PVC insulation for electrical conductors typically meets low temperature mandrelbend testing at temperatures approximately –15 °C lower than the predicted brittleness (TBby ASTM D 746) values of formulated PVC.

5.8 Permanence (Transience) of Plasticizers

Plasticizers have a strong affinity for PVC polymers, but do not undergo a chemical reactionthat causes bonding, or grafting, to the polymer. Note, however, that epoxy plasticizers are anexception, in that they undergo chemical grafting onto PVC in their role as stabilizers, replacinglabile chlorides [14] in addition to their role of acid absorption. Other functional additivesare known to graft and/or polymerize in the PVC matrix, but these are generally not consideredas traditional “external” plasticizers. Copolymers for example, can lower PVC’s Tg as plasticizersdo. But at the same time, any significant level of co-monomer will disrupt the syndiotacticPVC structure and disrupt the ability to form crystallites. The crystallites are the physicalcross-links that hold the structure together as a thermoplastic elastomer. Thus with copolymers,creep increases, compression set increases, and long-term elasticity is lost. Thus, the plasticizers,when not grafted or copolymerized, may be separated from the PVC matrix due to extractionby solvents, oils, water, surface rubbing, volatility, migration into adjacent media, or degradationmechanisms.

Investigations of plasticizer transience found that quantitative predictions were confoundedby “compatibility”, which was difficult to quantify [19, 20, 24, 25]. However, Quackenbossdetermined that two controlling mechanisms (other than effects of degradation) are at playunder conditions that contribute to loss of plasticizer. These are the rate of loss that occurs atthe surface of the specimen vs. the rate at which the plasticizer diffuses to the surface; theslowest rate is the controlling factor. For example, most plasticizers have extremely low solubilityin water, and therefore exhibit surface-controlled loss rates under aqueous environments.Plasticizer losses due to extraction by oily media (in which plasticizers are highly soluble) arecontrolled by diffusivity rates. Volatile losses of plasticizer are influenced by vapor pressure,solvency strength for the polymer and oxidative degradation, as well as the ambient airflowrate in the test chamber. It is known that test chamber atmospheres saturated with plasticizervapor allow for reversed absorption of plasticizer into the test specimens. Polymeric plasticizersof high molecular weight (≈1,000 to > 500,000) and of bulky molecular structures showexcellent permanence because of low diffusivity. However, this family of plasticizers is alsonoted for sensitivity to hydrolytic degradation in aqueous atmospheres.

Dialkyl phthalate plasticizers range in molecular weight from 278 (dibutyl) to about 530(ditridecyl). Commercial experience has shown that dibutyl is unacceptably volatile (exceptin some adhesive applications), while ditridecyl phthalate is useful for PVC applications under

191

high temperatures for extended periods; trimellitates and polyesters are typically even lowerin volatility. Preferred “General Purpose” plasticizers range in molecular weight from 362(DIHP) to 418 (DINP), while DOP has an intermediate molecular weight at 390. The volatilityof DINP is significantly less than that of DOP, while DIHP is significantly more volatile thanDOP in most applications. This characteristic often dictates the preferred choice of the generalpurpose plasticizer for given applications. Di(linear alkyl) phthalates impart lower volatilityand improved oxidative resistance vs. their branched counterparts. All phthalate, trimellitate,and aliphatic dicarboxylic diesters show excellent resistance to hydrolytic attack under exposureto aqueous environments. Diffusion-controlled transience is poor for the aliphatic dicarboxylicdiesters, but good for branched phthalates; linear dialkyl phthalates are measurably less resistantthan the branched phthalates, but significantly better than the aliphatic dicarboxylic diesters.Oil extraction resistance of trialkyl trimellitates is not good compared to that of phthalates.This is apparently due to their lower plasticizing efficiency as well as to the increased proportionof alkyl moieties in the molecular structure.

Two isomers of DOP have been found to have novel resistance to migration into F2nitrocellulose lacquer finishes [32, 33]. These are known as DOIP [di 2-ethylhexyl meta (called“iso”) phthalate] and DOTP [di 2-ethylhexyl para (called “tere”) phthalate].

Their overall performance in PVC Is similar to that of DOP, except for a slight indication ofbeing less compatible. The “mar resistant” feature may also be imparted to flexible PVC byalternate practices, such as top coating technology and/or the use of polyester plasticizers.However, this novel performance trait of DOIP and DOTP remains largely a technicalanomaly.

5.9 Solvency, Miscibility, or Compatibility

These terms are essentially interchangeable with respect to liquids and other lower molecularweight reagents added to PVC. Whether rigid or flexible, the systems behave as solid solutions,and abide by the three-dimensional solubility parameter concept of Hansen [10, 34]. Benchscale methods designed to measure the solvency, miscibility, or compatibility of plasticizers –and other reagents – in PVC are confounded by the simultaneous effect of diffusibility. Thisinterfering mechanism is present when attempting to measure plasticizer take-up, gelationtemperatures, or compatibility (phase separation). While the solubility forces are extremelysmall (units are (cal/cm3)1/2), their presence is responsible for the energy required to molecularlycombine plasticizers with PVC resin (up-take or swelling) and to hold them together(compatibility for duration of the application).

A statistical analysis of independent plasticizer variables versus take-up rates in PVC [11, 35]showed the following relationships. Equation 5.2 expresses dry blend time as a function ofplasticizer viscosity and specific gravity for phthalates, trimellitates, and aliphatic dicarboxylicdiesters that are used as plasticizers in PVC.

Dry Blend Times @ 88 °C = 10.05 + 0.218 · (Viscosity) – 10.08 · (Specific Gravity) (5.2)

5.9 Solvency, Miscibility, or Compatibility


Where:

Dry Blend Times are minutes using ASTM D 2396, viscosity is plasticizer viscosity at88 °C, centistokes (cS), specific gravity is specific gravity of the plasticizer at 20 °C.

That study found that the statistical confidence for evaluating plasticizer take-up is significantlyimproved by limiting the analyses to the eleven commercial grade phthalates tested. Equation5.3 expresses dry blend time as a function of plasticizer viscosity (distinctly) for dialkylphthalates:

Dry Blend Times @ 88 °C = –0.067 + 0.282 · (Viscosity) – 0.012 (Viscosity – 8)2 (5.3)

Similar relationships were developed for gelation temperatures. Equation 5.4 shows initialgelation temperatures are a function of plasticizer molecular weight and solvency strength fordialkyl phthalates, while Equation 5.5 shows that dialkyl phthalate plasticizers influence finalgelation temperature exclusively as a function of solvating strength.

Initial Gelation Temperature = –8.35 + 0.118 · (MW) + 0.001 · (MW – 450)2 + 21.4 · (δ) (5.4)

Final Gelation Temperature, °C = 71.48 + 39.30 · (δ) (5.5)

Where:

Initial gelation temperature for plastisols, °C, ref. [36], final gelation temperature forplastisols, °C, ref. [36], MW is molecular weight of the plasticizer, δ is Hansen’sInteraction Radius (HIR).

HIR is the distance between PVC resin and the plasticizer on Hansen’s three-dimensionalsolubility parameter grid. Smaller values of δ indicate stronger interaction forces. Theinvestigation that developed plastisol gelation values [36] indicated that the ultimate fusiontemperature is a function of the PVC resin, to the exclusion of plasticizer solubility parameters.

5.10 Processability

The ease with which various processes combine the liquid plasticizer with PVC polymer is afunction of the physical and chemical properties of the plasticizer as well as the polymercharacteristics. The mixing, fluxing, fusing, and shaping of the vinyl involves the applicationof elevated temperatures, up to about 160 °C to 170 °C. Thus, plasticizer fuming (volatility) isof concern. In addition, the rheology of plastisols (dispersions of PVC in plasticizer) criticallyimpacts the ease of shaping and controlling thickness of end products; further, initial andfinal gelation temperatures (influenced by plasticizer selection) influence processing ofplastisols.

As shown in Section 5.9, plasticizer selection influences dry blend rates. Melt viscosity, during“hot compounding” processes, is influenced by plasticizer characteristics, including solvency

193

for the PVC resin. Many investigators have studied these characteristics. Commercial practiceincludes the use of up to 10–20% of the plasticizer system as “strong solvating” type plasticizers,such as aryl-alkyl phthalates, benzoates, sulfonates, and so forth. Volatility is typically thelimiting factor on use levels of strong solvating plasticizers. Higher molecular weight plasticizerstypically offset volatility while imposing constraints on ease of processing.

5.11 Plasticizer Markets

Plasticizers are used to produce flexible PVC products for many different end uses or marketsegments. Figure 5.2 depicts a worldwide analysis of plasticizer consumption by PVC marketsegments.

The largest market segment is film, sheeting, and coated substrates. In this segment, the majorityof plasticizer consumed is for products produced in calendering operations. The primaryfactors in plasticizer selection are low cost and ease of processing, with DOP meeting thisrequirement in most parts of the world. In North America and Europe, DINP is the preferredplasticizer choice based on the above criteria and factoring in regulatory issues regarding theuse of DOP. If greater permanence is required, plasticizers such as DIDP, L9P, 911P, and DUPmay be used. For coated substrates prepared through a coating process, plastisol viscosity,gelation or fusion behavior, and emissions are all-important concerns. DINP is found to givea more consistent, stable viscosity than DOP while reducing emissions. For products thatneed a slight reduction in gelation or fusion temperature, DIHP (diisoheptyl phthalate) orBBP can be used to replace a small portion of the primary plasticizer.

Vinyl flooring is another major market for plasticized PVC. There are basically three types ofproducts: vinyl tile, resilient vinyl sheet flooring, and vinyl backed carpeting or carpet squares.Floor tiles are comprised of about 80% calcium carbonate held together by the fused flexiblePVC binder. The most commonly used plasticizers in floor tiles are DOP and DINP, while

W ire & Cable

19%

Film & Coated

Fabrics

35%Extruded &

Molded

12%

Food &

Regulated Uses

5%

Flooring

8%

Adhesives,

Sealants,

Coatings

7%

Miscellaneous

14%

Figure 5.2 End use markets for plasticized PVC

5.11 Plasticizer Markets

PVC Handbook


ISBN 3-446-22714-8

Leseprobe 2




10 Flexible PVC

WILLIAM COAKER

10.1 Origins

Polyvinyl chloride (PVC) homopolymer is a semi-crystalline polymer with a relatively highroom temperature tensile modulus of 2400 to 4140 MPa (3.5–6.0 · 105 psi), depending onformulation) that can be lowered by plasticizing entities to produce semi-rigid and flexibleitems. Many of these have turned out to be commercially useful and cost competitive. Use ofother additives, in addition to plasticizers, is essential to making successful flexible PVCproducts. These include stabilizers, pigments, fillers, lubricants, and many specialty additivessuch as fire retardants, anti-microbials, UV-screeners, and antistats for particular applications.

To put the development of flexible PVC in perspective, some historical facts are of interest. Ofthe seminal discoveries leading to successful flexible PVC products, some were empirical andothers the results of classical theory-driven research. Space allows only a few of these milestonesto be described here.

In Germany prior to World War I, the development of electric lighting resulted in a largeexcess of calcium carbide as acetylene lamps were phased out. Fritz Klatte, working forChemische Fabrik Griesheim-Elektron, found that acetylene can react with HCl to form vinylchloride monomer (VCM), which in turn, can be polymerized to PVC using free-radicalinitiators. Klatte was then commissioned to find uses for the hard, horny intractable PVCresin. He obtained several patents [1, 2], but Griesheim-Elektron let them expire in 1926.Klatte had shown promising leads but his team had not identified good enough plasticizers,heat stabilizers and process aids to make superior PVC replacements for celluloid, lacquers,oil cloth, coatings generally, fibers, and so forth.

Other German companies then took up the challenge and had developed commercial PVCformulations by 1939. Shortages of conventional materials during World War II led to practicaluses for both flexible and rigid PVC in wartime Germany. Post-war allied studies of Germanindustry publicized these world-wide, e.g., [3].

Between 1926 and 1933, Waldo Semon at BFGoodrich in the United States discovered thattricresyl phosphate (TCP) and dibutyl phthalate (DBP) were effective plasticizers for PVC. Inaddition, he determined that basic silicate of white lead was an adequate stabilizer to makeprocessable flexible PVC formulations plasticized with TCP or DBP. Goodrich then patentedsome of these formulations and commercialized coated fabrics and films made from themand trade-named them Koroseal® [4, 5].

Supplementing the antecedent German work, which sought volume uses for acetylene, Carbideand Carbon in the United States sought outlets for their large excess of ethylene dichloride

316 10 Flexible PVC [References on Page 361]

(EDC), which was a by-product of their ethylene chlorhydrin manufacture. In 1930, theyfound that they could produce pure VCM by treating EDC with caustic soda. They made PVChomo- and co-polymers from the VCM and supplied samples to Waldo Semon and others.

General Electric developed plasticized PVC insulation and jacketing for electric wires andcables. They trade-marked these Flamenol®, reflecting their resistance to ignition comparedto the then-standard flexible insulations and sheathings made from natural rubber.

In 1933, a patent covering the PVC plasticizer di-2-ethyl hexyl phthalate (known as DEHP ormore often DOP) was issued to Lucas Kyrides of Monsanto Chemical Company [6]. Monsantosold the patent to Union Carbide, because at the time Monsanto was more interested indeveloping plasticized polyvinyl butyral interlayers for automobile windshields and windowsthan in plasticized PVC.

Ironically, it was Monsanto’s air-oxidation process for making inexpensive phthalic anhydride,which later made low cost DOP a reality in the United States [7].

T. L. Gresham at BFGoodrich ran extensive tests and identified DOP as the best availableplasticizer for PVC homopolymers used for making flexible PVC.

Just as in Germany, World War II stimulated the development of uses for flexible and semi-rigid PVC in the United States, Great Britain, and other allied countries. This was driven byshortages of rubber, leather, and other naturally derived raw materials.

After World War II, it became obvious that many of the substituted plasticized PVC productshad performance properties and costs superior to those of the natural items they had replaced.Many became established as accepted items of commerce.

10.2 Types of PVC Resins Used in Flexible Applications

The largest volume PVC resin type used in flexible and semi-rigid applications is aqueoussuspension PVC homopolymer, made to have sufficient porosity in the particles to absorbenough plasticizer to meet the desired flexibility and hardness specifications of the intendedend-product. For most flexible uses, the resins range from medium to high molecular weight.By normally accepted conventions this means from approx. 30,000 to approx. 60,000 numberaverage molecular weight. In terms of commonly used tests based on dilute solution viscosity,this means from approx. 0.57 to 1.10 inherent viscosity, or from approx. 51 to 71 Fikentscher K[8].

Using a mercury intrusion porosimeter to measure resin porosity, it is common to requireabout 0.30ml/g or more for resins used for moderately flexible items made with monomericplasticizers; and about 0.40ml/g or more for highly flexible items made with monomericplasticizers and for items flexibilized with polymeric plasticizers. In addition to pore volume,pore diameter may be measured for PVC resins used in flexible products. Larger pore diameterscontribute to faster uptake of plasticizers.

Specialty soft plasticized PVC compounds requiring excellent compression-set properties callfor the use of ultra-high molecular weight PVC resins with high porosity. These may exhibitmolecular weights as high as 150,000 number average.

31710.2 Types of PVC Resins Used in Flexible Applications

Flexible PVC products injection-molded or extruded at very high shear rates may be madewith low molecular weight PVCs having less than 30,000 number average molecular weight.However, using PVC resins with such low molecular weights sacrifices physical properties,including compression set, elongation at failure, tensile strength, tear strength, and fatigueresistance. It is difficult to make ultra-low molecular weight PVC resins having high particleporosities. Very low molecular weight resins are not used for highly flexible items, becauseitems made with them are too weak.

When formulating flexible PVC compounds for specific exacting uses, many compromisesare often involved in selecting the best resin. For instance, other things being equal, the higherthe molecular weight of the chosen resin, the higher the processing temperature needs to beduring fabrication in order to achieve optimal fusion. Also, for a given processing temperature,use of higher molecular weight PVCs tends to give lower gloss on a finished product. If lowgloss is required on an extruded product,which otherwise calls for use of a medium molecularweight PVC, it is necessary to specify a specialty “low gloss” resin offered by a few PVCmanufacturers.

The particle shapes, size limits, size distribution, and internal porosities of the PVC grainsformed during suspension or mass polymerization processes are functions of many factorsdealt with in other chapters of this book (see, e.g., Chapters 3 and 11). For flexible applications,particle sphericity tends to provide good bulk flow behavior along with efficient particle packingand higher bulk density in dry blends. Irregular, knobby resin particles, broad particle sizedistributions, and high particle porosity tend to give low bulk density and poor bulk flowbehavior.

For flexible applications, a relatively narrow PVC particle size distribution is desirable and ismandated by many users’ specifications. In the United States, a typical specification for generalpurpose PVC aimed at flexible markets is ≥ 99.8% particles must pass through a U.S. 40 meshsieve, whose nominal openings are 420 microns (16.5 mils) square; 10% maximum by weightretained on a U.S. 60 mesh sieve, whose openings are 250 microns (9.8 mils) square; and 2%maximum through a U.S. 200 mesh sieve whose openings are 74 microns (2.9 mils) square.The aim point for average particle size (APS) is normally between 100 and 80 mesh. That isbetween 149 and 177 microns (5.9 and 7.0 mils). For special purposes, suspension and massPVC resins may have their APSs skewed to finer or coarser numbers.

The differences between mixing and processing methods for general-purpose flexible PVCand plastisol techniques for making flexible PVC products are described in Section 10.4 andin Chapter 9.

The distinctions between PVC resins made for use in plastisols and general purpose PVCresins are addressed here. Plastisol resins are sometimes called dispersion resins, or, mostly inEurope, paste resins. They are made by emulsion, microsuspension, or special proprietaryprocesses.Typically, they contain spherical, solid particles ranging in size from 0.1 to 1.1 micronsin diameter. In bulk resin, these resin particles provide large surface area due to their smalldiameters. They are readily wetted by plasticizers due to their surfactant content and absorbsome plasticizer during mixing. Because the particles are solid PVC, they absorb plasticizersquite slowly at typical ambient temperatures. The plastisol resins made by emulsion polyme-rization may be dewatered by spray drying or a combination of coagulation and conventionaldrying. Weak agglomerates of the basic emulsion resin particles are formed during drying.These are normally broken up to a substantial degree in grinding procedures before use of the


resins. Most PVC plastisol resins are fluffy, have relatively low bulk densities compared togeneral purpose PVCs, and poor bulk flow properties. In most cases, they have to be givenantistatic treatments to get them to flow satisfactorily in bulk handling systems. Consequently,plastisol resins are often bagged by the manufacturer before shipment to the customer. Thesystems used for bulk handling of plastisol resins are specially designed for this purpose andusually are dedicated to particular plastisol resins, and only used in large volume applications.

Microsuspension PVC resins used in plastisols differ from their emulsion-polymerizedcounterparts both in particle size distribution and emulsifier content. Their emulsifier contentis lower than that of most emulsion resins and this contributes to higher clarity in clearformulations, low moisture sensitivity of the end products, and little or no contribution tofogging in automobile interiors.

Another class of PVC resins made for use in plastisols is called plastisol-extender resins. Theseare solid particle resins made by suspension polymerization with particles finer than generalpurpose resins, but coarser than microsuspension plastisol resins. The rationale for the use ofsuch resins in plastisols is at least threefold. First, they are less expensive to make than trueplastisol resins. Second, due to their coarser particle size on average, they contribute to moreefficient particle packing in a plastisol when used in conjunction with a standard plastisolresin. This allows higher PVC content in a plastisol of given viscosity. Third, use of extenderresin in a plastisol usually gives a rougher surface finish with lower gloss when this is desirable.This is important in sheet flooring constructions and plastisol-coated metal siding, for instance.

Typical screen analyses for plastisol extender resins are: > 99.6% through 140 mesh, 5%maximum on 200 mesh and > 65% through 325 mesh, using U.S. standard screens. TheirAPS is thus equal to or less than 44 microns (1.7 mils), which is the size of the 325 meshscreen openings.

Specialty PVC resins are made for producing flexible PVC items by rotational molding frompowders and powder-coating of pre-heated parts. Some of them are specialty copolymersdesigned for blending with specialty homopolymers aimed to achieve particular performanceresults.

Graft copolymers constitute another type of resin used for making vinyl TPEs and unplasticizedflexible vinyl articles or lightly plasticized low extractibles flexible PVC. An example is VinnolVK 801 listed as having 50% EVA content, and offered by Vinnolit (which was formed by themerger of Hoechst and Wacker’s PVC operations).

Solution vinyl resins are a specialty dealt with in dedicated texts [9].

10.3 Particulate Architecture of PVC Resins Used in FlexibleProducts

This section deals with general purpose PVC resins made for use in flexible products. Theparticle architectures of other PVC resins are covered in Chapters 3, 9, and 11 of this book.

In the early years of the PVC industry, a common problem in the manufacture of flexible PVCitems by calendering and extrusion was the occurrence of gel particles, often referred to as

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“fisheyes”. These are relatively solid PVC particles, which do not absorb plasticizer as readilyas the other resin particles in the formulation and which do not fuse readily at the processingtemperatures used. Large fisheyes are the most objectionable.

As the technology of polymerizing VCM has improved, the occurrence of fisheyes in flexibleproducts has become a less frequent or severe problem. But it still exists, because the mostfrequent cause of fisheyes in PVC is inadequate cleaning of polymerization reactors betweenbatches. PVC particles that go through two or more polymerization batches tend to havereduced porosity for absorption of plasticizers and to contain enough cross-linked, insolublePVC to interfere with normal fusion. PVC manufacturers, who try to save money by reducingthe frequency and diligence of reactor cleaning, produce resins with unacceptable content ofgels.

A critically important part of PVC resin morphology is its semi-crystalline nature. This isresponsible for the resistance of flexible PVC to heat distortion, creep, and compression set.When plasticized PVC started replacing rubbers in insulation and sheathing for electricalwire and cable, rubber technologists asserted that no thermoplastic composition flexible andsoft enough to satisfy wire and cable flexibility requirements could meet the requiredspecifications for resistance to thermal distortion, cut-through, compression distortion,compression-set, agglomeration of multiple insulations within a cable, abrasion resistance,and so forth. They believed that only cross-linked rubbers could be soft and flexible enoughand still maintain their integrity under harsh conditions of use and testing. They asserted thatno thermoplastic such as plasticized PVC would replace cross-linked rubbers. But based on itsphysical and flammability properties, plasticized PVC took over most of the indoor wiringinsulation and jacketing market in developed countries. It was after this happened that manyof the reasons why plasticized PVC fulfilled these requirements were elucidated. Numerousthermoplastic elastomers (TPEs) have since been developed, which are both thermoplasticand resistant to compression-set, creep, abrasion, and thermal distortion.

The seminal discoveries of the micro-morphology of rigid and flexible PVC are dealt with indetail in Chapters 3, 9, and 12 of this book. However, a terminology rationalizing the behaviorof plasticized PVC is presented here. Rational terminology describing the morphology ofPVC suspension- and mass-polymerized resin particles was proposed by Geil [10]. The lowerend of Geil’s size hierarchy also applies to emulsion- and microsuspension-polymerized PVCresins. Table 10.1 is adapted from Geil.

The crystalline microdomains in plasticized PVC, which are interspersed between amor-phous regions, act like cross-links in rubbers in resisting creep, compression-set, and heatdistortion, except that they are thermally reversible. Most of these crystallites melt atprocessing temperatures and re-form during cooling of fused plasticized PVC melts. Themelting and recrystallization behavior, however, is complex because PVC contains severaldifferent kinds of crystallites, which melt and reform over a range of temperatures. Theirmelting and reformation is affected by the kind of plasticizer present. For instance, “fastfusing” plasticizers, of which butyl benzyl phthalate (BBP) is an example, depress the meltingtemperatures of the crystallites, thus promoting fusion at lower processing temperatures.“Low temperature” plasticizers, of which dioctyl adipate (DOA) is an example, primarilyplasticize the amorphous regions of the PVC and have little effect on crystallite meltingtemperature, thus requiring higher processing temperatures than fast fusion or generalpurpose plasticizers.

10.3 Particulate Architecture of PVC Resins Used in Flexible Products


Antiplasticization by plasticizers is a phenomenon that PVC formulators must take intoaccount. This is the stiffening effect of low levels of some plasticizers, which takes place afterfusing and cooling a formulation. It occurs most often with fast fusing plasticizers, to a moderateextent with general purpose plasticizers, and much less with low temperature plasticizers. It isobserved at plasticizer concentrations of approx. 1 to 15 parts per hundred resin (phr). Butthis range varies with plasticizer efficiency. Antiplasticization is explained as being mainlycaused by the promotion of crystallite formation by low levels of plasticizers without anoffsetting plasticization of the amorphous regions, which occurs at higher plasticizer levels. Itmust be noted that these extra crystallites are of the low melting type and do not raise the heatdistortion temperature of PVC, which is reduced by as little as 1 phr of most plasticizers orplasticizing stabilizers, such as the thiotins. Other factors discussed in the literature ascontributing to antiplasticization effects are hydrogen bonding, Van der Waals forces, sterichindrance, small, localized increases in molecular order, and decreased free volume [11, 12].

Many authors have contributed to understanding the relationships between PVC particlemorphology, fusion, and the processing rheology of PVC. Some of these are Collins and Krier[13], Berens and Folt [14], Singleton and Isner [15], Pezzin [16], Collins and Daniels [17],Lyngaae-Jorgensen [18], Summers [19, 20], and Rosenthal [21]. Their original findings relateto both rigid and flexible PVC.

Table 10.1 Size Hierarchy of PVC Particulate Phenomena

Term Size Description

Grain 70–420 µm diameter Free-flowing powder produced in suspension or mass polymerizations

Agglomerates ofprimary particles

3–10 µm diameter Formed during polymerization by coalescing ofprimary particles

Primary particles 1 µm diameter Formed at single polymerization sites by precipitation of newly-formed polymer into discrete molecular aggregates

Domains 0.1 µm diameter Observed after certain types of mechanical workingof the PVC

Microdomains 0.01 µm spacing Crystallites capable of holding portions of tie molecules, which maintain integrity of primaryparticles and span between primary particles in well fused PVC

Secondary crystallinity

0.01 µm spacing Crystallites re-formed during cooling of a fused PVC melt

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10.4 Favored Processing Methods for Flexible PVC

The natures of the PVC resins and other additives used in flexible PVC compounds haveconditioned the selection of efficient material handling procedures for making the primarymixes. The most common processing type is “dry blending”, also known as “powder mix-ing”. More than 85% of suspension and bulk PVC resins are initially processed by dryblending.

Historically, large volume, jacketed ribbon blenders and various other kinds of available mixingequipment were used. For flexible PVC formulations using liquid plasticizers, resins withsufficient porosity to absorb the liquids are intermixed with the other ingredients to get themmutually well dispersed down to the particulate level. The desired resulting product is a free-flowing powder. For most formulations, it is necessary to heat them during mixing to facilitateabsorption of the plasticizers into the resin and to avoid loss of product through formation ofundesirable lumps of wetted pigments and fillers. The main drawbacks to the large ribbonblenders were that cycle times were surprisingly long to achieve good blend uniformity and itrequired considerable technical effort of a largely empirical nature to determine the optimalorder and timing for addition of individual ingredients. It was not unusual to produce a fairamount of off-grade product during the process of mixing cycle optimization when newformulations were introduced.

Much theoretical work has been done on dry blending of flexible PVC formulations. Diffusionof liquid plasticizers into PVC resin particles is a kinetic process whose rate varies inverselywith the viscosity of the plasticizer [22]. During dry blending, liquid plasticizers penetrate theamorphous matrix surrounding the crystallites in the PVC resin particles. The crystallitesremain intact because of the thermodynamic barrier which prevents fusion until a certaintemperature is reached. This depends on the particular plasticizer or plasticizer mixture beingused and the molecular weight of the PVC resin. While studying the Flory-Huggins interactionparameter, χ, for various plasticizers with PVC, Anagnostopoulos et al. [23], developed amicroscope hot-stage fusion test, whereby the temperatures at which PVC resins of differentmolecular weights fuse in particular plasticizers are readily measured. These fusion tempera-tures are higher than dry blending drop temperatures.

A dry blend is a non-fused mixture of PVC, plasticizer, and other ingredients. Park publisheda method for running the pressure stain test for PVC dry blends [24], which determines thepoint in a commercial dry blending cycle at which the blend no longer stains brown paper orcigarette paper sheets between which it is pressed. This correlates with the small-scale laboratorydry time test ASTM D2396.

From a practical viewpoint, a dry blend cycle is complete when the dry blend, after cooling,flows well through a funnel or an extruder hopper and does not cake during storage. Thepressure stain test is a way of predicting when this state has been reached while the blend isstill hot and in the blender. Commercial blenders are stopped during a cycle and a “thief” isused to take small samples for testing.

Dry blends of plasticized PVC formulations are over-dried if they are dusty and fuse withmore difficulty in subsequent processing operations, such as extrusion, than a blend madewith a shorter dry blending cycle, which is slightly damper and less dusty, but still has goodbulk flow and non-caking behavior.

10.4 Favored Processing Methods for Flexible PVC


Low speed dry blending mixers are generally of the ribbon mixer type. These are bulky semi-cylindrical horizontal mixers supplied with a jacket surrounding the side and bottom surfacesthrough which heat or cooling can be applied. For mixing, radial blades are mounted on ahorizontal rotating shaft, which may turn at 40 to 70 rpm.

Intermediate speed dry blenders generally consist of a horizontal cylindrical jacketed chamberwith axially mounted blades, which can be run fast enough to achieve fluidization of the PVCduring blending.

Where the investment can be justified, batch high speed, high intensity, dry blending mixersare used for flexible PVC. Typically, these consist of a vertical drum with a dish bottom throughthe center of which a drive shaft penetrates. One or more sets of mixing blades are attached tothe drive shaft. A baffle penetrates the top of the mixer, which can be tightly sealed to thedrum. By running the drive shaft at speeds appropriate for the particular formulations beingmixed, mixing occurs with the generation of sufficient frictional heat to raise the batchtemperature to a desired drop temperature, such as 105 °C in 4 to 8 minutes. These intensivemixers all have Froude numbers* much greater than 1. Typical shaft speeds are 500 to 1200 rpmwith blade tip speeds between 10 and 50 m/s (33 and 164 ft/s). Typical batch capacities are 80to 200 kg (176 to 440 lb). Some of these mixers are designed to sustain high vacuum or airpurge during mixing for removing traces of unwanted volatiles such as moisture or residualvinyl chloride monomer [25]. The PVC and other ingredients are fluidized during blendingin these mixers.

The high speed mixers generally dump their batches into lower speed jacketed rotary coolershaving Froude numbers less than 1. The coolers introduce little frictional heat and cool byconduction into their cold water jackets. To avoid adventitious condensation of moisturefrom the ambient air, freezing brines are not used in these cooler jackets. Specialty dedicatedblending procedures are used for flexible PVC compounds made with solid plastifiers,polyolefin elastomers (POEs), and compatibilizers.

* Froude number = R ω2 g

where

R = blade radiusω = angular velocity, radians/sg = gravitational acceleration in consistent units

Trade names for typical intensive mixers are Henschel, Papenmeier, Welex, and Littleford.

To shorten mixing cycles, high viscosity plasticizers (polymeric liquids) are normally preheatedbefore addition to the mixer.

In many factories the cooled dry blend is air conveyed to interior storage before being fed tofabrication equipment or intermediate processing units. It must not “cake” during storage,and it must flow readily and uniformly in the feed hoppers of equipment such as extrudersand proportioning devices, for example, those used for color control operations.

From interim storage, dry blends are conveyed to fluxing devices, such as compoundingextruders, Banbury mixers, Farrel Continuous Mixers, Buss Ko-Kneaders, and fusion systemsmade by Coperion, Krauss-Maffei, Leistritz, Reifenhauser, and many others. The hot, fluxed

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PVC may then go to pelletizing units and be sold as compound pellets. Alternatively, it may befed directly to fabricating equipment such as extruders, calenders, or injection molding presses.

Another type of mixing for preparing flexible PVC goods involves compounding of liquidplastisols. This is done in liquids mixing equipment using plastisol grade PVC resins, suitableplasticizers, and other additives such as stabilizers, fillers, pigments, and other ingredients.These may include viscosity modifiers, air release agents and thinners, which are dedicatedfor use in plastisols, modified plastisols, and organosols, discussed later in Section 10.5 andChapter 11.

Inverted conical Nauta mixers, pony mixers, high speed Cowles Dissolvers, medium speedRoss Power Mixers, and three-roll mills are typical of the mixers used for manufacture ofplastisols.

In most cases, plastisols are de-aerated after mixing and stored at controlled temperatures,preferably at or below 23 °C (73 °F), to prevent heat-induced viscosity build-up and otherchanges in their desired rheology caused by aging.

Plastisols are fabricated into end-use items by liquids-forming procedures followed by gellingand fusion in ovens. Plastisol-derived sheet flooring is the largest volume use for plastisols.Many of these flooring constructions are sophisticated, involving a flexible backing coveredwith a foamable plastisol on which a design is printed with some inks containing a foaminginhibitor and overlaid by a clear plastisol wear layer.

10.5 Designing Flexible PVC Compounds

10.5.1 Formulation Development

There are two general approaches to formulating flexible vinyl materials. When a projectinvolves a novel untried concept for which the technical requirements of the product areunknown, a set of tentative needs is “guesstimated”. Trial formulations, whose properties bracketthe tentative needs, are developed and parts or items submitted to field trials. The process isiterated until a satisfactory product is developed or the project is abandoned as impractical ortoo costly.

When the technical and economic requirements for the new product are known and consideredfeasible using flexible PVC, these are listed and used as guidelines. They involve physical andoptical properties, stability to heat and light, decorative, electrical and toxicological require-ments, density, odor, allowable cost, and so forth. Specifications and necessary qualificationtests must be defined, including needs to run field trials at customers or evaluations at outsidetesting services, such as Underwriters’ Laboratories or suppliers’ or customers’ laboratories.The total cost of the development program and the potential profitability of the new productneed to be estimated to justify proceeding with development.

In developed countries, the existing markets for flexible PVC are defined and competitive. Insome cases, several plastics and plastic alloys, whose economics are close together, are competingand flexible PVC is simply defending or increasing its market share.


324 10 Flexible PVC [References on 1]

In undeveloped countries, flexible PVC has many opportunities to improve the standard ofliving of the people by satisfying unfulfilled needs and replacing natural materials, over whichflexible PVC has some clear-cut advantages.

When the technical requirements for a new product cannot be satisfied by flexible PVC, it isindustry practice to refrain from submitting a defective vinyl part for trial. There should be atleast a 50% probability of success to justify submittal of trial items.

10.5.2 General Problems in Formulation Development

In the flexible PVC industry, the term compounding has meanings specific to whether it isapplied to solid or liquid systems. For solids, the steps generally are mixing a dry blend fromsolid and liquid ingredients, or mixing a wet blend, then fluxing the dry or wet blend until itwill flow properly in forming equipment, shaping the melt into an end product, and coolingthe hot product before it loses its desirable shape.

In the case of plastisols and organosols, compounding means mixing the solid resin and othersolid ingredients uniformly into liquid plasticizers and other liquid ingredients so as to achievea targeted rheology suitable for follow-up liquids-forming operations (reverse roll coating,spread-coating, rotational molding, dip coating, spray coating, strand coating, injectionmolding, etc.), followed by fusion in a suitable oven or microwave treatment, and finallysucceeded by cooling before the product loses its desirable form and shape.

For PVC latexes, ingredients are normally added in waterborne solution, emulsion, ordispersion and mixed into the base latex under gentle agitation, so that it does not coagulatethe latex or mixture of latexes. The compounded latex is then applied to the substrate to betreated by common latex application methods, such as dipping, impregnation, coagulation,reverse roll coating, spraying, and so forth. The water is then evaporated and, if necessary, theproduct is fused and cooled.

When more than one company is involved in developing a product, the importance of accurateand complete inter-company communications cannot be over-emphasized. An example of aproduct failure, which was corrected, was the manufacture of a large number of electric alarmclocks with high-impact polystyrene (HIPS) cases around which the vinyl-jacketed cords weretightly wound before packaging them. After some of the clocks were sold it was found that theHIPS cases had all been marred by migration of plasticizer from the cord jackets. The makerof the cords had not been told that their jackets were going to be in direct contact with HIPS.The problem was solved at a modest increase in cost of the cords by replacing the commodityphthalate originally used with a blend of higher molecular weight phthalate and trimellitateand modifying the packaging procedure.

The vinyl industry has sometimes been victimized by callous formulators, who make itemsfrom the cheapest formulation that satisfies initial requirements without providing suitablein-use and aging behavior. An example with vinyl-coated bookbinders was one in which thevinyl coating was plasticized with di-hexyl phthalate. When placed in contact with photocopyinks this binder “lifted” the inks and ruined the copy’s appearance. A second example was a“non-migratory” binder plasticized with epoxidized soy bean oil (ESO) as sole plasticizer atabout 50 phr. Initially, this binder exhibited exemplary non-migratory behavior. But after

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about a year of exposure to fluorescent light and some sunlight, the binders with thiscomposition were ruined by tacky exudates which gave them a surface like fly paper. Theproblems were solved at some increase in formulation cost by using a combination of mediummolecular weight polymeric plasticizer and a high molecular weight phthalate for the non-migratory binder and just a high molecular weight phthalate for the regular binder.

Non-availability of optimal raw materials at competitive cost is a problem frequently faced byvinyl compounders. For instance, compounder A may have bulk storage for PVC resins J, K,and L along with plasticizers P, Q, and R. Compounder B has more extensive storage facilitiesand greater purchasing power for getting special deals on resins, plasticizers, and other rawmaterials. Compounder A, due to his lower overhead costs, has an advantage as long as heconfines himself to his niche markets. But compounder B’s products have better cost/performance in other markets due to his superior raw materials situation and more versatilecompounding equipment.

In competitive situations, compounders may need to modify their mixing and fluxingprocedures or upgrade their equipment. For instance, slow dry blending may be causing aproduction bottleneck caused by the use of a polymeric plasticizer or a slow dry blendingmonomeric plasticizer, such as DTDP. One approach is to reformulate, but the bottleneckmay be more readily eliminated by pre-heating the plasticizers before adding them to themixer and/or use of a faster blending, more porous PVC resin.

It is a luxury to work in a plant where the mixing, fluxing, and forming equipment are alloptimized for the products being manufactured. In older plants, it is common practice tooptimize throughput by selecting raw materials. For these situations, compounders use thelowest molecular weight PVC resin that satisfies end-use requirements, along with as muchprocess aid that economics allow, and as fast-fusing a plasticizer system that is consistent withend product requirements.

In calendering plants producing thin gauge films, it is normal to pass the fluxed feed throughan extruder-strainer to eliminate adventitious metal contamination, which could cause veryexpensive damage to the finishing rolls of the calender. These strainers need to be designedfor the rheology and throughput of the stocks run on the calender. A strainer designed forhighly flexible formulations tends to overheat rigid stocks at typical desired throughputs.This may limit the flexibility of calender lines with regard to switching back and forth fromflexible to rigid stocks. Similarly, continuous compounding mixers need to be designed forthe rheology and the throughput of the stocks they are handling. Equipment designated tothe manufacture of diced or pelletized compounds also needs to be suited to the rheology ofthe range of compounds being manufactured in order to achieve optimal through-puts.Versatility can be achieved by stocking a range of parts for compounders, pelletizers, anddicers.

End-product performance failures in terms of resistance to compression-set, retention ofelongation after oven aging, fatigue after repeated flexing, abrasion resistance, plasticizerextraction by oils or fats, and environmental stress-cracking of rigid plastics in contact with aflexible PVC part often can be corrected by switching to higher molecular weight PVC resinsin the rigid PVC, higher molecular weight plasticizers, and/or introducing specialty additivessuch as process aids or “plastifiers” in place of part of the plasticizer system in the flexiblePVC. To maintain throughput in spite of higher melt viscosity, the formulator usually has toadjust the stabilizer and lubricant systems to accommodate higher stock temperatures.



For composite products, such as coated fabrics or carpets backed with flexible vinyl materials,either by plastisol coating or laminating, the fusion temperature of the vinyl must not exceedthe tolerance of the fiber in the fabric or carpet. This is critical with some polyolefin fibers.Suitable low fusion temperatures can often be achieved in the vinyl composition by usingcopolymer resins and fast-fusing plasticizers such as BBP or DHP.

For vinyl automobile undercoats and sealants, paint oven temperatures dictate selection oflow-fusing resins and plasticizers. Fully fused vinyl undercoats survive considerable abuseand prolong the life of a vehicle, particularly in regions where roads are heavily salted inwinter. Poorly fused undercoats fail prematurely.

In flexible PVC products which are foamed during processing, whether they use hot melt orplastisol technology, the fusion characteristics of the resin-plasticizer system must be matchedto the behavior of the blowing agent-kicker-stabilizer in order to achieve manufacture ofgood quality uniform cell-size closed-cell foams. For consistently satisfactory results, excellentcontrol of the rheology and time-temperature profiles is essential.

Extrusion, calendering, injection molding, thermoforming, and compounding processes aresometimes marred or shut down by a phenomenon known as plate-out. This comprises theformation of sticky deposits on the hot surfaces of processing machinery, including mill rolls,calender rolls, extruder screws, dies, molds, and so forth. The surface appearance of finishedgoods deteriorates in the early stages of a plate-out problem. If this is not recognized andcorrected, it may lead to catastrophe such as “shipwreck” on a fast-running calender. Thisoccurs when the calender web stops releasing from the final calender roll and folds back intothe calender nip. This often causes severe degradation before the calender can be shut down.An expensive and labor-intensive clean-up always follows a calender “shipwreck”.

Plate-out is caused by precipitation and transfer of oxidation and/or hydrolysis products fromcomponents of the formulation (usually the stabilizer and lubricant system). The initial plate-out, if not observed and eliminated, then proceeds to build up by occluding solids from theformulation such as colorants, fillers, and smoke- and flame-retardants. If ignored, the plate-out eventually stops the plastic product from releasing from the hot/coated metal or releasesa badly marred product from a coated mold, calender roll, or die. Products marred by plate-out usually exhibit poor printability.

Sometimes, plate-out can be eliminated “on the run” by temporarily increasing the amount ofan abrasive, such as talc, or lubricant such as stearic acid, in the formulation. Plate-out isprevented in some formulations by including a small amount of a scouring agent such as talcor some grades of silica. Often, an operation plagued by plate-out has to be shut down and theaffected metal surface manually or operationally cleaned. Raw materials suppliers should beconsulted about persistent plate-out problems. Lippoldt [26] published an extensive study ofplate-out.

Pollution regulations differ so widely from country to country and region to region that generalrules for coping with them are meaningless. Processors need to dispose of solid, liquid, andgaseous wastes and vapors generated in the processing of flexible PVC in compliance withlocal regulations (see Chapter 18).

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10.5.3 Properties Often Specified for Semi-Rigid and Flexible PVC Products

Tensile strength and elongation at failure (ASTM D638) depend primarily on the level andtype of plasticizer or other flexibilizer in the formulation, but also on resin molecular weight.Higher molecular weight resins in fully fused formulations give higher tensile strength andelongation at failure. 100% Modulus, defined as tensile stress at 100% elongation, is a usefulmeasure of the stiffness of plasticized PVC, because it is relatively easy to measure accuratelyand reproducibly. For historical reasons, DOP is generally recognized as the benchmarkplasticizer for PVC. With a medium-high molecular weight PVC, at 23 °C, DOP at 25 phrgives a 100% modulus of about 22.8 MPa (3300 psi), which is classified as semi-rigid. Between35 phr DOP and approx. 85 phr DOP (100% modulus 4.48 MPa or 650 psi), PVC is consideredflexible Above 85 phr DOP, PVC is called highly flexible.

When comparing the efficiencies of different plasticizers, substitution factors (SFs) comparedto DOP are generally used. However, most authors calculate these from Shore hardnessmeasurements, which do not correlate exactly with 100% modulus, see Chapter 5 for moredetails.

The brittleness temperature of flexible PVC is generally measured by ASTM D-746, which isa cold impact test run on specimens punched from standard test sheets 1.9 ± 0.25 mm(75 ± 10 mils) thick. However, on calendered PVC films, some people prefer to use the MaslandImpact Test (ASTM D1790). This test is run on films 10 mils (0.25 mm) or less in thicknessunder specified impact conditions. In this test, the results are sensitive to the direction ofsampling and the direction of fold due to the molecular orientation effects of calendering.Outside the United States, local testing procedures may be preferred.

In commercial laboratories the low temperature properties of flexible PVC are often estimatedfrom stiffness measurements run by ASTM D 1043, which measures apparent modulus ofrigidity, G, at different temperatures. The way D1043 is run, the angular deflection may extendbeyond the elastic limits of the plastic at lower temperatures, so that the result is “apparent”rather than an actual modulus of elasticity, E, as measured by ASTM D 747. To convert G to E,the simplifying assumption is made that E = 3 G, which is only true if Poisson’s Ratio for thematerial under test conditions is 0.5. The temperature at which E = 931 MPa (135,000 psi) isreported as Tf, the flex temperature, which is the temperature at which the material is consideredto have lost most of its elastomeric properties. Sometimes T4, (E = 6.90 MPa or 10,000 psi),regarded as the upper end of a material’s useful temperature range, is also reported.

Academic laboratories generally use more precise methods of measuring moduli as functionsof temperatures.

Abrasion resistance of flexible PVC is often measured by the Taber Abrasion Test (ASTMD 4060). Results are reported as weight loss per 1000 cycles under conditions agreed to betweenthe interested parties. Results are important for automobile undercoatings, boot and shoesoles, floor coverings, mine belts, and electrical cords for use under harsh conditions.

The hardness of flexible PVC materials is commonly measured by Shore Hardness (ASTMD 2240) using the A scale. Sometimes the D scale is used on semi-rigid compounds withplasticizer levels at or below 40 phr DOP equivalent. Conditioning of test specimens at thetest temperature is critical. Aging after processing is also very important. This is explained asbeing due to the slowness with which PVC crystallites reform after processing. For accurate



results, at least one week of aging at 23 °C (73 °F) is recommended. Hardness readings increasewith age after molding or other processing. Note also that Shore hardness readings drop rapidlyduring the first several seconds after specimen contact. For vinyl plastics, ASTM specifiestaking Shore A hardness readings after 15 seconds. However, many commercial laboratoriesuse 10 second Shore hardness. Test sample thickness is critical.ASTM specifies using specimensmolded to 0.64 cm (0.25 in) thickness. On calendered or extruded films and sheets, manylaboratories stack several thicknesses, but this is less precise than using specimens molded to0.64 cm thickness.

In commercial quality control testing, operators never wait until Shore hardness has stabilizedbefore testing. They specify definite aging and conditioning periods of usually less than a day.

In the United Kingdom, British Standard Softness (B.S. 2782:32A) is generally specified. Thistest correlates well with 15 s Shore A hardness in the sense that a plot of Shore A hardnessagainst British Standard softness is a straight line. The same considerations regarding agingand conditioning of flexible PVC specimens apply as for Shore Hardness testing.

For flexible PVC compounds used as primary insulation on electrical wires and for electricaltapes, electrical properties are critical. Tests commonly used include dielectric constant (ASTMD150), dielectric strength (ASTM D149), and volume and surface resistivity (ASTM D 257).The Underwriters’ Laboratories Insulation Resistance Test is specified for insulation compoundsto be used on wires slated for use in wet locations.

The fire resistance of most flexible PVCs is less than that of rigid PVC. However, formulationscan be devised to meet stringent flammability requirements, such as those for plenum cables.These require enough flexibility for installation in confined spaces and must also pass theUL-910 (NFPA 262) test [27]. Other flammability tests often used on flexible PVC productsinclude: the UL-VW-1 Vertical Wire Flame Test; the Oxygen Index Test (ASTM D2863); theDOT 302 MVSS Test for materials used in automobile interiors; the UL-94 Test run in thehorizontal or vertical modes; the UL Vertical Tray Flame Test (UL 1581 for tray cables); andthe UL-1666 Test for riser cables.

The Cone Calorimeter Test, ASTM E1354, can be used to rank small samples of flexible vinylmaterials for rate of heat release after ignition, ease of ignition, and emission of obscurationalsmoke. The test is versatile, because the heat flux to which samples are exposed can be variedfrom about 10 to 100 kW/m2. Rate of heat release, sample mass loss rate, and smoke aremeasured or calculated from measured parameters.

For smoke evolution, the NBS Smoke Chamber Test (ASTM E662) is still used, because manylaboratories have the equipment. Cone calorimeter results are acknowledged to be moremeaningful.

Measuring the toxicity of smoke from burning PVC is complex and has been controversial. Itis discussed in Chapter 13, along with general flammability testing issues.

Other tests used on flexible PVC products include retention of elongation after oven aging,resistance to extraction of plasticizers by chemicals, weatherability, stain resistance, and effectson taste and odor of foods packaged in flexible PVC. For niche products, many other specializedtests are used. Physical and electrical testing of PVC are discussed in Chapter 12.

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10.6 Additives Used in Flexible PVC Compounds

10.6.1 Liquid Plasticizers and Solid Flexibilizers

Primary plasticizers are the principal additives responsible for flexibilizing PVC. These areclassified as monomeric, polymeric, epoxy, and specialty flame-retardant plasticizers. Theyare low volatility liquids whose polarity and other characteristics are such that they aresufficiently compatible with PVC not to be readily squeezed out of plasticized PVC by moderatepressure [28].

Secondary plasticizers are low volatility liquids whose compatibility with PVC is such thatthey can be used along with primary plasticizers as part of the plasticizer system, but whichexude if used as sole plasticizer. Chlorinated paraffins are common examples of secondaryplasticizers for PVC, used because they are low in cost and less flammable than most primaryplasticizers.

There are several types of solid flexibilizers for PVC, which include compatible nitrile rubbers,compatible polyurethanes, compatible polyesters, ethylene-carbon monoxide-vinyl acetateterpolymers, and some poly-acrylates. Many people refer to these materials as PVC “plastifiers”to distinguish them from liquid plasticizers. These solid materials are chiefly used in PVCthermoplastic elastomer (TPE) compounds and specialty PVC materials, some with lowflammability and low smoke evolution, for use in applications such as plenum cables. Thevolume cost of these plastifiers is higher than that of most plasticizers. When used as soleflexibilizer for PVC, plastifiers give compounds with higher melt viscosity than correspondingplasticized compounds of equivalent hardness and flexibility. Plasticizers for PVC and theoriesof plasticization are discussed in detail in Chapter 5.

A few additional practical comments will be offered in the following. The definition ofplasticizers adopted by IUPAC in 1951 is still generally accepted: a substance or materialincorporated in a material (usually a plastic or elastomer) to increase its flexibility, workability,or distensibility. A plasticizer may reduce the melt viscosity, lower the temperature of a secondorder transition, or lower the elastic modulus of a product. In comparison, a process aidimproves processability without necessarily reducing melt viscosity or the stiffness of theproduct.

Practical requirements for a successful plasticizer for PVC are that it be cost-effective, stable,low in color, compatible with PVC, readily dispersible in PVC, low in volatility, low in odor,low in toxicity, have good permanence, and must not interact unfavorably with other neededformulating ingredients or otherwise compromise the end-use properties of the product inwhich it is employed.

Plasticization theory works reasonably well in quantifying the behavior of single plasticizersin PVC. When mixtures of plasticizers of different chemical families are used, the correlationsbetween pragmatic performance parameters and scientific measurements on idealized systemsbecome too loose to maintain the latter as standards for predicting the performance ofplasticized PVC in the marketplace. However, on individual new plasticizer candidates,calculated or measured entities such as hydrogen bonding parameters, Flory-Hugginsinteraction parameters, dielectric constants, dipole moments, and solubility parameters canbe used to predict compatibility with PVC.



Primary plasticizers used in PVC fall into the followingchemical families: dialkyl ortho-phthalates, alkyl benzyl phthalates, dialkyl tere-phthalates, epoxides, aliphatic carboxylicdiesters, polyester-type polymerics, phosphate esters, trimellitate esters, benzoate anddibenzoate esters, alkyl sulphonic esters of phenol and cresol, and miscellaneous types.

The 2003 Modern Plastics World Encyclopedia lists 402 plasticizers of which 279 are indicatedto be compatible with PVC. Forty suppliers are listed.

Dialkyl ortho-phthalate esters are the most frequently used plasticizers in PVC applications.The alcohols range from hexyl (C6) to tridecyl (C13), and may be linear or branched. Increasingthe degree of branching in the alcohol gives a plasticizer with higher volatility, greatersusceptibility to oxidation, poorer low temperature brittleness in PVC, and higher volumeresistivity in formulated PVC. Di-2-ethyl hexyl phthalate, also known as DEHP or DOP, is theindustry standard general purpose (GP) plasticizer against which other dialkyl phthalatesand PVC plasticizers generally are compared via efficiency factors (EF).

In addition to making recommendations on how to use plasticizers based on their experience,several plasticizer suppliers calculate the exact concentrations of their plasticizers requiredwith a standard PVC resin to produce a desired set of physical properties, if it is attained inPVC plasticized with their products. The selection of the best phthalate plasticizer to use for aparticular application is guided by economics, toxicological regulation (if required), ease ofprocessing, and performance in end-use.

Aliphatic carboxylic diesters, such as the phthalates, are generally identified by acronyms.They are based on aliphatic dibasic acids esterified by alcohols ranging from C7 to C10. Thedibasic acids have carbon numbers varying from C5 (glutaric) to C10 (sebasic). Di-2-ethylhexyladipate is known as DOA. The azelates and the adipates do not lower the melting points ofPVC crystallites as much as the corresponding phthaltates do, but they flexibilize the amorphousregions of the PVC more efficiently, and they are lower in molecular weight and specific gravity.Hence, they impart higher flexibility weight-for-weight and better low temperature properties.DOA is less compatible with PVC than DOP and is considerably more volatile. DOA is regulatedby FDA for use in produce-wrap and meat-wrap films.

Most polyester-type polymeric plasticizers are condensation products of glycols with dibasicorganic acids. 1,3 buylene glycol and adipic acid are the most often used starting materials. C8or C10 alcohols are commonly used for terminating the polymerizations at average molecularweights between 1,000 and 8,000. Acid-terminated polymeric plasticizers are less environ-mentally stable than their alcohol-terminated analogs. The chief advantage of polymericplasticizers over general purpose monomeric plasticizers is greater permanence. The chiefdisadvantages are higher cost, lower plasticizing efficiency, poorer low temperature properties,and reduced environmental stability of end products exposed to combinations of warmth,humidity, UV light, and/or active microbial cultures. Practical formulations often containmixtures of polymeric and monomeric plasticizers.

Trimellitate ester plasticizers are made by reacting trimellitic anhydride with plasticizer-gradealcohols. Tri-2-ethyl hexyl trimellitate is known as TOTM. These esters represent the state-of-the-art in low volatility monomeric plasticizers. Their principal uses are in 90 °C- and 105 °C-rated electrical wire insulations and jackets and other applications requiring plasticizersvolatility lower than is attainable with higher molecular weight phthalates. Adams reviewedthe status of trimellitate plasticizer use in the United States [29].

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Epoxy plasticizers have oxirane oxygen groups in their molecules formed by the epoxidationof olefinic double bonds in their starting raw materials:

O O O// / \ //

R–CH = CH– + CH3–C–O–OH ⎯⎯→

cat. R–CH–CH– + CH3–C–OH

They are used as co-stabilizers along with suitable mixed metal stabilizers and some of thenewer types of stabilizers. Epoxidized soy bean oil (ESO) and epoxidized linseed oil (ELO) arethe most widely used epoxides. They have the disadvantage of being food nutrients for molds,some bacteria, and fungi. Sound formulators use epoxides at low levels because the oxiraneoxygen group has a strong compatibilizing action with PVC. Use of higher levels of ESO orELO risks formation of tacky “spew” resulting when the oxirane oxygen is photo-oxidized orhydrolyzed. To get the stabilizing action of oxirane oxygen without the risk of exudation ormicrobial attack, some formulators use epoxy resins even though these cost more than ESOor ELO.

Phosphate ester plasticizers made from phosphorus oxychlorides have the general structure:

(R1O)(R2O)(R3O)P = O

Where, R1, R2, and R3 are alkyl or aryl moieties. Numerous triaryl and alkylaryl phosphateplasticizers are available. They are more expensive than phthalate esters, have excellentcompatibility with PVC, and burn with lower heat release than phthalates. The principal useof phosphate esters is in flame-retarded and smoke-suppressed formulations.

Dipropylene glycol dibenzoate exemplifies the benzoate ester plasticizers, which are used mostlyin stain-resistant flooring.

Several miscellaneous plasticizers are used enough to be worth mentioning. Some citrate esters,such as acetyl tri-n-hexyl citrate and butyryl tri-n-hexyl citrate, find specialty uses in someblood bags and food wraps. Citrates are also used in toys produced by the plastisol process,where the toy is intended for use by young children. Polymerizable plasticizers are availablefor specialty applications such as insulation on electrical wires, which have to be connected bysoldering and where retraction of the insulation due to heat must not occur. Alkyl sulfonateesters of phenol are sold in Europe under the trade name Mesamoll®. Texanol Isobutyrate®(TXIB) is used as a volatile, viscosity-reducing plasticizer/diluent in plastisols for flooringsheet-goods and coil coatings. Specialty flame-retardant plasticizers are exemplified by GreatLakes DP-45, which is a tetrabromophthalate ester with outstanding fire-retardance and lowplasticizing efficiency due to its high molecular weight and high specific gravity. Secondaryplasticizers, extenders and diluents include chlorinated paraffins, naphthenic hydrocarbons,alkylated aromatics, and some linear paraffins.

10.6.2 Lead-Based Stabilizers

Stabilizers have been used in flexible PVC compositions to prevent degradation duringprocessing and forming into finished shapes. Mainly due to pressures from environmentalists,



but also partly due to the results of fundamental research, there have been more changes instabilizers during the last 20 years than in any other aspect of PVC technology.

Historically, lead-based stabilizer systems were the first commercially successful ones for PVC.They are generally fine particle size basic solids, which disperse readily in flexible PVCcompositions so that there are no significant unstabilized volume elements. Atomic chlorineand HCl released from degrading PVC, readily form basic lead chlorides which do not promotefurther degradation of PVC. Related theory is covered in Chapter 4.

A simple way to generalize the action of heat stabilizers in flexible PVC is the following: thermaldegradation of PVC molecules starts at defect structures which may take several forms but allinvolve labile chlorine atoms. Unless an active stabilizer molecule is close to the site fromwhich labile chlorine releases from PVC, a progressive “unzippering” of successive HClmolecules from the PVC is initiated. Stabilizers prevent this as follows:

| | —C–ClL + M–S— → —C–S— + M–Cl

| | labile chlorine stabilizer stabilized spent

on PVC PVC stabilizer

Desired features for stabilizers used in flexible PVC include that they should preferably becolorless, odorless, nontoxic, tasteless, non-staining, non-volatile, nonconductive, non-extractible, non-migrating, non-plasticizing, non-plating, resistant to oxidation and hydrolysis,non-exuding, non-chalking, and non-lubricating or only weakly lubricating. They shouldalso be low in cost, shelf stable, readily available, easily dispersed in PVC, compatible withPVC and other additives, homogeneous, heat stable, light stable, environmentally acceptable,chemically stable, easy processing, and efficient in stabilizing action.

Even though finely powdered litharge (PbO) was a fairly effective stabilizer for flexible PVC,Waldo Semon abandoned it early, because of its color, in favor of basic carbonate of whitelead (BCWL). Over the years, this has been replaced by tribasic lead sulfate (TBLS), dibasiclead phthalate, and dibasic lead phosphite, all manufactured as fine white powders. TBLS hasthe lowest cost of these three, but is sufficiently basic to hydrolyse some polymeric plasticizers.Dibasic lead phosphite is the most expensive of the three, but is favored in some applicationsbecause it has more light-stabilizing action than TBLS or dibasic lead phthalate.

All these lead stabilizers sulfur-stain on contact with mercaptides or hydrogen sulfide. Theyhave to be handled carefully due to their tendency to “dust”. When breathed or ingested byhumans, they are slightly toxic, but only slightly so due to their low solubilities in water orsaliva. They have refractive indices between 2.0 and 2.25, which are high enough to makethem unusable in transparent or translucent applications due to their pigmenting action.They are among the most cost-effective stabilizers for plasticized PVC, but are generally beingphased-out due to pressure from environmentalists on the PVC industry to stop using lead-containing stabilizers, pigments, or lubricants.

In the United States, problems of worker exposure to lead have been overcome by handlingthe powdered lead stabilizers in closed bulk air pallet systems, in pre-weighed batch charges(each in its own PVC bag), or in prilled stabilizer-lubricant one-packs. In the United States,the permissible exposure limit (PEL) for airborne lead is 0.05 mg/m3 [30].

333

For building wire insulations for use in damp or wet locations, lead stabilizers perform thebest. Many suppliers have qualified compounds, which pass long-term insulation-resistancetesting. This requires immersion in water at 75 or 90 °C for twenty-six (26) or more weekswithout significant loss of dielectric properties. Lead-replacement and low-lead systemscontinue to be actively evaluated for these uses.

TBLS and dibasic lead phthalate and dibasic lead phosphite have low solubilities in water atpH 6 to 8 (neutral). But, due to the amphoteric nature of lead, they are more soluble if theextractant is buffered to be acidic or alkaline. When lead-stabilized vinyl insulation or sheathingmaterials are ground to a fine particle size and subjected to EPA’s Toxic Characteristic LeachingProcedure (TCLP), which is run under acidic conditions and allows a maximum leadconcentration in the leachate of 5mg/l, marginal or failing results may be experienced.Therefore, lead-stabilized wire and cable PVC scrap is either recovered or sent to secure landfills,which are expensive. By contrast with lead, 100 mg/l of barium is allowed in TCLP leachates.Calcium and zinc are not regulated in this test.

Grossman has described low extractable lead stabilizers [31].

10.6.3 Mixed Metal Stabilizers

For many years, the most popular mixed metal stabilizers for flexible PVC were based onbarium and cadmium or barium-cadmium-zinc combinations, along with various phosphitesand epoxy plasticizers or resins. Cadmium has been phased out, because it is considered to bea toxicity hazard. But cadmium is present in much old flexible PVC rework. Today, manymixed metal stabilizers for flexible PVC use zinc compounds, which exchange their anions forlabile chorine atoms on PVC molecules. The zinc chloride formed in these exchanges is apotent Lewis acid capable of catalyzing catastrophic dehydrochlorination of PVC. Therefore,zinc is backed up by barium or calcium in the stabilizer at a higher level than the zinc. Thebarium and calcium compounds do not react with the labile chlorine atoms on PVC as activelyas the zinc compounds do. Then, by anion exchange, barium or calcium chlorides are formedin the mixed metal system, and the zinc ceases to be part of a strong Lewis acid. The bariumand calcium chlorides are weak Lewis acids and promote PVC degradation much less thanzinc chloride does. In 1993, Baker and Grossman presented work on cadmium-free mixedmetal stabilizers [32]. Today, use of cadmium has been phased out.

The barium-zinc and calcium-zinc stabilizers may be either solids or liquids. The workhorsesolids consist of barium or calcium stearate, plus some zinc stearate, together with varioussynergists. Mixed fatty acid salts, including palmitates and laureates, are also often used. Inliquid systems, barium alkyl phenates and zinc octoate may be used together, with high boilingsolvents compatible with PVC. Other synergistic ingredients include epoxides and phosphiteantioxidants, whose solubility parameters are close to those of PVC and other ingredientssuch as plasticizers in the formulation. Mixed metal stabilizers have been used for years inclear flexible PVC formulations. Alkyl aryl phophites improve clarity and help maintain “good,early color”. Pentaerythritol was found empirically to be beneficial. Phenolic antioxidantssuch as butylated hydroxytoluene (BHT) and Bisphenol A are included in many formulations.It is necessary to protect liquid mixed metal stabilizers from exposure to humid air by handlingthem in closed bulk or semi-bulk systems. Quite small amounts of water in many mixed



metal stabilizers are sufficient to cause phase separation and serious loss of properties byhydrolyzing some of the phosphite and adding to a portion of the epoxide.

Numerous calcium-zinc mixed metal stabilizers are sanctioned by FDA for use in flexiblePVC food contact films. Regulated phosphites and polyols are used as synergists in thesestabilizers, some of which are sold as one-pack systems. New calcium-zinc stabilizers weredescribed by Bacaloglu [33].

The compositions of most lead-replacement stabilizers are proprietary because of unresolvedpatent and technical issues. They are reported to contain combinations of primary andsecondary metals, metallic chloride deactivators, inorganic acid acceptors, metal coordinators,and antioxidants. Some of these use hydrotalcites similar to the well-known antacid Maalox®,which has aluminum, magnesium, hydroxyl, and carbonate functionalities. β-diketones, suchas Rhodiastab 83® or Rhodiastab 50®, are recommended to prevent early discoloration insome lead-replacement stabilizer systems. A novel approach using “latent mercaptides” wasdescribed by Conroy [34]. Promising early work on the stabilization of PVC by “plasticizerthiols” was described by Starnes [35]. Stabilizer technology is covered in Chapter 4.

Organotin stabilizers are very successful in the United States in rigid PVC, but are only used inspecialty flexible applications.

When foaming flexible PVC with azodicarbonamide blowing agents, it is advisable to use astabilizer recommended by the blowing agent manufacturer. For satisfactory foaming, thestabilizer needs to be matched to the desired temperature range for foam formation. Forinstance, some lead stabilizers are good “kickers” for blowing in the range 160 to 180 °C (320to 356 °F). Some zinc-containing stabilizers are effective kickers for blowing above 180 °C.

10.6.4 Fillers

Generically, filler may be any low cost solid, liquid, or gas which occupies volume in a part andreduces its volume-cost. The flexible PVC industry uses the term “fillers” to refer to inertparticulate solids incorporated into formulations for various reasons, including hardening,stiffening, and reduction of volume-cost. Functional fillers are added to improve specificproperties. Examples are calcined clays added to wire insulation formulas to raise electricalvolume resistivity, fumed silica or bentonite clay added to plastisols to increase their yieldvalue, and hollow microspheres used to lower specific gravity while achieving other desiredfiller effects. Particulate solids called fillers must not dissolve in the flexible PVC matrix. Sincemany flexible vinyl products are sold by volume rather than weight, their volume-cost is thedominant economic parameter. For use in volume-cost calculations, the specific gravity ofcalcite is 2.71; that of true dolomite is 2.85 and that of aragonite is 2.95

The most widely used fillers in flexible and semi-rigid PVC are grades of dry-ground, wet-ground, or precipitated calcium carbonate derived from limestone or marble, which arepredominantly calcite. This is the stable crystal structure of CaCO3 at ordinary temperaturesand pressures. Marble consists of small, interlocking crystals of calcite. Calcite is soft, havinga Mohs hardness of 3. Therefore, pure calcium carbonate fillers are low in abrasivity toprocessing equipment. Grades which contain significant fractions of hard silicates are muchmore abrasive. Recent work carried out in a PE carrier resin confirms this long-accepted factand shows that coarser grades are more abrasive than fine particle size fillers [36].

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Considerations in selecting a particular grade of calcium carbonate filler include the purity ofthe original ore, whether it has been dry-ground or wet-ground or precipitated, the averageparticle size and size distribution, and whether the particles have had a surface treatment. The“packing fraction” (PF) is a measure of how efficiently finer particles fill the voids betweencoarser particles. Presence of iron oxides such as Fe2O3 in the filler tends to color a compoundyellow-brown and will compromise its heat stability unless it is stabilized to withstand thepresence of the iron oxide.

The average size of filler particles is usually defined in terms of an equivalent spherical diameter(esd). The ratio of the average lengths of the major to minor axes of filler particles is called the“aspect ratio”. The most used fillers have aspect ratios of less than 4 : 1. Reinforcements suchas glass or metal fibers generally have aspect ratios in excess of 10 : 1.

Vinyl floor tile made by calendering tolerates filler particle sizes up to 99% through a U.S.Standard 50 mesh screen having 297 micron openings (11.7 mils). Typical electrical insulationsand cable jackets, which are extruded, require fillers with an average esd of 3 microns or lessand coarsest particles of 12 microns diameter (0.47 mils). Cable jackets designed to give lowHCl emission on burning generally use precipitated calcium carbonates having 0.6 micronesd.The best filler particle sizes for most flexible PVC applications are determined by experience,in optimizing end-use properties and minimizing cost.

The softest non-carbonate filler used in flexible PVC is talc represented as 3 MgO · 4 SiO2 · H2O.Zero or very low content of asbestos-related minerals is specified for talcs used with PVC. Talcis often added to calendering formulations to reduce plate-out on the rolls and to extrusion for-mulations to reduce plate-out on screws and dies. Talc may also be dusted at 0.1 to 0.25% ontoPVC compound cubes or pellets to improve flow in bulk handling systems and hopper cars.

Mica is added to PVC compounds to impart a non-blocking surface and to provide stiffeningwhen that is also desired. Typical grades used in non-blocking calendered films are fine-groundso that > 99% passes a 325 mesh screen (with openings of 1.7 mils or 44 microns).

Diatomite (amorphous silica) is added to PVC plastisols to increase viscosity and yield valueand to reduce surface gloss after fusion. Fumed silica may be added to hot-processedcompounds as a scrubbing agent and to plastisols to increase viscosity and yield value.

The refractive index (RI) of flexible PVC matrices usually ranges between 1.51 and 1.53 becausethe RI of PVC is 1.55 and that of typical phthalate plasticizers ranges between 1.48 and 1.50.TiO2, with an RI of 2.76 for rutile, is a strong pigment, which contributes a high degree ofopacity. Calcium carbonate (calcite), with an RI of 1.65, is a weak pigment as well as a filler forflexible PVC. Barium sulfate (Barytes), with a slightly lower RI (1.6) than calcite, may be usedin translucent flexible vinyl compounds, but allowance must be made for its high specificgravity (4.5). The high gravity is an advantage for use in sound-absorbing and visco-elasticdamping compounds. Clear vinyl compounds are generally unfilled.

The principal advantages of inorganic fillers in flexible PVC include cost reduction, stiffening,reducing coefficients of thermal expansion, and contributing to better flammability behavior.Specific heats per unit volume are comparable for most fillers and many polymers. Thedisadvantage of using high levels of fillers in flexible PVC is the reduction of tensile and tearstrength, elongation at failure, toughness at low temperatures, abrasion resistance, andresistance to attack by moisture and chemicals. High filler levels also compromise processabilityby increasing melt viscosity.


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