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The Economic and Technical Importance of PVC Stabilizers 427 3PVC Stabilizers Dr. R. Bacalogulu, Dr. M. H. Fisch, Polymer Additives, Crompton Corp., Tarrytown, NY,USA, Dipl. Chem. J. Kaufhold, Dipl. Chem. H. J. Sander * , Polymer Additives, Witco VinylAdditives GmbH, Lampertheim, Germany 3.1The Economic and Technical Importance of PVC Stabilizers Polyvinyl chloride (PVC) was one of the first thermoplastics developed. It has becomeworldwide a very important bulk plastic over its almost 70 year history. PVC consumptionin different geographic areas and expected demand through 2000 are shown in Fig. 3.1. Fig. 3.1 PVC consumption from 1980 to 2000 PVC – including the various copolymers of vinyl chloride and chlorinated PVC – is expectedto remain important among thermoplastics because of its compatibility with a large numberof other products (e. g., plasticizers, impact modifiers), in contrast to other plastics. BecausePVC’s mechanical properties can be adjusted over a wide range, yielding everything fromrigid to flexible end products, there are many different processing methods and applicationsfor PVC. The toxicological problems which at one time were major obstacles in themanufacture and processing of PVC were solved satisfactorily many years ago [1, 2]. * Recent address: Baerlocher GmbH, Unterschleissheim, Germany 0510152025301980 1985 1990 1995 2000 Year P V C c o n s u m p t i o n ( M i o . t ) World siaorth AmericaWestern Europe 428 PVC Stabilizers When PVC was first developed, flexible PVC was dominant, but rigid PVC production hasincreased continually and is now approximately two-thirds of total consumption in manycountries.The low thermal stability of PVC is well known. Despite this fact, processing at elevatedtemperatures is possible by adding specific heat stabilizers that stop the damage. This is oneof the main reasons PVC has become a major bulk plastic. The development and productionof suitable heat stabilizers followed the production of PVC from the beginning, and remainsa precondition for processing and application in the future. Consumption of heat stabilizersin Western Europe was approximately 150,000 tons in 1995 and is estimated to be 170,000tons by the year 2000 [3]. The consumption of thermal stabilizers for PVC worldwide isestimated to be 450,000 tons [4]. 3.2Thermal Degradation and Stabilization of PVC 3.2.1Mechanism of PVC Degradation When PVC is processed at high temperatures, it is degraded by dehydrochlorination, chainscission, and crosslinking of macromolecules. Free hydrogen chloride (HCl) evolves and dis-coloration of the resin occurs along with important changes in physical and chemical proper-ties. The evolution of HCl takes place by elimination from the polymer backbone; discolora-tion results from the formation of conjugated polyene sequences of 5 to 30 double bonds(primary reactions). Subsequent reactions of highly reactive conjugated polyenes crosslink or cleave the polymer chain, and form benzene and condensed and/or alkylated benzenes intrace amounts depending on temperature and available oxygen (secondary reactions). 3.2.1.1Dehydrochlorination of PVC in the Absence of Air (Primary Degradation) Any mechanism of degradation has to explain a series of experimental facts. Structuralirregularities, such as tertiary or allylic chlorine atoms, increase the degradation ratesmeasurably at the beginning of the process by a rapid dehydrochlorination that starts thedegradation process (Scheme 3.1). Initial rates of degradation are proportional to thecontent of these irregularities. However, PVC degrades even if these irregularities areeliminated by special polymerization conditions or treatments because of the dehydrochlori-nation of normal monomer units (random elimination) (Scheme 3.1).It is estimated that after allowing for the differences in concentrations and reaction rates,the rate of random degradation in commercial PVC because of normal chain secondarychlorine atoms has the same order of magnitude as does degradation that results fromstructural irregularities [5, 6, 7]. Cis -ketoallylic structures, although very reactive indehydrochlorination (Scheme 3.1), are not present in commercial PVC but can be generatedby thermal oxidative processes [7, 8]. After the reactive irregularities initially present areexhausted, degradation continues because of the elimination initiated from normalmonomer units [6, 7, 9].These findings indicate that thermal degradation in PVC is anintrinsic property of this polymer and that changes in synthesis conditions or specialtreatments that eliminate structural irregularities improve the stability of PVC, but can notcompletely eliminate its degradation. Stabilizers must be used. Thermal Degradation and Stabilization of PVC 429 Scheme 3.1 Not all allylic chlorine atoms preexisting and/or formed in the degradation process accel-erate degradation. Single double bonds can be identified in degraded PVC by NMR spec-troscopy. Double bond sequences, once formed, do not increase by continuation of degrada-tion [6]. There are allylic chlorides with some forms of alkenic double bonds that are stableunder degradation conditions [6].The conjugated polyene sequences are generated in apparently parallel processes from thefirst moment of degradation. For relatively low conversions, their concentrations increaselinearly with time. Zero order rate constants calculated as slopes of these lines decreaseexponentially with the increase of the number of double bonds in the sequence [6, 10].In the thermal degradation of solid PVC, an induction period is observed, and then forhigher conversions, the degradation rate increases with time, indicating an autocatalyticprocess. Hydrogen chloride formed in the degradation increases both the degradation rateand the mean number of double bonds in the polyene sequence, and consequently plays anessential catalytic role in PVC degradation [11, 12, 13].Some local configurations and conformations of the polymer chain of PVC, such as theconformation GTTG (G for Gauche T for Trans) at the end of certain isotactic sequences,favor degradation. These conformations exhibit a high local mobility relative to theremaining structures in PVC and possess some chlorine atoms with very high degrees of ClClClClClDehdrochlorination of structural Alllic chloridesTertiar chloridesOClOCis keto alllic chloridesDehdrochlorination of normal monomerClCl ClCl Dehvdrochlorination of normal monomer unitsDehvdrochlorination of structural irregularities 430 PVC Stabilizers freedom. Both features make possible the adoption of the conformation enabling theelimination reaction [14, 15]. It follows that dehydrochlorination is possible only forspecific local conformations. Along the same line, PVC molecules at the surface of primaryparticles in the solid state have a much higher conformational mobility than molecules inthe interior. PVC degradation consequently is expected to take place predominantly at thesurface of primary particles.It is well known that dehydrochlorination of PVC proceeds violently in the presence of Lewis acids such as FeCl 3 [111], ZnCl 2 , [112],AlCl 3 , [113] SiCl 4 , GeCl 4 , SnCl 4 , BCl 3 , andGaCl 3 [16, 17]. This process is responsible for the very fast discoloration of PVC in thepresence of Zn or Sn carboxylates that act as stabilizers till the corresponding halides areformed and fast dehydrochlorination starts.The reaction mechanism of a complex chemical process such as PVC degradation definesthe sequence of elementary reactions leading from reactants to products and describes eachof these reactions. The mechanism of PVC degradation should explain the abovefundamental observations and should also agree with the observations related to PVCstabilization that are discussed later in this chapter.The dehydrochlorination of PVC is a very specific chemical process because of the existenceof a long series of alternating CHCl and CH 2 groups in the polymer backbone that makespossible a chain of multiple consecutive eliminations. However, the parallel formation of conjugated polyene sequences containing 1 to 30 double bonds cannot be explained by asimple consecutive elimination. The chain reaction model from Scheme 3.2 can explain thisapparent contradiction [6]. Scheme 3.2 Cl Cl []n I 1 I Cl [][]2 2 n-1 I Cl [][] m-1 mn-m+1InitiationPropagationPropagationPropagationTerminationTerminationTermination-HCl-HCl-HCl-HCl........Cl []n I- active intermediates HCl catalyzedHCl catalyzedHCl catalyzedHCl catalyzed PVC -HClTermination-HClCl [3]]]][n-2k ik k k k -HCl-HClk'k'k'
