PVC Stabilisers

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<p>The Economic and Technical Importance of PVC Stabilizers</p> <p>427</p> <p>3</p> <p>PVC Stabilizers</p> <p>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 Vinyl Additives GmbH, Lampertheim, Germany</p> <p>3.1 The Economic and Technical Importance of PVC StabilizersPolyvinyl chloride (PVC) was one of the first thermoplastics developed. It has become worldwide a very important bulk plastic over its almost 70 year history. PVC consumption in different geographic areas and expected demand through 2000 are shown in Fig. 3.1.30</p> <p>25</p> <p>World</p> <p>PVC consumption (Mio. t)</p> <p>20</p> <p>15</p> <p>10</p> <p>Asia North America</p> <p>5</p> <p>Western Europe</p> <p>0 1980 1985 1990 1995 2000</p> <p>YearFig. 3.1 PVC consumption from 1980 to 2000</p> <p>PVC including the various copolymers of vinyl chloride and chlorinated PVC is expected to remain important among thermoplastics because of its compatibility with a large number of other products (e. g., plasticizers, impact modifiers), in contrast to other plastics. Because PVCs mechanical properties can be adjusted over a wide range, yielding everything from rigid to flexible end products, there are many different processing methods and applications for PVC. The toxicological problems which at one time were major obstacles in the manufacture and processing of PVC were solved satisfactorily many years ago [1, 2].* Recent address: Baerlocher GmbH, Unterschleissheim, Germany</p> <p>428</p> <p>PVC Stabilizers</p> <p>When PVC was first developed, flexible PVC was dominant, but rigid PVC production has increased continually and is now approximately two-thirds of total consumption in many countries. The low thermal stability of PVC is well known. Despite this fact, processing at elevated temperatures is possible by adding specific heat stabilizers that stop the damage. This is one of the main reasons PVC has become a major bulk plastic. The development and production of suitable heat stabilizers followed the production of PVC from the beginning, and remains a precondition for processing and application in the future. Consumption of heat stabilizers in Western Europe was approximately 150,000 tons in 1995 and is estimated to be 170,000 tons by the year 2000 [3]. The consumption of thermal stabilizers for PVC worldwide is estimated to be 450,000 tons [4].</p> <p>3.2 Thermal Degradation and Stabilization of PVC3.2.1 Mechanism of PVC DegradationWhen PVC is processed at high temperatures, it is degraded by dehydrochlorination, chain scission, and crosslinking of macromolecules. Free hydrogen chloride (HCl) evolves and discoloration of the resin occurs along with important changes in physical and chemical properties. The evolution of HCl takes place by elimination from the polymer backbone; discoloration 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 in trace amounts depending on temperature and available oxygen (secondary reactions). Dehydrochlorination of PVC in the Absence of Air (Primary Degradation) Any mechanism of degradation has to explain a series of experimental facts. Structural irregularities, such as tertiary or allylic chlorine atoms, increase the degradation rates measurably at the beginning of the process by a rapid dehydrochlorination that starts the degradation process (Scheme 3.1). Initial rates of degradation are proportional to the content of these irregularities. However, PVC degrades even if these irregularities are eliminated by special polymerization conditions or treatments because of the dehydrochlorination 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 secondary chlorine atoms has the same order of magnitude as does degradation that results from structural irregularities [5, 6, 7]. Cis-ketoallylic structures, although very reactive in dehydrochlorination (Scheme 3.1), are not present in commercial PVC but can be generated by thermal oxidative processes [7, 8]. After the reactive irregularities initially present are exhausted, degradation continues because of the elimination initiated from normal monomer units [6, 7, 9]. These findings indicate that thermal degradation in PVC is an intrinsic property of this polymer and that changes in synthesis conditions or special treatments that eliminate structural irregularities improve the stability of PVC, but can not completely eliminate its degradation. Stabilizers must be used.</p> <p>Thermal Degradation and Stabilization of PVC</p> <p>429</p> <p>Dehvdrochlorination of structural Dehydrochlorination of structural irregularities Cl Cl</p> <p>Cl Cl Allylic chlorides Cl</p> <p>Cl Tertiary chlorides</p> <p>O</p> <p>Cl</p> <p>O Cis keto allylic chlorides</p> <p>Dehvdrochlorinationof normal monomer units Dehydrochlorination of normal monomerCl Cl Cl</p> <p>Scheme 3.1</p> <p>Not all allylic chlorine atoms preexisting and/or formed in the degradation process accelerate degradation. Single double bonds can be identified in degraded PVC by NMR spectroscopy. Double bond sequences, once formed, do not increase by continuation of degradation [6]. There are allylic chlorides with some forms of alkenic double bonds that are stable under degradation conditions [6]. The conjugated polyene sequences are generated in apparently parallel processes from the first moment of degradation. For relatively low conversions, their concentrations increase linearly with time. Zero order rate constants calculated as slopes of these lines decrease exponentially 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 for higher conversions, the degradation rate increases with time, indicating an autocatalytic process. Hydrogen chloride formed in the degradation increases both the degradation rate and the mean number of double bonds in the polyene sequence, and consequently plays an essential catalytic role in PVC degradation [11, 12, 13]. Some local configurations and conformations of the polymer chain of PVC, such as the conformation 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 the remaining structures in PVC and possess some chlorine atoms with very high degrees of</p> <p>430</p> <p>PVC Stabilizers</p> <p>freedom. Both features make possible the adoption of the conformation enabling the elimination reaction [14, 15]. It follows that dehydrochlorination is possible only for specific local conformations. Along the same line, PVC molecules at the surface of primary particles in the solid state have a much higher conformational mobility than molecules in the interior. PVC degradation consequently is expected to take place predominantly at the surface of primary particles. It is well known that dehydrochlorination of PVC proceeds violently in the presence of Lewis acids such as FeCl3 [111], ZnCl2, [112],AlCl3, [113] SiCl4, GeCl4, SnCl4, BCl3, and GaCl3 [16, 17]. This process is responsible for the very fast discoloration of PVC in the presence of Zn or Sn carboxylates that act as stabilizers till the corresponding halides are formed and fast dehydrochlorination starts. The reaction mechanism of a complex chemical process such as PVC degradation defines the sequence of elementary reactions leading from reactants to products and describes each of these reactions. The mechanism of PVC degradation should explain the above fundamental observations and should also agree with the observations related to PVC stabilization that are discussed later in this chapter. The dehydrochlorination of PVC is a very specific chemical process because of the existence of a long series of alternating CHCl and CH 2 groups in the polymer backbone that makes possible a chain of multiple consecutive eliminations. However, the parallel formation of conjugated polyene sequences containing 1 to 30 double bonds cannot be explained by a simple consecutive elimination. The chain reaction model from Scheme 3.2 can explain this apparent contradiction [6].</p> <p>Cl</p> <p>Cl [</p> <p>Cl ]n -HCl -HCl k Termination Cl [ ][ ] 2 Cl [ ][ 3 ] n-2 ] ] n-1 [ ]n</p> <p>PVCInitiation ki HCl catalyzed</p> <p>I I</p> <p>-HCl1k</p> <p>Propagation k' HCl catalyzed</p> <p>Termination -HCl -HCl Termination</p> <p>2k</p> <p>Propagation k' -HCl HCl catalyzed ........ Propagation k' -HCl -HCl HCl catalyzed</p> <p>Cl [ [ ] m ] n-m+1</p> <p>I</p> <p>m-1</p> <p>k TerminationScheme 3.2</p> <p>I- active intermediates</p> <p>Thermal Degradation and Stabilization of PVC</p> <p>431</p> <p>The first elimination from a monomer residue from the chain (-CH2-CHCl-) or a structural irregularity such as a tertiary chlorine atom (-CH2-CClC=CH-CH2-CCl</p>


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