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Date Created: 10/06/15
THERMAL DEGRADATION OF POLYVINYL CHLORIDE D BRAUN Deutsches Kunststoff Institut Darmstadt Germany ABSTRACT A survey is given of the recent advances in the study of and the present knowledge of the thermal degradation of polyvinyl chloride The sites for initiation of the thermal degradation the mechanism of the dehydrochlorination the discoloura tion of PVC by heat and the in uence of plasticizers on the rate of degradation are discussed 1 INTRODUCTION For many years polyvinyl chloride PVC has been one of the most im portant technical polymers A great disadvantage of PVC is its rather low thermal stability It is well known that PVC splits 01f hydrogen chloride at high temperatures during this process polyene sequences are formed and the polymer is discoloured Up to about 220 hydrogen chloride is the only volatile degradation product in the presence of oxygen in addition to the dehydrochlorination oxidation reactions can occur which can also initiate chain scissions Nowadays it is possible to suppress these undesired degrada tion reactions by using stabilizers but the mechanisms of the dehydro chlorination of the PVC remained as obscure as the nature of the initiation sites from which the dehydrochlorination starts There is also very little known about the reactions between PVC and stabilizers Due to the fact that the dehydrochlorination of PVC is one of the most important technical reactions with polymers it would be important to have a better understanding of the above mentioned questions In recent years several detailed surveys on the degradation of PVC have been published 1 4 The present paper therefore is con ned to the discussion of newer developments and some open problems The main topic is the thermal degradation the degradation by radiation or chemical reagents will not be discussed After a short review of the experimental methods used for the 173 D BRAUN investigation of the dehydrochlorination of PVC the following topics will be discussed 1 Sites for initiation of thermal degradation 2 Mechanism of dehydrochlorination 3 Discolouration during degradation 4 Degradation in the presence of plasticizers 2 EXPERIMENTAL METHODS For investigations of the thermal degradation of PVC most authors use measurements of the hydrogen chloride split off Thus various apparatus have been described in which the HCl is measured after removal from the polymer sample by electrical conductivitys In a few cases the change in the electrical conductivity of the solid polymer due to the HCl splitting off 6 or the change in pressure due to the gaseous HCl7 is used The potentiometric measurement of the degradation can also be combined with viscosimetric investigations in a Brabender plastographe Recently the thermogravimetric analysis of the 39a 555 allyv1 L IIIl F igura 1 Apparatus for the measurement of the thermal HC Splitting off from PVC a valve for regulation of the gas rate b rotameter c vessel for the sample d thermostat e conductivity measurement cell f thermostat for e g conductivity meter h resistances i recorder 174 THERMAL DEGRADATION OF POLYVINYL CHLORIDE polymer in combination with the potentiometric determination of the HC19 1 and thermal volatilization analysis11 have also been applied Another possibility is the photometric determination of the discolouration during the degradation For practical purposes the discolouration of a PVC compound during processing can be used as a measure of its stability For basic research this method can be used only in combination with the measurement of the HCl splitting off Therefore most authors use for kinetic and mechanistic investi gations the direct determination of HCl an important factor is the exact con stancy of the temperature and the complete transfer of the HCl into the con ductivity cells The potentiometric determination of the HCl split off is possible with high sensitivity so that the initial part of the reaction conversions far below 1 per cent in relation to the HCl content of the undegraded PVC can also be measured exactly Figure I shows the scheme of such an apparatus which is suitable for the determination in an inert atmosphere or in air The vessel can be constructed in such a way that it can also be used for the degra dation of dissolved samples In contrast to such equipment measurement of the pressure of the HCl split off is not so sensitive At higher temperatures other volatile products besides HCl can also be formed so that additional dif culties can arise Thermal analysis up to now has been used mainly for physical investigations of PVC whereas it is useful for application to the chemical degradation only in special cases5 1 3 INITIAL SITES 0F DEHYDROCHLORINATION A very important part of the dehydrochlorination of PVC is the initial step which requires a relatively high activation energy From model investi gations with 24dichloroalkanes eg 24dichloropentane it follows that the normal undisturbed PVCchain is not very sensitive to heat13 14 Therefore in the literature various structural irregularities are discussed as initiation sites of the dehydrochlorination 1 Chain end groups with initiator residues or unsaturated end groups 2 Branch points with tertiary chlorine atoms 3 Random unsaturation with allylic chlorine atoms 4 Oxidation structures 5 Headto head units 31 Investigations with low molecular model compounds For clari cation of the nature of the initiation sites of the dehydrochlori nation experiments with low molecular models for PVC and for the various structural irregularities can be used In the literature there is much data which of course was not always determined with respectto PVC degradation1 5 From recent investigations with 2chlorobutane 24dichloropentane and 246trichloroheptane15 it can be concluded that the normal monomer units in PVC are thermally quite stable and that the dehydrochlorination of the models requires about 50 kcalmole activation energy for the pyrolysis at about 350 to 4000 In contrast 4chloro2pentene is thermally far less 175 D BRAUN stable than 3chloro1pentene which suggests that unsaturated chain end groups do not have an important in uence on the thermal stability of PVC whereas random isolated double bonds with allylic chlorine atoms are rather unstable However it should be mentioned that there are remarkable differences in the values for the activation energy and the rate constants between the39various investigators For the stability of the most important model compounds between 200 and 400 the following order is valid CH3 CH C CH2w CH3 gt CH3 CH CH2 CH3 m C c1 CH3 CH CH2 CH CH3 z C1 C1 CH3 CH CH2 CH CH2 CH CH3 gt Cl c1 Cl sz CH2CH CH CH2 CH3 gt CH3 C CH2 CH3 gt c1 c1 CH3 CHCH CH CH2 CH3 gt C1 CH 3 CHCH CHCH CH CH3 Cl From these investigations it follows that random allylic chlorine atoms or branches with tertiary chlorine atoms aremainly responsible for the initiation of the thermal dehydrochlorination of PVC 32 Branches in PVC For a long time branches in PVC were discussed as the reason for the low thermal stability Cotman39 was the rst who reduced PVC with lithium aluminium hydride and obtained a polyethylenelike product from irspectra using the ratio of methyl groups to methylene groups he determined the number of branches This method was later used by many authors but the results were not very exact because the peak of the methylgroup at 1378 cm 1 is seen just as a shoulder of the peak of the methylene group at 1370 cm 1 This may be the reason why the gures for branching in the literature differ between about 05 and 20 methyl groups per 1000 Catoms see example in reference 17 More exact values can be obtained by irspectroscopic com pensation of the reduced PVC against linear polymethylene1 8 19 Depending on the preparation of the polymer about 5 to 15 CH3 groups per 1000 C atoms are found The spectra of the reduced PVC also show random 176 THERMAL DEGRADATION OF POLYVINYL CHLORIDE transvinylene groups at 960 cm39 1 in about the same amount as the CH3 groups19 Figure 2 Whereas the occurrence of branches in PVC is recognised nowadays the structure of the branch points is not quite clear Caraculacu20 did not obtain any indications of tertiary chlorine atoms in PVC during his investi gations with copolymers of vinyl chloride and 2chloropropene Also investi gations with copolymers of vinyl chloride and 24dichloropentene1 led to the conclusion2 1 that due to steric reasons PVC should not contain branch points with tertiary chlorine atoms Braun and Weissn 23 confirmed these ndings by further investigations with copolymers from vinyl chloride and 1400 1200 1000 800 cm A 1 0 a 204 l 40 I E o 8 b C 0 20 E In S e l 0 l 60 39039 c 2039 1500 1200 1000 sob a Figure 2 IRspectra of reduced PVC a before b after treatment with bromine vapour 0 reduced PVC compensated against polymethylene 2chloropropene The thermal degradation of such copolymers with the same content of methyl groups as of branch points in radically prepared PVC is much faster also the distribution of the formed polyene sequences of different length is quite different from that of PVC after the same degree of conversion In the copolymers there is a remarkable shift to shorter polyene 177 D BRAUN sequences This together with the nding that there is no relationship between the number of branch points and the rate of degradation makes structures with tertiary chlorine atoms seem to be very unlikely Other groups such as CH2 CH g CH CH2 CH or C1 CIJHJCI 3931 IIHCl i H CH2 CHmCH2 C CH2 CHm c1 CH2 3931 Am should be much more stable to heat Therefore one can conclude that PVC contains branches but that these groups are not important for the initiation of the thermal degradation This is also in agreement with experiments of Gupta and St Pierre who found that copolymers of vinyl chloride and 2chloropropene A are degraded much faster than those with lchloro propene B This means that tertiary chlorine atoms in the polymer chains are much more sensitive to heat than tertiary hydrogen atoms H CH2 CHCl C CHCl CH2 CHCI CH3 B Cl l CH2 CHCl CH2 C CH2 CHC1 CH3 A It should be mentioned here that up to now nothing is known about the presence and the structure of long chain branches in PVC which could be formed during the polymerization at higher degrees of conversion by chain transfer 33 End groups In the older literature many authors discussed the possibility of the initiation of the dehydrochlorination at the chain ends of PVC macromole cules In some publications a reverse proportionality between molecular 178 THERMAL DEGRADATION OF POLYVINYL CHLORIDE weight and rate of degradation was found However these results are not conclusive as some authors did not nd any relationship between molecular weight and rate of degradation of fractions of PVC see Geddes reference 1 End groups can be formed from initiator residues or by chain transfer termination Although this is reported for different rates of thermal degrada tion of PVC prepared with various initiators the results of these investigations are not easily understood very often the molecular weights of the samples investigated are not comparable very often there is no information about other structural irregularities Some importance may be attached to the in uence of unsaturated end groups Bengough25 determined such groups qualitatively by irspectroscopy using an esterexchange method by reacting allylic chlorine atoms with cadmium acetate and pyridine However the role of unsaturated end groups as initiation sites for degradation is not supported by the experiments with low molecular model compounds see Section 31 Finally one should expect that with the dehydrochlorination starting at the end groups of the PVC molecules the polyene sequences should also be formed at the chain ends However the oxidative degradation of the original PVC results in a remarkable decrease in the average molecular weight which can not be due to the effect of the end groups The oxidative cleavage of heat degraded PVC also results in a decrease in the molecular weight by about the same amount as with the original PVC All these ndings do not support the theory of initiation of dehydrochlorination at the unsaturated end groups of the PVC macromolecules 34 Random unsaturation in PVC As the above mentioned investigations did not give clear results on the initiation of the dehydrochlorination of PVC at end groups or branches it was necessary to look for other initiation sites In this respect primarily un saturated groups with allylic chlorine have to be discussed The above mentioned experiments 31 with low molecular model compounds have shown that such structures are thermally much more labile than unsaturated end groups Also from degradation investigations with vinyl chloride vinyl bromide copolymers it was found that below 2000 the initiation of the dehydrochlorination cannot occur on normal vinyl chloride monomerunits but instead on positions with ally chloride structure Very recently a direct experimental proof for the relationship between the thermal stability of PVC and the content of random unsaturated groups could be obtained During the oxidative degradation of PVC using potassium permanganate in dimethylacetamide it was found viscosimetri cally that at 20 after about 100 hrs a constant final value for the molecular weight was reached It is possible from these gures to calculate the number of cleavages per molecule Figure 3 shows for fractions of a bulk PVC that the number of pcleavages is independent of the molecular weight In the lower part of this gure can be seen that this also holds for the rate of thermal degradation at 180 With technical suspension PVC samples of various sources a relationship between the number of cleavages or the rate of degradation and the molecular weight was found Figure 4 Here a clear connection between the number of cleavages and the rate of the 179 D BRAUN o A o o 9 E 303 D gt a 3302 39 o o 001 d 2 l i 1 l 0 2 10 11 18x10439 71 503 g02 o 01 o n o 4 i l l j 0 239 10 1 18x10quot Mv Figure 3 Number of oxidative cleavable sites in fractions of bulk PVC and rate of thermal dehydrochlorination at 180 under nitrogen 39 No of cleavages1000 C o HClh iv a O O CO 0 O O b 08 06 01 02 0 J C 39 I I l quota m 120m Mv C i l J L a m izmms MV Figure 4 Number of cleavable sites 1 and rate of thermal degredation 2 for varioustechnical suspension PVC types at 180 under nitrogen 180 THERMAL DEGRADATION OF POLYVINYL CHLORIDE dehydrochlorination can be seen Figure 5 which shows the great importance of unsaturated groups within the polymer chains After a careful chlorina tion of these double bonds an increase in the thermal stability was observed and the number of double bonds found by oxidation was reduced Further it can be shown that in the initial stage of the thermal dehydro chlorination one polyene sequence is formed from each isolated double bond M 08 39 0quot 06 OL 0 02 HClh No0f cleavages1000C 1 m o 02 04 05 018 10 12 Figure 5 Rate of dehydrochlorination of suspension PVC at 180 under nitrogen in relation to the number of cleavable sites After one hour of degradation at 180 the same decrease in molecular weight is shown after oxidation in solution by potassium permanganate as after oxidation of the thermically untreated material From the amount of HCI evolved and the number of cleavages the average length of the polyene sequences can be calculated which is in good agreement with the spectro scopically obtained values 35 Other Initiation Sites As alternative initiation sites of the dehydrochlorination considered in the literature oxidation structures are primarily discussed By oxidation with oxygen or ozone hydroperoxide or peroxide groups can be formed in PVC2939 30 As degradation products from these peroxide groups carbonyl bands can be seen in the irspectra of the polymer However up to now only very little is known about the in uence of such groups on the thermal stability of PVC It is possible that the radicals formed during the decomposi tion of the peroxides can in uence the degradation process and can possibly initiate a radical dehydrochlorination see Geddesl Another possibility for the initiation of the dehydrochlorination of PVC are head tohead units but up to now there is no experimental proof for the existence of such groups in PVC Investigations with chlorinated trans14 polybutadiene led to the conclusion that vicinal chlorine atoms in PVC should be less stable than those in 13 positions3239 33 The groups formed at the beginning of the degradation of headtohead units CH2uliH ZH CH2 CH CHc CH l C1 C1 C1 181 D BRAUN in analogy to the corresponding low molecular model compounds are more stable than allylic chlorine atoms Therefore the degradation begins at lower temperatures as in the case of PVC but also has a slower rate Finally it should be mentioned that up to now we do not have enough information about the in uence of the stereoregularity of PVC on its thermal behaviour It seems that with an increasing number of syndiotactic links the thermal stability is also increasing However this can also be due to the higher crystallinity and the higher melting temperature of these samples 4 DISCOLOURATION DURING THE DEHYDROCHLORINATION 0F PVC Nowadays it is quite clear that the discolouration during the thermal degradation of PVC is connected with the formation of sequences of con jugated double bonds within the polymer chains With increased splitting off of HCl the colour becomes more and more intense but exact quantitative relationships between colour and amount of evolved HCll are not yet known By spectroscopy in visible and uvrange conclusions can be drawn about the length and frequency distribution of the polyene sequences of different length 3 Thermally degraded PVC shows in the uv and visible part of the absorp tion spectra about ten to twelve not very well resolved absorption maxima 0 35000 30000 25000 20000 15000 cm I r 4 M 5 6 7 t 91912121 I I I A I L l 300 350 400 500 600 700nm Figure 6 Absorption spectrum of PVC in tetrahydrofuran after thermal degradation at 170 under nitrogen Degree of conversion a 019 b 035 A The absorption maxima correspond to polyene sequenceswith n 45 6 etc double bonds In Figure 6 the spectrum of a thermally degraded PVC in tetrahydrofuran is given Such spectra can be analysed assuming that the observed maxima are related to the main absorption band of the different polyene sequences The absorption spectrum of a polyene with double bonds consists of several sharp bands of which for n 2 5 the band of the longest wavelength has the highest intensity With increaSing number of double bonds the total system of bonds is shifted to longer wavelengths At the same time its intensity also increases which follows for example from the spectra of unsubstituted polyenes35 It is known that between the number of double bonds and the wavelength 182 THERMAL DEGRADATION OF POLYVINYL CHLORIDE 1 of the absorption maximum at the longest wavelength of a polyene the socalled square root law is valid Aknk In this equation the end groups of the polyene can be considered by adding socalled double bonding equivalents for example for the phenyl group 15 and for the carboxy group 08 For identifying the absorption maxima in the spectrum of degraded PVC the function A f n for dimethylpolyenes can be used in this relationship the absorption maxima of degraded PVC t very well at least in the range of longer sequences34 Because no spectra of model compounds for polyene sequences in PVC are known one can try to apply the regularities found on various classes of polyenes The following prerequisites can be used which are generally valid for polyenes a the relation between the absorption maxima and the number of double bonds is given by the square root law b the extinction coef cient 8 of the main band of a polyene is directly pro portional to the number of double bonds n 8n89gtltn whereas 89 1 molequot1 cm 1 is a constant and 8 1 molequot1 cm is the decadic extinction coef cient It has to be taken into account that the absorption at a maximum in the PVC spectrum is not only due to the main maximum of the corresponding polyene sequence In addition the absorptions of the following longer polyene sequences absorb at the same wavelength This amount is in the same order of magnitude as the absorption of the main maximum of the corresponding polyene sequence But Krauss and Grund3 6 found in the spectra of wphenyl polyenales in chloroform that in this case the proportionality between 3 and n is also given for the maximum of the complete undissolved system of hands If one presumes that the extinction of the absorption maxima in the spectra of degraded PVC are all caused by the band with the longest wavelength all concentrations of polyenes are calcu lated somewhat too high by about the same factor But the relative concentra tions of the polyene sequences of various lengths are quite exaCt The frequency of the polyene sequences of different lengths can be cal culated using the Lambert Beer law Thereby x is the conversion during the degradation ratio of the split off HCl to the HCl content of the undegraded polymer cpg1 is the concentration of the polymer in solution I 0 and I are the light intensities and c is the cell path Then the following equation can be deduced I log IOI dnxcp H is a relative measure for the frequency of the polyene sequences with n double bonds in the special degraded PVC All values in this equation are either known or can be directly measured It should be noted that due to the constants which are incorporated in H i only the values for a single polymer can be directly compared but not those of different polymer samples 183 D BRAUN Using this method the frequency of the polyene sequences was calculated 39 from the electron spectra of degraded PVC This is shown in Figure 7 for a sample whichwas degraded at 170 in nitrogen to a conversion of x 0165 x 10 It can be seen that the frequency of the polyene sequences rapidly becomes smaller with increasing number of conjugated double bonds It can be extrapolated that the longest sequences have about 25 to 30 double bonds 397 6 L Degradation Conversion I PVC 180 C 017 39E 5 PVBr 100 C 013 U 39r 4 U nu 3 t J 2 1 I l 1 l 1 O 5 10 15 20 25 30 35 40 Figure 7 Frequency distribution H of polyene sequences inpartially degraded polyvinyl chloride and polyvinyl bromide according to reference 34 In the same way also the degraded polyvinyl bromide can be investigated Here the average sequence length is much higher than in the case of PVC Sequences with about 12 to 14 double bonds are most frequently found the longest have about 40 to 45 double bonds Several other authors also reported about similar short average polyene sequences at the beginning of the PVC degradation 39 Geddes38 calculated from the amount of HCl split off and from the determination of the number of polyene sequences by ozonolyses average sequence lengths of about 10 to 15 From the analyses of the spectra it follows that polyene sequences of various length are formed during the degradation For an understanding of the mechanism of the dehydrochlorination it is important to know whether the sequence length distribution is changed with increasing conversion From the spectra of such PVC samples it follows that besides a small shift of the frequency distribution to shorter sequences with increasing conversion the spectroscopically found polyene concentration decreases It can be concluded from the change in the frequency distribution of the polyene sequences with increasing conversion that the polyene Sequences undergo further reactions during the thermal degradation Besides cross linking aromatic hydrocarbons such as benzene and toluene are formed and the irspectra of the polymers show aromatic structures The shift of the frequency distribution in favour of shorter sequence lengths makes it probable 184 THERMAL DEGRADATION OF POLYVINYL CHLORIDE that preferably the longer sequences undergo such secondary reactions Due to this shift of the frequency distribution no simple relationship between colour and degree of degradation of PVC can be expected With increasing temperature a shift of the frequency distribution to shorter sequences is also observed Under no circumstances are polyene sequences of remarkably greater lengths formed This means that for the mechanism of the dehydrochlorination of PVC the degradation must start on many sites simultaneously With increasing time of degradation or with increasing con version the number of polyene sequences becomes greater but not their length The zipperlike dehydrochlorination of PVC is stopped after the formation of sequences with up to 20 25 double bonds Also calculations of the dehydrochlorination kinetics of PVC and the length distribution of the polyene sequences by Kelen et al 42 are in good agreement with the experimental results If during the degradation the split off HCl is not completely removed deeply coloured complexes of HCl and the polyene are formed CH CH CH CH 2 CF CH CH CH2 C1 HCl CH2 CH CH CH Cl CH CH CH CH3 By the superposition of the blue coloured complexes with yellow to brown coloured free polyene sequences blue to olive discolourations of the polymers are obtained The complex formation is a reversible reaction after swelling in benzene the complexes can be destroyed with ammonia and can be regenerated with HCl Also basic stabilizers such as cadmium stearate react with the complexes bleaching the samples These results are also of prac tical importance because under technical conditions the split off HCl will not be removed completely from the samples This is only possible with small samples in solution or in very thin lms Thus Thallmaier and Braun40 observed on thin lms of suspensionPVC after the degradation at 170 the same spectra as on samples dissolved in tetrahydrofuran Also Onozuka and Asahina3 discussed chargetransfer complexes as the reason for the discolouration of PVC and have shown the analogue s complex formation with model compounds 5 MECHANISM OF THE THERMAL DEHYDROCHLORINATION 0F PVC For the thermal dehydrochlorination of PVC radical ionic and unimole cular elimination mechanisms have been discussed A clear decision is not yet possible and there are many contradictory results in the literature Also the experimental conditions and the history of the polymer sample play an important role and it is possible that in special temperature ranges various mechanisms are effective side by side 185 PAC 262 D D BRAUN 51 Radical mechanism In the older literature many formulations for the radical mechanism of the thermal degradation of PVC are given Experiments of Bengough and other authors48 have shown that inhibitors of radical reactions do not in uence the PVC degradation which is not in favour of a radical mechanism Thermally strongly degraded PVC gives an esr spectrum with one narrow line having a gvalue similar to that of thefree electron The reason for this paramagnetism must probably be seen in the conjugated structure of the net work formed at higher degrees of conversion However from the esrspectra no hints for a radical nature of the elimination process can be found Bamford and Fenton5 0 investigated the degradation of PVC in tritium labelled toluene with CH 2Tgroups They explained the observed incorporation of tritiu into the polymer by the following mechanism 39 CH2WCHCl gt CH2 CH C1 C1 C6H5CH2T TCl C6H5 CHzo gt HCl C6H5 CHT39 CHCH C6H5CH2T CH2 CHT C6H5CH2 W CH2 CH2 CGHS CHT From these and some other investigations the authors concluded that the mechanism of the PVC degradation is a radical one By thermal volatilization analysis of mixtures of PVC and some other polymers eg polymethyl methacrylate a retardation of the initial HCl Splitting off is always observed This was explained by a reaction between the radicals from PVC and radical fragments from other polymer due to a radical nature of the PVC degradation 52 Ionic mechanism The dehydrochlorination of PVC by bases such as lithium chloride and dimethyl formamide follows an ionic mechanism Also sulphuric acid or some heavy metal salts especially iron salts accelerate the dehydrochlori nation Baum54 and later on Rieche et 1155 also discussed an ionic mechanism for the pure thermal dehydrochlorination and thus explained the strong cata lytic effect of organic bases However up to now there is no direct proof for such a mechanism and the experimental ndings as well as theoretical considerations are contrary to this theory Imoto and Nakaya56 proposed an elimination mechanism with a cyclic transition state from calculations of the bonding energy of C Cl bondings 5 5 C1 C1 H C1 C 5 5 CH l3H CH2 ZH CHCH wCH2 CH 1 c1 H Cl C This mechanism is supported by many experiments 57 The thermal degrada tion of PVC in inert solvents is not in uenced by inhibitors for radical reactions hydroquinone gives no inhibition in the absence of oxygen 186 THERMAL DEGRADATION OF POLYVINYL CHLORIDE the autocatalysis of the thermal dehydrochlorination of PVC by HCl see Section 53 is scarcely in agreement with the radical mechanism There is a relationship between the rate of degradation in solution and the dielectric constant of the solvent2 which is characteristic for a nonradical reaction Also the kinetics of the degradation in inert solvents with a rst order rate laws 58 and the behaviour of PVC during the degradation in phenolic solvents59 are in line with the cyclic elimination mechanism Of course in the presence of basic compounds eg in dimethyl formamide solution other mechanisms are possible At higher temperatures or in the presence of oxygen or per oxides45 radical reactions are probable but up to now this has not yet been completely investigated 53 In uence of hydrogen chloride The in uence of hydrogen chloride on the thermal dehydrochlorination of PVC was for a long time a matter of dispute But in nearly all more recent publications a catalytic acceleration of the degradation by HCl was observed compare with the older literature see Geddesl Talamini et al showed this for the degradation of solid PVC61 Braun and Bender observed a higher rate of dehydrochlorination in ethyl benzoate in the presence of free HCl and also a more intensive discolouration without formation of polyen eHCl complexes The mechanism of the effect of HCl on the PVC degradation is not yet clear Below 200 a dissociation of HCl into free radicals is not very probable Van der Ven and de Witt62 therefore discuss the dissociation of HCl under formation of Cl or HCl2 39ions which as bases accelerate the dehydro chlorination HGaHCF 2 HCl H HC12 C1quot CH CH CH2 CH gt CHCH2 BC C1 1 a or HC12 quot Morikawa6 supposes that HCl reacts with double bonds in degraded PVC and influences the degradation in this way CHCH CH CHC1 3 H Cl H Thus the mechanism of the catalysis of PVC degradation by HCl is not completely clear but the accelerating effect of HCl is beyond doubt Under the conditions of technical PVC processing HCl is therefore important not only for the degradation but also for the discolouration 54 Termination of the growth of the polyene sequences The formation of polyene sequences during the dehydrochlorination can be understood on the basis of the rather high reactivity of the monomer units 187 D BRAUN adjacent to the allylic chlorine atoms which are much more reactive than a normal monomer unit of the PVC chain CHCH CH CH2 Cl It is much more dif cult to answer the question of why the polyene sequences normally only grow to a length of 5 to 10 conjugated double bonds Supposing a radical mechanism for the degradation this could be explained by a termination of the kinetic chain see reference 1 Such a formulation is not possible for a nonradical process It is also not very probable that the growth of the polyene sequences nishes at structural irregularities in the chains because such groups seem to be present only in a very small amount It can be thought that the allyl activation of the dehydrochlorination becomes smaller the longer and the more stabilized by resonance the adjacent polyene sequence is At a distinct length the energy for the dehydrochlorination and the energy content of the system by conjuga tion are about the same and the polyene formation does not continue This could also explain the observation that polyvinyl bromide under the same condition gives much longer polyene sequences than PVC Due to the weaker C Br bond compared with the C Cl bond the resonance stabiliza tion of the polyene requires longer sequences for the energetic balance At higher degrees of conversion the crosslinking of the chains may also be of importance but at lower conversions about 01 per cent no crosslinking is obtained and on average there is only one polyene sequence per macro molecule Thus up to now we have no nal explanation for the formation of relatively short polyene sequences 6 DEGRADATION IN PRESENCE OF OXYGEN During the thermal degradation of PVC in the presence of oxygen three effects are known1 l Acceleration of the dehydrochlorination 2 Bleaching of the degraded polymer 3 Lowering of the molecular weight Most authors explain these ndings by a superposition of radical and pure thermal reactions By autoxidation peroxidic macroradicals can be formed which by transfer reactions result in a branching of the kinetic chain Valko has given the kinetic scheme for the degradation of PVC in the presence of oxygen and checked this by experiments The rate of degradation is not only dependent on the reaction of oxygen with the formed double bonds but also on the number of cleaved C C bonds The importance of peroxidic structures for the degradation of PVC was shown by Geddes the hydro peroxidic groups obtained by ozonization result in a similar acceleration of PVC degradation as does the presence of cumene hydroperoxide or other peroxides Very probably under this condition the reaction follows a radical mechanism The bleaching effect of oxygen on degraded PVC is due to a shift of the sequence length distribution to shorter sequences This can be seen from 188 THERMAL DEGRADATION OF POLYVINYL CHLORIDE the spectra of polymers which have been degraded in the presence of oxygen These spectra do not show single absorption maxima which can be correlated to various polyene sequences Thus up to now it is not possible to discuss the polyene sequence length in PVC degraded in the presence of oxygen Figure 8 log 10 I x164 o under 02 IIx019 o under N2 c10 gl in ethyl benzocIte I m 1 I I I m L 360 380 100 125 150 175 500 525 550 PM Figure 8 Spectrum of PVC after degradation for 90 min at 170 in ethyl benzoate under oxygen and under nitrogen During the thermal oxidation of PVC at 1050 the molecular weight is increased due to the beginning of crosslinking here the presence of anti oxidants has a retarding in uence 7 INFLUENCE OF PLASTICIZERS ON THE PVC DEGRADATION The thermal stability of plasticized PVC is connected with the type and amount of the plasticizer Wolkober64 observed by simultaneous measure ment of the absorbed oxygen and the HCl evolution during the heating of plasticized PVC that there is a strong dependence on the oxidation stability of the plasticizer In many cases the oxygen uptake is faster than the dehydro chlorination65 and it must be concluded that the dehydrochlorination is influenced by peroxide products from the plasticizer66 67 68 Therefore many investigations were made recently on the thermal behaviour of plasti cizers in the presence of oxygen in part by use of the differential thermal analy sis The rate of the dehydrochlorination depends on the amount of plasti cizer in the PVCquot but there was found to be no linear relationship between plasticizer content and rate of dehydrochlorination For each plasticizer at a special concentration a minimum for the rate of degradation is found Figure 9 It is possible that at lower concentrations the interactions between the polar groups in PVC and the plasticizer molecules are stronger than be tween the PVC chains Therefore in this range a higher degree of order is obtained than in the absence of plasticizers and thus a higher energy for the HCl splitting off would be necessary Thus the solvation of the PVC chain by 189 D BRAUN J 05 J j 5 O l o O 03 o s 1 1 l g 0 00L 008 012 moles plusticizer primary moles of PVC Figure 9 Rate of dehydrochlorination of foils of plasticized PVC 180 under nitrogen 0 DOS Dioctyl sebacate O TCP Tricresyl phosphate A DBP Dibutyl phthalate the plasticizer molecule could have some stabilizing effect However a complete understanding of these interesting effects is still open 8 CONCLUSIONS In recent years remarkable progress was made in the eld of basic research about the thermal degradation of PVC From many experiments it is fairly clear nowadays that the degradation is initiated by allylic structures within the PVC chains However it is not clear up to now how these structural irregularities are formed and what can be done to avoid their formation It seems that the branches in PVC do not have such a great importance for the rate of degradation The mechanism of the thermal degradation is also not yet completely clear Under inert conditions a unimolecular cyclic mechanism is very prob able but only a little is known about the degradation under practical con ditions eg in the presence of oxygen The discolouration of PVC nowadays can be understood and described in a semiquantitative way by the length and and the frequency distribution of the polyene sequences Up to now we do not have quantitative relationships between colour and degree of degradation Finally the large eld of the stabilization of PVC should be mentioned Here we have very little knowledge about the reactions between PVC and stabilizers and we need much more information which could also be useful for the development of new stabilizers for PVC ACKNOWLEDGEMENT The author s own investigations in this field were supported by Arbeits 190 THERMAL DEGRADATION OF POLYVINYL CHLORIDE gemeinschaft Industrieller F orschungsvereinigungen EV which is ack REFERENCES 1 W C Geddes Rubber Chem Technol 40 177 1967 2 G C Marks 1 L Benton and C M Thomas Soc Chem Ind Iondon Monograph No 26 s 204 1967 3 M Onozuka and M Asahina J Macromol Sci C 3 235 1969 4 B Dolezel Materie Plast Elast 35 1514 1969 5 D Braun and M Thallmaier Kunststoffe 56 80 1966 6 J Novak Kunsts toffe 51 712 1961 7 H Luther and H Kruger Kunststo e 56 74 1966 8 G Schramm Kunststojjfe 58 697 1968 9 P Q Tho and P Roux Chim Anal Paris 48 448 1966 1 R Salovey and H E Bair ACS Div Polymer Chem Polymer Preprints 111 230 1970 1 P Smilek Plast Massen and Kautschuk Prag 6 203 1969 12 B Dolezel and M Pegoraro Materie Plast Elast 35 1259 1969 13 M Asahina and M Onozuka J Polymer Sci A 2 3505 3515 1964 14 V Chytry B Obereigner and D Lim Eur0pean Polymer J Suppl 379 1969 5 A Maccoll Chem Rev 69 33 1969 5 J D Cotman jr Ann NY Acad Sci 57 417 1963 J Amer Chem Soc 77 2790 1955 17 G Boccato H Rigo G Talamini and F ZilioGrandi Makromol Chem 108 218 1967 18 L Binder Dissertation T H Wien 1962 9 D Braun and W Schurek Angew Makromol Chem 7 121 1969 20 A Caraculacu J Polymer Sci A 1 4 1829 1839 1966 21 A Caraculacu E C Bezdadea and G Istrate J Polymer Sci Al 8 1239 1970 22 D Braun and F Weiss Angew Makromol Chem 13 55 1970 23 D Braun and F Weiss Angew Makromol Chem 13 67 1970 24 V P Gupta and L E St Pierre J Polymer Sci Al 8 37 1970 25 W I Bengough and M Onozuka Polymer London 6 625 1965 26 D Braun and W Quarg Unpublished 27 D Braun and M Thallmaier J Polymer Sci C 16 2351 1967 23 D Braun and W Quarg Unpublished 29 J Landler and P Lebel J Polymer Sci 48 477 1960 3 G Zeppenfeld Makromol Chem 90 169 1966 3 B C Achhammer Anal Chem 24 1925 1952 32 F E Bailey J P Henry R D Lundberg and J M Whelan J Polymer Sci B 2 447 1964 33 N Musayama and Y Amagi J Polymer Sci B 4 115 1966 34 D Braun and M Thallmaier Makromol Chem 99 59 1966 35 F Sondheimer D BenEfraim and R Wolovski J Amer Chem Soc 83 1675 1961 5quot W Kraus and H Grund Z Elekrrochem 58 767 1954 37 R R Stromberg S Straus and G B Achhammer J Polymer Sci 35 129 355 1959 33 W C Geddes European Polymer J 3 747 1967 39 G C Marks 1 L Benton and C M Thomas SCI Monograph No 26 S 204 4 M Thallmaier and D Braun Makromol Chem 108 241 1967 4 T Kelen G Balint G Galambos and F Tiidos European Polymer J 5 597 1969 42 T Kelen G Galambos F Tudos and G Balint European Polymer J 5 617 629 1969 6 127 1970 43 R Schlimper Plaste Kautschuk 13 196 1966 14 657 1967 4 L Valko J Polymer Sci C 16 5451979 1967 5 W C Geddes European Polymer J 3 733 1967 46 K S Minsker E O Kratz and I Pakhomova Vysokomolekul Soedin A 12 Nr 3 483 1970 47 K Kurzweil and P Kratochvil Collection Czech Chem Commun 34 1429 1969 48 W I Bengough and H M Sharp Makromol Chem 66 31 1963 49 I Ouchi J Polymer Sci A 3 2685 1965 5 C H Bamford and D F Fenton Polymer London 10 63 1969 51 I C McNeil and DNeil Makromol Chem 1172651968Eur0pean Polymer J6 143 1970 52 l P Roth P Rempp and l Parrod J Polymer Sci C 4 1347 1963 191 as D BRAUN 53 Z Wolkober J Polymer Sci 58 1311 1962 54 B Baum SPE J 17 71 1961 55 A Reiche A Grimm and H Miicke Kunststo e 52 265 1962 56 M Imoto and T Nakaya Kogyo Kagajaku Zasshi 68 2283 1965 57 D Braun and R F Bender European Polymer J Suppl 269 1969 58 W I Bengough and H M Sharpe Makromol Chem 66 31 1963 59 I K Varma and S S Grover Angew Makromol Chem 7 29 1969 6 W I Bengough and G F Grant European Polymer J 4 521 1968 61 G Talamini G Cinque and G Palma Materie Plastiche 30 317 1964 62 S van der Ven and W F de Witt Angew Makromol Chem 8 143 1969 53 T Morikawa Chem High Polymers Japan 25 505 1968 54 W Reicherdt Z Wolk ber and H Krause Plaste Kautschuk 13 454 1966 65 Z Wolk ber Angew Makromol Chem 3 38 1968 66 D Weichert Plaste Kautschuk 14 798 1967 67 J Stepek C Jirkal and J Menniker Plast Mod Elast 2010 119 1968 68 L Duch ne and R de Broutclles Rev Gen Caoutchouc Plast Edition Plast 5 315 1968 69 K Ogino and M Hirano J Chem Soc Japan 1nd Chem Sec 72 2337 1969 7 J Millan and D Braun Angew Makromol Chem 9 186 1969 STABILIZATION of POLYVINYL CHLORIDE OPTIMUM STABILIZER SELECTION George A Skip Thacker The process of recommending an optimum stabilizer for a particular vinyl application then selecting and evaluating that stabilizer can be a formidable task This is especially due to the vast numbers of stabilizers which are marketed to the PVC industry worldwide There are many well regarded stabilizer producers in North America Europe and the Far East as well as many more producers in Latin America and India Most of these offer many commercially available stabilizers in their product lines some in the dozens One reason so many stabilizers are offered to vinyl compounders is that the PVC compounding industry has become more astute and sophisticated over the years requiring development of stabilizers to fill many specific requirements other than simply to provide process heat stability for PVC compounds Another reason for the large selection of stabilizers centers on the wide diversity of vinyl applications and methods of processing PVC is truly the most versatile of all polymers The total HeatShear History of vinyl compounds can vary significantly in both amount and type among different applications This concept of Heat History and The total capacity of the stabilizer system to furnish the required protection is perhaps an oversimplified way of integrating the total energy input of all types to which a vinyl compound is subjected over a product s useful life span The shear and heat energy of mixing cycles dryblend banbury high speed plastisol dispersators processing calender extruder molder fabricating embossing thermoforming laminating scrap re work heat and light energy of outdoor exposure heat of a product s use environment auto interior hot air duct Gamma ray sterilization and other end use requirements all contribute to overall PVC degradation The stabilizer system must be able to furnish adequate protection at every stage during the production and useful service life of the vinyl product The amount and type of energy input varies considerably among the many different production methods and enduse applications of PVC Also the accumulated storage history of PVC resins before compounding can vary Resin degradation actually starts in the polymerization reactor and can continue under storage conditions through oxidation carbonyl formation etc even before use The purpose of this discussion is to present a logical sequence of factors to consider in selecting a stabilizer in order to provide a practical means first to select a generic group or class of stabilizers and secondly to choose a specific stabilizer within the class to fully optimize the selection Theoretical discussion of PVC stabilization and chemical mechanisms offered to explain the hows and whys of stabilizer behavior will not be presented in this treatise GENERAL STABILIZER GROUPS There are several basic groups of heat and light stabilizers currently offered to the vinyl industry 1 Mixed metalorganic acid salts liquid and solid consisting of any one or a combination of Barium Calcium Cadmiumdisappearing and Zinc Typically C8 to C18 straight chain or branched chain aliphatic carboxylic acids are used Aromatic alkyl benzoic acids once used are no longer in favor due to toxicity concerns 2 Organotin compounds liquid and solid mainly mercaptoester and mercapto alcohol types sulfur containing and carboxylate or dicarboxylate nonsulfur ester types 3 Lead salts and soaps liquid and solid 4 CalciumZinc carboxylate packages liquid paste powder containing USFDABGA sanctioned ingredients for nontoxic applications 5 Organic and miscellaneous types including alkylaryl phosphites epoxy compounds betadiketones amino crotonates nitrogen heterocyclic compounds organosulfur compounds le ester thiols hindered phenolics and polyolspentaerythritols These types currently are being heavily researched and their use at the expense of metalcontaining stabilizers is expected to grow significantly 6 A minor group consisting of carboxylic or mercaptoester salts of antimony strontium potassium Generally speaking flexible vinyl compounds which are calendered extruded or molded and plastisol compounds most commonly are stabilized with 1 the mixed metal BaZn CaZn systems Applications requiring USFDA or BGA sanction for direct or indirect food contact and most medical applications utilize the 4 nontoxic CaZn systems Rigid vinyl compounds for extrusion and molding are stabilized most frequently with 2organotin mercaptides in North and South America and parts of the Far East and 3Lead or1 Mixed Metal systems in Europe Flexible vinyl compounds used in electrical wire coating applications are mostly 3 Lead stabilized since leads offer the best electrical properties due primarily to the insolubility of lead chlorides formed during stabilization Currently lead is under pressure for possible replacement by special mixed metal systems in secondary and decorative wire insulation However primary insulation is still best stabilized by lead FACTORS GOVERNING OPTIMUM STABILIZER SELECTION The factors to consider in making an optimum stabilizer selection for a given vinyl process can be divided into three broad categories Formulation Variables Process Variables Enduse Properties Formulation variables such as PVC resin impact modifying resins processing aids plasticizers fillers pigments lubricants foaming agents and other miscellaneous ingredients can all have an effect upon or be affected by the stabilizer Process Variables which can be a determinant in stabilizer selection include consideration of compounding procedures Banbury Dryblend Plastisol Mixing calenderingextrusion single or twin screw injection molding blow molding coating molding or dipping of plastisols and organosols solution vinyl systems foamed vinyls and fluidized bed powder coating Other considerations of processing include melt rheology plastisol viscosity properties plateout scrap rework etc End Use Properties that affect stabilizer selection include clarity outdoor weathering toxicity stain resistance sulfide urethane asphalt impact strength heat distortion electrical properties odor effects of moisture haze dimensional stability heatsealing and printability and fogging automotive and food wrap film FORMULATION VARIABLES Vinyl Resins The wide variety of differing types of PVC resins which require stabilization and the large number of resins available within each variety probably is the single greatest factor which explains the large number of commercially available stabilizers confronting the vinyl compounder PVC homopolymer is produced by suspension bulk or mass and emulsion polymerization methods The amount and type of residual components on the resin shipped to users catalyst residues suspension agents emulsifying agents etc can differ to the extent that even vinyl resins manufactured by the same polymerization method from different producers can vary in their response to a given stabilizer system Moreover since the Vinyl Chloride Monomer crisis of the early 1970 s efforts to reduce residual VCM by steamstripping and more rigorous drying procedures added another dimension to resin variability since no two producers techniques were identical Pink or offwhite PVC sometimes found its way to customers especially during tight supply periods Postchlorinated PVC High Temp with a very narrow processing window offers additional stabilization challenges Vinyl acetate vinyl chloride copolymers and vinyl chloride with other comonomers propylene cetyl vinyl ether vinylidene chloride also differ widely in their response to a given stabilizer system One of the most striking differences in stabilizer response occurs with mixed metal BaZn CaZn and formerly BaCdZn stabilized formulations This phenomenon is known as Zinc Sensitivity and is seen as a drastic discoloration even burning in PVC compounds subjected to progressive exposure to heat PVC resins can vary considerably in their response to varying zinc levels of a stabilizer system Years ago most then commercially available vinyl resins were classified as to Zinc Sensitivity in addition to the ASTM D175560T 1960 cell classification covering other resin properties This classification unfortunately has fallen by the wayside over the ensuing years but is still a viable tool for the astute compounder In the figures Zinc Sensitivity we see the different response of four PVC resins each stabilized with a series of three BaCd systems differing only in the level of Zn and exposed in an oven at 350 degF for an increasing number of 10 minute intervals PVC resin A exhibits slight improvement as the zinc content is increased and we see that the presence of zinc in the stabilizing system for this resin is beneficial The stability of resin B is neither enhanced nor materially retarded with increasing levels of zinc Stability of resin C is moderately affected by increasing zinc levels of the stabilizer system while resin D is extremely sensitive to the presence of even small quantities of zinc Many vinyl acetate vinyl chloride copolymers are extremely zinc sensitive and would be comparable to resin D In fact the zinc sensitivity of a vinyl acetatevinyl chloride copolymer is directly proportional to the vinyl acetate content of the copolymer Ell E E H EFT WI 17 Rein A flat responsivequot l Harem E Err tolerant l t Elft EEHELTI uzlw I 1d u an an In an i l Home I 3 lift Wormnit 1 Emiln D w l u Eztn w ftLna g r E l A word should be said at this point about the contribution to stability of the various components in a mixed metal stabilizer system Our example is a BaCd Zn system although cadmium is no longer used by most As can be seen in the figure Effect of Stabilizer Components the barium component contributes long term stability with poor initial color as does the epoxy component Cadmium furnished extremely good initial color and intermediate stability while zinc provided very good initial color stability and very poor long term stability The zinc sensitivity phenomenon described above applies to a lesser extent to cadmium as well E FFEIEIT Elli ETa tEtLlEER G MH EHIE l i i a a t l E t fm 3 amp ETHEHG I39E EUMF ENEHT lm ll illTI CI HE El TIEi iiiEli 75393quot W W391 m Enig d Ba39li 39F E Bay 592 Epecrtfalr u loin Eat Emiri F Ba Etta 219 E may The catastrophic degradation which occurs with zinc sudden blackening amp burning is typical and is due to the fact that zinc chloride which is formed when the zinc salt s carboxylic acid displaces labile chlorine on the polymer chain is a strong Lewis Acid and a degradation catalyst for PVC The phosphite component provides a measure of long term stability by itself In the figure Synergy Component Combinations the significant improvements in heat stability which can be obtained by combining the various stabilizer ingredients is seen BaCd and BaCdZn systems benefit upon addition of the phosphite which is believed to chelate or tie up zinc and cadmium chlorides formed during the process of stabilization For this reason phosphites are known to retard the zinc burning effects described above The further addition of the epoxy component to a mixed metal system results in dramatic improvement in heat stability more than simply additive and truly synergistic recall epoxy performance alone It has been shown that a zinc sensitive resin can be made more zinc tolerant simply by washing the resin free of trace amounts of residual catalysts suspending agents or emulsifying agents Vinyl acetate vinyl chloride copolymers as mentioned previously are quite zinc sensitive depending on acetate content and use of zinc in the stabilizer system should be avoided The one exception to this recommendation will be discussed under the subject of Fillers Other types of copolymers generally respond to stabilizers in a manner similar to PVC suspension homopolymers exhibiting varying degrees of zinc sensitivity Most all of these copolymers propylene cetyl vinyl ether modified PVC have greater inherent heat stability than the acetate copolymers Bulk or mass polymerized PVC exhibits heat stability quite similar to corresponding Kvalue suspension PVC resins both responding well to a wide variety of mixed metal organotin and lead stabilizer systems Emulsion polymerized PVC resins of today are mainly the paste or plastisol dispersion resins having very small smooth surfaced particles Like suspension polymers most dispersion resins respond well to mixed metal and organotin stabilizer systems The older high soap emulsion resins mainly European did not respond as well to mixed metal stabilizers relying more on organotin and organic amine types of stabilizers Most of the emulsion resins of today have a much lower soap or residual emulsifier content This residual emulsifier probably explains why the use of minor amounts of emulsion resin in a general purpose suspension PVC calendering or extrusion formulation can help reduce plateout Modifying Resins Several different classes of thermoplastic resins are used with PVC to enhance strength andor processing and fusion of rigid PVC as well as to modify properties of flexible PVC such as the retention of embossing during postforming operations These modifying resins include chlorinated polyethylenes CPE s ethylene vinyl acetatecarbon monoxide terpolymers Modified EVA s acrylonitrile butadiene styrene ABS methacrylate butadiene styrene MBS and acrylic polymers Some of the newer thermoplastic elastomers that are PVC compatible Alcryn find occasional use in specialty flexible formulations When incorporated with PVC some of these modifiers can detract from heat and light stability to varying degrees Generally no basic change in the stabilizer system is required although a slightly higher level of stabilization may be needed depending on severity of the process The EVA terpolymers and chlorinated polyethylenes are perhaps the least detrimental towards heat and light stability and perform very well in rigid applications requiring good outdoor weathering Acrylic polymer modifiers also are recommended highly for outdoor exposure applications Some acrylic and ABS modifiers are good for clear vinyl applicationsminimal stresswhitening but detract somewhat from heat stability The nitrile portion of ABS especially is thought to detract from PVC s heat and light stability Impact modifiers can be classed as either matrix types functioning via chain entanglement with PVC molecules or discreet particle types functioning via resinrubber interfaces or shockabsorbers to block crack propagation EVA terpolymers and chlorinated polyethylene are examples of the former and acrylic MBS and ABS represent the latter types of modifiers The amount of impact strength obtained with these modifiers is as dependent upon processing conditions of extrusion or calendering as on the amount and type of modifier used The degree of chain entanglement or the presence of sufficient resinrubber interfaces to act as shock absorbers during the process of absorbing and dissipating energy of impact requires optimum dispersion of the impact modifier Too much or too little work applied during processing an impact modified PVC compound can furnish less than optimum impact strength Thus a stabilizerlubricant system which is quite lubricating may contribute to a condition of minimal work input while a stabilizer lubricant system which has poor lubricity may contribute to a condition of overwork and high frictional heat buildup Processing impact modified rigid PVC requires a delicate balance of internal external lubrication some of which may be contributed by stabilizer More will be said on this under the Lubricants topic The use of certain acrylic polymers as processing aids is widespread In addition to contributing towards smooth melt flow and good surface finish they tend to promote earlier faster fusion of PVC at a given temperature and can even lower the fusion or gelation temperature of the compound In addition to acrylics poly alpha methyl styrene has been used on occasion as a process aid requiring a higher use level to acheive equivalent performance to acrylic types Plasticisers In over simplified terms the word plasticiser can be used to describe any material which is incorporated into a vinyl formulation to impart elastomeric properties of flexibility elongation and elasticity to the compound Mostly liquid and sometimes solid in form these materials are generally organic compounds of fairly low volatility The most commonly used plasticisers include esters of aromatic and aliphatic dibasic acids glycol diesters of monobasic acids linear polyesters epoxidized glycerides and monoesters phosphate esters aromatic hydrocarbons and aliphatic chlorinated hydrocarbons Plasticisers are classified in terms of their efficiency permanence low temperature flexibility compatibility and solvating power on PVC resin Generally the greater the polarity aromaticity or degree of chain branching the greater will be the solvating power compatibility and efficiency of a plasticiser On the other hand low temperature flexibility performance is generally enhanced with a decrease in overall compatibility and solvating power within a series of plasticisers The older arbitrary definition of primary or secondary plasticisers is not entirely the case since there exists a complete spectrum of compatibility covering the entire range of plasticiser types There is not always a sharp distinct difference between a primary and a secondary plasticiser although the phthalates trimellitates adipates and azelates would be considered in the primary arena and hydrocarbons and chlorinated paraffins in the secondary arena by most With two notable exceptions choice of stabilizer is not materially affected by the type or amount of plasticiser present in a formulation Phosphates and chlorinated paraffins generally require the use of higher levels of epoxy costabilizer and additional phosphite chelator in a mixed metal bariumzinc or calciumzinc system Oddly enough both of these plasticiser types which are detrimental to heat stability also are among the few materials used to impart fireretardancy to flexible PVC compounds Epoxy plasticisers epoxidized soybean linseed oils and tallate esters are unique in the fact that they are almost universally used in flexible vinyl compounds at low 310 parts as an auxiliary costabilizer with mixed metal stabilizer systems Functioning as HCl acceptors primarily epoxy plasticisers significantly enhance the heat and light stability of most mixed metalphosphite stabilized compounds Fillers lnert solid inorganic mineral compoundsmany occurring as natural products are used in vinyl formulations as extenders for purposes such as reducing overall costs except that poundvolume costing must be considered if appropriate to the product s pricing providing opacity and acheiving certain desirable enduse properties Such enduse properties might include abrasion resistance tear strength dry hand or feel to the touch hardness and stiffness and even with alumina trihydrateATH fire retardency Included in this group of materials in addition to aluminum trihydrate are a variety of grades of calcium carbonates and silicates such as clay kaolin talc and years ago no longer used asbestos The most commonly used filler in PVC formulations is calcium carbonate in its various forms depending on scource ground limestone marble chalk or precipitated Although calcium carbonate itself does not detract from nor contribute to heat stability there is a definite need to alter the metal ratio of mixed metal stabilizer systems with increasing levels of calcium carbonate usage to acheive optimum results This effect is seen in the figure Effect of Filler on Zinc Sensitivity EFFEET tZiF EmilLlIEEH E mF ti Elil TE mired 1 39iiiil39iltift5 Has Stabilizer at n59quot Ellied and prams polishd Milled and twated 15 minutesquot at WET and press Catt actuate I 39 polished Ea 39ctcaatre HE E39 Milled and heated IEminuteE at in ewrate Wait and praise polished LE EFFECT GP FILLEE Gitl amt EEHSiTW T39t39 In Edda apnoea i i 5 mail l I E 1 313 i The low zinc containing stabilizer exhibits better heat stability than the higher zinc stabilizer in a clear unfilled PVC formulation a somewhat zinc sensitive formulation However when that same formulation also contains a significant amount of calcium carbonate filler the higher zinccontaining stabilizer furnishes better heat stability both initial color and long term than the low zinc stabilizer Thus we can see that with increasing levels of calcium carbonate filler the need for zinc in the stabilizer system also increases A further case in point to illustrate this is the recommended use of high zinc stabilizers with normally very zinc sensitive PVCacetate copolymers in highly filled flooring products homogeneous vinyl floor tiles Flexible PVC products can use the lower cost 36 micron average particle sized calcium carbonate fillers but rigid PVC benefits from use of finer 12 micronor less carbonate fillers primarily to maintain impact strength A 2 micron ground filler can have a small top size portion of 1015 microns and these rocks will kill impact The class of silicate fillers offers no great stability problem and usually an increase in the amount of epoxy plus inclusion of an additional 05 part of a phosphite will be sufficient to overcome any stability problem that might occur ATH alumina trihydrate behaves very much like calcium carbonate in terms of stabilizer considerations As a fire retardant smoke suppressant filler ATH can be used in flexible PVC which is processed below the temperature at which ATH gives off its water of hydration 425 degF However the rigid PVC processing window usually is close enough to ATH water emission temperatures to cause premature water release and porosity in the PVC product Pigments Much has been published about the comparative heat stability light stability chemical resistance oxidative resistance etc of the many organic and inorganic dyes and pigments commonly used in the vinyl industry If the PVC stabilizer system has been selected for its maximum properties in terms of resin plasticiser filler content process and end use properties it will generally suffice in protecting the pigment as well However a couple of special cases need to be mentioned Metallic pigments to acheive unusual decorative effects which are based upon finely divided aluminum copper gold or bronzeespecially the latter three generally retain best color stability in the presence of alkaline stabilizers such as those based on barium alkyl phenate and specifically those alkaline stabilizers which contain little or no zinc Lower epoxy use levels and the use of inert lubricants such as mineral oil or low molecular weight polyethylene are also recommended in place of acidic lubricants such as stearic acid Fluorescent pigments are best stabilized for both heat and light exposure with alkyltin mercaptoester stabilizers Good results are also obtained with high zinc mixed metalphosphite systems Lubricants One of the least understood for years and yet most important aspects of PVC technology is the phenomenon of lubricity The critical need to understand the nature of lubricity is especially important in rigid PVC technology since it is difficult to separate completely lubricity and stability considerations during a rigid PVC process Hillrglg l39IE illil i ihd ll Jihad l ghw 11 12M Jllr Lif r I iinil vtt Lnj39i139ti t i l g u w v iLuq Pvt hihtf 39 FFHTI T tn Hitmar withnu Fla DE39iquotigiiiIil39If l5IELI muh39rtitl Elf 394 H39E L39 quot3 I mnenacm H quot r w 39 Ful lle al urEE39fili39 Lubricants can be classified to some extent by relating chemistry and behavior the terms internal and external have been used to describe the nature of lubricants with respect to their use in PVC compounds However more realistically there is a spectrum of lubricating behavior from the internal lubricity of polar molecules such as stearic acid metal stearates fatty acid esters and glycerides to the materials containing both polar groups and long carbon chains giving balanced internalexternal characteristics and finally to the external lubricity of long chain hydrocarbon derivatives of paraffin oils paraffin waxes and low molecular weight polyethylenes oxidized and nonoxidized PE s lnternal lubricant behavior showing some PVC solubility contributes to lower melt viscosities and reduces internal friction between polymer molecules a plasticising effect Faster fusion is also seen with internal lubricity behavior External lubricants function essentially by their insolubility in PVC migrating to the surface where they can reduce frictional drag between PVC melt and hot metal surfaces of the extruder calender etc This type of lubricant behavior is very dependant on its molecular weight and melting point which determines where in the process ie barrel adapter die it will deliver the desired lubricity Current lubrication technology makes use of these internal and external characteristics in the form of specially designed lubricant onepacks designed for specific processing and product applications Stabilizer selection in light of lubricity requirements is especially critical in rigid PVC processing One exampleHigher stabilizer levels are needed when NN ethylene bisstearamide wax is used due to the degrading effects of amides on PVC resin Most all other lubricant types internal or external have no adverse effects upon PVC heat stability and respond well to normal stabilizer use levelslt is known that tin mercaptoester stabilizers contribute to lower melt viscosities than tin carboxylates bariumzinc calciumzinc and lead stabilizers Most tin mercaptide stabilizers are essentially nonlubricating but based on their compatibility and viscosity effects they might be thought to contribute some internal lubricity behavior Therefore tin mercaptides generally need more external lubricant in rigid PVC processing than the mixed metal or lead types Lubrication requirements for plasticized flexible PVC are not nearly so critical Stearic acid and stearyl alcohol are by far the most common lubricants in the industry and work well with a wide variety of stabilizers including most mixed metal CaZn BaZn systems Stearic acid is especially recommended with alkaline stabilizers based upon barium phenates Use of stearic acid should be avoided however with tin mercaptide stabilizers since highly incompatible alkyl tin stearates may be formed which can result in exudation or spew in plasticised formulations Miscellaneous Formulation Components Other additives used in many vinyl formulations include blowing foaming agents to be covered under the subject of vinyl foams wetting agents for plastisol viscosity control biocides antistatic agents antifog agents for food wrap film and UV absorbers to be covered under the subject of light stability and outdoor weathering Care must be taken in selecting a stabilizer for vinyl compounds containing any of these miscellaneous components since many of these additives may react with certain stabilizers to detract from heat stability or furnish colored reaction products For example some antistatic agents quarternary ammonium compounds can reduce heat stability significantly Some wetting agents also detract slightly from heat stability requiring a slightly increased stabilizer level Certain UV absorbers may form a yellow reaction product with alkaline stabilizers requiring a higher stearic acid level to counteract this problem Some UV absorbers triazoles may exhibit a pinking tendency with tin mercaptide stabilizers The best approach in most cases is to evaluate heat stability and color retention of the formulation with and without the miscellaneous additive to predetermine the extent of any potential problem PROCESS PRODUCTION VARIABLES Calendering Most general purpose plasticised calendered film and sheeting both unsupported and supported is extremely well stabilized with liquid mixed metal bariumzinc or where permitted barium cadmiumzincphosphiteepoxy systems the selection of which depends upon such factors as resin zinc sensitivity filler content clarity needs plateout tendencies etc The newer bariumzinc calciumzinc and zrnc cadmiumfree packages fortified with beta diketones nitrogen heterocycles or the new organosulfurZinc compounds also work very well in calendering of flexibles Special situations in flexible calendering would include production of high speed thin gauged calendered film which may require use of small amounts of powdered barium or zinc stearate boosters to the liquid system These will add a touch of lubricity to handle the higher frictional heat buildup that occurs in the calender roll nips at thinner below 10 mils gauges as far as I ve gone so far PL TE DDT C DLJGH ETAHDRD mga Watchung Filed Pigment 31hr o a 4 a e m if Ha 145 it EFFEET CH RESIN DH FLATE EDT EmutEiitan Smsenainri Ei nspension Ste l a ii iiiBrquot Lt tauEd Ettab liIer E ELE EdiEn A processing problemPlateout EEFEET EIHE Eli ULFiIEPE ST AIHINE Eag d BEu dEn Ea n Phil PEEP EFFECT EIF ETAELEIEE E GM ELAHFT aw A i M H H amingswam EaC d lmma t ht E th nyain Bil Ed Lanial ng rgtq Ehwpmna Enduse factors Sulfur Staining and Clarity
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