Effects of 6-anilino-1,3,5-triazine-2,4-dithiol,zinc stearate,and barium stearate on the thermal stabilization of PVC

Individual action and synergistic effect in the combination of 6-anilino-1,3,5-triazine-2,4-dithiol (AF), zinc stearate, and barium stearate on the color stabilization of PVC were investigated. In this system, AF selectively reacts with allylic chlorine atoms in PVC. Consequently, unstable allylic chlorine units were converted to thermally stable allylic structures, thus retarding the development of polyene sequences. Zinc stearate accelerated the reaction of AF with allylic chlorine atoms in PVC, forming the zinc salts of AF (AFZ_nSt, St?C₁–H₃₅COO? ) by reacting with AF. Barium stearate reacted with ZnCl₂ which is formed in the above reaction to give St₂Zn and BaCl₂. Consequently, barium stearate led to the selective reaction of AF with allylic chlorine atoms in PVC and the remarkable retarding effect of discoloration of PVC.     

New insights into the degradation mechanism of poly(vinyl chloride), based on the action of novel costabilizers. I.

Recent legislation introduced to limit the use of heavy metal stabilizers (cadmium based) in poly(vinyl chloride) (PVC) has necessitated the use of organic costabilizers as adjuncts to alternative main stabilizer systems (barium/zinc or calcium/zinc). It has been suggested in the literature that costabilizers substitute allylic chlorine by a C‐alkylation reaction; costabilizers act as reverse catalysts for the initiation of degradation by complexing the π‐electron sites that would otherwise have an activating affect on labile chlorines; and costabilisers destroy carbonylallyl active sites by proton donation. To rationalize this debate, the focus of this paper is to elucidate the type of interactions that occur between a model compound for PVC and the novel costabiliser N‐phenyl‐3‐acetylpyrrolidin‐2,4‐dione (P24D). The model compound chosen was 4‐chloro‐2‐hexene (4C2H), which simulates the in‐chain allylic chlorine impurities present in PVC and are considered the most labile defects present in the polymer. Results suggested that stabilisation involves concerted reactions involving metal complexes rather than a series of stepwise reactions. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2731–2743, 2004     

Structural and mechanistic aspects of the thermal degradation of poly(vinyl chloride)

A critical review of the title subject supports the following major conclusions. Thermal dehydrochlorination of poly(vinyl chloride) (PVC) begins with internal allylic chloride and tertiary chloride structural defects formed during polymerization. The tertiary chloride is associated with 2,4-dichloro-n-butyl, 1,3-di(2-chloroethyl), and chlorinated long branches. Mechanisms for the formation of all of the labile defects are well established. ‘Carbonylallyl’ structures and certain isotactic conformers of ordinary monomer units are unimportant as initiators of thermal dehydrochlorination. Both the initiation and the subsequent formation of conjugated polyene sequences occur via carbenium chloride ion pairs or by a closely related concerted four-center quasi-ionic route. Six-center concerted processes, pathways involving free radicals, and other mechanistic schemes suggested recently are not involved in polyene elongation. However, during thermal degradation, ordinary monomer units are converted into internal allylic chloride defects by a mechanism that may include the abstraction of hydrogen by triplet cation diradicals derived from polyene intermediates. Cyclization reactions seem likely to contribute to the termination of polyene growth. When PVC is thermolyzed in blends with other polymers, unusual kinetic phenomena are detected that remain to be fully explained.     

Use of N‐(N′‐arylamino)maleimides to improve the thermal properties of poly(vinyl chloride) through chemical modification and graft copolymerization

The reaction of poly(vinyl chloride) (PVC) with N‐(N′‐arylamino)maleimide derivatives was studied. The thermal stability of the modified polymer was improved markedly when compared with that of the unmodified polymer. The stability improvement was attributed to the replacement of the labile chlorine atoms by more stable organic groups. The modified polymer also showed a lower extent of discoloration when compared with that of unmodified PVC. In order to introduce a polymeric stabilizer into PVC, the dienophilic monomer was chemically grafted onto the polymeric chains. The mechanism of the chemical modification as well as that of the graft copolymerization are discussed. J. VINYL ADDIT. TECHNOL., 2008. © 2008 Society of Plastics Engineers.     

On the thermal degradation of poly(vinyl chloride). III. Structural changes during degradation in nitrogen

The structural changes in poly(vinyl chloride) during thermal degradation in nitrogen at 190°C have been investigated. From gel permeation chromatography analyses no chain scission, but only crosslinking reactions were observed. An increase in the molecular weight was measured even at 0.3% conversion. For longer polyene sequences and at higher conversions, a crosslinking reaction competed with the “zipper” propagation. The secondary reactions, were more extensive at longer polyene sequence lengths. The growing polyene sequences can be terminated not only by branching reactions but also at existing pendent chloromethylene groups. A decrease in the amount of short chain branching with conversion also indicated other types of secondary reactions. Such a decrease was also observed during thermomechanical degradation in a Brabender Plastograph. The average polyene sequence length was calculated to be around 10, depending somewhat on the type of analysis used. Although allylic chlorine atoms seem to be the main points of initiation, other sites cannot be excluded as the number of initiation points increases appreciably during the early stages of the degradation. Such an increase is, of course, also consistent with a radical mechanism.     

Modification of polyvinylchloride in solution or suspension by nucleophilic substitution

Nucleophilic displacement of chlorine from polyvinylchloride (PVC) suspended either in water or in solution can be achieved using a nucleophile such as a sodium thiolate (R? S^?, Na⁺). The nucleophilicity of the salt increases if an ether linkage exists in a β position to the thiol group [R? O? (CH₂)₂? S^?, Na⁺]. Addition of a solvent such as cyclohexanone (which is a good solvent for PVC) to the slurry increases substantially the degree of substitution. Elemental analysis shows that every chlorine displaced from the polymer is replaced by a thiolate group; thus almost no dehydrochlorination occurs. It is possible to obtain by this method a grafted polymer in which 33% of the chlorine atoms are replaced by the β-ether thiolate group. The resulting polymer behaves like an internally plasticized PVC. The higher the degree of substitution of chlorine, the greater the flexibility of the grafted polymer is. Other compounds, such as lauryl thiolate and diethyl dithiocarbamate, also were used as nucleophiles, the latter one resulting in a brittle crosslinked polymer whenever sulfur was above 2%. Grafting of polytetrahydrofuran from PVC also was achieved using tetrahydrofuran as solvent and silver-perchlorate as catalyst. The resulting flexible grafted polymer is believed to consist of longer chains of polytetrahydrofuran grafted from few displaced chlorine atoms along the PVC chain.     

Kinetics and mechanism of the thermal degradation of poly(vinyl chloride)

The kinetics of the thermal degradation of solid powdered poly(vinyl chloride) (PVC) under nitrogen was studied by thermogravimetry, rate of hydrogen chloride evolution, and rate of polyene sequence formation. These results are accommodated by a chain mechanism involving initiation by random dehydrochlorination at normal monomer residues of PVC, and a series of intermediates, each leaking to a stable conjugated polyene sequence. Structural irregularities such as allylic and tertiary chlorine are responsible for a fast initiation process at the very beginning of the degradation. Mean rate constants and activation parameters for random initiation, propagation, and termination reactions of the PVC degradation chain were calculated by simulation. Activation enthalpy/entropy correlations for the experimental data available for dehydrochlorination of chloroalkanes and chloroalkenes in the gas and in the liquid phase or nonpolar solvents and elementary reactions of PVC degradation show that initiation is an HCl elimination through a transition state of four centers requiring a synperiplanar conformation of the >CH–CCl< group, whereas propagation is a dehydrochlorination through a transition state of six centers requiring a cis configuration of the double bond.     

An investigation of mechanisms of synergistic interactions in PVC stabilization

Synergism in poly(vinyl chloride) stabilization has been studied by measuring rates of hydrogen chloride evolution from samples of polymer in the presence of stabilizers in di-(2-ethylhexyl) phthalate solution in an inert atmosphere. Barium, cadmium, calcium, and zinc laurates, when used alone, allow escape of hydrogen chloride well before stoichiometric uptake is achieved, whereas synergistic mixtures of calcium–zinc and barium–cadmium laurates absorb almost the theoretical quantity of hydrogen chloride. Cadmium and zinc laurates replace labile chlorine atoms in the polymer backbone by ester groups, reducing formation of long polyene sequences: barium and calcium laurates delay the formation of cadmium and zinc chlorides, apparently by reconverting them into their respective laurates. Polyols function by forming complexes with the prodegradant cadmium and zinc chlorides, but contrary to popular belief phosphites possess little activity in this respect. Instead, they slow down the rate of polymer degradation by removal of labile chlorine atoms, by reaction with hydrogen chloride, and by peroxide decomposition.     

Zinc maleate and calcium stearate as a complex thermal stabilizer for poly(vinyl chloride)

Zinc maleate (ZnMA) and calcium maleate (CaMA) were synthesized by reaction of maleic acid with the corresponding metal oxides and were characterized by X‐ray diffraction, thermal analysis, and Fourier‐transform infrared (FTIR) spectroscopy. The thermal stabilizing effects of ZnMA and CaMA on poly(vinyl chloride) (PVC) were investigated at 180°C in air by a static stability test. The stabilization mechanism of ZnMA and the synergism of ZnMA/CaSt₂ (St = stearate) were also studied by UV‐visible and FTIR spectroscopies, as well as a thermal stability test. The PVC with the ZnMA stabilizer exhibited good thermal and color stability caused by the ability of ZnMA to replace the labile chlorine atoms in PVC chains, absorb hydrogen chloride, and react with the polyene intermediates via a Diels–Alder mechanism. The gel content of the PVC/ZnMA samples reached 31% after 2 min of heating and 44% after 10 min, thereby indicating that crosslinking could easily occur with ZnMA, probably owing to catalysis by Zn species. The static and dynamic stability results showed that the synergistic effect of the ZnMA/CaSt₂ stabilizer was greater than that of ZnSt₂/CaSt₂. J. VINYL ADDIT. TECHNOL., 20:1–9, 2014. © 2014 Society of Plastics Engineers     

Synergistic combination of metal stearates and β‐diketones with hydrotalcites in poly(vinyl chloride) stabilization

A synergistic effect of synthetic hydrotalcites as long term stabilizer with metal soaps (the mixture of calcium and zinc stearate) and metal acetylacetonates on dehydrochlorination of PVC has been studied. A proper balance between color stabilization and HCl scavenging capacity has been obtained. Hydrotalcite was prepared by hydrothermal treatment and characterized by EDX, XRD, FTIR, TGA, and SEM techniques. The material is reasonably crystalline and suggests a relatively well ordered sheet arrangement with crystallite size 24.87 nm. The interlayer water content was calculated from the TGA curves and the suggested formula is Mg_0.76 Al_0.24(OH)₂(CO₃)_0.12·0.5H₂O. Synergism in PVC stabilization has been studied by measuring the HCl evolution during the processing at 180°C. Oven aging method was used to study the color stabilization at higher temperature. PVC sheet with different formulation was prepared using Labcoater and subjected to oven for different time interval. The color development (polyene formation) on oven ageing was recorded using UV–visible spectroscopy. UV–visible studies shows that an average sized polyene gives pale yellow color, whereas red or brown color was developed due to long range polyene (n = 10–14) sequences. Hence, the HCl evolution depends on the rate of dehydrochlorination but color depends on the kind of polyene formed. Mechanism of stabilization suggests that adsorption and ion exchange, both phenomenon, are responsible for hydrotalcites as long term stabilizers. The acetylacetonate complex too substitute allylic chlorides and inhibit formation of long polyene responsible for darkening. A clear effect of synergism has been observed. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Effect of metal stearate stabilizers on the thermal degradation of PVC in solution: The reversible blocking mechanism of stabilization

Systematic thermal degradation studies of PVC solutions have been carried out in the presence of different metal carboxylates (Pb-, Cd-, Ba-, Ca- and Zn- stearates). ZnSt₂ markedly accelerates degradation of the polymer. Significant induction periods (t_i) for the appearance of free HCI are obtained in the presence of the other salts. CaSt₂ acts mainly as an HCI-scavenger and has no direct influence on the elimination process. Mainly short polyenes are formed during the induction period when PbSt₂, CdSt₂, and BaSt₂ are used. After consumption of stabilizers rapid HCI loss occurs, and suddenly longer polyenes are formed, i.e., the amount of double bonds and the average length of polyenes sharply increases. At the same time, the rate of initiation (i.e., the rate of the polyene sequence formation) does not change at all around t_i. None of the investigated stabilizers retard effectively the random initiation of HCI loss. In the presence of PbSt₂, BaSt₂, and CaSt₂, and CaSt₂, the initiation rate is nearly identical to that of the unstabilized PVC. The concentration of labile sites (h₀) in PVC at the end of the induction period has been estimated by kinetic analysis. It has been found that in some cases h₀ is higher than the concentration of labile structures in the virgin PVC. These results indicate that the main role of metal soap stabilizers is the blocking of the fast zip-elimination of HCI from the PVC chain. It is likely that blocking occurs by attachment of a carboxylate group at the end of a propagating zip. This is a reversible process: the blocked structures become active again mainly after the consumption of stabilizers presumably by HCI-catalyzed hydrolysis. In contrast to other stabilization mechanisms by Frye and Horst, Minsker and coworkers, and Michell the reversible blocking mechanism of PVC stabilization is able to explain the experimental findings presented in this study. It also explains such facts of practical importance as color stability of the resin during the induction period and fast blackening after the consumption of stabilizers.     

Inhibition of the thermal degradation of rigid poly(vinyl chloride) using poly(N‐[4‐(N′‐phenyl amino carbonyl)phenyl]maleimide)

Poly(N‐[4‐(N′‐phenyl amino carbonyl)phenyl]maleimide), poly(PhPM), has been investigated for the inhibition of the thermal degradation of rigid poly(vinyl chloride) (PVC) in air, at 180°C. Its stabilizing efficiency was evaluated by measuring the length of the induction period, the period during which no detectable amounts of hydrogen chloride gas could be observed, and also from the rate of dehydrochlorination as measured by continuous potentiometric determination, and the extent of discoloration of the degraded polymer. The results have proved the greater stabilizing efficiency of poly(PhPM) relative to that of the DBLC commercial stabilizer. This is well demonstrated by the longer induction period values and by the lower rates both of dehydrochlorination and discoloration of the polymer during degradation relative to those of the DBLC reference stabilizer. The greater stabilizing efficiency of the poly(PhPM) is most probably attributed not only to its possession of various centers of reactivity that can act as traps for radical species resulting during the degradation process, and replacement of labile chlorine atoms on PVC chains by relatively more thermally stable poly(PhPM) moieties, but also due to the ability of its fragmentation products to react with the evolved hydrogen chloride gas. A radical mechanism is suggested to account for the stabilizing action of this polymeric stabilizer. A synergistic effect is achieved when the poly(PhPM) was blended in various weight ratios with DBLC. This synergism attains its maximum when poly(PhPM) and DBLC are taken at 3 : 1 weight ratio. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Synthesis of Bismuth(III)Neodecanoate and Its Application to Poly(Vinyl Chloride) as a Thermal Stabilizer

The bismuth(III)neodecanoate (Bi(Ne)₃) was synthesized via the method of co-reaction of bismuth oxide, neodecanoic acid, and acetic anhydride and then characterized by Fourier transform infrared spectroscopy (FTIR) and elemental analysis (EA). The effect of Bi(Ne)₃ as a thermal stabilizer on poly(vinyl chloride) (PVC) was assessed by thermal aging test, Congo red test, conductivity measurement, and thermogravimetric analysis, respectively. The results showed that Bi(Ne)₃ significantly provided PVC with a good initial color and long-term stability. Bi(Ne)₃ played a role in improving the stabilizing efficiency of PVC through absorbing hydrogen chloride (HCl) and displacing labile chlorine atoms in PVC molecular chains.     

Reaction mechanism of poly(vinyl chloride) degradation. Molecular orbital calculations

Semiempirical Molecular Orbital Calculations (MNDO AM1) support kinetic results concerning the molecular mechanism of thermal degradation of PVC and show that under special conditions radical and ionic mechanisms are also possible. The degradation of poly(vinyl chloride) is a complex chain dehydrochlorination that consists of an initiation process to generate an active intermediate followed by chain reactions that generate additional active intermediates with progressively increased numbers of double bonds. Each intermediate partitions between an intermediate with one more double bond and a stable conjugated polyene with the same number of double bonds. At low and moderate temperatures thermal degradation of PVC in an inert atmosphere is a succession of molecular concerted reactions. The initiation process is a 1,2-elimination through a four center transition state requiring a synperiplanar conformation. There are two main chain reactions: the first is a 1,4-elimination from allylic chlorine atoms and methylenes cis to a double bond through a transition state of six centers; the second is a 1,3-rearrangement of hydrogen atoms catalyzed by hydrogen chloride. The chain reaction is interrupted when a relatively stable trans double bond is formed and no hydrogen chloride is present to catalyze trans-cis isomerization or 1,3-rearrangement. Macro carbocations formed by heterolysis of carbon-halogen bonds in the presence of strong Lewis acids react much faster than does the original PVC in concerted elimination by 1,2-syn or 1,4-cis mechanisms, promoting a so-called catastrophic, very fast degradation. Macro radicals formed by thermal homolysis, irradiation or reaction with promoters can also promote very fast hydrogen chloride elimination because of a special mechanism consisting of a 1,2-rearrangement of a chlorine atom followed by a concerted 1,3-elimination through a five center transition state.     

Formation of anomalous structures in PVC and their influence on the thermal stability: 2. Branch structures and tertiary chlorine

Branch structures were determined in fractions of a commercial suspension of PVC (S-PVC) and experimental PVC samples obtained at subsaturation conditions (U-PVC). The analyses were performed with ¹³C n.m.r. spectroscopy at 50.31 MHz after reductive dehalogenation with tributyltinhydride. With increasing monomer starvation U-PVC was found to have an increasing amount of butyl and long chain branches (LCB). A polymer prepared at 55°C and 59% of the saturation pressure of vinylchloride had 3.4 butyl branches and 2.0 LCB per 1000 monomer units. In the S-PVC series the total content of these two structures varied between 0.5 and 1.0 per 1000 monomer units. By using tributyltindeuteride as reducing agent the structure of the butyl branches could be determined as ～CHClCH₂CCl(CH₂CHClCH₂CH₂Cl)CH₂CHCl～. A major part of the LCB points also contained tertiary chlorine. The formation of LCB is suggested as occurring after abstraction of hydrogen from the polymer chain by macroradicals and chlorine atoms. The latter will lead to LCB points with tertiary hydrogen and internal double bonds. The rate of dehydrochlorination at 190°C in nitrogen could be related to the amount of tertiary chlorine (correction coefficient=0.97). It was assumed that tertiary chlorine is the most important labile structure in PVC.     

Stabilization of rigid poly(vinyl chloride) by 5,6,7,8-Tetrahydro-2-mercapto-4-(p-methoxyphenyl)-3-quinolinecarbonitrile

The chemical 5,6,7,8-Tetrahydro-2-mercapto-4-(p-methoxyphenyl)-3-quinolinecarbonitrile (TMQC) has been investigated as a thermal stabilizer for rigid poly(vinyl chloride) (PVC) at 180°C in air. The results reveal the higher stabilizing efficiency of the investigated material, as shown by the longer induction period obtained in its presence relative to those produced by commonly used industrial stabilizers. Ultraviolet irradiation of the polymer stabilized by TMQC gives a lower extent of discoloration compared to that of unstabilized PVC. Reaction of TMQC with the polymer in solution also has been studied in order to investigate the mechanism of stabilization.     

Thermal dehydrochlorination of pure PVC polymer: Part I—thermal degradation kinetics by thermogravimetric analysis

Thermal degradation of PVC occurs in two stages, with each stage subdivided into two substages. The first refers to the dehydrochlorination, where hydrochloric acid is formed, and giving polyene structures. Hitherto, the degradation mechanism and action of hydrochloric acid as a catalyst during the dehydrochlorination stage are poorly known. Recently, the importance of the tacticity has gained attention for its influence on the dehydrochlorination mechanism. The present work focused on the dehydrochlorination stage, studying the molecular structure by FTIR analysis and the kinetic parameters by TGA analysis in Nitrogen atmosphere, based on three mathematical methods: Friedman, Kissinger, and Flynn-Wall-Ozawa. The sample was a pure homopolymer obtained by suspension polymerization. The dehydrochlorination kinetics follows a first order reaction model and occurs by nucleation and growth. The dehydrochlorination begins with the loss of very labile chlorine atoms present in defective and isotactic molecular segments. The formed HCl acts as a catalyst in the degradation. Following 40% conversion, a drop in Ea is observed. After that, chlorine atoms present in syndiotactic and atactic sequences, are released and, added to the large number of polyene chain sequences, and an increase in Ea is observed up to 60% conversion, where the dehydrochlorination stage is concluded.     

Inhibition of the degradation of poly(vinyl chloride) by its modification with 5-pyrimidine carbonitrile (1,2,3,4- tetrahydro-4-oxo-6-phenyl-2-thioxo)

The reaction of poly(vinyl chloride) (PVC) with 5-pyrimidine carbonitrile (1,2,3,4-tetrahydro-4-oxo-6-phenyl-2-thioxo) has been studied. The thermal stability of the modified polymer is improved markedly compared with the unmodified polymer. The stability improvement is attributed to the replacement of the labile chlorine atoms by more stable thio groups. The modified polymer also showed a lower extent of discoloration against ultraviolet rays compared with the unmodified PVC. © 1998 SCI.     

Formation of anomalous structures in PVC and their influence on the thermal stability: 3. Internal chloroallylic groups

Internal double bonds were determined by oxidative cleavage in fractions of a commercial suspension PVC (S-PVC) and experimental PVC samples obtained at subsaturation conditions (U-PVC). The changes in molecular weight were measured by g.p.c. and viscometry. The oxidation was performed by ozonolysis in tetrachloroethane solution at ?20°C. Oxidation by potassium permanganate in dimethylacetamide solution at ?10° to +50°C was also studied. However, this method was found to give erratic results. With increasing monomer starvation the number of internal double bonds increased. In the original S-PVC sample the internal double bond content was $\text{0.2}\text{1000}\text{VC}$. The formation of double bonds is assumed to be the result of an increased tendency, by chlorine atoms, to attack on the methylene groups in the chain. Hydrolytic cleavage and ¹H n.m.r. measurements did not give any evidence of ketoallylic groups. With increasing chloroallylic group content the U-PVC samples showed an increased rate of dehydrochlorination at 190°C in nitrogen. The S-PVC fractions, however, showed a decreased rate. However, the thermal stability in both series of samples could be related to the tertiary chlorine content. Butyl and long chain points with tertiary chlorine are more frequent than the internal chloroallylic groups. It was assumed that tertiary chlorine is the most important labile structure not only in U-PVC but also in ordinary PVC.     

Ways of poly (vinyl chloride) stabilization

Main results of research on poly (vinyl chloride) (PVC) stabilization are discussed. Stabilization is viewed from the standpoint of the modern notions of the reasons for PVC low thermal stability, the complicated nature of its dehydrochlorination, and the kinetics of its degradation. The internal unsaturated oxygen-containing groups of ～C(O)? CH?CH? CHCl～ type are regarded as the main source of the polymer instability. Typical processes resulting in PVC stabilization, such as the substitution of labile chlorine atoms and the destruction of initial active sites during reactions with various chemical agents, as well as the kinetic aspects of stabilizers' effect on HCl elimination and PVC macromolecules crosslinkage are considered. The influence of additives on the polymer coloration is estimated.