Stabilization of vinylidene chloride barrier resins

Vinylidene chloride copolymers have a number of superior properties, most notably a high barrier to the transport of oxygen and other small molecules. As a consequence, these materials have assumed a position of prominence in the packaging industry. At processing temperatures these copolymers tend to undergo degradative dehydrochlorination. Unsaturation generated via interaction of the polymer with a variety of agents commonly encountered during polymerization or processing introduces an allylic dichloromethylene unit which may function as a major defect (labile) site for the initiation of degradation. Three approaches to the potential stabilization of these materials have been examined. The first involved the addition of agents, e.g. metal formates, capable of converting labile dichlormethylene units into non-reactive groups which would interrupt propagation of the degradative dehydrochlorination. The second involved the incorporation into the polymer of a commoner capable of scavenging free chlorine atoms. The third involved the preparation of copolymers which contains units capable of reaction with (consumption of) a mole of hydrogen chloride to expose a good free radical stabilizer to scavenge chlorine atoms.     

Degradation of vinylidene chloride/phenylacetylene (VDC/PA) copolymers. Effect of internal unsaturation on poly(vinylidene chloride) stability

The thermal degradation of vinylidene chloride/phenylacetylene copolymers containing small but varying amounts of phenylacetylene has been examined in both the solid phase and in bibenzyl solution. Incorporation of phenylacetylene into the poly(vinylidene chloride) structure greatly facilitates degradative dehydrochlorination. Indeed, the presence of phenylacetylene promotes the formation of polyene segments during the polymerization process so that all the copolymers, even at very low phenylacetylene loading, are tan in color. The decreased stability of polymers containing interal unsaturation arises from an increased rate of initiation for degradative dehydrochlorination. The propagation rate is largely unaffected by the level of unsaturation initially present in the polymer. The ratio of hydrogen chloride to stilbene formed for degradation of these copolymers in bibenzyl solution is approximately 35:1. This suggests that the chlorine atom of the initially-formed radical pair preferentially abstracts an adjacent hydrogen atom rather than interacting with solvent, i.e., the chain-carrying radical pair does not dissociate appreciably as the unzipping dehydrochlorination occurs. Thus random double bonds introduced in a variety of ways may be identified as principal defect sites responsible for the initiation of the degradative dehydrochlorination of poly(vinylidene chloride). Species which promote the degradation of poly(vinylidene chloride) probably do so by facilitating the introduction of random double bonds into the structure.     

Degradation of vinylidene chloride/methyl acrylate/phenylacetylene (VDC/MA/PA) terpolymers. Effect of internal unsaturation on the stability of Saran copolymers

The thermal degradation of vinylidene chloride/methyl acrylate/phenylacetylene (VDC/MA/PA) terpolymers containing a constant 9 wt % methyl acrylate and small but varying amounts of phenylacetylene has been examined in the solid phase and in bibenzyl solution. Thermally promoted degradative dehydrochlorination, largely uncomplicated by methyl chloride formation, readily occurs at temperatures approaching 200°C. Incorporation of phenylacetylene into the polymer structure greatly facilitates degradative dehydrochlorination. Indeed, the presence of phenylacetylene induces the formation of polyene segments during the polymerization so that all the terpolymers, even at very low phenylacetylene loading, are tan in color. The decreased stability of polymers containing internal unsaturation arises from an increased rate of initiation for the degradation reaction. The propagation rate is largely unaffected by the level of unsaturation initially present in the polymer. Thus random double bonds have been identified as the principal defect sites responsible for the facile degradation of Saran copolymers. Species which promote the degradation of Saran polymers probably do so by facilitating the introduction of double bonds into the structure. The ratio of hydrogen chloride to stilbene formed for degradation of the terpolymers in bibenzyl solution is ca. 35:1. This is strongly reminiscent of PVDC degradation and suggests that for degradation of either the homopolymer or Saran copolymers the chain-carrying allylic radical pair does not dissociate to any appreciable extent as dehydrochlorination occurs.     

Thermal degradation of vinylidene chloride/4-vinylpyridine copolymers

Vinylidene chloride polymers are prominent in the barrier plastics packaging industry. They display good barrier to the transport of oxygen (to prevent spoilage of food items) and flavor and aroma constituents (to prevent 'scalping' on the supermarket shelf). However, these polymers undergo thermal dehydrochlorination during processing. This can lead to a variety of problems including the evolution of hydrogen chloride which must be scavenged to prevent its interaction with the metallic walls of process equipment. Such interaction leads to the formation of metal halides which act as Lewis acids to facilitate the degradation. A potentially effective means to capture hydrogen chloride generated might be to incorporate into the polymer a mild organic base. Accordingly, copolymers of vinylidene chloride and 4-vinylpyridine have been prepared and subjected to thermal aging. Results suggest that the pyridine moiety is sufficiently basic to actively promote dehydrochlorination in the vinylidene chloride segments of the polymer.     

The effect of thermal processing on PVC—II. The effect of lubricants and stabilizers

The effects of lubricants and a tin maleate stabilizer on the mechanochemical and thermal degradation of PVC during processing have been studied at 170° and 210°. The protective mechanism of the two types of additive were found to be complementary in that the lubricants reduced the severity of the mechano-chemical process whereas the tin maleate stabilizer reduced the rate of hydrogen chloride elimination during the second stage by reacting with allylic chlorine, with conjugated unsaturation and with hydrogen chloride.     

Composition of vinylidene chloride/phenylacetylene (VDC/PA) copolymers and vinylidene chloride/methyl acrylate/phenylacetylene (VDC/MA/PA) terpolymers from analysis by 13C-NMR spectroscopy

¹³C-NMR spectroscopy has been utilized to characterize Saran polymers containing polyene segments generated by incorporating phenylacetylene (PA) units into either poly(vinylidene chloride) (PVDC) or a typical Saran copolymer of 91% vinylidene chloride (VDC) and 9% methyl acrylate (MA). The incorporation of PA could not be defined in the usual statistical way because the presence of the PA double bond in the polymer backbone appeared to cause the dehydrohalogenation of units next to it. Thus, sequences of PA next to VDC were not observed. Rather, sequences of olefinic units next to VDC units were present at a level equal to the level of PA incorporation. The level of unsaturation in the PA copolymers is approximately four times the level of PA incorporation. These observations are consistent with the random incorporation into the copolymer of PA which then initiates the dehydrohalogenation in adjacent VDC units. This dehydrohalogenation reaction appears to propagate from one unit to the next along the backbone of the polymer such that polyene segments containing the PA unit are formed.     

Thermal degradation of vinylidene chloride/vinyl chloride copolymers in the presence of N-substituted maleimides

As a consequence of their excellent barrier properties vinyl chloride/vinylidene chloride copolymers have long been prominent in the flexible packaging market. While these polymers possess a number of superior characteristics, they tend to undergo thermally- induced degradative dehydrochlorination at process temperatures. This degradation must be controlled to permit processing of the polymers. Three series of N-substituted maleimides (N-alkyl-, N-aralkyl, and N-aryl) have been synthesized, characterized spectroscopically, and evaluated as potential stabilizers for a standard vinyl chloride/vinylidene chloride (85 mass%) copolymer. As surface blends with the polymer, these compounds are ineffective as stabilizers. However, significant stabilization may be achieved by pretreatment of the polymer with N-substituted maleimides. The most effective stabilization of the polymer is afforded by N-aralkyl- or N-arylmaleimides, most notably, N-benzylmaleimide and N-p-methoxyphenylmaleimide.     

Thermal decomposition of poly(vinyl chloride) and chlorinated poly(vinyl chloride). II. Organic analysis

A concerted study of poly(vinyl chloride), chlorinated poly(vinyl chloride), and poly(vinylidene chloride) polymers by spectroscopy, thermal analysis, and pyrolysis-gas chromatography resulted in a proposed mechanism for their thermal degradation. Polymer structure with respect to total chlorine content and position was determined, and the influence of these polymer units on certain of the decomposition parameters is presented. Distinguishing differences were obtained for the kinetics of decomposition, reactive macroradical intermediates, and pyrolysis product distributions for these systems. It was determined that chlorinated poly(vinyl chloride) systems with long-chain ? CHCI? units were more thermally stable than the unchlorinated precursor, exhibited increasing activation energy for the dehydrochlorination, and produced chlorine-containing macroradical intermediates and chlorinated aromatic pyrolysis products. The poly(vinyl chloride) polymer was relatively less thermally stable, exhibited decreasing activation energy during dehydrochlorination, and produced polyenyl macro-radical intermediates and aromatic pyrolysis products.     

Study of the thermal dehydrochlorination of poly(vinylidene chloride) and poly(vinyl chloride) by ESR spectroscopy

The evidence for a radical elimination of hydrogen chloride during the thermal degradation of homopolymers and copolymers of vinylidene chloride is summarized and confirmed by an ESR spectroscopic study of the degradation residues. However, sufficient differences in the degradation characteristics exist between these polymers and those of vinyl chloride to suggest that a radical process alone is not sufficient. No evidence of a radical process can be obtained from an ESR spectroscopic analysis of the elimination. The paramagnetic character of the degraded polymer is attributed to the polyene structure produced on dehydrochlorination.     

Grafting of Vinyudene Chloride. Model Studies with Poly(Vinyl Alcohol) as Substrate

The modification of coatings resins by graft polymerization of vinylidene chloride should produce a coatings binder with improved barrier properties. For superior color stability, vinylidene chloride must be copolymer-ized with other monomers such as alkyl acrylates and methacrylates. Ceric ion initiation was used to graft vinylidene chloride free-radically onto a model alcohol-containing polymer, polyvinyl alcohol. The effects of various reaction parameters on vinylidene chloride grafting were studied. Graft copolymers were characterized using selective solvent extraction, FTIR, SEM, XES, DSC, and x-ray diffraction.     

Thermal degradation of graft copolymers of PVC prepared by mastication with styrene and methyl methacrylate,and of further PVC mixtures and vinyl chloride copolymers

Graft copolymers prepared by mastication of PVC in the presence of styrene or of a styrene/ methyl methacrylate mixture, have been studied by thermogravimetry, estimation of hydrogen chloride, thermal volatilization analysis, and flash pyrolysis/g.l.c. The degradation behaviour of PVC/ polystyrene mixtures, vinyl chloride/styrene random copolymers, a random copolymer of methyl methacrylate and styrene, and PVC/poly-α-methylstyrene mixtures has also been studied. The graft copolymers resemble the PVC/methacrylate graft copolymers previously studied in showing retardation of the dehydrochlorination reaction, but contrast with them in yielding chain fragments but no monomer during HCl production. Some stabilization of the second component at higher temperatures is also found. PVC/polystyrene mixtures behave in the same way as the corresponding graft copolymers, but vinyl chloride/styrene copolymers show reduced stability towards both dehydrochlorination and monomer production compared with the homopolymers. PVC/poly-α-methylstyrene mixtures yield some monomer concurrently with HCl loss, and display marked retardation of the latter reaction. Stabilization of the second polymer at higher temperatures is again observed. Many of these results add further strong support to the view that chlorine atoms are involved as chain carriers in the thermal dehydrochlorination of PVC.     

Cationic polymerization of α-methylstyrene from polydienes. I. Synthesis and characterization of poly(butadiene-g-α-methylstyrene) copolymers

The preparation of poly(butadiene-g-α-methylstyrene) copolymers was investigated with three different alkylaluminum coinitiators. The alkylaluminum compounds in conjunction with polybutadiene which contained a low concentration of labile chlorine atoms initiated the polymerization of α-methylstyrene to produce graft copolymers. Trimethylaluminum gave higher grafting efficiencies than diethylaluminum chloride at comparable monomer conversions. Triethylaluminum produced only very low monomer conversions (<5%), even at long reaction times, and for this reason was not studied extensively. The number of grafts per polybutadiene backbone was determined for a number of copolymers and found to increase slightly as the allylic chlorine concentration in the polybutadiene backbone was increased. In all cases, however, only a low percentage of the available labile chlorine sites along the polybutadiene backbone resulted in grafted α-methylstyrene side chains. The addition of small quantities of water to the polymerization solvent greatly enhanced the grafting rate and ultimate monomer conversion during the synthesis of these poly(butadiene-g-α-methylstyrene) copolymers. The mechanistic role of water during these grafting reactions is unknown at the present time.     

Thermal degradation of vinylidene chloride/[4-(t-butoxycarbonyloxy)phenyl]methyl acrylate copolymers

Vinylidene chloride polymers containing comonomer units capable of consuming evolved hydrogen chloride to expose good radical-scavenging sites might be expected to display greater thermal stability than similar polymers containing simple alkyl acrylates as comonomer. Incorporation of a comonomer containing the phenyl t-butyl carbonate moiety into a vinylidene chloride polymer has the potential to afford a polymer with pendant groups which might interact with hydrogen chloride to expose phenolic groups. Copolymers of vinylidene chloride with [4-(t-butoxycarbonyloxy)phenyl]methyl acrylate have been prepared, characterized, and subjected to thermal degradation. The degradation has been characterized by thermal and spectroscopic techniques. The degradation of vinylidene chloride/[4-(t-butoxycarbonyloxy)phenyl]methyl acrylate copolymers is much more facile than the same process for similar copolymers containing either [4-(isobutoxycarbonyloxy)phenyl]methyl acrylate or methyl acrylate, a simple alkyl acrylate, as comonomer. During copolymer degradation, [4-(t-butoxycarbonyloxy) phenylmethyl acrylate units are apparently converted to acrylic acid units by extensive fragmentation of the sidechain. Thus, the phenyl t-butyl carbonate moiety does function as a labile acid-sensitive pendant group but its decomposition in this instance leads to the generation of a phenoxybenzyl carboxylate capable of further fragmentation.     

Studies in Cyclocopolymerization. VII. Copolymerization of Chloromaleic Anhydride with Divinyl Ether

Chloromaleic anhydride was found to copolymerize with divinyl ether to form soluble copolymers of 1:1 composition, devoid of residual unsaturation. A bicyclic structure is proposed in which the polymer backbone consists only of divinyl ether units. The ease with which the copolymers underwent dehydrohalogenation suggests that the hydrogen and chlorine atoms on the anhydride unit are in a trans configuration as a result of a stepwise cycli-zation process.

Oxidation of the hydrolyzed, dehydrohalogenated copolymers afforded the corresponding vic-diol copolymers. The absence of a significant decrease in copolymer molecular weight upon periodic acid cleavage of the vic-diol copolymers supported the proposed structure. Functional group analyses and softening points were in accord with the structures of the derived copolymers.     

Mechanism of the thermal stabilization of poly(vinyl chloride) with metal carboxylates and epoxy plasticizers

The chemical reactions responsible for the retardation of thermal discoloration in poly(vinyl chloride) (PVC) stabilized with a combination of an epoxy plasticizer and a heavy metal soap mixture of Group IIa and IIb metals have been studied. Allylic chlorides (a mixture of 4-chlorohexene-2 and 2-chlorohexene-3) are used as prototypes for the degrading segment of the polymer chain. The results confirm earlier reports that, when a mixture of a covalent and ionic metal soap is used as the stabilizer, the covalent moiety (e.g., Cd and Zn soaps) functions to esterify the allylic site of the degrading PVC model. A synergistic effect displayed by the ionic soap (e.g., Ca or Ba) is caused by a transfer of carboxylate ligands from the ionic soap to the depleting covalent species, which has been largely converted to the corresponding chloride. When an epoxy plasticizer model (cyclohexene oxide) is used in conjunction with the metal soap stabilizer, the preferred reaction is esterification. After a considerable build-up of ester, an α-chloroether, 2-hexenyl 4-(2-chlorocyclohexyl) ether, is formed by the reaction of cyclohexene oxide with the PVC model. This reaction was found to be catalyzed by cadmium chloride. The esterification and etherification reactions provide an explanation for the synergism observed in the stabilization of PVC containing a combination of an epoxy plasticizer with a covalent and an ionic metal soap.     

Electrochemical dehydrochlorination initiation for vinylidene chloride-vinyl chloride copolymers

Direct electrochemical initiation of dehydrochlorination is possible for polyvinylidine chloride VDC and copolymers of VDC with vinyl chloride VC. The ratio of the VDC and VC links in the initial polymer affects the rate of the dehydrochlorination and the structure of the products. Institute for Bioorganic Chemistry and Petrochemistry, National Academy of Sciences of Ukraine, 50 Kharkovskoe Shosse, 252660 Kiev-94, Ukraine. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 33, No. 3, pp. 180–183, May–June, 1997.     

Study of the effect of the chloroallyl group content on the thermal dehydrochlorination of PVC

The chloroallyl group contents of PVC and its low molecular weight fractions, soluble in chloroform or acetone, have been determined on the assumption that the reaction of AgNO₃ with labile chlorine atoms occurs only with the allylic chlorines. Their amount is highest in the low molecular weight fractions. The thermal stability of the polymer in inert atmosphere increases if these groups are removed. Neither the labile chlorine content nor the dehydrochlorination rate (in the subsequent degradation) of the initial polymer, or the fractions insoluble in the two solvents, is affected by heat treatment at 180 for 60 min, whereas the amount of labile chlorine in the low molecular weight fractions increases on heat treatment exceeding 30 min. No direct dependence of the dehydrochlorination rate on the amount of labile chlorine in the polymers under study has been established.     

Thiol-s-triazine as a Stabilizer for Thermal Discoloration

The formation of longer polyene chains (thermal discoloration) has been found to be avoidable by using thiol-s-triazines, such as 2-anilino-4,6-dithiol-s-triazine (AF), which hardly reacts with the original chlorine atoms but tends to replace the activated chlorine (allylic chloride) that plays an important role in triggering zipperlike dehydrochlorination. The optimum stabiliizing system is: PVC, 100; DOP, 50; AF, 0.18; Zn stearate, 0.5; Ba stearate, 1.5 parts.     

Polymerization products of p-benzoquinone as thermal stabilizers for rigid poly(vinyl chloride). Part II—Evaluation of the stabilizing efficiency

Quinone-tin polymers prepared by the cationic polymerization of p-benzoquinone with tin(II)chloride in the absence of solvent have been investigated as thermal stabilizers for rigid PVC at 180°C in air by measuring the rate of dehydrochlorination. The results reveal the higher stabilizing efficiency of these products relative to dibutyltin maleate, basic lead carbonate and barium-cadmium stearate stabilizers commonly used in industry. The induction period in the early stages of the dehydrochlorination process increases as a function of the metal content in the stabilizer molecule. The evidence indicates that the quinone and the metal part (

) of the stabilizer participate in the stabilization process by trapping the radical intermediates, as well as blocking the odd electron sites formed on the PVC chains. The mechanism of stabilization suggested to account for the results obtained supports a radical mechanism for the dehydrochlorination reaction.     

Polyvinylchloride stabilisation with organotin compounds—Part III: Effect of lewis acidity and co-ordination of organotin chlorides upon their catalytic activity towards dehydrochlorination

The catalysis of the E2 dehydrochlorination of a model compound for the allylic chlorine atoms in PVC involves the co-ordination of this chlorine to the tin atom. It is enhanced by an increase in the Lewis acidity of the tin chloride compound (scaled by Mossbauer spectroscopy), but may be inhibited if other groups, internal or external, are competing for the co-ordination. HCl may be one of these competitors, although, in some cases, it breaks the weakest tin—carbon bonds, giving rise to stronger prodegradant Lewis acids.