Thermal stability of PVC

Studies on model compounds also suggest that unsaturated chain-end groups should not have an important influence on the thermal stability of PVC [21]. In conclu-... 

The presence of allylic chlorines and tertiary chlorines and their influence on the thermal stability of PVC has now been established with some degree of confidence, and together they are considered to constitute the labile chlorine structures in the polymer. Numerous chemical modification methods involving the selective nucleophilic substitution of labile chlorines in PVC with other chemical moieties for identifying and quantifying labile structures have been reported in the literature. 

Tacticity or stereochemical arrangement of atoms in three-dimensional space in relation to each other along the polymer chain cannot really be termed a structural defect. But researchers have shown that tacticity has an important bearing on the reactivity and thermal stability of PVC. For this reason tacticity is being discussed under this section. 

Figure 2 Variation of thermal stability of PVC with acetate content.

This further interesting effect of very low levels of acetoxylation on the thermal stability of PVC may be explained on a basis similar to that applied to the copolymers, with specific reference to labile chlorines taken in isolation. But this can only be done with a certain amount of caution. 

Wirth and Andreas [141] studied the effect of octyl-tin chlorides on the thermal stability of PVC. They were found to significantly retard the dehydrochlorination in the following order ... 

Completely Reacted Material - The zinc borate contains no free zinc oxide. Thus, it does not have serious detrimental effects on the thermal stability of PVC or chlorinated paraffin as free zinc oxide does. 

Polyvinyl Chloride. It is well-known how various attempts have been made to stabilise PVC against dehydrochlorination by salts, usually of divalent metal ions - as long chain alkylcarboxylates of Cd(II), Ba(U), Zn(II) [96]. Biswas and Moitra [102] recently established that the 3d metal ions incorporated in PVC-DMG-complex enhance the thermal stability of PVC in the order. 

In conclusion, the thermal stability of PVC/EPR is at least equal to that o a PVC having a comparable molecular weight, and no negative effect may be attributed to the presence of the elastomer. 

This hypothesis has been confirmed by the greatly improved thermal stability of PVC as a result of the formation of a graft copolymer of d -l,4-polybutadiene onto poly (vinyl chloride). The improved thermal stability is demonstrated by the almost total absence of discoloration on molding the graft copolymer into a film at 200°C in air, the reduced rate of dehydrochlorination on heating in an inert atmosphere at 180°C, and higher onset and peak temperatures for hydrogen chloride evolution as determined by differential thermal analysis. 

Consequently, reduced or retarded hydrogen chloride evolution is a more reliable measure of the thermal stability of PVC. When the unmodified polymer was pressed to a film in air at 200°C on a black iron or untreated steel plate, the film was deeply colored, and the metal surface beneath the polymer film was corroded and became covered with a film of rust. In contrast, under the same conditions, the pressed film from the graft copolymer was very slightly discolored, and the metal surface remained free of corrosion and rust even after two years exposure to the atmosphere. 

Thus, the thermal stabilization of PVC which resulted from the heterogeneous grafting of as little as 3-5% cis-1,4-polybutadiene was more than a simple additive effect and indicates a synergistic interaction. This was demonstrated further by dissolving up to 10% cis-1,4-polybuta-diene in a chlorobenzene suspension or solution of PVC and isolating the polymer blend by precipitation with methanol. Films pressed from the polymer blend were generally deeply colored and contained incompatible, probably gelled or crosslinked, areas. 

To improve the thermal stability of PVC, it is common to add stabilizers, generally metalloorganic compounds, in concentrations of 1-5 parts per 100 parts of PVC. Although the cis-1,4-polybutadiene grafted PVC, in the absence of an added stabilizer, yielded essentially colorless or only faintly discolored films, completely colorless films were obtained when the conventional stabilizers were added in concentrations of 0.1-0.3 parts per 100 parts of modified PVC. Organotin stabilizers were not necessary and, in some cases, actually resulted in greater color development than when they were absent. 

Oligomer 163 represents a stabilizer having PA chain with pendant phenolic moiety [31], Pendant hindered piperidine or piperazine moieties were attached to oligomeric stabilizers having polysulfonamide, polyurea (e.g. 164), or PA (e.g. 165) unis [213]. Polyhydrazide 166 was tested as HD AO and copper deactivator in PP [214]. Poly (nitrophenylene-carbazide disulfide) 167 was prepared for thermal stabilization of PVC. Phosphorus containing crosslinkable polymers having polyamide, polyimide and polyurea chains were prepared for flame and heat resistant applications [215]. 

Thermal stability of PVC can be modified using crosslinking agents. For example, crosslinking can be done with amino silanes such as bis(Y-trimethoxysilylpropyl)amine followed by reaction with traces of water in the presence of an oxide used as catalyst [26]. The reactions taking place in this case are shown below ... 

In our work we have used the polymerization of vinyl chloride at pressures (P) below the saturation value (P ) as a way to produce polymers, subsaturaticn PVC (U-PVC), with increased amounts of defects. This system is also a model for the later stages in a conventional batch polymerization of vinyl chloride, i.e. after the pressure drop. With decreasing relative monomer pressure, P/P, the thermal stability of PVC deteriorates strongly (6-8. l8). In a series of investigations we have determined different structures in several U-PVC samples and, as a reference, in a series of fractions of a commercial suspension PVC (S-PVC) (6-8. 11). 

However, a comparison of PVC with different amounts of 1 and 2 indicated that neither 1 nor 2 has a major influence on the thermal stability of PVC. In a recent paper, van den Heuvel and Weber ( Li) showed that these structures are stable at 180°C. 

If the labile structures are the main reason to the low thermal stability of PVC, Figures 3 and 4 should also reflect the concentration of the defects. In our previous work (7. 8). we showed that the rate of dehydrochlorination could be related to the amounts of tertiary and internal allylic chlorine. However, it is also likely that random dehydrochlorination will contribute to a certain extent (2. 3. 33. 34). According to our estimation, random dehydrochlorination could account for 10-15% of the initiation during degradation of ordinary PVC C2). It has been suggested that the stereo-structure should influence dehydrochlorination from ordinary monomer units (33. 35-37). The present samples also cover a change in polymerization temperature, AS-SO C. A comparison between the content of labile structures and thermal stability might therefore reveal an eventual influence of the tacticity. 

PVC is a polymer characterized by its low thermal stability, hence numerous studies have examined its thermal behaviour.68-73 It has been observed that PVC is more susceptible to dehydrochlorination than Cl-containing low molecular weight compounds. The presence of different types of defect sites in the polymeric chains has been proposed to explain the low temperature release of HC1 from PVC 74-79 tertiary and allylic chlorines, internal unsaturations, head-to-head configurations, etc. Likewise, the low thermal stability of PVC has been related to the presence of residues of certain additives used in the polymerization reaction. The major role of the stabilizers added to PVC is to decrease the rate of dehydrochlorination. 

Owing to poor thermal stability of PVC, the high temperature blending must be avoided. Thus only few PVC/engineering resin blends are known. These are summarized in Table 1.31. 

Sheets of PVC/ABS blends are often used for thermoforming large parts such as automotive or aircraft interiors. These blends have also been formed by molding and extrusion processes as low cost alternatives to flame-retarded ABS. The low thermal stability of PVC requires strict control of the processing conditions. 

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