N-(Substituted phenyl)itaconimides as organic stabilizers for plasticized poly(vinyl chloride) against thermal degradation

Abstract

A series of N-(substituted phenyl)itaconimide derivatives, N-(RPh)II, (R: –NO 2, –COOH, –H, –OH, –OMe, –Me, –Cl, or –Br) has been investigated as organic thermal stabilizers for plasticized poly(vinyl chloride) (PVC) at 180 °C in air. Their stabilizing efficiencies are evaluated by measuring the length of the induction period ( T s), 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 on one hand, and the extent of discoloration of the degraded polymer on the other. Their stabilizing efficiencies were compared with that of dibasic lead carbonate (DBLC), barium–cadmium–zinc stearate (Ba–Cd–Zn stearate) and n-octyltin mercaptide ( n-OTM), which are commonly used industrial stabilizers. With the exception of the nitro derivatives of the itaconimide, the results clearly reveal the greater stabilizing efficiency of the N-(RPh)II derivatives relative to that of the industrially used thermal stabilizers. The higher efficiency of the investigated stabilizers is well illustrated by the longer T s values, and by the lower rates both of dehydrochlorination and discoloration of the polymer during degradation relative to those of the reference stabilizers. The results also demonstrate how the stabilizing efficiency is strongly affected both by the nature and position of the substituents in the phenyl ring of the itaconimide stabilizers together with the nature of the plasticizer used. While the stabilizing efficiency is higher for N-(RPh)II derivatives having substituents of positive resonance effect (+R) and lower for those having substituents of negative resonance effect (−R), the non-substituted N-PhII being in the middle. The stabilizing efficiency of N-(RPh)II derivatives is attributed to their radical trapping potency which intervenes with the radical degradation process of PVC and results in the replacement of the labile chlorines on the PVC chains by relatively more thermally stable stabilizer moieties. A radical mechanism is proposed which illustrates the stabilizing action of the products investigated. The better color stability of the degraded PVC in the presence of the itaconimide stabilizers may be ascribed by the ability of these materials to interact with the conjugated double bonds created on the PVC chains during degradation. The results also showed relatively higher stabilizing efficiency in the presence of DOP compared with the DBP plasticizer. This is most probably attributable to the greater size of the octyl groups which results in greater separation of the neighbouring polymeric chains and thus minimizes the possible cross-dehydrochlorination reaction. A synergistic effect was achieved when the materials under investigation were blended in various molar ratios with either of the reference stabilizers. This synergism attains its maximum when both the investigated and the reference stabilizers are taken in equivalent molar ratios.