Abstract
N-(Substituted phenyl)itaconimide derivatives,
N-(RPh)II, (R: –NO
2, –COOH, –H, –OH, –OMe, –Me, –Cl or –Br) have been investigated as organic photo-stabilizers for rigid poly(vinyl chloride) (PVC). Their stabilizing efficiencies were 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, the extent of discoloration and the change in the mechanical properties of the degraded polymer on the other. The efficiencies were also evaluated by determining the amount of gel formation as well as the intrinsic viscosity of the insoluble and of the soluble fractions of the degraded polymer, respectively. Their stabilizing efficiencies were compared with that of phenyl salicylate UV absorber which is a commonly used industrial stabilizer. The results have proved the higher stabilizing efficiency of almost all the investigated materials as compared with phenyl salicylate. This is well illustrated not only by the greater
T
s values, but also by the lower rates of dehydrochlorination at later stages of degradation. The results also reveal that the type and position of the substituent plays a major role in determining the stabilizing potency of these materials. It was found that the stabilizing efficiency increases with the increase in the ability of the substituent to increase the electron density around the carbon–carbon double bond of the itaconimide. The stabilizing efficiency of the itaconimides is attributed to their radical trapping potency which intervenes with the radical degradation of PVC. This most probably occurs not only through trapping the radical species in the degradation process, but also by blocking the newly formed radical sites on the polymer chains. The radical attack seems to occur firstly on the ethylenic carbon–carbon double bond of the itaconimide then by the imide linkages at later stages of degradation. Moreover, it was found that these materials lower the extent of discoloration of the polymer during degradation as compared with phenyl salicylate. This improvement in the colour stability is most probably attributable to the ability of the itaconimides to react by a Diels–Alder reaction with the conjugated double bonds created on the polymeric chains as a result of degradation, and responsible for the coloration of the polymer. Furthermore, the extent of cross-linking of the polymer in the presence of the investigated derivatives has been proved to be lower than that in the presence of phenyl salicylate, which implies a greater extent for preserving the mechanical properties of the polymer from being drastically changed. Finally, the results illustrate that blending of the itaconimide derivatives with phenyl salicylate improves both the
T
s values and the degree of discoloration, and this improvement attains its maximum when both the investigated stabilizers and phenyl salicylate are taken in equimolar ratios. The observed synergistic effect is most probably attributed to the combination of the mechanisms by which the itaconimide and phenyl salicylate work.