Inhibition of the thermal degradation of rigid poly(vinyl chloride) by dienophilic compounds

N-(N′-Arylamino)maleimide derivatives N-(N′-RPh)AMI (R = H, 2-NO2, 4-NO2, 2,4-di-NO2) were investigated as organic thermal stabilizers and costabilizers for rigid poly(vinyl chloride) (PVC) at 180°C in air. Their stabilizing efficiencies were evaluated by measuring the induction period (Ts, the period during which no evolved hydrogen chloride could be detected) and the dehydrochlorination rate as measured by continuous potentiometric determination, as well as the extent of discoloration. The stabilizing efficiencies were compared with those of some industrially used stabilizers. The results clearly revealed the greater efficiency of all of the investigated derivatives except the unsubstituted N-(N′-Ph)AMI, which was found to possess efficiency comparable with those of the reference stabilizers. The dinitro aminomaleimide [2,4-di(NO2)AMI] exhibited the highest efficiency. The stabilizing efficiencies of the AMIs are attributed to the replacement of the labile chlorine atoms on the PVC chains by more thermally stable stabilizer moieties. An ionic mechanism is proposed that explains the stabilizing action. Mixing the investigated stabilizers with cyanoguanidine, which is known to act as an HCl absorber, led to a true synergistic effect. The effect of the degradation process of adding small amounts of K2CO3 to PVC stabilized with the materials under investigation also is discussed. J. VINYL ADDIT. TECHNOL., 2008. © 2008 Society of Plastics Engineers