At elevated temperatures, poly(vinyl chloride) (PVC) undergoes dehydrochlorination to form polyene chain segments. These, in turn, produce hydrocarbons that burn in the vapor phase, leading to smoke and increased enthalpy. The effects of metal additives upon PVC at elevated temperature were studied. Three classes of metal compounds were used: (a) high-valent (+2, +3) metal chlorides, (b) high-valent metal complexes with oxidizable ligands, and (c) low-valent (+1, 0) metal complexes. Pyrolysis of PVC samples at 200°C in the presence of type (a), (b) or (c) metal compounds typically gave moderate to high degrees of polymer gelation due to crosslinking. Isothermal TGA studies of polymer/metal additive mixtures at 200°C also were conducted. These studies showed that metal additives of type (a) led to significant PVC mass losses due to dehydrochlorination. On the other hand, when additives (b) and (c) were used, early loss of mass due to the removal of volatile ligands was noted, but little dehydrochlorination was evident. The mechanism of polymer gelation promoted by low-valent metals appears to involve reductive coupling of allylic chloride defect sites. Complexes of copper(I) are especially attractive as reductive coupling agents and hence as potential PVC smoke suppressants.
展开▼
