Effect of hydrolysability and structural features of phosphites on processing stability of isotactic polypropylene

The performance of four commercially employed phosphites was examined in isotactic polypropylene. In combination with phenolic antioxidant the effects of hydrolysability, molecular structure and the rate of conversion on processing stability during multiple extrusion were studied. It was shown that besides the chemistry of trivalent phosphorus, the reactions of the secondary structure of the phosphite also influence the overall performance. Phosphites containing substituents able to form stable, non-propagating species during processing, such as hindered phenols, are able to perform also by the chain-terminating mechanism. The ability to release the chain-terminating species during radical attack or hydrolysis is controlled by the stability and steric size of the substituents as well as by their influence on the electron density at the phosphorus atom. The consumption rates of phosphites were found to be proportional to the performance. It was observed that phosphites may not only sacrifice themselves to protect phenols but up to a certain extent they may also protect one another. In some systems using equiportional combinations of hydrolysable and non-hydrolysable phosphite an improvement in performance was observed.