ANTIOXIDANT INTERACTION BETWEEN ORGANIC PHOSPHITES AND HINDERED AMINELIGHT STABILIZERS DURING PROCESSING AND THERMOXIDATION OF POLYPROPYLENE

Mixtures of hindered amine light stabilisers (HALS) and their radical analogues, with aromatic and aliphatic phosphites have been examined f or their effectiveness as melt and thermal antioxidants in polypropyle ne (PP. Stabilisers containing two different antioxidant functions in the same molecule (aromatic phosphite and either of the above HALS or related functions), referred to as `HALS-phosphites', were also examin ed and their antioxidant effectiveness compared with the optimum perfo rmance of the corresponding combination of mixtures of the individual antioxidants. Binary combinations of the phosphites Irgafos 168 (aroma tic) and TLP (aliphatic) with Tinuvin 770, its bis-nitroxyl analogue, or with Tinuvin 292 (tertiary amine) exhibited synergistic effects at different molar ratios; the level of synergism was generally higher du ring thermal ageing than during melt processing. The optimum combinati on, leading to the most effective synergism, varied with the chemical nature of the phosphite: under thermoxidative conditions, molar excess of the Irgafos 168 is required whereas molar excess of the HALS is ne eded for optimum of TLP. HALS-phosphites act autosynergistically and s how, in most cases, higher efficiency than their corresponding additiv e mixtures of stabilisers.