Synthesis and thermal and photo-oxidative behaviour of novel 4-substituted alicyclic amino-2-hydroxybenzophenone stabilizers in polypropylene films

Abstract

Four light stabilizers have been synthesized and characterized, based on the substitution of an alicyclic amine group into the 4-position of 2-hydroxybenzophenone. The compounds prepared were bis- N, N′-alkylpiperazines, N-propoxymorpholinyl and 4-amino-2,2,6,6-tetramethylpiperidinyl derivatives. The thermal and photo-oxidative stabilization activities of the three compounds have been examined and compared with those of 2-hydroxy-4-n-octoxybenzophenone (HOBZ) in polypropylene film at 0.1% w/w concentration using i.r. absorption spectroscopy, hydroperoxide analysis and yellowness index. Through both carbonyl index and embrittlement times, both alkylpiperazine derivatives were found to be ineffective on exposure to QUV irradiation but both the propoxymorpholinyl and the hindered piperidine compounds were found to be more effective than HOBZ. These stabilizing effects followed closely their ability to inhibit hydroperoxide formation before and during oven ageing. With the commercial hindered phenolic antioxidant, Irganox 1010, all the stabilizers were synergistic, especially HOBZ. On thermal ageing, all the light stabilizers including HOBZ behaved as antioxidants. The hindered piperidine derivative was the most effective, exhibiting a stabilizing effect close to that of the Irganox 1010. In the presence of Irganox 1010, weak antagonism was observed with HOBZ exhibiting the most effective stabilization. Both piperazine derivatives discoloured markedly on irradiation while the alkylmorpholinyl exhibited an initial photobleaching effect. The steric hindrance, molecular size and oxidizability of the cyclic amine are all concluded to be important factors in controlling the antioxidant behaviour of the stabilizer, with hydroperoxides playing a key role in the latter case. The results also demonstrate the importance of the ortho hydrogen bonded hydroxy group in 2-hydroxybenzophenones as an active chain-breaking donor and not the extinction coefficients. It is further concluded that the results show the potential for the development of a single stabilizer entity which will behave as an effective photothermal antioxidant.