Thermal oxidation of polyolefins by mild pro-oxidant additives based on iron carboxylates and lipophilic amines: Degradability in the absence of light and effect on the adhesion to paperboard

Abstract

Marine and inland pollution by non-degradable plastic bags and other plastic articles is a topic of great concern. Natural degradation processes based on oxidation of plastic pollutants could possibly contribute to limit the extent of pollution. Thermal degradation of polyolefins in the absence of light by non-polluting pro-oxidants has not been presented before. In this study, we show that two amines, stearyl amine and [(3-(11-aminoundecanoyl) amino) propane-1-] silsesquioxane (amino-POSS) in combination with ferric stearate (FeSt3) tremendously accelerate the thermal oxidation of polyolefins compared with reference samples. Both amines and FeSt3 are to a large extent based on renewable resources. Polyethylene and polypropylene samples containing less than 100 ppm of iron and 1% of amine were extremely brittle after 10 days in a circulation oven in the absence of light. No significant degradation could be seen with samples containing iron but no amine. In a different application, the initial oxidation of polyethylene can be used in order to increase its adhesion to cardboard. Excellent adhesion between polyethylene and cardboard is important for liquid packaging based on renewable resources. Amino-POSS has been chosen for food packaging applications due to its expected lower leakage from polyethylene (PE) compared with stearyl amine. Film samples of PE/amino-POSS/FeSt3 blends were partly oxidized in a circulation oven. The oxidation was documented by increased carbonyl index (CI) and melt flow index (MFI). The limited extent of oxidation has been proved by unchanged tensile strength and only moderate changes in elongation at break when compared to reference polyethylene films containing no FeSt3 or amino-POSS. The PE/amino-POSS/FeSt3 blends were compression moulded to paperboard. The adhesion of non-aged blends to paperboard decreased with increasing amino-POSS content which is in good compliance with an earlier reported lubricant effect of high amounts of POSS in PE. Thermal ageing of PE/amino-POSS/FeSt3 films prior to coating however led to a significant increase in adhesion. Improved physical interlocking due to increased MFI and interaction between C=O of the blends and OH of paperboard can explain the adhesion improvement. The films were not brittle after thermal ageing, which makes their use in industrial packaging feasible. A mechanism explaining the role of amines during thermal oxidation of polyolefins in the presence of iron is proposed.