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.
Highlights
Polyethylene (PE) coated paperboard has been widely applied for packaging foods, beverages and pharmaceuticals because of its low cost, lightweight, non-toxicity, excellent water barrier, good mechanical properties and processability
Amino-polyhedral oligomeric silsesquioxane (POSS) is due to its structure and molecular weight is likely to show lower leakage from PE compared with stearyl amine
carbonyl index (CI) is a measurement of the amount of carbonyl compounds formed during the thermal oxidation and is calculated as the ratio of the absorbance (A) of carbonyl peak in the region 1700–1780 cm1 and the CH2 scissoring peak at
Summary
Polyethylene (PE) coated paperboard has been widely applied for packaging foods, beverages and pharmaceuticals because of its low cost, lightweight, non-toxicity, excellent water barrier, good mechanical properties and processability. Suitable accelerators could increase the decomposition of hydroperoxides in PE and yield rapid thermal oxidation rates with iron as the only transition metal present. Amines derived from fatty acids, such as stearyl amine or polymer based amines are suitable candidates in order to accelerate the rate of thermal oxidation rate in polyethylene containing an iron based pro-oxidant. By using iron-based pro-oxidants in combination with stearyl amine or polymer based amines, it should be possible to improve the adhesion of polyethylene to paperboard. We chose [(3-(11-aminoundecanoyl) amino) propane-1-] silsesquioxane (amino-POSS) to examine the effect on the adhesion strength of PE films iron/amine pro-oxidants to paperboard. Amino-POSS is due to its structure and molecular weight is likely to show lower leakage from PE compared with stearyl amine It is the preferred amine for food packaging applications
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