Blown extruded films of low-density polyethylene (LDPE) have been subjected to climatic ageing in a subSaharan facility at Laghouat (Algeria) with direct exposure to sun. Samples were characterized by complementary techniques after prescribed amounts of time up to 8 months. It was shown by tensile testing that the mechanical properties are quite sensitive to ageing: (i) the elastic modulus increases and saturates, (ii) the tensile stress increases slightly, and (iii) the rupture energy decreases dramatically after 4 months weathering. Fourier-transform infrared spectroscopy (FTIR) and nuclear magnetic resonance ( 13 C NMR) were performed to identify the evolution of the polymer microstructure. The FTIR spectra reveal the initial presence of vinylidene groups that exhaust rapidly after 4 months ageing. Also, it detects the progressive multiplication of vinyl groups and oxidation products of many kinds. The NMR technique revealed specifically the carbon-carbon configurations in the polymer chains. By contrast to the original film that contained almost exclusively butyl chain branches, the aged specimens presented shorter ramifications, namely ethyl branches. Also, the presence of quaternary atoms was detected after long ageing times. The discussion of these complementary results in the light of current literature makes possible to identify the leading mechanisms that control the decay of LDPE film properties. Although these mechanisms are numerous and complex, they can be schematically summarized within three main classes: oxidation, scission, and crosslinking. Each class is discussed in details.
This work analyzes the influence of thermal degradation on the microstructure and the mechanical properties of low-density polyethylene subjected to aging at 70 C in the dark for times up to 21 months. It is found that the polymer shows a gradual increase of its elastic modulus and a dramatic reduction of its ductility, due to secondary crystallization. Infrared spectroscopy (FTIR) reveals the autoaccelerated oxidation of the polymer after 5 months aging. It is observed that the unsaturated vinylidene groups initially present in the material are gradually overridden by vinyl groups and, eventually, by t-vinylene groups. Nuclear magnetic resonance ( 13 C NMR) shows that the initial butyl chain branches are progressively completed by shorter ramifications, namely ethyl branches. These results are discussed in term of macromolecular mechanisms: (i) oxidation, (ii) chain scission, and (iii) crosslinking.
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