Effect of the Thermo-Oxidation and Natural Weather on the Structure, Morphology and Properties of Unstabilized and HALS-Stabilized LDPE Films

The aging of unstabilized low density polyethylene (LDPE) films and those stabilized with hindered amine light stabilizers (HALS) was studied as a function of the exposure time to thermooxidation at 90 degrees C and to natural weathering. The HALS tested was Tinuvin 783 from Ciba-Geigy and added to the polymer in the concentration of 0.6% (w/w). Modifications in the him characteristics were investigated by using Fourier transform infrared (FTIR), scanning electron microscopy (SEM), wide angle X-ray scattering (WAXS) and mechanical testing. The FTIR spectra showed that the thermo-oxidation Bed to the formation of ketone groups attributed to hydroperoxide decomposition, whereas in natural weathering ketones and vinyl unsaturations are formed due to Norrish II reactions. The mechanism of stabilization involved in thermo-oxidation of the HALS-stabilized samples may be attributed to the classical radical scavenging of alkyl and peroxy radicals formed during propagation by nitroxyl radicals. The stabilizing activity of Tinuvin 783 in natural weathering may be explained by its capacity to trap the alkyl radicals resulting from the charge transfer complexes reactions between the polymer and oxygen. The effectiveness of Tinuvin 783 as heat and UV stabilizer in LDPE film was confirmed by the slow decline of the elongation at break as found in the exposed samples. The thermo-oxidation and the natural weathering caused the formation of micro-voids on the surfaces of the unstabilized films; the presence of the HALS reduced the number and the size of the micro-voids. WAXS analysis showed that the crystallinity indexes of the unstabilized samples, both thermo-oxidated and exposed to natural weather, increase significantly with the time; for the stabilized samples the increases of the crystallinity resulted less pronounced.