Effect of photooxidative ageing at natural conditions on dynamic mechanical properties of PE-LD/PE-LLD blends

Abstract

The blends of low density polyethylene with linear low density polyethylene and pure polyolefins were blown for their use in agriculture. These films were stabilized with a commercial Hindered Amine Light Stabilized (HALS). Blends with this stabilizer were photodegraded under natural outdoor weathering conditions during two years. Photodegradation involves oxidation of the polymer chain and causes modifications of the mechanical properties of the exposed films that were monitored by dynamic mechanical spectroscopy. The mechanical relaxation spectra show the α, β and γ relaxation zones, in decreasing order of temperature. Their characterization was carried out using the deconvolution method proposed by Charlesworth, together with the Fuoss-Kirkwood equation. The results obtained for pure PE-LLD and PE-LD and their blends show that the storage modulus increases as a function of the exposure time. The maximum storage modulus is reached at different exposure times depending on the blend composition. The β relaxation zone is the most affected by weathering. A new relaxation was detected between -85 o C and -75 o C, which was modified as a function of the exposure time, and finally even disappeared. All the samples exhibit the same evolution, but at different time scales. It has been found that after two years of exposure, the degradation of the interfacial region is already initiated. The blends with 50 % of PE-LD are the most resistant to weathering.WP£YW FOTOOKSYDACYJNEGO STARZENIA W WARUNKACH NATURALNYCH NA DYNA-MICZNE W£AOECIWOOECI MECHANICZNE MIESZANIN PE-LD/PE-LLD Streszczenie -Przedmiotem badañ by³y mieszaniny polietylenów ma³ej gêstooeci (PE-LD/PE-LLD) w pe³nym przedziale ich sk³adu. Metod¹ dynamicznej analizy termomechanicznej (DMTA) okreoelono wp³yw sk³adu i czasu ekspozycji (do 24 miesiêcy) na przebieg fotooksydacyjnego starzenia tych uk³adów w warunkach naturalnych (Hiszpania, tabela 1). Scharakteryzowano zachodz¹ce w toku starzenia zmiany modu³u zachowawczego (E', rys. 1), tangensa k¹ta stratnooeci (tgδ, rys. 2) oraz modu³u stratnooeci (E", rys. 3). Na tej podstawie wyodrêbniono trzy obszary relaksacyjne (α, β, γ) okreoelaj¹c ich przedzia³y temperaturowe (tabele 2, 3, 4); w odniesieniu do nak³adaj¹cych siê obszarów relaksacji α i β zastosowano przy tym zaproponowan¹ przez Charleswortha metodê dekonwolucji wraz z równaniem Fuossa-Kirkwooda (rys. 4). Ustalono, ¿e zmiany spowodowane starzeniem uwi-daczniaj¹ siê najbardziej w obszarze relaksacji β. Najodporniejsze na starzenie s¹ mieszaniny zawieraj¹ce 50 % PE-LD. S³owa kluczowe: mieszaniny PE-LD/PE-LLD, fotooksydacja, dynamiczne w³aoeciwooeci mechaniczne, procesy relaksacyjne.Blends of polyethylenes with different branching degrees are commonly used in order to adjust the properties of these polymers, such as their modulus and toughness, or to improve their melt properties during processing. However, the miscibility or immiscibility of the components might affect the mechanical properties as a result of their weak interfacial adhesion, which may even induce ...