Thermal Degradation and Adhesion Studies of Ethylene Copolymers
The thermal stability of ethylene(vinyl acetate) (EVA), ethylene(n-butyl acrylate) (EBA), ethylene(vinyl trimethoxy silane) (EVS) and ethylene(vinyl trimetoxy, butylacrylate) (EVSBA) copolymers have been mapped out in this study. The different ester groups', i.e. VA and BA, influence on the copolymers adhesion behaviour towards aluminium as well as its response to corona treatment are reported. In most experiments Low Density Polyethylene (LDPE) has been used as a reference. The polymers have been degraded under nitrogen for 6 to 120 min. at temperatures between 150-390 °C. Volatile decomposition products have been determined by GC-MS and ion chromatography. The degraded polymers have been characterized by Size Exclusion Chromatography (SEC), IR, NMR, titration, decaline extraction and gravimetry. The adhesion behaviour has been studied by T-peel test and surfaces characterized by ESCA and Multiple Internal Relection IR (MIR) techniques.
EVA undergoes ester pyrolysis already at 150 °C. Acetic acid is split off and trans-vinylene groups are formed in the polymer. 30 min degradation at 333 °C leads to an almost complete removal of the acetate groups. EVA shows a strong tendency to molecular enlargements due to formation of allyl radicals at the internal double bonds. These radicals combine or add to unsaturated groups. To a minor extent acetaldehyde is formed giving ketone groups in the polymer. EBA undergoes ester pyrolysis at about 280 °C. Butene is split off and a carboxylic acid remains in the polymer. This is subsequently dehydrated to mainly intramolecular anhydrides in static degradation experiments. A slow decarboxylation reaction occurs as well. Due to the higher content of tertiary C-H bonds and resonance stabilization of radicals formed at the acrylate groups, EBA shows a stronger chain scission tendency than LDPE. The big difference in stability between the two investigated ester groups is mainly due to conformational reasons. The crosslinkable EVS copolymer does not give any gel during thermal degradation in the absence of water and crosslinking catalyst. In presence of water the crosslinking starts at about 200 °C. On the other hand, EVSBA starts to crosslink at about 300 °C, in spite of no external supply of water. This is explained by the above described degradation pattern of the BA group, i.e. formation of the carboxylic acid, which acts as a crosslinking catalyst, and its dehydration.
The adhesion studies showed that the peel force of EVA and EBA is linearly related to the comonomer content. The BA group is more effective than the VA group in improving the adhesion. Corona discharge treatment of these copolymers has a remarkably positive effect on the adhesion properties. The difference in adhesion behaviour between EVA and EBA can possibly be explained by their different degradation behaviours.