THERMAL-DEGRADATION OF EVA AND EBA - A COMPARISON .2. CHANGES IN UNSATURATION AND SIDE GROUP-STRUCTURE

This is the second in a series of papers in which the thermal degradation of ethylene-vinyl acetate (EVA) and ethylene-butyl acrylate (EBA) copolymers are compared. The EBA samples contain 0.8, 1.6, and 5.4 mol % butyl acrylate (BA), respectively, and the EVA samples 1.2 and 6.7 mol % vinyl acetate (VA). The samples were heated in nitrogen in a tubular oven at 285, 333, 350, 370 and 390-degrees-C for 6-120 min. The samples were analyzed with IR, NMR, gravimetry, and titration of carboxylic groups. The EVA samples were rapidly degraded by deacetylation, which was complete after about 30 min at 333-degrees-C. A linear relation between the loss of acetate groups and the formation of trans double bonds was found. A small amount of keto groups and traces of lactones were also observed. The data confirm the previously proposed mechanisms for deacetylation and the formation of acetaldehyde. A mechanism for lactone formation is suggested. The deacetylation rate is increasing with the VA content, presumably because of an increased amount of block sequences and an enhanced acid catalytic effect. The acrylate sidegroups are much more stable than the acetate groups, and are similar in stability to the main hydrocarbon chain. The BA decomposition results in carboxylic and anhydride groups. Decarboxylation also occur and increases with the thermal treatment. In LDPE and EBA the increase in unsaturation is small and mainly due to vinyl end groups formed via beta-cleavage or disproportionation. In EVA the formation of vinyl end groups is suppressed.