Crosslinking of Ethylene Copolymers

Abstract: The chemical reactions taking place when crosslinking poly(ethylene-co-vinyltrimethoxysilane) (EVS) have been studied. Films of EVS were stored in water at 90 °C or treated at processing temperatures in a tubular oven. The different chemical groups involved in the reactions were then followed by FTIR after different treatment times. Gel content was determined in parallel. The investigation concerning water treatment of films shows that a further formation of Si-O-Si crosslinks takes place also after the point at which maximum gel content has been reached. Mechanical measurements indicate that the further crosslinks are formed within the existing gel.

For samples treated at processing temperatures the great importance of temperature, crosslinking catalyst, surrounding atmosphere and supply of external water on the rate of crosslinking was demonstrated. Another EVS copolymer containing butyl acrylate in the main chain (EVSBA) as well, was treated at processing temperature in pure nitrogen without catalyst. A considerable amount of gel was obtained despite the absence of external water. It was shown that the butyl acrylate groups in EVSBA are degraded and form carboxylic acid groups, which form anhydride under a simultaneous formation of water, enabling the crosslinking reaction and the gel formation. In addition carboxylic acid groups were shown to be active as crosslinking catalyst in the polymer. The catalytic activity of both stearic acid and polymer bound carboxylic acid groups were demonstrated, although less active that the commonly used dibutyltin dilaurate (DBTDL) on a molar basis. Indications were found that carboxylic acids probably catalyse both the hydrolysis and the condensation step of the crosslinking reaction. Furthermore DBTDL was found to strongly catalyse the first step of the crosslinking reaction, that is, the hydrolysis.

The response of poly(ethylene-co-1,9-decadiene) to peroxide and electron beam irradiation was investigated and compared to a reference. These polymers were produced in a low pressure process using a catalyst giving a relatively high level of inherent unsaturations. The copolymerisation with 1,9-decadiene gave an additional 63 % vinyl end groups in the polymer and a considerable improvement in the crosslinking response was observed. The reason for the improved crosslinking response was found to be the overall higher amount of vinyl groups and the placement of vinyl groups along the molecular weight distribution. A certain amount of vinyl unsaturations still remained after irradiation whereas almost all double bonds were consumed at high peroxide levels. Determination of the crosslinking density, Mc, at a certain degree of crosslinking, exhibited relatively small differences between the material containing decadiene and the reference material. These results were found to support the idea that the crosslinked network is mainly built up of the entanglements, i.e. physical crosslinks. At industrially used levels of degree of crosslinking Mc was found to be higher for irradiated polymer than for peroxide crosslinked polymer. Micrographs obtained from transmission electron microscopy (TEM) show clear differences in morphological structure between the different crosslinking technologies and this is likely to explain some observed differences in mechanical behaviour between the materials.