Low-pressure plasma pretreatment of polyphenylene sulfide (PPS) surfaces for adhesive bonding

Polyphenylene sulfide (PPS) is a high-performance plastic with outstanding mechanical, physical and thermal characteristics for lightweight automobiles and rail-mounted vehicles. The adhesive behavior of PPS with aluminum plates by using epoxy (EP) and polyurethane (PU) adhesives is the subject of these investigations, whereby the activation of the PPS surfaces took place by means of low-pressure plasma treatment. The determination of the changes caused in terms of topography, surface tension and chemical composition of the PPS surfaces after low-pressure plasma treatment was carried out by means of scanning electron microscopy (SEM), contact angle measurements and energy dispersive X-ray analysis (EDX). The adhesion bond strength increased after oxygen plasma treatment by 103% (EP 1), 134% (EP 2) and 56% (PU). After argon plasma treatment, the increase in adhesive bond strength was only 79% (EP 1), 122% (EP 2) and 27% (PU). The failure took place with increasing adhesive strength with EP 1 predominantly cohesively in the PPS at the aluminum lap end, with EP 2 adhesively at the aluminum surface and with PU cohesively in the adhesive layer. The surface energy and the polar contributions increased significantly with treatment time with both plasma gases. This was more pronounced with oxygen than with argon plasma. The dispersive contributions to the surface free energy hardly changed with increasing treatment time. The SEM shows that the surface roughness also increases with treatment time in the case of the oxygen plasma more compared with the argon plasma. Furthermore, a deposit of reaction products on the surfaces was detected.