Durability of corona discharge and low-pressure air plasma-treated ethylene–vinyl acetate copolymers (EVAs)

Abstract

EVA (ethylene–vinyl acetate copolymer) containing 20 wt% vinyl acetate (VA)—EVA20—was treated with corona discharge and low-pressure air plasma, and the durability of these treatments was monitored for 12 months. The corona discharge treatment was carried out at corona energy of 0.3 J/cm 2 and the air plasma treatment at 300 W and 10 mbar for 3 s. Water contact angle values of EVA20 markedly decreased after treatment, and then were constant (about 60°) by increasing the time after treatment. On the other hand, a few hours after treatment the oxygen concentration on the treated EVA20 decreased, and after 12 months the surface chemistry of the corona discharge-treated EVA20 did not vary. However, for the air plasma-treated EVA20, a decease in oxygen concentration was obtained, becoming similar to that of the as-received EVA20. Peel strength values of treated EVA20/polychloroprene adhesive+5 wt% isocyanate joints increased from 1.5 kN/m (as-received EVA20) to 3.0–4.0 kN/m in the corona discharge and 4.5 kN/m for the air plasma-treated EVA20. On the other hand, the locus of failure in the joint produced with the as-received EVA20 was interfacial whereas a mixed failure (interfacial+cohesive failure in the EVA20) was produced in the joints produced with the corona discharge and air plasma treated EVA20. The adhesive strength of the corona discharge-treated samples remained unchanged for 12 months after treatment, whereas they markedly decreased 35 days after air plasma treatment.