ADHESIVE JOINING OF POLYMERS REPLACING RIVETTING AND WELDED JOINTS

Abstract

The surfaces of polypropylene (PP) have been exposed to DC glow discharge to enhance wettability of the polymers in order to improve their adhesive joint strength with steel. The surface modification of the polymers have been characterised by measuring contact angles of sessile drops of two test liquids of known surface tension components, such as deionised water and formamide, followed by estimation of their surface energy. The mechanism of surface modification has been studied using Fourier Transform Infrared Spectrophotometer (FTIR). When the polymers are exposed to glow discharge for 60 s at a power level of 25W, the contact angles are found to be reduced from 94 to 600 using deionised water for the PP sheet and 77° to 42° using formamide for the above polymer. Lowering in contact angle at this range results in enhancement in surface energy from 22.80 to 45.51 mJ/m2 for the PP sheet. It is found that surface energy increases due to the enhancement in polar component of surface energy as polar groups are formed on the polymer surface under exposure to glow discharge as evident from FTIR, which shows negligible oxygen peak for the unexposed polymers; however, glow discharge exposed polymers shows significant oxygen peak. The surface chemistry of the modified polymers, defined as their oxygen to carbon ratio, has been correlated to their adhesive joint strength with steel and it is found that adhesive joint strength of the polymers to steel increases with the increase of oxygen in the polymer surface.