WEAK BOUNDARY-LAYERS IN STYRENE-BUTADIENE RUBBER

In this paper two kinds of weak boundary layers (WBL) in synthetic vul canized styrene-butadiene rubber are described. i) WBL produced by the presence of antiadhesion compounds of the rubber formulation (zinc st earate, microcrystalline paraffin wax). These WBL cannot be effectivel y removed by solvent wiping, whether followed by washing with an ethan ol/water mix or not. Although this treatment allowed a significant rem oval of zinc stearate, the paraffin wax concentration on the surface w as not greatly reduced, thus, poor adhesion of rubber was obtained. Ch lorination with small amounts of ethyl acetate (EA) solutions of trich loro isocyanuric acid (0.5-5 wt% TCI/EA) and/or an extended halogenati on treatment increased the adhesion strength and effectively eliminate d the zinc stearate from the rubber surface. If an additional heat tre atment (50 degrees C/24 h) of the chlorinated rubber was also carried out, the WBL was more effectively eliminated and the resulting adhesio n was independent of the amount of chlorination agent applied to the r ubber surface. Furthermore, this heat treatment favoured the eliminati on of WBL in the untreated rubber and also contributed to the removal of WBL produced by an excess of halogenation agent. ii) WBL created by an excess of chlorination agent applied to the rubber surface. The ex cess of chlorination agent produced lack of adhesion in the rubber bec ause there was significant damage of the rubber surface and a non-rubb er surface layer was formed (mainly due to oxidized, chlorinating agen t residues and cyanuric acid), which contributed to the formation of W BL. To avoid the creation of WBL, a postchlorination treatment of rubb er with a solution of 25 wt% ethanol in water followed by a vacuum-dry ing process produced excellent results. The effectiveness of this trea tment relied on combining an adequate degree of chlorination with no e xternal surface deterioration of the rubber by the excess of chlorinat ion agent.