Weak boundary layers in styrene-butadiene rubber

In this paper two kinds of weak boundary layers (WBL) in synthetic vulcanized styrene-butadiene rubber are described. i) WBL produced by the presence of antiadhesion compounds of the rubber formulation (zinc stearate, microcrystalline paraffin wax). These WBL cannot be effectively removed by solvent wiping, whether followed by washing with an ethanol/water mix or not. Although this treatment allowed a significant removal of zinc stearate, the paraffin wax concentration on the surface was not greatly reduced, thus, poor adhesion of rubber was obtained. Chlorination with small amounts of ethyl acetate (EA) solutions of trichloro isocyanuric acid (0.5-5 wt% TCI/EA) and/or an extended halogenation treatment increased the adhesion strength and effectively eliminated the zinc stearate from the rubber surface. If an additional heat treatment (50 degrees C/24 h) of the chlorinated rubber was also carried out, the WBL was more effectively eliminated and the resulting adhesion was independent of the amount of chlorination agent applied to the rubber surface. Furthermore, this heat treatment favoured the elimination 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 excess of chlorination agent produced lack of adhesion in the rubber because there was significant damage of the rubber surface and a non-rubber surface layer was formed (mainly due to oxidized, chlorinating agent residues and cyanuric acid), which contributed to the formation of WBL. To avoid the creation of WBL, a postchlorination treatment of rubber with a solution of 25 wt% ethanol in water followed by a vacuum-drying process produced excellent results. The effectiveness of this treatment relied on combining an adequate degree of chlorination with no external surface deterioration of the rubber by the excess of chlorination agent.