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The effect of addition of isocyanate to a polyurethane adhesive has been investigated. Polyurethane adhesive was used for bonding a thermoplastic (TR) and a vulcanized (VR) synthetic butadiene-styrene rubber. CO 2 plasma treatment has oxidized the rubber surfaces, increasing surface energy (wettability), roughness, and favoring mechanical interlocking of rubber surface with the adhesive. Adhesion of TR rubber towards PU polyurethane adhesive was enhanced by these mechanisms; however, migration of antiadherent low molecular weight moieties to the VR rubber/polyurethane adhesive interface produced a lack of adhesion. In this case, it was necessary to produce the crosslinking with isocyanate of the polyurethane adhesive and of the VR rubber surface.

The effectiveness of nitrogen, oxygen and air Radio Frequency (RF) plasma treatments on two styrene-butadiene vulcanized rubbers with a different formulation has been studied. The presence of an antiadherent surface layer containing low-molecular weight ingredients (sulfur-rich vulcanization agents and wax) from SW (Sulfur-Wax) rubber formulation requires an extended plasma treatment capable of removing this surface layer. When the percentage of antiadherent moieties is reduced in ZS (Zinc Stearate) rubber formulation, shorter plasma treatment times are enough to modify rubber surface and increase its polarity by the creation of C-O and C=O polar groups that enhance adhesion towards a polyurethane adhesive. Air and oxygen plasma treatments are more aggressive than nitrogen plasma and therefore they are more effective in removing the antiadherent layer of the outermost rubber surface layer prior to oxidation of the rubber surface.