Shear strength of SMR-based pressure-sensitive adhesives

The shear strength of Standard Malaysian Rubber (SMR)-based pressure-sensitive adhesives was studied using coumarone-indene resin as the tackifier resin. Three grades of SMR, i.e., SMR L, SMR 10, and SMR 20 were used as the elastomers. The concentration of tackifier resin was varied from 0-80 parts per hundred parts of rubber (phr). Toluene was used as the solvent throughout the experiment to prepare the pressure-sensitive adhesives. A SHEEN hand coater was used to coat the adhesive on the polyethylene terephthalate substrate to give a coating thickness of 30, 60, 90, and 120 mu m. Shear strength of the adhesive was determined by using a Texture Analyzer. Results indicate that for a fixed coating thickness, shear strength decreases gradually with increasing resin content for all the rubbers studied. This observation is attributed to the decreasing cohesive strength of adhesive as resin loading is increased. However, for fixed resin content, shear strength increases with increasing coating thickness suggesting that shear strength is thickness-dependent. SMR L consistently shows higher shear strength than SMR 10 and SMR 20 for all coating thickness, an observation, which is attributed to higher purity of SMR L, compared with the latter two rubbers. The shear strength passes through a maximum at 5 min of mastication time, after which it decreases gradually with further mastication.