AbstractThe internal structure of pressure‐sensitive adhesives was studied using electron microscopy and measurements of mechanical loss, tensile modulus, viscosity, stress relaxation, and critical surface tension. The adhesives were blends of natural rubber and the pentaerythritol ester of hydrogenated rosin. Compositions containing up to 40 wt‐ resin are homogeneous mixtures. The temperature dispersion of mechanical loss shows a single peak, and the peak value remains almost constant. When the resin concentration exceeds 40 wt‐, phase separation occurs. The disperse phase is resin containing a small amount of rubber. Two peaks, or a peak and a shoulder, appear in the temperature–loss peak curves. It is postulated that one of the peaks corresponds to the phase in which resin is uniformly dispersed in the rubber and the other peak corresponds to the resin phase which contains a small amount of rubber. Evidence for homogeneity in compositions containing 40 wt‐ or less resin and of heterogeneity at higher concentrations of resin was also obtained from the electron microscope observations. The relationships between the internal structure of pressure‐sensitive adhesives and viscosity, Young's modulus, the relaxation spectrum, tack, and critical surface tension ar
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