The effect of crystallization on the modulus of thermally conductive silicone adhesives

Low modulus, thermally conductive silicones were evaluated as soft mount adhesives for substrate attachment. It was determined that due to the existence of a crystalline phase, above the glassy transition temperature (Tg≈−125°C), the modulus of the adhesives can be orders of magnitude higher than at room temperature. Differential scanning calorimetry shows that at a cooling rate of 5°C/min the crystallization temperature, Tc, is −75°C, which shifts to −66°C under a slower ramp rate of 0.5°C/min. Correspondingly, a sharp increase in modulus also occurs at Tc when the adhesive undergoes the same thermal processes. When the temperature is held isothermally below the crystalline melting temperature (Tm=−42°C), the modulus increases simultaneously with the increase in the degree of crystallization. The growth rate of the modulus increases exponentially as the temperature approaches −75°C from Tm. Once the silicones are in the high modulus state, the temperature must be increased above Tm to melt the crystalline phase and recover the normal low modulus phase. If the heating rate is faster than about 0.5°C/min, the recovery temperature can pass over Tm to about −35°C.