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

Low modulus, thermally conductive silicones were evaluated as soft mount ad hesives for substrate attachment. It was determined that due to the existen ce of a crystalline phase, above the glassy transition temperature (T(g)app roximate to - 125 degrees C), the modulus of the adhesives can be orders of magnitude higher than at room temperature. Differential scanning calorimet ry shows that at a cooling rate of 5 degrees C/min the crystallization temp erature, T-c, is -75 degrees C, which shifts to -66 degrees C under a slowe r ramp rate of 0.5 degrees C/min. Correspondingly, a sharp increase in modu lus also occurs at T-c when the adhesive undergoes the same thermal process es. When the temperature is held isothermally below the crystalline melting temperature (T-m= -42 degrees C), the modulus increases simultaneously wit h the increase in the degree of crystallization. The growth rate of the mod ulus increases exponentially as the temperature approaches -75 degrees C fr om T-m. Once the silicones are in the high modulus state, the temperature m ust be increased above T-m to melt the crystalline phase and recover the no rmal low modulus phase. If the heating rate is faster than about 0.5 degree s C/min, the recovery temperature can pass over T-m to about -35 degrees C (C) 2000 Elsevier Science B.V. All rights reserved.