Silicone hot melts are pressure-sensitive adhesives incorporating polysilicate resinous nanoparticles into a cross-linked dimethylsiloxane network. Unlike unfilled siloxane elastomers, these adhesives are viscoelastic with a pronounced dependence on time (rate of deformation) and temperature. The time-temperature correspondence of the dynamic mechanical properties of silicone hot-melt adhesives is adequately described using linear viscoelastic measurements at small strains. The response to monotonic, constant-rate deformations includes the appearance of a yield point. It was found that the stress response at larger deformations corresponding to the yield point and up to 50% shear strain obeyed the same time-temperature correspondence determined from linear viscoelastic measurements. This suggests that the same molecular relaxation processes governing its linear viscoelasticity also control its yielding behavior and rate-dependent nonlinear response at larger strains. It also serves to demonstrate that linear viscoelasticity can be a valuable tool as an engineering approximation for material applications in a nonlinear stress-strain regime.<br/>Like most particle-filled elastomers, silicone hot-melt adhesives exhibit stress softening and significant hysteresis under cyclic deformation, but will exhibit complete recovery of its stiffness. At elevated temperatures, longer rest periods barely restored the adhesive to its original state whereas recovery was more pronounced given the same rest time at lower temperatures. A mechanism to explain the effects of time and temperature on the stiffness recovery is proposed based on the reduction in molecular mobility of the dimethylsiloxane chains and the weakening of nanoparticle-polymer interactions. When polymer chain mobility is reduced with decreasing temperatures, a complex constrained network can be reestablished more easily after adequate rest time and the silicone hot-melt adhesive can become stiffer than its original, undeformed state.

Effects of time and temperature on mechanical properties of silicone hot-melt adhesives

253rd National Meeting (2017)

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