Effect of excess silane on the viscoelastic behavior of epoxy under hygrothermal conditions

This study investigates the effects of excess organofunctional silane on the elastic and viscoelastic properties of an epoxy resin. Samples were prepared by adding 3-glycidoxypropyl trimethoxysilane (GPTMS) in varying amounts ranging from 0.5 to 8.0 wt% to diglycidyl ether of bisphenol-F (DGEBF) epoxy cured using an amine-based hardener. Prepared samples were subjected to hygrothermal conditioning by immersion in water at 21 °C and 50 °C. Subsequently, instrumented nanoindentation was used to determine the elastic modulus and creep compliance. A Fickian model fitted to moisture absorption data indicates that after 48 h specimens were fully saturated to a depth of approximately 150 µm, which is significantly higher than the maximum indentation depth of 1 µm. This implies that the indentation material response was determined from a fully saturated region. It was found that the addition of silane leads to a reduction in the elastic modulus. In the presence of hygrothermal degradation, this reduction still occurs but is less than the non-conditioned case, especially for exposure at 50 °C. For example, the addition of 8.0 wt% silane reduces the elastic modulus by an average of 70% as compared to the pristine sample, and conditioning at 21 °C and 50 °C results in reductions of elastic modulus by 73% and 52%, respectively. The overall reduction in the elastic modulus value is due to the inclusion of flexible silane into the epoxy resin. Under hygrothermal conditions, however, the hydrolysis of silane mitigates the reduction in elastic modulus, especially at higher temperature conditioning. The viscous response of the material is sensitive to the inclusion of silane and hygrothermal conditioning. However, at very low wt% of silane (less than 2%), the material becomes creep resistant and at higher wt%, the behavior is similar to the elastic modulus. Fourier transform infrared (FTIR) spectra indicates that Si-Si interaction becomes predominant at higher wt. fractions leading to softening of the material. Inclusion of silane also reduces the glass transition temperature, which supports the increase in flexibility of the polymer chain.