Study on the model selection of water diffusion into silicone rubber by NMR and gravimetric technique

Moisture plays a major role in the aging phenomena of high temperature vulcanizing (HTV) silicone rubber material, which is always used as high voltage composite insulator sheds and sheath. In order to obtain an appropriate model to describe the process of water molecule diffusion into silicone rubber, the solid state nuclear magnetic resonance (NMR) was conducted to investigate the existence state of water molecule in silicone rubber. The gravimetric method was also employed to obtain the weight gain curves caused by the water molecule diffusion. The study showed that water molecules had two different state in silicone rubber, one was “free” state and the other one was “bound” state. By combining the results of NMR analysis and gravimetric method analysis, we suggested that the water molecules diffusion into silicone rubber could be explained quantitatively by a Non-Fick diffusion model, Langmuir diffusion model, instead of the classical Fick diffusion model. Furthermore, the water distribution curves in the silicone rubber were also predicted by solving the Fick diffusion equation and the Langmuir diffusion equation, the difference between these two kinds of water distribution curves had been distinguished and analyzed in depth.