Rubber vulcanization method for FBG pressure sensors

This paper presents the experimental analysis of the vulcanization process of nitrile rubbers for the diaphragms fabrication used in FBG-based pressure sensors. Tests using diaphragms with different rubber thicknesses (0.5mm, 1.0mm, and 1.5mm), vulcanization temperatures (125°C, 150°C, and 200°C), and vulcanization times (2.5min and 5.0min) were performed to analyse the chemical degradation of diaphragms. Degradation analysis was performed using digital macrography of the diaphragm surfaces and the diaphragms stiffness was analysed by tensile tests. An FBG was embedded in the diaphragm which was vulcanized using the temperature and time which resulted in the lowest rubber degradation, and a compression test was performed. Results of the digital macrography show that diaphragms vulcanized at 125°C presented smaller degradation areas, in which diaphragms with 0.5mm rubbers presented an area degradation mean of 17.5%, whereas 1.0mm rubbers presented 14.5%, and 1.5mm rubbers showed 11.0%. Moreover, greater thickness led to smaller degraded area for the same temperature and vulcanization time. Thus, for the same thickness in the rubber, the higher vulcanization temperature results in a higher material stiffness, where stiffness is directly related to the reduction of cross-links due to the degraded area. Results of the sensitivity test using diaphragms vulcanized at 125°C showed a pressure sensitivity of 11.67kPa/mm and wavelength sensitivity of 456.1pm/mm for 0.5mm rubber thicknesses, 18.04kPa/mm pressure sensitivity, and 112.3pm/mm wavelength sensitivity for 1.0mm rubber thickness and 31.55kPa/mm pressure sensitivity and 913.8pm/mm wavelength sensitivity for 1.5mm rubber thickness. It concludes that the 0.5mm rubber vulcanized diaphragm obtained higher sensitivity when comparing pressure and wavelength.