Water based blowing agents for rubber extrusion

Abstract

AbstractPhysiologically hazardous chemical blowing agents are state of the art for the foaming of extruded rubber profiles. Water is a potential alternative to these blowing agents and is incorporated into rubber compounds in the form of water‐leaded hygroscopic substances such as carbon blacks, silica, or hydrates. To achieve an optimum foaming result, the water desorption and cross‐linking reaction has to be coordinated. In thermogravimetric investigations it is found that the inertia of the evaporation process plays an important role in water desorption mechanisms. It turned out that manual conditioning of the hygroscopic fillers with water is not reproducible. As a consequence, measurement uncertainties in characterization resulted, so that statistical proof of the expected effects on water desorption due to the specific properties of the fillers (e.g., silanol group density, specific surface area) could not be provided. However, carbon blacks are less suitable than silica due to the lower desorption temperature. The precipitated silica Ultrasil 360 with a relatively high pH‐value showed higher water desorption rates than the other silica. The desorption temperature can reproducibly be adjusted as required by a suitable choice of hydrate. Since the absolute amount of water in the rubber compound is crucial to the foaming behavior during vulcanization Karl Fischer titration is used to determine the water content of a polar and a nonpolar rubber compound is determined over several weeks. Depending on the climatic conditions during storage, large deviations from the amount of incorporated water occur especially for polar rubber compounds such as nitrile butadiene rubber.