Study On The Step Rising Temperature Foaming Process Of Silicone Rubbers With Supercritical CO₂ As Blowing Agent

Supercritical carbon dioxide?CO₂?is an environmental benign blowing agent,it is green technology to foam silicone rubber by supercritical CO₂.In this paper,the effect of formula composition on rheological behavior of silicone rubber system,the effect of formula composition on vulcanization kinetics,the effect of pre-vulcanization on foaming process and the effect of foaming conditions on cell morphology were systematically studied.The effect of different molecular weights of methyl vinyl silicone rubber,dicumyl peroxide?DCP?content,filler silica content on rheological behavior of silicone rubber was studied.It was found that DCP content has the most significant effect on the rheological behavior of silicone rubber.Based on the orthogonal experimental results of loss factor,the optimum rubber formulation with raw rubber of molecular weight of 600,000 g·mol^-1 in 100 mass fraction,25 mass fraction of filler silica and 1 mass fraction of vulcanizing agent DCP should be suitable for foaming.Secondly,the non-isothermal vulcanization process of silicon rubber with different formulation compositions was investigated via differential scanning calorimeter method.The effect of vulcanizing agent dosage of dicumyl peroxide?DCP?,molecular weight of silicon raw rubber and filler content on vulcanization kinetics was systematically analyzed.The results show that this vulcanization reaction has a typical feature of autocatalysis reaction.The initial cross-linking temperature obviously decreases with the addition of DCP,but the rising rate of relative conversion in later stage is inhibited by its rather high cross-linking degree.Higher molecular weight of poly methyl vinyl silicone also results in lower initial cross-linking temperature,but it has little influence on the change of relative conversion.Due to the formation of bound rubber between silica filler and silicone rubber matrix,the effect of filler content on vulcanization process is non-monotonic.For silicon rubbers with DCP in their formulation,the apparent activation energy of vulcanization reaction obtained by Kissinger method is in the range of 138.77¹49.65kJ?mol^-1,and all kinetic curves fit?eatak-Berggren model well,the total reaction order is around 1.Thirdly,the effect of pre-vulcanization degree on foaming process was studied.It was found that the cell size of foamed non-vulcanized silicone rubber sample is larger.With the increasing of vulcanization degree,the elasticity and viscosity of silicone rubber both increase,which can result in good bubbles,but too high vulcanization degree may prevent CO₂ entering into polymer matrix as well as bubble growth,so there exists a proper range of pre-vulcanization degree which can foam silicone rubber with better cell morphology.For the preferred silicone rubber formulation system,this range of pre-vulcanization degree is about 36.0⁴3.8%.Finally,silicone rubber foams with cell size of 51.6⁴65.2?m,cell density of0.05⁷2.1×10⁵ cells·cm^-3 and expansion ratio of 1.8³.5 were prepared by step rising temperature process using supercritical CO₂ as blowing agent.The optimum condition,including CO₂ saturation pressure of 10 MPa,CO₂,saturation time of 120 min,foaming temperature of 170?and foaming time of 80 min,is beneficial to produce silicone rubber foams with high cell density and small cell size.