The New Synthesis Process For Polydimethylsiloxane

Polydimethylsiloxane was a fine chemical product that was widely used in mechanic, textile, electric, coatings, medicine, and so on. And the needs were growing day by day. Traditional method for the production of polydimethylsiloxane was mainly using sulfuric acid as the catalyst. However, the consumption of sulfuric acid was large. Therefore, the product was acidic which needed further treatment like washing and filtration. Thus, the cost was high and sulfuric acid was corrosive to the equipment. It was necessary to develop new process for the synthesis of polydimethylsiloxane which was environmental friendly, low cost and had better yields. Solid super acid was used instead of sulfuric acid as the catalyst in the synthesis of polydimethylsiloxane. Polydimethylsiloxane was synthesized using solid super acid SO₄^2-/TiO₂ as catalyst with octamethylcyclotetrasiloxane and hexamethyldisiloxane as raw materials. The research includes the characterization of solid super acid SO₄^2-/TiO₂ and the process optimization of polydimethylsiloxane synthesis which offered basic data for engineering application besides its academic meaning.In the synthesis of polydimethylsiloxane, self-made solid super acid SO₄^2-/TiO₂ was used as catalyst with octamethylcyclotetrasiloxane and hexamethyldisiloxane as raw materials Fixing the synthesis conditions of polydimethylsiloxane, with its viscosity as evaluating standard, influencing factors for catalyst activity such as the precipitating agent, the treating method of sulfuric acid, the concentration of sulfuric acid and pH when precipitating, was investigated through pre-experiments. Based on pre-experiment, orthogonal experiment and single factor experiment was carried out to optimize the synthesis conditions of polydimethylsiloxane as followed: the treating method for sulfuric acid was dipping, and the pH value of the solution when precipitating was 9, the baking temperature was 550℃, the baking time was 3 hours, and the concentration of dipping sulfuric acid was 0.75 mol/L. Repeated experiments under optimized conditions gave a stable product viscosity of 143 mPa.s which showed that the optimized conditions were reliable. The solid super acid catalyst prepared under different conditions was particularly characterized by BET, TPD, IR,XRD and so on. The influence of synthesis conditions on catalyst activity was illustrated as follows: Measurement of the .specific surface area of the catalyst found that the specific surface area of SO₄^2-/TiO₂ reduced with the rising of the temperature. The acid intensity of SO₄^2-/TiO₂ baked under different temperature was examined by Hammett indicator method, which found that the acid intensity was all under -12.14 (belonged to the solid supper acid category).The TPD test of the solid super acid showed that the central intensity of acid was biggest under the baking temperature of 550℃. Tested by IR spectrum, it indicated that on the surface of TiO₂, SO₄^2- combined with Ti both in the form of chelated biligand and bridge biligand, and the SO₄^2- which bonded with the carrier was stable and reached the largest amount when the baking temperature was 450-550℃, Moreover, through XRD analysis we can find that the solid super acid prepared was anatase. Lastly, TGA showed that solid super acid remarkably lost its weight when the temperature is above500℃, and the loss of SO₄^2- is distinguishable. All these have explained the inter-relationship between the syntheses condition of solid super acid and catalyst effect.When the catalyst synthesis process was assured, the material ratio was fixed, and the largest viscosity of product was treated as the aim, the optimal conditions for polydimethylsiloxane synthesis was as follows: the temperature for removing evaporative ingredients was 300℃,the pressure was 1.033KPa, the time for removing evaporative was 3hours. catalyst dosage is 2% the weight of the total reactants ,reaction temperature is 120℃, reaction time is 3h, theyield of product was above 90%. With the reacting conditions fixed as stated above, changing the raw material ratio, a multinomial was regressed to indicate the viscosity of product and a series of raw material ratio which was useful in guiding the industrial production.