Study On The Synthesis And Performance Of Boron-doped Silicone Resin

Due to its special structure of the main chain and good properties of heat resistance, chemical resistance and weather resistance, silicone resin has been applicated in many fields and attracted more and more attention.In this paper, a novel boron-doped silicone resin (BSR) was synthesized by hydrolysis-polycondensation method, with propyl-triethoxysilane(PTES), dimethyl-diethoxysilane (DMDES) and boric acid as starting materials, absolute ethyl alcohol as solvent and hydrochloric acid as catalyst. The structure of the BSR were characterized and studied by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The results showed that boron has been successfully introduced as doping element into the silicone resin as Si-O-B-O-Si chains. After we studied the reaction conditions including the proportion of monomer, the molar ration of B-OH and Si-OR, hydrolysis rate, the rotating speed and solvent dosage, the optimum technological conditions were obtained as following: R/Si is 1.22, n(B-OH):n(Si-OR) is 1:5, the hydrolysis rate 60%, the rotating speed 200 rpm, solvent dosage 20 mL, with hydrochloric acid as catalyst. TGA and DTG curves indicated that when the ratio of n(B-OH) and n(Si-OR) was 1:5, the heat resistant temperature of synthesized boron-doped silicone resin was up to 500～550℃,which was much better than general PSR(about 200～250℃) and showed good thermal stability.Based on the above results, we studied the heat resistence mechanism furthermore by means of infrared spectroscopy and thermal analysis methods. We proposed that during the heating process, small molecular groups, including-CH3,-C2H5 and -OH groups, cleaved firstly, followed by a small amount of decomposition of alkoxy (-OC2H5) substituents. Thermal decomposition experiments also showed that, the heat resistance of the products is different with the different size of molecules which is connected to the silicon atom, the smaller of the connected molecule, the better of the heat resistance properties. However, the smaller size of the groups will lead to the more hardness of the products, which is not good for the preparation of film-forming and the practical application.In the view of the above thermal decomposition results, we prepared additional boron-doped silicone resins with three kinds of siloxane as starting materials, such as MTMS, DMDES and PTES. The experiment results showed that, the heat resistance of the products was not superior to that of the above BSR with three siloxane monomers as reactants at the same time. The reasons of this may be related with the difference of the activation energy between methyl trimethoxysilane and triethoxysilane. Under the same reaction temperature and reaction conditions, methyltrimethoxysilane hydrolysised and polycondensed quickly, so the crosslinked structure of the BSR was not uniform and the heat-resistance performance was relatively poor. With MTMS, DMDES and boric acid as raw material, the obtained BSR showed better heat-resistant, but It is not solved in general organic solvents because of the high crosslinking degree, whch is not benefit to the practical application and industrial production. The prepared BSR showed excellent comprehensive properties with PTES, DMDES and boric acid as raw material, not only the heat-resistant of resin has been improved greatly, but also other performances is satisfactory,such as mechanical properties, resistance to alternating hot and cold performance and construction performance.