| In this paper, ways to improve the radiation resistance of silicone rubber (including HTV and RTV silicone rubber) are studied systematically. And a series of compounds, characterized by IR and HNMR analysis, are synthesized and used as anti-rays to improve silicone rubber's radiation resistance. The changes of crosslinking density and mechanical properties under different atmosphere (in vacuum, N2 and air) and different radiation dose are studied. Moreover, polymethylphenylsilicone rubber with excellent radiation resistance is also synthesized.Aromatic compounds (such as biphenyl,.naphthalene and phenanthrene) can be used as additives to improve HTV silicone rubber's radiation resistance. The bigger the aromaticity, the better the radiation protection effects are. Phenanthrene has the best radiation protection effects among them. When the aromatic compounds contain more conjugated atoms, their molecular weight becomes bigger and their melting points become higher. Once the melting points are higher than 170℃(the first cure temperature of silicone rubber), their dispersion in rubber system becomes worse, which affects silicone rubber's mechanical properties and the radiation protection effects of aromatic additives. Thus the radiation protection effects of p-terphenyl and 9,10-diphenylanthracene are worse than biphenyl, naphthalene and phenanthrene in spite of their bigger conjugated structure. Diphenylacetylene and benzophenone have good radiation protection effects due to their special radiation protection mechanism. And the suitable amount of aromatic compounds used in silicone rubber is 6 phr (weight parts). Since aromatic compounds are dispersed physically in rubber system, their radiation protection effects can be characterized as "external protection".When aromatic compounds are used to improve silicone rubber's radiation resistance, there exist two problems: their dispersion and compatibility in the rubber system. Introducing multi-phenylphenyl groups into silicone compound seems to be a good way to solve the problems. By the Diels-Alder reaction between tetraphenylcyclopentadienone and vinyl groups, two compounds containing tetraphenylphenyl groups are synthesized: (tetraphenylphenyl) trivinyltetramethyl cyclotetrasiloxane and tetramethylvinyl (tetraphenylphenyl) disiloxane. When the former is used in HTV silicone rubber, many vinyl groups are also introduced into rubber system, which can be easily broken by irradiation and then induce radiation crosslinking reactions. So, (tetraphenylphenyl) trivinyltetramethyl cyclotetrasiloxane can not be used to improve rubber's radiation resistance. While tetramethylvinyl (tetraphenylphenyl) disiloxane can be used to improve rubber's radiation resistance, and its suitable amount used is 4-6 phr.Therefore, in order to obtain compounds with good radiation protection effects, the compounds should:(1) Contain condensed aromatics with big conjugated structure;(2) Have lower melting point and have good compatibility with silicone rubber,(3) Contain less vinyl groups.So, by the Diels-Alder reaction of tetraphenylcyclopentadienone, acenaphthenecyclone and benzophenanthrenecyclone with the vinyl groups in C gum, three kinds of polymethylvinylsilicone oil with condensed aromatics are synthesized respectively: polymethylvinylsilicone oil with tetraphenylphenyl groups (called C1 gum for short), polymethylvinylsilicone oil with acenaphthenyl groups (C2 gum) and polymethylvinylsilicone oil with benzophenanthrene groups (C3 gum). And the content of vinyl groups and condensed aromatics in C1 gum, C2 gum and C3 gum is calculated by 1HNMR analysis according to the integral value of various H atoms.C1 gum, C2 gum and C3 gum are used as additives in silicone rubber to improve the radiation resistance. When vulcanizates are cured by peroxide (DBPMH), the condensed aromatics with big conjugated structure have stabilization on radicals forming during decomposition of initiator and inhibit the crosslinking reactions of silicone rubber, causing the vulcanizates to be in a state of "lack of cure". Even increasing the amount of peroxide used, vulcanizates with better vulcanization characteristics are not easy to be obtained. When vulcanizates are cured by hydrosilylation, which is catalysed by chloroplatinic acid catalyst, vulcanizates can be cured adequately when C1 gum, C2 gum and C3 gum are used. And their suitable amount used is 10-14 phr. When C1 gum, C2 gum and C3 gum are used in poly(dimethyl-diphenyl) silicone rubber, both the condensed aromatics and phenyl groups can provide radiation protection effects to silicone rubber with Synergistic effect. For example, after irradiated in N2, the tensile strength of polymethylvinyl silicone rubber is 5.7 MPa (350 kGy), 3.2 MPa (500 kGy) and 2.0 MPa (850 kGy); the tensile strength of polymethylvinyl silicone rubber containing 10 phr C1 gum is 6.7 MPa, 4.5 MPa and 2.6 MPa respectively; the tensile strength of poly(dimethyl-diphenyl) silicone rubber containing 10 phr C1 gum is 8.7 MPa, 5.7 MPa and 3.5 MPa respectively.The reason that C1 gum, C2 gum and C3 gum can obviously improve silicone rubber's radiation resistance is attributed to the condensed aromatics in polymethylvinyl silicone oil. When samples are irradiated, the absorbed energy could dissipate in the large conjugated structure of condensed aromatics before bond rupture occurs. So the radiation resistance is improved greatly. In addition, the vinyl groups in silicone oil are necessary. During the curing of silicone rubber, condensed aromatics could be introduced into the molecular network of silicone rubber by the reaction of vinyl groups in C1 gum, C2 gum and C3 gum. So, their radiation protection effects could be characterized as "internal protection".According to the experiment results, C2 gum has the best radiation protection effects among three gums. From the UV spectra, it can be found that the aromaticity of acenaphthenyl groups in C2 gum is the largest. But there are 30 carbon atoms in the conjugated structure of condensed aromatics in C1 gum and C3 gum, while 28 carbon atoms in acenaphthenyl groups in C2 gum. So, the aromaticity of acenaphthenyl groups in C2 gum should be the least. In order to explain the reason, quantum chemical calculation is utilized. The optimized geometrical structrures of condensed aromatics in C1 gum, C2 gum and C3 gum are calculated at B3LYP/6-31G* levels. We find that some phenyl groups in acenaphthenyl groups are coplanar and have the largest conjugated structure. While all the phenyl groups in tetraphenylphenyl groups or benzophenanthrene groups are not coplanar. At the same time, by density function theory (DFT), the electronic energy gap (Eg) between HOMO and LUMO frontier molecular orbitals of condensed aromatics are calculated and the Eg of acenaphthenyl groups in C2 gum is the smallest, which explains the reason why C2 gum has the best radiation protection effects.Methylphenyldichlorosilane is synthesized by Grignard reaction between methyltrichlorosiliane with bromobenzene. And the by-product with high boiling point in Grignard reaction is proved as methyldiphenylchlorosilane by 1HNMR analysis. By the hydrolyzation of MePhSiCl2 and Me2SiCl2, polymethylphenylsilicone rubber can be obtained. And the rubber containing 30 mol% phenyl groups has excellent radiation resistance, which is attributed to the phenyl groups in rubber system. For example, after 450 kGy radiation, the tensile strength decreased from 6.0 MPa to 5.4 MPa, only decreased 10%. When the content of phenyl groups in rubber system is the same, the radiation resistance of polymethylphenylsilicone rubber is a little better than that of poly(dimethyl-diphenyl)silicone rubber after higher dose radiation (900 kGy). This is attributed to the distribution of phenyl groups in rubber system.In order to improve the radiation resistance of condensed-type RTV silicone rubber, two kinds of crosslinking agents are synthesized successfully: triethoxysilane with acenaphthenyl groups (m.p. 49℃) and triethoxysilane with benzophenanthrene groups (m.p. 76℃), characterized by IR and 1HNMR analysis. When they are used as crosslinking agent in condensed-type RTV silicone rubber, the curing speed can be accelerated by improving the curing temperature (50-60℃). After radiation, the results show that they can effectively improve the radiation resistance of RTV silicone rubber and their suitable amount used is 14-18 phr.
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