Preparation Of Copper-carboxyl Terminated Siloxane Coordination Polymer And Its Effect On The Heat Resistance Of Addition Liquid Silicone Rubber

Due to its unique chemical structure,addition liquid silicone rubber?ALSR?has excellent heat resistance.However,with the development of science and technology,improvement of heat resistance of ALSR is required by automobiles,aerospace,machinery,construction and other aspects.At present,among the methods to improve the heat resistance of ALSR,adding heat resistant additives into ALSR is the most common and effective one,of which the metal oxide is a universal heat resistant additive.Adding metal oxide is practical,but its high surface energy and poor compatibility with silicone rubber,will result in easy agglomeration and bad dispersion in matrix.It is necessary to add a large amount of metal oxide into ALSR to achieve heat resistance.The mechanical properties and processing performance of ALSR will be damaged,and the application area of ALSR will be limited.Therefore,it is an urgent need to improve the heat resistance of ALSR by studying new heat resistant additives possessing good compatibility with silicone rubber.In this dissertation,copper-carboxyl terminated siloxane coordination polymer?PDMS-COO-Cu?was synthesized by coordination polymerization in methanol solution using carboxyl-terminated polydimethylsiloxane?PDMS-COOH₂?and Cu?Ac?₂ as raw materials.Then heat resistant ALSR was prepared with PDMS-COO-Cu as a heat resistant additive.The effect of PDMS-COO-Cu on transparency,vulcanizing properties and heat resistance of ALSR was investigated,and the mechanism of PDMS-COO-Cu for improving the heat resistance of ALSR was investigated.The main research contents and results are listed as follows:Firstly,copper-carboxyl terminated siloxane coordination polymer?PDMS-COO-Cu?was synthesized by coordination polymerization of carboxyl-terminated polydimethylsiloxane and Cu?Ac?₂ in methanol solution.The PDMS-COO-Cu was characterized by ultraviolet–visible spectroscopy?UV-vis?,Fourier transform infrared spectroscopy?FT-IR?,gel permeation chromatography?GPC?and thermogravimetric analysis?TGA?.The effect of reaction time,reaction temperature and molar ratio of Cu to COOH on the synthesis of PDMS-COO-Cu was investigated.And the effect of PDMS-COO-Cu synthesized with different conditions on the heat resistance of silicone rubber was also discussed.The results showed that PDMS-COO-Cu was synthesized by bidentate chelation method coordination between-COOH and Cu²⁺.As the amount of PDMS-COOH₂ increased,the mass fraction of Cu?Ac?₂ involved in the reaction increased.When the molar ratio of Cu to COOH is one to two,the PDMS-COOH₂ could coordinate with Cu?Ac?₂ fully.With the increase of reaction time,the content of copper in PDMS-COO-Cu increased,the number average molecular weight of PDMS-COO-Cu increased,and the molecular weight distribution index of PDMS-COO-Cu decreased.When reaction time was 15 h,the copper content of PDMS-COO-Cu was 4.98%,the number average molecular weight was 13400 g/mol,and the molecular weight distribution index was 1.37.As reaction temperature increased,the copper content of PDMS-COO-Cu decreased,the number average molecular weight of PDMS-COO-Cu decreased,and the molecular weight distribution index of PDMS-COO-Cu increased.The suitable conditions for the synthesis of PDMS-COO-Cu were that the molar ratio of Cu to COOH was one to two,reaction time was 15 h,and reaction temperature was40°C.PDMS-COO-Cu synthesized in the conditions was best for improving the heat resistance of ALSR.Secondly,heat resistant ALSR was prepared with PDMS-COO-Cu as a heat resistant additive.The effect of PDMS-COO-Cu on mechanical properties,transparency and heat resistance of ALSR was investigated,and the mechanism of PDMS-COO-Cu for improving the heat resistance of ALSR was studied.The results showed that PDMS-COO-Cu could significantly improve the heat resistance of ALSR.ALSR samples without PDMS-COO-Cu became completely brittle after treatment at 280°C for 24 h.ALSR with 0.03 phr PDMS-COO-Cu had excellent heat resistance.After treatment at 280°C for 24 h,the tensile strength retention rate of ALSR increased to 42.1%and the elongation at break retention rate increased to 28.6%.PDMS-COO-Cu had little effect on the vulcanizing properties,transparency and mechanical properties of ALSR.Finally,the heat resistance of ALSR/PDMS-COO-Cu was compared with that of ALSR/Cu?Ac?₂,and the effect of PDMS-COO-Cu on the crosslinking density of ALSR after heat treatment was studied.TGA and thermogravimetry-Fourier transform infrared spectrometry?TG-FTIR?were used to investigate the thermal stability and thermal degradation of ALSR.The mechanism of PDMS-COO-Cu for improving the heat resistance of ALSR was discussed.The results showed that after treatment at 280°C for 12 h,the aging coefficient of tensile product of ALSR/PDMS-COO-Cu was 0.19,and that of ALSR/Cu?Ac?₂was only 0.12.PDMS-COO-Cu content had important effect on the crosslinking density of ALSR.With the increase of the amount of PDMS-COO-Cu,the increase value of crosslinking density of ALSR became smaller after heat treatment.TGA results showed that PDMS-COO-Cu improved the heat resistance of ALSR.Under air and nitrogen atmosphere,the temperature of 10 wt%weight loss(T₁₀)of ALSR with 0.03 phr PDMS-COO-Cu increased to 473°C and 509°C from 433°C and 468°C of ALSR,respectively.TG-FTIR results showed that,under air atmosphere,PDMS-COO-Cu could delay and reduce the release of carbonyl compounds and methane,and delay the release of cyclic siloxanes.The mechanism of PDMS-COO-Cu for improving the heat resistance of ALSR was put forward.The Cu²⁺could capture the free radicals generated by the thermal oxidative reaction of the methyl group and Si-C bond cleavage,and then the Cu⁺was oxidized to Cu²⁺by the oxygen,which effectively eliminated radicals produced by the ALSR attacked by heat and oxygen.Thus cross-linking reactions of silicone rubber molecular chains caused by radicals was suppressed.Moreover,PDMS-COO-Cu with Si-O-Si backbone had good compatibility with ALSR,and was dispersed evenly in the ALSR,thereby effectively improving the heat resistance of ALSR.