Synthesis And Properties Of Fluorosilicone Rubber Via FD₃/D₃ Copolymerization

Fluorosilicone rubber exhibites excellent heat and cold resistance,solvent and oil resistance etc,and applied in some important fields like astronavigation,automobile, electron and so on.However,the higher price limits the widespread application of fluorosilicone rubber in civilian market.Introducing of the dimethylsiloxane unit into the main chain of fluorosilicone rubber is a useful approach to obtain fluorosilicone rubber with mediate oil resistance and relatively lower cost,and to overcome the disadvantages of dimethylsiloxane rubber/fluorosilicone rubber blend.It this thesis, fluorosilicone rubber with different compositions were prepared by anionic bulk ring-opening copolymerization of 1,3,5-tris(trifluoropropylmethyl)cyclotrisiloxane (FD₃)and hexamethyl cyclotrisiloxane(D₃).Influences of polymerization recipe, technological conditions on the ring-chain equilibrium and the molecular weight of copolymer formed were emphasisly investigated.In addition,the structure and properties of fluorosilicone rubber prepared by FD₃/D₃ copolymerization were studied.The FD₃/D₃ anionic bulk ring-opening copolymerizations were conducted in the presence of alkyl lithium catalyst and a promoter.The varition of the composition of FD₃/D₃ copolymerization system were analyzed by GPC method.It was found that the copolymerization system included,the high molecular weight copolymer,the small cyclo-molecules of trifluoropropylmethyl silicone(FDx,x=3～6)and dimethyl silicone(Dx,x=3～6).The weight fraction of copolymer was initially increased and then decreased and leveled off as polymerization proceeded.The content of FDx was also initially increased and then decreased as copolymerization proceeded,while the content of Dx was usually decreased as polymerization time increased.According to the varition of composition of copolymerization system,FD₃/D₃ copolymerization process could be divided into four stages:comonomers consumption stage,the first ring-chain equilibrium stage,back biting stage and the second ring-chain equilibrium stage.The changes of molecular weight of FD₃/D₃ copolymers and factors affecting the molecular weight of fluorosilicone copolymers were investaigted.It was found that the existence of water or oxygen in the reaction system had significant influences on the molecule weight of product.In order to obtain fluorosilicone copolymer with higher molecular weight,the concentration of water and oxygen should be controlled very low.In order to obtain FD₃/D₃ copolymer with homogeneous composition and high molecular weight,it was recommended to add and mix the FD₃ and D₃ before the copolymerization.The variations of molecular weight of FD₃ and D₃ ring-opening homopolymerization system with polymerization time had the similar behaviors.It was found that the molecular weight increased at the initial stage of polymerization, and level off or slightly decreased as polymerization proceeded for a longer time. Differently,the higher activity of FD₃ led the rapid variation of molecular weight of FD₃ homopolymerization system.The molecular weight of copolymers was usually lower than that of homopolymers.As the concentration of catalyst increased,the reaction rate was increased,the time to achieve equilibrium and the molecule weight of copolymer was decreased.The copolymerization rate could also be increased by increasing the concentration of promoter and temperature.Addition of vinyl comonomer to facilitate the cross-linking of copolymer,always led the decrease of molecular weight of copolymer.Base on the above results,the optimized copolymerization conditions are obtained as follows:the mole percentage of catalyst 1.7×10^-3,the mole percentage of promoter 0.2,polymerization temperature 80℃and polymerization time 300min.The structure analysis of FD₃/D₃ copolymers showed that the molar percentage of trifluoropropylmethyl silicone units in the copolymer chain was greater than that in the feeding monomers,due to the high reaction activity of FD₃ and the sublimation of D₃ at polymerization temperature.As the molar percentage of FD₃ in copolymer increased,the gel content of vulcanized fluorosilicone rubber was increased,while the degree of equilibrium swelling of vulcanized fluorosilicone rubber was decreased.The heat resistance of vulcanized FD₃/D₃ fluorosilicone rubber was superior than that of gum.The heat resistance of FD₃/D₃ copolymer rubber ranged between the silicone rubber and FD₃ homopolymr rubber.The FD₃/D₃ copolymer rubber vulcanized with bis 2,5 curing agent exhibited better heat resistance than the rubber vulcanized with BPO.