Synthesis And Investigation Of The Thermal,Reprocessable Properties Of The Silicon-Containing Thermosets

Silicone resins are one of many thermosets with excellent properties such as high heat resistance,electrical insulation and weather resistance,which makes them a kind of high performance materials.The harsher conditions of use require silicone resins with high thermal properties.Conventional methods such physically or chemically modification of silicone resins are used to improve their thermal properties,but the effect of this kind of modification is limited.Therefore,the new methods of modification are an important topic in this field.The three-dimensional cross-linked structure of conventional thermosets endows them with excellent properties such as heat resistance,dimensional stability and mechanical properties.However,they generally lose their reprocessability.The main solution is to introduce dynamic covalent bonds into thermosets in order to allow them to be reprocessed.The silyl ether（Si-O-C）bonds are one of the most prevalent chemical bonds in natural and synthetic materials.They can also be used to design reprocessable thermosets.The Si-O bond energy（460.5 k J/mol）is higher than that of C-C bonds（304.0k J/mol）and C-O bonds（358.0 k J/mol）,which can endow chemical and thermal stability to silicon-containing polymers.The specific work in this thesis is divided into four sections as follows:Part I.Synthesis and properties of silicone resins additionally cross-linked via Si-O-Ph bondsIn order to overcome the degradation of silicone resins caused by the"back-bite"reaction of the terminal silicone hydroxyl（Si-OH）groups at high temperatures.We have designed and synthesized the silicone resins containing phenolic hydroxyl（Ph-OH）groups.The additional active sites,Ph-OH groups,were used to react with the terminal Si-OH to form the silicone resins additionally cross-linked via Si-O-Ph bonds.The results show a substantial increase in the thermal stability of the silicone resin.The 5%weight loss temperature(T_d5),10%weight loss temperature(T_d10)and char yield at 800 ^oC(Y₈₀₀)in a nitrogen atmosphere are as high as 553 ^oC,617 ^oC and 83.1%,respectively.The corresponding data in a air atmosphere are 473 ^oC,532 ^oC and 81.2%,respectively.Those are superior to those of the normal methyl phenyl silicones.Part II.Synthesis and properties of silicone resin-based CANs additionally cross-linked via Si-O-Ph bondsConventional thermosetting silicone resins can not be reprocessed.The siloxane-containing benzoxazine monomers（P-mdes）and triethoxymethylsilanes were used as raw materials to produce silicone resin oligomers containing benzoxazine side groups.The reaction between Ph-OH groups and Si-OH groups in the system during the curing process resulted in silicone resin-based CANs additionally cross-linked via Si-O-Ph bonds.The results show that the dynamic Si-O-Ph bonds allow the silicone resins to be reprocessed.In addition,the Si-O-Ph bonds inhibit the"back-bite"reactions of terminal Si-OH and improve their thermal properties.The T_g determined from the tanδcurve is135 ^oC.In a nitrogen atmosphere,the T_d5,T_d10 and Y₈₀₀ of silicone resin-based CANs are389 ^oC,439 ^oC and 64.5%,respectively.The tensile strength of the original silicone resin-based CANs sample is 23.34 MPa.The retention of mechanical properties are 112.72%and 107.46%for the first and second reprocessed samples,respectively.This work provides a method to improve the thermal properties of silicone resins and allow them to be reprocessed,which is beneficial for the sustainable development of silicone resins.Part III.Synthesis and properties of monocyclic silicon-containing benzoxazine resin-based CANs based on metathesis reactions of Si-O-Ph bondsThe monocyclic siloxane-containing benzoxazine monomers（p-mtes）were synthesized by the Mannich reaction using phenol,paraformaldehyde and 3-aminopropyltriethoxysilane as raw materials.During the curing process,all the Ph-OH groups react with Si-O-CH₂CH₃ to form monocyclic silicon-containing benzoxazine resin-based CANs cross-linked via Si-O-Ph bonds.The glass transition temperature（T_g）determined by tanδis as high as 341 ^oC,much higher than that of the extant reprocessable thermosetting resins.The reprocessability of monocyclic silicon-containing benzoxazine resin-based CANs can be achieved by the metathesis reactions of Si-O-Ph bonds under hot pressing condition.The retention of mechanical properties after the second reworking is as high as 72.83%.Part IV.Synthesis and properties of bicyclic silicon-containing benzoxazine resin-based CANs based on metathesis reactions of Si-O-Ph bondsBisphenol A,paraformaldehyde and 3-aminopropyltriethoxysilane were used as raw materials to synthesize dicyclic siloxane-containing benzoxazine monomers（BA-mtes）via the Mannich reaction.During high temperature curing,all Ph-OH groups react with Si-O-CH₂CH₃ to form bicyclic silicon-containing benzoxazine resin-based CANs additionally cross-linked via Si-O-Ph bonds.The results show that the metathesis reaction of Si-O-Ph bonds stimulated by hot pressing allow the bicyclic silicone benzoxazine resin-based CANs to be reprocessed.The tensile strength of the original resin sample was 22.66 MPa,and after two reprocesses,the mechanical property retention rates of the reprocessed samples were 88.92%and 76.57%,respectively.In addition,the additional cross-linking of the Si-O-Ph bonds can improve their cross-linking density,and thus their thermal properties.Its T_g is much higher than 400 ^oC.The T_d5,T_d10 and Y₈₀₀ of the bicyclic silicon-containing benzoxazine resin-based CANs can reach 393 ^oC,448 ^oC and 51.2%under a nitrogen atmosphere.The present work provides a simple and facile method for the preparation of reprocessable silicon-containing thermoset resin materials.These materials can be potentially applied in the field of thermal stability and reprocessability.