Preparation,Structural And Performance Regulation Of Silicone Rubber Foam Composites Based On In-situ Self-assembly Of Two-dimensional Nanomaterials

Silicone rubber foam material is a polymer material with "Si-O-Si" as the main chain and organic groups(such as methyl,phenyl,etc.)as side chains.It is also called silicone rubber sponge.Such foam materials have the dual characteristics of silicone rubber and foamed porous materials with excellent properties such as non-toxicity,light weight,high elasticity,stability,high and low temperature resistance,and electrical insulation.As a result,these sponge materials are widely used in transportation,aerospace,electronic appliances and building field.However,under the high temperature/flame environments,the side organic groups of silicone rubber can be thermally degraded,and the main chain would be broken and rearranged,which will seriously affect its structural stability and mechanical reliability.In order to improve its high temperature resistance and flame retardant performance,the traditional strategies are mainly to incorporate a high content of inorganic flame retardants(such as 30-50wt% aluminum hydroxide)into the silicone rubber matrix and to surface-modify the foam with a flame retardant coatings.However,there are still many problems for these two strategies,such as negative effect of other important properties such as flame retardant properties at high filling contents,complex processing technology,and even the emission of organic solvents.In this work,inspired by the cross-linking/foaming reaction mechanism of silicone and the synergistic flame-retarding effect of silicone/GO,we use a homogeneous aqueous dispersion of 2D nanomaterials to react with the Si-H groups of silicone molecules and thus construct the in-situ assembly of 2D nanomaterials on the foam surface,and the silicone rubber foam materials with high-efficiency flame retardant and mechanical performances can be obtained.The main research contents are described as follows:(1)Based on MXene's unique structure(high specific surface area and hydroxyl groups on the surface)and properties(excellent mechanical and electrical properties),we synthesized MXene aqueous solution at high concentration.The dehydrogenation reaction between hydrosilylation groups induces the in-situ precise self-assembly of MXene(at low content of 0.1-0.2wt%)on the foam surface,thus forming a compact MXene protective layer with strong interface with the skeleton.The protective layer can significantly improve the thermal stability and high temperature mechanical stability of the silicone rubber sponge material and has little effect on the density of the material.Meanwhile,the green flame retardancy of the silicone rubber foam materials is achieved.For example,the presence of about 0.2wt% of MXene can increase the limiting oxygen index from 21.5% for pure foam to 27.1% for the composites,and the heat release rate and smoke release rate of the foam can be significantly reduced after addition of the MXene.This work provides a new design and development of lightweight,high-elastic and flame-retardant silicone rubber foam materials.(2)In order to further optimize the in-situ assembly of 2D nanomaterials,we selected high concentration of graphene oxide(GO)aqueous solution with large size,and used amphiphilic surfactant molecules to improve the dispersion stability of GO aqueous solution in the silicone matrix(i.e.emulsion method).At the same time,the in-situ self-assembly of GO sheets can be effectively tailored on the surface of the silicone rubber foam,which can avoid the movement of GO sheet into the inside matrix during the physical blending process.The results show that addition of ultra-low amount(0.04wt%)of large-layer GO can significantly improve the high and low temperature mechanical creep stability and thermal stability of silicone rubber foam.The flame retardant test results further demonstrate that,compared to simple physical blending,the self-assembly of GO sheet layers induced by amphiphilic molecules significantly inhibits the flame propagation height and speed during the combustion process,and thus improves the limiting oxygen index,heat release rate,smoke emission and other properties.And the vertical burning test demonstrates that the flame retardant grade of the composites containing in-situ assembled GO latyer reaches UL94-V0 level.The combustion cross-section analysis displays that during the thermal decomposition process,the surface assembled GO layer and the silicone molecules can form a compact nano-silica/graphene protective layer,which can effectively inhibits the oxygen by external oxygen and heat to attack the inside polymer chains,thus producing excellent synergistic flame-retardant effect.Clearly,this provides a new strategy and method for optimizing the structure and properties of silicone rubber foam materials containing the in situ assembled two-dimensional nanomaterials.