Study On Flame Retardancy Of Inorganic Filled Silicone Rubber Nanocomposites Prepared By Transient Normal Stress Strengthening

With the rapid promotion of silicone rubber（SR）in the electronics and aerospace industries,how to effectively improve the flame retardancy of silicone rubber has become one of the research problems that need to be solved urgently.Traditionally,metal hydroxides are commonly used to flame-retard of SR.But these fillers have low flame retardant efficiency and poor compatibility with the matrix,a large amount of addition will deteriorate the processing and mechanical properties of silicone rubber.In recent years,hexagonal boron nitride（h-BN）has great application potential in the flame retardant field due to its excellent heat resistance,stable chemical properties and unique layered structure.However,boron nitride has poor compatibility with polymers due to its inorganic properties,thus it is difficult to disperse in the matrix during mixing.At present,open mills,internal mixers and screw extruders are commonly used to prepare silicone rubber nanocomposites.Such equipment relies on shear force to mix rubber additives during processing.In order to ensure the mixing effect,it is necessary to increase the strength of shear force.But the main chain of SR will be broken when the shear force is too strong,which is not conducive to the subsequent preparation of high-performance silicone rubber products.Therefore,it is urgent to innovate a new rubber mixing method and develop new rubber mixing processing technologies and equipment.In this paper,based on the eccentric rotor internal mixer dominated by tensile deformation,the mixing processing method for preparing inorganic filled silicone rubber by transient normal stress strengthening was proposed for the first time,and a high-performance silicone rubber/boron nitride nanocomposite material was prepared on this equipment.Through a large number of experiments and analysis tests to explore the effect of the transient normal stress generated by the volume elongation flow on the strengthening of the hydrogen bond between the silicone rubber（SR）and hydroxy boron nitride（HOBN）,and the dispersion of nanofillers in the matrix.In addition,the mechanism of boron nitride to flame-retard silicone rubber have also been revealed.Firstly,this paper introduced the structural characteristics and working principle of the eccentric rotor internal mixer.By setting two straight sections and a special spiral section on the eccentric internal mixer rotor,the internal cavity of the internal mixer rotor can be connected to the internal cavity of the internal stator during the rotation and revolution.The meshing relationship makes the material subject to cyclic transient normal stress in the eccentric rotor internal mixer.Silicone rubber/boron nitride（SR/BN）nanocomposites were prepared by using an internal mixer（BM）based on the shear flow and an eccentric rotor internal mixer（ERM）based on the extension flow.The results showed that the molecular chain of polysiloxane in SR/BN-ERM was longer and the molecular weight distribution was narrower;the Payne effect was smaller,and the dispersion effect of BN in the matrix was better;the thermal stability and flame retardancy were also significantly improved.The results fully highlighted the advantages of preparing SR/BN nanocomposites by volume elongation flow.In order to further improve the dispersibility of boron nitride in silicone rubber and prepare nanocomposites with better properties,hydroxylated boron nitride was successfully prepared by high-temperature calcination,and SR/HOBN nanocomposites were prepared by ERM.The research results showed that the transient normal stress of ERM could enhance the hydrogen bonding between the SR macromolecular chain and the surface hydroxyl groups of HOBN,promoted the peeling and dispersion and lateral orientation of BN,thereby improving the thermal stability and flame retardancy of the matrix.SR/HOBN residue analysis showed that when exposed to an open flame,the layered structure of HOBN could play a role in isolating oxygen penetration and flame propagation on the surface of the substrate;in addition,the micro-nano confinement space constructed by HOBN in the substrate could inhibit the degradation of macromolecular chains and effectively improve the flame-retardant and smoke-suppressing properties of silicone rubber.In order to further improve the dispersibility of boron nitride in silicone rubber and prepare nanocomposites with better performance,hydroxylated boron nitride nanosheets were successfully prepared by high-temperature calcination and then added together with the raw silicone rubber in the ERM dominated by volume elongation flow.The results showed that the transient normal stress generated by ERM could enhance the hydrogen bonding between the SR macromolecular chain and the hydroxyl groups on the HOBN surface,and promote the peeling,dispersion and horizontal orientation of BN,thus improve the thermal stability and flame retardancy of the SR/HOBN-ERM nanocomposites.The flame-retardant mechanism of HOBN on SR was revealed:when exposed to flames and heat,on the one hand,HOBN could accumulate on the surface of the substrate during the burning,and effectively isolated the penetration of oxygen and the spread of flame by its layered structure;on the other hand,the micro-nano confined space constructed by HOBN in the matrix could inhibit the degradation of macromolecular chains,thus effectively improve the flame retardant and smoke suppressive properties of silicone rubber.Considering that the different process conditions have a greater impact on the blending effect of the internal mixer,based on the SR/HOBN system prepared by the transient normal stress strengthening hydrogen bonding effect,a large number of experiments were carried out to explore the influence of BN calcination temperature and HOBN filler particle size,mixing time,rotor speed and processing temperature on the interaction between SR and HOBN,the dispersion morphology of HOBN,and the thermal stability and flame retardancy of nanocomposites.The study found that when the added boron nitride was HOBN-d,the mixing time was 5 min,the processing temperature was 50℃,and the rotor speed was 45 rpm,the SR/HOBN-d had the smallest Payne effect and the strongest hydrogen bonding,the most uniform filler dispersion and the most excellent flame retardancy.In order to prepare silicone rubber products with higher thermal stability,based on the SR/HOBN system prepared by the above transient normal stress strengthening hydrogen bonding,a novel flame retardantγ-Fe₂O₃ modified hydroxy boron nitride（Fe BN）was added to SR via ERM.The effect of Fe BN on the mechanical properties,thermal stability and flame retardancy of silicone rubber was studied.FTIR,XRD,SEM and TEM test results shown thatγ-Fe₂O₃ had been decorated on the surface of HOBN.When the amount of Fe BN was only 4phr,SR/Fe BN nanocomposites with high thermal stability and excellent flame retardancy could be prepared.The composition analysis of SR/Fe BN combustion products shown that HOBN constructed a micro-nano confinement space in the matrix,andγ-Fe₂O₃ with free radical capture function could inhibit the degradation of macromolecular chains and promote they to transform into a ceramic-like layer with high thermal stability under high temperature inert environment.The barrier effect of the carbon layer was enhanced,thereby reducing the burning ability of silicone rubber.In order to prepare silicone rubber products with higher flame retardancy,layered double hydroxide（LDH）with a large number of hydroxyl groups,was introduced into silicone rubber.Firstly,a novel flame retardant PBS-d-LDH was prepared with polyborosiloxane（PBS）intercalation modification LDH,and then SR/PBS-d-LDH nanocomposite was prepared.FTIR,XRD and SEM test results proved that PBS-d-LDH was successfully prepared,and it was uniformly dispersed in the SR matrix.When 5 phr of PBS-d-LDH was added,the thermal stability and flame retardancy of SR/PBS-d-LDH were significantly improved.Based on the above experimental results of ERM dominated by volume elongation flow,it was found that the transient normal stress generated by volume extensional deformation could enhance the hydrogen bond between the polymer molecular chain and the nanofiller,which confirmed the transient normal stress strengthening hydrogen bond had an excellent mixing and dispersion effect on the silicone rubber nanocomposite system,thereby preparing a high-performance silicone rubber/boron nitride nanocomposite.The research results in this paper highlighted the significant advantages of transient normal stress strengthening hydrogen bonding for the processing of rubber nanocomposites,and provided a high-efficiency and innovative mixing method for the rubber processing field.