Modification Of Additon Type Silicone Rubber By Polyhedral Oligomeric Silsesquioxanes And Silicon Sol

Addition-type liquid silicone rubber(LSR)is one of the rapidly developed silicone materials with major features:(1)Biocompatibility remained favorable;(2)Suitable for deep vulcanization or injection Vulcanization;(3)The shrinking percentage of finished products is minimized.LSR is widely applied in medical,optical,electronic and aerospace fields at present,nonetheless,LSR without reinforcement treatment can hardly be used because of the poor mechanical properties,it’s thermal stability and flame retardant are unfavorable to meet the requirements of special high end applications.Polyhedral oligomeric Silsesquioxanes(POSS)is a kind of hybrid material between organic and inorganic materials,which has become one of the hottest spots in modification research field of organic materials.Many studies shown that to use nonreactive POSS to modified organic materials lead to POSS agglomeration and cause Material defect easily,which often limits the modification effect.Modification effect of reactive POSS is usually better than that of nonreactive POSS,but the reactive groups in POSS molecule should be selected strictly according to the principle of polymerization reaction,and the introduction of reactive POSS also cause agglomeration and compatibility issues as well.Some research on the Synergistic application of nanosilica material,usually fumed silica or precipitated silica and POSS to improve the physical properties of LSR has been reported in recent years.However,silicon sol with the tiniest particle size,high specific surface area and better dispersion has not been reported as silicone elastomer filler and modifier to control the agglomeration of POSS till now.In this work,a series of LSR/POSS/Silicon Sol composites were prepared by introducing Octavinyl-polyhedral oligosilsesquioxanes(OVPOSS)and modified Silicon Sol individually or jointly,and systematically studied the Synergistic effects of POSS and silicon sol on the thermal stability and mechanical property of LSR.The research of this paper is divided into six parts.A.Synthesis,surface treatment and characterization of POSS and high purity silicon solOVPOSS,OVPOSS modified PMHS and spherical silicon sol were prepared,their chemical structure,thermal stability were characterized and analyzed..Reactive Silicon sol and nonreactive silicon sol were prepared by the surface treatment using vinylsilazane and silazane to replace the silicon hydroxyl group on the surface of silicon sol.B.Preparation and Characterization of OVPOSS modified LSR(P)OVPOSS were introduce into LSR polymer system by two methods.(1)to use OVPOSS to modify PMHS first and obtained modified LSR by reaction between modified PMHS and VPDMS.(2)added OVPOSS into raw materials directly by physical blending before vulcanization,and obtained modified LSR.The experimental results show that the mechanical properties and thermal stability of modified LSR improved significantly compared with the unmodified sample by adding 0.5%-4%POSS directly,its tensile strength was enhanced by 423-508%,tear resistance increased from 22 to 44%,the residue at 600℃ in air increased by 8%-65%and 36-75%at 750℃ inN2 atmosphere.Some properties of modified LSR by adding 0.1%-0.8%POSS indirectly improved as well,its elongation rate at break improved by 177-677%,residue at 600 ℃ in air increased by19%-27%and 4-18%at 750 ℃ in N2 atmosphere,respectively.The results above confirm that the incorporation of the POSS cage improved the crosslinking density and structure stability of modified LSR polymer.The results show that thermal stability of LSR samples produced by two methods is extremely close in low loading rate,but mechanical property produced by method(1)is better.However,large amount of solvent is required in synthesis process of POSS modified PMHS,and it is difficult to increase the limitation amount of POSS cage structure because of over crosslinking of POSS modified PMHS.Method(2)could introduce high content OVPOSS into LSR system,but there are agglomeration and precipitation when the OVPOSS additive amount is increased.Method(2)is chosen to carry out the following study after comprehensive comparison.C.Preparation and Characterization of silicon sol modified LSR(S,RS)In order to solve the problem of POSS agglomeration,the silica sol was selected as the new modified material without introducing other elements.Two kinds of silicon sol modified VPDMS were prepared by kneading reactive and nomreactive silicon sols with VPDMS under heating and vacuum conditions respectively.Then,the nonreactive silicon sol modified LSR sample S and reactive silicon sol modified LSR sample VRS with different silica sol content were further prepared.The SEM image of the material section shows that silicon sol was uniformly distributed and no agglomeration was discovered.Characterization and analysis on the material structure,mechanical property and thermal performance by TGA and tensile machine shows that the introduction substantially improves the mechanical property and thermal performance of modified LSR.For example,by adding 5%-20%nonreactive silicon sol,the tensile strength of S samples was enhanced by 23%-215%compared with unmodified sample,tear resistance increased by 16%-80%,the residue at 600 ℃ in air increased by 11%-96%and 17-106%at 750℃ in N2 atmosphere,by adding 5%-20%reactive silicon sol,the tensile strength of RS samples was enhanced by 200%-523%compared with unmodified sample,tear resistance increased by 72%-302%,the residue at 600 ℃ in air increased by 66%-113%and 4%-66%at 750℃ in N2 atmosphere,respectively.It can be seen from the comparison of modification effect of two kinds of silicon sol,the mechanical property of reactive silicon sol modified LSR is significantly better than that of nonreactive silicon sol modified LSR,and the thermal stability was slightly higher than that of nonreactive modified LSR;the performance of the two modified LSRs showed a positive correlation with the addition ratio of silicon sol.D.Preparation and Characterization of nonreactive silicon sol and OVPOSS synergistic modified LSR(VPS)Nonreactive silicon sol was introduced into POSS modified LSR system to control and release the agglomeration of POSS.The SEM image of the material section shows that the POSS agglomeration quantity and size decreased greatly,and the dispersion of POSS is improved in contrast with the P series sample without the addition of silicon sol.By adding 10%-20%nonreactive silicon sol and 0.5%-2%POSS directly,the mechanical properties and thermal stability of VPSs improved significantly compared with unmodified sample,its tensile strength was enhanced by238%-631%,tear resistance increased by 70%to 163%,the residue at 600℃in air increased by 109%-161%up to 77.5%and 97-150%at 750℃ in N2 atmosphere up to 64.1%,even much superior to modified P series and S series,while the improvement of mechanical property is restricted.The test results proved that the introduction of silicon sol reduces the material defect caused by POSS agglomeration,which indicate that POSS and silicon sol are superimposed and synergistic jointly improved the system performance.E.Preparation and Characterization of reactive silicon sol and OVPOSS synergistic modified LSR(VPRS)In order to further improve the mechanical properties of LSR system,reactive silicon sol was introduced into POSS modified LSR system to prepare a new series of POSS/LSR/reactive silicon sol composite material VPRS with significantly improved crosslinking density.SEM image shows that the aggregation of POSS in VPRS is also reduced,silicon sol and POSS are uniformly dispersed in the material.Characterization analysis on the thermal stability and mechanical property by TGA and tensile machine shows that the mechanical property of VPRS system are obviously improved.By adding 10%-20%reactive silicon sol and 0.5%-2%POSS directly,the mechanical properties and thermal stability of VPRSs improved significantly compared with unmodified sample,its tensile strength was enhanced by 103 8%-1631%and up to 2.25MPa,tear resistance increased by 345%to 633%and up to 6.01 kN/m,its residue at 600℃ in air increased by 88%-158%and 9-101%at 750℃ in N2 atmosphere.The comprehensive properties of VPRS samples are superior than other modified LSR of(B),(C)and(D),and proved that POSS and silicon sol have significant synergistic effect on the enhancement of LSR system together with the study of Part D.The process has the potential of industrial application after further optimization.F.Thermal decomposition mechanism of silicon sol and OVPOSS modified LSRThermal decomposition mechanism of silicon sol and OVPOSS modified LSR was studied by online FTIR,results show that the decomposition of modified LSR samples mainly follow two mechanism:the backbone of irregular thermal depolymerization and free radical degradation.Infrared analysis showed that the decomposition residues were mainly SiO2 and SiC,but also a small amount of organic matter.The backbone of organic siloxane molecular chains mainly decomposed into cross-linked inorganic structure of silica and silicon carbide in thermal decomposition and thermal oxidative decomposition process,not mainly into the formation of volatile cyclic siloxanes.Methyl group on siloxane backbone mainly decomposed into methane or carbon dioxide volatilization,and the rest mainly in the form of silicon carbide into inorganic solid residue.The results show that the POSS cage structure and modified silicon sol has synergistic effect on improving thermal stability of LSR system by following ways:(1)to reduce the amount of methyl radicals,which led to the formation of methane or carbon dioxide gas and removal;(2)to inhibit the movement of siloxane polymer chain and reduce its fracture tendency to decompose into small molecular cyclic siloxanes,which led to improve the system decomposition temperature and inorganic residual rate;(3)blocking the heat and material exchange by steric hindrance and delaying the system decomposition process.