Study On The Preparation And Properties Of Magnesium Borate Whisker/Polycarbonate Composites

Polycarbonate?PC?is one of engineering thermoplastic widely used in the fields of mechanical transmission,building,electronic information and medical equipment,because it possesses several distinct properties,such as high impact strength,dimensional stability,flame resistance,transparency,and nontoxic and odorless.However,PC exhibits high notch sensitivity,poor wear resistance and processing liquidity,which limit its use in some applications.Magnesium borate whisker?Mg₂B₂O₅w?as a new type of reinforced material modified polymer research has been a great concern,because it has the advantages of low expansion coefficient,small size,stable chemical properties and high cost performance.In this paper,the surface of Mg₂B₂O₅ w was modified by borate ester?BE?coupling agent,unmodified Mg₂B₂O₅ w and BE-Mg₂B₂O₅ w filled in PC matrix were prepared by melt blending and injection molding,respectively.The effects of whisker before and after modification on the structure,morphology and mechanical properties of the composites were studied systematically,and the influence of Mg₂B₂O₅ w on the thermal properties of PC matrix was discussed.The major results are listed as follows:?1?The borate ester coupling agent was successfully grafted onto the surface of Mg₂B₂O₅ w by reaction,the grafting ratio about 2.1%.The surface modification of BE-Mg₂B₂O₅ w in PC matrix showed better dispersion and interfacial bonding.?2?The tensile strength and the flexural strength of Mg₂B₂O₅w/PC and BE-Mg₂B₂O₅w/PC composites reach the maximum when the content of the two whiskers is 3wt%,and the tensile strength of the two composites increases with the increase of the content of Mg₂B₂O₅ w.Compared with pure PC increased 12.5% and 17.2%,while the bending strength increased by 3.25% and 4.62%.When its content exceeds 3wt%,the local agglomeration in the matrix seriously weakened its toughening effect.The tensile and flexural modulus of the composites increased with the increase of the whisker content,and the tensile modulus was consistent with the Halpin-Tsai and Guth models.Besides,unexpected ductile-brittle transition for impact toughness happens after adding Mg₂B₂O₅ w.?3?With the increase of Mg₂B₂O₅ w content,the storage modulus ? of the composites increases.The storage modulus of composites increased observably as a function of whiskers content,especially for the composites with modified Mg₂B₂O₅ w due to the chain entanglement between BE and PC chains,but the addition of two kinds of whiskers did not improve the g of the composites.?4?When the content of Mg₂B₂O₅ w is low,the whiskers can be uniformly dispersed in the PC matrix and fully contact with it.The catalytic cracking effect on the PC molecular chain is stronger than that of the molecular chain free radical and thermal degradation barrier,which reduces the heat of the composites stability drastically.However,the effect on the latter two is obviously stronger than that of its catalytic cracking when the whisker content increased,which can effectively reduce the thermal stability of the composites and improve to a certain extent.The influence of Mg₂B₂O₅ w on PC composites degradation behavior can be grouped into three categories.At the beginning of the degradation reaction,the whiskers could capture the free radicals produced by the isopropylidene fragmentation at the terminal of the PC molecular chain and limit the occurrence of the action of the hindrance reaction,and the degradation initiation temperature is improved.The catalytic cracking of Mg₂B₂O₅ w on PC molecular chains at high temperature leads to the increase of reaction rate in the middle stage of thermal degradation reaction.In the middle and late stages of the reaction,Mg₂B₂O₅ w combined with the solid phase residue degraded by the outer PC to form a barrier structure,which hinders the conduction of heat and the escape of the volatiles,resulting in an increase in the mass of the degraded residue.?5?The addition of Mg₂B₂O₅ w improves the thermal activation energy of PC,and the activation energy of PC and its composites increases with the reaction.The degradation process is complex multi-step reaction,and the increase in the activation energy of the composites may be due to the addition of Mg₂B₂O₅ w to the formation of a stable intermediate product.