Flame-retardant Functionalized Graphene Filled Epoxy-based Thermal Conductive Composites

With the high-degree integration and miniaturization in the modern electronics,the high-efficiency thermal management system has become the key to ensure the high stability and working life of electronic components.Polymer-based thermally conductive composites?PTCs?have been regarded as the preferred thermally conductive materials in the next-generation thermal management system,because of their lightweight,electric insulation,corrosion resistance,high sealing,flowing process,etc.The current researchers are focusing on improving the thermal conductivity?TC?of PTCs under low filler loading level.However,the significant fire hazard of PTCs,exacerbated by the high energy losses and heat transference,is usually ignored.Therefore,it is close-necessary for preparing high-performance PTCs with simultaneously high TC and flame retardance.Two-dimensional graphene with ultra-high TC and aspect ratio has been widely used to prepare high-performance PTCs.Simultaneously,graphene and its derivatives are also believed to be the ideal flame retardant additives for polymers.For reasons above,the flame-retardant functionalized graphene sheets were used as flame retardant additives to address the fire hazard of PTCs for the first time in this dissertation.Moreover,the ultra-high TC of graphene and its synergistic effect with the conventional TC fillers were utilized to improve the TC property of PTCs simultaneously.Firstly,heteroatom doping strategy was used to improve the TC and fire resistance of reduced graphene oxide?RGO?.The resultant phosphorus and nitrogen co-doped graphene?PN-rGO?showed the considerable improvements in the thermal oxidation and fire resistance,due to the protective effect of the high bond energy groups with phosphorus and/or nitrogen atoms at defects and edges of RGO.Then,PN-rGO with surfactant?PVP?assistant was introduced into epoxy resin?EP?by solution blending.The partly defect restoration of RGO by the doping atoms,the catalytic carbonization of the doping groups and the barrier effect of graphene,resulted in the significant improvement in the TC,flame retardance and smoke suppression of the resulting EP/PN-rGO composites.Secondly,for further improving the TC of EP/RGO composite,the conventional thermal conductive materials as major fillers and flame retardant-functionalized RGO as a flame retardant additive,where the synergistic effect between RGO and conventional TC fillers was utilized to realize the composite with simultaneous improvement in TC and flame retardance.Including the following parts:?1?The polyphosphoramide oligomer with amino terminals was covalently grafted onto the surface of RGO.The resulted flame retardant-functionalized graphene?PFR-fRGO?as a flame retardant additive was then in situ introduced into EP/Al₂O₃ composites.The existence of PFR-fRGO in the ternary composite not only suppressed the settlement of Al₂O₃ particles but also enhanced the interfacial interaction between Al₂O₃ and matrix,both of which resulted in the significant enhancement of TC.The catalytic charring and barrier effects of PFR-fRGO due the enhanced interfacial interaction between RGO and EP matrix via grafting polyphosphoramide urged the formation of high strength and stable char layer based on Al₂O₃as a template,leading to the remarkable improvement in flame retardance for the EP/Al₂O₃/PFR-fRGO composites.?2?A facile“branch-like”strategy with a polymer of PGMA as the backbone and flame retardant molecule of DOPO as the branch was used to functionalize RGO.The grafted PGMA chains not only improved the grafting amount of DOPO molecule but also reinforced the compatibility of RGO in the polymer matrix.Then the obtained“branch-like”flame retardant functionalized RGO?GP-DOPO?was used as a flame retardant additive to incorporate into the EP/silver nanowires?EP/AgNW?composites.The introduced GP-DOPO suppressing effect for the aggregation of AgNWs and bridging the adjacent AgNWs,induced that the TC of ternary composite increased to 1.41 W/mK.Moreover,the catalytic charring effect of GP-DOPO restrained the endothermic melt and“wicking action”of AgNWs during combustion,further improved the flame retardance of composites.?3?Ni?OH?₂ nanoribbons,synthesized by a hydrothermal process,were used to decorate RGO by a further hydrothermal process.Such the metal hydroxide coating not only improves the interfacial interaction between RGO and matrix but also endows the capacities of heat absorption and free radical capture for RGO.Then the RGO@Ni?OH?₂ hybrid was incorporated into EP/hBN composites.As expected,the introduction of RGO@Ni?OH?₂hybrid can improve the TC of the resulting composites by effectively suppressing the stack behavior of h BN along the in-plane direction,and the bridging effect of hybrid between adjacent h BN sheets.Besides,the endothermic effect,catalytic carbonization and free radical capture of the RGO@Ni?OH?₂,together with the barrier effect of h BN resulted in the formation of a compact char layer during combustion,further improving the flame retardance of the ternary composite.Finally,hexachlorocyclotriphosphazene?HCCP?as a coupling agent and graphene oxide?GO?as a surfactant were employed to prepare GO coated Al₂O₃ microsphere?Al₂O₃@HGO?with high coating amount.The resultant Al₂O₃@HGO microspheres were used for preparing high-performance EP-based thermally conductive composites.The interfacial interaction between Al₂O₃ and matrix was improved drastically by the HGO coating layer,further weakened the interface thermal resistance.Combining the additional thermally conductive paths formed by GO coating layer,the TC property of composite was improved significantly.Moreover,both the restriction effect for polymer chains by the reinforcing interfacial interaction and the catalytic charring of HCCP led to a sharp reduction in the flame retardant of the EP/Al₂O₃@HGO.