Processing, thermal conductivity and flame retardant properties of silicone rubber filled with different geometries of thermally conductive fillers: A comparative study

With the development of highly integrated and miniaturized electronic devices, silicone rubber composites with good mechanical flexibility, high thermal conductivity and excellent flame resistance have aroused considerable interest in both industrial and research community. However, the effect of different geometries of thermally conductive fillers on the processing, mechanical and thermal properties as well as flame resistance of silicone rubber composites is still not clear. Herein, liquid silicone rubber (LSR) composites filled with three geometries of thermal conductive fillers i.e. spherical-like aluminum oxide (Al2O3), irregular aluminum nitride (AlN) and 2D boron nitride (BN) sheets were prepared by an extremely simple mechanical-mixing process. As expected, incorporation of BN sheets into LSR matrix produces much higher increase in the viscosity, tensile strength and hardness than the corresponding AlN and Al2O3 fillers, respectively. Typically, at a fixed filler loading of 120 phr, the increased thermal conductivity and heating/cooling efficiency of the BN/LSR composites display much better than the corresponding AlN/LSR and Al2O3/LSR composites, e.g. 316, 124 and 88% improvement in thermal conductivity, respectively. Interestingly, the thermal stability and flame resistance of the three composites present unexpected different phenomena. Compared with the pure LSR, the addition of AlN and Al2O3 exacerbates the thermal degradation of LSR chains, while the presence of BN sheets improves the flame resistance of LSR dramatically, which is attributed to the compact BN/silica residue after burning that effectively restricts the heat and oxygen to attack the inside LSR chains.