Polybenzoxazine/boron nitride foam: a promising low-k, flame-retardant and robust material

Herein, we report a facile strategy to prepare polybenzoxazine/boron nitride composites foams with simultaneously improved thermal resistance, flame retardance, mechanical and dielectric properties using a sol–gel method. It is found that the boron nitride (BN) granules are uniformly dispersed and tightly encapsulated within the polymer matrix without any modification. Due to the reinforcement of BN, the compressive stress of the composites foams nearly double, giving the highest value of 14.61 MPa. Moreover, the low densities of the composites foams lead to low dielectric constant and dielectric loss at both low and high frequencies, e.g., the composite foam with 30 wt% BN (F/30BN) show dielectric constant and dielectric loss of 1.42 and 0.0818 (1 MHz), and 1.72 and 0.0013 (1 GHz), respectively. Additionally, the glass transition temperature and char yield of F/30BN reach 190.5 °C and 55.4%, which is 48.6 °C and 33.4% higher than the pristine polybenzoxazine foam. The incorporation of BN also imparts the composites foams with better flame retardancy by delaying combustion and slowing the heat release rate.