Influences of high-impact polystyrene loading on the foaming behavior and flame-retardant properties of polyphenylene oxide composites blown with CO2

Polymeric foam with high expansion ratio, well-defined cell structure, and excellent flame retardant properties is essential for broadening its applications. Polyphenylene oxide (PPO) is a kind of cost-effective engineering plastic with excellent flame retardancy, anti-dripping behavior, and good mechanical strength, but suffers from its poor processability. In this study, microcellular PPO composite foams were fabricated by applying a solid-state foaming technology using compressed CO₂ as the blowing agent. High-impact polystyrene (HIPS) phase was introduced with the aim to improve the fluidity and foaming ability of PPO composites. It was interesting to find that the 18–48% HIPS loading significantly increased the expansion ratio, that is, 1.8–3.3 versus, 10.8–14.3, and broadened the optimum foaming temperature of PPO composite foams, attributing to the miscible character between PPO and HIPS and excellent foaming ability of HIPS. Furthermore, the as-prepared PPO/HIPS composite foams exhibited high limited oxygen index (LOI) of 22.0–29.9%, low horizontal flammability rate (HFR) of 60.5–141.2 mm/min, and anti-dripping behavior, and the void fraction was verified to be a critical parameter to determine the flame retardant performance of the composite foam. Besides its lightweight and excellent flame retardant properties, PPO composite foams also presented uncompromised tensile properties and well-defined thermal insulation properties.