Mechanistic Insights into the Non-Monotonic Flame Retardancy of CPVC/ABS Composite

The chlorinated polyvinyl chloride (CPVC)/acrylonitrile-butadiene-styrene (ABS) composite represents an important class of engineering thermoplastics, offering a strong balance of flame retardancy, chemical resistance, mechanical properties, processability, and cost efficiency. Despite its widespread application, the flame-retardant mechanism in the CPVC/ABS system remains poorly understood. This work systematically investigated the non-monotonic flame-retardant behavior of CPVC/ABS composites through comprehensive characterization. The combustion performance, as determined by limiting oxygen index (LOI), UL-94 vertical burning tests, and cone calorimeter tests (CCTs), showed an unexpected pattern of flame retardancy initially improving then decreasing with reduced ABS content, which contradicted conventional expectations. The optimal composition at a CPVC/ABS ratio of 2:3 demonstrated good performance, achieving a UL-94 5VA rating and 47.3% reduction in total heat release (THR) relative to CPVC. A more stable and compact structure was observed from the morphology analysis of the residual char, and the thermogravimetric analysis further revealed a synergistic effect in carbonization behavior. The above flame-retardant mechanism could be interpreted by the combined effects of accelerated char formation during the early decomposition stage and significantly enhanced char crosslinking degree. These findings provided fundamental insights for designing high-performance flame-retardant polymer composites and facilitating their industrial implementation.