Flame Retardancy and Heat Shielding in Green Polypropylene Composites Using Biohybrid Fibers and Agro-Waste Fillers

The current work presents the flame-retardant performance of hybrid polypropylene composites, reinforced with specific short woven flax fabrics (SWFs), short basalt fibers (BFs), and rice husk powder (RHP), using polypropylene grafted maleic anhydride (MAPP) as the coupling agent. Horizontal burning test (HBT), microcalorimeter test (MCT), and cone calorimeter test (CCT) were conducted on these composites. The formulations used were 25% SWF/PP, 25% SWF/20% BF/PP, and 25% SWF/20% BF/PP with 6% RHP and 25% SWF/20% BF/PP with varying RHP contents (6, 12, and 18%) in combination with 6% MAPP. A peak heat release rate (pHRR) reduction of 14.42% was recorded in microcalorimeter test results. However, cone calorimeter test results indicated a reduction of 80.95% in pHRR and 23.84% in total heat release rate (THR). Furthermore, the emissions of CO and CO₂ were reduced by 68 and 67%, respectively, in the 25SWF/20BF/PP.6MAPP-18RHP composite compared to pure PP, indicating an overall improvement in combustion efficiency. Fire performance index (FPI) also improved remarkably: FPI increased by 130% and fire growth index (FGI) improved by 81.34%. Moreover, the horizontal burning test showed an improved performance, with the burning rate further reduced by 11.10% compared to that of the 25% SWF/PP composite. These results highlight the synergy among natural fibers, agro-waste fillers, and MAPP in improving flame retardancy of polypropylene composites, and hence their potential application in automobiles and construction, where the demand for fire safety is very high.