The effect of inorganic flame-retardant fillers on the flammability properties of environmentally friendly biopolymer/sawdust fibre biocomposites

Abstract

The rapid accumulation of biomass waste, and the effect of non-degradable materials on the ecosystem have compelled scientific communities to explore sustainable alternatives. Moreover, growing concerns about fire safety and environmental sustainability have driven significant research into bio-based flame-retardant polymer composites. This thesis focuses on repurposing agro-industrial wastes —particularly sawdust (SD) and sugarcane bagasse (SB) —as reinforcing agents in biodegradable polymer matrices such as polybutylene succinate (PBS) and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH). It therefore addresses the challenges of environmental sustainability and performance through hybrid fibre systems and inorganic flame retardants, including halloysite nanotubes (HS) and expandable graphite (EG). Both natural fibres were incorporated at 10 wt% into PBS and PHBH to develop eco-friendly hybrid composites. These composites were melt-mixed using a TE-30 co-rotating twin-screw extruder, with the heating-zone temperatures ranging from 120 to 160 °C from the hopper to the die set. To counter the inherent flammability of these materials, flame-retardant fillers, EG (only in PBS), and HNT (in both systems) were incorporated at a concentration of 3 wt%. The effects of these additives on the morphology, thermal stability, flammability, dynamic mechanical properties, and rheological behaviour of the resulting composites were investigated. Two experimental studies were conducted on reinforcing PBS and PHBH with SD and SB. In the first study, PBS was reinforced via melt compounding with SD and SB, both individually and as a hybrid composite, together with HS and EG. The rheological analysis demonstrated an increase in complex viscosity, addressing PBS’s limitations in melt processing. The hybrid composite (PBS/SB/SD) exhibited improved morphology, as SD effectively encapsulated SB. More so, the hybrid composite (PBS/SB/SD) displayed enhanced morphology, as the sawdust effectively encapsulated the sugarcane bagasse. Meanwhile, HS and EG enhanced stiffness and crystallisation, although toughness and impact resistance decreased due to poor fibre-matrix adhesion. In the second study, PHBH was reinforced in a similar manner, revealing comparable trends with variations in matrix-fibre interactions. The SD exhibited better dispersion and compatibility with PHBH than the SB. Additionally, the inclusion of halloysite clay enhanced interfacial adhesion in the PHBH/SD/SB/HS hybrid composite. All composites showed improved stiffness, viscosity, and storage modulus compared to pristine PHBH, with the best thermal stability observed in the PHBH/SB/SD/HS system. Despite these enhancements in thermal and rheological properties, mechanical properties such as elongation at break and impact resilience were found to be reduced.