Soy protein and halloysite nanotubes-assisted preparation of environmentally friendly intumescent flame retardant for poly(butylene succinate)

Ammonium polyphosphate (APP) and soy protein (SP) were used to form biodegradable intumescent flame retardants (BIFR) and further combined with halloysite nanotubes (HNTs) to prepare environmentally friendly intumescent flame retardants (EIFR) to enhance the flame retardancy of poly(butylene succinate) (PBS). Thermogravimetric analysis (TGA), cone calorimeter test and the observation of char residue showed that SP acted as both char source and gas source in BIFR. BIFR with APP/SP (2/1) had the best char-forming capability for PBS, significantly increasing the limiting oxygen index (LOI), reducing peak heat release rate (PHRR) and total heat release (THR), but increasing the total smoke release (TSR). HNTs had a synergistic effect with BIFR to improve the flame retardancy of PBS when at an appropriate content. Addition of 1.0 wt% HNTs increased the LOI, and reduced the PHRR and THR, especially the TSR. However, the synergistic effect became week or diminished when the content of HNTs increased to 1.5 wt% and 3.0 wt%. The possible flame retardant mechanism of EIFR for PBS was proposed. Mechanical test showed that HNTs improved the tensile strength and impact strength of PBS/EIFR composites. The result of soil burial test indicated that BIFR accelerated the biodegradation of PBS. This work may offer a strategy for providing environmentally friendly flame retardants for biodegradable polymers such as PBS, and also addressing the environmental harmfulness of traditional intumescent flame retardants.