Fire resistance of flax fibre–reinforced polymer-balsa core sandwich structures modified by fire retardant

The fire resistance of flax fibre–reinforced polymer (FFRP)–balsa wood (BW) core sandwich structures was evaluated by the Underwriters Laboratories horizontal test (UL-94 HB). The influence of the FFRP layer number and BW thiwckness on the fire resistance was studied. Two types of fire retardant – ammonium polyphosphate (APP) and aluminium hydroxide (ALH) – were added to the polymer matrix of FFRP to improve the fire resistance of the FFRP-BW sandwich structures. The influence of the type and concentration of the fire retardant on the fire performance of the sandwich structures was investigated. The test results indicated that all FFRP-BW sandwich structures passed the UL-94 HB test regardless of the FFRP layer number and BW thickness. The horizontal flame spreading rate of the sandwich structures without fire retardant ranged from 11 to 20 mm/min. Concerning the fire retardant, both the APP (with concentrations of 10, 20 and 30%) and ALH (with concentrations of 20, 30 and 40%) improved the fire resistance of the sandwich structures. The sandwich structures with fire retardant stopped burning after flame removal.