Fire-resistant flax-reinforced polypropylene/polylactic acid composites with optimized fire and mechanical performances

Fabrics from commingled natural (flax), thermoplastic (polypropylene (PP)/polylactic acid (PLA)) polymeric fibres were fire retarded with an oragnophosphonate flame retardant (FR). The fire-retarded flax/PP and flax/PLA fabrics were melt pressed to prepare respective thermoplastic composites. The effect of FR concentration on the fire and mechanical properties of composites was studied and the results analysed in terms of concentration of FR elements in the composites. While the UL-94 test was used as a benchmark to achieve a V-0 rating, cone calorimetric parameters were used for in-depth analysis of fire performance. The results show that flax/PP and flax/PLA composites require a minimum 0.9 and 0.6% phosphorus, respectively, to achieve a V-0 rating in the UL-94 test. Mechanical properties, evaluated in tensile and flexural modes, are however slightly impaired, most probably due to the acidity of the FR solution (pH = 3.2). In order to improve the mechanical properties, a buffer solution was used to change the pH to 6.0, which while having a minimal effect on mechanical properties of flax/PP, significantly reduced those of flax/PLA and increased flammability of both composites and hence overall showed no benefit.