Flame retardancy of flax fibers by pre-irradiation grafting of a phosphonate monomer

The functionalization of natural fibers using a phosphorus-containing monomer to improve flame retardancy is presented. Flax fabrics were modified using a pre-irradiation grafting method requiring electron beam radiation treatment. A phosphorus-based dimethyl(methacryloxy)methyl phosphonate (MAPC1) was used as a flame retardant (FR) grafting agent. The grafting process consists of three main steps. First, flax fibers were irradiated at a dose ranging between 5 and 100 kGy and stored cold to preserve the radicals created over the long period before being immersed into an aqueous solution containing MAPC1 monomer. Finally, a washing step of the treated fibers was carried out to remove the unreacted monomer units as well as the free oligomers and polymers chains which were not covalently bonded to the flax structure. The presence of radicals on the flax fibers after irradiation was confirmed using electron paramagnetic resonance (EPR). Grafting efficiency was evidenced by infrared spectroscopy (FTIR) and quantified using inductively coupled plasma atomic emission spectroscopy (ICP-AES). The location of the grafted phosphorus polymer chains in the fiber structure was assessed using scanning electron microscopy coupled with energy dispersive X-ray spectrometry (SEM-EDX). The impact of grafting MAPC1 onto flax fibers on the thermal properties and fire behavior was evaluated. Thermogravimetric analysis (TGA), pyrolysis combustion flow calorimetry (PCFC) and a preliminary fire test were used for this assessment. Different parameters such as radiation dose, monomer concentration, temperature and reaction duration which might impact grafting efficiency were monitored. In particular, it was observed that MAPC1 was grafted in a homogeneous way into the bulk of the elementary flax fibers leading to high phosphorus content of around 2 wt% for a dose of 10 kGy and up to 7 wt% for a dose of 100 kGy.