Fire protection has been a major challenge in wood construction for many years. The demand for high-performance, environmentally friendly treatments respecting the physical and chemical characteristics of the material has been accentuated in recent years. Halogenated fire-retardant (FR) compounds were commonly used from the 70’s for their low-cost and high efficiency. They are able to form radicals when heated, which recombine with high energy radicals such as H or OH disturbing the chain reactions of the combustion, reducing oxidative character of the flame. However, evidence of the toxicity of some halogenated FR has limited their use in Europe since 2010, in Canada and in several states of USA. Therefore, non-toxic alternatives are developed and among them an increasing interest for phosphate compounds is noticed. In that frame, the study of polyelectrolyte complexes (PEC) is at its early stage for wood, but their versatility and eco-friendly character are already appreciated for the fire retardancy of fabrics. This study focuses on the study of the efficiency of polyelectrolyte complexes consisting of polyethylenimine (Mw = 600 g/mol and Mw = 25000 g/mol) and sodium hexametaphosphate to improve the fire behaviour of yellow birch. The samples were prepared by vacuum impregnation or by soaking, allowing relatively large weight gains in a very short time. Cone calorimeter tests revealed that a 20 % reduction in PHRR for a weight gain as low as of 3.5 wt.-% can be achieved. Small-scale Steiner tunnel confirmed the positive aspect of PEC, by reducing flame spreads. Brinell hardness and dimensional stability were also studied to ensure that the treatments are not detrimental on these properties and equilibrium moisture content of the samples were evaluated using dynamic vapor sorption. It was demonstrated that whereas fireproofing results are interesting, some improvements must be made to control the hydrophily of polyelectrolytes complexes that affects the dimensional stability of wood.

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