Physical Chemistry of the Interaction of Cellulose with Lignin during Wood Surface Treatment with a Phosphate Flame Retardant

A phosphate flame-retardant hardener (PFH) produced on the basis of industrial orthophosphoric acid and acetic anhydride was proposed for reducing the combustibility and revealing the natural texture of wood. Under fire conditions, phosphorus-containing compounds are able to decompose with formation of polyphosphoric acids providing a barrier coating on the combustion surface, as well as of substances inhibiting free-radical combustion, thereby reducing the rate of combustion and heat release. Wood surface treatment with the PFH was shown to reveal the natural texture of wood as a result of the reactions between the hydrolysis products of cellulose with the lignin components by the mechanism operating in the formation of triphenylmethane dyes. It was demonstrated that specifically the phenolic components of lignin participate in the formation of colored compounds, since cellulose with 90% lignin removed and cotton roving are not colored by the PFH. Softwoods are colored more strongly than hardwoods because of a higher lignin content. A complete scheme of reactions responsible for the appearance of colored structures in the wood texture in the presence of the PFH and atmospheric oxygen was proposed. A mechanism of thermal decomposition of cellulose during combustion in the presence of the PFH, resulting in predominant formation of a carbonaceous residue, was considered. From the quantitative data provided by the experiments on burning Whatman filter papers impregnated with the PFH, catalysis of the coke formation and enhancement of the coke yield were inferred. It was concluded that treatment with the PFH provides decorative appearance to wood and produces a protective effect thereon via reducing combustibility. A system of transparent decorative fire-resistant wood finishing was proposed, in which the PFH performs the function of a flame retardant and Porenbeize, and as well as of a hardener for epoxy varnish finish coating.