Chestnut tannin-furfurylic resins for wood preservation: influence of the proportion of tannins as a copolymer

Tannin, which is present in all plants, is the most abundant polyphenol globally and can be extracted using water or organic solvents such as acetone, ethanol and methanol. Tannins are classified into two categories: condensed tannins (catechin-based polymers) and hydrolysable tannins (sugar esters with galloyl units). Previous studies have demonstrated its potential as a raw material for modifying wood alongside furfuryl alcohol (FA). However, these studies utilised low concentrations of tannin and/or FA and did not consider the use of hydrolysable tannins. This research aimed to use substantial quantities of hydrolysable tannin from chestnut (Castanea sativa) to replace as much FA as possible for chemical modification of beech wood (Fagus sylvatica L). Impregnation was conducted using different concentrations and ratios of both FA and tannin through copolymerisation. The wood modification process employed tartaric acid as a catalyst, involving a vacuum cycle followed by a 24-hour polymerisation step at 120 °C. Properties such as weight percent gain (WPG), leachability (WPL), dimensional stability (ASE), thermal stability, and durability against brown rot (Coniophora puteana) and white rot (Coriolus versicolor) were analysed and compared to a furfurylation treatment without the addition of tannin. The results indicated that treatments with 50% organic concentration of tannin and FA achieved a weight gain percentage (WPG) of 49-55%, nearly double that of furfurylation alone. These new treatments exhibited an average leaching rate of 5%, slightly higher than the 1.5% observed for furfurylation. Additionally, they demonstrated similar leachability and dimensional stability to standard furfurylation. Furthermore, the new treatment showed improved resistance to wood-degrading fungi and enhanced thermal stability.