The properties of natural tannin-furfural resin to be used for Poplar wood modification

Tannin is a natural polymer that has recently been proposed for wood protection, but the structure of this polymer is not resistant to water and can be easily hydrolyzed or leached out from the wood. The use of various aldehydes to synthesize tannin-based resins and therefore improving the properties of these resins has already been studied. Furfural is a natural aldehyde and is the raw material for the production of furfuryl alcohol that is produced from the acidic hydrolysis of agricultural wastes, such as sugarcane. In this study, first, the effect of adding furfural on the tannin network features was investigated. Then, the ability of tannin/furfural natural resin was evaluated to impregnate and modify poplar wood to improve its physical properties and hygroscopic behavior.

Tannin-based resins were made with 20% m/m tannin and different ratios of furfural (2.5, 5, 10, 15, 20, 25, 40, and 50% of tannin dry weight) in different acidic conditions (pH 2, 4.5, 7, 9). For all formulations, the properties of resins were evaluated, including impregnation abilities, leaching resistance, viscosity, density and gel time. Optimal condition for making suitable resin to impregnate wood was selected by the complete factorial method using mini-tab software at a 95% confidence level. Then, the resin made under optimal condition was used to impregnate the poplar wood using the vacuum/pressure method. Heating was used to cure the resin and turn it into a polymer inside the wood. The physical properties of modified wood were measured according to ISO 13061 standard.

Examination of resin properties showed that with an increase in furfural ratio, the resin density increases. Moreover, the resin gel time and viscosity increased, with increasing furfural ratio and acidity respectively. The highest leaching resistance of resin was obtained with a resin made with 50% of furfural. Microscopic studies showed that resins containing furfural had a more porous structure. Thermogravimetric analysis showed that up to 580 °C, tannin / furfural modified polymer had higher thermal stability. According to the complete factorial method used, the optimal condition of resin manufacturing was adding 50% of furfural to tannin with an acidity of 4.5. Samples impregnated with the selected resins had less water absorption and swelling than the untreated ones. The volumetric swelling and water absorption of treated samples showed a considerable decrease with increasing resin concentration.

Extensive studies are underway worldwide to use natural polymers as wood preservatives or modifiers. The results of this study showed that the use of natural tannin / furfural resin has a promising prospect for improving the physical properties of wood. By accurately ‎adjusting the resin manufacturing conditions and the tannin-furfural pretreatment, a resin with suitable viscosity and leaching resistance can be achieved to be used in the wood impregnation industry. It is suggested that future studies investigate the conferred durability and mechanical properties of woods treated and modified with tannin / furfural polymer.