Effects of the Addition of Citric Acid on Tannin-Sucrose Adhesive and Physical Properties of the Particleboard

The effects of citric acid on the curing properties of tannin-sucrose adhesives and on the physical properties of particleboard utilizing the adhesives were investigated. The citric acid content of tannin-sucrose adhesive was adjusted to 0.4, 1.8, 4.6, 13.8, 20.0, and 33.3%, which corresponded to pH values of the adhesive 40 wt% solution at 3.8, 3.0, 2.5, 2.0, 1.8, and 1.5, respectively. Thermal analysis showed that with increasing of citric acid content, the temperature of significant weight loss and the endothermic reaction of the tannin-sucrose-citric acid adhesive was reduced. When the adhesives were heated at 200 °C, boiled for 4 h, and adjusted to 20.0 and 33.3% citric acid content, the insoluble matter was increased significantly, and an absorption band derived from ester linkages and another peak, possibly from dimethylene ether bridges, were observed by FT-IR. The particleboards bonded with 20.0 and 33.3% citric acid adhesives at 200 °C satisfied the physical requirements of the type 18 Japanese Industrial Standards A 5908 (2003). Consequently, the addition of citric acid promoted the reaction between tannin and sucrose at a lower temperature, and the optimal hot pressing temperature decreased from 220 to 200 °C. The mechanical properties and water resistance of the particleboards were also enhanced.