The development of natural adhesives derived from nonfossil resources is very important for the future. Besides, it is desirable to be safe adhesives without using harmful chemical substances. In this study, application of citric acid as a natural adhesive was investigated. Citric acid powder and bark powder obtained from Acacia mangium were used as raw materials. Citric acid powder was mixed with the bark powder, and the resulting powder mixture was poured into a metal mold. The mold was hotpressed at 180 C and 4 MPa for 10 min, and a bark molding was then obtained. The specific modulus of rupture and modulus of elasticity values of the molding containing 20 wt % citric acid were 18.1 MPa and 4.9 GPa, respectively. The molding did not decompose during a repeated boiling treatment. To clarify the effect of tannin on the adhesiveness of molding, bark was separated into tannin and residue. The molding was not obtained while using the tannin due to the marked fluidity, whereas it was obtained while using the residue, the same as while using the bark. It was considered that components other than tannin contributed to the adhesiveness. Based on the results of Fourier transform infrared spectra, the formation of ester linkages between carboxyl groups derived from citric acid and hydroxyl groups in the bark was confirmed. Accordingly, citric acid brought an adhesion by chemical bonding, and it could be used as a safe natural adhesive.

The wood-based moldings were fabricated by using only citric acid as an adhesive. The mechanical properties, water resistances, thermal properties and chemical structure were investigated. Wood powder obtained from Acacia mangium was mixed with citric acid under certain weight ratios (0-40 wt%), and each powder mixture was molded using two types of metal molds at 200°C and 4 MPa for 10 min. The modulus of rupture and the modulus of elasticity values of the wood-based molding containing 20 wt% citric acid were 35.8 MPa and 5.4 GPa, respectively. The maximum impact strength was 0.94 kJ/m 2 with the same citric acid content. The water resistance increased with increasing citric acid content, and the good resistance to boiling water was also recognized. However, some elution of substances derived from citric acid was observed at high levels of citric acid, causing a decrease in the thermal properties. The ester linkages were detected by Fourier transform infrared spectroscopy, indicating that the citric acid reacted with the wood. Our results demonstrated that citric acid brought about good adhesiveness for fabricating the wood-based molding.

Development of natural adhesives that do not depend on fossil resources is very important for the future of wood-based materials. We recently found that citric acid can be used as a natural adhesive for wood-based moulding. In this study, we investigated the effects of moulding temperature on the characterization of wood-based moulding bonded with citric acid. Wood powder obtained from Acacia mangium was mixed with 20 percent (by weight) citric acid, and the powder mixture was moulded at a temperature ranging from 140°C to 220°C. The press pressure and heating time were set at 4 MPa and 10 minutes, respectively. The bending properties were affected by the moulding temperature, and the moulding fabricated at 180°C showed excellent values. The average impact strength of the moulding at 180°C was 1.1 kJ/m2. Water resistance increased with moulding temperature, and good resistance against boiling water was achieved. Thermal properties were also clearly increased with moulding temperature. However, few significant differences were found in the mouldings after a repeated boiling treatment. Based on the results of Fourier transform infrared spectroscopy, ester linkages between wood and citric acid were confirmed, especially at high moulding temperatures.