- Original article
- Open Access
Characterization of wood-based molding bonded with citric acid
© The Japan Wood Research Society 2011
- Received: 27 April 2011
- Accepted: 12 August 2011
- Published: 27 December 2011
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/m2 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.
- Wood-based molding
- Citric acid
- Natural wood adhesive
- Physical properties
The demand for sustainable wood-based materials is expected to continue to increase. At present, synthetic resin adhesives derived from fossil resources are required when manufacturing wood-based materials. Considering the concerns about the future global environment and the potential shortage of fossil resources, it is desirable to reduce the consumption of synthetic resin adhesives. Recently, studies on natural adhesives using bio-resources have been performed with protein, tannin and lignin used as the main adhesive materials . However, when synthesizing these natural adhesives, chemical agents derived from fossil resources that cause health disorders and environmental problems are commonly necessary. In the further development of natural adhesives, the use of chemical agents should be restrained as much as possible.
In our previous research, we were able to make bark molding using only citric acid as an adhesive . The molding had good mechanical properties and water resistance. Citric acid (2-hydroxy-1,2,3-propanetricarboxylic acid) is an organic polycarboxylic acid containing three carboxyl groups. It is contained in citrus fruits such as lemons and limes and is commercially produced by fermentation of glucose or glucose- and sucrose-containing materials [3, 4]. It is widely used in food, beverages and pharmaceuticals. In addition, citric acid has been researched as a cross-linking agent for wood , plant fiber , paper , starch  and bio-based elastomers . In this study, wood-based molding was fabricated to clarify the possibility of using citric acid as a wood adhesive. The effects of citric acid content on the physical and chemical properties of the molding were investigated.
A wood block of Acacia mangium obtained from a Japanese wood company was pulverized, and wood powder was obtained by using a sieve of 250-μm mesh size. Citric acid was purchased from Nacalai Tesque, Inc. and was also pulverized into less than 250-μm mesh size. These materials were vacuum dried at 60°C for 15 h.
Formulation of the moldings
Citric acid content (wt%)
Weight ratio of citric acid to wood
Mold for JIS K 7139
Citric acid (g)
Citric acid (g)
Both edges of the dumbbell-shaped molding were cut, and rectangular specimens of 80 × 10 × 4–6 mm were prepared. The static 3-point bending test was carried out under 50 mm of span and 5 mm/min of loading speed. The test was performed in triplicate, and each average value with standard deviation of the modulus of rupture (MOR) and the modulus of elasticity (MOE) was calculated.
Charpy impact test
According to JIS K 7111, the Charpy impact test was carried out using a digital impact tester DG-CD (Toyo Seiki Seisaku-sho, Ltd). A rectangular specimen of 80 × 10 × 4–6 mm was prepared, and the impact strength in the flatwise direction was measured with 5 unnotched sample replicates. The average value with standard deviation was calculated.
Water immersion treatment
Water immersion treatment was performed at 20°C for 24 h using three rectangular specimens (50 × 20 mm) cut from the circular molding. After the treatment, thickness swelling (TS), linear expansion (LE) and weight increase were measured under wet condition. The wet specimens were then vacuum dried at 60°C for 15 h, and the weight change based on the weight before treatment was calculated.
Repeated boiling treatment
The weight and thickness changes caused by repeated boiling treatment were observed using the edge (about 20 × 20 mm) of the dumbbell-shaped molding. The treatment condition was boiling water immersion for 4 h, drying at 60°C for 20 h in an oven, boiling water immersion for 4 h and vacuum drying at 60°C for 15 h. The experiment was performed in triplicate, and the average value with standard deviations was calculated.
The molding obtained was pulverized into less than 150-μm mesh size, and the powder was vacuum dried at 60°C for 15 h. The thermogravimetric analysis (TGA) was carried out using a TGA 2050 (TA Instruments Japan). The powder was scanned from room temperature to 550°C at a rate of 10°C/min under nitrogen purging. The differential scanning calorimetry (DSC) measurement was conducted using a DSC 2910 (TA Instruments Japan). The powder was encapsulated in an aluminum pan and scanned from room temperature to 350°C at a rate of 10°C/min under nitrogen purging.
Fourier transform infrared spectroscopy (FT-IR)
All of the infrared spectra were obtained with a Fourier transform infrared spectrophotometer (FT/IR-4200, JASCO Corporation) using the KBr disk method and were recorded by means of an average of 16 scans at a resolution of 4 cm−1.
Density of the specimens for bending test
Citric acid content (wt%)
Thermal properties of the molding
Chemical structure of the molding
The physical and chemical properties of molding composed of wood powder and citric acid were investigated. The molding had good bending properties and impact strength. In particular, the molding with 20.0 wt% citric acid content showed excellent values. The water resistance increased with increasing citric acid content, and good resistance against boiling water was also recognized. However, marked elution caused by substances derived from citric acid was observed in samples with high citric acid content. The thermal properties of the molding decreased with increasing citric acid content. However, a significant difference of the thermal properties was scarcely observed in the moldings after a repeated boiling treatment, irrespective of citric acid content. Based on the results of FT-IR, it was clarified that the bonding mechanism of the molding was ester linkages between wood and citric acid. We therefore concluded that citric acid can be used as an adhesive for wood-based molding.
This work was financially supported by Grant-in-Aid for Scientific Research (C) (No.21580206) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and Special Coordination Funds for Promoting Science and Technology entitled “Creation of the paradigm of sustainable use of tropical rainforest by the intensive forest management and advanced utilization of forest resources”.
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