Adhesives formulated with chemically modified okara and phenol–resorcinol–formaldehyde for bonding fancy veneer onto high-density fiberboard

Abstract

With an abrupt increase of petrochemical prices and the critical environmental issue related to the volatile organic compounds emitted from the adhesive resins, the development of environmentally friendly new adhesive systems is common issue. In our study, the okara, which is a residue from the production of tofu, was introduced to develop competitive bio-based adhesives for the production of fancy-veneered floor boards. Okara (AC and AK) was hydrolyzed with 1% sulfuric acid solution and 1% sodium hydroxide solution. Phenol–resorcinol–formaldehyde (PRF) prepolymer was prepared as a crosslinker of AC and AK. Adhesive resins were formulated with AC, AK and PRF prepolymer. The adhesive resins were used to fabricate floor boards composed of oak veneers onto high-density fiberboard. The experimental variables were three weight ratio of AC/AK to PRF (45/45/10, 40/40/20 and 35/35/30), three assembly time (0, 10 and 20 min), and two press time (90 and 120 s), respectively. The fancy-veneered high-density fiberboards were prepared and subject to testing of the dry tensile strength, glueline failure by wetting and formaldehyde emission. Dry tensile strengths of the boards exceeded the requirement of KS standard. The formaldehyde emissions were satisfied with the limitation specified in KS standard. Based on these results, okara has a potential to be used as a renewable raw material of environmentally friendly adhesive resin systems for production of floor boards, but further researches, such as biological hydrolysis of okara and various formulations of PRF prepolymer, are required to improve the adhesive strength and to reduce formaldehyde emission of okara-based adhesive resins.