Abstract: [Objective] The phenolic resin was co-modified with boric acid and bio-oil to prepare a heat-resistant and toughening phenol formaldehyde resin, which can chronically satisfy the special requirements of the wood flooring used in a relatively high temperature and humid environment.[Method] The optimum technology of phenolic resin co-modified by boric acid and bio-oil(BBPF resin) was put forward based on examining index on the solid content, carbon residue rate, tensile strength and bond strength as well as examine factors on the substitute rate of bio-oil to phenol, the addition of boric acid and NaOH/P.[Result] 1) Range analysis of the orthogonal experiment showed that the substitute rate of bio-oil to phenol has the greatest influence on the solid content, tensile strength and bond strength of the BBPF resin, followed by the addition of boric acid to phenol and the molar ratio of NaOH/P among the main factors. While the substitute rate of bio-oil to phenol had the most pronounced influence on the residual carbon rate of the BBPF resin, followed by the molar ratio of NaOH/P and the addition of boric acid among the main factors. 2) With the increase of the substitute of bio-oil to phenol, the solid content and residual carbon rate of the BBPF resin both showed a downward trend, so as the bond strength of the plywood, but the tensile strength exhibited an increasing trend. Meanwhile, the solid content, residual carbon rate, tensile strength and bond strength increased first and then decreased along with the addition of boric acid. In addition, the solid content, residual carbon rate and bond strength increased initially and then decreased while the tensile strength increased due to the increase of NaOH/P molar ratio. 3) The optimum synthesis process of the BBPF resin was as follows:the substitute rate of bio-oil was 20%, the addition of boric acid to phenol was 4% and the molar ratio of NaOH/P was 0.5. Resin prepared under the optimum condition had a residual carbon rate of 58.10% at 800℃, and a tensile strength of 3.15 MPa, an elongation at break of 15.7%, the bond strength was 1.12 MPa. 4) The results of thermogravimetric analysis showed that the mass loss of the resins was divided into four stages in the range of 0-800℃. The first stage was 30-350℃. In this stage, the residual moisture in the resin evaporated, along with the post-curing of the resin. The hydroxymethyl group which was not involved in curing was oxidized and then removed. Meanwhile, the ether bond was split into methylene bonds which result ing the release of formaldehyde. In addition, the extremum of the weight loss rate of the BBPF resin moved to the high temperature zone. The methylene bond in the resin broke and decomposed and the mass loss was less in the second stage ranging from 350℃ to 450℃. The third stage of 450-600℃, the phenolic hydroxyl group underwent dehydration and cyclization and the resin was decomposed obviously. In this stage, the BBPF resin had a higher temperature on the fastest degradation which was higher than that of PF resin and phenolic resin modified by bio-oil(BOPF) but lower than phenolic resin modified by boric acid(BPF). In the fourth stage, the residual carbon rate of BBPF at 800℃ was higher than that of PF resin and BOPF resin, and lower than that of BPF resin. The infrared spectrum of the BBPF resin showed an absorption peak at 1 384 cm^-1 of B-O bond. Simultaneously, the intensity of the CH₂ characteristic peak at 2 924 cm^-1 was enhanced as well as peak at 876 cm^-1 which indicated the replacement of the phenolic active site.[Conclusion] The high-bonding B-O bonds wereintroduced into the BBPF resin owing to the reaction between boric acid and phenolic hydroxyl group. In addition, the flexible groups of bio-oil were also introduced to the BBPF resin. These improved the heat resistance and toughness of the BBPF resin.

Key words: phenolic resin, boric acid, bio-oil, wood flooring for ground with heating system, heat resistance, toughness