The effects of surface modification by using phytic acid-based hybrid complexes on the interfacial properties of heat-treated wood

Heat-treated wood is widely used in outdoors due to its good dimensional stability and biological durability, while the reduced surface hydrophilicity negatively affects its bonding performance with water-soluble adhesives. In this work, a convenient surface modification strategy was prepared on heat-treated wood by surface modification with phytic acid or phytic acid combined with Fe3+ to improve its interfacial bonding properties. Results showed that the phytic acid and phytic acid-Fe3+ chelate are distributed both in the cell cavity and cell wall, and phytic acid surface coating is smooth while phytic acid-Fe3+ seemed layer-by-layer. These coatings bring a large number of hydroxyl groups to improve the surface wettability of heat-treated wood. Therefore, the water contact angle of both phytic acid treated wood and phytic acid-Fe3+ treated wood at 60 s is smaller than that on control wood. The water contact angle in the earlywood of phytic acid treated wood even reaches 0°. The average penetration and effective penetration of phenol formaldehyde adhesive into heat-treated wood also increase after treatment. On average, the average penetration and effective penetration value of phytic acid treated wood increased from 870 to 997 μm, and 35 to 62 μm, respectively. The average penetration value of phytic acid-Fe3+ treated wood is decreased due to the accumulation of chelates. But its effective penetration value is more than twice that of heat-treated wood. The digital image correlation results that the tensile strength of phytic acid treated wood is much higher than that of heat-treated wood. The shear strain of phytic acid treated wood is distributed more homogeneously. However, the maximum tensile stress of phytic acid-Fe3+ treated wood is poor due to the glueline being too thick to closely connect with wood.