Creation of a high strength, hydrophobic and fireproof surface on wood by polyamide acid under mild and simple conditions

A high performance surface for balsa wood (BW) and delignified balsa wood (DW) was created using a two-step synergistic strategy of impregnation and layer by layer coating by polyamide acid (PAA). The results showed that the PAA synthesized from 3,3,4,4-biphenyltetracarboxylic acid dianhydride (S-BPDA) and 4,4 '-diaminodiphenyl ether (ODA) was the optimal choice for the surface of wood due to its exceptional comprehensive performance. Compared to BW, the bending strength of PAA coated BW (PBW) and PAA coated DW (PDW) was significantly increased by 193.1 % and 94.2 %, respectively. Meanwhile, the impact strength of PBW and PDW showed notable improvements of 68.2 % and 23.7 %, respectively. Due to the capability of PAA to block polar groups and undergo transformation into polyimide (PI) at high temperature, PBW and PDW exhibited enhanced hydrophobic property and flame retardancy. Compared to BW, the ignition time and residue mass after combustion of PBW and PDW increased by 230.0 %, 275.8 % and 250.0 %, 312.2 %, respectively. The improved hydrophobic capacity, flame retardance and mechanical strength of wood is of great significance for expanding its application fields and improving its safety in the fields of structure and decoration.