Simultaneously enhanced bonding strength, water resistance and flame retardancy of urea-formaldehyde adhesive using polymerized polyborosiloxane for plywood

Considering the numerous applications of urea-formaldehyde (UF) resin in wood-based building materials due to their low cost, light color, short hot-pressing time etc., simultaneously endowing UF with high water resistance, bonding strength and flame retardancy is imperative and challenged. In this study, polyborosiloxane (PBS) was polymerized by hydroxyl silicone oil (PDMS) with boric acid (BA) via solvent-free, one-pot method, and used to fabricate PBS/UF with efficient water resistance, bonding strength and flame retardancy for plywood. Since PBS exhibits dual effect of the unique viscoelasticity and contributes to form dense char layer during the combustion, PBS/UF adhesives can obtain excellent water resistance, bonding strength (dry/wet), and flame retardancy. The resultant heating rate from infrared thermal imaging of plywood with 5 % PBS/UF adhesives significantly slowed down, and corresponding burned through time delayed 20 s, as well as a high limiting oxygen index (LOI) value of 28.6 %. Meanwhile, the dry and wet bonding strength respectively increased by 44.0 % and 24.7 %, demonstrating the well bonding strength and enhanced water resistance. Overall, the addition of PBS can not only improve the flame retardancy but also maintain the excellent mechanical properties of UF, even improve the water-resistance performance. This work provides an effective approach for obtaining UF with water resistance, flame retardancy and mechanical properties meanwhile in wood-based building materials applications.