Preparation of bisDOPO-NH₂-POSS flame retardant and its application to plywood using modified urea-formaldehyde resin

A new type of polyhedral oligomeric silsesquioxanes bisDOPO-NH₂-POSS flame retardant consisting of one polyhedral oligomeric silsesquioxane (POSS), two units of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and reactive –NH₂ groups, was synthesized and characterized by FTIR, ¹H NMR, and ³¹P NMR. It was applied to modified conventional environmentally friendly E0 grade urea formaldehyde (UF) resin to prepare flame-retardant plywood. Two other POSS structure flame retardants were used for comparison. The results showed that the addition of bisDOPO-NH₂-POSS flame retardant to E0 UF resin improved bonding strength while lowering formaldehyde emission owing to the increase in cross-linking of the UF molecule chain in condensation. TGA curves showed that bisDOPO-NH₂-POSS-E0 UF exhibited the highest thermal stability. The flammability characteristics of the plywood were evaluated by a cone calorimeter test (CCT), and the results indicated that the heat release and smoke production of flame-retardant plywood were lower than those of the control E0-UF plywood. Moreover, compared to E0-UF plywood, the char residue remaining after the CCT test of the flame-retardant plywood with POSS structure increased. In particular, bisDOPO-NH₂-POSS-E0-plywood had a continual compact outer carbon layer, which was conducive to effectively forming a physical barrier to hinder the penetration of oxygen and heat and exhibiting the outstanding synergistic effect of phosphorous, silicon and nitrogen.