Solvent-free two-component polyurethane conjugated with crosslinkable hydroxyl-functionalized ammonium polyphosphate: Curing behaviors, flammability and mechanical properties

To simultaneously improve the compatibility, flame retardancy and mechanical properties of ammonium polyphosphate (APP) based polyurethane, an efficient and facile strategy was developed, in which hydroxyl-functionalized APP (HAPP) was synthesized through ion-exchange reaction between APP and 3-aminopropanol, and then utilized as a multiple-function excipient for solvent-free two-component polyurethane (2K-PU) by chemical cross-linking. The curing behavior, mechanical and flame-retardant performances of HAPP cured 2K-PU, designating as 2K-PU/HAPP, were elaborately evaluated. Cone calorimeter results indicated that the conjugation of 15 wt% HAPP into 2K-PU significantly reduced the peak heat release rate (PHRR), total heat release (THR) and total smoke production (TSP) by 68.7%, 42.8% and 9.3%, respectively. Intriguingly, mechanical tests illustrated that the chemical incorporation of HAPP definitely improved the tensile strength of 2K-PU/HAPP while well-maintained the elongation at break, highlighting its attractive potential for industrial application. In addition, a flame-retardant mechanism for 2K-PU/HAPP was proposed. All aforementioned results distinctly demonstrate a feasible yet promising paradigm for fabricating APP-containing polyurethane.