Effect of diphenyl methyl phosphonate and clay as additives on the mechanical and flame retardancy of crosslinked polyurethane

The diphenyl methyl phosphonate (DPMP) as flame retardant additive with or without clay was used for the crosslinking polyurethane (PU) in current study. The effects on the thermal properties, flame retardancy, crystallization and mechanical properties of PU were investigated, using various characterizations including thermogravimetric analysis/infrared spectroscopy (TGA-IR), differential scanning calorimetry (DSC), cone calorimetric test (CCT), X-ray diffraction analysis and mechanical tests. Results revealed that on the one hand DPMP could enhance flame retardancy of PU and mechanical properties on the other hand. Adding 10 wt% DPMP increases the limiting oxygen index value of PU from 23% to 28.3%. The Young's modulus, elongation at breaking and the tensile strength of PU/10DPMP were 1.37 MPa, 1042% and 13.84 MPa, respectively, while corresponding values for pure PU are 1.17 MPa, 811% and 11.32 MPa, respectively. DSC results showed that DPMP promoted the cold crystallization of PU, thus accounting for the enhancement in the mechanical properties. It was found that DPMP as flame retardant mainly acts in the gas phase and therefore quenches the active radical indicated by CCT results. Moreover, addition of clay further boosted the flame retardancy efficient of DPMP by acting in the condensed phase but depressed the crystallization of PU.