Abstract: A novel flame retardant， methyl diphenyl phosphine oxide （DPMP）， was compounded with polyurethane （PU） to obtain flame-retardant PU composites by using flake clay as a synergist. The effects of DPMP and clay on the flame retardancy of the composites were investigated through ultimate oxygen index （LOI）， vertical combustion （UL 94）， and micro calorimetry. The crystallization properties and thermal degradation behaviors of the composites were analyzed by using differential scanning calorimetry and thermogravimetric analyzer. The results indicated that DPMP could promote the cold crystallization of PU， thus improving its modulus and mechanical properties. The addition of 10 wt% DPMP increased the LOI value of PU from 23 to 28.3 vol%， and the resultant composite obtained a modulus， elongation at break， and tensile strength of 1.37 MPa， 1 042 % and 13.84 MPa， respectively， while the corresponding values for pure PU were 1.17 MPa， 811 % and 11.32 MPa， respectively. With a further addition of 1 wt% clay， the dripping phenomenon of the composites could be eliminated， enabling the composites to achieve a UL 94 V-0 classification in the vertical burning test.

Key words: methyldiphenylphosphine, polyurethane, cold crystallization, flame retardancy, composites