Flame retardancy and hydrolysis resistance of waterborne polyurethane bearing organophosphate moieties lateral chain

A novel halogen-free flame retardant dial bearing pendant organophosphate group, 2-ethyl-2-(2-oxo-5,5-dimethyl-1,3,2-dioxaphosphorinanyl-2-methylene)-1,3-propanediol (EPPD), was designed and synthesized, and then covalently conjugated into polyurethane as chain extender to prepare a series of phosphorus-containing flame retardant waterborne polyurethanes (PWPU). The chemical structures of the resultant EPPD and PWPU were confirmed by FTIR, H-1 and P-31 NMR, respectively. Simultaneously, the flammability and thermal behavior of PWPU films were systematically investigated by limiting oxygen index (LOI) test, UL-94 vertical burning experiment and thermogravimetric (TG) as well as residual char analysis. An LOI value of 26.6% and a UL-94 V-0 rating can be achieved when the PWPU sample conjugated with 12 wt% EPPD, showing good inherent flame retardancy. TGA and residual char analysis indicate that the incorporation of EPPD induces a slight decrease of the thermal stability due to the relatively weaker bond energy of O=P-O, whereas the formed phosphoric and polyphosphoric acid during combustion can impart PWPU with nonflammability owing to the promoting of rugged carbonaceous char in the condensed phase which shields the underlying polyurethane from further attacking of heat flux. Meanwhile, hydrolytic stability study demonstrates that PWPU with organophosphate moieties linking in the side-chain exhibits better hydrolysis resistance than that with phosphoester bond conjugated in the backbone. (C) 2015 Elsevier B.V. All rights reserved.