Simultaneously improving the fracture toughness and flame retardancy of soybean oil-based waterborne polyurethane coatings by phosphorus-nitrogen chain extender

Abstract Despite tremendous efforts have been dedicated to developing environmentally-friendly waterborne polyurethane (WPU) coatings from renewable biomass resources like soybean (Glycine max (L.) Merrill) oil, the fabrication of WPU coatings with high fracture toughness and flame retardancy is still challenging. In the present work, novel soybean oil-based waterborne polyurethane (SPU) coatings and the resultant films with significantly improved fracture toughness and flame retardancy as well as good deformation recovery were successfully synthesized via a chain extension technique by using [bis(2-hydroxyethyl)amino]-methyl- phosphonic acid dimethyl ester (BH) and 1,4-butane diol (BDO) as chain extenders. The structure and properties of the coatings and films were characterized systematically by FTIR, DMA, TGA, mechanical test and limiting oxygen index (LOI), etc. The results indicated that the mechanical properties of SPU films could be effectively improved by the introduction of BH, and SPU80 with 80 % BH molar ratio in the chain extenders showed remarkable improvements in mechanical properties like tensile strength (∼6.28 fold, 8.8 MPa), elongation at break (∼1.26 fold, 1510.9 %) and fracture toughness (∼4.70 fold, 64.8 MJ/m3) comparing to those of SPU20 with 20 % BH molar ratio. The incorporation of BH could delay the maximum decomposition speed of SPU films and increase char residue production. The flame retardancy of SPU films increased gradually with the BH molar content, and the LOI value of the SPU80 with 0.9 wt% phosphorus content could reach as high as 28.2 %. This work provides a new approach to develop multifunctional SPU coating with a tunable performance by incorporating phosphorus-nitrogen chain extender.