Synthesis of phosphorus-containing polyol and its effects on impact resistance and flame retardancy of structural epoxy adhesives

Abstract Structural epoxy adhesives are commonly used in many industries but are limited by their flammability and low impact resistance. In order to improve the flame retardancy of epoxy adhesives, phosphorus-containing species are mainly used as reactive flame retardants, but these generally cause a decrease in mechanical strength. In this study, an oligomeric phosphorus-containing polyol (P-polyol) was synthesized by step polymerization between phenylphosphonic dichloride and ethylene glycol. The curing behavior, viscoelastic properties, adhesion performance, impact resistance, and flame retardancy of an epoxy resin containing P-polyol were characterized. Especially, impact resistance was evaluated by the impact wedge-peel test, an industry-standard testing method. The flame retardancy of the structural epoxy adhesive improved steadily with the addition of p-polyol. Impact resistance also improved at moderate P-polyol contents (8–10 parts per 100 parts of resin (8–10 phr)) but drastically decreased at a high content (15 phr). Based on this study, by incorporating P-polyol, it is possible to simultaneously improve the impact resistance and flame retardancy of structural epoxy adhesives. We obtained both a high impact resistance (dynamic resistance to cleavage: 16.9 N/mm) and flame retardancy (heat release capacity: 255 J/(g∙°C)) at an optimum P-polyol content (8 phr).