Flame retardancy of dicyandiamide-crosslinked epoxy resins containing phenolphthalein structures and/or a phosphorus-containing additive

The feasibility of producing halogen-free epoxy moldings having a UL-94 V-0 flammability rating has been further supported. We have shown that the fire retarding properties (OI and UL-94) of the cured epoxy were strongly improved by using the commercial additive resorcinol bis(diphenyl phosphate) (FYROLFLEX RDP) but not improved merely by reacting phenolphthalein into the resin as a chain extender. However, a combination of the phosphate with the phenolphthalein as a chain extender gave substantially enhanced flame retardant results. A related system where the phenolphthalein structure was built into the phosphate additive rather than into the polymer backbone gave inferior results. Thermogravimetric analysis in N-2 showed that the system of the phosphate additive in the phenolphthalein-extended resin gave an increased char yield. Infrared studies an the system at different pyrolysis temperatures showed that the lactone carbonyl of the phenolphthalein structure underwent reaction during decomposition of the cured resin in the presence of the phosphate. Infrared evidence suggested that this lactone group may be involved in a crosslinking reaction forming amide links, which may lead to char enhancement. The phosphate depressed somewhat both the T-g and thermal stability but the presence of the phenolphthalein structure partially overcame the T-g depression.