Synergistic Flame Retardant Mixtures in Epoxy Resins

Abstract

Synergistic flame retardant mixtures were successfully tested as halogen-free flame retardants in two prepreg epoxies relevant in printed wired boards (PWB). The first epoxy system is composed of epoxy-novolac DOW DEN 438®, dicyandiamide (DICY) as hardener and fenurone as accelerator. The second one is based on the diglycidyl ether of bisphenol A (DGEBA) and uses the same hardener / accelerator combination. Reactive organophosphorus compounds were incorporated into these resins together with low-priced inorganic substances to give efficient flame-retardant systems. Samples without synergists or with tetrabromo bisphenol A (TBBA) were tested as well. Combinations of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and 30% of crystalline boehmite (Nabaltec®) showed outstanding flame retardant efficiencies, so that low phosphorus contents (0.4% in DEN 438® / DICY / fenurone, 1.3% in DGEBA / DICY / fenurone) were sufficient to reach UL 94 V0 rating. Melamine polyphosphate (MPP) amplifies the activity of organophosphorus flame retardants, too. Additionally, novel salt-like additives were found to be efficient fire-retardants in the epoxy resins examined. Their flame-retardant efficiencies could be further enhanced by combination with boehmite or MPP. Using these synergistic mixtures allows the maintainence of the glass transition temperature (Tg) of the cured samples and to reduce the loading of phosphorus compounds. Such synergistic mixtures surpass the flame retardant efficiency of TBBA.