Synergistic flame retardant mixtures in epoxy resins
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 (R), 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 (R)) showed outstanding flame retardant efficiencies, so that low phosphorus contents (0.4% in DEN 438 (R) / 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 (T-g) of the cured samples and to reduce the loading of phosphorus compounds. Such synergistic mixtures surpass the flame retardant efficiency of TBBA.