Epoxy resins and their utility are well known and have been described in numerous publications. Just as well known is the propensity of these resins to burn. To improve their flame retardant characteristics, epoxy resins have been compounded with phosphate and phosphonate esters including those containing haloalkyl groups as described in British Patent No. 1,487,609; U.S. Pat. No. 3,192,242 and South African Patent No. 18201/77. The use of triphenylphosphine as a flame retardant additive for epoxy resins was described by Martin and Price, J. Applied Polymer Science, 12, 143-158 (1968).
Tetrakishydroxymethylphosphonium chloride and trishydroxymethylphosphine oxide have also been used in epoxy resins as described in U.S. Pat. No. 2,916,473. Aminoalkylphosphonic acid esters are suggested as fire retardant hardeners for epoxy resins in U.S. Pat. No. 4,151,229.
U.S. Pat. Nos. 3,666,543 and 3,716,580 disclose 2-hydroxyalkylphosphine oxides and halogenated derivatives thereof that are said to have utility as flame proofing agents.
U.S. Pat. Nos. 4,345,059; 4,380,571 and 4,440,944 all disclose fire retardant epoxy resin compositions which contain 3-hydroxyalkylphosphine oxides to impart said fire retardant characteristics.
Addition of halogenated organic compounds alone to epoxy resins for imparting flame retardancy is well known and widely practiced. The most widely used method is to add a brominated bis-phenol acetone or a bis-epoxide adduct thereof to the resin. However, these brominated resins, when used alone, require bromine contents of up to 20% or even higher to be effective. These large organobromine levels increase the density of the resin, generate large amounts of smoke containing corrosive materials such as hydrogen bromide gas, and may lead to generation of highly toxic materials such as polybrominated dibenzodioxins. Formulations which reduce the bromine content required for flame retardancy would thus have significant value.
However, phosphorus based known flame retardant additives for use in epoxy resin compositions generally suffer from one or more deficiencies including low compatibility with the resin, low thermal stability or poor fire retardant behavior. Some fire retardant additives also negatively impact the glass transition temperature of the resin to an unacceptable extent.