Phosphorus flame retardants (PFRs) (organic and inorganic) are more environmentally friendly, good thermally stable and have superior performance, resulting from the synergistic effect of P-N. They do not tend to form toxic gases since phosphorus is mostly locked into the char. Phosphorous FRs may be reactive (chemically bound) and additive (physical mixing) [2]. Lanxess [3] has developed is a halogen-free phosphate ester. The product that combines plasticising and Flame Retardant properties has a low odour and can be used in many plastics, coated textile fibres, tarpaulins, furniture and automotive interiors etc. It was found that non-halogenated phosphorus flame retardants have an environmentally friendly profile e.g. 9, 10-dihydro-9-oxa-10- phosphaphenanthrene-10-oxide (DOPO), (Figure 1).
However, the chemical addition of DOPO to the epoxy resin significantly decreases the Tg of the cured epoxy resin. Hence, new multifunctional reactive DOPO derivatives, phosphorus containing oligomers and phosphorus-based curing agents that would not have a detrimental effect on the Tg of the cured resin are subject to extensive research [4]. Similar phosphorus contents are required for DOPO-based additives and for reactive DOPO derivatives to reach UL 94-V0 rating. Even though the reactive approach has yet to be adopted in industry, it has been subject to an ever-growing interest from the academic community. Indeed, the reactive P-H bond of hydrogen phosphonates or phosphinates enables to covalently bind the flame retardant to the polymer chain by reaction with the epoxy functionality [4].
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