polymers [57]. However, they did not analyze smoke and toxic gas evolution. Other
types of basic phosphorus, such as white phosphorus, are pyrophoric materials,
which can show FR activity; they can undergo structural reconfiguration from
one shape to another. Unconstrained organophosphorus FRs have attracted great
attention in polymer chemistry. These phosphorus-based FRs are classified into two
types, reactive type and non-reactive type. Red phosphorus is the best example of
additive FRs; it is non-reactive in nature and is widely used for FR polymer synthesis [58, 59]. Reactive phosphorus-based FR compounds are incorporated in
polymers as chain extenders or monomers and hence they become the part of the
polymer. Reactive organophosphorus FRs exists in different forms based on their
structure and phosphorus oxidation states.
4.3.1 DOPO-Based Phosphorus FRs
Polymer materials are being used in a variety of applications; however, most
polymers are highly flammable and the fire hazards associated with them can lead to
loss of life and property. Consequently, it is required to develop new synthesis
methods and techniques for low-cost highly efficient FRs to enhance the FR activity
of the synthesized polymers. Normally, FR polymers can be synthesized by introducing phosphorus oxide FR-derivative compounds in their formulations, which
decreases the flammability of the polymer and toxic gas evolution. Therefore, many
research institutes, primarily American, are focusing on the reactivity of different
phosphorus oxides to develop efficient FRs for aircraft applications. Organic
phosphorus compounds are popularly used for FR applications; these FRs exist
in different structural forms, but the synthesis of P,C-containing compounds is
difficult and expensive compared to P,O-containing compounds. Most of the
existing phosphorus-based FR compounds are P–O bond-containing compounds
and different organic moieties are attached to the oxygen. Sanko Chemical Co. Ltd.
developed and patented several novel manufacturing methods for cyclic
organophosphorus compounds in 1972 [59]. In general, these organophosphorus
compounds were obtained by heating and condensation of orthophenyl phenol
derivatives and PCl 3 . 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO)
was synthesized by reacting stoichiometric ratios of orthophenylphenol and PCl 3
in the presence of ZnCl 2 or AlC 3 at 210 °C [60]. Among the various types of
phosphorus-oxygen containing compounds, cyclic structured DOPO, which was
first synthesized by Saito [61], has the best flame retardant properties.
In recent years, the development of phosphorus-based FRs has generated much
research interest, especially DOPO and its derivatives. Further, several methods
have been developed for the synthesis of DOPO and its derivatives, with an aim to
increase the phosphorus content and aromatic groups in the structure; some of these
structures are shown in Scheme 4.5. Because DOPO is a phosphinate oxide compound, it contains P–H bonds and in solution easily converts P-OH bonds via
4.3 Phosphorus-Based FRs
23
types of basic phosphorus, such as white phosphorus, are pyrophoric materials,
which can show FR activity; they can undergo structural reconfiguration from
one shape to another. Unconstrained organophosphorus FRs have attracted great
attention in polymer chemistry. These phosphorus-based FRs are classified into two
types, reactive type and non-reactive type. Red phosphorus is the best example of
additive FRs; it is non-reactive in nature and is widely used for FR polymer synthesis [58, 59]. Reactive phosphorus-based FR compounds are incorporated in
polymers as chain extenders or monomers and hence they become the part of the
polymer. Reactive organophosphorus FRs exists in different forms based on their
structure and phosphorus oxidation states.
4.3.1 DOPO-Based Phosphorus FRs
Polymer materials are being used in a variety of applications; however, most
polymers are highly flammable and the fire hazards associated with them can lead to
loss of life and property. Consequently, it is required to develop new synthesis
methods and techniques for low-cost highly efficient FRs to enhance the FR activity
of the synthesized polymers. Normally, FR polymers can be synthesized by introducing phosphorus oxide FR-derivative compounds in their formulations, which
decreases the flammability of the polymer and toxic gas evolution. Therefore, many
research institutes, primarily American, are focusing on the reactivity of different
phosphorus oxides to develop efficient FRs for aircraft applications. Organic
phosphorus compounds are popularly used for FR applications; these FRs exist
in different structural forms, but the synthesis of P,C-containing compounds is
difficult and expensive compared to P,O-containing compounds. Most of the
existing phosphorus-based FR compounds are P–O bond-containing compounds
and different organic moieties are attached to the oxygen. Sanko Chemical Co. Ltd.
developed and patented several novel manufacturing methods for cyclic
organophosphorus compounds in 1972 [59]. In general, these organophosphorus
compounds were obtained by heating and condensation of orthophenyl phenol
derivatives and PCl 3 . 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO)
was synthesized by reacting stoichiometric ratios of orthophenylphenol and PCl 3
in the presence of ZnCl 2 or AlC 3 at 210 °C [60]. Among the various types of
phosphorus-oxygen containing compounds, cyclic structured DOPO, which was
first synthesized by Saito [61], has the best flame retardant properties.
In recent years, the development of phosphorus-based FRs has generated much
research interest, especially DOPO and its derivatives. Further, several methods
have been developed for the synthesis of DOPO and its derivatives, with an aim to
increase the phosphorus content and aromatic groups in the structure; some of these
structures are shown in Scheme 4.5. Because DOPO is a phosphinate oxide compound, it contains P–H bonds and in solution easily converts P-OH bonds via
4.3 Phosphorus-Based FRs
23