via a condensed-phase mechanism; a char layer is produced during polymer
combustion. Char formation and crosslinking are attributed to the strong phosphonic and phosphoric acids formed during initial pyrolysis. The studies also
reported that the strength of the acids also influenced the FR activity (apart from the
formation of char residue); phosphoric acid esterifies faster and forms a carbonaceous char layer more rapidly compared to phosphonic acid, due to it being a strong
acid. Price et al. [82] studied the thermal and FR properties of polymers with
different substituents containing phosphate and phosphonic derivatives; the FRs are
linked to the polymer via covalent bonds. They observed that the derivatives of
phosphorus acid-containing copolymers exhibit stronger FR activity than derivatives of phosphoric acid-containing copolymers. Lorenzetti et al. [83] studied the
FR mechanism of PU foams containing phosphorus oxides of various oxidation
states. Lower oxidation-state P-oxides exhibited a combination of gas-phase and
solid-phase FR activity, whereas high oxidation-state P-oxides exhibited only the
solid-phase mechanism. Further, Yang et al. [84] systematically investigated the
thermal and FR properties of polyesters containing melamine P-oxides of various
oxidation states; with a decrease in the oxidation state, the thermal stability of the
polyesters decreased, but their FR activity increased. At lower oxidation states,
P-oxides exhibit a combined gas-phase and solid-phase mechanism by diluting the
combustible gases and releasing non-flammable gases, such as CO 2 , CO, and NH 3 .
Several aryl groups containing different P-based compounds of different oxidation
states are shown in Scheme 4.7. Some aromatic substituent aryl phosphate and
phosphonate derivative-containing polymers show high FR and char residue formation compared to aliphatic substituent methyl phosphates and phosphonatecontaining polymers, as the extra crosslinking possible with aryl groups restricts the
release of volatile products.
Scheme 4.6 Synthesis of DOPO-based polyurethanes. Reproduced with permission from [72].
Copyright 2002, John Wiley Ltd
26
4 Types of Flame Retardants Used for the Synthesis of …
combustion. Char formation and crosslinking are attributed to the strong phosphonic and phosphoric acids formed during initial pyrolysis. The studies also
reported that the strength of the acids also influenced the FR activity (apart from the
formation of char residue); phosphoric acid esterifies faster and forms a carbonaceous char layer more rapidly compared to phosphonic acid, due to it being a strong
acid. Price et al. [82] studied the thermal and FR properties of polymers with
different substituents containing phosphate and phosphonic derivatives; the FRs are
linked to the polymer via covalent bonds. They observed that the derivatives of
phosphorus acid-containing copolymers exhibit stronger FR activity than derivatives of phosphoric acid-containing copolymers. Lorenzetti et al. [83] studied the
FR mechanism of PU foams containing phosphorus oxides of various oxidation
states. Lower oxidation-state P-oxides exhibited a combination of gas-phase and
solid-phase FR activity, whereas high oxidation-state P-oxides exhibited only the
solid-phase mechanism. Further, Yang et al. [84] systematically investigated the
thermal and FR properties of polyesters containing melamine P-oxides of various
oxidation states; with a decrease in the oxidation state, the thermal stability of the
polyesters decreased, but their FR activity increased. At lower oxidation states,
P-oxides exhibit a combined gas-phase and solid-phase mechanism by diluting the
combustible gases and releasing non-flammable gases, such as CO 2 , CO, and NH 3 .
Several aryl groups containing different P-based compounds of different oxidation
states are shown in Scheme 4.7. Some aromatic substituent aryl phosphate and
phosphonate derivative-containing polymers show high FR and char residue formation compared to aliphatic substituent methyl phosphates and phosphonatecontaining polymers, as the extra crosslinking possible with aryl groups restricts the
release of volatile products.
Scheme 4.6 Synthesis of DOPO-based polyurethanes. Reproduced with permission from [72].
Copyright 2002, John Wiley Ltd
26
4 Types of Flame Retardants Used for the Synthesis of …