Insights into Phosphorus-Containing Flame Retardants …
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in high oxidation states) mainly because of its characteristics such as (i) low water solubility, (ii) low volatility, (iii) low dose requirement, (iv) less degradation to possibly
hazardous substances, and (v) no toxic emissions [2, 6]. Mechanistically, phosphorous based flame retardants, during a fire form poly and meta-phosphoric acids which
form an oxygen-barrier layer [15] and commonly used due to the environmental
scrutiny halogenated and formaldehyde-free FRs.
The flame retardant mechanism described for phosphorus containing flame retardants includes both a condensed and a vapor phase mechanism depending on the type
of phosphorus compound and the polymer. Specific applications for red phosphorus,
organophosphates, chlorophosphates and bromophosphates are described. The use
of triarylphosphates in PVC, modified polyphenylene oxide, and polycarbonate/ABS
is described. The chlorophosphates are used in polyurethanes and the bromophosphates in engineering thermoplastics. Flammability and mechanical properties are
given for specific polymers [16].
Phosphorous based flame resistant materials have long been used since the 1940s–
1950s. P-based FRs exhibit excellent fire inertness ability and found effective both
in the vapour and condensed phases. They vary in oxidation states (0 to +5) and
can be classified into elemental, inorganic, and organic or organo-phosphorus [17]
categories.
1.1 Elemental Phosphorus as FR
Phosphorus (P 0 ) has several allotropic forms [18] out of which white phosphorus
(WP) and red phosphorus (RP) are most common. WP is a white, soft, waxy solid
consists of tetrahedral P 4 molecules, in which each atom is bound to the other three
atoms by a single bond. It ignites spontaneously and is very toxic and reactive in
nature, and therefore cannot be used as FR [17].
Samples of WP always contain red phosphorus in a very little amount and accordingly appear yellow. On heating, WP can be converted into RP in the absence of air.
It is harder, denser, more stable, less toxic, less reactive than WP and polymeric in
structure with P 4 units [19]. Although, it ignites easily but possesses thermal stability
up to 450 °C (approx.) and thus, the ability to be used as sufficient FR agent [18]. RP
is observed as an efficient FR especially for oxygen-containing polymers that work in
the vapor and condensed phases [19, 20]. Among the high performance flame retardants (HFFR) additives, RP is a type of powerful FR and has been significantly used
for polymeric moieties other than textiles such as polyethylene [21], poly(ethylene
terephthalate) [22], nylon [23, 24] etc.
Moreover, the combination of RP with other HFFR additives, metal hydroxide
or intumescent FR can improve the overall fire retardancy of highly flammable substrates like polyolefins (PO) blends have been investigated and reported as effective
[25]. However, the main disadvantages of RP are because of poor thermostability,
the evolution of highly toxic phosphine (PH 3 ) during the reaction with moisture
and the lack of compatibility with synthetic resins [26]. A novel technology was
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in high oxidation states) mainly because of its characteristics such as (i) low water solubility, (ii) low volatility, (iii) low dose requirement, (iv) less degradation to possibly
hazardous substances, and (v) no toxic emissions [2, 6]. Mechanistically, phosphorous based flame retardants, during a fire form poly and meta-phosphoric acids which
form an oxygen-barrier layer [15] and commonly used due to the environmental
scrutiny halogenated and formaldehyde-free FRs.
The flame retardant mechanism described for phosphorus containing flame retardants includes both a condensed and a vapor phase mechanism depending on the type
of phosphorus compound and the polymer. Specific applications for red phosphorus,
organophosphates, chlorophosphates and bromophosphates are described. The use
of triarylphosphates in PVC, modified polyphenylene oxide, and polycarbonate/ABS
is described. The chlorophosphates are used in polyurethanes and the bromophosphates in engineering thermoplastics. Flammability and mechanical properties are
given for specific polymers [16].
Phosphorous based flame resistant materials have long been used since the 1940s–
1950s. P-based FRs exhibit excellent fire inertness ability and found effective both
in the vapour and condensed phases. They vary in oxidation states (0 to +5) and
can be classified into elemental, inorganic, and organic or organo-phosphorus [17]
categories.
1.1 Elemental Phosphorus as FR
Phosphorus (P 0 ) has several allotropic forms [18] out of which white phosphorus
(WP) and red phosphorus (RP) are most common. WP is a white, soft, waxy solid
consists of tetrahedral P 4 molecules, in which each atom is bound to the other three
atoms by a single bond. It ignites spontaneously and is very toxic and reactive in
nature, and therefore cannot be used as FR [17].
Samples of WP always contain red phosphorus in a very little amount and accordingly appear yellow. On heating, WP can be converted into RP in the absence of air.
It is harder, denser, more stable, less toxic, less reactive than WP and polymeric in
structure with P 4 units [19]. Although, it ignites easily but possesses thermal stability
up to 450 °C (approx.) and thus, the ability to be used as sufficient FR agent [18]. RP
is observed as an efficient FR especially for oxygen-containing polymers that work in
the vapor and condensed phases [19, 20]. Among the high performance flame retardants (HFFR) additives, RP is a type of powerful FR and has been significantly used
for polymeric moieties other than textiles such as polyethylene [21], poly(ethylene
terephthalate) [22], nylon [23, 24] etc.
Moreover, the combination of RP with other HFFR additives, metal hydroxide
or intumescent FR can improve the overall fire retardancy of highly flammable substrates like polyolefins (PO) blends have been investigated and reported as effective
[25]. However, the main disadvantages of RP are because of poor thermostability,
the evolution of highly toxic phosphine (PH 3 ) during the reaction with moisture
and the lack of compatibility with synthetic resins [26]. A novel technology was