Most nitrogen- and phosphorus-based salt-containing polymers exhibit IFR
activity. Unfortunately, these IFR-active substances face some problems, such as
moisture sensitivity and poor compatibility with polymers, even though APP is
soluble in water. Therefore, to eliminate these limitations, a microencapsulation
technique was adopted. Microencapsulated di-ammonium hydrogen phosphate
(DAHP)-containing PUs were coated on textiles and it was found that they exhibited
improved compatibility and IFR activity; further, flame dripping was decreased
compared to virgin PU coatings [46, 47]. Microencapsulated APP/PU resins
showed significant improvement in thermal and FR activity compared to normal
APP/PU resins [48]. Microencapsulated ammonium polyphosphate (MCAPP) was
introduced into PUs at different percentages and it was found that 30% MCAPP
led to a V-0 rating in the UL-94 test and no dripping [49]. Using microencapsulation, microscopic amounts of additives can be incorporated in polymers.
Polyoxymethylene (POM) is considered an excellent engineering plastic material as
it exhibits high mechanical strength as well as self-lubrication and high wear
resistance. It is therefore used in several applications, but it faces the problem of high
flammability (low LOI value of 15%). Typically, FR polymers can be prepared by
the addition of FR additives, but this strategy is not applicable for POM polymers as
they exhibit a highly crystalline structure, which renders them incompatible with FR
additives. Many American and Japanese companies are involved in the preparation
of POMs with different FR additives, such as APP, MP, and triazine; they could
achieve high LOI values along with self-extinguishing ability, but the mechanical
properties of the resultant POMs are compromised [50, 51]. Studies on TPU
spherical particles, which are compatible with POM (through formation of H-bonds)
have been undertaken to overcome these limitations [52–55]. From the above discussion, it is clear that it is necessary to develop methods to improve FR compatibility to improve the FR activity of POM polymers.
4.3 Phosphorus-Based FRs
The possibilities for the application of phosphorus-containing FRs are versatile
because elemental phosphorus can exhibit flexible oxidation states; however, most
FR phosphorus compounds are in the 0, +3, and +5 oxidation states. Most
phosphorus-based FRs exist in the form of organic and inorganic compounds, such
as phosphines, phosphonium compounds, phosphine oxides, phosphonates, elemental red phosphorus, phosphites, and phosphates. Likewise, most inorganic
phosphorus FRs contain phosphates in their structures. Among them, red phosphorus with an oxidation state of zero has a unique structure and is often used as a
FR additive in polymers. Compared to normal red phosphorus, the stabilized form
of coated red phosphorus is effective with oxygen-containing polymers, such as
PUs, polyesters, and polyamides. In 1965, Piechota, for the first time, synthesized
FR PUs using red phosphorus and observed excellent FR activity [56]. Later,
Peters found that red phosphorus can show FR activity even in oxygen-containing
22
4 Types of Flame Retardants Used for the Synthesis of …
activity. Unfortunately, these IFR-active substances face some problems, such as
moisture sensitivity and poor compatibility with polymers, even though APP is
soluble in water. Therefore, to eliminate these limitations, a microencapsulation
technique was adopted. Microencapsulated di-ammonium hydrogen phosphate
(DAHP)-containing PUs were coated on textiles and it was found that they exhibited
improved compatibility and IFR activity; further, flame dripping was decreased
compared to virgin PU coatings [46, 47]. Microencapsulated APP/PU resins
showed significant improvement in thermal and FR activity compared to normal
APP/PU resins [48]. Microencapsulated ammonium polyphosphate (MCAPP) was
introduced into PUs at different percentages and it was found that 30% MCAPP
led to a V-0 rating in the UL-94 test and no dripping [49]. Using microencapsulation, microscopic amounts of additives can be incorporated in polymers.
Polyoxymethylene (POM) is considered an excellent engineering plastic material as
it exhibits high mechanical strength as well as self-lubrication and high wear
resistance. It is therefore used in several applications, but it faces the problem of high
flammability (low LOI value of 15%). Typically, FR polymers can be prepared by
the addition of FR additives, but this strategy is not applicable for POM polymers as
they exhibit a highly crystalline structure, which renders them incompatible with FR
additives. Many American and Japanese companies are involved in the preparation
of POMs with different FR additives, such as APP, MP, and triazine; they could
achieve high LOI values along with self-extinguishing ability, but the mechanical
properties of the resultant POMs are compromised [50, 51]. Studies on TPU
spherical particles, which are compatible with POM (through formation of H-bonds)
have been undertaken to overcome these limitations [52–55]. From the above discussion, it is clear that it is necessary to develop methods to improve FR compatibility to improve the FR activity of POM polymers.
4.3 Phosphorus-Based FRs
The possibilities for the application of phosphorus-containing FRs are versatile
because elemental phosphorus can exhibit flexible oxidation states; however, most
FR phosphorus compounds are in the 0, +3, and +5 oxidation states. Most
phosphorus-based FRs exist in the form of organic and inorganic compounds, such
as phosphines, phosphonium compounds, phosphine oxides, phosphonates, elemental red phosphorus, phosphites, and phosphates. Likewise, most inorganic
phosphorus FRs contain phosphates in their structures. Among them, red phosphorus with an oxidation state of zero has a unique structure and is often used as a
FR additive in polymers. Compared to normal red phosphorus, the stabilized form
of coated red phosphorus is effective with oxygen-containing polymers, such as
PUs, polyesters, and polyamides. In 1965, Piechota, for the first time, synthesized
FR PUs using red phosphorus and observed excellent FR activity [56]. Later,
Peters found that red phosphorus can show FR activity even in oxygen-containing
22
4 Types of Flame Retardants Used for the Synthesis of …