of the industrial importance of PUs, it is necessary to study diisocyanate chemistry
to understand their properties and reactivity. Due to the presence of cumulative
double bonds, isocyanates are highly reactive in forming urethane groups without
any by-product formation. Most of the existing isocyanate compounds containing
either two or more –NCO functional groups per one molecule are called as diisocyanates and polyisocyanates, respectively. An industrially important monoisocyanate compound is methyl isocyanate (MIC), which is primarily used for
pesticide manufacture, but this isocyanate is extremely hazardous and is held
responsible for the 1984 Bhopal disaster. Diisocyanates exist as either liquids or
solids and are highly reactive through the double bond –N=C– of the –NCO
group. Aromatic diisocyanates are more reactive than aliphatic diisocyanates
because of the possible resonance structures and decreased electron density on the
central carbon of isocyanates, which plays an important role in controlling their
reactivity. Hence, aromatic diisocyanates are more reactive than their aliphatic
counterparts and can exhibit a high degree of crosslinking, which is also an
important factor in improving the FR activity of polymers. Therefore, the reactivity
of diisocyanates also plays an important role in FR PU synthesis; diisocyanates can
lead to the formation of dimers and trimers and can also undergo self-condensation
[56]. In addition, a number of crosslinking reactions may take place depending on
the reactivity of the monomers and reaction conditions, such as temperature, catalysts, and the structures of the participating alcohols, amines, and isocyanates. The
commonly used diisocyanates are toluenediisocyanate (TDI), 4,4′-dicyclohexylmethane diisocyanate (H 12 MDI), 4,4′-methylene diphenyl diisocyanate (MDI),
isophorone diisocyanate (IPDI), and hexamethylene diisocyanate (HDI) [65]. The
isocyanates used most often industrially are TDI and IPDI; however, compared to
TDI, IPDI has some advantages as it is not toxic, has average reactivity, and is
aliphatic in nature. IPDI is widely used for the preparation of light-stable PU
coatings. The two isocyanate groups of IPDI exhibit different reactivities because of
differences in steric hindrance and chemical environment. Different types of
diisocyanates contribute to PU properties in different ways; for example, compared
to cycloaliphatic or aliphatic diisocyanates, aromatic diisocyanate and
polyisocyanate-containing PUs show high FR activity [119].
5.5 Effect of Segmental Separation in PUs on Their FR
Activity
PUs are used in a number of products we use in our day-to-day life, such as shoe
leather, rubbers, adhesives, seat cushions, insulation for walls and roofs, automotive
structural foams, refrigerators, automotive paints and coatings, textile coatings, and
surface coatings due to their good physical properties such as flexibility, durability,
impact resistance, and abrasion resistance [103]. PUs are a type of block
56
5 Flame-Retardant Polyurethanes
to understand their properties and reactivity. Due to the presence of cumulative
double bonds, isocyanates are highly reactive in forming urethane groups without
any by-product formation. Most of the existing isocyanate compounds containing
either two or more –NCO functional groups per one molecule are called as diisocyanates and polyisocyanates, respectively. An industrially important monoisocyanate compound is methyl isocyanate (MIC), which is primarily used for
pesticide manufacture, but this isocyanate is extremely hazardous and is held
responsible for the 1984 Bhopal disaster. Diisocyanates exist as either liquids or
solids and are highly reactive through the double bond –N=C– of the –NCO
group. Aromatic diisocyanates are more reactive than aliphatic diisocyanates
because of the possible resonance structures and decreased electron density on the
central carbon of isocyanates, which plays an important role in controlling their
reactivity. Hence, aromatic diisocyanates are more reactive than their aliphatic
counterparts and can exhibit a high degree of crosslinking, which is also an
important factor in improving the FR activity of polymers. Therefore, the reactivity
of diisocyanates also plays an important role in FR PU synthesis; diisocyanates can
lead to the formation of dimers and trimers and can also undergo self-condensation
[56]. In addition, a number of crosslinking reactions may take place depending on
the reactivity of the monomers and reaction conditions, such as temperature, catalysts, and the structures of the participating alcohols, amines, and isocyanates. The
commonly used diisocyanates are toluenediisocyanate (TDI), 4,4′-dicyclohexylmethane diisocyanate (H 12 MDI), 4,4′-methylene diphenyl diisocyanate (MDI),
isophorone diisocyanate (IPDI), and hexamethylene diisocyanate (HDI) [65]. The
isocyanates used most often industrially are TDI and IPDI; however, compared to
TDI, IPDI has some advantages as it is not toxic, has average reactivity, and is
aliphatic in nature. IPDI is widely used for the preparation of light-stable PU
coatings. The two isocyanate groups of IPDI exhibit different reactivities because of
differences in steric hindrance and chemical environment. Different types of
diisocyanates contribute to PU properties in different ways; for example, compared
to cycloaliphatic or aliphatic diisocyanates, aromatic diisocyanate and
polyisocyanate-containing PUs show high FR activity [119].
5.5 Effect of Segmental Separation in PUs on Their FR
Activity
PUs are used in a number of products we use in our day-to-day life, such as shoe
leather, rubbers, adhesives, seat cushions, insulation for walls and roofs, automotive
structural foams, refrigerators, automotive paints and coatings, textile coatings, and
surface coatings due to their good physical properties such as flexibility, durability,
impact resistance, and abrasion resistance [103]. PUs are a type of block
56
5 Flame-Retardant Polyurethanes