due to the catalytic effect of Cu 2 O, HCN was oxidized and converted into CO 2 , N 2 ,
H 2 O, and NO 2 . This clearly indicates that the fire toxicity of FR PUs is extremely
dependent on the FR used. Similarly, organic acid compounds also improve smoke
suppression and limit the release of toxic gases. Doerge et al. synthesized PU foams
using various organic acid compounds, such as fumaric acid, maleic acid, citric acid,
and oxalic acid, and found that they act as excellent smoke suppression agents; they
also decreased toxic gas evolution [99]. The evolution of smoke and toxic fumes
depends on the polymer structure, its chemical and physical composition, and
decomposition. Nitrogen-containing compounds, such as melamine, dicyandiamide,
and urea seem to be the best FRs for PU foams. According to the burning rate and
oxygen value, urea is a better FR additive for urethane foams compared to melamine
and dicyandiamide [100]. Owing to the greater polarity of urea, it may lead to
greater crosslinking density and thus decrease flammability. Studies indicate that
APP (ammonium polyphosphate) or EG (expandable graphite) highly reduced the
production of HCN, CO 2 , and CO during the thermal decomposition of PU polymers. Compared to PU/APP, PU/EG produced a high percentage of toxic gases with
less char yield [101]. Jiang et al. [102] prepared epoxy composites with
ZnS-decorated graphene sheets and studied their thermal decomposition behavior
and composition of the evolved materials. They observed that composites containing
2 wt% ZnS/graphene sheets showed significant inhibition in toxic gas evolution
when compared to individual ZnS and graphene sheet-containing epoxy composites
and pure epoxy resin. This is particularly true in the case of CO gas formation; this
phenomenon is attributed to the synergetic effect between ZnS and graphene nano
sheets. On the basis of the above discussion, it can be concluded that in the presence
of FR additives, the evolution of smoke and toxic gases, such as CO, CO 2 , and
HCN, is reduced when compared to the case of blank polymers. Further studies are
required on the production of smoke and toxic gases to optimize the conditions
necessary to eliminate these aspects, especially because more people die due to toxic
gas and smoke inhalation than burns.
5.2 Importance of Polyols in Flame-Retardant PUs
High-molecular-weight materials with hydroxyl groups at the terminal positions are
called polyols; these are often used as raw materials for PU synthesis. These polyols
with high molecular weight, high viscosity, and large number of reactive functional
groups improve the FR activity of the resulting PUs. The properties of the synthesized PUs mainly depend on the molecular weight of the starting polyols, viscosity, and degree of crosslinking. These polyols may also contain different
functionalities along with hydroxyl groups, such as esters, ethers, amides, acrylic,
or other functionalities [103]. Papageorgiou et al. [104] synthesized aromatic and
aliphatic ester copolymer diol-containing PUs. They noticed that the aliphatic part
confers elasticity to PU; conversely, the tensile strength and Young’s modulus of
the polymer decreased. Kashiwagi et al. [105] studied the effect of molecular weight
5.1 Thermal Degradation and Evolution of Components During PU …
53
H 2 O, and NO 2 . This clearly indicates that the fire toxicity of FR PUs is extremely
dependent on the FR used. Similarly, organic acid compounds also improve smoke
suppression and limit the release of toxic gases. Doerge et al. synthesized PU foams
using various organic acid compounds, such as fumaric acid, maleic acid, citric acid,
and oxalic acid, and found that they act as excellent smoke suppression agents; they
also decreased toxic gas evolution [99]. The evolution of smoke and toxic fumes
depends on the polymer structure, its chemical and physical composition, and
decomposition. Nitrogen-containing compounds, such as melamine, dicyandiamide,
and urea seem to be the best FRs for PU foams. According to the burning rate and
oxygen value, urea is a better FR additive for urethane foams compared to melamine
and dicyandiamide [100]. Owing to the greater polarity of urea, it may lead to
greater crosslinking density and thus decrease flammability. Studies indicate that
APP (ammonium polyphosphate) or EG (expandable graphite) highly reduced the
production of HCN, CO 2 , and CO during the thermal decomposition of PU polymers. Compared to PU/APP, PU/EG produced a high percentage of toxic gases with
less char yield [101]. Jiang et al. [102] prepared epoxy composites with
ZnS-decorated graphene sheets and studied their thermal decomposition behavior
and composition of the evolved materials. They observed that composites containing
2 wt% ZnS/graphene sheets showed significant inhibition in toxic gas evolution
when compared to individual ZnS and graphene sheet-containing epoxy composites
and pure epoxy resin. This is particularly true in the case of CO gas formation; this
phenomenon is attributed to the synergetic effect between ZnS and graphene nano
sheets. On the basis of the above discussion, it can be concluded that in the presence
of FR additives, the evolution of smoke and toxic gases, such as CO, CO 2 , and
HCN, is reduced when compared to the case of blank polymers. Further studies are
required on the production of smoke and toxic gases to optimize the conditions
necessary to eliminate these aspects, especially because more people die due to toxic
gas and smoke inhalation than burns.
5.2 Importance of Polyols in Flame-Retardant PUs
High-molecular-weight materials with hydroxyl groups at the terminal positions are
called polyols; these are often used as raw materials for PU synthesis. These polyols
with high molecular weight, high viscosity, and large number of reactive functional
groups improve the FR activity of the resulting PUs. The properties of the synthesized PUs mainly depend on the molecular weight of the starting polyols, viscosity, and degree of crosslinking. These polyols may also contain different
functionalities along with hydroxyl groups, such as esters, ethers, amides, acrylic,
or other functionalities [103]. Papageorgiou et al. [104] synthesized aromatic and
aliphatic ester copolymer diol-containing PUs. They noticed that the aliphatic part
confers elasticity to PU; conversely, the tensile strength and Young’s modulus of
the polymer decreased. Kashiwagi et al. [105] studied the effect of molecular weight
5.1 Thermal Degradation and Evolution of Components During PU …
53