stability to the PUs [40, 57–59]. Many studies indicate that compared to
solvent-based PUs, water-dispersible PUs show less thermal stability due to the
presence of thermally labile urethane and urea linkages [21, 60]. Kuruma et al. [61]
reported that with increasing hard segment content in water-dispersible PUs, thermal degradation is accelerated in a particular temperature region due to the large
number of labile urethane and urea linkages. The thermal stability of PUs also
depends on various characteristic features of CEs, such as their polarity, +I effect,
and presence of electron-donating groups [21, 62–64]. Coutinho et al. [21] synthesized PU dispersions using two different CEs, ethylene diamine and hydrazine,
and noticed that hydrazine-containing PU dispersions exhibited high thermal stability than their counterparts containing ethylene diamine owing to the more polar
nature of hydrazine. Qin et al. [64] synthesized PUs using two different aromatic
diamines as chain extenders; the CEs contained sulfone and diphenyl ethers as
functional groups. It was observed that the PUs prepared using sulfones exhibited
higher thermal stability due to the more polar nature of sulfone groups.
Chattopadhyay et al. [65] systematically evaluated the relationship between the
thermal decomposition of different types of PUs and flammability. However, they
did not discuss toxic gas evolution from PUs and halogenated FRs. The FR activity
of polymers also depends on their T m and T g . Therefore, polymers with lower T g
values melt easily and catch fire easily but those with higher T g values do not catch
fire easily, which implies higher FR activity. Liu et al. [66] synthesized
phosphorus-containing aryl alkyl novolac (Ar-DOPO-N) blended with phenol
formaldehyde novolac and melamine-modified phenol formaldehyde novolac and
used them as curing agents for o-cresol formaldehyde novolac epoxy resins; the
resulting cured epoxy resins showed increased T g values in both cases but in the
case of melamine-modified phenol formaldehyde novolac, higher FR activity was
observed. Tirumal et al. [67] evaluated the relationship between T g , thermal stability, and density of PU foams. Thermal stability decreases with a decrease in PU
foam density but the T g increased. To increase the thermal stability of the polymers,
PNCs with varieties of nanoparticles (NPs) have been prepared. However, transition
metals are more advantageous in improving the thermal stability and FR activity of
PUs. In their exhaustive investigation, Moroi et al. [68] analyzed the thermal
decomposition of various transition metal ion-containing composite PU polymers.
Generally, polymer combustion is accompanied by the release of CO and CO 2
and if the polymer contains nitrogen, NO, NO 2 , NH 3 , and HCN may also be
released. Chlorinated plastic materials can produce HCl and the hazardous gas
phosgene (COCl 2 ), while fluorinated plastics release HF [70]. Combustion of most
polymer materials yields only a small quantity of HCN, but some polymers such as
PUs, acrylates, polyamides, nitrocellulose, and other nitrogen-containing plastics
produce high quantities of HCN when subjected to flame exposure. Under normal
conditions, the toxicity of HCN by itself is not high, but when combined with other
toxic gases, it is considerable [70–72]. During combustion, PU polymers may emit
CO, CO 2 , NO, HCN, isocyanuric acid, isocyanates, hydrocarbons, amines, and
other potential hazardous components [72–75]. The evolution of these degradation
components and gases and their structures can be determined by TGA-Fourier
50
5 Flame-Retardant Polyurethanes
solvent-based PUs, water-dispersible PUs show less thermal stability due to the
presence of thermally labile urethane and urea linkages [21, 60]. Kuruma et al. [61]
reported that with increasing hard segment content in water-dispersible PUs, thermal degradation is accelerated in a particular temperature region due to the large
number of labile urethane and urea linkages. The thermal stability of PUs also
depends on various characteristic features of CEs, such as their polarity, +I effect,
and presence of electron-donating groups [21, 62–64]. Coutinho et al. [21] synthesized PU dispersions using two different CEs, ethylene diamine and hydrazine,
and noticed that hydrazine-containing PU dispersions exhibited high thermal stability than their counterparts containing ethylene diamine owing to the more polar
nature of hydrazine. Qin et al. [64] synthesized PUs using two different aromatic
diamines as chain extenders; the CEs contained sulfone and diphenyl ethers as
functional groups. It was observed that the PUs prepared using sulfones exhibited
higher thermal stability due to the more polar nature of sulfone groups.
Chattopadhyay et al. [65] systematically evaluated the relationship between the
thermal decomposition of different types of PUs and flammability. However, they
did not discuss toxic gas evolution from PUs and halogenated FRs. The FR activity
of polymers also depends on their T m and T g . Therefore, polymers with lower T g
values melt easily and catch fire easily but those with higher T g values do not catch
fire easily, which implies higher FR activity. Liu et al. [66] synthesized
phosphorus-containing aryl alkyl novolac (Ar-DOPO-N) blended with phenol
formaldehyde novolac and melamine-modified phenol formaldehyde novolac and
used them as curing agents for o-cresol formaldehyde novolac epoxy resins; the
resulting cured epoxy resins showed increased T g values in both cases but in the
case of melamine-modified phenol formaldehyde novolac, higher FR activity was
observed. Tirumal et al. [67] evaluated the relationship between T g , thermal stability, and density of PU foams. Thermal stability decreases with a decrease in PU
foam density but the T g increased. To increase the thermal stability of the polymers,
PNCs with varieties of nanoparticles (NPs) have been prepared. However, transition
metals are more advantageous in improving the thermal stability and FR activity of
PUs. In their exhaustive investigation, Moroi et al. [68] analyzed the thermal
decomposition of various transition metal ion-containing composite PU polymers.
Generally, polymer combustion is accompanied by the release of CO and CO 2
and if the polymer contains nitrogen, NO, NO 2 , NH 3 , and HCN may also be
released. Chlorinated plastic materials can produce HCl and the hazardous gas
phosgene (COCl 2 ), while fluorinated plastics release HF [70]. Combustion of most
polymer materials yields only a small quantity of HCN, but some polymers such as
PUs, acrylates, polyamides, nitrocellulose, and other nitrogen-containing plastics
produce high quantities of HCN when subjected to flame exposure. Under normal
conditions, the toxicity of HCN by itself is not high, but when combined with other
toxic gases, it is considerable [70–72]. During combustion, PU polymers may emit
CO, CO 2 , NO, HCN, isocyanuric acid, isocyanates, hydrocarbons, amines, and
other potential hazardous components [72–75]. The evolution of these degradation
components and gases and their structures can be determined by TGA-Fourier
50
5 Flame-Retardant Polyurethanes