temperature varies with PU structure [14]. For example, PUs used in industrial
thermal insulating applications start to degrade at temperatures above 250 °C.
A number of investigations have been carried out to understand the thermal
degradation and stability of PUs in terms of various parameters, such as different
polyols [15–21], diisocyanates [13, 17, 22–27], chain extenders [28–32], hard and
soft segments [13, 23, 27, 33–35], –NCO/–OH ratio [31, 36–40], and crosslinking
density [16, 18, 27]. Crosslinking density depends mainly on the molecular weight
of the polyol, –NCO/–OH ratio, and number of reactive functional groups in the
polyol and diisocyanate. A long carbon chain between hydroxyl groups indicates an
increasing molecular weight of the polyol in PUs, which in turn increases the
thermal stability of the synthesized PU [15]. It has also been reported that a large
number of reactive functional groups containing polyol or diisocyanates in the PU
polymer increases its thermal stability because of the high crosslinking density [41,
42]. A well-known and efficient method to enhance the thermal stability of PUs is to
introduce hetero atom-containing cyclic compounds and chemical cross linkers,
which can form 3D graphite-like structures, which prevent thermal degradation [43,
44]. A number of studies have been carried out on the structure of PU and it has
been reported that during PU synthesis, micro-phase separation occurs between its
hard and soft segments; this phenomenon also enhances the thermal stability of the
polymer [45–50]. The aggregation of hard segments in the PU polymer represents
the HS domains, which confer an ordered or semi-crystalline structure and stiffness
to the PUs. Kuruma et al. [27] synthesized PUs from hydroxyl-terminated
polybutadiene (HTPB) and HTPB-DNB (HTPB functionalized at the terminal
carbon atoms with dinitrobenzene) with different diisocyanates and demonstrated
an instantaneous increase in the tensile strength and elongation in HTPB-DNB-PUs.
Strong hydrogen bonding interactions between the –NO 2 groups of DNBs and the
soft segments of the PU backbone resulted in a highly ordered crystalline structure
and also a “fibrous-assembly” morphology, which is responsible for enhancing the
thermo-mechanical properties of the synthesized PUs. Rao et al. [13] studied the
nature and formation of HS domains in different modified HTPB-PUs using SAXS.
They observed that domains of different sizes are produced in different PUs due to
the differences in electrostatic interactions between the various triazine substrates
and the PU backbone. Diols and diamines are used as chain extenders during the
synthesis of PUs and polyurea, respectively. Compared to urethanes, the structure
of urea is more polar and hence capable of forming more hydrogen bonds, resulting
in an increase in the rigidity of the polymer, which promotes phase separation and
higher thermal stability in polyureas compared to PUs [34, 35, 51]. Butane diol is
widely used as a chain extender (CE) as it enhances phase separation; this is
because it contains an even number of carbons. Further, it has been noticed that
increasing branching in the CE structure reduces phase separation and the thermal
stability of PUs [30, 52–54]. During PU synthesis, if the ratio of –NCO/–OH is
greater than one; it means that in the presence of excess isocyanates, it is possible to
form side products such as allophanate and biuret. In addition, isocyanate compounds can react with each other to form dimer uretdione and trimer isocyanurates
[55, 56]; these isocyanurate linkages are capable of providing additional thermal
5.1 Thermal Degradation and Evolution of Components During PU …
49
thermal insulating applications start to degrade at temperatures above 250 °C.
A number of investigations have been carried out to understand the thermal
degradation and stability of PUs in terms of various parameters, such as different
polyols [15–21], diisocyanates [13, 17, 22–27], chain extenders [28–32], hard and
soft segments [13, 23, 27, 33–35], –NCO/–OH ratio [31, 36–40], and crosslinking
density [16, 18, 27]. Crosslinking density depends mainly on the molecular weight
of the polyol, –NCO/–OH ratio, and number of reactive functional groups in the
polyol and diisocyanate. A long carbon chain between hydroxyl groups indicates an
increasing molecular weight of the polyol in PUs, which in turn increases the
thermal stability of the synthesized PU [15]. It has also been reported that a large
number of reactive functional groups containing polyol or diisocyanates in the PU
polymer increases its thermal stability because of the high crosslinking density [41,
42]. A well-known and efficient method to enhance the thermal stability of PUs is to
introduce hetero atom-containing cyclic compounds and chemical cross linkers,
which can form 3D graphite-like structures, which prevent thermal degradation [43,
44]. A number of studies have been carried out on the structure of PU and it has
been reported that during PU synthesis, micro-phase separation occurs between its
hard and soft segments; this phenomenon also enhances the thermal stability of the
polymer [45–50]. The aggregation of hard segments in the PU polymer represents
the HS domains, which confer an ordered or semi-crystalline structure and stiffness
to the PUs. Kuruma et al. [27] synthesized PUs from hydroxyl-terminated
polybutadiene (HTPB) and HTPB-DNB (HTPB functionalized at the terminal
carbon atoms with dinitrobenzene) with different diisocyanates and demonstrated
an instantaneous increase in the tensile strength and elongation in HTPB-DNB-PUs.
Strong hydrogen bonding interactions between the –NO 2 groups of DNBs and the
soft segments of the PU backbone resulted in a highly ordered crystalline structure
and also a “fibrous-assembly” morphology, which is responsible for enhancing the
thermo-mechanical properties of the synthesized PUs. Rao et al. [13] studied the
nature and formation of HS domains in different modified HTPB-PUs using SAXS.
They observed that domains of different sizes are produced in different PUs due to
the differences in electrostatic interactions between the various triazine substrates
and the PU backbone. Diols and diamines are used as chain extenders during the
synthesis of PUs and polyurea, respectively. Compared to urethanes, the structure
of urea is more polar and hence capable of forming more hydrogen bonds, resulting
in an increase in the rigidity of the polymer, which promotes phase separation and
higher thermal stability in polyureas compared to PUs [34, 35, 51]. Butane diol is
widely used as a chain extender (CE) as it enhances phase separation; this is
because it contains an even number of carbons. Further, it has been noticed that
increasing branching in the CE structure reduces phase separation and the thermal
stability of PUs [30, 52–54]. During PU synthesis, if the ratio of –NCO/–OH is
greater than one; it means that in the presence of excess isocyanates, it is possible to
form side products such as allophanate and biuret. In addition, isocyanate compounds can react with each other to form dimer uretdione and trimer isocyanurates
[55, 56]; these isocyanurate linkages are capable of providing additional thermal
5.1 Thermal Degradation and Evolution of Components During PU …
49