**3. Effect of additives on PU thermal resistance**

In studying the complex structure and morphology of polymers modified by mineral fillers, some problems may arise concerning the character and extent of interaction at the polymer-filler interface, the homogeneity of filler distribution, the filler orientation in the case of filler anisometric particles, and the polymerfiller adhesion [42]. Polyurethanes do not get aside from this general rules. Thermal stability of PU has been reported to be improved via hybrid formation such as the incorporation of fillers, e.g., nanosilica, Fe2O3, and TiO2, silica grafting, nanocomposite formation using organically modified layered silicates (nanoclays), incorporation of Si-O-Si cross-linked structures via sol-gel processes, and the incorporation of polyhedral oligomeric silsesquioxane (POSS) structures into the PU backbone or side chain [43].

Nanoclays confer high barrier performance and improved thermal stability in composites with plastics, which make these compounds suitable for many applications [44, 45]. In a PU made from HTPB, PTMEG, and TDI, TGA results revealed that the thermal stability of PU was improved by nanoclay sepiolite, and the onset decomposition temperature for PU nanocomposites with a sepiolite content of 3 wt% was about 20°C higher than that for pure PU. Initial degradation temperature for nanocomposites was around 300°C [46] and when Cloisite was utilized with PTMEG-TDI-BDO PU, an exotherm at 370–375°C in differential scanning calorimetry studies [47].

Small amounts of nanoclays as modifier to polyurethane matrix led to an increase in degradation temperature. The clay plates acted as barrier to oxygen transfer causing the degradation temperature to move to higher temperatures [48]. Stefanovic and coworkers [49] have shown that that polyurethane nanocomposite (PUNC) began to degrade at a temperature 20–40°C higher than pure PU copolymers. PUNC were

prepared from α,ω-dihydroxy-poly(propyleneoxide)-b-poly(dimethylsiloxane)-bpoly(propylene oxide) (PPO-PDMS-PPO) and organo-montmorillonite nanoclay (Cloisite 30B®) and cured with diphenylmethane diisocyanate.

Rubbery modulus for PU based on PTMEG as soft segment, isophorone diisocyanate as diisocyanate, and 1,4-butanediol as chain extender reinforced with nanosilica increased to higher temperatures, enhancing mechanical and thermal properties [50].

For thermoplastic PU composites filled with huntite and hydromagnesite mineral fillers, thermal decomposition occurred through double step with maximum rates at 347 and 411°C, and two shoulders are seen at 300 and 466°C, leaving 1.3 wt% carbonaceous char [51]. The TGA analysis of synthetic silico-metallic mineral particles (SSMMP) based on talc added to PU made from polycaprolactone and hexamethylene diisocyanate showed a significant increase in the onset temperature of the nanocomposites evidencing that the thermal resistance increased with the increase in the amount of filler added. The degradation temperature of the pure PU was the lowest, with a value of 301°C, and the degradation temperature for nanocomposites with 3 wt% of SSMMP was the highest, with values of 337–340°C [52]. Polyester-type PU filled with talc produced a 7°C increase in temperature for 5% weight loss [53].

Silsesquioxane cage structure-like hybrid molecules produce nanostructured organic-inorganic hybrid polymers called polyhedral oligomeric silsesquioxane. The POSS chains act like nanoscale reinforcing fibers, producing extraordinary gains in heat resistance. Octaaminophenyl POSS was used as a cross-linking agent together with 4,4′-methylenebis-(2-chloroaniline) to prepare PU networks containing POSS. TGA results showed the thermal stability was improved with incorporation of POSS into the system. The results can be ascribed to the significant nanoscale reinforcement effect of POSS cages on the polyurethane matrix [54].

Together with fillers, fibers generally impart heat resistance to PU, or at least do not produce a deterioration effect. Thermoplastic PU elastomer nanocomposites (TPUC) filled with 15% carbon nanofiber submitted to the torch of the oxyacetylene test resisted up to 210°C for 5 seconds, while non-filled TPU resisted only up to 175°C [55]. Composites of PU made from HTPB and TDI with coir and sisal fiber showed a principal degradation peak at around 400°C. PU from HTPB and TDI displayed the same general behavior [56]. However, there are reports that stated that fiber loading decreased thermal stability of composites with TPU: main temperature peak of complicated decomposition of a TPU was around 363°C. At the TPU/Kenaf 20% fiber loading, the first peak occurred between 246 and 369°C, with a threshold at 346°C [57]. TPUs have been reinforced with synthetic fibers such as glass [58], aramid [59], and carbon fiber [60].

Flame retardants delay decomposition temperature of PU. A study of the effect of ammonium polyphosphate (APP) on the thermal stability of some N-H and N-substituted polyurethanes showed that degradation mechanism could differ markedly [61]. Phosphorus flame retardants augment thermal resistance of PU. In pure PU, the specimen surface gradually degrades to volatile oligomers, monomer, and some molecules, whereas the presence of phosphorous flame retardant additive causes delay in degradation of polymer matrix. Phosphorus flame retardant additive compounds have low thermal stability, are decomposed earlier, and protect underlying PU matrix [20, 62]. Also, a range of stabilizers, including both organic and inorganic additives for better stability against different types of degradation, are available, with a focus on their efficacy and mechanisms of action [18].

Blending with other polymers is another strategy to augment thermal resistance of PU. Thermal decomposition of blends of a polyester urethane and polyether sulfone with or without poly(urethane sulfone), taken as a compatibilizing agent, was studied by TGA under dynamic conditions. Polyester-urethane has a temperature for

**79**

**4. Conclusion**

**Figure 2.**

*Thermal Resistance Properties of Polyurethanes and Its Composites*

derived polymeric materials such as lignin in HTPB macrodiol [18].

Finally, the following figure is designed to summarize the main factors that

As a result of this review compilation, it was concluded that the two main general factors that determine thermal resistance of PUs are its structure from one side and the presence of additives on the other side. The structural factors that influence thermal stability of PUs are the chemical nature and composition of hard (isocyanate plus chain extender) and soft (macrodiol) segments, its segregation, and PU thermoplasticity (derived from characteristic of TPU's stable linear structure). The additives that have a marked effect on augmenting thermal stability of PUs are mineral fillers (e.g., nano-oxides, nanoclays, talcs) and specific modifiers like POSS, flame retardants (both as additive and as polyol modifier), and fibers (natural or synthetic). Also, blending and grafting with other polymers are strategies that are utilized for increasing thermal resistance of PU, both for improving processing in manufacture and for high demanding applications. However, it is necessary to state that this review did not attempt to cover all particular factors that need to be taken into account when studying thermal stability of PU. Complex PU structures will

5% mass loss of 328°C and poly(ethersulfone) was 500°C, while among blends the one with 80/20 poly(ether sulfone)/polyester urethane had the higher value of 360°C [63]. Thermal resistance of styrene-butadiene-styrene rubber (SBS) was improved before and after thermal aging as the amount of added TPU was increased in rubber blends obtained via melt blending [64]. Thermal stability of a polyetherbased TPU was found to be improved as a result of the incorporation of 5% polypropylene-graft-maleic anhydride (PP-g-MA) and 40% wollastonite: temperature for 50% weight loss increased from 380 to 416°C for composite compared to TPU [65]. From thermal degradation of polypropylene/TPU and ammonium polyphosphate blends, carbodiimide was generated, which, because of its unstability, also reacted with water to give urea. These several cross-linking reactions stabilize the urethane bonds until 400°C [66]. PP/TPU blends with fire retardants formed an intumescent char residue protecting the matrix which prevented first peak of thermal degradation up to 200°C [67]. Thermoplastic elastomers can be prepared by creating blends of an elastic polymer with a dimensional stabilizing polymer [68] and enhance thermal/mechanical properties. Also stabilization of PU elastomers against thermal degradation by polymer modification could be achieved by introducing natural

*DOI: http://dx.doi.org/10.5772/intechopen.87039*

affect thermal resistance of PU (**Figure 2**):

*Main factors that determine polyurethane thermal resistance.*

### *Thermal Resistance Properties of Polyurethanes and Its Composites DOI: http://dx.doi.org/10.5772/intechopen.87039*

5% mass loss of 328°C and poly(ethersulfone) was 500°C, while among blends the one with 80/20 poly(ether sulfone)/polyester urethane had the higher value of 360°C [63]. Thermal resistance of styrene-butadiene-styrene rubber (SBS) was improved before and after thermal aging as the amount of added TPU was increased in rubber blends obtained via melt blending [64]. Thermal stability of a polyetherbased TPU was found to be improved as a result of the incorporation of 5% polypropylene-graft-maleic anhydride (PP-g-MA) and 40% wollastonite: temperature for 50% weight loss increased from 380 to 416°C for composite compared to TPU [65]. From thermal degradation of polypropylene/TPU and ammonium polyphosphate blends, carbodiimide was generated, which, because of its unstability, also reacted with water to give urea. These several cross-linking reactions stabilize the urethane bonds until 400°C [66]. PP/TPU blends with fire retardants formed an intumescent char residue protecting the matrix which prevented first peak of thermal degradation up to 200°C [67]. Thermoplastic elastomers can be prepared by creating blends of an elastic polymer with a dimensional stabilizing polymer [68] and enhance thermal/mechanical properties. Also stabilization of PU elastomers against thermal degradation by polymer modification could be achieved by introducing natural derived polymeric materials such as lignin in HTPB macrodiol [18].

Finally, the following figure is designed to summarize the main factors that affect thermal resistance of PU (**Figure 2**):

**Figure 2.**

*Thermosoftening Plastics*

properties [50].

prepared from α,ω-dihydroxy-poly(propyleneoxide)-b-poly(dimethylsiloxane)-bpoly(propylene oxide) (PPO-PDMS-PPO) and organo-montmorillonite nanoclay

Rubbery modulus for PU based on PTMEG as soft segment, isophorone diisocyanate as diisocyanate, and 1,4-butanediol as chain extender reinforced with nanosilica increased to higher temperatures, enhancing mechanical and thermal

For thermoplastic PU composites filled with huntite and hydromagnesite mineral fillers, thermal decomposition occurred through double step with maximum rates at 347 and 411°C, and two shoulders are seen at 300 and 466°C, leaving 1.3 wt% carbonaceous char [51]. The TGA analysis of synthetic silico-metallic mineral particles (SSMMP) based on talc added to PU made from polycaprolactone and hexamethylene diisocyanate showed a significant increase in the onset temperature of the nanocomposites evidencing that the thermal resistance increased with the increase in the amount of filler added. The degradation temperature of the pure PU was the lowest, with a value of 301°C, and the degradation temperature for nanocomposites with 3 wt% of SSMMP was the highest, with values of 337–340°C [52]. Polyester-type PU filled with talc produced a 7°C increase in temperature for 5% weight loss [53]. Silsesquioxane cage structure-like hybrid molecules produce nanostructured organic-inorganic hybrid polymers called polyhedral oligomeric silsesquioxane. The POSS chains act like nanoscale reinforcing fibers, producing extraordinary gains in heat resistance. Octaaminophenyl POSS was used as a cross-linking agent together with 4,4′-methylenebis-(2-chloroaniline) to prepare PU networks containing POSS. TGA results showed the thermal stability was improved with incorporation of POSS into the system. The results can be ascribed to the significant nanoscale

(Cloisite 30B®) and cured with diphenylmethane diisocyanate.

reinforcement effect of POSS cages on the polyurethane matrix [54].

glass [58], aramid [59], and carbon fiber [60].

Together with fillers, fibers generally impart heat resistance to PU, or at least do not produce a deterioration effect. Thermoplastic PU elastomer nanocomposites (TPUC) filled with 15% carbon nanofiber submitted to the torch of the oxyacetylene test resisted up to 210°C for 5 seconds, while non-filled TPU resisted only up to 175°C [55]. Composites of PU made from HTPB and TDI with coir and sisal fiber showed a principal degradation peak at around 400°C. PU from HTPB and TDI displayed the same general behavior [56]. However, there are reports that stated that fiber loading decreased thermal stability of composites with TPU: main temperature peak of complicated decomposition of a TPU was around 363°C. At the TPU/Kenaf 20% fiber loading, the first peak occurred between 246 and 369°C, with a threshold at 346°C [57]. TPUs have been reinforced with synthetic fibers such as

Flame retardants delay decomposition temperature of PU. A study of the effect of ammonium polyphosphate (APP) on the thermal stability of some N-H and N-substituted polyurethanes showed that degradation mechanism could differ markedly [61]. Phosphorus flame retardants augment thermal resistance of PU. In pure PU, the specimen surface gradually degrades to volatile oligomers, monomer, and some molecules, whereas the presence of phosphorous flame retardant additive causes delay in degradation of polymer matrix. Phosphorus flame retardant additive compounds have low thermal stability, are decomposed earlier, and protect underlying PU matrix [20, 62]. Also, a range of stabilizers, including both organic and inorganic additives for better stability against different types of degradation, are

Blending with other polymers is another strategy to augment thermal resistance of PU. Thermal decomposition of blends of a polyester urethane and polyether sulfone with or without poly(urethane sulfone), taken as a compatibilizing agent, was studied by TGA under dynamic conditions. Polyester-urethane has a temperature for

available, with a focus on their efficacy and mechanisms of action [18].

**78**

*Main factors that determine polyurethane thermal resistance.*
