**2. Effect of structure on PU thermal resistance**

The first structural factor that greatly influences thermal resistance is phase microstructure; this is the nature, proportion, and segregation of soft and hard segments (SS and HS, respectively). At the same time, microphase is determined by the chemical structure of PU (polyol, isocyanate, and chain extender type), so the effect of phase microstructure on thermal resistance often overlaps with chemical structure. Therefore, individual effects are rather complex to analyze. Thermal degradation is mainly initiated within the HS, which has normally the faster degradation stage. When it comes to SS, as this is composed by macrodiol, which is typically the weakest link in the oxidation of PU elastomers, using macrodiols that have high oxidative stability could give PUs with better thermo-oxidative stability. A lower flexibility in chains of SS domains produced a lower thermal resistance threshold (temperature where 5% sample weight is lost) as a result of lower crystallinity [17]. The structure of the HS has more influence on thermal stability rather than SS structure. Interurethane hydrogen bonding plays a significant role in the thermal stability of segmented PUs, which can be enhanced by a higher degree of phase separation between SS and HS [18]. The higher the concentration of the urethane group, the lower are both the activation energy for thermal decomposition and the thermal stability of the PU [19].

The polyester polyol-based PUs are more stable than the materials obtained with polyether macrodiols [4, 18]. For example, onset decomposition of polyetherpolyurethane in air is about 245°C. This is anticipated to be about 13°C as compared with that in nitrogen atmosphere. Such anticipation suggested that polyesterpolyurethane is more stable thermally than polyether-polyurethane. It also suggested that the different soft segments will influence the thermal stability of PU [20]. Polycarbonate diols, cured with MDI and chain extended with 1,4-butanediol, showed a drop-off in the weight of samples at around 290°C [21]. Krol and Pilch

Pitera [22] studied the effect of increasing polyol chain length on TDI-cured polyoxyethyleneglycols; the heat effect of endothermic processes within 260–420°C becomes lower and lower, while the effect at 360–440°C becomes more important. They correlated this effect with the increasing share of ether-type bonds or estertype bonds with simultaneous reduction in the number of urethane groups; polyether PUs from polypropylenglycol (PPG), HDI, and BDO had melting temperatures of 223°C [23]. These few examples are representative of the general trend that polyester polyol-based PUs are more thermally stable than polyether macrodiol-based PUs. Hydroxy-terminated polybutadiene (HTPB) are a particular telechelic polyol in which PUs are utilized as liners for composite propellants in the manufacture of rockets. When reacted with TDI and cross-linked with small molecular weight diols as chain extenders, the final stage of decomposition was at 375°C [24].

The effect of isocyanates on thermal stability was seen early. The higher the symmetry of the isocyanate, the higher the thermal stability [25]. Aliphatic isocyanates give urethanes a higher thermal stability [4]. The decomposition of polymers made with 4,4′-diphenylmethane diisocyanate (MDI) occurred above 400°C and was at least a two-step process, while the decomposition of polymers containing toluene diisocyanate (TDI) occurred below 400°C and appeared to be a one-step reaction [26]. For PUs cured with 4,4′-dibenzyl diisocyanate (DBDI), based on polytetramethylene ether glycol (PTMEG) and chain extended with butylenglycol, three main degradation processes were seen: at approximately 340°C the decomposition of urethane groups occurred, at 420°C the destruction of ether groups took place, and at 560°C the destruction of carbon chains and rings began. In general, the DBDI material had a higher thermal oxidation stability than the similar polymer achieved with MDI [14]. Polyurethanes made from polyester-based PUs cured with MDI had a better thermal stability than those based on TDI, according to their higher degree of hard segment crystallinity [26].

Natural oil-based polyurethanes generally had better initial thermal stability (below 10% weight loss) in air than the polypropylene oxide-based PU, while the latter was more stable in nitrogen at the initial stage of degradation. If a higher weight loss (50%) is taken as the criterion of thermal stability, then oil-based polyurethanes appear to be more thermally stable material [27–30]. PU prepared from formiated soybean oil polyols and TDI with different OH functionalities showed an initial weight loss process at 210°C, while maximum weight loss was at 400°C [31]. An increase in NCO index of elastomeric PU samples prepared from soybean oil-derived polyol increased hydrogen bonds and consequently thermal stability [32]. PU from TDI, polycaprolactone, butanediol, and monoglyceride of sunflower oil had the first and second maximum peaks both linked to the degradation of urethane bonds in the rigid segment of PU. The third and fourth maximum peaks were the results of degradation of the ester bonds in the soft segments, which take place from 380°C, while the composition of the aromatic compounds begins at 480°C [33]. This findings support the fact that research on oil-based PUs is increasing, considering their natural origin and good thermal resistance properties.

PUs synthesized with the use of oligomeric α,ω-dihydroxy(ethylene-butylene adipate) (dHEBA) polyol, aliphatic 1,6-hexamethylene diisocyanate (HDI), and 1,4-butanediol (BDO) were stable until 428°C. Ten percent of initial mass was lost at 344°C [32] which is a higher temperature than TDI or other conventional polyether polyols such as polytetramethylene ether glycol-derived PUs [33]. A TPU made from polycaprolactone polyol cured with polymeric diphenyl diisocyanate prepolymer displayed a 5% weight loss at about 260°C [34]. The thermo-oxidative degradation of phosphorus-containing polyurethane based in polypropylene glycol, TDI, and 1,4-butanediol incorporating phenylbis(hydroxyethyl) phosphonate was studied by using TGA. The onset of thermal degradation is lowered to 360°C due

**77**

side chain [43].

etry studies [47].

*Thermal Resistance Properties of Polyurethanes and Its Composites*

increasing the polyurethane thermal stability [36].

to the lower thermal stability of phosphorous chain extender [35]. Although being cured with TDI, thermal resistance is much higher than for other TDI-cured PUs. Phosphonate PU displayed high thermal resistance: during the thermal degradation, the phosphorous molecules form a protective layer on the polymer surface,

Regarding chemical structure, biuret and allophanate linkages are the thermally

Finally, thermoplastic polyurethanes (TPU) are a relatively novel group of the family of PUs and have high comparative thermal resistance [38] which allows them to being easily processed. TPU were introduced by DuPont in 1954 and developed through the 1950s and 1960 [39]. In general, polyurethanes have no pronounced melting point endothermic peak on differential thermograms, which is characteristic of noncrystalline polymers. On the other side, TPU have a distinct behavior compared to conventional PU, exhibiting thermal patterns like thermoplastics. For example, TPUs synthesized from novel fatty acid-based diisocyanates were reported to display considerable thermal stability without any significant loss of weight at temperatures below 235°C [40]. Significant thermal decomposition was

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

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 calorim-

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

weakest chemical entities in PU networks. Dissociation of both types generally takes place above about 110°C and is completed by about 170°C [37]. These linkages appear when unbalanced ratios of NCO and OH groups are present in reactants and are not normally desired regarding improvement of physical properties. An increase in cross-link density, type of cross-linking, and introduction of isocyanurate ring structures in the polymer chain backbone also has a strong beneficial effect on the

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

thermal stability of polyurethanes [4].

observed only after 300°C [41].

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

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

*Thermosoftening Plastics*

Pitera [22] studied the effect of increasing polyol chain length on TDI-cured polyoxyethyleneglycols; the heat effect of endothermic processes within 260–420°C becomes lower and lower, while the effect at 360–440°C becomes more important. They correlated this effect with the increasing share of ether-type bonds or estertype bonds with simultaneous reduction in the number of urethane groups; polyether PUs from polypropylenglycol (PPG), HDI, and BDO had melting temperatures of 223°C [23]. These few examples are representative of the general trend that polyester polyol-based PUs are more thermally stable than polyether macrodiol-based PUs. Hydroxy-terminated polybutadiene (HTPB) are a particular telechelic polyol in which PUs are utilized as liners for composite propellants in the manufacture of rockets. When reacted with TDI and cross-linked with small molecular weight diols

as chain extenders, the final stage of decomposition was at 375°C [24].

higher degree of hard segment crystallinity [26].

The effect of isocyanates on thermal stability was seen early. The higher the symmetry of the isocyanate, the higher the thermal stability [25]. Aliphatic isocyanates give urethanes a higher thermal stability [4]. The decomposition of polymers made with 4,4′-diphenylmethane diisocyanate (MDI) occurred above 400°C and was at least a two-step process, while the decomposition of polymers containing toluene diisocyanate (TDI) occurred below 400°C and appeared to be a one-step reaction [26]. For PUs cured with 4,4′-dibenzyl diisocyanate (DBDI), based on polytetramethylene ether glycol (PTMEG) and chain extended with butylenglycol, three main degradation processes were seen: at approximately 340°C the decomposition of urethane groups occurred, at 420°C the destruction of ether groups took place, and at 560°C the destruction of carbon chains and rings began. In general, the DBDI material had a higher thermal oxidation stability than the similar polymer achieved with MDI [14]. Polyurethanes made from polyester-based PUs cured with MDI had a better thermal stability than those based on TDI, according to their

Natural oil-based polyurethanes generally had better initial thermal stability (below 10% weight loss) in air than the polypropylene oxide-based PU, while the latter was more stable in nitrogen at the initial stage of degradation. If a higher weight loss (50%) is taken as the criterion of thermal stability, then oil-based polyurethanes appear to be more thermally stable material [27–30]. PU prepared from formiated soybean oil polyols and TDI with different OH functionalities showed an initial weight loss process at 210°C, while maximum weight loss was at 400°C [31]. An increase in NCO index of elastomeric PU samples prepared from soybean oil-derived polyol increased hydrogen bonds and consequently thermal stability [32]. PU from TDI, polycaprolactone, butanediol, and monoglyceride of sunflower oil had the first and second maximum peaks both linked to the degradation of urethane bonds in the rigid segment of PU. The third and fourth maximum peaks were the results of degradation of the ester bonds in the soft segments, which take place from 380°C, while the composition of the aromatic compounds begins at 480°C [33]. This findings support the fact that research on oil-based PUs is increasing, considering their natural origin and good thermal resistance properties.

PUs synthesized with the use of oligomeric α,ω-dihydroxy(ethylene-butylene adipate) (dHEBA) polyol, aliphatic 1,6-hexamethylene diisocyanate (HDI), and 1,4-butanediol (BDO) were stable until 428°C. Ten percent of initial mass was lost at 344°C [32] which is a higher temperature than TDI or other conventional polyether polyols such as polytetramethylene ether glycol-derived PUs [33]. A TPU made from polycaprolactone polyol cured with polymeric diphenyl diisocyanate prepolymer displayed a 5% weight loss at about 260°C [34]. The thermo-oxidative degradation of phosphorus-containing polyurethane based in polypropylene glycol, TDI, and 1,4-butanediol incorporating phenylbis(hydroxyethyl) phosphonate was studied by using TGA. The onset of thermal degradation is lowered to 360°C due

**76**

to the lower thermal stability of phosphorous chain extender [35]. Although being cured with TDI, thermal resistance is much higher than for other TDI-cured PUs. Phosphonate PU displayed high thermal resistance: during the thermal degradation, the phosphorous molecules form a protective layer on the polymer surface, increasing the polyurethane thermal stability [36].

Regarding chemical structure, biuret and allophanate linkages are the thermally weakest chemical entities in PU networks. Dissociation of both types generally takes place above about 110°C and is completed by about 170°C [37]. These linkages appear when unbalanced ratios of NCO and OH groups are present in reactants and are not normally desired regarding improvement of physical properties. An increase in cross-link density, type of cross-linking, and introduction of isocyanurate ring structures in the polymer chain backbone also has a strong beneficial effect on the thermal stability of polyurethanes [4].

Finally, thermoplastic polyurethanes (TPU) are a relatively novel group of the family of PUs and have high comparative thermal resistance [38] which allows them to being easily processed. TPU were introduced by DuPont in 1954 and developed through the 1950s and 1960 [39]. In general, polyurethanes have no pronounced melting point endothermic peak on differential thermograms, which is characteristic of noncrystalline polymers. On the other side, TPU have a distinct behavior compared to conventional PU, exhibiting thermal patterns like thermoplastics. For example, TPUs synthesized from novel fatty acid-based diisocyanates were reported to display considerable thermal stability without any significant loss of weight at temperatures below 235°C [40]. Significant thermal decomposition was observed only after 300°C [41].
