**1. Introduction**

Polyurethanes (PUs) are characterized by excellent properties such as good resistance to abrasion and good oil and atmospheric resistance. Their main applications are very wide, as flexible foam in upholstered furniture and rigid foam in wall insulations, roofs, and appliances; thermoplastic PU resins in medical devices, automotive parts, and footwear industries; and last but not least their uses as coatings, adhesives, sealants, and elastomers (CASE) which are very important, for example, on floors and pipe protection and again in automotive parts.

It is not unusual that PU have to sustain very high temperatures in several uses, specially in applications such as defense [1]. For example, high-temperature resistant adhesives are required in advanced aircraft, space vehicles, missiles, and ground vehicles [2].

Thermal stability describes thermal durability as well as heat resistance. Polymers with higher thermal stability are characterized by higher melting points, softening and thermal decomposition, smaller mass loss during heating at high temperatures, and higher heat deflection temperature under load, without losing their basic properties which determine its functionality. In respect to analytical techniques, differential thermal analysis (DTA) and thermogravimetric analysis (TGA) were traditionally used to evaluate the thermal properties of several types of polyurethane and are still standard analytical techniques that are utilized. Thermal stability requirements can be summarized in the following statements: retention of mechanical properties (melting/softening point), high resistance to chemical attack, and high resistance to breakdown, specially under oxidative conditions.

The following figure introduces the general reactions involved in PU thermal decomposition:

The first reaction is fast. Flammable gases then react much faster with oxygen, producing more heat and small molecule gaseous degradation products (**Figure 1**). Finally, char reacts with oxygen but in a much slower rate, releasing heat but with a lesser rate. The first step of the degradation includes the scission of the urethane bonds to obtain the polyol and the isocyanate groups apart. In the second set reactions, dimerization gives off gaseous carbon dioxide and carbodiimide, and trimerization gives isocyanurates, while reactions with water render aromatic amines and carbon dioxide again. Heat is released in every reaction step, sustaining degradation until eventually a compact char is left.

PUs have unique properties derived from their two-phase microstructure composed of hard and soft segments. Soft segments (SS) are formed by polyols and have low glass transition temperatures, while hard segments (HS) are derived from diisocyanates and chain extenders and possess high glass transition temperature. PU can be considered as a block copolymer with alternating soft and hard segments along the macromolecule chain. The SS originates from the polyol and imparts extensibility to PU. The HS which is composed of urethane and aromatic rings aggregates into microdomains resulting from the hydrogen bonding, and the domains provide physical cross-linking points for materials [3].

Ingredients for manufacturing PU are polyisocyanate, polyester or polyether polyol, and a chain extender like a diol or diamine. The most reactive component is isocyanate due to its -NCO groups. The quality of PU obtained depends on the ratio of -NCO to -OH groups to obtain a good end product with the required properties. Insufficiency as well as an excess of -NCO groups will result in the formation of allophanate or biuret compounds, with different properties. On the other side, urea and isocyanurate linkages displayed higher thermal stability than polyurethanes [4].

Thermal stability of PU has been extensively studied for many decades. As introduced above, three general reactions can occur during the thermal degradation of polyurethane: (i) dissociation to the original polyol and isocyanate; (ii) formation of a primary mine, alkene, and carbon dioxide; and (iii) formation of a secondary amine and carbon dioxide [5]. The tendency for a particular mechanism depends on the chemical nature of the groups, adjacent to the urethane linkage, and the environmental conditions. Polyurethane degradation usually starts with dissociation of the urethane bonds and carbon dioxide and isocyanate evaporation [6]. The general consensus, however, is that decomposition occurs in three steps at the level

**75**

*Thermal Resistance Properties of Polyurethanes and Its Composites*

of the urethane group between 200 and 300°C [7]. The most important factors that determine thermal stability of PU are the nature of starting materials and the

When polyurethanes undergo thermal degradation, some potentially hazardous chemicals are released. These chemicals could not lead to visible warning. When PU is submitted to high temperatures, special health and safety precautions should be put in practice. It was early noticed that at temperatures above 600°C, cyanide is produced from PU decomposition and polyureas, giving off the so-called yellow smokes [8, 9] and emission of other toxic products [10]. The conditions of synthesis (polycondensation) and the nature of the reagents (initial prepolymers and monomers) influence the composition of the volatile compounds and residues arising from decomposition [7]. Health and safety, apart from material performance, is one of the reasons why it is important to establish heat stability ranges for materials

This review is intended to convey a brief compilation of research in the field of thermal resistance of non-foamed PUs and to identify strategies to augment stability to high temperatures of PUs and its composites. It is not focused in other aspects of PU which has been thoroughly covered by many other experts in reviews and books [6, 10–16]. We will concentrate on the effect of structural changes and on the effect of additives on PU thermal resistance. This contribution has in mind that the vast information about thermal properties of PU cannot be summarized in one single review but tries to present main factors that determine thermal resistance of

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

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

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

condition of polymer preparation [4].

with such a wide spectrum of utilization as PUs.

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

these important polymeric materials.

thermal stability of the PU [19].

**Figure 1.** *General mechanism of thermal decomposition of PUs.*

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

*Thermosoftening Plastics*

until eventually a compact char is left.

decomposition:

The following figure introduces the general reactions involved in PU thermal

PUs have unique properties derived from their two-phase microstructure composed of hard and soft segments. Soft segments (SS) are formed by polyols and have low glass transition temperatures, while hard segments (HS) are derived from diisocyanates and chain extenders and possess high glass transition temperature. PU can be considered as a block copolymer with alternating soft and hard segments along the macromolecule chain. The SS originates from the polyol and imparts extensibility to PU. The HS which is composed of urethane and aromatic rings aggregates into microdomains resulting from the hydrogen bonding, and the

Ingredients for manufacturing PU are polyisocyanate, polyester or polyether polyol, and a chain extender like a diol or diamine. The most reactive component is isocyanate due to its -NCO groups. The quality of PU obtained depends on the ratio of -NCO to -OH groups to obtain a good end product with the required properties. Insufficiency as well as an excess of -NCO groups will result in the formation of allophanate or biuret compounds, with different properties. On the other side, urea and isocyanurate linkages displayed higher thermal stability than polyurethanes [4]. Thermal stability of PU has been extensively studied for many decades. As introduced above, three general reactions can occur during the thermal degradation of polyurethane: (i) dissociation to the original polyol and isocyanate; (ii) formation of a primary mine, alkene, and carbon dioxide; and (iii) formation of a secondary amine and carbon dioxide [5]. The tendency for a particular mechanism depends on the chemical nature of the groups, adjacent to the urethane linkage, and the environmental conditions. Polyurethane degradation usually starts with dissociation of the urethane bonds and carbon dioxide and isocyanate evaporation [6]. The general consensus, however, is that decomposition occurs in three steps at the level

domains provide physical cross-linking points for materials [3].

The first reaction is fast. Flammable gases then react much faster with oxygen, producing more heat and small molecule gaseous degradation products (**Figure 1**). Finally, char reacts with oxygen but in a much slower rate, releasing heat but with a lesser rate. The first step of the degradation includes the scission of the urethane bonds to obtain the polyol and the isocyanate groups apart. In the second set reactions, dimerization gives off gaseous carbon dioxide and carbodiimide, and trimerization gives isocyanurates, while reactions with water render aromatic amines and carbon dioxide again. Heat is released in every reaction step, sustaining degradation

**74**

**Figure 1.**

*General mechanism of thermal decomposition of PUs.*

of the urethane group between 200 and 300°C [7]. The most important factors that determine thermal stability of PU are the nature of starting materials and the condition of polymer preparation [4].

When polyurethanes undergo thermal degradation, some potentially hazardous chemicals are released. These chemicals could not lead to visible warning. When PU is submitted to high temperatures, special health and safety precautions should be put in practice. It was early noticed that at temperatures above 600°C, cyanide is produced from PU decomposition and polyureas, giving off the so-called yellow smokes [8, 9] and emission of other toxic products [10]. The conditions of synthesis (polycondensation) and the nature of the reagents (initial prepolymers and monomers) influence the composition of the volatile compounds and residues arising from decomposition [7]. Health and safety, apart from material performance, is one of the reasons why it is important to establish heat stability ranges for materials with such a wide spectrum of utilization as PUs.

This review is intended to convey a brief compilation of research in the field of thermal resistance of non-foamed PUs and to identify strategies to augment stability to high temperatures of PUs and its composites. It is not focused in other aspects of PU which has been thoroughly covered by many other experts in reviews and books [6, 10–16]. We will concentrate on the effect of structural changes and on the effect of additives on PU thermal resistance. This contribution has in mind that the vast information about thermal properties of PU cannot be summarized in one single review but tries to present main factors that determine thermal resistance of these important polymeric materials.
