Elastomer applications are varied, and it is difficult to draw sweeping generalizations. It is especially difficult to provide meaningful geographical data regarding consumption since the applications are globally relevant to consumers, but manufacturing of
FIGURE 9.6 Volume fraction industrial usage of polyurethane elastomer in (a) North America in 2010 and (b) China in 2009.
specific products, such as footwear, is highly concentrated (in China). Another development has been the near disappearance of reactive injection molding (RIM) applications from the automotive segments, and its stagnant market development in nonautomotive applications.
Segmentation of applications by regional production exemplifies the changing global concentration of low-cost manufacturing of smaller devices and parts in low-cost economies, versus the high-cost manufacturing but dominant commercial markets in developed economies (Fig. 9.6). The dominance of transportation-related applications in North America reflects the compatibility of higher cost materials with more complex value-added assembly manufacturing. By contrast, the dominance of PU elastomers for footwear in China demonstrates the importance of low-cost manufacturing and the reduced impact of transportation of small items to final consumer cost. This macrotrend is further illustrated by the decline of PU footwear manufacturing in Western Europe as the opposite trend emerged in China (Fig. 9.7).
FIGURE 9.7 Consumption of microcellular polyurethane elastomer primarily used in footwear applications comparing data for Western Europe and China for the years 2000 and 2012 . (See insert for color representation of the figure.)
The footwear segment is a large and varied market and probably worthy of a more thoroughgoing treatment than can be afforded in this book. PU materials find uses throughout the footwear industry from work boots to ski boots, to running shoes, to high heels. Each segment has many optimized formulations based on cost and performance requirements. The footwear industry has moved quickly to provide ever greater performance in terms of low-temperature properties, abrasion resistance, and appearance. Certainly, one of the areas of greatest innovation has been the pursuit of excellent properties at lower densities resulting in lighter weight. Last, there has been significant progress in making TPUs with the required low modulus to result in adequate traction on all surfaces.
Relative to all polymer materials, PUs accounts for only about 6% of the total shoe-soling materials used. This still accounts for over 1 billion pounds of PU in 2011, but by comparison is dwarfed by materials such as poly vinylchloride (~5 billion pounds), natural vulcanized rubber (about 4 billion pounds), the tri-block copolymer of styrene-butadiene-styrene (SBS) also referred to as thermoplastic rubber, TPR (about 2 billion pounds), and ethylene vinylacetate ("EVA", about 1.5 billion pounds). For perspective, the same amount of PU is used as is for leather shoe soling. PU is favored by shoe designers for its excellent low-temperature flexibility, very good wear properties (low abrasion), and the ability to produce it at a wide variety of densities by introducing porosity.
The PU component is broken up into thermosetting PU produced by injection molding/microcellular methods, and those using TPU. The ratio for these is approximately 4:1 (Fig. 9.8).
Thermosetting PUs for footwear applications are usually made from preformulated urethane systems developed to make a specific product with a designed hardness,
FIGURE 9.8 Approximate volume fraction consumption of thermoset versus thermoplastic polyurethane elastomers for footwear.
density, cost, and wear performance. Many shoe sole manufacturers begin with a system called a quasiprepolymer [10, 11]. A quasiprepolymer is essentially a prepolymer (see Chapter 2 and Section 22.214.171.124) with excess isocyanate. To make a quasiprepolymer, isocyanate (usually monomeric methylene diphenyl isocyanate or MDI or an added uretonimine modified MDI to maintain fluidity; see Chapter 3) is reacted with a 1000-6000g/mol polyester or polyether polyol, or mixture of polyols with total NCO content range from 12 to 28% (see Chapter 2 for calculation of % NCO in a prepolymer). This makes one component of a two-component formulation. The other component will have a variable amount of polyol, chain extender, and additives such as blowing agent, stabilizers, dyes, and so on, to result in a final soling elastomer. Aliphatic polyester polyols certainly are the predominant choice for shoe sole systems. This is because of the higher toughness of polyester polyols in these systems, the attractive pricing of polyester polyols, and the ability to easily achieve the soft materials footwear applications demand for improved traction. The deficits of polyester systems relate to suboptimal low-temperature flexibility, the requirement to handle polyester polyols using heated lines to maintain adequate fluidity, and relatively poor polyester hydrolytic stability.
Some of the problems associated with shoe soles made using polyester-based PU formulations can be overcome using polyether formulations. In the past, polyether formulations were encumbered by an inability to fabricate soft elastomers that could meet durability requirements (see Table 9.1). Relatively long demolded times also hampered use of polyether systems from a manufacturing standpoint. Recent formulation and design advancements have solved some of these problems, and the attractive low-temperature performance, production handling, material feel, and price attributes of polyethers have led to significant growth. Although reliable data for the relative proportion of polyester versus polyether-based systems in shoe sole applications is
TABLE 9.1 Comparison of a polybutylene adipate ester and an ethylene oxide-capped polypropylene oxide polyether-derived shoe sole material
lacking, it is unquestionable that polyester systems represent more than 80% of the PU shoe sole volumes, and it is also quite likely that polyether systems will continue to gain ground as systems with better performance are created. The use of polyether systems will also increase as designers try to obtain the best of both polyester and polyether worlds by mixing the systems. There is evidence  that polyester/ polyether mixtures actually result in systems handicapped by the weaknesses of both systems rather than compensated by the strengths, but this will not stop attempts at innovation by artful blending of components.
PU footwear made from prepolymer or quasiprepolymer two-part formulations are pumped from their respective tanks and mixed by a high-speed (6,000-20,000 rpm) mixing mechanism. The mixed and reacting PU is then either injected or poured into a mold to form the desired shape. The mixing head may have numerous ports allowing for sequential injection of different formulations to get specific performance, or gas to introduce porosity and reduce density, or different dye packages to achieve specific cosmetics .
Until recently [ 14], footwear made from TPUs had been limited to injection molding of relatively hard footwear components such as ski-boots and base plates for soccer cleats (and the cleats as well), air bags for shoe cushioning, and decorative appliques for instance. This was because preparation of sufficiently soft TPU had been unobtainable in pellet form without the pellets melding upon sitting in a bag or a hopper. Minimum hardness conventionally obtained for TPUs had previously been about Shore A 70. Upon superficial inspection, a Shore A 70 TPU pellet will seem soft, but it is not soft or flexible enough for shoe sole application. To obtain adequate comfort and traction on a smooth surface, shoe soles typically require Shore A from 45 to 65. Breakthroughs in TPU formulation and production have in part eliminated these restrictions, and TPUs are becoming more freely employed for various footwear constructions. In some cases, this means use of conventional injection molding techniques. In other cases, to avoid degradation of the TPU due to overheating, manufacturers essentially pour the TPU melt into a standard casting mold rather than heating the plastic to a sufficiently high temperature that the viscosity is low enough to be injected.
Formulation and innovation of PU for footwear application must always be performed with respect to specified tests. Specified tests provide a common framework for comparison of different materials and also precisely prescribe the methods for measurement. In the footwear industry, certain tests and measurements are fundamental for understanding suitability. Such tests include density, hardness (Shore A for moderately soft to moderately hard, or Asker C for very soft), abrasion resistance, tear resistance, and compression force deflection. Other tests may be required by specific manufacturers or for specific applications, or more prevalent in one region than the other. Other tests such as "feel" are subjective and resistant to quantification. Table 9.2 lists down some of the commonly employed tests.
TABLE 9.2 Representative standard tests for characterizing, comparing, and ranking materials, including polyurethanes for footwear application
TABLE 9.3 Design guidelines for the effect of a change in material property on wet and dry coefficient of friction
For passing shoe manufacturer tests, formulation of shoe sole systems can be as much art as science . Shoe soles have become highly complex structures in the case of athletic shoes, and very high-performance materials in the case of work and safety shoes. It would be futile to offer a shoe sole formulation or table of performance attributes that could be understood as anything other than illustrative. However, there is little doubt that certain properties are exemplary of many shoe soles, and that certain formulations possess components common to the application. From a fundamental standpoint, one can begin by drawing common relationships between material properties and critical shoe sole requirements [16-19]. For instance, a shoe sole must possess a preferred coefficient of friction (COF) on dry and wet surfaces. While a high COF may be preferable, maximizing the COF for the material may entail an unacceptable tradeoff in other dimensions. Table 9.3 offers the effect of designing material properties on this particular variable. While not all aspects of this comparison are intuitively consistent (i.e., varying effects of resilience and hysteresis that are normally coupled), they are generally found to be true by experience. This suggests that the polymer structural characteristics, which are normally reflected in consistent polymer properties, are not being tested consistently and therefore provide relationships that are apparently at odds with each other .
An example of a PU shoe sole formulation based on polyester polyols and associated properties is given in Table 9.4. The effects of ester hydrolysis are pronounced on tests that stress the degree of network continuity that hydrolytic chain scission most effects. An example of a PU shoe sole formulation based on polyether polyols and associated properties is given in Table 9.5.
TPU for shoe soles only represents about 20% of the total PU shoe sole market, but the total PU market is only about half as large as the feedstock nonvulcanized SBS copolymer. This makes TPU only about one-seventh as large as the competitive TPR SBS (Fig. 9.9) [21-24]. While being a small fraction of the shoe sole market, it still represents about 380 million pounds of PU consumed—a fairly significant absolute volume of material. The general details of PU manufacture will be covered later in this chapter. TPU is commonly considered a high-performance material of shoe construction due to its low-temperature flexibility, good abrasion and scratch resistance, and its ability to fill even very complex shoe molds with very high fidelity resulting in desirable aesthetic values. These attributes have made TPUs
TABLE 9.4 Example shoes sole formulation based on a polyester quasiprepolymer and associated properties
desirable components in the construction of high-quality work, safety, and hiking shoes [25-27]. Limiting factors have been cost, and by relatively poor slip performance associated with difficulty making TPU at low enough hardness while still meeting customer requirements. To lower cost and reduce polymer hardness, plasticizers have been added.
Most TPU shoe soles are based on polybutylene adipate polyols with monomeric MDI and butanediol chain extender [28, 29]. These materials have very good properties, good clarity allowing for easy dyeability, and relatively low cost.
TABLE 9.5 Example polyether polyol shoe sole formulation based on a polyether soft segment
Quasrprepolymer/polyol ratio = 67/100.
FIGURE 9.9 Styrene-butadiene-styrene tnblock copolymer, the major component of TPR used in shoe soles.
TPR, which is compounded SBS copolymer (Fig. 9.9), is the dominant TPR within the shoe sole market. Compounding components vary, but sulfur and zinc oxide are common additives used to produce some crosslinking among the SBS chains through the butadiene moieties. Along with shoe soles, TPR finds common usage in tire construction. Several positive attributes of TPR include (i) price, (ii) durability, (iii) good aesthetic, and (iv) relatively easy processability . This last point is a very clear advantage in the eyes of manufacturers since much less can be asked or expected of machine operators filling molds, especially in operations in emerging economies . This is particularly evident when it is considered that along with beneficial properties, TPRs suffer inferior tensile properties, abrasion resistance, resilience, slip resistance at equivalent hardness in comparison with TPUs.
Trends in Footwear Applications
Although consumers rarely ask questions about their shoe sole materials, it would surprise many if informed about the level of innovation and technology that is invested into shoe soles. While patenting on shoe sole technology is a global pursuit, since 2000 the majority of patents have been filed from concerns located at the shoe manufacturing centers. Since the year 2000, of the 525 patents filed specifically on PU shoe soles, 145 were on behalf of companies entirely of Chinese origin. Among large global manufacturers, BASF filed 48 patents on shoe sole technology, Bayer 29, and Dow Chemical 12. The technology covered in these patents relates to improved hydrolysis, improved abrasion, improved cosmetics and design, improved manufacturing, electrically conductive shoe soles, shoe soles of complex density gradients, shoe soles with very low density, improved mold release agents, and odor mitigation. Certainly, the predominant technology trend is pursuit of high performance at reduced densities [32, 33]. The evidence of this trend is validated by casual inspection of shoes displayed at retail outlets.