TPUs: new material options for high performance footwear.

Polyurethane soling materials were introduced to the footwear industry in the late 1960s and gained wide acceptance in the 1970s (ref. 1). Because of their unique versatility in terms of hardness and density range, coupled with excellent physical properties and exceptional durability, these materials have gained acceptance in applications where performance is important. This versatility has allowed shoe manufacturers to develop a variety of designs and soling concepts for a number of applications. These include dual density versions, low weight midsole wedges for athletic footwear (refs. 2-4) and a variety of soles, inserts and insoles of different weight and flexibility for fashion footwear.

Almost without exception, the preferred processing method for polyurethane elastomers in the past has been via a two component reactive process, where the final article is formed in-situ either through direct injection or through an open mold casting operation. While this method of production is extremely versatile and results in materials of good quality and performance, it has some inherent drawbacks that do not allow the elastomer to reach its ultimate performance in terms of properties and part appearance.

Thermoplastic polyurethane (TPU) elastomers, on the other hand, because of their method of production and formulation approach, are capable of achieving properties that are significantly higher than those of liquid molded polyurethane elastomers. In the footwear industry, TPUs have been used for a long time in specialized segments where outstanding performance is needed. For applications such as sports shoes, ski boots and in-line skates, TPUs have been the materials of choice due to their excellent abrasion and cut resistance, high strength and outstanding flex life even at sub-freezing temperatures. In more conventional applications such as high performance soles, TPUs traditionally suffered from several drawbacks that limited their market penetration. These drawbacks included sole weight, material hardness and cost.

Aware of the above limitations. Huntsman Polyurethanes embarked on a research program about three years ago to develop a new generation of materials that would address the above limitations, increase further the performance of specialty footwear components and increase the total share of polyurethanes as soling materials. According to recent data, all polyurethane materials combined represent about 8% of the usage as measured by pairs of shoes produced. The total consumption of soling materials in 1999 was estimated at approximately 5.1 million tons (ref. 1). These data are shown in figure 1.

New developments

As previously discussed, there were three major restrictions to widespread acceptance of TPUs in footwear applications: hardness, weight and cost effectiveness. The remainder of this article details how these concerns have been overcome.

Soft TPUs

Prior to the advent of this program, the softest TPU commercially available for footwear applications had a durometer hardness of approximately 70A. At this hardness, a sole produced with a TPU is too hard and stiff for most applications. resulting in low comfort and decreasing slip resistance. A new family of materials has been developed with a hardness as low as 55A. Most soling applications require hardness in the range of 55 to 65A. The properties of three different TPU grades are shown in table 1.

These new materials have outstanding abrasion resistance. As a reference, a conventional polyurethane elastomer has values in terms of abrasion loss around 100 [mm.sup.3]. High quality rubbers display values between 120 and 180 [mm.sup.3]. All the new TPU grades consistently have abrasion loss values under 80 [mm.sup.3].

Another outstanding and important difference between TPUs and other materials is their flex life. Using a conventional flex life test, TPUs outperform the already acceptable polyurethane materials by a factor of two or better. It is important to notice that as the hardness of these materials decreases, their elastomeric behavior improves. As seen in figure 2, resilience increases steadily with decreasing hardness. Thus, softer TPUs have less tendency to dissipate energy and cause long term failure. Less energy dissipation results in less fatigue failure as measured by flex life.

[FIGURE 2 OMITTED]

The Avalon duo concept

To meet the specific market request for lightness and comfort, new soling concepts were developed. Figure 3 compares typical densities of soling materials under conditions of use. TPUs are similar to common materials such as PVC in terms of weight, and therefore, under normal conditions, part weight can only be managed via part design.

[FIGURE 3 OMITTED]

A successful approach to reduce part weight was the development of the Avalon duo concept. This concept takes advantage of the existing processing versatility, low weight and excellent comfort of conventional polyurethanes combined with the outstanding performance of TPU to produce a lightweight composite sole with excellent durability, comfort and appearance. Additional benefits delivered by the combination include excellent sole definition, improvements to slip, abrasion and shock absorption, and superior flex life performance.

Although the concept of a composite sole can be produced in conventional equipment, advances in processing technology have made it possible to increase efficiency and reduce overall cost. An example of this approach is the DESflow process by Desma (ref. 5) or the Incom Process (ref. 6) developed by Main Group. In this process, the thermoplastic polyurethane is dispensed into an open mold cavity. After this step, the mold is closed and the part is cooled. Two key advantages of this approach are the relatively low clamping pressure requirements and the ability to produce very thin parts, down to 1 mm. Conventional injection molding makes it very hard and costly to achieve this thickness in production because of the need to use high injection pressures and clamping forces, as well as more expensive molds.

Using these new molding techniques in combination with the novel range of Avalon TPU materials enables this process to succeed. The result is a sole product with excellent aesthetics and definition of detail. The ability to color this material uniformly or to produce translucent parts with special soft TPU grades enables the sole designer to produce effects difficult to achieve with conventional polyurethane technologies.

This novel approach also addresses the cost concerns associated with TPU in the past. The recyclable nature of the TPU material allows for a more efficient material usage; the amount of material used per sole is less due to the low thickness that can be achieved. Finally, the improved appearance and performance allow for a higher quality product to be obtained, thus adding value to the final product.

Special effects

Additional compounding developments have allowed extending the range of use of these materials to demanding applications such as safety shoes. One of the key challenges in this area is to develop suitable antistatic characteristics for the sole. An effective product has been developed that allows a shoe produced using the new TPUs to fulfill the stringent requirements of electrical resistance values between 50 and 100 MOhms achieved consistently. A summary of properties comparison with other common materials used in the safety shoe industry is given in table 2.

Light weight novel options

New developments in TPU formulation technology have also allowed the launch of a new proprietary technology that delivers high performance solutions to the designer of high quality soles. The new technology, known as A-Lite, is the result of intensive research and development in collaboration with leading footwear manufacturers and designers. The involvement at the design stage enables significant weight savings to be built into the final product. Under recommended conditions, soles with overall molded densities as low as 0.7g/[cm.sup.3] can be achieved. Unlike other light weight options, however, the new technology delivers a product of outstanding surface finish and superior physical performance.

A-Lite is able to offer the shoe designer more freedom to create exciting, imaginative designs. The lightweight nature of the product allows thicker styling without sacrificing performance, whereas the outstanding mold reproduction allows fine detail and texture to be built into the design. The end result is a high performance, light weight sole with exceptional surface finish and definition. The combination of these effects has led to an early adoption by several major branded shoe companies, where the overall quality of the sole enhances the overall value of the end product.

Typical properties for this new material are listed in table 3. As observed, all properties are in line with the superior performance expected from TPU-based products. But it is the combination of design freedom, lightness and performance that has provided an innovative solution to allow shoe producers to extend their product lines into the high quality, design led end of the market.

Conclusions

New developments in thermoplastic polyurethane elastomer formulation technology and production techniques have resulted in a new family of materials that allows further penetration into the soling materials market, particularly in those applications where performance, comfort and durability are important design factors.

A new concept in light weight materials has been developed without the restrictions often associated with other soling alternatives in terms of design freedom and properties. A-Lite will allow designers to achieve levels of performance, comfort and part appearance which have not been possible until now.

To summarize, the use of Avalon TPUs as footwear materials delivers a large number of benefits to the designer: Outstanding flex performance; excellent wear resistance; good adhesion to other materials; perfect surface definition; special surface effects (e.g., shiny, matt); multiple color and translucent options; and ability to create exciting designs

From the processing and manufacturing side. Avalon TPUs allow: Consistent production and low reject rates: no special treatment to create adhesion; no need to use release agents; and minimum material waste and reuse of the trimmings and reject pieces.

Table 1--comparison of properties for soft TPU
materials

Property                              55AE        60AE    65AE

Density, kg/[M.sup.3]                1,160       1,180   1,180
Hardness, durometer A                   55          60      65
Tensile strength, MPa                   12          15      18
Elongation, %                         >700        >700    >700
Tear strength, kN/M                     50          55      60
Abrasion loss, [mm.sup.3]               75          60      50
Ross flex (-10[degrees]C), kcycles    >250        >250    >250

Table 2--comparison of properties for safety
shoes

Property                             Rubber  Two comp.    65AE
                                                    PU

Electrical resistance, MOhm             70          70      90
Slip resistance (oil), degrees           8           7       9
Abrasion loss, [mm.sup.3]              170         120      50
Flex life (4mm), kcycles               >30         >30    >200

Table 3--properties of A-lite versus
conventional PU
                                                    Two
Property                              A-lite       comp.
                                                      PU

Density, kg/[M.sup.3]                        0.7     0.7
Hardness, durometer A                         60      62
Tensile strength, MPa                         10       7
Elongation, %                                750     450
Tear strength, kN/M                           40      15
Abrasion loss, [mm.sup.3]                    100     125
Flex life (-20[degrees]C), kcycles          >100      50

Figure 1--material share in footwear soling
applications

Other      1%
Rubber    40%
TR        14%
PVC       20%
PU/TPU     8%
EVA        9%
Leather    8%

Note: Table made from pie chart.

References

(1.) D. Price, Ed., 1999. "World soling markets," SATRA Technology Centre.

(2.) Mareschal, B. and H. Stilling, 1988. "Polyurethanes in sport shoes: Performance and efficiency," proceedings of the Utech '88 Exhibition and Conference, The Hague, The Netherlands, p. 157.

(3.) Stilling, H. and N.W. Lin, 1990. "Novel polyurethane systems for footwear as replacement for EVA and rubber," Cellular Plastics, 9:446

(4.) Indesteege, J., R.E. Camargo, P.W. Mackey G. Bagaglio and A. Dent, "Considerations on the selection of polyurethane systems for midsole wedges in athletic footwear application," J. Cellular Plastics, 34: 329.

(5.) Klockner Desma Schumaschinen GmbH, 1999, "Desma D-series: Trendsetter for direct-soling with polyurethane rubber and TPU," ref. 9.99..1/50 dPA.

(6.) Main Group SpA, Oct. 1999, "Le dual utilizzano la tecnologia dell' Incom process," Main News, 9:3.

This article is based on a paper given at the Alliance for the Polyurethane Industry meeting, October 2000.

Rafael E. Camargo and Nick Limerkens, Huntsman Polyurethanes, and Craig M. Roberts, Huntsman ICI (Brazil)