Thermoplastic rubber as a shoe soling.

Solings, like the other major components of the shoe, were traditionally made of leather but has now largely been replaced by polymer-based materials. These generally offer higher durability at lower cost. The soling can have a patterned or cleated surface; and can usually be produced in a range of densities, colors and surface finishes to meet market and fashion requirements.

Solings cut from natural crepe rubber were introduced in the 1920s, followed by soles molded from vulcanized natural rubber compounds. The years following World War II saw developments in solings based on synthetic rubbers such as styrene-butadiene rubber (SBR). These included molded and prefabricated sole units, also `resin robber' reinforced with high styrene resins which provided hard thin sheet solings leather-like in appearance and feel.

Thermoplastic solings - polyvinyl chloride (PVC) from the late 1950s and thermoplastic styrene-butadiene-styrene (SBS) rubber, from the late 1960s, allowed sole unit production by simpler molding processes than with rubber (figure 1). The late 1960s also saw the introduction of polyurethanes (PU) for shoe soles, most familiar in lightweight microcellular form. Other polymers used for solings include ethylene vinyl acetate (EVA), nylon and polyester.

[Figure 1 ILLUSTRATION OMITTED]

Current materials

Despite the introduction of the newer materials, most shoe soles continue to be produced from rubber, vulcanized or thermoplastic, or PVC. Figure 2 shows the estimated breakdown of solings worldwide; this has shown little change in recent years, but there are differences from region to region for economic and climatic reasons. For example, less PVC is used in cold northern countries due to increased risk of flex cracking. Overall, thermoplastics account for almost half of solings.

[Figure 2 ILLUSTRATION OMITTED]

In everyday footwear, the usage of soling materials is more or less in line with figure 2, but most of the resin rubber will be used in women's court shoes. Industrial and protective footwear usually has solings of vulcanized rubber (SBR or nitrile); PVC/nitrile rubber blends or polyurethane, de-pending on the intended wear environment. Composite soles with a rubber or PU wearing surface backed by low density PU or EVA have become popular, offering cushioning, lighter weight and greater durability. In sports shoes, rubber and polyurethane are most common, with thermoplastic rubber and EVA used on pseudo-sports footwear or trainers: Again, composite or dual density structures are now common.

Figure 3 shows typical ranges for hardness, density and durability of footwear solings. The durability values are on the SATRA scale of `specific durability' (sd) established from extensive testing and wear trials; a soling of sd 2 would be expected to wear half as rapidly as one of sd 1.

[Figure 3 ILLUSTRATION OMITTED]

Thermoplastic rubber for solings

Thermoplastic rubbers based on styrene-butadiene-styrene block copolymers were introduced for footwear in the late 1960s, offering rubbery appearance and properties with the simplicity of thermoplastics processing. Initially used to simulate natural crepe rubber, thermoplastic rubber has proved attractive for many styles of everyday and fashion shoes, especially with thicker or platform soles.

Compounding

Elastomers in a versatile hardness range can be produced by compounding. Extending the elastomeric matrix with processing oils improves flow during molding, softens the material and reduces its cost, generally at the expense of wear resistance. Extending the polystyrene domains with compatible polymers, such as polystyrene, hardens the base polymer and improves wear to some extent, but large amounts may inhibit adhesion. Including a third discreet mineral phase serves to harden and cheapen, but neither carbon black nor mineral fillers effective in vulcanized rubbers cause any marked reinforcement. Finally, an additional continuous polymeric phase is sometimes created, usually with ethylene vinyl acetate (EVA), and serves to produce a smooth surface amenable to lacquering.

Processing

Thermoplastic rubber is easily injection molded at 170-200 [degrees] C using simple screw or reciprocating screw machines, with a mold temperature of 30-50 [degrees] C and an in mold time of 1-2 minutes. The mold gate should be relatively large to minimize flow lines and orientation effects. Thermoplastic rubber is more hygroscopic than PVC, and damp compound may cause surface defects in molded soles, but it has excellent thermal stability in molding and is more tolerant of reprocessing in that up to 20% of granulated scrap may be used without problems.

Requirements of shoe solings

In order to provide adequate service during wear, it is necessary for a soling to have the following basic properties:

* Good adhesion to the upper part of the shoe;

* adequate wear resistance;

* resistance to flex cracking; and

* high coefficient of friction.

Adhesion

Adhesion problems were initially a serious handicap to the use of thermoplastic rubber in solings as established footwear bonding systems gave poor results. The breakthrough came with the development of the halogenation process (ref. 1) which chlorinates the butadiene in thermoplastic rubber, enabling good bonding with polyurethane adhesives as shown in table 1.

Table 1 - sole adhesion after surface chlorination

 Soling    Peel force   Type of f
compound     (N/mm)       ailure

   1          11.6         100R
   2          12.1         100R
   3          10.2       50AR 50SR

Bonds without surface chlorination < 1 N/mm.

R - rubber tear; SR - surface rubber failure;

AR - adhesion to rubber failure

The process originally used aqueous chlorine, but is now mostly done using solvent-borne halogenation primers, although a reversion to aqueous systems is a possibility to meet current restrictions on solvent emissions.

The adhesion mechanism is thought to rely on an increase in the polarity of the surface and the formation of hydrogen bonds between the polyurethane and the chlorine introduced into the butadiene molecule.

Care in carrying out the bonding process is needed as thermoplastic rubber is sensitive to solvents in both primers and adhesives, and gentle application and adequate drying times are essential to avoid surface weakening. On the other hand, an extended drying or open time may cause problems of poor tack due to migration of oil from the rubber to the adhesive surface.

Durability

SATRA measures wear resistance on the specific durability scale established from wear trials of materials against a standard soling originally assigned the arbitrary value 1. In trials to assess a material, weight loss is monitored, usually on heel top pieces, with one shoe of a pair carrying the test material and the other a control whose durability is known. The materials are reversed in a second wear period to eliminate bias due to wearers producing inherently more wear on one foot. Volume losses are calculated from weight losses and used to determine the specific durability. Thermoplastic rubber typically gives values in the range 0.8-1.5, which is acceptable for everyday footwear. Generally, it does not wear quite as well as PVC, but in contrast to PVC, durability tends to increase with hardness. A thermoplastic rubber may therefore give better wear than PVC in cold conditions, despite a reverse ranking at higher temperatures.

Laboratory abrasion tests are rather poor predictors of soling wear, although they may rank materials of the same general type. SATRA finds the drum abrasion test, DIN 53516, to be the most reliable, although with thermoplastic rubber the abrasion losses are relatively high in relation to wear performance. Durability prediction formulae for thermoplastic rubber have been derived, which combine abrasion values with hardness or tensile properties.

Resistance to flex cracking

Resistance of thermoplastic rubber to flex cracking is generally good, and, in contrast to PVC, improves as temperature is reduced. SATRA therefore carries out routine Ross flex tests ASTM D-1052 on thermoplastic rubber at 23 [degrees] C. rather than -5 [degrees] C as used for solings generally. SATRA has participated in a recent European project to evaluate current thermoplastic rubber materials, and with some compounds, a marked incidence of cracking occurred in shoes worn in Spain and Greece. The effects were reproduced by Ross flex tests at 40 [degrees] C (figure 4).

[Figure 4 ILLUSTRATION OMITTED]

These results tend to suggest that some thermoplastic rubber compounds are sensitive to changes in temperature. In practice, it is therefore important to select temperature insensitive compounds if they are to perform satisfactorily in service under different climatic conditions.

The Ross flex test is used to compare the flex performance of molded slabs or sheet thermoplastic rubber. However, the surface pattern on a sole unit can have a significant effect on the flex performance, and to evaluate the tendency of sole units to crack the Bata belt test, SATRA PM 133 is favored.

Slip resistance

SATRA originally measured slip resistance of solings by a walking ramp test, but now favors a laboratory test in which a shoe or sole is brought into contact with a flooring material under a vertical load representative of body weight, and the horizontal force to move the flooring is determined. The ratio of the forces gives the coefficient of friction. The normal form of the test presents the shoe heel at a contact angle of five degrees onto a dry or wet clay quarry tile.

Slip resistance of thermoplastic rubber (figure 5) is very good, but tends to decrease with increasing hardness. In this property, thermoplastic rubber is better than the other thermoplastics, and only bettered by the softer vulcanized rubbers. As with all materials, a well designed pattern with leading edges in many directions enhances the slip resistance. However, the good grip with thermoplastic rubber may aid kicking under of soft soles at the toe, causing sole bond failure, and makes it unsuitable for some sports footwear due to abrasion damage by frictional heat.

[Figure 5 ILLUSTRATION OMITTED]

Other performance factors

Thermoplastic rubber has a poor resistance to oils, fats, organic solvents and hot surfaces, which makes it unsuitable for most types of industrial footwear.

Stiffness is high relative to hardness, enhancing stability and ground insulation during walking, but to provide adequate stability in softer cored sole units, the rib width must be increased and spacing reduced, lessening material savings.

Cellular materials are feasible, but the density is quite high compared with polyurethane or EVA.

Hard thermoplastic rubber has seen some use for heel top lifts, but not for small sizes due to spreading in wear.

Summary and conclusions

Styrenic thermoplastic rubber has found a niche as a soling of rubbery appearance with adequate wear properties. It is favored in cold or seasonably cold climates as found in much of Northern Europe and North America, especially for its good flex crack and slip resistance. It is more expensive than PVC, but the price differential is more marked in some countries, including the U.K., than others. Usage is constrained by the availability of cheaper, well established alternatives such as PVC and sheet rubber for fashion and everyday footwear, by inferior properties to vulcanized rubbers and polyure-thanes for heavy duty applications, and by sporadic adhesion difficulties.

Acknowledgements

"Longevity of NR structural beatings" is based on a paper given at the October, 1997 International Rubber Conference.

"PNR for paper roller application" is based on a paper given at the October, 1998 Rubber Division meeting.

"Thermoplastic rubber as a shoe soling" is based on a paper given at the May, 1997 Rubber Division meeting.

Reference

(1.) "Behavior of urethane adhesives on rubber surfaces," D. Pettit and A.R. Carter, J. Adhesion, 1973, Volume 5, pages 333-349.

Alan R. Carter and Richard H. Turner, SATRA Footwear