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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.
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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.
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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.
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.
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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.
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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.
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.
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