Sole searching - Dinker Bajpai discusses sole-bonding systems

Leather was the first shoemaking material of note because it combined flexibility and moisture absorption with a degree of durability and water resistance. It could also be stitched and repaired, and coloured.

From the middle of the 19th century, outer soles of footwear have dramatically changed in materials. Leather has been the traditional soling material, but since 1980, only about 5% of shoes have leather soles. These days, most of the soles are made of rubber or plastics, which are classed as synthetics. So new types of soling need new adhesion methods or techniques with different types of upper materials; they are also different in bonding strength. Polyurethane is one that becomes a popular soling.

The unique versatility of polyurethanes as a soling material stems from the almost limitless chemical formulation combinations that give designers and manufacturers the freedom to create innovative designs that are in step with fashion and technological change. Polyurethanes can be made as light, tough, comfortable and hard-wearing as required, simply by varying the formulation. They can have an almost endless variety of shapes, surface textures and colours, and incorporate inserts or gels for extra comfort and support.

In recent times, footwear design has become something of a science, with specialist shoes being developed for a wide range of work and leisure applications. Using polyurethane, the manufacturer is well placed to address special needs such as energy management, anti-static properties, improved abrasion resistance, low-temperature flexibility and resistance to hydrolysis, microbial attack and premature ultraviolet yellowing. Thermoplastic (TPU), rather than reaction-moulded polyurethanes, offer footwear manufacturers a material even more flexible and abrasion resistant.

Available in different hardness, TPUs are especially suitable for applications such as safety boot outsoles, heels and top pieces for fashion shoes.

The properties that made leather an unparalleled material to be used as an upper material include availability, comfort and longevity among others. But it was also robust enough for soles.

Soling materials are available in a range of densities, weights, and chemical and physical properties. They may be cut from sheet, fabricated or available in complex mouldings containing several colours and types of material. Certain styles of footwear have traditionally used particular types of soling, and perhaps footwear manufacturers and bottom-material suppliers could benefit from taking a fresh look at these established uses - looking at newer areas for traditional materials and vice versa. The proportion of UK leather-uppered footwear with non-leather soles dropped about 90% by 1990. Nevertheless, leather is still sought after for its aesthetic appeal. Speciality leather soles also meet the needs of some hard-wear environments.

A fresh look at soling materials

The use of directly moulded-on rubber ranges from soles for high-quality safety boots to soles for less expensive and less durable canvas shoes for summer wear.

PVC soles are roughly divided into injection moulded-on and unit types, although both include straight PVC compounds and PVC blends - the most, common being PVC/nitrile rubber. The PU category is mainly microcellular polyurethane, although some thermoplastic PU is included. All the major suppliers of polyurethane soling compounds, like Bayer, Dow, Elastogran and ICI, have made strenuous efforts to replace CFCs as the blowing agent, without affecting the performance of the soling.

The main difference is claimed to be in the shrinkage characteristics. With CFC blown systems, there is about 1.5% shrinkage compared with 0.5% for the water-blown systems, which do not have a surface. For direct-moulded footwear, this should not present problems but fitting tolerances will be affected in unit sole production.

There is a big market out there for soling materials. Global footwear production is expected to push nearly $500 billion by 2020.

Through the ages, leather has maintained its hold on the shoe upper market, but in the last 50 years or so it has relinquished its grip in soling. With the exception of resin rubber, there appears to have been no attempt to simulate a leather soling material. Rather, new soling materials have stood on their own merits, being accepted for the aesthetic, performance and processing benefits they themselves offer.

Get a grip

The first real challenge to leather came with the availability of natural rubber in the 1930s. This offered good flexibility and greater durability than leather but was itself soon usurped by vulcanised rubber, introduced around 1940, which additionally offered considerable advantages in shoemaking. Its use provided, in fact, a genuine revolution in shoemaking. At a stroke, all the multiple sole attaching and finishing operations relating to leather soles, for example, were reduced to just one operation. Shoes made on the vulcanised process became the favoured method of production for work boots and shoes, uniform shoes and military footwear, safety footwear and school shoes.

The popularity and suitability of the vulcanising process to produce hard-wearing footwear for service, military and safety footwear continues to this day. Indeed, vulcanised rubber is still the most used soling material with an estimated 30% global market share. And most soles are attached to uppers by adhesive bonding, and it is inconceivable that this will not continue. The evolution of sole-attaching adhesives to date has fallen into three phases: introduction of solvent-based adhesives; adapting adhesives and pre-treatments to suit a wider range of materials; and development of solvent fine systems to reduce solvent usage. The next phase is likely to see developments in reactive liquid systems with the aim of rapid, simplified bonding. Adhesive costs may be relatively high, but offset by savings in processing.