A history of sole bond adhesion “

Sneha Bharti
6 min readSep 19, 2020

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HANNAH BOUGHEY investigates about the history of the sole that how the sole bonding process has developed over the years, enabling the endless design possibilities that exist today. Prior to adhesives being introduced for use in the footwear industry. The main method of attaching the sole to the upper assembly was by hand stitching and, eventually, by machine stitching. In the 1930s, the utilization of sole bonding adhesives began. These were required to adhered is similar materials, be strong, flexible, and, of course, suitable for the shoemaking process. In those early days, SATRA was heavily involved with the development of adhesives suitable for footwear applications. For relevant surface preparations and bonding procedures. Working alongside adhesive suppliers and footwear manufacturers. SATRA also developed test methods to establish the best adhesives to use, as well as the most reliable production processes to follow.

The bonding of a sole unit to the higher assembly of a shoe is currently a well- established method.

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Nonetheless, sole bond problems still occur and are literally.

Still a major cause of footwear failure. With constant changes in materials, constructions and production processes, it is important that the risks of any potential sole bond issues are considered. Sole bond quality should be regularly assessed and monitored. This can be done through pre-production testing with new materials, as well as product assessment to monitor production and due-diligence. Sole bond testing is not just about the numbers-it is extremely important to consider both materials and construction to ensure the correct bond is tested, as well as understanding what the results mean Poor sole bond can lead to quality issues, customer returns and an associated loss of reputation.

This is particularly important in the days of online product reviews and social media. A more serious consequence is that of a loose sole becoming a tripping hazard and, hence, a potential health and safety concern. The results of sole bond testing should therefore be utilised in order to achieve the best possible quality of adhesion.

Materials and constructions

In the early days of sole bond adhesives, common footwear materials included leather uppers and leather or vulcanised rubber sole materials — which are, of course, still very common today. Much of the early research and development was therefore based on these materials. The best possible adhesive combinations and guideline values for sole bond strength were established.

As more and more synthetic materials were introduced into footwear manufacture. Adhesives analysis continued to develop and so the bonding technique became more complex. Adhesives were required with higher strength and flexibility, heat and moisture-resistant properties, and improved ease of application.

Two types became the most commonly-used in the footwear industry — one was formulated from polychloroprene (often known as’neoprene’) and the other was from polyurethane (PU).

Solvent-based adhesives

These were solvent- based adhesives-a solution of individual polymer molecules in solvent with 12 to 20 per cent solids content. Being compatible with animal skin and most of the rubber family of sole materials. Polychloroprene adhesive was a powerful selection for the standard footwear materials. However, as new soling materials were developed. It was discovered that this adhesive was softened by the plasticiser in polyvinyl chloride (PVC) and so was incompatible with this type.

By contrast,PU adhesive could be used with a range of materials, including PU, PVC, leather and ethylene vinyl acetate(EVA). And proved to have a higher resistance towards greasy leathers.

The limitation of PU adhesive was its inability to bond to most rubber. In the late 1960s, SATRA pioneered the development of the halogenation process. A solvent- based solution to chemically treat the surface of rubber to make it compatible with PU adhesive. Halogenation primers, such as ‘SATREAT’, became widely used in the industry. PU adhesive then had the advantage of being virtually universal, provided that the correct surface preparation was employed.

Many other priming treatments were developed for improving the bonding capabilities of ‘difficult to bond’ materials, such as EVA primers, nylon primers and isocyanate primers.

Solid hot-melt adhesives

Solid hot-melt adhesives were also trialled, which were mainly used for less demanding tasks, such as laminating. They were initially less favoured for sole attaching than solvent-based adhesives, due to their higher viscosity and, therefore, poor wetting of substrates. Since the 1980s, hot-melts based on reactive PU have been used to overcome this issue. Reactive PU has a relatively low molecular weight and low viscosity upon application. It is fast setting, to give an adequate initial bond strength to hold the two materials together. Some hot-melts can be accurately sprayed, with a number of innovative processes involving this adhesive sprayed onto one adherent only during sole bonding.

The next major advancement came in the 1990s, with the introduction of water-based adhesives — polymers dispersed into a latex or emulsion with 40 per cent solids content. There were several reasons for this development, including legislation created to restrict solvent emissions in the workplace and to the external. Several studies also revealed a very large energy cost associated with the venting of solvent fumes from the sole bonding process. Water-based adhesives had many advantages — they were less hazardous and were much more pleasant to use in the footwear factory. The lower viscosity of the water dispersion allowed for spray application.

The white ‘milky’ appearance of the adhesive on application became clear when dry, making it easy to tell when the adhesive had fully dried. Despite these benefits, there was an initial reluctance to use water-based adhesives. Due to uncertainty over their effectiveness and perceived higher cost. However, savings were available as a result of the higher solids content reducing the number of adhesive coats required.

Direct moulding

A further innovation in sole attaching was direct moulding — in which the sole is both formed and attached to the upper in one single stage. This is a popular method used today, particularly for safety and sports footwear.

Despite initial investments in machinery and mould costs, direct moulding has several advantages over traditions. A major benefit is a simplified process (fewer steps involved in sole attaching), as well as the general advantage of moulded soles giving flexibility in design and colour. Not all materials that can be used for moulded sole units are suitable for direct moulding. The most suitable materials include thermoplastics, such as PVC and thermoplastic rubber (TR). PU can be direct moulded by reaction moulding and rubber can be direct moulded by direct vulcanisation.

PU reaction moulding can generally achieve sufficient bond strength without adhesive application. As long as the upper material has been suitably prepared. PU in its liquid pre-reacted state is highly effective in acting as its own adhesive, partly because it is highly penetrating into the microscopic pores and inter-fibre spaces in leathers and textiles. Following the liquid reaction, an elastic, rubbery material is formed. This technology makes PU ideal for use in multi-layer soles, with a distinct layer (midsole) between the upper and outsole.

The injection moulding of thermoplastics and direct vulcanising of rubber may still require the application of adhesive prior to moulding. However, for some upper materials an adequate bond can be achieved without applying adhesive. This is usually the case for a textile, if its structure is sufficiently open. To allow some penetration of the molten sole material.

Originally published at https://www.footwearsb.com on September 19, 2020.

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