Footwear Industry

From the early 19th century until the Second World War, footwear was exclusively bonded with nitrocellulose solutions. Polychloroprene rubber solutions were intro­duced in about 1949, and offered the possibility of bonding chemically dissimilar materials such as rubber and synthetics. Until then, only leather had been used in footwear production, and the introduction of a new adhesive resulted in an extremely dynamic development of new, adhesion-based technologies and special machinery that made production processes less difficult and more efficient.

Around 1970, the bonding technologies in industrial footwear manufacturing took a further development with the introduction of polyurethane adhesives, caused by the need for bonding new sole materials (mainly PVC).

Today, a wide variety of mostly solvent-based adhesives are employed in the manufacture of footwear. However, since 2004, the European Directive for Volatile Organic Compounds (VOC) has made solvent use more difficult and more expensive, and nowadays the REACH regulation continues to urge the industry to find organic solvent-free adhesives in shoe manufacturing. Large-scale footwear manufacturers are seeking waterborne and/or solvent-free alternatives to stay abreast of changes in the regulations stipulated by law, to consolidate their reputation as environmentally friendly companies, to save costs, and to optimize their processes.

The adhesives used in footwear manufacture are solvent-based systems derived from rubber, SBS or SIS block copolymers, dispersions of polyacrylates and polyvinyl acetates, lattices, PSAs and hot melts. Two-part contact adhesives or HF-weldable adhesives meet higher strength requirements, for example when sewn borders are replaced. EVA hot glues are used for inner uppers and stiffeners, while polyamide hot glues are employed for folds and corner joints. The parts to be joined, inner uppers, interlinings of the upper and other reinforcements, toe paddings and stiffeners can be primed with adhesives and joined by heat sealing.

In footwear manufacture, lasting — the fixing of the upper onto and around the rim of the insole — is the most demanding process with regards to structural require­ments. The upper is exposed to high stress during this process, but usually this does not involve any additional sewing or stitching. A typical lasting system is a combina­tion of pulling-over and fixation with automatic injection of hot glue, typically a rapidly solidifying polyester or a flexible polyamide; both are used in the form of sticks or granulated. For special designs only, polychloroprene contact adhesives or, for light shoes, natural rubber solutions or lattices are also employed.

The soles are bonded with solvent-based adhesives derived from polychloroprene or polyurethane. Different methods for the preparation of surfaces to be bonded are available. Substrates with pores on their surfaces are primed with a thin adhesive

film, while vulcanized rubber and thermoplastic rubber soles are halogenated (treatment with bleach or organic solvent solutions of trichloroisocyanuric) or sulfonated (treatment with sulfuric acid, also called cyclization) [62, 63]. Investiga­tions have been performed with physical surface preparations (corona discharge, low — and high-pressure plasma, UV radiation) of rubber to enhance adhesion [64]. Nylon and other synthetic polymers are primed with a primer coat [65, 66].

One-part contact adhesives are applied as a thin layer on the sole. The coated soles are then stored in a dust-free place until needed for the manufacturing process. When two-part isocyanate adhesives with hardeners are used, care must be taken not to exceed the open time. Bonding by heat activation is achieved with IR irradiation of coated soles for several seconds until the temperature of the surface is between 55 °C and 80 °C. As the temperature of the sole itself remains much lower, a strong bond is obtained within a relatively short period of time. Soft or flexible soles that tend to deform can be bonded with adhesives requiring low activation energy at room temperature.

Other manufacturing processes involved are the direct injection molding of ther­moplastic rubber or PVC, vulcanization of rubber, and low-pressure reaction molding of integral skin foam polyurethane. In this case, the fixed frame is coated with a two-part polyurethane adhesive, to which a latent hardener is frequently added, or with a self-vulcanizing synthetic rubber solution [67].

The application of hot glues derived from EVA is limited to the bonding of light footwear or textile uppers.

Adhesives are also used for shoe repair. One-part, polychloroprene-derived solvent — containing adhesives are universal adhesives that work with most different types of shoes, materials and material combinations. PVC and plastic materials are bonded with polyurethane adhesives.

Bonding strength in the footwear industry is defined by the European standard DIN EN 1392:2006 Adhesives for leather and footwear materials — Solvent-based and dispersion adhesives — Testing of bond strength under specified conditions, as well as the Specification and Testing of Adhesives and Materials for Shoe Production, as accredited by the European Association of Adhesives Manufacturers (FEICA).


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