Footwear Adhesives

One of the most important methods of assembling shoe components is adhesive bonding. In 1949 polychloroprene adhesives replaced the nitrocellulose solutions that had been introduced in 1906. The almost complete versatility of the polychloroprenes allowed leather, textiles, and other natural substances (until then the only shoe materi­als) to be supplemented by other materials, such as various rubbers and synthetics, or to be replaced altogether, as in the case of toe puffs, counter stiffeners, or heels. The way was thus opened for an astounding development in fashion and shoemaking tech­niques. Adhesive-based processes, supported by new machines and equipment, led to new bottom-attachment systems, simplified the production of footwear, and drastically reduced the number of production operations. Since 1970 a position of comparable importance has been reached by polyurethane adhesives.

Other adhesives for shoe manufacturing are solvent adhesives based on natural rubber or SBS or SIS block copolymers, dispersions of polyacrylates and poly(vinyl acetate), and a range of latex and hot-melt adhesives.

Closing Room Adhesives. These adhesives are light-colored and nonstaining solu­tions (or latices) of natural or synthetic rubber; other polymer dispersions are also possible. Wet-stick and pressure-sensitive bondings predominate. Also PSA hot-melt adhesives are of increasing importance for these applications. If higher strengths are demanded of the bonds, for instance, for replacing stitched structural seams, the requirements are met by two-way contact adhesives or by adhesives weldable in the high-frequency process. Ethylene-vinyl acetate hot-melt adhesives are used for apply­ing backings and for sticking-in toe puffs and counter stiffeners, whereas polyamide hot-melt adhesives are used for folding, edge binding, including french binding, etc.

The components to be bonded (linings, interlinings and other reinforcements, and also toe puffs and counter stiffeners) can be precoated with adhesive and then ironed

The Lasting Operation. Apart from the bottom attachment, the lasting operation produces the most vital structural bond in the shoe. It involves the fastening of the upper material over the margin of the insole and is subjected to considerable stress;

there is normally no supplementary stitching, tack-driving or stapling. A common lasting system is combined pulling-over and cement-lasting with automatic injection of hot-melt adhesive; this is mainly a rapid-setting crystalline polyester or the more flexible polyamide, both supplied in rod or granule form. Lasting by contact bonding with polychloroprene adhesives (or in light shoes also with natural rubber solutions and latices) is limited to particular constructions.

Подпись:Bonding Soles to the Lasted Uppers. Polychloroprene and polyurethane solvent adhesives, occasionally also dispersions of the same polymers, are used for this purpose. Various pretreatments are used to improve the bond: application of a thin first coat of adhesive, especially for absorbent surfaces; priming, in particular the halogenation of vulcanized rubber and thermoplastic rubber soling; the use of base-coat primers for nylon and other synthetics, as well as UV-curing primers for EVA and other substrates in athletic footwear.

Depending on the required productivity, quality, and performance a wide variety of processes are used in sole bonding. As an example for a contact bond, the single­component adhesive is applied to the sole, usually in advance. The soles can be stored (dust-free) until required in the production process. On the lasted margin of the shoe upper, the adhesive is usually a two-component mix containing isocyanate hardener; bonding must be carried out within the specified open-assembly time.

For bonding with heat activation, the sole is activated by IR irradiation or rapid flash activation: the activation time may be 2-6 s, and a surface temperature 55-80 °С can be reached. During the process it is the adhesive film that is heated, rather than the sole, which remains cooler. This ensures a more rapid development of bond strength. Soft soles that tend to deform when hot can be bonded at room temperature or with adhesives that have a low activation temperature.

For the various molding-on processes, such as the direct injection molding of thermoplastic rubber or PVC, the vulcanizing-on of rubber, and the low-pressure reaction molding of integral-skin cellular polyurethane, adhesives can seldom be dis­pensed with: the lasted margin of the upper is coated with a polyurethane adhesive, often containing a pre-mixed latent hardener, or with a self-vulcanizing synthetic rubber solution compounded to match the vulcanization system of the soling com­pound.

With moisture curing polyurethane hot-melt adhesives a new innovative technology has been introduced in the sole-bonding process. It can be used as a one-way bonding system or as a two-way contact adhesive, applied either to the sole or to the upper. In this process, an adhesive coat must be applied on the other substrate.

Adhesives for Shoe Repairs. These adhesives must be almost universally applicable, because widely differing shoe types and material combinations must be repaired. For this reason, one-component polychloroprene-based adhesives are used almost exclu­sively, though for PVC and some other plastics materials polyurethane adhesives are quite common.

Testing. In Europe adhesive properties are tested according to European standards (CEN) EN 522, EN 1391, and EN 1392. Specified standard test materials can be obtained from the various research institutes of the European shoe industry.

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