SCARLA

Initial situation & project description

Adhesive bonding has meanwhile become the dominant joining method in shoes, which are produced as fashion items in very large quantities. In the spirit of a circular economy, separating the different materials is indispensable—and it must be economically feasible. Therefore, the research project focuses on the development of specifically detachable adhesive bonds for use in shoes. Within the scope of the project, the two main components, the upper and the sole, are initially considered. These should be separated so that the materials can be fed into specific recycling processes (e.g., PET or PUR solvolysis and chromium recovery from leather). The model adhesives to be developed should be applicable like the adhesives commonly used today. The detachment is then achieved through a defined treatment, preferably one that can be carried out in bulk. Accidental detachment during shoe manufacturing or proper shoe use is excluded.

The goal of the project is to provide model adhesives on the basis of which commercial adhesives can be developed that are suitable for bonding shoe uppers to soles in a manner similar to current adhesives. The design principle is targeted debonding through the inclusion of thermally cleavable groups. Microwave heating is intended to be established as the preferred trigger for selective heating of the adhesive layer due to its scalability.

The research project focuses on the development of specifically deactivatable adhesive bonds for use in shoes. This creates the possibility to separate at least the main assemblies—the upper and the sole—and feed them into specific recycling processes. The adhesive meets the requirements of regular, industry-established products in the footwear sector and is deactivated through a defined treatment. Accidental detachment during shoe manufacturing or proper shoe use is excluded. The adhesives are being developed based on solvents, as their synthesis also takes place in solvents. A later transfer to solvent-free adhesives is intended. A benefit is projected for the entire German footwear industry, which is strongly characterized by SMEs, as current legislation affects both manufacturers and importers. The laws specifically target the “creation of an economic system for the collection, sorting, reuse, preparation for reuse, and recycling.” According to HDSL, an average of 428 million pairs of shoes are sold annually in Germany, all of which should ultimately be recycled. The prevailing material diversity has so far hindered mechanical-thermal recycling, where thermoplastics are remelted. In chemical recycling processes, some materials have negative effects—for example, metals in all processes, polyester with polyamides, and polyether with polyesters. The adhesive to be developed will facilitate the separation of composite materials. The project’s accompanying committee can map the entire product lifecycle. 66% of the committed participants are SMEs from shoe production, adhesive manufacturing, and recycling. Through networking the research institutions with other industries, knowledge transfer to sectors such as automotive, furniture, and mechanical engineering is highly likely, alongside teaching activities.