Footwear manufacturing involves the use of a large range of materials that are worked to the appropriate size and format by appropriate technological processes, thus resulting in a piece that can provide comfort, safety and pleasure to the consumer.

For simplicity, shoe may be divided in the upper and lower part. The first is actually made primarily of leather but, also, natural fibre textiles and man-made textiles. The second includes several components, the most relevant of them being the insock and insole made of leather, cellulose fibbers or composites; and the sole made of vulcanised rubber, thermoplastic rubber, thermoplastic polyurethane, expanded polyurethane or others.

Leather-goods manufacture involves the use primarily of leather. In these two industries, the materials are cut, processed to adequate shape and jointed mainly by stitching and bonding operations.

Leather and soles production involves specific processes in which hides, raw-materials, plastics, rubbers and others, are transformed to valuable materials and components. The materials, components and products obtained in these production processes are normally finished to enhance its aesthetics.

In any case, the sequential operations the materials, components and products are subjected to, give added value to numerous resources but result also in volatile organic compounds (VOC), liquid effluents and solid wastes, among other minor emissions. STEP TO SUSTAINABILITY training course Unit Environment introduces these aspects and best managing options in detail.

The VOC aspect can be addressed by updating production processes and using water based adhesives in the stitching rooms, water and solid based adhesives in the upper-sole bond; and water and solid base finishing systems. The liquid effluents, except in the leather production, are in small quantities, even though highly concentrated in organic matter, and can be treated separately in wastewater treatment plants or in industrial collective treatment centres at relatively low cost. Solutions to be implemented at each company have been studied but so far are in general costlier than the above proposed. Therefore, the main issue still lacking resolution is the solid waste generated by these industries, which is actually mainly managed by landfilling wasting the contained resources and causing environmental and image concerns.

 The solid wastes generated by these industries include leather in several stages (non-tanned, tanned shavings, and tanned and finished scraps and dusts), insocks, insoles and soles dust and scraps, and small quantities of other industrial wastes, like natural and man-made textiles, packaging, and lasts, among others. Taking in consideration that non-tanned leather wastes and leather shavings have been the object of hundreds of works, but not the leather dusts and mixes of leather, insole and sole materials dusts resulting from footwear buffing, roughing and carding operations, which present morphology that may facilitate their valorisation, the work here presented will focus the valorisation of these wastes.

Valorisation of leather fibers and footwear carding and roughing dusts in rubber-leather composites

Residues from footwear roughing and carding operations represent 5 % to 15 % (w/w) of the solid wastes generated by shoe-making companies. These wastes are mainly composed by chromium tanned leather and sole materials, and are mostly landfilled. A more sustainable end-of-life option for these wastes is recycling them in styrene butadiene rubber (SBR) and acrylonitrile butadiene rubber (NBR). With this objective we tested charging these rubbers with leather upper and soles industrial carding and roughing dusts in the range of 20 phr to 100 phr (w/w). The rubber composites tear strength increases till 100 phr, and tension and elongation decrease within the acceptable range till 20 phr. Thus the dusts from footwear roughing and carding operations, with any pre-treatment could be recycled as day-to-day footwear rubber soles additive up to 20 phr incorporation (look at the pictures).

The composite materials at the end of their life cycle may be considered inert or non-hazardous wastes.