The Initial Development and Evaluation of Cross-Linked Casein Films for Sustainable Footwear

To increase the sustainability of footwear and align it with the circular bioeconomy, there is a pressing need to develop novel bio-based materials to replace the synthetic polymers currently used. In this study, casein-based films were prepared by solution casting with a tannic acid cross-linker, and a glycerol plasticizer. The properties important to footwear materials were characterised, including tensile strength and elongation, stitch strength, hardness, grain cracking strength, water vapour permeability, and thermal properties. The tannic acid imparted a brown colour to the films with good resistance to light-induced fading. Their tensile strengths were 4-5 N/mm(2), elongation at break 45%-73%, and water vapour permeability 0.2-0.6 mg/(cm(2)h), depending upon the levels of the cross-linker and plasticiser. The strength of the films was close to those of some non-leather footwear materials such as compact rubbers and insock materials, but below that required for leather shoe vamps/uppers. The casein films were successfully used in sneaker- and ballerina-style shoes as components of the insock and decorative design elements on the vamp. This work has demonstrated the concept of using casein films in footwear. The properties of the films could be improved by further research, particularly with respect to cross-linking, plasticizing, and combining with bio-based fibers and fabrics.

Automated summary

In order to make footwear more sustainable and in line with the circular bioeconomy, new bio-based materials need to be developed to replace synthetic polymers. In this study, casein-based films were prepared using tannic acid as a cross-linker and glycerol as a plasticizer. The films showed properties suitable for footwear materials, such as tensile strength, elongation, stitch strength, hardness, grain cracking strength, water vapor permeability, and thermal properties. Further research can improve the properties of casein films by exploring cross-linking, plasticizing, and combining them with bio-based fibers and fabrics.