Running shoes and athletic footwear are usually made from a range of plastic foams and fabrics which are stitched or glued together, effectively preventing recycling at the end of life. Alarmingly many of the materials traditionally used for insoles and midsoles rely on chemical foaming agents, which can release VOCs (volatile organic compounds) during use, forming a smog at ground level and harming our environment. There is increasing pressure on the industry to find more sustainable footwear solutions but, given the rough treatment that most trainers sustain, finding replacement materials is challenging.
Celanese, a martial manufacturer have developed several ranges of materials that incorporate recycled plastics or biomass specifically developed to meet the needs of the footwear industry. Among these Hytel ECO-B grades are partially made from biomass (up to 72%, on a certified mass balance basis). For some grades, only nitrogen and carbon dioxide are used as foaming agents which avoids the VOC issue associated with traditional footwear materials. Finally, Hytle requires no material crosslinking, so it is recyclable. As the material is available as a foam and filament the same material can be used for many of the components used to make a shoe (uppers, insoles, midsoles, and outsoles to support components, fastening systems, and buckles) again increasing recycle potential. Importantly, as the material is produced on a certified mass balance basis it has the same chemical structure of the non-eco material – so it can be dropped into existing manufacturing processes, avoiding the costs of refining or retooling to meet the needs of a new polymer.
To their credit, Celanese are upfront about the fact that these eco grades of Hytel are produced on a mass-balanced basis. Proponents of the mass balance approach highlight the fact that this approach does reduce the overall demand for virgin fossil-based ingredients, which is a win for the environment. However, details of what mass balance actually means can get lost or diluted when presented by clients in their advertising material. The potential problem with this is that consumers might quickly become disillusioned if they purchase products believing them to be made from environmentally friendly materials, only to discover that they are actually effectively made from traditional petrochemicals. This will apply especially when a premium has been paid for those products. This issue is not unique to the footwear industry, all manufacturers need to take the time to explain the environmental benefits of the mass balance approach to ensure that consumers are not misled.