The key to sustainable development in the footwear industry through the principles of circular economy lies in taking care of the design, as well as the introduction of innovative and more resource efficient materials and processes to reduce or avoid the use of water, energy, hazardous chemicals and to minimise emissions and waste. In fact, the environmental footprint is already being considered as another requirement of the footwear through eco-design. In this sense, previous studies carried out by INESCOP regarding its environmental impact in terms of carbon footprint showed that 15% of it corresponds to the assembly processes, mainly by adhesive joints, due to their content on organic solvents, hazardous chemicals and polymers from fossil origin. Therefore, this paper focuses on recent developments carried out by INESCOP on more sustainable adhesives and adhesion processes for the upper-to-sole assembly in the footwear manufacturing process, through different approaches. Firstly, bio-based reactive polyurethane hot melt adhesives have been synthesised using polyols from different renewable sources. Secondly, the use of the low-pressure plasma surface treatment to improve the adhesion of polymeric materials used as soling materials was assessed in order to reduce volatile organic compounds emissions, as well as the use of hazardous chemicals for total automation of the bonding process.

The aim of this work is to develop sustainable reactive polyurethane hot melt adhesives (HMPUR) for footwear applications based on biobased polyols as renewable resources, where ma-croglycol mixtures of polyadipate of 1,4-butanediol, polypropylene and different biobased polyols were employed and further reacted with 4-4′-diphenylmethane diisocyanate. The different reactive polyurethane hot melt adhesives obtained were characterized with different experimental techniques, such as Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), softening temperature and melting viscosity. Finally, their adhesion properties were measured from T-peel tests on leather/HMPUR adhesives/SBR rubber joints in order to establish the viability of the used biobased polyols and the amount of these polyols that could be added to reactive polyurethane hot melt adhesives satisfactorily to meet the quality requirements of footwear joints. All biobased polyols and percentages added to the polyurethane adhesive formulations successfully met the quality requirements of footwear, being comparable to traditional adhesives currently used in footwear joints in terms of final strength. Therefore, these new sustainable polyurethane adhesives can be considered as suitable and sustainable alternatives to the adhesives commonly used in footwear joints.

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The implementation of a Circular Economy model, promoted by the increasingly stricter European policies, demands a comprehensive approach to resource efficiency. In this sense, polyurethanes, one of the most used polymers worldwide, are strongly dependent of non-renewable fossil resources. Thus, boosting the production of new polyurethanes / a new polyurethane based on more sustainable raw materials is crucial to move towards the footwear industry decarbonisation. INESCOP, aware of the footwear industry’s environmental impact, focuses on reducing or removing fossil-based raw materials and opts for eco-friendly ones. These sustainable raw materials provide polyurethane adhesives with additional beneficial non-toxicity and sustainable characteristics, without harming their properties during their useful life. Therefore, the aim of this study is to synthesise and characterise reactive hotmelt polyurethanes from biomass and CO2-based polyols as bioadhesives for the footwear industry. The influence of biobased polyols on the polyurethane structure, and therefore, on their final properties was analysed by different experimental techniques in order to assess their viability for the upper to sole bonding process.

In this study, functional nanocoatings for waterproof footwear leather materials were investigated by chemical plasma polymerization by implanting and depositing the organosilicon compound hexamethyldisiloxane (HMDSO) using a low-pressure plasma system. To this end, the effect of monomers on leather plasma deposition time was evaluated and both the resulting plasma polymers and the deposited leather samples were characterised using different experimental techniques, such as: Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). In addition, leather samples were tested by standard tests for color change, water resistance, surface wetting resistance and dynamic water contact angle (DWCA). The resulting polysiloxane polymers exhibited hydrophobic properties on leather. Furthermore, these chemical surface modifications created on the substrate can produce water repellent effects without altering the visual leather appearance and physical properties. Both plasma coating treatments and nanocoatings with developed water-repellency properties can be considered as a more sustainable, automated and less polluting alternative to chemical conventional processing that can be introduced into product-finishing processes in the footwear industry.

Polyurethanes, one of the most used polymers worldwide, are strongly dependent of non-renewable fossil resources. Thus, boosting the production of new polyurethanes based on more sustainable raw materials is crucial to move towards the footwear industry decarbonisation. The aim of this study is to synthesise and characterise reactive hotmelt polyurethanes from biomass and CO2-based polyols as bioadhesives for the footwear industry. The influence of biobased polyols on the polyurethane structure, and therefore, on their final properties was analysed by different experimental techniques such us Fourier transform infrared spectroscopy (FTIR), Differential scanning calorimetry (DSC), Thermogravimetric analysis (TGA), Melting viscosity, Softening temperature and T-peel strength test, in order to assess their viability for the upper to sole bonding process. The results obtained indicated that the incorporation of different amounts of the biobased polyols produces changes in the structure and final performance of the polyurethanes. Therefore, adhesion test carried out by the T-peel test 72 h after the upper -to- sole bonding of the sustainable adhesives show high final adhesion values. These sustainable raw materials provide polyurethane adhesives with additional beneficial non-toxicity and sustainable characteristics, without harming their properties during their useful life.

Advances in cleaner technologies have a critical role in reaching Europe’s climate and environmental goals. More precisely, in the case of the footwear bonding process its environmental impact is mainly related to the VOCs emissions resulting from the use of organic solvents in the most common adhesives used, as well as during the application of some surface treatments such as halogenation. Moreover, the use of raw materials of fossil origin also contributes to its carbon footprint. The aim of this work is to deploy cleaner technologies to minimise the environmental footprint of the footwear bonding process through several approaches at different steps of the process life cycle, also to contribute to footwear circularity. This work focuses on the synthesis and application of reactive hot melt polyurethanes from CO2-based polyols as biobased adhesives, and the study of plasma-based surface treatments (low-pressure plasma and atmospheric pressure plasma) to improve adhesion properties, using styrene-butadiene vulcanised (SBR) rubber as a representative footwear soling material. The influence of CO2-based polyols on the polyurethane structure and the physicochemical effects of the plasma treatments on the rubber surface, as well as the final adhesion properties have been evaluated by means of different experimental techniques. The results showed that the values obtained for the T-peel strength of the adhesive joints studied, with the combination of the application of the plasma surface treatments and the biobased reactive hotmelt, exceed the minimum quality requirements for footwear according to the standardised tests. As a result, the deployment of such cleaner technologies regarding adhesives and surface treatments in the footwear bonding process will contribute to improving footwear carbon footprint and its circularity.