Development of bio- and eco-composites for the footwear industry

Abstract

The evolution of consumers' environmental awareness, allied to the demand for bioproducts, is considered a challenge for the footwear sector. In this context, the development of novel composites, based on natural materials and by adopting green productive processes, is an area of great interest. Based on this motivation, the present work aims to develop cork composites where the toxic TDI-based binder traditionally used for cork agglomeration, is substituted by an aqueous water-based polyurethane adhesive reinforced with Melissa officinalis L. extract. The work was divided into three stages. The first one was dedicated to the synthesis of water-based polyurethane adhesives (PUD) following a modified pre-polymer method. Three adhesives were produced, namely a formulation with no added extract, and two formulations incorporating Melissa officinalis L. hydrophilic extract, at contents of 3 and 5% (w/w, prepolymer-basis). In the second stage of this study, based cork composites were produced using adhesives at contents of 20, 30 and 40% (w/w, corkbasis, adhesive weight based on its solids content). The final stage comprised the production of cork composites (30% of adhesives content of the three formulations were used) added with biomass residues (sugarcane bagasse (CA), malt bagasse (MA), coconut fibers (CO) and chia seeds (CH)) at contents of 5 and 10% (w/w, corkbasis). The obtained results pointed out for the viability of using PUDs to substitute the traditionally used TDI-based binders, which also added several advantages. Namely, a remarkable increment of the water absorption capacity was identified, particularly when the adhesives were incorporated with the extract. It was observed that all the produced composites fulfill the imposed value of 70 mg/cm2. For the water desorption capacity, and with the exception of the composite produced with the adhesive added with 5% of extract at a content of 20%, all of the adhesives were below to the limit of 80%. This means that the adhesive hydrophilic nature lead to an increase of the composite drying time. Regarding the dimensional stability, almost all the produced composites presented shrinkage lower or close to 2%. The results evidenced also the increasing of film’s flexibility due to the extract incorporation. In a general way, the adding of biomasses results in the decrease of both water adsorption and desorption properties, even very close to the corresponding base-composites. Concerning shrinkage values, and flexibility, better results were generally achieved. Namely, in terms of flexibility, the biomasses conducting to the best results were CA, CO or CH at contents of 5% combined with the PU3C adhesive at a content of 30%.