Ethylene vinyl acetate (EVA) and polyurethane (PU) are the primary constituents of shoe soles and midsoles. The use of these materials in shoe design provides high durability, good flexibility and excellent aging resistance (Sipaut et al., 2017). In 2019, 24.3 billion pairs of shoes were produced, consuming more than 1 million tonnes of EVA (ChemAnalyst, 2021) and 0.5 million tonnes of PU (2021). The relatively limited lifespan of shoes and fast-changing tastes of customers cause the generation of large amount of EVA and PU waste, containing also fillers, pigments, crosslinkers or blowing agents (Lee and Rahimifard, 2012). Due to waste mismanagement, over 300 million pairs of shoes accumulate yearly nearby or in the marine regions, posing threat to marine ecosystems (Thomas and Fiona, 2018).
The mechanical recycling of PU and EVA waste is highly challenging (recycling rate <5%) because of the permanent crosslinks formed in the molecular structure of waste EVA and PU, hampering the re-processing and re-melting (Correia et al., 2011; Lopes et al., 2015). As a result, mechanical recycling of such waste plastics produces lower-grade polymer products compared to the original materials (Pickering, 2006). To minimize the influence of crosslinks, the use of waste EVA and PU as fillers or modifiers was reported. For instance, waste EVA or PU could be incorporated into rubber to make soles or plates, improving their tensile strength and tear resistance (Lopes et al., 2015; Zia et al., 2007). Another way was to incorporate the waste EVA or PU into the cementitious composites to form lightweight concrete formulations characterized with only minor reduction of compressive strength and elastic modulus (Ben Fraj et al., 2010; Elgady and Supervisor, 2018). Another issue is that mechanical recycling often requires time-consuming sorting process to ensure retrieving uncontaminated waste streams (Lee and Rahimifard, 2012). Compared with mechanical recycling, chemical recycling enables the direct conversion of highly heterogeneous plastic mixtures into products that can be upgraded into new plastics or chemicals (Deng et al., 2016). By far, only a few studies have been conducted related to chemical recycling of waste EVA via crosslinking with virgin EVA resin into foam particles for commercial use (Sui-Chieh, 2015) or with triethyl borate via the transesterification reaction to form EVA vitrimers (Guo et al., 2020). While reported chemical recycling of waste PU via aminolysis, phosphorolysis or hydrolysis was found feasible only at research stage or for small scale treatment as a result of high costs associated with the use of high pressure (Simón et al., 2018). Only glycolysis was applied on industrial scale, however quality of end products (polyols) require further improvement (Simón et al., 2018).
Thermal waste processing, is one of the prospective recycling methods to convert plastics into value-added carbon nanomaterials such as carbon nanotubes (CNTs) via pyrolysis-chemical vapor deposition (CVD) process (Zhuo and Levendis, 2014). The advantages of using plastic feedstock for the growth of CNTs are associated with environmental benefits such as waste reduction and avoidance of fossil resources in their manufacturing process (Ahamed et al., 2020). To date, various types of plastics such as low density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET), poly (vinyl alcohol) (PVA) or mixed waste plastics, containing PE, PS and PP have been used for CNT synthesis (Williams, 2021). In fact, using these plastics (e.g., PE, PP, PS, PET and PVA) at a wide variety of operational parameters and with different catalysts primarily facilitates the production of multi-walled CNTs. It is likely that the composition of generated pyrolysis gas, which in turn, depends on the feedstock, plays a crucial role in the growth of multi-walled CNTs. For many applications, however, it is advantageous to have smaller diameter single-walled CNTs, few-walled CNTs (wall number ≤5) or a mixture of small and large diameter CNTs (Borghei et al., 2014; Kim et al., 2019). One recent study has reported the synthesis of single-walled CNTs (diameter: 0.9–1.5 nm) from LDPE and PP over a Co loaded catalyst (Zhao et al., 2022). However, the synthesis conditions and potential applications of produced CNTs were not disclosed in detail. Up to date, there are no studies addressing the conversion of EVA and PU waste into value-added carbon nanomaterials. Considering the large quantities of disposed PU and EVA, this could be a feasible approach for such waste management.
In this study, the thermochemical conversion of PU and EVA shoe waste plastics into CNTs was investigated using a two stage pyrolysis – CVD process. PU and EVA as well mixtures of both polymers were used as feedstock. The obtained CNTs were applied for construction of electrode material that exhibited electrocatalytic behavior towards CO2 reduction (CO2RR). CO2RR has a great potential to not only address excessive CO2 emissions but also the energy crisis simply by converting CO2 into fuels such as CO or hydrocarbons (Guo et al., 2017; Wang et al., 2020). To assess the suitability of CNTs from PU and EVA for CO2RR, the CNTs were mixed with ZIFs to prepare N-doped composites as electrodes. The properties and electrocatalytic activity of the composites made from EVA or PU-derived CNTs were compared with the composites made from commercial carbon black and commonly studied PE-derived CNTs. The role of PU and EVA feedstock in the growth of small diameter CNTs, their properties and electrocatalytic activity were discussed.