RESUMO
Diversification of polymer waste recycling is one of the solutions to improve the current environmental scenario. Upcycling is a promising strategy for converting polymer waste into molecular intermediates and high-value products. Although the catalytic transformations into small molecules have been actively discussed, the methods and characteristics of upcycling into new materials have not yet been addressed. Recently, the functionalisation of polymer wastes (polyethylene terephthalate bottles, polypropylene surgical masks, rubber tires, etc.) and their conversion into new materials with enhanced functionality have been proposed as an appealing alternative for dealing with polymer waste recycling/treatment. In this review, the term 'functional upcycling' is introduced to designate any method of post-polymerisation modification or surface functionalisation without considerable polymer chain destruction to produce a new upcycled material with added value. This review explores the functional upcycling strategy with detailed consideration of the most common polymers, i.e., polystyrene, poly(methyl methacrylate), polyethylene, polypropylene, polyurethane, polyethylene terephthalate, polyvinyl chloride, polycarbonate, and rubber. We discuss the composition of plastic waste, reactivity, available physical/chemical agents for modification, and the interconnection between their properties and application. To date, upcycled materials have been successfully applied as adsorbents (including CO2), catalysts, electrode materials for energy storage and sensing, demonstrating a high added value. Importantly, the reviewed reports indicated that the specific performance of upcycled materials is generally comparable or higher than that of similar materials prepared from virgin polymer feedstock. All these advantages promote functional upcycling as a promising diversification approach against the common postprocessing methods employed for polymer waste. Finally, to identify the limitations and suggest future scope of research for each polymer, we comparatively analysed the aspects of functional upcycling with those of chemical and mechanical recycling, considering the energy and resource costs, toxicity of the used chemicals, environmental footprint, and the value added to the product.
RESUMO
MAX phases (M = transition metal, A = A-group element, and X = C/N) are of special interest because they possess a unique combination of the advantages of both metals and ceramics. Most attention is attracted to the ternary carbide Cr2AlC because of its excellent high-temperature oxidation, as well as hot corrosion resistance. Despite lots of publications, up to now the influence of bias voltage on the chemical bonding structure, surface morphology, and mechanical properties of the film is still not well understood. In the current study, Cr-Al-C films were deposited on silicon wafers (100) and Inconel 718 super alloy by dc magnetron sputtering with different substrate bias voltages and investigated using Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), Atomic Force Microscopy (AFM), and nanoindentation. Transmission Electron Microscopy (TEM) was used to analyze the correlation between the growth of the films and the coating microstructure. The XPS results confirm the presence of Cr2AlC MAX phase due to a negative shift of 0.6-0.9 eV of the Al2p to pure aluminum carbide peak. The XRD results reveal the presence of Cr2AlC MAX Phase and carbide phases, as well as intermetallic AlCr2. The film thickness decreases from 8.95 to 6.98 µm with increasing bias voltage. The coatings deposited at 90 V exhibit the lowest roughness (33 nm) and granular size (76 nm) combined with the highest hardness (15.9 GPa). The ratio of Al carbide to carbide-like carbon state changes from 0.12 to 0.22 and correlates with the mechanical properties of the coatings. TEM confirms the columnar structure, with a nanocrystalline substructure, of the films.