When speaking about ‘‘sol–gel’’, the term ‘‘science’’ is often associated with the term ‘‘technology’’. The main advantage of this synthetic route is to allow easy shaping of the final oxide-based materials, and the sol–gel process thus appears as an important liquid-phase manufacturing techniques among the so-called bottom-up approaches, for a large range of nanomaterials (particles, thin films, coatings, membranes, fibres or bulk materials). The literature is rather scarce regarding life cycle assessment (LCA) of sol–gel derived products; however many researchers, who have addressed life cycle aspects of nanoproducts, agree that the manufacturing phase is a majorcontributor to the life cycle impacts.206The authors would like to illustrate through selected publications, which are not restricted to the case study of silica, the current analysis that can be found on the environmental impacts of nanomanufacturing methods.Indeed many concerns have been raised regarding the human and ecological health effects of nanoproducts, but little attention has been given to the manufacturing phase. Top-down productionmethods are the most commonly used approaches today for nanoproducts, and it is generally believed that such techniques are more waste-producing that bottom-up techniques, that are often considered as the ultimate tools for sustainable manufacturing as they allow for the customized design of reactions and processes at the molecular level, thus minimizing unwanted waste.206An interesting evaluation of a series of nano technological production methods (chemical vapour deposition (CVD), physical vapour deposition (PVD), flame-assisted deposition, sol–gel process, precipitation and lithography) was published.207Sol–gelprocesses performed very satisfactorily in terms of facility installation cost, since it involves rather basic chemical process engineering (compared for example to lithography) and energy input (considered as low). The potential for release of nanoparticle emissions during the production stage was estimated low-to-medium regarding the workplace since the process takes place within liquid medium, but also low-to-medium regarding the environment since discharge of nanomaterials is possiblevia polluted process media and wastewater. It is however noticed that discharge might be purified with adequate technology.During the product use, potential for release of nanoparticles is low if nanoparticles are encapsulated in the end-product within a fixed layer and medium if the end-product shows no long-term stability.In the course of the same study, life cycle assessment (LCA) was also performed to describe the ecological efficiency potentials of nano technology-based coating processes.208The case study concerned corrosion protective coatings on aluminium. Corrosionprevention has high economic as well as ecological relevance.209On aluminium, conventional coating technology requires the use of chromium, and chromium(VI) salts are known human carcinogens. A newly developed nanocoating based on sol–gel technology was compared to conventional coatings, such as waterborne, solventborne, and powder coat industrial coatings. In this case, thesol–gel coating was an organic–inorganic hybrid polymer, based on organosilanes. The assessment was carried out for the entire life cycle of the varnish, including surface pre-treatment: extraction of raw materials, production of basic components, varnish production, surface pre-treatment, varnishing, use stage application phase, disposal/recycling. The conclusion of this study was that sol–gel-based coatings show great potential for a very high degree of the improvement of eco-efficiency with respect to all emissions and environmental effects (VOCs, greenhouse gases). It also allows a simplified surface pre-treatment process, avoiding chromating.Additionally, the same level of functionality can be reached for much lower thicknesses. Sol–gel technology is also important for nanoparticle production for which exists a large offer of almost any composition at more and more competitive prices. But lower prices require more careful analysis of energy requirements in the possible process, and may enable both economically and ecologically safe choiceof the required technology. An interesting confrontation was published210 between liquid-based precipitation processes considered as traditional processes, and newer dry processes such as flame- or plasma assisted particle synthesis. Life cycle inventory was achieved using the emissions of CO2equivalents211and energy balances as indicators. Emergence of new nanoparticulate materials repeatedly speculated than dry processes are more economic and environmentally friendlier that their wet counterparts due to fewer process steps. But indeed, in terms of energy requirement, product composition strongly influences the selection of thepreferred method of manufacturing. The study focused on titania and zirconia nanoparticles. CO2 emission for TiO2 production was estimated to 4 kg kg1TiO2 starting from titaniumtetrachloride and to 15 kg kg1TiO2starting from titanium isopropoxide. For zirconia, these numbers are 5 and 9 kg kg1respectively. The authors of this study concluded that what they called traditional wet processes based on salts (chlorides or sulfates) excel in terms of efficiency over dry processes based on organic precursors, especially for metal oxide nanoparticles oflight elements with high valency. The purpose of this analysis that was to compare energy consumption during nanoparticle production, indeed demonstrated the importance of the choice of precursors in terms of energy requirements.Through those selected examples, it is clear that sustainable development of nanotechnology will inevitably require incorporation of life cycle thinking to analyze the environmental impact of nanomanufacturing
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