Advances in the assessment of the safety and sustainability of FreeMe’s plating-on-plastics processes

Following the Safe-and-Sustainable-by-Design (SSbD) framework, a voluntary approach to guide the innovation process for chemicals and materials in the European Union (EU) launched by the European Commission (EC) in 2022 [1], the University of Burgos (UBU) is currently evaluating the safety and sustainability of the plating-on-plastics (PoP) processes conceived and performed by three partners of the FreeMe project (CNANO, Polimi and Gaser). The objective is to determine how safe and sustainable those processes are and to inform recommendations for their improvement.

The safety assessment focuses on the risks associated with the chemicals/materials and the related processes and uses, arising from a combination of the evaluation of the chemicals’ intrinsic properties and of the occupational, consumer and environmental exposure. Accordingly, and following the SSbD framework, the safety evaluation for CNANO’s process performed by UBU began with a hazard identification (human, environmental and physical) of the chemical substances involved in the PoP process. Hazard selection was based on harmonised classifications from the ECHA CL Inventory, and appropriate QSAR models were applied to address missing data. Based on this hazard identification, substances were assigned risk levels (from Level 0 to Level 3), with Level 0 representing highly hazardous substances. Following the SSbD principle ‘Reduce exposure to hazardous substances’, the exposure assessment and risk characterisation related to human health (workers) and the environment were carried out for those Level 0 substances identified and for the specific CNANO’s PoP process operations in which these substances are employed.

The methodology used to prepare the safety assessment followed the ECHA guidance, the EU REACH Regulation and the JRC Technical report about the SSbD framework [2]. Data for hazard characterisation (or Dose–Response Assessment) and relevant exposure routes were obtained from the ECHA database. Information for exposure assessment (frequency, duration, use, operational conditions and risk management measures) was obtained from CNANO via questionnaires. To predict exposure to human health and environmental safety during processing, Tier 1-Tier 2 conservative models implemented in CHESAR v3.9 were applied. The severity of harm under the specified exposure conditions was expressed using Risk Characterisation Ratios (RCRs), which compared the predicted exposure with tolerable exposure limits. Finally, to evaluate environmental risks of CNANO’s intermediate products during storage, handling and environmental release, physico-chemical characterisation of the samples and in vitro ecotoxicity testing using Saccharomyces cerevisiae and Aliivibrio fischeri were conducted. Risk assessment provided early-stage insights for risk management, recommendations based on available information and predictive models, and identified data gaps to be addressed in subsequent tiers and higher TRLs (upscaling).

From the sustainability viewpoint, UBU has calculated the environmental footprint of the metallisation process conducted by CNANO, consisting of eight stages. One of them, the etching step, aimed at achieving adhesion between the polymeric surface and the metallic layer, is traditionally conducted with acidic solutions containing the highly toxic and carcinogenic CrO₃ [i.e., a Cr(VI) compound]. The alternative developed by CNANO, which replaces these solutions with H₂SO₄/H₂O₂ mixtures, was successfully applied to the etching of several plastic substrates (ABS, PC/ABS and PA12). To study not only this step, but the entire process from the environmental perspective, UBU has applied the Life Cycle Assessment (LCA) methodology, standardised through ISO 14040:2006/Amd 1:2020 [3] and ISO 14044:2006/Amd 2:2020 [4], and the Environmental Footprint (EF) method, recommended by the EC [5]. LCA considers the full life cycle of a given product, from the extraction of resources and processing of raw materials, through production, use and possible recycling, to the final disposal of the remaining waste. A ‘cradle-to-gate’ study, considering the impacts generated during the production phase, was carried out, and the outcome was compared with that of the conventional plating strategy, i.e., that relying on Cr(VI)-containing etching solutions. The results indicate that CNANO’s alternative could improve the traditional approach from the environmental angle; for instance, removal of Cr(VI), and also of Pd compounds (common reagents in the classical process and avoided in CNANO’s strategy), leads to a considerable decrease of impacts associated with the ecotoxicity and human toxicity categories, integrated in the EF method. Although this is a promising outcome, more research is required to confirm it.