Printed Electronics (PEs) is an emerging field in constant development for electronics manufacturing, which offers innovative solutions based on flexible, lightweight and cost-effective devices. Until now, PEs is mainly made of plastics on which a variety of materials are printed, including conductive inks, semiconductor and dielectric materials. Considering the eco-design, sustainability and environmental aspects, new alternative substrates such as paper have been studied to replace plastics. The use of paper offers several advantages such as low cost, flexibility, biodegradability, printability and functionalization, among others. Despite the sustainability gain, there is still a knowledge gap relative to the end-of-life of these paper-based devices, namely their recyclability. Then, this project aims to adapt conventional paper and board recycling lines to paper-based PEs recycling. This raises new challenges for recycling since the PEs composition differs from the composition of classical paper and board. Moreover, in conventional paper recycling processes, papermakers focus mainly on the obtention of clean and contaminants-free recycled fibers, and the rejected materials are not valorized. In the case of PEs, the printed materials contain added-value components such as expensive and critical metals which might be interesting to recover.
The project has been divided into three sections.
The first one is dedicated to the production and characterization of a simple paper-based prototype of an RFID antenna, used for the development of the recycling processes. Silver conductive ink is used as the functional material. To track silver during recycling, four methods of quantification have been compared, using both non-destructive image analysis and acid leaching followed by AAS analysis. The latter method was considered to be the most rigorous and precise, being further used to track Ag in different recycling streams.
The second section is devoted to the design of a recycling line using classical unit operations for paper and board to achieve an efficient separation of fibers and silver. During pulping, the paper disintegration was optimized, where longer operation times and the use of chemicals resulted in improved pulp homogeneity. Additionally, the effect of different pulping parameters (time, consistency, etc.) was also studied, using both coated and uncoated substrates separately or in a mixture. Screening, centrifugal cleaning and flotation were the tested unit operations. Among them, the centrifugal cleaning, driven by the centrifugal forces and the density differences between both materials (i.e. fibers and Ag particles), exhibited the highest separation efficiency (up to 83% of Ag, 97% of fibers). Screening was not efficient because the pulping did not generate big fiber aggregates. Flotation did not separate efficiently the silver ink due to the lack of hydrophobic character. Additionally, it was observed that the paper coating reduces the deinking efficiency compared to an uncoated substrate.
Finally, the last section is dedicated to the valorization of Ag recovered from the rejected hydrocyclone fraction. Ag leaching and recovery were demonstrated to be complex. The final direct synthesis of Ag particles of controlled size distribution was not achieved. Therefore, pre-purification steps are necessary to produce, in a closed-loop manner, Ag particles of suitable size to be used in new ink formulations for PE production.
To conclude, this PhD work demonstrated that paper-based PEs could be recycled using classical paper and board recycling lines, with some adjustments. Fibers could be separated and reused to make paper and silver particles coming from the printed material could be also recovered and a first step toward their valorization was proposed.
