Printed circuit boards are essential components for electrical and electronic equipment. Made of a substrate, typically an epoxy resin reinforced with glass fibers, and printed with a functional ink containing a conductive metal, these devices are often difficult to recycle.
The thesis is part of the European CircEl-paper project aiming to manufacture paper-based multilayer printed circuit boards using the principles of the circular economy, sustainable resources, and environmentally friendly materials. Paper is a substrate of choice for functional printing because it is bio-based, flexible, economical, and, above all, has high recycling potential. However, its rough, porous nature and dimensional instability in the presence of water require the incorporation of numerous additives during the design and production of the printed circuit. The objective of the PhD thesis is to adapt standard paper and cardboard recycling processes to these new complex printed electronic devices, in order to (1) recover cellulose fibers suitable for the production of recycled paper and (2) isolate the precious metals present in the functional ink for selective recovery. The separation of these two materials is therefore the key to their recycling.
To develop the recycling processes, the preliminary study focuses on simple RFID antennas printed with silver (Ag) flexographic ink. More complex models of printed electronic devices are studied subsequently. The conventional unit operations of paper and cardboard recycling are simulated at lab-scale using batch-operating pilot equipment. The recycling line starts with the disintegration of the printed devices in water, resulting in a “pulp” consisting of suspended cellulose fibers and detached elements. Separation operations are then applied (screening, centrifugal cleaning, flotation).
The manuscript is divided into four sections.
The first section establishes the sampling and analysis conditions for tracking the various separated material streams, particularly silver (Ag), throughout the recycling chain. Silver is quantified using atomic absorption spectrometry, following the calcination of pulp samples and the digestion of the resulting ash with nitric acid.
In the second part, the disintegration efficiency of the antennas is studied by varying operating conditions within standard ranges. Analysis of the fibrous flocs and the size of the ink fragments shows that this operation is efficient and significantly fragments the silver. Thus, screening is not efficient for separating silver from the fibers, unlike centrifugal cleaning and flotation, which concentrate silver in one fraction and cellulose fibers in another through different mechanisms. Thus, a complete recycling line comprising in series, the pulping, flotation, and centrifugal cleaning operations results in the separate recovery of over 90% of the fibers and 80% of the Ag, making the reuse of these materials feasible.
The following chapter addresses the recycling of more complex prototypes, including a resin and an adhesive. More severe operating conditions are required, particularly during disintegration. The cellulose fibers are poorly defibrated, but this does not prevent effective separation of the fibers from the Ag, with respective recovery rates averaging 70 to 80% for the organic material and 80% for the Ag.
Finally, the last chapter examines the synthesis of Ag particles from the separated silver, with the aim of reusing the recycled silver for the formulation of conductive inks. It has been demonstrated that it is possible to control the size and shape of the particles by using stabilizing polymers during synthesis.
