In recent years, the miniaturization of electronic assemblies has given rise to technologies - including in-mould and printed electronics - with which electronic components are incorporated directly into the non-conductive polymer carrier material or electronic structures are printed on. As in the entire electronics industry, the question arises as to how these assemblies can be broken down into their components at the end of their service life and how they can then be recycled. In the case of polymer-integrated electronics, this has been virtually impossible until now.
In recent years, the miniaturization of electronic assemblies has given rise to technologies - including in-mould and printed electronics - with which electronic components are incorporated directly into the non-conductive polymer carrier material or electronic structures are printed on. As in the electronics industry as a whole, the question arises as to how these assemblies can be broken down into their components at the end of their service life and then recycled. In the case of polymer-integrated electronics, this has been virtually impossible until now.
Electrical resistance during bending tests - increase in resistance during cyclic bending tests for samples without (orange) and with PVA layer (blue)In a project called 'Recyclable Integrated Electronics' (ReIn-E), the Leibniz Institute for New Materials (INM) in Saarbrücken is working with other partners to research recyclable designs and sustainable materials that enable the components to be recycled. To this end, the researchers have developed a layer that is applied between the polymer and metal. This separating layer not only improves the mechanical properties and therefore the service life of the component, but also allows the polymer and metal to be easily separated and recycled.
The project participants have investigated the manufacturing and recycling process of printed electronics, more specifically of silver conductors on polycarbonate on the one hand and in-mold electronics on the other. The release layer, which is applied directly to the polymer in both cases, consists of polyvinyl alcohol (PVA). PVA is harmless to health and is already used in food and medicine packaging.
Manufacturing and recycling process for silver conductors on polycarbonate (PC) as the base material:
- Before printing the conductive tracks, the PVA layer is applied over the entire surface using a slot die.
- Air drying for approx. 10 minutes
- The silver inks or pastes for the conductor tracks can then be printed using inkjet or screen printing.
- Thermal post-treatment at approx. 120 °C completes the process.
The researchers were able to show that the bending strength of the circuits produced in this way is better than that of circuits produced without the PVA release layer and that the structure of the print or the printed image on PVA does not change.
For recycling, the entire circuit is placed in a water bath. The silver conductor tracks detach from the polymer after a short time and can thus be reused.
Separation test - separation of conductor tracks and substrate in contact with water without PVA layer (left) and with PVA layer (right)
Process for integrated in-mold electronics:
- Full-surface application of the PVA layer to the substrate and drying as above
- Printing with silver conductor tracks using screen printing
- Full-surface application of a second PVA layer
- Thermal forming of the polycarbonate films with the PVA and the silver conductor tracks
- Application of a polycarbonate layer by injection molding to protect the conductor tracks and the integrated components
As the electronics in this case are encased in non-water-soluble polycarbonate, the following recycling process was developed, which can be integrated into established processes for the recycling of electronic waste with little effort:
- Shredding the components into approx. 10 mm x 10 mm pieces. The aim is to partially expose the water-soluble PVA layer
- Dissolving the PVA intermediate layer in a water bath (approx. 20 - 30 min)
- Separating sieving and filtering of the detached silver conductor tracks, other metallic components and the polycarbonate from the water bath. In this way, 90-95 % of the materials can be recovered.
In addition to the development of the separating layer, the INM is also working on the formulation of special pastes and inks for screen and inkjet printing. This complements the project tasks of the partners from Belgium and Germany: the Hahn-Schickard-Gesellschaft für angewandte Forschung prints electronics and tests the performance and stability of the printed and integrated materials. The Sirris Technology Research Center manufactures the integrated components based on the designs and materials developed. Finally, the Centre Terre et Pierre, which specializes in urban mining, is developing new processes for recovering metals and polymers.
A preliminary version of the publication on the project is available here: https://pubs.rsc.org/en/Content/ArticleLanding/2024/SU/D4SU00092G (accessed June 3, 2024).
