In the 1st period of WP1 (passive materials) the paper properties w.r.t. the individual processing steps were evaluated. Work on recyclable transfer paper started and the use of benzoxazine as glue was analyzed and compared to reference PU. The use as flame retardants showed that the rate of polymerization needs to be adapted. With all these results the 2nd period concentrated on new paper qualities. For this the paper was impregnated by Benzoxazines and containing flame retardants other than halogenide-based showing retarded/none flammability according to the UV test standards. As such the paper can be used for both: as substrate for printing but also for electroplating processes/VIA production. However, the recyclability was reduced. Another paper for printed electronics that still exhibited good recycling properties, showed good printing properties, but did not withstand the electroplating process. So, the project reveals, as expected that a proper trade-off between functionality and recyclability is needded. This also depends on the application and their requirements. Presumably, at the end of the project, there will be no one-fits-all-paper but several papers that meet different property profiles.
During the 1st period, the main results for WP 2 (Active materials), were with respect to silver inks that the commercial silver particles already contained a share of 87% of recycled silver. Thus, the investigations of further additives (solvents, binder) got into focus. These starting inks were shared between the partners for evaluation. First tests with base non-toxic metals (Sn, Al, Bi) were started. The evaluation of natural precursors for biobased hybrid polymers as dielectric materials was finished. First evaluations of new resistor materials with biobased solvents showed a lower conductivity but less dependency on temperature. During the 2nd period, major progress was accomplished for the different materials. Silver ink was generated based – additionally to the large amount of secondary raw materials on the metal component - on biological or sustainable additives/solvents; dielectrics were screened that were biobased. First biobased hybrid polymer dielectrics were realized on base of functionalized plant components. Mechanical sintering of base metal inks was a key condition to create first candidates for more sustainable conductive materials.
In WP 3 (Processes), the annealing process for inks and pastes on paper was investigated during the 1st period with respect to IR techniques to reduce thermal load. First galvanics on paper were very promising, however additional seed layers were needed. CO2 laser drilling caused ashing and non-controlled borders, but resolution of 200 µm was achieved finally. Mechanically drilled holes were successfully filled with metal inks. During the 2nd period. Printing, such as screen printing, ink-jet printing and aerosol printing, were utilized to deposit highly conductive lines with resolutions below 100µm. First promising results of upscale production using R2R-printing were achieved. Most results were achieved with inks developed in the project showing the good adaption of the processes towards the new materials. Printed resistors worked well within the limits of 5% accuracy after trimming. As paper is flexible, also folding might be employed. This was demonstrated using a simple battery holder.
In WP4 (Hybrid Integration) first discussions about the materials in WP1 and 2 were performed during the 1st period, to get a first idea how to adapt processes for new material. During the 2nd period the realization of flexible electronics by multilayer paper was abandoned as the paper after impregnation and gluing with a 2nd paper is already rather stiff. The complexity of the circuits, prints and metallization was increased and first reliability tests were done: The metallization as demonstrated here in this project is less reliable compared to FR4 metallization. The corrosion/oxidation of cupper / Ag is higher. The glues to combine 2 papers show low adhesion at high humidity. The metallization in VIAs might be also a weak point in terms of reliability but still has to be characterized.
In contrast to that, the reported discrete components on paper showed a reliable and stable behavior so far. Further investigations will be done next.
WP5 is dealing with the use cases. First demonstrator examples were started to be transferred to paper. For the sound generating electronic circuit, different circuitries and processing methods were already tested and pioneered. The demonstrator for medical sensors and logistics were designed and first mock-up versions were produced to check the function.
WP6 (LCA+ Recycling) Recycling trials of impregnated and printed paper were performed to compare the different technological options in terms of recyclability. This was important for the comparison of impregnation and lamination, which impact the pulpability of paper in standard conditions. Different disintegration conditions have been explored keeping the chemistry and process steps within the range of industrial paper recycling. The recovery of silver has been further developed. The recovery rate of silver is very high; LCA and circularity calculations confirmed that this step has the largest potential in terms of sustainability.
The list of materials employed in the technical WPs included about 60 materials, about half of it was not available in the LCA databases. Thus the synthesis routes of those chemicals had to be modelled and were integrated to fille the gaps in the databases. In parallel, the data platform used to exchange process and material data has been expanded to include also technical and economic parameters.
Given the large variety of chemicals employed in the project, the consortium agreed to extend the environmental assessment to include ecotoxicity. An expert partner was thus invited to join the consortium by the hop-on scheme, with the task of assessing ecotoxicity by means of literature research and computational models.
