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Demonstration and implementation of an integrated process for the Plasma-Enhanced Chemical Solution Deposition of PMMA-multicomponent coatings on wood and wood-based substrates

Periodic Reporting for period 1 - PlasmaSolution (Demonstration and implementation of an integrated process for the Plasma-Enhanced Chemical Solution Deposition of PMMA-multicomponent coatings on wood and wood-based substrates)

Reporting period: 2018-09-01 to 2020-08-31

This action will implement the new Plasma-Enhanced Chemical Solution Deposition (PECSD) technique for coating of wood and wood-based substrates. This technique synergistically employs plasma-chemistry in the gas phase and polymer chemistry in the liquid formulation, thus combining all benefits of conventional surface coatings and plasma-based deposition technologies.

This novel integrated process for the continuous coating of substrates includes the three steps: plasma pre-treatment, application of PMMA-based liquid formulations, and film formation via fast plasma curing.

This overall goal is divided into three main objectives:
Objective I: Building the integrated device,
Objective II: Optimization of the deposition parameters, and
Objective III: Demonstrating the technique’s capability and priming the industrial implementation.

These objectives can only be achieved in the proposed collaboration, i.e. with my knowledge of plasma deposition and design of suitable devices, the wood coating experts’ knowledge of surface wood finishing at the University of Ljubljana, as well as the knowledge of plasma and surface diagnostics at the Jožef Stefan Institute. This complementarity is unique in all of Europe and will ensure the success of the action. Further, the knowledge exchange will not only help me to implement a stable career path in research and innovation, but also yield a cross-border propagation of this new and innovative field of research. The cross-cutting objectives especially extend the new technique to contribute to the pan-European sustainable development by implementing a novel environmentally friendly production technique that will allow to use renewable (e.g. wood-based) resources in novel settings.
During the project, twelve different plasma devices with a total of eight different high voltage power supplies were successfully developed, implemented, characterized, and tested.

The adjustment of plasma pre-treatments for optimized wetting or liquid uptake were more complex on more levels than expected. In particular, previously unknown effects were observed, indicating influences by plasma surface treatment deeper inside the work pieces than previously reported. Changes in moisture content, conductivity, and dielectric parameters as well as with an apparent impact on the modulus of elasticity, which were found to occur at least 10 – 100 µm deep inside the material. In addition to that, these changes of substrates’ properties have an influence on the plasma parameters during the ongoing surface treatments. Although not apparent in chemical composition through infrared spectroscopy, these effects have strong implications on how to implement processes for functionalization of wood surfaces by plasma treatments, while also rendering possible additional potentials for applications, e.g. in microwave drying of wooden work pieces.

The preliminary results on plasma curing of liquid precursors were carried out in a closed reactor setup. For the envisioned applications, however, discussions with the mentor as well as with potential industrial users revealed that a closed batch reactor did not come into account. Despite testing various kinds of non-thermal plasma, no working solution for curing of precursor formulations in an open system was found. Instead, the plasma led to such enhanced desorption or evaporation even for the larger molecules, that plasma-induced reactions were not successful.

However, the works led to a new, innovative technical solution for conventional plasma treatments. A critical parameter during plasma treatments is the precise control and the repeatability of the working distance or discharge gap. Typically, this is achieved through elaborate positioning systems and includes additional safety mechanisms. It is, however, problematic for work pieces with changing dimensions or an unsteady feed mechanism. Based on the experiences with other plasma devices, the novel solution utilizes a double- or multiple-lever system that mechanically maintains the correct work distances without the need of an external control system. This "PlasmaSwing” was implemented and tested during the project, and a patent application was filed.
The Plasma-Enhanced Chemical Solution Deposition (PECSD) method is an entirely new concept, extending hitherto existing plasma deposition techniques by a whole dimension, i.e. all methods known from wet coatings. The approach was very promising particularly for exterior use and novel applications of wood-based and composite materials. In early trials using a small, enclosed plasma reactor, coating films with chemical and structural gradients were formed on a variety of different substrates (e.g. glass, metals, polymers). Unfortunately, the transfer into a scalable, industrially-suited open plasma device during the action was not successful, thus preventing the exploitation of the process for wood-based industrial applications.

Although cold plasma discharges have been successfully used for the surface finishing of wood, there has barely been any uptake in the wood-based industry sector. The collaborations with partners from the corresponding industrial sector were utilized during the project, to try and identify the obstacles, and to prime a more wide-spread utilization of the technology.

The development of simple implementations for various non-thermal plasmas has led to the establishment of a Plasma Center at the Biotechnical faculty, University of Ljubljana. The construction files are being openly published, in order to enable access to plasma technology for research groups, small companies, hobbyists and other interested parties, that were previously unable to use the plasma technology due to limited budgets. Moreover, the construction details are provided on collaborative platforms to allow a participation of interested citizens in advancing this field of science.
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