Advanced Polymers Surface Preparation and Electrochemical Metallization for Flexible Concentrator Mirrors
Authors: Oriani Andrea Vittorio , Liberale Francesco , Cojocaru Paula , Magagnin Luca 
Published: February 2020

For some added-value and specific applications, including alternative energy, a combination of polymers’ properties with those typical of metals is needed. The best way to obtain it is to cover the selected polymer with a metal layer, in such a way that polymer’s peculiarities, namely lightweight, flexibility and chemical resistance can be coupled and exploited with those specific of polymers, such as thermal and electrical conductivity, barrier to fluids, hardness, tribological resistance and reflectivity. This last property is peculiar when studying an industrially-scalable process for the production of flexible concentrator mirrors, which is the goal of this work.

The main concern of multilayer materials is adhesion, that is an enough strong bond between polymer and metal. Two main classes exist for polymer metallization, namely wet and dry processes1–3. The main polymer surface preparation techniques proposed in the last twenty years to guarantee strong adhesion between polymer and metal include wet chemical etching and dry chemical functionalization2,4–6. In the last twenty years, plasma technology has attracted much interest as a viable alternative to wet etching of polymer surfaces. This is due mainly to the possibility of inducing chemical modification of the polymer surface without affecting bulk properties7,8; moreover, roughness can be finely tuned according to the treatment conditions requested.

In the present work a roll-to-roll DBD equipment for surface modification of films is considered. The focus is that of implementing the laboratory results at industrial scale with the main application being the production of flexible concentrator mirrors for solar thermodynamic. A distinctive trait of this paper is also the use of special polymers instead of commodity ones, in particular fluorinated ECTFE. This polymer has a high transparency, which makes it suitable for the application in flexible concentrator mirrors. Moreover, it is particularly interesting due to its intrinsic chemical inertness.

The polymer is subjected to the plasma treatment to increase its wettability by introducing polar groups and thus allowing the deposition of a metallic layer with good adhesion to the substrate. Some studies on reflection properties of Cu and Ag-coated HALAR® films are performed, thus simulating the behavior of flexible concentrator mirrors. Films and thin plaques of HALAR®ECTFE (HALAR®350LC) were treated by means of a roll-to-roll dielectric barrier discharge plasma equipment operating in air.

Charaxterization was performed on the plasma treated samples in order to determine variation in wettability, chemistry and surface morphology. Different techniques were applied to verify the effectiveness of the plasma treatment, namely comparing the wettability of treated and pristine polymer substrates, their different morphology through SEM and AFM microscopes analysis. The next step in the characterization phase consisted in X-ray photoelectron spectroscopy, which allowed to confirm the presence of nitrogen bearing functionalities. After plasma treatment, metallization tests were executed to assess the effectiveness of the chemical modification with respect to palladium adsorption. Copper and silver layers were electroless deposited. Ahesion-bending tests were performed on the metallized samples, together with SEM and XRD characterization. Great attention was devoted to the evaluation of the applicability of metalized ECTFE films in the production of flexible concentrator mirrors. For this purpose some preliminary studies were performed on reflection properties of copper and silver coated Halar® films, obtained with the proposed process. ECTFE could be a good candidate for this application field thanks to its high transparency in the solar spectral range and for its chemical resistance. Moreover, plasma treatment on this polymer allows uniform, continuous and adherent metal deposition without affecting surface morphology and thus not impacting on the reflective properties of the assembly. The obtained results allow to conclude that this process, although needing further development and optimization, could allow producing flexible concentrator mirrors with good reflectance.

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