Water Wettability and Stability vs Outdoor Erosion of Superhydrophobic PU Films by Plasma Processes

What is it about?

Superhydrophobic surfaces have received a lot of research interest and have promising potential, e.g. for application as self-cleaning surfaces, as anti-corrosive surfaces, in microfluidics or as model surfaces for the study of wetting science. Although many different methods for microstructuring and coating have been successfully developed, it remains a challenge to produce superhydrophobic surfaces on a large scale and with enough durability against outdoor erosion. This could be achieved by using polyurethane films, which are known for their durability, and combining a microstructuring and different plasma processes into an efficient roll-to-roll polymer film functionalization process. In this article, the surface topography and chemistry, the stability against erosion, and the water wetting behavior of superhydrophobic polyurethane (PU) films are described and explained. A hot embossing technique with laser-drilled metal stamps is used to imprint conical microstructures with different geometrical parameters in PU films. Afterwards, about 200 nm thick hydrophobic plasma polymers are deposited by plasma-enhanced chemical vapor deposition (PECVD), using different fluorocarbons (CHF3, C3F6 or C4F8) or hexamethyldisiloxane (HMDSO) as precursors. The surface nanoroughness is altered by different Ar or O2 plasma etching processes. Ar etching of the plasma polymers or O2 etching of the PU films, followed by a thin plasma coating, results in an increased nanoroughness. The stability is characterized by different test methods. The plasma coatings show stability against sand abrasion and in a long-term outdoor test but are completely degraded by an industrial UV/water weathering test. The basics of wetting science and superhydrophobicity are recalled. Advancing and receding contact angle measurements (3 µl volume, 1.79 mm diameter, mean size in the range of raindrops) show that superhydrophobicity is reached on the coated microstructures with base diameters between 35 µm and 50 µm, top diameters between 14 µm and 20 µm and distances between 50 µm and 70 µm.

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Photo by John Thomas on Unsplash

Photo by John Thomas on Unsplash

Why is it important?

Superhydrophobic surfaces have received a lot of research interest and many different methods for microstructuring and coating have been successfully developed at lab scale. However, it remains a challenge to produce superhydrophobic surfaces on a large scale and with enough durability against outdoor erosion. Hot embossing for microstructuring and plasma processes for coating and etching are shown to be useful and efficient processes for large-area roll-to-roll surface functionalization of polymer films. The resulting microstructures are larger than commonly prepared microstructures (D > 35 µm, d > 14 µm, P > 50 µm) but still show superhydrophobicity with water drops of 3 to 13 µl. With polyurethane as the base material, these large microstructures and dense, hydrophobic plasma polymers reach higher stability against outdoor erosion than smaller microstructures or other polymer coatings.

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