Coatings that impart hydrophobic properties are of considerable interest. For applications such as aircraft windows, optical components, protective eyewear, and clothing, this type of surface is desired for the material to be soil-repellent and water-resistant. A prime model of a surface with these characteristics can be found in nature – the leaves of the lotus flower have super-hydrophobic properties as a means of self-cleaning.

A cylindrical atmospheric pressure plasma system was used to pretreat the UHMWPE. This system consists of two high-voltage electrodes and one grounded electrode.

To achieve a super-hydrophobic surface, the surface energy and surface roughness of a material play a key role and must be investigated. The lower the surface energy, the higher the hydrophobicity of that surface, and to further increase the hydrophobicity, an appropriate surface roughness is required. The goal for this study was to develop a low-cost, thin hydrophobic coating comprised of silica nanoparticles with pendant fluorocarbon chains extending toward the surface. To achieve this hydrophobic surface, wet chemistry and atmospheric pressure plasma treatment techniques were used on ultra-high-molecular-weight polyethylene (UHMWPE).

Through wet chemistry techniques, the surface roughness of the UHMWPE was increased physically by depositing silicon dioxide (SiO2) nanoparticles on the surface of the sample. Plasma-enhanced chemical vapor deposition under atmospheric pressure was used to deposit a fluorocarbon coating. This technique does not require the use of vacuum equipment, is capable of large area deposition, produces minimal waste, and is capable of controlling the coating chemistry.

Prior to any other treatment, a cylindrical atmospheric pressure plasma system was used to pretreat the UHMWPE. To ensure that silica particles adhered to the UHMWPE film, a pretreatment using the cylindrical atmospheric plasma system was used. The pretreatment serves to create reactive chemical groups on the surface of the inert UHMWPE for subsequent processing. After the pretreatment of UHMWPE films using the cylindrical atmospheric plasma system, silane coupling chemistry was used to incorporate nanoparticles for a textured surface to increase the surface roughness of the material. A micropulsed atmospheric pressure plasma jet (APPJ) system was used to deposit a fluorocarbon coating on the nanotextured UHMWPE.

The wetting behavior of UHMWPE films that were pretreated by the cylindrical plasma system and coated with different weight-percentages of LudoxTMA were studied through WCA. Generally, UHMWPE films exposed to He-H2O plasma at longer pretreatment times were found to contain the highest concentration of silica nanoparticles on the surface of the sample. This is a result of the higher surface concentration of functional groups created on the surface of UHMWPE films. In relation to the 0.5-wt% Ludox and 1-wt% Ludox, the 0.75-wt% Ludox was found as the weight-percent loading solution with the highest contact angle.

Microstructural analysis shows that the processes used to create the hydrophobic coating proved to be successful. A fluorocarbon coating was successfully deposited onto the UHMWPE. There was an approximately 25% increase in water contact angle (WCA) measurements of the bare substrate after 1 minute of exposure to the plasma. It was found that longer pretreatment times resulted in more Ludox silica nanoparticles adhering to the surface of the UHMWPE, leading to lower WCAs, and that longer deposition times resulted in a higher fluorine concentration in the coating.

This hybrid methodology of using combined plasma and wet chemistry was a very cost-efficient method for creating water repellency, by creating nanotextured features through SiO2 nanoparticles and depositing fluorocarbon-like coatings to achieve hydrophobicity on surfaces.

This work was done by Amanda S. Weerasooriya, Jacqueline Yim, Andres A. Bujanda, and Daphne Pappas of the Army Research Laboratory. ARL-0171

Aerospace & Defense Technology Magazine

This article first appeared in the December, 2014 issue of Aerospace & Defense Technology Magazine.

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