RESULTS AND DISCUSSION Water Contact Angle Water contact angle measurements as a function of the nutplate surface preparation are summarized below in Table 16,7. The nutplates with no surface preparation exhibited a WCA of 139±2°. This is a very large contact angle that suggests the surface of the nutplate is contaminated during storage resulting in a non-wetting and extremely hydrophobic surface. A solvent wipe of the nutplate surface results in a contact angle which is still hydrophobic. Plasma activation of the nutplate produces a hydrophilic surface by decreasing the WCA from 114±2° after DS-108 wiping to less than 5° after plasma activation. The nutplates which were activated by the NPC exhibit super hydrophilic wetting behavior which is also significantly lower than grit blasting which yielded a 33±3° WCA. Figure 5 shows images taken from the Krüss goniometer of water droplets on the stainless steel nutplates. The droplet on the left of Figure 5 was measured after solvent wiping with DS-108 and the droplet in the image on the right was after cleaning with the Nutplate Plasma Cleaner. The water droplet is observed to bead up on the solvent wiped sample. The water exhibits a very large contact angle that indicates a non-wetting, hydrophobic surface. This is in stark contrast to the behavior of the nutplate after cleaning and activation with the NPC. The water droplet spreads out on this surface due to the strong attractive forces between the water and the nutplate surface. This is indicative of complete wetting and is a necessary condition for producing a strong adhesive bond. The Aircraft Plasma Cleaner (APC) has been demonstrated to activate a wide variety of the materials that are typically used as the structural components for nutplate assemblies. This includes multiple composite resin systems and aluminum 70758-10. Data gathered previously using 7075 aluminum8 is presented alongside data for aluminum alloys 6061 and 2024. The effects of plasma surface preparation compared to hand abrasion and solvent wiping are summarized in Table 2. The water contact angle on the bare aluminum 7075 was reduced from 71±7° to 45±4° following surface abrasion. The contact angle was improved by a 5 second exposure to the APC in lieu of the abrasion process. This yielded a wetting angle of 10±1°. Anodized aluminum 7075 had its WCA reduced from 55±4° to 42±1° following surface abrasion compared to 25±1° after plasma treatment. It should be noted that the abrasion process has a damaging effect on the anodization layer which is removed during surface preparation. Meanwhile, cleaning and activation with the APC uses a non-ablative process which leaves the anodization intact. For the 6061 w w w. s a m p e . o r g Nutplate Preparation WCA (°) None 139±2° Solvent Wipe 128±2° Grit Blast 33±3° Plasma <5° Table 1. Water contact angle for nutplates following different surface preparation. Figure 5. Images of a 1 mL water drop on a stainless steel nutplate after DS108 solvent cleaning (left) and after cleaning and activation with the Surfx NPC (right). S E P T E M B E R /O C TO B E R 2 0 2 0 | SAMPE JOURNAL | 21http://www.heatcon.com http://www.sampe.org