Assembly - February 2009 - (Page 40) ASSEMBLY Adhesive Bonding Energizing LowSurface Energy Plastics Plasma treatment raises the surface energy of plastics to make them more amenable to bonding. P Plasma treatment makes plastic surfaces more wettable for adhesives, inks, paint and metallization processes. Here, a plastic cell phone housing undergoes plasma treatment prior to painting. Photo courtesy Plasmatreat North America Inc. By John Sprovieri Editor olypropylene is used in a wide range of assemblies, because of its toughness, flexibility and resistance to heat, moisture and chemicals. In the medical device industry, it’s used for syringe barrels and fluid containers. In the appliance industry, it’s used for motor housings in vacuum cleaners, pump housings in dishwashers, and bodies for coffeemakers and steam irons. In the automotive industry, it’s used for bumpers, body panels, exterior trim, dashboards, fuel tanks, intake manifolds and battery cases. However, the same properties that make polypropylene good for containing hot water and battery acid also make it difficult to bond with adhesives. The plastic has such low surface energy—just 30 dynes per square centimeter, depending on the formulation—that adhesives can’t adequately wet the surface to get a grip, whether chemical or mechanical. To bond polypropylene and other resistant plastics, engineers need to raise the material’s surface energy. One way to do that is by exposing the surface of the plastic to plasma, the so-called fourth state of matter. Plasma is a mixture of free electrons, ions, radicals and molecular fragments created when energy, such as electricity or microwaves, is applied to a gas. Treating plastic with plasma improves bondability in several ways. It removes grease and other organic contaminants that inhibit adhesion. It etches the surface of the plastic at a microscopic level, which improves the bond’s mechanical strength. And, most importantly, plasma chemically activates the surface of the plastic, making it more wettable and more likely to react with an adhesive. “Plasma breaks up any long-carbonchain contaminants on the surface,” explains Tim Smith, vice president of operations at Plasmatreat North America Inc. (Mississauga, ON). “Once these contaminants have been removed, the plasma can react directly with the surface of the substrate. Oxygen molecules from the ionized gas are inserted into the hydrocarbon substrate to produce hydroxyl and carbonyl functional groups. It is through these water-soluble functional groups that a polar surface with high energy is produced. “Most solvent- or water-based adhesives and coatings will form a strong chemical bond with these functional groups, resulting in adhesion properties that could not have been achieved without plasma treatment.” Besides polypropylene, plasma treatment is effective on such polymers as ABS, PVC, polycarbonate, polyethylene, polystyrene, Santoprene, silicone, Teflon, Mylar and nylon. However, energy, exposure time and other 40 ASSEMBLY / February 2009 www.assemblymag.com http://www.assemblymag.com
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