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FLEXO Magazine : January 2009
TECHNOLOGIES & TECHNIQUES mer, creating free radicals. In addition, plasma contains very high-energy UV radiation. This UV energy creates additional free radicals on the polymer surface. Free radicals, which are thermo- dynamically unstable, quickly react with the polymer backbone itself or with other free-radical species present at the surface to form stable covalently bonded atoms or more complex groups. Application of atmospheric plasma to finished films has been theorized and practiced to provide specific functionalities to the base film substrate adequate for improved adhesion relative to the corona treatment process. Since atmospheric plasma contains highly reactive species within a high density plasma at atmo- spheric pressure, it is proven to significantly increase surface area and to create polar groups on the surface of polymers, so that strong covalent bonding between the substrate and its interface (i.e., inks, coatings, adhesives), takes place. MEASURING SURFACE ENERGY The object of surface treating is to improve the wettability of the surface, thereby improving the ability to bond to solvents, ad- hesives, coatings, and extrusion coating. In order for a surface to be properly wet by a liquid, the surface energy of the plastic must be higher than the surface tension of the liquid. Surface energy is measured in dynes per centimeter. Ideally, the surface energy of the plastic should be 7 to 10 dynes/cm higher than the surface tension of the solvent or liquid. For ex- ample, a printing ink having a surface tension of 30 dynes/cm would not adequately wet or bond to a material having a surface energy less than 37 to 40 dynes/cm. One common method for quickly measuring surface energy is known as the Wetting Tension Test (ASTM D-2578). Surface energy testing is frequently done on post-treatment surfaces prior to follow-on converting processes. With this test, a series of mixed liquids (Formamide and Cellosolve™) with gradually increasing surface tensions are applied to a treated substrate surface until one is found that wets the surface for a minimum of 2 seconds. The surface energy of the plastic is then approximately equal to the surface tension of that particular mixture. Test solutions are avail- able from various manufacturers of corona treating equipment. The most accurate method of measuring surface energy, when properly performed, is contact angle measurement. The sessile drop method, the most often used in the converting industry, is measured by a contact angle goniometer using an optical sub- system to capture the profile of a pure liquid on a solid substrate. The angle formed between the liquid/solid interface and the liquid/vapor interface is the contact angle. Older systems used a microscope optical system with a back light. Current-generation systems employ high resolutions cameras and software to capture and analyze the contact angle. MEASURING ADHESION Bonding an ink, coating or adhesive to a surface produces a number of overlapping mechanical, physical, and chemical ef- fects that influence one another and that need to be tested prior to final usage. There are many adhesion tests such as tape pull tests, cross-hatched coatings with adhesive tape pulls, and T-peel tests such as ASTM F904–91, which can be used with many dif- ferent types of materials. These tests should be completed as an 36 FLEXO JANUARY 2009 intermediate step after printing, coating, adhesive application or metalizing as a measure of surface treatment optimization. TREATMENT OPTIMIZATION PROTOCOL Maintaining the previous assumption that all material, envi- ronmental and interfacial variables are controlled, the key to opti- mizing the surface treatment of films using corona, flame or at- mospheric plasma pretreatment technologies is to invest in tech- nology which provides the most applicable surface modification and functionalization with the most range of flexibility. So how do you determine which technology is most relevant and flexible to achieve this optimization? Here are the four key metrics: ??Inherent (native) surface tension of all processed substrate(s). ??Surface tension of the interfacial fluid (ink, coating, adhesive). ??Surface chemistry of the substrate(s). ??Bulk chemistry of the interfacial fluid. For example, if the surface tension differential between all pro- cessed substrates and the interfacial fluids applied can be easily bridged by corona treatment as opposed to opting to higher energy treatment processes, such as flame or plasma, then simply a succes- sive series of peel adhesion tests can be performed at progressively higher watt density levels until the optimal peel adhesion level is determined. If the determined levels for all substrates do not meet final converted product specifications, flame treatment should be considered as a possible pretreatment technology by first ascertain- ing if flame’s thermal discharge can match the full range of base material processing speeds without creating material damage. If the match can be made, a watt density/peel adhesion matrix should again be developed for optimizing treatment level. If the oxidative and hydrocarbon chemical surface effects of flame do not meet adhesion requirements, atmospheric plasma should be considered. Because atmospheric plasma systems can utilize variable gas chemistries to functionalize surfaces, knowing the chemical func- tionality of the substrate surface and the bulk chemistry of the interfacial fluid will predetermine gas chemistry protocols to pro- mote covalent interfacial bonding. Applying the same watt den- sity/peel adhesion evaluations across both primary and secondary gas-phase plasma chemistry trial protocols and all substrates should yield parameters for optimizing the surface pretreatment of polymer films. ? ABOUT THE AUTHOR: Rory A. Wolf is vice president of business develop- ment for Enercon Industries Corp. With 27 years of experience in the printing and packaging industry, Wolf is chair- man of the TAPPI (The Association of Pulp and Paper Industries) PLACE (Polymers Laminates Adhesives Coatings Extrusions) Division and chairman for the Society of Plastics Engineers (SPE). He has published numerous technical articles for FLEXO and other magazines throughout the packaging and printing industries. www. f le xography. org
End of Year 2008