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FLEXO Magazine : January 2009
TECHNOLOGIES & TECHNIQUES electrically grounded roll. In the corona treating system, the volt- age buildup ionizes the air in the air gap, creating a corona, which will increase the surface tension of the substrate passing over the electrically grounded roll. The controllable variables for optimiz- ing corona treatment are treatment gap (1.5mm is a norm) and watt density, a factor determined by the following equation: Watt Density = Watts Treat width (ft.) x Line Speed (fpm) x # of Sides Treated With flame treatment, a high-volume, low-pressure centrifugal air blower moves a column of air through a venturi mixer. The venturi section is adapted with a needle valve to pull gas (typically natural gas) according to demand from a pressure-regulated gas line. The resulting air/gas mixture (typically a stoichiometric ratio of 10 parts air to 1 part gas) is conveyed to the burner face and ignited. The flame’s oxidizing zone, which is optimum at 3/8in. to 1/2in. from the flame tip, impacts on the substrate surface and the excess oxygen activated by the high temperatures combines with carbon molecules to form the polar groupings generally as- sociated with an oxidized surface. form numerous reactive species. It is the interaction of these excited species with solid surfaces placed in opposition to the plasma that results in the chemical and physical modification of the material surface. The effect of plasma on a given material is determined by the chemistry of the reactions between the surface and the reactive species present in the plasma. At the low exposure en- ergies typically used for surface treatment, the plasma surface interactions only change the surface of the material; the effects are confined to a region only several molecular layers deep and do not change the bulk properties of the substrate. The result- ing surface changes depend on the composition of the surface and the gas used. Gases, or mixtures of gases, used for plasma treatment of poly- mers can include nitrogen, argon, oxygen, nitrous oxide, helium, water vapor, carbon dioxide, methane, ammonia, and others. Each gas produces a unique plasma composition and results in differ- ent surface properties. For example, the surface energy can be increased very quickly and effectively by plasma-induced oxida- tion, nitration, hydrolyzation, or amination. Depending on the chemistry of the polymer and the source gases, substitution of The difference between a treated surface that allows the dyne solution to wet out, and an untreated surface that repels the dyne level because of its low surface energy. The treated surface is approximately one molecular layer thick. Flame (residual oxygen) analyzers compensate for changes in am- bient temperature (gas composition and humidity) to maintain a proper oxidizing flame. Burner capacity or firing rate is automati- cally adjusted to changes in line speed. As such, the controllable process variables for optimizing flame treatment are: 1. Air-to-gas ratio. 2. BTU (kJ) output of the burner. 3. Distance of material surface from flame tips. 4. Dwell time of surface in oxidizing zone (line speed). The atmospheric plasma treatment process consists of ex- posing a polymer to a low-temperature, high-density glow discharge (i.e., a plasma). The resulting plasma is a partially ion- ized gas consisting of large concentrations of excited atomic, molecular, ionic, and free-radical species. Excitation of the gas molecules is accomplished by subjecting the gas, which is deliv- ered within an open station design, to an electric field, typically at high frequency. Free electrons gain energy from the imposed high-frequency electric field, colliding with neutral gas mol- ecules and transferring energy, dissociating the molecules to molecular moieties into the surface can make polymers very wet- table. The specific type of substituted atoms or groups determines the specific surface potential. For any gas composition, three surface processes simultane- ously alter flexible packaging substrates, with the extent of each depending on the chemistry and process variables: ablation, crosslinking, and activation. In the ablation process, the bom- bardment of the polymer surface by energetic particles (ie, free radicals, electrons, and ions) and radiation breaks the covalent bonds of the polymer backbone, resulting in lower-molecular- weight polymer chains. As long molecular components become shorter, the volatile oligomer and monomer byproducts vaporize off (ablate) and are swept away with exhaust. Crosslinking is done with an inert process gas (argon or helium). The bond breaking occurs on the polymer surface. But since there are no free-radical scavengers, it can form a bond with a nearby free radical on a dif- ferent chain (crosslink). Plasma activation is a process where surface polymer function- al groups are replaced with different atoms or chemical groups from the plasma. As with ablation, surface exposure to energetic species abstracts hydrogen or breaks the backbone of the poly- www. f le xography. org JANUARY 2009 FLEXO 35
End of Year 2008