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Surface Treatment - Page 10[Technical Publications][Back] Electrostatic discharge treatment. Plastic, as opposed to metal, substrates make good electrical insulators because they are electrically nonconductive and possess high electrical resistivity. The higher the surface resistivity, the lower the surface conductivity. However, those plastic insulating materials that have high dielectric constants can generate and store static electricity. Static electricity is generated when two materials in intimate contact are separated by a frictional force causing electrons to be preferentially stripped from one surface and transferred to the other surface. This causes the electron-rich and electron-deficient surfaces to assume positive and negative charges and this surface polarization results in the generation of static electricity. Unless this charge is dissipated, the static buildup can cause the attraction of dust, lint, sparks, materials-handling problems, shocks, and difficulty in wetting or adhering. Packaging substrates made from polyethylene, polypropylene, polyester, polystyrene, and other dielectric materials at some time during their manufacture are usually subjected to at least one of the many available static control techniques. These fall into two separate categories: charge dissipation and charge neutralization. With electrically conductive materials, dissipation of static charge can he accomplished by simply grounding the charged material. However, this is difficult with nonconducting materials such as polymer films, so one approach is to humidify the work area so that the exposed surface absorbs a thin layer of water that conducts the charge to ground. An alternative method is to shield the surface with antistatic organic compounds. Most antistatic agents fall under the following types: nonionic ethoxylated alkylamine, anionic aliphatic sulfonate/phosphates, and cationic quaternary ammonium compounds (17). Antistats can be applied topically or blended, and their purpose is to retard static buildup and also to rapidly discharge any accumulated charge. Another approach to static elimination is to neutralize the accumulated charge using devices capable of ionizing the surrounding air. This works by exposing electrically neutral atoms in air to an applied electric field of voltage high enough to create positively and negatively charged ions. Because of the bipolar nature of the ionized air, the static charge on a material can be neutralized by the oppositely charged ions present in the surrounding air. Basically, there are three types of air-ionizing devices available: nonpowered, powered, and self-powered. The nonpowered induction type of static eliminator consists of brass brushes mounted on ground straps that come in light contact with the charged material, causing the surrounding air to ionize. Electrically powered static eliminators are powered with a low-amperage high voltage power supply for the ionization of the air. Radioactive self-powered units are similar to electrical static eliminators in design and construction except for the source of power. Radioactive devices are self-propagating, usually consisting of a low-energy source of an alpha-emitting radioisotope such as polonium-210 (210Po). The alpha radiation interacts spontaneously with the air molecules, producing ionization of the surrounding environment. BIBLIOGRAPHY 1. E. Occhiello and E. Garbassi, "Surface Modifications of Polymers Using High Energy Density Treatments," Polym. News, 13, 365-368 (1988) 2. R. S. Gallagher, "Manual Cleaning Relies on Solvent Alternatives." Precision Cleaning, p. 29 (April 1995) 3. T. D. Held, Rinse Aid Technology for Improved Rinsing of Plastic Surfaces, Society of Manufacturing Engineers, 1992, EM92 182. 4. L. E. Rentz, "Proper Surface Preparation," Adhes. Age p. 10 (May 1987). 5. D. Briggs, "Surface Treatments for Polyolefins" in Coating Technology Handbook, Marcel Dekker, New York, 1991, Chapter 9, pp.216-218. 6. H. A. Willis and V. J. 1. Zichy in D. T. Clark and W. J. Feast, eds., Polymer Surfaces, Wiley, New York, 1978, p. 287. 7. R. M. Podhany, "Comparing Surface Treatments," Converting pp. 48-52 (Nov. 1990). 8. J. DiGiacomo, Flame Plasma Treatment-a Viable Alternative to Corona Treatment, Society of Plastic Engineers Regional Technical Conference on Decorating and Joining of Plastics, Sept. 1995, pp. 37-61. 9. G. W. Scott, "Flame Treatment Before Printing Enhances Ink Permanence," Microelectron. Mfg. pp. 60-61 (May 1990). 10. D. Briggs and C. R. Kendall, Polymer 20, 1053 (1979). 11. E. Finson, S. L. Kaplan, and L. Wood, "Plasma Treatment of Webs and Films" in 38th Annual Technical Conference Proceedings for the Society of Vacuum Coaters, Chicago, 1995. 12. S. L. Kaplan, and W. P. Hansen, "Plasma-the Environmentally Safe Method to Prepare Plastics and Composites for Adhesive Bonding and Painting," paper presented at SAMPE Environmental Symposium, San Diego, May 1991. 13. E. Finson, and J. Felts, "Transparent Si02 Barrier Coatings: Conversion and Production Status," TAPPI J. 79(l), 161-165 (Jan. 1995). 14. N. S. Mcintyre and M. J. Walzak, "New UV/Ozone Treatment Improves Adhesiveness of Polymer Surfaces," Modern Plast., 79-81 (March 1995). 15. D. G. Shaw and M. C. Langlois, "Some Performance Characteristics of Evaporated Acrylate Coatings" in 37th Annual Technical Conference Proceedings /br the Society of Vacuum Canters, Boston, MA, 1994, 16. R. Milker, in D. Satas, ed., Coating Technology Handbook, Marcel Dekker, New York, 1991, Chapt. 31, pp. 303-339. 17. R. Gidwani, "Fundamentals of Surface Treatment of Packaging Materials," Am. Lab., 81-89 (Nov. 1983). The Wiley Encyclopedia of Packaging Technology, Second Edition, Edited by Aaron L. Brody and Kenneth S. Marsh - ISBN 0-471-063975-5 © 1997 by John Wiley & Sons, Inc. |
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