Surface Treatment - Page 8

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Figure 4 illustrates the components of a typical plasma surface-treatment system. In a conventional plasma process, the chamber is evacuated to a specified pressure using a mechanical vacuum pump and gas is introduced into it through flow controllers. Once the gas flow has stabilized and the desired operating pressure has been reached, the rf power is applied to the electrodes and the gas is ionized. A capacitance-matching network tunes the chamber impedance to a constant load. During normal operation, gas is being continually introduced into the chamber and the unreacted species and byproducts are continuously evacuated. The chamber thus operates in a steady state. Cold-gas plasma offers the engineer a means of reengineering the polymer surface and introducing the desired functional groups in a controlled and reproducible manner. The nature of plasma surface modification lends itself to precise control and process repeatability. In a majority of applications, plasma surface treatment employs innocuous gases that allow the engineer or scientist to radically modify the surface while maintaining workplace and environmental cleanliness and safety (12). The treatment effect can be very long-lived and can be applied to a variety of configurations such as webs of plastic films or three dimensional containers.

Figure 4. Components of a typical plasma surface-treatment system.

Cold-gas-plasma processes can also be used to apply transparent thin-film silicon oxide-based gas-barrier depositions for flexible-packaging applications. Common polymer packaging films such as polyethylene terephthalate (PET) and biaixially oriented polypropylene (OPP) can be used as substrates for plasma-deposition processes. A common process involves the plasma decomposition of 1,1,3,3-tetramethyldisiloxane (TMDSO) or hexamethyldisiloxane (HMDSO) in a helium and oxygen plasma to form a gas and vapor barrier layer of SiO2 (Fig. 5). The process has been successfully commercialized into production equipment that can accommodate 1.5 meter web widths and run up to 100 m/min. (13). The resulting barrier films can then be processed through typical converting steps to form transparent high-barrier flexible packaging. This approach can improve the shelf life of packaged foods or products and offer an alternative to conventional packaging gas-barrier technologies.

Figure 5. Low-pressure SiO2 plasma deposition process.

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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.