Surface Treatment - Page 9

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Ultraviolet/ozone. For this process, the polymer surface is exposed to both uv light and ozone to increase the number of oxygen functional groups incorporated into the material. This approach can be useful in the surface modification of three dimensional parts. The process has been used on polypropylene and polyester substrates and has shown rapid and reproducible uptake of surface oxygen functional groups (14). The attachment of oxygen groups greatly changes the surface energy and chemistry, which can lead to improved adhesion of functional and decorative coatings. Most of the initial process development has been targeted at the treatment of three-dimensional plastic components, These are suspended or tumbled inside a reaction chamber at room temperature and pressure and exposed simultaneously to uv (mercury-source) light and various concentrations of ozone (O3) gas. After 5 min. of exposure, oxygen functional groups can be attached to as many as 30% carbon atoms on the outermost surface of polypropylene. The stability of this treatment is usually quite good. For example, relatively little change in the receding contact angle occurs on the treated surface of polypropylene after aging in air for a period of 28 days.

1. The coating forms a smooth layer on the polymer surface, eliminating any surface irregularities.

2. The coating has very good temperature stability which provides a thermally stable platform on which to apply a barrier material.

3. The acrylate surface is more chemically polar than many polymer films, and the density of the resulting film is higher. Barrier layers composed of metals and inorganic oxides tend to grow more readily on a polar substrate than on a nonpolar substrate.

Fluorination treatment. The fluorination process involves exposing polymeric webs continuously to fluorine gas (F2) diluted with an inert gas (eg, nitrogen) inside a reaction chamber (16). This process can greatly increase the surface energy of polymer materials such that excellent adherence can be attained to other materials such as lacquers and adhesive agents. Diatomic fluorine, an almost colorless gas, is one of the strongest oxidizing agents; it reacts with almost all organic and inorganic substances (except nitrogen and other inert gases). Fluorine's great reactivity is due to the interaction of the low dissociation energy of the molecule and the very strong bonds it forms with other atoms. Electron spectroscopy for chemical analysis (ESCA) data indicate that the activation of polymer surfaces using this process results from the partial fluorination of the hydrocarbon structure of the polymer molecules. An additional application would be the fluorination of high-density polyethylene gasoline tanks to provide hydrocarbon barrier. Fluorine is routinely transported in its liquid state and is commercially available because of use in the nuclear industry for the refinement of fuels. The safeguards used for this technology are similar to safety measures used and approved for ozone generation. Compared with other pretreatment processes, surface fluorination not only has a wide spectrum of applications but also doesn't require the use of electrical equipment such as corona or plasma treatment. Surfaces treated with fluorine exhibit long-lasting, if not irreversible, changes. This can be very important in practical applications in industry, since subsequent converting processes don't have to immediately follow the surface activation.

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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.