Surface Treatment - Page 5

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Chemical priming. Chemical primers can provide improved printing and adhesion characteristics by applying a chemically distinct layer on the substrate. This is usually accomplished by applying a liquid material in the form of a thin film and then drying off the solvents to leave a desired resin coating. Many polymer surfaces in the form supplied by the manufacturer can generate problems with respect to printability or the adhesion of decorative or functional coatings. Many packaging grade polymers are treated for improved adhesion, but chemical priming can also be used to improve productivity of converting processes. When primers are used on low-surface-energy substrates such as polyolefins, printing defects can be greatly reduced and issues such as screening, mottling, and "fisheyes" can be virtually eliminated. In the printing industry, press-speed limitations are seldom a function of solvent retention, but rather of adequate ink adhesion (7). As press speed increases the effectiveness of high-energy density treatments decreases. The fact that printing primers have the same surface tension characteristics at all press speeds provides a productive advantage as long as there is adequate drying capacity. As a result, maximum press or laminator speeds are attainable as long as the primer and subsequent printing inks can be dried. Unlike corona or flame treatment methods, primed surfaces tend to remain unaltered and the effect of additive migration to their surfaces appears to be limited. Primers can fall under various chemical classes such as polyethyleneimine, polyurethanes, acrylates, and chlorinated polymers. To prime foil substrates for printing or other subsequent converting operations, solvent-based solutions of nitrocellulose and shellac are still used. However, the trend is toward specific high-performance water-based primers such as ethylene acrylic acid. The main drawback to chemical priming is that there is no universal primer and different materials are needed for specific end-use requirements.

Flame treatment. In flame treatment, the polymer surface is passed through a flame generated by the combustion of a hydrocarbon (typically natural gas). Flame treatment can be conducted in a variety of configurations (illustrated in Fig. 1). Usually, containers or polymer webs are passed through a bank of flame jets at a given speed to provide the desired properties. In direct flame treatment, the high temperature (adiabatic flame temperature is approximately 33,000'F) is sufficient to dissociate nitrogen and oxygen molecules into free atoms (8). In addition, this high-temperature plasma contains carbon, free electrons, positively charged oxygen, and other ions and excited species. Because of this reaction, polar functional groups such as ether, ester, carbonyl, carboxyl, and hydroxyl are contained in a flame plasma; these are incorporated into the surface and affect the electron density of the polymer material. The result is that the polymer surface is polarized. By changing the polymer surface from nonpolar to polar, the ink adhesion, laminating, and metallizing characteristics are enhanced. Also, exposure to the open flame oxidizes the surface and burns off surface contamination such as material additives, processing aids, or organic contamination such as oils or grease (9). It is probable that some of the polymer chains actually undergo melting, which "locks" their positions on cooling with respect to the three-dimensional configuration of the substrate, restricting rotation of the polymer molecules. Polar functional groups tend to stay in place on the surface, which can explain why the surface change due to flame treatment does not decay like that due to corona treatment. This process is somewhat energy-intensive, and it may be difficult to reach recessed areas and to evenly treat complex shapes. Also, care must be taken to prevent thermal damage to sensitive materials such as thin-walled plastics or film substrates, and higher-energy output is necessary as production speed or throughput are increased.

Figure 1. (a) Ring burner for round-bottle treatment; (b) burner arrangement for treatment of round plastic bottles.

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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.