Polymer Engineering and Science is a publication providing coverage and analysis of the field of chemical engineering.
INTRODUCTION Packaging is an ideal application for digital printing because of the amount of variable information involved. Traditionally, packaging materials have been printed with offset, gravure, or flexography (1), but in this study the printing has been conducted with a digital printing technique, i.e., dry-toner electrophotography. In digital printing the information can be flexibly changed and easily updated. Digital printing allows customization and personalization for selected target groups. Compared with the traditional printing techniques, the volumes are quite small; digital printing is mainly suitable for short runs of highly personalized packages. The obvious benefit of digital printing is the low waste amount and print-on-demand. The status of digital printing in the packaging sector is still rather moderate, but the future looks bright, and estimates show that the use of this method will increase more than that of the traditional printing techniques (2). The quality of digital printing will also approach that of the traditional techniques. The extrusion coatings used in packaging applications include polyolefins (polyethylene, PE; polypropylene, PP), copolymers (e.g., ethylene methyl acrylate, E/MA), adhesives (e.g., acid copolymers), barrier polymers (e.g., polyamide, PA), and other polymers (e.g., polyethylene terephthalate, PET) (3). Based on a pilot trial that included a polymer from each of these groups, three polymers were chosen for more detailed examination: low-density polyethylene (PE-LD), ethylene methyl acrylate (E/MA), and polyethylene terephthalate (PET). PE-LD is the highest-volume polymer used in the extrusion coating industry. It is easy to process, it provides an adequate moisture barrier, and it has excellent chemical resistance and heat-sealing properties. In addition to these advantages is the low cost (3, 4). The only problem with PE-LD observed during the pilot trial is the low digital print quality. E/MA is a copolymer of ethylene and methyl acrylate containing typically 5%-20% methyl acrylate. E/MA adheres well to various substrates, and its heat-sealing properties are better than those of PE-LD (3). E/MA had the best print quality of all the polymers used in the pilot trial. PET films outstanding properties as a food-packaging material are its excellent chemical resistance and stability over a wide range of temperatures (-60[degrees]C to 220[degrees]C) (3, 4). PET also had good printability properties. With extrusion coatings, a common problem is the inadequate adhesion between a polymer coating and the printing ink. The most common methods to improve adhesion properties of polymer coatings are different surface treatments. These include flame, corona, and ozone treatments (3). The corona-discharge treatment is widely used to increase the wettability and the adhesion properties of extrusion coated boards (5, 6). With corona treatment, the surface energy of the printing material is increased by subjecting it to a high-voltage electrical charge. The surface molecules, previously arranged at random, are brought into a common pattern by this electrical charge. During corona treatment, three phenomena occur: (a) a chemical effect, (b) a physical effect (micropitting), and (c) an electrostatic effect (electret effect) (7-10). The chemical effect includes surface oxidation and formation of low-molecular-weight compounds. The corona-treatment effect is based on bombarding the surface of a polymer with electrons. When these come into contact with the polymer surface, they have so much energy that they break the molecular bonds between carbon-hydrogen or carbon-carbon, and, in consequence, very reactive free radicals form on the surface. At the same time, ozone and oxygen radicals form that react with the radicals on the surface. As a result, chemical functional groups form on the substrate surface. Polar functional groups that can form and enhance bondability include carbonyl (C=O), carboxyl (COOH), hydroperoxide (-OOH), hydroxyl (-OH), and amide (-CONH[.sub.2]) groups. Even a small amount of these reactive functional groups incorporated into polymers can be highly beneficial to improving surface characteristics and wettability (3, 7, 9, 11-15). Some polymers already contain these reactive groups in their basic structure, for example, the methyl acrylate groups of E/MA are highly polar (see Fig. 1) (3). In contrast, polyolefins, like PE-LD, need corona treatment to form these groups into their structure. With polyolefins, the treatment polarizes the long-chain polymer molecules into shorter-chain fragments and oxidizes the ends of these smaller sections, creating mainly ether and hydroxyl groups (16).
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