- July 01, 2008, By David Argent Contributing Editor
Last month's column focused on dyne levels relating to surface wetting. This column expands the concept to include surface modification to improve adhesion.
The introduction of plastic packaging films more than 50 years ago required surface treatment systems that would run at normal production speeds. In general, plastic films have chemically inert and nonporous surfaces with low surface energy, causing them to be nonbonding to other substrates, inks, coatings, and adhesives.
PE and PP have the lowest surface energy of commonly used plastics and are most often subjected to surface treatment to improve bonding. Surface treatment can be used to improve the bonding of virtually all plastics and some nonplastic materials such as foil and paper. All methods rely on combinations of chemical activation, surface roughening, and surface cleaning. Following is a review of the methods.
Acid etching/chemical treatment of a film involves cleaning, etching, and rinsing steps. The cleaning removes any surface contaminants. The etching involves the use of an acid or oxidizing agents, such as nitric acid (NHO3) or potassium chromate (K2Cr2O7), to change the polymer surface chemically. Finally, the film is rinsed clean of the etching chemicals and dried. This process usually is done following film manufacturing, significantly adding to the final cost of the film. This method often is slow and creates waste disposal issues.
Priming often is done in conjunction with corona treatment to increase surface energy and improve adhesion of a coating, ink, or adhesive. The film is corona treated to increase the surface energy enough to provide good adhesion for the primer coating. A primer is chosen that will provide a high surface energy for good adhesion to the film. Some primers bond chemically to the substrate. An example would be polyethyleneimine, which is a cationic chemical and bonds strongly with treated film surfaces, inks, and coatings that are anionic.
Flame treatment exposes a moving film surface to a gas-fired flame at a high enough temperature to create a plasma. The plasma reacts chemically with the film surface, which adds polar functional groups and increases surface energy.
Corona discharge converts the substrate surface from a nonpolar to a polar state. Ozone is generated during the process. It consists of a high-voltage electrical discharge across a fixed air gap between an electrode and a dielectric, usually a roller for web treatment applications. This discharge forms a corona in the gap between the electrode and the dielectric roller, thus treating the film surface toward the electrode. Corona treatment often is done during film manufacturing and again in-line with a secondary converting process such as printing to increase the film surface energy, often by 10 dynes and more.
Corona treatment of films supplied for further converting operations is not fixed. It can deteriorate over time or become masked by additives, especially slip additives, migrating to the surface. If treatment is too high, it can cause blocking of the wound substrate roll; if too low, it can cause adhesion and wetting problems in converting.
The amount of additives (ppm) in the film has a significant impact on film treatability and retention of corona treatment. The initial impact of higher additive loading is to require higher watt densities to raise the film's surface energy. In addition, higher additive loading will reduce the film's ability to retain corona treatment due to migration of additives to the surface and masking of corona treatment. Additives will migrate more readily to a film surface that has been treated. As a result, in-line retreatment may be required by the converter to achieve adequate wetting and bonding.
Two major developments have occurred to address this problem. Some film suppliers have devised much more stable additive packages, and newly available plasma treatment has created more stable surface energy levels (see PFFC March, p38).
Atmospheric plasma is similar to corona treatment. Like corona, plasma is the electrical ionization of a gas. In contrast, the plasma (glow) discharge creates a smooth cloud of ionized gas with no visible electrical filaments.
At this time plasma is more expensive, but benefits compared to corona include higher and long-lasting treatment levels; no backside treatment; elimination of ozone production; and longer-lasting treatments.
Corona is the most widely used surface treatment method, but developments in plasma are continuing. This method may find increased use in the future.
Process improvement expert David Argent has 30+ years of experience in process analysis with particular emphasis on ink and coating design and performance. Contact him at 636-391-8180; firstname.lastname@example.org.