PFFC - July 2008 - (Page 14)

PROCESS MANAGEMENT Dyne Levels—Part 2 L By David Argent Contributing Editor ast month’s column focused on dyne levels Corona discharge converts the substrate surface relating to surface wetting. This column from a nonpolar to a polar state. Ozone is generexpands the concept to include surface ated during the process. It consists of a high-voltage modification to improve adhesion. electrical discharge across a fixed air gap between The introduction of plastic packaging films an electrode and a dielectric, usually a roller for more than 50 years ago required surface treatment web treatment applications. This discharge forms systems that would run at normal production a corona in the gap between the electrode and the speeds. In general, plastic films have chemically dielectric roller, thus treating the film surface toward inert and nonporous surfaces with low surface the electrode. Corona treatment often is done during energy, causing them to be nonbonding to other film manufacturing and again in-line with a secondsubstrates, inks, coatings, and adhesives. ary converting process such as printing to increase PE and PP have the lowest surface energy the film surface energy, often by 10 dynes and more. of commonly used plastics and are most often Corona treatment of films supplied for further subjected to surface treatment to improve converting operations is not fixed. It can deteriorate bonding. Surface treatment can be used to over time or become masked by additives, especially improve the bonding of virtually all plastics and slip additives, migrating to the surface. If treatment some nonplastic materials such as foil and paper. is too high, it can cause blocking of the wound All methods rely on combinations of chemical substrate roll; if too low, it can cause adhesion and activation, surface roughening, and surface wetting problems in converting. cleaning. Following is a review of the methods. The amount of additives (ppm) in the film has a Acid etching/chemical treatment of a film significant impact on film treatability and retention involves cleaning, etching, and rinsing steps. The of corona treatment. The initial impact of higher cleaning removes any surface contaminants. The additive loading is to require higher watt densities etching involves the use of an acid or oxidizing to raise the film’s surface energy. In addition, agents, such as nitric acid (NHO3 ) or potassium higher additive loading will reduce the film’s ability chromate (K 2Cr 2O 7), to change the polymer to retain corona treatment due to migration of surface chemically. Finally, the film is rinsed clean additives to the surface and masking of corona of the etching chemicals and dried. This process treatment. Additives will migrate more readily to a usually is done following film manufacturing, film surface that has been treated. As a result, insignificantly adding to the final cost of the film. line retreatment may be required by the converter This method often is slow and creates waste to achieve adequate wetting and bonding. disposal issues. Two major developments have occurred to Priming often is done in conjunction with address this problem. Some film suppliers have corona treatment to increase surface energy devised much more stable additive packages, and and improve adhesion of a coating, ink, or newly available plasma treatment has created more adhesive. The film is corona treated to increase stable surface energy levels (see PFFC March, p38). the surface energy enough to provide good Atmospheric plasma is similar to corona adhesion for the primer coating. A primer is treatment. Like corona, plasma is the electrical chosen that will provide a high surface energy ionization of a gas. In contrast, the plasma (glow) for good adhesion to the film. Some primers discharge creates a smooth cloud of ionized gas bond chemically to the substrate. An example with no visible electrical filaments. would be polyethyleneimine, which is a At this time plasma is more expensive, cationic chemical and bonds strongly but benefits compared to corona include with treated film surfaces, inks, and higher and long-lasting treatment coatings that are anionic. levels; no backside treatment; Flame treatment exposes a elimination of ozone production; moving film surface to a gas-fired and longer-lasting treatments. flame at a high enough temperature Corona is COEFFICIENT OF used surface the most widely to create a plasma. The plasma reacts treatment method, FRICTION chemically with the film surface, but developments in plasma are which adds polar functional groups continuing. This method may find and increases surface energy. increased use in the future. COMING NEXT MONTH 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; djvargent@sbcglobal.net. 14 | JULY 2008 WWW.PFFC-ONLINE.COM http://www.nxtbook.com/nxtbooks/penton/pffc0308/index.php?startid=38 http://WWW.PFFC-ONLINE.COM

Table of Contents Feed for the Digital Edition of PFFC - July 2008

PFFC - July 2008 Contents First Glance Calendar of Events From the Editor Web Lines Process Management News Clips Narrow Web & Label Reporter Narrow Web & Label What’s New Products Cover: Special Report Start with Security 2008 TAPPI PLACE Conference Printing Partnerships Paper’s Pledge Edible Markers Flexography Exclusive Research: Critical Trends What’s New Products Services Directory Classified Marketplace Advertisers Index Experience Speaks

PFFC - July 2008

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