PFFC - February 2008 - (Page 40)

COATING LAMINATING several techniques. Two of these are the Wilhelmy plate method, in which the force necessary to withdraw a thin platinum plate from the solution is measured; and the duNuoy ring method, in which a ring made out of platinum wire is substituted for the Wilmhelmy plate and bubble techniques. Surface tension is important because it controls the wettability of a coating solution on a substrate. To obtain a coating that uniformly wets the entire substrate, the surface tension should be at least 5 dynes/cm lower than the surface tension of the substrate. Ongoing measurements are needed, particularly when defects are found, to ensure the proper balance is being maintained. The surface tension of the substrate is needed to optimize the surface tension correctly. It can be measured by contact angle measurements using a Special Report surements. If there is setting or agglomeration, mixing tests should be run to determine if they can be reconstituted. The stability of coating dispersions to the shear rates encountered in the feed system in the coating applicator also are important. Often the high shear rates can lead to particle agglomSolution Stability eration, which leads to defects There is often a lag between and loss of effectiveness. when a solution is first comThis can be measured by Solids Content pounded and when it is coated. setting up a small feed system A uniform solids content is This can occur because of a and running in a closed loop. needed to ensure constant vischange in the scheduling or if Monitoring pressure and visual cosity, maintain coating weight, there is excess solution from examination will indicate stabiland ensure coating can be dried a coating run and it must be ity. These results from solution at optimum line speed. The dry coated at a later time for ecostability measurements can coating weight is a function of nomic considerations. Therefore, be used to reformulate for imthe applied wet thickness and the solution stability over an proved stability and to set storthe solids content of that wet extended period of time should age limits. layer. Individual coatings can be determined. Purity have the same thickness wet This can be done by storA coating solution can contain layer, but dry coating wet will ing solutions in a controlled physical contaminants such as vary depending on the solid con- environment and monitoring dirt, oil, grease, lint, metals, tent. If the solids content varies, stability, which can be done human hair, and skin. These then the solvent level will vary, visually and by viscosity meagoniometer. There are also standard solutions that can be used to estimate substrate surface tension easily. In addition, the surface tension is important for the leveling of the wet coating after application and in the formation of defects such as convection cells, craters, dryer bands, fat edges, adhesive failure, and delaminating. which can lead to variations in drying rate and line speeds. Since viscosity and percent solids are correlated, the viscosity can be measured and used to control coating weight. In addition, in-line viscometers can be used as part of a loop to control viscosity (% solids). One tough sucker. As a former drill instructor, Jon Kuether respects “tough” … like the stainless steel screen used on our Isoten™ vacuum rolls. This rugged woven mesh distributes vacuum suction evenly across the roll face. Durable and smooth, it’s the perfect surface for mark-free web handling. Isoten vacuum rolls are available with a wide variety of shell coatings and coverings, including felt screens. With adjustable vacuum zone angles and widths, you’ll have complete control of every web. Time to get tough? Call Webex. Get more than just a roll. Get better web handling. www.webexinc.com 920-729-6666 PFFC-ASAP 131 40 | FEBRUARY 2008 WWW.PFFC-ONLINE.COM http://WWW.PFFC-ONLINE.COM

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

PFFC - February 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 Carton & Box Reporter Offset Printing A Triple Ply: Coating & Laminating Global Market Is Strong for Vacuum-Coated Products Melamine Innovation: A New Choice for Barrier Coatings How To Optimize Your Coating Solution’s Properties What’s New Products Contract Services Directory Custom Marketplace What’s New Products Services Directory Classified Marketplace Advertisers Index Experience Speaks

PFFC - February 2008

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