PFFC - February 2008 - (Page 36)

COATING LAMINATING Special Report Property Control A coating solution’s physical and chemical characteristics are critical factors in manufacturing a reproducible, high quality product. They must be optimized at each step of the product development cycle. By Edward D. Cohen, Edward D. Cohen Consulting Figure 1. COATING TECHNICAL REPORT Shear Rate Viscosity Behavior Viscosity, cP (Shear Stress/Shear Rate) Shear Thinning This article, along with future articles by other authors, is provided as a cooperative effort between PFFC and AIMCAL. Authors contribute to AIMCAL’s technical and education offerings, which include the association’s Fall Technical Conference, Summer School, and Ask AIMCAL. Shear Rate Curve Deﬁnition Newtonian Shear Thickening u A Newtonian fluid has a constant viscosity independent of shear rate. u A pseudo-plastic or shear-thinning behavior occurs when the viscosity decreases with increasing shear rate. u A dilatant or shearthickening behavior occurs when the viscosity increases with increasing shear rate. u A Bingham solid with yield-stress behavior occurs when the stress has to exceed some finite value before the fluid flows. u Thixotropic liquids, in which the viscosity decreases with time. u Rheopectic liquids, in which the viscosity increases with time. C Shear Rate [sec-1] (Viscosity/Thickness or Gap Width) Table I. Shear Rate Ranges for Web Coating Process Coating Operation Dip Roll, reverse Roll, forward Spray Slide Gravure, reverse Gravure, forward Slot die Curtain Blade Simple mixing High shear mixer Measurement Brookfield ICI Haake Fenske-Cannon 1-300 10,000 1-20,000 1-100 Shear Rate, sec -1 10-100 1,000-100,000 10-1,000 1,000-10,000 3,000-120,000 40,000-1,000,000 10-1,000 3,000-100,000 10,000-1,000,000 20-40,000 10-100 1,000-100,000 Ancillary Operations oater operability and the applicator’s ability to apply coating uniformly—as well as physical defects and final coating quality—all depend upon the properties of the coating solution. The solution properties also will affect drying behavior, the functional performance of the coating, and defect levels. These properties have become more important because of improved product quality requirements, thinner coatings, and the increased use of high-speed, multilayer coating methods. These methods are more sensitive to coating solution properties than older methods, such as Mayer rod, and require additional characterization methods. Coating solution properties must be characterized and optimized at each step of the product development cycle to ensure that product and process properties are within specifications and will produce a high quality, low-cost product. In addition, solution properties must be characterized and controlled in routine productions to ensure a successful web coating manufacturing process. This is often a problem area, because once a product is developed and manufacturing is demonstrated, these solution properties may be minimally monitored and/or controlled in routine manufacture. Some tests are considered for R&D use only and are not used in routine production because of test method complexity, even though they will help control the process. For example, surface tension is a key variable in controlling the wetting and coating quality of a solution or dispersion, yet it rarely is measured in routine manufacturing. The solution characterization test should be capable of predicting product performance in the coater and identifying defect causes. It is preferable to identify defective coating solutions and scrap them before coating, since the cost of scrap significantly increases when it is a coated product. Since raw materials are expensive and in limited supply, it is essential that all produced batches be used. The characterization should assure the solution meets all requirements for the coating application and drying process and is not defective in any way. This is important because none of the subsequent process steps can correct solution deficiencies in the original coating solution, and a defective product will result. Solution Properties There are several solution propWWW.PFFC-ONLINE.COM 36 | FEBRUARY 2008 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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