PFFC - February 2008 - (Page 38)

COATING LAMINATING erties, important for the web coating process, that must be characterized and optimized. These properties and how they affect the web coating process are defined below. Special Report tion of shear rate applied to the solution. This can result in a different type of flow behavior depending on the solution composition. Figure 1 (see p36) shows some of the possible general types of flow curves that can result and Viscosity provides a definition of various Viscosity is a measure of the types of behavior. resistance of a solution to flow Characterizing the shear rate under mechanical stress and is response is essential for several defined as the ratio of the shear reasons. The varying coating stress to the shear rate. It is a re- process components have a wide sult of the internal friction of the range of shear rates: Dip coatmaterial’s molecules. Materials ing shear rate is 10–100 sec-1, and slot die is 3,000–100,000 with a high viscosity do not sec-1. Table I (see p36) gives adflow readily; materials with a low viscosity are more fluid and ditional shear rate values for the coating process. flow easily. Therefore, the coating soluThe viscosity is the slope of the tions must be characterized shear stress vs. shear rate curve. There are a wide range of instru- as close to the process shear rate as possible so an accurate ments that can be used to make understanding of the solution the needed measurements. behavior under process condiThe viscosity level is dependent on the solution temperature, tions is obtained. Viscosity data solids concentration, solvent, and at low shear rate may not indicate process behavior. If the solution binder used. It is also a func- cosity, which also will lead to coating weight variability. u Low-viscosity solutions are easily disturbed by air velocity in the dryer. The air velocity at the entrance to an impingement dryer can distort the coating, leading to mottle, chatter, and partial removal u The viscosity always should of the coating. Increasing the be within the optimum range viscosity by concentrating soof the coating application prolution can cure these defects. cess to be used in order to enIn addition, if the viscosity is sure obtaining defect-free film. not monitored, normal variaIf the viscosity is not in the tion can lead to these defects optimum range, defects will and yield loss. result from chatter, ribbing, Surface Tension streaks, and air entrainment. u Variations in viscosity will Surface tension is a property of affect the applied coating liquids in which the exposed weight by two mechanisms. surface tends to contract to the The wet thickness is a funcsmallest possible area because of tion of viscosity and will unequal molecular cohesive forcchange as the viscosity chang- es near the surface. It is expressed es, resulting in coating weight as force per unit length, such as variations in the dried film. dynes/cm or mN/m (millinewAlso, changes in the solution’s tons per meter). The surface solid content will affect vistension can be measured by viscosity changes with time, the coating performance also will change and compensation must be made. There may be differences in the high shear rate and low shear rate measurements. Other considerations for measuring viscosity are: PFFC-ASAP 129 38 | FEBRUARY 2008 WWW.PFFC-ONLINE.COM http://www.protect-all.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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