- February 01, 2008, By Edward D. Cohen, Edward D. Cohen Consulting
Coater 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.
There are several solution properties, 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.
Viscosity is a measure of the resistance of a solution to flow under mechanical stress and is defined as the ratio of the shear stress to the shear rate. It is a result of the internal friction of the material's molecules. Materials with a high viscosity do not flow readily; materials with a low viscosity are more fluid and flow easily.
The viscosity is the slope of the shear stress vs. shear rate curve. There are a wide range of instruments that can be used to make the needed measurements.
The viscosity level is dependent on the solution temperature, solids concentration, solvent, and binder used. It is also a function 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 provides a definition of various types of behavior.
Characterizing the shear rate response is essential for several reasons. The varying coating process components have a wide range of shear rates: Dip coating shear rate is 10-100 sec-1, and slot die is 3,000-100,000 sec-1. Table I (see p36) gives additional shear rate values for the coating process.
Therefore, the coating solutions must be characterized as close to the process shear rate as possible so an accurate understanding of the solution behavior under process conditions is obtained. Viscosity data at low shear rate may not indicate process behavior. If the solution 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:
- The viscosity always should be within the optimum range of the coating application process to be used in order to ensure obtaining defect-free film. If the viscosity is not in the optimum range, defects will result from chatter, ribbing, streaks, and air entrainment.
- Variations in viscosity will affect the applied coating weight by two mechanisms. The wet thickness is a function of viscosity and will change as the viscosity changes, resulting in coating weight variations in the dried film. Also, changes in the solutions solid content will affect viscosity, which also will lead to coating weight variability.
- 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 of the coating. Increasing the viscosity by concentrating solution can cure these defects. In addition, if the viscosity is not monitored, normal variation can lead to these defects and yield loss.
Surface tension is a property of liquids in which the exposed surface tends to contract to the smallest possible area because of unequal molecular cohesive forces near the surface. It is expressed as force per unit length, such as dynes/cm or mN/m (millinewtons per meter). The surface tension can be measured by 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 goniometer. 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.
A uniform solids content is needed to ensure constant viscosity, maintain coating weight, and ensure coating can be dried at optimum line speed. The dry coating weight is a function of the applied wet thickness and the solids content of that wet layer. Individual coatings can have the same thickness wet layer, but dry coating wet will vary depending on the solid content. If the solids content varies, then the solvent level will vary, 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).
There is often a lag between when a solution is first compounded and when it is coated. This can occur because of a change in the scheduling or if there is excess solution from a coating run and it must be coated at a later time for economic considerations. Therefore, the solution stability over an extended period of time should be determined.
This can be done by storing solutions in a controlled environment and monitoring stability, which can be done visually and by viscosity measurements. 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 agglomeration, which leads to defects and loss of effectiveness.
This can be measured by setting up a small feed system and running in a closed loop. Monitoring pressure and visual examination will indicate stability. These results from solution stability measurements can be used to reformulate for improved stability and to set storage limits.
A coating solution can contain physical contaminants such as dirt, oil, grease, lint, metals, human hair, and skin. These contaminants can lead to a wide variety of spot and repellent defects. The coating solution also can contain gel particles from undissolved polymer in the mixing process.
An estimate can be made of contamination levels by visual examination of solution, making a small amount of coating for examination under a microscope. Chemical contaminants or the wrong ingredients occasionally are found in coatings. Thermal analysis and spectroscopic techniques can be used to estimate purity and take corrective action before coating, if needed.
Microscopy also can be used by coating on a transparent substrate and examining the coating. Examples of defects from contaminated solution are shown in Figure 2.
Many coating formulations are dispersions. In these formulations the particle size is important for the product function and coating quality. Therefore the particle size can be measured after dispersion preparation to ensure it is within specifications. Monitoring in the production unit will ensure quality and provide a reference database in the event particle size problems are incurred. Agglomeration of the particles encountered at the shear rates during the coating process can lead to defective product from agglomeration point defects and inadequate product performance.
There are several methods to consider: laser particle size analyzer, Coulter resistance particle counter, and microscopic image analysis techniques. In the last technique, image analysis software calculates particle size and distribution of sample particles being studied under microscope.
Edward D. Cohen is a technical consultant for the Assn. of Industrial Metallizers, Coaters & Laminators (AIMCAL). He has 40+ years of experience in research and manufacturing technology. Contact him at 480-836-9452; email@example.com.
The views and opinions expressed in Technical Reports are those of the author(s), not those of the editors of PFFC. Please address comments to author(s).
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.
|Coating Operation||Shear Rate, sec-1|
|High shear mixer||1,000-100,000|
Shear Rate Curve Definition
- A Newtonian fluid has a constant viscosity independent of shear rate.
- A pseudo-plastic or shear-thinning behavior occurs when the viscosity decreases with increasing shear rate.
- A dilatant or shear-thickening behavior occurs when the viscosity increases with increasing shear rate.
- A Bingham solid with yield-stress behavior occurs when the stress has to exceed some finite value before the fluid flows.
- Thixotropic liquids, in which the viscosity decreases with time.
- Rheopectic liquids, in which the viscosity increases with time.
For a more detailed discussion of viscosity and surface tension in the web coating process, see Coating and Drying Defects: Troubleshooting Operating Problems, second edition, by E.D. Cohen and E.B. Gutoff, John Wiley and Sons, New York, 2006.