- May 01, 2007, By Edward D. Cohen Edward D. Cohen Consulting Inc.
Coating defects are defined as the lack of something necessary for completeness or adequacy or as an imperfection that impairs worth or utility. Unfortunately, in normal operation the web coating process can generate a wide variety of defect classes that meet this definition.
Typical defect classes are physical defects, such as spots and streaks; process performance defects, such as low line speeds and productivity losses; and product performance defects, such as inadequate adhesion and haze. When these defects occur, effective characterization of the web coating process is essential to identify and eliminate them and prevent their recurrence.
In addition, since many coating defects can have a variety of causes, characterization will help determine the true cause. For example, mottle is an irregular pattern on the surface of a coating. It is a weak pattern that can vary in severity.
Slight variations in coating weight cause the mottle. This variation can result from several potential causes. Air trapped between the substrate and a smooth coating roll can cause the substrate to buckle, leading to coating weight variations. Non-uniform and high air velocity in the initial drying zone will lead to a disturbance of the coating and variations in coating weight and mottle. Low-viscosity coating solution from low-percent solids or high temperature will make the coating easier to distort. For this defect, characterization data from the process is needed to identify the cause.
A repellant spot is a round or elongated area in a coated product where there is either no coating or a reduced coating thickness. The reduction is caused by contamination, which prevents the coating solution from completely wetting the substrate.
The possible sources of contamination are oil, dirt, or grease from the substrate manufacturing process and from motors and bearings in the coater web transport system. Contaminants also come from raw materials in the coating solution, coater dryer air and air-handling systems, and ambient air in the coating room. Chemical analysis is needed to identify the composition of the contaminant and its source.
The first step in the defect reduction process is to characterize the defect and the process conditions under which the product was manufactured. The characterization should be done before any corrective actions are taken to reformulate the coating, change raw materials, or adjust process conditions. This characterization data should be analyzed to obtain information about the defect and to suggest corrective action. This approach offers the following advantages:
- Provides an accurate description of the defect properties
- Establishes process conditions when the defect was manufactured and when good material was being made
- Identifies potential causes for the defect
- Suggests corrective action
- Improves process economics by reducing defect losses
- Identifies regions on the web where a process cannot operate optimally
- Identifies opportunities for process improvement.
Determining Defect Causes
There are two basic causes of defects in the web coating process. The first is that something in the process, formulation, or coated product has changed.
This is the predominant cause of most defects in the manufacturing process. Defect-free material has been produced in the previous scale-up and manufacturing of the product. However, some process or product element has changed, which results in defects in current production. Examples of this cause are as follows:
- Coating contamination such as spots, repellents, comets, etc.
- Out-of-limits coating weight
- Coating is not drying
- Substrate is not being wetted by coating solution.
The second cause is that a fundamental scientific principle of the coating process is being violated. These causes usually are corrected during scale-up and they must be resolved for routine manufacturing.
This cause can lead to coating defects if process conditions are changed so that they violate a fundamental principle.
Following are examples of this type of cause:
- Coating remelt because drying temperatures are above the coating melt temperature, which results in a distorted coating
- Distorted substrate because the coater tension in the web transport system is above Young's Modulus
- Ribbing because changes in coating variables have resulted in a region of fundamental coating instability.
In both of these cases, the starting point to eliminating these coating defects is to characterize the product and process to determine the differences between the normal and defective product.
Identifying these differences will provide information as to the cause of the defect and the changes needed to return to defect-free product.
Once a representative sample of the defective coating has been obtained, there are a variety of analytical techniques that can be used to characterize the defective material.
A good starting point is to examine the sample visually and develop a “map” that shows the exact location of the defects.
The locations of defects on a long length of product are measured, and then a plot is made that shows their frequency and location.
This provides insight into the defect causes and often can lead to its rapid elimination. It also permits appropriate samples to be selected for further analysis.
Figure 1 is a map of a repeat spot defect, which is caused by contamination on a coater roll. Each point represents a spot location.
When plotted, there was a repeating spacing of 12.57 in. between spots located 12.0 in. from the web side.
The map indicates the spots are coming from a 4-in. roll, which has a circumference of 12.57 in., and the spot-causing material is 12.0 in. from the side. The defect-causing roll then can be located easily and the contamination removed. A defect map of a scratch in the coating will indicate possible streak-causing locations.
On-line instrumentation is effective in locating and characterizing defects. On-line surface vision systems inspect the entire moving web for physical defects. When defects are detected, a defect map is produced, which indicates the defect locations and frequency. In addition, images of the defect are obtained, which can be used to identify it. This characterization can be used to determine defect type and to start corrective action.
In-line coating weight measurements scan the entire moving web and produce defect maps, which show machine-direction and transverse-direction coating weight profiles along with calculation of average values. If product is out of limits, adjustment can be made — while coating — to obtain in-limits values without machine downtime. Spikes in the profile can indicate errors in coating applicator setup.
When physical defects such as spots, comets, repellents, dirt, etc., are found, the defect area and a good area should be characterized to determine the chemical and physical differences between them. The measured differences will be the basis for the corrective action to be taken.
These are the basic analytical techniques that can be used for this analysis:
- Video microscopy to produce visible images of structures or details too small to be seen by the human eye
- Scanning Electron Microscopy to produce high-resolution images of a sample surface at magnifications of 200-35,000
- Energy Dispersive X-ray to measure chemical composition from sodium to uranium
- Electron Spectroscopy for Chemical Analysis to provide both elemental and chemical bonding information about a sample surface (it can detect all elements from lithium to uranium)
- Infrared microscopy to view defects optically and obtain their IR absorption spectra.
The first analysis should be done with optical microscopy. Unknown particles in the defect can be identified by using available resources that have micrographs of standard particles such as the McCrone Atlas of Microscopic Particles (www.mccroneatlas.com).
Often this analysis will give sufficient information to determine the defect cause and start corrective action. The selection of the subsequent methods depends on what information needs to be determined and the instrument availability.
During normal production, key process variables are measured and controlled to ensure a stable, defect-free process. This data should be stored in a computer database so it can be accessed readily and analyzed to determine changes in the process that were present when defective product was manufactured.
Table I outlines process measure-ments needed to characterize a coating machine. The majority of these are part of the control loops used to maintain needed process conditions, i.e., line speed, dryer air conditions, etc.
In addition to the control loops, variables such as dryer nozzle velocity, ambient air conditions, in-coating-room measurements, and filter pressures are important to coater operation and defects and should be recorded. If these are not part of your standard coater measurement and control systems, there are portable instruments that can be used to measure them.
For a ribbing defect, the process variables that influence ribbing, such as viscosity, wet coating thickness, line speed, and coating roll speeds, should be obtained from the process database and an analysis run to determine the differences that led to the defect.
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; firstname.lastname@example.org.
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.
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 the author(s).
For more information on coating defects, see Ed Cohen's article, “Learning the Lingo,” in the February 2004 issue or visit us online at www.pffc-online.com.
|Tension levels through web path|
|Drive motors performance|
|Coated roll length|
|Unwind & Winder|
|Percent solids, temperature|
|pH, particle size|
|Gap between applicator and substrate|
|Vacuum levels for die coating|
|Flow rates to applicators|
|In all zones|
|Dry bulb, web bulb, or dew point|
|Slot velocity in select nozzles|
|Coater room conditions|
|Temperature & relative humidity|