By R.W. Stowe, director of applications engineering, Heraeus Noblelight America LLC
The variety of UV radiometers available today makes it difficult to determine which kind of instrument to select to measure a UV process. Its even more difficult to identify exactly what to measure. Ideally, if we had a measurement that would determine when a UV curing process is about to go out of control, or fall below the “red line,” that would be an invaluable management tool. Unfortunately, there is no easy or simple “online cure monitor,” so we monitor the exposure conditions using radiometry.
A radiometric monitoring method is valuable only if the measurements can be related to the operating limits of the process, or the “process window.” The most important limit is the point at which the end product will not pass its performance requirement. This is the “red line.” Because changes and deterioration in any part of the UV system can result in process failure, knowing in advance that the process is nearing “red line” can indicate the need for equipment maintenance or replacement.
When selecting instruments and methods, conflicting temptations face the user, including a desire to:
- find something simple, inexpensive, universal and quick;
- use sophisticated instruments with a credible underlying content of optical technology; and
- find a robust and valid quality control tool to use in production.
Are these compatible?
First, let’s distinguish process design from process monitoring. In the design of any UV application, it is essential to know the UV exposure conditions – irradiance, energy, wavelength range and surface temperature – that result in a successfully UV-cured product. Relating the key end-properties of the product to these conditions requires the ability to measure and quantify both. In a good design, optimization of these factors yields a more effective cure with reduced cost and less heat. A thorough characterization may involve more complex, sophisticated (potentially more expensive) instruments.
Once a process is designed and set, process monitoring seeks to maintain the “process window.” The primary purpose is to know when something has changed before it threatens the process. However, the measurements must be of key exposure conditions as determined in the design process. If the data collected don’t relate to the key factors affecting the cured properties, they don’t mean much.
If you are currently using a UV process, and the key exposure factors have not been identified (or communicated), or the instruments used to establish them are not known, you should find out now. This information allows you to determine the quantitative factors to monitor and the methods you should use.
- If depth of cure and adhesion are important, then the long UV wavelengths should be monitored.
- If surface properties such as tack-, stain-, chemical- or scratch-resistance are important, then the short UV wavelengths should be monitored.
- If the relative irradiance of short and long wavelengths is important, then a periodic check of the ratio of the two will be useful.
- If the photoinitiator wavelength response range is known, it will identify an important range to monitor.
- If curing with LEDs, you are limited to monitoring the principal wavelength. (Caution: It may be outside the UVA band.)
- Since the irradiance profile and peak are a function of lamp optics, specified in the design, then it’s not necessary to monitor these. Measurement of energy (exposure, mJ/cm²) is more meaningful.
Because process monitoring (compared to process development) is based on tracking changes in key exposure factors, the instrumentation used in production can be simpler and less expensive. Even so, the parameters measured must relate to the physical properties of the end product.
A quick method of determining the “Red Line”
Determining the “red line” involves intentionally varying the process until failure is detected, while measuring and recording the associated conditions. The easy way to do this is a technique called a “cure ladder,” using samples of production ink, coating or adhesive, along with samples of the actual production substrate.
- Incrementally increase the speed at which samples pass under the lamps.
- Measure or calculate the energy in the important wavelength range(s) at successive speeds until the process first fails to meet requirements. (This also is the marginal failure point – or MFP.)
- The energy at which failure occurs is the “red line.”
- Record this “red line” on the same data graphs on which periodic QC measurements are recorded.
- Establish a “caution line” to provide a margin of safety (typically 10 to 20% above the “red line”).
Measurements can be misused or misunderstood. We occasionally hear of production stopped because the UV measurements hit the “red line” – even though there was no problem with the product – only to learn that the “red line” limit was based on a measurement that had little to do with the end result. Worse yet is a process that has become marginal, but the radiometric measure provided no clue because it was measuring the wrong wavelength band or data from a completely different lamp or spectral distribution! A favorite bugaboo is a process measurement of exposure (“dose”) only, with complete disregard for irradiance as a key factor, or for the key wavelengths critical to the process.
Remember, process monitoring assumes that the key interactions between lamps and chemistries have been established – and are the same through design and production. This may be one of the worst assumptions in the technology. Many problems in radiometry arise from a lack of definition and reproduction of the key exposure factors to be measured.
Successful implementation of a regular measurement program of meaningful exposure factors, and periodic benchmarking against a properly set “red line,” can go a long way toward eliminating unnecessary “art” and “guesswork” from process control.