by R.W. Stowe, Heraeus Noblelight America LLC
A cure ladder is the fundamental laboratory tool for the design or analysis of a UV-curing system. It is used to optimize the design of a UV system, ranging from the selection and specification of the UV source to the optimization of a UV-curable formulation – ink, coating, paint or adhesive.
Determining the range of proper exposure of a material relies entirely on the ability to measure the resulting target physical properties of a cured material. In order to efficiently screen the properties of cured materials, the tests may be quite basic. For example, tack, scratch, MEK rubs, bulk cure and adhesion to a specified substrate are typical fundamental or initial tests. The tests may be few and simple, but they should relate to the end application. More complex or elaborate tests can be carried out, but they will be more efficient if less effective exposure conditions have been eliminated.
The ladder itself is ultimately simple. A series of identical samples is subjected to a progressively increasing (or decreasing) exposure with the objective of determining the maximum and minimum marginal success points of the material. A marginal failure point is the exposure at which one of the target physical properties requirements is not met, and the marginal success point is the next nearest condition. In the laboratory, one of the simplest variables is conveyor speed, making this screening fairly quick work. The number of samples can be minimized by using an exponential ladder in which the speed of each successful sample is doubled until failure, followed by speeds – for example – of 1.2, 1.5 and 1.7 times the last successful speed. With only a few samples, the limit is determined with good accuracy. Obviously, the four key exposure parameters must be recorded for each sample.
If all of the conditions of formulation, lamp power, peak irradiance, speed, substrate and film weight are identical to the production equipment, formulation and process of interest, we can stop there – because the measurable conditions that will be used to establish the quality control limits and safety margins have been determined.
However, the most likely reality is that the properties fail at different exposure conditions. By examining what in the formulation or in the exposure will affect that property, changes can be made – preferably one variable at a time – and a new ladder can be run.
Figure 1 illustrates a typical example for a UV-curable ink. Good adhesion can be achieved – but it requires a higher exposure compared to the exposure needed for good scratch resistance, so this system is not optimized. While the system will function at higher exposure, that may not be economical or competitive. A change in one or more of the exposure variables may improve the result. For this example, since depth of cure and adhesion are affected by both peak irradiance and longer wavelengths, the next cure ladder could be run at a higher peak irradiance or with a bulb that emits more energy in the long UV wavelengths. A thinner film weight would improve the adhesion but may not be an acceptable option.
Cure ladders for process development (Figure 2) can be a little more complicated, owing to properties that may behave in more complex ways. In this floor coating example, the object is to observe how the upper and lower limits of several properties combine to limit the overall “process window.” By examining which properties can be improved by changes in any of the four key exposure variables or in the formulation, a UV process can be optimized. An optimized system may be faster, more economical in operation or less expensive.
Many labs are limited in the exposure variables that can be independently varied, perhaps having only one type of lamp. Spectra can be varied with different types of bulbs; peak irradiance can be varied using a highly focused lamp at varying distances; and radiation-induced temperature can be controlled with optical methods. A few years ago, we visited a university UV lab that only had a single low-power lamp. Consequently, the lab was limited to a method of increasing exposure by sending a sample under the lamp in a number of “passes” at a fixed speed. That may have been fine for a demonstration of UV curing, but it provided little information for formulation, properties or lamp selection.
In an earlier column, we illustrated how the total energy expended for cure can be reduced by altering the UV exposure variables. Formulators have to deal with optimizing a formulation because a prospective customer may have almost any type of UV lamp and almost any kind of application.
R.W. Stowe is Director of Applications Engineering at Heraeus Noblelight America LLC. He can be reached at firstname.lastname@example.org.