Q&A: UV-Curing Technology
by R.W. Stowe
Heraeus Noblelight America LLC
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Why Don’t Technical Data Sheets Specify a UV-Curing System?
Product data sheets for UV inks, coatings, adhesives and paints provide a good amount of detail on the target physical properties of the product, as these are most important to the end use. Why is there so little information on the process of curing them? We know that UV exposure conditions can affect the properties of the cured material, so isn’t that important?
It is fairly well accepted that a high irradiance – photon flux rate – at a surface will affect depth of cure. This is evident with pigmented compositions and coatings that have a relatively high absorbance of the wavelengths required to activate the photoinitiators in them. Many materials will cure faster when exposed to higher power – and require less total energy – although this isn’t true for all. Some, like adhesives and elastomeric materials, are limited by their internal rate of propagation, and cure better under lower power and longer time of exposure.*
Some data sheets may be more descriptive than others, but most may include only a range of exposure (or “dose”), typically expressed in mJ/cm2 (millijoules per square centimeter). Many do not indicate the UV wavelength range for which the material is designed. The obvious difficulty with exposure as a specification is the fact that it is not an independent variable – it is the consequence of two independent variables: irradiance profile and time.
This might be analogous to sending me to the kitchen with instructions to bake something for a number of “degree-hours”– a number from which I could not determine either the proper temperature or time! Exposure is the time-integral of the irradiance profile. It can be useful as a quality control measure – but only if one or the other variable is known or doesn’t change.
As we have commented many times, four factors can be used to characterize UV-curing exposure systems:
- Irradiance – the profile of radiant power arriving at a surface, measured in W/cm2 or mW/cm2, in a specific wavelength band (often, only the peak value is reported);
- Time (or speed) – the time in seconds of exposure; inverse of speed;
- Spectral distribution – relative radiant power versus wavelength, in nanometers (nm); (typically, only the spectral type of the UV source is identified);
- Temperature of the film (ink or coating)
Selection of a UV-curable ink, coating, adhesive or paint is based on the ultimate performance properties it is designed to achieve. The ability to achieve those properties is significantly affected by the exposure conditions and the curing system design.
Irradiance and spectral distribution are factors intrinsic to the lamps applied. Time (or speed) is dependent on the mechanics of the application. Temperature of the work surface is primarily a product of its absorbance and all of the radiant energy delivered – not only infrared. It may be increased by a chemical exotherm but reduced by cooling systems.
So why are these factors not included on technical data sheets? Is it because formulators are reluctant to promote any one type or manufacturer of UV system? Is it because there is variation from type to type of radiometer, so a standard measurement is difficult? While these may be contributing reasons, there is something more.
The reason is that the formulator has no way of knowing what film weight will be applied, the surface to which it will be applied, nor the UV lamp system or configuration of lamps that will be used. We might note here that there are five or six types of UV lamps that are available for UV curing – all with different characteristics. So, perhaps a measure of exposure (“dose”) can – at least – get us in the ballpark.
Lab testing may be required for a more definitive evaluation of the best curing conditions. Lab tests typically involve a series of “cure ladders” – curing samples of controlled thickness at progressively decreasing exposure under a selected lamp type – usually by increasing process speed until failure of any of the required physical properties occurs.
The ladder is repeated with one of the exposure variables – for example, peak irradiance – increased or decreased. To be thorough, these ladders also are conducted with a range of film weights, depending on the method of application.
Selection of a curing system and its configuration requires some orderly experimentation. Lab studies can be invaluable for optimizing a system to produce the desired physical properties. In addition, the process of measuring the key factors leads to identifying the “process window” for production quality control. Optimizing the design of a curing system only begins with the technical data sheet.
* We may discuss “non-reciprocity of exposure” in a future column.