By R.W. Stowe, UV Applications Engineering Consultant, Heraeus Noblelight America LLC
Question: How does temperature affect UV Curing? How is it controlled, and how is it measured?
Answer: We have often said that temperature is one of the four key variables1 of UV curing. By this, we mean the temperature of the ink, coating, paint or adhesive. The temperature of the substrate also is important.
All radiant energy arriving at a surface generates heat in the film. The resulting temperature depends on the absorptivity of the surface and substrate. The film (ink, coating, etc.) and substrate have unique characteristics of spectral absorbance, specific heat, thermal conductivity and diffusivity.
Temperature can decrease the viscosity of the curable film and have an effect on flow-out, leveling, wetting and molecular mobility. It also is true that the reaction rate is increased primarily a result of the mobility of relatively large molecules in a viscous (even thixotropic) medium containing nonreactive particles.
Medium-pressure UV lamps typically operate at a quartz tube surface temperature of approximately 900°C. The infrared delivered to a surface is primarily a function of the surface area of the bulb and follows the Stefan-Boltzmann Law. Some bulbs generate three times the IR energy of others, primarily a function of their diameter. Although the radiant energy delivered by LEDs does not include any significant IR or visible radiation, we should remember that “a watt is a watt” in terms of the thermal effect of energy arriving at the surface.
The temperature rise of the film can have potentially beneficial (+) or detrimental (-) effects. Beneficial effects are:
+ increased rate of reaction;
+ improved leveling and gloss; or
+ improved wetting of the substrate, improving adhesion.
But, negative effects of excessive heat can include:
– reduced viscosity, causing ripples or runs;
– volatilization of low molecular weight molecules from the PR polymer, causing damage to its properties;
– increased effect of oxygen inhibition; or
– softening, deterioration, or damage to the substrate.
The practical answer depends entirely how the film and substrate respond to heat. This is why surface temperature should always be part of the specification of UV exposure. It will obviously be different for coatings or inks and different for substrates such as paper, glass, metal, plastic or wood. The following (in order of preference and roughly, cost) can reduce surface temperatures.
- Optimize the UV-curable system’s exposure/response efficiency2 and move faster (although some types of machine cannot increase speed).
- Select smaller diameter MP bulbs; they emit less IR.
- Use air to cool the surface. Use lamp air if the airflow is positive.
- If the coating doesn’t require high peak irradiance, use one of the methods of de-focusing the lamp. The diffusivity of the substrate may reduce the temperature rise.
- Use dichroic primary reflectors (as illustrated in Figure 1).
- Add a “hot mirror.” Otherwise avoid quartz windows when possible, as they reduce UV and block very important cooling air flow in MP systems.
- Use LED UV sources. This will usually require a change in formulation and photoinitiators.
- Add a chill roller or cold plate under the web. (effective for clear film substrates).
Temperature measurement under UV lamps
This has been a troublesome subject. As it relates to UV curing, the typical purpose of temperature measurement is to determine the instantaneous or peak temperature of a temperature-sensitive substrate or of an ink or coating as it passes under UV lamps. Surface temperature rise is affected by (a) radiant energy arriving at the surface, (b) spectral absorptivity of the ink or coating and (c) thermal conductivity or diffusivity of the substrate.
Several methods of measurement can be used, and all have deficiencies. We must understand the errors in each method, or we will not understand the data. As we know from radiometry, when a measurement is used as a specification, and the methods of making the measurement are not defined, they can be of little value and certainly misleading. This is even more of a problem with thermometry.
1. Temperature tabs
Devices are available that, by a nonreversible reaction, indicate the maximum temperature to which the tab was exposed. They do not function properly when exposed to infrared radiation, because they directly absorb radiant heat differently from the coating. Worse, they block the substrate from the radiation that would otherwise heat it, distorting the result.
Except for thick samples where the only interest is in the bulk average temperature (such as circuit boards or the insides of components), thermocouples are difficult to use reliably. A thermocouple reports only the temperature of its little metal bead – and it must be in nearly complete thermal contact with what it is measuring. If it is subjected to radiant heating, it will show a temperature that has little to do with the temperature of the surface or space – usually much higher.
Attempts to measure surface temperature of a radiated part often include various schemes of adhesives to improve conductive contact and over-taping to reduce the effects of radiation. All of this can interfere with an accurate measurement.
3. Noncontacting thermometers
Also called infrared thermometers, these noncontact electro-optical devices measure the infrared emittance of a warm or hot surface. They respond to surface temperature because the infrared emission from a surface is a function of the temperature and the emissivity of the surface.
To make measurements, the hand-held instrument is located at a convenient distance from the surface. Measurements can be taken in seconds. (Note that the purpose of the LED beam is to locate the target and is not part of the measurement.) Measurements also can be made continuously by connection of the instrument’s signal output to a chart recorder. While an optical thermometer is calibrated for emissivity, organic and many nonmetallic materials have an emissivity of approximately 95% (compared to a “blackbody,” or perfect emitter, having 100%). This allows readings of almost any material with good accuracy without recalibration by using a constant of 0.95.
One disadvantage of noncontact thermometers used with UV lamps is the difficulty of measuring the surface when it is directly under the lamp. This can be easily solved: Two measurements made in sequence at known times after exposure allow a backward extrapolation of the peak surface temperature. Most of the time, a quick measurement made as soon after exposure as possible is sufficient. A non-contacting IR thermometer is an accurate and reliable method of measuring surface temperature, although it is often limited to measuring immediately after exposure to the UV lamps.
Measuring the temperature of plastic films
IR thermometers respond to specific wavelengths in the IR region of the spectrum, typically over a wide range, of 4 to 14 micron (4,000 to 14,000 nm). This is where the difficulty of measuring the temperature of clear plastic films arises. A typical film may be somewhat transparent over most of the spectrum, so if measuring with a broad band detector we may not be measuring the temperature of the film, but the temperature of the floor or any other object behind it! Most clear plastic films exhibit molecular resonant absorption (opacity) at either or both 3.4 micron and 7.9 micron, so accurate measurement of temperature will require more precise instruments operating at only these wavelengths.3 Figure 2 illustrates the potential sources of measurement error when measuring the temperature of clear plastic films.
In UV curing, heat may be beneficial and can increase the effectiveness of the process, but only to a point. Excessive heat can be damaging. Methods exist for independently increasing or decreasing temperature. Temperature should be included as one of the process design variables, included in process specifications, and included in process control measurements.
- Four principal independent parameters are UV wavelength (λ1-λ2), irradiance profile (It), speed and temperature. Exposure (J/cm2), while fundamental to the curing process, is a secondary combination of two primary variables (irradiance profile and speed).
- UV+EB Technology, Vol 1, issue 3, 2015; “Cure Ladders” for Optimizing a UV Curing System
- “Plastic Film Measurement;” publication AN108, by Ircon, Inc.