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Understanding UV Output Technical Note

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Understanding UV Output Technical Note Understanding UV Output Technical Note Overview Curing with Ultraviolet (UV) energy is a complex process combining materials, chemistry and photons. There are many resources on the web that describe the process thoroughly (see References #4). This Technical Note will focus on developing an understanding of Irradiance, Energy Density and Power, which are three separate but inter-related items that impact an adhesive, coating, or ink’s ability to cure properly. Definitions This note uses the following definitions: Dwell Time (seconds): The total amount of time the substrate is exposed to UV energy. This can be effected by belt speed (or carriage speed) and emitting window size. Material running at 50m/min and exposed to a 20mm wide UV light exposure will have the same dwell time as material running at 100m/min exposed to a 40mm wide UV light exposure. Irradiance: The radiant power arriving at a surface per-unit area. With UV curing, the surface is most often the substrate and a square centimeter is the unit area. Irradiance is expressed in units of watts or milliwatts per square centimeter (W/cm2 or mW/cm2). In UV curing, the term intensity is also commonly used to describe irradiance; however, irradiance more correctly describes the concept of UV arriving at a two-dimensional substrate.1 Typically, irradiance is quoted at peak irradiance, meaning the maximum irradiance throughout the specified area. UV Power: Total power produced by a light source. Power is expressed in units of watts (W) and is an absolute number regardless of wavelength and time. Radiant Energy Density (Dose): The energy arriving at a surface per-unit-area during a defined time (dwell time). A square centimeter is again the unit area and radiant energy density is expressed in units of joules or millijoules per square centimeter (J/cm2 or mJ/cm2). Dose is irradiance multiplied by the dwell time. Light Source: Commonly referred to as a lamp. The unit that houses the LEDs and produces the UV energy. Material: The adhesive, coating or ink that is cured by the UV energy. Material is typically a combination of photo initiator, oligomer, monomer and sometimes pigment/dye for color. Substrate: The physical media that the adhesive, coating or ink adheres to once cured. This may be paper, films, plastics, glass, metals or a myriad of other materials; in the case of 3D printing/additive manufacturing, it is the resin or polymer recently applied. 1 | ©2017 Phoseon Technology Wavelength: The spectral output of the UV source, measured in nanometers (nm). When a UV source states a specific wavelength it refers to the peak wavelength producing the greatest amount of UV power. Generally speaking, the longer UV-A wavelengths provide for deeper penetration and good adhesion. Figure 1: Wavelength Note: Accurately measuring and interpreting power, irradiance and dose is beyond the scope of this technical note. Please contact a radiometer supplier for specific information.2 Impacts Each parameter of an LED lamp (irradiance, dose, UV power) needs consideration when specifying the criteria needed to cure a particular adhesive, coating, or ink. Diodes are natively a divergent source, meaning that without optics, the light is emitted at various angles. Without optics, irradiance decreases as distance away from the specified location increases. Said simply, irradiance will be lower the further the substrate is from the light source. Typically, irradiance is specified at the lamp’s emitting window, working distance (WD) equal to zero, and it reduces as distance increases. See the black line in Figure 2. Vendors compensate for this with different LED arrays, optical elements, or combination of both to create the blue line in Figure 2. Figure 2: Irradiance vs. Working Distance (with and without Optics) Dose (J/cm2) is impacted by the speed at which the substrate is moved under the lamp; or conversely the lamp is moved over the substrate. In a perfect scenario, if a material is curing sufficiently at 2 | ©2017 Phoseon Technology 50m/min, then increasing the speed to 100m/min while doubling the irradiance also cures the material sufficiently, since the substrate is exposed to the same amount of dose. Unfortunately, most scenarios are ever perfect. Some materials don’t cure faster when additional irradiance is provided. Increasing the belt speed to 100m/min sometimes requires double the dwell time, not an increase in irradiance. Viscous inks, like offset or flexographic inks, may require low irradiance to fully cure, whereas digital inks are very fluid and require higher irradiance in order to start the curing reaction. A rigid ink may cure faster with a shorter dwell time, whereas a flexible ink may need to be cured slower, with a longer dwell time. In Figure 3, the UV power under Area 1 and Area 2 of their respective curves is equal but each curve provides different peak irradiance. A material may actually cure better scanning across Area 2 with lower peak irradiance and a longer dwell time. Figure 3: Irradiance vs. Position For an individual lamp, power is not impacted by either distance or speed. It is an absolute number that does not vary. For instance, Figure 4 below shows products grouped by irradiance and UV power. Don’t assume a high irradiance lamp provides high UV power. Figure 4: Products Grouped By Irradiance and UV Power 3 | ©2017 Phoseon Technology So what is the most important; Irradiance, Dose, or Power? Unfortunately the answer is that it depends on the application. There are many applications where a ‘threshold’ irradiance is needed to start the polymerization process, and then a long dwell time of dose to finish the process. Other applications just need a sharp, short jolt of irradiance and curing is complete. The material and substrate have large impacts on what is needed. Working with your Phoseon representative will provide a starting point before beginning trials. Analogies While not perfect, the following analogy may help the reader better understand irradiance, power, and dose.3 Refer to a prescription (dose) described as - take one 50mg tablet (irradiance) 4 times a day for 10 days (dwell time). The same dose can also be provided two alternate ways: Take four 50mg tablets (200mg) once per day for 10 days Take one 50mg tablet twice a day for 20 days. Although the dose is the same for all three methods, according to the doctor the two alternatives are not optimal. UV curing is similar in that the goal is to optimize the dose to properly cure the material. Another analogy is baking a cake. The recipe (dose) calls for an oven temperature (irradiance) over a period of time (dwell time). A recipe instructs to heat the oven to 350°F and then bake the cake for 30 minutes, which results in a cake that is completely baked through, without burning. The cake experiences 350°F for 30 minutes, and this is the energy density, or dose, required to fully bake all of the cake batter. If the cake is baked at twice the temperature for half the time, the cake burns. Conversely, if the cake is baked at half the temperature for double the time, it may not be baked in the center (not fully cured). Measurement Irradiance and Dose can be measured using commercially available radiometers that are tuned for the specific wavelength to be measured. Often these radiometers also provide analytical software and are good for ensuring process control between shifts. See note 2 for a partial reference of suppliers. Power is best measured using a calibrated integrating sphere that captures all the energy emitted from a lamp. However, these devices are large, expensive, and not conducive to end-user applications so Power is not typically marketed. It can also be measured using a series of two- dimensional scans, stepping across the emitting window, and then totaling the scans. This is very time consuming and typically cannot capture all the light produced, especially at the edges as measuring devices are typically +/- 10% accurate. Considerations UV curing is application-specific. The substrate, material and UV energy must be combined together to maximize each contribution and minimize each shortcoming. For instance, increasing the material thickness from 10m to 20m drastically impacts the cure, as the photons have 2x the density to penetrate and polymerize the material. Increasing the belt speed from 50m/min to 100m/min requires double the Irradiance for the same curing reaction and dose, all else being equal. But as 4 | ©2017 Phoseon Technology with most things in life, doubling the speed may lead to other unknown or unforeseen challenges and may not be optimal. Each wavelength has unique properties and must be matched with the correct photo initiator to ensure proper cure. In general, longer wavelengths (UVA and UVV) result in deeper through-cure. Also, be aware of marketing claims, such as, Highest Irradiance. Figure 5 below shows product B with higher-peak irradiance vs. product A but drastically reduced UV Power (area under the curve). In a scanning application there is a high probability it will not solve the curing challenges due to reduced UV Power. Figure 5: Peak Intensity vs. Total Energy Each application needs to be considered for the type of material being cured and what type of light source will best match the curing needs. Working closely with the UV material supplier and the light source supplier will result in a well-matched total solution. Which lamp is right for my application? After understanding the difference between UV power, irradiance and dose, what determines the proper lamp to choose for a particular application? The first step is to collect as much information as possible about the material being cured.
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