Paint & Coatings Industry - February 2009 - (Page 19) Measurement useful for industrial applications. We think of “industrial” UV as covering a range from 200 nm to 400 nm. This region of the electromagnetic spectrum has no color equivalent names such as “green” or “orange”, and we often refer to the UV colors by letters (UVA, UVB, UVC) per ISO-DIS-21348. Individual spectral peaks are sometimes referred to in nanometers. Common UV bands and their ranges include: UVA 400 - 315 nm; UVB 315 – 280 nm; and UVC 280 – 200 nm. UV wavelengths shorter than 200 nm are sometimes described as Vacuum UV (VUV), while wavelengths that are longer and on the UV-visible border are sometimes referred to as UVV. Do not confuse the two terms. From a practical standpoint, UV sources, especially broad-spectra lamps like medium-pressure mercury lamps and those with additives such as iron or gallium have peaks that fall into these various regions. Figure 1 is a typical spectra of a UV lamp. Formulators ordinarily choose lamp sources that match the UV absorption properties of the photo package (photoinitiator, sensitizers, stabilizers and other materials). These materials have their own absorption spectra, which must coincide with the planned UV source to provide a workable system. In an ideal world, the UV source would be chosen based on what the formulator requests. In the real world, the formulator often needs to work with an existing UV source that the customer already has or plans to use with a particular formula. Measuring the individual wavelengths of a few nanometers from a UV source requires a sophisticated instrument called a spectrometer or spectral radiometer. Spectral radiometers, because they can measure individual peaks, can be useful tools for R&D of optical components or new bulb types. For formulators and end users, the output spectra of a UV source is well known. In a production environment, it’s not often practical to make detailed spectral measurements with a spectral radiometer. Simpler devices that measure UV emissions over a broader “band” are more common, affordable, practical and easier to use. FIGURE 2 | The absorption characteristics of the photoinitiator, shown in the graph, should complement the output spectra of the UV source. UV-Vis Spectrum of FIRSTCURE TPO Photoinitiator FIGURE 3 | Depiction of the relationship between measured irradiance and distance. 2 Absorbance 1.0 1 2 3 0 300 350 400 Wavelength(nm) 450 4 UV Intensity and Irradiance In simple terms, intensity is the output energy of the UV source. Imagine a UV source as though it were the dining room chandelier. More power is applied as the light dimmer is turned up and the visible light output increases. We can also increase the UV output by turning up the “dimmer”. Like a visible bulb, some UV sources have the capability to produce higher intensities than other UV sources. When more power is applied, UV sources generally behave the same way and produce more UV. Doubling the power does not mean that the UV output will double. UV sources are categorized by the amount of applied electrical power to the source. A 300 watt/inch bulb has 300 watts of electrical energy applied for each inch of the bulb. A 300 watt/inch system with a 10” bulb would have 3000 watts of applied power and a 300 watt/system with a 20” bulb would have 6000 watts of applied power. The applied power does not indicate (a) the amount and type of UV, (b) if the UV is matched to a particular formulation or (c) how much UV is arriving at the cure surface. The amount of UV arriving at the cure surface is called the irradiance. A square centimeter is the area that is used to track arriving UV from all arriving angles and it is generally determined by direct measurement. In theory we always like to measure at the cure surface. In the lab this is generally easier than the realities of measurement in the real world on process equipment. In the real world the terms intensity and irradiance are sometimes used interchangeably, but from a scientific standpoint irradiance is the UV arriving at a particular (cure) surface based on a specified area – in our case of a square centimeter (cm2). How and where to measure irradiance is important. Many UV lamps have reflectors to either focus or diffuse their UV energy. UV output may vary depending on the geometry of these reflectors and whether the lamp’s energy is concentrated to one area (focused) or whether it is diffused (non-focused), and will typically also fall PA I N T & C O A T I N G S I N D U S T R Y 19
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