UV-curable opened mystery

     Author: Richard W. Stowe (U.S. FUSION UV Systems, Inc., Gaithersburg, Maryland City)

   For screen printing, the UV-curable inks and curing technology there are advantages and disadvantages. Known the benefits of it. UV-curable faster speed does not need to use solvents, so it's a broad range of applications, is unbelievable. Such as CD-ROM industry, since the date of birth on the use of UV protection of plastic coatings and decorative printing. 100 percent as a "solid" ink, UV-curable inks do not emit VOC (volatile organic compound that is easy to adapt to environmental regulations), but also not as volatile (solvent and water-based) ink or coating due to dry or viscosity change screen blockage. Its disadvantage is that UV inks can not be printed as conventional ink or curing. For example, if the first pass printing inks do not completely solid, screen printing you ask how to deal with the staff? The answer may be reprinted again or lower the printing speed. Unfortunately, these traditional volatile inks are ideally suited for the idea of UV-curable but the disadvantage of this approach is likely to bake your substrates bad, give you trouble. Remember, you are dealing with the ultra-violet light, it has certain optical properties, which means you have to deal with the focal length and wavelength. This strange situation is that UV-curable with some of the causes of mystery. This is also in this article we will discuss the main content, and how effective use of light and excellent adhesion to reach the surface of curing effect. Another language?

   Terms it is more mystery. If we want to they get to know the working principle of ultra-violet lights, we can not avoid referring to spectral distribution and peak radiation these terms. Screen printing of staff rarely know or use of these terms, but they are correct but it is very key to curing. However, these terms and other relevant people confusing terminology UV expressed in the concept does not sound like they are less complex. Curing many problems because we do not take full advantage of UV curing some of the key elements of memory. In this article we will use language to talk about these elements. The basic formula of UV-curable material by any oligomer and monomer components such constitute - simply put, is the macromolecular and small molecule constitutes its basic formula. In general they could be likened to traditional ink resins and solvents, the difference is that the former occurred in the curing process of cross-linked to form a solid material. These mixtures can also contain many additives. For screen printing, the most obvious of course, paint additives. Only when we add a key element of "Photoinitiator", this chemical mixture can be UV-curable. When Photoinitiators when exposed to ultraviolet light, the mixture from the liquid into a solid. Many people do not know but not the least, we must use the correct ultra-violet light source.

     Key component

   Photoinitiator is the only directly affected by the impact of the composition of ultra-violet light. When Photoinitiator UV absorption, the reaction was triggered. Photoinitiator very high sensitivity, which explains why the UV-curable so rapid. In fact, only one ultraviolet photon (once again shown that UV light source is correct) hit Photoinitiator molecules, which can "inspire" This is the beginning of curing reaction elements. To remember that Photoinitiator molecules evenly distributed in the whole thin mixture. Let the mixture of ultra-violet light to penetrate, and arrived at the surface under the Photoinitiator molecules, particularly those in between ink and substrate adhesion points Photoinitiators elements, this is not always easy to do. The ink is concerned, the situation is even more so.

     Ray of light

   We first take a look at the characteristics of ultra-violet light. The best way to understand it from our more familiar with things proceed, that we perceived as the color of visible light. Screen printing staff to deal every day with the color, very clear that we see is actually a different color from certain wavelengths of light generated.

   Most of us are familiar with the chromatography, know that it is the so-called part of the electromagnetic spectrum (see Figure 1). All of the light - infrared, visible light and ultraviolet - in physics is similar, the difference lies in the wavelength. Usually nanometer wavelength (nm) to express. It is interesting that in the visible light (about 450 ~ 750nm), red and blue wavelength is only the difference, but the feelings of the human eye is very different. Therefore when we discuss the scope of ultraviolet light (200 ~ 450nm), we have to give up we will be ultra-violet light as a "non-visible but like the blue like" a vague concept. Although we can not see ultraviolet light, in the entire UV wavelength range, there are similar to visible light as blue, green, yellow, orange and red division. As the same as the visible spectrum, UV spectrum has a wide range of wavelengths. When different UV Photoinitiator under exposure to ultraviolet light, they are on different wavelengths of ultraviolet light have a reaction. We soon discovered why it is very, very important.
Source Exploration

   Ultra-violet lights there is a lamp - is actually a transparent quartz tube and often contains a small amount of mercury and a reflective shield. Although the lamp to stimulate different ways (see this article behind the arc or microwave part), the total work is the same: to impose mercury energy heating them and quickly evaporate. When the lamp to reach its operating temperature, the mercury vapor into plasma ionization concurrent shed mercury-specific wavelength of light - that is, ultra-violet light is also visible. At this point, common sense tells us that that these photon penetration of UV-curable material and curing fully, we need to produce a large number of photons. Indeed, we also found that ink arrived in the amount of light depends on the surface of UV light on many factors.

   First of all, the lamp output power is very important. This is usually used Watt / inch (W / in) to express. Unfortunately, despite W / in lamp can show the "horsepower" rating, it also shows that the lights can not be how to actual performance. We need to know is how efficient lights. In Figure 2, the attention of the three tubes 6KW power input is the same, but the D lamp (Figure 2b) of the output is better than other lights. From a practical point of view, the power to choose the kind of input into output maximum effective UV lamp energy - the most effective wavelength approach - in UV curing is the most effective.

     Mercury - UV curing power of the messenger is not the only issues to consider. Figure 2a of H lights histogram shows the mercury lamp UV fired. These histogram shows the range of ultraviolet light (450 ~ 200nm) for a band every 10nm energy output value. This characterization of the distribution of lamp output spectrum "map", a simple cut separation of the launch of each band of radiation energy. Note that this lamp does not have a whole range of ultra-violet spectrum of ultraviolet light. In fact, the only rely on mercury output UV lamp in a good long-wavelength peak output, but in a great extent, it's the strongest output in between 220 to 320nm short wavelength. We soon discovered why the two sections of curing is very important.

    Now, we have to return to UV-curable material, let us opt for a transparent varnish or varnished. It transparent, this is actually wrong. In fact, all the UV-curable material and are short-wavelength ultraviolet light absorption and prevent it from crossing. This is probably the most UV-curable were overlooked some areas. From the past situation, which led to the misconception that UV-curing can only be applied to very thin coating. In a sense it did not say anything wrong, because for many years the only tool we use an ordinary mercury lamp.

     Long-wave and short-wave UV

   UV lamp can add substance to become sometimes referred to as "doping" or add-style lamp. Be added the material can be evaporated and achieve plasma state. UV part from mercury, a part from these additives. However, additives to launch its unique wavelength. Figure 2b and 2c of the D spectrum lighting lamps and V distribution chart, we can see the output of additives change the lamp. D lamp in the range of 350 ~ 400nm strong output. It will also launch some short-wavelength ultraviolet light, but sometimes referred to as UV "UVA" the framework of band very effective (and sometimes the UV wavelengths are divided into "A", "B" and "C" three-band ). UV "A" band often refers to 320 ~ 400nm or 300 ~ 450nm. UV "B" band often refers to 280 ~ 320nm, and UV "C" band refers to 200 ~ 280nm. Because such a classification is not very accurate, I would prefer to use long-wave, medium wave and short-wave to distinguish.

   V lights add other substances, it is still fired a short-wave, but not many. But it is in range of 400 ~ 450nm there is a very strong, very effective output. People can design in the long-wave, medium or short-wave strong output UV lamp different. However it can not design all the band are effective UV lamp, but this is not what we want, because they can not activate Photoinitiator of the UV band can be null and void was a waste of energy. The choice of a particular lamp is the main reason it has issued to avoid UV-curable material to be absorbed, but its wavelength can activate Photoinitiator. Desired "Perspectives"
Let us now turn to a specific wavelength of UV Photoinitiator response. In Figure 3, we have seen Photoinitiators (in this case, benzophenone) absorption curve there are several "high-absorption point" - all have made so much Photoinitiator. For example Photoinitiators, UV absorption rate is a characterization of the Optical Active mode. Department of short-wavelength surface absorption is very important, and curing the material in question the underlying long-wavelength response should be very effective, because the short-wavelength surface has been absorbed. Therefore, the short-wave ultraviolet curing more influential on the surface. Because the question of curing the material to absorb short-wavelength ultraviolet light to prevent a long band of deep ultraviolet curing and adhesion on the essential. If we only containing Benzophenone Photoinitiator for curing the varnish, H lights or contain only mercury lamp (Figure 2a) is very suitable. Because Photoinitiators the spectral absorption curve with the launch tubes very much in line spectral distribution. Color and curing paint if we add what can happen? Also remember that we had likened paint "rock" it? In Figure 3, I set out the UV-absorbent blue pigment, its short-wave length and in the wavelength UV absorption are high, only very long wavelength would not absorb. Accordingly, this Photoinitiators of varnish is very suitable, but not suitable for ink. That is why the majority of screen printing ink using long-wavelength (350 ~ 400nm) Photoinitiator reasons. Figure 4 shows containing pigment in the depths of the film, UV energy absorption and attenuation due to the strong, almost no short-wavelength ultraviolet light, and only long-wavelength ultraviolet light to reach the bottom.

   Therefore, the screen printing inks with high optical thickness. Optical thickness of the coating thickness should not be confused. In fact, the thick layer of varnish UV absorbent low (with low optical thickness), may be more than a thin layer of high-absorbent ink (with a high optical thickness) more easily curable. This is because the ink components of short-, medium-and long-wavelength ultraviolet light energy absorption rate is not the same. This means that the ink may be of certain wavelength is of high optical thickness, while others with lower-wavelength optical thickness. To illustrate the effect of absorbent, let us imagine that will be cut into uniform ink film layer 100. Arrived at the top and bottom of the ultra-violet light can vary greatly. For example, UV varnish (no color) the absorption of low, or have a low optical thickness. Its top and bottom of the absorption of "ultra-violet light intensity" ratio of 3 to 1, the fact that usually the case. Now, if 2 / 3 of the photon can not arrived at the bottom of varnish, then we add paint what can happen? In fact, the top and bottom of the energy absorbed by the ratio can reach a few hundred to one (or higher). Only those who are not easy to be top-level photon absorption to reach the bottom.

     Modified for ink

   When the ink must be added when a lot of paint, ink manufacturers often have to provide the majority of ultraviolet optical thickness have ink. Therefore, in our previous note, the blue ink, we can see mercury lamp mercury are not well-curing screen printing inks, and more need to use doping lamp, for example, D lights. This explains why most screen ink manufacturers to use long-wavelength Photoinitiator. (Strict division of the formula will be adjusted according to the color Photoinitiator concentration, a good performance of color matching.)

     White monster

   Now, in Figure 5, the "mapping" out of titanium dioxide different UV absorption characteristics of titanium dioxide is a typical white pigment commonly used, it absorbs almost all visible light, ultraviolet light and reflection. This makes it difficult to use ultraviolet light to white curing. White material has a "window", about 400 ~ 430nm. If we use the V long-wavelength light, such lights in the window of the scope of very effective, so we can be successful in curing white ink. This is why we spend a lot of space to explain lamp spectral distribution with the UV curing material to absorb the reasons for phase matching. Highlights of UV-curable opened still has much room for the mystery: with the lamp output and spectral distribution is equally important that the focal length of the tube and cover the role of reflection.
At this moment, we have to be tube-launched with the arrival of radiation energy surface energy distinguish. May be the moment arrived in the surface of light known as irradiation. Irradiation is often inaccurately referred to as "intensity." The following radiation and radiation to make a distinction: The lamp to illuminate the walls. When the lamp away from the walls of the Agency, the lights of radiation has not changed, but the walls of irradiation reduced. (You will find, if you do not know we are talking about the lamp intensity or the strength to reach the surface, it may cause confusion.)

   Precise use of Reflector Lamp not increased in the case of input power to increase the workpiece surface radiation. Gather the most energy is the highest point or peak point where radiation.

   Fig.6 Reflector on the energy gathered in the workpiece surface. Some did not focus light can be arrived at the surface, but its not as good as the effect of curing ink focus light. Used as described in the oval is a reflection mask, which has about 75% of the energy collection effects (see the next question was about the content of Reflector). In other words, the lamp issued 75 percent of energy will be collected and focused Reflector. To maintain focus on the focal length, the increase in tube and the workpiece surface will reduce the distance between the peak radiation. Similarly, the focal length will be placed within the workpiece surface will reduce the peak radiation. Not to increase the input power in the circumstances, when the smaller diameter tubes, the higher the efficiency Reflector reflex, reflex cover the higher collection efficiency, the greater the radiation. Table 1: Comparison of various tube lamp curing curing rate of speed: m / min dose: J/cm2 speed / dose than the 9mm, oval (μ-wave), 120W/cm 21.2 140 151 23mm, oval (arc), 120W / cm 13.6 200 68 23mm, parabola (arc), 120W/cm 12.1 190 64 In order to understand the effects of radiation peak, Table 1 compares the three tubes in different 120W/cm transmission speed, the role of the polycarbonate substrate on a black screen printing ink (390 mesh). Their different manifestations What does this imply? Oh, oh, it seems a little wrong, because the highest radiation dose curing lamp is not the fastest lamp. In fact, the lowest radiation dose fastest curing lamp! Final if we look at the file content: speed dose ratio, we will find the smallest diameter (9mm) of the lamp tubes outperformed the other two. This is because the Oval Reflector can collect and gather more light from the bottom of the role of small tubes in the workpiece surface, a higher peak radiation. Why is it the best of curing? We'll find the rules from the Bill explained. The basic meaning of this rule is the surface layer was not immediately absorbed or reflected light will be transferred to a deeper level. The higher the radiation, then the better curing depth.

     On the reflector

   UV lamp reflector functions of some of the major by the aluminum because aluminum is the only reflection of the 200 ~ 450nm UV-band material. Aluminum outside to have a hard protective coating. Reflector (with visible light, like a mirror reflection), will light reflection to the surface in order to increase the surface to obtain the overall energy. At present, the majority of ultra-violet lights to use elliptical reflectors, and the reason is very simple: in all the reflectors, this form can provide the most energy to the reflector. Parabolic reflector will be scattered light to the workpiece surface of a larger area, such reflector collection efficiency is not high, but to create a good elliptical reflector to achieve 75 percent collection rate, which is said 75% of ultra-violet lights reflection energy was concentrated in the surface (the remaining 25 percent had direct exposure to radiation energy to the surface), and have little impact on the basic curing. Curing the difficulties we have a black car with a glass screen printing ink (which is famous for curing difficult) to make an interesting on the curing depth, as well as how the effects of radiation curing depth of the presentation. D with two lights to curing inks, these two lights on just a difference in the radiation, a light irradiation is another lamp twice. The use of the first light exposure time to reach the curing depth is a mil (mil). Re-irradiation does not significantly increase their curing depth. While the other lights can be issued twice the radiation exposure time it reached the curing depth of the previous two lights to reach the depth of the double exposure, although the radiation dose is the same. Here, we should be clear what the radiation dose is. Radiation dose is multiplied by the time or the accumulation of light energy. Slow down with speed, the dose increased, to speed up the belt speed, the dose decreased. If the ink tube under exposure to the longer, ink surface received the higher dose, but radiation is not higher. (We will find nothing to help not only increase the dosage, but harmful to the heat sensitivity of the substrate).

   The second re-exposure to a lamp - doubling the dose - but not a significant increase in curing depth. From this, we recognize that through repeated exposure to increase the dose not penetrate the photon arrived at the bottom of ink Photoinitiators effective method. Curing depth of more rather than the peak dose of radiation impact.