UV Chemistry Controls
Written October 17, 2019
Why is knowledge of UV chemistry so important to a screen printer who either uses UV inks right now or is going to use them? Simply because UV inks are part of the UV technology, and use of UV inks should be viewed rather as an application of UV technology in screen printing. UV technology consists of three major elements: UV ink, UV reactor (or UV processor) and substrate.
The chemistry of UV ink acts like a common solvent which mixes these elements together. It is my strong conviction, based on some experience, that an understanding of some UV chemistry will allow you to successfully implement UV technology in screen printing. UV technology has captured great interest in the screen-printing industry in the past few years.
Many reasons can be squeezed into two statements. First, UV inks provide many excellent physical/chemical properties for UV-cured film such as chemical resistance, gloss, abrasion resistance, etc. Second, these inks offer some savings in cost, energy and space in the screen-printing shop, and in speed of curing. Obviously, these reasons must be reviewed on an individual basis by a screen printer before he decides to go UV.
In the United States, there are two other very important factors that the screen printer must consider when deciding about UV: The constantly rising cost of energy and air pollution regulations. The screen-printing industry is constantly exposed to more and more severe legislative controls designed to elevate environmental concerns. First, Rule 66 was implemented in 1967 in California.In 1970, EPA was established, and starting in 1976, EPA has been issuing volatile organic emission guidelines for industrial and othercoatings. In 1980 the California Air Resources Board (CARB) proposed a new air pollution regulation which would limit the solvent content to 100 grams per liter of ink (one pint per gallon of ink, or 12% by volume).
Why, may I ask, do we suddenly have this air pollution regulatory bonanza? Well, recent air pollution studies have found that basically all organic materials in the atmosphere will photochemically (with the help of sunlight) form toxic smog. Furthermore, some common coatings and solvents such as petroleum ether, naphtha and mineral spirits can cause contact dermatitis and some nervous system depressions.
In addition to air pollution regulations, concern continues to grow over cost and availability of energy resources, such as energy required in manufacturing of raw materials, the rising cost of raw materials and the cost of natural gas used in heat-curing ovens. Presently, coating systems consist of solvent systems of less than 70% solvents. In 1970, approximately 97% of the United States coatings utilized solvent-based systems and 70%-by-volume solvents.
Today, environmental and energy regulatory demands are forcing coating and printing industries to develop more complex and sophisticated systems. It is very important to realize that the traditional concept of coat justification is no longer the sole basis for any final decision. A new system of ink will capture markets (in the United States) simply because it meets pollution control standards under conditions of reduced energy availability.
What choice do we have in screen printing? We call it choice of a high-solid coating system. First, a solvent system of more than 70% solids does not look very promising in the light of very limited availability of raw materials which would provide (in addition to low solvents content) good flow and working screen printability.Second is the two-part catalyzed system. Its concept is not new in screen printing; examples are epoxies and urethanes. But the two-part system, when mixed, has a very limited shelf life. Some families of catalyzed systems cure at about 149°C (300°F) and affect most plastic substrates adversely. Third is a water-based system. Most of these systems still contain about 20% of the conventional solvents. At present, water-based inks for plastics do not solve environmental and health problems. Fourth is powder coatings.
This system is not applicable in the screen-printing industry. Fifth are radiation-curable systems. To meet current air pollution standards and use solvent-based ink, one must consider installation of the incinerators to burn solvents before they are released to the atmosphere or a solvent recovery system based on the effective absorption of solvents by activated carbon filters. Both methods are very costly and will consume additional energy. But of the alternatives available to the screen-printing shop, none would come without cost.
Viable Radiation Systems
One additional alternative is a radiation curable system which seems to provide the screen printer with printable colored ink, good quality of cured film and relatively low capital investment, with fewer changes in traditional printing techniques. I would like to review three types of radiation curing. EB, electron beam, is based on particles of negatively charged radiation. UV, ultraviolet, consists of light waves; it is wave light. IR, infrared, is also wave light.The definition of radiation is “sending out energy”, so it shouldn’t be any problem to understand UV radiation, IR radiation, EB radiation, etc. Let’s review briefly these three types of radiation-curing technologies: Electron beam is based on radiolysis; it is the breaking down of chemicals by radiation. In this case electrons hit the compound, breaking it down to fragments which initiate curing. In UV we have photolysis; light is doing this job.
The compound in this case which absorbs light is called the photoinitiator. In IR we have a combination of thermolysis and solvent evaporation, most likely water or alcohol. Thermolysis means breaking the compound by means of temperature evolved in the system by the absorption of infrared light. Energy absorbed by the system in electron beam technology is the energy of electrons accelerated in the field of 100 to 500 kilowatt potentials. In UV we are talking about light in the range of 220 to 400 nanometers, (wavelengths of UV high-energy light). In IR we are talking about a range of 700 to 2,000 nanometers, which is 2 microns or micrometers, and a second range of 2 micrometers to 4 micrometers, which is related to high absorption by water.
Electron beam technology uses acrylated resins and monomers. In UV, we have in general the same system, plus a photoinitiator. In IR we have miscellaneous resins, plus solvent, plus a catalyst which eventually will help curing.
Film Thickness to be Cured
An electron beam penetrates very deeply; generally, a thickness of 1.5 mm (0.060 inches) can be cured. In UV, pigmented coating is about 13 microns (0.0005 inches), plus or minus maybe 2 or 4 microns (0.0001 or 0.0002 inches). In IR, we can estimate a few microns thickness to be cured.
Electron beam is a secondary X-ray, so its use may cause some safety problems. UV may leak ozone if there is not adequate ventilation or if some ozone-producing lamps are used in the UV processors. With IR, I can’t really say there is any safety problem.
Cost and Complexity
With electron beam, cost of the apparatus will be high, and it is very complicated. UV has moderate cost and is not very complex. IR is relatively low on both counts.
Solvent inks consist of three major components: Resin (in most cases a solid-type resin), solvent and additives (flow controls, wetting agents, etc.). The UV ink is similar, but the resin (sometimes called an oligomer) is a viscous liquid. UV monomers are liquid—related to solvent by the looks of them—and UV additives are very much the same as in solvent inks, but with a photoinitiator in amounts of 1% to 5% depending on what kind is used. In the curing process, solvent inks vs. UV inks can be characterized by Figures 1-3.If the solvent consists of about 50% of the ink, there obviously is a decrease in the thickness of the film compared to wet. So solid film is thinner; it’s maybe half the thickness of wet film (Figure 1).
In a solvent catalytic-type ink there may be less solvent; a baking process is used, so there is some loss of solvent and the thickness decreases slightly when going from wet film to solid film (Figure 2). In UV ink, the situation is different—wet film thickness is approximately the same in solid film (possibly less because of some slight shrinkage; however, we can say in comparing wet film with solid film that the thicknesses are similar). (See Figure 3)
Types of UV Curing Inks
Right now, the most popular UV ink available on the market is an acrylated system-type ink. There are other inks in developmental stages for specialized uses such as a UV-curable epoxy system, a polyester/styrene system used in particle board technology and a thiolene system.
Now I would like to discuss the chemistry of UV ink, emphasizing the acrylated system. In composition of UV inks, oligomers are polyesters, acrylated epoxies, urethanes or polyether polyols. For monomers we are talking about acrylated alcohols or polyols. The photoinitiator, which serves as the UV light obtainer (the most important part of the UV system), catches light and causes the ink to cure. UV additives are pigments, stabilizers, flow controls, etc.I would characterize the curing chemistry of UV ink in four stages. The first stage (see Figure 4A) exhibits the liquid film of the UV-curable coating before exposure to the UV light; the little circle is the monomer, the large circle the oligomer and the dot is the photoinitiator.
In the second stage (Figure 4B) the UV radiation shines vertically on the film from the top of the film. In exposure of the coating to UV light, UV energy is absorbed by the photoinitiator, which invokes formation of radicals. Radicals are nothing more than fragments of chemical—in this case, the chemical is the photoinitiator.
In Figure 4B, the little lines sticking out from the circles indicate free radicals.
This type of curing is called free-radical polymerization in chemical terminology. The third stage (Figure 4C) is propagation of the polymerization or curing reaction. Free radicals attack monomers and oligomers, causing fast growth of the polymer network. Those radicals attack monomers and oligomers, evoking new radicals. Those radicals react together, forming longer chains and turning liquid into solid. In the final stage (Figure 4D), completion of polymerization results in a solid cross-link network of the cured coating.
From a chemical point of view, there are a few controls in UV curing process. I would refer to them as determining factors. The speed of curing depends on the square root of photoinitiator concentration in UV ink, which means if you want to cut curing time in half you must increase the photoinitiator fourfold. In order to double speed of curing, you must also increase light intensity fourfold (see Figure 5).
Those formulas are obviously theoretical results of certain chemical discussions. In UV curing, determining factors also include film thickness, type of pigment and opacity. In Figure 6, the little equation on the righthand side indicates the light intensity penetrating the thick coating decreases exponentially; in other words, it decreases very fast with thickness. Penetration of UV light is vertical in UV curing.
It’s very important to realize this; it’s in one direction— it’s not like the heat cure systems where the heat follows the whole spherical geometry of curing. Thermal curing is from the bottom, from the sides and from the top. Here we’re dealing only with the top, vertical type. And because lighting intensity is decreasing exponentially, small increments in film thickness will rapidly affect curing.There is a relatively narrow margin for error in depositing film of proper thickness. For example, 13 microns (0.0005 inches) pigmented coating may cure completely, but 15 microns (0.0006 inches) might result in wet film. Pigment type and concentration — for example white, blue, black—may cure differently, may cure slower.
The type of pigment will affect UV light scattering and, in consequence, the availability of light to photoinitiator. The cure of UV ink would also be slower. White and black are obviously most difficult to cure when they are very opaque, and that’s often a problem in UV products right now.
Other UV curing determining factors are temperature, substrate sensitivity and atmosphere of curing. From a chemical point of view, temperature always helps chemical reaction. It speeds up chemical reaction, and because curing is a chemical reaction, temperature has some important effects on speed of curing. Each 11°C (20°F) increase will approximately double the speed of chemical reaction.
Of course, in screen printing we are limited by heat sensitivity of the substrate. If the substrate is not very heat-sensitive, temperature will help to cure the ink. Substrate of itself, with an ability to reflect light back, may enhance cure due to the secondary passage of UV light through ink when reflected.
For example, shiny white or transparent sheets have this ability. The same ink may cure differently on different substrates, not because they are different types of substrates but because their reflectivity differs. Inert atmosphere is a somewhat controversial point; however, UV curing (from the chemical point of view) in acrylated systems is sensitive to oxygen which is present in air, and by purging the UV processor with nitrogen you can eliminate oxygen and eliminate the retardation of curing—so the overall result will be faster curing of UV ink by inerting the system. All these chemical considerations are reflected in printing elements that will influence control of UV curing.
Preparation For Printing
A screen printer will have the chance to prepare the UV ink for screen printing by color mixing, color matching or thinning down with reactive diluent (e.g., one of the monomers provided by the manufacturer of the ink), adding mixing clear for opacity control, or even adding photoinitiator.The printer can increase the speed of curing by adding photoinitiator, or he can lower opacity or adjust opacity to the right ink thickness by adding thinner or mixing clears or even adding photoinitiator. The less opaque, the faster the curing. In short, a screen printer can influence curing and choose the right way to get the UV cure down as he wants it. The other effect on curing involves printing technique, and there are two major problems here. First is the film thickness deposited by screen printing and second is the choice of substrate. The proper choice of screen mesh, squeegee sharpeners, squeegee durometer, thickness of stencil layer, squeegee angle and pressure, and position of the squeegee relative to the center of the cylinder press all will affect thickness.
There is not very much margin for error here. The thickness must be really controlled closely in order to have a successful UV cure. In many cases, substrate may not be chosen for the UV ink. I’m referring here to adhesion of the UV ink to substrate and reflectivity of some substrates. UV ink has better or worse adhesion to certain substrates, and that must be realized in using different vinyls, polyesters, polyethylenes, etc.
Lastly, the use of the UV curing processor will have a very significant effect on curing. I would say use and choice. Here is why. We know that UV light intensity is affecting curing, so there is the question of using the UV processor with 100-watt-per-inch, 200-watt-per-inch or 300-watt-per-inch lamps and beyond. Exposure will relate to conveyor belt speed; high conveyor belt speed will shorten exposure.
The number of lamps is a factor; the more lamps, the longer the exposure. Length of the exposure box will affect curing, as will the temperature which is created by using certain types of lamps. Obviously, lamps with lower wattage (100) will give out less heat and a, say 300-watt lamp will give out more heat. So, the question might be whether to cool, and if you want a cool unit. The fourth element related to the UV curing processor will be the option of having an inerting system.
These are normally not needed except for specialty applications. UV lighting intensity cannot be increased by simply increasing the exposure time—it can only be increased by increasing the power of UV light, and by better focusing of the UV lamp on the surface of the substrate. The intensity also can be influenced to some degree by spectral output of UV lamps— spectral range can be varied to the degree that the intensity of light which is absorbed by photoinitiator can be substantially increased. Proper exposure time or belt speed necessary for the complete cure of the UV ink is determined by the cure speed of that ink.
If high belt speed is required for printing purposes when using the same UV ink and UV light intensity, you may consider increasing the number of UV lamps. The same curing effect can be obtained by approximately simultaneous increase of the belt speed and number of UV lamps.
According to general rules, all chemical reactions are accelerated at elevated temperature. Obviously, we are limited by heat sensitivity of the substrate; thin, plastic substrate and some papers will lose dimensional integrity in temperatures higher than 49°C (120°F), so there are certain temperatures to be used. This is very important in multicolor printing where close registration is vital.
Thick substrates, however, act as heat insulators—they will always cause buildup of temperature regardless of how the unit is cooled, because of the collectivity of temperature. Metal substrates, however, because of excellent heat conductivity, will sustain relatively high heat without any increase of surface temperatures, and can be used for so-called hot units and still have very low surface temperature.
Containing low temperature for heat-sensitive substrates in UV curing is a very serious matter in screen printing with UV inks. The low temperature of substrates can be effectively controlled in the following ways: The use of a UV processor with efficient cooling system; the use of a UV processor that utilizes lower-power UV lamps; and obviously, by influencing conveyor belt speed. The higher the conveyor belt speed, the lower the temperature.
An important aspect of deciding whether to use UV ink is testing the ink for applicability. The printer should test UV ink for screen printing using a UV processor he has in mind and employing a given substrate to check adhesion. Then check opacity of the ink. If opacity is right, next to be determined is proper curing speed. Then come the physical and chemical properties such as flexibility and chemical resistance.A key problem will be how to test the curing. Sometimes the curing is not done properly, so rejection of UV technologies is not justified. Testing for completeness of cure is very important. First, I suggest a 24-hour wait and post-cure. In acrylated systems, some UV inks have some post-cure effect, so even if they appear cured, properties may change after a few hours. All testing of UV inks is recommended in general after a few hours and preferably after 24 hours. It may not be desirable in production, but it’s important to know that adhesion will sometimes improve with time.
Proper choice of substrate will have some effect, too. Some substrates are not applicable to UV ink right now. One reason is that if the substrate is very porous, the deep ink penetration will increase the path of UV light, and it may happen that light won’t even penetrate deep enough in the substrate to get the part of the ink cured which is embedded in the substrate. I have in mind porous papers which may cure on the surface and be wet on the bottom.
Obviously, when they are wet beneath the surface there is not adhesion. How do you determine that UV ink is completely cured? There are some very sophisticated methods of chemical and physical analysis, but they are not applicable in the printing shop. I would suggest testing completeness of cure using three steps, and all three of them probably should be used to determine completeness of cure.
First, try tack. Second, check for wet ink beneath the surface by scratching the surface by cross hatch tape testing the cured ink. Third, by performing a thumb twist test, some inks will appear hard on the surface, yet remain at the ink/substrate interface. It is very important not to overcure UV inks. Most UV inks are designed to be print receptive after multiple exposures to UV light. Eventually, after many multiple exposures, overcure will result, and it will be difficult to get intercoat adhesion.
It is also very important to understand that UV dried is not equal to UV-cured. I would recommend not using terminology like “UV-dried” or “UV dryer.” We are not drying UV inks, we are curing, and curing refers to the chemical reaction, not the physical process of evaporation or drying. UV can be dried on the surface but wet beneath, and we are not talking about curing in this case.
So the UV ink can be dry but not cured. In conclusion I would like to say that practical knowledge of UV curing chemistry and its tools are essential for successful application of UV technologies in printing. A screen printer doesn’t have to be a chemist, but some knowledge of the chemistry will be very helpful. People using UV inks that know a little bit about chemistry can do a lot of things themselves to improve curing.