UV Coating vs. Conventional Coatings
Q. Our paint department is currently painting our metal transformer housing parts with conventional solvent-based coatings, and we want to possibly convert this process over to UV coating in an effort to speed up our dry time in production. We have researched this and while it shows promise for increased production we are not sure of how to go about setting up and applying this type of coating. We are also not sure which type of UV would be our best option between regular and water-based UV. Could you give us some insight as to what we should be looking for with the various types of UV technologies?—K.D.
A. Many companies are making transitions to powder or UV coatings as suitable alternatives to solvent-based coating products. So, first things first… What do you need to consider when determining if your operation is able to convert to a UV process? I am assuming that with your current finish system you are conventionally spraying and possibly oven drying/curing your transformer parts.
But unlike conventional solvent-based finish operations, UV coatings—whether 100 percent solids, dual cure or even waterborne—have a process that is much different than their conventional solvent counterparts. Yes, the UV coating is still applied in much the same fashion as solvent, but what happens beyond this point is much different.
As I previously mentioned, with most solvent systems, the part is sprayed and cured by air or in an oven, whereas with UV the part is instantly cured with one or more UV mercury (Hg) and/or gallium (Ga) lamps, depending on the chemistries and color densities. The output of either of these types of lamps is measured in millijoules output. The higher the lamps are adjusted for output, the higher the millijoules energy. This output information is crucial, as each manufacturer will have a recommended millijoule target to achieve proper curing. Too little output and the product will not cure, and too much can result in improper over-curing.
Typically, this output will range from 400-700 millijoules for most UV coatings. Millijoules output can be monitored with a radiometer or “puck” as referred to in the industry. This device is passed under the lamps at the proper production line speed to insure you are supplying the proper output to optimally cure the UV coating. So why are there two types of lamps you ask? Mercury lamps are used to cure clear and semi-transparent coatings, whereas gallium lamps are used to cure more deeply with opaque paint coatings. You can ask the coatings supplier for recommendations as to the proper configuration for its products.
I will try to further highlight the finer nuances of what to look at when determining which type of UV to use. First, many manufacturers are concerned about volatile organic compounds (VOCs) and the emissions related to this. As your company grows, these emissions can place you in a higher tonnage emission bracket and compliancy-related issues, often leading to expensive retrofitting of operations. So in an effort to fall under these thresholds, UV can be one of many types of coating processes that, implemented correctly, help with reductions in this area.
But which type of UV is best? First, let’s break down the various UV options available. “100 percent solids” UV is a combination of monomers, oligomers, and photoinitiators that requires no carrier solvents to flash as part of the drying and curing process. This type of UV coating is mechanically spray-applied to the substrate surface and passed under a lamp series of mercury (Hg), gallium (Ga) or even both to achieve an immediate cure.
I mentioned mechanical application because UV typically is applied with systems like vertical rotary bell atomizing spray equipment or robotics, fitted with airless guns, air-assist guns and/or rotary bell. So why is this? There are health issues related to UV coating materials for one, but the bigger issue is that for UV coating to properly cure, it needs to be applied with much more consistent mil application tolerances to ensure proper cure under the various lamps.
That said, let’s now look at dual cure UV. Much like its 100 percent solids counterpart, it has many of the same ingredients, but it incorporates some solvent to lower the cost and to ensure that the coatings will cure, even if the lamps do not efficiently do the job. It will still give you a very fast cure, but typically at a lower cost.
Lastly, you have waterborne UV which, if you are looking for a greener solution, works very well. However, the curing process is a little different. While the coating still utilizes and requires the same types of mercury and gallium lamps, it needs to have the water carrier driven out of the coating before it can pass under these lamps for final cure. Water-based UV must first pass through force-dry ovens for 6-10 minutes to drive water out. Once this has been achieved, it can be fully and instantly cured. The same general rule applies with clear coats using mercury lamps for cure, and painted material using gallium lamps.
One key thing to look at when considering water-based UV is that at the onset, it is typically more expensive than its 100 percent or dual cure counterparts. However, when you more closely look at its advantages, it is cost comparative. How is this possible, you ask? Both 100 percent and dual cure need to be cleaned up with costly solvents, and then hauled off as expensive hazardous waste, not to mention the additional insurance you need to carry for the flammability factor. With water-based UV coating, the coating is cleaned up with… you guessed it: water.
The zero-flammability attribute of water-based coatings will also help in reducing your insurance. As with any finishing operation, you’ll need to look at each UV process and its associated costs to determine what is best for your operation. The great benefit with most UV coatings is that they are fast, and allow for greater turnaround time in the finishing department, which is always a good thing.