UV coatings are formulated with 100% reactive liquid vehicles such as oligomers, monomers, photoinitiators and additives. Relating this to traditional coatings, the oligomers can be thought of as the resin, monomers as the solvent, photoinitiators as the catalyst and the additives as additives.
UV coatings are cured using various wavelengths of ultraviolet light. When exposed to high intensity UV light, the photoinitiators in the coating are transformed into free radicals. The free radicals react with the oligomers, monomers and other active chemicals in the coating. The reaction converts the liquid coating to a plastic in a fraction of a second. The UV lamps and the photoinitiators must be matched to ensure a proper cure that will attain the desired coating properties. This "quick" curing feature of UV coatings can offer an end-user significant overall savings in many areas.
Reduced Floor Space
One benefit is the reduction of plant floor space required for UV curing when compared to thermal curing used with traditional bake type coatings. Normal bake ovens may contain several hundred feet of conveyor based on the conveyor speed and required cure time. On the other hand, a typical UV curing chamber may only be 10 ft in length. This will vary based on the type and size of product coated. The smaller "oven" also means that there will be less work in process or inventory tied up in the coating system.
Environmentally Friendly Coatings
UV coatings offer an environmentally friendly alternative to traditional coatings. As with traditional coatings, UV coatings are designed to meet many different applications and processes. When formulated as 100% solids, there are no solvents (VOCs) in the coating.
The term 100% solids often conjures thoughts of solid objects and unpourable liquids. In actuality, 100% solid UV coatings are very user friendly with regards to handling and supplying to application equipment. Viscosity can be as low as 22 sec in a No. 2 Zahn cup (approximately 35 centipoise).
Although there are many applications using 100% solids UV materials, there are an equal number that use reduced solids. Many handheld consumer electronic devices are topcoated with UV materials that are approximately 60% solids. These coatings are applied over traditional base coats and are used to obtain desired abrasion and stain resistance. Some solvents are required in the coating in order to obtain adhesion to the base coat material. Prior to exposure to UV light for curing, the solvent must be allowed to evaporate from the coating. Exposure to infrared (IR) light before the UV light is a common method used to speed up the solvent evaporation time.
Quite often in the decorative plastics market, high-volume, low-pressure (HVLP) applicators are used in conjunction with a chain-on-edge type conveyor system. This type of setup can successfully apply lower solids UV coatings.
If the intent of using UV is environmental compliance, then the end-user may want to apply a 100% solids coating formulation. At lower flow rates (under 50 cc/min) success can be achieved.
As flow rates increase and pigments are added to the formulation, experience has taught us that a rotary atomizer often yields the best application results. A rotary atomizer, or bell as it is often called, uses centrifugal and mechanical forces to atomize the coating material. This method of atomization provides uniform particle size and distribution. A rotary atomizer is a type of electrostatic applicator. During the atomization process, a negative electrostatic charge is induced on the atomized coating particle. The product to be coated is at ground potential and appears to be an opposite charge. The principal is simple-opposites attract-and the UV coating is drawn to the grounded part.
Electrostatics is commonly used with conductive or groundable substrates. It can also be used with nonconductive substrates using various methods such as conductive primers, adhesion promoters, prep coats or the inherent conductivity/moisture level within the substrates. An electrostatic application offers several additional benefits. Since the coating is attracted to the product being coated, better overall uniformity can be obtained. Hidden edges that can be difficult to coat will be coated by a process called electrostatic wrap. Coating that would typically be carried past the part and into the spray booth in the form of overspray is now drawn back onto the part. An electrostatic application will also yield higher transfer efficiency. In other words, more of the coating applied will end up on the product coated. Typical transfer efficiencies with non-electrostatic applicators are 15-35%. With electrostatics, transfer efficiencies of 45-85% are possible depending on the application equipment. It is common to reduce material use 30-50% when switching over to an electrostatic application system.
Fluid Handling Considerations
Through experience, we have learned that there are some unique characteristics associated with the spray application of UV coatings.
All wetted components in the fluid delivery system should be stainless steel or Teflon. Fittings made of brass may be attacked by the UV coatings, causing defects in the final film.
The fluid supply lines and tubes should be opaque to minimize exposure of the UV coating to any ambient light or sunlight in the application area. By reducing light exposure to the fluid delivery lines, premature curing of the materials will be eliminated, and clogging in the delivery system will be reduced.
UV coatings are shear sensitive. Excessive shear induced on the coating may cause premature polymerization and the material will cause the device to seize. Examples of fluid delivery equipment to avoid are gear and piston pumps. This phenomenon may not be as prevalent with lower solids UV materials.
Fluid supply to the application equipment should be from a pressure pot, diaphragm or peristaltic type pump. None of these devices induce shear on the coating.
Pigmented 100% solid UV coatings tend to be higher in viscosity than clearcoats. For higher viscosity coatings, an inline paint heater will lower the viscosity. A lower viscosity material is easier to pump as well as atomize. For example, a coating that measures 58 sec in a No. 3 Zahn at 77F measures 28 sec in a Zahn No. 3 at 115F. A fluid heater should be used in conjunction with a diaphragm pump to create a recirculating fluid delivery system. This will ensure consistent heat.
The importance of controls to monitor the UV coating process is critical. Knowing all the variables that can affect the application process is crucial. Operators must have a good understanding of what it takes to repeat the process in a production environment. To make the coating process repeatable one must determine what parameters will be valuable to monitor. Application parameters to be monitored should include atomization air pressures or speeds, shaping air, pattern air, fluid flow rates, voltage levels, part fixturing, grounding and target distance. Environmental conditions to monitor should include temperature, humidity and airflow.
The most important factor is the fluid delivery rates. Excessive fluid delivery rates will cause increased coating costs and may create curing problems. This would be most evident with pigmented UV systems.
A unique feature of 100% solid UV coatings is that wet film thickness is equal to dry film thickness. Coating thickness can be built very quickly on the product coated. Because of this, it is often recommended that a fluid monitoring or closed loop fluid delivery system be used. A monitoring system can be as simple as inserting a fluid flow meter in the fluid stream. The flow meter sends a signal to a display and allows the operator to monitor and adjust the fluid delivery rate (this is similar to having a speedometer in your car). A more advanced system may use the same flow meter and make adjustments using a transducer to maintain an operator selected fluid delivery rate. Systems like these continually monitor, compare and adjust the fluid flow rate in an effort to maintain a desired set point (similar to cruise control). The ability to monitor and control the application process is critical in maintaining transfer efficiencies, material savings, accurate film builds and ultimately VOC compliance.
If you want to evaluate the validity of UV coatings for your process, it is important that you assemble the right team consisting of the coating, lamp and equipment suppliers. A good first step is to submit samples of your substrate to the coating suppliers and have them generate coated samples in their facilities for you to review. These samples should be evaluated to ensure that they meet the physical requirements of your finished product. These requirements may include gloss, orange peel, hardness, abrasion resistance, corrosion resistance and adhesion. Once you have qualified the coating on your substrate, a trial can be coordinated online or at the equipment or lamp supplier's facility. This trail should simulate your existing or proposed process as closely as possible.