Coatings have a variety of curing options, including UV and LED mechanisms. In this article, we discuss the types of resins utilized in those same UV-curable coatings as well as Electron Beam (EB) curing.
As mentioned in the previous article, UV-LED curable coatings offer a high-speed light curing process with a number of advantages over more conventional cure processes. In addition to fast line speed (higher productivity) and solvent-free compositions, UV/EB curable coatings offer many more important benefits. These include:
- Reduced floor space – UV/EB curing equipment has a much smaller footprint and is more compact than conventional drying ovens, and the solvent-free compositions require less storage space than solvent-based coatings, providing comparable dry film weight.
- Suitable for heat-sensitive substrates – The absence of thermal drying and fast line speeds achieved with UV/EB curable coatings result in a relatively cool coating process which can be used for coating heat-sensitive substrates, such as plastic, wood and paper.
- Reduced in-process inventory – A conventional thermal curing coating manufacturing process, requiring intermediate drying stages, can be converted into a single-step, in-line process with UV/EB curable coatings.
- Lower insurance costs and reduced handling hazards – Solventless UV/EB curable coatings are rated as non-flammable liquids. This means reduced insurance costs, less stringent storage requirements and a reduction in handling hazards compared to flammable solvent-based coatings.
- Compliant technology – Federal, state and local governments recognize the many advantages offered by UV/EB curable coatings in complying with volatile organic compounds (VOC) and hazardous air pollutants (HAP) restrictions1 2. For example, UV curable coatings for metal can application have been reported by the EPA to contain less than 0.01 VOC/gallon of coating. Coors reported no significant emission of ozone or other undesirable emissions from a UV can line for one billion cans per year.
- Reduced costs – According to several studies, switching from conventional thermal curing coatings to UV/EB curable coatings can result in a significant reduction in energy costs. Additional studies show that switching to UV/EB curable coatings is less expensive than converting an existing solvent- based coating operation into a VOC and HAP compliant operation.
- Proven technology – UV/EB curable coatings are a proven technology in commercial use worldwide since the 1960s.
One can formulate with bio-based raw materials instead of ones derived from petrochemicals.1 With a focus on sustainable and renewable raw materials, using plant-derived materials to formulate products is quite attractive, and they often perform as well as or better than their petrochemically-derived counterparts.
The types of unsaturation used in UV/EB cured coatings are provided in Table I, with acrylate the largest type by far.
UV Curable Monomers and Oligomers
UV Curable monomers and oligomers are other key ingredients that determine the performance of UV cure formulations. Figure 2 illustrates typical monomers that are used and their performance characteristics.
Generally, a mixture of monofunctional (one acrylate group) and polyfunctional (more than one acrylate group) acrylates is used in order to optimize cured film properties and liquid coating cure speed. Monofunctional monomers tend to reduce viscosity more effectively than polyfunctional acrylates. The monofunctional monomers also reduce cured film shrinkage and increase the elasticity of the cured film. However, a high concentration of monofunctional monomer severely reduces the coating cure speed.
Highly functionalized monomers increase coating cure speed and cured film resistance to abrasion. Unfortunately, these two desirable cured film features are achieved at the sacrifice of embrittling the cured film and reducing adhesion to the substrate. Optimized coating properties are achieved by systematically balancing the oligomer and monomer concentrations. Eyeglass lenses are coated for abrasion resistance and other attributes, which can easily be accomplished utilizing UV coatings based on these materials.
There are a number of UV curable oligomer types available as well depending on the type of performance desired. Figure 3 lists some of the common oligomer types available along with an overview of performance characteristics.
Waterborne UV and Powder UV
In addition to 100% solid liquid UV coatings, other UV types include waterborne UV and powder UV. Waterborne UV curables have advantages over conventional UV cure as no reactive diluent is necessary to control viscosity. Also, as opposed to conventional UV cure formulations, the viscosity of the coating is independent of the molecular weight of the resin and for spray application viscosity; solids are adjusted by adding water rather than low viscosity reactive monomer.
In addition, since there are fewer double bonds to cure, shrinkage is lower and can thus improve adhesion. The main disadvantage is that the water needs to be removed by passing through an oven at about 80°C prior to UV curing.
In powder UV cure coatings, the part is sprayed electrostatically. Automatic guns are recommended over manual application to ensure an even, consistent film thickness is applied. Next, the applied coating is baked in a convection, IR or oven to melt and flow the powder. This step is at a much lower temperature and less time (175-280°F for a few seconds, instead of 320-390°F for 5 to 20 minutes) for conventional powder coating. Once the powder is melt flowed, the parts enter a UV cure chamber that cures the coating in seconds instead of minutes, as with traditional thermal powder.
References and resources:
- “Synthesis of bio-based unsaturated polyester resins and their application in waterborne UV-curable coatings”
- Image: Ciba–Geigy literature
- “An Introduction to Ultraviolet Light (UV) and Electron Bean (EB) Curable Coating Technology“
- “UV-LED Curable Coatings – Why Wait When You Can Cure at the Speed of Light“
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