UV-LED Curable Coatings offer a high-speed light curing process with a number of advantages over more conventional cure processes. Multiple advantages include High speed, lower energy requirements, little or no VOC, less production space, less dirt collection, high quality finish, rapid processing as well as instant on-off with some UV light technologies also expedite production and energy savings. UV Curable paint finishes have existed since the 1960’s and are based on polymerization reactions including free radical and cation-initiated chain-growth polymerization. As the majority of coatings for UV cure coating utilize free radical polymerization (>90% of market), this article will focus primarily on free radical polymerization initiated by a photoinitiator (Fig. 1):
The types of unsaturation used in UV/EB cure coatings are provided in Table I, with by far the largest type being acrylate.
Photoinitiator considerations primarily include two different characteristics of the photoinitiator’s absorption curve. First, is the maximum wavelength (Lambda Max) of light that is absorbed by the PI and second, the strength of this absorption (molar extinction coefficient). Photoinitiators developed for curing pigmented films normally have higher molar extinction coefficients at longer wavelengths between 300 nm to 450 nm than those for curing clear formulations. To maximize cure and efficiency, the PI’s absorbance must match the light output of the lamp as different lamps have different spectral outputs (see Table I). Longer wave- length light is also essential to enhance cure in thicker coatings. Newer PI’s have also enabled the formulation of pigmented coatings in addition to that of clear coatings. The general cure considerations influenced by color, PVC, pigment particle size and film thickness are summarized in Fig. 2:
There are two main types of free radical photoinitiators, Type I and Type II. Type I photoinitiators undergo cleavage upon irradiation to form two free radicals. Normally only one of these free radicals is reactive and thus initiates polymerization. 1-hydroxy-cyclohexylphenyl-ketone is a widely used Type I PI. Type II photoinitiators form an excited state upon irradiation, and abstract an atom or electron from a donor molecule (synergist). The donor molecule in turn initiates polymerization. An example of a widely used Type II photoinitiator is benzophenone. Tertiary amines are typically used as synergists as they react with benzophenone, and also retard the inhibition of polymerization by oxygen. Acrylated tertiary amine compounds are used when odor and extractables are of concern. Oxygen can also inhibit cure especially in thin films; to counteract oxygen inhibition, coatings can use amine synergists, be cured under a nitrogen atmosphere, employ the addition of wax, high initiator concentration, more intense UV Light, and/or surface active initiators.
Other key ingredients that determine the performance of UV Cure formulations include UV Curable Monomers and Oligomers. Figure 3 illustrates typical monomers that are used along with performance characteristics.
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There are a number of UV curable oligomer types available as well depending on the type of performance desired, please refer to Fig. 4 for a listing of some of the common oligomer types available along with an overview of performance characteristics.
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 cure the coating in seconds instead of minutes, as with traditional thermal powder.
Any discussion of UV-LED cure coatings is remiss without at least a short overview of UV-LED bulbs as well as the characteristics of each type. As illustrated in Table III.
A final consideration of UV cure coatings is that they are normally line of sight. In other words, for complex three-dimensional surfaces, where the light does not shine, the coating will not cure. Also, most UV cure technologies provide optimum uniform cure on a two dimensional surface using focused light. LED curing has multiple advantages over more traditional UV cure technology such as low heat generation. This is ideal for curing heat sensitive substrates. In addition LED offers an ozone free environment, energy efficiency, and ultra long bulb life and the stable spectral output means consistent quality.
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Dear Ron,
Interesting information, specially those parts about photo-initiators and UV light sources.
Hello Nader, thank you very much for your comments. I hope the article was helpful!
Dear Ron,
Thank you for your information. I have been trying a formulation for UV curable primer using acrylate monomers, oligomers and photoinititaor . It is used as coating on EVA plastic sheet before application the adhesive to EVA sheet. It shoud be very fast cured with UV lamb. But I could not obtain cure rate up to now. Do you have recommendation about this subjects. Thank you in advance.
Sincerely yours
Enver Demirhan, PhD, Chem.Eng.
Hello Enver,
Please provide some additional information such as PI and bulb type you are using as the PI wavelength of activation should overlap with the spectral output of the lamp for proper activation. Also, if you are curing in air and applying a thin film you may need an amine synergist.
Kind regards,
Ron Lewarchik
Dear sir Good day
I am very interested with uv cured paint for wood and mdf.
I have wood workshop and I need to start this kind of paints.
I hope you supply me .
Kindly give me the price list for liquid uv paints with primer,fillers,putty ( any materials need for finishing)
I hope you reply me soon.
Thank you
Hi Abed,
Thanks for your question. We don’t provide supply prices – you’ll need to contact the manufacturer directly for those, to better address your specific needs.
Thanks!
Angie, Prospector Knowledge Center content manager
Hello sir i want to produce uv curable spray paint for metal components
There are tons of articles about this new technology (LED-cured wood coating) but almost no information about suppliers of the coatings themselves. I found 3: deVesting from the Netherlands (USA office in CA) that has an LED-cured oil, various colors: Sherwin Williams AcromaPro – solid paints only sold through only a very few distributors; Rubiocoat but they say not available in N America until sometime 2018.
do you have any other sources for the actual finishes? there are dozens of providers of the light equipment, both domestic and abroad.
Thanks!
Dear sir I am in to production of uv led curable fingernail cosmetic coatings
I want to produce tack free high gloss top coat but not getting right inputs so need consultation for the same
Hi Ron
thanks for a good read is there a uv cure paint in Australia that you know of
i would like more info if you can help me on uv cure paint for zinc coated steel
thanks from David
The demand for the UV LED across ink, adhesive, and coating industries for the process of curing plays a significant role in the development of the UV LED market.