by Scott Sabreen and Norman Robool
There are many benefits with utilizing ultraviolet (UV)-curable coating as a versatile and cost-effective process in decorating plastic products. It can offer significant benefits versus conventional thermal curing methods (using solvent-based coatings), providing critical advantages for manufacturers:
- Increased productivity
- Rapid drying time with minimal heat
- Reduced floor space requirements
- Lower energy costs (up to 75 percent)
- Reduced solvent content and VOC emissions
- One-part formulations
- Indefinite pot life
Optimal processing conditions of UV coatings can achieve excellent product quality and cosmetic appearance. End-use performance attributes demonstrate robust adhesion (ASTM tape test), abrasion-mar resistance/durability (Taber and RCA test methods) and chemical resistance. Custom formulations can be developed to achieve unique physical requirements as well.
The application of ultraviolet (UV) coatings is a photopolymerization process formation of molecular chains by fusion. This category of coatings contains various accelerators or catalysts that are dormant until acted upon by ultraviolet light. The UV light or electron bombardment triggers a free-radical reaction among chemical groups that results in cross-linking (curing) of the paint resins. UV coatings consist of liquid oligomers (polyester resins are very common and cost effective), monomers (generally acrylates as dilution agents), photoinitiators, and various additives and pigments as required. UV A electromagnetic radiation, approximately 300-450 nanometers in wavelength, is a very efficient range for curing most applications.
The chemical photoinitiators are sensitive to UV light, which changes the chemical bond structure of the photoinitiators, forming free-radical groups that trigger resin cross-linking. Curing happens in a 2-step sequence; first a photoinitiator absorbs UV rays and forms free radicals. These interact with resin molecules to form resin free radicals, then the small amount of heat from the infrared (IR) component in UV lamps accelerates the polymerization crosslinking reactions of the resin molecule free radicals. This IR heat is minimal due to the brief dwell time of parts in the UV cure zone, but it is enough to give a fully-cured coating. Some radicals often remain for a brief time (1-2 minutes) after UV exposure which gives a minor degree of added postcuring to the film.
UV coatings may or may not require solvent (or other fluidizing media) to reduce their viscosity and promote flow-out. If solvent is used, a flash-off time is allowed after application prior to UV cure. If the fluidizing media is also a cross-linker, it is called a reactive diluent. For reactive diluents, no flash-off time is required since they become part of the cured film. Rapid, extensive resin cross-linking can be initiated with UV light, so that often extremely low-molecular weight resins with very low viscosities are possible in the coating formulation. For this reason, the UV coating cures to a more stress-free and smoother film with less orange peel than possible with most heat-cured coatings. The UV resins may flow out so well by themselves that little solvent is required, allowing a low VOC coating, and in some cases even a zero VOC coating. The minimal solvent content in UV coatings results in only minor shrinkage from wet to dry film and considerably less induced film stresses compared to 2-component forced-curing and heat-curing coatings. Forced thermal-curing coatings most often contain 40 percent or more solvent content.
UV curing is fast usually in 10-60 seconds, which permits UV ovens to be confined and compact, and which enables faster production rates than cure methods that require substantial oven dwell times. The quick cure also minimizes substrate heating, which is a great advantage when curing films on heat-sensitive thermoplastic substrates. Since the UV lamps become hot, it is necessary that they be turned off whenever the production line stops to avoid harming the product being coated. In the past, many UV lamps could not be restarted quickly once they had been turned off. Fortunately, modern technology irradiator systems utilize fast on-off UV lamps that cool off rapidly to enable starting and stopping the coating line quickly.
Cure by UV is accomplished in shielded and enclosed chambers saturated with high intensity electrically generated UV light. For total curing to take place, the UV light must activate all of the photoinitiator molecules, which means that the light must see them. Therefore, the UV light source must be kept close to the painted part. Todays advanced irradiator systems utilizing highly polished parabolic reflectors enable a variety of 3-dimensional components to be UV-coated.
Despite all of the advantages of UV curable coatings there are potential process considerations that require proper application development. Pigmented coatings can be more difficult to cure because the pigment molecules will block UV light rays from some of the photoinitiator. Typical dry-film thickness for pigmented coatings is about 1-mil (25 microns). To resolve any issues, manufacturers of UV coatings & inks can adjust the photoinitiator concentration and spectral absorption of the material to match the proper bulb spectra distribution. Mercury-only wavelength bulbs (strongest output in the range of 220-320 nm) are well suited for curing clear coatings. Heavily pigmented coatings and inks typically require longer wavelengths to cure which is achieved by utilizing a doped or additive bulb (strongest output in the range of 350-400 nm). Another issue relates to the potential shrinkage of some coating formulations during photopolymerization. Solutions to these issues can be resolved via optimal coating formulation and curing parameters.
In addition to spray-applied UV coatings (atomization, electrostatic, etc.), UV inks are commonly used in transfer pad printing, screen printing and off-set printing decorating processes. Some applications (keyboard industry) apply a UV clear coat over pigmented inks for superior abrasion resistance. In general, UV applications have narrower process windows than thermal drying. Many tools are available to establish process control limits, including radiometers, which provide valuable information relative to radiation dose, peak irradiance and speed/dose ratio. Regardless of the decorating method, the key to robust UV manufacturing operations relates directly to proper development engineering and Six-Sigma quality control techniques.
Scott R. Sabreen is Founder & President of The Sabreen Group (TSG). TSG is an engineering company specializing in secondary plastics manufacturing operations surface pretreatments, bonding decorating and finishing, and laser marking. For more information call toll-free at 888-SABREEN or visit: www.sabreen.com.