"I have an application where I am dome coating thin polyester labels. I’m experiencing some curling when I cure the dome coating. Do you have any recommendations on how to eliminate this?"
To eliminate curl when doming on thin polyester labels, try pre-curing the labels with a black light. The black light should have a maximum intensity at 365 nm. You can order one from most industrial electrical lighting supply houses. Regular purple-colored bulbs or germicidal bulbs will not work.
Pre-curing the label with the black light polymerizes the coating below the coating’s surface. A second lamp can then be used to cure the surface to a scratch-resistant state. As the label gets larger, softer coatings are necessary to prevent warping. You can also use of
thicker polyester stock or stiffer backing sheets to
minimize curling of the domed decals.
“Considerations for Selecting a Dome Coating”
With hundreds of dome coating options available on the market, selecting the right dome coating for your application can seem like a daunting task. When selecting the best coating for your application, it is important to consider five main criteria: viscosity, substrate flexibility, substrate adhesion, hardness, and environmental durability.
- Viscosity. How thick does the coating need to be? Decorative coatings are clear and usually thin. Dome coatings have curved domes, which optically magnify and enhance the appearance of a label. Higher-viscosity, thicker resins generally produce taller domes.
- Substrate Flexibility. Flexible substrates, like thin polyester labels, paper, or soft plastics, may require careful selection of coating and enhanced curing equipment. Flexible substrates may bow or warp when thick coatings cure. Soft coatings shrink less and are designed to reduce or eliminate warpage during cure. Rigid substrates don’t bend when stressed, so they can be coated with all of the products.
- Substrate Adhesion. Individual formulations may have excellent adhesion to some inks (or substrates) and little adhesion to others. The right coating should have good adhesion. Adhesion should be tested after product lifetime testing, as well as after cure. Inks and substrates from second source suppliers may improve or worsen adhesion. Some plastics require surface treatment to enhance adhesion.
- Hardness. Both soft and hard coatings can be scratch resistant. Hard coatings resist scratching because of their hardness. Soft coatings resist scratching because they momentarily dent and then spring back when the scratching object is withdrawn. Hard coatings are measured on the D-hardness scale. Soft coatings are measured on the hardness scale. On both scales, higher numbers imply harder coatings.
- Environmental Durability. Coatings should be lifetime tested on your product. Some properties, like UV weatherability, provide a relative guideline to distinguish endurance of clarity and durability among the available products offered.
Coatings, Dome Coatings
Tackiness or stickiness may be noticed on the surface of some ultraviolet (UV) light curable adhesives and coatings. This phenomenon, known as oxygen inhibition, is the result of atmospheric oxygen inhibiting the cure on the surface layer of the polymerizing material. This condition is present anytime free radical polymerization occurs. However, the ability of a UV resin to be cured "tack-free or to a slick, dry finish" is dependent on the composition of the adhesive or coating formulation and the intensity and wavelength of the UV light.
Overcoming the effects of oxygen inhibition and producing a tack-free surface cure is dependent upon several factors:
- Heat generated by the UV curing system
- Intensity generated over the entire UV spectrum (200-390 nm)
- Exposure time
- Specific formulation of the adhesive or coating
Typically, short and medium wavelength (220-320 nm) UV light generated by mercury vapor lamps achieve more efficient surface cures. Short and medium wavelength curing systems, however, do have depth of cure and safety issues associated with them making them an undesirable option for many UV curing applications. Longer wavelength (320-390 nm) systems, which usually emit a small fraction of UV light in the lower wavelengths, will provide fast, tack-free curing while achieving better depth of cure.
Time to cure “tack free” should not be confused with full cure time. It is only an indication of the material’s ability to overcome oxygen inhibition, at the surface of the curing material, when the material is exposed to a given level of light intensity for a specific period of time. It has been demonstrated that the higher the intensity of the UV light the lower the total energy level needed to achieve a "tack free" surface. For example, to produce a tack-free surface cure of a DYMAX conformal coating (984-LVUF) using a 200 mW/cm2 light source, the coating should be exposed for 20-30 seconds. This equals approximately 7 J/cm2 of energy. This same coating cures in 1-2 seconds when exposed to 2500 mW/cm2 equaling 2.5 J/cm2 of energy.
Even though removing oxygen from the surface will also work at achieving a tack-free surface, this tends to be the least desirable method since it can be logistically challenging to implement.
Adhesives, Coatings, Conformal Coatings, Dome Coatings, Electronic