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UV Curable Inks and Special Effect Decorative Pigments
by Scott Sabreen
Ask the Expert
Question: UV-curable Inks
We are considering changing from heat curing inks to UV to speed up our printing process lines. Will UV ink stick to polyethylene, and how does the process work?
UV inks can achieve excellent adhesion to polyethylene using a surface pretreatment such as corona discharge or flame treatment. Note that some pigment colors may be more difficult to print than others, but your ink supplier can assist you. Basically, the application of ultraviolet (UV) inks is a photopolymerization process or formation of molecular chains by fusion. Various chemical accelerators or catalysts are dormant in the ink until acted upon by ultraviolet light. UV inks generally 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 post-curing to the ink.
Question: Special-effect Decorative Pigments
What’s the difference between pearlescent and metallic effect pigments?
Pearlescent pigments used in paint, inks, and plastic are based on mica flakes coated with titanium dioxide. As the coating thickness increases, the color varies from silvery white to yellow, red, blue, and green. Different colors can be achieved by adding a second coating of iron oxide (gold and beige) or chrome oxide (green), and a range of metallic colors (bronze and copper) is achieved by replacing the titanium dioxide with iron oxide. Pearl-like appearances can be “tuned” by adjusting the size of the flakes. Small flakes (about 5 microns) give rise to a satiny appearance with good opacity. Larger flakes (about 25 microns) give a lustrous effect with lower hiding power. Typically you would blend different particle sizes to achieve a desired combination of luster and opacity.
Metallic pigments are made from copper alloys and aluminum, and are available in a variety of colors and particle shapes. The copper versions vary in color from bright greenish gold to red gold. Aluminum versions are silver and silver-gray in color. Originally, the vast majority of offerings were in flake form to maximize luster. As with pearlescents, the plastics processor has to take care to minimize shear forces to preserve the particle shape. Because metallic pigments conduct heat and electricity, they are often used to impart other functional properties aside from decoration. Examples include antistatic properties and electromagnetic shielding, microwave absorption to promote heating, and thermal management (conducting and radiating heat). A reference source is “Coloring of Plastics, Fundamentals”, by Robert A. Charvat.