|The UV & EB Curing Process|
UV & EB curing typically describes the use of electron beam (EB), ultraviolet (UV) or visible light to polymerize a combination of monomers and oligomers onto a substrate. The UV & EB material may be formulated into an ink, coating, adhesive or other product. The process is also known as radiation curing or radcure because UV and EB are radiant energy sources. The energy sources for UV or visible light cure are typically medium pressure mercury lamps, pulsed xenon lamps, LEDs or lasers. EB--unlike photons of light, which tend to be absorbed mainly at the surface of materials--has the ability to penetrate through matter.
Three Compelling Reasons to Convert to UV & EB Technology
Energy Savings and Improved Productivity: Since most systems are solvent-free and require less than a second of exposure, the productivity gains can be tremendous compared to conventional coating techniques. Web line speeds of 1,000 ft/min. are common and the product is immediately ready for testing and shipment.
Suited for Sensitive Substrates: Most systems do not contain any water or solvent. In addition, the process provides total control of the cure temperature making it ideal for application on heat sensitive substrates.
Environmentally and User Friendly: Compositions are typically solvent-free so emissions and flammability are not a concern. Light cure systems are compatible with almost all application techniques and require a minimum of space. UV lamps can usually be installed on existing production lines.
UV & EB Curable Compositions
Monomers are the simplest building blocks from which synthetic organic materials are made. A simple monomer derived from petroleum feed is ethylene. It is represented by: H2C=CH2. The symbol "=" between the two units or atoms of carbon represent a reactive site or, as chemists refer to it, a "double bond" or unsaturation. It is sites like these which are capable of reacting to form bigger or larger chemical materials called oligomers and polymers.
A polymer is a grouping of many (i.e. poly-) repeat units of the same monomer. The term oligomer is a special term used to designate those polymers which often can be further reacted to form a large combination of polymers. The unsaturation sites on oligomers and monomers alone will not undergo a reaction or crosslinking.
In the case of electron beam cure, the high energy electrons interact directly with the atoms of the unsaturated site to generate a highly reactive molecule. If UV or visible light is utilized as the energy source, a photoinitiator is added to the mixture. The photoinitiator, when exposed to light, generates free radical or actions which initiate crosslinking between the unsaturation sites.ponents of UV &ude
Oligomers: The overall properties of any coating, ink, adhesive or binder crosslinked by radiant energy are determined primarily by the oligomers used in the formulation. Oligomers are moderately low molecular weight polymers, most of which are based on the acrylation of different structures. The acrylation imparts the unsaturation or the "C=C" group to the ends of the oligomer.
Monomers: Monomers are primarily used as diluents to lower the viscosity of the uncured material to facilitate application. They can be monofunctional, containing only one reactive group or unsaturation site, or multifunctional. This unsaturation allows them to react and become incorporated into the cured or finished material, rather than volatilizing into the atmosphere as is common with conventional coatings. Multifunctional monomers, because they contain two or more reactive sites, form links between oligomer molecules and other monomers in the formulation.
Photoinitiators: This ingredient absorbs light and is responsible for the production of free radicals or actions. Free radicals or actions are high energy species that induce crosslinking between the unsaturation sites of monomers, oligomers and polymers. Photoinitiators are not needed for electron beam cured systems because the electrons are able to initiate crosslinking.
Additives: The most common are stabilizers, which prevent gelation in storage and premature curing due to low levels of light exposure. Color pigments, dyes, defoamers, adhesion promoters, flatting agents, wetting agents and slip aids are examples of other additives.
RadTech International publishes and distributes a variety of books and other publications on UV & EB curing technology. Please refer to the RadTech Publications Catalog for additional resources, including the recently revised UV & EB Curing Primer and the association's bimonthly magazine RadTech Report.