Technical Support at the Speed of Light
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Overview
Radiation curing is a process for creating and using a desired coating or adhesive.
There are many methods of applying a coating or adhesive to products that we are all
familiar with. Perhaps the simplest examples are the use of glue and paint. Suppose we
are painting two blocks of wood, one with a water based paint and the other with a solvent
based paint. The process for drying the paint or specialty chemical formulation, is called
"curing". The paint is brushed or sprayed onto the wood and allowed time to cure. In
the case of water based paint, allowing time for the water to evaporate from the paint
solution cures the paint. Solvent based products cure in the same manner; however, the
water is replaced with a VOC (Volatile Organic Compound). Solvents allow for improved
or different final properties for the paint (toughness, gloss, hardness, etc.) and because
solvents typically evaporate at lower temperatures than water, they can cure faster.
Usually, the process is a result of exposure to ambient room temperature and humidity
however, the process for both water based and solvent based methods can be sped up
by creating higher temperatures and employing other methods of altering ambient
conditions. Since VOC's are pollutants and are usually highly flammable, adequate
ventilation is always required for solvent based products.
In contrast, radiation curing does not give off VOC's like solvent based systems and
does not consume as much energy and time to dry as water based formulations. As a
result of being both environmentally friendly and cost effective, radiation curing is a highly
preferred and growing method. Limitations to radiation curing growth include lack of
awareness and understanding, and the expense required for industries to replace current
solvent and water based manufacturing equipment and processes.
How does Radiation Curing Work?
As with the examples above, the process of radiation curing requires a source of
energy (radiation). When a specialty chemical formulation is exposed to radiation, it is
cured not by driving off water or VOC's, but by causing a chemical reaction within the
formulation itself. As a result, the process is typically very fast. A simplistic diagram
depicting the process is shown below.
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About Specialty Chemical Formulations
Radiation cured formulations are solutions made up of photoinitiators, oligomers,
and monomers and additives. Photoinitiators are the active ingredient that when
exposed to radiation, start a chain reaction that hardens the solution (cross linking or
polymerization). Oligomers are low molecular weight polymers and make up the bulk of
the formulated solution accounting for most of the final physical properties. Monomers
are used to dilute or "thin" the formulated solution which can make application and
handling prior to curing easier. As an example, a solution could be formulated for
spraying onto a product surface or substrate receiving the finish, and would need an
appropriate viscosity to be suitable for injection through the spray nozzle.
About Radiation
Radiation can come in many forms and from many sources. Some examples
include UV (Ultraviolet) light lamps, heat, lasers and electron beams. Each of these
sources emit "waves" of energy. The energy when transferred to a photoinitiator
causes the initiator to react. In the case of a radiation cured specialty chemical
formulation, exposure starts the curing process. The science of obtaining proper
radiation sources, source location and intensity is critical to obtaining system
optimization.
By categorizing radiation waves according to their characteristic wavelength and
frequency, wave types can be named and in turn, used by a formulating chemist or
manufacturer to determine what type of radiation optimizes their process. There are
many factors to be included to properly match a radiation source to a formulation, one
of which is proper wave length. Below is a diagram showing names and categorization
of some forms of radiation including the visible light spectrum shown as rainbow colors
on the chart. Radiation sources may be very targeted to a wavelength as with a LASER
however most include a mix of wavelengths and intensities which must be factored
when trying to establish an optimal system for radiation curing. Source suppliers such
as FusionUV have spent significant research and development into providing clients
optimization tools for radiation source selection and placement.
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