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May282014

Fundamental Principles of UV Reactive Manufacturing Processes


While the UV systems technology and the appropriate chemistry have been developing continuously, the principle of irradiation curing has widely remained unchanged: High-energy UV irradiation causes chemical curing of UV reactive coatings within seconds.

UV irradiation

Discharge Lamps
The heart of a UV discharge lamp is the silica tube with fused-in electrodes on both ends. With the ignition of the UV lamp a high-voltage arc develops between the electrodes and the mercury, which is in the tube becomes activated and evaporates. In this activated state the mercury vapour emits the typical UV spectrum of medium pressure radiator.




The short-wave, high-energy UV irradiation in the spectrum between 200nm and 400nm is able to convert a liquid UV reactive substance into a solid film within a split second.

 

















DIN 5031, part 7 classifies the UV range of the electromagnetic spectrum into four sub-groups, with significant characteristics in each.
















By adding different dopings to the mercury, e.g. iron or gallium, the wavelength can be shifted into a longer-wave range within the respective spectrum. Due to the chemistry the curing parameters can be optimized by the use of a doped spectrum.







































LED

Compared to conventional discharge lamps UV-LEDs do not produce a broad UV spectrum, but a narrow band with specific emission peak. Furthermore LEDs do not emit IR irradiation.

When using LEDs even temperature-sensitive materials can be irradiated, as only marginal heat affects the substrate. The different spectra ensure safe and fast curing.



















LEDs are characterized by a very long life cycle. Typical application fields for LED curing are in the bonding, potting and digital printing industries.

Terminology

The performance of an UV bulb is classified by the specific lamp power in W/cm. Typical values for a specific lamp power are 80W/cm to 200W/cm. This classification indicates the electrical power supplied to the bulb per cm length. The power specification W/cm offers no real meaningful indication of the power or energy density actually present at the curing surface. Therefore information such as reflector geometry or distance to the substrate has to be considered. The actual existing intensity and energy values on the surface cannot be calculated, they must be measured!

Here the intensity or irradiancy refers to the measured power in Watt [W] per surface [cm2].

Intensity I = mW/cm2

The dose or energy density is indicated in joule [J] per surface [cm2]. It results from the integral of the intensity, thus it considers the irradiation period.

Energy density E = mw*s/cm2 = mJ/cm2

In order to be able to determine a UV process accurately, both data as well as information about the measuring system are necessary.

Chemical curing

Contrary to a thermal drying process, which works by evaporating the solvent contained in the coating, curing initiates a chemical reaction within the coating compound, which leads to a polymerization reaction.
























As soon as the reaction is activated by the UV irradiation the fluid layer “cross-links” to an inert film within a split-second. The majority of UV coatings offer a 100% coating residual, i.e. they cure almost without loss of coat thickness or VOCemissions.

A UV formulation consists of the following main parts:

  • Oligomere/Prepolymere (cross-linking main component)
  • Monomers (cross-linking low viscosity component)
  • Pigments (in paints and other pigmented systems)
  • Photo-initiators (UV sensitive component)
  • Additives (stabilizers, de-foamers,…)


The irradiation-sensitive element of the coating formulation is the photo initiator. Influenced by the UV irradiation, the photo initiator - at a radical polymerization - forms free radicals, which are able to split the double bonds within the oligomeres and monomers. This is the start of a polymerization reaction, which transforms the fluid varnish film into a three-dimensional structure.

Advantages



 

Wide Variety of UV Applications

Hönle products are used in the following markets: Printing and coating industry, automotive-, aviation- and furniture industry, packaging industry, medical technology, electronic industry, photovoltaic and material testing.

Areas of UV application:
Inkjet
Flexo/Offset Printing
Sun Simulation
Adhesives Curing
Disinfection/Sterilization
UV/LED Measurement
Fluorescence Test

Typical application fields for the utilization of UV technology are in sheet-fed offset printing, inkjet printing, web offset, flexo printing, in the coating and finishing, bonding and potting for electronic and opto-electronic components, in the surface sterilization, sun simulation and photovoltaic industries.

Each industry needs special UV reactive substances, whose characteristics meets the requirements of the specific application. The chemical industry resolved this issue and over recent years has developed a broad product range of UV reactive varnishes, inks and adhesives.

UV technology grew out of its infancy in the furniture industry a long time ago. Today it is indispensable for high technology applications.


Interactions

This makes UV curing a unique, high-quality and safe process.























With over 35 years of experience and know-how in active process facilitation, we willingly support you with:

  • arranging contacts with chemistry suppliers
  • initial testing in our UV laboratories
  • the process development in your production line
  • training about UV in general
  • expert knowledge in research & development

 

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