II. What The UV Radiations Do?

The drying of the solvent and water-based inks is a change of state - the transition from liquid to solid state -, which first results in a loss.

We then speak of "polymerization", "fusion with the substrate" (in the case of enamels for glass or different ceramics) or "absorption" (in the case of printing on textiles with dyes).

The UV inks - schematically - are composed, for their "base", of oligomers (polyester, polyester-urethane and most of all polyester-acrylate), of very liquid monomers (in general acrylates) as "diluents" - to reduce the viscosity. Then of pigments for the color part, plus diverse additives (stabilizing agents, thixotropic agents, tension corrector, etc.) and most of all of the "photo-initiators".

These last elements are extremely important because they generate the change of state.

Behavior and action of the photo-initiators.

A photo-initiator is a chemical component absorbing the energy emitted by a source of ultraviolet radiations -particularly the energy emitted between 200 and 270 Nm.

It transforms the oligomers into cross-linked polymers
This reaction "snowballs" very rapidly.
This is a change of state "without loss of material" that results in a stable polymers.

• The photo-initiators absorb the UV energy.
• Their percentage of concentration is very important.

The medium of this reaction.

• either reaction in a partially active environment, chemically speaking, that is in the ambient air composed of 18% of oxygen (active), 81% of nitrogen (N, inert) and 1% of rare gas;
• or reaction in a totally inert environment, that is composed of 100% of nitrogen, this colorless, odorless and tasteless gas that is the principal mass component (even though inactive) of the air we breathe.

Some peoples speaks of "oxygen inhibition"...But in my opinion, the real question is: "does it have a real importance in screenprinting?"

My persora answer is NO!

The additives

• correctors of tension,
• thixotropic agents designed for decreasing the inter-molecular cohesion of the ink. This property is particularly useful in halftone printing.
• "attenuator of viscosity"

What is the influence of these additives on the reactivity of the ink?

All together, their influence is very feeble.

Other factors having an influence on the reaction : the pigments.

• reflective power, sometimes "competition" with the photo-initiators,
• "absence of transparency" then, the "incidental reactivity".

Nevertheless, a basic principle should be taken into account, as follows :

- the thicker the coat of ink is,   the slower the rapidity of reaction.

The whole of the factors of "relative accumulated opacity" of the pigments is more important than their factors of absorption and their reflexive power.

This idea, which we will see again in the context of commercial  (or economic) and technical purposes, implies the need for a perfect control of the thickness of the ink coat.

N.B.: the "flash" UV systems work correctly only with very transparent inks Impact of the UV factor on the characteristics of the inks : what is the flexibility of the UV inks? what is their resistance to sun   or atmospheric conditions? their physical and chemical solidity? the ink mileage? the gloss or mat finish? the absorption by the substrate?

a)  What does not depend on the UV factor.
The resistance to the solar light,

b)  What mainly depends on the UV factor.

• The resistance to atmospheric conditions :
• The hardness, the resistance to abrasion, to solvents :
• The suppleness, the flexibility: old UV systems new UV systems
• The adhesion to the substrate :

The factor "superficial tension" - the "wetability" if you prefer of the substrate to print or the already printed underlying coat of ink are of vital importance.

Indeed, with the UV inks, no solvents therefore no loss by evaporation. Everything that goes through the screen is deposited and stays

c) What is in total relation with the UV factor.

The reflectivity of the substrate to print acts a certain role. The movement under the lamp generates the formation of an angle of reflection.

On an "indirectly reflecting" substrate - the rapidity of polymerization will be improved of about 30%. Finally, in the case of the printing of "dots" - the improvement of the rapidity of polymerization can reach 50%.