Archive for the ‘UV Curing’ Category

In addition to print tests done with a densitometer or a spectrocolorimeter, it is imperative to check the degree of UV curing.

In fact, a incomplete cure due to several different parameters (such as an excessive ink filmweight, no interdeck dryers, a failing dryer or non-optimised printing conditions) can cause the following problems:

• Excessive residual smell of the printed item;

• A tacky printed surface,

• Lack of substrate adhesion

• Poor mechanical resistance

• A risk of candling (or flaking)

• A high sensitivity to humidity

When they exist, the specified defects are often discovered too late. A second pass under the UV dryer rarely recovers the poor initial cure. To be able to guarantee the consistency of printing, it is essential to implement quality control tests of the print when it comes out of the press.

Obviously, tests such as the analysis of nonreacted monomers by chromatography in liquid phase or humidity resistance control by using a climatic oven, can only be carried out by specialised equipped laboratories. Nevertheless, some simple and efficient tests are available: their application helping in mastering UV printing, and in overcoming eventual curing faults.

The following tests are examples, which can be carried out at the end of the press.

Solvent resistance tests

These are designed to check the curing of UV inks and varnishes. Ethanol is used for testing the cure of UV inks; while Methyl Ethyl Ketone (M.E.K.) is used to test the curing of UV varnishes.

• Ethanol resistance test:

Principle:

Ethanol has a dissolving action, especially on the non-reacted components of the ink. This test assesses the degree cure of the ink film, and consists of rubbing over the print sample with an ethanol soaked cotton pad.

Operation mode:

• Soak cotton (or similar) with ethanol. Then place the cotton on the face of the print sample and rub backwards and forwards at constant pressure and at regular speed and amplitude.

• Count the necessary numbers of cycles needed to register:

• the beginning of the ink film deterioration,

• the complete destruction of the ink film.

The results of the test will depend on:

• the size of the cotton pad,

• quantity of solvent used,

• exerted pressure,

• deplacement speed on the print sample,

• ambient temperature (solvent evaporation).

A specification for the number of cycles to produce this deterioration (initial or complete) can be established after having performed the test on validated prints.

• MEK resistance test:

Like Ethanol, MEK has a dissolving power action on varnishes, especially on its nonreacted components. MEK is used to test the curing of UV varnishes only, since it would be too aggressive on UV inks (a test using MEK would not give conclusive results on UV inks). Refer to the Ethanol resistance test for the application mode.

It must be noted that a negative test result can not only be due to a lack of cure, but also due to an insufficient varnish coating weight.

The polymerisation reaction can be via a radical or cationic mechanism; the radical process is the most current and is based on acrylates chemistry. UV inks and varnishes contain a blend of monomers and oligomers, and photo initiators.

With the radical mechanism, photo initiators are fragmented by UV rays and form free radicals during a phase called initiation. These free radicals then react with the double bonds present on the acrylate functionalities of the oligomers and monomers; this second stage is known as the propagation phase. The termination of this chain reaction produces the hardening of the film in 3 dimensions.

UV drying of printing inks (and varnishes) is based on radical polymerizable vehicles. UV inks with appropriate UV dryers are suitable for sheet-fed printing presses and web presses. Drying between the print units – interunit drying  can be used to prevent a reversal of the ink splitting in the following inking unit. In flexographic and gravure printing, drying has to be carried out after each inking unit (e.g., straight (recto) printing and perfecting) because of the ink properties (ink trapping, etc.). Very often an overall drying becomes necessary after the last inking unit, possibly at a higher output rate.

UV inter-unit dryers for sheet offset printing press after printing unit and coating unit; UV dryer (blue) can be replaced by IR dryer (red),(IST Strahlungstechnik metz)

In case of UV drying, the ink film polymerizes and dries completely as soon as radiation occurs. Polymerization takes fractions of a second. The UV drying method, however, requires special inks  containing completely different binders (vehicles) and additional photoinitiators.  The color black prevents UV radiation from penetrating in the ink layer and the curing effect is less than with chromatic colors or varnishes. 

Conventional UV dryers work with one or several mercury vapor lamps.  The wave length range lies between 100 and 380 nm. The system is enclosed by a reflector housing. Optimum cooling and extraction of generated ozone is necessary for the complete system.The units are designed in such a way that the permissible threshold limit value of 0.ppm (parts per million = one millionth of the volume of the substance in question, e.g., air) is not exceeded and damage (e.g., irritation to the mucous membrane) to one’s health is prevented.

UV drying system.
a) Ranges of the UV spectrum and their effect;
b) UV radiator reflector system (Dr. Hönle)