UV - oxygen reduced – direct ionization process
Challenge
IST METZ has launched a new compact curing unit at drupa 2000, called the “BLK-U” unit.
The operating range reaches from atmospheric conditions up to oxygen reduced operation with lowest O2-residual proportions. The unit doesn’t require any quartz screen separating the cooling gaz from the inert gaz flow thus raising the efficiency of the short-wave output onto the object. In addition, the heat impact onto the object is reduced by 40 - 50 % using cold mirrors adjusted to the required lamp spectrum.
BARCO Graphics resently made use of this new development.
BARCO Graphics is the first company to take ultra-high speed, variable data - digital color printing into industrial applications with the launch at drupa 2000 of a new printer pilot line called “the.factory” (pronounced the-dot-factory). This new printer pilot line - the.factory - is a unique and extremely powerful digital inkjet press tailored to the growing IVIP (Industrial Variable Inkjet Printing) markets.
BARCO Graphics is currently analyzing curing properties of different UV inkjet ink compositions for this “the.factory”. These tailor made inks have a low viscosity and a printed layer thickness between 3 and 18 µm. A full curing of these inks is not possible without an inert gas blanket, preferably nitrogen for economic reasons. BARCO Graphics therefore researches new ink compositions that have higher reactivity and a highly reduced photoinitiator content.
Test results
Cooperative curing tests were done at IST METZ recently, under supervision of the BARCO Graphics Strategic Research department.
The following issues were tackled, and outcome could be shown during the common trials BARCO/IST METZ early this month:
1. Complete curing of Ink Jet colors with a coating thickness up to 14 µm without photo initiator under oxygen reduced conditions.
2. The same result has been obtained using a low photo initiator proportion with a considerably higher residual oxygen content.
Economical relevance
The combinations of ink with lower Photo Initiator (PI) content, and UV curing under inert gas atmosphere evens out or even lowers the total cost per m2 and reduces the ROI period.
Industrial relevance
The new digital press “ the.factory “, targeted for the market of Industrial and Variable Information Printing (or IVIP), runs single pass full process color and additional spot color printing on a single side continuous feed roll to roll configuration.
It is capable of printing up to 800 square meters an hour, of fully variable images at 360 dpi. BARCO Graphics has created a very modular system, allowing customers to select from a range of print widths (14 cm to 63 cm) and colors (up to six colors), plus additional spot colors.
Technical background
When the energy richest wavelength range of 200 to 240 nm, emitted by the UV-lamp and known as UVC, isn‘t absorbed by the atmospheric oxygen and by ozone formation in an oxygen reduced atmosphere, it can lead to the so called direct ionization process.
This carries out the radical chain polymerization, as its energy is equal or larger than a carbon-carbon molecular double bond (C=C). The result is ink curing without or with minimal photo initiator, which withstands all industry tests and is odorless with regard to photo initiator.
Also, the chain formation isn’t interrupted by atmospheric oxygen, which leads to a complete curing of the upper ink coating.
The requirement for a 2% or less residual oxygen atmosphere for industrial applications already implies a certain expenditure of the unit design.
A compromise between the photo initiator proportion and the residual oxygen concentration can fulfill the demands of a safe curing process in everyday use.
At the same time an optimum cost-profit relation is obtained maintaining a wide range of production safety. This method can also be used in those processes where an indirect contact with food exists.
The inert gas consumption during the multiple color ink curing can be reduced to a minimum when using a UV-curing unit with circular lamp cooling to which nitrogen is added. The short-wave UV-light consuming quartz screen can be dropped.
With the use of high-quality dielectric coatings in the UV-unit, for the special reflection of energetic short-wave light from 200 nm to max. 450 nm, the substrate heating is reduced by approx. 40 – 50 % compared to conventional reflectors, while the light proportion required for direct ionization is completely exploited.
A combination between an UV-unit with an increased proportion of long-wave UV-light (Ga or Fe) and a classical UV-lamp used for the end-of-press-drying (Hg) will result in safe results for the wet-on-wet curing of thick coatings.
