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Disadvantages are getting less

The new BLK-U lamp designed by IST METZ: improved conditions for UV-curing with oxygen reduction
By Dr. Georg Bolte

For some time the use of inert gases, particularly of oxygen, has been under discussion with regard to UV-curing of inks and varnishes. This technology has been already successfully practised for radiation curing of silicone coatings. Technical development made it possible to adapt the UV-lamp design in such way that the demands of both general and high-quality graphic applications are fulfilled by this technology. During drupa 2000 the IST METZ company has introduced the BLK-U system designed for the curing under oxygen reduced conditions the benefits of which shall be presented here. For silicone coatings UV-lamps have been used for some years working under so-called inert (oxygen reduced) atmosphere. Because of the air streams required for cooling, the UV-lamp unit has been usually separated from the web by a quartz screen (refer to figure no. 1).

Due to these separating screens, however, there is an energy loss of 25% to 30%. Thanks to the BLK-U system developed by IST METZ, this screen can now be dropped: a circumstance considerably increasing radiation efficiency. What means UV-curing in oxygen-reduced atmosphere? It is not difficult to show the difference between conventional UV-curing and the new technology of radiation curing in oxygen-reduced atmosphere. Essentially, the atmosphere around the material to be cured is altered. For conventional UV-curing the reaction takes place in an atmosphere consisting of - approx. 20% oxygen - approx. 79% nitrogen - ozone and - atmospheric moisture. In oxygen-reduced atmosphere using nitrogen the process is effected in an atmosphere consisting of - less than 1% oxygen - more than 99% of nitrogen - no ozone - without atmospheric moisture. The content of residual oxygen depends on the used UV-materials. As opposed to the well-known UV-curing of silicone coatings under inert conditions with an oxygen content of less than 50 ppm, a clear effect can be recognized for other materials when the oxygen proportion is reduced to less than 10 000 ppm. Logically the amount of the required inert gas is reduced making the process much more economical. A change of ambiency has considerable effects on reaction processes.

Oxygen molecules are usually contained in gaseous oxygen, in ozone and in the atmospheric moisture. They react extremely quickly with the photo-initiators and binders contained in UV-varnishes, UV-inks and UV-adhesives. This leads to incomplete cross-linking when exposed to UV-light (refer to figure 2a). Short molecule chains and molecule fractions result from this. Only an overdose of photo-initiators and so-called oxygen catchers or coinitiators provide a sufficient cross-linking. If atmospheric moisture and oxygen are displaced or considerably reduced, the materials are easily cross-linked (refer to figure 2b) and the product quality is much improved. Simultaneously, the reduction of photo-initiators provides more possibilities for the formulation of e.g. UV-flexo inks. Photo-initiators are much responsible for shrinking tendencies. A reduction of photo-initiators from 10% to 1% considerably reduces material shrinking. New solution with oxygen reduction During drupa 2000 the UV-specialists from IST Metz have presented for the first time their new BLK-U lamp concept (figure 3). The system is a consequent development of the well-known BLK-lamp. The most important benefits that can be achieved with the BLK-U concept are as follows: - As the quartz screen separating the UV-radiation chamber from the substrate surface is dropped, UV-radiation directly reaches the medium to be cured.

Compared to unit concepts with a quartz screen, radiation efficiency is considerably improved. - The system is equipped with a complete water cooling and doesn’t require any air extraction therefore. There is no ozone formation either. - The introduction of nitrogen prevents ozone formation and provides a dry ambiency. - The heat exchanger cooling the nitrogen and the filter unit serving for the cleaning of internal parts like lamp and reflector are integrated in the lamp housing. Because of the internal circulation no additional piping is required. The continuous filtering of atmosphere has a positive effect on the lifetime of UV-lamps and reflectors. The placement of the nitrogen circulation inside the lamp housing considerably reduces the size of the lamp modules compared to the previous placement in an additional box (figure 4). The reduction of size opens new possibilities of fitting this technology into various press systems. Oxygen reduction offers many chances UV-curing in oxygen-reduced atmosphere has a complex effect on the process and on the used materials. The possibilities of intervening into UV-technology start from formulating radiation curing inks, coatings and adhesives. The reduced proportion of photo-initiators offers more space for binders, pigments and additives and has a positive effect on shrinking, substrate adhesion and odour formation. While UV-curing of silicone application is effected under inert gas conditions (oxygen proportion less than 50 ppm), IST METZ has examined more thoroughly the effects with diverse residual oxygen proportions. Many investigations were made over a broad spectrum with diverse adjustments of residual oxygen proportions in the presence of users and manufacturers of inks and raw materials. Positive effects for curing were clearly visible with values of less than 5% (50 000 ppm). As soon as lower degrees of purity are acceptable, the nitrogen consumption can considerably be reduced. When the residual oxygen proportion is adjusted to the formulation and the lamp during UV-exposure, consumption costs can considerably be reduced.

Tests confirm the effectiveness of this technology The following example taken from the UV-coating sector shall present the possibilities which arise for the formulation of UV-curing products. The described tests were carried out on a UV-varnish system used in practise. By means of a colouring reaction of potassium permanganate making non-crosslinked coating parts visible, the cross-linking degree was evaluated in relation to diverse speeds but with constant lamp output. The results can be seen on figure 5. The expected deterioration of cross-linking which is related to increasing speed has been confirmed. The relative cross-linking degree went back from 64% at 60 m/min to only 42% at 200 m/min. For this test the UV-varnish contained 8% of photo-initiators (FI). This proportion was reduced to 2% for a further series of tests. The strong decrease of the cross-linking degree is marked on the figure at a speed of 100 m/min. If the ambient oxygen proportion of the medium to be cured is reduced to a value of 0,5%, the cross-linking quality can be raised to more than 80% at a speed of 100 m/min. At 200 m/min the relative cross-linking degree is slightly inferior to 80%.

Even with a further reduction of photo-initiators up to 0,8%, the values remain much superior to the cross-linking degrees evaluated under ambient conditions. A further potential for improvement is contained in the proportion of amines used as so-called “oxygen-catchers”. If their concentration is reduced to one tenth of the previous amount, the cross-linking quality can be additionally improved. Step by step towards economical use Figure 6 shows the influence of diverse oxygen concentrations and formulations on the cross-linking degree. A standard UV-varnish was used, the photo-initiator proportion of which was reduced from 8% to 2% in a first step. In both cases curing is done at 100 m/min with ambient air. The obvious quality loss at cross-linking can be more than compensated when reducing the ambient oxygen proportion from 20% to 1,0% in a second step. In the range between 1,0% and 0,1% of oxygen proportion a fine adjustment of the photo initiator proportion and of the used amine quantity can be done in a third step. By adjusting materials like nitrogen, photo-initiators and amines the required cross-linking degrees can be achieved considering the aspect of economy.

The technology of nitrogen supply There are two possibilities of supplying nitrogen serving as inert gas for UV-curing in oxygen-reduced atmosphere: the installation of a nitrogen tank (figure 7) or the use of a nitrogen-generator. In any case, the decision should be taken in relation to the required oxygen proportion. The following economical aspects should be considered in view of a nitrogen tank: - tank capacity in cubic metres - consumption rate - quality degree - frequency of the tank car supply The investment of a nitrogen-generator should be preceeded by the following considerations: - capacity in cubic metres per hour - consumption in cubic metres per hour - the required quality level (e.g. 95,0 up to 99,999% nitrogen proportion) Especially for quality classifications with a content of residual oxygen of 0,5% to 1,0% the investment and consumption costs of a nitrogen generator should be calculated. In this case an own nitrogen production might be more economic. Users interested in a detailed analysis of conditions may consult the experts from IST METZ. The main application fields of the new development The following benefits result from UV-curing under oxygen-reduced conditions: - higher production speeds - cost saving - quality increase and - odour reduction The following application fields can obviously benefit from this technology: - packaging industry (especially food packaging made of foils, paper, carton, aluminium foil, tin plate) - furniture industry (decorative foils, wood coatings, laminate) as well as - metal coatings As opposed to the above mentioned application fields those sectors are problematic which require labour-intensive shieldings of the UV-lamp-sphere. In this respect coatings of three-dimensional parts or sheet transport within offset printing have to be mentioned. Many experts predict a broad application field for UV-curing in oxygen-reduced atmosphere.

At present this development is at the beginning of it’s possibilities. A large number of tests recently carried out has shown that already a slight reduction of the residual oxygen proportion (much less than originally assumed) leads to very good results. An intense exchange of experience between all parties involved in the process should therefore take place as soon as possible. The following parties should participate in the discussion: - unit manufacturers (coating units, UV-radiation, nitrogen dosing) - manufacturers of UV-curing products (inks, coatings, adhesives) and - users. Only through an intensive co-operation between the mentioned groups all partners will benefit from the new technology. The IST METZ GmbH therefore propagates to start within short an open exchange of experience between the several interest groups.