Drying and gas or vapor contact with solids – Material treated by electromagnetic energy – Ultraviolet energy
Reexamination Certificate
1998-01-26
2001-02-13
Bennett, Henry (Department: 3744)
Drying and gas or vapor contact with solids
Material treated by electromagnetic energy
Ultraviolet energy
C034S275000
Reexamination Certificate
active
06185840
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an apparatus for hardening a layer applied to a substrate.
The invention is concerned with the treatment of substances, more especially of dyestuffs, which mainly have a double bond as the monomer, which double bond is to be polymerised by means of UV light. It is known to polymerise double bonds with electrons or cationically.
To enable the UV radiation to start the photo reaction, a so-called photo initiator is needed in the mixture to be treated, e.g. of a dyestuff. This photo initiator is used to excess so that the polymerisation reaction, once started by UV light, is not interrupted by the dyestuff radical reacting with the diradical oxygen. Accordingly, until now, a relatively high concentration of photo initiator is used, so that the likelihood of the dyestuff radical encountering an oxygen radical and being penetrated thereby becomes minimal compared with the likelihood of its encountering another monomer with a double bond and radicalising such monomer.
Central cylinder machines are known with which various dyes are applied one after the other to a paper web or to a plastics material film, each layer being dried before the next dye layer is applied. UV emitters are used, and cooled by means of air, to dry these individual dye layers. For this, a UV lamp with an external temperature of approx. 800° C. is cooled by the induction of air which is conducted past the lamp The disadvantages in this arrangement are the constant production of ozone, the movement of large quantities of dirt particles and the heating of the coated substrate which, especially with heat-sensitive plastics material films, can lead to serious imperfections.
Alterations to the known cooling system by water-cooling around or in front of the UV lamp lead to performance losses. Meanwhile, arrangements with a water-cooled housing and reflector and possibly also with a water-cooled counter-pressure cylinder are being used successfully. This structural arrangement is indeed usable in heat technology; no dirt particles are moved and no ozone is produced, but large performance losses are to be expected with a water-cooling system which encases the UV lamp.
Since, in fact, the photo initiators have the disadvantage of, on the one hand, having a relatively strong inherent smell arid, on the other hand, being very expensive, the technical problem of the invention resides in producing an apparatus in which the quantity of photo initiators can be considerably reduced.
According to the invention, this problem is solved with an apparatus according to the claims.
SUMMARY OF THE INVENTION
According to the invention, in the case of printing ink, the dyestuff comprising a maximum of only 20% photo initiators is applied to an aluminium foil or plastics material film, or respectively a paper web, and hardened/dried by means of UV light, two essential steps being taken. The first step resides in the fact that the film, which is generally very heat-sensitive and is between 10 and 50 &mgr;m thick, has to be cooled during the UV irradiation. However, the present invention does in fact reside in effecting the second step, i.e. carrying out the printing and drying/hardening operation in an atmosphere of scavenging gas. If inert gas is used, nitrogen or carbon dioxide is preferred.
The scavenging gas does not necessarily have to be an inert gas, but it may also be dry air, moist air or a different reactive gas, depending on the layer to be hardened. For example, there are chemical systems which are not sensitive to oxygen but are sensitive to moisture. If, however, adhesives are applied to a paper web or a plastics material film, or respectively an aluminium foil, for example, said adhesives require water in order to be able to react better and to harden. A polyamide film, however, is inclined to bond a considerable amount of moisture to the surface. As a result, the gas must be selected in such a way that this film has the moisture removed before the printing operation, that is to say before the dye is applied, so that substantially more advantageous conditions prevail than is the case if the dye is applied to a monomolecular film of moisture.
The proposal according to the invention can be used with particular advantage in a flexo-printing cylinder machine working at high speed, on which machine, for example, films for food packaging are produced or laminating must be performed. It allows not only a remarkable adhesion of dye on paper/plastics material films or on aluminium foils, but the previously serious problem of the unpleasant odour is also removed by the considerable reduction in the number of photo initiators, because the hardening/drying of the individual dye layers now occurs in a protected atmosphere and rapidly. The laminar flow, including oxygen, can, more especially, also be replaced by nitrogen before the substrate is passed to the hardening stage in the form of the UV drier, the dye on the surface also being released by the quickly absorbed oxygen. According to the invention, therefore, up to more than 80% of the photo initiators previously required can be eliminated, so that considerable savings are also made with the UV hardening printing inks.
The invention is explained more fully hereinafter by embodiments with reference to the accompanying drawing.
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Bolte Georg
Noelle Lutz
Bennett Henry
Drake Malik N.
Jordan and Hamburg LLP
Noelle GmbH
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