Coating processes – Direct application of electrical – magnetic – wave – or... – Electrostatic charge – field – or force utilized
Reexamination Certificate
2000-12-29
2002-12-31
Parker, Fred J. (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Electrostatic charge, field, or force utilized
C427S482000, C427S484000, C427S485000
Reexamination Certificate
active
06500494
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for matting the radiation-sensitive layer of a printing plate precursor. In particular, this invention relates to a method for spray coating a matting layer on the radiation-sensitive layer of a printing plate precursor wherein the method excludes the necessity of a dampening step, drying step or both after the spray coating.
2. Background Information
Lithographic printing plates are prepared from plate precursors by imagewise exposing the precursor to radiation such as UV light and a subsequent developing step. Conventionally, the imagewise exposure is done in a vacuum frame; under a glass plate a mask is put on top of the precursor and the exposure is done through the mask. The vacuum removes the air and allows a close contact between the mask and plate precursor. In the case of a positive working printing plate precursor, the radiation-sensitive layer comprises a quinonediazid compound which releases nitrogen during exposure; the nitrogen is also removed by the vacuum.
When the surface of the printing plate precursor is too smooth gas can remain between the mask and precursor thus preventing close contact and causing undercutting, i.e. fine elements get lost or shifted in the tonal value after exposure and development. It is therefore preferred that the surface of the light-sensitive layer is roughened. This can, for instance, be done through dusting the surface before exposure to radiation. However, such a procedure usually results in irregularly roughened surfaces so that irregular copy results are obtained. The producers of radiation-sensitive elements such as printing plate precursors therefore make efforts to develop products which already have a rough surface. There are several methods for putting small protrusions on the surface of the printing plate to prevent the inclusion of air. This treatment of the precursor is called matting. Matting can be done by either incorporating particles into the radiation-sensitive coating solution or by using an additional treatment step after applying the radiation-sensitive coating and drying it. The first method is lacking mainly because fine filters cannot be used on the radiation-sensitive coating solution because these would remove the particles out of the coating solution. Therefore, an additional step after the coating step (i.e. radiation-sensitive coating) is advantageous. Another advantage is that the particles are insoluble in the developer and can cause sludging whereas the post-treatment can use polymers which are easily soluble in the developer.
The matting may, for instance, be done by a powder application as described in DE-A-29 28 396. The drawback of this method is that the powder must be a polymer which has to be melted by an oven process for adhesion, which restricts the choice of polymer because of the required low Tg. These polymers are usually not developer soluble.
The preferred matting method is one which is described for instance in U.S. Pat. Nos. 4,626,484 and 4,781,941. This method comprises the steps of spraying an aqueous solution of polymer by use of an electrostatically supported high speed bell followed by dampening and then drying.
Furthermore, EP-A-545458 describes a spraying method without mentioning dampening and drying. Spraying is done on a web which moves slowly, i.e. at a line speed of up to 10 m/min in order to obtain a dry matting layer. The abundance of time by the low line speed might allow air drying without an oven. However, modern line web speeds are at least four times as fast and the required length for an oven grows with the speed. For aqueous solutions infrared dryers are used frequently.
DE-A-43 35 425 describes a printing plate precursor which is matted by spraying-on a dispersion of a resin; subsequently, the plate requires infrared-drying or convection air drying.
The necessity of dampening and/or drying, of course, is not economical due to the need for additional equipment and the prolonged time for producing the precursor.
Besides the additional need of energy for a drying oven the saving of line space—i.e., no dampening nor drying section—is an important factor: the compactness provides room for additional and/or improved other process steps. In addition to these advantages in production the plate quality and the yield can be improved as well. For instance the cosmetic appearance is free from vapor pattern and there are no droplets of dripping water, a problem which is associated with the vaporization process, especially during machine start-up. The heat of an oven step can cause an additional strain to the coating, especially in case of negative working plates.
Therefore, it is the object of the present invention to provide a single step process for matting a printing plate precursor by spraying a matting layer on the radiation-sensitive layer of the precursor without the need for post-dampening, post-drying, or both. In addition, the process should be applicable to high web speed production lines.
SUMMARY OF THE INVENTION
The above object is achieved by a process in which a matting composition having a solid content of more than 10% by weight and no more than 50% by weight is spray-coated on the radiation-sensitive layer of a printing plate precursor using an electrostatically aided rotary atomizer with a bell having a distance of less than 50 cm from the radiation-sensitive layer of the printing plate precursor to be coated and a bell speed of more than 10,000 rpm.
DETAILED DESCRIPTION OF THE INVENTION
The process of the present invention can be used for matting any kind of radiation-sensitive layer of conventional positive and negative working lithographic printing plate precursors. The term “conventional plate precursor” refers to precursors which are imaged by exposing through a mask. Digitally imageable plate precursors (for instance thermo plates) are directly imaged by means of a laser without the use of a mask and therefore do not need a matting layer. In the case of positive working plate precursors, scattered light is often used for exposure and therefore especially positive plate precursors need a matting layer. Printing plate precursors which can be matted by the process of the present invention are for instance described in EP-A-0 544 264, GB-A-2081919 and U.S. Pat. No. 5,948,595 which are all incorporated herein by reference. However, the presently claimed process is not limited to these printing plate precursors.
As the matting composition to be sprayed on the radiation-sensitive layer of a printing plate precursor any known matting composition can be used as long as the solid content of the composition is adjusted to more than 10% by weight and less than 50% by weight based on the total composition. Hereafter the term “solid content” refers to the amount of polymer or polymers in the composition. The composition comprises at least one polymer (or copolymer) and at least one solvent. The composition has to meet the following requirements: (1) the polymer has to be soluble in the solvent; (2) the polymer has to be soluble in the alkaline developer which will be used for developing the imagewise-exposed printing plate precursor later; (3) the polymer should not have a negative effect on the print-out; (4) the composition should dry in air; (5) the matting layer obtained should be hard but not brittle; (6) the matting layer obtained should not be sticky at high humidity; and (7) the matting layer obtained should not adhere to the radiation-sensitive layer to such a degree that rolling up of the web causes tearing out of the radiation-sensitive layer.
Suitable solvents are water, lower aliphatic alcohols, aqueous ammonia and mixtures thereof. The use of lower alcohols improves the drying properties; however lower alcohols are not preferred due to their inflammability. The most preferred solvent is water.
Suitable polymers are water-soluble polymers such as polyacrylic acid, polymethacrylic acid, polymethacrylamide, polyvinylpyrrolidone, soluble amides, soluble carbox
Hauck Gerhard
Nessler Nils
Rudolph Heinfried
Baker & Botts L.L.P.
Kodak Polychrome Graphics LLC
Parker Fred J.
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