Radiation imagery chemistry: process – composition – or product th – Nonradiation sensitive image processing compositions or... – Process of preparing composition from plural preformed...
Reexamination Certificate
2000-11-03
2002-05-21
Le, Hoa Van (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Nonradiation sensitive image processing compositions or...
Process of preparing composition from plural preformed...
Reexamination Certificate
active
06391530
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a process for developing imagewise exposed radiation-sensitive devices, especially radiation-sensitive printing plates. In particular, it relates to a process for developing exposed radiation-sensitive devices where the efficiency of the partially exhausted alkaline developer is lower than it appears from the measured electrical conductivity. The invention also relates to a method for the production of a replenisher composition, and to its use as a replenisher composition for partially exhausted alkaline developers having a lower efficiency than it appears from the measured electrical conductivity.
BACKGROUND OF THE INVENTION
The art of lithographic printing is based upon the immiscibility of oil and water, in which oily material or ink is preferentially retained by the image areas and the water or fountain solution is preferentially retained by the non-image areas of the printing plate. When a suitably prepared surface is moistened with water and ink is applied, the background or non-image areas retain the water and repel the ink while the image areas accept the ink and repel the water. The ink on the image areas is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth, and plastic materials. Commonly, the ink is transferred to an intermediate material called the blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Lithographic printing plates, sometimes known as printing plates precursors, can be either negative-working or positive-working, and comprise one or more radiation-sensitive layers on a suitable substrate, such as metal or polymeric support. The radiation-sensitive layer generally includes one or more radiation-sensitive components that may be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material.
In order to obtain a printing plate with imagewise distribution of printable areas, it is necessary to remove the non-image areas of an exposed printing plate precursor. The most common method for removing the undesired areas is to contact the exposed precursor with a developer solution.
A negative-working printing plate generally has a light sensitive layer composed of a radiation-sensitive component such as an unsaturated resin, a negative diazo resin dissolved in a polymeric binder, or an initiator system together with monomers and/or oligomers, on a suitable substrate. Upon exposure to light, the exposed areas are hardened, leaving non-exposed areas removable during development. Certain negative-working printing plates contain novolac resins, a cross-linking agent, and a radiation-sensitive component that produces acid on exposure. By subsequently heating the plate, only the exposed areas are cured and the unexposed areas can be removed by a developer. The exposed, hardened areas are therefore oleophilic and will accept ink while the non-exposed underlying areas of the substrate are hydrophilic.
Many alkaline developable positive-working printing plates generally have a light sensitive layer comprising a novolac resin and a radiation-sensitive component such as an o-diazoquinone or diazonaphthoquinone compound. Upon exposure to light the radiation-sensitive component is converted to the corresponding carboxylic acid. The use of an alkaline developer will remove only the exposed areas of the radiation-sensitive layer, leaving the surface of the support. Because the surface of the support is hydrophilic, the uncovered non-image area attracts water and repels the oily ink. The image area remaining after development is oleophilic thereby repelling water and attracting the printing ink.
Various aqueous solutions are known for use as developers. Yamasue, U.S. Pat. No. 4,259,434, describes use of a dispersion of a silicate to develop positive-working printing plates. These solutions include alkali metal silicates, in which the ratio of SiO
2
to M
2
O is from 0.5 to 0.75 (“M” being the alkali metal) at 1-4% SiO
2
concentration.
Seino, U.S. Pat. No. 4,452,880 describes silicate-containing developers in which the SiO
2
to alkali metal oxide ratios are much higher, that is between 1.6 and 2.0, and the % SiO
2
concentration is from 2 to 9%.
Miller, U.S. Pat. No. 5,851,735, discloses an aqueous alkaline developing composition that is useful in developing either positive-working or negative-working lithographic printing plates. The composition has a pH of at least 12 and comprises an alkali metal silicate and at least 6 wt % of a water-soluble or water-dispersible thickener, such as glycerol.
Toyama, GB-A-2,276,729 (DE-A-4 411 176) discloses a developer or replenisher composition for both positive-working and negative-working lithographic printing plates comprising an alkali metal silicate and a water-soluble ethylene oxide adduct, obtained by addition of ethylene oxide to a sugar alcohol having not less than 4 hydroxy groups. This developer is stable and shows a high developing efficiency.
During development of exposed printing plate precursors, the developer is weakened as the throughput (i.e., m
2
of the exposed printing plate precursors) increases. For alkaline developers, the alkalinity of the developer decreases as additional exposed precursors are developed. The weakening of the developer leads to a darker or fuller image reproduction; which does not meet the requirements for a standardized printing. In the case of conventional printing plates, i.e. plates that are imagewise exposed to radiation through a mask, the change of efficiency of the developer and, therefore, of the image reproduction can be monitored by simultaneously copying test elements.
However, this is not possible for digital plates directly exposed by a laser without using a mask. If the developer has a measurable electrical conductivity, as is usually the case with aqueous alkaline developers, the users often monitor the electrical conductivity of the developer during the development process to determine the efficiency of the developer. As soon as a weakening of the developer efficiency is established by means of a lower electrical conductivity, the developer is refreshed by adding either fresh developer or a so-called replenisher composition that has a higher concentration of hydroxide ions than the developer, to the weakened developer until the electrical conductivity of the developer again reaches its original value.
As described in EP-A-0 556 690, the electrical conductivity of a developer does not always decrease when the alkalinity decreases during the- development process. Depending on the kind of developer system and also on the kind of plate being developed, the electrical conductivity of the developer may be constant, may increase, or may decrease, even though the efficiency of the developer decreases in each case. According to one theory, it is assumed that ingredients of the removed radiation-sensitive layer of the developed printing plates and/or side reactions are responsible for the production of new ions. These ions at least partially compensate or even overcompensate the loss of electrical conductivity due to consumption of hydroxide ions (OH
−
) of the developer, thus producing a measured electrical conductivity that is not as low as it would be expected due to the consumption of hydroxide ions by the developing process. Measuring the electrical conductivity does therefore not in every case allow a conclusion on the efficiency (i.e. activity) of the developer. Constant conductivity values therefore do not guarantee the constant developer activity required for the desired constant developing result. If there is a developer system and/or plate where the activity of the developer decreases while the electrical conductivity only slightly decreases, keeps constant or even increases the addition of a highly alkaline replenisher composition for increasing the activity results in an increase of the electrical conductivity.
EP-A-0 556 690 discloses a computer control
Fiebag Ulrich
Timpe Hans-Joachim
Kodak Polychrome Graphics LLC
Le Hoa Van
Ratner & Prestia
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