Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Radiation sensitive composition or product or process of making
Reexamination Certificate
2000-05-11
2002-07-16
Hamilton, Cynthia (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Radiation sensitive composition or product or process of making
C430S278100
Reexamination Certificate
active
06420089
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to printing plate precursors which can be processed under daylight or similar conditions; in particular on press developable printing plate precursors.
BACKGROUND OF THE INVENTION
During the last few years, the practical requirements for printing plates have been increasing continuously. So far, the bulk of printing forms is being produced by an image-wise exposure step followed by subsequent chemical development. In the past few years, printing forms were introduced to the market which are developed directly on the press by means of applying a fountain solution and printing ink (and paper), i.e. in negative working printing forms the non-exposed areas of the photosensitive layer are removed by applying the fountain solution, printing ink and paper while in positive working printing forms the exposed areas are removed. This considerably facilitates the manufacture of the printing forms and saves costs for the purchase and disposal of developing chemicals. Such on press developable printing forms are described for example in EP-A-768 172, EP-A-771 645, EP-A-778 499, EP-A-769 724, U.S. Pat. Nos. 5,258,263, 5,514,522, 5,620,827, 5,516,620, 5,407,764, 5,607,816, 5,677,110, 5,677,108, 5,811,220, 5,368,973 and WO 96/34315.
Most of the described on press developable printing forms are sensitive to light in the near-ultraviolet range. Due to this sensitivity in the proximity of the visible spectral range, the portion of short-wave radiation contained in daylight or daylight-like light sources, such as white fluorescent tubes, partially exposes the printing forms and thus renders them useless. Therefore, these printing forms have to be handled under yellow light or should only be exposed to white light for a very short period of time. These restrictions are not acceptable to users who develop the printing form on press. There is therefore a strong demand for printing forms which can be processed under daylight or similar conditions and which can be developed on press.
In practical applications, attempts have been made to allow the processing of printing forms under daylight or similar conditions by selecting photoactive components whose spectral absorption in the near-ultraviolet range is limited, for example WO 96/34315. However, there are limits to this method. Powerful light sources for UV exposure emit in the range of from 350 to 430 nm, i.e. directly on the short-wave border of the visible spectral range, and the radiation-absorbing components in printing forms generally possess relatively wide absorption bands. Thus, if the radiation-absorbing system of the printing form is selected such that it falls within the emission range of the radiation source of from 350 to 430 nm, the material cannot be processed under daylight or similar conditions since it will be pre-exposed. In order to safely exclude the influence of daylight, the absorption maximum of the photoactive component has to be clearly below 350 nm. However, as a consequence, there is only a slight overlap between absorption of the printing form and emission of the radiation. This means that only little energy can be absorbed by the printing form and the printing form has a low light sensitivity. This entails long exposure times which is undesired in practical applications.
It is therefore an object of the present invention to provide a printing form precursor which can be processed under daylight or similar conditions and which at the same time possesses a high degree of light sensitivity.
In particular, it is an object of the present invention to provide printing form precursors for offset printing which can be processed under daylight or similar conditions and which possess a high degree of light sensitivity without affecting other properties such as developability, ink acceptance and storage stability.
It is a special object of the present invention to provide printing form precursors for offset printing which can be developed on the printing press by means of a fountain solution and ink (and paper), which can be processed under daylight or similar conditions while avoiding a marked decrease in the light sensitivity of the printing form precursor or other negative properties such as impaired developability, ink acceptance and storage stability.
SUMMARY OF THE INVENTION
According to the present invention, this object is achieved by the use of an overcoat on the radiation-sensitive layer wherein the overcoat comprises
(a) at least one polymeric organic binder, soluble in water or in organic solvents having a water content of at least 50 wt.-% based on the total solvent content,
(b) at least one radiation-absorbing component wherein the absorption maximum with the longest wavelength of the radiation-absorbing component of the overcoat is at or below 550 nm, the radiation-absorbing component absorbs in the range of the flank with the longest wavelength of the absorption band of the photoactive system of the printing plate precursor and the difference between the absorption maximum with the longest wavelength of the radiation-absorbing component of the overcoat and the absorption maximum with the longest wavelength of the radiation-absorbing layer of the printing plate precursor is in the range of 5 to 200 nm, and
(c) optionally at least one additive selected from an adhesive, a wetting agent, an inhibitor, a filler, a plasticizer, a flow improver, a thickener and an antifoaming agent.
DETAILED DESCRIPTION OF THE INVENTION
Overcoats on organic radiation-sensitive layers of printing form precursors are in principle known, e.g. U.S. Pat. No. 3,458,311. They are used as temporary coatings for offset printing plate precursors, letterpress plates, screen printing forms, printed circuit boards, reliefs etc. In these applications, the overcoats have the function of protecting the radiation-sensitive layer from atmospheric oxygen during storage, exposure and particularly between exposure and subsequent processing (developing and the like). During that time, the temporary coating has to adhere sufficiently to the radiation-sensitive layer in order to guarantee safe handling of the plate precursor (during manufacture, packaging, transportation, exposure etc.) without tearing the layers.
U.S. Pat. No. 3,458,311 describes the coating of the water-soluble overcoat in the presence of a water-miscible organic solvent for rendering it adhesive.
In U.S. Pat. Nos. 4,072,527 and 4,072,528 it is claimed that in addition to the overcoat polymers dissolved in water, water-insoluble polymers are dispersed to improve the adhesion:
EP-A-275 147 describes amphoteric compounds as additives to water-soluble layers for improving adhesion.
DE-A-197 32 902 claims adhesive polymeric additives to overcoats on radiation-sensitive layers.
EP-A-403 096 claims overcoats comprising at most 20 wt.-% of the photoinitiators used in the radiation-sensitive layer. This serves to improve thermal stability and light-sensitivity.
EP-A-354 475 claims dyes in water-soluble overcoats which absorb light in the range of from 300 to 700 nm and have an absorption gap within this range corresponding to the emission range of the desired copying light source. It is demonstrated that such overcoats improve the resolution of the printing form. A similar effect is described in EP-A-465 034 with respect to polymerization inhibitors in the overcoat.
EP-A-352 630 protects the combination of a water-soluble polymer with a low degree of permeability to atmospheric oxygen and a water-soluble polymer which binds atmospheric oxygen for the improvement of storage stability. As polymers which bind atmospheric oxygen, this patent mentions polymers having aliphatic amino groups, preferably of the polyalkylene imine type.
As polymeric inorganic binders for the overcoat according to the present invention, water-soluble polymers such as for example polyvinyl alcohol, partly saponified polyvinyl acetate which may also contain vinylether and vinyl acetate units, polyvinyl pyrrolidone and its copolymers with vinyl acetate and vinyl ethers, hydrox
Baumann Harald
Dwars Udo
Baker & Botts L.L.P.
Hamilton Cynthia
Kodak Polychrome Graphics LLC
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