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
2002-03-01
2003-11-04
Baxter, Janet (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
C430S302000, C430S348000, C430S138000, C430S434000, C430S494000, C430S944000, C430S945000, C101S453000, C101S463100, C101S467000
Reexamination Certificate
active
06641976
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of preparing a negative-working printing plate precursor having a hydrophilic substrate and a heat-sensitive image-recording layer provided thereon as well as a method of making a printing plate using such a material.
BACKGROUND OF THE INVENTION
Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the so-called computer-to-film method wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an image-setter. After processing, the film can be used as a mask for the exposure of an imaging material called plate precursor and after plate processing, a printing plate is obtained which can be used as a master.
A typical printing plate precursor for computer-to-film methods comprise a hydrophilic support and an image-recording layer of a photosensitive polymer layers which include UV-sensitive diazo compounds, dichromate-sensitized hydrophilic colloids and a large variety of synthetic photopolymers. Particularly diazo-sensitized systems are widely used. Upon image-wise exposure, typically by means of a film mask in a UV contact frame, the exposed image areas become insoluble and the unexposed areas remain soluble in an aqueous alkaline developer. The plate is then processed with the developer to remove the diazonium salt or diazo resin in the unexposed areas. So the exposed areas define the image areas (printing areas) of the printing master, and such printing plate precursors are therefore called ‘negative-working’.
In addition to the above photosensitive materials, also heat-sensitive printing plate precursors are known. Such materials offer the advantage of daylight stability and are especially used in the so-called computer-to-plate method wherein the plate precursor is directly exposed, i.e. without the use of a film mask. The material is exposed to heat or to infrared light and the generated heat triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross-linking of a polymer, decomposition, or particle coagulation of a thermoplastic polymer latex. Especially the latter imaging mechanism allows to obtain a daylight-stable material with high lithographic performance and typical prior art examples of such heat-sensitive materials will now be discussed.
Research Disclosure no. 33303 of January 1992 discloses a heat-sensitive imaging element comprising on a support a cross-linked hydrophilic layer containing a latex of thermoplastic polymer particles and an infrared absorbing pigment such as e.g. carbon black. By image-wise exposure to an infrared laser, the thermoplastic polymer particles are image-wise coagulated thereby rendering the exposed areas ink-receptive without any further development.
EP-A-514145 discloses a heat-sensitive imaging element including a coating comprising core-shell particles having a water insoluble heat softenable core component and a shell component which is soluble or swellable in aqueous alkaline medium. Red or infrared laser light directed image-wise at said imaging element causes selected particles to coalesce, at least partially, to form an image and the non-coalesced particles are then selectively removed by means of an aqueous alkaline developer. Afterwards a baking step is performed.
EP-A-800928 discloses a heat sensitive imaging element comprising on a hydrophilic surface of a lithographic base an image forming layer comprising hydrophobic thermoplastic polymer particles dispersed in a water insoluble and alkali soluble or swellable resin and a compound capable of converting light into heat, wherein said alkali swellable or soluble resin comprises phenolic hydroxy groups and/or carboxyl groups.
The major problem associated with the prior art compositions which work according to heat-induced latex coalescence is the ease of mechanical damage of the image-recording layer of such materials which may cause a low run length of the printing plate and/or ink-acceptance in the non-printing areas (toning), e.g. due to some pressure applied thereto.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a composition that enables to make a lithographic printing plate precursor which works according to heat-induced coalescence or fusing of hydrophobic thermoplastic polymer particles and which allows to obtain a high run length without toning. This object is realized by the method defined in claim 1. Specific features for preferred embodiments of the invention are set out in the dependent claims. The use of a polymer B which has a softening temperature that is lower than the glass transition temperature of the hydrophobic thermoplastic particles of polymer A allows to heat the composition up to a temperature above the softening temperature of polymer B without substantially triggering the image mechanism of heat-induced fusing or coalescence of the particles of polymer A.
Further advantages and embodiments of the present invention will become apparent from the following description.
DETAILED DESCRIPTION OF THE INVENTION
According to the method of the present invention, an aqueous dispersion of at least two polymers is prepared, referred to herein as polymer A and polymer B. The glass transition temperature of polymer A is higher than the softening temperature of polymer B. The softening temperature is the temperature at which the polymer begins to deform from a rigid state to a soft state, which normally occurs at a rapid rate over a narrow temperature interval. For amorphous polymers the softening temperature is near the glass transition temperature, whereas for highly crystalline polymers it is close to the melting point. The term “aqueous” shall be understood as meaning that more than 50 wt. % of the solvent is water. Organic liquids which are miscible with water can be present, e.g. alcohols, ketones, or derivatives thereof, but preferably only water is used as a solvent.
Polymer A is a hydrophobic thermoplastic polymer that is not soluble or swellable in an aqueous alkaline developer. Specific examples of suitable hydrophobic polymers are e.g. polyethylene, poly(vinyl chloride), poly(methyl (meth)acrylate), poly(ethyl (meth)acrylate), poly(vinylidene chloride), poly(meth)acrylonitrile, poly(vinyl carbazole), polystyrene or copolymers thereof. Polystyrene and poly(meth)acrylonitrile or their derivatives are highly preferred embodiments of polymer A. According to such preferred embodiments, polymer A comprises at least 50 wt. % of polystyrene, and more preferably at least 65 wt. % of polystyrene. In order to obtain sufficient resistivity towards organic chemicals, such as the hydrocarbons used in plate cleaners, polymer A preferably comprises at least 5 wt. %, more preferably at least 30 wt. % of nitrogen containing units or of units which correspond to monomers that are characterized by a solubility parameter larger than 20, such as (meth)acrylonitrile. According to the most preferred embodiment, polymer A consists of styrene and acrylonitrile units in a weight ratio between 1:1 and 5:1 (styrene:acrylonitrile), e.g. in a 2:1
Damme Marc Van
Vermeersch Joan
AGFA-Gevaert
Baxter Janet
Gilliam Barbara
Leydig , Voit & Mayer, Ltd.
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