The present invention relates to a method for making a lithographic printing plate from a presensitized plate having a photopolymerizable photosensitive layer on an aluminum substrate. More specifically, the present invention relates to a method for making a lithographic printing plate which shows an improved development performance thus show a resistivity to printing contamination and has an increased strength of image with decreased development damage.
When printings are prepared from a lithographic printing plate, it is important to obtain printings with no background contamination. Generally, the occurrence of background contamination in printings increases with the storage time of a presensitized plate used for making a lithographic printing plate (hereinafter, referred to as PS plate) after production. In particular, this phenomenon becomes more intense when the PS plate has been stored under a high temperature and high humidity. It is particularly difficult to stably store a PS plate having a photopolymerizable photosensitive layer for a long time since free radical easily generates to initiate a chain polymerization reaction in the photopolymerizable photosensitive layer when the PS plate is exposed to light. Accordingly, there has been a demand to obtain a PS plate that can be stably kept for a long time, especially under a high temperature and high humidity and that can provide a lithographic printing plate which does not cause any background contamination.
Many attempts have been made to eliminate the background contamination. For instance, the following PS plates or methods disclosed in various publications are known; a PS plate comprising a basecoat layer consisting of polyvinylphosphonic acid coated on a surface of an anodized aluminum plate and a photosensitive layer coated thereon comprising a diazo compound (German Patent No. 1,621,478), a PS plate comprising a basecoat layer of polyacrylic acid that is coated on an aluminum substrate and a layer of diazo resin coated on the basecoat layer (German Patent No. 1,091,433), a PS plate comprising a basecoat layer of polyacrylamide and a photosensitive layer coated thereon (U.S. Pat. No. 3,511,661), and a method to add an organic acid polymer compound to a photosensitive layer comprising a diazo compound and organic polymer carrier to improve the stability with time of a PS plate comprising the photosensitive layer, thereby inhibiting generation of a background contamination of the PS plate (Japanese Un-examined Patent Publication (hereunder referred to as xe2x80x9cJ.P. KOKAIxe2x80x9d) No. Sho 56-107238). However, these PS plates are still insufficient and a further improvement is required. J.P. KOKAI No. Sho 57-5042 discloses a photo-addition composition comprising a combination of a diazo resin having plural diazonium groups on the side chain and a sulfonated polymer comprising plural sulfonate groups (for example, sulfonated polyurethane and sulfonated polyester). However, the effect of the composition to inhibit the background contamination is not sufficient and further, the utility of the PS plate is restricted since the performance of the plate is highly influenced by the sulfonated polyurethane or sulfonated polyester used in the photosensitive layer.
J.P. KOKAI No. 59-101651 discloses a method to improve the background contamination of a PS plate having a photosensitive layer comprising a diazo compound and a photopolymerizable compound when the plate is kept for a long time, which the method comprises coating a polymer comprising a repeating unit having a sulfonic acid as an intermediate layer. The storage stability of the PS plate prepared by the method is enough when the PS plate has a relatively low sensitive photopolymerizable photosensitive layer of which sensitivity which can be treated with the conventional PS plate-exposing device. However, the effect is not sufficient when the PS plate has a highly sensitive photopolymerizable photosensitive layer that is exposed to laser light as used for CTP technique.
The method to deposit a basecoat layer comprising a compound highly soluble in a developer or the method to add such a compound to the photopolymerizable photosensitive layer is effective for improving the background contamination property of a PS plate. But an irradiated portion of the photopolymerizable photosensitive layer of the PS plate is severely damaged by development and the printing durability of the PS plate is also deteriorated.
As a developer for a PS plate having a photopolymerizable photosensitive layer on an aluminum substrate, an aqueous solution of silicate, phosphate, carbonate or hydroxide of an alkali metal or organic amine compound has been proposed.
For example, J.P. KOKAI No. Hei 8-248643 discloses a developer having a high pH of at least 12 and comprising an alkali silicate and an amphoteric surfactant. J.P. KOKAI No. Hei 11-65129 discloses a developer having a pH of 12 or lower, wherein the ratio SiO2/M2O (M is alkali metal) is specified.
The former developer having such a high pH has a further problem, in addition to the handling problem, that an image area of the PS plate to be developed is easily damaged. The latter developer has a problem that the contained silicate easily gelates and becomes insoluble due to the decrease of the pH of the developer during the development process.
A developer comprising no alkali silicate is described, for example, in J.P. KOKAI No. Sho 61-109052. The patent application discloses a developer comprising an alkaline agent, a complexing agent, an anionic surfactant, an emulsifying agent and n-alkanoic acid. Another example is described in German Patent No. 1984605 which discloses a developer comprising an alkaline agent, a complexing agent, an anionic surfactant, amyl alcohol, and N-alkoxyamine. However, it is difficult to obtain a good printing performance such as printing durability since both the developers have high pH and comprise an organic solvent which causes damages of image area.
A developer having a relatively low pH (12 or less) with no alkali silicate is described, for example, in J.P.KOKAI No.2000-81711 which discloses a developer comprising an aqueous solution of potassium hydroxide comprising an anionic surfactant, and in J.P. KOKAI No. Hei 11-65126 that discloses a developer comprising an aqueous solution of alkali metal carbonate having a pH ranging from 8.5 to 11.5.
However, such a developer having a relatively low pH poorly dissolves photopolymerizable photosensitive layer and thus, may cause problems such as a remain of a film in a non-image area of the photosensitive layer, particularly when the utilized PS plate is an aged one (that is, the plate has been stored for a long time after production). In order to solve the problem, it is necessary to improve the development performance by increasing an acid number of a binder polymer in the photosensitive layer of the PS plate, or by use of an additional monomer having an acid group in the binder polymer. But, when such a binder polymer having a higher acid number is used, it often causes a problem in a printing process such as a blinding, that is, ink does not adhere to the PS plate during the printing process.
Thus, the purpose of the present invention is to provide a method for making a lithographic printing plate which has a stability with time and does not cause any background contamination when the plate is used to make printings after exposed to light and developed, and even when the plate is kept for a long time under a high temperature and high humidity. More specifically, the purpose of the present invention is to provide a PS plate useful for making a lithographic printing plate and to provide a preparation method therefor, wherein the lithographic printing plate shows a stable and good development performance even if the PS plate has been developed with an alkali developer having a relatively low pH preferably used from the viewpoint of environmental protection and safety, and the lithographic printing plate shows an increased image strength and causes no background contamination during printing process and no damage in the image area during development process.
The inventors of the present invention have found that a lithographic printing plate shows a resistivity to background contamination when the PS plate comprises an intermediate layer comprising a polymer comprising a structure unit having an acid group and a structure unit having an onium group and the PS plate is developed with an developer comprising a specific nonionic surfactant in an aqueous alkali solution having a relatively low pH. In addition, dissolution rate of non-exposed area of the photopolymerizable photosensitive layer is increased by the method and penetration of the developer is inhibited around the photo-crosslinked image portion to thus give a strong film.
Thus, the present invention provides a method for preparing a lithographic printing plate which comprises imagewise exposing to light a presensitized plate useful for making a lithographic printing plate having an intermediate layer and a photosensitive layer on an aluminum substrate in this order and developing the imagewise exposed plate with a developer, wherein said intermediate layer comprises a polymer compound comprising a structure unit having an acid group and a structure unit having an onium group and said developer comprises an inorganic alkali salt and a nonionic surfactant having polyoxyalkylene ether group and pH of the developer ranges from 11.0 to 12.7.
By such a method, it becomes possible to obtain a lithographic printing plate that does not cause any background contamination even it is kept for a long time and image portion of the plate becomes strong film to thus give a good printing durability and a resistance to a solvent.