Radiation-sensitive plates suitable for lithographic printing are well known. Such plates typically consist of a substrate such as aluminum that may be grained and/or anodized, or of zinc, magnesium, copper or steel or a combination such as a bimetal or laminate, and a layer of radiation-sensitive material deposited on the substrate.
Image-wise exposing the radiation-sensitive layer to actinic radiation through a transparency causes the solubility of the radiation-exposed areas to change relative to that of the unexposed areas. Upon treatment of the exposed plate with a suitable developer, the more soluble areas can be readily removed to reveal the underlying substrate and leave an image on the substrate constituted by the less soluble areas. The areas of the substrate revealed upon development constitute the non-image areas.
A type of radiation-sensitive materials known as photopolymers become less soluble after exposure to radiation and therefore a negative transparency is used in the exposure. In this case, it is the non-radiation-exposed areas that are removed upon development and the radiation-exposed areas that remain on the substrate form the image. Examples of suitable photopolymers include diazo resins, chromium-sensitized colloids, diazonium or azide-sensitized resins or polymers bearing such groups. Plates having a radiation-sensitive layer based on such materials are known as negative-working plates.
Radiation-sensitive materials such as those based on orthoquinone diazides become more soluble after exposure to radiation and therefore a positive transparency is used in the exposure. In this case, it is the radiation-exposed areas that are removed by development and the non-radiation-exposed areas that remain on the substrate form the image. Plates having such radiation-sensitive layers are known as positive-working plates.
The life, in terms of the number of copies it can produce, of a printing plate can often be increased by ("burning-in") the image areas, provided, of course, that the material of the image areas is suitable. "Burning-in" is a well-established practice in the art of producing lithographic printing plates from radiation-sensitive plates. The "burning-in" causes extensive crosslinking to occur in the polymeric structure of the material comprising the image area. The limiting temperature and time of the "burning-in" is that at which the aluminum anneals, resulting in a loss of strength required for a printing plate.
U.K. Patent 669,412 discloses the burning-in of images based on naphthoquinone diazides. In accordance with the teachings of this patent, a radiation-sensitive plate including a layer of the diazide is image-wise exposed, developed with an alkaline solution to remove selectively those areas of the layer exposed to radiation, and then placed in an oven to heat the image constituted by those areas of the layer that were not exposed to radiation. Thereafter, it is necessary to treat the plate with an alkaline solution again in order to remove contaminating residues from the plate and make the plate ready for printing.
In many cases, the image areas to be heated may be reinforced by incorporating reinforcing material in the radiation sensitive layer and/or by applying the reinforcing material in the form of a reinforcing lacquer to the image areas after development. Novolak resins and/or resol resins are examples of commonly used reinforcing materials. However, as disclosed in U.K. Patent 1,154,749, heating at a temperature sufficient to harden resin-reinforced image areas causes those areas of the substrate revealed on development to be at least partially covered with a contaminating layer which is ink accepting and which would therefore cause scumming and yield a soiled background during printing. This layer must therefore be removed before printing is initiated and this is achieved in accordance with the teachings of the patent by treating the plate with aqueous alkaline solution.
U.S. Pat. No. 4,294,910 discloses the use of various aqueous compositions known as "gumming" or "pre-bake" solutions to avoid problems resulting from the burning-in process. Such solutions contain materials such as sodium dodecyl phenoxy benzene disulphonate, alkylated naphthalene sulphonic acid, sulphonated alkyl diphenyl oxide, methylene dinaphthalene sulphonic acid, etc.
U.S. Pat. No. 4,786,581 discloses the use of "gumming" solutions for protecting plates during the burning-in process; these aqueous solutions contain a hydrophilic polymer component and an organic acid component. The organic acid component (or water-soluble salt thereof) contains di- or greater acid functionality and encompasses the benzene carboxylic acids, sulphonic acids and phosphonic acids including alkane phosphonic acids. In contradistinction to the materials recited in the '581 patent, the present invention does not require the use of a hydrophilic polymer. Furthermore, the diphosphonic acid (present as the di-potassium salt) employed in the present invention provides superior results in protecting the surface of aluminum plates to be burned-in. Such superior protectant properties would be related to the ability of the di-potassium salt of the hydroxyethylidene diphosphonic acid to complex with the aluminum oxide surface of the printing plate, whereas the alkane phosphonic acids disclosed in the '581 patent do not possess such a property.
The contaminating layer produced as the result of the burning-in process is not, as a rule, discernible to the naked eye and it is difficult to ensure that all the contamination has been removed. Moreover, in the case of those substrate surfaces that are porous, as is the case of an anodized aluminum plate, contamination may be present in the pores. Such contamination is likely to cause scumming during long printing runs as the substrate surface is gradually worn away. The alternative of redeveloping the plates after burning-in in order to remove the contaminating layer is costly and inconvenient since the plates have to be returned to the plate fabrication facility after they have been removed from the oven.
In view of the difficulties associated with the removal of the contamination which is produced by the burning-in procedure, it is desirable to prevent such contamination from occurring in the first instance. It has been found that the contamination apparently arises as a result of some component of the image material subliming from the image areas during heating and subsequently being redeposited on the areas of the substrate revealed on development. Even plates that contain no substances that could generate contamination during heating nevertheless become contaminated by deposition of contaminating material previously deposited on the internal surfaces of the burning-in oven as a result of prior usage.