This invention relates to the processing of lithographic printing
plate precursors and is principally concerned with the wash water used in the
production of a lithographic printing plate by means of the system known as silver
The well-known comparatively high sensitivity to light of silver
halides over conventional photopolymeric materials and their ability to respond
to light from ultra-violet and infra-red make them ideally suited for use in printing
plate applications where direct-to-plate exposure, rather than exposure through
a contact film intermediate, is required.
Silver halides in this context can be used in a variety of ways.
Printing plates, in general, include at least one layer of photosensitive material;
such a layer may comprise a silver halide in combination with gelatin or other
matrix binder, so providing a tough, ink-receptive image. This technique is often
referred to as tanning development. Alternatively, a silver halide emulsion layer
can be overcoated onto a conventional photopolymerisable layer of a printing plate.
The consequence of the difference in sensitivity between the layers is that, at
a given intensity of radiation, a short light exposure can be used to imagewise
expose the top silver halide layer which, on development, forms a mask for a longer,
blanket light exposure to convert the polymeric layer into a soluble or insoluble
form depending on the nature of the photopolymer.
A third general use of silver halide in printing plate applications
is the well known technique of diffusion transfer reversal (DTR). The principles
of the DTR process have been described in US-A-2352014 and in "Photographic Silver
Halide Diffusion Processes" by Andre Rott and Edith Weyde, The Focal Press, London
and New York, 1972. In this method, a developer is used which chemically develops
exposed areas of the photosensitive coating whilst at the same time dissolving
the unexposed areas. The developer contains a so-called silver halide solvent,
such as sodium thiosulphate or sodium thiocyanate, and the complexes formed by
these solvents with the dissolved silver halide from the unexposed areas diffuse
to an image-receiving element, typically a nucleation layer containing physical
development nuclei, and are reduced therein with a developing agent to form a silver
image (DTR image) having reversed image density values with respect to the black
silver image obtained in the exposed areas of the photographic material. An assembly
including a DTR image may be used as a planographic printing plate, the silver
image areas being the water-repellent, ink-receptive areas on an otherwise water-receptive,
ink repellent background. The oleophilicity of the silver image areas may be improved
by treatment with a suitable oleophilising agent, such as a mercapto compound.
Two different diffusion transfer systems are known. The two sheet
system includes a silver halide layer and a receiving layer which are on separate
substrates, and the system relies on the diffusion of silver halide from the former
layer to the latter when the two are placed in contact in the presence of a developer.
In the single sheet system, however, the silver halide layer and the receiving
layer are both coated on the same substrate, and a water permeable relationship
exists between the layers, allowing for image formation on application of a developer,
following exposure. It is the latter, single sheet, system which is preferred for
the preparation of offset lithographic printing plates via the diffusion transfer
There are two different types of single sheet diffusion transfer
printing plates currently provided by the known art. In the first instance, plates
are disclosed in, for example, US-A-4722535 and GB-A-1241661, wherein a support
is coated, in order, with a silver halide emulsion layer and an image receiving
layer containing physical development nuclei. Following imagewise exposure and
development, the plate is used for printing without removal of the spent emulsion
layer. A second type of plate, however, includes the coatings applied in a different
order, such that a support is coated first with a layer of physical development
nuclei, then subsequently with a silver halide emulsion layer. The assembly is
imagewise exposed and developed, then washed with water or an aqueous wash-off
solution to remove the spent emulsion layer, thereby leaving a support carrying
a silver image which may be used as a printing plate. Plates of this type are disclosed,
for example, in EP-B-278766 and EP-A-410500.
During exposure of a two sheet diffusion transfer plate, all the
silver halide in the exposed areas is converted to silver, which is dispersed throughout
a binder material, this generally comprising gelatin. Water has been most commonly
used as the wash-off solution for removal, or decoating, of the spent emulsion
In order to improve the ink receptivity of lithographic printing plates
having image areas of metallic silver derived from silver halide emulsions, a finishing
composition is described in EP-A 0 131 462 wherein a proteolytic enzyme is included
to catalyse the breakdown of proteins such as the gelatine binder, and wherein
mercapto compounds or cationic surfactants are added as oleophilising compounds.
Ecological, economic and general practical considerations dictate
that re-use of the wash-off solution to the maximum extent possible is particularly
desirable, and silver is most preferably removed from this recirculated wash water
by means of a filtration system, such as that described in EP-A-651063. This document
also discloses flocculation of the silver via the addition of materials such as
cationic surfactants - for example, Empigen BCB50 from Albright and Wilson - or
cationic polymers - for example, Merquat 100 supplied by Chemviron - thereby facilitating
the more efficient removal of the silver by filtration.
Nevertheless, it has still been found that there are difficulties
associated with this method. Most particularly, the use of such materials tends
to lead to stickiness of the flocculated silver which, in turn, can result in problems
due to the accumulation of this flocculated silver on the walls of the wash tank
and in the pipe work, and may even cause seizure of pumps to occur. Furthermore,
redeposition of the flocculated silver on the processed plates is an additional
hazard, creating the potential for the appearance of black marks on the plate surfaces
and, consequently, on the printed images.
It is, therefore, an object of the present invention to provide a
wash liquor, the use of which does not lead to such stickiness of the flocculated
silver halide and, thereby, to provide a means of achieving cleaner plate processing.
It has now been found that very significant improvements can be effected
in this regard by the addition of non-ionic surfactants to the wash water. Moreover,
in reducing or eliminating stickiness of the flocculated silver and facilitating
cleaner processing in this way, no deleterious effect is provided in terms of actual
flocculation of the silver, or its subsequent removal from the wash water by filtration.
According to the present invention, there is provided a process for
the decoating of a silver halide diffusion transfer printing plate, said process
including the step of treating the plate with a wash liquor comprising an aqueous
solution containing at least one non-ionic surfactant in an amount of from 0.1
to 10 g/l.
Various types of non-ionic surfactant are available, and all are
effective, at least to some extent, in reducing the stickiness of the flocculated
silver. The types of non-ionic surfactant are detailed in "Surfactants Europa -
A Directory of Surface Active Agents available in Europe", published by The Royal
Society of Chemistry.
In practice, the wash water in such systems tends to be alkaline due
to the carry over of developer from the plate development stage. Consequently,
it is desirable that the non-ionic surfactant should be stable in alkaline conditions.
Additionally, surfactants which generate low levels of foam are preferred.
Typical low foam non-ionic surfactants which are alkali-stable include
ethylene oxide/propylene oxide block copolymer type surfactants; unfortunately,
however, such products are less effective than other non-ionic surfactants in preventing
stickiness of the flocculated silver. Improved results in this regard are shown
by alcohol ethoxylates, alkylphenol ethoxylates and alkylamine ethoxylates, all
of which are alkali stable materials and are particularly effective in the wash
liquors of the present invention. Especially preferred surfactants are the low
foam modified alcohol ethoxylates such as Ethylan CPG660 and Ethylan CPG945 (Akcros
The non-ionic surfactant may be added to the wash water either as
a separate aqueous solution, or in combination with the silver flocculating agent,
such as a cationic surfactant or cationic polymer. Either a single non-ionic surfactant
or mixtures of said surfactants may be employed. The surfactant is preferably present
in the wash water to the extent of from 0.1 to 10g/l. The temperature of the wash
water is generally controlled in the range between 20°C and 50°C, although it
preferably falls between 30°C and 40°C.
Optionally, the wash water may also contain additional additives
including, for example, enzymes, biocides and anti-foams, all of which may be added
to the liquor in combination with the non-ionic surfactant. Preferably, the wash
liquor will also contain at least one organic or inorganic acid salt, such as a
silicate or pyrophosphate - as disclosed in our co-pending Application No. GB
9723026.2 - which is added in combination with the non-ionic surfactant and serves
to provide improved plate decoating properties. Most preferably, the wash water
further includes an enzyme which is capable of degrading gelatin, for example trypsin,
pepsin, papain or a bacterial proteinase, such as Alcalase®2.5L (supplied
by Novo Enzymes Limited).
The decoating process is most successfully carried out by treating
the plate with the wash liquor whilst, at the same time, applying mechanical forces.
Thus, the use of high pressure jets or scrubbing rollers produces the most satisfactory
The printing plate used in conjunction with the wash liquor of the
present invention is, most preferably, a single sheet diffusion transfer printing
plate comprising a substrate, an image receiving layer and a silver halide emulsion
The substrate used is generally aluminium, which may be pure aluminium
or, alternatively, may comprise an aluminium alloy having an aluminium content
of at least 95%. The thickness of the substrate preferably lies in the range between
0.13mm and 0.5mm. In order to enhance its lithographic properties, the aluminium
is electrochemically grained and anodised on at least one surface. Graining of
the substrate may be achieved by treating the surface with an aqueous acid or a
mixture of acids; typically, hydrochloric acid, or a mixture of hydrochloric and
nitric acids may be employed. Anodising is preferably carried out by treating the
grained aluminium substrate in an aqueous mineral acid or a mixture of such acids.
Most satisfactory results are obtained by the use of sulphuric or phosphoric acids
or their mixtures. Typical graining and anodising conditions are disclosed in US-A-3861917.
Most preferably, the graining and anodising conditions are selected such that
the substrate has an anodic weight (g/m2) to surface roughness (microns)
ratio greater than 6, more particularly greater than 8, according to the disclosures
Optionally, the grained and anodised aluminium may be laminated to
other materials, such as paper or various plastics materials, in order to enhance
its flexibility whilst retaining the good dimensional stability associated with
The image receiving layer preferably comprises a metal sol, most
preferably colloidal silver nuclei prepared by the Carey Lea method at a coating
weight in the region of 3mg/m2. The colloidal nuclei are optionally
dispersed in a suitable binder, most preferably gelatin. Alternative colloidal
nuclei which may be employed include sulphides of heavy metals, such as silver
sulphide or palladium sulphide.
The silver halide emulsion layer may be any photosensitive silver
halide emulsion incorporating a hydrophilic colloid binder. The photosensitive
silver halide may comprise, for example, silver chloride, silver bromide, silver
bromoiodide or silver chlorobromide or their mixtures. The use of an emulsion containing
in excess of 50% silver chloride is preferred in order that a sufficiently high
rate of dissolution of the silver halide may be achieved during development, and
that satisfactory gradation may be obtained for lithographic purposes. It is also
desirable that the emulsion should include a minimum of 20% silver bromide, thereby
ensuring adequate stability on the grained, anodised aluminium substrate.
The silver halide emulsion may include coarse or fine grains, and
can be prepared by any of the standard procedures well known to those skilled
in the art. Optionally, the emulsion may be chemically and spectrally sensitised.
The available techniques for the preparation and coating of the emulsion are detailed
in Product Licensing Index, Volume 92, December 1971, publication 9232.
In addition to the preferred negative working silver halide emulsions,
which exhibit particularly high photosensitivity, direct positive silver halide
emulsions, producing a positive silver image in the emulsion layer and a negative
silver image on the aluminium substrate, may also be employed.
The emulsion layer also includes a hydrophilic colloid binder, as
previously disclosed. Generally, the binder comprises a protein, preferably gelatin.
However, partial replacement of the gelatin with suitable synthetic, semi-synthetic
or natural polymers is possible.
Optionally, the emulsion may also include further components such
as antifogging agents, development agents, development accelerators, wetting agents,
stabilisers, acutance dyes and pigments, matting agents and the like.
Additionally, it is possible to include a further, intermediate,
water-swellable layer between the image receiving layer and the silver halide emulsion
layer. Suitable intermediate layer formulations are detailed in EP-A-483415.
The diffusion transfer plate containing the above elements is imagewise
exposed to a beam of energy, and the exposed plate is developed by treatment with
an aqueous alkaline solution in the presence of at least one developing agent and
at least one silver halide solvent. The developing agent or agents and the silver
halide solvent or solvents may be incorporated in the aqueous alkaline solution
and/or in the actual imaging element itself.
The most suitable developing agents for use in conjunction with the
present invention are hydroquinone-type compounds in combination with secondary
developing agents. Preferably, the hydroquinone-type compound is hydroquinone itself,
methyl hydroquinone or chlorohydroquinone.
The secondary developing agent comprises p-N-methylaminophenol, 1-phenyl-3-pyrazolidone,
or a derivative of the latter, such as 4-methyl-1-phenyl-3-pyrazolidone, 4,4-dimethyl-1-phenyl-3-pyrazolidone,
4-hydroxymethyl-4-methyl-1-phenyl-3-pyrazolidone or 4-methyl-1-tolyl-3-pyrazolidone.
Typical silver halide solvents for use in relation to the present
invention include thiosulphates and thiocyanates which are able to form complexes
with silver halides, for example ammonium thiosulphate, potassium thiosulphate
or, most preferably, sodium thiosulphate pentahydrate, which is used at a level
of 5 to 150 g/l, preferably 10 to 80 g/l. Alternative silver halide solvents are
disclosed in the prior art; for example polythioethers are described in US-A-5200294,
EP-A-554585 mentions the use of meso-ionic compounds, whilst cyclic imides and
thiosalicylates are the subjects of US-A-4297430 and US-A-2857276, respectively.
Optionally, combinations of silver halide solvents may be employed, and it is
possible to incorporate one or more silver halide solvents into a suitable layer
of the plate, and also include one or more silver halide solvents in the developing
The aqueous alkaline developing solution typically incorporates common
alkaline materials such as sodium hydroxide, potassium hydroxide, sodium carbonate
or alkali metal salts of phosphoric and/or silicic acid, eg trisodium phosphate
and sodium metasilicate. The solution may also include other ingredients, examples
being oxidation preservatives, eg sodium sulphite, bromide salts, calcium sequestering
agents, anti-sludge agents, antifoggants and thickening agents.
Preferably, the alkaline developing solution also contains amines
or alkanolamines which act as development accelerators and, in addition, function
as silver halide solvents; typical examples include 2-amino-2-methyl-1-propanol,
2-amino-2-methyl-1,3-propanediol, 2-amino-2-ethyl-1,3-propanediol, tris(hydroxymethyl)
aminomethane, 2-amino-2-ethyl-1-propanol, 1-amino-2-propanol, 2-methylaminoethanol,
ethanolamine, diethanolamine, triethanolamine and N,N-diethylaminoethanol.
The development and diffusion transfer operations may be carried out
by means of any of a number of standard techniques, for example, by dipping the
material to be treated in to the liquid composition. Said treatment is generally
carried out at a temperature in the range 15-30°C and over a period of around 5-60
seconds. Any excess of alkaline developer remaining following the development process
and formation of the silver image may be removed by passing the plate through
a pair of squeezing rollers.
The surface of the silver image thereby produced in the layer of
physical development nuclei may then be exposed by washing the plate with water,
such that removal of all the layers above this layer takes place.
Improved printing performance may be achieved by chemical treatment
of the imaged surface of the aluminium with a formulation which increases the
hydrophilicity of the background areas and also enhances the oleophilicity of the
silver image. Said formulation for after treatment of plates is generally referred
to as a fixer or finisher and, in the present case, would typically comprise an
enzyme and a hydrophobising (oleophilising) compound. Typical such compositions
are disclosed in EP-B-131462.
After said finishing treatment, the plate is ready for use in printing
operations and may be used on a printing press to produce high quality prints.
The invention will now be illustrated, without limitation, by reference
to the following experiments:
A silver chlorobromide emulsion in a gelatin binder having a silver
to gelatin weight ratio of 1:1 was coated at a coat weight of 4.0g/m2
grained and anodised aluminium substrate which had been previously coated with
a Carey Lea silver sol. The plate was exposed to white light, and then processed
in an Autolith SLT70 automatic processor comprising a development section, a wash
section and a finisher section.
Development was for 20 seconds at 21°C in the following developer
Water to 1 litre
The wash section contained a scrubbing roller and 25 litres of fresh
water at 32°C. A total of 15.6m2 of plate was processed. The wash water
was collected. The silver content of the water was 1.0g/1.
To 1 litre samples of the above wash water at 32°C were added various
non-ionic surfactants and 1.0ml of the flocculent solution (example 4 of EP-A-651,063)
(Merquat 100 is a solution of a cationic polymer supplied by Chemviron; Alcalase®
2.5L is an enzyme supplied by Novo Enzymes Limited; Bacteron B6 is a biocide supplied
by Bactria Biocides.)
Victoria Blue B
The mixture was stored for 5 minutes in the presence of a piece of
clean polypropylene sheet. The mixture was then gently washed off with water and
the amount of flocculated silver adhered to the polypropylene was noted. The results
were graded from 0 to 5, a value of 5 indicating a significant amount of silver
adhered to the polypropylene sheet and a value of 0 indicating perfectly clean
A silver chlorobromide emulsion in a gelatin binder having a silver
to gelatin weight ratio of 1:1 was coated at a coat weight of 4.0g/m2
grained and anodised aluminium substrate which had been previously coated with
a Carey Lea silver sol. After imagewise exposure the plate was processed in an
Autolith SLT70 automatic processor comprising a development section, a wash section,
and a finisher section. Development was for 30 seconds at 20°C in the developer
described in experiment 1.
This gave chemically reduced silver in the exposed areas and physically
reduced silver attached to the anodised aluminium in the non-exposed areas. After
developing, the plate was transferred to a wash section comprising scrub rollers
and water jets, and containing 25 litres of water at 32°C. This removed loose material,
which comprised silver metal, gelatin, silver complexes and developer, from the
plates. The flocculent mixture of experiment 1 was then added to the wash water
at a rate of 1.6ml/m2 of plate. The wash water was pumped through a
5 micron, 50cm Purtrex polypropylene filter (supplied by Osmonics Inc, Minnetonka,
USA) and recycled. Substantially all of the silver was removed from the wash water.
After 100m2 of plate had been processed the filter became blocked with
silver. A sticky deposit of silver was present on the sides of the wash water tank.
The above procedure was repeated but this time the wash water contained
1.0g/1 Ethylan CPG660, added at the start. Silver was removed from the wash water
as before but the sides of the wash water tank were now substantially free of any