Treating developed lithoplate with oleophilic composition

A positive working radiation sensitive plate is image-wise exposed, developed and desensitized, and dried. Thereafter an oleophilic composition including a solvent capable of softening the image is applied to the plate and the plate is then washed to remove the oleophilic composition from the non-image areas. The image accepts ink readily even when the image-wise exposure is effected using a continuous tone original in the absence of a half-tone screen.

This invention relates to the processing of radiation sensitive plates in 
lithographic printing plate production. 
As is well known, lithographic printing plates are conventionally produced 
from radiation sensitive plates comprising a substrate coated with a layer 
of radiation sensitive material which on exposure to actinic radiation 
becomes more or less soluble in suitable developers than the unexposed 
material. Thus, in the case of the so called positive-working plates the 
radiation-struck portions of the layer become more easily removable from 
the substrate than the non-radiation-struck portions and in the case of 
the so called negative-working plates, the non-radiation-struck portions 
of the layer remain more easily removable from the substrate than the 
radiation-struck portions. In each case, the portions remaining on the 
substrate after development form the image areas and the underlying 
substrate revealed on development constitute the non-image areas. 
To produce prints from continuous tone originals using such 
radiation-sensitive plates, it is the usual practice to create a master in 
the form of a photographic positive or negative reproduction of the 
original in which the tones are represented by regularly spaced dots of 
various sizes. The plate is then exposed whilst it is in contact with said 
master whereby the plate is provided with image areas comprising regularly 
spaced dots, known as half-tone dots, which vary in size in direct 
relationship to the tones being matched. The dots are normally so small 
that the presence of the individual dots is not readily distinguishable to 
the naked eye. However their size variations create the optical illusion 
of variation in tonal value. 
This system suffers from the inherent and limiting disadvantage that the 
resultant regularly disposed dot image sometimes clashes with the detail 
and form of the subject matter and results in "patterning." Moreover, when 
two or more similarly disposed images are super-imposed, as occurs when 
reproducing multicoloured originals, moire patterns may occur. Also, a 
special half tone screen has to be employed at one stage in the 
reproduction method, and this is not only difficult to make and hence 
expensive but also requires considerable skill and expenditure of time by 
the user. 
Recently there have been attempts to carry out so-called continuous-tone or 
screenless lithography in which the printing plate is prepared without 
using a half-tone screen. One particular type of radiation sensitive plate 
which has proved particularly suitable for this process comprises a 
substrate formed of electrograined and anodised aluminium coated with a 
positive-working radiation sensitive material comprising a suitable 
phenolformaldehyde resin such as a novolak resin and a suitable sensitiser 
such as a quinone diazide or a diazonium salt. After exposure through only 
a continuous tone positive the plate is treated with an appropriate 
alkaline developer and desensitised with gum arabic. This process depends 
on the fact that during the development of the plate, the 
radiation-sensitive material is removed in proportion to the amount of 
radiation to which it has been subjected, such removal taking place from 
the free surface of the material which obviously receives most light. 
Thus, the image areas of the plate corresponding to the lighter tones of 
the original comprise only microscopically thin layers of material lodged 
in the recesses in the surface grain of the plate. Although this process 
can be tolerably successful, the image areas consisting of these thin 
layers of material corresponding to the lighter tones do not accept ink 
easily at the onset of the printing operation. More than 100 copies 
frequently have to be taken before acceptable results are obtained and 
normal manual inking has proved to be both difficult and unreliable. 
It has now been surprisingly found that the ink acceptability of the image 
areas can be greatly improved by treating the developed and desensitised 
plate with a special liquid composition. 
Accordingly the present invention provides a method of processing a 
radiation sensitive plate comprising a substrate coated with a layer of a 
positive-working radiation sensitive material, which method comprises: 
(i) image-wise exposing the layer to actinic radiation so that the layer 
includes radiation-struck portions and non-radiation-struck portions, 
(ii) developing the image-wise exposed layer to selectively remove the 
radiation-struck portions of the layer and reveal the substrate underlying 
these portions and desensitising the revealed substrate to render the same 
oleophobic 
(iii) drying the developed and desensitised plate, 
(iv) coating the whole of the dried plate with a liquid composition 
comprising an oleophilic film-forming material and an organic solvent 
liquid which is capable of softening the surface of the 
non-radiation-struck portions, 
(v) drying the coated plate to form an oleophilic layer on the 
non-radiation struck portions and on said revealed substrate, and 
(vi) washing the plate to remove the oleophilic layer from said revealed 
substrate. 
The method of the present invention is particularly suitable in the case 
where the radiation sensitive plate is exposed using a continuous tone 
positive (i.e. without recourse to the use of a half-tone screen) in the 
manner above described. In this case, the substrate is a grained 
substrate, preferably an aluminium substrate electrochemically grained 
using alternating current and dilute hydrochloric acid as electrolyte. 
The method of the present invention may, however, also be practised in the 
case where a half tone screen is used in the preparation of the plate. 
The radiation sensitive material may be any suitable positive-working 
material such as a diazonium salt or quinone diazide optionally in 
admixture with an alkali soluble resin such as a novolak resin or other 
phenol-formaldehyde resin. 
When carrying out the method of the present invention it is preferred to 
effect the development of the image-wise exposed layer and the 
desensitisation of the revealed substrate in a single processing step. 
This is effected by utilising a developer which also has a desensitising 
effect. Examples of such developers can be found in UK patent 
specification No. 881,593 and U.S. Pat. No. 3,110,596. The preferred 
developer is an alkali silicate such as sodium silicate but other 
desensitising developers such as trisodium phosphate or trisodium citrate 
may be used. 
The liquid composition applied to the dried plate in accordance with the 
present invention should be of low viscosity in order to facilitate 
applying the same and the nature and amount of the organic solvent liquid 
present in the composition will be dependent on the nature of the 
radiation sensitive material of the plate. Examples of organic solvent 
liquids which have been found to be useful in conjunction with radiation 
sensitive materials based on novolak resins sensitised with quinone 
diazides or diazonium salts are alcohols such as ethanol, iso-propanol, 
n-butanol, 2-ethyl hexanol or 1-octanol; esters such as n-hexyl acetate, 
diethyl phthalate or diethyl carbonate; ketones such as 2,6-dimethyl 
heptanone, 3-methyl-2-pentanone or 4-methyl-2-pentanone; a lactone such as 
.gamma.-butyrolactone or a lactam such as 1-methyl-2-pyrrolidinone. The 
nature and amount of organic solvent liquid should be such that the 
organic solvent liquid only softens the surface of the image material and 
does not result in the removal of any significant amount of the material. 
The film-forming oleophilic material may be, for example, a normally solid 
hydrocarbon or fat in solution in a liquid hydrocarbon. Examples of 
suitable normally solid materials are asphaltum (gilsonite), naturally 
occurring waxes, paraffin waxes and/or fats such as tallow and examples of 
suitable liquid hydrocarbons are mineral oils, turpentine and/or white 
spirit. The liquid composition may include a conventional pigment and/or 
dye such as carbon black, or Phthalocyanine Blue Pigment and can readly be 
formulated so that it is of low viscosity. Typically, the liquid 
composition comprises from 0.5 to 5% by weight of normally sold 
hydrocarbon and/or fat, from 60 to 95% by weight of liquid hydrocarbon, 
from 0.1 to 30% by weight of the organic solvent liquid and up to 5% by 
weight of pigment and/or dye. Whilst the pigment and/or dye is not 
essential in respect of increasing the ink acceptance of the image areas, 
its presence allows the treated plate to be visibly inspected to check 
whether the lighter tones have become ink accepting before the plate is 
put on the press. The oleophilic layer is preferably a grey-black colour, 
but maybe any other suitable colour. It is preferable but not essential 
that the oleophilic layer is non-drying i.e. does not react with the air 
to harden or polymerise. The plate obtained in accordance with the present 
invention may be heated to harden the image areas to increase the length 
of the printing run if desired. 
In a particularly preferred embodiment, the washing of the plate is 
effected using an aqueous solution containing an organic solvent liquid 
such as an alcohol and an alkaline material such as trisodium phosphate or 
an acidic material such as phosphoric acid to aid removal of the 
oleophilic layer from the revealed substrate (th non-image areas); 
polymeric desensitising materials such as gum arabic, dextrin or sodium 
alginate; and/or wetting agents. If desired the plate may be simply 
protected by applying a thin layer of inert colloidal material and drying, 
after which it can be fastened to a suitable printing press and used to 
produce copies. Alternatively the plate may be further wet-inked with 
black ink in the traditional manner, or it may be washed out and wet-inked 
with the coloured ink to be used to obtain the required copies.

The following Examples illustrate the invention. In these Examples all 
percentages and parts are expressed on a weight basis. 
EXAMPLE 1 
A composition was made containing: 
White spirit: 88%; 
Mineral oil: 6%; 
4-Methyl pentane-2-one: 2%; 
Asphaltum: 2.5%; 
Carbon black: 1.25%; 
Tallow: 0.25%; 
A positive-working printing plate was prepared comprising a substrate 
formed of aluminium which had been electrochemically grained in 
hydrochloric acid electrolyte using alternating current and anodised in 
sulphuric acid electrolyte using direct current and a positive working 
radiation sensitive coating comprising a mixture of 1 part of the 
.alpha.-naphthol ester of naphthoquinone 1,2-diazide 2,5-sulphonic acid 
and 4 parts of a cresol-based novolak resin (VL 6859 Bakelite GmbH). The 
coating weight was 2 g/m.sup.2. The coating was exposed to actinic light 
for 3 minutes through a continuous tone positive and developed with a 
desensitising developer comprising an aqueous solution of a mixture of 
trisodium phosphate (5%) and sodium silicate (2%) having a pH of 13.6. The 
sodium silicate used was Grade H120 of Joseph Crossfield and Sons Ltd, 
Warrington. The developed plate was washed and then dried. The above 
composition was applied and burnished down to form an even layer having a 
thickness less than the depth of the grain. When the resultant oleophilic 
layer was dry, the plate was cleaned with an aqueous solution of dilute 
gum arabic (1%) to selectively remove the oleophilic layer from the areas 
of the substrate revealed on development. The plate was finally protected 
by applying gum arabic solution in the usual way. 
Visible inspection of the plate showed that the lighter tones had accepted 
the liquid composition and this suggested that they would print correctly 
and without difficulty. When placed on an offset printing press and washed 
out in the normal manner, good screenless copies of the tone subject 
represented on the continuous tone positive were obtained within 15 
revolutions of the press. 
EXAMPLE 2 
Example 1 was repeated except that the developer was an aqueous solution of 
0.2% sodium hydroxide and 2% sodium silicate. The sodium silicate was that 
marketed by Joseph Crossfield & Sons Ltd, under the designation Number 1. 
Similar results were obtained 
EXAMPLE 3 
Example 1 was repeated using as the developer a 5% solution of sodium 
metasilicate containing 10% polyethyleneglycol and as the aqueous cleaning 
solution a 3% aqueous solution of dextrin to which 0.5% phosphoric acid 
and 5% isopropanol had been added. Similar results were obtained. 
EXAMPLE 4 
Example 1 was repeated using water to clean the plate. Whilst similar 
results were obtained as regards ink acceptance of the lighter tones of 
the plate, a greater time was required to clean the plate, than when the 
aqueous solution of Example 3 was used. 
EXAMPLE 5 
Example 1 was repeated using as the radiation sensitive coating a mixture 
of 1 part of 4'-methoxy-diphenylamino-4-diazonium chloride and 4 parts of 
a cresol based novolak resin (VL 6859) and, as the liquid composition, the 
following mixture: 
n-Butanol: 5%; 
White Spirit: 66%; 
Turpentine: 25.5%; 
Paraffin wax: 2%; 
Phthalocyanine; 
Blue pigment: 1.5%; 
The plate was cleaned with the aqueous solution of Example 3 and wet-inked 
with black ink in the normal manner. Results similar to those of Example 1 
were obtained. 
EXAMPLE 6 
Example 5 was repeated using, as the radiation sensitive coating, a 
naphthoquinone 1,2-diazide 2,5-sulphonic acid ester of a novolak resin (VL 
6859). Similar results were again obtained. 
EXAMPLE 7 
Example 1 was repeated except that the treatment with the liquid 
composition and the final cleaning treatment were omitted. It was 
impossible to tell from a visible inspection of the plate whether the 
lighter tones would print correctly and when the plate was placed on the 
press several hundred revolutions of the press were necessary before these 
tones began to print correctly. 
EXAMPLE 8 
Examples 1-6 were repeated except that the radiation sensitive plates were 
exposed through a half-tone screen. In all cases plates were found to 
accept ink readily when placed on the press. 
EXAMPLE 9 
Example 7 was repeated except that the radiation sensitive plate was 
exposed through a half-tone screen. The resultant plate was reluctant to 
accept ink.