Currently the commonest method of preparing a lithographic plate is to image a photosensitive lithographic plate using an image mask, such as a photographic negative, and to prepare the plate therefrom using an aqueous developing solution. This procedure is time consuming and requires facilities and equipment to support the necessary chemistry.

Thus recently various methods have been proposed for preparing lithographic plates on the press which is to be used to produce prints from the plate. These methods prepare the image using a digitally controlled laser image head. As described in E.P.A. 580393 such methods include ink-jet methods digitally controlled, spark-discharge methods and the production of electromagnetic-radiation pulses that create chemical changes of the plate blank. Also etching methods have been described as well as blank plates which are ablated by the laser to form an ink-receptive image. But such methods require expensive apparatus.

We have discovered a method of processing a lithographic plate precursor wherein the plate is processed whilst on the press.

According to the present invention there is provided a method of preparing a lithographic plate which comprises coating on a metal plate having a hydrophilic surface a layer of a radiation sensitive coating, imaging the coating then acting on the plate whilst on the press with aqueous fount solution to remove the unexposed areas of the coating to reveal the hydrophilic surface of the plate and to leave an ink receptive image, wherein the radiation sensitive coating comprises a diazo salt of the following formula: wherein R, is an anion, R2 and R3 represents optional substitution, R4 is -N-or -S and R5 is a group which after exposure of the plate renders the residue of the diazo salt oleophilic and fount insoluble.

Usefully R, is sulphate, nitrate or chloride or it may be a more complex anion for example perfluorooctyl. R2 and R3 if present are preferably selected from the alkoxy, halogen, alkyl or amine. The presence of a substituent group in R2 or R3 helps to stabilise the diazo compound.

When R4 is -N- it may be represented by where R7 and R6 may be part of a polymer chain or may be part of a morpholino functional group.

Preferably however R6 is hydrogen and R7 is a group where R8 is the residue of a condensing agent and n is 1 to 100. Thus the groups R7 converts the diazo compound to a diazo resin and this helps to prevent the exposed diazo from washing off during processing.

Examples of diazo salts used in the examples which follow are: (CaF,7SO2)2CH I at, N + X~ N C n=l-ll NH NH y½CH} n Perfluorooctyl diazo A Sulfate diazo B and (HSO4 0.5 ZnC12) CH2 W "CH2f Diazo C The plate whilst on the press may have fount solution applied to it by a sponge or a cloth.

Preferably however the dampening roller of the press applies it. In this case printing ink may be applied to the plate at the same time.

The plate may be imaged on a lightframe using U.V. light. Preferably it is imaged using a digital laser imaging head which comprises a laser which scans in an imagewise manner across the plate.

The laser may be one which emits in the UV region; diazo salts being naturally sensitive to UV light. However preferably the laser emits at above 800nm and the diazo coating composition comprises an infra-red absorbing compound.

Preferably the infra-red absorbing compound is one whose absorption spectrum is significant at the wavelength output of the laser which is to be used. For example gallium arsenide diode lasers emit at 830nm and Nd YAG lasers emit at 1064nm.

Preferably the digital laser imaging head is in essence an image setter attached to the printing press.

Several types of support can be used for the manufacturing of a diazo sensitised lithographic printing plate. Common supports are metal supports like Al or Zn, polyester film supports and paper bases. These supports if not sufficiently hydrophilic by themselves, are first coated with a hydrophilic layer to form the background of the printing plate and a top layer containing the diazo compound is then applied.

The lithographic support may be a flexible support which can be attached to the printing press.

However preferably the lithographic support is a metal sleeve or cylinder which has a hydrophilic surface and which forms part of the printing surface of a printing press.

Thus in the preferred method of the present invention a metal sleeve or cylinder which has a hydrophilic surface and which forms part of the printing surface of a printing press is coated with a diazo coating solution, the metal sleeve is disengaged from the roller drive of the printing press and is caused to rotate at a speed suitable for imaging, the digital laser head attached to the printing press images the diazo layer on the metal sleeve, after imaging the metal sleeve is re-engaged to the roller drive of the printing press and the rollers of the press rotate and act as fount-dampening rollers, thus removing the unexposed areas of the diazo on the surface of the sleeve and to reveal the hydrophilic surface of the sleeve in the unexposed areas of the sleeve, the rollers of the press are then inked up and the printing press prints onto paper fed to it. After the print run has finished a plate washer can be employed to remove all the coating from the sleeve which can then be re-used.

Preferably the metal sleeve can be removed from the press to clean it thoroughly and also to renew it periodically.

In a modification to the method of the present invention there is present between the hydrophilic surface of the metal sleeve and the dried diazo coating solution a hydrophilic layer. It helps the fount solution to remove the unexposed diazo coating more cleanly after the imagewise exposure. Also the presence of the layer makes it easier to thermally image the diazo layer.

Examples of such hydrophilic layers are layers composed of hydrophilic polysaccharides for example dextran or pullulan.

The following examples will serve to illustrate the invention.

Example 1 A solution containing 0.5g of Diazo resin (Diazo B) sulphate and 0.95g water:methanol (80:20) was prepared and coated onto a Horsell standard grained/anodised aluminium plate using a bar coating technique.

This was then dried at 800C for 2 minutes in an oven and gave a coating film weight of 0.6gum~2.

The plate was imaged to U.V. light using a Montakop light frame at 200 units and 50:50 vacuum (with a stouffer wedge). Approximately 50mls of fount solution was then applied to the plate and rubbed gently with cotton wool for 1 minute. Rapid ink was then applied (no removal of fount).

Assessment was made of the stouffer reading and any background staining.

Using a Stouffer Wedge the readings were: Solid 1, Slight trace 11 A similar test was carried out using the same quantity of the perfluorooctyl diazo (diazo A) instead of the sulphate diazo (diazo B).

Using a Stouffer Wedge the readings were: Solid 11, Slight Trace 17 In both cases when the plates were inked up over 5000 copies were obtained before background staining occurred.

Example 2 Laser imaged.

The diazo compound used was diazo C.

The 1. R dye used had the formula:- A binder ROC 15 was present of the formula: A solution containing 0.88g of an ROC15 resin solution at 40% (w/w) in methoxypropanol 1.79g of a 5% (w/w) diazo C solution in water, 0.02g of IR dye 1, 7.07g of methoxypropanol and 0.24g of water was prepared and coated onto a substrate consisting of a sheet of aluminium that had been electrograined and anodised and given a coating film weight of 1.3gm3 after drying at 800C in an oven for 2 minutes.

The coated substrate to be imaged was cut into a circle of 1 05mm diameter and placed on a disc that could be rotated at a constant speed of 2500 revolutions per minute. Adjacent to the spinning disc a translating table held the source of the laser beam so that the laser beam impinged normal to the coated substrate, while the translating table moved the laser beam radially in a linear fashion with respect to the spinning disk. The exposed image was in the form of a spiral whereby the image in the centre of the spiral represented slow laser scanning speed and long exposure time and the outer edge of the spiral represented fast scanning speed and short exposure time. The laser used was a single mode 830nm wavelength 200mW laser diode which was focused to a 10 micron spot. The laser power supply was a stabilised constant current source.

The exposed plate was then rubbed with a cotton wool swab with printing ink and fount solution applied which removed the parts of the coating on the plate that were not struck by the laser beam giving an image. The imaging energy density required to give a suitable image was 500mJcm2.

Example 3 Laser imaged A printing plate made according to example 2 was also imaged on a commercially available image setter, the Trendsetter, supplied by Creo Products of Vancouver, Canada. The imaging energy density required to give a suitable image was 500mJcm2.

As the invention relies on a heating process, any suitable light of sufficient power and is suitably absorbed by components in the system to generate heat in the composition, may be used.

Comparative A solution containing 0.88g of an ROC15 resin solution at 40% (w/w) in methoxypropanol 1.79g of a 5% (w/w) diazo C solution in water, 7.07g of methoxypropanol and 0.24g of water was prepared and coated onto a substrate consisting of a sheet of aluminium that had been electrograined and anodised and given a coating film weight of 1.3gm3 after drying at 800C in an oven for 2 minutes.

The resulting plate was imaged using a 200mW laser diode at a wavelength of 830nm using the imaging device described previously. The plate was then rubbed with a cotton wool swab with printing ink and fount solution applied. This process removed the parts of the coating that had not reacted by being struck with the laser beam. As no image remained on the plate, the imaging reaction had not taken place.