IMPROVEMENTS IN OR RELATING TO THE PROCESSING OF RADIATION SENSITIVE DEVICES

Abstract: Changes occurring in the electrical conductivity of liquids used to process image-wise exposed radiation sensitive devi­ ces are used as a measure of the deterioration in effectiveness of the liquids. This deterioration is compensated for by varying the processing conditions, such as temperature, time, scrub­ bing action and processing liquid composition, in accordance with the change in conductivity.

Description: This invention relates to the processing of radiation sensitive devices comprising a radiation-sensitive coating on a substrate. Such radiation sensitive devices are used in the production of, for example, printing plates, in particular lithographic printing plates, printed circuits and integrated circuits. Radiation sensitive devices for use in the production of lithographic printing plates usually consist of a metallic support sheet, which is mechanically or chemically treated to provide a suitable hydrophilic surface and which carries the radiation sensitive coating. In use, the device is image-wise exposed to actinic radiation using either a negative or positive transparency of an appropriate subject. The effect of the actinic radiation is to alter the solubility of the radiation-sensitive coating. The image-wise exposed device is then processed. The processing step involves contacting the image-wise exposed device with a developer to selectively remove unwanted areas of coating from the support sheet to leave an image constituted by the areas of coating remaining on the support sheet. Other types of printing plates and printed and integrated circuits are produced in a similar manner. After the developing step, the device is washed and in the case of a lithographic plate, is treated with a finisher/densitiser whose main purpose is to protect and/or render the non-image areas hydrophilic. The precise processing routine and processing liquids used, depend on the solubility and chemical characteristics of the radiation sensitive coating being processed. Whilst the processing may be done manually, it is increasingly being carried out in automatic processors. There are three types of liquid commonly used for subtractive developing namely alkaline developers, solvent developers and aqueous developers. Alkaline developers are used for positive-working coatings based on quinone diazides and consist of an aqueous solution of a suitable alkali, for example a silicate, a phosphate or a hydroxide. Solvent developers are used for negative-working coatings based on photo-crosslinkable materials, for example polyvinyl cinnamate and consist of a suitable solvent, for example a glycol ether or butyrolactone, a surfactant and possibly a mineral acid. Aqueous developers are used for negative working coatings based on diazo resin and consist of an aqueous solution of a surfactant. A problem arising when processing exposed radiation sensitive devices with developer liquid is that the developer liquid gradually deteriorates with use until it is no longer capable of adequately removing the coating from the support. This deterioration is progressive and, therefore, in the case of a lithographic plate incorrect development can occur well before the developer is completely exhausted. It has now been found that the effectiveness of developer liquids for image-wise exposed radiation sensitive devices can be monitored by measuring their electrical conductivities. JP-A-53 110532 discloses a wet type diazo copying machine for developing diazo copies which are produced from an original by exposing diazo paper directly to the original to be copied. A latent image is produced on the diazo paper during the exposure step and the diazo paper is then contacted with an aqueous developer liquid which renders the latent image visible. The electrical resistance of the developer liquid is measured and the developer liquid is diluted with water in dependence on the resistance so as to compensate for the evaporation of water from the developer liquid. The developer process is not a subtractive developer process. According to one aspect of the present invention, there is provided a method of processing a plurality of image-wise exposed radiation sensitive devices each comprising a substrate carrying an image-wise exposed radiation sensitive coating which method comprises (i) contacting the devices in turn under given processing conditions with a developer liquid, (ii) monitoring the electrical conductivity of the developer liquid during the processing of the devices, and (iii) varying the processing conditions under which the devices are contacted with the developer liquid in dependence upon the changes in the electrical conductivity so as to maintain the efficacy of the developer liquid characterised in that the developer liquid is one which selectively removes the more soluble areas of the coating from the substrate and whose efficacy for this purpose changes as the number of devices processed increases and in that the processing conditions are varied by varying the time for which the devices are in contact with the developer liquid and/or by adding further developer liquid to the developer liquid and/or by varying the degree to which the developer liquid is agitated in contact with the devices and/or by varying the temperature of the developer liquid. According to another aspect of the present invention there is provided an apparatus for developing image-wise exposed radiation sensitive devices, which apparatus comprises:- (i) a container for developer liquid, (ii) a means for moving the devices along a path through the apparatus so that they are contacted by the developer liquid under given processing conditions, (iii) a means for measuring the electrical conductivity of the developer liquid and for producing an output signal in dependence upon said conductivity, and (iv) a means of varying the processing conditions which is controlled by said output signal, characterised in that said means of varying the processing conditions comprises a variable speed motor for driving the device moving means so that the period of time for which the devices are in contact with the developer liquid is dependent on the conductivity, a reservoir for storing further developer liquid and having a valve to enable further developer liquid to be fed to the container from the reservoir in dependence on the conductivity, a variable speed motor for driving an agitating roller for agitating developer liquid in contact with the devices so that the degree to which the developer liquid is agitated in contact with the devices is dependent on the conductivity, and a unit immersed in the developer liquid to vary the temperature thereof so that the temperature of the developer liquid is dependent on the conductivity. In a further embodiment, the apparatus includes a heating and cooling unit for varying the temperature of the developer liquid in dependence on the output signal. In yet a further embodiment, the apparatus includes a reservoir for containing developer liquid replenisher, ie. the same, a different or a more concentrated solution, the flow of replenisher to the main body of developer liquid in the apparatus being controlled by the output signal by means of, for example, a solenoid valve. In the case where the developer liquid does not have sufficiently high conductivity, a lithographically inert material which ionises in solution, eg. potassium nitrate, may be added to it. For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:- Figure 1 is a schematic diagram of an apparatus in accordance with the present invention; Figure 2 is a block diagram of the control circuit of the apparatus of Figure 1; Figure 3 is a circuit diagram of a part of the control circuit shown in Figure 2; and Figure 4 is a schematic diagram of another apparatus in accordance with the present invention. Referring to Figure 1 the apparatus comprises a tank 1 for containing developer liquid, a pair of rubber covered input rollers 2 and 2 a , a pair of rubber covered output rollers 3 and 3 a , and a variable speed dc electric motor 4 connected to drive the roller 2. Roller 2 a is driven by contact with roller 2. (The rollers 3, 3 a may also be driven from the motor 4 if desired). A conductivity cell 5 is mounted in the tank 1 so as to lie in the developer liquid. A reservoir 1 a for storing developer replenisher is mounted on the tank 1. A heater/cooler unit 8 may also be provided. Referring to Figure 2, the electrical control circuit consists of a constant voltage source 9 in the form of an integrated circuit voltage regulator, a voltage converter 10 incorporating the conductivity cell and a servo-device 11 in the form of a dc thyristor controller, the output of which has a control function. As shown in Figure 3, the regulated voltage from the source 9 is fed through the conductivity cell 12 to a pair of dc amplifiers 13 and 14. Variation in the conductivity of the developer produces a change in the input voltage to the amplifier 13. The output of the amplifier 14 is connected to the servo device 11. A variable resistor 15 is provided to vary the gain of the amplifier 13. In use, image-wise exposed radiation sensitive devices are fed in succession into the input rollers 2 and 2 a which move the same along path 6 through the apparatus and then out of the apparatus via output rollers 3 and 3 a . During passage along the path 6, the devices are submerged in the developer in tank 1 whereby the more soluble areas of the image-wise exposed radiation sensitive coating of the devices are selectively removed. Depending on the conductivity of the developer, the output signal from the servo device can be used to vary the speed of the motor 4, vary the temperature of the developer by actuating the heater/cooler unit, and/or operate a control valve 1 b on the replenisher reservoir. Clearly, the conductivity parameters of one type of developer liquid may not be the same as another type. Suitable alteration of the measurement range may be obtained by adjusting the potentiometer 15. Referring to Figure 4, the apparatus includes a pair of rubber covered input rollers 20 and 20 a , a pair of rubber covered output rollers 30 and 30 a , and a variable speed dc electric motor 40 for driving the rollers 20 and 20 a . The apparatus includes a separate reservoir 21 for developer liquid and a pump 25 for delivering developer liquid to a spray bar 26 located between a pair of plush covered agitator rollers 23 and 23 a driven by a separate variable speed motor 24. A planar member 22 is located under the rollers 23 and 23 a and bar 26 and a catch tray 27 is provided to return processing liquid to the reservoir 21. A storage reservoir 21 a is provided for developer replenisher. A heater/cooler unit 29 is provided in the reservoir 21. The apparatus includes a conductivity cell 28 similar to that of the apparatus of Figure 1 and this is preferably located in the reservoir 21 as shown. The apparatus also includes an electrical control circuit of the type shown in Figures 2 and 3 and the output from the servo device of the circuit is fed to motor 40 and/or motor 24, and/or the heater/cooler unit 29, and/or a control valve 21 b in the reservoir 21 a . In use, an image-wise exposed radiation sensitive device is fed face upwards along a path between the input rollers 20 and 20 a , between the rollers 23 and 23 a and the member 22, and then between the output rollers 30 and 30 a . The exposed radiation sensitive coating of the device is contacted by the rollers 23 and 23 a and development is carried out by a combined scrubbing and solvent action. The degree to which the device is processed (ie. its residence time in the apparatus and/or the extent to which the developer liquid is agitated in contact with the device by the roller 23) and/or the strength of the developer liquid are controlled in dependence on the variation in the conductivity of the developer liquid during processing. The following examples illustrate the invention. Example 1 A processor of the type shown in Figure 1 was filled with developer liquid consisting of an aqueous solution containing sodium metasilicate, disodium phosphate and a surfactant at 22 degC. A number of positive working presensitised plates each consisting of a grained and anodised aluminium substrate coated with a radiation sensitive mixture of a naphthoquinone diazide sulphonic acid ester and a novolak resin, was exposed to UV light beneath a continuous tone step-wedge and a Fogra Precision Measuring strip PMS1. The plates were passed through the processor. The conductivity was monitored using a conductivity meter having a full scale deflection of 60,000 microsiemens/cm. A note was made of the step-wedge and PMS1 readings for each decrease in conductivity of 1,000 microsiemens. The results were as follows: The processor was refilled with fresh developer and further plates processed until the conductivity had dropped to 45,000 microsiemens. The developer was then heated until the conductivity was again 50,000 microsiemens. A further plate was processed and gave step-wedge and PMS1 readings of 3/9 and 8,8 respectively. The developer was cooled to 22 degC (conductivity 45,000 microsiemens). Further plates were processed at successively slower speeds until readings of 3/9 and 8,8 were obtained. It was found that a plate had to be given 50% greater development time. Finally, developer replenisher was added to the processor to bring the conductivity back to 50,000 microsiemens whereupon a plate passed through the processor again gave readings of 3/9 and 8,8. Example 2 Example 1 was repeated except that the developer consisted of an aqueous solution containing sodium hydroxide, and a surfactant. The original conductivity was 60,000 microsiemens/cm and gave step-wedge and PMS1 readings of 3/9 and 8.8 respectively. The conductivity of effectively exhausted developer was 50,000 microsiemens/cm (step-wedge 3.7, PMS1 10.8). The developer was heated until the conductivity was 60,000 microsiemens/cm which restored the step-wedge and PMS1 readings. On cooling the developer until the conductivity was 50,000 microsiemens/cm, a plate had to be given a 45% greater development time to produce the correct readings. A plate developed with developer replenished to 60,000 again gave the same readings of 3/9 and 8.8. Example 3 A number of plates were exposed and developed in accordance with Example 1 of GB Patent No. 1591988 to produce so called screenless or continuous tone plates. The conductivity of the developer initially was 32,000 microsiemens/cm and the initially developed plate had a density range of 1.65. After a number of plates had been processed, the conductivity had dropped to 25,000 microsiemens/cm and the density range produced was only 1.3. Heating the developer until the conductivity was again 32,000 microsiemens/cm restored the density range of 1.65. Cooled developer (conductivity 25,000 microsiemens/cm) required 55% extra development time. The density range of 1.65 could be regained by replenishing the developer back to a conductivity of 32,000 microsiemens/cm. Example 4 The reservoir of a processor of the type shown in Figure 4 was filled with an aqueous developer consisting of an aqueous solution containing a surfactant, sodium benzoate and sodium octanoate. The conductivity of the fresh developer was 24,500 microsiemens/cm. Unexposed plates having a diazo resin coating were passed through the processor and the conductivity was continuously monitored until the plates showed signs of scumming when inked-in. The conductivity of the developer was then 15,200 microsiemens/cm. Further plates were processed at successively slower speeds and it was found that 35% more development time had to be given to produce clean plates. Example 5 This example deals with solvent developers which deteriorate mainly due to contamination with water from the atmosphere. A processor of the same type as used in Example 4, was filled with a developer consisting of 2-methoxy ethyl acetate, a surfactant and phosphoric acid. The conductivity of the developer was 14,100 microsiemens/cm. To simulate contamination, water was added in amounts of 0.5% and the conductivity was monitored whilst unexposed plates having a coating based on poly vinyl cinnamate were passed through the processor. At a water content of 6%, the conductivity was 46,500 microsiemens/cm and the developed plates showed signs of scumming. Further plates were processed at successively slower speeds and it was found that 40% more development time had to be given to produce clean plates.

Claims: A method of processing a plurality of image-wise exposed radiation sensitive devices each comprising a substrate carrying an image-wise exposed radiation sensitive coating which method comprises (i) contacting the devices in turn under given processing conditions with a developer liquid, (ii) monitoring the electrical conductivity of the developer liquid during the processing of the devices, and (iii) varying the processing conditions under which the devices are contacted with the developer liquid in dependence upon the changes in the electrical conductivity so as to maintain the efficacy of the developer liquid characterised in that the developer liquid is one which selectively removes the more soluble areas of the coating from the substrate and whose efficacy for this purpose changes as the number of devices processed increases and in that the processing conditions are varied by varying the time for which the devices are in contact with the developer liquid and/or by adding further developer liquid to the developer liquid and/or by varying the degree to which the developer liquid is agitated in contact with the devices and/or by varying the temperature of the developer liquid. A method according to claim 1 wherein the developer liquid includes a lithographically inert material which ionises in solution to increase the electrical conductivity. An apparatus for developing image-wise exposed radiation sensitive devices, which apparatus comprises:- (i) a container (1) (21) for developer liquid, (ii) a means (2, 2a, 3, 3a) for moving in devices along a path (6) through the apparatus so that they are contacted by the developer liquid under given processing conditions, (iii) a means (5) (28) for measuring the electrical conductivity of the developer liquid and for producing an output signal in dependence upon said conductivity, and (iv) a means of varying the processing conditions which is controlled by said output signal, characterised in that said means of varying the processing conditions comprises a variable speed motor (4) (40) for driving the device moving means so that the period of time for which the devices are in contact with the developer liquid is dependent on the conductivity, a reservoir (1a) (21a) for storing further developer liquid and having a valve (1b) (21b) to enable further developer liquid to be fed to the container from the reservoir in dependence on the conductivity, a variable speed motor (24) for driving an agitating roller (23) (23a) for agitating developer liquid in contact with the devices so that the degree to which the developer liquid is agitated in contact with the devices is dependent on the conductivity, and a unit (8) (29) immersed in the developer liquid to vary the temperature thereof so that the temperature of the developer liquid is dependent on the conductivity.

Inventors: INGHAM, MICHAEL, LAWSON, LESLIE EDWARD
Applicants: VICKERS PLC
Application year: 1983
Main classification: G03F7
IPC codes: G03F7, G03D3