GB2081916A - Method of producing a positive acting photographic element - Google Patents

Abstract

The invention relates to a method of producing a positive acting lithographic element by means of changes in the exposure and development parameters applied to a normally negative acting coated substrate. The substrate has a first coating comprising a water soluble lithographically suitable, light sensitive negative acting aromatic diazo composition and a second layer applied over said first layer of a water insoluble, ink receptive, water permeable negative working lithographic photo-sensitive composition which comprises a water insoluble, water permeable oleophilic resin and the reaction product of a diazo diphenylamine-formaldehyde condensate with a benzophenone sulphonic acid. By subjecting the coated substrate to only 3% to 15% of the light exposure normally necessary to produce a negative acting lithographic element, and subsequently developing the exposed coating with water a positive acting element is produced. The developed element is then further exposed to light to render the remaining coatings insoluble in water.

Description

SPECIFICATION Method of producing a positive acting photographic element This invention relates to a novel method of producing a photographic element, such as a lithographic printing plate, which, may optionally be employed as either a positive working or a negative working element merely by altering the relevant exposure and developing parameters.

Heretofore lithographic printing plates were capable of use solely as a positive working or a negative working plate. In a negative plate system, a substrate was coating with a diazonium salt admixed in an oleophilic resin. Such a plate was then exposed through a photographic transparency and the light struck areas became highly insoluble. As a result of this solubility differential, the unexposed areas, which remain soluble in a developer are able to be washed away and the exposed areas on the plate remain adhered to the substrate. In a positive working system, an aromatic diazo oxide composition is applied to a substrate and exposed. In this type of system the exposed portion of the element is removed by a developer while the unexposed portion remains adhered to the substrate.It has not been previously possible to convert a negative plate type to operate positively or vice versa at least without elaborate treatment techniques which are not economically feasable. By the present invention a negative plate may be made to operate positively with development by tap water solely by altering the exposure and developer parameters.

U.S. Patent 4,104,072 issued to Golda et al, provides lithographic printing plates which are developable solely by the use of water. However, whilst this patent describes the manufacture of both positive and negative working plates the photosensitive compositions employed are positive working for a positive plate and negative working for a negative plate. Thus U.S. Patent 4,104,072 does not teach a method of making a positive printing plate using a negative working photosensitizer.

The present invention provides a method of producing a positive working photographic element using a normally negative working light sensitive material.

It has been found that a satisfactory plate which can be used in this method of the invention may be produced by first applying a layer of a lithographically suitable, light sensitive, water soluble negative acting diazonium salt to the surface of a substrate and subsequently coating the diazonium salt layer with a layer comprising a water insoluble, ink receptive, water permeable normally negative working lithographic photosensitive composition which comprises a water insoluble, water permeable oleophilic resin and the reaction product of a diazo diphenylamine4ormaldehyde condensate with a benzophenone sulphonic acid. This upper level coating composition is characterised as water insoluble, ink receptive, and water permeable. Water permeability means the composition is insoluble in water, yet water may filter through the composition.

When such a plate is exposed in the normal manner to a substantial quantity of ultraviolet radiation through a photographic transparency and washed with water a negative reproduction of the transparency image is obtained. It has now been unexpectedly found that when such a plate is given an exposure equivalent to from 3% to 15% of a normal negative producing exposure and is washed with water, a positive reproduction of the transparency image is obtained.

In a negative working format the upper coating is initially water permeable and upon imagewise exposure through a suitable mask by methods known to the skilled worker the exposed areas are rendered water impermeable and remain ink receptive. In such a negative working system the lower layer is initially water soluble but is rendered water insoluble at the exposed areas while remaining water soluble at the unexposed areas. Therefore, upon washing an imagewise exposed plate with water, the water permeates the negative working photosensitive composition at the unexposed non image area, and reaches and dissolves the water soluble unexposed photosensitive substance under the non image area. These areas which lack adequate foundation may be quickly and easily swept away by a combination of water and a slight rubbing action.

Since the imaged areas were rendered non-water permeable upon exposure, water could not reach the firm foundation of the now water insoluble substance beneath the image areas. These areas are therefore not undermined nor lifted away. The water insolubility of the lower image areas adds resistance to the undesirable loosening of the image areas. The imaged areas are thus ink receptive and the bare metal substrate which coincided with the removed non-image areas is by its nature ink repelling. The result is a commercially satisfactory negative working lithographic printing plate.

After exposure in the manner of the present invention the upper coating is of course still initially water permeable but the very short exposure duration is not sufficient to render the photosensitive layers water insoluble nor water impermeable.

Rather, although the mechanism is not completely understood, the exposure is believed to be only sufficient to release nitrogen gas from the diazonium compounds. This nitrogen, it is theorized, attempts to free itself from the coatings and forms channels in the layers. This in effect renders the upper level coating more water permeable due to such channels at the exposed areas. This permeability differential is sufficient to allow the quick undermining and removal of the exposed areas. As a result, the exposed areas are effectively removed and the unexposed areas remain adhered to the substrate thus forming a developed, commercially acceptable photographic element.

It must be remembered that the unexposed areas of the positive plate are still light sensitive and water permeable although not as permeable as the exposed areas which were removed. As this plate now exists it would quickly deteriorate on a printing press. In order to prevent this, in the method of the present invention the positive plate is given a substantial exposure to ultraviolet light so as to render it insoluble. Since this plate has already been developed, no differential is needed at this point between the exposed and unexposed areas.

As the first step in the production of a photographic element according to the present invention, a sheet substrate, preferably aluminum and the alloys thereof, especially those aluminum compositions suitable for the manufacture of lithographic printing plates such as Alcoa 3003 and Alcoa 1100, which may or may not have been pre-treated by standard graining and/or etching and/or anodizing techniques as are well known in the art and also may or may not have been treated with a composition suitable for use as an interlayer for lithographic plates is coated with a layer of a lithographically suitable, light sensitive, water soluble diazonium salt. Said coated substrate is then further coated with a stratum of the photosensitive composition referred to above.Said upper layer composition is characterized as water insoluble, ink receptive and water permeable prior to exposure to ultraviolet or actinic radiation. The upper layer composition is preferably not a film former which would impede water permeability. In the known negative working system upon imagewise exposure through a mask by methods known to the skilled worker, the aforementioned top layer substance which was initially water permeable is rendered water impermeable at the exposed areas while remaining water permeable at the unexposed areas. This is true only for negative working systems. In the method of the invention the upper coating is also initially water permeable and upon exposure the unexposed areas will remain water permeable and the exposed areas are rendered more water permeable than the unexposed portions.

Standard metal substrate pretreatments include electrolytically anodizing in sulfuric, chromic, hydrochloric and/or phosphoric acids or electrolytically etching in hydrochloric or phosphoric acid, and chemical or mechanical graining by methods which are all known to the skilled worker.

Interlayer compositions employable in the practice of this invention include those which may be applied as aqueous solutions, such as aqueous solutions of alkali metal silicate, such as sodium silicate, silicic acid, the Group IV-B metal fluorides, the alkali metal salts or the acids thereof, polyacrylic acid, the alkali metal salts or the acids thereof, the alkali zirconium fluorides, such as potassium zirconium hexafluoride, or hydrofluozirconic acid which are applied in concentrations of 0.5% to 20% by volume.

It has been found in the practice of the instant invention that satisfactory results are obtained when the lower level photosensitive substance comprises a water soluble, minimally or non-organic solvent soluble, lithographically suitable photosensitizer which may or may not be mixed with a water soluble resin, dissolved in water or a combination of water and a water miscible organic solvent.

Water miscible organic solvents usable in the context of the present invention include those having a boiling point of up to 250 degrees F, preferably up to 212 degrees F and most preferable between 50 degrees F and 150 degrees F. These include: methanol, ethanol, isopropyl alcohol, acetone, methyl ethyl ketone and methyl cellosolve Registered Trade Mark.

Water soluble photosensitizers employable as part of the invention include: 4-diazo-diphenylamine sulfate 1-diazo-4-N,N-dimethylamino-benzene zinc chloride 1-diazo-4-N,N-diethylamino-benzene zinc chloride 1 -diazo-4-N-ethyl-N-hydroxyethylamino-benzene 1/2 zinc chloride 1 -diazo-4-N-Methyl-N-hydroxyethylaminobenzene 1/2 zinc chloride 1 -diazo-2,5-diethoxy-4-benzoylamino-benzene, 1/2 A zinc chloride 1 -diazo-4-N-benzylamino-benzene, 1/2 zinc chloride 1-diazo-4-N,N-dimethylamino-benzene borofluoride 1-diazo-4-morpholino-benzene, 1/2 zinc chloride 1 -diazo-4-morpholino-benzene-borofluoride 1 -diazo-2,5-dimethoxy-4-p-tolylmercaptobenzene, 1/2 zinc chloride 1 -diazo-2-ethoxy-4-N,N-dimethylaminobenzene 1/2 zinc chloride p-diazo-dimethyl aniline, 1/2 zinc chloride 1-diazo-4-N,N-diethylamino-benzene, 1/2 zinc chloride 1-diazo-2,5-dibutoxy-4-morpholino-benzene sulfate 1 -diazo-2,5-diethoxy-4-morpholino-benzene, 1/2 zinc chloride 1 -diazo-2,5-dimethoxy-4-morpholino-benzene, zinc chloride 1 -diazo-2,5-diethoxy-4-morpholino-benzene, 1/2 zinc chloride 1 -diazo-2,5-diethoxy-4-morpholino-benzeneborofluoride 2-diazo-1-naphthol-5-sulfonic acid, sodium salt 1-diazo-4-N,N-diethylamino-benzene, borofluoride 1 -diazo-2,5-diethoxy-4-p-tolylmercapto-benzene, 1/2 zinc chloride 1 -diazo-3-ethoxy-4-N-methyl-N-benzylaminobenzene, 1/2 zinc chloride 1-diazo-3-chloro-4-N,N-diethylamino-benzene, 1/2 zinc chloride 1-diazo-3-methyl-4-pyrrolidino-benzene chloride, zinc chloride 1-diazo-3-methyl-4-pyrrolidino-benzene- borofluoride 1 -diazo-2-chloro-4-N,N-dimethylamino-5methoxy-benzene, borofluoride 1-diazo-3-methoxy-4-pyrrolidino benzene, zinc chloride condensation product of 4-diazodiphenylamine sulfate and formaldehyde zinc chloride.

Water soluble resins which may comprise the bottom layer as previously described may include, polyvinyl pyrollidone, polyvinyl alcohols, polyacrylamides, and copolymers, synthetic gums such as, starabic, dextrines, natural gums such as, gum arabic and hydroxyethyl cellulose gum.

When a water soluble resin is included as part of the lower level composition the ratio of photosensi timer to resin may broadly range from 100:1 to 1 :100 parts by weight.

When a water miscible organic solvent is employed as part of the solution which comprises the lower coating layer, the amount of organic solvent included may be up to that amount which causes a precipitate to form when added to an aqueous solution of the photosensitive substance employed in the lower level.

In a preferred embodiment, the ratio of water to organic solvent may be from about 1:9 to about 9:1 parts by weight. A most preferred ratio is 1:1 parts by weight.

Development speed may be controlled by the relative amounts of water and organic solvent employed. When a greater proportion of water is employed a more continuous film of the lower level substance is formed upon drying which provides stronger bonding to the substrate but a thusly produced plate requires a longer development time.

If a greater proportion of organic solvent is used a more discontinuous film of lower level substance is formed upon drying which does not bond as strongly to the substrate but allows quicker development.

A A balance of these factors must be selected according to the specific characteristics desired.

The lower level substance is applied to the substrate at a coating weight of from about 5 to about 200 mg/sq.ft., preferably from about 10 to about 100 mg/sq.ft., and most preferably from about 25 to about 60 mg/sq.ft.

Water permeable oleophilic resins usable in the upper coating substance include, epoxy resins (such as, Epon 1031, 1001-1009-Shell Oil Company); polyurethane (such as Goodrich's Estane Series 5714,5715); polyester (for example, DV 521 available from Polychrome Corp.); formvar (such as 12/85 available from Monsanto); urethanes of low molecularweight (such as DV 53, DV 532 available from Polychrome Corp.); butryl resins (commercially available product of Monsanto); polyox (polyethylene oxide series, available from Union Carbide); and polyvinyl hydrogen phthalate.

The water insoluble photosensitive compositions which may be satisfactorily employed in forming the upper layer of this invention are those which are lithographically suitable and are actinic and/for ultraviolet light reactive. The photosensitive compositions which may be used as part of this invention are those which are negative acting and include such negative acting photosensitive agents such as, the reaction product of paradiazo diphenyl aminepara-formaldehyde condensate and 2-hydroxy-4methoxy benzophenone sulfonic acid.

The optimum proportion of each ingredient and selection of particular compositions naturally depends on the specific properties desired in the final lithographic plate. Although there is virtually an infinite range to the ratio of the amount of photosensitizer to resin in the upper layer, a practical, operable ratio of sensitizer to resin in the top coating is from about 1 to 10 parts by weight sensitizer to from about 50 to 1 part by weight resin. A preferred ratio is from about 1 to 5 parts by weight sensitizer to from about 10 to 1 part by weight resin. A most preferred ratio is from about 1 to 3 parts by weight sensitizer to from about 4 to 1 part by weight resin.

The coating weight of the photosensitizer-resin upper level composition on the plate is from about 5 to about 300 mg/sq.ft., with a preferred range of from about 35 to about 200 mg/sq.ft., and a most preferred range of from about 50 to about 100 mg/sq.ft.

For the optimum practice of the present invention, the upper coating composition should possess a certain degree of water permeability. The following test has been devised to determine if its water permeability is preferred for the present application.

Water Permeability Test Procedure 1. Dip a sample of .008" thick fluted filter paper in a sample of the solution to be tested. A preferred filter is S & S fluted filter #588 available commercially from the Schleicher & Schuell company.

2. Dry the coated filter paper in a 600F oven for one hour without allowing exposure to light.

3. Line a funnel with the paper and place the funnel over a flask. Pour 25 mis. of water into the filter paper and record the time required for the water to pass through the filter.

Coatings which require 200 seconds or more to pass through the filter will perform most satisfactorily as positive/negative printing plates.

Obviously the permeability characteristics can be varied with the amount and type of ingredients in the coating.

The photographic element so produced may now be used in the method of the invention to produce a positive working image.

Producing a negative working printing plate using a normally negative working photosensitizer is well known to the skilled worker. A given plate is covered with a photographic transparency while mounted on a vacuum frame and exposed to a certain quantity of ultraviolet radiation with a subsequent exposure. In order to test the properties of such a plate, the transparency employed is a 21 Step Sensitivity Guide. This is a clear plastic strip with an optically varying degree of opacity ranging in numbered steps from clearthrough gray to black. Such a 21 Step Sensitivity Guide is made in accordance with specifications published by the Graphic Arts Technical Foundation in GATF Research Bulletin No. 215.

Such a Guide is available commercially by the Stouffer Graphic Arts Equipment Company of South Bend, Indiana.

Naturally, plate ingredients and exposures are subject to a myriad of variations. It is therefore necessary to first determine the optimum quantity of light exposure for a given lithographic plate. This is done by exposing a test plate through a 21 Step Sensitivity Guide until a reading in the range of a step 7 through step 9 reading is obtained. For example, a step 9 means that after exposure and development, the highest number which has a visible quantity of coating still remaining on the plate is a step 9 on the Sensitivity Guide reproduction. Those skilled in the art have determined that for a negative working plate, optimum reproduction quality and press life is obtained when a reading of step 7 through step 9 is obtained. If the number is higher, the plate is overexposed with resulting halation, bridging between halftone dots and dot enlargement.If the reading is lower, there is a reduced length of press run and dot sharpening.

Once the optimum number of lumens of light exposure for a negative working step 7-9 has been determined, we have found that an exposure to 20% or more, and preferably 100% of the determined optimum lumen value produces a negative working plate and an exposure in the range of from 3% to 15% of the optimum lumen value will produce a positive working plate. It has furthermore been determined that the developer may be tap water in both cases. Whilst a negative plate should preferably be developed in water at a temperature preferably in the range of 700F to 1 000F a positive plate should preferably be developed in water at a temperature of 500 to 80 F.

When a positive plate is produced by the method of the invention it is given a subsequent overall post exposure to harden the previously unexposed areas preferably at the determined optimum lumen value of exposure.

The method of the invention is further illustrated by the following Example.

Example A sheet of grade 3003 aluminum was chemically etched in 3% sodium hydroxide at 800C to obtain a 300 mg/dm2 aluminum etch loss. This was then desmutted in an 8% nitric acid solution and anodized with sulfuric acid in D.C. current at 16 amp. min./ft.2.

The sheet was then rinsed in water. The sample was submersed in 5% sodium silicate for 30 seconds at 800C to provide an interlayer and rinsed in deionized water. The following solutions were then prepared.

All ingredients are in parts by weight.

Formula (A) 2% condensation product of paraformaldehyde and paradiazo diphenylamine 1% phosphoric acid 97% deionized water Formula (Bj 2% condensation product of paraformaldehyde and paradiazo diphenylamine reacted with 2 hydroxy, 4-methoxy-5-sulfonic acid.

2.5% bisphenol A epoxy resin (available as Epon 1007 from Shell Chemical Co.) 0.5% polyvinyl acetal resin (available as Formvar 7/95 from Monsanto Chemical Co.) 0.2% Orasol blue G.N. dye 90% ethylene glycol monomethyl ether 4.8% N,N-dimethylformamide The aluminum sheet prepared as above was then first coated with Formula (A) via dip coating to attain a coating weight of 3.4 mg/dm2 and dried for 3 minutes at 55 C. This was subsequently coated with Formula (B) via dip coating to attain a coating weight of 4.0 mg/dm2 and dried for 15 minutes at 65 C. The sample so prepared was cut into 2 sections.

A photographic transparency was placed over one of the sections and was exposed for 30 seconds in a 5 K.W. mercury vapor/metal halide exposure unit.

The unexposed area was then removed with a tap water developer at 38 C.

A negative reproduction of the transparency image was produced.

The same photographic transparency was placed over the other plate section and it was exposed for 3 seconds in a 5 KW mercury vapor/metal halide exposure unit. The exposed area was then removed with a tap water developer maintained at a temperature of 18 C. After development the sample was post exposed for 30 seconds with the exposure unit described above. A positive reproduction of the transparency image was produced.

The two samples were then mounted on a Chief 22 Press and produced 25,000 acceptable impressions.

Claims (14)

1. A method for producing a positive acting photographic element which comprises: a) coating at least one surface of a substrate with a first layer comprising a water soluble lithographically suitable, light sensitive, negative acting aromatic diazo composition; and b) applying to said first layer, a second layer comprising a water insoluble, ink receptive, water permeable negative working lithographic photosensitive composition which comprises: i) a water insoluble, water permeable, oleophilic resin; and ii) the reaction product of a diazo diphenylamine- formaldehyde condensate with a benzophenone sulfonic acid; and c) exposing the element to a light source at an intensity and for a duration to provide from 3% to 15% of the optimum exposure required to produce a negative working element; d) developing the element by washing with a developer comprising water; and e) post exposing the entire surface of the elementto render the remaining coatings insoluble in water.

2. A method as claimed in claim 1 wherein the reaction product (ii) of step (b) is the reaction product of the condensation of paradiazo diphenyl amine with formaldehyde or paraformaldehyde and 2-hydroxy 4-methoxy benzophenone sulfonic acid.

3. A method as claimed in claim 1 or 2 wherein said substrate is comprised of aluminum.

4. A method as claimed in claim 3 wherein said substrate has been grained.

5. A method as claimed in claim 4 wherein said grained substrate has been anodized.

6. A method as claimed in claim 5 wherein said grained and anodized substrate has applied thereto between the upper surface of said substrate and the first layer, an interlayer composition which compris es a substance selected from the group consisting of alkali metal silicate, silicic acid, the Group IV B metal fluorides, polyacrylic acid, the alkali zirconium fluorides and hydrofluozirconic acid.

7. A method as claimed in any of claims 1 to 6 wherein the oleophilic resin comprises one or more compositions selected from the group consisting of epoxy resins; polyurethane; polyester; urethane; polyethylene oxides, polyvinyl hydrogen phthalate, polyvinyl acetals, and polyvinyl butyral resins.

8. A method as claimed in any of claims 1 to 7 wherein the coating weight of the first layer is from about 5 to about 200 mg/sq.ft. and the coating weight of said second layer is from 5 to about 300 mg/sq.ft.

9. A method as claimed in any of the preceding claims, wherein the ratio of lithographically suitable photosensitizer to oleophilic resin in said second layer is from about 1 to about 10 parts photosensitizer to from about 50 to about 1 part by weight oleophilic resin.

10. A method as claimed in any of the preceding claims wherein said first layer further comprises a composition selected from the group consisting of polyvinyl pyrollidone; polyvinyl alcohols; polyacrylamides and copolymers; synthetic gums and natural gums.

11. A method as claimed in any of the preceding claims wherein the first layer comprises a compound selected from 4-diazo-diphenylamine sulfate 1-diazo-4-N,N-dimethylamino-benzene zinc chloride 1-diazo-4-N,N-diethyiamino-benzene zinc chloride 1 -diazo-4-N-ethyl-N-hydroxyethylamino-benzene 1/2 zinc chloride 1 -diazo-4-N-Methyl-N-hydroxyethylamino- benzene 1/2 zinc chloride 1 -diazo-2,5-diethoxy-4-benzoylami no-benzene, 1/2 zinc chloride 1-diazo-4-N-benylamino-benzene, 1/2 zinc chloride 1-diazo-4-N,N-dimethylamino-benzene borofluoride 1-diazo-4-morpholino-benzene, 1/2 zinc chloride 1-diazo-4-morpholino-benzene borofluoride 1 -diazo-2,5-dimethoxy-4-p-tolylmercaptobenzene, 1/2 zinc chloride 1 -diazo-2-ethoxy-4-N,N-dimethylaminobenzene, 1/2 zinc chloride p-diazo-dimethyl aniline, 1/2 zinc chloride 1-diazo-4-N,N-diethylamino-benzene, 1/2 zinc chloride l-diazo-2,5-dibutoxy-4-morpholino-benzen su 1- fate 1 -diazo-2,5-diethoxy-4-morpholino-benzene, 1/2 zinc chloride 1-diazo-2,5-dimethoxy-4-morpholino-benzene, zinc chloride 1 -diazo-2,5-diethoxy-4-morpholino-benzene, 1/2 zinc chloride 1-diazo-2,5-diethoxy-4-morpholino-benzene- borofluoride 2-diazo-1 -naphthol-5-sulfonic acid, sodium salt 1-diazo-4-N,N-diethylamino-benzene, borofluoride 1 -diazo-2,5-diethoxy-4-p-tolyl mercapto-benzene, 1/2 zinc chloride 1 -diazo-3-ethoxy-4-N-methyl-N-benzylaminobenzene, 1/2 zinc chloride 1 -diazo-3-chloro-4-N,N-diethylamino-benzene, 1/2 zinc chloride 1-diazo-3-methyl-4-pyrrolidino-benzene chloride, zinc chloride 1 -diazo-3-methyl-4-pyrrolidino-benzene- borofluoride 1-diazo-2-chloro-4-N,N-dimethylamino-5- methoxy-benzene, borofluoride 1-diazo-3-methoxy-4-pyrrolidino benzene, zinc chloride condensation product of 4-diazodiphenylamine sulfate and formaldehyde zinc chloride.

12. A method as claimed in any of the preceding claims wherein said first layer has been coated from a solvent selected from the group consisting of water, and water mixed with a water miscible organic solvent having a boiling point of up to 250 F.

13. A method as claimed in claim 12 wherein said water miscible organic solvent is comprised of a composition selected from the group consisting of methanol, ethanol, isopropyl alcohol, acetone, methyl ethyl ketone and methyl cellosolve.

14. A method of producing a positive acting photographic element as claimed in claim 1, substantially as hereinbefore described in the Example.