GB1586574A - Recording process using diazonium compounds - Google Patents

Description

(54) RECORDING PROCESS USING DIAZONIUM COMPOUNDS (71) We, AMERICAN HOECHST CORPORATION, a Corporation organized according to the laws of the State of Delaware, having a place of business at Route 202/206 North, Somerville, New Jersey 08876, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a process for recording images by means of laser radiation, in particular for preparing printing plates, and a material for use in the process.

A process for the preparation of printing plates which is able to generate letterpress printing plates directly from computer generated "soft" images is the "Laser-Graph" system of Laser Graphic Systems, Inc. This type of system is described in U.S. Patent No. 3,832,948, and U.S. Patent No. 3,461,229. The process has been used to prepare plates from images either stored on magnetic tape or generated at another location by means of a laser scanning reading unit and transmitted electrically by wire; similar arrangements are described in U.S. Patent No. 3,506,779.

The use of laser light to expose photosensitive coatings also has been reported in several publications and patents. For instance, U.S. Patent 3,664,737 describes the exposure to UV emitting lasers of sensitized aluminum offset plates such as Litho-Chemical and Supply Company's Kem-Lon Pre-cote and Minnesota Mining and Manufacturing's "R" plate. Both of these plates use diazo sensitizers. In this case, the object is the production of high quality printing plates.

German Offenlegungsschrift No. 25 00 906 uses a YAG or Argon laser to remove the carbon particle plus nitrocellulose portion of a coating composition by infrared radiation. The diazo part of the composition is then photohardened in a conventional manner by overall exposure to the ultraviolet radiation of a carbon arc lamp, indicating that the radiation energy of these lasers was insufficient for direct imaging of a diazo coating.

Printing plates presensitized with negative working diazos have attained high acceptance in the lithographic printing industry under standard noncoherent light sources because of their high resolution and excellence of images obtained, easy processing, trouble-free printing, and the great length of press runs which are obtainable.

The above mentioned use of these materials for laser beam recording has up to now not been possible on a commercial scale because of the mismatch of laser light output and diazo coating spectral sensitivity. Even with the most powerful practical laser (argon ion lasers operating at between 1020 watts), the UV output (at 370 nm) is only 1.2% of its total energy when operating in the all-line mode and with optics that do not filter out the UV. The balance of the output is above 450 nm (in the region of 457.9 to 514.5 nm) with the bulk of power in two lines at 488.0 and 514.5 nm. In contrast, diazo coatings are primarily sensitive below 420 nm, with negligible sensitivity above 450 nm, as described in TAGA Proceedings Preprint "Spectral Sensitivity of Offset Printing Plates" by Robert E. Gesullo and Peter G.

Engeldrum. With argon ion laser UV emission so low, diazo coating exposure in a practical time due to available UV radiation was not to be expected.

It has been estimated by printing industry experts that a direct exposure system to be economically justifiable must be at least equivalent in cost to known methods of imaging: i.e. preparing photographic transparencies followed by exposing sensitized plates and accomplished within a reasonable period of time. A practical speed has been considered to be 2-3 minutes to expose a newspaper page or 0.3--0.5 secs/in2 when scanning techniques are to be used.

In copending Application No. 23903/77, Serial No. 1,586,573 it is disclosed that negative working diazonium compounds comprising a coating on a support are sensitive to laser light from which substantially all UV light has been removed.

However, surprising and valuable this finding, greater exposure speed is desirable.

It is well known that dyes or pigments which absorb actinic light generally decrease the sensitivity of diazo coating to actinic radiation. Nonetheless, certain thio- and selenopyronine dyes are claimed to increase the speed of colored, polar diazonium compounds, as described in German Patent No. 745,595 (1944) assigned to I. G. Farbenindustrie AG. Also, some colorless "optical brighteners" (compounds absorbing ultraviolet light which fluoresce in the actinic region of diazonium compounds) are said to attain a similar effect with diazonium compounds, as reported in Belgian Patent No. 661,789 (1965) assigned to Kalle AG.

Finally, a number of colorless diazonium compounds are claimed to have been improved in the solid state by means of ultraviolet light-absorbing compounds, as reported in German Patents Nos. 906,405(1954), 903,061 (1954) and 763,388(1952), all assigned to Kalle AG. and U.S. Patent No. 1,972,323 (1934) to Shiraeff & Jacobs. In no instance, as far as the applicants are aware, has it been possible to sensitize a diazo compound to respond rapidly to actinic light outside the region in which diazonium compounds are known to be sensitive; e.g., to make it responsive to electromagnetic radiation in the range of between 450 to 550 nm. It thus has come as a surprise to find that inexplicably certain coloring matter absorbing light in the region of 450 to 550 nm can be used to render certain diazo based coatings sensitive to exposure to radiation outside the known sensitivity range of these diazos. This unexpected finding makes it practical for the first time to use, e.g., an argon ion laser scanning device to prepare printing members at an adequate speed directly from copy with no intervening photographic steps.

The present invention provides a recording process which comprises imagewise exposing to visible laser radiation and subsequently developing a photosensitive material comprising a negative working diazonium compound and a colourant that absorbs light in the wavelength range of from 450 to 550 nm and which sensitizes the diazonium compound for exposure to the laser light to which the compound is exposed, the said laser light containing at least some radiation in the wavelength range of from 450 to 550 nm.

The photosensitive material is normally on a carrier, e.g., a metal carrier, for example aluminum.

The process of the invention may be carried out using laser radiation without any ultraviolet component.

The diazo coating comprises a negative working photosensitive diazo compound for example, a polymeric condensation product of a substituted benzene diazonium-salt with a dialkoxyaromatic compound. As the sensitizing dye capable of absorbing light in the spectral range of from 450550 nm, dyes from the azo, triarylmethane, xanthene, or methine classes are preferred. In the waiting the diazo compound and the dye are in intimate admixture. Optionally, the material may also contain a resin, e.g., a styrenemaleic anhydride copolymer or a polyvinyl acetal resin, a mineral or strong organic acid and an indicating dye, for example 4 phenylazodiphenylamine.

It is surprising to find that an argon ion laser from which virtually all UV light, to which the diazos are sensitive, has been filtered out, can be used as an exposure source. For example a photosensitive material used in accordance with this invention is exposable by a 15 watt argon ion laser in only 0.16 second per square inch, or one minute for a 16x24 inch plate.

The invention is particularly concerned with the preparation of printing members such as printing plates. Such printing plate materials comprise a carrier and a homogeneous light-sensitive coating containing a negative working diazonium compound and an azo, triarylmethane, xanthene, or methine dye. The coating may also contain resins such as phenolics resins, polyvinylformal and vinyl copolymers containing carboxylic acid groups or other aqueous alcohol- or base soluble resins, a small quantity of mineral acid e.g., sulfuric or phosphoric acid or an organic acid e.g., alkyl or aryl sulfonic, sulfuric, phosphoric, or phosphonic acids. An indicator dye also may be used.

The carrier may be plastic film or of metal such as magnesium or aluminum.

Aluminum, with a well developed art of preparation, is a preferred carrier for lithographic purposes, and magnesium is a preferred carrier for letterpress purposes. The preparation of the carriers includes the steps of cleaning, mechanical graining, if desired, and/or etching, if desired, which may be performed purely chemically as by the use of acids and bases or electrochemically. If aluminum is used, further optional preparation steps include anodizing. The last step in preparation of an aluminum carrier prior to coating with a light-sensitive coating may include treating with alkali metal silicate or with a phosphonic acid as described in U.S. Patent No. 3,220,832. It is evident that dependent upon the graining, etching, anodizing, and like procedures used, the surface area will vary. In the statements made below regarding coating weights, the surface area referred to is merely that of the gross dimensions of the plate.

Negative working benzenediazonium compounds suitable include, for example, those described in U.S. Patents Nos. 3,849,392, 3,867,147, 3,679,419, and 3,255,384.

Examples of such negative working diazos are the condensation products of 3-methoxydiphenylamine-4-diazonium salt with bis-(4 methoxymethylphenyl) ether, of diphenylamine-4-diazonium salt with bis-(4-methoxymethylphenyl) ether, of diphenylamine-4-diazonium salt or 3-methoxydiphenylamine-4-diazonium salt with formaldehyde.

These negative working diazonium compounds may be used in an amount from about 0.01 to about 0.8 gram per square meter and preferably from about 0.1 to about 0.5 gram per square meter. When the amount of the negative working diazonium compounds in the coating is increased, the photosensitivity in conventional exposure decreases, as is well known.

Surprisingly, under laser exposure, the photo-sensitivity increases.

Mineral acids or organic acids may be used in an amount of up to about 0.005 gram per square meter.

A resin, e.g., a styrene maleic acid anhydride copolymer or a polyvinyl acetal, may be added to the coating to improve mechanical strength. However, the addition thereof is not necessary to the practice of the invention, which has as its purpose to greatly increase the speed of negative working diazo-type coatings to laser radiation. When desired, a resin may be used in an amount of up to about 3 grams per square meter.

An indicator dye may be added to the coating to show a color change directly upon exposure. The addition of such a dye is optional and, as its presence will not increase the speed of the plate, it should be selected with care in order not to reduce it unnecessarily. Examples of suitable indicating dyes are paraphenylazodiphenylamine, Metanil Yellow, C.I. #13065, Methyl Orange, C.I. 13025 and 4-(p-anilinophenylazo) benzenesulfonic acid(sodium salt). Such a dye may be used in an amount of up to about 0.1 gram per square meter.

Not all colorants are suited for sensitizers. However, useful colorants include the classes of Azo, Triarylmethane, Xanthene, and Methine, as defined in The Colour Index, Third Edition, Volume 4. Such colorants are generally used in an amount from about 0.01 gram per square meter to about 0.5 gram per square meter and preferably from about 0.05 to 0.1 gram per square meter.

Preferred xanthenes are Acridine Red 3B (Colour Index #45000), Pyronine G(#45005), Rhodamine Scarlet G (#45015), C.I. Basic Red 1 (Rhodamine 6G) (#45160), Rhodine 2G (#45165), Rhodamine 4G (#45166), C.I. Basic Violet 10 (Rhodamine B) (#45170), Rhodamine 12 GF (#5315), and Spirit Soluble Fast Pink B.

Preferred triarylmethanes are C.I. Basic Red 9 (#42500), Tryparosan (#42505), C.I. Basic Violet 14 (#42510), or C.I. Basic Violet 2 (Remacryl Magenta B) (#42520).

Preferred methines such as C.I. Basic Violet 16 (Sandocryl Red B-6B) (#48013), C.I. Basic Violet 7 (#48020) and Astrazone Violet R (#48030).

Preferred azos are Sudan Red BV (#1 i 125), C.I. Solvent Red 3 (#12010), C.I.

Solvent Yellow 14 (Sudan Yellow) (#12055), C.I. Solvent Orange 7 (#12140), C.I.

Solvent Red 8 (#12715), C.I. Solvent Red 100 (Neozapon Red BE) (#12716), and C.I. Acid Red 14 (#14720) may be used.

The laser source used in the Examples is an argon ion laser, linked to a suitable scanning system wherein, for instance, a laser beam scanner and modulator deliver impulses directly to the coating. This equipment is by way of example only as it is evident that the invention resides in the interaction of the laser beam and the photosensitive coating described herein. Laser beams driven and modulated by other mechanisms are equally suitable and are within the purview of this invention.

The following examples are given in order to illustrate the invention in greater detail. Laser exposures were made with the use of a scanner (Scan Scriber) made by Laser Graphic System, Inc.

The sole figure is a diagram of this device, and shows: A laser, in this embodiment an argon ion laser 1, such as the Coherent Radiation model CR 8 laser or the Spectra Physics model 164 laser, which produces a coherent, nearly parallel beam of light 2, which is reflected by the first surface mirrors 3, which, in this embodiment, are strictly selective for reflection between 450 to 530 nm only so that only 5.10-5% of light below 450 nm is allowed to pass.

The intensity of the laser beam 2 is modulated by the modulator 4 which in this embodiment is an acousto/optical modulator such as a Spectra Physics model LGS 100--5B. The amplitude-modulated beam 5 is then focused into a converging beam 12 by a Cassegrainian type reflective optical system 6 such as Spectra Physics model ADS 1006. The beam 12 is scanned across the imageable surface or plate 10 by means of the planar, first-surface mirror 13, the curved, first surface mirror 9 and the rotatable (driven by motor 7) truncated-pyramidal, first-surface mirror 8, to arrive at the plate 10 as indicated by the arrows, while the plate 10 is being transported under the line of scan as indicated by the arrow. The image on the plate results from the intensity of the laser beam being modulated in accordance with an input source of information 11, which may be a computer output, magnetic tape output, modified signal from an image reading device with optical arrangement similar to that of Figure 1 or other suitable means.

Data in all Examples are based upon the use of a 6 watt argon ion laser.

Identical plates were exposed with another scanner differing essentially in using a commercially available laser with an output of 15 watts. From these trials, the exposure rate appeared to be proportional to the laser output wattage. This relationship was subsequently verified for five wattages in the range of 3 watts to 15 watts. Laser recording speed data in the last column of Table 1 has been normalized to the 15 watt laser.

EXAMPLE 1 To a 2000 ml Erlenmeyer flask equipped with a mechanical stirrer containing 990 g (13.2 moles) of 2-methoxyethanol (available from Union Carbide as "methyl Cellosolve": "Cellosolve is a trade mark) were added sequentially over several hours the following components: 4.9 g of a polyvinyl formal resin (commercially available from Monsanto as "Formvar 12/85": "Formvar" is a trade mark), 4.9 g of a polymeric condensation product of 1 mol of 4-(phenylamino)-2-methoxybenzene diazonium salt and 1 mol of bis-(4-methoxymethylphenyl) ether, prepared in 85% phosphoric acid and isolated in the form of a mesitylene-sulfonate, as described in U.S. Patent No. 3,849,393 0.049 g (0.18 mmol) 4-phenylazodiphenylamine, and 0.111 g (0.96 mmole) of 85% aqueous phosphoric acid. After stirring at room temperature for one hour, the solution was filtered through coarse filter paper. This solution will hereafter be ca led Stock Solution A.

To 100 g of Stock Solution A, 136.0/mg Remacryl Magenta B (Colour Index #42520), (available from American Hoescht Corporation) were added. The solution was stirred for 30 minutes and filtered through coarse filter paper.

Fifty ml of the above-dyed solution were whirler-coated at 90 rpm on hydrophilic, anodized grained aluminum treated with an 0.10/, aqueous solution of polyvinyl phosphonic acid in accordance with U.S. Patent No. 3,220,832. In the coating the diazonium compound . and the colorant were in homogeneous admixture. In the same manner, a control plate (containing no dye) was prepared by whirler-coating 50 ml of Stock Solution A on the above-described aluminum. The control plate was used as reference for the exposure speed changes noted in columns 4 and 5 of Table 1. In both cases, the coating weight of the dried plates was 0.25g/m2.

The plates were first exposed conventionally in a Berkey/Ascor 30x40 inch exposure unit Model No. 161840, to 20 units of light (approximately 20 seconds) as measured by means of the attached integrator. The plates were exposed through a standard Stauffer 21 Stepwedge, developed manually for 45 seconds using an aqueous developer containing 20% by weight of n-propanol about 1% of surfactant, rinsed with tap water, squeegeed, and finished with an aqueous solution containing about 10% of hydrolyzed starch and 0.5 /" of phosphoric acid. The plates were then inked in a conventional manner using Imperial Triple Ink available from Lithoplate, Inc. The solidly inked steps on the stepwedge images were then compared, and from this the relative exposure speed was calculated from the property of the Stauffer 21 Stepwedge that each successively denser step on the wedge is 1.41 (the square root of 2) times optically denser than the previous step. In column 4, line 1, of Table 1 the observation is entered that the dyed plate is 75 /n slower than the control.

In an analogous manner, the laser exposure speed of the plates was measured by subjecting sections of each plate to argon laser scanning by means of a Scan Scriber, described above, with dwell times successively decreasing by a factor of 0.71 (the square root of 0.5); i.e. referring to Figure 1, the transport of the plate 10 was varied stepwise so that the first inch to be scanned was scanned in 50 seconds; the second, 36 seconds; the third, 25 seconds. The rotational rate of the mirror 8 was varied so as to be proportional to the transporting speed. The image projected onto the plates was an 85 line per inch screen pattern with densities in fifteen steps from solid (100%) to zero (00/,). After developing, finishing, and inking the plates as above, the images were compared for retention of highlight dots, shadow plugging, and density of solid areas, and the minimum exposure times were compared. The results are entered in Table 1, column 5, which shows that the control plate requires 70% more exposure than the dyed plate under argon laser exposure.

EXAMPLE 2 Example 1 was repeated, except that 64.0 mg of Neozapon Red BE (C.I.

#12716) available from BASF, was used instead of the Remacryl dye of Example 1.

The results are also recorded in Table 1.

EXAMPLE 3 To a 2000 ml Erlenmeyer flask, equipped with a mechanical stirrer, containing 990 g (13.0 moles) of 2-methoxyethanol, were added sequentially the following components: 5.3 g of a styrene-maleic acid copolymer having an acid number of about 180 and an average molecular weight of about 20.000 (available from Monsanto as "Lytron 820" "Lytron" is a trade mark), 0.50 g (2.9 mmole) of ptoluenesulfonic acid, and 4.2 g of the diazonium compound used in Example 1.

After stirring at room temperature for one hour, the solution was filtered through coarse filter paper. This solution will hereafter be called Stock Solution B. Example 1 was then repeated, except that Stock Solution B was used both for the dyed plate and for the control plate, and 136.0 mg of Rhodamine 6GDN Extra (C.I. #45160) available from Du Pont, were used instead of the Remacryl dye of Example 1. The results are recorded in Table 1. In this case, the laser exposure speed for the plate made from the dyed solution was 0.40 sec/in2 or 2.50 min. per 16"x24" newspaper page when exposed with a laser with an output of 6 watts.

EXAMPLE 4 Example 1 was repeated, except that 136.0 mg of Sandocryl Red B-6B (C.I.

#48013), available from Sandoz, were used instead of the Remacryl dye of Example 1. The results are recorded in Table 1.

EXAMPLE 5 Example 1 was repeated except that 136.0 mg of Sudan Yellow (C.I. #12055) available from BASF, were used instead of the Remacryl dye of Example 1. The results are recorded in Table 1.

EXAMPLE 6 Example 1 was repeated except that 136.0 mg of Spirit Soluble Fast Pink B, formerly available from BASF, were used instead of the Remacryl dye of Example 1. The results are recorded in Table 1.

EXAMPLE 7 Example 1 was repeated except that 136.0 mg of Rhodamine FB (C.I. #45170) available from BASF were used instead of the Remacryl dye of Example 1. The results are recorded in Table 1.

EXAMPLE 8 Example 1 was repeated except that 136.0 mg of Oracet Red B (Colur Index Solvent Red 16) available from CIBA-Geigy were used instead of the Remacryl dye of Example 1. The results are recorded in Table 1.

EXAMPLE 9 Example 3 was repeated, except that 136.0 mg of Azo Eosin G (C.I. #14710) available from Du Pont were used instead of the Rhodamine dye of Example 3. The results are recorded in Table 1.

EXAMPLE 10 Example 1 was repeated except that 136.0 mg of Rhodamine 6 GDN (C.I.

#45160) available from Du Pont were used instead of the Remacryl dye of Example 1 and Hostaphan H polyester film, available from American Hoescht Corporation, silicated according to the published German Patent Application DT-AS 1,228,414 for lithographic printing plates was used instead of the aluminum carrier of Example 1. The results are recorded in Table 1.

EXAMPLE Il A coating solution was prepared as in Example 1 from the diazonium compound of Example I, "Formvar 12/85", 1.22 g of Thodamine 6 GDN, 0.045 g of 85% aqueous phosphoric acid and 0.04 g of 4-phenylazodiphenylamine in a mixture of 79.35 g of 2-methoxyethanol and 11.34 g of ethylene glycol monomethyl ether acetate varying the ratio of the first two ingredients as shown below. Four plates were prepared from these solutions using the method of Example 1, and exposed and developed as in Example 1. Plate 1 lea was the control plate.

gof Plate Diazonium Compound g of Resin lla 1.59 6.37 llb 2.63 5.33 llc 3.98 3.98 Ild 5.33 2.63 The results are recorded in Table 1.

EXAMPLE 12 Example 11 was repeated except that instead of the diazonium compound of Example 11, a polymeric condensation product of 4-(phenylamino)-2methoxybenzene diazonium sulfate with bis-(4-methoxymethylphenyl) ether was used, in the same four proportions to give four plates 12a, 12b, 12c, and 12d, of which 12a was the control plate. The exposure results are recorded in Table 1.

EXAMPLE 13 Example 11 was repeated, except that, instead of the diazonium compound of Example 11, a polymeric condensation product of diphenylamine-4-diazonium chloride with formaldehyde was used. The proportions of diazonium compound and resin were as follows: g of Plate Diazonium Compound g of Resin 13a 2.63 5.33 13b 3.98 3.98 13c 5.33 2.63 Plate 13a was the control; the results are recorded in Table 1.

EXAMPLE 14 Example 13 was repeated except that instead of the diazonium compound of Example 12, a polymeric condensation product of 3-methoxydiphenyl-4-diazonium chloride with formaldehyde was used. Plate 14a was the control; the results are recorded in Table 1.

EXAMPLE 15 A coating solution of 0.5 g of "Lytron 820", 0.05 g of p-toluene-sulfonic acid, 0.06 g of Rhodamine 6 GDN extra, 100 g of 2-methoxyethanol and 0.4 g of the diazonium compound of Example 1 was prepared and coated as in Example 1. The plate was exposed as in Example 1, but was developed with a dilute aqueous alkaline developer containing surfactant, rather than alcohol containing developer used before. The exposure was satisfactory. The effective speed was 0.19 secs/in.

(normalized to a 15 watt Argon Ion laser).

EXAMPLE 16 Example 1 was repeated, except that Dowetch Deadline magensium engraving plates suitable for the preparation of shallow relief plates for letterpress printing available from Dow Chemical were used instead of the aluminum printing plates of Example 1. The exposed and developed plates were then etched with Dowetch etchant to give an image in relief where these raised areas corresponded to the laser hardened areas which remained after development.

TABLE 1 Relative Photosensitivity of Experimental Plates Absolute Laser VS. Control Plates Exposure Rate Example Dye colour Diazonium Conventional Laser Normalized to No. Index No. Compound Exposure Exposure 15W (Seconds/In2) 1 42520 1 -75 +70% 0.27 2 12716 1 -30 +100 0.23 3 45160 1 0 +185 0.16 4 48013 1 -30 +40 0.33 5 12055 1 -30 +40 0.33 6 Spirit Soluble Fast Pink B 1 0 +100 0.23 7 45170 1 0 +100 0.23 8 Oracet Red B 1 -30 +40 0.33 9 14710 1 -50 +40 0.33 10 45160 1 -60 +100 0.23 lla 45160 1 - - 0.38 llb 45160 1 -30 +20 0.31 c 45160 1 -65 +100 0.19 d 45160 1 -75 +140 0.16 12a 45160 2 - - 0.38 b 45160 2 -50 +20 0.31 c 45160 2 -75 +100 0.19 d 45160 2 -85 +140 0.16 13a 45160 3 - - 0.88 b 45160 3 0 +180 0.31 c 45160 3 -30 +180 0.31 14a 45160 4 - - 1.25 b 45160 4 -30 +60 0.79 c 45160 4 -30 +80 0.69 WHAT WE CLAIM IS: 1. A recording process which comprises imagewise exposing to visible laser radiation and subsequently developing a photosensitive material comprising a negative working diazonium compound and a colorant that absorbs light in the wavelength range of from 450 to 550 nm and which sensitizes the diazonium compound for exposure to the laser light to which the compound is exposed, the said laser light containing at least some radiation in the wavelength range of from 450 to 550 nm.

2. A process as claimed in claim 1, wherein the diazonium compound is a diazonium salt condensation product.

3. A process as claimed in claim 2, wherein the condensation product is the product of 3-methoxydiphenylamine-4-diazonium salt with bis-(4 methoxymethylphenyl) ether the product of diphenylamine-4-diazonium salt with bis-(4-methoxymethylphenyl) ether, or the product of diphenylamine-4-diazonium salt or 3-methoxydiphenylamine-4-diazonium salt with formaldehyde.

4. A process as claimed in any one of claims 1 to 3, wherein the colorant is an azo, triarylmethane, methine or xanthene dye.

5. A process as claimed in any one of claims I to 4, wherein the photosensitive material is a coating on a support.

6. A process as claimed in claim 5, wherein the support is aluminum or magnesium.

7. A process as claimed in any one of claims 1 to 6, wherein the photosensitive material also comprises a resin, a mineral or strong organic acid or an indicating dye.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (11)

**WARNING** start of CLMS field may overlap end of DESC **. EXAMPLE 16 Example 1 was repeated, except that Dowetch Deadline magensium engraving plates suitable for the preparation of shallow relief plates for letterpress printing available from Dow Chemical were used instead of the aluminum printing plates of Example 1. The exposed and developed plates were then etched with Dowetch etchant to give an image in relief where these raised areas corresponded to the laser hardened areas which remained after development. TABLE 1 Relative Photosensitivity of Experimental Plates Absolute Laser VS. Control Plates Exposure Rate Example Dye colour Diazonium Conventional Laser Normalized to No. Index No. Compound Exposure Exposure 15W (Seconds/In2)

1 42520 1 -75 +70% 0.27

2 12716 1 -30 +100 0.23

3 45160 1 0 +185 0.16

4 48013 1 -30 +40 0.33

5 12055 1 -30 +40 0.33

6 Spirit Soluble Fast Pink B 1 0 +100 0.23

7 45170 1 0 +100 0.23

8 Oracet Red B 1 -30 +40 0.33

9 14710 1 -50 +40 0.33

10 45160 1 -60 +100 0.23 lla 45160 1 - - 0.38 llb 45160 1 -30 +20 0.31 c 45160 1 -65 +100 0.19 d 45160 1 -75 +140 0.16 12a 45160 2 - - 0.38 b 45160 2 -50 +20 0.31 c 45160 2 -75 +100 0.19 d 45160 2 -85 +140 0.16 13a 45160 3 - - 0.88 b 45160 3 0 +180 0.31 c 45160 3 -30 +180 0.31 14a 45160 4 - - 1.25 b

45160 4 -30 +60 0.79 c 45160 4 -30 +80 0.69 WHAT WE CLAIM IS: 1. A recording process which comprises imagewise exposing to visible laser radiation and subsequently developing a photosensitive material comprising a negative working diazonium compound and a colorant that absorbs light in the wavelength range of from 450 to 550 nm and which sensitizes the diazonium compound for exposure to the laser light to which the compound is exposed, the said laser light containing at least some radiation in the wavelength range of from

450 to 550 nm.

2. A process as claimed in claim 1, wherein the diazonium compound is a diazonium salt condensation product.

3. A process as claimed in claim 2, wherein the condensation product is the product of 3-methoxydiphenylamine-4-diazonium salt with bis-(4 methoxymethylphenyl) ether the product of diphenylamine-4-diazonium salt with bis-(4-methoxymethylphenyl) ether, or the product of diphenylamine-4-diazonium salt or 3-methoxydiphenylamine-4-diazonium salt with formaldehyde.

4. A process as claimed in any one of claims 1 to 3, wherein the colorant is an azo, triarylmethane, methine or xanthene dye.

5. A process as claimed in any one of claims I to 4, wherein the photosensitive material is a coating on a support.

6. A process as claimed in claim 5, wherein the support is aluminum or magnesium.

7. A process as claimed in any one of claims 1 to 6, wherein the photosensitive material also comprises a resin, a mineral or strong organic acid or an indicating dye.

8. A process as claimed in any one of claims I to 7, wherein the laser is an

argon ion laser.

9. A process as claimed in claim 1, carried out substantially as described with reference to and as illustrated by the accompanying drawing.

10. A process as claimed in claim 1, carried out substantially as described in any one of the examples herein.

11. An image-bearing recording material whenever made by a process as claimed in any one of claims 1 to 10.