GB1586573A - Recording process using a diazonium compound - Google Patents

Description

(54) RECORDING PROCESS USING A DIAZONIUM COMPOUND (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, especially for preparing printing plates.

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 Graphics Systems, Inc. This type of process is described in U.S. Patent No. 3,832,948, and in 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, e.g., by wire; similar arrangements are described in U.S.

Patent No. 3,506,779.

It has been proposed to use laser light to expose photosensitive coatings. For instance, U.S. Patent No. 3,664,737 describes the use of UV emitting lasers to expose sensitized aluminum offset plates, e.g., Litho-Chemical and Supply Company's Kem-Lon Pre-cote and Minnesota Mixing and Manufacturing's plate. Both of these plates use diazo sensitizers, and the patent is concerned with the production of higher quality printing plates.

German Offenlengungsschrift 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 photohardened in a conventional manner by overall exposure to the ultraviolet radiation of a carbon arc lamp, indicating that the radiation energy of such lasers to be insufficient for direct imaging of a diazo coating.

Printing plates presensitized with negative working diazonium compounds have attained high acceptance in the lithographic printing industry under standard noncoherent light sources because of their high resolution and the 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 commerical 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 120 watts), the UV output (at 370 nm) is only 1.20/, 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/in.2 when scanning techniques are to be used.

Dyes or pigments which absorb actinic light generally decrease the sensitivity of so dyed, diazo coatings to actinic radiation. Nonetheless, certain thio and selenopyronine dyes are stated to increase the photosensitivity of colored, polar diazonium compounds, as described in German Patent No. 745,595. 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. A number of colorless diazonium compounds are also stated to have been improved in the solid state by means of ultraviolet light-absorbing compounds. However, in no instance, as far as is known, has it been possible to sensitize a diazo compound to respond rapidly to actinic light outside of the region in which diazonium compounds are known to be sensitive, for instance to electromagnetic radiation in the range of between 450 to 550 nm. It is, therefore, surprising that certain colorants absorbing light in the region of 450 to 550 nm render certain diazo based coatings sensitive to exposure to radiation outside the known sensitivity of these diazo compounds.

The subject matter of the invention is a process for recording laser radiation, in which a photosensitive material comprising a copying coating and a carrier is exposed image-wise by means of visible laser light which needs no UV component and is then developed into an image, in which the coating comprises a negative working diazonium compound and a binder, with the diazonium compound comprising more than 25 /" of the total coating weight.

The term "imagewise" includes, without limitation, processes in which the radiation itself carries a signal, or is passed through an original to be copied.

The diazo coating advantageously comprises a diazo type negative working photosensitive compound such as a polymeric condensation product of a benzene diazonium salt and, optionally, a sensitizing dye capable of absorbing light in the spectral range between 450-550 nm. Azo, triarylmethane, xanthene, or methine dyes are preferred. Optionally, the composition may also comprise mineral or strong acids or indicating dyes such as 4-phenylazodiphenylamine.

It has been found that contrary to all previous experience, coatings containing higher percentages of diazonium compounds have an increased photosensitivity when exposed to laser light from which virtually all UV in the range of sensitivity of these diazos has been excluded, as compared to identical coatings containing lower percentages of diazonium compounds. This finding affords a practical means of increasing exposure speeds to laser radiation or of shortening exposure times.

Using the process of the invention, photosensitive diazo coatings may be satisfactorily exposed to laser light to which the diazonium compounds themselves are substantially insensitive. The invention is particularly concerned with the preparation of printing members such as printing plates. Such printing plate materials comprise a carrier and a light-sensitive coating containing a negative working diazonium compound and optionally, an azo, triarylmethane, xanthene, or methine dye. The coating also contains a binder, and may also contain one or more other components, for example a small quantity of mineral acid, for example sulfuric or phosphoric acids or an organic acid, for example alkyl or aryl sulfonic, sulfuric, phosphoric, or phosphonic acids. An indicator dye also may be used.

The carrier may be plastic film or metal, e.g., magnesium or aluminum.

Aluminum, with a well developed art of preparation, is a preferred carrier for lithography, and magnesium is a preferred carrier for letterpress. 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 the 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 according to 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 diazonium compounds are used, e.g., those described in U.S. Patents Nos. 3,849,392, 3,867,147, 3,679,419, and 3,235,384. 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 photosensitivity increases.

The photosensitive composition used according to the invention comprises a binder, which may improve the mechanical strength. Examples of suitable binders are resins such as phenolic resins, polyvinylformal and vinyl copolymers containing carboxylic acid groups or other aqueous alcohol- or base-soluble resins. One particularly suitable polyvinylformal is Formvar 12/85 (Monsanto Corporation), and further examples of resins which may be used are a styrene maleic acid anhydride copolymer and a polyvinyl acetal. The resin is preferably used in an amount of up to about 3 grams per square meter.

The amount of diazonium salt should of course be at least 25% of the total coating weight. No practical increase of sensitivity is achieved above a diazonium compound content of 85% of the total coating weight; ranges between about 35 and 55 / > have proved suitable.

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

As indicated above, the composition may also contain a colorant that sensitizes the diazo compound to light in the visible region; processes employing such combinations are described and claimed in our copending Application No.

23904/77 (Serial No. 1,586,574).

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, 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, sensitize selectively to laser light having no UV component.

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 12GF (#45315), and Spirit Soluble Fast Pink B.

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

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

Preferred azos are Sudan Red BV (#11125), 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).

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 Graphics 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 CR8 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 of radiation between 450 to 530 nm only so that only 5.10-50/, 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 100-6. 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 commerically 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 79.35 g of 2-methoxyethanol and 11.34 g of 2-methoxy ethyl acetate were added sequentially over several hours, the following components: a polyvinyl-formal resin, Formvar 12/85 (Monsanto Corp.), a polymeric condensation product of 1 mol of 4-(phenylamino)-2-methoxybenzene diazonium salt and 1 mol of bis-(4methoxymethylphenyl) ether, produced in 85 per cent. Phosphoric acid and isolated in the form of a mesitylene sulfonate, 1.22 g of Rhodamine 6 GDN, 0.045 g of 85 /,, aqueous phosphoric acid, and 0.04 g of 4-phenyl azodiphenylamine, varying the ratio of the first two ingredients as shown below. After stirring at room temperature for one hour, the solutions were filtered through coarse filter paper.

Four plates were prepared from these solutions by whirler-coating fifty ml of each solution at 90 rpm onto anodized, grained aluminum treated with an 0.1 per cent aqueous solution of polyvinyl phosphoric acid. The coating weight in each case was approximately 0.5 g/m2. Plate la was the control plate.

g of Diazonium Plate Compound g of Resin la 1.59 6.37 lob 2.63 5.33 lc 3.98 3.98 Id 5.33 2.63 The plates were first exposed conventionally for 8 seconds in a commerically available exposure unit, through a standard Stauffer 21 Stepwedge. They were 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 photosensitivity was calculated from the property of the stepwedge that each successively denser step on the wedge is 1.4 times optically denser than the previous step.

In analogous manner, the sensitivity to laser light of the plates was measured by subjecting sections of each plate to argon laser scanning by means of a recorder described above, with dwell times successively decreasing by a factor of 0.7 (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 (0%). 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. The results are recorded in Table 1.

EXAMPLE 2 Example 1 was repeated except that instead of the diazonium compound of Example 1 the corresponding diazonium sulfate was used, in the same amounts to give four plates, 2a, 2b, 2c, and 2d, of which 2a was the control plate. The exposure results are recorded in Table 1.

EXAMPLE 3 Example 1 was repeated, except that, instead of the diazonium compound of Example 1, 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 Diazonium Plate Compound g of Resin 3a 2.63 5.33 3b 3.98 3.98 3c 5.33 2.63 Plate 3a was the control; the results are recorded in Table 1.

EXAMPLE 4 Example 3 was repeated except that instead of the above-mentioned diazonium compound a polymeric condensation product of 3-methoxydiphenyl-4diazonium chloride with formaldehyde was used. Plate 4a was the control; the results are recorded in Table 1. The constituent amounts were the same as in Example 3.

TABLE 1 Relative Photosensitivity Absolute Laser of Experimental Plates Recording Rate VS. Control Plates Normalized Example Dye colour Diazonium Conventional Laser to 15W No. Index No. Compound Exposure Exposure (Seconds/in2) la 45160 1 - - 0.38 b - 45160 1 -30 +20 0.31 c 45160 1 -65 +100 0.19 d 45160 1 -75 +140 0.16 2a 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 3a 45160 3 - - 0.88 b 45160 3 0 +180 0.31 c 45160 3 -30 +180 0.31 4a 45160 4 - - 1.25 b 45160 4 -30 +60 0.79 c 45160 4 -30 +80 0.69 WHAT WE CLAIM IS: 1. A process for recording laser radiation which comprises imagewise exposing to visible laser radiation and subsequently developing a photosensitive composition comprising a negative working diazonium compound in a binder, the diazonium compound constituting more than 25% of the total weight of the composition.

2. A process as claimed in claim 1, wherein the photosensitive composition is a coating on a carrier.

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

4. A process as claimed in any one of claims 1 to 3, wherein the composition also comprises a colorant that absorbs in the wavelength range of 450 to 550 nm

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

Claims (7)

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

   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 (0%). 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. The results are recorded in Table 1.

   EXAMPLE 2 Example 1 was repeated except that instead of the diazonium compound of Example 1 the corresponding diazonium sulfate was used, in the same amounts to give four plates, 2a, 2b, 2c, and 2d, of which 2a was the control plate. The exposure results are recorded in Table 1.

   EXAMPLE 3 Example 1 was repeated, except that, instead of the diazonium compound of Example 1, 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 Diazonium Plate Compound g of Resin 3a 2.63 5.33 3b 3.98 3.98 3c 5.33 2.63 Plate 3a was the control; the results are recorded in Table 1.

   EXAMPLE 4 Example 3 was repeated except that instead of the above-mentioned diazonium compound a polymeric condensation product of 3-methoxydiphenyl-4diazonium chloride with formaldehyde was used. Plate 4a was the control; the results are recorded in Table 1. The constituent amounts were the same as in Example 3.

   TABLE 1 Relative Photosensitivity Absolute Laser of Experimental Plates Recording Rate VS. Control Plates Normalized Example Dye colour Diazonium Conventional Laser to 15W No. Index No. Compound Exposure Exposure (Seconds/in2) la 45160 1 - - 0.38 b - 45160 1 -30 +20 0.31 c 45160 1 -65 +100 0.19 d 45160 1 -75 +140 0.16 2a 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 3a 45160 3 - - 0.88 b 45160 3 0 +180 0.31 c 45160 3 -30 +180 0.31 4a 45160 4 - - 1.25 b 45160 4 -30 +60 0.79 c 45160 4 -30 +80 0.69 WHAT WE CLAIM IS: 1. A process for recording laser radiation which comprises imagewise exposing to visible laser radiation and subsequently developing a photosensitive composition comprising a negative working diazonium compound in a binder, the diazonium compound constituting more than 25% of the total weight of the composition.
2. 2. A process as claimed in claim 1, wherein the photosensitive composition is a coating on a carrier.
3. 3. A process as claimed in claim 1 or claim 2, wherein the diazonium compound is a diazonium salt condensation product.
4. 4. A process as claimed in any one of claims 1 to 3, wherein the composition also comprises a colorant that absorbs in the wavelength range of 450 to 550 nm

   and is capable of sensitizing the diazonium compound to the visible laser radiation to which the composition is exposed, the said radiation containing at least some radiation in the wavelength range of from 450 to 550 nm.
5. 5. A process as claimed in claim 4, wherein the colorant is an azo, methine, xanthene or triarylmethane dye.
6. 6. A process as claimed in claim 1, carried out substantially as described with reference to and as illustrated by the accompanying drawing.
7. 7. A process as claimed in claim 1, carried out substantially as described in any one of the Examples herein.