CA1094860A - Process for recording images by means of laser radiation - Google Patents

Abstract

Abstract of the Disclosure A process for recording laser radiation is disclosed. A photo-sensitive material comprising a copying coating and a carrier is exposed to visible laser light having no ultra-violet component and is then developed.
The process is characterised by the use of a negative working diazonium com-pound and a binder which together form the copying coating. The diazonium compound comprises more than 25 percent of the total coating weight. The re-latively high concentration of diazonium compounds causes an increased photo-sensitivity when the material is exposed to laser light from which virtually all the ultra-violet in the range of sensitivity of the diazonium compound has boon excluded loading to increased exposure speeds or of shortened exposure times.

Description

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This invention relates to a process for recording images by means of laser radiation, in particular for preparing printing plates.
A process for the preparation of printing plates which is able ~o generate letterpress printing plates directly from computer generated "soft"
images is the Laser-Graph6~ system of Laser Graphics Systems, Inc. This type of process is described in United States Patent No. 3,832,948, and in United States Patent No. 3,461,229. The method 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 United States Patent No. 3,506,779.
The concept of using laser light to expose photosensitive coatings is a known concept. For instanceS United States Patent No. 3,664,737 des-cribes the use of UV emitting lasers to expose sensitized aluminum offset ~! ~
plates such as Litho-Chemical and Supply Company's Kem-Lon Pre-cote and Minne-sota Mining and Manufacturing's "R" plate. Both of these plates use diazo sensitizers. Although in this case production of high quality printing plates ~, is aimed at, increase in speed of exposure to compete with conventional ex-posure coupled with chemical etching is not an avowed object, nor is improved photosensitivity clear from this process.
20 ~ DT 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 now photohardened in a conventional manner by overall exposure to the ultra-violet radiation of a carbon arc lamp. This ; patent shows that the inventors found the radiation energy of these lasers to be insufficient for direct imaging of a diazo c02ting.
Printing plates presensitized with negative working diazonium compounds have attained high acceptance in the lithographic printing industry under stan-dard noncoherent light sources because of their high resolution and excellence of images obtained, easy processing, trouble-free printing~ and the great lengthof press runs which are obtainable.
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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 mis-match of laser light output and diazo coating spectral sensitivity. Even with the most powerful, yet practical laser ~argon ion lasers operating at between 10-20 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 sensi~ive 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. ~ngel-drum. 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 ec-onomically 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. when scanning techniques are to ~0 be used.
Dyes or pigments which absorb actinic light generally decrease the sensitivity of so dyed, diazo coat mgs to actinic radiation. Nonetheless, certain thio- and selenopyronine dyes are claimed to increase the photosensi-tivity of colored, polar diazonium compounds, as described in German Patent No. 745,595. Also, some colorless "optical brighteners" ~compounds absorbing ultra-violet light which fluoresce in the actinic region of diazonium compounds) are said to attain a similar effect with diazonium compounds, as reported in Belgian Pa*ent No. 661,789. Finally, a number of colorless diazonium compounds are claimed to have been improved in the solid sta~e by means of ultra-violet light-absorbing compounds. However, in no instance, as far as is known, has it 8~

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 coloring matter absorbing light in the region of 450 to 550 nm render certain diazo based coat-ings sensitive to exposure to radiation outside of the known sensitivity of these diazo compounds.
It has been an object of the invention to provide a process for re-cording laser radiation, which requires only a relatively short irradiation or a relatively lo~ radiation intensity, resp.
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 having no UV
component and is then developed into an imageO
The process according to the invention is characterised in that ~he copying coating is composed of a negative working diazonium compound and a binder, with the diazonium compound comprising more than 25% of the total coat-îng weight.
The diazo coating is composed of a diazo type negative working photo-sensitive compound such as a polymeric condensation product of a benzene diazonium salt, and optionally, a sensltizing dye capable of absorbing light in the spectral range between 450 550 nm. Suited are, eOgO azo, triarylmethane, xanthene, or methine dyesO Optionally, there may be also added mineral or strong organic acids or indicating dyes such as 4-phenylazodiphenylamine.
It has been found that contrary to all previous experience, coatings containing higher percentages o diazonium compounds have an increased photo-sensitivity when exposed to laser light from which virtually all UV in the ~ange of sensitivity o these diazos has been excluded, as compared to identi-cal coatings containing lower percentages of diazonium compoundsO This finding a~fords a practical means of increasing exposure speeds to laser radiation or o shortening exposure times, respO

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The invention is the use of certain photosensitive diazo coatings particularly suitable for exposure to laser light to which the diazonium compounds themselves are substantially insensitive~ The invention is particu-larly 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 optionally, an azo, triarylmethane, xanthene, or methine dye. The coating may also contain resins such as phenolics, polyvinylformal and vinyl copoly-mers containing carboxylic acid groups or other aqueous alcohol-or base-soluble resins7 a small quantity of mineral acid such as sulfuric or phosphoric acid or an organic acid such as alkyl or aryl sulfonic, sulfuric, phosphoric, or phosphonic. An indicator dye also may be used.
The carrier may be plastic ~ilm 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 electro-chemically. If aluminum is used, further optional preparation steps include ~0 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 according to United States Patent No. 3,220, 832. It is evident that dependent upon the graining, etching, anodizing, and like proc~dures used~ the surface area will vary. In the statements made be-low regarding coating weights, the surface area referred to is merely that of the gross dimensions of the plate.
Negative working benzenediazonium compounds must be used, such as those described in United States 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-methoxymethyl-~L09~

phenyl) ether, of diphenylamin0-~-diazonium salt with bis-~4-methoxymethylphenyl) ether, of diphenylamine-4-diazonium salt or of 3-methoxydiphenylamine-4-dia-zonium 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 work-ing diazonium compounds in the coating is increased, the photosensitivity in conventional exposure decreases, as is well known.
Surprisingly, under laser exposure, the photosensitivity increases.
When binders such as polyvinyl formal, Formvar~12/85, product of Monsanto Corporation, or other components are used in addition to the diazonium salt, the amount of diazonium salt ~should be at least 25% of the total coating weight.
The upper limit can be about 85% of total coating weight and more can be used but produces no practical increase of sensitivity.
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 poly-vinyl acetal, may be added to the coating to improve mechanical strength.
However, the addition thereof is not necessary to the practice of the inven~ion, which has as its purpose to greatly increase the speed of negati~e 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 para-phenylazodiphenylamine, Metanil Yellow~ C.I. #13065, Methyl Orange, C.I. 13025 and ~-(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.

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Not all colorants are suited for sensitizers. However, useful color-ants include the classes of Azo, Triarylmethane, Xanthene, and Me~hine, as de-fined in the Colour Index, Third Edition, Volume 4. Such colorants are gener-ally 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, selectively sensitize to laser light having no UV component.
XANTHENES such as Acridine Red 3B (Colour Index #45000), ~yronine 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 (#45315), Spirit Soluble Fast Pink B, and other xanthene type colorants or TRIARYLMETHANES such as C.I. Basic Red 9 (#42500), Tryparosan (#42505), -C.I. Basic Violet 14 (#42510), C.I. Basic Violet 2 (Remacryl Magenta)B (#42520) and other triarylmethane-type colorants or METHINES such as C.I. Basic Violet 16 (Sandocryl Red B-6B) (#48013), C.I. Basic Violet 7 (#48020), Astrazone Violet R (#48030), and any other methine-type colorant or AZOS such as 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 ~E) (#12716), C.I. Acid Red 14 (#14720~ and others 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 to the coating directly. This equipment is by way of example only as it is evident that the invention resides in the interac~ion of the laser beam and the photosensi~ive coating described herein. Laser beams driven and modulated by other mechanisms are equally suitable and are within the 8~19 purview of this invention.
The following examples are given in order to illustrate the inven-tion in greater detail. Laser exposures were made with the use of a scanner (Scan Scribers) made by Laser Graphics System, Inc, -Figure 1 is a diagram o this device.
The drawing shows: A laser, in ~his embodiment an argon ion laser 1, such as the Coherent Radiation model CR 8 laser or the spectra Physics model 164 laser. A coherent, nearly parallel beam of light 2 is reflected by the irst surface mirrors 3 and 9, which in this embodiment, are strictly selective for reflection between 450 to 530 nm only so that only 5010 5% of light below 450 nm are 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 Cassegranian type reflective optical system 6 such as Spectra Physics model ADS 100-6. The beam 12 is scanned across the image-able surface or plate 10 by means of the planar, first-surface mirror 13, the curved, first-surface mirror 9 and the rotating motor 7 driven, truncated-pyramidal, first-surface mirror 8~ ~o arrive at the plate lQ as indicated by the arrows, while the plate 10 is being transported under the line of scan as indicated by ~he arrow. The image on the plate results from the intensity of the laser beam being modulated in accordance with an input source of informa-tion ll, 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 scanner equipped with a 6 watt argon ion laser. Identical plates were exposed with another scanner difering essentially in using a commercially a~ailable laser with an output of 15 watts. ~rom these trials, the exposure rate appeared to be pro-portional to the laser output wattage. This relationship was subsequently 86~1 verified for five wattages in the range of 3 watts to 15 watts. Laser record-ing speed data in the last column of Table 1 has been normalized to the 15 watt laser.
Example 1 To 100 grams of 2-methoxyethanol were added the following components:
1.35 g of a polyvinyl-formal resin ~Formvar ~ 12/85 of Monsanto Corp.) 0.20 g of a polymeric condensation product of 1 mol of 4-(phenylamino)-2-methoxy-ben~ene diazonium salt and 1 mol of bis-(4-methoxymethylphenyl) ether, prod~ced in an 85 percent phosphoric acid and isolated in the form of a mesitylene sulfonate, 0.01 g 4-phenylazodiphenylamine and 0.02 g H3P04. After stirring at room temperature the solution was filtered through coarse filter paperO
Fifty ml of the above solution was whirler-coated at 90 rpm on anodized, grained aluminum treated with an 0~1 percent aqueous solution of polyvinyl phosphonic acid. A second solution was prepared and coated in the same manner except that the weight of dia~onium salt was OolO gO The coating weight in each case was approximately 005 g/m O
The pla~es were first exposed conventionally or 8 seconds in a com-mercially available exposure unit through a standard S~auffer 21 Stepwedge.
They were developed manually for 45 seconds using an aqueous developer contain-ing 20% by weight of n-pxopanol, about 1% of surfactant, rinsed with tap water~
squeegeed, and finished with an aqueous solution containing about 10% of hydro-ly~ed starch, and 0.5% of phosphoric acid. The plates were then inked in a conventional manner using Imperial Triple In~ 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. It was observed that the coating with the higher concentration of diazonium salt showed a solid 8 while the other coating showed a solid 9. Th0 difference of one step means that the coating with the higher concentration o diazonium salt is relatively less photo-sensitive.

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In an analogous manner, the sensitivity to laser light of the plates was measured by subjecting sections of each plat~ to argon laser scanning by means of a recorder described above, with dwell times successively decreasing by a factor of O.i (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.
After six minutes exposure, the plate with the higher diazonium salt concentration lost only a few high light dots while the other plate lost most of its high light dots. In fact, it required a nine minute exposure for the plate with the lower diazonium salt concentration to be able to retain its high light dots. This showed an increased photosensitivity for the plate with the higher diazonium salt concentration.
EXA~PLE 2 To a 2000 ml Erlenmeyer flask equipped with a mechanical stirrer con-taining 79.35 g of 2-methoxyethanol and 11.34 g of 2-methoxy ethyl acetate were added sequentially over several hours, the following components: the diazonium compound and polyvinyl-formal resin of Example 1, 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 using the method of Example 1 and exposed and developed as in Example 1. Plate 2a was the control plate.

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Plate ~ of Diaionium Compound g of Resin 2a 1.59 6.37 2b 2.63 5.33 2c 3.98 3.98 2d 5.33 2.63 The results are recorded in Table 1.

Example 2 was repea~ed except that instead of the diazonium compound of Example 1 the corresponding diazonium sulfate was used, in the same amount to give four plates, 3a, 3b, 3c ~ 3d, of which 3a was the control plate. The exposure results are recorded in Table 1.

Example 1 was repeated, except that, instead of the diazonium com-pound of Example 1, a polymeric condensation product of diphenylamine-4-dia-zonium chloride with formaldehyde was used. The proportions of diazonium compound and resin were as follows:
Plate g of Diazonium Cdmpound g of Resin 4a 2.63 5.33 4b 3.98 3.98 ` ~Q 4c 5.33 2.63 Plate 4a was the control; the results aTe recorded in Table 1.

Example 4 was repeated except that instead of the above-mentioned diazonium compound a polymPric condensation product of 3-methoxydiphenyl-4-diazonium chloride with formaldehyde was used. Plate 5a was the control; the results are recorded in Table 1. The constituent amounts were the same as in Example 4.

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It will be obvious to those skilled in the art that many modifica-tions may be made within the scope of the presen~ invention without departing from the spirit thereof, and the invention includes all such modifications.

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Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for recording laser radiation, in which a photosensitive material comprising a copying coating and a carrier is exposed image-wise to visible laser light having no UV component and is then developed into an image, characterised in that the copying coating is composed of a negative working diazonium compound and a binder, with said diazonium compound com-prising more than 25 percent of the total coating weight.

2. A process according to claim 1, characterised in that the source of radiation used is an argon ion laser.

3. A process according to claim 1, characterised in that the diazonium compound used is a diazonium salt condensation product.