FIELD OF THE INVENTION
The invention relates to new radiation sensitive coating compositions, suitable for coating substrates, particularly aluminum or polyester lithographic printing plates, for color proofing films and photoresist.
Compositions used for heat sensitive lithographic printing plates are well-known in the art. Imaging on such plates occurs through the action of infrared radiation which, upon striking on the radiation sensitive composition, changes its solubility in the developer, the non-exposed area solubility remaining unchanged. In the case of a positive plate, the area exposed to radiation becomes developer-soluble while in a negative plate the exposed area becomes insoluble.
As examples of patents disclosing radiation sensitive compositions and printing plates containing the same it can be mentioned U.S. Pat. Nos. 4,708,925; 5,286,612; 5,372,915; 5,491,046; 5,466,557; PCT/GB97/01117; PCT/GB95/02774 and U.S. Pat. No. 6,060,218.
U.S. Pat. No. 5,491,046 describes an example of printing plate containing a radiation sensitive composition, such composition containing a novolak phenolic resin, a resol phenolic resin, a Broensted acid, and an infrared absorber. However, a resin for such radiation sensitive composition requires a combination of a resol resin and a novolak resin. If, for example, novolak resin is omitted, there will be no imaging and when the layer is put into contact with the developer it will be removed together with that portion which was not exposed. Accordingly, the plate thus processed is no good for use. In the compositions disclosed in that patent, the area exposed to radiation in order to imaging requires a heating step before it can be developed. Moreover, such plates require highly alkali developers which are subject to react with carbon dioxide.
PCT/GB97/01117 disclose a composition for use with a printing plate comprised of an alkali developer-insoluble complex, made up from a phenolic resin and quinoline, benzothiazole, pyridine and imidazoline. When this complex is exposed to infrared radiation, its solubility changes because of the heat absorbed, the non-exposed area remaining unaffected. The agents making insoluble the polymer mentioned in that patent are dyes which formulae are described therein.
PCT/GB95/02774 is another example of imaging from a composition for positive plates containing naphtoquinone diazide ester and a phenolic resin. In the method described therein, the photo-sensitive composition is firstly uniformly exposed to ultraviolet radiation in order to make the composition soluble in an alkali developer. The plate is placed on a device where it receives infrared radiation so as to image and changes the composition solubility on those areas. The areas not exposed to radiation are then removed by the developing alkali solution.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide new radiation sensitive coating compositions, especially suitable for use on aluminum or polyester printing plates, color proofing films and photoresists.
It is another object of the present invention the products manufactured with the use of radiation sensitive coating compositions of the present invention.
Another object of the present invention relates to a process for manufacturing offset aluminum or polyester lithographic printing plates, color proofing films, photoresist and related products using the new coating compositions of the present invention.
It still refers to the use of said coating compositions for preparing the products mentioned herein. Additional embodiments of the invention are represented by a bilayer product, bilayer coating per se and the system itself comprising the suitable components so as to provide the radiation sensitive superposing layer product/coating.
DETAILED DESCRIPTION OF THE INVENTION
The invention refers to radiation sensitive compositions, suitable for coating substrates, particularly aluminum or polyester lithographic printing plates, for color proofing films and photoresist.
The printing plates of the present invention contain two superposing layers which make up the radiation sensitive coating, the first layer containing soluble alkali phenolic resin such as novolak as the binding polymer and one or more infrared-absorbing dyes. The second layer, coated on the first one, contains a different polymer for the purposes of protecting and insolubilize the phenolic resin of the first layer. After infrared exposure, the energy absorbed by the infrared-absorbing dye is transferred as heat and destroys the phenolic resin protection, allowing dissolution thereof in an alkali solution.
The printing plate containing radiation sensitive coating compositions of the present invention has many advantages as compared to the conventional printing plates made from other compositions One of the advantages of the present invention is that preheating of the binding polymer system so to image prior to development is not required. Another advantage of this invention is that pre-exposure to ultraviolet radiation prior to infrared image is not needed. Still another advantage of this invention is that these compositions use a low concentration of infrared sensitive dye since the second layer polymer protects and insolubilize the resin, allowing development thereof in high pH developers, about 14, of the kind used for conventional positive plates (PD2 IBF developer), after exposure. An additional advantage is that a plate containing the infrared sensitive coating of the present invention may be processed in different radiating devices, at wavelengths such as from 830 nm to 1064 nm. Still another advantage is that the coating compositions of the present invention do not emit particles or vapors (ablation), avoiding the formation of precipitates on the infrared-emitting devices and the evolution of harmful vapors to the environment during exposure.
Other polymers can be associated to the binding polymer, increasing both the abrasion resistance of the layer and the acceptability to printing paint so as to improve plate performance during the printing process. In particular, compositions employed for coating the printing plates of the present invention are not limited to the use of only one resin as the binding polymer; they are sensitive to radiation in the range from 750 to 1200 nm of the radiation spectrum and can be manipulated in the visible region; they can be used in commercially available infrared image setters and with different wavelengths; and they employ developers for conventional positive plates. Therefore, the radiation sensitive coating composition of the present invention provides a wide range of processing (e.g., temperature, dip time etc.) and a high degree of adequacy of the printing plates to the printer real needs.
The printing plates containing the compositions of the present invention exhibit a large number of prints in the printing process, about 100,000 copies in regular processing and over 600,000 copies if they are subject to heat (cure).
The term “radiation sensitive composition” as used herein encompasses not only the composition (combination) of layer(s) as defined herein but also the product manufactured from such specific compositions of each of said layers.
The new radiation sensitive composition of the present invention comprises two layers which are coated onto an aluminum surface (most preferred substrate), as known to one skilled in the art of forming a substrate for a printing plate, the first layer being applied and dried and then the second being applied and dried soon after.
The first layer comprises a binding polymer; one or more infrared-absorbing compound and the second layer comprises one or more polymeric compound which adheres to the first layer binding polymer making it insoluble in the conventional developer for positive plates.
If applied to a polyester support, it may be used as a color proofing film or for printing.
When used as a printing plate, the composition is sensitive to energy in the infrared region and is not sensitive in the visible region of the spectrum. Depending on the infrared absorber selected, a composition may be made to respond in the region between 750 and 1200 nm.
The compositions of the present invention may contain one or more infrared-absorbing dye, even of different wavelengths, A printing plate containing a radiation sensitive composition comprising infrared absorbers of different wavelengths has the advantage of making possible the use thereof in different commercially available devices for receiving radiation and imaging. Presently, two wavelengths are used. An array of laser diodes emitting at 830 nm is commercially available, which is manufactured and sold by Creo, Vancouver, Canada. The other is the YAG laser outputting at the range of 1064 nm also in the market, which is manufactured and sold by Gerber, a Barco division, Gent, Belgium. Each wavelength has its advantages and disadvantages and both are able to generate acceptable images according to the specific manufacturing mode. Digital information is then used for modulating the laser output. Energy is directed to the printing plate surface where energy absorption by an infrared-absorbing dye occurs, then it is transferred as heat, changing solubility at the interface between the two layers, the binding polymer then becoming unprotected and developer-soluble. That portion of the coating which is struck by energy is removed in the developing process while the non-radiated area remains insoluble as the imaging area. This process is known as “Write-the-non-image area”.
All coating compositions described herein are all developed using a developing composition, which is aqueous and alkaline. Developers typically used for positive printing plates may be employed in the present compositions. Developer takes advantage from the differentiation created with the radiation exposure, to remove, the coating from the non-image area and allow the image area to remain. At this point, the plate is able of performance on the printing machines, and may print 100,000 copies. If necessary, such performance may be enhanced by subjecting the coating to heat cure. The step of full cure completes the polymers cross-linking resulting in an image able to provide over 600,000 copies. The curing time usually employed is in the range of 1 to 10 minutes and the temperature is from 180 to 260° C. Curing is usually carried out with conveyor oven such as those sold by Wisconsin Oven.
As described above, the components necessary to the compositions sensitive to radiation in the range from 750 and 1200 nm comprise two superposing layers: one layer containing a binding polymer and one layer containing one or more infrared-absorbing dye. The second layer, applied over the first, comprises one or more polymeric compound which adheres to the binding polymer, making it insoluble in the developer for conventional positive plates.
The binding polymer is a condensation product of phenol, ortho-chiorophenol, o-, m- or p-cresol, p-hydroxy benzoic acid, 2 naphthol or other aromatic monohydroxy monomer with an aldehyde such as formaldehyde, acetaldehyde, fural, benzaldehyde, or any other aliphatic or aromatic aldehyde. This polymer is preferred to have a molecular weight in the range from 2,000 to 80,000, more preferably in the range of 4,000 to 40,000 and most preferably in the range of 7,000 to 20,000.
The binding phenolic polymer is preferably used in the range from about 85% to about 99%, more preferably in the range from about 90% to about 95%, based on the total solids in the composition.
In case a higher number of copies is needed, other polymers can be optionally added to the system above, such as the condensation polymer of methylated melamine formaldehyde
Resimene 735 manufactured by Monsanto, polymer of butyiated urea formaldehyde—Cymel U216-8—manufactured by Cytec Industries, copolymer of vinyl pirridone/vinyl acetate—Luviskol Va—manufactured by BASF Fine Chemicals. These polymers can be added in the ratio of 0.5 to 20% based on the total weight of the binding polymer, preferably from 2.0 to 10%.
The infrared absorber may be one or more dye or insoluble material such as carbon black. Preferred dyes are those from the classes including, but not limited to, pyridyl, quinolinyl, benzoxazolyl, thiazolyl, benzothiazolyl, oxazolyl and selenazolyl. Carbon black is useful in that it is a panchromatic absorber and works well with energy sources in the full infrared spectrum used for the application of imaging coating films and it is inexpensive and readily available. This region begins in the near infrared (nir) at 750 nm and goes up to 1200 nm. The disadvantage of the carbon black is that it is unable to participate in an image differentiation. In contrast, dyes are just appearing as commercial products and are very expensive. They must be carefully selected so that the maximum absorption (λmax) closely matches the output wavelength of the laser used in the image setter. Dyes will advantageously improve differentiation between image and nonimage areas created when laser images in the medium being employed.
The infrared-absorbing medium is preferably used in the composition in the range of 0.5% to 10% by weight based on the total weight of solids in the composition. More preferably is ranges from about 2.0% to about 5.0%.
It is always desirable to add a dye to the compositions. The purpose of using dyes in the compositions is that of distinguishing an image area after development and increasing the layer oleophily thus enabling a higher amount of printing paint to be received. The amount of dye to be added ranges from 0.5 to 3.0% by weight based on the total weight of solids in the composition. Dyes which may be employed are Malachita green, methylene blue, Victoria Blue, Crystal Violet and Rhodamine B. Other types of dyes that can be used are Orasol Blue, Orasol Red and Orasol Violet, manufactured by Ciba-Geigy.
The composition of the first layer is dissolved in a suitable solvent (or suitable solvents). Examples of such solvents include, but are not limited to: 1-methoxy-2-ethanol, 1-methoxy-2-propanol, acetone, methyl ethyl ketone, diisobutyl ketone, methyl isobutyl ketone, n-propanol, isopropanol, tetrahydrofuran, butyrolactone and methyl lactate.
The compound forming the second layer coating the binding polymer, making it developer-resistant, may be one or more polymers including polymeric amines, polyacetals (polyvinyl butyral, polyvinyl formal etc.), polyethylene glycol, butylated urea formaldehyde, copolymers of vinyl pirrolidone and vinyl acetate, methylated melamine formaldehyde, cellulose esters, or mixtures thereof. This second layer is preferably dissolved in solvents not dissolving the binding polymer of the first layer, which form a film evenly adhered to the first layer, and which are readily evaporated, such as methylene chloride, toluene and xylene. The dry coating weight must range between 5 and 200 mg/m2. The following commercially available polymers may be mentioned:
Polymeric amines—Solsperse, manufactured by Avecia Pigments and Additives
Polyvinyl butyral—Mowital B30 H, manufactured by Hoescht Celanese
Polyethylene glycol—Carbowax, manufactured by Union Carbide
Cellulose butyrate/acetate and cellulose propionate acetate—Eastman Chemical
Butylated urea formaldehyde—Cymel U216-8—manufactured by Cytec Industries
Methylated melamine formaldehyde—Resimene 735 manufactured by Monsanto.
According to need, surfactants may be added to the compositions so as to obtain characteristics required by a printing plate. Surfactants are employed in order to enhance the coating application to aluminum or polyester supports. Surfactants to be employed include fluorocarbonated surfactants such as FC-430 by 3M Corporation or Zonyl Ns by DuPont, block polymers of ethylene oxide and propylene oxide known as Pluronic and manufactured by Basf and silicone surfactants such as BYK 307 manufactured by BYK Chemie. These surfactants improve the composition cosmetics during application thereof to the substrate, avoiding imperfections and the appearance of voids on the layer. The amount of surfactant employed ranges from 0.01 to 0.5% by weight base on the total weight of solids in the composition.
The coating components may be added at several solids levels depending on the technique used to apply the coating to the substrate being coated. Therefore, the ratios of components nay be the same, but percentages differ. The disclosure of the percentage ranges of the components as that of the solution is not significant. The amount of each coating component will therefore be described herein as a percentage of total solids. The coating components are dissolved in a desired solvent system. The solution of the first coating layer is applied to the selected substrate. The coating is applied such as to have a dry coating weight in the range from about 0.5 g/m2 to about 2.0 g/m2. More preferably, from about 0.8 g/m2 to about 1.4 g/m2 is used. The coating is dried under conditions which will effectively remove all solvent therein, but not so harmful to cause degradation of the polymers with themselves or with others. Then, the second coating layer is applied so as to have a dry coating weight from 5 to 200 mg/m2, preferably from 10 to 100 mg/m2.
The following examples illustrate the invention but do not limit the same at all.