Dye-sensitive photopolymerizable compositions.

0001The present invention relates to dye-sensitive photo-polymerizable compositions, and photosensitive materials employing the same.

0002U.S. Patents 4,399,209 and 4,440,846 to The Mead Corporation describe imaging materials and processes in which images are formed through exposure controlled release of an image-forming agent from a microcapsule containing a photohardenable composition. The imaging material is image-­wise exposed to actinic radiation and subjected to a uniform rupturing force such as pressure. Typically, the image-­forming agent is a colour precursor which is released from the microcapsule and reacts with a developer to form a visible image.

0003One of the problems which has been encountered in designing commercially acceptable full colour imaging materials has been the relatively short wavelength band to which most photohardenable compositions are sensitive. In most cases, compositions are sensitive to ultravlolet radiation or blue light. Compositions which are sensitive to green or red light are few in number.

0004Full colour photosensitive materials employing photohardenable microcapsules are described in British Patent Specification 2113860 and U.S. Patent 4,576,891. These materials include a photosensitive layer containing three sets of photohardenable microcapsules. Each set of microcapsules is sensitive to a different band of radiation and contains a cyan, magenta or yellow image-forming agent. The absorption spectra of the initiators employed in these microcapsules are never perfectly distinct. There is always some degree of overlap in the absorption curves and, as a result, exposure conditions must be carefully controlled to avoid cross-exposure.

0005It would at least be desirable to extend the sensitivity of photohardenable compositions to longer wavelengths and it would be particularly desirable to extend the sensitivity of photohardenable compositions throughout the visible spectrum. By extending sensitivity deeper into the visible spectrum, the amount of overlap in the absorption spectra of the initiators can be reduced. When sensitivity is extended throughout the visible spectrum, a panchromatic system results.

0006Dye-sensitized photopolymerizable compositions are well known in the art. An excellent summary of them is provided by Eaton, "Dye Sensitized Photopolymerization", Advances in Photochem, Vol. 13, pgs. 427-487. Typically, dye-sensitized photopolymerizable compositions include a photoreducible or photoxidizable dye and a reducing agent or an oxidizing agent for the dye. An early example of a dye-­sensitized photopolymerizable composition is provided by Oster in U.S. Patent 2,875,047 which teaches a composition comprising vinyl compounds, a photoreducible dye and a reducing agent. Typical dyes include Rosebengal, Rhodamine B, Brilliant Green, and thionine. Typical reducing agents are stannous chloride, ascorbic acid, glutathione, thiourea, allyl thiourea and hydrazine. Other examples of dye sensitized compositions are provided in U.S. Patents 3,488,269 to Allen et al.; 3,573,922 to Rust; 3,531,281 to Rust; and 3,650,927 to Livinos disclosing a combination of a photoreducible dye and a sulfone.

0007According to a first aspect of the present invention, there is provided a dye-sensitized photopolymerizable composition characterised in comprising a free radical addition polymerizable or cross-linkable material, a photoreducible dye, and a thiol.

0008Such dye-sensitized photopolymerizable compositions can be used in those applications in which photopolymerizable compositions have been used previously including as photoresists, photoadhesives, and in the imaging systems described in U.S. Patents 4,399,20-9; 4,440,846; and in full colour panchromatic imaging systems described in British Patent Specification 2113860.

0009In accordance with a second and alternative aspect thereof, the invention provides a photosensitive material comprising a support having a layer of a dye-­sensitized photopolymerizable composition on the surface thereof, characterised in that said composition comprises a free radical addition polymerizable or cross-linkable material, a photoreducible dye, and a thiol.

0010Such a photosensitive material is useful in forming polymer images for use in printing or in forming printed circuits.

0011The invention provides in a third alternative aspect thereof a photosensitive material comprising a support having a layer of microcapsules on one surface thereof, said microcapsules having an image-forming agent associated therewith and containing an internal phase, characterised in that said internal phase includes a dye-­sensitized photopolymerizable composition comprising a free radical addition polymerizable or cross-linkable compound, a photoreducible dye, and a thiol.

0012In still another alternative aspect of the present invention, there is provided a photosensitive material adapted for full colour imaging, comprising: a support having a layer of photosensitive microcapsules on the surface hereof; the microcapsules comprising a first set of microcapsules having a cyan image-forming agent associated therewith, a second set of microcapsules having a magenta image-forming agent associated herewith, and a third set of microcapsules having a yellow image-­forming agent associated therewith; characterised in that at least one of the first, second and third sets contains a dye-sensitized photopolymerizable composition including a free radical addition polymerizable or cross-linkable material, a photoreducible dye, and a thiol.

0013In accordance with a more preferred embodiment, the first, second and third sets of microcapsules are respectively sensitive to red, green and blue light and the dye-sensitized photopolymerizable composition of the present invention is used in at least one of the first, second and third sets of microcapsules. Typically, the dye-sensitized composition will be used in at least the red-sensitive or green-sensitive microcapsules and possibly in all three of the red, green and blue-­sensitive microcapsules.

0014Photoreducible dyes are well known in the photographic art and theoretically any photoreducible due should be useful in practice of the present invention including photoreducible methine, polymethine, triarylmethane, indoline, thiazine, acridine, xanthane, oxazine and acridine dues. However, cyanine, carbocyanine and hemicyanine dyes are particularly preferred. Representative examples of dyes useful in the practice of the present invention are 1,1′-dialkyl­3,3,3,′3′-tetramethylindocarbocyanine halides and 1,1′-­dialkyl-3,3,3,′3′-tetramethylindodicarbocyanine halides.

0015Useful in practice of the present invention are thiols generally represented by the formula (I) <chemistry id="chem0001" num="0001"><img file="EP0318336A2_D0001.tif" /></chemistry> where z represents the atoms necessary to complete a 4 to 10 membered monocyclic or bicyclic heterocyclic ring such as a benzoxazole, benzimidazole, benzothiazole, tetrazole, etc.

0016Representative examples of thiols useful in practice of the present invention are mercaptobenzoxazole, ethoxymercaptobenzothiazole, mercaptobenzothiazole and t-phenyl-5-mercaptotetrazole.

0017Typically, the photoreducible dye is used in the photopolymerizable composition in an amount of approximately 0.05 to 0.20 parts to 100 parts ethylenically unsaturated compound. The thiol is typically used in a stoichiometric amount based upon the amount of dye present in the composition.

0018Free radical addition polymerizable ethylenically unsaturated compounds are well known in the art. Such compounds include monomers having one or more ethylenically unsaturated groups such as vinyl or allyl compounds or prepolymers having terminal or pendant ethylenic unsaturation. Representative examples include acrylic and methacrylic esters of polyhydric alcohols such as trimethylolpropane, pentaerythrithol, and the like: acrylate or methacrylate terminated epoxy resins, acrylate or methacrylate terminated polyesters, etc. Representative examples include ethylene glycol diacrylate, ethylene glycol dimethacrylate, trimethanolpropane triacrylate (TMPTA), pentaerythritol tetraacrylate, pentaerythritol tetramethacrylate, diepentaerythritol hydrozypentacrylate (DPHPA), hexanediol-1,6-dimethacrylate and diethyleneglycol dimethacrylate.

0019While the dye and thiol can be used alone as the initiator system of the photopolymerizable compositions, film speeds tend to be quite low and oxygen inhibition is observed. It is desirable to use a compound in combination with an initiator which functions as an autoxidizer. An autoxidizer is a compound which is capable of consuming oxygen in a free radical chain process. Examples of useful autoxidizers are N,N-dialkylanilines. The autoxidizers are preferably used in a concentration of about 0.5 to 10 parts per 100 parts ethylenically unsaturated compound.

0020Examples of preferred N,N-dialkylanilines are dialkylanilines substituted in one or more of the <u style="single">ortho</u>-, <u style="single">meta</u>-, or <u style="single">para</u>-position by the following groups: methyl, ethyl, isopropyl, <u style="single">t</u>-butyl, 3,4-tetramethylene, phenyl, trifluoromethyl, acetyl, ethoxycarbonyl, carboxy, carboxylate, trimethylsilylmethyl, trimethylsilyl, triethylsilyl, trimethylgermanyl, triethylgermanyl, trimethylstannyl, triethylstannyl, <u style="single">n</u>-butoxy, n-pentyl, penoxy, hydroxy, acetyl-oxy, methylthio, ethylthio, isopropylthio, thio-(mercapto-), acetylthio, fluoro, chloro, bromo and iodo.

0021Representative examples of N,N-dialkylanilines useful in practice of the present invention are 4-cyano-N,N-dimethyl­aniline, 4-acetyl-N,N-dimethylaniline, 4-bromo-N,N-­dimethylaniline, ethyl 4-(N,N-dimethylamino) benzoate 3-chloro-N,N-dimethylaniline, 4-chloro-N,N-dimethylaniline, 3 ethoxy-N,N-dimethylaniline, 4-fluoro-N,N-dimethylaniline, 4-methyl-N,N-dimethylaniline, 4-ethoxy-N,N-dimethylaniline, N,N-dimethylthioanicidine, 4-amino-N,N-dimethylaniline, 3-hydroxy-N,N-dimethylaniline, N,N,N′,N′-tetramethyl-­1,4-dianiline, 4-acetamido-N,N-dimethylaniline, etc.

0022Preferred N,N-dialkylanilines are substituted with an alkyl group in the <u style="single">ortho</u>-position and include 2,6-diisopropyl-N,N-dimethylaniline, 2,6-diethyl-N,N-­dimethaniline, N,N,2,4,6-pentamethylaniline (PMA) and p-t-­butyl-N,N-dimethylaniline.

0023The photohardenable compositions can be coated upon a support in a conventional manner and used as a photoresist or in photolithography to form a polymer image; or they can be encapsulated as described in U.S. Patents 4,399,209 and 4,440,846 and used to control the release of an image-forming agent. The latter processes typically involve image-wise exposing the photosensitive material to actinic radiation and subjecting the layer of microcapsules to a uniform rupturing force such as pressure, abrasion, or ultrasonic energy whereupon the image-forming agent is released from the microcapsules for reaction with a developer.

0024Several processes can be used to form colour images as explained in British Patent Specification 2113880. If the microcapsules contain photosensitive compositions which are sensitive to red, green and blue light, images can be formed by direct transmission or reflection imaging or by image processing. Image processing may involve forming colour separations (colour-seps) corresponding to the red, green, and blue component images and sequentially exposing the photosensitive material to three distinct bands of radiation hereinafter designated λ-1, λ-2, and λ-3 through each colour separation. λ -1, λ-2, and λ-3 may be red, green, and blue light. Otherwise, it may involve electronic processing in which the image or subject to be recorded is viewed through a Dunn or matrix camera and the output from the camera electronically drives three exposure sources corresponding to λ-1, λ-2, and λ-3. Alternatively, the image may be produced synthetically, e.g., a computer-generated image.

0025While the discussion herein relates to forming 3-colour full colour images, 4-colour images are also possible. For example, microcapsules containing cyan, magneta, yellow, and black image-forming agents can be provided which have distinct sensitivities at four wavelengths, e.g., λ-1, λ-2, λ-3, and λ-4.

0026In such a full colour system at last one set of the microcapsules contains a dye-sensitive photohardenable composition as described above. The other sets also may contain such a composition, or they may contain a different type of photoinitiator.

0027In preferred embodiments, a full colour imaging system is provided in which the microcapsules are sensitive to red, green, and blue light respectively. The photosensitive composition in at least one and possibly all three microcapsules are sensitized by an ionic dye compound. For optimum colour balance, the microcapsules are sensitive (λmax) at about 450 nm, 550 nm, and 650 mn, respectively. Such a system is useful with visible light sources in direct transmission or reflection imaging. Such a material is useful in making contact prints or projected prints of colour photographic slides. They are also useful in electronic imaging using lasers or pencil light sources of appropriate wavelengths.

0028Because the ionic dye compounds absorb at wavelengths greater than 400 nm, they are coloured. Typically, the unexposed dye compound is present with the image-forming agent in the image areas and, thus, the colour of the compound must be considered in determining the colour of the image. However, the compound is used in very small amounts compared to the image-forming agent and exposure sometimes bleaches the compound.

0029The photohardenable compositions can be encapsulated in various wall formers using techniques known in the area of carbonless paper including coacervation, interfacial polymerization, polymerization of one or more monomers in an oil, as well as various melting, dispersing, and cooling methods. To achieve maximum sensi­tivities, it is important that an encapsulation technique be used which provides high quality capsules which are responsive to changes in the internal phase viscosity in terms of their ability to rupture.

0030Oil soluble materials have been encapsulated in hydrophilic wall-forming materials such as gelatin-type materials (see U.S. Patent Nos. 2,730,456 and 2,800,457 to Green et al) including gum arabic, polyvinyl alcohol, carboxy-methylcellulose; resorcinol-formaldehyde wall-­formers (see U.S. Patent No. 3,755,190 to Hart, et al); isocyanate wall-formers (see U.S. Patent No. 3,914,511 to Vassiliades); isocyanate-polyol wall-formers (see U.S. Patent No. 3,796,669 to Kirintani et al); urea-formaldehyde wall-formers, particularly urea-resorcinol-formaldehyde in which oleophilicity is enhanced by the addition of resorcinol (see U.S. Patent Nos. 4,001,140; 4,087,376 and 4,089,802 to Foris et al); melamine-formaldehyde resin; hydroxypropyl cellulose (see U.S. Patent No. 4,025,455 to Shackle); and UF capsules formed using pectin as a system modifier as discussed in U.S. Patent 4,608,330 to Marabella.

0031Urea-resorcinol-formaldehyde and melamine-­formaldehyde capsules with low oxygen permeability are preferred.

0032A capsule size should be selected which minimizes light attenuation. The mean diameter of the capsules used in our compositions typically ranges from approximately 1 to 25 microns. As a general rule, image resolution improves as the capsule size decreases. If the capsules become too small, they may become inaccessible in the pores or the fiber of the substrate. These very small capsules may therefore be screened from exposure by the substrate. They may also fail to rupture when exposed to pressure or other rupturing means. In view of these problems, we have determined that a preferred mean capsule diameter range is from approximately 5 to 10 microns. Technically, however, the capsules can range in size up to the point where they become visible to the human eye.

0033An open phase system may also be used instead of an encapsulated one. This can become by dispersing what would otherwise be the capsule contents throughout a binder on the substrate as discrete droplets. Suitable coatings for this embodiment include polymer binders whose viscosity has been adjusted to match the dispersion required in the coating. Suitable binders are gelatin, polyvinyl alcohol, polyacrylamide, and acrylic lattices. Whenever reference is made to "capsules" and "encapsulation" without reference to a discrete capsule wall in this specification or the appended claims, those terms are intended to include the alternative of an open phase system.

0034The photosensitive material can be used to control the interaction of various image-forming agents.

0035In one embodiment of the material the capsules may contain a benign visible dye in the internal phase in which case images are formed by contacting the exposed imaging material under pressure with a plain paper or a paper treated to enhance its affinity for the visible dye. A benign dye is a colored dye which does not interfere with the imaging photochemistry, for example, by relaxing the excited state of the initiator or detrimentally absorbing or attenuating the exposure radiation.

0036In a preferred embodiment images are formed through the reaction of a pair of chromogenic materials such as a color precursor and a color developer, either of which may be encapsulated with the photohardenable composition and function as the image forming agent. In general, these materials include colorless electron donating type compounds and are well known in the art. Representative examples of such color formers include substantially colorless compounds having in their partial skeleton a lactone, a lactam, a sultone, a spiropyran, an ester or an amido structure such as triarylmethane compounds, bisphenylmethane compounds, xanthene compounds, fluorans, thiazine compounds, spiropyran compounds and the like. Crystal Violet Lactone and Copikem X, IV and XI (products of Hilton-Davis Chemical Co., Cincinnati, Ohio), React Yellow (a product of BASF) are often used. The color formers can be used alone or in combination.

0037The developer materials conventionally employed in carbonless paper technology are also useful in practice of the present invention. Illustrative examples are clay minerals such as acid clay, active clay, attapulgite, etc.; organic acids such as tannic acid, gallic acid, propyl gallate, etc.; acid polymers such as phenol-formaldehyde resins, phenol acetylene condensation resins, condensates between an organic carboxylic acid having at least one hydroxy group and formaldehyde, etc.; metal salts or aromatic carboxylic acids such as zinc salicylate, tin salicylate, zinc 2-hydroxy naphthoate, zinc 3,5 di-tert butyl salicylate, zinc 3,5-di-(o-methylbenzyl)salicylate, oil soluble metal salts or phenol-formaldehyde novolak resins (e.g., see U.S. Patent Nos. 3,672,935; 3, 732,120 and 3,737,410) such as zinc modified oil soluble phenol-formaldehyde rcsin as disclosed in U.S. Patent No. 3,732,120, zinc carbonate etc. and mixtures thereof. A particularly useful developer is described in EP-A-0260129.

0038As indicated in U.S. Patents 4,399,209 and 4,440,846, the developer may be present on the photo­sensitive sheet (providing a so-called self-contained system) or on a separate developer sheet.

0039In self-contained systems, the developer may be provided in a single layer underlying the microcapsules as disclosed in U.S. Patent No. 4,440,846. Alternatively, the color former and the color developer may be individually encapsulated in photosensitive capsules and upon exposure both capsule sets image-wise rupture releasing color former and developer which mix to form the image. Alternatively, the developer can be encapsulated in non-photosensitive capsules such that upon processing all developer capsules rupture and release developer but the color former containing capsules rupture in only the unexpected or under-­exposed area which are the only areas where the color former and developer mix. Still another alternative is to encapsulate the developer in photosensitive capsules and the color former in non-photosensitive capsules.

0040Such systems are not necessarily limited to embodiments where the image-forming agent is present in the internal phase. Rather, this agent may be present in the capsule wall of a discrete capsule or in the binder of an open phase system or in a binder or coating used in combination with discrete capsules or an open phase system designed such that the image-wise ruptured capsules release a solvent for the image-forming agent. Embodiments are also envisioned in which a dye or chromogenic material is fixed in a capsule wall or binder and is released by interaction with the internal phase upon rupturing the capsules.

0041The most common substrate for the microencapsulated imaging material is a transparent film since it assists in obtaining uniform development characteristics, however, paper may also be used. Another preferred substrate is a reflective substrate such as metallized polymeric films like aluminized PET. The paper may be a commercial impact raw stock, or special grade paper such as cast-coated paper or chrome-rolled paper. Transparent films such as polyethylene terephthalate can be used. Translucent substrates can also be used, Conventional printing plate supports can be used in printing applications.

0042The present invention is illustrated in more detail by the following non-limiting examples:

0043To illustrate the initiator system, a series of "slide tests" were conducted as below:

0044A stock solution was made which consisted of 400g TMPTA and 0.80g of 1,1′-dibutyl-­3,3,3′,3′ -tetramethylindocarbocyanine iodine . To 10g portions of this stock was added 0.4g of a number of potentially efficient quenching agents. The resultant solution was then stirred at 60°C until the quencher was dissolved. After the solution was cooled, a drop was placed between two glass microscope slides. A light source (consisting of one 15 Watt F15T8-D G.E. Daylight tube six inches from the sample with a 0.3 Wratten neutral density filter placed between the light source and sample) was used to expose the sandwiched photo sensitive composition. The times required for first noticable polymerization and for total slide immobilization were noted. <tables id="tabl0001" num="0001"><table frame="all"><tgroup cols="3" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="52.50mm" /><colspec colnum="2" colname="col2" colwidth="52.50mm" /><colspec colnum="3" colname="col3" colwidth="52.50mm" /><thead valign="top"><row><entry namest="col1" nameend="col1" /><entry namest="col2" nameend="col3" align="center"><u style="single">AVERAGE TIME (sec.)</u></entry></row><row><entry namest="col1" nameend="col1" align="center"><u style="single">THIOL</u></entry><entry namest="col2" nameend="col2" align="center"><u style="single">FIRST POLYMERIZATION</u></entry><entry namest="col3" nameend="col3" align="center"><u style="single">IMMOBILIZATION</u></entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">2-Mercaptobenzoxazole</entry><entry namest="col2" nameend="col2" align="right">41</entry><entry namest="col3" nameend="col3" align="right">44</entry></row><row><entry namest="col1" nameend="col1" align="left">1-Phenyl-5-Mercaptotetrazole</entry><entry namest="col2" nameend="col2" align="right">35</entry><entry namest="col3" nameend="col3" align="right">37</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">No added Thiol</entry><entry namest="col2" nameend="col2" align="right">>60</entry><entry namest="col3" nameend="col3" align="right">>60</entry></row></tbody></tgroup></table></tables>

0045In a second round of tests, a known efficient autooxidant, N,N,2,4,6-pentamethylaniline, was added to each of the above three solutions. This addition greatly enhanced the polymerization rate, as shown below: <tables id="tabl0002" num="0002"><table frame="all"><tgroup cols="3" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="52.50mm" /><colspec colnum="2" colname="col2" colwidth="52.50mm" /><colspec colnum="3" colname="col3" colwidth="52.50mm" /><thead valign="top"><row><entry namest="col1" nameend="col1" /><entry namest="col2" nameend="col3" align="center"><u style="single">AVERAGE TIME (sec.)</u></entry></row><row><entry namest="col1" nameend="col1" align="center"><u style="single">THIOL</u></entry><entry namest="col2" nameend="col2" align="center"><u style="single">FIRST POLYMERIZATION</u></entry><entry namest="col3" nameend="col3" align="center"><u style="single">IMMOBILIZATION</u></entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">2-mercaptobenzoxazole</entry><entry namest="col2" nameend="col2" align="right">13</entry><entry namest="col3" nameend="col3" align="right">15</entry></row><row><entry namest="col1" nameend="col1" align="left">1-phenyl-5-mercaptotetrazole</entry><entry namest="col2" nameend="col2" align="right">10</entry><entry namest="col3" nameend="col3" align="right">11</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">No added Thiol</entry><entry namest="col2" nameend="col2" align="right">>60</entry><entry namest="col3" nameend="col3" align="right">>60</entry></row></tbody></tgroup></table></tables>