Encapsulated photosensitive compositions

Abstract: A process for producing microencapsulated photosensitive compositions having an extended dynamic range is disclosed. An emulsion of a multiplicity of droplets of an internal phase in a continuous phase is provided. The internal phase includes a photohardenable or photosoftenable composition and an image-forming agent, and the composition of the internal phase incrementally or continuously varies. A wall is formed around the droplets.

Description: The present invention relates to a process for producing an encapsulated photosensitive composition. Imaging systems employing photosensitive microcapsules are described in U.S. Patent Nos. 4,399,209 and 4,440,846, assigned to The Mead Corporation. These patents teach the formation of images by the exposure-controlled release of an image-forming agent from a microcapsule containing a photohardenable composition. The image-forming agent is typically a substantially colourless colour precursor which reacts with a developer to form a visible image. Our U.K. Patent Specification No. 2,113,860 and U.S. Patent 4,576,891 disclose a full colour imaging system wherein three sets of microcapsules which are sensitive to different bands of actinic radiation are employed. These microcapsules respectively contain cyan, magenta and yellow colour precursors Panchromatic, full colour, imaging systems wherein a panchromatic imaging sheet is exposed to visible light and subsequently developed are described in our European Application Publication No. 0233567. Although the above described inventions have significantly advanced imaging technology, the systems therein described still suffer from minor drawbacks. For example, for some photosensitive materials, the resulting H&D curve has a high gamma, causing a relatively narrow dynamic range. For applications which require or prefer continuous tone images, it is desired to extend the dynamic range of the materials. In addition, for full color or panchromatic imaging systems, it is desirable that the systems be color balanced. Color imbalance may result when the H&D curves of respective cyan, magenta and yellow microcapsules are not properly aligned. Accordingly, it is desirable to devise a system wherein the H&D curves are aligned. Several alternative means have been proposed to cure the above-described defects. For example, our British patent Specification 2163563 discloses photosensitive microcapsules wherein a solid diluent, typically a wax or fatty acid or alcohol, is incorporated into the internal phase of microcapsules. The incorporation of the solid diluent extends the dynamic range of the microcapsules. Our U.S. Application Serial No. 164,653, filed March 7, 1988 (for which the corresponding Japanese Application is published under No. 54859/89) discloses a photosensitive imaging method utilizing a heating step. In some photosensitive compositions, heating the microcapsule imaging sheet prior to exposure to actinic radiation functions to extend the dynamic range of the photosensitive microcapsules. Our European Patent Application No. 89302941.3 (Publication No. 0334663) describes a microcapsule imaging system having an extended dynamic range. The dynamic range is extended by utilizing a mixture of two or more independently prepared sets of microcapsules having the same spectral sensitivity but different film speeds. To obtain different film speeds, various techniques may be used including: different concentrations of photoinitiators, different photoinitiators or different concentrations of photopolymerizable monomers, incorporating a radiation absorber in one set of the microcapsules and a different amount or no amount of absorber in the other set of microcapsules, utilizing different wall formers for preparing the respective microcapsules, or varying the size of the microcapsules. US-A-4587194 discloses an imaging material having at least two sets of microcapsules having distinctly different film speeds to enable the production of multicoloured images by exposure at two or more different intensities or exposure times. Although the above-described methods have helped to remedy the above-described defects, they require the preparation of two or more types of microcapsules, the incorporation of additional materials into the internal phase of the microcapsules, or altering imaging process conditions. Further, undesirable photographic characteristics sometimes result. For example, when producing a composition from two sets of microcapsules having H&D curves with different gammas an unacceptable hump occasionally develops in the resulting H&D curve. Although the dynamic range of the mixture of different speed microcapsules is extended, the hump in the H&D curve introduces problems in achieving colour balance. As will become clear from what follows, we have now found a way of forming an encapsulated photosensitive composition having an extended dynamic range and improved photographic properties wherein the photosensitive composition may be produced in a single batch and wherein process conditions during imaging do not have to be altered. According to the present invention, there is provided a process for producing microencapsulated photosensitive composition comprising the steps of: (a) providing an emulsion of a multiplicity of droplets of an internal phase in a continuous phase, and (b) forming a wall around said droplets; characterised in that said internal phase includes a photohardenable or photosoftenable composition and an image-forming agent; the concentration of one of the constituents of said internal phase in said emulsion incrementally or continuously varying. Thereby it is possible to produce a single set of microcapsules having internal phases which exhibit different film speeds. The different film speed microcapsules may be employed to control the shape of the H&D curve and improve the photographic characteristics of the photosensitive material, including the dynamic range. While the present process and photosensitive materials carrying the said compositions upon a substrate can be used to produce images for a monochromatic imaging system, they may also be used in a full colour system, and more particularly a panchromatic system, as a means to match the gamma of the H&D curves of the cyan, magenta and yellow microcapsules and thereby improve colour balance. For example, it may be useful in slowing the toe speed of magenta microcapsules to bring the magenta H&D curve into alignment with the cyan and yellow H&D curves. In the preferred arrangement the internal phase contains a photohardenable monomer and a photoinitiator and the concentration of the photoinitiator varies in the internal phase of the microcapsules to provide a continuous or incremental concentration gradient. In addition, or in the alternative, the process may be utilized to provide microcapsules in which the internal phases contain varying concentrations of an image-forming agent, an absorber, photosensitive monomer, a co-initiator, or a photoinhibitor. The key feature is that the variable concentration of the internal phase is produced prior to forming microcapsule walls around the microcapsules. The term "incrementally or continuously varies" relates to the concentration of a constituent of the internal phase of the resultant microcapsules. The term "incrementally" indicates that the variance in the constituent is produced by a process which stepwise alters the concentration of the characteristic. The term "continuously" indicates that the variance in the constituent is produced by a process which continuously alters the concentration of the constituent. The invention is hereinafter more particularly described with reference to the accompanying drawings, in which: Fig. 1 is a schematic diagram of apparatus adapted for putting into effect the teachings of the instant invention; Fig. 2 is a schematic diagram of an alternative apparatus for performing the instant invention; Fig. 3 is an H&D curve of a photosensitive material produced in accordance with the instant invention. Reference can be made to U.S. Patents 4,399,209 and 4,440,846, U.K. Patent 2,113,860 and European Published Application 0 233 587 for examples of imaging systems, including full colour and panchromatic imaging systems which can be modified in accordance with the present invention to provide microcapsules having controlled gamma, H&D curve shape, and improved dynamic range and film speed. An encapsulated photosensitive composition having an extended dynamic range is produced by altering the composition of the internal phase prior to encapsulation. To produce the photosensitive composition, a continuous phase, an initial internal phase, and an internal phase modifying agent may be used and the interrelationship between these phases is best shown in Figure 1. The description of the system 10 and process as shown in Figure 1 is made with respect to a preferred embodiment wherein the modifying material is used to incrementally dilute the concentration of the photoinitiator to produce a photosensitive composition having a broadened dynamic range. However, the process and system described with respect to Figure 1 may be used similarly in connection with any of the modifying materials as described below. As shown in Fig. 1, tank 12 having stirrer 14 contains the internal phase modifying agent 13. In communication with tank 12 through pipe 16 is tank 20. Regulation of the flow of modifying phase 13 through pipe 16 is accomplished by flow regulation means such as valve 18, which is actuated and controlled by controller 32, typically a computer. Tank 20 having stirrer 22 contains internal phase 21 as defined above. In communication with tank 20 through pipe 24 is tank 28. Regulation of the flow of internal phase 21 through pipe 24 is accomplished by flow regulation means such as valve 26, which is actuated and controlled by controller 32. Tank 28 having stirrer 30 contains continuous phase 29 as defined above. To produce photosensitive microcapsules tank 12 is filled with modifying phase 13, tank 20 is filled with internal phase 21, and tank 28 is filled with continuous phase 29. Valves 18 and 26 are in a closed position. At this stage of the process, internal phase 21 should have a high concentration of image-forming agent and photoinitiator present. In one example the solution in phase 21 contains approximately .0053 parts photoinitiator per part monomer. To incrementally dilute the concentration of photoinitiator in the emulsified internal phase, controller 32 then opens valve 26 to enable a portion of internal phase 21 to pass through pipe 24 and into tank 28 where it is dispersed by forming droplets in continuous phase 29 and is stirred by stirrer 30. After a portion of internal phase 21 has been dispersed, controller 32 closes valve 26 to prevent any additional internal phase 21 from passing through pipe 24 into tank 28. At this stage in the process, tank 28 has dispersed in its continuous phase droplets of an internal phase having a relatively high concentration of photoinitiator. While stirrer 30 continues to operate in tank 28, controller 32 opens valve 18 to enable a predetermined amount of modifying phase 13 to pass through pipe 16 and into tank 20 where it mixes with internal phase 21 with the aid of agitation from stirrer 22 to cause the concentration of the image-forming agent and photoinitiator in internal phase 21 to be reduced. Once a desired concentration of colour-forming agent and photoinitiator is achieved, controller 32 closes valve 18 to prevent any additional modifying phase 13 from passing through pipe 16 into tank 20. The concentration of the resulting internal phase 21 contains approximately .003 parts photoinitiator per part monomer, in the said one example. With valve 18 closed, controller 32 opens valve 26 to allow the dilute internal phase 21 to pass through pipe 24 and disperse and form droplets in continuous phase 29. At this point in time continuous phase 29 contains droplets of an emulsified high photoinitiator concentration internal phase and droplets of an emulsified dilute photoinitiator concentration internal phase. Reaction of a pre-wall former initially present in phase 21 prevents the emulsified internal phases from completely intermixing, although some intermixing may be desirable. When the desired amount of dilute internal phase 21 has been dispersed in continuous phase 29, controller 32 closes valve 26 to prevent any additional dilute internal phase 21 from passing through pipe 24. For many applications, the maintenance of droplets of two different concentration internal phases may be sufficient to produce the extended dynamic range microcapsules. If, however, additional lower concentration internal phases 21 are desired to be dispersed in continuous phase 29, the above process may be repeated for as many times as required to produce a desired dispersion. Alternatively, the process described in connection with Figure 1 may be performed on a continuous basis. To accomplish this, valves 18 and 26 are maintained in an open position with controller 32 controlling the rate of flow of liquid through valves 18 and 26. In this embodiment, the resultant droplets dispersed in continuous phase 29 should have a photoinitiator concentration gradient ranging from the concentration of photoinitiator in phase 13 to the concentration of photoinitiator in phase 21. In an embodiment wherein two independently prepared internal phases are dispersed in the same aqueous phase to form a photosensitive material having a varying concentration of internal phase within the dispersed droplets, the system as shown in Figure 2 may be utilized. As seen in Figure 2, tank 112, containing stirrer 114 has contained within it internal phase 113. Internal phase 113 typically contains a photohardenable monomer, an image-forming agent and a photoinitiator having a first concentration. Tank 120, containing stirrer 122 has contained within it internal phase 121. Internal phase 121 differs from internal phase 113 in at least one aspect. For example, internal phase 121 may contain identical concentrations of monomer and image-forming agent but may contain a different concentration of photoinitiator. Alternatively, internal phase 121 may contain a co-photoinitiator and a photoinhibitor. To produce the photosensitive composition having a variable concentration internal phase, valves 118 and 126 are opened by controller 132 in a manner similar to controller 32 of Figure 1 to enable internal phase 113 to pass through line 116 into tank 128 and enable internal phase 121 to pass through line 124 into tank 128. Depending on the operation of valves 118 and 126, internal phase 113 and internal phase 121 can be incrementally or continuously added to tank 128 to form phase 129 which is agitated by stirrer 130 and contains droplets having variable concentrations of internal phase in a continuous phase. It is believed that in addition to the droplets of phase 129 containing the internal phase concentration of phases 113 and 121, there may be an amount of intermixing of the respective emulsions as a result of droplet coalescence prior to wall formation. In still another embodiment, instead of mixing together two different internal phases, two different emulsions of a similar type (e.g. oil-in-water) may be mixed together to form a photosensitive material having a varying concentration of internal phase within the dispersed droplets. To accomplish this, a system such as that shown in Fig. 2 is used wherein composition 113 is a first emulsion having droplets of a first internal phase concentration dispersed in a continuous phase and composition 121 is a second emulsion having droplets of a second internal phase concentration dispersed in a continuous phase. To mix emulsions 113 and 121, valves 118 and 126 are opened to enable emulsions 113 and 121 respectively to pass into tank 128 and stirrer 130 is actuated. In most common encapsulation processes, the continuous phase is aqueous and an oil-in-water emulsion is used for producing the inventive photosensitive composition. However, as will be recognized in the art, the present invention may be practiced by using an organic phase and a water-in-oil emulsion. The continuous phase preferably includes agents known as emulsifiers and system modifiers to control the size and uniformity of the microcapsules and to produce individual mononuclear capsules in preference to clusters of microcapsules. Useful emulsifiers and system modifiers are well known in the art. Their selection will depend on the type of microencapsulation process used and the nature of the wall formers. For making melamine-formaldehyde microcapsules a combination of methylated polygalacturonic acid (e.g., pectin) and sulfonated polystyrenes is preferred. The polygalacturonic acid acts as a stabilizer for the aqueous phase, and the sulfonated polystyrenes aid in emulsification. See U.S. Patent No. 4,608,330. In addition, the concentration of the emulsifiers and modifiers in the aqueous phase may be selected to correspond to the amount of desired coalescence, and hence, intermixing of the dispersed droplets. As will be discussed below, for some applications an amount of droplet intermixing improves the photographic properties of the resultant microcapsules. The initial internal phase is designed so that it may be readily dispersed in the aqueous phase. The internal phase typically includes an image-forming agent, and a photohardenable or photosoftenable composition. Various image-forming agents can be used in the internal phase. For example, images can be formed by the interaction of color formers and color developers of the type conventionally used in the carbonless paper art. In addition, images can be formed by the color producing interaction of a chelating agent and a metal salt or by the reaction of certain oxidation-reduction reaction pairs, many of which have been investigated for use in pressure-sensitive carbonless papers. Alternatively, an oil soluble dye can be used and images can be formed by transfer to plain or treated paper. The internal phase itself has its own image-forming capability. For example, it is known that many of the toners used in xerographic recording processes selectively adhere to the image areas of an imaging sheet exposed and developed as in the present invention. Typical color-forming agents useful in the aforesaid embodiments include colorless electron donating type compounds. Representative examples of such color formers include substantially colorless compounds having in their partial skeleton a lactone, a lactam, a sulfone, a spiropyran, an ester or an amido structure such as triarylmethane compounds, bisphenylmethane compounds, xanthene compounds, fluorans, thiazine compounds and spiropyran compounds. Cyan, magenta and yellow colour precursors useful in practice of the present invention are particularly preferred and are commercially available. A positive working photohardenable or a negative working photosoftenable radiation sensitive composition is also present in the dispersed internal phases. Photohardenable compositions such as photopolymerizable and photocrosslinkable materials increase in viscosity or solidify upon exposure to radiation and yield positive images. Ethylenically unsaturated organic compounds are useful radiation curable materials. These compounds contain at least one terminal ethylene group per molecule. Typically, they are liquid. Polyethylenically unsaturated compounds having two or more terminal ethylene groups per molecule are preferred. An example of this preferred subgroup is ethylenically unsaturated acid esters of polyhydric alcohols, such as trimethylol propane triacrylate (TMPTA) and dipentaerythritol hydroxypentaacrylate (DPHPA). The internal phase also typically includes a photoinitiator. A preferred photoinitiator system for use in the present invention includes ionic dye-counter ion compounds described in European Application Publication No. 0 233 587. A preferred class of ionic dye-counter ions is cationic dye borates and still more particularly cyanine dye borates. Typically the borate is a triphenylalkyl borate such as a triphenylbutyl borate. Other dye complexes such as Rose Bengal iodonium and Rose Bengal pyryllium complexes may also be used. Examples of other photoinitiators useful in practice of the present invention include diaryl ketone derivatives, quinones and benzoin alkyl ethers. Where ultraviolet sensitivity is desired, suitable photoinitiators include alkoxy phenyl ketones, O-acylated oximinoketones, polycyclic quinones, phenanthrenequinone, naphthoquinone, diisopropylphenanthrenequinone, benzophenones and substituted benzophenones, xanthones, thioxanthones, halogenated compounds such as chlorosulfonyl and chloromethyl polynuclear aromatic compounds, chlorosulfonyl and chloromethyl heterocyclic compounds, chlorosulfonyl and chloromethyl benzophenones and fluorenones and haloalkanes. In many cases it is advantageous to use a combination with imaging photoinitiators. Initiators including the ionic dye complexes may preferably include an autoxidizer. Suitable examples include N,N,-dialkylanilines as described in the European Publication. It is preferred that the droplets of the internal phase do not completely mix after dispersing them in the continuous phase. Intermixing can be controlled through appropriate choice of the emulsifier, through the use of a pre-wall former as discussed below, or by controlling agitation conditions. The internal phase preferably contains a compound which is able to form a pre-wall material around the initial concentration internal phase when emulsified in the continuous phase. For example, a polyisocyanate may be contained in the internal phase. Upon dispersing the internal phase in an aqueous continuous phase, the polyisocyanate is believed to react with water at the interface of the internal phase in the aqueous medium and form a thin layer of a polyurethane polymer around the internal phase. The internal phase concentration modifying phase is used to incrementally or continuously modify the concentration of the internal phase prior to encapsulation. The material used to modify the concentration is miscible with the internal phase. In the preferred embodiment, the modifying material is used to dilute the concentration of the photoinitiator in the internal phase to produce a photosensitive material having a photoinitiator containing a concentration gradient. However, as will be discussed in greater detail below, the modifying material may function to alter other constituents of the internal phase. The modifying material in the preferred embodiment includes a photopolymerizable monomer and it is preferable that the monomer is identical to the monomer contained in the internal phase. Trimethylolpropane triacrylate is particularly preferred. The incremental or continuous addition of the monomer to the internal phase functions to lower the overall concentration of photoinitiator and image-forming agent in the modified internal phase. Alternatively, the modifying material may take the form of an inert liquid diluent which is miscible with the internal phase and is capable of lowering the concentration of the image-forming agent and photoinitiator when added to the internal phase. Examples of such inert liquids include mineral oil and propylene carbonate. In another embodiment, the modifying material may take the form of a co-initiator or autoxidizers. Co-initiators are known in the art and are compounds which interact with an absorber and directly or indirectly generate free radicals. Autoxidizers interact with the absorber and oxygen to increase film speed. When added to the internal phase, the co-initiator functions to increase the film speed of the internal phase. For example, when using a dye-borate system as defined in European Published Application 0 233 587, the co-initiator can be an N,Ndialkylaniline compound which enhances the film speed of the resultant composition. In yet another embodiment, the modifying material may take the form of a photoinhibitor or radical scavenger. When added to the internal phase, the photoinhibitor or radical scavenger functions to limit the rate of photopolymerization of the internal phase, and lower the film speed. Examples of photoinhibitors are radical scavengers are set out in commonly assigned U.S. Patent No. 4,587,194. In yet another embodiment, the modifying material may be an image-forming agent. It has been found that certain image-forming agents enhance or reduce the film speed of the photosensitive composition. Hence, by changing the concentration of the image-forming agent, the dynamic range can be improved. From the foregoing discussion, and particularly as discussed with respect to Figure 2, those skilled in the art will appreciate that an equivalent result may be achieved by forming a second emulsion containing the modified internal phase composition and adding this emulsion to the first emulsion containing droplets of an initial internal phase composition. After the concentration modifying procedures have been completed and the desired internal phases are dispersed in the continuous phase, microcapsule walls are formed around the internal phases. The discrete walled microcapsules formed around the different concentration internal phases in the present process can be produced using known encapsulation techniques including coacervation, in situ polymerization, interfacial polymerization, polymerization of one or more monomers in an oil. Representative examples of suitable wall-formers are gelatin materials (see U.S. Patent Nos. 2,730,456 and 2,800,457 to Green et al.) including gum arabic, polyvinyl alcohol, carboxy-methyl-cellulose; 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 Kiritani 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.); and melamine-formaldehyde resin and hydroxpropyl cellulose (see U.S. Patent No. 4,025,455 to Shackle). A melamine-formaldehyde capsule is particularly preferred. The mean microcapsule size used in the present process generally ranges from about 1 to 25 microns. In the preferred embodiment, melamine-formaldehyde capsule walls are formed around the internal phases. This process is now described in more detail. Formation of the melamine-formaldehyde capsule walls is obtained by adding solutions of melamine and formaldehyde or a solution of a precondensate thereof to the oil in water emulsion. On reaction, any amine-formaldehyde condensate separates from the solution which wets and enwraps the dispersed internal phase droplets. After the walls have been solidified, the capsules may be separated from the medium and washed. The condensation reaction proceeds under acid conditions, e.g., pH of 7 or less; however, the reaction is preferably carried out at a pH in the range of 2.5 to 6.0. The temperature of the encapsulation medium should be maintained at about 10 to 90°C, preferably about 25 to 85°C and more preferably about 45 to 75°C. Among the acid catalysts that may be used are low molecular weight carboxylic acids, e.g., formic acid, acetic acid; inorganic acids, e.g., sulfuric acid, hydrochloric acid, phosphoric acid; and acidic or easily hydrolyzable salts such as aluminum sulfate, ammonium nitrate, ammonium chloride, ammonium sulfate. Ammonium salts are preferred as they seem to provide a stronger and less permeable capsule. The ammonium salts are usually employed in an amount of about 2 to 20% by weight based on the amount of urea. The melamine and formaldehyde are preferably present, whether as a free monomer or as a precondensate, in a molar ratio of formaldehyde to melamine of at least 1.5 and preferably 2.0 to 3.0. To reduce malodor and the incidence of skin irritation, when the polymerization reaction is complete, it is desirable to remove or react the excess formaldehyde. This can be accomplished by using any one of several known techniques such as the addition of phosphoric acid, urea, sulfite, bisulfite, ammonium salts, 2-imidazolidone, ethylene-n-methylolacrylamide-vinylacetate copolymer, defatted soybean powder, isobutylene/maleic anhydride copolymer/ammonium salt, 5-methylbenzimidazole, dimethyloldihydroxyethyleneurea, carbohydrazide, water insoluble amino group containing polymers such as Amberlite IR45, 2-oxazolidone and dihydroxyacetone (dimer form). These materials react with the formaldehyde to form a product which does not interfere with the performance of the medium. The addition of urea or sodium sulfite to scavenge formaldehyde is preferably performed in a single step after encapsulation and prior to storage. The pH and temperature of the medium should be adjusted for this reaction. The sulfite is preferably reacted at a pH of 6 to 9 from room temperature to 85°C for about two hours. The urea can be reacted at a pH of 3 to 5 or 8 to 12 at a temperature of 30-60°C for four hours. To produce an imaging sheet the photosensitive composition is coated onto a substrate. The most common substrate for imaging sheets in accordance with the present process is a transparent film or paper. The paper may be a commercial impact raw stock, or special grade paper such as cast-coated paper or chrome-rolled paper. Transparent substrates such as polyethylene terephthalate and translucent substrates can also be used. Another preferred substrate for the microcapsules is aluminized polyethylene terephthalate. The microcapsules can be located on either the top or bottom surface of a transparent substrate to form an imaging sheet. To form an image, the substrate is exposed to actinic radiation such that the microcapsules are image-wise exposed. Typically, exposure of the microcapsules to actinic radiation (ultraviolet, visible or infrared wavelengths) causes the internal phase of the microcapsules to polymerize, thereby preventing escape of the image-forming agent from the imaging sheet. Thereafter, the microcapsules on the imaging sheet are subjected to a uniform rupturing force in the presence of a developer material to rupture the unexposed microcapsules and form an image. Where the image-forming agent is not a dye or pigment, a developer material is selected to react with the image-forming agent to form an image. Illustrative examples of color developers useful in conjunction with the embodiment employing the aforesaid color precursors are clay minerals such as acid clay, active clay, attapulgite; organic acids such as tannic acid, gallic acid, propyl gallate; 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; metal salts or aromatic carboxylic acids such as zinc salicylate, tin salicylate, zinc 2-hydroxynaphthoate, zinc 3,5-di-tert-butyl 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 resin as disclosed in U.S. Patent No. 3,732,120, zinc carbonate and mixtures thereof. A preferred class of glossable developers is described in our European Patent Application No, 87307985.9 (Publication No. 0260129). The extended dynamic range microcapsules may be used to produce monochromatic or full colour images. Full-colour imaging systems are described in U.S. Patent No. 4,576,891, European Published Application No. 0233587 and U.K. Patent No. 2,113,860. These systems employ imaging sheets having three sets of microcapsules which respectively contain cyan, magenta and yellow colour precursors. As explained in more detail in the above-listed references, each set of microcapsules is primarily sensitive in a different wavelength band such that the microcapsules can be individually exposed with minimum cross-talk. In panchromatic systems, the cyan, magenta, and yellow forming photosensitive microcapsules are respectively sensitive to red, green and blue light. The photosensitive microcapsules resulting from the present process may have an extended dynamic range. It is hypothesized that by emulsifying the different concentration internal phases prior to encapsulation, the different concentration internal phases may coalesce and hence slightly intermix. This provides a batch of microcapsules having a concentration gradient of a characteristic of the internal phase of the microcapsules. For example, in the preferred embodiment the microcapsules may contain a concentration gradient of photoinitiator within their internal phases. As will be shown in the examples, this results in a continuous H&D curve having an extended dynamic range. By comparison, when mixing two independently prepared batches of different speed, same image-forming microcapsules to obtain microcapsules having an extended dynamic range because there is no droplet coalescence, the internal phases of the microcapsules will never combine. As a result, a concentration gradient is not demonstrated and the slope of the resulting H&D curve is less continuous. Thus, in addition to producing extended dynamic range microcapsules in a single batch versus a multiple batch process, the photographic qualities of similar microcapsules produced according to the present process are superior. Intermixing may be controlled by optimizing the amount and presence of a pre-wall material, emulsion stabilizers and modifiers, and agitation conditions. Droplet intermixing is particularly preferred when the inventive process is performed on a batch-wise basis. In addition, when the photosensitive microcapsules are blended with two other sets of microcapsules to produce a full-colour imaging material, improved colour balance can be obtained. A problem associated with full-colour imaging materials is that the H&D curves for the cyan, magenta and yellow colour-forming microcapsules do not always properly align, resulting in images having colours which are not balanced. If one or two of the H&D curves need to be aligned, photosensitive microcapsules produced according to the present process may be utilized and selected so that their corresponding H&D curve aligns with the H&D curves which were previously in alignment. Once the H&D curves of the three colour-forming microcapsules are properly aligned, the colours in the produced images will be balanced. An example of the use of our microcapsules for this application is set forth in Example 2. The present invention is illustrated in more detail by the following non-limiting examples. Example 1 To produce magenta colour-forming microcapsules, the following internal phase was added to a 80 litre tank equipped with a stirrer operating at 550 rpm: Trimethylolpropane triacrylate (TMPTA) 14.54 kg 1,1′-di-n-heptyl-3,3,3′,3′-tetramethyl indocarbocyanine triphenyl-n-butylborate 63 grams N,N-dimethyl-2,6-diisopropylanaline (DIDMA) 145 grams Magenta color former 4.32 kg Desmodur N-100 isocyanates .97 kg 2,6-di-tert-butyl-4-methylphenol (BHT) 3.63 grams 61.90% by weight of the above internal phase was added to an emulsion tank containing the following continuous phase: Water 51.6 kg Pectin 1.52 kg Versa 503 0.96 kg and was stirred between 1210 and 3450 rpm at 25°C. The dispersed internal phase constitutes approximately 50% of the total internal phase to be emulsified and has a relatively fast film-speed. In a separate tank having a volume of 51, and being equipped with stirrer operating at 1000 rpm, the following modifying phase was added: TMPTA 3.42 kg DIDMA 34.63 grams Desmodur N-100 Isocyanates .23 kg The modifying phase was then added in its entirety to the tank containing the remaining internal phase to dilute the concentration of photoinitiator in the internal phase and was stirred at 350 rpm. The remainder of the now diluted internal phase was then added to the emulsion tank and was stirred between 66 and 4300 rpm at 36°C. The emulsion tank contained approximately 50% of a higher film speed, higher concentration magenta color-forming internal phase (high concentration of color-former and initiator) and approximately 50% of a lower film speed, lower concentration magenta color-forming internal phase (low concentration of color-former and initiator). Melamine formaldehyde microcapsules were formed around the dispersed internal phase by in situ condensation The microcapsules were coated on an aluminized polyethylene terephthalate film exposed to green light (λ= 550 nm) and developed in the presence of a developer material. The H&D curve for the magenta color-forming microcapsules was obtained and is shown in Fig. 3. In comparison to magenta color-forming microcapsules blended from two independent batches of microcapsules, the slope of the H&D curve for the microcapsules is smooth and does not include a hump which characterizes the H&D curve of a blend. Example 2 The magenta color-forming microcapsules produced according to Example 1 were mixed with cyan color-forming microcapsules and yellow color-forming microcapsules and coated onto polyethylene terephthalate film. The microcapsules were exposed to a broad band visible white light source and ruptured in the presence of a developer material to produce a full color image. The RGB curve produced had the following characteristics. RGB Curve White Light Data R G B log 10% 3.29 3.29 3.29 log 90% 2.86 2.88 2.86 log 1.0 3.05 3.06 3.04 As demonstrated by the RGB curve data, the H&D curves for the three color-forming microcapsules were properly aligned and the microcapsules produced color balanced images. In comparison to H&D curves produced when using three colour-forming microcapsules wherein the magenta capsules comprised a mixture of two photoinitiator concentration capsules, the curves produced by the present microcapsules were much more in alignment.

Claims: A process for producing microencapsulated photosensitive composition comprising the steps of: (a) providing an emulsion of a multiplicity of droplets of an internal phase in a continuous phase, and (b) forming a wall around said droplets; characterised in that said internal phase includes a photohardenable or photosoftenable composition and an image-forming agent; the concentration of one of the constituents of said internal phase in said emulsion incrementally or continuously varying. A process according to Claim 1, further characterised in that said internal phase includes a photohardenable composition and said photohardenable composition includes a photopolymerizable monomer and a photoinitiator. A process according to Claim 2, further characterised in that the concentration of said photoinitiator incrementally or continuously varies in said droplets. A process according to Claim 3, further characterised in that the step of providing said emulsion comprises the steps of: dispersing a portion of a first internal phase comprising a first concentration of monomer, said image-forming agent, and said photoinitiator in said continuous phase; diluting a second portion of said first internal phase with an internal phase modifying material to form a second internal phase having a second concentration of said photohardenable monomer, said image-forming agent, and photoinitiator; and dispersing said second internal phase in said continuous phase. A process according to Claim 4, further characterised in that the step of providing said emulsion comprises incrementally or continuously diluting and dispersing said internal phases. A process according to any of Claims 1 to 3, further characterised in that the step of providing an emulsion comprises: dispersing a portion of a first internal phase composition in a continuous phase; adding an internal phase modifying material to a second portion of said first internal phase to form a second internal phase; and dispersing said second internal phase into said continuous phase. A process according to Claim 6, further characterised in that the step of providing an emulsion comprises incrementally or continuously modifying and dispersing said internal phases. A process according to Claims 6 or 7, further characterised in that said internal phase modifying material is an absorber compound, a co-initiator, or an autoxidizer. A process according to Claims 6 or 7, further characterised in that said modifying material is a monomer. A process according to Claim 9, further characterised in that said monomer is the same as said photohardenable monomer. A process according to Claims 6 or 7, further characterised in that said internal phase modifying material is an image-forming agent. A process according to any of Claims 1 to 3, further characterised in that the step of providing an emulsion comprises: providing a first emulsion comprising a multiplicity of droplets of a first internal phase having a first composition in a continuous phase; providing a second emulsion comprising a multiplicity of droplets of a second internal phase having a second composition in a continuous phase; and mixing said first emulsion and said second emulsion. A process according to any of Claims 1 to 3, further characterised in that the step of providing an emulsion comprises: dispersing a first internal phase having a first composition in a continuous phase; and dispersing a second internal phase having a second composition in said continuous phase. A process according to any preceding claim, further characterised in comprising the additional step of: (c) mixing said microcapsules produced in step (b) with a second set of microcapsules containing a second image-forming agent and with a third set of microcapsules containing a third image-forming agent; the photosensitive composition produced by step (c) being capable of providing full colour images. A process according to Claim 14, further characterised in that the three different image-forming agents comprise a cyan colour-forming agent, a magenta colour-forming agent and a yellow colour-forming agent. A process according to any preceding claim, further characterised in compromising the additional step of permitting coalescence of said droplets to allow an amount of droplet intermixing; said additional step occurring prior to said wall forming step. Photosensitive material comprising a substrate formed of paper or of a polymeric film and a photosensitive composition coated thereon as a layer of microcapsules, characterised in that said composition is formed by a process according to any preceding claim, and in that said varying internal phase composition provides for broadened dynamic range. Material according to Claim 17, further characterised in that said composition comprises microcapsules having variable film speeds. Material according to Claims 17 or 18, further characterised in that said composition is sensitive to visible light.

Inventors: HAMMANN WILLIAM A, KOVACS KERRY, HAMMANN, WILLIAM A., KOVACS, KERRY
Applicants: MEAD CORP, THE MEAD CORPORATION
Application year: 1989
Main classification: G03F7
IPC codes: B01J13, G03F7