GB2159973A - Microcapsules - Google Patents

Abstract

Photosensitive microcapsules have an internal phase including a photohardenable composition and a photo inhibitor precursor. A photosensitive imaging sheet comprises a support with a layer of the said microcapsules on one surface thereof. Imaging is performed in one arrangement by image-wise exposing such an imaging sheet to a first radiation, the imaging sheet having an image-forming agent associated with the microcapsules. The first radiation image-wise converts the precursor to a photohardening inhibitor. The image-wise exposed sheet is then uniformly exposed to a second radiation different from the first. This second radiation causes the photohardenable composition to harden in those areas in which hardening is not inhibited by the inhibitor. The imaging sheet is then subjected to a uniform rupturing force. The image-forming agent is activated and a negative image is formed.

Description

SPECIFICATION Microcapsules The present invention relates to microcapsules.

More particularly it relates to photosensitive microcapsules and to their use in imaging systems.

Photosensitive imaging systems employing microencapsulated radiation sensitive compositions are described in U.S. Patents 4,399,209 and 4,416,966 assigned to the Mead Corporation. These imaging systems are characterized in that the imaging sheet includes a layer of microcapsules containing a photosensitive composition in the internal phase. In the most typical embodiments, the photosensitive composition includes an ethylenically unsaturated compound, such as trimethylolpropane triacrylate (TMPTA), and a photoinitiator, and a colour former is encapsulated with the photosensitive composition. Images are formed by imagewise exposing the imaging sheet to actinic radiation and subjecting the microcapsules to a uniform rupturing force in the presence of a developer material. In the exposed areas the internal phase is hardened and the colour former is not released.In the unexposed areas, the internal phase remains liquid and, upon rupturing the microcapsules, they release the colour former whereupon it reacts with the developer material and forms a coloured image. In areas of intermediate exposure, the color former may be released to an intermediate extent depending upon the degree of exposure.

U.S. Patent No. 4,399,209 discloses a transfer imaging system in which the imaging sheet is assembled with a developer sheet following exposure and passed through the nip between a pair of pressure rollers to rupture the microcapsules and develop the image.The images are formed by image-wise transfer of the color former from the imaging sheet to the developer sheet where the color former reacts to form an image.

U.S. Patent No. 4,416,966 discloses a self-contained imaging system in which the developer material is present on the same surface of the support as the photosensitive microcapsules. After exposure, upon passing the imaging sheet through the pressure rolls, the microcapsules rupture and image-wise release the internal phase. Thereupon, the color former migrates to the developer, which is usually provided in a separate layer, where it reacts and forms a color image.

Imaging systems of the aforementioned type, which employ ethylenically unsaturated compounds or other photohardenable materials in the internal phase are, by nature, positive- working systems. By "positive-working" is meant that the image areas in the copy and the original correspond to one another. In certain applications it would be desirable to use a negative-working system and form negative images. One negativeworking system is described in the aforementioned patents wherein it is disclosed that photosoftenable materials can be incorporated in the internal phase as the photosensitive composition. Photosoftenable materials soften upon exposure to actinic radiation and thereby produce an image in areas corresponding to the transparent or non-image areas of the original. Our aforementioned U.S.

Patent No. 4,416,966 also discloses a negative working microencapsulated imaging system in which the microcapsules contain a decolourizing agent instead of a colour former. The decolourizing agent bleaches or otherwise prevents the formation of a colour image in the exposed areas.

We describe below a novel system employing microcapsules containing a combination of a photohardenable composition and a photoinhibitor precursor in the internal phase. The precursor does not undesirably inhibit photohardening reactions such as free-radical polymerization. However, upon exposure of the precursor to predetermined radiation, the precursor generates a compound which inhibits photohardening of the internal phase of the microcapsules. Thus, negative images can be formed by preparing an imaging sheet comprising a substrate having a layer of the aforementioned microcapsules on the surface thereof and, first, image-wise exposing the microcapsules to radiation which converts the precursor to a photoinhibitor.

Photohardening of the internal phase is thereby inhibited in the exposed areas. In the unexposed areas, the precursor remains inactive and photohardening can proceed in the usual manner upon exposure to actinic radiation.

After subjecting the layer of microcapsules to the image- wise exposure, the microcapsules are uniformly exposed to a second radiation. This radiation is selected such that it does not activate the photoinhibitor but photohardens the internal phase of the microcapsules. After the second exposure, the microcapsules in the areas which received the first exposure remain rupturable because the photoinhibitor is generated in these areas and hardening is prevented. On the other hand, the microcapsules in the areas which did not receive the first exposure are hardened. Thus, upon subjecting the twice exposed imaging sheet to a uniform rupturing force, the microcapsules in the first exposed areas rupture and release the internal phase.In those embodiments in which the microcapsules contain a color former, the color former is released in the first exposed areas and reacts with the developer (which may be present on the same or a different support) to form a negative image. In the areas which were not exposed during the first exposure, the microcapsules harden and do not release the internal phase. In areas of intermediate exposure, the microcapsules may be hardened to an intermediate extent and release the color former in proportion to the degree of first exposure.

Above, the imaging sheet is used to produce negative images. It can also be used in a positive mode. That is, if the imaging sheet is not subjected to an exposure which converts the precursor to an inhibitor, the imaging sheet can be used in the previous manner and simply image-wise exposed to radiation which hardens the internal phase to form positive images.

Thus, in one aspect thereof, the present invention resides in photosensitive microcapsules hav ing an internal phase including a photohardenable composition and a photoinhibitor precursor.

In a second and alternative aspect thereof, the invention provides a photosensitive imaging sheet comprising a support, and a layer of photosensitive microcapsules on a surface thereof, said microcapsules including an internal phase containing a photohardenable composition and a photoinhibitor precursor.

The microcapsules preferably have an imageforming agent associated therewith.

According to a third alternative aspect of the invention, there is provided a process for imaging which comprises the steps of: image-wise exposing an imaging sheet to a first radiation, said imaging sheet including a support having a layer of microcapsules on a surfact therefor, said microcapsules having an image-forming agent associated therewith and including an internal phase containing a photohardenable composition and a photoinhibitor precursor, said first radiation image-wise converting said precursor to an inhibitor of said photohardening; uniformly exposing said image-wise exposed imaging sheet to a second radiation different than said first, said second radiation causing said photohardenable composition to harden in those areas in which hardening is not inhibited by said inhibitor; and subjecting said imaging sheet to a uniform rupturing force wherein said image-forming agent is activated and a negative image is formed.

The photohardenable composition is preferably a photooolyermizable composition containing an ethylenically unsaturated compound and the precursor generates an inhibitor for free radical polymerization.

The term "microcapsule" as used herein includes both microcapsules having a discrete capsule wall and open phase microcapsules formed by dispersing the photohardenable composition in combination with the photoinhibitor in an appropriate binder.

The term "harden" as used herein means to increase the viscosity of the internal phase of the microcapsules and is not necessarily limited to solidifying the internal phase.

The invention is hereinafter more particularly described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a schematic cross-sectional view of an embodiment of imaging sheet in accordance with the present invention; Figure 2 is a cross-sectional schematic view of the imaging sheet of Figure 1 which has been image-wise exposed in a negative-working fashion; Figure 3 is a cross-section schematic view of the imaging sheet of Figure 2 following uniform exposure; and Figure 4 is a cross-sectional schematic view of a developer sheet bearing a negative image obtained by transfer development from the sheet of Figure 3.

The embodiment of imaging sheet 10 shown in Figure 1 includes a support 12, which may be transparent or opaque, and a layer 14 of photosensitive microcapsules 16 containing at least a photohardenable composition and a precursor designated by the letter P in the internal phase 18.

An image-forming agent such as a colour former or a colored dye may be associated with the capsules such that when the capsules rupture and release the internal phase, the image-forming agent is mobilized or otherwise activated. Prior to imaging, the internal phase 18 is typically liquid.

In accordance with one embodiment of the present invention, negative images are formed by first image-wise exposing the imaging sheet 10 to radiation which converts the precursor P to an inhibitor (I). Figure 2 illustrates an imaging sheet which has been image-wise exposed to radiation which activates the agent P. The exposed areas of the imaging sheet are designated by the reference numeral 20 and the unexposed areas are designated by the reference numeral 22. In the exposed areas 20 the precursor is converted by the exposure radiation to an inhibitor designated by the letter I. In the unexposed areas 22, the precursor remains in its non-inhibiting form.

The image-wise exposure is carried out using a radiation source, intensity, or wavelength which generates the inhibitor I without initiating photohardening of the internal phase. Usually, a precursor and a photoinitiator are selected which are active at distinct wavelengths. For example, the extinction coefficient of the precursor may be substantially higher than the extinction coefficient of the photoinitiator at the wavelength of the first exposure whereas the reverse may be true at the wavelength of the second exposure. Thus, by appropriately selecting the precursor and inhibitor and controlling the intensity and wavelength of the exposures, the inhibitor can be generated without hardening the internal phase and the internal phase can be hardened without generating the inhibitor.

The image-wise exposure can be conducted in a conventional manner such as by exposure through a photomask, by reflection imaging, or by using a laser or a cathode ray tube. In accordance with one embodiment of the invention, a thermally sensitive precursor which can be activated by exposure with an infrared laser is used. Infrared lasers are advantageous because they are less expensive than other laser sources such as blue or ultraviolet emitting lasers.

Following the first exposure, the imaging sheet is subjected to uniform exposure with radiation which is capable of hardening the photohardenable composition in the capsules without converting the precursor P to the inhibitor l.As shown in Figure 3, in the areas 22 in which the precursor P remains in its non-inhibiting form, the uniform exposure hardens or otherwise increases in viscosity of the internal phase of the microcapsules.While the microcapsules are illustrated as containing a solid internal phase in the areas 22, in fact, the internal phase may be highly viscous or semi-solid provided it is sufficiently viscous to prevent image-formation by release of the internal phase from these areas.In the areas 20, where the precursor is converted to its inhibiting form I, photohardening of the internal phase 18 is prevented and the microcapsules remain capable of being ruptured and releasing the internal phase.

Following uniform exposure, the microcapsules are subjected to a uniform rupturing force whereupon the microcapsules image- wise release the internal phase 18 from the areas 20. This may be accomplished by passing the imaging sheet of Figure 3 through a pressure nip in contact with a developer sheet. Alternatively, the microcapsules can be designed to be ruptured by other forces such as ultrasonic vibration, burnishing, friction, peeling development, heating, etc. Figure 4 illustrates a developer sheet 30 containing a negative image produced in this manner. The developer sheet 30 includes a support 32 and a developer layer 34 in which negative images 36 are formed in the areas corresponding to the exposed areas 20 of the imaging sheet.

Figures 1-4 illustrate just one of the ways in which microcapsules and imaging systems in accordance with the present invention may be employed. The imaging system can also be used in a positive mode by omitting the first exposure and shielding the imaging material from radiation which converts the precursor to the inhibitor form.By image-wise exposing the imaging sheet as described in U.S. Patents 4,399,209 and 4,416,966, positive images can then be formed. This could be particularly valuable in reader-printers where the user could have the option of producing a positive or negative image.

Our microcapsules include a precursor for a photoinhibitor in the internal phase. As previously noted, this is a compound which does not inhibit photohardening of the microcapsules but which generates a compound which inhibits photohardening of the microcapsules upon exposure to appropriate radiation. In the most typical case, the agent generates a compound which inhibits free radical polymerization. Many inhibitor precursors exhibit a small inhibitory effect on photohardening.

As long as this effect is small, however, such precursors can be used in practice of the present invention.

The precursors used in practice of the present invention can be selected from among those precursors which are known in the art and have been previously used in photopolymerizable compositions. Typical examples of such agents are disclosed in U.S. Patents Nos. 4,029,505; 4,168,981; 4,168,982; 4,198,242; and 4,269,933. Precursors having high extinction coefficients, e.g., greater than 100 M-' cm-' and more preferably greater than 1000 M-5cm ' are preferred.

An example of a useful class of photoinhibitor precursors is nitroaromatic compounds in which the nitro group is ortho to a hydrogen-bearing alpha-carbon substituent. These compounds photochemically rearrange to nitrosoaromatic inhibitors of free radical polymerization upon exposure to radiation having a wavelength of about 200-380 nm.

Representative examples include 2-nitro-4,5-dimethoxybenzaldehyde, 3-methoxy-2-nitrobenzaldehyde, 4-methoxy-2-nitrobenzaldehyde, 3,4-dimethoxy-2-nitrobenzaldehyde, 4-cyano-2 nitrobenzaldehyde, 6-nitroveratraldehyde, 6-n itropi- peronal etc. Other examples of such o-nitroaromatic compounds are described in U.S Patent 4,198,242 the disclosure of which is herein incorporated by reference.

Nitroaromatic compounds are preferably used in combination with photoinitiators which have an active radiation absorption band with a molar extinction coefficient of at least 50 in the range of 400 to 600 nm, such as phenanthraquinones, and the ketocoumarins as reported in U.S. Patent No.

4,147,552. They are preferably used in an amount of 0.04 to about 0.15 parts per part by weight of the photohardenable species in the photohardenable composition.

The nitroso compounds formed by irradiation of the nitroaromatic compounds with short wavelength radiation interfere with the normal free-radical induced polymerization process. Thus, when using the shorter wavelength region of the spectrum in the presence of a nitrosoaromatic compound, an insufficient number of initiating and propagating free radicals is available, and polymerization does not occur. When a microcapsule of this invention is exposed to radiation of wavelength greater than about 380 nm, the nitroaromatic compound is relatively unaffected, and the photoinitiator system operates to produce initiating radicals. These radicals are able to effect chain propagation in the usual way and polymerization occurs.

Another group of useful precursors are nitroso dimers which are non-inhibitors of free-radical polymerization but thermally dissociate to nitroso monomers which inhibit free-radical polymerization. Typical examples of such compounds are described in U.S. Patent No. 4,168,982, which is incorporated herein by reference. In general, these nitroso dimers have nitroso groups attached to primary or secondary aliphatic or alicyclic carbon atoms, to a six-membered aromatic ring or to the beta carbon atom of a vinyl group attached to a six-membered aromatic ring.Representative examples include 1-nitrosooctadecane dimer, nitrosocyclohexane dimer, nitrosododecane dimer, 2nitroso-2,4-dimethyl-3-pentanone dimer, di-t-butylnitrosomethane dimer, and the like.These compounds are converted to inhibitors upon exposure to infrared radiation or heat and are preferably used in an amount of 0.1 to 10 parts per 100 parts of the photohardenable composition. These compounds can be incorporated into microcapsules to provide an imaging system which is useful in forming negative images by first exposing the microcapsules to infrared radiation such as emitted by an infrared laser. Thereafter, the microcapsules can be uniformly exposed to visible or ultraviolet radiation which image-wise photohardens the internal phase.

Another group of useful precursors are bis(substituted amino) monosulfides or polysulfides. Typical examples are bis(substituted amino)sulfides which are substituted with alkyl groups containing 1 to 12 carbon atoms and the like. Examples of such compounds are described in U.S. Patent No. 4,168,981, which is incorporated herein by reference. Representative examples include bis(2,2,6,6-tetramethylpiperidino) disulfide, bis(dicyclohexylamino) -disulfide, bis(diphenylamino)disulfide, bisfpiperidino)-disulfide, bis(morpholino)disulfide, bis(piperidino) trisulfide, and bis(4-oxo-2,2,6,6-tetramethyl piperidino) tetrasulfide.These compounds generate radicals upon heating which inhibit free radical polymerization.

The precursor must be used in an amount such that the amount of the inhibitor generated upon its image-wise exposure is sufficient to prevent photohardening. The amount will vary with the efficiency with which the precursor generates the inhibitor, the intensity of the first exposure radiation that is desired, the time between the image-wise and uniform exposures, and the half-life of the inhibitor.

Higher amounts of precursor within the aforementioned ranges are generally preferred.

The photopolymerizable compositions, imageforming agents, wall formers, encapsulation and development techniques described in U.S. Patents No. 4,399,209 and 4,416,966 are useful in the present systems. Those patents are hereby incorporated by reference.

The photohardenable compositions used in of the present system can be designed to be sensitive to ultraviolet, infrared, visible, X-ray, ion beam radiation or the like. This is accomplished through a judicious selection of the photohardenable composition and the photoinitiator.

The photohardenable compositions used in the present system can be selected from among photohardenable compositions which are well known in the art.The most typical examples of such compositions are compositions including ethylenically unsaturated compounds.These compounds contain at least one terminal ethylene group per molecule.

Typically, liquid ethylenically unsaturated compounds having two or more terminal ethylene groups per molecule are preferred. Examples of this preferred subgroup are ethylenically unsaturated acid esters of polyhydric alcohols such as ethylene glycol dimethacrylate, triethylene glycol dimethacrylate, trimethylolpropane triacrylate (TMPTA) and trimethyolpropane trimethacrylate (TMPTMA). Another example of a useful radiation sensitive composition is an acrylate prepolymer derived from the partial reaction of pentaerythritol with acrylic acid, methacrylic acid, or acrylic or methacrylic acid esters.Another group of substances useful as photohardenable compositions include isocyanate prepolymers modified with acrylic, methacrylic and itaconic acid esters of polyhydric alcohols.

Photopolymerizable prepolymers are also useful in the present system Suitable prepolymers can be selected from commercially available acrylate terminated polyesters and polyethers. Typically, these compounds are prepared by end capping isocyanate terminated prepolymers with acrylic or methacrylic acid. The prepolymers can range up to about 16,000 in molecular weight, but in most cases do not exceed about 1,000 to 3,000 in molecular weight. If the molecular weight of the prepolymer is too high, it may be too viscous to be adequately emulsified in a continuous phase such as water for encapsulation. However, higher molecular weight prepolymers can be used in the present system if they are diluted with low molecular weight reactive monomers such as TMPTA.

Representative examples of acrylate terminated urethane prepolymers that should be useful in the present system include Chempol 19-4832 and Chempol 19-4833 available from Freeman Chemical Corporation; Uvithane 893, 788, 782 and 783 available from Thiokol Corporation; Ebecryl 220, 204, 210 and 240 available from Virginia Chemicals, Inc.; etc. Examples of epoxy acrylate prepolymers include Chempol 19-4824 and Chempol 19- 4825 from Freeman Chemical Corp.; Celrad 3200, 3700 and 3701 from Celanese Corp.; Ebecryl 600 series prepolymers from Virginia Chemicals, Inc.; etc.

In most cases, the photohardenable composition includes a photoinitiator. It is possible to use either homolytic photoinitiators which are converted to an active species by radiation and generate a radical by abstracting hydrogen from a hydrogen donor, or photoinitiators which complex with a sensitizer to produce a free radical generating species, or photoinitiators which otherwise generate radicals in the presence of a sensitizer. If the system relies upon ionic polymerization, the photoinitiator may be the anion or cation generating type, depending on the nature of the polymerization.

Examples of photoinitiators useful in the present system include diary ketone derivatives, and benzoin alkyl ethers. The photoinitiator is selected based on the sensitivity of the system that is desired. Where ultraviolet sensitivity is desired, suitable photoinitiators include alkoxy phenyl ketones, O-acylated oximinoketones, polycyclic quinones, 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 of imaging photoinitiators. 3-substituted Coumarin compounds such as described in U.S. Patent 4,147,552 are also useful herein. For ultraviolet sensitivity a combination of Michler's ketone and benzoin methyl ether (ratio 2:5) may be used. Another useful combination is 2,2'-dimethoxy-2-phenylacetophenone, isopropylxanthone and ethyl paradimethylaminobenzoate. The latter is preferably used with TMPTA to provide a radiation sensitive composition.

The amount of photoinitiator used in the photosensitive composition depends on the particular photosensitive material selected. It must be present in an amount sufficient to initiate the photochemistry within a short exposure time. The photoinitiator may also be used to sequester oxygen, which is present in the microcapsules and in hibits photopolymerization, by conducting a nonimaging, oxygen sequestering pre-exposure or coexposure. When the photoinitiator is also relied upon to sequester oxygen, it must be used in amounts sufficient to fulfill both this function and its imaging function.

To obtain improved film speed, it may be desirable to incorporate certain prepolymers in the photosensitive microcapsules.One such prepolymer is diallyl-O-phthalate prepolymer.

Prepolymers enhance film speed by accelerating the rate with which the viscosity of the internal phase increases upon exposure.

It has been found that technically an imageforming agent is not obligatory in the present system because there is a change in the light transmitting properties of photohardened microcapsules which results in a detectable image. In the most typical embodiments, however, an image-forming agent is associated with the microcapsules. The image-forming agent may be a colored dye such as a benign dye which does not detrimentally interfere with exposure and which produces a color (including black) image upon transfer to a receptor sheet such as plain paper. In the most typical case, however, the image- forming agent is a chromogen, such as a substantially electron donating colorless compound, which reacts with a developer to produce a color image.

The image-forming agent may be associated with the microcapsules in a variety of ways such that upon exposure of the microcapsules, the image-wise release of the internal phase controls the mobilization or activation of the image-forming agent.ln the most typical case, the image-forming agent is incorporated in the internal phase of the microcapsule. However, other constructions are possible. For example, the image-forming agent may be incorporated into the capsule wall of a microcapsule having a discrete wall or it may be incorporated in a layer contiguous with the microcapsule layer such that the image-forming agent is dissolved by the internal phase as it is released from the microcapsule and transported to the developer layer.

In the most typical case, the image-forming agent is a substantially colorless electron donating compound. Representative examples of such compounds 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, Xl, and XX (products of Hilton-Davis Chemical Co.) are often used alone or in combination as color precursors in the present system.

Images can also be formed by encapsulating a chelating agent, as a chromogenic material, which reacts with a metal salt, as a developer, to generate a color image upon being released from the microcapsules. Some typical examples of useful image- forming pairs of this type are nickel nitrate and N,N' bis(2- octanoyloxethyl)-dithiooxamide, and alum [ Fe(lll) ] and yellow prussiate.

The internal phase may also include a diluent oil.

Inclusion of the oil will often improve half tone gradation. Examples of carrier oils are alkylated biphenyls (e.g., monoisopropylbiphenyl), polychlorinated biphenyls, castor oil, mineral oil, deodorized kerosene, naphthenic mineral oils, dibutyl phthalate, dibutyl fumerate, brominated paraffin and mixtures thereof. Alkylated biphenyls are generally less toxic and preferred.

The photosensitive microcapsules used in the present system are easily formed using conventional techniques such as coacervation, liquid-liquid phase separation, interfacial polymerization and the like. Various melting, dispersing and cooling methods may also be used.

The photosensitive compositions are usually oleophilic and therefore preferably encapsulated in hydrophilic wall-forming materials such as gelatintype materials (see U.S. Patent Nos. 2,730,456 and 2,800,457 to Green et al) including gum arabic, polyvinyl alcohol, carboxymethyl-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 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); and melamine-formaldehyde resin and hydroxypropyl cellulose (see United States Patent No. 4,025,455 to Shackle).

When the image-forming agent is an electron donating compound, a developer which accepts electrons must be associated with the imaging system. The developer can be carried on the same support as the microcapsules or on a separate support.

Reactive developers can be selected from among the developers conventionally used in carbonless paper including acid clay, active clay, attapulgite, etc.; organic acids such as tannic acid, gallic acid, propyl gallate; aromatic carboxylic acids such as benzoic acid, p-tert-butyl-benzoic acid, 4-methyl- 3nitrobenzoic acid, salicylic acid, 3-phenyl salicylic acid, 3- cyclohexyl salicylic acid, 3-tert-butyl-5methyl salicylic acid, 3,5-di-tert-butyl salicylic acid, 3-methyl-5-benzyl salicylic acid, 3-phenyl-5-(#, -di- methylbenzyl)salicylic acid, 3- cyclohexyl-5- a, methylbenzyl)salicylic acid, 3-(a, a-dimethylbenzyl)5-methyl salicylic acid, 3,5-di-cyclohexyl salicylic acid, 3,5-di-(cu-methylbenzyl) salicylic acid, 3,5- di (cow, ee-dimethylbenzyl)salicylic acid, 3-(a-methylben zyl)-5-((x, cu-dimethylbenzyl) salicylic acid, 4-methyl5-cyclohexyl salicylic acid, 2-hydroxy-1-benzyl- 3naphthoic acid, 1 -benzoyl-2-hyd roxy-3-naphthoic acid, 3- hydroxy-5-cyclohexyl-2-naphthoic acid and the like, and polyvalent metallic salts thereof such as zinc salts, aluminum salts, magnesium salts, calcium salts and cobalt salts as disclosed in U.S.

Patents 3,864,146; 3,924,027 and 3,983,292; phenol compounds such as 6,6' -methylenebis-(4-chlorom-cresol) as disclosed in Japanese Patent Publica tions 9,309 of 1965 and 20,144 of 1967, and Japanese Laid Open Patent Publication No. 14,409 of 1973; phenol resins such as phenol-aldehyde resins e.g., p-phenyl-phenol-formaldehyde resin and phenol-acetylene resins, e.g., p-tert-butylphenolacetylene resin, and polyvalent metallic salts thereof such as zinc modified phenyl formaldehyde resin as disclosed in U.S. Patent 3,732,120; acid polymers such as maleic acid-rosin resin and copolymers of maleic anhydride with styrene, ethylene or vinylmethylether; and aromatic carboxylic acid-aldehyde polymers, aromatic carboxylic acidacetylene polymers and their polyvalent metallic salts as disclosed in U.S. Patents 3,767,449 and 3,772,052.The aforementioned developers are applied to the developer sheet support or the imaging sheet in a conventional manner to obtain transfer of self-contained imaging systems. They may be mixed with a binder latex, polyvinyl alcohol, maleic anhydride styrene copolymer, starch, gum arabic, etc., and coated on a substrate such as paper or coated directly.

The present invention is illustrated in more detail by the following non-limiting examples: Example 1 Urea-formaldehyde (UF) microcapsules were produced which contained the following internal phase: 50 g TMPTA; 0.1 g 7- diethylamino-3-cinnamoylcoumarin; 1.0 g ethyl-p-dimethylaminobenzoate (Quanticure EDP); 1.0 g 6nitroveratraldehyde; 3 g crystal violet lactone. The microcapsules were coated with a g10 Meyer bar on 80 1 b B & W raw stock (a product of the Mead Corporation) after a 1:1 dilution of the emulsion with 10% Triton X-100 in water to prepare an imaging sheet. 6-Nitroveratraldehyde acts as a precursor for the photogenerated inhibitor. The UVvisible absorption spectrum of this compound exhibits maximum absorption at 349 nm (E = 5000 M-cm ') in TMPTA. 7-Diethylamino-3-cinnamoylcoumarin acts as the photoinitiator. Its UV-visible absorption spectrum exhibits an absorption maximum at 453 nm (E = 52,000 M-lcm 1) in TMPTA.

The imaging sheet was image-wise exposed through a step wedge (21 step, 0.15 density increments) using two black-light tubes (GE F 15T8 BLB; 15 watt) situated six inches from the sample. This image-wise exposure was immediately followed by uniform exposure to visible light. This exposure was performed using two GE F 15T8 CW 15 watt cool white bulbs situated six inches from the sample and covering the samples with two thicknesses of Rite-Lite (U.V. Process Supply, Inc.) UV-cutoff filter. Both the image-wise and uniform exposures were conducted for 64 seconds. Pressure development against a developer sheet resulted in a negative image. The photographic characteristics of the material were: Dmax 1.36 D min 0.07 Gamma 3.79

Claims (26)

1. Photosensitive microcapsules having an internal phase including a photohardenable composition and a photoinhibitor precursor.

2. Photosensitive microcapsules according to Claim 1, wherein each said microcapsule possesses a discrete capsule wall.

3. Photosensitive microcapsules according to Claim 1 or Claim 2, wherein said photohardenable composition includes an ethylenically unsaturated compound and a photoinitiator and said precursor does not detrimentally inhibit free radical polymerization.

4. Photosensitive microcapsules according to any preceding Claim, wherein said precursor is an o-nitroaromatic compound, a nitrosodimer, or a bis (substituted amino)sulphide.

5. Photosensitive microcapsules according to any preceding Claim, wherein said internal phase additionally includes an image-forming agent.

6. Photosensitive microcapsules according to Claim 5, wherein said image-forming agent is a substantially colourless electron donating compound.

7. Photosensitive microcapsules substantially as herein described with reference to the Examples and/or as shown in the accompanying drawing.

8. A photosensitive imaging sheet comprising a support and, on a surface thereof, a layer of microcapsules according to any preceding Claim.

9. A photosensitive imaging sheet comprising a support, and a layer of photosensitive microcapsules on a surface thereof, said microcapsules including an internal phase containing a photohardenable composition and a photoinhibitor precursor.

10. A photosentive imaging sheet according to Claim 9, wherein said microcapsules have discrete capsule walls.

11. A photosensitive imaging sheet according to Claims 9 or 10, wherein said microcapsules have associated therewith an image- forming agent.

12. A photosensitive imaging sheet according to Claim 11, wherein said image-forming agent is present in said internal phase.

13. A photosensitive imaging sheet according to Claim 11 or Claim 12, wherein said image-forming agent is a substantially colourless electron donating compound.

14. A photosensitive imaging sheet according to any of Claims 9 to 13, wherein said photohardenable composition includes an ethylenically unsaturated compound and a photoinitiator and said precursor does not detrimentally inhibit free radical polymerization.

15. A#hotosensftive imaging sheet according to any of Claims 9 to 14, wherein said precursor is an o-nitroaromatic compound, a nitrosodimer, or a bis (substituted amino)sulphide.

16. A photosensitive imaging sheet according to Claim 11 or any Claim appendent thereto, wherein said sheet further comprises a developer material codeposited upon said substrate with said microcapsules.

17. A photosensitive imaging sheet substantially as herein described with reference to the Examples and/or as shown in the accompanying drawing.

18. A process for imaging which comprises the steps of: image-wise exposing an imaging sheet to a first radiation, said imaging sheet including a support having a layer of microcapsules on a surface thereof, said microcapsules having an image-forming agent associated therewith and including an internal phase containing a photohardenable composition and a photoinhibitor precursor, said first radiation image-wise converting said precursor to an inhibitor of said photohardening; uniformly exposing said image-wise exposed imaging sheet to a second radiation different than said first, said second radiation causing said photohardenable composition to harden in those areas in which hardening is not inhibited by said inhibitor; and subjecting said imaging sheet to a uniform rupturing force wherein said image-forming agent is activated and a negative image is formed.

19. A process according to Claim 18, wherein said microcapsules have discrete capsule walls.

20. A process according to Claims 18 to 19, wherein said photohardenable composition includes an ethylenically unsaturated compound and a photoinitiator.

21. A process according to any of Claims 18 to 20, wherein said image-forming agent is a substantially colourless electron accepting compound and a developer material is associated with said imaging sheet such that after subjecting said imaging sheet to said uniform rupturing force, said electron accepting compound image-wise reacts with said developer and forms a colour image.

22. A process according to any of Claims 18 to 21, wherein said photohardenable composition is sensitive to ultraviolet or visible radiation and said precursor is activated by infrared radiation.

23. A process according to Claim 22, wherein said image- wise exposure is conducted using an infrared laser.

24. A process according to any of Claims 18 to 23, wherein said precursor is a nitrosodimer.

25. A process for imaging which comprises the steps of: image-wise exposing an imaging sheet according to Claim 11 or any Claim appendent thereto, to a first radiation effective to convert said precursor to an inhibitor of said photohardening; uniformly exposing said imaging sheet to a second radiation different from said first and effective to cause said photohardenable composition to harden in those areas in which hardening is not inhibited by said inhibitor; and subjecting said imaging sheet to a uniform rupturing force whereby said image-forming agent is activated and a negative image is formed.

26. A process for imaging substantially as hereinbefore described with reference to the Examples and/or the accompanying drawing.