GB2163563A - Imaging material - Google Patents

Abstract

Broader dynamic range and superior tonal quality are obtained in an imaging material in which a radiation sensitive composition is encapsulated in microcapsules present on a support by incorporating a crystalline organic compound in the internal phase.

Description

SPECIFICATION Imaging material The present invention relates to an imaging material.

U.S. Patent Nos 4399209 and 4440846, assigned to The Mead Corporation, describe transfer and self-contained imaging systems in which the imaging sheet comprises a support carrying a layer of photosensitive microcapsules. The microcapsules contain an internal phase which includes a photosensitive composition which undergoes a change in viscosity upon exposure to actinic radiation. The photosensitive composition is typically a photohardenable composition such as a composition which undergoes free radical addition polymerization. The imaging materials can be used to form dye images or light scattering images, but an image-forming agent, which is a substantially colourless compound and generates a coloured dye upon reaction with a developer, is typically encapsulated with the photosensitive composition in the internal phase of the microcapsules.In self-contained imaging systems, the co-reactive developer material is provided on the surface of the imaging sheet with the photosensitive microcapsules. In transfer imaging systems the developer is provided on a separate sheet.

To form images, the above described imaging sheets are image-wise exposed to actinic radiation and subjected to a uniform rupturing force whereupon the microcapsules rupture and image-wise release the internal phase. In the case of microcapsules containing a photohardenable composition, the internal phase is released in the areas which are not exposed to actinic radiation or areas which are underexposed. In these areas, the microcapsules rupture and the internal phase remains sufficiently fluid to be released from the microcapsules. Thus, in these areas the imge-forming agent associated with the microcapsules can react with the developer material and produce a colour image. In the fully exposed areas, the microcapsules are either incapable of rupturing or, if they do rupture, the internal phase is too viscous to be released from the capsules.A detailed explanation of these imaging materials and the process whereby images are formed can be found in both of the aforementioned patents.

According to the present invention, we provide an imaging material comprising a support having on the surface thereof a layer of photosensitive microcapsules and an image-forming agent, said microcapsules containing an internal phase including a radiation sensitive composition and a crystalline organic compound.

We describe below a number of examples of imaging materials which have a lower gamma value and a broader dynamic range than our aforementioned materials described in U.S. Patents 4399209 and 4440846 and which are useful in producing images having improved tonal characteristics. Our present imaging materials are particularly useful in those applications requiring a continuous tonal quality.

In our present arrangements, a crystalline organic compound is added to the internal phase of the photosensitive microcapsules. The crystalline organic compound has the effect of increasing shoulder speed and, at the same time, decreasing toe speed and thus broadening the dynamic range of the imaging material. While not desiring to be bound by this explanation, we believe that in the present microcapsules, the photosensitive composition is entrapped within a crystalline array in the microcapsules where it is protected from oxygen and thus less susceptible to oxygen inhibition. We believe that this is responsible for the increased shoulder speed obtained in these imaging materials. The decreased toe speed appears to be the result of requiring more photosensitive composition to react to immobilize the image-forming agent when the unpolymerizable crystalline organic compound is present.The mid-tone speed of our present imaging materials does not appear to be affected by the crystalline organic compound. Thus, the addition of a crystalline organic compound to the internal phase of photosensitive microcapsules can be used to convert an imaging material which otherwise has a relatively high gamma to an imaging material having a lower gamma without sacrificing film speed. These materials are useful in those applications requiring better tonal quality.

In preferred arrangements, the microcapsules have discrete capsule walls and the internal phase additionally includes an image-forming agent such as a substantially colourless electron donating compound which forms a colour upon reaction with an electron accepting compound such as acid clay or a salicyclic acid derivative.

In the most preferred arrangements, the radiation sensitive composition is a photohardenable composition and, still more particularly, a composition which undergoes free radical addition polymerization such as a composition including an ethylenically unsaturated compound and a photoinitiator.

The term "microcapsule" as used herein includes both microcapsules having a discrete capsule wall and so-called open phase microcapsules formed by dispersing the internal phase in a binder.

The term "actinic radiation" as used herein includes the entire electromagnetic spectrum including visible light, ultraviolet and infrared radiation, and particle radiation such as X-ray and ion beam radiation. The preferred forms of actinic radiation are ultraviolet radiation and visible light having a wavelength of about 190-800 nm and, more preferably, 380-480 nm.

The reader is referred to our U.S. Patent Nos 4399209 and 4440846, the disclosures of which are accordingly to be regarded as incorporated herein by reference.

The imaging material of the present invention is characterized in that the internal phase of the microcapsules includes a crystalline organic compound. Substantially any crystalline organic compound which does not inhibit the reaction of the radiation sensitive composition and which does not substantially absorb exposure radiation can be used in the present materials. The crystalline compound is described herein as being "organic" because the internal phase of the microcapsules including the radiation sensitive composition is oleophilic and, as such, water soluble inorganic compounds cannot be incorporated into the internal phase and encapsulated using conventional encapsulation techniques. Thus, the term "organic" simply implies that the crystalline compound is compatible with the internal phase of the microcapsules.

The crystalline organic compounds used in the present invention preferably have a melting point less than 100 C and, still more preferably, less than 80 C. At the same time, the compounds must be crystalline at room temperature (23 C). Preferably the crystalline organic compounds are miscible in the radiation sensitive composition and, more particularly, the crystalline organic compounds are miscible in monomers, such as trimethylolpropane triacrylate, such that upon melting the crystalline organic compound and admixing it with the monomer/radiation sensitive composition, a homogeneous phase is produced which, upon cooling, provides a crystalline matrix having microvoids containing the radiation sensitive composition.Preferably, the crystalline organic compounds are substantially insoluble in water such that they can be dispersed in a continuous aqueous phase for encapsulation without being removed from the internal phase by extraction.

Representative examples of useful crystalline organic compounds include 1,2-diphenoxyethane, cyclododecanol, 1 -docosanol, 1 -octadecanol, p-diethoxybenzene, octacosane, pentamethyl benzene, bibenzyl, and biphenyl.

The crystalline organic compound is typically used in an amount of about 0.1 to 1 parts by weight per part by weight of the radiation sensitive composition.

The crystalline organic compound can be encapsulated in the internal phase of the microcapsules by melting the crystalline organic compound, admixing it with the photosensitive composition and forming a wall therearound using conventional encapsulation techniques such as coacervation or interfacial polymerization or by dispersing the internal phase in an appropriate binder. As previously indicated, it is particularly desirable that the crystalline organic compound be miscible with the radiation sensitive composition such that a crystalline array having photosensitive regions or microvoids is provided. A preferred microcapsule is one having a discrete capsule wall formed from a urea-formaldehyde or urea-resorcinol-formaldehyde copolymer.

The radiation sensitive compositions are suitably photohardenable compositions which undergo an increase in viscosity upon exposure to actinic radiation. The photohardenable compositions described in the patents which have been incorporated by reference may be used herein.

Preferred compositions comprise an ethylenically unsaturated compound and a photoinitiator system. Typically, the ethylenically unsaturated compounds contain two or more terminal or pendant vinyl or allyiic groups. Such compounds are well-known in the art and include acrylic and methacrylic esters of polyhydric alcohols, trimethylolpropane, pentaerythritol, and the like.

Representative examples include ethylene glycol diacrylate, ethylene glycol dimethacrylate, trimethylolpropane triacrylate (TMPTA), trimethylolpropane trimethacrylate (TMPTMA), pentaerythritol tetraacrylate, pentaerythritol tetramethacrylate, hexanediol- 1 6-dimethacrylate, diethyleneglycol dimethacrylate, and dipentaerythritol hydroxypentaacrylate. Commercially available photopolymers such as acrylate and methacrylate terminated polyesters and polyethers are also useful in the practice of the present invention.

The radiation sensitive compositions used in the present materials usually include a photoinitiator system. The photoinitiator system may include a sensitizer. An absorber may also be used in combination with the system to adjust the sensitivity of the photosensitive composition. Photoinitiators which generate free radicals upon photochemical cleavage (homolytic initiators) such as certain benzoin ethers and initiators which function via hydrogen abstraction are useful. Diary ketone derivatives and benzoin alkyl ethers are particularly useful. Specific examples of useful initiator systems include benzophenone, Michler's ketone, benzoin methyl ether, and 2,2'-dimethoxy-2-phenylacetophenone, isopropylxanthone, isopropylthioxanthone, (ethyl para-dimethyl-aminobenzoate), and ketocoumarin compounds as described in U.S. Patent 4,147,552, etc.

The photoinitiator is present in the photosensitive composition an an amount effective to initiate polymerization or crosslinking. For example, isopropylthioxanthone is typically present in an amount up to about 10% by weight based on the weight of the photocrosslinkable or photopolymerizable material present in the photosensitive composition. The exact amount of photoinitiator used will vary with the nature of the photosensitive composition. It is also possible to reduce the exposure time by incorporating a scattering agent such as magnesium oxide in the capsule layer. The scattering agent increases the mean free path and thereby intensifies expo sure.

Ultraviolet sensitive microcapsules are generally preferred because they can be handled in room light for short periods of time. They are also useful for copying from a CRT screen. One disadvantage of ultraviolet sensitive systems, however, is that many printed documents are on a paper which includes optical brighteners which absorb ultraviolet radiation. Accordingly, to copy printed documents, blue light sensitive systems are advantageous.

In accordance with certain embodiments of the present invention (particularly those embodiments in which the photosensitive composition contains a photopolymerizable monomer such as TMPTA), the photosensitive composition may include an oligomeric and/or a polymeric material to further increase the film speed of the microcapsules. Typically, these materials range from about 800 to 3,000 in average molecular weight in the case of oligomers and up'two 40,000 in molecular weight in the case of polymers. The oligomer or polymer may be reactive, i.e., curable or polymerizable by free radical initiated polymerization, or not reactive. In both cases, it enhances the film speed by increasing the rate with which the viscosity of the composition reaches a level at which the microcapsules can be differentially ruptured.

Representative examples of some commercially available oligomers which are useful in the present invention include Ebecryl 240, Ebecryl 270, Ebecryl 810 (Virginia Chemicals Inc.); DER 662, DER 663U, DER 664U (Dow Chemical Co.); Cargill 1570 (Cargill); Uvithane 893 (Morton Thiokol Inc.); Diallyl-o-phthalate prepolymer (Polysciences); polyvinylpyrrolidone (GAF).

In some cases it is advantageous to include a polythiol in the photosensitive composition to improve sensitivity (film speed). Useful polythiols contain 2 or more terminal or pendant -SH groups. Examples of polythiols that are desirable for use in the present invention are esters of thioglycolic acid and fi-mercaptopropionic acid.Representative examples of the preferred polythiols include ethylene glycol bis (thioglycolate), ethylene glycol bis (#-mercaptoprnpionate), trimethylolpropane tris (thioglycolate), pentaerythritol tetrakis (thioglycolate) and the most preferred pentaerythritol tetrakis (p-mercaptopropionate), dipentaerythriol hexa (p-mercaptopropionate), and trimethylolpropane tris (fl-mercaptopropionate), and mixtures thereof. These compounds are commercially available. Certain polymeric polythiols such as polypropylene ether glycol bis (p-mercap- topropionate) which is prepared by esterification of polypropylene ether glycol may also be useful.

One example of an image-forming agent useful in the invention is a colorless electron donating compond. 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, Copikem X, IV Xl, XX (Hilton-Davis Co.), and Reakt Yellow (BASF Aktiengesellschaft) may be used alone or in combination as color precursors in the present invention.

Illustrative examples of color developers useful in conjunction with the electron donating type color precursors 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 (see U.S. Patents 3,864,146 and 3,934,070), zinc 3,5-di(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 modififed oil soluble phenol-formaldehyde resin as disclosed in U.S.Patent No. 3,732,120, zinc carbonate etc., and mixtures thereof.

Images can also be formed using as the image-forming agent, a chelating agent 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(lil) ] and yellow prussiate.

Substantially any color-forming agent which can be encapsulated and which will react with a developer material to form an image can be used in the present invention. Furthermore, either the color former or the color developer may be associated with the microcapsules. It is not always necessary to encapsulate the color former, as is the conventional practice.

As a further alternative, the capsules may contain a visible dye or pigment. Substantially any benign colored dye, i.e., a dye which does not detrimentally attenuate the exposure radiation can be used in this embodiment. A few examples are Sudan Blue and Rhodamine B dyes. Certain pigments or toners can also be used.

The image-forming agent may be associated with the microcapsules in various ways such that upon release of the internal phase, the image-forming agent is able to react and/or migrate to produce an image. It is typically encapsulated with the photosensitive composition, in the microcapsules, however, it may also be incorporated in the walls of the microcapsules. Those skilled in the art will appreciate that various arrangements can be used provided that the activation or mobilization of the image-forming agent is controlled by rupturing the microcapsules and releasing the internal phase.

The internal phase may additionally include a diluent oil. Inclusion of the oil will often improve half tone gradation in visual images. Preferred diluent oils are weakly polar solvents having boiling points above 1 700C and preferably in the range of 180 to 300 C. 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 and kerosene are generally less toxic and preferred. The amount of diluent oil incorporated in the microcapsules will depend upon the photographic characteristics that are desired in the photosensitive materials.Typically. the diluent oil is used in an amount of approximately 10 to 20% by weight based on the weight of the internal phase.

The photosensitive microcapsules may be easily formed using known encapsulation techniques.

The photosensitive composition and associated agents can be encapsulated in hydrophilic wallforming 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-methyl-cellulose; resorcinolformaldehyde wall formers (see U.S. Patent No. 3,755,190 to Hart et al); isocyanate wallformers (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 commonly assigned U.S. Patent No. 4,025,455 assigned to The Mead Corporation).

The mean size of the microcapsules we employ generally ranges from approximately 1 to 25 microns. As a general rule, image resolution improves as capsule size decreases except that if the capsule size is too small, the capsules may disappear in the pore or fibre structure of some substrates.

The image-forming agent is used in an amount sufficient to produce a visible image of the desired density upon reaction with a developer or upon transfer. In general, the image-forming agent is present in an amount of approximately 0.5 to 25% by weight based on the weight of the photopolymerizable or photocrosslinkable species. A preferred range is about 2 to 10% by weight.

The most common substrate for the image-receiving sheet is paper. The paper may be a commercial impact raw stock, or a special grade paper such as cast-coated paper or chromerolled paper. Transparent substrates such as polyethylene terephthalate and translucent substrates can also be used.

The teachings of the present invention can be used in conjunction with both transfer and selfcontained imaging systems as described in U.S. Patent Nos. 4,399,209 and 4,440,849. In addition, the teachings herein can be used in conjunction with full colour imaging systems as described in our British Patent Specification 2113860 and in our European Patent Application 85303484.1 (Publication No: ) to which the reader is referred and which are also to be regarded as incorporated herein by reference.

The present invention is illustrated in more detail by the following nonlimiting examples. Unless otherwise indicated, all percentages are by weight.

Example 1 Microcapsules containing, as the internal phase, 6 g Quanticure BMS (Ward-Blenkinsop, Ltd.), 2 g Quanticure EPD (Ward-Blenkinsop, Ltd.), and 1 g Crystal Violet Lactone, and amounts of trimethylolpropane triacrylate (TMPTA) and 1,2-diphenoxyethane to total 50 g, as shown in Table 1 below were prepared by the following encapsulation procedure: Capsule Preparation: 1. Into a 600 ml stainless steel beaker, 104 g water and 22.3 g isobutylene maleic anhydride copolymer (20.6%) are weighed.

2. The beaker is clamped in place on a hot plate under an overhead mixer. A six-bladed, 450 pitch, turbine impeller is used on the mixer.

3. After thoroughly mixing, 3.1 g pectin is slowly sifted into the beaker. This mixture is stirred for 20 minutes.

4. The pH is adjusted to 4.0 using a 20% solution of H2SO4, and 0.2 g Quadrol (2hydroxypropyl ethylenediamine with propylene oxide from BASF) is added.

5. The mixer is turned up to 3000 rpm and the internal phase is added over a period of 10-15 seconds. Emulsification is continued for 10 minutes.

6. At the start of emulsification, the hot plate is turned up so heating continues during emuisification.

7. After 10 minutes, the mixing speed is reduced to 2000 rpm and 16.6 g urea solution (50% w/w), 0.8 g resorcinol in 10 g water, 21.4 g formaldehyde (37%), and 0.6 g ammonium sulfate in 10 ml water are added at two-minute intervals.

8. The beaker is covered with foil and a heat gun is used to help bring the temperature of the preparation to 65 C. When 650C is reached, the hot plate is adjusted to maintain this temperature for a two to three hour cure time during which the capsule walls are formed.

9. After curing, the heat is turned off and the pH is adjusted to 9.0 using a 20% NaOH solution.

10. Dry sodium bisulfite (2.8 g) is added and the capsule preparation is cooled to room temperature.

Imaging Sheet Preparation The microcapsule preparation, so obtained, was diluted 1:1 with water containing 0.5% Triton X-100 surfactant (Rohm & Haas Co.) and coated on 80 pound Black and White paper (The Mead Corporation) using a number 12 wire wound rod.

After drying, the sheets were exposed for 4 seconds with a light source consisting of one 15T8/BL and one 15T8/D fluorescent tube through a 2 step wedge at a distance of 6 inches from the light source. The gamma values and the step number corresponding to 90%, 50%, and 10% Dmax are shown in Table 1 below: Table 1 Internal Phase Gamma 90% 50% 10% TMPTA (50 g) 3.1 10.41 9.07 7.64 TMPTA (34 g) 1,2-Diphenoxyethane (16 g) 1.7 11.72 9.90 7.53 TMPTA 25 g 1,2-Diphenoxyethane (25 g) 1.0 12.69 9.87 5.93 As Table 1 shows, the addition of 1,2-diphenoxyethane reduces the gamma value of the imaging material from 3.0 to 1.0. Similarly, the dynamic range (the number of steps between 90% and 10% Dmax) increases from approximately 3 steps to approximately 7 steps. Thus, the addition of crystalline organic compounds such as 1,2-diphenoxyethane represents a viable means of reducing gamma and, at the same time, providing a high speed photographic system.

Example 2 Microcapsules containing 6 g Quanticure BMS (Ward-Blenkinsop, Ltd.), 2 g Quanticure EPD (Ward-Blenkinsop, Ltd.), 1 g Crystal Violet Lactone, 25 g trimethylpropanetriacrylate and 25 g cyclododecanol, as the crystalline organic compound, were prepared by following the same procedure outlined in Example 1. Imaging sheets were prepared and image-wise exposed as in Example 1. The imaging material exhibited the gamma value and the dynamic range shown in Table 2 below: Table 2 Internal Phase Gamma Speed (Step #) 90% 50% 10% TMPTA (25 g) 0.74 11.75 8.29 3.53 Cyclododecanol (25 g)

Claims (15)

1. An imaging material comprising a support having on the surface thereof a layer of photosensitive microcapsules and an image-forming agent, said microcapsules containing an internal phase including a radiation sensitive composition and a crystalline organic compound.

2. An imaging material according to Claim 1, wherein said microcapsules have discrete capsule walls.

3. An imaging material according to Claim 1 or Claim 2, wherein said image-forming agent is present in the internal phase of said microcapsules.

4. An imaging material according to any preceding claim, wherein said image-forming agent is a substantially colourless electron donating compound.

5. An imaging material according to any preceding claim, wherein said radiation sensitive composition includes an ethylenically unsaturated compound and a photoinitiator system.

6. An imaging material according to Claim 5, wherein said crystalline organic compound, as a melt, is miscible with said ethylenically unsaturated compound.

7. An imaging material according to any preceding claim, wherein said crystalline organic compound has a melting point less than about 100 C.

8. An imaging material according to any preceding claim, wherein said crystalline organic compound does not substantially inhibit reaction of said radiation sensitive composition.

9. An imaging material according to any preceding claim, wherein said crystalline organic compound is substantially insoluble in water.

10. An imaging material according to any preceding claim, which further comprises a developer material on the same surface of said support as said photosensitive microcapsules.

11. An imaging material according to any of Claims 1 to 9, wherein said imaging material additionally includes a developer material provided on a support separate and distinct from said support carrying said photosensitive microcapsules.

12. An imaging material according to Claims 10 or 11 as appendent to Claim 4, wherein said developer material is an electron accepting compound.

13. An imaging material according to any preceding claim, wherein said crystalline organic compound is 1,2-diphenoxyethane or cyclododecanol.

14. An imaging material according to any preceding claim, wherein said radiation sensitive composition is present in microvoids in said crystalline organic compound.

15. An imaging material substantially as hereinbefore described with reference to the Examples.