Imaging materials employing microparticles including a silver initiator

An imaging material comprising a support and a layer of photosensitive microparticles on one surface of said support, said microparticles including an image-forming agent and a photosensitive composition containing a polymer which is capable of undergoing cationically-initiated depolymerization and a photoinitiator including a silver halide and an organo silver salt, wherein, after exposing said microparticle to radiation, said microparticles, directly or with additional processing, release said image-forming agent or become permeable to a developer which reacts with said image-forming agent to form a visible image.

BACKGROUND OF THE INVENTION 
The present invention relates to an imaging system which is capable of 
forming monochromatic or full color images; it is more particularly 
related to an imaging system employing photosensitive microparticles 
containing an image-forming agent in which exposure of the microparticles 
to actinic radiation controls the release of the image-forming agent from 
the particles or its reaction with a developer. 
Imaging systems employing photosensitive microcapsules are known in the 
art. An early version of these systems is described in U.S. Pat. No. 
3,219,446 to Berman. More recently, Sanders et al. have described an 
imaging system in U.S. Pat. Nos. 4,399,209 and 4,440,846 in which a 
chromogenic material is encapsulated with a photohardenable or 
photosoftenable composition. In this system, the microcapsules are 
image-wise exposed to actinic radiation and subjected to a uniform 
rupturing force whereupon they image-wise release a chromogenic material 
which reacts with a developer to form a full color or a monochromatic 
image. Japanese Patent Publication No. 34488/1977 also discloses a 
photosensitive material including microcapsules. Therein, a chromogenic 
material is encapsulated in a wall material containing a photosensitive 
substance which generates a gas by photodecomposition. Upon exposure, the 
microcapsules release the chromogenic material whereupon it reacts with a 
developer, which is present in the microcapsule layer, to form the image. 
SUMMARY OF THE INVENTION 
The present invention provides an imaging material which comprises a 
support having a layer of photosensitive microparticles on the surface 
which contain an imageforming agent. The microparticles are formed from a 
photosensitive composition containing a depolymerizable polymer which has 
an image-forming agent admixed therewith. The microparticles are designed 
such that they release the image-forming agent or permit the image-forming 
agent to react with a developer if they are exposed to actinic radiation. 
In one embodiment of the invention, the image-forming agent is a 
chromogenic material and a developer material is associated with the 
photosensitive material such that, after exposure, the chromogenic 
material reacts with the developer and forms a visible image. 
The photosensitive material of the present invention is advantageous 
because it is a negative-working material. It is only necessary to expose 
the material to radiation in the areas in which an image is desired, i.e., 
the image areas. Non-image or background areas in which no color is 
desired remain white in the absence of exposure. These materials are 
particularly advantageous for imaging by laser scanners and the like, 
because the scanner can pass over the non-image areas. By contrast, the 
most typical of the photosensitive materials described in the 
aforementioned Sanders' patents employ photohardenable compositions. These 
materials are positive-working. To achieve Dmin, they must be fully 
exposed in the non-image areas in order to lock the chromogenic material 
in the capsule and prevent its reaction with the color developer. These 
materials are not desirable for use with scanning-type exposure devices 
because both image and non-image areas must be exposed. The image areas 
require exposure to the extent that only a partial release of the 
chromogenic material in the image areas is desired. The non-image areas 
require a full exposure to achieve Dmin. Furthermore, the photohardenable 
compositions which have been developed for use in the Sanders' materials 
have comparatively low sensitivity. As a consequence of the need to fully 
expose a large area of the photosensitive material and the comparatively 
low sensitivity of the photohardenable compositions, long exposure periods 
are required to expose the positive-working embodiment of the Sanders' 
material using a scanner device. 
A further advantage of the photosensitive material of the present invention 
is its handleability. Because the microparticle is solid, inadvertent 
release of the image-forming agent in the course of handling is not nearly 
as likely to occur as it is using photosensitive microcapsules which 
readily rupture. The photosensitive material of the present invention is 
also advantageous because it can provide full color images from a single 
photosensitive layer as contrasted with silver halide materials which 
include a plurality of layers. 
Another advantage of the microparticles of the present invention is their 
relatively insensitivity to oxygen. In many cases depolymerization is 
initiated by a photogenerated acid which reacts directly with the 
depolymerizable polymer. In systems in which imaging occurs via free 
radical polymerization, the oxygen in the photosensitive composition must 
be consumed by free radicals before polymerization proceeds. Oxygen does 
not affect the depolymerization chemistry relied upon in most embodiments 
of the present invention. 
The photosensitive material of the present invention is distinguished from 
the Sanders' materials and, more particular, Sanders' materials employing 
photosoftenable compositions, in that the microparticle used herein does 
not include a microcapsule wall. Furthermore, in accordance with the 
preferred embodiments of the invention, the microparticles are formed from 
a photosensitive composition of a polyaldehyde. 
In accordance with one embodiment of the present invention, the imaging 
material is used to form full color images. This can be accomplished two 
ways. One method is to form full color transfer images using three 
separate imaging sheets on which the microparticles respectfully contain 
cyan, magenta, and yellow color-forming chromogenic materials. By 
sequentially exposing the three sheets and conducting successive transfers 
in register to an image-receiving sheet carrying a developer material, the 
cyan, magenta, and yellow color-forming chromogenic materials are 
image-wise transferred to the surface of the developer sheet where they 
react to form a full color image. 
In another full color photosensitive imaging material in accordance with 
the present invention, the layer of microparticles includes three, and in 
some cases four, sets of microparticles. Each set of microparticles 
contains a different chromogenic material and is selectively sensitive to 
a different band of actinic radiation. The distinct sets of microparticles 
respectively contain cyan-, magenta-, and yellow-forming chromogenic 
materials or cyan-, magenta-, yellow-, and black-forming chromogenic 
materials. The photosensitive compositions from which the microparticles 
are formed are designed with distinctly different wavelength sensitivities 
such that, at selected wavelengths, each set of microparticles can be 
exposed such that it releases the image-forming agent and/or permits it to 
react with a developer without cross-exposing the other sets of 
microparticles. 
The latter full color imaging material is advantageous because the 
microparticles are image-wise exposed on the surface of a single imaging 
sheet. The visible image can be developed using any of the development 
techniques described later including one embodiment in which the developer 
material is present on the same surface of the image sheet as the 
microparticles and one embodiment in which the developer material is 
provided on a separate image-receiving sheet. After image-wise exposing 
the imaging material to actinic radiation sequentially or simultaneously 
in each of the regions in which the respective sets of microparticles are 
sensitive, the microparticles image-wise release the chromogenic materials 
(directly or with additional processing) or otherwise enable the 
chromogenic material to react with a developer. 
Definitions 
The term "microparticle" is used herein to define a particle formed from an 
admixture of an image-forming agent and a photosensitive composition 
containing a depolymerizable polymer. The term "microparticle" is to be 
distinguished from the term "microcapsule" which is defined in U.S. Pat. 
Nos. 4,399,209 and 4,440,846 as a capsule having a discrete capsule wall 
or an encapsulated dispersion of a photosensitive composition in a binder. 
The term "radiation" as used herein includes the full spectrum of 
electromagnetic radiation including actinic radiation such as ultraviolet, 
infrared, and visible light, as well as ionizing and particle radiation 
The term "depolymerizable polymer" as used herein includes polymers which 
either become depolymerized or decrosslinked in the presence of products 
generated upon exposing the photosensitive composition to radiation. The 
polymers themselves may be photosensitive. That is, the polymers may be 
photolyzable and depolymerize directly upon exposure to radiation. 
Otherwise, the polymers may be non-photosensitive, in which case another 
photosensitive species such as a photoinitiator must be present in the 
photosensitive composition.

DETAILED DESCRIPTION OF THE INVENTION 
A photosensitive material in accordance with the present invention, in its 
simplest form 10, comprises a support 12 and a layer 14 of microparticles 
16. The microparticles 16 are formed from a photosensitive composition 
containing a depolymerizable polymer which is admixed with an 
image-forming agent as discussed below. 
In FIG. 2, the photosensitive material 10 is shown being exposed to actinic 
radiation (designated h.lambda.) through a mask M to form a latent image 
in the form of unexposed and exposed microparticles 16A and 16B. After 
exposure, the manner of processing will depend on the nature of the 
microparticle and the image-forming agent. The microparticles 16A in the 
unexposed areas 18 are unchanged. The microparticles 16B in the exposed 
areas 20 undergo a change which enables the image-forming agent to be 
released or exuded from the microparticle or simply to react with a 
developer to form a visible image. The change may occur directly upon 
exposure or, depending upon the composition of the microparticle, the 
change may occur upon heating, application of pressure, or like treatment. 
Images are formed by one of two mechanisms. The image-forming agent may be 
released from the microparticles or the microparticles may become 
permeable to a developer material. Where transfer images are formed, the 
image-forming agent (e.g., a benign dye or a chromogenic compound) must be 
released from the microparticles. In self-contained systems or systems in 
which the developer is applied externally, either mechanism is useful. 
FIG. 3 represents the photosensitive material 10 after exposure and, in 
most cases, after an optional step of treating the microparticles to 
facilitate the release of the image-forming agent or its accessibility to 
a developer. In FIG. 3, the unexposed microparticles 16A are shown as 
retaining their original form, whereas, the exposed microparticles 16B are 
shown as having lost their form. The change in the morphology of 
microparticles 16B is shown for purposes of illustration only. In fact, 
there may be no visible change in the exposed microparticles. 
Where the image-forming agent is a chromogenic material, that is, a 
material which becomes colored upon reaction with a developer, it will be 
necessary to provide a developer. The developer can be provided using a 
number of techniques depending upon its nature and that of the chromogenic 
material. The developer may be incorporated into the layer of 
microparticles or into a contiguous layer to provide a self-contained 
imaging material. Images can be obtained using this type of material by 
simply exposing the material to radiation or exposing it to radiation and 
heating or otherwise inducing the reaction of the chromogenic material and 
the developer. The developer may penetrate the microparticle or the 
chromogenic material may exude from the microparticle in this type of 
development. Color development processing, in accordance with the 
foregoing embodiment, is illustrated in FIG. 4. There, the image 20 is 
shown in the area of the exposed microparticle 16B. Where desirable, the 
developer can be microencapsulated or provided in a separate 
microparticle. 
Another technique that can be used is to provide the developer on a 
separate support to which the chromogenic material is transferred. Images 
are formed by contacting the photosensitive material of FIG. 2 or FIG. 3 
with the developer sheet such that the image-forming agent transfers to 
the developer sheet to form the image. In this case, heating and/or 
pressure can be combined with the transfer step to simultaneously cause 
the image-forming agent to be released from the microparticle and transfer 
to the developer sheet. 
Alternatively, various external applications of developers can be used. The 
developing agent can be a gas with which the photosensitive material is 
contacted. Furthermore, while "dry" development processing (i.e., 
processing without external application of a developer) is clearly 
favored, images can also be formed by contacting the photosensitive 
material of FIG. 3 with a solution of a developer. When the developer is 
applied externally, the developer may be a common developer which is 
capable of reacting with all the chromogenic materials, or a combination 
of developers may be used which respectively reacts with the different 
chromogenic materials. 
In accordance with another embodiment of the invention, a toner may be 
applied which selectively adheres to the microparticles in the exposed 
areas. 
When the microparticles contain a colored image-forming agent, such as a 
dye or pigment, transfer may be the only step required to produce the 
image, i.e., no developer reaction is required. The exposed photosensitive 
material of FIG. 2 simply may be assembled with a transfer sheet, which 
could be plain paper or a paper treated to enhance its absorption of the 
dye or pigment, and transfer effected. In most cases, transfer will be 
effected simultaneously with the application of pressure and optionally 
heat to release and transfer the dye or pigment. 
Those skilled in the art will appreciate from the foregoing discussion that 
while the process of the present invention has been illustrated in FIGS. 
2-4 as involving separate steps, the development steps need not be 
performed separately. Indeed, if the microparticle releases the 
image-forming agent directly upon exposure without additional processing 
and a developer is incorporated on the surface of the photosensitive 
material with the microparticle, the steps illustrated in FIGS. 2-4 occur 
simultaneously. Typically, the steps illustrated in FIGS. 3 and 4 will 
occur simultaneously. 
The microparticles used in the present invention are formed from a 
photosensitive polymer composition containing a depolymerizable polymer 
which breaks down upon exposure to radiation, such that an image-forming 
agent incorporated in the polymer composition can react with a developer 
or be released from the microparticles. Useful depolymerizable polymers 
undergo free radical initiated or cationic depolymerization. 
Representative examples of photosensitive compositions useful in the 
present invention are described in U.S. Pat. Nos. 4,108,839; 3,984,253; 
3,917,483; 3,915,704 and 3,127,811. These compositions contain 
polyaldehydes, but other depolymerizable compositions such as compositions 
containing polycarbonates as disclosed by Crivello, "Applications of 
Photoinitiated Cationic Polymerization to the Development of New 
Photoresists," Polymers in Electronics, ACS 242, p3 (1984) and Frechet et 
a1., J. Imaging Science, 30 (2). p. 59 (1986); compositions containing 
polyethers ad disclosed by Goethals, E. J., "The Formation of Cyclic 
Oligomers in the Cationic Polymerization of Heterocyclics," Adv. Poly. 
Sci., Vol. 23, p. 103; compositions containing poly(olefin sulfones) as 
disclosed by Hiraoka, H. "Deep U.V. Photolithography with Composite 
Photoresists Made of Poly(olefin sulfones)," Polymers in Electronics, ACS 
242, p. 55 (1984); Bowden, M. J., et al., ibid, pg. 135 and 153; and U.S. 
Pat. No. 3,935,331 to Foliniak et al,; and compositions containing 
poly(3-oximino-2-butanone methacrylate) or poly(4'-alkyl acylophenones) as 
disclosed in Reichmanis, E., Am. Chem. Soc. Div. Orq. Coat. Plast. Chem. 
Prepr., 1980, 43, 243-251 and Lukac, I., Chmela, S., Int Conf. on Modif. 
Polym. 5th Bratislave, Czech., July 3-6, 1979, I.U.P.A.C. Oxford, England 
1979, 1, 176-182) may be also be useful. Polysulfones of the type used in 
thermal transfer systems are also potentially useful. 
Other potentially useful depolymerizable systems are the metal-crosslinked 
polymeric gels described in U.S. Pat. No. 3,097,097 to Oster et al.; the 
compositions containing bichromated gelatin described in U.S. Pat. Nos. 
2,484,451 and 2,500,028 to Griggs; the photodegradable polyolefin 
compositions described in U.S. Pat. No. 3,968,095 to Freeman et al.; 
compositions containing polymers having acid cleavable C--O--C groups as 
disclosed in U.S. Pat. No. 4,421,844 to Buhr et al. such as polyalkylaryl 
ethers disclosed in U.S. Pat. No. 4,435,496 to Walls et al.; compositions 
containing polyketones as described in U.S. Pat. Nos. 3,923,514 to Marsh 
and 4,419,506 to Nate et al. and by Tsuda et al. in U.S. Pat. No. 
4,297,433; compositions containing polymethacrylate such as those 
described in U.S. Pat. Nos. 4,125,672 to Kakuchi and 3,779,806 to Gipstein 
et al. 
Of the aforementioned depolymerizable systems, those which hold the most 
potential are those which combine an initiator which generates an acid 
upon exposure and an acid degradable polymer. For example, compositions 
employing initiators such as onium salts which undergo photolysis to 
produce strong acids which catalyze main chain scission of a polymer may 
be used. These systems are preferred to the others because a single 
photochemical event generates the acid which produces a number of bond 
transformations leading to complete or nearly complete reversion to the 
monomer. Included within this class of photodepolymerizable systems are 
compositions of acid degradeable polyaldehydes, polycarbonates and 
polyethers. 
Another useful polymer is one which is crosslinked by an acid cleavable 
linking group. Expousre generates an acid as above which cleaves the 
linking group. Still another class of useful polymers are copolymers 
having acid degradable units or blocks in the polymer backbone. 
The polyaldehydes which are useful in the present invention include 
poly(aromatic 1,2-dialdehydes), poly(aliphatic monoaldehydes), and 
copolymers thereof. These polymers undergo cationc depolymerization. 
Polyaldehydes may be end-capped with a group which stabilizes the polymer 
to depolymerization. The end-capping group may be one which is photolabile 
nd which separates fromthe polymer directly upon exposure to radiation, or 
be one which is acid cleavable, such as an ester or an ether group, and 
which separates from the polymer in the presence of a photogenerated acid. 
Alternatively, the polymer maybe sufficiently stable to be processable 
into the microparticle without end-capping. 
Polycarbonates which are potentially useful in the present invention are 
described in the aforementioned papers of Crivello and Frechet. One useful 
polycarbonate is formed from the reaction of 2,5-dimethyl-2,5-hexanediol 
and 4,4-isopropyllidenediphenol. 
Each microparticle includes an initiator which generates an acid, a cation, 
or a free radical which initiates depolymerization. The acid, cation, or 
free radical may be generated directly upon exposure or upon processing 
such as by heating. Where the system is silver catalyzed, a latent image 
in the form of the exposed silver halide may be formed and the cation 
generated subsequently upon heating or application of a developing agent. 
The initiator may b incorporated into the polymer chain, appended to the 
polymer chain, or simply mixed with the polymer. 
Useful initiators for polyaldehydes include photogenerated acid precursors 
such as (i) triarylsulphonium hexafluorophosphates, triarylsulphonium 
arsenates and triarylsulphonium antimonates, (ii) diaryliodonium 
hexafluorophosphates, diaryliodonium arsenates and diaryliodonium 
antimonates, (iii) dialkylphenacylsulfonium tetrafluoroborates and 
dialkylphenacylsulfonium hexafluorophosphates, (iv) 
dialkyl-4-hydroxyphenylsulfonium tetrafluoroborates and 
dialkyl-4-hydroxyphenylsulfonium hexafluorophosphates. Other useful 
initiators include halogen-containing compounds such as carbon 
tetrabromide, hexachloroethane, tribromoacetophenone, etc. and diazo 
compounds such as diazonium salts and o-quinonediazides, etc. 
The foregoing compounds may be used alone or in combination with a 
sensitizer. Useful sensitizers for diaryliodonium compounds include 
Acridine Orange, Acridine Yellow, Phosphine R, Benzoflavin and Setoflavin 
T. Anthracene, perlene, phenothiazine, 1,2-benzanthracene, coronene, 
pyrene, and tetracene are useful sensitizers for triarylsulphonium 
arsenates, diaryliodonium arsenates and dialkylphenacyl sulphonium 
compounds. Ketocoumarins are also useful sensitizers. These sensitizers 
are used in conventional amounts. 
In accordance with one embodiment of the present invention, cationic 
depolymerization is photoinitiated using an initiator system including a 
silver halide, an organo-silver salt, and, optionally, a reducing agent. 
Examples of each are provided in U.S. Pat. No. 4,629,676. In the presence 
of the exposed silver halide, the silver salt releases an acid which 
initiates depolymerization. 
Typical examples of silver salts useful in this embodiment of the present 
invention are silver behenate, silver alkanesulfonic acid salts, silver 
perfluoroalkanesulfonic acid salts, and silver 
.beta.-hydroperfluoroalkanesulfonic acid salts. Examples are provided in 
U.S. Pat. Nos. 3,347,676 and 4,504,575. More specific examples are silver 
dodecylsulfonate, silver hexadecylsulfonate, silver 
trifluoromethylsulfonate, silver pentafluoroethylsulfonate, silver 
perfluoropropylsulfonate, silver perfluoroctylsulfonate, (CF.sub.3).sub.2 
CHCF.sub.2 SO.sub.3 Ag, and n-C.sub.3 F.sub.7 CFHCF.sub.2 SO.sub.3 Ag. 
Hydroquinones, m-dimethylaminophenol and m-diethylaminophenol, are useful 
reducing agents. 
Conventional photographic silver halides are useful herein including silver 
chloride, silver bromide, silver iodide, silver chlorobromide, silver 
chloroidide, silver iobobromide, and silver chloroiodobromide, and, more 
particularly, silver halides associated with a sensitizing dye. These 
reactions are accelerated by heating after exposure. 
The microparticles of the present invention can be used to control the 
release of various image-forming agents. 
In a more preferred embodiment of the present invention, the microparticles 
can contain a benign visible dye and images are formed by contacting the 
exposed imaging material under pressure with a plain paper or a paper 
treated to enhance its affinity for the visible dye. A benign dye is a 
colored dye which does not interfere with the imaging photochemistry, 
(e.g., by relaxing the excited state of the initiator or detrimentally 
absorbing or attenuating the exposure radiation). 
In another embodiment of the invention, images are formed through the 
reaction of a pair of chromogenic materials such as a color precursor and 
a color developer. In general, color precursors include colorless 
electrondonating 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, xanthane compounds, 
fluorans, thiazine compounds, spiropyran compounds and the like. These 
materials are conventionally used in carbonless paper. Crystal Violet 
Lactone, 2,6-diphenyl-4-(4'-dimethylaminophenyl)-pyridine, and Copikem X, 
IV, and XI are a few examples. 
The foregoing compounds are acid developable and therefore inherently 
basic. As such, they may compete with the polymer for the photogenerated 
acid in certain embodiments. Another type of chromogenic material which 
may be preferred for use in the present invention is a base-developable 
chromogenic material. These materials are also leuco compounds and include 
the phenolphthaleins such as sulfobromophthalein sodium tetrahydrate, 
phenolphthalein, bromophenol blue, bromocresol green, bromocresol purple, 
and bromothymol blue. 
In addition to carbonless paper-type color precursors, color photographic 
dye couplers can also be used as image-forming agents in accordance with 
the present invention. These materials may be developed using conventional 
phenolic or anilino photographic developers. 
Diazonium salts are particularly useful image-forming agents because they 
react with unoxidized color couplers to provide azo dye images. Examples 
of useful diazonium salts are: 4-diazo-N,N-diethylamino fluoroborate, 
2-methoxy-4-morpholino benzene-diazonium chloride zinc chloride double 
salt, (3-chloro-6-methoxylienzene diazonium chloride zinc chloride double 
salt, 2,5-dethoxy-4-benzoylamide-benzenediazonium chloride zinc chloride 
double salt, (4-chloro-2-methylbenzene diazonium chloride zinc chloride 
double salt, 5-nitrothiazole diazonium hexafluorophosphat, and 
4-nitrobenzene diazonium hexafluorophosphate. 
Useful couplers include: 1-naphthol, 2-naphtol, 
3-methyl-1-phenyl-2-pyrazoline-5-one, 3-methoxyphenol, 4-methoxyphenol, 
1,3,5,-trimethoxybenzene, 1,3,-dimethoxybenzene, 2,3,-dihydroxynaphtalene, 
1,3-dihydroxynaphthalene, 1-methoxynaphthalene, 2-methoxynaphthalene, 
2-(.alpha.-cyanoacetyl)benzofuran. 
Examples of cyan, magenta and yellow image-forming dyes are disclosed in 
U.S. Pat. No. 4,500,624. 
Illustrative examples of the developer materials conventially employed in 
carbonless paper technology which are also useful electron donating color 
precursors in the present invention are clay minerals such as acid clay, 
active clay, attapulgite, etc.; organic acids such as tannic acid, gallic 
acid, propyl gallate, etc.; acid polymers such as phenol-formaldehyde 
resins, phenol acetylene condensation resins, condensates between an 
organic carboxylic acid having at least one hydroxy group and 
formaldehyde, etc.; metal salts or aromatic carboxylic acids such as zinc 
salicylate, tin salicylate, zinc 2-hydroxy naphthoate, zinc 3,5 di-tert 
butyl salicylate, zinc 3,5-di(.alpha.-methylbenzyl) salicylate, 
oil-soluble metal salts or phenol-formaldehyde novolak resins (e.g., see 
U.S. Pat. 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. Pat. No. 
3,732,120, zinc carbonate, etc., and mixtures thereof. Because these 
developers are acidic, in a self-contained system, they must be isolated 
from the microparticle. Otherwise, the developer may trigger undesirable 
depolymerization. One means to provide a self-contained material is to 
microencapsulate the developer in a separate microcapsule. The capsule 
containing the developer is then ruptured for development. Heatrupturable 
or pressure-rupturable capsules could be used. 
To develop the base-developable chromogenic materials, weak bases such as 
sodium carboxylate or basic resins may be used. 
Various methods can be used to form the microparticles of the present 
invention. A solution of the depolymerizable polymer and other additives 
(e.g., the initiator, sensitizer, and image-forming agent) in a 
water-miscible or a water immiscible solvent can be added to an aqueous 
solution of a stabilizing agent (e.g., an anionic, amphoteric or ionic 
surfactant such as sodium lauryl sulfate; pectin; or polyvinyl alcohol) 
under high sheer mixing and the dispersion coated on a support and the 
water removed through drying. Where the solvent used is immiscible in 
water, it is preferably removed prior to coating. 
Alternatively, a polymer melt containing the other additives can be 
dispensed into an aqueous solution of an appropriate surfactant without a 
solvent and the dispersion coated on an appropriate support. Another 
method which can be used to form the microparticles is spray drying. In 
one useful spray drying technique, a solution of the polymer and additives 
is aspirated into a heated air space. 
To prepare silver catalyzed microparticles, a silver halide emulsion (e.g., 
stabilized with polyvinylbutyral) containing an organo silver salt may be 
dispersed into a solution or melt of the depolymerizable polymer and the 
microparticles formed by either of the methods described above. 
The composition of the microparticles used in the present invention will 
vary depending upon the nature of the photosensitive composition and the 
image-forming agent. In particular, in forming full color images, the 
composition of the mixture of microparticle will be adjusted to provide 
the appropriate color balance. Generally, microparticles in accordance 
with the present invention contain approximately 0.1 to 25 parts by weight 
of the image-forming agent per 100 parts by weight of the photosensitive 
composition and preferably 0.1 to 10 parts by weight. 
A microparticle size should be selected which minimizes light attenuation. 
The mean diameter of the micrparticles used in this invention typically 
ranges from approximately 1 to 25 microns. As a general rule, image 
resolution improves as the size decreases. If the microparticles become 
too small, they may disappear in the pores of the fiber of the substrate. 
These very small microparticles may therefore be screened from exposure by 
the substrate. It has been determined that a preferred mean microparticle 
diameter range is from approximately 3 to 15 microns, and particular, 3 to 
10 microns. 
The most common substrate for use in this invention is paper. The paper may 
be a commerical impact raw stock, or special grade paper such as 
cast-coated paper or chrome-rolled paper. Alternatively, transparent 
substrates such as poly(ethylene terephthalate) can be used. Using a 
transparent substrate, the imaging material can be exposed from either the 
coated or uncoated side. A particularly preferred substrate is a thin 
transparent film. 
The imaging materials of the present invention can be designed to provide 
monochromatic or full color images. Image processing techniques are 
desirably used to form full color images. 
FIGS. 5 and 6 are block diagrams illustrating two processes in accordance 
with the present invention for forming full color images. As shown in FIG. 
5, a color original is resolved into its red, green, and blue component 
images. Because, in accordance with the most typical embodiment of the 
present invention, the photosensitive compositions are 
ultraviolet-sensitive or sensitive to blue or green light; the red, green, 
and blue component images are shown translated into radiation to which the 
microparticles are sensitive for exposure of three separate imaging 
sheets, each bearing microparticles which respectively contain cyan-, 
magenta-, and yellow-forming chromogenic materials. In accordance with 
this embodiment of the invention, apart from the chromogenic materials, 
the microparticles on the three imaging sheets can be identical. Thus, the 
red, green, and blue component images of the subject to be copied are 
inverted to produce reciprocal image information (i.e., -red, -green, and 
-blue) which are translated into the same wavelength band of actinic 
radiation. 
As shown in FIG. 5, in addition to transforming the red, green, and blue 
component images into active radiation, image processing is used to 
correlate the component image with the exposure of the microparticles and 
the release of the image-forming agent. Since the microparticles are 
negative working, this involves an electronic inversion of the subject 
imge to produce the reciprocal image. Thus, in areas corresponding to the 
red image, the magenta and yellow color formers must be released to form a 
red image. In areas corresponding to the green image, yellow and cyan 
color formers must be released, and in areas corresponding to the blue 
image, cyan and magenta color formers must be released. To form full color 
images, an original may be viewed with a Dunn or matrix camera and the 
red, green and blue channel outputs electronically inverted to provide 
reciprocal red (-red), reciprocal green (-green), and reciprocal blue 
(-blue) image information. This information is then used to drive the 
radiation source such that the color formers are released by exposure of 
the three imaging sheets as described above. 
In accordance with the foregoing embodiment of the invention, full color 
images are formed by transfer processing. Thus, each of the exposed 
imaging sheets is contacted in face-to-face registration with a developer 
sheet to form the full color image. As explained above, either directly as 
a consequence of exposure, or the combination of exposure and an 
additional treatment such as the application of heat or pressure, the 
microparticles image-wise release the chromogenic materials. Upon contact 
with the developer sheet, the cyan-, magenta-, and yellow-forming 
chromogenic materials are transferred to the developer sheet where they 
react to provide a full color image. 
The more preferred method for forming full color images utilizes an imaging 
material in which the layer of microparticles is an admixture of three or 
four sets of microparticles respectively containing cyan-, magenta-, 
yellow-, and optionally, black-forming chromogenic materials. In 
accordance with this embodiment of the invention, the photosensitive 
compositions from which the microparticles are formed have distinctly 
different sensitivities to actinic radiation such that each set of 
microparticles can be exposed and caused to release the chromogenic 
material or become permeable to a developer without exposing the other 
sets of microparticles. In accordance with this embodiment of the 
invention, the microparticles may be formed from different photosensitive 
compositions containing different initiator systems. Imaging in accordance 
with this embodiment of the invention is shown in FIG. 6. 
In FIG. 6, an original image is shown resolved into its red, green, and 
blue component images which, in accordance with the most typical 
embodiments of the invention, are processed (inverted) to produce 
reciprocal image information. In this embodiment, however, three exposure 
is at three wavelengths which are respectively indicated in FIG. 6 as 
.lambda.-1, .lambda.-2, and .lambda.-3. In areas exposed to .lambda.-1, 
the microparticles sensitive to .lambda.-1 radiation release the cyan 
color precursor. The microparticles sensitive to .lambda.-2 and .lambda.-3 
radiations, however, do not release the color precursors. In a parallel 
fashion, in areas exposed to .lambda.-2, the microparticles sensitive to 
.lambda.-2 release the magenta color precursor; whereas, the 
microparticles sensitive to .lambda.-1 and .lambda.-3 radiations do not. 
In areas exposed to .lambda.-3 radiation, the microparticles sensitive to 
.lambda.-3 radiation release the yellow color precursor; whereas, the 
microparticles sensitive to .lambda.-1 and .lambda.-2 radiation do not. In 
this manner, the microparticles image-wise release the color precursors 
and, upon development processing as described above, full color images are 
formed. 
Color images can also be formed in accordance with the present invention by 
using negative color seps or separations. Each color sep is mounted on the 
imaging sheet and an exposure is made at one of the wavelengths to which 
the microparticles are sensitive. 
When the imaging material employs silver initated microparticles, the 
imaging material is visible light sensitive. In this case, the imaging 
material may be exposed to visible light through a color negative to form 
full color positive images. Electronic image processing is not required. 
The present invention will be illustrated in more detail by the following 
non-limiting Example. 
EXAMPLE 
Polyphthalaldehyde is prepared by the method of C. Aso and S. Tagami 
(Macromolecular Synthesis, Collective Volume I, J. A. Moore, ed., 505 
(1977)). Generally polymerizations are done in a 250 ml round bottom 
three-necked flask equipped with septum, overhead stirrer and a nitrogen 
inlet/thermometer adapter. The reaction vessel is first purged with 
nitrogen with septum removed and dried with hot air gun until air 
temperature reaches 50.degree. C. The reaction vessel is cooled, charged 
with phthalaldehyde (12.5 g). The septum is replaced and approximately 125 
ml of methylene chloride, freshly distilled, is siphoned into the reaction 
vessel. The phthalaldehyde is dissolved and then cooled to dry ice-acetone 
temperatures. Some precipitation of monomer is always observed at this 
stage. With the solution stirring, BF.sub.3 OEt.sub.2 catalyst in 
methylene chloride (1.3 to 1.4 percent on a molar basis of monomer) is 
added dropwise from a syringe. Some coloration is observed at this stage 
and precipitated monomer redissolves as polymerization proceeds. 
Polymerization at -75.degree. C. is continued for three hours at slow 
stirring rates. The polymerization is quenched with dry pyridine (3 g) and 
warmed to room temperature. Polyphthalaldehyde is purified by repeated 
precipitation into two parts methanol using a Waring blender. 
The microparticles are prepared by an emulsification technique. The typical 
solvent and water phase used is: 
______________________________________ 
Solvent phase 
Polyphthalaldehyde 5.0 g 
Diphenyliodonium hexafluorophosphate 
0.5 g 
Isopropyl thioxanthone 0.05 g 
Methylene chloride 50.0 g 
Water phase 
Pectin 2.5 g 
Water 100 g 
______________________________________ 
The solvent phase is added rapidly to the water phase with high speed 
stirring (T-Line Laboratory Stirrer, Model 104 rated at 7500 rpm.) using a 
three bladed propeller type stirrer. The size of the dispersion is checked 
to insure particles less than 10 microns in size. A fritted glass sparger 
with nitrogen is introduced to evaporate the methylene chloride solvent. 
This requires approximately 1.5 hrs. The microparticle suspension is 
coated as is with a number 12 Meier rod on paper and dried with an air 
gun. Scanning electron photomicrographs of the coated surface confirm the 
presence of microparticles. Coulter counter analysis of the emulsion 
solution gave results of a 3.4 micron average particle size (1.3 to 16 
micron range). 
The coated sheet is baked for 1 minute at 100.degree. C.-120.degree. C. A 
half-tone mask is placed over the sheet and exposed with a bank of three 
BLK-BP fluorescent bulbs (.lambda.max-390 nm) for 8 sec. The imaged sheet 
is developed in a chamber saturated with ammonium hydroxide vapors to 
produce a negative of the original mask. 
Having described the invention in detail and by reference to preferred 
embodiments thereof, it will be apparent that modifications and variations 
are possible without departing from the scope of the invention defined in 
the appended claims.