U.S. patent number 3,899,334 [Application Number 05/405,345] was granted by the patent office on 1975-08-12 for photoimaging procedures and compositions.
This patent grant is currently assigned to Monsanto Company. Invention is credited to Floyd B. Erickson.
United States Patent |
3,899,334 |
Erickson |
August 12, 1975 |
Photoimaging procedures and compositions
Abstract
This invention concerns a photoimaging procedure in which the
image is characterized by a differential concentration of sulfate
groups, depending upon the degree of light exposure, and is
developable by selective absorption of dyes and other procedures.
The sulfate groups can be obtained by SO.sub.2 treatment of
hydroperoxy groups produced in a photooxidation imaging
procedure.
Inventors: |
Erickson; Floyd B. (Webster
Groves, MO) |
Assignee: |
Monsanto Company (St. Louis,
MO)
|
Family
ID: |
26886004 |
Appl.
No.: |
05/405,345 |
Filed: |
October 11, 1973 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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190337 |
Oct 18, 1971 |
3801320 |
|
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Current U.S.
Class: |
430/292; 430/294;
430/390 |
Current CPC
Class: |
G03C
1/733 (20130101); G03C 1/73 (20130101) |
Current International
Class: |
G03C
1/73 (20060101); G03c 005/24 () |
Field of
Search: |
;96/48,63,66,87,98,115
;260/314 ;204/159.23,158,162 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Torchin; Norman G.
Assistant Examiner: Kimlin; Edward C.
Parent Case Text
This application is a division of my application Ser. No. 190,337,
filed Oct. 18, 1971, now U.S. Pat. No. 3,801,320.
Claims
What is claimed is:
1. The method of treating an image comprised of hydroperoxy groups
attached to a polymeric substate comprising a polymer with a
hydrocarbon backbone which comprises contacting same with sulfur
dioxide.
2. The method of treating an image comprised of hydroperoxy groups
attached to a polymeric substrate comprising a polymer with a
hydrocarbon backbone which comprises contacting same with
sulfolene.
3. The method of obtaining a visible image which comprises adding a
coloring material to a polymeric substrate comprising a polymer
having a hydrocarbon backbone containing an image in the form of a
differential distribution of sulfate groups attached to said
substrate.
4. The method of claim 3 in which a dye is added to selectively
color areas containing sulfate groups.
5. The method of claim 4 in which a polar dye is added in an
alcoholic solvent.
6. The method of claim 3 in which a dye in non-polar solvent is
used to selectively color areas not containing sulfate groups.
7. The method of obtaining a visible image which comprises treating
a polymer having aliphatic carbon-to-carbon unsaturation containing
an image in the form of a differential distribution of sulfate
groups with a liquid to selectively remove portions of said polymer
based on such differential distribution to form a visible
image.
8. The method of claim 7 in which a liquid is employed to
selectively remove areas not having sulfate groups.
9. The method of claim 8 in which an aliphatic hydrocarbon is
employed for selective removal.
10. The method of claim 8 in which the developed image is
transferred from one support to another support.
11. The method of claim 3 in which the substrate comprises a
polymer of a diolefin.
Description
The present invention is concerned with photoimaging and relates to
photosensitive surfaces useful in photography and photocopy
reproduction processes.
In the graphic arts industry represented by photography,
photoengraving, photolithography, collotype, etc. silver halides
and diazo compounds have been used as photosensitive materials to
absorb energy from the light spectrum in processes for making
photographic images and in reprography. Such photosensitive agents
have usually required close control of chemicals used, time and
amount of treatment to develop the images of objects replicated on
the surface by exposure of the object to the photosensitive surface
in the light spectrum. Such processes are both expensive and
impractical, however, when it is desired to prepare numerous copies
or replicas of an object or image on an inexpensive surface such as
a celulosic web for example cellulosic films, paper sheets and
boards, and closely woven cellulosic textiles. In addition, the
chemicals often used to develop and fix the silver halide or diazo
photosensitized surfaces often need close temperature and
concentration contro, or the use of water solutions of effect
acceptable reproduction on the photosensitized surface.
It is therefore desirable to find photosensitive materials and
substrates which can be placed on smooth surfaces to effect simple,
inexpensive photoreproduction of images exposed thereon without the
need for special facilities such as dark rooms, and for extensive
controls on time of exposure to light, developer type, or
concentration.
It has recently been found, as described in patent applications
referred to hereinbelow, that substrates, particularly polymeric
substrates, can be photoimaged so as to have chemical groups, i.e.
hydroperoxy groups, suitable for development of visible images, and
that the photosensitive surfaces and procedures involved can be
used directly for replica or image reproduction and copy work.
SUMMARY OF THE INVENTION
It has now been found that latent images can be composed of
materials containing differential distributions of non-migrating
sulfate groups and that such images can be developed to form
visible images. The latent image can be rendered visible by various
techniques dependent upon differences in properties occasioned by
the presence of such groups.
In general it is preferred that the sulfate groups be present as
substituents attached to polymer structures, although sufficient
stability as to the location can be obtained by attachment to other
coating or film layer materials. Such sulfate substituents can
conveniently be obtained by conversion of image-forming hydroperoxy
substituents to sulfate groups. Procedures for effecting imagewise
formation of hydroperoxy groups in unsaturated materials,
especially in ethylenically unsaturated polymers, are taught in a
copending application of Robert A. Hemisch and Eric T. Reaville,
Ser. No. 644,121 filed June 7, 1967, and a continuation-in-part
thereof Ser. No. 115,727, filed Feb. 16, 1971, and any of the
procedures of the aforesaid applications can be utilized in
preparing the sulfate-group images in accordance with the present
invention. In general the imagewise formation of hydroperoxy groups
is accomplished by imagewise exposure to light of an unsaturated
substrate in the presence of an oxidation photosensitizer and
light. The resulting latent image can then be treated with SO.sub.2
to cause conversion of hydroperoxy groups to sulfate groups.
In one aspect the invention involves the use of sulfur dioxide for
treatment of the image after exposure. In another aspect, the
sulfur dioxide can be used to treat the coating or components
thereof before exposure. The sulfur dioxide can, for example, be
incorporated into a solution with the photosensitizer, apparently
forming an adduct or complex with porphyrin sensitizer. If the
sulfur dioxide is used to directly treat the photosensitive coating
before exposure, it may complex with the sensitizer in such coating
or otherwise remain in the coating to have a subsequent effect upon
hydroperoxy groups. It is to be understood that materials which
generate or provide SO.sub.2, as well as SO.sub.2 itself, can be
used for the SO.sub.2 treatments or reactions herein.
The present invention involves use of a photosensitive layer
containing carbon-to-carbon double bond unsaturation in the
photoimaging procedure. Such layers are capable of undergoing
oxidation reactions under the influence of light to effect a change
in properties which can produce a latent image which is subject to
development and treatment as taught herein. The photosensitive
layer should be relatively stable in form to prevent undue
migration of compounds containing the unsaturated group, if such
compound should be of relatively low molecular weight or liquid.
ordinarily any film-forming materials will provide a stable matrix
to prevent migration of the sulfate substituents with polymeric
materials generally being used. Of course, some of the layer
materials disclosed herein have both the unsaturated groups and the
properties for forming stable films or coatings, e.g. high
molecular weight polymers containing residual unsaturation, such as
styrene/butadiene copolymers. Thus in general there will be no need
for using separate binder materials, but the use of such are fully
consonant with the present invention. Even though a particular
polymer in itself has all the properties necessary to serve as a
photosensitive substrate, it may nevertheless also be used in
conjunction with other polymeric binder materials which do not have
unsaturated groups, or in conjunction with other materials which do
have such groups.
Photooxidation photoimaging is convenient for use in the present
invention because of versatility, convenient work-ability, and
various other features of such procedures as pointed out in the
present application. In such procedures the various layer materials
containing unsaturation referred to herein can be employed. Any of
the materials disclosed in the aforesaid applications Ser. No.
644,121 and (C-11-21-0189) can be employed. The photooxidizable
layer may be any natural or synthetic material containing suitable
carbon-to-carbon unsaturated, which material is spreadable on a
suitable base support such as a glass or metal plate, a plastic
solid or sheet, or a paper sheet or board surface, etc. and is
sufficiently non-volatile at the temperature used. For use of this
invention at ordinary room temperature the substrate material
should have a molecular weight above about 140 so that it will not
be removed from the surface or from the reaction site by migration
in the oxidizing form or by evaporation from the treated surface.
The photooxidizable substrate may contain the suitable
carbon-to-carbon unsaturation as part of its structure or molecules
containing suitable carbon-to-carbon unsaturation may be added
thereto. Higher molecular weight polymers or other materials maybe
used as a binder for low molecular weight materials containing the
unsaturation to provide a suitably stable layer or film for
imaging.
Natural materials which may be used in photooxidation include rosin
and the double bond containing components thereof, terpenes such as
abietic-acid, neoabietic acid, maleopimaric acid, levopimaric acid,
.alpha.-pinene, camphene, 3-carene, citronellol, aldehyde modified
rosin materials such as formaldehyde modified rosins, and fortified
rosin materials such as those obtained by reacting the rosin with
alpha, beta-olefinically unsaturated polycarboxylic acids and
anhydrides thereof, and partial and complete esters of such acids
as maleic acid, fumaric acid, itaconic acid aconitic acid,
citraconic acid, etc., both saponified or unsaponified with an
alkaline material. Other examples include the use of unsaturated
fatty oils either in the glyceride ester form or in the free acid
form. A few examples of such oils include olive, peanut, almond,
neat's foot, pecan nut, lard, tung, safflower, cottonseed and
soybean oils. Non-glyceride source unsaturated oils such as tall
oil may also be used.
Unsaturated hydrocarbons of natural and synthetic origin may also
be used in photooxidation. Examples of such materials include the
aliphatic olefinically unsaturated hydrocarbons having an average
of at least about 10 carbon atoms, e.g. 1-dodecene, 1-tridecene,
1-tetradecene, 1-pentadecene, 1-hexadecene, 1-docosene,
1-pentacosene and the internally unsaturated olefins such as 7
heptadecene, 7,10-heptadecadiene, etc. the aromatic olefinically
unsaturated hydrocarbons in the unpolymerized form such as
isopropenyl toluene, phenylisobutene, phenylhexadiene and
isopropenyl naphthalene.
Natural and synthetic polymeric materials containing unreacted
carbon-to-carbon double bonds therein may also be used as the
photooxidizable substrate material in practicing this invention.
The carbon-to-carbon unsaturation may be intralinear e.g.
--CH.sub.2 --CH=CH--CH.sub.2 --, a vinylene linkage, terminal, e.g.
--CH.sub.2 --CH=CH.sub.2 vinyl, ##SPC1##
vinylidene and the like. Attached groups to the aforedescribed
entities may be linear or branched. In general, the polymeric
backbone will be hydrocarbon in structure with any halide, ester
ether, hydroxyl, nitrile, phenyl or other group present in the
polymer molecule appended to the polymeric backbone.
It will be understood that the vinyl compounds are a species of
vinylidene compounds since they contain the characteristic CH.sub.2
=CH-- group, the indicated free carbon valence being satisfied by
another atom in the polymer molecule. The term "vinylidene" is used
herein to include both vinylidene and vinyl unsaturation.
Illustrative examples of these olefinically unsaturated polymers
useful in photooxidation include natural rubbers; homopolymers,
copolymers, and polymers from three or more monomers, prepared from
diolefins such as butadiene, isoprene, 2,3-dimethyl-1,3-butadiene,
piperylene, chloroprene, bromoprene, 2-acetoxybutadiene-1,3,
2-methyl-pentadiene, 2-ethylhexadiene; and polymers prepared from
diolefins such as those aforementioned and compounds containing a
vinyl or a vinylidene group such as
a. Vinyl ethers, e.g. vinyl alkyl ethers such as vinyl ethyl ether,
vinyl butyl ether, vinyl octyl ether, vinyl dodecyl ether, vinyl
tetradecyl ether, vinyl hexadecyl ether, vinyl octadecyl ether and
vinyl alkenyl ethers e.g. vinyl ether, vinyl octenyl ether, vinyl
tetradecenyl ether, vinyl octadecenyl ether;
b. Vinyl esters, e.g. vinyl acetate, vinyl butyrate, vinyl
caprylate, vinyl caprate, vinyl laurate, vinyl myristate, vinyl
palmitate, vinyl stearate;
c. Vinyl halides, e.g. vinyl chloride, vinyl bromide;
d. Vinyl ketones, e.g. vinyl methyl ketone;
e. Vinyl sulfides, sulfoxides and sulfones, e.g. vinylethyl
sulfide; vinyl propyl sulfoxide, vinyl tert-butyl sulfone;
f. Vinylidene compounds, e.g. vinylidene chloride;
g. Acrylic, methacrylic acids or crotonic acids and their
derivatives, e.g. acrylic acid, acrylonitrile, methacrylamide,
crotonamide;
h. Acrylic, methacrylic esters or crotonic esters, e.g. methyl
methacrylate, ethyl acrylate, propyl acrylate, amyl acrylate,
heptyl acrylate, octyl methacrylate, nonyl acrylate, undecyl
acrylate, tetradecyl acrylate, hexadecyl acrylate, octadecyl
acrylate, ethenyl acrylate, hexenyl methacrylate, dodecenyl
acrylate, octadecenyl acrylate, ethyl crotonate;
i. Allyl esters, e.g. allyl acetate, allyl butyrate, allyl
caprylate, allyl caprate, allyl laurate, allyl myristate, allyl
palmitate, allyl stearate;
j. Allyl alkyl ethers, e.g. allyl ethyl ether, allyl octyl ether,
allyl dodecyl ether, allyl tetradecyl ether, allyl hexadecyl ether,
allyl octadecyl ether, and vinyl alkenyl ethers, e.g. allyl ethenyl
ether, allyl octenyl ether, allyl tetradecenyl ether, allyl
octadecenyl ethers;
k. Cycloaliphatic vinyl compounds, e.g. vinyl cyclohexane;
l. Aryl vinyl compounds, e.g. styrene, vinyltoluene, vinylbiphenyl,
vinyl naphthalene and the ar-chloro substituted styrenes;
m. Heterocyclic vinyl compounds, e.g. vinyl pyridine and vinyl
dihydropyran;
n. Alpha-olefins, e.g. ethylene, propylene, butene-1, octene-1,
dodecene-1-tetradecene-1-hexadecene-1-and heptadecene-1,
dichloroethylenes, tetrafluoroethylene; and
o. Branched olefins, e.g. isobutylene, isoamylene,
2,3,3-trimethyl-1-butene.
It is to be understood that the unsaturated polymers which are used
in the practice of this invention can also be prepared by
copolymerization of two or more different diolefins, e.g. from a
mixture of butadiene and piperylene, either in the presence or
absence of one or more non-dienic copolymerizable monomers.
The physical characteristics of the olefinically usaturated
polymers which can be photooxidized in accordance with the present
invention may vary from low molecular weight polymer oils
containing relatively few olefinic bonds to high molecular weight
rubbers and resins such as those resulting from the polymerization
or copolymerization of diolefins in the presence or absence of one
or more non-dienic copolymerizable monomers.
The photoimaging by photooxidation and SO.sub.2 treatment utilized
in the present invention depends upon the change in properties
caused by substitution of sulfate groups on molecular structures of
the photosensitive substrate, and appears to a considerable extent
to be due to the polarity of sulfate groups in such molecules. The
procedure makes possible differentially profound point-by-point
property changes in a film due to differential distribution of
non-migrating sulfate groups in such film. The differential
distribution of sulfate accurately reproduces the intensity of
light to which each differential area was exposed and the
difference in chemical and physical properties of the sulfated
regions compared to the non-exposed regions results in a latent
image which can be rendered visible by difference in dye sorption,
solubility, surface tack or other properties. For the differences
caused by the sulfation to be most evident, it is desirable that
the original photosensitive composition not have groups with
properties similar to sulfates, although some such groups can be
tolerated, even though contributing to an objectionable background
in some photoimaging procedures. Such polar groups as carboxy,
hydroxyl, nitrile, etc. may mask the photoimaging effect of
hydroperoxy groups to some extent. For this reason some of the
preferred polymers for use herein are hydrocarbon polymers with
residual unsaturation, e.g. styrene/butadiene copolymers, etc.
However, for the effect upon the original solvent solubility, or
upon mechanical film properties it may at times be desirable to use
polymers having various substituents, such as halogen, nitrile,
carboxyalkyl, etc. and polymers containing such substituents can be
used if some loss in photoimaging sensitivity can be tolerated in
the particular application in view. In general it will be preferred
that polar monomers not constitute more than three-fourths of the
monomer content of a copolymer used as the photooxidizable
substrate in the present invention.
Aside from characteristics of the photosensitive compositions for
forming latent photoimages in accordance hereinwith, various other
mechanical or other properties of the polymers will have
significance with respect to ease of coating application,
durability and handling properties of the film, but in general
those skilled in the art will be able to select appropriate
materials for particular applications, particularly in view of the
present disclosure.
In general solid polymers having carbon-to-carbon double bonds with
allylic hydrogen adjacent thereto are suitable for photooxidation
use herein. Such polymers will preferably be substantially linear
and have a weight average molecular weight of at least 50,000, and
at least one such carbon-to-carbon double bond for about every 50
carbon atoms; and more limited class of such polymers are
hydrocarbon in structure to avoid possible adverse effects of polar
or other substituents.
Photosensitizers used herein for catalyzing the preferred
photooxidation type of reaction are referred to generally as being
of the prophyrin type. The porphyrin type of photosensitizer may be
described as any compound having the prophin structure, i.e. four
pyrrole rings connected by single carbon or nitrogen atoms, which
includes related compounds such as the porphyrazines,
phthalocyanines or chlorophylls.
Particular photosensitizers useful in practicing photo-oxidation in
the present invention are the aromatic group meso-substituted
porphin compounds. Among such aromatic substituted porphins are the
ms-tetraarylporphins (ms-meso). These compounds are those porphins
in which aryl groups having from 6 to 24 carbon atoms are
substituted on the bridging carbon atoms of the porphin ring
structure which contains four pyrrole nuclei linked together in a
circular pattern by four bridging carbon atoms to form a great
ring. Examples of aryl groups which may be substituted in the
meso-position of these compounds are phenyl, chlorophenyl,
dichlorophenyl, methylphenyl, N,N-dimethylaminophenyl,
hydroxyphenyl, naphthyl, biphenyl, anthracyla, phenanthryl, etc. In
addition to the substituents in the aryl group substituents noted
above, the aryl groups can also have any or a combination of such
substituents, e.g. as alkyloxy (1 to 20 carbon atoms) substituents
such as methoxy, ethoxy, isopropoxy, butoxy, hexyloxy, etc., as
well as other substituents, particularly those which do not change
the fundamental aromatic character of the groups. These porphin
sensitizers including the above exemplified aryl-porphins, can have
various other substituents, particularly at the beta and beta'
positions of the pyrrole rings, e.g., such substituents as lower
alkyl (1-20 carbon atoms) such as vinyl or allyl or alkanoic acid
groups such as methylcarboxy or ethylcarboxy.
Examples of porphin compounds which are useful as photochemical
sensitizers in practicing this invention are the arylporphins such
as the tetraphenyltetrazoporphins and the complexes thereof, such
as diamagnetic complexes, e.g., magnesium
tetraphenyltetrazoporphin, tetraphenyl tetrazoporphin acetate,
tetraphenyltetrazoporphin sulfate, zinc tetraphenyltetrazoporphin;
the meso-aryl porphins including alpha, beta, gamma,
delta-naphthylporphin, and the diamagnetic metal chelates
thereof,
tetraphenylporphin
tetrakis(2,4-dichlorophenyl)porphin
tetrakis(2-furyl)porphin
tetrakis(4-methoxyphenyl)porphin
tetrakis(4-methylphenyl)porphin
tetrakis(2-thienyl)porphin
tetraphenylporphin zinc complex
tetrakis(4-nitrophenyl)porphin
tetrakis(4-dimethylaminophenyl)porphin
zinc complex;
the tetrabenzomonoazo- and tetrabenzodiazo porphins, the
1,2,3,4,5,6,7,8-octaphenylporphins and azoporphins such as
octaphenylporphyrazine, the tetrabenzoporphins, e.g.
tetrabenzoporphin and the zinc complex of tetrabenzoporphin.
Other useful porphin types of photosensitizing materials which may
be used include chlorophyll, such as chlorophyll a and chlorophyll
b, hemin, the tetrazoporphins, chlorophyllin salt derivatives such
as the reaction product of an alkaline metal chlorophyllin salt and
sodium bisulfite, hematoporphin, mercury proto- and
hemato-porphins; vitamin B.sub.12 and its derivative
tetrakis(1-naphthyl)porphin, porphin, ms-tetramethylporphin,
ms-tetraethylporphin and other ms-alkyl and alkoxy porphins.
Related porphin type materials which may be used include the
phthalocyanines including the metal-free phthalocyanine and metal
complexes of phthalocyanine such as the zinc and magnesium,
complexes of phthalocyanine, as well as phthalocyanine derivatives
such as the barium or calcium salts of the phthalocyanine sulfonic
acid, acetylated phthalocyanine, alkoxy- and
aryloxy-benzo-substituted phthalocyanines, 5,5', 5",
5'"-tetraamino-metal phthalocyanine-4,4', 4", 4'"-tetrasulfonic
acid; magnesium tetra(4)methylthiophthalocyanine, arylthioethers of
phthalocyanines, vinyl group containing tetraazoporphins and
polymers thereof, mercaptoamino phthalocyanine derivatives and
phthalocyanine.
Other useful photosensitizers which can be used include fluorescein
type dyes and light absorber materials based on a triarylmethane
nucleus. Such compounds are well known and include Crystal Violet,
Malachite green, Eosin, Rose Bengal and the like.
Another group of photosensitizers particularly useful in the
ultraviolet region include the aromatic compounds such as
acetophenone, benzophenone, benzoin, benzil and triphenylene.
Light necessary in the practice of this invention can vary
considerably in wave lengths, depending on the sensitizer in this
system. The light can be monochromatic or polychromatic. Light of
wave lengths in the range of 3600 to 8000 Angstroms has been found
very suitable.
Light sources suitable for use in the practice of this invention
include carbon arcs, tungsten and mercury-vapor arcs, fluorescent
lamps, argon glow lamps, electronic flash units, photographic flood
lamps and sunlight.
The present invention is particularly concerned with imaging
procedures utilizing photosensitized oxygen transfer reactions in
which light in the presence of a sensitizer causes to oxygen to
oxidize the carbon-to-carbon unsaturated substrate by being added
to one carbon atom of a double bond with shift of the double bond
to the adjacent position and movement of the allylic hydrogen atom
to the oxygen atom of the oxygen molecule which is not attached to
the carbon atom. The reaction can be postulated: ##SPC2##
The aforesaid oxygen transfer reaction does not include
autooxidations, proceeding by a free radical mechanism in which
irradiation with light serves to initiate free radicals and cause
the formation of free radical sites in the substrate by hydrogen
abstraction. The true photosensitized oxidation or oxygen transfer
reactions used herein are characterized by the fact that they can
proceed using wavelengths of light which may be ineffective for
autooxidation and by the fact that in general ordinary oxidation
inhibitors do not retard the reaction.
The photosensitized oxygen utilized herein involves what is
referred to by Gollnick and Schenck (K. Gollnick and G. O. Schenck,
Pure and Applied Chemistry, Vol. 9. 507 [1954]) as a Type 2
reaction, or "photosensitized oxygen transfer." The reaction
involves some excited oxygen species, whether pictured as an oxygen
molecule itself in an excited singlet state, or an excited
sensitizer-oxygen adduct. Irradiation with light appears to
transform the sensitizer to an excited state, such as a triplet
state: ##SPC3##
The sensitizer then transfers its energy to oxygen. This can be
postulated as ##SPC4##
and the excited singlet delta oxygen can then add to the double
bond as pictured above. It should be understood that if the triplet
energy of the sensitizer is above 37 kcal singlet sigma oxygen may
also be produced. It may react directly or decay to the lower
energy delta species. The reaction involves light energy to excite
the sensitizer and produce an excited state oxygen which reacts
with the substrate to cause addition of oxygen to one of the
doubly-bonded carbons thereof. The reaction ordinarily does not
include any chain propagation, but only one oxygen addition per
photon absorbed at quantum yield of unity. The photosensitized
reaction can proceed using wave lengths of light other than
ultraviolet, and ordinary oxidation inhibitors do not inhibit the
reaction. A senstizer-oxygen adduct is presumably formed but is
apparently short-lived; however it should be understood that the
oxidation is effective regardless of what the mechamism and exact
contribution of the adduct to the oxidation of the olefin may
be.
In another aspect, the present invention can utilize an oxidation
reaction in which a hydrogen atom is abstracted to give a radical
and oxygen is then added. For example the Type 1 reaction in the
foregoing Gollnick and Schenck article involves a reaction in which
the allylic hydrogen atom at C-3 is abstracted to give a mesomeric
mono-radical ##SPC5##
and oxygen is then attached at either of radical sites C.sub.1 or
C.sub.3, and the peroxyradicals thus formed extract hydrogen from
the C-3 position of the olefin to give hydroperoxides and a new
mesomeric radical, thus permitting chain propagation. Sensitizers
which are voracious hydrogen abstractors when excited by light
energy are suitable for use in such reaction. One group in such a
class is the carbonyl compounds, as for example, benzophenone,
acetophenone, etc. Aside from olefins containing allyl hydrogen,
other types of materials containing labile hydrogen can undergo
reactions similar to that of Type 1 to produce materials containing
hydroperoxy groups, and such materials can be used for photoimaging
in accordance with this aspect of the invention, for example, such
materials as ditetrahydrofurfuryl phthalate, di-2-ethylhexyl
phthalate, compounds containing tertiary hydrogen, etc. If such
materials are liquid they will generally be used in conjunction
with a polymer or some other high molecular weight binder to
provide a suitable matrix. in oxidations involving abstraction of
labile hydrogen to generate free radicals, other reactions such as
crosslinking can also occur and may interfere with the desired
photoimaging reaction, and such oxidations, while usable in the
present invention and useful to some extent, are not in accord with
the preferred aspects of the present invention.
Sensitizers which can function in both Type 1 and Type 2 reactions
include benzophenone, acetophenone, benzil, benzoin, etc. These
carbonyl type sensitizers requre ultraviolet light of suitable wave
length for excitation. Sensitizers which also function in both Type
1 and Type 2 reactions, but are active in the visible region as
well as the ultraviolet region of the spectrum include eosin,
fluoroescein, rose bengal, etc. Sensitizers which are not hydrogen
abstracters and which function largely or wholly by energy
interchange as in Type 2 reactions, include the various porphyrin
type photosensitizers. In general photosensitizers capable of Type
2 sensitization are much preferred, and ordinarily the better ones
of this Type are not very effective as Type 1 photosensitizers.
Moreover, the absence of Type 1 activity may make it easier to
minimize side reactions. There will be variations in the
effectiveness of sensitizers with the physical type of phase
involved and it will be understood that the sensitizers utilized
herein will be those effective under the conditions of use. The
sensitizers described above are known to function in liquid phase.
In the practice of this invention some systems have involved the
post-application of a sensitizer solvent system to an already
pre-formed film. The resulting system can be called a quasi-solid
phase as it consists of a solid polymer lightly swollen with a
swelling solvent system. In such a quasi-solid phase, the
sensitizers listed above and similar photosensitizers are found to
effect photochemical addition of oxygen, although with great
variation in effectiveness. In the practice of the invention it is
also feasible to use solid phase systems in which the sensitizer
has been incorporated into a polymeric film composition which has
not been swollen by solvent. In the solid phase such sensitizers as
rose bengal, fluorescein, eosin, methylene blue, etc. did not
exhibit significant photoimaging capability in reasonable exposure
times. Benzophenone, acetophenone, benzil, etc., were found to
require high level of application, circa 10%, and prolonged
exposure, circa 30 minutes at ultraviolet flux of 7 .times.
10.sup.5 ergs/cm.sup.2 /sec. in order to show substantial
photoimaging capability. In contrast to this, porphyrins, such as
tetraphenyl porphin are very active at levels as low as 0.1% at a
flux of 2.5 .times. 10.sup.5 ergs/cm.sup.2 /sec. for 1 to 2
minutes. Triphenylene in the solid phase seems to function largely
by crosslinking, making it of little value for continuous tone
image production.
The photosensitizers can be placed in solid phase by incorporation
into a coating solution, emulsion, melt, or suspension, etc. and
application to support. After evaporation, drying, or other means
of removing volatile solvents or other liquid medium, the
photosensitizer remains in the residual coating composition
dispersed in solid form in the solid composition. Even if the
photosensitizer is incorporated by post-application with a solvent,
it can be converted to the solid state by permitting the solvent to
evaporate. For ease of handling and reproducibility, it is
preferred that the photosensitive compositions be dry at the time
of photoimaging. It will be recognized that in referring to the
photosensitizer as being in a solid medium, it is not meant to
exclude such resilience, elasticity or other properties as may be
desirable in photosensitive films for various purposes, and that
plasticizers or low molecular weight materials may be present for
such purposes. Thus the solid state contrasts with the quasi solid
state in which liquid is present to swell the polymer structure as
in a plastisol or organosol and to provide a liquid medium in which
the photosensitizer can be present, aside from how much of the
photosensitizer is actually present in such medium.
The photosensitizer as employed will generally be of a type
suitable for producing a desired amount of oxygen addition in the
particular photosensitive composition upon degrees of light
exposure within ranges of practicality for some photoimaging
applications. For most applications it will be desirable that a
required energy absorption be no greater than 10.sup.7
ergs/cm.sup.2, or possibly 10.sup.8 ergs/cm.sup. 2 in some
applications, and preferred that such absorption be sufficient with
a concentration of photosensitizer no greater than 1% by weight of
the photosensitive composition, for example with a flux sufficient
to obtain such absorption over a 2-minute exposure period. The
energy required for imaging will generally lie in the range of 1 to
10 .times. 10.sup.6 ergs/cm.sup.2. Absorptions with usual imaging
procedures are frequently of the order of 2.5 .times. 10.sup.4
ergs/cm.sup.2 /sec. The foregoing energy ranges are those generally
employed with the concentrations of photosensitizer generally used,
but increasing the concentration makes it possible to use lower
energy input for the photoimaging as the imaging requirements are
approximately in inverse proportion to the photosensitizer
concentration. The use of various amplifying means also makes it
possible to lower the energy requirements. In order to have
practical value in a photoimaging system, a photosensitizer as used
should produce an oxygen uptake of at least about 10.sup..sup.-9
moles/cm.sup.2 with an absorbed energy of 10.sup.7 ergs. A
styrene-butadiene polymer (40 wt. parts styrene/60 wt. parts
butadiene) film can suitably be used for determining oxygen uptake
at particular energy absorptions.
Conversion of the hydroperoxy groups to sulfate groups can be
considered an amplification technique. Thus the acidic sulfate
groups are strongly polar and exert a stronger effect than
hydroperoxy groups on most properties of the substrate upon which
developability of an image depends. For this reason the exposures
needed when SO.sub.2 treatment is involved tend more toward the
lower portion of the cited ranges than is the case when only
hydroperoxy groups are involved.
Only a portion of the applied light flux will be absorbed by the
photoimaging substrate, with that portion varying with the
absorption characteristics of such substrate, as well as with the
wave length of the light. Absorptions may, for example, be less
than 10%, or of the order of 7% in some cases. Visible light can
effectively be used in the present invention, and this is a
definite advantage as it avoids the cost and loss in efficiency
which results from having to produce light in particular ranges,
such as the ultra violet range. In the present invention it is not
necessary to utilize the ultra violet range, and in fact, the range
of 4000 to 4500 angstroms appears most efficient. If desired, light
in ranges above 4000 angstroms can be used to avoid possible ultra
violet catalysis of competing reactions, although this is not
ordinarily necessary. While visible light is effective, the present
materials can generally be handled in ordinary daylight, such as by
removing the photosensitive material from the intense light used
for exposure, and carrying out development or other steps without
special precaution to avoid further exposure to ordinary ambient
light.
The preferred sensitizers for use herein belong to the class of
porphyrins which are compounds with pyrrole rings linked together
by carbon atoms to form a conjugated double bond structure, and in
which one or more of the carbon atoms, i.e. methine groups, can be
replaced by a nitrogen atom, and which class also includes the
phthalocyanines and benzoporphyrins, as well as the
meso-arylporphyrins, or other compounds having various substituents
on the basic porphyrin structure. The tetraaryl porphyrins,
particularly tetraphenyl porphyrin, are characterized by the
capability to utilize energy from three of the main areas of the
visible light spectrum, i.e. the violet, yellow and red areas and
therefore to make efficient use of light energy. Such sensitizers
can be termed panchromatic photooxidation sensitizers and are
particularly valuable. Some other sensitizers are active only in
limited areas of the spectrum, for example chlorophyll absorbs
mainly in the red region. Aside from efficiency, the panchromatic
sensitizers are advantageous for multi-color work in that it makes
it possible to use a single sensitizer for exposures to reproduce
different colors from an original, rather than having to change
sensitizers for each of the main segments of the color spectrum.
Tetraphenyl porphyrin absorbs most strongly in the violet, and less
in the yellow and least in the red, which is advantageous in that
visible light is the reverse, being strong in the red, etc., and
therefore the sensitizer compensates for the variation in
light-intensity and tends to equalize the effect of different
intensities. Utilizing the panchromatic sensitizers, the present
invention produces images from multi-color transparencies. If
polychromatic light is used with a single exposure, the resultant
image will not ordinarily exhibit appreciable differentiation
between the colors, but the image produced will have parts
corresponding to the various colors. The image can be developed by
applying a single dye to give a monochromatic image e.g. red and
white, or by using a solvent to remove areas either corresponding
to the colored regions or the non-colored regions.
The sulfate materials used herein are materials having sulfate
groups, ##SPC6##
chemically bonded thereto. Such groups can be represented as
--OSO.sub.3 M, where the oxygen bond is generally to a carbon atom
of a substrate material, and M represents hydrogen, metal, or other
moiety capable of forming a salt with the sulfate anion. It will be
recognized that the sulfate group has ionic character, so the group
can also be represented as --OSO.sub.3.sup.- M.sup.+. M can, for
example, be such ions as hydrogen, sodium, potassium, ammonium,
ethyl ammonium, dimethylammonium, trimethylammonium,
tetraethylammonium, etc., as well as polyvalent cations such as
calcium, aluminum, etc. In the case of polyvalent cations, the
number of sulfate groups will be such as to balance the valence of
the cation, e.g., to sulfate grups for one calcium ion. It is also
feasible to use partially neutralized cations effectively as
monovalent cations, e.g. to use ##SPC7##
Ordinarily the present invention will employ the --OSO.sub.3 H
groups which is readily obtained in accord with procedures taught
herein. However, at some stages of development or for other reasons
it may be desirable to neutralize the acidic sulfate group, and
this can conveniently be done by general procedures for
neutralizing sulfate or other acidic groups. Ammonia, being a
volatile base, can be conveniently used for such purpose and
converts the sulfate groups from hydrogen sulfate groups to
ammonium sulfate groups. It will be recognized that the cation will
have an influence upon solubility properties, and that some
neutralization procedures may effect development by washing away
portions of the coating as taught herein.
With the unsaturated polymers and the photoimaging procedures
utilized herein, the imaged coatings will be characterized by the
presence of groupings represented by ##SPC8##
However, the illustrated double bond does not have a marked effect
upon properties ordinarily utilized for development of the images,
and the presence of such unsaturation is not essential as the
sulfate groups give the desired properties for developing visible
images, regardless of whether the unsaturation is present. Thus the
photoimaging procedures are effective even if some or all of the
unsaturation is lost in the photoimaging, or if a method of
obtaining the imagewise distribution of sulfate groups does not
utilize an unsaturated polymer as a starting point.
It will also be recognized that the procedures herein for
converting hydroperoxy groups to sulfate groups need not convert
all hydroperoxy groups, and are unlikely to give 100% conversion in
all cases. Thus the photoimages will often involve the differential
distribution of both hydroperoxy and sulfate groups. As the
response of these groups to various development procedures is
similar, the development procedures taught herein can suitably be
employed with latent images in which both groups are present. It is
advantageous that the sulfate group be firmly bound to a large
polymer moiety in order to have a stable location. However, it is
recognized that the sulfate group is reactive, and potential
reactions may involve rupture of the bond to the polymer.
In converting hydroperoxy groups to sulfate groups in the present
invention, any methods effective to achieve such conversion can be
employed. Peroxy groups readily react with sulfur dioxide to
produce sulfate groups. It is generally convenient to employ sulfur
dioxide gas for such treatment. However, any source of sulfur
dioxide can be employed, including generation in situ. For example,
an aqueous bath of meta bisulfite salts can be utilized to generate
SO.sub.2, and the peroxidized coating can be exposed to vapors over
the bath. NaHSO.sub.3 is convenient for use, but other bisulfite
salts which ionize can be used. Solutions containing SO.sub.2 can
be used, although the effect of the solvent medium on the coating
and support must then be considered, in accordance with the
teachings herein concerning liquids for use in development by
coating removel. Aqueous solutions containing or generating
SO.sub.2 can be used for direct treatment of the coatings, although
this will have disadvantages when the coating is on a paper support
and the strength and dimensional stability of such support are
adversely affected by water. Treatment of the coatings with
solfolene, before or after exposure, or before or after
development, produces substantially the same results as treatment
with sulfur dioxide gas.
The sulfate containing photoimages will generally be produced by
converting --OOH groups to sulfates. However, it is also
contemplated that the hydroperoxy groups first be converted to
alkali peroxy, e.g., sodium peroxy, groups or to other alkali or
alkaline metal intermediates, and the latter then converted to
sulfates through treatment with sulfur dioxide.
The latent sulfate-group containing photoimages can be developed in
general by applying any of the development procedures taught in the
aforesaid copending applications of Heimsch and Reaville for
photooxidation images.
The dyes used in this invention to fix or develop the images
defined by differential distributions of sulfate groups, as may be
produced by photo-oxidation and sulfation, may be any dye which has
varying affinities for sulfated and non-sulfated sites on the light
exposed treated surface. Dyes generally found useful in this
invention are the organic solvent or oil soluble dyes such as the
kerosene or alcohol soluble dyes, e.g., the triphenylmethane type,
azo dyes and disperse dyes. It has been found that dyes dissolved
in a solvent such as deodorized or highly refined kerosene are
directed generally to the non-sulfated portion of the exposed
treated surface and that dyes dissolved in an alcohol such as
2-ethylhexanol are directed chiefly to the sulfated portions of the
exposed treated surface. The particular site at which the dye
locates itself appears to depend on the selective swelling
characteristics of the solvent used, the effect of the dye on the
solubility parameter of the solvent, and on the polarity of the
dye. Although these factors appear to control the direction of the
dye, the actual chemical or physical mechanism of the direction and
image application brought by the dye is not fully understood.
The solvents or dispersants which are used for the dye may be any
organic material which will (1) dissolve or disperse the dye and
which will aid in selectively directing the dye to the sulfated or
non-sulfated regions of the exposed treated surface so that the
differences in sulfation in the various areas of the surface can be
readily made apparent thereby. Suitable solvent or diluents for the
dyes which can be used include low melting molten waxes, liquid
alkanes, cycloalkanes, alkanes mixed with aromatic compounds such
as benzene, toluene, xylene, chlorobenzene, etc., aliphatic fatty
acids having from 6 to 24 carbon atoms, molten unsaturated fatty
acids such as palmitic acid, higher liquid aliphatic alcohols
having from 6 to about 20 carbon atoms, and such higher alcohols
mixed with up to about 50% of lower alcohols, aliphatic esters
which are liquid or low melting (below 100.degree.C.) solids at
room temperatures such as triacetin, ethyl hexanoate, methyl
oleate. The dyes may be used in any desired concentration in the
solvent or diluent but a solution containing about 0.1 to about 6
percent of dye by weight in the selected solvent is generally
sufficient for most fixing or developing purposes of this
invention. The dyes may also be applied as a solid.
Examples of dyes found in the Colour Index which are useful for
amplifying or developing latent images produced with a positive
image forming object, e.g., a typewritten opaque white sheet,
include Sudan Brown, Sudan Red, or Calco Oil Red dissolved in
deodorized kerosene. Examples of dyes useful for amplifying or
developing latent images produced with a negative image forming
object, e.g., a photographic negative film, include Crystal Violet,
Malachite Green, Victoria Blue or Nigrosine B dissolved in
2-ethylhexanol.
The site to which the dye is directed depends both on the dye and
the solvent. For example, Sudan Brown dissolved in kerosene goes
preferentially to the non-sulfated site. A strongly basic dye like
Crystal Violet goes preferentially to the sulfated sites. In some
cases inversion appears to occur at sites of too high a degree of
sulfation with the dye being rejected at such sites.
The dyes may be simply wiped on the exposed photooxidized element
as with a rag, brush, or sprayed on or applied by other
conventional methods, and then dried as by wiping the dye treated
surface with a dry cloth or tissue. The result is a clear, useful
print or copy of the object to be copied or reproduced. While
various dyes are selective in that they absorb in differential
concentrations in various exposed and non-exposed areas, it will be
recognized that some dyes will absorb to some extent in areas other
than those intended for development by such dye, and the
acceptability of this depends upon the degree of absorption,
compared to absorption in intended areas, and whether background
color is objectionable in the particular application. Such effects
can be regulated by control of exposure times and dye application
procedures.
It will be recognized that the acidic sulfate groups can effect the
color of some dyes, and this must be taken into account in the
selection of dyes for image development. In general the solvents
used herein for development by coating removal will be relatively
poor solvents for the polymer substrate involved, as a selective
removal is desired. For selective removal of non-exposed areas,
solvents will be selected which are effective to soften or dissolve
the non-exposed areas, but which are not sufficiently effective to
remove exposed areas in which sulfate groups or other groups
affecting solubility are present. Solubility parameters present a
suitable guide for choosing appropriate solvents, although there
will be some variance with individual members of such classes. Such
parameters .delta. based on cohesive energy density, are described
in Solubility Parameters by Harry Burrell, Parts I and II,
Interchemical Review, Vol. 14, No. 1, pages 3-16, and Vol. 14, No.
2, pages 31-46 (1955). As the sulfate groups increase the
solubility parameter, the solvent will generally be selected to
have a parameter slightly less than that of the polymer employed.
Thus with a diene rubber having a solubility parameter around 8.5,
a suitable solvent could have a solubility parameter less than 8,
such as methylcyclohexane with a paramter of 7.8. Many of the
hydrocarbon diene polymers suitable for use herein have solubility
parameters in the range of 8 to 9.5, and accordingly, the
developing solvents usually will have solubility parameters below
8, although higher parameters may at times be suitable. Many of the
solvents suitable for use herein for removal of non-exposed areas
will have solubility parameters in the range of 7 to 8 or 8.2.
Poorer solvents when used sometimes require longer development
times, but this may not be objectionable in some applications.
Aliphatic hydrocarbons, including cycloaliphatics, provide a number
of suitable solvents for use with particular polymer systems, e.g.,
kerosene, VM and P naphthas, methylcyclohexane, octane, etc. Some
aliphatic, i.e. non-benzenoid, unsaturation may be present in such
solvents, but aromatic solvents may have too high a solubility
parameter for use with the preferred polymer systems herein.
Solvents such as benzene, toluene, etc. may dissolve both the
exposed and non-exposed areas in the photooxidation imaging system.
Thus it is not preferred to use really good solvents for the
polymer for the development purposes, but to select solvents of
lower solubility parameter than the polymer, e.g. 0.2 to 0.5 or
more units lower. However, sulfate groups have a strong effect on
solubility, so fairly good solvents can often be tolerated for
development. If the starting polymer is readily soluble in toluene
or benzene, as is usually the case herein, the exposed areas after
photooxidation imaging may still be soluble as the imaging does not
crosslink or otherwise change such polymer so as to render it
completely insoluble in such solvents. It will be recognized that
the suitability of particular development solvents will vary with
such factors as the molecular weight of polymers in the film
substrate, characteristics of other additives or components in the
film, desired speed of development, film thickness and strength,
etc. The sulfate groups are strongly polar, and therefore to avoid
dissolution of the exposed areas, solvents lacking strong polar
groups are ordinarily employed. Hydroxy, keto, etc. groups tend to
increase the solubility parameter, and therefore not generally used
unless with a high solubility parameter polymer system, or if such
groups are present in such proportions in the solvent as not to
unduly affect the parameter.
For the development of images by dissolution of the exposed
sulfated areas, solvents of higher solubility parameter, such as
polar solvents, are employed. In broad terms, solvents will
dissolve polymers if the solubility parameters match. Thus if a
high solubility parameter solvent is used, it can dissolve the
sulfated region having a similar solubility parameter, but has too
high a parameter to dissolve the non-exposed regions of the
polymer. The precise relationship between the number of sulfate
groups and the solubility parameter is not known, but the solvents
chosen should have a solubility parameter above that of the
original polymer so as not to dissolve non-exposed areas. Polar
solvents, such as hydroxyl containing compounds, e.g., various
alcohols, glycols, cellosolves, etc. can be used. Solvents will be
selected so as to give the desired degree of solubility without
unduly affecting the non-exposed area, and may at times have
solubility parameters considerably above that of the original
polymer substrate. It is to be recognized that the solvents herein
can include mixtures of various solvents, and that descriptions
herein are mainly concerned with the overall characteristics of
solvents, rather than with characteristics of individual components
thereof. For example, a particular naphtha may be considered as a
low solubility parameter aliphatic hydrocarbon solvent even though
it has some aromatics content. With some of the solvents for the
sulfated areas it may be desirable to use small amounts of bases,
for example, alkalies such as sodium hydroxide, or chemical
reagents to aid in the dissolution, as the --SO.sub.3 H groups is
acidic, and forming salts may result in solubility.
Another useful guide in choice of development fluids is the KB
value. The KB value, i.e., Kauri-butanol value, is a measure of the
solvent power of a solvent as determined by dissolving Kauri resin
in butanol, and titrating the amount of a given solvent necessary
to titrate to incipient insolubility. In general, with
hydrocarbons, the stronger the solvent, the higher the KB value.
The KB value has a fairly good correlation with solubility
parameter for aliphatic solvents. Reported KB values are available
for many solvents. When a solvent of particular KB value has been
found suitable for developing a particular photosensitive coating,
with a particular application method, other solvents of similar KB
value will also be suitable. The initial selection can be made by
trying solvents on a scale of gradually changing KB value,
generally in half-tone work using one which is just sufficiently
effective to remove non-exposed areas well, but without disturbing
the exposed areas. In the case of mixed solvents of vastly
different character, e.g., hydrocarbons and polar solvents,
effective KB value must be used, as dependent upon the KB values
and relative amounts, as the ordinary method of determining KB
values would not give the proper value in such case. With the
unsaturated hydrocarbon resins preferred herein, particularly
styrene/butadiene polymers, useful KB values are usually in the
range of about 32 to 38, or up to 39 to 40 with some application
methods.
The latent images obtained herein are at times visible to some
extent after the SO.sub.2 treatment without further treatment.
Moderate heating will generally render the image clearly visible,
e.g., as a black or brown image in the exposed areas against a
light background. The color of the image can be affected, of
course, by pigments or dyes present initially in the photosensitive
coating. Heating when used, can, for example, be in the range of
50.degree. to 150.degree.C. or higher for a short time, such as a
few seconds up to 5 minutes. The heating apparently causes
reactions of the sulfate groups, and of any peroxy groups still
present. The images may also develop a different background color
in non-exposed areas, such as a yellow green color from the
porphyrin catalyst or effect of the sulfur dioxide. This background
color can ordinarily be removed by treatment with alkaline
materials, such as ammonia, with further heating if desired. The
ammonia treatment may lighten the exposed area changing it to a
brown from black.
The sulfated photoimages and procedures described herein can be
utilized in the photoimaging and transfer procedures described in
my copending application Ser. No. 119,911 filed Mar. 1, 1971, and
can be employed as described therein for preparation of four-color
composite pictures or for other purposes. In following such
procedures, or in the general practice of the present invention,
pigments can be employed in the photosensitive coatings. The
pigments employed herein can in general be any pigments which have
the desired colors. For the most part the pigments will be organic
compounds and these are most suitable although inorganic colorants,
for example chrome yellows, can also be used. As used herein the
term pigment refers to solid, colorant materials which are
insoluble, or at least of very limited solubility, in the polymer
solvent systems employed in coating and development. The term dye
is used to designate colorant materials employed in solution. The
pigments as used are generally in very finely divided,
nonagglomerated form, such as in ranges up to 0.5 micron, say 0.1
to 0.5 micron, and seldom exceed 1 micron in diameter. The
advantage of the small particle size can be seen with reference to
obtaining fine resolution, and also for production of thin, smooth
coatings. If not already in fine particle size, the pigments should
be reduced to such size, and it may be necessary to break up any
agglomerates in preparing the coating formulations. The pigment
loading will be regulated to give the desired color depth with the
particular coating thickness and other factors. However, the
pigment also has an effect upon the developability of the coatings
as it tends to make the coatings fracture upon washing with
appropriate fluids. A 5 to 10% by weight amount of pigment, based
on polymer, is usually sufficient for this purpose, and loadings up
to the amount the polymer can bind can be employed, but it will
generally be convenient to operate in the range of 10 to 50% by
weight, based on polymer. If for some reason a particular coating
is to have little or no color, and is to be developed by liquid
wash, it may be advisable to add some insoluble solids to the
coatings for the effect upon developability, the amounts possibly
being sufficient to provide 5 to 10% or so of solids, taking any
pigments into account as well as any white, colorless, or low-color
solids. For example, various amounts of silica or other fillers can
be added along with pigments.
In preparing four-color proofs in accord with the present
invention, color separation exposures are formed on photosensitive
polymeric coatings on supports. The coated supports are prepared by
coating supports with a material comprising polymer,
photosensitizer and pigment. Various procedures can be employed for
coating the support, but the following exemplifies a suitable
procedure. Mastergrinds of pigments are prepared containing in
parts by weight, 30 parts pigment solids, 170 parts toluene, 0.2
parts of a surfactant and 25 parts of a 20% solution of
styrene/butadiene copolymer in benzene. The pigments are gound for
a number of hours with ceramic balls. The polymer to be employed is
placed in solution as a 20% solution in benzene by rolling pieces
of the polymer in benzene on a ball mill. The polymer solution is
sensitized by adding sensitizer thereto, e.g. tetraphenylporphin
for photooxidation photoimaging. The desired amount of sensitizer
can be added dissolved in 60 parts by weight of chloroform which is
added to 50 parts of the polymer solution, along with 50 additional
parts benzene. Varying amounts of sensitizer can be used, for many
applications a suitable amount is 0.25% by weight of
tetraphenylporphin on the total polymer for the yellow, magenta and
cyan coatings, and 0.5% for the black coating. Coating slurries are
made from the pigment grinds and sensitizer, polymer solution,
conveniently by adding the polymer solution slowly with stirring to
the pigment grind in a brown bottle, and adding benzene to obtain
the desired solids content. The slurries can conveniently be used
as 6% solids concentrations (polymer and pigment). Suitable pigment
loadings, are, for example, 25 parts per hundred yellow and cyan,
and 20 parts per hundred of magenta and black, the parts being by
weight on the polymer. It may be desirable to filter the slurry to
insure absence of large particles, e.g. through a seven micron
opening nylon screen. The coating slurries are then used to coat a
support appropriate for transfer, e.g., a release paper. The
coating can be accomplished by usual procedures for applying liquid
coatings, for example, by using wire wound rods, such as a No. 10
for yellow and cyan, No. 14 for magenta and No. 12 for black. If
the coated papers are to be stored before use, they can be faced
with another sheet of release paper to prevent damage to the
coating.
The color separation exposures are formed on the coated release
papers using the appropriate color separation transparency. A 4000
watt pulsed Xenon light can conveniently be used. During the
exposure when a photooxidation is involved, the photosensitive
coating may have access to oxygen. This can conveniently be
provided for by having an air gap between the transparency and the
photosensitive surface. For example, the coated support can be held
flat on a vacuum plate. The transparency is on top of the coated
support and held in register by register pins. The transparency is
topped by a glass plate which is supported by spacers one-half mil
thicker than the transparency. An air stream is then forced between
the transparency and the coated support, forcing the transparency
against the glass plate, thereby providing uniform spacing from the
photosensitive coating, and providing oxygen for the photooxidation
reaction. With the above light coil held 32 inches from the coated
film, the flux as measured by a radiometer was 5.2 .times. 10.sup.5
ergs/cm.sup.2 /sec. Typical exposure times for photooxidation
imaging with this flux are 2.5 minutes for yellow and magenta
coatings, 2.75 minutes for cyan coatings, and 7 minutes for black
coatings.
For development of the images, areas of the image can be
selectively removed by treatment with fluids. For negative working,
the non-exposed areas are removed. This is generally accomplished
by applying a liquid having some affinity for the polymeric coating
but not very great solvent power for it, such as an aliphatic
hydrocarbon when the polymer is a styrene/butadiene copolymer. For
example, a varnish makers and painters naphtha with a KB value of
36 (Skellysolve V) can be used with a styrene/butadiene copolymer.
Any type of washing procedure with such fluids can be employed,
provided that care is exercised to avoid excessive mechanical
stress on the coating. Spraying with the liquid has been found
suitable, whether by hand with a nozzle, or mechanically with a
gang of nozzles. Spray nozzles can, for example, be about 5 inches
from the coating and operated from a tank under 25 psi. Passing
back and forth before the nozzles for about 20 seconds is usually
sufficient. In general the fluids employed in the development step
will be liquids. However, materials having the proper affinity for
the coating can also be employed in gaseous or vapor form,
employing mild heating if desired. The spray nozzles can also
employ air or other gases under pressure as an aid in spraying, or
various other expedients employed in spraying paints or cleaning
solutions. It is also feasible to employ other washing methods,
such as immersing or dipping the coatings in the solutions, with
mild agitation. This can at times be combined with blotting or
wiping dry, although care must be exercised to avoid damaging the
image and it is generally advisable to avoid rubbing the coating.
In the dipping procedure the wash solvent becomes discolored with
the pigment, and presents some redeposition problem, but this can
ordinarily be overcome by changing the wash solution in the later
stages.
When the image is to be transferred, the support from which the
image is to be transferred should have a surface to which the
photosensitive coating has low to moderate adherence. Release
papers used in casting of plastics in general have suitable
surfaces if they can be coated with the photosensitive material.
Such papers can have matt, dull, gloss or patent finishes, so long
as the proper release properties are present. In paper or other
materials, the release properties are generally provided by release
or parting agents, such as waxes, silicones, fluorocarbons,
metallic stearates, or other soaps, paraffins or other
hydrocarbons, e.g., polyethylenes, or inorganic materials such as
silica or silicates, e.g., calcium silicate. Such materials can be
applied as a solution or suspension in a volatile solvent which
evaporates, leaving a film of the release material on the surface.
Such materials need not necessarily be a completely contiguous film
as such materials can frequently be effective if incorporated into
the body of the support material, as practiced with antiblocking
materials in plastics manufacture, or of various agents in paper
manufacture. There are a number of proprietary silicones used as
release agents, some of which are dimethyl siloxane resins. In
general such resins, or lower molecular weight materials, having
alternating silicon and oxygen atoms, with hydrocarbon substituents
on the silicon, have release properties. The series of silicones
manufactured by Dow Corning as release agents give release
surfaces, including those of the 200 series, e.g., Dow Corning 203
fluid, or Dow Corning 230 Fluid, which is an alkylaryl polysiloxane
fluid, or emulsions such as Dow Corning HV-490 emulsion which is a
silicone emulsion prepared from 100,000-centistoke dimethyl
polysiloxane fluid. Possibly, the foregoing will be modified, e.g.,
as Dow Corning No. 23 silicone release coating has its release
properties degraded by Dow Corning C-4-2109 release additive. The
desirable coatings will vary somewhat with the properties of the
photosensitive coatings employed, the surface properties of the
support to which the image is to be transferred, and the
development liquids and washing procedure employed. However in
general the release surface will have a relatively low surface
energy so that the developed image can be transferred to the higher
surface energy receptive support. At the same time the
photosensitive coating has to be sufficiently adherent to the
release surface that it is not completely removed therefrom in the
washing development. Any surface can be used for the receptive,
i.e., transfer support, provided only that the photosensitive
coating can be bonded to it. Paper, including paperboard, can be
conveniently used for the transfer surface, either as uncoated
paper stock, or coated paper. For example, high quality papers,
bond paper, etc. are suitable, and in general most papers having a
glossy surface. Cast coat, a high quality paper provided by the
Abermarle Paper Company is satisfactory. Papers having the usual
titanium dioxide, clay and synthetic binder, e.g.,
styrene-butadiene resin, coatings are suitable. Various other
surfaces can be used for transfer surfaces, e.g., plastic films,
metals in general, etc. For reasons of economy and convenience,
paper surfaces will generally be preferred.
After development, the images can be dried and transferred. When a
composite proof is being made, a number of images are separately
transferred in register from their supports to a single transfer
support, and the images superimposed thereon constitute the
composite proof. The transfer is accomplished by contacting the
second support with the image, and stripping the original support
from the image. Simple lamination procedures can be employed for
the contacting, such as pressing the materials together, and, if
desired, applying heat. A platen press can conveniently be employed
with pressures of the order of 100 to 500 lbs/in.sup.2. Heat can
conveniently be used by heating the platen upon which the receptive
transfer support rests, while the upper platen is cooled by tap
water. Temperatures employed will be well below the melting point
of the coating materials, but can make the surface more tacky or
soften it to promote bonding to the receptive surface. A rubber
blanket or other resilient material may be used between the upper
platen and the release support to aid in equal distribution of the
pressure. For styrene/butadiene copolymer materials, a bottom
platen temperature, for example, around 80.degree.C. is
satisfactory. A short sojourn under pressure is sufficient, say
five seconds or so. The sheets are then cooled and the original
support is separated, leaving the image on the receptive transfer
support. Ordinarily a clean transfer is obtained and no residue on
the release support is observed. At times there may be some
residual color left, or even some background color in areas where
the coating has been removed by development, but this is ordinarily
not harmful as it does not usually transfer. In the transfer the
heating is ordinarily very brief and limited. Heating can affect
the photoreaction products, e.g., hydroperoxy or sulfate groups,
and promote further reactions thereof, or affect the colorants, and
this should be taken into consideration when employing heat in the
development or transfer steps. In some cases such further efforts
may be desirable, but if not, only mild heating conditions should
be used. Platen temperatures are usually in the range of 50.degree.
to 150.degree.C.
In view of the effectiveness of the sulfate groups in improving
resistance to certain solvents, as compared to hydroperoxy groups,
the air gap in the photoimaging step can be dispensed with if
desired. It is still necessary to have oxygen present for the
photooxidation imaging, but sufficient can diffuse to the
photosensitive surface for imaging in practical exposure times if
sulfur dioxide and ammonia treatments are also employed. In
general, however, to insure good results, an air gap or other means
of providing oxygen will be employed.
The procedures herein are suitable for producing either continuous
tone on half-tone images. The procedures herein involving solvent
removal of portions of the coating and subsequent transfer of the
developed image are particularly suited to the production of
half-tone prints, in procedures which generally involve the
transfer of coatings in the form of dot areas.
The procedures herein produce differential film properties,
resulting from differential light exposure and consequent
differential distribution of sulfate groups, and differential
permeability to fluids is among the properties produced. Thus
fluids can be passed through a film or coating containing such
sulfate groups in imagewise fashion to produce an image on an
adjacent substrate or support, by dying, deposition, etching or
otherwise. The exposure time or other imaging variables can be
adjusted in conjunction with the coating thickness to obtain the
desired degree of photooxidation penetrating to the opposite
coating surface to define the image and to permit selective passage
of fluids through the coating in imagewise manner. In general,
fluid permeability can be used herein in the manner taught herein
with respect to dye development, those carriers and dyes suitable
for development being suitable for selective passage through the
photoimaged film. However some dyes can be selectively passed
through the film in a suitable carrier to form an image on a
support surface, even though the areas of the film itself do not
absorb the dye. For example polar and similar fluids, such as
alcohols, can be caused to selectively penetrate the sulfated areas
of a film. Similarly, non-polar and similar type fluids can be
utilized to penetrate the areas which have not been sulfated. The
solubility parameters as discussed herein can be used as a general
guide, with fluids which are capable of dissolving areas being
suitable for passage therethrough. Complete disintegration of the
film can be avoided by using only moderate amounts of solvent at a
time to achieve penetration and by avoiding mechanical stress, for
example by just wiping a small amount of solvent and dye on the
film. Moreover, complete solubility of the polymer is not required,
but generally only that the polymer in the desired areas
sufficiently absorb or be swelled by the fluid to permit
penetration therethrough, so the solvents for this purpose may at
times be poorer solvents than would be used for development of an
image by selective removal of coating areas. The fluids will be
selected to give the desired permeability through selected areas in
a desired time while not passing through other areas in such
time.
In carrying out the present invention there are advantages in
utilizing polymer systems which give uniform homogeneous coatings.
In particular, solution coatings systems give homogeneous coatings,
whereas emulsion or other dispersion systems give coatings in which
the coating film is composed of fairly large particles, possibly
formed by agglomeration together of polymer molecules. While
emulsion coatings can be usefully employed, the solution coatings
are definitely preferred. Also it is advantageous to have the
photosensitizer well-dispersed in the polymer coating, at least on
a horizontal basis to the depth in the film to which photoimaging
is desired. This can advantageously be accomplished by
incorporating the sensitizer in a good solvent therefor, e.g.,
chloroform or other solvents for tetraphenylporphin, which is
compatible with the solvents used for applying the polymer coating.
The solution, of course, refers to the polymer, as the pigments are
ordinarily insoluble in the coating solvents, so the pigmented
polymer coatings are slurries in a solvent for the polymer.
The sulfated images herein may involve concentrations of
10.sup..sup.-10 sulfate groups per cm.sup.2 or greater and perhaps
usually is in a range from 10.sup..sup.-9 to 10.sup..sup.-6
sulfates/cm.sup.2. The sulfates may be more concentrated on the
surface with generally decreasing concentration with depth into the
film, e.g., the concentration per cm.sup.3 measured for a 5 micron
thickness may be about double that measured for a 10 micron
thickness, although the relationship is generally a nonlinear
curve. If the 10.sup..sup.-9 groups is considered as distributed in
a 10 micron thickness, the concentration is 10.sup..sup.-6
groups/cm.sup.3, or about 10.sup..sup.-6 groups/gram of film,
assuming the film has a density near one. The 10.sup..sup.-9 to
10.sup..sup.-6 groups/cm.sup.2 amounts to about 1 sulfate group for
every 50,000 carbon atoms in a polymer up to about one sulfate
group for every 50 carbon atoms in the polymer. While there will be
variances in the degree of desired sulfation with different
polymers, the foregoing is largely a matter of the concentration of
sulfate groups in the quantity of material (in the 10 micron film),
and will be generally valid regardless of the particular polymer.
If the film contains non-photooxidizable polymers or other
materials, as well as the photooxidizable polymer, the above ratios
of sulfate groups can be considered on the basis of the carbon
atoms in the total composition, so as to have about 10.sup..sup.-6
to 10.sup..sup.-3 groups per gram of film. While the amount of
sulfate groups will generally be within the foregoing ranges, it is
to be understood that higher degrees of sulfation can be employed.
Also in view of the strong effect of sulfate groups on various
properties, and amplification effects, very minute sulfate
concentrations can give desired imaging effects.
The photoimaging in the present polymers is due to sulfate groups,
and possibly residual hydroperoxy groups, in the coating materials,
and hence crosslinking, polymerization and other reactions are not
necessary. The preferred polymer systems utilized herein will
generally be substantially linear and soluble in such aromatic
hydrocarbons as toluene, but the sulfate groups still have a marked
effect in crosslinked polymers upon properties amenable to image
development.
EXAMPLE 1
A solution of a styrene/butadiene (60/40) block copolymer (Solprene
406) containing 5 mg tetraphenylporphin per gram of polymer was
used to draw coatings on a release paper. The coatings were imaged
through a step-wedge (Stauffer 21 .times. 0.15) employing a 1000
watt tungsten halogen light delivering a flux of 1.4 .times.
10.sup.5 erg/cm.sup.2 /sec. The images were then developed by
rolling on a dye solution with a wire-wound rod, and using a
squeege to remove excess after a 2-minute contact. The coating was
then transferred to a receptive paper support, and the optical
density was measured, employing a red or green filter with a
densitometer to measure reflection density. In some of the
procedures the dye development was preceded by treatment with an
So.sub.2 atmosphere for 30 seconds, and followed by NH.sub.3
-treatment for 30 seconds. In one procedure a cyan dye was employed
for negative working development, and a red filter was used in the
density measurement. With a 30 second light exposure, the first
step on the step-wedge produced an optical density of 0.46 without
the subsequent gas treatments and 0.92 with the SO.sub.2 and
NH.sub.3 treatments; the 0.24 value for the third step without gas
treatment compared to 0.24 for the 13th step with the gas
treatment. The light transmission through the third step of the
step-wedge is 44.7%, compared to only 1.41% through the 13th step.
Thus the use of SO.sub.2 and NH.sub.3 improved, the imaging speed
by a factor of about 31. For the development, the dye was a
solution of cyan dye, Astra Blue 3R Conc., as a 4% (wt.-volume)
solution in a 1 to 1 mixture of 2-ethylisohexanol and ethanol. The
step-wedge used had light transmissions varying from 89.1% for the
first step, 22.4% for the fifth step, 3.98% for the tenth step,
0.71% for the fifteenth step, and known intermediate values for the
other steps. With a one minute exposure and development with
crystal violet as a 1% wt/volume solution in a 3:1 mixture of
2-ethylisohexanol and ethanol, the measured optical density (green
filter) without gas for the second step exposure was equivalent to
that of the 16th step with the SO.sub.2 and NH.sub.3 treatments,
for a factor of about 90. With a 30 second light exposure and a
magenta dye (Astra Violet 3RA Extra) as a 4% (wt-volume) solution
in 1:1 2-ethylisohexanol and ethanol, the measured optical density
(green filter) of the step two exposure without gas treatment was
0.50, the same as the step 12 exposure with SO.sub.2 and NH.sub.3
treatment. Light transmission through step 2 is 63.1, compared to
1.99 through step 12, for a factor of about 30, i.e., with the gas
treatments only about one-thirtieth the exposure is necessary to
obtain equivalent optical density.
For a positive working system, a styrene/butadiene 50/50 copolymer
was employed in the foregoing procedures, with development by a
sudan red dye (Sudan Red O) as a 2% (wt.-volume) solution in
kerosene, or a blue dye (Calco Oil Blue V) as a 2% (wt.-volume)
solution in kerosene. The dyes solutions are primarily adapted to
absorb on non-exposed areas, but optical density is measured to
determine the effect of exposure in causing rejection of dye in
exposed areas, with higher rejection, i.e., lower optical density,
being useful for achieving lower background and greater contrast in
reproduction of positives. For a three minute exposure and blue dye
development, density values (measured with a red filter) varied
from 0.40 for step 1 to 1.40 for step 5, with no gas treatment,
while the SO.sub.2 and NH.sub.3 treatment gave values of 0.28 for
step 1, 1.28 for step 5, and 1.35 for step 7, for an amplification
factor of about 1.5. With the same exposure, and Sudan red dye, the
optical density (green filter) varied from 0.40 for step 1 to 1.01
for step 5, without gas treatment, and 0.34 for step 1, to 1.04 for
step 5 with the SO.sub.2 and NH.sub.3 treatments, for an
amplification factor of about 1.5.
EXAMPLE 2
A paper board was coated with a styrene/butadiene 60/40 latex (Dow
636) to which a 2:1 pigment loading, based on polymer, had been
added. The virtually colorless pigment was 9 parts clay to 1 part
TiO.sub.2. The coating was sensitized by wiping it with a solution
of tetraphenylporphin in chloroform, chlorinated paraffins of 40 to
70% chlorine content (Chlorowax, grade 40) and a plasticizer,
methylcarbityl benzyl phthalate. The coating was exposed to light
through a positive transparency and developed by wiping with a
kerosene solution of Sudan Brown dye. The non-exposed areas pick up
the dye. A one minute exposure gave a satisfactory picture with
good contrast, while a 15 second exposure gave poor contrast
because the non-exposed areas picked up some dye. With the light
exposure was followed by 5 minutes treatment with SO.sub.2 gas, a
15 to 30 second light exposure was sufficient to produce a picture
with good contrast. With a 1 minute light exposure and the SO.sub.2
treatment, the resulting picture appeared over-exposed.
Substantially equivalent results were obtained when the SO.sub.2
treatment preceded the light exposure.
EXAMPLE 3
The sensitized coating used in Example 2 was exposed to light
through a negative transparency for 5 minutes, treated with
SO.sub.2 for 5 minutes, and heated at 120.degree.C. for 2 to 3
minutes. A very definite picture was obtained which was black upon
a yellow background. The procedure was repeated, but with a
chloroform wash before the SO.sub.2 treatment. A black picture upon
a white background was produced.
EXAMPLE 4
A paper board coated with styrene/butadiene block copolymer (Kraton
101 marketed by Shell Chemical Company) was sensitized with
tetraphenylporphin solution, exposed to light through a negative
for 15 seconds, then exposed to SO.sub.2 gas, and developed by
application of polar dye. In an alternate procedure, the coating
was exposed to SO.sub.2 prior to the light exposure. Both SO.sub.2
treatments produced darker pictures than were obtained in controls
without the SO.sub.2 treatment. Two different dyes were used for
the development of separate samples, 2-ethylhexanol solutions of
Crystal Violet and Basacryl Red.
EXAMPLE 5
A styrene/butadiene block copolymer coating was sensitized with a
tetraphenylporphin solution. The coating was exposed through a
stencil for 10 to 15 minutes. The coating was then treated with
sulfur dioxide. The light-exposed area was gray to colorless, while
the non-exposed area was yellow-green. The coating was heated at
about 120.degree.C for 2 minutes to turn the exposed area black,
while the non-exposed area was yellow-green. The coating was then
contacted with ammonia to gradually lighten the yellow-green area
until it disappeared, while the black area turned brown. An
additional SO.sub.2 contact produced the yellow-green color again,
but this disappeared upon heating, while the brown color remained
in the exposed area. Another sample of the coating was similarly
light-exposed and then heated for 1 to 2 minutes at 120.degree.C.
The coating was then contacted with SO.sub.2, which turned the
non-exposed area a yellow-green, while the exposed area remained
colorless. Upon additional heating, the exposed area turned gray,
while the non-exposed area remained yellow-green. Any moderate heat
can be used in accelerating the effect of the sulfur dioxide, such
as 50.degree. to 150.degree.C for a few seconds up to 5 minutes or
more, with longer times generally being used as the heating
temperature is lowered.
EXAMPLE 6
A paper board was coated with a styrene/butadiene block copolymer
and sensitized with a tetraphenylporphin solution of
tetraphenylporphin. The solution contained 500 mg.
tetraphenylporphin, 70 grams chloroform, 30 grams chlorinated
paraffin and 70 grams plasticizer, methylcarbityl benzyl phthalate.
The sensitized coating was exposed to light through a negative
transparency, and developed by wiping with a solution of Crystal
Violet in 2-ethylhexanol. A 1 minute light exposure gave a good,
positive picture. A small portion of the above sensitizer solution
was placed in a small bottle, filling about one-fourth of its
volume, and SO.sub.2 gas was added to fill the free space. The
solution turned green. When this sensitizer was used with the same
styrene/butadiene coating, a 30-second light exposure gave a darker
picture than was obtained above, and a 1 minute exposure gave a
much darker picture. Thus the addition of SO.sub.2 to the
sensitizer shortened the light-exposure time necessary to obtain a
picture of a given shade. The SO.sub.2 apparently forms an adduct
with the tetraphenylporphin forming some kind of complex which has
limited stability. The pyrrole rings of the porphin contain amino
nitrogen. Since it is not necessary to form an adduct with all
tetraphenylporphin molecules present, it is not necessary to have
one SO.sub.2 molecule for each porphin molecule, but such amount,
as well as greater or lesser amounts, will be effective. The
SO.sub.2 groups may then react with the hydroperoxy groups, alone
or accompanied by a porphin moiety.
EXAMPLE 7
A photosensitive coating was made from a styrene/butadiene random
copolymer (Solprene 303, Phillips Pet.) of 48 parts butadiene to 52
parts butadiene with tetraphenyl porphyrin sensitizer, and a 15
parts per hundred pigment loading, based on polymer. The pigment
was a 9 to 1 mixture to cyan and green pigments (Monastral Blue B
BT284-D and Monastral Green Y GT 805-D, both available from
duPont). The coatings on release paper were exposed through
half-tone tints of varying coverage, treated with SO.sub.2 gas for
30 seconds developed by washing with an aliphatic naphtha
(Skellysolve V, KB value of 36), treated with ammonia gas for 30
seconds, and transferred to another paper. In an alternative
procedure, the ammonia treatment directly followed the SO.sub.2
treatment. The light exposure times varied from 15 to 150 seconds,
and the sensitizer concentrations from 0.01% to 0.1%, based on
polymer. The equivalent dot areas obtained were closer to that of
the tints used at the lower ends of the light exposure and
sensitizer concentration scales, with a tendency to considerable
dot gain at the upper ends of the scale. In general the ammonia
treatment before development resulted in greater dot areas than
were obtained when the ammonia treatment followed development.
EXAMPLE 8
A styrene/butadiene polymer coated paper board was wiped with a
solution of sulfolene in ethanol and sensitized with the
tetraphenylporphin solution described in Example 2. The coating was
exposed to light through a positive transparency for 1 minute,
heated in a 120.degree.C. oven for 5 minutes, and wiped with a 1%
solution of Sudan Brown dye in kerosene. A picture was obtained
which was much lighter than one produced in a control procedure
with no sulfolene treatment, i.e., the exposed areas picked up less
dye when the sulfolene treatment was used, and gave a picture of
greater contrast. Similar procedures with 30 second exposures, and
adding the sulfolene either before or after the exposure, gave
suitable pictures. Sulfolene, C.sub.4 H.sub.6 O.sub.2 S, i.e.,
2,4-dihydrothiophene-1,1-dioxide, is a sulfone as it has an
SO.sub.2 moiety.
In accordance with one aspect of the present invention an image can
be formed by image-wise exposure in the presence of oxygen of a
photosensitive coating comprising a photochemically oxidizable
substrate and an oxidation photosensitizer capable of absorbing
light energy to effect a transfer of oxygen from the surroundings
to form a chemical bond between oxygen and the coating composition,
and treating the coating with sulfur dioxide. The image thus formed
can be developed by adding coloring materials to selectively color
areas, or by selective removal of areas by treatment with
liquids.
* * * * *