Note: Descriptions are shown in the official language in which they were submitted.
MEAA:044
IMPROVED IMAGING SYSTEM
The present invention relates to the production
of photographs, photocopies, or other fixed images. Mor~
specifically, it is an i~aging sy~em which employs a
coated substrate whi~h contains a chromogenic ma~erial and
a mic~oencapsulated photosensiti~e composition. In the
most preferred embodimen of this invention the
chromogenic material and photosensi~ive composition are in
the same microcapsules~
Several known imagi~g ~ystems employ
photosensi~ive encapsulates. One such imaging syste~
which has significant advan~ages over all previou~ly known
-ones is referred to generically as the Sanders process.
This process employs a coating composition which is
usually applied to a substrate. The coating includes a
photosensitive composition which is encapsula~ed and a
chromogenic material which may or may not ~e within the
microcapsules. (~Encapsulat~d~ is used herein to refer to
bo~h open phase systems in ~hi~h the photosensitive
composition is dispersed as droplets throughout ~
dispersing medium and sys~ems in which there is a discre~e
capsular wall.) The microcapsules generally ha~e a mean - -
diameter of 1 to 25 microns~ Images aee formed by
imagewise exposure of ~he coating composition ~o actini~
radiation. ~Actinic radiation~ i5 used herein ~o
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designate the entire spectrum of ele~tromagneti~
radiationO3 ~Imagewise~ exposure means tha~ radia ion is
_ applied in a pat~ern such that areas which are t~ be dark
receive the most radiation while areas which are ~o be
ligh~ receive little or no radia~ion~ ~r vice versa. This
can be accomplished, for example, by placing a s~encil
between ~he radia~ion source and ~he coa ing. Exposure
can be through either dir~ct transmission or reflection
imaging.
After exposure, the ~icrocapsules, or at leas~
those in the image areas, c~n be rup~ured by calendering
or other suitable means. In the ~ase of a photohardenable
photosensitive composition, the viscosity of the
photosensitive composition i5 increased ~ubs~antially upon
exposure to actinic radiation, through mechanisms such as
cross-linking or simple polymeÆization. Therefore, when
the capsules are broken, the photosensitive composition
which received a strong exposure will flow very little, if
at all, while the unexposed or weakly exposed
photosensitive composition can flow relatively freely. As
a direct result, the chromogenic materi~l react~ imagewise
with the developer according to ~he degree of exposure to
form a color in the form of the desired image. This can
occur several different ways.
In one embodiment, ~he chromogenic material i5
encapsulated with the photosensitive composition. Outside
the capsules, a developer is contained in ~he co~ing,
whi~h is applied to a sub~trate. When the ~apsules are _~-
ruptured~ the chormogenic material is available to flow,
but its movement from the exposed capsules is restricted
by the increased viscosity of the photosensitive
5~
compositions in those capsules. ~s a resul~, the
accessibility of chromogenic material ~o developer depends
on the exposure rec~ived locally. The developer and
chromogenic material react according to the exposure to
form the desired image. When this embodiment of a coating
composition is applied to a substra~e, the result is a
self-contained imaging sheet.
In another embodiment, the photosensitive
composition is encapsulated and the chromo~enic material
is wi~hin the coating inside or outside ~he ~apsules. A
developer can be located as a separate layer rom the
chormogenic material in the coating, or can be on a
separate substrate altogether. In the ormer situation,
capsule rupture releases imagewise the photosensitive
composition. The chromogenic material now reacts wi$h the
developer to form a color generally in the form o an
image. In the lat~er situation~ the ~wo substrates are
superimposed during capsule rupture so that the dissolved
chromogenic material flows onto the developer sheet and
reacts imagewise there.
In an alternative embodiment, the photosensitive
composition can be a high viscosity substance which
depolymerizes upon exposure to actinic radiation. In that
case~ the chromogenic material located in or near expos~d
capsules, instead of unexposed ones, i~ made accessible ~o
the developer. This changes the imaging system Erom a
posi~ive one to a negative one.
The photosensitive composition includes a ---
photoinitiator and a substance which undergoes a change in
viscosity upon exposure to light in the presence ~f the
photoinitiator. That substance may be a monomer, dimer,
~ ~,
or oligomer which is polymerized to a higher molecular
weight compound or it may be a polymer which is
- cross-linkedO Alternatively i~ may be a compound which is
depolymerized or oth~rwis~ lysed upon exposu~e. ~adia~ion
curable material~ that are often used are matexials
curable by free radical initiated chain propagated
addition polymerization or ionic polymeriza~ion.
Representative radiation curable ~aterial are
ethylenically unsaturated organic compounds. These
compounds contain at least one terminal ethylenic group
per molecule. Typi~ally they are liquid at room
temperature and can also double as a carrier oil .or the
chromogenic material in ~he internal phase. A preferred
group of radiation curable materials is ethylenically
unsaturated compounds having two or more ~erminal
ethylenic groups per molecule~ Representative examples of
these compounds include ethylenically unsaturated acid
esters of polyhydric alcohols such as trimethylol propane
triacrylate or trimethacrylate7 acrylate prepolymers
derived from the partial reaction of pentaerythritol with
acrylic or methacrylic acid or acrylic or me~hacrylic acid
esters; isocyanate modified acrylate, methacrylic and
itaconic acid esters of polyhydric alcohols, etc.
Some typical examples of photosoftenable
2s materials useul in other embodiments are photolysable
compounds such as certain diazonium compounds,
3-oximino-~-butanone methacrylate which undergoes main
chain scission upon U.V. exposure and poly 4'-alkyl - .
acylo-phenones~
Various photoinitiators are used. These
compounds absorb ~he exposure radiati~n and generate a
-- 5
free radical alone or in conjunction with a sensitizer.
Suitable photoinitiators include ~-alkoxy phenyl ketones,
Michler's ketone, O-acylated ~-oximinoketones, polycyclic
quinones, benzophenones and substituted benzophenones,
xanthones, thioxanthones, halogenated compounds such as
chlorosulfonyl and chloromethyl polynuclear aromatic
compounds, chlorosulfonyl and chloromethyl heterocyclic
compounds, chlorosulfonyl and chloromethyl benzophenones
and fluorenones, haloalkanes, ~-halo-~-phenylacetophenones
photoreducible dye-reducing agent redox couples, halo-
genated paraffins (e.g. bromina-ted or chlorinated paraffin)
benzoin alkyl ethers, etc.
The above described embodiments are only a few
of the possible variations on the Sanders process. The means
and methods of each comprise an imaging system which has
substantial practical, commercial and functional advantages
over prior art imaying systems. Various aspects of this
process are disclosed in the following U.S. Patent No.
4,399,209 and Canadian Patent Applications Serial No. 409,388
and 409,389.
After exposure and capsule rupture, -the oil
phase (i.e., the photosensitive composition and the chromogenic
material) migra-tes to the developer layer which is on the
same substrate as the microcapsules in a self-contained
imaging sheet and on a separate substrate in a transfer
sheet.
Some of the color formed is retained in the
photosensitive oil phase in a mobile solution after image
~ .
formation. Thl5 i5 particularly true where large amounts
of heavier photosensitive oils form the internnl phase.
If the: pho~osens1~ive composition does not quickly react
to visible ligh~ followlng capsule rupture ana image
formation9 in some cases the image may bleed dswn into and
across the imaging shee~, blurring and reducing ~he
intensity of the image. This effect $s ~ometimes referred
~o as ~featheringr.
The present invention solves the problem of image
diffusion in the Sanders process imaging system by
including an agent in the coating composition which reacts
with the photosensitive composition released from the
microcapsules following exposure (more particularly the
photohardenable or photosof~enable species) and cures or
otherwise hardens it. This agent Ihereinafter referred to
as ~a curing agent~) is typically a free radical
generating species such as a thermal initiator. While one
might expec~ that the photosensitive composition released
from the microcapsules in the image areas could be
hardened by a subse~uent exposure with actinic radiation,
this has not been found to be effective~ ~n instances
where image diffusion occurs, it has been found desirable
to incorporate a curing agent in the composition for
subsequent reaction with ~he released photosensitive
composition.
In one aspect, the invention pertains to a coating
eomposition for use in photoimaging the coating composition
comprising a chromogenic material, rupturable microcapsules
whieh contain a photosensitive composition, and a curing
agent. The coating composition is characterized in that
- 6a - f
images are formed by imagewise exposing a coating of the
composition to actinic radiation and rupturing the micro-
capsules in the image areas such that the chromogenic
material can react with an associated co-reactive developer
material to form a color image. The curing agent reacts
with the photosensitive composition following capsule
rupture and hardens the same to provide image stability
upon aging.
Another aspect of the invention pertains to an imaging
sheet comprising a substrate, with such coating composition
on cne surface of the substrate. The imaging sheet is
characterized in that images are formed by imagewise exposing
the substrate to actinic radiation and rupturing the micro
capsules in the image areas such that the chrpmogenic
material reacts with a co-reactive developer material
associated with the imaging sheet. The curing agent reacts
with the photosensitive composition released from the
capsules to improve image stability upon aging.
The curing agent is preferably encapsulated. It
may be contained in the same capsules as the
photosensitive composition or in separate photoinert
capsules. The latter is preferred. If included in the
`;`
same photoactiYe capsules, the effect will be the same at
first, but precuring of the capsule conten s ~y begin to
_occur in some cases, and all photosensitiYi~y may be lost
in a few days. An inert oil carrier ean be included in
S the capsules containing the ~uring agent to facilita~e
`~ flow and curing on rupture,
~ he curing agent is preferabl~ selec~ed on the
basis of shelf-lifet reactivity with ~he pho~osensitive
composition, lack of solubility in water, solubility in an
1~ inert oil carrier, low hydrolysis ra~e in the presence of
water phases having a pH ranging from about 3.5 to 9.5,
ease of encapsulation, and other ch~racteristics
consistent with known encapsulation techniques.
By exposing a coating of the composition,
lS rupturing the capsules alone (in the case of a
self-contained system) or in contact with a developer
shee~, and preferably heating th~ image to activate ~he
curing agent oe accelera~e curing, ~able images can be
formed. The chromogenic material reacts with the
2Q developer ~o form a color image and the curing agent
hardens the photosensitive composition released from the
capsules to prevent image diffusion.
Besides greatly reducing image diffusion, ~his
will improve smudge resistance and solvent resistance and
make the images more scuff proof.
The present invention is an improvemen~ of an
imaging ~ystem which employs a coating of a
microencapsulated photosensi~ive composition, a
chromogenic material which may or may not be inside ~he
capsule3, and a developer which c~n be ~n luded ~n the
coating or provided on a separa~e substra e wherein images
are ~ormed by imagewi~e exposing the coated substrate to
ac~inic radi~tion and ~hen rupturing the capsulesO I the
deYeloper is on ~ separa~e substr~te, the two ~ubstrates
~re superimposed while the capsules are ruptured. In
these systems after the image develops, some color remains
dissolved in the photosensitive phase released from the
capsules, and can bleed through and across the substrate,
ha~ming imaqe definition and intensity unless ~he
photosensitive pha~e is reacted~
The preferred embodiment of this invention
comprises including ~ chemical ~essen~ially
non-photosensitive) curing agent for the photosensitive
composit$on in the coat~ng composition. The function of
the curing age~t is to cure the photosensitive ~o~position
after developing. Once the color has been fsrmed, ~he
rapid hardening of the photosensitive composition prevents
over~development and difusion~ The chemically cured
photosensitive composition is thus ~frozen~ and the color
remains in place a~d does not diffuse. The curing agent
is suitably encapsulated with an inert carrier in capsules
separate from those containing the photosensitive
composition. Maintaining separate encapsulation prevents
precuring, which could interfere wi h the imaging system's
photosensitivity in a matter of days.
Selecting the curing agent and inert carrier oil
for encapsulation requires consideration of the capsule
wall material, conditions prevailinq in the waterless
phase outside the capsules~ and storage temperatures.
Further bases for selecting a curing ~gent include lack o
5~
solubility in water, solubility in the inert carrier oil
(which itself should be compatible with the oil phase in
the photosensitive composition~s capsuiès) a relatively
low hydrolysis rate in water phases having a pH between
3.5 and 9.5 for up to two hours at 40 to 50C, ease of
encapsulation, and other considerations with respect to
encapsulation. It is important that the encapsulated
curing agent have an adequate shelf life.
~referred curing agents are free radical
generators such as thermal initiators, which upon reacting
with the photosensitive composition cause it to polymerize
or crosslink. By exposing the coating composition to
actinic radiation, and rupturing the capsules in the
presence of a developer material, the chromogenic material
and the developer react to produce color in the form of an
in~age and the curing agent reacts with the photosensitive
composition and ~ardens it, thereby preventing image
diffusion. In the case of certain curing agents, lt may
be desirable to heat the image to accelera-te the cure. A
curing agent is preferably selected which is relatively
inactive at room temperature Ifor good shelf life) and
which is readily activated by heating to tempera-tures in
excess of room temperature.
A particularly useful class of thermal initiators
reactive with ethylenically unsaturated compounds are
organic peroxides. Suitable peroxides include diacyl
peroxides, ketone peroxides, peroxydicarbonates, alkyl
peroxides, alkyl hydroperoxides and sulfonyl peroxides.
Also useful as thermal initiators are bisazides, perborates
and diazo compounds.
`,.
~2~ 35
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Carrier oil~ conventionally use~ in forming
pressure-sensi~ive microcapsules can be used in the
preserlt invention provided they are essentially ~nert to
the curing agent. Deodorized kero~ense, alkylated
5 biphenyls and alkylated ph~chalates are examples,.
The encapsulation proce~s s:hosen mu~t give a high
yield of thermal initiator retained over extended periods
of storage, wheSher in water dispersions or after being
applied to a subs~rate~ Also, the temperature during
manufacture and storage mu~t be maintained below levels
tha~ would cause rapid deeomposition of ~he ~hermal
initiator~ For this reason, ini~iators which are no~
activated below about 25C are preferably used~
In some situations, peroxides can pre ent a fir~
lS hazard~ Th~s problem can be avoided by limi~ing the
amount used~ Preferably, the amount of peroxide used
should equal abou~ 0.1 ~o 1.0 percent of ~he weight of
photQsensitive composition used. Thus, if three pounds of
photosensitive composi~ion ~apsules are used fOE 3, 300
square feet of ~ubstrate paper7 only 0.03 pounds of
peroxide would be present at th~ 1~ level of use~ This
level is about equal to 0.6 pound~ of peroxide oil
solution which i~ not explosi~e and can be burned safely.
The following example~ illustrate the manufacture
of the present invention and its effectivenes~ in s~opping
image diffusion.
The peroxide containing microcapsules used in the
following examples were prepared as follows~
A batch o~ peroxide containing capsules was
prepared by incorporating 5 grams of commercial peroxide
into a dioctyl phthalate oil phase which is made from
~2~
Lupersol PDV (made by ~ucidol, Pennwalt). The total
solution amounted to 50 grams. After heating to 60C the
oil phase was added to a Waring blender, which contained a ~'
water phase at 60~C already stirring at full power. The
water phase contained 26.g grams of a 17~1% Isobam
solution whose pH was adjus~ed to 4.0 9 38~3 grams of
distilled wa~er, ~5 grams of a 10~ a~ueous gum arabic
solution, ~.6 grams of dissolved urea and 0.8 grams of
resorcinol. After the dispersion of the oil phase
averaged a~.out 5 microns, the power was reduced to about
40 volts and 17 milliliters of 37% formaldehyde solution
was added. The combined solutions were mixed at 40 volts
and 60C for two more hours. Then a solution of 0,6 grams
of ammonium sulfate and 1~.1 grams of water was added and
mixed for fifteen additional minutes~ The mix~ure was
transferred to a beaker and urther mixed at 60C for one
hour~ The pH was adjusted to 7~0 by drop-wise addition o~
a 19% sodium hydroxide solution, and the capsules were
reserved for further use.
The photosensitive microcapsules used in the
following examples were prepared as follows:
A batch o photosensitive capsules was prepared
under darkroom conditions in the same way as the batch of
peroxide capsules with two exceptions. The ingredients
were doubled in amount and the oil phase compris~d 100
grams of trimethylolpropenetriacrylate, 6 grams of Copikem*
IV (a product of Hilton-~avis Co.), 5 grams of benzoin
methyl ether, and 2 grams of Michler's ketone. The final
pH was adjusted to 9.0 and 5.6 grams of sodium bisulfite
was added and dissolved. The capsules were placed in foil
wrapped bottles to prevent premature exposure.
*trademarks
`'~ 2
Com~rison ~x_m~e~
A portion of the photosensitive capsule ba~ch 2
was diluted with an equal weight of a 10% sodium o~ gum "
arabic and then mixed well. Next the mixture was poured
across the coa~ed side of a sheet of ~ead Transrite impact
paper under darkroom conditions. The wet mix~ure was
drawn in the machine direction by using a number 10 Mayer
bar. The coating was dried in a circulating air oven for
2 minutes at 90C.
Two strips of the dried coa~ed surface were
cooled, aged one hour in darkroom conditions and then each
exposQd imagewise to a Stouffer transparent sensitivity
guide, which is a step wedge gray scale. The two strips
were covered by Gilclear and then developed by running
them through a laboratoEy super calender se~ at ~0 pounds
gauge~ which is approximately equivalent to 600 pounds per
linear inch of face contact. One of the developed sheets
was placed in ~he 90C oven for 5 minutes. Thi~ produced
a gray scale on each sample. The samples were placed in a
heavy gray envelope to age. The effect of aging on the
2~ image density lthe last gray scale step) is shown in Table
1 below.
Exam~le 1
The coating composition was prepar2d by mixing 10
~5 grams of peroxide capsules with 90 grams of photosensitive
capsules ~nd 100 grams of a 10% solution of gum arabi~.
TnP pH was adjusted to 7Ø
This composition was used to coat paper and
prepare exposed and developed samples as in the Comparison
Example. The two resulting samples were placed in the
same envelope as those from the Comparison. The effect of
aging on image density is shown in Table lo
~ 13 -
A coating composikion was prepared by mixing ~0
grams of peroxide capsules with 80 grams of photosensi~ive
capsules and 100 grams of a 10~ .~olu~ion of gum arabic.
~he pH was adjusted to 7O0~ The procedure was o~herwise
the same as in Example 1~ The results ~re shswn in Table
1.
Table 1 shows the results o these tests measured
by the last gray scale s~ep of ~he sample i~ages before
vi~ible gray scale change.
TABLE 1
Last Gray Scale Step
15 min. after 60 days after
Example _magin~ Aqin~ Develo~ment Development
CompariSon 26C 26C g 7
Comparison 26C 5 min. at 90C~ 8 7
then cooled to
26C
1 26C 26C 9 9
1 2SC 5 min. at 90C, 9 g
then cooled to
26C
2 2~C ~6C g
2 26C S min. at 90C 9 9
then cooled to
26C
These results clearly indicate that the
incorporation of peroxide capsules into the coating
provides a precise and stable imagP over a long period of
time. Although no tests were run, it is believed that
- 14 -
improved smudge and solvent resistance and scuff proofness
will also result. Additional testing did c~nfirm that the
organic peroxide remained ac~ive and that a very large
fraction continued to be present in both the
water-suspended encapsula~ion and on the dried paper~
prepared as in examples ~ and 5 for at least several
months.
The foregoing descriptions are inteneded to be
illustrative. They are not intended to define the scope
Of the invention, which i5 limi~ed only by the claims that
foll~w.
