U.S. patent application number 10/764411 was filed with the patent office on 2005-07-28 for process of producing microcapsules and product thereof.
Invention is credited to Hoderlein, Paul M., Rochester, Peter D., Smith, Dennis E., Wang, Yongcai.
Application Number | 20050161843 10/764411 |
Document ID | / |
Family ID | 34795278 |
Filed Date | 2005-07-28 |
United States Patent
Application |
20050161843 |
Kind Code |
A1 |
Wang, Yongcai ; et
al. |
July 28, 2005 |
Process of producing microcapsules and product thereof
Abstract
This invention relates to a process for preparing microcapsules
containing a hydrophobic liquid core material, the process
comprising: (1) mixing an organic liquid phase which comprises the
hydrophobic liquid core material with an aqueous phase comprising a
stabilizer to form a premix; (2) homogenizing the premix by forcing
the premix under pressure through a high pressure passage into a
low pressure area to produce a microparticle dispersion, said
microparticles having a mean size of greater than 1.0 micron, (3)
adding an encapsulating material prior to step (4); and (4) curing
the encapsulating material associated with the microparticles to
form the microcapsules.
Inventors: |
Wang, Yongcai; (Webster,
NY) ; Hoderlein, Paul M.; (Rochester, NY) ;
Smith, Dennis E.; (Rochester, NY) ; Rochester, Peter
D.; (Rochester, NY) |
Correspondence
Address: |
Paul A. Leipold
Patent Legal Staff
Eastman Kodak Company
343 State Street
Rochester
NY
14650-2201
US
|
Family ID: |
34795278 |
Appl. No.: |
10/764411 |
Filed: |
January 23, 2004 |
Current U.S.
Class: |
264/4.1 |
Current CPC
Class: |
B01J 13/04 20130101;
G03F 7/002 20130101 |
Class at
Publication: |
264/004.1 |
International
Class: |
B65B 001/00 |
Claims
What is claimed is:
1. A process for preparing microcapsules containing a hydrophobic
liquid core material, the process comprising: (1) mixing an organic
liquid phase which comprises the hydrophobic liquid core material
with an aqueous phase comprising a stabilizer to form a premix; (2)
homogenizing the premix by forcing the premix under pressure
through a high pressure passage into a low pressure area to produce
a microparticle dispersion, said microparticles having a mean size
of greater than 1.0 micron, (3) adding an encapsulating material at
any time prior to step (4); and (4) curing the encapsulating
material associated with the microparticles to form the
microcapsules.
2. The process of claim 1 wherein the microparticles have a mean
size of greater than 2.0 microns.
3. The process of claim 2 wherein the microparticles have a mean
size of greater than 2.0 and less than 50 microns.
4. The process of claim 2 wherein the microparticles have a mean
size of greater than 2.0 and less than 20 microns.
5. The process of claim 2 wherein the microparticles have a mean
size of greater than 2.0 and less than 15 microns.
6. The process of claim 1 wherein the encapsulating material is
added only prior to or during step (1).
7. The process of claim 1 wherein the encapsulating material is
added only after step (2).
8. The process of claim 1 wherein the encapsulating material is
added prior to or during step (1) and after step (2).
9. The process of claim 8 wherein the encapsulating materials added
prior to or during step (1) and after step (2) are different.
10. The process of claim 1 wherein the pressure differential
between the high pressure passage and the low pressure area is
greater than 2000 psi.
11 The process of claim 10 wherein the pressure differential is
greater than 4000 psi.
12. The process of claim 1 wherein the encapsulation material is
cured by heat.
13. The process of claim 1 wherein the encapsulation material is
cured by a change in pH.
14. The process of claim 1 wherein the encapsulation material is
cured by a condensation polymerization reaction.
15. The process of claim 1 wherein the stabilizer is a polymeric
stabilizer.
16. The process of claim 1 wherein the stabilizer is a particulate
stabilizer.
17. The process of claim 16 wherein the particulate stabilizer is a
colloidal inorganic oxide.
18. The process of claim 16 wherein the particulate stabilizer is a
latex.
19. The process of claim 1 wherein the stabilizer is a water
soluble polymer.
20. The process of claim 19 wherein the stabilizer is pectin,
sodium polystyrene sulfonate, polyvinyl alcohol, alginate, xanthan
gum, poly(vinyl methyl ether), or poly(vinyl pyrrolidone).
21. The process of claim 1 wherein the stabilizer is an anionic
polymer mixture comprising a mixture of a first sulfonated
polystyrene polymer and a second sulfonated polystyrene polymer
wherein the ratio of the weight average polymer molecular weight of
the first polymer to the second polymer is greater than 2.
22. The process of claim 21 wherein the ratio of the weight average
polymer molecular weight of the first polymer to the second polymer
is greater than 4.
23. The process of claim 21 wherein the weight average molecular
weight of the first polymer is greater than 500,000.
24. The process of claim 21 wherein the weight average molecular
weight of the first polymer is greater than 1,000,000.
25. The process of claim 1 wherein the stabilizer is other than
pectin and further comprises pectin.
26. The process of claim 1 wherein the microcapsules are
photohardenable.
27. The process of claim 1 wherein the liquid core material is a
color precursor which can react with a developer material to form
color.
28. The process of claim 1 wherein the encapsulating material is
polyurethane, polyurea, polyamide, polyester, polycarbonate, a
urea/formaldehyde resin, a melamine resin, polystyrene, a
styrene/methacrylate copolymer, or a styrene/acrylate
copolymer.
29. The process of claim 1 wherein the encapsulating material is
polyurethane, polyurea, polyamide, polyester, or polycarbonate.
30. The process of claim 1 wherein the encapsulating material is
polyurethane or polyurea.
31. The process of claim 1 wherein the weight average molecular
weight of the second polymer is less than 300,000.
32. Microcapsules containing a hydrophobic liquid core material
made by the process comprising: (1) mixing an organic liquid phase
which comprises the hydrophobic liquid core material with an
aqueous phase comprising a stabilizer to form a premix; (2)
homogenizing the premix by forcing the premix under pressure
through a high pressure passage into a low pressure area to produce
a microparticle dispersion, said microparticles having a mean size
of greater than 1.0 micron, (3) adding an encapsulating material at
any time prior to step (4); and (4) curing the encapsulating
material associated with the microparticles to form the
microcapsules.
33. The microcapsules of claim 32 wherein the microparticles have a
mean size of greater than 2.0 microns and less than 20 microns.
34. An imaging element comprising a support and an image forming
unit comprising a developer and microcapsules containing a
hydrophobic liquid core material, said microcapsules made by a
process comprising: (1) mixing an organic liquid phase which
comprises the hydrophobic liquid core material with an aqueous
phase comprising a stabilizer to form a premix; (2) homogenizing
the premix by forcing the premix under pressure through a high
pressure passage into a low pressure area to produce a
microparticle dispersion, said microparticles having a mean size of
greater than 1.0 micron, (3) adding an encapsulating material at
any time prior to step (4); and (4) curing the encapsulating
material associated with the microparticles to form the
microcapsules.
35. The imaging element of claim 34 wherein the imaging element is
light sensitive and heat or pressure developable.
36. The imaging element of claim 34 wherein the imaging element is
light sensitive and pressure developable.
37. The imaging element of claim 34 wherein the microcapsules are
photohardenable.
38. The imaging element of claim 34 wherein the microparticles have
a mean size of greater than 2.0 microns.
39. The imaging element of claim 34 wherein the microparticles have
a mean size of greater than 2.0 and less than 50 microns.
40. The imaging element of claim 34 wherein the microparticles have
a mean size of greater than 2.0 and less than 20 microns.
41. The imaging element of claim 34 wherein the microparticles have
a mean size of greater than 2.0 and less than 15 microns.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a method of making microcapsules
containing a hydrophobic core material. It more specifically
relates to a light sensitive and heat or pressure developable
imaging element comprising an image forming unit comprising
photosensitive microcapsules.
BACKGROUND OF INVENTION
[0002] Microencapsulation is the envelopment of an active agent or
a core material within a solid coating. The active or core material
can be in the form of a solid particle, a liquid droplet, or a gas
bubble. The solid coating used to form the capsule may be, for
example, an organic polymer, a wax, or an inorganic oxide. A
capsule is characterized in general by parameters such as particle
size and distribution, particle geometry, active contents and
distribution, release mechanism, and storage stability.
[0003] Many encapsulation processes have been reported in the
literature; only a few, however, have been commercialized. These
include interfacial and in-situ polymerization, complex
cocervation, spray drying, and fluidized-bed coating (the Wurster
process). Others are used in low volume specialty applications.
Interfacial polymerization is by far the most successful commercial
process.
[0004] Microcapsule-based products are used in the graphic arts,
adhesives, pharmaceutical, food, and pesticide industries.
Carbonless copy paper is by far the largest use for microcapsules.
Microcapsules containing solvents, liquid epoxy, or acrylate
monomers are also manufactured commercially and used in adhesive
formulations.
[0005] The patent literature has described imaging systems that
utilize microcapsules as the key component for developability and
color/tone scale differentiation by heat or pressure. These systems
are very useful as they do not use conventional photographic wet
processing. Heat or pressure developable photographic products,
such as Thermo-Autochrome (Fuji) and Cycolor Dry Media (Cycolor
Inc.), have been commercially available.
[0006] Microcapsules described in the art for use in imaging
applications are almost exclusively prepared by interfacial and
in-situ polymerization processes. In interfacial polymerization,
the materials used to form the capsule wall are in separate phases,
one in the aqueous phase and the other in the oil phase.
Polymerization occurs at the phase boundary. Wall formation of
polyester, polyamides, and polyurea proceeds by interfacial
polymerization. Polyurea capsule walls can also be made by
dissolving a polyisocyanate adduct in the oil phase. Hydrolysis of
the isocyanate groups at the phase boundary form amine groups that
in turn react with isocyanate groups to form urea linkages. In
in-situ polymerization, the capsule wall forming materials are
dissolved in the aqueous phase as resin precursors that, upon
further polymerization reaction, form the walls of the
microcapsules. Resin precursors used in this process include
melamine-formaldehyde, urea-formaldehyde, and
urea-melamine-formaldehyde polymers.
[0007] In the art of microencapsulation, the particle size and size
distributions are controlled by mechanical shear, aqueous phase
viscosity, and oil phase viscosity. The degree of shear and amount
of shear energy produced depend significantly on the geometry of a
particular shear device and residence time. For example, a higher
shear rate and longer residence time would produce a finer
microcapsule size. U.S. Pat. No. 5,643,506 describe a continuous
process of generating microcapsules using a conventional LP. Gaulin
colloid mill device. Such a device is capable of generating a high
shear rate by driving the conical motor at a very high rpm. The
final microcapsule size is controlled by how fast the motor
rotates, the viscosity of the oil and aqueous phases, and the ratio
of the organic phase to aqueous phase. It is well known in the art
that microcapsules generated by the above process have a broad size
distribution and poor batch-to-batch reproducibility. There is a
broad distribution of the shell thickness within the same batch of
microcapsules especially when the shell forming materials are added
to the oil phase. Larger particles have a thicker shell, and
smaller particles have a thinner shell. This undoubtedly produces a
distribution in the microcapsule permeability or the degree of
impermeability.
[0008] When microcapsules are used in imaging systems such as
carbonless paper or light sensitive pressure developable or heat
developable image media, the microcapsule shell must be impermeable
to the core materials. They must also have very low permeability to
oxygen if the physical characteristics of the microcapsules are
changed by free radical initiated reactions, since oxygen is an
inhibitor. The microcapsule shell functions as a barrier material
to prevent oxygen from infiltrating the light sensitive
composition. Upon exposing the material to light, free radicals
consume the oxygen present inside the capsule and the
polymerization reaction proceeds. If the oxygen re-infiltrates the
light sensitive composition, the photographic speed of the media is
very poor.
[0009] Microcapsules need to be resistant to low pressure during
normal storage and handling process, otherwise premature release of
the core material will occur. In addition, microcapsules used for
imaging applications need to be capable of withstanding
temperatures up to 100.degree. C. since during the manufacturing
process the coating may be dried by heating. It is believed that
the ability to control microcapsule size and size distribution is
crucial to meet those requirements.
[0010] Therefore, there is a need for a process that is capable of
generating microcapsules having a narrow size distribution and good
imaging capabilities.
SUMMARY OF THE INVENTION
[0011] This invention provides a process for preparing
microcapsules containing a hydrophobic liquid core material, the
process comprising:
[0012] (1) mixing an organic liquid phase which comprises the
hydrophobic liquid core material with an aqueous phase comprising a
stabilizer to form a premix;
[0013] (2) homogenizing the premix by forcing the premix under
pressure through a high pressure passage into a low pressure area
to produce a microparticle dispersion, said microparticles having a
mean size of greater than 1.0,
[0014] (3) adding an encapsulating material at any time prior to
step (4); and
[0015] (4) curing the encapsulating material associated with the
microparticles to form the microcapsules. It further provides
microcapsules made by the above process and an imaging material
comprising said microcapsules. The microcapsules produced by the
process of the invention have a narrow size distribution, wherein
the size is controlled not by the amount of shear, but rather by
the type and amount of stabilizers utilized. The process is capable
of producing microcapsules which are very robust and have excellent
resistance to low pressure during normal storage and handling
process, and which have excellent high temperature resistance to
premature release of encapsulated materials. The process has good
manufactuability with excellent batch to batch reproducibility.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 depicts a pictomicrograph of a drop of microcapsule
solution made by the comparative process of Example 1.
[0017] FIG. 2 depicts a pictomicrograph of a drop of microcapsule
solution made by the comparative process of Example 2.
[0018] FIG. 3 depicts a pictomicrograph of a drop of microcapsule
solution made by the inventive process of Example 3.
[0019] FIG. 4 depicts a pictomicrograph of a drop of microcapsule
solution made by the inventive process of Example 4.
[0020] FIG. 5 depicts a pictomicrograph of a drop of microcapsule
solution made by the inventive process of Example 5.
[0021] FIG. 6 depicts a pictomicrograph of a drop of microcapsule
solution made by the inventive process of Example 6.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0022] In a preferred embodiment of the invention, the process is
used to produce microcapsules used in imaging materials including,
for example, carbonless papers, heat sensitive imaging materials,
light sensitive and heat developable imaging materials, light
sensitive and pressure developable imaging materials, and ink jet
image recording materials. In a second preferred embodiment of the
invention, the process is used to produce microcapsules used in
optical and electronic display applications, such as
electrophoretic display, ferroelectric liquid crystal display, or
any display based on glass or plastic or paper-like flexible
substrates.
[0023] In accordance with the invention, the process of forming
microcapsules comprises the steps of
[0024] (1) mixing an organic liquid phase which comprises the
hydrophobic liquid core material with an aqueous phase comprising a
stabilizer to form a premix;
[0025] (2) homogenizing the premix by forcing the premix under
pressure through a high pressure passage into a low pressure area
to produce a microparticle dispersion, said microparticles having a
mean size of greater than 1.0 micron;
[0026] (3) adding an encapsulating material at any time prior to
step (4);
[0027] (4) curing the encapsulating material associated with the
microparticles to form microcapsules.
[0028] The first step of the process is the mixing of an organic
liquid phase comprising a hydrophobic core material with an aqueous
phase comprising a stabilizer to form a premix. This step is
preferably carried out in a mixing device which is capable of
imparting intense agitation to the mixture. The mixing can be done
in a batch process or in a continuous fashion. Any type of
propeller mixers or ultrasonic mixers can be used in the batch
process. The organic liquid phase and aqueous phase can also be fed
to a mixer continuously by a dosing apparatus. Mixers that can be
used include impingement mixers, stator rotor mixers, colloid mill
mixers, and the like. To effectively practice the present
invention, the volume ratio of the organic liquid phase to the
aqueous phase is preferably less than 60:40, more preferably less
than 50:50.
[0029] Any hydrophobic core materials can be used. If the
hydrophobic core material is liquid it may itself form the organic
liquid phase. If the hydrophobic core materials are solid, they can
be dissolved in an organic solvent to form the organic liquid
phase. Organic solvent can also be used to modulate the organic
phase viscosity. Examples of useful organic solvents, preferably
low boiling, include; propyl acetate, isopropyl acetate, ethyl
acetate, acetone, methyl ethyl ketone, dichloroethane, methyl
isobutyl ketone, isopropanol, isobutanol, toluene, xylene,
dichloromethane, high boiling aromatic hydrdrocarbons, phthalate
ester, cholorinated paraffins, alkylnaphthalenes, alkylated
biphenyls, and the like. The hydrophobic core materials to be
encapsulated can be dyestuff precursors such as leuco dyes, perfume
oils, scents, flavors, foodstuffs, colorants, paints, catalysts,
nutritional formulations for plants or animals, adhesives, paraffin
oils, pharmaceuticals, insecticides, fungicides, herbicides and
repellents. In one preferred embodiment the hydrophobic core
material is a color precursor which can react with a developer to
form color, such as a leuco dye.
[0030] The stabilizer useful for the practice of the present
invention is dissolved in the aqueous phase by methods known to
those skilled in the art. The amount of the stabilizer typically
ranges from 0.01% to 20% of the organic phase, and preferably from
0.01% to 10% by weight. The stabilizers are preferably polymeric
stabilizers. In one embodiment the stabilizer is a water soluble
polymer. Stabilizers that can be used in the present invention
include, for example, a sulfate, a sulfonate, a cationic compound,
an amphoteric compound, and a polymeric protective colloid.
Specific examples are described in "McCUTCHEON'S Volume 1:
Emulsifiers & Detergents, 1995, North American Edition" and
include, for example, alkali polyvalent metal salts of alkylbenzene
sulfonic acids, substituted napthalene sulfonic acids,
alkylsulfosuccinic acids, alkyl diphenyl oxide sulfonic acids,
alpha olephin sulfonic acids, alkyl polyglycosides, ethoxylated
alkyl phenols, ethoxylated alcohols, polyglycidols, block
copolymers of ethoxylated/propoxylated alcohols, polyacrylamide,
polyvinyl alcohol, polyvinyl pyrrolidone, sulfonated polyvinyl
alcohol, carboxylated polyvinyl alcohol, sulfonated polystyrene,
polyacrylic acid, maleic anydride-vinyl copolymers,
carboxymethylcellulose, poly(vinyl methyl ether),
hydroxyethylcellulose, gelatin, pectin including citrus and apple
pectin, gum Arabic, sulfonated cellulose, xanthan gums, alginates
or any water soluble starches, and the like. Preferred stabilizers
include is pectin, polystyrene sulfonate, polyvinyl alcohol,
alginate, xanthan gum, poly(vinyl methyl ether), or poly(vinyl
pyrrolidone). The more preferred stabilizers used for the practice
of the invention are sulfonated polystyrene (particularly sodium
polystyrene sulfonate), maleic acid/sulfonated styrene copolymers,
and pectin. The most preferred stabilizer for the practice of the
invention is a mixture of sulfonated polystyrenes. In one
embodiment the stabilizer is an anionic polymer mixture comprising
a mixture of a first sulfonated polystyrene polymer and a second
sulfonated polystyrene polymer wherein the ratio of the weight
average polymer molecular weight of the first polymer to the second
polymer is greater than 2 and preferably greater than 4. Preferably
the weight average molecular weight of the first polymer is greater
than 500,000 and more preferably the molecular weight of the first
polymer is greater than 1,000,000. Preferably the weight average
molecular weight of the second polymer is less than 300,000.
[0031] Stabilizers useful for the practice of the invention also
include colloidal particles, such as latex particles and colloidal
inorganic oxide particles. The most preferred inorganic oxide
particles are colloidal silica particles having a mean size of less
than 100 nm.
[0032] In one embodiment the stabilizer is other than pectin and
further comprises pectin. In a preferred embodiment the preferred
stabilizers for the practice of the invention are a mixture of
sulfonated polystyrenes and pectin.
[0033] The second step of the process is the homogenizing of the
premix by forcing the premix under pressure through a high pressure
passage into a low pressure area to produce a microparticle
dispersion having a mean size of greater than 1.0 micron. As for
the high-pressure homogenizer which may be used in the present
invention, it is considered that the dispersion into fine particles
is generally achieved by dispersion forces such as (a) "shear
force" generated at the passage of a dispersoid through a narrow
slit under a high pressure at a high speed, and (b) "cavitation
force" generated at the time of the release of the dispersoid from
the high pressure so as to be under normal pressure. The high
pressure passage may be, but is not limited to, a hole, a gap, a
slit, a pipe or tube, or a channel. Generally the passage is
narrower than the low pressure (low pressure includes normal
atmospheric pressure) area in order to provide the pressure
differential. The low pressure area may be, but is not limited to,
a container, or a wider pipe, tube or channel. There are various
configurations that can be used to force the premix under pressure
through a high-pressure passage into a low-pressure area.
[0034] A typical high pressure homogenizer consists of a pump and a
homogenizing valve. An example of such as apparatus has been
described in U.S. Pat. No. 4,383,769, incorporated herein by
reference. In such a case, the premix is forced through a narrow
gap between a valve seat and a valve plate. Through the gap, the
premix undergoes extremely rapid acceleration as well as an extreme
drop in pressure. The pressure drop occurs in a very short time,
for example, less than 50 microseconds, which produce a large
amount of energy in the liquid. The high energy density produced in
the premix causes the premix emulsion droplet to disrupt fairly
uniformly into primary particles of less than 1 micron in size
provided that the homogenization pressure is sufficiently high and
that the organic phase has a viscosity of less than, for example,
200 cps. The primary particles then coalesce in a controlled manner
to form particles having a mean size greater than 1.0 micron, and
preferably greater than 2.0 microns. In the present invention, the
homogenization pressure is preferably higher than 4000 psi, and
more preferably higher than 5000 psi. Preferably the pressure
differential between the high pressure passage and the low pressure
area is greater than 2000 psi and more preferably the pressure
differential is greater than 4000 psi. If the viscosity of the
organic phase is high, for example, greater than 200 cps, a higher
homogenization pressure is needed to disrupt the droplets of the
premix to particles of less than 1 micron.
[0035] Another example of a suitable apparatus includes the Gauline
homogenizer. By using this apparatus, the solution to be dispersed
is transported under a high pressure and converted into a
high-speed flow through a narrow slit on a cylinder surface, and
the energy of the flow allows collision of the flow against the
peripheral wall surface to achieve emulsification and dispersion.
In order to increase the dispersion efficiency, some apparatuses
are designed wherein a part of a high flow velocity is formed into
a serrated shape to increase the frequency of collision.
Apparatuses capable of dispersion under a higher pressure and at a
higher flow velocity have been developed in recent years, and
examples include Microfluidizer (manufactured by Microfluidex
International Corporation) and Nanomizer (manufactured by Tokusho
Kika Kogyo KK).
[0036] Examples of other dispersing apparatus which can be suitably
used in the present invention include Microfluidizer M-110S-EH
(with G10Z interaction chamber), M-110Y (with H10Z interaction
chamber), M-140K (with G10Z interaction chamber), HC-5000 (with
L30Z or H230Z interaction chamber) and HC-8000 (with E230Z or L30Z
interaction chamber), all manufactured by Microfluidex
International Corporation. By using these apparatuses, the premix
is transported under a positive pressure by means of a
high-pressure pump or the like into the pipeline, and the solution
is passed though a narrow slit provided inside the pipeline to
apply a desired pressure. Then, the pressure in the pipeline is
rapidly released to the atmospheric pressure to apply a rapid
pressure change to the dispersion to obtain an optimal dispersion
for use in the present invention.
[0037] There are a number of ways which can be used to measure the
microcapsule size and size distribution. A preferred way is to use
the Coulter Multisizer manufactured, for example, by Beckman.
Preferably the microcapsules have a mean size (volume average) of
less than 50 microns, preferably less than 20 microns and most
preferably less than 15 microns.
[0038] In the present invention, the size distribution index of
microcapsules is measured by the ratio of the volume average size
to the number average size. Preferably the microcapsules of the
invention has a size distribution index of less than 2, more
preferably less than 1.8, most preferably less than 1.6.
[0039] In a preferred embodiment of the invention, the stabilizers
used in the aqueous phase are "slow moving" stabilizers. By "slow
moving" stabilizer it means that the interfacial tension (or
dynamic surface tension) drops slowly with interfacial age for a
newly created interface. This type of stabilizer will allow smaller
particles (<1 micron) to have a sufficient amount of time to
coalesce to form larger particles (>1 micron) in a controlled
fashion. Typical "slow moving" stabilizers include particulate
stabilizers such as latex particles and colloidal inorganic oxide
particles such colloidal silica particles. Other slow moving
stabilizers include salted egg yolk, lacprodan-60, pectin, and the
like.
[0040] The types of encapsulating materials (also known as
wall-forming materials) useful for the invention depend on the
intended application, which in turn dictates the releasing
mechanism of the encapsulated core materials. The capsule wall can
be formed by a coacervation process utilizing a hydrophilic
wall-forming material described in U.S. Pat. Nos. 2,800,457 and
2,800,458; an interfacial polymerization process as described in
U.S. Pat. No. 3,287,154, U.K. Patent 990,443, and JP-B Nos.
38-19574, 42-446, and 42-771; a polymer deposition process as
described in U.S. Pat. Nos. 3,418,250 and 3,660,304; a process
utilizing isocyanate-polyol wall forming material such as described
in U.S. Pat. No. 3,796,669; a process utilizing an isocyanate wall
forming material such as described in U.S. Pat. No. 3,914,511; a
process utilizing urea-formaldehyde and
urea-formaldehyde-resorcinol wall-forming materials such as
described in U.S. Pat. Nos. 4,001,140, 4,087,376, and 4,089,802; a
process utilizing wall-forming materials such as a
melamine-formaldehyde resin and hydroxypropylcellulose such as
described in U.S. Pat. No. 4,025,455; an in-situ method utilizing a
polymerization of monomers as described in JP-B No. 36-9168 and
JP-A No. 51-9079; a method utilizing electrolytic dispersion
cooling such as described in U. K. Patents 952,807 and 965,074; and
a spray-drying method such as described in U.S. Pat. No. 3,111,407
and U. K. Patent 930,442, all incorporated herein by reference.
[0041] The encapsulating method is not limited to the methods
listed above. However, for use in the imaging material of the
present invention, it is particularly preferable to employ the
interfacial polymerization method wherein the reactants that form
the capsule wall polymers, the encapsulating materials, are added
to the liquid organic phase prior to forming of the premix (inside
the microparticle) or to the mixture after the homogenization step
(outside of the droplets). Examples of the capsule wall polymers
(encapsulating materials) include polyurethane, polyurea,
polyamide, polyester, polycarbonate, urea/formaldehyde resins,
melamine resins, polystyrene, styrene/methacrylate copolymers,
styrene/acrylate copolymers, and so on. Among these substances,
polyurethane, polyurea, polyamide, polyester, and polycarbonate are
preferable, and polyurethane and polyurea are particularly
preferable. The above-listed polymeric substances may be used in
combinations of two or more kinds.
[0042] As noted above the encapsulating material may be added at
any time prior to the curing step. It is preferably added prior to
or during the formation of the premix, after the homogenizing step,
or at both times. The encapsulating material may the same or
different when it is added at two different times. A mixture of
encapsulated materials may be utilized at any of the steps noted
above. The encapsulation material is cured using any suitable
method, such as heat, pH change or a chemical reaction. In one
embodiment the encapsulation material is cured by a condensation
polymerization reaction. In a typical process, a wall forming
material or a reactant such as a polyisocyanate, optionally
together with a chain extender, is added to the liquid organic
phase prior to forming the premix, and a polyamine soluble in the
aqueous phase is added to the homogenized mixture. A polyurea wall
is formed by heating the mixture for a period of time. Optionally a
second wall forming material can be added during or after the first
wall formation. For example, melamine formaldehyde precondensate
can be added to the above mixture to form a melamine-formaldehyde
shell by controlling pH and reaction temperature.
[0043] The invention further comprises an imaging element
comprising a support having a light sensitive and heat developable
image forming unit or a light sensitive and pressure developable
image forming unit provided thereon, wherein the image forming unit
comprises microcapsules made by the method of the invention. In a
preferred embodiment the element comprises an image forming unit
which is light sensitive and pressure developable i.e. it is
exposed by light and developed by applying pressure. The image
forming unit of the various element types may comprise one layer or
more than one layer. At least one layer comprises a color-forming
component that is preferably enclosed in the microcapsule of the
invention. At least one layer comprises a color developer. The
microcapsules and the developer may be in the same layer or in
different layers. Preferably the microcapsules are light sensitive.
More preferably the microcapsules are both light and pressure
sensitive.
[0044] Preferably the microcapsules are photohardenable. The
hydrophobic core of the light sensitive microcapsules of the
invention comprises a color-forming component, a polymerizable
compound, and a photopolymerization initiator. In the light
sensitive and pressure developable imaging element, exposure to
light according to a desired image causes the polymerizable
compound present inside the microcapsules to harden the
microcapsule interior by a polymerization reaction due to the
radical generated from the photopolymerization initiator upon
exposure so that a latent image in a desired shape is formed. That
is, in the exposed portions, the color-forming reaction with the
developer particles present outside the microcapsules is inhibited.
Next, when pressure is applied to the imaging element, the
microcapsules which have not hardened (the unexposed microcapsules)
are broken which cause the color-forming component to move within
the unexposed area to react with the developer particles to develop
a color. Accordingly, the light sensitive and pressure developable
image-imaging element is a positive-type, light sensitive and
pressure developable imaging element in which the image formation
is performed such that color formation is not made in exposed
portions but color formation is made in the unexposed portions that
do not harden.
[0045] In a preferred embodiment of the invention, the
color-forming component is mixed together with a
photopolymerization composition to form the microcapsule core, or
microcapsule internal phase. The microcapsule shell or the
microcapsule wall material is a polyurea, or polyurethane-urea. The
microcapsule shell or the microcapsule wall material comprises a
polyurea shell or a polyurethane-urea shell and a
melamine-formaldehyde or urea-formaldehyde shell.
[0046] Preferably the microcapsule containing the color-forming
component is prepared by the steps of dissolving the color-forming
component (hydrophobic core) and a wall forming material such as a
polyisocyanate in an auxiliary organic solvent such as ethyl
acetate, or a thermal solvent, to form a solution, mixing the
solution with an aqueous phase comprising a stabilizer to form a
premix; homogenizing the premix by forcing the premix under
pressure through a high pressure passage into a low pressure area
to produce a microparticle dispersion, adding a curing agent to
react with the wall forming material; and curing the wall forming
materials at an elevated temperature to form microcapsules.
[0047] If it is desirable to form a second shell, an aqueous
solution of melamine and formaldehyde or a precondensate is added
to the above microcapsule dispersion. The melamine-formaldehyde
shell is formed by raising the temperature of the resulting mixture
at neutral or acidic pH, e.g. pH of 7 or less. The temperature of
encapsulation is maintained at about 20 to 95.degree. C.,
preferably about 30 to 85.degree. C., ad more preferably about 45
to 80.degree. C.
[0048] The mean particle diameter of the microcapsules for use in
the imaging material of the present invention is preferably 20
.mu.m or less, more preferably 10 .mu.m or less and most preferably
6 .mu.m or less from the standpoint of obtaining high resolution.
The mean particle diameter is preferably 1.0 .mu.m or greater
because, if the average particle diameter of the microcapsules is
too small, the surface area per unit amount of the solid components
becomes larger and a lager amount of wall-forming materials is
required.
[0049] The color-forming components useful for the practice of the
invention include an electron-donating, colorless dye such that the
dye reacts with a developer (i.e. compound B, compound C, or
compound E) to develop a color. Specific examples of these
color-forming components include those described in Chemistry and
Applications of Leuco Dye, Edited by Ramaiah Muthyala, Plenum
Publishing Corporation, 1997. Representative examples of such color
formers include substantially colorless compounds having in their
partial skeleton a lactone, a lactam, a sultone, a spiropyran, an
ester or an amido structure. More specifically, examples include
triarylmethane compounds, bisphenylmethane compounds, xanthene
compounds, thiazine compounds and spiropyran compounds. Typical
examples of the color formers include Crystal Violet lactone,
benzoyl leuco methylene blue, Malachite Green Lactone,
p-nitrobenzoyl leuco methylene blue,
3-dialkylamino-7-dialkylamino-fluora- n,
3-methyl-2,2'-spirobi(benzo-f-chrome),
3,3-bis(p-dimethylaminophenyl)ph- thalide,
3-(p-dimethylaminophenyl)-3-(1,2 dimethylindole-3-yl)phthalide,
3-(p-dimethylaminophenyl)-3-(2-methylindole-3-yl)phthalide,
3-(p-dimethylaminophenyl)-3-(2-phenylindole-3-yl)phthalide,
3,3-bis(1,2-dimethylindole-3-yl)-5-dimethylaminophthalide,
3,3-bis-(1,2-dimethylindole-3-yl).sub.6-dimethylaminophthalide,
3,3-bis-(9-ethylcarbazole-3-yl)-5-dimethylaminophthalide,
3,3-bix(2-phenylindole-3-yl)-5-dimethylaminophthalide,
3-p-dimethylaminophenyl-3-(1-methylpyrrole-2-yl)-6-dimethylaminophthalide-
, 4,4'-bis-dimethylaminobenzhydrin benzyl ether, N-halophenyl leuco
Auramine, N-2,4,5-trichlorophenyl leuco Auramine,
Rhodamine-B-anilinolact- am, Thodamine-(p-nitroanilino)lactam,
Rhodamine-B-(p-chloroanilino)lactam,
3-dimethylamino-6-methoxyfluoran, 3-diethylamino-7-methoxyfluoran,
3-diethyl amino-7-chloro-6-methylfluoro an,
3-diethylamino-6-methyl-7-ani- linofluoran,
3-diethylamino-7-(acetylmethylamino)fluoran,
3-diethylamino-7-(dibenzylamino)fluoran,
3-diethylamino-7-(methylbenzylam- ino)fluoran,
3-diethylamino-7-(chloroethylmethylamino)fluoran,
3-diethylamino-7-(diethylamino)fluoran,
3-methyl-spiro-dinaphthopyran, 3,3'-dichloro-spiro-dinaphthopyran,
3-benzyl-spiro-dinaphthopyran,
3-methyl-naphtho-(3-methoxybenzo)-spiropyran,
3-propyl-spirodibenzoidipyr- an, etc. Mixtures of these color
precursors can be used if desired. Also useful in the present
invention are the fluoran color formers disclosed in U.S. Pat. No.
3,920,510, which is incorporated by reference. In addition to the
foregoing dye precursors, fluoran compounds such as disclosed in
U.S. Pat. No. 3,920,510 can be used. In addition, organic compounds
capable of reacting with heavy metal salts to give colored metal
complexes, chelates or salts can be adapted for use in the present
invention.
[0050] The polymerizable compound is an addition polymerizable
compound selected from among the compounds having at least one,
preferably two or more, ethylenically unsaturated bond at
terminals. Such compounds are well known in the industry and they
can be used in the present invention with no particular limitation.
Such compounds have, for example, the chemical form of a monomer, a
prepolymer, i.e., a dimer, a trimer, and an oligomer or a mixture
and a copolymer of them. As examples of monomers and copolymers
thereof, unsaturated carboxylic acids (e.g., acrylic acid,
methacrylic acid, itaconic acid; crotonic acid, isocrotonic acid,
maleic acid, etc.), and esters and amides thereof can be
exemplified, and preferably esters of unsaturated carboxylic acids
and aliphatic polyhydric alcohol compounds, and amides of
unsaturated carboxylic acids and aliphatic polyhydric amine
compounds are used. In addition, the addition reaction products of
unsaturated carboxylic esters and amides having a nucleophilic
substituent such as a hydroxyl group, an amino group and a mercapto
group with monofunctional or polyfunctional isocyanates and
epoxies, and the dehydration condensation reaction products of
these compounds with monofunctional or polyfunctional carboxylic
acids are also preferably used. The addition reaction products of
unsaturated carboxylic esters and amides having electrophilic
substituents such as an isocyanato group and an epoxy group with
monofunctional or polyfunctional alcohols, amines and thiols, and
the substitution reaction products of unsaturated carboxylic esters
and amides having releasable substituents such as a halogen group
and a tosyloxy group with monofunctional or polyfunctional
alcohols, amines and thiols are also preferably used. As another
example, it is also possible to use compounds replaced with
unsaturated phosphonic acid, styrene, vinyl ether, etc., in place
of the above-unsaturated carboxylic acids.
[0051] Specific examples of ester monomers of aliphatic polyhydric
alcohol compounds and unsaturated carboxylic acids include, as
acrylates, ethylene glycol diacrylate, triethylene glycol
diacrylate, 1,3-butanediol diacrylate, tetramethylene glycol
diacrylate, propylene glycol diacrylate, neopentyl glycol
diacrylate, trimethylolpropane triacrylate, trimethylolpropane
tri(acryloyloxypropyl)ether, trimethylolethane triacrylate,
hexanediol diacrylate, 1,4-cyclohexanediol diacrylate,
tetraethylene glycol diacrylate, pentaerythritol diacrylate,
pentaerythritol triacrylate, pentaerythritol tetraacrylate,
dipentaerythritol diacrylate, dipentaerythritol hexaacrylate,
sorbitol triacrylate, sorbitol tetraacrylate, sorbitol
pentaacrylate, sorbitol hexaacrylate,
tri(acryloyloxyethyl)isocyanurate, polyester acrylate oligomer,
etc. As methacrylates, examples include tetramethylene glycol
dimethacrylate, triethylene glycol dimethacrylate, neopentyl glycol
dimethacrylate, trimethylolpropane trimethacrylate,
trimethylolethane trimethacrylate, ethylene glycol dimethacrylate,
1,3-butanediol dimethacrylate, hexanediol dimethacrylate,
pentaerythritol dimethacrylate, pentaerythritol trimethacrylate,
pentaerythritol tetramethacrylate, dipentaerythritol
dimethacrylate, dipentaerythritol hexamethacrylate, sorbitol
trimethacrylate, sorbitol tetramethacrylate, and
bis[p-(3-methacryloxy-2-hydroxy-propoxy)phenyl]dimethylmethane,
bis[p-(methacryloxyethoxy)-phenyl]dimethylmethane. As itaconates,
examples include ethylene glycol diitaconate, propylene glycol
diitaconate, 1,3-butanediol diitaconate, 1,4-butanediol
diitaconate, tetramethylene glycol diitaconate, pentaerythritol
diitaconate, and sorbitol tetraitaconate. As crotonates, examples
include ethylene glycol dicrotonate, tetramethylene glycol
dicrotonate, pentaerythritol dicrotonate, and sorbitol
tetradicrotonate. As isocrotonates, examples include ethylene
glycol diisocrotonate, pentaerythritol diisocrotonate, and sorbitol
tetraisocrotonate. As maleates, examples include ethylene glycol
dimaleate, triethylene glycol dimaleate, pentaerythritol dimaleate,
and sorbitol tetramaleate. Further, the mixtures of the
above-described ester monomers can also be used. Further, specific
examples of amide monomers of aliphatic polyhydric amine compounds
and unsaturated carboxylic acids include methylenebis-acrylamide,
methylenebis-methacrylamide, 1,6-hexamethylenebis-acrylamide,
1,6-hexamethylenebis-methacrylamide,
diethylenetriaminetris-acrylamide, xylylenebis-acrylamide, and
xylylenebis-methacrylamide.
[0052] Further, urethane-based addition polymerizable compounds
which are obtained by the addition reaction of an isocyanate and a
hydroxyl group are also preferably used in the present invention. A
specific example is a vinyl urethane compound having two or more
polymerizable vinyl groups in one molecule, which is obtained by
the addition of a vinyl monomer having a hydroxyl group represented
by the following formula (V) to a polyisocyanate compound having
two or more isocyanate groups in one molecule.
CH.sub.2.dbd.C(R)COOCH.sub.2CH(R')OH
[0053] wherein R and R' each represents H or CH 3.
[0054] Other examples include polyfunctional acrylates and
methacrylates, such as polyester acrylates, and epoxy acrylates
obtained by reacting epoxy resins with (meth)acrylic acids.
Moreover, photo-curable monomers and oligomers listed in Sartomer
Product Catalog by Sartomer Company Inc. (1999) can be used as
well.
[0055] The details in usage of the addition polymerizable compound,
e.g., what structure is to be used, whether the compound is to be
used alone or in combination, or what an amount is to be used, can
be optionally set up according to the final design of the
characteristics of the photosensitive material. For example, the
conditions are selected from the following viewpoint. For the
photosensitive speed, a structure containing many unsaturated
groups per molecule is preferred and in many cases bifunctional or
more functional groups are preferred. For increasing the strength
of an image part, i.e., a cured film, trifunctional or more
functional groups are preferred. It is effective to use different
functional numbers and different polymerizable groups (e.g.,
acrylate, methacrylate, styrene compounds, vinyl ether compounds)
in combination to control both photosensitivity and strength.
Compounds having a large molecular weight or compounds having high
hydrophobicity are excellent in photosensitive speed and film
strength, but may not be preferred from the point of development
speed and precipitation in a developing solution. The selection and
usage of the addition polymerizable compound are important factors
for compatibility with other components (e.g., a binder polymer, an
initiator, a colorant, etc.) in the photopolymerization composition
and for dispersibility. For example, sometimes compatibility can be
improved by using a low purity compound or two or more compounds in
combination. Further, it is also possible to select a compound
having specific structure for the purpose of improving the adhesion
property of a support and an overcoat layer. Concerning the
compounding ratio of the addition polymerizable compound in a
photopolymerization composition, the higher the amount, the higher
the sensitivity. But, too large an amount sometimes results in
disadvantageous phase separation, problems in the manufacturing
process due to the stickiness of the photopolymerization
composition (e.g., manufacturing failure resulting from the
transfer and adhesion of the photosensitive material components),
and precipitation from a developing solution. The addition
polymerizable compound may be used alone or in combination of two
or more. In addition, appropriate structure, compounding ratio and
addition amount of the addition polymerizable compound can be
arbitrarily selected taking into consideration the degree of
polymerization hindrance due to oxygen, resolving power, fogging
characteristic, refractive index variation and surface adhesion.
Further, the layer constitution and the coating method of
undercoating and overcoating can be performed according to
circumstances.
[0056] Various photoinitiators can be selected for use in the
above-described imaging systems. However by far the most useful
photoinitators consist of an organic dye and an organic borate salt
such as disclosed in U.S. Pat. Nos. 5,112,752; 5,100,755;
5,057,393; 4,865,942; 4,842,980; 4,800,149; 4,772,530 and
4,772,541. The photoinitiator is preferably used in combination
with a disulfide coinitiator as described in U.S. Pat. No.
5,230,982 and an autoxidizer which is capable of consuming oxygen
in a free radical chain process.
[0057] The amount of organic dye to be used is preferably in the
range of from 0.1 to 5% by weight based on the total weight of the
photoplymerization composition, preferably from 0.2 to 3% by
weight. The amount of borate compound contained in the
photopolymerization composition of the invention is preferably from
0.1% to 20% by weight based on the total amount of
photopolymerization composition, more preferably from 0.3 to 5% by
weight, and most preferably from 0.3% to 2% by weight.
[0058] The ratio between the organic dye and organoborate salt is
important from the standpoint of obtaining high sensitivity and
sufficient decolorization by the irradiation of light in the fixing
step of the recording process described later. The weight ratio of
the organic dye to the organoborate salt is preferably in the range
of from 2/1 to 1/50, more preferably less than 1/1 to 1/20, most
preferably from 1/1 to 1/10.
[0059] The organic dyes for use in the present invention may be
suitably selected from conventionally known compounds having a
maximum absorption wavelength falling within a range of 300 to 1000
nm. High sensitivity can be achieved by selecting a desired dye
having the wavelength range within described above and adjusting
the sensitive wavelength to match the light source to be used.
Also, it is possible to suitably select a light source such as
blue, green, or red, or infrared LED (light emitting diode), solid
state laser, OLED (organic light emitting diode) or laser, or the
like for use in image-wise exposure to light.
[0060] Specific examples of the organic dyes include 3-ketocoumarin
compounds, thiopyrylium salts, naphthothiazolemerocyanine
compounds, merocyanine compounds, and merocyanine dyes containing
thiobarbituric acid, hemioxanole dyes, and cyanine, hemicyanine,
and merocyanine dyes having indolenine nuclei. Other examples of
the organic dyes include the dyes described in Chemistry of
Functional Dyes (1981, CMC Publishing Co., Ltd., pp. 393-416) and
Coloring Materials (60[4], 212-224, 1987). Specific examples of
these organic dyes include cationic methine dyes, cationic
carbonium dyes, cationic quinoimine dyes, cationic indoline dyes,
and cationic styryl dyes. Examples of the above-mentioned dyes
include keto dyes such as coumarin dyes (including ketocoumarin and
sulfonocoumarin), merostyryl dyes, oxonol dyes, and hemioxonol
dyes; nonketo dyes such as nonketopolymethine dyes, triarylmethane
dyes, xanthene dyes, anthracene dyes, rhodamine dyes, acridine
dyes, aniline dyes, and azo dyes; nonketopolymethine dyes such as
azomethine dyes, cyanine dyes, carbocyanine dyes, dicarbocyanine
dyes, tricarbocyanine dyes, hemicyanine dyes, and styryl dyes;
quinoneimine dyes such as azine dyes, oxazine dyes, thiazine dyes,
quinoline dyes, and thiazole dyes.
[0061] Preferably the organic dye useful for the invention is a
cationic dye-borate anion complex formed from a cationic dye and an
anionic organic borate. The cationic dye absorbs light having a
maximum absorption wavelength falling within a range from 300 to
1000 nm and the anionic borate has four R groups, of which three R
groups each represents an aryl group which may have a substitute,
and one R group is an alkyl group, or a substituted alkyl group.
Such cationic dye-borate anion complexes have been disclosed in
U.S. Pat. Nos. 5,112,752, 5,100,755, 5,075,393, 4,865,942,
4,842,980, 4,800,149, 4,772,530, and 4,772,541, which are
incorporated herein by reference.
[0062] When the cationic dye-borate anion complex is used as the
organic dye in the photopolymerization compositions of the
invention, it does not require to use the organoborate salt.
However, to increase the photopolymerization sensitivity and to
reduce the cationic dye stain, it is prefered to use an
organoborate salt in combination with the cationic dye-borate
complex. The organic dye can be used singly or in combination.
[0063] Specific examples of the above-mentioned water insoluble
phenols are given below. However, it should be noted that the
present invention is not limited to these examples. 12345678
[0064] The borate salt useful for the photosensitive composition of
the present invention is represented by the following general
formula (I).
[BR.sub.4].sup.-Z.sup.+ [I]
[0065] where Z represents a group capable of forming cation and is
not light sensitive, and [BR.sub.4].sup.- is a borate compound
having four R groups which are selected from an alkyl group, a
substituted alkyl group, an aryl group, a substituted aryl group,
an aralkyl group, a substituted aralkyl group, an alkaryl group, a
substituted alkaryl group, an alkenyl group, a substituted alkenyl
group, an alkynyl group, a substituted alkynyl group, an alicyclic
group, a substituted alicyclic group, a heterocyclic group, a
substituted heterocyclic group, and a derivative thereof. Plural Rs
may be the same as or different from each other. In addition, two
or more of these groups may join together directly or via a
substituent and form a boron-containing heterocycle. Z.sup.+ does
not absorbe light and represents an alkali metal, quaternary
ammonium, pyridinium, quinolinium, diazonium, morpholinium,
tetrazolium, acridinium, phosphonium, sulfonium, oxosulfonium,
iodonium, S, P, Cu, Ag, Hg, Pd, Fe, Co, Sn, Mo, Cr, Ni, As, or
Se.
[0066] Specific examples of the above-mentioned borate salts are
given below. However, it should be noted that the present invention
is not limited to these examples. 9101112
[0067] Various additives can be used together with the
photoinitiator system to affect the polymerization rate. For
example, a reducing agent such as an oxygen scavenger or a
chain-transfer aid of an active hydrogen donor, or other compound
can be used to accelerate the polymerization. An oxygen scavenger
is also known as an autoxidizer and is capable of consuming oxygen
in a free radical chain process. Examples of useful autoxidizers
are N,N-dialkylanilines. Examples of preferred N,N-dialkylanilines
are dialkylanilines substituted in one or more of the ortho-,
meta-, or para-position by the following groups: methyl, ethyl,
isopropyl, t-butyl, 3,4-tetramethylene, phenyl, trifluoromethyl,
acetyl, ethoxycarbonyl, carboxy, carboxylate, trimethylsilymethyl,
trimethylsilyl, triethylsilyl, trimethylgermanyl, triethylgermanyl,
trimethylstannyl, triethylstannyl, n-butoxy, n-pentyloxy, phenoxy,
hydroxy, acetyl-oxy, methylthio, ethylthio, isopropylthio,
thio-(mercapto-), acetylthio, fluoro, chloro, bromo and iodo.
Representative examples of N,N-dialkylanilines useful in the
present invention are 4-cyano-N,N-dimethylaniline,
4-acetyl-N,N-dimethylaniline, 4-bromo-N,N-dimethylaniline, ethyl
4-(N,N-dimethylamino)benzoate, 3-chloro-N,N-dimethylaniline,
4-chloro-N,N-dimethylaniline, 3-ethoxy-N,N-dimethylaniline,
4-fluoro-N,N-dimethylaniline, 4-methyl-N,N-dimethylaniline,
4-ethoxy-N,N-dimethylaniline, N,N-dimethylaniline,
N,N-dimethylthioanicidine, 4-amino-N,N-dimethylanili- ne,
3-hydroxy-N,N-dimethylaniline, N,N,N',N'-tetramethyl-1,4-dianiline,
4-acetamido-N,N-dimethylaniline,
2,6-diisopropyl-N,N-dimethylaniline (DIDMA),
2,6-diethyl-N,N-dimethylaniline, N,N, 2,4,6-pentamethylaniline
(PMA) and p-t-butyl-N,N-dimethylaniline. In accordance with another
aspect of the invention, the dye borate photoinitiator is used in
combination with a disulfide coinitiator.
[0068] Examples of useful disulfides are described in U.S. Pat. No.
5,230,982 which is incorporated herein by reference. Two of the
most preferred disulfides are mercaptobenzothiazo-2-yl disulfide
and 6-ethoxymercaptobenzothiazol-2-yl disulfide. By using these
disulfides as described in the referenced patent, the amount of the
photoinitiators used in the microcapsules can be reduced to levels
such that the background coloration or residual stain can be
reduced significantly. At these low levels, the low-density image
area coloration of the imaging layer does not detract unacceptably
from the quality of the image. In addition, thiols, thioketones,
trihalomethyl compounds, lophine dimer compounds, iodonium salts,
sulfonium salts, azinium salts, organic peroxides, and azides, are
examples of compunds useful as polymerization accelerators.
[0069] Other additives which can be incorporated into the
photopolymerization composition of the invention include various
ultraviolet ray absorbers and hindered amine light stabilizers,
photostabilizers as described in detail by J. F. Rabek in
"Photostabilization of Polymers, Principles and Applications"
published by Elsevier Applied Science in 1990.
[0070] The substantially colorless compound, which reacts with the
color-forming component to develop a color, may or may not have a
polymerizable group. Color developers useful for the invention
include inorganic solids such as clay and attapulgite, substituted
phenols and biphenols, polyvalent metal salts of modified
p-substituted phenol-formaldehyde resins, and polyvalent metal
salts of aromatic carboxylic acids. Preferably the color developers
used to practice of the invention are metal salts of modified
p-substituted phenol-formaldehyde resins and polyvalent metal salts
of aromatic carboxylic acid derivatives such as multivalent
polyvalent metal salts of 3,5-disubstituted salicylic acid
derivatives or multivalent polyvalent metal salts of a salicylic
acid resin obtained by reacting salicylates with styrene.
[0071] In a most preferred embodiment of the invention, the color
developer is a polyvalent metal salt of salicylic acid/styrene
copolymer developer which comprises multivalent salt of a salicylic
acid derivative and a styrenic compound. Specific examples of the
salicylic acid derivative include, but not limited to, salicylic
acid, 3-methylsalicylic acid, 6-ethylsalicylic acid,
5-isopropylsalicylic acid, 5-sec-butylsalicylic acid,
5-tert-butylsalicylic acid, 5-tert-amylsalicylic acid,
5-cyclohexylsalicylic acid, 5-n-octylsalicylic acid,
5-tert-octylsalicylic acid, 5-isononylsalicylic acid,
3-isododecylsalicylic acid, 5-isododecylsalicylic acid,
5-isopentadecylsalicylic acid, 4-methoxysalicylic acid,
6-methoxysalicylic acid, 5-ethoxysalicylic acid,
6-isopropoxysalicylic acid, 4-n-hexyloxylsalicylic acid,
4-n-decyloxylsalicylic acid, 3,5-di-tert-butylsalicylic acid,
3,5-di-tert-octylsalicylic acid, 3,5-diisononylsalicylic acid,
3,5-diisododecylsalicylic acid, 3-methyl-5-tert-nonylsalicylic
acid, 3-tert-butyl-5-isononylsalicylic acid,
3-isononyl-5-tert-butylsalicylic acid,
3-isododecyl-5-tert-butylsal- icylic acid,
3-isononyl-5-tert-amylsalicylic acid, 3-isononyl-5-tert-octyl-
salicylic acid, 3-isononyl-6-methylsalicylic acid,
3-isododecyl-6-methylsa- licylic acid,
3-sec-octyl-5-methylsalicylic acid, 3-isononyl-5-phenylsalic- ylic
acid, 3-phenyl-5-isononylsalicylic acid,
3-methyl-5-.alpha.-methylben- zyl)salicylic acid,
3-methyl-5-(.alpha.,.alpha.-dimethylbenzyl)salicylic acid,
3-isononyl-5-(.alpha.-methylbenzyl)salicylic acid,
3-(.alpha.-methylbenzyl)-5-tert-butylsalicylic acid,
3-benzylsalicylic acid, 5-benzylsalicylic acid,
3-(.alpha.-methylbenzyl)salicylic acid,
5-(.alpha.-methylbenzyl)salicylic acid,
3-(.alpha.,.alpha.-dimethylbenzyl- )salicylic acid,
4-(.alpha.,.alpha.-dimethylbenzyl)salicylic acid,
5-(.alpha.,.alpha.-dimethylbenzyl)salicylic acid,
3,5-di(.alpha.-methylbe- nzyl)salicylic acid,
3,5-di(.alpha.,.alpha.-dimethylbenzyl)salicylic acid,
3-(.alpha.-methylbenzyl)-5-(.alpha.,.alpha.-dimethylbenzyl)salicylic
acid, 3-(1',3'-diphenylbutyl)salicylic acid,
5-(1',3'-diphenylbutyl)salic- ylic acid,
3-[.alpha.-methyl-4'-(.alpha.'-methylbenzyl)benzyl]-salicylic acid,
5-[.alpha.-methyl-4'-(a'-methylbenzyl)benzyl]-salicylic acid,
3-(.alpha.-methylbenzyl)-5-(1',3'-diphenyl-butyl)salicylic acid,
3-(1',3'-diphenylbutyl)-5-.alpha.-methylbenzyl)salicylic acid,
3-phenylsalicylic acid, 5-phenylsalicylic acid,
3-.alpha.-methylbenzyl)-5- -phenylsalicylic acid,
3-(.alpha.,.alpha.-dimethylbenzyl)-5-phenylsalicyli- c acid,
3-phenyl-5-(.alpha.-methylbenzyl) salicylic acid,
5-(4'-methylphenyl)salicylic acid, 5-(4'-methoxyphenyl) salicylic
acid, 5-fluorosalicylic acid, 3-chlorosalicylic acid,
4-chlorosalicylic acid, 5-chlorosalicylic acid, 5-bromosalicylic
acid, 3-chloro-5-(.alpha.-methyl- benzyl)salicylic acid,
3-(.alpha.-methylbenzyl)-5-chlorosalicylic acid, and the like.
Specific examples of the styrenic compound include, but not limited
to, styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene,
o-ethylstyrene, p-ethylstyrene, o-isopropylstyrene,
m-isopropylstyrene, p-isopropylstyrene, p-ter-butylstyrene, and
.alpha.-methylstyrene, divinylbenzene, and styrene dimmers having
the chemical formula: 13
[0072] Wherein R.sub.3 is a hydrogen or an alkyl group having 1 to
4 carbon atoms, and R.sub.4 to R.sub.6 represent a hydrogen or a
methyl group.
[0073] There are many processes known in the art for making
salicylic acid/styrene compounds. For example the multivalent
polyvalent metal salt of salicylic acid resin can be produced by
reacting salicylic acid with a benzyl alcohol derivative at
elevated temperature as disclosed in U.S. Pat. No. 4,754,063. Or
they can be produced by reacting salicylic acid with a styrene
derivative at elevated temperature as disclosed in U.S. Pat. No.
4,929,710, or reacting salicylate ester with a styrene derivative
at low temperature as disclosed in U.S. Pat. No. 4,952,648. Some of
the processes form small molecules having a ratio of styrene to
salicylic acid of 1:1 to 2:1. Others result in a mixture of
copolymers having a ratio of styrene to salicylic acid of 1:1 to
very large molecules with a molecular weight of 10,000 or more. The
developer composition depends on the stoichiometry of the styrene
derivative and salicylate used in the process. It may also depend
on the type of reaction method utilized. It is preferred that the
mole ratio of styrene derivative to salicylate used to make the
salicylic acid/styrene polyvalent metal salt utilized in the
invention be 2:1 to 7:1, and more preferably 3:1 to 6:1. In a
preferred process salicylate ester is reacted with a styrene
derivative at low temperature as disclosed in U.S. Pat. No.
4,952,648, incorporated herein by reference.
[0074] It is preferred that the salicylic acid/styrene polyvalent
metal salt be a zinc salt, although other multivalent metals such
as aluminum, barium, lead, cadmium, calcium, chromium, iron,
gallium, cobalt, copper, magnesium, manganese, molybdenum, nickel,
mercury, silver, strontium, tantalum, titanium, vanadium, tungsten,
tin and zirconium may be utilized. Other preferred metals are
aluminum, titanium, vanadium, and tin.
[0075] The composition may further comprise additives that are
compatible with the salicylic acid/styrene polyvalent metal salt.
Examples of such additives include antooxidants, light stabilizers
such as UV absorbers, hindered amine light stabilizers, singlet
oxygen quenchers, inorganic fillers, water insoluble resins such as
epoxy resin, flow promoters or rheology modifiers, a hydrophobe
such as hexadecane, and the like.
[0076] Preferably the color developer is incorporated into the
imaging forming unit of the invention as particles which have a
mean size from about 0.5 microns to about 5 microns, more
preferably from about 0.7 microns to about 3 microns. Many methods
of forming particles of a polyvalent polyvalent metal salt of
salicylic acid/styrene copolymer are known in the art. Preferably
the composition is made by the method of forming an aqueous
dispersion of the developer composition by means of an organic
solvent dispersion, which comprises the following steps.
[0077] (a) preparing an organic phase comprising one or more
auxiliary solvents, a polyvalent polyvalent metal salt of salicylic
acid/styrene developer, and a surfactant;
[0078] (b) preparing a separate aqueous phase containing a water
soluble polyermeric dispersant;
[0079] (c) dispersing the organic phase into the aqueous phase
using a high sheer method to form a dispersed composition; and
[0080] (d) removing the auxiliary solvent from the dispersed
composition;
[0081] wherein the pH maintained during the process is greater than
6.
[0082] The auxiliary organic solvent may be any solvent which will
dissolve the polyvalent polyvalent metal salt of salicylic
acid/styrene copolymer developer. The amount of low boiling organic
solvent used to dissolve the developer composition is not
particularly limiting, however a minimum amount of solvent is
preferred in order to facilitate evaporation of the solvent after
droplet formation. Useful ranges of organic solvent to developer
composition on a weight basis varreis from about 0.2:1 to 20:1,
more preferably, from about 0.5:1 to 10:1 and most preferably, from
about 0.5:1 to about 5:1.
[0083] Examples of useful organic solvents, preferably low boiling,
include; propyl acetate, isopropyl acetate, ethyl acetate, acetone,
methyl ethyl ketone, dichloroethane, methyl isobutyl ketone,
isopropanol, isobutanol, toluene, xylene, dichloromethane, and the
like. Preferred solvents include propyl acetate, isopropyl acetate,
ethyl acetate, methyl ethyl ketone, dichloroethane, toluene,
dichloromethane. Any combination of low boiling organic solvents
may be used to dissolve the developer composition and the mixture
may be heated to below the boiling point of the organic solvent to
achieve complete dissolution of the developer composition.
[0084] The surfactant may be dissolved in the organic to control
the average particle size, width of the distribution of particles,
and colloidal stability of the aqueous suspension. The amount of
dispersant used to prepare the aqueous dispersion is not
particularly restricted. Typical amount ranges from 0.01% to 10% of
the organic phase, and prefereably from 0.01% to 5%, and more
preferably from 0,1% to 5%. Surfactants that can be used include,
for example, a sulfate, a sulfonate, a cationic compound, or an
amphoteric compound, and an oil soluble polymeric protective
colloid. Specific examples are described in "McCUTCHEON'S Volume 1:
Emulsifiers & Detergents, 1995, North American Edition" and
include, for example, alkali polyvalent metal salts of alkylbenzene
sulfonic acids, substituted napthalene sulfonic acids,
alkylsulfosuccinic acids, alkyl diphenyl oxide sulfonic acids,
alpha olephin sulfonic acids, alkyl polyglycosides, ethoxylated
alkyl phenols, ethoxylated alcohols, polyglycidols, block
copolymers of ethoxylated/propoxylated alcohols. The preferred
surfactant is an alkali salt of an alkylsulfosuccinic acid.
[0085] The water soluble polymeric dispersants include, but are not
limited to, polyacrylamide, polyvinyl alcohol, polyvinyl
pyrrolidone, sulfonated polyvinyl alcohol, carboxylated polyvinyl
alcohol, sulfonated polystyrene, polyacrylic acid, maleic
anydride-vinyl copolymers, carboxymethylcellulose,
hydroxyethylcellulose, gelatin, and the like. The preferred water
soluble polymeric dispersant is polyvinyl alcohol.
[0086] The organic phase may be dispersed into the aqueous phase
using any known high sheer method, preferably by means of a
mechanical mixer such as a rotor-stator mixer, a homogenizer, a
microfluidizer, and the like. There is no restriction on the
addition of phases as the organic phase may be added to the aqueous
phase or the aqueous phase may be added to the organic phase,
provided that sufficient agitation is applied during mixing.
[0087] The pH utilized in the process for the developer dispersion
making is preferably greater than 6. Preferably the pH value of the
finished dispersion is greater than 6. The organic solvent is then
removed using suitable temperature and pressure so as to evaporate
the solvent from the aqueous dispersion. It is highly preferred
that there be nearly complete removal of the organic solvent in
order to achieve good stability of the particles of the developer
composition of the present invention. The residual volatile organic
solvent must be less than about 2%, more preferably less than 1%
and most preferably less than about 0.5% by weight of the final
aqueous dispersion.
[0088] Prefereably a pH adjustment step follows the solvent
evaporation step whereby the pH of the resulting aqueous dispersion
of the developer composition is raised to above 9.0. This may be
accomplished with any suitable base including, for example, sodium
hydroxide, potassium hydroxide, triethanol amine, N,N-dimethyl
ethanolamine, triethylamine, and the like. The final concentration
of solids in the aqueous dispersion is about 50% solids or less and
can be achieved by further distillation of water from the
dispersion once the volatile organic solvent is removed.
[0089] The imaging element of the invention comprises a support and
above the support a light sensitive and heat developable image
forming unit or light and pressure developable image forming unit.
In one embodiment, a multicolor image can be realized using an
imaging element produced by producing a plurality of single-color
image forming layers within the image forming unit, each of which
contains microcapsules enclosing a color-forming component designed
to form a different color, and irradiating the imaging element with
a plurality of light sources each having a different
wavelength.
[0090] That is, the light sensitive and heat developable imaging
layer or light sensitive and pressure developable imaging layer has
a structure produced by providing on a support a first imaging
layer which contains microcapsules containing a color-forming
component for developing a yellow color and a photopolymerization
composition sensitive to a light source having a central wavelength
of .lambda..sub.1, providing on top of the first imaging layer a
second imaging layer which contains microcapsules containing a
color-forming component for developing a magenta color and a
photopolymerization composition sensitive to a light source having
a central wavelength of .lambda..sub.2, and providing on top of
second imaging layer a third imaging layer which contains
microcapsules containing a color-forming component for developing a
cyan color and a photopolymerization composition sensitive to a
light source having a central wavelength of .lambda..sub.3. In
addition, if necessary, the imaging layer may have an intermediate
layer between the different colored imaging layers. The
above-mentioned central wavelengths .lambda..sub.1, .lambda..sub.2,
and .lambda..sub.3 of the light sources differ from each other.
[0091] The light sensitive and heat developable image forming unit
layer or light sensitive and pressure developable image forming
unit of the present invention may have any number of the imaging
layers. Preferably, the imaging layer may contain first to i th
layers, each layer is sensitive to light having a central
wavelength different from the light having a central wavelength to
which other layers are sensitive, and each layer develops a color
different from that of other layers. For example, the first imaging
layer is sensitive to light having a central wavelength of
.lambda..sub.1 and develops a color, a second imaging layer is
sensitive to light having a central wavelength of .lambda..sub.2
and develops a color different from the color of the first imaging
layer, and an ith imaging layer is sensitive to light having a
central wavelength of .lambda..sub.i and develops a color different
from the colors of i-1 th imaging layer.
[0092] The multicolor image can also be realized using an imaging
element by producing a multicolor image forming unit in which all
of the microcapsules are in one layer. The layer contains
microcapsules of which each type contains a color-forming component
of a different color, is sensitive to light having a central
wavelength different from the light having a central wavelength to
which other types of microcapsules are sensitive, and develops a
color different from the color other types develop. For example,
the first type of microcapsule is sensitive to light having a
central wavelength of .lambda..sub.1 and develops a color, a second
type is sensitive to light having a central wavelength of
.lambda..sub.2 and develops a color different from the color of the
first type of microcapsules, and an i th type of microcapsules is
sensitive to light having a central wavelength of .lambda..sub.i
and develops a color different from the colors of i-1 th type of
microcapsules. In the present invention, i is preferably any
integer selected from 1 to 10, more preferably any integer selected
from 2 to 6, and most preferably any integer selected from 2 to 4.
When images are formed using an imaging material having a
multicolor image forming unit like the one for use in the present
invention, the exposure step consists of image-wise exposure using
plural light sources whose wavelengths match the absorption
wavelengths of the imaging layers, respectively, and are different
from each other. This exposure enables the imaging layers whose
absorption wavelengths match the wavelengths of the respective
light sources to form latent images selectively. Because of this,
multicolor images can be formed with a high sensitivity and in high
sharpness. Furthermore, since the background, which is colored with
such compounds as a spectral sensitizing compound and a
photopolymerization initiator, can be decolorized by irradiating
the imaging layer surface with light, high-quality images having a
high contrast can be formed.
[0093] The light sensitive and heat developable or light sensitive
and pressure developable image forming unit or imaging layers of
the invention also contain a binder material. There is no
limitation on the choice of the binder material as far as it is
compatible with other components incorporated in the layer or unit.
The binder material includes, for example, water-soluble polymers,
water dispersible polymers, and latex. Specific examples include
proteins, protein derivatives, cellulose derivatives (e.g.
cellulose esters), polysaccharides, casein, and the like, and
synthetic water permeable colloids such as poly(vinyl lactams),
acrylamide polymers, poly(vinyl alcohol) and its derivatives,
hydrolyzed polyvinyl acetates, polymers of alkyl and sulfoalkyl
acrylates and methacrylates, polyamides, polyvinyl pyridine,
acrylic acid polymers, maleic anhydride copolymers, polyalkylene
oxide, methacrylamide copolymers, polyvinyl oxazolidinones, maleic
acid copolymers, vinyl amine copolymers, methacrylic acid
copolymers, acryloyloxyalkyl sulfonic acid copolymers, vinyl
imidazole copolymers, vinyl sulfide copolymers, and homopolymer or
copolymers containing styrene sulfonic acid. Binder also include
dispersions made of solvent soluble polymers such as polystyrene,
polyvinyl formal, polyvinyl butyral, acrylic resins, e.g.,
polymethyl acrylate, polybutyl acrylate, polymethyl methacrylate,
polybutyl methacrylate, and copolymers thereof, phenol resins,
styrene-butadiene resins, ethyl cellulose, epoxy resins, and
urethane resins, and latices of such polymers.
[0094] The binder is preferably cross-linked so as to provide a
high degree of cohesion and adhesion. Cross-linking agents or
hardeners which may effectively be used in the coating compositions
of the present invention include aldehydes, epoxy compounds,
polyfunctional aziridines, vinyl sulfones, methoxyalkyl melamines,
triazines, polyisocyanates, dioxane derivatives such as
dihydroxydioxane, carbodiimides, chrome alum, zirconium sulfate,
and the like.
[0095] The light sensitive and heat developable or light sensitive
and pressure developable image forming unit or imaging layer
thereof may also contain various surfactants for such purposes as a
coating aid, an antistatic agent, an agent to improve sliding
properties, an emulsifier, an adhesion inhibitor. Examples of the
surfactant that can be used include nonionic surfactants such as
saponin, polyethylene oxide, and polyethylene oxide derivatives,
e.g., alkyl ethers of polyethylene oxide; anionic surfactants such
as alkylsulfonates, alkylbenzenesulfonates,
alkylnaphthalenesulfonates, alkylsulfuric esters,
N-acyl-N-alkyltaurines, sulfosuccinic esters, and
sulfoalkylpolyoxyethylene alkylphenyl ethers; amphoteric
surfactants such as alkylbetaines and alkylsulfobetaines; and
cationic surfactants such as aliphatic or aromatic quaternary
ammonium salts.
[0096] Furthermore, if necessary the light and heat sensitive or
light sensitive and pressure developable image forming unit or an
imaging layer thereof may contain additives other than those
described above. For example, dyes, ultraviolet absorbing agents,
plasticizers, fluorescent brightenesr, matting agents, coating
aids, hardeners, antistatic agents, and sliding property-improving
agents. Typical examples of these additives are described in
Research Disclosure, Vol. 176 (1978, December, Item 17643) and
Research Disclosure, Vol. 187 (1979, November, Item 18716).
[0097] Examples of the support for use in the imaging material of
the present invention include paper; coated paper; synthetic paper
such as laminated paper; films such as polyethylene terephthalate
film, cellulose triacetate film, polyethylene film, polystyrene
film, and polycarbonate film; plates of metals such as aluminum,
zinc, and copper; and these supports whose surface is treated with
a surface treatment, a subbing layer or metal vapor deposition. A
further example is the support described in Research Disclosure,
Vol. 200 (1980, December, Item 20036 XVII). These supports may
contain a fluorescent brightener, a bluing dye, a pigment, or other
additives. Furthermore, the support itself may be made of an
elastic sheet such as a polyurethane foam or rubber sheet. Between
a support and the light sensitive and heat developable or the light
sensitive and pressure developable image forming unit, a layer,
which comprises a polymer such as gelatin, polyvinyl alcohol (PVA),
or the like having a low oxygen transmission rate, can be provided.
The presence of this layer makes it possible to effectively prevent
the fading due to photooxidation of the images formed.
[0098] The image element of the present invention can contain at
least one electrically conductive layer, which can be either
surface protective layer or a sub layer. The surface resistivity of
at least one side of the support is preferably less than
1.times.10.sup.12 {overscore (.OMEGA.)}/square, more preferably
less than 1.times.10.sup.11 .OMEGA./square at 25.degree. C. and 20
percent relative humidity. To lower the surface resistivity, a
preferred method is to incorporate at least one type of
electrically conductive material in the electrically conductive
layer. Such materials include both conductive metal oxides and
conductive polymers or oligomeric compounds. Such materials have
been described in detail in, for example, U.S. Pat. Nos. 4,203,769;
4,237,194; 4,272,616; 4,542,095; 4,582,781; 4,610,955; 4,916,011;
and 5,340,676.
[0099] The image element of the invention can contain a curl
control layer or a backing layer located opposite of the support to
the imaging forming unit for the purposes of improving the
machine-handling properties and curl of the recording element,
controlling the friction and resistivity thereof, and the like.
Typically, the backing may comprise a binder and a filler and
optionally a lubricant. Typical fillers include amorphous and
crystalline silicas, poly(methyl methacrylate), hollow sphere
polystyrene beads, micro-crystalline cellulose, zinc oxide and
talc. The filler loaded in the backing is generally less than 5
percent by weight of the binder component and the average particle
size of the filler material is in the range of 1 to 30 .mu.m.
Examples of typical binders used in the backing are polymers such
as polyacrylates, gelatin, polymethacrylates, polystyrenes,
polyacrylamides, vinyl chloride-vinyl acetate copolymers,
poly(vinyl alcohol), gelatin and cellulose derivatives. Lubricants
can be same as those incorporated in the outer protective layer
located in the opposite side to the backing layer. Additionally, an
antistatic agent also can be included in the backing to prevent
static hindrance of the image element. Particularly suitable
antistatic agents are compounds such as dodecylbenzenesulfonate
sodium salt, octylsulfonate potassium salt, oligostyrenesulfonate
sodium salt and laurylsulfosuccinate sodium salt, and the like. The
antistatic agent may be added to the binder composition in an
amount of 0.1 to 15 percent by weight, based on the weight of the
binder. An image forming unit may also be coated on the backside,
if desired.
[0100] Visible images can be made by heat development if the
imaging element of the present invention is a light sensitive and
heat-developable imaging element or by pressure development if the
imaging element of the present invention is a light sensitive and
pressure developable imaging material. The heat or pressure
development can be carried out either simultaneously with the
exposure for latent image formation or after the exposure.
[0101] A conventionally known heating method can be employed for
the heat development. Generally, the heating temperature is
preferably 80 to 200.degree. C., more preferably 83 to 160.degree.
C. and most preferably 85 to 130.degree. C. The duration of heating
is preferably in the range of 3 seconds to 1 minute, more
preferably in the range of 4 to 45 seconds and most preferably in
the range of 5 to 30 seconds.
[0102] The pressure development can be accomplished with a pressure
applicator device. For example, the imaging material is developed
by passing an exposed imaging media between a pair of calendar
rollers that rupture the microcapsules, thereby allowing contact
between the color-forming component and a developer that react to
develop the image. The imaging material can also be developed by
moving a point contact which is resiliently biased into engagement
with the imaging sheet. Typically, the imaging sheet is secured to
a cylinder and the point contact is positioned in resilient
pressure contact with the imaging sheet. As the cylinder is
rotated, the point contact is simultaneously moved along the
cylinder in synchronism with the rotation of the cylinder to
rupture the microcapsules and develop the image in the imaging
sheet, or the imaging sheet may be mounted on a planer platform and
the point contact is moved across the surface of the sheet using a
screw thread in an X-Y transport device. The pressure that is to be
applied is preferably 10 to 300 kg/cm.sup.2, more preferably 80 to
250 kg/cm.sup.2 and most preferably 130 to 200 kg/cm.sup.2. If the
pressure is less than 10 kg/cm.sup.2, sufficient density of
developed color may not be obtained, whereas, if the pressure
exceeds 300 kg/cm.sup.2, the discrimination of the images may not
be sufficient because even the hardened microcapsules are
broken.
[0103] The imaging element of the present invention comprises a
photopolymerization initiator or the like such as a spectral
sensitizing. Therefore, the imaging element of the present
invention is colored with the photopolymerization initiator or the
like. Since background is also colored with the compound, it is
very important for the method of the present invention that the
colored background is decolorized by irradiation after heat
development.
[0104] Accordingly, it is preferable that, after the heat
development, the image forming unit surface is irradiated with
light to fix the images formed and to decolorize, decompose, or
deactivate the components such as a spectral sensitizing compound
which remain in the imaging layer and decrease the whiteness of the
background. By carrying out the irradiation, it is possible to
inhibit the coloration reaction. As a result, the density variation
in the images can be inhibited and the image storability can be
largely enhanced.
[0105] The imaging element of the invention is exposed image-wise
to light according to the pattern of a desired image shape so that
the photopolymerization forms a latent image. The color development
step is accomplished by heat or/and pressure so that the
color-forming components develop colors according to the latent
image to thereby produce images. The fixing step in which the
imaging layer surface is irradiated with light so as to fix the
image formed and decolorize the organic dyes.
[0106] In the exposure step, it is possible to employ, for example,
a means for exposing the whole face to an amount of light which has
wavelengths corresponding to the sensitive regions of respective
colors and can provide a desired density of the developed color.
The light source for use in the exposure step may be any light
source selected from the light sources having wavelengths ranging
from ultraviolet to infrared light if the light sensitive and heat
developable imaging layer contains a light-absorbing material such
as a spectral sensitizing compound that exhibits an absorption in a
specific wavelength region. More specifically, a light source
providing maximum absorption wavelengths ranging from 300 to 1000
nm is preferable. It is preferable to select and use a light source
whose wavelength matches the absorption wavelength of the
light-absorbing material such as an organic dye to be used. The
selective use of such light-absorbing material enables the use of a
blue to red light source and the use of a small-sized, inexpensive
infrared laser device and consequently not only broadens the use of
the imaging material of the present invention but also raises
sensitivity and image sharpness. Among the light sources, it is
particularly preferable to use a laser light source such as a blue,
green, or red laser light source or an LED from the viewpoint of
simplicity, downsizing, and low cost of the device.
[0107] After the color development step, the image forming unit
surface is subjected to a fixing step in which the whole imaging
layer surface is irradiated with light from a specific light source
to fix the images formed and to decolorize photopolymerization
initiator components remaining in the imaging layer. As for the
light source that can be used in the fixing step, a wide range of
light sources, such as a mercury lamp, an ultrahigh pressure
mercury lamp, an electrodeless discharge-type mercury lamp, a xenon
lamp, a tungsten lamp, a metal halide lamp, and a fluorescent lamp,
can be suitably used. The method of irradiating the image forming
unit with light from the light source in the fixing step is not
particularly limited. The whole image forming unit surface may be
irradiated with light at one time or the image forming unit surface
may be gradually irradiated with light by scanning or the like
until the irradiation of the surface finally ends. That is, any
method that finally enables the irradiation of the entire surface
of the image forming unit material after image formation with
nearly uniform light may be employed. The irradiation of the entire
image forming unit layer is preferable from the standpoint of the
enhancement of the effects of the present invention. The duration
of the irradiation with light from the light source needs to be the
time period that allows the produced images to be fixed and the
background to be sufficiently decolorized. In order to perform
sufficient fixing of images and decolorization, the duration of the
irradiation is preferably in the range of several seconds to tens
of minutes and more preferably in the range of several seconds to
several minutes.
[0108] The following examples illustrate the practice of this
invention. They are not intended to be exhaustive of all possible
variations of the invention. Parts and percentages are by weight
unless otherwise indicated.
EXAMPLES
[0109] The following organic phase and aqueous phase are used to
form microcapsules using different dispersion equipments (Example 1
through 6). The organic phase is formed by mixing together 50 grams
of trimethylolpropane triacrylate, 4 grams of desmodur N-100 from
Mobay (hexamethylene-1,6-diisocyanate (HMDI), and 0.02 grams of
dibutyltin dilaurate. The aqueous phase is formed by mixing
together 110 grams of water, 4 grams of pectin, and 2 grams of a
mixure of sodium polystyrene sulfonate TL502 and TL130 (National
Starch Chemical) at a weight ration of 100/0, 30/70, 20/80, and
0/100. The aqueous phase formed was pH adjusted to 6 with sodium
carbonate.
Example 1 (Comparative)
[0110] The organic phase and aqueous phase (Versa TL 502/TL130:
100/0) were mixed using a Cowles mixer at 3000 rpm for 10 minutes
at room temperature. The mixing speed was then dropped to and
maintained atl 500 rpm. The resultant mixture was heated in a
60.degree. C. bath for 10 minutes before a melamine formaldehyde
prepolymer solution was added. The prepolymer was formed by
reacting 3.9 grams of melamine and 6.5 grams of 37% formaldehyde
solution in 44 grams of water (pH>8). The pH was adjusted to pH
6 with H.sub.3PO4 and the reaction mixture was heated to 70 C for 2
hours while mixing at 1500 rpm. A solution of 2.5 grams of urea in
7 grams of water was then added to the reaction mixture and
reaction was allowed to continue at 70.degree. C. for 40 minutes.
The stirring was adjusted to 500 rpm. The pH was adjusted to 9
using a 10% NaOH solution.
[0111] A drop of microcapsule solution was place on a cover glass
and its photomicrograph was taken. The microcapsules made by this
process have a very broad size distribution (FIG. 1)
Example 2 (Comparative)
[0112] The organic phase and aqueous phase (Versa TL 502/TL130:
100/0) were mixed using a propeller mixer at 1000 rpm for 10
minutes at room temperature to form a premix. The premix was then
passed through a Gaulin mill at a speed of 3200 rpm three times.
The resultant mixture was heated in a 60.degree. C. bath for 10
minutes before a melamine formaldehyde prepolymer solution was
added. The prepolymer was formed by reacting 3.9 grams of melamine
and 6.5 grams of 37% formaldehyde solution in 44 grams of water
(pH>8). The pH was adjusted to pH 6 with H.sub.3PO4 and the
reaction mixture was heated to 70 C for 2 hours while mixing at
1500 rpm. A solution of 2.5 grams of urea in 7 grams of water was
then added to the reaction mixture and reaction was allowed to
continue at 70.degree. C. for 40 minutes. The stirring was adjusted
to 500 rpm. The pH was adjusted to 9 using a 10% NaOH solution.
[0113] A drop of microcapsule solution was place on a cover glass
and its photomicrograph was taken. The microcapsules made by this
process have a very broad size distribution. (FIG. 2).
Example 3: (Invention)
[0114] The organic phase and aqueous phase (Versa TL 502/TL130:
100/0) were mixed using a propeller mixer at 1000 rpm for 10
minutes at room temperature to form a premix. The premix was then
passed through a homogenizer (Microfluidizer) once at a pressure
greater than 6000 psi. The resultant mixture was heated in a
60.degree. C. bath for 10 minutes before a melamine formaldehyde
prepolymer solution was added. The prepolymer was formed by
reacting 3.9 grams of melamine and 6.5 grams of 37% formaldehyde
solution in 44 grams of water (pH>8). The pH was adjusted to pH
6 with H.sub.3PO4 and the reaction mixture was heated to 70 C for 2
hours while mixing at 1500 rpm. A solution of 2.5 grams of urea in
7 grams of water was then added to the reaction mixture and
reaction was allowed to continue at 70.degree. C. for 40 minutes.
The stirring was adjusted to 500 rpm. The pH was adjusted to 9
using a 10% NaOH solution.
[0115] A drop of microcapsule solution was place on a cover glass
and its photomicrograph was taken. The microcapsules made by this
process have a narrow size distribution. (FIG. 3).
Example 4: (Invention)
[0116] The organic phase and aqueous phase (Versa TL 502/TL130:
30/70) were mixed using a propeller mixer at 1000 rpm for 10
minutes at room temperature to form a premix. The premix was then
passed through a homogenizer once at a pressure greater than 6000
psi. The resultant mixture was heated in a 60.degree. C. bath for
10 minutes before a melamine formaldehyde prepolymer solution was
added. The prepolymer was formed by reacting 3.9 grams of melamine
and 6.5 grams of 37% formaldehyde solution in 44 grams of water
(pH>8). The pH was adjusted to pH 6 with H.sub.3PO4 and the
reaction mixture was heated to 70 C for 2 hours while mixing at
1500 rpm. A solution of 2.5 grams of urea in 7 grams of water was
then added to the reaction mixture and reaction was allowed to
continue at 70.degree. C. for 40 minutes. The stirring was adjusted
to 500 rpm. The pH was adjusted to 9 using a 10% NaOH solution.
[0117] A drop of microcapsule solution was place on a cover glass
and its photomicrograph was taken. The microcapsules made by this
process have a narrow size distribution. (FIG. 4).
Example 5 (Invention)
[0118] The organic phase and aqueous phase (Versa TL 502/TL130:
20/80) were mixed using a propeller mixer at 1000 rpm for 10
minutes at room temperature to form a premix. The premix was then
passed through a homogenizer once at a pressure greater than 6000
psi. The resultant mixture was heated in a 60.degree. C. bath for
10 minutes before a melamine formaldehyde prepolymer solution was
added. The prepolymer was formed by reacting 3.9 grams of melamine
and 6.5 grams of 37% formaldehyde solution in 44 grams of water
(pH>8). The pH was adjusted to pH 6 with H.sub.3PO4 and the
reaction mixture was heated to 70 C for 2 hours while mixing at
1500 rpm. A solution of 2.5 grams of urea in 7 grams of water was
then added to the reaction mixture and reaction was allowed to
continue at 70.degree. C. for 40 minutes. The stirring was adjusted
to 500 rpm. The pH was adjusted to 9 using a 10% NaOH solution.
[0119] A drop of microcapsule solution was place on a cover glass
and its photomicrograph was taken. The microcapsules made by this
process have a narrow size distribution. (FIG. 5).
Example 6 (Invention)
[0120] The organic phase and aqueous phase (Versa TL 502/TL130:
0/100) were mixed using a propeller mixer at 1000 rpm for 10
minutes at room temperature to form a premix. The premix was then
passed through a homogenizer once at a pressure greater than 6000
psi. The resultant mixture was heated in a 60.degree. C. bath for
10 minutes before a melamine formaldehyde prepolymer solution was
added. The prepolymer was formed by reacting 3.9 grams of melamine
and 6.5 grams of 37% formaldehyde solution in 44 grams of water
(pH>8). The pH was adjusted to pH 6 with H.sub.3PO4 and the
reaction mixture was heated to 70 C for 2 hours while mixing at
1500 rpm. A solution of 2.5 grams of urea in 7 grams of water was
then added to the reaction mixture and reaction was allowed to
continue at 70.degree. C. for 40 minutes. The stirring was adjusted
to 500 rpm. The pH was adjusted to 9 using a 10% NaOH solution.
[0121] A drop of microcapsule solution was place on a cover glass
and its photomicrograph was taken. The microcapsule made by this
process have a narrow size distribution. (FIG. 6).
[0122] Invention Examples 3 to 6 have clearly demonstrated that the
size of microcapsules can be easily controlled by the process of
the invention using different combination of stabilizers in the
aqueous phase. The size distribution is much narrower than the
microcapsules prepared using processes disclosed in the prior
art.
[0123] Microcapsules Having a Core Composition Comprising a Color
Former
[0124] The following organic phase and aqueous phase are used to
form microcapsules at different homogenization conditions and
stabilizer concentrations. The organic phase was formed by mixing
together 198.2 grams of trimethylolpropane triacrylate, 23.8 grams
of Pergascript Red from Ciba-Geigy, 0.6 grams of Altax from J. D.
Vanderbilt, and 10 grams of Irganox 1010 from Ciba-Geigy at
85.degree. C., followed by cooling down to 70.degree. C. before 10
grams of Desmodur N-100 and 10 grams of Desmodur from Mobay were
added. The aqueous phase was formed by mixing together 440 grams of
water, pectin, and a mixture of sodium polystyrene sulfonate TL502
and poly(styrenesulfonic acid-co-maleic acid) (3:1) sodium salt at
different concentrations and weight ratios which will be described
in the following examples. The mixture was heated to 85.degree. C.
for an hour, the pH was adjusted to 5.5 with a 10% sodium carbonate
solution, and cooled down to room temperature.
Example 7
[0125] The prepared organic phase and aqueous phase were mixed
using a propeller mixer at 1000 rpm for 10 minutes to form a
premix. The aqueous phase comprised 6 grams of pectin, 6 grams of
Versa TL 502, and 5 grams of poly(styrenesulfonic acid-co-maleic
acid) (3:1) sodium salt (MW 20,000). The premix was then passed
through a homogenizer once at a pressure of 8000 psi. The resultant
mixture was stirred at 500 rpm for 20 minutes before a mixture
containing 15.2 grams of diethylene tetraamine (DETA) in 120 grams
of water was added, which was followed by addition of a mixture
containing 5 grams of poly(styrenesulfonic acid-co-maleic acid)
(3:1) sodium salt, 0.16 grams of NaOH, and 16 grams of water. After
curing for an hour at 40.degree. C., the reaction mixture was
heated to 70 C for curing for an additional 40 minutes before a
melamine-formaldehyde prepolymer solution was added over 20
minutes. The melamine-formaldehyde prepolymer solution was formed
by reacting 19.5 grams of melamine, 12.6 grams of paraformaldehyde
in 196 grams of water in the presence of a trace amount of NaOH.
The reaction mixture was stirred at 70.degree. C. for another 2
hours followed by addition of 100 grams of 10% aqueous Airvol 205
(Air Product) solution and 48.6 grams of 26% aqueous urea solution.
After curing for an additional 40 minutes, the reaction mixture of
cooled down to room temperature. The pH was adjusted to 9 using a
10% NaOH solution.
[0126] The microcapsules prepared had a mean size of about 4 micron
and a size distribution index of about 1.26 as measured by Beckman
Coulter Multisizer. The size distribution index is expressed as the
ratio of volume average size to number average size.
Example 8
[0127] The microcapsules were prepared in a similar manner as in
Example 7 except that the homogenization pressure was dropped to
4000 psi.
[0128] The microcapsules had a mean size of about 4 microns and a
size distribution index of about 1.36 as measured by Beckman
Coulter Multisizer.
Example 9
[0129] 30 The microcapsules were prepared in a similar manner as in
Example 8 except that the aqueous phase comprised 6 grams of
pectin, 3.6 grams of Versa TL 502, and 4.5 grams of
poly(styrenesulfonic acid-co-maleic acid) (3:1) sodium salt (MW
20,000).
[0130] The microcapsules had a mean size of about 5 microns and a
size distribution index of about 1.26 as measured by Beckman
Coulter Multisizer.
[0131] The results from Example 7, Example 8, and Example 9 clearly
demonstrate that the particle size and size distribution of
microcapsules prepared by the process of the invention is
controlled by the stabilizer composition and is insensitive to
changes in homogenization pressure.
[0132] The invention has been described in detail with particular
reference to certain preferred embodiments thereof, but it will be
understood that variations and modifications can be effected within
the spirit and scope of the invention.
* * * * *