Title: Organosol including amphipathic copolymeric binder made with Soluble High Tg Monomer and liquid toners for electrophotographic applications
Abstract: The invention provides liquid toner compositions in which the polymeric binder is chemically grown in the form of copolymeric binder particles dispersed in a liquid carrier. The polymeric binder includes one amphipathic copolymer that comprises the residue of a Soluble High Tg Monomer. The toners described herein exhibit surprisingly low fusion temperatures, yet are surprisingly resistant to blocking, are non-tacky and are resistant to marring and undesired erasure.
Patent Number: 7,014,973 Issued on 03/21/2006 to Qian, et al.
| Inventors:
|
Qian; Julie Y. (Woodbury, MN);
Herman; Gay L. (Cottage Grove, MN);
Baker; James A. (Hudson, WI)
|
| Assignee:
|
Samsung Electronics Company (Suwon, KR)
|
| Appl. No.:
|
612533 |
| Filed:
|
June 30, 2003 |
| Current U.S. Class: |
430/114; 430/117; 430/137.22 |
| Current Intern'l Class: |
G03G 9/08 (20060101) |
| Field of Search: |
430/114,117,137.22
|
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| |
Other References
U.S. Appl. No. 10/612,243, filed Jun. 30, 2003, entitled "Organosol Including
Amphipathic Copolymeric Binder and Use of the Organosol to Make Dry Toners for
Electrographic Applications" (30 pgs.).
U.S. Appl. No. 10/612,535, filed Jun. 30, 2003, entitled "Organosol Including
Amphipathic Copolymeric Binder Having Crystalline Material, and Use of the Organosol
to Make Dry Toners for Electrographic Applications" (32 pgs.).
U.S. Appl. No. 10/612,534, filed Jun. 30, 2003, entitled "Organosol Liquid Toner
Including Amphipathic Copolymeric Binder Having Crystalline Component" (35 pgs.).
U.S. Appl. No. 10/612,765, filed Jun. 30, 2003, entitled "Organosol Including
High Tg Amphipathic Copolymeric Binder and Liquid Toners for Electrophotographic
Applications" (26 pgs.).
European Search Report EP1422575 A1 issued on Mar. 15, 2004.
|
Primary Examiner: Chapman; Mark A.
Attorney, Agent or Firm: Kagan Binder, PLLC
Parent Case Text
This application claims the benefit of U.S. Provisional Application Ser. No.
60/425,467, filed Nov. 12, 2002, entitled "ORGANOSOL INCLUDING AMPHIPATHIC COPOLYMERIC
BINDER MADE WITH SOLUBLE HIGH T
G MONOMER AND LIQUID TONERS FOR ELECTROPHOTOGRAPHIC
APPLICATIONS," which application is incorporated herein by reference in its entirety.
Claims
What is claimed is:
1. A liquid electrophotographic toner composition comprising:
a) a liquid carrier having a Kauri-Butanol number less than about 30 mL; and
b) a plurality of toner particles dispersed in the liquid carrier, wherein the
toner particles comprise polymeric binder comprising at least one amphipathic copolymer
comprising one or more S material portions and one or more D material portions,
wherein the S material portions and the D material portions have respective solubilities
in the liquid carrier that are sufficiently different from each other such that
the S material portions tend to be more solvated by the carrier while the D material
portions tend to be more dispersed in the carrier, and wherein one or more of the
S or D material portions comprises the residue of a Soluble High T
g
Monomer having a T
g at least about 20° C., wherein:
the absolute difference in Hildebrand solubility parameters between the Soluble
High T
g Monomer and the liquid carrier is less than about 3 MPa
1/2; and
the D portions of the amphipathic copolymer each have a T
g at least
about 30° C.
2. The liquid electrophotographic toner composition according to claim 1 further
comprising at least one visual enhancement additive.
3. The liquid electrophotographic toner composition according to claim 2 wherein
the Soluble High T
g Monomer has a T
g at least about 40° C.
4. The liquid electrophotographic toner composition according to claim 2 wherein
the Soluble High T
g Monomer has a T
g at least about 60° C.
5. The liquid electrophotographic toner composition according to claim 2 wherein
the Soluble High T
g Monomer has a T
g at least about 100° C.
6. The liquid electrophotographic toner composition according to claim 2 wherein
the D portions of the amphipathic copolymer each have a T
g at least
about 40° C.
7. The liquid electrophotographic toner composition according to claim 2 wherein
the D portions of the amphipathic copolymer each have a T
g at least
about 45° C.
8. The liquid electrophotographic toner composition according to claim 2 wherein
the absolute difference in Hildebrand solubility parameters between the Soluble
High T
g Monomer and the liquid carrier is less than about 2.2 MPa
1/2.
9. The liquid electrophotographic toner composition according to claim 2 wherein
the Soluble High T
g Monomer is selected from the group consisting of
t-butyl methacrylate, n-butyl methacrylate, isobornyl (meth)acrylate, TCHMA, and
combinations thereof.
10. The liquid electrophotographic toner composition according to claim 2 wherein
the Soluble High T
g Monomer is present at a concentration of between
about 5 and 30% by weight of the amphipathic copolymer.
11. The liquid electrophotographic toner composition according to claim 1 wherein
the S portions and the D portions of the amphipathic copolymer each have a Tg greater
than about 45° C.
12. The liquid electrophotographic toner composition according to claim 1 wherein
the Soluble High T
g Monomer is in the D material portion of the amphipathic copolymer.
13. The liquid electrophotographic toner composition according to claim 1 wherein
the Soluble High T
g Monomer is in the S material portion of the amphipathic copolymer.
14. The liquid electrophotographic toner composition according to claim 1 wherein
the Soluble High T
g Monomer is TCHMA.
15. The liquid electrophotographic toner according to claim 1, wherein the S
portion has a glass transition temperature calculated using the Fox equation (excluding
grafting site components) of at least about 90° C.
16. The liquid electrophotographic toner according to claim 1, wherein the S
portion has a glass transition temperature calculated using the Fox equation (excluding
grafting site components) of from about 100° C. to about 130° C.
17. The liquid electrophotographic toner according to claim 1, wherein the S
portion has a glass transition temperature calculated using the Fox equation (excluding
grafting site components) of at least 90° C., and wherein the absolute difference
in Hildebrand solubility parameter between the S portion and the liquid carrier
is from about 2 MPa
1/2 about 3 MPa
1/2.
18. The liquid electrophotographic toner according to claim 1, wherein the S
portion (excluding grafting site components) has a calculated Hildebrand solubility
parameter of from about 16 MPa
1/2 to about 17.5 MPa
1/2.
19. The liquid electrophotographic toner according to claim 1, wherein at least
about 75% of the S portion (excluding grafting site components) is derived from
ingredients selected from the group consisting of trimethyl cyclohexyl methacrylate;
t-butyl methacrylate; n-butyl methacrylate; isobornyl (meth)acrylate; 1,6-Hexanediol
di(meth)acrylate and combinations thereof.
20. The liquid electrophotographic toner according to claim 1, wherein at least
about 90% of the S portion (excluding grafting site components) is derived from
ingredients selected from the group consisting of trimethyl cyclohexyl methacrylate;
t-butyl methacrylate; n-butyl methacrylate; isobornyl(meth)acrylate; 1,6-Hexanediol
di(meth)acrylate and combinations thereof.
21. A method of making a liquid electrophotographic toner composition comprising
steps of:
a) providing a dispersion of amphipathic copolymer in a liquid carrier having
a Kauri-Butanol number less than about 30 mL, wherein said amphipathic polymeric
comprises one or more S material portions and one or more D material portions,
wherein the S material portions and the D material portions have respective solubilities
in the liciuid carrier that are sufficiently different from each other such that
the S material portions tend to be more solvated by the carrier while the D material
portions tend to be more dispersed in the carrier, and wherein one or more of the
S or D material portions comprises the residue of a Soluble High T
g
Monomer having a T
g at least about 20° C., wherein:
the absolute difference in Hildebrand solubility parameters between the Soluble
High T
g Monomer and the liquid carrier is less than about 3 MPa
1/2; and
the D portions of the amphipathic copolymer each have a T
g at least
about 30° C.; and
b) mixing the dispersion with one or more ingredients comprising at least one
visual enhancement additive under conditions effective to form a plurality of toner particles.
22. A method of electrophotographically forming an image on a substrate surface
comprising steps of:
a) providing a liquid toner composition of claim 1;
b) causing an image comprising the toner particles to be formed on the substrate surface;
c) fusing said image on the substrate surface.
Description
FIELD OF THE INVENTION
The present invention relates to liquid toner compositions having utility in
electrophotography. More particularly, the invention relates to amphipathic copolymer
binder particles that include Soluble High T
g Monomer components.
BACKGROUND OF THE INVENTION
In electrophotographic and electrostatic printing processes (collectively electrographic
processes), an electrostatic image is formed on the surface of a photoreceptive
element or dielectric element, respectively. The photoreceptive element or dielectric
element may be an intermediate transfer drum or belt or the substrate for the final
toned image itself, as described by Schmidt, S. P. and Larson, J. R. in Handbook
of Imaging Materials Diamond, A. S., Ed: Marcel Dekker: New York; Chapter 6, pp
227-252, and U.S. Pat. Nos. 4,728,983; 4,321,404; and 4,268,598.
In electrostatic printing, a latent image is typically formed by (1) placing a
charge image onto a dielectric element (typically the receiving substrate) in selected
areas of the element with an electrostatic writing stylus or its equivalent to
form a charge image, (2) applying toner to the charge image, and (3) fixing the
toned image. An example of this type of process is described in U.S. Pat. No. 5,262,259.
In electrophotographic printing, also referred to as xerography, electrophotographic
technology is used to produce images on a final image receptor, such as paper,
film, or the like. Electrophotographic technology is incorporated into a wide range
of equipment including photocopiers, laser printers, facsimile machines, and the like.
Electrophotography typically involves the use of a reusable, light
sensitive, temporary image receptor, known as a photoreceptor, in the process of
producing an electrophotographic image on a final, permanent image receptor. A
representative electrophotographic process involves a series of steps to produce
an image on a receptor, including charging, exposure, development, transfer, fusing,
and cleaning, and erasure.
In the charging step, a photoreceptor is covered with charge of a desired polarity,
either negative or positive, typically with a corona or charging roller. In the
exposure step, an optical system, typically a laser scanner or diode array, forms
a latent image by selectively discharging the charged surface of the photoreceptor
in an imagewise manner corresponding to the desired image to be formed on the final
image receptor. In the development step, toner particles of the appropriate polarity
are generally brought into contact with the latent image on the photoreceptor,
typically using a developer electrically-biased to a potential opposite in polarity
to the toner polarity. The toner particles migrate to the photoreceptor and selectively
adhere to the latent image via electrostatic forces, forming a toned image on the photoreceptor.
In the transfer step, the toned image is transferred from the photoreceptor to
the desired final image receptor; an intermediate transfer element is sometimes
used to effect transfer of the toned image from the photoreceptor with subsequent
transfer of the toned image to a final image receptor. In the fusing step, the
toned image on the final image receptor is heated to soften or melt the toner particles,
thereby fusing the toned image to the final receptor. An alternative fusing method
involves fixing the toner to the final receptor under high pressure with or without
heat. In the cleaning step, residual toner remaining on the photoreceptor is removed.
Finally, in the erasing step, the photoreceptor charge is reduced to a substantially
uniformly low value by exposure to light of a particular wavelength band, thereby
removing remnants of the original latent image and preparing the photoreceptor
for the next imaging cycle.
Two types of toner are in widespread, commercial use: liquid toner and dry toner.
The term "dry" does not mean that the dry toner is totally free of any liquid constituents,
but connotes that the toner particles do not contain any significant amount of
solvent, e.g., typically less than 10 weight percent solvent (generally, dry toner
is as dry as is reasonably practical in terms of solvent content), and are capable
of carrying a triboelectric charge. This distinguishes dry toner particles from
liquid toner particles.
A typical liquid toner composition generally includes toner particles suspended
or dispersed in a liquid carrier. The liquid carrier is typically nonconductive
dispersant, to avoid discharging the latent electrostatic image. Liquid toner particles
are generally solvated to some degree in the liquid carrier (or carrier liquid),
typically in more than 50 weight percent of a low polarity, low dielectric constant,
substantially nonaqueous carrier solvent. Liquid toner particles are generally
chemically charged using polar groups that dissociate in the carrier solvent, but
do not carry a triboelectric charge while solvated and/or dispersed in the liquid
carrier. Liquid toner particles are also typically smaller than dry toner particles.
Because of their small particle size, ranging from about 5 microns to sub-micron,
liquid toners are capable of producing very high-resolution toned images.
A typical toner particle for a liquid toner composition generally comprises a
visual
enhancement additive (for example, a colored pigment particle) and a polymeric
binder. The polymeric binder fulfills functions both during and after the electrophotographic
process. With respect to processability, the character of the binder impacts charging
and charge stability, flow, and fusing characteristics of the toner particles.
These characteristics are important to achieve good performance during development,
transfer, and fusing. After an image is formed on the final receptor, the nature
of the binder (e.g. glass transition temperature, melt viscosity, molecular weight)
and the fusing conditions (e.g. temperature, pressure and fuser configuration)
impact durability (e.g. blocking and erasure resistance), adhesion to the receptor,
gloss, and the like.
Polymeric binder materials suitable for use in liquid toner particles typically
exhibit glass transition temperatures of about -24° C. to 55° C., which
is lower than the range of glass transition temperatures (50-100° C.) typical
for polymeric binders used in dry toner particles. In particular, some liquid toners
are known to incorporate polymeric binders exhibiting glass transition temperatures
(T
g) below room temperature (25° C.) in order to rapidly self fix,
e.g., by film formation, in the liquid electrophotographic imaging process; see
e.g. U.S. Pat. No. 6,255,363. However, such liquid toners are also known to exhibit
inferior image durability resulting from the low T
g (e.g. poor blocking
and erasure resistance) after fusing the toned image to a final image receptor.
In other printing processes using liquid toners, self-fixing is not required.
In such a system, the image developed on the photoconductive surface is transferred
to an intermediate transfer belt ("ITB") or intermediate transfer member ("ITM")
or directly to a print medium without film formation at this stage. See, for example,
U.S. Pat. No. 5,410,392 to Landa, issued on Apr. 25, 1995; and U.S. Pat. No. 5,115,277
to Camis, issued on May 19, 1992. In such a system, this transfer of discrete toner
particles in image form is carried out using a combination of mechanical forces,
electrostatic forces, and thermal energy. In the system particularly described
in the '277 patent, DC bias voltage is connected to an inner sleeve member to develop
electrostatic forces at the surface of the print medium for assisting in the efficient
transfer of color images.
The toner particles used in such a system have been previously prepared using
conventional polymeric binder materials, and not polymers made using an organosol
process. Thus, for example the '392 patent states that the liquid developer to
be used in the disclosed system is described in U.S. Pat. No. 4,794,651 to Landa,
issued on Dec. 27, 1988. This patent discloses liquid toners made by heating a
preformed high T
g polymer resin in a carrier liquid to an elevated temperature
sufficiently high for the carrier liquid to soften or plasticize the resin, adding
a pigment, and exposing the resulting high temperature dispersion to a high energy
mixing or milling process.
Although such non self-fixing liquid toners using higher T
g (T
g
generally greater than or equal to about 60° C.) polymeric binders should
have good image durability, such toners are known to exhibit other problems related
to the choice of polymeric binder, including image defects due to the inability
of the liquid toner to rapidly self fix in the imaging process, poor charging and
charge stability, poor stability with respect to agglomeration or aggregation in
storage, poor sedimentation stability in storage, and the requirement that high
fusing temperatures of about 200-250° C. be used in order to soften or melt
the toner particles and thereby adequately fuse the toner to the final image receptor.
To overcome the durability deficiencies, polymeric materials selected for use
in both nonfilm-forming liquid toners and dry toners more typically exhibit a range
of T
g of at least about 55-65° C. in order to obtain good blocking
resistance after fusing, yet typically require high fusing temperatures of about
200-250° C. in order to soften or melt the toner particles and thereby adequately
fuse the toner to the final image receptor. High fusing temperatures are a disadvantage
for dry toners because of the long warm-up time and higher energy consumption associated
with high temperature fusing and because of the risk of fire associated with fusing
toner to paper at temperatures approaching the autoignition temperature of paper
(233° C.).
In addition, some liquid and dry toners using high T
g polymeric binders
are known to exhibit undesirable partial transfer (offset) of the toned image from
the final image receptor to the fuser surface at temperatures above or below the
optimal fusing temperature, requiring the use of low surface energy materials in
the fuser surface or the application of fuser oils to prevent offset. Alternatively,
various lubricants or waxes have been physically blended into the dry toner particles
during fabrication to act as release or slip agents; however, because these waxes
are not chemically bonded to the polymeric binder, they may adversely affect triboelectric
charging of the toner particle or may migrate from the toner particle and contaminate
the photoreceptor, an intermediate transfer element, the fuser element, or other
surfaces critical to the electrophotographic process.
In addition to the polymeric binder and the visual enhancement additive, liquid
toner compositions can optionally include other additives. For example, charge
control agents can be added to impart an electrostatic charge on the toner particles.
Dispersing agents can be added to provide colloidal stability, aid fixing of the
image, and provide charged or charging sites for the particle surface. Dispersing
agents are commonly added to liquid toner compositions because toner particle concentrations
are high (inter-particle distances are small) and electrical double-layer effects
alone will not adequately stabilize the dispersion with respect to aggregation
or agglomeration. Release agents can also be used to help prevent the toner from
sticking to fuser rolls when those are used. Other additives include antioxidants,
ultraviolet stabilizers, fungicides, bactericides, flow control agents, and the like.
One fabrication technique involves synthesizing an amphipathic copolymeric binder
dispersed in a liquid carrier to form an organosol, then mixing the formed organosol
with other ingredients to form a liquid toner composition. Typically, organosols
are synthesized by nonaqueous dispersion polymerization of polymerizable compounds
(e.g. monomers) to form copolymeric binder particles that are dispersed in a low
dielectric hydrocarbon solvent (carrier liquid). These dispersed copolymer particles
are sterically-stabilized with respect to aggregation by chemical bonding of a
steric stabilizer (e.g. graft stabilizer), solvated by the carrier liquid, to the
dispersed core particles as they are formed in the polymerization. Details of the
mechanism of such steric stabilization are described in Napper, D. H., "Polymeric
Stabilization of Colloidal Dispersions," Academic Press, New York, N.Y., 1983.
Procedures for synthesizing self-stable organosols are described in "Dispersion
Polymerization in Organic Media," K. E. J. Barrett, ed., John Wiley: New York,
N.Y., 1975.
Liquid toner compositions have been manufactured using dispersion polymerization
in low polarity, low dielectric constant carrier solvents for use in making relatively
low glass transition temperature (T
g≦30° C.) film-forming
liquid toners that undergo rapid self-fixing in the electrophotographic imaging
process. See, e.g., U.S. Pat. Nos. 5,886,067 and 6,103,781. Organosols have also
been prepared for use in making intermediate glass transition temperature (T
g
between 30-55° C.) liquid electrostatic toners for use in electrostatic stylus
printers. See e.g. U.S. Pat. No. 6,255,363 B1. A representative non-aqueous dispersion
polymerization method for forming an organosol is a free radical polymerization
carried out when one or more ethylenically-unsaturated monomers, soluble in a hydrocarbon
medium, are polymerized in the presence of a preformed, polymerizable solution
polymer (e.g. a graft stabilizer or "living" polymer). See U.S. Pat. No. 6,255,363.
Once the organosol has been formed, one or more additives can be incorporated,
as desired. For example, one or more visual enhancement additives and/or charge
control agents can be incorporated. The composition can then subjected to one or
more mixing processes, such as homogenization, microfluidization, ball-milling,
attritor milling, high energy bead (sand) milling, basket milling or other techniques
known in the art to reduce particle size in a dispersion. The mixing process acts
to break down aggregated visual enhancement additive particles, when present, into
primary particles (having a diameter in the range of 0.05 to 1.0 microns) and may
also partially shred the dispersed copolymeric binder into fragments that can associate
with the surface of the visual enhancement additive.
According to this embodiment, the dispersed copolymer or fragments derived
from the copolymer then associate with the visual enhancement additive, for example,
by adsorbing to or adhering to the surface of the visual enhancement additive,
thereby forming toner particles. The result is a sterically-stabilized, nonaqueous
dispersion of toner particles having a size in the range of about 0.1 to 2.0 microns,
with typical toner particle diameters in the range 0.1 to 0.5 microns. In some
embodiments, one or more charge control agents can be added after mixing, if desired.
Several characteristics of liquid toner compositions are important to provide
high quality images. Toner particle size and charge characteristics are especially
important to form high quality images with good resolution. Further, rapid self-fixing
of the toner particles is an important requirement for some liquid electrophotographic
printing applications, e.g. to avoid printing defects (such as smearing or trailing-edge
tailing) and incomplete transfer in high-speed printing. Another important consideration
in formulating a liquid toner composition relates to the durability and archivability
of the image on the final receptor. Erasure resistance, e.g. resistance to removal
or damage of the toned image by abrasion, particularly by abrasion from natural
or synthetic rubber erasers commonly used to remove extraneous pencil or pen markings,
is a desirable characteristic of liquid toner particles.
Another important consideration in formulating a liquid toner is the tack
of the image on the final receptor. It is desirable for the image on the final
receptor to be essentially tack-free over a fairly wide range of temperatures.
If the image has a residual tack, then the image can become embossed or picked
off when placed in contact with another surface (also referred to as blocking).
This is particularly a problem when printed sheets are placed in a stack. Resistance
of the image on the final image receptor to damage by blocking to the receptor
(or to other toned surfaces) is another desirable characteristic of liquid toner particles.
To address this concern, a film laminate or protective layer may be placed over
the surface of the image. This laminate often acts to increase the effective dot
gain of the image, thereby interfering with the color rendition of a color composite.
In addition, lamination of a protective layer over a final image surface adds both
extra cost of materials and extra process steps to apply the protective layer,
and may be unacceptable for certain printing applications (e.g. plain paper copying
or printing).
Various methods have been used to address the drawbacks caused by lamination.
For example, approaches have employed radiation or catalytic curing methods to
cure or crosslink the liquid toner after the development step in order to eliminate
tack. Such curing processes are generally too slow for use in high speed printing
processes. In addition, such curing methods can add significantly to the expense
of the printing process. The curable liquid toners frequently exhibit poor self
stability and can result in brittleness of the printed ink.
Another method to improve the durability of liquid toned images and address
the drawbacks of lamination is described in U.S. Pat. No. 6,103,781. U.S. Pat.
No. 6,103,781 describes a liquid ink composition containing organosols having side-chain
or main-chain crystallizable polymeric moieties. At column 6, lines 53-60, the
authors describe a binder resin that is an amphipathic copolymer dispersed in a
liquid carrier (also known as an organosol) that includes a high molecular weight
(co)polymeric steric stabilizer covalently bonded to an insoluble, thermoplastic
(co)polymeric core. The steric stabilizer includes a crystallizable polymeric moiety
that is capable of independently and reversibly crystallizing at or above room
temperature (22° C.).
According to the authors, superior stability of the dispersed toner particles
with respect to aggregation is obtained when at least one of the polymers or copolymers
(denoted as the stabilizer) is an amphipathic substance containing at least one
oligomeric or polymeric component having a weight-average molecular weight of at
least 5,000 which is solvated by the liquid carrier. In other words, the selected
stabilizer, if present as an independent molecule, would have some finite solubility
in the liquid carrier. Generally, this requirement is met if the absolute difference
in Hildebrand solubility parameter between the steric stabilizer and the solvent
is less than or equal to 3.0 MPa
1/2.
As described in U.S. Pat. No. 6,103,781, the composition of the insoluble resin
core is preferentially manipulated such that the organosol exhibits an effective
glass transition temperature (T
g) of less than 22° C., more preferably
less than 6° C. Controlling the glass transition temperature allows one to
formulate an ink composition containing the resin as a major component to undergo
rapid film formation (rapid self-fixing) in liquid electrophotographic printing
or imaging processes using offset transfer processes carried out at temperatures
greater than the core Tg, preferably at or above 22° C. (Column 10, lines
36-46). The presence of the crystallizable polymeric moiety that is capable of
independently and reversibly crystallizing at or above room temperature (22°
C.) acts to protect the soft, tacky, low T
g insoluble resin core after
fusing to the final image receptor. This acts to improve the blocking and erasure
resistance of the fused, toned image at temperatures up to the crystallization
temperature (melting point) of the crystallizable polymeric moiety.
In attempting to address tack of the image on a final receptor, one must also
consider film strength and image integrity. As described in U.S. Pat. No. 6,103,781,
for liquid electrophotographic toners (particularly liquid toners developed for
use in offset transfer processes), the composition of the insoluble resin core
is preferentially manipulated such that the organosol exhibits an effective glass
transition temperature (Tg) of less than 22° C., more preferably less than
6° C. Controlling the glass transition temperature allows one to formulate
an ink composition containing the resin as a major component to undergo rapid film
formation (rapid self-fixing) in printing or imaging processes carried out at temperatures
at least the core Tg, preferably at or above 22° C. (Column 10, lines 36-46).
SUMMARY OF THE INVENTION
The present invention relates to liquid electrophotographic toner compositions
comprising a liquid carrier and toner particles dispersed in the liquid carrier.
The toner particles comprise at least one visual enhancement additive and a polymeric
binder. The binder comprises at least one amphipathic copolymer comprising one
or more S material portions and one or more D material portions. One or more of
the S or D material portions comprises the residue of a Soluble High T
g
Monomer having a T
g at least about 20° C. The absolute difference
in Hildebrand solubility parameters between the Soluble High T
g Monomer
and the liquid carrier is less than about 3 MPa
1/2. The S portions and
the D portions of the amphipathic copolymer each have a T
g at least
about 30° C.
The toner particles of the liquid toner composition advantageously include at
least one visual enhancement additive, for example, a colorant particle, and a
polymeric binder that comprises an amphipathic copolymer. As used herein, the term
"amphipathic" refers to a copolymer having a combination of portions having distinct
solubility and dispersibility characteristics in a desired liquid carrier that
is used to make the organosol and/or used in the course of preparing the liquid
toner particles. Preferably, the liquid carrier is selected such that at least
one portion (also referred to herein as S material or portion(s)) of the copolymer
is more solvated by the carrier while at least one other portion (also referred
to herein as D material or portion(s)) of the copolymer constitutes more of a dispersed
phase in the carrier.
In preferred embodiments, the copolymer is polymerized in situ in the desired
liquid carrier, as this yields substantially monodisperse copolymeric particles
suitable for use in toner compositions. The resulting organosol is then preferably
mixed with at least one visual enhancement additive and optionally one or more
other desired ingredients to form a liquid toner. During such combination, ingredients
comprising the visual enhancement particles and the copolymer will tend to self-assemble
into composite particles having solvated (S) portions and dispersed (D) portions.
Specifically, it is believed that the D material of the copolymer will tend to
physically and/or chemically interact with the surface of the visual enhancement
additive, while the S material helps promote dispersion in the carrier.
The liquid toner compositions according to the invention provide a system wherein
an image can surprisingly be provided having excellent transfer under relatively
low fusion temperature conditions, and yet be surprisingly resistant to blocking.
Images made using the compositions of the present invention are surprisingly non-tacky
and are resistant to marring and undesired erasure.
More specifically, the incorporation of a Soluble High T
g Monomer
in a toner particle as described in more detail herein surprisingly provides liquid
toner compositions that exhibit lower fusing temperatures. For example, the liquid
toner composition incorporating a Soluble High T
g Monomer preferably
can fuse at temperatures of about 140° C., as compared to fusing temperatures
of about 150° C. that are seen with otherwise identical liquid toner compositions
that lack Soluble High T
g Monomer in the D material. As a result, printing
equipment used in conjunction with preferred liquid toner compositions of the invention
do not require as much energy to fuse the toner composition on the substrate.
Soluble High T
g Monomers may be provided in either the D portion
or the S portion of the amphipathic copolymer. The benefits of the incorporation
of Soluble High T
g Monomers is particularly surprising when the Soluble
High T
g Monomers is located in the D portion of the polymeric constituent
of the organosol. First because these monomers are soluble in the carrier liquid,
it is surprising that they can be incorporated in an effective amount at this portion
of the amphipathic copolymer. Additionally, the physical location of the D portion
in the toner particle is generally considered to be at the internal part of the
particle, and thus it would not be expected that this monomer at this location
would have a meaningful impact on the fusion temperature of toner particles. After
fusing, the binder material of the toner particle solidifies, and excellent blocking
resistance is observed at temperatures up to about the melting temperature (T
m)
of the amphipathic copolymer.
While not being bound by theory, it is believed that the Soluble High T
g
Monomer component of the amphipathic copolymer has an affinity for the liquid carrier
of the toner composition, and therefore tends to retain at least a small amount
of the liquid carrier in the particle during the printing process. This liquid
carrier is believed to have a plasticizing effect during the printing and image
formation process, thereby reducing the fusing temperature when the toner is fused
on the substrate as compared to an otherwise identical liquid toner composition
that lacks a Soluble High T
g Monomer.
The typical fusing temperature is thus preferably reduced from about 170-180°
C. to about 140-150° C. The application costs are thus reduced because less
energy is used in the printing process. It is believed that the liquid carrier
associated with the Soluble High T
g Monomer component is driven off
during the heating/fusing process. The resulting toner after it is in place on
the substrate in the form of an image exhibits a high T
g, and therefore
is resistant to blocking, etc. Surprisingly, inclusion of Soluble High T
g
Monomer in the amphipathic copolymer provides a toner composition that exhibits
improved resistance against blocking (reduced tackiness), as compared to otherwise
identical liquid toner compositions that lack the Soluble High T
g Monomer.
The Soluble High T
g Monomer described herein are selected to be soluble
in liquid carriers. Thus, it is surprising that these Soluble High T
g Monomer
can be included in the D material without affecting properties of the amphipathic
copolymer. Moreover, placement of the Soluble High T
g Monomer in the
D material of the copolymer provides more flexibility in formulating the amphipathic
copolymer. As described herein, preferred embodiments of the invention comprise
an amphipathic copolymer having a relatively larger amount of D material than S
material. By including Soluble High T
g Monomer in the more abundant
D material, flexibility is provided in formulating the S material of the copolymer.
Previously, it has been taught that organosols with core T
g's
above room temperature (22° C.) typically do not form cohesive films, resulting
in poor image transfer in offset printing. It was taught that the integrity of
the toned image during partial removal of the solvent also depended upon the core
T
g, with lower T
g promoting film strength and image integrity
at the cost of additional image tack. See U.S. Pat. No. 6,103,781 (column 11, lines
18-23). Thus, the U.S. Pat. No. 6,103,781 patent describes that preferably the
minimum film forming temperatures are between about 22-45° C. and the organosol
core T
g is below room temperature to allow the toner to form a film
and maintain good image integrity during solvent removal and good cohesive strength
during image transfer from the photoconductor onto either a transfer medium or
receptor. (U.S. Pat. No. 6,103,781, column 11, lines 23-31).
However, it has been surprisingly been found that providing a Soluble High
T
g Monomer in the insoluble portion of the polymeric constituent of
the organosol provides excellent image quality, with reduced tack. In other words,
inclusion of materials having a T
g above room temperature provides surprising
benefits as described herein.
DETAILED DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS
The embodiments of the present invention described below are not intended to
be exhaustive or to limit the invention to the precise forms disclosed in the following
detailed description. Rather, the embodiments are chosen and described so that
others skilled in the art can appreciate and understand the principles and practices
of the present invention.
Preferably, the nonaqueous liquid carrier of the organosol is selected
such that at least one portion (also referred to herein as the S material or portion)
of the amphipathic copolymer is more solvated by the carrier while at least one
other portion (also referred to herein as the D material or portion) of the copolymer
constitutes more of a dispersed phase in the carrier. In other words, preferred
copolymers of the present invention comprise S and D material having respective
solubilities in the desired liquid carrier that are sufficiently different from
each other such that the S blocks tend to be more solvated by the carrier while
the D blocks tend to be more dispersed in the carrier. More preferably, the S blocks
are soluble in the liquid carrier while the D blocks are insoluble. In particularly
preferred embodiments, the D material phase separates from the liquid carrier,
forming dispersed particles.
From one perspective, the polymer particles when dispersed in the liquid carrier
may be viewed as having a core/shell structure in which the D material tends to
be in the core, while the S material tends to be in the shell. The S material thus
functions as a dispersing aid, steric stabilizer or graft copolymer stabilizer,
to help stabilize dispersions of the copolymer particles in the liquid carrier.
Consequently, the S material may also be referred to herein as a "graft stabilizer."
The core/shell structure of the binder particles tends to be retained when the
particles are dried when incorporated into liquid toner particles.
The solubility of a material, or a portion of a material such as a copolymeric
portion, may be qualitatively and quantitatively characterized in terms of its
Hildebrand solubility parameter. The Hildebrand solubility parameter refers to
a solubility parameter represented by the square root of the cohesive energy density
of a material, having units of (pressure)
1/2, and being equal to (ΔH/RT)
1/2/V/
1/2,
where ΔH is the molar vaporization enthalpy of the material, R is the universal
gas constant, T is the absolute temperature, and V is the molar volume of the solvent.
Hildebrand solubility parameters are tabulated for solvents in Barton, A. F. M.,
Handbook of Solubility and Other Cohesion Parameters, 2d Ed. CRC Press,
Boca Raton, Fla., (1991), for monomers and representative polymers in
Polymer
Handbook, 3rd Ed., J. Brandrup & E. H. Immergut, Eds. John Wiley, N.Y., pp
519-557 (1989), and for many commercially available polymers in Barton, A. F. M.,
Handbook of Polymer-
Liquid Interaction Parameters and Solubility Parameters,
CRC Press, Boca Raton, Fla., (1990).
The degree of solubility of a material, or portion thereof, in a liquid carrier
may be predicted from the absolute difference in Hildebrand solubility parameters
between the material, or portion thereof, and the liquid carrier. A material, or
portion thereof, will be fully soluble or at least in a highly solvated state when
the absolute difference in Hildebrand solubility parameter between the material,
or portion thereof, and the liquid carrier is less than approximately 1.5 MPa
1/2.
On the other hand, when the absolute difference between the Hildebrand solubility
parameters exceeds approximately 3.0 MPa
1/2, the material, or portion
thereof, will tend to phase separate from the liquid carrier, forming a dispersion.
When the absolute difference in Hildebrand solubility parameters is between 1.5
MPa
1/2 and 3.0 MPa
1/2, the material, or portion thereof,
is considered to be weakly solvatable or marginally insoluble in the liquid carrier.
Consequently, in preferred embodiments, the absolute difference between
the respective Hildebrand solubility parameters of the S portion(s) of the copolymer
and the liquid carrier is less than 3.0 MPa
1/2, preferably less than
about 2.0 MPa
1/2, more preferably less than about 1.5 MPa
1/2.
In a particularly preferred embodiment of the present invention, the absolute difference
between the respective Hildebrand solubility parameters of the S portion(s) of
the copolymer and the liquid carrier is from about 2 to about 3.0 MPa
1/2.
In a particularly preferred embodiment of the present invention, the absolute difference
between the respective Hildebrand solubility parameters of the S portion(s) of
the copolymer and the liquid carrier is from about 2 to about 3.0 MPa
1/2.
Additionally, it is also preferred that the absolute difference between the respective
Hildebrand solubility parameters of the D portion(s) of the copolymer and the liquid
carrier is greater than 2.3 MPa
1/2, preferably greater than about 2.5
MPa
1/2, more preferably greater than about 3.0 MPa
1/2, with
the proviso that the difference between the respective Hildebrand solubility parameters
of the S and D portion(s) is at least about 0.4 MPa
1/2, more preferably
at least about 1.0 MPa
1/2. Because the Hildebrand solubility of a material
may vary with changes in temperature, such solubility parameters are preferably
determined at a desired reference temperature such as at 25° C.
Those skilled in the art understand that the Hildebrand solubility parameter
for a copolymer, or portion thereof, may be calculated using a volume fraction
weighting of the individual Hildebrand solubility parameters for each monomer comprising
the copolymer, or portion thereof, as described for binary copolymers in Barton
A. F. M.,
Handbook of Solubility Parameters and Other Cohesion Parameters,
CRC Press, Boca Raton, p 12 (1990). The magnitude of the Hildebrand solubility
parameter for polymeric materials is also known to be weakly dependent upon the
weight average molecular weight of the polymer, as noted in Barton, pp 446-448.
Thus, there will be a preferred molecular weight range for a given polymer or portion
thereof in order to achieve desired solvating or dispersing characteristics. Similarly,
the Hildebrand solubility parameter for a mixture may be calculated using a volume
fraction weighting of the individual Hildebrand solubility pa
