Gel organosol including amphipathic copolymeric binder having crosslinking functionality and liquid toners for electrophotographic applications Number:7,029,814 from the United States Patent and Trademark Office (PTO) owispatent 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 comprising one or more S material portions and one or more D material portions, wherein the components of the composition comprise sufficient crosslinking functionality to provide a three dimensional gel of controlled rigidity which can be reversibly reduced to a fluid state by application of energy. The toners as described herein surprisingly provide compositions that are particularly suitable for electrophotographic processes wherein the transfer of the image from the surface of a photoconductor to an intermediate transfer material or directly to a print medium is carried out without film formation on the photoconductor.<H electrophotographic, functionality, Gel, organosol, i, 7029814.html, Patent, pto, 7029814

Title: Gel organosol including amphipathic copolymeric binder having crosslinking functionality 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 comprising one or more S material portions and one or more D material portions, wherein the components of the composition comprise sufficient crosslinking functionality to provide a three dimensional gel of controlled rigidity which can be reversibly reduced to a fluid state by application of energy. The toners as described herein surprisingly provide compositions that are particularly suitable for electrophotographic processes wherein the transfer of the image from the surface of a photoconductor to an intermediate transfer material or directly to a print medium is carried out without film formation on the photoconductor.

Patent Number: 7,029,814 Issued on 04/18/2006 to Baker, et al.

Inventors: Baker; James A. (Hudson, WI); Qian; Julie Y. (Woodbury, MN)
Assignee: Samsung Electronics Company (Suwon, KR)

Appl. No.: 612444

Filed: June 30, 2003

Current U.S. Class: 430/114 ; 430/117; 430/137.22

Current International Class: G03G 9/00 (20060101)

Field of Search: 430/114,117,137.22

References Cited [Referenced By]

U.S. Patent Documents


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5302482	April 1994	Elmasry et al.
5384226	January 1995	Kanakura et al.
5650253	July 1997	Baker et al.
5652282	July 1997	Baker et al.
5698616	December 1997	Baker et al.
5886067	March 1999	Li et al.
5916718	June 1999	Kellie et al.
5965314	October 1999	Herman et al.
6088560	July 2000	Zenk et al.
6103781	August 2000	Li et al.
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6221545	April 2001	Tran et al.
6255363	July 2001	Baker et al.
6546221	April 2003	Baker et al.
6647234	November 2003	Herman et al.
6649316	November 2003	Baker et al.
2002/0110390	August 2002	Park et al.
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Foreign Patent Documents


0 852 746	Mar., 2000	EP
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WO 01/79363	Oct., 2001	WO
WO 01/79364	Oct., 2001	WO

Other References

US. Appl. No. 10/612,182, filed Jun. 30, 2003, entitled "Gel Organosol Including Amphipathic Copolymeric Binder Having Selected Molecular Weight and Liquid Toners for Electrophotographic Applications" (58 pgs.). cited by other .
U.S. Appl. No. 10/612,058, filed Jun. 30, 2003, entitled "Gel Organosol Including Amphipathic Copolymeric Binder Having Acid/Base Functionality and Liquid Toners for Electrophotographic Applications" (74 pgs.). cited by other .
U.S. Appl. No. 10/612,448, filed Jun. 30, 2003, entitled "Gel Organosol Including Amphipathic Copolymeric Binder Having Hydrogen Bonding Functionality and Liquid Toners for Electrophotographic Applications" (74 pgs.). cited by other.

Primary Examiner: Chapman; Mark A.
Attorney, Agent or Firm: Kagan Binder, PLLC

Claims

The invention claimed is:

1. A liquid electrophotographic toner composition comprising: a) a liquid carrier having a Kauri-butanol number less than 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 liquid carrier while the D material portions tend to be more dispersed in the liquid carrier, and wherein the amphipathic copolymer comprises covalent crosslinking functionality in an amount sufficient to provide a three dimensional gel of controlled rigidity which can be reversibly reduced to a fluid state by application of energy; and wherein the electrophotographic toner composition does not form a film under Photoreceptor Image Formation conditions.

2. The liquid electrophotographic toner composition according to claim 1, wherein the crosslinking functionalities are located in the S material portion of the amphipathic copolymer.

3. The liquid electrophotographic toner composition according to claim 1, wherein the crosslinking functionalities are located in the D material portion of the amphipathic copolymer.

4. The liquid electrophotographic toner composition according to claim 1, wherein the crosslinking functionalities are located in both the S material portion and the D material portion of the amphipathic copolymer.

5. The liquid electrophotographic toner composition according to claim 1, said composition comprising a polyfunctional bridging compound having at least two crosslinking functionalities to assist in gel formation.

6. The liquid electrophotographic toner composition according to claim 1, wherein crosslinking functionalities comprise isocyanate functionalities and hydroxyl functionalities that react to form polyurethane linkages.

7. The liquid electrophotographic toner composition according to claim 1, wherein crosslinking functionalities are provided by incorporation of one or more bifunctional polymerizable compounds in the amphipathic copolymer, wherein the bifunctional polymerizable compound is selected from the group consisting of divinyl benzene; 1,3 butanediol diacrylate; 1,4 butanediol diacrylate; 1,3 butanediol dimethacrylate; diethylene glycol diacrylate; diethylene glycol dimethacrylate; ethoxylated Bisphenol A diacrylate; ethoxylated Bisphenol A dimethacrylate; ethylene glycol dimethacrylate (EGDMA); 1,6 hexanediol diacrylate; 1,6 hexanediol dimethacrylate; neopentyl glycol diacrylate; neopentyl glycol dimethacrylate; polyethylene glycol diacrylate; polyethylene glycol dimethacrylate; propoxylated neopentyl glycol diacrylate; tetraethylene glycol diacrylate; tetraethylene glycol dimethacrylate; triethylene glycol diacrylate; triethylene glycol dimethacrylate; tripropylene glycol diacrylate; tripropylene glycol dimethacrylate; zinc diacrylate; zinc dimethacrylate and 1,4 phenylene diisocyanate (PDI).

8. The liquid electrophotographic toner composition according to claim 1, wherein crosslinking functionalities are provided by incorporation of one or more bifunctional polymerizable compounds in the amphipathic copolymer, wherein the bifunctional polymerizable compound is selected from the group consisting of t-butylaminoethyl methacrylate; diethylaminoethyl acrylate; diethylaminoethyl methacrylate; 2-diisopropylaminoethyl methacrylate; 2-dimethylaminoethyl methacrylate; dimethylaminopropyl methacrylamide; dipentaerthritol monohydroxypentaacrylate; 2,3-epoxypropyl methacrylate (glycidyl methacrylate); 4-hydroxybutyl acrylate; 2-hydroxyethyl acrylate; 2-hydroxyethyl methacrylate; 2-hydroxypropyl acrylate; cinnamyl alcohol; allyl mercaptan, methallylamine; azlactones, such as 2-alkenyl-4,4-dialkylazlactone; 2-hydroxypropyl methacrylate; meta-isopropenyldimethylbenzyl isocyanate (TMI); isocyanatoethylmethacrylate (IEM); trimethylsilylmethacrylate; (trimethylsilylmethyl)methacrylate; n-vinyl caprolactam; 2-vinyl pyridine; 4-vinyl pyridine and N-vinyl-2-pyrrolidone.

9. The liquid electrophotographic toner composition according to claim 1, wherein crosslinking functionalities are provided by incorporation of one or more trifunctional polymerizable compounds in the amphipathic copolymer, wherein the trifunctional polymerizable compound is selected from the group consisting of ethoxylated trimethylolpropane triacrylate; glyceryl propoxy triacrylate; pentaerythritol triacrylate; trimethylolpropane triacrylate; trimethylolpropane trimethacrylate (TMPTMA); and tris(2-hydroxyethyl)isocyanurate triacrylate).

10. The liquid electrophotographic toner composition according to claim 1, wherein crosslinking functionalities are provided by incorporation of one or more tetrafunctional polymerizable compounds in the amphipathic copolymer, wherein the tetrafunctional polymerizable compound is pentaerythritol tetraacrylate.

11. The liquid electrophotographic toner composition according to claim 1, wherein crosslinking functionalities are capable of carrying out polymerization crosslinking reactions selected from the group consisting of reaction of polyfunctional free radicals; group transfer polymerizations, ring-opening polymerization of cyclic ethers, esters, amides or acetals; epoxidations; reactions of hydroxyl or amino chain transfer agents with terminally-unsaturated end groups; esterification reactions and condensation reactions.

12. The liquid electrophotographic toner composition according to claim 1, wherein crosslinking functionalities comprise isocyanate functionalities and amine functionalities that react to form polyurea linkages.

13. The liquid electrophotographic toner composition according to claim 5, wherein said polyfunctional bridging compound is selected from the group consisting of divinyl benzene; 1,3 butanediol diacrylate; 1,4 butanediol diacrylate; 1,3 butanediol dimethacrylate; diethylene glycol diacrylate; diethylene glycol dimethacrylate; ethoxylated Bisphenol A diacrylate; ethoxylated Bisphenol A dimethacrylate; ethylene glycol dimethacrylate (EGDMA); 1,6 hexanediol diacrylate; 1,6 hexanediol dimethacrylate; neopentyl glycol diacrylate; neopentyl glycol dimethacrylate; polyethylene glycol diacrylate; polyethylene glycol dimethacrylate; propoxylated neopentyl glycol diacrylate; tetraethylene glycol diacrylate; tetraethylene glycol dimethacrylate; triethylene glycol diacrylate; triethylene glycol dimethacrylate; tripropylene glycol diacrylate; tripropylene glycol dimethacrylate; zinc diacrylate; zinc dimethacrylate; 1,4 phenylene diisocyanate (PDI); t-butylaminoethyl methacrylate; diethylaminoethyl acrylate; diethylaminoethyl methacrylate; 2-diisopropylaminoethyl methacrylate; 2-dimethylaminoethyl methacrylate; dimethylaminopropyl methacrylamide; dipentaerthritol monohydroxypentaacrylate; 2,3-epoxypropyl methacrylate (glycidyl methacrylate); 4-hydroxybutyl acrylate; 2-hydroxyethyl acrylate; 2-hydroxyethyl methacrylate; 2-hydroxypropyl acrylate; cinnamyl alcohol; allyl mercaptan, methallylamine; azlactones, such as 2-alkenyl-4,4-dialkylazlactone; 2-hydroxypropyl methacrylate; meta-isopropenyldimethylbenzyl isocyanate (TMI); isocyanatoethylmethacrylate (IEM); trimethylsilylmethacrylate; (trimethylsilylmethyl)methacrylate; n-vinyl caprolactam; 2-vinyl pyridine; 4-vinyl pyridine; N-vinyl-2-pyrrolidone; ethoxylated trimethylolpropane triacrylate; glyceryl propoxy triacrylate; pentaerythritol triacrylate; trimethylolpropane triacrylate; trimethylolpropane trimethacrylate (TMPTMA); tris(2-hydroxyethyl)isocyanurate triacrylate); and pentaerythritol tetraacrylate.

14. The liquid electrophotographic toner composition according to claim 1, wherein the D material portion of the amphipathic copolymer has a total calculated T.sub.g greater than or equal to about 30.degree. C.

15. The liquid electrophotographic toner composition according to claim 1, wherein the D material portion of the amphipathic copolymer has a total calculated T.sub.g of from about 50 60.degree. C.

16. The liquid electrophotographic toner composition according to claim 1, wherein the amphipathic copolymer has a total calculated T.sub.g greater than or equal to about 30.degree. C.

17. The liquid electrophotographic toner composition according to claim 1, wherein the amphipathic copolymer has a total calculated T.sub.g greater than about 55.degree. C.

18. The liquid electrophotographic toner composition according to claim 1, the toner particle comprising at least one visual enhancement additive.

19. 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 in a carrier liquid to be formed on a surface of a photoreceptor; and c) transferring the image from the surface of the photoconductor to an intermediate transfer material or directly to a print medium without film formation on the photoreceptor.

20. A method of making a liquid electrophotographic toner composition, comprising the steps of: a) providing a plurality of free radically polymerizable monomers, wherein at least one of the monomers comprises a first reactive functionality; b) free radically polymerizing the monomers in a solvent to form a first reactive functional polymer, wherein the monomers and the first reactive functional polymer are soluble in the solvent; c) reacting a compound having a second reactive functionality that is reactive with the first reactive functionality and free radically polymerizable functionality with the first reactive functional polymer under conditions such that at least a portion of the second reactive functionality of the compound reacts with at least a portion of the first reactive functionality of the polymer to form one or more linkages by which the compound is linked to the polymer, thereby providing an S material portion polymer with pendant free radically polymerizable functionality; d) copolymerizing ingredients comprising (i) the S material portion polymer with pendant free radically polymerizable functionality, (ii) one or more free radically polymerizable monomers, and (iii) a liquid carrier in which polymeric material derived from ingredients comprising the one or more additional monomers of ingredient (ii) is insoluble; said copolymerizing occurring under conditions effective to form an amphipathic copolymer having S and D portions and to incorporate crosslinking functionality in the copolymer, 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 liquid carrier while the D material portions tend to be more dispersed in the liquid carrier; and wherein the toner composition comprises crosslinking functionality in an amount sufficient to provide a three dimensional gel of controlled rigidity which can be reversibly reduced to a fluid state by application of energy; and wherein the electrophotographic toner composition does not form a film under Photoreceptor Image Formation conditions.

21. The method of claim 20, wherein the first reactive functionality is selected from hydroxyl and amine functionalities, and the second reactive functionality is selected from isocyanate and epoxy functionalities.

22. The method of claim 20, wherein the first reactive functionality is a hydroxyl functionality, and the second reactive functionality is an isocyanate functionality.

23. The method of claim 20, wherein the first reactive functionality is selected from isocyanate and epoxy functionalities, and the second reactive functionality is selected from hydroxyl and amine functionalities.

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 provided in a gel composition.

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, discharged area development, involves a series of steps to produce an image on a receptor, including charging, exposure, development, transfer, fusing, cleaning, and erasure.

In the charging step, a photoreceptor is substantially uniformly covered with charge of a desired polarity to achieve a first potential, 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 to achieve a second potential 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 of the same polarity as the toner polarity and intermediate in potential between the first and second potentials. 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. The image may be transferred by physical pressure and contact of the toner, with selective adhesion to a target intermediate or final image receptor as compared to the surface from which it is transferred. Alternatively, the toner may be transferred in a liquid system optionally using an electrostatic assist as discussed in more detail below. 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 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 sub-micron to about 5 microns, 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.degree. C. to 55.degree. C., which is lower than the range of glass transition temperatures (50 100.degree. 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.sub.g) below room temperature (25.degree. 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.sub.g (e.g. poor blocking and erasure resistance). In addition, such toners, while suitable for transfer processes involving contact adhesive forces, are generally unsuitable for transfer processes involving an electrostatic transfer assist due to the extreme tackiness of the toner films after fusing the toned image to a final image receptor. Also low T.sub.g toners are more sensitive to cohesive transfer failure (film split), and are more difficult to clean (sticky) from photoreceptors or intermediate transfer elements.

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 (described in more detail below). 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.sub.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.sub.g (T.sub.g generally greater than or equal to about 60.degree. 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.degree. 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.sub.g of at least about 55 65.degree. C. in order to obtain good blocking resistance after fusing, yet typically require high fusing temperatures of about 200 250.degree. 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 above the autoignition temperature of paper (233.degree. C.).

In addition, some liquid and dry toners using high T.sub.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.sub.g.ltoreq.30.degree. 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.sub.g between 30 55.degree. 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 before or 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. For example, in organosol toner compositions that exhibit low T.sub.gs, the resulting film that is formed during the imaging process may be sticky and cohesively weak under transfer conditions. This may result in image splitting or undesired residue left on the photoreceptor or intermediate image receptor surfaces. 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.degree. 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.sup.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.sub.g) of less than 22.degree. C., more preferably less than 6.degree. 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 T.sub.g. Preferably, the offset transfer process is carried out at a temperature at or above 22.degree. 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.degree. C.) acts to protect the soft, tacky, low T.sub.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.

Liquid inks using gel organosol compositions have been described in U.S. Pat. No. 6,255,363, and also in WO 01/79316, WO 01/79363, and WO 01/79364. These systems are designed to provide toner compositions that will form films at room temperature and without specific drying procedures or heating elements. See, for example the U.S. '363 patent at column 15, lines 50 63. Thus, the T.sub.g of the toner materials described in these patents and applications specifically are described to be low as part of ability to form a film at room temperature.

SUMMARY OF THE INVENTION

The present invention relates to gel liquid electrophotographic toner compositions comprising a liquid carrier and toner particles dispersed in the liquid carrier. The liquid carrier has a Kauri-butanol number less than 30 mL. The toner particles comprise a polymeric binder comprising at least one amphipathic copolymer with one or more S material portions and one or more D material portions. The amphipathic copolymer comprises covalent crosslinking functionality in an amount sufficient to provide a three dimensional gel of controlled rigidity which can be reversibly reduced to a fluid state by application of energy. The electrophotographic toner composition substantially does not form a film under Photoreceptor Image Formation conditions.

For purposes of the present invention, a "gel" is a three dimensional matrix of controlled rigidity which can be reversibly reduced to a fluid state by application of energy. Gel formation in particular is believed to result from particle-particle interactions that cause reversible agglomeration of the particles. These particle-particle interactions, however, are weak enough to be broken down by the application of shear energy, sonic energy, heat energy, and/or the like.

As noted above, the compositions of the present invention are formulated so that the toner substantially does not form a film under Photoreceptor Image Formation conditions, as defined below. Because of the unique formulation, essentially no film is formed on the photoconductor during the printing process. Instead, the image is transferred from the surface of a photoconductor to an intermediate transfer material or directly to a print medium without substantial film formation on the photoconductor. Film formation may occur after transfer from the photoconductor, preferably at or before the time of final fusing of the image on the final receptor.

"Photoreceptor Image Formation conditions" for purposes of the present invention means that a composition substantially does not form a film when at a solids content of from about 30% to about 40%, and at a temperature between 23.degree. C. and 45.degree. C., and more preferably does not form a film when at a solids content of less than 70% at a temperature between 23.degree. C. and 45.degree. C. As a primary consideration, the T.sub.g of the amphipathic polymer strongly influences whether a film is formed by the organosol gel composition of the present invention. Additional factors, however, may be brought to bear to influence the film formation properties of the composition, such as selection of carrier solvent, location of homogenous regions of polymer components having lower or higher T.sub.g as compared to the balance of the amphipathic copolymer, and the incorporation of various functional groups, particularly at the S material portion of the amphipathic copolymer. The skilled artisan is able to prepare organosol compositions meeting such identified film forming properties by manipulation of these and other factors that will be understood in the art.

Gel toner compositions that do not substantially form a film under Photoreceptor Image Formation conditions provide specific advantages, including excellent image transfer from the photoreceptor, with low or no back transfer of the image to the photoreceptor during the printing process. Additionally, the gel toner compositions exhibit exceptional storage stability without the need to incorporate dispersant, surfactant, or stabilizer additives in an amount deleterious to image quality, although these additional components can be used if desired. Because amphipathic copolymers are used, the S portion of the copolymer may easily comprise covalently bonded stabilizing functionalities that further assist in stabilization of the overall liquid toner composition. Superior final image properties are also observed relative to erasure resistance and blocking resistance.

Additionally, toner particles comprising the amphipathic copolymers as described herein are consistent in size and shape, and therefore provide substantial benefit in uniformity in image formation. Such uniformity of size and shape is difficult or impossible to achieve in conventionally milled toner binder polymers. The liquid toner compositions according to the invention provide a system wherein an image can surprisingly be provided having excellent image transfer, and additionally are resistant to blocking. Images made using the compositions of the present invention are surprisingly non-tacky and are resistant to marring and undesired erasure. The gels impart useful properties to the liquid ink, notably improved sedimentation stability of the colorant, without compromising print quality or ink transfer performance. The inks formulated with the gels also exhibit improved redispersion characteristics upon settling, and do not form dilatant sediments such as those formed by non-gelled organosol inks. These characteristics of gel inks facilitate preparation and use of high solids ink concentrates (greater than 2% by weight solids, more preferably greater than 10% by weight solids, and most preferably >20%), thus providing an increased number of printed pages or images from a given volume of ink. Surprisingly, the organosols of the present invention exhibit effectively larger particle size of gels, thereby exhibiting low to intermediate charge per mass (Q/M) suitable for high optical density development, but additionally exhibiting a break up of the gel under image development field to yield fine particles for high resolution imaging.

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 copolymer and/or used in the course of preparing the liquid toner particles. Preferably, the liquid carrier (also sometimes referred to as "carrier liquid") is selected such that at least one portion (also referred to herein as S material or block(s)) of the copolymer is more solvated by the carrier while at least one other portion (also referred to herein as D material or block(s)) of the copolymer constitutes more of a dispersed phase in the carrier.

The gel is formed by incorporating a low percentage (typically less than 1.6% w/w) of monomer having a crosslinkable functionality in the amphipathic copolymer, and crosslinking the amphipathic copolymer to form intermolecular covalent bonds in an amount sufficient to form a gel. The crosslinking functionalities may be provided in the S material portion, in the D material portion, or in both the S material portion and D material portion of the amphipathic copolymer. Upon crosslinking, the amphipathic copolymer particles are connected by intermolecular covalent bonds, thereby forming a gel organosol.

In an alternative embodiment of the present invention, a bridging compound having at least two crosslinkable functionalities is additionally provided in the organosol composition to assist in crosslinking the amphipathic copolymer.

The gel organosols provide a new approach to improving the sedimentation and redispersion properties of pigmented inks. The method of inducing gelation does not require manipulation of the relative difference in solubility parameter between the amphipathic copolymer and the carrier solvent into a range (solubility parameter difference greater than 2.5 MPa.sup.1/2) that acts to reduce agglomeration stability of the amphipathic copolymer. This allows the ink formulator increased flexibility in selection of monomer components of the amphipathic copolymer, as well as greater flexibility in carrier fluid selection.

For example, side-chain crystallizable monomers that have a high degree of solubility in the carrier solvent may be incorporated into the amphipathic copolymer without sacrificing gelation characteristics. The use of crystallizable polymeric moieties to improve the durability of non-gel organosol inks has been disclosed in U.S. Pat. No. 5,886,067. Heretofore, the use of such crystallizable polymeric moieties at high weight percentages in an amphipathic copolymer has prevented the formation of gel organosols owing to the relative solubility parameter difference between the amphipathic copolymer and the carrier solvent falling in the range of good solubility (0 2.5 MPa.sup.1/2). It would be advantageous to combine the characteristics of a gel organosol and a controlled-crystallinity organosol into a single composition.

Preferably, the toner particles additionally comprise at least one visual enhancement additive.

In preferred embodiments, the copolymer is polymerized in situ in the desired liquid carrier. The use of the carrier liquid as the reaction solvent facilitates the formation of 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.

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).sup.1/2, and being equal to (.DELTA.H-RT).sup.1/2/V.sup.1/2, where .DELTA.H 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.sup.1/2. On the other hand, when the absolute difference between the Hildebrand solubility parameters exceeds approximately 3.0 MPa.sup.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.sup.1/2 and 3.0 MPa.sup.1/2, the material, or portion thereof, is considered to be weakly solvatable or marginally insoluble in the liquid carrier. While not being bound by theory, it is believed that the amphipathic copolymer is covalently crosslinked to such an extent that it behaves as an extremely high molecular weight copolymer near its incipient phase separation point in the dispersant liquid.

Gel organosols are dispersions in which the attractive interactions between the elements of the dispersed phase are so strong that the whole system develops a rigid network structure and, under small stresses, behaves elastically. The characteristic of organosol gelation is visibly apparent to one skilled in the art. The crosslinked gel organosols rapidly gel to form a voluminous polymer sediment and a substantially clear supernatant layer of carrier liquid upon standing.

While not being bound by theory, it is believed that gelation of the amphipathic copolymer organosol is induced by forming covalent bonds between portions of the amphipathic copolymer. Crosslinking is effected using a polyfunctional crosslinking agent, i.e. a crosslinkable polymerizable compound containing a plurality of polymerizable moieties. The crosslinking moiety may be incorporated in the S material portion, the D material portion, or in both the S material portion and the D material portion. The crosslinkable polymerizable compound may have all of its reactive groups comprising the same chemical moiety, or alternatively, one or more of the reactive groups may be different or distinct from the others. For convenience, we will refer to crosslinkable polymerizable compounds having a single type of chemical moiety comprising its reactive groups as a symmetrical crosslinkable polymerizable compound. We will refer to crosslinkable polymerizable compounds having at least two different and distinct chemical moieties comprising its reactive groups as an asymmetrical crosslinkable polymerizable compound. The composition may also be provided with an additional polyfunctional bridging compound having at least two reactive groups to assist in gel formation.

The strength of the gel (and hence sedimentation resistance of the ink) can be readily manipulated by controlling the extent to which the amphipathic copolymer is crosslinked. Greater gel strength (greater sedimentation resistance) is obtained by increasing the crosslink density (percentage of crosslinker) of the graft stabilizer.

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.degree. 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 parameters for each component of the mixture.

In addition, we have defined our invention in terms of the calculated solubility parameters of the monomers and solvents obtained using the group contribution method developed by Small, P. A., J. Appl. Chem., 3, 71 (1953) using Small's group contribution values listed in Table 2.2 on page VII/525 in the Polymer Handbook, 3rd Ed., J. Brandrup & E. H. Immergut, Eds. John Wiley, New York, (1989). We have chosen this method for defining our invention to avoid ambiguities which could result from using solubility parameter values obtained with different experimental methods. In addition, Small's group contribution values will generate solubility parameters that are consistent with data derived from measurements of the enthalpy of vaporization, and therefore are completely consistent with the defining expression for the Hildebrand solubility parameter. Since it is not practical to measure the heat of vaporization for polymers, monomers are a reasonable substitution.

For purposes of illustration, Table I lists Hildebrand solubility parameters for some common solvents used in an electrophotographic toner and the Hildebrand solubility parameters and glass transition temperatures (based on their high molecular weight homopolymers) for some common monomers used in synthesizing organosols.

TABLE-US-00001 TABLE I Hildebrand Solubility Parameters Solvent Values at 25.degree. C. Kauri-Butanol Number by ASTM Method D1133-54T Hildebrand Solubility Solvent Name (ml) Parameter (MPa.sup.1/2) Norpar .TM. 15 18 13.99 Norpar .TM. 13 22 14.24 Norpar .TM. 12 23 14.30 Isopar .TM. V 25 14.42 Isopar .TM. G 28 14.60 Exxsol .TM. D80 28 14.60 Source: Calculated from equation #31 of Polymer Handbook, 3.sup.rd Ed., J. Brandrup E. H. Immergut, Eds. John Wiley, NY, p. VII/522 (1989). Monomer Values at 25.degree. C. Hildebrand Solubility Glass Transition Monomer Name Parameter (MPa.sup.1/2) Temperature (.degree. C.)* 3,3,5-Trimethyl 16.73 125 Cyclohexyl Methacrylate Isobornyl Methacrylate 16.90 110 Isobornyl Acrylate 16.01 94 n-Behenyl acrylate 16.74 <-55 (58 m.p.)** n-Octadecyl Methacrylate 16.77 -100 (45 m.p.)** n-Octadecyl Acrylate 16.82 -55 Lauryl Methacrylate 16.84 -65 Lauryl Acrylate 16.95 -30 2-Ethylhexyl Methacrylate 1