CA1212854A - Liquid electrographic developers - Google Patents

Abstract

LIQUID ELECTROGRAPHIC DEVELOPERS
Abstract of Disclosure Liquid electrographic developers are dis-closed comprising an electrically insulating liquid carrier, toner, a charge-control agent and a charging agent. The charge-control agent is a carrier-soluble, addition copolymer of a quaternary ammonium salt monomer, a monomer having -COOH, -SO3H or -PO3HR acidic function wherein R is hydrogen or alkyl, and a solubilizing monomer. The charging agent is a carrier-soluble, addition polar copoly-mer. The disclosed developers exhibit improved replenishability as evidenced by reduced buildup of charge in the developers during the course of use and repeated replenishment.

Description

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LIQUID ELECTROGRAPHIC DEVELOPERS
The present invention relates to a liquid electrographic developer composition comprising an electrically insulating carrier liquid, a toner dis-persed in the carrier, a quaternary ammonium polymerto control the electrical charge on the pigment, and a polar copolymer to impart charge to the toner dis-persion.
In the art of electrography, charge images are formed on a substrate and thereafter rendered visible by contact with an electrographic developer composition. These compositions are formulated in dry or liquid form, each presentin~ its own unique requirements and advantages.
Liquid electrographic developers are a dis-persion of toner particles, usually of a pigment and a binder resin, in an electrically insulating carrier liquid such as a volatile hydrocarbon fractlon. The binder resin serves to bind ~he pigment to the copy 20 paper on which the latent electrostatic image is developed. The developer can also contain a stabi-lizer additive to maintain a stable dispersion of ~he toner particles. Liquid developers also often con-tain a charge-control agent to main~ain a constant charge level.
A particularly useful liquid developer is disclosed in US Patent 4,273,849 issued June 16, 1981, to S. H. Merrill et al. This developer com-prises an electrically insulating carrier liquid hav-ing stably dispersed therein (1) toner particles, andhaving dissolved therein (2) a copolymer of a quater-nary ammonium salt monomer and a solubilizing mono-mer, and (3) a copolymer of an addition polar monomer and a solubilizing monOmer~ the amounts of solubiliz-ing monomer units in each copolymer being sufficientto make the copolymer soluble in the carrier liquid alone.

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The aforementioned liquid developers of US
'849 have certain drawbacks, however, relating to the stability of their charge as they are used through a number of copy sequences. In particular, the charge of the developer per unit of mass of dispersed toner increases, indicating that the quaternary ammonium charge-control copolymer deposits on an electrostatiC
image at ~ lower rate than the toner. This uneven depletion rate and consequential incr~ase in charge per unit mass in the developer presents difficulty in developer replenishment and causes nonuniform image density from copy ~o copy-In accordance with the present invention,the charge of ~he developer per unit mass of toner is stabilized so as to reduce the rate at which charge per unit of mass increases during use. Such sta-bility is achieved when the quaternary ammonium salt charge-control polymer in the above-described devel oper composition contains an insolubilizing monomer having an acidic function selected from the group consisting of -COOH, -S03H and -P03HR wherein R is hydrogen or alkyl.
The present invention provides, therefore, electrographic liquid developer composition~, and 25 processes for their use, comprising an electricallyi insulating carrier liquid having dispersed toner par-ticles and (1) a charge-control agent comprising a carrier-soluble addition copolymer of a quaternary ammonium salt monomer, a solubilizing monomer and an insolubilizing monOMer having an acidic function selected from the group consisting of -COOH~ -S03H
and P03HR wherein R is hydrogen or alkyl, and (2) a carrier-soluble charging addition copolymer compris-ing a polar monomer and a solubilizing monomer. When R is alkyl in the -P03HR group, it is preferably 1-10 carbon alkyl such as methyl, propyl, amyl or hexyl. Most preferably, alkyl is 1-4 carbon alkyl.

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In a preferred embodiment of the invention, the charge of the developer per unit mass is further stabilized against change when the toner contains a neutral or basic pigment.
The present invention constitutes an improvement of the prior-art liquid developers dis-closed ln the US Patents 4,273,849 and 4,252,921.
These developers comprise a carrier liquid, dispersed toner particles, a carrier-soluble charge-control addition polymer and a carrier-soluble polar addition polymer to stabilize the dispersion of toner. The improvement is provided by incorporating the acidic monomer defined above into the charge- control poly-mer.
The liquid carrier employed in our developer has a low dielectric constant and a high electrical resistance such that it will not disturb or destroy the electroststic charge pa~tern being developed.
Carrier liquids should have a dielectric constant of less than about 3, have a volume resistivity greater than about 10l ohm-cm and be chemically stable.
Suitable carrier liquids include halogenated hydro-carbon solvents such as ~richlorotrifluoroethane;
hydrocarbon solvents are useful, such as isoparaf-finic hydrocarbons and cyclohydrocarbons such ascyclohexane. Preferably, the liquid carrier is a combination of two different elec~rically insulating liquids.
Toner is dispersed in ~he carrier and serves as the marking particle congtituent which is attracted to an electrostatic charge image on an ele-ment being developed. The toner comprises a colorant and a binder polymer, if necessary, for adhesion of the colorant to the image-bearing element. Colorants can be selected from one or more of a variety of dyes and pigments. Carbon black is a preferred colorant, but virtually any of the compounds in the "Colour Index", Second Edition, 1956, Volumes I and II, may, in principle, be used.
Preferred toners comprise neutral or basic pigments such as neutral or basic carbon black pig-ments. The neutrality or basicity of a pigment isdetermined as the pH of the pigment by the ASTM D
1512-60(76) method, details of which are reported in the "Analysis of Carbon Black" by B. Schubert et al, Encyclopedia of Industrial Chemical Analysi_, Volume B (reprint pages 51-523 pH), John Wiley and Sons, Inc.. Neutral or basic pigments according to this method exhibit a pH of 7 and higher.
Binder polymers which can be employed with the toner particles~ if desired, include a variety of 15 materials such as halogenated polyolefins, addition polymers such as acrylic polymers and condensa~ion polymers. Preferably, such polymers are polyester-ionomers such as disclosed in US Patent 4,252,921 to D. Santilli et al.
The dPveloper of ~he present invention includes, as a charge-control agent, a carrier-soluble addition copolymer. "Carrier-soluble" refers to its ability to dissolve in the electrically insu-lating carrier, in the absence of the toner constitu-25 ents, at the concentration level at which it is employed in the developer. The monomer constituents of the polymer include at least the following: a qua-ternary ammonium salt monOmer3 a solubilizing mono-mer, and a monomer having an acidic function selected from the group consisting of -COOH, -S03H and P03H~.
Examples of suitable quaternary ammonium salt monomers include:
methacryloyloxyethyltrimethylammonium ~-tolu-ene sulfonate;
dimethyloctadecylvinylbenzylammonium chlo-ride;

2-vinyl-N-methylpyridinium methyl sulfate;
N-methyl-N-methacryloyloxyethylpiperidinium bromide;
N-methacryloyloxyethylpyridinium nitrate.
Representative solubilizing monomers include the following: alkylstyrenes and alkoxystyrenes hav-ing about 3 to about 10 carbon atoms in the alkyl group; alkyl acrylates and methacryla~es having about 8 to about 22 carbon atoms in the alkyl group; vinyl alkyl ethers having about 8 to about 22 carbon atoms in the alkyl group; and vinyl esters of alkanoic acids having about 6 ~o about 22 carbon atoms in the alkyl group.
Specific solubilizing monomers which can be so used include: ~-butylstyrene, 4-pen~yls~yrenP, 4-hexylstyrene, 4-octylstyrene, p~amyloxystyrene, lauryl acrylate, lauryl methacrylate, hexadecyl meth-acrylate, octadecyl methacrylate, eicosyl acrylate, docosyl methacrylate, vinyl caprate, vinyl l~urate, vinyl palmitate, vinyl stearate, vinyl eicosate and vinyl docosate.
Suitable insolubilizing monomers having an acidic function include addition polymerizable poly-mers such as olefinic and acrylic monomers having the requisite acidic function. Representative acidic monomers include:
Monomers containing -COOH:
acrylic acid methacrylic acid vinyyl benzoic acid styryl acetic acid Monomers containing -S03H:
sulfoe~hyl methacrylamide sulfoethyl methacrylate styrene sulfonic acid Monomers containing -PO3HR:
vinylbenzylethylphosphate vinylbenzylphosphosphonic ~cid vinylphosphonic acid vinylethylphosphonate In addition to the quaternary ammonium, solubilizing and acidic monOmers, the charge control polymer set forth above can include additional other monomers such as to adjust the degree of solubility of the polymer in the carrier, as well as to provide other desirable properties.
Quaternary ammonium salt polymers useful in the liquid developers of this invention typically have an inherent viscoslty in ~he range from about 0.1 to about 0.8. The determination is made at ~
concentraton of 0.25 gram of polymer in 100 ml of chloroform at a temperature of 25 C. The resul-tant polymeræ contain recurring units derived from one or more solubilizing monomers, from one or more quaternary ammonium salt monomers and from one or more insolubilizing acidic monomers. A typical qua-ternary ammonium salt copolymer used in the liquid developers of the invention contains from about 85 to about 96 mole weight percent of solubilizing monomer units, from about 10 to about 2 mole weight percent acidic insolubilizing monomer units, and from about 5 to about l mole weight percen~ of quaternary ammonium salt monomer units.
Th~ solubility of ~he quaternary ammonium salt copolymer in the carrier liquid can be con-trolled by ad~usting the amount of solubilizing and insolubilizing acidic monomers which are used. If the æolubilizing monomer has a very long alkyl group, rendering a polymer containing it very soluble in hydrocarbon carrier liqulds~ the insolubilizing mono-mer may have a relatively short alkyl group. On the other hand, a relatively short alkyl group on ~he s~

solubilizing monomer requires a somewhat longer alkyl group on the insolubilizlng monomer if one is used.
The relative amount of quaternary ammonium salt monomer can be varied to provide polymers having different charge properties when ~ncorporated into a liquid developer.
The developer of the present invention also contains, as a charging agent, a carrier-soluble polar addition copolymer of the type disclosed in US
Patents 4,273,849 by Merrill et al issued June 16, 1981, and 3,849,165 by Stahly et al issued November 19, 1974. As above~ "carrier-soluble" refers to the ability of th~ charging copolymer to dissolve in the liquid carrier, in the absence of the toner consti~u-ents, at the concen~ration level which is employed inthe developer.
The soluble polar copolymer is an addition copolymer of a polar monomer and at least one solu-bilizing monomer. The amount of polar monomer is at least about 1.5 X 10-4 moles/gm of copolymer.
Monomers for addition copolymerizing with the polar monomer to obtain a soluble polar copolymer are the solubilizing monOmers previously described and, if desired, insolubilizing monomers such as sty-rene, vinyl toluene, ethyl acrylate, methyl acrylate,butyl acrylate, ethyl metha~rylate, propyl methacry-late, butyl methacrylate, vinyl acetate, vinyl pro-pionate and vinyl butyra~e~ as described by Stahly et al, US Patent 3,849,165. The relative amounts of the 30 comonomers are selerted ~o produce a polar addition copolymer of the desired solubility, as described above with respect to the quaternary ammonium salt copolymers.
Suitable polar monomers include sulfoalkyl 35 acrylates and methacrylates, metal salts of sulfoal-kyl acrylates and methacrylates, and amlne salts of sulfoalkyl acrylates and methacrylates. Preferably, s~

the sulfoalkyl groups contain from 1 to about 4 car-bon atoms. Also suitable are metal salts and ami~e salts of acrylic and methacrylic acids, and of vinyl-benzoic acid, vinylphenylacetic acid, 9(10)-acryl-amidostearic acid, monoalkylphthalic acid, and sty-renesulfonic acid, the metals of said salts being from Groups I, IIa, IIb and VIII of the periodi~
table, and mixtures thereof.
Preferred polar monomers are acrylic mono-mers. In this sense, the term "acrylic" broadly includes methacrylic, and salts and esters of such acrylic, acids.
Specific examples of such acrylic polar monomers include sulfoethyl methacrylate; sulfoe~hyl acrylate; sulfopropyl methacrylate; sulfobutyl meth-acrylate; ~ulfoethyl methacryla~e, sodium salt; sul-foethyl methacryl~te, partial sodium salt; sulfopro-pyl methacrylate, sodium salt; sulfobutyl methacry-late, potassium salt; sulfoethyl methacrylate, lithium salt; sulfoethyl methacrylate, copper salt;
sulfopropyl methacrylate, dimethylammonium salti sul-foethyl methacrylate diethylammonium salt; sulfo-methyl acrylate, dimethylammonium ~alt; sodium meth-acrylate; sodium acrylate; li~hium methacrylate;
potassium acrylate; barium methacrylate; zinc methac-rylate; cobalt methacrylate; ferrous acrylate; magne-sium methacrylate; zinc acrylate; and mixtures thereof.
The liquid electrographic developers of this invention contain from about 0.01 percent to about 5 percent by weight of soluble polar copolymer, from about 0.01 percent to about 5 percent by weight of quaternary ammonium salt copolymer, and from 0.01 percent to about 5 percent by weight of toner. Pre-ferred developers contain from about 0.02 percent to about 0.5 percent by weight of the soluble acrylic polar copolymer, from about 0.02 percent to about 0.5 _g_ percent by weight quaternary ammonium salt copolymer, and from about 0.02 percent to about 0.5 percent by weight toner.
In the following examples, developers of the present invention are evaluated for replenishability, R or charge distributions. Replenishability R is defined for the purpose of the present invention as the ratio of initial charge¦mass to final charge/mass after a 1/2 liter reservoir of the developer has been employed to form electrographic images and replen-ished. The extent of imaging and replenishment was predetermined so as to use and replenish three times the number of marking particles in the original developer. (For example, if the developer contains 0.2 gram toner, D.6 gram is developed and replen-ished.) The replenisher employed was a 5X concen-trate of the original developer. The charge before and after in coulombs was measured with a Keithly Instruments Capaci~ance Bridge (available from Tettex Instruments 3 Inc. 9 Pennsylvania). The charge mea-sured is divided by the toner weight in kilograms to give charge per mass in coulombs per kilogram tone.
The following ~eveloper comp~ ents were employed in an Isopar ~fSolvesso lO~liquid car-rier system with a pigment concent ~tion of 0.7 gramper liter developer. (Solve~o 10 is an alkylated aromatic liquid and Isopar ~ is an is~paraffinic hydrocarbon liquid.) The Solvesso 100W concentra-tion was approximately 21 grams per liter of devel-oper.

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Parts Devel- by oper Component Type Weight A (1) toner pigment A~idic Raven 1255~ 1 carbon black binder poly~neopentyl-4- 1.2 methylcyclohexene-1,2-dicarboxylate-co-terephthalate-co-5 (N-potassio-p-toluene sulphon-amidosulphonyl)iso-phthalate]
(2) addition poly(_-butyl styrene-polar co-lithium methacry-copolymer late) 97/3 charging agent

(3) quaternary poly(t-butyl styrene- 0.2 ammonium co-methacrylic acid charge- co-methacryloyloxy-control ethyltrimethyl ammo-agent nium-p-toluene sulfo-nate 5~

B (1~ toner pigment Acidic Raven 1255~ 1 carbon black binder poly~neopentyl-4- 1.2 methylcyclohexene-1,2-dicarboxylate-co-terephthalate-co-5-(N-potassio-~-toluene sulphon-amidosulphonyl)iso-phthalate]

(2) addition poly(t-butyl styrene- 1.5 polar co-lauryl methacry-copolymer late-co-lithium meth-charging acrylate co-meth-agent acrylic acid~
60/36/3.6/0.4 (3) quaternary poly(t-butyl æty- 0.3 ammonium rene-co-meth- and 0.2 charge- acrylic acid-co-control methacryloyloxy-agent ethyl trimethyl ammonium ~-toluene sulfonate) C (1) toner pigment Regal 300- carbon black binder poly[neopentyl-4- 1.2 methylcyclohexene-1,2-dicarboxylate-co-~erephthalate-co-5-(N-potassio-~-toluene sulphon-amidosulphonyl)iso-phthalate]

(2) addition poly(t-butyl styrene- 1.5 polar co-lauryl methacry-copolymer late-co-li~hium meth-charging acrylate-co-meth-agent acrylic acid) 60/36/3.6/0.4 (3) quaternary poly(~-butyl sty- 0.3 ammonium rene-co-meth- and 0.2 charge- acrylic acid-co-control methacryloyloxy-agent ethyltrimethyl ammonium-~-toluene sulfonate3 Each of the developers also contain~d 0.5 30 part plasticizer, 0.25 part wax and 0.125 part dis-persing agent for the wax.
Examples 1-4:
In this example 9 replenishability R of con-trol liquid developers containing qua~ernary ammonium charge-control polymers as described by Merrill, US
: Patent 4,273,B49, column 5, was compared with R for liquid developers within the scope of the invention.

The developer of the invention corresponded to Developer A above. The charge-control polymer employed in the control developer Wa6 either (a) an addition terpolymer of a solubilizing monomer, an insolubilizing monomer which contained no acidic groups and a quaternary ammonium monomer or (b3 an addition copolymer of a monomer and a quaternary ammonium polymer. In all other respects, the control developers and developers of the invention were identical~
The results in terms of R values are reported in Table 1.

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These results indicate that, when the insolubilizing monomer is acidic, developer replen-ishability, R, is improved relative to charge-control polymers containing either nonacidic insolubilizing monomer or no insolubilizing monomer Examples 5-7:
These examples illustrate that replenisha-bility, R, is further enhanced by the use of non-acidic coloran~s.
The procedure of Examples 1-4 was repeated employing developer A with either Raven 1255~ carbon black, which was neutralized by washing, or neutral Regal 300~ carbon black. The concentration of addition polar copolymer and quaternary ammonium charge-control polymer was also varied as indicated Replenishability values, R, are reported in Table 2, ~ ,, oo 0 o~ o~
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The R values for Examples 5-7 are to be com-pared with the R value for the developer in Example 2, an otherwise identical developer. The results indicate that basic or neu~ral pigments improve the replenishability in the developers of the present inven~ion compared with acidic pigments.
Example 8 Developers B and C, when subjected to the procedure of Examples 1-7, are expected to show increased replenishability, R9 compared with controls analagous to those above.
The present invention specifies that the charge-control polymer contain monomers with a qua-ternary ammonium group. Other lonium groups such as sulfonium or phosphonium are expected also to result in comparable developers with enhanced replenisha-bility.
"Electrography" and "electrographic" as used herein are broad terms which include image-forming processes involving the development of an electro-static charge pattern formed on a surface with or without light exposure, and thus include electropho-tography and other processes.' Although the inven-tion has been described in considerable detail with 25 particular reference to certain preferred embodlments thereof, variations and modifications can be effected within the spirit and scope of the invention.

Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A liquid electrographic developer compo-sition comprising an electrically insulating liquid carrier containing (a) dispersed toner particles, (b) a carrier-soluble charge control agent comprising an addition copolymer of a quaternary ammonium salt monomer, a solubilizing monomer and an acidic insolu-bilizing monomer having an acidic function selected from the group consisting of -COOH, -SO3H and -PO3HR wherein R is hydrogen or alkyl, and (c) a carrier-soluble charging addition copolymer compris-ing a polar monomer and a solubilizing monomer which may be the same as or different from the solubilizing monomer in said charge-control agent.

2. A developer as in Claim 1 wherein said acidic insolubilizing monomer is selected from the group consisting of acrylic acid, methacrylic acid, vinyl benzoic acid, styrene acetic acid, sulfoethyl methacrylamide, sulfoethyl methacrylate, styrene sulfonic acid, vinylbenzylethylphosphate, vinyl-benzylphosphosphonic acid, vinylphosphonic acid and vinylethylphosphonate.

3. A developer as in Claim 2 wherein said solubilizing monomer in (b) and (c) are independently selected from the group consisting of t-butylstyrene, 4-pentylstyrene, 4-hexylstyrene, 4-octylstyrene, p-amyloxystyrene, lauryl acrylate, lauryl methacrylate, hexadecyl methacrylate, octadecyl methacrylate, eico-syl acrylate, docosyl methacrylate, vinyl caprate, vinyl laurate, vinyl palmitate, vinyl stearate, vinyl eicosate and vinyl docosate, said quaternary ammonium salt monomer is selected from the group consisting of methacryloyloxyethyltrimethylammonium p-toluene sul-fonate, dimethyloctadecylvinylbenzylammonium chlo-ride, 2-vinyl-N-methylpyridinium methyl sulfate, N-methyl-N-methacryloyloxyethylpiperidinium bromide, and N-methacryloyloxyethylpyridinium nitrate, and said polar monomer is selected from the group con-sisting of sulfoethyl methacrylate, sulfoethyl acrylate; sulfopropyl methacrylate; sulfobutyl methacrylate; sulfoethyl methacrylate, sodium salt;
sulfoethyl methacrylate, partial sodium salt;
sulfopropyl methacrylate, sodium salt; sulfobutyl methacrylate, potassium salt; sulfoethyl methacry-late, lithium salt; sulfoethyl methacrylate, copper salt; sulfopropyl methacrylate, dimethylammonium salt; sulfoethyl methacrylate diethylammonium salt;
sulfomethyl acrylate, dimethylammonium salt; sodium methacrylate; sodium acrylate; lithium methacrylate;
potassium acrylate; barium methacrylate; zinc methacrylate; cobalt methacrylate; ferrous acrylate;
magnesium methacrylate; and zinc acrylate.

4. A developer as in Claim 1 wherein the concentrations of said toner of said charge-control agent, and of said charging agent, are each about 0.01 to about 5 percent, by weight of total developer.

5. A developer as in Claim 4 wherein said concentrations are about 0.02 to about 0.5 percent.

6. A developer as in Claim 1 wherein said toner comprises a neutral pH or basic pH colorant.

7. A developer as in Claim 6 wherein said toner further comprises a polyesterionomer as a binder for said colorant.

8. A developer as in Claim 6 wherein said charge-control agent is poly(t-butyl styrene-co-meth-acrylic acid-co-methacryloyloxyethyltrimethylammo-nium-p-toluene sulfonate) and said charging agent is poly(t-butyl styrene-co-lithium methacrylate) or poly(t-butyl styrene-co-lauryl methacrylate-co-lithium methacrylate-co-methacrylic acid).

9. A developer as in Claim 1 comprising, in addition, a second electrically insulating carrier liquid.

10. A developer as in Claim 9 wherein said charging agent is poly(t-butyl styrene-co-lithium methacrylate) and said electrically insulating carri-ers are an isoparaffinic hydrocarbon liquid and an alkylated aromatic liquid.