CA2026212C

CA2026212C - Charge director composition

Info

Publication number: CA2026212C
Application number: CA002026212A
Authority: CA
Inventors: Benzion Landa; Yaacov Almog
Original assignee: Indigo BV
Current assignee: HP Indigo BV
Priority date: 1989-02-06
Filing date: 1990-01-16
Publication date: 1997-12-02
Anticipated expiration: 2010-01-16
Also published as: KR0180733B1; CN1041462C; EP0456659A1; CA2026212A1; JPH04503262A; DE69034151T2; DE69034151D1; MX173774B; EP0456659B1; WO1990008983A1; CN1045304A; JP3076921B2; KR920701871A; HK1012439A1

Abstract

A method for stabilizing a charge director solution, and a charge director composition made by this method, whereby a charge director is mixed with a solvent and a polar monomer species and a polymerization reaction is initiated and allowed to progress to completion.

Description

WO 90/089B3 2 ~ 2 PCI'/US~ 15 C~ DIREC~OR COM~08ITION

P~ R~ OF T~E l~ lON
.
The presen~ i~vention r lates to liquid de~eloper electrostatic photocopying an~ mo~ particularly to a method o~ stabilizing charge director ~olut~ons and a new stabilized charg~ director co~position.
: Proc~sses ~or for~ing electrostati~ i~ages, existing as electros~atlc c~arge patterns upon a substxate, are well known. I~ el~ctrostatic printing or ~ copying, a photo~on~ucti~a imaging ~ur~ac~ irst :~ provided with a uni~orm ~l~ctro atic charge, typically by moving the i~aging sur~ace past a charge ~orona at a uni~orm veloci~y.~ The imaging surf ace is then exposed to an optical imag~ o~ an original to~be copied. ~This optical image ~electively ~ hAr~es the imaging sur~ace in a pattern to ~orm a latent electrostatic lmage. In the case of an original bearing dark print on a light backy~ou..~, ~this~la~ent imag~ consists::of substantially ~ l~n~isrh~rg d ~print~ poxtions corresponding to :the :~ graphic matter on ~the original,~ a~idst a ~backgroundn portion that has been s~bstantially ~is~h~rged by : exposur~ to light. The latent i~age is developQd by exposure to oppositély: charged t pigmented, toner SUBS I I~UTIE SHEET

,' : - - - - -, -.. .
i - -- . - " :
, ~, v~J~v~ v~

2~26~ 2-part1cles, which deposit on the print ~ortions of the latent image in a pattern correspondin~ to that of the origi~al.
In liquid d~veloper photocopiers, these charged toner particles are suspended in a liquid developer comprising a carrier liquid, toner particles and charge directors. The entire latent electrostatic image is covered with a thin film of liquid developer from a liquid developer reservoi~. The charged toner particles in the liquid developer migrate to the oppositely charged ~print" portion~ of the latent image to form a pattern on the photoconductive surface. This pattern, and the cor~esponding t~ner particles, are then transferred to a sheet to produce a visible image. Any liquid developer remaining on the photoconductive surface after this process i~ recy~led bac~ into the liquid d~veloper reservoir. : .
Char~e director plays an important role in the developing process described above. The eharge.director is a chemical species, either molecular or ionic, which acts to control the polarity and charge on the toner particles~ The charge director creates charged species causing charging o~ th~ imaging material t~ ~nsure that the toner particles will b~ deposited and ~igrate in such a way as to form the desired i~aga on the imaging surface. Counter ions are al50 created ~o keep the liguid developer sub tantially- electrically neutral overall. The present inv~ntion may be practiced with any number of charge directors, of which lecithin and barium petrona~e are examples.
one of the ~ajor problems conc~rning the material used as charge directors is the de~radation of the charge carrying species under the application ~f the electric ~ield created during the electrophoxetic development process. Degradation of the charge carrying species also occurs during repl~nishmPnt of developer with carrier liquid due to dilution of the charge SUB~TI~UTE SHEE~

W~ ~/0~3 PCT/U~/~15~
202~12 director. De~r~dation of the charge carrying species destabiliz~s the liquid developer electrically. Since stable electrical characteristics o~ the liquid developer are important to achieve a high quality image, S particularly when a large number o~ i~pressio~s are to be produced without changing the liquid developer dispersion, degradation o~ the charge carrying ~pecies results in poor copy quality.
It is believed that in many liquid developers the charge director molecules form inverse micelles. An example of these micelles is shown in Figure 1. The micelles are Pormed by aggregation ~uch that the polar portions of the charge direckor ~olecules point inside, and the nonpolar portions point outside to decrease the overall surface energy of the system. These ~icelles may solubilize ions gener~ted by the disassociati~n of the charge director molecules. ~t is beliaved that the solubilization o~ ions by the charge director micelles is due to the Pormation within and arou~d the micelles, of a microenvironment havi~g a higher dielectric constant. ~he solubiliz~tion o~ ions by ~he charge director micelles results in micelles containing a charged species in their center. So~e o~ the ~icellQs have a positiv~ species in the center and others have a negative species in the center. We believe that during t~e electrophoretic developing process tAese mirelles rupture under the influence o~ the electric field created by the charged photocsnductive sur~ace. The exact ~ ism of the ~Lu~ing is not known. The ~y~ of the ~icelles ch~nges - the electrical properties of the liquid developer solution by ~reeing the charged species in the center o~ the micelles which, due to th~ir relatively strong positive and negative charges and the low dielectric constant o~ the carrisr liquid, tend to reassociate with each other to Porm ~lectrically neutral compounds. The formation of these electrically neutral compounds changes the overall SUBSTITUTE~ SHEE~

~.....

.~ v ;~v~ 70 ~ J/iJ~ I S5 ~2~ 4-electrical propertie~ o~ the liquid davel~per~- Th~
change in electrical propexties of the liquid developer changes the toner partic~e dispersion in tha liquid developer and the number of the charge-carrying species r~sultin~ in a degradation in copy quality.
We also ~elieve that t~e micelles rupture when the liquid developer dispersion in a photocopier is replenished by the addition of new carrier liquid~
Again, the exact mech~ni~m ~is not known. The ~ffect of this rupturing i8 mani~2sted in an instability of th~
charge-carrying spQcies in ths syste~. Again, the overall result is a degradation in copy quality.
Accordingly, one object of the present invention is a charge director ~ o~i~ion which will resis~
degradation und~r the influence of a~ electric field.
Another object o~ the pre~ent inven~lon is a charge dir2ctor ~ Fition wbieih w~ll resist: de~radation during the replP~ t of carrier liguid in a liquid . devQloper di~p~r~ion.
20A further ob~ect of the present lhventlon ls a chargQ director ~olution which will resist destabiliza-tio~
. . ' i j B~F ~ PT~TION OF ~B D~AWIN~B
Figure 1 i8 an ideali~ed depiction o~ charge director micell~s.
Figure 2 i~ a gx~phic representation of th~ current in a lecithin solution ~or 4 successivC elec~ric pulses.
30Figurz 3 is a graphic representation o~ the conductivity kinetics under dilution of lecithin and the materi~l of the present invention.
Figure 4 is a ~raphic representation o~ the s~ability o~ various charge director compositions of the present invention .
!

SllP~S~lTUTE SHE~

Wo ~OB983 PCT/US~/~I~S
2~2~12 Figurs 5 ~hows th~ a~solute change in cond~lctivity during a long developin~ run for a 21% coverage target for lecithin and a charge direr~or o~ the present invention.
s ~UMMARY OF ~ Nv~c..lON

The pre~ent inv~ntion is directed to a method of stabili~ing a charge director solution wherein a charge lo director, a solvent, and a polar monomer ~pecles are ~ixed, and subsequently the monomer molecules are polymerized. An initiator species is used to begin the polymerizat~on and the reac~ion i~ allowed to proceed to substantial completion. We believe that the result is a chemical inco~poration of a polar polymer, ~pe~ies into the core o~ the charge direct~r ~icell~.~. The polar species ~tabilizes the core o~ the mic~lles and reduces the possibility of the micelle rupturing~
In ac~ordance wlth the present invention, charg~
director micelles are Associated w~th insoluble pol~mer molecules so that th~ charged species are ~ore stable and less su~c~ptible to degradation. It will be apprec~ed that by re~u~i ng ~he d~gradatlon c~ the charged specie~ Or the liquid developer ~o~po~ition the images fo~med by the developer will be denser over a longer period o~ usage, since the pr~sence of the charged species is essent~al to the ele¢trophoretic i~aging pxocess.

- ~et~ D0~a~iptio~

In our invention, a charge director, a solvPnt, and a polar monomer ~pecies are ~ixed, and subsequently the monomer molecules are polymerized. An initiator species is used to hegin the polymerization and the reaction is allowed to proceed to substantial completion. While the polymer species which are formed are not soluble, the iàUB~;TlTUTE SHE~

, . , : ., ~

Wo ~/0~983 PCTtUS~/OOlS~
% ~

monomeric species o~ the pre~ent in~ention are solub~
in the solvent containing the charge director~ The charge director, which is at least partially present as micelles, acts as a surfactant for the polymerization of the monomer species. It is believed the monomer species clings to the micelle and polymerizes in the core of the micelle.
T~e selected solvent may be any suitable solvent in which the nP~pcs~ry polymerization may occur. Many nonpolar solvents will work well in the present invention, including: Isopar ~a trade~arked product of the ~xxon Corporation), Isoparafine, hexane, cyclohexane, t-butylbenzene, 2,2,4-~rimethylpentane, and normal para~fins. The monomer species chosen mày be any lS unsaturated monomer that ls soluble in the selected solvent and poly~rize~ in the solvent in the presence of an appropriate initiator. It i8 belleved a large "u~h~r of un~aturated molecules ~ill ~ork well in the present invention as a monomer, but c~rtain species should work espPc;~lly well, including ~l-vinyl-2-pyrrolidone, 2 vinyl pyridine, vinylfuran, and methylmethacrylat~
It is believed that the i~itiator m~y b~ any one o~
a large . number o~ ~pec~s which wi~l initiate a polymerization reaction, including azobis~u~onitrile, benzoyl peroxide, triph~nylazobe~n~ ne hydroperoxide, and t butyl peracetate.- - ~
In one preferred ~o~; ~n~ of t~e present invention, Isopar is heated to approximately 50 degrees C in a reac~ion vessel fitted with a re~lux condenser The reaction is run under a nitrogen atmosphere.
Lecithin is slowly mixed into the Isopar. The solution is heated to about 80 90 degrees C and 1-vinyl-2-pyrrolidon~ is added, ~ollowed by a polymerization initiator, e.~., azobisbutyronitrile. ~he temperature is kept constant, and the reaction is allowed to proceed for about 24 hours. The charge director composition SllBSTl ~ UTE SHE~ET

.. ,, .. . ~

W~ ~/~ ~3 ~ ~ 2 ~ PCT/U$90/OOlg~

formed by thi~ process will ~e le~s subject to dQgradation of the charge-carrying species than a composition lac~ing the stabilizing polymer molecules.
This superior resistance to degradation will be exhibited both when an electric current is applied to the composition, and when the composition is diluted with solvent (Isopar).
It is pr~erred to use a nonpolar solvent in which the-l-vinyl-2-pyrrolidone monomQr is soluble, but the polymer is insoluble. The ~olvent should boil ~t a significantly hi~her temperatura than 90 C, 50 ~hat it will remain liquid under the reaction conditions. It is believed that, as the polymeri~ation reaction progresses, the polymer molecule~ will reach a crit$cal len~th above which they are ~nsolubl~ i~ the sol~en~; a very fine dispersion of these poly~er molecules in the solvent results, and the charge director micelles ~orm around the polymer molecules. Th~ micelles $n turn are rigidized and ~tabilized by the polymer molecules. .The critical percent o~ vinyl pyrrolidone polymer nee~ed to obtain a large stabilizati~ e~ect is b~tween about 5-9~ on a weight-to-weight basie wikh respect to the chaxgeidirertor solids. ~ith ~ polymer concentration o~
9% or more, very littl~ d~gradation of -the charged species occurs upon dilution wit~ solv~n~ or the i~position of an electric field. Below-a 5% polymer concentratio~, however, a ~ignificant amount of degradatisn will occur. The present invention is furth~x illustrated by, but not limited to, the following examples.

EXAMPLE I

Under a nitrogen atmosphere, 1400 grams of Isopar-H
was heated to 50 deg. in a 4-necked, 2 liter, mechanically stirred glass reactor fitted with a reflux condenser. 600 grams o~ lecithin was dissolved in the SUB~i ~ UTE S5 I~ET

:

W(J ~U~Y~3 PCr/U59t~ 5~i 202621~
Isopar-H by slow addition and ~itirrirlq. The Isopar H/Lecithin solution was then heated to 806 C and then 102 grams of l-vinyl-2-pyrrolidone was added to the solution. Three grams of azobisbutyronitrlle suspended 5 in 10-20 ml, of Isopar-H was ~hen added, and the reaction allowed to proceed for 2~ hours ~o completion.
EX~NPLE ~

500 ~ram~ Isopar~H, ~0 grams o~ lecithin, and 1.7 grams l-vinyl-2-pyrrolido~e were mixed at 90 dey. C in a .-4-necked glass roundbottom fl~sk under an N2 atmosphere.
0.5 gra~s azobisbutyronitrile was dispersed in 20 grams of Isopar and added~ The reaction was allowed to proceed for 17-1/2 hours~ ~h2 resulting solution was clear, and somewhat darker t~an a solutlon o~ lecithin -in Isopar.
The ~dvantages of the pxesent in~ent~on are illustrated by the ~ollowing exper~mental results.
20.. Table ~ and ~igure 2 s~ow thQ result~ of our experiment on the effect o~ an appliad electric field to a common unstabilized. charg director, lecithin, ~olution. .In the experiment 800 V. DC pulses were sequentially applied to a cell con~ a ~ecithin solution for 4 seCon~C and:the charge transport of the lecithin solution for ea~h pulse was measured. Ta~le 1 shows the charge ~ransport in the solution ~or each pulse. Figure 2 is a graphic representati3n o~ the current in th~ ~antern solution during the ti~e period of the pulse. As shown in Table 1 and Figure 2 the application o~ an electric ~ulse to a cha~ge director solution changes the electrical properties of the solution. The applied electric pulss of the experimen~
is similar to the electric field reated during the copying process. Thus, the ef~ect of the electric pulse ;

SUE;S~lTUTa SHEEr WO ~/08983 PCT/~S~/~155 9 2~2l~

on the lecithin solution resembl~ ~he ~ect of the electric ~ield creat~d during the copying pro~-ess of the liquid developer solution.
Figure 3 show the conductivity o~ a composition comprising 17% monomer stabilized species by weight with respect to charge director solids, according to the prese.nt invention as co ~red to a lecithin control, in both case~ after addition of a carrier liquid such as Isopar H. As shown in Figure 3, the conductivity of the ~tabilized composition in Isopar remains relatively constant wi~h tlme, whil~ that o~ the control decreases with tim~. Thus, the stabilized composition o~ the present invention is advan~ageous ~or ~se in a photocopier 6ince the conductivity will not change appreciably with time~
Figure 4 shows the results o~ a similar experiment on various stabilized charge director cr ~o~itions accordi~g to th~ present invention. In this experiment 4, 800 V. DC pulses were sequentially applied to a cell cont~;~;n~ a charge director solution and the total charge transport in th~ cell was measured for each pulse. The control charga director solution was an unstabilized lecithin solution a~ used in the above ~ ention~d expex~ment. Fiv~ stabilized ~harge director solutions made ~ccording to the present invention were also tested. ~ach charge director s~lution was ~ade with a di~ferent p~rcentage of the mono~er stabilizing ~pecies. As shown in Figure 4, ~h~ charge director should co~se between 5% and 9% by weight with respect to charge director solids or ~ore ~ the monomer stabilizing species to achieve a high degree of charge transport constancy. As also show~ in Figure 4, little degradation in charge transport is maintained by a charge director composition comprising 17% monome~
stabilizi~g species by weight with respect to oharge director solids.

SUBSTITUTE S5~EET

. ~ ; .
.

.

., .

WO ~/08~B3 PCT/US~/~1S5 2 -lo~
Figur~ 5 shows the results of a~ experiment on th~
decreas~ in conductivity o~ a charge director ~olution during continuous electrophotocopier operation with no paper feed. Ths lecthin charga director solution shown on ~he chart is an un~tabilized ordinary charge director solution. The other charge dlrector i~ mada according to example 1 o~ the present invention comprising 17%
monomer stabilizing species by weight with. re~pect to charge director solids. As ~i~cllcse~ in a preceding section, we believe that during the electrophotographic process unstabilizQd charge director micelles rupture, causing the decrease in the numb~r of charge species, and ~hus a decrease in bulk conductivity o~ the liquid developer and a degradation in copy quali~y. As shown in Figure 5, ~he unstabilized lecithin ~olution had a decr~as~ of an 18picomho/cm in conauctivity-~uring the ele~y okhotocopier op~ration~ The solution comprising 17% monomer ~tabilizing ~pecies by we~ght with respect to charge direc~or solids, ~ad~ ac~o~ding to example 1 o~ the present invention, howe~er, showed only a 4pico~ho~cm decrease in conducti~ity during continuous electropho~ocopier op~ration.
It should be understood that th~ foregoing descriptions are for the purpose o~ illustration only and that the inYention includes all ~odi~ications ~alling within ~he scope of the ~ollowing:claims.:

SU~TUTE SHE~T

Claims

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

1. An improved charge director composition formed by a process comprising the steps of:
dissolving a charge director in a non-polar solvent and a monomer species;
initiating a polymerization reaction among molecules of said monomer species; and allowing said polymerization reaction to progress to completion to thereby associate the polymer thus formed with said charge director thereby producing said improved charge director composition.

2. A charge director composition as in claim 1 wherein the non-polar organic solvent is an isoparaffinic material having a specific gravity of 0.759@60/60° F., a Saybolt Color + 30, a viscosity of 1.72 cST@25°C. and an auto-ignition temperature of 349° C.

3. A charge director composition as in claim 1 wherein the charge director is lecithin.

4. A charge director composition as in claim 1 wherein the polymer is polyvinylpyrrolidone.

5. A charge director as in claim 1 wherein the polar monomer species comprises at least 5% by weight with respect to charge director solids.

6. A charge director as in claim 1 wherein the polar monomer species comprises 5% to 9% by weight with respect to charge director solids.

7. A charge director as in claim 1 wherein the polar monomer species comprises between 10% and 17% by weight with respect to charge director solids.

8. A charge director composition as in claim 1 wherein said solvent is a nonpolar organic compound or mixture of compounds.

9. A liquid developer comprising:
a carrier liquid, toner particles, and a stabilized charge director including:
a polymer insoluble in said carrier liquid and a charge director, soluble in said carrier liquid and at least partially present in the form of micelles, wherein said polymer is chemically incorporated into said micelles.

10. The liquid developer of claim 9, wherein the charge director is lecithin.

11. The liquid developer of claim 9, wherein the polymer is polyvinylpyrrolidone.

12. A liquid developer comprising:
a carrier liquid, toner particles, and a stabilized charge director composition formed by mixing a charge director, at least partially present in the form of micelles, with a solvent and a monomer species and then initiating a polymerization reaction among molecules of said monomer species and allowing said polymerization reaction to progress to completion, to produce a polymer which is chemically incorporated into said micelles.

13. The liquid developer of claim 12 wherein the stabilized charge director is at least partially present in the form of micelles and said charge director acts as a surfactant for the polymerization of the monomer species.

14. The liquid developer of claim 12, wherein the solvent is the carrier liquid.

15. The liquid developer of claim 12, wherein the charge director is lecithin.