CA2140524C - Electrically stabilized liquid toners - Google Patents

Abstract

A homogeneous liquid composition, for use in the preparation of liquid toners containing at least one charge director and in which the electrical properties of the charge director(s) is/are stabilized, comprises: (1) insulating non-polar carrier liquid compatible with liquid toners for electrostatic imaging; (2) at least one charge director other than an amine salt; and (3) at least one stabilizing component in an amount effective to stabilize the electrical properties of said at least one charge director, said component being selected from non-quaternary amine salts and being soluble in the carrier, e,g. diethylammonium chloride and isopropylamine dodecylbenzenesulfonate. The invention further relates to liquid toners for electrostatic imaging comprise thermoplastic resin particles dispersed in an insulating non polar carrier liquid, preferably a hydrocarbon having a volume resistivity above 109 ohm-cm and a dielectric constant below 3.0, colorant particles micro-dispersed in said carrier liquid, at least one charge director, and at least one stabilizing component (3), as defined above; a method for producing the liquid toners; and an electrostatic imaging process utilizing the liquid toners.

Description

yyv ELECTRICALLY STAI3ILh2ED L)EQIJID TQI~iGItS
FIELD AND BACKGitOUND OF'fFl~E 1NVlrN"fION
'this invention relates to the field of electrostatic imaging and, more particularly, to the preparation of liquid toners containing components for improving the stabilization of the S electrica) properties due to the charge directors contained therein.
In the art of electrostatic photocopying or photo-printing, a latent electrostatic image is generally produced by first providing a photoconductive imaging surface with a uniform electrostatic charge, e.g. by exposing the imaging surface to a change corona.
The uniform electrostatic charge is then selectively discharged by exposing it to a modulated beam of light 0 corresponding, c.g., to an optical image of an original to be copied or to a computer generated image, thereby forming an electrostat9c charge pattern an the phntoeonduetive imaging surface, i.e. a latent electrostatic image having a background portion at one potential and a "print" portion at another potential. The latent electrostatic image can then be developed by applying to it charred pigmented toner particles, which adhere to the "print" portions of the photoconductive 1 S surface to farm a toner image which is subsequently transferred by various techniques to a copy sheet (e.g. paper).
it will be understood that other methods may be employed to farm an tlectrostatic image, such as, for ex~unple, providing a carrier with a dielec>ric surface and transferring a prefarmed electrostatic charge to the surface. The charge may be formed from an array of styluses. This 20 invention will he described in respect ofo.ffice copiers and the like, though it is to be understood that it is applicable to other uses involving elactrosta#ographics including eleetrostatographie printing.
In liquid-developed electrostatic imaging, the toner particles arc generally dispersed in an insulating non-polar liquid carrier, generally an aliphatic hydrocarbon fraction, which generally 25 has a high-volume rtsistivlty above 10~ ohm cm, a dielectric canstam below 3,4 and a low vapor pressure (less then 10 tort, at 25°C). The liquid developer system further comprises so-called charge directors, i_e. compounds capable of imparting to the toner particles an electrical charge of the desired polarity and uniform magnitude so that the particles may be electrophoretieally deposited on the photaconductive surface to form a toner image.
30 In the course of the process, liquid developer is applied to the photoconductive imaging surface, Under the influence of the electrical potential present in the latent image and a developing electrode which is usually present, the charged toner particles in the liquid developer film migrate to the "print" portions of the latent electrostatic image, thereby forming the developed toner image.
35 Charge director molecules play an important role in the above-described developing process in view of their function of controlling the polarity and magnitude of the charge on the toner particles. The choice of a particular charge director for use in a specific liquid developer system, wiiE depend on a comparatively large number of physical characteristics of tlye charge . director compound, inter alia its solubility in the earner liquid, its ehargeabiiity, its high electric field tolerance, its release properties, its time stability, the particle:
mobility, ctc., as well as on characteristics of the developer. All these characteristics are crucial to achieve high quality imaging, particularly when a large number of impressions ace to be produced.
A wide range of charge director compounds for use in liquid-developed electrostatic imaging are known from the prior art. Examples of charge director compounds are ionic compounds, particularly metal salts of fatty acids, metal salts of sulfo-succinates, metal salts of oxyphosphates, metal salts of alkyl-benzencsulfanic acid, metal salts of aromatle carboxylic acids or sulfonic acids, as well ac zwitterionic and non-ionic compounds, such as polyoxycthylated alkylamines, lecithin, polyvinyl-pyrrolidone, organic acid esters of polyvalent alcohols, ctc.
Notwithstanding the undoubted utility of charge directors, however, the charging caused thereby is generally unstable. In particutar, lecithin, basic barium petronate (8BP) and calcium petronate (CP), which are used as negative charge directors, are unstable under high voltage conditions. Thus, when a solution of charge director (or a dispersion of toner particles in carrier liquid and containing charge director) is subjected to a high electric field, e.g. during the development process, the charge transport characteristics and conductivity sut~'er from transient suppression, and it may take several minutes far these characteristics to recover. This Leads to unstable printing performance when long print runs are undertaken. Further, such solutions or dispersions containing particularly BBP, CP and to a lesser extent lecithin, tend to lose conductivity in the course of time (after dilution with Isopar~ or other carrier liquids), sa that, e.g., solutions or dispersions containing BBP or CP, when diluted with .lsopar~, will change their conductivity by about one order of magnitude in a day and a half. In this connection, it may be noted that in US Patent No. 4,897,332 (Gibson), there is described the use of alkylated polyvinylpyrralidones in liquid toners, for the purpose of promoting their electrical stability under high voltage conditions.
In an attempt to imprnvc the quality of the image Formed, particularly when using liquid toners containing charge directors, it has been suggested to use adjuvants in the toner compositions, such as polyhydmxy compounds, aminoaicohols, polybutylene succinimide, an aromatic hydrocarbon, a metallic soap or a salt o.f a Group Ia, Ila, or Ilia metal.
US Patent No. 4,924,766 (Jrlrnasry et al), while mentioning in passing possible use of oligomers containing amine groups as charge dtreetors, is principally concerned with stcrieally stabilising the thermoplastic resin in liquid tons; this patent dues not address the subject of electrical stabilization.
Two US Patents of Kose1 (3,753,760 and 3,991,226) list many components of liquid toners, including charge directors. The isopropylamine salt of dodecylbenzencsulfonic acid is one of many charge directors disclosed in these two patents. The present inventors have found, however, that this salt is a poor charge director, This finding is not entirely surprising, because the utility of an amine as charge director is believed to depend on its ability to form salts with the palycarboxylic resin particles, from which eatians may be split off leaving a charged residue;
amine salts could thus not readily perfonn this function.
Use of an alkylhydroxybenzylpalyamine as an adjuvant in liquid toners, in addition to S chargt director, is disclosed in US Patent No. 4,977,OS6 (El-Sayed), while in US Patent No.
4,783,388 (El-Sayed et al), there are disclosed liquid toners containing charge directors and an additional component which is a quaternary ammonium hydroxide. There is no su~estion in either patent that the additive stabilizes the electrical stabi lity of the toners.
Sl3MMARY OF THE 1NVENTIQN
14 It is an object of the present invention to provide a method far producing improved liquid toner compositions containing charge directors and components which stabilize the electrical properties ofthe charge directors. Other objects ofthe invention will appear from the description which follows.
'fhe present invention accordingly provides in one aspect, a homogeneous liquid 15 composition, for use in the preparation of liquid toners Gont:~ining at least one charge director and in which the electrical properties of the charge directors) is/are stabilized, which composition wm~prises: (1) insulating non-polar carrier liquid compatible with liquid toners for electrostatic imaging; {2) at least one charge director other than an amine salt; and (3) at least one st<~bilizing component in an amount ei1'cctive to stabilize the electrical properties of Said at least one charge 2(1 director, said component being selected from non-quaternary amine salts and being soluble in the carrier.
In another aspect, the invention provides a liquid toner for electrosratic imaging which comprises thermoplastic resin particles dispersed in an insulating non-polar carrier liquid 'having a volume resistivity above lb9 ohm-cm and a dielectric constant bElow 3.0;
colorant particles ZS micro-dispersed in said carrier liquid; at least one charge director other than an amine sally and at least one stabili2ing cpmponent (3) as just defined, in an amount effective to stabilize the electrical properties of said at Ieast one charge director_ In yet another aspect, the invention provides a method for producing liquid toner compositions containing charge directors other than amine salts and in which the electrical 30 properties of the charge directors are stabilized, which method comprises the steps of (A) first making a homogeneous liquid composition comprising components (1), (2) and (3), as defined above, and (I3) mixing the with homogeneous 1 iquid composition from step (A) in any order colored thermoplastic resin particles, and optionally further non-polar carrier liquid, in such manner that the colorant particles and the thermoplastic resin particles are micro-dispersed in the 35 toner compasition_ fn still another aspect, the present invention provides an eicctrostatic imaging process which comprises the steps of: farming a charged latent electrostatic image on a photacanductive surface; applying to said surface oppositely charged colorant particles from a liquid toner of the invention as defined herein {or as prepared by the method defined in the preceding paragraph};
and transferring the resulting toner image to a substrate.
The carrier liquid, i.e, component {1) as defined above, is preferably an insulating non polar carrier liquid hydrocarbon having a volume resistivity above 109 ohm-cm and a dielectric constant below 3Ø
The amine salts utilized in accordance with the present invention as component (3), as described above, may be for example a salt of an amine with an inorganic acid such as HCl or NHr, or with an organic acid such as a carboxylic acid or a suifonic acid.
Presently preferred organic acids are sulfonic acids, more preferably alkyl-substituted aromatic sulfon.ic acids and particularly where there is a total within the range of 8-18 carbon atoms in Such alkyl substituent(s), The amine component of the amine salt may be far example an aliphatic amine, must preferably a primary, secondary or tertiary aliphatic amine containing from 3 to $ carbon atoms in tho molecule. Non-limiting examples of amine salts which may bc.
utilized in accordance with the present invention arc diethylammonium chloride and isopropylarnine dodccylbenzencsulfonate.
It has been fo~md that the present taper compositions exhibit excellent time stability of charge and reduction of conductivity loss under high voltage conditions, as well as excellent recovery of charge after subjection to high voltage conditions, Also, use of such toner compositions results in images of very good copy quality and long term stability.
BRIEF DESC121'PTION OF THIDRAWINGS
Figs. 1-4 show the effect of the stabilizing component of the invention on the eleelrical stability of carrier liquid compositions containing charge directors; and Fig.
5 shows the effect of an exemplary snrblllzing component of the invention on the conductivity of compositions containing charge director.
Df'"fAILED DESGRIp'fION Of HE INVENTION
The thennoplastic resins, insulating non-polar carrier liquids, colorant particles and charge directors, which ma.y suitably be used in the toner compositions of the invention are known in the art. Illustratively, the insulating non-polar liquid carrier, which should preferably also serve as the solvent for the charge directors, is most suitably an aliphatic hydrocarbon 34 fraction having suitable electrical and other physical properties.
Preferred solvents are the series of branched-chain aliphatic hydrocarbons and mixtures thereof, e.g, the isoparafGnie hydrocarbon fractions having a boiling range above about 155°C, which are commercially available under the name lsopar~ (a trademark of the Exxon Corporation).
As set forth above, the at least one component for stabilizing the electrical properties of the at least one charge director is selected from carrier soluble pan-quaternary amine salts.
The weight ratio of stabilizing component to charge director in the liquid toners preferably falls within the range of 0.041-2.0:1.
1'he invention will be illustrrtted by the following non-limiting Lxample, in which all "parts" are parts by weight.
EXAMPLE
(a) 'I"en parts of Llvax~ I l' 5950 (E.I. du Pont) and five parks of Lsopar~ L
(Exxon) are mixed at law speed in a jacketed double planetary mixer connected to an ail heating unit set at 130°C far one hour. Five parts of Isopar~ I. arc added to the mix in the double planetary mixer and the whole is mixed far a further hour at high speed. Ten parts of Isopar~
L, preheated to 110°C are added, and the mixing is continued without heating until the temperature of the mixture drops to 40°C.
(b) Ninety grams of the product of part {a) is transferred to a Union Process 41 attritor together with 7.S g of Mogul L carbon black (Cabot) and 120 g Isopar~ .L. 'fhe mixture is ground for 24 hours with water cooling ("20°C) using 3116" stainless steel media. The resuhant toner particles have an average (weight) diameter of about 2.1 pm.
(c) Three charge directors are used, namely, basic barium petranate (BBP), lecithin, and 50-50 mixtures of lecil;hin with >3BP. 600 g. Isopar~ L, is used to dissolve 60 g. charge dircctor(s) until a homogeneous solution is obtained, to which was added the stabilizing components) according to the invention to obtain a homogeneous solution. '1"he amount of stabilizing component{s) may be, e.g., 0.25-10 wt.% of tdc Isopar~ solution, but up to 1 wt.°/a is usually adequate. It is noted that BI3P when added to lecithin improves its humidity tolerance {which is poor when used atone); the stabilizing components do not appear to aBcct the humidity stability of any of the charge dircCtors including the mixtures.
(d) The toner concentrate from part (b) is diluted to a non-volatile solids content of 1.5%, using Isopar~ L. Charge director solution including stabilizing component, from part (c}, is added in an amount of e.g. 5-100 mg. charge director solids per g. oftoner solids.
The toners thus produced were tested in a Savin 870 copier and in a high speed electrophotographic printer in which the image is developed on a photoreceptor with a reverse roller developer and then transferred to the final copy sheet via an intermediate transfer member.
Erint quality was at least equal to that of toner without the stabilizers and was stable under high speed printing conditions, consistent with the following experiments based on the stabilized charge director alone.
Electrical stabjlity under high yolts eat3plications These measurements are made for solutions containing charge d irectors alone (as control), or with the addition of stabilizing components, prepared according to part (c) of the l;xample, above, and diluted with the same carrier liquid. A solution of O.l wt.% charge director (arid, when present, stabilizing component in the cancentratinns described below) is placed in an electrical cell having a one mm, separation between plate electrodes. A first pulse of 1500 volts having a duration of 8 seconds is applied to the electrodes and the total charge transported is measured.
This charge represents the "basis" value for comparison. Ailcr a I second delay a second pulse of 1500 volts having a duration of 68 seconds is applied; this pulse is desibmed to cause depletion of the charge; director by high voltage loading. Ali;cr a further 1 second delay a third pulse of 1500 volts having a duration of 8 seconds is applied and the total charge transported is measured. 'this charge represents the diminished charge transport capability ofthe material after being subjected to a high voltage. After a t minute wait an additional pulse of 1500 volts having a duration of 8 seconds is applied and the total charge transported is measured; this charge is a measure of the recovery of the charge director after being subjected to high voltage.
The results of this study are shown in Figs. 1--0, which show clearly that addition of the stabilizing components in accordance with the invention improved both the pulse loading and recovery characteristics of the charge director. In the Figures, the other abbreviations used. have the following meanings:
GB = isoprapylamine dodccylbenzenesulfonate (ICI G3300B); CC-42 =
diethylammonium chloride {WfrCO-EMC4L PPG-40); P10-SJ, S-93 = arnin~ dodecylbenzenesulfonate.
(WffCONATf P10-59, 593); PC-20U, PC-205, fC-210 ~ amine alkylarylsulfonates of increasing molecular weights (VVITCOR); WIT-918 = amine sulfonate (WIT'FLOW
91$).
The charge director compositions in the Figures (conce;ntratians of stabilizing components shown in parentheses) were prepared as follows:
Fig, 1: 0.05 g lecithin (LAN) and 0.05 g BI3P, with U.al g of the seated stabilizing component according to the invention, were dissolved in Isopar~ L to make 100 g total solution.
Fig. 2: 0.1 g DBP, with varying amounts of isopmpylamine dadecylbenzenesulfonate, were dissolved in lsopar~ i. to make 1.00 g total solution.
Fig. 3: 0.05 g lecithin (i,A~ and 0.05 g BBl', with varying amounts of isopropylamine dodecylbenzenesulfonatc, were dissolved in lsopar~ L to make 100 g total solution.
Fig. 4: 0,1 g lecithin (LAN), with varying amounts of isopropylamine dodecylbenzenesulfanatc, were dissolved in lsopar~ L to make 100 g total solution.
2S As is clearly seen from these Figures, the addition of stabilizing ccrrnponents to solutions of charge director material substantially improves the stability of these solutions.
Conductivity kinetics {stability of conductivity with time) The effect of an exemplary stabilizer of the invention {isopropylamine dodecyltx;nzenesuifonate) on conductivity degradation, following dilution to 0.1 wt.% charge director, from a stock solution of 10% BBP or lecithin or a 5%+5%
.13BP..lecithin mixtura, in ,Isopar~ L to a 0,1 % solution in Isopar~ L, is presented in Fig. 5. This H
igurc shows that the conductivity of these solutions was sl;abilized when they conta.incd {hosed on charge director) at least approximately 7.5 wt.%a stabilizer.
While the present invention has been particularly described, persons skilled in the art will 3~ appreciate thatrnany variations and modircations can be made. Therefore, the invention is not w be construed as restricted to the particularly described embodiments, rather the scope, Spirit and concept of the invention will be more readily understood by reference to the claims which follow.

Claims (10)

1. A homogeneous liquid composition, for use in the preparation of liquid toners containing at least one charge director and in which the electrical properties of the charge director(s) is/are stabilized, which composition comprises:
(1) insulating non-polar carrier liquid compatible with liquid toners for electrostatic imaging;
(2) at least one charge director other than an amine salt; and (3) at least one stabilizing component in an amount effective to stabilize the electrical properties of said at least one charge director, said component being selected from non-quaternary amine salts and being soluble in the carrier.

2. A homogeneous composition according to claim 1, wherein component (3) is selected from non-quaternary salts of aliphatic amines with an acid selected from inorganic acids and alkyl-substituted aromatic sulfonic acids.

3. A homogeneous composition according to either claim 1 or claim 2, wherein said insulating non-polar carrier liquid comprises an insulating non-polar liquid hydrocarbon raving a volume resistivity above 10 9 ohm-cm and a dielectric constant below 3Ø

4. A homogeneous composition according to any one of claims 1-3, wherein the weight ratio of stabilizing component to charge director falls within the range of 0.001-2.0:1.

5. A homogeneous composition according to any one of claims 1-4, wherein said at least one charge director is selected from lecithin, basic barium petronate and calcium petronate.

6. A homogeneous composition according to any one of claims 1-5, wherein component (3) is selected from diethylammonium chloride and isopropylamine dodecylbenzenesulfonate.

7. A liquid toner for electrostatic imaging which comprises thermoplastic resin particles dispersed in a homogeneous composition according to any one of claims 1-6, wherein the at least one charge director is operative to charge the particles and the stabilizing compound is present in an amount effective to stabilize the electrical properties of the at least one charge director.

8. A liquid toner according to claim 7, wherein the weight ratio of stabilizing component to charge director falls within the range of 0.001-2.0:1.

9. A method for producing liquid toner compositions containing charge directors and in which the electrical properties of the charge directors are stabilized, which method comprises the steps of: (A) first making a homogeneous liquid composition as defined in any one of claims 1 to 6, and (B) mixing the homogeneous liquid composition from step (A) in any order with colored thermoplastic resin particles, and optionally further non-polar carrier liquid, in such manner that the colored thermoplastic resin particles are micro-dispersed in the toner composition.

10. An electrostatic imaging process which comprises the steps of: forming a charged latent electrostatic image on a photoconductive surface; applying to said surface oppositely charged colorant particles from a liquid toner as defined in claim 7 or claim 8 or as prepared by the method of claim 9; and transferring the resulting toner image to a substrate.