Reversible developer for electrostatic latent imaging method
United States Patent Machida et al.  
 [451 *Apr. 1,1975  
 REVERSIBLE DEVELOPER FOR ELECTROSTATIC LATENT IMAGING METHOD Inventors: l-Iazime Machida; Akira Tsurugi;  
 Zenjiro Okuno, all of Tokyo, Japan Kabushiki Kaisha Ricoh, Tokyo, Japan Assignee:  
 Notice: The portion of the term of this patent subsequent to Jan. 12, 1988, has been disclaimed.  
 Filed: Oct. 22, 1971 Appl. No.: 191,894  
 Related U.S. Application Data Continuation-in-part of Ser. No. 176,727, Aug. 31, 1971, abandoned, which is a continuation of Ser. No. 838,640, July 2, 1969, abandoned.  
 Foreign Application Priority Data July 11, 1968 Japan 43-48108 U.S. Cl. 117/37 LE, 96/1 LY, 252/621 Int. Cl B05c l/16, 844d 1/02, G03g 9/04 Field of Search 252/621; 117/37 LE;  
 Primary Examiner-Norman G. Torchin Assistant E.\&#39;aminer-Jack P. Brammer Attorney, Agent, or FirmWoodhams, Blanchard and Flynn [57] ABSTRACT A liquid developer for electrostatic latent images, in which the polarity (either positive or negative) of the toner particles dispersed in the carrier liquid can be reversibly changed by altering the temperature of the liquid developer at the time of application of the developer to the electrostatic latent image.  
 7 Claims, No Drawings REVERSIBLE DEVELOPER FOR ELECTROSTATIC LATENT IMAGING METHOD CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of our copending application Ser. No. 176,727, filed Aug. 31, 1971, now abandoned, which in turn is a continuation of our application Ser. No. 838,640, filed July 2, 1969, now abandoned.  
 BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a liquid developer for developing electrostatic latent images in which the polarity of the pigment particles can be selectively reversed by changing the temperaature of the liquid developer.  
 2. Description of the Prior Art A liquid developer for electrostatic latent images comprises, in general, pigment particles dispersed in a carrier liquid of high electric resistance and low dielectric constant. As the pigment particles, various organic pigments as well as carbon black have been widely used. Almost all these pigment particles, with very few exceptions, when dispersed by themselves in a carrier liquid, are electrically charged with positive polarity. Generally speaking, however, the amount of their electrical charge is not necessarily sufficient for electrophotographic development purposes and in some cases it is necessary to charge the dispersed particles with negative polarity. It has so far been more practical to adhere an appropriate synthetic resin (usually called a polarity controlling agent) onto the pigment particles and thereby control the polarity of dispersed particles by suitable selection of the synthetic resin used.  
  The present invention provides a novel liquid developer for electrostatic latent images, wherein the technique of controlling the polarity of the dispersed particles by means of synthetic resins has been further advanced and the polarity of the dispersed pigment particles can be changed reversibly merely by altering the temperature of the liquid developer.  
 SUMMARY OF THE INVENTION A reversible liquid developer for electrostatic latent images according to the present invention is prepared by applying a coating layer consisting of a copolymer of a specific class, as set forth later, onto pigment partcles and then dispersing the coated pigment particles in a carrier liquid having a relative resistance of more than (Lem and a dielectric constant less than 3. The copolymer not only controls the polarity of the pigment particles, but also the amount thereof that adheres onto the pigment particles varies depending on the temperature of the carrier liquid. To be more precise, the amount of said copolymer that adheres to the pigment particles decreases with an increase of the temperature of the carrier liquid, and when the temperature of the carrier liquid rises to a level higher than normal room temperature (20C) by an amount in the range of 20-40C, the amount of the copolymer adhered onto the pigment particles decreases to such an extent that the polarity-controlability effect of said copolymer is substantially hampered. However, the copolymer which has come off the pigment particles can be made to adhere again to the pigment particles by cooling the carrier liquid to return the temperature thereof to normal room temperature whereby the polarity-controlability effect is again displayed. Therefore, with the liquid developer according to the present invention, it is possible to reversibly change the polarity of the developer pigment particles by merely changing the temper- 5 ature of the liquid.  
  The copolymers applicable in the present invention may be classified into the following two groups by the types of monomers of which they are comprised. That is, the group I copolymers consist of copolymers comprising two monomers and capable of adhering onto pigment particles to thereby control the polarity of said particles so as to charge them with positive polarity at normal room temperature, while the group II copolymers consist of copolymers comprising two monomers and capable of adhering onto pigment particles to thereby control the polarity of said particles to charge them with negativepolarity at normal room temperature.  
 Group I R R1 01-1125 CHI=JII 00R: X monomer (a) monomer (b) Group II R R GH il CH=+ OORz Y monomer (a) monomer (c) wherein:  
  R1 CH3 01&#39; H on. N02 Q r OH ooocmcn om or -COOR@ wherein: R (C,,,H )OI-I or Accordingly, a reversible developer capable of reversibly changing the polarity of pigment particles by heating or cooling the carrier liquid can be prepared either (1) by dispersing pigment particles on which is adsorbed a copolymer of the group I, when the pigment particles employed are capable of being charged with negative electricity when they are dispersed independently (without polarity control agent), in a carrier liquid having a relative resistance of more than 0.0m and a dielectric constant less than 3, or (2) by dispersing pigment particles on which is adsorbed a copolymer of the group II, when the pigment particles employed are capable of being charged with positive electricity when they are dispersed independently (without polarity control agent), in the same carrier liquid as above. In the case of reversible developer (1) the coated pigment particles have a positive charge at room temperature, because of thepolarity controlling effect of the group I copolymer, but they are charged negative at a temperature 2040C higher than room temperature, because the group I copolymer will have been removed. Conversely, in the case of developer (2), the coated pigment particles will be charged negative at room temperature and will be charged positive at the higher temperature. Moreover, when pigment particles are first made to adsorb a copolymer of group I (or group II) and next made to adsorb a copolymer of the group II (or the group I) and are then dispersed in a carrier liquid, there will be obtained a reversible developer which is not affected by the normal polarity of the pigment particles when they are free of polarity control agent. That is, in the latter case, the two different copolymer layers themselves provide the reversible polarity effect and the pigment particles do not affect same. Whereas, in the former case, the single coating layer provides one polarity at room temperature and the pigment particles provide the opposite polarity at the higher temperature.  
  The respective copolymers of group I and group II can be prepared by conventional polymerization of the specified monomers under such conditions as a reaction temperature of 60l 10C and a polymerization period of 3l5 hours in the presence of a polymerization initiator such as azobisisobutyronitrile, benzoyl peroxide, etc. and a solvent such as toluene. Both the group I and group II copolymers contain acrylic ester or methacrylic ester as one of the copolymeric constituents thereof (monomer (a)). It can generally be said that regarding the copolymer of group I, when applied as a single coating layer or as an outer coating layer in a two layer coating, the mole ratio of monomer (b) to monomer (a) is in the range of 0.1-0.7:1, whilst, as regards the copolymer of group II, when applied as a single coating layer or as an outer coating layer in a two layer coating, the mole ratio of monomer (c) to monomer (a) is in the range of from 0.01 to 0.5:1. When group I copolymer is applied as the inner coating layer in a two layer coating, the mole ratio of monomer (b) to monomer (a) is in the range of 1-421. When the group II copolymer is applied as the inner coating layer in a two layer coating, the mole ratio of monomer (c) to monomer (a) is in the range of 0.7-4/ 1. The mole ratio of said acrylic ester or methacrylic ester to the copolymer and the length of the alkyl group of said ester have a comparatively great influence on the hardness of the adhesion of said copolymer to the pigment. In other words, in the case of copolymers (a)/(b) or (a)/(c), the larger the percentage of acrylic ester or methacrylic ester corresponding to (a) contained in the copolymer, the less is the adhesion of said copolymer to the pigment in response to an increase of the developer temperature. Further, in copolymers containing the same amounts of the acrylic ester or methacrylic ester (monomer (a)), but in which the carbon atom number of the alkyl group of the monomer (a) is different, the longer the carbon atom number of the alkyl group of said ester, the less is the adhesion of the copolymer to the pigment in response to an increase of the developer temperature. Accordingly, when it is intended to make two layers of copolymers adhere to the pigment, it is desirable to select the copolymers so that the one which is most adhesive to pigment in response to the increase of the developer temperature adhere first, as an inner layer, onto the pigment particles taking into consideration the effect of the carbon atom number of the alkyl group of monomer (a) as set forth above.  
  The following table summarizes the foregoing description concerning the types of pigments and coating agents used to provide liquid developers according to the invention and the charge properties thereof.  
 Table A Char e of Deve oper Materials at 20-40 above room temperature room temperature Copolymer Coating Agent Charge of pi ent WI I I I QUI polant contro agent Type of Coating Single Layer I mol ratio of b/a=0.IlI0.7/ 1  
  mol ratio of c/a=0.0l-0.5/l Double Layer 3. or Inner liiyer mol ratio of b/a=l-4/l Outer layer II mol ratio of c/a=0.0l-0.5/l Inner llayer mol ratio of c/a=0.7 4.0/ 1 Outer llayer dition to carbon black (C.I. No. 77266) are applicable. They include Fast Yellow G (C.l. No. 13135), Benzidine Yellow TR (C.I. No. 21090), Cyanine Blue BM (C.1. No. 74160), Fast Red 2B (C.l. No. 37115), Pigment Scarlet TR (C.1. No. 16105), Brilliant Carmine FB (C.l. No. 16105) and Brilliant Carmine 6B (C.I. No. 24830), (These are the products manufactured by Sanyo Colour Works Ltd.); Aizen Spilon Yellow (C.l. No. 41000), Aizen Spilon Red (C.I. No. 22310), Aizen Spilon Blue (C.l. No. 22610), and Aizen Victoria Blue (C.l. No. 44045), (These are the products manufactured by Hodogaya Chemical Co., Ltd.); and Spirit Black (C.l. No. 50415), Vali Fast Yellow (C.l. No. 11680), Vali Fast Red (C.l. No. 12120), Vali Fast Brown (C.I. No. 20195), Vali Fast Blue (C.1. No. 51175), Vali Fast Black (C.l. No. 51215), Oil Blue B (C1. No. 77450), Oil Green BG (C.1.No. 77601), Oil Black (C.1. No. 77011), (These are the products manufactured by Orient Chemical Industries Ltd.). These substances have hitherto been utilized as pigment particles for use in preparing conventional liquid developers for electrostatic latent images. As the carrier liquid in the present invention, there is employed a liquid having a relative resistance of more than l0 Q.cm and a dielectric constant of less than 3. Typical carrier liquids include organic hydrocarbon solvents such as Naphtha No. 6 (a product manufactured by ESSO Standard Oil Co., Ltd.), SOLVESSO (a product manufactured by ESSO Standard Oil Co., Ltd.), Isopar H (a product manufactured by ESSO Standard Oil Co., Ltd.), etc.  
 EXAMPLE 1 A mixture comprising g of a pigment capable of being charged with negative polarity when dispersed independently in a carrier liquid, to wit, Cyanine Blue GM (a product manufactured by Sanyo Dye K.K.), 5g  
 EXAMPLE 2 A mixture comprising 5g of a pigment capable of being charged with positive electricity when dispersed independently in a carrier liquid, to wit, Cyanine Blue SM (a product manufactured by Sanyo Dye K.l(.), 5g of diethylaminoethylmethacrylate-butylmethacrylate copolymer (mol ratio 1:9) and 50g of a blended liquid comprising Naphtha No. 6 and SOLVESSO (mixing ratio 1:1) was kneaded with a ball-mill for 20 hours. Then, 1 part by weight of a resultant kneaded mixture was disperesed in 100 parts by weight of Naphtha No. 6, whereby there was prepared a liquid developer. The pigment particles dispersed in thus obtained liquid developer were charged with negative polarity at normal room temperature, but the polarity of said dispersed particles was reversed to be positive when the liquid temperature was maintained at 60C for 2 hours, whereas the polarity reverts to be negative again when the liquid is cooled down.  
 EXAMPLE 3 The seven liquid developers numbered 3 to 9 in the following Table 1 were prepared in a manner identical to the preparation of the developer of Example 2 except that there were used the pigment particles, coating layers and carrier liquid as indicated in the Table I. The polarity of the toner particles dispersed in the carrier liquid, depending on a change of the temperature thereof, was investigated and the results thereof were as shown in the following Table I.  
 Table l Copolymer Coating No. T M of laurylmethacrylatehydroxyethylmethacrylate coype onomer (a) Monomer (b) or (c) gag polymer (mol ratio 6:4) and 40g of Naphtha No. 6 was (c)/(a) kneaded with a ball-mill for 20 hours. Then, 1 part by 4O 3 l Lauryl- Hydroxyethyl- 3:7 weight of a resultant kneaded mixture was dlspersed 1n methacrylate methacrylate (=0.43;1) 100 parts by weight of Naphtha No. 6, whereby there 4 l zg i Pmilmslyrene 317 was prepared a liquid developer. The thus obtained liq- 5 me ate p dimethy|amino i :1 uid developer contained pigment particles which were 6 megiacry late 4 stylrened (=0 eary- -viny ri ine 2:8 charged with positive polarity at normal room tempera methacrylate W @0152) ture, but the polarity of sa1d dispersed particles 1s re- 7 1 Tridecyl- Glycidyl- 3:7 versed to be negative when the liquid temperature is methacrylate melbacrylate (=0.43:1)  
  o 8 ll Tetradecylp-ammostyrene l :9 maintained at C for 3 hours, whereas the same partlmethacrylate (=0.l 1:1 cles are charged with positive polarity again when the 9 n p-dlmelhylamino 213 liquid is cooled down to normal room temperature. acrya e styrene @0257) No. Pigment Carrier Polarity of Pigment Liquid Particles 20C C 20C Initial Rccoolcd 3 Statex B-l2 lsopar H (CI. 77266) 4 Mogal A lsopar G (Cl. 77266) 5 Vulcan XC72R lsopar H (CI. 77266) 6 Acetyrcne Black lsopar M (CI. 77266) 7 Pearls Mark 11 Naphtha No. 6  
  (CI. 77266) 8 Neo Spectra lsopar H Mark 11 (C .1. 77266) 9 Conductcx SC Naphtha No, 5  
 EXAMPLE 4 A mixture comprising g of a pigment, to wit, XC- 550 (a product manufactured by Asachi Carbon K.K.), 5g of styrenebutylmethacrylate copolymer (mol ratio 1:1) and 40g of toluene was kneaded with a ball-mill for 15 hours. Subsequently, l part by weight of thus kneaded mixture, 1 part by weight of laurylmethacrylate-dimethylaminoethylmethacrylate copolymer (mol ratio 9:1 and 50g of Naphtha-SOLVESSO isoquantitative mixture was mixed and kneaded with a ball-mill for hours. Then, 1 part by weight of thus kneaded mixture was dispersed in a blended liquid comprising Isopar H and Naphtha No. 6 (mixing ratio 2:8), whereby there was prepared a liquid developer. The pigment particles dispersed in the thus obtained liquid developer are charged with negative polarity at normal room temperature, but the polarity of said dispersed particles is reversed to be positive when the liquid temperature is maintained at 60C for 3 hours.  
 EXAMPLE 5 The six liquid developers numbered 1 l to 16 in the following Table II were prepared in the same manner as the developer of Example 4 except that in place of the pigment, coating layer and carrier liquid employed in Example 4, there were used those indicated in the Table II. The polarity of the toner particles dispersed in the carrier liquid was changed by elevating and reducing the temperature thereof as shown in the Table ll.  
 Table ll Inner Copolymer Layer No. Mol  
 Type Monomer (a) Monomer Ratio of or )/(a) l l l Lauryl- Styrene 8:2  
 methacrylate (=4zl Hydroxymethylmethacrylate (mol ratio=lzl 12 ll do. p-chlorostyrene 8:2  
  =411 p-dimethylaminostyrene (mol ratiofi) 13 l Z-ethylhexyl- Styrene 7:3  
  methacrylate (=2.3:l l4 l Laurylp-nitrostyrene 5:5  
 methacrylate (=1 :l  
  Methylmethacrylate (mol ratio-4:l) 15 l Octyl- Styrene 5 :5  
 methacrylate (=1 :l  
  Glycidylmethacrylate (mol ratio=3:2) 16 ll Butyl- (p-dimethylamino- 5:5  
 methacrylate styrene (=1 :l  
 Outer Copolymer Layer No. Type Monomer (a) Monomer Mol Ratio of or )/(a) or 1 I ll Lauryl- Dimethylaminol :9  
  methacrylate ethylmethacrylate (=O.l l :l 12 l Octyl- Styrene 4:6  
  methacrylate (=0.67:l Hydroxyethylmethacrylate (mol ratio=2:2) 13 ll Lauryl- 3-vinylpyridine 1:9  
  methacrylate (=0. l M 14 ll Tridecylp-aminostyrene [.5 28.5  
  methacrylate (=0. l 8: l 15 ll Ethylp-dimethyl- 2:8  
  methacrylate actetamide styrene (=0.25 l) 16 l Stearyl- Glycidyl- 3:  
 methacrylate methacrylate (=0.43 1) Carrier Polarity of Pigment No. Pigment Liquid Particles 20C 60C 20C Initial Rccoolcd ll Vulcan XC72R lsopar G (Cl. 77266) l2 Statcx Dcncss Naphthu No. 6  
  (C .l. 77266) 13 Pcgcnt Super lsopar H (C .l. 77266) l4 Printcx 60 lsopur G (C .l. 77266) 15 Spirit Black lsopur L ((1.50415) 16 Cyanine Blue Naphth&#39;tl No. 6  
 BM (Cl. 7-1l60) In this context, it is noted that the determination of the polarity with respect to the liquid developers in the foregoing Examples was conducted on the basis of images developed from the sensitive paper RICOH BS-l (a sensitive paper for electrostatic latent image manufactured by RICOH K.K.) by means of RlCOl-I FAX M4 (a photographic apparatus manufactured by RICOl-l K.K.).  
 EXAMPLE 6 A series of liquid developers were prepared in order to compare the results obtained by the use of a developer according to the invention with a variety of control developers.  
  The first purpose was to investigate whether or not there occurs a polarity change of the dispersed pigment particles in response to a change of the temperature of the liquid developer, with regard to three kinds of developers, namely, developers in which (1) an acrylate ester homopolymer (control) (2) a polystyrene homopolymer (control), and (3) a copolymer according to the present invention are employed as polarity regulating agents, respectively, other conditions being the same.  
  The second purpose was to study the effect of changes in the mo] ratio of the monomers constituting the copolymer employed as a polarity regulating agent.  
 Developers were prepared as follows:  
 DEVELOPER A 5 gr. of Cyanine Blue GM (manufactured by Sanyo Dye K.K.), i.e., a pigment which is charged with negative electricity when it is dispersed by itself in a carrier liquid, gr. of butylmethacrylate/styrene copolymer (mol ratio 2:1), and 40 gr. of Naphtha No. 6 were kneaded together in a ball-mill for 15 hours.  
  One part by weight of the kneaded mixture thus obtained was dispersed in 100 parts by weight of Isopar H, in order to prepare a liquid developer A. This is a liquid developer according to the present invention.  
 DEVELOPER B Developer B was prepared by the same procedure as Developer A, except that in place of the copolymer employed in Developer A, there was used a methacrylate homopolymer.  
 DEVELOPER C Developer C was prepared by the same procedure as Developer A, except that in place of the copolymer and Naphtha No. 6 employed in Developer A, there were used polystyrene and toluene, respectively.  
 DEVELOPER D Developer D was prepared by the same procedure as Developer A, except that in place of the copolymer employed in Developer A, there was used butylmethacrylate/styrene copolymer (mol ratio 9:1  
 DEVELOPER E Developer E was prepared by the same procedure as Developer A, except that in place of the copolymer employed in Developer A, there was used butylmethacrylate/styrene copolymer (mol ratio 1:1).  
 The testing procedure was as follows:  
  In a beaker of 250 ml capacity there was placed an anode plate and a cathode plate, both made of aluminum (20mm X 30mm) and spaced apart a distance of 35mm. A copying paper for use in electrophotography (a copying paper for use in Electron Recopy BS-I, manufactured by Kabushiki Kaisha Ricoh) having dimensions of 20mm X 30mm was fitted to the cathode plate and, thereafter, 150 ml of Developer A at a temperature of 25C. was poured into the beaker. Then a direct current voltage of 200 V was applied between the anode and cathode plates for 15 seconds. Thereafter, the copying paper was peeled off the cathode plate. The concentration of toner existing on the surface of the copying paper was determined using a densitometer of Narumi NSG-III Type (manufactured by Narumi &amp; Co.).  
  Next, a copying paper was fitted to the anode plate alone and electroplating was conducted in the same manner as described above. Thereafter the concentration of toner existing on the surface of said copying paper was determined in the same way.  
  Next, the above-described tests were repeated, but with the Developer A heated to a temperature of 60C., for a period of 3 hours.  
  Then, the same testing procedures were repeated with Developers B, C, D and B, respectively.  
 III. Results The results of the tests were as follows:  
  The toner particles in a developer liquid are determined to be charged with positive electricity when the measured concentration of the toner on a copying paper fitted to the anode plate is high and the measured concentration of the toner on a copying paper fitted to the cathode plate is low, and that the toner particles are charged with negative electricity in the reverse case.  
 Accordingly, from the results tabulated in Table Ill,  
 the following results can be deduced.  
 Table IV Developer Polarity of Toner Particles As is apparent from Table IV, only Developer A, namely a liquid developer according to the presentinvention, displays a change of the polarity of the dispersed toner particles, in response to a change of temperature.  
  The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:  
  1. In a method for developing electrostatic latent images, in which a copy material bearing the latent image is immersed in a liquid developer, the improvement which comprises employing a liquid developer comprising pigment particles having a charge-modifying copolymer coating and dispersed in a hydrocarbon carrier liquid having a relative resistance of more than 10 0 cm. and a dielectric constant of less than 3, said pigment being selected from the group consisting of carbon black (C.I. No. 77266), Fast Yellow G (C.I. No. 13135), Benzidine Yellow TR (C.I. No. 21090), Cyanine Blue BM (C.I. No. 74160), Fast Red 2B (C.I. N0. 371 15), Pigment Scarlet TR (C.I. No. 16105), Brilliant Carmine FB (C.I. No. 16105), Brilliant Carmine 6B (C.I. No. 24830), Aizen Spilon Yellow (C.I. No. 41000), Aizen Spilon Red (C.I. No. 22310), Aizen Spilon Blue (C.I. NO. 22610), Aizen Victoria Blue (C.I. No. 44045), Spirit Black (C.I. No. 50415), Vali Fast Yellow (C.I. No. 11680), Vali Fast Red (C.I. No. 12120), Vali Fast Brown (C.I. No. 20195), Vali Fast Blue (C.I. No. 51175), Vali Fast Black (C.I. No. 51215), Oil Blue BO (C.I. No. 77450), Oil Green (C.I. No. 77601) and Oil Black (C.I. No. 77011), in which said copolymer coating reverses the charge of the coated pigment disperesed in said carrier liquid as compared with pigment free of said copolymer coating and dispersed in said carrier liquid, said copolymer being selected from the group consisting of (I) a copolymer of monomer (a) and monomer (b) in which the mol ratio of monomer (b) to monomer (a) is in the range of about 010.711, to give positively charged coated pigment particles at room temperature, and (II) a copolymer of monomer (a) and monomer (c) in which the mol ratio of monomer (c) to monomer (a) is in the range of about 0.010.5:1, to give negatively charged coated pigment particles at room temperature,  
 monomer (.2)  
 CH C  
 monomer (b) v R crr monomer (c) R CH3 N02 OH COOCH;CH-CHzor -ooR3 wherein:  
 R C,,,H ,,,OH or (1 S m s 4) N N NH2 aw-(ra N(CH3)2 Cl CONHZ CON(CH;)  
  4. In a method for developing electrostatic latent images, in which a copy material bearing the latent image is immersed in a liquid developer, the improvement which comprises employing a liquid developer com- 5 prising pigment particles having two charge-modifying copolymer coating layers and dispersed in a hdyrocarbon carrier liquid having a relative resistance of more than Qcm. and a dielectric constant of less than 3, said pigment being selected from the group consisting of carbon black (C.1. No. 77266), Fast Yellow G (C.l. No. 13135), Benzidine Yellow TR (C.l. No. 21090), Cyanine Blue BM (C.l. No. 74160), Fast Red 28 (Cl. No. 37115), Pigment Scarlet TR (C.I. No. 16105), Brilliant Carmine PB (G1. No. 16105), Brilliant Carmine 68 (Cl. No. 24830), Aizen Spilon Yellow (C.l. No. 41000), Aizen Spilon Red (C.l. No. 22310), Aizen Spilon Blue (C.l. No. 22610), Aizen Victoria Blue (C.I. No. 44045), Spirit Black (C.l. No. 50415), Vali Fast Yellow (C.1. No. 11680), Vali Fast Red (C.l. No. 12120), Vali Fast Brown (C.I. No. 20195), Vali Fast Blue (C.1. No. 51175), Vali Fast Black (C.l. No. 51215), Oil Blue BO (C.l. No. 77450), Oil Green (C.I. No. 77601 and Oil Black (C.l. No. 7701 1 one of said copolymers being a copolymer 1 of monomer (a) and monomer (b), the other copolymer being a copolymer ll of monomer (a) and monomer (c),  
 monomer (a) 1 CHg=JJ monomer (b) monomer (c) wherein:  
 R -CI-I or --H R2 C H2 n g -CH1CHCHa CON(CH3)2 wherein:  
 the two copolymer layers being selected from the group consisting of:  
 1. an inner coating layer of copolymer I in which the mol ratio of monomer (b) to monomer (a) is in the range of 1-111, and an outer coating layer of copolymer II in which the mol ratio of monomer (c) to monomer (a) is in the range of 0.01-0.5zl, to give negatively charged pigment particles at room temperature, and  
 2. an inner coating layer of copolymer II in which the mol ratio of monomer (c) to monomer (a) is in the range of 0.7-4.0:1 and an outer coating layer of copolymer I in which the mol ratio of monomer (b) to monomer (a) is in the range of 0.1-0.7:1, to give positively charged pigment particles at room temperature, and raising the temperature of the liquid developer to from 20 to 40C above room temperatuure so that the pigment particles then have a charge of a polarity opposite to the polarity of the charge they possess at room temperature.  
  5. A method according to claim 4, in which said inner coating layer is a copolymer of styrene and butyl methacrylate and said outer coating layer is a copolymer of lauryl methacrylate and dimethylaminoethyl methacrylate.  
  6. A process as claimed in claim 4, in which said two copolymer layers consist of l.  
 7. A process as claimed in claim 4, in which said two copolymer layers consist of 2.  
 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 874 896 Dated April 1, 1975 Inventor($) Hazime Machida, Akira Tsurugi and Zeniiro Okuno It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:  
  N Col. 11, line 47; change to --Y Q g Col. 12, line 60; change &#34;Y to Y Col. 12, line 63;-change &#39;Q H to GCONH Q CON (CH3) 2 Col. 12 line 67- aft R er C00 4 or add CON (C H 2 Col. 13, line 8; change &#34;l-l:l&#34; to ---l-4:l.  
 Col. 14, line 4; change &#34;temperatuure&#34; to temperature-.  
 Signed and sealed this 1st day of July 1975.  
 (SEAL) Attest C. i-IARSI IALL DANN- RUTH C. MASON Commissioner of Patents attesting Gfficer and Trademarks