GB2124792A - Xerographic developers - Google Patents

Abstract

An electrostatic developer comprises a mixture of (a) toner particles an (b) carrier particles, containing about 2% by weight of the toner particles based on the weight of the mixture of toner particles and carrier particles, in which: the toner particles (a) comprise from 88 to 90% by weight of a partially cross-linked polyester resin having a glass transition temperature of about 58 DEG C and an intrinsic viscosity, measured in chloroform at 25 DEG C, of about 0.55, and from 8 to 12% by weight of carbon black; and the carrier particles (b) comprise solid core particles having a diameter of from 30 to 1000 micrometres coated with about 0.1% by weight, based on the weight of the core particles, of an organic solvent-soluble miscible rubber of (i) a butadiene-acrylonitrile mixture derived from 20 to 40% by weight of acrylonitrile and (ii) a polyurethane elastomer. <IMAGE>

Description

SPECIFICATION Xerographic developers This invention is concerned with improvements in and relating to developers for use in electrostatographic (xerographic) reproduction systems.

The formation and development of images on the surface of photoconductive materials by electrostatic means is well-known. Thus, for example, the basic xerographic process is described in U.S. Patent No. 2,297,691 which discloses a method comprising placing a uniform electrostatic charge on a photoconductive layer, exposing the layer to a light and shadow image to dissipate the charge on the areas of the layer exposed to the light, and developing the resulting light and electrostatic image by despositing on the image a finely divided electroscopic material referred into the art as "toner". The toner will normally be attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the latent electrostatic image. This powder image may then be transferred to a support surface such as paper.The transferred image may subsequently be permanently affixed to the support surface, for example by heating.

Instead of latent image formation by uniformly charging the photoconductive layer and then exposing the layer to a light and shadow image, one may also form the latent image by directly charging the layer in an imaged configuration. The powder image may be fixed to the photoconductive layer if elimination of the powder image transfer step is desired. Other suitable means such as solvent or overcoating treatment may be used in place of the heat fixing step.

Several methods are known for applying the electroscopic particles to the latent electrostatic image to be developed. One such method is known as the "cascade" development method, and is disclosed in U.S. Patents Nos. 2,618,551 and 2,618,552. In the cascade method, a developer material comprising relatively large carrier particles having fine toner particles electrostatically coated thereon is conveyed to and rolled or cascaded across the electrostatic image-bearing surface. The composition of the carrier particles is so chosen as to triboelectrically charge the toner particles to the desired polarity. As the mixture cascades or rolls across the image-bearing surface, the toner particles are electrostatically deposited and secured to the charged portion of the latent image and are not deposited on the uncharged or background portion of the image.Most of the toner particles accidentally deposited in the background areas are removed by a rolling carrier, apparently due to the greater electrostatic attraction between the toner and carrier than between the toner and discharge background.

A second technique for developing electrostatic images is the "magnetic brush" process, as disclosed in U.S. Patent No. 2,874,063. In this method, a developer material containing toner and magnetic carrier particles is carried by magnets. The magnetic field of the magnet causes alignment of the magnetic carrier in a brush-like configuration. This magnetic brush is engaged with an electrostatic image-bearing surface, and the toner particles are drawn from the brush to the electrostatic image by electrostatic attraction.

In common commercial processes, the cascade technique is carried out in automatic machines. For example, a machine may consist of small buckets on an endless belt conveyor which scoop the developer material from a sump and convey it to a point above an electrostatic image-bearing surface where the developer mixture is allowed to fall and cascade or roll by gravity across the image-bearing surface. The carrier beads together with any unused toner particles are then returned to the sump for recycling through the developing system. This process is repeated for each copy produced in the machine and is ordinarily repeated many thousands of times during the usable life of the developer. The great deal of mechanical agitation causes degradation of both the toner and carrier particles and such degradation is manifested by print deletion and poor print quality on the copies.Also, the triboelectric and flow characteristics of many carriers are adversely effected when the relative humidity is high. For example, the triboelectric values of some carrier coatings fluctuate with changes in relative humidity and thus such coatings are not desirable for use in xerographic systems, particularly in automatic machines which require carriers having stable protectible triboelectric values. Furthermore, many carrier coating materials are difficult to apply to carrier cores because they tend to form thin filaments rather than smooth continuous coatings.Since developer materials must flow freely to facilitate accurate metering and even distribution during the development and developer recycling phases of the electrostatic process, the presence of filaments and carriers having rough outer surfaces and developer materials is unsuitable because the developer materials tend to cake, bridge and agglomerate. Some carrier coating materials have acceptable triboeelectric and coating properties but are unacceptable on a commercial scale because they cannot be economically mass produced. For example, quality control for the triboelectric value of some resin biends is difficult to maintain because a slight deviation in component percentages causes the triboelectric value of the resulting product to change drastically. Therefore, it is evident that there is a continuing need for better systems for developing latent electrostatic images.

It is, therefore, an object of the present invention to provide developer materials with improved dark image density with good background, which developers may have improved solid area fill characteristics, improved fixing properties, diminished humidity sensitivity and are readily manufactured on a commercial scale.

According to the invention there is provided a developer comprising a mixture of (a) toner particles and (b) carrier particles and containing about 2% by weight of the toner particles based on the weight of the mixture of toner particles and carrier particles, in which: the toner particles (a) comprise from 88 to 90% by weight of a partially cross-linked polyester resin having a glass transition temperature of about 58"C and an intrinsic viscosity, measured in chloroform at 25to, of about 0.55, and from 8 to 12% by weight of carbon black; and the carrier particles (b) comprise solid core particles having a diameter of from 30 to 1000 micrometers coated with about 0. 1% by weight, based on the weight of the core particles, of an organic solvent-soluble miscible mixture of (i) a butadiene-acrylamide rubber derived from 20 to 40% by weight of acrylonitrile and (ii) a polyurethane elastomer, suitably in a weight ratio of butadiene rubber to polymethene elastomer of from 10:90 to 30:70.

The developer may also contain certain additives to the toner. These additives are preferably conventional iubricants, such as metal stearates or silica. Most preferably the additives will comprise about 0.3 to 0.6% by weight of the toner, of a metal stearate (preferably zinc stearate) and about 0.3 to 0.6%, by weight of the toner, of silica.

The solid cores of the carrier particles may be conventional solid materials such as glass or ferrous metals. Preferably the solid cores are of oxidized steel. The carrier particles may be from 30 to about 1000 microns in diameter; however it is preferred that the carrier particles be provided such that greater than about 80% of the particles are of a diameter of about 100 micrometers, contains from 88 to 90% by weight of a partially cross-linked polyester resin having a glass transition temperature of about 58"C and an intrinsic viscosity of 0.55 in chloroform at 25"C. Such resins are known and are commercially available.

The preferred resin for use according to the present invention is Toner Resin RES-D-0031, manufactured by Hercules Incorporated, Wilmington, Delaware, U.S.A. This resin is typically characterized by an acid number of 23, initial penetration point of 58"C (by Thermal Analysis 10 C/min., 5 gram load, 0.025 inches (0.6035 mm) diameter probe), initial flow point of 70"C and glass transition temperature of 58"C (by Thermal Analysis, 20"C/minutes, second melt extrapolated onset of heat capacity step).

The toner according to the present invention also contains from 8 to 12% by weight of carbon black. Conventional carbon blacks may be used which are well-known in the art. The preferred carbon black is Black Pearls L Carbon Black. A preferred toner composition consists essentiaily of about 90% by weight of the above described partially cross-linked polyester resin and about 10% by weight of carbon black.

The carriers used in the developer according to the present invention are solid core particles coated with a coating forming about 0. 1% by weight of the core particles and comprising a miscible mixture of a butadiene-acrylonitrile rubber. Such carriers are disclosed in U.S. Patent No. 4,331,756. The diameter of such core material may be generally from 30 to 1000 microns, usually from 100 to 600 microns. Any of a wide variety of materials may be used as the core. For use with a magnetic brush, the core should be a ferromagnetic material such as iron, steel, ferrite, etc. These same materials may be used with non-magnetic cores as also sand, glass beads, or other known conventional materials. The carrier coatings are miscible mixtures of a butadiene-acrylonitrile rubber containing from about 20% to 40% acrylonitrile with a polyurethane elastomer.Coatings produced with these mixtures are tough, tenacious, non-tacky, and do not abrade away from the cbre and tend to be non-adhesive to additives, toner, or other surfaces with which they come into contact during the development process. The coatings are easily applied by solvent coating, and do not require pretreatment of the carrier or a high temperature curing cycle.

The preferred butadiene-acrylonitrile rubbers are typically characterized by having a specific gravity of from 0.95 to 1.00 and an average Mooney viscosity of from 30 to 80. Such rubbers are available commercially from B.F. Goodrich under the trademark Hycar. These rubbers are soluble in organic solvents, especially polar organic solvents such as methyl ethyl ketone, acetone, tetrahydrofuran and dimethyl formamide.

The following Table 1 gives the code number and some of the characteristics of those Hycar rubbers which are useful according to the present invention.

TABLE 1 SPECIFIC AVERAGE MOONEY TYPE % ACRYLONITRILE GRAVITY VISCOSITY 1031 41 1.00 60 1042 31 0.98 80 1042F 33 0.98 80 1094-80 21 0.95 80 1432 33 0.98 80 Hycar 1432 which is dusted with a ketone soluble polyvinyl chloride resin is the rubber of choice because of the excellent results obtained through its use.

The polyurethanes useful in this invention are thermoplastic elastomers which are the reaction products of polyesters or polyethers with diisocyanates. These polyurethanes are film forming resins and are soluble in organic solvents such as those mentioned above. The polyurethanes are prepared by the reaction of linear hydroxy terminated polyestes, usually adipates; or polyethers, usually poly(oxytetramethylene)-glycol with glycols such as ethylene glycol and 1,4butanediol and diisocyanates, most commonly diphenylmethane-4,4'-diisocyanate.

The presently preferred elastomers are available from B.F. Goodrich under the trademark Estane. These elastomers are believed to be the reaction products of adipic acid, 1,4-butanediol and diphenylmethane-4,4'-diisocyanate. The presently preferred Estane is sold as Estane 5715.

This is preferred because it is readily soluble in methyl ethyl ketone, a common inexpensive solvent which also dissolves the Hycar rubbers, and because it may be formed into an extremely hard, flexible film with excellent abrasion resistance. The physical fproperties of Estane 5715 are shown below in Table 2.

TABLE 2 TYPICAL VALUE ASTM TEST PHYSICAL PROPERTIES (1) PROCEDURE Specific Gravity 1.20 D-792 Hardness, Durometer A 97 D-2240 (2) Durometer D 63 Tensile strength (psi) 6150 D-882 (3) Modulus 100% Elongation (psi) 1400 Elongation (%) 300 Graves Tear (Ibs/in) 260 D-1004 Tear Propagation (Ibs/in) 120 Solution Viscosity (20% TS Concentration in MEK BROOKFlELD RVF Viscometer No. 2 Spindle, 20 rpm, 25"C), cps (1) Representative solution cast film data on typical production material.

(2) Test conducted on 75 mil tensile sheets.

(3) Test conducted on dumbell film samples/ (4) Actual specification value.

The butadiene-acrylonitrile rubbers used in the practice of the present invention possess extremely high positive triboelectric characteristics. Carrier coatings prepared from a solution of Hycar 1 432 exhibit a triboelectric charge of over 60 microcoulombs per gram of toner in a developer mixture containing two parts of toner for every 98 parts of coated carrier beads. The polyurethanes, on the other hand, have relatively low to moderate positive triboelectric characteristics. For Estane 5715, the comparable charge is 15 microcoulombs. It has been found that the two polymers may be mixed in varying proportions and coated on core materials to produce carrier particles with selected triboelectric characteristics.The carrier particles when mixed with toner will manifest triboelectric characteristics between the high of the butadiene acrylonitrile rubber and the low of the polyurethane elastomer. The selected coating may be applied to the core by any suitable means to produce carrier particles which may be used with the toner particles described above in accordance with the present invention. For example, the coating may be applied by dipping, spraying or tumbling the cores with a coating solution in a barrel or through a fluidized bed. The fluidized bed process is preferred since it permits the production of a uniform coating on the core particles. This process is described in U.S. Patents Nos. 2,648,409, 2,799,241, 3,253,944, 3,196,827 and 3,241,520.

A preferred method of forming the toner compositions according to the present invention is as follows. Preweighed amounts of the partially cross-linked polyester resin and carbon black are blended by first mixing the preweighed amounts into a barrel tumbler and premixing for 30 minutes, then compounding the mixture in a twin-extruder under the desired temperature and feeding rate. Preferably one pass through the extruder is made at a rate of four pounds per hour at a temperature of 102"C.

The resulting blend is then ground and classified to meet requirements for the toner particle size. The toner particle size may be measured on the Coulter Counter TA-II to control the particle size distribution of toner particles. The secondary jet-pulverization and secondary classification methods described below may be applied if the measurements on the counter deem it necessary. The secondary jet-pulverization is a method of pulverizing toner particles two or three times at a relatively higher feeding rate, instead of pulverizing once at a low feeding rate. Using this approach, the pulverization effect mainly occurs on large particles and has a lesser effect on fine particles. Similarly, secondary classification is a method of classifying toner particles twice at the same settings to improve screening efficiency.To fabricate the toner particles according to the present invention, secondary jet-pulverization may not be necessary if the initial extrusion is done with a jetting setting of nine.

The developer compositions according to the present invention are particularly useful in xerographic copiers wherein the developer is exposed to temperature above about 100"C within a developing and fixing zone of the copier. Copiers which expose a toner to somewhat lower temperatures may not obtain the advantages of the present developing composition. It has been found that the developing compositions according to the present invention are particularly advantageous when used in Ricoh Copier Models 6200 and 6600 because of the adequate temperatures obtained within the developing and fixing zones of those machines.

In order that the invention may be well understood the following Examples are given by way of illustration only.

EXAMPLE 1 To compare the image density of various developer compositions with that of the present invention, five developer compositions were made using the toners identified as follows: A: Styrene/ > butyl methacrylate and partially crosslinked pentapolymer (available from Diamond Shamrock), designated as Dialec S308 and Dialec L657, respectively.

B: Partially crosslinked polyester polymer RES-D-0031 (Hercules).

Both toners comprised 90% by weight polymer and 10% by weight of Black Pearls L Carbon Black, and were jetted an classified to particle size average of 8 microns, population 85% in excess of 5 microns. The two types of the toners (2 parts by weight of each toner) were combined with three types of carrier, (1) methyl metilacrylate coated steel, (2) Hycar-Estane coated oxidized steel, and (3) non-coated oxidized steel (100 parts by weight of each carrier) to give five developer compositions as shown in Table 3. For each developer composition, 1,000 copies were run on a FT-6200 (Ricoh) copier; developer Q/M and density were measured.

(Q/M is the charge divided by mass). The results are tabulated below in Table 3. Table 3 clearly shows that the composition according to the present invention, B, has triboelectric properties similar to the control on oxidized surfaces as well as on Hycar Estane coatings. (MMA is methyl methacrylate homopolymer; H.E. is Hycar-Estane coating; weight ratio 20:80-which weight ratio was used for all Hycar-Estane coatings in the following Examples). All tests were performed on a Ricoh Copier FT-6200.

TABLE 3 % Toner Image Developer Composition Q/M Conc. Density A+ 100 micron steel (MMA) 12.0 1.7 1.1 A+ 100 micron steel ox. (H.E.) 22.0 1.7 1.0 B+100 micron steel ox. (H.E.) 24.0 1.9 1.3 B+100 micron steel (MMA) 15.0 1.7 1.1 B + 200 micron oxidized steel 27.2 1.7 0.9 EXAMPLE 2 The degree of fix for each toner composition in Example 1 was measured for percent loss of image density on test copies run on a FT-6200 copier at each of the designated fuser test temperatures. One thousand copies were run on the copier using an oxidized 200 micron steel carrier. The results are shown in Table 4. The toner according to the present invention, B, shows an advantageous degree of fix and low percentage loss. It should be noted that the lower % loss represents the higher fix level.

TABLE 4 % Image Density Loss* Fuser Roll Temperature, "C Toner 165 175 185 195 205 215 225 A 55 37 29 31 26 23 19 B 31 14 13 12 9 7 6 %Image Density Loss refers to the change in density when a controlled subject is subjected to abrasion in a Taber Abrader, manufactured by Teledyne Taber.

EXAMPLE 3 The electrical resistivity of the carrier beads is critical in preventing bead carryover to the photoreceptor drum. Carrier resistivity can be controlled via surface treatment-oxidation or to those skilled ion the art via polymeric coatings. The data of Table 5 clearly illustrates the effects of surface treatment on resistivity. Furthermore, the type of surface treatment has an effect on triboelectric charging, ranging from 34 esu. to 19.5 esu. The first four carriers in Table 5 consist of oxidized iron 100 micron cores. The last three carriers in Table 5 consist of unoxidized steel cores, 100 micron.

TABLE 5 Oxidized Carrier Mat. y-cm Q/M (relative to A) UC-1 1011 7.6 UC-1 (.1% H.E.) 1011 13.9 UC-1 (.5% MMA) 1015 15.0 UC-1 (.5% PKHH) 1015 3.4 Unoxidized NM 105 8.9 NM (.1% H.E.) 105 19.5 NM (.5% MMA) 1014 9.0 (NM = Nuclear Metals, Inc.: H.E. = Hycar-Estane coating; MMA = methyl methacrylate (Dupont); PKHH = Phenoxy resin (Union Carbide)).

EXAMPLE 4 In order to function properly on the FT-6200 copier, both the carrier and toner must react in a mutually compatible manner with the cleaning additives. The carrier employed in this example was Hycar-Estane coated oxidized steel.

TABLE 6 Additive Level by % (by wt.) Weight of Toner Toner Zinc Stearate Silica A B O 0 9.3 3.7 1 0 6.7 5.8 0 1 24.2 26.4 5 5 16.4 16.4 EXAMPLE 5 A test of toner B containing 0.3% silica and 0.32% zinc stearate, with two carriers, 100 microns unoxidized steel shot coated with 0.5 MMA and 100 microns oxidized steel shot, was run to determine bead carry over properties. There is shown below in Table 7 that using oxidized carrier bead carryover occurs at 1.7% toner concentration but at 1.9% toner concentration carryover is minimized. On the unoxidized carrier, running the toner concentration in excess of 1.8% did not improve the image density. There was no bead carryover even at 1.5% toner concentration for the carrier coated with methyl methacrylate.

TABLE 7 B Toner/Oxidized Carrier, 0.3% Silica, 0.32% Zinc Stearate T.C.% Density Q/M Bead Carryover 1.7 1.3 28.2 Yes 1.9 1.4 35.1 Yes, less than 1.7% 2.9 1.4 31.6 Minimal B-Toner/ N M -1 00 Coated 0.5% MMA, 0.3% Silica and 0.32% Zinc Stearate 1.5 1.03 14.6 No 1.6 1.01 11.9 No 1.8 1.06 10.2 No EXAMPLE 6 Toner B was tested in the FT-6200 copier with 2 different polymeric coatings. From the data of Table 8, it is apparent the developer mixture of B and Hycar-Estane coating has a higher image density at equivalent toner concentration and has a flatter, non-glaring image than the standard commercially available developer.

TABLE 8 Image Core Coating Q/M T.C Density Gloss 100 micron steel shot MMA 11 2.0 1.1 flat 100 micron Hycaroxidized shot Estane 21 2.0 1.3 flat EXAMPLE 7 Humidity sensitivity results at 30"C are shown below in Table 9. From the data in Table 9 it can be seen that the toner B Hycar-Estane mixture gives good image fill and image density and is superior.

TABLE 9 Humidity Sensitivity Results at 30"C Material B/Toner/H.E.

Copies Start 1K Humidity % RH 80 80 Q/M 24.4 21.0 % T.C. 2.2 2.1 Image Density 1.4 1.4 Humidity % RH 25 25 Q/M 25.2 27.0 % T.C. 2.0 2.0 Image Density 1.4 1.4

Claims (8)

1. An electrostatic developer comprising a mixture of (a) toner particles an (b) carrier particles and containing about 2% by weight of the toner particles based on the weight of the mixture of toner particles and carrier particles, in which: the toner particles (a) comprise from 88 to 90% by weight of a partially cross-linked polyester resin having a glass transition temperature of about 58"C and an intrinsic viscosity, measured in chloroform at 25"C, of about 0.55, and from 8 to 12% by weight of carbon black; and the carrier particles (b) comprise solid core particles having a diameter of from 30 to 1000 micrometres coated with about 0.1% by weight, based on the weight of the core particles, of an organic solvent-soluble miscible rubber of (i) a butadiene-acrylonitrile mixture derived from 20 to 40% by weight of acrylonitrile and (ii) a polyurethane elastomer.

2. A developer as claimed in claim 1 in which the toner particles (a) further comprise from 0.3 to 0.6% by weight of a metal stearate and from 0.32 to 0.64% by weight of silica.

3. A developer as claimed in claim 2 in hch the metal stearate is zinc stearate.

4. A developer as claimed in any one of the preceding claims in which the core particles of the carrier (b) are of glass or oxidized ferrous metal.

5. A developer as claimed in claim 4 in which the said particles are of oxidized steel.

6. A developer as claimed in any one of the preceding claims in which the core particles have a diameter of about 100 microns.

7. A developer as claimed in any one of the preceding claims in which the toner particles comprise about 90% by weight of the polyester resin and about 10% by weight of carbon black.

8. A developer as claimed in claim 1 substantially as hereinbefore described with reference to the Example.