CA1082025A - Coated carrier particles with magnitude of triboelectric charge controlled and method of making same - Google Patents

Abstract

COATED CARRIER PARTICLES WITH MAGNITUDE
OF TRIBOELECTRIC CHARGE CONTROLLED
AND METHOD OF MAKING SAME
Abstract of the Disclosure An electrophotographic development carrier comprising core particles coated with a carboxylated polyethylene ionomer. When utilized with appropriate electroscopic toner materials, the carrier, which is coated with a carboxylated polyethylene ionomer is effective in producing a desired polarity and magnitude of the triboelectric charge. on the toner particles. Control of the polarity and magnitude of the triboelectric charge characteristic of the carrier, which has its particles coated with the carboxylated polyethylene ionomer is obtained by controlling the curing temperature of the carboxylated polyethylene ionomer.

Description

17 Specification 18 In electrophotography, a photoconductor is charged 19 and then exposed imagewise to light. In the area of the photoconductor exposed to light, the charge dissipates 21 or decays while the dark areas retain the electrostatic 22 charge.
23 The difference in the charges between the areas 24 exposed to light and the dark areas produces electrical 25 fields therebetween. Accordingly, the resultant latent .
26 electrostatic image on the photoconductor is developed ,~ '' ~1. ' ~ . .

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1 by depositing small colored pàrticles~ which are known

2 as toner particles, over the surface of the photoconductor -- -

3 with the toner particles having a charge so as to be

4 directed by the electrical fields to the image areas of the photoconductor to develop the electrostatic image.
6 A number of means are known for developing the 7 latent electrostatic image by the application of the 8 toner particles. One of these is known as cascade 9 development. Another means is known as the magnetic brush process.
11 In each of the cascade and magnetic brush development 12 processes, a two-component developer material is utilized.
13 The developer material comprises a mixture of small toner 14 particles and relatively large carrier particles. The toner particles are held on the surfaces of the 16 relatively large carrier particles by electrostatic 17 forces, which develop from the contact between the toner 18 and carrier particles producing triboelectric charging 19 of the toner and the carrier to opposite polarities.
These polarities are determined by the relative positions 21 of the materials in the triboelectric series. When the 22 developer material is moved into contact with the latent 23 electrostatic image of the photoconductor, some of the 24 toner particles are detached from the carrier particles by a combination of mechanical forces and electrostatic 26 forces and are attracted to the latent image.

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10~20Z5 1 The toner and carrier particles of the developer material are specially made and processed so that the toner obtains the correct charge polarity and magnitude of charge to insure that the toner particles are preferentially attracted to the desired image areas of the photo-conductor. For a given developer-hardware system, the magnitude of the triboelectric charge is important in that if such charge is too low, the copy will be characterized by high print density but heavy background, if the charge is too high, the background is good but the print density will tend to be low. Thus, there is an optimum range of toner charge for best overall results.
Prior art carriers have been particularly sensitive to toner filming, with the exception of carriers such as those disclosed in the copending Canadian application of C.A. Queener, et al, entitled "Coated ~ -Carrier Particles, Method of Making Same, and Improved Electrophotographic Process," Serial No. 133,097, filed January 25, 1972 and Canadian appli-cation of William J. Kukla, et al, entitled "Coated Carrier Particles with Polarity and/or Magnitude of Triboelectric Charge Controlled, Method of Making Same, and Improved Electrophotographic Process," Serial No. 133,096, filed January 25, 1972.

1 The carrier disclosed herein exhibits some of the 2 characteristics of the carriers disclosed in the above 3 applications while possessing better toner filming 4 resistance than prior art carriers other than those above. There are three primary modes of carrier failure:
6 abrasion, spalling, and toner filming. Spalling occurs ~ -7 when the bond between the coating and the core breaks 8 and coating material breaks up and separates from the 9 core.
Toner filming is the phenomena of toner particles 11 fuslng to the carrier coating and thus reducing the -12 capacity of the coating to triboelectrically charge the 13 toner particles.
14 Because of the contact between the carrier particles and between the carrier particles and parts of the 16 machine, there is abrasion of the coating of the carrier 17 particles. This abrasion of the coating also may 18 reduce the effectiveness of the triboelectric charging ~-19 between the carrier and the toner by exposing the toner to the core material of the carrier.
21 Thus, if the coating is not sufficiently resistant 22 to abrasion, early replacement of the entire developer 23 material is required. This replacement of the entire 24 developer material is costly and tlme consuming~
especially in high speed copy machines.

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10820~5 1 Furthermore, even if the coating of the carrier 2 particle resists abrasion, the coating also must have 3 good adhesion to the core of the carrier particle.
4 Otherwise, the coating-can chip, flake, or spall, even ~ ~-if the coating is of a material that is not subject to 6 abrasion, due to the rubbing or contact between the 7 various carrier particles and between the carrier 8 particles and parts of the machine. This also requires 9 early replacement of the developer material.
Therefore, in addition to having the proper 11 triboelectric characteristics, the coating of a carrier 12 particle must have good anti-stick (low surface energy) 13 properties to prevent filming of the carrier particle 14 by the toner. The coating also must have good adherence to the core and be resistant to abrasion.
16 In addition to having the foregoing desired 17 properties, the carrier particles must have the charac-18 teristic of having a triboelectric charge of a desired 19 magnitude and polarity when used with a particular electrbscopic toner. This is because the magnitude 21 of the triboelectric charge of the toner is controlled by 22 the magnitude and polarity of the carrier charge when 23 the toner and the carrier are mixed.
24 The magnitude of the charge of the toner is important for the electrophotographic system to produce copies of 26 a high quality. If the magnitude of the charge of the ' - ,.;~ . .- . - , . . - - : - ~:
, . . . .: .: . : . , ~ 108Z(~Z5 1 toner lS too low, the copies have poor edge definition 2 and lack contrast since the non-image or background 3 areas possess an unacceptable level of toner as previously 4 mentioned. If the magnitude of the charge of the toner is too high> then there is poor image fill since the 6 toner would tend to stay with the carrier. Accordingly, 7 the magnitude of the charge of the toner must be within 8 a range above that in which poor edge definition is 9 produced and below that in which poor image fill occurs. 1` -Thus, if the coating of the carrier particle has 11 the characteristic of imparting a triboelectric charge 12 to the toner when mixed therewith so that the toner charge 13 does not fall within the range in which copies of high 14 quality can be obtained, the coating cannot be employed lS for a carrier for the toner even though it meets the 16 abrasion, anti-stick, and adhesion requirements.
17 Accordingly, for a carrier coating to be satisfactory, 18 it not only must meet the abrasion, anti-stick, and 19 adhesion requirem0nts but also must be capable of causing the carrier to have the characteristic of triboelectrically 21 charging the toner when mixed with the toner so that 22 the charge of the toner is within the desired range that ¦ 23 enables copies of high quality to be produced.
24 From the location of polyethylene in various 25 published triboelectric series, and experimentation with ~ -26 polyethylene, one skillel~ in the art is led to reject ~08~02S

1 it as a carrier coating even if it could meet the other 2 requirements, because the charge magnitude capability 3 of the polyethylene is so low as to provide 4 degraded copy qualities.
The present invention has overcome the above problems 6 by using a carboxylated ionomer o~ polyethylene and then 7 curing it under controlled conditions. The carboxylation 8 makes the polyethylene generally more electronegative 9 and moves its position in the triboelectric series toward the negative end. By controlling the curing 11 conditions of the carboxylated polyethylene ionomer 12 coated carrier, the magnitude of the charge characteristic 13 may be controlled. Also, if desired, by controlling the 14 curing conditions, the carrier coating may be forced to a more electropositlve position in a triboelectric series.
16 Thus, by controlling the curing condition and the extent 17 of carboxylation of the polyethylene, it is possible to 18 select a toner and then match the carrier to it rather 19 than having to develop a toner and carrier simultaneously and in parallel as a developer composition.
21 Accordingly, even if the potential of the image 22 areas in an electrophotographic system were changed 23 significantly so that the charge and/or polarity of the ~-24 toner must be changed for the toner to continue to be ~ -employed in the electrophotographic system, this invention 26 allows the particular toner to still be used. Additionally, ~, , 1 this invention permits the selection of any one of a ; 2 plurality of toners for use with a particular electro- -3 photographic system.
4 At the same time that thls flexibility of the toner charge through controlling the triboelectric characteristic 6 of the carrier material is available by employing the 7 method of the present invention, the present invention 8 also insures that the carrier particles have a relatively 9 long life. This is because the coating of the carrier particles has the required anti-stick, abrasion, and 11 adhesion properties necessary for a long life coating 12 for a carrier.
13 An object of this invention is to provide a new 14 coated carrier particle having the magnitude and/or polarity of its triboelectric charge controlled in 16 accordance with the toner and the electrophotographic 17 system with which it is used.
18 Another object of this invention is to provide a 19 new electrophotographic development carrier having a relatively long life.
21 Still another object of this invention is to 22 provide an improved electrophotographic process.
23 The foregoing and other objects, features, and 24 advantages of the invention will be more apparent from the following more particular description of the 26 preferred embodiments of the invention.

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1 The core of the carrier particle or bead formed 2 by the present invention may be any material to which 3 the coating can adhere and which can withstand the curing 4 temperature. Thus, the material of the core of the carrier particle may be sand, glass beads, metallic 6 beads, or metal alloys, for example, or any of the many 7 accepted carrier particle materials.
8 When the carrier particles of the present invention 9 are employed in a developer utilized in the magnetic brush process, the material of the core is a ferromagnetic 11 material such as iron or steel, for example. Other 12 suitable ferromagnetic materials such as magnetic oxides 13 and alloys (copper-nickel-iron), for example, may be 14 employed.
The size of the core generally may be between 50 16 and 1,000 microns. The preferred size range is between 17 100 and 600 microns.
18 The material of the coating of the core of the 19 carrier particle of the present invention is formed of a carboxylated polyethylene ionomer. Carboxylated 21 polyethylene ionomers are sold by E. I. DuPont Corporation 22 under the trademark Elvax D which are dispersions in 23 water of ionomer resins.
24 In applying t~e coating to the core, the coating may be applied by any suitable means such as dipping, 26 spraying, tumbling the cores with a coating solution in g - -' - ~: - . : : . - .~ : .

1 a barrel, or through a controlled fluidized bed. The 2 fluidized bed process is preferred since it enables a 3 uniform coating to be applied to the cores of the 4 particles. The fluidized bed process is well known in the prior art as a coating technique.
6 In a fluidized bed process, for example, the cores 7 are suspended and circulated in an upwardly flowing 8 stream of heated gas such as air, for example, in a 9 manner such that the particles move upwardly and are sprayed by the coating material in a first zone. Then, 11 in a second zone, the particles settle through the air 12 stream in a zone of lower air velocity. The liquid, 13 which is a solvent and/or a dispersant, of the sprayed 14 coating evaporates to leave a thln solid coating on the particles. The particles recirculate to the first zone 16 so that successive layers of the coating material are 17 built up on the core in a uniform manner.
18 After the core has been coated to form the 19 carrier particle, the coating is cured so as to possess the desired triboelectric properties. The curing 21 process depends upon the material of the coating and the 22 composition of the toner with which the carrier particles 23 of the present invention are to be employed, as well as 24 the triboelectric requirements of the system in which it -is used.
Z6 Thus, if the coating is a carboxylated polyethylene 27 ionomer, for example, then curing of the coating occurs 28 by heating the carrier particles at a selected temperature .

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1 for the selected period of time; the carrier particles 2 are then allowed to cool to room temperature, thereby 3 affecting the magnitude of the triboelectric charge of the 4 carrier particle when mixed with a particular toner.

5 Thus, for a particular toner, the carboxylation of the

6 polyethylene and the curing produces substantlal

7 increase in the magnitude of the triboelectric charge

8 over the uncarboxylated polyethylene. This enables the g triboelectric charge of the toner to fall within a desired range in which high quality copies are produced.
11 It should be understood that the curing temperature 12 must be sufficiently high so that the coating will 13 soften or melt and adhere to the core. The desired 14 magnitude and polarity of the toner charge for the particular toner determines the actual temperature 16 selected.
17 The thickness of the coating may vary from less 18 than 1 to about 20 microns. However, it is preferably 19 about 1 to 5 microns. The coating thickness must be sufficient to impart the desired triboelectric effect 21 to the carrier with the upper limit of the thickness ~ -~
22 being determlned by physical integrity of the coating.
23 The following are examples of the present invention 24 in detail. The examples are included merely to aid in the understanding-of the invention, and variations may 26 be made by one skilled in the art without departing from 27 the spirit and scope of this invention. -. .

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108;20Z5 1 The particles or the core materials in practice are 2 selected or size and material and placed within a 3 coating apparatus which may, for example, be a fluidized 4 bed coating apparatus or other apparatus well known S in the art or coated by tumbling, spraying, dipping, 6 or other recognized techniques. For the sake of illustra-7 tion, the following examples will utilize the fluidized 8 bed technique of coating the core materials.

9 The core particles are placed in a chamber, the chamber being such that a spray of the material to 11 be coated is injected into an upward moving air stream 12 and is carried by the air stream into contact with 13 the core materials. The spray of material is injected 14 in the center of the air stream such that the materials are carried up with the air stream to an equilibrium 16 height and then fall back through the air flow to 17 a lower height where they re-enter the upward air 18 flow where it is again combined with the coating material 19 and receive subsequent coats until the desired coating thickness is obtained. The following examples will 21 more clearly illustrate the characteristics and preparations 22 of a carrier material made from a core material and 23 a form of carboxylated polyethylene ionomer.
24 Example 1 1~ pounds of S-70 spherical steel shot were loaded 26 into a fluidized bed coating apparatus. A water dispersion " .

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of carboxylated polyethylene ionomer designated by 2 E. I. DuPont as Elvax D-1236 was diluted with two parts 3 of de-ionized water to one part of the Elvax D-1236 -4 dispersion such that the solid percentage was approximatley 14 to 16 percent to yield 600 ml of dilute dispersion.
6 Air was then forced under pressure into the fluidized 7 coating apparatus at 150 to 170 F. and the resulting 8 dispersion is pumped through an atomizing spray nozzle 9 ,at a rate such that uniform coating occurs and is slow enough to allow minute quantities to coa~ the steel 11 shot and partially dry prior to the steel shot being 12 caught in the fluidized bed for a second pass of coating.
13 This repetitive coating of a steel shot continues 14 until all shot in the batch are coated to the desired thickness slze which approximates 3 to 3 1/2 microns 16 of coating thickness. The approximate rate that the 17 diluted dispersion is atomized and injected into the 18 air :Elow is 10 ml per minute. A total volume of coating 19 material after dilution is approximately 600 ml for a 16 pound batch of S-70 spherical steel shot. After 21 the coating was completed, the steel shot coated with 22 Elvax D-1236 is then circulated in a heated air current 23 for approximately 15 minutes at a temperature of 24 approximately 150 to 170 F. inlet temperature. This drying step drives off most of the remaining water 26 and leaves a batch of coated steei shot which is , 27 essentially dry and not agglomerated.
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~08Z~25 1 As was discussed above, the triboelectric charging 2 capability and polarity of the carrier is partially 3 dependent upon the cure temperature and cure time. The 4 shot, coated as above, was then cured for approximately 20 minutes at a temperature of approximately ? F.
6 This additional curing after coating dries off residual 7 solvent material, in this case, de-ionized water, 8 and further cures the carboxyliated polyethylene ionomer 9 to acquire desired triboelectric charge capabilities.
The curing, as described above, in addition to driving ll off the remaining solvent or carrier fluid, aids in 12 acquiring the best possible bond between the coated 13 material and the steel shot. Since the temperature 14 to which it is heated during the cure period is above lS the glass transition temperature of the coating, any 16 surface irregularities, roughness, or particles of 17 carboxylated polyethylene ionomer which have heretofore 18 not been thoroughly melted and/or softened to allow 19 a uniform spherical coating are then heated to a temperature which allows the coating to become a uniform thickness, 21 smooth surface, and spherical.
22 The resulting carrier was then tested to determine its 23 triboelectric characteristics and its capabilies for making 24 acceptable photocopies and the results of these tests indicated that triboelectric characteristics of the ., .
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1 carboxylated polyethylene ionomer marketed under the 2 trade name of Elvax D-1236, when coated and cured as 3 above, produced good copies. The test copies had 4 excellent background, i.e., low background, and excellent line definition.
6 The test copies were made utilizing the magnetic brush 7 development technique wherein a magnetic brush attracts the 8 steel core of the carrier beads and carries them, 9 together with the triboelectrically charged toner particles ~-onto the surface of the photoconductor, thus toning and 11 developing the latent electrostatic image carried by the 12 photoconductor.
13 Example 2 14 A water dispersion of a carboxylated polyethylene ionomer marketed by DuPont under the trade name Elvax 16 D-1070 was prepared by diluting the dispersion in 17 proportions of one part dispersion to two parts de-ionized 18 water thereby reducing the minimum sollds to the approximate 19 range of 14 to 16~, and it was coated upon S-70 shot in the same quantities as in Example 1. The conditions 21 of the coating process were the same as described 22 in Example 1 as was the curing procedure.
23 The resulting carrier was then tested for its 24 triboelectric characteristics and indications were that the triboelectric charge capabilities of this carrier were good 26 to excellent. ~ . -" ~, .

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. - . .. : - . , : ,-. -, 1 Example 3 2 In this example, a water dispersion Elvax D-1220 3 carboxylated polyethylene ionomer having a cross linkable 4 capability was diluted two parts de-ionized water 5 to one part dispersion of Elvax D-1220. This material 6 was applied in the same manner as that described in 7 Example 1 to a total of 14 pounds of S-70 steel shot.
8 The total quantity of diluted dispersion used in coating 9 the 14 pounds of shot was 450 ml. The input - -temperature of the fluidized coating chamber was 130 11 to 150 F. and the coating rate was approximately 12 15 ml per minute. After the coating was completed, 13 the coated core material was then dried in a heated 14 air flow for approximately 10 minutes at 115 to 120 F. The carrier particles were then removed from the 16 fluidized bed coating apparatus and oven cured at 17 a temperature of approximately 250 F. for a period 18 of approximately 20 minutes. The resulting material 19 was broken up and screened to result in relatively uniform carrier bead material. The resulting carrier 21 beads were then tested for triboelectric charge capability.
22 The resulting carrier beads were then used in the 23 production of electrostatic copies. The copies were 24 excellent having excellent line and half-tone qualities : .
and some low background.

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. -1 Example 4 2 The same general process as described in Example 1 was 3 performed using Elvax D-1249 carboxylated polyethylene 4 ionomer having a very high molecular weight and havlng ~
5 been diluted two parts de-ionized water to one part -6 of carboxylated polyethylene ionomer dispersion. This 7 mixture was then introduced into a fluidized bed coating 8 apparatus where a total volume of 800 ml of the above 9 dispersion was coated onto 24 pounds of S-70 spherical steel shot. The coating was performed in an air stream 11 having an inlet temperature of 170 to 190 F. at a :
12 coating rate of 30 ml per minute. When coating ~~
13 was completed, the shot was continued to be circulated 14 in the fluidized bed coatlng apparatus for a period lS of 30 minutes at a temperature of 180 to 200 F. thereby 16 drying and curing the carrier beads. The combined 17 curing and drying performs the same function as a 18 drying in the fluidized bed apparatus and subsequent 19 oven curing or a subsequent curing in the coating Z0 apparatus. The curing of the carrier particles in 21 the fluidized bed apparatus prevents agglomeration 22 of the particles and obviates the need to break up an 23 agglomerated mass such as is formed by the technique 24 of oven curing as described in Example 3. Thus, the - -particle coatings are uniform, smooth, and spherical 26 with properly cured in the fluidized bed coating apparatus.

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1 The carrier material thus formed was tested for its 2 triboelectric charge capabilities and produced excellent 3 copies with excellent line definition, excellent half 4 tone qualities, and 1QW background toning. As can be seen from the foregoing examples, it is apparent that the 6 cure temperature is selected to be as high as possible 7 to minimize the time required to remove the last water 8 or carrier liquid but yet is not so high as to deteriorate 9 the triboelectric charge capabilities or cause agglomeration within the fluidized bed apparatus.
11 In all of the above examples and tests, the copies 12 were produced-with a-toner designated as IBM Part No.
13 1162013 and a magnetic brush developer. IBM Part No.
14 1162013 is a toner comprlsing carbon black pigment, polyvinyl stearate, a copolymer of styrene/n-butyl 16 methacrylate resin, and a non-phenolic modified maleic 17 rosin. The toner concentration in the tests referred 18 to above was .5 to 1.0 percent concentration by weight 19 and was used in an otherwise conventional developer system.
It should be understood that the core material in the 21 above examples was selected as S-70 steel shot. However, -22 the S-70 steel shot should not be considered as the only 23 acceptable substrate material. The other acceptable types 24 of material would be S-llO steel shot, 250 micron worked ~ -steel grit 450 micron worked steel grit, sand, and other 26 well-known carrier materials. The selection of the core LE9-70-~25 . ~

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- - - ~, . , - . :: . .- . . -1()8ZOZ5 1 material will depend upon the type of developer mechanism 2 employed with the carrier developer mix. It likewise 3 depends on process speed and configuration of the 4 photocopying machines. For example, if a magnetic brush developing mechanism is used, the selection 6 of the core material must be a magnetic material. - .-7 However, it should be recognized that if the developing 8 mechanism is a cascade mechanism, sand, steel shot, 9 worked grit, or other core material may be used so long as its density, size, and other properties are 11 acceptable from a dynamics standpoint. One further 12 requirement would be that any core material selected 13 must be coatable with the carboxylated polyethylene 14 ionomer in order to become a satisfactory carrier material.
16 As a preliminary to the treatment of the core 17 material with carboxylated polyethylene ionomer, the 18 core material may be pretreated with a primer material 19 or other pretreatment process to improve the adhesion to the core of the carboxylated polyethylene ionomer.
21 Examples of primers which might be used as above are 22 polyvinyl butyral resin, vinyl chloride terpolymer, and 23 a polymethylmethacrylate.
24 Pretreatments may by way of example including cleaning, pickling, etching, and roughening or other 26 surface treatments which would enhance adhesion between 27 the core and outer layer.
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' 1082~325 1 Purther, pigments or other additives may be added, 2 if desired, to the carboxylated polyethylene ionomer . 3 coating. Also, other polymers may be added to modify the 4 triboelectric behavior of the coating as is well known in the prior art, if desired.
6 While the invention has been particularly shown and 7 described with reference to preferred embodiments thereof, 8 it will be understood by those skilled in the art that 9 the foregoing and other changes in form and details may be made therein without departing from the spirit ana 11 scope of the invention.
12 What is claimed is:
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Claims (9)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A carrier material comprising a plurality of bead-like particles, each having a core portion and an outer surface layer wherein the improvement comprises:
said outer surface layer comprises a major portion of carboxylated polyethylene ionomer.

2. A carrier material of claim 1 wherein said outer surface layer comprises essentially carboxylated polyethylene ionomer.

3. The carrier material of claim 1 wherein said outer surface layer has a negative triboelectric charge with respect to the toner particles with which said carrier particles are used.

4. A carrier material as defined in claim 1 or 2 mixed with toner particles in a developer composition for use in electrophotographic apparatus to develop a latent image with the toner particles.

5. A carrier material as defined in claim 1, 2 or 3 wherein the diameter of the core portion is from about 100 to about 600 microns and the surface layer is from about 1 to about 5 microns in thickness.

6. A carrier material as defined in claim l or 2 wherein the material of the core portion is selected from the group consisting of sand, glass beads, metallic beads, metal alloys and ferromagnetic materials including iron, steel, magnetic oxides and alloys thereof.

7. The method of making an electrophotographic development carrier for use with toner particles comprising the steps of:
coating core materials with a thin layer of a water dispersion of carboxylated polyethylene ionomer;
heating the coated core materials to such a temperature as to partially dry said core material outer layer;
repeating said coating step and said heating step until a desired thickness of carboxylated polyethylene ionomer is deposited on said core material;

heating said coated core materials to a temperature and for a time sufficient to remove the carrier liquid of said dispersion and heat the core materials and core coating to a temperature such that said coating of said carboxylated polyethylene ionomer is coalesced into a smooth uniform film.

8. The method as defined in claim 7 wherein the core materials are selected from the group consisting of sand, glass beads, metallic beads, metal alloys and ferromagnetic materials including iron, steel, magnetic oxides and alloys thereof.

9. The method as defined in claim 7 or 8 wherein the diameter of the core material is from about 100 to about 600 mircrons and the core coating is from about 1 to about 5 microns in thickness.