US3595794A

US3595794A - Electrostatographic developer

Info

Publication number: US3595794A
Application number: US631193A
Authority: US
Inventors: Robert J Hagenbach; Myron J Lenhard
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1967-04-17
Filing date: 1967-04-17
Publication date: 1971-07-27
Anticipated expiration: 1988-07-27
Also published as: GB1222471A; NL6805320A; FR1582855A; AT290986B; BE713752A; DE1772219A1; GB1227471A; US3730707A; CH505410A; LU55894A1

Abstract

AN ELECTROSTATOGRAPHIC DEVELOPER MIXTURE COMPRISING FINELY-DIVIDED ELECTROSCOPIC TONER PARTICLES HAVING A SIZE LESS THAN ABOUT 30 MICRONS AND SUBSTANTIALLY HOMOGENEOUS GLASS CARRIER PARTICLES HAVING AN AVERAGE PARTICLE SIZE BETWEEN ABOUT 30 MICRONS AND ABOUT 1000 MICRONS, EACH OF THE CARRIER PARTICLES HAVING A SPECIFIC GRAVITY BETWEEN ABOUT 3.0 AND ABOUT 7.6 AND CONSISTING ESSENTIALLY OF FROM ABOUT 12 PERCENT TO ABOUT 30 PERCENT BY WEIGHT OXIDES OF SILICON AND FROM ABOUT 70 PERCENT TO ABOUT 88 PERCENT BY WEIGHT OXIDES OF LEAD AND LESS THAN ABOUT 5 PERCENT BY WEIGHT OXIDES OF SODIUM, POTASSIUM, LITHIUM AND MIXTURES THEREOF.

Description

United States Patent 3,595,794 Patented July 27, 197i 3,595,794 ELECTROSTATOGRAPHIC DEVELOPER Robert J. Hagenbach and Myron .1. Lenhard, Rochester, N.Y., assignors to Xerox Corporation, Rochester, N.Y. No Drawing. Filed Apr. 17, 1967, Ser. No. 631,193 Int. Cl. G03g 9/02 US. Cl. 252-621 3 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention relates in general to an imaging system and, more particularly, to an electrostatographic material.

Electrostatography is best exemplified by the process of xerography as first described in US. Pat. 2,297,691 to C. F. Carlson. In this process a photoconductor is first provided with a uniform electrostatic charge over its surface and is then exposed to an image of activating electromagnetic radiation which selectively dissipates the charge in illuminated areas of the photoconductor while the charge in the non-illuminated areas is retained thereby forming a latent electrostatic image. This latent electrostatic latent image is then developed or made visible by the deposition of finely-divided electroscopic marking particles referred to in 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 electroscopic image. This powdered image may then be transferred to a support surface such as paper. The transferred image may subsequently be permanently affixed to the support surface as by fusing. Instead of forming latent images by uniformly charging a photoconductive layer and then exposing the layer to a light and shadow image, an image may be formed by directly charging the layer or an insulating member in image configuration. The powder image may be fixed to the imaging layer if elimination of the powder image transfer step is desired. Other suitable means such as solvent or overcoating treatment may be substituted for the foregoing heat fixing steps.

Several methods are known for applying the electroscopic particles to the latent electrostatic image to be developed. One well-known commercial method for developing electrostatic images is the cascade process disclosed by L. E. Walkup in U.S. Pat. 2,618,551 and E. N. Wise in US. Pat. 2,618,552. In this method a developer material comprising relatively large carrier beads having fine toner particles electrostatically coated thereon is conveyed to or 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 image cascades or rolls across the image bearing surface, the toner particles are electrostatically deposited and secured to the charged portion of a 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 the rolling carrier, due apparently to the greater electrostatic attraction between the toner and carrier than between the toner and the discharged background. The carrier and excess toner are then recycled.

In most commercial processes the cascade technique is carried out in automatic machines. In these machines small buckets on an endless belt conveyor scoop the developer mixture comprising relatively large carrier beads and smaller toner particles and convey it to a point above an electrostatic image bearing surface where the developer mixture is allowed to fall and roll by gravity across the image bearing surface. The carrier beads along with any unused toner particles are then returned to the sump for recycling through the developing system. Small quantities of toner material are periodically added to the developer mixture to compensate for the toner depleted during the development process. 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 mixture. It is apparent that in this process, as well as in other development techniques, the developer mixture is subjected to a great deal of mechanical attrition which tends to degrade both the toner and carrier particles. This degradation, of course, occurs primarily as a result of shear and impact forces due to the tumbling of the developer mixture on the image bearing plate and the movement of the bucket conveyor through the developer material in the sump.

In prior art processes both coated and uncoated carrier beads were employed with varying degrees of success. Coated carrier beads are subjected to deterioration or degradation characterized by the separation of portions of or the entire carrier coating from the carrier core. The separation may be in the form of chips, flakes, or entire layers and is primarily caused by poorly adhering coating materials which fail upon impact and abrasive contact with machine parts and other carrier particles. Carriers having coatings tend to chip and otherwise separate from the carrier core and must be frequently replaced, thereby increasing expense and consuming time. Print deletion and poor print quality occur when carrier particles having damaged coatings are not replaced. Fines and grit formed by the carrier coating disintegration tend to drift and form unwanted deposits on critical machine parts. In addition, the triboelectric properties of the carrier material varies with deterioration of the coating resulting in poor print quality.

Uncoated carrier beads on the other hand have three main deficiencies. First, often they lack the weight required to insure against adherence of the granular carrier material to the charged plate. Desirably, the specific gravity of the carrier material should be between about 3 and about 8. Heavier carrier bead materials cause impact damage to the surface of the image bearing layer. Secondly, the prior art uncoated carrier materials lacked the triboelectric qualities required of an electrostatographic material. Problems encountered when carrier materials lack these properties are set out in the following discussion. In the reproduction of high contrast copies such as letters, tracings and the like, it is desirable to select the electroscopic powder and carrier materials so that their mutual electrification is relatively large; the degree of such electrification being determined in most cases by the distance between their relative positions in the triboelectric series. However, when otherwise compatible electroscopic powder and carrier materials are removed from each other in the triboelectric series by too great a distance, the resulting images are very faint because the attractive forces between the carrier and toner particles compete with the attractive forces between the latent electrostatic image and the toner particles. Although the image density described in the immediately preceding sentence may be improved by increasing the toner concentration in the developer mixture, undesirably high background toner deposition as well as increased toner impaction and agglomeration is encountered when the developer mixture is overtoned. The initial electrostatographic plate charge may be increased to improve the density of the deposited powdered image but the plate charge would ordinarily have to be excessively high in order to attract the electroscopic powder away from the carrier particle. Excessively high electrostatographic plate charges are not only undesirable because of the high power consumption necessary to maintain the electrostatographic plate at high potentials but also because a high potential causes the carrier particles to adhere to the electrostatographic plate surface rather than merely roll across and off the electrostatographic plate surface. Print deletion and massive carryover of carrier particles often occur when carrier particles adhere to reusable electrostatographic imaging surfaces. Massive carrier carryover problems are particularly acute when the developer is employed in solid area coverage machines where excessive quantities of toner particles are removed from carrier particles thereby leaving many carrier particles substantially bare of toner particles. Further, adherence of carrier particles to reusable electrostatographic imaging surfaces promotes the formation of undesirable scratches on the surfaces during image transfer and surface cleaning operations. It is, therefore, apparent that many materials which otherwise have suitable properties for employment as carrier particles are unsuitable because they possess too high a triboelectric value. Desirably, the triboelectric value for conventional electrostatography measured in micro-coulom-bs per gram of toner should be between 8 and 30.

Finally, the triboelectric value of a carrier material should not be significantly affected by ambient humidity conditions since such effect would destroy print quality at higher humidities and complicate machine design and operation, prior art uncoated glass materials were never commercially successful because of their great humidity sensitivity.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide a system for developing electrostatic latent images which overcome the above noted deficiencies.

It is another object of this invention to provide a system for developing electrostatic latent images which is relatively insensitive to humidity conditions.

It is another object of this invention to provide a carrier materal which is relatively resistant to abrasion.

It is another object of this invention to provide a homogeneous carrier material which has relatively improved triboelectric properties.

It is another object of this invention to provide a homogeneous carrier material which has a relatively high density.

The foregoing objects and others are accomplished in accordance with this invention by an electrostatic latent image developing system wherein an electrostatic latent image is formed on an imaging surface and the electrostatic image is then contacted with a developer mixture comprising finely-divided particles of toner electrostatically adhering to the surface of uncoated glass carrier beads. The carrier beads in general are a homogeneous mixture of a glass composition comprising from about to about 45 parts oxides of silicon and from about 20 parts to about 95 parts oxides of lead. A portion of the toner particles are attracted to and held on the surface of the image bearing member in accordance with the electrostatic latent image forming a visible image.

The preferred composition comprises about 13 to 17 parts oxides of silicon and from about 83 parts to 87 parts oxides of lead.

The uncoated carrier beads of this invention have a specific gravity of from about 3.0 to about 7.6 and produce high quality images over a wide range of ambient humidity conditions.

Although other components may be present to aid in manufacture of the glass beads or as natural impurities resulting from the manufacture of the beads, it has been found that the presence of over about 5 percent by weight, either singularly or in combination, of the oxides of sodium, potassium, and lithium cause the carrier bead material to be sensitive to high ambient humidity. That is, the presence of these materials imparts to the carrier beads humidity sensitivity which prevents the use of such beads in electrostatographic processes which are exposed to high ambient humidity.

The carrier bead diameter of from about 30 microns to about 1,000 microns is preferred for xerographic use because the bead then possesses sufficient inertia to avoid adherence to the latent electrostatic images.

Any suitable pigmented or dyed electroscopic toner material may be employed with the uncoated carriers of this invention. Typical toner materials include: gum copal; gum sandarac; rosin; cumaromeindene resin; asphaltum; uintaite; phenol formaldehyde resins; rosin modified phenol formaldehyde resins; methacrylic resins; polystyrene resins; polypropylene resins; epoxy resins; polyethylene resins and mixtures thereof. The particular toner material to be employed depends upon the separation of the toner particles from the treated carrier beads in the triboelectric series and whether a negatively or positively charged image is to be developed. Among the patents describing electroscopic toner compositions are US. Patent 2,659,670 to Copley; U.S. Pat. 2,753,308, to Landrigan; U.S. Pat. 3,079,342 to Insalaco; US. Pat. Reissue 25,136 to Carlson, and US. Pat. 2,788,288 to Rheinfrank et a1. These toners generally have an average particle diameter between about 1 and 30 microns. A toner comprising a styrene-N-butyl methacrylate copolymer, polyvinylbutyral and carbon black produced by the method disclosed by M. A. Insalaco in Example I of US. Pat. 3,079,342 is preferred because of its excellent tribo electric qualities and its deep black color.

In conventional xerography a uniform positive charge is placed on the surface of a photoconductive member. Subsequent exposure to a light image discharges the exposed areas of the plate. A powder material is then chosen which is triboelectrically negative in respect to the carrier bead material of this invention. However, the advantages of this invention apply equally to the development of latent electrostatic images which are negative. In the case where the latent image is comprised of negative electrostatic charges, an electroscopic powder and carrier com bination should be selected in which the powder is triboelectrically positive to the carrier material. The selection of suitable toner materials can be made by anyone skilled in the art from the many materials that have been tested and occupy recognized positions in the triboelectric series. The dimension of the charge acquired by any particular toner through triboelectric contact with any designated carrier material is easily and readily determined by the test method described more fully herein.

DESCRIPTION OF PREFERRED EMBODIMENTS The following examples further specifically illustrate the present invention. The examples below are intended to illustrate the various preferred embodiments of the improved carrier materials. The parts and percentages are by weight unless otherwise indicated.

In the following examples the triboelectric values are determined as follows: The relative triboelectric values generated by contact of carrier beads with toner particles are measured by means of a Faraday Cage. The device comprises a brass cylinder having a diameter of 1 inch and a length of 1 inch. A mesh screen is positioned at each end of the cylinder. The cylinder is weighed, charged with about 2 grams of a mixture of carrier and toner particles and connected to ground through a capacitor and an electrometer connected in parallel. Dry compressed air is then blown through the brass cylinder to drive all the toner from the carrier. The charge on the capacitor is then read on the electrometer. Next, the chamber is reweighed to determine the weight loss. The resulting data is used to calculate the toner concentration and the charge in micro-coulombs per gram of toner. Since triboelectric measurements are relative, the measurements should for comparative purposes be conducted under substantially identical conditions. Thus, a toner comprising a styrene-nbutyl methacrylate copolymer, polyvinylbutyral and carbon black produced by the method disclosed by M. A. Insalaco in Example I of US. Pat. 3,079,342 is used as a contact triboelectrification standard and as toner in all of the examples. Obviously, other suitable toners such as those listed above may be substituted for the toner used in the examples.

EXAMPLE I A developer mixture is produced by mixing one part colored styrene copolymer toner particles having an average particle size of about 10 to about 20 microns with about 100 parts glass carrier bead particles having an average particle size of about 600 microns. The glass bead composition comprises about 71% S10 about 2% A1 about 13% CaO and about 14% Na O and has a specific gravity of about 2.4.

The following tests are run at an ambient temperature of about 70 F. and an ambient relative humidity of about 40%.

The relative triboelectric value of the carrier measured by means of a Faraday Cage is about 9 micro-coulombs per gram of toner. The developer mixture is cascaded across an imaging surface bearing a positively charged electrostatic image. The resulting developed image is transferred by electrostatic means to a sheet of paper whereon it is fused by heat. The resulting image is dense and substantially free of background toner deposits.

EXAMPLE II The experiment of Example I is repeated except that the tests are performed in the presence of an ambient temperature of about 70 F. and an ambient relative humidity of about 80%. The relative triboelectric value of the carrier measured by means of a Faraday Cage is about 3 micro-coulombs per gram of toner. The resulting fused image prepared as in Example I is characterized by a faded, washed out appearance and has high background toner deposits.

EXAMPLE III A developer mixture is produced by mixing one part colored sytrene copolymer toner particles described in Example I with about 100 parts glass carrier bead particles having an average particle size of about 600 microns.

The glass bead composition comprises about 71 SiO about 10% Na O, about 7% K 0 and about 12% PbO and has a specific gravity of about 2.8.

The following tests are run at an ambient temperature of about 70 F. and an ambient relative humidity of about 40% The relative triboelectric value of the carrier measured by means of a Farady Cage is about 10 micro-coulombs per gram of toner. The developer mixture is cascaded across an imaging surface bearing, a positively charged electrostatic image. The resulting developed image is transferred by electrostatic means to a sheet of paper whereon it is fused by heat. The resulting image is dense and is substantially free of background toner deposits.

EXAMPLE IV The experiment of Example III is repeated except that the tests are performed in the presence of an ambient temperature of about 70 F. and an ambient relative humidity of about 80%. The relative triboelectric value of the carrier measured by means of a Faraday Cage is about 4 micro-coulombs per gram of toner. The resulting fused image prepared as in Example II is dense but possesses a relatively high background toner deposition.

EXAMPLE V A developer mixture is produced by mixing one part colored styrene copolymer toner particles described in Example I with about 75 parts glass carrier bead particles having an average particle size of about 600 microns.

The glass bead composition comprises about SiO about 13% Na O, about 30% BaO and about 7% TiO and has a specific gravity of about 3.5.

The relative triboelectric value of the carrier measured by means of a Faraday Cage is about 4.8 microcoulombs per gram of toner. The developer mixture is cascaded across an imaging surface bearing a positively charged electrostatic image. The resulting developed image is transferred to a sheet of paper whereon it is fused by heat. The resulting fused image is characterized by very high background and poor resolution.

EXAMPLE VI The experiment of Example V is repeated except that the tests are performed in the presence of an ambient temperature of about 75% and an ambient relative humidity of about 85. The resulting fused image prepared as in Example V is characterized by extremely poor resolution and very high background.

EXAMPLE VII A developer mixture is produced by mixing one part colored styrene copolymer toner particles described in Example I with about 200 parts glass carrier bead particles having an average particle size of about 400 microns. The glass bead composition comprises about 35 percent Si0 and about percent PhD and has a specific gravity of about 4.7.

The following tests are run at an ambient temperature of about F. and about 40 percent ambient relative humidity.

The relative triboelectric value of the carrier measured by means of a Faraday Cage is about 20.0 micro-coulombs per gram of toner. The developer mixture is cascaded across an imaging surface bearing a positively charged electrostatic image. The resulting developed image is transferred to a sheet of paper whereon it is fused by heat. The resulting fused image has good resolution, good density and substantially no background.

EXAMPLE VIII The experiment of Example VII is repeated except that the images are produced in the presence of an ambient temperature of about F. and an ambient relative humidity of about percent. The resulting fused image prepared as in Example VII has good resolution, good density and substantially no background.

EXAMPLE IX A developer mixture is produced by mixing one part colored styrene copolymer toner particles described in Example I with about 200 parts glass carrier bead particles having an average particle size of about 250 microns.

The glass bead composition comprises about 15 percent SK); and about 85 percent PbO and has a specific gravity of about 6.0.

The following tests are run at an ambient temperature of about 70 F. and an ambient relative humidity of about 60 percent.

The relative triboelectric value of the carrier measured by means of a Faraday Cage is about 15.0 micro-coulombs per gram of toner. The developer mixture is cascaded across an imaging surface bearing a positively charged electrostatic image. The resulting developed image is transferred by electrostatic means to a sheet of paper whereon it is fused by heat. The resulting fused image has excellent resolution, excellent density and no background.

EXAMPLE X The experiment of Example IX is repeated except that images are produced in the presence of an ambient temperature of about 90 F. and an ambient relative humidity of about 85 percent. The resulting fused image as prepared in Example IX has excellent resolution, excellent density and substantially no background.

EXAMPLE XI A developer mixture is produced by mixing one part colored styrene copolymer toner particles described in Example I with about 200 parts glass carrier bead particles having an average particle size of about 250 microns.

The glass bead composition comprises about 32 percent SiO and about 42 percent PbO and has a specific gravity of about 5.2. The relative triboelectric value of the carrier measured by means of a Faraday Cage is about 23.5 micro-coulombs per gram of toner.

The following images are produced in the presence of relative humidity of about 40 percent.

The developer mixture is cascaded across an imaging surface bearing a positively charged electrostatic image. The resulting developed image is transferred by electrostatic means to a sheet of paper whereon it is fused by heat. The resulting fused image has good resolution, good density and substantially no background.

EXAMPLE XII The experiment of Example XI is repeated except that images are produced in the presence of an ambient temperature of about 70 F. and an ambient relative humidity of about 85 percent. The resulting fused image prepared as in Example XI has good resolution, good density and substantially no background.

EXAMPLE XIII A developer mixture is produced by mixing one part colored styrene copolymer toner particles described in Example I with about 200 parts glass carrier bead particles having an average particle size of about 600 microns.

The glass bead composition comprises about 25 percent SiO about 70 percent PbO and has a specific gravity of about 5.5.

The following images are produced in the presence of an ambient temperature of about 75 F. and an ambient relative humidity of about 40 percent. The developer mixture is cascaded across an imaging surface bearing a positively charged electrostatic image. The resulting developed image is transferred by electrostatic means to a sheet of paper whereon it is fused by heat. The resulting fused image has excellent resolution, excellent density and no background.

EXAMPLE XIV The experiment of Example XIII is repeated except that the images are produced in the presence of an ambient temperature of about 80 F. and an ambient relative humidity of 85 percent. The resulting fused image prepared as in Example XIII has excellent resolution excellent density and no background.

EXAMPLE XV A developer mixture is produced by mixing one part colored styrene copolymer toner particles described in Example I with about 100 parts glass crushed frit particles having an average particle size of about 500 microns. The glass composition comprises about 8 percent SiO 8 and about 92 percent PbO and has a specific gravity of about 7.5.

The following tests are run at an ambient temperature of about F. and about 60 percent ambient relative humidity.

The developer mixture is cascaded across an imaging surface bearing a positively charged electrostatic image. The resulting developed image is transferred to a sheet of paper whereon it is fused by heat. The resulting fused image has excellent resolution, excellent density and substantially no background.

EXAMPLE XVI The experiment of Example XV is repeated except that the images are produced in the presence of an ambient temperature of about 75 F. and an ambient relative humidity of about 85 percent. The resulting fused image prepared as in Example XV has good resolution, good density and substantially no background.

EXAMPLE XVII A developer mixture is produced by mixing one part colored styrene copolymer toner particles described in Example I with about 100 parts glass carrier bead particles having an average particle size of about 450 microns.

The glass bead composition comprises about 15 percent SiO and about 85 percent PhD and has a specific gravity of about 6.6.

The following tests are run at an ambient temperature of about 75 F. and an ambient relative humidity of about 60 percent.

The relative triboelectric value of the carrier measured by means of a Faraday Cage is about 14 microcoulombs per gram of toner. The developer mixture is cascaded across an imaging surface bearing a positively charged electrostatic image. The resulting developed image is transferred by electrostatic means to a sheet of paper whereon it is fused by heat. The resulting fused image has excellent resolution, excellent density and no background.

EXAMPLE XVIII The experiment of Example XVII is repeated except that images are produced in the presence of an ambient temperature of about F. and an ambient relative humidity of about percent. The resulting fused image as prepared in Example XVII has excellent resolution, excellent density and substantially no background.

EXAMPLE XIX A developer mixture is produced by mixing one part colored styrene copolymer toner particles described in Example I with about parts glass carrier bead particles having an average particle size of about 5 50 microns.

The glass bead composition comprises about 23 percent Si0 and about 77 percent PhD and has a specific gravity of about 5.9.

The following images are produced in the presence of an ambient temperature of about 75 F. and an ambient relative humidity of about 60 percent.

The relative triboelectric value of the carrier measured by means of a Faraday Cage is about 14.2 micro-coulombs per gram of toner. The developer mixture is cascaded across an imaging surface bearing a positively charged electrostatic image. The resulting developed image is transferred by electrostatic means to a sheet of paper whereon it is fused by heat. The resulting fused image has good resolution, good density and substantially no background.

EXAMPLE XX The experiment of Example XIX is repeated except that images are produced in the presence of an ambient temperature of about 75 F. and an ambient relative humidity of about 85 percent. The resulting fused image prepared as in Example XIX has good resolution, good density and substantially no background.

Although specific components and proportions have been stated in the above description of preferred embodiments of the invention, other typical materials as listed above Where suitable may be used With similar results. In addition, other materials may be added to the mixture to syner-gize, enhance, or otherwise modify the properties of the carrier beads. For example, a material to improve the sphericity of the beads may be incorporated during manufacture.

Other modifications and ramifications of the present invention will occur to those skilled in the art upon a reading of this disclosure. These are intended to be included within the scope of this invention.

What is claimed is:

1. An electrostatographic developer mixture comprising fine1y-divided electroscopic toner particles having an average particle size less than about 30 microns electrostatically adhering to the surface of substantially homogeneous glass carrier particles size between about 30 microns and about 1000 microns, each of said toner particles comprising a resin and a colorant selected from the group consisting of dyes and pigments and each of said homogeneous glass carrier particles having a specific gravity between about 3.0 and about 7.6 and consisting essentially of from about 12 percent to about 30 percent by weight oxides of silicon, from about 70 percent to about 10 88 percent by Weight oxides of lead and less than about 5 percent by weight of oxides of metals selected from the group consisting of sodium, potassium, lithium and mixtures thereof.

2. An electrostatographic developer mixture according to claim ll wherein said carrier particles consist essentially from about 13 percent to about 17 percent by Weight oxides of silicon and from about 83 percent to about 87 percent oxides of lead.

3. An electrostatographic developer mixture according to claim 1 wherein said toner particles comprise colored styrene copolymer particles.

References Cited UNITED STATES PATENTS 3,326,848 6/1967 Clemens et a1. 26041 2,618,551 11/1952 Walkup 252-62.l

FOREIGN PATENTS 557,577 5/1959 Canada 252--62.1

GEORGE F. LESMES, Primary Examiner I. P. BRAMMER, Assistant Examiner UJS. Cl. X.R.

2 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 5 I 794 d July 27 I 19 71 Invefitofls) Robert JL Hagenbach et al It is' certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

IN THE CLAIMS; Column 9, line 22, after "particles" insert -1 1avin g an average particle- Signed and sealed this 16th day of May 1972.

(SEAL) Attest:

ROBERT GOITSCHALK EDWARD M.FLETCHER,JR.

Comissioner of Patents Attesting Officer