US 3278439 A
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Description (OCR text may contain errors)
@et H, 1966 R. G. BLANCHETTE ETAL Bgg DEVELOPER MIX Filed sept. 1o. 1963 6 Patented Oct. 11, 1966 3,278,439 DEVELOPER MIX Robert George Blanchette, Des Plaines, and Loren lE.
Sheltie, Palatine, Ill., assignors to Addressograph-Multigraph Corporation, a corporation of Delaware Filed Sept. 10, 1963, Ser. No. 307,951 7 Claims. (Cl. 252-621) This invention relates to improved developer mixes for developing electrostatic prints and more particularly to improvements in magnetically attractable carrier particles useful in the magnetic brush type development of electrostatic latent images.
Electrophotographic processes involve the conversion of a light image into an electrostatic latent image orl charge pattern on a suitable photoconductive insulating material, such as, for example zinc oxide particles suspended in a resin binder. The latent electrostatic image is rendered visible by a development step in which the charged surface is brought into intimate physical contact with a suitable developer mix and the charged surface can be accomplished by means of the well known magnetic brush type applicator. Suitable developer mix compositions are also well known in the art and generally contain dyed or colored pigmented thermoplastic powders, known either as toner particles, or as electroscopic powders. Said toner particles are admixed with larger carrier particles such as, for example, iron powder to produce a satisfactory magnetic brush type developer mix.
Developer mixes can be formulated so that the toner carries a negative or positive charge. A typical positive developer is formulated from toner particles comprising a carbon black pigmented polystyrene resin admixed with carrier particles which can be iron powder, steel filings, magnetite or ferrite particles, or other finely divided magnetic material. The thermoplastic materials and the carrier particles are selected on the basis of their relative positions in the triboelectric series. In this way, the toner particles acquire the proper charge with respect to the latent electrostatic image. When the developer brush is brought into rubbing contact with the charged surface of the insulating layer of the electrophotographic member, the greater attractive force of the electrostatic image causes the toner particles t leave the carrier particles of the brush and adhere to the image areas of the electrophotographic surface.
Apparatus for applying the developer mix to the latent electrostatic image is set forth in U.S. Patent No. 3,003,- 462 wherein the magnetic brush technique is employed. A magnetic brush typically consists of a non-magnetic rotatably mounted tube or cylinder having fixed magnetic means mounted inside. The tube can be arranged to rotate with part of its surface immersed in a supply of developer mix, or the mix can be supplied to the tube surface by other means. The granular mass comprising the mix is magnetically attracted to the surface of the tube. As the developer mix comes within the influence of the field generated by the magnetic means within the tube, the particles thereof arrange themselves in bristle-like formations resembling a brush. The bristle formations of developer mix tend to conform to the lines of magnetic flux, standing erect in the vicinity of the poles and lying substantially fiat when said mix is outside the environment of the magnetic poles. Within one revolution the continually rotating tube picks up developer mix from a supply in a trough and returns part or all of this material to the trough. This mode of operation assures that fresh mix is always available to the copy sheet surface at its point of contact with the brush. In a typical rotational cycle, the roller performs the successive steps of developer mix pickup, brush formation, brush contact with the copy sheet, brush collapse, and finally mix release.
The electrostatic printing process as carried out by the above described brush development technique leaves much to be desired in the quality and fidelity of the finished print. Electrostatic reproductions of originals which have large solid image areas thereon are susceptible to a defect known as edge effect. The electroscopic powder is not uniformly deposited; most 0f it adheres to the edges of these solid areas leaving the central zones of poor density.
Another serious defect of prior art developer mixes relates to the mix life or the useful life of the mix. By mix life is meant the total number of reproductions that may be put through the developer apparatus before the developer mix becomes inoperable. The latter is defined as a spent mix or a fatigued mix.
Various attempts have been made to correct these disadvantages. Among these are the addition of large, filamentary, electrically conductive particles of the mix to correct edge effect. These are particularly unsuitable for magnetic brush type development where micro sized particles are required and where the unique properties of the brush must not be disrupted.
In accordance with the present invention there has been discovered a new carrier particle for developing electrostatic images, said particle having certain physical characteristics relating to particle shape and size as well as certain electrical properties. The newly discovered carrier particles as disclosed herein give rise to improved developer mixes when combined with suitable toners for electrostatic printing in general.
Accordingly, it is a general object of this invention to provide an improved developer mix for developing latent electrostatic images.
Another object of this invention is to provide an irnproved developer mix which forms a magnetic brush capable of developing uniformly dense electrostatic images.
A further object of this invention is to provide an irnproved developer mix suitable for magnetic brush type developing techniques capable of uniformly developing large solid image areas.
A specific object of this invention is to provide a new carrier particle which when incorporated with an electroscopic powder gives rise to a developer mix capable of forming a magnetic brush having soft, dense, long, flexible bristles for developing dense and high fidelity reproductions.
Additional objects and advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description in connection with the accompanying drawing in which:
FIGURE 1 illustrates the brush formation in accordance with the present invention;
FIGURE 2 illustrates the brushing action of the magnetic brush during development of the photocon- -ductive member;
FIGURE 3 illustrates the front or top view of the carrier particles comprising this invention;
FIGURE 4 illustrates the side view of the carrier particles of FIGURE 3; and
FIGURE 5 is a greatly enlarged diagrammatic view of the individual bristle formation comprising the brush of FIGURE l.
In ac-cordance with the instant invention, carrier particles of magnetically attractable material are employed which are irregular in shape. For the purpose of this invention an irregular shape is defined as having surfaces which are indented, pitted, and fssured, including surfaces having open areas therethrough.
The carrier particles are in the form of thin, irregular chips or broken turnings which are preferably curved in more than one direction. The particles are similar to broken potato chips which have curled-up edges, but are more irregular in shape in that they contain holes, indentations, jagged edges, tears and rips. It is intended that particles having an irregular shape include acicular and ribbon-like shapes and in all instances the particles are of foil-like thickness (i.e., 3 to 20 microns), the edges of which are jagged with sharp protrusions extending therefrom at random angles. Such irregular shaped particles are produced from certain grinding and polishing operations carried out on metal surfaces as well as cutting and trimming metal parts. Irregularly shaped particles may also be produced by processing certain powders in a ball mill or other crushing equipment.
It has been found that the magnetically attractable material should have a specific resistivity less than rl ohmcm. of a measured volume of material after agitation for 40 hours in a rotary mill; preferably less than 103 ohm-cm. The resistivity measurement as a parameter is taken of a 16 cubic centimeters volume of granular mass that is compacted by a force of 125 grams/ cm?. Characteristic of developer mixes which have exhibited poor mix life is a bulk electrical resistivity in excess of 1012 ohm-cm. where the carrier is present in the range of -25 parts per part of toner. Different concentrations of carrier and toner will understandably have different resistivities. Although initial resistivity might be in the preferred range, this high value is attributed to the gradual increase in resistivity of the carrier particle as the mix is agitated by the apparatus.
Magnetically attractable powders having an apparent density in the range of 0.75 to 1.75 when measured in accordance with ASTM Method B-212-48 are preferred. The low apparent density is indicative of the open and irregular shape required for the carrier particles of this invention.
Referring to FIGURE 1, the magnetic brush applicator is identified generally as 20 with the developer mix of this invention 22 being formed in a brush configuration 24 comprising bristle-like formations 26 which emanate from a solid base 28 of developer mix clinging to the periphery of the non-magnetic tube 30. A magnet 21 is fixedly mounted inside the tube 30 on a supporting member 23 to which it is affixed with tape 25. The member 23 is supported by a shaft 27 and the tube 30 is supported by an end ring 29 and a bearing 31. As the tube rotates the brush 24 is formed along the lines of magnetic fiux.
Referring to FIGURE 2, the preferred brush as illustrated by the bristles 26 is brought into engagement with the photoconductive member 32, having a photoconductive layer 33 such as a ZnO in a resin binder, moving in the direction of rotation of the applicator 20, but at a much slower speed. The greatly improved development due to the irregular shape of the carrier particles is best explained by considering FIGURES 3 and 4. FIGURE 3 shows several typical irregularly shaped carrier particles 34, 36, 38 and 40, each comprising a main body portion A having generally laterally extending face portions 42, 44, 46 and 48, having edge formations 50, 52 and 56, surrounding each of the respective particles. Particle 34 is typical of an irregular carrier particle having Widely distributed indentations B throughout its face portion 42, as well as pitted areas C and fissures D going through the main body portion A. Particle 36 is illustrative of an irregularly shaped particle having a large opening E in its face portion 44, as well as protrusions F extending outward from its edge portions 52 and from its main body portion A. It will be noted that the protrusions F and the edge portions 52 are jagged and sharp. Particle 38 is dendritic in character with its face portion 46 having indentations B, pitted areas C and ssures D extending through its thickness. Irregular particle has a main body portion A which is ribbon-like in character and having the indentations B, pitted areas C and fissures through its face portion 48 and additionally having hooklike protrusions F extending from its main body portions as well as its edges 56. It should be pointed out that the main body portions A in all cases do not lie in one plane nor are the face portions planar with one another, but rather tend to be `dished out with the edge formations turning up away from the respective face portions of each particle. In a similar fashion the protrusions F extend in a random fashion from the main body and edge portions as best shown in FIGURE 4.
Irregular shape contributes significantly to the ability of the developer mix to form a brush having elongated bristle-like structures 26 as shown in FIGURE 5. The carrier particles combine with one another tending to interlock through the various protrusions F through the fissures C and the apertures D permitting the formed bristle a high degree of flexibility without breaking as it engages and brushes against the photoconductive paper 32.
Referring to FIGURE 5, the presence of the profusion of indentations in the face portions 42, 44, 46 and 48 permits the carrier particle to carry a greater concentration of toner particles 56 on its surface, and hence on the bush overall, than would be expected for an equivalent mass of magnetically attractable particles that are spheroidal, cubical or otherwise regular in shape. It is believed that this capability contributes significantly to the ability of the developer mix of the instant invention to provide imatges having greater density.
It has been suggested that the surprising and unexpecting yresults obtained in print quality using irregularly shaped particles is attributable to .the interlocking that occurs between the carrier particles themselves. Accordingly, the photoconductive layer when in engagement with such a brush comprised of the interlocking carrier particles is thoroughly brushed in one pass with the more iiexible bristle formations 26 as shown in FIGURE 2. With greater flexibility and with better anchorage of the individual bristles 26 and the base layer 28, there is achieved a much more dense and softer brush producing a suitably dense image without abrasion of the sensitive zinc oxide-resin binder layer 33.
-It Will -be appreciated that the developer mix in a magnetic brush apparatus experiences continual agitation and mixing between the magnetically attractable particles and the thermoplastic resin toner such that the carrier will acquire a covering or deposit of the thermoplastic material thereon. Since the specific resistivity of the thermoplastic layer is in the order of 109 to 1013 ohm-cm. the electrical resistance of the magnetically attractable mate-rial (e.g., iron) is somewhat aliected. Where the surface of the carrier particle becomes coated with a layer of the resinous material ythe triboelectric attractive force iswdiminished since the triboelectric force of attraction depends on contact between the similar materials. In the circumstance where irregularly shaped particles are employed, the developer mix life is greatly extended since the tendency for the carrier particle to become coated with the resinous material occurs over a much longer period of time as opposed .to generally uniformly shaped particles such as spheres or rods and the like, which would collect a uniform `film of resin on one or more smooth surfaces.
The apparent density described above, that is, the low mass per unit volume of the irregularly shaped carrier particles, is believed to aid in transmitting the fine toner throughout the mixture. This permits the two components of the developer mix to be uniformly distributed, particularly when fresh toner is added to replenish that amount which is removed by printing. The low apparent density of the carrier particles also permits them to form longer bristle formations, said formations being more strongly held onto the periphery of the tube 30 for a given strength of magnetic field due to their lower mass. Here again the quality of the brush due to the irregularity of the carrier particles is significantly enhanced.
The foregoing theories of operation 'have been presented in order to better explain the unique results relating to the use of irregularly shaped carrier particles. However, it is not intended that they represent the only explanation for these observed improvements. 'It is intended that this invention not be limited by a possible theory of operation since there may be other explanations not considered here for the advancements achieved in the use of irregular shaped particles.
In carrying out the principles of the invention, a magnetically attractable powder which is comprised predominantly of iron, nickel or cobalt-steel and having the following characteristics was mixed with a suitable thermoplastic resin in the ratio of 30 parts carrier to 1 part toner:
Average particle size-25 microns to 150 microns.
-Percent non-magnetic material-less than 1%.
Apparent density-0.75 to 1.75.
Specific resistivity-ess than 10I ohm-cm.
The thermoplastic resin constituent of the developer mix can be any resin hav-ing a specic resistivity greater than l `105 ohm-cm. which can be fused at a temperature below lthe char point of paper. Suitable resins are polystyrene resins and blends thereof, acrylic resins, rosin, asphalt, tgilsonites, polyvinyl resins, and other thermoplastic resins having a softening point within the range of 50 C. to 140 C., preferably 95 C. to 140 C. and having the indicated resistance characteristics. The resins are pigmented or colored with carbon black or other suitably colored dyes or pigments. In the case of polystyrene, an amount of pigment as much as 6% of the amount of resin can be used.
In the preparation of the electroscopic powder, the resin constituents of the toner are rendered molten, and the pigment and/ or dye are added in an amount ranging from about 1% to about 17% of the weight ofthe amount of resin employed, preferably in the range of from 3% to 6% pigment and/ or dye by weight of resin. The pigment and/ or dye are distributed throughout the molten resin so that a homogeneously colored solution or dispersion is achieved. After cooling the material is ground or milled to the particle size range of from l to 50 microns, preferably in the range of from to 25 microns in size. In the final step, the pigmented resin particles are combined in the ratio of 1 part electroscopic powder to from 5 to 50 parts by weight of carrier particles, preferably in the range of from 10 to 20 parts by weight of carrier per part of electroscopic powder.
The resulting developer mix employing the irregularly shaped carrier particles of this invention produced electrostatic prints whose image density corresponded to the density of the image on the original and produced in excess of 50,000 electrostatic prints such that the prints at the 50,000 level were of a quality comparable to the rst print produced from said material. All of the advantages and desirable properties such as improved brush formation were present when using the above developer mix employing the irregularly shaped carrier particles.
The detail characteristics embodying the preferred carrier particles of this invention has been set forth in terms of the particle shape, size and certain of its electrical properties. Magnetically attractable irregularly shaped particles in accordance with this invention comprise unique carrier particles for electrostatic development procedures affording uniformly dense reproductions of large areas by virtue of the improved type of magnetic brush which is created; the true scope of the invention being defined in the appended claims.
What is claimed as new and desired to be secured as Letters Patent of the United States is:
1. A developer mix for developing a visible image on an electrophotographic member having an electrostatic charge pattern thereon, said mix comprising a granular mass of 1 part of electroscopic thermoplastic powder in physical mixture with from 5 to 50 parts of magnetically attractable carrier particles, said carrier particles having irregularly shaped main body portions of Ifoil-like thickness in the range from about 3 to 20 microns, said main body portions having ssured, dented, and pitted surfaces and protrusions extending from said main body portion whereby said particles link, interlock, intertwine and otherwise form into a strong flexible bristle-like magnetic brush formation.
2. A developer mix in accordance with claim 1 in which the carrier particle is a magnetically attractable material selected from the group consisting of iron, nickel, cobalt, magnetites, and ferrites.
3. A developer mix in accordance with claim 1 wherein the carrier particles range from 25 microns to 150 microns in average size dimension.
4. A developer mix for developing a visible image on an electrophotographic member having an electrostatic charge pattern thereon, said mix comprising a granular mass including l part electroscopic thermoplastic powder in physical mixture with from 5 to 50 parts of magnetically attractable carrier particles, said carrier particles being of an irregular shape consisting essentially of a main body portion having tissured face portions and -being of a foillike thickness in the range `from about 3 to 20 microns with protrusions extending in random directions from said main body portion, said carrier particles having an apparent density in the range of from 0.75 to 1.75, whereby said carrier particles form into a strong flexible bristle-like magnetic brush.
5. A developer mix for developing a visible image on an electrophotographic member having an electrostatic charge pattern thereon, said mix comprising a granular mass including 1 part electroscopic thermoplastic powder in physical mixture with from 5 to 50 parts of magnetically attractable carrier particles, said carrier particles being of an irregular shape consisting essentially of a main body portion having fissured, dented, and pitted face portions and being of a foil-like thickness in the range from about 3 to 20 microns with protrusions extending in a random fashion from said main body portions, said carrier particles having an apparent density in the range of from 1.3 to 1.75 and a specic resistance of less than 107 ohm-cm. whereby said carrier particles form into a strong exible bristle-like magnetic brush.
6. A developer mix in accordance with claim 5 wherein the resistance of said carrier is in the range of from 50 ohm-cm. to 103 ohm-cm.
7. A developer mix in accordance with claim 4 wherein the apparent density of the carrier particles is about 1.5.
References Cited by the Examiner UNITED STATES PATENTS 2,456,313 12/ 1948 Pratt. 2,846,333 8/1958 Wilson. 2,874,063 2/ 1959 Greig 117-17.5
FOREIGN PATENTS 686,466 1/ 1953 Great Britain.
LEON D. ROSDOL, Primary Examiner.
JULIUS GREENWALD, ALBERT T. MEYERS,
Examiners. I. D. WELSH, Assistant Examiner.