Classifications

• • G—PHYSICS
  • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
  • G03G—ELECTROGRAPHY; ELECTROPHOTOGRAPHY; MAGNETOGRAPHY
  • G03G5/00—Recording members for original recording by exposure, e.g. to light, to heat, to electrons; Manufacture thereof; Selection of materials therefor
  • G03G5/02—Charge-receiving layers
  • G03G5/04—Photoconductive layers; Charge-generation layers or charge-transporting layers; Additives therefor; Binders therefor
  • G03G5/05—Organic bonding materials; Methods for coating a substrate with a photoconductive layer; Inert supplements for use in photoconductive layers
  • G03G5/0528—Macromolecular bonding materials
  • G03G5/0532—Macromolecular bonding materials obtained by reactions only involving carbon-to-carbon unsatured bonds
  • G03G5/0535—Polyolefins; Polystyrenes; Waxes

Definitions

• This invention relates to a new coating composition, to a coated base adapted for use as a recording element in electrostatic printing processes, to a process for preparing said base and to a process for electrostatic printing using such recording element.
• a latent electrostatic image is produced on a photoconducting insulating material by any of a number of procedures recognized in the art. This may be accomplished by first producing on the surface, a substantially uniform electrical charge, as for example, by exposure to a Corona discharge. The charged sensitive recording element is then exposed to a pattern of light and shadow illumination whereby the illuminated areas are discharged and a charge image remains which corresponds to the pattern presented to the sensitive recording element. Visible images are then produced on the charge image surface by the electrostatic attraction of finely divided developer particles to the charge surface.
• Such developer particles may be presented tothe charge image surface of the recording element by a number of methods known to the art, for example, by the use of dry solid material, such as carbon particles, iron filings, etc., known as either cascade development or powder cloud development, or by applying a liquid or solid developer containing finely divided developer particles to the charge image surface.
• dry solid material such as carbon particles, iron filings, etc.
• cascade development or powder cloud development or by applying a liquid or solid developer containing finely divided developer particles to the charge image surface.
• Recording elements previously used in electrostatic printing processes have comprised a backing member such as paper, coated with an electrically insulating photoconductive layer such as photoconductive Zinc oxide suspended in an insulating binder.
• binders there have been used liquid dispersions of various resin binders, for example styrene-butadiene copolymers and resinous polysiloxane (silicone) binders.
• the resinous binders including the silicone binders and the styrene-butadiene copolymers are relatively viscous, thermoplastic materials, and consequently must be formulated as aqueous latices or with solvents or thinners such as liquid hydrocarbons in order to render them sufficiently fluid for application to the backing material.
• the silicone materials in addition are expensive and thus are not adapted for the production of an economic electrostatic printing system.
• the success of the electrostatic printing process depends to a large extent, first, on the "ability of the recording element to accept a relatively high overall voltage charge, second, on its ability to give up the charge immediately on exposure to light, and to discharge quantitatively in proportion to the amount of light to which it is exposed, third, to retain the full charge for substantial periods, sufiicient to efiect their development, in the portions which have been unexposed or only partially discharged by exposure to light, and fourth, to produce a final visible image of good contrast and definition.
• a still further object is to provide an improved process for electrostatic printing wherein a high degree of contrast and definition in reproduction is obtained.
• a composition comprises at least about 60% of a finely divided photoconductor dispersed in a normally solid polyethylenewax vehicle of average molecular weight between about 1,000 and about 3,000, said composition being solid at temperatures up to about 100 C. and having melt viscosity and thixotropic properties providing sufiicient fluidity for coating by the curtain coating method.
• the polyethylene wax may be composed of polyethylene wax components entirely Within the 1,000- 3,000 molecular weight range or it may be composed of a blend of polyethylene wax component of molecular weight below this range with polyethylene of molecular weight above this range or a blend of the polyethylene Wax with either of the other components in suitable proportions to provide the specified average molecular weight.
• a particularly desirable composition of the above type comprises at least about 60% of photoconductive finely divided zinc oxide dispersed in normally solid vehicle comprising at least about of a polyethylene wax of molecular weight between about 1,000 and about 3,000, said composition having a melt viscosity at a temperature of 300 F., sufiiciently low for application by hot melt coating in a conventional curtain and/ or roller coater.
• compositions may exhibit a melt viscosity. of 6000 centipoises or more when tested by the Brookfield viscometer at spindle speeds of 60 r.p.m., they canbe used effectively in conventional manner as hot melt coatings in curtain and/or roller coaters, which usually are considered capable of hand-lingcoatings of viscosities only up to 2,000 or 3,000 centipoises.
• compositions may be applied to a paper or other backing as a hot melt coating to deposit at least about 20 pounds of coating per ream of paper, and the resulting coated paper is cooled to solidify the coating.
• the polyethylene wax is melted in a suitable container equipped with agitation means.
• Photosensitive zinc oxide in finely divided form is added to the melt, preferably gradually, while agitating the mixture. Agitation is continued until the components are thoroughly Patented Sept. 5, 1967 blended.
• a colloid mill or other suitable blender may be used for this purpose. After blending, it may be desirable to allow the mixture to stand for a short period to permit release of any bubbles which may have become entrapped in the mixture.
• Other additives may be blended at this point if desired, but should not exceed about of the total vehicle portion of the composition and should not be of a character to adversely affect the electrical properties of the resulting composition or to unduly increase its melt viscosity.
• compositions consisting solely of photosensitive zinc oxide and polyethylene wax are satisfactory for many electrostatic printing applications.
• additional components such as dyes, toners, etc., tackifiers, such as polyisobutylene or adhesive resins, such as, for example, the terpene resins.
• tackifiers such as polyisobutylene
• adhesive resins such as, for example, the terpene resins.
• Flow modifiers and other additives may be included in small proportions as necessary.
• the polyethylene wax used as the binder in my electrostatic printing composition is a normally solid polymer of ethylene having an average molecular Weight between about 1,000 and about 3,000, melting point between about 95 C. and about 115 C., and viscosity at 140 C. of not more than about 1,000 centipoises, preferably between about 50 centipoises and about 250 centipoises.
• the polyethylene waxes having average molecular weights between about 1,000 and about 3,000 prepared according to US. Patents of Michael Erchak, Jr., No. 2,504,400, issued Apr.- 18, 1950, and No. 2,683,141, issued July 6, 1954, are especially suitable.
• waxes prepared by polymerizing ethylene in contact with isopropanol are described. Studies of the reactions of these products indicate that the structural formulae of the compounds making up the waxes formed in isopropanol may be written wherein n is an integer. These waxes are a mixture of individual homologs having varying individual values for n, substantially all of said individual values for n being in the Wax range.
• Any finely divided, pulverulent photosensitive zinc oxides can be used, and those having -a particle size in the range between about 0.39 micron and about 0.41 micron are especially suitable.
• the proportions of components in my electrostatic printing composition are important, and for assuring the acceptance and maintenance of the required voltages, and for providing satisfactorily reproduced images I find that the photosensitive zinc oxide must be present in 'at least about 60% by Weight of the total composition, preferably in amounts between about 65% and about 85%. Compositions containing less than about 60% zinc oxide fail to provide reproduced final images of satisfactory contrast and definition, while compositions containing more than about 85% zinc oxide become increasingly thixotr'opic and too thick for application to substrates by conventional coating processes.
• the polyethylene wax employed surprisingly provides an excellent suspending medium for even the high loadings of zinc oxide required for electrostatic purposes. While such compositions tend to be somewhat thixotropic, they readily break down under the agitation imposed by the application techniques of the coaters and yield homogeneous, free-flowing, low viscosity compositions at temperatures above the melting point of the wax, adapted for hot melt coating of paper or other substrate through the use of conventional hot melt coating equipment.
• the coatings of my invention have apparent viscosities, as measured in a Brookfield rotation viscometer, which reflect to some extent the thixotropic character of the compositions. I have found that coatings having apparent viscosities up to about 10,000 centipoises measured in a Brookfield viscometer at 300 F. using a 30 rpm. spindle are satisfactory for application by means of the standard hot melt coating process, and are broken down to an actual viscosity in the vicinity of 1,000 to 2,000 centipoises at the point of application.
• the coatings of my invention are applied to the substrate, which may be paper, cloth, or the like, by any of the conventional hot melt coating devices, for example, by means of a Mayer Coater, or a Steinemann Coater.
• the paper or other substrate is led over a roll dipping in a trough of the hot, molten coating composition.
• the roll picks up coating from the pool and applies it to the traveling web, the excess being removed by'a doctor roll.
• the coated Web is then passed over a chilled roll which sets the coating on the substrate.
• the molten coating is passed through a slot positioned above the traveling web of substrate, forming a curtain of molten coating composition which flows onto the substrate. Excess may be doctored off if desired.
• the latent image is developed by applying a pigment powder, which may be pigment alone, such as carbon black, iron filings, etc., or a pigmented liquid resin or a pigmented resin powder.
• a pigment powder which may be pigment alone, such as carbon black, iron filings, etc., or a pigmented liquid resin or a pigmented resin powder.
• the powder carries an electrostatic charge and is attracted to the oppositely charged image and held thereon.
• the powder image is then fixed by suitable means, for example as by pressure or by melting the resin powder so that it fuses to the paper and produces a durable, lightfast image.
• Examples 1-5 Five coating compositions were prepared by melting portions of polyethylene wax of molecular weight about 1,500, melting point (ASTM E-28-51-T) 210-217 F., hardness 6-9 (ASTM D-1321-55-T), specific gravity 0.91 and viscosity at 140 C. of centipoises, and mixing therewith in a colloid mill at a temperature between 300 F. and about 350 F. portions of photoconductive zinc oxide (New Jersey Zinc Co. Photox 801) of average particle size, about 40 microns, in the proportions shown in the table below. Each of the coating compositions was heated to 300 F.
• the resulting coated papers were tested for efliciency of reproduction in an electrostatic printing machine in the following manner.
• the coated papers were givenan electrical charge by passing a bar at about 500 volts across the sheets. Light was then passed through a printed sheet to be copied, onto the charged paper, dissipating the charge in proportion to the light reaching the paper, and forming a latent electrostatic image thereon.
• Iron filings dispersed in a low melting resin were then spread on the paper and adhered to the charged portions of the paper, rendering the latent image visible. Excess filings were brushed off and the filings were fused to the image on the sheet by heating at about 160 F. for 30-60 seconds.
• Compositions used in the tests, as well as manner of application and results are shown in Table I below:
• the coatings which contained only 50% of zinc oxide produced recording papers which yielded unsatisfactory images on reproduction, even when applied at a total coat weight of 25 pounds per ream.
• the coatings of only 11 pounds per ream gave especially poor reproduction.
• Those deposited at and pounds of total coating per ream gave slightly improved, but still unsatisfactory coatings.
• a coating containing 60% zinc oxide applied at 25 pounds per ream gave satisfactory reproduction, and a coating containing 65% zinc oxide, applied at 35 pounds per ream gave excellent reproduction.
• the several coated papers were also checked for voltage decay and rate of discharge on an oscilloscope.
• the coatings were first given a 500 volt charge.
• the papers were placed in the instrument and held for 2 minutes and voltage decay noted. All the coated papers maintained their charge of 500 volts substantially unchanged during the entire length of the test.
• the sheets Were then given a reverse charge, and all discharged substantially instantaneously, i.e., in substantially less than 1 second.
• Example 6 Five samples of paper coated with a composition consisting solely of polyethylene Wax of 2,000 molecular weight and various proportions of photochemical zinc oxide ranging from 50% to 80% of the total composition are charged with a 500 volt charge and placed in an oscilloscope wherein voltage is measured as a function of time. The coatings are held in the oscilloscope for 2 minutes (about the maximum time required for reproduction of an image in electrostatic printing machines) and voltage decay determined. The charge on the papers is then reversed and the time required to discharge to zero is noted. To test the quality of image reproduction, other samples of the same papers are charged with 500 volts, exposed to identical light patterns to produce an electrostatic image and the image is then developed by brushing with a resin powder pigmented with iron filings.
• the paper is inverted to remove non-adhering particles.
• the paper is then heated for to 60 seconds at 160 F. whereupon the filings are fused to the image portions of the paper.
• Each coated sample was tested by writing thereon with a ballpoint pen to test its markability.
• Example 7 A coating composition was prepared by melting in a colloid mill, 35 parts of a polyethylene wax having an average molecular weight of 2,000, melting point about 2l9-226 F. and viscosity at C. of centipoises. The temperature of the melted wax was raised to 300- 350 F. and maintained in this temperature range during gradual addition with agitation, of 65 parts of photosensitive Zinc oxide of particle size 0.40 1. After completion of addition of zinc oxide, agitation and temperature were maintained for about an hour to thoroughly blend the components. After blending, the coating composition was allowed to stand quiescent at 300 F. until gas bubbles, which had become entrapped during mixing, were substantially dissipated. The resulting composition was tested for apparent viscosity in a Brookfield viscometer at various temperatures and spindle speeds with results shown in Table III below.
• compositions of the invention thus have excellent electrical properties and they maintain the zinc oxide in excellent suspension. They have fair adhesion to the paper substrate which can be improved by the addition of small quantities of compatible tacky modifiers such as polybutene and tacky resins of the terpene type. Usually proportions of 510% or less of such tackifiers are sufiicient. If desired, small proportions of toners or other additives may be used, but these should be of such character as not to detract from the desirable electrical properties of the coating or unduly increase its melt viscosity. In general, not more than about 10% of the vehicle of the coating composition will be made up of such additives, i.e. at least about 90% of the total binder will consist of polyethylene wax.
• a photoconductive insulating coating composition suitable for electrostatic printing purposes comprising at least about 60% by Weight of a finely divided photoconductor dispersed in a normally solid polyethylene wax vehicle, said polyethylene wax being a mixture of individual homologs having the structural formula wherein n is an integer having an average value appropriate to provide an average molecular weight between about 1,000 and about 3,000, said composition being solid at temperatures up to about 100 C. and having melt viscosity and thixotropic properties providing sufiicient fluidity for coating by the curtain coating method.
• composition according to claim 1 wherein said vehicle comprises at least about 90% polyethylene wax having a viscosity at 140 C. between about 50 and about 1,000 centipoises.
• a photoconductive insulating coating composition comprising at least about 60% by weight of a photoconductive zinc oxide dispersed in a normally solid vehicle Comprising polyethylene wax, said polyethylene Wax being a mixture of individual homologs having the structural formula wherein n is an integer having an average value appropriate to provide an average molecular weight between about 1,000 and about 3,000 and viscosity at 140 C. of not more than about 1,000 centipoises, said polyethylene wax constituting at least about 90% of the vehicle portion of said coating composition, said coating composition having a viscosity at 350 F. sufficiently low for application by hot melt coating techniques.
• composition according to claim 1 wherein the photoconductive Zinc oxide has an average particle size between about 0.39 micron and about 0.41 micron.
• a process for electrostatic printing which comprises producing a latent electrostatic image on the surface of a photoconductive insulating material comprising a layer constituting at least about 20 pounds per ream of substrate of a composition comprising at least about of a finely divided photoconductive zinc oxide dispersed in a vehicle comprising polyethylene wax, said polyethylene wax being a mixture of individual homologs having the structural formula wherein n is an integer having an average value appropri-- ate to provide an average molecular weight between about 1,000 and about 3,000, said polyethylene Wax constituting at least about of said vehicle, and thereafter developing said latent image.

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• Physics & Mathematics (AREA)
• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Spectroscopy & Molecular Physics (AREA)
• General Physics & Mathematics (AREA)
• Photoreceptors In Electrophotography (AREA)
• Paints Or Removers (AREA)
• Paper (AREA)

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Citations (10)

• 0
  • NL NL301678D patent/NL301678A/xx unknown
• 1962
  • 1962-12-12 US US243982A patent/US3340057A/en not_active Expired - Lifetime
• 1963
  • 1963-11-27 GB GB46931/63A patent/GB1008126A/en not_active Expired
  • 1963-11-29 BE BE640632A patent/BE640632A/xx unknown
  • 1963-12-09 SE SE13649/63A patent/SE310457B/xx unknown
  • 1963-12-11 DE DE19631472902 patent/DE1472902A1/de active Pending

Patent Citations (10)

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