US 3676350 A
The triboelectric properties and minimum caking temperature of particulate toner material can be altered by glow discharge treatment in the presence of an inert gas, air or a gaseous polymerizable material.
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Description (OCR text may contain errors)
United States Patent Wright et al.
[ 1 July 11,1972
 GLOW DISCHARGE POLYMERIZATION COATING OF TONERS FOR ELECTROPHOTOGRAPHY  .lnventors: John F. Wright; James R. Olson, both of Rochester, NY.
Eastman Kodak Company, Rochester, NY.
22 Filed: Feb. 3, 1970 21 App1.No.: 8,424
 U.S.Cl. ..252/62.1, 117/175,117/93.1GD,
 References Cited UNITED STATES PATENTS 3,090,755 5/1963 Erchak et a1 ..252/6 1.1 3,392,139 7/ 1968 Dengmon ..260/41 3,507,686 4/1970 Hogenboch... 1 17/100 3,387,991 6/1968 Erchak 1 17/93. 1 3,247,014 4/1966 Goldberger et al. ..1 17/ 1 00 3,447,950 6/1969 Evans et al 1 17/100 3,526,533 9/1970 Jacknow et al. ..1 17/100 3,440,085 4/1969 Baker et a1 ..1 l7/l00 3 ,461,092 8/1969 Story ..260/28 3,533,829 10/1970 Quanquin ..11/62 2 3,326,177 6/1967 Taylor ..1 18/49.
Primary Examiner-George F. Lesmes Assistant Examiner-John C. Cooper, [I] Attomey-W. H. J. Kline, J. R. Frederick and T. Hiatt 57 ABSTRACT The triboelectn'c properties and minimum caking temperature of particulate toner material can be altered by glow discharge treatment in the presence of an inert gas, air or a gaseous polymerizable material.
8 Claims, No Drawings GLOW DISCHARGE POLYMERIZATION COATING OF TONERS FOR ELECTROPHOTOGRAPHY such as by a corona source and an imagewise light exposure that discharges the photoconductor in the exposed areas, an electrostatic charge image remains. This electrostatic image, as well as electrostatic images produced by other techniques, can be rendered visible by treatment with an electrostatic developing composition or developer. Conventional dry developers include a carrier that can be either a magnetic material such as iron filings, powdered iron or iron oxide, or a triboelectrically chargeable, non-magnetic substance like glass beads or crystals of inorganic salts such as sodium or potassium chloride. As well as the carrier, electrostatic developers include a toner that is usually a resinous material suitably colored or darkened for image viewing purposes with a colorant such as dyestuffs or pigments, for example, carbon black.
To develop an electrostatic image, the dry developer can be applied imagewise to the electrostatically charged surface by various techniques. One such technique is known as cascade development and is described in U.S. Pat. No. 2,618,552. This development technique is carried out by rolling or cascading across the electrostatic latent image bearing surface, a developing mixture composed of relatively large carrier particles, each having a number of electrostatically adhering fine marking particles, known as toner particles, on its surface. As this mixture rolls across the image-bearing surface, the toner particles are electrostatically deposited on the charged portions of the image.
Another suitable developing technique is known as magnetic brush development and is described in U. S. Pat. No. 3,003,462. This development technique involves the use of a magnetic means in connection with a developing mixture composed of magnetic carrier particles having a number of smaller electrostatically adhering toner particles. In this technique the developer composition is maintained during the development cycle in a loose, brushlike orientation by a magnetic field surrounding, for example, a rotatable non-magnetic cylinder having a magnetic means fixedly mounted inside. The magnetic carrier particles are attracted to the cylinder by the described magnetic field, and the toner particles are held to the carrier particles by virtue of their opposite electrostatic polarity. Before and during development, the toner acquires an electrostatic charge of a sign opposite to that of the carrier material due to triboelectric charging derived from their mutual frictional interaction. When this brushlike mass or mag netic brush of carrier and toner particles is drawn across the photoconductive surface bearing the electrostatic image, the toner particles are electrostatically attracted to an oppositely charged latent image and form a visible toner image corresponding to the electrostatic image.
in typical electrophotographic applications, the developed image formed on a photoconductive element is transferred to a receiving sheet. The image thus transferred is then made permanent by heating to fuse the transferred image. Thus, the resin of the toner material must be capable of being fused under temperature conditions which will avoid any. charring, burning or other physical damage to the receiver sheet which is typically formed of paper. A variety of resin combinations have been suggested in the art which allegedly will provide suitable fusion properties. However, if the fusion temperature is sufficiently low, it is often found that the caking temperature of the toner material is also very low. This latter property is undesirable in that the toner material can readily be sintered or caked during storage or shipment. If caking of the toner occurs, it can render the material totally unusable.
In addition, when toner materials are prepared using various resin compositions which have suitable fusion properties, it is quite common that the resultant toner materials have poor triboelectric charging properties. It is extremely important that the resultant toner material be capable of being triboelectrically charged either positively or negatively depending upon the particular use and particular carrier with which it is mixed. If the toner material does not charge properly, it will result in a poor quality developed image. A further problem which can result is that the toner material will not be triboelectrically attracted to the carrier material with which it is mixed. This latter instance will result in the toner material settling to the bottom of the containerv of developer mixture rather than being carried to the element tobe developed. Thus, the end result in this instance could be that no image at all will be developed using a mixture of this type.
Accordingly, there is a need in the art for toner materials which have good fusion properties, a reduced tendency for the toner to cake and improved triboelectric charge properties.
It is, therefore, an object of this invention to provide new dry toner compositions for use in developing electrostatic charge patterns.
An additional object of this invention is to provide new electrostatic toner compositions which exhibit reduced caking tendencies.
A further object of this invention is to provide new dry toner materials which have improved triboelectric charging properties.
Still another object of this invention is to provide new dry developer compositions for use in development of electrostatic images.
A still further object of this invention is to provide novel processes for treating dry toner material for use in developing electrostatic charge patterns.
These and other objects are accomplished in accordance with this invention by the glow-discharge treatment of dry particulate toner material typically comprised of a colorant and a thermoplastic resin. Particles useful for forming the toner materials of the present invention can be prepared by various methods. Two convenient techniques for producing these particles are spray-drying or melt-blending followed by grinding. Spray-drying involves dissolving the polymer and a colorant in a volatile organic solvent such as dichloromethane. This solution is then sprayed through an atomizing nozzle using a substantially nonreactive gas such as nitrogen as the atomizing agent. During atomization, the volatile solvent evaporates from the airborne droplets, producing particles of uniformly dyed resin. The ultimate particle size is determined by varying the size of the atomizing nozzle and the pressure of the gaseous atomizing agent. Conventionally, particles of diameter between about one-half and about 25p. are used, with particles between about 2p. and l5p. being preferred, although both larger and smaller particles can be used where desired for particular developer conditions or developer compositions. As mentioned above, suitable particles can also be prepared by melt-blending. This technique involves melting a powdered form of polymer or resin and mixing it with a suitable colorant and any other desired additives. The resin can readily be melted on heated compounding rolls which are also useful for stirring or otherwise blending the resin and other addenda in order to promote the complete intermixing of the various ingredients. After thorough blending, the mixture is cooled and solidified. The resultant solid mass is then broken into small pieces and finely ground to form a free-flowing powder. The resultant particles usually range in size from about one-half to about 25 u.
The choice of starting polymeric materials is quite large in that a variety of materials may be used which heretofore would have been unsuitable. A wide variety of materials can be used and the choice of one material is not strictly limited by the caking temperature of the material or its triboelectric charging properties. Thus, materials which previously were not suitable for use in toner compositions can in accordance with the present invention be utilized. Useful resins have an inherent viscosity of from about 0.10 to about 0.4. The inherent viscosity is determined at C in accordance with the followwherein 1; solution is the viscosity of the solution, '1 solvent is the viscosity of the solvent and C is the concentration in grams of the polymer in 100 ml. of chloroform. Useful resin materials can include natural and synthetic resins. Styrene and styrene-containing polymers, alkyd resins, including modified alkyd resins such as rosin-modified maleic alkyd resins, various polyolefins, such as polyethylene, polypropylene, etc, and the like are among the many polymeric materials which can be utilized in accordance with the present invention.
The colorants useful in the practice of this invention can be selected from a variety of materials such as dyestufis or pigments. Such materials serve to color the toner and thus render it more visible. Suitable toner materials having appropriate caking and charging properties can, of course, be prepared without the use of a colorant material where it is desired to have a developed image of low optical opacity. ln those instances where it is desired to have high optical opacity, the colorants used, can, in principle, be selected from virtually all of the compounds mentioned in the Color Index, Vol. I and ll, Second Edition. Included among the vast number of useful colorants would be such materials as Hansa Yellow G (CI. 11680), Nigrosine Spirit soluble (CI. 50415), Chromogen Black ETOO (CI. 14645), Rhodamine B (CI. 45170), Solvent Black 3 (CI. 26150), Fuchsine N (CI. 42510), C]. Basic Blue 9 (CI. 52015), etc. Another useful class of colorants is comprised of nigrosine salts such as nigrosine salts of mono and difunctional organic acids having from about 2 to about 20 carbon atoms such as chloroacetic acid, stearic acid, sebacic acid, lauric acid, azelaic acid, adipic acid, abietic acid and the like. Nigrosine salts of this type are disclosed in copending application Ser. No. 736,552, filed June 13, 1968, now abandoned, in the name of James R. Olson and entitled UNIFORM POLARITY RESIN ELECTROSTATIC TONERS.
After suitable resin-containing particles having the proper size are made, these particles are subjected to a glowdischarge treatment in accordance with the present invention. The term glow discharge treatment" in connection with this invention has reference to any suitable means of subjecting the particles to activating radiation to cause surface crosslinking or polymerization. Useful means of activation would include direct current, electrodeless radio frequency, microwave glow discharge, as well as ultraviolet radiation and electron bombardment.
The apparatus involved in the glow discharge treatment of the present invention is relatively simple. The actual treatment takes place within a reaction vessel which can readily be evacuated. The reaction vessel or chamber usually contains two parallel, closely spaced plate electrodes. When an ac. or do field of the order of several hundred v/cm is imposed on the parallel electrodes, a uniform discharge occurs between the plates and a thin, polymeric film is deposited on any material maintained between the electrodes. In one useful arrangement, the electrodes are connected to a power source capable of maintaining at least a 10 kc. a.c. field sufficient to produce a glow between the electrodes. Also present is a means for containing the particles to be treated and maintaining them in a position between the electrodes. A suitable means for this purpose is a filter paper diaphram mounted in a vibratory holder and out of contact with the electrodes. The vibratory holder is activated during the reaction to vibrate or otherwise agitate the particles and maintain them in a relatively fluidized state. The electrodes are generally maintained in close proximity to the particles being treated. Typically the electrodes are at a distance of about 9% to 2% cm. from one another depending on the potential applied and the configuration of the apparatus used.
In accordance with one embodiment of this invention, a suitable reaction vessel containing particles to be treated is evacuated to a pressure of the order of about 0.5 to 3 mm. of mercury. After evacuation, a non-reactive or inert gas such as helium or nitrogen is bled into the apparatus to increase the pressure to about 1 to 5 mm. of mercury. Air can also be used as the gas. The vibratory holder is activated such that the particles are maintained in a thoroughly agitated condition while they are being treated. The power source is activated to produce an even glow between the two substantially parallel plate electrodes. This glow discharge treatment results in the formation of a substantially crosslinked outermost shell of the polymeric binder material of the toner particles. This crosslinking of the polymer results in a substantial increase in the minimum caking temperature of the toner particles. In general, the increased minimum caking temperature is at least about 5 to 10 C over that of the untreated toner particles. The term minimum caking temperature" has reference to the temperature at which the particles become sintered together to such an extent that they cannot be broken apart by gently shaking. Typical materials treated in accordance with this invention can be stored for periods in excess of ten hours at temperatures of at least 50 to 55 C with no substantial sintering occurring. Preferred materials can be stored at temperatures of about 60 to 75 C without substantial caking.
In another embodiment of this invention, a plurality of suitable particles of resin and colorant are provided in a reaction vessel as described above. Once again, the reaction vessel is evacuated and this time a gaseous polymerizable material is bled into the apparatus until the pressure is of the order of l to 5 mm. of mercury. Then a potential is applied to the electrode to cause glow discharge. During the glow discharge treatment, the particles are maintained between the electrodes in an agitated or fluidized state. This glow discharge treatment results in the formation of a thin, continuous layer of glow discharge polymerized material on said particles. This layer changes both the triboelectric properties of the toner and the minimum caking temperature of the toner. In general, this layer is extremely thin and difficult to measure. The thickness is believed to be less than about 0.2 .1. and possibly even as thin as about 0.01 2.
Useful gaseous polymerizable materials can be selected from a wide variety of vaporizable monomers or polymer precursors. Suitable materials would include such monomers as trifluoromonochloroethylene, hexafiuoropropylene, tetrafluoroethylene, octafluorobutene-2, vinyl fluoride, vinylidene fluoride, hexafluoroacetone, acrylonitrile, styrene, ethylene, vinyl chloride, methyl methacrylate, divinylbenzene, carbon tetrachloride, hexafluoroethane, vinyl ferrocene, etc, as well as materials which are not generally considered as polymer precursors such as benzene, naphthalene, anthracene, etc. In addition, mixtures of these or any other vaporizable polymer precursors which undergo polymerization in the presence of activating radiation can be used. In general, it hasbeen found that when a gaseous polymer precursor as described above is polymerized in accordance with the techniques described herein, it is preferred to keep the particles in motion. Any suitable means forming any such motion can be used. By maintaining the particles in a relatively fluidized condition, a relatively uniform layer of glow discharge polymerized material is insured. Similarly, when toner materials are subjected to glow discharge in the presence of an inert atmosphere or air, it is preferred that the particles be thoroughly agitated to insure uniform crosslinking of the outermost shell of the individual particles. However, useful results can be obtained without thorough agitation of the particles.
Developer compositions for developing electrostatic charge patterns can be prepared by admixing the present toner particles with a suitable carrier material. Typical developer compositions contain from about 1 to 10 percent by weight of the present toner with from about 99 to percent by weight of carrier vehicle. The carrier vehicle which can be used with the present toners to form new developer compositions can be selected from a variety of materials. Suitable carriers useful in this invention include various non-magnetic particles such as glass beads, crystals of inorganic salts such as sodium or potassium chloride, hard resin particles, metal particles, etc. In addition, magnetic carrier particles can be used in connection with this invention. Suitable magnetic carriers are particles of ferromagnetic materials such as iron, cobalt, nickel and alloys thereof. Other magnetic carriers that can be used are resin particles coated with a thin, continuous layer of a ferromagnetic material as disclosed in Miller, U. S. application Ser. No. 699,030, filed Jan. 19, 1968, entitled METAL SHELL CAR- RlER PARTlCLES, now abandoned. The carrier particles used typically have an average particle size between about 1,200to 30p. depending on the size of the toner particles used. Preferred carriers have a particle size of about 600 to 4011..
The toner and developer compositions of this invention can be used in a variety of ways to develop electrostatic charge patterns or electrostatic latent images. Such developable charge patterns can be prepared by a number of means and can be carried on either an electrophotographic element or on a non-sensitive element such as a receiver sheet. One suitable technique involves cascading the developer composition across the electrostatic charge pattern; while another technique involves applying toner particles from a magnetic brush. This latter technique requires the use of a magnetically attractable carrier vehicle in forming the developer composition. After imagewise deposition of the toner particles, the image can be fixed by heating the toner to cause it to fuse to the substrate carrying the toner. If desired, the unfused image can be transferred to another support and then fused to form a permanent image.
The following examples are included for a further understanding of the invention.
EXAMPLE 1 A toner material is prepared by mixing 62 percent by weight of Piccolastic D125 (a polystyrene resin, Pennsylvania industrial Chemical Co.) and 20.6 percent by weight of a rosinmodified maleic resin by melt-blending. During the meltblending procedure, 9.0 percent by weight of black colorant is incorporated into the mixture. The melt-blended mixture is allowed to cool to room temperature and is ground such that it will pass through a 20 mesh U. S. Standard Sieve Series screen. The 20 mesh material is then ground in a fluid energy mill to a maximum particle size of about 20 microns. Next, a developer composition is prepared by mixing 3 grams of this control toner with 97 grams of iron carrier particles having an average particle size of about 100 to 140 microns. An electrophotographic element is provided which comprises a conductive layer having coated thereon a polycarbonate binder containing an organic photoconductor. This element is subjected to a negative polarity corona discharge while maintained in the dark. The negatively charged element is then given an imagewise exposure to a photographic positive which results in the formation of an electrostatic charge pattern. The developer composition prepared as above is placed on a small magnet to form a magnetic brush. The magnetic brush is lightly rubbed over the electrostatic charge pattern and toner is deposited only in the unexposed areas of the photoconductor. This method of deposition indicates that few, if any, particles have acquired a negative charge. The measurement of the charge on this toner indicates the net charge is +l microcoulombs per gram.
EXAMPLE 2 A sample of the control toner prepared as in Example 1 is placed on a piece of filter paper contained within a polymeric holder capable of vibrating the toner material so as to maintain it in a fluidized state. This holder is placed between two parallel electrodes comprised of circular stainless steel discs about 2%. cm. in diameter and mounted about 2% cm. apart such that they are above and below the fluidized bed of toner.
This apparatus is contained within an evacuable reaction vessel. The vessel is evacuated to a pressure of about 0.6 mm. of mercury and the pressure is then increased to 1.5 mm. of mercury by bleeding helium into the reaction vessel followed by bleeding tetrafluoroethylene into the vessel to increase the pressure to 2.5 mm. of mercury. A 10 kc. a.c. field sufficient to produce a glow at a current of 60 ma. is applied across the electrodes for a period of 8 minutes. The electrical equipment used to produce this field is comprised of an audio oscillator, a ZOO-watt audio amplifier and a step-up transformer. A 1,000- ohm current limiting resistor is placed in the line to one of the electrodes and the voltage drop across this resistor is recorded and used to calculate the current. The glow discharge treatment in the helium-tetrafluoroethylene atmosphere causes the toner particles to attain a net negative charge of 5.4 microcoulombs per gram. This treated toner material is then mixed with carrier particles as in Example 1 and used to develop an electrostatic charge pattern prepared by negatively charging a photoconductive element and imagewise exposing to a photographic negative. Toner particles are deposited in the exposed (image) areas of the photoconductor indicating that the toner charge is predominantly negative.
EXAMPLE 3 A toner is prepared using about 30 grams of polystyrene dissolved in about 390 cc. of dichloromethane containing about 2% grams of Nigrosine Spirit Jet added as a colorant. The resultant combination is then spray dried through a pneumatic spray nozzle using nitrogen as the atomizing gas at ambient temperature and at a pressure of about 10 psig. to produce uniform particles having a maximum diameter of about 10 microns. This control toner is used as in Example 1 to develop an electrostatic charge pattern formed on a negatively charged photoconductive element which has been exposed to a photographic positive. As in Example 1, no negatively charged particles are observed in that the toner deposits only in the areas of no exposure.
EXAMPLE 4 The toner material prepared as in Example 3 is placed within a reaction vessel and treated with a glow discharge in the presence of a heliumtetrafluoroethylene atmosphere in the manner described in Example 3 above. This treatment results in the formation of particles which, when mixed with the iron carrier, attain a negative charge with respect to the carrier. This negative charge is evident by the deposition of toner particles only in the areas of exposure (non-image areas) when applied to a negative polarity electrostatic charge pattern as described in Example 1.
EXAMPLE 5 The control toner of Example 3 is exposed to glow discharge in the presence of a nitrogen atmosphere at a pressure of about l.5 mm. of mercury in accordance with the general procedure described in Example 2. When this toner is used to develop a negative electrostatic charge pattern, no difference is seen between this treated toner and the control toner of Example 3 as far as toner charge is concerned. However, the treated toner has a significant increase in resistance to caking on storage at elevated temperatures. On storage for 15 hours at about 61 C no caking occurred with this treated toner. The untreated toner of Example 3 exhibits severe caking when stored under these conditions.
EXAMPLE 6 A toner composition is prepared by melt-blending about 48 grams of a polyamide resin binder with about 2.88 grams of carbon black as a colorant. The mixture is cooled to room temperature and ground to a size such that it will pass through a 20 mesh U. S. Standard Sieve Series screen. The ground material is then further ground in a fluid energy mill at a 50 psig. air pressure which reduces the particle size such that the maximum diameter is about 20 microns. About 0.5 gram of this toner material is then placed in a reaction vessel such as described in Example 1. The toner is placed on a piece of filter paper covering an electrically grounded horizontal square stainless steel electrode. A high voltage electrode having the same dimensions as the grounded electrode is positioned about I cm. above the grounded electrode. The reaction vessel is evacuated to a pressure of about 0.6 mm. of mercury and helium is bled into the vessel to a pressure of about 2.4 mm. of mercury. Next, a kc. a.c. field sufficient to produce a glow at a current of 60 ma. is applied between the electrodes for 5 minutes. The particles are removed, placed in a container and stored for 18 hours at about 65 C. A similar amount ofidentical toner which has not been subjected to glow discharge is also stored in the same condition. The treated sample shows some slight clumping after storage; however, the clumps are easily broken by light shaking of the container. The untreated control shows considerable caking after storage in these conditions. Similar treated and untreated materials are stored for l5 hours at about 7 l C. The treated sample shows some caking but can easily be broken up. However, the control shows formation of a hard cake which is very difficult to break up.
EXAMPLE 7 A 0.5 gram portion of the toner material of Example 6 is placed in the glow discharge apparatus and the apparatus is evacuated to a pressure of 0.5 mm. of mercury. The pressure is increased to 1 mm. of mercury by the addition of helium and then increased further to 2.5 mm. of mercury by introducing tetrafluoroethylene in the reaction vessel. A glow discharge is produced as in the precedingexample for a period of two minutes. The resultant treated toner particles and a similarly constituted untreated control are stored for 18 hours at about 66 C. The untreated control shows considerable clumping; whereas, the treated sample shows only slight clumping and is readily broken up by shaking. A similar treated toner material and a corresponding control are stored for l5 hours at about 71 C. Again, the control toner is severely caked; whereas, the treated sample shows only slight caking and can readily be broken up by gently shaking.
EXAMPLE 8 A sample of the control toner of Example 3 and a sample of the treated toner of Example 4 are stored for hours at about 60 C. The treated sample of Example 4 shows some settling, but no caking whatsoever. On the other hand, the control toner of Example 3 shows severe caking and is not readily broken up.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
1. A method of treating toner particles for use in the development of electrostatic charge patterns comprising the steps of:
a. providing a plurality of particles comprised of a colorant and a thermoplastic polymeric binder material,
b. agitating said particles between two substantially parallel electrodes contained within an evacuated reaction vessel,
c. introducing a gaseous polymerizable material into said vessel and d. establishing a glow discharge between said electrodes while maintaining said particles in an agitated state whereby a thin, continuous film of polymer is deposited over each particle.
2. A particulate, electroscopic toner composition comprising particles containing a colorant and a thermoplastic polymeric binder material, each of said particles having coated thereon a thin, continuous layer of a glow discharge polymerized material.
. A toner composition as descrtbed in claim 2 wherein said layer of polymerized material is formed from a gaseous polymerizable material selected from the group consisting of trifluoromonochloroethylene, hexafluoropropylene, tetrafluoroethylene, octafluorobutene-Z, vinyl fluoride, vinylidene fluoride, hexafluoroacetone, acrylonitrile, styrene, ethylene, vinyl chloride, methyl methacrylate, divinylbenzene, vinyl ferrocene, carbon tetrachloride, hexafluoroethane, benzene, naphthalene, anthracene and mixtures thereof.
4. A toner composition for use in the development of electrostatic charge patterns comprising particles of a thermoplastic binder having therein a colorant, each of said particles having coated thereon a thin, continuous layer of glow discharge polymerized tetrafluoroethylene.
5. A toner composition as described in claim 4 wherein said layer of glow discharge polymerized tetrafluoroethylene has an average thickness of less than about 0.2 microns.
6. A developer composition for use in developing electrostatic charge patterns comprising a mixture of about to 99 percent by weight of a particulate carrier vehicle and about 10 to 1 percent by weight of a toner material comprised of particles of a thermoplastic polymeric material having therein a colorant, each of said particles having coated thereon a thin, continuous layer of glow discharge polymerized material.
7. A developer composition as described in Claim 6 wherein said toner has an average particle size of about /2 to about 25 microns and has a minimum caking temperature in excess of about 55C.
8. A developer composition for use in developing electrostatic charge patterns comprising a mixture of about 90 to 99 percent by weight of a magnetically responsive particulate carrier vehicle and about 10 to 1 percent by weight of a toner material comprised of particles of a thermoplastic polymeric material having therein a colorant, each of said particles having coated thereon a thin, continuous layer of glow discharge polymerized material, said layer formed from a gaseous polymerizable material selected from the group consisting of trifluoromonochloroethylene, hexafluoropropylene, tetrafluoroethylene, octafluorobutene-2, vinyl fluoride, vinylidene fluoride, hexafluoroacetone, acrylonitrile, styrene, ethylene, vinyl chloride, methyl methacrylate, divinylbenzene, vinyl ferrocene, carbon tetrachloride, hexafluoroethane, benzene, naphthalene, anthracene and mixtures thereof.