US3441505A - Developing powder - Google Patents

Description

United States Patent Oflice 3,441,505 Patented Apr. 29, 1969 43,9 rm. (:1. G03g 9/00, 9/02, 13/08 17.5. CI. 25262.1 8 Claims ABSTRACT OF THE DISCLOSURE Development of latent electrostatic images is accomplished by use of a thermoplastic, conductive, non-electroscopic, polarizable developer powder containing an acceptor-doner complex such as an addition product of a polynuclear hydrocarbon and a halogen, for example.

This invention relates to the development of electrostatic charge images. More particularly it relates to the development of electrostatic charge images by means of finely divided electrostatically attractable solid substances.

The electrophotographic process described in United States Paten No. 2,297,691 and known as xerography, is a process according to which an electrostatic charge pattern is formed corresponding to an image to be reproduced. The electrostatic image is made visible by means of an electroscopic material which is fixed, for instance, by heating to the developed material surface itself or to a second material surface, after transfer of the electroscopic material thereto.

In the conventionally applied development methods for electrostatic charge images an insulating powder is used to which a charge is imparted generally by rubbing against carrier particles or a brush, before bringing it into contact with the electrostatic charge image. By this rubbing contact an electron exchange takes place between the materials that are rubbed against each other. The materials are selected in the triboelectric series such that the powder will be charged to a polarity opposite to that of the latent charge image. In all these development methods it is to be understood that the powder is charged before being brought into contact with the latent image and that it remains charged for some time i.e., that it is sufiiciently insulating. Suchlike powder is commonly referred to as electroscopic powder.

In the electrophotographic processes known as xerography, wherein use is made of photoconductive layers composed of selenium and the like, the charge image is to be found, after charging and exposure of the layer, on the surface of the photoconductive layer. Subsequently, there has been a change in the art to the use of recording layers wherein the charge images are not formed on the surface of the layer but in the layer itself. The internal charge images may be formed for instance by captured charge carriers or by internal polarization. It has been found that the internal charge images formed by captured charge carriers or internal polarization for instance in photoconductive polymer layers, in the known photoconductive layers containing zinc oxide and in persistent internal polarization layers cannot be removed by bringing their surface into contact with a conductive material for instance by conducting their surface over well conductive metal rollers. Consequently, charge images that are not formed on the surface of a layer, can be developed by means of powder having a fairly good conductivity. Amony these powders we may mention the metallic developers described in United States patent specifications No. 2,357,809 and No. 2,297,691.

It was found that such metallic powders are only of minor use for electrophotography since they cannot be fixed according to the fixing methods conventionally applied in electrophotography, i.e., by melting on the material, and moreover give rise to the formation of rough feeling images.

The present invention relates to solid, thermoplastic, conductive, polarizable developing powders that are suitable for the development of electrostatic charge images and that are capable of being fixed by heat. These developing powders are not electroscopic since they are too strongly conductive for being charged electroscopically.

The present invention further relates to the production of visible images from latent electrostatic charge images by means of said developing powders.

Developing powders for electrostatic charge images according to the present invention are prepared with an acceptor-donor complex as the basic substance.

By acceptor-donor complex is understood a complex compound of organic or organic-inorganic electron donors and electron acceptors, the total electron configuration of which possesses an intermolecular mesomerism and a corresponding bond energy.

For more data about electron-acceptor-donor complexes we refer to Giinther-Briegleb Elektronen-Donator- Akzeptor Komplexe 1961, Springer Verlag Berlin-Gottingen, Heidelberg, W. Germ-any; Mulliken, J. Am. Chem. Soc., 74 (1952), 811; Mulliken, J. Am. Chem. Soc., 19 (1951), 514; Mulliken, J. Chem. Phys, 56 (1952), '801.

Polarizable acceptor-donor complexes are among others the so-called organic molecular complexes, among which may be mentioned: the coloured molecular complexes of polynitro compounds such as picric acid, 1,3,5- trinitrobenzene, 2,4,6-trinitrotoluene and the like with polynuclear aromatic hydrocarbons and their derivatives such as naphthalene, addition compounds of the type NC CN =0 -qulnoline (5=0.01 ohm cm.)

For details of the structure of molecular complexes we refer to M. J. S. Dewar, The Electronic Theory of Org-anic Chemistry, Oxford, Clarendon Press 1949), pp. 184-185.

The stable charge complexes known as intermolecular addition compounds of an electron-donor (e.g., an aromatic hydrocarbon) and an electron-acceptor (e.g., a halogen) are favoured (see among others J. Chem. Ed., October 1963, vol. 40, pp. 550-551).

Developing powders according to the present invention comprises preferably at least 50% by weight of acceptordonor complex and possess a specific resistivity lower than 10" ohm/cm.

Fine aluminium powder and other conductive material in powder form may be added to the composition of the developing powder in such an amount that the volume resistivity of the powder particles comprising the acceptor-donor complex is brought below 10 ohm/cm. and yet the powder still can be fixed by melting.

The powder particles may also be coated with a thin layer of metal for instance with silver metal as illustrated in Example 3 hereinafter.

For the manufacture of a developing composition according to the present invention the acceptor-donor complex is preferably dissolved or homogeneously dispersed in a meltable high molecular weight compound. The presence in the developing powder of preferably not more than 50% by weight of a meltable high molecular weight compound offers the possibility of allowing the powder to be fixed by melting exactly as is the case with an electroscopic resin powder.

Meltable polymers which can be mixed with the acceptor-donor complex should have a softening point preferably above 40 C. and a melting point preferably lower than 200 C.

Examples of suitable high molecular weight compounds are: natural resins such as dammar resin, elemi resins, gum mastic, gum manila, sandarac resin, microcrystalline waxes, modified natural substances such as ethyl cellulose stearate, pentaerytrite polyesters, colophony resins, polymers and copolymers of ethylene, styrene and vinyl acetate, polyvinyl acetals of formaldehyde, acetaldehyde and butyraldehyde, polyacrylates, polymethacrylates and coumarone indene resins, polycondensates such as glycerol phthalate resins or other glycerol polyesters, alkyd resins, polyethylene glycol esters, formaldehyde resins, silicone resins and polyamides.

Ferromagnetic substances may also be mixed with said resins to make the powder magnetically attractable so that it can be applied to the material to be developed by means of a magnet.

The diameter of the powder particles is of the same order of magnitude as that of the known xerographic developing powders. Good results are obtained with particles the diameter of which is comprised between 0.020 and 0.15 mm.

The developing process wherein use is made of a developing powder according to the present invention is quite distinguishable from the conventional electrophotographic dry developing methods in that no additional material has to be mixed with the developing powder, to permit triboelectrical charging.

A further advantage of this developing process is that the developer does not become exhausted by the consumption of toner (as compared with a toner which is mixed with iron particles or glass beads as carrier particles).

In contrast to chargeability, the polarizability of an electroscopic powder is not a surface property but a mass property so that the development power of the powder according to the invention is independent of atmospheric circumstances such as humidity and temperature and in consequence thereof good results are obtained even in the case of high relative humidity.

It has been found that by means of powder developers according to the invention large charge areas too, are developed uniformly and with good blackening whereas with electroscopic powders said areas are almost always developed at the edges. The developing only powder according to the invention need not be dusted in a dusting chamber onto the layer to be developed as described in United States Patent 2,357,809 or applied by sifting through a screen as described in United States Patent 2,297,691. On the contrary it is desirable to bring the developing powder as a compact mass into contact with or within the zone wherein the electrostatic attraction of the powder to the electrostatic charge pattern is possible. This may be achieved by conducting the recording material comprising the electrostatic latent image through a container filled with such powder.

According to another developing embodiment the powder can also be poured out onto the layer carrying the latent image whereupon the excess of powder is shaken off and then sucked off.

A preferred method for applying the developing powder according to the present invention is pouring the powder as a relatively thick layer onto an endless belt or drum against which the material to be developed is passed in contact but without counter-pressure. Since the electricity induced by rubbing with the endless belt is immaterial in the mechanism of this process, and the triboelectric properties are of no importance it is possible to choose for said belt a rough material. This material must be able to absorb and carry along a layer of developing powder which is not too thin. The use of a compact amount of developing powder according to the present invention is especially useful when wide charged zones are to be developed. By compact amoun is understood that the powder particles are closely united.

For an explanation of the developing process, it may be assumed that the electrostatic forces, that are present in the photoconductive layer, act upon the powder particles and polarize them. By charge separation in the interior of the powder particles, adhesion of these particles to electrostatically charged surfaces is realized.

From this explanation another advantage of the present invention, viz. development to sharp contrast, can be explained, i.e., by the fact that the electrostatic field itself achieves the charging of the powder and that uncontrollable supplemental charging caused by rubbing is avoided.

The following examples illustrate the preparation of developing powder according to the invention.

EXAMPLE 1 A mixture (a) of 10 parts of anthracene and 1 part of iodine is melted, stirred for 5 minutes and then cooled. The mass obtained is pulverized.

A mixture (b) of: Parts Pitch 2 Pentalyn A (trade name for an ester of pentaerytrite and natural resins marketed by The Hercules Powder Company Inc., Wilmington,

Del, U.S.A.) 2 Russ 810 (trade name for soot marketed by the firm Lummerzheim) 1.5

is melted. To the melted mixture (b) 1.1 parts of the pulverized mixture (a) is added. After cooling the mass is ground to powder.

is melted, stirred for 5 minutes, cooled and pulverized. The particle size required for rendering the powder suitable for being applied as developing powder is reached after grinding for 2 hours in a ball mill.

EXAMPLE 3 The following ingredients are mixed while being dry, by grinding for 1 hour in a ball mill:

Parts Hydroquinone 10 Quinone 10 Parts Hobimal P59 (trade name for a modified maleic resin esterified with polyethylene terephthalate marketed by Adriaan Honigs Kunsthars-Industrie Zaandam, Netherlands) Nigrosine (C.I. 50,415B) Then, mixtures A and B are mixed by melting very slowly for 30 minutes. After cooling the mixture is pulverized and ground for 2 hours in a ball mill.

The powder obtained is brought while strongly stirring for 1 minute in a 1% ammoniacal silver nitrate solution. Then the powder is filtered off and rinsed one time with distilled water. The powder thus treated is dried in an oven at 50 C. By this treatment the surface resistivity of the powder is decreased with a factor I claim:

1. 'Electrically-conductive, heat-fusible developing powder for development of electrostatic chanrge patterns present in an insulating material consisting essentially of an intimate pulverulent admixture of a thermoplastic high molecular weight organic polymeric binder and an acceptor-donor complex being a molecular addition product selected from the class consisting of:

(l) a polynuclear aromatic hydrocarbon and a member of the group consisting of a halogen, tetracyano ethylene, and a organic polynitro mononuclear compound,

(2) quinoline and tetracyano quinomethylene, and

(3) quinone and hydroquinone.

2. Developing powder according to claim 1, wherein at least 50% by weight of said acceptor-donor-complex is present in said developing powder.

3. Developing powder according to claim 1, having a specific resistivity of less than 10" ohm cm.

4. Developing powder according to claim 1, including a minor amount of powder conductive metal.

5. Developing powder according to claim 1, wherein the said thermoplastic high molecular weight substance has a softening point higher than C. and a melting point lower than 200 C.

6. Developing powder according to claim 1, wherein the said acceptor-donor-complex is a stable intermolecular addition compound of anthracene and iodine.

7. Developing powder according to claim 1 wherein said acceptor-donor complex is the stable molecular addition product of hydroquinone and quinone.

8. The developing powder of claim 1, wherein said halogen is iodine and said polynitro compound is a member of the group consisting of 1,3,5-trinitrobenzene, 2,4,6-trinitrotoluene and picric acid.

References Cited UNITED STATES PATENTS 2,892,794 6/1959 'Insalaco 25262.1 2,919,247 12/1959 Allen 252--62.1

OTHER REFERENCES Labes et 211., Organic Semiconductors ill, I. of Chem.

Physics, vol. 33, No. 3, September 1960, pp. 868-872.

LEON D. ROSDOL, Primary Examiner.

J. D. WELSH, Assistant Examiner.

US. Cl. X.R.