EP0046960B1 - Electrophotographic recording material - Google Patents

Description

The invention relates to an electrophotographic recording material composed of an electrically conductive layer support, optionally an insulating intermediate layer, a photoconductive layer system of organic materials composed of a layer optionally containing a binder with a charge-generating compound and a layer with charge-transporting compound and a protective transparent cover layer.

In the electrophotographic process for the production of copies known from US-A-2 297 691 and widely used today, a thorough cleaning of the photoconductor layer is always necessary after the transfer of the toner image developed on the photoconductor layer in a dry way to the copy carrier. The cleaning is usually done by brushing or wiping the layer with suitable brushes or fabrics. In copying machines that work with liquid developers, the effect of mechanical cleaning is often enhanced by the use of a cleaning liquid. In addition to these cleaning operations, the photoconductive layer is also exposed to other influences that damage it. For example, it is subject to the action of the dry developer and the developer station (counter-voltage) and, in the case of liquid development, to the action of the developer liquid.

It is also exposed to the ionized air generated in the charging station. It is known that the required cleaning processes and the other influences mentioned lead to impairment or even mechanical damage to the photoconductor layer and thus to a reduction in its service life.

Recording materials with a photoconductive layer of at least one layer with a charge-generating compound and charge-transporting compound are known. For example, highly sensitive, organic photoconductor layers (DE-B-2 314 051) are used on conductive carrier films or tapes due to their high elasticity.

In particular, very highly sensitive photoconductor systems according to, for example, DE-A-2 734 288 can be used as endless belts because of their great flexibility, which can be guided over deflecting rollers with a relatively small diameter.

It is also known (US-A-2 901 348, US-A-4148 637, DE-A-2 452 622) to protect photoconductor layers with an additional cover layer made of inorganic or organic materials. The disadvantage of this is that they are used either on the less flexible layers consisting of or containing selenium or on generally less sensitive organic photoconductor systems. In addition, these cover layers must be mixed with additives such as silane couplers for better durability or contain substances that increase the electrical conductivity.

GB-A-918 234 discloses an electrophotographic recording material made of an electrically conductive base with a layer of photoconductive, insulating material, such as selenium or similar photoconductive material, which is coated with a thin transparent layer of an inorganic material, such as sulfide, silicon dioxide, titanium dioxide , Silicate and alkaline earth fluoride. The cover layer has a thickness that should begin with significant protection of the photoconductor layer up to a maximum of 2.5 µm (0.1 mil). A range from approximately 1.25 to 25.4 μm is specified as the photoconductor layer thickness. A completely different system is presented as suitable from DE-B-1 109 031 corresponding to this patent specification. There, the photoconductor layer has a thickness in the range from 0.0008 to 0.04 cm and the inorganic cover layer has a thickness from 0.0004 to 0.004 cm. It was therefore not at all clear to the reworking specialist which thickness ranges he should now use, let alone that he should go from this system of inorganic photoconductor / inorganic cover layer to an organic material, as is the case according to the invention.

From DE-A-2 220 270 a xerographic storage area is also known which has an anti-reflection and protective coating made of silicon oxide. Based on the physical information, the disclosure is clearly based on selenium coatings as a photoconductor layer. There is no question of an electrically conductive base. The thickness of the vapor deposition layer is only given insofar as it is determined optically and the evaporation is ended when the reflectivity passes its first minimum. An electrophotographic recording material of the type described in the introduction does not emerge from this teaching.

The object of the invention was to provide the photoconductor systems which are made of organic materials and which are highly light-sensitive in the photoconductor double-layer arrangement, with a cover layer made of materials which are transparent to visible light and which protect them from mechanical or possibly other adverse influences and which do not significantly impair the function of the photoconductor layer and generally the useful life of such Increase systems.

The object is achieved by an electrophotographic recording material mentioned in claim 1, which is characterized in that the cover layer consists of 1 to 10- ³ Pa at (10- ⁵ to 10- ⁸ bar) applied silicon monoxide, magnesium fluoride or calcium fluoride and 0, 5 to 10 microns thick.

The result of this is that an electrophotographic recording material can be made available in which the abrasion resistance and the service life are significantly improved while the photosensitivity remains virtually the same.

By increasing the abrasion resistance and thus the service life, it is also achieved that the multi-layer arrangement can be used more profitably not only on flexible conductive substrates, but also on drums.

The electrophotographic recording material according to the invention is shown schematically by FIGS. 1 to 3. The photoconductive layer is in the form of a double-layer arrangement which consists of a layer 2 containing charge carrier-producing compounds, as is expressed in FIGS. 1 and 2, and a layer containing charge-transporting compounds under the respective position 3. The conductive layer support is indicated with 1 in each case. An insulating intermediate layer is indicated at position 4, position 5 shows a layer of charge-generating compound in dispersion. Position 7 indicates the protective cover layer according to the invention.

For example, aluminum foil, optionally transparent, aluminum-vapor-coated or laminated polyester foil, is used as the conductive layer support, but any other layer support made sufficiently conductive can be used.

The insulating intermediate layer can be produced by a thermally, anodically or chemically produced aluminum oxide intermediate layer. It can also consist of organic materials. For example, different natural or synthetic resin binders are used that adhere well to a metal or aluminum surface and dissolve little when the other layers are subsequently applied, such as polyamide resins, polyvinylphosphonic acid, polyurethanes, polyester resins or specifically alkali-soluble binders, such as Example styrene-maleic anhydride copolymers. The thickness of such organic intermediate layers can be up to 5 gm, that of the aluminum oxide layer is mostly in the range of 0.01-1 µm.

• Perylen-3,4,9,10-tetracarbonsäureanhydrid bzw. Perylen-3,4,9,10-tetracarbonsäureimidderivate nach DE-A-2 237 539;
• oder gemäß DE-B-2 232 513 N,N'-Bis-(3-methoxy-propyl)-3,4,9,10-perylentetracarbonsäureimid;
• polynukleare Chinone nach DE-A-2 237 678; cis- bzw. trans-Perione nach DE-A-2 239 923;
• Thioindigo-Farbstoffe nach DE-A-2 237 680; Chinacridone nach DE-A- 2 237 679;
• Kondensationsprodukte aus Benzo-4,10-thioxanthen-3,1'-dicarbonsäureanhydrid und Aminen nach DE-A-2 355 075;
• Phthalocyanin-Derivate nach DE-A-2 239 924 und Farbstoffe, die durch Kondensation nach der Vorschrift Bull. Chem. Soc. Japan, 25, 411-413/1952, aus
• Perylen-3,4,9,10-tetracarbonsäureanhydrid und o-Phenylendiamin oder 1,8-Diaminophthalin hergestellt werden, gemäß DE-A-2 314 051.

Layer 2 or 5 has the function of a charge carrier generation layer. The compounds used to generate charge carriers determine to a particular degree the spectral sensitivity of the photoconductive system. Dyes of various classes are available as charge-generating substances. Examples include:

• Perylene-3,4,9,10-tetracarboxylic anhydride or perylene-3,4,9,10-tetracarboximide derivatives according to DE-A-2 237 539;
• or according to DE-B-2 232 513 N, N'-bis- (3-methoxypropyl) -3,4,9,10-perylenetetracarboximide;
• polynuclear quinones according to DE-A-2 237 678; cis- or trans-perione according to DE-A-2 239 923;
• Thioindigo dyes according to DE-A-2 237 680; Quinacridones according to DE-A-2 237 679;
• Condensation products from benzo-4,10-thioxanthene-3,1'-dicarboxylic acid anhydride and amines according to DE-A-2 355 075;
• Phthalocyanine derivatives according to DE-A-2 239 924 and dyes obtained by condensation according to the Bull. Chem. Soc. Japan, 25, 411-413 / 1952
• Perylene-3,4,9,10-tetracarboxylic anhydride and o-phenylenediamine or 1,8-diaminophthalene can be prepared, according to DE-A-2 314 051.

Dyes according to DE-A-2 246 255, 2 253 639 and 2 356 370 can also be used, for example.

The application of a homogeneous, densely packed layer 2 is preferably obtained by vacuum deposition. An advantageous layer thickness range of layer 2 is between 0.005 and 3 μm, since the adhesive strength and homogeneity of the vapor-deposited compound are particularly favorable here.

In combination with the intermediate layer or as a replacement for the intermediate layer, homogeneous, well-covering dye layers with thicknesses of the order of 0.1―3 µm can also be obtained by grinding the dye with a binder, in particular with highly viscous cellulose nitrates and / or crosslinking binder systems, for example polyisocyanate-crosslinkable acrylic resins. Lacquers based on polyisocyanates and hydroxyl-containing polyester or ether and by subsequently applying these dye dispersions 5 to the substrate can be produced, as can be seen from FIG. 3.

Organic materials which have an extensive z-electron system are particularly suitable as the material for the charge transport in layer 3. These include, in particular, monomeric aromatic or heterocyclic compounds which have at least one dialkylamino group or two alkoxy groups. Oxdiazole derivatives, which are mentioned in German Patent 1,058,836, have proven particularly useful. These include in particular 2,5-bis- (p-diethylaminophenyl) -oxdiazole-1,3,4. Other suitable monomeric electron donor compounds are, for example, triphenylamine derivatives, more highly condensed aromatic compounds such as pyrene, benzo-condensed heterocycles, and also pyrazoline or imidazole derivatives (DE-B-1 060 714, 1 106 599), which also include triazole, thiadiazole and especially oxazole derivatives, for example 2-phenyl-4- (2-chlorophenyl) -5- (4-diethylamino) oxazole, as are disclosed in German patents 1,060,260, 1,299,296, 120,875.

The charge transport layer 3 has practically no photosensitivity in the visible range (420-750 nm). It preferably consists of a mixture of an electron donor compound with a resin binder if negative charging is to be carried out.

Layer 3 is preferably transparent. However, it is also possible that it does not need to be transparent, for example in the case of a transparent, conductive layer support. It has a high electrical resistance (≥10 ¹² Ω) and prevents the discharge of electrostatic charge in the dark. When exposed, it transports the charges generated in the organic dye layer.

In addition to the charge generating and transporting compounds described so far, the added binder influences both the mechanical behavior such as abrasion, flexibility, film formation etc. and to a certain extent the electrophotographic behavior such as photosensitivity, residual charge and cyclical behavior.

Film-forming compounds such as polyester resins, polyvinyl chloride / polyvinyl acetate copolymers, styrene / maleic anhydride copolymers, polycarbonates, silicone resins, polyurethanes, epoxy resins, acrylates, polyvinyl acetals, polystyrenes, cellulose derivatives such as cellulose acetobutyrates etc. are used as binders. In addition, post-crosslinking binder systems such as DD coatings (eg Desmophen / Desmodur ^®, Bayer AG), polyisocyanatvernetzbare acrylate resins, melamine resins, resins of unsaturated polyesters, etc. applied successfully.

Because of the combined advantages (high photosensitivity, flash sensitivity, high flexibility), the use of cellulose nitrates, in particular the highly viscous types, is preferred.

The mixing ratio of the charge transporting compound to the binder can vary. However, the requirement for maximum photosensitivity, i. H. the largest possible proportion of charge-transporting compound and after avoiding crystallization and increasing flexibility, d. H. as large a proportion of binders as possible, relatively certain limits. A mixing ratio of about 1: 1 parts by weight has generally been found to be preferred, but ratios between 4: 1 to 1: 2 are also suitable.

The thickness of the layer 3 is preferably between about 3 and 20 gm.

Leveling agents such as silicone oils, wetting agents, in particular nonionic substances, plasticizers of different compositions, such as, for example, based on chlorinated hydrocarbons or based on phthalic acid esters are added to the layer as customary additives. If necessary, sensitizers and / or acceptors can also be added to the charge transport layer, but only to the extent that the optical transparency of the charge transport layer is not significantly impaired.

The protective cover layer formed on the organic photoconductor system has a uniform thickness of 0.5-10 µm, preferably 0.5-5 µm. This range of thicknesses is suitable because, on the one hand, this does not significantly influence the charge flow during the exposure and discharge process, and on the other hand, a significantly improved abrasion behavior is achieved.

The selection of the inorganic vapor deposition materials depends primarily on their evaporation temperature and rate, their resistance to evaporation and the properties of the vapor-deposited protective layers, such as homogeneous film formation, flexibility, surface conductivity (also depending on the air humidity) and the spectral Translucency.

Materials such as silicon monoxide or magnesium fluoride can improve the anti-reflective behavior of the photoconductor layer, which is reflected in better utilization of the incident light. They are therefore used with particular preference.

For the most part, these materials can be vapor-deposited in vacuum from electrically heated tungsten, molybdenum or tantalum boats at a substrate temperature of ≤50 ° C. Electron beam evaporators may also be necessary for certain materials.

These methods have the advantage that no organic solutions are required to apply a cover layer, as in a coating process followed by drying, so that there are also no problems of dissolution of the photoconductive layer or layers to be protected. Rather, the kinetic energy of the molecules to be evaporated ensures a homogeneous, adherent cover layer on the surface; compatibility with the photoconductive layer is practically always present. In the lower covering layer thickness range of up to approx. 3 μm, the electrophotographic recording material with covering layer proves to be largely flexible, with already significantly improved abrasion resistance.

Furthermore, the cover layers according to the invention have the advantage that they prevent surface effects which arise during handling of the photoconductor, for example crystallization effects by hand contact.

The cover layers according to the invention are preferably used as layer supports on photoconductor drums.

The invention is explained in more detail with the aid of the examples, without being restricted thereto.

example 1

On an electrophotographic recording material which, in the order given, consists of aluminum-coated 75 μm thick polyester film as an electrically conductive substrate, a dye layer of N, N'-dimethylperylimide (CI 71 129) as a charge-generating compound How a charge transport layer is made up of 2,5-bis (4-diethylaminophenyl) oxdiazole-1,3,4 (= To) and the binders polyester and polyvinyl chloride / polyvinyl acetate resin is used in a vacuum evaporation system at 2.7 × 10 ^-7 bar at a distance evaporator boat ― substrate of 20-25 cm light-grained silicon monoxide evaporated homogeneously. The evaporation takes place from tungsten boats, which are heated in a direct current passage. The material to be vapor-coated on one side for comparison purposes is clamped onto a rotating drum and varied from 5 to 15 minutes by means of different evaporation times in order to produce different SiO layer thicknesses.

• Zur Ermittlung der Hellentladungskurven bewegt sich die Meßprobe auf einem sich drehenden Teller durch eine Aufladevorrichtung hindurch zur Belichtungsstation, wo sie mit einer Xenonlampe kontinuierlich belichtet wird. Ein Wärmeabsorptionsglas und ein Neutralfilter mit 15% Transparenz sind der Lampe vorgeschaltet. Die Lichtintensität in der Meßebene liegt im Bereich von 50―100 µW/ cm², sie wird unmittelbar nach Ermittlung der Hellabfallkurve mit einem Opto-Meter gemessen. Die Aufladungshöhe (U_o) und die photoinduzierte Hellabfallkurve werden über ein Elektrometer durch eine transparente Sonde oszillographisch aufgezeichnet. Die Photoleiterschicht wird durch die Aufladungshöhe (U₀) und diejenige Zeit (T_1/2) charakterisiert, nach der die Hälfte der Aufladung (Uo/2) erreicht ist. Das Produkt aus Ti/₂ und der gemessenen Lichtintensität I (uW/cm2) ist die Halbwertsenergie E_i/2 (pJ/cm2). Gemäß dieser Charakterisierungsmethode wird die Photoempfindlichkeit mit und ohne SiO-Schutzschicht bestimmt:
  

The photosensitivity is measured as follows:

• To determine the light discharge curves, the test sample moves on a rotating plate through a charging device to the exposure station, where it is continuously exposed to a xenon lamp. A heat absorption glass and a neutral filter with 15% transparency are installed in front of the lamp. The light intensity in the measuring plane is in the range of 50-100 µW / cm ² , it is measured with an opto-meter immediately after determining the light decay curve. The charge level (U _o ) and the photo-induced light decay curve are recorded oscillographically using an electrometer using a transparent probe. The photoconductor layer is characterized by the charge level (U ₀ ) and the time (T _1/2 ) after which half the charge (Uo / 2) has been reached. The product of Ti / ₂ and the measured light intensity I (uW / cm2) is the half-value energy E _i / 2 (pJ / cm2). The photosensitivity with and without SiO protective layer is determined according to this characterization method:
  

The residual charge (U _R ) after 0.1 sec, determined from the above bright discharge curves, is a further measure of the discharge of a photoconductor layer.

It can be seen from this that the applied silicon monoxide layer of different thicknesses in this area essentially does not impair the photoconductor properties.

Example 2

A recording material vaporized on one side with SiO at 2 g / m ² according to Example 1 is exposed to the copying process in a copier (25 copies / min.) In order to demonstrate the improved surface properties. A magnetic brush device with a two-component toner mixture is used for development. To clean residual toner, the material is guided past a rotating brush. It can be seen that surface filming, resulting from the toner / photoconductor surface interaction, limits the service life of the unprotected surface. After 2000 copies, clear surface filming is already visible, but not at all on the protected surface.

After 10,000 copies, the aperture margin (max. Opening) is used for the production of perfect copies of the unprotected material by filming the surface, while the material according to the invention can be copied further in the initial aperture position.

Example 3

As in Example 1, magnesium fluoride is evaporated as a protective layer instead of silicon monoxide. Here, too, it can be seen that the photosensitivity is not yet reduced:

Example 4

In the same way as in Example 3, calcium fluoride is evaporated as a cover layer in a thickness of 0.5 g / m ² . In copying experiments according to Example 2, equally good photosensitivity is demonstrated. There was practically no surface filming on the top layer.

Example 5

On a recording material that consists of an aluminum-vaporized 125 µm thick polyester film, a layer of charge-generating compound (N, N'-bis (3-methoxy-propyl) perylimide according to DE-B-2 451 781) and a charge transport layer thickness 8.6 g / m ² ) consists of To and highly viscous cellulose nitrate, a top layer of silicon monoxide is evaporated to a thickness of about 3 g / m ² , as indicated in Example 1. To test the abrasion behavior of this material compared to an unprotected material, the abrasion of both layers is measured on a standard abrasion device (Taber Abraser Type 352) under the following conditions:

The abrasion in g / m ² is the quotient of the gravimetrically determined abrasion and the abrasion area.

The abrasion of the unprotected material results in approx. 2.5 g / m ² , the material provided with a covering layer has an abrasion of 0.9 g / m ² . Abrasion is therefore reduced by a factor of 3.

Claims (4)

1. An electrophotographic recording material, composed of an electrically conductive support, an insulating interlayer, if appropriate, and a photoconductive layer system of organic materials, comprising a layer which contains a binder, if appropriate, and a compound which generates charge carriers, and a layer containing a compound which transports charges, as well as a protective transparent covering layer, wherein the covering layer is composed of silicon monoxide, magnesium fluoride or calcium fluoride, applied under 1 to 10-³ Pa, and has a thickness of 0.5to 10 um.

2. A recording material as claimed in claim 1, wherein the compound which generates charge carriers is an organic dyestuff.

3. A recording material as claimed in claim 1, wherein the compounds which transport charges are monomeric aromatic or heterocyclic compounds which posses at least one dialkylamino group or two alkoxy groups.

4. A recording material as claimed in claim 1, wherein cellulose nitrate is present as the binder for the photoconductive layer.

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