GB2108694A - Photoconductive layer and process for the preparation thereof - Google Patents

Abstract

A photoconductive material, which is preferably in the form of a layer in an electrophotographic material, comprises (1) a polymerised heterocyclic vinyl compound, (for example poly-N-vinylcarbazole) or a polycondensed carbocyclic aromatic compound (for example a bromopyrene resin) as photoconductor, (2) an electron acceptor, (3) a subsequently crosslinkable binder and, (4) optionally, additives which improve the adhesion and/or the surface structure of the layer; wherein, the electron acceptor is present in an amount from about 0.1 to 0.5 mol per monomer unit of the photoconductor and the binder is present in an amount from about 30 to 120 parts by weight per 100 parts by weight of the photoconductor. A process for the preparation of the photoconductive layer is also described.

Description

SPECIFICATION Photoconductive layer and process for the preparation thereof This invention relates to a photoconductive layer and to a process for the preparation of such a layer.

Organic photoconductors, particularly polymeric photoconductors, such as polyvinylcarbazole and derivatives of polyvinylcarbazole or polycondensed carbocylic aromatic compounds are known and have been described, for example, in German Patents Nos. 10 68 11 5 (=U.S. Patent No.3,037,861), 11 73797 (=U.S. Patent No.

3,155,503), 1472 982 (=U.S. Patent No.

3,232,755), in German Auslegeschrift 11 09 032 (=U.S. Patent No. 3,206,306) and in German Patent No. 21 37 288 (=U.S. Patent No.

3,842,038).

It is known to improve the sensitivity of certain organic photoconductors in the visible light range, by adding substances which act as electron acceptors: German Patent No. 11 11 935 (=U.S.

Patent No. 3,037,861), German Patent No.

11 27218 (=U.S. Patent No.3,287,120) German Patent No. 12 33 265 (=British Patent Specification No. 988,361).

In the process described in the abovementioned patents, the electron acceptors are used in concentrations of up to about 20 percent by weight, relative to the photoconductor (German Patent No. 11 935=U.S. Patent No.

3,037,661). Photoconductive layers with considerably higher contents of electron acceptors, such as, for example, up to about 1.23 mols of electron acceptor per mol of monomer unit of the photoconductor have, however, also been disclosed (German Auslegeschrift 15 72 347=U.S. Patent No. 3,484,237 and German Auslegeschrift No. 20 59 540=British Patent Specification No. 1,375,719). These photoconductive layers are applied to an electrically conductive support to form a film, which has a dry thickness of about 0.5 to 50 ym, on the support and the material so prepared is used as an electrographic or electrophotographic reproduction material.If the layer comprising the polymeric photoconductor is transparent and flexible and has a high dark-resistivity, a corresponding reproduction material can be substantially light transmissive and flexible, with a high charge-acceptance. The transparent and/or flexible reproduction material is then not only suitable for use in a conventional office copier, but may, for example, also be used for an electrophotographic microfilm and/or for a reflex exposure system, in which light passes through the photoconductive layer.

It is furthermore known that a layer comprising a polymeric photoconductor, for example, polyvinylcarbazole, is brittle and has to be combined with inactive additives, such as plasticisers and/or binders, if a mechanically tough and flexible layer is required. Suitable plasticisers are, for example, chlorinated diphenyl, epoxy resin, dioctyl phthalate and tricresyl phosphate. Binders which may be used for this purpose include ketone resins, polycarbonates and cyanoethyl cellulose. As disclosed in German Offenlegungsschrift 22 38 425 (=U.S. Patent No.

3,850,629), it is also possible to add isocyanate compounds in an amount of less than 30 parts by weight per 100 parts of the photoconductor. The plasticiser and binder constituents are mixed homogeneously with the polymeric photoconductor and the photoconductive layer which is finally obtained is substantially flexible and can be used in practice as a reproduction material in a commercial copier. As far as mechanical toughness is concerned, the photoconductive layer so prepared has proved to be satisfactory for use.

The addition of such additives is, however, disadvantageous so far as the electrographic characteristics (light-decay rate and residual potential) of the photoconductor are concerned, since the photoconductive molecules are diluted with these inactive additives. By increasing the amounts of inactive additives, the light-decay rate and the residual potential of the photoconductive layer are reduced and increased, respectively, in an undesirable manner.

It is also a disadvantage of previously proposed photoconductive layers containing plasticisers and/or binders that they have a relatively low resistance to abrasion. If a photoconductive layer of that kind is used in a conventional copier and is, for example, cleaned by means of a scraper blade, scratch marks will very soon develop.

These scratch marks will give rise to streaks appearing on the copies and it will then be necessary to replace the photoconductive material.

The present invention provides a photoconductive material comprising at least one photoconductor selected from polymerised heterocyclic vinyl compounds and polycondensed carbocyclic aromatic compounds, a total of about 0.1 to 0.5 mol per photoconductor monomer unit of at least one electron acceptor, and a total of about 30 to 1 20 parts by weight per 100 parts by weight of photoconductor of at least one subsequently crosslinkable binder, the photoconductive material being opaque.

The photoconductive material is advantageously in the form of a photoconductive layer, and the invention thus also provides such a layer, which is advantageously on a support.

More particularly, the invention provides a photoconductive layer comprising a polymerised heterocyclic vinyl compound or a polycondensed carbocyclic aromatic compound as the photoconductor, an electron acceptor, at least one subsequently crosslinkable binder and, optionally, additives which improve adhesion and/or surface structure, the photoconductive layer containing the electron acceptor in an amount ranging from about 0.1 to 0.5 mol per monomer unit of the photoconductor, and the binder in an amount ranging from about 30 to 120 parts by weight per 100 parts by weight of the photoconductor, and the photoconductive layer being opaque.

The photoconductive material of the present invention is preferably suitable for use in an electrographic or electrophotographic copying process, and the invention thus also provides an electrographic or electrophotographic reproduction material which comprises a photoconductive material, particularly a photoconductive layer, according to the invention. At least when the photoconductive material is to be used in a reproduction material, the crosslinkable binder in the photoconductive material of the invention is advantageously crosslinked.

The photoconductive material may if desired contain one or more photoconductors in addition to the polymerised heterocyclic vinyl compound(s) and/or polycondensed carbocyclic aromatic compound(s). If this is the case, the amounts of electron acceptor and binder are calculated on the total amount of photoconductor present.

The photoconductive material preferably contains the binder in an amount ranging from about 40 to 100 parts by weight per 100 parts by weight of the photoconductor. The subsequently crosslinkable binder preferably comprises an aliphatic isocyanate in admixture with a lowmolecular weight compound containing hydroxyl groups, the compound which contains the hydroxyl groups being selected from polyesters, acrylates and epoxides. The isocyanate and the compound containing hydroxyl groups are preferably prereacted with each other to give the crosslinkable binder. The low molecular weight compound containing hydroxyl groups has a molecular weight of about 5,000 or below, for example frem 500 to 5000.

The present invention makes it possible to provide a photoconductive layer for an electrographic or electrophotographic reproduction material, which comprises a polymeric or polycondensed photoconductor and appropriate additives and which is not only flexible, tough and resistant to abrasion, but has also a lightdecay rate and a residual potential which approach the values obtained with the photoconductor without additives.

In particular it is possible to provide a photoconductive layer which is flexible, tough and resistant to abrasion and which is capable of adhering firmly to a support to which it is applied and then has values of a light-decay rate and residual potential which substantially correspond to those of the plain photoconductor when applied to a support. It was completely surprising that such a good result could be obtained, in spite of an increase in the binder content and a change from the transparent layer, hitherto called for, to an opaque layer.

Preferred polymerised heterocyclic vinyl compounds for use as the photoconductor include polymers of vinylcarbazole, of vinyldibenzofuran and/or of vinyldibenzothiophene, particularly polymers of N-vinylcarbazole or of one of its derivatives or copolymers with other heterocyclic vinyl compounds. It is also possible to use mixtures of heterocyclic polyvinyl compounds with vinylcarbazole polymers and copolymers.

Poiy-N-vinylcarbazole is preferably used.

Preferred carbocyclic aromatic compounds are pyrene and perylene, and the substitution products and derivatives thereof, which are polycondensed with formaldehyde or paraformaldehyde. Of these aromatic compounds, halogen derivatives of pyrene have proved to be particularly useful because of their satisfactory solubility characteristics and their relatively good electrophotographic sensitivity which manifest itself in the polycondensate. Bromine derivatives are more readily available because it is easier to prepare these compounds and, therefore, a polycondensate of 3-bromopyrene and formaldehyde or paraformaldehyde is preferably used. Where a polycondensed carbocyclic aromatic compound is used as the or a photoconductor, the only monomer units in this compound taken into consideration when calculating the amount of electron acceptor are the units derived from the aromatic compound.

Substances which are advantageously used as electron acceptors which allow a sensitisation in specific spectral regions and shift the maximum of sensitivity from the short-wave to the long-wave region of the spectrum include fluorene, fluorenone and naphthalic acid, these compounds being substituted by nitro and/or cyano groups.

Particularly suitable compounds are, for example, 2,4,7-trinitrofluorenone and 3,6-dinitronaphthalic anhydride.

Suitable subsequently crosslinkable binders include, for example, systems comprising one or more saturated or unsaturated polyesters or polyethers which polyester or polyethers contain hydroxy groups and are capable of reacting with one or more aliphatic and/or aromatic polyisocyanate resins to give a crosslinked product; acrylic and methacrylic resins or epoxy resins which contain hydroxy groups and are capable of reacting with polyisocyanates to give crosslinked products; and air-drying polyurethane resins or temporarily blocked polyisocyanates with isocyanate groups which are capable of crosslinking upon heating. The subsequently cross linkabie binder preferably comprises a low molecular weight compound carrying hydroxy groups with an aliphatic isocyanate, the compound which carries hydroxy groups being selected from polyesters, acrylates and epoxides.

To improve adhesion and/or the surface structure of the photoconductive material plasticisers may be added. Such additives can also improve the flexibility of the polymeric or polycondensed photoconductor which is admixed with the subsequently crosslinkable binder.

Plasticisers which may be employed include commercially available products which are readily compatible with the photoconductor. Suitable plasticisers are, for example, terphenyl, chlorinated diphenyls, chlorinated naphthalenes and epoxy resins. Further substances that may be added include, for example, a silicone oil or a fluorocarbon oil, which imparts a smooth surface to the photoconductive material. The quantities of the additives used are chosen such that the mechanical properties and the photoconductivity of the material can be adjusted appropriately.

A photoconductive layer according to the invention may be supported temporarily or permanently on a support. Thus, the photoconductive layer may either be in the form of a self-supporting film or, in its preferred form, is applied to a support to give, for example, a flexible web, a flat plate or a photoconductor drum. The support is preferably made of an electrically conductive material, for example, of brass, aluminium, or steel, or of a dielectric or insulating material which is provided with a conductive coating. The support may have any appropriate thickness and may be rigid or flexible. Metallised paper and plastics films, such, for example, as polyester films which are provided with a thin metal layer comprising, for example, aluminium or copper iodide, or glass which is coated with a thin layer of chromium oxide or tin oxide, may also be used as supports.The support may be provided with an insulating, optinally adhesionpromoting, barrier layer which is an organic intermediate layer, for example, of a polyamide resin or polyvinyl phosphonic acid, or a metaloxide layer, for example, of aluminium oxide, which barrier layer preferably has a thickness of about 0.01 to 1 ssm.

The photoconductive layer of the invention may additionally be provided with a protective covering layer, as is known in the art.

The invention is explained in further detail with reference to the accompanying drawings. In Figure 1 of the drawings the relative sensitivity of a photoconductive layer comprising 100 parts by weight of polyvinylcarbazole (PVCa), 33 parts by weight each of isocyanate and polyester as in Examples 3 to 7 and 3,6-dinitronaphthalic anhydride (DNNA) as electron acceptor, is plotted as a function of the indicated mixing ratios. As can be seen, a good sensitivity is achieved in the range from about 0.1 to about 0.5 mol of electron acceptor per mol of monomer unit of polyvinylcarbazole.

Figure 2 shows the relative sensitivity of a photoconductive layer according to the invention, comprising polyvinylcarbazole (100 g), 3,6- dinitronaphthalic anhydride (20 g) and, as the binder, a polyester and polyisocyanate which are capable of reacting to give a crosslinked product and which are as specified in Example 3 below (curve II), compared with the relative sensitivity of a known photoconductive layer composed of polyvinylcarbazole (100 g) and 3,6-dinitronaphthalic anhydride (20 g), and a low-molecular weight polyester (as specified in Example 2 below) as the polymeric binder (curve 1). As indicated, curve I cannot be continued beyond a binder content of about 20 percent by weight, relative to the photoconductor, since the polymeric photoconductor and the polyester are no longer compatible and it is thus impossible to prepare further useful layers. In mixtures which contain polyester and polyvinylcarbazole in ratios over 20:100, the components are incompatible and will precipitate in the solutions prepared therewith. These solutions can no longer be used for coating purposes. Precipitation can be avoided by adding a further quantity of the solvent, but it is then impossible to apply a coating of, for example, 10 g/m2, which is required to obtain uniform comparative values.

Contrary to the prevailing opinion that sensitivity decreases continuously with a rising binder content, an increase of the binder content only causes an initial drop in the sensitivity of the photoconductive layer of this invention (curve II).

When the amount of subsequently crosslinkable binder is raised to over about 30 parts by weight per 100 parts by weight of the photoconductor, a steep rise in sensitivity is observed and, at the same time, the photoconductive layer of the invention becomes opaque while retaining its glossy surface (curve II). The term "opaque" is intended to denote the phenomenon of light scattering, the wavelength of the light involved being that of the visible spectrum. The reason for the above change to opacity is not yet fully understood, but it appears that the mixture of the photoconductor and the subsequently crosslinkable binder is no longer entirely compatible.

This is, however, not noticeable in the coating solution which remains clear and can thus be used for coating. It may be the case that some separation occurs during drying of the coating solution, which would account for the turbid appearance of the photoconductive layer. At a ratio of about 100 to 120 parts by weight of subsequently crosslinkable binder per 100 parts by weight of the photoconductor, a drop in sensitivity takes place, which is also recognisable by a certain roughness on the surface of the photoconductive layer. The surface then becomes porous and useless for electrographic and/or electrophotographic applications, since a uniform electric charge can no longer be applied and, in addition, the toner which is required to render the latent charge images visible, will stick in the pores.

The invention also provides a process for the preparation of a photoconductive layer according to the invention and a layer produced by the process. In the process, a coating solution is prepared by dissolving the constituents of the layer in a solvent. This solution is then applied to a support in an atmosphere of less than about 40% relative humidity and is dried at a temperature in the range of from about 60 to 1 200C and then post-cured at temperatures ranging from about 1 1 OOC to 1 600C, preferably about 140"C to 1 600 C. When the subsequently crosslinkable binder comprises an isocyanate compound and a compound containing hydroxy groups, these are preferably first dissolved in a solvent (preferably each is first dissolved separately in a solvent) and the solution is allowed to prereact with the exclusion of water or water vapour, e.g., atmospheric humidity. This reaction may be continued over a certain period which, however, should normally not exceed 4 days at ambient temperature. Preferably, prereaction is allowed to take place for 24 hours at ambient temperatures.

Vessels containing the solution of the subsequently crosslinkable binder are preferably closed to exclude atmospheric humidity.

Post-curing of the dried photoconductive layer is effected at temperatures in the range from about 11 00C to 160"C, preferably about 1 400C to 1 600 C, advantageously within a period of a few minutes up to one hour. Preferably, the postcuring operation takes about 30 minutes at the above-identified temperatures. The post-curing step may if desired be carried out simultaneously with the drying step.

Suitable solvents for use in the process of the invention include, for example, benzene, toluene, chlorobenzene, methylene chloride, ethylene dichloride, ethylene trichloride, ethylene tetrachloride and other halogenated aliphatic and/or aromatic hydrocarbons, used alone or as mixtures. Preferably, tetrahydrofuran or a mixture of tetrahydrofuran with, for example, cyclohexanone is used as the solvent. A paricularly preferred solvent mixture comprises tetrahydrofuran containing 5 percent by weight of cyclohexanone.

Any suitable coating method may be used to apply the coating solution to the support, for example, a doctor-blade coating, roll coating or spray coating method.

The applied solution is dried to form a layer which preferably has a thickness in the range of from about 5 to 50 ym.

Of the following examples, Examples 4-7, 11-13 and 15-18 illustrate the invention, while Examplesl-3, 8-10, 14 and 19 are comparative examples.

Example 1 A solution is prepared of 10 g of poly-N-vinylcarbazole (PVCa) 2 g of 3,6-dinitronaphthalic anhydride (CNNA), and 0.1 g of silicone oil, in 130 g of tetrahydrofuran.

The solution is applied to an aluminium sheet in a thickness which corresponds to a dry-layer weight of about 10 g/m2 and is then dried for 30 minutes at 11 00C. The resulting layer is transparent and has a glossy surface. When sprayed with a positive corona discharge, the layer shows a very low charge acceptance. Its flexibility and resistance to abrasion are extremely poor. In accordance with known measures, a plasticiser is added, typically in a concentration of 5 to 20 percent by weight, relative to the PVCa.

This plasticiser improves the flexibility of the layer; an improved resistance to abrasion is, however, not obtained by the use of this additive.

Example 2 A solution is prepared of 10gofPVCa, 2 g of DNNA, 2 g of a low-molecular weight polyester (Resin 49,000, Du Pont), and 0.1 g of silicone oil, in 1 50 g of tetrahydrofuran.

The weight ratio between the polyester binder and the PVCa is 20:100. The solution is applied to an alumnium sheet, in a thickness which corresponds to a dry-layer weight of about 10 g/m2 and is then dried for 30 minutes at 11 00C.

The layer obtained is substantially transparent and has a glossy surface. Its electrophotographic sensitivity corresponds to about 30% of the value of the PVCa-DNNA layer of Example 1, as can be seen from curve I in Figure 2. The flexibility of the layer is definitely improved over that of the layer of Example 1. It is, for example, possible to bend the layer around a bar which has a radius of 1 cm, without cracks appearing. Resistance to abrasion is, however, still poor. As in the layer of Example 1, distinct abrasion marks occur in the layer after 5,000 cycles in an abrasion tester which simulates the copying process; these abrasion marks become visible as streaks on the copy and it is consequently necessary to replace the photoconductive material.

Example 3 A solution is prepared of 10gofPVCa, 2 9 of DNNA, 0.75 g of a low-molecular weight, branched chain polyester which is capable of cross linking with a polyisocyanate (Desmophen 651, Bayer AG-Desmophen is a trade mark), 1.0 g of hexamethylene diisocyanate, reaction product with water.

(NCO-CH2)6-NH-CO)2-N-(CH2)6-NCO, 75 percent strength, and 0.01 g of silicone oil, in 130 g of tetrahydrofuran.

10 mol-percent strength solutions are first prepared from the polyester and diisocyanate components, which are mixed and are allowed to prereact for 24 hours, with the exclusion of humidity. This procedure, although not specifically mentioned in each case, is also employed in the Examples which follow. The weight ratio between the subsequently cross linkable binder and the PVCa is 15:100. At a relative atmospheric humidity of about 30%, the solution is applied to an aluminium sheet in a thickness which corresponds to a dry-iayer weight of about 10 g/m2; it is dried at 1000C immediately after appiication and is then post cured at 1 600C for 30 minutes. The resulting layer is transparent and has a glossy surface.

The charge-acceptance of the layer is considerably improved by the addition of the binder component. The layer is, however, brittle and has an insufficient resistance to abrasion. Its electrophotographic sensitivity is less than 10% of the sensitivity value of the PVCa/DNNA layer of Example 1 (curve II in Figure 2).

Example 4 A solution is prepared of 10gofPVCa, 2 9 of DNNA, 1.5 g of the polyester specified in Example 3, 2 g of the 75% strength diisocyanate reaction product specified in Example 3, and 0.01 g of silicone oil, in 1 50 g of tetrahydrofuran.

The weight ratio between the subsequently crosslinkable binder and the PVCa is 30:100. At a relative humidity of about 30%, the solution is applied to an aluminium sheet, in a thickness which corresponds to a dry-layer weight of about 10 g/m2; it is immediately dried at 1000C and then post-cured at 1 600C for 30 minutes. The layer thus prepared is not definitely transparent and is a layer in accordance with the invention; it has a glossy surface.

The charge-acceptance of the layer is greatly improved by the addition of the binder component. Flexibility and resistance to abrasion of the layer are satisfactory. The electrophotographic sensitivity is less than 20% of the sensitivity obtained in Example 1.

Example 5 A solution is prepared of 10gofPVCa, 2 9 of DNNA, 3.75 g of polyester specified in Example 3, 5 g of the 75% strength diisocyanate reaction product specified in Example 3, and 0.01 g of silicone oil, in 1 90 g of tetrahydrofuran.

The weight ratio between subsequently crosslinkable binder and the PVCa is 75:100. The solution is applied to an aluminium sheet, at a relative humidity of about 30%, in a thickness which corresponds to a dry-layer weight of about 10 g/m2; it is immediately dried at 1000C and is then post-cured at 1 600C for 30 minutes. The resulting layer is opaque and has a glossy surface.

The electrophotographic sensitivity is about 80% of the value obtained with the PVCa-DNNA layer of Example 1, in spite of its high binder content, as can be seen from curve II in Figure 2. The charge-acceptance of the layer, its flexibility and resistance to abrasion are considerably improved by the specified additives.

Upon bending around a bar which has a radius of 1 cm, the plain PVCa-DNNA layer of Example 1 develops severe cracks and begins to detach itself from the support. In contrast to this, the layer comprising the photoconductor and the subsequently crosslinkable binder as described in this example does not show any cracks when it is treated in the same manner. While the layer of Example 1 and also that of Example 2 show distinct abrasion marks after 5,000 cycles in an abrasion tester simulating the copying procedure, the first signs of abrasion marks can only be detected after 50,000 cycles when testing the layer of the present example.

Example 6 A solution is prepared of 10gofPVCa, 2gofDNNA, 4.5 g of the polyester specified in Example 3, 6.0 g of the 75% strength diisocyanate reaction product specified in Example 3, and 0.01 9 of silicone oil, in 1 80 g of tetrahydrofuran.

The weight ratio between the subsequently crosslinkable binder and the PVCa is 90:100. The solution is applied to an aluminium sheet, at a relative humidity of about 30%, in a thickness which corresponds to a dry-iayer weight of about 10 g/m2; it is dried at 1000C immediately after application and is then post-cured at 1 600C for 30 minutes. The layer is opaque and has a glossy surface. In spite of the high content of inactive binder, the electrophotographic sensitivity amounts to about 60% of the value obtained with the PVCa-DNNA layer of Example 1 (curve Il in Figure 2). Charge-acceptance, flexibility and resistance to abrasion of the layer are greatly improved by the specified addition of the binder component.

Example 7 A solution is prepared of 10gofPVCa, 2gofDNNA, 6 g of the polyester specified in Example 3, 8 g of the 75% strength diisocyanate reaction product as specified in Example 3, and 0.01 g of silicone oil, in 220 g of tetrahydrofuran.

The weight ratio between the subsequently crosslinkable binder and the PVCa is 120:100. At a relative humidity of about 30%, the solution is applied to an aluminium sheet, in a thickness which corresponds to a dry-layer weight of about 10 g/m2; it is immediately dried at 1000C and it then post-cured at 1 600C for 30 minutes. The layer thus prepared is opaque and has a glossy surface. Its electrophotographic sensitivity is about 50% of the value obtained with the PVCa DNNA layer of Example 1. The chargeacceptance, flexibility and resistance to abrasion of the layer are considerably improved by the specified addition of the binder component.

A further increase in the binder content over a ratio of about 130:100 results in layers which have a rough and porous surface and a low charge-acceptance. Such layers are not suitable for use in an electrophotographic copying process.

Example 8 A solution is prepared of 10gofPVCa, 2 g of 2,4,7-trinitro-9-fluorenone (TNF), and 0.1 g of silicon oil, in 1 30 g of tetrahydrofuran.

The solution is applied to an aluminium sheet to give a thickness of the dry-layer corresponding to a weight of about 10 g/m2. It is then dried for 30 minutes at 1 1 OOC. The layer thus obtained is transparent and has a glossy surface. The layer has a very low charge-acceptance when it is sprayed with a positive corona discharge. Its flexibility and resistance to abrasion are very poor.

A plasticiser is therefore added in a concentration of, typically, 5 to 20 percent by weight, relative to PVCa. By this means, the flexibility of the layer is improved; an improved resistance to abrasion is, however, not obtained.

Example 9 A solution is prepared of 10gofPVCa, 2 g of TNF, 2 g of a low-molecular weight polyester, as indicated in Example 2, and 0.1 g of silicone oil, in 1 50 g of tetrahydrofuran.

The weight ratio between the polyester binder and the PVCa is 20:100. The solution is applied to an aluminium sheet to give a dry-layer thickness corresponding to a weight of about 10 g/m2. It is then dried for 30 minutes at 1 1 OOC. The layer thus obtained is generally transparent and has a glossy surface. The electrophotographic sensitivity corresponds to about 60% of the value of the PVCa-TNF layer of Example 8. Flexibility is markedly improved over that of the layer of Example 8. It is, for example, possible to bend this layer around a bar which has a radius of 1 cm, without cracks appearing. Resistance to abrasion is, however, still poor. As in the layer of Example 8, disadvantageous abrasion marks are already clearly perceivable after 5,000 cycles in an abrasion tester which simulates the copying process.It is then necessary to replace the photoconductive material.

Example 10 A solution is prepared of 10 g of PVCa, 2 g of TNF, 0.75 of the polyester specified in Example 3, 1.0 g of the 75% strength diisocyanate reaction product as specified in Example 3, and 0.01 g of silicon oil, in 1 30 g of a solvent (tetrahydrofuran mixed with 5 percent by weight of cyclohexanone).

The weight ratio between the subsequently crosslinkable binder and the PVCa is 15:100. At a relative humidity of about 30%, the solution is applies to an aluminium sheet, in a thickness which corresponds to a dry-layer weight of about 10 g/m2; it is immediately dried at 1000C and is then post-cured at 1 600C for 30 minutes. The resulting layer is transparent and has a glossy surface.

The charge-acceptance of the layer is considerably improved by the added binder component; its elasticity and resistance to abrasion are, however, still insufficient. The electrophotographic sensitivity is less than 70% of the sensitivity of the layer according to Example 8.

Example 11 A solution is prepared of 10 g of PVCa, 2 g of TNF, 1.5 g of the polyester of Example 3, 2.0 g of the 75% strength diisocyanate reaction product according to Example 3, and 0.01 g of silicone oil, in 1 50 g of the solvent mixture of Example 10.

The weight ratio between the subsequently crosslinkable binder and the PVCa is 30:100. At a relative humidity of about 30%, the solution is applied to an aluminium sheet, in a thickness which corresponds to a dry-layer weight of about 10 g/m2; it is immediately dried at 1000C and is then post-cured at 1 600C for 30 minutes. The layer is not definitely transparent and is a layer in accordance with the invention; it has a glossy surface.

The charge-acceptance of the layer is considerably improved by the added binder component, and its elasticity and resistance to abrasion are satisfactory. The electrophotographic sensitivity is less than 50% of the sensitivity obtained in Example 8.

Example 12 A solution is prepared of 10 g of PVCa, 2 g of TNF, 3.75 g of the polyester of Example 3, 5 g of the 75% strength diisocyanate reaction product according to Example 3, and 0.01 g of silicone oil, in 180 g of the solvent mixture of Example 10.

The weight ratio between the subsequently crosslinkable binder and the PVCa is 75:100. At a relative humidity of about 30%, the solution is applied to an aluminium sheet, in a thickness which corresponds to a dry-layer weight of about 10 g/m2; it is immediately dried at 1000C and is then post-cured at 1 600C for 30 minutes. The layer thus prepared is opaque and has a glossy surface. In spite of the high content of inactive binder, the electrophotographic sensitivity is about 80% of the value obtained with the plain PVCa-TNF layer of Example 8. The chargeacceptance of the layer, its elasticity and resistance to abrasion are greatly improved by the specified addition of the binder component.

The PVCa-TNF layer of Example 8, for example, develops severe cracks, and begins to detach itself from the support, when it is bent around a bar which has a radius of 1 cm. In the present layer comprising a photoconductive compound and a subsequently crosslinkable binder, on the other hand, no cracks are detected when it is treated in the same way. Furthermore, whereas the PVCa-TNF layer of Example 8 and also the layer of Example 9 show clear abrasion marks after 5,000 cycles in an abrasion tester which simulates the copying process, abrasion marks are not observed in the layer described in this Example until it has performed 50,000 cycles.

Example 13 A solution is prepared of 10 g of PVCa, 2 g of TNF, 4.5 g of the polyester used in Example 3, 6 g of the 75% strength diisocyanate reaction product according to Example 3, and 0.01 f of silicone oil, in 1 90 g of the solvent mixture of Example 10.

The weight ratio between the subsequently crosslinkable binder and the PVCa is 90:100. At a relative humidity of about 30% the solution is applied to an aluminium sheet, in a thickness which corresponds to a dry-layer weight of about 10 g/m2; it is immediately dried at 1000C and is then post-cured at 1 600C for 30 minutes. The resulting layer is opaque and has a glossy surface.

In spite of the high content of inactive binder, the electrophotographic sensitivity is about 80% of the value of the PVCa-TNF layer of Example 8.

Charge-acceptance, elasticity and resistance to abrasion of the layer are considerably improved by the specified addition of the binder component.

A further increase in the binder content over a ratio of about 100:100 leads to layers which have a rough and porous surface and a lower chargeacceptance.

Example 14 A solution is prepared of 10 g of PVCa, 2gofDNNA, 1.0 g of acrylate resin which is capable of crosslinking with a polyisocyanate (Desmophen A 360, Bayer AG Desmophen is a trademark), 0.67 g of the 75% strength diisocyanate reaction product according to Example 3, and 0.01 g of silicone oil, in 1 30 g of tetrahydrofuran.

10 mol percent strength solutions are first prepared from the polyacrylate and diisocyanate components. The solutions are then mixed and allowed to preact for 24 hours at ambient temperature, while preventing any access of humidity. This procedure is also employed in the following examples.

The weight ratio between the subsequently crosslinkable binder and the PVCa is 15:100. At a relative humidity of about 30%, the solution is applied to an aluminium sheet to give a final layer thickness corresponding to about 10 g/m2; it is immediately dried at 1000C and is then postcured at 1 600C for 30 minutes. The layer is transparent and has a glossy surface. The chargeacceptance of the layer is considerably improved (compared with the layer in Example 1) by the addition of the binder component; its elasticity and resistance to abrasion are, however, still insufficient. The electrophotographic sensitivity is less than 10% of the sensitivity obtained in a layer without a binder, according to Example 1.

Example 15 A solution is prepared of 10gofPVCa, 2 9 of DNNA, 2.04 g of the acrylate of Example 14, 1.8 g of hexamethylene diisocyanate, according to Example 14, and 0.01 g of silicone oil, in 1 50 g of tetrahydrofuran.

The weight ratio between the subsequently cross linkable binder and the PVCa is 30:100. At a relative humidity of about 30%, the solution is applied to an aluminium sheet, in a thickness which corresponds to a dry-layer weight of about 10 g/m2; it is immediately dried at 1000C and is then postcured at 1 600C for 30 minutes. The layer is in a state of beginning opacity, and is a layer in accordance with the invention.

The charge-acceptance of the layer is considerably improved by the binder component; its elasticity and resistance to abrasion are satisfactory. The electrophotog raphic sensitivity is less than 10% of the sensitivity value obtained in a layer without addition of a binder component, according to Example 1.

Example 16 A solution is prepared of 10 g of PVCa, 2 sq of DNNA, 5.1 g of acrylate used in Example 14, 3.27 g of the isocyanate of Example 14, and 0.01 g of silicone oil, in 1 80 g of tetrahydrofuran.

The weight ratio between the subsequently cross linkable binder and the PVCa is 75:100. At a relative humidity of about 30%, the solution so prepared is applied to an aluminium sheet, in a thickness which corresponds to a dry-layer weight of about 10 g/m2; it is immediately dried at 1000C and is then post-cured at 1 600C for 30 minutes. The resulting layer is opaque and has a glossy surface. In spite of the high content of inactive binder, the electrophotographic sensitivity is about 80% of the value of the binder free PVCa-DNNA layer of Example 1. The charge acceptance, elasticity and resistance to abrasion of the layer are satisfactory.

As already mentioned, the PVCa-DNNA layer of Example 1 develops severe cracks and begins to detach itself from the aluminium support when it is bent around a bar which has a radius of 1 cm.

In the present layer which contains the subsequently cross-linkable binder, cracks cannot be detected when it is treated in the same way.

The PVCa-DNNA layer of Example 1 and also the layer of Example 2 show distinct abrasion marks after 5,000 cycles in an abrasion tester which simulates the copying process, while the layer described in this Example does not show abrasion marks until it has performed 50,000 cycles.

Example 17 A solution is prepared of 10 g of PVCa, 2 9 of DNNA, 6.1 g of the acrylate used in Example 14, 3.8 of the isocyanate used in Example 14, and 0.01 g of silicone oil, in 190 g of tetrahydrofuran.

The weight ratio between the binder and the PVCa is 90:100. At a relative humidity of about 30%, the solution is applied to an aluminium sheet, in a thickness which corresponds to a dry layer weight of about 10 g/m2; it is immediately dried at 1000C and is then post-cured at 1 600C for 30 minutes. The layer thus obtained is opaque and has a glossy surface. In spite of the high content of inactive binder, the electrophotographic sensitivity of the layer is about 70% of that of the PVCa/DNNA layer of Example 1. The charge-acceptance, elasticity and resistance to abrasion of the layer are considerably improved by the specified addition of the binder component.

A further increase in the binder content over a ratio of about 100:100 results in layers which have a rough surface and a lower chargeacceptance. Such layers are not suitable for use in an electrophotographic copying process.

Example 18 A solution is prepared of 10 g of a bromopyrene resin (prepared as indicated below), 1.0 g of 3,6-dinitronaphthalic anhydride, 3.57 g of the acrylate resin used in Example 14, 1.90 g of the diisocyanate used in Example 14, and 0.01 g of silicone oil, in 100 g of tetrahydrofuran.

The weight ratio between the subsequently crosslinkable binder and the photoconductor is 50:100. The solution is applied to an aluminium sheet in a thickness corresponding to a dry-layer weight of about 5.5 g/m2. It is then dried and cured for 30 minutes at 1 1 OOC. The resulting layer is opaque and has a glossy surface. When it is sprayed with a negative corona discharge, the layer shows a very good charge-acceptance.

Flexibility and resistance to abrasion of this layer are excellent.

The bromopyrene resin was prepared in the following manner: 40 g of 3-bromopyrene are dissolved, together with 4.5 g of para-formaldehyde, in 400 ml of glaciai acetic acid at 95-1 000C, 2 g of anhydrous ZnCI2 are added and the formulation is further stirred for 3 to 4 hours. The precipitating polycondensate is drawn off by suction, washed with water and ethanol, dried, and then purified by dissolving the polycondensate in tetrahydrofuran, filtering, and then reprecipitating the polycondensate by introducing the solution dropwise into methanol. This purification process is repeated. A colourless powder results which has a softening range of 130-1 400C and a degree of polycondensation between 6 and 8.

Yield: 9095%.

Example 19 A solution is prepared of 10 g of the bromopyrene resin of Example 18, 1.0 g of 3,6-dinitronaphthalic anhydride, 5.0 g of a copolymer of vinyl chloride and vinyl acetate, and 0.01 g of silicon oil, in 1 00 g of tetrahydrofuran.

The weight ratio between the polymeric binder and the photoconductor is 50:100. The solution is applied to an aluminium sheet in a thickness which corresponds to a dry-layer weight of about 5.5 g/m2. It is then dried and cured for 30 minutes at 1 1 OOC. The layer obtained is almost transparent and has a glossy surface.

The flexibility of this layer is very good. Chargeacceptance and resistance to abrasion are, however, poorer than in the layer according to Example 18. The electrophotographic sensitivity of the layer is only 70% of that of the layer according to Example 18.

Claims (36)

Claims

1. A photoconductive material comprising at least one photoconductor selected from polymerised heterocyclic vinyl compounds and polycondensed carbocyclic aromatic compounds, a total of about 0.1 to 0.5 mol per photoconductor monomer unit of at least one electron acceptor, and a total of about 30 to 120 parts by weight per 100 parts by weight of photoconductor of at least one subsequently crosslinkable binder, the photoconductive material being opaque.

2. A material as claimed in claim 1, which comprises a total of about 40 to 100 parts by weight of the subsequently crosslinkable binder(s) per 100 parts by weight of the said photoconductor(s).

3. A material as claimed in claim 1 or claim 2, wherein the binder comprises a polyisocyanate and a hydroxyl group-containing compound capable of reacting with the polyisocyanate to give a crosslinked product.

4. A material as claimed in any one of claims 1 to 3, wherein the binder comprises an aliphatic isocyanate and a low molecular weight compound containing hydroxyl groups.

5. A material as claimed in claim 4, wherein the low molecular weight compound containing hydroxy groups is a polyester, an acrylate or an epoxide.

6. A material as claimed in any one of claims 3 to 5, wherein the hydroxyl group containing compound and the polyisocyanate are prereacted with each other.

7. A material as claimed in any one of claims 1 to 6, wherein the photoconductor is a polymer of a vinylcarbazole, a vinyldibenzofuran, and/or a vinyldibenzothiophene.

8. A material as claimed in any one of claims 1 to 7, wherein the photoconductor is a homopolymer of N-vinylcarbazole or a derivative thereof, or a copolymer of N-vinylcarbazole or a derivative thereof with another vinyl compound.

9. A material as claimed in any one of claims 1 to 6, wherein the photoconductor is pyrene or perylene, or a substitution product or derivative of pyrene or perylene, polycondensed with formaldehyde or paraformaldehyde.

10. A material as claimed in any one of claims 1 to 6 and 9, wherein the photoconductor is a halogen derivative of pyrene polycondensed with formaldehyde or paraformaldehyde.

11. A material as claimed in claim 10, wherein the halogen derivative of pyrene is 3bromopyrene.

12. A material as claimed in any one of claims 1 to 11, wherein the electron acceptor is nitro and/or cyano substituted fluorene, fluorenone or naphthaiic acid.

13. A material as claimed in any one of claims 1 to 12, wherein the electron acceptor is 2,4,7trinitrofluorenone or 3,6-dinitronaphthalic anhydride.

14. A material as claimed in any one of claims 1 to 13, which also comprises one or more additives which improve adhesion and/or surface structure.

1 5. A material as claimed in any one of claims 1 to 13, which also comprises a plasticiser.

1 6. A material as claimed in claim 15, wherein the plasticiser is terphenyl, a chlorinated diphenyl a chlorinated naphthalene, or an epoxy resin.

17. A material as claimed in any one of claims 1 to 16, which also comprises a silicone oil or a fluorocarbon oil.

18. A material as claimed in claim 1, substantially as described in any one of Examples 4-7, 11-l3and 15-18.

19. A material as claimed in any one of claims 1 to 18, wherein the crosslinkable binder has been crosslinked.

20. A photoconductive layer which comprises a photoconductive material as claimed in any one of claims 1 to 18.

21. A photoconductive layer as claimed in claim 20, which is on a support.

22. A photoconductive layer comprising a polymerised heterocyclic vinyl compound or a polycondensed carbocyclic aromatic compound as the photoconductor, an electron acceptor, at least one subsequently crosslinking binder and, optionally, additives which improve adhesion and/or surface structure, the photoconductive layer containing the electron acceptor in an amount ranging from about 0.1 to 0.5 mol per monomer unit of the photoconductor, and the binder in an amount ranging from about 30 to 120 parts by weight per 100 parts by weight of the photoconductor, and the photoconductive layer being opaque.

23. A photoconductive layer as claimed in any one of claims 20 to 22, wherein the crosslinkable binder has been crosslinked.

24. A process for the preparation of a photoconductive layer as claimed in claim 23 on a support, which comprises preparing a coating solution by dissolving the layer components in a solvent, applying the solution to the support in an atmosphere having a relative humidity of less than about 40%, drying the solution at a temperature in the range of from about 60 to 1 200 C, and then post-curing at a temperature in the range of about 11 00C to 1 600C.

25. A process as claimed in claim 24, wherein the binder is as specified in claim 3 and wherein, before addition of the binder to the coating solution, the hydroxyl group-containing compound and the polyisocyanate are allowed to prereact in a solvent with the substantial exclusion of water or water vapour.

26. A process as claimed in claim 25, wherein separate solutions are prepared of the hydroxyl group-containing compound and the polyisocyanate and these separate solutions are then mixed.

27. A process as claimed in any one of claims 24 to 26, wherein the solvent is tetrahydrofuran.

28. A process as claimed in any one of claims 24 to 26, wherein the solvent is a mixture of tetrahydrofuran and 5% by weight of cyclohexa none.

29. A process as claimed in any one of claims 24 to 28, wherein curing is carried out at a temperature of from 1400Cto 1600C.

30. A process as claimed in claim 24, carried out substantially as described in any one of Examples 4 11-13 and 15-18.

31. A photoconductive layer on a support whenever prepared by a process as claimed in any one of claims 24-30.

32. A photoconductive layer as claimed in any one of claims 21 to 23 and 31, wherein the support is made of an electrically conductive material or of a dielectric or insulating material with a conductive coating.

33. A photoconductive layer as claimed in any one of claims 21 to 23, 31 and 32, wherein the support is provided with an insulating barrier layer.

34. A photoconductive layer as claimed in any one of claims 21 to 23 and 31 to 33, wherein the layer and the support together are in the form of a flexible web, a flat plate, or a photoconductor drum.

35. A photoconductive layer as claimed in any one of claims 20 to 23 and 31 to 34, which is provided with a protective covering layer.

36. An electrographic or electrophotographic reproduction material which comprises a photoconductive layer as claimed in any one of claims 20 to 23 and 31 to 35.