GB2112403A - Charge-transporting vinyl metallocene copolymer - Google Patents

Abstract

A charge-transporting organic substance for use in electrophotography, is a copolymer of a vinyl metallocene and a styrene compound. Preferably the copolymer comprises vinyl ferrocene, styrene and, optionally, hydroxyethyl acrylate. Such substances can be deposited, for example, as layers onto metals to produce recording (copying) materials. The brittleness of such layers can be almost completely avoided by adding about 12 mole per cent of ferrocene. The adhesive power is improved by the copolymerized acrylates.

Description

SPECIFICATION Charge-transporting organic substance This invention relates to charge-transporting organic substances as used particularly in electro photography. In electrophotography, pairs of charge carriers are produced by rays such as by light or X-rays absorbed in a semiconductor. In an electric field, the pairs of charge carriers are separated, and a charge pattern is produced on the surface of a recording material, of which the irradiated semiconductor forms a constituent part.

Accordingly, in the entire process it is necessary, that charges are produced at all and that the charges produced can move towards boundary surfaces.

Moreover, it is necessary that the substance on which the charge pattern is stored has a high dark resistance.

Selenium possesses all of the above properties which a substance suitable for use in electrophotography needs. However, in future, selenium is to be replaced to an increasing extent by organic materials. It has proved, however, that with organic substances it is not possible up to now to meet all needs with respect to properties in an optimum way. Thus hand in hand with the development of organic substances, there have also been developed new structures of electrophotographic recording materials.

Frequently one uses a two-layer structure, of which the one layer contains a charge-producing substance, and of which the second layer contains a charge-transporting organic substance.

Charge-transporting organic substances of the type of particular interest in this case are known, e.g. from US Patent No. 3,71 1,280. A metallocene or metallocene derivative is used as the actual charge-transporting substance. To a dispersion or solution of such a metallocene derivative there is added a film-forming, hydrophobic, polymeric binder. A large number of suitable binders is stated in the cited specification.

With such charge-transporting organic substances containing a dissolved or dispersed metallocene derivative in a film-forming compound it has proved, however, that with increased temperatures, e.g. at 500 C, the metallocene or the inserted metallocene derivative evaporates out of the film-forming compound and that, in connection therewith, especially when such a substance is deposited on a layer of selenium, the chargeability of the electrophotographic recording material is more and more reduced as the duration of the temperature loading increases.

It is an object of the invention, to provide a charge-transporting organic substance containing a metallocene derivative and a film-forming compound which, also after a longer lasting temperature loading at about 500C still has electrophotographic properties which are unchanged compared to the initial properties.

According to the invention, there is provided a charge-transporting organic substance, especially for use in electrophotography, which contains a metallocene derivative and a film-forming compound, wherein the substance is a copolymer of a metallocene derivative and a phenyl compound.

Using copolymerisation instead of dissolving or dispersing the metallocene derivative with the film-forming compound, ensures that the metallocene derivative is firmly bound and, in the event of increased temperatures, no longer evaporates out of the film-forming compound.

From among the various binder substances stated in the US Patent No. 3,711,280 polystyrene and other phenyl compounds, such as ones with alkyl groups, have proved particularly advantageous.

Nitrated phenyl compounds, such a nitrated polystyrene, however, are less suitable.

Copolymers of a metallocene derivative, namely of vinyl ferrocene and a film former, such as vinyl pyridine and further substances, are known, for example from "Chemical Abstracts", vol. 82 (1975) No. 163013 p. The copolymers stated therein, are based on vinyl ferrocene, a filmforming monomer, an electron-acceptor compound and a complex-forming compound. In such substances, exposed areas become nonconductive in the long run, which permits many copies to be made from a once produced charge pattern. The substance according to the invention, however, behaves completely different, for it can transport charges also after the exposure.

As a metallocene derivative there can be used, in particular, ferrocene derivatives or such metallocene derivatives which have a similar proportion of ionic linkage energy as the ferrocene derivatives. For example, cobaltocene derivatives have an excessive proportion of ionic linkage energy. Among the ferrocene derivatives, vinyl ferrocene as one of the most simple structure and most inexpensive, is the most suitable ferrocene derivative.

A copolymer found to be particularly advantageous has a 1:7 ratio of ferrocenyl groups to phenyl groups. In the case of a larger phenyl proportion, the film-forming properties of the phenyl polymers become advantageously evident, but at the expense of the charge-transporting properties due to the ferrocenyl groups. By contrast, however, an increase of the ferrocenyl contents has a good effect upon the transporting properties, but with an excessively high ferrocenyl content it is no longer possible to make the continuous layers needed to make an electrophotographic recording material. On the whole, useful results are, achieved at a relationship of about 1:5 to about 1:9 between ferrocenyl groups and phenyl groups.

As mentioned above, a high content of ferrocenyl groups is desirable, but this high content causes deterioration of the film-forming properties of the copolymer. The polymer deposited as a layer on a carrier becomes slightly cracked (flawy) in the case of a high ferrocenyl content. This, however, can be counteracted by adding to the copolymer a softening agent (plasticizer), i.e. preferably the metallocene whose metallocene derivative is copolymerised. When ferrocenyl vinyl is used as the metallocene derivative it is preferable to use about 12 per cent ferrocene by wt. as a softening agent.Below about 5 per cent ferrocene by wt., any noticeable softening effect is no longer detectable, and about 20 per cent ferrocene by wt. it has to be expected that ferrocene dissolved as a softening agent, evaporates at increased temperatures, thus having negative influences upon other, neighbouring layers.

With a decreasing phenyl content in the copolymer, there is a deterioration of the adherence of a layer of polymer to a carrier or supporting material. In this case, an improvement can be achieved by copolymerising an additional hydroxyacrylate to improve adhesive power.

however, above a certain limit this hydroxyacrylate has a bad influence upon the residual potential of a recording material. The residual potential is that potential which recording material which has been as highly charged as is possible has after having been exposed. When a 2-hydroxyethylacrylate is used, not more than one 2-hydroxyethylacrylate group should be used for approximately each time 20 ferrocentyl groups, so as not to degrade the residual potential.

Copolymers of the type proposed herein may be suitable for use as layers without any further additives, e.g. on charge-producing substances, or it may be possible to make copolymer layers which also contain a charge-producing substance either dissolved or dispersed. As such dissolved, charge-producing substances there may be used organic dyes (colorants) such as copper phthalocyanine, while as dispersed substances there may be used, for example, either cadmium sulphide or cadmium selenide powder.

The copolymers are not only very temperatureresistant as far as their electrophotographic properties are concerned, but also have more favourable residual-potential values than noncopolymerized substances. Thus, for example, a two-layer recording (copying) material consisting of a substrate, of a copper phthalocyanine layer and of a layer of ferrocene, dissolved in polystyrene, is found to have a residual potential of -200 volts when first charged to -800 volts and thereafter exposed. The corresponding structure using a copolymer of ferrocenyl vinyl and styrene, however, only has a residual potential of -100 volts.

We now discuss some examples.

Example 1 Vinyl ferrocene and styrene in the form as commercially available are used as starting substances.

The vinyl ferrocene is at first cleaned by sublimation at a temperature of 600C under vacuum. From commercially available styrene, the stabilising agent (stabiliser) is removed by distillation under vacuum, which stabiliser is assumed to prevent polymerisation into polystyrene. After these purification processes, 1.1 5 g vinyl ferrocene, 2.7 g styrene and 40 mg azobisisobutyronitrile are carefully mixed and heated up to 800C for 4 hours, until a solid, clear, reddish-brown block is obtained. During this, vinyl ferrocene partly sublimates at colder points of the reaction vessel, so that the content of ferrocenyl groups in the copolymerization is always smaller than expected from the inserted molar ratios. The actual ratio of ferrocenyl to phenyl groups is ascertained by nuclear resonance spectroscopy.

The copolymer biock is dissolved in 20 ml benzene, and by using about 100 ml methanol, the copolymer is precipitated again. The resulting sediment is washed with methanol until the supernatant solution has become colourless.

Thereafter, the copolymer is dried at a temperature of 800C and 1 Torr. is given 1.8 g copolymer, in which 9 phenyl groups come on each ferrocenyl group.

0.4 g copolymer are dissolved in 30 ml toluene (toluole) and deposited on a plate of aluminium by a spiral type film-casting apparatus.

After the solvent has evaporated, this gives a defect-electron-(hole)-transporting layer having a thickness of about 11 ,um.

Example 2 The copolymer is made as in Example 1.

Thereafter, 0.4 g copolymer and 0.05 g ferrocene as a softening agent are dissolved in 30 ml toluene and, as stated in Example 1, deposited on a substrate. Such layers, unlike those of Example 1, no longer tend to crack.

The copolymer containing ferrocene as the softening agent according to Example 2 was deposited onto a layer evaporated on to a foil of aluminium. This deposited layer consists a) of amorphous selenium with 7.5 per cent tellurium and b) of copper phthalocyanine, having a thickness of 1 jum. The thus-produced recording (copying) materials can be charged up to a maximum of -800 volts with a corona but, owing to the different charge-producing substances, are differently sensitive to light at different wavelengths.In case a) subsequent to the exposure at 450 nm with 2 XuJ/cm2, there was a residual potential of -120 volts and, in the case b), subsequent to the exposure at 530 nm with 50 MJ/cm2 there was a residual potential of -160 volts.

Example 3 1 g (4.7 m mol) sublimated vinyl ferrocene, 3.359 (32 m mol) freshly distilled styrene, 0.026 g (0.23 m mol) 2-hydroxyethylacrylate and 65 mg (0.4 m mol) cox., al-azobisisobutyronitrile are converted and purified (cleaned) as in Example 1.

The nuclear resonance spectrum shows that the copolymer contains eight phenyl groups per ferrocenyl group.

0.8 g of the copolymer is dissolved in 4 ml toluene and deposited on a plate of aluminium by a spiral type film-casting apparatus. After the toluene has vaporised, there results a layer having unchanged electrophotographic properties, but a much improved power of adhering to the aluminium as compared to the layer of Example 1.

Example 4 Into the copolymer-toluene solution of Example 3, there is stirred 1.6 g cadmium selenide power and which is dispersed by an ultrasonic disintegrator. By using a spiral type filmcasting apparatus, the resulting dispersion is deposited onto an aluminium foil which was previously rubbed with toluene, and is dried for two hours at 1200 C. To obtain a greater layer thickness, a second layer of the same dispersion is deposited in the same way, and dried. The total layer thickness is then about 34 m. With a corona, it is then possible to achieve a negative charge of -700 volts.Following the exposure with 10 lug sec from a tungsten-filament lamp, the residual potential of the recording (copying) material still only amounts to -90 volts. This is much lower than obtainable when using dissolved ferrocene in polystyrene.

Layers of various copolymers have been produced in which the ratio of the number of ferrocenyl groups to the phenyl groups has been varied between 1:7 and 1:25, and in which the effective mobility of the defect electrons (holes) was measured according to the transient photoconductivity method. At a ratio of 1:25, the effective mobility was 6.10-11, at a ratio of 1:13 it was3.10-9,ata ratioof 1:9 itwas2.10~8andata ratio of 1:7 it was 7.10-9, with the dimension of the effective mobility each time being cm2/V sec.

Layers with a proportion of ferrocenyl groups greater than 1:13, however, have showed during the electrophotographic measurements to have excessively high residual potentials after the exposure, so that they are of little use in practice.

Layers with a proportion of ferrocenyl groups less than 1:7 were too brittle; this disadvantage, however, can be avoided at first by adding a softening agent such as farrocene. Only at a relationship of about 1:5 between ferrocenyl and phenyl groups have layers to which a softening agent has been added proved to embrittle.

The examples given only set forth processes suitable for making a layer of copolymer on a substrate. Layers of copolymer, however, can also be produced, for example, by manufacturing compact mouldings in a moulding press, by being melted on and cast, or by evaporation coating under vacuum. When using compact mouldings, however, it is easily possible to realise individual layers, but it is hardly possible to produce a layer structure. In evaporation coating and in melting on its is likely that the layers will crystallise, which leads to non-reproducible results. Moreover, some of the copolymers decompose when heated.

When employing the evaporation coating method it is also difficult to produce large-surface layers.

On the whole, the layer production method from the solution offers the most advantages.

The adherence of the layers of copolymer to a metal base can not only be improved by copolymerising a hydroxyacrylate, but it is also possible to employ other acrylates, such as methacrylates.

Claims (12)

1. A charge-transporting organic substance, especially for use in electrophotography, which contains a metallocene derivative and a filmforming compound, wherein the substance is a copolymer of a metallocene derivative and a phenyl compound.

2. A substance as claimed in claim 1, and in which the phenyl compound is styrene.

3. A substance as claimed in claim 1 or 2, and in which the metallocene derivative is a ferrocene derivative.

4. A substance as claimed in claim 3, and in which the ferrocene derivative is vinyl ferrocene.

5. A substance as claimed in claim 3 or 4, and in which the ratio of ferrocenyl groups to phenyl groups is between 1:5 and 1:9.

6. A substance as claimed in claim 3 or 4, and in which the ratio of ferrocenyl groups to phenyl groups is 1:7.

7. A substance as claimed in any preceding claim, and which contains a metallocene as a softening agent (plasticiser).

8. A substance as claimed in claim 7, and which contains 5 to 20 mole per cent, preferably 12 mole per cent of ferrocene.

9. A substance as claimed in any preceding claim, and in which a hydroxyacrylate is also copolymerized.

10. A substance as claimed in claim 9, and in which a maximum of about 0.05 mole per cent hydroxyacrylate is copolymerized.

11. A substance as claimed in any preceding claim, and which also contains a charge-producing substance.

12. A charge-transporting organic substance, especially for use in electrophotography, substantially as herein described.