ELECTROPHOTOGRAPHIC ELEMENTS CONTAINING POLYAMIDE INTERLAYE&S The present invention relates to electropho¬ tography. In particular, the invention relates to electrophotographic elements containing a polyamide interlayer between a photoconductive layer and a sup¬ port.

In the field of electrophotography, as dis¬ closed in US Patent 4,175,960, electrophotographic elements comprising, in sequence, 'one or more photo- conductive and charge-transport layers, an electri¬ cally conducting layer and a support are employed to form electrostatic images by photodecay of an elec¬ trostatic charge uniformly applied on the surface of the photoconductive layer. The resulting electro¬ static image is rendered visible by development with a suitable toner composition. Many of these ele¬ ments, particularly those with metal electrically conducting layers, do not charge uniformly and this defect ultimately passes to the copy desired in the form of nonuniform toner density. This nonunifor ity stems from the propensity of such metal conducting layers to inject unwanted charge carriers into the photoconductive layers from various defect sites in the conducting layer. The nonuniformity is measured as the standard deviation from the desired level of charge imposed on the photoreceptor and is referred to as electrical granularity.

The effect of unwanted injection of charge carriers can be alleviated in some applications by the use of a barrier layer between the photoconduc¬ tive layers and the electrically conducting layer. Japanese o ai Patent No. 58[1983]-63945 published April 16, 1983, for example, discloses the use of caprolactam polyamide layers above a carbon- containing, electrically conducting support to pro¬ vide such barrier protection. Unfortunately, the use

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of barrier layers in such locations of the element interferes with the ability of the element to be regenerated in a copy process wherein the element is repeatedly charged, exposed, developed and erased.

Efforts to improve the uniformity of charge of elements, while preserving their regeneration capability, moreover, have been less than success¬ ful. For example, various interlayers have been evaluated for use in the element in a location

10 between the metal conducting layer and the support. While some polymers improved the sensito etric prop¬ erties of the element, they also produced undesirable haze. Such haze interferes with light exposures through the rear side of the element. Still other

-**5 polymer interlayers studied displayed unsatisfactory coatability and/or poor adhesion between adjacent layers.

In accordance with the present invention, therefore, an electrophotographic element is provided 0 comprising, in sequence, a photoconductor layer, a metal electrically conducting layer, an interlayer comprising a polyamide resin having repeating units derived from caprolactam, and a support. This ele¬ ment exhibits improved electrical granularity and 5 high optical clarity (i.e., freedom from haze) in addition to other desirable properties. In a pres¬ ently preferred embodiment, the element comprises, in sequence, the photoconductive layer, a vacuum- deposited metal conducting layer, the defined poly- amide interlayer, a screen layer and a transparent support.

Use of the electrophotographic element of the present invention offers several advan ages. For example, when the photoconductor surface of the ele- 5 ent is charged to an initial uniform level, Vo, the standard deviation from Vo of such charge is signifi¬ cantly decreased compared with an otherwise identical

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element without an interlayer. Thus, when the ele¬ ment is exposed and developed, the resulting image exhibits less image granularity. The polyamide interlayer in this element, moreover, exhibits reduced optical haze, thus facilitating light trans¬ mission for rear-side exposures. The adhesion of the polyamide interlayer to adjacent layers, furthermore, is high, and the polyamide interlayer is readily coated uniformly on the support, thereby providing element integrity.

Preferred elements of the invention having an incorporated halftone screen layer, furthermore, exhibit reduced dark decay and a capability of being charged to higher and more stable initial charge lev- els, Vo, compared with elements without an interlayer. The present element includes, as the photo¬ conductive portion thereof, any of a variety of pho¬ toconductive compositions such as arylalkane leuco bases, arylamines, terphenyls, quaterphenyls, zinc oxide, selenium and the like. Preferably, one or more aggregate photoconductive layers as described, for example, in US Patents 3,615,414 and 4,350,751 are employed. An aggregate photoconductive layer comprises a co-crystalline complex of (a) a polymer having an alkylidene diarylene unit in a recurring unit and (b) at least one pyrylium dye salt. The co- crystalline complex is dispersed as a discontinuous phase in a continuous polymeric phase. Other useful types of aggregates comprise co-crystalline complexes of pyrylium dye salts with themselves or with other pyrylium dye salts.

In addition to the aggregate-containing pho¬ toconductive layer, the present invention contem¬ plates the optional use of a charge-transport layer in electrical contact with the aggregate layer. In such embodiments, the aggregate layer is referred to

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by various synonyms such as a charge-generating or emitter layer.

Elements containing charge-transport layers in electrical contact with the aggregate layer are referred to in the art as multiactive. US Patent 4,175,960 describes such elements.

The conducting layer of the element of the present invention comprises an electrically conduct¬ ing metal, such as nickel or chromium or other con¬ ducting metal, lying between the photoconductive layer and the polyamide interlayer. The conducting layer can be sufficiently thin to allow exposure of the aggregate photoconductor layer through the sup¬ port side of the element, if desired. In use, the conducting layer is usually electrically grounded to facilitate charging.

Vacuum-deposited metal conducting layers are preferred for use in the present element, as such layers are extremely thin and thereby transmit light to facilitate rear-side exposure of the element.

Preferably, the vacuum-deposited metal has a thick¬ ness in the range from about 20 A(10" ¹⁰ m) to about 40 A(10 ^"10 m) so as to provide an optical density no greater than 0.4 and a resistivity of less than 8 x 10* Ω/square.

In accordance with the present invention, a polyamide interlayer containing repeating units derived from caprolactam is incorporated between the metal conducting layer and the support. Sucft poly- amides can be made by well-known nylon-type syntheses involving, for example, alkaline polymerization of caprolactam:

into a polyamide having recurring units of the gen¬ eral structure:

Homopolymers, commonly referred to as "nylon 6", are useful in the present Invention, as well as block or random copolymers in which additional recurring units are derived from hexamethylene adipamide or hexameth- ylene sebacamide. Preferably, the caprolactam- derived polyamides are soluble In lower alcohols such as aliphatic alcohols having 1 to 6 carbon atoms.

A useful polyamide is poly(caprolactam-co- hexamethylene adipamide-co-hexamethylene sebacamide) . The thickness of the polyamide layer can vary widely to reduce the electrical granularity (as defined below) of the element compared with an other¬ wise identical element without an interlayer. A use¬ ful thickness can range from about 0.25 micrometer to about 2 micrometers when coated over an integral screen layer. If there is no screen layer, the poly¬ amide layer thickness can be less than 0.25 microme¬ ter.

The support for the present element under¬ lies the polyamide interlayer. Opaque, as well as transparent, supports can be employed, but transpar¬ ent ones are preferred to allow exposures through the support. In the latter case, conventional photo-

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graphic transparent film bases such as cellulose ace¬ tate or poly(ethylene terephthalate) are useful. Optionally, the element of the present invention contains a halftone screen layer interposed between the polyamide interlayer and the support. In a preferred embodiment, the screen layer is inter¬ posed between the polyamide interlayer and a trans¬ parent support.

The halftone screen is made up of a number of finely divided, alternating, opaque and transpar¬ ent areas. The screen pattern of opaque and trans¬ parent areas may be a conventional dot pattern or line pattern of the type used for the fabrication of halftone plates for newspaper printing. The alter- nating opaque and transparent areas of the screen pattern may be of almost any shape, including round dots, elliptical dots, lines and the like. The spac¬ ings of the pattern may also vary so that the pattern is regular, irregular, or random. The pattern may also be varied in size from dot-to-dot or line-to- line. Particularly useful results are obtained with halftone tint screens having a frequency of about 32 to about 80 dots/cm and a percent tint, i.e., percent opaque areas, of about 10 to 90 percent. The halftone screen layer can be applied to the support by any suitable technique such as by off¬ set or direct gravure printing, ink jet printing or the like.

The materials employed as the screen layer _C n also vary, but generally any opaque material is useful. Preferred materials include pigmented inks for maximum opacity. In this regard, photoconductive elements having pigmented ink screen layers between the metal conducting layer and support exhibit unde- sirably high dark-decay levels. With the polyamide interlayer, however, such dark decay is substantially reduced or avoided.

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The following examples are provided to aid in the practice of the present invention. Examples 1-4:

This illustrates elements of the present _^ invention and the reduced electrical granularity which such elements exhibit with respect to control elements. Granularity was determined as the standard deviation from an applied _, electrostatic charge, Vo. The element was not exposed to actinic radiation in this example.

A multiactive electrophotographic control element was prepared containing, in sequence, a 12- to 13-micrometer-in-thickness charge-transport layer, a 5- to 6- micrometer aggregate charge-generation layer, a 30-A, vacuum-deposited nickel conducting layer and a 4-mil (100-micrometer) transparent poly¬ ethylene terephthalate support.

The charge-transport layer and charge- generation layer can be prepared as in Example 2 of Berwick et al US Patent 4,175,960.

Similar elements were prepared containing a polyamide interlayer, between the nickel layer and the support, of varied coating coverage in milligrams per meter ² . The polyamide employed was Elvamide 8061 (a trademark) , a copolyamide of caprolactam, hexamethylene adipamide and hexamethylene sebacamide soluble In methanol.

Each of the elements was charged to a Vo of -500 volts. The standard deviation in volts from Vo (electrical granularity) was determined as follows: The apparatus employed contained a corona charger, a Trek Microprobe (a trademark of Trek, Inc.) for mea¬ suring small-area surface potential, and a sample holder capable of holding a film sample flat by vacuum. All measurements and steps took place in the dark.

Each of the elements was uniformly charged to a Vo of 500 volts. Portions of the charged sur¬ face 0.01 cm in diameter were measured with the microprobe at 0.005-cm spacings. After the elements , were erased to 0, the procedure was repeated 5 times. From the voltage readings, the standard deviation from Vo was determined and the percentage reduction from the standard deviation of the control calculated. Results are shown in Table 1. Examples 2-4 show reduced granularity in elements containing a polyamide interlayer compared with the control Example 1. Examples 5-8:

This illustrates the reduced electrical granularity of elements of the invention which contained an integral screen layer between the poly¬ amide interlayer and the support.

The control element and elments in Examples

1-4 were modified to include an integral screen layer between the Elvamide 8061 (trademark) polyamide layer and the support. The integral screen was applied by gravure-printing the support with a dioctyl-phthalate plasticized ink formulation containing the following:

Weight Component Percent

1-acrylic varnish (50% solids, 50% solvent 30

2-cellulose nitrate (40% solids, 60% solvent 10

3-solvents 10

4-pigment Bon Red (a trademark) base (21% 50 pigment, 25% cellulose solution, 54% sol¬ vents)

The procedure employed in Examples 1-4 was repeated. Results are shown in Table 1. The Exam¬ ples 5-8 results show especially reduced granularity in integral-screen-layer elements containing a poly¬ amide interlayer.

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Table 1

Percent Reduction

Polyamide from

Interlayer Vo Standard

Coverage Integral Standard Deviation

Example (mg/m ² Screen Deviation of Control

1 control ___— none 1.51 --

2 2.32 none 1.17 23

3 4.65 none 1.11 26

4 6.97 none 1.27 16

5 control yes 2.67 ' --

6 2.32 yes 1.27 52

7 4.65 yes 1.12 58

8 6.97 yes 1.34 50

Example 9:

This example illustrates elements of the present invention using another caprolactam polyamide in the defined interlayer.

The formulations and the procedures described in connection with Examples 1-8 were repeated using the polyamide Ultramid IC 018081 (a trademark of BASF-Wyandotte Chem. Co. for a polyamide derived from adipic acid, caprolactam, 1,6-hexanedi- amine and 4,4'-methylenebis(cyclohexylamine)) in place of the Elvamid interlayer of the previous exam¬ ples. The elements containing the Ultramid interlay¬ ers and gravure dot screen on the support exhibited from 31 to 70% reduction in charge standard deviation compared with a control without the interlayer. Ele¬ ments containing the Ultramid interlayers without the integral screen exhibited from 21 to 24 percent reduction in charge standard deviation compared with a control without the interlayer. This indicates that the electrical granularity of elements is

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enhanced when this caprolactam polyamide interlayer is employed. Example 10:

This illustrates the dark decay and chargea- bility, Vo, of an element of the invention having both the polyamide interlayer and an integral screen layer after 36,000 and 54,000 electrical cycles, respectively. Chargeability, as determined after such repeated cycling in this example, refers to the level to which an element is charged after each cycle of charging and discharging and Is, therefore, one indication of the regeneration capability of elements of the present Invention. A control element without the polyamide interlayer was used for comparison with elements of the Invention.

The control element of Example 5 containing an integral screen layer but no interlayer was used. The element of Example 8 was used as the element of the invention. Each element was subjected to 36,000 and 54,000 electrical cycles, each consisting of electrically charging to a preselected Vo and dis¬ charging to a preselected level. Immediately after each cycle, the element was recharged and the Vo mea¬ sured. (In each recharging step, including the final recharging conducted after each cycle, the apparatus charging conditions remained unchanged.)

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The initial dark decay of each element was also measured separately.

Results are shown in Table 2.

Table 2

Initial

Dark Decay 36,000 54,000 Polyamide Initial (picoamps/- Cycles Cycles Example Interlayer Vo cm ² •sec) Vo Vo

control none 550 568 515 510

9 yes 590 306 585 575

From Table 2, it can be seen that elements of the invention exhibit reduced initial dark decay and a chargeability level, Vo, after repeated cycles, which is relatively stable, i.e. Vo varies less from the initial Vo of the element compared with an other¬ wise identical element without the polyamide inter- layer.

Example 11:

This illustrates the lack of optical haze of elements of the invention compared with otherwise identical elements with different interlayers. All elements contained an integral screen layer.

Elements were prepared having the following interlayer:

(a) Elva ide 8061 (a trademark) (as in Examples 1-8)

(b) polyurethane ( _c ) poly(methylacrylate-co-vinylidene chloride-co- itaconic acid)

(d) poly(vinylidene chloride-co-acrylonitrile-co- acrylic acid)

(e) cellulose nitrate Each element appeared hazy compared with the element containing the Elvamide 8061 (trademark) interlayer.

Example 12:

This example illustrates the effect of placing the polyamide interlayer in an incorrect location of an electrophotographic element. In this example, an element similar to that of the element in Example 8 was prepared except that the Elva id inter¬ layer was applied over the nickel conducting layer; i.e., the Elvamid layer was located between the charge-generating layer and the nickel conducting layer. After 500 cycles of charging and erasing, the residual voltage on the charge-transport layer after the final erase was 15 to 25 volts, whereas the residual voltage for a control element without any interlayer was less than 5 volts. This residual vol- tage also evidences a problem in regenerating such elements in repeated copy cycles and is further mani¬ fested in a rise in the toe of a VlogE curve for the element. Elements of the invention do not experience this problem of regeneration as shown in Example 10 above.

Similar results can be obtained in terms of reduced electrical granularity and stable regenera¬ tion capability when other caprolactam polyamides are employed in the interlayer, and when metals other than nickel are employed in the electrically- conducting layer.

Photoconductive elements of the invention containing a caprolactam polyamide interlayer between a support and a layer of an electrically conducting metal are particularly suited to applications in which the element is subjected to repeated copy cycles. In such applications, the elements exhibit minimum electrical granularity and reproducible regeneration from cycle to cycle. In other respects, the polyamide interlayer in the element of the inven¬ tion also exhibits good coatability and adhesion to adjacent layers to provide resistance to layer sepa-

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ration. Furthermore, when the metal electrically conducting layer is vacuum-deposited to provide opti¬ cal transparency In combination with the haze-free characteristics of the defined polyamide interlayer, the present element is also well-suited to imagewise exposure of the photoconductive layer through the rearside (i.e., through the support) of the element.

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