US3397086A - Photoconductive composition and coated article - Google Patents

Description

United States Patent 3,397,086 PHOTOCONDUCTIVE COMPOSITION AND COATED ARTICLE John J. Bartfai, Schenectady, N. assignor to General Electric Company, a corporation of New York No Drawing. Filed Mar. 12, 1965, Ser. No. 439,467 8 Claims. (Cl. 117-218) ABSTRACT OF THE DISCLOSURE A photoconductive composition comprising a substantially non-photoconductive organic polymer containing a dispersion of photoconductive pigment which is phthalocyanine or its metal derivatives.

This invention relates to photoconductive compositions and to information recording media containing these materials. More particularly, this invention relates to photoconductive composite structures which comprise a dispersion of phthalocyanine pigment particles in a matr x of an otherwise non-photoconductive organic polymeric material. Specifically, the present invention relates to photoconductive structures having the described composition which are particularly suitable for sensing and recording information in the form of an electrostatic charge pattern which may then be retrieved or developed by more or less conventional techniques which may provide a permanent record for the storage of information.

A number of information recording techniques utilizing photoconductive materials are known. One such recording technique has involved the provision of a relatively thin film of a photoconductive material supported upon a relatively thin electrically conductive film which is in turn supported upon an electrically non-conductive substrate. A uniform electrical charge is produced upon the surface of the photoconductive film by, for example, a corona discharge. The so-charged surface may then be exposed to an incident pattern of electro-magnetic radiation. Those areas of the photoconductor which are exposed become electrically conductive and permit the electrical charge in the exposed areas to leak off leaving a retained or latent image pattern of charges associated with those areas which were not illuminated. It should be stated at this point that there are other ways in which such a charge pattern may be created on the photoconductive surface, but since the particular method for forming such a charge-pattern image does not constitute the present invention, it is not believed that a further discussion of charging and exposure techniques beyond the example given is necessary.

Several methods of retrieval of the information or image which may involve the production of a permanent record of the charge pattern image may be employed. For example, the charge pattern image may be produced by an electron beam with subsequent exposure to light and may be read out by means of an electron beam scanner similar to that used in conventional television cameras, the modulated signal produced by the charge pattern being converted to, for example, a visible image on the screen of a cathode ray tube of which a photographic record, for example, may be made. Alternatively, such a charge pattern might be retrieved by other techniques such as thermal development, powder development, or the like.

The so-called powder techniques entail the use of a finely-divided powder which is either attracted to the latent charge areas or repelled therefrom when applied to the surface of the photoconductive layer after it has been exposed. Depending upon the choice of powder used and the initial polarity of the charge induced upon "ice the surface of the photoconductive layer, the powder image may be selectively a positive or a negative image. If desired, the polymeric photoconductive layer, if it is a suitable thermoplastic, may then be heated to its softening point to fuse the powder pattern in place to form a stable, permanent powder image. If the photoconductive layer is not thermoplastic, a resinous material may be incorporated in the powder which will act to thermally fuse the powder in place. It is possible, of course, to form a hard copy image from the powder pattern. In this, a sheet of paper is charged with the opposite polarity of the charge pattern areas on the photoconductive surface, the paper applied to the powdered surface and the charge on the paper acts to transfer the powder from the photoconductive surface to the surface of the paper. If the powder is composed of a pigment and resin mixture, the pattern may be permanently afiixed to the paper by heating thepowdered paper to the softening point of the resin and permitting it to cool. Other variations involving the use of powders are well known and the foregoing are merely examples of known techniques.

A yet further method of development and retrieval of the information or image pattern on a thermoplastic photoconductive layer is physical deformation of the layer by the latent image charge pattern when the layer is heated to its softening point. This method will be discussed in greater detail in the following disclosure.

The recording of information on a deformable photoconductive medium in the form of minute light-modifying deformations has become known as photoplastic recording and is described in the copending patent application of Joseph Gaynor, entitled Information Storage on Deformable Medium, Ser. No. 79,260, filed Dec. 29, 1960, and now Patent Number 3,291,601, and assigned to the assignee of the present invention. In that invention, information to be stored is transmitted by activating radiation onto a uniformly charged deformable photoconductive member and by reason thereof, the photoconductive medium becomes selectively discharged according to the pattern of the activating radiation. The selectively charged member is developed by softening the deformable medium so that the forces of the electrostatic charge pattern deform the medium in accordance therewith. Upon cooling the medium, the deformations become permanently fixed on the surface of the storage member and are thereby permanently stored unless deliberately erased by reheating. The information stored in the form of these deformations is visible and may be viewed by reflecting a beam of light from the deformed surface. However, because light is refracted or diffracted by the deformations, depending on their nature, such an image can be projected only by a Schlieren optical system. The addition of a flying spot scanner permits conversion to electrical signals by means of such known light-sensing devices as photomultipliers and the like. A complete recordlng apparatus and system of recording is disclosed in the copending application for production of the informatron-bearing deformations corresponding to a radiation lmage pattern.

' The information storage member of the copending invention may comprise a deformable photoconductive medium, an electrically conductive inner layer and dielectr1c support layer. In a preferred embodiment of the medium, a thin metallic film is vapor-deposited on a dielectric support layer and the vapor-deposited metallic film thereafter coated with an organic solvent solution of deformable photoconductive film-forming material. The final storage member may be obtained by removing solvent from the surface recording layer. It will, of course, be apparent that the deformable photoconductive layer may be formed directly on a metal substrate which may take the form of a flexible tape, for example. The class of useful photoconductive materials used in the practice of the copending invention includes such diverse materials as thermoplastic photoconductive polymers, mixtures of inorganic photoconductive solids with thermoplastic nonphotoconductive polymers, and mixtures of organic photoconductors with thermoplastic non-photoconductive polymers. Suitable thermoplastic non-photoconductive polymers for the recording layer of the deformable storage member are characterized by substantially infinite room temperature viscosity and a relatively low viscosity at a temperature of about IOU-150 0, together with a relatively high electrical resistivity in ohms-centimeter in the dark. Useful polymers include acetals, acrylics, polyesters, silicones, and vinyl resins having these properties. It will, of course, be appreciated that as a general rule, photoconductive films formed from mixtures of inorganic photoconductive solids with thermoplastic non-photoconductive polymers are more stable and less volatile than organic photoconductive polymers or organic mixtures. Usually, this greater stability is reflected by a greater degree of reusability. Since the photoconductive layer also performs the recording function, these recording media and their mode of use [have been referred to as a single layer system.

It will, of course, be recognized that while these thermoplastic photoconductive films are advantageously developed by thermal means, the information or image of the latent charge pattern may be retrieved or developed by other techniques as pointed out previously. It should also be noted that if the method of retrieval of the information or image formed by the charge pattern does not require heat deformation, a thermosetting polymeric material may be used in place of the previously discussed thermoplastic, if desired. For example, if the information or image pattern is to be retrieved electronically by the electron beam technique or by the hard copy" charge transfer powder technique referred to previously, either a thermoplastic or a thermoset member may be employed. After use, the residual charge pattern may be removed by uniform exposure of the surface of the recording member to light or, alternatively, the recording surface may be exposed to a strong corona field to produce a uniform change, thereby rendering the recording member suitable for reuse.

The image pattern achieved by the foregoing photoconductive organic materials consists of relatively highly charged surface areas which have not been exposed to light and 'areas having less-charge where exposure has occurred. When the image :pattern is thermally developed to produce a deformation pattern as in the previously described photoplastic recording technique, the resulting Schlieren image is a black and white negative image. These images do not contain tone or gray scale unless the image is sampled during exposure to break up the light to produce scattering. As is well known, sampling may be achieved by projecting the image pattern through a fine mesh screen to impinge upon the photoconductive surface during exposure.

It is, therefore, a principal object of this invention to provide a new and useful class of photoconductive materials.

It is a further object of this invention to provide a novel and useful class of photoconductive materials having substantial stability when subjected to thermal cycling.

It is a still further object of this invention to provide novel and useful photoconductive materials having improved spectral response, sensitivity, and resolution.

It is a yet further object of this invention to provide novel and useful photoconductive materials capable of providing positive deformation image patterns having continuous tone without image sampling.

Other and specifically different objects of this invention will become apparent to those skilled in the art from the following detailed description.

Briefly stated, the photoconductive materials of this invention comprise organic polymeric materials as a matrix containing a dispersion of very small, of the order of 0.1 micron diameter, particles of phthalocyanine pigments. As is well known, these compounds constitute a series of organic pigments having as a structural unit four isoindole groups (C H )C N linked by four nitrogen atoms so as to form a conjugated chain. They include many modifications of the basic compound, metal-free phthalocyanine, e.g., copper phthalocyanine, iron phthalocyanine, lead phthalocyanine, molybdenum phthalocyanine, cobalt phthalocyanine, nickel phthalocyanine, and manganese- (ous) phthalocyanine, among others, in which a metal atom is held by secondary valences of the isoindole nitrogen atoms. According to one embodiment of this invention, a dilute dispersion of these materials in a suitable organic thermoplastic non-photoconductive polymer may be employed in a manner similar to that of the previously described known organic thermoplastic photoconductive materials. In addition, they are responsive to a wide range of electromagnetic radiation including X-rays. Under certain conditions, they may behave differently upon thermal development and the characteristics of the thermally developed image pattern of deformations is diiferent from those of the previously described organic photoconductive thermoplastic polymers. As stated, after exposure the previous photoplastic recording materials are left with a pattern of relatively high electrostatic charge in those areas which were not exposed. Upon heating to develop the image, these remanent charges act to deform the softened plastic to form the characteristic ripple pattern. The areas Which had been exposed do not so deform. Conversely, when a similar surface layer of the material of this invention is charged and exposed to an image pattern of radiation for relatively short periods of time and heated to the softening point of the thermoplastic polymer vehicle, those areas which had been exposed to the illuminated areas of the image pattern deformed in a pattern of ripples. Under certain conditions, the size of the deformations so formed were very much smaller than those formed by the usual photoplastic recording media. The resulting image, therefore, has a matte appearance due to the small size of the individual deformations and upon Schlieren projection produce a continuous tone image. Thus, these materials have been used to produce continuous tone photographic images without the use of a screen or other sampling techniques. Under different conditions, however, these materials may be caused to produce a ripple pattern corresponding to the usual photoplastic recording pattern. As will be pointed out later, this may be accomplished by increasing the exposure time, thickness, and concentration. As might be expected, this latter ripple pattern corresponds to the unexposed areas. Use of a screen is effective to produce continuous tone images as described previously.

In order to more completely disclose the present invention, the following specific examples are set forth.

Example 1 A deformable photoconductive composition was pre pared from a coating composition comprising an approximately 25 percent solids solution of polystyrene having an average molecular Weight of 20,000 dissolved in a mixture of benzene, toluene, and xylene and which contained a suspension of 5 percent by weight metal-free phthalocyanine pigment particles. The solution containing the suspension was filtered through filter paper and through a glass frit filter which removed virtually all the phthalocyanine particles leaving a solution which was only faintly tinted blue-green indicating the presence of a colloidal suspension. A film approximately 10 to 25 microns in thickness was cast upon a thin transparent electrically conductive film of stannic oxide supported upon a 2 in. by 2 in. glass plate by coating the conductive film with the solution and removal of the solvent by evaporation. The so-prepared film was charged by corona discharge to about 500 to 600 volts and exposed to an image pattern of white light of about 5 milliwatts/cmfor about 1 second. Heat was applied by passing 500 watts through the electrically conductive coating for 0.2 second. An image pattern having a point-to-point correspondence with the projected light image pattern was thereby formed in the surface of the polymeric film by the formation of a pattern of very fine deformations in those areas which had been exposed to light with no deformation occurring in the areas which had not been lighted.

Example 2 A number of recording members were made in the same manner set forth in Example 1 except copper phthalocyanine was substituted for the metal-free phthalocyanine. These were severally exposed in the described manner to an image pattern of light having wave lengths ranging from the ultraviolet through the visible to the infrared. Upon heating in the described manner, the same sort of image described in Example 1 was developed.

Example 3 A coating composition was prepared from a 28 to 30 percent solids trichloroethylene solution of a diphenyl siloxane polymer with a phenylene oxide polymer present in a weight ratio of 9 to l siloxane to oxide, the type polymer being disclosed in Example 2 and elsewhere in US. Patent 3,063,872 to E. M. Boldebuck, dated Nov. 13, 1962, entitled Recording Medium and Polysiloxane and Resin Mixture Therefor assigned to the assignee of the present invention. The softening point of the particular solid polymer mixture of polydiphenyl siloxane and poly (2,6-dimethyl-l,4-phenylene) ether selected was around IOU-110 C., at which temperature the solids become a mobile sirupy liquid. Into this polymer solution was added 5 percent by weight of copper phthalocyanine, based on the weight of the polymer solids. The bulk of the phthalocyanine was removed by filtering, leaving only a colloidal suspension in the polymer solution. Films were cast, exposed, and heat-developed as set forth in Example 1 to produce similar images.

Example 4 A cross-linked therrnosetting polymeric film containing an effective amount of copper phthalocyanine was prepared as follows.

A 4 to 1 ratio of an epoxy resin (Epon 815, Shell Chemical Corporation, 460 Park Avenue, New York, N.Y.) to copper phthalocyanine powder was mixed in a mortar and pestle and catalyzed with DETA catalyst supplied by the resin vendor in a ratio of 100 parts resin to 10 parts of catalyst. The resulting mixture was applied to the electrically conductive coating of a glass plate similar to the plate of Example 1 and cured to form a thermoset resin coating about 2 mils in thickness. After curing the coating was charged to about 600 volts and exposed to an image pattern as set forth in Example 1. A triboelectric powder was dusted over the surface and an image having continuous tone was formed by the powder adhering to the remanently-charged areas. When the same procedure was repeated using reversed polarity of the initial charge, a similar but reverse image was produced.

Example 5 A photoconductive fiLm containing a very small but effective amount of copper phthalocyanine was prepared as set forth in Example 1. In this example, the exposure time to white light was increased to 5 seconds. Upon heating, an image was developed which was similar to the usual image thermally developed from the one-layer system previously described, i.e., the ripples constituting the image pattern were very much larger than the matte pattern of Example 1 and the Schlieren projected image was the reverse of that of Example 1. Upon inspection under a microscope, the surfaces of these gross deformations were found to have the smaller deformations characteristic of the image patterns observed in Examples 1 t0 3.

Other phthalocyanines have been found to render suitable organic polymers photoconductive in a similar manner. For example, the iron, lead, molybdenum, cobalt, nickel, and manganous phthalocyanines have been found to behave in an analogous manner to produce polymeric films capable of accepting an image pattern of electrostatic charges which may then be recovered or developed by a variety of means. Furthermore, recording members made according to Example 2 have been successfully charged, exposed, developed, and erased repeatedly for more than 25 times with no evidence of image deterioration. Additionally, when a copper phthalocyanine pigment was used which did not tend to agglomerate in the liquid resin-solvent suspension, virtually none was removed by filtration and films formed by the evaporation of the solvent from such suspensions containing up to 10 percent by weight of the phthalocyanine pigment responded equally as well as those containing the lower concentrations. In fact, those films containing the higher phthalocyanine concentrations appear to have a faster response to illumination.

While for the purposes of this disclosure, specific working examples have been set forth, it will be apparent to those skilled in the art that many other and specifically different variants of the invention are possible. For example, while glass plates have been disclosed as the supporting media, any suitable substrate may be employed, such as for example, flexible polymeric tape such as commercially available polyethylene terephthalate condensation polymer photographic film base, flexible metallic tape, other rigid bases suclh as metallic plates, other opaque thermoset polymers such as the polyester resins or the phenol aldehyde type resins as well as many other suitable substrates. Where electrically non-conductive substrates are employed, any of the many well-known electrically conductive films may be employed in place of the disclosed stannic oxide film as the ground layer and for electrical heating. Many other variants may be employed within the scope of the invention and, therefore, it is intended to limit the invention only to the scope of the following claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A photoconductive composition comprising a matrix of a substantially nonphotoconductive organic polymer containing a substantially uniform dispersion of particles of a photoconductive pigment selected from the group consisting of phthalocyanine and metal derivatives of phthalocyanine.

2. The composition of claim 1 which is thermally deformable.

3. An information storage medium which comprises a recording layer formed from the composition of claim 1, an electrically conducting inner layer, and a dielectric support.

4. An information storage medium according to claim 3 wherein the dielectric support is an organic polymer having a higher deformation temperature than the nonphotoconductive organic polymer of the photoconductive composition.

5. The medium of claim 3 wherein said composition is thermally deformable.

6. An information storage medium according to claim 3 which is in the form of a tape.

7. The tape of claim 6 wherein said composition is thermally deformable.

8. An information storage medium according to claim 4 which is optically transparent.

(References on following page) 7 References Cited UNITED STATES PATENTS 11/ 1964 Cassiers 961 8/1965 Gaynor et a1 117-201 X 5 8/ 1966 Gaynor 117201 8 OTHER REFERENCES Grei'g: An Organic Ehotoconductive System, RCA Review, September 1962, vol. 23, pp. 413-419.

WILLIAM L. JARVIS, Primary Examiner.