US 3469979 A
Description (OCR text may contain errors)
3,469,979 ELECTROPHOTOGRAPHIC RECORDING ELEMENT WITH INCREASED SPEED Louis James Ferrarini, Wakefield, R.I., Harold J. Homonolf, Saxonville, and George R. Nelson, Holliston, Mass, assignors to Dennison Manufacturing Company,
Framiugham, Mass., a corporation of Nevada No Drawing. Filed Nov. 26, 1965, Ser. No. 510,089
Int. Cl. G03g 7/00 US. Cl. 96--1.7 8 Claims ABSTRACT OF THE DISCLOSURE where each of R R R and R is either hydrogen, lower aliphatic, lower cycloaliphatic, lower acyl, aryl or heterocyclic monovalent radicals and R and R are lower divalent aliphatic radicals.
This invention relates to electrophotography and has for its principal object provision of photoconductive recording elements and recording processes with increased speed of discharge upon exposure to light.
Electrophotography is well known and involves generally the electrostatic charging in the dark of a photoconductive layer of a recording element, and exposing the layer to a pattern of light to which the photoconductor is sensitive, thereby forming a latent image as an electrostatic charge pattern in areas not receiving light. The resulting image can be electronically scanned or displayed but is usually developed to a visible image with electroscopic colored particles. The particles can then be fixed, if desired, either to the recording element itself or to a receiving surface to which they are transferred.
Recording elements comprising finely divided photoconductive particles dispersed in insulating resinous binders are known and are disclosed in US. Patent No. 3,121,006. Photoconductive zinc oxide in a binder is generally used and such recording elements are more specifically disclosed in US. Patent No. 3,052,539. Such recording elements are known to be sensitive to ultraviolet radiation at wavelengths of 3-800 angstroms or lower. Sensitivity refers to the capacity of the photoconductor of the recording element to dissipate electrostatic charges on its surface by a lowering of its resistivity at specified wave lengths of light.
In commercial practice, it is generally preferred that the recording elements be sensitive to light at visible Wavelengths includingSOOO to 5500 angstroms. This can be accomplished by adding dye of known type as disclosed for example in the above two patents. However, dyes or a United States Patent 0 3,469,979 Patented Sept. 30, 1969:
combinations of dyes which provide good sensitivity and speed to the photoconductive layer tend to be highly colored and to impart color to the photoconductive layer. This is a disadvantage in copying processes Where dark prints on a white background are desired.
It has now been found that inclusion, together with sensitizing dye, within the photoconductive layer or coating of dispersed thiourea or thiourea derivatives having a carbon atom bonded to two nitrogen atoms and. one sulfur atom, materials which are not themselves sensitizing dyes, increases the speed of discharge of the coating, referred to herein as hypersensitization. In addition to the increase in speed which it makes possible, hypersensitization permits use of a lesser amount of dye, a greater variety of dyes, and permits formulation of zinc oxide coatings which are more nearly White in appearance. As will appear hereinafter, the amount of the hypersensitizer additive is not critical and the nature of the N substituents, provided they are substantially non-reactive in the coating, is not important, a large number including substantially all types currently available having been found to be useful.
Thus in one aspect, the present invention comprises an improved electrophotographic recording element comprising a backing and a coating of finely-divided zinc oxide and finely divided sensitizing dye therefor dispersed in an insulating binder wherein the improvement consists in the dispersion within the coating of an eflective amount of a hypersensitizer for increasing the speed of discharge of the coating when electrostatically charged and exposed to light, said sensitizer consisting essentially of (a) thiourea dioxide, or (b) a compound of the structure wherein each of R R R3 or R is selected from the group consisting of hydrogen, lower aliphatic, lower cycloaliphatic, lower acyl, aryl, and heterocyclic monovalent radicals and R and R are lower divalent aliphatic radicals. Thiourea dioxide, also known as formamidinesulforic acid, has the structure In a further aspect, the invention comprises the method of producing a latent electrostatic image comprising applying a uniform electrostatic charge to the above improved recording element in the dark, that is, the absence of radiation to which the photoconductor is sensitive, and exposing the element to a pattern of light to which the photoconductor is sensitive.
The effect of the hypersensitizer is shown in the following examples, the compositions stated being coated onto paper 0.0022 inch in thickness and having a resistivity at 20% relative humidity of 1000 megohms. The coating compositions were applied and dried to a thickness of about 0.0008 inch. After storage in the dark at 20% R.H., they were charged to approximately 500 volts using a negative corona at 7.4 kv. grounded with a positive corona of 3.0 kv. The discharge time was the time in seconds to discharge to 5 volts upon exposure to 2.0 foot candles of light from a General Electric Photo flood lamp EPR of 375 watts. The time lapse between charging' and exposure in each case was about 8 seconds. Voltages were measured as apparent surface voltage with the equipment and procedure described in E. Giamo, RCA Review, volume XXII, No. 4, pages 780-790 of December 1961. All parts are by weight except where otherwise indicated.
EXAMPLES 1-24 Zinc oxide 100. 100.0 Aeryloid 13-82 (40%) 37. 8 37.8 Poly-alpha methyl styrene. 1. 4 1. 4 Toluene 70.0 70.0 sensitizing dye solution, ml 1. 2 mercapto imidazoline. 1. 0 Hypersensitizer, one of 1.0
Exposure Example Hypersensitizer time N,N -ethylenethiourea. N methyl thiourea N,N dimethyl thiourea 1,3 diethyl, 2 thiourea 1,3 dibutyl thiourea N acetyl thiourea. 1 allyl, 2 thiourea 3 allyl1,1 diethyl, 2 thiourea 1 phenyl, 2 thiourea 1,1 diphenyl, 2 thiourea N,N dicyclohexyl thiourea. 1 cyclohexyl, 3 (2 morpholinoethyl) th10urea Thiourea 1,3 bis (carboxypentyl) thiourea N,N, bis (4 fluorophenyl) thiourea N,N, di tert-butyl thiourea N,N', di-isopropyl thiourea N,N dimethyl N phenyl thiourea. N,N dimethyl N (O tolyl) thiourea N,N dimethyl N (M tolyl) thiourea. N,N dimethyl N (p tolyl) thiourea Thiourea dioxide 23-- O methoxyphenyl thiourea 24 N,N dimethyl thiourea In the above examples the zinc oxide was Florence Green Seal No. 8 of the New Jersey Zinc Co., Acryloid B-82 as a 40% solution in toluene of an acrylic polymer from Rohm and Haas Co., the alpha-methyl styrene was 4 Resin 276 V2 of the Dow Chemical Company, and the sensitizing dye solution had the following composition:
Eosin 0], Acid Red 87 (a hydroxyphthalein dye), 1% Calco fiuorescein, Acid Yellow 73, 1% Brom phenol blue (3,3,3,5-tetrabrorno-phenol sulphionphthalein), 1% 0.26 Brilliant Yellow 6G, 1% 0.84
The samples were prepared by dispersing the zinc oxide in the solvent, adding the resins, dye solution, and hypersensitizer, the resulting suspension coated onto the paper base and the coating dried, for example for one minute at C.
To show the wide variation in useful amounts of hypersensitizer additive, Examples 25-33 below were prepared using the Zinc oxide and resin formulation and procedure of Examples 1-24 but substituting the indicated dyes and additive in the quantity of toluene solvent indicated in ml. Exposure time was the time in seconds to discharge to 5 volts apparent surface voltage.
Hypersensi- Initial Exposure Solvent Dye tizer charge time The dye employed in Examples 25-33 was a 1% solution of Calco Florescein of the American Cyanamid Co. in methyl Cellosolve, the amounts indicated being in ml. The hypersensitizer was 2 mercapto imidazoline, the amount being in grams. At 3 ml. of dye solution, the zinc oxide coating exhibited some color. While these data show that amount is not critical, Example 29 with 1.0 part by weight hypersensitizer per parts zinc oxide with minimum dye is preferred.
To show that the eifect of the hypersensitizer additive is applicable to a wide variety of dye formulations, Examples 34 to 61 below were prepared using Formula B of Examples 1 to 24 together with the dyes indicated in the following Examples 34 to 61, the dye amounts being in ml. of a 1% solution of the dye in methyl Cellosolve.
EXAMPLES 34-01 Dye Class Amount Example:
3 Bromophenol blue Sulphonphthaleimuu 1,0 Eosin OJ Hydroxyphthalein. 1.0 Calco fluorescein 1 0 Brilliant yellow 66 base. 1,0 Methylene blue 2. 5 Auramiue O 2. 0 Azo eosine. 2. 0 Anthroquinone blue 2. 0 HLR 2.0 Acndme orange Aerrdme. 2. 0 2. 0 2. 0 2. 0 47.- Rapidogen red CF j 48 Niagara brilliant blue BFL. 6 0 49 Sulfidone brilliant blue 0 G cone 6, 0 50 Stilbene yellow GX Stilbene 6, 0 51 Thiofiavine. Thiozole 6. 0 52 Fluorescent purp Polyazo 6, 0 53 Solophenyl green.. Trisazo 6. 0 54.- Fast acid violet 10B Triarylmethane 6.0 55 Brilliant sulpho fiavm Aminoketone 6. 0 56.. Ciba blue 2B Indigoid 6.0 57 Quinoline yellow ba Quinoline 6.0 58 Nigrosine base Azine... 6.0 59.- Genacryl yellow SGF Methine 6. 0 60.- Zapon fast green GG Oxazine and nitro- 6. 0 61 Variamine blue salt BD Azoie 6.0
In Table I below, the initial voltages obtained as stated above and final voltages obtained after the stated exposure time in seconds for each of Examples 34 to 61 are given. Control figures are initial and final voltages for the same formulation but without the hypersensitizer.
TABLE I Control Example Exposure time Initial Final Initial Final As sensitizer, 2 mercapto imidazoline is preferred, about 1 part per 100 parts zinc oxide, although amount is not critical.
It should be understood that the foregoing disclosure is for the purpose of illustration and that the invention includes all modifications within the scope of the appended claims.
1. An improved electrophotographic recording element comprising a backing and a coating thereon of finely divided photoconductive zinc oxide and sensitizing dyes therefor dispersed in an insulating binder wherein the improvement consists in the dispersion within the coating of an effective amount of a hypersensitizer for increasing the speed of discharge of the coating when electrostatically charged and exposed to light, said hypersensitizer selected from the group consisting essentially of (a) thiourea dioxide or (b) a compound having the structure wherein each of R R R and R is selected from the group consisting of hydrogen, lower aliphatic, lower cycloaliphatic, lower acyl, aryl, and heterocyclic monovalent radicals and R and R are lower divalent aliphatic radicals, said heterocyclic radical consisting essentially of a six-membered ring of carbon, nitrogen and oxygen.
2. A recording element according to claim 1 wherein said backing is paper and said hypersensitizer is selected from the group consisting of N,N-ethylenethiourea; N methyl thiourea; N,N dimethyl thiourea; 1,3 diethyl, 2 thiourea; 1,3 dibutyl thiourea; N acetyl thiourea; 1 allyl, 2 thiourea; 3 allyl 1, 1 diethyl, 2 thiourea; 1 phenyl, 2 thiourea; 1,1 diphenyl, 2 thiourea; N,N dicyclohexyl thiourea; 1 cyclohexyl, 3(2 morpholinoethyl) thiourea; thiourea; 1,3 bis (carboxypentyl) thiourea; N,N bis (4 fluorophenyl) thiourea; N,N di tert-butyl thiourea; N,N di-isopropyl thiourea; N,N dimethyl N' phenyl thiourea; N,N dimethyl N (o tolyl) thiourea; N,N dimethyl N (m tolyl) thiourea; N,N dimethyl N (p tolyl) thiourea; thiourea dioxide; 0 methoxyphenyl thiourea; and N,N dimethyl thiourea.
3. A recording element according to claim 2 wherein said hypersensitizer is present in an amount between about 0.01 and 25 parts by weight per parts zinc oxide.
4. A recording element according to claim 3 wherein said hypersensitizer is N,N-ethylene thiourea.
5. The method of producing a latent electrostatic image which comprises applying a uniform electrostatic charge in the dark to the coating of the recording element of claim 1 and exposing the coating to a pattern of light.
6. The method of producing a latent electrostatic image which comprises applying a uniform electrostatic charge in the dark to the coating of the recording element of claim 2 and exposing the coating to a pattern of light.
7. The method of producing a latent electrostatic image which comprises applying a uniform electrostatic charge in the dark to the coating of the recording element of claim 3 and exposing the coating to a pattern of light.
8. The method of producing a latent electrostatic image which comprises applying a uniform electrostatic charge in the dark to the coating of the recording element of claim 4 and exposing the coating to a pattern of light.
References Cited UNITED STATES PATENTS 3,031,301 4/1962 Agens 96-10l X 3,352,670 11/1967 Coles 961.7
OTHER REFERENCES Inoue et al., Hypersensitization of Photoconduction in Microcrystalline Zinc Oxide, Journal of Physical Chemistry, vol. 69, No. 3, pp. 767-779 (March 19-65). GEORGE F. LESMES, Primary Examiner C. E. VAN HORN, Assistant Examiner U.S. Cl. X.R. 96-1