US 3775105 A
A photoconductor of the organic type is disclosed where a sensitizing disazo pigment is milled to a submicron particle size in an alkyl substituted benzene or alkyl substituted benzene blended with another solvent resulting in a more light sensitive photoconductor with good stability characteristics.
Claims available in
Description (OCR text may contain errors)
United States Patent 1191 Kukla Nov. 27, 1973 [5 1 DISAZO PIGMENT SENSITIZED 3,622,341 11/1971 Lee 96/1.6 PHOTOCONDUCTOR 3,384,632 5/1968 Solodar 96/1.6 X 3,597,196 8/1971 Jones et a1. 96/].6  Inventor: William J. Kukla, Lexington, Ky.
D FOREIGN PATENTS OR APPLICATIONS  Asslgnee' lmemamfnal Busmes; gfi 964,873 1 7/1964 Great Britain 96/].6 cm'pm'ammArmm 964,877 7/1964 Great Britain 96/1.6  Filed: Dec. 26, 1972 Primary ExaminerRoland E. Martin, Jr.  Appl 317393 AttorneyLaurence R. Letson et a1.
 US. Cl. 96/16, 96/15 57 ABSTRACT  Int. Cl 603g 5/06 58 Field 61 Search 96/15, 1.6 A the 9 dscmsed where a sensitizing dlsazo pigment is milled to a sub- 56 R f d micron particle size in an alkyl substituted benzene or 1 UNITE]; gig LZ alkyl substituted benzene blended with another SOI- vent resulting in a more light sensitive photoconductor 3,484,237 12/1969 Shattuck et a1 96/1.5 with good stability characteristics 3,287,121 11/1966 Hoegl 96/1.5 3,634,079 H1972 Champ 96/l.5 9 Claims, No Drawings DISAZO PIGMENT SENSITIZED PHOTOCONDUCTOR RELATED CO-PENDING APPLICATION Electrophotographic Process Using Disazo Pigments, Ser. No. 129,635, filed Mar. 30, 1971, inventors R. B. Champ and M. D. Shattuck.
BACKGROUND OF THE INVENTION It has been discovered in working with the photoconductor described in US. Pat. No. 3,484,237 to Shattuck, et al., Organic Photoconductive Compositions and Their Use in Electrophotographic Processes, that the sensitivity of the photoconductor may be enhanced when sensitized with a disazo pigment such as is disclosed in the co-pending application referenced above.
It was further discovered that additional light sensitivity was secured when the coating was not completely dried and there was a retained solvent level in the coating approximating 1.5 percent to 2.0 percent.
Under ambient conditions this retained solvent level or residual solvent level was not sufficiently high to create an' instability phenomena. However, when this photoconductive structure with a high residual solvent level was placed in an electrophotographic machine environment, where the photoconductor temperature may reach as high as 140 to 150 F. due to'energy dissipated from electrical components, light being generated by the illumination system, and the heat'energy generated by the fuser mechanism, the residual solvent in the photoconductor was driven off with time and the enhancement of the light sensitivity present previously, was at least partially destroyed.
SUMMARY OF THE INVENTION A stable photoconductor may be enhanced in light sensitivity while obtaining the advantages of unstable solvent levels by milling the sensitizing disazo pigment in an alkyl substituted benzene or a blend thereof with another solvent, prior to adding it to the photoconductor matrix material. This procedure eliminates the need to retain unstable residual solvent levels in the. final photoconductor structure.
It is an object of the invention to eliminate problems associated with high residual solvent levels in organic photoconductive coatings.
Itis a further object of this invention to alleviate the detrimental effect on the sensitizing capability of disazo pigments when said pigments are prepared in 'tetrahydrofuran alone.
It is an additional'object of this invention to enhance the sensitivity of the photoconductive structure comprising a polyvinylcarbazole/2,4,7-trinitro-9- fluorenone charge transfer complex sensitized with a disazo pigment.
The foregoing objects and benefits will be more easily understood from the following more specific examples and detailed description.
DETAILED DESCRIPTION It should be understood at the outset that the exact mechanism by which this invention functions is not presently understood, but that there is aclearly discemable pattern evident which supports the thesis that the milling and preparation of a disazo pigment to submicron sized particles in a solvent selected from the family of alkyl substituted benzenes, which have a boiling point less than about 250 C. and a melting point below 20 C., is advantageous. The subsequent addition of the pigment paste formed by milling the disazo pigment in the alkyl substituted benzene to a photoconductive charge transfer complex of polyvinylcarbazole (PVCz) and 2,4,7, trinitro-9-fluorenone (TNF) results in a more light sensitive photoconductive member compared to such a photoconductor when the identical materials are prepared in a straight, single solvent system where the single solvent is tetrahydrofuran.
It has been found that when a disazo pigment, and more particularly when chlorodiane blue pigment is added to an approximately 1:1 molar weight ratio of an organic photoconductive charge transfer complex, particularly polyvinylcarbazole and 2,4,7 trinitro-9- fluorenone, the light sensitivity of the photoconductor is greatly enhanced. Further light sensitivity is evident when the solvent from which the photoconductive layer is coated is not completely dried out of the resulting structure. It has been found that residual tetrahydrofuran solvent levels of about 1.5 percent yield an increased speed or light sensitivity, however, this level is unstable at elevated temperatures.
The temperature of a photoconductor rises due to the normal operating temperatures of an electrophotographic copying machine or apparatus, and thus the residual solvent is driven off with time. When the residual solvent is evaporated to below about 1 percent, most of the speed advantage observed as a result of the high solvent level, is dissipated.
Some of the benefits of the high residual solvent level may be obtained by using a solvent blend where one of the solvents is tetrahydrofuran and the second solvent is an alkyl substituted benzene. The best results are obtained when the chlorodiane blue pigment is milled to a submicron particle size in the alkyl substituted benzene alone and slightly less effectively in a tetrahydrofuran/toluene blend.
It is believed that there is a detrimental effect generated on the sensitizing properties of the disazo pigment by the contact of the pigment with other ingredients in the photoconductive structure when tetrahydrofuran solvent is used.
It is believed, insofar as the invention is understood, that the use of alkyl substituted benzene eliminates or at least minimizes any undesirable effects on the disazo pigment when the alkyl substituted benzenes are used either in a blend with tetrahydrofuran or by themselves, during the step of milling the pigment paste for addition to the photoconductive organic solution.
It is believed that thereis a surface wetting of the pigment particles with the alkyl substituted benzenes and that this tends to act as a barrier between pigment and other materials in the solution/dispersion which are insoluble in the alkyl substituted benzenes but otherwise advantageous to the final product.
EXAMPLE 1 To an abrasion resistant container is added a quantity of well cleaned steel shot. To the container and the shot is added chlorodiane blue pigment and toluene, in the proportion of 6 grams of pigment to 94 grams of tolu ene. The mixture of pigment, toluene and shot is agitated on a Red Devil paint shaker for about minutes to mill the pigment to a submicron size and wet the particles with solvent. The agitation results in a pigment paste which is removed from the steel shot and container. A quantity of pigment paste is added to 250 grams of a 1:1 molar weight ratio of PVCz:TNF charge transfer complex solution containing approximately 8 percent DuPont 49000 adhesive to yield a 6 weight percent pigment concentration based on total solids. The 1:1 molar ratio PVCz:TNF and 49000 adhesive, are dissolved in tetrahydrofuran. The amount of pigment paste added depends upon the percent solids of the charge transfer complex solution. The above blend is then placed on the Red Devil paint shaker and vigorously agitated for about minutes. The blended mixture is then allowed to cool to approximately 78 F. and the viscosity of the blended solution is adjusted with tetrahydrofuran to 60 centipoise seconds at 75 F. The solution/dispersion is then coated onto a conductive substrate within 1 hour after the pigment paste has been added to the organic photoconductive charge transfer complex solution.
The above procedure will result in a solvent ratio of about 83 percent tetrahydrofuran and 17 percent toluene prior to adjusting the viscosity. After viscosity adjustment, the solvent ratio will generally be in the range of 84 to 88 percent tetrahydrofuran and approximately 16 to 12 percent toluene. The toluene content of the final blend may be up to about 30 percent of the solvent and still produce improved results. The above procedure was performed with a polyvinylcarbazole designated Luvican M170 acquired from Badische Analin & Soda-Fabrik AG.
A photoconductive structure was made by coating a substrate with at least one electrically conductive face with a layer of the above formulation. This structure was negatively charged by a corona and the quantity of light necessary to discharge the charged photoconductive structure to a 200 volt charge level was measured. The exposure was the converted to a relative sensitivity advantage expression resulting in a sensitivity advantage of from 2.31: to 2.5x, this figure being a relative measure of sensitivity as compared to a photoconductor without chlorodiane blue pigment and using only a tetrahydrofuran solvent, meaning that the sensitivity was 2.3 to 2.5 times that of the standard which has a value of 1.0x.
EXAMPLE 2 A photoconductor coating liquid was prepared by adding to an abrasion resistant container 200 grams of clean steel shot, 57.81 grams of tetrahydrofuran and 57.81 grams of toluene. To this solvent blend, 9.38 grams of Formvar 15/95S, a polyvinylformaldehyde resin from Monsanto Chemical Company, is added to the solvent blend and the container is agitated on a Red Devil paint shaker for approximately 20 minutes. After a 20 minute agitation on the Red Devil paint shaker, 7.37 grams of chlorodiane blue pigment is added to the container and the container is again agitated on the Red Devil paint shaker for approximately 75 minutes. The container is removed from the shaker and immediately 63.6 grams of the solvent/adhesive/pigment paste is added to 250 grams of a 1:1 molar ratio PVCz/T NF solution having 28 percent solids. This mixture is placed on the Red Devil paint shaker for 15 minutes and the solution is then allowed to cool to approximately 78 F. This mixture is then adjusted in viscosity with tetrahydrofuran solvent to 41 centipoise seconds at 75 F. This solution is then coated onto a conductive substrate within 1 hour after the paste has been added to the organic photoconductive charge transfer complex solution.
The polyvinylcarbazole used in this example was secured from the DeSoto Chemical Company. The differences between Example 1 and Example 2 are primarily the stage at which the adhesive is added and the viscosity to which the solution is adjusted. The above procedure will result in a solvent ratio of approximately 87 percent THF and 13 percent toluene prior to viscosity adjustment and approximately 90 to 94 percent THF and 10 to 6 percent toluene after the viscosity has been adjusted.
With respect to solids, the pigment concentration will be 4.5 weight per cent and the adhesive concentration will be 6.0 weight percent.
The solution is coated onto an electrically conductive substrate, negatively charged by a corona, and the quantity of light necessary to discharge the charged photoconductive structure to a 200 volt charge level, measured.
This exposure, is converted into a relative sensitivity advantage, which is based upon a photoconductive structure as discussed in FIG. 1. The relative speed advantage is from about 1.98x to about 2.24x.
EXAMPLES 3 THROUGH 9 Photoconductive structures were formulated and constructed according to the procedure of Example 2, using the polyvinylcarbazole acquired from DeSoto Chemical Company and the Formvar 15/95S adhesive. The only deviation from the procedure of Example 2 was the substitution of various alkyl substituted benzene solvents to illustrate that results approximately equal to or better than that secured by the use of pure tetrahydrofuran can be secured in the case of all alkyl substituted benzene solvents tested which have a melting point less than 20 C. and a boiling point of less than approximately 250 C. The results of the formulations are tabularly set forth in Table 1, together with repetitive results of Examples 1 and 2.
TABLE I Example Relative Coat wt. Residual No. Solvent Sensitivity mg/in Solvent 1 Toluene 2.38X N/A 0.18 1a Toluene 2.32X N/A 0.25 lb Toluene 2.58X N/A 0.21 2 Toluene 2.24X 9.7 0.55 221 Toluene 1.98X 11.7 1.08 2b Toluene 2.18X 13.4 0.68 2c Toluene 2.01X 10.6 0.68 2d Toluene 2.10X 10.6 0.46 2c Toluene 2.16X 10.5 0.41 3 Benzene 1.60X 9.7 0.15 4 Ethyl benzene 1.80X 10.6 0.52 5 Xylene 1.54X 10.8 0.865 6 Propylbenzene 1.99 X 8.7 0.405 7 Mesitylene 1.97X 9.9 1.375 8 Tertiary butyl 1.50X 8.4 1.245
benzene 9 Cumene 1.67 12.9 1.02
Examples la and 1b, and 2a through 2e are repetitive tests using the procedures of Examples 1 and 2 respectively.
"Used in Addition to THF As Per Examples 1 and 2.
"Of The Alkyl Substituted Benzene Solvent.
N/A Not Available.
Table 11 illustrates the results using only THF in the two procedures of Examples 1 and 2, for comparative purposes.
TABLE 11 Relative Residual Procedure Sensitivity Solvent (THF) Example I 1.54X 0.58 1.6OX 0.25 Example ll 1.52X 0.18
As can be seen from the tables above, seven additional solvents were utilized and tested in addition to toluene and the results indicate that the sensitivity of a photoconductive structure formulated using the technique of Example 2 andthe various solvents, yields a photoconductor which is at least approximately as sensitive or more sensitive than the straight tetrahydrofuran solvent system. It also appears that from the different tests using the toluene, the preferred solvent is toluene but at least some of the other solvents such as propylbenzene, mesitylene, and ethylbenzene have a substantial sensitivity advantages over a'pure tetrahydrofuran solvent. Differences of 0.1 or more between relative sensitivity figures are considered significant.
The residual solvent levels indicated in the table above, are the residual solvent levels for the solvent indicated in the leftmost column of Table I and tetrahydrofuran in Table II. The tests showed no detectable tetrahydrofuran residual solvent in any of the samples tabulated in Table I. It appears that when a solvent blend of tetrahydrofuran and an alkyl substituted benzene is used and the alkyl substituted benzene is used in a range of up to about percent and preferably between about 6 to about 16 percent, any residual alkyl substituted benzene solvent level below about 1.4 percent is insufficient to exhibit a correlation between that and enhanced photoconductive properties. It appears that by the use of the solvent blend, the tetrahydrofuran may be virtually eliminated from the resulting photoconductor structure and a relatively low and stable quantity of residual solvent of the alkyl substituted benzene group may remain in the photoconductor and have little if any effect on the relative sensitivity of the photoconductor.
Another pigment tested in the co-solvent system, diane blue, showed sensitivity advantage over the standard and also over the pigmented photoconductor formulated using tetrahydrofuran alone. The relative speedadvantage over the standard, was 1.7X. No residual solvent data is available with respect to this formulation.
As can be seen from the foregoing, the use of an alkyl substituted benzene solvent in a proportion of approximately 50 percent to 100 percent of the solvent used in milling the disazo pigment and particularly chlorodiane blue pigment into a paste is advantageous. This improvement yields an ultimate solvent blend of from 6 to about 16 percent alkyl substituted benzene and from about 84 to 94 percent tetrahydrofuran. This photoconductor results in most cases in improved light sensitivity over the similar type structure which was constructed using only a tetrahydrofuran solvent.
By using the solvent blend techniques as set forth above, it is believed that the light sensitivity of the photoconductive structure will be enhanced to a point where it more nearly approaches the potential light sensitivity of the chlorodiane blue pigmented coating.
The differences between Example 1 and Example 2 above, particularly in the formulating technique and the viscosity adjustment is due, it is believed, to the difference in the molecular weights between the Luvican M170 and the DeSoto Chemical Company polyvinylcarbazole. Secondly, it has been found that the Formvar will dissolve in the THF but is partially incompatible with the PVCzzTNF complex. Therefore, it is formed as part of the pigment paste necessitating the use of a solvent blend to adequately dissolve the material and accomplish the objects of this invention.
The following invention has been particularly shown and described with reference to preferred embodiments thereof, to be understood by those skilled in the art that the foregoing and other changes in form and detail may be made therein without departing from the spirit and scope of the invention.
1. An electric photoconductive member comprising:
a substrate with at least one electrically conductive surface;
a photoconductive matrix medium overlying and in electrical contact with said electrically conductive surface;
' a disazo pigment dispersed in particulate form throughout said photoconductive matrix medium; the improvement comprising:
said photoconductive matrix medium together with said disazo pigment particles,-bei'ng the residue of a coating solution/dispersion comprising a charge transfer complex of polyvinyl-carbazole and trinitrofluorenone dissolved in tetrahydrofuran; and
a sensitizing quantity of said disazo pigment, said pigment having been milled to submicron size particles in a solvent comprising an alkyl substituted benzene, said alkyl substituted benzene being liquid at below about 20 C. and having a boiling point of less than about 250 C.
2. The photoconductive structure of claim 1, wherein said disazo pigment is chlorodiane blue.
3. The photoconductive structure of claim 1, wherein said alkyl substituted benzenes are selected from the group consisting of benzene, toluene, ethylbenzene, xylene, propylbenzene, mesitylene, tertiary butylbenzene, and cumene.
4. The photoconductive structure of claim 2 wherein said chlorodiane blue is present in a percentage of from about 1 to about 8 weight percent.
5. The photoconductive structure of claim 1, wherein said alkyl substituted benzene solvent is present in the solution/dispersion, prior to coating, in an amount up to about 30 weight percent of said solvents.
said alkyl substituted benzene is ethylbenzene.