US3799911A - Binder for electrophotographic coatings - Google Patents

Abstract

A POLYMERIC BINDER FOR ZINC OXIDE FOR USE AS AN ELECTROPHOTOSTATIC COATING COMPOSITION CONTAINS A TERPOLYMER OF ABOUT 50-70% BY WEIGHT ETHYL ACRYLATE, ABOUT 30-50% BY WEIGHT STYRENE, AND ABOUT 1-5% BY WEIGHT MALEIC ACID WHICH IS PARTIALLY MOND-ESTERIFIED PRIOR TO AND DURING POLYMERIZATION.

Description

Patented Mar. 26, 1974 United States Patent Oflice v ABSTRACT OF THE DISCLOSURE A polymeric binder for zinc oxide for use as an electrophotostatic coating composition contains a terpolymer of about 50-70% by weight ethyl acrylate, about 30-50% by weight styrene, and about l% by weight maleic acid which is partially mono-esterified prior to and during polymerization.

The present invention relates to improved resin binders for photoconductive pigments, and more particularly, to terpolymer binder resins which possess highly desirable electrical properties when used as a photoconductive insulating material. The role of the binder resin is extremely important in achieving optimum performance of the electrophotographic coating composition. The binder resin serves a multiple function. Its most basic function is simply to hold together the dyed zinc oxide grains and to bond the photoconductive layer to the substrate. Its second function is as an insulator to aid in the retention of electrostatic charges on the surface of the coating. The resin insulates the zinc oxide grains from one another but the degree of insulation or physical separation must be carefully controlled. Grain-to-grain contact is necessary to provide a conduction path to ground to carry off the negative charges duringexposure and to allow for charge transport through the layer during charging. Insufiicient resin will allow too much grain-to-grain contact and the layer will short out during charging, resulting in a very low level of charge acceptance and high dark decay rate. Too much resin will cause too little grain-to-grain contact. This will usually give a high charge acceptance with very slow photoresponse. A resin which does not have a high enough dielectric constant can break down I under the influence of the high field strength during charging, causing the layer to short-out. v

' Optimum pigment-to-binder ratios for commercial electrophotographic paper is 6/ 1 to'9/l. Ch'arge'acceptance and'speed are a direct function of coating weight or coating thickness'The highest coating weights commercially employed are in the range of 20-25 lb./ 3000 square feet. This will yield the highest image density. Most commercial Electrofax paper runs 15-20 lb./3000 square feet. New light weight paper is now on the market with coating weights of -12 lbs./ 3000 square feet. The image quality of this type of paper is not as good as that of the heavier paper but this is "offset by the light weight of the sheet to compete with plain paper copiers. l

' Much of the-binderresin research has been empirical in nature. Th'e'reason why some resins work well while others do not is still not clearly understood. It has been found, for example, that a'high degree'of-aromaticity is important for good performance in oil-typeresins. This is usually accomplished by incorporating a dibasic acid, such as phthalic, or TDI into the resin. In polyvinyl acetate and'acrylic resins, ca-rboxy functionality is usually necessary for'good charge acceptance. Optimum monomer ratios are often established by Edisonian trial-and-error methods. Deviation from these optimum ratios usually results in a loss of properties. If one were to draw up a list of properties for the ideal resin binder, it would probably be something like this:

(1) High charge acceptance.

(2) Low dark decay rate.

(3) Very rapid light response.

(4) High image density.

(5 Clean background.

(6) Broad exposure latitude.

(7) Good performance at extremes of humidity.

(8) Immunity to pre-exposure efiects.

(9) Reduceable in toluene or in systems which comply with air pollution control ordinances.

(10) Coatable at high solids.

(11) Lacquer dry or quick-cure.

(12) 'Good package stability.

(13) Maximum performance at low coating weights.

(14) Low cost.

The terpolymers of the present invention provide for improved light sensitivity, good adhesion properties, improved optical density in the images, as well as slower dark decay rates. A slow dark decay rate is desirable in order to provide the necessary time after charging of the electrostatic member to advance it to the exposure station. The light sensitivity is defined interms ofthe average voltage drop in seconds, which is important to permit lower exposure time and operation at lower light intensity sources. Print quality in the form of sharp, dense images is highly desirable for obvious reasons. Finally, the binder resin must have a strong adhesion to the base support and provide a base member with good flexibility and good electrical properties at reasonably low coating weights.

The present invention is directed to a binder resin which yields unexpectedly good electrophotostatic properties when mixed with a photoconductive pigment, appropriate dyes and sensitizers, and applied as'a'thin coating to suitable conducting base support. The binder resins of the present invention comprise a terpolymer formed from ethyl acrylate, styrene, and a maleic acid component. More particularly, the present invention contemplates a terpolymer containing from about 50 to about by weight of ethyl acrylate, from about 30 to about 50% by weight of styrene, and from about 1 to about 5% by weight of a maleic acid component. Preferably the ethyl acrylate to styrene ratio is about 6 to 4 parts by weight, with the maleic acid component making up about 22.5% by weight of the resinous composition. The maleic acid component must include both a partial mono-ester of maleic acid as well as some free maleic acid. It is preferred that from about 30 to about of the acid be converted to the mono-ester, leaving from about 70 to 20% of the acid as free acid, in the finished polymer. It has been found that the desired properties are achieved if the binder resins have an acid number between 6 and 25 (on a solids basis). It is preferred that the acid number be between about 12 and 20; The maleic acid 3 must be partially esterified with a lower alkyl alkanol, with propanol being the preferred alkanol.

The binder of the present invention, when incorporated with zinc oxide photoconductive systems gives improved lithographic plates. Further, the resins of the present invention can be used as binders for ofi-set masters in conventional duplicating processes.

The binder resins of the present invention, when combined with zinc oxide and other conventional materials may be applied to various electrically-conducting substrates which have resistivity in the range of 10 to 10 ohms centimeters and preferably in the range of 10 to 10 ohms centimeters. While the primary interest is in the application of these materials to paper substrates, other substrates such as plastic, cloth, metal foil such as aluminum and tin, and metal such as aluminum, copper, steel or lead, provided these materials have the prope electrical resistivity.

The binder resins of the present invention are used by forming a suspension of finely divided particles of photo! conductive material in the binder resin, which coating is applied as a thin film or layer to the surface of the substrate or the base. Generally the coatings include an organic solvent which is evaporated following the coating of the substrate. The resulting product is an electrophotographic plate which can be used in various duplicating processes as described above.

The photoconductive materials which may be used include vitreous selenium, sulphur, oxides of zinc, aluminum, titanium, lead, antimony, bismuth, cadmium, mercury, molybdenum, and copper, as well as the selenides and tellurides of these metals, with zinc oxide being the preferred photoconductor. The amount of photoconductive material used in the binder resins of the present invention may vary over wide limits. When zinc oxide is used, a pigment to binder ratio should be from about 5:1 to :1 on a weight basis, with a pigment to binder ratio of 6:1 to 8:1 being preferred.

The resins of the present invention show unexpectedly high efiiciency at low coatings weights. While the resins of the present invention may be used as coating weights as high as 30 lbs. per 3000 square feet, it has been found that at coating weights less than 10 lbs. per. 3000 square feet give good, fine-line copy. The resins of the present invention retain, the image quality at low coating weight, thereby demonstrating unexpected properties.

As is known to those skilled in the art, the particle size of the zinc oxide used is important with respect to the electrical properties found. It has been found that finely divided or small size zinc oxide produces a higher charge acceptance with consequent high contrast and tonal qualities but there is a corresponding loss of speed and light sensitivity. The larger. size-zinc oxide particles on the other hand are more light sensitive, but give a lower charge acceptance.

The resins of the present invention are produced with conventional free radical solution polymerization. In this polymerization process the various monomeric materials are combined at a proper ratio and in a suitable solvent such as xylene or toluene, with the maleic acid compoent being dissolved in the alcohol-half ester solution. The amount of solvent employed should be roughly equal to the total weight of the solids in order to prepare a solution containing from about 40 to about 60% by weight of solids. The amount of solvent used is not critical, since solvent can be added or distilled off prior to shipment. It is also contemplated that the resin may be mixed with additional solvent before it is actually coated.

Various solvents can be used in the preparation of the solution polymer used for the polymeric binder. Representative of the various solvents are solvent mixtures comprising'from about 50 to 90 percent or more aromatic hydrocarbons such as xylene, toluene and benzene. It is contemplated that from about 50 to about 10 percent by weight aliphatic hydrocarbons, aliphatic alcohols or alcohol ethers may be used. Representative of various aliphatic hydrocarbons are the pentanes, the hexanes, the heptanes and the octanes. Representative of the various aliphatic alcohols are ethyl alcohol, propyl alcohol, and butyl alcohols. Representative of alcohol ethers is the mono methyl ether of ethylene glycol. The solvents used in the polymerization process may serve as the solvents for the coating operation, or the solvents may be supplemented as required.

The solution polymerization reaction can be promoted by various free radical initiators well known to those skilled in the art for promoting free radical solution polynierizations. Representative examples of the various free radicalinitiators are benzoyl peroxide, ditertiary butyl peroxide, lauroyl peroxide, capryloyl peroxide, t-butyl perbenzoate, and azo-bis-isobutyronitrile (hereinafter referred to as Vazo catalyst).

The amount of free radical initiator employed in the polymerizations may be varied over wide concentrations. Of course, at least a catalytic amount of the initiator must be employed to cause polymerization of the monomers. The optimum amount of initiator depends on a number of factors such as temperature, reactants used, purity of reactants, reaction times desired and the like. Those skilled in the art will readily determine the optimum catalytic ranges.

A wide range of temperatures can be used for the polymerization such as from about 70 C. to about 120 C. although higher or lower temperatures can be used.

- Usually the time for the polymerization reaction to reach at least about a 97 percent conversion can be various times depending upon the initiator, solvent, and reaction temperature such as from about 5 to about 20 hours. The polymerization reaction can be conducted at atmospheric pressure or, if desired, it can be carried out at sub-atmospheric pressure or super-atmospheric pressure.

The monomers may be premixed before charging into the reaction vessel or may be separately charged.

The terpolymers of the present invention contain from about 30 to about 50% by weight of styrene. It has been found that formulations based on resins containing substantially more than about 50% by weight of styrene tend to seal the face of the paper, which then curls up upon contact with high ambient humidity. Curled paper is very undesirable with respect to automatic feeding apparatus used in copying machines. On the other hand, low styrene resins, those containing substantially less than 30% by weight of styrene, tend to produce a soft limp coating. Since ethyl acrylate is an expensive monomer, as compared to styrene, economics dictate that the ethyl acrylate should be kept as low as possible in the system.

Generally the maleic acid component preferably comprises from about,2.3 to 2.4% by weight of the solids content of the binder resins. However, from about 1 .to about 51% by weight of the maleic component can be used.

As was mentioned above, the binder resins of th present invention have. acid numbers in the 10-20 range which requires the presence of carboxylic acid groups of the maleic acid component. In preparingthe maleic acid component, it is necessary to esterify a portion of the maleic acid and produce at least some mono-ester of the maleic acid. Preferably, from about 30 to about of the acid is converted to the mono-ester before or during the polymerization. Free maleic acid will not give adequately high concentrations in its better organic solvents, such as the lower molecular weight alcohols. It has been found, however, that if a portion of the maleic acid is converted into the mono-maleic ester, the residual free maleic acid is adequately soluble. With respect to the electrical properties of the resin, it is necessary to have at least some non-esterified or free dibasic acid as a portion .of the terpolymer, in order to meetthe requirement that the acid number of the finished terpolymer place so that the acid component willdissolve in the al-' cohol in order to facilitate polymerization with the. other monomers and that the acid number of the resulting ter polymer be within the desired range. v

I With respect to forming the esters, propanol is generally preferred since it is a good solvent for the acidhalf ester and it has a reasonably high boiling point and therefore does not depress the boiling point of the solvent used as the reaction medium for the polymerization of the terpolymer. Methanol can be similarly used. Normal butanol is adequate, but the bad odor caused by butanol in the resin system is undesirable.

Experimental work has shown that the diesters of maleic acid, when used in corresponding terpolymers, do not produce the improved electrical properties achieved by the resins of the present invention. It has been discovered that some carboxylic acid groups must be present in order to achieve the desired electrical properties. On the other hand, if the acid number is too high, the viscosity of the resin is increased, and various rheological problems are encountered.

After the binder resin is coated on the substrate, the alcohol is evaporated with the other solvents as the coating dries. Generally it is desirable to keep the alcohol level in the coating composition as slow as possible in order to permit the operation of the solvent recovery systems which will not tolerate large quantities of alcohol.

The following examples will serve to illustrate the preparation of several electrophotographic binder resins, but it is understood that these examples are set forth merely for illustrative purposes and many other electrophotographic binder resins are within the scope of the present invention.

EXAMPLE 1 A five-liter flask, equipped with a condenser, an a'gitator, a heating mantel, a nitrogen blanket tube, a thermometer and two dropping funnels was charged with 1330 grams of xylene and the xylene was heated to reflux.

In a second container, 150 grams (2.5 moles) of n-prov panol and 62 grams (0.53 mole) of maleic acid were mixed together. The mixture was heated, with stirring, to 120-130 F. and held at this temperature for about minutes in order to form a partial half ester of maleic acid. The resulting material was a solution containing half ester, maleic acid and propanol, which was charged into one of the dropping funnels of the five liter flask.

In a separate vessel, the following materials were mixed:

Grams Ethyl acrylate (13.6 moles) 1364 Styrene (9.2 moles) 960 Tert-butyl perbenzoate 9 Vazo catalyst (azo bis-isobutyral nitrile) 9 n-Dodecyl mercaptan 6 These materials were mixed together and charged to the other dropping funnel of the five-liter flask. The two mixtures were metered into the five-liter flask simul-' 'taneously over about three hours while the refluxing 6 number of 20 on a solid basis, and 58.72% nonvolatile material. The product hereinafter referred to as Resin was then cooled and packaged.

EXAMPLE 2 A five-liter flask equipped with a reflux condenser, an agitator, a heating mantle, a nitrogen blanket tube, a thermoineter and a dropping funnel was charged with 798 grams of xylne, which was heated to reflux.

"In asecon'd vessel a mixture of 66 grams (0.89 mole) of n-propanol and 37.2 grams (0.32 mole) maleic acid were heated, with stirring, to -130 F. and held for 10 minutes to'form a partial half ester solution. The partial through a dropping funnel, over a three-hour period while the solvent was held at the reflux.

Then a mixture of 120 grams of toluene, 3.6 grams of Vazo catalyst and 7.2 grams of tert-butyl perbenzoate were added to the flask over a 2-hour period. After the final addition, the flask was held at the reflux for an additional 3 hours, then cooled and packaged to yield a m'aterialhereinafter known as Resin B, which had a viscosity of Y%, an acid number of 20.2 on a 100% solids basis, and 58.50% non-volatile material.

EXAMPLE 3 Monobutyl maleate was prepared by charging maleic anhydried to a three-liter flask equipped with a condenser, agitator, thermometer, nitrogen blanket tube, heating mantel and dropping funneL'The maleic anhydride was heated to ISO- F. and a 5% excess over the stoichiometric amount of n-butanol was added at a rate sufficiently slow so the temperature did not exceed F. Completion-of reaction was determined by infrared spectroscopy, with the absence of the anhydride indicating the complete mon-esterification of the maleic anhydride.

A 5-liter flask equipped with'the accessories described in Example 2 was charged with 798 grams of xylene and the xylene was heated to reflux. In a second vessel a mixture of 12 grams (0.103 mole) of maleic acid and 66 grams'(1.1 moles) of a propanol was heated with stirring to' 120-130 F. and held for 10 minutes. The resulting solution was mixed withthe following materials:

Grams Ethyl acrylate "(8.2 moles) 818 Styrene (5.5 moles)..- 576 Vazo catalyst 5.4 n-Dodecyl merea-ptan 3.6 Tert-butylj 'perbenzoate 5.4 Mono-butylmaleate ..-(0.44 mole) 76 7 EXAMPLE 4 A 5-liter flask equipped with the auxiliary equipment described in Example 2 was charged with 1275.0 grams of toluene and the toluene was heated to reflux.

In a separate container 65 grams (1.08 moles) of n-propanol and 50 grams (0.31 mole) of maleic acid were heated with stirring to 160 F. and held for approximately one-half hour to form the partial haifester and then cooled to room temperature. I

A mixture of the following materials were added to the half ester solution:

Grams Ethyl acrylate 13.6 moles) 1364 Styrene ....(9.2 moles)..- 960 Vazo catalyst 12 n-Dodecyl mercaptan 6 EXAMPLE 5 Samples of Resins A, B, and D, produced as described above, were subjected to an electrophotographic evaluation to measure the performance of each sample in standard formulation. A typical formulation is as follows:

Grams Resin 50% weight solids in toluene 100 Toluene 500 Photoconductive zinc oxide 400 Bromophenol Blue, 1% soltuion in methanol, 60

p.p.m. Uranine,- 1% solution in methanol, (40 p.p.m. based on thezinc oxide weight) 1.6

The above formulation has a pigment/binder ratio of 8/1 and is 45% weight solids.

The resin is weighed into a container and the'toluene is added thereto. When the resin has been completely dissolved in the toluene, the solution is charged to a laboratory size Kady mill. The zinc oxide is added followed by the dye solutions. The mill is operated for three minutes. The resultant, dispersion has a typical viscosity of 20-50 cps. and a Hegman grind of 4.0-7.0 N.S. The dispersion is coated on a specially treated paper substrate (such as conductive coating base 1 manufactured by Weyerhaeuser-Paper Company) .with a wire wound applicator rod and allowed to air dry. Typical coating weights range from 8-24 pounds per 3000 square feet although the preferred test weight is 1 4-15 poundsper 3000 square feet.

The coated sheets are conditioned at 72 F. and 45 R.H. in the. dark for 18 hours (overnight). The dark adapted samples are imaged in an electrostatic copying machine such as an SCM 211,,A. B. Dick 675 or A-M 2000. The relative speed of the sample and maximum image density of the print are measured against a known standard. Samples are also tested for electrical properties in a Victoreen Electrostatic Paper Analyzer. Properties measured are maximum charge acceptance, dark decay rate and light decay rate. I

Table 1, shown herewith, shows the exact formulations (in terms of weight) used for each resintested, .aswe'll as the viscosity of the resulting coatingcompositiom-The lower portion of the table shows the electrical properties.

TABLE I A B D 100. 0 85. 5 85. 5 500. 0 514. 5 514. 5 Photox (zinc oxide) 400. 0 400. 0 400. 0 Bromophenol Blue, 1% solution 1 methanol 0.006% 2. 4 2. 4 2. 4 Uranine, 1% solution in methanol,

g 1.6 1.6 1.6 Hegman, NS 6. 0 6.0 6.0 Viscosity, #2 Ford, 75 F. sec., Inst 27. 5 32. 2 27. 7 Viscosity, #2 Ford, 73 1*. sec., 24 hours- 27. 7 35. 2 28. 0 Charge accept., V0, (volts) 375. 0 385.0 375. 0 Dark decay, Yn, (v./sec.).. 1.0 1.0 1. 0 Light decay Yr, (v-lsec. 310 250 260 Speed, Yr. 1,, (sec- 0.830 0.645 0.690 Coating weight, u/3,000 it. 15. 4 14. 4 15. 1 Volts per lb 24. 4 26.8 24. 8 A.B. Dick 675 speed..." 5 5 5 Average maximum image 0. 38 0. 35 0. 36 Background 0. 11 0. 11 0. 12 Back. cond. (ohms) 1. 2X10 1 1X10 1 0X10 "Wedge end density 0. 0. 85 0. 79

EXAMPLE 6 Samples of Resin A, C, and D were prepared as described above and made up into coatings using the formula described in Example 5 to determine their comparative performance. Table II below, shows the formulations in which they were used, along with the coating viscosity thereof. The electrical properties are shown in the lower portion of the table.

TABLE II A C D 100.0 85. 5 85. 5 500. 0 514. 5 514. 5 400. 0 400. 0 400. 0 Bromophenol e,

methanol 0.006% 2. 4 2. 4 2. 4 Uranine, 1% solution in mehtanol,

l. 6 1. 6 1. 6 Hegman, NS 5.5 6.0 6.0 Viscosity, #2 Ford, 78 F., sec. inst- 27. 7 28. 4 28. 2 Viscosity, #2 Ford, 78 F., 24 hours. Y 27. 9 28. 4 28.4 Charge accept., V0, (volts) 390. 0 395. 0 390. 0 Dark decay, Yd, (v./sec.) 1. 0 1. 0 1. 0 Light decay Yr, (v.lsec.) 845 305 275 Speed, YL/v (seer 0.880 0.775 0.705 Coating Weight, lb./3,000 f 14. 2 14. 9 15. 5 Volts/lb 27. 4 26. 5 25. 2 A.B. Dick 675 speed 5 5 6 Average max. image density 0. 37 0. 35 0. 36 Background 0. 10 0. l0 0. 11 Back. cond. (ohms).. 1. 2 l0 1 1X10 1 1X10 Wedge end" density 0. 78 0. 83 0. 80

EXAMPLE 7 In order to demonstrate the effect of variations in monomer content as between ethyl acrylate and styrene, a series of resins were made, using the method described in Example 4, and the resulting resins were formulated into coating compositions according to the procedures of Example 5. The exact formulations are shown, below, in Table III, and the electrical properties achieved by the coatings are also shown.

. In this series of resin comparisons Resin F is essentially the same as Resin D, described in Example 4. Resin G is a 50% blend of Resin F and Resin H. Resin I is a 50% blend of Resin H and Resin I It is believed that blends of similarly produced resins will produce approximately the same electrical properties as would a terpolymer made to precisely the same monomer content. As is the case in the resin of Example 4, each resin contains about 2.3%

by weight of maleic acid which had been partially esterified.

70% by weight ethyl acrylate, from about 30 to about 50% by weight styrene, and from about 1 to about 5% TABLE III Resin E F G H I J K L Ethyl acrylate (relative percent) 80 70 65 60 55 50 40 .3 Styrene (relative percent) 60 7 Charge acceptance, v0 (volts)- 400 430 420 410 895 365 340 31 Dark decay, YD (volts/sec.) 3. 5 2. 0 2. 5 2. 0 3. 0 3. 5 7. 5 11. 0 Light decay, Y1. (volts/sec.) 190 170 175 210 250 215 205 18 Speed, S YLIVO (sec: 0. 475 0. 395 0. 420 0. 510 0. 635 0. 590 0. 605 0. 580 Coating weight (lbS./3,000 sq ft 14. 4 14. 5 14. 5 14. 7 15. 0 15. 1 14. 9 14. 9 Volts/lb. (Yo/it) 27. 8 29. 6 29. 0 27. 9 26. 4 24. 2 22. 8 20. 8

eat ng viscos ty, #2 Ford cup, initial 81 R, see 26. 4 29.8 28. 5 27. 5 27. 5 26. 1 27.3 27. 7 Coating viscosity, #2 Ford cup, 24 hrs. 80 R, see 26. 7 30. 4 28. 9 27. 8 28. 5 27. 9 55. 0 33. 3 Hegman grind, NS initial 6. O 6. 0 6. 0 6. 0 6.0 6. 0 6.0 6. 5 A.B. Dick 675 Speed 9 10+ 10 8 6 4 3 2 Average maximum image density.- 0.34 0.35 0. 33 0. 33 0. 32 0. 31 0. 29 0. 25 Background 0. 12 0. 13 0. l2 0. 12 0. 11 0. 12 0. 13 0. 12 Backside conductivity, ohms-.. 9 2X10 B 0. 2X10 5 0. 1X10 7. 7X10 B 8 0X10 B 8 1X10 8 0X10 l 8 1X10 l Wedge end" density 0. 77 0. 76 0. 76 0. 76 0. 67 0. 62 0. 54 0. 42

The data shown above indicate that maximum charge acceptance and minimum speed both occur at about 70% ethyl acrylate on a relative basis. The blends prepared to give a 55% ethyl acrylate on a relative basis and 65% ethyl acrylate on a relative basis exhibit properties which were expected from these levels of monomer, based on the balance of the data obtained. The monomer ratio of about ethyl acrylate and 40% styrene represent the optimum compromise for charge acceptance and speed.

It is known to those skilled in the art that the sensitivity of photoconductive materials may be broadened over a wider spectrum by mixing small amounts of various dyes, which are generally known as dye sensitizers, with the photoconductive materials. The dye sensitizers cause the photoconductive materials to be more sensitive, for example, to visible light. Many dye sensitizers can be used and are well known to those skilled in the electrostatic printing art. When dye sensitizers are used they are normally mixed with the photoconductive material in an amount of 0.1 to about 5 percent and preferably from about 0.1 to about 3 percent. Representative of the many suitable dye sensitizers are, for example, sensitizers of the phenolsulphonephthalein group, phthalein dyes, triphenylmethane dyes, Rhodamine, Eosine, Rose Bengal, the fluorescein group and cyanine dyes.

The resins of the present invention, when made up into electrophotographic coating compositions, can be improved somewhat by the addition of certain inorganic salts. While the prior art lists many different salts which may be helpful in improving light sensitivity, it has been found that many of the inorganic salts described in the prior art are ineffective when used with the resin of the present invention. However, it has been found that certain calcium salts, and particularly calcium naphthenate produce an increased light sensitivity when used with the resins of the present invention. In addition to the naphthenate, it has been found that calcium soaps, including calcium octoate, calcium neodecanoate and the like may be also used. Also similar salts of zinc, manganese, lead and cobalt may be used.

In testing the resins of the examples, certain electrical properties are established as the minimum useful values. It will be understood by those skilled in the art that the values can be varied somewhat by changing the formula by which the binder resin is tested.

The forms of the invention herein shown and described are to be considered only as illustrative. It will be apparent to those skilled in the art that numerous modifications may be made therein without departure from the spirit of the invention or the scope of the appended claims.

What is claimed is:

1. A polymeric resin binder which is suitable for the production of a photoconductive zinc oxide coating for producing electrophotographic members, said resin binder comprising a terpolymer of from about 50 to about by weight of an acidic component, said acidic component comprising the reaction product of maleic acid and an excess of a lower alkyl alkanol, whereby from about 30 to about of the maleic acid is converted to the mono-ester.

2. A resin as described in claim 1, wherein said maleic acid is esterified with n-propanol.

3. A resin as described in claim 1, wherein the resin, at solids, has an acid number from about 6 to about 25.

4. A resin as described in claim 3, wherein the resin has an acid number between about 14 and 20.

5. A resin as described in claim 1, wherein the resin is dissolved in a mixture of xylene and a small amount of a lower alkyl alkanol.

6. A resin as described in claim 1, wherein the resin is dissolved in a mixture of toluene and a small amount of a lower alkyl alkanol.

7. A polymeric resin binder, suitable for producing electrophotographic coatings with zinc oxide, said resin binder consisting essentially of a terpolymer containing about 60% by weight of ethyl acrylate, about 40% by weight of styrene and about 2.5% by weight of a maleic acid component, which maleic acid component comprises maleic acid reacted with a lower alkyl alkanol to the extent that a portion of the maleic acid is converted to the mono-ester, said binder resin having an acid number, measured at 100% solids, of between 6 and about 25.

8. A resin binder as described in claim 7, wherein the acid number is from about 14 to 20.

9. A binder resin as described in claim 7, wherein said maleic acid is esterified with n-propanol.

10. A binder resin as described in claim 7, in which from about 30 to about 80% of the maleic acid component is mono-esterified.

11. A binder resin as described in claim 7, wherein said resin is dissolved in a mixture of xylene and a small amount of a lower alkyl alkanol.

12. A binder resin as described in claim 7, wherein said resin is dissolved in a mixture of toluene and a small amount of a lower alkyl alkanol.

References Cited UNITED STATES PATENTS 3,672,889 6/ 1972 Baltazzi et al. .1 961.8 3,620,729 11/1971 Ray-Chaudhuri et al. 96--1.8 3,388,106 6/1968 Muskat 260785 3,244,655 4/ 1966 Sullivan et al 26029.6

JOSEPH L. SCHOFER, Primary Examiner J. KIGHT HI, Assistant Examiner US. Cl. X.R.

96-1.8; 26078.5E, HC