Light-sensitive sheet for use in electrophotography

BACKGROUND OF THE INVENTION

This invention relates to light-sensitive sheets for use 30 in electrophotography. More particularly, it relates to light-sensitive sheets which are useful for the production of offset masters with a high degree of preservability and durability by the electrophotographic process.

Conventionally, light-sensitive sheets designed to be 35 developed electrophotographically in the production of offset masters comprise a paper sheet one side of which is provided with an electroconductive layer, the opposite side of which is provided with a precoat layer topped with a photoconductive layer. Precoat layers typically ⁴⁰are prepared from polyvinyl alcohol, polyvinyl acetate and the like. The photoconductive layer usually comprises a photoconductive material such as zinc oxide, selenium, cadmium sulfide, and the like, together with a sensitizer such as rose bengal in an insulating resin binder. ⁴⁵For the production of offset masters an image is developed on the light sensitive sheet and the non-image areas are thereafter treated with a desensitizing solution.

The precoat layer is designed to serve several functions and must meet several prerequisites. Since the sheet is ⁵⁰to be treated with a desensitizing solution and washed with water at the time of printing the precoat must be water resistant. It must be capable of being deposited as a smooth layer and must have sufficient adhesiveness with respect to the paper substrate and the photoconduc- ^5;ltive layer so that the offset master is able to withstand the rigors of the printing process in the production of a large number of prints. It must be substantially unaffected by the electrostatic treatment of the photoconductive layer. Further it must be sufficiently stable so that the light- ⁶⁰sensitive sheet will not deteriorate subsequent to manufacture and before use.

Such precoat layers as have heretofore been proposed for use have not met these criteria in a satisfactory manner. 65

SUMMARY OF THE INVENTION

The present invention substantially alleviates the above problems and does so in a novel manner. It provides light-sensitive sheets for the production of offset masters and other products by the electrophotographic process. The sheets are characterized by their superior stability and durability.

In its broadest aspect this invention provides novel light-sensitive sheets of the class described comprising a support, usually paper, coated on one side with an electroconductive layer and on the opposite side with a novel precoat composition on which the photoconductive layer is superposed. The precoat layer comprises at least one member selected from the group consisting of poly(Nmethylol acrylamide) polymers, copolymers of Nmethylol acrylamide and at least one vinyl monomer, and copolymers derived by chemical combination of polyethylenetetramine, adipic acid, urea and formaldehyde; a water soluble high molecular weight substance having at least one functional group which is an hydroxyl or carboxyl radical; and a water dispersible resin.

The copolymers of N-methylol acrylamide useful in the practice of this invention include copolymers formed with vinyl monomers such as acrylic and methacrylic esters in which the ester moiety is, for example, an alkyl group containing up to about 4 carbon atoms, styrene, acrylonitrile, vinyl acetate, butadiene, vinyl chloride acrylic and methacrylic acid, maleic and phthalic anhydride and itaconic acid, the copolymers normally contain from about 5 to 15 mol percent of N-methylol acrylamide, preferably 7 to 15 mol percent.

Copolymers formed from polyethylene tetramine, adipic acid, urea and formaldehyde can be used in place of poly (N-methylol acrylamide) polymers and copolymers or in addition to such products. The preparation of one such copolymer is illustrated in the examples where, for convenience it is identified as tetramer A. Other tetramers containing different relative proportions of the monomers can be similarly prepared and utilized. As is pointed out in the examples, such tetramers are commercially available. A typical example is Epinox P9007Y.

The water soluble high molecular weight substance forming one component of the precoat composition of the invention may be either a natural or synthetic water soluble high molecular weight substance characterized by the presence of hydroxyl or carboxyl functional groups which will react with the methylol groups of the foregoing compounds at an elevated temperature to form complicated, cross-linked, three dimensional structures which have a high degree of water resistance. Typical examples of such water soluble substances are starch, methyl cellulose, hydroxypropyl cellulose, carboxymethyl cellulose, casein, and polyvinyl alcohol. Typical examples of carboxyl group containing compounds include polymers prepared from acrylic acid, methacrylic acid, maleic acid or phthalic acid, and copolymers of such monomers with vinyl monomers such as ethylene, propylene, butadiene, acrylic and methacrylic acid and esters, acrylonitrile, styrene, vinyl chloride, vinylidene chloride, vinyl acetate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, and the like. The water solubility of these products can be enhanced by at least partial neutralization (10-60%) with an appropriate alkaline reagent such as alkali metal hydroxide.

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An additional ingredient in the unique compositions of this invention is a water dispersible resin. This resin may be derived from the same monomers mentioned in the preceding paragraph. Its distinguishing characteristic, however, is that it is dispersible in water, but soluble in organic solvents, especially the hydrocarbon solvents such as toluene and the like used to prepare the photoconductive layer. This specially selected ingredient of the compositions of the invention increases the adhesion of the photoconductive layer to the substrate because of its special solubility characteristics.

Suitable water dispersible resins include those selected from the group (a) polymers of acrylic esters, methacrylic esters, styrene, vinyl chloride or vinyl acetate, and (b) copolymers prepared by reaction of monomers such as maleic acid, phthalic acid, itaconic acid, acrylic acid, acrylic esters, methacrylic acid methacrylic esters, hydroxyethyl acrylate, glycidyl methacrylate, butadiene, acrylonitrile, styrene, vinyl chloride, vinylidene chloride, vinyl acetate, ethylene or propylene.

Poly (N-methylol acrylamide) can be prepared as follows:

Solid N-methylol acrylamide together with a small amount of ammonium persulfate and isopropanol is placed in water at 50° C. and a pH of 5.75 under an atmosphere of nitrogen. The temperature is increased to 60° C. and held for 2 hours. The pH of the resulting solution is adjusted to 7.5 with caustic soda to stabilize the polymer and inhibit coagulation of the polymer.

A general method for the production of N-methylol acrylamide copolymer is as follows:

A total of 10 to 20 parts by weight of N-methylol acrylamide, 60 to 10 parts by weight of acrylic or methacrylic acid and 30 to 60 parts by weight of an alkyl ester of the selected α,/3-unsaturated monocarboxylic acid such as methyl acrylate are mixed together and subjected to redox polymerization in either water or a water-alkanol mixture. The water soluble ammonium or amine salt can be obtained by treating the resultant copolymer with the appropriate base.

The precoat layer can be prepared by mixing from about 10 to 50 parts by weight of poly(N-methylol acrylamide), a copolymer thereof, or a polymer generically referred to herein as tetramer A with from 50 to 90 parts by weight of the above defined high molecular weight water soluble material in water and thereafter adding from about 10 to 60 parts by weight of the water dispersible resin and coating the resultant dispersion on the selected support. The dispersion may be dried at about 150° C. for from about 1 to 2 minutes.

It is known to produce light sensitive sheets in which the precoat layer contains a melamine formaldehyde resin or a urea formaldehyde resin. The products of this invention unexpectedly have the following advantages when compared with such previously known products:

(1) They are highly water resistant despite the small quantity of precoat composition employed.

(2) They are highly stable especially with respect to light fatigue.

(3) There is little change in viscosity of the precoat dispersions at room temperature. Therefore, they can be stored for an appreciable period of time before use and there is no difficulty in such use.

(4) The cross-linking reaction is rapidly accelerated by heat.

(5) They can be employed over a relatively wide pH range.

(6) They can be used in both the dry developing and wet developing process to produce distinct copies.

(7) The preservability of offset masters produced from them is remarkably improved compared to known lightsensitive sheets.

(8) Offset masters prepared from them can be used to produce as many as 500 to 10,000 prints because of their remarkable durability.

The light sensitive sheets of this invention normally weigh from about 76 to 205 g./m.². The electroconductive coating weighs from about 1 to 15 g./m.². The weight of the precoat is from about 5 to 20 g./m.², and of the photoconductive layer from about 20 to 40 g./m.².

The following non-limiting examples are given by way of illustration only.

Example 1

A dispersion of Composition 1 described below was coated onto one side of a slick paper weighing 80 g.Zm.²and dried at 150° C. for 2 minutes as a precoat layer weighing 11 g./m.².

Composition 1: Parts by weight

Ethyl acrylate-N-methylol acrylamide copolymer (resin content—37%) ¹ 27

Polyvinyl alcohol (resin content=10%) ²250

Polyacrylic ester (an aqueous emulsion containing 38% of resin) ³40

Ammonium chloride0.75

Water__________,___________________________ 200 ¹A copolymer containing 15 parts by weight N-methyM acrylamide, 60 parts by weight ethyl acrylate and 25 parts by weight acrylic acid sold by High Polymer Chemical Co., Ltd., under the trade name Kogum HW-IO.

² A product of Kurashiki Rayon Co., Ltd., sold under the trade name PVA—117.

³ A copolymer of methyl acrylate, butyl methacrylate, ethyl acrylate and acrylic acid sold by Saiden Chemical Co., Ltd. under the trade name EK-4.

The other side of the paper was coated with a dispersion containing Composition 2 described below and dried at 150° C. for 2 minutes, to produce an electroconductive layer weighing 15 g./m.².

Composition 2: Parts by weight

Polyacrylic ester emulsion (resin content =45%) i22.2

Butyl methacrylate/acrylic acid copolymer (35% aqueous solution) ² 14

Clay 30

Water 50 ¹A copolymer of methyl methacrylate and acrylic acid sold by Nippon Acryl Chemical Co., Ltd. under the trade name Primal HA—16.

² A copolymer sold by Toa Gosel Chemical Industry Co., Ltd. under the trade name Alon A-21.

By subjecting thus coated paper to spar calendering, a substrate for use in electrophotography was prepared. The side of the substrate was coated with the photoconductive layer described below as Composition 3 and dried at 130° C. for 2 minutes to produce a light-sensitive sheet for use in electrophotography. The weight of the photoconductive layer was 32 g./m.².

Composition 3: Parts by weight

Zinc oxide100

Silicone (a xylene solution containing 70% of resin) 5.3

Acrylic resin (a toluene solution containing 50% of resin) 32

Rose bengal (a 5% methanol solution, 1.2 cc.

Toluene, 120 cc.

The thus prepared light-sensitive sheet when subjected to humidity adjustment in the dark having a 65% relative humidity for 12 hours, displayed good surface characteristics under standard tests. There was firm adhesion of the photoconductive layer. Satisfactory images could be produced by the wet-developing process. Similarly satisfactory images could be produced even under conditions such that the temperature was 32° C. and the relative humidity was 80%.

The foregoing light-sensitive sheet, when used as an offset master, was not affected by the etching liquid or by the dipping water. Even after producing 500 prints, no frilling or exfoliation of the light-sensitive layer was observed and it was still possible to produce satisfactory prints.

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Example 2

The procedures utilized in this example were the same as Example 1 except for the employment of the dispersion of Composition 4, described below, for the precoat layer.

Composition 4: Parts by weight

N-methylol-acrylamide polymer (an aqueous solution containing 10% of resin)___ 100

Carboxymethyl cellulose (a 5% aqueous solution) (a manufacture of Nippon CMC, Ltd., trade name CMS-AL)500

EK-4 40

Ammonium chloride__,0.75

Water 500

This light-sensitive sheet displayed good surface characteristics and firm adhension of the photoconductive layer similar to Example 1, and when subjected to wet-developing process, could produce satisfactory images at a temperature of 30° C. and a relative humidity of 80%, as well as at room temperature. This light-sensitive sheet, when employed for offset printing, showed no frilling or exfoliation of the light-sensitive layer even after the production of 500 prints, and the prints obtained were satisfactory.

Example 3

A dispersion according to Composition 5, described below, was applied onto one side of a slick paper weighing 130 g./m.² (pulp ratio of N material to L material=50:50) and dried at 150° C. for 2 minutes, to form a precoat layer weighing 130 g./m.².

Composition 5: Parts by weight

HW-16 27

PVA-117 33

EK-4 40

Ammonium chloride0.75

Water 200

The other side of the paper was coated with a dispersion according to Composition 6, below, and dried at 150° C. for 2 minutes to form an electroconductive layer weighing 3 g./m.². Subsequently, by subjecting thus coated paper to calendering, a support for use in electrophotography was prepared.

Composition 6: Parts by weight

CMC-AL 160

Melamine resin (resin content—80%) (a manufacture of Sumitomo Chemical Co., Ltd.; trade name Sumitex Resin M-3) 2.5

Ammonium chloride 0.1

Water 100

The precoat layer was coated with the photoconductive mixture of Composition 3 and dried at 130° C. for 2 minutes, to produce a light-sensitive sheet for use in electrophotography. The weight of the photoconductive coating was 20 g./m.².

This light-sensitive sheet was subjected to humidity adjustment at a 65% relative humidity for 24 hours. The thus processed light-sensitive sheet displayed good surface characteristics and firm adhesion of the photoconductice layer, and could form satisfactory images using the dry-developing process.

Further, the foregoing light-sensitive sheet, when used as an offset master, was not affected by the etching liquid or the dipping water even after the production of 3000 prints. It was found to be free from frilling, exfoliation of the light-sensitive layer and similar deficiencies.

Example 4

The procedures applied to this example were the same as Example 3 except for the employment of the dispersion of Composition 7, described below, for the precoat layer.

Composition 7: Parts by weight

HW-16 27

Methyl cellulose (a 5% aqueous solution) (A manufacture of Shrinestu Chemical Ind. Co., Ltd.; trade name; Metolose) 300

CMC-AL 300

EK-4 40

Ammonium chloride0.75

Water 300

The thus prepared light-sensitive sheet for electro photography could produce satisfactory images at ordinary humidity and even under a high humidity. When used as an offset master, no frilling or exfoliation of the light-sensitive layer was observed even after producing 3000 prints, and the printing was satisfactory.

Example 5

By coating a dispersion according to Composition 8, described below, on one side of a slick paper weighing 130 g./m.² (pulp ratio of N material to L material =50:50) and drying at 150° C. for 1 minute, and subsequently calendering, a support provided with a conductive weighing 5 g./m.² was prepared.

Composition 8: Parts by weight

CMC-AL 800

Tetramer A¹ 33

Ammonium chloride0.25

Water 400 ¹ Prepared by following process : A solution is prepared by dissolving 300 parts by weight of polyethylene tetramine in 120 parts by weight of water. Adipic acid (292 parts by weight) is added and the mixtures stirred for one hour at 210° C. The resulting mixture is cooled to 120° C., 480 parts by weight of urea is added and the mixture stirred for two hours. A 37% aqueous solution of formaldehyde is added and the resulting mixture stirred for 2 hours at 70° C. The pH is adjusted to 8 utilizing 35% hydrochloric acid. The product Is a 30% aqueous composition containing a copolymer of polyethylene tetramlne/adipic aeid/urea/ formaldehyde. This polymer Is readily available on the market, for example, as Epinox P9007Y, a product of DIC Hercules Chemical Industries Inc.

Next, to the side opposite to that provided with the conductive layer, there was applied a dispersion according to Composition 9, below. It was dried at 150° C. for 1 minute to provide a precoat layer weight of 13 g./m.².

Composition 9: Parts by weight

10% aqueous solution of amino-denatured starch (a manufacture of Sucorten Chemical Inc.; trade name: Nylgum)400

PVA-117 400

Tetramer A¹ 67

EK-4 83

Ammonium chloride 0.7

Water 400 ¹ See footnote in table above.

The precoat layer was coated with the photoconductive layer of Composition 10 and dried at 130° C. for 2 minutes to form a photoconductive layer weighing 20 g./m.²on a light-sensitive sheet for use in electrophotography.

Composition 10:

Zinc oxide____________________________g100

Acrylic resin (a toluene solution containing 50% of resin)___________________________g__32

Silicone (a toluene solution containing 70% of resin) _____________________________g__8

Rose Bengal (a 5% methanol solution)___cc1.9

Toluene______________________________g120

The thus obtained light-sensitive sheet for electrophotography did not show any cracks, exfoliation or similar deficiencies of the photoconductive layer even when dipped in water for 24 hours, and was shown to have satisfactory water-resisting properties, surface characteristics, and adhesive properties for the photoconductive layer by standard tests.

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This light-sensitive sheet, when subjected to dry-developing process to form images, could form distinct images having a good contrast even under conditions of high temperature and humidity (such as 35’ C. and 85% R.H.), as well as at room temperature and ordinary humidity. When this light-sensitive sheet was placed in a polyethylene bag and subjected to the usual preservation test at 50° C. for one month within a thermostatic tank it proved still capable of forming distinct images which were almost as satisfactory as those produced with the untreated sheet.

Moreover, the light-sensitive sheet prepared as above, when used as an offset master, could further produce distinct prints, without showing any such frilling or exfoliation of the photoconductive layer such as would hamper the printing operation, even after producing 10,000 prints.

For the purpose of comparison, light-sensitive sheets were prepared employing the same procedures as that for the present example except for the use of melamine formaldehyde resin (namely Sumitex Resin manufactured by Sumitomo Chemical Co., Ltd.) as the bridging agent in lieu of a 30% aqueous solution of tetramer A employed as the bridging agent in the Examples 1, 4, 5, 7 and 9. The results of the comparison are shown in the following table.

Product of the Comparpresent ative Items invention product

Water-resisting property of light-sensitive sheet 1;

Immediately after coating____________________ fi4

After being left standing for 50 days__________ B5

Light fatigue 2 (V)_____________ 300200

Image-forming efficiency (50° C.; after 1 month). _ Good(

Printing durability (number of prints) 10,000 B, 000

Determined by the Improved Finger Method in which the dispersion for a precoat layer is applied to the support provided with an electroconductive layer and dried at 150° C. for 1 minute. Then, 2 drops of water are dripped onto the dried coating, and 30 seconds later, the surface of said coating is rubbed with a finger 7 times, and the waterresistivity is graded by observing the state of exfoliation of the coating layer, as follows: B=Excellent; 4=Good; 3=Fair; 2=Bad.

The initial potential subsequent to 10 seconds pre-exposure to 500lux.

³ Lack of uniformity of image is observed.

As seen from the foregoing results, the product of the present invention is superior in its electrophotographic characteristics. For instance, the comparative product displays a slow increase in water resisting properties while the product of the present invention demonstrates the desirable effect immediately after its preparation. This fact is attributable to the difference of reactivity of the respective product. Moreover, in case of the product of the present invention, the light fatigue is less than that of the comparative product, and its image-forming efficiency is superior to that of the latter. Additionally, the printing durability of the former was found to be twice as good as the latter.

Example 6

By employing the same method and materials as in Example 5 except for the application of Composition 11, described below, in lieu of Composition 9 applied in forming the precoat layer in said Example 5, a light-sensitive sheet for use in electrophotography was prepared.

Composition 11: Parts by weight

CMC-AL ______________________________1600

Acrylic resin solution (an aqueous solution containing 45% of resin; a copolymer of 5% acrylic acid—50% methyl methacrylate—45% ethyl acrylate)______________ 80

EK-4 83

Tetramer A 33

Ammonium chloride 0.8

Water 800

This light-sensitive sheet displayed surface smoothness, interlayer adhesive property and preservability as good as that of the light-sensitive sheet prepared in Example 5, and could form distinct images under a high temperature and high humidity.

When this light-sensitive sheet was employed as an offset master, it was possible to produce satisfactory prints without showing any frilling or exfoliation of the photoconductive layer even after the production of 7000 prints.

Example 7

By employing the same method and materials as in Example 5 except for the use of Composition 12 in lieu of Composition 9 for the dispersion to form the precoat layer, a light-sensitive sheet for use in electrophotography was prepared.

Composition 12: Parts by weight

Kogum HW-12 (22% aqueous solution)273

Alon A-21 57

Tetramer A 67

Acrylic resin emulsion (an aqueous emulsion resin) ¹ 80 containing 45% of resin) ¹ 80

EK-4 83

Ammonium chloride 0.2

Water 300 ¹A methyl methacrylate-aerylic acid copolymer sold by Nippon Acryl Chemical Co., Ltd. under the trade name Primal HA—16.

This light-sensitive sheet displayed good surface smoothness, interlayer adhesive property and preservability as good as that of the light-sensitive sheet prepared in Example 5, and could form satisfactory images ©ven under conditions of high temperature and humidity.

When employed as an offset master, it could produce distinct prints without showing any frilling or exfoliation of the photoconductive layer even after the production of 5000 prints.

Further, in case where light-sensitive sheets were prepared by employing styrene-butadiene resin emulsion (a manufacture of the Dow Chemical Co.; trade name: Dow Latex-612, -620, -630, -636) in lieu of acrylic ester resin emulsion employed in Examples 1 through 7, or employing such water dispersible resin (styrene-butadiene resin) as a manufacture of Sumitomo Chemical Co., Ltd. sold under the brand name 2752-E, manufacture of Nippon Synthetic Rubber Ind. Co. sold under the brand names JSR-0660, JSR-0667, etc., it was possible to attain a similar efficiency.

The ammonium chloride of the above examples may be replaced with ammonium acetate or equivalent compound.