Method of processing porous materials

Abstract

The present invention relates to a method for processing porous materials. The porous material is coated with a surface-coating material. The surface-coating material substantially completly prevents mass penetration of liquid into the porous material, but since the surface-coating material is not entirely cohesive or entirely film-forming, tiny pinholes are formed in the surface of the surface-coating material. This causes problems when a liquid is later applied to the porous material coated iwth the surface-coating material because the liquid penetrates into the porous material. This problem is solved by, after applying the surface-coating material and drying the same, applying thereto a "pinhole-filling dispersion," which comprises a liquid and a finely dispersed substance therein, whereby the liquid easily penetrates through the pinholes in the surface-coating material, carrying the finely dispersed substance, whereafter, the finely dispersed substance "coagulates" in the pinholes and blocks them completely, whereby the combined surface-coating material/pinhole-clogging material forms a substantially impermeable film.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for processing a porous material, and, more particularly, to a method for removing pinholes through which a liquid penetrates into the porous material.

2. Description of the Prior Art

For convenience's sake, the requirements for such a porous material will be described below, taking an electrophotographic recording paper as exemplary.

Electrophotographic light-sensitive materials comprising a support, such as paper, having provided thereon a zinc oxide/resin dispersion type photoconductive insulating layer or an organic photoconductive layer mainly comprising polyvinyl carbazole, are widely used in the copying field.

However, when a so-called liquid developing method is used to develop such materials, the following problems are encountered. In many cases, the back surface or surface free of light-sensitive layer of such a material is also provided with a resin layer, and fine pinholes unavoidably lie scattered throughout the layer. A liquid developer is liable to penetrate through these pinholes into the interior of the paper support, resulting in the formation of undesirable spot-like stains on the back side of a finished print, as well as increasing the drying load and consumption of developer.

When the back surface is not coated in any manner to permit the free penetration (absorption) of liquid over the whole surface and the processing solution is squeezed at the end of processing followed by forced drying, stains are uniformly formed over the whole surface since the liquid is freely volatilized from the paper, and hence here are comparatively less problems regarding the drying load and the wastage rate of the liquid.

However, where a film capable of preventing a liquid from penetrating into a light-sensitive material to a considerable degree is provided, the liquid is difficult to volatilize once it penetrates into the light-sensitive material, which results in spot-like, non-dryable regions which remain over a long period of time.

As one solution to these problems, methods of reducing the number of pinholes have been suggested, e.g., making a film thick to such an extent that almost no liquid penetration is observed during processing with a liquid, and conducting processing in two steps wherein the back surface is first processed with a carrier liquid for a developing agent to penetrate a colorless, toner-free liquid into the support.

The former method, however, is economically disadvantageous since the film must be thick. For example, in order to try to attain the above-described object, resins such as polyvinyl alcohol must be applied in an amount of about 5 g/m.sup.2 to the back side of the light-sensitive material. This seriously affects the mechanical properties of the resulting paper and imposes additional restrictions on the processing of the opposite surface from the point of curling balance or the like, as well as lowering the flexibility and feel which is characteristic of paper. Thus, the application of a resin film in such an amount is not preferred. In addition, even at such a coating amount, resins capable of constituting a closed layer are restricted as to kind, which seriously narrows the degree of freedom in setting up production conditions.

The latter method involves the defects that it requires complicated apparatus since the processing is conducted in two steps and that non-dryable spots remain as in the first method, though no stains are left after drying since the penetrating liquid contains no toner. These non-dryable spots cause problems in the case of, e.g., obtaining color images and the like by a superposition development.

SUMMARY OF THE INVENTION

The present invention relates to a method for providing a coating film containing extremely few pinholes and, expressed in connection with the above-described use, it provides a base paper for use in electrophotography where liquid penetration is prevented.

However, the method of the present invention relates to processing in general, as will become apparent from the following descriptions.

The method of the invention comprises: (a) subjecting the surface of a base material, where penetration of liquid is to be prevented, to processing to prevent a liquid from penetrating therethrough over the almost the whole surface (say, for instance, on the order of 98%); and (b) applying to the surface subjected to processing (a) a pinhole-filling dispersion prepared by dispersing fine particles in a liquid which easily penetrates into the interior of the base material through the pinholes, the particles having a diameter less than or approximately the same as that of the pinholes and being capable of stopping up the pinholes.

In step (b), the pinhole-filling dispersion penetrates into the interior of the base material through the pinholes, and the finely dispersed particles agglomerate at the pinholes to prevent the further penetration of the liquid.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be more specifically described.

The first step of the invention (hereinafter referred to as "step (a)" ) can be accomplished by various techniques so far known. Step (a) does not constitute an essential concept of the invention. As a typical example, there is illustrated applying a resin solution or a resin dispersion to a base material such as paper. Resins possess a film-forming property and form an approximately continuous film on the surface of the base material. Of course, there remain microscopically discontinuous portions on the surface, which portions form pinholes.

The type of element with which the present invention finds particular application can be described with reference to the analysis method described in TAPPI, volume 48, No. 10, page 97 A middle and right columns. Pinholes, or penetration points, can be classified into various degrees. A great number of pinholes is more than 20 penetration points/cm.sup.2. An intermediate level is from 5 to 19 penetration points/cm.sup.2. A very low level of penetration points is less than 5, preferably less the 4, per cm.sup.2.

In the present invention, coated porous materials which have more than 20 penetration points per cm.sup.2 are particularlly amenable to the penetration problem discussed. Using the pinhole/filling dispersion of this invention, the number of penetration points can be lowered to less than 4 penetration points/cm.sup.2. Thus, assuming 5 to 30 penetration points/cm.sup.2, the pinhold filling dispersion of the present invention can reduce the number of penetration points per cm.sup.2 to less than 4, and offers excellent affects when the number of pinholes prior to treatment is from 5 to 25 per cm.sup.2.

As will be understood from the context of the present invention, the term "pinhole" refers to any small hole through which a solution can penetrate. For most practical purposes, layers which are subject to pinholing (and yet which are commercially used in the industry), have pinholes of a size less than about 10 microns.

Through holes of this size, the pinhole filling dispersion (or the penetration of the dispersion which is to be prevented) rapidly penetrates initially, for instance, at an penetration rate of about 2 .times. 10.sup.-.sup.4 mm.sup.3 to about 60 .times. 10.sup.- .sup.4 mm.sup.3 per second, more commonly from 4 .times. 10.sup.-.sup.4 to 32 .times. 10.sup.-.sup.4 mm.sup.3 /sec. After treatment, the rate of penetration is less than one-fifth such a penetration rate.

It will be understood from the above that the surface-coating material of the present invention is not limited in any special manner. However, practically speaking, the surface-coating materials which are subject to the pinholing problem solved by the present invention are resins, and most common of such surface-coating material used to surface coat porous materials are (c) polyalkyl acrylates, methacrylates, alkylacrylates, alkyl methacrylates, copolymers thereof with compatible materials (where any alkyl group in the proceeding materials preferably has 1-4 carbon atoms), polystyrene, polyvinyl acetate, linear polyesters unsaturated polyesters, alkyd epoxy resins, polyvinyl alcohol, polyvinylbenzyl trimethyl ammonium chloride, copolymers of styrene and butadiene, and the like. It will be apparent to one skilled in the art that it is completely impossible to list every conceivable resin which could be used as a surface-coating material, and the above listing shall only be taken as representative.

The second step (hereinafter referred to as step (b) ) constitutes the basis of the invention. As the processing dispersion, those which meet the following requirements suffice:

i. No detrimental influence is exerted on the base material.

ii. The dispersion medium of the processing dispersion is volatile.

The "carrier liquid" for the finally dispersed pinhole-filling particles is best an easily volatilized petroleum hydrocarbon solvent, an ester, ketone, alcohol, halogenated hydrocarbon or the like. As will be appreciated by one skilled in the art, it is important that the carrier liquid not attack the coating layer since, essentially, one would then be enlarging the pinholes. High volatility is useful because this can be used as one procedure for the coagulation of the finally dispersed pinhole-filling material.

Specific examples of such materials are petroleum hydrocarbons such as kerosene, hexane, heptane, cyclohexane, benzene, toluene, xylene, isoparaffins and ligroin; esters such as carboxilic acid alkyl esters where the carboxylic acid and alkyl group having from 1 to 4 carbon atoms such as methyl acetate, ethyl acetate, butyl acetate, ethyl lactate and butyl lactate; ketones such as acetone, methyl ethyl ketone and methyl isobutyl ketone; alcohols having from 1 to 3 carbon atoms, such as methanol, ethanol and propanol; halogenated hydrocarbons such as methylene chloride, ethylene chloride, trichloroethylene, tetrachloroethane, carbon tetrachloride and chloroform; mixture thereof and the like.

The material which is finally dispersed in the carrier liquid to serve as a pinhole-fitting or clogging material is not especially limited. However, practically speaking, useful materials will comprise colorless pigments, plastic particles, and plastic coated pigment particles, which are able to coagulate in the pinholes and fill the same.

Preferred plastic particles are thermoplastic resins having a molecular weight of from about 10,000 to about 200,000 such as polyvinyl chloride, vinyl chloride-vinyl acetate copolymers, styrene butadiene copolymers, polyacrylic acid-alkyl esters (ester moiety having from 1 to 4 carbon atoms), polymethacrylic acid alkyl esters (ester moiety having from 1 to 4 carbon atoms), copolymers of such esters and vinyl chloride, styrene, acrylic acid or crotonic acid, or mixtures thereof and the like.

Preferred colorless pigments are pigments such as titanium oxide, zinc oxide, magnesium oxide, aluminum oxide, silicon dioxide, aluminum hydroxide, calcium hydroxide, calcium silicate, potassium silicate, mixtures thereof and the like.

Preferred plastic coated pigments are the above pigments coated with the above plastics.

While not especially limited, the finally dispersed material should comprise from about 1 to about 20% by volume of the carrier liquid, more preferably 1 to 5% of the carrier liquid. This permits a good transportation rate of the finally dispersed material into the pinholes.

From the above discussion it will be apparent that the size of the finally dispersed particles in the carrier liquid can vary greatly but, however, cannot be greater than the pinhole diameter. Generally speaking, as size increase much above 10 microns the pinhole-filling effect tends to fall off, with the same tendency being seen with particles below about 0.1 micron. Considering both of these factors, it is thus most preferred to use finally dispersed particles which have a size of 0.5 micron to 5 microns.

The present invention will now be explained taking, for instance, the processing of the back surface of a support for use in electrophotography.

Assuming the surface of the base material to be the support, which is the surface on which there is to be provided a light-sensitive layer, is not yet processed, a resin layer is provided on the other surface or back side thereof.

As a procedding dispersion, a dispersion prepared by dispersing fine particles of a thermoplastic resin in a petroleum solvent is utilized. This dispersion is brought into contact with the back side surface for a sufficient period of time, for instance, on the order of 2 seconds to 40 seconds, more generally contact for about 5 to 20 seconds being sufficient. Thereafter, the liquid is completely squeezed out by means of squeezing rollers, followed by drying. Prior to drying, the liquid penetrates into the squeezed surface through the pinholes. The projected area of the penetrated liquid is from about 10 to 20 times to about 100 times that of the pinholes. It has been found that once the thus processed support is dried, the liquid does not penetrate thereinto any more. That is, when the liquid passes through the pinholes the fine particles agglomerate in the pinholes and function to plug the pinholes. If a lot of excess liquid is left in a small amount on the film, no trouble is encountered in the following steps and it can be completely be removed by air-squeezing or the like.

The time of drying and the temperature of drying are, of course important, since if one drys too fast the particles do not have an opportunity to fill up the pinholes. On the other hand, if one drys for too long of a period, the process time is increased which leads to lower process efficiency. The time and temperature of drying must be decided on a case by case basis, but the acceptability of any drying procedure is generally easily determined. For instance, drying will, of course, be sufficient to drive off the solvent but at a temperature insufficient to actually degrade any of the porous material, coating material or pinhole-filling material. Generally, an appropriate temperature is selected which ensures carrier liquid drive-off and no harm to the other components and then, after cutting a sample into several strips, different drying times are used until one which provides optimum results is reached. If a desired pinhole filling effect cannot be achieved, then the temperature is either raised or lowered, depending on whether the trend of the results indicate the temperature initially selected was too high or too low. Commonly used temperatures are about 60.degree. to about 120.degree.C. In the present invention, the pressure of treatment is unimportant and, for practical purposes, will always be atmospheric pressure. The temperature of treatment is also unimportant, so long as, of course, the particulate nature of the finally dispersed particles is not altered prior to the coagulating effect desired. Due to the complicated nature of pinhole filling dispersion which depend on an exotic chemical reaction to perform their pinhole filling function, the general rule in the process of the present invention is that the "coagulation" or setting of the particles in the pinholes is accomplished by the drying step itself, i.e., carrier liquid removal. The results obtainable by such a simple procedure are excellent, and little need exists to go to complicated intrapinhole filling particle reactions. Obviously, a pinhole-filling substance such as titanium dioxide is not coagulated, but with the drying such particles are firmly set into the layer treated.

It is very convenient in this instance to use as the pinhole-filling dispersion one resembling the processing solution to be applied to the material during use, i.e., one which is similar in composition and viscosity to the processing solution to be applied to the material during practical use. For example, in the case of electrophotography, the liquid applied to the light-sensitive material at use is a liquid developer which is prepared by dispersing fine carbon black particles or the like of not more than 1 .mu. in particle size in a non-polar carrier liquid having a low viscosity and a low surface tension. Therefore, it is desirable to utilize as the pinhole-filling liquid a liquid having a similar surface tension to that of the liquid developer. Also, the dispersed particles are preferably about 1 .mu. or less in particle size, and are preferably particles of colorless pigment or resin.

It is to be noted that generally the surface tension of aqueous pinhole-filling liquids is relatively high, and they will tend to fail to adequately penetrate into very small pinholes. Accordingly, the general rule is that aqueous pinhole-filling systems are to be avoided. In short, the main point of the present invention lies in utilizing the phenomenon that when a filling liquid flows through pinholes fine particles contained therein are caught at the pinholes and agglomerate to plug or stop up the pinholes. Such a phenomenon is experienced in the familiar case of, e.g., filtrating a precipitate in a chemical experiment in which case a filter paper becomes impassable in the course of filtration, though the precipitate passes through the filter paper during the first stage.

Although the present invention has been explained above referring to the case of electrophotography, the method of the invention can be commonly applied to processing various porous base materials.

The merits of the method of the present invention are clear from the data given in the following Examples. For example, polyvinyl alcohol is applied in an amount of about 2 g/m.sup.2 as an aqueous solution to one surface of a fine quality paper of about 100 g/m.sup.2 in weight. The thus formed layer contains a number of pinholes and, when a liquid developer for electrophotography is applied thereto without the processing of the invention, there are observed 20 to 30 pinholes per 1 cm.sup.2. In order to reduce the number of pinholes to several pinholes per 1 cm.sup.2, it is necessary to again apply thereto polyvinyl alcohol in an amount of 2-3 g/m.sup.2. On the other hand, when the method of the invention is applied to the surface containing 20 30 pinholes per 1 cm.sup.2, there can be obtained an almost completely pinhole-free surface, almost without an increase in the coating amount, as confirmed by measurement. That is, the method of the invention enables one to greatly reduce materials cost and coating amount.

Additionally, it is of course effective to repeat the method of the present invention one or more times.

The pinhole-filling liquid used in the method of the invention may be optionally selected, depending upon the kind of porous material used.

The present invention will now be illustrated in greater detail by several non-limiting examples of preferred embodimens of the present invention. The present invention is not limited by the examples in any way, however.

Unless otherwise indicated, all percentages in the examples are weight percentages, and all processings were at room temperature and at atmospheric pressure.

EXAMPLE 1

A 100 g/m.sup.2 paper of fine quality was used as a base material, and a 15% aqueous solution of polyvinyl alcohol (degree of polymerization ca.700) was applied to the wire-coated surface thereof in a dry amount of 2 g/m.sup.2. After drying, it was cut into two pieces. A commercially available electrophotographic liquid developer (prepared by dispersing carbon black in a carrier liquid of isoparaffinic hydrocarbons, the particle size of the carbon black being considered to be about 0.1 micron of less) was poured onto the resin-coated surface of one piece, it was left for 10 seconds and then washed well with an isoparaffinic solvent. The whole wire-coated surface was found to be dotted with black spots. The frequency of the spots was 22 to 25 per 1 cm.sup.2.

Thereafter, a dispersion was prepared by dispersing a vinyl chloride/vinyl acetate copolymer (copolymerization ratio: 65:35; polymerization degree: about 260) as fine particles in kerosene based solvent in the following manner.

30 Grams of a long oil-type alkyd resin (Solid Beckosol 18, made by Japan Reichhold Chemicals, Inc.; containing 21% by weight phthalic acid anhydride, 70% by weight of linseed oil, based on 100% of non-volatile matter; acid value less than 9%; specific gravity from 1.02 to 1.03) and 10 g of vinyl chloride/vinyl acetate copolymer (Denka Lac 61, made by Electro Chemical Industry Co., Ltd.; 45% by weight toluene solution of vinyl chloride/vinyl acetate copolymer having a solution viscosity of 1800-2800 cps/25.degree. C, the copolymer consisting of 35% by weight of vinyl chloride and 65% by weight of vinyl acetate, and having a polymerization degree of 260) were dissolved in 40 cc of acetone and added to 5 liters of kerosene while stirring and applying ultrasonic waves. The alkyd resin was soluble in the kerosene while the vinyl chloride/vinyl acetate copolymer was insoluble and dispersed as fine particles, whose particle size was considered to be not more than 1 micron.

The second piece of the processed paper surface was brought into contact with the resulting dispersion for 10 seconds and washed with pure kerosene. Spots were seen as in the case of the aforesaid developer due to the penetration of the liquid. The thus processed paper was dried at 70.degree.C and passed between calender rolls. When the above-described developer was then poured thereon, it did not penetrate.

EXAMPLE 2

A 50 g/m.sup.2 office paper was used as a base material, and an aqueous coating solution prepared by mixing 20 parts of an electroconductive resin [ECR-34, made by Dow Chemical Co.; poly-vinylbenzyltrimethyl ammonium chloride in water, containing about 30% non-volatile components ] and 50 parts of a 10% aqueous solution of polyvinyl alcohol (degree of polymerization ca. 600) was applied to the felted surface thereof in a dry amount of 1.5 g/m.sup.2. The sample was cut into two pieces.

When the same developer as in Example 1 was poured onto one piece, there were observed 50 to 60 black spots per 1 cm.sup.2.

As the processing dispersion to remove pinholes, there was used an organic solvent dispersion of polyvinyl acetate. This processing solution was prepared as follows.

A first solution was prepared by dissolving 10 g of polyvinyl acetate (degree of polymerization ca. 550) in 100 cc of acetone. Separately, a second solution was prepared by adding to 200 ml of cyclohexane 20 ml of a 20% toluene solution of varnish obtained by thermally condensing a rosin-modified phenolformaldehyde resin and linseed oil. 10 cc of the first solution and 200 cc of the second solution were mixed with each other to prepare a processing dispersion.

When the resin-coated surface of the second piece was brought into contact with the resulting processing dispersion, the same spots as before were observed on the office paper. When the thus processed paper was dried at 60.degree.C and the same developer as in Example 1 was poured thereon, no penetration of the developer took place. The finely dispersed substance had a size of less than 10 .mu. in the Example.

EXAMPLE 3

A carbon black-containing original paper of 80 g/m.sup.2 in weight was used as a base material, and an acetone solution of vinyl methyl ether/maleic anhydride copolymer (molecular ratio: 1:1; degree of polymerization ca. 250) was applied thereto in a dry amount of 1.5 g/m.sup.2. The sample was cut into two pieces. The thus processed surface contained a number of pinholes. Therefore, upon immersing in an isoparaffinic solvent (Isopar H C Esso Co., Ltd. ; boiling range 174.degree.-186.degree.C; drying point 189.degree.C, density 0.757, saturated hydrocarbon 99.3% aromatic hydrocarbons 0.2%, aniline point 83.degree.C), random spots were produced due to the penetration of the solvent.

On the second piece of the thus coated copolymer surface there was poured a commercially available electrophotographic developer (developer for Elefax made by Iwasaki Communication Apparatus Co.; alkyd resin having carbon black adsorbed thereon carried in Isopar H) and, after standing for about 10 seconds, the surface washed with cyclohexane and dried at 80.degree.C. When isoparaffinic solvent was poured on the thus treated surface the isoparaffins did not penetrate into the surface. The developer was self-fixing and contained as a toner the mixture of carbon black and the alkyd resin. The alkyd resin is considered to harden and stop up the pinholes.

EXAMPLE 4

5 Grams of gelatin for photographic use (bone gelatin) was weighed and added to 95 ml of distilled water. The gelatin was swollen by allowing it to stand for 30 minutes and then dissolved by heating to 60.degree.C while stirring. The thus obtained 5% by weight gelatin aqueous solution was coagulated by placing it in a refrigerator kept at 5.degree.C. 10 Grams of the thus coagulated gelatin was charged into a ball mill together with 100 ml of acetone and milled for 10 hours. There was thus obtained a milky white acetone dispersion of gelatin. This acetone dispersion was subjected to centrifuging to collect the gelatin particles. Thereafter, 100 ml of fresh acetone was added to the gelatin particle and the gelatin particles further milled in a ball mill for 1 hour. This gelatin dispersion was again subjected to centrifuging to collect the gelatin particles, which were then washed with acetone. Thus, there was finally obtained 5 ml of an acetone paste containing gelatin particles.

To the resulting paste was added 5 ml of soybean oil, and the mixture was stirred and dispersed in 500 ml of acetone by ultrasonic waves.

A 50 g/m.sup.2 thin paper of fine quality was used as a base material, and an electroconductive resin [Calgon Conductive Polymer 261, made by Calgon Corp.; poly (N,N-dimethyl-3,5-methylene piperidinium chloride) having the formula below; 1000 cps viscosity and a 1.085 specific gravity] ##EQU1## (wherein n is a positive integer)] was applied as an aqueous solution to the wire-coated surface thereof in an amount of 1 g/m.sup.2. The thus processed surface possessed numerous pinholes through which xylol or toluol could pass. Therefore, when a coating solution prepared by dispersing zinc oxide in a toluene solution of silicone resin (KR-211, made by Sin-etsu Chemical Industry Co., Ltd.; methylphenylpolysiloxane) was applied to the processed surface, the solution penetrated thereinto over the whole surface.

Then the above-described gelatin-soybean oil-acetone, pinhole-filling liquid was applied to the Conductive Polymer 261 resin-coated surface, there was observed penetration of the liquid over the entire surface in a spot-like form. When the same toluene coating solution as above was applied thereto after drying, no penetration of the solution was observed.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

Claims

What is claimed is:

1. A method for processing a porous paper coated with a layer of a surface coating of a film-forming resin to reduce or remove pinholes which permit the penetration of a liquid and which exist in the surface coating of a film-forming resin which is capable of nearly preventing penetration of a liquid to the porous material; which method comprises bringing said layer of surface-coating resin into contact with a pinhole-filling dispersion comprising a liquid and a finely dispersed substance selected from the group consisting of a colorless pigment, a plastic particle or a pigment coated with a plastic therein, said liquid easily penetrating into the interior of the porous paper through pinholes formed in said surface-coating resin and carrying said finely dispersed substance into said pinholes, whereby said finely dispersed substance fills and seals said pinholes and wherein the rate of penetration of said pinhole-filling dispersion is from about 2 .times. 10.sup.-.sup.4 mm.sup.3 to about 60 .times. 10.sup.-.sup.4 mm.sup.3 /sec.

2. The method of claim 1 wherein prior to the treatment with the pinhole-filling dispersion from 5 to 30 pinholes per cm.sup.2 are present and after the treatment with the pinhole-filling dispersion less than 5 pinholes per cm.sup.2 are present.

3. The method of claim 2 where the pinholes have a diameter less than about 10 microns and the finely dispersed substance has a size equal to or less than the size of the pinholes.

4. The method of claim 3 wherein the size of the finely dispersed substance is from about 0.5 micron to about 5 microns.

5. The method of claim 3 wherein the liquid carrying the finely dispersed substance therein is selected from the group consisting of one or more volatile petroleum hydrocarbons, esters, ketones, alcohols and halogenated hydrocarbons.

6. The method of claim 1 wherein the surface coating resin is a polyalkyl acrylate, polymethacrylate, copolymer of an alkylacrylate, copolymer of an alkylmethacrylate, copolymer of a methacrylate, polystyrene, polyvinyl acetate, linear polyester, unsaturated polyester, alkyd epoxy resin, polyvinyl alcohol, polyvinylbenzyl trimethylammonium chloride or a copolymer of styrene and butadiene.

7. The method of claim 1 wherein the finely dispersed substance comprises from about 1 to 20% by volume of the said liquid.

8. The method of claim 1 wherein the liquid carrying said finely dispersed substances therein is contacted with the surface-coating resin for about 2 seconds to about 40 seconds, after which drying is conducted.

9. The method of claim 8 wherein drying is at about 60.degree. to about 120.degree.C.

10. The method of claim 8 which consists essentially of the recited steps.

11. The method of claim 1 where the finely dispersed substance is a colorless pigment.

12. The method of claim 1 wherein the finely dispersed substance is a plastic particle.

13. The method of claim 1 wherein the finely dispersed substance is a pigment coated with a plastic.

14. The method of claim 1 where the pinholes have a diameter of less than about 10 microns and the finely dispersed substance has a size equal to the pinholes.

15. The method of claim 1 wherein said liquid is an organic liquid.

16. The method of claim 1 wherein the rate of penetration of said pinhole-filling dispersion of from 4 .times. 10.sup.-.sup.4 to 32 .times. 10.sup.-.sup.4 mm.sup.3 /sec.