Process For The Manufacture Of Silver Halide Photosensitive Materials

Abstract

A process for the manufacture of silver halide photosensitive materials wherein one or more hydrophilic colloid layers are applied to a support, at least one layer being a silver halide emulsion layer, and then the layer is dried, characterized in that when the moisture content is about 300% or less, based on average dry weight of the solids content of the hydrophilic colloid containing layer(s), drying is performed by microwave heating under air at a relative humidity of 55-85%.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a process for the manufacture of silver halide photosensitive materials, more particularly, in a process for preparing silver halide photosensitive materials to the drying of a colloidal layer applied to a support.

DESCRIPTION OF THE PRIOR ART

Silver halide photosensitive materials are manufactured by preparing a silver halide emulsion, applying the thus prepared emulsion to a support after adding additives such as a hardening agent, a coating auxiliary, etc., to the emulsion, and then drying the thus coated support. Two or more silver halide emulsion layers may be applied to the support and, further, the emulsion layer(s) may be applied together with (an) auxiliary layer(s) such as a protective layer, a backing layer, etc.

For example, silver halide photosensitive substances which comprise a gelatino-silver halide emulsion layer and a protective layer containing gelatin which have been applied to support are subjected to cold solidification using cold air having a dry-bulb temperature ranging from -10.degree.C to 10.degree.C, and thereafter air having a dry-bulb temperature of 15.degree.-45.degree.C and a relative humidity of 10-50% is applied to the material thereto as it is being conveyed on a support, for example, by a straight type conveying, an arch type conveying, a zigzag type conveying, a suspension type conveying, etc. The amount of air supplied depends upon the dry state of the coating layer, and after conveying the thus treated materials are then subjected to a moisture adjustment by means of air having a dry-bulb temperature of 20.degree.-26.degree.C and a relative humidity of 50.degree.-70.degree.%. In general, the above steps from the cold solidification to moisture adjustment are called a drying operation, and due to such drying the silver halide photosensitive materials contain an appropriate moisture content suitable for a long periods of storage.

The above drying operation may impart good effects to photographic substances prepared using a low content of silver halide relative to gelatin, but is suitable for silver halide photosensitive materials as are used for attaining rapid treatments, high sensitivity and high image-resolution. That is, if photosensitive materials containing a high content of silver halide relative to a hydrophilic colloid such as gelatin, i.e., the weight ratio of silver halide to a hydrophilic colloid is more than about 0.6, (hereafter referred to as high-silver halide emulsions or high-silver halide photosensitive materials) are dried according to the above drying operation, the final high-silver halide photosensitive materials show an extreme increase in fog, and thus the materials have a fatal defect. Fog means silver in unexposed parts of the emulsion which is reduced by development.

The above defect is particularly severe when a protective layer containing gelatin is provided on a high-silver halide emulsion layer as well as when films as having low elasticity and hygroscopicity, for example, polyester, polycarbonate, polystyrene, etc., are used as a support. Although the reason therefor has not yet been determined, it is believed that due to the high silver halide content in the emulsion layer the gelatin containing the silver halide is rapidly contracted by the action of air having a low moisture content whereby the silver halide is strained by the contraction of gelatin. The same phenomena occurs in the case of using a film support having low elasticity and hygroscopicity.

In an effort to eliminate such defects, two methods have hitherto been proposed. One is to add to the emulsion, before coating, compounds which are hygroscopic or able to absorb strain, and the other is to dry using air having a dry-bulb temperature of 20.degree.-35.degree.C and a relative humidity of 60-80% when the total moisture content in a gelatino-silver halide emulsion layer and a gelatin-containing protective layer on the emulsion layer, that is,

moisture content as % = (weight of moisture in layer(or layers) which contains hydrophilic colloid on the support being dried/dry weight of solids in such layer (or layers)being dried) .times. 100.

is smaller than 300 %.

According to the former method while fog may be eliminated, due to the action of the added compounds the photographic properties of the coating layer are badly affected, e.g., adhesive ability, film strength, etc. On the other hand, according to the latter high moisture drying method the drying speed is decreased as compared with other conventional low moisture drying methods, and thus the production rate for manufacturing photosensitive materials is decreased. Further, the latter method requires large scale apparatus, and thus the construction expenses, repair expense and operational expenses are increased.

SUMMARY OF THE INVENTION

It is a primary object of the present invention to provide an improved process for manufacturing silver halide photosensitive materials.

Another object of the present invention is to provide a method for drying silver halide photosensitive materials in a short period of time and to eliminate fog without enlarging the drying apparatus used.

The present invention is based, in part, upon microwave heating within a certain moisture content range. In the present invention the moisture content is given as a percentage, which is calculated as follows: (weight of moisture in a photographic layer(or layers) which contains a hydrophilic colloid/weight of total dry solids content in such layer (or layers) .times. 100. Usually, the moisture content in a photographic coating layer or layers will be above 300% immediately after coating. In the first step of the present invention the moisture content is reduced from its initial value down to about 300% or less using a drying method other than microwave drying. After the moisture content in the layers present on the support is reduced to about 300% or less, then drying is completed using microwave drying under air having a relative humidity of 55 - 85%.

DETAILED DESCRIPTION OF THE INVENTION

Microwaves as are used in the present process are, as is known, electromagnetic waves having a wave length shorter than 1 m, preferably from several 10 cm to several cm. Although the microwaves most useful in this invention are from about 300 MHz (.lambda.= 1 m) to about 30 GHz (.lambda.= 1 cm), the microwaves generally used in industrial heating are 915 MHz and 2,450 MHz, and these will most commonly be the microwave frequencies selected by users of the present invention.

A great deal of literature describes the heating and drying of sheet materials utilizing microwaves, e.g., the Japanese magazine entitled "Insatsu Zasshi (Printings)" published in Dec. 1968, "Microwave Dryer;" "Tappi" Vol. 53, No. 6, "Microwave Paper Drying Experience and Analysis;" U.S. Pat. No. 3,475,827; etc. In the process of the present invention, apparatus as is described in these publications may be used. With respect to the structures of the heating furnace used, for example, oven type furnaces such as electronic ranges, combination type furnaces consisting of an air bearing (often called an air roll) and microwave radiation apparatus (see, e.g., U.S. Pat. No. 3,426,439), or a serpentine wave guide type furnace which is provided with slits in the wave guide between which the materials to be heated are continuously fed can all be used. The use of the serpentine wave guide type furnace for heating sheet materials is advantageous in many aspects in that uniform heating may be attained, heat efficiency is high and any electric wave leakage can be decreased. Such apparatus is described in "Review of Drying of Silver Halide Photographic Film, " Journal of the SMPTE 73 (1), 1969, "What is the Outlook for Drying Paper with Micro-Waves," Paper Trade Journal, Sept. 28, 1970, and in U.S. Pat. No. 3,475,827.

The silver halide photosensitive materials which are efficiently manufactured according to the present invention comprise a silver halide emulsion layer on a support. In the present process, any type of silver halide emulsion may be used. As silver halides commonly used in manufacturing photosensitive materials there can be mentioned silver chloride, silver bromide, silver iodide, silver bromochloride, silver bromoiodide, silver bromoiodochloride, etc., all of which can be processed in accordance with this invention.

As to the protective colloids used in forming the emulsion, gelatin is most commonly used, but gelatin derivatives such as gelatin phthalide, and gelatin malonate, cellulose derivatives such as hydroxyethyl cellulose and carboxymethyl cellulose hydrophilic polymers such as colloidal albumin, polyvinylalcohol, etc., may also be processed singly or as mixtures thereof in accordance with the present invention.

The weight ratio of the protective colloid and the silver halide in the silver halide photographic emulsion can vary within a broad range, but in a high silver halide photosensitive material having weight ratios of more than about 0.6, preferably more than 0.8 weight parts of silver halide per weight part of protective colloid an effect on the prevention of fog is especially attained.

The silver halide photographic emulsions which can be processed in accordance with the present invention can include compounds containing unstable sulfur such as sodium thiosulfate, allyl thiocarbazide, etc., metal compounds such as a thiocyanategold(I) complex, reducing agents such as stannous chloride, polyalkyleneoxide derivatives as well as mixtures of these substances, and can be chemically sensitized according to any known method in this art. The silver halide photographic emulsions can be sensitized by these chemical sensitizers in a usual manner. Further, the silver halide photographic emulsions may be subjected to dye sensitization, for example, by means of a cyanine dye or a mixture of cyanine dyes, e.g., 1,1'-diethylcyanine-iodide, 1,1'-diethyl-9 -methyl-carbocyanine-bromide, anhydro-5,5'-diphenyl-9-ethyl-3,3'-di(2-sulfoethyl)-benzoxazolcarbocyanine -hydroxide, etc. A sensitizing amount of a dye can be in a range of from about 1 .times. 10.sup..sup.-6 mole to about 5 .times. 10.sup..sup.-3 mole per mole of silver halide. Moreover, the silver halide photographic emulsions can contain a developing agent which can release a restrainer (development-inhibiting substance) such as 2-iodo-5-pentadecylhydroquinone, 2-methyl-5-(1 -phenyl-5-tetrazolylthio)-hydroquinone, etc., a stabilizer such as 4-hydroxy-6-methyl-1,3,3a, 7-tetrazaindene, benzimidazole, 1-phenyl-5-mercaptotetrazole, a hardening agent such as formaldehyde, mucobromic acid, and a coating auxiliary such as saponin, sodium alkylbenzenesulfonate, etc.

The silver halide emulsion is applied to a support, for example, a plastic film such as a polyester (e.g., polyethylene terephthalate), a polycarbonate, polystyrene, cellulose acetate, cellulose acetate butyrate, polypropylene, etc., or glass in the form of a layer, and particularly in the case of supports having a lower elasticity and hygroscopicity than that of cellulose acetate the effect of the present invention is remarkable.

A protective colloid layer, particularly a protective layer containing gelatin, can be applied on the emulsion layer. In this case, the content of protective colloid is preferably 3-10% by weight of the protective layer coating liquid. Similar values are used for other non-silver halide hydrophilic colloid layers. The protective layer can be applied simultaneously with an emulsion layer or sequentially to emulsion layer application.

The present invention, of course, is not limited to the drying of photographic elements containing only these two layers, but can be used to dry photographic elements containing other normal layers as are used in this art, e.g., an intermediate layer, subbing layer, etc. The moisture content denotes the total value in the layers containing hydrophilic colloid present in the light-sensitive material, e.g., emulsion layer, protective layer, intermediate layer, subbing layer, etc.

For an emulsion layer -- protective layer element, the moisture content in the emulsion and protective layers can be calculated by the formula: ##SPC1##

In coating the layer or layers of photographic materials on the support, the moisture content thereof is usually in excess of 300% for ease of coating. While the initial moisture content may vary greatly, the majority of commercial coating processes involve a hydrophilic coating layer or layers having a moisture content of about 600% to about 1200%.

The first step of the present invention involves reducing the moisture content in the hydrophilic layer (s) to a value of about 300% or less using any conventional drying method, i.e., non-microwave drying, for instance, by blowing heated air on the element. In those rare instances where the initial moisture content is about 300%, this initial drying can be omitted. Such will seldom be the case, however, with commercial coating operations.

Once the moisture content is reduced to about 300% or less, based on dry solids content, drying to the desired level is conducted by microwave heating under air at a relative humidity of 55 to 85%, preferably 60 - 80%. Drying in the moisture content range of less than 300% is called "drying in the falling rate range of drying."

The microwave application under air of a relative humidity of 55 - 85%, usually a flowing air stream to ensure sufficient moisture contact, can be used to per se provide an element of a relatively high moisture content by appropriately selecting a high relative humidity of the air, i.e., near 85%. Alternatively, the element can be "overdried" with microwaves and air having a low relatively humidity, i.e., near 55%, and then treated with air having a controlled (temperature) humidity to adjust the moisture content to the desired value. For instance, certain photographic materials require a moisture content of about 6% to about 12%, and such is achieved after microwave drying by storing the element under air which will provide such a moisture content to the hydrophilic colloid layers of the element; as a general rule, air of a moisture content of 55 to 70% is used, but this is non-critical.

It will be apparent to one skilled in the art that the microwave/controlled humidity drying can be used to dry from any moisture content from about 300% or less to the desired final moisture content, and conventional drying can be used to some intermediate value, e.g., 250% moisture.

The temperature of drying can vary greatly in the present invention. Two criteria generally set the temperature which is used. First, if too low a temperature is used, drying is very slow and commercially uneconomical. On the other hand, a temperature which renders the hydrophilic colloid layer which is being dried molten so that it flows from the support also is obviously unacceptable. One skilled in the art will be able to select an appropriate drying temperature considering the above criteria, and generally speaking a temperature of about 15.degree. to about 30.degree.C is used, with 18.degree. to 27.degree.C being preferred.

The moisture content is calculated according to the formula given above, and the values in the formula may be obtained, for example, by measuring according to any conventional method, the weights involved and the temperature on the surface of the coating layer.

The time of microwave irradiation may range from several seconds to several minutes, which in practice is determined depending upon the desired moisture content.

The microwave heating of the present invention is different from a conventional external heating which is conducted by heating a substance from its surface by means of radiation, convection, transmission, etc., and the mechanism of heat generation with microwave heating consists of molecular movement in the interior of the heated substance. In the present case, accordingly, the heated substance itself becomes a heat generating substance, and thus the temperature of both the surface and the interior of the substance are uniformly elevated in temperature. Therefore, the heat efficiency is high, and generally parts which contain a higher moisture content have a higher heating efficiency and thus a rapid drying is possible without uneven drying even if there exists an uneven moisture content distribution. Accordingly, the drying period of a photosensitive material may be reduced by removing moisture present in an amount of less than 300% on the basis of the average dry weight of the solids content of the photographic material or, in other words, a moisture content nearly in the falling rate range of drying, by means of the microwave heating.

Further, according to the drying step of the present invention, an uniform drying is possible without over-drying, which results in a favorable effect against fog. The effects of the present invention are greatly augmented by conducting microwave heating while blowing air having a relative humidity of 55-85%, preferably 60-80%, on the material to be dried so as to eliminate any over-drying of the surface of the coating layer thereof. This embodiment of the invention is especially suitable for the manufacture of high-silver halide photosensitive materials using elastic and hygroscopic supports which are apt to yield fog due to the contraction of gelatin.

An uneven coating thick at some points and thin at other points often occurs upon starting coating and can occur due to any accident during the course of coating. Such an uneven coating results in an extremely low drying-speed(retardation)in the drying step to follow. In the case where air relative humidity is kept at 55-85% in and around the falling rate range of drying, for the purpose of preventing overdrying on the surface of the material which has been unevenly coated, the above mentioned tendency of low drying speed is further promoted. However, according to the process of the present invention since microwaves are absorbed to a greater extent in high moisture areas, the coated material is firstly dried from the parts containing high moisture then from those containing low moisture, and thus a film surface which is unevenly coated never is affected badly. Accordingly, the drying operation may be accelarated and an extremely efficient drying attained.

The present invention will now be illustrated in more detail by the following non-limiting examples which illustrate several preferred embodiments of the invention.

EXAMPLE 1

A silver bromoiodide emulsion consisting of silver bromoiodide containing 2 mol% of silver iodide (100 mg AgBr/100cm.sup.3 of solution) and gelatin (30 mg/100cm.sup.3) and a protective layer containing gelatin (16 mg/100 cm ), were applied onto a polyester film in the form of two layers, and after being subjected to cooling and solidification the thus coated film was divided into three pieces, Sample A, Sample B and Sample C, having an initial moisture content of 700%.

Sample A was dried for 4.5 minutes by air having a dry-bulb temperature of 25.degree.-30.degree.C and a relative humidity of 25-30% and was then subjected to cooling and moisture adjustment by air having a dry-bulb temperature of 22.degree.C and a relative humidity of 65%.

Sample B was treated as follows: Moisture in excess of 150% based on the average dry weight of the solids content was dried with air of a dry-bulb temperature of 25.degree.-30.degree.C and a relative humidity of 30%, and moisture in an amount less than 150% based on the average dry weight of the solids content was dried with air of a dry-bulb temperature of 28.degree.C and a relative humidity of 70%, the total drying period being 6.5 minutes. After drying, Sample B was then subjected to cooling and moisture adjustment by means of air having a dry-bulb temperature of 22.degree.C and a relative humidity of 65%.

Sample C was treated according to the process of the present invention as follows: Moisture present in an amount above 150% based on the average dry weight of the solids content was dried with air of a dry-bulb temperature of 25.degree.-30.degree.C and a relative humidity of 30%, and the moisture present in an amount less than 150% based on the average dry weight of the solids content was subjected to air drying with air of a dry-bulb temperature of 28.degree.C and a relative humidity of 70% with a simultaneous heat treatment for 0.5 minutes by microwave heating apparatus of the serpentine type wave guide having a frequency of 2,450 MHz, the total drying period being 4.0 minutes in all. After being dried, sample C was subjected to cooling and moisture adjustment by blowing air having a dry-bulb temperature of 22.degree.C, and a relative humidity of 65% onto the film for about 5 minutes.

Each of Samples A, B and C thus subjected to drying and moisture adjustment was given an identical exposure through an optical wedge, and then each sample was subjected to development under the same conditions. The sensitivity and fog of the thus developed Samples A, B and C were measured, and the following results obtained. For Samples A, B and C, the fog was 0.10, 0.06, respectively; the sensitivity and other characteristics such as .gamma., Dmax, etc. and film stiffness strength, melting point, reticulation point, etc., were almost the same for each sample.

Air or initial drying was performed on the above samples by passing the samples in a straight line under a hood provided with slits through which the air of controlled temperature/moisture was blown perpendicularly on the samples.

The processing sequence for each sample is summarized below:

25-30.degree.C 28.degree.C Microwave + 28.degree.C 25-30% RH* 70% RH 70% RH Total ______________________________________ A 4.5 min. -- -- 4.5 min. B 3.5 do. 3.0 min. -- 6.5 do. C 3.5 do. -- 0.5 min. 4.0 do. ______________________________________ *RH Relative Humidity

EXAMPLE 2

A silver bromoiodide emulsion having the same composition as in Example 1 and consisting of silver bromoiodide (100 mg/100 cm.sup.3) and gelatin (35 mg/100 cm.sup.3 and a protective layer the same as that of Example 1 were applied onto a polystyrene film support by the same procedure as in Example 1 to obtain Samples D, E, and F. These samples were subjected to exposure development, etc., as in Example 1 resulting in almost the same results as in Example 1.

EXAMPLE 3

A silver bromoiodide emulsion consisting of silver bromoiodide containing 5 mol% of silver iodide (50 mg/100 cm.sup.3) and gelatin (40 mg/100 cm.sup.3) and a protective layer containing gelatin (14 mg/100 cm.sup.2) were applied onto a polyester film support in the form of superposed layers, and after being subjected to cooling and solidification the thus coated film was divided into two pieces, Sample G and Sample H of an initial moisture content of 850%.

Sample G was subjected to a conventional treatment as follows:

Moisture in excess of 250% based on the average dry weight of the solids content was dried with air of a dry-bulb temperature of 28.degree.-32.degree. C and a relative humidity of 25-30%, and moisture present in an amount of less than 250% (same basis) dried with air of a dry-bulb temperature of 30.degree.C and a relative humidity of 70%, the total drying period being 6.8 minutes. After being dried, Sample G was subjected to cooling and moisture adjustment by means of air having a dry-bulb temperature of 25.degree.C and a relative humidity of 65% as in Example 1.

Sample H was treated according to the process of the present invention as follows:

Moisture in excess of 250% was removed as for Sample G and moisture less than 250% was removed by heat treatment for 0.5 minute by means of a microwave heating apparatus of the serpentine type wave guide and having a frequency of 2,450 MHz while supplying air to the surface of the film to be dried having a dry-bulb temperature of 25.degree.C and a relative humidity of 70%. After 4.2 minutes of drying, drying was completed.

Sample H thus treated was subjected to cooling and moisture adjustment under the same conditions as Sample G. Each of Samples G and H thus subjected to drying and moisture adjustment was then subjected to exposure and development as in Example 1.

The results were as follows:

Fog appeared in Sample H to a degree identical to that of Sample G despite the extreme reduction of the drying period in Sample G as compared with Sample H.

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 process for the manufacture of silver halide photosensitive materials wherein one or more hydrophilic colloid layers are coated on a support, at least one hydrophilic colloid layer comprising a silver halide in a hydrophilic colloid, and the layer or layers are dried, the improvement wherein the moisture content is reduced to about 300% or less by non-microwave drying and the moisture present at a moisture content of about 300% or less in the hydrophilic layer or layers is removed by microwave heating under air of a relative humidity of 55 to 85%, moisture content being calculated by the formula:

moisture content = (weight of moisture in layer or layers which contain hydrophilic colloid on the support being dried/dry weight of solids in such layer or layers being dried) .times. 100,

the weight of silver halide contained in the silver halide emulsion being more than 0.6 parts per part of hydrophilic colloid.

2. A process as claimed in claim 1 wherein the support is a film having a lower elasticity and/or a lower hygroscopicity than that of cellulose acetate.

3. A process as claimed in claim 1 wherein the weight of silver halide contained in the silver halide emulsion is more than 0.8 part per part of hydrophilic colloid.

4. A process as claimed in claim 1 wherein the hydrophilic colloid comprises at least gelatin.

5. A process as claimed in claim 1 wherein a protective layer is applied onto the silver halide emulsion layer.

6. A process as claimed in claim 1 wherein a protective layer containing at least gelatin is applied onto the silver halide layer and then dried.

7. A process as claimed in claim 1 wherein the initial moisture content of the layer is about 600 to about 1,200%.

8. A process as claimed in claim 1 wherein microwave drying is to a moisture content less than 6% and the moisture content is then elevated to 6 to 12% by maintaining the photosensitive material in air having a moisture content of 55-70%.