US3446648A - Reactive copying sheet and method of using - Google Patents

Description

United States Patent 3,446,648 REACTIVE COPYING SHEET AND METHOD OF USING Wesley R. Workman, St. Paul, Minn., assignor to Minnesota Mining and Manufacturing Company, St. Paul, Minn., a corporation of Delaware No Drawing. Filed Sept. 27, 1965, Ser. No. 490,662 Int. Cl. G03c 1/06 U.S. Cl. 11736.8 5 Claims ABSTRACT OF THE DISCLOSURE Copy-sheets containing reducible silver soap and requiring an image toning agent for making intense blueblack images contain an acyl derivative of a toning agent and a Weak organic acid, which slowly hydrolyzes to provide the required toning action.

This invention relates to improvements in copypapers useful in the copying of graphic intelligence by heatinitiated reduction of silver compounds, and has particu lar reference to improvements in the toning of the silver image.

Heat-sensitive copy-sheets capable of undergoing permanent visible change on being briefly heated are described in U.S. Patent No. 2,910,377. The color change typically requires the heat-initiated reduction of a silver compound with an organic reducing agent. Silver behenate and dihydroxybenzoic acid are representative of reactants disclosed in the patent. A similar reaction occurs between a silver compound and a photosensitive organic reducing agent in the copy-sheet products described in U.S. Patents Nos. 3,094,417 and 3,094,619. In all instances the image formed is intensified and improved by the incorporation, in intimate admixture or association with the silver compound, of an image toning agent. Although other toning agents for the heat-developed silver image are known, e.g. as described in U.S. Patent No. 3,074,809, best results in terms of intense blue-black images have been obtained with phthalazinone as the toner, as described in U.S. Patent No. 3,080,254.

Under suitably controlled conditions of storage before use, copy-sheet products as above described and containing small amounts of phthalazione as the toner for the silver image are found to retain for long periods their ability to form dense black or blue-black images. Under some conditions, and particularly under storage at high humidity and at moderately elevated temperatures, e.g. as in many tropical or semi-tropical locations, the imageforming properties of the copy-sheets are found to deteriorate slowly. Copies made with such sheets, while still legible, do not provide the expected dense black images. The difiiculty is not effectively overcome by increasing the initial toner content, since any excesses in this direction are found to result in unwanted darkening of the sheet at the unreacted background areas.

It has now been found possible to effectually suppress the decay in obtainable image intensity normally occasioned by wet heat aging of the silver-containing copysheet, by introducing with the phthalazinone toner as a toner progenitor an acyl derivative of phthalazinone which on hydrolysis will yield phthalazinone and an innocuous acid.

An example of an effective toner progenitor is 2-acetylphthalazinone. This compound is readily prepared by refluxing 31.2 g. (0.214 mol.) of phthalazinone in 175 ml. of acetic anhydride for 2 hours. The mixture is cooled overnight and the solid product is collected on a filter and recrystallized from benzene. The white solid melts at Patented May 27, 1969 Parent acid: Deriv. M.P., C. Lauric 65 Pivalic 99-101 Benzoic 118 p-Methozybenzoic 141-143 Ethoxyformic 77-78 The derivatives of oxalic acid (M. 274-275) and of m-nitrobenzoic acid (M. 229-232) are less effective. The image formed in the sheet after wet heat aging shows little if any improvement in appearance over that obtained in the absence of the derivative. Oxalic acid is known to have a bleaching or decolorizing effect on many materials. The dissociation constants in water of oxalic and m-nitrobenzoic acid are orders of magnitude greater than those of the others named. The melting point of the oxalyl and m-nitrobenzoyl derivatives is significantly above the temperature of C. frequently considered as an approximate upper temperature limit in commercial thermographic copying processes.

On the contrary, compounds such as 2-phthalyl phthalazinone oifer additional improvement in toning of the silver image due to the independent toning action of the acidic component which is obtained, together with phthalazinone, on hydrolysis of the derivative.

The acyl derivatives have the structure 0 N-il R 1 is the acyl radical and R may be alkyl, aryl, substituted aryl, aralkyl, or alkoxy. ,f.

The several acyl derivatives have very slight if any toning influence on the silver image as first introduced. They develop toner properties during exposure to wet heat, and at different rates depending on the specific acyl substituent. Thus, for example, the derivative based on pivalic acid generates a toning action more rapidly than any of the others named; the acetyl, lauryl and benzoyl derivatives are intermediate in rate of toner generation; 2(3'4'-dimethoxybenzoyl) phthalazinone generates the toner at a relatively slow rate. Since the rate of deterioration of the toning action provided by phthalazinone incorporated in the original coating may also vary extensively depending on such factors as the specific backing, silver salt, binder, or other components of the sheet, the relative proportions and the extent of compatibility of the several components, the storage conditions employed, and perhaps other factors, it will be seen that some selection of particular acyl derivatives may be necessary to achieve the highest order of uniformity in toner performance or the most extended useful life of the copy-sheet.

3 Example 1 A master mixture is first prepared as described in US. Patent No. 3,107,174 and containing, in parts by weight, 2.8 parts of a mixture of equal mol percent of silver behenate and behenic acid, 2.25 parts of terpene resin, 11.25 parts of zinc oxide, 1.69 parts of polyvinylacetate as a film-forming binder, and optionally 0.05 part of tetrachlorophthalic anhydride, smoothly dispersed in 77.5 parts of acetone.

To IO-gram portions of the mixture are then added 0.1 gram of an acyl derivative of phthalazinone as a toner progenitor, and the portions are smoothly knife coated through a 2-rnil coating orifice onto white paper and dried to produce a series of copy-sheets. I

An intermediate sheet with latent image areas containing trace amounts of 4-methoxy-l-hydroxynaphthalene,a reducing agent for silver ion, is placed against the coated surface and exposed to heat. The resulting oxidationreduction reaction produces a visible image on the copysheet corresponding to the latent image of the intermediate sheet. The procedure is described in more detail in US. Patent No. 3,094,417. Another portion of the coated copysheet is placed in a test chamber and held at 90% relative humidity and 53 C. for 75 minutes, and is then similarly treated to form an image. In all instances the image formed on the sheet which has been exposed to wet heat is darker and more dense in appearance. The specific derivatives tested are 2-acetyl-, 2-benzoyl-, 2- (p-methoxybenzoyl)-, and 2-(3',4-dimethoxybenzoyl)- phthalazinone.

Example 2 To four 10-gram portions of the master mixture of Example 1 is added .05 gram of 2,6-di-tertiary-butyl-4- methoxyphenol and amounts of phthalazinone (I) and of the 3,4-dimethoxybenzoyl derivative of phthalazinone (II) as specified in the following tabulation. The portions are coated on white paper using a knife coater at an orifice of 3 mils, and dried. Portions of the resulting copysheets are subjected to wet heat aging at 53 C. and 90% RH. for various times. All portions are then subjected to heating in contact with an intermediate sheet bearing trace amounts of 4-methoxy-1-hydroxynaphthalene at latent image areas, to form visible images on the copy-sheets. The image optical density of each of the four samples is then determined by reflection. A commercially acceptable visibly dense image has an optical density value of at least about .50.

Toner material, Density value after aging for In s (hours)- II 1 4 l6 which is itself heat-sensitive and requires no added trace amounts of 4-methoxy 1 hydroxynaphthalene or other reducing agent in order to undergo visible darkening when heated briefly at elevated temperatures of the order of to C. as in thermographic copying processes. The presence of the significant small amount of 2-acyl pthala- Zinone together with the phthalazinone insures the formation of a dense dark image both with the sheet as initially prepared and after it has been subjected to prolonged exposure to high humidity and elevated temperature storage conditions. 1

What is claimed is as follows:

1. Sheet material containing, in intimate association, a silver salt of an organic acid and a significant small amount of 2-acyl phthalazinone having the structure wherein o H OR is an acyl radical which is gradually removable by hydrolysis under exposure to wet heat atmospheric conditions and R is alkyl, aryl, substituted aryl, aralkyl, or alkoxy.

2. Sheet material of claim 1 wherein is included a significant small quantity of phthalazinone.

3. Sheet material of claim 1 which is capable of undergoing permanent visible change on being briefly heated and including an organic reducing agent for silver ion.

4. Sheet material of claim 3 wherein is included a significant small quantity of phthalazinone.

5. The method of achieving the toning of silver images formed in a copy-sheet containing a reducible silver salt of an organic acid, com rising including with said silver salt an acyl deriverative of a phthalazinone toner for said silver image, the acyl radical being gradually removable by hydrolysis under exposure to wet heat atmospheric conditions, and permitting at least partial hydrolysis of said derivative prior to formation of said image whereby to liberate a significant amount of said toner.

References Cited UNITED STATES PATENTS 2,910,377 10/1959 Owen l17-36.8 3,074,809 1/1963 Owen 11736.8 3,080,254 3/1963 Grant 11736.8 3,094,417 6/1963 Workman 11736.9 3,094,619 6/1963 Grant 117-36.8 3,107,174 10/1963 Wartman 117-36.8 3,218,166 11/1965 Reitter 11736.8

MURRAY KATZ, Primary Examiner.

US. Cl. X.R.