US3628980A - Heat-sensitive copy sheet - Google Patents

Abstract

The reaction temperature of a copy sheet containing bis(triphenylphosphine)borohydridocopper(I) as heat-decomposable image-forming component is significantly reduced by incorporating certain nonnitrogenous nickel compounds as catalysts.

Description

United States Patent inventor Joseph A. Wiese, Jr.

Saint Paul, Minn.

Appl. No, 870,536

Filed Oct. 30, 1969 Patented Dec. 21, 1971 Assignee Minnesota Mining and Manufacturing Company St. Paul, Minn.

HEAT-SENSITIVE COPY SHEET 10 Claims, No Drawings 117/138.8 A, 117/155 UA, 260/41 C, 260/922 lnt.Cl B41m 5/18 Field of Search 1 l7/36.8, 36.9

[56] References Cited I UNlTED STATES PATENTS 2,813,042 11/1957 Gordonet a1... l17/36.8 3,505,093 4/1970 Schultz l l7/36.8

Primary Examiner-Murray Katz Attorney-Kinney, Alexander, Sell, Steldt & Delahunt HEAT-SENSITIVE COPY SHEET This invention relates in a major aspect to the copying of differentially radiation-absorptive graphic originals by thermographic methods involving brief exposure of the original to intense radiant energy while in heat conductive contact with a heat-sensitive copy sheet, and, while not limited thereto, has particular reference to novel heat-sensitive copy sheet articles and methods of making. The heat-sensitive coatings or coated articles are also useful in thermal telltales or temperature indicator devices.

In U.S. application, Ser. No. 687,126 filed Dec. 1, 1967, now U.S. Pat. No. 3,505,093 there is described a heat-sensitive copy sheet containing as the visibly heat-sensitive material the compound bis(triphenylphosphine)borohydrid ocopper(l). Localized heating of the copy sheet under maximum exposure in a thermographic copying machine causes decomposition of the compound, with formation of a stable dark colored residue which provides a visible record of the thermographically applied heat pattern.

Many commercially available thermographic copying machines operate at optimum temperatures which are significantly lower than that required for the sheet materials just described, so that such copy sheets either fail to produce a fully developed image, or require that the machine be operated above its normal rating or at less than its recommended speed, with attendant overheating of machine components and with other difficulties. A particular difficulty is the formation of fuzzy or filled letters in the copy, caused by excessive lateral diffusion of the heat pattern under the highoperating temperatures.

In accordance with the present invention it has now been found possible to cause permanent visible change in the bis(triphenylphosphine)borohydridocopper(l) coatings at significantly lower temperatures by incorporating catalytically small amounts of certain nitrogen-free nickel compounds. Where heat-sensitive copy sheets are involved, overheating of machine and of printed original is avoided, lamp life is increased, wrinkling of copy is prevented, and sharply defined copies, with letters free of fuzziness or filling, are obtained.

As described in application, Ser. No. 687,126, the copy sheet will normally comprise a heat-resistant carrier or backing having a coating of the visibly heat-sensitive material in a heat-resistant film-forming binder, although other constructions may also be used. Thin paper and transparent plastic film are typical carriers. Polyvinyl acetate is a preferred binder, but others, such for example as ethyl cellulose, vinyl acetate-vinyl chloride copolymer, and vinylidene chlorideacrylonitrile copolymer are also useful. The compositions are ordinarily applied from solution or suspension in a volatile organic liquid vehicle which is then removed by evaporation at a temperature below the reaction temperature. Pigments, fillers,

opacifrers, stabilizers and other additives may be included with the reactant materials, and additional subcoatings or protective surface coatings may be added if desired. 7 v

The amount of bis(triphenylphosphine)borohydrid ocopper(l) applied per unit area of coating is at least sufficient to provide a readily visible image when locally heated but for reasons of economy should not be greatly in excess of such amount. Good results are obtained at a concentration of about 1 to 1% grams of the copper compound per square meter, i.e., at least about 0.0016 gram mol per sq.m.

Surprisingly small amounts of nickel compound are fully effective in significantly lowering the imaging temperature, but much larger amounts may be tolerated. As an example, a sheet containing 1% grams of the copper compound per sq.m. and showing only a faint darkening at image areas when subjected to normal thermographic copying procedures is found to produce sharp images of high density under the same copying conditions when formulated with but 0.005 gram/sq.m. of nickel acetylacetonate.

Many nonnitrogenous nickel compounds are found similarly to catalyze the image-forming reaction; but others are ineffective. It has not been found possible adequately to distinguish the two classes in terms of chemical structure.

However it has been found possible to distinguish the two in terms of a simple chemical test which will now be described.

Ten grams of the bis(triphenylphosphine)borohydrid ocopper(l) is dissolved in ml. of chloroform to provide a clear stock solution. One milligram of the nickel compound under test is dissolved or dispersed in 5 ml. of chloroform. One milliliter of the stock solution is added to the test mixture and the combination is briefly heated to boiling. A distinct darkening of the solution indicates that the nickel compound is an effective catalyst.

Tested in the manner described, nickel acetylacetonate produces a dark brown solution, as does nickel acetate, nickel oxalate, nickel trifluoromethanesulfonate, nickel formate, nickelocene, and nickel diethyl dithiocarbamate. A less intense but still distinctly brown color is obtained with nickel tartrate and with nickel bromide. Each of these compounds is useful in lowering the imaging temperature of the bis(triphenylphosphine)borohydridocopper(l) copy sheet, as are nickel stannate, nickel oleate, nickel trifluoroacetylacetonate and nickel hypophosphite. Surprisingly, nickel chloride does not cause a visible change; and nickel chloride is found to be ineffective as an additive for reducing the image-forming temperature of the copy sheet. The same is true of nickel sulfate.

EXAMPLE A coating composition is prepared by uniformly mixing mg. of bis(triphenylphosphine)borohydridocopper(l) into 5 grams of a 5 percent solution of polyvinyl acetate in methylene chloride. The solution is coated on thin paper, using a knife coater set at an orifice of three mils, and the coating is permitted to dry. This control sheet produces black images of typewritten correspondence and other originals in the thermographic copying process under maximum temperature conditions but the images are fuzzy" and the block letters are largely filled.

Another composition is prepared and coated in identical manner except for the addition of 0.5 mg. of nickel acetylacetonate. The copying machine is adjusted to an operating condition in which the control sheet produces only a dim partial image of the original. Under these conditions the augmented sheet produces complete and sharp images of high image density.

Excellent images are obtained with sheets prepared with compositions containing 0.5, 2,5,10 and 20 mg. of the nickel compound. With the addition of 40 mg. of the compound the mixture becomes gelatinous and cannot be coated.

What is claimed is as follows:

1. An article capable of being heated and having on a surface thereof a coating of a visibly heat-sensitive composition comprising the compound bis(triphenylphosphine)borohydrid ocopper (l) and a catalytic amount sufficient to significantly depress the decomposition temperature of said compound, of a nonnitrogenous nickel compound characterized by its ability to cause distinct darkening of a solution of bis( triphenylphosphine )borohydridocopper(l) in boiling chloroform.

2. The article of claim 1 wherein said coating includes a polymeric film-forming binder.

3. The article of claim 2 in the form of a heat-sensitive copy sheet comprising a thin flexible backing with said coating extending over one entire major surface.

4. The copy sheet of claim 3 wherein the copper compound is present in an amount of at least about 1 gram per sq. meter and said nickel compound is present to the extent of about one-half to about 20 parts by weight for each 150 parts of said copper compound.

5. The copy sheet of claim 4 wherein the backing is paper.

6. The copy sheet of claim 4 wherein the backing is thin transparent plastic film.

7. The copy sheet of claim 4 wherein said nickel compound is nickel acetylacetonate.

8. The copy sheet of claim 4 wherein said nickel compound is nickel formate.

9. The copy sheet of claim 4 wherein said nickel compound is nickel trifluoromethanesulfonate.

10. The copy sheet of claim 4 wherein said nickel compound is nickelocene.

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Claims (9)

1. 2. The article of claim 1 wherein said coating includes a polymeric film-forming binder.
2. 3. The article of claim 2 in the form of a heat-sensitive copy sheet comprising a thin flexible backing with said coating extending over one entire major surface.
3. 4. The copy sheet of claim 3 wherein the copper compound is present in an amount of at least about 1 gram per sq. meter and said nickel compound is present to the extent of about one-half to about 20 parts by weight for each 150 parts of said copper compound.
4. 5. The copy sheet of claim 4 wherein the backing is paper.
5. 6. The copy sheet of claim 4 wherein the backing is thin transparent plastic film.
6. 7. The copy sheet of claim 4 wherein said nickel compound is nickel acetylacetonate.
7. 8. The copy sheet of claim 4 wherein said nickel compound is nickel formate.
8. 9. The copy sheet of claim 4 wherein said nickel compound is nickel trifluoromethanesulfonate.
9. 10. The copy sheet of claim 4 wherein said nickel compound is nickelocene.