US3028252A - Protective coating for facsimile prints - Google Patents

Description

United States Patent 3,028,252 Patented Apr. 3, 1962 3,028,252 PROTECTIVE COATING FOR FACSIMILE PRINTS Arthur H. Mones, Brooklyn, and Gerald Mandel, Flushing, N.Y., and Arthur W. Sperling, Emerson, N.J., assignors, by mesne assignments, to Hogan Faximile Corporation, a corporation of Delaware No Drawing. Filed July 26, 1957, Ser. No. 674,304 Claims. (Cl. 117-333) This invention concerns a coating for preventing discoloration of electrically recorded facsimile prints.

Electrolytic recording papers, such as described in US. Patents 2,339,267 and 2,358,839, have come into rather widespread use. These recording papers contain polyhydroxy aromatics, such as catechol, as the marking compounds. Graphic representations are recorded on such paper by passing electric currents representative of graphic copy therethrough via a positive electrode containing iron or other metal. The marks on the paper occur in areas where the metal has reacted with the marking compound. The recorded copy may be black on a white background or may include half-tones ranging through various shades of gray from white to black on the white paper recording sheet.

It has been found that such graphic copy tends to discolor with the passage of time. These changes occur as follows:

(1) The areas of the copy through which current has passed to form gray or black marks, turn brown.

(2) White areas of the paper through which electric current was not passed so that the paper remained unmarked, assume various shades of tan or light brown.

(3) All areas through which electric current has passed darken perceptibly, that is the percentage of reflectance decreases, or stated in another way, the optical density increases.

Each of these changes in the recorded copy is objectionable. When the copy represents reproduced photographs, the esthetic appearance of the copy is reduced. Also the reduction in contrast between dark and light areas in the copy reduces the general utility of the half-tone reproductions. As such copy ages high density marks which originally had different observable densities become substantially distinguishable in tone. In effect the tone scale range from white to black becomes compressed into the fewer discrete steps. Reproduction quality of the copy as pictures in newspapers and other publications is reduced.

It is desirable that the densities of the original marks remain constant or that the various White, gray, and black areas retain their same relative densities over long periods of time of the order of several years.

The invention is directed to reducing or eliminating the discoloration and darkening effects and to provide means for protecting the recorded copy against such effects. It is therefore a principal object of the invention to provide a coating which can be applied to the electrolytically recorded copy for inhibiting changes in color tone over long periods of time. It is a further object to provide a coating containing a protective agent which inhibits color degradation of electrolytically recorded facsimile copy. It is a further object to provide a preservative coating material for facsimile copy which contains a substituted compound containing a thiocarbonyl group as a protective agent.

A symbolic diagram of an embodiment of the coated and marked facsimile print is as follows:

Coating (ultraviolet light absorber, binder, and thiocarbonyl compound) Porous Sheet Paper It has been noted that if recorded copy containing catechol or a similar polyhydroxy phenolic compound is exposed to light, particularly short wave length ultraviolet light, the degradation of the copy is accelerated greatly.

Attempts have been made to inhibit the color tone degradation by washing the recorded paper. This method results in more stable copy but is objectionable because of the washing process required. Also the washed copy must be ironed fiat. During the washing the paper shrinks irregularly so that the copy becomes distorted. When the copy is washed the unmarked marking compound is dissolved out of the paper. Since the paper is more stable after washing this had led to the conclusion that some of the degradation is sube to subsequent oxidation of the unmarked catechol or its iron complex which may to an extent be water soluble.

It has been further noted that the browning and darkening is reduced when the copy is stored in an ambient atmosphere of carbon dioxide, helium, or vacuum. It has also been noted that once the copy is exposed to light, the degradation or color tones continues more rapidly after the copy is covered, than if the copy had not been exposed to light.

The color tone degradation may be attributed largely to oxidation of the marking compound. The process may be a light initiated oxidation which is propagated by a free radical mechanism common to phenolic oxidation reactions. Over-all the darkening of the copy appears to be caused by a combination of the degradation of catechol in unmarked areas which results in the brown products, and by the oxidation of the recorded marks which results in loss of contrast and general darkening.

Various ultraviolet light absorbers have been used in coating according to the invention and effectively inhibited in the color tone degradation, but when the ultraviolet light absorbers were used alone the copy still tended to develop a brown cast. Such ultraviolet light absorbers were used as benzo-phenone derivatives, as described in US. Patent 2,693,492.

Various coatings were made using compounds which are known free radical acceptors. These free radicals are odd electron species capable of combining with one another or with other free radicals to form more stable electron pair bonds. The free radicals can initiate formation of other free radicals by removing an electron from another species. When these reactions proceed in succession with regeneration of the reactive species they constitute chain reactions. Since the degradation process appears to be at least in part a free radical mechanism, coatings have been made including various free radical acceptors. They may be acrylamide, acrylic acid, acrylonitrile and the like. These acrylics result in moderate inhibition of copy degradation.

Acrylamide can react with a free radical to become a radical having the form:

The radical thus produced can then successively react with other acrylamide molecules to form progressively a long chain molecule, i.e., polyacrylamide. When acrylamide is incorporated in a coating for facsimile copy containing catechol an inhibition effect attributable to the acrylamide has been noted. However over a period of time a brownish cast does develop. Similar results were obtained with the other acrylics mentioned.

Various antioxidants were incorporated in coatings and applied to recorded paper on the theory that they would prevent oxidation of the catechol to catechol complexes at the unmarked areas of the paper. They were found to be ineffective in preventing copy degradation. Among the antioxidants which were tested and found ineffective in this particular system were the following:

(1) Propyl gallate (2) Butylated hydroxy anisole (3) Butylated hydroxy toluene (4) Hydroquinone (5) Di-tert, butyl para cresol It was finally discovered that the most effective color tone inhibitors were coatings containing substances having in common the ability to form disulphide linkages, particularly substituted compounds containing a thiocarbonyl radical. This is a bivalent radical or group of the form C=S.

When a coating is applied to recorded paper including such a compound the increase in density (or darkening) of both marked and unmarked areas of the recorded print is reduced to negligible amounts.

It has been known heretofore that a small amount of thiourea which is a thiocarbonyl derivative may be incorporated in an impregnant containing catechol to stabilize the paper before marking occurs. This small amount of thiourea does not interfere with the electrolytic marking process and is principally used for stabilizing the paper in storage before marking. After marking the paper containing thiourea exhibits the degradation effects described. If the quantity of thiourea in the impregnant is increased the initial density of the recorded marks is reduced and a distinct purple cast is imparted to the copy which is undesirable. Furthermore the copy containing high concentration of thiourea does turn brown in time. If the thiourea or other thiocarbonyl derivative is dissolved in a solvent with a binder a solution is obtained which can be applied as a coating to either the printed side of the facsimile print or less preferably to the reverse side of the facsimile print. This coating when dry provides an effective means for inhibiting the tendency of the print to darken in time, whether exposed to light or not. If the print is to be exposed to light it is preferred that the coating be applied to the printed side of the copy and an ultraviolet light absorber compound is preferably added to the coating solution. Preferred formulations for the coating solution are:

Ultraviolet light absorber compound gram 0.25

Optimum effects according to the invention are obtained by use of thiourea as the protective agent. However other thiocarbonyl derivatives provide satisfactory equivalent results. These thiocarbonyl compounds have the general form i Ra-C-N where R and R can be hydrogen or hydrocarbons and derivatives thereof, and R can be hydrogen,

or hydrocarbons and derivatives thereof.

In these compounds identified as thiocarbonyl derivatives, C=S is the thiocarbonyl group. It is believed that the thiocarbonyl group is the active agent in the protective mecahnism. Substituted compounds in general containing such a group prove effective according to the invention.

Other effective thiocarbonyl derivatives have the form:

8 R4i 3Rs where R; and R are substitution groups.

Such compounds which may be specifically mentioned are thiourea, allyl thiourea, phenyl thiourea, tetramethyl thiourea, thioacetamide, dithiobiuret, and dithiooxamide. The criteria for useful compounds are that they are essentially colorless in the concentrations used, have low volatility and are themselves reasonably stable to reactions other than those including the protective mechamsm.

The quantities of the protective agent specified in the above formulations are preferred but are not critical. The effectiveness of the coating solution depends to a large extent on the thickness of the resulting dried coating and the concentration of the protective agent. Thus a thick coating with a low concentration of protective agent may be as effective as a thin coating containing a high concentration of the protective agent.

In general a solution containing 0.03 mole of the protective agent applied in a very thin coating has been found to provide a minimum acceptable inhibiting effect for darkening of color tones of a facsimile print. Higher concentration may be used up to the maximum quantity of the protective agent soluble in the carrier solvent. For reasons of economy and convenience about .06 mole of the thiocarbonyl protective agent in cc. of solution may be taken as a preferred concentration in formulation of a solution according to the invention. The higher concentrations of protective agent may impart a purple or other colored cast to the recorded facsimile print which may not be desirable.

The binder used in the formulation of the solution must be a substance having good adhesive properties with respect to the paper base of the print. It should be substantially colorless. In addition it should serve as a spreading agent for the protective material and finally should have some light absorbing qualities. Suitable binders are rosin, glyceryl phthalate, and polyvinyl aceate.

The solvent may be any material in which the thiocarbonyl derivatives and the binder will dissolve. The solvent should volatilize rapidly without shrinking the paper or weakening its fibrous structure. Suitable solvents are acetone, methanol, etc.

The ultraviolet light absorber should be a substantially colorless material fully soluble in the solvent for the other ingredients in the solution. Suitable ultraviolet light absorbers are benzophenone derivatives (such as described in Patent 2,693,492) and cinnamalazine.

The coating is applied as a liquid to the recorded paper and allowed to dry. Only one side of the recorded sheet (preferably the printed side) need be coated. The dried coating is then efiective to inhibit color tone degradation where exposed to light for many months and even years.

The effectiveness of the coatings according to the invention have been verified by accelerated aging tests done under intense light conditions as well as by natural aging tests in bright sunlight over many months.

It has been suggested that a free radical interceptor mechanism may be significant in producing the protection effect desired. However certain compounds known to oxidize by free radical mechanism do not inhibit degradation, such as the phenolics and hydroquinone above mentioned. The reason for their ineffectiveness is not known but it may be a function of the conditions present in the system.

The marked effectiveness of thiourea, and other thio carbonyl derivatives in preventing oxidation of the cate chol and catechol complexes is believed due to reaction of the thiocarbonyl radicals with the free radicals produced in the recorded paper in the course of degradation. The efiectiveness of the thiocarbonyl compounds may be due to their ability to form disulphide linkages. For example, thiourea is oxidized to:

The mechanism of the protective process is believed to involve the sulphur in the molecules. It is known too that thiocarbonyls are capable of complexing metallic ions and this property may play a part in the protective mechanism.

The binder, such as rosin, used in the coating acts as a spreading agent and to some extent acts as a light absorber, thus in the presence of the hinder the number of photons reaching the copy per unit time is decreased. The number of active molecules (radicals) formed per unit time from photon interaction is reduced and the protective agent available is better able to cope with the smaller quantity of active molecules that are produced.

The ultraviolet light absorber is especially useful if the facsimile print is exposed to sunlight, daylight or ultraviolet light. It supplements the light absorbing effect of the rosin.

What is claimed is:

1. An electrolytically marked black and white facisimile print coated after marking and drying with a transparent protective agent selected from the group of thiocarbonyl derivatives consisting of thiourea, thioacetamide, allyl thiourea, tetramethyl thiourea, phenyl thiourea, dithio- & Q Wa /a) biuret and dithiooxamide and a binder adhering said agent to said print, said agent being dispersed in said bnder.

2. In an electrolytically marked black and white facsimile print comprising a substantially white porous sheet impregnated with a polyhydroxyphenolic compound combined with a metal in selective areas of the sheet forming marks thereon ranging in tone from white to black, a transparent coating on at least one side of the said sheet, said coating having been applied after the print was marked and containing protective agent for inhibiting color tone degradation of the marks and darkening of white areas of the sheet, said agent being a thiocarbonyl derivative selected from the group consisting of thiourea, thioacetamide, allyl thiourea, tetramethyl thiourea, phenyl thiourea, dithiobiuret, and dithiooxamide, and a binder for adhering protective agent to the sheet.

3. An electrolytically marked black and white facsimile print coated after marking and drying in accordance with claim 2 and wherein the binder is selected from the group consisting of rosin and polyvinylacetate.

4. In an electrolytically marked black and white facsimile print comprising a substantially white porous sheet impregnated with a polyhydroxy phenol compound combined with a metal in selective areas of the sheet forming marks thereon ranging in tone from white to black, a transparent coating on at least one side of said sheet, said coating having been applied after the print was marked and containing a protective agent for inhibiting color tone degradation of the marks and darkening of white areas of the sheet, said agent being a thiocarbonyl derivative selected from the group consisting of thiourea, thioacetamide, allyl thiourea, tetramethyl thiourea, phenyl thiourea, dithiobiuret, and dithiooxamide, a binder for adhering said protective agent to the sheet, and an ultraviolet light absorber compound.

5. An electrolytically marked black and white facsimile print coated after marking and drying with a transparent protective agent consisting of an ultraviolet light absorber and a thiocarbonyl compound selected from the group consisting of thiourea, thioacetamide, allyl thiourea, tetramethyl thiourea, phenyl thiourea, dithiobiuret and dithiooxamide; and a binder adhering said agent to said print, said agent being dispersed in said binder.

References Cited in the file of this patent UNITED STATES PATENTS 1,958,683 Stand May 15, 1934 1,973,488 Kocher Sept. 11, 1934 1,977,642 Moss Oct. 23, 1934 2,339,267 Hogan et al Jan. 18, 1944 2,358,839 Wagner Sept. 26, 1944 2,440,070 Blout et al Apr. 20, 1948 2,440,202 Klaber Apr. 20, 1948 2,546,724 Coe Mar. 27, 1951 2,555,321 Dalton et al. June 5, 1951 2,568,894 Mackey Sept. 25, 1951 2,625,494 Morrison Jan. 13, 1953 2,749,253 Shoemaker et a1. June 5, 1956 2,876,210 Wynn et al. Mar. 3, 1959 UNITED STATES PATENT OFFICE CERTIFICATE OF CGRREETIGN Patent N0o 3 O28 252 April 3 1962 Arthur Ho Mones et al0 It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read,as corrected below Column 1 line 441 for distinguishable -read indistinguishable column 2 line 26 for subev read due line 47, strike out "in"; column 4 line 31 for "mecahnism" read mechanism -g column line 2, for "bnder" read binder Signed and sealed this 17th day of July 1962o (SEAL) Attest:

ERNEST w. SWIDER DAVID L D Attesting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,028,252 April 3, 1962 Arthur H. Mones et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should reedvas corrected below.

Column 1 line 44. for "distinguishable" read indistinguishable column 2, line 26 for "sube" read due line 47, strike out "in"; column 4, line 31, for "mecahni sm" read mechanism column 6, line 2. for "bnder" read binder Signed and sealed this 17th day of July 1962.

(SEAL) Attest:

ERNEST w. SWIDER DAVID L i ing Officer Commissioner of Patents

Claims (1)

1. AN ELECTROLYTICALLY MARKED BLACK AND WHITE FACSIMILE PRINT COATED AFTER MARKING AND DRYING WITH A TRANSPARENT PROTECTIVE AGENT SELECTED FROM THE GROUP OF THIOCARBONYL DERIVATIVES CONSISTING OF THIOUREA, THIOACETAMIDE, ALLYL THIOUREA, TETRAMETHYL THIOUREA, PHENYL THIOUREA, DITHIOBIURET AND DITHIOOXMAIDE AND A BLINDER ADHERING SAID AGENT TO SAID PRINT, SAID AGENT BEING DISPERSED IN SAID BLINDER.