US3477850A - Light sensitive heat erasable copying system - Google Patents

Description

United States Patent 3,477,850 LIGHT SENSITIVE HEAT ERASABLE COPYING SYSTEM Elliot Barman, Braintree, Mass., assignor to. Itek Corporation, Lexington, Mass., a corporation of Delaware No Drawing. Filed July 8, 1965, Ser. No. 470,579

Int. Cl. G03c 5/24 U.S. Cl. 96-48 7 Claims ABSTRACT OF THE DISCLOSURE A photographic process of recording an image pattern of activating radiation comprising exposing imagewise a copy medium comprising at least one member of the group consisting of the 6',8'-dinitro and the 5-chloro-6,8'- dinitro derivatives of l,3,3-trimethyl-spiro[(2H-1-benzopyran) -2,2-indoline] to form a visibleimage which is heat erasable.

The present invention relates to improved data storage systems and to methods of making the same. In particular, the present invention relates to improved data storage systems adaptable to the storage of binary-coded information in a novel data storage arrangement having colorless and colored states.

Data storage systems employing compounds capable of being converted between colored and colorless states are known in the prior art, for example, US. Patent 2,953, 454. For the most part, the systems employ'compounds whose stable or unexcited state at room'temperature (70 F.) is the colorless form. On the application of activating energy, usually ultraviolet light, the stable colorless state is converted to the activated colored form. Conversion from the colored to the colorless 'form, for purposes of erasing stored information, is usually effected in such prior art systems by irradiation of the colored form with light. However, a return to the stable colorless form will also occur simply with the passage of time because of the inherent instability of the colored form. The rate of this fading is variable depending on the compound employed.

The present invention relates to data storage systems employing materials for which the form stable at room temperature is the colored form. The colored form can be converted to a colorless or substantially colorless material by irradiation with visible light. The colorless form can only be reconverted to a colored state of high optical density by heating, and has a particularly desirable low rate of spontaneous conversion to the colored state in the absence of the application of heat. Irradiation of the colorless state with ultraviolet light only eifects a partial conversion to the stable colored state in the absence of heat.

The sensitivity of the system of the present invention to irradiation with visible light is of importance. Prior art systems, employing colorless compounds as the room temperature stable form, are obviously insensitive to visible light since the compounds transmit such light. For conversion to the colored state, prior art systems have always required irradiation with wavelengths outside the visible, usually ultraviolet light. The employment in the present invention of a system sensitive to visible light "ice permits the use of modulated laser beams for conversion of the colored compound to its colorless counterpart. As is known in the art, lasers permit a high concentration of energy in light beams of extremely small area in a fashion which is as yet impossible with ultraviolet wavelengths. Thus the data storage systems of the present invention are adaptable to many uses, in combination with laser beams, which must be excluded when employing systems sensitive only to ultraviolet light. Systems for writing with modulated laser light are disclosed in commonly owned copending patent application Ser. No. 423,- 801, for example.

Another feature of the systems of the invention is the necessity for heating the activated colorless or substantially colorless state above room temperature to return it to a stable colored form of an otpical density sufficient for easy discrimination between the colored and colorless forms. For such easy discrimination by the eye or by light-sensitive apparatus such as photomultiplier tubes, the colored and colorless states preferably should diifer in their absorbance by at least about ten fold ,e.g., preferably by a diiference in optical density between about 0.1 or less for the substantially colorless or colorless state and about 1 or greater for the colored state.

Subjecting the substantially colorless activated form of the compounds of the invention to radiation, such as ultraviolet radiation, for long periods of time at high intensities, but at room temperature, fails to convert the material to a colored form having an optical density greater than 1. That is, in the absence of heating at a temperature between F. and 200 F., the colored state induced by such irradiation is not sufl'iciently colored to discriminate it clearly from the colorless state. When the initial colored state has an optical density of 2 or more before decolorization by exposure to light, irradiation of the substantially colorless or colorless form at room temperature is compounds of the prior art to their colorless forms. Stated otherwise, the half-life of the activated colorless forms of the compounds used in the present invention is considerably greater than the half-life of the activated colored forms of compounds used in a number of data storage systems known in the prior art. Thus, the systems of the invention are more stable in their activated state than are those of the prior art.

The data storage systems of the present invention employ, as the compounds which can be converted from a colored to a colorless state by irradiation. with visible light, l,3,3-trimethyl-2-[(2-oxo 3,5' dinitrocyclohexadienylidene)-ethylidene]-indoline and its S-chloro substituted derivative. The unsubstituted compound is prepared, as described by Koelsch et al. in Journal of the American a 1 3 hemia ,-S99i tY.74 38 (1952) by condensing F schers base with 2-hydroXy-3,S-nitro-benzaldehyde:

The chloro compound is prepared analogously from the S-chlordsubstituted Fischers base, available commercially from CIBA.

Both of these condensation products are colored at room temperature, and can be classified as cyanine dyes. The materials have a Wine color, and on irradiation with visible light, for example light from a photofiood lamp,

are quickly converted to a colorless state of long half-life.

Any source of visible light can be employed for changing the state of the colored compounds. As mentioned earlier, the systems are particularly adaptable to use with laser beams, for example high energy (1 to watt) argon ion lasers. These lasers produce light with peak intensity at 4880A and about 5150A. However, alternative light sources, including sunlight, tungsten bulbs, xenon flash lamps, etc., can be employed.

For conversion from the unstable colorless or substantially colorless state to the colored state, the data storage systems of the invention are heated to temperatures above room temperature, namely to temperatures between about 100 C. and 200 C. Increasing the temperature within this range speeds the reversal reaction, but it is preferred to use lower temperatures within this range to prolong the life of the compounds employed.

To prepare a data storage medium according to the invention the dyes are suitably dispersed in a solvent. The color conversion reaction occurring in the compound occurs only when the compounds are in solution, but this term must be understood as including both liquid and solid solutions of the compounds. Solutions of the dyes can be dispersed in a liquid solvent according to the invention. However, most conveniently, the compounds of the present invention are dissolved in a liquid solvent together witha resinous binder, and are then cast as films or into other convenient shapes. Upon evaporation of the fluid solvent, the dye remains dispersed as a solute in the resinous binder as solvent.

For example, the dyes are conveniently dissolved in a liquid solvent together with a resinous material and are thenapplied to an opaque or pellucid (transparent or translucent) substrate. Since it is convenient to read stored information by transmission, the colored compounds of the invention are preferably cast on a pellucid base, such as of glass or plastic. Upon irradiation with visible light of sufiicient intensity, even by flashing for periods as short as microseconds, the colored compounds can be converted totheir colorless state. On heating to the temperatures earlier mentioned, an optically dense colored state is quickly regenerated.

- The presence or absence of color in such a data storage system can be employed as a binary code for the storage of information. Alternatively, the information may be in the form of pictorial images and can be stored in a data storage system of the present invention as a pictorial image.

A better understanding of the invention and of its many adv nta e wi l be hadv y re e ng t th l ow SP6.

cific examples given by way of illustration.

EXAMPLE 1 0.01 gm. of 1,3,3-trimethyl-2-[(2'-oxo-3',5'-dinitrocyclohexadienylidene)-ethylidene]-indoline was dissolved in 20 gms. of a 20% solution of polyvinylidene chloride (Saran F 220)in dimethylformamide. The resulting mixture was coated to a wet thickness of 2 mils on a transparent polyethylene terephthalate (Mylar) film.

The resulting red-purple colored film was given successive 30 second exposures tol'a wavelength of 520 millimicrons having an intensity of about 835 microwatts/cmP. Table I below shows the .absorbance or optical density of the material as a function of time.

TABLE I t (seconds) Absorbance O Q -3 1.40 30 1.16 0.94 0.76 0.625 0.515 210 0.375 Infinity 0.240

The reversion ofa bleached sample of this material to the colored state was measured as a function of time at a temperature of about 74 C. (about F.) by measuring the absorbance of the sample as a function of time at a wavelength of 540 millimicrons. Table IIbelovv shows the redevelopment of color in the sample as a function of time.

A thin film of polyvinylidene chloride containing 1,3,3- trimethyl-2[(2"-oxo 3',5 dinitrocyclohexadienylidene)- ethylidene]-5-chloroidoline on a polyethylene terephthalate substrate was prepared as in Example 1.

The conversion of the colored form of the dye in the resulting film to the bleached form was observed on exposure of the material to a Wavelength of 533 millimicrons at an intensity of about '835 microwatts/cm. Thirty-second exposureswere made. The results have been tabulated below in Table III.

' TABLE III t(seconds) Absorbance 0 1.74

450 0.51 Infinity 0.28

As in Example 1, measurements were made on the rate of conversion of the bleached form to the colored form at a temperature of about 74 C. (about-465 F.) by mounting the films in a spectrophotometer holder having a thermostat control. The results are reported in Table IV below.

A thin fil-m of the 5-chloroindoline compound of Example 2 was dispersed in polyvinylidene chloride on a polyethylene terephthalate carrier had an initial absorbance of 1.60 measured at 540 millimicrons. The film was rendered substantially colorless (absorbance=0.055) by exposure to a high intensity source of visible light (Sylvania Sun Gun).

The colorless film was then exposed to ultraviolet light at a wavelength of 366 millimicrons while maintained at room temperature (about 70 F.). The absorbance of the sample after 20 minutes of such exposure asymptotically approached a maximum value of about 0.450.

However, on heating for 15 minutes at 70 C. (159 F.) the samples absorbance reached a value of 1.75.

EXAMPLE 4 For the purpose of comparing the half-lives of the activated states of a typical prior art compound and a typical compound according to the present invention, solid solutions of (A) 1,3,3-trimethylindoline-5',7'-dichloro-6'- nitrobenzopyrylspiran (taught in US. Patent 3,100,778 as convertible from a stable colorless state to an activated colored compound) and (B) of the 1,3,3-trimethyl-2[(2- oxo 3'5 dinitrocyclohexadienylidene) ethylidene] 5- chloroindoline of the present invention, were prepared in polyvinylidene chloride in the form of a film on a pellucid carrier.

Sample (A) was exposed to ultraviolet light for a pro longed period until it had darkened to maximum absorbance. The absorbance of the sample was then measured at regular intervals at the absorption peak wavelength of 555 millimicrons with the same kept at 25 C. From the observed rate of decrease of absorbance, a half-life for the activated colored state of 18.5 hours at 25 C. was calculated on the basis of first order kinetics.

Sample (B), which was colored when prepared, was bleached by exposure to a high intensity source of visible light. With the sample thermostatically kept at 25 C., the rate of increase in the absorbance of the sample was measured by periodic observation at 540 millimicrons, the wavelength of peak absorption. A first-order half-life of 2.26 days was calculated for the activated uncolored state from the measurements.

Although the various films described in the specific examples have been made using polyvinylidene chloride as the solvent in the preparation of solid solutions, because the films using this substance are particularly easy to work with, other liquid and solid solvents including dimethyl formamide, methylene chloride, pyridine, polystyrene polymers, polycarbonate polymers (Lexan), butadiene-styrene copoly-mers, partially hydrolyzed polyvinyl acetate, etc., can be used according to the invention in the preparation of data storage systems.

What is claimed is:

1. A process of recording an image pattern of activating radiation comprising exposing imagewise a copy medium comprising a solution of at least one of 1,3.3-trimethyl-2-[(2 oxo 3,5' dinitrocyclohexadienylidene)- ethylidene]-indoline of the general structure:

H3O on,

N03 [ore-on: II 0 N0:

or its S-chloro substituted derivative, which exposure bleaches exposed portions of the copy medium.

2. A process as in claim 1 wherein the bleached image is erased by heating.

3. A process as in claim 2 wherein the heating is from a temperature of about 100 F. to a temperature of about 200 F.

4. A process as in claim 1 wherein the bleached image is stored at room temperature.

5. A process of recording an image pattern of activating radiation comprising exposing to an image pattern of visible light a copy medium comprising a solution of at least one of 1,3,3-t1imethyl-2-[(2-oxo-3,5'-dinitrocyclohexadienylidene)-ethylidene] -indoline or its S-chloro substituted derivative which forms a visible image which is heat erasable.

6. A process as in claim 5 comprising the additional step of heating the copy medium which erases the visible image previously formed by exposure to visible light.

7. A process as in claim 5 wherein the photosensitive compound is in a solid solution and wherein the imaged copy medium is stored at a temperature below about 100 F.

References Cited UNITED STATES PATENTS 3,134,674 5/1964 Brown. 3,242,122 3/ 1966 Cheng. 3,102,027 8/1963 Sprague et al. 96--89 OTHER REFERENCES Day, I. H.: Thermochromism, Chem. Rev., vol. 63, 1963, pp. 65-70.

Koelsch, C. F., et al.: Some Thermochromic Spirans, J. Am. Chem. Soc., vol. 74, 6288-89, 1952.

Berman, E.: Photochromic Spiropyrans, J. Phys. Chem., vol. 66, No. 11, Nov. 1962, p. 2275.

Heller, C. A. et al.: Photochromism, J. Phys. Chem. 65, 1908 (1961).

NORMAN G. TORCHIN, Primary Examiner R. E. FICHTER, Assistant Examiner US. Cl. X.R. 96-89,