NO794130L - CHROMOGENT MATERIALS AND PROCEDURES FOR ITS MANUFACTURING - Google Patents

Description

The present invention relates to chromogenic material, methods for its production and pressure-sensitive copying material containing such a chromogenic material.

The color-forming systems used in pressure-sensitive copying material generally employ a substantially colorless chromogenic material or compound, a color developer capable of reacting with the chromogenic material to develop a color, and a solvent in which the color-forming reaction can happen. The reactive components of the color-forming system are kept apart until the system is to be used, and this is normally achieved by microencapsulating a solvent solution of one of them, usually the chromogenic material. When the copying material is to be used, an application of pressure will cause the microcapsules to burst and release the solvent solution. This means that the colour-forming components come into reactive contact and a colored image is developed which exactly corresponds to the print pattern. In this way, pressure-sensitive copying material can be used to provide copies without the need to use blue paper.

There are usually two types of systems used

in pressure-sensitive copying material. In the one type which is usually referred to as a self-supporting copying system, the copying material will consist of a sheet which has a coating of a color developer mixed with a microencapsulated solution of a chromogenic material. Alternatively, the color developer and the microencapsulated chromogenic solution can be dispersed within the sheet itself.

In the second type, which is referred to as a transfer copying system, at least two copying sheets or materials are used. In the normal and most commonly used transfer copying system, one sheet will have a coating of a microencapsulated chromogenic solution (the CB sheet), while the other sheet will have a coating of a color developer (the CF sheet).

In the reverse transfer copying system, it will be the color developer and not the chromogenic material that is microencapsulated as a solution. Consequently, one sheet in such a system will have a coating of a microencapsulated color developer solution (the CB sheet), while the other sheet has a coating of a water-insoluble, essentially neutral pigment in which the chromogenic material is adsorbed. The CB and CF sheets are put together in sets where the different coatings lie next to each other so that the transfer of the solution can take place from the CB sheet to the CF sheet.

If additional copies are desired, the set can also contain a third sheet which has a CB coating on one side and a CF coating on the other. One or more of these sheets (CFB sheets) are placed between the CF and CB sheets in the set and where each CB coating is in contact with a CF coating.

In order to obtain an image with special optical properties, different chromogenic materials can be used in combination. One of the most common known compounds in this regard is crystal violet lactone (CVL;|. 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide) which is used in combination with other chromogenic materials or compounds to thereby obtain a blue or black image. However, crystal violet lactone has a poor resistance to bleaching, i.e. poor ability to resist color dilution of the produced image. Furthermore, after being exposed to light, crystal violet lactone will show a significant weakening with respect to color-forming reactivity, i.e. its ability to produce an image of acceptable intensity upon reaction with a color developer. This weakening or lowering of reactivity, which occurs whether the material is encapsulated or not, and which is often referred to as a CB weakening in connection with normal transfer copying systems, can also be greater if the material is used as a chromogenic solution in certain solvents such as 2,2, 4-trimethyl-1,3-pentanediol-di-i-butyrate (TXlB). Despite these disadvantages, crystal violet lactone has been widely used because there has been no suitable alternative.

It is an aim of the present invention to provide a chromogenic material which can completely or partially replace crystal violet lactone and which has improved resistance to bleaching and improved ability to withstand a weakening with regard to reactivity.

The present invention provides a chromogenic material with the following formula

In addition to having improved resistance to bleaching and an improved ability to resist a weakening with respect to reactivity, the chromogenic material according to the present invention, which is hereinafter referred to as pyridyl blue, also has an ability to rapidly color and which also gives a blue color with excellent intensity and stability. Furthermore, the compounds have very little reactivity with respect to the paper itself. This is important, because when used in a normal transfer system there is minimal or no CB discoloration, i.e. a discoloration caused by a reaction between the paper itself in the CB sheet and any chromogenic solution that has not been transferred.

Either of the above two isomers can be used as the chromogenic compound, although it is preferred to use the pyridine-5-one isomer. In practice, however, it is more appropriate to use a mixture of isomers where the pyridine-5-one isomer predominates. The reason for this is that a method used for the production of the chromogenic material produces a mixture of this type, and because the subsequent separation of the isomers in a commonly known manner is difficult, time-consuming and expensive.

The present invention also provides a method for producing the above-defined chromogenic compounds which includes reacting a keto acid with the following formula: with a compound of the formula:

wherein one of the groups A and B is 4-diethylamino-2-ethoxyphenyl, while the other is 1-ethyl-2-methylindol-3-yl, in the presence of a dehydrating agent.

A suitable type of dehydrating agent is an acid anhydride such as acetic anhydride, and the reaction is preferably carried out at an elevated temperature, e.g. from 65-75°C.

When group A is l-ethyl-2-methylindol-3-yl, the keto acid can be prepared by reacting quinolinic anhydride with l-ethyl-2-methylindole, and this reaction is usually carried out at temperatures from 60-70°C.

When A is 4-diethylamino-2-ethoxyphenyl, then the keto acid can be prepared by reacting quinolinic anhydride with an N,N-diethyl-m-phenetidine. This reaction is usually carried out in a solvent in the presence of a catalyst such as aluminum chloride, and at temperatures from 35-40°C.

It is preferred that A is 1-ethyl-2-methylindol-3-yl, as the keto acid can then be prepared without using a solvent and a catalyst. Furthermore, the resulting two-step process will give a greater amount of the pyridine-5-one isomer than the two-step process where A is 4-diethylamino-2-ethoxyphenyl. In the preferred method, it is possible to prepare a mixture in which the pyridin-5-one isomer constitutes at least 90% or even 95% of the mixture.

The present invention also provides another method for producing the above-defined chromogenic compounds which involves reacting quinolinic anhydride, 1-ethyl-2-methylindole and N,N-diethyl-m-phenetidine in the presence of a dehydrating agent, e.g. of the type mentioned earlier.

However, this method is less preferred.

The chromogenic material according to the present invention can of course be combined with other chromogenic compounds to thereby achieve a particular intensity and color. The resulting combination will be advantageous in that it includes a chromogenic compound of the present invention, as this gives the combination a better ability to resist loss of reactivity. Examples of other compounds that can be used are one or more of the following: 3,7-bis(dimethylamino)-10-benzoyl-phenothiazine-(benzoyl-leuco-methylene blue, BLMB) 2<1->anilino-6<1 ->diethylamino-3'-methylfluorane (N102)-3,3-bis(1-ethyl-2-methylindol-3-yl)-phthalide (indolyl red); 3,3-bis(1-butyl-2-methylindol-3-yl)phthalide; spiro-7-chloro-2,6-dimethyl-3-ethylaminoxanthene-9,2-(2H)naphthol-(1,8-bc)furan; 7-chloro-6-methyl-3-diethylaminofluorane; 3-diethylaminobenzo(b)fluorane; 3-(4-diethylamino-2-ethoxy)-3-(2-methyl-1-ethylindol-3-yl)phthalide; 3-(4-diethylamino-2-butoxy)-3-(2-methyl-1-ethylindol-3-ylphthalide and 3,7-bis(diethylamino)-10-benzoyl-benzoxazine. In addition, the chromogenic material "according to the present invention is used together with crystal violet lactone or as a substitute for it.

In a self-supporting or normal transfer copying system, the chromogenic material according to the present invention can be used alone or in combination as a microencapsulated chromogenic solution. Examples of suitable organic solvents that can be used include de-alkyl phthalates where the alkyl group has from 4-13 carbon atoms, such as dibutyl, dioctyl, dinonyl and ditridecyl phthalates, 2,2,4-trimethyl-1,3-pentanediol-di-i-butyrate ( TXIB, US patent 4027065); ethyldiphenylmethane (US patent 3996405); alkylbiphenyls such as mono-i-propylbiphenyl (US patent 3627581); C 1 -C 3 -alkyl benzenes such as dodecylbenzene, diaryl ethers such as diphenyl ether, di(aralkyl) ethers such as dibenzyl ether and aryl aralkyl ethers such as phenyl benzyl ether; liquid dialkyl ethers having at least 8 carbon atoms, liquid alkyl ketones having at least 9 carbon atoms; alkyl or aralkyl benzoates such as benzyl benzoate, alkylated naphthalenes, partially hydrogenated terphenyls. Preferred solvents include ethyl diphenylmethane and 2,2,4-trimethyl-1,3-pentanediol-di-i-butyrate.

These solvents, which are essentially odorless, can be used alone or in combination. They can also be used together with a diluent to thereby reduce their price. Of course, the diluent must not be chemically reactive either with the solvent or any chromogenic material and must at least be partially miscible with the solvent to give a single phase. The diluent must be used in a sufficient amount to obtain an economic advantage without impairing the solubility of the chromogenic material.

Such diluents are already known, and

a preferred example is Magnaflux oil which is a mixture of saturated aliphatic hydrocarbon oils with a distillation range of from 160-290°C.

The amount of chromogenic material used in the chromogenic solution will of course depend on the requirements, but generally an amount of between 0.6 and 3% by weight will be used (i.e. parts by weight of the chromogenic material per 100 parts by weight of the solvent).

Microencapsulation of the chromogenic solution can be carried out in any way, e.g. by using gelatin as described in US patents 2800457 and 3041289, by using a urea-formaldehyde resin as described in US patents 4001140, 4087376 and 4089802 and by using various melamine-formaldehyde resins as described in US patent 4100103.

Color developers suitable for use with chromogenic compounds according to the present invention are well known and include acid clays and oil-soluble metal salts of phenol-formaldehyde novolak resins of the type described in US patents 3672935, 3732120 and 3737410. A preferred example of a suitable resin is a zinc-modified, oil-soluble phenol-formaldehyde resin, such as the zinc salt of a p-octylphenol-formaldehyde resin or the zinc salt of a p-phenylphenol-formaldehyde resin.

In an opposite transfer system, the chromogenic material according to the present invention will not be microencapsulated as a chromogenic solution, but will be adsorbed in a water-insoluble, essentially chemically neutral pigment, as described in British patent 1337924. The color developer, which can be a resin - color developer

however, as noted above, is microencapsulated as a soln. Suitable solvents and encapsulation processes include those described above.

The present invention also provides a pressure-sensitive copying material which contains as a component in its color-forming system the above-defined chromogenic compounds.

Compositions for coating as well as methods for producing pressure-sensitive copying material are well known, see e.g. British patent 1337924 and US patents 3627581, 3775424 and 3853869.

The following examples illustrate the invention, and all parts are parts by weight.

Example 1 - Production of chromogenic material

Quinolinic anhydride (0.21 mol) and 1-ethyl-2-methyl-indole (0.33 mol) were mixed in a reaction flask at temperatures between 65 and 70°C for 3 hours. The mixture was then cooled and then washed with benzene or chlorobenzene to give (1-ethyl-2-methylindol-3-yl)(3-carboxypyridin-2-yl)-ketone and its isomer (0.19 mol.

(1-ethyl-2-methylindol-3-yl)(3-carboxypyridin-2-yl)-ketone and its isomer (total 58.0 g, 0.188 mol) were stirred for 2 hours at 60-65°C with 35.3 g (.0.188 mol) of N,N-diethyl-m-phenetidine and 250 ml of acetic anhydride. The reaction mixture was poured into 500 ml of water, and said acetic anhydride was slowly hydrolysed by adding 29% ammonium hydroxide (450 ml). After stirring for 2 hours, the resulting solid was filtered off and washed with water, 200 ml of 40% methanol/water and 50 ml of petroleum ether (boiling point 60-110°C). The solid was then dried to constant weight in an oven at 75°C to give a mixture (9:1 respectively) of the pyridine-5-one compound and the pyridine-7-one compound (80.5 g, 90%, melting point 134-137°C).

Example 2 - Production of chromogenic material

A mixture of (1-ethyl-2-methylindol-3-yl)(3-carboxypyridin-2-yl)ketone and its isomer (total 3.62 g, 0.0117 mol) and 2.26 g (0.0117 mol) N,N-diethyl-m-phenetidine was stirred and for 4 hours kept at 60°C with 17 ml of acetic anhydride. The mixture was poured into 150 ml of water containing 18.4 g of sodium hydroxide. The hot reaction mixture was stirred vigorously for one hour and then filtered. The resulting product was washed several times with 30% methanol/water (10 mL) and finally with petroleum ether (bp 60-110°C). A mixture was obtained (20:1 respectively)

of the pyridine-5-one compound and the pyridine-7-one compound (5.66 g, m.p. 148-150°C).

Example 3 - Production of chromogenic material

A mixture of 2'-carboxypyrid-3'-yl-(4-diethyl-amino-2-ethoxyphenyl)ketone (3.75 g, 0.01096 mol), 1-ethyl-2-methylindole (1.74 g, .0.01096 mol) and 35 ml of acetic anhydride were kept at 75°C for 5 hours. The reaction mixture was poured into 250 ml of water containing 38.8 g of sodium hydroxide. After stirring for several hours, the product was filtered off. It was washed several times with water and finally with petroleum ether. The chromogenic material was obtained in a yield of 3.37 g, 65%, and thin layer chromatography on silica showed the presence of only a very small amount of the pyridine-7-one compound.

The reaction occurs in a similar way when using mixed keto acid isomers, but the resulting product then contained more of the pyridine-7-one compound.

Example 4 - Preparation of chromogenic solutions

A number of chromogenic solutions were prepared by mixing the following components in the indicated parts by weight:

Example 5 - Preparation of microencapsulated chromogenic solutions

Each of the chromogenic solutions 1-13 was microencapsulated using the method described in US patent 4001140. This method is briefly described as follows: 180 parts of the chromogenic solution were emulsified in a mixture of 35 parts 10% EMA 31 (ethylene-maleic anhydride copolymer with a molecular weight ranging from 75,000 to 90,000, manufactured by Monsanto Chemical Co.) in water, 32 parts 20% EMA 1103 (ethylene-maleic anhydride copolymer with a molecular weight from 5,000 to 7,000, manufactured by Monsanto Chemical Co. .) in water, 133 parts water, 10 parts urea and 1 part resorcinol, adjusted to pH 3.5. After emulsification, 29 parts of 37% formaldehyde were added,

and the mixture was stirred at 55°C. After two hours, the mixture was allowed to cool to room temperature with stirring.

Example 6 - Preparation of microcapsule compositions

Each of the microcapsule portions 1 to 13 was processed into compositions for coating paper, using the materials and parts indicated below:

Example 7 - Production of CB copying material

Each of the coating compositions 1 to 13 was dispersed, applied to a paper sheet and drawn with a No. 12 wire-round coating rod and the coatings were dried using hot air.

Example 8 - Comparative tests on CB attenuation

Each of the CB copying materials 1 to 13 was subjected to a typewriter intensity test in which a standard pattern was written on a set consisting of a CF and a CB sheet (the CF sheet in this sample was coated with a zinc-modified phenolic resin as described in US patents 3732120 and 3737410). One colored image corresponding to the writing pattern on the CF sheet and the intensity of this image was determined using an opacimeter.

The intensity is a measure of the color development and the ratio of reflection from the colored image in relation to the paper where there was no image (I/Io) was expressed as a percentage. A high value indicates that there was little color development, while a low value indicates that there was strong color development.

Such intensity tests were carried out before exposing the CB sheet to irradiation with fluorescent light and after one and two hours of exposure of the CB sheets to such irradiation. In all cases, the intensity measurements were carried out 20 minutes after the image was formed.

The device used for the fluorescent light tests consisted of a light box containing 18 daylight fluorescent lamps (53.3 cm long and 13 nominal lamp watts) placed vertically on central bases whose diameter was 2.5 cm. The CB sheets were placed from 2.5 - 3.8 cm from the lamps.

The results obtained with the 13 CB copy sheets are listed below:

The degree of CB attenuation that is accepted is of course dependent on the duration of light exposure on the CB sheet. After a 2 hour exposure, a loss of more than 26 units would be too high. Thus, the copy material 2, 3, 4, 6, 7, 10, 11 and 13 which contained pyridyl blue thus gave sufficiently little CB attenuation for them to be accepted. The replicates 1, 5, 8, 9 and 12 were controls without pyridyl blue, and showed too high a degree of CB attenuation.

Example 9 - Preparation of chromogenic solutions

A number of chromogenic solutions were prepared by mixing the following components in the indicated parts:

Example 10 - Preparation of microencapsulated chromogenic solutions

Each of the chromogenic solutions 14 to 16 was microencapsulated using the method described in US patent 4100103. The procedure is as follows:

5400 parts of a chromogenic solution were emulsified

in a mixture of 10% EMA 31 in water and 5600 parts water adjusted to pH 3.7. The pH of the emulsion was then adjusted to 4.0. A mixture of 1000 parts 10% EMA 1103 was added to the emulsion

in water adjusted to pH 4.0, 1000 parts water and 1000 parts Resimene 714. The mixture was then held at 55°C for 2 hours. It was then set aside to cool to room temperature while stirring.

Example 11 - Preparation of microcapsule composition for application

Each of the microcapsule portions 14 to 16 was made up into a coating composition using the materials indicated below:

Sufficient water was added to obtain a composition containing 17.4% solids.

Example 12 - Production of CB copying material

Each of the compositions 14 to 16 was dispersed and coated on a paper using an air knife coater and then dried.

Example 13 - Comparative tests of CB attenuation The CB copy material 14 to 16 was subjected to the same tests as described in Example 8.

The results obtained are as follows:

Using the acceptance guidelines set forth in Example 8, it was found that materials 14 and 15 containing pyridyl blue exhibited excellent resistance to CB deterioration, while the control without pyridyl blue, i.e., material 16, gave unsatisfactory results .

Example 14 - Preparation of pyridyl blue CF copying material for use in a reverse transfer system

(a) 1 g of pyridyl blue was dissolved in 150 ml of acetone. 70 g of precipitated calcium carbonate, 20 g of Cabolite 100 urea-formaldehyde resin pigment (US patent 3988522) and 10 g of zinc oxide (Green Seal 8 from New Jersey Zinc Co. in the USA) were then added to the solution and the dispersion was set aside for drying. (b) The product from step (a) was worked up into a coating composition using the following ingredients:

paper sheet (15.42 kg) with a No. 12 Mayer rod and then dried. The weight of the dry coating was approx. 2.04 kg per rice of 500 sheets that measured 63.5 x 96.5 cm.

Example 15 - Preparation of CVL CF copying material for use in a reverse transfer system The procedures of steps (a), (b) and (c) of Example 14 were repeated except that crystal violet lactone was used instead of pyridyl blue.

Example 16 - Alternative method for producing pyridyl blue CF copying material

(a) 300 g of pyridyl blue, 600 g of calcium carbonate, 300 g of a Penford gum 230 with 10% solids (modified corn starch), 30 g of Tamol 731 with 25% solids and 1200 g of water were ground in a mill in 45 minutes together with a few drops of octanol to reduce foaming. (b) The product from step (a) was worked up into a coating composition using the following ingredients. (c) The resulting composition was coated onto a paper weighing 15.42 kg using an air knife coater and then dried. The dry coating weighed approx.

2.04 kg per rice of 500 sheets that measured 63.5 x 96.5 cm.

Example 17 - Alternative preparation of CVL CF copying material

The procedures from steps (al, (b) and (cl i

Example 16 was repeated except that crystal violet lactone was used instead of pyridyl blue, and 300 g of calcium carbonate was replaced in step (al) with 300 g of zinc resinate.

The solids in the composition as well as the viscosity

the peak was 27% and 57 eps respectively.

Example 18 - Production of acid resin CB and CFB sheets

(al 1200 g of p-phenylphenol resin (PPP resin) was

dissolved in 3200 g of dibenzyl ether and 1600 g of Magnaflux oil by heating and stirring. EMA 31 (ethylene-maleic anhydride copolymer with a molecular weight from 75,000 to 90,000,

200 g) was dissolved in 1800 g of deionized water using heat and stirring. The resulting EMA solution was diluted with 6000 g of deionized water and pH adjusted to 4.0 with 20% sodium hydroxide solution. 01jedissolution of PPP resin

It was then emulsified in the EMA solution using a Cowles dissolver at 25°C. The emulsification was continued until an average oil droplet size of approx. 2 microns and a total particle size distribution ranging from 0.5 to 15 microns.

The resulting emulsion was then transferred to a water bath

at 55°C and with rapid stirring, 1000 g of 80% Resloom 714 (etherified methylol-melamine diluted with 1000 g of deionized

hard water. The mixture was kept at 55°C for 2 hours with constant stirring to produce a capsule formation. After 2 hours, the mixture was allowed to cool to room temperature. Stirring was continued for a further 16 hours.

(bl) The microencapsulated solution of PPP was then worked up into an application composition using the following ingredients.

(c) The resulting composition was coated on a paper whose weight was 15.42 kg using an air knife coater and then dried. The dry weight of the e-coating was approx. 1.7 kg per rice of 500 sheets that measured 63.5 x 96.5 cm.

In addition to this, the composition was coated on the reverse side of a copy sheet prepared as described in Examples 16 and 17 to thereby prepare CFB sheets.

Example 19 - Preparation of alternative acid resin CB sheet (a) An oil solution of p-octylphenol resin (1400 g, POP) in 3200 g of dibenzyl ether and 1600 g of Magnaflux oil was prepared by means of sufficient heat and stirring. The oil solution was then microencapsulated using the method described in step (a) of Example 18. (b) and (c) The procedure in steps (b) and (c) of Example 18 was repeated, except that used 27.30 (instead of 26.60) parts by weight of the POP stage capsules and that the composition was only used to prepare CB sheets.

Example 20 - Comparison of pyridyl blue and CVL copying material

Pyridyl blue and crystal violet lactone copying material were tested using the aforementioned typing intensity test where a standard pattern was written down on a copying set consisting of a CF and a CB sheet, optionally with an intermediate CFB sheet. A colored image was thus produced which corresponded to the print pattern on the CF sheet or on the CF side of a CFB sheet, and the intensity of the image was determined using an opacimeter.

The intensity is a measure of the color development and the ratio of reflection from the written image and that in an area without an image (I/Io) was expressed as a percentage. A high value indicates little color development, while a low value indicates strong color development.

To compare pyridyl blue and the CVL copying material, the following intensities were measured: A - The original intensity, i.e. the intensity of a

image 24 hours after development,

B - The intensity of an image after it had been exposed to:

(i) fluorescent lights

(ii) natural sunlight

(iii) room temperature or

(iv) inside a furnace,

C - The intensity of an image produced on a sheet as on

was previously exposed to:

(i) fluorescent lights

(ii) natural sunlight

(iii) room temperature or

(iv) inside a furnace.

The device in which the fluorescent light tests were carried out was a light box containing a total of 18 daylight fluorescent lamps (53.3 cm long and with 13 nominal lamp watts) vertically placed on a central base with a diameter of 2.5 cm. The sheets with or without image were placed 2.5 to 3.8 cm from the lamps for 48 hours. Image formation was carried out using an IBM Executive typewriter.

Exposure of the sheets with or without an image to natural sunlight was carried out by placing them in a south-facing window for 48 hours. The imaging was done with an IBM Memory typewriter by using the letter X.

Exposure of the sheets with or without image to room temperature conditions was carried out by hanging the sheets on a laboratory wall for 7 and 9 weeks, where the sheets were exposed to air, natural and fluorescent room light as well as air temperature and normal humidity. Image formation was carried out with an IBM Selectric typewriter using a type that produced a square.

Exposure of the sheets inside an oven was carried out for 3 weeks at 60°C. The image formation was here carried out using an IBM Executive typewriter.

The following results were obtained:

It appears from the preceding examples, as examples 8, 13 and 20, that the chromogenic material according to the present invention shows a significant improvement compared to crystal violet lactone. In particular, an image produced using the present material has far better resistance or resistance to bleaching, and the material itself is far less sensitive to light and therefore retains its reactivity significantly longer. In opposite transfer systems, it also turns out that copying materials containing pyridyl blue are able to produce an image with greater intensity than copying materials containing crystal violet lactone.

Claims (25)

1. Chromogenic material, characterized by the following formula: 2. Chromogenic material according to claim 1, characterized by including a mixture of 7-(1-ethyl-2-methylindol-3-yl)-7-(4-diethylamino-2-ethoxyphenyl)-5,7-dihydrofuro{3 ,4-b}pyridin-5-one and 5-(1-ethyl-2-methyl-indol-3-yl)-5-(4-diethylamino-2-ethoxyphenyl)-5,7-dihydrofuro-{3, 4-b}pyridin-7-one. 3. Chromogenic material according to claim 2, characterized in that the pyridine-5-one isomer predominates. 4. Chromogenic material according to claim 3, characterized in that the pyridine-5-one isomer constitutes at least 90% of the mixture. 5. Chromogenic material according to claim 4, characterized in that the pyridine-5-one isomer constitutes at least 95% of the mixture. 6. 7-(1-ethyl-2-methylindol-3-yl)-7-(4-diethylamino-2-ethoxyphenyl)-5,7-dihydrofuro{3,4-b}pyridin-5-one. 7. 5-(1-ethyl-2-methylindol-3-yl)-5-(4-diethylamino-2-ethoxyphenyl)-5,7-dihydrofuro{3,4-b}pyridin-7-one. 8. Process for producing chromogenic material according to any of the preceding claims, characterized in that a keto acid is reacted with the formula with a connection with the formula wherein one of the groups A and B is 4-diethylamino-2-ethoxyphenyl while the other is 1-ethyl-2-methylindol-3-yl, in the presence of a dehydrating agent. 9. Method according to claim 8, characterized in that the dehydrating agent is an acid anhydride. 10. Method according to claim 9, characterized in that the acid anhydride is acetic anhydride. 11. Method according to any one of claims 8 to 10, characterized in that the reaction is carried out at temperatures between 65 and 75°C. 12. Method according to any one of claims 8 to 11, characterized in that A is 1-ethyl-2-methylindol-3-yl and the keto acid is prepared by reacting quinolinic anhydride with 1-ethyl-2-methylindole. 13. Method according to any one of claims 8 to 11, characterized in that A is 4-diethylamino-2-ethoxyphenyl and the keto acid is prepared by reacting quinolinic anhydride with N,N-diethyl-m-phenetidine. 14. Process for producing chromogenic material according to any one of claims 1 to 7, characterized in that quinolinic anhydride, 1-ethyl-2-methylindole and N,N-diethyl-m-phenetidine are reacted in the presence of a dehydrating agent. 15. Chromogenic material, characterized by being produced using a method according to any one of claims 8 to 14. 16. Pressure-sensitive copying material, characterized by containing as a component in its color-forming system a chromogenic material produced according to any one of claims 8 to 14. 17. Used as a chromogenic material and as an agent to prevent loss of color-forming reactivity on crystal violet lactone or other chromogenic material, characterized by using the compound 7-(1-ethyl-2-methylindol-3-yl)-7 -(4-diethylamino-2-ethoxy-phenyl)-5,7-dihydrofuro{3,4-b}pyridin-5-one and/or 5-(1-ethyl-2-methylindol-3-yl)-5 -(4-diethylamino-2-ethoxyphenyl)-5,7-dihydrofuro{3,4-b}pyridin-7-one. 18. Use as stated in claim 17, characterized in that chromogenic material produced by any of the methods 8 to 14 is used. 19. Used as a chromogenic material and as an agent to prevent loss of resistance to bleaching in crystal violet lactone or other chromogenic material, characterized by using the compound 7-(1-ethyl-2-methylindol-3-yl)-7 -(4-diethylamino-2-ethoxyphenyl)-5,7-dihydrofuro{3,4-b}pyridin-5-one and/or 5-(1-ethyl-2-methyl-indol-3-yl)-5 -(4-diethylamino-2-ethoxyphenyl)-5,7-dihydrofuro {3,4-b}pyridin-7-one. 20. The chromogenic material according to claim 19, characterized by being produced using a method according to any of claims 8 to 14. 21. Method for improving a copying material's ability to resist a weakening of its color-forming reactivity, characterized in that in the material's color-forming system together with crystal violet lactone and/or one or more other chromogenic materials, the compound 7-(l-ethyl) is used -2-methylindol-3-yl)-7-(4-diethyl-amino-2-ethoxyphenyl)-5,7-dihydrofuro{3,4-b}pyridin-5-one and/or 5-(1-ethyl -2-methylindol-3-yl)-5-(4-diethylamino-2-ethoxy-phenyl)-5,7-dihydrofuro{3,4-b}pyridin-7-one. 22. Method according to claim 21, characterized in that said material is prepared using a method according to any of claims 8 to 14. 23. Method for improving a copying material's ability to withstand a weakening of the material's resistance to bleaching, characterized in that the compound 7-(l- ethyl-2-methylindol-3-yl)-7-(4-diethyl-amino-2-ethoxyphenyl)-5,7-dihydrofuro{3,4-b}pyridin-5-one and/or 5-(l- ethyl-2-methylindol-3-yl)-5-(4-diethylamino-2-ethoxyphenyl)-5,7-dihydrofuro{3,4-b}pyridin-7-one. 24. Method according to claim 23, characterized in that said material is produced by a method according to any of claims 8 to 14. 25. Copying material, characterized by being produced by a method according to any one of claims 21 to 24.