JPH07214908A - Reversible thermal recording material - Google Patents

Abstract

PURPOSE:To obtain a recording material capable of forming and erasing an image in good contrast and capable of holding an image stable with the elapse of time by adding an electron donating dye precursor and an electron acceptive compd. represented by a specific formula generating a reversible hue change by heating. CONSTITUTION:A reversible thermal recording material is obtained by providing a thermal recording 'layer based on a usually colorless or light-colored electron donating dye precursor and an electron acceptive coupler generating a reversible hue change, that is, color development and erasure in the dye precursor by heating on a support. In this case, as the electron acceptive coupler, a compd. represented by general formal (R1-R3 are a hydrogen atom or a hydroxy group and not simultaneously a hydrogen atom Y is a divalent group represented by -S-, -NHCONH-, -CONH-, or -NHCO-, X is a divalent group represented by -S-, -NH-, -COO-or -CONHNH-, n is 0 or 1 and m and l are an integer of 0-12) is used.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reversible thermosensitive recording material capable of forming and erasing an image by heating.

[0002]

2. Description of the Related Art A heat-sensitive recording material generally comprises a support and a heat-sensitive recording layer containing an electron-donating, usually colorless or light-colored dye precursor and an electron-accepting developer as main components. By heating with a thermal head, a heating pen, a laser beam or the like, a dye precursor and a developer react instantaneously to obtain a recorded image. JP-B-43-4160 and JP-B-45-
No. 14039 is disclosed.

In general, such a heat-sensitive recording material cannot erase the portion of the image once formed and return to the state before the image is formed. Therefore, when further information is recorded, the image is not recorded. There was no choice but to add to the formation part. Therefore, when the area of the heat-sensitive recording portion is limited, there is a problem that the recordable information is limited and it is impossible to record all the necessary information.

In recent years, in order to deal with such a problem, a reversible thermosensitive recording material has been devised which can repeat image formation and image deletion. For example, JP-A-54-11937
7, JP-A-63-39377, JP-A-63
No. 41186 discloses a heat-sensitive recording material composed of a resin base material and organic low-molecular substances dispersed in the resin base material. However, since this method reversibly changes the transparency of the heat-sensitive recording material by heat energy, the contrast between the image-formed area and the image-unformed area is insufficient.

Further, in the methods described in JP-A-50-81157 and JP-A-50-105555, since the image to be formed changes according to the ambient temperature, the image forming state and the erasing state. The holding temperatures are different, and these two states cannot be held for an arbitrary period at room temperature.

Further, in Japanese Patent Laid-Open No. 59-120492, the hysteresis characteristic of the color-developing component is utilized to keep the recording material in the hysteresis temperature range, thereby forming an image.
Although a method of maintaining the erased state is described, this method requires a heating source and a cooling source for image formation and erasure, and the temperature range in which the image formation state and the erased state can be maintained is limited to the hysteresis temperature region. However, it is still insufficient for use in the temperature environment of daily life.

On the other hand, JP-A-2-188293, JP-A-2-188294, and International Publication No. WO90 /
No. 11898 describes a reversible thermosensitive recording medium composed of a leuco dye and a color-developing and decoloring agent for coloring and decoloring the leuco dye by heating. The color-developing agent is an amphoteric compound having an acidic group for coloring the leuco dye and a basic group for decoloring the colored leuco dye, and the coloring effect by the acidic group or the decoloring effect by the basic group is controlled by controlling thermal energy. One of them is preferentially generated, and coloring and erasing are performed. However, with this method, it is not possible to completely switch the coloring reaction and the erasing reaction only by controlling the thermal energy, and since both reactions occur at a certain ratio at the same time, sufficient coloring density cannot be obtained and the erasing Color cannot be done completely.
Therefore, sufficient image contrast cannot be obtained, and since the decolorizing action of the basic group also acts on the color-developing portion at room temperature,
The phenomenon that the density of the color-developed portion decreases with time cannot be avoided.
Furthermore, JP-A-5-124360 discloses a reversible thermosensitive recording medium that develops and decolorizes a leuco dye by heating, and has an aliphatic group having 12 or more carbon atoms as the electron-accepting compound. Organic phosphoric acid compounds, fatty acid carboxylic acid compounds and specific phenolic compounds (alkylthiophenols, alkyloxyphenols, alkylcarbamoylphenols, gallic acid alkyl esters) are disclosed. However, even with such a recording medium, the two problems of low color density or incomplete decolorization cannot be solved at the same time, and the temporal stability of the image is also at a practically satisfactory level. Not in

As described above, according to the conventional technique, a practical reversible thermosensitive recording material having a good image contrast, capable of forming and erasing an image, and retaining a stable image over time in the environment of daily life. Was not known.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to provide a reversible thermosensitive recording material capable of forming and erasing an image with good contrast and capable of holding a stable image over time in an environment of daily life. Is to provide.

[0010]

As a result of various investigations to solve this problem, the present inventors usually found that a colorless or light-colored electron-donating dye precursor was reversibly changed in color tone by heating, that is, The present invention has been completed by finding an electron-accepting compound represented by the following general formula 2 which causes color development and decolorization.

[0011]

[Chemical 2]

(In the formula 2, R ₁ , R ₂ and R ₃ each independently represent a hydrogen atom or a hydroxy group.
₁ , R ₂ and R ₃ are not hydrogen atoms at the same time. Y is
X is -S-, -NH-, -COO-, a divalent group represented by -S-, -NHCONH-, -CONH- or -NHCO- bond.
Alternatively, it represents a divalent group represented by a -CONHNH- bond. n
Represents 0 or 1, and m and l each independently represent 0 to 1.
Represents an integer of 2. )

Specific examples of the electron-accepting compound used in the present invention which causes a reversible color change in the usually colorless or light-colored electron-donating dye precursor are listed in Table 1. The present invention is not limited to these specific examples. It is not limited to.

[0014]

[Table 1]

The electron-accepting compound according to the present invention may be used alone or in admixture of two or more, and usually, the amount of the electron-accepting compound according to the present invention to the colorless or light-colored dye precursor is 5 or less. ˜5000 wt%, preferably 10-3000 wt%.

The usually colorless or light-colored electron-donating dye precursor used in the present invention is represented by those generally used for pressure-sensitive recording paper, heat-sensitive recording paper, etc., but is not particularly limited. Specific examples include the followings, but the present invention is not limited thereto.

(1) Triarylmethane compound 3,3-bis (p-dimethylaminophenyl) -6-dimethylaminophthalide (crystal violet lactone), 3,3-bis (p-dimethylaminophenyl) phthalide, 3- (p-dimethylaminophenyl) -3-
(1,2-dimethylindol-3-yl) phthalide,
3- (p-dimethylaminophenyl) -3- (2-methylindol-3-yl) phthalide, 3- (p-dimethylaminophenyl) -3- (2-phenylindole-
3-yl) phthalide, 3,3-bis (1,2-dimethylindol-3-yl) -5-dimethylaminophthalide, 3,3-bis (1,2-dimethylindole-3-)
Yl) -6-Dimethylaminophthalide, 3,3-bis (9-ethylcarbazol-3-yl) -5-dimethylaminophthalide, 3,3-bis (2-phenylindol-3-yl) -5 -Dimethylaminophthalide, 3-p
-Dimethylaminophenyl-3- (1-methylpyrrol-2-yl) -6-dimethylaminophthalide and the like,

(2) Diphenylmethane compound 4,4'-bis (dimethylaminophenyl) benzhydryl benzyl ether, N-chlorophenyl leuco auramine, N-2,4,5-trichlorophenyl leuco auramine, etc.

(3) Xanthene compound Rhodamine B anilinolactam, rhodamine B-p-chloroanilinolactam, 3-diethylamino-7-dibenzylaminofluorane, 3-diethylamino-7-octylaminofluorane, 3- Diethylamino-7-phenylfluorane, 3-diethylamino-7-chlorofluorane, 3-diethylamino-6-chloro-7-methylfluorane, 3-diethylamino-7- (3,4-dichloroanilino) fluorane, 3 -Diethylamino-7
-(2-chloroanilino) fluorane,

3-Diethylamino-6-methyl-7-anilinofluorane, 3- (N-ethyl) tolylamino-
6-methyl-7-anilinofluorane, 3-piperidino-6-methyl-7-anilinofluorane, 3- (N-ethyl) tolylamino-6-methyl-7-phenethylfluorane, 3-diethylamino-7 -(4-Nitroanilino) fluorane, 3-dibutylamino-6-methyl-7
-Anilinofluorane, 3- (N-methyl) propylamino-6-methyl-7-anilinofluorane, 3- (N
-Ethyl) isoamylamino-6-methyl-7-anilinofluorane, 3- (N-methyl) cyclohexylamino-6-methyl-7-anilinofluorane, 3- (N-
Ethyl) tetrahydrofurylamino-6-methyl-7-
Anilino Fluoran, etc.

(4) Thiazine-based compound benzoyl leuco methylene blue, p-nitrobenzoyl leuco methylene blue, etc.

(5) Spiro compound 3-methylspirodinaphthopyran, 3-ethylspirodinaphthopyran, 3,3'-dichlorospirodinaphthopyran, 3-benzylspirodinaphthopyran, 3-methylnaphtho- (3 -Methoxybenzo) spiropyran, 3-propylspirobenzopyran and the like.

The usually colorless to light-colored dye precursors may be used either individually or in combination of two or more.

Next, a specific method for producing the reversible thermosensitive recording material of the present invention will be described, but the present invention is not limited thereto.

As a specific example of the method for producing the reversible thermosensitive recording material of the present invention, usually a colorless or pale color dye precursor and the electron-accepting compound according to the present invention are main components, and these are coated or printed on a support. And a reversible thermosensitive recording layer is formed.

As a method for incorporating a colorless or light-colored dye precursor and the electron-accepting compound according to the present invention into the reversible thermosensitive recording layer, each compound is dissolved in a solvent or dispersed in a dispersion medium. A method of mixing each compound, a method of dissolving each compound in a solvent or dispersing in a dispersion medium, a method of heating and dissolving each compound to homogenize and then cooling, and a method of dissolving in a solvent or dispersing in a dispersion medium. Can be mentioned but not specified.

A binder may be added to the reversible thermosensitive recording layer for the purpose of improving the strength of the reversible thermosensitive recording layer. Specific examples of the binder include starches, hydroxyethyl cellulose, methyl cellulose, carboxymethyl cellulose, gelatin, casein, polyvinyl alcohol, modified polyvinyl alcohol, sodium polyacrylate, acrylic acid amide / acrylic acid ester copolymer, acrylic acid amide / acrylic. Water-soluble polymer such as acid ester / methacrylic acid terpolymer, alkali salt of styrene / maleic anhydride copolymer, alkali salt of ethylene / maleic anhydride copolymer, polyvinyl acetate, polyurethane, polyacrylic acid Examples include, but are not limited to, latex such as ester, styrene / butadiene copolymer, acrylonitrile / butadiene copolymer, methyl acrylate / butadiene copolymer, ethylene / vinyl acetate copolymer, and the like. No.

A heat-fusible substance may be contained in the reversible thermosensitive recording layer as an additive for controlling the color developing sensitivity and the decoloring temperature of the reversible thermosensitive recording layer. 60 ° C
Those having a melting point of ~ 200 ° C are preferable, and especially 80 ° C.
Those having a melting point of ˜180 ° C. are preferred. It is also possible to use a sensitizer which is used in general thermal recording paper. For example, waxes such as N-hydroxymethylstearic acid amide, stearic acid amide and palmitic acid amide, naphthol derivatives such as 2-benzyloxynaphthalene, biphenyl derivatives such as p-benzylbiphenyl and 4-allyloxybiphenyl, 1,2. -Bis (3-
Polyether compounds such as methylphenoxy) ethane, 2,2'-bis (4-methoxyphenoxy) diethyl ether, bis (4-methoxyphenyl) ether, diphenyl carbonate, dibenzyl oxalate, bis (p-methylbenzyl oxalate) Carbonic acid such as ester or oxalic acid diester derivative may be used in combination.

The support used in the reversible thermosensitive recording material of the present invention is paper, various non-woven fabrics, woven fabrics, synthetic resin film, synthetic resin laminated paper, synthetic paper, metal foil, glass, etc., or a combination thereof. The composite sheet can be arbitrarily used according to the purpose, and may be transparent, semitransparent or opaque. Moreover, it is not limited to these.

The layer structure of the reversible thermosensitive recording material of the present invention is as follows:
Only the reversible thermosensitive recording layer may be used. If necessary,
A protective layer may be provided on the reversible thermosensitive recording layer, or an intermediate layer may be provided between the reversible thermosensitive recording layer and the support. In this case, the protective layer and / or the intermediate layer may be composed of a plurality of layers of 2 layers or 3 layers or more. Further, information can be recorded electrically, magnetically, or optically in the reversible thermosensitive recording layer and / or on the surface provided with another layer and / or the reversible thermosensitive recording layer and / or on the opposite surface. The material may be included. Further, a back coat layer may be provided on the surface opposite to the surface on which the reversible thermosensitive recording layer is provided for the purpose of preventing curling and preventing electrification.

The reversible thermosensitive recording layer is prepared by mixing each dispersion obtained by finely pulverizing each color forming component and coating the mixture on a support.
It can be obtained by a method of printing and drying, a method of dissolving each color forming component in a solvent, mixing each solution, coating on a support, printing and drying. in this case,
For example, each color-forming component may be contained in one layer to form a multilayer structure.

The reversible thermosensitive recording layer and / or the protective layer and / or the intermediate layer include diatomaceous earth, talc, kaolin, calcined kaolin, calcium carbonate, magnesium carbonate, titanium oxide, zinc oxide, silicon oxide and hydroxide. Aluminum, pigments such as urea-formalin resin, and other higher fatty acid metal salts such as zinc stearate and calcium stearate, waxes such as paraffin, oxidized paraffin, polyethylene, oxidized polyethylene, stearamide, and castor wax, A dispersant such as sodium dioctylsulfosuccinate, a surfactant, a fluorescent dye and the like can be added.

[0033]

The electron-accepting compound according to the present invention has a specific decoloring effect, that is, a reversible effect, although it has the ability to develop the color of the leuco dye. This is totally unexpected, and it is an electron-accepting compound used in a conventional heat-sensitive recording material, that is, 2,2-bis (4-hydroxyphenyl) propane, bis (4-hydroxyphenyl) sulfone, 4- No such reversible effect is observed with benzyl hydroxybenzoate and the like.

The principle of image formation and erasure of the heat-sensitive recording material of the present invention is not clear yet, but it is considered as follows. When a colorless or light-colored dye precursor is heated together with an electron-accepting compound such as a phenolic compound, electron transfer from the dye precursor to the electron-accepting compound occurs to develop a color.
At this time, it is considered that the electron-accepting compound molecule exists in the extremely close vicinity of the dye molecule that has developed color. Further, when the electron-accepting compound molecule is separated from the color-developed dye molecule, the color-developed dye molecule receives the electron again and becomes in the state of the dye precursor before color development. It is considered that the present invention changes the distance between the molecule of the electron-accepting compound and the molecule of the dye by heating to perform coloring and decoloring.

More specifically, since the electron-accepting compound according to the present invention has a cholesterol residue in its structure, it has low compatibility with the dye precursor molecule and the colored dye molecule, and in the coagulated state. It is considered that they are hardly compatible with each other. Further, in a state where the dye precursor molecule and the electron-accepting compound molecule according to the present invention can freely move like a heating and melting state, the dye precursor molecule and the electron-accepting compound molecule according to the present invention are mixed with each other at a certain ratio to form a color. It becomes a state. Therefore, when the colored mixture in the molten state is slowly cooled, the electron-accepting compound according to the present invention and the dye molecule become insoluble in each other and phase-separate and disappear the color as the temperature decreases. On the other hand, when rapidly cooled, it solidifies before phase separation, that is, in the color-developed state, so that the color-developed state is fixed and the color-developed state is stably maintained even after the solidification.

As described above, the reversible thermosensitive recording material according to the present invention produces a compatible state and a phase separated state between the dye molecule and the electron-accepting compound according to the present invention, and develops a coloring and decoloring state. Conceivable. In order to develop a color, rapid cooling may occur after heating, and for example, heating with a thermal head, laser light or the like is possible. Also, if heated and slowly cooled, the color disappears, for example, heat roll, heat stamp, thermal head, high frequency heating,
It can be performed by using radiant heat from a hot air, an electric heater or a light source such as a halogen lamp.

Next, representative examples of specific synthetic examples of the electron-accepting compound represented by Chemical Formula 2 are shown below.

Synthesis Example 1 (Synthesis of Exemplified Compound 1) 9.3 g of cholesterol monochloroacetate and 70 parts of toluene
After dissolution by adding ml, p-hydroxythiophenol 2.
8 g was added, and triethylamine (3.1 ml) was added dropwise with stirring at room temperature, and the mixture was stirred at the same temperature overnight. Water was added to the reaction solution, the precipitate was collected by filtration, washed with water three times, and the crystals were washed with methanol to obtain the desired product. Yield: 7.9 g Melting point: 196-198 ° C

Synthesis Example 2 (Synthesis of Exemplified Compound 13) To 3.1 g of p-hydroxybenzoic acid hydrazide, 40 ml of dimethylformamide was added and dissolved, and then 9.0 g of cholesterol chloroformate and 50 ml of toluene were added and stirred at room temperature. 2.8 ml of triethylamine was added dropwise,
The mixture was stirred overnight at the same temperature. After toluene was distilled off under reduced pressure from the reaction solution, water was added and the precipitated crystals were collected by filtration and washed with water and methyl ethyl ketone to obtain the desired product. Yield: 10.1 g Melting point: 248-251 ° C

Synthesis Example 3 (Synthesis of Exemplified Compound 14) After dissolving 1.8 g of [3- (p-hydroxyphenyl) propiono] hydrazide in 10 ml of dimethylacetamide, 1.1 g of triethylamine was added and stirred at room temperature. A solution prepared by dissolving 4.26 g of cholesterol chloroformate in 15 ml of chloroform was added dropwise. After completion of dropping, the mixture was stirred at the same temperature for 2 hours. After the reaction was completed, the reaction solution was poured into a 1N-HCl aqueous solution and left overnight, and then the upper layer was removed by decantation. Alcohol was added to the remaining layer to crystallize the desired product, and then the crystals were collected by filtration and washed with alcohol to obtain the desired product. Yield: 2.9g Melting point: 227.7 ° C

Synthesis Example 4 (Synthesis of Exemplified Compound 8) After dissolving 3.3 g of [11- (p-hydroxyphenylthio) undecano] hydrazide in 15 ml of dimethylacetamide, 1.1 g of triethylamine was added and the mixture was stirred at room temperature. A solution prepared by dissolving 4.48 g of cholesterol chloroformate in 10 ml of chloroform was added dropwise. After completion of dropping, the mixture was stirred at the same temperature for 3 hours. After completion of the reaction, chloroform was distilled off under reduced pressure, the obtained residue was poured into a 1N-HCl aqueous solution, and the precipitated crystals were collected by filtration, washed with water, and recrystallized from isopropanol to obtain the desired product. Yield: 2.8 g Melting point: 131.7 ° C

[0042]

The present invention will be described in more detail with reference to the following examples.

Example 1 (A) Preparation of reversible heat-sensitive coating liquid 40 parts of a dye precursor, 3-di-n-butylamino-6-methyl-7-anilinofluorane, was added to a 2.5% polyvinyl alcohol aqueous solution. It was crushed together with 90 parts by a ball mill for 24 hours to obtain a dye precursor dispersion liquid. Next, 1100 parts of the exemplified compound represented by Chemical formula 2 was pulverized with 400 parts of a 2.5% aqueous solution of polyvinyl alcohol in a ball mill for 24 hours to obtain a dispersion liquid. After mixing the above two dispersions, 10
% Polyvinyl alcohol aqueous solution (200 parts) and water (400 parts) were added and mixed well to prepare a reversible heat-sensitive coating liquid.

(B) Preparation of reversible heat-sensitive recording material The reversible heat-sensitive coating liquid prepared in (A) was applied to a polyethylene terephthalate (PET) sheet to give a solid content of 4 g / m ^2.
To obtain a reversible heat-sensitive recording material.

Example 2 A reversible thermosensitive recording material was obtained in the same manner as in Example 1 except that the Exemplified compound 5 was used in place of the Exemplified compound 1 represented by Chemical formula 2 used in Example 1.

Example 3 A reversible thermosensitive recording material was obtained in the same manner as in Example 1 except that the Exemplified compound 8 was used instead of the Exemplified compound 1 represented by Chemical formula 2 used in Example 1.

Example 4 A reversible thermosensitive recording material was obtained in the same manner as in Example 1 except that the Exemplified compound 13 was used in place of the Exemplified compound 1 represented by Chemical formula 2 used in Example 1.

Example 5 A reversible thermosensitive recording material was obtained in the same manner as in Example 1 except that the Exemplified compound 14 was used in place of the Exemplified compound 1 represented by Chemical formula 2 used in Example 1.

Example 6 A reversible thermosensitive recording material was obtained in the same manner as in Example 1 except that the Exemplified Compound 16 was used in place of the Exemplified Compound 1 represented by Chemical formula 2 used in Example 1.

Comparative Example 1 The same procedure as in Example 1 was carried out except that a salt of gallic acid and stearylamine was used instead of the exemplified compound 1 represented by Chemical formula 2 used in Example 1.

Comparative Example 2 The same procedure as in Example 1 was repeated except that 2,2-bis (p-hydroxyphenyl) propane was used instead of the exemplified compound 1 represented by Chemical formula 2 used in Example 1. It was

Comparative Example 3 The procedure of Example 1 was repeated except that p- (n-octadecylthio) phenol was used in place of the exemplified compound 1 represented by Chemical formula 2 used in Example 1.

Test 1 (coloring density = thermoresponsiveness) The thermal recording materials obtained in Examples 1 to 6 and Comparative Examples 1 to 3 were applied to the Kyocera print head KJT-256-8MGF1 with Okura Denki's thermal fax printing tester. TH-PMD
Was applied under the condition of an applied voltage of 26 V for an applied pulse of 1.1 milliseconds, and the density of the obtained color image was measured using a densitometer Macbeth RD918. The results are shown in Table 1.

Test 2 (Change in color density over time = image stability) The thermal recording materials obtained in Examples 1 to 6 and Comparative Examples 1 to 3 were used as thermal printers manufactured by Okura Electric Co. with a print head KJT-256-8MGF1 manufactured by Kyocera. Printing tester TH-PMD
Was applied under the condition of an applied voltage of 26 V for an applied pulse of 1.1 milliseconds and stored in an atmosphere of a temperature of 50 ° C. and a relative humidity of 30% for 14 hours. The density was measured, and the image residual ratio was calculated by the following formula 1. The results are shown in Table 1.

[0055]

[Equation 1] A = (C / B) × 100 A: Image residual ratio (%) B: Image density before test C: Image density after test

Test 3 (Image erasability) The thermal recording materials obtained in Examples 1 to 6 and Comparative Examples 1 to 3 were used as a thermal facsimile printing tester TH-manufactured by Okura Electric Co. with a print head KJT-256-8MGF1 manufactured by Kyocera. PMD
Was applied at a voltage of 26 V for an applied pulse of 1.1 ms, and the resulting color image part was heated for 1 second at 120 ° C. using a heat stamp, and then the density was changed in the same manner as in Test 1. It was measured. The results are shown in Table 1.

[0057]

[Table 2]

In Table 1, ∘ indicates that the density of the erased portion is less than 30% of the density of the colored portion and the contrast between the colored portion and the erased portion is good,
Δ indicates that contrast is insufficient when the density of the erased portion is 30% or more and less than 80% of the density of the colored portion, and x indicates that reversibility is not observed when the density of the erased portion is 80% or more of the density of the colored portion.

[0059]

As shown in Table 1, it is usually represented by a colorless or light-colored electron-donating dye precursor and the general formula 2 which causes the dye precursor to reversibly change its color tone by heating. A reversible thermosensitive recording material containing an electron-accepting compound, capable of forming and erasing an image with good contrast, and capable of holding a stable image over time in the environment of everyday life. I was able to get it.

Claims (1)

[Claims] 1. A colorless or light-colored electron-donating dye precursor, and an electron-accepting compound represented by the following general formula 1 which causes a reversible color change in the dye precursor by heating. Reversible thermosensitive recording material. [Chemical 1] (In Formula 1, R ₁ , R ₂ and R ₃ each independently represent a hydrogen atom or a hydroxy group, provided that R ₁ , R ₂ and R ₃
Are not hydrogen atoms at the same time. Y is -S-, -NH
CONH-, -CONH- or -NHCO- is a divalent group represented by a bond, X is -S-, -NH-, -COO-, or -C
It represents a divalent group represented by an ONHNH- bond. n represents 0 or 1, m and l each independently represent an integer of 0 to 12. )