DE69229292T2 - Reversible, heat-sensitive recording material and a recording material using this material - Google Patents

Description

Background of the invention 1. Field of the invention:

The present invention relates to a thermosensitive recording material which can reversibly record and erase information by means of heat, and a recording medium using the same. The present invention is applicable to a memory card which can re-record information and, if desired, has a display function, the memory card being used as a season ticket, an ordinary ticket, a section ticket or a prepaid ticket, an IC card, a recording sheet for a facsimile machine, a thermosensitive recycled paper, an optical disk, etc.

2. Description of the state of the art:

Examples of a reversible thermosensitive recording material include dye-type thermosensitive recording materials which contain a leuco dye, a developer and a color erasing agent in combination and which can reversibly develop and erase a color, thermosensitive recording materials having organic crystal particles dispersed in a matrix polymer, in which the recording and erasing of information is carried out by utilizing the change in transparency of the recording material in accordance with the melting and solidification of the particles in the matrix, and thermosensitive recording materials comprising a liquid crystal polymer such as cholesterol liquid crystals, wherein the transparency of the recording material by changing the molecular orientation of the polymer by applying heat. Of the above-mentioned various types of reversible thermosensitive recording materials, in the case of the recording materials having organic crystal particles dispersed in a matrix polymer, recording and erasing of information is carried out by changing the transparency of the recording material in accordance with the melting and solidification conditions of the particles. That is, when the above-mentioned organic crystal particles are melted by heat and then solidified by lowering the temperature, the particles assume various kinds of solidified states according to the melting and solidification conditions, that is, a polycrystalline state, a single-crystalline state, an amorphous state or a non-crystalline state. Each state has different transparency characteristics. Thus, recording and erasing of information is achieved by utilizing this phenomenon.

DE-A-40 19 683, for example, discloses low molecular weight compounds comprising, among others, -OH, -COOH or -CONH. Specifically mentioned are compounds such as behenic acid, stearic acid, hexadecanone dioic acid, eicosane dioic acid, stearyl stearate or phthalate as recording media.

For example, the transparent matrix polymer film comprising organic crystal particles in a polycrystalline state is opaque at ordinary temperature because the organic crystal particles scatter light. As shown in Fig. 3, when the polymer film is gradually heated and the temperature exceeds T₀ (which is approximately equal to the glass transition point (Tg) of the matrix polymer), the polymer film begins to change from the opaque state to a transparent state. When the temperature reaches T₁, the polymer film becomes completely transparent. At this point, the organic crystal particles are in a single-crystalline state and are translucent. When the temperature is further increased to T₂ or above, the light transmittance of the organic crystal particles gradually decreases and the polymer film becomes semitransparent at T₃ (which is approximately equal to the melting point). point of the organic crystal particles). When the organic crystal particles which had previously been heated to a temperature between T₁ and T₂ are cooled to room temperature, the organic crystal particles are transparent to light and thus the polymer film remains transparent. When the organic crystal particles which had previously been heated to T₃ or above are cooled to room temperature, the organic crystal particles scatter light and thus the polymer film becomes opaque. Therefore, for example, the state of the transparent film which has been cooled to room temperature after being heated to a temperature between T₁ and T₂ is taken as an initial state. The information is written on the film at a temperature of T₃ or above, thereby recording the information. Alternatively, the state in which the film is opaque is taken as an initial state and the information can be written on the film so that the film becomes transparent.

Japanese Patent Laid-Open No. 54-119377 discloses the combination of the above-mentioned matrix polymer and the organic crystal particles. Examples of the materials of the organic crystal particles disclosed therein include aliphatic and aromatic alcohols, carboxylic acids, amines and amides, and halides and sulfides thereof. Examples of the matrix polymer disclosed therein include polyesters, polyamides, polyacrylic acid, polystyrene, silicone resins, polyvinyl chloride, polyvinylidene chloride and polyacrylonitrile. Improved recording materials additionally containing carbon black or an antioxidant are disclosed in Japanese Patent Laid-Open Nos. 57-82087 and 57-82088.

In general, in the reversible thermosensitive recording medium having the organic crystal particles dispersed in the above-mentioned matrix polymer, a reversible thermosensitive recording layer is formed on a substrate by coating a recording material, and a hard protective layer is formed on the recording layer. This protective layer is provided so that the recording layer is not damaged due to contact with a thermal head for recording and erasing to protect the recording layer. In particular, the protective layer is designed to prevent thermal deformation of the recording layer by contact with the head, adhesion of the molten matrix polymer to the head and mechanical damage to the recording layer by pressure from the head. Mechanical strength and flexibility as well as thermal stability are required for the protective layer. In addition, the protective layer must be transparent for satisfactory reading of the information on the recording layer.

In general, the materials used for the protective layer include cellulose type resins, polystyrene or styrene copolymer resins, acrylic or methacrylic resins including homopolymers and copolymers, polyester resins, butyral resins, polyvinyl chloride, polyvinyl acetate, vinyl chloride/vinyl acetate copolymer resins, polyurethane resins, radiographically curable acrylate type resins, etc. The protective layer also prevents the adhesion of debris and dirt to the recording layer.

This type of recording medium has a problem that waste and dirt adhere to the thermal head, causing irregularities in a recorded image. To overcome this problem, Japanese Patent Laid-Open No. 2-258287 describes a method for preventing the adhesion of waste and dirt to the thermal head by mixing fine particles into the protective layer. These particles form a minimal unevenness with a roughness of 0.5 to 3 µm on the surface of the protective layer and cause movement and elimination of waste and dirt from the surface of the protective layer in accordance with the movement of the recording medium.

As described above, various reversible thermosensitive recording materials and recording media having organic crystal particles dispersed in the matrix polymer are known, but they have the following disadvantages. In this type of recording medium, the relatively low temperature at which the organic crystal particles dispersed in the matrix are transparent to light is approximately between 70ºC and 75ºC, so that the medium is poor in stability for preserving information. Moreover, since the temperature range (T₁ to T₂ in Fig. 3) in which the particles are transparent to light is a relatively small number of degrees in ºC, the temperature at which the particles become transparent to light should be strictly regulated or controlled.

For example, a conventional reversible thermosensitive recording material is obtained by using behenic acid as a material for the organic crystal particles and a vinyl chloride/vinyl acetate copolymer as a matrix polymer. The behenic acid has 22 carbon atoms and a melting point of 80°C, which is a representative saturated straight-chain aliphatic acid that can provide high resolution and contrast. The temperature range of the thus-obtained material in which the particles are transparent to light is between about 68°C and 74°C. Thus, the relevant temperature range has a width of about 6°C. Therefore, it is difficult to perform stable recording.

In order to improve such a low and narrow temperature range in which the particles become transparent, new organic crystal particles have been researched, and the new combination of the organic crystal particles with the matrix polymer has been studied. For example, Japanese Patent Laid-Open Nos. 2-1363 and 3-2089 describe a method for forming an organic crystal particle by mixing an aliphatic dicarboxylic acid with a higher aliphatic acid having 16 or more carbon atoms. In Japanese Patent Laid-Open No. 2-1363, it is described that a higher aliphatic acid is mixed with a dicarboxylic acid or a derivative thereof having 4 to 16 carbon atoms and a weight ratio of 95:5 to 20:80 to form organic crystal particles. If the resulting particles are heated in a matrix polymer, the higher aliphatic acid and the dicarboxylic acid are melted and mixed, resulting in a eutectic mixture, thereby broadening the temperature range in which the particles are transparent to light (see also DE-A-40 19 683).

Japanese Patent Laid-Open No. 3-2089 describes a higher aliphatic acid mixed with an aliphatic dicarboxylic acid having 20 or more carbon atoms in a weight ratio of 95:5 to 50:50 to form organic crystal particles. When the resulting particles are heated in a matrix polymer, the higher aliphatic acid and the dicarboxylic acid melt and are mixed, resulting in a eutectic mixture, thereby broadening the temperature range in which the particles are transparent to light. However, the temperature (T1 in Fig. 3) at which the particles become transparent to light is 100°C or lower, and the width of the temperature range in which the particles are transparent to light is broadened to about 20°C at the maximum. In order to perform stable recording and preserve the recorded information, a material with a higher temperature at which the particles become translucent and a wider temperature range in which the material is transparent is required, particularly in view of the temperatures commonly used for recording or erasing information.

In addition, for the conventional reversible thermosensitive recording materials, the relationship between the crystallinity of the organic crystal particles and the compatibility, dispersibility, etc. of the particles with respect to the matrix polymer and the relationship between the crystallinity and the recording characteristics have not been sufficiently clarified. Thus, there is a problem that recording with excellent contrast cannot be performed.

In addition, this type of reversible thermosensitive recording material has many other problems. For example, since the organic crystal particles are repeatedly melted and solidified as a result of recording and erasing information, the shape of the particles begins to deteriorate during such repetitions. As a result, the contrast between the recorded and unrecorded areas decreases and the recording stability is poor in the course of repeated use. In addition, Furthermore, the recording layer is deformed due to the pressure of the thermosensitive recording head. This means that the durability and longevity are low.

The above-mentioned protective layer helps prevent the mechanical and physical deterioration of the recording layer as mentioned above. However, the protective layer itself is subjected to heat stress such as thermal contraction due to the repetition of heating and cooling for recording and erasing information, and the surface of the protective layer is deformed. As a result, irregular optical reflection is caused on the surface of the protective layer, and therefore satisfactory recording and erasing cannot be performed.

In the protective layer having a slight unevenness on the surface as described in the above-mentioned Japanese Patent Laid-Open No. 2-258287, there is a problem that garbage and dirt are accumulated in a concave part of the protective layer according to the scanning of the thermal head. As a result of the garbage and dirt thus accumulated, failures in recording by the thermal head are generated. For this reason, further improvement in the durability of the surface of the protective layer is required.

Summary of the invention

The reversible thermosensitive recording material according to this invention, which overcomes the above-mentioned and numerous other disadvantages and deficiencies of the prior art, is made from a composition comprising a transparent matrix polymer and organic crystal particles dispersed therein;

wherein the crystal state of the organic crystal particles is variable according to the applied temperature, resulting in a reversible change in the transparency of the recording material;

wherein each of the organic crystal particles comprises at least one material selected from the group consisting of:

(1) at least one hydroxycarboxylic acid or derivatives thereof having a melting point in the range of 60ºC to 120ºC;

(2) at least one compound selected from the group consisting of an aliphatic amide compound and an aliphatic urea compound, and wherein each of the aliphatic amide compound and the aliphatic urea compound has at least one straight-chain hydrocarbon group each containing at least 10 carbon atoms, and wherein the melting point of each of the compounds is in the range of 70°C to 150°C;

(3) a compound having an amide or urea group and an aliphatic dicarboxylic acid, wherein the compound having an amide or urea group is at least one selected from the group consisting of saturated aliphatic monocarboxamides having a hydrocarbon group having at least 12 carbon atoms, saturated aliphatic biscarboxamides having a hydrocarbon group having at least 12 carbon atoms, and ureas substituted with a hydrocarbon group having at least 12 carbon atoms, wherein the aliphatic dicarboxylic acid corresponds to the formula HOOC(CH₂)n8COOH (wherein n8 is an integer from 14 to 24) and wherein the weight ratio of the compound having an amide or urea group to the aliphatic dicarboxylic acid is in the range of 60:40 to 10:90; and

wherein the matrix polymer and the organic crystal particles each have a group capable of forming a hydrogen bond.

In a preferred embodiment, each of the organic crystal particles comprises at least one hydroxycarboxylic acid or a derivative thereof having a melting point in the range of 60°C to 120°C as defined under (1) in claim 1 and

the matrix polymer is selected from the group consisting of polyesters with a hydroxyl group, partially saponified vinyl acetate/vinyl chloride copolymers, polyamides, polyurethanes, thermoplastic phenolic resins, vinyl alcohol copoly mers, acrylic copolymers, acrylamide copolymers, maleic acid copolymers, urea resins, epoxy resins and melamine resins.

Preferably, the hydrocarboxylic acid or derivative thereof is at least one selected from the group of compounds corresponding to the following formulas Ia, Ib, Ic, Id and Ie:

where ml and n1 are each integers and the sum of ml and n1 is 6 to 24;

HO-Ph-(CH₂)n2-COOH (Ib)

where Ph is a phenylene group and n2 is an integer in the range of 0 to 18 ;

HO-Ph-COO(CH2)n3-COOH (Ic)

where Ph is a phenylene group and n3 is an integer in the range of 1 to 18;

HO-Ph-OCO(CH2)n4-COOH (Id)

where Ph is a phenylene group and n4 is an integer in the range of 1 to 18; and

HO-Ph-COO(CH₂)n5-H (Ie)

where Ph is a phenylene group and n5 is an integer in the range of 0 to 18.

Preferably, the above hydroxycarboxylic acid is an α-hydroxyalkylcarboxylic acid.

In another embodiment according to (2) above, the aliphatic amide compound is at least one selected from the group consisting of the compounds corresponding to the following formulas IIa, IIb and IIc:

R¹-CONH-R² (IIa)

wherein R¹ is a straight-chain hydrocarbon group having 1 to 25 carbon atoms, R² is hydrogen, a straight-chain hydrocarbon group having 1 to 26 carbon atoms or a methylol group and at least one of the radicals R¹ and R² is a straight-chain hydrocarbon group having at least 10 carbon atoms;

R³-CONH-(CH₂)n6-NHCO-R³ (IIb)

where R³ is a straight-chain hydrocarbon group having 10 to 25 carbon atoms and n6 is an integer in the range of 1 to 8, and

R4 -NHCO-(CH2 )n7-CONH-R4 (IIc)

where R4 is a straight chain hydrocarbon group having 10 to 25 carbon atoms and n7 is an integer in the range of 1 to 8.

In a further embodiment, the aliphatic urea compound (2) corresponds to the following general formula III:

R⁵-NHCONH-R⁵ (III)

where R⁵ and R⁶ independently represent hydrogen or a straight-chain hydrocarbon group having 1 to 26 carbon atoms and at least one the radicals R⁵ and R⁶ are a straight-chain hydrocarbon group having at least 10 carbon atoms.

Preferably, each of the organic crystal particles has a particle size of 0.1 µm or less.

Preferably, the preparation comprises an antioxidant with active hydrogen.

Preferably, each of the organic crystal particles is microencapsulated in a transparent matrix polymer for encapsulation, wherein the matrix polymer for encapsulating the organic crystal particles microcapsules is different from the matrix polymer of the recording material itself.

Preferably, the transparent matrix polymer for encapsulation comprises a coloring or color-imparting material.

According to a further aspect of the invention, there is provided a reversible, thermosensitive recording medium comprising a substrate, a recording layer made of a reversible, thermosensitive material according to any one of claims 1 to 10, and a protective layer stacked in this order,

wherein the recording layer comprises transparent spacer particles having a thickness approximately equal to that of the recording layer and having a particle size below the thickness of the recording layer, in a proportion of 10 wt.%.

According to yet another aspect of the invention, there is provided a reversible thermosensitive recording medium comprising a substrate, a recording layer made of a reversible thermosensitive material according to any one of claims 1 to 10, and a protective layer stacked in this order,

wherein the protective layer is selected from the group consisting of:

(i) a protective layer of a soluble polyimide which is soluble in organic solvents,

(ii) a protective layer comprising ultrafine particles of an oxide, the primary particles of which have an average particle size of 100 nm or less;

(iii) a protective layer made of a plurality of layers, the hardness of the plurality of layers becoming higher toward a surface layer from the substrate, a protective layer of at least the outermost surface layer comprising the ultrafine particles of an oxide whose primary particles have an average particle size of 100 nm or less; and

(iv) a protective layer made of at least one resin selected from the group consisting of polyethylene terephthalate, fluorine-containing polymers, polysulfones, polyethylene naphthalate, polyphenylene sulfide, polyarylates, polyimides and polyamides, wherein in the case of the protective layer being (iv), an adhesive layer is additionally disposed between the substrate and the recording layer.

Finally, a reversible, thermosensitive recording material is provided, comprising a substrate, a recording layer made of a reversible, thermosensitive material as defined above, and a protective layer, arranged in this order,

wherein a reflection layer is arranged between the substrate and the recording layer and the information is recorded or erased by means of a laser beam and

wherein each of the organic crystal particles has a particle size of 0.1 µm or less.

Further preferred embodiments are as set out in the claims.

Thus, the invention described here enables the objects of (1) providing a reversible thermosensitive recording material made from a composition comprising a matrix polymer and organic crystal particles dispersed therein, which can record and erase information easily and reliably because the temperature at which the organic crystal particles become transparent to light is high and the temperature range in which the organic crystal particles are transparent to light is wide, (2) providing a reversible thermosensitive recording material which is excellent in recording resolution and gives a high contrast in recording information, (3) providing a reversible thermosensitive recording material which is excellent in durability and can nevertheless maintain recording properties constant even though information is repeatedly recorded and erased by means of a thermal head, and (4) providing a reversible thermosensitive recording medium which comprises a recording material having the above-mentioned excellent properties and which can be used for a card with a display function, a film for a facsimile machine or an optical storage device.

Short description of the drawings

The invention will be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings, as follows:

Fig. 1 is a cross-sectional view schematically illustrating an example of a reversible thermosensitive recording medium according to the present invention.

2a and 2b are views schematically showing a crystalline state of organic crystal particles in the reversible thermosensitive recording medium according to the present invention. Specifically, Fig. 2a illustrates an organic crystal particle 6a in a single-crystalline state in which the particle is light-transmissive. Fig. 2b shows an organic crystal particle 6b in a polycrystalline state in which the particle can scatter light.

Fig. 3 is a graph showing the light transmittance of the organic crystal particles as a function of temperature of the reversible thermosensitive recording sheet according to the present invention.

Fig. 4 is a diagram schematically showing an example of the reversible thermosensitive recording material according to the present invention.

Figs. 5a and 5b are views schematically showing a crystalline state of an organic crystal particle in the reversible thermosensitive recording material according to the present invention.

Fig. 5a shows an organic crystal particle 6A in a single-crystalline state in which the particle is transparent to light. Fig. 5b shows an organic crystal particle 6B in a polycrystalline state in which the particle can scatter light.

Fig. 6 is a cross-sectional view schematically showing an example of the reversible thermosensitive recording medium according to the present invention.

Fig. 7 is a cross section schematically showing an example of the reversible thermosensitive recording medium according to the present invention.

Fig. 8 is a cross section schematically showing an example of the reversible thermosensitive recording medium according to the present invention.

Fig. 9 is a cross-sectional view schematically showing an example of the reversible thermosensitive recording medium according to the present invention having an adhesive layer.

Fig. 10 is a view schematically illustrating a magnetic card which is an example of the recording medium using the re reversible thermosensitive recording material according to the present invention.

Fig. 11 is a graph showing the recording characteristics of the reversible thermosensitive recording material obtained in Example 4 of the present invention.

Fig. 12 is a graph showing the recording characteristics of the reversible thermosensitive recording material obtained in Example 6 of the present invention.

Fig. 13 is a cross section schematically showing an example of the optical recording medium using the reversible thermosensitive recording material according to the present invention.

Fig. 14a, 14b and 14c are graphs showing the recording characteristics of the reversible thermosensitive sheets obtained in Example 11 of the present invention. Fig. 14a is a graph showing the recording characteristics of the reversible thermosensitive recording material according to the present invention.

Fig. 14b is a graph showing the recording characteristics of the reversible thermosensitive recording material comprising eicosanedicarboxylic acid as a material for the organic crystal particles. Fig. 14c is a graph showing the recording characteristics of the reversible thermosensitive recording material comprising docosanol as a material for the organic crystal particles.

Fig. 15 is a cross-sectional view schematically showing another example of the reversible thermosensitive recording medium according to the present invention.

Description of preferred embodiments

A matrix polymer used for the reversible thermosensitive recording material of the present invention has a group capable of forming a hydrogen bond. When this polymer is heated, the bonds between the polymer and the organic crystal particles dispersed therein can be changed. It is preferred that the polymer be transparent and colorless. Examples of suitable matrix polymers include polyesters, polyacrylates, polyvinyl chloride, polyvinyl chloride/vinyl acetate copolymers, cellulose acetate, polyvinyl butyral, polystyrene, and styrene/butadiene copolymers. Specifically, adhesive polyesters having a hydroxyl group, partially saponified vinyl acetate/vinyl chloride copolymers, polyamides, polyurethanes, thermoplastic phenol resins, vinyl alcohol copolymers, acrylic acid copolymers, acrylamide copolymers, maleic acid copolymers, urea resins, epoxy resins, melamine resins and the like are preferred. In the recording material of the present invention, this matrix polymer is used as a thin film so that excellent transparency can be obtained.

Among the above matrix polymers, with respect to the copolymers having vinyl chloride and vinyl acetate as repeating units, when the recording medium to be obtained is heated, a temperature at which the organic crystal particles dispersed therein become transparent to light and the width of the temperature range in which the particles are transparent to light can be adjusted according to the difference in the ratio of the above-mentioned units in the copolymer. Moreover, a vinyl acetate component is partially saponified to form vinyl alcohol units, resulting in a three-component copolymer comprising vinyl chloride, vinyl acetate and vinyl alcohol. Thus, the proportion of a group of the matrix polymer capable of forming hydrogen bonds is adjusted, whereby the above-mentioned temperature and the width of the temperature range can be adjusted.

The organic crystal particles used in the present invention are made of a compound having a group capable of forming a hydrogen bond as defined in claim 1. Examples of the group capable of forming a hydrogen bond include a carboxyl group, a hydroxyl group, an amino group, an amide group, etc. The above-mentioned compound has at least one of these groups, and the organic crystal particles have a melting point in the range of 60°C to 120°C.

The hydroxycarboxylic acid, which is a first material for the organic crystal particles, is also called a hydroxy acid and has at least one hydroxyl group and at least one carboxyl group in the molecule, each group being able to form a hydrogen bond. As hydroxycarboxylic acids, compounds corresponding to the following formulas Ia - Ie, as defined above, are particularly preferred.

The compound of formula Ia is a hydroxyalkylcarboxylic acid. The compound of formula Ib is an alkylphenol in which the alkyl group is carboxylated, the position of the carboxyl group being arbitrarily chosen. The compound of formula Ic is an ester of a hydroxybenzoic acid and a hydroxyalkylcarboxylic acid. The compound of formula Id is an ester of a hydroquinone with an alkylenedicarboxylic acid. The compound of formula Ie is a hydroxybenzoic acid ester. These compounds of formulae Ia - Ie are materials used for antiseptics, food additives and plasticizers for industrial purposes. Due to their low toxicity, they are particularly suitable for use in a recording material which is likely to come into direct contact with the consumer. By mixing these hydroxycarboxylic acids with other compounds such as higher alcohols, aliphatic acids, alkylamines, other hydroxycarboxylic acids, dicarboxylic acids, diamines and alkylene glycols, a eutectic crystal and a complex can be formed, thereby producing organic crystal particles with a melting point in the range ranging from 60 to 120ºC can be obtained by using the above-mentioned mixture. Examples of such hydroxycarboxylic acids include gallic acid, mandelic acid, tropic acid, malic acid, tartaric acid and citric acid and their derivatives.

In hydroxycarboxylic acid, a hydroxyl group and a carboxyl group in the molecule, both of which are capable of forming hydrogen bond, greatly affect the melting and crystallization behavior of organic crystal particles. This hydroxycarboxylic acid has two groups capable of forming hydrogen bond, so its crystallinity is high and the strength of the crystal is also high. Most hydroxycarboxylic acids have a suitable melting point of 60ºC to 120ºC and their melting and crystallization behavior is highly reliable. In contrast, alkylenediamines, alkylenedicarboxylic acids and alkylene glycols are molecules that have two groups capable of forming hydrogen bond in their molecule. However, these molecules are not suitable for the present invention because the melting point varies depending on whether the number of carbon atoms in their alkyl chain is even or odd, and the melting point is low compared to that of the hydroxycarboxylic acids.

The crystallization of the hydroxycarboxylic acid particles in the matrix polymer is essentially influenced by the hydrogen bonding present at the interface between the matrix polymer and each hydroxycarboxylic acid particle. Therefore, the number of groups capable of forming hydrogen bonds present at the interface between crystal particles and matrix polymer determines the behavior of crystallization. Likewise, the characteristics of the group capable of forming hydrogen bonds determines the crystallization.

The aliphatic amide compound according to (2), which is a second material for the organic crystal particles, corresponds to the following formula IIa, IIb or IIc, as defined above.

The aliphatic urea compound according to (2), which is also a second material for the organic crystal particles, corresponds to the above formula III.

These compounds have an amide group or a urea group as the group capable of forming hydrogen bonds. The amide group has two moieties capable of forming hydrogen bonds due to its structure of -NHCO-; thus, an amide group produces a strong association between molecules. The strength of this association or attraction affects the melting point of the compound (for example, the melting point of organic crystal particles), the crystallinity of the compound (this affects the recording properties, particularly the contrast while information is being recorded), and the dispersibility of the compound in the matrix. The urea group has a structure of -NHCONH- which includes another moiety capable of forming a hydrogen bond, compared with the amide group. Therefore, it produces a stronger association or attraction between molecules. The amide compound represented by formula IIb or IIc has hydrogen bond forming moieties twice as many as those represented by formula IIa, so that the compounds represented by formula IIb or IIc show a stronger attraction than that of the compound represented by formula IIa. Due to these properties, the organic crystal particles have a high melting point in the range of 70 to 150°C. As a result, satisfactory crystallinity can be obtained. These hydrogen bond forming moieties show a strong interaction with the matrix polymer and improve the reversible thermosensitive recording properties. These hydrogen bond forming moieties associate with long-chain hydrocarbon groups present in the aliphatic amide compound or the aliphatic urea compound. The long-chain hydrocarbon moiety influences the compatibility between the matrix polymer and the compound molecule and the melting point of the compound molecule. In particular, when the aliphatic hydrocarbon moiety represents a hydrocarbon chain with 10 or more carbon atoms, the organic crystal particles are easily dispersed in the matrix polymer as fine particles. Thus, this case is preferred. As described above, most of these compounds have a high melting point in the range of 70 to 150°C and their behavior in melting and crystallization is reliable, so that these compounds are preferred.

A mixture obtained by adding at least one compound selected from the group consisting of higher alcohols, aliphatic acids, alkylamines, dicarboxylic acids, alkylenediols, alkylenediamines, alkylene glycols, hydroxycarboxylic acids, derivatives of alkyl benzoate, carboxyalkylphenols, aminoalkylphenols and aminoalkyl alcohols to the aliphatic amide or aliphatic urea compound has excellent properties.

A third material for the organic crystal particles is a mixture of an aliphatic dicarboxylic acid and an aliphatic compound containing an amide group or a urea group as defined in (3) above.

Examples of the saturated aliphatic monocarboxamides include lauramide (melting point = mp: 86ºC), palmitinamide (mp: 100ºC), stearamide (mp: 101ºC), behenamide (mp: 110ºC), hydroxystearamide (mp: 110ºC), N-stearylstearamide (mp: 94ºC), N-stearyloleamide (mp: 67ºC), oleylstearamide (mp: 74ºC), methylolstearamide (mp: 111ºC) and methylolbehenamide (mp: 110ºC). Examples of the saturated aliphatic bisamide include methylenebisstearamide (mp: 143ºC), ethylenebislauramide (mp: 157ºC), ethylenebisstearamide (mp: 143ºC), ethylenebishydroxystearamide (mp5 144ºC), hexamethylenebisstearamide (mp: 146ºC), N,N'-distearyladipamide (mp: 144ºC), m-xylenebisstearamide (mp: 123ºC) and N,N'-distearylisophthalamide (mp: 129ºC). Examples of the hydrocarbon group-substituted urea include N,N'-distearylurea (mp: 114ºC), stearylurea (mp: 109ºC), xylenebisstearylurea (mp: 163ºC) and diphenylmethanebisstearylurea (mp: 210ºC).

The aliphatic dicarboxylic acid has a carboxyl group at both ends of its hydrocarbon chain, and this carboxyl group contributes to a hydrogen bond between the molecules. For this reason, the aliphatic dicarboxylic acid itself has satisfactory crystallinity and a high melting point of 120°C. In addition, when the organic crystal particles are prepared and mixed with the matrix polymer, the temperature at which the particles become transparent to light is relatively high. The length of the hydrocarbon chain of the aliphatic dicarboxylic acid is appropriately determined in view of the melting point of the organic crystal particles, the interaction of the molecules in the organic crystal particles, the solubility of the organic crystal particles in the matrix polymer, and the dispersibility of the organic crystal particles in the matrix polymer.

The aliphatic compound having an amide group or a urea group has a hydrocarbon chain having almost the same length as that of the aliphatic dicarboxylic acid. When a mixture of this compound and the aliphatic dicarboxylic acid is heated to a eutectic, the interaction between the molecules is enhanced, resulting in the formation of an association or a complex, thereby forming a composite such as a mixed crystal or a eutectic. In this way, a plurality of crystal forms are formed, and the temperature range in which the particles are transparent to light can be broadened due to the difference in the temperature characteristics of each form. In addition, a variety of complicated fine crystals are present, so that the degree of white turbidity of the recording material when the particles scatter light is high. As a result, it is believed that high contrast can be obtained. In addition, it is assumed that the aliphatic compound having an amide group or a urea group exists as ultrafine particles each dispersed in the organic crystal particles, and that these ultrafine particles serve as a crystal nucleus in the organic crystal particle. Since the melting point of each crystal nucleus is high, it acts as a nucleus during the step of melting and crystallization of the organic crystal particles. even at higher temperatures. As a result, the organic crystal particles have a high melting point and a wide range of temperatures at which the particles are transparent to light can be achieved. Also, since the matrix polymer has a group capable of forming hydrogen bonds, more remarkable effects due to the strong interaction between the matrix and the organic crystal particles are exhibited and excellent recording characteristics are exhibited.

The dicarboxylic acid and the compound having an amide group or a urea group are mixed in a ratio in the range of 60:40 to 10:90 (by weight). When the aliphatic dicarboxylic acid is present in excess, the aliphatic dicarboxylic acid is not effectively eutectic to be complexed with the aliphatic compound having an amide group or urea group. An excess amount of dicarboxylic acid causes a strong interaction with the matrix polymer and thus the excess amount of dicarboxylic acid migrates from the organic crystal particle and is dispersed in the matrix. As a result, the resulting recording medium does not have high contrast. In contrast, when the compound having an amide group or urea group is present in excess, the excess amount of the compound migrates from the organic crystal particle and is mixed with the matrix and becomes inhomogeneous. As a result, the resulting recording medium does not provide high contrast and the temperature range in which the particles are transparent to light becomes narrow.

The particle size of the organic crystal particles made of the above-mentioned first to fourth materials is 0.1 µm or less. As described below, these organic crystal particles are obtained by dissolving the above-mentioned materials and the matrix polymer in a protic organic solvent and coating the resulting mixture on a substrate to form particles dispersed in the matrix polymer. For example, a recording layer 3 comprising organic crystal particles dispersed therein is formed on the substrate 1 and then on the surface of which a protective layer 4 (described below) is formed, thereby obtaining a recording medium as shown in Fig. 6. Alternatively, the recording medium can be obtained by dispersing the organic crystal particles in an organic solvent solution of the matrix polymer and coating this dispersion on the surface of the substrate, followed by drying to form a recording layer, and then forming a protective layer on the recording layer.

As shown in Fig. 4, it is advantageous that the organic crystal particles 6 are microencapsulated in a matrix polymer 8 different from the matrix polymer 5 in which the organic crystal particles are dispersed. A material for the matrix polymer 8 (hereinafter referred to as matrix polymer for encapsulation) used for microencapsulation is appropriately selected from the group of polymers used as the matrix polymer in which the organic crystal particles are dispersed. As the matrix polymer for encapsulation, compounds having appropriate compatibility with the material for the organic crystal particles and the matrix polymer are selected. As described above, even if the compatibility between the compound constituting the organic crystal particles and the matrix polymer is relatively low, this compatibility can be increased by interposing the matrix for encapsulation therebetween. Therefore, the microencapsulated organic crystal particles enable high reliability and stable crystal properties.

For example, when the organic crystal particles are a compound having a group capable of hydrogen bonding and have either an alkyl group or an alkylene group having 10 or more carbon atoms, and when the matrix polymer 5 has a repeating unit polymer having an alkylene component having 10 or more carbon atoms, styrene-butadiene copolymers, ethylene/vinyl acetate copolymers, ethylene/acrylic ester copolymers, olefinic copolymers and the like are preferable as the matrix polymer for encapsulation 8. When encapsulation is effected with such a polymer, the compound constituting the core part of the organic crystal particle and the matrix polymer for encapsulation form a micelle of the opposite type of hydrogen bond due to the affinity of the alkylenes. On the other hand, the matrix polymer for encapsulation and the matrix polymer form a hydrogen bond. As a result, the encapsulated organic crystal particles and the matrix polymer have a suitable affinity (ie, compatibility) with each other, thereby obtaining the above-mentioned effects.

The microcapsule can be produced by forming a micelle via an aqueous or non-aqueous type coacervation method. As the coacervation method, a simple coacervation method in which a polymer solution and either a non-solvent or an electrolyte are used in combination and a complex coacervation method in which electrical phase separation, i.e., phase separation of polycations and polyanions, is caused are mentioned. Examples of the method for producing a microcapsule by a coacervation method include chemical manufacturing methods such as an interfacial polymerization method, in-situ polymerization, and an interfacial curing coating method in which a curing agent is used. And physical manufacturing methods such as a method using phase separation, a spray drying method, and a fluid coating method.

The particle size of the obtained microcapsule is usually 0.5 to 50 µm, and the thickness of the coating is 0.1 to 5 µm. The content of the microcapsule 6 in the matrix polymer 9 is 10 wt% to 70 wt% based on the total weight of the matrix polymer 5 and the microcapsule 6, and is preferably 20 wt% to 40 wt%, although the content may vary depending on the size of the microcapsule and the thickness of the coating on the microcapsule. If the content of the microcapsules exceeds this range, the bonding strength of the recording layer is reduced. As a result, a uniform or even recording layer cannot be obtained. In contrast, if the As the content of the matrix polymer increases, the content of microcapsules decreases, so that the contrast of the recording is deteriorated.

A colored medium can be obtained by using a colored polymer containing a dye as a matrix polymer for encapsulation in the production of the above-mentioned microcapsule. A multi-color recording medium can be obtained by using various kinds of microcapsules each having different colors. For example, a reversible thermosensitive recording material of the present invention comprising a plurality of differently colored microcapsules is formed on a substrate (e.g., a polymer film) 1 on which a reflection layer 2 as shown in Fig. 1 is formed, and a protective layer 4 is provided thereon, thereby obtaining a multi-color recording film optically scattered in various colors due to opacity caused by thermal writing.

As shown in Fig. 15, it is possible that spacer particles 43 are contained in a recording layer 3 in order to improve the durability of the recording medium of the present invention. The spacer particles or particles 43 are light-transmitting particles having a particle size approximately equal to or smaller than the thickness of the recording layer 3. It is desirable that the spacer particles are formed of spherical particles made of glass or a polymer and having an average particle size of 1 to 100 µm with a narrow particle size distribution. As the material for the copolymer particles, melamine resins, acrylic resins, nylon, polycarbonates and the like can be used. The spacer particles are contained in the recording layer in an amount of 10% by weight or less. The material, particle size and content of the spacer particles are selected in consideration of the material of the matrix polymer used in the recording medium and in consideration of the accuracy of recording. As shown in Fig. 15, when the spacer particles are contained in the recording layer, the hard spherical particles function even though the recording material likely to be thermally deformed, as a cushion in the recording layer and withstand the pressure of a thermal head. Thus, the recording layer is not deformed during melting in recording and durability is significantly improved. As described above, according to the present invention, problems regarding deformation of the recording material due to melting of the recording layer, which cannot be avoided in this type of recording, can be easily overcome by a simple method.

The organic crystal particles (including the microencapsulated organic crystal particles) are contained in an amount of 5 to 50 parts by weight, preferably 15 to 40 parts by weight, based on 100 parts by weight of the matrix polymer. If the content of the organic crystal particles exceeds the above amount, the bonding strength of each component constituting the recording layer decreases and it becomes difficult to form a uniform or even recording layer by coating. In contrast, if the amount of the matrix polymer is increased, the amount of the organic crystal particles is reduced, so that it becomes difficult to make the recording layer opaque and the contrast of the recording is deteriorated.

Examples of the materials for the substrate 1 used in the recording medium according to the present invention include polymers, metals and ceramics. However, there are no particular limitations. Examples of the polymers include polyester resins, polycarbonate resins and acrylic resins. Examples of the metal include aluminum and stainless steel. Examples of the ceramics include glass. The polymer material commonly used is in the form of a film, and the substrate is selected based on properties such as strength and rigidity of the material. Plastics such as nylon, cellulose acetate type resins, polystyrene, polyethylene, polypropylene, polyester, polyimides, polycarbonates and polyvinyl chloride are used alone or in combination. Polyester and polyvinyl chloride are preferred. Regarding the film structure, a sufficient thickness is required to maintain the configuration of the substrate. The thickness is preferably about 0.005 to 5 mm.

The reversible thermosensitive recording medium of the present invention is obtained by sequentially forming a recording layer 3 and a protective layer 4 (described below) on a base material 1 as shown in Fig. 6. As described above, the recording layer is formed by dissolving the above-mentioned materials capable of forming the organic crystal particles and the matrix polymer in the organic solvent; adding, if necessary, the above-mentioned spacer particles, a plasticizer, a leveling agent, a dispersant, a nucleus forming agent having a group capable of forming a hydrogen bond, an antioxidant, etc.; coating the mixture on a substrate 1, followed by drying. It is also desirable that the nucleus forming agent and the antioxidant have a group capable of forming a hydrogen bond. For this reason, a recording medium can be obtained which has a high temperature for crystallization of the organic crystal particles and a wide temperature range in which the particles are transparent to light, and which provides excellent contrast when information is recorded. As the antioxidant, phenol type antioxidants are preferred. Examples of methods for coating the mixture include conventional coating methods such as gravure coating, calender coating and screen coating. The reversible thermosensitive recording medium of the present invention can be manufactured in various kinds of configurations in accordance with the intended use. For example, in order to increase the contrast when recording the information, a reflection layer 2 (described below) is provided between the substrate 1 and the recording layer 3 (Fig. 1). Other examples of the recording medium of the present invention are shown in Figs. 7 to 9.

In the recording medium as shown in Fig. 1 or 4, the organic crystal particles are dispersed in the matrix polymer 9. The organic crystal particles are present in two kinds of crystal states as shown in Figs. 2a, 2b and 5a, 5b. Figs. 2a and 5a show a single-crystal state in which the particles are transparent to light. Figs. 2b and 5b show a polycrystalline state in which the particles scatter light. A recording layer containing the organic crystal particles capable of scattering light appears opaque as a whole.

As shown in Fig. 3, when the recording layer is gradually heated and the temperature exceeds T₀ (where T₀ is almost equal to the glass transition point (Tg) of the matrix polymer), the recording layer begins to change from an opaque state to a transparent state; when the temperature reaches T₁, the recording layer becomes completely transparent. At this point, the organic crystal particles are in a single-crystal state and are translucent. When the temperature is further increased to T₂ or above, the light transmittance of the organic crystal particles gradually decreases and the recording layer becomes semi-transparent at T₃ (T₃ is approximately equal to the melting point of the organic crystal particles). When the organic crystal particles are heated to a temperature between T₁ and T₃, the recording layer begins to change from an opaque state to a transparent state; when the temperature reaches T₁, the recording layer becomes completely transparent. and T₂, are cooled to room temperature, the organic crystal particles are transparent to light and thus the recording layer remains transparent. When the organic crystal particles heated to T₃ or above are cooled to room temperature, the organic crystal particles scatter light and thus the recording layer becomes opaque.

Recording is performed by setting the above-mentioned transparent or opaque state as an initial state and changing the state as described above. For example, first, a recording medium having a recording layer containing organic crystal particles is heated to a temperature in the range of T₁ to T₂, thereby obtaining a transparent recording layer. Then, as shown in Fig. 7, information can be recorded on the recording medium. By selecting appropriate materials for the organic crystal particles and the matrix polymer, the recording speed can be controlled due to the delay in the change of the crystal structure in a supercooled portion at the time when the recording layer is cooled after heating. As the means for heating 19, a thermal head, a heat roller and a laser beam are used; however, various other means or devices may be used without limitation.

In the recording medium of the present invention, a protective layer 4 is provided to protect the surface of the recording layer. Any transparent film can be used as the protective layer as long as it has the appropriate strength and rigidity for supporting the recording layer and resistance to abrasion. Examples of materials for the protective layer include polyethylene terephthalate, fluorine-containing polymers, polysulfones, polyethylene naphthalate, polyphenylene sulfide, polyarylates, polyimides and polyamides. The thickness of the protective layer should be such that heat for recording and erasing information generated by a heating agent is transmitted to the recording layer. Preferably, the thickness of the protective layer is about 0.001 to 0.05 mm.

As the protective layer according to the present invention, a protective layer made of a soluble polyimide or a protective layer made of a polymer comprising ultrafine particles or particles of an oxide whose primary particles have an average particle size of 10 nm or less is preferred. The soluble polyimide means a polyimide which is soluble in an organic solvent and is a polymer having repeating units represented by the following formula IV:

wherein X is O, CO, C(CF₃)₂ or a single bond; Y is O, CO, C(CF₃)₂ or a single bond and R⁷ represents a group having an aromatic ring and m₂ and m₃ are independently 0, 1 or 2.

In a polymer comprising the above-mentioned repeating unit of formula IV as the molecular structure of an aromatic portion of the tetracarboxylic anhydride component, ie:

there are biphenyl, biphenyl ether, benzophenone and di(trifluoro)diphenylmethane; and as the structure (-NR⁷-N-) of an aromatic diamine, there are phenylenediamine, oxidianiline, methylenedianiline, diaminobiphenyl and tolidine. A soluble polyimide resin obtained by combining these components and a soluble polyimide resin which is a copolymer comprising these components and other components such as pyromellitic anhydride can be used for the protective layer. These polyimides are formed by polycondensation of aromatic tetracarboxylic anhydrides with aromatic diamine. These polyimides are dissolved in a solvent which can dissolve the polyimide and coated on the recording layer as described above. As organic solvents which can dissolve these soluble polyimide resins, dimethylformamide, Dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide and cresol are suitable. However, any organic solvent capable of dissolving the polyimide resins can be used for the formation of the protective layer. A solvent capable of dissolving a soluble polyimide resin is selected as an organic solvent for preparing the soluble polyimide by polymerization, whereby a polyimide resin solution can be obtained. In addition, there is another method for obtaining a polyimide resin solution in which a polyamic acid as a precursor is taken out from the reaction mixture by precipitation. The polyamic acid thus obtained is subjected to heat treatment to obtain an imide and then dissolved in an organic solvent.

The recording medium of the present invention as shown in Fig. 6 is obtained by forming a recording layer 3 made of a matrix polymer comprising organic crystal particles on a substrate 1 and then coating a solution comprising a soluble polyimide thereon, followed by drying. When the protective layer 4 is formed of the above-mentioned polyimide resins, the protective layer is excellent in heat resistance and mechanical strength, so that repeatable properties of highly reliable recording and erasing can be obtained. As for the temperature for thermal deformation of the protective layer caused by deterioration due to the thermal head, it is 230°C in the case of polyethylene terephthalate, while it is 300°C or more in the case of polyimides, which is an excellent property as can be seen. When a polyimide is used for the protective layer, a decrease in recording conditions due to repetition of heating in the process of recording and erasing and mechanical damage caused by the thermal head can be avoided. As a result, reversible thermosensitive recording with satisfactory repeatability characteristics can be realized. In the method for producing a recording medium of the present invention using a soluble polyimide resin as a protective layer, a solution of the soluble polyimide resin is coated on the recording layer. In this method, a step of applying heat for imidization is not required. In this method, only heating to a relatively low temperature and reducing the pressure to remove the organic solvent are required. Thus, deterioration of the matrix polymer of the recording layer can be prevented. In addition, condensed water resulting from imidization is not generated, and thus defects are unlikely to occur and a homogeneous or uniform protective layer can be formed.

Furthermore, according to the preferred embodiment in which the soluble polyimide resin component comprises a repeating unit represented by the formula IV, deterioration in recording conditions due to repetition of heating during recording and erasing as well as mechanical damage caused by the thermal head can be avoided. As a result, reversible thermosensitive recording with satisfactory repeatability properties can be realized.

On the other hand, in the case where poly(pyromellitic)imide, which is commercially available as "Kapton" (trade name, manufactured by Dupont Co., Ltd.), is formed into a protective layer, heat treatment is required for the following reasons. Since the poly(pyromellitic)imide is insoluble in organic solvents, polyamic acid, which is the precursor of the imide and is soluble in organic solvents, is coated on the recording layer as an organic solvent solution, and the coated layer is heat-treated to form a polyimide. In this case, a temperature of 300°C or more is required to sufficiently cause the imidation reaction, so that it is necessary to pay great attention to the thermal deterioration of the recording layer caused during the heat treatment.

In addition, there is a problem that a void is formed by water generated during a condensation reaction of imidization, and thus a uniform film cannot be easily formed.

In another preferred embodiment of the protective layer, the protective layer 4 is made of a polymer comprising ultrafine particles 18 of an oxide whose primary particles have an average particle size (i.e., diameter) of 100 nm or less. As a resin that can form the matrix of this protective layer, thermosetting resins such as acrylic resins, epoxy resins and unsaturated polyester resins are preferred because they have high hardness. Since ultrafine particles or particles of an oxide are contained, thermoplastic resins such as polyester resins or polyamide type resins can also be used. Among them, when an energy beam-curable acrylic resin is used, the resin can be easily cured with an energy beam after coating, so that productivity is satisfactory. The obtained protective layer has sufficient transparency. As an energy beam curable acrylic resin, transparent resins having an acryloyl group or a methacryloyl group are suitable; for example, urethane acrylate resins, epoxy acrylate resins, acrylic resin oligomers, methacrylate resins and acrylate resins can be used alone or in combination. As a method for curing with an energy beam, conventional methods such as UV irradiation and electron beam irradiation can be used.

The material of the ultrafine or microfine particles 18 of an oxide contained in the protective layer 4 is a metal oxide or silicon oxide. Such a material includes silicon oxide, aluminum oxide, titanium oxide, chromium oxide, zinc oxide, tantalum oxide, niobium oxide and manganese oxide. Among them, silicon oxide, aluminum oxide and titanium oxide are preferred because they are white and have satisfactory compatibility with the resin of the protective layer. Particles having a narrow particle size distribution are preferred and they can be easily produced. Particles of silicon oxide, aluminum oxide or titanium oxide having a narrow particle size distribution Molecular size distributions can be produced, for example, by hydrolysis of a metal halide gas at high temperature.

As microfine particles of an oxide, particles whose primary particles have an average particle size of 100 nm or less are used, and particles whose primary particles have an average particle size of 20 nm or less are preferred.

When the ultrafine particles of an oxide are contained in the protective layer, (1) the durability of the protective layer is increased and a flat configuration of the surface of the protective layer is maintained, and (2) a reduced friction between the means for providing heat energy (for example, the thermal head) and the surface of the protective layer is provided. The ultrafine particles used in the present invention are fine particles which have high hardness and do not wear off by themselves. Since the average particle size of the primary particles of the ultrafine particles is preferably 100 nm or less, the surface of the protective layer is flat and its durability is increased. However, although the primary particles are small, a very fine unevenness of less than 100 nm is formed on the surface of the protective layer. Due to this unevenness, the contact area between the thermal head and the protective layer decreases, so that the friction between the thermal head and the protective layer also decreases. As a result, mechanical and thermal damage can be prevented. In addition, the apparent heat resistance of the protective layer itself is improved because the ultrafine particles are made of an inorganic oxide which is heat resistant. The ultrafine particles of an oxide can satisfactorily conduct the heat generated by the thermal head. In addition, since ultrafine particles are used, light scattering due to the unevenness of the surface hardly occurs. As a result, the transparency of the protective layer is maintained. Furthermore, the fine particles present on the surface of the protective layer regulate the surface energy so that the surface does not get dirty as easily and waste and dust do not stick to it as easily.

The ultrafine particles of an oxide are contained in the protective layer in an amount of 0.1 to 50 wt%, preferably about 10 wt% or less. When the ultrafine particles of an oxide are contained in an amount of 5 wt%, the dispersibility is particularly satisfactory and the unevenness on the surface of the protective layer becomes 50 nm or less, thereby obtaining a flat surface.

Furthermore, in order to improve the lubricity and abrasion resistance of the surface of the oxide ultrafine particles, it is possible to form an organic chemical adsorption film on the surface of the fine particles to provide a water-repellent property, a lipophilic property and a fouling resistance property thereon. As a chemical adsorbent used to form such a film, a silane coupling agent having a hydrocarbon chain and/or a fluorocarbon chain can be used. When the surface of the oxide ultrafine particles is treated with this type of chemical adsorbent, an organic chemical adsorption film is formed on the surface. In this adsorption film, an alkyl group, a perfluoroalkyl group and/or a partially fluorinated alkyl group having different chain lengths derived from the chemical adsorbent are present. These groups are chemically bonded to the oxide ultrafine particles via a siloxane group, an ether group or the like. When the ultrafine particles of oxide treated as described above are used, for example, in the case of the lipophilic organic chemical adsorbent film, the compatibility between the film and the resin of the protective layer is improved and the bonding strength between them is increased. Also, in the case of a fluorine type organic chemical adsorbent film, a particularly excellent lubricity can be provided on the surface of the ultrafine particles of oxide, so that damages caused by the thermal head can be prevented and excellent durability performance can be achieved. In addition, the surface can be protected from dirt, debris and dust so that image irregularity does not occur. In addition, the compatibility between the organic chemical adsorption film and the resin of the protective layer and the contamination resistance of the surface of the ultrafine particles can be adjusted by designing and selecting a hydrocarbon chain and a group that binds to the hydrocarbon chain in the organic chemical absorbent.

In order to improve the durability of the protective layer, it is preferable that two or more protective layers are laminated. In this case, a structure in which the hardness of the protective layer increases toward the outermost surface layer of the protective layer is desired, and the ultrafine particles of an oxide are contained at least in the outermost surface layer. When the protective layer is made of a multilayer structure, the hardness increases toward the outermost surface layer by selecting a desired resin or a desired mixing ratio of resins for the respective layers.

In the recording medium of the present invention, as shown in Figs. 1 and 7, for example, it is possible to provide a reflection layer 2 between the base material 1 and the recording layer 3 in order to increase the contrast between transparent and opaque portions when information is recorded. As for the reflection layer, a metal having a high reflection index in the visible light region such as Al, Au and Te can be used directly in the form of a film. Alternatively, a layer in which their powders are dispersed in a binder resin can be used in the form of a film. Organic thin films having metallic luster, for example, methine dyes or xanthene dyes can be used. The reflection layer can be formed directly on a support film by vacuum deposition or sputtering, or can be coated thereon by casting or plating. It is It is also possible to use a reflective film (designated by reference numeral 29 in Fig. 9) in which a reflective layer is formed on the surface of a carrier film. The carrier film is made of any material that can be used for a substrate.

With the recording medium provided with the reflective layer, clear recording with high resolution can be easily performed. Therefore, excellent, high-density optical recording or erasure of information can be performed using a laser beam. This recording medium can be used as a low-cost optical disk in which information can be repeatedly recorded.

In the recording medium of the present invention, it is recommended that an adhesive layer 22 is provided between the recording layer and the substrate in order to prevent deterioration in performance due to deformation of the recording layer caused by repeated recording and erasing of information. In the case that the reflection layer is provided between the recording layer and the substrate, the adhesive layer may be provided between the substrate and the reflection layer and/or between the reflection layer and the recording layer. Examples of the adhesive used for the adhesive layer include thermosetting resins such as phenol resins and epoxy resins, thermoplastic resins such as polyamide resins, polyurethane resins, vinyl chloride/vinyl acetate copolymer resins, and butyral resins, and elastomers such as butadiene/acrylonitrile rubber.

Next, a recording medium having an adhesive layer and a recording medium having an adhesive layer and a reflection layer will be described with reference to Figs. 8 and 9. Fig. 8 shows a recording medium in which an adhesive layer 22 is provided between a substrate 1 and a recording layer 3. This recording medium is produced, for example, by laminating a recording film 24 in which a reversible thermosensitive recording layer 3 is formed on a protective layer 4 by means of coating, on a substrate 1 with an adhesive layer 22 such that the surface of the recording layer 3 is opposite to that of the adhesive layer 22.

Fig. 9 shows a structure in which the adhesive layer 22, a reflection film 29, and a layer of a silane coupling agent 28, another adhesive layer 22 and a protective layer 24 are laminated sequentially on a substrate 1. The layer of a silane coupling agent 28 is provided to improve the adhesion between a reflection layer 2 on the reflection film 29 and the adhesive layer 22. The silane coupling agent may, for example, correspond to the following formula V:

Y-(CH2 )n11-Si(R8 )3-m4(X)m4 (V)

wherein Y represents a functional group that can react with an adhesive material in the adhesive layer. For example, Y is an amino group, a vinyl group, an epoxy group, a mercapto group, a chlorine atom or a lipophilic methyl group. Furthermore, n11 is an integer in the range of 0 to about 30. R8 is an inactive group such as a methyl group or ethyl group. X is a functional group that can be hydrolyzed and react with a material of the reflective layer. For example, X is a methoxy group, an ethoxy group or a chlorine group. The term m4 is an integer in the range of 1 to 3.

The recording medium in Fig. 9 can be manufactured, for example, as follows. First, a reflection layer 2 made of aluminum or the like is formed on a carrier film 26 by vapor deposition to obtain a reflection film 29. The surface of the reflection layer of this reflection film is treated with a silane coupling agent and adhered to the substrate 1 via the adhesive layer 22. Separately, a polymer film serving as a protective layer is provided, and a recording layer 3 made of a matrix polymer comprising organic crystal particles. The polymer film thus obtained having the recording layer 3 is laminated on the surface of the base material 1 with the reflection film 29 so that the recording layer 3 is in contact with the reflection layer 2 via the adhesive layer 22, thereby obtaining a recording medium.

As described above, the recording media shown in Figs. 8 and 9 are excellent in durability due to the formation of the adhesive layer. By using these media, high contrast recording can be performed due to the presence of the reflection layer.

Generally, according to the description of the prior art, the following points were required for reversible thermosensitive recording materials containing organic crystal particles dispersed in a matrix polymer: (1) the temperature to which the particles are heated to be transparent is high, and the temperature range in which the particles are in the transparent state is wide; (2) a contrast between transparent areas and opaque areas is high when information is recorded; and (3) stability in terms of retention and repeatability are excellent. The inventors have studied the above-mentioned points and found that the melting point of the organic crystal particles and the strength of the interaction generated between the compound constituting the organic crystal particles and the matrix polymer are important. In particular, it was found that the type and number of groups capable of forming hydrogen bonds in the organic crystal particles and in the matrix polymer are essential for obtaining satisfactory recording properties and contrast.

In the reversible thermosensitive recording material of the present invention, both the organic crystal particles and the matrix polymer are composed of a compound having a group capable of forming hydrogen bonds. As a result, they are compatible with each other and will have their melting and Softening properties are affected by their interactions. The recording material of the present invention exhibits crystallization behavior which is highly reliable, and the above-mentioned requirements can be satisfied. Also, the organic crystal particles and the matrix polymer are not substantially separated from each other, and large crystal particles and cracks are not generated.

In the recording material of the present invention, the compatibility between the matrix polymer and the organic crystal particles is appropriate so that the size of the crystal particles does not change when heating and cooling steps (i.e., recording and erasing) are repeated, and a recording material having high resolution and excellent durability characteristics can be provided.

Examples

In the following, the present invention will be described with the help of illustrative examples.

example 1

First, as a recording material, 2 g of 16-hydroxyhexadecanoic acid, 0.1 g of a phenol type antioxidant and 8 g of a partially saponified vinyl chloride/vinyl acetate copolymer having a hydroxyl group were dissolved in 100 ml of tetrahydrofuran. The solution thus obtained was coated on a reflection layer 2 formed by vapor deposition of aluminum on a polyester film (substrate 1) to a thickness of 0.2 mm and dried, thereby forming a recording layer 3 to a thickness of 13 µm. Then, a UV-curable acrylic resin prepolymer was coated on the recording layer 3 to a thickness of 10 µm. Thereafter, the prepolymer was irradiated with ultraviolet rays to cure, thereby forming a protective layer 4 (see Fig. 1). Thus, a reversible thermosensitive recording film was obtained. The coating The recording layer of the reversible thermosensitive recording sheet was opaque at room temperature. The sheet was heated using a thermal head and the temperature range in which the recording layer was transparent was determined by visual observation and using a Macbeth densitometer. The temperature range was found to be about 73ºC to 95ºC.

Then, starting from the film in an initial state in which the recording layer was opaque at room temperature, this film was heated to a temperature in the above-mentioned range and cooled to room temperature. This film, in which the recording layer was then transparent, was energized using a thermal head and information was recorded in the recording layer at 100ºC and erased from the recording layer at 90ºC.

The opaque region of the recording layer on which information was recorded and the transparent region of the recording layer on which information was erased were analyzed by X-ray scattering, and the diffraction peaks in both regions were observed. It was found that fine crystal particles were present in both regions. As a result, the particles in the transparent region were found to be in a single-crystal state. This state is schematically shown as 6A in Fig. 2a. As a result, the particles in the opaque region were also found to be in a polycrystalline state. This state is schematically shown as 6B in Fig. 2b. Furthermore, observation of these regions using a scanning electron microscope revealed that the interface between the organic crystal particles and the matrix polymers was not clear and that no cracks were observed. Furthermore, observation of the organic crystal particles using a scanning electron microscope revealed that the particles had a size of 0.1 µm or less.

The recording film was cut as shown in Fig. 10 and a magnetic recording layer 30 and the recording film 3 were placed on a Part of a substrate is formed so that a recording card with display function is produced.

Example 2 (comparative example)

First, as a recording material, 2 g of 1,16-hexadecanediol, 0.1 g of a phenol type antioxidant, 2 g of an adhesive polyester (Vyron, manufactured by Toyobo Co., Ltd.), and 5 g of polyurethane were dissolved in 100 ml of tetrahydrofuran. The solution thus obtained was coated on a reflection layer 2 made of aluminum formed on a substrate 1 and dried to form a recording layer 3 having a thickness of 20 µm. Then, a protective layer 4 made of a UV-cured urethane acrylate resin was formed on the recording layer 3 to have a thickness of 5 µm, thereby obtaining a reversible thermosensitive recording sheet (Fig. 1). The recording layer of the reversible thermosensitive recording sheet was opaque at room temperature. The film was heated using a thermal head and the temperature range in which the recording layer was transparent was determined by this visual observation and using a Macbeth densitometer. The temperature range was found to be approximately 65ºC to 95ºC.

Then, starting from the film in an initial state in which the recording layer was opaque at room temperature, this film was heated to a temperature in the above-mentioned range and cooled to room temperature. This film in which the recording layer was then transparent was energized using a thermal head and information was recorded in the recording layer at 100°C and erased from the recording layer at 90°C. As a result, satisfactory recording properties were obtained using the film.

Example 3

First, as a recording material, 2 g of α-hydroxystearic acid, 0.1 g of a phenol type antioxidant and 8 g of a partially saponified vinyl chloride/vinyl acetate copolymer having a hydroxyl group were dissolved in 100 ml of tetrahydrofuran. The thus obtained solution was coated on a reflection layer 2 formed by vapor deposition of aluminum on a polyester film and dried to form a recording layer 3 having a thickness of 13 µm. Then, a protective layer 4 of an oligomer containing an acrylic ester at both ends of the molecule as a component, which was cured with ultraviolet rays, was formed on the recording layer to obtain a reversible thermosensitive recording sheet. The recording layer of the reversible thermosensitive recording sheet was opaque at room temperature. The film was heated using a thermal head and the temperature range in which the recording layer was transparent was determined by visual observation and using a Macbeth densitometer. The temperature range was found to be about 90ºC to 110ºC.

Then, starting from the film in an initial state in which the recording layer was opaque at room temperature, this film was heated to a temperature in the above-mentioned range and cooled to room temperature. This film in which the recording layer was then transparent was energized using a thermal head and information was recorded in the range of 120ºC and erased from the recording layer at 100ºC.

Example 4

First, 2.5 g of butyl p-hydroxybenzoate, 0.1 g of a hindered phenol type antioxidant and 7.5 g of a vinyl chloride/vinyl acetate/acrylamide copolymer were dissolved in 100 ml of tetrahydrofuran as a recording material. The solution thus obtained was coated in a thickness of 20 µm on a film provided on the surface a blue colored layer formed on a hard polyvinyl chloride film 1 (1 mm thick), thereby forming a recording layer 3. Then, a protective layer 4 made of a urethane acrylate resin cured with ultraviolet rays was formed on the resulting recording layer 3 to a thickness of 5 µm, thereby obtaining a reversible thermosensitive recording film. The recording layer of the reversible thermosensitive recording film was opaque at room temperature. The film was heated using a thermal head, and the temperature range in which the recording layer was transparent was determined by visual observation and using a Macbeth densitometer. The temperature range was as a result from about 70°C to 90°C. The recording characteristics measured by a Macbeth densitometer are shown in Fig. 11. It was found that the width of the temperature range was wide and that the film provided high contrast during recording.

Then, starting from the film in an initial state in which the recording layer was opaque at room temperature, this film was heated to a temperature in the above-mentioned range and cooled to room temperature. Then, this film in which the recording layer was then transparent was energized using a thermal head and information was recorded in the recording layer at 100ºC and erased from the recording layer at 80ºC.

Example 5

First, as a recording material, 1 g of stearamide (melting point: 101°C) having a saturated hydrocarbon chain with 17 carbon atoms and 3 g of a partially saponified vinyl chloride/vinyl acetate copolymer were dissolved in 15 g of tetrahydrofuran. The solution thus obtained was coated on a reflection layer 2 formed by vapor deposition of aluminum on a polyester film 1 having a thickness of 0.188 mm as shown in Fig. 1 and dried, thereby obtaining a Recording layer 3 was formed to have a thickness of 10 µm. Then, a UV-curable acrylic resin prepolymer was coated on the recording layer 3 to have a thickness of 8 µm. Thereafter, the obtained recording layer 3 was irradiated with UV rays to be cured, thereby forming a protective layer 4. As a result, a reversible thermosensitive recording sheet was obtained. The recording layer of the reversible thermosensitive recording sheet was opaque at room temperature. The sheet was heated using a thermal head, and the temperature range in which the recording layer was transparent was determined by visual observation and using a Macbeth densitometer. As a result, the temperature range was from about 80°C to 105°C, and thus the width of the temperature range was about 26°C.

Then, starting from the film in an initial state in which the recording layer was opaque at room temperature, this film was heated to a temperature in the above-mentioned range and cooled to room temperature. Then, this film in which the recording layer was then transparent was energized using a thermal head and information was recorded in the recording layer at 120ºC and erased from the recording layer at 95ºC.

Example 6

First, as a recording material, 2 g of erucamide (melting point: 81°C) having a hydrocarbon chain containing one unsaturated bond and 21 carbon atoms and 4 g of a partially saponified vinyl chloride/vinyl acetate copolymer (copolymerization ratio: vinyl chloride/vinyl acetate = 97/13) were dissolved in 15 g of tetrahydrofuran. Then, a reversible thermosensitive recording sheet was obtained by using this solution in the same manner as in Example 5 except that the thickness of the recording layer 3 was 15 µm. The recording layer of the reversible thermosensitive recording sheet The film was opaque at room temperature. The film was heated using a thermal head, and the temperature range in which the recording layer was transparent was determined by visual observation and using a Macbeth densitometer. The temperature range was as a result from about 68°C to 81°C, and thus the width of the temperature range was 14°C. The recording characteristics measured by a Macbeth densitometer are shown in Fig. 12. It was found that the width of the temperature range was wide and that the contrast between the recorded area and the unrecorded area after recording was high.

Next, 2 g of erucamide and 4 g of a partially saponified vinyl chloride/vinyl acetate copolymer (copolymerization ratio: vinyl chloride/vinyl acetate = 50/50) were dissolved in 15 g of tetrahydrofuran, whereby a reversible thermosensitive recording sheet was prepared in the same manner as in the above-mentioned method using this solution. This sheet was heated using a thermal head, and the temperature range in which the recording layer was transparent was determined by visual observation and using a Macbeth densitometer. The temperature range was as a result about 57°C to 81°C. Since the number of hydroxyl groups of the copolymer used was larger than that of the above-mentioned copolymer, it was likely that the hydroxyl group was bonded to the amide group of the erucamide (which is an aliphatic carboxamide) through a hydrogen bond. For this reason, the width of the temperature range was large (25ºC). Due to this wide temperature range, the recording conditions can be set within a wide range.

Then, starting from the film in an initial state in which the recording layer was opaque at room temperature, this film was heated to a temperature in the above-mentioned range (ie 68ºC to 81ºC) and cooled to room temperature. This film, in which the recording layer was then transparent, was energized using a thermal head and information recorded in the recording layer at 90ºC and erased from the recording layer at 70ºC. At this time, a clear display was obtained. In addition, measurement of the durability by repeating the above-mentioned recording and erasing steps revealed that the reversible thermosensitive recording sheet could withstand 500 or more repetitions.

Example 7

First, as a recording material, 1.5 g of N,N'-dioctadecyl urea (melting point: 114°C) having two hydrocarbon chains each having 18 carbon atoms and 4 g of a partially saponified vinyl chloride/vinyl acetate copolymer were dissolved in 15 g of tetrahydrofuran. Then, a reversible thermosensitive recording sheet was held using this solution in the same manner as in Example 5. The recording layer of the reversible thermosensitive recording sheet was opaque at room temperature. The sheet was heated using a thermal head, and the temperature range in which the recording layer was transparent was determined by visual observation and using a Macbeth densitometer. The temperature range was from about 90°C to 120°C.

Then, starting from the film in an initial state in which the recording layer was opaque at room temperature, this film was heated to a temperature in the above-mentioned range and cooled to room temperature. This film in which the recording layer was then transparent was energized using a thermal head, and information was recorded in the recording layer at 130ºC (the recorded area was opaque) and erased from the recording layer at 100ºC (the erased area was transparent). As a result, satisfactory recording characteristics were obtained.

Example 8

First, as a recording material, 2 g of erucamide (melting point: 81°C), 1 g of docosanol (melting point: 69°C) and 8 g of a partially saponified vinyl chloride/vinyl acetate copolymer were dissolved in 20 g of tetrahydrofuran. Then, a recording layer 3 having a thickness of 20 µm was formed on a glass substrate 1 having a thickness of 1.2 mm using the solution thus obtained. On the recording layer 3, a carbon film having a thickness of 0.1 µm was formed as a layer 34 which is heated due to light absorption. As a reflection layer 2 on the layer 34, aluminum having a thickness of 0.2 µm was vapor-deposited, resulting in a reversible thermosensitive recording sheet as shown in Fig. 13. Information was recorded in or erased from the recording layer by irradiation of a semiconductor laser beam 37 having an oscillation wavelength of 780 nm through a centering lens 36 from the side of the glass substrate 1. The temperature range in which the recording layer was transparent was determined by visual observation using a Macbeth densitometer. The temperature range was found to be from about 60°C to 80°C. Under these conditions, the heat provided by the layer 34 was adjusted by controlling the energy of the semiconductor laser beam so that the temperature of the recording layer was changed. As a result, information was recorded in the recording layer at 100°C and erased from the recording layer at 70°C. Observation of the dot formed by the light irradiation showed that satisfactory bit recording was performed with a clear edge of the dot.

Example 9

First, as a material for the organic crystal particles, 0.6 g of eicosanedicarboxylic acid (HOOC(CH₂)₁₈COOH, melting point: 127ºC), which is an aliphatic dicarboxylic acid, and 0.4 g of stearamide (melting point: 101ºC) were added to 15 g of tetrahydrofuran. Then, 3 g of a partially saponified vinyl chloride/vinyl acetate copolymers (the number of hydroxy groups was about 5% based on the total number of hydroxy groups and acetoxy groups) as a matrix polymer was added to the mixture thus obtained to provide a solution. As shown in Fig. 1, a substrate 1 made of a polyester film having a thickness of 0.2 mm on which a reflection layer 2 was formed by vapor deposition of aluminum was provided. The tetrahydrofuran solution was coated on the surface of the reflection layer and dried in an isothermal bath at 150 ° C to form a recording layer 3 having a thickness of 14 µm. A UV-curable acrylate resin prepolymer was coated on the recording layer 3 to a thickness of 10 µm, and the resulting recording layer 3 was then irradiated with ultraviolet rays to cure, thereby forming a protective layer 4 and obtaining a reversible thermosensitive recording sheet. The recording layer of the reversible thermosensitive recording sheet was opaque at room temperature. The sheet was heated using a thermal head, and the temperature range in which the recording layer was transparent was determined by visual observation and using a Macbeth densitometer. The temperature range was found to be from about 80ºC to 110ºC. And thus, the width of the temperature range was about 30ºC.

Then, starting from the film in an initial state in which the recording layer was opaque at room temperature, this film was heated to a temperature in the above-mentioned range (i.e., 80°C to 110°C) and cooled to room temperature. This film in which the recording layer was then transparent was energized using a thermal head, and information was recorded in the recording layer at 115°C (the recorded portion was opaque) and erased from the recording layer at 100°C (the erased portion was transparent). As a result, satisfactory recording properties were obtained. Using the Macbeth densitometer, the contrast between the recorded portion and the unrecorded portion was 0.73 and satisfactory recording was performed. Repeating the above recording and erasing steps resulted in the film withstanding 500 or more repetitions.

As a comparative example, a reversible thermosensitive recording sheet comprising organic crystal particles formed from 0.7 g of eicosanedicarboxylic acid and 0.3 g of stearamide was prepared. The temperature range in which the recording layer was transparent was determined by visual observation using a Macbeth densitometer. The temperature range was as a result from about 80°C to 85°C, and thus the width of the temperature range was 6°C. Accordingly, the transparency of the recording layer was unsatisfactory and the sheet could only withstand 10 repetitions.

Example 10

First, as a material for organic crystal particles, 0.7 g of octadecanedicarboxylic acid (HOOC(CH₂)₁₆COOH, melting point: 125°C) which is an aliphatic dicarboxylic acid, 0.3 g of m-xylenebisstearylurea (melting point: 163°C) and 0.3 g of docosanol (melting point: 70°C) were mixed. This mixture was added to 15 g of tetrahydrofuran, and 3 g of a partially saponified vinyl chloride/vinyl acetate copolymer (the number of hydroxy groups was about 5% based on the total number of hydroxy groups and acetoxy groups) was added as a matrix polymer to the resulting mixture, whereby the mixture was dissolved to prepare a solution of a recording material. A substrate 1 made of a polyester film having a thickness of 0.2 mm was provided on which a reflection layer 2 was formed by vapor deposition of aluminum, and the tetrahydrofuran solution was coated on the surface of the reflection layer 2 and dried in an isothermal bath at 150°C to form a recording layer 3 having a thickness of 15 µm. A protective layer 4 made of a urethane acrylate resin cured with ultraviolet rays was formed on the recording layer 3 to prepare a reversible thermosensitive recording film. The recording layer of the reversible thermosensitive The recording film was opaque at room temperature. The film was heated using a thermal head and the temperature range in which the recording layer was transparent was determined by visual observation and using a Macbeth densitometer. The temperature range was found to be from about 78ºC to 110ºC, thus the width of the temperature range was about 33ºC.

Then, starting from the film in an initial state in which the recording layer was opaque at room temperature, this film was heated to a temperature in the above-mentioned range and cooled to room temperature. This film in which the recording layer was then transparent was energized using a thermal head, and information was recorded in the recording layer at 115ºC (the recorded area was opaque) and erased from the recording layer at 100ºC (the erased area was transparent). As a result, satisfactory recording characteristics were obtained.

Example 11 (comparative example)

First, as a material for organic crystal particles, 0.6 g of eicosanedicarboxylic acid (HOOC(CH₂)₁₈COOH, melting point: 127°C) which is an aliphatic dicarboxylic acid and 0.4 g of docosanol (melting point: 70°C) were added to 15 g of tetrahydrofuran. Then, 3 g of a partially saponified vinyl chloride/vinyl acetate copolymer (the number of hydroxyl groups was about 12% based on the total number of hydroxyl groups and acetoxy groups) was added to the resulting mixture as a matrix polymer to obtain a solution. A reversible thermosensitive recording sheet was prepared in the same manner as in Example 9. The recording layer of the reversible thermosensitive recording sheet was opaque at room temperature. The film was heated using a thermal head and the temperature range in which the recording layer was transparent was determined by visual observation and using a Macbeth densitometer. The temperature range was found to be about 70°C to 110°C, thus the width of the temperature range was about 40°C. The recording characteristics measured using the Macbeth densitometer are shown in Fig. 14a. The Macbeth optical density, which served as a standard of contrast, was 1.48 when the recording layer was transparent and 0.6 when the recording layer was opaque. Such contrast was considered satisfactory. In addition, measurement of the life characteristics by repeating the above steps of recording and erasing showed that the film endured 500 or more repetitions.

As a comparative example, eicosanedicarboxylic acid or docosanol alone was used as a material for an organic crystal particle, thereby forming a reversible thermosensitive recording film (the thickness of the recording layer was about 10 µm). The results obtained by measuring the Macbeth optical density are shown in Figs. 14b and 14c. From Figs. 14b and 14c, it was found that when eicosanedicarboxylic acid was used, the temperature range in which the recording layer was transparent was from about 80°C to 120°C and thus the width of the temperature range was about 40°C. However, the Macbeth optical density was about 0.8 and the transparency of the recording layer was relatively high when the recording layer was opaque, and the recording property of the recording was not high. In contrast, when docosanol was used, the Macbeth optical density was 0.6 or less when the recording layer was opaque, and the contrast between the recorded area and the unrecorded area was high. However, the temperature range in which the recording layer was transparent was as a result from about 61ºC to 69ºC, and thus the width of the temperature range was 9ºC. As a result, the available temperature range in recording was narrow and the recording conditions could not be set in a wide range.

Example 12

First, as a material for organic crystal particles, 0.5 g of eicosanedicarboxylic acid (HOOC(CH2)18COOH, melting point: 127°C), which is an aliphatic dicarboxylic acid, and 0.5 g of erucamide (melting point: 81°C) were mixed. This mixture was added to 15 g of tetrahydrofuran. Then, 3 g of a partially saponified vinyl chloride/vinyl acetate copolymer (the number of hydroxyl groups was about 5% based on the total number of hydroxyl groups and acetoxy groups) as a matrix polymer was added to the thus obtained mixture to provide a solution. The solution was coated on a substrate and dried in an isothermal bath at 150°C, thereby forming a recording layer having a thickness of 10 µm. Further, a protective layer was formed of an oligomer containing an acrylic ester at both ends of the molecule as a component, cured with UV rays, thereby producing a reversible thermosensitive recording sheet. The recording layer of the reversible thermosensitive recording sheet was opaque at room temperature. The sheet was heated using a thermal head, and the temperature range in which the recording layer was transparent was determined by visual observation using a Macbeth densitometer. The temperature range was as a result from about 67°C to 110°C, and thus the width of the temperature range was about 44°C.

Then, starting from the film in an initial state in which the recording layer was opaque at room temperature, this film was heated to a temperature in the above-mentioned range and cooled to room temperature. This film in which the recording layer was then transparent was energized using a thermal head, and information was recorded in the recording layer at 120°C (the recorded area was opaque) and erased from the recording layer at 100°C (the erased area was transparent). As a result, satisfactory recording characteristics were obtained. Furthermore, repetition of the above recording and erasing steps showed that the film could withstand 500 or more repetitions.

Example 13

First, as a material for organic crystal particles, 0.3 g of sebacic acid (HOOC(CH2)8COOH, melting point: 134°C), which is an aliphatic dicarboxylic acid, 0.3 g of oleamide (melting point: 70°C) and 0.4 g of behenic acid (melting point: 81°C) were mixed. To this mixture was added 15 g of tetrahydrofuran. Then, 3 g of an adhesive polyester resin (having a hydroxyl group) as a matrix polymer was added to the thus-prepared mixture to obtain a solution. The thus-obtained solution was coated on a reflection layer formed by vapor deposition of aluminum on a polyester film having a thickness of 0.2 mm and dried in an isothermal bath at 150°C to form a recording layer having a thickness of 15 µm. Further, a protective layer of urethane acrylate resin cured with UV rays was formed on the recording layer, thereby producing a reversible thermosensitive recording film. The recording layer of the reversible thermosensitive recording film was opaque at room temperature. The film was heated using a thermal head, and the temperature range in which the recording layer was transparent was determined by visual observation using a Macbeth densitometer. As a result, the temperature range was from about 65°C to 98°C, and thus the width of the temperature range was about 34°C.

Then, starting from the film in an initial state in which the recording layer was opaque at room temperature, this film was heated to a temperature in the above-mentioned range and cooled to room temperature. This film in which the recording layer was then transparent was energized using a thermal head and information was recorded in the recording layer at 110ºC (the recorded area was opaque) and was erased from the recording layer at 90ºC (the erased portion was transparent). As a result, satisfactory recording characteristics were obtained. Furthermore, repetition of the above recording and erasing steps showed that the film could withstand 500 or more repetitions.

Example 14

First, 4.5 g of microcapsules having an average particle diameter of 20 µm comprising the organic crystal particles having a composition of Example 12 as a core and a cross-linked polyurethane as a coating and 6 g of a polyvinyl alcohol were added to 100 ml of ethyl alcohol, thereby dispersing the microcapsules and dissolving the polyvinyl alcohol. Then, this mixture was coated on the surface of a reflection layer 2 formed by vapor deposition of aluminum on a polyester film 1 having a thickness of 0.2 mm. A UV-curable acrylate resin prepolymer was coated on the recording layer 3 to a thickness of 0.15 µm and cured with UV rays, thereby forming a protective layer 4. The recording layer of the reversible thermosensitive recording film was opaque at room temperature. The film was heated using a thermal head and the temperature range in which the recording layer was transparent was determined by visual observation and using a Macbeth densitometer. The temperature range was found to be from about 70ºC to 110ºC.

Then, starting from the film in an initial state in which the recording layer was opaque at room temperature, this film was heated to a temperature in the above-mentioned range and cooled to room temperature. This film in which the recording layer was then transparent was energized using a thermal head, and information was recorded in the recording layer at 120ºC (the recorded area was opaque) and erased from the recording layer at 100ºC (the erased area was transparent). As a result, satisfactory recording properties were obtained. Since Furthermore, repetition of the above recording and erasing steps showed that the film could withstand 740 or more repetitions.

Example 15

First, 1.5 g each of microcapsules of three different colors comprising an organic crystal particle of 2-hydroxypalmitic acid (melting point: 86°C) as a core and a modified ethylene/vinyl acetate copolymer colored with a dye as a coating and 5 g of polyvinyl butyral were added to 100 ml of ethyl alcohol. As a result, the microcapsules were dispersed and the polyvinyl butyral was dissolved. Then, the ethyl alcohol mixture was coated on a coating having a thickness of 0.02 µm which had been formed by vapor deposition of gold on a hard polyvinyl chloride film and had a thickness of 1 mm, thereby forming a recording layer having a thickness of 25 µm. A UV-curable acrylate resin prepolymer was coated on the recording layer to a thickness of 10 µm and cured with UV rays to form a protective layer. The recording layer of the reversible thermosensitive recording film was opaque at room temperature. The film was heated using a thermal head and the temperature range in which the recording layer was transparent was determined by visual observation and using a Macbeth densitometer. The temperature range was found to be about 65ºC to 85ºC.

Then, starting from the film in an initial state in which the recording layer was opaque at room temperature, this film was heated to a temperature in the above-mentioned range and cooled to room temperature. This film, in which the recording layer was then transparent, was energized using a semiconductor laser and information was recorded in and erased from the recording layer.

Example 16

First, 2 g of 12-hydroxystearic acid, 0.1 g of a phenol type antioxidant and 8 g of a partially saponified vinyl chloride/vinyl acetate copolymer having a hydroxyl group were dissolved in 100 ml of tetrahydrofuran. Then, 0.8 g of glass beads 43 having an average particle size of 15 µm was dispersed in the mixture. As shown in Fig. 15, the above dispersion solution was coated on a reflection layer 2 formed on a substrate 1 (a polyester film) having a thickness of 0.2 mm by vapor deposition of aluminum and dried. As a result, a recording layer 3 having a thickness of 13 µm was formed. A UV-curable acrylate resin was coated on the recording layer 3 to a thickness of 10 µm and cured with UV rays to form a protective layer 4. Thus, a reversible thermosensitive recording sheet was obtained. The recording layer of the reversible thermosensitive recording sheet was opaque at room temperature. The sheet was heated using a thermal head, and the temperature range in which the recording layer was transparent was determined by visual observation and using a Macbeth densitometer. The temperature range was found to be about 70ºC to 90ºC.

Then, starting from the film in an initial state in which the recording layer was opaque at room temperature, the film was heated to a temperature in the above-mentioned range and cooled to room temperature. This film in which the recording layer was then transparent was energized using a thermal head and information was recorded in the recording layer at 100°C (the recorded area was opaque) and erased from the film at 80°C (the erased area was transparent). As a result, no deterioration of the recorded image quality due to the glass beads was visually observed. Furthermore, the durability test by repeating the above recording and erasing showed that the film was 1500 or more repetitions. The lifetime is about seven times that obtained in the case where no glass beads are added.

Example 17

First, 2 g of the organic crystal particles having the composition of Example 12, 0.1 g of a phenol type antioxidant, 2 g of an adhesive polyester (Vyron, manufactured by Toyo Boseki Co., Ltd.) and 5 g of polyurethane were dissolved in 100 ml of tetrahydrofuran. Further, 0.3 g of melamine resin 43 having an average particle size of 20 µm was dispersed therein. The above dispersion solution was coated on a reflection layer 2 formed on a substrate 1 (a polyester film) having a thickness of 0.2 mm by vapor deposition of aluminum, and dried, thereby forming a recording layer 3 having a thickness of 20 µm. In the same manner as Example 16, a UV-curable acrylate resin prepolymer was coated on the recording layer to a thickness of 10 µm and cured with UV rays to obtain a protective layer 4, thereby obtaining a reversible thermosensitive recording sheet. The recording layer of the reversible thermosensitive recording sheet was opaque at room temperature. The sheet was heated using a thermal head, and the temperature range in which the recording layer was transparent was determined by visual observation using a Macbeth densitometer. The temperature range was as a result from about 90°C to 110°C.

Then, starting from the film in an initial state in which the recording layer was opaque at room temperature, this film was heated to a temperature in the above-mentioned range and cooled to room temperature. This film in which the recording layer was then transparent was energized using a thermal head, and information was recorded in the recording layer at 120°C (the recorded area was opaque) and erased from the recording layer at 100°C (the erased area was transparent). As a result, satisfactory recording characteristics were obtained. Furthermore, the durability characteristics determined by repeating the above recording and erasing steps showed that the film withstood 1800 repetitions.

Example 18

As a substrate 1 shown in Fig. 6, a white polyethylene terephthalate film having a thickness of 200 µm on which a magnetic recording layer of dark brown color made of γ-Fe₂O₃ was formed was used. A recording layer 3 was formed by coating the dispersion solution comprising the reversible thermosensitive recording material of the composition in Example 12 on the substrate so that the thickness after drying was 12 µm.

As a soluble polyimide resin used for a protective layer 4, a polyimide (Upilex R, manufactured by Ubekosan Co., Ltd.) corresponding to the following formula VI was used:

where n is about 5 or more.

Next, 5 g of the polyimide was dissolved in 100 ml of m-cresol and the resulting solution was coated on the surface of the recording layer 8 to a thickness of 12 µm to form a pale yellow and transparent protective layer 4. Then, the solvent was removed at 120°C for 5 h and further 12 h in a vacuum dryer, resulting in a reversible thermosensitive recording film as shown in Fig. 6. The recording layer of the reversible thermosensitive recording film was opaque at room temperature. The film was heated using a thermal head and the temperature range in which the recording layer was transparent was determined by visual observation and using a Macbeth densitometer. The temperature range was found to be from about 75°C to 110°C.

Then, starting from the film in an initial state in which the recording layer was opaque at room temperature, this film was heated to a temperature in the above-mentioned range and cooled to room temperature. The images of the numbers 0 and 9 were recorded in the recording layer at a temperature of over 110ºC. As a result, the numbers were displayed in white color on the dark brown surface of the magnetic recording layer and were satisfactorily recognizable. The numbers were repeatedly recorded in and erased from the film. The film withstood 3000 or more repetitions. Furthermore, on the surface of the protective layer 13 in contact with the thermal head, residual numbers usually resulting from thermal or mechanical damage were not observed after erasing the numbers.

As a comparative example, a reversible thermosensitive recording sheet was prepared in the same manner as the above-mentioned recording medium except that a material of the protective layer was changed. A UV-curable acrylate resin prepolymer was coated on the recording layer in a thickness of 10 µm and cured with UV rays to form a protective layer 4, thereby obtaining a reversible thermosensitive recording. Images of letters were repeatedly recorded and erased from the protective layer, and the sheet endured 1000 or more repetitions. However, residual letters having a white color were observed on the surface of the protective layer 4. This was apparently due to irregular reflection due to mechanical abrasion by the thermal head or due to surface surface disturbances of the recording layer itself caused by thermal stress thereof or of the protective layer caused by thermal contraction thereof.

Example 19

As the substrate 1 shown in Fig. 6, a polyethylene terephthalate film having a thickness of 180 µm was used, on which a reflection layer 2 having a thickness of 0.2 µm was formed by vapor deposition of aluminum. As the compound capable of forming organic crystal particles, 2.5 g of p-dodecyloxybenzoic acid and 1.5 g of erucamide were used; as the antioxidant, 0.1 g of a phenol type antioxidant and as the matrix polymer, 10 g of vinyl chloride/vinyl acetate/maleic acid copolymer were used. The materials were evenly dissolved in 100 ml of tetrahydrofuran. The solution thus obtained was coated on the reflection layer 2 on the substrate 1 to form a recording layer 3 having a thickness of 20 µm.

As a soluble polyimide resin for a protective layer 4 having heat resistance, a polyimide resin containing a fluorine atom and corresponding to the following formula VII was used:

where n is about 5 or more.

This polyimide resin was dissolved in N-methylpyrrolidone, and the solution was coated on the surface of the recording layer 3 to a thickness of 10 µm. Thereafter, the resulting recording layer 3 was heat-treated at 150°C for 30 minutes and dried in vacuum for a further 12 hours to obtain a reversible thermosensitive recording film with a protective layer 4. A defect was not present in the protective layer of the film thus formed. The recording layer of the reversible thermosensitive recording film was opaque at room temperature. The film was heated, and the temperature range in which the recording layer was transparent was determined by visual observation and using a Macbeth densitometer. The temperature range was found to be about 70°C to 95°C.

Then, starting from the film in an initial state in which the recording layer was opaque at room temperature, this film was heated to a temperature in the above-mentioned range and cooled to room temperature. This film in which the recording layer was then transparent was energized using a thermal head and information was recorded in the recording layer at 100ºC and erased from the recording layer at 75ºC.

This film was cut to the size of a telephone card (54 mm x 86 mm) and the repetition of recording and erasure was evaluated using a card reader and writer. The film withstood 5000 or more repetitions of recording and erasure. Furthermore, residual information after erasure was not observed on this film.

Example 20

The present example will be described with reference to Fig. 7. A recording layer 3 was formed on the surface of a reflection layer 2 disposed on a substrate 1. Then, a protective layer 4 was formed on the surface of the recording layer 3 using a recording material having the composition of Example 12. This protective layer 4 was obtained by thoroughly mixing 5 g of urethane acrylate which is an energy ray curable resin, 5 g of acrylate resin oligomer, 0.5 g of a photopolymerization initiator and 0.8 g of titanium dioxide powder as ultrafine particles 18 whose primary particles had an average particle size of about 21 nm, then coating the resulting mixture as a prepolymer on the surface of the recording layer 3 to a thickness of 3 µm and curing the mixture with ultraviolet rays.

The protective layer 4 thus formed was a transparent hard coating and its surface had an average roughness of 50 nm. The recording layer of the reversible thermosensitive recording film was opaque at room temperature. The film was heated using a thermal head 19 and the temperature range in which the recording layer was transparent was determined by visual observation and using a Macbeth densitometer. The temperature range was found to be about 70°C to 110°C.

Then, starting from the film in an initial state in which the recording layer was opaque at room temperature, this film was heated to a temperature in the above-mentioned range and cooled to room temperature. This film in which the recording layer was then transparent was energized using a thermal head 19 and information was recorded in the recording layer at 120°C (the recorded area was opaque) and erased from the recording layer at 90°C (the erased area was transparent). The endurance test was carried out by repeating recording and erasing and showed that the film withstood 1800 or more repetitions. The service life was thus about three times that in the case where no ultrafine particles of an oxide 18 are added to the protective layer 4.

Separately, 3 g of titanium dioxide as ultrafine particles of an oxide 18, the primary particle of which had an average particle size of 100 nm, was dispersed in that of the above comparable resin component to produce a comparable reversible thermosensitive recording sheet. The protective layer 4 had an average roughness of 90 nm. When a comparable endurance test was conducted for this reversible thermosensitive recording sheet, satisfactory recording characteristics were obtained as above, and this sheet endured 1000 repetitions. The reason for the difference in recording characteristics between this case and the above is not apparent; However, it is considered that this is due to the increase in the degree of unevenness on the surface of the protective layer 4 rather than the increase in the average roughness of the surface of the protective layer 4. Even when titanium dioxide with an average particle size of 100 nm was used, the lifetime was about twice that of the prior art.

Example 21

As a recording material, 2 g of erucamide having a hydrocarbon chain with an unsaturated bond of 21 carbon atoms and 4 g of vinyl chloride/vinyl acetate/acrylamide copolymer were dissolved in 15 g of tetrahydrofuran. This solution was coated on a reflection layer 2 formed on a substrate 1 made of a polyethylene terephthalate having a thickness of 0.2 mm by vapor deposition of aluminum. The solution was dried to form a recording layer 3 having a thickness of 15 µm. A protective layer 4 was formed on the recording layer 3 as follows.

Next, 7 g of urethane acrylate as an energy beam curable resin, 3 g of oligomer containing an acrylic acid ester at both ends of the molecule as a grain component, and 0.5 g of photopolymerization initiator. As a prepolymer, this mixture was coated on the surface of a recording layer 3 to a thickness of 3 µm. Thereafter, the obtained recording layer 3 was cured with UV rays to form a first protective layer. Then, 5 g of urethane acrylate as an energy ray curable resin, 5 g of oligomer containing an acrylic ester at both ends of the molecule as a component, 0.5 g of photopolymerization initiator and 0.7 g of silica powder as ultrafine particles of an oxide 18 whose primary particle had an average particle size of 12 nm and whose surface was treated with perfluorooctyltrichlorosilane were thoroughly mixed.

This mixture was coated as a prepolymer on the first protective layer in a thickness of 3 µm and cured with UV rays to form a second protective layer. The protective layer thus formed with a two-layer structure was a transparent hard coating and its surface had an average roughness of 50 nm. The recording layer of the reversible thermosensitive recording sheet was opaque at room temperature. The sheet was heated using a thermal head and the temperature range in which the recording layer was transparent was determined by visual observation and using a Macbeth densitometer. The temperature range was found to be about 68ºC to 81ºC, and the width of the temperature range was thus 14ºC.

Then, starting from the film in an initial state in which the recording layer was opaque at room temperature, this film was heated to a temperature in the above-mentioned range and cooled to room temperature. This film in which the recording layer was then transparent was energized using a thermal head 19 and information was recorded in the recording layer at 90°C and erased from the recording layer at 75°C. Clear display was obtained without irregularity of the image due to the stains of the thermal head caused by the stains of the surface. The endurance test conducted by repeating the recording and erasing revealed that this film could withstand 2000 or more repetitions. Such a lifetime was about three or more times that found in the case when no ultrafine particles of an oxide 18 are added to the second protective layer.

Example 22

The present example will be described with reference to Fig. 8.

As a substrate 1, a white polyethylene terephthalate film having a thickness of 200 µm was used, on which a magnetic recording layer of dark brown color made of γ-Fe₂O₃ was formed on its surface. A transparent polyethylene terephthalate film having a thickness of 15 µm was used as a protective film 4. The recording material having the same composition as in Example 12 was coated on the transparent film 4 to a thickness of 12 µm, thereby obtaining a recording film 24 having a recording layer 3. As an adhesive layer 22, a polyester resin adhesive was coated on the surface of the recording layer 3 of the recording film 24 to a thickness of about 1 µm. Thereafter, the magnetic recording layer on the substrate 1 and the recording layer 3 on the recording film 24 were laminated so that the respective surfaces faced each other, thereby forming a reversible thermosensitive recording film as shown in Fig. 8. The recording layer of the reversible thermosensitive recording film was opaque at room temperature. The film was heated and the temperature range in which the recording layer was transparent was determined by visual observation using a Macbeth densitometer. The temperature range was found to be from about 65°C to 81°C.

Images representing the alphabet were recorded in the recording layer of the reversible thermosensitive recording film. As a result, the alphabet was recorded in white color on the dark brown surface of the magnetic recording drawing layer and the 26 letters of the alphabet could be recognized satisfactorily.

The alphabet was repeatedly recorded and erased from the recording layer, and it was found that the film could withstand 3000 or more repetitions. Furthermore, no residual letters resulting from thermal or mechanical damage were observed on the surface of the transparent film contacted by the thermal head after the letters were erased.

Example 23

As a substrate 1, a white polyethylene terephthalate having a thickness of 180 µm was used. As a transparent thin film 4 having abrasion resistance and as a film 26 for supporting a reflection layer 2, a polyethylene terephthalate having a thickness of 15 µm was used.

Then, 2 g of eicosanedicarboxylic acid, 2 g of erucamide, 0.5 g of docosanol, 0.1 g of a phenol type antioxidant as an antioxidant and 10 g of vinyl chloride/vinyl acetate acrylamide copolymer as a matrix polymer were uniformly dissolved in 100 ml of tetrahydrofuran. This solution was coated on the transparent film 4 as a protective layer so that it had a thickness of 12 µm after drying, thereby obtaining a recording film 24 having a recording layer 3.

Next, a method for producing a reflection sheet 29 will be described. A reflection layer 2 made of aluminum was formed to a thickness of 0.2 µm on the sheet 26 by vapor deposition. Then, γ-glycidoxypropyltrimethoxysilane (a silane coupling agent) in isopropyl alcohol was chemically adsorbed in the reflection layer 2 to form a monomolecular layer. This silane coupling agent layer 28 improves the adhesion between an adhesive layer 22 and the reflection sheet 29. An epoxy resin as an adhesive was applied as an adhesive layer 22 to the surface of the sheet 26 of the reflection sheet 29. 29 to a thickness of about 1 µm. Thereafter, the reflection film 29 was laminated on the substrate 1 via the adhesive layer 22. Then, an epoxy resin adhesive as an adhesive layer 22 was coated to a thickness of about 1 µm on the reflection layer 2 which had been treated with the silane coupling agent, and then the recording layer 3 of the recording film 24 and the adhesive layer 22 were laminated so that the respective surfaces faced each other to form a reversible thermosensitive recording film as shown in Fig. 9. The recording layer of the reversible thermosensitive recording film was opaque at room temperature. The film was heated using a thermal head, and the temperature range in which the recording layer was transparent was determined by visual observation and using a Macbeth densitometer. The temperature range was as a result from about 70°C to 110°C.

Then, starting from the film in an initial state in which the recording layer was opaque at room temperature, the film was heated to a temperature in the above-mentioned range and cooled to room temperature. The film in which the recording layer was then transparent was energized using a thermal head, information was recorded in the recording layer at 115ºC (the recorded area was opaque) and erased from the recording layer at 85ºC (the erased area was transparent).

This reversible thermosensitive recording sheet was cut to the size of a telephone card (i.e., 45 mm x 86 mm). Recording and erasing of information was repeated using a card reader/writer with a thermal head. It was found that the sheet could withstand 2000 or more repetitions with stable performance and that no residual information was observed.

Claims (19)

1. A reversible, heat-sensitive or thermosensitive recording material made from a composition comprising a transparent matrix polymer and organic crystal particles dispersed therein; wherein the crystal state of the organic crystal particles is variable according to the applied temperature, resulting in a reversible change in the transparency of the recording material; wherein each of the organic crystal particles comprises at least one material selected from the group consisting of: (1) at least one hydroxycarboxylic acid or derivatives thereof having a melting point in the range of 60ºC to 120ºC; (2) at least one compound selected from the group consisting of an aliphatic amide compound and an aliphatic urea compound, and wherein each of the aliphatic amide compound and the aliphatic urea compound has at least one straight-chain hydrocarbon group, each containing at least 10 carbon atoms, and wherein the melting point of each of the compounds is in the range of 70°C to 150°C; (3) a compound having an amide or urea group and an aliphatic dicarboxylic acid, wherein the compound having an amide or urea group is at least one selected from the group consisting of saturated aliphatic monocarboxamides having a hydrocarbon group having at least 12 carbon atoms, saturated aliphatic biscarboxamides having a hydrocarbon group having at least 12 carbon atoms and having a hydrocarbon group having at least 12 carbon atoms substituted ureas, wherein the aliphatic dicarboxylic acid corresponds to the formula HOOC(CH₂)n8COOH (wherein n8 is an integer from 14 to 24) and wherein the weight ratio of the compound having an amide or urea group to the aliphatic dicarboxylic acid is in the range from 60:40 to 10:90; and wherein the matrix polymer and the organic crystal particles each have a group capable of forming a hydrogen bond. 2. A recording material according to claim 1, wherein each of the organic crystal particles comprises at least one hydroxycarboxylic acid or a derivative thereof having a melting point in the range of 60°C to 120°C as defined under (1) in claim 1, and wherein the matrix polymer is selected from the group consisting of polyesters having a hydroxyl group, partially saponified vinyl acetate/vinyl chloride copolymers, polyamides, polyurethanes, thermoplastic phenolic resins, vinyl alcohol copolymers, acrylic copolymers, acrylamide copolymers, maleic acid copolymers, urea resins, epoxy resins and melamine resins. 3. A recording material according to claim 2, wherein the hydroxycarboxylic acid or its derivative is at least one selected from the group of compounds corresponding to the following formulas Ia, Ib, Ic, Id and Ie: where ml and n1 are each integers and the sum of ml and n1 is 6 to 24 ; HO-Ph-(CH₂)n2-COOH (Ib) where Ph is a phenylene group and n2 is an integer in the range of 0 to 18; HO-Ph-COO(CH2)n3-COOH (Ic) where Ph is a phenylene group and n3 is an integer in the range of 1 to 18; HO-Ph-OCO(CH2)n4-COOH (Id) where Ph is a phenylene group and n4 is an integer in the range of 1 to 18; and HO-Ph-COO(CH₂)n5-H (Ie) where Ph is a phenylene group and n3 is an integer in the range of 0 to 18. 4. Recording material according to claim 3, wherein the hydroxycarboxylic acid is an α-hydroxyalkylcarboxylic acid. 5. A recording material according to claim 1, wherein the aliphatic amide compound is at least one selected from the group consisting of the compounds corresponding to the following formulas IIa, IIb and IIc: R¹-CONH-R² (IIa) wherein R¹ is a straight-chain hydrocarbon group having 1 to 25 carbon atoms, R² is hydrogen, a straight-chain hydrocarbon group having 1 to 26 carbon atoms or a methylol group and at least one of the radicals R¹ and R² is a straight-chain hydrocarbon group having at least 10 carbon atoms ; R³-CONH-(CH₂)n6-NHCO-R³ (IIb) where R³ is a straight-chain hydrocarbon group having 10 to 25 carbon atoms and n6 is an integer in the range of 1 to 8, and R4 -NHCO-(CH2 )n7-CONH-R4 (IIc) where R4 is a straight chain hydrocarbon group having 10 to 25 carbon atoms and n7 is an integer in the range of 1 to 8. 6. Recording material according to claim 1, wherein the aliphatic urea compound corresponds to the following formula III: R⁵-NHCONH-R⁵ (III) wherein R⁵ and R⁶ independently represent hydrogen or a straight-chain hydrocarbon group having 1 to 26 carbon atoms and at least one of R⁵ and R⁶ is a straight-chain hydrocarbon group having at least 10 carbon atoms. 7. The recording material according to claim 1, wherein each of the organic crystal particles has a particle size of 0.1 µm or less. 8. Recording material according to claim 1, wherein the preparation comprises an antioxidant with active hydrogen. 9. A recording material according to claim 1, wherein each of the organic crystal particles is microencapsulated in a transparent matrix polymer for encapsulation, wherein the matrix polymer for encapsulating the organic crystal particles in microcapsules is different from the matrix polymer of the recording material itself. 10. Recording material according to claim 9, wherein the transparent matrix polymer for encapsulation comprises a coloring material. 11. A reversible thermosensitive recording material comprising a substrate, a recording layer made of a reversible thermosensitive material according to any one of claims 1 to 10, and a protective layer stacked in this order, wherein the recording layer comprises a transparent spacer particle having a thickness approximately equal to that of the recording layer and a particle size below the thickness of the recording layer, in a proportion of 10 wt.%. 12. The recording medium according to claim 11, wherein the spacer particle is made of glass or a polymer and is a spherical particle having an average particle size of 1 to 100 µm and a narrow particle size distribution. 13. A reversible thermosensitive recording medium comprising a substrate, a recording layer made of a reversible thermosensitive material according to any one of claims 1 to 10, and a protective layer stacked in this order, wherein the protective layer is selected from the group consisting of: (i) a protective layer of a soluble polyimide which is soluble in organic solvents, (ii) a protective layer comprising ultrafine particles of an oxide whose primary particles have an average particle size of 100 nm or less; (iii) a protective layer made up of a plurality of layers, wherein the hardness of the plurality of layers is higher in the direction of a surface layer from the substrate wherein a protective layer of at least the outermost surface layer comprises the ultrafine particles of an oxide whose primary particles have an average particle size of 100 nm or less; and (iv) a protective layer made of at least one resin selected from the group consisting of polyethylene terephthalate, fluorine-containing polymers, polysulfones, polyethylene naphthalate, polyphenylene sulfide, polyacrylates, polyimides and polyamides, wherein in the case of the protective layer being (iv), an adhesive layer is additionally disposed between the substrate and the recording layer. 14. A recording medium according to claim 13(i), wherein the soluble polyimide has a repeating unit corresponding to the following formula IV: wherein X represents O, CO, C(CF₃)₂ or a single bond, Y represents O, CO, C(CF₃)₂ or a single bond, R⁷ represents a group having an aromatic ring and m₂ and m₃ are independently 0, 1 or 2. 15. The recording medium according to claim 13 (ii), wherein the protective layer is an acrylic type energy beam curable resin. 16. The recording medium according to claim 13 (ii), wherein the ultrafine particles are made of silica, alumina or titanium oxide. 17. A recording medium according to claim 16, wherein each of the ultrafine particles is a particle of silicon oxide, aluminum oxide or titanium oxide having a Surface treatment with an organic chemical adsorbent. 18. The recording medium according to claim 13, wherein an adhesive layer is additionally disposed between the substrate and the recording layer. 19. Reversible, thermosensitive recording medium comprising a substrate, a recording layer made of a reversible, thermosensitive material according to any one of claims 1 to 10 and a protective layer, arranged in this order, wherein a reflection layer is arranged between the substrate and the recording layer and the information is recorded or erased by means of a laser beam, and wherein each of the organic crystal particles has a particle size of 0.1 µm or less.