Reversible photochromic microcapsule pigment for use in offset printing ink composition, offset printing ink composition containing the same, and reversible photochromic printed matter produced using the ink composition

Abstract

To provide a novel material that can retain reversible photochromic properties when subjected to offset printing.SOLUTION: A reversible photochromic microcapsule pigment for use in an offset printing ink composition, the pigment encapsulating a reversible photochromic composition containing a photochromic compound.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a reversibly photochromic microencapsulated pigment for use in an ink composition for offset printing, an offset printing ink composition containing the same, and a reversibly photochromic printed matter using the ink composition. [Background technology]

[0002] Photochromic compounds have the characteristic of developing color when irradiated with ultraviolet light or the like and losing color when the irradiation is stopped (i.e., they reversibly change color depending on the presence or absence of light irradiation), and can be used for a variety of applications and purposes. Patent Document 1 discloses that printed matter can be produced by letterpress, offset, gravure, and screen printing processes using ink containing a photochromic compound. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 3-114872 Summary of the Invention [Problem to be solved by the invention]

[0004] In recent years, offset printing has become popular due to various improvements in offset printing plates. In offset printing, the colorant used is a dye or a pigment, and its particle size is relatively small due to its printing method. For example, offset printing is used to decorate containers that require large quantities of printing, such as metal and plastic cans for beverages or food, taking into account production efficiency and cost.

[0005] On the other hand, the inventors' investigations have revealed that when an ink composition containing a photochromic compound is subjected to offset printing, there is a problem in that the ink composition may no longer exhibit the reversible photochromism characteristic of the photochromic compound.

[0006] The present disclosure has been made in light of these circumstances, and one of its objectives is to provide a novel material that can maintain reversible photochromism even when offset printing is performed, an ink composition containing the material, and a printed matter using the ink composition. [Means for solving the problem]

[0007] Aspect 1 of the present invention is The reversibly photochromic microcapsule pigment is used in an ink composition for offset printing, and contains a reversibly photochromic composition containing a photochromic compound.

[0008] Aspect 2 of the present invention is 2. The microencapsulated pigment of embodiment 1, wherein the reversible photochromic composition further comprises an oligomer.

[0009] Aspect 3 of the present invention is An ink composition for offset printing, comprising the microcapsulated pigment according to Aspect 1 or 2 and a vehicle.

[0010] A fourth aspect of the present invention is A reversibly photochromic printed matter comprising a substrate and a reversibly photochromic layer that is a dried product of the ink composition according to embodiment 3.

[0011] A fifth aspect of the present invention is Aspect 5 is the reversibly photochromic printed matter according to aspect 4, wherein the substrate is a container containing a metal material.

[0012] A sixth aspect of the present invention is The container is the reversibly photochromic printed matter according to aspect 4 or 5, which contains food or beverage. [Effects of the Invention]

[0013] According to an embodiment of the present invention, it is possible to provide a novel material that can maintain reversible photochromism even when offset printing is performed, an ink composition containing the material, and a printed matter using the ink composition. [Brief explanation of the drawings]

[0014] [Figure 1] Figure 1 is a photograph showing the color change of each printed matter before (top) and after (bottom) light exposure. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present inventors conducted extensive research to develop a novel material that can maintain reversible photochromism even after offset printing. As a result, they discovered that by microencapsulating a reversible photochromic composition containing a photochromic compound, reversible photochromism can be maintained even after offset printing. Although the detailed mechanism is unclear, it is believed that photochromic compounds are susceptible to degradation caused by various factors that can inhibit reversible photochromism during offset printing (e.g., various components of the vehicle, the printing process (accompanying pH changes and temperature changes)), and that microencapsulating a reversible photochromic composition containing the compound protects the reversible photochromic composition from these degradation factors. The details of each requirement defined by the embodiments of the present invention are described below. In this specification, "ink composition for offset printing" means a composition in which the colorant is a dye, or in which the colorant contains a pigment (including a microencapsulated pigment) and the average particle size is 3 μm or less, which is relatively small compared to other printing compositions. In addition, in this specification, "dried product of composition" means a composition that has been dried and solidified, and may include, for example, a composition in which at least a portion has reacted.

[0016] <1. Reversible Photochromic Microencapsulated Pigments for Use in Offset Printing Ink Compositions> The reversibly photochromic microencapsulated pigment used in the ink composition for offset printing according to an embodiment of the present invention contains a reversibly photochromic composition containing a photochromic compound encapsulated therein. This microencapsulated pigment is a novel material that can maintain its reversible photochromic properties even when incorporated into an ink composition and used for offset printing.

[0017] The reversibly photochromic composition exhibits a characteristic of developing a color when irradiated with sunlight, ultraviolet light, or blue light having a peak emission wavelength in the range of 400 to 495 nm, and decolorizing when the irradiation is stopped. In an embodiment of the present invention, the reversibly photochromic composition contains a photochromic compound. The photochromic compound may be a known compound, for example, one or more selected from the group consisting of spirooxazine derivatives, spiropyran derivatives, and naphthopyran derivatives. Examples of the photochromic compound include compounds described in JP 2021-120493 A and WO 2020 / 137469 A.

[0018] As the photochromic compound, a photochromic compound having optical memory properties (color memory photochromic properties) can also be used. Examples of such photochromic compounds include diarylethene derivatives, such as those described in JP-A-2021-120493.

[0019] The reversible photochromic composition may contain an oligomer, and a reversible photochromic composition in which the above-mentioned photochromic compound is dissolved in an oligomer can also be used. When an oligomer is contained, the oligomer can be present around the photochromic compound and can protect the photochromic compound from various degradation factors in offset printing. Therefore, by containing an oligomer, the reversible photochromic property of the photochromic compound can be more easily maintained. Furthermore, by containing an oligomer, it is possible to increase the color density as well as the light resistance, and further to adjust the color change sensitivity. The oligomer may be, for example, one or more selected from the group consisting of styrene-based oligomers, acrylic-based oligomers, terpene-based oligomers, and terpene-phenol-based oligomers.

[0020] The styrene-based oligomer preferably has a mass-average molecular weight of 200 to 6000, more preferably 200 to 4000. By setting the mass-average molecular weight to 6000 or less, color retention during decolorization is less likely to occur, the color density can be easily increased, and the discoloration sensitivity can be more easily adjusted. On the other hand, by setting the mass-average molecular weight to 200 or more, the amount of contained monomer is reduced, improving stability and facilitating improvement of light resistance. Styrene oligomers are compounds having a styrene skeleton or hydrogenated products thereof, and examples thereof include low-molecular-weight polystyrene, styrene-α-methylstyrene copolymer, α-methylstyrene polymer, and α-methylstyrene-vinyltoluene copolymer.

[0021] The acrylic oligomer preferably has a mass-average molecular weight of 12,000 or less, more preferably 1,000 to 8,000, and even more preferably 1,500 to 6,000. By setting the mass-average molecular weight to 12,000 or less, it becomes easier to adjust the discoloration sensitivity. On the other hand, by setting the mass-average molecular weight to 1,000 or more, the amount of contained monomer decreases, improving stability, making it easier to increase the color density and improve lightfastness. Examples of the acrylic oligomer include acrylic acid ester copolymers.

[0022] The terpene oligomer preferably has a mass-average molecular weight of 250 to 4000, more preferably 300 to 4000. By setting the mass-average molecular weight to 4000 or less, color retention during decolorization is reduced, the color density can be easily increased, and the color change sensitivity can be more easily adjusted. On the other hand, if the mass-average molecular weight is 250 or more, the amount of contained monomer is reduced, improving stability and facilitating improved light resistance. The terpene oligomer is a compound having a terpene skeleton, and examples thereof include α-pinene polymer, β-pinene polymer, and d-limonene polymer.

[0023] The terpene phenol oligomer preferably has a mass average molecular weight of 200 to 2000, more preferably 500 to 1200. By setting the mass average molecular weight to 2000 or less, it becomes easier to adjust the discoloration sensitivity. On the other hand, by setting the mass average molecular weight to 200 or more, the amount of contained monomer decreases, improving stability and making it easier to increase the color density. Terpene phenol oligomers are compounds obtained by copolymerizing cyclic terpene monomers with phenols or their hydrogenated products, such as α-pinene-phenol copolymers.

[0024] The mass average molecular weight of the above styrene-based oligomer, acrylic-based oligomer, terpene-based oligomer, and terpene-phenol-based oligomer can be measured by gel permeation chromatography (GPC).

[0025] The oligomers can be used alone or in combination of two or more.

[0026] In the reversibly photochromic composition, the mass ratio of the photochromic compound to the styrene-based oligomer or the photochromic compound to the acrylic oligomer is preferably 1:1 to 1:10000, more preferably 1:5 to 1:500. In the reversibly photochromic composition, the mass ratio of the photochromic compound to the terpene oligomer is preferably 1:1 to 1:5000, more preferably 1:5 to 1:500. In the reversibly photochromic composition, the mass ratio of the photochromic compound to the terpene phenol oligomer is preferably 1:1 to 1:50, more preferably 1:2 to 1:30. When the mass ratio of the photochromic compound to the oligomer is within the above range, the reversible photochromic property of the photochromic compound is more easily maintained, and sufficient color density is more easily exhibited.

[0027] The reversibly photochromic composition is encapsulated in microcapsules and used as a reversibly photochromic microencapsulated pigment (hereinafter sometimes referred to as a "microencapsulated pigment"). By encapsulating the reversibly photochromic composition in microcapsules, a chemically and physically stable pigment can be formed, and it can be protected from various deterioration factors in offset printing. Furthermore, under various usage conditions, the reversibly photochromic composition is more likely to maintain a predetermined composition and exhibit predetermined properties.

[0028] Microencapsulation can be performed by known methods such as isocyanate-based interfacial polymerization, in situ polymerization such as melamine-formalin polymerization, in-liquid curing coating, phase separation from an aqueous solution, phase separation from an organic solvent, melt-dispersion cooling, air suspension coating, and spray drying, and can be selected appropriately depending on the application.

[0029] Depending on the purpose, the microcapsulated pigment may further have a resin film or the like provided on its surface, thereby imparting durability or modifying the surface properties.

[0030] The reversible photochromic microcapsule pigment comprises a microcapsule wall (wall material) and an inclusion (including a reversible photochromic composition) encapsulated therein, with the mass ratio of inclusion to wall being preferably 7:1 to 1:1. By having the mass ratio of inclusion to wall within the above range, it is possible to prevent a decrease in color density and clarity during color development. More preferably, the mass ratio of inclusion to wall is 6:1 to 1:1.

[0031] Furthermore, by incorporating a non-color-changing colorant such as a general dye and / or pigment during microencapsulation, the microencapsulated pigment can be made to undergo an alternating color change from a first color to a second color.

[0032] The particle size of the reversibly photochromic microcapsule pigment is the equivalent sphere diameter of equal volume measured using a laser diffraction / scattering particle size distribution analyzer (manufactured by Horiba, Ltd., product name: LA-960V2) that has been calibrated as specified, and the average particle size is the average value of the equivalent sphere diameters of equal volume (the particle size D50 that corresponds to a frequency of 50% when particle size distribution is calculated on a volume basis, i.e., the median diameter). The above-mentioned predetermined calibration will now be described. If the particle diameter of all particles exceeds 0.2 μm, measure the average equivalent diameter of spheres of equal volume using the Coulter method using a particle size distribution analyzer (Beckman Coulter, Inc., product name: Multisizer 4e), and perform calibration based on this value. In cases other than those mentioned above, the particle area is determined using image analysis particle size distribution measurement software (manufactured by Mountec Co., Ltd., product name: MacView), the diameter equivalent to a circle with a projected area (Heywood diameter) is calculated from the area of ​​the particle area, and calibration is performed based on the average value of the diameter equivalent to a sphere with the same volume.

[0033] In an embodiment of the present invention, the reversibly photochromic microencapsulated pigment preferably contains particles that are not perfectly spherical, and preferably contains particles with depressions on their surfaces. For example, the reversibly photochromic microencapsulated pigment preferably contains particles in which double circles (an outer circle corresponding to the diameter and an inner circle corresponding to the outer shape of the depression) are observed in a surface SEM image. It is believed that such particles can have a larger contact area with a substrate, etc., than perfectly spherical particles in printed materials, as described below, and can improve transferability to the substrate.

[0034] <2. Ink composition for offset printing> An ink composition for offset printing according to an embodiment of the present invention comprises the above-described reversibly photochromic microcapsule pigment and a vehicle.

[0035] In an embodiment of the present invention, the vehicle is composed of components such as a resin, a solvent, and, if necessary, various additives. Resins used in the vehicle include non-drying oil alkyd resins, semi-drying oil alkyd resins, drying oil alkyd resins, urethane-modified alkyd resins, styrene-modified alkyd resins, acrylic-modified alkyd resins, epoxy-modified alkyd resins, phenol-modified alkyd resins, oil-free alkyd resins, acid-cured amino alkyd resins, rosin-modified alkyd resins, rosin-modified phenolic resins, rosin-modified maleic acid resins, acrylic resins, acrylic polyols, epoxy resins, epoxy-modified alkyd resins, etc. The above resins can be used alone or in combination of two or more.

[0036] Examples of solvents include aliphatic hydrocarbons, aromatic hydrocarbons, alcohols, esters, ketones, etc. From the viewpoint of on-machine stability of ink in offset printing, relatively slow-drying solvents are used, such as industrial solvent, kerosene, Solvesso 100, Solvesso 150, xylene, mineral spirits, n-butanol, anone, isophorone, cellosolves, and cellosolve acetates.

[0037] Furthermore, the resins can be crosslinked as needed. In the case of a baking type, crosslinking can be carried out at high or medium temperatures by blending an amino resin such as a butylated urea resin, a butylated urea melamine resin, a butylated melamine resin, or a butylated benzoguanamine resin as a crosslinking agent, adding a metal dryer, or adding an isocyanate crosslinking agent.

[0038] Furthermore, ultraviolet-curable vehicles and electron beam-curable vehicles can also be applied to embodiments of the present invention, and the following prepolymers, oligomers, and / or monomers can be used in combination as needed. Examples of acrylate-based radical polymerization types include photopolymerizable prepolymers such as epoxy acrylate, urethane acrylate, oligoester acrylate, and polyester acrylate. Examples of acrylate compounds having a molecular weight of 100 to 800 and a relatively low viscosity, which are classified as photopolymerizable monomers or diluent monomers, include 2-ethylhexyl acrylate, butanediol diacrylate, diethylene glycol diacrylate, tetraethylene glycol diacrylate, 1,6-hexanediol diacrylate, neopentyl glycol diacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, dipentaerythritol hexaacrylate, neopentyl glycol acrylic acid benzoate, 2-acryloyloxyethyl acid phosphate, 2-hydroxyethyl acrylate, isooctyl acrylate, and benzyl acrylate.

[0039] Acrylate-based vehicles can be used by combining the above materials as needed and adding a photoinitiator and sensitizer. For cationic polymerization vehicles, a cationic polymerization vehicle can be used that combines a liquid epoxy resin with an initiator that generates protons or acids upon exposure to light. Other applicable photocurable vehicles include ene-thiol addition reaction vehicles.

[0040] The vehicle may further contain, as needed, thixotropic agents, extender pigments, general diluting solvents, antioxidants, UV absorbers, polymerization inhibitors for dark-place stabilization, non-polymerizable polymers, leveling agents, defoamers, adhesion promoters, etc. The inclusion of extender pigments in the vehicle can improve the fluidity, hiding power, gloss, and colorability of the ink. Examples of extender pigments include clay (activated clay), clay minerals such as bentonite, calcium carbonate, and silica (white carbon). Preferably, the vehicle contains at least one selected from the group consisting of clay, clay minerals such as bentonite, and silica. However, these extender pigments may be solid acids that inhibit the reversible photochromism of the photochromic compound. However, by encapsulating the photochromic compound in microcapsules, the effects of the extender pigment can be achieved while maintaining the reversible photochromism. The content of the extender pigment in the vehicle is not particularly limited, but is preferably between 1% and 30% by mass.

[0041] The ink composition according to the embodiment of the present invention may contain other components as long as they do not deviate from the scope of the present disclosure. Examples of other components include other non-discoloring colorants (dyes and / or pigments, excluding extender pigments). When the ink composition includes other colorants, it is possible to provide the ink composition with an enantiotropic color change from a first color to a second color.

[0042] <3. Reversible photochromic prints> A reversibly photochromic printed matter according to an embodiment of the present invention comprises a substrate and a reversibly photochromic layer which is a dried product of the ink composition. In the reversibly photochromic printed matter according to the embodiment of the present invention, the reversibly photochromic layer may be provided by offset printing, or in some cases, the reversibly photochromic layer may be provided by other printing methods.

[0043] In an embodiment of the present invention, the reversible photochromic layer is formed by drying and solidifying the ink composition. The reversible photochromic layer may be formed by printing the ink composition once on a substrate and then drying and solidifying it. However, the density of the reversible photochromic layer can be improved by printing the ink composition multiple times. The drying conditions for the ink composition are not particularly limited, and known conditions can be used. The reversible photochromic layer may be a solid pattern, or may be a figure such as a circle, ellipse, square, or rectangle; various letters, symbols, patterns, etc.; or an image (reversible photochromic image) of a person, animal, plant, fruit, food, vehicle, building, celestial body, etc. The reversible photochromic layer may be in contact with the surface of the substrate. The reversible photochromic layer does not have to be in contact with the surface of the substrate; for example, another layer may be provided between the reversible photochromic layer and the surface of the substrate.

[0044] The substrate is not particularly limited as long as it can form the reversible photochromic layer, and can be made of various materials and in various shapes, such as paper, synthetic paper, fiber, fabric, synthetic leather, leather, plastic, foam, glass, ceramic material, metal material, wood, stone, etc.

[0045] In an embodiment of the present invention, the substrate preferably contains a metal material. Metal materials have light reflectivity and can provide a unique aesthetic feel to the appearance of printed matter. The content of the metal material in the substrate is not particularly limited, but is preferably 10% by mass or more, 50% by mass or more, or 90% by mass or more. The metal material is not particularly limited, but preferably contains at least one selected from the group consisting of aluminum, aluminum alloys, iron, and iron alloys, and more preferably contains at least one selected from the group consisting of aluminum and aluminum alloys.

[0046] In one embodiment of the present invention, the substrate is preferably a container capable of accommodating various contents. This allows various contents to be accommodated in the container and offered as a product, and the reversible photochromic layer can provide the product with an aesthetic appearance. The container preferably has a cylindrical shape with an openable top and a bottom, allowing the contents to be accommodated and removed and easily carried by hand. The contents are not particularly limited, but may include industrial products such as paints, inks, and solvents, tools, electronic devices, cosmetics, pharmaceuticals, food, or beverages. Beverages may include alcoholic beverages such as beer, or soft drinks such as juice. Since aesthetics are particularly important for products such as food or beverages, the contents are preferably food or beverages. That is, in one embodiment of the present invention, the substrate is preferably a container for accommodating food or beverages. When the substrate is a container, the reversible photochromic layer is preferably provided on the side surface (outer peripheral surface) of the container, which makes the reversible photochromic layer more visible.

[0047] The reversibly photochromic printed matter according to the embodiments of the present invention may include other components, such as a layer that does not exhibit reversible photochromism (an irreversibly photochromic layer), as long as such components do not deviate from the scope of the present disclosure. In such cases, the reversibly photochromic layer and the irreversibly photochromic layer may be laminated on the surface of the substrate, or may be formed in parallel without being laminated. For example, when a non-reversible photochromic layer is provided between the surface of the substrate and the reversible photochromic layer, a reversibly photochromic printed matter can be caused to undergo an alternating color change from a first color to a second color.

[0048] A transparent protective layer may be provided on the reversible photochromic layer to impart durability, or a layer containing a light stabilizer or a transparent metallic luster pigment may be provided on the reversible photochromic layer to impart light resistance. The transparent protective layer has the role of protecting the reversibly photochromic microcapsule pigment contained in the adjacent reversibly photochromic layer from physical impact, and can be provided by printing or applying a solution containing a resin or a resin emulsion, or by adhering a plastic, elastomer, rubber, etc. in the form of a plane, sheet, film, etc. The transparent protective layer can be provided by a conventional lamination process, such as dry lamination or hot melt lamination, or by interposing a heat-sealing film between the transparent protective layer and the adjacent reversible photochromic layer and then heat-pressing them together.

[0049] Examples of the light stabilizer include an ultraviolet absorber, an antioxidant, an antiaging agent, a singlet oxygen quencher, a superoxide anion quencher, an ozone quencher, a visible light absorber, and an infrared absorber. Examples of transparent metallic luster pigments include pigments having a core material such as natural mica, synthetic mica, glass flakes, alumina, or transparent film flakes whose surface is coated with a metal oxide such as titanium oxide. Light resistance can be imparted by providing a layer in which these light stabilizers or transparent metallic luster pigments are fixed in a dispersed state.

[0050] The reversibly photochromic printed matter according to the present invention may include a white, non-reversibly photochromic layer formed by printing a white ink composition that does not exhibit reversible photochromic properties (so-called white printing) in the area where the reversibly photochromic layer will be formed, in order to increase the density of the reversibly photochromic layer. That is, the reversibly photochromic printed matter may be configured such that the reversibly photochromic layer is laminated on a white, non-reversibly photochromic layer. Alternatively, the reversibly photochromic layer may be formed by blending a white, non-color-changing colorant (e.g., titanium oxide) with the ink composition according to the present invention, printing the ink composition, and then solidifying the ink composition. [Example]

[0051] The following examples are provided to more specifically describe the embodiments of the present invention. The embodiments of the present invention are not limited to the following examples, and may be modified as appropriate within the scope of the above-described and below-described aims, and all such modifications are within the technical scope of the embodiments of the present invention.

[0052] A reversible photochromic composition was obtained by uniformly dissolving 5 parts by weight of the photochromic compound 1,3,3-trimethyl-6'-(1-morpholino)-spiroindoline naphthoxazine in 50 parts by weight of a styrene-α-methylstyrene copolymer (Eastman Chemical Company, product name: Picolastic A-5) oligomer under warming. This reversible photochromic composition was then microencapsulated by interfacial polymerization. Specifically, the reversible photochromic composition was added to a mixed solution consisting of 20 parts by weight of an aromatic isocyanate prepolymer and 20 parts by weight of ethyl acetate as wall materials, followed by emulsification and dispersion in a 15% by weight aqueous gelatin solution. The mixture was heated and stirred to prepare a microcapsule dispersion. A reversible photochromic microcapsule pigment (hereinafter referred to as "Material 1") for use in an offset printing ink composition was obtained from the microcapsule dispersion by centrifugation. The average particle size of Material 1 was measured using the method described above and found to be 1.9 μm. A reversible photodiscoloration test was conducted on Material 1 by irradiating it with sunlight (UVA) for 1 minute. Before exposure, the material was colorless, but when exposed to light, it turned purple. After leaving it indoors for a while, it returned to its colorless state, and this color change was reversible.

[0053] An ink composition for offset printing (hereinafter referred to as "ink composition 1") was obtained by mixing 30 parts by mass of the above material 1 with 70 parts by mass of a commercially available offset printing vehicle for metal (the vehicle contains 1 to 30% by mass of silica as an extender pigment). For comparison, ink composition 2 was prepared by mixing 1.5 parts by mass of the above photochromic compound, 15 parts by mass of the above oligomer, and 70 parts by mass of the above vehicle, and ink composition 3 was prepared by mixing 1.5 parts by mass of the above photochromic compound and 70 parts by mass of the above vehicle.

[0054] Printed Matters 1 to 3 were produced by applying ink compositions 1 to 3 to the surface of aluminum foil as a substrate and drying at 190°C for 4 minutes. The results of the reversible photodiscoloration test for printed matters 1 to 3 are shown in Figure 1. Figure 1 is a photograph showing the color change of each printed matter before (top) and after (bottom) light exposure. To better understand the color change in Figure 1, a color original was submitted with this application as a document for submission. Please refer to this original as needed. As shown in Figure 1, printed matter 1, produced using a microencapsulated pigment (and an ink composition containing the same) satisfying all the requirements of the embodiment of the present invention, maintained reversible photodiscoloration even after a heating process simulating offset printing. Specifically, in the color original, printed matter 1 was colorless before light exposure, turned purple after light exposure, and then returned to colorless after being left indoors for a while (not shown). On the other hand, printed matter 2 and 3, which were produced using ink compositions 2 and 3 that do not satisfy the requirements of the embodiment of the present invention, did not turn purple after light irradiation. Although printed matter 2 and 3 did not turn purple, they did turn slightly, and the degree of discoloration in printed matter 2 was greater than that in printed matter 3. This is thought to be because printed matter 2, unlike printed matter 3, contains an oligomer, which protects the photochromic compound to some extent.

Claims

1. A reversibly photochromic microcapsule pigment for use in an ink composition for offset printing, which contains a reversibly photochromic composition containing a photochromic compound.

2. The microencapsulated pigment of claim 1 , wherein the reversible photochromic composition further comprises an oligomer.

3. An ink composition for offset printing, comprising the microcapsule pigment according to claim 1 or 2 and a vehicle.

4. A reversibly photochromic printed matter comprising a substrate and a reversibly photochromic layer which is a dried product of the ink composition according to claim 3.

5. The reversibly photochromic printed matter according to claim 4 , wherein the substrate is a container containing a metal material.

6. 6. The reversible photochromic printed matter according to claim 5, wherein the container contains food or drink.

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