Reversible photochromic microcapsule pigments, offset printing ink compositions, and reversible photochromic printed materials

Abstract

To provide microcapsule pigments with excellent color development, dispersibility, and heat resistance. [Solution] A collection of microcapsules comprising a wall film and a reversible photochromic composition encapsulated within the wall film, wherein the reversible photochromic composition comprises a photochromic compound, the average particle diameter (X) of the microcapsules on a volume basis is 0.1 μm to 3.0 μm, and the average thickness (Y) of the wall film is 0.02 μm to 0.40 μm, a reversible photochromic microcapsule pigment for use in offset printing ink compositions.

Description

Technical Field

[0001] The present disclosure relates to a reversible photochromic microcapsule pigment, an offset printing ink composition, and a reversible photochromic printed matter.

Background Art

[0002] Photochromic compounds have the characteristic of developing color upon irradiation with ultraviolet light or the like and fading when the irradiation is stopped, that is, reversibly changing color depending on the presence or absence of light irradiation, and are used for various applications and purposes. Patent Document 1 discloses producing a printed matter by a method such as letterpress printing, offset printing, gravure printing, screen printing, etc. using an ink mixed with a photochromic compound.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Offset printing is suitable for mass printing on paper from the viewpoints of production efficiency and cost. In recent years, offset printing has also been used for decorating containers such as metal cans or plastic cans for beverages or food.

[0005] In offset printing, the ink composition adhered to the plate is first transferred to a resin or rubber transfer roller called a blanket, and then transferred to the printed material. Offset printing has the advantage that since it passes through the blanket, the plate and the printed material do not come into direct contact, and wear of the plate is suppressed even when rotating at high speed.

[0006] Offset printing has the following characteristics. (I) Printing is carried out at high speed. (2) The process includes a step of transferring the ink composition to a blanket. (3) The process includes transferring the ink composition from the blanket to the material to be printed. (4) The system includes a baking process (heating process) for fixing the printed layer to the material to be printed.

[0007] Therefore, it is desirable for ink compositions used in offset printing to be highly viscous. This is because highly viscous ink compositions are less likely to splatter during high-speed printing and are more easily retained on the blanket.

[0008] The following qualities are desirable for pigments used in offset printing: • Excellent color reproduction. This is because high-speed printing tends to make the printed layer thin (about 1-2 μm). • High dispersibility is important. In high-viscosity ink compositions, pigments can aggregate or settle, making redispersion difficult. Furthermore, coarse particles formed by pigment aggregation can accumulate on the blanket, causing printing defects. In addition, these coarse particles can reduce the glossiness of the printed layer, potentially lowering quality. Moreover, these coarse particles accelerate plate wear. • It must be heat-resistant. This is because pigments are easily degraded by heating.

[0009] This disclosure has been made in view of the above circumstances, and one of its objectives is to provide a microcapsule pigment that is excellent in color development, dispersibility and heat resistance and is suitable for use in offset printing. [Means for solving the problem]

[0010] One aspect of the present invention is: An aggregate of microcapsules comprising a wall film and a reversible photochromic composition encapsulated within the wall film, The aforementioned reversible photochromic composition comprises a photochromic compound, The average particle size (X) of the microcapsules, based on volume, is 0.1 μm to 3.0 μm. The aforementioned wall film has an average thickness (Y) of 0.02 μm to 0.40 μm, and is a reversible photochromic microcapsule pigment used in offset printing ink compositions.

[0011] Aspect 2 of the present invention is, The above-mentioned reversible photochromic microcapsule pigment, This is an offset printing ink composition containing a vehicle.

[0012] A third aspect of the present invention is: The printed material and, A reversible light-changing printed material comprising a reversible light-changing layer formed on the substrate, which is a dried-on version of the offset printing ink composition. [Effects of the Invention]

[0013] The present invention provides microcapsule pigments that are excellent in color development, dispersibility, and heat resistance, and are suitable for use in offset printing. [Modes for carrying out the invention]

[0014] The microcapsule pigments of this disclosure have color development, dispersibility, and heat resistance suitable for offset printing. To improve color development, one approach is to increase the primary particle size of the microcapsules, thereby encapsulating a larger amount of photochromic compound within the microcapsules. However, if the primary particle size of the microcapsules is excessively large, their dispersibility decreases, such as settling in the ink composition, leading to uneven printing. One way to improve heat resistance is to thicken the microcapsule wall. However, if the wall is excessively thick, the internal volume of the microcapsule will relatively decrease, and the amount of photochromic compound that can be encapsulated will also decrease.

[0015] That is, increasing the primary particle diameter of the microcapsules is in conflict with improving the dispersibility of the microcapsules and microcapsule pigments (hereinafter, these may be collectively referred to as microcapsule pigments). Increasing the wall film thickness of the microcapsules is in conflict with improving the color development property of the microcapsule pigments.

[0016] In the present disclosure, the volume-based average particle diameter (X) of the microcapsules is 0.1 μm to 3.0 μm, and the average thickness (Y) of the wall film is 0.02 μm to 0.40 μm. By simultaneously satisfying this range for the average particle diameter (X) and the average thickness (Y) of the wall film, it is possible to balance heat resistance and color development property while suppressing aggregation and sedimentation of the microcapsules.

[0017] Color development property can be expressed by the color density (also referred to as color development density). Excellent color development property can be rephrased as a dark color (high color development density). In this specification, the color development property is relatively evaluated visually. The color development property can also be evaluated by a spectrophotometer.

[0018] 1. Reversible photochromic microcapsule pigment The reversible photochromic microcapsule pigment of the present disclosure (hereinafter, simply referred to as microcapsule pigment) is an aggregate of microcapsules (particles) including a wall film and a reversible photochromic composition encapsulated in the wall film.

[0019] The microcapsule pigment of the present disclosure is particularly preferably used in an ink composition for offset printing. The microcapsule pigment of the present disclosure may be blended in an ink composition used for other printing methods.

[0020] The reversible photochromic composition contains a photochromic compound. The photochromic compound develops color when irradiated with sunlight, ultraviolet light, or light having a peak emission wavelength in the range of 400 to 495 nm (typically, purple light with a wavelength of 405 nm), and fades when the irradiation is stopped. Therefore, an ink composition containing the reversible photochromic composition reversibly changes color.

[0021] The reversible photochromic composition is encapsulated within the wall film. Therefore, the reversible photochromic composition is less susceptible to external influences and can exhibit the desired reversible photochromic properties over a long period. External influences include changes in pH, temperature, and pressure that occur during offset printing. Temperature, in particular, has a significant impact on offset printing.

[0022] The volume-based average particle size (X) of the microcapsules used in this disclosure is 0.1 μm to 3.0 μm. When the average particle size (X) is 0.1 μm or larger, a sufficient amount of photochromic compound can be encapsulated, improving the color development of the microcapsule pigment. In addition, aggregation of microcapsules is suppressed, making it difficult for coarse particles to form, thus reducing printing defects. When the average particle size (X) is 3.0 μm or smaller, the dispersibility of the microcapsule pigment in the ink is improved. As a result, the quality of the printed layer formed by offset printing is improved.

[0023] The average particle size (X) may be 0.5 μm or larger, or 0.7 μm or larger. From the viewpoint of dispersibility, the average particle size (X) may be 2.0 μm or smaller, or 1.5 μm or smaller. From the viewpoint of color development, the average particle size (X) may be 1.5 μm or smaller, or 1.0 μm or smaller. The average particle size (X) can be said to be the average diameter of the primary particles of the microcapsules.

[0024] In the printing industry, a white ink layer (white undercoat layer) is typically added beneath the printed layer to improve color reproduction. The white ink layer reduces the influence of the substrate, thereby enhancing color vibrancy.

[0025] The printed layer (reversible light-changing layer) containing the microcapsule pigments of this disclosure has a whitish appearance. This is thought to be because the average particle size (X) of the microcapsules is small, making it easier for visible light to be diffusely reflected. As a result, the reversible light-changing layer can exert a so-called white underlay effect, further improving color development.

[0026] The whiteness of the reversible photochromic layer is thought to be caused by visible light being reflected (i.e., scattered) in various directions from the outer surface of the microcapsules. Since the average particle diameter (X) is about the same as or larger than the wavelength of visible light, much of the above scattering is thought to be Mie scattering. In Mie scattering, light of all wavelengths is scattered in the same way, so the whiteness is strongly expressed.

[0027] The average particle size (X) of the microcapsules is approximately the same as or larger than the wavelength of visible light, while being smaller than conventional microcapsules. Therefore, Mie scattering is more likely to occur. In addition, scattering toward the incident light side (in this embodiment, the side opposite to the printed material) is increased. Consequently, the reversible photochromic layer has a stronger whiteness.

[0028] While visible light is scattered on the outer surface of the microcapsules, some of the visible light is transmitted through the wall film. The visible light that enters the inside of the microcapsules acts on the photochromic compound, exhibiting reversible photochromicity.

[0029] (Method for calculating average particle size (X)) The particle size of the microcapsules is the equivalent diameter of an isovolume sphere, measured using a laser diffraction / scattering particle size distribution analyzer (e.g., LA-960V2, manufactured by Horiba, Ltd.) that has undergone a predetermined calibration.

[0030] The average particle size (X) is the average value of the equivalent diameter of an equal-volume sphere (the particle size D50, i.e., the median diameter, which corresponds to a frequency of 50% when the particle size distribution is determined based on volume).

[0031] The prescribed calibration will be explained. If the particle size of all microcapsules exceeds 0.20 μm, the average value of the equivalent diameter of an equivolute sphere is measured using the Coulter method with a particle size distribution analyzer (e.g., Multisizer 4e, manufactured by Beckman Coulter, Inc.), and calibration is performed based on that value.

[0032] In cases other than those described above, the microcapsule region is determined using image analysis-based particle size distribution measurement software (for example, MacView, manufactured by Mountec Co., Ltd.), the projected area equivalent diameter (Heywood diameter) is calculated from the area of ​​the microcapsule region, and calibration is performed based on the average value of these equivalent diameters of equivolute spheres.

[0033] The average wall thickness (Y) of the microcapsules used in this disclosure is 0.02 μm to 0.40 μm. When the average wall thickness (Y) is 0.02 μm or more, the heat resistance of the microcapsules is improved. When the average wall thickness (Y) is 0.40 μm or less, sufficient heat resistance can be obtained while maintaining the internal volume of the microcapsules. In addition, light transmission is less likely to be hindered, and the color change effect of the photochromic compound is easily exhibited.

[0034] The average thickness (Y) of the wall film may be 0.05 μm or more, or 0.10 μm or more. The average thickness (Y) of the wall film may be 0.25 μm or less, or 0.20 μm or less.

[0035] Heat resistance refers specifically to the resistance of microcapsules to heat. In this specification, heat resistance is evaluated by the color development of the ink composition after being placed in a high-temperature environment (e.g., an environment with a temperature of 150 to 250°C) for a predetermined time (e.g., 2 to 5 minutes).

[0036] (Method for calculating the average thickness (Y) of the wall film) The average thickness (Y) of the membrane can be obtained by image analysis from cross-sectional images of microcapsule pigments embedded in epoxy resin. The specific procedure is as follows:

[0037] (i) Microcapsule pigments (aggregates of microcapsules) are embedded in a resin (typically epoxy resin), and the epoxy resin is cured. (ii) The cross-section of the hardened material obtained is exposed using a microtome.

[0038] (iii) Observe the cross-section using a field emission scanning electron microscope to determine the field of view that contains 20 to 50 microcapsules. The above number of microcapsules is the count of only those that can be seen in their entirety and whose wall cross-section can be clearly seen.

[0039] (iv) Determine the circularity of all microcapsules within the field of view. Circularity is an index that represents the complexity of the external shape of a figure (in this case, the cross-sectional shape of a microcapsule). The maximum value of circularity (a perfect circle) is 1, and it decreases as the external shape becomes more complex. Circularity ranges from greater than 0 to less than or equal to 1. Circularity is calculated using the formula: 4π × (area of ​​the figure) / (perimeter of the figure). 2 It can be calculated using this method.

[0040] (v) Arrange the microcapsules in order of circularity and select the 20 with the highest circularity. (vi) For the 20 selected microcapsules, calculate the area of ​​the region enclosed by the outer circumference of the membrane (outer area) and the area of ​​the region enclosed by the inner circumference of the membrane (inner area).

[0041] (vii) The diameter of a circle (equivalent circle) having the same area as the outer area is considered to be the outer diameter of the microcapsule. The diameter of a circle (equivalent circle) having the same area as the inner area is considered to be the inner diameter of the microcapsule.

[0042] (viii) The following formula: Wall thickness = (outer diameter - inner diameter) / 2 The thickness of the microcapsule wall is determined using the method described above. The thickness of all wall membranes of the selected 20 microcapsules is determined in the same manner. The average of the wall membrane thicknesses of the 20 microcapsules is defined as the average thickness of the microcapsule wall (Y).

[0043] In steps (iv), (vi), and (vii), the circularity, area, and diameter can be obtained using image analysis software (for example, MacView, manufactured by Mountec Co., Ltd.).

[0044] The average particle diameter (X) and the average wall thickness (Y) are given by the following formula: Y / X < 0.3 (1) The following relationship may be satisfied. Microcapsules that satisfy this range are easy to manufacture. Microcapsules that satisfy this range also have a particularly good balance between heat resistance and color intensity. Y / X may be 0.25 or less, or 0.2 or less. Y / X may be 0.02 or more, 0.03 or more, or 0.04 or more.

[0045] (wall membrane) The wall film has a microscopic capsule shape (a shape with a closed space inside). The wall film forms the aforementioned internal space and demarcates the internal space from the outside. The reversible photochromic composition is contained within the aforementioned internal space.

[0046] The barrier film improves the chemical and physical stability of the reversible photochromic composition and protects it from various degradation factors in offset printing. This allows the desired reversible photochromic properties to be achieved.

[0047] The material of the wall film is not particularly limited. Examples of wall film materials include polyurea, polyamide, polyurethane, epoxy resin, melamine resin, urea resin, urea urethane resin, isocyanate resin, vinyl resin, gelatin, ethylcellulose, polyvinyl alcohol, and carboxymethylcellulose. These can be used individually or in combination of two or more.

[0048] The membrane (and microcapsules) are obtained by known methods. The membrane is manufactured by a method appropriate to its material, etc. For example, the membrane can be manufactured by interfacial polymerization, in situ polymerization, liquid curing coating, phase separation from aqueous solutions, phase separation from organic solvents, melt-dispersion-cooling, air suspension coating, or spray drying.

[0049] For example, microcapsules can be obtained using interfacial polymerization as follows. First, a system (oil phase) containing each component of the reversible photochromic composition to be contained within, along with the wall film material (wall film forming material), is added to an aqueous medium (aqueous phase) all at once or in stages and stirred to prepare an emulsion. As stirring continues, the wall film material polymerizes at the interface between the oil phase and the aqueous phase (interfacial polymerization reaction). At this time, the wall film closes to hold the reversible photochromic composition inside, forming a microcapsule. Finally, the microcapsules are separated from the aqueous phase, washed, and dried to isolate them.

[0050] A curing agent may be added to the reaction system. The reaction system may be heated. Desolvent removal may be performed during or after the interfacial polymerization reaction.

[0051] The average particle size (X) and the average wall thickness (Y) can be controlled by the manufacturing conditions of the microcapsules. For example, in interfacial polymerization, increasing the stirring speed or lengthening the stirring time tends to decrease the average particle size (X). In interfacial polymerization, increasing the amount of wall material tends to increase the average wall thickness (Y).

[0052] To control the average particle size (X) to 0.1 μm to 3.0 μm, for example, microcapsules can be manufactured by interfacial polymerization using 0.5 to 25 parts by mass of wall film material per 1 part by mass of reversible photochromic composition. To control the average wall film thickness (Y) to 0.02 μm to 0.40 μm, for example, microcapsules can be manufactured by interfacial polymerization using a stirring speed of 4000 to 13000 rpm.

[0053] A coating layer may be provided on the surface of the wall film, depending on the purpose. The coating layer can further improve heat resistance and modify surface properties. Examples of materials for the coating layer include those similar to those of the wall film. The materials of the wall film and the coating layer may be the same or different.

[0054] The mass ratio (wall film:inclusions) of the wall film and its inclusions (including reversible photochromic compositions) may be, for example, 1:1 to 1:7. This mass ratio makes it easier to obtain the desired color intensity and clarity. The mass ratio (wall film:inclusions) may also be 1:1 to 1:6.

[0055] (Reversible light-changing composition) Reversible photochromic compositions contain photochromic compounds. Photochromic compounds change their molecular structure in response to light. Photochromic compounds develop color when exposed to sunlight, ultraviolet light, or light with a peak emission wavelength in the range of 400-495 nm (typically violet light with a wavelength of 405 nm), and lose their color when the irradiation stops.

[0056] • Photochromic compounds Known photochromic compounds can be used. Examples of photochromic compounds include one or more selected from the group consisting of spirooxazine derivatives, spiropyran derivatives, and naphthopyran derivatives. Examples of photochromic compounds include those described in Japanese Patent Publication No. 2021-120493 and International Publication No. 2020 / 137469.

[0057] Photochromic compounds may have photomemory properties (color memory photochromic properties). Examples of photochromic compounds having photomemory properties include diarylethene derivatives. Examples of photochromic compounds having photomemory properties include the compounds described in Japanese Patent Application Publication No. 2021-120493.

[0058] • Oligomer Reversible photochromic compositions may contain oligomers. Photochromic compounds may dissolve in oligomers. Oligomers may be present around photochromic compounds. Oligomers protect photochromic compounds from various degradation factors in offset printing. Oligomers can maintain the reversible photochromic properties of photochromic compounds. Oligomers can assist in molecular structural changes of photochromic compounds and adjust their color change sensitivity. Oligomers improve the color intensity of microencapsulated pigments. Oligomers also enhance the lightfastness of microencapsulated pigments.

[0059] Examples of oligomers include one or more selected from the group consisting of styrene-based oligomers, acrylic-based oligomers, terpene-based oligomers, and terpene phenol-based oligomers.

[0060] The styrene oligomer may have a mass-average molecular weight (Mw) of 200 to 6000. If the Mw of the styrene oligomer is 200 or higher, stability and light resistance may be improved. If the Mw of the styrene oligomer is 6000 or lower, the molecular structure of the photochromic compound changes more easily, allowing it to quickly revert to its original molecular structure when light irradiation stops. Therefore, color retention during fading is less likely to occur. Similarly, the molecular structure of the photochromic compound changes more easily due to light irradiation, making it easier to achieve higher color intensity. In addition, the color change sensitivity is easier to adjust. The Mw of the styrene oligomer may be 4000 or lower.

[0061] Styrene oligomers are compounds having a styrene skeleton or their hydrogenated derivatives. Examples of styrene oligomers include low molecular weight polystyrene, styrene-α-methylstyrene copolymers, α-methylstyrene polymers, and α-methylstyrene-vinyltoluene copolymers. These can be used individually or in combination of two or more.

[0062] Acrylic oligomers may have an Mw of 12000 or less. When the Mw of acrylic oligomers is 12000 or less, the color change sensitivity is easily adjustable. Acrylic oligomers may have an Mw of 1000 or more. When the Mw of acrylic oligomers is 1000 or more, stability is improved. In addition, the color intensity tends to increase and lightfastness may be further improved. Acrylic oligomers may have an Mw of 1000 to 12000. Acrylic oligomers may have an Mw of 1500 or more. Acrylic oligomers may have an Mw of 8000 or less, and may also have an Mw of 6000 or less.

[0063] Examples of acrylic oligomers include acrylic acid ester copolymers.

[0064] Terpene oligomers may have an Mw of 250 to 4000. If the Mw of the terpene oligomer is 250 or higher, stability and lightfastness may improve. If the Mw of the terpene oligomer is 4000 or lower, color retention during fading is less likely, and the color intensity tends to be higher. In addition, the color change sensitivity is easier to adjust. The Mw of the terpene oligomer may be 300 or higher.

[0065] Terpene oligomers have a terpene skeleton. Examples of terpene oligomers include α-pinene polymers, β-pinene polymers, and d-limonene polymers. These can be used individually or in combination of two or more.

[0066] Terpene phenol oligomers may have an Mw of 200 to 2000. If the Mw of terpene phenol oligomers is 200 or higher, stability improves and the color intensity may increase. If the Mw of terpene phenol oligomers is 2000 or lower, the color change sensitivity is easier to adjust. The Mw of terpene phenol oligomers may be 500 or higher. The Mw of terpene phenol oligomers may be 1200 or lower.

[0067] Terpene phenol oligomers are copolymers or hydrogenated products of cyclic terpene monomers and phenols. Examples of terpene phenol oligomers include α-pinene-phenol copolymers. These can be used individually or in combination of two or more.

[0068] The Mw of the above oligomers can be measured by gel permeation chromatography (GPC).

[0069] The mass ratio of the photochromic compound to the styrene-based oligomer or acrylic-based oligomer (photochromic compound:styrene-based oligomer / acrylic-based oligomer) may be, for example, 1:1 to 1:10000, or 1:5 to 1:500.

[0070] The mass ratio of the photochromic compound to the terpene oligomer (photochromic compound:terpene oligomer) may be, for example, 1:1 to 1:5000, or 1:5 to 1:500.

[0071] The mass ratio of the photochromic compound to the terpene phenol oligomer (photochromic compound: terpene phenol oligomer) may be, for example, 1:1 to 1:50, or 1:2 to 1:30.

[0072] When the mass ratio of the photochromic compound to the various oligomers is within the above range, the reversible photochromic properties of the photochromic compound are more easily maintained, and sufficient color intensity can be obtained.

[0073] (others) The inclusions may contain non-coloring agents such as general dyes and / or pigments, along with the reversible photochromic composition. The non-coloring agents may cause an intermutation color change from a first color to a second color.

[0074] Microencapsulated pigments may contain particles that are not perfectly spherical. Microencapsulated pigments may also contain particles with depressions on their surface. Particles with depressions on their surface may be observed, for example, as a double circle (an outer circle corresponding to the shape of the microcapsule and an inner circle corresponding to the shape of the depression) in a planar SEM image. In offset printing, particles with depressions on their surface may have a larger contact area with the substrate than perfectly spherical particles, and thus may have improved transferability to the substrate.

[0075] 2. Ink compositions for offset printing The offset printing ink composition of this disclosure (hereinafter simply referred to as the ink composition) comprises the above-mentioned reversible photochromic microcapsule pigment and vehicle. The ink composition is prepared by mixing and stirring each component.

[0076] The ink compositions disclosed herein are particularly suitable for use in offset printing. The ink compositions disclosed herein may also be used in other printing methods.

[0077] The viscosity of the ink composition is not particularly limited. Even at relatively high viscosity, the microcapsule pigments of this disclosure have high dispersibility. The viscosity of the ink composition can be evaluated by the "spread diameter (mm)" determined by a parallel plate viscometer (spread meter) according to JIS K 5701. A spread meter is a device that measures fluidity by observing over time the characteristic of a sample sandwiched between two horizontally placed parallel plates spreading concentrically due to the weight of the load plates.

[0078] The above-mentioned expansion diameter (mm) is measured at 25°C using the following procedure. (i) Fill the sample hole of the parallel plate viscometer with the ink composition, and make the top surface of the sample and the top surface of the fixing plate the same plane. (ii) The piston below the sample hole is pushed up. At this time, the ink composition moves to the upper surface of the fixed plate and the load plate (115±1g) falls, and the ink composition is spread out concentrically on the fixed plate by the weight of the load plate. (iii) Read the diameter of the spread 60 seconds after the load plate comes into contact with the sample.

[0079] The spreading diameter of the ink composition may be, for example, 10 mm or more and 50 mm or less. The spreading diameter of the ink composition may be 20 mm or more and 25 mm or more. The spreading diameter of the ink composition may be 40 mm or less and 30 mm or less.

[0080] The content of microcapsule pigment may be, for example, 5% by mass or more and 45% by mass or less of the mass of the ink composition. The above content of microcapsule pigment may be 10% by mass or more and 15% by mass or more. The above content of microcapsule pigment may be 40% by mass or less and 35% by mass or less.

[0081] The vehicle includes, for example, a binder, a solvent, and various additives as needed. The vehicle may be curable (polymerizable). The vehicle may be oxidative polymerizable, thermosetting, ultraviolet curable, or electron beam curable.

[0082] (binder) The binder is an organic compound. The binder contains at least one of a polymerizable monomer, a polymerizable oligomer, and a polymer. The polymer may be polymerizable or nonpolymerizable.

[0083] Examples of polymers 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, silicone-modified alkyd resins, rosin-modified phenol resins, rosin-modified maleic acid resins, acrylic resins, silicone-modified acrylic resins, alkyd-modified acrylic resins, cellulose acetate butyrate (CAB)-modified acrylic resins, acrylic polyols, epoxy resins, acrylic-modified epoxy resins, amine-modified epoxy resins, fluororesins, polycarbonate resins, amino resins, melamine resins, benzoguanamine resins, urea resins, isocyanate resins, chlorinated polyolefin resins, vinyl chloride resins, vinyl chloride copolymer resins, and vinylidene chloride resins. These can be used individually or in combination of two or more.

[0084] Examples of polymerizable monomers include epoxy acrylates, urethane acrylates, oligoester acrylates, and polyester acrylates. Other examples of polymerizable monomers include acrylate compounds with molecular weights of 100 to 800. Examples of acrylate compounds 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 acrylate benzoate, 2-acryloyloxyethyl acid phosphate, 2-hydroxyethyl acrylate, isooctyl acrylate, and benzyl acrylate. These can be used individually or in combination of two or more.

[0085] Examples of polymerizable oligomers include dimers, trimers, tetramers, and low molecular weight polymers (Mw 100-10000) of one or more of the polymerizable monomers mentioned above.

[0086] The solid content of the binder may be, for example, 30% by mass or more and 60% by mass of the mass of the vehicle. The above content of the binder may be 35% by mass or more and 40% by mass or more. The above content of the binder may be 55% by mass or less and 50% by mass or less.

[0087] (solvent) Examples of solvents include aliphatic hydrocarbons, aromatic hydrocarbons, alcohols, esters, and ketones. For offset printing, the solvent may be relatively slow-drying. Examples of slow-drying solvents (high-boiling point solvents) include industrial solvents, kerosene, Solvesso 100, Solvesso 150, xylene, mineral spirits, n-butanol, anone, isophorone, cellosolve, and cellosolve acetate. These can be used individually or in combination of two or more.

[0088] The solvent content may be, for example, 30% by mass or more and 60% by mass of the vehicle's mass. The solvent content may be 35% by mass or more and 40% by mass or more. The solvent content may be 55% by mass or less and 50% by mass or less.

[0089] (others) The vehicle may include, for example, thixotropy-inducing agents, wetting agents, coupling agents, curing agents, viscosity modifiers, extender pigments, general solvents for dilution, antioxidants, UV absorbers, light stabilizers, polymerization inhibitors for dark-time stabilization, leveling agents, defoamers, adhesion-inducing agents, antistatic agents, antiseptics and antifungal agents, flame retardants, and rust inhibitors.

[0090] The vehicle may contain solid matter. Since the photochromic compound is encapsulated within the wall film, its function (reversible photochromicity) is unlikely to be impaired by solid matter in the vehicle.

[0091] Vehicles containing polymerizable binders may contain polymerization initiators and sensitizers. It is desirable that polymerization inhibitors and polymerization initiators do not coexist.

[0092] The vehicle may contain extender pigments. Extender pigments improve the fluidity, opacity, gloss, and colorability of the ink. Examples of extender pigments include clay minerals such as clay (activated clay) and bentonite, calcium carbonate, and silica (white carbon). The extender pigment may contain at least one of clay minerals and silica. The extender pigment content may be between 1% and 30% by mass of the vehicle's mass.

[0093] (others) The ink composition may contain non-coloring colorants such as general dyes and / or pigments. The ink composition may also contain non-coloring white pigments such as titanium dioxide.

[0094] 3. Reversible light-changing printed materials The reversible photochromic printed material of this disclosure (hereinafter simply referred to as "printed material") comprises a substrate and a reversible photochromic layer formed on the substrate, which is a dried form of the ink composition.

[0095] The reversible photochromic layer changes color upon light irradiation due to the action of a photochromic compound. For example, the color of the printed material changes between indoors and outdoors. The printed material of this disclosure has excellent design and high appeal.

[0096] (Reversible light-changing layer) The reversible photochromic layer is the dried and solidified portion of the ink composition. The reversible photochromic layer is formed when the ink composition dries and solidifies. Solidification refers to the change of a substance (in this embodiment, the ink composition) from a liquid to a solid. The solidified ink composition is the dried and solidified portion, which in this embodiment corresponds to the reversible photochromic layer. The reversible photochromic layer may contain components (typically binders) that were included in the ink composition or their reaction products.

[0097] The reversible light-color-changing layer may be formed by offset printing. Because the microcapsules exhibit excellent color development, dispersibility, and heat resistance, the above ink composition is particularly suitable for offset printing. The reversible light-color-changing layer may also be formed by other printing methods. Examples of other printing methods include screen printing, process printing, gravure printing, flexographic printing, and transfer printing.

[0098] The number of print cycles is not particularly limited and can be set appropriately according to the desired color density. The number of print cycles may be one or two or more.

[0099] After printing, volatile components (typically solvents) contained in the vehicle are removed, drying and solidifying the ink composition.

[0100] Hereinafter, solidification without intentional chemical changes will be referred to as "drying solidification." After printing, the chemical hardening (polymerization) and solidification of a polymerizable vehicle will be referred to as "hardening drying." From the viewpoint of improving the heat resistance of the reversible photochromic layer, a polymerizable vehicle may be used to form the reversible photochromic layer by hardening drying.

[0101] The drying and solidification method is not particularly limited. Examples of drying and solidification methods include natural drying, heat drying, and hot air drying. The curing and drying method is appropriately selected depending on the properties of the vehicle. Examples of curing and drying methods include the addition of an oxidizing agent, heating, ultraviolet irradiation, and electron beam irradiation. Curing and drying may be carried out by heating.

[0102] When forming a reversible photochromic layer by heating using a polymerizable vehicle, the heating temperature may be between 150°C and 250°C. The heating process may be performed multiple times. For example, the first heating step (primary drying) removes the solvent contained in the ink composition to obtain a touch-dry or semi-dried reversible photochromic layer. Primary drying suppresses color transfer of the ink composition and improves the handling properties of the printed material. Subsequently, in the second heating step (secondary drying), the vehicle within the reversible photochromic layer is chemically cured.

[0103] The primary drying temperature and the secondary drying temperature may be the same or different. The secondary drying temperature may be higher than the primary drying temperature. This facilitates the curing of the vehicle and improves the heat resistance of the reversible photochromic layer.

[0104] The reversible color-changing layer may be formed on the entire surface of the substrate or on only a portion of it. The reversible color-changing layer may be formed to represent, for example, circles, ellipses, squares, rectangles, and other shapes; shapes of people, animals, plants, fruits, food products, vehicles, buildings, celestial bodies, and so on; various letters; symbols; and geometric patterns. These may be used individually or in combination of two or more types.

[0105] The thickness of the reversible color-changing layer is not particularly limited and is set appropriately so that reversible color changeability is achieved. For example, the thickness of the reversible color-changing layer is 0.5 μm or more and 2 μm or less. The thickness of the reversible color-changing layer may be 0.7 μm or more and 1 μm or more. The thickness of the reversible color-changing layer may be 1.5 μm or less and 1.3 μm or less.

[0106] (Printing material) The material and shape of the substrate are not particularly limited, as long as a reversible light-color-changing layer can be formed on it. Examples of substrate materials include paper, synthetic paper, textiles, fabrics, synthetic leather, genuine leather, plastics, foams, glass, ceramics, metals, wood, or stone. These can be used individually or in combination of two or more.

[0107] The substrate may contain metal. Metals generally have light-reflecting properties, giving the printed material a metallic aesthetic. The metal content may be, for example, 10% or more by mass of the substrate, 50% or more by mass, 90% or more by mass, or 100% by mass.

[0108] Because substrates containing metal are harder than paper, the pressure on microcapsule pigments during printing tends to be higher. Furthermore, substrates containing metal do not absorb ink compositions well, making it difficult to apply a large amount of ink. In addition, because substrates containing metal have a luster and color derived from the metal, the color development of the printed layer is easily impaired.

[0109] Microencapsulated pigments exhibit excellent heat resistance and can produce strong color even in small amounts. The reversible photochromic layer formed by the microencapsulated pigments provides a so-called white underlay effect. Therefore, by using microencapsulated pigments, the above-mentioned problems related to printed materials containing metals can be solved.

[0110] Examples of metal types include one or more selected from the group consisting of aluminum, aluminum alloys, iron, and iron alloys (typically steel). The substrate may contain at least one of aluminum and aluminum alloys.

[0111] The shape of the object to be printed may be flat or three-dimensional. The object to be printed may have a three-dimensional shape. The object to be printed may be a container. The container has a shape that allows it to contain and remove various contents. Specifically, the container may be a bottomed cylindrical shape.

[0112] The contents are not particularly limited. Examples of contents include liquid industrial products such as paints, inks, and solvents, tools, electronic equipment, cosmetics, pharmaceuticals, food, and beverages. The contents may be food or beverages. Food or beverages may be served in public places. Food or beverages are also expected to enhance the atmosphere and provide conversation starters. Therefore, the appearance of the containers for food or beverages greatly influences purchasing intent. Containers equipped with a reversible light-changing layer have a high level of design appeal and can meet the above expectations. Examples of beverages include alcoholic beverages such as beer and soft drinks such as juices.

[0113] The substrate may be a container containing metal as a material, a container for food or beverage, or a container containing metal as a material and for food or beverage. The reversible light-coloring layer is formed, for example, on the outer surface of the container (or the outer surface if it is cylindrical).

[0114] (Other layers) The printed material may include a base layer interposed between the substrate and the reversible light-changing layer. The base layer may be a layer that does not exhibit reversible light-changing properties (an irreversible light-changing layer). Examples of the base layer include a white ink layer, an adhesive layer, and a sealing layer. The thickness of the base layer is not particularly limited and is set appropriately according to the purpose, etc. The white ink layer contains a white pigment. The base layer is formed, for example, by the printing method, lamination method, or heat-pressing method described above.

[0115] The printed material may include a transparent protective layer provided on a reversible light-color-changing layer. The transparent protective layer protects the microcapsule pigments from physical impact. Transparency means that the total light transmittance is 70% or higher. The total light transmittance can be measured by a method in accordance with JIS K 7361-1.

[0116] The transparent protective layer may contain a light stabilizer or a transparent metallic luster pigment from the viewpoint of light resistance. Examples of light stabilizers include ultraviolet absorbers, antioxidants, anti-aging agents, singlet oxygen quenchers, superoxide anion quenchers, ozone quenchers, visible light absorbers, and infrared absorbers. The transparent metallic luster pigment has, for example, a base material such as natural mica, synthetic mica, glass fragments, alumina, or transparent film fragments, and a metal oxide such as titanium oxide that coats its surface.

[0117] The thickness of the transparent protective layer is not particularly limited and is set as appropriate depending on the purpose. The transparent protective layer is formed, for example, by the printing method, lamination method, or heat-pressing method described above.

[0118] The printed material may include a layer that does not exhibit reversible light-changing properties (an irreversible light-changing layer) formed on the substrate. The irreversible light-changing layer may be located below and adjacent to the reversible light-changing layer, or above and adjacent to the reversible light-changing layer, or may be formed on the same plane as the reversible light-changing layer. The irreversible light-changing layer may cause the printed material to undergo an intermutation color change from a first color to a second color.

[0119] The thickness of the irreversible light-coloring layer is not particularly limited and can be set as appropriate depending on the purpose. The irreversible light-coloring layer can be formed, for example, by the printing method described above. [Examples]

[0120] The embodiments of the present invention will be described in more detail below with reference to examples. The embodiments of the present invention are not limited by the following examples, and can be implemented with appropriate modifications within the scope that is consistent with the spirit described above and below, and all such modifications are included within the technical scope of the embodiments of the present invention.

[0121] [Example 1] (1) Preparation of reversible photochromic composition Five parts by mass of a photochromic compound (1,3,3-trimethyl-6′-(1-morpholino)-spiroindoline naphthoxazine) and 50 parts by mass of a styrene oligomer (styrene-α-methylstyrene copolymer, Mw: 317, manufactured by Eastman Chemical Co., Ltd., product name: Picorastic A-5) were mixed, and the photochromic compound was dissolved by heating to obtain a reversible photochromic composition.

[0122] (2) Fabrication of microcapsules Microcapsules were fabricated using the obtained reversible photochromic composition by interfacial polymerization. First, 5 parts by mass of a reversible photochromic composition was added to a mixed solution of 40 parts by mass of a wall film material (aromatic isocyanate prepolymer) and 70 parts by mass of ethyl acetate. This was emulsified and dispersed in a 15% by mass gelatin aqueous solution, and stirred at a speed of 8000 rpm while heating to prepare a microcapsule dispersion. Finally, microcapsule aggregates (microcapsule pigments) were obtained from the above microcapsule dispersion by centrifugation.

[0123] The average particle size (X) and average wall thickness (Y) of the obtained microcapsules were calculated as described above. A Hitachi High-Tech Corporation SU8220 (magnification 2000x) field emission scanning electron microscope was used. MacView, a Mountec Co., Ltd. image analysis software was used. The results are shown in Table 1.

[0124] (Confirmation of reversible photochromicity) Two parts by mass of the obtained microcapsule pigment and four parts by mass of water were mixed to prepare an aqueous dispersion of the microcapsule pigment, which was then placed in a screw-cap bottle. Using a light irradiation device (Optcode Co., Ltd., product name: Select 100 LED Stand Light), the screw-cap bottle was irradiated with violet light at a wavelength of 405 nm for one minute. The distance between the light source and the screw-cap bottle was 10 cm. The microcapsule pigment was colorless before irradiation with violet light, but changed to violet after irradiation with violet light. After being left in the room for a while, it returned to being colorless. This confirmed that the microcapsule pigment changes color upon irradiation with violet light, and that this color change is reversible.

[0125] (3) Preparation of ink composition An ink composition was obtained by mixing 30 parts by mass of microcapsule pigment with 70 parts by mass of a commercially available offset printing vehicle for metal. The vehicle contained 1 to 30% by mass of silica as an extender pigment, 40 to 50% by mass of polymer as a binder, and 40 to 50% by mass of a high-boiling point solvent as a solvent. The spread diameter of the ink composition, measured at 25°C according to JIS K 5701 using a parallel plate viscometer (spread meter), was 27 mm.

[0126] (4) Production of printed materials An aluminum plate was prepared as the substrate for printing. After applying the ink composition to the aluminum plate, it was heated at 210°C for 2 minutes, simulating offset printing, to form a reversible photochromic layer.

[0127] [Examples 2-60, Comparative Examples 1-9] (2) Microcapsule pigments were obtained in the same manner as in Example 1, except that the amount of wall film material and the stirring speed were changed as shown in Tables 1 to 3. Furthermore, ink compositions and printed materials were obtained in the same manner as in Example 1. In Tables 1 to 3, the amount of wall film material is the amount relative to 5 parts by mass of the reversible photochromic composition. The spread diameter of these ink compositions was 18 mm to 35 mm.

[0128] [evaluation] (1) Dispersibility The presence or absence of microcapsule aggregates (coarse particles) in the ink compositions prepared above was observed using an optical microscope (Olympus Corporation, product name: System Biological Microscope BX53, magnification 100x) and evaluated according to the following criteria. The results are shown in Tables 1-3.

[0129] The fewer coarse particles there are, the higher the dispersibility of the microcapsules can be evaluated. If the evaluation is C or higher, the dispersibility of the microcapsule pigment is high, and the ink composition can be used without problems in offset printing.

[0130] • Evaluation criteria A: No coarse particles were detected. B: A small amount of coarse particles are detected. C: Some coarse particles are observed. D: Many coarse particles are observed.

[0131] (2) Color development The printed material obtained above was irradiated with 405nm wavelength violet light for 1 minute using a light irradiator (Optcode Co., Ltd., product name: Select 100 LED Stand Light). The distance between the light source and the printed material was 10cm.

[0132] The color changes of the printed materials were visually inspected and evaluated on a 7-point scale according to the following criteria. The results are shown in Tables 1-3. A higher number indicates a higher color density. A rating of 3 or higher indicates high color development of the microcapsule pigments, and the ink composition can be used without problems in offset printing.

[0133] (1) Ink compositions with a dispersibility rating of D are difficult to print with, therefore, (2) color development evaluation was not performed.

[0134] • Evaluation criteria 7. The color change is very clearly visible when illuminated with light. 6. The color change is clearly visible when illuminated with light. 5. The color change is clearly visible upon light irradiation. 4. The color change is clearly visible upon light irradiation. 3. A color change is visible upon light irradiation. 2: A very slight color change is visible upon light irradiation. 1: When exposed to light, almost no color change is visible.

[0135] (3) Heat resistance The printed material obtained above was divided into two parts. One of the printed materials was further heated at 210°C for 3 minutes to obtain a heat-treated sample. Heat-treated and untreated samples were irradiated with 405 nm wavelength violet light for 1 minute using a light irradiation device (Optcode Co., Ltd., product name: Select 100 LED Stand Light). The distance between the light source and the sample was 10 cm.

[0136] The color intensity of the samples was visually confirmed and evaluated according to the following criteria. The results are shown in Tables 1-3. If the evaluation is C or higher, the microcapsule pigment has high heat resistance, and the ink composition can be used without problems in offset printing.

[0137] • Evaluation criteria A: There is absolutely no difference in color intensity between the heat-treated sample and the untreated sample. B: There is almost no change in color intensity between the heat-treated sample and the untreated sample. C: The color intensity of the heat-treated sample is slightly lower than that of the untreated sample, but the color change is clearly visible upon light irradiation. D: The color intensity of the heat-treated sample is significantly lower than that of the untreated sample, and almost no color change is visible even when irradiated with light.

[0138] [Table 1]

[0139] [Table 2]

[0140] [Table 3]

[0141] To solve the above problems, the present invention provides the following embodiments. [1] An aggregate of microcapsules comprising a wall film and a reversible photochromic composition encapsulated within the wall film, The aforementioned reversible photochromic composition comprises a photochromic compound, The average particle size (X) of the microcapsules, based on volume, is 0.1 μm to 3.0 μm. The average thickness (Y) of the aforementioned wall film is 0.02 μm to 0.40 μm, and the reversible photochromic microcapsule pigment is used in an offset printing ink composition. [2] The average particle diameter (X) and the average thickness of the wall film (Y) are given by the following formula: Y / X < 0.3 (1) A reversible photochromic microcapsule pigment according to [1] above, which satisfies the relationship. [3] The reversible photochromic composition further comprises an oligomer, and is a reversible photochromic microcapsule pigment according to [1] or [2] above. [4] A reversible photochromic microcapsule pigment from any of the above [1] to [3], An offset printing ink composition comprising a vehicle. [5] The printed material and, A reversible photochromic printed material comprising: a reversible photochromic layer formed on the substrate, which is a dried product of the offset printing ink composition [4] described above. [6] The printable material is a container containing metal as a material, as described in [5] above, which is a reversible photochromic print. [7] The container is a reversible light-changing printed material as described in [6] above, for holding food or beverages. [Industrial applicability]

[0142] This invention provides microcapsule pigments with excellent color development, dispersibility, and heat resistance. The microcapsule pigments of this invention are particularly suitable for use in offset printing.

Claims

1. An aggregate of microcapsules comprising a wall film and a reversible photochromic composition encapsulated within the wall film, The aforementioned reversible photochromic composition comprises a photochromic compound, The average particle size (X) of the microcapsules, based on volume, is 0.1 μm to 3.0 μm. A reversible photochromic microcapsule pigment for use in an offset printing ink composition, wherein the average thickness (Y) of the wall film is 0.02 μm to 0.40 μm.

2. The average particle diameter (X) and the average thickness of the wall film (Y) are given by the following formula: Y / X<0.3 (1) A reversible photochromic microcapsule pigment according to claim 1 that satisfies the relationship.

3. The reversible photochromic composition further comprises an oligomer, according to claim 1 or 2, the reversible photochromic microcapsule pigment.

4. A reversible photochromic microcapsule pigment according to claim 1 or 2, An offset printing ink composition comprising a vehicle.

5. The printed material and, A reversible light-changing printed material comprising a reversible light-changing layer formed on the substrate, which is a dried product of the offset printing ink composition according to claim 4.

6. The reversible photochromic printed material according to claim 5, wherein the printed material is a container containing metal as a material.

7. The container is for holding food or beverages, the reversible light-changing printed material according to claim 6.

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