Ink for inkjet printer, and temperature indicator using ink

Abstract

To provide ink for inkjet printer which is capable of marking under a normal temperature environment by using an inkjet printer, and applicable to marking on refrigerated products or frozen products, and a temperature indicator using the ink.SOLUTION: Ink for inkjet printer according to the present invention is ink in which temperature indicator is dissolved into solvent, where the temperature indicator contains leuco dye, developer and decoloring agent, the decoloring agent is a mixture of a plurality of kinds of cholesterol derivative, each of the plurality of kinds of cholesterol derivative has predetermined polar group, polar site and / or polar structure by 1 or more and 3 or less, and the solvent is a mixture of main solvent having relatively high volatility and sub solvent having relatively low volatility.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a technology for a temperature indicator for checking temperature changes of an object under temperature control, and more particularly to ink for inkjet printers for marking a temperature indicator, and a temperature indicator using said ink. [Background technology]

[0002] Fresh foods, frozen foods, and cryogenically preserved pharmaceuticals (e.g., vaccines and biopharmaceuticals) must be kept within a specified temperature range throughout the distribution process, from transportation and storage to sales after production and processing. For this reason, a method has often been adopted in the past to install a data logger capable of continuously recording time and temperature in shipping containers to constantly measure and record the temperature throughout the distribution process.

[0003] The method using a data logger has the advantage that if there is temperature-related damage to a product, it is possible to clarify when the temperature problem occurred. However, this method is suitable for managing a large number of products at once, and has a drawback in terms of cost when managing each product individually.

[0004] On the other hand, one method for managing the temperature of individual products is to use a temperature indicator rather than a data logger. A temperature indicator is a component that displays a marker (for example, letters, symbols, one-dimensional barcodes, or two-dimensional barcodes) that changes color when a preset temperature is exceeded or dropped, allowing users to know changes in the surrounding temperature. Although temperature indicators do not have the same recording accuracy as data loggers, they are suitable for managing individual products because they can be affixed to products or marked directly on them. Temperature-sensing ink is used for the color-changing marker.

[0005] Markings on the temperature indicator can be made by various printing methods (e.g., gravure printing, screen printing, dispenser printing, inkjet printing), among which inkjet printing is considered to be the preferred printing method because it is highly adaptable to a wide variety of markings and has relatively low printing costs.

[0006] For example, Patent Document 1 (JP 2022-52104 A) discloses an ink containing temperature sensing particles and an organic dispersion medium, wherein the temperature sensing particles contain a temperature sensing material containing a leuco dye, a color developer, and a decolorizer, a first surfactant, and a second surfactant, the organic dispersion medium is an organic solvent having 5 or less carbon atoms in its molecular formula, and the second surfactant is resistant to the organic dispersion medium. Patent Document 1 states that it is possible to provide a temperature sensing oil-based ink that is stable and can be used in inkjet printers.

[0007] Furthermore, Patent Document 2 (JP 2022-87590 A) discloses an irreversible thermosensitive color developing ink that contains a leuco dye, a color developer, a decolorizer, and a solvent, wherein at least one of the leuco dye, the color developer, and the decolorizer is not microencapsulated, and the volatility satisfies the relationship of the solvent > the decolorizer > the leuco dye and the color developer, and the ink develops color upon volatilization of the decolorizer. Patent Document 2 states that it is possible to provide an irreversible thermosensitive color developing ink that can be printed with a charge-controlled inkjet printer, and that changes color upon the accumulation of temperature and time when the printed dots reach a predetermined temperature or higher. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2022-52104 [Patent Document 2] Japanese Patent Publication No. 2022-87590 Summary of the Invention [Problem to be solved by the invention]

[0009] The ink taught in Patent Document 1 is a dispersion in an organic dispersion medium of temperature-sensing particles (so-called microencapsulated particles) in which a temperature-sensing material (also called a temperature indicator) is coated with a film of a first and a second surfactant. In inkjet printing, it is necessary to reduce the diameter of the ink particles to prevent clogging of the printer nozzle or ink flow path. Patent Document 1 also teaches that in inkjet printing, the ink particles preferably have a median diameter (D50) of 0.1 to 1 μm.

[0010] However, such small-diameter capsules contain a very small amount of temperature-sensing material, making it difficult to ensure sufficient color intensity. In addition, the capsule shell is thin (for example, the thickness of the capsule shell is thought to be approximately 0.03 to 0.3 μm or less), which raises concerns that the capsule shell may swell or break in the organic dispersion medium, causing the temperature-sensing ability to deteriorate over time.

[0011] On the other hand, the ink taught in Patent Document 2 does not contain a microencapsulated temperature-sensing material, but is dissolved in a solvent. The marker is fixed by the evaporation of the solvent during printing, and the color is developed by the evaporation of the decolorant as the temperature rises. In other words, the evaporation temperature of the decolorant is the upper deviation temperature of the product to be controlled. Furthermore, in order to fix the marker, printing must be performed at a temperature above the evaporation temperature of the solvent.

[0012] Here, it is assumed that the printing machine used for marking is normally operated at room temperature (defined as 15 to 25° C. in the present invention), so the volatilization temperature of the ink solvent is preferably less than 15° C., and the volatilization temperature of the decolorizing agent is preferably more than 25° C. Patent Document 2 teaches that the volatilization temperature of the decolorizing agent is about 40 to 180° C. at normal pressure.

[0013] Such inks are difficult to use for marking refrigerated goods (for example, those that should be stored at 3 to 5°C) or frozen goods (for example, those that should be stored at -18 to -20°C). Furthermore, if the ink taught in Patent Document 2 is to be used for marking frozen goods, the printer must be operated in an environment lower than -20°C, which causes the inconvenience of dramatically increasing the equipment cost, running cost, and maintenance cost of the printer.

[0014] Therefore, an object of the present invention is to provide ink for inkjet printers that can be used to mark in a room temperature environment using an inkjet printer and that can also be used to mark refrigerated or frozen products, and a temperature indicator that uses said ink. [Means for solving the problem]

[0015] (I) One aspect of the present invention is an ink for inkjet printers in which a temperature indicator is dissolved in a solvent, the temperature indicator includes a leuco dye, a developer that causes the leuco dye to develop color, and a decolorizer that suppresses the color development of the leuco dye; the decolorizing agent is a mixture of a plurality of cholesterol derivatives, each of the plurality of cholesterol derivatives has one to three of a carboxy group, a chlorine group, a bromine group, an amino group, a hydroxyl group not bonded to an aromatic ring, an ester moiety, a ketone moiety, an ether moiety, an amide moiety, and a sodium carboxylate structure; the solvent is selected from an ether-based solvent, a ketone-based solvent, and an ester-based solvent, and is a mixture of a main solvent having a vapor pressure of 2 kPa or more and 65 kPa or less at 20°C and a sub-solvent having a vapor pressure of more than 0 kPa and 0.5 kPa or less at 20°C; The present invention provides an ink for inkjet printers, characterized in that

[0016] In the present invention, the following improvements and modifications can be freely combined in the ink for ink-jet printers (I) according to the present invention. (i) The mass ratio of the main solvent to the auxiliary solvent in the solvent is within the range of "main solvent:auxiliary solvent=75:25" to "main solvent:auxiliary solvent=99:1." (ii) The co-solvent has a dissolution rate of 50 g or less of water per 1 L of the co-solvent at 20°C. (iii) The ink for inkjet printers has a transmittance of 70% or more at room temperature for light with an optical path length of 1 cm and a wavelength of 400 nm or more and 700 nm or less.

[0017] (II) Another aspect of the present invention is a temperature indicator having an ink marker printed on a printing substrate, The present invention provides a temperature indicator, wherein the ink is any one of the inks for ink-jet printers described above. [Effects of the Invention]

[0018] According to the present invention, it is possible to provide ink for inkjet printers that can be used to perform marking in a room temperature environment using an inkjet printer and that can also be used to mark refrigerated or frozen products, and a temperature indicator that uses this ink.

[0019] Problems, configurations, and effects other than those described above will become clear from the description of the embodiments below. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a diagram showing an example of a differential scanning calorimetry curve of a temperature indicator used in the present invention. [Figure 2] FIG. 1 is a diagram showing an example of the relationship between temperature and color intensity of a marker when an object subject to temperature control is marked with the ink of the present invention. [Figure 3] 1A and 1B are chemical structural diagrams showing examples of a colorless state of a leuco dye and a color-developed state of a leuco dye. [Figure 4] FIG. 2 is a chemical structural diagram showing an example of an association state between a leuco dye and a decolorizing agent.

[0021] [Basic concept of the present invention] Aiming to create an ink for inkjet printers that can be used to mark in room temperature environments and can also be used to mark refrigerated or frozen products, the inventors investigated an ink in which the temperature sensing material / thermochromic agent is directly dissolved in a solvent without being microencapsulated, as in Patent Document 2. Here, in order to overcome the weaknesses of Patent Document 2, the present inventors went back to the manufacturing process of the object to be marked and carefully examined the requirements for a temperature sensing ink.

[0022] If an example of a refrigerated product is fresh food (which should be stored at 3 to 5°C), and an example of a frozen product is frozen food (which should be stored at -18 to -20°C), frozen food is manufactured under stricter conditions, so if you can handle frozen food, it will be easier to handle fresh food. Therefore, the following explanation will use frozen food as an example.

[0023] Most frozen foods are manufactured and packaged in a temperature environment of around 20-30°C, and then flash-frozen after the necessary information is marked on the packaging. Frozen products are grouped together in designated quantities, packed sequentially, temporarily stored, and then shipped as appropriate. The time from marking on the packaging to flash-freezing naturally varies depending on the type of product and manufacturer, but is said to be roughly 10 seconds to 3 minutes.

[0024] As described above, an object of the present invention is to provide an ink for inkjet printers that can be used to perform marking in a room temperature environment using an inkjet printer and that can also be used to mark refrigerated or frozen products.

[0025] The room temperature environment during printing before flash freezing is a temperature that is significantly higher than the temperature control temperature for the temperature-controlled item (in this case, frozen food). However, if the temperature detection marker develops color during this printing stage (before flash freezing), the temperature control for the frozen food will be impossible. In other words, it is required that the color does not develop from the time the marking is applied to the packaging until freezing is complete.

[0026] On the other hand, if the printed marker is not dried and fixed to the packaging, there is a risk that the marker will be transferred to other products when the manufactured frozen food is packaged, or will be rubbed off and become unreadable when needed. In other words, the marker is required to dry and fix to the packaging between the time of marking on the packaging and the time of packaging. However, the drying speed of ink drops sharply at low temperatures. Therefore, in practice, the marker is required to dry and fix to the packaging between the time of marking on the packaging and the time of quick freezing.

[0027] As can be seen from the above requirements, the difficulty with this technology lies in achieving both the marker drying and fixing between marking on the packaging and flash freezing, and the marker not developing color between marking on the packaging and flash freezing.

[0028] The present inventors have conducted extensive research into a configuration that satisfies the above requirements. As a result, they have found that it is important to use a specific combination of a decolorizing agent that constitutes a temperature indicator and a solvent that dissolves the temperature indicator. They have also found that it is important to use a mixed solvent of a highly volatile main solvent and a low-volatile auxiliary solvent as the solvent. The present invention was completed based on these findings.

[0029] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the present invention is not limited to the specific embodiments described, and can be appropriately combined with or improved on known techniques without departing from the technical concept of the invention.

[0030] [Inkjet printer ink] The ink for inkjet printers according to the present invention is a temperature indicator dissolved in a solvent without being microencapsulated. The temperature indicator includes a leuco dye, a color developer that causes the leuco dye to develop color, and a decolorizer that suppresses the color development of the leuco dye. The decolorizer is a mixture of multiple cholesterol derivatives, each of which has one to three predetermined polar groups, polar moieties, and / or polar structures. The solvent is a mixture of a relatively highly volatile main solvent and a relatively less volatile auxiliary solvent.

[0031] Although not an essential component of the present invention, a leveling agent may be added to the ink in order to control the print quality of the marker.

[0032] The ink of the present invention is an ink for a temperature indicator, and changes color when an object under temperature control deviates from the controlled temperature. For this reason, it is desirable that the ink be colorless and transparent before changing color. Specifically, when the ink is placed in a quartz cell with an optical path length of 1 cm and the transmittance of light with a wavelength of 400 to 700 nm is measured using an ultraviolet-visible spectrophotometer, it is desirable that the transmittance be 70% or more over the entire region. The transmittance is more preferably 80% or more.

[0033] The inkjet printers used are industrial inkjet printers, but there are no particular limitations on the inkjet method. For example, it may be a piezo method that uses a piezoelectric element to eject ink from a nozzle, or a thermal method that uses an increase in volume caused by heater heating to eject ink from a nozzle. It may also be a charge control method that charges ink particles with a voltage proportional to pixel position information and deflects them in an electrostatic field to print. Note that when used in a charge control type inkjet printer, it is preferable to add a conductive agent to the ink to adjust the ink's electrical resistivity to 2000 Ω·cm or less.

[0034] The mixing ratio of the leuco dye, developer, and decolorizer in the temperature indicator is preferably 0.5 to 10 parts by weight of developer and 20 to 100 parts by weight of decolorizer per 1 part by weight of leuco dye. If the mixing ratio of the decolorizer is below the lower limit, color development tends to occur at temperatures outside the set color development temperature (for example, at temperatures lower than the set temperature). If the mixing ratio of the decolorizer is above the upper limit, the luminescence intensity during color development weakens.

[0035] The solvent content of the ink is preferably 65% ​​to 90% by mass, and more preferably 70% to 80% by mass. If the solvent content is below the lower limit, the inkjet printer nozzles are more likely to clog. If the solvent content is above the upper limit, the color intensity during development is weaker.

[0036] Although a leveling agent is not an essential component of the ink of the present invention, its addition contributes to improving the flatness of the printed marker (preventing localized swelling of printed dots). When added, the mixing ratio of the leveling agent is preferably 0.1% by mass or more and 1% by mass or less of the total ink (1 / 30 to 1 / 10 parts by mass of the temperature indicator). Addition of an excessive amount reduces the physical strength of the printed marker.

[0037] The ink for ink-jet printers of the present invention will now be described in more detail.

[0038] (thermoindicator) As mentioned above, the temperature indicator contains a leuco dye, a developer that causes the leuco dye to develop color, and a decolorizer that suppresses the color development of the leuco dye. The color change of the temperature indicator used in the present invention will be briefly explained. Figure 1 shows an example of a differential scanning calorimetry (DSC) curve of the temperature indicator used in the present invention. In Figure 1, the vertical axis represents heat flux, the horizontal axis represents temperature, and T a is the color development start temperature, T d is the decolorization starting temperature, and M is the molten state temperature.

[0039] As shown in FIG. 1, the temperature indicator used in the present invention is in a molten state (a color-disappearing state in which the leuco dye and the color developer are separated) at a temperature M, and the color development starting temperature T a When rapidly cooled (in the direction of the left-pointing arrow in the figure), the decolorizer solidifies in an amorphous state, and the leuco dye and developer are frozen in a separated state (i.e., a decolorized state). Furthermore, if crystallization does not occur due to rapid cooling, no exothermic peak due to crystallization is observed.

[0040] When the temperature is gradually increased from this frozen state (in the direction of the right-pointing arrow in the figure), the color development starts at the temperature T a The decolorizer begins to crystallize at this temperature, and as the molecules rearrange for crystallization, the leuco dye and the developer bond to develop color. The developed state is maintained while the decolorizer is crystallizing. As the temperature rises further, the decolorization starting temperature T d At this temperature, the decolorizing agent crystals begin to melt, an endothermic peak due to melting is observed, and the leuco dye and the developer separate, resulting in decolorization. In other words, the color development / decolorization shows hysteresis with respect to temperature changes.

[0041] By utilizing this color development / decolorization property, the color development start temperature T a By using a color developer prepared so that the temperature reaches the upper limit temperature of the object to be temperature controlled, it is possible to detect whether the temperature of the object to be temperature controlled has reached the upper limit temperature from the presence or absence of color development.

[0042] Next, the relationship between color intensity and temperature in the ink of the present invention will be explained. As described above, the ink of the present invention is a material in which a temperature indicator is dissolved in a solvent. The color-developing function is exhibited when the solvent evaporates. In other words, if a certain amount of solvent remains, the solvent gets between the leuco dye and the color developer, preventing them from meeting, thereby inhibiting the color-developing function (the decolorized state is maintained). The present invention focuses on this property and utilizes it by controlling it.

[0043] FIG. 2 is a diagram showing an example of the relationship between the color intensity of the marker and temperature when the ink of the present invention is used to mark an object subject to temperature control (here, frozen food). In FIG. 2, the vertical axis represents color intensity, the horizontal axis represents temperature, and Tm is the temperature at the time of marking, T s indicates storage temperature, and other symbols are the same as in Figure 1.

[0044] During the ink manufacturing process, the temperature is raised to the molten state temperature M of the color developer to put the ink into a decolorized state (also called initialization). Before marking, the color developing function of the ink is inhibited even at room temperature because the temperature indicator is dissolved in the solvent (there is sufficient solvent present), and the decolorized state is maintained.

[0045] Temperature T to temperature controlled object m When the marking is performed, the local temperature of the marker (not the temperature of the entire object under temperature control) drops due to the heat of vaporization when the solvent evaporates from the printed marker. Also, after marking, the entire object under temperature control is quickly frozen within a specified time, and the storage temperature T s At this time, the printed marker has a much smaller heat capacity than the object to be temperature controlled, so the temperature drops faster than the object to be temperature controlled.

[0046] Marking temperature T m is the ink color development start temperature T a Although the temperature is higher than that of the marking process, the decolorized state can be maintained as long as the solvent remains in the printed marker. In other words, between marking and quick freezing, the evaporation rate of the solvent (the drying rate of the ink) is controlled so that the solvent in the marker does not completely evaporate while fixing the marker.

[0047] If the time from marking to quick freezing is approximately 10 seconds to 3 minutes, most of the solvent must evaporate quickly to fix the marker. To achieve this, it is desirable for the solvent to have a high vapor pressure.

[0048] Typical printing conditions for industrial inkjet printers are several tens of picoliters (pL) / dot and several thousand dots / cm. 2 Assuming this, the amount of ink to be marked on one temperature control object is several tens of nanoliters (nL) / cm 2To simplify the calculation, the ink density is set to 1 g / cm 3 Assuming this, the thickness of the printed marker is calculated to be several μm.

[0049] As a result of detailed research by the present inventors, it was found that solvents with a vapor pressure of 2 kPa or more at 20°C volatilize almost entirely in about 10 seconds. It was also found that solvents with a vapor pressure of 3 kPa or more at 20°C volatilize almost entirely within 10 seconds. On the other hand, if a solvent with a vapor pressure of more than 60 kPa at 20°C is used, the ink will begin to solidify near the inkjet nozzle due to its excessive volatility, which can lead to nozzle clogging. For this reason, the ink of the present invention uses a solvent with a vapor pressure of 2 kPa or more and 60 kPa or less at 20°C as the main solvent.

[0050] Each dot of the printed marker usually begins to solidify to form a film from the heterogeneous interface region (surface region, contact region with the packaging). The above-mentioned main solvent is preferable from the viewpoint of fixing the ink in a short time, but if each dot of the printed marker is completely dried before quick freezing, the environment will be such that the color development temperature T a The marker may begin to develop color due to the higher temperature.

[0051] Therefore, in the present invention, a solvent with a relatively low vapor pressure at 20°C is mixed as a co-solvent to prevent the solvent inside each dot of the printed marker from completely volatilizing before quick freezing. As a result of detailed research by the inventors, it was found that if a solvent with a vapor pressure of more than 0 kPa and 0.5 kPa or less at 20°C is mixed, the marker will not begin to develop color before quick freezing.

[0052] The mixing ratio of the main solvent to the auxiliary solvent in the solvent can be adjusted appropriately depending on the time from marking to quick freezing. For example, the mass ratio is preferably in the range of "main solvent:auxiliary solvent = 75:25" to "main solvent:auxiliary solvent = 99:1." The mass ratio is more preferably in the range of "main solvent:auxiliary solvent = 80:20" to "main solvent:auxiliary solvent = 98:2."

[0053] After quick freezing, the solvent in each dot of the printed marker is completely evaporated during storage before shipping, allowing the color-developing function to be realized. After shipping, the temperature-controlled object reaches the color-developing start temperature T a When this temperature is exceeded, the marker begins to develop color to indicate a temperature excursion.

[0054] As a result, the ink of the present invention has the effect of being capable of marking in a room temperature environment using an inkjet printer, and is also applicable to marking on refrigerated or frozen products.

[0055] Next, specific examples of the leuco dye, color developer, decolorizer, and solvent that constitute the ink of the present invention will be described.

[0056] (leuco dye) The leuco dye is an electron-donating compound, and known dyes for pressure-sensitive copying paper or thermal recording paper can be used, such as triphenylmethanephthalide-based dyes, fluorans-based dyes, phenothiazine-based dyes, indolylphthalide-based dyes, leucoauramine-based dyes, spiropyran-based dyes, rhodamine lactam-based dyes, triphenylmethane-based dyes, triazenes-based dyes, spirophthalane xanthene-based dyes, naphtholactam-based dyes, and azomethine-based dyes.

[0057] Specific examples of leuco dyes include: 9-(N-ethyl-N-isopentylamino)spiro[benzo[a]xanthene-12,3'-phthalide], 2-methyl-6-(Np-tolyl-N-ethylamino)-fluoran-6-(diethylamino)-2-[(3-trifluoromethyl)anilino]xanthene-9-spiro-3'-phthalide, 3,3-bis(p-diethylaminophenyl)-6-dimethylaminophthalide, 2'-anilino-6'-(dibutylamino)-3'-methylspiro[phthalido-3,9'-xanthene], 3-(4-diethylamino-2-methylphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide, 1-ethyl-8-[N-ethyl-N-(4-methylphenyl)amino]-2,2,4-trimethyl-1,2-dihydrospiro[11H-chromeno[2,3-g-]quinoline-11,3'-phthalide], 2'-anilino-6'-[ethyl(3-methylbutyl)amino]-3'-methylspiro[isobenzofuran-1(3H),9'-[9H]xanthin]-3-one, 2-anilino-6-dibutylamino-3-methylfuran, 2-(phenylamino)-6-(diphenylamino)-3-methylspiro[9H-xanthine-9,3'-phthalide], Dibutylamino-6-methyl-7-anilinofluoran, 2'-anilino-6'-[ethyl(p-tolyl)amino]-3'-methylspiro[isobenzofuran-1(3H),9'-[9H]xanthin]one, 9-(diethylamino)spiro[12H-benzo[a]xanthin-12,1'(3'H)-isobenzofuran]-3'-one, 2'-chloro-6'-(diethylamino)spiro[isobenzofuran-1(3H),9'-[9H]xanthin]-3-one, 3-[4-(dimethylamino)phenyl]-3-(1-ethyl-2-methyl-1H-indol-3-yl)phthalide, 2'-Bromo-6'-(dibutylamino)-3'-methyl-spiro[isobenzofuran-1(3H),9'-[9H]xanthin]-3-one, 3-(1,2-dimethyl-3-indolyl)-3-[4-(diethylamino)-2-methylphenyl]phthalide, 7-[4-(dimethylamino)-2-hexoxyphenyl]-7-(1-ethyl-2-methylindol-3-yl)furo[3,4-b]pyridin-5-one, 6'-[ethyl(p-tolyl)amino]-2'-(methylphenylamino)spiro[isobenzofuran-1(3H),9'-[9H]xanthin]-3-one, The temperature indicator may be a combination of two or more leuco dyes.

[0058] (developer) The color developer is an electron acceptor, a compound that changes the structure of an electron-donating leuco dye upon contact with the leuco dye to cause color development, and is generally an acidic substance. Known color developers used in thermal recording paper, pressure-sensitive copying paper, etc. may be used. Metal salts of carboxylic acid derivatives, salicylic acid, metal salicylate salts, sulfonic acids, sulfonates, phosphoric acids, metal phosphate salts, acidic phosphate esters, metal salts of acidic phosphate esters, phosphorous acids, metal phosphites, etc. may also be used.

[0059] Specific examples of the developer include: benzyl 4-hydroxybenzoate, 2,2'-biphenol, 1,1-bis(3-cyclohexyl-4-hydroxyphenyl)cyclohexane, 2,2-bis(3-cyclohexyl-4-hydroxyphenyl)propane, 2,2-bis(4-hydroxyphenyl)propane, 1,1-bis(4-hydroxyphenyl)-1-phenylmethane, 2,2-bis(4-hydroxyphenyl)hexafluoropropane, 2,2-bis(4-hydroxyphenyl)butane, bis(4-hydroxyphenyl)diphenylmethane, 2,2-bis(3-methyl-4-hydroxyphenyl)propane, bis(4-hydroxyphenyl)-2,2-dichloroethylene, 1,1-bis(4-hydroxyphenyl)ethane, bis(4-hydroxyphenyl)methane, 2,2-bis(4-hydroxy-3-isopropylphenyl)propane, 1,3-bis(2-(4-hydroxyphenyl-2-propyl)benzene), bis(4-hydroxyphenyl) sulfone, 1,4-bis(2-(4-hydroxyphenyl)-2-propyl)benzene, bis(4-hydroxyphenyl) sulfide, 4,4-sulfonylbis[2-(2-propenyl)phenol], 4-{[4-(allyloxy)phenyl]sulfonyl}phenol, 2,4'-dihydroxydiphenyl sulfone, parahydroxybenzoic acid esters, Furthermore, phenols such as gallic acid esters may also be used.

[0060] In particular, those having high compatibility with leuco dyes and decolorizing agents described below are preferred, and organic color developers such as benzyl 4-hydroxybenzoate, 2,2'-bisphenol, 2,2-bis(4-hydroxyphenyl)propane, and gallic acid esters are preferred.

[0061] The temperature indicator may be a combination of two or more color developers. By combining color developers, the color intensity of the leuco dye when developed can be adjusted. The mixing ratio of the color developers may be appropriately set according to the desired color intensity.

[0062] (decolorizing agent) The decolorizer is a compound that can suppress the encounter between the leuco dye and the developer, and can control the color development temperature of the leuco dye and the developer. In the temperature range where the leuco dye is in the developed state, the decolorizer is solidified in a phase-separated state. On the other hand, in the temperature range where the leuco dye is in the decolorized state, the decolorizer is melted, which has the effect of reducing the probability of the leuco dye and the developer encountering each other.

[0063] For this reason, the state change temperature of the decolorizing agent becomes important for controlling the temperature of the temperature indicator. Naturally, it is preferable that the decolorizing agent is a substance that is highly compatible with the leuco dye and the developer described above.

[0064] As mentioned above, the present invention is directed to a temperature indicator in which a marker changes color when the temperature deviates from a preset temperature range, and assumes that refrigerated or frozen foods are the objects to be temperature controlled. At first glance, it would seem correct for a temperature indicator to change color immediately when the temperature deviates from the set temperature range.

[0065] However, in the case of temperature deviations at a level that has absolutely no adverse effect on the quality of temperature-controlled items (refrigerated foods or frozen foods), for example, if the color were to appear immediately due to a slight temperature deviation or time when transferring products to refrigerated or frozen shelves for product display, there is a concern that this would make display work extremely difficult and meanwhile damage the value of the products (products that have no problems would be displayed as if they were not acceptable).In other words, from the perspective of workability and proper quality control, it is desirable for the color to appear at a temperature and / or time deviation that would have an adverse effect on the quality of the temperature-controlled items.

[0066] As a result of detailed research into the above-mentioned problems, the inventors have found that by using a mixture of multiple cholesterol derivatives as a decolorizer, the temperature at which the decolorizer crystallizes can be expanded from a single point to a wide range (i.e., the color development onset temperature can be expanded from a single point to a wide range). This leads to a gradual change / increase in the color intensity of the marker. More specifically, when the temperature deviation and / or deviation time is at a level that does not adversely affect the quality of the temperature-controlled object, the color can be adjusted to a very low level of visibility.

[0067] Cholesterol derivatives are a group of compounds with a four-membered steroid skeleton consisting of three six-membered chair rings and one five-membered ring. Cholesterol derivatives are typically classified into five types based on differences in side chains: estrane, androstane, pregnane, cholane, and cholestane. Other types include plant cholesterol derivatives with a substituent at the 24th position of the steroid skeleton, and unclassified cholesterol derivatives that do not fall into the above five types or plant cholesterol derivatives.

[0068] Each cholesterol derivative used in the present invention has a specific polar group, polar moiety, and / or polar structure in its molecular structure, specifically, a carboxyl group, a chlorine group, a bromine group, an amino group, a hydroxyl group not bonded to an aromatic ring, an ester moiety, a ketone moiety, an ether moiety, an amide moiety, or a sodium carboxylate structure.

[0069] The polar groups, polar moieties, and polar structures in the decolorizer suppress the color development of the leuco dye. To function as a decolorizer, the molecular structure of the cholesterol derivative must contain at least one of these polar groups, polar moieties, and / or polar structures. On the other hand, if a cholesterol derivative is used that contains four or more polar groups, polar moieties, and polar structures in total, the color development becomes too small, making it difficult to sense temperature deviations. From the perspective of color development (color change, color development), the total number of polar groups, polar moieties, and polar structures in the molecular structure is preferably one to three, and more preferably one to two.

[0070] Here, the colorless state of the leuco dye, the colored state of the leuco dye, and the association state of the leuco dye and the decolorizer will be briefly described. Figure 3 is a chemical structural diagram showing an example of the colorless state of the leuco dye and the colored state of the leuco dye, and Figure 4 is a chemical structural diagram showing an example of the association state of the leuco dye and the decolorizer.

[0071] As shown in Figure 3, protons (H + ) bonds to the leuco dye, which changes into a color-forming structure and develops color, and returns to a colorless state when the proton dissociates. As shown in Figure 4, when a colorless leuco dye encounters a decolorizer that has an ester moiety, which is a polar moiety, the leuco dye and the decolorizer form an association via the polar moiety, inhibiting the color development of the leuco dye.

[0072] Acidic cholesterol derivatives (i.e., cholesterol derivatives that supply protons) are not preferable as decolorants in the present invention because they may act as color developers at undesired times. For example, cholesterol derivatives having a hydroxyl group bonded to an aromatic ring in their molecular structure can be mentioned.

[0073] Specific examples of cholesterol derivatives having an estrane structure that can be used as decolorizing agents include: Allylestrenol, Orthrenogest, 9(10)-dehydronandrolone, Desogestrel, Dienogest, 6-dehydronandrolone acetate, estradiol benzoate, estrone-3-methyl ether, estra-4,9-diene-3,17-dione, Ethylgonendione, estradiol dipropionate, gestodene, gastrinon (gestrinone), Lynestrenol, nordistron (norethisterone), norethisterone acetate, Nandrolone, norgestrel, quinestrol, Trenbolone, Ethynodiol diacetate, estra-5(10),9(11)-diene-3,17-dione-3-ethylene ketal, Mifepristone, These have 1 to 3 preferred polar groups, polar moieties and / or polar structures.

[0074] Specific examples of cholesterol derivatives having an androstane structure that can be used as decolorizers include: stanolone, Δ 4 -Andelosterone-3,17-dione (Δ 4 -androstene-3,17-dione), abiraterone, abiraterone acetate, clostebol acetate, Dehydroepiandrosterone, Dehydroepiandrosterone acetate, ethisterone, Epiandrosterone, Dianabol, Methylandrosteronediol, Methyltestosterone, mesterolone, mestaline, methenolone enanthate, 6-methyleneandrost-4-ene-3,17-dione, Testosterone, Testosterone propionate, Adrenosterone, stanozolol, trilostane, These have 1 to 3 preferred polar groups, polar moieties and / or polar structures.

[0075] Cholesterol derivatives with a pregnane structure are classified into progestogens and corticoids, but progestogens are more preferred in terms of the number of polar groups, polar sites and / or polar structures in the molecular structure.

[0076] Specific examples of cholesterol derivatives of the pregnane structure progestogen type used as decolorizers include: drospirenone, 16-dehydropregnenolone acetate, pregnenolone acetate, Progesterone, 5β-pregnane-3α,20α-diol, 5α-pregnane-3,20-dione, prognenolone, Δ 6 -Progesterone (Δ 6-progesterone), Esonogestrel, cyproterone acetate, chlomadinone acetate, 16,17-epoxyprogesterone, fluorometholone, 11α-hydroxyprogesterone, 17α-hydroxyprogesterone caproate, 17α-hydroxyprogesterone, 17α-hydroxyprogesterone acetate, Mygestrol acetate, medroxyprogesterone acetate, medroxyprogesterone, 4-prognene-3,11,20-trione, fluorogestone acetate, These have 1 to 3 preferred polar groups, polar moieties and / or polar structures.

[0077] Specific examples of pregnane-structure corticoid cholesterol derivatives used as decolorizing agents include: corticosterone, Deoxycorticosterone acetate, spironolactone, These have three preferred polar groups, polar moieties and / or polar structures.

[0078] Specific examples of cholesterol derivatives having a cholane structure that can be used as decolorizers include: Sodium deoxycholate, sodium choleate, Methyl hyodeoxycholate, Sodium chenodeoxycholate, These have three preferred polar groups, polar moieties and / or polar structures.

[0079] Specific examples of cholesterol derivatives having a cholestane structure that can be used as decolorizers include: Cholestane steroid derivatives include: β-cholestanol, Cholesterol, cholesteryl chloride, cholesteryl bromide, cholesterol margarate, (+)-4-cholesten-3-one, cholesterol lauryl carbonate, These have one to two preferred polar groups, polar sites and / or polar structures.

[0080] Specific examples of plant cholesterol derivatives used as decolorizing agents include: Plant steroid derivatives include: ergosterol, β-sitosterol, Stigmasterol, β-sitosterol acetate, Sarsasapogenin, Diosgenin, digitoxigenin, peiminine, Peimine, (20S)-protopanaxadiol, Panaxadiol, These have 1 to 3 preferred polar groups, polar moieties and / or polar structures.

[0081] Specific examples of unclassified cholesterol derivatives used as decolorizers include: Finasteride, 3-Oxo-4-aza-5α-androstan-17β-carboxylic acid, Dutasteride, These have one to two preferred polar groups, polar sites and / or polar structures.

[0082] Two or more decolorizing agents may be used in combination. The mixing ratio of the decolorizing agents may be appropriately determined depending on the desired temperature deviation and / or deviation time.

[0083] (solvent) As mentioned above, the solvent used in the present invention controls the evaporation rate (ink drying rate) so that the solvent in the marker does not completely evaporate while fixing the marker between marking and quick freezing. It is a mixture of a main solvent with relatively high volatility and a sub-solvent with relatively low volatility.

[0084] Naturally, it is preferable to select a combination of main solvent and auxiliary solvent that are highly compatible with each other, and it is also preferable to select a combination of solvents that are highly compatible with the temperature indicator (leuco dye, color developer, decolorizer). Considering the compounds that make up the temperature indicator, the solvents (main solvent and auxiliary solvent) used in the present invention are preferably selected from ether-based solvents, ketone-based solvents, and ester-based solvents.

[0085] Particularly preferred ether solvents are those with a cyclic ether structure or multiple ether bonds in the molecule.Particularly preferred ketone solvents are those with a cyclic ketone structure or multiple carbonyl moieties in the molecule.Particularly preferred ester solvents are those with a structure in which a carboxylic acid having 1 to 3 carbon atoms or an alcohol having 1 to 3 carbon atoms is ester-bonded.

[0086] (main solvent) In order to fix the marker within the time between marking and quick freezing, the main solvent must volatilize quickly, and preferably has a vapor pressure of 2 kPa or more and 60 kPa or less at 20°C.

[0087] Specific examples of ether solvents used as the main solvent include: Diethyl ether (vapor pressure at 20°C: 58.7 kPa), Dipropyl ether (vapor pressure at 20°C: 10 kPa), Tetrahydrofuran (vapor pressure at 20°C: 19.3 kPa), 2-methyltetrahydrofuran (vapor pressure at 20°C: 13.6 kPa), Tetrahydropyran (vapor pressure at 20°C: 7.2 kPa), 1,3-dioxolane (vapor pressure at 20°C: 16 kPa), 1,4-dioxane (vapor pressure at 20°C: 4.2 kPa), 1,3-dioxane (vapor pressure at 20°C: 4.3 kPa), Dimethoxymethane (vapor pressure at 20°C: 48 kPa), Diethoxymethane (vapor pressure at 20°C: 6 kPa), Dimethoxyethane (vapor pressure at 20°C: 6.4 kPa), The following can be mentioned:

[0088] Specific examples of ketone solvents used as the main solvent include: Acetone (vapor pressure at 20°C: 24.7 kPa), 2-butanone (vapor pressure at 20°C: 10.5 kPa), Cyclobutanone (vapor pressure at 20°C: 3.6 kPa), 2,3-butanedione (vapor pressure at 20°C: 9 kPa), 2,3-pentanedione (vapor pressure at 20°C: 3 kPa), The following can be mentioned:

[0089] Specific examples of ester solvents used as the main solvent include: Methyl acetate (vapor pressure at 20°C: 20.8 kPa), Ethyl acetate (20°C vapor pressure: 10 kPa), Propyl acetate (vapor pressure at 20°C: 3.3 kPa), Methyl propionate (vapor pressure at 20°C: 7.5 kPa), Ethyl propionate (vapor pressure at 20°C: 4.8 kPa), The following can be mentioned:

[0090] (co-solvent) It is preferable to use a co-solvent having a vapor pressure of more than 0 kPa and not more than 0.5 kPa at 20°C so that the solvent inside each dot of the printed marker completely evaporates before quick freezing and the marker does not start to develop color at an undesired time.

[0091] Specific examples of ether solvents used as auxiliary solvents include: Dipentyl ether (vapor pressure at 20°C: 0.1 kPa), Diisopentyl ether (vapor pressure at 20°C: 0.19 kPa), Dihexyl ether (vapor pressure at 20°C: 0.01 kPa), The following can be mentioned:

[0092] Specific examples of ketone solvents used as auxiliary solvents include: Cyclohexanone (vapor pressure at 20°C: 0.45 kPa), Cycloheptanone (vapor pressure at 20°C: 0.14 kPa), 1-methylcyclohexanone (vapor pressure at 20°C: 0.3 kPa), 1,1-dimethylcyclohexanone (vapor pressure at 20°C: 0.3 kPa), 2,2-dimethylcyclohexanone (vapor pressure at 20°C: 0.2 kPa), 3,5-heptanedione (vapor pressure at 20°C: 0.2 kPa), 2-octanone (vapor pressure at 20°C: 0.2 kPa), The following can be mentioned:

[0093] Specific examples of ester solvents used as auxiliary solvents include: Hexyl acetate (vapor pressure at 20°C: 0.18 kPa), Hexyl propionate (20°C vapor pressure: 0.1 kPa), The following can be mentioned:

[0094] Two or more main solvents and two or more auxiliary solvents may be used in combination. The mixing ratio of the main solvent and the auxiliary solvent may be appropriately determined depending on the time required from marking to quick freezing and / or the desired degree of dryness of the marker.

[0095] As mentioned above, the temperature T m When marking with a marker, the local temperature of the marker drops due to the heat of vaporization when the solvent evaporates from the printed marker. In this case, if the marker is used in a high humidity environment such as in summer, condensation may occur on the marker.

[0096] If the resulting condensation water penetrates into the printed dots and dissolves in the solvent (especially the auxiliary solvent) remaining in the printed dots, there is a possibility that the hydrophobic decolorant will precipitate and crystallize within the printed dots. The precipitation and crystallization of the decolorant will lead to an encounter between the leuco dye and the developer (i.e., color development). This is not desirable because it will not develop at the correct time (it will result in abnormal color development).

[0097] In order to prevent such abnormal color development (color development at undesired times), it is preferable to select a solvent with low water solubility as the auxiliary solvent. As a result of detailed studies by the present inventors, it was found that a solvent in which the amount of water dissolved per liter of auxiliary solvent at 20°C is 50 g or less is suitable.

[0098] Specific examples of such co-solvents include: Cyclohexanone (solubility in water at 20°C: 25 g / L), Cycloheptanone (solubility in water at 20°C: 20 g / L), 1-methylcyclohexanone (solubility in water at 20°C: 1.5 g / L), 1,1-dimethylcyclohexanone (solubility in water at 20°C: 1 g / L), 2,2-dimethylcyclohexanone (solubility in water at 20°C: 1 g / L), The following can be mentioned:

[0099] It should be noted that the above-mentioned problems / solutions are for environments where condensation is likely to occur, and are not always necessary.

[0100] (Leveling agent) Although a leveling agent is not an essential component of the ink of the present invention, its addition contributes to improving the flatness of the printed marker (preventing localized swelling of printed dots). There are no particular limitations on the leveling agent used in the present invention, as long as it is soluble in the solvent used. Note that the leveling agents exemplified below are all liquids with relatively low vapor pressures and high boiling points, and therefore also have the secondary effect of suppressing drying of printer nozzles and the resulting clogging.

[0101] [ka]

[0102] [ka]

[0103] [ka]

[0104] [ka]

[0105] The leveling agents shown in Compound Groups 1 to 4 are a group of compounds that have a polydimethylsiloxane chain as the main chain and have a polyalkoxy group at the end of this main chain or on a side chain.

[0106] [ka]

[0107] [ka]

[0108] The leveling agents shown in Compound Groups 5 and 6 are a group of compounds that have a polydimethylsiloxane chain as the main chain and an amino group connected to this main chain via an alkyl chain. [Example]

[0109] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0110] [Experiment 1] (Preparing leuco dye) In preparing the inks of the examples and comparative examples, three types of leuco dyes L-1 to L-3 shown in Table 1 were prepared.

[0111] [Table 1]

[0112] (Preparing the developer) In preparing the inks of the examples and comparative examples, three types of color developers C-1 to C-3 shown in Table 2 were prepared.

[0113] [Table 2]

[0114] (Preparing the decolorizer) In preparing the inks of the Examples and Comparative Examples, 21 types of decolorizing agents D-1A to D-9A were prepared as shown in Table 3. Table 3 also lists the number of polar moieties (polar groups, polar sites, and polar structures) contained in the molecule of each decolorizing agent.

[0115] [Table 3]

[0116] (Preparing the solvent) In preparing the inks of the examples and comparative examples, six main solvents Sm-1 to Sm-6 and six sub-solvents Ss-1 to Ss-6 were prepared as shown in Table 4. Table 4 also lists the vapor pressure of each solvent at 20°C.

[0117] [Table 4]

[0118] [Experiment 2] (Preparation of inks in Examples 1 to 6 and Comparative Examples 1 to 3) Inks for Examples 1 to 6 and Comparative Examples 1 to 3 were prepared using the leuco dye, developer, decolorant, main solvent, and auxiliary solvent prepared in Experiment 1. The mixing ratio of "leuco dye: developer: decolorant" in the temperature indicator of each ink was "1 part by mass: 1 part by mass: 10 parts by mass."

[0119] To the prepared temperature indicator, 108 parts by mass of the main solvent and 10 parts by mass of the auxiliary solvent were added, and the mixture was thoroughly stirred to dissolve the solids, thereby preparing the inks of Examples 1 to 6 and Comparative Examples 1 to 3. The specifications of each ink are shown in Table 5. Table 5 also lists the number of polar moieties contained in the molecule of each decolorizing agent, as well as the measurement results of the color intensity, which will be described later.

[0120] [Table 5]

[0121] [Experiment 3] (Investigation of ink properties in Examples 1 to 6 and Comparative Examples 1 to 3) The properties of the inks of Examples 1 to 6 and Comparative Examples 1 to 3 prepared in Experiment 2 were investigated.

[0122] First, each ink was placed in a quartz cell with an optical path length of 1 cm, and the transmittance of light with wavelengths of 400 to 700 nm was measured using an ultraviolet-visible spectrophotometer (manufactured by JASCO Corporation, model: V-770). As a result, it was confirmed that the transmittance was 80% or higher for all of the inks in Examples 1 to 6 and Comparative Examples 1 to 3.

[0123] Next, a marking and color development test was conducted using an inkjet printer. An ice pack (made of polypropylene with a white surface) was used as a sample simulating frozen food. Marking was performed on the surface of a room-temperature ice pack (not frozen) using an inkjet printer in a room-temperature environment. After waiting one minute after marking, the marked area was lightly rubbed with a finger, and after waiting another four minutes, the ice pack was placed in a freezer at -20°C for rapid cooling.

[0124] After leaving the ink in the freezer for one week, the color of the marker was checked and it was found that all the inks remained colorless. This confirmed that the inks of the present invention do not immediately develop color even when used for marking at room temperature, which is higher than the color development initiation temperature.

[0125] Next, the sample was transferred to an incubator at 25°C, and the color intensity was measured by observing the color change of the marker while measuring the temperature of the sample surface with a thermocouple.

[0126] Here, we will briefly explain how to measure color intensity. After adjusting the brightness of the printed marker area to 5000 lux with an LED white light source, we photograph it with a digital camera. The RGB values ​​of this photographed image are calculated, and the distance (L1) from the white coordinate of the area where no marker is printed is determined. The calculated L1 is divided by the distance (L0) between the white coordinate and the black coordinate, and the value "L1 / L0" is defined as the color intensity.

[0127] According to visual inspection by 10 subjects, when the color intensity was 0.08 or more, 8 subjects were able to visually determine whether or not color was present. Furthermore, when the color intensity was 0.1 or more, all 10 subjects were able to visually determine whether or not color was present. Based on this, in the present invention, inks with a color intensity of 0.1 or more were judged to be "passed," and inks with a color intensity of less than 0.1 were judged to be "failed."

[0128] After transferring the samples to a 25°C incubator, the surface temperature of the samples reached 0°C in about 30 minutes, and the color intensity of the samples in Examples 1 to 6 became approximately 0.1. Approximately 3 hours after transferring to the 25°C incubator, the color intensity reached its maximum. The color intensity at its maximum was 0.2 for Example 1, 0.3 for Example 2, 0.2 for Example 3, 0.3 for Example 4, 0.5 for Example 5, and 0.5 for Example 6, and all were judged to be "passed." The measurement results of the color intensity at its maximum are also shown in Table 5.

[0129] In contrast, in Comparative Examples 1 to 3, after the samples were transferred to a 25°C incubator, the surface temperature of the samples reached 0°C in about 30 minutes, but no color development was visible. As in Examples 1 to 6, the color development intensity was measured approximately 3 hours after being transferred to the 25°C incubator. As a result, the color development intensity of Comparative Examples 1 to 3 was "0.03" for Comparative Example 1, "0.03" for Comparative Example 2, and "0.05" for Comparative Example 3, and all were judged to be "fail." The color development intensity measurement results are also shown in Table 5.

[0130] The difference between the Examples and Comparative Examples is the number of polar moieties in the molecule of the decolorizing agent used. In Examples 1 to 6, the number of polar moieties in the molecule of the decolorizing agent used is all 3 or less. On the other hand, in Comparative Examples 1 to 3, the number of polar moieties in the molecule of the decolorizing agent used is included as 4. From these results, it is confirmed that it is preferable to use a decolorizing agent constituting a temperature indicator that has 3 or less polar moieties in the molecule.

[0131] It was also confirmed that the shapes of the printed markers were not distorted in the samples of Examples 1 to 6. These results confirmed that the ink of the present invention can be sufficiently fixed in a short time after marking on an object subject to temperature control.

[0132] The above-described embodiments and experiments have been described to aid in understanding the present invention, and the present invention is not limited to the specific configurations described. For example, it is possible to replace part of the configuration of the embodiments with configurations within the technical common sense of those skilled in the art, and it is also possible to add configurations within the technical common sense of those skilled in the art to the configuration of the embodiments. In other words, it is possible to delete, replace, or add other configurations to part of the configuration of the embodiments in this specification without departing from the technical spirit of the invention.

Claims

1. An ink for inkjet printers in which a temperature indicator is dissolved in a solvent, the temperature indicator includes a leuco dye, a developer that causes the leuco dye to develop color, and a decolorizer that suppresses the color development of the leuco dye; the decolorizing agent is a mixture of a plurality of cholesterol derivatives, each of the plurality of cholesterol derivatives has one to three of a carboxy group, a chlorine group, a bromine group, an amino group, a hydroxyl group not bonded to an aromatic ring, an ester moiety, a ketone moiety, an ether moiety, an amide moiety, and a carboxylate sodium salt structure; the solvent is selected from ether-based solvents, ketone-based solvents, and ester-based solvents, and is a mixture of a main solvent having a vapor pressure of 2 kPa or more and 65 kPa or less at 20°C and a sub-solvent having a vapor pressure of more than 0 kPa and 0.5 kPa or less at 20°C; 1. An ink for an inkjet printer.

2. 2. The ink for inkjet printers according to claim 1, An ink for an inkjet printer, wherein the mass ratio of the main solvent to the auxiliary solvent in the solvent is within a range of "main solvent:auxiliary solvent=75:25" to "main solvent:auxiliary solvent=99:1."

3. 2. The ink for inkjet printers according to claim 1, The ink for inkjet printers is characterized in that the amount of water that dissolves in 1 L of the auxiliary solvent at 20°C is 50 g or less.

4. 3. The ink for inkjet printers according to claim 2, The ink for inkjet printers is characterized in that the amount of water that dissolves in 1 L of the auxiliary solvent at 20°C is 50 g or less.

5. The ink for inkjet printers according to any one of claims 1 to 4, An ink for inkjet printers characterized by a transmittance of 70% or more at room temperature for light with an optical path length of 1 cm and a wavelength of 400 nm to 700 nm.

6. A temperature indicator having an ink marker printed on a printing substrate, A temperature indicator, wherein the ink is the ink for an ink-jet printer according to any one of claims 1 to 4.

7. A temperature indicator having an ink marker printed on a printing substrate, 6. A temperature indicator, wherein the ink is the ink for ink-jet printers according to claim 5.

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