Inkjet ink

Abstract

According to the present invention, provided is an inkjet ink which is for a glass substrate, and with which a beautiful image having a high concealing property can be formed on the surface of the glass substrate. The inkjet ink disclosed herein contains: an inorganic solid content including an inorganic pigment exhibiting black color, and glass frit; a monomer component having photocurability; and a photopolymerization initiator. In such an inkjet ink, the volume percentage of the inorganic solid content is at most 35 volume% when the total volume of the ink is 100 volume%, the volume percentage of the inorganic pigment is 15-90 volume% (exclusive of 90) when the total volume of the inorganic solid content is 100 volume%, and the volume ratio of the inorganic pigment to the photopolymerization initiator is at most 6. Accordingly, the ejection property during printing, the photocurability after printing, the concealing property of the image after firing, and the fixability to a glass substrate are exhibited at a high level, and thus, a beautiful image can be drawn precisely on the glass substrate.

Description

[Technical Field]

[0001] The present invention relates to an inkjet ink. Specifically, the present invention relates to an inkjet ink for glass substrates used to print images on transparent glass substrates. This application claims priority to Japanese Patent Application No. 2020-054528, filed on March 25, 2020, the entire contents of which are incorporated herein by reference. [Background technology]

[0002] Inkjet printing has traditionally been used as a printing method for drawing desired images, such as patterns and letters, on a printing target. Inkjet printing is used in a variety of fields because it can draw highly accurate images using simple and inexpensive equipment. In recent years, the use of inkjet printing has been considered when drawing images on inorganic substrates, such as glass substrates, ceramic substrates (e.g., porcelain, ceramic tiles), and metal substrates. Specifically, in the field of inorganic substrates, handwriting and plate-based printing have traditionally been used to draw images, such as patterns and letters. However, inkjet printing has attracted attention from the perspective of improving productivity because it does not require the skilled artisanal skills of handwriting and, unlike plate-based printing, allows for rapid printing on demand.

[0003] However, it is difficult to directly apply inkjet printing techniques used in other fields, such as paper and cloth, to the field of inorganic substrates, and there is still much room for improvement in inkjet printing in the field of inorganic substrates. For example, in products using inorganic substrates (inorganic products), the inorganic substrate on which an image has been drawn may be subjected to a baking process at 450°C or higher (e.g., 450°C to 1200°C). If an inkjet ink used for paper, cloth, etc. is used in this process, there is a risk that the pigment will discolor (or fade) during the baking process. For this reason, inkjet inks used on inorganic substrates that require baking (inkjet inks for inorganic substrates) are required to have a composition that takes baking into consideration. Examples of such inkjet inks for inorganic substrates include the inks described in Patent Documents 1 to 3. Furthermore, unlike paper, cloth, etc., inorganic substrates do not absorb ink. For this reason, photocurable inks containing a photocurable component (e.g., a photocurable monomer) are usually used for inkjet inks for inorganic substrates.

[0004] Among the inorganic substrates mentioned above, the required ink performance (such as fixability) differs between glass substrates, ceramic substrates, and metal substrates. For this reason, in recent years, in the field of inkjet inks for inorganic substrates, studies have been conducted to further finely modify the ink composition depending on the printing target. For example, Patent Document 4 discloses an inkjet ink for printing on glass substrates (inkjet ink for glass substrates). It is presumed that the ink described in Patent Document 4 has excellent adhesion, particularly to glass substrates, because the crosslinker and silicone resin contain siloxane bonds. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2007 / 20779 [Patent Document 2] Japanese Patent Application Publication No. 2017-75251 [Patent Document 3] Japanese Patent Application Publication No. 2009-154419 [Patent Document 4] Japanese Patent Application Publication No. 2016-069390 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the above-mentioned prior art, although adhesion to glass substrates has been considered, the aesthetics of images formed using the inks have not been sufficiently considered. Specifically, with a transparent glass substrate, if the image drawn on its surface has high transparency (low hiding power), the opposite side of the image can be seen through, which can significantly impair the aesthetics of some types of images.

[0007] The present invention has been made in view of the above circumstances, and a main object of the present invention is to provide an ink-jet ink for glass substrates that can form a beautiful image with high hiding power on the surface of a glass substrate by ink-jet printing. In another aspect, the present invention provides a method for manufacturing a glass product using the ink-jet ink for glass substrates. [Means for solving the problem]

[0008] In consideration of the above-mentioned problems, it is necessary to increase the content of inorganic pigment in the ink in order to form an image with high hiding power. However, inks with increased inorganic pigment content have not been easily used because they cause various new problems.

[0009] First, inks containing increased amounts of inorganic pigments have significantly increased viscosity, making inkjet printing difficult and making it impossible to print precise images. In response to this, the inventors believed that since the viscosity of inkjet inks is affected by the total amount of inorganic solids, including inorganic pigments and glass frit, increasing the amount of inorganic pigments and reducing the amount of glass frit to keep the total amount of inorganic solids below a certain level could maintain low ink viscosity. On the other hand, because glass frit is a component that fixes inorganic pigments to the substrate surface, reducing its content too much could result in the image not being fixed to the glass substrate surface after firing. Taking these points into consideration, the inventors devised a method for adjusting the "ratio of inorganic solids content to the total amount of ink" and the "ratio of inorganic pigment content to the total amount of inorganic solids" to achieve a high level of balance between the ejection performance during printing, the hiding power of the image after firing, and the fixation performance to the glass substrate. It is the "volume" of the inorganic solids, not the "weight," that directly affects ink viscosity. For this reason, the technology disclosed herein specifies the respective contents of inorganic solids and glass frit as volume ratios.

[0010] Second, inks containing increased amounts of inorganic pigments are difficult to cure after printing on the surface of a glass substrate, causing bleeding and making it impossible to form clear images. The inventors of the present invention believe that the reason for this decrease in clarity due to ink bleeding is that when the amount of inorganic pigment is increased, light transmittance is reduced (hiding ability is improved), and a sufficient amount of light is not supplied to the photocurable components inside the ink, preventing the ink from curing immediately after printing. Based on this knowledge, the inventors of the present invention investigated the optimum range of the volume ratio of inorganic pigment to photopolymerization initiator so that the ink can be photocured with a small amount of light. They found that by adjusting the volume ratio to a predetermined range, bleeding due to poor curing of the ink is eliminated and clear images are formed.

[0011] The inkjet ink disclosed herein has been made based on the above findings. This inkjet ink is an inkjet ink for glass substrates used to print images on transparent glass substrates. This inkjet ink contains inorganic solids including a black inorganic pigment and glass frit, a photocurable monomer component, and a photopolymerization initiator. In the inkjet ink disclosed herein, the volume ratio of the inorganic solids is 35% or less by volume when the total volume of the inkjet ink for glass substrates is taken as 100% by volume, the volume ratio of the inorganic pigment is 15% or more but less than 90% by volume when the total volume of the inorganic solids is taken as 100% by volume, and the volume ratio of the inorganic pigment to the photopolymerization initiator is 6 times or less.

[0012] As described above, in the inkjet ink disclosed herein, when a black inorganic pigment is used, the "volume ratio of inorganic solids to the total ink amount," the "volume ratio of inorganic pigment to the total inorganic solids amount," and the "volume ratio of inorganic pigment to photopolymerization initiator" are adjusted within predetermined ranges. This allows for high levels of ejection performance during printing, photocuring performance after printing, image hiding performance after baking, and fixation to glass substrates. Note that black inorganic pigments have lower light transmittance than inorganic pigments of other colors, and therefore the required content of photopolymerization initiator for black inks differs from that for inks of other colors. For this reason, the technology disclosed herein is limited to inks using black inorganic pigments.

[0013] In a preferred embodiment of the inkjet ink disclosed herein, the inorganic pigment is spinel black, which has excellent color development and is therefore particularly suitable as a black inorganic pigment.

[0014] In a preferred embodiment of the inkjet ink disclosed herein, the volume ratio of the inorganic solid content is 15% by volume or more and 30% by volume or less, when the total volume of the inkjet ink for glass substrates is taken as 100% by volume, thereby achieving higher levels of ejection properties during printing, hiding power of the image after baking, and fixation to the glass substrate.

[0015] In a preferred embodiment of the inkjet ink disclosed herein, the volume ratio of the inorganic pigment is 25% by volume or more and 85% by volume or less when the total volume of the inorganic solids is taken as 100% by volume, thereby enabling the ink to exhibit higher levels of hiding power and fixation to glass substrates after baking.

[0016] In a preferred embodiment of the inkjet ink disclosed herein, the volume ratio of the inorganic pigment to the photopolymerization initiator is 5 or less, which allows for higher levels of hiding power after baking and photocurability after printing.

[0017] In a preferred embodiment of the inkjet ink disclosed herein, the monomer component contains at least a monofunctional acrylate monomer containing one acryloyl or methacryloyl group in the molecule, a monofunctional N-vinyl compound monomer in which one vinyl group is bonded to the nitrogen (N) atom of a nitrogen-containing compound, and a multifunctional vinyl ether monomer containing at least two vinyl ether groups in the molecule. By using a photocurable monomer component containing these three types of monomers, it is possible to print an image that is well fixed to the surface of a printing target and has excellent flexibility after fixing.

[0018] In an embodiment containing the above three types of monomers, the volume ratio of the monomer components is preferably 50% by volume or more and 70% by volume or less when the total volume of the inkjet ink for glass substrates is taken as 100% by volume. This allows for a high level of both fixation to the surface of the printing object and flexibility after fixation, and also allows for the formation of an image with excellent gloss and color development after baking.

[0019] Another aspect of the present invention provides a method for manufacturing a glass product having a decorative portion, which includes the steps of inkjet printing the inkjet ink disclosed herein onto the surface of a glass substrate, irradiating the surface of the glass substrate with ultraviolet light to cure the inkjet ink for glass substrates adhered to the surface of the glass substrate, and firing the glass substrate under conditions in which the maximum firing temperature is set within the range of 450°C to 1200°C.

[0020] The method for manufacturing glass products disclosed herein uses the inkjet ink described above. This allows inkjet printing with excellent ejection properties, making it possible to print precise images on the surface of a glass substrate. Furthermore, because such inks have excellent photocuring properties, they can prevent adhesion of the coating film and bleeding of the image after printing, allowing for the formation of clear images. Furthermore, the image (decorative portion) after firing has high levels of both hiding power and fixation, allowing for the maintenance of excellent aesthetic appearance for a long period of time. In other words, the manufacturing method disclosed herein makes it possible to easily manufacture glass products bearing beautiful images.

[0021] The present invention also provides a method for manufacturing transfer paper for glass substrates (hereinafter simply referred to as "transfer paper") for use with glass substrates. The method for manufacturing such transfer paper includes the steps of inkjet printing the inkjet ink described above onto the surface of a mount, and irradiating the surface of the mount with ultraviolet light to cure the inkjet ink for glass substrates adhered to the surface of the mount. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a cross-sectional view that schematically shows an agitator / pulverizer used in the production of inkjet ink. [Figure 2] FIG. 2 is a schematic overall view showing an example of an inkjet device. [Figure 3] FIG. 3 is a cross-sectional view schematically showing an inkjet head of the inkjet device in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0023] Preferred embodiments of the present invention will be described below. It should be noted that matters necessary for carrying out the present invention other than those specifically mentioned in this specification can be understood as design matters for a person skilled in the art based on the prior art in the relevant field. The present invention can be carried out based on the contents disclosed in this specification and the common general technical knowledge in the relevant field.

[0024] 1. Inkjet ink The inkjet ink disclosed herein is an inkjet ink for glass substrates, which is used to print images on transparent glass substrates. Such inkjet ink contains at least an inorganic solid component, a photocurable monomer component (photocurable monomer component), and a photopolymerization initiator. Each component will be described below.

[0025] (1) Inorganic solids The inorganic solid content is a component that constitutes the base material of the image (decorative portion) after firing, and includes inorganic pigments and glass frit.

[0026] (a) Inorganic pigments The inorganic pigment is added to develop a desired color on the surface of the substrate after firing. The inorganic pigment may contain, for example, a metal compound. Such inorganic pigments have excellent heat resistance. Therefore, when a glass substrate with ink attached thereto is subjected to a firing treatment at 450°C or higher (for example, 450°C to 1200°C), the pigment can be prevented from discoloring (or fading). Specific examples of such inorganic pigments include composite metal compounds containing at least one metal element selected from the group consisting of Cu, Mn, Zr, Ti, Pr, Cr, Sb, Ni, Co, Al, and Cd.

[0027] The technology disclosed herein is directed to inks using black inorganic pigments. As will be described in detail later, black inorganic pigments have lower light transmittance than inorganic pigments of other colors, and therefore the content of photopolymerization initiator required for black inks differs from that for inks of other colors. As such black inorganic pigments, FeCr-based composite metal compounds (e.g., spinel black) are preferably used.

[0028] The inorganic pigment may typically be in the form of particles. The particle size of such particulate inorganic pigments is preferably adjusted appropriately taking into consideration the diameter of the discharge port of the inkjet device, which will be described later. If the particle size of the inorganic pigment is too large, the inorganic pigment may clog the discharge port, resulting in a decrease in ink discharge performance. Since the diameter of the discharge port of a typical inkjet device is about 15 μm to 60 μm (for example, 25 μm), the diameter of the discharge port of the inkjet device is about 15 μm to 60 μm (for example, 25 μm), and the diameter of the discharge port of the inkjet device is about 15 μm to 60 μm (for example, 25 μm), which corresponds to a cumulative 100% by number of particles from the smallest particle size. 100 It is preferable to microparticulate the inorganic pigment so that the particle size (maximum particle size) is 5 μm or less (preferably 1 μm or less). 100 The particle size may be a value measured based on particle size distribution measurement by dynamic light scattering.

[0029] (b) Glass frit The glass frit melts when the glass substrate to which the ink is attached is baked, and then solidifies as the substrate cools, thereby fixing the inorganic pigment to the substrate surface. The glass frit of the inkjet ink disclosed herein preferably contains a glass material that coats the inorganic pigment after cooling and imparts a beautiful luster.

[0030] Examples of glass materials that can have such properties include SiO2-B2O3-based glass, SiO2-RO (RO represents an oxide of a Group 2 element, such as MgO, CaO, SrO, or BaO; the same applies hereinafter), SiO2-RO-R2O (RO represents an oxide of an alkali metal element, such as Li2O, Na2O, KO, Rb2O, Cs2O, or Fr2O, particularly Li2O; the same applies hereinafter), SiO2-B2O3-R2O-based glass, SiO2-RO-ZnO-based glass, SiO2-RO-ZrO2-based glass, SiO2-RO-Al2O3-based glass, SiO2-RO-Bi2O3-based glass, SiO2-R2O-based glass, SiO2-ZnO-based glass, SiO2-ZrO2-based glass, SiO2-Al2O3-based glass, RO-R2O-based glass, and RO-ZnO-based glass. These glass materials may contain one or more components in addition to the main components indicated in the names above. Furthermore, the glass frit may contain not only general amorphous glass but also crystallized glass containing crystals.

[0031] In a preferred embodiment, when the entire glass material is taken as 100 mol %, SiO2 accounts for more than half (50 mol %). The proportion of SiO2 can be approximately 80 mol % or less. Furthermore, from the viewpoint of improving the melting property of the glass frit, components such as RO, R2O, and B2O3 may be added. In a preferred embodiment, when the entire glass material is taken as 100 mol %, RO accounts for 0 to 35 mol %. In another preferred embodiment, when the entire glass material is taken as 100 mol %, R2O accounts for 0 to 10 mol %. In another preferred embodiment, when the entire glass material is taken as 100 mol %, B2O3 accounts for 0 to 30 mol %.

[0032] In a preferred embodiment, the glass material is composed of a multi-component system of four or more components (e.g., five or more components). This improves the physical stability of the image after firing. For example, components such as Al2O3, ZnO, CaO, and ZrO2 may be added in a proportion of, for example, 1 mol % or more. This improves the chemical durability and abrasion resistance of the decorative portion. In a preferred embodiment, when the entire glass material is taken as 100 mol %, Al2O3 accounts for 0 to 10 mol %. In a preferred embodiment, when the entire glass material is taken as 100 mol %, ZrO2 accounts for 0 to 10 mol %.

[0033] A preferred example of the glass frit disclosed herein is a glass frit having the following composition in terms of oxide molar ratios when the entire glass material is taken as 100 mol %: SiO2 40-70 mol% (e.g., 50-60 mol%); B2O3 10-40 mol% (e.g., 20-30 mol%); R2O (at least one of Li2O, Na2O, K2O, Rb2O) 3-20 mol% (e.g., 5-10 mol%); Al2O3 0-20 mol% (e.g., 5-10 mol%); ZrO2 0-10 mol% (e.g., 3-6 mol%); Examples of suitable glass frits include borosilicate glass containing borosilicate glass. The proportion of SiO in the entire glass matrix of such borosilicate glass may be, for example, 40 mol% or more, and typically 70 mol% or less, for example, 65 mol% or less. The proportion of BO in the entire glass matrix may be typically 10 mol% or more, for example, 15 mol% or more, and typically 40 mol% or less, for example, 35 mol% or less. The proportion of RO in the entire glass matrix may be typically 3 mol% or more, for example, 6 mol% or more, and typically 20 mol% or less, for example, 15 mol% or less. In a preferred embodiment, the borosilicate glass contains LiO, NaO, and KO as RO. The proportion of LiO in the entire glass matrix may be, for example, 3 mol% or more and 6 mol% or less. The proportion of KO in the entire glass matrix may be, for example, 0.5 mol% or more and 3 mol% or less. The proportion of NaO in the entire glass matrix may be, for example, 0.5 mol% or more and 3 mol% or less. The proportion of Al2O3 in the entire glass matrix is ​​typically 3 mol% or more and typically 20 mol% or less, for example, 15 mol% or less. The proportion of ZrO2 in the entire glass matrix is ​​typically 1 mol% or more and typically 10 mol% or less, for example, 8 mol% or less. The borosilicate glass may also contain additional components other than those described above. Examples of such additional components include, in the form of oxides, BeO, MgO, CaO, SrO, BaO, ZnO, Ag2O, TiO2, VO5, FeO, Fe2O3, Fe3O4, CuO, Cu2O, Nb2O5, PO5, La2O3, CeO2, Bi2O3, and Pb2O3. The additional components may be contained in a total amount of up to 10 mol% when the entire glass matrix is ​​taken as 100 mol%.

[0034] Another example of the glass frit disclosed herein is a glass frit having the following composition in terms of oxide molar ratio, when the entire glass is taken as 100 mol %: SiO2 45-70 mol% (e.g., 50-60 mol%); SnO2 0.1-6 mol% (e.g., 1-5 mol%); ZnO 1-15 mol% (e.g., 4-10 mol%); RO (at least one of BeO, MgO, CaO, SrO, BaO) 15-35 mol% (e.g., 20-30 mol%); R2O (at least one of Li2O, Na2O, K2O, Rb2O) 0-5 mol% (e.g., 1-5 mol%); B2O3 0-3 mol% (e.g., 0-1 mol%); Examples of glass frits include glass materials composed of: The proportion of SiO2 in the entire glass matrix of a glass material having such a composition may be, for example, 50 mol% or more and typically 65 mol% or less, for example 60 mol% or less. The proportion of SnO2 in the entire glass matrix may be typically 0.5 mol% or more, for example 1 mol% or more, and typically 5.5 mol% or less, for example 5 mol% or less. The proportion of ZnO in the entire glass matrix may be typically 2 mol% or more, for example 4 mol% or more, and typically 12 mol% or less, for example 10 mol% or less. The proportion of RO in the entire glass matrix may be typically 18 mol% or more, for example 20 mol% or more, and typically 32 mol% or less, for example 30 mol% or less. The proportion of R2O in the entire glass matrix may be approximately 0.1 mol% or more, for example 1 mol% or more, and typically 3 mol% or less. The proportion of B2O3 in the entire glass matrix may be typically 1 mol% or less, for example 0.1 mol% or less. The glass frit may also contain additional components other than those mentioned above. Examples of such additional components include, in the form of oxides, Ag2O, Al2O3, ZrO2, TiO2, VO5, FeO, Fe2O3, Fe3O4, CuO, Cu2O, Nb2O5, PO5, La2O3, CeO2, and Bi2O3. The additional components may be contained in a total amount of 10 mol% or less, assuming that the entire glass matrix is ​​100 mol%.

[0035] The coefficient of linear thermal expansion of the glass frit (average coefficient of linear expansion measured in the temperature range from 25°C to 500°C using a thermomechanical analyzer; the same applies hereinafter) is, for example, 10.0 x 10 -6 K -1 It is preferable that the temperature is equal to or lower than this. This reduces the difference in shrinkage rate between the glass frit and the object to be decorated (glass substrate) during firing, making it less likely that peeling or cracks will occur in the decorative portion. The deformation point of the glass frit is not particularly limited, but may be, for example, 400°C to 700°C. The glass transition point (Tg value based on differential scanning calorimetry; the same applies hereinafter) of the glass frit is not particularly limited, but may be, for example, 400°C to 700°C.

[0036] Furthermore, the glass frit typically contains a particulate glass material. The particle size of the glass frit affects the ink viscosity, so it is preferable to adjust it appropriately in consideration of the ejection properties from the inkjet device. Specifically, if the ink contains glass frit with a large particle size, clogging of the ejection port is likely to occur, and the ejection properties may be reduced. For this reason, the maximum particle size of the glass frit (D corresponding to 100% by number of particles from the smallest particle size side) is set to 1. 100 It is preferable to control the particle size of the glass frit so that the particle size is 1 μm or less (preferably 0.85 μm or less).

[0037] (2) Photocurable monomer component The inkjet ink disclosed herein is a photocurable inkjet ink containing a photocurable monomer component. In this specification, the term "photocurable monomer component" refers to a material that is typically liquid and contains at least one resin monomer that polymerizes (or crosslinks) and hardens when irradiated with light (e.g., ultraviolet light). As the photocurable monomer component, any monomer that can be used in a general photocurable ink can be used without particular limitation, as long as the effects of the present invention are not significantly impaired.

[0038] A suitable example of the photocurable monomer component is a photocurable monomer component containing (a) a monofunctional acrylate monomer, (b) a monofunctional N-vinyl compound monomer, and (c) a polyfunctional vinyl ether monomer. The photocurable monomer component containing the monomers (a) to (c) has excellent fixability (photocurability) to the printing target, and therefore can be suitably used for various printing targets. Furthermore, the photocurable monomer component containing the monomers (a) to (c) also has the advantage of excellent flexibility after photocuring, and therefore can be suitably used for printing targets that need to be curved during use (for example, transfer paper for inorganic substrates).

[0039] (a) Monofunctional acrylate monomer A monofunctional acrylate monomer is a compound that contains one acryloyl group (CH2=CHCOO-) or one methacryloyl group (CH2=CCH3COO-) in the molecule. Such monofunctional acrylate monomers have excellent dispersibility of inorganic solid components and can suppress an increase in ink viscosity, which contributes to the preparation of inks with favorable ejection properties. Furthermore, among photocurable monomers, monofunctional acrylate monomers also have the property of having relatively low rigidity (high flexibility) after photocuring. From the viewpoint of further improving the ejection property and flexibility, when the total volume of the photocurable monomer components is taken as 100% by volume, the volume ratio of the monofunctional acrylate monomer is preferably 40% by volume or more, more preferably 45% by volume or more, even more preferably 50% by volume or more, particularly preferably 55% by volume or more, for example 60% by volume or more. On the other hand, since the monofunctional acrylate monomer tends to have relatively low photocurability, from the viewpoint of ensuring the content of the monomer having excellent photocurability described later, it is preferably 96% by volume or less, more preferably 90% by volume or less, even more preferably 85% by volume or less, particularly preferably 80% by volume or less, for example 78% by volume or less.

[0040] Specific examples of monofunctional acrylate monomers include benzyl acrylate, cyclic trimethylolpropane formal acrylate, phenoxyethyl acrylate, isobornyl acrylate, tetrahydrofurfuryl acrylate, methoxyethyl acrylate, cyclohexyl acrylate, ethyl carbitol acrylate, (2-methyl-2-ethyl-1,3-dioxolan-4-yl)methyl acrylate, hydroxyethyl acrylate, hydroxypropyl acrylate, 4-hydroxybutyl acrylate, methyl (meth)acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, pentyl acrylate, n-stearyl acrylate, butoxyethyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, isobornyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate. acrylate, 2-hydroxybutyl (meth)acrylate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, t-butylcyclohexyl (meth)acrylate, isoamyl acrylate, lauryl (meth)acrylate, octyl acrylate, isooctyl (meth)acrylate, isononyl acrylate, decyl acrylate, isodecyl acrylate, tridecyl (meth)acrylate, isomyristyl acrylate, isostearyl acrylate, 2-ethylhexyl acrylate, 2-ethylhexyl-diglycol acrylate, 4-hydroxybutyl acrylate, methoxydiethylene glycol acrylate, methoxytriethylene glycol acrylate, ethoxydiethylene glycol acrylate, 2-(2-ethoxyethoxy)ethyl acrylate, 2-ethylhexyl carbitol acrylate, phenoxyethoxyethyl acrylate, etc. The above-mentioned (meth)acrylate compounds can be used alone or in combination of two or more. Among these, benzyl acrylate, phenoxyethyl acrylate, and cyclic trimethylolpropane formal acrylate are particularly excellent in flexibility after photocuring, and therefore can suitably prevent cracks from occurring when the transfer paper is bent.

[0041] (b) Monofunctional N-vinyl compound monomer A monofunctional N-vinyl compound monomer is a compound in which one vinyl group is bonded to the nitrogen (N) atom of a nitrogen-containing compound. The "vinyl group" here refers to the group CH2=CR 1 -(where R 1 is a hydrogen atom or an organic group). Such monofunctional N-vinyl compound monomers have high stretchability, which can prevent cracks from occurring in the printed image. In addition, monofunctional N-vinyl compound monomers have excellent photocuring properties, which improve the fixation of the printed image to the surface. From the viewpoint of further improving fixability, the volume ratio of the monofunctional N-vinyl compound monomer, when the total volume of the photocurable monomer components is taken as 100% by volume, is preferably 2% by volume or more, more preferably 3% by volume or more, even more preferably 4% by volume or more, and particularly preferably 5% by volume or more. On the other hand, the addition of the monofunctional N-vinyl compound monomer tends to reduce the flexibility of the ink after curing. For this reason, when printing on transfer paper for inorganic substrates, etc., it is preferable to reduce the content of the monofunctional N-vinyl compound monomer. From this viewpoint, the volume ratio of the monofunctional N-vinyl compound monomer is preferably 20% by volume or less, more preferably 17% by volume or less, even more preferably 15% by volume or less, particularly preferably 13% by volume or less, for example, 10% by volume or less.

[0042] The N-vinyl compound monomer is represented by, for example, the following general formula (1). CH2=CR 1 -NR 2 R 3 (1) In the above general formula (1), R 1 R is a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, a phenyl group, a benzyl group, or a halogen group. Among these, a hydrogen atom or an alkyl group having 1 to 4 carbon atoms is preferred, and a hydrogen atom is particularly preferred. 2 ,R 3can be a group selected from a hydrogen atom, an alkyl group which may have a substituent, an alkenyl group, an alkynyl group, an aralkyl group, an alkoxy group, an alkoxyalkyl group, an alkylol group, an acetyl group (CHCO-), and an aromatic group. 2 ,R 3 may be the same or different. The total number of carbon atoms in the alkyl group, alkenyl group, alkynyl group, aralkyl group, alkoxy group, alkoxyalkyl group, alkylol group, and acetyl group, which may have a substituent, may be 1 to 20. The alkyl group, alkenyl group, alkynyl group, aralkyl group, alkoxy group, alkoxyalkyl group, alkylol group, and acetyl group, which may have a substituent, may be linear or cyclic, but linear is preferred. The aromatic group is an aryl group, which may have a substituent. The total number of carbon atoms in the aromatic group is 6 to 36. The substituents that the alkyl group, alkenyl group, alkynyl group, aralkyl group, alkoxy group, alkoxyalkyl group, alkylol group, acetyl group, and aromatic group may have include, for example, a hydroxyl group and a halogen atom such as a fluorine atom or a chlorine atom. In the general formula (1), R 2 and R 3 may be bonded to each other to form a cyclic structure.

[0043] Suitable examples of the monofunctional N-vinyl compound monomer include N-vinyl-2-caprolactam, N-vinyl-2-pyrrolidone, N-vinyl-3-morpholinone, N-vinylpiperidine, N-vinylpyrrolidine, N-vinylaziridine, N-vinylazetidine, N-vinylimidazole, N-vinylmorpholine, N-vinylpyrazole, N-vinylvalerolactam, N-vinylcarbazole, N-vinylphthalimide, N-vinylformamide, N-vinylacetamide, N-methyl-N-vinylformamide, N-methyl-N-vinylacetamide, etc. Among these, N-vinyl-2-caprolactam has high photocurability among monofunctional N-vinyl compound monomers and can more suitably improve fixation to the surface of the printing target.

[0044] (c) Polyfunctional vinyl ether monomer A polyfunctional vinyl ether monomer is a compound containing at least two vinyl ether groups in the molecule. The "vinyl ether group" here refers to a -O-CH=CHR 1 (where R 1 represents a hydrogen atom or an organic group). Such multifunctional vinyl ether monomers containing at least two vinyl ether groups have a fast photocuring rate when irradiated with UV light and excellent photocuring properties, which improve the fixation to the surface of the printed object. Furthermore, among monomers with excellent photocuring properties, multifunctional vinyl ether monomers have the property of low rigidity after curing and excellent flexibility. From the viewpoint of achieving both fixability to the printing target and flexibility after photocuring, the volume ratio of the polyfunctional vinyl ether monomer, when the total volume of the monomer components is taken as 100% by volume, is preferably 2% by volume or more, more preferably 5% by volume or more, even more preferably 7% by volume or more, and particularly preferably 10% by volume or more, for example, 15% by volume or more. On the other hand, if too much polyfunctional vinyl ether monomer is added, the amount of monofunctional acrylate monomer added tends to be reduced, resulting in reduced flexibility after photocuring. For this reason, the upper limit of the volume ratio of the polyfunctional vinyl ether monomer is preferably 40% by volume or less, more preferably 35% by volume or less, even more preferably 30% by volume or less, and particularly preferably 25% by volume or less, for example, 20% by volume or less.

[0045] Suitable examples of the polyfunctional vinyl ether monomer include ethylene glycol divinyl ether, diethylene glycol divinyl ether, triethylene glycol divinyl ether, tetraethylene glycol divinyl ether, polyethylene glycol divinyl ether, propylene glycol divinyl ether, dipropylene glycol divinyl ether, tripropylene glycol divinyl ether, polypropylene glycol divinyl ether, butanediol divinyl ether, neopentyl glycol divinyl ether, hexanediol divinyl ether, nonanediol divinyl ether, 1,4-cyclohexanedimethanol divinyl ether, etc. Among these, diethylene glycol divinyl ether, triethylene glycol divinyl ether, and 1,4-cyclohexanedimethanol divinyl ether are particularly preferred because they can achieve both high levels of fixation to the substrate surface and flexibility after photocuring.

[0046] When using a photocurable monomer component containing the monomers (a) to (c) described above, the volume ratio of the monomer component, when the total volume of the inkjet ink is taken as 100 volume %, is preferably 50 volume % or more, more preferably 52 volume % or more, even more preferably 58 volume % or more, and particularly preferably 60 volume % or more. This allows for a high level of both fixation to the surface of the printing object and flexibility after fixation. Furthermore, from the viewpoint of ensuring a sufficient content of inorganic solids and forming an image (decorative portion) with excellent gloss and color development, the volume ratio of the monomer component is preferably 85 volume % or less, more preferably 80 volume % or less, even more preferably 75 volume % or less, and particularly preferably 70 volume % or less.

[0047] (d) Other Monomers As described above, the photocurable monomer component in the inkjet ink disclosed herein can be any monomer component that can be used in a general photocurable inkjet ink, and is not limited to the monomers (a) to (c) described above. An example of a monomer (other monomer) other than the above (a) to (c) is a polyfunctional acrylate monomer containing at least two acryloyl or methacryloyl groups in the molecule. Suitable examples of this polyfunctional acrylate monomer include 1,9-nonanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, tricyclodecane dimethanol diacrylate, hydroxypivalic acid neopentyl glycol diacrylate, triethylene glycol di(meth)acrylate, tetramethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, and 1,3-butanediol di(meth)acrylate. , neopentyl glycol di(meth)acrylate, hexanediol di(meth)acrylate, cyclohexane-1,4-dimethanol di(meth)acrylate, cyclohexane-1,3-dimethanol di(meth)acrylate, 1,4-cyclohexanediol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, dipentaerythritol di(meth)acrylate, neopentyl glycol di(meth)acrylate, polytetramethylene glycol di(meth)acrylate, bisphenol AEO3.8-mol adduct diacrylate, trimethylolpropane tri(meth)acrylate, trimethylolethane tri(meth)acrylate, trimethyloloctane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, trimethylolpropane polyethoxy tri(meth)acrylate, dipentaerythritol tri(meth)acrylate, dipentaerythritol propionate tri(meth)acrylate, tris(2-hydroxyethyl)isocyanurate tri(meth)acrylate, sorbitol tri(meth)acrylate, ditrimethylol Examples include propane tetra(meth)acrylate, pentaerythritol polyethoxytetra(meth)acrylate, pentaerythritol polyproxytetra(meth)acrylate, sorbitol tetra(meth)acrylate, dipentaerythritol propionate tetra(meth)acrylate, ethoxylated pentaerythritol tetra(meth)acrylate, sorbitol penta(meth)acrylate, dipentaerythritol penta(meth)acrylate, dipentaerythritol hexa(meth)acrylate, and sorbitol hexa(meth)acrylate. Furthermore, examples of other monomers other than the polyfunctional acrylate monomers include butyl vinyl ether, butyl propenyl ether, butyl butenyl ether, hexyl vinyl ether, ethylhexyl vinyl ether, phenyl vinyl ether, benzyl vinyl ether, phenyl allyl ether, vinyl acetate, acrylamide, methacrylamide, trimethylolpropane tri((meth)acryloyloxypropyl)ether, tri((meth)acryloyloxyethyl)isocyanurate, and bisphenol A diglycidyl ether acrylic acid adduct.

[0048] (3) Photopolymerization initiator Next, the inkjet ink disclosed herein contains a photopolymerization initiator. The photopolymerization initiator absorbs light and becomes activated, generating reaction initiators such as radical molecules and hydrogen ions. These reaction initiators act on the photocurable monomer, accelerating the polymerization reaction and crosslinking reaction of the photocurable monomer. In other words, by increasing the content of the photopolymerization initiator, it is possible to prepare an ink that can be easily cured with even a small amount of light. Note that, as the photopolymerization initiator, any conventionally used photopolymerization initiator can be used without any particular restrictions. Examples include radical photopolymerization initiators such as alkylphenone-based photopolymerization initiators and acylphosphine oxide-based photopolymerization initiators. As such alkylphenone-based photopolymerization initiators, for example, α-aminoalkylphenone-based photopolymerization initiators (e.g., 2-methyl-1-(4-methylthiophenyl)-2-morpholinopropan-1-one, 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1, 2-(dimethylamino)-2-[(4-methylphenyl)methyl]-1-[4-(4-morpholinyl)phenyl]-1-butanone, etc.) are preferably used. Other examples of alkylphenone-based photopolymerization initiators that can be used include α-hydroxyalkylphenone-based photopolymerization initiators (1-hydroxycyclohexylphenylketone, 2-hydroxy-2-methyl-1-phenylpropan-1-one, 1-[4-(2-hydroxyethoxy)phenyl]-2-hydroxy-2-methyl-1-propan-1-one, 2-hydroxy-1-{4-[4-(2-hydroxy-2-methylpropionyl)benzyl]phenyl}-2-methylpropan-1-one, etc.). Among the various photopolymerization initiators described above, α-aminoalkylphenone-based photopolymerization initiators such as 2-methyl-1-(4-methylthiophenyl)-2-morpholinopropan-1-one are particularly preferred because they exhibit high reactivity, can improve the ink curing rate, and have excellent thin film curing properties and surface curing properties.

[0049] (4) Other ingredients The inkjet ink disclosed herein may further contain, as necessary, known additives (e.g., dispersants, polymerization inhibitors, binders, viscosity modifiers, etc.) that can be used in inkjet inks (typically, inkjet inks for glass substrates and photocurable inkjet inks), provided that the effects of the present invention are not impaired. The content of the above additives may be appropriately set depending on the purpose of their addition, and detailed description thereof will be omitted as they do not characterize the present invention.

[0050] (a) Dispersant The inkjet ink disclosed herein may contain a dispersant. Examples of dispersants include cationic dispersants. Such cationic dispersants efficiently adhere to the surface of inorganic pigments through an acid-base reaction. Therefore, unlike other dispersants such as phosphate-based dispersants, they can suppress aggregation of the inorganic pigments and effectively disperse them. An example of such a cationic dispersant is an amine-based dispersant. Such an amine-based dispersant can prevent aggregation of inorganic pigments due to steric hindrance and stabilize the inorganic pigment. Furthermore, by imparting the same charge to inorganic pigment particles, aggregation of the inorganic pigment can also be effectively prevented. This effectively reduces the viscosity of the ink, significantly improving printability. Examples of such amine-based dispersants include fatty acid amine-based dispersants and polyester amine-based dispersants. For example, DISPERBYK-2013 manufactured by BYK Japan K.K. is a preferred example.

[0051] (b) Polymerization inhibitor The inkjet ink disclosed herein may contain a polymerization inhibitor. Addition of such a polymerization inhibitor can inhibit polymerization and curing of the photocurable monomer component before use, thereby facilitating storage of the ink. Any polymerization inhibitor conventionally used in the field of photocurable inkjet inks can be used without particular limitation, as long as it does not significantly reduce the photocurability of the photocurable monomer component containing the monomers (a) to (c) above and does not reduce the effects of the technology disclosed herein. Examples of such polymerization inhibitors include hydroquinone, methoquinone, di-t-butylhydroquinone, p-methoxyphenol, butylhydroxytoluene, and nitrosamine salts. Among these compounds, N-nitrophenylhydroxylamine aluminum salt is particularly preferred due to its excellent long-term storage stability.

[0052] (5) Content of each ingredient The inkjet ink disclosed herein is characterized in that (a) the volume ratio of the inorganic solid content to the total amount of ink, (b) the volume ratio of the inorganic pigment to the total amount of the inorganic solid content, and (c) the volume ratio of the inorganic pigment to the photopolymerization initiator are controlled within predetermined ranges. Each element will be explained below.

[0053] (a) Volume ratio of inorganic solids to the total amount of ink First, in the inkjet ink disclosed herein, the volume ratio of the inorganic solids is 35% by volume or less when the total volume of the inkjet ink is taken as 100% by volume. This "volume of inorganic solids" refers to the combined volume of the inorganic pigment and glass frit described above. As the volume of the inorganic solids increases, the ink viscosity tends to increase. Because there are many types of inorganic pigments and glass frits contained in the inorganic solids and their specific gravities vary, in this embodiment, the "volume" of the inorganic solids is adjusted rather than the "weight" of the inorganic solids. As will be described in detail later, the inkjet ink disclosed herein increases the amount of inorganic pigment to form an image with high hiding power. Even with inks containing such a large amount of inorganic pigment, a low ink viscosity (typically less than 110 Pa·s, preferably 70 Pa·s or less) suitable for inkjet printing can be achieved by setting the volume ratio of the inorganic solids to the total ink volume to 35% by volume or less. From the perspective of more suitably reducing the ink viscosity, the volume ratio of the inorganic solids is preferably 32% by volume or less, more preferably 30% by volume or less, even more preferably 28% by volume or less, and particularly preferably 25% by volume or less. On the other hand, from the viewpoint of ensuring sufficient hiding power and fixation power of the image after baking, the lower limit of the volume ratio of the inorganic solid content is preferably 10% by volume or more, more preferably 12% by volume or more, even more preferably 15% by volume or more, and particularly preferably 16% by volume or more.

[0054] (b) Volume ratio of inorganic pigment to total inorganic solids Next, in the inkjet ink disclosed herein, the volume of the inorganic pigment is adjusted to 15% by volume or more when the total volume of the inorganic solids is taken as 100% by volume. By incorporating such a large amount of inorganic pigment, an image with excellent hiding power can be formed. From the viewpoint of forming an image with even better hiding power, the volume ratio of the inorganic pigment is preferably 17.5% by volume or more, more preferably 20% by volume or more, and particularly preferably 25% by volume or more. On the other hand, if the volume ratio of the inorganic pigment relative to the total amount of inorganic solids is increased too much, the glass frit content may decrease, resulting in a decrease in the fixation of the image after firing. From this viewpoint, the upper limit of the volume ratio of the inorganic pigment is set to less than 90% by volume. From the viewpoint of more reliably ensuring the fixation of the image after firing, the upper limit of the volume ratio of the inorganic pigment is preferably 85% by volume or less, more preferably 80% by volume or less, and particularly preferably 70% by volume or less.

[0055] (c) Volume ratio of inorganic pigment to photoinitiator In the technology disclosed herein, the volume ratio of inorganic pigment to photopolymerization initiator is specified so that appropriate photocuring action can be achieved even in inks containing increased amounts of inorganic pigment. Experiments conducted by the present inventors have confirmed that even in inks with the same volume ratio of inorganic solids to photopolymerization initiator, the ink with a larger volume of inorganic pigment exhibits lower photocurability. While not intending to limit the technology disclosed herein, this phenomenon is presumed to occur because an insufficient amount of light is supplied to the photocurable monomer in inks with a high volume ratio of inorganic pigment and high hiding power. In contrast, in the inkjet ink disclosed herein, the amount of photopolymerization initiator added is determined taking into account the volume ratio of inorganic pigment, which is a factor in reducing photocurability. Specifically, in the inkjet ink disclosed herein, the volume ratio of inorganic pigment to photopolymerization initiator is adjusted to 6 times or less. This ensures sufficient photocurability despite the ink containing a large amount of inorganic pigment, allowing for the formation of clear images without bleeding. Note that black inks are prone to reduced photocurability due to the absorption of ultraviolet light by the pigment. For this reason, in order to obtain sufficient photocurability, it is necessary to increase the content of photopolymerization initiator and decrease the content of inorganic pigment compared to inks of other colors (typically the three primary colors). From the viewpoint of obtaining more suitable photocurability, the volume ratio of inorganic pigment to photopolymerization initiator is preferably 5.5 times or less, more preferably 5 times or less, and even more preferably 4 times or less. Meanwhile, the lower limit of the volume ratio of inorganic pigment to photopolymerization initiator is not particularly limited, and may be 0.4 times or more, 1.0 times or more, 1.1 times or more, or 1.4 times or more.

[0056] As described above, in the inkjet ink disclosed herein, (a) the volume ratio of the inorganic solids to the total amount of ink, (b) the volume ratio of the inorganic pigment to the total amount of the inorganic solids, and (c) the volume ratio of the inorganic pigment to the photopolymerization initiator are controlled within predetermined ranges. Such inkjet inks exhibit high levels of ejection properties during printing, photocurability after printing, image hiding properties after baking, and fixability to glass substrates, making it easy to produce glass products bearing beautiful images.

[0057] 2. Inkjet Ink Preparation Next, a procedure for preparing (manufacturing) the inkjet ink disclosed herein will be described. The inkjet ink disclosed herein can be prepared by mixing the above-described materials in a predetermined ratio, followed by crushing and dispersing the inorganic solids. FIG. 1 is a cross-sectional view that schematically shows a stirring and grinding machine used in the production of inkjet ink. Note that the following description is not intended to limit the inkjet ink disclosed herein.

[0058] When producing the ink-jet ink disclosed herein, first, the above-mentioned materials are weighed and mixed to prepare a slurry, which is a precursor of the ink. Next, using an agitator grinder 100 as shown in FIG. 1, the slurry is agitated and the inorganic solids (inorganic pigment and glass frit) are pulverized. Specifically, grinding beads (e.g., zirconia beads with a diameter of 0.5 mm) are added to the slurry, and the slurry is then supplied from a supply port 110 into an agitator vessel 120. A shaft 134 having a plurality of agitator blades 132 is housed within the agitator vessel 120. One end of the shaft 134 is attached to a motor (not shown), and by operating the motor to rotate the shaft 134, the plurality of agitator blades 132 agitate the slurry while sending it downstream in the liquid-feeding direction A. During this agitation, the inorganic solids are pulverized by the grinding beads added to the slurry, and the finely divided inorganic solids are dispersed throughout the slurry.

[0059] The slurry sent downstream in the liquid sending direction A then passes through filter 140. As a result, grinding beads and inorganic solids that have not been atomized are collected by filter 140, and inkjet ink in which the atomized inorganic solids are sufficiently dispersed is discharged from outlet 150. By adjusting the pore size of filter 140 at this time, the maximum particle size of the inorganic solids in the inkjet ink can be controlled.

[0060] 3. Inkjet ink applications Next, applications of the inkjet ink disclosed herein will be described. As described above, the inkjet ink disclosed herein is used to draw an image on a transparent glass substrate. In this specification, the phrase "used to draw an image on a transparent glass substrate" refers not only to an embodiment in which the ink is directly applied to the surface of the glass substrate, but also to an embodiment in which the ink is indirectly applied to the surface of the glass substrate via transfer paper or the like. In other words, the inkjet ink disclosed herein can be used for printing on transfer paper for glass substrates (manufacturing transfer paper) or for printing on the surface of glass substrates (manufacturing glass products).

[0061] (1) Manufacture of transfer paper A method for producing transfer paper for glass substrates (a printing method for drawing an image on the surface of transfer paper) using the inkjet ink disclosed herein will be described. Fig. 2 is a general view showing a typical example of an inkjet device. Fig. 3 is a cross-sectional view showing a typical inkjet head of the inkjet device shown in Fig. 2.

[0062] The inkjet ink disclosed herein is stored in an inkjet head 10 of an inkjet device 1 shown in FIG. 2. The inkjet device 1 includes four inkjet heads 10. Each inkjet head 10 stores one of four different colors of ink: black (K), cyan (C), yellow (Y), and magenta (M). The inkjet ink disclosed herein is stored in the black (K) inkjet head 10. Each inkjet head 10 is housed inside a print cartridge 40. The print cartridge 40 is inserted into a guide shaft 20 and is configured to reciprocate along the axial direction X of the guide shaft 20. Although not shown, the inkjet device 1 also includes a moving means for moving the guide shaft 20 in the vertical direction Y. This allows ink to be ejected from the inkjet head 10 toward a desired position on a backing sheet W of transfer paper.

[0063] The inkjet head 10 shown in Fig. 2 may be, for example, a piezoelectric inkjet head as shown in Fig. 3. Such a piezoelectric inkjet head 10 has a storage section 13 for storing ink in a case 12, and the storage section 13 is connected to a discharge section 16 via a liquid supply path 15. The discharge section 16 has a discharge port 17 that opens to the outside of the case 12, and a piezoelectric element 18 is arranged opposite the discharge port 17. In such an inkjet head 10, the piezoelectric element 18 is vibrated to discharge the ink in the discharge section 16 from the discharge port 17 toward the backing paper W (see Fig. 2).

[0064] 2, a UV irradiation means 30 is attached to the guide shaft 20. The UV irradiation means 30 is disposed adjacent to the print cartridge 40, and moves in conjunction with the reciprocating movement of the print cartridge 40, irradiating the ink-adhered backing paper W with ultraviolet light. This causes the ink to harden immediately after adhering to the surface of the backing paper W, allowing a sufficient thickness of ink to be fixed on the surface of the transfer paper (backing paper W).

[0065] As described above, in the inkjet ink disclosed herein, the volume of inorganic solids relative to the total volume of the inkjet ink is adjusted to 35% by volume or less. This allows the ink viscosity to be maintained at a low level, enabling the ink to be ejected from the ejection orifices 17 with high precision and to print a precise image on the surface of the printing target (here, transfer paper). Furthermore, in the inkjet ink disclosed herein, the volume ratio of the inorganic pigment content to the photopolymerization initiator content is adjusted to 6 times or less. This ensures high photocurability, allowing the ink to cure immediately after UV printing and preventing ink bleeding.

[0066] In addition, it is preferable to use a photocurable monomer component containing the above-mentioned monomers (a) to (c) in the production of this transfer paper, which allows for the printing of an image (cured ink) with sufficient flexibility, thereby suitably preventing the image from cracking when the transfer paper is bent.

[0067] (2) Manufacturing methods for glass products Next, a method for manufacturing a glass product using the inkjet ink disclosed herein will be described, which includes the steps of applying the inkjet ink disclosed herein to the surface of a glass substrate and firing the glass substrate.

[0068] The glass product produced by this manufacturing method is not particularly limited as long as it has an image formed on the surface of a glass substrate. For example, the glass product is not limited to everyday items such as tableware, window glass, and cooking equipment, but may also be industrial products such as electronic devices and displays. The glass substrate to be printed is not particularly limited, and commonly used glass members can be used without any particular restrictions. In consideration of the firing process described below, it is preferable to use a glass substrate with a softening point of 500°C or higher (more preferably 600°C or higher, and even more preferably 700°C or higher). On the other hand, the upper limit of the softening point of the glass substrate is not particularly limited. For example, the upper limit of the softening point of the glass substrate may be 1600°C or lower, 1200°C or lower, or 1000°C or lower.

[0069] In the manufacturing method disclosed herein, first, an inkjet ink is applied to the surface of a glass substrate. The means for applying the ink to the glass substrate is not particularly limited, and the ink may be applied directly to the surface of the glass substrate using an inkjet device, or the ink may be applied indirectly via the above-mentioned transfer paper. When using an inkjet device to directly apply the ink to the surface of the glass substrate, it is preferable to eject the ink toward the surface of the glass substrate according to the same procedure as in the above-mentioned "manufacturing of transfer paper."

[0070] In the manufacturing method disclosed herein, the glass substrate with the ink attached is then baked under conditions in which the maximum baking temperature is set within the range of 450°C to 1200°C (preferably 500°C to 1000°C, more preferably 550°C to 850°C). This burns off the resin component formed by curing the monomer, and melts the glass frit in the inorganic solids. Then, by cooling after baking, the melted glass frit solidifies, and the inorganic pigment is fixed to the surface of the substrate. In this manufacturing method disclosed herein, the ink used is adjusted so that the volume of the inorganic pigment relative to the total volume of the inorganic solids is 15% by volume or more, thereby forming a beautiful image with excellent hiding power. Furthermore, because the volume of the inorganic pigment relative to the total volume of the inorganic solids is adjusted to less than 90% by volume, the inorganic pigment can be properly fixed to the surface of the glass substrate.

[0071] [Test example] Test examples relating to the present invention will be described below, but these test examples are not intended to limit the present invention.

[0072] <Inkjet ink> In this test, 14 types of inkjet inks (Examples 1 to 14) containing inorganic solids, photocurable monomers, and photopolymerization initiators were prepared. Specifically, a slurry was prepared by mixing the raw materials in the volume ratios shown in Tables 1 and 2, and the inks in Examples 1 to 14 were obtained by grinding and dispersing them using grinding beads (zirconia beads with a diameter of 0.5 mm). The volume ratios in the tables are based on the total volume of the ink being 100% by volume, unless otherwise specified. In addition, in these test examples, a dispersant (DISPERBYK-2013, manufactured by BYK Japan Co., Ltd.) and a polymerization inhibitor (Q-1301 (N-nitroso-N-phenylhydroxylamine aluminum), manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were also added as other additives. The volume ratios of these additives are also shown in Tables 1 and 2. The volume ratios of each component are rounded to one decimal place.

[0073] Regarding the inorganic solid content used in this test example, the black inorganic pigment in Tables 1 and 2 is spinel black, and the "glass frit" is borosilicate glass with a softening point of 550°C.

[0074] The "photocurable components" in Tables 1 and 2 are mixtures of a monofunctional acrylate monomer, a monofunctional N-vinyl compound monomer, a polyfunctional acrylate monomer, and a polyfunctional vinyl ether monomer in a predetermined volume ratio. The monofunctional acrylate monomers used were a mixture of isobornyl acrylate (manufactured by Osaka Organic Chemical Industry Ltd.), benzyl acrylate (manufactured by Osaka Organic Chemical Industry Ltd.), phenoxyethyl acrylate (manufactured by Osaka Organic Chemical Industry Ltd.), and cyclic trimethylolpropane formal acrylate (manufactured by Osaka Organic Chemical Industry Ltd.). The monofunctional N-vinyl compound monomer used was N-vinylcaprolactam (manufactured by Tokyo Chemical Industry Co., Ltd.). The polyfunctional acrylate monomer used was 1,9-nonanediol diacrylate (manufactured by Osaka Organic Chemical Industry Ltd.). As the polyfunctional vinyl ether monomer, a mixture of triethylene glycol divinyl ether (manufactured by Nippon Carbide Corporation), diethylene glycol divinyl ether (manufactured by Nippon Carbide Corporation), and 1,4-cyclohexanedimethanol divinyl ether (manufactured by Nippon Carbide Corporation) was used.

[0075] As the photopolymerization initiator, an acylphosphine oxide-based photopolymerization initiator (Omnirad 819, manufactured by IGM RESINS) was used.

[0076] In this test example, the "volume ratio of inorganic solids to the total amount of ink," the "volume ratio of inorganic pigment to inorganic solids," and the "volume ratio of inorganic pigment to photopolymerization initiator" were calculated for each example. The "volume ratio of inorganic solids to the total amount of ink" is a value when the total amount of ink is 100% by volume, and the "volume ratio of inorganic pigment to inorganic solids" is a value when the total amount of inorganic solids is 100% by volume. The "volume ratio of inorganic pigment to photopolymerization initiator" is a value (multiple) obtained by dividing the volume of inorganic pigment by the volume of photopolymerization initiator.

[0077] <Evaluation test> (1) Evaluation of ink viscosity The ink viscosity of each example prepared was measured using a Brookfield viscometer. The ink temperature during measurement was set at 25°C, and the spindle rotation speed was set at 5 rpm. Inks with a viscosity of less than 70 mPa·s were rated as "excellent," inks with a viscosity of 70 mPa·s or more but less than 110 mPa·s were rated as "fair," and inks with a viscosity of 110 mPa·s or more were rated as "poor." The evaluation results are shown in Tables 1 and 2.

[0078] (2) Printing images Using inkjet printing, the ink of each example was printed on the surface of a 5 mm thick glass substrate (softening point: 820°C). Specifically, an ink was ejected onto the surface of the glass substrate using an inkjet device (Fujifilm Corporation: Material Printer (DMP-2831)), and then an image 5 to 50 μm thick was drawn on the surface of the glass substrate by irradiating it with UV light (wavelength: 395 nm) for 1 second. The glass substrate was then fired at 700°C to produce a glass product with a decorative portion.

[0079] (3) Fixation evaluation The adhesive strength of the decorative part after firing was measured to evaluate the ink fixability to the glass substrate. Specifically, a scratch hardness test was conducted on the decorative part using a pencil method based on JIS K5600-5-4. A pencil hardness of 3H or higher was evaluated as "excellent," and a pencil hardness of less than 3H was evaluated as "poor." The evaluation results are shown in Tables 1 and 2.

[0080] (4) Concealment evaluation The hiding power of the decorative part formed on the glass substrate was evaluated by visual observation. Specifically, a piece of paper with writing on it was placed underneath the glass substrate, and the decorative part was observed from the decorated side. If the writing was completely invisible, it was rated as "excellent," if the writing was visible but unreadable, it was rated as "fair," and if the decoration was visible enough that the writing was readable, it was rated as "poor." The evaluation results are shown in Tables 1 and 2.

[0081] (5) UV curing evaluation Here, the UV curing properties of the ink were evaluated after UV irradiation but before baking. Specifically, a wipe was lightly pressed against the glass substrate after UV irradiation. Those that showed no ink transfer to the wipe were rated "excellent," those that showed slight ink transfer but no disruption (bleeding) to the appearance of the image were rated "fair," and those that showed disruption to the appearance of the image due to a large amount of ink transfer were rated "poor." The evaluation results are shown in Tables 1 and 2.

[0082] [Table 1]

[0083] [Table 2]

[0084] As shown in Tables 1 and 2, in Examples 1 to 9, the evaluation results for ink viscosity, adhesion strength, hiding power, and photocurability were all "fair" or better. This confirms that, for inkjet inks using black inorganic pigments, inks with a well-balanced improvement in these performances can be prepared by setting the volume ratio of inorganic solids to the total ink volume to 35% by volume or less, the volume ratio of inorganic pigment to inorganic solids to 15% by volume or more and less than 90% by volume, and the volume ratio of inorganic pigment to photopolymerization initiator to 6 times or less.

[0085] Although specific examples of the present invention have been described above in detail, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific examples exemplified above. [Explanation of symbols]

[0086] 1. Inkjet device 10 Inkjet head 12 cases 13 Storage 15 Liquid transfer path 16 Discharge part 17 Discharge port 18 Piezo element 20 Guide shaft 30 UV irradiation means 40 print cartridges 100 Stirring mill 110 Supply port 120 Stirring vessel 132 stirring blade 134 Shaft 140 filters 150 Outlet A Flow direction X Guide shaft axial direction Y guide axis vertical direction

Claims

1. An inkjet ink for glass substrates used to print images on transparent glass substrates, comprising: an inorganic solid content including a black inorganic pigment and glass frit; a photocurable monomer component; Photopolymerization initiator and Contains a volume ratio of the inorganic solid content to the total volume of the inkjet ink for glass substrates is 15% by volume or more and 30% by volume or less, The volume ratio of the inorganic pigment is 25% by volume or more and 85% by volume or less when the total volume of the inorganic solid content is 100% by volume, and an inkjet ink for glass substrates, wherein a volume ratio of the inorganic pigment to the photopolymerization initiator is 6 times or less.

2. The inkjet ink for glass substrates according to claim 1 , wherein the inorganic pigment is spinel black.

3. 3. The inkjet ink for glass substrates according to claim 1, wherein a volume ratio of the inorganic solid content is 15% by volume or more and 30% by volume or less when the total volume of the inkjet ink for glass substrates is 100% by volume.

4. 4. The inkjet ink for glass substrates according to claim 1, wherein a volume ratio of the inorganic pigment is 25% by volume or more and 85% by volume or less when the total volume of the inorganic solid content is 100% by volume.

5. 5. The inkjet ink for glass substrates according to claim 1, wherein a volume ratio of the inorganic pigment to the photopolymerization initiator is 5 times or less.

6. 6. The inkjet ink for glass substrates according to claim 1, wherein the monomer components contain at least a monofunctional acrylate monomer containing one acryloyl group or one methacryloyl group in the molecule, a monofunctional N-vinyl compound monomer in which one vinyl group is bonded to the nitrogen (N) atom of a nitrogen-containing compound, and a polyfunctional vinyl ether monomer containing at least two vinyl ether groups in the molecule.

7. 7. The inkjet ink for glass substrates according to claim 6, wherein a volume ratio of the monomer component is 50% by volume or more and 70% by volume or less when the total volume of the inkjet ink for glass substrates is 100% by volume.

8. A method for manufacturing a glass product having a decorative portion, comprising: a step of inkjet printing the inkjet ink for glass substrates according to any one of claims 1 to 7 onto a surface of a glass substrate; a step of irradiating a surface of the glass substrate with ultraviolet light to cure the inkjet ink for glass substrates attached to the surface of the glass substrate; Firing the glass substrate under conditions in which a maximum firing temperature is set within a range of 450°C to 1200°C; A method for manufacturing a glass product, comprising:

9. A method for manufacturing a transfer paper for a glass substrate to be used on a glass substrate that involves baking, a step of inkjet printing the inkjet ink for glass substrates according to any one of claims 1 to 7 onto a surface of a mount; a step of irradiating a surface of the mount with ultraviolet light to cure the inkjet ink for glass substrates attached to the surface of the mount; A method for producing transfer paper for glass substrates, comprising:

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