Inkjet ink and image forming method

Abstract

The objective is to provide an inkjet ink that hardens with an active ray, which can achieve both the pinning properties of the inkjet ink and the varnishability of the formed image, and an image forming method using the same. [Solution] An inkjet ink that hardens by irradiation with an active ray, comprising two or more types of wax and a polymerizable compound, wherein the total mass of the two or more types of wax is 1.0% to 10.0% by mass relative to the total mass of the inkjet ink, and of the wax with the highest mass content and the wax with the second highest mass content, the melting point of the high-melting-point wax is 5°C or more higher than the melting point of the low-melting-point wax, and the mass content of the high-melting-point wax is 1.0% to 10.0% by mass relative to the mass content of the low-melting-point wax.

Description

[Technical Field]

[0001] The present invention relates to an inkjet ink and an image forming method. [Background technology]

[0002] Inkjet recording methods are used in various printing fields because they allow for easy and inexpensive image formation. Among the inks used in inkjet recording methods, inkjet inks that cure with active rays (hereinafter referred to as inkjet inks) are known, and whose liquid component is a polymerizable compound that polymerizes with active rays (hereinafter referred to as polymerizable compound). When irradiated with active rays, active-ray curable inkjet inks harden through polymerization of the active-ray polymerizable compound, firmly adhering the colorant to the substrate. This hardened film formation allows for the creation of a desired image.

[0003] One type of inkjet ink is one that contains a gelling agent (wax).

[0004] For example, Patent Document 1 discloses a radiation-curable inkjet ink composition comprising a first ester compound and a second ester compound. Patent Document 1 states that the gelation of the ink suppressed excessive wetting and spreading of ink droplets (improved pinning properties). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] International Publication No. 2016 / 097180 [Overview of the project] [Problems that the invention aims to solve]

[0006] In inkjet inks containing wax as a gelling agent, the wax is dissolved by heating the inkjet ink during ejection, and then the wax crystallizes as the liquid temperature drops upon impact with the substrate, causing the inkjet ink to gel. By sufficiently increasing the gelling properties of the inkjet ink, the inkjet ink thickens sufficiently upon cooling after impact with the substrate, making it easier to improve the pinning properties of the inkjet ink.

[0007] On the other hand, with inkjet inks containing wax, varnish is sometimes applied to the resulting image. Images obtained using such inkjet technology sometimes exhibited poor wettability or adhesion to the varnish layer (i.e., poor varnishability) when varnished.

[0008] According to the inventors' research, the inkjet ink described in Patent Document 1 has the problem of being difficult to achieve both the pinning properties of the inkjet ink and the varnishability of the image obtained from the inkjet ink.

[0009] The present invention has been made in view of the above circumstances, and aims to provide an inkjet ink that hardens with an active ray, and that can achieve both the pinning properties of the inkjet ink and the varnishability of the formed image. Another object of the present invention is to provide an image forming method using the inkjet ink. [Means for solving the problem]

[0010] One aspect of the present invention, for solving the above problems, relates to the following inkjet inks [1] to [3].

[0011] [1] An inkjet ink that hardens upon irradiation with an active ray, comprising two or more types of wax and a polymerizable compound, The total mass of the two or more waxes is 1.0% to 10.0% by mass relative to the total mass of the inkjet inks. In the case where, among the two or more waxes mentioned above, the wax with the highest mass content and the wax with the second highest mass content are designated as the high-melting-point wax and the wax with the low-melting-point wax, The melting point of the high-melting-point wax is 5°C or more higher than the melting point of the low-melting-point wax. The mass content of the high-melting-point wax is 1.0% to 10.0% by mass relative to the mass content of the low-melting-point wax. Inkjet ink.

[0012] [2] The HSP distance between the high melting point wax and the polymerizable compound and the HSP distance between the low melting point wax and the polymerizable compound are both within the range of 2.5 to 6.5. [1] The inkjet ink described above.

[0013] [3] The melting point of the high-melting-point wax is 10°C or more higher than the melting point of the low-melting-point wax. The mass content of the high-melting-point wax is 2.0% to 6.0% by mass relative to the mass content of the low-melting-point wax. The HSP distance between the high-melting-point wax and the polymerizable compound, and the HSP distance between the low-melting-point wax and the polymerizable compound are 3.5 to 5.5. The inkjet ink described in [1] or [2].

[0014] Another aspect of the present invention for solving the above problems relates to the image forming method described in [4] below.

[0015] A process of ejecting the inkjet ink described in any of [4][1] to [3] from the inkjet head and depositing it on the surface of the substrate, The process involves irradiating the applied inkjet ink with an active light to cure the inkjet ink, An image forming method including, The temperature of the substrate when the inkjet ink lands is 5°C to 25°C lower than the gelation temperature of the inkjet ink. Image forming method.

Advantages of the Invention

[0016] According to the present invention, there can be provided an inkjet ink that is cured by actinic rays and can achieve both the pinning property of the ink and the varnish suitability of the formed image, and an image forming method using the same.

Brief Description of the Drawings

[0017] [Figure 1] FIG. 1 is a flowchart showing an image forming method according to the present embodiment. [Figure 2] FIG. 2 is a schematic diagram showing the configuration of an image forming apparatus capable of implementing the image forming method according to the present embodiment.

Modes for Carrying Out the Invention

[0018] Hereinafter, embodiments of the present invention will be described in detail. Note that the present invention is not limited to the following embodiments.

[0019] In this specification, a numerical range represented using "~" means a range including the numerical values described before and after "~" as the lower limit value and the upper limit value.

[0020] [[ID=,35]] Also, in this specification, "(meth)acrylate" means either one or both of acrylate and methacrylate, and "(meth)acrylic" means either one or both of acrylic and methacrylic.

[0021] 1. Inkjet Ink The inkjet ink (hereinafter referred to as inkjet ink) cured by actinic rays in the present embodiment contains two or more kinds of waxes.

[0022] The inkjet ink according to this embodiment, based on the above findings, will be described in more detail below.

[0023] 1-1. Wax Wax is a compound that dissolves in polymerizable compounds contained in inkjet ink when the inkjet ink containing it is heated (e.g., to 80°C). Furthermore, wax is a compound that crystallizes in the inkjet ink and gels it when the liquid temperature drops (e.g., to 40°C) as the inkjet ink is ejected from the inkjet head and lands on the surface of the substrate.

[0024] Of the two or more waxes mentioned above, the wax with the highest mass content and the wax with the second highest mass content are designated as the high-melting-point wax and the wax with the lower melting point as the low-melting-point wax. In this case, each wax satisfies the following three requirements. (Requirement 1) The total mass of the two or more types of wax is 1.0% to 10.0% by mass relative to the total mass of the inkjet ink. (Requirement 2) The melting point of the high-melting-point wax is 5°C or higher than the melting point of the low-melting-point wax. (Requirement 3) The mass content of the high-melting-point wax is 1.0% to 10.0% by mass relative to the mass content of the low-melting-point wax.

[0025] The reason why the aforementioned two or more types of wax can satisfy both (Requirement 1) to (Requirement 3) and achieve both the pinning properties of inkjet ink and the varnishability of the formed image is not entirely clear, but it can be considered as follows.

[0026] Generally, polymerizable compounds that impart reactive curability to inkjet inks include highly polar compounds such as acrylates and methacrylates. Furthermore, since waxes need to be compatible with polymerizable compounds when heated and crystallize when cooled, they are often modified to have a somewhat lower compatibility with polymerizable compounds by introducing less polar functional groups, such as long-chain hydrocarbon groups.

[0027] Among waxes, high-melting-point waxes crystallize more easily in inkjet ink at relatively high temperatures compared to low-melting-point waxes, and thus facilitate the gelation of the inkjet ink, thereby improving the pinning properties of the inkjet ink. On the other hand, high-melting-point waxes tend to have worse compatibility with polymerizable compounds compared to low-melting-point waxes, for example, because they have a higher molecular weight due to an increase in the number of hydrocarbon groups introduced, or their polarity is reduced due to an extension of the hydrocarbon chain length. Therefore, when using two or more types of wax, increasing the addition ratio of high-melting-point wax makes it easier for it to precipitate near the surface of the image during image formation. When applying varnish to the obtained image, the varnish may contain highly polar compounds such as acrylates and methacrylates, similar to the polymerizable compounds mentioned above. In this case, if there is a large amount of crystallized wax near the image surface, repelling is more likely to occur when applying varnish (the suitability of the varnish tends to deteriorate).

[0028] Therefore, in order to satisfy (Requirement 1), the total amount of wax in the inkjet ink is adjusted so as not to be too large, and in order to satisfy (Requirement 3), the addition ratio of high-melting-point wax, which has relatively low compatibility with polymerizable compounds, is reduced relative to low-melting-point wax, which makes it easier to reduce the amount of wax near the surface of the image during image formation and improves varnishability.

[0029] Furthermore, by adjusting the total amount of wax in the inkjet ink so that it does not become too low in order to satisfy (Requirement 1), the gelling properties of the inkjet ink can be easily improved. And, even when the addition ratio of high-melting-point wax is low in order to satisfy (Requirements 2) and (Requirements 3), the small amount of high-melting-point wax, which has a different structure from the low-melting-point wax, prevents the low-melting-point wax from agglomerating too densely when it crystallizes into a plate-like form, making it easier to create voids (three-dimensional spaces). As a result, it becomes easier to form a structure (cardhouse structure) that contains polymerizable compounds, the inkjet ink becomes more viscous, and the pinning and ejection properties of the inkjet ink can be easily improved.

[0030] Furthermore, even when the addition ratio of high-melting-point wax is reduced to satisfy requirements 2 and 3, the high-melting-point wax crystallizes first as the temperature of the low-inkjet ink decreases, acting as a seed crystal and promoting the crystallization of the low-melting-point wax. Therefore, even with a relatively small amount of high-melting-point wax added, it is possible to sufficiently raise the gelation temperature of the inkjet ink and easily improve the gelability of the inkjet ink.

[0031] Based on the above, it is considered that by using two or more of the aforementioned waxes that satisfy requirements 1 to 3, it becomes easier to achieve both the pinning properties of the inkjet ink and the varnishability of the formed image.

[0032] Examples of waxes include aliphatic ketones, aliphatic esters, glycerol compounds, pentaerythritol compounds, petroleum waxes, plant waxes, animal waxes, mineral waxes, hydrogenated castor oil, modified waxes, higher fatty acids, higher alcohols, hydroxystearic acid, fatty acid amides including N-substituted fatty acid amides and special fatty acid amides, higher amines, esters of sucrose fatty acids, synthetic waxes, dibenzylidene sorbitol, dimer acid, and dimer ol. Of these, from the viewpoint of increasing solubility in polymerizable compounds at high temperatures and increasing crystallinity in polymerizable compounds at low temperatures, the wax is preferably aliphatic ketone, aliphatic ester, higher fatty acid, and higher alcohol, with aliphatic ketone and aliphatic ester being more preferred.

[0033] Examples of aliphatic ketones include dibehenyl ketone, diheptadecyl ketone (stearone), distearyl ketone, dieicosyl ketone, dipalmysyl ketone, dilauryl ketone, dimyristyl ketone, myristylpalmysyl ketone, and palmitylstearyl ketone.

[0034] Examples of aliphatic esters include, Behenyl behenate, eicosyl eicosanoate, stearyl stearate, palmityl stearate, behenyl stearate, myristyl myristate, cetyl myristate, oleyl palmitate, cetyl palmitate, and other monoalcohol fatty acid esters. Fatty acid esters of polyhydric alcohols such as glycerin fatty acid esters, sorbitan fatty acid esters, propylene glycol fatty acid esters, ethylene glycol fatty acid esters, polyoxyethylene fatty acid esters, and pentaerythritol fatty acid esters. It includes.

[0035] Examples of pentaerythritol fatty acid esters include pentaerythritol tetrastearate, pentaerythritol distearate, and pentaerythritol tetrapalmitate.

[0036] Examples of higher fatty acids include behenic acid, arachidic acid, stearic acid, palmitic acid, myristic acid, lauric acid, oleic acid, and erucic acid.

[0037] Examples of higher alcohols include stearyl alcohol and behenyl alcohol.

[0038] From the viewpoint of enhancing gelatinization, the hydrocarbon chains contained in aliphatic ketones and aliphatic esters preferably have at least one carbon chain with 12 to 22 carbon atoms flanking the ketone group or ester group, and more preferably both carbon chains with 12 to 22 carbon atoms. When the number of carbon atoms is 12 or more, the crystallinity of the wax tends to increase, and more space is created in the cardhouse structure. As a result, polymerizable compounds tend to be sufficiently encapsulated in the space, and the pinning properties of the inkjet ink tend to increase. Furthermore, when both carbon chains have 12 to 22 carbon atoms, the crystallinity tends to increase even further, and the pinning properties tend to increase even more. In addition, when the number of carbon atoms is 22 or less, the solubility of the wax tends to increase, and the suitability for varnishing tends to improve.

[0039] Furthermore, the hydrocarbon chains contained in aliphatic ketones and aliphatic esters are preferably straight-chain hydrocarbon chains. When the hydrocarbon chains are straight-chain, the polarity of the wax can be reduced, which tends to decrease its compatibility with polymerizable compounds and increases the crystallinity of the wax.

[0040] Examples of aliphatic ketones containing hydrocarbon chains with 12 to 22 carbon atoms include dibehenyl ketone (21-22 carbon atoms), diheptadecyl ketone (stearone) (17-17 carbon atoms), distearyl ketone (17-18 carbon atoms), dieicosyl ketone (19-20 carbon atoms), dipalmityl ketone (15-16 carbon atoms), dimyristyl ketone (13-14 carbon atoms), dilauryl ketone (11-12 carbon atoms), and lauryl myristyl ketone. This includes lucetones (11-14 carbon atoms), lauryl palmityl ketones (11-16 carbon atoms), myristyl palmityl ketones (13-16 carbon atoms), myristyl stearyl ketones (13-18 carbon atoms), myristyl behenyl ketones (13-22 carbon atoms), palmityl stearyl ketones (15-18 carbon atoms), palmityl behenyl ketones (15-22 carbon atoms), and stearyl behenyl ketones (17-22 carbon atoms), etc. The carbon numbers in parentheses above represent the number of carbon atoms in each hydrocarbon chain bonded to the carbon atom of the carbonyl group.

[0041] Examples of aliphatic esters containing hydrocarbon chains with 12 to 22 carbon atoms include behenyl behenate (21-22 carbon atoms), eicosyl eicosanoate (19-20 carbon atoms), stearyl stearate (17-18 carbon atoms), palmityl stearate (17-16 carbon atoms), lauryl stearate (17-12 carbon atoms), behenyl stearate (17-22 carbon atoms), cetyl palmitate (15-16 carbon atoms), stearyl palmitate (15-18 carbon atoms), and myristate. This includes stil (13-14 carbon atoms), cetyl myristate (13-16 carbon atoms), octyldodecyl myristate (13-20 carbon atoms), stearyl oleate (17-18 carbon atoms), stearyl erucate (21-18 carbon atoms), stearyl linoleate (17-18 carbon atoms), behenyl oleate (18-22 carbon atoms), arachidyl linoleate (17-20 carbon atoms), pentaerythritol tetrastearate (17-17-17-17 carbon atoms), etc. The carbon numbers in parentheses above represent the number of carbon atoms in the hydrocarbon chain bonded to the carbon and oxygen atoms of the ester group.

[0042] Regarding (Requirement 1), the total mass of the two or more types of wax is preferably 2.00% to 8.00% by mass relative to the total mass of the inkjet ink. Furthermore, the total mass of the two or more types of wax is more preferably 3.00% to 6.00% by mass, and even more preferably 3.00% to 4.00% by mass, relative to the total mass of the inkjet ink. Setting it to 2.00% by mass or more makes it easier for the inkjet ink to gel, and thus easier for the ink to pinch. Setting it to 3.00% by mass or more makes it easier for the ink to pinch. Setting it to 8.00% by mass or less makes it easier to reduce the amount of wax near the surface of the image during image formation, and thus easier for the varnishability to improve. Setting it to 6.00% by mass or less makes it easier for the varnishability to improve even further. In addition, setting it to 4.00% by mass or less reduces the amount of wax added, which is relatively difficult to dissolve in polymerizable compounds, so wax is less likely to precipitate near the nozzle during or before ejection, thus making it easier for the ejection performance to improve.

[0043] Regarding requirement 2, the following methods can be used to obtain waxes with a melting point 5°C or higher: increasing the number of hydroxyl groups (functional groups) of the alcohol used as a raw material for aliphatic esters, increasing the number of carbon atoms in the hydrocarbon chains contained in aliphatic ketones and aliphatic esters, reducing the number of branched structures and using linear hydrocarbon chains, and improving the symmetry of the molecular structure. The melting points of each wax were obtained using a differential scanning calorimeter "Diamond DSC" (manufactured by PerkinElmer). The melting point was measured under the following conditions (heating and cooling conditions): a first heating process in which the temperature is raised from room temperature (25°C) to 110°C at a heating rate of 10°C / min and held isothermally at 110°C for 5 minutes; a cooling process in which the temperature is cooled from 110°C to 0°C at a cooling rate of 10°C / min and held isothermally at 0°C for 5 minutes; and a second heating process in which the temperature is raised from 0°C to 110°C at a heating rate of 10°C / min. The above measurement is performed by sealing 3.0 mg of the sample in an aluminum pan and setting it in the sample holder of a differential scanning calorimeter "Diamond DSC". An empty aluminum pan is used as a reference. In the above measurement, analysis was performed on the endothermic curve obtained during the first heating process, and the top temperature of the endothermic peak (full width at half maximum within 15°C) originating from the crystalline polyester resin was taken as the melting point (Tm) of the wax.

[0044] Regarding requirement 2, the melting point of the high-melting-point wax is preferably 10°C or more higher than the melting point of the low-melting-point wax, and more preferably 10°C to 45°C higher. By setting the melting point difference to 10°C or more, sufficient precipitation (seed crystals) of the high-melting-point wax occurs at the time the low-melting-point wax precipitates, making it easier to improve gelability. By setting the melting point difference to 45°C or less, it is possible to suppress excessive crystallization of the high-melting-point wax at the time the low-melting-point wax precipitates, which can lead to excessive particle size formation of seed crystals, thus making it easier to improve interaction with the low-melting-point wax.

[0045] The melting points of each of the two or more waxes are preferably 105°C or lower, more preferably 89°C or lower, and even more preferably 82°C or lower. When the melting point is 105°C or lower, the wax is more easily dissolved by polymerizable compounds. When the melting point is 89°C or lower, the crystallization rate of the gelling agent slows down, making it easier to form a stronger cardhouse structure and increasing the viscosity of the inkjet ink. Furthermore, the melting points of each of the two or more waxes are preferably 30°C or higher, and more preferably 40°C or higher. When the melting point is 30°C or higher, the crystallinity of the wax can be further increased, making it easier to increase gelling properties.

[0046] Regarding requirement 3, the mass content of the high-melting-point wax is preferably 2.0% to 6.0% by mass, and more preferably 3.5% to 5.5% by mass, relative to the mass content of the low-melting-point wax. Setting it to 2.0% by mass or more allows for sufficient precipitation (seed crystals) of high-melting-point wax, making it easier to improve gelability and thus improving pinning properties. Setting it to 6.0% by mass or less reduces the proportion of high-melting-point wax, which is relatively difficult to dissolve in polymerizable compounds, making it less likely for wax to precipitate on the image surface and thus improving varnishability. Furthermore, setting it to 6.0% by mass or less makes it less likely for wax to precipitate near the nozzle during or before ejection, thus improving ejection performance.

[0047] The inkjet ink may contain two types of wax, or three or more types, but it is preferable to contain two types to facilitate the effects of the present invention. If there are three or more types, the total mass of the two waxes consisting of the wax with the highest mass content and the wax with the second highest mass content is preferably 85% to 100% by mass, more preferably 90% to 100% by mass, and even more preferably 95% to 100% by mass, relative to the total mass of wax contained in the inkjet ink.

[0048] The wax may or may not have polymerizable groups within its molecule. Specifically, the wax may or may not be polymerizable by active radiation. If it has polymerizable groups, the wax molecules are incorporated into the polymerization chain of the polymerizable compound, making it difficult for the wax to move and thus improving its varnishing properties.

[0049] When the wax has polymerizable groups, the number of polymerizable groups is preferably 1 to 3. From the viewpoint of making it easier to form linear molecules of the wax and thereby further improving the crystallinity of the wax, the number of polymerizable groups is more preferably 1 to 2.

[0050] Examples of polymerizable groups include (meth)acryloyl groups, vinyl groups, and ethynyl groups. Of these, (meth)acryloyl groups are preferred.

[0051] The weight-average molecular weight of the gelling agent is preferably 150 to 1300, and more preferably 450 to 1000. The above molecular weight can be measured using gel permeation chromatography.

[0052] 1-2.Polymerizable compound Polymerizable compounds are compounds that polymerize and crosslink upon irradiation with active rays. Preferably, polymerizable compounds are radical polymerizable compounds.

[0053] Furthermore, the polymerizable compound is preferably liquid at 30°C, from the viewpoint of adjusting the viscosity of the inkjet ink at or near room temperature and improving its ejection performance.

[0054] Examples of the active rays mentioned above include electron beams, ultraviolet rays, alpha rays, gamma rays, and X-rays. Of these, ultraviolet rays and electron beams are preferred, with ultraviolet rays being more preferred.

[0055] Radical polymerizable compounds are compounds (monomers, oligomers, polymers, or mixtures thereof) that have ethylenically unsaturated bonds that can be radically polymerized. Radical polymerizable compounds may be used alone or in combination of two or more types.

[0056] Examples of compounds having ethylenically unsaturated bonds that can be radically polymerized include unsaturated carboxylic acids and their salts, unsaturated carboxylic acid ester compounds, unsaturated carboxylic acid urethane compounds, unsaturated carboxylic acid amide compounds and their anhydrides. Other examples of compounds having ethylenically unsaturated bonds that can be radically polymerized include acrylonitrile, unsaturated polyesters, unsaturated polyethers, unsaturated polyamides, and unsaturated urethanes. Examples of unsaturated carboxylic acids include (meth)acrylic acid, itaconic acid, crotonic acid, isocrotonic acid, and maleic acid.

[0057] The radical polymerizable compound is preferably an unsaturated carboxylic acid ester compound, and among these, it is more preferably a (meth)acrylate from the viewpoint of facilitating the dissolution of the gelling agent in the ink.

[0058] Examples of monofunctional (meth)acrylates include isoamyl (meth)acrylate, stearyl (meth)acrylate, lauryl (meth)acrylate, octyl (meth)acrylate, decyl (meth)acrylate, isomirsutyl (meth)acrylate, isostearyl (meth)acrylate, 2-ethylhexyl-diglycol (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 2-(meth)acryloyloxyethylhexahydrophthalic acid, butoxyethyl (meth)acrylate, ethoxydiethylene glycol (meth)acrylate, methoxydiethylene glycol (meth)acrylate, and methoxypolyethyl This includes ethylene glycol (meth)acrylate, methoxypropylene glycol (meth)acrylate, phenoxyethyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, isobornyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, 2-(meth)acryloyloxyethyl succinic acid, 2-(meth)acryloyloxyethyl phthalic acid, 2-(meth)acryloyloxyethyl-2-hydroxyethyl phthalic acid, and t-butylcyclohexyl (meth)acrylate.

[0059] Examples of polyfunctional (meth)acrylates include triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, 1,10-decanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, tricyclodecanedimethanol di(meth)acrylate, and neopentyl glycol hydroxypivalate. This includes difunctional (meth)acrylates comprising di(meth)acrylate, polytetramethylene glycol di(meth)acrylate, polyethylene glycol diacrylate, and tripropylene glycol diacrylate, as well as trifunctional or more (meth)acrylates comprising trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, glycerin propoxy tri(meth)acrylate, and pentaerythritol ethoxytetra(meth)acrylate.

[0060] Furthermore, the polymerizable compound preferably contains a (meth)acrylate having at least one (meth)acroyl group, and preferably contains a polymerizable compound having two or more (meth)acryloyl groups. It is preferable that the polymerizable compound does not contain a polymerizable compound having 10 or more (meth)acroyl groups. When there are two or more (meth)acrylate groups, the polarity of the polymerizable compound tends to increase, the crystallinity of the wax tends to increase, and the pinning properties of the inkjet ink tend to increase. By not containing a polymerizable compound having 10 or more (meth)acrylate groups, a moderate cross-linked structure is formed, which increases the toughness of the image, suppresses cohesive breakdown between the image and the varnish layer when varnish is applied, and tends to increase varnish adhesion.

[0061] Furthermore, from the viewpoint of improving compatibility with waxes and further enhancing the solubility of waxes, it is preferable that the radical polymerizable compound contains a (meth)acrylate having an ethylene oxide (EO) group or a propylene oxide (PO) group. Generally, (meth)acrylates have ester groups and tend to be highly polar. On the other hand, EO groups or PO groups have lower polarity compared to ester groups, and thus tend to have higher compatibility with relatively low-polarity waxes, such as those having long-chain alkyl groups.

[0062] The number of EO groups or PO groups contained in the (meth)acrylate having an EO group or PO group is preferably 1 to 14, and more preferably 2 to 12 or less.

[0063] Examples of (meth)acrylates having an EO group or a PO group include polyethylene glycol diacrylate, EO-modified 1,6-hexanediol di(meth)acrylate, EO-modified nonylphenol (meth)acrylate, EO-modified cresol (meth)acrylate, EO-modified pentaerythritol tetraacrylate, EO-modified dipentaerythritol pentaacrylate, EO-modified dipentaerythritol hexaacrylate, EO-modified bisphenol A diacrylate, EO-modified ditrimethylolpropane tetraacrylate, EO-modified trimethylolpropane tri(meth)acrylate, PO-modified nonylphenol (meth)acrylate, PO-modified neopentyl glycol diacrylate, PO-modified trimethylolpropane tri(meth)acrylate, and PO-modified bisphenol A diacrylate.

[0064] The radical polymerizable compound may include modified acrylates such as urethane-modified acrylate, epoxy-modified acrylate, and polyester acrylate. Furthermore, the radical polymerizable compound may also include oligomers such as polyester oligomers.

[0065] Polymerizable compounds may include cationic polymerizable compounds. Examples of cationic polymerizable compounds include epoxy compounds, vinyl ether compounds, and oxetane compounds.

[0066] It is preferable that the HSP distance between the high-melting-point wax and the polymerizable compound, and the HSP distance between the low-melting-point wax and the polymerizable compound, are both within the range of 2.5 to 6.5. More preferably, the HSP distances are both within the range of 3.0 to 6.5, and even more preferably, both within the range of 3.5 to 5.5. When the HSP distances are both 2.5 or higher, the compatibility between the wax and the polymerizable compound can be moderately lowered, making it easier for crystallization to occur during cooling and improving pinning properties. When the HSP distances are both 3.5 or higher, the compatibility with the polymerizable compound is moderately low, making it easier for the wax to crystallize and reducing the mobility of the wax. As a result, uneven distribution of the wax on the image surface is suppressed, and varnishing properties are improved. When the HSP distances are both 6.5 or lower, the compatibility between each wax and the polymerizable compound is not reduced too much, making it difficult for the wax to precipitate near the image surface, and thus improving varnishing properties. In addition, moderately increasing the compatibility makes it easier for the wax to thicken due to crystallization, which improves injection molding and pinning properties.

[0067] The HSP distance between each wax and the polymerizable compound is calculated as follows.

[0068] First, the HSP values ​​(Hansen solubility parameters: dispersion term (dD), polarity term (dP), and hydrogen bonding term (dH)) for each wax and polymerizable compound are calculated using the computer software Hansen Solubility Parameters in Practice 5th Edition 5.0.13 (HSPiP, manufactured by Tegara Co., Ltd.) by inputting the chemical structural formula into the software. The HSP value is based on the idea that "two substances with similar intermolecular interactions readily dissolve in each other" (as described in "Chemical Industry Co., Ltd., Chemical Industry, March 2010 issue, Hiroshi Yamamoto, Steven Abbott, Charles M. Hansen"). The HSP value consists of the following three parameters, and these three parameters can be considered as coordinates in three-dimensional space (also called "Hansen space"). The distance between these coordinates of two substances is called the HSP distance, and the closer the HSP distance, the higher the affinity and the more readily the substances dissolve in each other. δD: Energy due to intermolecular dispersion forces δP: Energy due to intermolecular dipole interaction δH: Energy due to intermolecular hydrogen bonding

[0069] The HSP distance between a high-melting-point wax or low-melting-point wax and a polymerizable compound (or a mixture of polymerizable compounds if multiple polymerizable compounds are included) is calculated using the following formula. In the following formula, the dispersion, polarity, and hydrogen bonding terms of one component of the wax and polymerizable compound (or mixture thereof) for which the HSP distance is calculated are denoted as dD, dP, and dH, respectively. The dispersion, polarity, and hydrogen bonding terms of the other component are denoted as dD', dP', and dH', respectively. If multiple polymerizable compounds are included, the parameters (dD, dP, and dH) of the polymerizable compound mixture are obtained by multiplying each parameter (dD, dP, and dH) of each polymerizable compound by the molar ratio of each compound in the inkjet ink and adding these values ​​together. HSP distance = (4 × (dD - dD') 2 +(dP-dP') 2 +(dH-dH') 2 ) 1 / 2

[0070] The polymerizable compound content is preferably 1% to 97% by mass, and more preferably 30% to 95% by mass, based on the total mass of the inkjet ink. Furthermore, the polymerizable compound content is even more preferably 50% to 95% by mass, and most preferably 70% to 95% by mass, based on the total mass of the inkjet ink.

[0071] 1-3. Other ingredients 1-3-1. Colorants In this embodiment, the inkjet ink may contain a colorant as needed.

[0072] The coloring agent is either a dye or a pigment, but pigments are preferred because they have good dispersibility with the components of inkjet ink and excellent weather resistance. Depending on the colors of the image to be formed, the pigment can be selected from, for example, yellow pigment, red or magenta pigment, blue or cyan pigment, black pigment, and white pigment.

[0073] Examples of yellow pigments include CIPigment Yellow (hereinafter also simply referred to as "PY") 1, PY3, PY12, PY13, PY14, PY17, PY34, PY35, PY37, PY55, PY74, PY81, PY83, PY93, PY94, PY95, PY97, PY108, PY109, PY110, PY137, PY138, PY139, PY153, PY154, PY155, PY157, PY166, PY167, PY168, PY180, PY185, and PY193, among others.

[0074] Examples of red or magenta pigments include CIPigment Red (hereinafter also simply referred to as "PR") 3, PR5, PR19, PR22, PR31, PR38, PR43, PR48:1, PR48:2, PR48:3, PR48:4, PR48:5, PR49:1, PR53:1, PR57:1, PR57:2, PR58:4, PR63:1, PR81, PR81:1, PR81:2, PR81:3, PR81:4, PR88, PR104, PR108, PR112, PR122, PR123, PR144, PR146, PR149, PR166, PR168, PR169, PR170, PR177, PR178, PR179, PR184, PR185, PR208, PR216, PR226, and PR257, CIPigment This includes Violet (hereinafter also simply referred to as "PV") 3, PV19, PV23, PV29, PV30, PV37, PV50, and PV88, as well as CIPigment Orange (hereinafter also simply referred to as "PO") 13, PO16, PO20, and PO36, etc.

[0075] Examples of blue or cyan pigments include CIPigment Blue (hereinafter also simply referred to as "PB") 1, PB15, PB15:1, PB15:2, PB15:3, PB15:4, PB15:6, PB16, PB17-1, PB22, PB27, PB28, PB29, PB36, and PB60, among others.

[0076] Examples of green pigments include CIPigment Green (hereinafter also simply referred to as "PG") 7, PG26, PG36, and PG50.

[0077] Examples of black pigments include CIPigment Black (hereinafter simply referred to as "PBk"). This includes 7, PBk26, and PBk28, etc.

[0078] The white pigment can be any pigment that causes the cured film formed by the hardening of white ink to exhibit a white color. Examples of white pigments include inorganic pigments such as titanium dioxide, zinc oxide, calcium carbonate, barium sulfate, and aluminum hydroxide. Of these, titanium dioxide is preferred.

[0079] The crystalline form of the titanium dioxide described above may be rutile, anatase, or blue kite. From the viewpoint of making it easier to reduce the particle size of the white pigment, the anatase type, which has a lower specific gravity, is preferred, and from the viewpoint of further improving the opacity of the formed image, the rutile type, which has a higher refractive index in the visible light region, is preferred.

[0080] The colorant content is preferably 0.1% to 10.0% by mass, and more preferably 1.0% to 5.0% by mass, relative to the total mass of the inkjet ink. If a white pigment is included, the white pigment content is preferably 3.0% to 8.0% by mass.

[0081] 1-3-2. Pigment Dispersant The above inkjet ink may contain a pigment dispersant for dispersing the pigment.

[0082] Examples of pigment dispersants include hydroxyl group-containing carboxylic acid esters, salts of long-chain polyaminoamides and high molecular weight acid esters, salts of high molecular weight polycarboxylic acids, salts of long-chain polyaminoamides and polar acid esters, high molecular weight unsaturated acid esters, polymer copolymers, modified polyurethanes, modified polyacrylates, polyether ester-type anionic surfactants, naphthalene sulfonic acid formalin condensate salts, aromatic sulfonic acid formalin condensate salts, polyoxyethylene alkyl phosphate esters, polyoxyethylene nonylphenyl ether, and stearylamine acetate. Examples of commercially available pigment dispersants include the Azisper series (manufactured by Ajinomoto Fine Techno Co., Ltd.).

[0083] The pigment dispersant content is preferably 10% to 200% by mass, and more preferably 20% to 100% by mass, relative to the total mass of the pigment. When the dispersant content is 10% or more by mass relative to the total mass of the pigment, the dispersion stability of the pigment is enhanced, and when the dispersant content is 200% or less by mass relative to the total mass of the pigment, the ink ejection performance from the inkjet head becomes more stable.

[0084] 1-3-3. Polymerization Initiators The inkjet ink according to this embodiment may contain an active ray polymerization initiator (hereinafter simply referred to as "polymerization initiator"). The polymerization initiator should be capable of initiating the polymerization of the active ray polymerizable compound described above by irradiation with an active ray. For example, when the inkjet ink contains a radical polymerizable compound, the polymerization initiator can be a radical polymerization initiator. Also, for example, when the inkjet ink contains a cationic polymerizable compound, the polymerization initiator can be a cationic polymerization initiator (photoacid generator). Note that when the inkjet ink can be sufficiently cured without a polymerization initiator, such as when curing the inkjet ink by electron beam irradiation, the polymerization initiator is not necessary.

[0085] Radical polymerization initiators include intramolecular bond cleavage type radical polymerization initiators and intramolecular hydrogen abstraction type radical polymerization initiators.

[0086] Examples of intramolecular bond cleavage type radical polymerization initiators include: Acetophenone-based initiators including diethoxyacetophenone, 2-hydroxy-2-methyl-1-phenylpropan-1-one, benzyldimethylketal, 1-(4-isopropylphenyl)-2-hydroxy-2-methylpropan-1-one, 4-(2-hydroxyethoxy)phenyl-(2-hydroxy-2-propyl)ketone, 1-hydroxycyclohexyl-phenylketone, 2-methyl-2-morpholino(4-methylthiophenyl)propan-1-one, and 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone, etc. Benzoins including benzoin, benzoin methyl ether, and benzoin isopropyl ether, Acylphosphine oxide - based initiators including phenylbis(2,4,6 - trimethylbenzoyl)phosphine oxide (product name: Omnirad 819, manufactured by IGM Resins B.V.), And benzyl and methyl phenyl glyoxy esters are included.

[0087] Examples of intramolecular hydrogen abstraction - type radical polymerization initiators include Benzophenone - based initiators including benzophenone, methyl o - benzoylbenzoate, 4 - phenylbenzophenone, 4,4’ - dichlorobenzophenone, hydroxybenzophenone, 4 - benzoyl - 4’ - methyl - diphenyl sulfide, acrylated benzophenone, 3,3’,4,4’ - tetra(t - butylperoxycarbonyl)benzophenone, and 3,3’ - dimethyl - 4 - methoxybenzophenone, Thioxanthone - based initiators including 2 - isopropylthioxanthone, 2,4 - dimethylthioxanthone, 2,4 - diethylthioxanthone, 2,4 - dichlorothioxanthone, Aminobenzophenone - based initiators including Michler's ketone, 4,4’ - diethylaminobenzophenone, 10 - butyl - 2 - chloroacridone, 2 - ethylanthraquinone, 9,10 - phenanthrenequinone, and camphorquinone are included.

[0088] Examples of cationic polymerization initiators include photoacid generators. Examples of photoacid generators include B(C6F5)4 salts of aromatic onium compounds including diazonium, ammonium, iodonium, sulfonium, and phosphonium, - PF6 - AsF6 - SbF6 - CF3SO3 - Salts, etc., Sulfonides that generate sulfonic acid, Halides that generate hydrogen halide upon light irradiation, This also includes iron allene complexes, etc.

[0089] The content of the polymerization initiator is not particularly limited, as long as the inkjet ink is sufficiently cured by irradiation with active rays (e.g., ultraviolet light) without reducing its applicability to the substrate surface. For example, the content of the polymerization initiator is preferably 0.1% to 20% by mass, and more preferably 1% to 10% by mass, based on the total mass of the inkjet ink.

[0090] 1-3-4. Polymerization inhibitors In this embodiment, the inkjet ink may contain a polymerization inhibitor.

[0091] Examples of polymerization inhibitors include (alkyl)phenol, hydroquinone, catechol, resorcinol, p-methoxyphenol, t-butylcatechol, t-butylhydroquinone, pyrogallol, 1,1-picrylhydrazyl, phenothiazine, p-benzoquinone, nitrosobenzene, 2,5-di-t-butyl-p-benzoquinone, dithiobenzoyl disulfide, picric acid, cuperone, aluminum N-nitrosophenylhydroxylamine, tri-p-nitrophenylmethyl, N-(3-oxyanilino-1,3-dimethylbutylidene)aniline oxide, dibutylcresol, cyclohexanone oxime cresol, guaiacol, o-isopropylphenol, butyraldoxime, methyl ethyl ketoxime, and cyclohexanone oxime.

[0092] The content of the polymerization inhibitor is not particularly limited, but is preferably 0.1% to 10% by mass relative to the total mass of the inkjet ink.

[0093] 1-3-5. Surfactants In this embodiment, the inkjet ink may contain a surfactant for adjusting the surface tension.

[0094] Examples of surfactants include, Anionic surfactants including dialkyl sulfosuccinates, alkylnaphthalene sulfonates, and fatty acid salts, Nonionic surfactants including polyoxyethylene alkyl ethers, polyoxyethylene alkyl allyl ethers, acetylene glycols, and polyoxyethylene / polyoxypropylene block copolymers, Cationic surfactants containing alkylamine salts and quaternary ammonium salts, It contains silicone-based surfactants as well as fluorine-based surfactants.

[0095] Examples of commercially available silicone-based surfactants include KF-351A, KF-352A, KF-353, KF-354L, and KF-355A (all manufactured by Shin-Etsu Chemical Co., Ltd.).

[0096] The surfactant content is not particularly limited, but is preferably 0.001% to 10% by mass, and more preferably 0.001% to 1.0% by mass, relative to the total mass of the inkjet ink.

[0097] In this embodiment, the inkjet ink may, in addition to the above components, optionally contain a fixing resin, viscosity modifier, resistivity modifier, film-forming agent, ultraviolet absorber, antioxidant, fade inhibitor, mold inhibitor, rust inhibitor, and the like.

[0098] 1-4. Physical properties of inkjet inks The viscosity of the inkjet ink at 80°C is preferably 3 mPa·s to 20 mPa·s, and more preferably 5 mPa·s to 15 mPa·s. This improves the ejection performance when the inkjet ink is heated and ejected in the inkjet head.

[0099] The viscosity can be measured using a rheometer. For example, the pre-coat agent is heated to 100°C, and while measuring the viscosity with a stress-controlled rheometer (Anton Paar, Physica MCR301 (cone plate diameter: 75 mm, cone angle: 1.0°)), the inkjet ink is cooled to 20°C under conditions of a shear rate of 11.7 (1 / s) and a cooling rate of 0.1°C / s to obtain a viscosity temperature dependence curve. The viscosity can then be determined by reading the viscosity at 80°C from the obtained temperature dependence curve.

[0100] Furthermore, the viscosity of the above inkjet ink at 35°C is preferably 3 Pa·s or more, more preferably 10 Pa·s or more, and even more preferably 15 Pa·s or more. The upper limit of the viscosity is not particularly limited, but for example, it is 300 Pa·s. The viscosity can be determined by reading the viscosity at 35°C from the temperature change curve described above.

[0101] 1-5. Method for preparing inkjet ink Inkjet ink can be prepared by mixing the aforementioned wax, polymerizable compound, and any other components under heating. It is preferable to filter the resulting mixture through a predetermined filter. When preparing an ink containing a pigment, it is preferable to prepare a pigment dispersion containing the pigment and an activated polymerizable compound, and then mix the pigment dispersion with the other components. The pigment dispersion may further contain a dispersant.

[0102] The above-mentioned pigment dispersion can be prepared by dispersing a pigment in a polymerizable compound. Pigment dispersion can be performed using, for example, a ball mill, sand mill, attritor, roll mill, agitator, Henschel mixer, colloid mill, ultrasonic homogenizer, pearl mill, wet jet mill, or paint shaker. A dispersant may also be added at this time.

[0103] Furthermore, when using multiple polymerizable compounds, the polymerizable compounds may be mixed first to prepare a polymerizable composition, and then the polymerizable composition, wax, and any other components may be mixed under heating.

[0104] 2. Image forming method Figure 1 is a flowchart showing the image forming method according to this embodiment. The image forming method according to this embodiment includes the steps of ejecting droplets of inkjet ink from an inkjet head and applying them to the surface of a substrate (step S10), and curing the applied inkjet ink droplets by irradiating them with an active ray (step S20).

[0105] 2-1. Process of applying inkjet ink to the substrate (Process S10) In this process, the aforementioned inkjet ink is ejected from the inkjet head and applied to the surface of the substrate (at a position corresponding to the image to be formed).

[0106] The inkjet head can use either an on-demand or continuous ejection method.

[0107] On-demand inkjet heads are Electromechanical conversion methods such as single-cavity type, double-cavity type, bender type, piston type, shear mode type, and sheared wall type, Furthermore, electrical-to-thermal conversion methods such as thermal inkjet type and bubble jet ("bubble jet" is a registered trademark of Canon Inc.) Any of the above is acceptable. Furthermore, the inkjet head can be either a scanning type or a line-type inkjet head.

[0108] Since inkjet ink droplets are ejected in a heated, sol-like state, it is preferable to set the temperature of the inkjet ink during inkjet head loading to be between the gelation temperature of the inkjet ink + 10°C and 30°C. When the temperature of the inkjet ink inside the inkjet head is 10°C or higher than the gelation temperature, a decrease in ejection performance due to ink gelation inside the inkjet head or on the nozzle surface is less likely to occur. On the other hand, when the temperature of the ink inside the inkjet head is 30°C or lower than the gelation temperature, degradation of components due to high temperature is less likely to occur. The gelation temperature of the inkjet ink is measured by the following method: The inkjet ink is heated to 100°C, and while measuring the viscosity using a stress-controlled rheometer, Physica MCR301 (cone plate diameter: 75 mm, cone angle: 1.0°), manufactured by Anton Paar, the ink is cooled to 20°C under conditions of a shear rate of 11.7 (1 / s) and a cooling rate of 0.1°C / s to obtain a viscosity temperature dependence curve. The gelation temperature is defined as the temperature at which the viscosity changes to 200 mPa·s in the viscosity temperature dependence curve.

[0109] The method for heating inkjet ink is not particularly limited. For example, at least one of the ink supply system components of the head carriage, such as the ink tanks, supply pipes, and pre-chamber ink tanks directly in front of the head, as well as filtered piping and piezo heads, can be heated with a panel heater, ribbon heater, and warm water.

[0110] From the viewpoint of further improving recording speed and image quality, the amount of inkjet ink droplets ejected is preferably between 2 pL and 20 pL.

[0111] The base material is not particularly limited and can be ordinary uncoated paper, coated paper, synthetic paper YUPO ("YUPO" is a registered trademark of YUPO Corporation), various plastics used in flexible packaging, and their films. Examples of various plastic films include PP film, PET film, OPS film, OPP film, ONy film, PVC film, PE film, and TAC film. Other plastics that can be used include polycarbonate, (meth)acrylic resin, ABS, polyacetal, PVA, and rubbers.

[0112] The substrate temperature at the time of inkjet ink deposition is preferably 1°C to 25°C lower than the gelation temperature of the inkjet ink, more preferably 5°C to 25°C lower, and even more preferably 10°C to 25°C lower. By setting the substrate temperature within this range, it becomes easier to achieve both the pinning properties of the inkjet ink and the varnishability of the image obtained from the inkjet ink.

[0113] 2-2. Process for curing inkjet ink (Process S20) In this process, the droplets of inkjet ink applied to the substrate in step S10 are irradiated with an active ray to cure the droplets. This forms an image consisting of a cured film of the inkjet ink.

[0114] The active ray can be selected from, for example, electron beams, ultraviolet rays, alpha rays, gamma rays, and X-rays, but ultraviolet rays or electron beams are preferred. The ultraviolet ray is preferably light having a peak wavelength of 360 nm to 410 nm. Furthermore, the ultraviolet ray is preferably irradiated from an LED light source. LEDs emit less radiant heat compared to conventional light sources (e.g., metal halide lamps). Therefore, when irradiated with an active ray, LEDs make it less likely for the ink to melt and less likely to cause uneven gloss.

[0115] 3. The process of applying varnish to the image. In the process of applying varnish to the image layer, varnish is applied to the image formed by the image forming method for aesthetic and protective purposes. If drying is not required with the inkjet ink, varnish can be applied immediately after image formation.

[0116] Varnishing is a technique that involves coating the image surface of a recording with varnish. The purpose of this technique is to give the image surface a glossy finish, thereby enhancing its premium appearance, and to improve its scratch resistance and chemical resistance.

[0117] Commercially available varnishes can be used, including: Digital Printing Varnish PL-LV, KM-EP Varnish KM-2, KM-3, UV Roll Coat Varnish RI-13, RI-13-K2, RI-16, RI-FX-3, RI-XG33, CX-1, CX-2, CX-3, UV Coat Varnish AT-B, UV VECTA Coat Varnish PC-3KW2 (manufactured by T&K TOKA), UV Gloss Varnish ULTRASHEEN UV-9021A (manufactured by ACTEGA), UV Matte Varnish 5070E, Soft Touch ULTRASHEEN UV-XT3037 (manufactured by KUSTOM&GROU), Plus Size (registered trademark) OP-5267, OP-5275 (manufactured by Go-O Chemical Co., Ltd.), FD Clear Coat PC, C-YS (manufactured by Toyo Ink Co., Ltd.), Brightone (registered trademark) TUV (manufactured by Sakata Inx Co., Ltd.), and DC POD Clear UV. Examples include SP-001~003 (manufactured by DIC Corporation).

[0118] In particular, from the standpoint of adhesion, an oil-based varnish is preferable. Specific examples of oil-based varnishes include UV VECTA Coat Varnish PC-3KW2 (manufactured by T&K TOKA), Plus Size (registered trademark) OP-5267, OP-5275 (manufactured by Go-o Chemical Co., Ltd.), and FD Clear Coat C-YS (manufactured by Toyo Ink Co., Ltd.).

[0119] The method of applying the varnish is not particularly limited and may be by inkjet or other methods. Methods other than inkjet include bar coating, spray coating, curtain coating, roll coating, screen printing, offset printing, gravure printing, methods using plates such as letterpress and intaglio plates, and other methods that do not use plates. Among these, screen printing, offset printing, gravure printing, or bar coating are preferred from the viewpoint of ease of work and uniform application.

[0120] From the viewpoint of simplifying the apparatus configuration and reducing the cost of image formation, the method for applying varnish is particularly preferably an inkjet method.

[0121] Furthermore, if the varnish is an active-ray curing type varnish, after applying the varnish to the image layer, the varnish is irradiated with active rays to cure it. From the viewpoint of simplifying the setup of the device and efficiently forming images, it is preferable that the conditions for irradiating the varnish with active rays be the same as the conditions for irradiating the inkjet ink with active rays.

[0122] The varnishing process may be performed using different machines for the image formation process and the varnishing process on the image layer, or it may be performed at different locations on the same machine. A step of transporting the substrate on which the image layer has been formed may be included between the image formation process and the varnishing process on the image layer. From the viewpoint of high speed, the transport speed of the substrate is preferably in the range of 30 to 120 m / min.

[0123] 4. Image forming apparatus The following describes an image forming apparatus 100 capable of performing the image forming method described above.

[0124] Figure 2 is a schematic diagram showing the configuration of the image forming apparatus 100 according to this embodiment. As shown in Figure 2, the image forming apparatus 100 has an inkjet head 110, a transport unit 120, and an irradiation unit 130. In Figure 2, the arrows indicate the transport direction of the substrate.

[0125] The inkjet head 110 has a nozzle surface 113 on which the nozzle 111 ejection ports are located, which faces the transport unit 120 when forming an image, and ejects inkjet ink onto the substrate 200 transported by the transport unit 120. From the viewpoint of improving the ejection performance of the inkjet ink, the inkjet head 110 may have a temperature adjustment means for adjusting the ink temperature to lower the viscosity of the ink. Examples of temperature adjustment means include a panel heater, a ribbon heater, and a heating means using warm water.

[0126] The inkjet head 110 may be a scanning type inkjet head whose width in the direction perpendicular to the substrate transport direction is smaller than the substrate 200, or it may be a line type inkjet head whose width in the direction perpendicular to the substrate transport direction is larger than the substrate 200.

[0127] The nozzle 111 has an outlet on the nozzle surface 113. The number of nozzles 111 can be equal to or greater than the number of inks used for image formation (e.g., 4).

[0128] The transport unit 120 transports the substrate 200 so that, when forming an image, the substrate 200 facing the inkjet head 110 moves directly below the inkjet head 110 in the vertical direction. For example, the transport unit 120 has a drive roller 121, a driven roller 122, and a transport belt 123.

[0129] The irradiation unit 130 irradiates the upper surface of the transport unit 120 with an active ray. This irradiates the inkjet ink droplets that have landed on the transported substrate 200 with the active ray, thereby curing the droplets. The irradiation unit 130 can be positioned downstream of the inkjet head 110 and directly above the transport unit 120.

[0130] In addition to the above configuration, the image forming apparatus 100 may also have an ink tank (not shown) for storing inkjet ink before ejection, and an ink channel (not shown) that allows inkjet ink to flow between the ink tank and the inkjet head 110. Furthermore, the image forming apparatus 100 may also have a control unit (not shown) that controls the operation of the inkjet head 110, the transport unit 120, and the irradiation unit 130.

[0131] The image forming apparatus 100 may also have an intermediate transfer body and a transfer unit (neither of which are shown). In this case, the inkjet head 110 ejects inkjet ink onto the intermediate transfer body, causing it to land on the surface of the intermediate transfer body, and forms an intermediate image on the surface of the intermediate transfer body, which is made up of aggregated droplets of inkjet ink. Subsequently, the transfer unit transfers the intermediate image from the surface of the intermediate transfer body to the surface of the substrate. Then, the irradiation unit 130 irradiates the intermediate image transferred to the surface of the substrate with an active ray to harden the droplets of inkjet ink. [Examples]

[0132] The present invention will be described below with reference to examples. The scope of the present invention is not to be limited by the examples.

[0133] 1. Preparation / Synthesis of Materials The materials used to prepare the inkjet ink are listed below.

[0134] 1-1. Wax Stearyl stearate (melting point: 50.0℃) Cetyl palmitate (melting point: 40.0℃) Behenyl behenate (melting point: 79.0°C) Stearon (melting point: 62.0℃) Behenyl stearate (melting point: 67.0°C) Pentaerythritol tetrastearate (melting point: 64.0℃)

[0135] The melting points of each wax were obtained using a differential scanning calorimeter "Diamond DSC" (manufactured by PerkinElmer). The melting point was measured under the following conditions (heating and cooling conditions): a first heating process where the temperature was raised from room temperature (25°C) to 110°C at a heating rate of 10°C / min and held isothermally at 110°C for 5 minutes; a cooling process where the temperature was cooled from 110°C to 0°C at a cooling rate of 10°C / min and held isothermally at 0°C for 5 minutes; and a second heating process where the temperature was raised from 0°C to 110°C at a heating rate of 10°C / min. The above measurements were performed by sealing 3.0 mg of the sample in an aluminum pan and setting it in the sample holder of the differential scanning calorimeter "Diamond DSC". An empty aluminum pan was used as a reference. In the above measurement, analysis was performed on the endothermic curve obtained during the first heating process, and the top temperature of the endothermic peak (full width at half maximum within 15°C) originating from the crystalline polyester resin was defined as the melting point (Tm) of the wax.

[0136] 1-2.Polymerizable compound 3EO-modified trimethylolpropane triacrylate Tricyclodecane dimethanol dimethacrylate 1,10-Decanediol dimethacrylate Tricyclodecanedimethanol diacrylate Neopentyl glycol diacrylate Lauryl acrylate

[0137] 1-3. Other ingredients 1-3-1. Pigment dispersion 9 parts by mass of pigment dispersant (EFKA-7701, BASF) and 71 parts by mass of tripropylene glycol diacrylate were placed in a stainless steel beaker and heated on a hot plate at 65°C for 1 hour while stirring. After cooling to room temperature, 20 parts by mass of Pigment Black 7 (#52, Mitsubishi Chemical Corporation) were added to the stirred stainless steel beaker, and then the mixture was placed in a glass bottle with 200 g of zirconia beads (0.3 mm in diameter, Nikkatoh Co., Ltd.) and sealed tightly. This pigment-containing solution was dispersed using a paint shaker, and then the zirconia beads were removed to obtain a pigment dispersion. The dispersion treatment time was 4 hours.

[0138] 1-3-2. Polymerization Initiators Polymerization initiator: Omnirad 819 (manufactured by IGM Resins BV)

[0139] 1-3-3. Polymerization inhibitors, surfactants Polymerization inhibitor: Irgasutab UV-10 (manufactured by BASF) Surfactant: KF-352A (manufactured by Shin-Etsu Chemical Co., Ltd.)

[0140] 1-4. Preparation of inkjet inks Wax, polymerizable compound, pigment dispersion, polymerization initiator, and other additives were placed in a stainless steel beaker to obtain the compositions shown in Tables 1 and 2, and stirred at 105°C for 45 minutes. The inkjet inks of Examples 1-10 and Comparative Examples 1-4 were then obtained by filtering through an ADVANTEC Teflon® 3μm membrane filter.

[0141] 1-5. Calculation of HSP distance The HSP values ​​(dispersion term (dD), polarity term (dP), and hydrogen bonding term (dH)) for each wax and polymerizable compound were calculated using the computer software Hansen Solubility Parameters in Practice 5th Edition 5.0.13 (HSPiP, manufactured by Tegara Co., Ltd.) by inputting the chemical structural formula into the software. When multiple polymerizable compounds were included, the parameters (dD, dP, and dH) for each polymerizable compound were multiplied by the molar ratio of each compound in the inkjet ink and added together to obtain the parameters (dD, dP, and dH) for the polymerizable compound mixture.

[0142] The HSP distance between a high-melting-point wax or low-melting-point wax and a polymerizable compound (or a mixture of polymerizable compounds if multiple polymerizable compounds are included) was calculated using the following formula. In the following formula, the dispersion, polarity, and hydrogen bonding terms of one component of each wax and polymerizable compound (or mixture thereof) were denoted as dD, dP, and dH, respectively, while the dispersion, polarity, and hydrogen bonding terms of the other component were denoted as dD', dP', and dH', respectively. HSP distance = (4 × (dD - dD') 2 +(dP-dP') 2 +(dH-dH') 2 ) 1 / 2

[0143] Tables 1 and 2 show the HSP distance between the high-melting-point wax or low-melting-point wax and the polymerizable compound (or mixture thereof) in each inkjet ink.

[0144] 1-6. Gelation temperature For the inkjet inks of Examples 1-10 and Comparative Examples 1-4, the viscosity was measured after heating to 100°C using a stress-controlled rheometer, Physica MCR301 (cone plate diameter: 75 mm, cone angle: 1.0°), manufactured by Anton Paar. The ink was cooled to 20°C under conditions of a shear rate of 11.7 (1 / s) and a cooling rate of 0.1°C / s to obtain a viscosity temperature dependence curve. The gelation temperature was determined as the temperature at which the viscosity reached 200 mPa·s in the viscosity temperature dependence curve. The gelation temperatures for each inkjet ink are shown in Tables 1 and 2.

[0145] 2. Image formation Monochromatic images were formed using a line-type inkjet recording device with one of the inkjet inks from Examples 1-10 or Comparative Examples 1-4. The inkjet head temperature of the inkjet recording device was set to 80°C. A 5cm x 5cm solid image was printed on a substrate (OK Topcoat + 127g, manufactured by Oji Paper Co., Ltd.). After the image was formed, the ink was cured by irradiating the image with ultraviolet light using an LED lamp (Phoseon Technology, 395nm, water-cooled LED) placed downstream of the recording device. A piezo head was used as the ejection head. The ejection conditions were such that the volume of one drop was 9.0 pl, and the liquid velocity was approximately 6 m / s, and the image was recorded at a resolution of 1200 dpi x 1200 dpi. The recording speed was 500 mm / s. Image formation was performed in an environment of 23°C and 55% RH, and the substrate temperature at the time of inkjet ink impact was adjusted to 40°C. dpi refers to the number of dots per inch (2.54 cm).

[0146] 3. Evaluation 3-1. Injection properties Inkjet recording devices were used to eject ink, and the presence or absence of nozzle defects and misaligned ejection was visually observed. Each inkjet ink was evaluated according to the following criteria. A score of 2 or higher was considered to be at a level suitable for practical use.

[0147] 4. No nozzle failures were observed at all. 3: Of the 1024 nozzles tested, 1 to 5 nozzles were found to be missing. 2: Out of a total of 1024 nozzles, nozzle defects were found in 6 to 9 nozzles. 1: Nozzle defects were found in 10 or more nozzles out of a total of 1024 nozzles.

[0148] 3-2. Pinning properties Dots were printed using the image forming method described above, with the inkjet inks of Examples 1-11 and Comparative Examples 1-4, respectively. When the 400 formed dots were observed using an optical microscope, it was confirmed that the active-ray polymerizable compound seeped out from the outer edge of the dots. Here, the longest distance from the center of the dot to the outer edge of the area where the active-ray polymerizable compound seeped out was defined as the "outer diameter" of the dot, and the distance from the center of the dot to the outer edge of the dot body (the part where the active-ray polymerizable compound begins to seep out) was defined as the "inner diameter" of the dot. The percentage of dots in which the difference between the outer diameter and the inner diameter was 10% or more relative to the inner diameter was determined, and the pinning performance was evaluated according to the following evaluation criteria. A score of 2 or higher was considered to be a level that is practically usable.

[0149] 4: The above quantity percentage was between 0 and 5. 3: The above percentage was between 6 and 10 percent. 2: The above percentage was between 11 and 20 percent. 1: The above percentage of droplets was 21% or more, confirming that adjacent droplets had merged.

[0150] 3-3. Varnish suitability For the solid image obtained by the image formation method described above, DC POD Clear UV SP-001 (manufactured by DIC Corporation) was applied as a varnish to a thickness of 10 μm using a wire bar. The resulting coating was exposed and cured using a conveyor-equipped UV irradiator (manufactured by Iwasaki Electric Co., Ltd.) (output 120 W / cm, cold mirror focusing type, irradiation distance 100 mm, conveyor line speed 15 m / min, maximum illuminance 220 mW / cm). 2 , light intensity 300mJ / cm 2The following procedure was performed. The resulting solid images after varnish application were magnified 100 times, and the presence of varnish repellency was checked and evaluated according to the following criteria. A score of 2 or higher was considered to be a level that is practically usable.

[0151] 4: No pinholes or streaks are present in the image. 3: The area ratio of pinholes to the image is less than 1%. 2: The area ratio of pinholes to the image is 1% or more and less than 10%. 1: The area ratio of the pinhole to the image is 10% or more.

[0152] The evaluation results are shown in Tables 1 and 2.

[0153] [Table 1]

[0154] [Table 2]

[0155] As shown in Tables 1 and 2, by using two or more of the above-mentioned waxes and satisfying requirements 1 to 3, it was possible to achieve both the pinning properties of the inkjet ink and the varnishability of the formed image. [Industrial applicability]

[0156] The inkjet ink of the present invention significantly enhances the pinning properties of the inkjet ink and improves the varnishability of the formed image. Therefore, the present invention is useful in the field of image formation. [Explanation of Symbols]

[0157] 100 Image forming apparatus 110 Inkjet Heads 120 Conveying section 130 Irradiation area

Claims

1. An inkjet ink that hardens upon irradiation with an active ray, comprising two or more types of wax and a polymerizable compound, The total mass of the two or more waxes is 1.0% to 10.0% by mass relative to the total mass of the inkjet inks. In the case where, among the two or more types of wax mentioned above, the wax with the highest mass content and the wax with the second highest mass content are designated as the high-melting-point wax and the wax with the low-melting-point wax, The melting point of the high-melting-point wax is 5°C or more higher than the melting point of the low-melting-point wax. The mass content of the high-melting-point wax is 1.0% to 10.0% by mass relative to the mass content of the low-melting-point wax. Inkjet ink.

2. The HSP distance between the high-melting-point wax and the polymerizable compound, and the HSP distance between the low-melting-point wax and the polymerizable compound, are both within the range of 2.5 to 6.

5. The inkjet ink according to claim 1.

3. The melting point of the high-melting-point wax is 10°C or more higher than the melting point of the low-melting-point wax. The mass content of the high-melting-point wax is 2.0% to 6.0% by mass relative to the mass content of the low-melting-point wax. The HSP distance between the high-melting-point wax and the polymerizable compound, and the HSP distance between the low-melting-point wax and the polymerizable compound are 3.5 to 5.

5. The inkjet ink according to claim 2.

4. A step of ejecting the inkjet ink described in any one of claims 1 to 3 from an inkjet head and depositing it on the surface of a substrate, The process involves irradiating the inkjet ink that has been deposited with an active ray to cure the inkjet ink, An image forming method including, The substrate temperature at the time of inkjet ink deposition shall be 5°C to 25°C lower than the gelation temperature of the inkjet ink. Image forming method.

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