Patent Description:
Photopolymerizable monomers used in photopolymerizable inkjet inks are, for example, (meth)acrylic acid ester compounds, acryl amide compounds, cation polymerizable oxetane compounds and epoxy compounds. Among them, preferred are (meth)acrylic acid ester compounds and acryl amide compounds since they are available in large amounts, inexpensive, easily procurable, and capable of being used in combination with a radical polymerizable photopolymerization initiator that is inexpensive and commonly used among polymerization initiators.

However, many of (meth)acrylic acid ester compounds and acryl amide compounds have toxicity. Although some of them are relatively mild in oral toxicity and skin irritation, those with low viscosity used in inkjet inks are not safe materials in skin sensitization where they come into contact with the skin to cause allergy.

The present inventor conducted extensive studies and previously found some materials safe in skin sensitization. As a result of further studies, the present inventor previously found that there were some (meth)acrylic acid ester compounds and acryl amide compounds usable as photopolymerizable monomers and having a Stimulation Index (SI value) of less than <NUM> where the Stimulation Index indicates the extent of sensitization as measured by a skin sensitization test based on the LLNA (Local Lymph Node Assay). However, when those compounds are formulated so as to attain practically usable curing property, the curing property of the resultant inks becomes inferior to those of commonly used photopolymerizable inkjet inks. Thus, a photoradical polymerization initiator has to be incorporated in a larger amount than usual. Such a large amount of the photoradical polymerization initiator incorporated was found to prevent the resultant inks from showing sufficient storage stability in high-temperature environments such as summer.

Also, the present inventor previously found a photopolymerizable inkjet ink which is negative for skin sensitization, improves the coated film in curing property, enables possible curing failures in the coated film to be visually confirmed, and facilitates quality control in the production process. Specifically, the photopolymerizable inkjet ink contains: one or more types of photopolymerizable monomers each having a SI value of less than <NUM>; a self-cleaving photopolymerization initiator; a hydrogen-abstracting polymerization initiator; and an amine compound serving as a polymerization accelerator. However, even this photopolymerizable inkjet ink is difficult to attain sufficient storage stability as described above.

As has widely been known, the storage stability of the photopolymerizable inkjet ink can be improved by the addition of a material called a polymerization inhibitor. Examples of known useful polymerization inhibitors include phenol compounds and quinone compounds such as methoquinone and benzoquinone, and aromatic secondary amine compounds such as diphenyl amine and phenothiazine (see, for example, PTLs <NUM> and <NUM>; reference may also be made to inks disclosed in PTLs <NUM> to <NUM>).

However, even when such a known polymerization inhibitor as methoquinone is added to the above newly found compound, it is not possible to prevent the resultant inks from thickening or solidification resulting from polymerization. In addition, it was found that aromatic secondary amine compounds such as phenothiazine cannot be used due to their considerable coloring properties, although they have higher polymerization inhibiting effects
Furthermore, many of the photopolymerizable monomers having a SI value of less than <NUM> have high viscosity as inkjet inks, and the addition of a polymerization initiator increases the inks in viscosity. Thus, it is important to take proper measures to reduce the inks in viscosity. Here, it is easy to reduce their viscosity by incorporating a diluent solvent thereinto. However, use of the diluent solvent is not desirable since it volatizes to be released to the air to give adverse effects to the environment. Therefore, the incorporation of a solvent into inks should be avoided. Alternatively, water may be incorporated into inks containing water-soluble monomers to reduce their viscosity. In this case, when non-permeable base materials such as plastic materials are used, it may not possible to obtain the effect that water penetrates into the non-permeable base materials to lead to drying. Thus, in an attempt to attain high-speed printing process, it is necessary to volatize water instantly for drying. In order to do so, a heat source must be provided in many cases, which is not preferred in terms of energy saving.

An object of the present invention is to provide a photopolymerizable inkjet ink that is safe in skin sensitization and good in storage stability at high temperatures.

Means for solving the problems are as follows. Specifically, a photopolymerizable inkjet ink of the present invention contains: photopolymerizable monomer(s) consisting of one or more photopolymerizable monomers each having a Stimulation Index (SI value) of less than <NUM>, where the Stimulation Index indicates the extent of sensitization as measured by a skin sensitization test (LLNA); and a phenol aromatic compound having two hydroxyl groups in the molecule thereof, wherein the photopolymerizable monomer is at least one of polyethylene glycol dimethacrylate represented by the following General Formula (<NUM>) where n is nearly equal to <NUM>, <NUM> or <NUM>, CH<NUM>=CH-CO-(OC<NUM>H<NUM>)n-OCOCH=CH<NUM> where n is nearly equal to <NUM>, γ-butyrolactone methacrylate, trimethylolpropane trimethacrylate, tricyclodecanedimethanol dimethacrylate, caprolactone-modified dipentaerythritol hexaacrylate, polypropylene glycol diacrylate [CH<NUM>=CH-CO-(OC<NUM>H<NUM>)n'OCOCH=CH<NUM>, where n is nearly equal to <NUM>], diacrylates of caprolactone-modified neopentylglycol hydroxypivalate, polyethoxylated tetramethylol methane tetraacrylate, ethylene oxide-modified bisphenol A diacrylate, neopentyl glycol dimethacrylate, stearyl acrylate, <NUM>,<NUM>-butanediol dimethacrylate, hydroxyethyl acrylamide, acryloylmorpholine, t-butyl methacrylate, n-pentyl methacrylate, n-hexyl methacrylate, ethylene oxide-modified phenol acrylate, isostearyl acrylate, ethylene oxide-modified trimethylolpropane trimethacrylate, stearyl methacrylate, glycerin dimethacrylate, triethylene glycol divinyl ether and isobutyl vinyl ether
<CHM>
General Formula (<NUM>), as defined in claims <NUM> to <NUM>.

Also provided according to the present invention are an ink cartridge as defined in claim <NUM>, and a printer as defined in claim <NUM>.

The present invention can provide a photopolymerizable inkjet ink that is safe in skin sensitization and good in storage stability at high temperatures.

A photopolymerizable inkjet ink of the present invention contains: photopolymerizable monomer(s) consisting of one or more photopolymerizable monomers each having a Stimulation Index (SI value) of less than <NUM>, where the Stimulation Index indicates the extent of sensitization as measured by a skin sensitization test (LLNA); and a phenol aromatic compound having two hydroxyl groups in the molecule thereof, wherein the photopolymerizable monomer is as defined above. The photopolymerizable inkjet ink preferably contains at least one selected from the group consisting of a photopolymerization initiator and an amine compound serving as a polymerization accelerator; and, if necessary, further contains other ingredients.

The present inventor has found that by incorporating the phenol aromatic compound having two hydroxyl groups in the molecule thereof into a photopolymerizable inkjet ink (hereinafter may be referred to as "ink") containing a safe photopolymerizable monomer having a SI value of less than <NUM> where the SI value indicates the extent of sensitivity as measured by the skin sensitization test (LLNA), the resultant ink can show good storage stability even at high temperatures.

Notably, the "LLNA" is the skin sensitization test defined as OECD test guideline <NUM>. As described in literatures (for example, "<NPL>), the compound having a Stimulation Index (SI value) of less than <NUM>, where the Stimulation Index indicates the extent of skin sensitization, is judged as being negative for skin sensitization. Also, the compounds evaluated as "negative for skin sensitization" or "no skin sensitization" in their MSDS (Material Safety Data Sheet) and/or literatures (for example, van der<NPL>) each have the above SI value which is less than <NUM>, and therefore they are encompassed by the present invention. The lower SI value means lower skin sensitization. Thus, in the present invention, a monomer or an oligomer having lower SI value is preferably used. The SI value of the monomer or the oligomer used is preferably <NUM> or lower, more preferably <NUM> or lower.

The phenol aromatic compound having two hydroxyl groups in the molecule thereof is not particularly limited and may be appropriately selected depending on the intended purpose. Examples thereof include hydroquinone, methylhydroquinone, tert-butylhydroquinone, <NUM>,<NUM>-di-tert-butylhydroquinone, <NUM>,<NUM>'-methylenebis(<NUM>-methyl-<NUM>-tert-butylphenol), <NUM>,<NUM>'-methylenebis(<NUM>-ethyl-<NUM>-tert-butylphenol) and <NUM>,<NUM>'-methylenebis[<NUM>-(<NUM>-methylcyclohexyl-p-cresol)]. These may be used alone or in combination.

Among them, hydroquinone, methylhydroquinone, tert-butylhydroquinone and <NUM>,<NUM>-di-tert-butylhydroquinone are preferred since they are particularly excellent in storage stability at high temperatures.

A proper amount of the phenol aromatic compound having two hydroxyl groups in the molecule thereof varies with properties required for inks depending on their applications. When the amount of the phenol aromatic compound having two hydroxyl groups in the molecule thereof is too small, satisfactory effects cannot be obtained in some cases, whereas when it is too large, the curing property of the resultant ink may be degraded. Thus, the amount of the phenol aromatic compound having two hydroxyl groups in the molecule thereof contained in the ink is preferably <NUM> parts by mass to <NUM> part by mass per <NUM> parts by mass of the photopolymerizable monomer. However, the amount of the phenol aromatic compound having two hydroxyl groups in the molecule thereof is necessarily limited to this range. In particular, considering a generally known mechanism of showing storage stability in which the phenol aromatic compound having hydroxyl groups in the molecule thereof traps radicals generated in inks to form stable radicals, the amount of the phenol aromatic compound having two hydroxyl groups in the molecule thereof is preferably larger for improving storage stability. However, a larger amount of the phenol aromatic compound having two hydroxyl groups in the molecule thereof is not preferred for obtaining sufficient curing property. Therefore, the amount of the phenol aromatic compound having two hydroxyl groups in the molecule thereof should appropriately be determined considering required curing property.

The photopolymerizable monomer involving no problems in skin sensitization, inexpensive and easily procurable is as defined above but otherwise not particularly limited and may be appropriately selected depending on the intended purpose. Examples thereof include polyethylene glycol dimethacrylates represented by the following General Formula (<NUM>) where n is nearly equal to <NUM>, <NUM> or <NUM>, γ-butyrolactone methacrylate, trimethylolpropane trimethacrylate, tricyclodecanedimethanol dimethacrylate, caprolactone-modified dipentaerythritol hexaacrylate, polypropylene glycol diacrylate [CH<NUM>=CH-CO-(OC<NUM>H<NUM>)n-OCOCH=CH<NUM> (n is nearly equal to <NUM>)], diacrylates of caprolactone-modified neopentylglycol hydroxypivalate, polyethoxylated tetramethylol methane tetraacrylate, ethylene oxide-modified bisphenol A diacrylate, neopentyl glycol dimethacrylate, stearyl acrylate, <NUM>,<NUM>-butanediol dimethacrylate, hydroxyethyl acrylamide, acryloylmorpholine, t-butyl methacrylate, n-pentyl methacrylate, n-hexyl methacrylate, ethylene oxide-modified phenol acrylate, isostearyl acrylate, ethylene oxide-modified trimethylolpropane trimethacrylate, stearyl methacrylate, glycerin dimethacrylate, triethylene glycol divinyl ether and isobutyl divinyl ether. These may be used alone or in combination.

Among them, preferred are polyethylene glycol dimethacrylates represented by the following General Formula (<NUM>) where n is nearly equal to <NUM>, <NUM> or <NUM>, γ-butyrolactone methacrylate, trimethylolpropane trimethacrylate, tricyclodecanedimethanol dimethacrylate, caprolactone-modified dipentaerythritol hexaacrylate, polypropylene glycol diacrylate [CH<NUM>=CH-CO-(OC<NUM>H<NUM>)n-OCOCH=CH<NUM> (n is nearly equal to <NUM>)], diacrylates of caprolactone-modified neopentylglycol hydroxypivalate, polyethoxylated tetramethylol methane tetraacrylate, ethylene oxide-modified bisphenol A diacrylate, neopentyl glycol dimethacrylate, steary acrylate, <NUM>,<NUM>-butanediol dimethacrylate, hydroxyethyl acrylamide, t-butyl methacrylate, n-pentyl methacrylate, n-hexyl methacrylate, ethylene oxide-modified phenol acrylate, isostearyl acrylate, ethylene oxide-modified trimethylolpropane trimethacrylate, stearyl methacrylate and glycerin dimethacrylate, since they are excellent in storage stability at high temperatures.

In General Formula (<NUM>), n is nearly equal to <NUM>, <NUM> or <NUM>.

Notably, when using a mixture containing two or more polyethylene glycol dimethacrylates each represented by General Formula (<NUM>), the average of the numbers indicated by "n," which are calculated through analysis such as measurement of their molecular weights, preferably falls within the range of <NUM> to <NUM>.

Also, other (meth)acrylates, (meth)acryl amides and compounds may be used in combination, so long as the amount of them falls within such a range that does not raise any problems as inks even if they somewhat cause problems in skin sensitization when used alone or they have not been confirmed for skin sensitization.

Examples thereof include ethylene glycol di(meth)acrylate, neopentylglycol hydroxypivalate di(meth)acrylate, γ-butyrolactone acrylate, isobornyl acrylate, formalized trimethylolpropane mono(meth)acrylate, polytetramethylene glycol di(meth)acrylate, trimethylolpropane (meth)acrylate benzoate, diethylene glycol diacrylate, triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, polyethylene glycol diacrylates [CH<NUM>=CH-CO-(OC<NUM>H<NUM>)n-OCOCH=CH<NUM> where n is nearly equal to <NUM>], [CH<NUM>=CH-CO-(OC<NUM>H<NUM>)n-OCOCH=CH<NUM> where n is nearly equal to <NUM>], [CH<NUM>=CH-CO-(OC<NUM>H<NUM>)n-OCOCH=CH<NUM> where n is nearly equal to <NUM>], and [CH<NUM>=CH-CO-(OC<NUM>H<NUM>)n-OCOCH=CH<NUM> where n is nearly equal to <NUM>], dipropylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, polypropylene glycol dimethacrylate [CH<NUM>=C(CH<NUM>)-CO-(OC<NUM>H<NUM>)n-OCOC(CH<NUM>)=CH<NUM> where n is nearly equal to <NUM>)], <NUM>,<NUM>-butandiol diacrylate, <NUM>,<NUM>-butandiol diacrylate, <NUM>,<NUM>-hexanediol di(meth)acrylate, <NUM>,<NUM>-nonanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, tricyclodecanedimethanol diacrylate, di(meth)acrylate of bisphenol A propylene oxide adduct, polyethylene glycol di(meth)acrylate, dipentaerythritol hexa(meth)acrylate, methacryloyl morpholine, <NUM>-hydroxypropyl methacrylamide, ethylene oxide-modified tetramethylolmethane tetramethacrylate, dipentaerythritol hydroxypenta(meth)acrylate, caprolactone-modified dipentaerythritol hydroxypenta(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, pentaerythritol tetra(meth)acrylate, trimethylolpropane triacrylate, ethylene oxide-modified trimethylolpropane triacrylate, propylene oxide-modified trimethylolpropane triacrylate, caprolactone-modified trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, tris(<NUM>-hydroxyethyl)isocyanurate tri(meth)acrylate, neopentyl glycol diacrylate, ethoxylated neopentyl glycol di(meth)acrylate, propoxylated neopentyl glycol di(meth)acrylate, propoxylated glyceryl tri(meth)acrylate, polyester di(meth)acrylate, polyester tri(meth)acrylate, polyester tetra(meth)acrylate, polyester penta(meth)acrylate, polyester poly(meth)acrylate, vinylcaprolactam, vinylpyrrolidone, N-vinylformamide, polyurethane di(meth)acrylate, polyurethane tri(meth)acrylate, polyurethane tetra(meth)acrylate, polyurethane penta(meth)acrylate, polyurethane poly(meth)acrylate, N-dimethylaminoethyl acrylamide, N-isopropyl acrylamide, N-dimethyl acrylamide, N-diethyl acrylamide and N-dimethylaminopropyl acrylamide.

The amount of these photopolymerizable monomers contained in the ink is not particularly limited and may be appropriately selected depending on the intended purpose. It is preferably <NUM>% by mass to <NUM>% by mass, more preferably <NUM>% by mass to <NUM>% by mass. Although incorporating a colorant and other additives into the ink can provide the ink with various functions, the polymerization reaction of the monomers may be inhibited depending on the amount thereof. Thus, it is not preferred to incorporate an unnecessarily excessive amount of a colorant and other additives.

The ink may further contain a photopolymerization initiator. The photopolymerization initiator is preferably a self-cleaving photopolymerization initiator or a hydrogen-abstracting photopolymerization initiator. The photopolymerization initiator used is preferably negative for skin sensitization similar to the photopolymerizable monomers. Also, the following compounds, which somewhat sensitize the skin when used alone or which have not been confirmed for skin sensitization, would be used so long as the amount of them falls within such a range that does not raise any problems as inks.

The self-cleaving photopolymerization initiator is not particularly limited and may be appropriately selected depending on the intended purpose. Examples thereof include <NUM>,<NUM>-dimethoxy-<NUM>,<NUM>-diphenylethan-<NUM>-one, <NUM>-hydroxycyclohexyl phenyl ketone, <NUM>-hydroxy-<NUM>-methyl-<NUM>-phenylpropan-<NUM>-one, <NUM>-[<NUM>-(<NUM>-hydroxyethoxyl)-phenyl]-<NUM>-hydroxy-<NUM>-methyl-<NUM>-propan-<NUM>-one, <NUM>-hydroxy-<NUM>-{<NUM>-[<NUM>-(<NUM>-hydroxy-<NUM>-methylpropionyl)benzyl]phenyl}-<NUM>-methyl-<NUM>-propan-<NUM>-one, phenylglyoxylic acid methyl ester, <NUM>-methyl-<NUM>-[<NUM>-(methylthio)phenyl]-<NUM>-morpholinopropan-<NUM>-one, <NUM>-benzyl-<NUM>-dimethylamino-<NUM>-(<NUM>-morpholinophenyl)butanone-<NUM>,<NUM>-dimethylamino-<NUM>-(<NUM>-methyl-benzyl)-<NUM>-(<NUM>-morpholin-<NUM>-yl-phenyl)butan-<NUM>-one, bis(<NUM>,<NUM>,<NUM>-trimethylbenzoyl)phenylphosphine oxide, bis(<NUM>,<NUM>-dimethoxybenzolyl)-<NUM>,<NUM>,<NUM>-trimethyl-pentylphosphine oxide, <NUM>,<NUM>,<NUM>-trimethylbenzoylphosphine oxide, <NUM>,<NUM>-octanedion-[<NUM>-(phenylthio)-<NUM>-(o-benzoyloxime)], ethanone-<NUM>-[<NUM>-ethyl-<NUM>-(<NUM>-methylbenzoyl)-<NUM>-carbazol-<NUM>-yl]-<NUM>-(O-acetyloxime) and [<NUM>-(methylphenylthio)phenyl]phenylmethanone. These may be used alone or in combination.

In particular, <NUM>-dimethylamino-<NUM>-(<NUM>-methylbenzyl)-<NUM>-(<NUM>-morpholin-<NUM>-yl-phenyl)-butan-<NUM>-one is preferred since it exhibits good curing performance. Also, <NUM>-methyl-<NUM>-[<NUM>-(methylthio)phenyl]-<NUM>-morpholinopropan-<NUM>-one is preferred since it is inexpensive.

The hydrogen-abstracting photopolymerization initiator is not particularly limited and may be appropriately selected depending on the intended purpose. Examples thereof include: thioxanthone compounds such as <NUM>,<NUM>-diethylthioxanthone, <NUM>-chlorothioxanthone, isopropylthioxanthone and <NUM>-chloro-<NUM>-propylthioxanthone; and benzophenone compounds such as benzophenone, methylbenzophenone, methyl-<NUM>-benzoylbenzoate, <NUM>-benzoyl-<NUM>'-methyldiphenyl sulfide and phenylbenzophenone. These may be used alone or in combination.

With the increased requirements for awareness to envitonmental protection, the recent interest has focused on LED light sources as light sources for curing photopolymerizable inks since the LED light sources exhibit higher energy efficiency than conventional mercury lamps and metal halide lamps and realize energy saving. However, the wavelength of the LED light sources free from practical problems is <NUM> or greater. Thus, when the current LED light sources are used, it is difficult to expect benzophenone compounds to satisfactorily serve as an initiator, judging from their specific absorption spectra. For this reason, thioxanthone compounds are more suitable. Among them, chlorine-free <NUM>,<NUM>-diethylthioxanthone and isopropylthioxanthone are preferred from the viewpoint of environmental protection.

The polymerization accelerator is not particularly limited, so long as it is an amine compound, and may be appropriately selected depending on the intended purpose. Examples thereof include benzoic acid ester compounds containing an N,N-dimethylamino group, such as ethyl p-dimethylaminobenzoate, <NUM>-ethylhexyl p-dimethylaminobenzoate, methyl p-dimethylaminobenzoate, <NUM>-dimethylaminoethyl benzoate and butoxyethyl p-dimethylaminobenzoate. These may be used alone or in combination.

Among them, ethyl p-dimethylaminobenzoate is preferred since it is inexpensive, easily procurable and has the highest viscosity-reducing effect.

The amine compound serves as a source for supplying hydrogen to the hydrogen-abstracting photopolymerization initiator. In particular, a benzoic acid ester compound having a N,N-dimethylamino group can more efficiently accelerate the curing reaction.

When the amount of the photopolymerization initiators (the self-cleaving photopolymerization initiator and/or the hydrogen-abstracting photopolymerization initiator) and the polymerization accelerator is too small, the photopolymerization reaction hardly proceeds whereby sufficient curing cannot be attained in some cases. When it is too large, the polymerization reaction proceeds, but the polymerization degree does not increase so that the cured product may be brittle; or the photopolymerization initiator may excessively increase the viscosity of the ink to cause failures in inkjet ejection.

Therefore, the total amount of the photopolymerization initiators and the polymerization accelerator is preferably <NUM> part by mass to <NUM> parts by mass, particularly preferably <NUM> parts by mass to <NUM> parts by mass, per <NUM> parts by mass of the photopolymerizable monomer. When high-energy light sources such as α rays, β rays, γ rays, X rays or electron beams are used, the polymerization reaction proceeds without photopolymerization initiator or polymerization accelerator. This is a widely known matter, and these high-energy light sources have not been commonly used yet since they require high cost for their safety and for maintenance of necessary facilities. Thus, detail explanation therefor is not given here.

The other ingredients are not particularly limited and may be appropriately selected depending on the intended purpose. Examples thereof include colorants; higher-fatty-acid esters having, for example, a polyether, an amino group, a carboxyl group, and/or a hydroxyl group in the side chain or the end thereof; polydimethylsiloxane compounds having, for example, a polyether, an amino group, a carboxyl group, and/or a hydroxyl group in the side chain or the end thereof; surfactants such as fluoroalkyl compounds having, for example, a polyether, an amino group, a carboxyl group, and/or a hydroxyl group; and polar group-containing polymeric pigment dispersing agents. These may be used alone or in combination.

The colorant of the ink is not particularly limited and may be appropriately selected from known inorganic pigments and organic pigments considering, for example, physical properties of the ink.

As for black pigments, those such as carbon black produced by the furnace method or the channel method can be used. These may be used alone or in combination.

As for yellow pigments, for example, the following Pig. Yellow series pigments can be used: Pig. Yellow <NUM>, Pig. Yellow <NUM>, Pig. Yellow <NUM>, Pig. Yellow <NUM>, Pig. Yellow <NUM>, Pig. Yellow <NUM>, Pig. Yellow <NUM>, Pig. Yellow <NUM>, Pig. Yellow <NUM>, Pig. Yellow <NUM>, Pig. Yellow <NUM>, Pig. Yellow <NUM>, Pig. Yellow <NUM>, Pig. Yellow <NUM>, Pig. Yellow <NUM>, Pig. Yellow <NUM>, Pig. Yellow <NUM>, Pig. Yellow <NUM>, Pig. Yellow <NUM>, Pig. Yellow <NUM>, Pig. Yellow <NUM>, Pig. Yellow <NUM>, Pig. Yellow <NUM>, Pig. Yellow <NUM>, Pig. Yellow <NUM>, and Pig. Yellow <NUM>. These may be used alone or in combination.

As for magenta pigments, for example, the following Pig. Red series pigments can be used: Pig. Red <NUM>, Pig. Red <NUM>, Pig. Red <NUM>, Pig. Red <NUM> (Ca), Pig. Red <NUM> (Mn), Pig. Red <NUM> (Ca), Pig. Red <NUM>:<NUM>, Pig. Red <NUM>, Pig. Red <NUM>, Pig. Red <NUM>, Pig. Red <NUM>, Pig. Red <NUM>, Pig. Red <NUM>, and Pig. Violet <NUM>. These may be used alone or in combination.

As for cyan pigments, for example, the following Pig. Blue series pigments can be used: Pig. Blue <NUM>, Pig. Blue <NUM>, Pig. Blue <NUM>, Pig. Blue <NUM>, Pig. Blue <NUM>:<NUM>, Pig. Blue <NUM>:<NUM>, Pig. Blue <NUM>, Pig. Blue <NUM>, Pig. Blue <NUM>, Vat Blue <NUM>, and Vat Blue <NUM>. These may be used alone or in combination.

As for white pigments, for example, sulfuric acid salts of alkaline earth metals such as barium sulfate, carbonic acid salts of alkaline earth metals such as calcium carbonate, silica such as fine silicic acid powder and synthetic silicic acid salts, calcium silicate, alumina, alumina hydrate, titanium oxide, zinc oxide, talc and clay. These may be used alone or in combination.

An inkjet cartridge of the present invention includes at least the above-described photopolymerizable inkjet ink of the present invention.

In other words, the ink of the present invention can suitably be used as an ink cartridge where the ink is housed in a container. With this form, users do not have to directly touch the ink during works such as exchange of the ink, and thus they are not concerned with staining of their fingers, hands or clothes. In addition, it is possible to prevent interfusion of foreign matter such as dust into the ink.

The container is not particularly limited, and the shape, structure, size and material thereof may be appropriately selected depending on the intended purpose. For example, the container is preferably selected from those having an ink bag formed of an aluminum laminate film, or a resin film.

The ink cartridge will be described referring to <FIG> schematically illustrates an example of an ink bag <NUM> of an ink cartridge. <FIG> schematically illustrates an ink cartridge <NUM> containing the ink bag <NUM> illustrated in <FIG> and a cartridge case <NUM> which houses the ink bag <NUM>.

As illustrated in <FIG>, the ink bag <NUM> is filled with the ink by injecting the ink from an ink inlet <NUM>. After removal of air present inside the ink bag <NUM>, the ink inlet <NUM> is sealed by fusion bonding. At the time of use, a needle attached to the main body of the device is inserted into an ink outlet <NUM> formed of a rubber member to supply the ink to the device therethrough. The ink bag <NUM> is formed of a wrapping member such as an air non-permeable aluminum laminate film. As illustrated in <FIG>, the ink bag <NUM> is typically housed in a plastic cartridge case <NUM>, which is then detachably mounted in use to various inkjet recording devices as the ink cartridge <NUM>.

The ink cartridge of the present invention is preferably detachably mounted to inkjet recording devices (e.g., a printer). The ink cartridge can simplify the refill and exchange of the ink to improve workability.

A printer of the present invention includes at least the above-described ink cartridge of the present invention mounted thereto.

Here, <FIG> schematically illustrates one exemplary printer.

The printer illustrated in <FIG> forms a color image as follows. Specifically, printing units <NUM> (i.e., printing units 3a, 3b, 3c and 3d having ink cartridges 200a, 200b, 200c and 200d for respective colors (e.g., yellow, magenta, cyan and black)) eject color inks (yellow, magenta, cyan and black) on a base material to be printed <NUM> (which is conveyed from left to right in <FIG>) fed from a base material feed roller <NUM>, and light (UV rays) is applied from UV light sources (curing light sources) 4a, 4b, 4c and 4d to the corresponding color inks for curing.

The base material <NUM> used is, for example, paper, a film, a metal or a composite material thereof. The base material <NUM> illustrated in <FIG> is a roll but may be a sheet. In addition, the base material may be subjected to double-side printing as well as single-side printing.

When UV rays are applied to each of the color inks for every printing process, the color inks are satisfactorily cured. In order to achieve high-speed printing, the UV light sources 4a, 4b and 4c may be lowered in output power or may be omitted, so that the UV light source 4d is made to apply a sufficient dose of UV rays to a composite printed image formed of a plurality of colors. By doing so, energy saving and cost reduction can also be realized.

In <FIG>, reference numeral <NUM> denotes a processing unit and reference numeral <NUM> denotes a wind-up roll for printed products.

The present invention will next be described in more detail by way of Examples and Comparative Examples. However, the present invention should not be construed as being limited to the Examples.

Mixtures of Preparation Examples <NUM> to <NUM> were prepared by mixing the following photopolymerizable monomers A1 to A23 (i.e., (meth)acrylic acid ester compounds or acrylamide compounds each being negative for skin sensitization or having a SI value of less than <NUM>) with at least one of the following self-cleaving photopolymerization initiators B1-<NUM> and B1-<NUM>, the following hydrogen-abstracting photopolymerization initiator B2-<NUM> and B2-<NUM>, and the following polymerization accelerator B3 in the compositions and the amounts (parts by mass) shown in Tables <NUM> to <NUM>. Then, <NUM> parts by mass of each of the mixtures was mixed with <NUM> parts by mass of each of the following phenol aromatic compounds C1 to C4 having two hydroxyl groups in the molecule thereof shown in Tables <NUM>-<NUM> to <NUM>-<NUM>, to thereby produce inks (photopolymerizable inkjet inks) of Examples <NUM> to <NUM>.

Notably, the units of the amounts of the photopolymerizable monomers, the photopolymerization initiators, and the phenol aromatic compound having two hydroxyl groups in the molecule thereof shown in Tables <NUM> to <NUM> are "parts by mass.

Inks of Comparative Examples <NUM> to <NUM> were produced in the same manner as in each of Examples <NUM> to <NUM>, except that <NUM> parts by mass of the phenol aromatic compound C1, C2, C3 or C4 having two hydroxyl groups in the molecule thereof was changed to <NUM> parts by mass of each of the following phenol aromatic compounds C5 and C6 having one hydroxyl group in the molecule thereof as shown in Tables <NUM>-<NUM> to <NUM>-<NUM>.

Notably, the units of the amounts of the photopolymerizable monomers, the photopolymerization initiators, and the phenol aromatic compound having one hydroxyl group in the molecule thereof shown in Tables <NUM> to <NUM> are "parts by mass.

The following compounds were used as A1 to A23 (photopolymerizable monomers), B1-<NUM> to B2-<NUM> (photopolymerization initiators), B3 (polymerization accelerator), C1 to C4 (phenol aromatic compounds having two hydroxyl groups in the molecule thereof) and C5 and C6 (phenol aromatic compounds having one hydroxyl group in the molecule thereof) shown in Tables <NUM> to <NUM>-<NUM>.

The value in parentheses after each of A1 to A23 is "SI value" and the description "negative" or "none" after each of A1 to A23 means that the compound is evaluated as "negative for skin sensitization" or "no skin sensitization" in the MSDS (Material Safety Data Sheet) or literatures (for example, van der <NPL>). The description "negative" or "none" is equivalent to the SI value of less than <NUM>.

Each of the inks of Examples <NUM> to <NUM> and Comparative Examples <NUM> to <NUM> was placed in a container, which was then hermetically sealed in the dark and left to stand at <NUM> for <NUM> week. In the below-described manner, the ink was evaluated for "rate of change in viscosity at <NUM>" and "inkjetting property. " Notably, since evaluating the storage stability of an ink at ordinary temperature requires considerably long-term observation and complicated evaluation process, the inks were subjected to the acceleration test at <NUM>.

In addition, the mixtures of Preparation Examples <NUM> to <NUM> and the inks of Examples <NUM> to <NUM> and Comparative Examples <NUM> to <NUM> were subjected to "measurement of curing light dose" and "observation of the appearance of cured and uncured coated films" in the below-described manner.

Before and after subjected to the acceleration test, each of the inks of Examples <NUM> to <NUM> and Comparative Examples <NUM> to <NUM> was measured for viscosity at <NUM> (mPa·S) and the obtained value was used to calculate the rate of change (%) in viscosity at <NUM> from the following equation.

The viscosity at <NUM> was measured with a cone-plate-type rotary viscometer (TV-<NUM>, product of TOKI SANGYO CO. ) with the temperature of circulating water being constantly set to <NUM>. The temperature of <NUM> is a temperature set considering the specification of a commercially available inkjet ejection head able to be heated, such as GEN4 (product of Ricoh Printing Systems, Ltd.

The results are collectively shown in Tables <NUM>-<NUM> to <NUM>-<NUM>. All of the inks of Comparative Examples <NUM> to <NUM> gellated after the acceleration test. Here, "gellation" described in Tables <NUM>-<NUM> to <NUM>-<NUM> means a state where part or the whole of the ink turned into a solid state after the acceleration test and the ink could not be measured for viscosity. In other words, it means that the ink had considerably bad storage stability.

In contrast, the rates of change in Examples were up to +<NUM>%, indicating that the inks of Examples had high storage stability at high temperatures.

An aluminum pouch bag having a shape illustrated in <FIG> was charged with each of the inks of Examples <NUM> to <NUM> and Comparative Examples <NUM> to <NUM> before and after the acceleration test, and hermetically sealed so as to avoid inclusion of air bubbles. The hermetically sealed pouch bag containing the ink was housed in a plastic cartridge as illustrated in <FIG>. This cartridge was mounted to a casing adapted for housing it. In the casing, an ink flow channel was provided from the cartridge to an inkjet head (GEN4, product of Ricoh Printing Systems, Ltd. Then, each of the inks of Examples <NUM> to <NUM> and Comparative Examples <NUM> to <NUM> before and after the acceleration test was ejected through the ink flow channel to form a solid coated film (i.e., a uniformly and entirely coated film) on a commercially available polyethylene terephthalate film (product name: COSMOSHINE A4300, product of TOYOBO CO. ) so that the solid coated film had a thickness of about <NUM>.

As a result, the inks of Examples <NUM> to <NUM> could be ejected without any particular problems both before and after the acceleration test. The solid coated films formed of the inks of Example <NUM> to <NUM> both before and after the acceleration test were cured without any problems. Specifically, by applying a light dose twice or less times of that required for curing the initial solid coated films formed of the inks of Example <NUM> to <NUM> before the acceleration test, the solid coated films formed of the inks of Example <NUM> to <NUM> after the acceleration test were cured to be in a non-sticky state which was judged by touching them with a finger.

The inks of Comparative Examples <NUM> to <NUM> partially or totally gelated after the acceleration test to lose properties as inkjet inks. Thus, these inks could not be evaluated for inkjetting property. Also, their curing properties were not evaluated since they could not form the above-described solid coated film.

Each of the mixtures of Preparation Examples <NUM> to <NUM> shown in Tables <NUM> to <NUM> was measured for required curing light dose for the disappearance of tackiness in the following manner.

An aluminum pouch bag having a shape illustrated in <FIG> was charged with each of the mixtures of Preparation Examples <NUM> to <NUM>, and hermetically sealed so as to avoid inclusion of air bubbles. The hermetically sealed pouch bag containing the mixture was housed in a plastic cartridge as illustrated in <FIG>. This cartridge was mounted to a casing adapted for housing it. In the casing, an ink flow channel was provided from the cartridge to an inkjet head (GEN4, product of Ricoh Printing Systems, Ltd. Then, the mixture was ejected through the ink flow channel to form a solid coated film (i.e., a uniformly and entirely coated film) on a commercially available polyethylene terephthalate film (product name: COSMOSHINE A4300, product of TOYOBO CO. ) so that the solid coated film had a thickness of about <NUM>, whereby an uncured film was formed.

The thus-formed uncured film was irradiated with light using a UV curing device (LH6, product of Fusion UV Systems Co. ), with the light dose being changed stepwise to <NUM>,<NUM> mJ/cm<NUM>, <NUM> mJ/cm<NUM>, <NUM> mJ/cm<NUM>, <NUM> mJ/cm<NUM>, <NUM> mJ/cm<NUM>, <NUM> mJ/cm<NUM>, <NUM> mJ/cm<NUM> and <NUM> mJ/cm<NUM>. After the irradiation of the above predetermined light dose, the state of the coated film was confirmed by touching it with a finger. The coated film that had lost tackiness was judged as being cured. The minimum light dose required that the coated film had been cured was used as a curing light dose required that the coated film had lost tackiness. The curing light dose required that the coated film had lost tackiness is shown as "curing light dose" in Tables <NUM> to <NUM>.

In addition, the uncured coated film was irradiated with the curing light dose required that the coated film would lose tackiness to prepare a cured coated film. The above-prepared uncured and cured coated films were observed for appearance. The results are shown in Tables <NUM> to <NUM>.

In the same manner, the inks of Examples <NUM> to <NUM> and Comparative Examples <NUM> to <NUM>, each containing each of the mixtures of Preparation Examples <NUM> to <NUM> together with the phenol aromatic compound having two hydroxyl groups in the molecule thereof (any one of C1 to C4) or the phenol aromatic compound having one hydroxyl group in the molecule thereof (C5 or C6), were measured for curing light dose, and the appearance of cured and uncured coated films made of each ink was observed.

Although the results are not shown in Tables <NUM> to <NUM>, they were comparable to the results shown in Tables <NUM> to <NUM> (those of the mixtures of Preparation Examples <NUM> to <NUM>); i.e., remained unchanged.

The inks containing the mixtures of Preparation Examples <NUM> to <NUM>, each containing the self-cleaving photopolymerization initiator only as the photopolymerization initiator, together with the phenol aromatic compound having two hydroxyl groups in the molecule thereof (any one of C1 to C4) were found to exhibit good storage stability (Examples <NUM> to <NUM>).

The inks containing the mixtures of Preparation Examples <NUM> to <NUM>, each containing the self-cleaving photopolymerization initiator only as the photopolymerization initiator, together with the phenol aromatic compound having only one hydroxyl group in the molecule thereof (C5 or C6) were found to gelate; i.e., not to exhibit sufficient storage stability (Comparative Examples <NUM> to <NUM>).

As described above, the present inventor previously found a photopolymerizable inkjet ink which is negative for skin sensitization, improves the coated film in curing property, enables possible curing failures in the coated film to be visually confirmed, and facilitates quality control in the production process. Specifically, the photopolymerizable inkjet ink contains: one or more types of photopolymerizable monomers each having a SI value of less than <NUM>; a self-cleaving photopolymerization initiator; a hydrogen-abstracting polymerization initiator; and an amine compound serving as a polymerization accelerator. When the mixtures of Preparation Examples <NUM> to <NUM> each containing, as in the above-described ink, the self-cleaving photopolymerization initiator, the hydrogen-abstracting polymerization initiator, and the amine compound serving as a polymerization accelerator as photopolymerization initiators were mixed with the phenol aromatic compound having two hydroxyl groups in the molecule thereof (any one of C <NUM> to C4), the obtained inks were found to exhibit good storage stability similar to the mixtures of Preparation Examples <NUM> to <NUM> each containing the self-cleaving photopolymerization initiator only as the photopolymerization initiator (Examples <NUM> to <NUM>). However, when they were mixed with the phenol aromatic compound having only one hydroxyl group in the molecule thereof (C5 or C6), all of the obtained inks were found to gelate; i.e., not to exhibit sufficient storage stability (Comparative Examples <NUM> to <NUM>).

An ink of Example <NUM> was produced in the same manner as in Example <NUM>, except that the amount of the phenol aromatic compound having two hydroxyl groups in the molecule thereof C3 was changed from <NUM> parts by mass from <NUM> parts by mass per <NUM> parts by mass of the mixture of Preparation Example <NUM>.

The thus-produced ink was measured for rate of change in viscosity at <NUM> in the same manner as described above. The rate of change of the ink was +<NUM>%, indicating good storage stability.

An ink of Example <NUM> was produced by adding <NUM> parts by mass of phenol aromatic compound having two hydroxyl groups in the molecule thereof C7 [<NUM>,<NUM>'-methylenebis(<NUM>-methyl-<NUM>-tert-butylphenol) (NONFLEX MBP, product of Seiko Chemical Co. )] to <NUM> parts by mass of the mixture of Preparation Example <NUM>.

An ink of Example <NUM> was produced by adding <NUM> parts by mass of phenol aromatic compound having two hydroxyl groups in the molecule thereof C8 [<NUM>,<NUM>'-methylenebis(<NUM>-ethyl-<NUM>-tert-butylphenol) (NONFLEX MBP, product of Seiko Chemical Co. )] to <NUM> parts by mass of the mixture of Preparation Example <NUM>.

An ink of Example <NUM> was produced by adding <NUM> parts by mass of phenol aromatic compound having two hydroxyl groups in the molecule thereof C9 [<NUM>,<NUM>'-methylenebis[<NUM>-(<NUM>-methylcyclohexyl-p-cresol) (NONFLEX MBP, product of Seiko Chemical Co. )] to <NUM> parts by mass of the mixture of Preparation Example <NUM>.

A mixture of Preparation Example <NUM> was prepared in the same manner as in Preparation Example <NUM>, except that <NUM> parts by mass of A20 and <NUM> parts by mass of A24 [diethyleneglycol diacrylate (SR230, product of Sartomer Co. )] were added instead of <NUM> parts by mass of A20.

An ink of Example <NUM> was produced by adding <NUM> parts by mass of the phenol aromatic compound having two hydroxyl groups in the molecule thereof C1 to <NUM> parts by mass of the mixture of Preparation Example <NUM>.

A mixture of Preparation Example <NUM> was prepared in the same manner as in Preparation Example <NUM>, except that <NUM> parts by mass of A20 and <NUM> parts by mass of A25 [trimethylolpropane triacrylate (M-<NUM>, product of Toagosei Chemical CO. )] instead of <NUM> parts by mass of A20.

The present invention can provide: a photopolymerizable inkjet ink safe in skin sensitization and good in storage stability at high temperatures; an ink cartridge housing the ink; and a printer containing the ink cartridge mounted thereto.

Also in the photopolymerizable inkjet ink, curing failures can easily be determined visually, not requiring dangerous actions such as inserting fingers into printers in operation nor providing printers with expensive detectors.

Claim 1:
A photopolymerizable inkjet ink comprising:
photopolymerizable monomer(s) consisting of one or more photopolymerizable monomers each having a Stimulation Index of less than <NUM>, where the Stimulation Index indicates the extent of sensitization as measured by a skin sensitization test based on Local Lymph Node Assay; and
a phenol aromatic compound having two hydroxyl groups in the molecule thereof,
wherein the photopolymerizable monomer is at least one of polyethylene glycol dimethacrylate represented by the following General Formula (<NUM>) where n is nearly equal to <NUM>, <NUM> or <NUM>, CH<NUM>=CH-CO-(OC<NUM>H<NUM>)n-OCOCH=CH<NUM> where n is nearly equal to <NUM>, γ-butyrolactone methacrylate, trimethylolpropane trimethacrylate, tricyclodecanedimethanol dimethacrylate, caprolactone-modified dipentaerythritol hexaacrylate, polypropylene glycol diacrylate [CH<NUM>=CH-CO-(OC<NUM>H<NUM>)n-OCOCH=CH<NUM>, where n is nearly equal to <NUM>], diacrylates of caprolactone-modified neopentylglycol hydroxypivalate, polyethoxylated tetramethylol methane tetraacrylate, ethylene oxide-modified bisphenol A diacrylate, neopentyl glycol dimethacrylate, stearyl acrylate, <NUM>,<NUM>-butanediol dimethacrylate, hydroxyethyl acrylamide, acryloylmorpholine, t-butyl methacrylate, n-pentyl methacrylate, n-hexyl methacrylate, ethylene oxide-modified phenol acrylate, isostearyl acrylate, ethylene oxide-modified trimethylolpropane trimethacrylate, stearyl methacrylate, glycerin dimethacrylate, triethylene glycol divinyl ether and isobutyl vinyl ether
<CHM>