Patent Publication Number: US-9891546-B2

Title: Ultraviolet-curable liquid developer

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a liquid developer for use in image-forming apparatuses that utilize an electrophotographic system, for example, electrophotography, electrostatic recording, and electrostatic printing. 
     Description of the Related Art 
     An electrophotographic system is a method in which printed material is obtained by uniformly charging the surface of an image bearing member such as a photosensitive member (charging step), forming an electrostatic latent image by photoexposure on the surface of the image bearing member (photoexposure step), developing the thus formed electrostatic latent image with a developer that contains colored resin particles (development step), transferring the developer image to a recording medium such as paper or plastic film (transfer step), and fixing the transferred developer image to the recording medium (fixing step). 
     The developers here are broadly classified into dry developers and liquid developers: colored resin particles constituted of a material that contains a binder resin and a colorant such as a pigment are used in a dry state in the former, while the colored resin particles are dispersed in an electrically insulating liquid in the latter. 
     The need for color output and high-speed printing from image-forming apparatuses that use an electrophotographic system, e.g., copiers, facsimile machines, printers, and so forth, has been increasing in recent years. In the realm of color printing, the demand for high-resolution, high-quality images has resulted in demand for developers that can accommodate high-speed printing while having the ability to form high-resolution, high-quality images. 
     Liquid developers are known to be developers that offer advantages with regard to color image reproducibility. With a liquid developer, the occurrence during storage of aggregation of the colored resin particles in the liquid developer is suppressed, and due to this, a microfine toner particle can be used. As a consequence, excellent properties with regard to the reproducibility of fine line images and the reproducibility of gradations are readily obtained with a liquid developer. Development is becoming quite active with regard to high-image-quality, high-speed digital printing apparatuses that exploit these excellent features by utilizing electrophotographic technologies that use liquid developers. In view of these circumstances, there is demand for the development of liquid developers that have even better properties. 
     A dispersion of colored resin particles in an electrically insulating liquid, e.g., a hydrocarbon organic solvent or silicone oil, is already known as a liquid developer. However, when the electrically insulating liquid remains present on the recording medium, e.g., paper or plastic film, this ends up causing a substantial decline in the appearance of the image, and due to this the electrically insulating liquid must be removed. In a method generally used to remove the electrically insulating liquid, thermal energy is applied to volatilize and remove the electrically insulating liquid. However, this has not necessarily been preferred from an environmental and/or an energy-savings perspective when the emission of an organic solvent vapor from the apparatus has been a possibility at this point and/or when large amounts of energy have been required. 
     As a countermeasure to this, a method has been introduced in which the electrically insulating liquid is cured by photopolymerization. A photocurable liquid developer used here uses a reactive functional group-bearing monomer or oligomer as the electrically insulating liquid and has a photoinitiator dissolved therein. This photocurable liquid developer can also accommodate high speeds because it is cured by the reaction of the reactive functional groups upon exposure to light, e.g., ultraviolet light. Such a photocurable liquid developer is proposed in Japanese Patent Application Laid-open No. 2003-57883. Acrylate monomer, e.g., urethane acrylate, is provided in Japanese Patent Application Laid-open No. 2003-57883 as an example of the reactive functional group-bearing monomer. 
     Japanese Patent No. 3,442,406 proposes the use, as a curable electrically insulating liquid, of a curable liquid vehicle that has a prescribed range for its viscosity and a prescribed range for its resistance value. Epoxy compounds, vinyl ethers, and cyclic vinyl ethers are given as examples of the curable liquid vehicle. 
     SUMMARY OF THE INVENTION 
     However, the aforementioned acrylate monomer has a low volume resistivity and facilitates a drop in the potential of the electrostatic latent image during the development step, and due to this the ability to obtain a high image density is impaired and image blurring is produced (the image sharpness is degraded). 
     Vinyl ether compounds, on the other hand, support the generation of a high volume resistivity and provide a fast reaction rate and are thus favorable curable electrically insulating liquids; however, they generally require the use of an ionic photoacid generator in combination with a photoinitiator for cationic polymerization. 
     When, however, a vinyl ether compound is mixed with an ionic photoacid generator, the volume resistivity ends up undergoing a large decline from that for the vinyl ether compound by itself. 
     Due to this, liquid developers that contain a vinyl ether compound and an ionic photoacid generator have suffered from the problems of an impaired ability to provide a high image density and/or the ready appearance of image blurring. 
     The present invention provides a liquid developer that solves these problems. 
     That is, the present invention provides an ultraviolet-curable liquid developer that yields a high image density, that resists the occurrence of image blurring, and that has a satisfactory fixing performance and an excellent storability. 
     The present invention is an ultraviolet-curable liquid developer that contains a cationically polymerizable liquid monomer, a photoinitiator, and a toner particle, wherein the cationically polymerizable liquid monomer contains a vinyl ether compound; the molar average SP value of the cationically polymerizable liquid monomer is not more than 9.0; the molar average number of functional groups for the cationically polymerizable liquid monomer is at least 1.8; the photoinitiator contains a compound represented by the following formula (1); and the content of the compound represented by the following formula (1) is at least 0.01 mass parts and not more than 5.00 mass parts per 100 mass parts of the cationically polymerizable liquid monomer. 
                         
[In formula (1), R 1  and R 2  are bonded to each other to form a ring structure; x is an integer that is at least 1 and not more than 8; and y is an integer that is at least 3 and not more than 17.]
 
     The present invention thus provides an ultraviolet-curable liquid developer that yields a high image density, that resists the occurrence of image blurring, and that has a satisfactory fixing performance and an excellent storability. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments. 
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The present invention is described in detail in the following. 
     The ultraviolet-curable liquid developer of the present invention contains a cationically polymerizable liquid monomer, a photoinitiator, and a toner particle. 
     Each of the constituent components present in the ultraviolet-curable liquid developer of the present invention is described in the following. 
     [The Photoinitiator] 
     The photoinitiator used by the present invention contains a compound represented by the following formula (1). 
                         
[In formula (1), R 1  and R 2  are bonded to each other to form a ring structure; x represents an integer that is at least 1 and not more than 8; and y represents an integer that is at least 3 and not more than 17.]
 
     Under exposure to ultraviolet radiation, the photoinitiator with formula (1) undergoes photolysis and generates a sulfonic acid, a strong acid. When a photopolymerization sensitizer is co-used therewith, the absorption of ultraviolet radiation by the photopolymerization sensitizer can also act as a trigger to bring about degradation of the photoinitiator and production of the sulfonic acid. 
     The use of the photoinitiator with formula (1), while making possible an excellent fixing performance, also provides a high-resistance liquid developer, unlike the case for the use of an ionic photoacid generator. 
     The ring structure formed by the bonding of R 1  to R 2  can be exemplified by five-membered rings and six-membered rings. Specific examples of the ring structure formed by the bonding of R 1  to R 2  are, for example, the succinimide structure, phthalimide structure, norbornenedicarboximide structure, naphthalenedicarboximide structure, cyclohexanedicarboximide structure, and epoxycyclohexenedicarboximide structure. 
     These ring structures may also have an alkyl group, alkyloxy group, alkylthio group, aryl group, aryloxy group, or arylthio group as a substituent. Another ring structure, e.g., a possibly substituted alicycle, heterocycle, aromatic ring, and so forth, may also be condensed. 
     The C x F y  group, which has a strong electron-withdrawing character, is a fluorocarbon group and is a functional group for bringing about decomposition of the sulfonate ester moiety upon exposure to ultraviolet radiation. The number of carbon atoms here is at least 1 and not more than 8 (x is at least 1 and not more than 8), and the number of fluorine atoms is at least 3 and not more than 17 (y is at least 3 and not more than 17). 
     Synthesis of the strong acid proceeds readily when the number of carbon atoms is at least 1, while the storage stability is excellent when the number of carbon atoms is not more than 8. The number of carbon atoms is preferably at least 1 and not more than 4. 
     Function as a strong acid is possible when the number of fluorine atoms is at least 3, while synthesis of the strong proceeds readily when the number of fluorine atoms is not more than 17. The number of fluorine atoms is preferably at least 3 and not more than 9. 
     The C x F y  group in formula (1) can be exemplified by linear-chain alkyl groups in which the hydrogen atom has been substituted by the fluorine atom (RF1), branched-chain alkyl groups in which the hydrogen atom has been substituted by the fluorine atom (RF2), cycloalkyl groups in which the hydrogen atom has been substituted by the fluorine atom (RF3), and aryl groups in which the hydrogen atom has been substituted by the fluorine atom (RF4). 
     The linear-chain alkyl groups in which the hydrogen atom has been substituted by the fluorine atom (RF1) can be exemplified by the trifluoromethyl group (x=1, y=3), pentafluoroethyl group (x=2, y=5), heptafluoro-n-propyl group (x=3, y=7), nonafluoro-n-butyl group (x=4, y=9), perfluoro-n-hexyl group (x=6, y=13), and perfluoro-n-octyl group (x=8, y=17). 
     The branched-chain alkyl groups in which the hydrogen atom has been substituted by the fluorine atom (RF2) can be exemplified by the perfluoroisopropyl group (x=3, y=7), perfluoro-tert-butyl group (x=4, y=9), and perfluoro-2-ethylhexyl group (x=8, y=17). 
     The cycloalkyl groups in which the hydrogen atom has been substituted by the fluorine atom (RF3) can be exemplified by the perfluorocyclobutyl group (x=4, y=7), perfluorocyclopentyl group (x=5, y=9), perfluorocyclohexyl group (x=6, y=11), and perfluoro(1-cyclohexyl)methyl group (x=7, y=13). 
     The aryl groups in which the hydrogen atom has been substituted by the fluorine atom (RF4) can be exemplified by the pentafluorophenyl group (x=6, y=5) and 3-trifluoromethyltetrafluorophenyl group (x=7, y=7). 
     Among C x F y  groups with formula (1), the linear-chain alkyl groups (RF1), branched-chain alkyl groups (RF2), and aryl groups (RF4) are preferred from the standpoint of the ease of acquisition and the decomposability of the sulfonate ester moiety, and the linear-chain alkyl groups (RF1) and aryl groups (RF4) are more preferred. The trifluoromethyl group (x=1, y=3), pentafluoroethyl group (x=2, y=5), heptafluoro-n-propyl group (x=3, y=7), nonafluoro-n-butyl group (x=4, y=9), and pentafluorophenyl group (x=6, y=5) are particularly preferred. 
     In addition, the compound represented by the formula (1) is a compound represented by the following formula (2) in a preferred embodiment. 
                         
[In formula (2), x represents an integer that is at least 1 and not more than 8; y represents an integer that is at least 3 and not more than 17; R 3  and R 4  each independently represent an alkyl group, an alkyloxy group, an alkylthio group, an aryl group, an aryloxy group, or an arylthio group; o and p each independently represent an integer that is at least 0 and not more than 3; when o is equal to or greater than 2, a plurality of R 3  may be bonded to each other to form a ring structure; when p is equal to or greater than 2, a plurality of R 4  may be bonded to each other to form a ring structure; and R 3  and R 4  may be bonded to each other to form a ring structure.]
 
     Preferably R 3  and R 4  each independently represent a C 1-18  alkyl group, C 1-18  alkyloxy group, C 1-18  alkylthio group, C 1-14  aryl group, C 1-14  aryloxy group, or C 1-14  arylthio group. 
     Specific examples of the photoinitiator with formula (1) are given below [example compounds A-1 to A-27], but the present invention is not limited to or by these examples. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Among the preceding, (A-23), (A-24), (A-25), (A-26), and (A-27) are preferred because in combination with a photopolymerization sensitizer they facilitate the generation of a high fixing performance. 
     A single one of these photoinitiators can be used or a combination of two or more can be used. In addition, a photoinitiator other than a compound with formula (1) may also be incorporated to the extent that the effects of the present invention are not impaired. 
     The content of the compound represented by the formula (1) in the ultraviolet-curable liquid developer of the present invention, expressed with reference to 100 mass parts of the cationically polymerizable liquid monomer, is at least 0.01 mass parts and not more than 5.00 mass parts. At least 0.05 mass parts and not more than 1.00 mass part is preferred and at least 0.10 mass parts and not more than 0.50 mass parts is more preferred. 
     When the content of the compound with formula (1) is less than 0.01 mass parts, the amount of sulfonic acid generated under exposure to ultraviolet radiation is deficient and the fixing performance then declines. 
     When, on the other hand, the content of the compound with formula (1) exceeds 5.00 mass parts, the volume resistivity of the ultraviolet-curable liquid developer declines and the developing performance then declines. In addition, the storability also declines due to an increase during storage in sulfonic acid produced by thermal decomposition. 
     [The Cationically Polymerizable Liquid Monomer] 
     The cationically polymerizable liquid monomer is selected in the present invention from liquids that have a high volume resistivity, that are electrically insulating, and that have a low viscosity at around room temperature. 
     In addition, the cationically polymerizable liquid monomer is preferably selected from liquids that do not dissolve the binder resin that is present in the toner particle. 
     Specifically, selection is preferably made from cationically polymerizable liquid monomer/binder resin combinations for which not more than 1 mass parts of the binder resin dissolves at a temperature of 25° C. in 100 mass parts of the cationically polymerizable liquid monomer. 
     The volume resistivity of the cationically polymerizable liquid monomer here is preferably approximately at least 1×10 9  Ω·cm and not more than 1×10 15  Ω·cm and is more preferably approximately at least 1×10 1C  Ω·cm and not more than 1×10 15  Ω·cm. 
     A volume resistivity of less than 1×10 9  Ω·cm facilitates a drop in the potential of the electrostatic latent image and sets up a trend of impeding the generation of a high optical density and/or a trend of facilitating the occurrence of image blurring. 
     The viscosity of the cationically polymerizable liquid monomer at 25° C., on the other hand, is preferably approximately at least 0.5 mPa·s and less than 100 mPa·s and is more preferably at least 0.5 mPa·s and less than 20 mPa·s. 
     The cationically polymerizable liquid monomer in the ultraviolet-curable liquid developer of the present invention contains a vinyl ether compound. In addition, a cationically polymerizable liquid monomer other than a vinyl ether compound may also be incorporated to the extent that the effects of the present invention are not impaired. The cationically polymerizable liquid monomer is preferably formed of one or two or more vinyl ether compounds. 
     The use of a vinyl ether compound in the present invention makes it possible to obtain an ultraviolet-curable liquid developer that has a high volume resistivity, a low viscosity, and a high sensitivity. 
     The present inventors hypothesize that this expression of favorable characteristics is caused by the small intramolecular polarization of the electron density in vinyl ether compounds. 
     Here, the vinyl ether compound refers to a compound that has a vinyl ether structure (—CH═CH—O—C—). 
     This vinyl ether structure is preferably given by R—CH═CH—O—C— (R is hydrogen or C 1-3  alkyl and is preferably hydrogen or methyl). 
     Acrylic monomers and cyclic ether monomers, e.g., epoxides and oxetanes, are also widely used as the aforementioned cationically polymerizable liquid monomer. However, acrylic monomers exhibit intramolecular polarization of the electron density and, due to the operation of intermolecular electrostatic interactions, it is difficult to obtain a low-viscosity liquid developer and a declining trend is also assumed for the volume resistivity. 
     With cyclic ether monomers, on the other hand, it is also difficult to obtain a high volume resistivity and in addition the polymerization reaction rate is prone to be significantly lower than for vinyl ether compounds, and as a consequence their use amount is very limited when they are used in an ultraviolet-curable liquid developer. 
     In the present invention, a small amount of a cyclic ether monomer may also be mixed and used with the vinyl ether compound within a range in which the volume resistivity and polymerization reaction rate are not significantly reduced. 
     In one preferred embodiment in the present invention, the vinyl ether compound is also a compound that does not contain a heteroatom outside the vinyl ether structure. 
     Here, “heteroatom” denotes an atom other than the carbon atom and hydrogen atom. 
     When it is a compound that does not contain a heteroatom outside the vinyl ether structure, intramolecular polarization of the electron density is suppressed and a high volume resistivity is readily obtained. 
     In another preferred embodiment in the present invention, the vinyl ether compound also does not contain a carbon-carbon double bond outside of the vinyl ether structure in the vinyl ether compound. Polarization of the electron density is suppressed and a high volume resistivity is readily obtained with a vinyl ether compound that does not contain a carbon-carbon double bond outside of the vinyl ether structure. 
     The vinyl ether compound in the present invention is preferably given by the following formula (C).
 
(H 2 C═CH—O   n R  formula (C)
 
[In formula (C), n represents the number of vinyl ether structures in one molecule and is an integer that is at least 1 and not more than 4. R is an n-valent hydrocarbon group.]
 
     n is preferably an integer that is at least 1 and not more than 3. 
     R preferably is a group selected from C 1-20  linear-chain or branched, saturated or unsaturated aliphatic hydrocarbon groups, C 5-12  saturated or unsaturated alicyclic hydrocarbon groups, and C 6-14  aromatic hydrocarbon groups, and these alicyclic hydrocarbon groups and aromatic hydrocarbon groups may have a C 1-4  saturated or unsaturated aliphatic hydrocarbon group. 
     R is more preferably a C 4-18  linear-chain or branched saturated aliphatic hydrocarbon group. 
     Specific examples of vinyl ether compounds are given below [example compounds B-1 to B-30], but the present invention is not limited to or by these examples. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     The following are preferred examples among the preceding: dicyclopentadiene vinyl ether (B-8), cyclohexanedimethanol divinyl ether (B-17), tricyclodecane vinyl ether (B-10), trimethylolpropane trivinyl ether (B-24), 2-ethyl-1,3-hexanediol divinyl ether (B-25), 2,4-diethyl-1,5-pentanediol divinyl ether (B-26), 2-butyl-2-ethyl-1,3-propanediol divinyl ether (B-27), neopentyl glycol divinyl ether (B-23), pentaerythritol tetravinyl ether (B-28), and 1,2-decanediol divinyl ether (B-30). 
     The molar average SP value of the cationically polymerizable liquid monomer is not more than 9.0 in the present invention and is preferably not more than 8.8 and more preferably not more than 8.6. 
     This molar average SP value is also at least approximately 7.6. 
     In addition, the cationically polymerizable liquid monomer in the present invention is preferably formed of cationically polymerizable liquid monomer having an SP value of not more than 9.0 and is more preferably formed of cationically polymerizable liquid monomer having an SP value of not more than 8.9. 
     This cationically polymerizable liquid monomer is also desirably formed of cationically polymerizable liquid monomer that has an SP value of at least approximately 7.6. 
     The SP value here is a value introduced by Hildebrand and defined by a regular solution theory. It is given by the square root of the cohesive energy density of the solvent (or solute) and is a measure of the solubility in a two-component system solution. 
     In the present invention, the SP value is the value determined by calculation from the vaporization energy and molar volume of the atoms and atomic groups in accordance with Fedors as described in Coating Basics and Engineering (page 53, Yuji Harazaki, Converting Technical Institute). 
     The molar average SP value of the cationically polymerizable liquid monomer is determined as follows for the use of a mixture of (Awt) mass parts of a cationically polymerizable liquid monomer A having an SP value of (Asp) and a molecular weight of (Amw) with (Bwt) mass parts of a cationically polymerizable liquid monomer B having an SP value of (Bsp) and a molecular weight of (Bmw).
 
molar average  SP  value={( Asp×Awt/Amw )+( Bsp×Bwt/Bmw )}/{( Awt/Amw )+( Bwt/Bmw )}
 
     The determination is also similarly carried out for the use of a mixture of three or more species of cationically polymerizable liquid monomers. 
     Cationically polymerizable ultraviolet-curable resins are quite susceptible to moisture-induced cure inhibition, and as a consequence the cure of an ultraviolet-curable resin is impaired by an increase in the humidity of the curing environment. 
     However, by having the molar average SP value be not more than 9.0, the water content of the cationically polymerizable liquid monomer can be reduced and the moisture that permeates post-transfer into the liquid developer from the liquid developer/air interface can also be reduced, and a high fixing performance is then obtained. 
     The cationically polymerizable liquid monomer in the present invention is more preferably formed of a vinyl ether compound that has an SP value of not more than 9.0. 
     The molar average number of functional groups for the cationically polymerizable liquid monomer in the present invention is at least 1.8, preferably at least 2.0, and more preferably at least 2.2. 
     This molar average number of functional groups is also not more than approximately 6.0. 
     In addition, the cationically polymerizable liquid monomer in the present invention is preferably formed of cationically polymerizable liquid monomer for which the number of polymerizable functional groups per one molecule is at least two. 
     The cationically polymerizable liquid monomer is desirably formed of cationically polymerizable liquid monomer for which the number of polymerizable functional groups per one molecule is not more than approximately 6.0. 
     Here, when one polymerizable functional group is present in one molecule of the cationically polymerizable liquid monomer, the number of polymerizable functional groups per one molecule is given by “1” (or monofunctional), while when n are present the number of polymerizable functional groups per one molecule is given by “n” (or n-functional). 
     In the case of a vinyl ether compound, when n vinyl ether structures (—CH═CH—O—C—) are present in one molecule of the vinyl ether compound, this is indicated as n-functional. 
     For the case of the use of a mixture of (Awt) mass parts of a cationically polymerizable liquid monomer A having a number of polymerizable functional groups (Af) per one molecule and a molecular weight (Amw) with (Bwt) mass parts of a cationically polymerizable liquid monomer B having a number of polymerizable functional groups (Bf) per one molecule and a molecular weight (Bmw), the molar average number of functional groups for the cationically polymerizable liquid monomer is determined as follows in the present invention.
 
molar average number of functional groups={( Af×Awt/Amw )+( Bf×Bwt/Bmw )}/{( Awt/Amw )+( Bwt/Bmw )}
 
     The determination is similarly carried out when a mixture of three or more species of cationically polymerizable liquid monomers is used. 
     A cationic polymerization reaction is generally considered to be a polymerization reaction in which the acid produced by degradation from the polymerization initiator upon exposure to light reacts with monomer to produce a cationic active species, with the polymerization reaction proceeding successively as long as this cationic active species is present. When a water molecule is present in the vicinity of the cationically polymerizable liquid monomer at this time, this cationic active species is trapped and polymerization does not occur beyond this. 
     That is, the chain extension reaction of one polymerizable functional group in the cationically polymerizable liquid monomer is stopped per one water molecule. 
     Accordingly, the moisture-induced cure inhibition can be suppressed and a high fixing performance can then be obtained by having the molar average number of functional groups for the cationically polymerizable liquid monomer be at least 1.8. 
     The cationically polymerizable liquid monomer is more preferably composed in the present invention of a vinyl ether compound that has at least two polymerizable functional groups per one molecule. 
     As necessary, a photopolymerization sensitizer may also be added to the ultraviolet-curable liquid developer of the present invention with the goals of, for example, improving the acid-generating efficiency of the aforementioned photoinitiator and extending the photosensitive wavelengths to longer wavelengths. 
     There are no particular limitations on this photopolymerization sensitizer as long as it is capable of sensitizing the photoinitiator through an electron transfer mechanism or energy transfer mechanism. 
     Specific examples are as follows: aromatic polycondensed ring compounds such as anthracene, 9,10-dialkoxyanthracene, pyrene, and perylene; aromatic ketone compounds such as acetophenone, benzophenone, thioxanthone, and Michler&#39;s ketone; and heterocyclic compounds such as phenothiazine and N-aryloxazolidinone. 
     Preferred examples among the preceding are anthracene compounds such as 9,10-diethoxyanthracene, 9,10-dipropoxyanthracene, and 9,10-dibutoxyanthracene, as well as thioxanthone compounds such as 2,4-diethylthioxanthone and 2-isopropylthioxanthone. 
     A single one of these photopolymerization sensitizers can be used or a combination of two or more can be used. 
     The content of the photopolymerization sensitizer is selected as appropriate in correspondence to the goal, but, expressed per 1 mass parts of the photoinitiator, is preferably at least 0.1 mass parts and not more than 10.0 mass parts and is more preferably at least 1.0 mass parts and not more than 5.0 mass parts. 
     A photopolymerization sensitizing aid may as necessary also be incorporated in the ultraviolet-curable liquid developer of the present invention with the goal of improving the electron transfer efficiency or energy transfer efficiency between the aforementioned photopolymerization sensitizer and photoinitiator. 
     The photopolymerization sensitizing aid can be exemplified by the following: naphthalene compounds such as 1,4-dihydroxynaphthalene, 1,4-dimethoxynaphthalene, 1,4-diethoxynaphthalene, 4-methoxy-1-naphthol, and 4-ethoxy-1-naphthol, and benzene compounds such as 1,4-dihydroxybenzene, 1,4-dimethoxybenzene, 1,4-diethoxybenzene, 1-methoxy-4-phenol, and 1-ethoxy-4-phenol. 
     The naphthalene compounds are preferred examples among the preceding. 
     A single one of these photopolymerization sensitizing aids can be used or a combination of two or more can be used. 
     The content of the photopolymerization sensitizing aid is selected as appropriate in correspondence to the goal, but, expressed per 1 mass parts of the photopolymerization sensitizer, is preferably at least 0.1 mass parts and not more than 10.0 mass parts and is more preferably at least 0.5 mass parts and not more than 5.0 mass parts. 
     [The Toner Particle] 
     The ultraviolet-curable liquid developer of the present invention contains a toner particle. 
     The toner particle preferably contains a binder resin and a colorant. 
     &lt;Binder Resin&gt; 
     A known binder resin can be used—as long as it is insoluble in the aforementioned cationically polymerizable liquid monomer and exhibits a fixing performance for the adherend, e.g., paper or plastic film—as the binder resin present in the toner particle. 
     Here, this “insoluble in the cationically polymerizable liquid monomer” is provided as an indicator that not more than 1 mass parts of the binder resin dissolves at a temperature of 25° C. in 100 mass parts of the cationically polymerizable liquid monomer. 
     Specific examples of this binder resin are resins such as epoxy resins, ester resins, (meth)acrylic resins, styrene-(meth)acrylic resins, alkyd resins, polyethylene resins, ethylene-(meth)acrylic resins, and rosin-modified resins. As necessary, a single one of these can be used or two or more can be used in combination. 
     The content of the binder resin is not particularly limited, but is preferably 50 to 1,000 mass parts per 100 mass parts of the colorant. 
     &lt;Colorant&gt; 
     There are no particular limitations on the colorant incorporated in the toner particle, and, for example, any generally commercially available organic pigment, organic dye, inorganic pigment, or pigment dispersed in, e.g., an insoluble resin as a dispersion medium, or pigment having a resin grafted to its surface can be used. 
     These pigments can be exemplified by the pigments described in, for example, “Industrial Organic Pigments”, W. Herbst and K. Hunger. 
     The following are specific examples of pigments that present a yellow color: 
     C. I. Pigment Yellow 1, 2, 3, 4, 5, 6, 7, 10, 11, 12, 13, 14, 15, 16, 17, 23, 62, 65, 73, 74, 83, 93, 94, 95, 97, 109, 110, 111, 120, 127, 128, 129, 147, 151, 154, 155, 168, 174, 175, 176, 180, 181, and 185, and C. I. Vat Yellow 1, 3, and 20. 
     Pigments that present a red or magenta color can be exemplified by the following: 
     C. I. Pigment Red 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 21, 22, 23, 30, 31, 32, 37, 38, 39, 40, 41, 48:2, 48:3, 48:4, 49, 50, 51, 52, 53, 54, 55, 57:1, 58, 60, 63, 64, 68, 81:1, 83, 87, 88, 89, 90, 112, 114, 122, 123, 146, 147, 150, 163, 184, 202, 206, 207, 209, 238, and 269; C. I. Pigment Violet 19; and C. I. Vat Red 1, 2, 10, 13, 15, 23, 29, and 35. 
     Pigments that present a blue or cyan color can be exemplified by the following: 
     C. I. Pigment Blue 2, 3, 15:2, 15:3, 15:4, 16, and 17; C. I. Vat Blue 6; C. I. Acid Blue 45; and copper phthalocyanine pigments in which the phthalocyanine skeleton is substituted by 1 to 5 phthalimidomethyl groups. 
     Pigments that present a green color can be exemplified by the following: 
     C. I. Pigment Green 7, 8, and 36. 
     Pigments that present an orange color can be exemplified by the following: 
     C. I. Pigment Orange 66 and 51. 
     Pigments that present a black color can be exemplified by the following: 
     carbon black, titanium black, and aniline black. 
     The following are specific examples of white pigments: 
     basic lead carbonate, zinc oxide, titanium oxide, and strontium titanate. 
     A dispersing means adapted to the toner particle production method may be used to disperse the pigment in the toner particle. Devices that can be used as this dispersing means are, for example, a ball mill, sand mill, attritor, roll mill, jet mill, homogenizer, paint shaker, kneader, agitator, Henschel mixer, colloid mill, ultrasonic homogenizer, pearl mill, wet jet mill, and so forth. 
     A pigment dispersing agent may also be added when pigment dispersion is carried out. The pigment dispersing agent can be exemplified by hydroxyl group-bearing carboxylate esters, the salts of long-chain polyaminoamides and high molecular weight acid esters, the salts of high molecular weight polycarboxylic acids, high molecular weight unsaturated acid esters, high molecular weight copolymers, modified polyacrylates, aliphatic polybasic carboxylic acids, naphthalenesulfonic acid/formalin condensates, polyoxyethylene alkyl phosphate esters, and pigment derivatives. The use of commercially available high molecular weight dispersing agents such as the Solsperse series from The Lubrizol Corporation is also preferred. 
     A synergist adapted to the particular pigment may also be used as a pigment dispersing aid. 
     These pigment dispersing agents and pigment dispersing aids are added preferably at 1 to 50 mass parts per 100 mass parts of the pigment. 
     The ultraviolet-curable liquid developer of the present invention may as necessary contain a charge control agent. 
     A known charge control agent can be used without particular limitation as this charge control agent as long as it provides little reduction in the volume resistivity of the ultraviolet-curable liquid developer and provides little increase in the viscosity of the ultraviolet-curable liquid developer. 
     Examples of specific compounds are as follows: 
     fats and oils such as linseed oil and soy oil; alkyd resins; halogen polymers; aromatic polycarboxylic acids; acidic group-containing water-soluble dyes; oxidative condensates of aromatic polyamines; metal soaps such as zirconium naphthenate, cobalt naphthenate, nickel naphthenate, iron naphthenate, zinc naphthenate, cobalt octoate, nickel octoate, zinc octoate, cobalt dodecanoate, nickel dodecanoate, zinc dodecanoate, aluminum stearate, and cobalt 2-ethylhexanoate; metal sulfonates such as petroleum-based metal sulfonates and metal salts of sulfosuccinate esters; phospholipids such as lecithin and hydrogenated lecithin; metal salts of salicylic acid, e.g., metal complexes of tert-butylsalicylic acid; polyvinylpyrrolidone resins; polyamide resins; sulfonic acid-containing resins; and hydroxybenzoic acid derivatives. 
     The content of the charge control agent in the present invention, expressed per 100 mass parts of the toner particle (solids fraction), is preferably at least 0.01 mass parts and not more than 10 mass parts and more preferably at least 0.05 mass parts and not more than 5 mass parts. 
     &lt;Charge Adjuvant&gt; 
     A charge adjuvant can as necessary be incorporated in the toner particle. A known charge adjuvant can be used as this charge adjuvant. 
     Examples of specific compounds are as follows: metal soaps such as zirconium naphthenate, cobalt naphthenate, nickel naphthenate, iron naphthenate, zinc naphthenate, cobalt octoate, nickel octoate, zinc octoate, cobalt dodecanoate, nickel dodecanoate, zinc dodecanoate, aluminum stearate, aluminum tristearate, and cobalt 2-ethylhexanoate; metal sulfonates such as petroleum-based metal sulfonates and the metal salts of sulfosuccinate esters; phospholipids such as lecithin and hydrogenated lecithin; metal salts of salicylic acid, e.g., metal complexes of tert-butylsalicylic acid; polyvinylpyrrolidone resins; polyamide resins; sulfonic acid-containing resins; and hydroxybenzoic acid derivatives. 
     &lt;Cationic Polymerization Inhibitor&gt; 
     The ultraviolet-curable liquid developer of the present invention may also contain a cationic polymerization inhibitor. The cationic polymerization inhibitor can be exemplified by alkali metal compounds and/or alkaline-earth metal compounds and by amines. 
     The amines can be exemplified by alkanolamines, N,N-dimethylalkylamines, N,N-dimethylalkenylamines, and N,N-dimethylalkynylamines. 
     The amines can be specifically exemplified by triethanolamine, triisopropanolamine, tributanolamine, N-ethyldiethanolamine, propanolamine, n-butylamine, sec-butylamine, 2-aminoethanol, 2-methylaminoethanol, 3-methylamino-1-propanol, 3-methylamino-1,2-propanediol, 2-ethylaminoethanol, 4-ethylamino-1-butanol, 4-(n-butylamino)-1-butanol, 2-(t-butylamino)ethanol, N,N-dimethylundecanolamine, N,N-dimethyldodecanolamine, N,N-dimethyltridecanolamine, N,N-dimethyltetradecanolamine, N,N-dimethylpentadecanolamine, N,N-dimethylnonadecylamine, N,N-dimethylicosylamine, N,N-dimethyleicosylamine, N,N-dimethylheneicosylamine, N,N-dimethyldocosylamine, N,N-dimethyltricosylamine, N,N-dimethyltetracosylamine, N,N-dimethylpentacosylamine, N,N-dimethylpentanolamine, N,N-dimethylhexanolamine, N,N-dimethylheptanolamine, N,N-dimethyloctanolamine, N,N-dimethylnonanolamine, N,N-dimethyldecanolamine, N,N-dimethylnonylamine, N,N-dimethyldecylamine, N,N-dimethylundecylamine, N,N-dimethyldodecylamine, N,N-dimethyltridecylamine, N,N-dimethyltetradecylamine, N,N-dimethylpentadecylamine, N,N-dimethylhexadecylamine, N,N-dimethylheptadecylamine, and N,N-dimethyloctadecylamine. In addition to these, for example, a quaternary ammonium salt may also be used. The cationic polymerization inhibitor is particularly preferably a secondary amine. 
     The content of the cationic polymerization inhibitor is preferably 1 to 5,000 ppm on a mass basis in the ultraviolet-curable liquid developer. 
     &lt;Radical Polymerization Inhibitor&gt; 
     The ultraviolet-curable liquid developer of the present invention may also contain a radical polymerization inhibitor. 
     In the case of an ultraviolet-curable liquid developer that contains a vinyl ether compound, during storage the photoinitiator may undergo a trace decomposition and thereby convert into a radical compound and a polymerization caused by this radical compound may then be induced. A radical polymerization inhibitor is preferably added to prevent this. 
     Usable radical polymerization inhibitors can be exemplified by phenolic hydroxy group-containing compounds; quinones such as methoquinone (hydroquinone monomethyl ether), hydroquinone, and 4-methoxy-1-naphthol; hindered amine antioxidants; 1,1-diphenyl-2-picrylhydrazyl free radical; N-oxyl free radical compounds; nitrogen-containing heterocyclic mercapto compounds; thioether antioxidants; hindered phenol antioxidants; ascorbic acids; zinc sulfate; thiocyanates; thiourea derivatives; saccharides; phosphoric acid-type antioxidants; nitrites; sulfites; thiosulfates; hydroxylamine derivatives; aromatic amines; phenylenediamines; imines; sulfonamides; urea derivatives; oximes; polycondensates of dicyandiamide and polyalkylenepolyamine; sulfur-containing compounds such as phenothiazine; complexing agents based on tetraazaannulene (TAA); and hindered amines. 
     Phenolic hydroxy group-containing compounds, N-oxyl free radical compounds, 1,1-diphenyl-2-picrylhydrazyl free radical, phenothiazine, quinones, and hindered amines are preferred from the standpoint of preventing the ultraviolet-curable liquid developer from undergoing a viscosity increase due to polymerization of the vinyl ether compound, and N-oxyl free radical compounds are more preferred. 
     The content of the radical polymerization inhibitor is preferably 1 to 5,000 ppm on a mass basis in the ultraviolet-curable liquid developer. 
     &lt;Other Additives&gt; 
     In addition to those described above, the ultraviolet-curable liquid developer of the present invention may as necessary contain various known additives to respond to the goals of improving the compatibility with recording media, improving the storage stability, improving the image storability, and improving other characteristics. For example, the following can be selected as appropriate and used: surfactant, lubricant, filler, antifoaming agent, ultraviolet absorber, antioxidant, anti-fading agent, fungicide, anticorrosion agent, and so forth. 
     The method of producing the ultraviolet-curable liquid developer in the present invention is not particularly limited and can be exemplified by known methods, for example, the coacervation method and the wet pulverization method. 
     An example of a general production method is a production method in which a colorant, a binder resin, other additives, and a dispersion medium are mixed; pulverization is carried out using, e.g., a bead mill, to obtain a dispersion of toner particles; and the obtained toner particle dispersion, a photoinitiator, cationically polymerizable liquid monomer, and so forth are mixed to obtain the ultraviolet-curable liquid developer. 
     The details of the coacervation method are described in, for example, Japanese Patent Application Laid-open No. 2003-241439, WO 2007/000974, and WO 2007/000975. 
     In the coacervation method, a pigment, resin, solvent that dissolves the resin, and solvent that does not dissolve the resin are mixed and the solvent that dissolves the resin is then removed from the mixture to cause the resin that had been dissolved to precipitate, thereby creating a dispersion of pigment-enclosing toner particles in the solvent that does not dissolve the resin. 
     The details of the wet pulverization method, on the other hand, are described in, for example, WO 2006/126566 and WO 2007/108485. 
     In the wet pulverization method, the pigment and binder resin are kneaded at or above the melting point of the binder resin; this is followed by a dry pulverization; and the obtained pulverized material is subjected to a wet pulverization in an electrically insulating medium, thereby creating a dispersion of toner particles in the electrically insulating medium. 
     Known methods such as these can be used in the present invention. 
     Viewed from the perspective of obtaining a high-definition image, the volume-average particle diameter of the toner particle is preferably at least 0.05 μm and not more than 5 μm and is more preferably at least 0.05 μm and not more than 1 μm. 
     The toner particle concentration in the ultraviolet-curable liquid developer in the present invention is not particularly limited, but is desirably made approximately at least 1 mass % and not more than 70 mass %, is preferably made approximately at least 1 mass % and not more than 50 mass %, and is even more preferably made approximately at least 2 mass % and not more than 40 mass %. 
     &lt;Properties of the Ultraviolet-Curable Liquid Developer&gt; 
     The ultraviolet-curable liquid developer of the present invention is preferably used having been prepared so as to have the same property values as common liquid developers. 
     Thus, viewed from the perspective of obtaining a suitable toner particle electrophoretic mobility, the viscosity of the ultraviolet-curable liquid developer at 25° C. for a toner particle concentration of 2 mass % is preferably at least 0.5 mPa·s and not more than 100 mPa·s. In addition, in terms of not causing a drop in the potential of the electrostatic latent image, the volume resistivity of the ultraviolet-curable liquid developer is preferably at least 1×10 9  Ω·cm and not more than 1×10 15  Ω·cm and is more preferably at least 1×10 10  Ω·cm and not more than 1×10 13  Ω·cm. 
     The present invention makes possible the preparation of an ultraviolet-curable liquid developer that exhibits a high ultraviolet curability while also satisfying the property values indicated above. 
     [The Image-Forming Apparatus] 
     The ultraviolet-curable liquid developer of the present invention can be advantageously used in typical image-forming apparatuses that use an electrophotographic system. 
     &lt;Light Source&gt; 
     The image is fixed by curing the ultraviolet-curable liquid developer of the present invention through its exposure to ultraviolet radiation immediately after transfer to a recording medium. 
     The light source here for carrying out ultraviolet irradiation is suitably, for example, a mercury lamp, metal halide lamp, excimer laser, ultraviolet laser, cold cathode tube, hot cathode tube, black light, or light-emitting diode (LED). A strip-shaped metal halide lamp, cold cathode tube, hot cathode tube, mercury lamp, black light, or LED is preferred. 
     The ultraviolet dose is preferably from 0.1 to 1,000 mJ/cm 2 . 
     The measurement methods used in the present invention are given in the following. 
     &lt;Method of Measuring the Volume Resistivity&gt; 
     With regard to the volume resistivity of the ultraviolet-curable liquid developer, the volume resistivity of the liquid developer is measured using an R8340A digital ultrahigh resistance/microcurrent meter (Advantest Corporation). The measurement is carried out by introducing 25 mL of the liquid developer into an SME-8330 liquid sample electrode (Hioki E.E. Corporation) and then applying 1,000 V direct current at a room temperature of 25° C. 
     &lt;Compositional Analysis&gt; 
     The following procedures are used for structural identification of the compounds. 
     The  1 H-NMR and  13 C-NMR spectra are measured using an ECA-400 (400 MHz) from JEOL Ltd. 
     The measurements are carried out at 25° C. in a deuterated solvent containing tetramethylsilane as the internal standard. The chemical shift values are given as the shift value (δ value) in ppm assigning 0 to the tetramethylsilane internal standard. 
     &lt;Methods for Measuring the SP Value and Molar Average SP Value of the Cationically Polymerizable Liquid Monomer and the Number of Polymerizable Functional Groups Per One Molecule of the Cationically Polymerizable Liquid Monomer and the Molar Average Number of Functional Groups for the Cationically Polymerizable Liquid Monomer&gt; 
     The SP value and molar average SP value of the cationically polymerizable liquid monomer in the ultraviolet-curable liquid developer and the number of polymerizable functional groups per one molecule of the cationically polymerizable liquid monomer and the molar average number of functional groups for the cationically polymerizable liquid monomer are determined using the following methods in the present invention. 
     (1) The ultraviolet-curable liquid developer is subjected to centrifugal separation to sediment the toner particles and extract a supernatant. 
     (2) Through measurement of the supernatant by gel permeation chromatography, the molecular weight and content of the contents are determined and fractionation into each molecular weight component is carried out. 
     (3) The chemical structure of each component is identified by measuring the  1 H-NMR and  13 C-NMR spectra of each fractionated component and the molecular weight, content, and number of polymerizable functional groups per one molecule are determined for the contained cationically polymerizable liquid monomer. 
     Here, while the chemical structure is identified by measurement of the  1 H-NMR and  13 C-NMR spectra, as necessary known analytical procedures such as infrared spectroscopy and gas chromatography may also be used in combination therewith. 
     (4) The SP value is calculated using Fedors method for each cationically polymerizable liquid monomer for which the chemical structure has been identified in accordance with (3). 
     (5) Using the equations given below, the molar average SP value and the molar average number of functional groups are determined from the molecular weight, content, number of polymerizable functional groups per one molecule, and SP value for each component that have been calculated in the preceding (3) and (4). 
     The molar average SP value of the cationically polymerizable liquid monomer is determined as follows for the case of the use of a mixture of (Awt) mass parts of a cationically polymerizable liquid monomer A having an SP value of (Asp) and a molecular weight of (Amw) with (Bwt) mass parts of a cationically polymerizable liquid monomer B having an SP value of (Bsp) and a molecular weight of (Bmw).
 
molar average  SP  value={( Asp×Awt/Amw )+( Bsp×Bwt/Bmw )}/{( Awt/Amw )+( Bwt/Bmw )}
 
     The molar average number of functional groups for the cationically polymerizable liquid monomer is determined as follows for the case of the use of a mixture of (Awt) mass parts of a cationically polymerizable liquid monomer A having a number of polymerizable functional groups per one molecule (Af) and a molecular weight (Amw) with (Bwt) mass parts of a cationically polymerizable liquid monomer B having a number of polymerizable functional groups per one molecule (Bf) and a molecular weight (Bmw).
 
molar average number of functional groups={( Af×Awt/Amw )+( Bf×Bwt/Bmw )}/{( Awt/Amw )+( Bwt/Bmw )}
 
     The preceding is the equation for the case in which the cationically polymerizable liquid monomer contains two species, i.e., the cationically polymerizable liquid monomer A and the cationically polymerizable liquid monomer B, but the determination can be similarly carried out for cases in which three or more species are present by extending the equation to a system of three or more components. 
     EXAMPLES 
     The present invention is more specifically described in the following using examples, but the present invention is not limited to or by these examples. Unless specifically indicated otherwise, the “parts” and “%” in the following description denote “mass parts” and “mass %”, respectively. 
     Example 1 
     Toner Particle Production 
     25 parts of Nucrel N1525 (ethylene-methacrylic acid resin, Du Pont-Mitsui Polychemicals Co., Ltd.) and 75 parts of cyclohexanedimethanol divinyl ether (example compound B-17) were introduced into a separable flask and the temperature was raised over 1 hour to 130° C. on an oil bath while stirring at 200 rpm using a Three-One motor. After holding for 1 hour at 130° C., gradual cooling was carried out at a ramp down rate of 15° C. per 1 hour to produce a binder resin dispersion. The obtained binder resin dispersion was a white paste. 
     59.40 parts of this binder resin dispersion, Pigment Blue 15:3 (4.95 parts) as pigment, 0.20 parts of aluminum tristearate as a charge adjuvant, and 35.45 parts of cyclohexanedimethanol divinyl ether were filled into a planetary bead mill (Classic Line P-6, Fritsch) along with zirconia beads having a diameter of 0.5 mm, and pulverization was carried out at 200 rpm for 4 hours at room temperature to obtain a toner particle dispersion (solids fraction=20 mass %). 
     The toner particles present in the obtained toner particle dispersion had a volume-average particle diameter of 0.85 μm (measured with a Nanotrac 150 from Nikkiso Co., Ltd., a particle size distribution analyzer based on dynamic light scattering (DLS)). 
     (Preparation of Liquid Developer) 
     An ultraviolet-curable liquid developer was obtained by mixing 10.00 parts of the aforementioned toner particle dispersion; 0.10 parts of hydrogenated lecithin (Lecinol S-10, Nikko Chemicals Co., Ltd.) as a charge control agent; 1.66 parts of cyclohexanedimethanol divinyl ether, 28.99 parts of diethylpentanediol divinyl ether (example compound B-26), and 57.98 parts of pentaerythritol tetravinyl ether (example compound B-28) as cationically polymerizable liquid monomers; example compound A-26 (0.29 parts) as a photoinitiator; 0.49 parts of 2,4-diethylthioxanthone (referred to hereafter as DETX) as a photopolymerization sensitizer; and 0.49 parts of 1,4-diethoxynaphthalene (referred to hereafter as DEN) as a photopolymerization sensitizing aid. 
     The following evaluations were performed on the obtained ultraviolet-curable liquid developer, and the results are given in Tables 1 and 2. The items evaluated and the results of the evaluation are as follows. 
     (Developing Performance: Evaluation Concerning the Image Density and Image Blurring) 
     An electrostatic pattern was formed at a surface charge of 500 V on electrostatic recording paper, and development was performed using the liquid developer at a process speed of 20 mm/sec using a roller developing device that used a metal roller. The gap between the roller and the electrostatic recording paper (the development gap) was set to 34 μm. The quality of the obtained image was visually inspected. 
     5: the obtained image had a high density and a high definition 
     4: a slight image density non-uniformity is present, or slight image blurring is seen 
     3: image density non-uniformity or image blurring is seen in spots, but a generally good development is recognized 
     2: severe image density non-uniformity and/or image blurring was produced and development was unsatisfactory 
     1: development could not be carried out 
     (Fixing Performance) 
     The liquid developer was dripped onto a polyethylene terephthalate film in a 25° C. room temperature/50% humidity environment and was bar-coated (a film with a thickness of 13.7 μm was formed) using a wire bar (No. 6) [supplier: Matsuo Sangyo Co., Ltd.], and a cured film was formed by exposure (illuminance=1000 mW/cm 2 , exposure gap=15 mm) from an LED having an emission wavelength of 385 nm. The amount of irradiated light was measured for the point at which there was no surface tack (stickiness) and complete curing had occurred and was evaluated using the following criteria. 
     10: 100 mJ/cm 2    
     9: 150 mJ/cm 2    
     8: 200 mJ/cm 2    
     7: 300 mJ/cm 2    
     6: 400 mJ/cm 2    
     5: 800 mJ/cm 2    
     4: 1,000 mJ/cm 2    
     3: 1,500 mJ/cm 2    
     2: 2,000 mJ/cm 2    
     1: curing does not occur 
     The effects of the present invention with regard to the fixing performance were considered to be operative at 5 or larger. 
     (Storability) 
     The storability was evaluated by placing the ultraviolet-curable liquid developer in a closed container at 50° C. The evaluation criteria are given below. 
     The viscosity change percentage (%) was calculated using the following formula.
 
viscosity change percentage (%)={(viscosity post-storage)−(viscosity pre-storage)}/(viscosity pre-storage)×100
 
     The viscosity was measured as follows using a viscoelastic measurement instrument (Physica MCR300, Anton Paar GmbH). Approximately 2 mL of the sample was filled into the measurement instrument fitted with a cone/plate measurement fixture (75 mm diameter, 1°) and adjustment to 25° C. was carried out. The viscosity was measured while continuously varying the shear rate from 1000 s −1  to 10 s −1 , and the value at 10 s −1  was used as the viscosity. 
     5: the viscosity change percentage after storage for 1 month was not more than ±5%, and a change in the developing performance or fixing performance was also not seen after storage for 1 month 
     4: the viscosity change percentage after storage for 1 month was not more than ±10%, and a large change in the developing performance or fixing performance was also not seen after storage for 1 month 
     3: the viscosity change percentage after storage for 1 month was not more than ±100%, and, while a deterioration in the developing performance and/or fixing performance was seen after storage for 1 month, a good developing performance and fixing performance were obtained upon adjusting the developing conditions or the fixing conditions
 
2: the viscosity change percentage after storage for 1 month was at least ±100%; the developing performance and/or fixing performance was substantially worse after storage for 1 month; and a good developing performance and fixing performance were not obtained upon adjusting the developing conditions or the fixing conditions
 
1: curing and conversion into a solid occurred within 1 month
 
     Examples 2 to 11, Examples 15 to 17, and Comparative Examples 1 to 12 
     Ultraviolet-curable liquid developers were obtained proceeding as in Example 1, but blending the cationically polymerizable liquid monomer, photoinitiator, photopolymerization sensitizer, and photopolymerization sensitizing aid as in Example 1 to provide the compositions given in Tables 1 and 2. 
     Example 12 
     A toner particle dispersion (solids fraction=20 mass %) was obtained proceeding as in (Toner particle production) in Example 1, but changing the cyclohexanedimethanol divinyl ether (example compound B-17) to dodecyl vinyl ether (example compound B-3). 
     The toner particles present in the obtained toner particle dispersion had a volume-average particle diameter of 0.82 μm (measured with a Nanotrac 150 from Nikkiso Co., Ltd., a particle size distribution analyzer based on dynamic light scattering (DLS)). 
     (Preparation of Liquid Developer) 
     An ultraviolet-curable liquid developer was obtained by mixing 10.00 parts of the aforementioned toner particle dispersion; 0.10 parts of hydrogenated lecithin (Lecinol S-10, Nikko Chemicals Co., Ltd.) as a charge control agent; 1.62 parts of dodecyl vinyl ether, 4.81 parts of dicyclopentadiene vinyl ether (example compound B-8), and 81.76 parts of butylethylpropanediol divinyl ether (example compound B-27) as cationically polymerizable liquid monomers; example compound A-3 (0.96 parts) as a photoinitiator; 0.48 parts of 2,4-diethylthioxanthone (referred to hereafter as DETX) as a photopolymerization sensitizer; and 0.48 parts of 1,4-diethoxynaphthalene (referred to hereafter as DEN) as a photopolymerization sensitizing aid. 
     Examples 13 and 14 
     Ultraviolet-curable liquid developers were obtained proceeding as in Example 12, but blending the cationically polymerizable liquid monomer, photoinitiator, photopolymerization sensitizer, and photopolymerization sensitizing aid as in Example 12 to provide the compositions given in Tables 1 and 2. 
     The same evaluations as in Example 1 were carried out using the resulting liquid developers. The results of these evaluations are given in Tables 1 and 2. 
     In addition, the cured film was formed in Examples 15 to 17 and Comparative Examples 5 to 12 by exposure to light with a wavelength of 365 nm using a high-pressure mercury lamp having a lamp output of 120 mW/cm 2  rather than the LED having an emission wavelength of 385 nm. The amount of irradiated light was measured for the point at which there was no surface tack (stickiness) and complete curing had occurred, and the fixing performance was then evaluated using the criteria given above. 
     The following polymerizable liquid monomers and polymerization initiators are also used in the comparative examples. 
     &lt;Radical Polymerizable Monomer (C-1)&gt; 
     1,6-hexanediol diacrylate (radical polymerizable monomer, Osaka Organic Chemical Industry Ltd.) 
     &lt;Photoinitiator (D-1)&gt; 
     CPI-110P (triarylsulfonium salt-type cationic photoinitiator, San-Apro Ltd.) 
     &lt;Photoinitiator (D-2)&gt; 
     WPI-113 (diphenyliodonium salt-type cationic photoinitiator, Wako Pure Chemical Industries, Ltd.) 
     &lt;Photoinitiator (D-3)&gt; 
     IRGACURE (registered trademark) 369 (α-aminoalkylphenone-type radical photoinitiator, BASF Japan Ltd.). 
     &lt;Photoinitiator (D-4)&gt; 
     Lucirin TPO (acylphosphine oxide-type radical photoinitiator, BASF Japan Ltd.) 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                   
                 photo 
                 photo 
                 polymerizable 
                 polymerizable 
                 polymerizable 
                   
               
               
                   
                 photo 
                 polymerization 
                 polymerization 
                 liquid 
                 liquid 
                 liquid 
               
               
                   
                 initiator 
                 sensitizer 
                 sensitizing aid 
                 monomer 1 
                 monomer 2 
                 monomer 3 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 cont. 
                   
                 cont. 
                   
                 cont. 
                   
                 cont. 
                   
                 cont. 
                   
                 cont. 
                   
                   
                   
                   
                   
               
               
                   
                 type 
                 (parts) 
                 type 
                 (parts) 
                 type 
                 (parts) 
                 type 
                 (parts) 
                 type 
                 (parts) 
                 type 
                 (parts) 
                 C 
                 D 
                 E 
                 F 
                 G 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Example 1 
                  A-26 
                 0.30 
                 DETX 
                 0.50 
                 DEN 
                 0.50 
                 B-17 
                 10.00 
                  B-26 
                 30.00 
                 B-28 
                 60.00 
                 8.4 
                 3.1 
                 5 
                 10 
                 5 
               
               
                 Example 2 
                  A-26 
                 0.30 
                 DETX 
                 0.50 
                 DEN 
                 0.50 
                 B-17 
                 10.00 
                  B-26 
                 30.00 
                 B-24 
                 60.00 
                 8.3 
                 2.6 
                 5 
                 10 
                 5 
               
               
                 Example 3 
                  A-26 
                 0.30 
                 DETX 
                 0.50 
                 DEN 
                 0.50 
                 B-17 
                 10.00 
                  B-26 
                 60.00 
                 B-24 
                 30.00 
                 8.2 
                 2.3 
                 5 
                 10 
                 5 
               
               
                 Example 4 
                 A-3 
                 0.30 
                 DETX 
                 0.50 
                 DEN 
                 0.50 
                 B-17 
                 10.00 
                  B-26 
                 60.00 
                 B-24 
                 30.00 
                 8.2 
                 2.3 
                 5 
                 9 
                 5 
               
               
                 Example 5 
                 A-3 
                 0.30 
                 DETX 
                 0.50 
                 DEN 
                 0.50 
                 B-17 
                 10.00 
                 B-8 
                 10.00 
                 B-24 
                 80.00 
                 8.6 
                 2.7 
                 5 
                 8 
                 5 
               
               
                 Example 6 
                 A-3 
                 0.30 
                 DETX 
                 0.50 
                 DEN 
                 0.50 
                 B-17 
                 10.00 
                 B-3 
                 10.00 
                 B-24 
                 80.00 
                 8.3 
                 2.7 
                 5 
                 7 
                 5 
               
               
                 Example 7 
                 A-3 
                 0.30 
                 DETX 
                 0.50 
                 DEN 
                 0.50 
                 B-17 
                 10.00 
                 B-3 
                 50.00 
                 B-24 
                 40.00 
                 8.3 
                 1.9 
                 5 
                 7 
                 5 
               
               
                 Example 8 
                 A-3 
                 0.10 
                 DETX 
                 0.50 
                 DEN 
                 0.50 
                 B-17 
                 10.00 
                 B-3 
                 10.00 
                 B-24 
                 80.00 
                 8.3 
                 2.7 
                 5 
                 7 
                 5 
               
               
                 Example 9 
                 A-3 
                 1.00 
                 DETX 
                 0.50 
                 DEN 
                 0.50 
                 B-17 
                 10.00 
                 B-3 
                 10.00 
                 B-24 
                 80.00 
                 8.3 
                 2.7 
                 5 
                 7 
                 5 
               
               
                 Example 10 
                 A-3 
                 0.03 
                 DETX 
                 0.50 
                 DEN 
                 0.50 
                 B-17 
                 10.00 
                 B-3 
                 10.00 
                 B-24 
                 80.00 
                 8.3 
                 2.7 
                 5 
                 5 
                 5 
               
               
                 Example 11 
                 A-3 
                 5.00 
                 DETX 
                 0.50 
                 DEN 
                 0.50 
                 B-17 
                 10.00 
                 B-3 
                 10.00 
                 B-24 
                 80.00 
                 8.3 
                 2.7 
                 4 
                 6 
                 3 
               
               
                 Example 12 
                 A-3 
                 1.00 
                 DETX 
                 0.50 
                 DEN 
                 0.50 
                 B-3  
                 10.00 
                 B-8 
                 5.00 
                 B-27 
                 85.00 
                 8.2 
                 1.8 
                 5 
                 7 
                 5 
               
               
                 Example 13 
                 A-3 
                 0.03 
                 DETX 
                 0.50 
                 DEN 
                 0.50 
                 B-3  
                 10.00 
                 B-8 
                 5.00 
                 B-27 
                 85.00 
                 8.2 
                 1.8 
                 5 
                 5 
                 5 
               
               
                 Example 14 
                 A-3 
                 5.00 
                 DETX 
                 0.50 
                 DEN 
                 0.50 
                 B-3  
                 10.00 
                 B-8 
                 5.00 
                 B-27 
                 85.00 
                 8.2 
                 1.8 
                 4 
                 6 
                 3 
               
               
                 Comparative 
                 A-3 
                 0.008 
                 DETX 
                 0.50 
                 DEN 
                 0.50 
                 B-17 
                 10.00 
                 B-3 
                 10.00 
                 B-24 
                 80.00 
                 8.3 
                 2.7 
                 5 
                 3 
                 5 
               
               
                 Example 1 
               
               
                 Comparative 
                 A-3 
                 7.00 
                 DETX 
                 0.50 
                 DEN 
                 0.50 
                 B-17 
                 10.00 
                 B-3 
                 10.00 
                 B-24 
                 80.00 
                 8.3 
                 2.7 
                 3 
                 6 
                 2 
               
               
                 Example 2 
               
               
                 Comparative 
                 A-3 
                 0.30 
                 DETX 
                 0.50 
                 DEN 
                 0.50 
                 B-17 
                 10.00 
                 B-8 
                 40.00 
                 B-24 
                 50.00 
                 9.3 
                 2.0 
                 3 
                 3 
                 5 
               
               
                 Example 3 
               
               
                 Comparative 
                 A-3 
                 0.30 
                 DETX 
                 0.50 
                 DEN 
                 0.50 
                 B-17 
                 10.00 
                 B-3 
                 60.00 
                 B-24 
                 30.00 
                 8.2 
                 1.7 
                 5 
                 3 
                 5 
               
               
                 Example 4 
               
               
                   
               
               
                 In the Table 1, 
               
               
                 C: molar average SP value of the cationically polymerizable liquid monomer 
               
               
                 D: molar average number of functional groups of the cationically polymerizable liquid monomer 
               
               
                 E: developing performance 
               
               
                 F: fixing performance 
               
               
                 G: storability 
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                   
                 photo 
                 photo 
                 polymerizable 
                 polymerizable 
                 polymerizable 
                   
               
               
                   
                 photo 
                 polymerization 
                 polymerization 
                 liquid 
                 liquid 
                 liquid 
               
               
                   
                 initiator 
                 sensitizer 
                 sensitizing aid 
                 monomer 1 
                 monomer 2 
                 monomer 3 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 cont. 
                   
                 cont. 
                   
                 cont. 
                   
                 cont. 
                   
                 cont. 
                   
                 cont. 
                   
                   
                   
                   
                   
               
               
                   
                 type 
                 (parts) 
                 type 
                 (parts) 
                 type 
                 (parts) 
                 type 
                 (parts) 
                 type 
                 (parts) 
                 type 
                 (parts) 
                 C 
                 D 
                 E 
                 F 
                 G 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Example 15 
                 A-26 
                 0.30 
                 DETX 
                 0.50 
                 DEN 
                 0.50 
                 B-17 
                 10.00 
                 B-26 
                 30.00 
                 B-28 
                 60.00 
                 8.4 
                 3.1 
                 5 
                 10 
                 5 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Example 16 
                 A-26 
                 0.30 
                 DETX 
                 0.50 
                 none 
                 B-17 
                 10.00 
                 B-26 
                 30.00 
                 B-28 
                 60.00 
                 8.4 
                 3.1 
                 5 
                 8 
                 5 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Example 17 
                 A-26 
                 0.30 
                 none 
                 none 
                 B-17 
                 10.00 
                 B-26 
                 30.00 
                 B-28 
                 60.00 
                 8.4 
                 3.1 
                 5 
                 6 
                 5 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Comparative 
                 none 
                 none 
                 none 
                 B-17 
                 10.00 
                 B-26 
                 30.00 
                 B-28 
                 60.00 
                 8.4 
                 3.1 
                 5 
                 1 
                 5 
               
               
                 Example 5 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Comparative 
                 D-1 
                 1.00 
                 none 
                 none 
                 B-17 
                 10.00 
                 B-26 
                 30.00 
                 B-28 
                 60.00 
                 8.4 
                 3.1 
                 1 
                 2 
                 5 
               
               
                 Example 6 
               
               
                 Comparative 
                 D-1 
                 5.00 
                 none 
                 none 
                 B-17 
                 10.00 
                 B-26 
                 30.00 
                 B-28 
                 60.00 
                 8.4 
                 3.1 
                 1 
                 10 
                 5 
               
               
                 Example 7 
               
               
                 Comparative 
                 D-2 
                 1.00 
                 none 
                 none 
                 B-17 
                 10.00 
                 B-26 
                 30.00 
                 B-28 
                 60.00 
                 8.4 
                 3.1 
                 1 
                 2 
                 5 
               
               
                 Example 8 
               
               
                 Comparative 
                 D-2 
                 5.00 
                 none 
                 none 
                 B-17 
                 10.00 
                 B-26 
                 30.00 
                 B-28 
                 60.00 
                 8.4 
                 3.1 
                 1 
                 10 
                 5 
               
               
                 Example 9 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Comparative 
                 D-4 
                 2.50 
                 none 
                 none 
                 C-1  
                 100 
                 none 
                 none 
                 9.6 
                 2.0 
                 2 
                 1 
                 5 
               
               
                 Example 10 
               
               
                 Comparative 
                 D-4 
                 10.00 
                 none 
                 none 
                 C-1  
                 100 
                 none 
                 none 
                 9.6 
                 2.0 
                 1 
                 3 
                 5 
               
               
                 Example 11 
               
               
                 Comparative 
                 D-3 
                 10.00 
                 none 
                 none 
                 C-1  
                 100 
                 none 
                 none 
                 9.6 
                 2.0 
                 1 
                 3 
                 5 
               
               
                 Example 12 
               
               
                   
               
               
                 In the Table 2, 
               
               
                 C: molar average SP value of the cationically polymerizable liquid monomer 
               
               
                 D: molar average number of functional groups of the cationically polymerizable liquid monomer 
               
               
                 E: developing performance 
               
               
                 F: fixing performance 
               
               
                 G: storability 
               
            
           
         
       
     
     Table 3 gives the SP values and the number of polymerizable functional groups per 1 molecule for the polymerizable liquid monomers in Tables 1 and 2. 
     The “content (parts)” in Tables 1 and 2 gives the proportion of the particular material as mass parts per 100 mass parts of the total polymerizable liquid monomer. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
                   
                 number of 
               
               
                   
                   
                   
                 polymerizable 
               
               
                   
                 polymerizable 
                   
                 functional 
               
               
                   
                 liquid 
                   
                 groups per 1 
               
               
                   
                 monomer 
                 SP value 
                 molecule 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 B-3 
                 8.1 
                 1 
               
               
                   
                 B-8 
                 10.4 
                 1 
               
               
                   
                 B-17 
                 8.8 
                 2 
               
               
                   
                 B-24 
                 8.3 
                 3 
               
               
                   
                 B-26 
                 8.1 
                 2 
               
               
                   
                 B-27 
                 8.1 
                 2 
               
               
                   
                 B-28 
                 8.5 
                 4 
               
               
                   
                 C-1 
                 9.6 
                 2 
               
               
                   
                   
               
            
           
         
       
     
     The ultraviolet-curable liquid developers in Tables 1 and 2 that had a fixing performance of rank 5 or greater, a developing performance of rank 3 or greater, and a storability of rank 3 or greater provided a satisfactory image density, exhibited little image blurring, and had a satisfactory fixing performance and an excellent storability. 
     A comparison of Examples 1 to 14 with Comparative Examples 1 to 4 shows that high levels can be achieved for the developing performance, fixing performance, and storability by using the ultraviolet-curable liquid developer of the present invention. 
     It is additionally shown that even better levels are achieved for the developing performance, fixing performance, and storability when the following conditions are satisfied: 
     a content of the compound with formula (1) of at least 0.05 mass parts and not more than 1.00 mass part per 100 mass parts of the cationically polymerizable liquid monomer; 
     the cationically polymerizable liquid monomer is formed of cationically polymerizable liquid monomer for which the number of polymerizable functional groups per 1 molecule is at least 2; 
     the cationically polymerizable liquid monomer is formed of cationically polymerizable liquid monomer having an SP value of not more than 9.0; and 
     the photoinitiator is a compound with formula (2) above. 
     The results for the comparative examples in Table 2 demonstrate that the developing performance and fixing performance are unable to coexist when, in order to obtain a satisfactory fixing performance, a polymerization initiator that facilitates a decline in the volume resistivity is used in large amounts. 
     In contrast to this, it is shown in Example 17 of the present invention that an excellent developing performance can be obtained even while a good fixing performance is obtained. 
     It is further shown that an excellent fixing performance is obtained—without impairing the developing performance—through the use of a thioxanthone compound as the photopolymerization sensitizer and a naphthalene compound as the photopolymerization sensitizing aid. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2015-107305, filed May 27, 2015, Japanese Patent Application No. 2016-43296, filed Mar. 7, 2016, which are hereby incorporated by reference herein in their entirety.