ULTRAVIOLET CURABLE INKJET INK SET AND PRODUCTION METHOD OF PRINTED MATTER

Abstract

By the use of an ultraviolet curable inkjet ink set for a line head printer comprising a lower layer ink containing a polymerizable monomer having a specific structure, and, a specific photopolymerization initiator, and an upper layer ink containing a polymerizable monomer having a specific structure and a specific photopolymerization initiator, wherein among the lower layer ink and the upper layer ink, one of the inks is a white ink and the other ink is a black ink, a good printed matter having excellent adhesion between a recording medium and the ink layer, and, between different ink layers can be obtained without generating curing wrinkles and curling over time even during high speed printing.

Claims

1. An ultraviolet curable inkjet ink set comprising a lower layer ink and an upper layer ink, and used in a line pass inkjet printer having a temporary curing step, wherein among the lower layer ink and the upper layer ink, one of the inks is a white ink, and the other ink is a black ink, the lower layer ink contains a first polymerizable monomer, a first photopolymerization initiator, and a first colorant, the first polymerizable monomer comprises a compound represented by the following General formula (A), the first photopolymerization initiator contains a first acylphosphine oxide photopolymerization initiator, a first benzophenone photopolymerization initiator, and 2-hydroxy-1-{1-[4-(2-hydroxy-2-methylpropionyl) phenyl]-1,3,3-trimethylindane-5-yl}-2-methylpropane-1-one, the upper layer ink contains a second polymerizable monomer, a second photopolymerization initiator, and a second colorant, the second polymerizable monomer comprises a compound represented by the following General formula (A), and the second photopolymerization initiator contains a second acylphosphine oxide photopolymerization initiator, and a second benzophenone photopolymerization initiator.
CH.sub.2═CH—CO—(O—CH.sub.2CH.sub.2).sub.n—O—R   General formula (A): (In General formula (A), R represents an acryloyl group or a vinyl group, and n represents an integer from 2 to 10. Further, the compound represented by General formula (A) in the first polymerizable monomer and the compound represented by General formula (A) in the second polymerizable monomer may be the same or different.)

2. The ultraviolet curable inkjet ink set according to claim 1, wherein R of the compound represented by General formula (A) in the first polymerizable monomer and R of the compound represented by General formula (A) in the second polymerizable monomer are both vinyl groups.

3. The ultraviolet curable inkjet ink set according to claim 1, wherein the upper layer ink further contains a photopolymerization initiator having an amino group.

4. The ultraviolet curable inkjet ink set according to claim 1, wherein the lower layer ink is substantially free of a thioxanthone-based photopolymerization initiator, and, a photopolymerization initiator having an amino group.

5. A production method of a printed matter which uses the ultraviolet curable inkjet ink set according to claim 1, comprising Step 1 for discharging and imparting the lower layer ink on a substrate, Step 2 for irradiating an ultraviolet ray from an ultraviolet light emitting diode to the substrate imparted with the lower layer ink obtained in Step 1, Step 3 for discharging and imparting the upper layer ink to the substrate imparted with the lower layer ink obtained in Step 2 in order to at least partially overlap with the lower layer ink, and Step 4 for irradiating an ultraviolet ray from an ultraviolet lamp to the substrate imparted with the lower layer ink and the upper layer ink obtained in Step 3.

6. The ultraviolet curable inkjet ink set according to claim 2, wherein the upper layer ink further contains a photopolymerization initiator having an amino group.

7. The ultraviolet curable inkjet ink set according to claim 2, wherein the lower layer ink is substantially free of a thioxanthone-based photopolymerization initiator, and, a photopolymerization initiator having an amino group.

8. A production method of a printed matter which uses the ultraviolet curable inkjet ink set according to claim 2, comprising Step 1 for discharging and imparting the lower layer ink on a substrate, Step 2 for irradiating an ultraviolet ray from an ultraviolet light emitting diode to the substrate imparted with the lower layer ink obtained in Step 1, Step 3 for discharging and imparting the upper layer ink to the substrate imparted with the lower layer ink obtained in Step 2 in order to at least partially overlap with the lower layer ink, and Step 4 for irradiating an ultraviolet ray from an ultraviolet lamp to the substrate imparted with the lower layer ink and the upper layer ink obtained in Step 3.

9. The ultraviolet curable inkjet ink set according to claim 3, wherein the lower layer ink is substantially free of a thioxanthone-based photopolymerization initiator, and, a photopolymerization initiator having an amino group.

10. A production method of a printed matter which uses the ultraviolet curable inkjet ink set according to claim 3, comprising Step 1 for discharging and imparting the lower layer ink on a substrate, Step 2 for irradiating an ultraviolet ray from an ultraviolet light emitting diode to the substrate imparted with the lower layer ink obtained in Step 1, Step 3 for discharging and imparting the upper layer ink to the substrate imparted with the lower layer ink obtained in Step 2 in order to at least partially overlap with the lower layer ink, and Step 4 for irradiating an ultraviolet ray from an ultraviolet lamp to the substrate imparted with the lower layer ink and the upper layer ink obtained in Step 3.

11. The ultraviolet curable inkjet ink set according to claim 6, wherein the lower layer ink is substantially free of a thioxanthone-based photopolymerization initiator, and, a photopolymerization initiator having an amino group.

12. A production method of a printed matter which uses the ultraviolet curable inkjet ink set according to claim 4, comprising Step 1 for discharging and imparting the lower layer ink on a substrate, Step 2 for irradiating an ultraviolet ray from an ultraviolet light emitting diode to the substrate imparted with the lower layer ink obtained in Step 1, Step 3 for discharging and imparting the upper layer ink to the substrate imparted with the lower layer ink obtained in Step 2 in order to at least partially overlap with the lower layer ink, and Step 4 for irradiating an ultraviolet ray from an ultraviolet lamp to the substrate imparted with the lower layer ink and the upper layer ink obtained in Step 3.

13. A production method of a printed matter which uses the ultraviolet curable inkjet ink set according to claim 6, comprising Step 1 for discharging and imparting the lower layer ink on a substrate, Step 2 for irradiating an ultraviolet ray from an ultraviolet light emitting diode to the substrate imparted with the lower layer ink obtained in Step 1, Step 3 for discharging and imparting the upper layer ink to the substrate imparted with the lower layer ink obtained in Step 2 in order to at least partially overlap with the lower layer ink, and Step 4 for irradiating an ultraviolet ray from an ultraviolet lamp to the substrate imparted with the lower layer ink and the upper layer ink obtained in Step 3.

14. A production method of a printed matter which uses the ultraviolet curable inkjet ink set according to claim 7, comprising Step 1 for discharging and imparting the lower layer ink on a substrate, Step 2 for irradiating an ultraviolet ray from an ultraviolet light emitting diode to the substrate imparted with the lower layer ink obtained in Step 1, Step 3 for discharging and imparting the upper layer ink to the substrate imparted with the lower layer ink obtained in Step 2 in order to at least partially overlap with the lower layer ink, and Step 4 for irradiating an ultraviolet ray from an ultraviolet lamp to the substrate imparted with the lower layer ink and the upper layer ink obtained in Step 3.

15. A production method of a printed matter which uses the ultraviolet curable inkjet ink set according to claim 9, comprising Step 1 for discharging and imparting the lower layer ink on a substrate, Step 2 for irradiating an ultraviolet ray from an ultraviolet light emitting diode to the substrate imparted with the lower layer ink obtained in Step 1, Step 3 for discharging and imparting the upper layer ink to the substrate imparted with the lower layer ink obtained in Step 2 in order to at least partially overlap with the lower layer ink, and Step 4 for irradiating an ultraviolet ray from an ultraviolet lamp to the substrate imparted with the lower layer ink and the upper layer ink obtained in Step 3.

16. A production method of a printed matter which uses the ultraviolet curable inkjet ink set according to claim 11, comprising Step 1 for discharging and imparting the lower layer ink on a substrate, Step 2 for irradiating an ultraviolet ray from an ultraviolet light emitting diode to the substrate imparted with the lower layer ink obtained in Step 1, Step 3 for discharging and imparting the upper layer ink to the substrate imparted with the lower layer ink obtained in Step 2 in order to at least partially overlap with the lower layer ink, and Step 4 for irradiating an ultraviolet ray from an ultraviolet lamp to the substrate imparted with the lower layer ink and the upper layer ink obtained in Step 3.

Description

EXAMPLES

[0131] The present invention will be described in further detail below. The following examples do not limit the scope of rights of the present invention in any way. Further, unless otherwise stated, the term “parts” indicates parts by mass and “%” indicates % by mass.

(Preparation Example of White Ink W1)

[0132] A white pigment dispersion was prepared prior to the white ink preparation. 50 parts of titanium oxide (“Tipaque PF740” manufactured by Ishihara Sangyo Kaisha, Ltd.) as the pigment, 3.75 parts of a pigment dispersant (“Solsperse 32000” manufactured by Lubrizol Corporation), and, 21.25 parts of 2-(2-vinyloxyethoxy)ethyl acrylate (“VEEA” manufactured by NIPPON SHOKUBAI CO., LTD.) as the polymerizable monomer (dispersion medium), and 25 parts of dipropylene glycol diacrylate (“DPGDA” (product name: M222) manufactured by Miwon Specialty Chemical Co., Ltd) were charged in a tank, and stirred with a high speed mixer until uniform, and then, a white pigment dispersion WB1 was obtained by dispersing with a horizontal sand mill for approximate 5 hours.

[0133] Subsequently, a polymerizable monomer, a photopolymerization initiator, the surface modifier, and a polymerization inhibitor were added sequentially while stirring in the white pigment dispersion WB1 obtained as stated above so as to form the blending formulation described in the row “W1” of Table 3 which will be described later, and mixed until the photopolymerization initiator dissolved. Moreover, filtration was performed using a depth type filter having a 0.5 μm pore size, and the white ink W1 was obtained by removing the coarse particles. Note that, the addition of each material may be performed in any sequence.

(Preparation Example of Black Ink K1)

[0134] Subsequently, a black pigment dispersion was prepared prior to the black ink preparation. 20 parts of carbon black (“Special Black 350” manufactured by Orion Engineered Carbons) as the pigment, 8 parts of pigment dispersant(“Solsperse 32000” manufactured by Lubrizol Corporation), and, 32 parts of 2-(2-vinyloxyethoxy)ethyl acrylate(“VEEA” manufactured by NIPPON SHOKUBAI CO., LTD.) as the polymerizable monomer (dispersion medium), 40 parts of dipropylene glycol diacrylate (“DPGDA” (product name: M222) manufactured by Miwon Specialty Chemical Co., Ltd) were charged in a tank, and stirred with a high speed mixer until uniform, and then, a black pigment dispersion BB1 was obtained by dispersing with a horizontal sand mill for approximate 5 hours.

[0135] Subsequently, a polymerizable monomer, a photopolymerization initiator, a surface modifier, and a polymerization inhibitor were added while stirring to the black pigment dispersion BB1 obtained as stated above so as to form the blending formulation described in the example of “K1” in Table 4 which will be described later, and mixed until the photopolymerization initiator dissolved. Moreover, filtration was performed using a depth type filter having a 0.5 μm pore size and the black ink K1 was obtained by removing the coarse particles.

<Production of Printed Matter>

[0136] The obtained white ink W1 was combined with the black ink K1 to make an ink set. Further, the white ink W1 was made as the lower layer ink and the black ink K1 was made as the upper layer ink, and the printed matter was produced by the following method, and the evaluations which will be described later were performed.

[0137] Two inkjet heads (KJ4A-AA) manufactured by KYOCERA Corporation were mounted, an ultraviolet light emitting diode (“G5A” manufactured by KYOCERA Corporation, emission wavelength 395 nm and set to 80% output) for temporary curing the lower layer ink was provided between the two inkjet heads, and furthermore, the printed matter was produced using an inkjet discharge apparatus (“OnePass JET” manufactured by Tritek Co., Ltd.) in which an ultraviolet lamp (240 W/cm metal halide lamp manufactured by GEW, Ltd., set to 80% output) was disposed further on the downstream side than the inkjet head (inkjet head arranged on the downstream side with respect to the transport direction of the recording medium) for discharging the upper layer ink, and passing through all of the aforementioned Steps 1 to 4.

[0138] Note that, a PET substrate (“PET50(K2411)” manufactured by Lintec Corporation) was used as the recording medium, and the printing was performed for each of the lower layer ink and the upper layer ink at a droplet volume of 14 pl and a print rate of 100%. Further, by printing so that the area of the ink layer having a print rate of 100% with the upper layer ink is smaller than the area of the ink layer having a print rate of 100% with the lower layer ink, there is a portion where only the lower layer ink was printed and a portion in which the ink layer made of the upper layer ink was laminated on the ink layer made of the lower layer ink in one printed matter.

Example 1

[0139] The following evaluations were performed with regards to the printed matter produced by the aforementioned method. The evaluation results are as illustrated in Table 5.

<Evaluation of Adhesion between Lower Layer Ink Layer and Recording Medium>

[0140] Among the produced printed matters, the portion where only the lower layer ink was printed was used and the adhesion between the ink layer made by the lower layer ink and the recording medium was evaluated in accordance with JIS K 5600-5-6 (Cross cut method). Moreover, the evaluations were based on the following scale of 1 to 5, and grades of 3 or more were deemed to be in the practicable region.

[0141] 5: no peeling was observed

[0142] 4: no squares were completely peeled, but there was partial peeling only at the cut end

[0143] 3: less than 10% of the squares were completed peeled

[0144] 2: 10% to less than 50% of the squares were completed peeled

[0145] 1: 50% or more of the squares were completed peeled

<Evaluation of Adhesion between Lower Layer Ink Layer and Upper Layer Ink Layer>

[0146] Among the produced printed matters, the portion in which the ink layer made of the upper layer ink was laminated on the ink layer made of the lower layer ink was used to evaluate the adhesion between the ink layer made by the lower layer ink and the ink layer made by the upper layer ink in accordance with JIS K 5600-5-6 (Cross cut method). Moreover, the evaluations were based on the following scale of 1 to 5, and grades of 3 or more were deemed to be in the practicable region.

[0147] 5: no peeling was observed

[0148] 4: no squares were completely peeled, but there was partial peeling only at the cut end

[0149] 3: less than 10% of the squares were completed peeled

[0150] 2: 10% to less than 50% of the squares were completed peeled

[0151] 1: 50% or more of the squares were completed peeled

<Whiteness Evaluation of White Ink>

[0152] Among the produced printed matters, “X-Rite eXact” manufactured by X-Rite, Inc. was used and the measurements for the portion where only white ink was printed were performed under the conditions of a viewing angle of 2°, a D50 light source, and a CIE color system. Moreover, based on the results of the measurements, the evaluations were based on the following scale of 1 to 5, and grades of 3 or more were deemed to be in the practicable region.

[0153] 5: b value was less than 2.0

[0154] 4: b value was 2.0 to less than 3.0

[0155] 3: b value was 3.0 to less than 4.0

[0156] 2: b value was 4.0 to less than 5.0

[0157] 1: b value was 5.0 or more

<Evaluation of Curing Wrinkles>

[0158] The evaluation of the curing wrinkles in the laminated portions of the lower layer ink and the upper layer ink was performed. Among the produced printed matters, a Micro-gloss manufactured by BYK Chemie Japan KK was used and the measurement of the glossiness was performed at a measurement angle of 60° for the portion in which the ink layer made of the upper layer ink was laminated on the ink layer made of the lower layer ink. Moreover, based on the results of the measurement, the evaluations were based on the following scale of 1 to 5, and grades of 3 or more were deemed to be in the practicable region.

[0159] 5: gloss value was 75 or more

[0160] 4: gloss value was 60 to less than 75

[0161] 3: gloss value was 50 to less than 60

[0162] 2: gloss value was 40 to less than 50

[0163] 1: gloss value was less than 40

<Evaluation of Substrate Curling Over Time>

[0164] Among the produced printed matters, the portion in which the ink layer made of the upper layer ink was laminated on the ink layer made of the lower layer ink was cut into 10 cm.sup.2 square shapes, and placed in an oven set to a temperature of 60° C. and a humidity of 20% RH for 14 days. Further, after being removed from the oven, the samples were left standing for one hour under an environment having a temperature of 25° C. and a humidity of 50% RH. Moreover, the curling over time of the printed matter was evaluated by placing the cut printed matter on a flat surface, and measuring the length floating from the flat surface for the respective four vertices, and then summing the floating lengths of the four vertices. Based on the results of this measurement, the evaluations were based on the following scale of 1 to 5, and grades of 3 or more were deemed to be in the practicable region.

[0165] 5: sum of the floating lengths of the four vertices was less than 25 mm

[0166] 4: sum of the floating lengths of the four vertices was from 25 mm to less than 40 mm

[0167] 3: sum of the floating lengths of the four vertices was from 40 mm to less than 60 mm

[0168] 2: sum of the floating lengths of the four vertices was from 60 mm to less than 80 mm

[0169] 1: sum of the floating lengths of the four vertices was 80 mm or more, or the printed matter was deformed into a cylindrical shape and could not be measured

<Preparation Examples of White Inks W2 to 23 and Black Inks K2 to 17>

[0170] Except for changing the type and the blending amount of the polymerizable monomer as shown in Table 1, the white pigment dispersions WB2 to WB7 were prepared in the same manner as the white pigment dispersion WB1. Further, except for changing the type and the blending amount of the polymerizable monomer as shown in Table 2, black pigment dispersions BB2 to BB7 were prepared in the same manner as the black pigment dispersion BB1.

TABLE-US-00001 TABLE 1 White pigment dispersion WB1 WB2 WB3 WB4 WB5 WB6 WB7 Pigment PF740 50 50 50 50 50 50 50 Pigment disspersant Solsperse 32000 3.75 3.75 3.75 3.75 3.75 3.75 3.75 Polymerizable Compound represented VEEA 21.25 21.25 31.25 monomer by General formula (A) PEG200DA 21.25 DEGDA 21.25 Other polymerizable EGDA 21.25 monomer PEA 21.25 HDDA 25 15 DPGDA 25 25 25 25 25

TABLE-US-00002 TABLE 2 Black pigment dispersion BB1 BB2 BB3 BB4 BB5 BB6 BB7 Pigment Special Black350 20 20 20 20 20 20 20 Pigment disspersant Solsperse 32000 8 8 8 8 8 8 8 Polymerizable Compound represented VEEA 32 monomer by General formula (A) PEG200DA 32 DEGDA 32 Other polymerizable EGDA 32 monomer PEA 40 BzA 32 32 32 HDDA 40 DPGDA 40 40 40 40 40

[0171] Moreover, the white ink and the black ink were prepared according to the respective blending formulations of Table 3 and Table 4.

TABLE-US-00003 TABLE 3 W1 W2 W3 W4 W5 W6 Pigment White pigment dispersion WB1 WB1 WB2 WB4 WB1 WB1 dispersion Blending amount 40 40 40 40 40 40 Polymerizable Compound represented VEEA 31.5 49.5 31.5 31.5 monomer by General formula (A) PEG200DA 13 49.5 13 9 PEG400DA DEGDA 49.5 Other polymerizable EGDA monomer PEA BzA HDDA DPGDA 5 5 5 TMP (EO)TA DPHA Photopolymerization Acylphosphine oxide- Omnirad TPO H initiator based Omnirad 819 4 4 4 4 4 4 Benzophenone-based Omnirad BMS 2 2 2 GENOPOL BP-2 2 2 2 tMeInd SpeedCure XFs01 4 4 4 4 4 8 Thioxanthone-based Omnirad ITX Amino group- Alkylaminoacetophenone- Omnirad 369 containing based photopolymerization Aminobenzoate-based Omnirad EDB initiator Esacure A198 Surface modifier BYK UV3510 0.4 0.4 0.4 0.4 0.4 0.4 Polymerization inhibitor BHT 0.1 0.1 0.1 0.1 0.1 0.1 Total amount 100 100 100 100 100 100 W7 W8 W9 W10 W11 W12 Pigment White pigment dispersion WB1 WB1 WB5 WB6 WB5 WB6 dispersion Blending amount 40 40 40 40 40 40 Polymerizable Compound represented VEEA 31.5 31.5 1.5 47.5 41.5 42.5 monomer by General formula (A) PEG200DA 15 PEG400DA 9 DEGDA Other polymerizable EGDA monomer PEA 15 BzA 15 2 HDDA 17 DPGDA 5 5 TMP (EO)TA 8 DPHA 5 Photopolymerization Acylphosphine oxide- Omnirad TPO H 2 4 initiator based Omnirad 819 4 4 4 2 4 Benzophenone-based Omnirad BMS 2 2 3 2 2 GENOPOL BP-2 4 tMeInd SpeedCure XFs01 8 2 4 4 4 4 Thioxanthone-based Omnirad ITX Amino group- Alkylaminoacetophenone- Omnirad 369 containing based photopolymerization Aminobenzoate-based Omnirad EDB initiator Esacure A198 Surface modifier BYK UV3510 0.4 0.4 0.4 0.4 0.4 0.4 Polymerization inhibitor BHT 0.1 0.1 0.1 0.1 0.1 0.1 Total amount 100 100 100 100 100 100 W13 W14 W15 W16 W17 W18 Pigment White pigment dispersion WB1 WB1 WB1 WB1 WB1 WB7 dispersion Blending amount 40 40 40 40 40 40 Polymerizable Compound represented VEEA 31.5 31.5 31.5 31.5 31.5 monomer by General formula (A) PEG200DA 6 6 6 6 8 PEG400DA DEGDA Other polymerizable EGDA monomer PEA 4.5 BzA 40 HDDA 5 5 5 5 DPGDA 6 6 5 6 10 5 TMP (EO)TA DPHA Photopolymerization Acylphosphine oxide- Omnirad TPO H 4 4 4 4 initiator based Omnirad 819 4 4 Benzophenone-based Omnirad BMS 2 2 2 2 4 2 GENOPOL BP-2 tMeInd SpeedCure XFs01 5 4 4 4 4 Thioxanthone-based Omnirad ITX 1 Amino group- Alkylaminoacetophenone- Omnirad 369 1 containing based photopolymerization Aminobenzoate-based Omnirad EDB 2 initiator Esacure A198 2 Surface modifier BYK UV3510 0.4 0.4 0.4 0.4 0.4 0.4 Polymerization inhibitor BHT 0.1 0.1 0.1 0.1 0.1 0.1 Total amount 100 100 100 100 100 100 W19 W20 W21 W22 W23 Pigment White pigment dispersion WB3 WB1 WB1 WB1 WB1 dispersion Blending amount 40 40 40 40 40 Polymerizable Compound represented VEEA 31.5 31.5 31.5 31.5 monomer by General formula (A) PEG200DA 11 11 13 6 PEG400DA DEGDA Other polymerizable EGDA 49.5 monomer PEA BzA HDDA 5 DPGDA 5 5 5 6 TMP (EO)TA DPHA Photopolymerization Acylphosphine oxide- Omnirad TPO H 8 4 4 initiator based Omnirad 819 4 4 Benzophenone-based Omnirad BMS 2 4 2 GENOPOL BP-2 tMeInd SpeedCure XFs01 4 8 4 3 Thioxanthone-based Omnirad ITX Amino group- Alkylaminoacetophenone- Omnirad 369 4 containing based photopolymerization Aminobenzoate-based Omnirad EDB initiator Esacure A198 Surface modifier BYK UV3510 0.4 0.4 0.4 0.4 0.4 Polymerization inhibitor BHT 0.1 0.1 0.1 0.1 0.1 Total amount 100 100 100 100 100

TABLE-US-00004 TABLE 4 K1 K2 K3 K4 K5 Pigment Black pigment dispersion BB1 BB1 BB2 BB4 BB1 dispersion Blending amount 12.5 12.5 12.5 12.5 12.5 Polymerizable Compound VEEA 36 73 36 monomer represented PEG200DA 73 by General PEG400DA formula (A) DEGDA 73 Other EGDA polymerizable PEA monomer BzA HDDA DPGDA 41 41 TMP (EO)TA DPHA Photopolymerization Acylphosphine oxide- Omnirad TPO H initiator based Omnirad 819 4 4 4 4 4 Benzophenone- Omnirad BMS 6 6 based GENOPOL BP-2 6 6 6 tMeInd SpeedCure XFs01 Amino group- Alkylaminoacetophenone- Omnirad 369 containing based photopolymerization Aminobenzoate-based Omnirad EDB initiator Esacure A198 4 4 4 GENOPOL AB-2 Surface modifier BYK UV3510 0.4 0.4 0.4 0.4 0.4 Polymerization inhibitor BHT 0.1 0.1 0.1 0.1 0.1 Total amount 100 100 100 100 100 K6 K7 K8 K9 Pigment Black pigment dispersion BB1 BB1 BB1 BB1 dispersion Blending amount 12.5 12.5 12.5 12.5 Polymerizable Compound VEEA 36 36 36 36 monomer represented PEG200DA by General PEG400DA 9 formula (A) DEGDA Other EGDA polymerizable PEA monomer BzA HDDA DPGDA 39 28 37 37 TMP (EO)TA DPHA Photopolymerization Acylphosphine oxide- Omnirad TPO H 6 initiator based Omnirad 819 4 4 4 Benzophenone- Omnirad BMS 6 6 6 based GENOPOL BP-2 6 tMeInd SpeedCure XFs01 Amino group- Alkylaminoacetophenone- Omnirad 369 2 containing based photopolymerization Aminobenzoate-based Omnirad EDB 2 initiator Esacure A198 4 4 GENOPOL AB-2 Surface modifier BYK UV3510 0.4 0.4 0.4 0.4 Polymerization inhibitor BHT 0.1 0.1 0.1 0.1 Total amount 100 100 100 100 K10 K11 K12 K13 K14 K15 K16 K17 Pigment Black pigment dispersion BB1 BB5 BB6 BB1 BB3 BB7 BB1 BB1 dispersion Blending amount 12.5 12.5 12.5 12.5 12.5 12.5 12.5 12.5 Polymerizable Compound represented VEEA 36 13 60 36 36 36 monomer by General formula (A) PEG200DA PEG400DA DEGDA 20 Other polymerizable EGDA 73 monomer PEA 5 BzA 16 6 36 HDDA 29 DPGDA 37 11 39 39 39 TMP (EO)TA 20 DPHA 6 Photopolymerization Acylphosphine oxide- Omnirad TPO H 4 6 initiator based Omnirad 819 4 4 4 6 4 4 4 Benzophenone-based Omnirad BMS 6 5 6 6 6 6 6 GENOPOL BP-2 tMeInd SpeedCure XFs01 4 4 Amino group- Alkylaminoacetophenone- Omnirad 369 containing based photopolymerization Aminobenzoate-based Omnirad EDB 2 2 2 initiator Esacure A198 4 GENOPOL AB-2 4 Surface modifier BYK UV3510 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 Polymerization inhibitor BHT 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Total amount 100 100 100 100 100 100 100 100

[0172] Note that, “tMeInd” in Tables 3 and 4 means “2-hydroxy-1-{1-[4-(2-hydroxy-2-methylpropionyl) phenyl]-1,3,3-trimethylindane-5-yl}-2-methylpropane-1-one”. Further, the details of the abbreviations of the materials described in Tables 1 to 4 are as illustrated below.

[0173] VEEA: 2-(2-vinyloxyethoxy)ethyl acrylate(manufactured by NIPPON SHOKUBAI CO., LTD.)

[0174] PEG200DA: polyethylene glycol 200 diacrylate(n≈4 in General formula (A))

[0175] PEG400DA: polyethylene glycol 400 diacrylate(n≈9 in General formula (A))

[0176] EGDA: ethylene glycol diacrylate

[0177] DEGDA: diethylene glycol diacrylate

[0178] PEA: 2-phenoxyethyl acrylate

[0179] BzA: benzyl acrylate

[0180] HDDA: 1,6-hexanediol diacrylate

[0181] DPGDA: dipropylene glycol diacrylate

[0182] TMP(EO)TA: trimethylolpropane EO-modified triacrylate

[0183] DPHA: dipentaerythritol hexaacrylate

[0184] Omnirad TPO H: 2,4,6-trimethylbenzoyl-diphenylphosohine oxide

[0185] Omnirad 819: bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide (manufactured by IGM RESINS B.V.)

[0186] Omnirad BMS: [4-(methylphenylthio)phenyl]-phenylmethane (manufactured by IGM RESINS B.V.)

[0187] GENOPOL BP-2: benzophenone compound (manufactured by Rahn AG)

[0188] SpeedCure XFs01: 2-hydroxy-1-{1-[4-(2-hydroxy-2-methylpropionyl) phenyl]-1,3,3-trimethylindane-5-yl}-2-methylpropane-1-one (manufactured by LAMBSON LIMITED)

[0189] Omnirad ITX: 2-isopropylthioxanthone (manufactured by IGM RESINS B.V.)

[0190] Omnirad 369: 2-(dimethylamino)-1-(4-morpholinophenyl)-2-benzyl-1-butanone (manufactured by IGM RESINS B.V.)

[0191] Omnirad EDB: ethyl p-(dimethylamino) benzoate (manufactured by IGM RESINS B.V.)

[0192] Esacure A198: aminobenzoate-based compound (manufactured by IGM RESINS B.V.)

[0193] GENOPOL AB-2: aminobenzoate-based compound (manufactured by Rahn AG)

[0194] BYK UV3510: silicone-based surface tension modifier manufactured by BYK Chemie Japan KK

[0195] BHT: 2,6-di-tert-butyl-4-methylphenol (“BHT SWANOX” manufactured by Seiko Chemical Co., Ltd.)

Examples 2 to 23 and Comparative examples 1 to 10

[0196] The obtained white ink and black ink were combined as described in Table 5 to make an ink set. Moreover, the printed matters were produced by the aforementioned method and were evaluated. The evaluation results are as illustrated in Table 5. Note that, with respect to Comparative example 1 and Comparative examples 3 to 5, the adhesion between the lower layer ink and the substrate was remarkably weak, thus, the adhesion between the lower layer ink layer and the upper layer ink layer evaluation could not be carried out.

TABLE-US-00005 TABLE 5 Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- ple 1 ple 2 ple 3 ple 4 ple 5 ple 6 ple 7 ple 8 ple 9 ple 10 Lower layer ink W1 W2 W3 W4 W5 W1 W1 W1 W1 W1 Upper layer ink K1 K2 K3 K4 K5 K2 K3 K4 K6 K7 Adhesion between lower layer1 ink layer 5 5 4 4 5 5 5 5 4 5 and substrate Interlayer adhesion between lower layer ink 4 5 4 4 5 5 5 5 5 5 layer and upper layer ink layer Whiteness of white ink 5 5 5 5 5 5 5 5 5 5 Curing wrinkles in laminated portions of lower 4 5 5 5 4 4 4 5 5 5 layer ink layer and upper layer ink layer Substrate curl over time 5 5 5 4 5 5 5 5 5 5 Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- ple 11 ple 12 ple 13 ple 14 ple 15 ple 16 ple 17 ple 18 Lower layer ink W1 W6 W7 W8 W9 W10 W11 W12 Upper layer ink K8 K6 K7 K8 K11 K9 K10 K12 Adhesion between lower layer1 ink layer 4 5 5 5 3 5 5 5 and substrate Interlayer adhesion between lower layer ink 5 5 5 5 4 5 5 5 layer and upper layer ink layer Whiteness of white ink 5 5 5 5 5 4 5 4 Curing wrinkles in laminated portions of lower 4 5 5 4 3 5 5 5 layer ink layer and upper layer ink layer Substrate curl over time 4 5 5 3 3 5 5 5 Compar- Compar- Compar- ative ative ative Exam- Exam- Exam- Exam- Exam- exam- exam- exam- ple 19 ple 20 ple 21 ple 22 ple 23 ple 1 ple 2 ple 3 Lower level ink W13 W14 W15 W16 K13 W18 W19 W20 Upper level ink K9 K9 K9 K9 W17 K1 K14 K1 Adhesion between lower layer1 ink layer 5 5 5 5 5 1 2 2 and substrate Interlayer adhesion between lower layer ink 5 5 5 5 3 X 2 X layer and upper layer ink layer Whiteness of white ink 5 3 3 3 5 5 5 5 Curing wrinkles in laminated portions of lower 5 5 5 5 5 3 3 1 layer ink layer and upper layer ink layer Substrate curl over time 5 5 5 5 5 4 2 3 Compar- Compar- Compar- Compar- Compar- Compar- Compar- ative ative ative ative ative ative ative exam- exam- exam- exam- exam- exam- exam- ple 4 ple 5 ple 6 ple 7 ple 8 ple 9 ple 10 Lower level ink W21 W22 W23 W1 W1 W1 W1 Upper level ink K1 K1 K9 K14 K15 K16 K17 Adhesion between lower layer1 ink layer 1 1 3 4 4 4 4 and substrate Interlayer adhesion between lower layer ink X X 4 2 3 1 2 layer and upper layer ink layer Whiteness of white ink 5 5 1 5 5 5 5 Curing wrinkles in laminated portions of lower 1 1 2 3 2 1 2 layer ink layer and upper layer ink layer Substrate curl over time 1 1 4 2 1 3 3 (X): Evaluation not possible

[0197] The results of the evaluation are as shown in Table 5, the combinations of the lower layer ink and the upper layer ink which satisfy the aforementioned conditions could obtain the printed matter having a good quality for all of the adhesion between the lower layer ink layer and the recording medium, the adhesion between the lower layer ink layer and the upper layer ink layer, the whiteness of the white ink, curing wrinkles, and the substrate curling over time.

[0198] For example, when compared with Examples 2, 3 and 4, it was verified that the adhesion between the lower layer ink layer and the recording medium, and, the adhesion between the lower layer ink layer and the upper layer ink layer improved by containing 2-(2-vinyloxyethoxy)ethyl acrylate as the compound represented by General formula (A).

[0199] Further, for example, when compared with Example 2 and Example 18, it was verified that the whiteness of the white ink improved by use of bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide as the acylphosphine oxide photopolymerization initiator contained in the white ink.

[0200] Furthermore, for example, when compared with Example 1 and Example 10, it was verified that the adhesion between the lower layer ink layer and the upper layer ink layer was maintained while the curing wrinkles in the laminated portion improved by the upper layer ink containing the aminobenzoate-based compound.

[0201] On the one hand, when the lower layer ink was free of the compound represented by General formula (A) as in Comparative example 1 and Comparative example 2, the adhesion between the lower layer ink layer and the recording medium, and between the lower layer ink layer and the upper layer ink layer was insufficient, and it became clear that the curling property decreases over time.

[0202] Further, in the case when the lower layer ink was free of the acylphosphine oxide photopolymerization initiator (Comparative example 3), and the case when the lower layer ink was free of the benzophenone photopolymerization initiator (Comparative example 4), not only the adhesion between the lower layer ink and the recording medium, but the curling wrinkles in the laminated portion and the curling over time deteriorate, and these qualities cannot be simultaneously satisfied. As can also be verified from the comparison with Example 1, it was verified that all of these problems can be solved by the lower layer ink containing all of the acylphosphine oxide photopolymerization initiator, the benzophenone photopolymerization initiator, and 2-hydroxy-1-{1-[4-(2-hydroxy-2-methylpropionyl) phenyl]-1,3,3-trimethylindane-5-yl}-2-methylpropane-1-one.

[0203] Further, in the case in which the lower layer ink was free of 2-hydroxy-1-{1-[4-(2-hydroxy-2-methylpropionyl) phenyl]-1,3,3-trimethylindane-5-yl}-2-methylpropane-1-one (Comparative example 5), with the exception of the whiteness of the white ink, none of the characteristics reached the practical level. In order to improve the adhesion between the lower layer ink and the recording medium, there is also a method for using an alkylaminoacetophenone-based photopolymerization initiator in the lower layer ink as in Comparative example 6, but in this case, the whiteness of the white ink and the curing wrinkles deteriorate, and accordingly, all of the problems cannot be solved simultaneously.

[0204] On the one hand, when the upper layer ink was free of the compound represented by General formula (A) (Comparative example 7 and Comparative example 8), it was obvious that one or more items among the adhesion between the lower layer ink layer and the upper layer ink layer, the curing wrinkles, and the substrate curling over time do not reach the practical level.

[0205] Further, as in Comparative example 9 and Comparative example 10, when the upper layer ink was free of the acylphosphine oxide photopolymerization initiator and the benzophenone polymerization initiator, it was verified that the adhesion between the lower layer ink layer and the upper layer ink layer, and, the curing wrinkles do not reach the practical level. From the above, in order to simultaneously solve all of the problems of the present invention, it is clear that it is necessary for the upper layer ink to contain the acylphosphine oxide photopolymerization initiator and the benzophenone photopolymerization initiator.