JP2021008114A - Inkjet recording method, ultraviolet curable ink, and inkjet recording device - Google Patents

Abstract

To provide an inkjet recording method excellent in durability of a head, and discharge stability and discharge quantity stability of ink.SOLUTION: An inkjet recording method includes a discharge step for discharging ultraviolet-curable ink whose viscosity at 28°C is 8 mPa-s or higher, from a head toward a medium to be recorded, and a curing step for curing the ultraviolet-curable ink adhering to the medium to be recorded, in which, in the discharge step, a temperature of the discharged ultraviolet-curable ink is 28-40°C, and a viscosity of the ultraviolet-curable ink at the temperature is 15 mPa-s or lower.SELECTED DRAWING: None

Description

The present invention relates to an inkjet recording method, an ultraviolet curable ink, and an inkjet recording device.

Conventionally, various methods have been used as a recording method for forming an image on a recording medium such as paper based on an image data signal. Of these, the inkjet method is an inexpensive device that ejects ink only to the required image portion to form an image directly on the recording medium, so that the ink can be used efficiently and the running cost is low. Further, since the inkjet method has low noise, it is excellent as a recording method.

In recent years, an inkjet recording method using an ultraviolet curable ink in which a monomer photopolymerizes (cures) by irradiating with ultraviolet rays forms an image having excellent water resistance and abrasion resistance on the recording surface of a recording medium. Therefore, it is used for manufacturing color filters, printing on printed substrates, plastic cards, vinyl sheets, large signs, and plastic parts, as well as printing bar codes and dates.

Examples of the ink used for inkjet recording include solvent-based water-based inks and solvent-free ultraviolet curable inks (UV inks). Of these, solvent-free UV-curable inks have a significantly higher viscosity than solvent-based water-based inks, so viscosity fluctuations due to temperature fluctuations during ejection are large, and these viscosity fluctuations cause changes in droplet size and droplet ejection. It has a great effect on changes in speed, which in turn causes deterioration of image quality. Therefore, a technique is disclosed in which, when ejecting ultraviolet curable ink, the ink is heated to reduce its viscosity and then ejected.

For example, in Patent Document 1, since UV ink has a higher viscosity than general ink at room temperature, it is heated in a recording head to a target set temperature (a set temperature required for the ink to have a viscosity that can be ejected). ), And the ink viscosity should be 7,000 to 500 mPa · s under the condition of 5 ° C., and the ink viscosity should be 20 to 3 mPa · s under the condition of 80 ° C. A UV ink is disclosed, which is characterized by being changed by heating and temperature control (paragraphs 0034, 0041, and 0042 of Patent Document 1).

Japanese Unexamined Patent Publication No. 2003-200559

However, the UV ink disclosed in Patent Document 1 has a problem that the member of the head is deteriorated by heating. Further, since the UV ink has a very high viscosity, if it is attempted to be ejected without heating, there arises a problem that the ejection stability and the ejection amount stability are inferior.

Therefore, one of the objects of the present invention is to provide an inkjet recording method having excellent head durability, ejection stability and ejection amount stability of ultraviolet curable ink.

In order to solve the above problems, the inventors of the present application conducted diligent studies and obtained the following findings. First, a method of preparing an ultraviolet curable ink having a very low viscosity (hereinafter, the ultraviolet curable ink is also simply referred to as “ink”) and ejecting the ink without heating was examined. However, according to this method, it has been found that the ink temperature tends to fluctuate due to changes in the environmental temperature, and the ink ejection stability and the ejection amount stability cannot be improved. Furthermore, it has been found that due to the composition of the ultraviolet curable ink having a very low viscosity, the members of the head are deteriorated, the durability of the head is deteriorated, and cured wrinkles are likely to occur. Therefore, the inventors of the present application have attempted to heat an ultraviolet curable ink having a relatively low viscosity and a viscosity in a predetermined range at a temperature in a predetermined range at a relatively low temperature. As a result, it was found that deterioration of the head member can be prevented. In addition to this, it was found that the temperature fluctuation at the time of ejection can be reduced, so that the viscosity fluctuation can be suppressed, and the ejection stability and the ejection amount stability of the ultraviolet curable ink are also improved.

As a result of further diligent studies by the inventors of the present application based on the above findings, the ultraviolet curable ink having a viscosity of 8 mPa · s or more at 28 ° C. is heated to determine the temperature of the ejected ultraviolet curable ink. We have found that the above problems can be solved by an inkjet recording method in which an ultraviolet curable ink having a viscosity of 28 to 40 ° C. and a viscosity at the temperature of 15 mPa · s or less is ejected and cured, and the present invention has been completed.

That is, the present invention is as follows.
[1]
An ejection process in which UV curable ink having a viscosity at 28 ° C. of 8 mPa · s or more is ejected from the head toward the recording medium, and
A curing step of curing the ultraviolet curable ink adhering to the recording medium is included.
An inkjet recording method in which the ultraviolet curable ink is heated so that the temperature of the ultraviolet curable ink ejected is 28 to 40 ° C., and the viscosity of the ultraviolet curable ink at the temperature is 15 mPa · s or less.
[2]
The inkjet recording method according to [1], wherein the head includes a line head having a length equal to or longer than the width of the recording medium, and recording is performed using a line-type inkjet recording device.
[3]
Recording is performed using an inkjet recording device, wherein at least a part of the ink path for supplying the ultraviolet curable ink from the ink container to the head is an ink circulation path for circulating the ultraviolet curable ink, [1] or The inkjet recording method according to [2].
[4]
The inkjet according to [3], wherein the ink inflow amount of the ultraviolet curable ink supplied to the head from the ink circulation path is twice or more the maximum ink ejection amount of the ultraviolet curable ink discharged from the head. Recording method.
[5]
The inkjet recording method according to [3] or [4], wherein a heating mechanism for heating the ultraviolet curable ink is provided at least at a position other than the position connected to the head in the ink circulation path.
[6]
The inkjet recording according to any one of [3] to [5], wherein there are a plurality of heads to which the ultraviolet curable ink is supplied from the ink circulation path, and the ultraviolet curable ink is ejected from the plurality of heads. Method.
[7]
The inkjet recording method according to any one of [1] to [6], wherein the ultraviolet curable ink contains vinyl ether group-containing (meth) acrylic acid esters represented by the following general formula (I).
CH ₂ = CR ^1- COOR ^2- O-CH = CH-R ³ ... (I)
(In the formula, R ¹ is a hydrogen atom or a methyl group, R ² is a divalent organic residue having 2 to 20 carbon atoms, and R ³ is a hydrogen atom or a monovalent organic residue having 1 to 11 carbon atoms. Is the basis.)
[8]
The ultraviolet curable ink further contains a monofunctional (meth) acrylate (excluding vinyl ether group-containing (meth) acrylic acid esters represented by the general formula (I)), according to [7]. Ink recording method.
[9]
The inkjet recording method according to any one of [1] to [8], wherein the light source used in the curing step is a light emitting diode.
[10]
The inkjet recording method according to [9], wherein the light emitting diode irradiates ultraviolet rays having a peak intensity of 800 mW / cm ² or more.
[11]
The inkjet recording method according to any one of [1] to [10], wherein an epoxy resin is used in the head.
[12]
An inkjet recording device that utilizes the inkjet recording method according to any one of [1] to [11].
[13]
An ultraviolet curable ink used in the inkjet recording method according to any one of [1] to [11] or the inkjet recording apparatus according to [12].

It is a block diagram which shows an example of the structure of the inkjet recording apparatus of this invention. It is schematic cross-sectional view which shows an example around the head unit, the transport unit, and the irradiation unit in the line printer which is an example of the inkjet recording apparatus of this invention. It is a schematic front view which shows an example of the ink supply apparatus provided in the inkjet recording apparatus of this invention.

Hereinafter, embodiments for carrying out the present invention will be described in detail.
As used herein, the term "recorded material" refers to a material in which ink is recorded on a recording medium to form a cured product. The cured product in the present specification means a cured substance including a cured film and a coating film.

Further, in the present specification, "curing" means that when an ink containing a polymerizable compound is irradiated with light, the polymerizable compound polymerizes and the ink solidifies. "Curability" refers to the property of being cured by being sensitive to light, and is also referred to as photopolymerizable. "Curing wrinkles" are coatings after curing as a result of the polymerization volume shrinkage rate increasing due to the uncured ink existing inside the coating film to be cured flowing irregularly before curing. It means wrinkles generated on the film surface.

Further, in the present specification, the “ejection stability” refers to a property of ejecting a stable ink droplet from a nozzle without clogging of the nozzle. The “ejection amount stability” refers to a property that when ink is ejected from a nozzle for a predetermined time, there is little variation in the ink ejection amount over the time. More specifically, the discharge rate stability can be mainly affected by fluctuations in heating temperature in the short term, while it can be mainly affected by fluctuations in environmental temperature in the long term. , The inventors of the present application have found. Therefore, in the present specification, the former discharge amount stability is referred to as "short-term discharge amount stability" or "discharge amount stability (short-term)", and the latter discharge amount stability is referred to as "long-term discharge amount stability". Alternatively, it may be referred to as "discharge rate stability (long-term)".

Further, in the present specification, the "durability of the head" includes deterioration such as swelling when a head member (specifically, an adhesive among the head members) constituting the recording device comes into contact with ink. It is a property that does not easily deteriorate.

Further, in the present specification, the “storage stability” refers to a property that the viscosity of the ink before and after storage does not change easily when the ink is stored. "Abrasion resistance" refers to the property that when a cured product is rubbed, the cured product is not easily peeled off and scratched.

Further, in the present specification, "(meth) acrylate" means at least one of acrylate and its corresponding methacrylate, and "(meth) acrylic" means at least one of acrylic and its corresponding methacrylic. However, "(meth) acryloyl" means at least one of acryloyl and its corresponding methacryloyl.

[Inkjet recording method]
One embodiment of the present invention relates to an inkjet recording method. The inkjet recording method includes a ejection step in which ultraviolet curable ink having a viscosity in a predetermined range at 28 ° C. is heated to a viscosity in a predetermined range and discharged from a head toward a recording medium, and adheres to the recording medium. It includes at least a curing step of curing the UV-curable ink. In this way, the ink cured on the recording medium forms a cured product of the ink.

[Viscosity of UV curable ink at 28 ° C]
The ultraviolet curable ink used in the above recording method has a viscosity of 8 mPa · s or more at 28 ° C. By using the ultraviolet curable ink having such a viscosity, it is possible to effectively prevent the occurrence of cured wrinkles in the obtained cured product. The principle of hardening wrinkles is presumed as follows, but the scope of the present invention is not limited by the following speculation. Cured wrinkles are caused when the surface of the coating film is cured first and then the inside of the coating film is cured later than the surface of the coating film, the surface of the coating film cured first is deformed or later. It is presumed that this is caused by the irregular flow of ink inside the coating film before it is cured. Further, the ultraviolet curable ink having a low viscosity has a polymerization shrinkage rate due to curing (difference between the volume of the ink and the volume of the ink (cured product) after curing with respect to the volume of the ink having a predetermined mass before curing). There is a tendency for the ink to be large, and it is presumed that the occurrence of hardening wrinkles is remarkable. Further, the ultraviolet curable ink containing a monofunctional (meth) acrylate described later, particularly a vinyl ether group-containing (meth) acrylate represented by the general formula (I), tends to cause curable wrinkles, and in particular, It is presumed that the low-viscosity ultraviolet curable ink containing the vinyl ether group-containing (meth) acrylate represented by the general formula (I) has remarkable curable wrinkles. The ultraviolet curable ink used in the inkjet recording method of the present embodiment can effectively prevent the occurrence of cured wrinkles by setting the viscosity within the above range even when these components are contained. .. As the viscosity in the present specification, a value measured by the method performed in the examples described later is adopted, but this does not mean that the method for measuring the viscosity is limited, and a conventionally known measuring method can be used. ..
Among them, the viscosity of the ink in this embodiment can be measured particularly using an E-type viscometer. When using an E-type viscometer, it is a general common sense that the measurement should be performed according to the instruction manual of the viscometer. Therefore, the type of rotor and the rotation speed should be measured according to the instruction manual. It goes without saying that the viscosity of the ink is set to one that can be measured normally, and the viscosity of the ink to be measured is determined according to the instruction manual in this embodiment as well. It is self-evident to set it to something that can be measured normally.

[Recording medium]
Examples of the recording medium include a non-ink-absorbing or low-absorbing recording medium. Among the recording media, the ink-non-absorbable recording medium is, for example, on a base material such as a plastic film or paper that has not been surface-treated for inkjet recording (that is, does not form an ink absorbing layer). There are those coated with plastic, those coated with plastic film, and the like. The plastics referred to here are not limited to the following, but are, for example, polyvinyl chloride (vinyl chloride), polyethylene terephthalate (PET), polycarbonate (PC), polystyrene (PS), polyurethane (PU), polyethylene (PE), and polypropylene. (PP) can be mentioned. Examples of the recording medium having low ink absorption include, but are not limited to, printing main paper such as art paper, coated paper, and matte paper.

[Discharge process]
The ejection process in the present embodiment ejects the ultraviolet curable ink from the head toward the recording medium. The temperature of the ultraviolet curable ink to be ejected is 28 to 40 ° C., and the viscosity of the ultraviolet curable ink at the temperature is 15 mPa · s or less.

The above temperature of 28 to 40 ° C. is a relatively low temperature as the temperature raised by heating. As described above, when the temperature of the ejected ink (hereinafter, also referred to as “ejection temperature”) is relatively low, deterioration of the head member can be prevented, so that the durability of the head is excellent and the head is durable. Since there is almost no variation in temperature, an advantageous effect that the ink ejection stability and the ejection amount stability are good can be obtained.

Here, the "temperature of the ultraviolet curable ink to be ejected" in the present specification means that the ink is continuously ejected from the head for 60 minutes, and the temperature of the nozzle is measured every 5 minutes during that period, and each of the measured temperatures is measured. It shall be expressed by the average value of.

Hereinafter, the above discharge temperature will be described more specifically. When the temperature is 28 ° C. or higher, the fluctuation of the discharge amount over a long period of time can be reduced. In other words, the long-term ejection rate stability is excellent due to the suppression of fluctuations in the environmental temperature (in the ink path described later). In addition to this, UV curable inks that can be ejected below 28 ° C have a very low viscosity, but the problem caused by this low viscosity is that the head members deteriorate and the durability of the head deteriorates. There is a problem that hardened wrinkles are likely to occur. On the other hand, the ink in this embodiment can solve the problem.

On the other hand, when the temperature is 40 ° C. or lower, the fluctuation of the discharge amount in a short period of time can be reduced. In other words, the short-term discharge rate stability is excellent due to the suppression of fluctuations in the heating temperature. In addition to this, the ultraviolet curable ink having an ink viscosity of 15 mPa · s or less when the heating temperature exceeds 40 ° C. can prevent the occurrence of curing wrinkles, but is caused by the heating temperature being too high. , The durability of the head is deteriorated, and the discharge rate stability is also inferior. On the other hand, the ink in this embodiment can solve the problem.

Further, when the viscosity of the ink at the above-mentioned ejection temperature is 15 mPa · s or less, an advantageous effect that the ejection stability and the ejection amount stability of the ink are excellent can be obtained. There may be a problem that the ejection stability deteriorates when the viscosity of the ink is high, but if it is 15 mPa · s or less, such a problem does not occur and the ejection stability becomes excellent. On the other hand, as for the discharge amount stability, the lower the viscosity, the smaller the fluctuation range of the discharge amount, and when it is 15 mPa · s or less, the fluctuation range of the discharge amount is sufficiently small, so that the discharge amount stability is excellent.

Further, the discharge temperature is preferably 34 to 40 ° C. in order to further enhance the above effect and more reliably solve the above problem. The upper limit of the viscosity of the ink at a predetermined ejection temperature is preferably 12 mPa · s or less. The lower limit of the viscosity is preferably 5 mPa · s or more, more preferably 7 mPa · s or more, and even more preferably 8 mPa · s or more. When the lower limit of the viscosity of the ink at a predetermined ejection temperature is the above value, the durability of the head due to the composition of the ink becomes better, and the occurrence of curing wrinkles due to the composition of the ink is effectively prevented. It is possible to prevent instability of discharge due to low viscosity. Being able to prevent discharge instability due to low viscosity means that the discharge stability and the discharge amount stability are further excellent.

Further, as described above, the ultraviolet curable ink has a higher viscosity than the water-based ink used in ordinary inkjet inks, so that the viscosity fluctuates greatly due to the temperature fluctuation at the time of ejection. Such a variation in the viscosity of the ink has a great influence on the change in the droplet size and the change in the droplet ejection speed, which may cause deterioration in image quality. Therefore, it is preferable to keep the temperature of the ejected ink (ejection temperature) as constant as possible. The ink in the present embodiment has a relatively low ejection temperature, and the ejection temperature can be kept substantially constant by adjusting the temperature by heating. Therefore, the ink in this embodiment is also excellent in image quality.

Here, an example of an ink design method for setting the viscosity of the ink within a desired range will be described.

The mixed viscosity of the entire polymerizable compound contained in the ink can be estimated from the viscosity of each polymerizable compound used and the mass ratio of each polymerizable compound to the ink composition.

It is assumed that the ink contains N kinds of polymerizable compounds such as polymerizable compounds A, B ... (Omitted in the middle) ..., N. Let VA be the viscosity of the polymerizable compound A, and MA be the mass ratio of the polymerizable compound A to the total amount of the polymerizable compound in the ink. The viscosity of the polymerizable compound B is VB, and the mass ratio of the polymerizable compound B to the total amount of the polymerizable compound in the ink is MB. Similarly, the viscosity of the Nth polymerizable compound N is defined as VN, and the mass ratio of the polymerizable compound N to the total amount of the polymerizable compound in the ink is defined as MN. As a confirmation, the formula "MA + MB + ... (omitted in the middle) ... + MN = 1" holds. Further, the mixed viscosity of the entire polymerizable compound contained in the ink is defined as VX. Then, it is assumed that the following mathematical formula (1) is satisfied.

MA x LogVA + MB x LogVB + ... (omitted in the middle) ... + MN x LogVN = LogVX ... (1)

For example, when two kinds of polymerizable compounds are contained in the ink, the mass ratio of the polymerizable compounds after MB is set to zero. The number of types of the polymerizable compound can be any number of one or more.

Next, an example of the procedure (steps 1 to 7) for setting the ink viscosity within a desired range will be described.

First, information on the viscosity of each polymerizable compound to be used at a predetermined temperature is obtained (step 1). Examples of the acquisition method include obtaining from a manufacturer's catalog and measuring the viscosity of each polymerizable compound at a predetermined temperature. Since the viscosity of a single polymerizable compound may differ depending on the manufacturer even if it is the same polymerizable compound, it is preferable to adopt the viscosity information from the manufacturer of the polymerizable compound to be used.
Subsequently, a target viscosity is set in VX, and the composition ratio (mass ratio) of each polymerizable compound is determined so that VX has the target viscosity based on the above formula (1) (step 2). The target viscosity is the viscosity of the ink composition to be finally obtained, and is set to a certain viscosity in the range of 8 to 15 mPa · s. The predetermined temperature is a certain temperature in the range of 28 to 40 ° C.

Subsequently, the polymerizable compound is actually mixed to prepare a composition of the polymerizable compound (hereinafter, referred to as “polymerizable composition”), and the viscosity at a predetermined temperature is measured (step 3).
Subsequently, when the viscosity of the polymerizable composition is approximately close to the above target viscosity (in this step 4, the target viscosity may be ± 5 mPa · s), the polymerizable composition and the photopolymerization are carried out. An ink composition containing components other than the polymerizable compound such as an initiator and a pigment (hereinafter, referred to as “components other than the polymerizable compound”) is prepared, and the viscosity of the ink composition is measured (step 4). In step 4, when there is a component other than the polymerizable compound that is mixed in the ink composition in the form of a pigment dispersion such as a pigment, the polymerizable compound previously contained in the pigment dispersion is also included. Since it is carried into the ink composition, the ink is obtained by subtracting the mass ratio of the polymerizable compound that is brought into the ink composition as a pigment dispersion from the composition ratio of each polymerizable compound determined in step 2. The composition needs to be adjusted.

Subsequently, the difference between the measured viscosity of the ink composition and the measured viscosity of the polymerizable composition is calculated and used as VY (step 5). Here, usually "VY> 0". VY depends on the content conditions such as the type and content of components other than the polymerizable compound, but in the examples described later, VY = 3 to 5 mPa · s.

Subsequently, "target viscosity-VY of step 2" is set for VX, and the composition ratio of each polymerizable compound is set so that VX becomes "target viscosity-VY of step 2" determined above from the above formula (1). Is decided again (step 6).

Subsequently, each polymerizable compound having a composition ratio determined in step 6 and a component other than the polymerizable compound are mixed to prepare an ink composition, and the viscosity at a predetermined temperature is measured (step 7). If the measured viscosity is the target viscosity, it means that the ink composition adjusted in step 7 is obtained as the ink composition having the target viscosity.

On the other hand, if the measured viscosity of the prepared polymerizable compound composition is not within the range of "target viscosity ± 5 mPa · s" in step 3, the following fine adjustments are made and the process is repeated from step 3. First, when the measured viscosity is too high, the content of the polymerizable compound whose viscosity as a simple substance is higher than the target viscosity is reduced, and the content of the polymerizable compound whose viscosity as a simple substance is lower than the target viscosity is increased. Make fine adjustments. On the other hand, when the measured viscosity is too low, the content of the polymerizable compound whose viscosity as a simple substance is lower than the target viscosity is reduced, and the content of the polymerizable compound whose viscosity as a simple substance is higher than the target viscosity is increased. Make fine adjustments. If the measured viscosity of the prepared ink composition does not reach the target viscosity in step 7, the same adjustment as the above fine adjustment is performed, and then the process is repeated from step 7.

[Ink supply process]
In the recording method of the present embodiment, recording may be performed using an inkjet recording device in which at least a part of the ink path for supplying ink from the ink container to the head is an ink circulation path for circulating ink. .. In other words, the recording method includes an ink circulation path for circulating ink in at least a part of an ink path for supplying ink to the head of the inkjet recording apparatus, and an ink supply step for circulating ink in the ink circulation path. Further may be included. Since the ink flowing out from the head circulates in at least a part of the ink path, the temperature of the ink in the ink circulation path becomes easy to stabilize, and the ejection amount stability becomes more excellent.

In the ink supply step, the inflow amount (ink inflow amount) of the ultraviolet curable ink supplied from the ink circulation path to the head may be adjusted to supply the ink of the ink inflow amount to the head. The ink supply step may be performed during the above ejection step. In the ink supply step, it is preferable that the amount of ink inflow is larger than the amount of ink ejected from the head during recording (during printing) because ink flows out and ink circulates. Further, the ink inflow amount is more preferably larger than the maximum value (maximum ink ejection amount described later) of the ink ejection amount from the head, and further preferably twice or more of the maximum ink ejection amount. It is even more preferable that the maximum ink ejection amount is 2.5 times or more. When the ink inflow amount is within the above range, the ejection amount stability becomes further excellent. On the other hand, the upper limit of the ink inflow amount is not particularly limited, but is preferably 4 times or less of the maximum ink ejection amount. The amount of ink ejected from the head, that is, the ink inflow amount and the maximum ink ejection amount are both volume-based quantities.

The ink supply step can be performed, for example, by providing a device for adjusting the ink supply amount (hereinafter, also simply referred to as “ink supply device”) in the inkjet recording device described later. The ink supply device will be described later.

[Curing process]
The curing step included in the recording method of the present embodiment cures the ultraviolet curable ink adhering to the recording medium by irradiating the light source with ultraviolet rays (light). In this step, the photopolymerization initiator contained in the ink is decomposed by irradiation with ultraviolet rays to generate starting species such as radicals, acids, and bases, and the polymerization reaction of the photopolymerizable compound depends on the function of the starting species. Be promoted. Alternatively, in this step, the polymerization reaction of the photopolymerizable compound is started by irradiation with ultraviolet rays. At this time, if a sensitizing dye is present together with the photopolymerization initiator in the ink, the sensitizing dye in the system absorbs ultraviolet rays to be in an excited state, and contacts with the photopolymerization initiator to promote decomposition of the photopolymerization initiator. It is possible to achieve a more sensitive curing reaction.

A mercury lamp, a gas / solid-state laser, or the like is mainly used as a light source (ultraviolet source), and a mercury lamp or a metal halide lamp is widely known as a light source used for curing an ultraviolet curable ink. On the other hand, mercury-free is strongly desired from the viewpoint of environmental protection, and replacement with a GaN-based semiconductor ultraviolet light emitting device is very useful industrially and environmentally. Further, LEDs (light emitting diodes) such as ultraviolet light emitting diodes (UV-LED) and ultraviolet laser diodes (UV-LD) are small in size, have a long life, have high efficiency, and are low in cost, and are expected as light sources for ultraviolet curable ink. ing.

As described above, the ultraviolet curable ink in the present embodiment can be preferably used regardless of whether the light source is an LED or a metal halide lamp, but the LED is particularly preferable.

The emission peak wavelength of the above light source (ultraviolet source) is preferably in the range of 360 to 420 nm, more preferably in the range of 380 to 410 nm. When the emission peak wavelength is within the above range, it is preferable because the UV-LED is easily available and inexpensive.

The light source having an emission peak wavelength in the range (preferably LED) peak intensity (illumination peak intensity) of ultraviolet light emitted from, preferably 800 mW / cm ² or more, more preferably 1,000 mW / cm ² That is all. The upper limit of the irradiation peak intensity is not particularly limited, but it is preferably 3,000 mW / cm ² or less. When the irradiation peak intensity is within the above range, the curability is further excellent and the occurrence of cured wrinkles can be suppressed more effectively. The principle of curing wrinkles is presumed as described above, but if the irradiation peak intensity is within the above range, the coating film surface can be cured at the same time as the inside, and ultraviolet rays can effectively generate curing wrinkles. It is presumed that it can be suppressed. When the viscosity of the ultraviolet curable ink in the present embodiment at 28 ° C. is 8 mPa · s or more, the occurrence of cured wrinkles can be prevented more effectively. In particular, when the ultraviolet curable ink contains vinyl ether group-containing (meth) acrylic acid esters represented by the general formula (1) described later and the irradiation peak intensity is within the above range, the curability is further excellent. , The occurrence of hardening wrinkles can be suppressed more effectively.

For the irradiation peak intensity in the present specification, values measured using an ultraviolet intensity meter UM-10 and a light receiving unit UM-400 (both manufactured by KONICA MINOLTA SENSING, INC.) Are adopted. .. However, this does not mean that the method for measuring the irradiation peak intensity is limited, and a conventionally known measuring method can be used.

Further, from a light source having an emission peak wavelength in the above range, an ultraviolet curable ink that can be cured with an irradiation energy of preferably 600 mJ / cm ² or less, more preferably 200 to 500 mJ / cm ² , is used in the recording method of the present embodiment. Good to use. In this case, it becomes easy to increase the output of the LED, and low-cost printing and high printing speed can be realized. Here, the above-mentioned irradiation energy is the total irradiation energy which is the sum of the irradiation energies when the irradiation is performed a plurality of times.

The irradiation energy in the present specification is calculated by multiplying the time from the start of irradiation to the end of irradiation by the irradiation peak intensity. Further, when the irradiation is performed over a plurality of times, the above-mentioned irradiation energy is represented by the total irradiation energy amount of the plurality of irradiations. The emission peak wavelength may be one or a plurality within the above-mentioned preferable wavelength range. Even if there are a plurality of them, the total amount of irradiation energy of ultraviolet rays having an emission peak wavelength in the above range is defined as the above irradiation energy.

Such an ink is obtained by containing at least one of a photopolymerization initiator that decomposes by ultraviolet irradiation in the wavelength range and a polymerizable compound that initiates polymerization by ultraviolet irradiation in the wavelength range.

Further, the amount of ink ejected (adhesion amount, injection amount) per unit area at the time of ejection to the recording medium is preferably 5 to 16 mg / inch ^{2 in} order to prevent wasteful use of the ink.

The amount of ink ejected per unit area varies depending on the recording resolution and the amount of ink injected per recording unit area (pixel) defined by the recording resolution, but the recording resolution (printing resolution) is defined as the "sub-scanning direction". In terms of "resolution x resolution in the direction intersecting the sub-scanning direction (main scanning direction)", 300 dpi x 300 dpi to 1500 dpi x 1500 dpi are preferable. Then, it is preferable to adjust the nozzle density and the discharge amount of the head according to the recording resolution.
The amount of ink ejected per pixel is preferably 2 to 200 ng / pixel, more preferably 3 to 160 ng / pixel. The nozzle density (distance between nozzles in the nozzle row) is preferably 180 to 720 dpi, more preferably 300 to 720 dpi.

As described above, according to the present embodiment, the durability of the head, the ejection stability and the ejection amount stability (short-term and long-term) of the ultraviolet curable ink are excellent, and the photopolymerization initiator contained in the ultraviolet curable ink is further excellent. It is possible to provide an inkjet recording method having excellent solubility, curability of an ultraviolet curable ink, and suppression of cured wrinkles.

[Inkjet recording device]
One embodiment of the present invention relates to an inkjet recording device, that is, an inkjet printer. The recording device utilizes the inkjet recording method of the above embodiment. A recording device (printer) for carrying out the recording method will be described in detail.

FIG. 1 is a block diagram showing an example of the configuration of the inkjet recording device of the present embodiment. A printer driver is installed in the computer 130, and print data corresponding to the image is output to the printer 1 in order for the printer 1 to record an image. The printer 1 includes a transport unit 20, a head unit 30, an irradiation unit 40, an ink supply unit (not shown), a detector group 110, a memory 123, an interface (I / F) 121, and a controller 120. The printer 1 that has received the print data from the computer 130, which is an external device, controls each unit by the controller 120 and records an image on the recording medium according to the print data. The situation in the printer 1 is monitored by the detector group 110, and the detector group 110 outputs the detection result to the controller 120. The controller 120 controls each unit based on the detection result output from the detector group 110. The controller 120 stores the print data input via the interface 121 in the memory 123, and has a CPU 122 and a unit control circuit 124. Control information for controlling each unit is also stored in the memory 123.

The printer of the present embodiment can record (form an image) inks of various colors on a recording medium, and for example, an image using four color inks of CMYK (cyan, magenta, yellow, black). Or use white ink to form an image of the background that imparts excellent concealing properties to the recording medium.

Examples of the type of printer of this embodiment include a line printer and a serial printer, and either of them can be used. These have different printer methods.

A line printer, which is a line-type inkjet recording device, includes a line head having a length equal to or longer than the width of the recording medium as a head. Ink from the line head to the recorded medium, that is, the recorded medium to be scanned relative to the line head while moving the position relative to the scanning direction in which the line head and the recording medium intersect the width direction. Is discharged. Then, in the line printer, the head is fixed without (almost) moving, and recording is performed in one pass (single pass). Line printers have an advantage over serial printers in that they have a high recording speed.

Here, the above-mentioned "line heads having a length corresponding to the width of the recording medium" are not limited to the case where the width of the recording medium and the length (width) of the line head completely match each other. It may be different. As a case where they may be different from each other, for example, the length (width) of the line head corresponds to the width (recording width) of the recording medium on which the ink should be ejected (the image should be recorded). There are cases where it is.

On the other hand, a serial printer, which is a serial type inkjet recording device, performs a main scan (pass) in which ink is ejected while moving in a main scan direction in which the head intersects the sub scan direction of the recording medium, and usually has two or more passes. Recording is performed by (multi-pass).

[Inkjet head]
The head unit 30 included in the inkjet recording device (printer 1) includes a head (inkjet head) that ejects ultraviolet curable ink toward a recording medium to perform recording. The head includes a cavity for discharging the contained ink from a nozzle, a discharge drive unit provided for each cavity to apply a driving force for ejection to the ink, and a head provided for each cavity. It has at least a nozzle for ejecting ink to the outside. A plurality of cavities, and a plurality of discharge drive units and nozzles provided for each cavity may be provided independently of each other in one head. The discharge drive unit is formed by using an electromechanical conversion element such as a piezoelectric element that changes the volume of the cavity by mechanical deformation, or an electronic heat conversion element that generates air bubbles in the ink to discharge the ink by generating heat. be able to. The inkjet recording device may be provided with one head or a plurality of heads for each color of ink. Above all, when a plurality of heads are provided, the line heads may be formed by arranging a plurality of heads in the width direction of the recording medium, whereby the above-mentioned recording width can be made longer. When recording using a plurality of colors of ink, the inkjet recording device includes a head for each ink. Here, the head provided in the serial printer or line printer which is the printer of the present embodiment is preferably a head in which epoxy resin is used for at least a part of a portion in contact with ink such as the inside or the surface of the head. The epoxy resin can be used, for example, as an adhesive for adhering the members of the head to each other when manufacturing the head. By adopting the head using such an epoxy resin adhesive, it is possible to maintain a strong adhesive force between the head members even when the head has a temperature change. The above-mentioned "contact with ink" includes direct contact with ink or indirect contact by permeation of constituent components of ink. At this time, since the ultraviolet curable ink in the present embodiment can prevent the adhesive of the epoxy resin from swelling, deterioration including deterioration is unlikely to occur, so that the durability of the head is excellent. As described above, the ultraviolet curable ink can be suitably discharged from the head using the epoxy resin adhesive.

Examples of the epoxy resin adhesive include, but are not limited to, conventionally known adhesives in which a main agent containing a compound having an epoxy group is cured with a curing agent. The compound having an epoxy group contained in the above-mentioned main agent is not limited to the following, and for example, a bisphenol type epoxy such as bisphenol A type and bisphenol F type, a novolak type epoxy such as phenol novolac type and cresol novolac type, and an epoxy polyol. Examples include type epoxies, urethane-modified epoxies, chelate-modified epoxies, and rubber-modified epoxies. Examples of the above-mentioned curing agent include, but are not limited to, amines such as amines and polyamines, acid anhydrides, amides such as amides and polyamides, imidazoles, and polymercaptans. Among the above, a combination of a bisphenol type epoxy as a main agent and amines as a curing agent is preferable because of its excellent adhesive strength. The mixing ratio of the main agent and the curing agent (main agent: curing agent) is preferably 10: 1 to 1:10 in terms of mass because the adhesive has excellent curability. The head can be configured, for example, as shown in FIG. 3 of JP-A-2009-279830.

Hereinafter, the printer of this embodiment will be described in more detail with reference to the drawings. The scope of the present invention is not limited to the drawings below. Further, in each drawing used in the following description, the scale of each member is appropriately changed in order to make each member a recognizable size.

[Line printer]
FIG. 2 is a schematic cross-sectional view showing an example around the head unit, the transport unit, and the irradiation unit in the above-mentioned line printer, which is an example of the printer of the present embodiment.

A transfer motor (not shown) rotates a transfer roller including an upstream roller 25A and a downstream roller 25B, and the transfer drum 26 is driven. The recording medium S is conveyed along the peripheral surfaces of the transfer roller and the transfer drum 26 as the transfer roller rotates. Each head composed of a head K, a head C, a head M, and a head Y is arranged around the transport drum 26 so as to face the transport drum 26, and ink is ejected toward the recording medium S facing each head. Recording is performed by the discharge process to be attached. Irradiation portions 42a, 42b, 42c, and 42d for temporary curing are arranged on the downstream side in the transport direction of each head, and irradiate the recording medium S with ultraviolet rays. The main curing irradiation unit 44 is arranged further downstream in the transport direction. Such a recording device can be configured as shown in FIG. 11 of Japanese Patent Application Laid-Open No. 2010-269471, for example.

In the present specification, "temporary curing" means temporary fixing (pinning) of ink, and more specifically, curing is performed before the main curing in order to prevent bleeding between dots and control the dot diameter. means. In general, the degree of polymerization of the polymerizable compound in the temporary curing is lower than the degree of polymerization of the polymerizable compound by the main curing performed after the temporary curing. Further, "main curing" means curing the dots formed on the recording medium to a cured state necessary for using the recorded material. Here, the term "curing" in the present specification simply means "curing" unless otherwise specified.

Since it is sufficient that the ink is main-cured by irradiating the ultraviolet rays from the main-curing irradiation unit 44, the ultraviolet rays are not irradiated from a part or all of the temporary curing irradiation units 42a, 42b, 42c, and 42d. The curing step may be completed by irradiating ultraviolet rays from the main curing irradiation unit 44. As described above, the curing step may be performed only by the main curing without performing the temporary curing.

[Ink supply device]
As described above, the ink supply unit of the inkjet recording device of the present embodiment may include a device (ink supply device) for adjusting the ink supply amount. The above-mentioned step of adjusting the ink supply amount can be performed, for example, by providing an ink supply device in the inkjet recording device described later. The ink supply device is provided between the head and an ink container such as an ink tank or an ink cartridge, for example. Then, the ink supply device can adjust the amount of ink flowing into the head by having an ink circulation path in at least a part of the ink path for supplying ink from the ink container to the head. More specifically, the ink supply device first regulates the amount of ink inflow supplied from the ink circulation path to the head. Secondly, the ink supply device ejects ink from the head for at least a part of the flow rate (the ejected amount is the ejection amount), and returns the remaining amount of the flow rate (ink outflow amount) from the head to the ink circulation path. Can be done. Therefore, for example, if the amount of ink flowing into the head is equal to or greater than the amount of ink ejected from the head (ink ejection amount), the ink flows out of the head and returns to the ink circulation path, and the ink circulates. If the ink inflow amount is twice or more the ink ejection amount, the ink outflow amount is one or more times the ink ejection amount.

Hereinafter, the above ink supply device will be described with reference to the drawings. FIG. 3 is a schematic front view showing an example of the ink supply device 10 included in the inkjet recording device of the present embodiment.

(1. Device configuration)
The ink supply device 10 is located between the ink cartridge 50 and the head 60 of the inkjet recording device. The ink supply device 10 includes an ink cartridge 50, an ink path 51 including an ink circulation path 80, a sub tank 70, a heating mechanism 90, a degassing mechanism 100, and a head 60. The head 60 also belongs to the above-mentioned head unit 30.

The ink cartridge 50 is for accommodating ultraviolet curable ink. The holder 52 is for mounting the ink cartridge 50. The ink path 51 is a flow path through which ink passes from the ink cartridge 50 to the head 60, including the ink circulation path 80. In other words, at least a part of the ink path 51 for supplying ink from the ink cartridge 50, which is an ink container, to the head 60 is an ink circulation path 80 for circulating ink. A holder 52, a valve 53, a supply pump 54, and a filter 55 are provided in the pipe between the ink cartridge 50 and the sub tank 70 in the ink path 51.
The valve 53 opens when the ink cartridge 50 containing the ink is mounted on the holder 52. When the valve 53 is opened, the supply pump 54 pushes ink from the ink cartridge 50 into the ink path 51. The ink path 51 supplies the ink extruded from the ink cartridge 50 by the supply pump 54 to the sub tank 70 via a filter 55 that filters foreign matter in the ink. The pressurizing pump 56 pressurizes the sub tank 70 and supplies ink from the sub tank 70 to the ink circulation path 80.

In the sub tank 70, the liquid amount sensor 71 detects the liquid amount of the ink in the sub tank 70, and when the liquid amount becomes equal to or higher than the predetermined first liquid amount, the ink is supplied to the ink circulation path 80 and the liquid amount is predetermined. When the amount is equal to or less than the second liquid amount of the above, ink is received from the ink cartridge 50.

The ink circulation path 80 leads to the sub tank 70 and the head 60, and ink is supplied from the sub tank 70 to supply the ink to the head 60. The ink circulation path 80 is a pipe having a filter 81, a circulation pump 82, and a head filter 83. The ink supplied from the sub tank 70 is circulated in the ink circulation path 80 by the circulation pump 82. The filter 81 is provided downstream of the circulation pump 82 of the ink circulation path 80 and filters foreign matter in the ink. A part of the ink circulation path 80 is provided in the head 60, and at least a part of the circulating ink is discharged from the head 60 through a head filter 83 that filters foreign matter in the ink.

The heating mechanism 90 and the degassing mechanism 100 are located in the middle of the ink circulation path 80, that is, between the sub tank 70 and the head 60, respectively.

The heating mechanism 90 is provided at a position other than at least a position connected to the head 60 in the ink circulation path 80. Here, the above-mentioned "position of the ink circulation path 80 connected to the head 60" corresponds to the joint portion of the ink circulation path 80 outside the head 60 in FIG. The heating mechanism 90 heats the ink in the ink circulation path 80 by the temperature control module 94 while circulating the hot water in the hot water tank 91 between the temperature control module 94 and the hot water tank 91 by the hot water circulation pump 92. is there. The heater 93 of the hot water tank 91 adjusts the temperature of the circulating ink to the target temperature.

The degassing mechanism 100 is provided on the downstream side of the temperature control module 94 of the ink circulation path 80 and on the upstream side of the head 60. The degassing module 102 includes a degassing chamber (not shown) into which ink flows, and a decompression chamber (not shown) in contact with the degassing chamber via a separation membrane that does not allow a liquid such as ink to pass through. The negative pressure pump 101 decompresses the decompression chamber. When the pressure reducing chamber is depressurized, the amount of dissolved air in the ink in the ink circulation path 80 is reduced and bubbles are removed. In this way, the degassing mechanism 100 degass the ink in the ink circulation path 80.

The head 60 is for ejecting ink toward the recording medium. The head 60 has a nozzle for ejecting ink (not shown), a nozzle plate having a nozzle surface on which the nozzle is formed, a cavity communicating with the nozzle and accommodating ink (not shown), and backflow of ink. A reservoir to prevent (not shown) and an ejection drive unit (FIG.) that applies an ejection driving force to the ink contained in the cavity to form droplets of ink suitable for ejection and ejects the droplets from the nozzles. Not shown). In FIG. 3, for example, the cavity is a pressure generating chamber, and the discharge driving unit is a piezoelectric element. The cap 61 protects the nozzle of the head 60 in order to prevent the ink adhering to the vicinity of the nozzle from drying when the recording device is not used.

In FIG. 3, four heads 60 are provided in parallel in the ink circulation path 80. As described above, it is preferable that there are a plurality of heads 60 to which ink is supplied from the ink circulation path 80, and ink is ejected from the plurality of heads 60. In this case, as will be described later, since there is only one ink circulation path 80 for the plurality of heads 60, the ink circulation path 80 and the temperature control module 94 are shared and the ink supplied to the four heads 60 is shared. The temperature of the ink can be made uniform, and the cost of the recording device can be reduced.
As demonstrated in the examples described later, according to the recording device of the present embodiment, the ink inflow amount is set to a predetermined value even when a plurality of heads are provided and the recordable width is increased. As a result, the discharge rate stability is excellent.

(2. Operation of the device)
First, the initial filling of ink is performed. When the ink cartridge 50 containing the ink is mounted on the holder 52, the valve 53 is "opened" and the ink is supplied to the sub tank 70 by the supply pump 54 via the filter 55 that filters foreign matter in the ink. .. When the liquid level sensor 71 detects that the ink liquid amount in the sub tank 70 is equal to or higher than the predetermined first liquid amount, the supply pump 54 is stopped and the valve 53 is "closed". The sub tank 70 is pressurized by the pressurizing pump 56, and ink is supplied from the sub tank 70 to the ink circulation path 80. Here, once the liquid amount in the sub tank 70 falls below a predetermined second liquid amount (that is, an amount less than the predetermined first liquid amount) before the ink circulation path 80 is completely filled with ink, once The pressurizing pump 56 is stopped and the sub tank 70 is returned to normal pressure. Then, in the same manner as described above, the ink is supplied from the ink cartridge 50 to the sub tank 70 again, and the ink is supplied from the sub tank 70 to the ink circulation path 80 again. By repeating such an operation, when the ink circulation path 80 is completely filled with ink, the pressurizing pump 56 is stopped, the sub tank 70 is returned to the atmospheric pressure, and the liquid volume of the sub tank 70 is a predetermined first liquid. Ink is supplied from the ink cartridge 50 to the sub tank 70 again so as to exceed the amount. In this way, the initial filling of the ink is completed.

When the initial filling of the ink is completed, the ink is subsequently circulated in the ink circulation path 80 by the circulation pump 82. The heating mechanism 90 in which the heater 93 is turned on in advance circulates the hot water in the hot water tank 91 between the temperature control module 94 and the hot water tank 91 by the hot water circulation pump 92. Then, the temperature control module 94 heats the ink circulating in the ink circulation path 80. A filter 81 provided downstream of the circulation pump 82 of the ink circulation path 80 filters foreign matter of ink. The degassing module 102 in the degassing mechanism 100 is in contact with the degassing chamber via a degassing chamber (not shown) into which ink flows and a separation membrane that allows gas such as air to pass through and liquid such as ink to pass through. (Not shown) and. When the decompression chamber is decompressed by the negative pressure pump 101, air bubbles and dissolved air contained in the ink in the degassing chamber escape to the decompression chamber through the separation membrane, so that the amount of dissolved air in the ink in the ink circulation path 80 decreases. Bubbles are removed. Since four degassing modules 102 are provided in parallel, the degassing efficiency is high, and the ink can be degassed while circulating the ink. By providing the degassing mechanism 100 on the downstream side of the temperature control module 94, degassing can be performed at the position where the ink temperature is the highest in the ink circulation path 80. Therefore, the degassing efficiency of the ink is very high, and since the degassing module 102 is located downstream of the circulation pump 82, the degassing can be performed at a position where the ink pressure is high, and the degassing efficiency is remarkably high. can do.

In the ink circulation path 80, four heads 60 are provided in parallel. The ink circulation path 80 in the head 60 is provided with a reservoir (not shown) on the downstream side of the head filter 83 for filtering foreign matter of ink. The ink that has flowed into the head and has passed through the reservoir flows out of the head 60 again, and the ink that has flowed out of each head 60 merges at the joint portion of the ink circulation path 80 and returns to the sub tank 70. The reservoir is connected to 600 pressure generating chambers (not shown) provided for each head, and each pressure generating chamber is individually driven by a piezoelectric element (not shown) provided for each chamber. Therefore, the volume of the pressure generating chamber can be changed. Further, a nozzle (not shown) is provided for each pressure generating chamber, and ink can be ejected from the nozzle to the outside. Since the ink supply device is common to the four heads, the temperature of the ink supplied to the four heads 60 can be made uniform by sharing the ink circulation path 80 and the temperature control module 94, and the recording device can be used. It is also possible to reduce the cost. The ink that has returned to the sub tank 70 is circulated again to the ink circulation path 80. When the head 60 is assembled by adhering the members constituting the reservoir, the above-mentioned epoxy resin adhesive is used. In FIG. 3, the ink circulation path 80 passes through the head 60, but the ink circulation path passes outside the head without passing through the inside of the head, and passes outside the head. Ink may be supplied to the reservoir in the head. In this case, the ink circulates up to the ink circulation path passing outside the head. However, also in this case, the ink flowing into the ink circulation path is the ink flowing into the head, and the ink flowing out from the ink circulation path is the ink flowing out from the head.

Then, preparations are made before the start of printing. Ink circulation is performed for 15 minutes to stabilize the ink temperature in the ink circulation path 80. The ink temperature is detected as the nozzle temperature by a temperature sensor (not shown) provided near the nozzle, and is adjusted to the target temperature before and during printing by controlling the heater 93 of the hot water tank 91. ing. When the printing preparation is completed, the piezoelectric elements are individually driven to eject ink from the nozzles and start printing.

As described above, the amount of ink flowing into the ink circulation path 80 is preferably larger than the amount of ink ejected from the head during printing. In order to have such a relationship between the ink inflow amount and the ink ejection amount, it is preferable that the ink inflow amount is larger than the above-mentioned maximum ink ejection amount. Further, as described above, the ink inflow amount is more preferably larger than the maximum ink ejection amount, and further preferably twice or more the maximum ink ejection amount. The amount of ink flowing into the head 60 from the ink circulation path 80 is A (mL / min), and all the nozzles of all the heads 60 are driven at the maximum drive frequency at the time of printing, and per drive that can be ejected during printing. The maximum ink ejection amount B (mL / min), which is the ejection amount when the ink is ejected with the maximum amount of ink in the above, and the ink circulation path 80 from the head 60 when the head 60 is ejected at the maximum ink ejection amount. Let C (mL / min) be the amount of ink flowing out to. At this time, it is set so as to satisfy the following mathematical formula (a).

A ≧ 2B = 2 (AC) ・ ・ ・ (a)

When the ink inflow amount is set so as to satisfy the above formula (a) and the ink circulates, the ink temperature and the degree of degassing can be further stabilized. Further, by performing such printing preparation, it is possible to stabilize the ink temperature and the degree of degassing even before the start of printing. It should be noted that the discharge amount of the head can be the maximum ink discharge amount described above. However, the actual ejection amount during printing can change depending on the ejection state, such as whether or not each nozzle is driven according to the image to be recorded, and the actual ink outflow amount can also change according to this change. .. The setting information regarding the ink inflow amount is determined in advance based on the maximum ink ejection amount of the inkjet recording device and is stored in the memory 123 or the like described above, and the ink inflow amount is controlled by the controller 120 based on the information. Will be. The maximum ink ejection amount may be grasped by ejecting under the above conditions. Further, when the maximum ink ejection amount per head 60 is D (mL / min), “B = 4D”. During printing, the amount of liquid in the sub tank 70 is gradually consumed as the ink is ejected. Therefore, ink is always supplied to the sub tank 70 by the supply pump 54 during printing so that the liquid amount in the sub tank 70 is always equal to or higher than the predetermined first liquid volume. Further, although the ink supply device 10 shown in FIG. 3 is not provided, a temperature control module is provided at an arbitrary position in the ink path 51 between the supply pump 54 and the sub tank 70 in order to stabilize the ink temperature during printing. May be further provided.

As described above, according to the present embodiment, the durability of the head, the ejection stability and the ejection amount stability (short-term and long-term) of the ultraviolet curable ink are excellent, and the photopolymerization initiator contained in the ultraviolet curable ink is further excellent. It is possible to provide an inkjet recording apparatus that utilizes an inkjet recording method having excellent solubility, curability of an ultraviolet curable ink, and suppression of cured wrinkles.

[UV curable ink]
Further, one embodiment of the present invention relates to an ultraviolet curable ink that can be used in the inkjet recording method and the inkjet recording apparatus of the above embodiment. As described above, the ultraviolet curable ink is characterized in that the viscosity at 28 ° C., the ejection temperature and the viscosity at the temperature are each within a predetermined range. The ink for setting the viscosity within a predetermined range can be designed by the ink design method as described above.

Hereinafter, additives (components) that can be contained in the ultraviolet curable ink of the present embodiment (hereinafter, also simply referred to as “ink”) or can be contained as desired will be described.

[Polymerizable compound]
The polymerizable compound contained in the ink of the present embodiment can be polymerized by light irradiation alone or by the action of a photopolymerization initiator described later to cure the printed ink. As other polymerizable compounds, various monomers and oligomers known conventionally, such as monofunctional, bifunctional, and trifunctional or higher polyfunctional, can be used. Examples of the monomer include unsaturated carboxylic acids such as (meth) acrylic acid, itaconic acid, crotonic acid, isocrotonic acid and maleic acid, salts or esters thereof, urethane, amide and its anhydride, acrylonitrile and styrene. Examples include unsaturated polyesters, unsaturated polyethers, unsaturated polyamides, and unsaturated urethanes. Examples of the oligomer include oligomers formed from the above monomers such as linear acrylic oligomers, epoxy (meth) acrylate, oxetane (meth) acrylate, aliphatic urethane (meth) acrylate, and aromatic urethane (meth). Acrylate and polyester (meth) acrylate can be mentioned.

Among the above, esters of (meth) acrylic acid, that is, (meth) acrylates are preferable. Among the (meth) acrylates, at least one of vinyl ether group-containing (meth) acrylic acid esters represented by the general formula (I) and other monofunctional (meth) acrylates is preferable, and the vinyl ether group-containing (meth) acrylate is preferable. ) Acrylic acid esters are more preferable, and the vinyl ether group-containing (meth) acrylic acid esters and other monofunctional (meth) acrylates are even more preferable.
Hereinafter, the polymerizable compound will be described in detail with a focus on these (meth) acrylates.

(1. Vinyl ether group-containing (meth) acrylic acid esters)
The ink of the present embodiment preferably contains vinyl ether group-containing (meth) acrylic acid esters represented by the following general formula (I).
CH ₂ = CR ^1- COOR ^2- O-CH = CH-R ³ ... (I)
(In the formula, R ¹ is a hydrogen atom or a methyl group, R ² is a divalent organic residue having 2 to 20 carbon atoms, and R ³ is a hydrogen atom or a monovalent organic residue having 1 to 11 carbon atoms. Is the basis.)

By containing the vinyl ether group-containing (meth) acrylic acid esters in the ink, the viscosity of the ink can be reduced, the curability of the ink can be improved, and curable wrinkles can be generated. It can be effectively prevented. Furthermore, it is better to use a compound having both a vinyl ether group and a (meth) acrylic group in one molecule than to use a compound having a vinyl ether group and a compound having a (meth) acrylic group separately. It is preferable for improving the curability.

In the above general formula (I), the divalent organic residue having 2 to 20 carbon atoms represented by R ² is a linear, branched or cyclic substituted organic residue having 2 to 20 carbon atoms. May be an alkylene group, an optionally substituted alkylene group having an oxygen atom due to an ether bond and / or an ester bond in the structure, an optionally substituted divalent group having 6 to 11 carbon atoms. Aromatic groups are preferred. Among these, an alkylene group having 2 to 6 carbon atoms such as an ethylene group, an n-propylene group, an isopropylene group and a butylene group, an oxyethylene group, an oxyn-propylene group, an oxyisopropylene group, an oxybutylene group and the like. An alkylene group having 2 to 9 carbon atoms having an oxygen atom due to an ether bond in the structure of is preferably used.

In the above general formula (I), the monovalent organic residue having 1 to 11 carbon atoms represented by R ³ is substituted in a linear, branched or cyclic form having 1 to 10 carbon atoms. Alkyl groups may be suitable, and aromatic groups having 6 to 11 carbon atoms which may be substituted are suitable. Among these, an aromatic group having 6 to 8 carbon atoms such as an alkyl group having 1 to 2 carbon atoms, a phenyl group and a benzyl group, which is a methyl group or an ethyl group, is preferably used.

When each of the above organic residues is a optionally substituted group, the substituent is divided into a carbon atom-containing group and a carbon atom-free group. First, when the substituent is a group containing a carbon atom, the carbon atom is counted in the number of carbon atoms of the organic residue. Examples of the group containing a carbon atom include, but are not limited to, a carboxyl group and an alkoxy group. Next, examples of the group containing no carbon atom include, but are not limited to, a hydroxyl group and a halo group.

The vinyl ether group-containing (meth) acrylic acid esters are not limited to the following, and include, for example, 2-vinyloxyethyl (meth) acrylate, 3-vinyloxypropyl (meth) acrylate, and 1-methyl (meth) acrylate. -2-Vinyloxyethyl, 2-vinyloxypropyl (meth) acrylate, 4-vinyloxybutyl (meth) acrylate, 1-methyl-3-vinyloxypropyl (meth) acrylate, 1-vinyloxymethyl (meth) acrylate Propyl, 2-methyl-3-vinyloxypropyl (meth) acrylate, 1,1-dimethyl-2-vinyloxyethyl (meth) acrylate, 3-vinyloxybutyl (meth) acrylate, 1-methyl-methyl (meth) acrylate- 2-Vinyloxypropyl, 2-vinyloxybutyl (meth) acrylate, 4-vinyloxycyclohexyl (meth) acrylate, 6-vinyloxyhexyl (meth) acrylate, 4-vinyloxymethylcyclohexylmethyl (meth) acrylate, 3-vinyloxymethylcyclohexylmethyl (meth) acrylate, 2-vinyloxymethylcyclohexylmethyl (meth) acrylate, p-vinyloxymethylphenylmethyl (meth) acrylate, m-vinyloxymethylphenyl (meth) acrylate Methyl, (meth) acrylate o-vinyloxymethylphenylmethyl, (meth) acrylate 2- (vinyloxyethoxy) ethyl, (meth) acrylate 2- (vinyloxyisopropoxy) ethyl, (meth) acrylate 2 -(Vinyloxyethoxy) propyl, (meth) acrylate 2- (vinyloxyethoxy) isopropyl, (meth) acrylate 2- (vinyloxyisopropoxy) propyl, (meth) acrylate 2- (vinyloxyisopropoxy) Isopropyl, 2- (vinyloxyethoxyethoxy) ethyl (meth) acrylate, 2- (vinyloxyethoxyisopropoxy) ethyl (meth) acrylate, 2- (vinyloxyisopropoxyethoxy) ethyl (meth) acrylate, ( 2- (Vinyloxyisopropoxyisopropoxy) ethyl (meth) acrylate, 2- (vinyloxyethoxyethoxy) propyl (meth) acrylate, 2- (vinyloxyethoxyisopropoxy) propyl (meth) acrylate, (meth) 2- (Vinyloxyisopropoxyethoxy) propyl acrylate, 2- (vinyloxyisopropoxyisopropoxy) propyl (meth) acrylate, 2- (vinylo) acrylate Xiethoxyethoxy) isopropyl, 2- (meth) acrylate 2- (vinyloxyethoxyisopropoxy) isopropyl, 2- (meth) acrylate 2- (vinyloxyisopropoxyethoxy) isopropyl, 2- (meth) acrylate 2- (vinyloxyisopropoxy) Isopropoxy) isopropyl, 2- (meth) acrylate 2- (vinyloxyethoxyethoxyethoxy) ethyl, (meth) acrylate 2- (vinyloxyethoxyethoxyethoxy) ethyl, (meth) acrylate 2- (isopropenoxyethoxy) ) Ethyl, 2- (isopropenoxyethoxyethoxy) ethyl (meth) acrylate, 2- (isopropenoxyethoxyethoxyethoxy) ethyl (meth) acrylate, 2- (isopropenoxyethoxyethoxyethoxy) (meth) acrylate Examples thereof include ethoxy) ethyl, polyethylene glycol monovinyl ether (meth) acrylate, and polypropylene glycol monovinyl ether (meth) acrylate.

Among these, 2- (meth) acrylate 2- (vinyloxyethoxy) ethyl, that is, 2- (vinyloxy) acrylate, is available because the ink can be made to have a lower viscosity, has a high ignition point, and has excellent curability of the ink. At least one of ethoxy) ethyl and 2- (vinyloxyethoxy) ethyl methacrylate is preferable, and 2- (vinyloxyethoxy) ethyl acrylate is more preferable. In particular, 2- (vinyloxyethoxy) ethyl acrylate and 2- (vinyloxyethoxy) ethyl methacrylate both have a simple structure and a small molecular weight, so that the ink can be remarkably reduced in viscosity. Examples of 2- (vinyloxyethoxy) ethyl (meth) acrylate include 2- (2-vinyloxyethoxy) ethyl (meth) acrylate and 2- (1-vinyloxyethoxy) ethyl (meth) acrylate. Examples of 2- (binyloxyethoxy) ethyl acrylate include 2- (2-vinyloxyethoxy) ethyl acrylate and 2- (1-vinyloxyethoxy) ethyl acrylate. In addition, 2- (vinyloxyethoxy) ethyl acrylate is superior to 2- (vinyloxyethoxy) ethyl methacrylate in terms of curability.

The vinyl ether group-containing (meth) acrylic acid esters may be used alone or in combination of two or more.

The content of the vinyl ether group-containing (meth) acrylic acid esters, particularly 2- (vinyloxyethoxy) ethyl (meth) acrylate, is 10 to 70% by mass with respect to the total mass (100% by mass) of the ink. Preferably, 10 to 60% by mass is more preferable, and 20 to 50% by mass is further preferable. When the content is at least the above lower limit value, the viscosity of the ink can be reduced and the curability of the ink can be further improved. On the other hand, when the content is not more than the above upper limit value, the storage stability of the ink can be maintained in a good state, and the occurrence of cured wrinkles can be more effectively prevented.

The method for producing the vinyl ether group-containing (meth) acrylic acid esters is not limited to the following, but is a method for esterifying (meth) acrylic acid and a hydroxyl group-containing vinyl ether (Production Method B), (meth) acrylic acid halide. And a method of esterifying a hydroxyl group-containing vinyl ether (manufacturing method C), a method of esterifying a (meth) acrylic acid anhydride and a hydroxyl group-containing vinyl ether (manufacturing method D), an esterifying a (meth) acrylic acid ester and a hydroxyl group-containing vinyl ether. Exchange method (manufacturing method E), method of esterifying (meth) acrylic acid and halogen-containing vinyl ether (manufacturing method F), method of esterifying (meth) acrylic acid alkali (earth) metal salt and halogen-containing vinyl ether. (Production method G), a method of exchanging vinyl between a hydroxyl group-containing (meth) acrylic acid ester and vinyl carboxylate (Production method H), and a method of ether exchange of a hydroxyl group-containing (meth) acrylic acid ester with an alkyl vinyl ether (Production method I). Can be mentioned.

Among these, the production method E is preferable because the desired effect can be further exerted in the present embodiment.

(2. Monofunctional (meth) acrylate)
The ink of this embodiment preferably contains a monofunctional (meth) acrylate. Here, when the ink of the present embodiment contains the above-mentioned vinyl ether group-containing (meth) acrylic acid esters (however, it is limited to those that are monofunctional (meth) acrylates), the vinyl ether group-containing (meth) acrylic acid. Esters are also included in the monofunctional (meth) acrylate, but the description of the vinyl ether group-containing (meth) acrylic acid esters will be omitted. Hereinafter, monofunctional (meth) acrylates other than the above-mentioned vinyl ether group-containing (meth) acrylic acid esters will be described. When the ink contains the monofunctional (meth) acrylate, the viscosity of the ink can be reduced, and the solubility of the photopolymerization initiator and other additives and the curability of the ink are both excellent. .. Furthermore, the ink ejection stability is excellent due to the excellent solubility of the photopolymerization initiator and other additives, and the toughness, heat resistance, and chemical resistance of the coating film are also excellent. Increases sex.

Examples of the monofunctional (meth) acrylate include phenoxyethyl (meth) acrylate, isoamyl (meth) acrylate, stearyl (meth) acrylate, lauryl (meth) acrylate, octyl (meth) acrylate, decyl (meth) acrylate, and iso. Myristyl (meth) acrylate, isostearyl (meth) acrylate, 2-ethylhexyl-diglycol (meth) acrylate, 2-hydroxybutyl (meth) acrylate, 4-hydroxybutyl (meth) acrylate, 2-methoxyethyl (meth) acrylate , Butoxyethyl (meth) acrylate, ethoxydiethylene glycol (meth) acrylate, methoxydiethylene glycol (meth) acrylate, methoxypolyethylene glycol (meth) acrylate, methoxypropylene glycol (meth) acrylate, tetrahydrofurfuryl (meth) acrylate, isobornyl (meth) Acrylate, 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 2-hydroxy-3-phenoxypropyl (meth) acrylate, lactone-modified flexible (meth) acrylate, t-butylcyclohexyl (meth) Acrylate, dicyclopentanyl (meth) acrylate, dicyclopentenyloxyethyl (meth) acrylate, benzyl (meth) acrylate, ethoxylated nonylphenyl (meth) acrylate, alkoxylated nonylphenyl (meth) acrylate, p-cumylphenol EO modified (meth) acrylates can be mentioned.

Among the above, monofunctional (meth) acrylate having an aromatic ring skeleton in the molecule is preferable because it is more excellent in curability, storage stability, and solubility of the photopolymerization initiator. The monofunctional (meth) acrylate having an aromatic ring skeleton is not limited to the following, and includes, for example, phenoxyethyl (meth) acrylate, benzyl (meth) acrylate, 2-hydroxyphenoxypropyl (meth) acrylate, and phenoxydiethylene glycol (meth). ) Acrylate is preferably mentioned. Among these, phenoxyethyl (phenoxyethyl) (because the viscosity of the ink can be reduced and the curability, abrasion resistance, adhesion, and solubility of the photopolymerization initiator can all be excellent. At least one of meta) acrylate and benzyl (meth) acrylate is preferable, and phenoxyethyl (meth) acrylate is more preferable.

The monofunctional (meth) acrylates other than the vinyl ether group-containing (meth) acrylic acid esters may be used alone or in combination of two or more.

The content of the monofunctional (meth) acrylate other than the vinyl ether group-containing (meth) acrylic acid esters is preferably 10 to 75% by mass, preferably 10 to 55% by mass, based on the total mass (100% by mass) of the ink. More preferably, 10 to 40% by mass is further preferable. When the content is at least the above lower limit value, the solubility of the photopolymerization initiator is further excellent in addition to the curability. On the other hand, when the content is not more than the above upper limit value, the adhesiveness is further excellent in addition to the curability.

When the ink contains the vinyl ether group-containing (meth) acrylic acid esters (however, it is limited to those that are monofunctional (meth) acrylates), the total content of the monofunctional (meth) acrylates containing the same. Is preferably 35 to 90% by mass, more preferably 40 to 70% by mass, based on the total mass (100% by mass) of the ink. When the content is within the above range, both the ink viscosity, specifically, the ink viscosity at 28 ° C. and the ink viscosity at the ejection temperature are likely to be within the above-mentioned desired ranges.

(3. Polymerizable compounds other than the above)
The ink of the present embodiment may further contain a polymerizable compound other than the above (hereinafter, referred to as “other polymerizable compound”). Examples of other polymerizable compounds include the above-mentioned monomers and oligomers, and among them, bifunctional or higher functional (meth) acrylates are preferable.

Examples of the bifunctional (meth) acrylate include diethylene glycol di (meth) acrylate, triethylene glycol di (meth) acrylate, tetraethylene glycol di (meth) acrylate, polyethylene glycol di (meth) acrylate, and dipropylene glycol di (meth) acrylate. ) Acrylate, tripropylene glycol di (meth) acrylate, polypropylene glycol di (meth) acrylate, 1,4-butanediol di (meth) acrylate, 1,6-hexanediol di (meth) acrylate, 1,9-nonanediol Di (meth) acrylate, neopentyl glycol di (meth) acrylate, dimethylol-tricyclodecanedi (meth) acrylate, EO (ethylene oxide) adduct of bisphenol A di (meth) acrylate, PO (propylene oxide) of bisphenol A Additives Di (meth) acrylates, neopentyl glycol di (meth) acrylates of hydroxypivalate, and polytetramethylene glycol di (meth) acrylates can be mentioned.

Examples of the trifunctional or higher functional polyfunctional (meth) acrylate include trimethylolpropane tri (meth) acrylate, EO-modified trimethylolpropane tri (meth) acrylate, pentaerythritol tri (meth) acrylate, and pentaerythritol tetra (meth) acrylate. , Dipentaerythritol hexa (meth) acrylate, ditrimethylolpropane tetra (meth) acrylate, glycerin propoxytri (meth) acrylate, cowprolactone-modified trimethylolpropane tri (meth) acrylate, pentaerythritol ethoxytetra (meth) acrylate, and Caprolactam-modified dipentaerythritol hexa (meth) acrylate can be mentioned.

Other polymerizable compounds may be used alone or in combination of two or more.

When other polymerizable compounds are contained in the ink, the content thereof is preferably 5 to 50% by mass with respect to the total mass (100% by mass) of the ink. In particular, when the ink contains a bifunctional (meth) acrylate, the content of the bifunctional (meth) acrylate is preferably 5 to 50% by mass, preferably 10 to 45% by mass, based on the total mass (100% by mass) of the ink. More preferably by mass. When the content is within the above range, the curability of the ink and the abrasion resistance of the cured product are excellent, and it is easy to design the viscosity of the ink to a desired viscosity. In addition, monofunctional (meth) acrylates having a relatively low viscosity as a simple substance of the polymerizable compound, particularly the vinyl ether group-containing (meth) acrylic acid esters having a low viscosity, and other polymerizable compounds having a relatively high viscosity, It is preferable to combine. This makes it easier to design the viscosity of the ink to the desired viscosity.

By using a photopolymerizable compound as the polymerizable compound, it is possible to omit the addition of the photopolymerization initiator, but it is easier to adjust the initiation of the polymerization by using the photopolymerization initiator. Can be and is suitable.

[Photopolymerization initiator]
The ink of the present embodiment may contain a photopolymerization initiator. The photopolymerization initiator is used to cure the ink existing on the surface of the recording medium by photopolymerization by irradiation with ultraviolet rays to form a print. By using ultraviolet rays (UV) among the light, the safety is excellent and the cost of the light source lamp can be suppressed. As long as an active species such as a radical or a cation is generated by the energy of ultraviolet rays and the polymerization of the above-mentioned polymerizable compound is started, there is no limitation, but a photoradical polymerization initiator or a photocationic polymerization initiator should be used. Of these, it is preferable to use a photoradical polymerization initiator.

Examples of the photoradical polymerization initiator include aromatic ketones, acylphosphine oxide compounds, aromatic onium salt compounds, organic peroxides, thio compounds (thioxanthone compounds, thiophenyl group-containing compounds, etc.), and hexaarylbi. Examples thereof include imidazole compounds, ketooxime ester compounds, borate compounds, azinium compounds, metallocene compounds, active ester compounds, compounds having a carbon halogen bond, and alkylamine compounds.

Among these, acylphosphine oxide compounds are particularly preferable because the curability of the ink can be further improved.

Specific examples of the photoradical polymerization initiator include acetophenone, acetophenone benzyl ketal, 1-hydroxycyclohexylphenyl ketone, 2,2-dimethoxy-2-phenylacetophenone, xanthone, fluorenone, benzaldehyde, fluorene, anthraquinone, triphenylamine, and carbazole. , 3-Methylacetophenone, 4-Chlorobenzophenone, 4,4'-dimethoxybenzophenone, 4,4'-diaminobenzophenone, Michler ketone, benzoin propyl ether, benzoin ethyl ether, benzyl dimethyl ketal, 1- (4-isopropylphenyl)- 2-Hydroxy-2-methylpropane-1-one, 2-hydroxy-2-methyl-1-phenylpropane-1-one, thioxanthone, diethylthioxanthone, 2-isopropylthioxanthone, 2-chlorothioxanthone, 2-methyl-1 -[4- (Methylthio) phenyl] -2-morpholino-propan-1-one, bis (2,4,6-trimethylbenzoyl) -phenylphosphine oxide, 2,4,6-trimethylbenzoyl-diphenyl-phosphine Oxide, 2,4-diethylthioxanthone, and bis- (2,6-dimethoxybenzoyl) -2,4,4-trimethylpentylphosphine oxide can be mentioned.

Examples of commercially available photoradical polymerization initiators include IRGACURE 651 (2,2-dimethoxy-1,2-diphenylethane-1-one), IRGACURE 184 (1-hydroxy-cyclohexyl-phenyl-ketone), and DAROCUR 1173. (2-Hydroxy-2-methyl-1-phenyl-propane-1-one), IRGACURE 2959 (1- [4- (2-hydroxyethoxy) -phenyl] -2-hydroxy-2-methyl-1-propane- 1-on), IRGACURE 127 (2-hydroxy-1-{4- [4- (2-hydroxy-2-methyl-propionyl) -benzyl] phenyl] -2-methyl-propan-1-one}, IRGACURE 907 (2-Methyl-1- (4-methylthiophenyl) -2-morpholinopropan-1-one), IRGACURE 369 (2-benzyl-2-dimethylamino-1- (4-morpholinophenyl) -butanone-1) ), IRGACURE 379 (2- (dimethylamino) -2-[(4-methylphenyl) methyl] -1- [4- (4-morpholinyl) phenyl] -1-butanone), DAROCUR TPO (2,4,6) -Trimethylbenzoyl-diphenyl-phosphine oxide), IRGACURE 819 (bis (2,4,6-trimethylbenzoyl) -phenylphosphin oxide), IRGACURE 784 (bis (η5-2,4-cyclopentadiene-1-yl)) -Bis (2,6-difluoro-3- (1H-pyrrole-1-yl) -phenyl) titanium), IRGACURE OXE 01 (1.2-octanedione, 1- [4- (phenylthio)-, 2- ( O-benzoyloxime)]), IRGACURE OXE 02 (Etanone, 1- [9-ethyl-6- (2-methylbenzoyl) -9H-carbazole-3-yl]-, 1- (O-acetyloxime)), IRGACURE 754 (mixture of oxyphenylacetic acid, 2- [2-oxo-2-phenylacetoxyethoxy] ethyl ester and oxyphenylacetic acid, 2- (2-hydroxyethoxy) ethyl ester) (above, BASF trade name), KAYACURE DETX-S (2,4-diethylthioxanthone) (trade name manufactured by Nippon Kayaku Co., Ltd.), Speedcure TPO (2,4,6-trimethylbenzoyl-diphenyl-phosphine oxide), Speedcure DETX (2,4-diethylthioxanthene-9-one) (hereinafter, trade name manufactured by Lambson), Lucirin TPO, LR8883, LR8970 ( As mentioned above, BASF's product name), Yubekrill P36 (UCB's product name), and the like can be mentioned.

The photopolymerization initiator may be used alone or in combination of two or more.

The content of the photopolymerization initiator is such that the ultraviolet curing rate can be improved to improve the curability, and the undissolved residue of the photopolymerization initiator and the coloring derived from the photopolymerization initiator are avoided. It is preferably 20% by mass or less with respect to the total mass (100% by mass) of the ink.

In particular, when the photopolymerization initiator contains an acylphosphine oxide compound, the content thereof is more preferably 5 to 15% by mass and 7 to 13% by mass with respect to the total mass (100% by mass) of the ink. It is more preferably%. When the content is at least the above lower limit value, the curability is further excellent. More specifically, it is more excellent in curability because a sufficient curing rate can be obtained particularly when curing by an LED (preferable emission peak wavelength: 360 nm to 420 nm). On the other hand, when the content is not more than the above upper limit value, the solubility of the photopolymerization initiator is further excellent.

[Optical brightener]
The ink of the present embodiment may contain a fluorescent whitening agent (sensitizer). The fluorescent whitening agent is a colorless or weakly colored compound capable of absorbing light having a wavelength of about 300 to 450 nm, which is visible in ultraviolet to short waves, and capable of emitting fluorescence having a wavelength of about 400 to 500 nm. Optical brighteners are also known as Fluorescent Whitening Agents. A description of the physical principles and chemistry of optical brighteners is given in Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, Electronic Release, Wiley-VCH 1998.

When the ink of the present embodiment contains a fluorescent whitening agent, the curability can be further improved.

Examples of the fluorescent whitening agent include, but are not limited to, naphthalene benzoxazoyl derivatives such as 1,4-bis- (2-benzoxazoyl) naphthalene, and 2,5-thiophenedylbis (5-tert-butyl-). 1,3-Benzoxazole) and other thiophenebenzoxazoyl derivatives, stilbene benzoxazoyl derivatives, coumarin derivatives, styrene biphenyl derivatives, pyrazolone derivatives, stilbene derivatives, benzene and biphenyl styryl derivatives, bis (benzazole-2-yl) derivatives , Carbostilyl, naphthalimide, derivatives of dibenzothiophene-5,5'-dioxide, pyrene derivatives, and pyridotriazole.

Commercially available fluorescent whitening agents include, for example, Hostalx KCB (Clariant brand name, 1,4-bis- (2-benzoxazoyl) naphthalene), TINOVAL OB (BASF trade name, 2). , 5-Thiophendiylbis (5-tert-butyl-1,3-benzoxazole)) and the like.

The fluorescent whitening agent may be used alone or in combination of two or more. The fluorescent whitening agent is preferably 0.1 to 0.5% by mass with respect to the total mass (100% by mass) of the ink. When the content is within the above range, the influence of the fluorescent whitening agent itself on the hue of the cured film can be reduced.

[Color material]
The ink of this embodiment may contain a coloring material. As the coloring material, at least one of a pigment and a dye can be used.

(1. Pigment)
By using a pigment as a coloring material, the light resistance of the ink can be improved. As the pigment, either an inorganic pigment or an organic pigment can be used.

As the inorganic pigment, carbon black (CI pigment black 7) such as furnace black, lamp black, acetylene black, and channel black, iron oxide, and titanium oxide can be used.

Examples of organic pigments include azo pigments such as insoluble azo pigments, condensed azo pigments, azolakes and chelate azo pigments, phthalocyanine pigments, perylene and perinone pigments, anthraquinone pigments, quinacridone pigments, dioxane pigments, thioindigo pigments, isoindolinone pigments and quinophthalone pigments. Polycyclic pigments, dye chelate (for example, basic dye type chelate, acidic dye type chelate, etc.), dyeing lake (basic dye type lake, acidic dye type lake), nitro pigment, nitroso pigment, aniline black, daylight Fluorescent pigments can be mentioned.

Pigments used in white ink include C.I. I. Pigment Whites 6, 18 and 21.

Examples of the pigment used in the yellow ink include C.I. I. Pigment Yellow 1,2,3,4,5,6,7,10,11,12,13,14,16,17,24,34,35,37,53,55,65,73,74,75, 81, 83, 93, 94, 95, 97, 98, 99, 108, 109, 110, 113, 114, 117, 120, 124, 128, 129, 133, 138, 139, 147, 151, 153, 154, 167, 172, 180 can be mentioned.

Pigments used in magenta ink include C.I. I. Pigment Red 1,2,3,4,5,6,7,8,9,10,11,12,14,15,16,17,18,19,21,22,23,30,31,32, 37, 38, 40, 41, 42, 48 (Ca), 48 (Mn), 57 (Ca), 57: 1, 88, 112, 114, 122, 123, 144, 146, 149, 150, 166, 168 , 170, 171, 175, 176, 177, 178, 179, 184, 185, 187, 202, 209, 219, 224, 245, or C.I. I. Pigment Violet 19, 23, 32, 33, 36, 38, 43, 50.

Examples of the pigment used in the cyan ink include C.I. I. Pigment Blue 1, 2, 3, 15, 15: 1, 15: 2, 15: 3, 15:34, 15: 4, 16, 18, 22, 25, 60, 65, 66, C.I. I. Bat blue 4, 60 can be mentioned.

Examples of pigments other than magenta, cyan, and yellow include C.I. I. Pigment Green 7, 10, C.I. I. Pigment Brown 3, 5, 25, 26, C.I. I. Pigment Orange 1,2,5,7,13,14,15,16,24,34,36,38,40,43,63.

The pigment may be used alone or in combination of two or more.

When the above pigments are used, the average particle size thereof is preferably 300 nm or less, more preferably 50 to 200 nm. When the average particle size is within the above range, reliability such as ejection stability and dispersion stability of the ink is further improved, and an image with excellent image quality can be formed. Here, the average particle size in the present specification is measured by a dynamic light scattering method.

(2. Dye)
A dye can be used as the coloring material. The dye is not particularly limited, and acid dyes, direct dyes, reactive dyes, and basic dyes can be used. As the dye, for example, C.I. I. Acid Yellow 17, 23, 42, 44, 79, 142, C.I. I. Acid Red 52,80,82,249,254,289, C.I. I. Acid Blue 9,45,249, C.I. I. Acid Black 1,2,24,94, C.I. I. Hood Black 1, 2, C.I. I. Direct Yellow 1,12,24,33,50,55,58,86,132,142,144,173, C.I. I. Direct Red 1,4,9,80,81,225,227, C.I. I. Direct Blue 1,2,15,71,86,87,98,165,199,202, C.I. I. Dilekdo Black 19, 38, 51, 71, 154, 168, 171, 195, C.I. I. Reactive Red 14, 32, 55, 79, 249, C.I. I. Reactive Black 3, 4, 35 can be mentioned.

The above dyes may be used alone or in combination of two or more.

The content of the coloring material is preferably 1 to 20% by mass with respect to the total mass (100% by mass) of the ink because excellent hiding property and color reproducibility can be obtained.

[Dispersant]
When the ink of the present embodiment contains a pigment, a dispersant may be contained in order to improve the dispersibility of the pigment. The dispersant is not particularly limited, and examples thereof include dispersants commonly used for preparing a pigment dispersion such as a polymer dispersant. Specific examples thereof include polyoxyalkylene polyalkylene polyamines, vinyl polymers and copolymers, acrylic polymers and copolymers, polyesters, polyamides, polyimides, polyurethanes, amino polymers, silicon-containing polymers, sulfur-containing polymers, fluorine-containing polymers, and epoxies. Examples thereof include resins containing one or more of the main components. As commercial products of polymer dispersants, Ajinomoto Fine-Techno's Ajispar series (trade name), Solspers series available from Avecia Co. (Solsperse 32000, 36000, etc. [trade name]) , BYK Chemie's Disperbic Series (trade name) and Kusumoto Kasei's Disparon Series (trade name).

The dispersant may be used alone or in combination of two or more. The content of the dispersant is not particularly limited, and a preferable amount may be appropriately added.

[Polymerization inhibitor]
The ink of the present embodiment may contain a polymerization inhibitor. When the ink contains a polymerization inhibitor, it is possible to prevent the polymerization reaction of the above-mentioned polymerizable compound before curing.

The polymerization inhibitor is not particularly limited, and examples thereof include a phenolic polymerization inhibitor. The phenolic polymerization inhibitor includes, but is not limited to, p-methoxyphenol, cresol, t-butylcatechol, di-t-butylparacresol, hydroquinone monomethyl ether, α-naphthol, 3,5-di-. t-butyl-4-hydroxytoluene, 2,6-di-t-butyl-4-methylphenol, 2,2'-methylene-bis (4-methyl-6-t-butylphenol), 2,2'-methylene -Bis (4-ethyl-6-butylphenol) and 4,4'-thio-bis (3-methyl-6-t-butylphenol) can be mentioned.

Commercially available products of phenolic polymerization inhibitors include, for example, p-methoxyphenol (trade name manufactured by Tokyo Chemical Industry Co., Ltd., p-methoxyphenol) and non-flex MBP (Seiko Kagaku Co., Ltd. (Seiko Kagaku Co., Ltd.). Seiko Chemical Co., Ltd.), 2,2'-methylene-bis (4-methyl-6-t-butylphenol)), BHT Swanox (Seiko Chemical Co., Ltd., 2,6-di-t) −Butyl-4-methylphenol).

The polymerization inhibitor may be used alone or in combination of two or more. The content of the polymerization inhibitor is not particularly limited, and a preferable amount may be added as appropriate.

[Surfactant]
The ink of this embodiment may contain a surfactant. The surfactant is not particularly limited, and examples thereof include a silicone-based surfactant. As the silicone-based surfactant, polyester-modified silicone and polyether-modified silicone are preferably mentioned, and among these, at least one of polyether-modified polydimethylsiloxane and polyester-modified polydimethylsiloxane is more preferable. Examples of commercially available slip agents include BYK-347, BYK-348, BYK-UV3500, 3510, 3530, and 3570 (all manufactured by BYK).

The surfactant may be used alone or in combination of two or more. The content of the surfactant is not particularly limited, and a preferable amount may be added as appropriate.

[Other additives]
The ink of the present embodiment may contain additives (components) other than the additives listed above. Such components are not particularly limited, and may include, for example, conventionally known polymerization accelerators, penetration accelerators, wetting agents (moisturizers), and other additives. Examples of the above-mentioned other additives include conventionally known fixing agents, fungicides, preservatives, antioxidants, ultraviolet absorbers, chelating agents, pH adjusters, and thickeners.

As described above, according to the present embodiment, the inkjet recording method is excellent in head durability, ejection stability and ejection amount stability (short-term and long-term) of ultraviolet curable ink, and is also excellent in suppressing cured wrinkles. It is possible to provide an ultraviolet curable ink having excellent curability and solubility of a photopolymerization initiator used in the above.

Hereinafter, the present embodiment will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.

[Material used]
The materials used in the examples and comparative examples are as shown below.
[Polymerizable compound]
-2-MTA (2-methoxyethyl acrylate, trade name manufactured by OSAKA ORGANIC CHEMICAL INDUSTRY LTD., Monofunctional (meth) acrylate)
4-HBA (4-hydroxybutyl acrylate, trade name manufactured by Osaka Organic Chemical Industry Co., Ltd., monofunctional (meth) acrylate)
-VEEA (2- (2-vinyloxyethoxy) ethyl acrylate, trade name manufactured by Nippon Shokubai Co., Ltd., monofunctional (meth) acrylate, hereinafter referred to as "VEEA")
-New Frontier PHE (Phenoxyethyl acrylate, trade name manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., monofunctional (meth) acrylate, hereinafter referred to as "PEA")
-V # 160 (benzyl acrylate, trade name manufactured by Osaka Organic Chemical Industry Co., Ltd., monofunctional (meth) acrylate, hereinafter referred to as "BZA")
-A-DPH (tripropylene glycol diacrylate, bifunctional (meth) acrylate, trade name manufactured by Shin-Nakamura Chemical Industry Co., Ltd. (LTD.), Hereinafter referred to as "TPGDA")
-SR295 (Pentaerythritol tetraacrylate, tetrafunctional (meth) acrylate, trade name manufactured by Sartomer Company Inc.)
[Photopolymerization initiator]
-Lucirin TPO (Product name manufactured by BASF, hereinafter referred to as "TPO")
[Optical brightener]
・ Hostalx KCB (1,4-bis- (2-benzoxazoyl) naphthalene, product name manufactured by Clariant)
[Polymerization inhibitor]
-P-methoxyphenol (trade name manufactured by Tokyo Chemical Industry Co., Ltd., p-methoxyphenol, hereinafter referred to as "MEHQ")
[Surfactant]
BYK-UV3500 (polyester-modified polydimethylsiloxane, manufactured by BYK, hereinafter referred to as "BYK3500")
[Color material]
-Cyanine Blue KRO (CI pigment blue 15: 3 (phthalocyanine pigment), trade name manufactured by SANYO COLOR WORKS, Ltd.), pigment particle size 80 nm, hereinafter referred to as "blue 15: 3". )
[Dispersant]
-Solspece 32000 (Product name manufactured by Avecia, hereinafter referred to as "SOL32000")

[Preparation of UV curable inks A to L]
Each material shown in Table 1 below is added so as to have the content (unit: mass%) shown in Table 1, and this is stirred with a high-speed water-cooled stirrer to obtain UV-curable inks A to L. Got

[Measurement / evaluation items]
[Ink viscosity rank at 1.28 ° C]
The viscosity of each ink prepared above was measured at 28 ° C. using a DVM-E type rotational viscometer (manufactured by Tokyo Keiki Co., Ltd.).
As the rotor, a DVM-E type cone having a cone angle of 1 ° 34'and a cone radius of 2.4 cm was used. The rotation speed was 10 rpm.
The evaluation criteria are as follows. The evaluation results are shown in Table 1 below.
A: It was less than 8 mPa · s.
B: 8 mPa · s or more and 10 mPa · s or less.
C: It was more than 10 mPa · s and less than 12 mPa · s.
D: It was more than 12 mPa · s and less than 15 mPa · s.
E: It exceeded 15 mPa · s.

[2. Evaluation of solubility of photopolymerization initiator]
Each ink prepared above was stirred at room temperature for 30 minutes. After stirring, the presence or absence of undissolved residue of the photopolymerization initiator was visually observed.
The evaluation criteria are as follows. The evaluation results are shown in Table 1 below. In Table 1, this evaluation column is abbreviated as "initiator solubility".
A: No undissolved residue of the photopolymerization initiator was observed.
B: Undissolved residue of the photopolymerization initiator was observed.

[3. Ink curability evaluation]
Each of the above inks was applied to Lumirror # 125-E20 (trade name manufactured by Toray Industries, Inc., PET film) with a bar coater manufactured by Tester Sangyo CO., LTD. The applied film thickness was 10 μm after curing. Next, the applied ink was irradiated with ultraviolet rays having an irradiation peak intensity of 1,000 mW / cm ² from an LED having a peak at a wavelength of 395 nm (Firefly [trade name], manufactured by Phoseon) for a predetermined time, and the cured ink was applied. A coating film was obtained. After irradiation, the surface of the ink coating film was rubbed 20 times with a cotton swab manufactured by Johnson & Johnson under a load of 100 g, and the irradiation energy required until no scratch marks were formed was measured by changing the predetermined time. ..
The evaluation criteria are as follows. The evaluation results are shown in Table 1 below.
A: It was 300 mJ / cm ² or less.
B: 300mJ / cm ² Beyond was 400 mJ / cm ² or less.
C: It exceeded 400 mJ / cm ² .

The UV curable inks A, B, C, D, E, F, G, K, and L correspond to the inks that can be used as examples, and the UV curable inks H, I, and J are used as comparative examples. Corresponds to the ink used.
Hereinafter, the recording method in each Example and each Comparative Example will be described.

[Examples 1 to 13, Comparative Examples 2, 4, 5, 9]
The ink supply device of FIG. 3 is provided, and the four heads 60 having a length substantially corresponding to the width (recording width) at which the image of the recording medium to be recorded shown in FIG. 2 should be recorded are arranged in the width direction. A line printer equipped with a line head to be used was used. The nozzle density of the head was 600 dpi.

The heater provided in the ink circulation device measures the temperature of the nozzle plate with a thermocouple provided on the nozzle plate of the head, and the temperature of the discharged ink (discharge temperature) is shown in Tables 2 to 4 below. The heating temperature of the heater is adjusted for each of the examples and the comparative examples so as to reach the temperature, and the temperature is measured every 5 minutes in 60 minutes of continuous discharge from the head, and the average temperature becomes each temperature of the examples and the comparative examples. The average temperature was taken as the ink ejection temperature.

In the ink supply device, the diameter of the ink supply pipe that supplies ink to the sub tank and the diameter of the ink pipe of the ink circulation path connecting the sub tank and the head are both 6 mm, the total length of the ink circulation path is 1 m, and the capacity of the sub tank is 100 mL. And said. Since the maximum ink ejection amount D per head is 10 mL / min and four heads are provided, the maximum ink ejection amount B of the ink supply device is 40 mL / min. The ink inflow amount A was set to 80 mL / min, and the ink was circulated at the ink inflow amount. The ink outflow amount C when the ink supply device is ejecting ink at the maximum ink ejection amount is 40 mL / min.

The head Y of the recording device shown in FIG. 2 was filled with each ink. The other heads shown in FIG. 2 were not used.

[Comparative Examples 1, 3, 6 to 8]
Recording was performed in the same manner as in Example 1 above, except that the heater was turned off and the temperature was not adjusted. At this time, the nozzle temperature was 25 ° C. (ink temperature 25 ° C.).

[Examples 14 to 16, Comparative Example 11]
Recording was performed in the same manner as in Example 1 above, except that the ink inflow amount A was changed and "ink inflow amount A / maximum ink ejection amount B" was set to the values shown in Tables 3 and 4 below. ..

[Comparative Example 10]
In the above embodiment, except that the heater is turned off, the temperature is not adjusted, the ink inflow amount A is changed, and the "ink inflow amount A / maximum ink ejection amount B" is set to the numerical value shown in Table 4 below. Recording was performed in the same manner as in the first magnitude. At this time, the nozzle temperature was 25 ° C. (ink temperature 25 ° C.).

[Example 17]
The same as in Example 1 above, except that the heating device in the circulation path was turned off and instead a heater was mounted on the head to heat the head so that the ink temperature became 33 ° C. And recorded.

As the heater of the head, as shown in FIG. 2 of Japanese Patent Application Laid-Open No. 2003-200559, the heater is attached to the head to heat the head including the reservoir.

[Example 18]
Recording was performed in the same manner as in Example 1 and the like, except that the number of heads was changed from four to one. Specifically, only one head was ejected, and three heads were not ejected to block the inflow of ink. The ink inflow amount A was twice the discharge amount (10 mL / min) of one head. Therefore, the ink inflow amount A in Example 18 was 20 mL / min.

[Examples 19 to 21]
Recording was performed in the same manner as in Example 1 above, except that the irradiation peak intensity in the curing wrinkle evaluation was changed from 1,000 mW / cm ² to 500 mW / cm ² .

[Measurement / evaluation items]
[1. Ink viscosity rank at the time of ejection]
The viscosity of each ink at the time of ejection was measured in the same manner as the above-mentioned ink viscosity rank at 28 ° C. except that the measurement temperature was the ejection temperature shown in Tables 2 to 4 below.
The evaluation criteria are the same as the viscosity rank of the ink at 28 ° C. described above. The evaluation results are shown in Tables 2 to 4 below.

[2. Discharge stability evaluation]
The evaluation was performed based on the number of non-ejection nozzles when all the nozzles of one head were ejected for 5 minutes.
The evaluation criteria are as follows. The evaluation results are shown in Tables 2 to 4 below.
A: The number of non-ejection nozzles was 2 or less.
B: There were 3 to 5 non-ejection nozzles.
X: The number of non-ejection nozzles was 6 or more.

[3. Evaluation of short-term discharge rate stability]
All nozzles were used to eject the maximum amount of ink for 60 minutes. An ink receiver is installed at the bottom of the head, the mass of the ink ejected from the mass of the ink receiver is measured every 5 minutes during ejection, and the ejection mass per ink droplet is calculated from the number of ink droplets ejected to the ink receiver. The difference between the minimum and maximum discharge mass for 60 minutes was calculated by mass% with respect to the average discharge mass for 60 minutes.
It should be noted that four heads are provided, each of the four heads is provided with 600 nozzles, and all the nozzles of all the heads are used for ejection. However, in Example 18, one head was evaluated. In addition, the non-discharge nozzle due to poor discharge was not treated as a discharge nozzle, and the discharged nozzle was used as the measurement target of the discharge mass. The evaluation criteria are as follows. The evaluation results are shown in Tables 2 to 4 below.
A: It was 3% by mass or less.
B: It was more than 3% by mass and less than 5% by mass.
C: It exceeded 5% by mass.

[4. Evaluation of long-term discharge rate stability]
The short-term evaluation was performed once a day for 10 days, that is, 10 times. Then, the difference between the minimum and maximum discharge mass during the 10 evaluations (tests) with respect to the average of the 10 discharge masses was calculated in mass%.
The evaluation criteria are as follows. The evaluation results are shown in Tables 2 to 4 below.
A: It was 3% by mass or less.
B: It was more than 3% by mass and less than 5% by mass.
C: It exceeded 5% by mass.

[5. Head durability evaluation]
The durability of the head was evaluated by measuring and calculating the swelling rate of the adhesive.
Approximately 0.2 g of epoxy resin adhesive (a mixture of equal amounts of EPIKOTE 828, an epoxy resin manufactured by Shell, and VERSAMID 125, a curing agent manufactured by COGNIS) is cured to form an adhesive piece. Was created and weighed. Then, the above-mentioned adhesive piece was dipped in each ink put in the screw tube, covered, and left for 6 months. The temperature during standing was the ejection temperature of each ink shown in Tables 2 to 4 below. After being left to stand, the adhesive piece was taken out, the ink was thoroughly washed away, and then the weight was measured. Then, the swelling rate was calculated from the following formula.
Weight change rate (%) = {(weight after loading-weight before loading) / weight before loading} x 100
The evaluation criteria are as follows. The evaluation results are shown in Tables 2 to 4 below.
A: It was less than 50%.
B: It was 50% or more.

[6. Hardened wrinkle evaluation]
The head of FIG. 2 and the light sources 42a to 42d for temporary curing are not used, and an LED having an irradiation peak intensity of 1000 mW / cm ² having a peak wavelength at 395 nm is arranged in the light source for main curing to evaluate the curability of the above-mentioned ink. In the same manner as above, the ink was applied to the film with a bar coater, the film was conveyed to the main curing light source, and the ink was irradiated. The irradiation time was adjusted so that the irradiation energy was set so as to irradiate until curing by the same test method as the curability test. However, the film thickness of the ink after curing was 12 μm.
Then, the surface of the cured film was visually observed. The evaluation criteria are as follows. The evaluation results are shown in Tables 2 to 4 below.
A: No wrinkles occurred.
B: Wrinkles were generated in a part of the cured film.
C: Wrinkles were generated on the entire surface of the cured film.

Based on the above results, the ejection step includes a ejection step of ejecting an ultraviolet curable ink having a viscosity at 28 ° C. of 8 mPa · s or more and a curing step of curing the ink, and the ejection step is the ejected ultraviolet curable type. The inkjet recording method (each example) in which the ink temperature is 28 to 40 ° C. and the viscosity of the ultraviolet curable ink at the temperature is 15 mPa · s or less is compared with the other recording methods (comparative examples). It was found that the ink is excellent in ejection stability, ejection amount stability, and head durability, and is also excellent in solubility of the photopolymerization initiator contained in the ink, curability of the ink, and suppression of cured wrinkles. Here, the curability and the curable wrinkles did not differ depending on the heating temperature. Further, the ejection amount stability is superior as the value of "ink inflow amount / maximum ink ejection amount" in the ink supply device is larger, and in particular, by doubling or more the value, fluctuations in the ejection amount are effectively suppressed. It turned out to be. Hereinafter, consideration will be made based on the above results, but the consideration does not limit the scope of the present invention in any way.

First, when the viscosity of the ink at the time of ejection is 8 to 12 mPa · s, that is, when the evaluation result of the viscosity is “B” or “C”, the recording method using the ink has further improved ejection stability. It is presumed to be excellent. However, as shown in Table 1 above, the ultraviolet curable ink K tended to take a long time to dissolve the photopolymerization initiator, and the solubility of the photopolymerization initiator was slightly inferior. Therefore, it is presumed that the recording method using the ultraviolet curable ink K resulted in slightly inferior ejection stability due to the precipitation of the photopolymerization initiator.

Further, from the comparison between Example 9 and Comparative Example 9, it is presumed that the head durability is excellent when the viscosity is 8 mPa · s or more at 28 ° C. Further, from the comparison of Examples 2 and 14, the comparison of Examples 8 and 15, and the comparison of Examples 11 and 16, the ink used and the ejection temperature are the same, but "ink inflow amount / It is presumed that the example in which the value of "maximum ink ejection amount" is larger is superior in ejection amount stability. On the other hand, in Comparative Example 3 and Comparative Example 10, the ink used and the ejection temperature were the same, while the "ink inflow amount / maximum ink ejection amount" was different, but the evaluation results were the same. From this, it is presumed that when the ink is not heated, the evaluation result is not affected even if the "ink inflow amount / maximum ink ejection amount" is different.

Further, in Comparative Example 5 and Comparative Example 11, the ink used and the ejection temperature were the same, but the values of "ink inflow amount / maximum ink ejection amount" were different, and the evaluation results were the same. This is because when the ejection temperature is high enough to exceed 40 ° C, the ink temperature fluctuation in the circulation path is large, and the evaluation result is better even if the "ink inflow amount / maximum ink ejection amount" is 2.5 times. It is presumed that it is not enough. Therefore, when the ejection temperature is 40 ° C. or lower, it is presumed that the larger the “ink inflow amount / maximum ink ejection amount”, the better the ejection amount stability (short-term, long-term).

Further, in the second and 17th embodiments, the heating positions of the ink are the heaters mounted on the circulation path and the head, respectively. It was found that the ejection amount fluctuation was better when the ink heating position was set to the circulation path.

Further, in Example 1 and Example 18, the number of heads is 4 and 1, respectively. It was found that the one head was more stable in the discharge amount (short-term, long-term), but the recordable width was narrowed. That is, it was found that the recording method of the present invention is excellent in ejection amount stability by setting the ink inflow amount to a predetermined value even when a plurality of heads are provided and the recordable width is lengthened.

Further, Example 2 and Example 19 are different from each other in that the irradiation peak intensity is different, but it is added that the larger the irradiation peak intensity, the more effectively the occurrence of hardening wrinkles can be prevented. Further, Examples 11 and 20 are also different from each other in that the irradiation peak intensity is different, but unlike the cases of Examples 2 and 19 described above, since high-viscosity ink is used, irradiation is performed. It is presumed that the occurrence of hardened wrinkles can be prevented regardless of the magnitude of the peak intensity. Further, Examples 7 and 21 are also different from each other in that the irradiation peak intensities are different, but the ink containing vinyl ether group-containing (meth) acrylic acid esters represented by the general formula (I) has an irradiation peak. It is presumed that when the strength is high, the occurrence of hardening wrinkles can be prevented.

Further, Comparative Examples 6 to 9 using any of the inks H, I, and J having an ink viscosity rank of A at 28 ° C. were inferior in the evaluation of cured wrinkles.
Further, from the viewpoint of the light source from Examples 19 to 21, when the LED in which the irradiation peak intensity was changed from 1,000 mW / cm ² to 500 mW / cm ² was used, the curing wrinkle evaluation was inferior.
Although not shown as an example, curing was performed using a metal halide lamp having an irradiation peak intensity of 1,000 mW / cm ² instead of the LED as the light source. As a result, in the example in which the evaluation result of the cured wrinkle was B or C among the examples and the comparative examples, the cured wrinkle evaluation was improved to one rank superior, and the curability result was further improved. Do you get it. However, the film was deformed due to the heat generated by the metal halide lamp, and the light source was larger than the LED, so installation space was required. That is, it was found that the use of LEDs is preferable in terms of low heat generation and space-saving recording equipment, and it is more preferable to increase the irradiation peak intensity of LEDs in terms of hardening wrinkles.

Further, although not shown as an embodiment, recording is performed in the same manner as in the first embodiment except that a serial printer having a peak intensity of 500 mW / cm ² mounted on the side of the carriage is used as a light source instead of the line printer. Was done. The serial printer used is the inkjet printer described in FIG. 2 of JP-A-2010-167677. The nozzle density of the head is 300 dpi, the recording resolution is 600 dpi × 600 dpi, but the recording resolution per pass is 300 dpi × 300 dpi, and dots are formed in the same recording area of the recording medium in 4 passes (main scanning direction 2). Pass x 2 passes in the sub-scanning direction). As a result, the evaluation result of the cured wrinkles was A, but it was found that the recording speed was low because it was a serial printer. That is, according to the recording method of the present invention, even when high-speed printing is performed by a line printer, it is possible to effectively prevent the occurrence of cured wrinkles by using LEDs and increasing the irradiation peak intensity thereof. It turned out that it is possible to make a good record.

1 ... printer, 10 ... ink supply device, 20 ... transfer unit, 25A ... upstream roller, 25B ... downstream roller, 26 ... transfer drum, 30 ... head unit, 40 ... irradiation unit, 42a, 42b, 42c, 42d ... Temporary curing irradiation unit, 44 ... main curing irradiation unit, 50 ... ink cartridge, 51 ... ink path, 52 ... holder, 53 ... valve, 54 ... supply pump, 55 ... filter, 56 ... pressurizing pump, 60 ... head , 61 ... cap, 70 ... sub tank, 71 ... liquid level sensor, 80 ... ink circulation path, 81 ... filter, 82 ... circulation pump, 83 ... head filter, 90 ... heating mechanism, 91 ... hot water tank, 92 ... hot water circulation Pump, 93 ... heater, 94 ... temperature control module, 100 ... degassing mechanism, 101 ... negative pressure pump, 102 ... degassing module, 110 ... detector group, 120 ... controller, 121 ... interface, 122 ... CPU, 123 ... Memory, 124 ... unit control circuit, 130 ... computer, K, C, M, Y ... head, S ... recording medium.

Claims (13)

An ejection process in which UV curable ink having a viscosity at 28 ° C. of 8 mPa · s or more is ejected from the head toward the recording medium, and
A curing step of curing the ultraviolet curable ink adhering to the recording medium is included.
An inkjet recording method in which the ultraviolet curable ink is heated so that the temperature of the ultraviolet curable ink ejected is 28 to 40 ° C., and the viscosity of the ultraviolet curable ink at the temperature is 15 mPa · s or less. The inkjet recording method according to claim 1, wherein the head includes a line head having a length equal to or longer than the width of the recording medium, and recording is performed using a line-type inkjet recording device. Claim 1 or claim 1, wherein at least a part of the ink path for supplying the ultraviolet curable ink from the ink container to the head is an ink circulation path for circulating the ultraviolet curable ink, and recording is performed using an inkjet recording device. 2. The inkjet recording method according to 2. The inkjet according to claim 3, wherein the ink inflow amount of the ultraviolet curable ink supplied from the ink circulation path to the head is twice or more the maximum ink ejection amount of the ultraviolet curable ink discharged from the head. Recording method. The inkjet recording method according to claim 3 or 4, further comprising a heating mechanism for heating the ultraviolet curable ink at a position other than the position connected to the head in the ink circulation path. The inkjet recording according to any one of claims 3 to 5, wherein there are a plurality of heads to which the ultraviolet curable ink is supplied from the ink circulation path, and the ultraviolet curable ink is discharged from the plurality of heads. Method. The inkjet recording method according to any one of claims 1 to 6, wherein the ultraviolet curable ink contains vinyl ether group-containing (meth) acrylic acid esters represented by the following general formula (I).
CH ₂ = CR ^1- COOR ^2- O-CH = CH-R ³ ... (I)
(In the formula, R ¹ is a hydrogen atom or a methyl group, R ² is a divalent organic residue having 2 to 20 carbon atoms, and R ³ is a hydrogen atom or a monovalent organic residue having 1 to 11 carbon atoms. Is the basis.) The ultraviolet curable ink according to claim 7, further containing a monofunctional (meth) acrylate (however, excluding vinyl ether group-containing (meth) acrylic acid esters represented by the general formula (I)). Ink recording method. The inkjet recording method according to any one of claims 1 to 8, wherein the light source used in the curing step is a light emitting diode. The inkjet recording method according to claim 9, wherein the light emitting diode irradiates ultraviolet rays having a peak intensity of 800 mW / cm ² or more. The inkjet recording method according to any one of claims 1 to 10, wherein an epoxy resin is used in the head. An inkjet recording device that utilizes the inkjet recording method according to any one of claims 1 to 11. An ultraviolet curable ink used in the inkjet recording method according to any one of claims 1 to 11 or the inkjet recording apparatus according to claim 12.

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