JP7654514B2 - Manufacturing method for printed matter - Google Patents

Description

The present invention relates to a method for producing printed matter using a specific inkjet device.

In recent years, there has been a demand for highly decorative painted metal materials, and there is a demand for the formation of coating films of various colors on metal sheets and the application of fine patterns to the surface of metal sheets. There is also a demand for the formation of coating films on demand according to needs, and for example, the application of active energy ray-curable inks on metal sheets by the inkjet method is being considered.

Here, the printing methods of the inkjet printing device include a serial method and a line method. In the serial method, a process of applying ink while scanning the inkjet head in the width direction of the metal substrate to be printed (also referred to as the "main scanning direction" in this specification) and a process of relatively moving the inkjet head and the recording medium in a direction perpendicular to the main scanning direction (also referred to as the "sub-scanning direction" in this specification) are alternately repeated. On the other hand, in the line method, an inkjet head (line head) is arranged to cover the recording medium in the width direction, and an image is formed by applying ink while moving the recording medium and the inkjet head relatively.

Among these methods, the serial method allows the length of the inkjet head (hereinafter also referred to as the "head") to be relatively short, and the cost of the inkjet device (hereinafter also simply referred to as the "printing device") can be reduced. Therefore, it is expected that this serial method inkjet device will be applied to the production of various coated metal materials.

On the other hand, active energy ray curable inks are cured by irradiation with active energy rays. Therefore, they can be applied to surfaces such as metals that do not absorb solvents and cured. However, active energy ray curable inks shrink significantly when cured. Therefore, when active energy ray curable inks are used for image formation, there is an issue that the ink layer, which is the cured product, is easily peeled off from various substrates.

In response to these problems, several methods have been proposed to improve the adhesion of the ink layer to the substrate. For example, Patent Document 1 describes a method in which an active energy ray-curable ink is applied to a substrate, which is then irradiated with active energy rays, and then heated with a heater. It is believed that this method improves the curability of the ink layer, thereby improving the adhesion between the ink layer and the substrate. Patent Document 2 describes a method in which an active energy ray-curable ink is applied to a preheated substrate. It is believed that this method allows the active energy ray-curable ink to be sufficiently wetted and spread over the substrate, improving the adhesion between the ink layer and the substrate.

Furthermore, Patent Documents 3 and 4 describe a method in which the resin film is subjected to a corona discharge treatment before applying an active energy ray-curable ink onto the resin film to increase the adhesion between the resin film and the ink layer.

JP 2001-009367 A JP 2008-087242 A International Publication No. 2018/163941 Japanese Patent Application Publication No. 9-300477

However, when applying active energy ray-curable ink to a metal plate, it was difficult to sufficiently increase the adhesion between the metal plate and the ink layer, regardless of which of the methods described in Patent Documents 1 to 4 was used. In particular, when the applied thickness of the active energy ray-curable ink was increased, the amount of cure shrinkage that occurred during curing increased, making the ink layer even more susceptible to peeling.

It has also become clear that when active energy ray-curable ink is applied using a serial inkjet device, the resulting ink layer is prone to color unevenness and streaking (sometimes called the "banding phenomenon"). The reasons for this are as follows.

An example of the configuration of a conventional serial inkjet printing device for printing active energy ray-curable ink is shown in Figures 1A and 1B. Figure 1A is a plan view of a portion of the configuration of the printing device 900, and Figure 1B is a side view of the configuration. As shown in Figures 1A and 1B, a printing device typically includes a head 911 for ejecting ink and a light source 912 for curing the ink ejected from the head 911, which are attached to a carriage 910. Then, the carriage 910 moves in the main scanning direction X to apply and cure the ink.

The head 911 typically includes multiple heads (in FIG. 1, a black head 911K, a cyan head 911C, a magenta head 911M, and a yellow head 911Y) corresponding to the multiple colors of ink. Each head 911 has a nozzle (not shown) for ejecting ink, and ejects ink from the nozzle. Meanwhile, only one light source 912 is disposed behind the multiple heads 911 in the traveling direction, and cures the multiple colors of ink applied from the head 911 collectively.

In the printing device 900, the position of the nozzles (not shown) may deviate slightly from the desired position due to errors during manufacturing of the head 911 or errors when attaching the head 911 to the carriage 910. When such an error occurs, the droplets (ink) ejected from the head 911 will all land in positions that are different from the desired locations. This causes white streaks in areas where the distance between adjacent rows is long, and color unevenness in areas where the distance between adjacent rows is short.

Furthermore, when all the inks of each color are applied and then cured together, as in the conventional printing device 900, the time from application to curing is long, and the inks coalesce during the time until curing. In particular, the greater the amount of uncured ink, the more likely the inks are to coalesce. When this coalescence occurs, the ink does not spread sufficiently in the width direction (sub-scanning direction Y) due to the surface tension, and the distance between adjacent dots in that direction becomes longer. As a result, streaky unevenness occurs along the main scanning direction X.

Another problem is that it is difficult to form high-resolution images with the conventional printing device 900. As described above, with the conventional printing device 900, after each color of ink is applied, the inks are photocured all at once. This means that there is a difference in the time it takes for the ink applied first (black ink in FIG. 1A) to harden and the time it takes for the ink applied later (yellow ink in FIG. 1) to harden. As a result, the dot diameter becomes larger the farther the color is from the light source, making it more likely to appear smudged and reducing image quality.

In response to these problems, the present invention aims to provide a method for producing a printed material in which the cured product of an actinic energy ray-curable ink adheres to a metal substrate to be printed on without uneven color or streaks, and the cured product has good adhesion to the metal substrate to be printed on.

The present invention is a method for producing a printed matter, which includes a printing process in which a serial type inkjet printing device has a carriage in which a plurality of inkjet heads, each including a plurality of nozzles arranged along the sub-scanning direction, and a plurality of LED light sources are arranged alternately along the main scanning direction, and a plurality of types of active energy ray curable inks are applied to a metal substrate to be printed on, and the ink layers are formed by photo-curing the inks. The printing process includes a step in which the inkjet head applies the ink while the carriage is scanned in the main scanning direction, and the LED light source adjacent to the inkjet head in the traveling direction detects the ink applied by the inkjet head. The method includes a scanning step of irradiating light onto the printed metal substrate, and in the printing step, the scanning step is performed two or more times on the same region of the printed metal substrate while shifting the carriage in the sub-scanning direction, and the printed metal substrate has a metal plate and a printed layer disposed on the metal plate, the printed layer containing a hardened product of a paint containing a polyester resin having a number average molecular weight of 12,000 to 30,000 and a melamine resin, and when the composition is analyzed in the depth direction by X-ray photoelectron spectroscopy using AlKα rays as an X-ray source for a region from the surface of the printed layer to a depth of 10 nm, the maximum value of N atoms relative to the total amount of N atoms, C atoms, and O atoms is 5 to 30 atom %.

According to the method for producing a printed product of the present invention, a printed product can be obtained in which the cured product of the actinic energy ray curable ink adheres to the metal substrate to be printed on without uneven color or streaks, and the cured product has good adhesion to the metal substrate to be printed on.

FIG. 1A is a plan view that shows a schematic configuration of a conventional inkjet printing device, and FIG. 1B is a side view of the same configuration. FIG. 2A is a plan view that shows a schematic configuration of an inkjet printing apparatus used in the method for producing a printed matter of the present invention, and FIG. 2B is a side view of the configuration. 3A and 3B are plan views that diagrammatically show the position of the carriage in the method for producing a printed matter of the present invention.

In the present invention, an inkjet printing device having a specific carriage is used to apply an active energy ray curable ink onto a specific metal substrate to be printed on, and then photocuring the ink to produce a printed product. Therefore, below, the inkjet printing device and the method for producing a printed product will be described first, and then the method for producing a printed product using a metal substrate to be printed on will be described.

(Inkjet printing device)
An example of the configuration of an inkjet printing apparatus 100 (hereinafter also referred to as a "printing apparatus") used in the method for producing a printed matter of the present invention is shown in Figures 2A and 2B. Figure 2A is a plan view of a portion of the configuration of the printing apparatus 100, and Figure 2B is a side view of the configuration.

The printing device 100 has a serial carriage 110. The carriage 110 has a plurality of inkjet heads 111K, 111C, 111M, and 111Y (hereinafter, when it is not necessary to distinguish between them, they will be collectively referred to as "heads 111") and a plurality of LED light sources 112K, 112C, 112M, and 112Y (hereinafter, when it is not necessary to distinguish between them, they will be collectively referred to as "LED light sources 112"). In the carriage 110, the plurality of heads 111 and the plurality of LED light sources 112 are alternately arranged along the main scanning direction X of the carriage 110.

Here, the number of heads 111 and the number of LED light sources 112 arranged on the carriage 110 are not particularly limited, and are appropriately selected according to the number of types of ink to be applied. The carriage 110 of the printing device 100 shown in FIG. 2A is arranged with heads 111 and LED light sources 112 corresponding to four types of ink. Specifically, a black head 111K, a black LED light source 112K, a cyan head 111C, a cyan LED light source 112C, a magenta head 111M, a magenta LED light source 112M, a yellow head 111Y, and a yellow LED light source 112Y are arranged in this order.

Each of the heads 111 is a member for applying each color ink to the metal substrate 10 to be printed, which will be described later, and is equipped with an ink tank (not shown) for storing the ink, a nozzle (not shown) for ejecting the ink, a flow path connecting the nozzle and the ink tank, etc. There are no particular limitations on the structure of these, and they have the same structure as the ink tank, nozzle, and flow path of a known inkjet head.

Each head 111 has a number of nozzles arranged at approximately equal intervals along the sub-scanning direction Y. The nozzle density of the head is preferably 200 nozzles/inch to 1600 nozzles/inch, more preferably 300 nozzles/inch to 1440 nozzles/inch. The wider the head width (total head width when multiple heads are connected in the sub-scanning direction), the more droplets can be ejected in one scan of the carriage 110 (head 111), and the more efficiently the printed matter can be produced. In addition, as will be described in detail later, in the method for producing printed matter of this embodiment, the carriage 110 is shifted in the sub-scanning direction Y while scanning the same area multiple times. Therefore, the more nozzles there are, the more finely the carriage 110 can be shifted to apply ink to the same area multiple times. Note that the number of nozzles and the intervals between the nozzles in each head 111 may be different from each other, but are usually the same.

In addition, in the printing device 100, the head 111 is positioned at a height such that the distance between the tip of the nozzle and the surface of the metal substrate 10 to be printed is 0.5 to 10 mm. The distance between the nozzle and the metal substrate 10 to be printed is more preferably 0.5 to 5 mm. When the distance between the nozzle and the metal substrate 10 to be printed is within this range, the light emitted from the LED light source 112 is less likely to get between the head 111 (nozzle) and the metal substrate 10 to be printed when the carriage 110 is scanning, and the ink can be prevented from hardening near the nozzle.

The LED light source 112 is a member for irradiating light onto the active energy ray curable ink that has landed on the metal substrate 10 to be printed, and photocuring the ink. The LED light source 112 may be equipped with not only an LED, but also a lens, cover, mirror, etc. for concentrating light or reflecting it in a desired direction.

Each LED light source 112 may have a structure capable of irradiating light onto all areas into which ink has been ejected from the nozzles of the corresponding head 111, and may, for example, have a structure in which multiple LEDs are arranged side by side along the sub-scanning direction Y or the main scanning direction X. Also, multiple types of LEDs may be combined. Each light source 112 may be capable of emitting the same wavelength of light with the same intensity, or may be capable of emitting light with different wavelengths of light or different intensities.

The wavelength of light emitted by each LED light source 112 is selected appropriately depending on the type of ink to be applied, but ultraviolet light or visible light is preferred, and the peak wavelength is preferably 350 to 420 nm, and more preferably 380 to 410 nm. If the peak wavelength is within this range, the ink can be cured efficiently.

The intensity of the light emitted by each LED light source 112 may be sufficient to completely cure the ink, or sufficient to partially cure the ink. The light intensity is preferably, for example, 0.5 to 25 W/ ^cm2 , and more preferably 2 to 15 W/ ^cm2 . Within this range, the ink can be efficiently cured while the carriage 110 is moved in the scanning direction X.

In addition, in the printing device 100, it is preferable that each LED light source 112 is arranged so that the distance between the end (bottom end) of each LED light source 112 facing the metal substrate 10 to be printed and the surface of the metal substrate 10 to be printed is 5 to 15 mm, and this distance is more preferably 7 to 12 mm. When the distance between the LED light source 112 and the metal substrate 10 to be printed is within this range, the ink can be cured with energy efficiency.

In addition, the distance between each head 111 and the corresponding LED light source 112 (the adjacent LED light source 112 at the rear of the traveling direction) is not particularly limited, but the shorter these distances are, the more the ink ejected from the head 111 can be cured before it spreads excessively. In addition, the shorter these distances are, the more advantageous it is that the carriage 110 can be made smaller. The distance between the end of the nozzle located at the rearmost side of the main scanning direction X of each head 111 and the end of the LED light source 112 at the front side of the main scanning direction X is preferably about 20 to 60 mm. If the distance is 20 mm or more, light is less likely to get between the head 111 (nozzle) and the metal substrate 10 to be printed, and the ink can be prevented from curing near the nozzle. On the other hand, if the above distance is 60 mm or less, the carriage 110 can be made smaller. Since the LED light source 112 generates little heat, the above distance is 60 mm or less, and is less likely to affect the head 111 and the ink inside it. In addition, such LED light sources 112 can be turned on and off frequently.

Here, the method of attaching each head 111 and each LED light source 112 to the carriage 110 is not particularly limited, and in this embodiment, the head 111 and the LED light source 112 are attached to the holder 113 of the carriage 110, but the method is not limited to this.

In addition, in this embodiment, the carriage 110 is attached to a rail 120 that extends in the main scanning direction X, and the carriage 110 moves along the rail 120.

Furthermore, in addition to the carriage 110 and rail 120, the inkjet printing device 100 has a transport mechanism (not shown) that moves the metal substrate 10 to be printed and the carriage 110 relatively in the sub-scanning direction Y. The transport mechanism can be the same as the transport mechanism of a typical serial printing device. In addition to the above, the inkjet printing device 100 may have various configurations within the scope that does not impair the purpose and effect of this embodiment.

The ink used in the method for producing a printed matter of this embodiment is not particularly limited as long as it is curable by irradiation with active energy rays, more specifically, by light irradiated from the LED light source 112, and is appropriately selected depending on the type of metal substrate 10 to be printed and the use of the resulting printed matter. Examples of active energy rays in this specification include electron beams, ultraviolet light, visible light, alpha rays, gamma rays, X-rays, etc., but it is preferable that the ink is curable by ultraviolet light and/or visible light.

The ink may be a cationic polymerization ink or a radical polymerization ink. Radical polymerization ink is easier to cure using the LED light source and is also preferred from the standpoint of cost, etc.

The composition of the radical polymerization ink is not particularly limited, and typically contains a radical polymerizable compound and a radical polymerization initiator. The radical polymerizable compound may have any photopolymerizable functional group, and is preferably a compound having a (meth)acrylate group. The compound having a (meth)acrylate group is not particularly limited, and any known monofunctional or higher (meth)acrylate monomer can be used. In this specification, (meth)acrylate refers to methacrylate, acrylate, or both.

The radical polymerization initiator is not particularly limited as long as it generates radicals by the light from the LED light source 112 and initiates polymerization of the radically polymerizable compound. Known compounds can be used as the radical polymerization initiator. Examples of radical polymerization initiators include acylphosphine oxide compounds, thioxanthone compounds, aromatic ketones, aromatic onium salt compounds, organic peroxides, thio compounds (such as compounds containing a thiophenyl group), α-aminoalkylphenone compounds, hexaarylbiimidazole compounds, ketoxime ester compounds, borate compounds, azinium compounds, metallocene compounds, active ester compounds, compounds having a carbon-halogen bond, and alkylamine compounds.

The ink may also contain other components than those mentioned above. Examples include coloring materials such as pigments and dyes; and various additives such as dispersants, surfactants, penetrants, moisturizers, dissolution aids, viscosity adjusters, pH adjusters, antioxidants, preservatives, fungicides, corrosion inhibitors, polymerization inhibitors, and chelating agents.

(Method of manufacturing printed matter)
The method for producing a printed matter of this embodiment includes a printing step in which a plurality of types of active energy ray curable inks are applied to a metal substrate to be printed by the above-mentioned inkjet printing device, and the ink layer is formed by photo-curing the ink. The printing step includes a scanning step in which the inkjet head applies ink while the above-mentioned carriage is scanned in the main scanning direction, and an LED light source adjacent to the inkjet head behind the inkjet head in the traveling direction irradiates light onto the ink applied by the inkjet head. In the printing step, the carriage is shifted in the sub-scanning direction, and the scanning step is performed two or more times on the same region of the metal substrate to be printed. This will be described in detail below.

In the scanning process, first, as shown in FIG. 3A, the carriage 110 is placed at a specific position (usually the end of the metal substrate 10 to be printed). Then, while moving the carriage 110 in the main scanning direction X, ink is ejected from the head 111 onto the metal substrate 10 to be printed, and the ink that has landed on the metal substrate 10 to be printed is cured by light from the LED light source 112. At this time, the ejection and curing of the ink is performed for each ink.

For example, when applying all colors to a specific area using the carriage 110 of the printing device 100 shown in FIG. 2A, black ink is applied from the black head 111K and the black ink is cured by the black LED light source 112K. Next, cyan ink is applied to the area from the cyan head 111C and the cyan ink is cured by the cyan LED light source 112C. Furthermore, magenta ink is applied to the area from the magenta head 111M and the magenta ink is cured by the magenta LED light source 112M. Then, yellow ink is applied to the area from the yellow head 111Y and the yellow ink is cured by the yellow LED light source 112Y. In this way, by applying and curing each ink, the dot diameter of each ink can be made uniform, making it easier to obtain a high-definition ink layer. In addition, when an ink layer is formed by this method, uncured inks are less likely to coalesce on the metal substrate to be printed, and the resulting ink layer is less likely to have streaks or color unevenness.

The amount of ink applied during one scan of the carriage in the main scanning direction X and the application pattern are determined by how many scanning steps are performed on the same area. For example, if two scanning steps are performed on the same area, half the amount of ink required to form the desired pattern is applied in each scanning step. There are no particular restrictions on the pattern in which the ink is applied, but it is preferable to print every other dot, for example. Applying in this way not only halves the amount of ink, but also makes streaks and color unevenness much less noticeable in the resulting ink layer.

On the other hand, when performing four scanning processes on the same area, 1/4 of the amount of ink required to form the desired pattern is applied in each scanning process. There are no particular restrictions on the pattern in which the ink is applied, but it is preferable to print every 3 dots. In this case, not only can the amount of ink be reduced to 1/4, but streaks and color unevenness are also much less noticeable in the resulting ink layer.

Then, the carriage 110 is shifted in the sub-scanning direction, and a scanning process is performed again on the area that was scanned by the carriage 110. The width by which the carriage 110 is shifted (the distance indicated by W in FIG. 3) is determined based on how many scanning processes are performed on the same area. For example, if two scanning processes are performed on the same area, the width W is set to half the length of the carriage in the sub-scanning direction Y. Also, if four scanning processes are performed on the same area, the width W is set to 1/4 the length of the carriage in the sub-scanning direction Y.

In this way, by shifting the carriage 110 and then performing the scanning process on the same area, the nozzles used for the first scanning process can be different from the nozzles used for the second and subsequent scanning processes. This makes it difficult for dot misalignment to be noticeable, and makes it difficult for streaks or color unevenness to occur, even if there is an installation error in one nozzle.

In addition, in the second and subsequent scanning steps, it is preferable to apply ink so as to fill in the spaces between the dots formed in the previous step.

The number of scanning steps performed on the same area should be at least two, and preferably between two and eight. The more times the steps are performed, the longer it takes to produce a printed product, but the less likely streaks or color unevenness will occur in the resulting ink layer. Note that streaks are particularly noticeable when the ink layer formed is dark. Therefore, fewer scanning steps may be performed in areas where the ink layer is light in color, and more scanning steps may be performed in areas where the ink layer is dark in color.

The size of each dot formed in each scanning process is selected appropriately depending on the amount of ink ejected from the nozzle, but for example, if the printing resolution is 600 dpi or more, a diameter of 75 μm or more is preferable, 95 μm or more is preferable, and 115 μm or more is even more preferable.

Here, in a typical printing method, in order to improve productivity, printing may be performed in each direction of the carriage in a serial manner. However, when printing is performed in the reciprocating direction in this way, the printing order of black, cyan, magenta, and yellow changes between the forward and backward passes. As a result, the color tone changes and the design quality is reduced. Therefore, in this embodiment, it is preferable to apply and harden the inks when the carriage 110 is moved in only one direction (main scanning direction X). In other words, unidirectional printing is preferable.

The thickness of the ink layer formed in the printing process is not particularly limited, and can be, for example, about 5 to 150 μm, and more preferably 10 to 100 μm. The printing process may be repeated to achieve the above thickness of the ink layer. If the ink layer becomes thicker, the curing shrinkage during curing increases, and the resulting ink layer becomes more likely to peel off. However, by using the metal substrate to be printed (particularly when the substrate has a printable layer that has been subjected to a frame treatment) described below, the adhesion between the metal substrate to be printed and the ink layer is high, and peeling is unlikely to occur even when an ink layer having a thickness of, for example, 40 μm or more is formed.

In the printing process, printing may be performed so that the thickness of the ink layer changes continuously or intermittently. When ink layers of different thicknesses are formed on a general metal plate or printable layer, adhesion changes between thick and thin ink layer areas, and peeling may occur from areas with low adhesion. In contrast, when a metal substrate for printing (particularly one having a printable layer that has been frame-treated) is used as described below, adhesion between the ink layer and the printable layer is high in both cases, whether the ink layer is thin or thick. Therefore, peeling is unlikely to occur even when an ink layer with a variable thickness is formed.

Here, after the printing process, if necessary, a process of irradiating the ink with light (hereinafter also referred to as a "post-curing process") may be carried out. This can further increase the curability of the ink layer, resulting in an ink layer with high strength.

The wavelength of the light irradiated in the post-curing process is not particularly limited, and may be, for example, ultraviolet light, visible light, infrared light, etc. It is selected appropriately depending on the type of ink. In addition, the light source is not particularly limited, and may be an LED light source or a general light source such as a halogen lamp. In addition, the atmosphere of the space into which the light is irradiated preferably has an oxygen concentration of 5% by volume or less, and more preferably an oxygen concentration of 1% by volume or less, in order to prevent a decrease in the surface curing degree due to oxygen inhibition of the ink layer.

The intensity of the light irradiated in the post-curing step is not particularly limited, and is preferably, for example, 0.5 to 25 W/cm ² , and more preferably 10 to 25 W/cm ^2. When the intensity is within this range, the curability of the ink tends to be good.

In addition, in this embodiment, after the printing process, a process may be further performed in which the ink layer is heated to relieve distortion caused in the printing process, i.e., distortion caused when the ink is cured (hereinafter, also referred to as a "distortion relaxation process"). By performing the distortion relaxation process, the adhesion of the ink layer to the metal substrate to be printed is improved.

The heating temperature in the distortion relaxation process is not particularly limited, but is usually preferably 80 to 270°C, more preferably 100 to 230°C. The heating time is preferably about 0.5 to 60 minutes, more preferably about 1 to 20 minutes. If the heating temperature and time are within the above ranges, the distortion can be relaxed without affecting or damaging the metal substrate to be printed or the ink layer.

In addition, before the printing step, the method may further include a step of forming a printable layer of the metal substrate to be printed, which will be described later, more specifically, a step of applying a paint containing a polyester resin and a melamine resin having a number average molecular weight of 12,000 to 30,000 onto the metal plate and curing the paint to form the printable layer, or a step of subjecting the printable layer formed on the metal plate to frame treatment or corona treatment.

(Metal substrate for printing)
The metal substrate to be printed used in the method for producing a printed matter of the present embodiment is sufficient to have at least a metal plate and a specific printing layer disposed on the metal plate, and may further include other layers as necessary. The substrate to be printed may be a precoated metal plate or a postcoated metal plate.

As mentioned above, coating a metal plate with active energy ray-curable ink has been considered in the past, but it is difficult to improve the adhesion of the ink layer, which is the cured product of the ink, and peeling is particularly likely to occur at the interface when the ink layer becomes thick. This is thought to be the reason why the ink layer peels off because it shrinks significantly when curing, which creates residual stress in the ink layer after curing and stress at the interface between the metal plate and the ink layer during curing.

In contrast, the metal substrate for printing used in this embodiment has a printing layer on a metal plate. The printing layer contains a cured product of a paint containing a polyester resin and a melamine resin having a number average molecular weight of 12,000 to 30,000. In addition, when the composition is analyzed in the depth direction from the surface to a depth of 10 nm by X-ray photoelectron spectroscopy (hereinafter also referred to as the "XPS method"), the maximum value of N atoms relative to the total amount of N atoms, C atoms, and O atoms is 5 to 30 atom%. According to the inventors' intensive research, the maximum value of N atoms relative to the total amount of N atoms, C atoms, and O atoms in the region from the surface of the printing layer to a depth of 10 nm is closely related to the peeling of the ink layer, and by setting the maximum value to 5 to 30 atom%, the adhesion with the ink layer is increased. In addition, by setting the maximum value in the above-mentioned range, the workability and scratch resistance of the metal substrate for printing are also improved. The reasons for this are considered to be as follows.

The N atoms observed when the printed layer is analyzed by XPS are mainly N atoms derived from the melamine resin, and the above-mentioned maximum value is an index showing the degree of concentration of the melamine resin on the surface. A printed layer with a maximum value of N atoms of more than 30 atom% has a high crosslink density of the polyester resin near the surface of the printed layer, and the surface of the printed layer is said to be high in hardness. Having such a printed layer improves the scratch resistance of the metal substrate to be printed, but a printed layer with high hardness is likely to have low adhesion to the above-mentioned ink layer and is also likely to have poor processability.

On the other hand, a printable layer in which the maximum value of N atoms is less than 5 atom % has a low crosslink density of the polyester resin near the surface of the printable layer, and the surface is soft. Having such a printable layer improves adhesion between the ink layer and the printable layer described above, but the ink layer cannot maintain its shape when it is formed, resulting in color unevenness in the ink layer and reduced scratch resistance of the metal substrate to be printed, making it difficult to use for various applications.

In contrast, if the maximum value of the above-mentioned N atoms is set to 5 atom% or more and 30 atom% or less, the crosslink density of the polyester resin becomes appropriate, and even if the active energy ray curable ink shrinks when cured, the stress can be alleviated by the printed layer. In other words, residual stress is unlikely to occur in the ink layer. In addition, the printed layer can deform in accordance with the cure shrinkage of the active energy ray curable ink. Therefore, stress is unlikely to act between the printed layer and the ink layer, and the adhesion between them can be improved. In addition, because the crosslink density of the polyester resin is in a moderate range, the processability and scratch resistance of the printed layer are also guaranteed.

It has been difficult to achieve such a concentration of N atoms in a coating film obtained from a paint containing a conventional polyester resin and a melamine resin. For example, methylated melamine resin, butylated melamine resin, etc. have been known as melamine resins in the past. However, methylated melamine resin is difficult to concentrate on the surface, and when used in a general blending amount, the maximum value of the above-mentioned N atoms tends to be less than 5 atom%. On the other hand, butylated melamine resin tends to concentrate on the surface, and when used in a general blending amount, the maximum value of the above-mentioned N atoms tends to exceed 30 atom%. Therefore, in this embodiment, the amount of melamine resin is adjusted or multiple melamine resins are combined to adjust the maximum value of the above-mentioned N atoms to be within the desired range. The metal substrate to be printed used in this embodiment will be described in detail below.

Metal Plate The metal plate may have any shape as long as it has a structure capable of forming a printing layer described later. For example, it may be flat or may have a three-dimensional structure. The metal plate may also be strip-shaped. The thickness of the metal plate is not particularly limited and is appropriately selected depending on the application of the coated metal material.

The material of the metal sheet is not particularly limited, and includes plated steel sheets such as hot-dip Zn-plated steel sheets, hot-dip Zn-55% Al alloy-plated steel sheets, hot-dip Zn-Al-Mg alloy-plated steel sheets, hot-dip Al-plated steel sheets, electrolytic Zn-plated steel sheets, and electrolytic Cu-plated steel sheets; steel sheets such as ordinary steel sheets and stainless steel sheets; aluminum sheets; copper sheets, and the like. The metal sheet may have a chemical conversion coating or an undercoat coating formed on its surface, as long as the effect of the present invention is not impaired. Furthermore, the metal sheet may be subjected to uneven processing such as embossing or drawing, as long as the effect of the present invention is not impaired.

The type of chemical conversion treatment used to form the chemical conversion coating is not particularly limited. Examples of chemical conversion treatments include chromate treatment, chromium-free treatment, phosphate treatment, etc. The amount of the chemical conversion coating applied is not particularly limited as long as it is within a range that is effective in improving corrosion resistance.

Furthermore, the undercoat coating is a layer that is formed directly on the metal plate or on the above-mentioned chemical conversion coating, and improves the adhesion of the printed layer and the corrosion resistance of the metal plate.

The undercoat coating film is formed, for example, by applying a resin-containing undercoat paint to the surface of a metal plate or a chemical conversion coating film and then drying (or curing). There are no particular limitations on the type of resin contained in the undercoat paint. Examples of resins include polyester, epoxy resin, acrylic resin, etc. Epoxy resin is particularly preferred because it has high polarity and good adhesion to the metal plate. The thickness of the undercoat coating film is selected appropriately according to the final application and type of painted metal material, and is, for example, about 5 μm.

Printable layer The printable layer is a layer containing a cured product of a paint containing a polyester resin and a melamine resin having a number average molecular weight of 12,000 to 30,000. The printable layer is formed by applying a printable layer-forming paint containing a polyester resin and a melamine resin having a number average molecular weight of 12,000 to 30,000 onto the above-mentioned metal plate, and then curing the paint.

The layer made of the cured product of the coating material for forming the printable layer may be used as the printable layer as is, but it is more preferable to use the layer that has been further subjected to frame treatment or corona discharge treatment as the printable layer. By performing frame treatment or corona discharge treatment, the adhesion of the ink layer described below to the printable layer is significantly improved, and even if a thick ink layer is formed, it is less likely to peel off. Among these, printable layers that have been subjected to frame treatment are particularly preferable from the viewpoint of adhesion of the ink layer, etc.

There are no particular limitations on the method for applying the paint for forming the printable layer onto the metal plate, and the method is appropriately selected according to the shape of the metal plate, the pattern of the printable layer to be formed, the area of the printable layer to be formed, etc. Examples of application methods include inkjet printing, gravure printing, offset printing, screen printing, roll coating, bar coating, spin coating, air spraying, airless spraying, immersion and pulling, etc. Two or more of these methods may be combined when applying the paint for forming the printable layer.

The coating material for forming the printable layer may contain a polyester resin having a number average molecular weight of 12,000 to 30,000 and a melamine resin. The polyester resin and the melamine resin are appropriately combined so that when the cured product (printable layer) of the coating material for forming the printable layer is measured by the XPS method, the amount of N atoms is 5 to 30 atom% relative to the amount of N atoms, C atoms, and O atoms in a region 10 nm from the surface. For example, the degree of concentration of the melamine resin on the coating surface side varies depending on the number average molecular weight of the polyester resin. When the number average molecular weight of the polyester resin is large, the melamine resin is less likely to concentrate on the coating surface. Therefore, it is preferable to increase the amount of butylated melamine resin, which is easily concentrated on the surface. On the other hand, when the number average molecular weight of the polyester resin is small, the butylated melamine resin is easily concentrated on the coating surface, so the amount of butylated melamine resin is reduced and the amount of melamine resin other than butylated melamine resin is increased, so that the amount of the above-mentioned N atoms is adjusted to be within the desired range. In addition, the coating material for forming the printable layer may contain other components such as catalysts, amines, extender pigments, and coloring pigments as necessary.

Polyester resins are any resin that has multiple ester structures in its molecular chain, and are usually prepared by polymerizing polycarboxylic acids and polyhydric alcohols.

Examples of polycarboxylic acids include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, 2,6-naphthalenedicarboxylic acid, and 2,7-naphthalenedicarboxylic acid, and their anhydrides; aliphatic dicarboxylic acids such as succinic acid, adipic acid, azelaic acid, sebacic acid, dodecanedicarboxylic acid, and 1,4-cyclohexanedicarboxylic acid, and their anhydrides; lactones such as γ-butyrolactone and ε-caprolactone; and trivalent or higher polyvalent carboxylic acids such as trimellitic acid, trimesic acid, and pyromellitic acid. The polyester resin may contain only one type of carboxylic acid structural unit derived from the above polycarboxylic acids, or may contain two or more types.

In the polyester resin, the ratio of the amount of structural units derived from trivalent or higher polycarboxylic acids to the total amount of alcohol structural units derived from polyhydric alcohols and carboxylic acid structural units derived from polycarboxylic acids is preferably 20 mol% or less, and more preferably 10 mol% or less, to prevent the crosslinking density of the coating film from becoming excessively high.

On the other hand, examples of polyhydric alcohols include ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 1,2-pentanediol, 1,4-pentanediol, 1,5-pentanediol, 2,3-pentanediol, 1,4-hexanediol, 2,5-hexanediol, 1,5-hexanediol, 1,6-hexanediol, 3-methyl-1,5-pentanediol, 2-methyl-1,5-pentanediol, glycols such as ethyl-1,3-propanediol, diethylene glycol, triethylene glycol, 1,2-dodecanediol, 1,2-octadecanediol, neopentyl glycol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, bisphenol A alkylene oxide adducts, and bisphenol S alkylene oxide adducts; and trihydric or higher polyhydric alcohols such as trimethylolpropane, glycerin, and pentaerythritol. The polyester resin may contain only one type of alcohol structural unit derived from the above polyhydric alcohols, or may contain two or more types.

The number average molecular weight of the polyester resin may be 12,000 to 30,000, preferably 12,000 to 25,000, and more preferably 14,000 to 23,000. The number average molecular weight of the polyester resin is a styrene-equivalent value determined by gel permeation chromatography (GPC). If the number average molecular weight of the polyester resin is excessively small, the strength of the printed layer is low and the hardness of the printed layer is likely to be low. In contrast, if the number average molecular weight of the polyester resin is 12,000 or more, a printed layer having sufficient strength (scratch resistance) can be obtained. On the other hand, if the number average molecular weight is 30,000 or less, the viscosity of the paint containing the polyester resin does not increase excessively, making it easier to form a coating film using a paint for forming a printed layer containing the polyester resin. Furthermore, the processability of the printed layer is also improved.

The hydroxyl value of the polyester resin is preferably 3 to 40 mgKOH/g, more preferably 5 to 20 mgKOH/g. The hydroxyl value is determined by potentiometric titration as specified in JIS K0070 using a 0.5 mol/L KOH alcohol solution. When the hydroxyl value of the polyester resin is within this range, the OH groups on the metal plate surface and the OH groups in the polyester resin (printable layer) tend to form hydrogen bonds, etc., and the adhesion between the metal plate and the printable layer is improved. Similarly, the OH groups in the polyester resin (printable layer) tend to form hydrogen bonds with hydrophilic groups in the ink layer, further improving the adhesion between them.

The glass transition temperature of the polyester resin is preferably -10 to 80°C, and more preferably 0 to 50°C. When the glass transition temperature of the polyester resin is in the above range, the resulting printed layer has good processability.

On the other hand, the melamine resin contained in the coating material for forming the printable layer is a thermosetting resin obtained by reacting melamine with aldehyde. The melamine resin is appropriately selected so as to realize the above-mentioned maximum value of N atoms when the printable layer is analyzed by the XPS method. The melamine resin has three reactive functional groups -NX ¹ X ² per one molecule of triazine nucleus. X ¹ and X ² usually represent CH ₂ OR (R is an alkyl group), CH ₂ OH, or a hydrogen atom. However, they are not limited to this structure.

Specific examples of melamine resins include four types: a fully alkyl melamine resin in which all three reactive functional groups are -N-(CH ₂ OR) ₂ ; a methylol group type melamine resin in which at least one reactive functional group is -N-(CH ₂ OR)(CH ₂ OH); an imino group type melamine resin in which at least one reactive functional group is -N-(CH ₂ OR)(H); and a methylol/imino group type melamine resin containing -N-(CH ₂ OR)(CH ₂ OH) and -N-(CH ₂ OR)(H) as reactive functional groups, or containing -N-(CH ₂ OH)(H). The melamine resin may be a melamine polymer in which reactive functional groups are condensed together. The coating material for forming the printable layer may contain only one type of melamine resin, or may contain two or more types.

The R of the reactive group of the melamine resin, i.e., the alkyl group, is preferably an alkyl group having 1 to 4 carbon atoms, and the alkyl group may be linear or branched. Among these, a methyl group or a butyl group is particularly preferred.

The melamine resin may be a commercially available resin. Examples of commercially available melamine resins include fully alkylated methyl/butyl mixed etherified melamine resins such as Cymel 232, Cymel 232S, Cymel 235, Cymel 236, Cymel 238, Cymel 266, Cymel 267, and Cymel 285 manufactured by Allnex Japan Co., Ltd.; methylol group-type methyl/butyl mixed etherified melamine resins such as Cymel 272 manufactured by Allnex Japan Co., Ltd.; imino group-type methyl/butyl mixed etherified melamine resins such as Cymel 202, Cymel 207, Cymel 212, Cymel 253, and Cymel 254 manufactured by Allnex Japan Co., Ltd.; and Cymel 300 and Cymel 301 manufactured by Allnex Japan Co., Ltd. fully alkylated methylated melamine resins (sometimes referred to as "fully alkylated methyl etherified melamine resins") such as Cymel 303 and Cymel 350; imino group type methylated melamine resins such as Cymel 325, Cymel 327, Cymel 703, Cymel 712, Cymel 253, Cymel 212 and Cymel 1128 manufactured by Allnex Japan; methylol group type methylated melamine resins such as Cymel 370 manufactured by Allnex Japan; butylated melamine resins (sometimes referred to as "butyl etherified melamine resins") such as U-BAN 20SE60 manufactured by Mitsui Chemicals and Super Beckamin J830 manufactured by DIC.

In this specification, "methylated melamine resin" refers to fully alkylated methyl etherified melamine resin, imino group type methyl etherified melamine resin, methylol group type methyl etherified melamine resin, or a mixed resin of these. "Butylated melamine resin" refers to fully alkylated butyl etherified melamine, butyl etherified melamine resin whose main structure is imino group type butyl etherified melamine, or a mixed resin of these. Melamine resins that have both methyl groups and butyl groups, such as fully alkylated methyl/butyl mixed etherified melamine resin, methylol group type methyl/butyl mixed etherified melamine resin, and imino group type methyl/butyl mixed etherified melamine resin, are also referred to as "methyl/butyl mixed melamine resin".

The melamine resin may contain only one type of melamine resin selected from the above, but from the viewpoint of being able to realize the above-mentioned amount of N atoms when the printed layer is analyzed by the XPS method, it is preferable that the melamine resin contains two or more types of melamine resin, and it is more preferable that the melamine resin is a mixture of butylated melamine resin and other melamine resin, i.e., methylated melamine resin or methyl/butyl mixed melamine resin. Furthermore, it is particularly preferable that the melamine resin is a mixture of butylated melamine resin and methylated melamine resin. In this case, the mass ratio of the butylated melamine resin to the methylated melamine resin is preferably 1:1 to 1:8, and more preferably 2:3 to 1:6. The melamine resin may contain three or more types of melamine resin.

The ratio (mass ratio) of polyester resin and melamine resin contained in the paint for forming the printable layer is preferably 90:10 to 60:40, and more preferably about 85:15 to 70:30. When the mass ratio of polyester resin and melamine resin is within this range, a printable layer with excellent weather resistance and impact resistance is obtained.

The total amount of polyester resin and melamine resin per 100 parts by mass of solids in the paint for forming the printable layer is preferably 30 to 80 parts by mass, more preferably 50 to 70 parts by mass. When the total amount of polyester resin and melamine resin is within this range, the strength of the printable layer obtained from the paint for forming the printable layer is likely to be sufficiently high. Furthermore, the adhesion between the printable layer and the ink layer is likely to be good.

The coating material for forming the printable layer may further contain a catalyst. Examples of catalysts include dodecylbenzenesulfonic acid, paratoluenesulfonic acid, benzenesulfonic acid, etc. The amount of catalyst used is preferably 0.1 to 8 mass% based on the total solid content of the coating material for forming the printable layer.

The coating material for forming the printable layer may further contain an amine. The amine is a compound for neutralizing the catalyst, and examples thereof include triethylamine, dimethylethanolamine, dimethylaminoethanol, monoethanolamine, isopropanolamine, and the like. The amount of amine used is not particularly limited, but is preferably 50 mol% or more of the acid (catalyst) equivalent.

The coating material for forming the printable layer may further contain extender pigments (including beads) and color pigments. Examples of extender pigments include silica, calcium carbonate, barium sulfate, aluminum hydroxide, talc, mica, resin beads, glass beads, etc. Examples of resin beads include acrylic resin beads, polyacrylonitrile beads, polyethylene beads, polypropylene beads, polyester beads, urethane resin beads, epoxy resin beads, etc.

These resin beads may be manufactured using known methods, or commercially available products may be used. Examples of commercially available acrylic resin beads include "Tuftic AR650S (average particle size 18 μm)", "Tuftic AR650M (average particle size 30 μm)", "Tuftic AR650MX (average particle size 40 μm)", "Tuftic AR650MZ (average particle size 60 μm)", "Tuftic AR650ML (average particle size 80 μm)", "Tuftic AR650L (average particle size 100 μm)" and "Tuftic AR650LL (average particle size 150 μm)" by Toyobo Co., Ltd. Examples of commercially available polyacrylonitrile beads include Toyobo Co., Ltd.'s "Tuftic A-20 (average particle size 24 μm)," "Tuftic YK-30 (average particle size 33 μm)," "Tuftic YK-50 (average particle size 50 μm)," and "Tuftic YK-80 (average particle size 80 μm)." The coating material for forming the printable layer may contain only one of these, or may contain two or more of them.

On the other hand, examples of color pigments include carbon black, titanium oxide, iron oxide, yellow iron oxide, phthalocyanine blue, cobalt blue, etc. The amount of pigment is appropriately selected depending on the type of pigment, particle size, etc. The paint for forming the printable layer may contain only one of these, or may contain two or more types.

The coating material for forming the printable layer may further contain a solvent as necessary. The solvent is not particularly limited as long as it can sufficiently dissolve the polyester resin or melamine resin and uniformly disperse the extender pigment or color pigment. Examples of the solvent include hydrocarbon solvents such as toluene, xylene, Solvesso (registered trademark) 100 (trade name, Exxon Mobil Corporation), Solvesso (registered trademark) 150 (trade name, Exxon Mobil Corporation), and Solvesso (registered trademark) 200 (trade name, Exxon Mobil Corporation); ketone solvents such as methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, and isophorone; ester solvents such as ethyl acetate, butyl acetate, and ethylene glycol monoethyl ether acetate; alcohol solvents such as methanol, isopropyl alcohol, and n-butyl alcohol; ether alcohol solvents such as ethylene glycol monoethyl ether and diethylene glycol monobutyl ether; n-methylpyrrolidone; and ethyl-3-ethoxy-propionate. The coating material for forming the printable layer may contain only one of these, or may contain two or more of these. Among these, from the viewpoint of compatibility with polyester resins and melamine resins, xylene, Solvesso (registered trademark) 100, Solvesso (registered trademark) 150, cyclohexanone, and n-butyl alcohol are preferred.

There are no particular limitations on the method for preparing the coating material for forming the printable layer, and the polyester resin, melamine resin, and other components, if necessary, may be mixed by a known method.

Then, after the paint for forming the printable layer is applied onto the metal plate, the paint for forming the printable layer is heated (hereinafter also referred to as "baking") to harden it. There are no particular restrictions on the baking method, but it is preferable to heat the metal plate so that the plate temperature reaches a range of 150 to 250°C.

In addition, if a process of framing the printable layer is carried out after the paint for forming the printable layer is baked, the metal plate on which the printable layer is formed is placed on a conveying machine such as a belt conveyor. Then, while moving in a certain direction, a flame is emitted onto the printable layer using a flame treatment burner.

Here, the frame treatment amount is preferably 100 to 600 kJ/ ^m2 . In this specification, the "frame treatment amount" refers to the amount of heat per unit area of the metal substrate calculated based on the supply amount of combustion gas such as LPG. The frame treatment amount can be adjusted by the distance between the burner head of the frame treatment burner and the surface of the printed layer, the conveying speed of the printed layer, etc. If the frame treatment amount is 100 kJ/m2 or ^more , the entire surface of the printed layer is more likely to be treated uniformly. On the other hand, if the frame treatment amount is 600 kJ/ ^m2 or less, the printed layer is less likely to yellow, etc.

Prior to the above frame treatment, a preheating treatment may be performed to heat the surface of the printed layer to 40°C or higher. When a flame is irradiated onto a printed layer formed on the surface of a metal substrate with high thermal conductivity (e.g., a metal substrate with thermal conductivity of 10 W/mK or more), the water vapor generated by the combustion of combustible gas is cooled and turns to water, which temporarily accumulates on the surface of the printed layer. This water may then absorb the energy during the frame treatment and turn into water vapor, which may impede the frame treatment. In response to this, the generation of water during flame irradiation can be suppressed by preheating the surface of the printed layer.

The means for preheating the printed layer is not particularly limited, and is appropriately selected according to the shape of the metal plate, etc. For example, a heating device generally called a drying oven can be used. For example, a batch-type drying oven (also called a "safe oven") can be used, and specific examples include a low-temperature incubator manufactured by Isuzu Manufacturing Co., Ltd. (model Mini Catalina MRLV-11), an automatic discharge type dryer manufactured by Tojo Thermal Engineering Co., Ltd. (model ATO-101), and a simplified explosion-proof dryer manufactured by Tojo Thermal Engineering Co., Ltd. (model TNAT-1000).

On the other hand, when performing a step of performing a corona discharge treatment on the printing layer, the metal plate on which the printing layer is formed is placed between an insulated electrode and a grounded counter electrode dielectric roll. Then, a high frequency of 1 to 600 KHz and a high voltage of 5 to 30 KV are applied between them to generate a corona discharge. Corona discharge treatment devices include spark gap type, vacuum tube type, solid state type, etc., and any of them can be used. The corona discharge treatment conditions are preferably 200 w/m ² /min or more, and more preferably 200 to 800 w/m ² /min. If it is less than 200 w/m ² /min, the amount of corona discharge treatment may be insufficient, and if it exceeds 800 w/m ² /min, the treatment becomes excessive, which is not economically preferable.

Here, the thickness of the printable layer is not particularly limited, but is preferably 10 to 40 μm, and more preferably 12 to 25 μm. When the thickness of the printable layer is 10 μm or more, the durability of the printable layer is likely to be good. In addition, the printable layer is likely to be flexible, and the adhesion of the ink layer described below is likely to be improved. On the other hand, when the thickness is 40 μm or less, popping is unlikely to occur during the above-mentioned heating, and the surface condition is likely to be good. Furthermore, when the surface condition of the printable layer is good, the adhesion between the ink layer formed in the above-mentioned printing process and the printable layer is likely to be improved.

As described above, when the composition is analyzed in the depth direction by XPS using AlKα radiation as the X-ray source for the region from the surface of the printed layer to a depth of 10 nm, the maximum value of N atoms relative to the total amount of N atoms, C atoms, and O atoms should be 5 to 30 atom%, more preferably 12 to 28 atom%, and even more preferably 15 to 25 atom%. The concentration of each atom from the surface of the printed layer to a depth of 10 nm is measured by an XPS analyzer under the following conditions while etching the printed layer from the surface to a depth of 10 nm.
(Measurement conditions)
Measurement equipment: ULVAC-PHI Versaprobe II scanning X-ray photoelectron spectrometer X-ray source: AlKα (monochrome: 50 W, 15 kV) 1486.6 eV
Analysis area: 1.0 x 1.0 mm ²
Pass Energy: 5.85eV (C1s, O1s), 187.85eV (N1s)
Charge neutralization (electron neutralization + ion gun)
Analysis chamber vacuum level: 1.0× ^10-7 Pa
(Etching conditions)
Etching conditions: Ar-gas cluster ion beam Acceleration voltage: 5 kV
Etching rate: Approximately 1 nm/min (polymethyl methacrylate resin (PMMA) equivalent)

Here, in order to perform the above-mentioned printing process on the surface of the print layer and allow the ink to spread sufficiently, it is preferable that the surface of the print layer is made uniformly hydrophilic. Whether the surface of the print layer is made uniformly hydrophilic or not can be evaluated by the methylene iodide sliding angle described below. For example, when the methylene iodide sliding angle of the print layer is 10° or more and 40° or less, it can be said that the surface is made uniformly hydrophilic. It is more preferable that the methylene iodide sliding angle is 35° or less.

The methylene iodide sliding angle is likely to fall within the above range when the hydrophilicity of the printed layer surface is sufficiently high or when the surface roughness of the printed layer is high. However, when the hydrophilicity of the printed layer is non-uniform, the methylene iodide sliding angle will be higher than 40°. For example, when the surface is treated by corona treatment, the methylene iodide sliding angle is likely to exceed 40°. In contrast, when frame treatment is performed, the surface is uniformly hydrophilized and the methylene iodide sliding angle is 40° or less.

The reason why the methylene iodide sliding angle becomes larger than 40° when the hydrophilicity of the surface of the printed layer becomes non-uniform due to corona discharge treatment, etc., is considered as follows. It is assumed that there are two types of coating films with the same number of hydrophilic and hydrophobic groups on the surface, one of which has no bias in the distribution of hydrophilic and hydrophobic groups, and the other has a bias in the distribution of hydrophilic and hydrophobic groups. In this case, the static contact angles of both are not easily affected by the distribution of hydrophilic and hydrophobic groups and are approximately the same. In contrast, the dynamic contact angles of both (methylene iodide sliding angles) are affected by the distribution of hydrophilic and hydrophobic groups and are different values. When measuring the methylene iodide sliding angle, if the distribution of hydrophilic and hydrophobic groups is non-uniform, methylene iodide droplets are adsorbed to the areas with a high density of hydrophilic groups. In other words, if there is an uneven distribution of hydrophilic and hydrophobic groups, the methylene iodide droplets will be less likely to move than if there was no uneven distribution, and the sliding angle will be larger. Therefore, if a large number of hydrophilic groups are introduced to the surface of the printing layer, as in corona discharge treatment, but there is unevenness in their distribution, the methylene iodide sliding angle will be a high value exceeding 40°.

The methylene iodide sliding angle is measured as follows. First, 2 μl of methylene iodide is dropped onto the printing layer. Then, using a contact angle measuring device, the inclination angle of the printing layer (the angle between the plane perpendicular to gravity and the printing layer) is increased at a speed of 2 degrees/second. At this time, the methylene iodide droplet is observed using a camera attached to the contact angle measuring device. The inclination angle at the moment when the methylene iodide droplet slides down is identified, and the average value of five measurements is taken as the methylene iodide sliding angle of the printing layer. The moment when the methylene iodide droplet slides down is the moment when both the end point of the methylene iodide (droplet) in the downward direction of gravity and the end point in the upward direction of gravity start to move.

(effect)
As described above, the carriage of a conventional serial type inkjet printing device has only one light source for multiple inkjet heads. Therefore, multiple types of ink are ejected from each inkjet head and then cured together, which tends to cause problems such as streaks and color unevenness as described above. Furthermore, there is also a problem that the dot diameters of the resulting ink layers differ for each color, which tends to reduce image quality.

In contrast, in the printing device used in the method for producing printed matter of this embodiment, an LED light source is arranged for each head, and ink is ejected and cured separately for each ink. Therefore, the amount of ink that is cured at one time is small, and the ink that has landed on the metal substrate to be printed is less likely to coalesce. As a result, the ink that has landed can spread sufficiently in the sub-scanning direction, making it possible to maintain a constant distance between dots.

In addition, in the above-mentioned method for producing printed matter, the carriage of the inkjet printing device is scanned at least twice over the same area to form an ink layer. In addition, ink is applied from different nozzles during the first and second scans. Therefore, even if there is an error in the installation of the nozzles of the inkjet head, misalignment of the dots is less noticeable, and streaks and color unevenness are less likely to occur.

As described above, when applying multiple types of ink, each type of ink is ejected and cured separately. Therefore, the dot diameters of the inks of each color can be made uniform in the resulting ink layer. This allows a highly precise ink layer to be formed. According to the above method, the maximum dot diameter and the minimum dot diameter on the metal substrate to be printed can be kept within ±20% of the average dot diameter, and can also be kept within ±10%. The ink dot diameter is the diameter of the main dot excluding the satellite component when observed under a microscope, and the maximum and minimum values in a specific direction are identified for each dot. The average value of these values is taken as the diameter of each dot.

Furthermore, in the method for producing a printed matter of this embodiment, by using the above-mentioned metal substrate for printing, even if a thick ink layer is formed, the adhesion between the print layer of the metal substrate for printing and the ink layer is high, and peeling is unlikely to occur at the interface between them. In addition, by using the above-mentioned metal substrate for printing, even if an ink layer with a partially different thickness is formed, peeling of the ink layer can be made unlikely to occur.

(Use of printed matter)
The uses of the above-mentioned printed matter are not particularly limited, and it can be used for a wide variety of components and applications that use a metal substrate. Examples of applications include home appliances such as refrigerator outer panels, microwave oven outer panels, personal computer housings, and air conditioner housings; interior decorative building materials such as partitions, doors, ceiling materials, floor materials, elevator doors, and elevator interior panels; various residential equipment such as range hoods and bathroom interior components; furniture such as desks, chairs, lockers, and shelves; and vehicle components such as passenger car interiors and railway car interiors. However, the uses of coated metal materials are not limited to these.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

1. Preparation of metal substrate for printing [Preparation of metal plate]
A hot-dip Zn-plated steel sheet with a thickness of 0.3 mm and an A4 size with a coating weight of 45 g/ ^m2 per side was used. After alkaline degreasing of the plated steel sheet, a paint-type chromate (NRC300NS: manufactured by Nippon Paint Industrial Coatings Co., Ltd.) was applied to the steel sheet so that the coating weight of Cr was 50 mg/ ^m2 , and a primer composition (FL641EU primer, manufactured by Nippon Paint Industrial Coatings Co., Ltd.) was applied to the steel sheet with a roll coater so that the dry film thickness was 5 μm. The steel sheet was then baked to a maximum plate temperature of 215° C. to obtain a metal plate.

[Formation of Printing Layer]
Preparation of paint for forming printable layer The polyester resin and melamine resin shown below were mixed in the mass ratio shown in Table 1. Furthermore, the catalyst and color pigment shown below were also mixed to prepare paints P1 to P8 for forming printable layer.

(1) Regarding polyester resin: The following two types of polyester resin were used. The polyester resin was dissolved in a solvent and mixed with other components. The solvent was one of n-methylpyrrolidone, cyclohexanone, an aromatic solvent (Solvesso (registered trademark) 150, surface tension 30 mN/m), and ethyl-3-ethoxy-propionate (surface tension 25 mN/m), or a mixture of multiple solvents.

Polyester resin A (manufactured by Toyobo Co., Ltd., Vylon (registered trademark) GK140, Tg: 20°C, number average molecular weight: 14,000, amorphous)
Polyester resin B (manufactured by Toyobo Co., Ltd., Vylon (registered trademark) 630, Tg: 7°C, number average molecular weight: 23,000, amorphous)

(2) Melamine resin The following two types of melamine resin were used: Methylated melamine (Allnex Japan, Cymel 303, fully alkylated methylated melamine resin)
-Butylated melamine (DIC Corporation, Super Beckamin (registered trademark) J830, butylated melamine resin)

(3) Catalyst The catalyst used was dodecylbenzenesulfonic acid. More specifically, dodecylbenzenesulfonic acid was neutralized with triethylamine so that the amine equivalent was 1.25 times the acid equivalent, and the neutralized catalyst was added in an amount of 1 mass % based on the resin solid content of the coating material for forming the printable layer.

(4) Coloring Pigment Titanium oxide (Tipake (registered trademark) CR-95, manufactured by Ishihara Sangyo Kaisha, Ltd.) was used as the coloring pigment. The amount of the pigment added was 50% by mass based on the total amount of the polyester resin and the melamine resin.

Coating and curing of coating material for forming a printable layer Coating materials P1 to P8 for forming a printable layer were each applied to one side of the metal plate having the above-mentioned primer layer with a roll coater so that the dry film thickness was 18 μm. Then, the plate was baked for 60 seconds so that the maximum plate temperature reached was 225° C., and a printable layer was formed on the metal plate.

[Frame Processing]
The printed layer was subjected to frame treatment under the following conditions as necessary. The printed metal substrate on which the printed layer was formed was placed on a conveyor, and the printed layer was subjected to frame treatment. The flame treatment burner used was F-3000 manufactured by Flynn Burner (USA). In addition, a mixed gas (LP gas: clean dry air (volume ratio) = 1:25) in which LP gas (combustion gas) and clean dry air were mixed with a gas mixer was used as the combustible gas. In addition, the flow rate of each gas was adjusted so that LP gas (combustion gas) was 1.67 L/min and clean dry air was 41.7 L/min per cm ² of the burner flame port. The length of the burner head flame port in the conveying direction of the printed layer was 4 mm. On the other hand, the length of the burner head flame port in the conveying direction and perpendicular to the conveying direction was 450 mm. In addition, the distance between the burner head flame port and the surface of the printed layer was 50 mm according to the desired amount of frame treatment. Furthermore, the conveying speed of the printed layer was set to 30 m/min, and the frame processing amount was adjusted to 212 kJ/ ^m2 .

2. Preparation of Ink (Active Energy Ray Curable Ink) A radical polymerization type inorganic ink of DNT Digital Coating System manufactured by Dai Nippon Toryo Co., Ltd. was used as the inkjet ink (active energy ray curable ink).
Cyan ink: NS-inorganic cyan 002
Magenta ink: NS-inorganic magenta 002
Yellow ink: NS-Inorganic Yellow 002
Black ink: NS-Black 002

3. Formation of Ink Layer (Example 1)
A black head, a black LED light source, a cyan head, a cyan LED light source, a magenta head, a magenta LED light source, a yellow head, and a yellow LED light source were arranged in this order on the carriage of an inkjet printing device. For each head, a Kyocera KJ4A-TA (2656 nozzles, effective recording width 108.25 mm) was used. For each LED light source, a Phoseon Technology Japan UV-LED light source (peak wavelength: 395 nm, FE410, maximum UV output: 4.8 W/cm ² ) was used. The distance between the nozzle of each head and the corresponding LED light source was 40 mm. The output of the LED light source was 70% of the maximum output. The amount of ink droplets from each nozzle of each head was set to 14 pl. The ejection frequency was set to form an image of 600 dpi at a speed of about 45 m/min.

Then, while scanning the carriage in the main scanning direction at about 45 m/min, the ink was applied to the above-mentioned metal substrate to be printed and light was irradiated for each color. The printing pattern at this time was appropriately selected according to the evaluation method described below. In Example 1, the scanning process was performed twice (two passes) on the same area, so in the first scanning process, ink was applied to every other dot in the desired pattern. Then, the relative position between the carriage and the metal substrate to be printed was shifted by 1/2 the head width (total head width when multiple heads are connected in the sub-scanning direction). Then, the second scanning process was performed in the same manner as above to obtain the desired ink layer. In the second scanning process, ink was also applied to every other dot (however, in the areas where no dots were formed in the first scanning process).

Thereafter, the ink layer was further irradiated with light (post-curing step) while the obtained print was moved at a speed of 3 m/min. The light irradiation was performed in an atmosphere with an oxygen concentration of 5% by volume or less. The light irradiation was performed using a UV-LED light source unit (FL400, manufactured by Phoseon Technology Japan, maximum UV output: 24 W/cm ² ) with a peak wavelength of 395 nm. The output at this time was 70% (16.8 W/cm ² ) of the maximum output. Thereafter, the print was placed in a 230°C heating furnace (manufactured by Takeda Rika Kogyo Co., Ltd.) for 1 minute for heat treatment, and the shrinkage distortion of the ink layer was alleviated to obtain a print (coated metal plate).

(Examples 2 to 12)
Printed matter was produced in the same manner as in Example 1, except that the type of printed metal substrate was changed as shown in Table 1.

(Examples 13 to 24)
Printed matter was produced in the same manner as in Examples 1 to 12, except that the scanning process was performed four times for the same area. For the second and subsequent scanning processes, the relative positions of the carriage and the metal substrate to be printed were shifted from the initial positions by ¼ of the head width (total head width when multiple heads are connected in the sub-scanning direction). Furthermore, in each scanning process, ink was applied every 3 dots in the desired pattern.

(Comparative Examples 1 to 12)
Printed matter was produced in the same manner as in Examples 1 to 12, except that the ink layer was formed in a single scanning step.

(Comparative Examples 13 to 24)
Printed materials were produced in the same manner as in Examples 13 to 24, except that a black head, a cyan head, a magenta head, a yellow head, and a common LED light source were arranged on the carriage and the inks of each color were cured together.

(Comparative Examples 25 to 28)
A printed matter was produced in the same manner as in Example 13, except that P7 or P8 was used as the coating material for forming the printable layer when forming the printable layer.

4. Evaluation The printing layer of the metal substrate for printing was subjected to elemental analysis by XPS method and measurement of methylene iodide sliding angle by the following method. In addition, the dot size of the printed matter, the ink adhesion to the metal substrate for printing, bleeding, and streaking were evaluated. The results are shown in Table 2 or 3.

4-1. Elemental analysis by XPS method (determining the amount of N atoms, C atoms, and O atoms in the printed layer)
The concentrations of N atoms, C atoms, and O atoms in the depth direction (the total amount of N atoms, C atoms, and O atoms is taken as 100 atom%) were measured while etching from the surface of the printed layer to a depth of 10 nm under the following conditions using an XPS analyzer. Then, the maximum value of N atoms relative to the total amount of N atoms, C atoms, and O atoms was identified.
(Measurement conditions)
Measurement equipment: ULVAC-PHI Versaprobe II scanning X-ray photoelectron spectrometer X-ray source: AlKα (monochrome: 50 W, 15 kV) 1486.6 eV
Analysis area: 1.0 x 1.0 mm ²
Pass Energy: 5.85eV (C1s, O1s), 187.85eV (N1s)
Charge neutralization (electron neutralization + ion gun)
Analysis chamber vacuum level: 1.0× ^10-7 Pa
(Etching conditions)
Etching conditions: Ar-gas cluster ion beam Acceleration voltage: 5 kV
Etching rate: Approximately 1 nm/min (polymethyl methacrylate resin (PMMA) equivalent)

4-2. Measurement of the methylene iodide sliding angle of the printed layer 2 μl of methylene iodide was dropped onto the printed layer held horizontally. Then, using a contact angle measuring device (DM901 manufactured by Kyowa Interface Science Co., Ltd.), the inclination angle of the printed layer (the angle between the horizontal plane and the printed layer) was increased at a speed of 2 degrees/second. Then, the methylene iodide droplet was observed with a camera attached to the contact angle measuring device. The inclination angle of the printed layer at the moment when the methylene iodide droplet fell was specified, and the average value of five times was taken as the methylene iodide sliding angle of the printed layer. The moment when the methylene iodide droplet fell was taken as the moment when both the end point of the methylene iodide droplet in the downward direction of gravity and the end point in the upward direction of gravity began to move.

4-3. Evaluation of dot diameter (ink wetting and spreading property) in printed matter Using the method described above, dots were printed on a metal substrate to be printed with black ink. At this time, the distance between adjacent dots was set to 500 μm so that the dots would not overlap each other. The dot diameter was measured using a LEXT OLS3000 confocal laser scanning microscope manufactured by Olympus Corporation. The dot diameters of eight dots were measured by enlarging the area (200 times) so that only one dot was visible, and the average value was shown. When the spread of the dots was close to an ellipse, the average value of the major axis and minor axis was taken as the dot diameter, and the evaluation was performed as follows. △ or higher was considered to be acceptable.
◎: Dot diameter is 115 μm or more and less than 160 μm ○: Dot diameter is 95 μm or more and less than 115 μm △: Dot diameter is 75 μm or more and less than 95 μm ×: Dot diameter is less than 75 μm

4-4. Evaluation of ink adhesion A color patch of 3.5 cm x 10 cm and 300% density was created using the above printing method. Note that density refers to the area of ink adhesion relative to the area of the color patch. 300% refers to a state in which cyan ink, magenta ink, yellow ink, and black ink are evenly layered so that the total applied area of cyan ink, magenta ink, yellow ink, and black ink is three times the area of the color patch.

A cross-cut test was carried out on the color patch in accordance with JIS K5600-5-6. Specifically, grid-like cuts were made on the surface of the ink layer so as to create 25 grids spaced 2 mm apart, and tape was applied to the corresponding areas. After the tape was peeled off, the remaining rate of the coating was observed. Evaluation was performed according to the following criteria. Evaluation of △ or higher was considered to be a pass.
○: The peeled area of the ink layer is 0%.
△: Peeled area is more than 0% and less than 20%. ×: Peeled area is more than 20%.

4-5. Evaluation of color unevenness (bleed) Using the above printing method, color patches were prepared with densities varying from 0% to 300% in 30% increments. Then, the patches were observed from a distance of 50 cm, and the highlight area (density 30%), medium density area (density 90%), and high density area (density 180%) were visually observed for color unevenness (bleed) within the patches, and evaluated according to the following criteria. The number of evaluators was 10, and the average of the 10 was used as the evaluation value. Evaluation points of △ or higher were within the acceptable range.
○: No bleeding △: Slight bleeding ×: Heavy bleeding

4-6. Evaluation of streaky unevenness A color patch was prepared in the same manner as in the evaluation of color unevenness (bleed) described above. The color patch was then observed from a distance of 50 cm, and the presence or absence of periodic streaky unevenness (streaky unevenness) in the highlight area (density 30%), medium density area (density 90%), and high density area (density 180%) was evaluated on the following 5-point scale. The number of evaluators was 10, and the average of the evaluation scores of the 10 evaluators was rounded off to the nearest whole number to obtain the evaluation value. A rating of 3 or higher is within the acceptable range.
5: No streaks are observed.
4: Slight streak unevenness is observed, but not bothersome.
3: Streaks are observed but acceptable.
2: I'm concerned about uneven streaks.
1: Streaks are very noticeable.

5. Results The evaluation results are shown below.

As shown in Tables 2 and 3 above, when ink was applied and cured for each type (color) of ink and the scanning process was performed multiple times in the same area by shifting the carriage position (Examples 1 to 24), streaks and color unevenness were less likely to occur compared to when the inks of all colors were cured at once (Comparative Examples 13 to 24).

Even if the ink was applied and cured for each type (color) of ink, streaks were likely to occur when the ink layer was formed by performing the scanning process only once (Comparative Examples 1 to 12). This was thought to be largely influenced by the state of the inkjet head nozzles.

As shown in Tables 2 and 3, when the composition was analyzed in the depth direction of the printed layer using the XPS method, the ratio of N atoms changed significantly in the region from the surface to a depth of 10 nm depending on the amount, type, and combination of melamine resin.

When the maximum value of N atoms relative to the total amount of N atoms, C atoms, and O atoms in the region from the surface of the printable layer to a depth of 10 nm was 5 to 30 atom%, the adhesion of the ink layer was better than when the maximum value was more than 30 atom% (Comparative Examples 25 and 26) (Examples 1 to 24 and Comparative Examples 1 to 24). On the other hand, when the maximum value of N atoms relative to the total amount of N atoms, C atoms, and O atoms in the region from the surface of the printable layer to a depth of 10 nm was less than 5 atom%, the adhesion of the ink layer was good, but the crosslink density of the printable layer was too low, so ink bleeding was likely to occur (Comparative Examples 27 and 27).

The method for producing printed matter of the present invention makes it possible to form an ink layer on a metal substrate for printing without uneven color or streaks and with good adhesion. This makes it possible to produce a variety of products with highly aesthetic designs.

100, 900 Inkjet printing device 110, 910 Carriage 111, 111K, 111M, 111C, 111Y, 911, 911K, 911M, 911C, 911Y Inkjet head 112, 112K, 112M, 112C, 112Y LED light source 113 Holder 120 Rail 912, 912K, 912M, 912C, 912Y Light source

Claims (8)

A method for producing a printed matter, comprising a printing step of applying a plurality of types of active energy ray curable inks to a metal substrate to be printed on, and photo-curing the inks to form an ink layer, by using a serial type inkjet printing device having a carriage in which a plurality of inkjet heads, each of which includes a plurality of nozzles arranged in a sub-scanning direction, and a plurality of LED light sources are arranged alternately in a main scanning direction,
the printing step includes a scanning step in which the inkjet head applies the ink while scanning the carriage in a main scanning direction, and the LED light source adjacent to the inkjet head at a rear side in a traveling direction irradiates light onto the ink applied by the inkjet head,
In the printing step, the scanning step is performed two or more times on the same region of the metal substrate to be printed while shifting the carriage in a sub-scanning direction;
The metal substrate to be printed is
A metal plate;
a printable layer disposed on the metal plate, the printable layer including a cured product of a coating material including a polyester resin having a number average molecular weight of 12,000 to 30,000 and a melamine resin;
having
When a composition is analyzed in a depth direction by X-ray photoelectron spectroscopy using AlKα radiation as an X-ray source for a region from the surface of the printing layer to a depth of 10 nm, the maximum value of N atoms relative to the total amount of N atoms, C atoms, and O atoms is 5 to 30 atom %.
A method for producing printed matter. The scanning step is performed eight times or less on the same area of the metal substrate to be printed while shifting the carriage in a sub-scanning direction.
A method for producing the printed matter according to claim 1 . The method further includes a step of irradiating the ink layer formed by the printing step with light.
A method for producing the printed matter according to claim 1 or 2. The method for producing a printed matter according to any one of claims 1 to 3, wherein the melamine resin is a mixture of at least a butylated melamine resin and a melamine resin other than the butylated melamine resin. The methylene iodide sliding angle on the surface of the printing layer is 10° or more and 40° or less.
The method for producing the printed matter according to any one of claims 1 to 4. Before carrying out the printing process,
The method further includes a step of applying a coating material containing a polyester resin having a number average molecular weight of 12,000 to 30,000 and a melamine resin onto a metal plate and curing the coating material to form the printable layer.
The method for producing the printed matter according to any one of claims 1 to 5. Before carrying out the printing process,
The printing layer may further include a step of subjecting the printing layer to a frame treatment or a corona discharge treatment.
The method for producing the printed matter according to claim 6 . The thickness of the ink layer is 40 μm or more.
The method for producing the printed matter according to any one of claims 1 to 7.