JP7035612B2 - Magnetic ink printing method - Google Patents

Description

In the present invention, after a print pattern is formed on a paper substrate by an inkjet printing method using magnetic ink, even when the printed matter is immersed in water, the magnetic output (reading) is lowered or discolored due to the precipitation of the magnetic material. Regarding the improvement of the printing method that eliminates the concern of.

With variable information recording media such as gift certificates, coupon tickets, securities, and lottery tickets, information patterns and barcodes are printed using magnetic ink as print contents that differ for each individual, and information is recorded by magnetically detecting them. Information recording media that satisfy the decryption and anti-counterfeiting functions are provided.

As a method of forming a print pattern using magnetic ink, the inkjet printing method is regarded as promising and is widely used in order to meet the demand for shortening the drying time of ink and the fixing time on a paper substrate for the purpose of realizing high-speed printing. There are many examples of improvements in ink formulations related to the method using water-based magnetic ink.

The water-based magnetic ink is composed of a magnetic material and a dispersion, and the magnetic material includes, for example, Fe ₃ O ₄ (ferrous oxide tetraoxide) and Fe ₂ O ₃ (γ-type ferric oxide) having a particle size of about 0.1 to 10 μm. , Mn—Zn ferrite, Ni—Zn ferrite, magnetite, magnetic powder of iron garnet crystal of bismuth rare earth, and water-soluble polymers such as polyacrylic acid sodium salt are used as the dispersion.

In Magnetic ink character recognition (hereinafter referred to as MICR), characters are magnetized when printed on paper, and the right end of each character becomes the north pole.

The MICR character can be read through another print or stamp stamped on the character, and the waveform read differs depending on the shape of the character, so that the character can be recognized as a character.

When MICR characters are formed by the inkjet printing method, in order to ensure the dispersion stability of the magnetic material in the magnetic ink, a three-dimensional repulsive force is applied between the magnetic ink particles, and the polymer is used for the purpose of eliminating aggregation and precipitation. Measures have been taken to cover the magnetic ink particles with a dispersant. The polymer dispersant acts like a surfactant, providing hydrophilicity to the magnetic ink particles, but with a negative charge. A proposal relating to an aqueous magnetic ink in which monovalent metal ions and ammonium ions serving as counter ions are added to a solvent in order to electrically stabilize the magnetic ink particles is known. (Patent Document 1)
In Patent Document 1, the first layer made of a copolymer containing styrene and acrylic acid and the second layer made of an amphipathic compound containing polyethylene oxide and an aromatic ring adsorbed on the first layer are magnetic ink particles. It becomes a coating and contributes to ensuring dispersion stability. By applying at least one type of counter ion, monovalent metal ion and ammonium ion, resolubility in water is improved, solidification (increased viscosity) is suppressed even when the magnetic ink is concentrated, and clogging during printing is eliminated. In addition to contributing, the scratch resistance of printed characters is improved.

Refer to the drawings to the effect that the inkjet printed matter using the water-based magnetic ink of Patent Document 1 has the following problems due to the "improvement of resolubility of magnetic ink particles in water". explain.

Immediately after printing, the surface of the magnetic ink particles 20 of the printed matter 10 (FIG. 2A) in which the MICR characters are formed on the paper substrate 11 is swollen with a solvent, and a three-dimensional repulsive force (obstacle to aggregation) is generated. It is maintained.

When the printed matter is immersed in water 30, the counter ions (Na ⁺ and NH ₄ ⁺ ) on the surface of the magnetic ink particles 20 work to attract the electrons of the water molecules, and the magnetic ink particles 20 are redissolved to cause the magnetic ink particles 20 to become the paper substrate 11. It has been confirmed that the MICR function disappears when it dissolves in water. The above phenomenon is more prominent in the case of purified water than in the case of tap water containing cations in which the MICR characters are immersed (or adhered). (Fig. 2 (b))
In the printed matter 10 several days after the print formation of the MICR characters, the phenomenon of remelting of the magnetic ink particles 20 is not confirmed by the immersion test in water 30 (tap water and purified water). (Fig. 2 (c))
It is presumed that the solvent on the surface of the magnetic ink particles volatilizes, the steric repulsive force disappears, and the magnetic ink particles aggregate with each other while maintaining the printed MICR character pattern.

In this state, the functions required for MICR printed matter are secured, but it takes several days experimentally to reach a acceptable product as MICR printed matter after waiting for solvent volatilization due to natural drying. This goes against the "realization of high-speed printing" that the inkjet printing method advocates.

Japanese Unexamined Patent Publication No. 2001-354883

INDUSTRIAL APPLICABILITY In the production of MICR printed matter by an inkjet printing method using a water-based magnetic ink to which positive counter ions are added, the present invention has eliminated the concern of harmful effects due to rediscovery of magnetic ink particles in water and improved water resistance. The purpose is to present a method for providing printed matter more quickly.

The magnetic ink printing method according to the present invention is
On one surface of the paper substrate, a water-based magnetic ink containing a solvent, a magnetic substance composed of magnetic particles, a surfactant composed of a resin having a negatively charged functional group, and a water-soluble copolymer was used. A magnetic ink printing method that forms a magnetic ink character recognition (MICR) pattern by inkjet recording.
Immediately after printing with magnetic ink in which positive counter ions for the surfactant are added to the solvent , heat-drying treatment is performed with a drum heater and an air dryer at a temperature equal to or higher than the glass transition point of the surfactant. It is characterized by.

As the additional drying treatment, a treatment of 1 minute or more at a heating temperature of 140 ° C. or higher or 10 minutes or longer at a heating temperature of 120 ° C. or higher and lower than 140 ° C. is preferable.

According to the present invention, the polymer dispersant (surfactant) that coats the magnetic ink particles is not swollen, the magnetic ink pattern is firmly fixed on the paper substrate, and the printed matter is immersed in water (the printed matter is immersed in water (). Alternatively, even if the printed matter adheres to water), the magnetic ink particles do not redissolve in water, and the printed matter having improved water resistance is provided more quickly without fear of MICR dysfunction.

The graph which shows the result of having measured the change of the magnetic output after immersing the sample which concerns on this invention in purified water. Explanatory drawing which shows the behavior of a magnetic ink particle on a paper base material.

Hereinafter, the present invention will be described in more detail.

The present invention relates to an improvement of an inkjet printing method that realizes a "fixed state of water-based magnetic ink particles on a paper substrate" as shown in FIG. 2C in a short time.
<Inkjet printing method>
In the present invention, as a method for ejecting ink particles required for printing, any of "on-demand type" and "continuous type" printers can be adopted.

In an on-demand printer, a plurality of ink ejection ports (nozzles) are arranged vertically, and ink is selectively ejected from nozzles required for printing and sprayed onto an object. There are piezo method, thermal method, and bulb method as methods for applying pressure to the ink particles to eject them, and any of these can be adopted.

In a continuous printer, ink particles continuously ejected from a nozzle are charged, and ink particles used for printing are selectively bent by a deflection electrode and sprayed onto a printing surface. This is the current mainstream method for industrial inkjet printers, which reuses the collected ink that is not used for printing and is not biased.
<Paper substrate>
The ink used for inkjet printing is required to have the aptitude for ejection from a nozzle, the aptitude for charging in a continuous type, and the aptitude for magnetizing unique to the present invention, and the aptitude for drying and fixing on a paper substrate to be printed. In many cases, a special paper base material (image receiving paper) is prepared due to aptitude.

In inkjet printing, the ink particles permeate the paper substrate to be fixed. When the sprayed ink particles come into contact with the paper substrate, they are absorbed and permeate into the fibrous voids, but the spread on the paper surface (surface) results in bleeding and coarse-resolution image quality, resulting in high resolution. In order to obtain a clear image of, it is necessary to select a fibrous paper base material with less ink particle size and bleeding.

Porous silica is often coated on the special image receiving paper. Porous silica, which is industrially designed and manufactured, has various pore sizes and densities, and it is easy to realize ink absorption at a finer level than that of paper fiber. If the pore size is fine and the surface roughness of the paper surface is small (high glossiness), the time required for ink drying and the processing increase.
<Aqueous magnetic ink>
Since a magnetic powder that is originally low in suitability as a pigment for ink is blended, various improvements in ink formulation have been seen in application to inkjet printing.

As mentioned above, aggregation in the ink due to the magnetism of the pigment → improvement of dispersibility (addition of surfactant) → addition of electrical stability (addition of counter ion) have been tackled so far, but new As a problem to be solved, after printing on a paper substrate, the printed matter comes into contact with (or is immersed in) purified water that does not contain cations, resulting in "redissolution of magnetic ink particles in water (deficiency of printing pattern)". In the present invention, particular attention is paid to the countermeasures against. As the water-based magnetic ink, the formulation disclosed in Patent Document 1 is preferably adopted.

The magnetic ink according to the present invention is not particularly limited as long as it is composed of at least a magnetic material and a binder resin and penetrates into a paper substrate, but a water-based ink can be preferably used. The magnetic material is not particularly limited as long as it is a magnetic particle having a holding power, but Co-Mn ferrite is particularly preferable. The particle size of the magnetic particles is preferably in the range of 0.1 to 50 nm. If the particle size of the magnetic particles exceeds 50 nm, the particles tend to settle, and it is not possible to obtain a magnetic ink having high storage stability. It is preferable that the magnetic particles are synthesized by an aqueous reaction that does not include a drying step, are in the state of an aqueous suspension, and are crystalline. The magnetic particles that have undergone the drying step are unsuitable for producing a dispersion because the primary particles are hard and aggregated to form secondary particles that are extremely difficult to redisperse, and the surface property tends to be hydrophobic.

As the binder resin, a known water-soluble copolymer composed of styrene and acrylic acid can be used, but the present invention is not limited to this. Further, in order to improve the dispersibility and storage stability of the magnetic ink, other additives such as a dispersant and a surfactant are appropriately added. The molar ratio of the monomer units of styrene and acrylic acid in the copolymer is preferably in the range of 1: 1 to 3: 1 in order to form a uniform adsorption layer on the surface of the magnetic particles.

The total amount of monovalent metal ion and ammonium ion as counter ions should be set within the range of 1 × 10 ^-5 mol / g or more and 5 × 10 ^-4 mol / g or less. It is preferable for ensuring the solubility of the coalescence and the abrasion resistance of the printed matter. The composition ratio of NaOH and aqueous ammonia (25 Wt%) is appropriately designed and added to the copolymer containing magnetic particles to prepare a magnetic ink.

Immediately after printing MICR characters, the dispersant and surfactant are softened by performing a drying treatment under heating conditions at a temperature above the glass transition point of the dispersant that coats the magnetic particles and the polymer that functions as a surfactant. As a result, the adjacent magnetic particles are entangled with each other, the binding between the pigments is strengthened, and the fixation on the paper substrate is also strengthened. It will be an effective measure. For polystyrene and acrylic, the glass transition point is approximately 100 ° C., but in the present invention, additional drying treatment is performed under heating conditions of 120 ° C. or higher or 140 ° C. or higher. Drying treatment of 1 minute or more at a heating temperature of 140 ° C. or higher and 10 minutes or longer at a heating temperature of 120 ° C. or higher and lower than 140 ° C. is preferable. do.

After the MICR characters are dried and fixed, a protective layer may be formed on the print pattern to impart further resistance. The protective layer may be formed in a large similar figure so as to cover the entire surface of the paper substrate or the print pattern. Examples of the protective layer include ultraviolet curable resins having an acryloyl group having high reactivity in the main chain such as epoxy, polyester, and urethane. A composition containing a dilutable reactive monomer and a photopolymerization initiator is applied to an ultraviolet curable resin, and the composition is instantly cured by irradiating with ultraviolet rays, imparting excellent durability as a non-aqueous cured film. can do.

Immediately after the printing unit, an inkjet printing device equipped with a heating / drying mechanism (drum heater + air dryer) with a pass length of less than 10 m is used to prepare a print sample of MICR characters, which is then immersed in purified water to obtain "magnetic ink particles. Resolution in water (deficiency of print pattern) ”was evaluated.
<Inkjet printing-heat drying conditions>
-Magnetic ink: Aqueous magnetic ink for inkjet containing Co-Mn ferrite with a particle size of 40 to 50 nm (DNC: X-nano)
-Composition of NaOH and 25 Wt% ammonia water: 1: 1
-Inkjet head: 600 x 600 dpi
-Printing speed: 150 m / min
-Paper substrate: High-quality paper (not compatible with inkjet)
・ Base printing: Partial tint block (magenta only)
・ Drum temperature: 140 ℃
・ Air temperature: 140 ℃
・ Additional drying: Oven ・ Protective layer formation: None ・ Immersion test water: Japanese Pharmacopoeia purified water (JIS K0557 equivalent to A2)
After performing additional drying temperature (2 types of 120 ° C and 140 ° C) and additional drying time (6 types of 1 sec, 3 sec, 1 min, 5 min, 10 min, 30 min) in an oven, the sample is immersed in purified water and then magnetically output. The result of measuring the change of is shown in the graph of FIG. (In the graph, "Ref" is not additionally dried by the oven.)
When "maintenance of magnetic output of 80% or more" is set as the target value for judging a acceptable product, additional drying of 120 ° C. (FIG. 1 (a)) requires drying of 10 minutes or more, and additional drying of 140 ° C. (1). In FIG. 1 (b)), the pass is judged by drying for 1 min or more.

10: Printed matter 11: Paper base material 20: Magnetic ink particles 30: Water

Claims (2)

On one surface of the paper substrate, a magnetic substance made of magnetic particles containing a solvent, a surfactant made of a resin having a negative charge, and a water- based magnetic ink made of a water-soluble copolymer are applied. A magnetic ink printing method that uses ink ink character recognition (MICR) to form a magnetic ink character recognition (MICR) pattern by inkjet recording.
Immediately after printing with the magnetic ink in which positive counter ions for the surfactant are added to the solvent , heat-drying treatment is performed with a drum heater and an air dryer at a temperature equal to or higher than the glass transition point of the surfactant. After that
The magnetic ink printing method according to claim 1, wherein an additional drying treatment is performed at a heating temperature of 140 ° C. or higher for 1 minute or longer, or at a heating temperature of 120 ° C. or higher and lower than 140 ° C. for 10 minutes or longer. The magnetic ink printing method according to claim 1 , wherein the counter ion is Na + ionized from sodium hydroxide or ammonia added in the solvent and NH4 + ionized from ammonia.

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