Ink for Ink-Jet Printer Containing Dichloromethane as an Organic Solvent and a Basic Dye as a Coloring Agent

1. Field of the Invention

The present invention relates to an ink for an ink-jet printer.

2. Description of the Prior Art

An ink-jet printing process has attracted intense interest recently for printing letters or symbols on the surface of mass-produced industrial products or packed boxes, etc.

The ink-jet printing process has heretofore been used in terminal printing of an electronic computer, etc., but the substrate has been paper. Therefore, water-soluble inks have been used in the process (U.S. Pat. No. 3,687,887).

According to the ink-jet printing method, an ink sent from a storage tank by the action of compressed air, etc. enters an ink-jet apparatus having an orifice of 20 to 150 .mu.m in diameter, where the ink is converted into a continuous flow of fine particles by the action of a vibrator operated magnetically or electrically. In the course from said jet apparatus to a printing substrate, the flow of fine particles is controlled by a video signal from an electronic character generator and an electrostatic deflection system and prints letters and symbols continuously on the printing substrate.

In the above-mentioned process, the ink particles must always be fine particles of a definite size and must respond sensitively to the action of said control system in order to keep the uniform density of the printed letters and symbols. Therefore, the properties such as viscosity, surface tension, specific gravity, electroconductivity, etc. of the ink are strictly controlled according to the shape, size and material of the nozzle and applied voltage, etc. Since prior art inks are printed on paper as described above, however, proposals on improvement in the process have exclusively been directed to water-soluble inks.

Therefore, an object of the present invention is to provide a novel ink for an ink-jet printer which can print on a hydrophobic substrate such as plastic, glass, metal, etc. according to an ink-jet printing process.

Another object of the invention is to provide a fire-proof ink required for the ink-jet printing process.

The other objects and advantages of the present invention will be apparent from the following description.

According to the present invention, there is provided an ink for an ink-jet printer comprising a vehicle and a coloring agent and having a sepcific resistance of 1 k.OMEGA.-cm or less wherein (1) a substantially anhydrous organic solvent in the vehicle comprises 100 parts by weight of

(a) dichloromethane or

(b) a mixed solvent obtained by replacing a part of dichloromethane by at least one member selected from the group consisting of halogenated lower alkanes except for dichloromethane and halogenated lower alkenes and

(2) the coloring agent comprises 0.2 part by weight or more, preferably 0.5 to 6 parts by weight, of

(a) a basic dye or

(b) a mixture obtained by replacing a part of a basic dye by at least one member selected from the group consisting of oil-soluble dyes and organic pigments

which is soluble in the solvent as shown in (1) above.

The desired ink for an ink-jet printer must also be applicable to prior art ink-jet printing processes and must be able to print rapidly on not only ink-absorbent substrates such as paper but also hydrophobic and non-absorbent substrates such as plastics, metals, etc. It is particularly desirable therefor that the ink has a specific resistance of 1 k.OMEGA.-cm or less, a viscosity of 5 centipoises or less at room temperature, a surface tension of 25 dyne/cm or more, a specific gravity of 0.9 to 1.3, etc. Also, it is required for said ink to be excellent in adhesive property, drying property, water proofness and particularly flame proofness as compared with prior art aqueous inks for paper. The reason therefor is that a DC voltage of about 100 to 7,000 V is applied in the ink-jet printing process owing to the charge of ink particles and the deflection of the ink. Even if discharge occurs in the case owing to dust in the air, the ink must not catch fire. Also, it must be avoided that printed letters or symbols become illegible or contaminate the other articles owing to poor dryness of the ink when the printed surface is rubbed after printing. Further, it must be absolutely avoided that a coloring agent or vehicle components separate as crystals, since an orifice of 20 to 150 .mu.m in diameter provided on the flow path for the ink in the ink-jet printer is clogged thereby. In order to satisfy the above-mentioned requisites, the following components of an ink for an ink-jet printer have been selected:

(1) As an organic solvent component in the vehicle,

(a) dichloromethane

or

(b) a mixture of dichloromethane and at least one member selected from halogenated lower alkanes except for dichloromethane and halogenated lower alkenes

(2) As a coloring material,

(a) a basic dye

or

(b) a mixture of a basic dye and at least one member selected from oil-soluble dyes and organic pigments.

The present invention is characterized by this selection of the components.

The solvent in said vehicle plays an important part in the present invention in that it dissolves the vehicle solid component (or a so-called binder) and said coloring agent, provides the ink with flame resistance and controls the drying velocity of the ink. As such a solvent, dichloromethane is most suitable. Further, according to the drying velocity, at least one member selected from the group consisting of trichlorofluoromethane, 1,1,1-trichloroethane, 1,1,2-trichloro-1,2,2-trifluoroethane, 1,1,2,2-tetrachloro-1,2-difluoroethane, trichloroethylene, tetrachloroethylene, .alpha.-bromopropane, etc. may be used in admixture with dichloromethane. The proportion of dichloromethane in the mixed solvent system in preferably 20% by weight or more. In order to maintain the specific resistance stably and to prevent the mixed solvent from instability due to vaporization, more preferable proportion of dichloromethane in the mixed solvent is 30-60% by weight. If the proportion of dichloromethane is less than 20% by weight, the dissolving power of the mixed solvent for the vehicle and the coloring agent and the electroconductivity of the ink become unsatisfactory.

In the present invention, a basic dye is an important component for controlling the electroconductivity of the ink together with said solvent and is also a coloring material (or agent) and resistance controller. Such basic dyes are exemplified by:

A specific resistance of 1 k.OMEGA.-cm or less can be obtained by adding 0.2 part by weight or more, and preferably 0.5-6 parts by weight, more preferably 1 to 2 parts by weight, of the above-mentioned dyes to 100 parts by weight of the solvent containing at least 20% by weight of dichloromethane.

The following Table shows influence of the kind of a dye and the kind of a solvent on a specific resistance of the resulting ink.

As is clear from the above Table, an ink having a specific resistance of 1 k.OMEGA.-cm or less can only be obtained when a basic dye is added to dichloromethane or a mixed solvent containing at least 20% by weight of dichloromethane.

The above-mentioned basic dyes are an essential component in the present invention, but are generally poor in weather proofness. This defect can be obviated by adding an oil-soluble dye or organic dye which is soluble in said solvents to the basic dyes are exemplified by: ##STR31##

Also, as the organic pigments, pigments which are soluble in the above-mentioned solvents such as Pigment Red 81 ##STR32## or C.I. 70460 (an organic pigment of phthalocyanine series such as Heliogen Blue 7080) may be used.

The amount of the oil-soluble dyes and organic pigments used in the present invention in order to supplement the deficient weather proofness of the basic dyes may be varied within a range of 20 to 95% by weight of a basic dye and 80 to 5% by weight of an oil-soluble dye and/or an organic pigment based on the total weight of the mixed coloring agent. The lower limit for the basic dye content depends upon the specific resistance value of the ink.

The solid component or a so-called binder constituting a vehicle together with the organic solvent in the present invention is a component making a remarkable contribution to the properties of the ink, and particularly the drying property after printing, adhesive property and durability of the ink. As the solid component, various resins and rubbers which have been used in the production of printing inks and paints may be used as in prior art inks. Specifically, epoxy resins, phenol resins, xylene resins, ketone resins, alkyd resins, rosin resins, terpene resins or modified resins thereof, homopolymers and copolymers of styrene, acrylate, vinyl chloride or vinyl acetate, polyvinyl acetals, polyesters, polyamides, cellulose derivatives and rubbers such as natural rubber, polybutadiene rubbers, polysioprene rubbers, chloroprene rubbers and isobutylene-isoprene rubbers may be used. These resins and rubbers may be used alone or in a mixture of two or more thereof.

In order to keep the voscosity of the ink at 5 centipoises or less as a condition necessary to form a smooth flow of ink particles, these resins and rubbers are preferably used in an amount of up to 10 parts by weight per 100 parts by weight of the solvent. On the one hand, in order to secure the adhesion of the coloring agent, the resins and rubbers are used in an amount of 0.5 to 6 parts by weight per 100 part by weight of the organic solvent.

Further, the inks according to the present invention may contain, if desired, an ultraviolet absorber, an antioxidant, a plasticizer, etc. which are soluble in said solvents.

The following examples, in which all parts are expressed by weight unless otherwise indicated, will serve to illustrate the practice of the invention in more detail. Also, in the preparation of inks, the following coloring agents, resins, etc. manufactured by the respective companies were used.

Malachite Green: Tokyo Kasei or Sumitomo Chemical Company Limited

Methyl Violet: Tokyo Kasei (Methyl Violet) or Hodogaya Chemical Co., Ltd. (Aizen Methyl Violet pure special)

Crystal Violet: Tokyo Kasei

Aniline Blue: Tokyo Kasei

Rhodamine B: Tokyo Kasei (Rhodamine B), Sumitomo Chemical Company Limited (Rhodamine B) or Badische Anilin & Soda Fabrik A.G. (Rhodamine F4G)

Rhodamine 6G: Tokyo Kasei

Acridine Orange NO: Tokyo Kasei (Acridine Orange)

Bismarck Brown R: Tokyo Kasei

Methylene Blue: Tokyo Kasei

Sudan I: Tokyo Kasei

Oil Red: Tokyo Kasei and Hodogaya Chemical Co., Ltd. (Aizen Oil Red XO)

Oil Red B: Hodogaya Chemical Co., Ltd. (Aizen Orange Red)

Oil Brown B: Sumitomo Chemical Company Limited (Oleosol Brown B)

Oil Brown BB: Badische Anilin & Soda Fabrik A.G. (Sudan Brown BB)

Induline: Badische Anilin & Soda Fabrik A.G. (Indulin N base)

Solvent Red 122: Badische Anilin & Soda Fabrik A.G. (Neozapon Red GE) (C.I. 12716)

Solvent Blue 35: Badische Anilin & Soda Fabrik A.G. (Sudan Blue II) (C.I. 61554)

Solvent Blue 70: Badische Anilin & Soda Fabrik A.G. (Neozapon Blue FFR) (C.I. 74400)

Solvent Green 4: Badische Anilin & Soda Fabrik A.G. (Fluorol 5 G)

Badische Anilin & Soda Fabrik A.G. (Heliogen Blue 7080 (C.I. 70460) and Fanal Blue PTM 6340 (C.I. 42595), 6390 (C.I. 42595))

Phenol resins: Hitachi Chemical Co., Ltd. Hitanol 40G, 40P, 1501, 1502, 2501

Terpene-modified phenol resins: Sumitomo Bakelite Co., Ltd. Sumilite Resin PR220

Alkyd resins: Hitachi Chemical Co., Ltd. Phthalkyd 265, 375

Ketone resins: Hitachi Chemical Co., Ltd. Hilac 222

Xylene resins: Mitsubishi Gas-Chemical Co., Inc. Nikanol H

Polyamide resins: Hitachi Chemical Co., Ltd. Hitamide 420, 503

Rosin resins: Hercules Powder Co. Staybelite Ester

Terpene resins: Esso Standard Sekiyu K.K. Piccolyte 125

Epoxy resins: Shell Chemical Co. Epikote 828, 834, 871, 1001

Polyester resins: Toyo Spinning Co., Ltd. Bairon 300

Polyvinyl butyral: Sekisui Chemical Co., Ltd. Esulex BL-1

Chloroprene rubbers: Asahi Chemical Industry Co., Ltd. Neoprene WRT

Polybutadiene rubbers: Japanese Geon Co., Ltd. Nipol BR1220

1,1,2-Trichloro-1,2,2-trifluoroethane: Daikin Kogyo Co., Ltd. Daiflon 113

As the other solvents, commercially available reagents were used.

The above-mentioned solvent was charged into a ground stopper Erlenmeyer flask. The resins were added to the solvent with ultrasonic stirring to dissolve the resins completely. The coloring agent was then added to the resulting solution to dissolve it. Thus, an ink for an ink-jet printer was prepared.

The ink showed a specific resistance of 950 .OMEGA.-cm, a viscosity of 2.5 centipoises, a specific gravity of 1.2 and a surface tension of 28.4 dyne/cm at 20.degree. C. and was non-combustible. Further, the formation of ink particles and the examination of printing by a Hitachi DP-I type ink-jet printer were carried out by the use of said ink. Thus, satisfactory results were obtained such as smooth formation and control of particle flow, rapid drying property, distinctness of printing, etc.

Also, inflammability test were carried out according to JIS K2274 by measuring the flash point of the ink by the use of a Cleveland Open Cup and judging a flash point of 200.degree. C. or more as flame retardant. Also, glass fiber cord of about 7 mm in diameter was impregnated enough with the ink. The impregnated cord was exposed directly to a flame to ignite it and the flame was then removed. When combustion did not last, the ink was judged as incombustible.

In the same manner as in Example 1, inks for an ink-jet printer were prepared by blending a coloring agent (II) and resins, etc. (III) with a solvent (I) in the ratio as described below.

The specific resistance, viscosity, specific gravity and surface tension of the inks obtained were as shown in the following table (measured at 20.degree. C.).

The weather resistance of the printings were evaluated by exposing to light from a 100V, 400W mercury-vapor lamp (20,000 lux) and measuring the time required for the color density to decrease to one half of its initial value (half-value period). The half-value period was 100 hours in the printing of the inks of Examples 1-11, 26 and 32, and 150 hours in the printing of the inks of Examples 12-21, 24, 25, 27, 29, 30, 33, 35 and 36 using an oil-soluble dye together with a basic dye or basic dyes. In the printing of the inks of Examples 22, 23, 28, 31, 34 and 37 using an organic pigment together with a basic dye or with basic dyes and an oil-soluble dye, a decrease in the color density was hardly observed even after irradiation for 240 hours.