JP2015004022A - Thermally decolorable ink composition for writing instrument and writing instrument containing the ink composition - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thermally decolorable ink composition excellent in rewriting property, with which sharp handwriting can be formed without thinning even when writing is done once again on erased handwriting, and to provide a writing instrument containing the ink composition.SOLUTION: The thermally decolorable ink composition for a writing instrument comprises a colorant that is decolored by heating, water, and trisiloxane type polyether-modified silicone. The writing instrument contains the above thermally decolorable ink composition for a writing instrument. A writing instrument including a friction member is also provided, in which the friction member generates frictional heat to decolorize handwriting by the writing instrument. A thermally decolorable composition included in the colorant comprises (1) an electron-donating coloring organic compound, (2) an electron-accepting compound, and (3) a reaction medium that controls the coloring reaction of (1) and (2).

Description

  The present invention relates to a thermodecolorable writing instrument ink composition. Furthermore, the present invention relates to a thermally decolorable writing instrument ink composition that can be rewritten on a handwriting that has been decolored by frictional heat, and a writing instrument incorporating the same.

  Conventionally, writing instruments that can erase the handwriting written on the paper surface by frictional heat generated by a friction body have been widely used by storing ink that becomes transparent by heating in the writing instrument. An irreversible type ink that can be reversible or an irreversible type that does not recolor even at extremely low temperatures has been proposed (see, for example, Patent Documents 1 to 3).

JP 2010-247358 A JP 2010-229332 A JP 2012-180412 A

In the writing instrument using the thermo-erasable ink, in addition to the usual writing application, it is used for the purpose of thermally erasing the wrong place, and writing again in the erased place, but is mixed in the ink. Depending on the type of additive such as resin, the handwriting at the time of rewriting may be interrupted or blurred, and rewriting may not be possible.
The present invention is a thermo-erasable ink composition excellent in re-writing property and capable of forming a clear handwriting without causing blurring or the like even when writing again on the handwriting once erased. The writing instrument which built-in is provided.

The heat decolorable writing instrument ink composition of the present invention is required to contain a colorant that is decolored by heating, water, and trisiloxane-type polyether-modified silicone.
Further, the colorant is a microcapsule pigment in which a thermochromic composition is encapsulated in microcapsules, and the thermochromic composition comprises (a) an electron-donating color-forming organic compound, and (b) an electron. It is a requirement that the composition be a reversible thermochromic composition comprising a receptive compound and (c) a reaction medium that controls the color reaction of (i) and (b) above.
Furthermore, it is a requirement that a writing instrument containing the thermodecolorable writing instrument ink composition is provided, and that a friction member that discolors the handwriting by the writing instrument with frictional heat is provided.

  The thermo-erasable writing instrument ink composition of the present invention is a re-writing property capable of forming a clear handwriting without discontinuity or blurring in the handwriting even when the handwriting is rewritten on the handwritten part which has been erased by heat. It will be excellent. In addition, even when overcoated, it is excellent in applicability so that a clear handwriting can be formed on the previously formed ink surface.

〔式中ＺはＨ又は炭素数１〜４の直鎖又は分岐アルキル基を表し、ａとｂは、ａ／ｂ＝２０／８０〜１００／０の比率を表す。〕 The writing instrument ink composition imparts re-writing property to a heat-erased portion by blending a trisiloxane-type polyether-modified silicone in a water-based ink using a colorant that is decolored by heating.
The trisiloxane-type polyether-modified silicone is a compound given by the following general formula, and has a hydrophilic polyether chain with respect to a short trisiloxane skeleton, so that it has excellent leveling when added to water-based ink. Property (re-writing property) and lubricity are imparted.

[Wherein Z represents H or a linear or branched alkyl group having 1 to 4 carbon atoms, and a and b represent a ratio of a / b = 20/80 to 100/0. ]

Including the trisiloxane-type polyether-modified silicone may be added in an amount of 0.01 to 3% by weight, preferably 0.05 to 1% by weight in the ink composition.
If it is less than 0.01% by weight, it is difficult to obtain the desired effect, and even if it exceeds 3% by weight, no further effect is obtained, so no further addition is required.

As the colorant to be decolored by heating, the one in which the handwriting is decolored (transparent) by heating the handwriting with a heat source or a friction body can be selectively applied regardless of reversible or irreversible. The composition can also be changed in hue by using a general-purpose colorant in combination.
Examples of the colorant to be decolored by heating include, for example, the reversible type disclosed in JP 2010-247358 A, JP 2010-229332 A, JP 2012-180412 A, and the like exemplified as cited references. The irreversible type disclosed in (1) is applicable.
In particular, the change of the handwriting is preferable because the thermochromic composition is encapsulated in microcapsules and can be stably expressed for a long period of time without causing a composition change. The thermochromic composition encapsulated in the microcapsule pigment includes (a) an electron-donating color-forming organic compound and (b) an electron-accepting compound from the viewpoints of repeated usability, temperature change accuracy, and the like. (C) A reversible thermochromic composition comprising a reaction medium that determines the temperature at which the two color reactions occur is preferred.

Examples of the reversible thermochromic composition include those described in Japanese Patent Publication No. 51-44706, Japanese Patent Publication No. 51-44707, Japanese Patent Publication No. 1-29398, etc., at a predetermined temperature (discoloration point). Discolored before and after, decolored state in the temperature range above the high temperature side discoloration point, color development state in the temperature range below the low temperature side discoloration point, only one specific state exists in the normal temperature range among the two states, The other state is maintained as long as the heat or cold required to develop the state is applied, but when the heat or cold is no longer applied, the hysteresis width returns to the state exhibited in the normal temperature range. A heat decolorable microcapsule pigment in which a reversible thermochromic composition having relatively small characteristics (ΔH = 1 to 7 ° C.) is encapsulated in a microcapsule can be applied.
Furthermore, comparatively large hysteresis characteristics (ΔH = 8 to 50) described in JP-B-4-17154, JP-A-7-179777, JP-A-7-33997, JP-A-8-39936, and the like. ° C), and exhibit large hysteresis characteristics described in JP-A-2006-137886, JP-A-2006-188660, JP-A-2008-45062, JP-A-2008-280523, In other words, the shape of the curve plotting the change in color density due to temperature change differs greatly between when the temperature is raised from the lower temperature side than the color change temperature range and when the temperature is lowered from the higher temperature side than the color change temperature range. The color changes following the path, and the color development state in the low temperature range below the complete color development temperature, or the color erase state in the high temperature range above the complete color erase temperature is in the specific temperature range. Microcapsule pigment was contained a reversible thermochromic composition heated decolorizable having chroma memory property can be applied.
In addition, as the reversible thermochromic composition having color memory, which is applied to writing instrument inks, specifically, a complete coloring temperature is a temperature that can be obtained only in a freezing room, a cold region or the like, that is, −50 to 0 ° C. , Preferably −40 to −5 ° C., more preferably −30 to −10 ° C., and a complete decoloring temperature obtained from a frictional heat generated by a friction body, a heating body such as a hair dryer, that is, 50 to 95 ° C. , Preferably 50 to 90 ° C., more preferably 60 to 80 ° C., and by specifying ΔH value 40 to 100 ° C., effective in maintaining colors exhibited in normal conditions (daily living temperature range) Can function.

As general-purpose colorants that can be used in combination with the colorant that is heated and decolored, all dyes and pigments that can be dissolved or dispersed in an aqueous medium can be used, and specific examples thereof are illustrated below.
As the dye, an acid dye, a basic dye, a direct dye, or the like can be used.
Examples of the acid dye include New Coxin (CI. 16255), Tartrazine (CI. 19140), Acid Blue Black 10B (CI. 20470), Guinea Green (CI. 42085), Brilliant Blue FCF. (CI.42090), Acid Violet 6B (C.I.42640), Soluble Blue (C.I.42755), Naphthalene Green (C.I.44025), Eosin (C.I.45380), Phloxin (C.I. 45410), erythrosine (C.I. 45430), nigrosine (C.I. 50420), acid flavin (C.I. 56205) and the like are used.
Examples of basic dyes include chrysoidine (C.I. 11270), methyl violet FN (C.I. 42535), crystal violet (C.I. 42555), malachite green (C.I. 42000), Victoria blue FB ( CI 44045), rhodamine B (C.I. 45170), acridine orange NS (C.I. 46005), methylene blue B (C.I. 52015) and the like are used.
As direct dyes, Congo Red (C.I. 22120), Direct Sky Blue 5B (C.I. 24400), Violet BB (C.I. 27905), Direct Deep Black EX (C.I. 30235), Kaya Las Black G Conch (C.I. 35225), Direct Fast Black G (C.I. 35255), Phthalocyanine Blue (C.I. 74180) and the like are used.

Examples of the pigment include inorganic pigments such as carbon black and ultramarine, and organic pigments such as copper phthalocyanine blue and benzidine yellow, as well as water dispersion finely and stably dispersed in an aqueous medium in advance using a surfactant or resin. Pigment products and the like are used. I. Pigment Blue 15: 3B [Product name: Sandy Super Blue GLL, pigment content 24%, manufactured by Sanyo Dye Co., Ltd.], C.I. I. Pigment Red 146 [Product name: Sandy Super Pink FBL, pigment content 21.5%, manufactured by Sanyo Dye Co., Ltd.], C.I. I. Pigment Yellow 81 [Product name: TC Yellow FG, pigment content about 30%, manufactured by Dainichi Seika Kogyo Co., Ltd.], C.I. I. Pigment Red 220/166 [Product name: TC Red FG, pigment content of about 35%, manufactured by Dainichi Seika Kogyo Co., Ltd.].
As the resin for dispersing the pigment, polyamide resin, polyurethane resin, polyester resin, epoxy resin, melamine resin, phenol resin, silicone resin, polyvinyl alcohol, polyvinyl pyrrolidone, polyvinyl acetate, polyvinyl chloride, polyvinylidene chloride, Examples thereof include polystyrene, acrylic acid resin, maleic acid resin, gum arabic, cellulose, dextran, casein, and derivatives thereof, and copolymers of the above-described resins.
As the fluorescent pigment, a fluorescent pigment in the form of fine particles of synthetic resin in which various fluorescent dyes are formed into a solid solution in a resin matrix can be used.
Also, white pigments such as titanium oxide, metal powder pigments such as aluminum, natural mica, synthetic mica, alumina, pearl pigments coated with a metal oxide such as titanium dioxide on the surface of a core material, cholesteric liquid crystal type A bright pigment or the like can also be used.

  The colorant can be used singly or in combination of two or more, and is used in the range of 1 to 35% by weight, preferably 2 to 30% by weight in the ink composition.

In addition, a conventional general-purpose water-soluble organic solvent compatible with water can be used, for example, ethanol, propanol, butanol, glycerin, sorbitol, triethanolamine, diethanolamine, monoethanolamine, ethylene glycol, diethylene glycol, thiodiethylene glycol. , Hexylene glycol, 1,3-butanediol, neoprene glycol, polyethylene glycol, propylene glycol, butylene glycol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, ethylene glycol monomethyl ether acetate, Propylene glycol monomethyl ether, propylene glycol Monoethyl ether, 2-pyrrolidone, N- methyl-2-pyrrolidone.
In addition, the said water-soluble organic solvent can also use 1 type, or 2 or more types together, It is 2-60 weight% in an ink composition, Preferably it is used in 5-35 weight%.

In addition, if necessary, pH adjusters such as inorganic salts such as sodium carbonate, sodium phosphate and sodium acetate, organic basic compounds such as water-soluble amine compounds, rust preventives such as benzotriazole, tolyltriazole and saponin , Carboxylic acid, sodium salt of 1,2-benzthiazolin-3-one, sodium benzoate, sodium dehydroacetate, potassium sorbate, propyl paraoxybenzoate, 2,3,5,6-tetrachloro-4- (methylsulfonyl) Preservatives or antifungal agents such as pyridine, urea, sorbit, mannitol, sucrose, glucose, reduced starch hydrolysates, wetting agents such as sodium pyrophosphate, antifoaming agents, fluorine-based interfaces that improve ink permeability An activator or a nonionic surfactant may be used.
Further, a lubricant can be added, such as metal soap, polyalkylene glycol fatty acid ester, ethylene oxide addition type cationic surfactant, phosphate ester type surfactant, N-acyl amino acid type surfactant, dicarboxylic acid type surfactant. , Β-alanine type surfactant, 2,5-dimercapto-1,3,4-thiadiazole and its salts and oligomers, 3-amino-5-mercapto-1,2,4-triazole, thiocarbamate, dimethyl A dithiocarbamate, α-lipoic acid, a condensate of N-acyl-L-glutamic acid and L-lysine, a salt thereof, or the like is used.
Further, oligomers of N-vinyl-2-pyrrolidone, oligomers of N-vinyl-2-piperidone, N-vinyl-2-pyrrolidone, N-cyclohexyl-2-pyrrolidone, ε-caprolactam, N-vinyl-ε-caprolactam By adding a thickening inhibitor such as an oligomer, it is possible to enhance the function in the intruding form.

  In addition, one or two or more water-soluble resins such as alkyd resin, acrylic resin, styrene maleic acid copolymer, cellulose derivative, polyvinyl pyrrolidone, polyvinyl alcohol, dextrin may be added in a category that does not hinder drying resistance, urea, One or more wetting agents such as nonionic surfactants, sorbitol, mannitol, sucrose, glucose, reduced starch hydrolyzate and sodium pyrophosphate can be added.

A shear thinning agent can be added to the water-based ink.
As the shear thinning agent, a substance that is soluble or dispersible in water is effective, and xanthan gum, welan gum, zetasy gum, dieutan gum, macrohomopsis gum, and a constituent monosaccharide is an organic acid-modified heterogeneous glucose and galactose Succinoglycan, which is a polysaccharide (average molecular weight: about 100 to 8 million), guar gum, locust bean gum and its derivatives, hydroxyethyl cellulose, alkyl alginate, alkyl ester of methacrylic acid, molecular weight 100,000 to 150,000 Polymers, glucomannan, carbohydrates having gelling ability extracted from seaweed such as agar and carrageenan, cross-linked poly N-vinyl-carboxylic acid amide, benzylidene sorbitol and benzylidene xylitol or their derivatives, cross-linkable acrylic acid Coalescence Inorganic fine particles, nonionic surfactants having an HLB value of 8 to 12, can be exemplified metal salts and amine salts of dialkyl sulfosuccinate. Furthermore, N-alkyl-2-pyrrolidone and an anionic surfactant may be used in combination in the ink composition.
The shear thinning agent can be used in the range of 0.1 to 20% by weight in the ink composition.

  The thermodecolorable writing instrument ink composition of the present invention is filled in a marking pen or ballpoint pen having a fiber tip, felt tip, plastic tip, or ballpoint tip attached to the tip of the writing. The marking pen and the ballpoint pen have a cap type with a cap that covers the pen tip, a knock type, a rotary type, a slide type, etc. It may be.

When filling the marking pen, the structure and shape of the marking pen itself are not particularly limited. For example, a pen tip for a marking pen such as a fiber chip, a felt chip, a plastic chip (a shell type, a chisel type, a brush pen type) Etc.) is attached to the writing tip, and an ink occlusion body consisting of a fiber bundle housed in the shaft tube is impregnated with ink, and ink is supplied to the writing tip, ink is stored directly in the shaft tube, and comb A structure that supplies a predetermined amount of ink to the writing tip by interposing a groove-like ink flow adjusting member or an ink flow adjusting member made of a fiber bundle, directly storing the ink inside the shaft tube, and writing the tip by a valve mechanism And a marking pen having a structure for supplying a predetermined amount of ink.
Further, in addition to a single nib, a double-headed form in which nibs having different thicknesses and shapes are provided at both ends of the shaft cylinder may be used. In the double-headed form, one end may be a ballpoint pen.

  When filling the ball-point pen, the structure and shape of the ball-point pen itself are not particularly limited. For example, an ink occlusion body composed of a fiber bundle housed in the shaft tube is impregnated with ink and supplied to the writing tip. Structure, ink is stored directly inside the shaft cylinder, and there is an ink storage tube filled with the ink composition in the shaft cylinder, with a comb groove-shaped ink flow control member and an ink flow control member consisting of fiber bundles interposed The ink storage tube communicates with a chip having a ball attached to the tip, and a ballpoint pen having a structure in which a liquid stopper for backflow prevention is in close contact with the end surface of the ink. A solid stopper can be used together with the liquid stopper.

The ballpoint pen tip will be described in more detail. A tip formed by holding a ball in a ball holding portion obtained by pressing and deforming the vicinity of the tip of a metal pipe inward from the outer surface, or a metal material is cut by a drill or the like A chip formed by holding the ball in the ball holding part formed by the above, or a ball held in a chip formed by cutting a metal pipe or metal material, and the like that is urged forward by a spring body, etc. Can be applied.
The ball may be cemented carbide, stainless steel, ruby, ceramic, or the like.

For example, a molded body made of a thermoplastic resin such as polyethylene, polypropylene, or polyethylene terephthalate is preferably used as the ink storage tube for storing the thermodecolorable ink in terms of low ink evaporation and productivity. Further, by using a transparent, colored transparent, or translucent molded body as the ink containing tube, the ink color, the remaining amount of ink, and the like can be confirmed.
In addition to directly connecting the chip to the ink storage tube, the ink storage tube and the chip may be connected via a connecting member.
The ink storage tube may be a refill type in which the refill is stored in a shaft tube, or the shaft tube itself having a tip attached to a tip is used as an ink container, and ink is directly applied to the shaft tube. It may be filled.

The back end of the ink composition housed in the ink housing tube is filled with an ink backflow prevention body.
The ink backflow preventer can be either liquid or solid, and the liquid ink backflow preventive includes non-volatile media such as polybutene and silicone oil. Aluminum silicate or the like can also be added.
Moreover, a resin molding is mentioned as a solid ink backflow prevention body.
The liquid and solid ink backflow preventers can be used in combination.

Furthermore, the friction member for decoloring or discoloring a handwriting with frictional heat can be used with the said writing instrument.
As the friction member, an elastic body such as an elastomer or plastic foam which is rich in elasticity and can generate frictional heat by generating appropriate friction during friction is suitable. Although the handwriting can be rubbed using an eraser, the above-mentioned friction member is preferably used because erase scraps are generated at the time of friction.
As the material of the friction member, silicone resin, SEBS resin (styrene ethylene butadiene styrene block copolymer), SBS resin (styrene butylene styrene copolymer), polyester resin, or the like is used.
The friction member can be obtained by combining a writing instrument and a member of arbitrary shape (friction body) separately to obtain a writing instrument set. However, by attaching the friction member to the exterior of the writing instrument, the form becomes excellent in portability.
In the case of a cap-type writing instrument, the location where the friction member is provided is not particularly limited. For example, when the cap itself is formed of a friction member, the shaft tube itself is formed of a friction member, or a clip is provided. It can be formed of a friction member, or a friction member can be provided at the tip end (top) of the cap or the rear end of the shaft tube (the portion where the writing tip is not provided).
In the case of a retractable writing instrument, the location where the friction member is provided is not particularly limited. For example, the shaft cylinder itself is formed of the friction member, or when the clip is provided, the clip itself is formed of the friction member, A friction member can be provided in the vicinity of the cylinder opening, the rear end of the shaft cylinder (the part where the writing tip is not provided) or the knock.

Examples will be described below, but the present invention is not limited to these examples.
Table 1 shows the compositions of water-based inks for writing instruments of Examples and Comparative Examples. In addition, the numerical value of a composition in a table | surface shows a weight part.

The contents of the raw materials in the table will be described along the note numbers.
(1) 4.5 parts of 2- (2-chloroanilino) -6-di-n-butylaminofluorane as component (a), 1,1-bis (4'-hydroxyphenyl) n- as component (b) Reversible thermochromic composition comprising 4.5 parts of decane, 7.5 parts of 2,2-bis (4'-hydroxyphenyl) hexafluoropropane, and 50.0 parts of 4-benzyloxyphenylethyl caprate as the component (c) Microcapsule pigment encapsulating a product (T ₁ : −20 ° C., T ₂ : −9 ° C., T ₃ : 40 ° C., T ₄ : 57 ° C., ΔH: 63 ° C., average particle size: 2.5 μm, black to colorless Change color)
(2) 2.0 parts of 3- (4-diethylamino-2-hexyloxyphenyl) -3- (1-ethyl-2-methylindol-3-yl) -4-azaphthalide as component (a), (b) A reversible thermochromic composition comprising 8.0 parts of 2,2-bis (4'-hydroxyphenyl) hexafluoropropane as a component and 50.0 parts of 4-benzyloxyphenylethyl caprate as a component (c) was included. Microcapsule pigment (T ₁ : −14 ° C., T ₂ : −6 ° C., T ₃ : 48 ° C., T ₄ : 60 ° C., ΔH: 64 ° C., average particle size: 2.3 μm, color change from blue to colorless )
(3) 2.0 parts of 3-bis (p-dimethylaminophenyl) -6-dimethylaminophthalide, 6.0 parts of 2,2-bis (4′-hydroxyphenyl) propane, 50.0 parts of diphenyl phthalate Was stirred at 80 ° C. for 1 hour and then cooled to obtain a dye compound. Thereafter, 58.0 parts of a dye compound and 10.0 parts of a cyclohexanone-based ketone resin are blended, and 25.0 parts of a blend pulverized to a particle size of 0.5 mm or less by a pulverizer is used to form a β-naphthalenesulfonic acid formalin condensation. Pigment compound dispersion obtained by dispersing 3.0 parts of sodium salt, 51.8 parts of ion-exchanged water, 20.0 parts of propylene glycol, and 0.2 parts by weight of a silicone-based antifoaming agent with a ball mill for 1 hour (4 ) Compound of Z = H, a / b = 40/60 in the general formula of paragraph 0007 (5) Compound of Z = CH ₃ , a / b = 100/0 in the general formula of paragraph 0007 (6) Phosphate ester system Surfactant, Daiichi Kogyo Seiyaku Co., Ltd., trade name: Prisurf AL
(7) Product name: Hokuside R-150, manufactured by Hokuko Chemical Co., Ltd.
(8) Product name: FS Antiform 013A, manufactured by Toray Dow Corning Co., Ltd.

Preparation of ball-point pen ink The raw materials excluding the thickener were mixed in the blending amounts of Examples 1 to 3 and Comparative Examples 1 to 3, and after stirring at 20 ° C. with a disper at 400 rpm for 1 hour, the thickener was added. The mixture was further stirred for 1 hour to obtain a ballpoint pen ink composition.

Preparation of Ink Backflow Preventing Body After adding 1.5 parts of fatty acid amide as a thickener to 98.5 parts of polybutene as a base oil, an ink backflow preventing body was obtained by kneading with 3 rolls.

Preparation of ballpoint pen Each of the above ballpoint pen ink compositions is filled in a ballpoint pen refill in which a stainless steel cutting tip holding a ball of 0.5 mm diameter cemented carbide is fitted to one end of a transparent polypropylene pipe. After the ink backflow prevention body was disposed at the end, the ballpoint pen refill was assembled into a shaft cylinder (equipped with a friction body made of SEBS resin at the rear end), and a sample ballpoint pen was manufactured by attaching a cap.

Preparation of marking pen ink Each raw material was mixed in the blending amounts of Examples 4 to 5 and Comparative Examples 4 to 5, stirred and dissolved at 20 ° C. for 3 hours, and then filtered to obtain a marking pen ink composition.

Preparation of a marking pen An ink occlusion body in which a polyester sliver is coated with a synthetic resin film is impregnated with the marking pen ink composition, accommodated in a shaft tube made of polypropylene resin, and a polyester fiber is placed on the tip of the shaft tube through a holder. A sample marking pen was prepared by assembling a resin-processed chip (bullet type) in a connected state and attaching a cap made of SEBS resin friction at the tip.

Writing test Using each of the sample ballpoint pens and sample marking pens, the alphabet from A to J is written by hand on the old JIS P3201 writing paper A, and the frictional heat is generated by rubbing with a friction body attached to each writing instrument. Discolored. Thereafter, the state of the handwriting when the spiral circle was continuously written in the decolored portion was visually confirmed.
The results of the test are shown below.

In addition, the evaluation regarding the symbol in the said table | surface is as follows.
Writing test ○: Good handwriting was obtained and rewriting was possible.
X: Scratch and repellency are seen in the handwriting.

Claims (5)

  A thermally decolorable writing instrument ink composition comprising a colorant that is decolored by heating, water, and a trisiloxane-type polyether-modified silicone.   The thermochromic writing instrument ink composition according to claim 1, wherein the colorant is a microcapsule pigment in which a thermochromic composition is encapsulated in microcapsules.   The thermochromic composition comprises (a) an electron-donating color-forming organic compound, (b) an electron-accepting compound, and (c) a reaction medium that controls the color reaction of (a) and (b). The thermodecolorable writing instrument ink composition according to claim 1 or 2, which is a reversible thermochromic composition.   A writing instrument incorporating the thermodecolorable writing instrument ink composition according to any one of claims 1 to 3.   The writing instrument according to claim 4, further comprising a friction member that discolors the handwriting by the writing instrument with frictional heat.