There is a compound represented by the following formula (I): ##STR00001##
in which R1 and R2, identical or different, represent a C.sub.2-C.sub.30 alkyl group, a C.sub.2-C.sub.30 alkenyl group, a C.sub.2-C.sub.30 alkynyl group, or a C.sub.2-C.sub.30 alkoxy group, the alkyl, alkenyl, alkynyl, or alkoxy groups being optionally substituted with at least one hydroxy, halogen, amino, C.sub.1-C.sub.8 alkyl, or C.sub.1-C.sub.8 alkoxy group, as a temperature change regulating agent in a thermochromic ink composition. There also are thermochromic pigment microcapsules having a compound of formula (I) according to the disclosure, to ink compositions having such thermochromic pigment microcapsules, and to writing instruments comprising such ink compositions.

[Problems] To provide a reversibly thermochromic composition having excellent contrast between a colored state and a decolored state, and excellent light resistance, and a reversibly thermochromic microcapsule pigment encapsulating the reversibly thermochromic composition.

[Solution] Disclosed is a reversibly thermochromic composition including: (a) an electron-donating color-developing organic compound; (b) a combination of a compound having a specific structure as an electron-accepting compound; and (c) a reaction medium which reversibly induces an electron transfer reaction between the component (a) and the component (b) in a specific temperature range, and a reversibly thermochromic microcapsule pigment encapsulating the reversibly thermochromic composition.

[Problems] To provide a reversibly thermochromic composition having excellent contrast between a colored state and a decolored state, and excellent light resistance, and a reversibly thermochromic microcapsule pigment encapsulating the reversibly thermochromic composition.

[Solution] Disclosed is a reversibly thermochromic composition including: (a) an electron-donating color-developing organic compound; (b) a combination of a compound having a specific structure as an electron-accepting compound; and (c) a reaction medium which reversibly induces an electron transfer reaction between the component (a) and the component (b) in a specific temperature range, and a reversibly thermochromic microcapsule pigment encapsulating the reversibly thermochromic composition.

There is a non-thermochromic irreversible color changing aqueous writing ink including core-shell dye microparticles and a coloring agent, in which the core of the core-shell dye microparticles includes an organic solvent immiscible with water and at least one dye soluble in the organic solvent and the shell includes a resin component. There also is a writing instrument containing it and the process of preparation of this ink.

There is a non-thermochromic irreversible color changing aqueous writing ink including core-shell dye microparticles and a coloring agent, in which the core of the core-shell dye microparticles includes an organic solvent immiscible with water and at least one dye soluble in the organic solvent and the shell includes a resin component. There also is a writing instrument containing it and the process of preparation of this ink.

An object of the present invention is to provide: a developer that can provide a thermosensitive recording material excellent in color development sensitivity and storage stability; and a thermosensitive recording material obtained using the developer. The object is achieved by a developer including a compound represented by the following Formula (1).

##STR00001##

In Formula (1), a, A, and L are as follows: a is an integer selected from 1 to 5, A is each independently a fluoro group or a perfluoroalkyl group, and L is any of functional groups represented by the following Formula (2):

##STR00002##

wherein in Formula (2), R is a C.sub.1-C.sub.4 alkyl group, and * represents a binding site to a sulfur atom in Formula (1).

Provided are a reversibly thermochromic aqueous ink composition having excellent writability and few problems such as aggregation of a colorant, an increase in viscosity of the composition, or a decrease in handwriting density, and a refill or an aqueous ballpoint pen using the reversibly thermochromic aqueous ink composition.

A reversibly thermochromic composition is a reversibly thermochromic aqueous ink composition containing a reversibly thermochromic microcapsule pigment encapsulated in a microcapsule formed from an organic resin, an N-vinyl-2-pyrrolidone polymer, glycerin, and water, in which when contents (% by mass) of the reversibly thermochromic microcapsule pigment, the N-vinyl-2-pyrrolidone polymer, and the glycerin with respect to a total mass of the ink composition are P.sub.MC, P.sub.PVP, and P.sub.G, respectively, 0.3≤P.sub.MC/(P.sub.PVP+P.sub.G)≤4, and 0.2≤P.sub.PVP/P.sub.G≤5, and a refill or an aqueous ballpoint pen incorporating the reversibly thermochromic aqueous ink composition.

Provided are a reversibly thermochromic aqueous ink composition having excellent writability and few problems such as aggregation of a colorant, an increase in viscosity of the composition, or a decrease in handwriting density, and a refill or an aqueous ballpoint pen using the reversibly thermochromic aqueous ink composition.

A reversibly thermochromic composition is a reversibly thermochromic aqueous ink composition containing a reversibly thermochromic microcapsule pigment encapsulated in a microcapsule formed from an organic resin, an N-vinyl-2-pyrrolidone polymer, glycerin, and water, in which when contents (% by mass) of the reversibly thermochromic microcapsule pigment, the N-vinyl-2-pyrrolidone polymer, and the glycerin with respect to a total mass of the ink composition are P.sub.MC, P.sub.PVP, and P.sub.G, respectively, 0.3≤P.sub.MC/(P.sub.PVP+P.sub.G)≤4, and 0.2≤P.sub.PVP/P.sub.G≤5, and a refill or an aqueous ballpoint pen incorporating the reversibly thermochromic aqueous ink composition.

There is a non-aqueous gel writing ink having an organic solvent, a coloring agent and a gelling agent, and the gelling agent is non-oxidized cellulose nanofibers. There also is a use of non-oxidized cellulose nanofibers as a gelling agent in a non-aqueous gel writing ink. There is finally a writing instrument having the ink according to the present disclosure.

There is a non-aqueous gel writing ink having an organic solvent, a coloring agent and a gelling agent, and the gelling agent is non-oxidized cellulose nanofibers. There also is a use of non-oxidized cellulose nanofibers as a gelling agent in a non-aqueous gel writing ink. There is finally a writing instrument having the ink according to the present disclosure.