1. Field of the Invention
The present invention relates to a fluoran compound which is useful as color forming compound in recording materials such as a pressure-sensitive recording material and heat-sensitive recording materials. More particularly, the invention relates to a crystal of the fluoran compound, isolation process of said crystal, and recording materials comprising said crystal.
2. Description of the Related Art
Pressure-sensitive recording, heat-sensitive recording and electroheat-sensitive recording have conventionally been used as systems for recording transfered information through the mediation of external energy, such as pressure, heat or electricity by utilizing color reaction between a colorless or pale colored electron donor compound (color forming compound) and an organic or inorganic electron acceptor (developer).
In these systems, fluoran compounds have widely been used as the color forming compound.
The fluoran compound having the formula (I): ##STR2## is disclosed as a color forming compound in Japanese Laid-Open Patent SHO 59-65053(1984), 60-47066(1985) and 60-141762(1985). The compound is described in these patent publications to have a melting point of about 180.degree. C. (181.2 to 182.6.degree. C. in Japanese Laid-OPen Patent SHO 59-65053, 177 to 178.degree. C. in 60-47066, and 178 to 180.degree. C. in 60-141762).
The crystal of the fluoran compound represented by the formula (I) is difficult to ground, requires a long time for pulverization and leads to a problem that a dispersion of the color forming compound for the heat-sensitive recording material is difficult to prepare.
When the crystal of the fluoran compound represented by the formula (I) and having the stated melting point is used as a color forming compound of a recording material, for example, a heat-sensitive recording material using bisphenol A as a developer, the compound leads to a high color-developing temperature of the recording material and is unsatisfactory for use in high speed recording which is desired today.
Japanese Laid-open Patent SHO 54-34909(1979) discloses only the compound having the formula (]I). However, no description is found at all on the properties of the compound. Japanese Laid-Open Patent SHO 60-35053(1985) practically discloses the compound of the formula (II): ##STR3##
The crystal of the fluoran compound having the formula (II) prepared 5Y the Process described in Japanese Laid-Open Patent SHO 60-35053(1985) is also difficult to ground, requires a long time for pulverization and has a problem that a dispersion of color forming compound for use in the heat-sensitive recording material is difficult to prepare.
Further, the crystal of fluoran compound having the formula (II) isolated by the process described in Japanese Laid-Open Patent SHO 60-35053 is poor in light stability and has a disadvantage of quick yellowing by irradiation of light.
Consequently, in cases where the crystal is used, for example, for a color forming compound of the heat-sensitive recording paper, the crystal has a disadvantage that the uncolored portion of the resulting paper causes yellowing by light irradiation, and serious problems have frequently occurred in practical use.