Phthalide compounds, which are a type of colorless dye (color former), develop color via a ring-opening reaction when interacted with acids (developer), as shown in the following scheme. Because this reaction is reversible, decoloration occurs in a reversible manner under basic conditions.
Pressure and heat sensitive recording systems utilize this color developing mechanism in which color formers, developers and sensitizers are intimately incorporated. In pressure sensitive recording, the coloration occurs as follows: Capsules containing color former are destroyed by pressure and the color former makes contact with the developer coated support material. In heat sensitive recording, both color former and developer, which are dispersed on the support material, are thermally melted, thus contacting each other to initiate the coloring reaction. Furthermore, a low melting point sensitizer is also added to achieve a high sensitivity. Recently, the technique has been applied to re-writable recording paper, which can be used for repeated writing and erasing. This technique has been applied to print on commuter passes or prepaid cards. By combining acid-generating methods with the coloring mechanism, new applications can be expected for these materials.