Effect of lake formation on the light stability of geranium lake pigment

Abstract

In this study, we have investigated the effect of lake formation on the light stability of lake pigment, comparing structural and spectroscopic properties of eosin dye and its aluminum lake, i.e., geranium lake pigment. XRD and SEM observation confirmed that they both have crystalline structure, but their particle size is smaller in geranium lake pigment than in eosin. Their UV-VIS reflectance spectra showed slight differences, such as a slight shift of absorption edge wavelength, while no significant difference was observed by FT-IR and Raman spectra. In the light stability test of their oil paint, the color change of geranium lake paint was about four times less than eosin paint. These results imply that, in geranium lake pigment, the bond formation of eosin with aluminum ion causes the modification of its electronic state, which contributes to the improvement of light stability. We have also investigated their discoloration mechanism. FT-IR analysis showed the dissociation of C-Br bonds and the structural modification of xanthene ring in both samples. Their cross-section and micro-FT-IR analyses revealed that a thin layer was formed at the paint surface, and the photodegradation occurred only at this thin top layer.