Open Access

A small series of indigo derivatives were mixed with palygorskite or sepiolite, respectively, and heated at 150 °C overnight in order to prepare stable pigments. While mixtures with violet halo-indigos such as Tyrian Purple (6,6′-dibromoindigo) and its difluoro- and dichloro-derivatives formed blue pigments with a slight greenish hue upon heating, alkoxy-indigo derivatives turned olive green during the heating process when mixed with palygorskite or sepiolite. Mixtures of isatin and indigo with palygorsite led to the formation of green pigments. The pigments were analyzed by spectroscopy (UV-VIS and Raman) and electron microscopy. Thermal studies were carried out and the obtained pigments were stable at temperatures of up to 250 °C. Maya Blue and pigment samples derived from 6,6′-dichloroindigo were tested for their flame retarding properties in mixtures with cellulose acetate and both additives led to distinct heat release rate reductions.