Kinetic analysis of the multistep thermal decomposition of Maya Blue-type pigments to evaluate thermal stability

This study aimed to evaluate the practical usefulness of kinetic deconvolution analysis (kDa) as a means to obtain the kinetic information on specific reaction steps that characterize the thermal properties of materials for various purposes. The partially overlapping multistep thermal decomposition of Maya Blue (MB)-type pigments was used as an example reaction. Red and yellow MB-type pigment materials, composed of a fibrous clay mineral and an organic dye, were synthesized using palygorskite and sepiolite as the clay minerals and Methyl Red and Alizarin as red and yellow dyes, respectively. The multistep thermal decompositions of the MB-type pigments were investigated using thermogravimetry. The thermoanalytical data were deconvoluted into individual component reaction steps using an empirical kDa technique based on a cumulative kinetic equation that considers the contribution of each reaction step to the overall thermal decomposition. By comparing the kDa results for the thermal decomposition of the composites with those for the decomposition of pure palygorskite and sepiolite, the thermal decomposition steps for the incorporated organic dyes were extracted from the multistep thermal decompositions of the MB-type pigments. Finally, the thermal stabilities of MB-type pigments comprising different clay minerals and organic dyes were compared using the kinetic results extracted for the reaction step associated with the decomposition of the organic dyes.