Desenvolvimento e caracterização de pigmentos frios à base de óxidos de alumínio e cobalto

Abstract

Urban temperature rise has intensified the search for coatings with higher solar reflectance, highlighting cool pigments as an alternative to reduce the heating of exposed surfaces. This study aimed to develop and describe cool blue pigments based on aluminum and cobalt oxides. The pigments were synthesized using an adapted Pechini method, with a Rotational Central Composite Design (RCCD) applied to define cobalt content and calcination temperature. Colorimetric analysis was performed to evaluate hue and chromatic stability, revealing variations from violet to intense blue predominantly influenced by calcination temperature, along with good stability of the L*, a*, and b* parameters over two years. X-ray diffraction (XRD), used to determine crystalline phases and crystallinity, indicated spinel phase formation in 8 of the 11 syntheses and, at higher temperatures, the presence of α-alumina, with increasing crystallite size associated with higher thermal treatment. Scanning electron microscopy (SEM) showed irregular particle morphology and heterogeneous size distribution, while FTIR analysis confirmed characteristic Al–O bonds and structural evolution with temperature. Diffuse reflectance and photoacoustic spectroscopies, employed to investigate electronic transitions and thermal absorption behavior, revealed characteristic Co²⁺ and Co³⁺ transitions and reflectance values close to 80% in the 700–1000 nm range for most samples, indicating promising potential for cool pigment applications. Overall, the results enabled the identification of suitable calcination conditions to obtain blue pigments with desirable tonalities and optical properties compatible with energy-efficient coating applications.