Novel eco-friendly hybrid pigment with improved stability as a multifunctional additive for elastomer composites with reduced flammability and pH sensing properties

Optical pH sensors are generally fabricated using pH sensitive dyes immobilized on a solid inorganic carrier. In this work, the natural indicator alizarin (C.I. 58000) was incorporated in vermiculite to obtain a pH sensitive organic-inorganic pigment with potential fire-retardant properties. Fourier transform infrared spectroscopy (FTIR), time-of-flight secondary ion mass spectrometry (TOF-SIMS), powder X-ray diffraction analysis (PXRD), thermogravimetric analysis (TGA), and UV–Vis spectrophotometry were used to characterize the colored powder. The color change after modification and the presence of C14H7O4Mg+ and C14H6O5Al- (detected by TOF-SIMS) confirmed successful fixation of the organic molecules on the phyllosilicate. The novel colorant was then used to prepare acrylonitrile-butadiene rubber (NBR) composites, which were assessed in terms of their allochroic behavior and flammability in acid and basic environments. The thermal, mechanical, and rheological properties of the composites were also evaluated. A polymer sensor was obtained that is sensitive to pH variation, with color changes from yellow (acid environment) to violet (basic environment). The application of 10 wt% of the organic-inorganic pigment decreased the heat release rate parameter by 49% and the total heat release by 44% in comparison to the reference sample. The method presented here offers a new approach to imparting low flammability and pH sensing properties to NBR polymer materials.