Sustainable Natural Lake Pigments from Caesalpinia sappan: Improving Stability through Inorganic Support Morphologies...

Red-pink natural lake pigments were prepared from Caesalpinia sappan L. extract colorant powder (S-Alum, without inorganic supports) and as colorant-adsorbed inorganic supports: silica (S-Si) and kaolinite (S-Kaol). Among these, S-Kaol demonstrated the highest stability and color strength (K/S), supported by its superior initial thermal degradation at 450 °C, high pH and UV stability, and consistent hue (H° = 19-22). X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) analyses confirmed the formation of a stable structure in S-Kaol, involving strong electrostatic interactions between brazilein and the kaolinite surface through Al³⁺ chelation. The DFT results further revealed an Al adsorption energy of -0.97 eV and a strong orbital hybridization between the Al (s) and the O (p) orbitals. The resulting pigments were incorporated into PLA to produce pinkish-red composite films. PLA/S-Kaol demonstrated superior pigment dispersity, thermal stability, and photostability, showing only a minor hue shift (H° shifting from 33.12° to 32.21°) after 168 h of UV exposure, attributed to the UV-shielding effect of kaolinite layers. In contrast, spherical S-Si particles introduced film defects, while amorphous S-Alum yielded moderate improvements. Overall, the inorganic supports acted as nucleating agents, enhancing PLA crystallization and thermal performance. These findings highlight S-Kaol as a renewable, nontoxic with heavy metal-free alternative to synthetic dyes for biodegradable polymer applications.