Decolorization of cashew nut shell liquid (CNSL) using kaolin clay: a facile refinement strategy for a sustainable bio-based resource

Abstract

Industries are progressively turning to sustainable alternatives due to rising environmental pollution and the depletion of fossil fuels. Over the past decades, significant efforts have been made to replace petroleum-based raw materials like phenol and formaldehyde with bio-based alternatives such as cardanol, derived from cashew nut shell liquid (CNSL). This study focuses on addressing the major limitation of crude CNSL—its dark brown color, which restricts its industrial utility. We have developed an effective and economical decolorization method using kaolin and metakaolin clays, examined for the first time as decolorization agents. By combining these clays with organic solvents, we achieved selective removal of the dark hue from CNSL. Hexane was found to be the most efficient medium, with a decolorization efficiency of 99%, and metakaolin outperformed kaolin in decolorizing CNSL at room temperature. The resulting decolorized CNSL resin was transparent and maintains a low viscosity of 55 cP, while its chemical integrity remains intact, as confirmed by UV and FTIR analyses. Additionally, the process yielded hydrophobic kaolin with a contact angle of 142°, indicating its potential as a valuable functional filler. This innovative physisorption technique is a zero-discharge process, facilitating the recovery and reuse of solvents and kaolin. The decolorized CNSL resin was successfully blended with polyurethane (PU) resin, proving its applicability in paints and coatings. This study presents a significant advancement in the direct use of CNSL for various industrial applications, showcasing the dual benefits of decolorized resin and hydrophobic kaolin.

Graphical Abstract