Bio-Based Guar-Borate Hydrogels: Processing Effects and Rheological Insights for High-Temperature Applications

The increasing emphasis on environmentally sustainable practices in industry has intensified interest in natural biopolymers for use in advanced material applications. Hydrogel systems composed of natural polymers, such as guar, xanthan gum, or carboxymethyl cellulose, crosslinked with borate or transition metal complexes are particularly relevant due to their renewability and tunable rheological properties. In this study, bio-based guar-borate hydrogels were prepared and rheologically characterised over a temperature range of 25 to 140 °C, using a rheo-reactor apparatus that enables measurements above the solvent's boiling point. Various geometries were employed depending on the viscosity of the formulation, allowing a broad range of shear rates to be explored. Gelation was found to occur rapidly, typically within 10 min; however, inefficient distribution of borate ions in highly viscous guar solutions delayed network formation. The gels exhibited significant increases in elastic properties and shear thickening upon crosslinking. Rheological properties, including gel strength and elasticity, decreased exponentially with temperature. Despite this, the gels retained high viscosity and viscoelasticity up to 100 °C, beyond which a discontinuous gel phase dominated the response. These findings contribute to the understanding of structure–property–processing relationships in natural polymer systems and highlight the potential of guar–borate gels for sustainable, high-temperature applications.