Structuring and Stability of Biopolymer‐Thickened Greases: Macromolecular Design for Sustainable Lubrication

ABSTRACT The transition toward sustainable lubrication has necessitated the development of eco‐friendly alternatives to conventional lithium‐based greases. This study aims to evaluate the macromolecular structuring behavior and physical stability of castor oil‐based lubricating greases formulated with three distinct organic thickeners: polypropylene (PP), cellulose, and chitosan. Each thickener represents a different class of polymeric material, offering varied interaction mechanisms with the oil matrix. Grease samples were prepared via thermal dispersion and evaluated using standardized procedures set by the American Society for Testing and Materials (ASTM), including consistency grading (ASTM D217), oil bleeding (ASTM D4425), and oil separation (ASTM D1742). Consistency was quantified using the cone penetration test and reported using the National Lubricating Grease Institute (NLGI) grading system, where lower grades (e.g., 000–00) indicate fluid‐like textures, and higher grades (e.g., 1–2) indicate firmer, mechanically stable greases. Results showed that greases thickened with polypropylene consistently achieved NLGI Grade 1 across all concentrations (25–35 wt.%), while cellulose and chitosan‐based greases remained within the softer NLGI 000–00 range. Oil bleeding was significantly lower for polypropylene‐based samples (as low as 7%) compared to chitosan and cellulose systems (>18% at lower loadings). Similarly, oil separation remained below 1% for polypropylene at higher concentrations, indicating excellent thermal and storage stability. These findings underscore the critical role of macromolecular architecture, polarity, and thickener–oil compatibility in forming robust 3D grease networks. The work provides a comparative structure–property framework for selecting organic thickeners in green grease formulations, advancing the design of biodegradable, high‐performance lubricants for industrial and environmental applications.