Macromolecular Structuring and Structure–Property Relationships of Sodium Soap‐Based Grease Derived from Treated Waste Cooking Oil

The development of sustainable lubrication systems requires a clear understanding of the macromolecular interactions governing grease structure and performance. In this study, sodium soap‐based greases were formulated using treated waste cooking oil (TWCO) as a renewable base fluid and sodium stearate as an amphiphilic thickener. Unlike conventional grease formulations that rely on synthetic additives, this work adopts a fully additive‐free approach to isolate intrinsic structure–property relationships within the grease system. Grease samples with varying oil‐to‐thickener ratios (SG40–SG75) were prepared via in situ saponification and evaluated using standardized methods, including consistency (ISO 2137), oil separation (ASTM D6184), oil bleeding, dropping point (ASTM D2265), and corrosion resistance (ASTM D4048). The results show that increasing sodium soap concentration promotes the formation of a denser three‐dimensional fibrous network, enhancing oil immobilization, mechanical strength, and thermal stability. Among the formulations, SG60 (60 wt% TWCO, 40 wt% sodium stearate) achieved the optimal balance between structural integrity and oil mobility, exhibiting NLGI Grade 2 consistency, low oil separation (<1.5%), controlled oil bleeding, and a high dropping point (>225°C). The improved performance is attributed to effective oil confinement within the interconnected soap network. This study establishes a structure–property framework for designing sustainable, additive‐free grease systems and provides a mechanistic basis for developing bio‐based lubricants with predictable performance characteristics.