lubricant systems, matching conventional lubricants such as polyalphaolefin (PAO-09), polyethylene glycol (PEG400), and an 86 % glycerol solution (G-86). Sheardependent viscosity was characterized and modeled using a Carreau-Yasuda approach. Friction properties were evaluated with a Mini Traction Machine (MTM2) tribometer, complemented by optical interferometry measurements to determine central and minimum lubricant film thickness under various conditions. Results and Discussion Rheological investigations revealed that original high molecular weight HEC derivatives (HCL and HCS) exhibited pronounced shear-thinning behavior, resulting in thin lubricant films. Enzymatic hydrolysis significantly reduced the molecular weight, eliminating shear-thinning and enhancing suitability for EHD lubrication. Resultant Science and Research 38 Tribologie + Schmierungstechnik · volume 72 · issue 2/ 2025 DOI 10.24053/ TuS-2025-0011 Introduction The development of environmentally friendly lubricants is a key concern in modern tribology. In view of the ecological challenges posed by fossil lubricants and increasing regulatory pressure, the use of bio-based, watersoluble systems is becoming an increasingly important focus of development ii,iii . Lubricants with a functional water content offer an attractive alternative to mineral oil-based products due to their environmental friendliness, high availability, and, last but not least, their nonflammable nature iv,v,vi . The combination of water with polymeric viscosity modifiers makes it possible to create hydrodynamic lubricating films even in highly stressed tribological contacts. The present study deals with the development and investigation of such systems based on hydroxyethyl cellulose (HEC) and glycerin, with a particular focus on their behavior under elastohydrodynamic (EHD) lubrication. Materials and Methods Commercially available hydroxyethyl celluloses, specifically adjusted to various molecular weights through enzymatic hydrolysis, were used in this study. The weight-average molecular mass was reduced to approximately 19 kg/ mol (see Figure 1). Additionally, glycerol (G) was used in weight proportions between 40 % and 70 % to adjust the viscosity of the Cellulose in Motion: Enzymatically Modified Biopolymers and Glycerol in Tribological Interaction Sandra Kiese, Daniela Leistl, Jan Ulrich Michaelis, Stefan Hofmann, Thomas Lohner* environmentally friendly, bio-based, water-soluble, hydroxyethyl cellulose, elastohydrodynamic Keywords * Dr. Sandra Kiese Dr.-Ing. Daniela Leistl M.Sc. Jan Ulrich Michaelis Fraunhofer Institute for Process Engineering and Packaging IVV Giggenhauser Straße 35, 85354 Freising, Germany M.Sc. Stefan Hofmann Dr.-Ing. Thomas Lohner Gear Research Center (FZG), Department of Mechanical Engineering School of Engineering and Design Technical University of Munich Boltzmannstraße 15, 85748 Garching, Germany Note: This article is an abridged and adapted version of the original article published under the title “Elastohydrodynamic lubrication of aqueous hydroxyethyl cellulose-glycerol lubricants” in the journal *Tribology International* i . Figure 1: Differential distribution as a function of molecular weight for HCL, HCS, and HCH, adapted from Michaelis et al. i . hydroxyethyl cellulose hydrolysates (HCH) showed stable viscosity across relevant shear rates, substantially improving film formation capabilities. Glycerol’s role is notable as it serves not only as a viscosity modifier but also increases the pressure-viscosity coefficient. Figure 2 shows the influence of different HEC types with varying molecular weights and glycerin content on the coefficient of friction in aqueous formulations as a function of the mean velocity v m . As expected, friction decreases with increasing speed in all formulations - albeit to very different degrees. Compared to the reference PEG400, all HEC-glycerol mixtures show improved friction performance at medium to high speeds. Figure 3 shows the influence of different glycerol concentrations in combination with hydrolyzed hydroxyethyl cellulose (HCH) on the coefficient of friction. The results show that all glycerin-HCH systems have significantly lower friction coefficients than the reference oils PAO-09 and PEG400 and the non-hydrolyzed samples. Compared to PAO-09, friction was reduced by more than 90 %. While PEG400 and PAO-09 show only moderate friction reductions with increasing speed, the glycerin-containing systems - especially those with glycerin contents above 50 wt% - exhibit a clearly pronounced speed dependence and a transition from mixed friction to fluid friction at speeds above approximately 1000 mm/ s. In combination with HCH, a friction coefficient of less than µ = 0.01 was achieved in formulations with a glycerin content of 50 % by weight - a range that is considered liquid superlubrication according to the current definition. Even with higher glycerin contents, favorable friction behavior is achieved, although a slight increase in friction coefficients was observed from approx. 70 % glycerin by weight, which is attributed to an increasing pressure viscosity coefficient. In addition, the central lubricating film thickness was measured in EHD contact (Figure 4). This showed that the hydrolyzed HCH types in combination with glycerin were able to form significantly higher lubricating film thicknesses than the non-hydrolyzed starting materials, whose lubricating film thicknesses were below 50 nm. The formation of a lubricating film correlates strongly with the effective viscosity in the inlet area of the EHD Science and Research 39 Tribologie + Schmierungstechnik · volume 72 · issue 2/ 2025 DOI 10.24053/ TuS-2025-0011 Figure 2: Friction coefficients as a function of mean velocity for HEC-glycerol formulations, adapted from Michaelis et al. i . Figure 3: Friction coefficients as a function of mean velocity for HCH-glycerol formulations, adapted from Michaelis et al. i . Figure 4: Central lubricant film thicknesses at varying mean velocities for different formulations, adapted from Michaelis et al. i . i Michaelis, J. U., Kiese, S., Hofmann, S., Lohner, T., & Eisner, P. (2025). Elastohydrodynamic lubrication of aqueous hydroxyethyl cellulose-glycerol lubricants. Tribology International, 110563. Volltext unter: https: / / www. sciencedirect.com/ science/ article/ pii/ S0301679X25000581 ii Shah, R., Woydt, M., & Zhang, S. (2021). The economic and environmental significance of sustainable lubricants. Lubricants, 9(2), 21. iii Pichler, J., Maria Eder, R., Besser, C., Pisarova, L., Dörr, N., Marchetti-Deschmann, M., & Frauscher, M. (2023). A comprehensive review of sustainable approaches for synthetic lubricant components. Green Chemistry Letters and Reviews, 16(1), 2185547. iv Schmidt, R., Klingenberg, G., & Woydt, M. (2006). Thermophysical and viscosimetric properties of environmentally acceptable lubricants. Industrial Lubrication and Tribology, 58(4), 210-224. v Shetty, P., Mu, L., & Shi, Y. (2020). Polyelectrolyte cellu lose gel with PEG/ water: Toward fully green lubricating grease. Carbohydrate polymers, 230, 115670. vi Rahman, M. H., Warneke, H., Webbert, H., Rodriguez, J., Austin, E., Tokunaga, K., ... & Menezes, P. L. (2021). Water-based lubricants: Development, properties, and performances. Lubricants, 9(8), 73. Science and Research 40 Tribologie + Schmierungstechnik · volume 72 · issue 2/ 2025 DOI 10.24053/ TuS-2025-0011 contact, with the glycerine concentration playing a particularly dominant role here. In the systems examined, the range for a good balance between low friction and sufficient lubricating film thickness was around 50 - 60 % glycerine in combination with 3 - 5 % HCH. Summary and Outlook The presented findings demonstrate that lubricants with functional water content based on hydroxyethyl cellulose and glycerol hold significant potential for technical applications, contributing substantially to bio-based tribological systems development and their use in numerous fields of application in which environmental aspects are playing an increasingly central role. The targeted adjustment of the polymer structure via enzymatic hydrolysis enables significant friction reduction, improved lubricating film formation, and favorable rheological behavior. At the same time, glycerin allows flexible control of viscosity and supports lubricating film formation even under high pressure and shear loads.