in 2020 [2, 3, 4]. A large proportion of the appropriately declared biolubricants are characterized by the use of vegetable oils and animal fats as raw materials. In Germany, this share in the lubricants sector amounted to about 48,000 tons in 2017. The application is mainly in the areas of hydraulic oils and engine, compression and turbine oils with 30 % and 28 % respectively [5]. The demand for bio-based lubricants is due to increasing awareness and understanding of the environment by users and acceptance by the industry, as well as the introduction of legal regulations [1]. Here, the United States of America with the “Vessel General Permit (VGP)” and Europe with the “EU Ecolabel” are the most advanced regions. Against this background, the use of vegetable, animal or used oils and greases offers various advantages over petroleum-based lubricants in terms of biodegradability, cost efficiency, renewability and environmental impact [1]. Aus Wissenschaft und Forschung 28 Tribologie + Schmierungstechnik · 70. Jahrgang · 6/ 2023 DOI 10.24053/ TuS-2023-0037 Bio-lubricants: A market on the rise due to environmental awareness and legal regulations Bio-lubricants can replace mineral oil-based lubricants to make manufacturing operations more sustainable and ecofriendly. The global market for bio-lubricants was estimated at around USD 2.92 billion in 2021. Assuming an average growth rate of 4.7 %, the market will grow to around USD 4.3 billion by 2029 [1]. Despite the projected growth, the market share of bio-lubricants was 4 -5 % Plant raw materials for bio-based lubricants and additives Daniela Leistl, Melanie Platzer, Jan Ulrich Michaelis, Sandra Kiese* Dieser Beitrag wurde im Rahmen der 64. Tribologie-Fachtagung 2023 der Gesellschaft für Tribologie (GfT) eingereicht. Biobasierte Schmierstoffe aus pflanzlichen Ressourcen gewinnen als nachhaltige Alternativen zu Mineralölen an Bedeutung. Aktuelle Forschungsarbeiten untersuchen Deodestillate aus der Produktion von Speiseölen als vielversprechende Basisöle. Diese Destillate zeigen nach enzymatischer Behandlung einen deutlichen Rückgang freier Fettsäuren, was die Oxidationsbeständigkeit verbessert. Parallel dazu werden pflanzliche Extrakte, reich an Antioxidantien, als potenzielle Bio-Additive eingesetzt, welche die oxidative Stabilität der Schmiersysteme weiter erhöhen. Zusätzlich zeigen Forschungsarbeiten, dass Zellulose-Derivate wie Natriumcarboxymethylzellulose als biologisch hergestellte Verdickungsmittel für wässrige Schmierstoffe dienen können, um die Herausforderungen der geringen Viskosität von Wasser zu adressieren. Diese Erkenntnisse unterstreichen das Potenzial biobasierter Schmierstoffe und Additive, Leistungsfähigkeit mit Umweltverträglichkeit zu vereinen. Schlüsselwörter Bioschmierstoffe, deodorierte Destillate, Bio-AW- Additive, Zellulose Derivate, Viskositäts-Modifier Bio-lubricants, derived from plant resources, are gaining prominence as sustainable alternatives to mineral oils. Recent research highlights deodorized distillates from edible oil production as promising bio-lubricant base fluids. These distillates, upon enzymatic treatment, reveal a profound decrease in free fatty acids, improving oxidation resistance. Plant extracts, rich in secondary plant metabolites especially in antioxidants, emerge as potential bio-additives, enhancing the oxidative stability of these lubricant systems. Additionally, research sheds light on cellulose derivatives like sodium carboxymethyl cellulose, as potential biologically-sourced thickeners for aqueous lubricants, addressing the challenges posed by water’s inherent low viscosity. These findings accentuate bio-lubricants’ potential to combine superior performance with environmental sustainability. Keywords Bio-lubricants, deodorized distillate, vegetable AW additives, cellulose derivatives, viscosity modifiers Kurzfassung Abstract * Dr. Daniela Leistl Dr. Melanie Platzer Jan Ulrich Michaelis Dr. Sandra Kiese Orcid-ID: https: / / orcid.org/ 0000-0001-8770-5641 Fraunhofer Institut für Verfahrenstechnik und Verpackung (IVV), Giggenhauser Straße 35, 85276 Freising TuS_6_2023.qxp_TuS_6_2023 01.02.24 14: 18 Seite 28 Valorization of deodorized distillate from vegetable oil refining as a base fluid A previously largely unused by-product from the field of edible oil production - deodorized distillates (DODs), which has great potential for use as a base fluid in biobased lubricant systems, was investigated. Deodorization is the final step in the refining process. It removes various undesirable substances from the vegetable oil, for example, ingredients that can lend it an off-odor or offflavor. The process of refining vegetable oil leaves byproducts that have hardly been used to date. They contain many valuable bioactive compounds. In addition to its favorable composition, deodestillate has the advantage of being a plant-based feedstock that does not compete with food production. Recovering these by-products with efficient and cost-effective technologies is a challenge. By means of specifically controlled enzymatic reactions, the free fatty acids in the distillate are esterified with glycerol - a by-product in biodiesel production - and are transformed into a blend of mono-, di-, and triglycerides. The enzymatic approach to neutralize DODs achieved over 99 % reduction in free fatty acids (FFA) content (Figure 1). By this way, rheological properties, such as viscosity (Figure 2), can be adjusted in a targeted and optimal manner with regard to the area of application. The reduction in FFA content leads to an increase in oxidation stability to equal level with rapeseed oil (Figure 3). In addition, tribological properties of the samples were measured using an Anton Paar tribology measurement cell with a ball-on-three-plates geometry a temperature of 80 °C and a normal force of 23.57 N, equaling a maximum Hertz contact pressure of 1.01 GPa (Figure 4). In the tribological characterization, the neutralized and additivated distillates obtained from the chemical refining of olive pomace oil and sunflower oil showed comparable results to standard oils and in some cases even better friction and wear behavior. Especially, DOD from Aus Wissenschaft und Forschung 29 Tribologie + Schmierungstechnik · 70. Jahrgang · 6/ 2023 DOI 10.24053/ TuS-2023-0037 Figure 1: FFA content as a function of time during the enzymatic neutralization reaction. Rapeseed oil DOD Modified DOD 0 5 10 15 20 25 30 35 40 45 Viscosity [mPas*s] Figure 2: Viscosity of native and enzymatically modified DOD in comparison to rapeseed oil. Rapeseed oil DOD Modified DOD 0 5 10 15 20 Oxidation induction time [h] Figure 3: Oxidation induction time in Rancimat test of native and enzymatically modified DOD in comparison to rapeseed oil. 0.001 0.01 0.1 1 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 DOD Modified DOD Rapeseed oil Friction coefficient [1] Sliding speed [m/ s] Figure 4: Stribeck graph of native and enzymatically modified DOD in comparison to rapeseed oil. TuS_6_2023.qxp_TuS_6_2023 01.02.24 14: 18 Seite 29 tion focused on side streams from food and agricultural industries to avoid competition with food production and to achieve a holistic use of raw materials. The secondary plant substances (SPS) in the residues are the focus here, which possess functional properties that make them attractive for use as bio-based antioxidant additives. In nature, there are over 10,000 distinct SPS that plants don’t require for their growth and development. However, they serve various ecological functions, such as protection against pathogens, pests, herbivores, and UV radiation [6, 7, 8]. Due to their disease-preventing and health-promoting properties, SPS are commonly used in the pharmaceutical and dietary supplement industries. In addition to these attributes, they are particularly renowned for their potent antioxidant effects, enabling them to neutralize reactive oxygen species and other free radicals. As a result, they are used in various applications such as food and cosmetics and can also be formulated into bio-lubricants. The extracts obtained were incorporated into various lubricant model systems in order to determine their effectiveness using the Rancimat method, which accelerates the aging process of a plant oil at elevated temperatures with continuous air intake. The airflow carries the volatile oxidation products from the sample vessel into distilled water, increasing its conductivity. The inflection point of the obtained curve marks the induction time and is automatically determined by the device. By varying the measurement temperature, conditions can be adjusted to different application scenarios [9]. The results are shown in Figure 5. The extracts (with solubilizer) were formulated into rapeseed oil at concentrations of 1 wt.%. The results show that an increase in oxidation stability was achieved by both SPS extracts. The highest result was obtained by flavonoid-rich extracts, which showed an increased value of over 300 % compared to rapeseed oil [9]. These values are comparable to standard-antioxidants like tocopherol or butylated hydroxytoluene. A Aus Wissenschaft und Forschung 30 Tribologie + Schmierungstechnik · 70. Jahrgang · 6/ 2023 DOI 10.24053/ TuS-2023-0037 olive pomace oil has shown great potential as an alternative to harmful fossil-based raw materials like mineral oils in the lubricant sector. From the results, it can be concluded that deodistillates are suitable as an alternative base medium for the formulation of biodegradable lubricant systems and thus represent a potential alternative to fossil raw materials, which are harmful to health and the environment. Alongside the mentioned technological benefits, their use paves the way for reduced dependency on fossil additives, promoting sustainability. Functional additives from secondary plant substances Often, the direct use of vegetable raw materials as base fluid is limited by their low oxidative properties, lack of stability, poor low-temperature properties or a narrow range of accessible viscosities. These disadvantages can be mitigated by chemical modifications, formulations with additives, or combination with mineral oil. However, these measures lead to increased costs, impurities as well as the decrease of biodegradability [1]. Conventional, fossil-based lubricants typically contain highly functional, synthetic additives that aren’t always environmentally friendly. Especially in bio-lubricants, these should be replaced with bio-based additives. By utilizing natural and renewable raw materials, the environmental compatibility of lubricants can be significantly increased. Furthermore, expensive disposal costs associated with conventional products can be minimized. To develop additives, we used natural antioxidants in plant extracts to increase the oxidative stability of lubricant systems. Furthermore, the wear protection effect of these components was investigated. Extracts rich in antioxidants were obtained from various raw and residual plant materials. The raw material selec- Figure 5: Oxidation induction time in the Rancimat test of different plant extracts as well as standard antioxidants compared to pure rapeseed oil. Figure 6: Stribeck graphs of rapeseed oil formulated with different plant extracts: Extract 1 is rich in polyphenols, extracts 2 and 3 are rich in phytic acid and extract 4 is rich in glucosinolates. TuS_6_2023.qxp_TuS_6_2023 01.02.24 14: 18 Seite 30 further increase in the efficacy of the plant extracts by purification and thus concentration of the active ingredients to reach the level of Additin or higher is currently being investigated. In addition to the antioxidant properties, another focus of the project was on the effect of SPS in various tribological tests (Anton Paar tribology measurement cell, MicroScratch, SRV). Figure 6 shows the results of the tribological measurements at 23.57 N normal force and a temperature of 80 °C of different extracts in rapeseed oil. Here, all extracts were able to reduce the friction coefficient. However, the extract rich in polyphenols (extract 1) provided the lowest reduction compared to the samples containing phytic acid (extracts 2 and 3) or even glucosinolates (extract 4). Thus, similar to standard additives, the phosphor and sulfur compounds are able to provide a reduction in friction [10]. Samples were also prepared with mineral oil and polyalphaolefin (PAO) as base oils. Tribological measure ments were performed as described above and the results are shown in Figure 7. Unfortunately, the measurement method developed for rapeseed oil was not suitable for mineral oil and PAO. Therefore, seizure occurred in each of the second and third runs of the velocity ramps. Therefore, only the first runs are shown in the figure. However, the extracts were also able to significantly reduce the coefficient of friction or prevent seizure in mineral oil and PAO. It could be demonstrated that especially glucosinolatecontaining plant extracts with high sulfur content show typical AW properties and thus have the potential to replace or at least reduce conventional additives. However, a certain friction-reducing effect was also observed with polyphenol-rich extracts. Since these can additionally be used as antioxidants, anticorrosives and even antibacterial/ antimicrobial agents [11], these extracts are interesting especially with regard to an application as multifunctional additives. Plant-based thickeners for aqueous lubricant systems In terms of environmental and political challenges, water-based lubricants are also of great interest. The high thermal conductivity of water and the low coefficients of friction of aqueous solutions can enhance the tribological properties of lubricant systems. The low viscosity of water alone proves to be a disadvantage for the formation of a sufficient minimum lubricant film thickness [12, 13, 14, 15, 16 ,17]. In order to set application-specific viscosities and to obtain an aqueous lubricant system, high-molecular polymer additives are generally added, so-called thickeners. Some of these are relatively expensive and make the lubricant system susceptible to oxidation processes. In addition, thickeners used to date are predominantly fossil based [13, 18]. Polymers of biological origin were investigated for their suitability as thickening agents for the formulation of sustainable, aqueous transmission fluids. The investigations focused on hydroxyethyl and sodium carboxymethyl cellulose (CMCs) as heat-stable cellulose derivatives [19, 20]. The aim of the studies was to investigate the influence of the molecular weight, concentration and degree of substitution of CMC on the rheological and tribological properties of aqueous CMC solutions [18]. It was possible to set all viscosity classes relevant for gear and bearing applications. The viscosity indices determined are comparable with those of the reference media or higher. A shear stability test assessed the viscosity changes under intense shear conditions. This test is based on the tapered roller bearing methodology. The test conditions, adjusted for water-based lubrication, adhered to standards found in DIN 51350-6. The apparatus was powered at a rotational speed of 1000 rpm for 24 h, temperature was regulated at 40 °C, and axial load of 50 N was applied. Figure 8 shows the initial (dark colored) and final (light colored) viscosities of the solutions. The rheological investigations showed that the reversible temporary as well as the permanent viscosity loss increased with increasing molecular weight of the CMCs. The highest molecular weight derivative showed the greatest losses under shear, with a maximum of 73.3 % from an infinite shear rate of 46.07 mPas to 12.30 mPas, whereas the low MW CMC only lost 13.4 %. Further, the use of CMCs with a low degree of substitution promoted thixotropic behavior and reduced temperature stability [21]. Additional investigations with rolling contact were carried out with a disc-on-ball tribology system (Mini traction machine from PCS Instruments). During testing, the coefficient of friction was evaluated over a velocity range of 1 to 3500 mm/ s while maintaining a constant Aus Wissenschaft und Forschung 31 Tribologie + Schmierungstechnik · 70. Jahrgang · 6/ 2023 DOI 10.24053/ TuS-2023-0037 Figure 7: First runs of the speed ramps of glucosinolate-rich and polyphenol-rich extracts in different lubricant base oils. TuS_6_2023.qxp_TuS_6_2023 01.02.24 14: 18 Seite 31 Conclusion Bio-based additives and lubricant base-fluids made from plants have the potential to replace mineral oil-based products in many areas of application. By using natural and renewable raw materials, the environmental compatibility of lubricants can be increased. Moving away from fossil resources not only secures our future energy needs but also harnesses the benefits of a broader spectrum of active ingredients, thanks to the natural variance found in alternative resources. This shift significantly enhances sustainability aspects, pushing industries towards greener practices. Additionally, enzymatic or chemical modifications are an opportunity for customization, tailoring ingredients to specific needs. These highperforming ingredients are not just efficient but also biobased, merging the best of both worlds. Impressively, they can be integrated seamlessly into existing product formulations and production facilities, ensuring a smooth transition towards more sustainable operations. We support companies in integrating bio-based additives into their products. In doing so, we match the additives and lubricants to the target application depending on their functionality. As a prerequisite for characteristic formulation adaptations, we perform specific analysis of the raw materials, taking into account the interactions of individual formulation components. We offer the development of all process steps, from the extraction to the production of ready-to-use formulations. Upon request, we provide our customers with our facilities for their own developments. All work is possible from laboratory to pilot plant scale. If you have any questions or are interested in our research services, please feel free to contact us: https: / / www.ivv.fraunhofer.de/ en/ recycling-environment/ biolubricants.html. Funding This research was partly funded by the Bavarian Research Foundation (grant number Az-1314-17), by the German Federal Ministry of Education and Research (grant number 031B0360A) and by the European Union H2020 (ID: 887407). References [1] Fortune Business Insights, Bulk-Chemikalien - „Markt für Bio-Schmierstoffe“ Bericht-ID: FBI104654 [online] https: / / www.fortunebusinessinsights.com/ de/ markt-f-rbio-schmierstoffe-104654, Abruf 18. August, 2023. [2] Bauer N., „Biobasierte Schmier- und Verfahrensstoffe in der nachhaltigen öffentlichen Beschaffung“ [online] https: / / www.ifas.rwth-aachen.de/ cms/ IFAS/ Forschung/ Fluide/ Abgeschlossene-Forschungsprojekte/ ~spqv/ Noebio, Abruf 18. August, 2023. [3] Fachagentur Nachwachsende Rohstoffe e. V., „Anbau und Verwendung nachwachsender Rohstoffe in Deutschland (Statistikbericht)“ Gülzow-Prüzen, 2022. Aus Wissenschaft und Forschung 32 Tribologie + Schmierungstechnik · 70. Jahrgang · 6/ 2023 DOI 10.24053/ TuS-2023-0037 temperature of 40.0 °C and a normal load of 30 N, corresponding to a peak hertz contact pressure of 0.95 GPa. In all tests, the slide-to-roll ratio remained at 30 %. Results are shown in Figure 9. Friction consistently reduced as sliding speed increased. Regardless of temperature, the coefficient of friction of low M W CMC was roughly 35 % lower than other tested derivatives, bottoming out at 0.01 at 3500 mm/ s. Fluctuations at high speeds can be attributed to factors like polymer orientation. These results indicate that cellulose derivatives with a lower molecular weight should be used in future investigations in order to fully exploit the potential of the CMCs as presented [18]. Figure 8: Viscosities of CMCs of different molecular weights before (dark colored) and after (light colored) 24h shear stability test. Reproduced and adapted with permission from [21]. Figure 9: Coefficient of friction as a function of speed for low, medium and high M W CMCs at 40 °C (ballon-disc, normal load 30 N, SRR = 30 %). Reproduced with permission from [21]. TuS_6_2023.qxp_TuS_6_2023 01.02.24 14: 18 Seite 32 [4] Fachagentur Nachwachsende Rohstoffe e. V., „Marktanalyse Nachwachsende Rohstoffe: Schriftenreihe Nachwachsende Rohstoffe“, Gülzow-Prüzen, 2014 [5] Fachagentur Nachwachsende Rohstoffe e. V., „Rohstoffquellen für Bioschmierstoffe in Deutschland 2017“ Gülzow-Prüzen, 2019 [online], Basisdaten: Bioschmierstoffe (fnr.de), Abruf 18. August, 2023. [6] F. Shahidi und M. Naczk, Food phenolic: sources chemistry effects applications, Lancaster PA, USA: Technomic Publishing Company Co., 1995. [7] R. Bennett und R. Wallsgrove, „Sesondary metabolites in plant defence mechanism,“ New Phytol., Bd. 127, Nr. 4, pp. 617-633, 1994. [8] A. Crozier, M. Clifford und H. Ashihara, Plant Metabolies: Occurence, Structure and Role in the Human Diet, NJ, USA: John Wiley & Sons, 2008. [9] Platzer, M.; Kiese, S. „Pflanzliche Antioxidantien für Bioschmierstoffanwendungen“, Tribologie + Schmierungstechnik, 69. Jahrgang, 2022, 3, 39- 43. [10] Rudnick, L. R. Synthetics, mineral oils, and bio-based ubricants: chemistry and technology Second (ed Rudnick, L. R.) isbn: 9781138068216 (CRC Press, London, UK, 2013). [11] Tiwari, R., & Shukla, A. K. (2020). Plant metabolites and their role in health benefits: A brief review. Adv. Pharma. J, 5(2), 47-53. [12] Ma, L.; Zhang, C.; Liu, S. Progress in experimental study of aqueous lubrication. Chin. Sci. Bull. 2012, 57, 2062- 2069. [13] Yilmaz, M.; Mirza, M.; Lohner, T.; Stahl, K. Superlubricity in EHL contacts with water-containing gear fluids. Lubricants 2019, 7, 46. [14] Yilmaz, M.; Lohner, T.; Michaelis, K.; Stahl, K. Minimizing gear friction with water-containing gear fluids. Forsch. Ing. 2019, 83, 327-337. [15] Sagraloff, N.; Dobler, A.; Tobie, T.; Stahl, K.; Ostrowski, J. Development of an oil free water-based lubricant for gear applications. Lubricants 2019, 7, 33. [16] Rahman, M.H.; Warneke, H.; Webbert, H.; Rodriguez, J.; Austin, E.; Tokunaga, K.; Rajak, D.K.; Menezes, P.L. Water-based lubricants: Development, properties, and performances. Lubricants 2021, 9, 73. [17] Martin, J.M.; De Barros-Bouchet, M.I. Water-like lubrication of hard contacts by polyhydric alcohols. In Aqueous Lubrication; IISc Research Monographs Series; Co-Published with Indian Institute of Science (IISc): Bangalore, India, 2011; Volume 3, pp. 219-235. [18] Michaelis, J.U.; Kiese, S.; Amann, T.; Folland, C.; Asam, T.; Eisner, P. Thickening Properties of Carboxymethyl Cellulose in Aqueous Lubrication. Lubricants 2023, 11(3), 112. [19] Naik, S.C.; Pittman, J.F.T.; Richardson, J.F.; Lansdown, A.R. Evaluation of hydroxyethyl cellulose ether as a thickener for aqueous lubricants or hydraulic fluids. Wear 1978, 50, 155-168. [20] Jain, S.; Sandhu, P.S.; Malvi, R.; Gupta, B. Cellulose derivatives as thermoresponsive polymer: An overview. J. Appl. Pharm. Sci. 2013, 3, 139-144. [21] Michaelis, J.U.; Kiese, S.; Amann, T.; Folland, C.; Asam, T.; Eisner, P. Thickening Properties of Carboxymethyl Cellulose in Aqueous Lubrication. Lubricants 2023, 11, 112. https: / / doi.org/ 10.3390/ lubricants11030112 Aus Wissenschaft und Forschung 33 Tribologie + Schmierungstechnik · 70. Jahrgang · 6/ 2023 DOI 10.24053/ TuS-2023-0037 TuS_6_2023.qxp_TuS_6_2023 01.02.24 14: 18 Seite 33