**4. Conclusions**

The production of biolubricants (kinetics and conversion) was influenced by temperature, concentration of catalyst (titanium isopropoxide), and alcohol/FAME molar ratio. Thus, temperature slightly increased the final yield, with notorious kinetic changes at higher temperatures. Equally, the final conversion was higher, as catalyst concentration increased from low values to intermediate ones, apparently reaching a point (1% *w*/*w*) where it was not dependent on concentration. Finally, as the molar ratio alcohol/FAME was higher, the yield was better. In order to meet both yield and economic factors, intermediate values for temperature and catalyst concentration (T = 160 ◦C, [Catalyst] = 1% *w*/*w*) and the highest alcohol/FAME ratio (alcohol:FAME = 3:1) were considered to be suitable for biolubricant production. Once the main parameters were optimized for biolubricant production, the yield obtained for rapeseed and castor oils was acceptable, exceeding 93% in both cases.

The raw materials studied, rapeseed and castor oils, presented different fatty acid profiles. For the former, the majority fatty acid was oleic acid, whereas for the latter, it was ricinoleic acid. Therefore, the kind of seed selected, among other factors such as pre-harvest conditions, make this initial characterization necessary. Consequently, the proportion of fatty acids (especially the majority one), plays an important role in biolubricant features. Thus, the structure of these fatty acids, including unsaturations, length of the chain, and hydroxyl groups, influenced many parameters of the final biolubricant. For example, the presence of a hydroxyl group in ricinoleic acid could explain the high viscosity of castor oil and its corresponding biodiesel and biolubricant, compared to rapeseed oil. Both biolubricants showed (in general) suitable properties, sometimes better than in the case of the commercial lubricants that were studied.

The kind of alcohol used in biolubricant production seemed to influence some of its characteristics, especially viscosity. The longer and more branched the molecule chain, the higher the viscosity. Consequently, the right choice of the alcohol could optimize the performance of the biolubricant, especially concerning tribology.

For further studies on this subject, analyses to complete characterization of the final biolubricant produced are necessary, paying special attention to IR, mass spectrometry, etc.

**Author Contributions:** Conceptualization, J.M.E., S.N.-D., and N.S.; methodology, S.N.-D. and N.S.; formal analysis, S.N.-D. and N.S.; investigation, J.M.E., S.N.-D. and N.S.; resources, J.M.E. and J.F.G.; data curation, S.N.-D. and N.S.; writing—original draft preparation, S.N.-D.; writing—review and editing, J.M.E., N.S. and J.F.G.; visualization, J.M.E. and J.F.G.; supervision, J.M.E. and J.F.G.; project administration, J.M.E. and J.F.G.; funding acquisition, J.M.E. and J.F.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Junta de Extremadura and FEDER "Fondos Europeos de Desarrollo Regional", gran<sup>t</sup> numbers GR18150 and IB18028.

**Acknowledgments:** We would like to thank Junta de Extremadura and FEDER "Fondos Europeos de Desarrollo Regional" for the financial support, and the Research Center "La Orden-Valdesequera" (from Cicytex) for the raw material provided for this research work.

**Conflicts of Interest:** The authors declare no conflict of interest.
