# Studying the Tribological Properties of Coffee Oil-Loaded Water-Based Green Lubricant
## 2 Materials and Methods
### Extraction and Analysis of Coffee Oil  
The coffee oil (CO) was extracted from the collected spent coffee grounds. First, the obtained coffee grounds were dried to constant mass at 40 \({}^{\circ}\)C in a Binder dryer. Then, the dry coffee grounds (50 \(\pm\) 5 g) were placed in a filter paper tube, and the extraction was carried out in a Soxhlet apparatus using hexane solvent. A 24 h extraction duration was chosen to obtain the maximum yield of coffee oil. After the extraction, the solvent was removed using the IKA RV 06-ML 1-B vacuum rotary distiller. The resulting coffee oil was collected in a glass container, sealed, and stored in a refrigerator (+4 \({}^{\circ}\)C) for further investigation.  
Table 1 summarises the properties of extracted coffee oil, comparing it with the results published by other researchers. The yield of extracted coffee oil was 11 wt.% of the dry coffee grounds. This result was similar to that reported by [PERSON] et al. [21] (12.70%) and [PERSON] et al. [22] (10-13%). The difference in coffee grounds' moisture, particle size, brewing method, and coffee bean type could affect the extraction yield.  
The acid number is a property of the oil, directly indicating its quality. The acid number corresponds to the amount of free fatty acids in the oil and is defined as the KOH required to neutralise the free fatty acids present in the oil. The acidity of extracted coffee oil was 31.14%, corresponding to 61.94 mg KOH/g acid number. This result is much higher than that observed in coffee oils analysed in other studies, i.e., 5-1.7 times higher [14; 21; 22]. The higher acid number is probably the result of direct contact between grounded coffee beans and hot water during the coffee production process.  
The fatty acid composition depends on the type of coffee beans. Therefore, the properties and performances also will vary from type to type. The fatty acid composition observed in the extracted coffee oil is presented in Table 2. It was found that investigated coffee oil is mainly composed of saturated fatty acids. As a result, the extracted CO possess a high pour point of 25 \({}^{\circ}\)C. The CO mainly contains palmitic (C16:0) 37.26% and linoleic (C18:2) 36.59% fatty acids. In addition, oleic (C18:1) 9.26% and stearic (C18:0) 8.79% acids were found. Table 2 also summarises CO composition results published by other researchers.  
\begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Properties** & **This Study** & **[PERSON] et al. [14]** & **[PERSON] et al. [21]** & **[PERSON] et al. [22]** \\ \hline Acid value, mg KOH/g & 61.94 \(\pm\) 0.23 & 15.42 & 35.62 \(\pm\) 0.30 & 12.20 \\ Acidity, \% & 31.14 \(\pm\) 0.42 & & & 6.14 \\ Oil yield, \% (dry wt) & 11 \(\pm\) 0.25 & & 12.70 \(\pm\) 0.18 & 10–13 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Acidity and yield of extracted coffee oil and their comparison with the results obtained by other researchers.  
It was found that the coffee oil composition can vary in broad intervals. The coffee oil extracted in this study has a composition similar to that observed by [PERSON] et al. [21]. However, [PERSON] et al. [14] presented results showing a much higher amount of unsaturated fatty acids, while the results obtained by [PERSON] et al. [22] show the opposite--a very high amount of saturated fatty acids.