Thermal and rheological properties of soapberry Sapindus saponaria L. (Sapindaceae) oil biodiesel and its blends with petrodiesel

- Biodiesel is an alternative fuel that is steadily gaining attention and significance.
- The yield of oil extracted from Sapindus saponaria L. seed is higher than 40% (w/w).
- TGA and DSC are tools aids for finding cold flow properties and economic viability.
- Rheological assessment is useful for characterization of vegetable oils and biofuels.
- A protocol for thermal and rheological analysis of fuels was well established.

The search for alternative energy sources encompasses increasing the capacity of renewable energy, while reducing the amount of carbon dioxide emissions. The nontoxic and biodegradable character of biomass justifies its exploitation as a fuel to generate power. Specifically, Sapindus saponaria L. oil, with its high lipid content, thermal stability, suitable saponification values, and acid and iodine content, seems to be a promising source for the production of biocombustibles. In this work, biodiesel was produced from the methyl esterification of fatty acids from S. saponaria L. oil. In addition, blends were prepared from the mixture of petrodiesel and biofuel in proportions of 5, 10, 15, 20, and 25% (v/v). The biodiesel and its blends were evaluated for their rheological properties, thermal profile (thermogravimetric and differential scanning calorimetric analyses), density, and refractive index. The biodiesel presented a Newtonian behavior and a kinematic viscosity value of 5.029 mm2/s at 40 °C, in accordance with the ASTM D6751, EN 14214, and CNS 15072 standards. The slight increase in the biodiesel's viscosity, compared to that of petrodiesel (3.442 mm2/s), is advantageous because it provides an increased lubricity and reduces engine wear. Blends B5 to B25 also presented kinematic viscosity values in accordance with the ASTM D976, ASTM D7467, EN 590, and CNS 1471 standards. The obtained biodiesel (Tonset = 188.14 °C) showed a superior thermal stability to petrodiesel (Tonset = 102.17 °C). Differential scanning calorimetry revealed the phase transition between the liquid state to the crystallization of fuels at low temperatures. The compilation of these results highlights the potential of S. saponaria L. oil to generate a biofuel suitable for future commercialization.