Stearic Acid as Additive for Bio Lubricant Formulations in Neem Seed Oil

Abstract

Vegetable oils are emerging as a promising source of renewable and eco-friendly lubricants due to their excellent lubricity, high thermal stability, and biodegradability. This study evaluates and compares the lubricant properties and environmental impacts of Neem seed oil with and without stearic acid additive to commercially available mineral oil SAE20W40. Key lubricant properties such as tribological, thermal, and rheological properties, as well as environmental effects like biodegradability, are examined. Neem seed oil is mixed with varying proportions of stearic acid.

The results indicate that the coefficient of friction and wear scar diameter of Neem seed oil combined with stearic acid are superior to plain Neem oil and mineral oil separately. While the addition of stearic acid has a negligible impact on viscosity, it acts as a viscosity index booster in Neem seed oil. Furthermore, stearic acid improves the pour point of the base oil. Biodegradability and toxicity of Neem seed oil with and without stearic acid are evaluated and compared with SAE20W40.

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Keywords

Neem seed oil, Stearic acid, Thermal stability, Biodegradability, Viscosity

1. Introduction

The lubricants commonly used today are primarily mineral-based and derived from petroleum products. These lubricants are predominantly used in automotive applications and gear lubrication. Vegetable oils offer an environmentally friendly, non-toxic, renewable, and biodegradable alternative to conventional mineral oils. Growing concerns about the environmental impact of mineral oils, which can accumulate and pollute the environment over time, have spurred interest in vegetable oils as a basis for eco-friendly bio lubricant formulations [1]. Vegetable oils possess low toxicity, anti-wear properties, and biodegradability, making them a promising choice for lubrication applications.

Efforts to develop and maintain these desirable properties in vegetable oils are crucial for their continued adoption.

Past research has focused on the development of vegetable oils as potential bio lubricants. Studies have explored the anti-wear properties of oils such as soybean oil and olive oil with various additives, comparing them to conventional lubricants [5]. Viscosity is a critical parameter influencing lubricant performance. Vegetable oils typically have lower viscosity compared to mineral oils, and chemical additives can be used to modify viscosity. Biodegradability and toxicity have also been evaluated for vegetable oils, along with their thermal properties [2] [3].

2. Experiments

2.1 Materials

The Neem oil used in this study was extracted from Neem seeds and obtained from National Oil Traders in Coimbatore, Tamil Nadu, India. Stearic acid pellets with a density of 0.847 g/cm³ and a melting range of 54-56°C were sourced from Nice Chemicals (P) LTD in Edappally, Kochi, Kerala, India.

2.2 Preparation of Bio Lubricating Oil Formulations

The main sample formulation of Neem seed oil was prepared using varying proportions of stearic acid, ranging from 0.5% to 2.5% by weight. Stearic acid was heated with Neem seed oil at 70ºC for 10 minutes and thoroughly mixed using an ultrasonic probe (Vibra-cell VCX750) for an additional 10 minutes, resulting in a homogeneous clear solution.

2.3 Measurement of Coefficient of Friction and Wear

The tribological properties, including the coefficient of friction and wear scar diameter, were determined using a four-ball tester. This tester consists of one rotating ball in contact with three fixed balls within a ball pot. The rotating ball was spun at 1200 rpm for 3600 seconds at a temperature of 75ºC. A chromium steel ball with a diameter of 12.7mm and a hardness of 64HN was used. The standard ASTM D4172 B was followed for evaluating the coefficient of friction, and wear scar diameter was measured using an image acquisition system.

2.4 Measurement of Viscosity

Dynamic viscosity of the samples was measured using a Brookfield LVDV2T viscometer according to ASTM D2983-09 standard. The viscosity index was also evaluated following ASTM D2270.

2.5 Measurement of Pour Point

Thermal analysis, specifically pour point evaluation, was performed using Differential Scanning Calorimetry (DSC) for the best-formulated sample, Neem seed oil with 2% stearic acid, as well as plain Neem oil and SAE20W40.

2.6 Biodegradability

The biodegradability test was conducted using a bacterial growth medium. Pseudomonas aeruginosa bacteria were cultured in the oil samples with dimethyl sulphoxide (DMSO) at a concentration of 100µg/ml. Incubation was carried out at 28ºC with 40% RH, and the increase in biodegradability indicated bacterial growth by oxidizing the carbon chains in the samples.

3. Results and Discussions

The tribological properties, coefficient of friction, and wear scar diameter, were measured for the formulated oils. Table 1 shows the variation in the coefficient of friction for Neem seed oil, Neem seed oil with various proportions of stearic acid, and SAE20W40. Notably, a significant reduction in the coefficient of friction is observed with the addition of 2% stearic acid to Neem oil.

Table 1: Coefficient of Friction for Different Lubricants

Table 2 exhibits a similar trend as the coefficient of friction, where the wear scar diameter decreases up to a concentration of 2% stearic acid with the base oil and then increases. This initial decrease and subsequent increase can be attributed to the additive concentration.

Table 2: Wear Scar Diameter for Different Lubricants

The presence of fatty acids in Neem seed oil results in a lower coefficient of friction due to the carboxylic groups' ability to adsorb on metallic surfaces. This decreases the rate of friction and is influenced by oil structure and content. Neem oil + 2% stearic acid provides the best results in terms of both coefficient of friction and wear scar diameter, outperforming mineral oil SAE20W40.

Viscosity measurements in Table 3 reveal that the addition of stearic acid has a minimal impact on the oil's viscosity. Mineral oil, on the other hand, exhibits more than double the viscosity of Neem oil + stearic acid samples.

Table 3: Viscosity Measurements for Different Lubricants at Various Temperatures

Table 4 illustrates that Neem oil + 2% stearic acid exhibits a superior viscosity index compared to mineral oil SAE20W40.

Table 4: Viscosity Index Comparison

Differential Scanning Calorimetry (DSC) results  demonstrate that the addition of anti-wear additive stearic acid reduces the pour point of Neem seed oil, making it more thermally stable.

Table 5 presents the pour points of the oil samples, further highlighting the improved thermal properties of Neem oil + 2% stearic acid.

Table 5: Pour Points of the Oil Samples

5. Conclusion

This study systematically examined the feasibility of developing a bio lubricant from Neem seed oil using an anti-wear additive, stearic acid. The addition of stearic acid significantly improved the wear resistance and coefficient of friction, making the formulated oil superior to mineral oil SAE20W40. Although the viscosity of the formulated sample remained consistent and lower than SAE20W40 in the temperature range of 40°C - 100ºC, the viscosity index was enhanced, making stearic acid a viable viscosity improver alongside an effective anti-wear additive. Furthermore, the addition of stearic acid lowered the pour point of plain Neem oil, although it did not reach the level of SAE20W40. Neem seed oil demonstrated remarkable biodegradability compared to mineral oil.

6. References

1. M. Farooq, A. Ramli, "The study of wear behavior of 12-hydroxystearic acid in vegetable oil," Journal of Applied Science, 11(8), 1381-1385, 2011.
2. S. Rani, "The evaluation of lubricant properties and environmental effect of bio lubricant developed from rice bran oil," International Journal of Surface Engineering, 11(5), 2017.
3. S. Rani, M. L. Joy, K. Prabhakaran Nair, "A comparative study of polymeric additives as biodegradable viscosity boosters for biolubricant formulations," Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology.
4. A. Adhvaryu, S. Z. Efhan, "Tribological studies of thermally and chemically modified vegetable oils for use as environmentally friendly lubricants," Wear, 257, 2004, 359-367.
5. U.S. Choi, Y.J. Chun et al., "Tribological behavior of some antiwear additives in vegetable oil," Tribology International, vol. 30, no. 9, pp. 677-683, 1997.