Synthesis of ester-based biolubricant from waste cooking oil, water-miscible metalworking fluid blending applications

ABSTRACT:

This study explores the application of bio-lubricants synthesized from trimethylolpropane (TMP) esters derived from waste cooking oil (WCO) in the formulation of water-soluble metalworking fluids. A variety of performance-enhancing additives were selected and blended with bio-lubricants and mineral base oils in different proportions. The resulting formulations were evaluated for physicochemical properties and stability. Analytical results indicated that a formulation containing 12.5% TMP ester and 87.5% 150N mineral base oil (by weight) met the quality standards required for water-soluble metalworking fluids. These findings highlight the potential of replacing conventional mineral base oils with environmentally friendly bio-lubricants, thereby offering a sustainable solution that balances both performance efficiency and environmental responsibility.

Keywords: biolubricant, waste cooking oil, water-miscible metalworking fluids, TMP ester, green synthesis.

1. Introduction

In recent years, the demand for bio-based lubricant products has continued to grow. According to a report by Grand View Research, in 2024 the global biolubricants market was estimated at USD 2.95 billion, and it is projected to reach USD 5.04 billion by 2030, with a compound annual growth rate (CAGR) of 13.7% during the forecast period from 2025 to 2030[1].Biolubricants have been studied for applications across various sectors including engine oils [2][3] hydraulic fluids [4] and neat cutting oils [5]. Research on incorporating biolubricants into water-miscible metalworking fluid (MWF) formulations is still relatively novel and limited. Unlike conventional lubricants, water-miscible MWFs are used in diluted water forms that form stable emulsions, making their recovery and reuse after application highly complex. Currently, most water-miscible MWFs are derived from mineral oils, which have low biodegradability and pose notable environmental risks. The transition from mineral-based oils to bio-based oils in MWF formulations is a new direction that promotes sustainability and reduces ecological impact. Studies on bio-based MWFs derived from vegetable oils have demonstrated superior performance and safety for both the environment and human health [6].

However, vegetable oil–based lubricants have also faced criticism due to concerns about disrupting food supply chains and contributing to deforestation[7]. Data from Global Data indicates that in 2022, the total volume of global waste cooking oil (WCO) reached approximately 3.7 billion gallons and is forecast to hit 10 billion gallons by 2030[8]. Asia is the largest supplier region. With such abundant reserves, choosing WCO to produce sustainable alternatives to non-renewable mineral oils is a viable and eco-conscious solution.

The synthesis technology using CaO catalyst offers a green and heterogeneous pathway that allows for easy recovery, abundant raw material availability, low cost, and simplified processing - opening the door to industrial-scale production and commercialization in the near future. In water-miscible MWFs formulations, substituting 12.5% of mineral base oil with TMP ester-based biolubricant yields emulsions that remain stable under storage conditions from 10 - 40°C, demonstrate excellent emulsifiable, and deliver up to 3% rust inhibition on iron chips - surpassing the performance of conventional products currently available on the market.

2. Experimental

2.1. Material

WCO was collected from the canteen of AP SAIGON PETRO Joint Stock Company, located in Cat Lai Ward, Ho Chi Minh City, Vietnam. Calcium oxide (CaO), methanol (CH₃OH), and potassium hydroxide (KOH) were obtained from Xilong Scientific. 1,1,1-Tris-(hydroxymethyl)-propane (98%) was supplied by Thermo Scientific. The emulsion additive package was sourced from Additiv Chemie Luer, while the biocide additive came from Troy Corporation. The mineral base oil 150N Group II was manufactured by GS Caltex. All chemicals used in the analysis met purity standards and were produced by Merck, Germany.

In this study, the first step involved converting the waste cooking oil into fatty acid methyl esters (WCO-FAME) through methanol esterification, followed by purification steps to isolate WCO-FAME. This intermediate was then used to synthesize WCO-TMP ester via esterification with TMP using CaO as a catalyst. The effects of various reaction parameters on the conversion of WCO to WCO-FAME (step one) and WCO-FAME to WCO-TMP ester (step two). The WCO-TMP ester was subsequently blended with 150N Group II mineral oil at different ratios, along with fixed additive components, to formulate a water-miscible metalworking fluid (Soluble Oil). The physicochemical properties and formulation stability were evaluated. Details of the physicochemical tests related to both WCO-TMP ester and Soluble Oil are presented in the results and discussion sections.

2.2. Methodology

Step 1: Transesterification of WCO into WCO-FAME

The initial esterification step was carried out in a 2000 mL three-neck round-bottom flask equipped with a thermometer, a magnetic stirrer with a heating base, a 2 L water bath for thermal stabilization of the reaction, and a condenser column. The reaction was conducted under atmospheric pressure. Prior to the reaction, WCO was preheated in an oven at 110 °C to remove moisture. The reaction flask was dried to eliminate residual humidity, then charged with a reaction mixture consisting of WCO, methanol, and KOH catalyst. The methanol-to-WCO ratio was 6:1, and the amount of KOH used was 1 wt%.

Warm water in the external bath and heating equipment were used to maintain the reaction temperature at 55 °C, with a stirring rate of 1200 rpm and a reaction duration of 60 minutes. After completion, the reaction mixture was poured into a separating funnel to isolate the WCO-FAME phase from glycerol.

The WCO-FAME layer was then washed repeatedly with warm distilled water (50–70 °C) until a neutral pH was reached. Finally, the product was dried in an oven at 110 °C to remove residual water. Each experiment was conducted three times, and the data are reported as mean values accompanied by their corresponding standard deviations.

Step 2: Transesterification of WCO-FAME into WCO-TMP ester

The reaction was conducted in a 500 mL three-neck round-bottom flask equipped with a reflux condenser, thermometer, heated magnetic stirrer, and glass wool to maintain a consistent reaction temperature.

A vacuum pump fitted with a gas release valve was connected to the top of the condenser to regulate the vacuum pressure at 0.5 atm. A defined amount of TMP was introduced into the reaction flask and heated to 80 °C, stirred at 1200 rpm under a vacuum of 0.5 atm.

The temperature was then raised to 90 °C for 10 minutes to remove moisture. Subsequently, a calculated amount of WCO-FAME, based on a molar ratio of TMP to WCO-FAME of 3:1, was added to the flask, and the reaction mixture was heated to 140 °C. A catalyst was then added at 2 wt% of the total mass, and the reaction was carried out under vacuum conditions (0.5 atm) for 3.5 hours. It is important to adjust the stirring speed to prevent foam overflow. Upon completion, the post-reaction mixture was extracted using ethyl acetate to obtain WCO-TMP ester [9].

Each experiment was conducted three times, and the data are reported as mean values accompanied by their corresponding standard deviations. FTIR analysis was performed to identify the functional groups in the molecular structure. The physicochemical properties of the ester were also evaluated, including of kinematic viscosity, viscosity index, acid value and density at 15 °C

2.3. Formulation of Water-Miscible Metalworking Fluid

The synthesized WCO-TMP ester was gradually blended with 150N Group II mineral oil, along with fixed proportions of emulsion additive package and biocide additive. These two additives were maintained at constant weight percentages in the formulation to evaluate the compatibility of WCO-TMP ester as a substitute for 150N Group II mineral oil in cutting fluid formulations. The properties of the formulated oil were assessed, including of kinematic viscosity at 40 °C (ASTM D445) and pH value (ASTM D1287). The properties of the 5% emulsion solution derived from the formulated oil were evaluated as follows: rust test (IP287), pH value, emulsion stability, and solubility based on the NF T60-187 method.

3. Results and discussions

3.1. Composition of WCO

The waste cooking oil (WCO) used as a feedstock for the esterification reaction in this study possesses the properties listed in Table 1.

Table 1. Physicochemical properties of the waste cooking oil used in the study

It is evident that the composition of WCO contains a high proportion of unsaturated fatty acids. Therefore, the physicochemical properties of the synthesized WCO-FAME and WCO-TMP ester will be influenced by the fatty acid profile present in the original WCO feedstock.

3.2. Transesterification of WCO into WCO-FAME

Determination of the composition and relative content of volatile compounds in the WCO FAME sample using GC-MS. The oil sample was esterified with 3% sulfuric acid (H₂SO₄) and refluxed for 1.5 hours.

GC-MS conditions: Gas chromatography GC-2030 (Shimadzu, Japan) coupled with GC-MS-QP2020 (Shimadzu, Japan). A capillary column Rxi-5MS (length 30 m, inner diameter 0.25 mm, film thickness 0.25 µm, Shimadzu, Japan) was used for the analysis. Temperature program: Initial temperature: 50 °C, held for 4 minutes, increased to 80 °C at 2 °C/min, then to 150 °C at 5 °C/min, continued to 200 °C at 10 °C/min and finally raised to 300 °C at 20 °C/min and held for 3 minutes. 

Other conditions: ion chamber temperature: 230 °C, Carrier gas: Helium at a flow rate of 1.69 mL/min, Split ratio: 1:10, column head pressure: 100 kPa. The results are presented in Figure 2.

The GC-MS results indicate that most of the fatty acids have been successfully converted into methyl esters.

The physicochemical properties of WCO-FAME synthesized from WCO are presented in Table 2 below.

Table 2. Physicochemical properties of WCO-FAME

The viscosity of WCO-FAME is significantly lower than that of the original WCO feedstock, and the viscosity index as well as acid value parameters have shown remarkable improvement.

Transesterification of WCO-FAME into WCO-TMP Ester

FTIR spectra (Fig. 3, Fig. 4) showed that the C=O bond stretching of the ester was also observed at 1742.98 cm⁻¹ in the WCO-TMP ester. The –CH₂ functional group was identified with a peak at 1464.76 cm⁻¹, and the peak at 1172.40 cm⁻¹ was attributed to the C–O bond stretching in the ester.

The physicochemical properties of WCO-FAME are presented in Table 3 below.

Table 3. Physicochemical properties of WCO-TMP Ester

The properties of the synthesized WCO-TMP ester are comparable to those of ISO VG 22 mineral base oil; however, the ester exhibits a significantly higher viscosity index, indicating superior thermo-viscous characteristics.

Formulation of Water-Miscible Metalworking Fluid

The compatibility of WCO-TMP ester with mineral base oil in water-miscible cutting fluid formulations is evaluated through the survey table 4.

Table 4. Surveyed oil formulations

The physicochemical properties of the water-soluble metalworking fluid (concentrate) and its 5% emulsion in water are evaluated in detail in Table 4.

Table 4. Evaluation of the physicochemical properties of the oil formulations

The results show that a 12.5% conversion rate of 150N base oil in the water-miscible MWF formulation maintains the stability of the formulation, meeting usage requirements.

4. Conclusions

The WCO-TMP ester was successfully synthesized through a two-step transesterification process at a temperature of 140 °C, with a molar ratio of WCO-FAME to TMP of 3:1, using 1.5 wt% of green CaO catalyst. The reaction conditions were maintained under a vacuum pressure of 0.5 atm for 3.5 hours. The successful formulation of water-miscible metalworking fluid using TMP ester derived from waste cooking oil at a substitution ratio of 12.5% not only confirms its technical feasibility, but also reinforces the strategic value of circular economy principles in lubricant development.

REFERENCES

[1]     “https://www.grandviewresearch.com/industry-analysis/biolubricants-industry#.”

[2]     M. Gul et al., “Effect of TMP-based-cottonseed oil-biolubricant blends on tribological behavior of cylinder liner-piston ring combinations,” Fuel, vol. 278, 2020.

[3]     S. K. Tulashie and F. Kotoka, “The potential of castor, palm kernel, and coconut oils as biolubricant base oil via chemical modification and formulation,” Therm. Sci. Eng. Prog., vol. 16, 2020, doi: 10.1016/j.tsep.2020.100480.

[4]     K. Kamalakar, A. K. Rajak, R. B. N. Prasad, and M. S. L. Karuna, “Rubber seed oil-based biolubricant base stocks: A potential source for hydraulic oils,” Ind. Crops Prod., vol. 51, pp. 249–257, 2013, doi: 10.1016/j.indcrop.2013.08.058.

[5]     S. Edla, A. Krishna, G. V. S. Karthik, M. M. Arif, and S. Rani, “Potential use of transesterified vegetable oil blends as base stocks for metalworking fluids and cutting forces prediction using machine learning tool,” Biomass Convers. Biorefinery, vol. 13, no. 12, pp. 10665–10676, 2023, doi: 10.1007/s13399-021-01952-6.

[6]     J. John, M. Bhattacharya, and P. C. Raynor, “Emulsions containing vegetable oils for cutting fluid application,” Colloids Surfaces A Physicochem. Eng. Asp., vol. 237, no. 1–3, pp. 141–150, 2004, doi: 10.1016/j.colsurfa.2003.12.029.

[7]     J. A. Cecilia, D. B. Plata, R. M. A. Saboya, F. M. T. de Luna, C. L. Cavalcante, and E. Rodríguez-Castellón, “An overview of the biolubricant production process: Challenges and future perspectives,” Processes, vol. 8, no. 3, 2020, doi: 10.3390/pr8030257.

[8]     GlobalData, “UCO Supply Outlook,” no. August, pp. 1–6, 2023, [Online]. Available: www.globaldata.com

Tổng hợp chất bôi trơn sinh học gốc este từ dầu ăn thải, ứng dụng pha trộn chất lỏng

gia công kim loại hòa tan trong nước

Lê Văn Sinh¹

 Lê Nguyễn Phúc Thiên¹

Trần Tấn Việt²

¹Khoa Kỹ Thuật Hóa học, Trường Đại học Bách Khoa

²Trường Đại học Bách Khoa, Đại học Quốc Gia TP.Hồ Chí Minh

Tóm tắt:

Nghiên cứu trình bày về việc ứng dụng dầu bôi trơn sinh học gốc este TMP có nguồn gốc từ dầu ăn thải (WCO) vào công thức dung dịch dầu gia công kim loại hòa tan trong nước. Các chất phụ gia tăng cường hiệu suất phù hợp đã được lựa chọn và phối trộn với dầu bôi trơn sinh học và dầu gốc khoáng theo các tỷ lệ khác nhau. Các đặc tính lý hóa và độ ổn định của sản phẩm sau pha trộn đã được phân tích và đánh giá. Kết quả nghiên cứu cho thấy tỷ lệ este TMP với dầu gốc khoáng 150N là 12,5/87,5 (khối lượng/khối lượng) đã đáp ứng các tiêu chuẩn chất lượng cần thiết cho dầu gia công kim loại hòa tan trong nước. Nghiên cứu này mở ra một hướng đi đầy hứa hẹn cho việc thay thế dầu gốc khoáng bằng dầu bôi trơn sinh học trong công thức pha chế dung dịch gia công kim loại hòa tan trong nước. Chất bôi trơn sinh học thân thiện với môi trường sẽ giúp nâng cao tính bền vững và bảo tồn sinh thái.

Từ khóa: dầu nhờn sinh học, dầu ăn thải, chất lỏng gia công kim loại hòa tan trong nước, este TMP, tổng hợp xanh.

[Tạp chí Công Thương - Các kết quả nghiên cứu khoa học và ứng dụng công nghệ, Số 23 năm 2025]