Epoxidation of mixture of waste cooking oil and soybean oil with different compositions

Epoxidation of soybean oil (SO) blended with waste cooking oil (WCO) of different mass ratio (90:10, 80:20, 70:30, 60:40 and 50:50) was carried out at reaction temperature of 65 °C and reaction time of 7 h. Physicochemical properties of the reagent and product such as viscosity at 40 °C and 100 °C, viscosity index, iodine index as well as FT-IR spectral analysis are studied to investigate the effect of the mixing ratio of SO and WCO on C=C conversion and physical properties of epoxidized product. The experiment results showed that conversion of C=C in the feed oil of different blending ratio through epoxidation at 65 °C and 7 h could reach high value (almost 98 %) with corresponding Iodine Value of all products of nearly 2. The viscosity of epoxidized oil at both 40 °C and 100 °C increased remarkably compared to the values of soybean oil while viscosity index decreased. However, the VI of epoxidized products are in the range of requirement for lubricating oil. Among all samples studied, the mixture of SO and WCO of 90:10 (mass ratio) had the best lubricating properties, with the highest viscosity at both 40 °C and 100 °C (138.22 and 18.50 cSt) and viscosity index (151).


Introduction
Mineral oil is the main base oil used in manufacturing lubricants besides synthetic and vegetable oil [1].However, the depletion of crude oil and environmental concerns lead to increasing interest in renewable and biodegradable lubricants such as vegetable oils.Triglycerides in vegetable oils are the esters of glycerol with fatty acids of different Carbon chain lengths (14-22 Carbon atoms) and unsaturation degree which causes low oxidation stability.Triglycerides with similar lengths of fatty acids and different degrees of unsaturation are the main components of most vegetable oils [2].The presence of C=C bond in vegetable oils provides reaction sites for the preparation and modification of various polymers [3,4] or epoxidation to form value-added products, epoxidized vegetable oil [5].Among various vegetable oils, soybean oil is available with high proportion which has lower iodine value and higher kinematic viscosity [6].However, conversion of soybean oil into epoxidized product, applied in lubricants' industry, could compete with the use of soybean oil as food for human.Therefore, waste cooking oil (WCO) originating from animal and vegetable oil becomes the interest of scientists in research to partially or totally replace edible oil in renewable energy generation.This is also significant in reducing environmental pollution from the direct disposal of this waste cooking oil [7].
IOP Publishing doi:10.1088/1755-1315/1340/1/012006 2 Epoxidation of vegetable oils has a high potential for various applications, such as usage as a plasticizer or as an additive to lubricating base oils to increase viscosity and lubricity index [8].There are two kinds of catalysts used in the epoxidation of vegetable oil: homogeneous (inorganic acid like H2SO4, H3PO4, HNO3 and HCl [9,10]) and heterogeneous (sulfonated ion-exchange resins [11] and sulfonated metal oxides [12]).Although using heterogeneous catalysts could provide a feasible way to separate the reaction mixture, homogeneous catalysts are still applicable because of their high performance.It has been reported that the type of inorganic acids has different efficiency in epoxidation.The catalytic effectiveness of inorganic acid decreases in the following order: sulphuric acid > phosphoric acid > nitric acid > hydrochloric acid.H2SO4 is reported to booster epoxidation and yield high content of epoxy rings in the product [10].Epoxidation of waste cooking oil using H2SO4 to generate performic acid shows the best yield of product at 55°C [13].From the literature, there has been no report on the effect of the mixing ratio of WCO and vegetable oil on epoxidation and properties of epoxidized product.
Hence, in this study, epoxidation of mixture of soybean oil and WCO of various blending ratio was investigated and some properties such as viscosity, viscosity index and functional group of epoxidised oil were analysed.

Epoxidation reaction
2.1.1.Materials.soybean oil purchased from Tuong An company, household waste cooking oil, H2O2 (XiLong, 30%), CH3COOH (XiLong, 99.5%).2.1.2.Proceduce.Epoxidation of blended soybean oil and waste cooking oil with different mass ratio, varying from 90:10 to 50:50 was carried out.The amount of feed oil and other reactants were calculated to have C=C (oil)/CH3COOH/H2O2 molar ratio of 1:2:4.Reaction temperature was set at 65 °C and epoxidation duration was 7 h.Reaction mixture was then neutralized by water and diethyl ether to enhance separation of epoxidized oil [13].

Viscosity measurement and viscosity index determination
Kinematic viscosity was determined using a Redwood viscometer by measuring the time of flowing (t) through a glass capillary tube of definite diameter, kept at 40 °C and 100 °C of the sample.The kinematic viscosity value (in cSt) was obtained by multiplying the flow time t (seconds) and the viscometer constant C of the viscometer used.Viscosity index is deduced from the viscosity of the sample at 40°C and 100°C according to ASTM D2270.

Determination of Iodine index
Iodine index of the samples was determined by titration method according to ASTM D5768.

Fourier Transform Infrared Spectroscopy (FTIR)
The analysis of the chemical structure of the samples was performed using Fourier Transform Infrared Spectroscopy (FTIR).

Epoxidation of blended soybean oil with waste cooking oil of different ratio 3.1.1. Iodine value of samples.
Iodine value (IV) indicates the number of C=C bond in the sample and C=C conversion could be deduced by measuring IV change through epoxidation.Initial IV of soybean oil (SO), waste cooking oil (WCO) and their mixture are presented in Table 1.As could be seen, IV of these sample are so high that they have low oxidation stability and their structure could be changed in applications at high temperature.Through epoxidation, IV of all blended samples is as low as almost 2 and C=C conversion of all the samples is as high as 98%.There is almost no obvious difference in C=C conversion of all blended sample with different composition of soybean oil and waste cooking oil.This fact indicates that epoxidation conditions used in this study are effective to convert majority of C=C bonds in the feed oil, being independent on the blending ratio of SO and WCO from 90:10 to 50:50.Epoxidized oil is an epoxidized derivative of a mixture of esters of glycerol with various saturated and unsaturated fatty acids.The viscosity of the unmodified oils decreases as the level of oil unsaturation increases.However, the viscosity of the epoxidized oils increases slightly as the level of epoxidation increases.Besides, an increase in the viscosity of epoxidised oil could be explained by an increase in the molecular weight of epoxidised oil compared to initial oil due to addition of oxygen atoms at C=C bonds [12].Moreover, the higher polarity of epoxidised oil, leading to a stronger intermolecular force between epoxidised oil molecule, is another reason for the higher viscosity of the obtained product.The viscosity at 40°C, 100°C and viscosity index of SO, epoxidised oil obtained through epoxidation of blended SO with WCO are presented in Table 2 and Figure 1.As it is shown, the viscosity of epoxidised oils with different blending ratio between SO and WCO at both 40°C and 100°C are enhanced strongly compared to initial soybean oil.Moreover, the viscosity of the epoxidized oils increases slightly with the blending ratio between SO and WCO.The viscosity index (VI) was found to be 133 to 150 for epoxidized oils with different blending ratio of SO and WCO in the feed obtained after 7 hours of reaction at 65°C (Table 2).Although VI of epoxidized oil is lower than the one of soybean oil, the VI of these products was found to be within the range of a standard lubricant [14].These VI values are consistent with plant-based bio-lubricants, such as epoxidized fatty acid waste cooking oil methyl esters (157) [14], epoxidized soybean oil open-loop (137-149) [15], the methyl ester of 9,10-palmitoyloxy-acetoxy stearic acid (171), and the methyl ester of stearic acid 9,10-lauroyloxy-capronoyloxy (137) [16].According to data reported by Rudnick et al [17], epoxidized oils obtained in this study could meet the requirements of ISO VG 32 and ISO VG 46 based on the average range of viscosity values and viscosity index compared to International Standards Organization Viscosity Grade (ISO VG).Hernández-Sierra et al [18] revealed that bio-based lubricant showed lower friction value thicker of the lubricating film because of higher viscosity and pressure-viscosity coefficient than mineral lubricating oil.Bio-lubricants derived from vegetable oils have better lubricating and anti-wear properties than those used in mechanical systems.Those characteristics indicate that the prepared lubricants have no shortcomings for technical applications, making vegetable oil-based lubricants a good choice for the production of green lubricants.In order to obtain a higher viscosity of lubricating oils for engine, automotive, and industrial gear applications, various additive could be added to the prepared sample.

FTIR spectra
The FT-IR spectra of WCO, SO, epoxidised oil obtained from epoxidation of blended SO with WCO of 50:50 and 90:10 (mass ratio) are presented in Figure 2.
It could be asserted that absorption peak at 1376 cm -1 , 1461 cm -1 and 2854-2924 cm -1 characterising the presence of CH3, CH2 and CH groups in all the samples.Moreover, the ester functional group (O-C=O) is confirmed by the absorption peaks at 722 and 1743 cm -1 in all samples.It had been reported that the presence of C=C bond is proved by the absorption peak at 3007 cm -1 in the FT-IR spectrum of canola oil.Besides, the presence of epoxy rings in epoxy canola oil structure was claimed by the absorption peak at 831 cm -1 [19].In this study, soybean oil and waste cooking oil had the peak of C=C at 3008 cm -1 (Figure 2a and 2b) while both epoxidised products of different blending ratio (50:50 and 90:10) did not have this peak (Figure 2c and 2d).This fact confirms that epoxidation with C=C/CH3COOH/H2O2 ratio of 1:2:4, reaction temperature of 65°C and duration of 7 h could convert almost C=C, being consistent with conversion of 98 % as listed in Table 1.In addition, the presence of O-H bond in the epoxidised product with blending ratio of SO and WCO of 50:50 and 90:10 was confirmed by the appearance of an absorption peak in the range of 3260-3570 cm -1 in the FT-IR spectra, caused by the bending and stretching of O-H bond.This evidence supported the fact that some oxirane ring in the epoxidised oil have been opened in the side reaction -oxirane ring opening.Figure 2c and 2d exhibits the FT-IR spectrum of two epoxidized products resulted through epoxidation of soybean oil at C=C/H2O2/CH3COOH of 1:2:4 within 7 h at 65°C.As could be seen, epoxy band is noted to appear at around 849 cm -1 confirming conversion of C=C in soybean oil blended with WCO into epoxide.

Conclusions
Among different blending ratio of soybean oil and waste cooking oil, the sample with 90% SO and 10% WCO could undergo epoxidation at 65°C within 7 h with 98% conversion of C=C bonds to yield epoxidized product with the highest viscosity and medium viscosity index.These results revealed that the blending ratio did not significantly affect C=C conversion through epoxidation.Meanwhile, from a point of view on the physicochemical properties of epoxidized product, blended oil of 90:10 mass ratio could release a product with the best lubricating properties among studied samples.

Table 1 .
Iodine value of SO, WCO, blended oil, epoxidized oil and C=C conversion through

Table 2 .
Viscosity at 40°C, 100°C and viscosity index of SO, epoxidised oil