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. 2021 Feb 10;6(7):5046–5055. doi: 10.1021/acsomega.0c06254

Interactions between Used Cooking Oil Biodiesel Blends and Elastomer Materials in the Diesel Engine

Zhi-yuan Hu 1,*, Jun Luo 1, Zhang-ying Lu 1, Zhuo Wang 1, Pi-qiang Tan 1, Di-ming Lou 1
PMCID: PMC7905940  PMID: 33644613

Abstract

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Used cooking oil (UCO) biodiesel may be one of the most potential alternative fuels in China to lower the dependency on crude oil for transportation. An experimental study has been conducted to assess the interactions between biodiesel produced from UCO in Shanghai and elastomer materials on high-speed marine diesel engines by immersing elastomer materials into conventional fossil diesel, 5, 10, and 20%, of a volumetric blending ratio of UCO biodiesel and pure UCO biodiesel. The test duration is 168 h at different temperatures of 25, 50, and 70 °C. Meanwhile, the effects of the mixing ratio of UCO biodiesel and the immersion temperature on the compatibility of elastomer materials with UCO biodiesel were analyzed. The results revealed that elastomer materials such as nitrile butadiene rubber (NBR), ethylene propylene diene monomer (EPDM), fluororubber (FKM), and silicone rubber (SR) exposed to biodiesel blends would reveal worse but acceptable changes than those exposed to petroleum diesel, including the slight increase of mass and volume and decline of tensile strength and hardness. FKM, NBR, and SR represented better compatibility with pure UCO biodiesel than diesel, and EPDM showed worse compatibility with UCO biodiesel as the blend ratio rises. In general, the recommended volumetric mixing ratio of UCO biodiesel should be no larger than 20%. The present study could be helpful for the investigation of UCO biodiesel blends as a potential fuel to satisfy the energy demand.

Introduction

In 2020, China’s dependency on foreign crude oil reached 73.5% and the transportation industry remained in the first rank in crude oil consumption.1 To alter the energy consumption structure in China and lower the external dependency on crude oil, utilizing some sustainable biofuels such as ethanol and biodiesel in transportation sectors is becoming increasingly prominent. Biodiesel is one of the most optimum sustainable biofuels and has been widely used in European Union (EU), Brazil, the United States (US), and some Southeast Asia countries. Moreover, Brazil, European Union, and the United States have set a probable target of 20, 10, and 25% of biodiesel blends in the total fuel consumption, respectively, by 2020.2 The traditional feedstock of biodiesel includes soybean oil,3 rapeseed oil,4 palm oil5 and other edible oils,6 jatropha,7 castor8 and other nonedible oils,9,10 and some renewable sources such as used cooking oil (UCO) and animal fats.11 In 2019, over 33 million tons of edible oil were consumed in China, of which about 11.5 million tons are imported from foreign countries.12 There exists no adequate edible oil to manufacture biodiesel and 20–30% of consumed edible oil becomes UCO ultimately in China, hence UCO becomes a potential raw material to produce biodiesel. The high consumption of edible oil ensures large quantities of raw materials for biodiesel, especially in densely populated cities such as Beijing, Shanghai, Shenzhen, and Chongqing. Therefore, UCO biodiesel can be used as one of the alternative transitional fuels and to reduce the dependency on crude oil in China.13,14

Biodiesel consists of many C14–C20 fatty acid alkyl esters (FAMEs). The free fatty acids which are derived from the oxidation process of FAME would cause the fuel delivery elastomeric components swelling up and also could generate a series of problems including fuel line leakage, hose rupture, and seal breakage.15,16 These problems are related with the composition and molecular structural properties of biodiesel, which could also be the major causes of the increased corrosiveness of biodiesel17,18 and the failure of the engine parts.19 At the same time, the properties of biodiesel blend also exhibit significant changes after the contact with the engine parts, including density, kinematic viscosity, and the total acid number.20 Whether the biodiesel can be used in a diesel engine, an experimental study needs to be conducted to illustrate the compatibility between biodiesel and elastomer materials of the engine fuel delivery system. There have been many studies on the compatibility between materials in automotive engine fuel supply systems and single feedstock-based biodiesel, such as canola oil21 and palm oil;22−26 only Zhu27 has investigated the compatibility between elastomer materials of a diesel engine and UCO biodiesel.

Most of the biodiesel compatibility studies are conducted at room temperature (25 °C). Haseeb et al.28 studied the effects of palm oil biodiesel on the mass, volume, hardness, and tensile properties of several commonly used elastomers of diesel engines. They observed that the aforesaid properties of the ethylene propylene diene monomer (EPDM) and chloroprene rubber varied remarkably. Kass29 assessed the compatibility of biodiesel blends with five common elastomers using Hansen solubility parameters and found that aldehydes and short-chain acids are associated with biodiesel degradation and could further affect its compatibility with elastomers. Also, formic acid could generate higher swelling in nitrile butadiene rubber (NBR), fluorocarbon, neoprene, and silicone than what acetaldehyde did. Zhu27 investigated the effects of the molecular structure and feedstock of biodiesel on its compatibility between biodiesel and NBR. It has been reported that the carbon chain length, number of double bonds, and alcohol moiety chain length of the fatty acid ester also have great impact on its compatibility with the NBR. Chandran30 adopted a novel immersion method to investigate the compatibility of biodiesel with fuel delivery system materials under different diesel engine operating conditions. Compared to the typical immersion method, the new methodology can simulate the real working conditions of diesel engines including the variation of pressure and temperature, which are critical to estimate the compatibility of biodiesel with fuel delivery system materials under diesel engine working conditions.

In the fuel supply system of a diesel engine, the temperature of fuel varies greatly from the fuel tank to the fuel injector. With the deepening of research, some scholars have begun to pay attention to the compatibility of fuel system elastomer materials with biodiesel under different temperatures. Coronado31 analyzed the resistance of nitrile rubber (NR) fuel hoses at 25 and 70 °C by utilizing gravimetric tests, tensile strength measurements, and scanning electron microscopy analysis. It has been concluded that the fuel temperature did not result in a significant mass loss of NR fuel hose, while the higher blend ratio of biodiesel weakened the performance of elastomers and caused the phenomenon of swelling. The exposure of hoses to different fuels with the increasing blend ratio of biodiesel led to the degradation of tensile strength. Meenakshi32 investigated the compatibility of B0, B10, B20, and BD100 palm biodiesel blends with automotive polymeric materials including polypthalamide and polyarylamide at 25, 50, and 70 °C. Both polymeric materials revealed a significant mass increase at higher temperature.

The components of FAMEs produced from different feedstocks are also diverse. The ingredients of UCO biodiesel are strongly dependent on the genres of UCO during the production process. There is little research about the compatibility of UCO biodiesel with elastomers at high temperature. Moreover, the blend ratio of biodiesel could be flexible for high-speed marine diesel engines according to the guidelines for marine alternative fuels of the China Classification Society.33 Therefore, this study investigated the effects of conventional fossil diesel, three UCO biodiesel blends, that is, 5, 10, and 20%, and pure UCO biodiesels which are named as B0, B5, B10, B20, and BD100 on NBR, FKM, EPDM, and silicone rubber (SR) at 25, 50, and 70 °C, which is intended to assess the compatibility between UCO biodiesel blends and elastomer parts on high-speed marine diesel engines systematically. This study will be of help in providing valuable information for the usage of UCO biodiesel.

Results and Discussion

Mass Changes

Generally, the mass of elastomer materials could be enhanced when dipped into diesel or biodiesel blends due to the diffusion effect of diesel or biodiesel. Figure 1 exhibits the mass changes of the four tested elastomer materials, that is, NBR, FKM, EPDM, and SR after being exposed to different blend ratios of UCO biodiesel at 25, 50, and 70 °C for 168 h, respectively.

Figure 1.

Figure 1

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Mass changes of different elastomer materials after immersion.

Obviously, the mass of FKM was enlarged slightly as the blend ratio of UCO biodiesel increases; the EPDM reveals a slight mass decrement when the UCO biodiesel blend ratio is less than 20%, while the mass of EPDM increased remarkably in BD100. The mass of NBR and SR increased obviously when the blend ratio of UCO biodiesel is less than 20%, but the amount of increase is much smaller in BD100. Among the four tested elastomer materials, NBR was subjected to the maximal swelling, whereas FKM showed minimum swelling. According to the general principle of “like dissolves like”, the polar substances in elastomers are much more likely to dissolve in polar solvents, while the nonpolar substances are more likely to dissolve in nonpolar solvents.34 This indicates that the polarity of NBR is closer to UCO biodiesel blends, whereas that of FKM is different from UCO biodiesel. Concerning the influence of immersion temperature, as shown in Figure 1, there exist greater mass changes for NBR, EPDM, and SR at 50 than 70 and 25 °C. This can be attributed to the fact that the swelling of rubber has peaked at 50 °C. Komariah35 has reported that the swelling of rubber will increase in the first contact of 300 h and then decrease obviously and finally remain constant at room temperature. Though the immersion time in this paper is 168 h, this phenomenon has illustrated that the higher diffusion with temperature will speed up the peak of swelling. To summarize, the swelling of engine parts can be suppressed effectively under the typical diesel engine operation conditions because the swelling of elastomer materials will be lower than the peak at higher temperature. In any case, the FKM may be an ideal material which can be employed in seal or pipe line systems near the injection pump.

Volume and Hardness Changes

The hardness of elastomers would be lower due to the swelling caused by biodiesel, whereas cross-linking, one of the dissolution mechanisms between biodiesel and elastomer, can inhibit this trend. Figures 2 and 3 display the volume and hardness changes of four tested elastomer materials after dipping into different test fuels at 25, 50, and 70 °C to keep the static immersion for 168 h, respectively. It can be concluded from Figures 3 and 4 that the volume of NBR and SR increased obviously and their hardness correspondingly reduced when immersed into diesel and UCO biodiesel blends. The numerical value of volume and hardness changes for NBR and SR slightly increased when the blend ratio of UCO biodiesel is less than 20% and then decreased sharply in BD100. For EPDM, the trends of volume and hardness changes were almost the same when the blend ratio of UCO biodiesel is less than 20%, and then, the volume increased moderately in BD100 but the hardness decreases obviously. For the FKM, the change of the volume and hardness waved slightly with the increase of the blend ratio of UCO biodiesel. From the perspective of volume and hardness changes, NBR was also subjected to the maximal swelling, whereas FKM showed minimum swelling compared with other tested elastomer materials.

Figure 2.

Figure 2

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Changes of volume for different elastomer materials after immersion.

Figure 3.

Figure 3

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Changes of hardness for different elastomer materials after immersion.

Figure 4.

Figure 4

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Changes of tensile strength for different elastomer materials after immersion.

It can be seen from Figures 2 and 3 that the value of changes for the volume and hardness is not further reduced when the immersion temperature rises from 50 to 70 °C. There is a negligible difference of volume changes for NBR, EPDM, and FKM between 50 and 70 °C. In general, the volume reduction related to the chain softening results from chain scission and the volume increment could be attributed to the chain hardening caused by the cross-linking mechanism.36 The dissolution of the molecular structure in the elastomer, mostly caused by the cross-linking mechanism, plays a dominant role in preventing the volume from sustainable growth, as shown in Figure 2, and causing the reduction of softening, as shown in Figure 3, even though higher temperature leads to an increase in mass. The decline of hardness for NBR, SR, and FKM is reduced with the enlargement of the blend ratio of UCO biodiesel, which means that more molecular structures are dissolved into elastomers in the form of the cross-linking mechanism instead of chain scission. For EPDM, the different performance can be attributed to the reason that the chain scission has played a more important role.

Tensile Strength Changes

Figure 4 shows the tensile strength changes of four tested elastomer materials after being exposed to different UCO biodiesel blends at 25, 50, and 70 °C for 168 h, respectively. It can be summarized from Figure 4 that the tensile strength of NBR and SR decreased deeply than EPDM and FKM upon exposure to UCO biodiesel. With the increase of the blend ratio, the degree of decline of tensile strength for NBR, SR, and FKM decreased, while EPDM reveals the opposite trend.

Among four tested elastomer materials, EPDM is nonpolar and SR, NBR, and FKM have strong polarity, thus they can absorb the solvent with similar polarity gradually by the cross-linking mechanism or chain scission. The major fatty acid methyl esters of biodiesel are more easily absorbed by rubber which has similar polarity. The highest resistance to swelling for FKM could be explained by the small part of unsaturation and metal oxide/hydroxide particles in the elastomer structure.37 Silva38 used comprehensive multiphase (CMP NMR) to obtain information regarding the effect of biodiesel on the elastomer structure and observe the entrapped fuel into the elastomer material cavities and discovered that the higher constraint caused by the elastomer structure is related to the smaller diffusion coefficient, thereby the low diffusion coefficient of FKM may suppress the swelling. Linhares39 also found that the choice of the accelerator occupied an important position in the resistance of the rubber to the biofuel. If the solvent–polymer interactions are more dominant than polymer–polymer interactions, maximum swelling can be obtained.40 In short, it seems that the high mixing ratio of UCO biodiesel would not promote the decline of tensile strength for SR, NBR, and FKM.

Summary for Tested Elastomer Materials

According to the above discussions of mass, volume, hardness, and tensile strength changes of the four tested elastomer materials, that is, NBR, EPDM, FKM, and SR that are exposed to UCO biodiesel blends at different temperatures, the swelling behavior of elastomer materials is affected by the cross-linking and chain scission at the same time. The four tested elastomer materials are more easily subjected to swelling when immersed in UCO biodiesel than in diesel, especially when the blend ratio is larger than 20%. A higher temperature can also promote the swelling of elastomer materials, whereas the swelling process slowed down when the temperature further rises from 50 to 70 °C. FKM shows smaller swelling behavior than NBR, EPDM, and SR in B0, B5, B10, B20, and BD100 among all immersion elastomer materials, thus it is suitable for manufacturing any parts, particularly fuel hose, seals in fuel supply systems of high-speed marine diesel engines fueled by all blend ratios of UCO biodiesel. According to the swelling phenomenon in BD100 compared with B0, B5, B10, and B20, NBR and SR can be more suitable to be applied in fuel supply systems when the engine is fueled with an extremely high blend ratio of UCO biodiesel, even the pure UCO biodiesel. EPDM requires technical improvement when it is employed in a high blend ratio of UCO biodiesel due to its higher swelling in BD100.

In summary, the recommended volumetric blend ratio of UCO should not be larger than 20% when adopting no strongly antiswelling elastomer materials in the engine fuel supply system.

Fuel Property Changes

The acidity and water content of tested UCO biodiesel blends before and after the immersion test were measured to assess the impact of elastomer materials on the physical and chemical properties of the UCO biodiesel blends.Figure 5 represents acidity and water content changes of different UCO biodiesel blends exposed to NBR, EPDM, FKM, and SR at 25 °C. All the acidity and water content of UCO biodiesel blends after the immersion test were lower than the limits of diesel, B5, and BD100 standard in China. It can be concluded that the acidity of biodiesel blends decreased after the immersion test of NBR; this may be the reason why the NBR was subjected to the highest swelling after the immersion test. Also, the water content reveals a significant decrease for four elastomer materials. The diffusion of water from blends to the molecular structure of the elastomer can lower the water content.

Figure 5.

Figure 5

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Changes of acidity and the water content of UCO biodiesel blends at 25 °C.

Figure 6 represents acidity and water content changes of different UCO biodiesel blends exposed to NBR, EPDM, FKM, and SR at 50 °C. The water content still decreases almost after the immersion, which strongly supports the diffusion of water from blends to the molecular structure of the elastomer. The acidity of diesel, B5, B10, and B20 UCO biodiesel blends slightly changed after the immersion test, and the acidity of diesel of BD100 remains significantly decreased. This may be attributed to the fact that the free fatty acid components of UCO biodiesel can be absorbed by the elastomer.

Figure 6.

Figure 6

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Changes of acidity and the water content of UCO biodiesel blends at 50 °C.

Figure 7 shows acidity and water content changes of different UCO biodiesel blends exposed to NBR, EPDM, FKM, and SR at 70 °C. The acidity of diesel, B5, B10, and B20 UCO biodiesel blends reveals moderate changes, even though the decline of acidity for BD100 is lower than that at 25 or 50 °C. This may be attributed to the fact that the higher temperature caused more hydrolysis of FAMEs. The trend of the water content still decreases for most of the immersion.

Figure 7.

Figure 7

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Changes of acidity and the water content of UCO biodiesel blends at 70 °C.

Conclusions

Many experimental investigations have been reported to support the potential of biodiesel. However, UCO biodiesels have not gained much importance as an easily available feedstock due to the lack of reliable scientific evidence. This paper assessed the interactions between UCO biodiesel blends and elastomer materials, that is, NBR, EPDM, FKM, and SR on high-speed marine diesel engines at different temperatures, and the effects of the mixing ratio of UCO biodiesel and immersion temperature on the compatibility of elastomer materials were analyzed. The main conclusions can be summarized as follows.

Elastomer materials such as NBR, EPDM, FKM, SR, and so forth on high-speed marine diesel engines would reveal worse but acceptable changes than those exposed to petroleum diesel, including a slight increase of the mass and volume and decline of tensile strength and hardness than that exposed to petroleum diesel when the volumetric blend ratio of UCO biodiesel is less than 20%. A higher temperature can also promote the swelling of elastomer materials, whereas the swelling process slowed down when the temperature further rises from 50 to 70 °C.

FKM shows better compatibility with UCO biodiesel than NBR, EPDM, and SR. The compatibility of NBR and SR with UCO biodiesel blends becomes better when the blend ratio of UCO biodiesel is high, even pure UCO biodiesel. EPDM shows poor compatibility with pure UCO biodiesel and requires technical improvement before utilizing high-speed marine diesel engines fueled with a high mixing ratio of UCO biodiesel.

There is no significant effect of elastomer materials swelling on the acidity of UCO biodiesel blends, especially when the blend ratio of UCO biodiesel is less than 20%, whereas the water content reveals a significant decrease. The recommended volumetric blend ratio of UCO biodiesel is no large than 20% when strong antiswelling elastomer materials are not utilized in the engine fuel supply system.

This paper managed to study the compatibility between elastomer materials and UCO biodiesel blends and can provide theoretical evidence to support the reliability of UCO biodiesel. Due to the complexity of UCO sources, the properties and composition of UCO biodiesel could be different in different areas. Therefore, the results obtained herein could improve casing support for theoretical discussion. The compatibility of UCO biodiesel with elastomer materials in different areas needs a further study. Future studies can be conducted by collecting the properties and composition of UCO biodiesel in other cities and areas that can provide evidence to the applicability. A relevant verification experiment can be conducted so that the results can be suitable for the UCO biodiesel in the broader region.

Experimental Setup and the Procedure

Tested UCO Biodiesel Blends

In this paper, B0, B5, B10, B20, and BD100, respectively, were selected as tested fuels. The UCO biodiesel was purchased from a company named Shanghai Zhongqi Environmental Protection Technology Co., Ltd. According to the research of Mert Gulum,41 the influences of main transesterification reaction variables on the properties of biodiesel cannot be ignored. Transesterification reaction parameters are as follows.

The esterified oil (acidity lower than 3 mg KOH/g, measured according to ASTM D664) after the esterification reaction is transferred to the transesterification reactor, and the methanol alkali solution is added, in which the solid KOH is 0.55–0.70% of the weight of esterified oil and methanol is 10–16% of that. The temperature of the heating reaction is controlled at 60–64 °C for 0.5–1 h. The mixture after the reaction is separated by sedimentation for 8–12 h to obtain biodiesel. The physical and chemical properties of diesel and UCO biodiesel blends are displayed in Table 1.

Table 1. Physical and Chemical Properties of Diesel and UCO Biodiesel Blends.

item test method B0 B5 B10 B20 BD100
density (20 °C)/(kg/m3) ISO 3675 811.8 821.7 823.5 826.9 875.1
acidity/(mg KOH/g) ASTM D664 0.056 0.061 0.071 0.093 0.280
sulfur/(mg/kg) ASTM D7039 5.0 5.2 5.3 5.6 7.3
kinematic viscosity (20 °C)/(mm2/s) ISO 3104 3.794 4.394 4.530 4.610 4.099
water content/(mg/kg) ISO 12937 106.6 131.8 156.1 178.8 226.9
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The feedstock of UCO biodiesel would be a mixture of different types of edible oils coming from all regions of Shanghai. The composition analysis of the tested UCO biodiesel has been investigated by gas chromatography–mass spectrometry (GC–MS) according to EN 14078. Figure 8 represents the FAME components of UCO biodiesel in Shanghai. As shown in Figure 8, UCO biodiesel in Shanghai mainly contains methyl palmitate (C16:0), methyl stearate (C18:0), methyl oleate (C18:1), and methyl linoleate (C18:2). The FAME components of UCO biodiesel were obviously different from those of biodiesel manufactured from single feedstock such as soybean, rapeseed, and palm oil, as listed in Table 2. The unsaturated components of palm biodiesel are lower than those of other biodiesels, and the feedstock of UCO biodiesel consists of lots of waste oil in restaurant, so it can be concluded that the palm oil may be one of the major components of the UCO in Shanghai.

Figure 8.

Figure 8

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GC–MS analysis of UCO biodiesel in Shanghai.

Table 2. Chemical Composition of Biodiesel.

FAMEs C16:0 C18:0 C18:1 C18:2 others
tested UCO 23.4 6.7 33.8 27.7 6.8
palm42 37.18 0.91 47.51 13.33 1.05
rice bran42 17.91 0.35 43.95 36.03 1.76
karanja42 19.48 4.90 45.99 27.77 1.86
soybean43 14.1 5.2 25.2 48.7 6.8
rapeseed44 3.73 1.8 64.84 18.47 11.16
Jatropha(45) 14.22 8.39 43.14 31.42 2.83
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Tested Elastomer Materials and Immersion Temperatures

Elastomer materials were commonly used to manufacture many parts of high-speed marine diesel engines such as fuel tanks, oil pipes, fuel hoses, and seals. Among these materials, NBR, FKM, EPDM, and SR were widely used. Therefore, this research selected four elastomer materials, that is, NBR, FKM, EPDM, and SR according to ASTM D471. Each elastomer material is divided into square- and dumbbell-shaped samples, respectively. The size of square- and dumbbell-shaped samples is demonstrated in Table 3. The square-shaped sample was used to evaluate the change of mass, volume, and hardness; the dumbbell-shaped sample was used to assess the change of tensile strength.

Table 3. Size of Square- and Dumbbell-Shaped Samples.

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The fuel temperature should gradually rise during the transportation process when the fuel was pumped out of the tank and supplied into the fuel injector in turn through the fuel pump, filter, hose, and pipe. Thus, this paper chose 25, 50, and 70 °C to reflect the realistic temperature of the fuel in tank, high pressure pipe, and fuel injector position, respectively, to simulate the actual temperature variation in the fuel supply system as accurately as possible.

Experimental Procedure

To assess the compatibility of four typical elastomer materials, that is, NBR, FKM, EPDM, and SR of high-speed marine diesel engines with UCO biodiesel, several static immersion tests were conducted in the case of 25, 50, and 70 °C for 168 h. The physical property changes including the mass, volume, hardness, and tensile strength would be obtained by a sample measurement before and after the immersion test. As per described in ASTM D471, a 5-decimal place analytical balance was employed to measure the mass changes and the variation of volume can be calculated by eq 1

graphic file with name ao0c06254_m001.jpg 1

In this formula, mo and mow are the samples’ masses in the air and water before the immersion test, while mt and mtw, respectively, correspond samples’ masses in air and water after the test.

The sample hardness was measured by a durometer (model LX-A) according to ASTM D224, and tensile strength was obtained by several strength tests of the dumbbell-shaped samples with a gauge length of 30 mm on a DLD 2500 electronic universal testing machine according to ASTM D412. In addition, acidity and the water content of the tested UCO biodiesel blends before and after the immersion test were also measured to assess the impact of elastomer materials on the UCO biodiesel blends’ physical and chemical properties according to the test methods in Table 1.

Acknowledgments

We appreciate the support provided by the project supported by the Science and Technology Commission of Shanghai Municipality (grant no. 18DZ1202802) and the State Laboratory Open Project of Automotive Biofuel Technology (CN) (grant no. 2013010).

The authors declare no competing financial interest.

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