13th International Colloquium Fuels - September 2021 109 Plasma cross-linked vegetable oil as fuels additives: effect of vegetable oil nature, viscosity and Functionalization. Dr. Rémi Absil Green Frix, Blandain, Belgium Frédéric Danneaux Green Frix, Blandain, Belgium Summary Plasma reactors enable Green Frix to cross-link oils and substances to generate bio-based oils and additives for the industry with a large improvement of their oxidative stability, lubricity, friction modification, rheology behavior, ... The process developed and patented by Green Frix offers a large panel of non-Newtonian products with various viscosities and rheologies (, shear-thinning, thixotropy, adaptive relaxation time,…) and can offer tailor-made solution. Previous studies on the effects of vegetable oil methyl esters on diesel fuel lubricity have shown an increase in lubricity associated with the addition of these esters. [1] In this study, we examine the effect of various additives based on cross-linked vegetable oils. The effect of their nature, viscosity and functionalization are investigated. Additive levels of 0-0.1% of specific polymerized vegetable oils were added to diesel fuel and the resulting lubricity was measured using the High Frequency Reciprocating Rig method. 1. Introduction Automotive Fuels are complex formulation of chemicals where compositions highly impact the lubricity of the diesel Fuels. In the 90s’, the concern of keeping the environment green, regulations change to lower the value of sulfur content in diesel up to maximum 500 ppm. More recently, several countries are going further and limit the concentration of sulfur to a maximum of 50 ppm. As so, different processes have been applied to get rid off the sulfur. Unfortunately, during the process, several other components involved in the lubricity get also removed. Therefore, low sulfur diesel required higher concentrations of additives or blending with other substances to achieve sufficient lubricity. [2] Biofuels gain increasing interest over the years. Underlying, there is a large trend to develop and promote sustainable, non-toxic and biodegradable additives. Plant-based oils such as canola, linseed, soy, sunflower and palm are common sources of sustainable feedstocks. These oils present several advantages such as biodegradability, lubricity, high flash point and good solvency (ability to dissolve additives and disperse contaminants). However, their poor oxidative and thermal stabilities, low viscosities are drawback limiting their application. Large number of papers have been published on vegetable oils and vegetable oil derivatives as additives or based fuels to substitute petroleum-based products by fixing their above-mentioned downsides. It is known that poor oxidation stability and poor lubricity affecting bio-based products are challenges that need to be settled. Several studies demonstrates that the used of well know fatty acid methyl ester (FAME) increased diesel fuel lubricity at concentration lower than 1%. [3],[4] Moreover, fatty acid composition, presence of functional group, molecular weight of the FAME are factor impacting the lubricity of the respective diesel. [5],[6] Hydroxyl functionalized FAME based on castor oil present more lubricity compared to their respective counterpart without any hydroxyl function. [7] Later, investigation confirmed that the structure of fatty acid presents a high importance. Indeed, it was showed that the lubricity respectively increased with the unsaturation of the FAME. Moreover, the paper suggests that vegetable oil based FAME mixtures consistently perform better as lubricity additives than their respective single fatty acid counterpart making the fatty acid distribution an important key factor fort the efficiency of the lubricity additives. [1] Plasma techniques applied to oils is a century old story. With its fundamental research program, Green Frix investigated the link between plasma energy and non-newtonian behaviors. Plasma energy will impact the cross-linking resulting in different level of shear-thinning. With low sulfur fuels, increasing admission pressure, risk of failure increases. Envision a fuel containing of a non-newtonian component will contribute to absorb the increase of load related to this evolution. The process enables to create low molecular weight vegetable oil (oligomers). 110 13th International Colloquium Fuels - September 2021 Plasma cross-linked vegetable oil as fuels additives: effect of vegetable oil nature, viscosity and Functionalization. The cross-linking process can be carried on; hence products will move from oligomers to polymers to achieve viscosity up to 6000mm²/ s at 40°C. Herein, we describe for the first time the use of plasma technologies to promote vegetable oligomers and polymers. In the following section we will detail the effect of the plasma treatment on the properties of the additives used for the fuel lubricity. The new polymers additives will be examined by several techniques including surface tension measurements, thermo-gravimetric analysis to study their physicochemicals properties. Finally, we will discuss the rheological properties of the plasma polymerized oils. Several oils will be tested in diesel fuels as additives. Effect of molecular weight, oil nature and functional group on lubricity of the diesel fuel will be highlighted by HFRR test following ISO 12156. 2. Materiel and method 2.1 Materials Base oils were purchased at Aveno NV (Nieuwelandenweg 32 / 1 2030 Antwerp Belgium. Oils present a yellow color and a specific gravity of 0.92-0.93 at 15°C. Col- Fadol products used in this study were prepared using the Green Frix plasma process reported in the patent.[8] Un-additivated Diesel fuel was received from Total (Belgium, Antwerpen) and used as received. 2.2 Viscosity measurement Viscosity measurement were realized using a Lauda viscosimeter according to ASTM D 445-06 Method. 2.3 SEC Size exclusion chromatography (SEC/ GPC) was performed in CHCl 3 at 23°C using either a Polymer Laboratories liquid chromatograph equipped with Rheodin manual injection (loop volume = 200 µl, solution conc. = 1 mg/ ml), a Pl-DRI refractive index detector and two PLgel columns. 2.4 FTIR analysis The FTIR spectra were recorded on a Bruker IFS 66v/ S spectrometer in Attenuated Total Reflectance (ATR) mode with 4 cm -1 resolution. The background and sample spectra were obtained in the 500-4000 cm-1 wave-number range. 2.5 Surface tension measurement Surface tension measurements were realized at room temperature with a Krüss tensiometer K10ST Krüss using De Nouy Ring method. 2.6 TGA analysis The measurements were performed on a TA instrument Q 500 Thermogravimatric analyser. A ramp of temperature of 10°C/ per minutes was applied. 2.7 Rheological behavior The measurements were performed on an Anton Paar MCR302 rheometer equipped with a plate/ plate shear module. The gap used is one millimeter. Shear thinning ISO 2884-1were used as guideline fort the measurements. 2.8 Lubricity determination Lubricity determinations were performed at 60 °C (controlled to 1 °C), according to the standard method ISO 12156 with an HFRR lubricity tester from PCS Instruments. 3. Results 3.1 Synthesis of polymer Synthesis of the whole polymer of vegetable oil in this study have been synthesized by plasma cross-linking. Polymerisation take place in the plasma reactor has described earlier in the patent. The oil present several interesting properties like controlled viscosity, non-newtonien behavior (shear thinning, thixotropy,…) and controlled relaxation time depending on the parameters used in the plasma. Proof of polymerisation can be attested by size exclusion chromatography as depicted in Figure 1. Figure 1: Evolution of the molecular weight distribution of polymerized oil with time of treatment. As depicted in Figure 1, for a fixed level of Plasma energy, the highest the time of treatment, the highest the molecular weight will become and therefore it will be accompanied by an increase of viscosity. The plasma technologies allow good control over the broad viscosity and provide several 13th International Colloquium Fuels - September 2021 111 Plasma cross-linked vegetable oil as fuels additives: effect of vegetable oil nature, viscosity and Functionalization. advantages. First, the process is metal-free, non-oxidative and provide a decreased of the iodine value of the oil offering better oxidation stability compared to pure vegetable oil. The decrease of iodine value is confirmed by FTIR. It can be noticed in Figure 2 the decrease of the signals corresponding to alkene stretch and alkene bend respectively around 3100 cm-1 and 720 cm -1 in FTIR. Secondly, it does provide high molecular weight vegetable with viscosity index > 200 up to 380 for the highest viscosity. Viscosity data can be found in the Table 1 below. Finally, plasma polymerisation technologies can offer non-newtonian biobased polymer depending on the viscosity and plasma parameters used. Moreover, functionalisation of the oil is possible as depicted in the Table 1. ColFadol 700-P is a phosphate organic functionalized oil and SunFadol 800 OH is a hydroxyl functionalized oil. All these results indicates that vegetable oils can be functionalized, and polymers of vegetable oil has been obtained by plasma polymerization. Good control of molecular weight and viscosity can be achieved depending on the plasma parameters and time of reaction. Figure 2: FTIR comparison of pure vegetable oil and ColFadol 800D Table 1: viscosity measurements of additives diluted in diesel for the HFRR test. Product V40 (mm²/ s) V100 (mm²/ s) VI ColFadol 68 S 73.35 14.40 206.0 ColFadol 700-P 705.91 116.30 266.2 SunFadol 800 OH 772.87 114.02 249.3 ColFadol 800 D 792.43 125.74 264.5 ColFadol 3000 D 3162.36 478.75 334.2 3.2 TGA analysis TGA analysis demonstrate good thermal stability of the polymerized oil over the process from the low molecular weight oil to high molecular weight oil. The thermal stability of the product is > to 300°C as attested by Figure 3. Moreover, there is very limited amount of remaining solid at the end of the analysis suggesting that the biobased additives will not form ash after its used in the fuels. Figure 3: TGA analysis of low to high polymerized vegetable oil. 3.3 Surface tension characterization Atomization of fuels is considered as the initial process in combustion. It has been studied for more than 40 years. [9],[10],[11] These studies showed the importance of surface tension on fuels properties. Surface tension of additives for fuels is suggested to highly important role within the final fuels properties as surface tension impact drastically the atomization phenomena in the combustion of the fuels. Larges number of papers has been published on modification of vegetable oil with various chemistries to provide higher lubricity by introducing polar group like epoxide [12] or by varying the oil nature,… [13,14,15,16,1] Therefore, as surface tension is a critical factor for fuels properties, we studied the effect of the plasma treatment on the surface tension as molecular weight change occurs, the effect of the oil nature (unsaturation) and presence of functional group (hydroxyl) on the final surface tension. The surface tension of each compound was measured at 25°C and value are depicted in Figure 4. 112 13th International Colloquium Fuels - September 2021 Plasma cross-linked vegetable oil as fuels additives: effect of vegetable oil nature, viscosity and Functionalization. Figure 4: Evolution of surface tension with oil nature and viscosity of the thickened oil. On Figure 4, several behaviours can be noticed for the plasma polymerized oils. First, the viscosity increased is accompanied with an increased-on surface tension. This increased of surface tension with viscosity (or molecular weight) has been already noticed for polymer. [17,18,19] Based on this data, we can say that the molecular weight dependence of plasma polymerized oils can be explained according to literature. It has been showed that surface tension for medium to high molecular weight polymer is proportional to Mn -1 . The dependence is attributed to the fact that polymer can be seen as two different kind of building blocks: the repetitive units which represent the majority of the mass of the polymer and the end group. Since end group concentration is low for high molecular weight polymer, it is expected that bulk properties like surface tension followed the dependence of Mn -1 . [20,21] Interestingly, surface tension of plasma polymerized oils showed the same trend as cited in literature. Moreover, the presence of functional group (hydroxyl) increases surface tension of the oil for similar viscosity i.e. molecular weight. Interestingly, OH functionalized Plasma polymerized oils present higher surface tension compared to their unfunctionalized polymer (same range of Mw). As surface tension is a bulk property and largely dominated following literature by the cohesive energy density [22] , it does make sense that increases the content of hydrogen-bond increases this cohesive energy density and therefore the surface tension. Overall, the magnitude of the surface tension changes is small compared to their unpolymerized oil. The small surface tension increase is likely to cause little if any change in the atomization characteristics of a fuel. 3.4 Rheological characterization It is known that polymers can modify viscosity and rheological behavior of fluids. [23] Typical example are the multigrade oils where viscosity temperature improvement has been made during the past years. Currently most polymers used in such application are petrobased polymers. [24,25] In this section, we are interested to demonstrate the possibility of using Biobased polymerized oils as VI improver but also rheological modifier. In this section, we will study the effect of the addition of polymerized vegetable oil on the improvement of viscosity for several oil. VI comparison and viscosity evolution will be compared to demonstrate the positive effect of the addition of the plasma biobased additive. In regards of fuel economy, there is a large trend to decrease the motor oil viscosity for both passage car motor oil (PCMO) and heavy-duty diesel engine oil (HDDEO). It is believed that additives will play an important role for the development of new and better fluid with fuel economy characteristic. [26] In this section, we look first at the effect of the addition of several plasma polymerized oil as additives on final viscosity and how it improves the viscosity. Results are showed in Figure 5. It can be noticed that the addition of 5 to 10% of the additives can lead up to a viscosity increase of 51 % compared to the reference oil. Interestingly, the use of 5 % of high viscosity plasma polymerized oils lead to similar viscosity at 100°C but present different viscosity temperature behavior as depicted in Figure 6. This data demonstrated the effect of the polymer as effective VI improver. Figure 5: viscosity improvement of a PAO 2 fluid with addition of two different plasma polymerized oil. 13th International Colloquium Fuels - September 2021 113 Plasma cross-linked vegetable oil as fuels additives: effect of vegetable oil nature, viscosity and Functionalization. Figure 6: VI evolution with addition of ColFadol polymer in PAO. In the next paragraph, we will investigate the shear thinning behavior of the plasma polymerized oil. Formulators favored multigrade oil for specific reason. As mentioned, they present better temperature response but also multigrade oil present lower viscosity compared to monograde oil under high shear rates encountered in most engine components. This can be explained by the shear thinning of the lubricants where the polymer align in the shear field. This fact also explains why multigrade lubricants produce lower fuel consumption than monograde lubricants. [27] In this section, several plasma polymerized oils has been studied to investigate their shear-thinning behavior. Oil viscosity, oil nature and plasma mode has been investigated. Results are respectively depicted in Figure 8 and Figure 9. The shear thinning has been studied by applying low and high shear over time. Temporary viscosity loss is recordded by following the evolution of viscosity over time taking into account the shear applied. In figure 7, it can be noticed that the higher the viscosity i.e. the higher the molecular weight, the higher is the shear thinning for a fixed shear. This observation is consistant with litterature. Moreover, polymerized oils based on sunflower oil present higher shear thinning compared to rapeseed oils as depicted in Figure 8. Figure 7: shear thinning behavior evolution with viscosity of the plasma polymerized oil oils additives Figure 8: shear thinning behavior evolution with the oil nature within the plasma polymerized oil additives Figure 9: shear thinning behavior evolution with the plasma parameters within the plasma polymerized oil additives It has been showed that the oils nature (i.e. fatty acid distribution) impact the respective shear thinning behavior of the finals polymers produced by plasma. Finally, we investigated the plasma effect on the shear thinning be- 114 13th International Colloquium Fuels - September 2021 Plasma cross-linked vegetable oil as fuels additives: effect of vegetable oil nature, viscosity and Functionalization. havior. As depicted in Figure 9, shear thinning behavior can be tailored for a target viscosity from around 5-6 % to up to 39 % of shear thinning for a viscosity ranging from 750 to 900 Mpa.s. The plasma technologies offer new possibilities of the technique to promote new types of rheological modifiers with tailor made possibilities depending on the application and used of the final product. The capability to choose the appropriate shear-thinning level, relaxation time, … open new ways to address issues related to the increasing load in application like higher pressure in injectors, increased torque in shafts, axles, bearings, … because it enables the fluid (or grease) to absorb part of the load without being compressed (heated up). 3.5 HFRR Test: diesel lubricity Due to the polar nature of the plasma polymerized oil oil, investigation of their lubricity properties as diesel additives is investigated. In this section several oil and their effect on lubricity will be discussed depending on their nature, concentration, and viscosity. The oils characteristics used in this section are detailed in Table 1. Lubricity was assessed using Iso 12156. Lubricity, as determined per HFRR test, can be evaluated using the lubricity specification in the petrodiesel standard EN 590. The reference diesel used here (ULSD) showed poor lubricity in the neat form (Table 2) with scar value around 580 µm. Table 2: results obtained by HFRR with petrobased diesel as reference fluid Additives added Additive concentration (ppm) Wear scar (um) Reference 0 580 ColFadol 68 S 1000 530 ColFadol 700-P 1000 400 SunFadol 800 OH 1000 430 ColFadol 800 D 1000 400 ColFadol 800 D 500 540 ColFadol 800 D 250 560 ColFadol 3000 D 1000 420 All sample additives with 1000 ppm showed good lubricity except for the low molecular weight additives (Col- Fadol 68 S). Addition of Colfadol 800D and ColFadol 700-P showed the best results in term of lubricity even though the difference is little compared with other samples. The additives reduced the wear scar value up to 31 %. The results are in accordance with the previous articles as polar ester groups in vegetable oils enable to adhere to metal surfaces and, therefore, possess good boundary lubrication properties. Addition of concentration below 1000 ppm led to a decrease of lubricity. Interestingly, the functionalization of the oils does not seem change drastically the wear scar. This observation is surprising as it has been referred that castor oil FAME showed better lubricity compared to their counterparts FAME. [28] Two explanations are emphasized. The functionalization content is to low leading to insufficient functional group in contact with the surface or the functionalized of the side chain of the polymer does not play a role and adsorption of the surface is mainly done by the ester group present in the triglycerides. Viscosity range of the additives play an important role as to low does not offer good lubricity and to high does not improve more the wear scar as depicted by the results of ColFadol 68 (S) and ColFadol 3000 (D). 4. Conclusions - Polymerization of vegetable oil can be achieved by plasma treatment leading to molecules of high viscosity, high molecular weight and high viscosity index. - Polymer produced are liquid polymers making them more easily manipulable. - Plasma polymerized oil can be used as viscosity improver and rheological modifiers increased viscosity at 100°C up to 51 %. - TGA analysis demonstrates that low residue offering potential ashless additives for fuel. - Plasma polymerized oil at low additive level significantly improved the lubricity of ULSD. Addition of 1000 ppm decreased the wear scar of 31 %. The additives concentrations are 10 times lower compared to additivation with FAME (1%) Our future work will be dedicated to improve the lubricity by increasing the polar functionality within the additive and optimized the molecular weight range for optimum effect for the fuel lubricity. 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