Polyacrylate-magnetite nanocomposite as a potential multifunctional additive for lube oil

The application of polymer nanocomposites (PNCs) in lubricant industry has attracted considerable interest due to their much enhanced properties compared to neat polymers. In this study, magnetite (Fe3O4) nanoparticles (NPs) were synthesized. Then PNCs were prepared by reinforcing these NPs in the homopolymer of dodecyl acrylate in different percentages. The characterization of the prepared NPs and PNCs was done by different analytical techniques. Thermal stability is determined through thermogravimetric analysis (TGA). Performance evaluation of the PNCs as viscosity index improver, pour point depressant and antiwear additive was carried out by blending them with a mineral base stock at different percentage ratios. Standard ASTM methods are followed to carry out the evaluations. It is found that with increasing the percentage of nanocomposites in the base stock, the overall performance of the furnished lubricant is enhanced.

Lubricants are the widely utilized material in automotive industry. Lubricants form a protective layer on the surface of machinery parts and thus reduce friction during turbo-chemical process. Except that, it improves the efficiency of engines, prolong their lifetime as well as economize energy. In the last few decades, use of oil-soluble additives in lubricant as effective friction reducer and anti-wear have been extensively studied in lubrication engineering [1][2][3] . But, the use of these additives increases pollution, toxicity, waste disposal in the environment. The application of nano materials as additive mitigates these limitations and opens new horizon in lubricant industry [4][5][6] . Nanoparticles were incorporated into lubricating oils to improve their tribological properties. In recent years, a large number of studies have been carried out to measure the potentiality of a range of inorganic nanoparticles as friction and wear reducer for lubricating oil [7][8][9][10] . Owing to excellent tribological and environmentally benign property, the nanoparticles have been preferred as an exceptional candidate in replace of traditional lubricating oil additives, particularly at higher load, higher sliding speed and higher frictional conditions [11][12][13] . The NPs adhere to the friction surfaces, resulting in the modification of the friction surfaces and the improvement of the tribological properties. Use of graphite nanosheets as lubricating oil additives improves tribological properties of paraffin oil 14 . The anti-wear property of paraffin oil was significantly improved by the incorporation of MoS 2 nanoparticles 15,16 . Oxide based nanoparticles such as CuO nanoparticles exhibit good anti-wear and friction reduction properties 17 . Cai-Xiang et al. 18 reported the anti-wear and friction reducing properties of CeO 2 and TiO 2 nanoparticles in lubricating oil. All the nanoparticles mentioned above are non-magnetic. The use of non-magnetic compounds containing elements like sulfur, phosphorus, lead, etc. as anti-frictional additive for lubricants also has some adverse effect on the environment. Magnetic NPs, on the other hand, is environmentally friendly and therefore has attracted considerable attention in this application area. Another advantage of using magnetic NPs as lubricant additive is its magnetic effect which originates in its remanent magnetization 19 . The lubricant with Fe 3 O 4 NP additives forms a protective film adhering on the friction steel pairs and filled the gaps and cracks of the surface due to possible magnetic interaction between the lubricant and friction surface and also tribo-chemical reactions on the metal surface 20,21 . This results in significant improvement of antiwear properties of the lubricating oil.
Huang et al. 22 reported the effect of magnetite nanoparticle on tribological property of paraffin oil which showed improvement of load carrying capacity and anti-wear property of the formulated lubricant compared to pure paraffin oil. Ziang et al. 23 described the tribological and tribochemical properties of magnetite (Fe 3 O 4 ) nanoflakes as additive in mineral base fluids. From the above discussion it is revealed that magnetite nanoparticle and other oxide based nanoparticles are used only as anti-wear and friction reducing additives in lubricating oil. Furthermore, acrylate based polymers are known to perform as good viscosity index improver (VII) [24][25][26] , pour point depressant (PPD) 27 . They were not known to act as good anti-wear and friction reducing additives. Hence Preparation of dodecyl acrylate. Dodecyl acrylate (DDA) was prepared by esterification of acrylic acid with dodecyl alcohol in 1.1:1 mol ratio. The reaction was performed in a resin kettle in presence of concentrated sulfuric acid as a catalyst, 0.25% hydroquinone with respect to the reactants as polymerization inhibitor, and toluene as a solvent. The reaction was carried out under nitrogen atmosphere. The reaction mixture was heated gradually from room temperature to 403 K using a well-controlled thermostat. The extent of reaction was followed by monitoring the amount of water liberated during reaction. After completion of the reaction, the ester dodecyl acrylate (DDA) was collected.
Purification of the prepared ester (DDA). To purify the product, a desired amount of charcoal was added to the ester, followed by reflux for 3 h and then filtered. The filtrate was washed repetitively with 0.5 N sodium hydroxide solution to ensure complete removal of unreacted acid. To remove traces of sodium hydroxide, purified ester was washed several times with distilled water. The ester was then left on calcium chloride overnight and recollected by distillation under reduced pressure. This purified ester was then used in the polymerization process.
Synthesis of homo polymer of DDA. The polymerization was carried out in a four-necked round bottom flask fitted with a condenser, stirrer, thermometer and an inlet for the nitrogen insertion. Required amounts of dodecyl acrylate and initiator (AIBN, 0.5% w/w) were taken in the flask and toluene was also added as solvent.
The reaction temperature was controlled at 353 K for 6 h. Then the reaction mixture was poured into methanol solvent with stirring to cease the polymerization and a precipitate was appeared. The precipitated polydodecyl acrylate (A), PDDA, was further purified by frequent precipitation of its hexane solution with methanol followed by drying under vacuum at 313 K.

Preparation of magnetite (Fe 3 O 4 ) nanoparticle.
Magnetite nanoparticles were synthesized following the method reported by Bruce et al. 28 Solutions of iron(II) sulphate heptahydrate (1.67 g, 6 × 10 -3 mol) in 50 ml deionized water, potassium nitrate (1.01 g, 1 × 10 -2 mol) in 10 ml of deionized water and 2.5 M potassium hydroxide solution were prepared. 1% (w/w) of surfactant (CTAB) was mixed with the iron salt solution under vigorous stirring for 2 h. Solution of potassium nitrate was added to this solution and stirred for another half an hour. Then 10 ml of 2.5 M potassium hydroxide (2.5 × 10 -2 mol) was slowly added to the above solution. The reaction mixture was heated to 100 °C under nitrogen atmosphere and maintained at this temperature for 2 h. The nitrogen flow was then turned off and the mixture was cooled down to room temperature. After cooling, the black precipitate was repeatedly washed with deionized water, centrifuged and allowed to dry under vacuum at 323 K overnight 29 .  Table 1. Thermo gravimetric analysis (TGA). The thermal stabilities of the prepared homo polymer and polymer nanocomposite were determined by a thermo gravimetric analyzer (Shimadzu TGA-50) using an alumina crucible in air. The system was run at a heating rate of 10 °C/min. The percentage of weight loss (PWL) of the samples with rise in temperature was calculated. Evaluation of tribological properties. The anti-wear and friction modifier performance of the lubricant compositions were evaluated in terms of wear scar diameter (WSD) by Four-ball wear test apparatus (FBWT) following ASTM D 4172-94 method 31 . In this experiment, 392 N (40 kg) load at 75 °C for 30 min. was employed to measure the wear scar diameter (WSD). The diameter and rotating speed of the ball were 12.7 mm and 1200 rpm respectively. The details procedure is described in our previous publication 32 .

Results and discussion
The FT-IR spectrum of homopolymer of dodecyl acrylate (A) and one of the PNCs (polymer composite F-3) is shown in Fig. 1    www.nature.com/scientificreports/ and 68.12% respectively. The degradation of the polymer was inhibited by the addition of nano-Fe 3 O 4 and as a result the polymer composites showed improved thermal stabilities. The decrease of mobility of polymer chain and the tendency of magnetite nanoparticle to eliminate free radicals may be the key effects accountable for this enhancements 33 . Figure 5 showed the X-ray diffraction pattern of the prepared magnetite nanoparticle. It was taken within the range of 20°-70° (2θ). The six most intense peaks at 30.3°, 35.6°, 43.2°, 53.6°, 57.1° and 62.8° respectively were markedly observed and was found very similar as obtained for magnetite nanoparticles elsewhere 29 . The purity of magnetite nanoparticles was confirmed by the absence of peaks due to other forms of iron oxides like maghemite or hematite in the sample. Hematite nanoparticles shows nine intense peaks in the diffraction angle from 6° to 70°3 4 . Figures 6 and 7 showed the scanning electron micrograph of the prepared magnetite nanoparticle and the composite respectively. Shapes of the particles marked in Fig. 6 showed that the particles were nearly spherical. It can be seen from the figure that the particles have an average size of about 29 ± 2 nm. The formation of the nanoparicles is further confirmed by TEM image mentioned in the supplementary materials.
Magnetic behavior of these nanoparticles depends on temperature and particle size. The magnetic characterization was done using a vibrating sample magnetometer (VSM) at 300 K with a magnetic field − 10 kOe to + 10 kOe 35 . The symmetric hysteresis loop indicated a superparamagnetic behaviour of the nano particles in the applied field with zero coercivity and remanance values. The saturation magnetization (M s ) of the magnetic nanoparticles is 74.23 emu/g. The saturation magnetization value is related with the size of the NPs. It increases with increasing size of the NPs. The higher value of Ms in this study give evidence about the average size of NPs as obtained by SEM study. The diagram of the magnetic behavior of the nanoparticles as obtained by the experiment was mentioned in the supplementary material.   www.nature.com/scientificreports/ additives, there is always a steady increase of VI values with the increase in additive concentration in base stock. The lubricant blended with 5% F-3 additive showed highest increment (42.7%) in viscosity index compared to pure base oil. This is due to increase in total volume of polymer micelles in lubricant with increase of concentration of the additives. The nanoparticles separate the polymer chains in the matrix which is responsible for increase of the volume also. The greater volume of composite units in solution contributes higher VI of the lubricant. The PPD of lubricant compositions at different percentage of additives varying from 1 to 5 wt% was tested and the results are depicted in Fig. 9. The results showed that the additives (A, F-1, F-2 and F-3) are efficient as PPD and the efficiency decreases with increasing the concentration of additives. The PPD of all the lubricants blended with composites is very similar with that of pure polymer. That means incorporation of nanoparticles into the polymer matrix does not affect the PPD property compared to neat polymer (A).
The tribological properties of all the lubricant compositions (A, F-1, F-2 and F-3) were tested through measuring WSD by FBWT apparatus employing 40 kg load and the results were depicted in Fig. 10. The AW performance of the base oil is significantly enhanced when the additives are blended with it and is indicated by the gradual decrease in WSD values with increasing the percentage of the additives. The nano-iron particles in lubricant composition interact with the metal surface during tribochemical process which decreases wear 36 . The lubricant containing 5% nano-additive (F-3) exhibited lowest WSD (35% decrease) compared to pure base oil.

Conclusion
In this study nano-Fe 3 O 4 was synthesized and used as filler particles in the formation of polydodecyl acrylate composites. The characterizations of both the polymer and composites were studied by SEM, TEM, XRD and spectral analysis techniques (FTIR, NMR). Through TGA data we showed that thermal stability of polymer was www.nature.com/scientificreports/ improved significantly by the incorporation of nanomagnetite into the polymer matrix. During their evaluation as additive for lubricant, it was found that all the nano-blended composites showed improved performance as viscosity modifier and anti-wear additives for lube oil. Therefore, the above study is definitely a potential approach to design multifunctional additives for lubricating oil. www.nature.com/scientificreports/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.