Dodecyl methacrylate and vinyl acetate copolymers as viscosity modifier and pour point depressant for lubricating oil
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The article presents application of homo polymer of dodecyl methacrylate (DDMA) and its copolymers with vinyl acetate (VA) as multifunctional additives for lubricant formulation. Homo polymer of DDMA and five copolymers of DDMA with VA at different molar ratios were synthesized by free radical polymerization method using azobisisobutyronitrile (AIBN) as initiator. The characterization of the polymers was carried out through FTIR, NMR and GPC (gel permeable chromatography) analysis. The performance of all the polymers as viscosity index improver (VII) or viscosity modifier and pour point depressant (PPD) additive in two different base oils (mineral) were evaluated. The mechanism of action of the polymers as pour point depressant was studied by photo micrographic analysis. Rheological study of the formulated lubricant was also carried out and reported. The thermal stability of the polymers was determined by thermogravimetric analysis (TGA). It was found that thermal stability, VI and molecular weights of copolymers are higher than the homopolymer which showed better PPD property.
KeywordsCopolymer Molecular weight Viscosity modifier Pour point depressant Rheology
Lubricating oils play a very important role in automobile industry. It keeps the moving parts lubricated and protects them against rust and corrosion. Lubricating oils alone cannot satisfy all the requirements of modern engines. Some additives are to be blended with lube oil to improve the overall performance of the lubricant . The role of additives in lubricant is very significant. They optimize the performance of lubricant and generally fall into two major categories viz. surface active additive and performance enhancing additive . The first one protects the metal surfaces of the engine from corrosion, such as antiwear, anti-rust and extreme pressure additives . The second type reinforces the base stock performance, such as viscosity index improver (VII) [4, 5], pour point depressant (PPD) [6, 7], antioxidant , and detergent–dispersant . Generally, in the formulation of a high performance lubricant, different types of additives at different percentages are blended with the base stocks. This increases the overall cost of the furnished lubricant. The addition of additives having multifunctional character to the base fluid may lead to formulate a cost effective as well as better performing lubricant. Therefore, research in this area has attracted much attention. In this work we have synthesized multifunctional additives which showed excellent VII and PPD performances. Lot of works in this direction has carried out so far. Kamal and his group  in their work have shown the application of copolymers of vinyl acetate (VA) and esters of acrylic acid as viscosity index improver for lubricating oil. They also studied the rheological properties of lube oil with and without polymeric additives. Al-Sabagh and his group  have mentioned the application of copolymers of VA with n-alkyl itaconate as pour point depressants in lubricating oil. The rheological properties were also studied by the same research team. Abdel-Azim et al.  has synthesized the copolymers of dialkyl fumarate with vinyl acetate and recognized that the copolymer of didodecyl fumarate with VA is the most effective as PPD for lubricating oil. In 2008, Nassar  had prepared six polymers at different molar ratios of 2-ethylhexyl methacrylate and vinyl acetate and studied the VI property of the lubricants. He reported that most efficient VI property is obtained when the ratio of acrylate and vinyl acetate is 1:0.2. The potential application of copolymer of vinyl acetate (VA) as pour point depressant for crude oil is also well documented. VA—α olefin copolymers  and VA—methacrylate copolymers  were used as pour point depressant for crude oil. The copolymers of vinyl acetate, styrene and n-butyl acrylate having different monomer ratios were used to study the rheological behaviours of Mexican crude oil . Borthakur and his group  have shown the application of alkyl fumarate and vinyl acetate copolymer in combination with alkyl acrylate as a flow improver for high waxy Borholla crude oil. Machado et al.  studied the influence of ethylene vinyl acetate copolymers on viscosity and pour point of a Brazilian crude oil. Al-Shafy and Ismail prepared an ester of polyethylene acrylic acid with 1-docosanol and finally it was grafted with vinyl acetate to produce graft ester. The grafted product was used as flow improver for Egyptian waxy crude oils .
Although there exists lot of works on the additive performance of copolymers of VA and different acrylates, reports regarding their application as multifunctional additive like PPD and VII for lube oil are scanty. Therefore, in this work copolymers of DDMA with VA at different molar ratio were synthesized and their performances are evaluated. Here efficiencies of the polymers as viscosity index improver and pour point depressant in two different types of mineral base oils (SN150 and SN500) were carried out according to ASTM standards. Photo micrographic images were taken to study their mechanism of action as pour point depressant. Rheological properties of the lubricants were also studied during the work by a rheometer. Homo polymer of dodecylmethacrylate (DDMA) was also synthesized along with the copolymers for comparison of the results.
Physical properties of the mineral base oil
Density (g cm−3) at 40 °C
Viscosity at 40 °C in cSt
Viscosity at 100 °C in cSt
Cloud point, °C
Pour point, °C
Preparation of ester and its purification
Dodecylmethacrylate (DDMA) was prepared by reacting methacrylic acid with dodecyl alcohol in 1.1:1 molar ratio in presence of conc. H2SO4 as catalyst, 0.25% (w/w) hydroquinone (with respect to the total amount of the reactants) as polymerization inhibitor and toluene as solvent in a Dean Stark apparatus. The process of esterification was carried out by the procedure as reported in the earlier publication . The purification of the ester was carried out by adding a suitable amount of charcoal to the prepared ester and refluxed for 3 h and then filtered off. The filtrate was treated with dilute (0.5 N) NaOH solution in a separating funnel to remove the unreacted acid and hydroquinone. The process was repeated several times and finally washed with distilled water. The purified ester was passed through sodium sulphate and left overnight over calcium chloride to dry completely and used in the polymerisation process.
Preparation of copolymers and homopolymer
Molar ratio and molecular weights of the prepared polymers
Molar ratio of monomers
Average molecular weights
IR spectra were recorded by Shimadzu FTIR 8300 spectrometer using 0.1 mm KBr cell at room temperature within the wave number range of 400–4000 cm−1. NMR spectra were recorded in Bruker Avance 300 MHz FT-NMR spectrometer using a 5 mm BBO probe. CDCl3 was used as solvent and tetramethylsilane (TMS) as reference material.
Determination of the molecular weight
The number average molecular weight (Mn) and weight average molecular weight (Mw) were measured by GPC instrument (polystyrene calibration) equipped with a 2414 detector, waters 515 HPLC pump and 717 plus auto sampler. Sample solutions (0.4% w/v in HPLC grade THF) are prepared by dissolving ~4 mg of polymer per ml THF and filtering (0.45-μm Millipore PTFE) to remove suspended particulates. The pump flow rate is 1.0 mL/min with THF as the carrier solvent, and injection volumes are set to 20 μL. The polydispersity index  which indicates the nature of the distribution of the molecular weights in the polymers was also calculated.
Determination of thermo gravimetric analysis (TGA) data
The thermo-oxidative stability of all the polymers was determined by a thermo gravimetric analyzer (Shimadzu TGA-50) in air using an alumina crucible at a heating of rate of 10 °C/min. A comparison of thermal stabilities of homo polymer with the copolymers was explained by this study.
Performance evaluation as viscosity index improvers
Viscosity indices of the lubricating oils (SN150 and SN500) blended with polymers at different concentration levels [ranging from 1% to 5% (w/w)] were calculated by measuring kinematic viscosity (KV) values at 313 and 373 K. The ASTM D445 method was applied to determine the KV values and ASTM D 2270-10 method was applied to determine the VI values.
Performance evaluation as pour point depressants
Pour points were determined using the cloud and pour point tester (model WIL-471, India) according to ASTM D 97-09 method. The performance of additives as PPD was investigated through variation of their concentration [from 1 to 5% (w/w)] in the formulated lubricants.
The photomicrograph images showing wax behaviour of the lube oil (SN150, pour point = −6 °C) without and with polymers (4%, w/w) have been recorded at 0 °C. A Banbros polarizing microscope (model BPL-400B) was used for this purpose and the adopted magnification was 200X.
Rheological study of the homo polymer and copolymers at 5% (w/w) concentration in SN150 oil was performed using Brookfield rheometer (Model DV-III ultra). Dynamic viscosity (cp) and shear rate (s−1) were measured at two temperatures, 40°C and 100 °C.
Results and discussion
The homopolymer of DDMA exhibited IR absorption band at 1722.3 cm−1 for the ester carbonyl group. Peaks at 2853.5 and 2924 cm−1 were for the alkyl (CH3CH2–) groups. Peaks at 1456.2, 1435.9, 1377.3, 1330.2, 1296.1 and 1164.0 cm−1 were due to CO stretching vibration and absorption bands at 1013.5, 937.3, 812.9, 715.5, and 649.0 cm−1 were due to bending of C–H bonds.
In the 1H NMR of homopolymer, methyl protons appeared in the range of δH 0.881–1.027 ppm, methylene protons in the range of 1.283–1.812 ppm for all alkyl groups. A broad peak at 3.930 ppm indicated the protons of –OCH2 group. Absence of any significant peaks in the range of 5–6 ppm in the spectrum confirmed completion of the polymerisation process. In the 13C NMR of homopolymer, peaks at δC 176.73–177.84 ppm indicated the presence of ester carbons. The peaks at 63.06–65.40 ppm confirmed the presence of –OCH2 carbon. Peaks in the range of 14.13–45.22 ppm represent all sp3 carbon atoms of alkyl groups. No significant peaks in the range of 120–150 ppm indicated the absence of sp2 carbon atoms and therefore supported formation of the polymers.
The spectral data (IR and NMR) of all the five copolymers (P-2 to P-6) are similar. In the IR spectra, peaks at 1732.9–1735.2 and 1716.5–1720 cm−1 indicated the presence of ester carbonyl groups in the copolymers due to vinyl acetate and DDMA moiety, respectively. Peaks at 2853.6–2854 and 2922.9–2925.4 cm−1 were for the CH3CH2– groups. The peaks at 1456.2–1456.8 cm−1, 1377.1–1377.8 cm−1, 1368.4–1369 cm−1, 1321.1–1321.9 cm−1, 1296.1–1296.8 cm−1, 1238.2–1238.8 cm−1, 1163.0–1164.2 cm−1, 1065.5–1066 cm−1, and 1011.6–1012 cm−1 were due to CO stretching vibration and absorption bands at 814.9–816 and 721.3–721.9 cm−1 were due to bending of C–H bond. It is observed from the IR data of five copolymers that with increasing the percentage of vinyl acetate moiety in the copolymers the peak intensity olefinic groups gradually decreases.
In 1H NMR of copolymers, a broad peak δ 3.926–4.158 ppm indicated the protons of –OCH2 and –OCH3 groups. The hydrogen attached to sp3 carbons appeared in the range of 0.858–2.637 ppm. Absence of any significant peaks in the range of 5–6 ppm indicated the disappearance of C=C bonds and confirmed formation of the copolymers.
In 13C NMR, peaks at δC 176.60–176.70 ppm indicated the presence of ester carbonyl groups. The peaks appeared from 64.66 to 65.06 ppm indicated the presence of –COCH3 methyl carbons and –OCH2 carbons. The peaks ranging from 14.08 to 45.09 ppm represented all other sp3 carbons. No significant peaks in the range of 120–150 ppm indicated the absence of sp2 carbons and confirmed the formation of the copolymers. (All IR and NMR spectra are given in supporting information).
Analysis of molecular weight data
The experimental values of Mn and Mw of the homo and copolymers are given in Table 2. From the values, it is found that molecular weight of homopolymer is less than the copolymers. The molecular weights of copolymers increase with increasing the percentage of vinyl acetate moiety in the prepared copolymers. The comparatively higher PDI values of P-2 and P-3 indicated that their molecular weight distributions are wider and the copolymers are more branching compared to others. On the other hand, the lowest PDI value of P-6 indicated that the molecular weight distribution of the copolymer is narrow compared to others and the polymer is expected to be more linear which is also reflected of its highest thermal stability.
Analysis of thermo gravimetric data
Analysis of viscosity index data
Comparison of viscosity index values of the synthesized polymers with other polymers published earlier 
Molar ratio of monomers
av. mol. wt.
Analysis of pour point data
Analysis of rheological study
Analysis of photo micrographic image
From the above study it is found that polymers are effective as viscosity index improver and pour point depressant for lube oil. Viscosity index property of homo polymer is lower than copolymers. VI property increases with increase in the percentage of vinyl acetate in the copolymers. The efficiency as pour point depressant was found higher in case of homo polymer compared to copolymers. Among the copolymers, the polymers which have higher PDI values, results better PPD property. From rheological study, it was found that pure lube oil is a Newtonian fluid at any shear rate but polymer doped lube oil is non-Newtonian fluid at low shear and Newtonian fluid at high shear rate.
Authors thank UGC, New Delhi for financial support. Thanks also to IOCL, India for supplying base oil.
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