production and application properties of dispersive viscosity index improvers

Production and Application Properties of Dispersive Viscosity IndexImproversIvana Soljic Jerbic,† Jelena Parlov Vukovic,‡ and Ante Jukic*,†

†Faculty of Chemical Engineering and Technology, Department of Petroleum Refining and Petrochemisty, University of Zagreb,Savska c. 16/II, 10000 Zagreb, Croatia‡Product Development Department, INA Oil Industry Ltd., Refining & Marketing Business Division, Lovinciceva bb, 10002 Zagreb,Croatia

ABSTRACT: Polymeric dispersive viscosity index improvers of lubricating mineral oils based on styrene, dodecyl-methacrylate,octadecyl methacrylate, and N,N-dimethylaminoethyl methacrylate (d-PSAMA) were produced by performing copolymerizationsisothermally up to the high conversion in mineral base oil solution, using monofunctional or bifunctional peroxide initiator. Theobtained kinetics results reveal the benefits of the usage of a bifunctional peroxide initiator over a monofunctional, becausecomplete conversion of monomers was accomplished in the shorter reaction time, performing the process in a full batchwisemode. When the bifunctional initiator was applied, the required polymerization temperature was slightly higher (105 °C), andcopolymers of higher average molecular weight values (Mw = 60−120 kg mol−1) were obtained, while in case of themonofunctional peroxide initiator, the reaction temperature was 100 °C, and average molecular weight values of copolymerswere Mw = 30−100 kg/mol. Investigated application properties demonstrated that d-PSAMA additives were fully comparablewith conventional pure methacrylate additives, and also it provided other advantages such as higher viscosity index and kinematicviscosity, lower values of pour point temperatures, as well as better dispersant and detergent properties. Thus, by increasing theN,N-dimethylaminoethyl methacrylate share in copolymers from 2 to 10 mol %, their weight average molecular weight decreasedfrom 120 to 60 kg mol−1, while kinematic viscosity values at 100 °C remain high and amounted to 14.5 ± 0.5 mm2 s−1.

1. INTRODUCTION

Poly n-alkyl methacrylates (PMAs) of specific composition andarchitecture are among the polymers most commonly used asviscosity modifiers, mainly for lubricating mineral oils.1,2 Mostfrequently, they are copolymers of alkylmethacrylate monomerswith optimized share of lateral alkyl groups because each ofthese groups contributes to the different application property.Methacrylates with medium-size lateral alkyl groups (C10−C14)enhance the viscosity index; the long-chained groups (C16−C18) mostly contribute to the lowering of the pour point ofsolutions, while the methyl group contributes to the stiffness ofthe polymer chain.3 Recently, the styrene was used as acomonomer for modifying the poly(alkyl methacrylate)additive to increase its thermal and oxidation stability.4−6

These improvements are facilitated by increasing the styrenecontent, but application of copolymers containing high styrenecontent is limited by their relatively low solubility in mineraloils.3,7−9

New standards regarding emissions of gases and resultingengine design changes such as implementation of the EGR(exhaust gas recirculation) system in diesel engines led to thechanges in lubricant requirements and formulations.10 Whilethe EGR system effectively reduces NOx emissions to theatmosphere, soot load in the lubricant can be expected toincrease dramatically, causing increased temperature andviscosity, dispersancy failure, fouling, deposits, and wear.11

This has directed modern lubrication science toward thedevelopment of a new type of polymer additives withmultifunctional activity and ability to improve their dispersancyin addition to the control of viscosity properties.12 Introducing

the ability of dispersancy into a polymer additive demands acarefully engineered incorporation of a strongly polar functionalgroup to the main polymer backbone. The most commonlyemployed functional groups are amines, alcohols, oramides.13−16 Such formulated polymer additives will have theability to keep insoluble combustion debris and oil oxidationproducts dispersed in the oil, which will prevent theirdeposition on the main part of the engine. This will have adirect effect on minimizing harmful engine exhaust emissions,increasing engine life, and controlling oil consumption bymaintaining clean engine operation.10

A free-radically initiated copolymerization is the mostcommonly used technique for the synthesis and productionof the polymethacrylate viscosity index improvers.17,18 Themain limitation of a such mechanism is its small ability toinfluence the molar mass distribution and structural propertiesof the synthesized copolymers.19 Such synthesized polymeradditives need to fulfill certain structural requirements becausehigh values of weight-average molecular weights and narrowmolecular weight distribution are needed for better resistanceagainst shear rates developed in lubricating conditions.20 Also,from the free radical theory, it is well-known that molecularweight is inversely proportional to the rate of polymerization.17

As such, it is not possible to simultaneously obtain highreaction rates and high polymer molecular weights for bulk,

Received: April 18, 2012Revised: July 20, 2012Accepted: August 22, 2012Published: August 22, 2012

Article

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© 2012 American Chemical Society 11914 dx.doi.org/10.1021/ie301010n | Ind. Eng. Chem. Res. 2012, 51, 11914−11923

pubs.acs.org/IECR

suspension, and solution processes with a usage of aconventional monofunctional initiator.19 In recent studies,mostly regarding the bulk homopolymerization of styrene,21−24

it was shown that this problem can be overcome with a usage ofinitiators with two or more functional peroxide groups ofdifferent thermal stabilities in comparison with conventionalmonoperoxide initiators.25 Beside, the complete conversion ofmonomers can be achieved, and reaction time can besignificantly reduced with no need for any modification ofreactor equipment.26−28

In this work, high-conversion copolymerization kinetics ofstyrene, dodecyl methacrylate, octadecyl methacrylate, andN,N-dimethylaminoethyl methacrylate was investigated. Reac-tions were performed in a mineral base oil SN-150 solution byusing both bifunctional or monofunctional peroxide initiators,under isothermal conditions. All of the synthesized copolymerswere characterized with respect to composition (1H NMR) andmolar mass distribution (SEC). The main application proper-ties of the prepared polymer solution samples in mineral baseoil SN-200 such as viscosity, pour point, viscosity loss, andviscosity index were investigated by using standardized testmethods.

2. EXPERIMENTAL SECTION2.1. Materials. Polymerization grade monomers, styrene

(Sty), dodecyl methacrylate (DDMA), octadecyl methacrylate(ODMA), and N,N-dimethylaminoethyl methacrylate(DMAEM), were used as purchased (RohMax Chem. Co.).Two types of peroxide initiators, monofunctional tert-butylperoxy-2-ethylhexanoate (Mm = 216.3 g mol−1) (Trigonox 21,70 wt % solution, Akzo Chemie) and bifunctional 1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane (Mm = 302.4 g mol−1)(Trigonox 29, 90 wt % solution, Akzo Chemie), were used asreceived as well as mineral base oils SN-150 and SN-200 (INALubricants, Zagreb). Mineral base oil SN-150 with kinematicviscosity of 30.5 mm2 s−1 at 40 °C, viscosity index of 98, andpour point of −12 °C was used as a solvent for polymerizationreactions. Mineral base oil SN-200 with kinematic viscosity of40.6 mm2 s−1 at 40 °C, viscosity index of 104, and pour point of−9 °C was used for preparation of diluted polymer solutionsamples for examination of main application properties.Initiator decomposition rate coefficients were given as temper-ature functions:29 kd/s

−1 = 1.54 × 1014 exp[−124 900/(RT)]and kd/s

−1 = 7.59 × 1013 exp[−127 520/(RT)] for monofunc-tional and bifunctional initiators, respectively.To get insight into temperature influence on initiator

decomposition rate, additional calculations were made.Reaction temperatures, 100 °C for monoperoxide and 105°C for diperoxide, are chosen because the initiator half-lives(t1/2) of approximately 60 min for both initiator types result inconcentration curves that are comparable (Figure 1). Highertemperatures (105 °C for mono- and 115 °C for diperoxide)correspond to the initiator half-lives of around 20 min, whilelower temperatures (85 °C for mono- and 95 °C fordiperoxide) correspond to those around 3 h, respectively.On the basis of calculated data that present initiator

concentration (ci) as a function of reaction time (tp) atdifferent temperatures, it can be observed for both initiatortypes that decomposition is optimal at temperatures that suitthe initiators half-lives (t1/2) of approximately 60 min.Therefore, those temperatures were chosen for furtherinvestigation. Decomposition at higher and lower temperaturesresults in initiator efficiency decrease.

2.2. Polymerizations. All additive samples were preparedby free radical copolymerization of four different monomers inmineral base oil SN-150 solution by using two types of peroxideinitiators, monofunctional (Trigonox 21) or bifunctional(Trigonox 29), respectively. Polymerizations were performedat 100 °C when monomer mixture was initiated withmonoperoxide (system 1) and at 105 °C when monomermixture was initiated with diperoxide (system 2). Thosetemperatures were chosen to conform to the initiators half-livesof approximately 1 h. Experiments were carried out for 5 h in adouble jacket glass reactor (0.50 L) connected to a thermo-statted bath, equipped with a mechanical stirrer (200 rpm)under nitrogen atmosphere. The total monomer concentrationwas 50 wt %, and the concentration of the initiators was 1.0 wt% relative to the monomers. Recent kinetic studies show thatdiperoxide initiator efficiency increases with its concentration,and very often attempts where the amount of monoperoxidewould be replaced with half of the amount of diperoxideinitiator do not yield a complete conversion of monomers.26−28

Therefore, the molar ratio of monoperoxide to diperoxideinitiator was 3:2.Compositions of monomers in initial feed were carefully

selected to design the new type of additive with improvedapplication properties. The styrene content in the initialmonomer mixture was limited at optimal 15 wt % due to therelatively low solubility in mineral oils.3,7−9 Because recentstudies10,15,16 have shown that even a small fraction of a certainfunctional monomer may have a strong impact on desireddispersant properties of the synthesized polymer additives,DMAEM was added in a small amount to the reaction mixturewithin the range 0−10 mol % (in increments of 2 mol %).12−14

Ratios of long chain alkyl methacrylates (DDMA, ODMA)were kept at a weight ratio of 1:1 with respect to their well-known favorable effect on viscosity and pour point depressionof lubricating mineral oils.30−32 Described process conditionsfor the performed polymerization experiments are summarizedand given in Table 1.

2.3. Characterization. Reaction mixture samples weretaken directly from the reactor at exact time intervals (10, 20,40, 60, 120, 180, 240, and 300 min) whereat the final

Figure 1. Initiator concentration (ci) as a function of polymerizationtime (tp) at three different temperatures for both initiated systems(calculated data).

Industrial & Engineering Chemistry Research Article

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conversions of monomers were determined by 1H NMR usinga Bruker Avance model. The 1H NMR spectra were recorded at300 MHz with deuterated chloroform, CDCl3, as a solvent.Tetramethylsilane (TMS) was used as an internal standard.Weight-average molecular weight and number-average

molecular weight were determined at room temperature usinga GPC-20 Polymer Laboratories size exclusion chromatograph.Measurements were performed in toluene as an eluent with aflow rate of 1.0 mL min−1. The calibration curve was based onpolystyrene standards (EasyCal PS-1B, 580−2 560 000 gmol−1) of narrow distribution.Application properties of diluted polymeric additive solutions

in the mineral base oil SN-200 were determined by thestandardized test methods. Kinematic viscosity (ν) of thepolymer solution samples was determined using ASTM D-445test method. Measurements were carried out at 40 and 100 °C,using the calibrated Cannon−Fenske capillary viscometersimmersed in a constant temperature bath. Viscosity index (VI)was calculated from obtained kinematic viscosities at 40 and100 °C by using ASTM D2270 standardized practice. Theshear stability of polymer-containing oil was measured by usingthe DIN-51382 test method. This test method measures thepercent viscosity loss at 100 °C of investigated fluids whenevaluated by a diesel injector apparatus procedure that usesEuropean diesel injector test equipment. The viscosity lossreflects polymer degradation due to the shear at the nozzle.Pour points of the polymer solution samples were determinedby the ISO 3016 test method. Measurements were carried outby placing a test jar with 50 mL of the sample submerged into acooling medium. The sample temperature was measured in 1°C increments at the top of the sample until the liquid stoppedpouring. All viscosity and pour point measurements were run induplicate, and the average values were reported.

3. RESULTS AND DISCUSSION3.1. Composition Analysis. Determination of composition

of synthesized copolymers in mineral base oil is performed by1H NMR. To determine the amount of residual monomers andoverall molar monomer conversion during the polymerizationtime, the 1H NMR spectra for all monomers present in theinitial reaction mixture such as styrene (Sty), dodecylmethacrylate (DDMA), octadecyl methacrylate (ODMA), andN,N-dimethylaminoethyl methacrylate (DMAEM) were re-

corded and interpreted.33 The chemical shifts for certain typesof protons in the chemical groups that appear in the monomersinvolved in polymerization reactions are given in Table 2.

The content of the residual styrene monomer (mol %) inexamined samples was determined from the corresponding 1HNMR spectra by integrating the areas of characteristic peaksthat appear at ∼5.25 ppm (doublet) and at ∼5.75 ppm(doublet). On the basis of the intensities of the signals thatappear at ∼5.50 ppm (singlet) and at ∼6.10 ppm (singlet), thecontents (mol %) of total methacrylate monomers such asdodecyl, octadecyl, and N,N-dimethylaminoethyl methacrylatewere determined because they behave quite similarly in theapplied NMR field and cannot be distinguished within thespectra. The content of the formed polymer (mol %) wasdetermined by integrating the area, corresponding to the twooxymethylene protons that appear at ∼3.75 ppm (Figure 2).

3.2. Kinetic Study: Monomer Conversion versusPolymerization Time (Xp vs tp). The free radicalcopolymerizations of styrene, dodecyl-, octadecyl-, and N,N-dimethylaminoethyl methacrylate were performed in mineralbase oil SN-150 solution using monofunctional initiator, tert-butylperoxy-2-ethyl-hexanoate, at 100 °C (system 1) orbifunctional initiator, 1,1-di(tert-butylperoxy)-3,3,5-trimethylcy-clohexane, at 105 °C (system 2). Monoperoxide initiator wasadded gradually to the reaction mixture (modified batchprocess) to maintain constant radical flux.17,18 To promote andsimplify the production procedure, polymerization processinitiated with diperoxide was performed in a fully batchwise

Table 1. Process Conditions for the PolymerizationsPerformed at 100 °C Using Monoperoxide (System 1) and at105 °C Using Diperoxide Initiator (System 2)a

experiment

0-1(0-2) 1-1(1-2) 2-1(2-2) 3-1(3-2) 4-1(4-2) 5-1(5-2)

Content, wt %DMAEM 0.0 1.4 2.8 4.2 5.7 7.2Sty 15.0 15.0 15.0 15.0 15.0 15.0DDMA 42.5 41.8 41.4 40.4 39.65 38.9ODMA 42.5 41.8 41.4 40.4 39.65 38.9Content, mol %DMAEM 0.0 2.0 4.0 6.0 8.0 10.0Sty 33.0 32.7 32.4 32.1 31.8 31.5DDMA 38.2 37.3 36.4 35.4 34.4 33.4ODMA 28.8 28.0 27.2 26.5 25.8 25.1

aConcentration of monomer mixture in mineral base oil SN-150, CM =50 wt %; concentration of initiator, CI = 1 wt % relative to monomers;overall reaction time, tp = 5 h.

Table 2. Assignment of the Chemical Shifts for DifferentChemical Groups That Appear in the Monomers Involved inPolymerization Reactions

no. chemical group chemical shifts (δ/ppm)

1 −C6H5 (from styrene) 6.57−7.502 R2CCH2, singlet (from methacrylates) 6.103 RCHCH2, doublet (from styrene) 5.754 R2CCH2, singlet (from methacrylates) 5.505 RCHCH2, doublet (from styrene) 5.256 −O−CH2− (from monomers) 4.207 −O−CH2− (from polymer) 3.75

Figure 2. Characteristic 1H NMR spectrum of one of the synthesizedcopolymer samples in mineral base oil SN-150 solution.



mode where a mixture of monomers and whole amount ofdiperoxide initiator was added simultaneously into the reactorat the beginning of the experiments. Sequential decompositionof the second peroxide group allows for repeated initiation(reinitiation) of the macromolecular species produced in theearlier stage of the process and, thereby, maintains an optimalratio between the amount of formed radicals and residualmonomers in the reaction mixture during the whole polymer-ization reaction.26−28

The decomposition mechanism of diperoxide initiator ispresented in Scheme 1. In a first stage (I), two different primaryradicals are formed: one similar to the monofunctional initiator(II) and the other bearing an undecomposed peroxide group(III). This unreacted peroxide group in polymers (IV)decomposes further, and new radical species are generated(II, V) and distributed via elementary chain polymerizationreactions.The kinetic results (Xp vs tp) obtained for both investigated

systems are compared and discussed. The overall molarmonomer conversion (Xp) versus polymerization time (tp)relationships established by 1H NMR measurements are shownin Figures 3 and 4.

From the presented results, it can be observed that with anincrease of the content of DMAEM in the initial monomermixture within a range 0−10 mol %, higher values of monomerconversions were achieved in both investigated systems. Thedistinctions between the performed experiments with differentshares of DMAEM are more pronounced in system 1. Using adiperoxide initiator in system 2, a higher rate of polymerizationand complete conversion of monomer were accomplished incomparison with results obtained when the conventionalmonofunctional initiator was used. Also, it can be observedthat styrene monomer converts more gradually as a function oftime in system 1 where monoperoxide initiator was employed,while in system 2 in the first 20 min more than 40% of styreneconverts and incorporates into copolymer chain during thecourse of the synthesis (see Figure 5). According to thepreliminary results for homopolymerization of styrene insolution and initiator manufacturer data,29 thermal self-initiation of styrene is neglected because the differencesbetween operating temperatures when monoperoxide (100°C) and diperoxide initiators (105 °C) were employed are verylittle (ΔT = 5 °C), and the amount of the styrene among othermonomers in initial reaction mixture is only 15 wt % relative tothe total mass of monomers.

Scheme 1. Sequential Decomposition of Diperoxide Initiator

Figure 3. Overall molar monomer conversion as a function ofpolymerization time for copolymerization of Sty, DDMA, ODMA, andDMAEM in mineral base oil SN-150 solution with monoperoxideinitiator at 100 °C (system 1).

Figure 4. Overall molar monomer conversion as a function ofpolymerization time for copolymerization of Sty, DDMA, ODMA, andDMAEM in mineral base oil SN-150 solution with diperoxide initiatorat 105 °C (system 2).



Presented results demonstrate practical benefits of the usageof bifunctional initiator in the manufacturing process ofpolymer additives for lubricating mineral oils because completeconversions of monomers in shorter reaction time wereaccomplished. It should be emphasized that market require-ments regarding the polymer grades are very specific becauseeven a small amount of residual monomer may have an adverseimpact on the quality and classification of the producedpolymer materials.34

3.3. Conversion Heterogeneity. In most cases, thecomposition of copolymer differentiates from the compositionof the corresponding mixture of monomers in initial feed due tothe differences in their reactivity. Also, it is difficult to achievehomogeneity of their chemical composition at differentconversion levels. In our previous work, a low-conversionterpolymerization of styrene, dodecyl, and N,N-dimethylami-noethyl methacrylate in toluene solution was investigatedwhere it was found that styrene content in the terpolymers isalways higher than in the initial monomer feeds and thatcompositional drift was significant, more than 10 mol %.35 Thiswas a consequence of notable differences in copolymerizationreactivity ratios between styrene and methacrylates (see Table3). Because preserving a homogeneous composition of the

polymer product can be important for most applications,36,37 inthis chapter conversion heterogeneity for both investigatedsystems was studied.In Figure 6, the dependence between experimentally

determined average molar fraction of styrene (Sty) and totalmethacrylates (TMA) in a copolymer chain and overall molarmonomer conversion achieved at exact polymerization timeintervals (10, 20, 40, 60, 120, 180, 240, and 300 min) for thefour selected experiments are presented.From the obtained result, it can be observed that at lower

conversion level up to 50 mol %, average copolymercomposition is significantly different from the initial monomerfeed mixture in both investigated systems. With the increase ofoverall monomer conversion, the average composition ofsynthesized copolymers approaches the initial feed mixture.

Figure 5. Molar monomer conversion of styrene (Sty) and total methacrylates (TMA) as a function of polymerization time for the four selectedcopolymerization experiments in Table 1: (a) 1-1, (b) 1-2, (c) 2-1, and (d) 2-2.

Table 3. Copolymerization Reactivity Ratios, r, for theDMAEM/Sty/DDMA System

monomer 1 monomer 2 r

DMAEM-1 Sty-2 (r12) 0.45 (r21) 1.77DMAEM-1 DDMA-3 (r13) 0.79 (r31) 0.74Sty-2 DDMA-3 (r23) 2.19 (r32) 0.45



Although in most cases it is better to avoid compositionheterogeneity, in this particular system where preparation ofpolymer additives was performed in mineral base oil solution,occurrence of conversion heterogeneity does not have anadverse impact on the final outcome of the polymerizationprocess. In some way, it even contributes to a better miscibilityand compatibility of the examined polymeric additive withmineral base oil, which itself is also a mixture of polar andnonpolar components such as alkanes, cyclic paraffins,aromatics, and hybrid hydrocarbons.3.4. Molar Mass Distribution. Molar mass distributions of

synthesized copolymers selected at final conversion weredetermined by size exclusion chromatography. The obtainedweight-average molecular weight (Mw) and number-average(Mn) molecular weight for both investigated systems arepresented in Table 4 and Figures 7 (system 1) and 8 (system2). Additionally, degrees of polymerization for both inves-tigated systems were calculated and presented in Table 4 andFigure 9.From the obtained results, it can be observed that with an

increase of the content of DMAEM in the feed mixture,number-average (Mn) and weight-average (Mw) molecularweight values decrease from 64.5 (103.7) to 20.5 (31.6) kgmol−1 in system 1 and from 68.4 (119.2) to 29.8 (60.6) kgmol−1 in system 2, respectively (see Table 4). Also, Figure 9shows that DP as a function of mole fraction of DMAEM in the

initial feed in both investigated systems decreases significantly,and, as expected, this is more pronounced in system 1. Despitethe higher reaction temperatures in systems initiated by a

Figure 6. Average molar fraction of styrene (Sty) and total methacrylates (TMA) versus overall molar monomer conversion achieved at exactpolymerization time interval (10, 20, 40, 60, 120, 180, 240, and 300 min) for the four selected copolymerization experiments in Table 1: (a) 1-1, (b)1-2, (c) 2-1, and (d) 2-2.

Table 4. Number-Average (Mn), Weight-Average (Mw)Molecular Weights, Degree of Polymerization (DP), andPolydispersity Index (PI) Values Obtained forPolymerization Systems 1 and 2

experiment

0-1 1-1 2-1 3-1 4-1 5-1

Mw/kg mol−1

103.66 91.88 63.69 49.49 39.41 31.57

Mn/kg mol−1

64.48 53.91 38.21 31.57 23.57 20.45

PI = Mw/Mn 1.61 1.70 1.67 1.57 1.67 1.54DP = Mn/Mm

281.56 237.63 170.03 141.80 106.87 93.62

experiment

0-2 1-2 2-2 3-2 4-2 5-2

Mw/kg mol−1

119.24 108.19 105.80 85.42 83.36 60.59

Mn/kg mol−1

68.42 56.96 44.80 42.71 32.50 29.77

PI = Mw/Mn

1.75 1.90 2.36 2.00 2.55 2.04

DP = Mn/Mm

298.74 251.07 199.39 191.86 147.36 136.31



bifunctional initiator, the average molecular weight values arehigher in comparison with the values obtained withmonoperoxide initiator. This enables an increase of productivitythrough modification of the manufacturing process of well-defined polymeric additives for lubricating mineral oils.Established polydispersity index values (PI = Mw/Mn) for

synthesized copolymers show that bifunctional initiator (system2) produced copolymers with somewhat broader molar massdistribution (see Table 4 and Figure 10). This is not in linewith literature findings,21−28 although most recent studiescarried out with diperoxide initiator referred to a bulkhompolymerization of styrene. However, the obtained PI

values between 1.75 and 2.55 are still sufficient for theirpractical application as viscosity index improvers.38

3.5. Application Properties. Some application propertiesof polymer additives samples synthesized in mineral base oilSN-150 with a usage of diperoxide initiator at 105 °C (system2) were investigated, that is, viscosity, viscosity index, viscosityloss, and pour point. Synthesized polymer samples from series 2are chosen for further investigation because completeconversion of monomers to polymer was accomplished. Forexperiments where conventional monofunctional initiator wasused, the highest conversion achieved was around 85% at 10mol % of the DMAEM in the initial feed mixture.Kinematic viscosity was measured for the polymerization

batches diluted by the SN-200 base oil to the polymerconcentration of 5 wt %, at 40 and 100 °C. Pour point

Figure 7. Number-average (Mn) and weight-average (Mw) molecularweight values for copolymers at final conversion as a function of molefraction of N,N-dimethylaminoethyl methacrylate in the initial feed forpolymerization system 1.

Figure 8. Number-average (Mn) and weight-average (Mw) molecularweight values for copolymers at final conversion as a function of molefraction of N,N-dimethylaminoethyl methacrylate in the initial feed forpolymerization system.

Figure 9. Degree of polymerization (DP) for copolymers at finalconversion as a function of mole fraction of N,N-dimethylaminoethylmethacrylate in the initial feed for polymerization systems 1 and 2.

Figure 10. Polydispersity index values (PI) for copolymers at finalconversion as a function of mole fraction of N,N-dimethylaminoethylmethacrylate in the initial feed for polymerization systems 1 and 2.



measurements are performed at polymer concentrations of 0.5wt % in mineral base oil. The data are given in Table 5.From the obtained application properties given in Table 5, it

can be observed that viscosities of polymer solutions thatcontain DMAEM are significantly higher than those ofsolutions without DMAEM, despite lower values of molarmasses (see Table 4 and Figure 9). Also, kinematic viscosityvalues at 100 °C before the shear stability test for all polymersolutions that contain DMAEM are higher than 14.0 mm2 s−1

and remain around that value, although with increase ofDMAEM in the initial feed mixture, molar masses and degreeof polymerization significantly decrease. This also confirmstheir application as viscosity index improvers. Therefore, theabrupt increase of viscosity of polymer solutions with theDMAEM is not a consequence of the differences in molarmasses of polymers, but of the change of solvent/polymermolecular interactions.9,39 By incorporation of DMAEM into acopolymer, mineral base oil becomes thermodynamically amuch better solvent, and thereby the volume of hydrodynamiccoil of polymeric molecules in the solutions is larger, whichresults in the viscosity increase. This assumption wasinvestigated and described in more detail in our previouswork on viscometric behavior of diluted solutions ofhomopolymeric constituents of investigated additives.40

Toluene was used as a model solvent, because somehomopolymeric constituents of investigated additives such asPMMA and PS are not soluble in the mineral base oil.Monomer with a strongly polar functional group such asDMAEM incorporated in a small amount (from 2 to 10 mol %)into the polymer additive contributes to the better solubility inthe investigated temperature range from 40 to 100 °C becauseits homopolymer is soluble in mineral base oil. Furthermore, itwas found that among methacrylates the highest viscosity isdisplayed by the DMAEM solutions (see Table 6), significantlyhigher than solutions of the short- and long-chain poly(alkylmethacrylates). This behavior is in line with the viscosity resultsobtained for polymer solutions in base oil. Also, theexperimentally determined Huggins’ constant, kH, obtainedfor the PDMAEM/toluene system was 0.45, which ischaracteristic for a polymer in a thermodynamically goodsolvent. Toluene is a particularly poor solvent for the PMMA,displaying kH ≈ 0.7. In case of PDDMA and PODMA solutions,kH values above 1 are noticed, characteristic for the poly(alkylmethacrylates) solutions as systems prone to the associations of

polymeric molecules.9,41 Values of limiting viscosity numbers,[η], which are a measure of polymer/solvent interactions anddirectly proportional to the size of hydrodynamic coil of thepolymer molecules in solution, support these findings. Theslope of Huggins’ straight line, b, which is a measure ofpolymer/polymer interaction, has highest values for PDMAEMsolutions, followed by the long-chain poly(alkyl methacrylates),PDDMA and PODMA, and finally the lowest values forsolutions of methyl methacrylate polymer.The viscosity index values of the polymer solutions that

contain DMAEM are slightly higher as compared to theconventional solutions without DMAEM, while the pour pointsof all investigated oil solutions are in the narrow temperaturerange between −18.0 and −21.0 °C, proving the well-knownuse of the investigated class of copolymers as excellent mineraloil pour point depressants.30−32

With the increase of the macromolecular coil volume,viscosity increases, but the shear stability of polymer solutionsdecreases. However, obtained values of kinematic viscosity at100 °C after shear stability test are above a limiting value of12.5 mm2 s−1, and they meet the requirements of the newACEA and API international specifications for multigradeengine oils42−46 (see Table 5).

4. CONCLUSIONSIn this work, free radical copolymerization kinetics of long-chain alkyl methacrylates (dodecyl-methacrylate, DDMA; andoctadecyl methacrylate, ODMA) and functional alkyl mono-mer, N,N-dimethylaminoethyl methacrylate (DMAEM), withstyrene (Sty) was investigated. Reactions were conducted

Table 5. Summarized Application Properties of Polymer Additives Synthesized in Mineral Base Oil SN-150 (5 wt % Solutions)with a Usage of Diperoxide Initiator at 105 °C (System 2)a

experiment

01-2 1-2 2-2 3-2 4-2 5-2

Viscosity at 40 °Cνi/mm

2 s−1 84.64 100.46 94.49 94.94 95.23 98.01νf/mm

2 s−1 79.48 86.97 84.72 84.41 84.30 84.20Δν at 40 °C 6.10 13.43 10.34 11.09 11.48 14.09Viscosity at 100 °Cνi/mm

2 s−1 12.65 15.11 14.22 14.32 14.41 14.96νf/mm

2 s−1 11.87 13.06 12.70 12.66 12.68 12.71Δν at 100 °C 6.17 13.56 10.70 11.67 12.00 15.04Viscosity IndexVIi 147 158 155 156 157 160VIf 143 150 148 148 149 149Tp/°C −18.0 −18.0 −18.0 −18.0 −21.0 −21.0

aKinematic viscosity (i, initial; f, sheared); viscosity index, VI; viscosity loss, Δν; and pour point temperature (Tp).

Table 6. Experimental Values of the Slope of Huggins’Straight Line (b), Limiting Viscosity Numbers ([η]), andHuggins’ Constants (kH) of Investigated Diluted Solutions ofPolymers in Toluene at 30 °C40

polymer b/cm6 g−2 [η]/cm3 g−1 kH r2

PDMAEMa 2601 75.8 0.45 0.98PMMAb 225 17.7 0.72 0.95PDDMAc 757 24.2 1.30 0.99PODMAd 714 15.0 3.17 0.90

aPDMAEM: poly(dimethylaminoethyl methacrylate). bPMMA: poly-(methyl methacrylate). cPDDMA: poly(dodecyl methacrylate).dPODMA: poly(octadecyl methacrylate).



isothermally up to high-conversion in mineral base oil solution,using two types of peroxide initiator, monofunctional (system1) and bifunctional (system 2), respectively. For bothinvestigated systems, the influence of process conditions onthe composition and average molecular weights and molecularweight distribution was examined. The compositions ofsynthesized polymers were determined by 1H NMR andstructural properties by SEC. Application properties of theprepared polymeric additive solutions in mineral base oil SN-200 were determined by standardized test methods. Obtainedkinetics results demonstrate the practical advantage of the usageof bifunctional peroxide initiator in manufacturing of thelubricating mineral oil polymeric additives in comparison withmonoperoxide, because the process was performed by a simple,entirely batch routine, and complete conversion of monomerswas accomplished. Also, it was found that bifunctional initiatorproduced copolymers with higher average molecular weightsand somewhat broader molar mass distribution (higher PI).Investigated application properties of dispersant polymethacry-late additives (d-PSAMA) demonstrated that this new type ofadditive was fully comparable to existing commercial products,and also it provided some other advantages such as higherkinematic viscosity, lower values of pour point temperatures,and satisfying values of kinematic viscosity at 100 °C after theshear stability test.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] authors declare no competing financial interest.

■ ACKNOWLEDGMENTSFinancial support from the Ministry of Science, Education andSports of the Republic of Croatia (Grant No. 125-1251963-1980) is acknowledged.

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