Production and Application Properties of Dispersive Viscosity Index

Article pubs.acs.org/IECR

Production and Application Properties of Dispersive Viscosity Index Improvers Ivana Šoljić Jerbić,† Jelena Parlov Vuković,‡ and Ante Jukić*,† †

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, Lovinčićeva 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 copolymerizations isothermally up to the high conversion in mineral base oil solution, using monofunctional or bifunctional peroxide initiator. The obtained kinetics results reveal the benefits of the usage of a bifunctional peroxide initiator over a monofunctional, because complete conversion of monomers was accomplished in the shorter reaction time, performing the process in a full batchwise mode. When the bifunctional initiator was applied, the required polymerization temperature was slightly higher (105 °C), and copolymers of higher average molecular weight values (Mw = 60−120 kg mol−1) were obtained, while in case of the monofunctional peroxide initiator, the reaction temperature was 100 °C, and average molecular weight values of copolymers were Mw = 30−100 kg/mol. Investigated application properties demonstrated that d-PSAMA additives were fully comparable with conventional pure methacrylate additives, and also it provided other advantages such as higher viscosity index and kinematic viscosity, lower values of pour point temperatures, as well as better dispersant and detergent properties. Thus, by increasing the N,N-dimethylaminoethyl methacrylate share in copolymers from 2 to 10 mol %, their weight average molecular weight decreased from 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 and architecture are among the polymers most commonly used as viscosity modifiers, mainly for lubricating mineral oils.1,2 Most frequently, they are copolymers of alkylmethacrylate monomers with optimized share of lateral alkyl groups because each of these 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 of solutions, while the methyl group contributes to the stiffness of the polymer chain.3 Recently, the styrene was used as a comonomer for modifying the poly(alkyl methacrylate) additive to increase its thermal and oxidation stability.4−6 These improvements are facilitated by increasing the styrene content, but application of copolymers containing high styrene content is limited by their relatively low solubility in mineral oils.3,7−9 New standards regarding emissions of gases and resulting engine design changes such as implementation of the EGR (exhaust gas recirculation) system in diesel engines led to the changes in lubricant requirements and formulations.10 While the EGR system effectively reduces NOx emissions to the atmosphere, soot load in the lubricant can be expected to increase dramatically, causing increased temperature and viscosity, dispersancy failure, fouling, deposits, and wear.11 This has directed modern lubrication science toward the development of a new type of polymer additives with multifunctional activity and ability to improve their dispersancy in addition to the control of viscosity properties.12 Introducing © 2012 American Chemical Society

the ability of dispersancy into a polymer additive demands a carefully engineered incorporation of a strongly polar functional group to the main polymer backbone. The most commonly employed functional groups are amines, alcohols, or amides.13−16 Such formulated polymer additives will have the ability to keep insoluble combustion debris and oil oxidation products dispersed in the oil, which will prevent their deposition on the main part of the engine. This will have a direct effect on minimizing harmful engine exhaust emissions, increasing engine life, and controlling oil consumption by maintaining clean engine operation.10 A free-radically initiated copolymerization is the most commonly used technique for the synthesis and production of the polymethacrylate viscosity index improvers.17,18 The main limitation of a such mechanism is its small ability to influence the molar mass distribution and structural properties of the synthesized copolymers.19 Such synthesized polymer additives need to fulfill certain structural requirements because high values of weight-average molecular weights and narrow molecular weight distribution are needed for better resistance against shear rates developed in lubricating conditions.20 Also, from the free radical theory, it is well-known that molecular weight is inversely proportional to the rate of polymerization.17 As such, it is not possible to simultaneously obtain high reaction rates and high polymer molecular weights for bulk, Received: Revised: Accepted: Published: 11914

April 18, 2012 July 20, 2012 August 22, 2012 August 22, 2012 dx.doi.org/10.1021/ie301010n | Ind. Eng. Chem. Res. 2012, 51, 11914−11923

Industrial & Engineering Chemistry Research

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suspension, and solution processes with a usage of a conventional 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 of initiators with two or more functional peroxide groups of different thermal stabilities in comparison with conventional monoperoxide initiators.25 Beside, the complete conversion of monomers can be achieved, and reaction time can be significantly reduced with no need for any modification of reactor equipment.26−28 In this work, high-conversion copolymerization kinetics of styrene, dodecyl methacrylate, octadecyl methacrylate, and N,N-dimethylaminoethyl methacrylate was investigated. Reactions were performed in a mineral base oil SN-150 solution by using both bifunctional or monofunctional peroxide initiators, under isothermal conditions. All of the synthesized copolymers were characterized with respect to composition (1H NMR) and molar mass distribution (SEC). The main application properties of the prepared polymer solution samples in mineral base oil SN-200 such as viscosity, pour point, viscosity loss, and viscosity index were investigated by using standardized test methods.

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

2. EXPERIMENTAL SECTION 2.1. 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-butyl peroxy-2-ethylhexanoate (Mm = 216.3 g mol−1) (Trigonox 21, 70 wt % solution, Akzo Chemie) and bifunctional 1,1-di(tertbutylperoxy)-3,3,5-trimethylcyclohexane (Mm = 302.4 g mol−1) (Trigonox 29, 90 wt % solution, Akzo Chemie), were used as received as well as mineral base oils SN-150 and SN-200 (INA Lubricants, Zagreb). Mineral base oil SN-150 with kinematic viscosity of 30.5 mm2 s−1 at 40 °C, viscosity index of 98, and pour point of −12 °C was used as a solvent for polymerization reactions. Mineral base oil SN-200 with kinematic viscosity of 40.6 mm2 s−1 at 40 °C, viscosity index of 104, and pour point of −9 °C was used for preparation of diluted polymer solution samples for examination of main application properties. Initiator decomposition rate coefficients were given as temperature functions:29 kd/s−1 = 1.54 × 1014 exp[−124 900/(RT)] and kd/s−1 = 7.59 × 1013 exp[−127 520/(RT)] for monofunctional 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 in concentration curves that are comparable (Figure 1). Higher temperatures (105 °C for mono- and 115 °C for diperoxide) correspond to the initiator half-lives of around 20 min, while lower temperatures (85 °C for mono- and 95 °C for diperoxide) 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) at different temperatures, it can be observed for both initiator types that decomposition is optimal at temperatures that suit the initiators half-lives (t1/2) of approximately 60 min. Therefore, those temperatures were chosen for further investigation. Decomposition at higher and lower temperatures results in initiator efficiency decrease.

2.2. Polymerizations. All additive samples were prepared by free radical copolymerization of four different monomers in mineral base oil SN-150 solution by using two types of peroxide initiators, monofunctional (Trigonox 21) or bifunctional (Trigonox 29), respectively. Polymerizations were performed at 100 °C when monomer mixture was initiated with monoperoxide (system 1) and at 105 °C when monomer mixture was initiated with diperoxide (system 2). Those temperatures were chosen to conform to the initiators half-lives of approximately 1 h. Experiments were carried out for 5 h in a double jacket glass reactor (0.50 L) connected to a thermostatted bath, equipped with a mechanical stirrer (200 rpm) under nitrogen atmosphere. The total monomer concentration was 50 wt %, and the concentration of the initiators was 1.0 wt % relative to the monomers. Recent kinetic studies show that diperoxide initiator efficiency increases with its concentration, and very often attempts where the amount of monoperoxide would be replaced with half of the amount of diperoxide initiator do not yield a complete conversion of monomers.26−28 Therefore, the molar ratio of monoperoxide to diperoxide initiator was 3:2. Compositions of monomers in initial feed were carefully selected to design the new type of additive with improved application properties. The styrene content in the initial monomer mixture was limited at optimal 15 wt % due to the relatively low solubility in mineral oils.3,7−9 Because recent studies10,15,16 have shown that even a small fraction of a certain functional monomer may have a strong impact on desired dispersant properties of the synthesized polymer additives, DMAEM was added in a small amount to the reaction mixture within 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 wellknown favorable effect on viscosity and pour point depression of lubricating mineral oils.30−32 Described process conditions for the performed polymerization experiments are summarized and given in Table 1. 2.3. Characterization. Reaction mixture samples were taken directly from the reactor at exact time intervals (10, 20, 40, 60, 120, 180, 240, and 300 min) whereat the final 11915

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


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corded and interpreted.33 The chemical shifts for certain types of protons in the chemical groups that appear in the monomers involved in polymerization reactions are given in Table 2.

Table 1. Process Conditions for the Polymerizations Performed at 100 °C Using Monoperoxide (System 1) and at 105 °C Using Diperoxide Initiator (System 2)a experiment 0-1(0-2) Content, wt % DMAEM 0.0 Sty 15.0 DDMA 42.5 ODMA 42.5 Content, mol % DMAEM 0.0 Sty 33.0 DDMA 38.2 ODMA 28.8

1-1(1-2)

2-1(2-2)

3-1(3-2)

4-1(4-2)

5-1(5-2)

1.4 15.0 41.8 41.8

2.8 15.0 41.4 41.4

4.2 15.0 40.4 40.4

5.7 15.0 39.65 39.65

7.2 15.0 38.9 38.9

2.0 32.7 37.3 28.0

4.0 32.4 36.4 27.2

6.0 32.1 35.4 26.5

8.0 31.8 34.4 25.8

10.0 31.5 33.4 25.1

Table 2. Assignment of the Chemical Shifts for Different Chemical Groups That Appear in the Monomers Involved in Polymerization Reactions no.

chemical group

chemical shifts (δ/ppm)

1 2 3 4 5 6 7

−C6H5 (from styrene) R2CCH2, singlet (from methacrylates) RCHCH2, doublet (from styrene) R2CCH2, singlet (from methacrylates) RCHCH2, doublet (from styrene) −O−CH2− (from monomers) −O−CH2− (from polymer)

6.57−7.50 6.10 5.75 5.50 5.25 4.20 3.75

a

Concentration 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.

The content of the residual styrene monomer (mol %) in examined samples was determined from the corresponding 1H NMR spectra by integrating the areas of characteristic peaks that appear at ∼5.25 ppm (doublet) and at ∼5.75 ppm (doublet). On the basis of the intensities of the signals that appear at ∼5.50 ppm (singlet) and at ∼6.10 ppm (singlet), the contents (mol %) of total methacrylate monomers such as dodecyl, octadecyl, and N,N-dimethylaminoethyl methacrylate were determined because they behave quite similarly in the applied NMR field and cannot be distinguished within the spectra. The content of the formed polymer (mol %) was determined by integrating the area, corresponding to the two oxymethylene protons that appear at ∼3.75 ppm (Figure 2).

conversions of monomers were determined by 1H NMR using a Bruker Avance model. The 1H NMR spectra were recorded at 300 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 using a GPC-20 Polymer Laboratories size exclusion chromatograph. Measurements were performed in toluene as an eluent with a flow rate of 1.0 mL min−1. The calibration curve was based on polystyrene standards (EasyCal PS-1B, 580−2 560 000 g mol−1) of narrow distribution. Application properties of diluted polymeric additive solutions in the mineral base oil SN-200 were determined by the standardized test methods. Kinematic viscosity (ν) of the polymer solution samples was determined using ASTM D-445 test method. Measurements were carried out at 40 and 100 °C, using the calibrated Cannon−Fenske capillary viscometers immersed in a constant temperature bath. Viscosity index (VI) was calculated from obtained kinematic viscosities at 40 and 100 °C by using ASTM D2270 standardized practice. The shear stability of polymer-containing oil was measured by using the DIN-51382 test method. This test method measures the percent viscosity loss at 100 °C of investigated fluids when evaluated by a diesel injector apparatus procedure that uses European diesel injector test equipment. The viscosity loss reflects polymer degradation due to the shear at the nozzle. Pour points of the polymer solution samples were determined by the ISO 3016 test method. Measurements were carried out by placing a test jar with 50 mL of the sample submerged into a cooling medium. The sample temperature was measured in 1 °C increments at the top of the sample until the liquid stopped pouring. All viscosity and pour point measurements were run in duplicate, and the average values were reported.

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

3.2. Kinetic Study: Monomer Conversion versus Polymerization Time (X p vs t p ). The free radical copolymerizations of styrene, dodecyl-, octadecyl-, and N,Ndimethylaminoethyl methacrylate were performed in mineral base oil SN-150 solution using monofunctional initiator, tertbutylperoxy-2-ethyl-hexanoate, at 100 °C (system 1) or bifunctional initiator, 1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane, at 105 °C (system 2). Monoperoxide initiator was added gradually to the reaction mixture (modified batch process) to maintain constant radical flux.17,18 To promote and simplify the production procedure, polymerization process initiated with diperoxide was performed in a fully batchwise

3. RESULTS AND DISCUSSION 3.1. Composition Analysis. Determination of composition of synthesized copolymers in mineral base oil is performed by 1 H NMR. To determine the amount of residual monomers and overall molar monomer conversion during the polymerization time, the 1H NMR spectra for all monomers present in the initial reaction mixture such as styrene (Sty), dodecyl methacrylate (DDMA), octadecyl methacrylate (ODMA), and N,N-dimethylaminoethyl methacrylate (DMAEM) were re11916



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Scheme 1. Sequential Decomposition of Diperoxide Initiator

mode where a mixture of monomers and whole amount of diperoxide initiator was added simultaneously into the reactor at the beginning of the experiments. Sequential decomposition of the second peroxide group allows for repeated initiation (reinitiation) of the macromolecular species produced in the earlier stage of the process and, thereby, maintains an optimal ratio between the amount of formed radicals and residual monomers in the reaction mixture during the whole polymerization reaction.26−28 The decomposition mechanism of diperoxide initiator is presented in Scheme 1. In a first stage (I), two different primary radicals 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 polymerization reactions. The kinetic results (Xp vs tp) obtained for both investigated systems are compared and discussed. The overall molar monomer conversion (Xp) versus polymerization time (tp) relationships established by 1H NMR measurements are shown in Figures 3 and 4.

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

From the presented results, it can be observed that with an increase of the content of DMAEM in the initial monomer mixture within a range 0−10 mol %, higher values of monomer conversions were achieved in both investigated systems. The distinctions between the performed experiments with different shares of DMAEM are more pronounced in system 1. Using a diperoxide initiator in system 2, a higher rate of polymerization and complete conversion of monomer were accomplished in comparison with results obtained when the conventional monofunctional initiator was used. Also, it can be observed that styrene monomer converts more gradually as a function of time in system 1 where monoperoxide initiator was employed, while in system 2 in the first 20 min more than 40% of styrene converts and incorporates into copolymer chain during the course of the synthesis (see Figure 5). According to the preliminary results for homopolymerization of styrene in solution and initiator manufacturer data,29 thermal selfinitiation of styrene is neglected because the differences between operating temperatures when monoperoxide (100 °C) and diperoxide initiators (105 °C) were employed are very little (ΔT = 5 °C), and the amount of the styrene among other monomers in initial reaction mixture is only 15 wt % relative to the total mass of monomers.

Figure 3. Overall molar monomer conversion as a function of polymerization time for copolymerization of Sty, DDMA, ODMA, and DMAEM in mineral base oil SN-150 solution with monoperoxide initiator at 100 °C (system 1). 11917



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

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

Presented results demonstrate practical benefits of the usage of bifunctional initiator in the manufacturing process of polymer additives for lubricating mineral oils because complete conversions of monomers in shorter reaction time were accomplished. It should be emphasized that market requirements regarding the polymer grades are very specific because even a small amount of residual monomer may have an adverse impact on the quality and classification of the produced polymer materials.34 3.3. Conversion Heterogeneity. In most cases, the composition of copolymer differentiates from the composition of the corresponding mixture of monomers in initial feed due to the differences in their reactivity. Also, it is difficult to achieve homogeneity of their chemical composition at different conversion levels. In our previous work, a low-conversion terpolymerization of styrene, dodecyl, and N,N-dimethylaminoethyl methacrylate in toluene solution was investigated where it was found that styrene content in the terpolymers is always higher than in the initial monomer feeds and that compositional drift was significant, more than 10 mol %.35 This was a consequence of notable differences in copolymerization reactivity ratios between styrene and methacrylates (see Table 3). Because preserving a homogeneous composition of the

monomer 1

monomer 2

DMAEM-1 DMAEM-1 Sty-2

Sty-2 DDMA-3 DDMA-3

r (r12) 0.45 (r13) 0.79 (r23) 2.19

(r21) 1.77 (r31) 0.74 (r32) 0.45

polymer product can be important for most applications,36,37 in this chapter conversion heterogeneity for both investigated systems was studied. In Figure 6, the dependence between experimentally determined average molar fraction of styrene (Sty) and total methacrylates (TMA) in a copolymer chain and overall molar monomer conversion achieved at exact polymerization time intervals (10, 20, 40, 60, 120, 180, 240, and 300 min) for the four selected experiments are presented. From the obtained result, it can be observed that at lower conversion level up to 50 mol %, average copolymer composition is significantly different from the initial monomer feed mixture in both investigated systems. With the increase of overall monomer conversion, the average composition of synthesized copolymers approaches the initial feed mixture. 11918



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Figure 6. Average molar fraction of styrene (Sty) and total methacrylates (TMA) versus overall molar monomer conversion achieved at exact polymerization 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.

Although in most cases it is better to avoid composition heterogeneity, in this particular system where preparation of polymer additives was performed in mineral base oil solution, occurrence of conversion heterogeneity does not have an adverse impact on the final outcome of the polymerization process. In some way, it even contributes to a better miscibility and compatibility of the examined polymeric additive with mineral base oil, which itself is also a mixture of polar and nonpolar 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 were determined by size exclusion chromatography. The obtained weight-average molecular weight (Mw) and number-average (Mn) molecular weight for both investigated systems are presented in Table 4 and Figures 7 (system 1) and 8 (system 2). Additionally, degrees of polymerization for both investigated systems were calculated and presented in Table 4 and Figure 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) molecular weight values decrease from 64.5 (103.7) to 20.5 (31.6) kg mol−1 in system 1 and from 68.4 (119.2) to 29.8 (60.6) kg mol−1 in system 2, respectively (see Table 4). Also, Figure 9 shows that DP as a function of mole fraction of DMAEM in the

Table 4. Number-Average (Mn), Weight-Average (Mw) Molecular Weights, Degree of Polymerization (DP), and Polydispersity Index (PI) Values Obtained for Polymerization Systems 1 and 2 experiment M̅ w/ kg mol−1 M̅ n/ kg mol−1 PI = M̅ w/M̅ n DP = M̅ n/ Mm

0-1

1-1

2-1

3-1

4-1

5-1

103.66

91.88

63.69

49.49

39.41

31.57

64.48

53.91

38.21

31.57

23.57

20.45

1.61 281.56

1.70 237.63

1.67 170.03

1.57 141.80

1.67 106.87

1.54 93.62

experiment M̅ w/ kg mol−1 M̅ n/ kg mol−1 PI = M̅ w/ M̅ n DP = M̅ n/ Mm

0-2

1-2

2-2

3-2

4-2

5-2

119.24

108.19

105.80

85.42

83.36

60.59

68.42

56.96

44.80

42.71

32.50

29.77

1.75

1.90

2.36

2.00

2.55

2.04

298.74

251.07

199.39

191.86

147.36

136.31

initial feed in both investigated systems decreases significantly, and, as expected, this is more pronounced in system 1. Despite the higher reaction temperatures in systems initiated by a 11919



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

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

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

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

values between 1.75 and 2.55 are still sufficient for their practical application as viscosity index improvers.38 3.5. Application Properties. Some application properties of polymer additives samples synthesized in mineral base oil SN-150 with a usage of diperoxide initiator at 105 °C (system 2) were investigated, that is, viscosity, viscosity index, viscosity loss, and pour point. Synthesized polymer samples from series 2 are chosen for further investigation because complete conversion of monomers to polymer was accomplished. For experiments where conventional monofunctional initiator was used, the highest conversion achieved was around 85% at 10 mol % 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 polymer concentration of 5 wt %, at 40 and 100 °C. Pour point

bifunctional initiator, the average molecular weight values are higher in comparison with the values obtained with monoperoxide initiator. This enables an increase of productivity through modification of the manufacturing process of welldefined polymeric additives for lubricating mineral oils. Established polydispersity index values (PI = Mw/Mn) for synthesized copolymers show that bifunctional initiator (system 2) produced copolymers with somewhat broader molar mass distribution (see Table 4 and Figure 10). This is not in line with literature findings,21−28 although most recent studies carried out with diperoxide initiator referred to a bulk hompolymerization of styrene. However, the obtained PI 11920



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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 Viscosity at 40 °C νi/mm2 s−1 νf/mm2 s−1 Δν at 40 °C Viscosity at 100 °C νi/mm2 s−1 νf/mm2 s−1 Δν at 100 °C Viscosity Index VIi VIf Tp/°C a

1-2

2-2

3-2

4-2

5-2

84.64 79.48 6.10

100.46 86.97 13.43

94.49 84.72 10.34

94.94 84.41 11.09

95.23 84.30 11.48

98.01 84.20 14.09

12.65 11.87 6.17

15.11 13.06 13.56

14.22 12.70 10.70

14.32 12.66 11.67

14.41 12.68 12.00

14.96 12.71 15.04

147 143 −18.0

158 150 −18.0

155 148 −18.0

156 148 −18.0

157 149 −21.0

160 149 −21.0

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

measurements are performed at polymer concentrations of 0.5 wt % 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 that contain DMAEM are significantly higher than those of solutions without DMAEM, despite lower values of molar masses (see Table 4 and Figure 9). Also, kinematic viscosity values at 100 °C before the shear stability test for all polymer solutions that contain DMAEM are higher than 14.0 mm2 s−1 and remain around that value, although with increase of DMAEM in the initial feed mixture, molar masses and degree of polymerization significantly decrease. This also confirms their application as viscosity index improvers. Therefore, the abrupt increase of viscosity of polymer solutions with the DMAEM is not a consequence of the differences in molar masses of polymers, but of the change of solvent/polymer molecular interactions.9,39 By incorporation of DMAEM into a copolymer, mineral base oil becomes thermodynamically a much better solvent, and thereby the volume of hydrodynamic coil of polymeric molecules in the solutions is larger, which results in the viscosity increase. This assumption was investigated and described in more detail in our previous work on viscometric behavior of diluted solutions of homopolymeric constituents of investigated additives.40 Toluene was used as a model solvent, because some homopolymeric constituents of investigated additives such as PMMA and PS are not soluble in the mineral base oil. Monomer with a strongly polar functional group such as DMAEM incorporated in a small amount (from 2 to 10 mol %) into the polymer additive contributes to the better solubility in the investigated temperature range from 40 to 100 °C because its homopolymer is soluble in mineral base oil. Furthermore, it was found that among methacrylates the highest viscosity is displayed by the DMAEM solutions (see Table 6), significantly higher than solutions of the short- and long-chain poly(alkyl methacrylates). This behavior is in line with the viscosity results obtained for polymer solutions in base oil. Also, the experimentally determined Huggins’ constant, kH, obtained for the PDMAEM/toluene system was 0.45, which is characteristic for a polymer in a thermodynamically good solvent. 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(alkyl methacrylates) solutions as systems prone to the associations of

Table 6. Experimental Values of the Slope of Huggins’ Straight Line (b), Limiting Viscosity Numbers ([η]), and Huggins’ Constants (kH) of Investigated Diluted Solutions of Polymers in Toluene at 30 °C40 polymer PDMAEM PMMAb PDDMAc PODMAd

a

b/cm6 g−2

[η]/cm3 g−1

kH

r2

2601 225 757 714

75.8 17.7 24.2 15.0

0.45 0.72 1.30 3.17

0.98 0.95 0.99 0.90

a

PDMAEM: poly(dimethylaminoethyl methacrylate). bPMMA: poly(methyl methacrylate). cPDDMA: poly(dodecyl methacrylate). d PODMA: poly(octadecyl methacrylate).

polymeric molecules.9,41 Values of limiting viscosity numbers, [η], which are a measure of polymer/solvent interactions and directly proportional to the size of hydrodynamic coil of the polymer molecules in solution, support these findings. The slope of Huggins’ straight line, b, which is a measure of polymer/polymer interaction, has highest values for PDMAEM solutions, followed by the long-chain poly(alkyl methacrylates), PDDMA and PODMA, and finally the lowest values for solutions of methyl methacrylate polymer. The viscosity index values of the polymer solutions that contain DMAEM are slightly higher as compared to the conventional solutions without DMAEM, while the pour points of all investigated oil solutions are in the narrow temperature range between −18.0 and −21.0 °C, proving the well-known use of the investigated class of copolymers as excellent mineral oil pour point depressants.30−32 With the increase of the macromolecular coil volume, viscosity increases, but the shear stability of polymer solutions decreases. However, obtained values of kinematic viscosity at 100 °C after shear stability test are above a limiting value of 12.5 mm2 s−1, and they meet the requirements of the new ACEA and API international specifications for multigrade engine oils42−46 (see Table 5).

4. CONCLUSIONS In this work, free radical copolymerization kinetics of longchain alkyl methacrylates (dodecyl-methacrylate, DDMA; and octadecyl methacrylate, ODMA) and functional alkyl monomer, N,N-dimethylaminoethyl methacrylate (DMAEM), with styrene (Sty) was investigated. Reactions were conducted 11921



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isothermally up to high-conversion in mineral base oil solution, using two types of peroxide initiator, monofunctional (system 1) and bifunctional (system 2), respectively. For both investigated systems, the influence of process conditions on the composition and average molecular weights and molecular weight distribution was examined. The compositions of synthesized polymers were determined by 1H NMR and structural properties by SEC. Application properties of the prepared polymeric additive solutions in mineral base oil SN200 were determined by standardized test methods. Obtained kinetics results demonstrate the practical advantage of the usage of bifunctional peroxide initiator in manufacturing of the lubricating mineral oil polymeric additives in comparison with monoperoxide, because the process was performed by a simple, entirely batch routine, and complete conversion of monomers was accomplished. Also, it was found that bifunctional initiator produced copolymers with higher average molecular weights and somewhat broader molar mass distribution (higher PI). Investigated application properties of dispersant polymethacrylate additives (d-PSAMA) demonstrated that this new type of additive was fully comparable to existing commercial products, and also it provided some other advantages such as higher kinematic viscosity, lower values of pour point temperatures, and satisfying values of kinematic viscosity at 100 °C after the shear stability test.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Ministry of Science, Education and Sports of the Republic of Croatia (Grant No. 125-12519631980) is acknowledged.

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REFERENCES

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Production and Application Properties of Dispersive Viscosity Index

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