a-olefin copolymers as viscosity modifier additives in lubricants

Transcription

1 IndianJournalof ChemicalTechnology Vol. 5, November998,pp Alkyl methacrylate: a-olefin copolymers as viscosity modifier additives in lubricants P Ghosh,A V Pantilr& A S Sarma R&DCentre,LubrizolIndiaLimited,9/3,ThaneBelapurRoad,Turbhe,NaviMumbai ,India Received9MayI998;accepted20 August.998 The potential for use of copolymers of iso-decyl methacrylate and I-decene as viscosity moditying additives in lubricant compositionshas been investigated.three copolymers were prepared by free-radical initiated polymerization in toluene solvent, using AIBN as initiator and employing different levels of decene (0-40 wt %) in the monomer mixture. The copolymers were characterized employing GPC, DSC/TGA, FTIR and NMR techniques for determining the thermal characteristics, average molecular weights (Mw and Mn), and copolymer composition. The composition of the copolymers with respect to relative proportion of two monomers was determined by two independent methods described in literature based on FTIR and Proton NMR spectroscopy. The level of incorporation of I-decene monomer in the copolymers as determined by these methods was only of the order of7-3%, while literaturereports suggest scope for incorporating upto 2-5% a-olefm under free-radical polymerization conditions. The resulting copolymers were evaluated for potential use as viscosity modifiers in lubricant compositions through viscometric and bench screen tests, in comparisonto a normal poly (alkyl methacrylate)(pma). The results demonstrated a moderate improvement in the performance of one of the experimental copolymers as compared to standard PMA in certain respects. Viscosity modifiers (VM), also often called as viscosity index improvers (VII), are used as additives in lubricant compositions for improving the viscometric and rheological properties of the lubricant\' 2. Occasionally, VMs are looked upon to provide additional performance characteristics (multi-functionality) such as improved low temperature fluidity and dispersancy (i.e. keep the. insoluble materials produced during usage of the lubricant in dispersed form in the oil). Some of the types of polymers generally used commercially as VII, and known to possess certain inhergot advactages and disadvantages in performance, are--(i) Poly Isobutylenes (PIBs), (ii) Hydrogenated Styrene - Isoprene copolymers, (iii) Olefin copolymers (OCP); e,g. Ethylene - Propylene copolymer, (iv) Esters of Styrene - Maleic anhydride copolymers and (v) Poly alkyl methacrylates (PMA). Although PMAs are preferred type of VII in certain applications, they often contribute to enhanced formation of deposits in the engine due to thermal instability of these additives under high temperature conditions. The usual approach to overcome this shortcoming is to incorporate a dispersancy property into the VII. Dispersant versions of PMA type VIIs are usually derived from a nitrogen-containing methacrylate monomer or by grafting the PMA polymer with a nitrogen compound. Although dispersant - PMAs function as better VIIs as compared to parent PMAs, there is a risk of affecting certain beneficial properties associated with normal PMAs such as pour point depressancy and good shear stability; particularly in the case of grafted PMAs. Thus, within the category of PMA type VMs, considerable need and potential exists to develop polymers with improved performance. As a part of the on-going studies on development of PMA-based VMs with improved performance3, it became a matter' of interest to explore alkyl methacrylate and a-olefin copolymers as potential VIIs. Exploratory studies on copolymers of a-olefins with alkyl methacrylates4,5 or with alkyl acrylates6 towards their potential application as synthetic base stocks\

2 372 INDIAN J. CHEM. TECHNOL., NOVEMBER 998 VIIs5 or as pour point depressants6 are reported in literature. However, these being preliminary studies, the information available in these reports about the suitability of these polymers for the intended applications is quite limited. On the basis of these literature reports, as well as from a consideration of the structure and morphology of the polymers, authors envisaged that these copolymers can exhibit improved performance in terms of shear stability, mineral oil solubility and VI properties as well as deposit forming tendency in the engine, as compared to PMAs of corresponding molecular weights. It was also anticipated from literature reports4,7 that suitably tailored copolymers of this type can exhibit pour point depressancy s an additiollal performance, and accordingly employing of a ten-carbon chain length for the alkyl group in the two monomers was preferred. Studies on synthesis of a few copolymers of isodecyl methacrylate and I-decene, characterization of these copolymers and their evaluation as viscosity modifiers is being reported here. Three copolymers from these two monomers were prepared by varying the I-decene concentration in the monomer mixture from 0% wt. to 40% wt. and employing free-radical polymerization technique using AlBN as initiator in toluene solvent. The polymerization conditions were 'selected following literature reports4,6. Characterization of the copolymers was carried out employing GPC, DSC/TGA, IR and NMR techniques. Performance evaluation of the copolymers as VII were carried out as solutions in mineral oil at known concentrations, using a homopolymer of iso-decyl methacrylate as reference for comparisons. The results presented herein indicate that the present copolymers contain I-decene constituent only to an extent of about 8 0%, and they exhibit moderately improved VII performance in some respects as compared to a normal PMA. Experimental Procedure Materials--Commercial I-decene (GC Purity: 96% for CIO) procured from Ethyl Corporation, USA, and iso-decyl methacrylate (GC Purity : 98%), prepared in the laboratory from commercial iso-decyl alcohol alld methacrylic acid and purified by standard procedures, were used. The initiator, AlBN, obtained from Aldrich Chemicals, was purified by crystallisation from m?thanolchloroform mixture. Laboratory grade toluene (Purity: 98%) was used as solvent in polymerization experiments. Commercial paraffinic mineral oils corresponding to HVI 50 Neutral and HVI 500 Neutral grades, having kinematic viscosities (at 00 C) of 5.2 cst and 0.7 cst, respectively, were used for preparing the polymer solutions in base stocks for evaluating their performance as VIIs. Polymerisation-- The polymerization conditions (Initiator concentration: 0.5%; reaction time: 4 h; reaction temperature: 70 C; monomers - to toluene ratio : : wt/wt) were selected on the basis of literature on optimi:z;ation of these experimental parameters in related polymerization studies4,6. The monomers and the toluene were taken in a 4-necked round bottomed flask fitted with a condenser and a stirrer, and the reaction flask was kept under dry nitrogen atmosphere. The solution was heated to 70 C under stirring, and after attaining this temperature the required quantity of initiator (AIBN) dissolved in minimum amount of toluene was added. The reaction mixture was stirred at 70 C for 4 h. At the end of the reaction time, the reaction mixture was poured into methanol with stirring to terminate the polymerization and precipitate the polymer. The polymer was further purified by repeated precipitation of its hexane solution by methanol followed by drying under vacuum at 40 C. A homopolymer of iso-decyl methacrylate was similarly prepared under the same conditions for use in reference experiments. Measurements-IR Spectra were obtained on a Mattson 2040 FT-IR Spectrometer using 0. mm KBr cells. Hand l3c NMR spectra were recorded on a Bruker 80 MHz spectrophotometer using deuterated chloroform as solvent. Average molecular weights (Mw and Mn) were obtained by gel permeation chromatography (GPC) using THF as mohile phase in a Waters GPC system. Thermogravimetric analyses were performed using TA-2 OO/TGA 950 equipment under nitrogen atmosphere. The kinematic viscosities were measured for the polymer-oil blends, on calibrated capillary viscometer according to ASTM D-445. Shear stability of the polymer blends in mineral 0 'I II '"'''" 'I "'!"I'" "I'I'lI""

3 GHOSH et al.: ALKYL METHACRYLATE 373 Table I-Polymers synthesised and their characterization Expt. No. (Polymer No.) %wt. of I-decene in monomer mix. IH-NMR signals (ppm) due to Mol. wt. by GPC -O-CHz -CH -C-CH3 Mw Mn x 0-3 X 0-3 TGAData Decom. Temp,oC PWL Nil /362 24/77 34/365 22/78 306/346 29/7 297/344 54/46 PWL = Percent Weight Loss Expt. No Table 2-Monomer Composition in the copolymers M - A : Monomer A = Iso decyl methacrylate, M - B : Monomer B = l-decene % wt. in the feed M-A M-B % wt. of (M-B) in copolymer By PMR method By FTIR method oils were studied according to ASTM D-3945 method using Bosch Diesel Injector Rig. Results and Discussion Polymer characterization--the previous studies on copolymerization of decyl methacrylate with tetradecene had shown that a true copolymer with specific alternating monomeric chains is formed during copolymerization of this monomeric pair4 Accordingly, the copolymers prepared here can be structurally depicted as. The IR spectra of the co-polymers from the present studies exhibited absorptions due to ester function around 720 cm-!, and absorption bands at 2730 and 2870 cm-l characteristic of CH2 groups in decene chain. Other characteristic absorptions were at 465 and 379 cm-! (C-CH3) and at 240 and 066 cm-i due to asymmetric and symmetric C-O-C stretchings. The average molecular weights (Mw and Mn) of the polymers as determined shown in Table. by GPC technique are Thermogravimetric analysis (TGA) of the copolymers carried out under nitrogen atmosphere showed a two-step decompositi.on (with Tmax values at C and 365 C), as in the case of copolymers of methyl methacrylate and iso-decyl methacrylate in contrast to single step degradation observed8 for poly (iso-decyl methacrylate). The TGA data shows (Table ) that the copolymer from Expt. No.2 is better in thermal stability than other copolymers as well as the homo polymer of isodecyl methacrylate. Accordingly, this copolymer is expected to result in less deposits formation in the engine than a normal PMA, when used as a VII in a lubricant formulation. The extent of incorporation of I-decene in the polymer chain was determined from analysis of the proton NMR spectra through a comparison of the area of the signal due to OCH2 group appearing around 4 ppm with the area of signals due to other protons (Table 2) based on an earlier report6 The absence of signals between 5 and 6 ppm indicated the absence ofvinylic protons. The proton-decoupled 3C-NMR spectrum of one of the copolymers from the present studies, in.;omparison with the 3C-NMR spectrum of poly (iso-decyl methacrylate), was in complete agreement with the assigned structure. The copolymer compositions as determined by an analysis of proton NMR signal due to -OCH2 group as mentioned above was further verified through an analysis of FTIR spectral data

4 374 INDIAN. CHEM. TECHNOL, NOVEMBER 998 Table 3-Performance Characteristics of Polymers as VIIs Polymer No. Test I: VI increase of 50N 0% wt. treat Test 2: SSI Test 3: PPD 0.2% wi. treatment in: t'9 ISON oil* SOONoil* Test 4: CCS vis. in formulated - O"C: Test 5: Panel Coker Test; weight of deposits, mgs ISO ~ *Pour Point (PP) of ISON is -3 C and PP of SOONis DoC, t VI of ISDN'" 99 following a method described in literature9 for determining the composition in copolymers of methyl methacrylate and dodecyl methacrylate. The intensity of C=O absorption in FTIR spectra of present copolymers, as compared to the carbonyl absorptions in a standard, were analysed for quantitative determination of the content of methacrylate component in the copolymer. For quantitative analysis of the carbonyl absorption, a calibrated FTIR curve.of carbonyl absorptions was generated using blends of known quantities of an ethylene-propylene copolymer and poly (iso-decyl methacrylate) in which varied quantities of poly (iso-decyl methacrylate) was present. The comparative results obtained on compositions of copolymers by the Proton NMR method and FTIR method are presented in Table 2. Performance of copolymers as VIIs-The copolymers were evaluated for their potential as VIIs in comparison with the iso-decyl methacrylate homopolymer (PMA) through the following standard visco metric tests and bench screen tests. In these studies, known concentrations of polymers in mineral oils (HVI neutral base stocks) were used without compounding the blends with other usual additives norma;;; present in fully fonnulated lubricants. Test : Viscosity index (VI) detennination (measure of kinematic viscosity change with temperature) by ASTM D-2270 method. Test 2: Shear stability index (SSI) (stability towards mechanical shear) by ASTM D 3945 method using Bosch Injector Rig. (The mt thod is based on determining the extent "f loss in kinematic viscosity of the oil after su~jecting the polymercontaining oil to!iechanical shear). Test 3: Determination cf pour point depression of a mineral oil (HVI ISON or HVI 500N) by ASTM D-97 method. Test 4: Effect on low temperature viscosity viz. CCS (Cold Cranking Simulator) of an engine lubricant determined at -0 C by ASTM D-2602 method. Test 5: Screen test for evaluating engine deposit forming tendency (thermal stability) of lubricants (Panel Coker Test). The results obtained on the polymers when subjected to above tests are summarized in Table 3. The results demonstrate that the copolymer No. 2 from the present studies has marginally better performance than the corresponding normal PMA (iso-decyl methacrylate homopolymer) in tests at serial numbers 2 (shear stability) and 5 (deposit foffiling tendency), while it is almost comparable to the PMA performance in other tests. This indicates that suitably designed copolymers of alkyl methacrylates and a-olefins can serve as superior VIIs as compared to normal PMAs. A further understanding of the envisaged superiority of copolymers of PMA and cc-olefins as VMs as compared to PMA can be had from an investigation of the performance of such copolymers containing higher levels of a-olefin content in the copolymer. However, copolymers containing more than around 3% of oc-olefin component under varied experimental conditions with the application of frel:-radical polymerization, could not be obtained. Conclusion In a search for developing modified polymers of poly (alkyl methacrylate) (Pfv'jA) category which can serve as superior VIIs than the normal PMAs,., 'I II

5 GHOSH et al.: ALKYL METHACR YLA TE 375 few copolymers of!so-decyl methacrylate and decene were prepared, characterised and their potential for use as VIIs through several standard tests was investigated. Results on these copolymers, reported here, demonstrate that properly designed copolymers of this category can serve as better VIIs than usual PMAs. Acknowledgement The authors thank Mr Ravi Kelkar, R & T Centre, ICI India Limited, Mumbai, for recording some of the NMR spectra for these studies. References I Mortier R M & Orszulik S T, Chemistry and Technology of Lubricants. Second Edn (Blackie Academic & Professional, London), 997, 44 2 Briant J, Denis J & Parc G, Rheological Properties of Lubricants (Editions Technip, Paris), a) Desai N M, Sarma A S, Mallik K L & Rudra P K, Indian Pat (to Lubrizol India Limited), 988; b) Mammen A, Sarma A S, Mallik K L & Rudra P K, Indian Pat (to Lubrizol India Limited), Akhmedov A I & Levishina A M, Inst Khim Prisdok, 26 (986) 24 5 Beyer C & Jelitte R, Lubr Eng, 48 (992) Chatterjee A K, Murthy P S N & Joshi G C, Macromolecules Current Trends Vol I, 995,3. 7 EI - Gamal I M & Gobie! S, J Appl Polym Sei, 6 (996) a) Varma I K, Nair M V & Karan V K, J Thermal Anal, 35 (989) 989; b) Varma I K, Nair M V, Karan V K & Varma D S, ThermochimActa, 42 (989) Kleinova A Borsig E, Schulze U & Pionteck J, Macromol Chem Phys, 97 (996) 2289.