Shear Stability Dependence of Molecular Weight Distribution in Viscosity Index Improvers Curt Lindhe A B Nynzs-Petroleum, Nynzshamn, Sweden HIGHMOLECULAR WEIGHT POLYMERS are used as lubricating oil additives in three applications : pour point depressants, viscosity index improvers, and polymeric dispersants. Viscosity index improvers, as well as polymeric dispersants, make possible the formulation of high quality multigrade oils. Viscosity index improvers are oil-soluble polymers with molecular weights which range from 25,000 to over 1,000,000. Two types are available, the polyisobutylenes and the polyalkyl methacrylates. The polyalkyl methacrylates are obtained by the free-radical solution polymerization of the monomer at temperatures on the order of 200 O F . The polymerization is stopped and the products are chilled by injection of oil. The obvious function of the viscosity index improver is to improve the temperature-viscosity relationship of the finished oil. The polymer molecule in solution exists as a polymer coil which is swollen by the hydrocarbon solvent. The degree of swelling or voluminosity of this coil determines the degree to which the polymer increases viscosity. In addition to thickening power and viscosity index blending characteristics, the shear stability is critical in determining the performance of a polymer. When a polymer thickened oil is subjected to high shear rates under conditions of turbulent flow, polymer backbone may be physically cleaved. The shear stability of a polymer, its ability to resist such cleavage, is measured by subjecting the oil solution to cavitation in an ultrasonic generator. The result of polymer cleavage is a loss in viscosity. While it is economically attractive to use high molecular weight polymers because of their high thickening power, their inferior shear stability results in a rapid loss of viscosity in service. It is therefore necessary to geX an optimal relation between viscosity index improving qualities and viscosity loss. It is often stated from the practical viewpoint of polymer design and lubricating oil formulation that the critical factor that controls shear stability is the average molecular weight of the particular class of polymers used. This paper shows, through gel permeation chromatography (1-4), that the critical factor is the molecular weight distribution and not merely the average molecular weight. EXPERIMENTAL
Apparatus. The gel permeation chromatograph was obtained from Waters Associates Inc. A nonpulsating chromo syringe pump with constant flow was used. The columns are closed in by a transparent plastic cover of polymethyl methacrylate with a constant temperature (25 "C) air stream flowing through. The detector used is Waters Associates Model R 4 differential refractometer with a 0.01-ml flowing reference cell and a 0.01-ml sample cell. Solvent is siphoned through the reference cell. The signal from the refractometer
(1) J. C . Moore, J . Polym. Sci. Part A , 2, 835 (1964). (2) L. E. Maley, J. Polym. Sci. Part C, 8,253 (1965). (3) J. C . Moore and J. G. Hendrickson, ibid., p 233. 38, 997 (1966). (4) J. C. Giddings and K. L. Mallik, ANAL.CHEM.,
1
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ID5-
1D4
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Figure 1. Calibration curve for columns 1oj lo5 showing molecular weight us. distance in centimeters from benzene peak to the peak of known molecular weight is amplified and connected to a recorder. Because variations in temperature effect changes in refractive index, a constant temperature circulating system is used to keep the temperature differential between sample and reference at a minimum. Reagents. The tetrahydrofuran (boiling range 65.8-66.0 "C) used was analytical grade (J. T. Baker Chemical Co.). The influence of impurities in tetrahydrofuran has been studied in detail. In this study the curves are marked with dashes where impurities due to air dissolved in the sample solution (3,water, peroxides, and stabilizer may have affected the results. Procedure. Samples are dissolved in the tetrahydrofuran to a concentration of 1.3z by weight. Two milliliters of solution are necessary for the experiment. The column systems consist of three l o 4 columns and two l o 5 columns. Flow rate is 0.30 ml/min and the columns are held at 25 "C. ( 5 ) D. A. Alliet, J . Polym. Sci. Parr. A - I , 5 , 1783 (1967).
VOL. 41, NO. 11, SEPTEMBER 1969
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6p
5.5
50
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49
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30 Oislance
2.5
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In crn from benzene peak
Figure 2. Molecular weight distributions of hydraulic oil methacrylic polymer additives with different shear stability. Columns lo4 lo4 lo4
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Distance in cm from benzene peak
Figure 3. Molecular weight distributions of hydraulic oil methacrylic polymer additives with different shear stability. Columns lo5 105
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The same sensitivity (4X) and the same recorder speed (12.65 cm/hour) have been used in all experiments. Exactly 35 minutes after injection of the methacrylate sample, a sample with benzene is injected to get a reference peak. Under these conditions, it takes about eight hours to elute a sample through the three columns in series. RESULTS AND DISCUSSION Calibration Curves. Calibration curves were made by using polystyrene samples with known nominal molecular weights ranging from 860.000 to 900 (Pressure Chemical Co.). For lower molecular weights stearic acid, n-decene, and benzene were used. Calibration curves of molecular weight us. distance in centimeter from the benzene peak to the peak of known molecular weight were plotted (Figure 1). Because polystyrenes were used for the calibration, all the molecular weights are for polystyrenes. However, they can be corrected so that the hydrodynamic volume of the methacrylate will equal that of the polystyrene (6). Hydraulic Oils. Hydraulic oil additives are concentrates of methacrylic polymer in low viscosity petroleum oils. They differ in molecular weight, in concentration of the polymeric ingredient, and in the type of carrier oil in concentrate. Before running the Raytheon Shear Stability Test (7) the concentrate, which itself contains low viscosity petroleum oils from the manufacturing process, was diluted with 7 6 x by weight of base oil. In the following table viscosity loss is given for three difference methacrylic polymers at a temperature of 100 O F .
Curves in Figure 2 1 2 3 Viscosity loss 29.5 10.0 1.1% (6) H. Benoit, 2.Grubisic, P. Rempp, D. Decker, and I-G Zilliox, J. Chem. Phys.,63, 1507 (1966). (7) Amer. SOC.Testing Mater., ASTM Std., D 2603-67 T, Philadelphia, Pa. 17,991 (1969). 1464
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The height of the curve shows differential refractive index R.I. and the area under the curve is consequently an approximate measure of the quantity of the sample. From Figure 2 it can be seen that the polymer appears between 25 and 60 cm from the benzene peak and the oil, injected whenmanufactured for purposes of stopping polymerization and chilling, appears between 8 and 20 cm. It is obvious from curves I and 2 that the compounds are of such high molecular weight that they are not completely separated in this column system. It is also shown that the higher the degree of polymerization (DP) and the wider the molecular weight distribution, the higher is the cleavage of the polymer (curve I = 29.5%, curve 2 = l o x , and curve 3 = 1.1 Figure 2 also demonstrates that the higher the DP, the greater the amount of oil that is used. The oil apparently has a higher viscosity when the D P is higher. To show that these results are correct, the same additives were run in another column system with two lo5 columns coupled in a series. The result is given in Figure 3 and the calibration curve is given in Figure 1. Figure 3 shows that there is a complete separation of all additives. If it is desired to know an approximate molecular weight at a certain point, you can read the centimeter scale and go into the calibration curve Figure 1. If the area below the curve, between 35 and 44 cm from the benzene reference, is studied, it is easily found that the area is approximately proportional to the degradation. ACKNOWLEDGMENT The author thanks A. Bergholm for his discussion and remarks and B. RBnby for skilled technical and theoretical assistance. RECEIVED for review February 25, 1969. Accepted May 12, 1969. Part of a thesis for the Swedish grade “Teknologie licentiat” at the Royal Institute of Technology in Stockholm.
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