Polymer-Thickened Lubricating Oils Laboratory Test t o Predict Mechanical Shear Stability Maurice E. Le Pera and Jules Pigliacampi Coating & Chemical Laboratory, U . S . A r m y Aberdeen Research & Development Center, Aberdeen Prouing Ground, M d . 21005 The mechanical shear stability of polymer-thickened lubricating oils in relation to laboratory bench-scale techniques was investigated. In subsequent studies using standard laboratorytype paint blending-mixing devices, a kinetic dispersion mill (Model 1) provided a mechanical shear environment demonstrating a directional trend of degradation for polymer thickeners. Further refinement of this technique led to the establishing of a correlation with single and multicylinder engine tests.
Lubricating oils designed for use in automotive internal combustion engines can be classified into one of two types: conventional (a single grade) or polymer-thickened (a multigrade) oils. The conventional type crankcase oils are formulated using combinations of base stock blending fractions (light or medium neutrals, bright stocks, etc.) to fulfill the viscosity requirements specified for the individual SAE grade designations. However, multigrade engine oils are formulated using selected polymer thickeners in concentrations up to approximately 20% in a light base stock lubricating oil. The polymer concentrates (polymethacrylates, polyacrylates, polyisobutylenes, etc.) exert a thickening action which increases the viscosity by means of their solubility in the base lubricant (Lyman and Kavanagh, 1959; Smalheer and Smith, 1967; Stewart, 1963). The use of these multigrade crankcase oils has increased significantly to the present time, when they account for approximately 35% of the total oil market, with an anticipated increase to 50% by 1970 (Enjay Chemical Co., 1969; Smalheer and Smith, 1967). Although polymer-thickened oils exhibit distinct advantages in performance characteristics over conventional crankcase lubricants, one serious problem area exists: the susceptibility of the polymer concentrates toward mechanical shear degradation. When these oils are subjected to the mechanical shear environment of an internal-combustion engine, the degradation of the polymer “chains” can be a function of molecular weight range, polymer type, amount of polymer present, type of engine operation, and extraneous contaminants. This over-all degradation process results in a loss in the viscosity of the lubricant a t engine operating temperatures, thereby reducing its film strength properties (Braithwaite, 1967). Lyman and Kavanagh (1959) reported that since the polymer’s thickening power is proportional to molecular weight, a single break in the center of the polymer can reduce its thickening power to one half the original value. The importance of minimizing the mechanical shear degradation of polymer thickeners has been realized by oil and additive suppliers, since oil economy and oil film strength protection are the two critical lubrication param-
eters. In support of this, recent publications (Denherder. 1966; Heinen, 1967; Jeffrey and Boschma, 1964; Miller, 1968; Sholts, 1968) have stated that considerable attention should be directed toward improving the shear stability of current and future multigrade engine lubricants. As a result of these tendencies toward mechanical shear degradation, considerable efforts have been expended to develop reliable laboratory tests which correlate with field service performance. A laboratory procedure currently in use for evaluating shear stability is the sonic shear tester (ASTM D-2603-67T, American Society for Testing Materials, 1969) which utilizes a 10-kc sonic oscillator to shear or rupture the polymer chains. However, a degree of unreliability has existed when using this device to predict potential shear degradation encountered in an internal combustion engine environment (Pearce, 1968; Preuss et al., 1968; Vick and Goodson, 1964). This unreliability has been attributed to the following discrepancies which exist from using the sonic device: The sonic tester exhibits a distinct sensitivity to polymer type. Polymer breakdown in the sonic test decreases as oil blend viscosity increases, whereas the reverse effect exists for field performance. At the present time, several nonstandard bench or laboratory devices currently are being employed for evaluating this property, with a majority of these techniques utilizing hardware items such as diesel fuel injectors, power steering pumps, throttled valve devices, and truncated cones, all of which provide a high degree of shear stress to effect the mechanical shear degradation. One such technique, the power steering pump apparatus, was recently reported (Pearce, 1968) to correlate with results obtained from several test programs. Another report (LeMar, 1967) compared the shear stability of four hydraulic oils using three laboratory test devices: a hydraulic pump, a diesel injector, and the sonic shear tester. Excellent correlation with field data was attained using the hydraulic pump technique, whereas the other two laboratory techniques did not give satisfactory correlation. However, a technique such as the hydraulic pump or the power steering pump (mentioned above) was not considered to exhibit characteristics which are desirable for a bench-type laboratory Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 4, 1970
525
device-simplicity of component ( s ) involved, ease of operation, and minimum time required. Since this laboratory has the custodial responsibility for internal combustion automotive engine lubricant specifications in which multigrade oils (Military Specification MIL-L-2104B, 1964; MIL-L-l0296B, 1968a; MIL-L-21260A. 1968b) are currently being qualified, an investigation was initiated to develop a bench-scale technique for predicting the mechanical shear stability of these oils. This paper describes the subsequent development of a bench-scale procedure for defining the mechanical shear stability of multigrade crankcase lubricants Discussion
Development of Test. I n an effort to fill the need for a bench-scale apparatus to define mechanical shear susceptibilities, specific hardware devices were viewed in light of existing laboratory devices available for experimentation. Using a commercially available SAE 1OW-30 engine oil as the test fluid, different paint dispersion-blending devices were evaluated to determine whether or not their mechanical environments would induce the desired degradation of the polymer thickener. The per cent loss in kinematic viscosity at 210" F was selected as the criterion of effectiveness for each candidate device apparatus. A majority of these devices did not significantly degrade the test oil without necessitating extensive modifications t o the standard units. However, one device produced significant loss in viscosity in a relatively straightforward manner. This unit. known as a "Kady Model L kinetic dispersion mill" (Kinetic Dispersion Corp.. Buffalo, N. Y.), consisted of a motor-driven mixing impeller, fitted into a water-jacketed 1-gallon blending container (Figure 1). The test oil was placed in the blending container and after positioning of the impeller, the motor was engaged t o rotate the impeller a t approximately 16,000 rpm. The temperature of the test oil, increasing as a result of applied work, was maintained a t 100°F by control of water flow through the jacketed container. In the initial experimentation with this device. considerable attention was directed
toward controlling operating variables such as belt slippage affecting impeller speed, water and temperature control, oil vortexing and frothing, impeller positioning. and minimum sample requirements. Initial experiments with the Kady mill had been conducted, using an operating temperature of 100"F. Additional tests were subsequently conducted using operating temperatures of 125" and 150"F to define any increase in test severity. The results of this temperature study (Table I ) confirmed the initial selecting of 100"F as the operating test temperature. The feasibility of this Kady mill as a laboratory technique was established after evaluating several commercial MS multigrade crankcase oils and selected Coordinating Research Council (CRC) reference oils described in the literature (Vick et al.. 19663. The results obtained are presented in Table 11. Although other dispersion units1 e , colloid mills. roller mills. ball and pebble mills, etc.function by shear, attrition, and impact t o effect the necessary mixing of ingredients. the Kady mill utilizes only the impact and attrition forces. However. this environment produced the directional shear stress effect on the different polymer thickeners, polymethacrylates exhibiting greater viscosity loss than polyisobutylenes. Since data demonstrated the Kady mill's ability to differentiate between polymer thickeners, it was decided to determine whether a correlation existed with field service performance. The operating procedure, described in the Appendix, was standardized using the per cent viscosity loss after 6 hours' operation to define mechanical shear stability. Correlation of Kady Mill with Engine Tests. The correlation program was contingent upon obtaining multigrade crankcase oils having engine shear data. However, since the problems of conducting full-scale engine tests usually relate t o economics and adequate facilities (test stands, dynamometers. operators. etc.) , an alternate approach was initially considered. A review of the Qualified Products List (QPL) for Military Specification MIL-L-2104B lubricating oil. internal combustion engine (heavy duty) (1964) revealed a significant number of multigrade crankcase oils approved under Grade OE-30. The qualification acceptance engine test results on the approved products for the CRC L-38 engine test (Federal Test Method Standard No. 3405, 1969) usually reported a loss in viscosity a t 210" F for the multigrade crankcase oils. This acceptance engine test is designed to evaluate the oxidation stability and bearing corrosion tendencies of candidate lubricating oils, The operating conditions for the L-38 test are as follows: Engine model Test duration Engine speed Brake horsepower BMEP Water outlet temperature Oil sump temperature Fuel
fa
526
+ 3 ml TEL
Oh Viscosity toss a t 210' F after
Test Oil" Temp, 'F
1
2
3
4
5
6
Oil feeder
100
11.1
Shaft and driving quill
126 150
9.5 8.9
12.1 10.8 9.2
13.8 12.7 10.9
13.9 13.1 12.0
14.8 13.6 13.5
16.4 14.4 14.3
A. B. C.
Motor
D. E. f.
40 hours 3150 rpm 5 hp 29.5 psi 2000F 290' F Isooctane
Table I. Effect of Operating Temperature on Kady Mill Test Severity
Figure 1. Kady Model L kinetic dispersion mill
G.
CLR Labeco, single-cylinder engine
Pulley arrangement
Jacketed blending container Mixing impeller (rotor and stator assembly) Hydraulic lift
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 4, 1970
" 011 samde. commercial SAE 1OW-30 designated as "test fluid."
Table II. Effect of Kady Mill Environment" on Multigrade Lubricating Oils
Designation
Kinematic Viscosity a t 210" F., Cs
1OW-30 (A) 10W-30 (B) 10W-30 (C) 1OW-30 ( D ) 1OW-30 ( E ) 5w-20 REO-155 REO-156 REO-157 REO-159 REO-160
12.49 10.66 12.27 12.12 11.68 6.07 11.70 10.98 11.84 11.95 12.10
Oil Sample
VI Improver
N.A." N.A. PIB N.A. N.A.
PIB PIB
vca PMA vc PMA
Yo Viscosity Loss a t 210" F after 1
2
3
4
5
6
7.69 12.01 2.68 9.41 11.13 4.61 2.12 4.80 10.39 10.35 14.01
8.57 12.66 3.09 10.23 13.10 5.11 3.30 6.32 13.85 12.60 15.92
9.80 14.35 3.59 11.30 14.55 5.43 4.75 8.90 15.03 14.05 18.24
11.21 15.38 3.99 11.96 15.92 5.93 5.80 10.05 16.81 15.39 18.82
12.17 16.23 4.56 12.46 16.78 6.09 6.10 11.10 18.16 16.25 19.65
12.30 16.70 4.63 13.36 17.21 6.75 6.23 11.50 18.33 16.50 20.48
' I-gallon mixing container used for six commercial oils; 1-quart container used for five CRC reference oils (designated as " Type of VI improver unknown. ' Polyisobutylene. Vinyl copolymer. ' Polymethacrylate.
REO).
Table Ill. Characteristics of Multigrade MIL-L-2 1048 Engine Oils Sample No.
A B
C D E
F G H I J
Kinematic Viscosity
100" F
210" F
Viscosity Index
65.3 75.5 75.6 73.1 76.4 73.0 121.9 72.0 61.9 63.0
11.52 11.92 12.10 11.60 11.45 11.07 12.46 12.03 11.59 10.63
146 138 142 138 153 153 102 176 196 171
Gravity API
29.2 29.8 30.4 28.6 30.6 29.0 27.2 28.0 29.7 27.8
The qualification acceptance criterion for this engine test permits a 50.0-mg maximum bearing weight loss, whereas the viscosity change, given as a per cent increase due to oil thickening (oxidation-condensation polymerization reactions), supplements the bearing weight loss value. However, the viscosity increase resulting from oil oxidation is defined using the per cent viscosity change a t 100°F. Since oil oxidation was determined by viscosity change a t 10O0F, the change in viscosity a t 210°F could be interpreted to indicate shear degradation of the VI improver. With this objective in mind, retain samples of several multigrade engine oils qualified under MILL-2104B (Military Specification, 1964) were obtained. The physical characteristics of these selected MIL-L-2104B oils are presented in Table 111. The ten retain samples were then evaluated in the Kady mill and the results obtained as well as those from qualification acceptance L-38 tests are presented in Table IV. The viscosity loss data obtained from the L-38 were interpreted to relate directly t o shear stability. since the viscosity a t 100"F did not indicate any measurable oxidation. The agreement between data obtained from the Kady mill us. the L-38 engine test was calculated using the "r-square method" and the correlation coefficient was found to be 0.99. In addition to results obtained from the single-cylinder engine test, additional data from multicylinder engine tests were sought to supplement the correlation established thus far. T o accomplish this, oil samples having field-engine dynamometer experience were requested from several companies. Samples with developed engine shear data were
Flash Point,
Pour Point,
"F
435 460 415 460 420 480 455 445 420 430
VI Improver
"F
Sulfated Ash, W t Yo
Type
Concentration, h wt O
-25 -30 -40 -30 -35 -45 - 0 -40 -25 -25
1.15 0.97 0.90 0.95 0.61 0.93 0.60 1.19 1.40 2.03
PMA PMA PMA PMA PMA PMA PIB PMA PMA PMA
4.7 5.0 6.7 4.9 2.8 5.0 6.5 6.0 6.3 5.8
obtained from three companies, each utilizing a different engine system; the following test parameters were given Company Designation
Engine Test Environmeni
230 CID, 6-cylinder engine operated under no load a t 2000 rpm for 7.5 hours (oil sump a t 205O F.) 283 CID, 8-cylinder engine dynamometer motored a t 2000 rpm for 14 hours 2000-mile road fleet test consisting of 4 passenger cars (obtained results were average of four cars)
A
B C
Table IV. Effects of Operating Environments on Multigrade MIL-1-21046 Oils Yo Loss in Kinematic Viscosity a t 210" F after
Sample
A
B C
D E F G H I J
No
Kody mill test,
CRC 1-38 test,
6 hours a t looo F, oil temperature
40 hours a t 290" F,
10.6 12.4 21.6 13.9 9.6 8.7 3.4 11.3 14.1 8.6
oil sump temperature
10.5 12.5 19.3 13.8 8.3 "I
.a
2.3 9.8 14.3 7.0
~
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 4, 1970
527
Table V. Effects of Operating Environments on Multigrade Oils Oo /
Oil
viscosity Loss a t 210° F after
Sample Designation
SA€
VI
Grade
Improvern
Kady mill
A-1 A-2 A-3
N.A. N.A. 1OW-30
PMA PIB PMA
22.3 9.5 13.4
23 8 14
B-1 B-2 B-3
1OW-40 5W-30 1OW-40
PIB PMA PMA
11.3 13.4 11.6
13.2 15.9 13.5
c-l
1OW-30 1OW-30 1OW-30 1OW-40 1OW-40 1OW-30 1OW-30
PA PMA PMA PIB PMA PIB PMA
13.4 12.3 10.2 7.0 15.8 3.7 13.0
11.3 9.1 7.4 6.6 14.4 1.6 9.7
Engine environment'
2 Lo lii
z i
c-2 c-3 c-4 C-5 C-6 C-7
20
.
B
5 .
PMA, Polymethylacrylate. PA, Polyacrylate. PIB, Polyisobutene. bEngine environment described as: A. 230 C I D engine, no load at 2000 rpm for 7.5 hours. B, 283 C I D engine, motored at 2000 rpm for 14 hours. C, 2000-mile road fleet test.
Table VI. Mechanical Shear Stability of Commercial and Specification Multigrade Oils Sample Type
MIL-L-2104B
MIL-L-10295A MIL-L-7808E Commercial premium (a) Commercial premium (b) Commercial premium (c) Commercial premium (d) Factory-fill oil Type A, suffix A, automatic transmission fluid
1
I
I
I
5
10
15
2G
':
VISCOSITY LOSS FRO'.I KADI V I L L
Figure 2. Correlation of laboratory with engine shear data
Yo viscosity
0
Kady vs. engine shear d a t a
SAE Grade
Loss"
A
Kady vs. L-38 data
1OW-30 1OW-30 1OW-30 1OW-30 1OW-30 1OW-30
6.3 12.0 13.5 15.1 2.7 9.4
5w-20 5W-20
16.6
N.A. 1OW-30 1OW-40 1OW-40 1OW-30 5W-20
5.6 20.6 16.9 14.8 8.6 5.7
...
10.9
12.7
'Viscosity loss at 210a F determined after 6-hour Kady mill test.
Conclusions
The ability t o define mechanical shear stability in terms of predictive laboratory technique has been attained by the use of a Kady dispersion mill. This approach has merit in its simplicity and absence of parameters adversely affecting the reliability of test results. Engine test data establish its validity for rating the shear stability of polymer thickeners. Acknowledgment
The authors gratefully acknowledge the helpful suggestions and discussions offered by the late C. F. Pickett, director of this laboratory. Appendix. Method of Test for Defining Mechanical Shear Stability of Polymer-Thickened Lubricating Oils
The oil samples submitted were evaluated in the Kady mill and the results obtained as well as those furnished by the individual companies are presented in Table V. The correlation existing between the two sets of data was calculated, giving a coefficient of 0.97. In response to this value, attention is directed to the distinct variation between engine and Kady mill test environments. Ideally, one engine system should be selected to establish the final correlation; however, this was impractical because of limited sample data. T o illustrate the similar trends evidenced in this study of shear stability, a graphical representation of viscosity loss comparing Kady mill with both single- and multicylinder engine tests is presented in Figure 2 . The two plots show a minor divergence from the 45" line. The capability of this technique as a laboratory test method was further demonstrated by evaluating additional oil samples containing polymer thickeners (Table VI). 528
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 4,1970
Scope. This method describes a test precedure for evaluating the mechanical shear stability of polymer thickeners in crankcase engine oils. Outline of Method. A sample of test lubricant is subjected to mechanical agitation at 100" F for 6 hours using a Kady Model L dispersion mill. The degree of shear instability is rated by determining the kinematic viscosity at 210°F before and after the 6 hour period. Specimen. Approximately 1 quart of the oil to be tested. 8pparatus. TESTUNIT.The test unit consists of a Kady Model L dispersion mill fitted with a water-jacketed stainless steel dispersion vessel having a 1-quart total capacity. The mill is equipped with a l12-hp explosion-proof motor capable of driving the impeller (rotor-stator assembly) at a speed of 16.000 i 100 rpm. TACHOMETER. A direct reading dial-type hand tachometer is used to monitor the belt drive speed of the impeller periodically.
Procedure. Elevate impeller-motor assembly. Place 900 ml of the test oil in the 1-quart blending pot. Lower impeller-motor assembly until unit is approximately inch from bottom of container. Replace container covers and insert thermometer so that the end is approximately halfway immersed in the test oil. Turn on cooling water and activate motor switch. Adjust cooling water to maintain a test oil temperature of 100’ i 2” F during the 6-hour agitation period. Periodically check the small drop-feed oiler to ensure proper lubrication to the driving quill and shaft spindle. Periodically. monitor the impeller speed by placing the hand tachometer against the flat belt drive. After 6 hours of continuous operation, turn off the motor and cooling water supply. Remove a sample of stressed oil and determine the kinematic viscosity at 210” F. Concurrent with this, determine the kinematic viscosity a t 210°F on a sample of new or fresh-fill oil. Calculation. Determine the mechanical shear stability of the test oil by the following calculation:
Per cent viscosity loss = (visc. new oil
-
visc. stressed oil) / visc. new oil x 100 Literature Cited
Amer. Soc. Testing Materials, Philadelphia, Pa., ASTM Standards 1969, Part 17,1969. Braithwaite E. R., “Lubrication and Lubricants,” pp. 131-2, Elsevier, New York, 1967. Denherder, M. J . . Society of Automotive Engineers, Paper 660593 (1966). Enjay Chemical Co.. Kew York. “1968 Lubricating Oil Survey.” 1969.
Federal Test Method Standard No. 791B, “Lubricants, Liquid Fuels, and Related Products. Methods of Testing,” Jan. 15, 1969; Federal Test Method Standard No. 3405, Jan. 15, 1969. Heinen, C. M., Seventh World Petroleum Congress, Mexico City, P.D. 28, 7 (1967). Jeffrey, R. E.. Boschma, L. G., Proc. Amer. Petrol. Inst. 111, 491-5 (1964). LeMar, R. L., “Factors Affecting Sonic Degradation of Polymer Solutions,” Rock Island Arsenal, 1967. Lyman, A. L., Kavanagh, F. W., Proc. Amer. Petrol. Inst. 111, 296-316 (1959). Military Specification MIL-L-2104B Lubricating Oil, Internal-combustion Engine (Heavy Duty), Dec. 1, 1964. Military Specification MIL-L-10295B Lubricating Oil, Internal-combustion Engine (Sub-zero), March 18, 1968a. Military Specification MIL-L-21260A Lubricating Oil, Internal-Combustion Engine (Preservative and BreakI n ) , June 6, 1968b. Miller, D. F., National Petroleum Refiners Association, Paper FL-68-67 (1968). Pearce, A. F., Society of Automotive Engineers, Paper 680069 (1968). Preuss, A. F.. Stambaugh, R. L.: Radtke, H. H.. Society of Automotive Engineers, Paper 680070 (1968). Sholts, R. A., National Petroleum Refiners Association, Paper FL-68-68 (1968). Smalheer, C. V., Smith, R . K., “Lubricant Additives,” p. 7. Lezius-Hiles. Cleveland, 1967. Stewart, W. T., Advan. Petrol. Chem. Refining 7, 6 (1963). Vick, G. W., Goodson, R . M., American Society for Testing and Materials, Philadelphia, (ASTM STP-382, 34-5) 1964. Vick, G. W., Meyer, W. A. P., Selby, T. W., Society of Automotive Engineers, Paper 650441 (1965). RECEIVED for review May 25, 1970 ACCEPTED September 4, 1970
Solid Lubricating Films Friction-Wear Improvement with Poly (benzothiazole) Binders Bobby D. McConnell Fluid and Lubricant Materials Branch, Air Force Materials Laboratory, Wright Patterson A F B , Ohio 45433 Selection of organic resins as binder materials i s based on thermal and oxidative stability, toughness, adhesive properties, a n d ease of handling. Poly(benzothiazo1e) (PBT) was chosen from among several candidates for investigation as a binder for solid film lubricant formulations. The polymer, its structure, a n d properties are described. Procedures for formulating solid lubricants a n d wear behavior obtained are presented. Several formulations have potential for improved performance. One formulation, MoSj
+
-
S b O PBT, was selected for detailed study. Evaluation of friction and wear properties in several test devices is discussed. Improved performance i s demonstrated at 500” F and higher, compared to the performance exhibited by other available films.
S o l i d film formulations based on the use of phenolic and epoxy resins, and mixtures of these two, have been the mainstay of the solid lubricant industry for many years. These formulations consist primarily of MoS? and/ or graphite as the lubricating mediums which, with the resin binders, formed solid lubricating systems that met aircraft requirements for over two decades. However, the
advancing temperature and lifetime requirements for aircraft systems require constant improvement and reformulation of these films. The thermal and oxidative stability of the phenolic and epoxy-type resins limited the solid film formulations to about 500” F for useful long-term operation. Interest in high temperature ( > 1000”F ) lubrication prompted the Ind. Eng. Chern. Prod. Res. Develop., Vol. 9, No. 4, 1970
529
