Focus on Viscosity - Analytical Instrumentation
this thinning occurs, the engine is more susceptible to corruption and damage. When the lubricant can no longer control the heat generated by friction, and the surrounding high temperatures, the engine can no longer function properly.
There are two mechanisms for shear loss in polymer-thickened lubricants. One is permanent viscosity loss (PVL) due to the polymers breaking into smaller polymers. PVL can also occur from thermal degradation or oxidation [6], but this usually does not signifi cantly contribute to viscosity loss as the polymers degrade from the ends. In mechanical shearing, the breakage occurs towards the middle of the polymer chain length causing a signifi cant viscosity loss. In automotive engines, this typically occurs in the pistons and bearings. The other mechanism for viscosity loss is temporary viscosity loss (TVL) which occurs under high shear rates and as the name implies is reversible with the viscosity returning to a higher value when the shear rate is lowered [7]. TVL occurs when polymers are stretched but not broken under the applied shear stresses. Oil viscosities can be tested with High Temperature High Shear (HTHS) viscosity measurements and these values are then included in engine oil specifi cations. The amount or treat rate of polymer added to any engine oil dictates the degree of PVL or TVL. For example, synthetic engine oils tend to have naturally higher viscosity indexes (a measure of the rate of change of viscosity vs. temperature where a higher index indicates a lower slope) so they require less and sometimes no polymer to achieve their multi- grade viscosity ratings. HTHS viscosity are measured with methods such as ASTM D4683 (Tapered Bearing Simulator), D4741/CEC L-36-90 (Tapered Plug Viscometer), D5481 (Multicell Capillary Viscometer) or with an ultrahigh shear rate viscometer (USV) [14]. The minimum SAE J300 -2015 HTHS ratings for engine oils are:
Table 1. SAE J300 -2015 HTHS ratings for engine oils. SAE Viscosity Grade 8
12 16 20 30
40 - Winter
(0W-40, 5W-40, 10W-40 grades) 40 – Heavy Duty
(15W-40, 20W-40, 25W-40 40 grades)
50 60
High Temperature High Shear Rate @1500
C (mPa·s) 1.7 2
2.3 2.6 2.9
3.5 3.7
3.7 3.7
Three methods, Kinematic viscosity (KV) loss, percent loss, and PSSI are often used to evaluate the PVL performance of multi- grade, polymer-thickened lubricants. Kinematic viscosity (KV), is a measure of a fl uid’s resistance to fl ow under gravitational forces. It is measured as the ratio of dynamic viscosity to fl uid mass density [8]. Kinematic viscosity is measured by calculating the time it takes for oil to travel through an orifi ce of a capillary with the force of gravity acting upon it. A capillary tube viscometer is an instrument used to measure the kinematic viscosity of a fl uid.
A prominent procedure for analyzing kinematic viscosity measurements is ASTM D445 which requires the use of a calibrated viscometer. The kinematic viscosity loss of various fl uids is measured using a procedure such as ASTM D445 with a viscometer in a temperature-controlled environment. The kinematic viscosity of oils and lubricants can be lost in environments with high temperatures. To enhance the performance of a
to increase kinematic viscosity and is often recorded as the kinematic viscosity of a blend of a polymer-containing product at a certain weight percent in a standard base oil. For shear stability, most oil and lubricant analysis measure and report the kinematic viscosity before and after shearing occurs.
Another measure of viscosity loss is percent loss. The percent loss of viscosity is defi ned as:
(1)
33 where Vi is the initial KV of the lubricant with polymer, and Vf is
the KV of the lubricant after shearing. The percent loss of viscosity measured in lubricants dictates the performance of the lubricants. A signifi cant percent loss indicates to the manufacturing company that their lubricant may not be a good performer [10].
Permanent shear stability index (PSSI) is another type of viscosity loss indicator that can be measured. The SSI is defi ned as:
(2) where Vo is the KV of the base oil before addition of polymer.
PSSI is a measure of the loss of viscosity due to shearing contributed by an additive [11]. It is affected by the base fl uid, the chemistry and concentration of the additive, and the presence of other additives. The PSSI is a measure of the effectiveness of the polymer additives in maintaining their contribution to the fi nal fl uid blend viscosity. The property is important to the suppliers of the polymers and to the lubricant formulators who use them. The percent viscosity loss is the important measure for the lubricant supplier and end user.
The three previously mentioned properties require some kind of shear procedure. Sonic shear, diesel injector shear, KRL, and Sequence VIII (L-38) and are examples of laboratory bench test also used to evaluate the performance of multi-grade lubricants. Sonic shear tests are often used in the hydraulic fl uid industry. They can also be applied to transmission fl uids and tractor fl uids [7]. ASTM D2603, Sonic Shear for Polymer-Containing Oils, and ASTM D5621, Sonic Shear for Hydraulic Fluids, are the two test methods commonly used in the industry. A sonic oscillator is used to irradiate or shear the lubricant sample for a certain amount time at power levels calibrated with specifi ed Reference Fluids in both methods. The viscosity change between the initial and sheared lubricant is then measured [11]. ASTM D2603 can be run at different severity levels. At its lower levels, using Reference Fluid A, the test is known to be polymer chemistry dependent, and so is mostly used for polymer manufacturing and their specifi cations. At higher shear levels, using Reference Fluid B, it is less polymer dependent and in ASTM D5621, which also uses Reference Fluid B and shears for a longer time, no polymer chemistry dependence is observed.
Figure 2. Capillary tube viscometer.
lubricant and to be economically effi cient, it is preferable to limit the number of polymers needed to make a good lubricant [8]. Therefore, quantifying the effects of molecular weight, molecular weight distribution, and thickening effi ciency are important. The thickening effi ciency is defi ned by the amount of polymer needed
The diesel injector shear test or ASTM D6278 and CEC L-14-A uses a high shear nozzle or diesel injector apparatus to evaluate the percent viscosity loss of polymer- thickened lubricants due to shearing [12]. This test method is mostly used for quality control. The oils are passed through the shearing injector 30 times, or cycles, and the severity of the test is comparable to the shearing severity found in
The KRL (Kegelrollenlager, German for Tapered Rolling Bearing) test is also used to test the shear stability and performance of multi- grade, polymer-thickened lubricants. This test utilizes a tapered rolling bearing in a cup fi tted to a four-ball instrument [7]. Load is applied to the bearings as it rotates at a specifi c speed for a certain period of time. The test usually takes up to 20 hours to complete with the bearings rotating at around 1,450 revolutions per minute. After the test is over, the viscosities before and after the test are compared to calculate the percent viscosity loss. This test method is aggressive and considered to be one of the most severe shear tests as it forces the lubricants to undergo extreme shearing for an extended period of time. The method is relatively severe because it operates in the elastohydrodynamic lubrication regime (EHD) and causes the bearings produce high shear stresses on the fl uids. CEC L45-A-99 is one of the procedures that uses the KRL test.
Figure 4. KRL tester and tapered bearing assembly.
Sequence VIII (Roman numeral eight, also known as the Labeco 38 or L-38) is a common test used to specify the allowed viscosity loss of a multi-grade, polymer-thickened passenger car engine lubricants. The Sequence VIII or L-38 test is used to test the quality of a lubricating fl uid under high-temperature conditions.
Figure 5. Sequence VIII (L-38) test stand.
This test uses a copper/tin/lead rod bearing and measures the presence and extent of corrosion in the bearings [13]. The shear stability is also measured in this test. The test uses a single- cylinder, spark ignition, lubricant test engine with an external lubricant heater circuit. The engine runs a 3150 revolutions per minute for 40 hours with the tested fl uid. After the 40 hours are over, the bearings are visually examined to see if there is any corrosion. When there’s visible corrosion on the bearings, that means that the quality and the performance of the lubricant tested is poor. Furthermore, the viscosity of an oil sample taken after 10 hours of testing and stripped of any fuel that may have diluted the oil is also measured to calculate percent loss or stay-in-grade kinematic viscosity. To summarize, the severity of permanent shear stability tests can be ranked as follow: KRL > HF sonic shear > 90-cycle fuel injector > (30-cycle fuel injector, low power sonic shear, L-38).
Figure 3. Diesel injector tester.
passenger cars. A 90-cycle version of the test, ASTM D7109, is a more severe test and may better approximate shear loss found in heavy duty Diesel engines.
Different kinds of multi-grade, polymer-thickened lubricants are formulated for different applications. Engine oil, also known as motor oil, is a type of multi-grade, polymer-thickened lubricant made from mineral oil base stocks and other additives [14]. The base stock takes up to 70 to 90% of the solution and the additives take up 10 to 30% of the solution. Engine oil is used to lubricate internal combustion engines, to reduce friction in moving parts of the engine, and to clean, cool, and protect the engine. Multi-grade engine oils are formulated with viscosity index improvers (VIIs) to improve the viscosity vs. temperature characteristics (VI). VIIs are polymer additives capable of
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