Shear Stability & Viscosity Index Improver in Lubricant.

Shear Stability is the ability of a polymer to resist shear stress, which is a crucial performance of Viscosity Index Improvers. When a viscosity index improver has poor shear stability, the main chain of the polymer will break under the action of shear stress, leading to a drop in viscosity. As a result, the blended lubricating oil cannot maintain its original viscosity grade, which causes increased wear and fuel consumption.

Shear stability is related to the degree of polymerization (relative molecular weight), degree of dispersion (relative molecular weight distribution), and the hydrodynamics of the polymer in solution for viscosity index improvers.

The longer the polymer chain (the greater the relative molecular weight) and the greater the dispersion, the more easily it will break, resulting in poorer shear stability.

Conversely, the smaller the relative molecular weight of the polymer, the better the shear stability, but the treat rate will be larger, which is detrimental to cleanliness. Therefore, it is only necessary to achieve the required shear stability, rather than blindly pursuing the smallest possible shear stability.

 

How to express the Shear Stability?

 

Mineral base oil is a Newtonian fluid whose viscosity has nothing to do with the shear rate. The lubricating oil (multi-grade oil) added with a viscosity index improver changes the flow properties of the base oil and becomes a non-Newtonian fluid. The viscosity changes with the increase or decrease of the shear rate.

Under the action of shear stress, the viscosity of multi-grade oil will decrease, which is expressed by the viscosity decrease rate or viscosity loss rate (Shear Stability Index, SSI).

When the polymer is subjected to a high shear rate, a temporary viscosity loss will occur. At this time, the shear rate is usually around 10⁴s^-1, and its molecules will flow along the axial direction and become orderly. The shape of the polymer changes from a spherical line The agglomerates change into elongated structures that occupy less hydrodynamic volume, resulting in a decrease in viscosity. This viscosity loss is reversible and can be recovered once the shear stress disappears, so this viscosity loss is temporary, expressed by Temporary Shear Stability Index (TSSI).

As the shear rate is further increased, more and more molecules are deformed, resulting in greater loss of viscosity, until the maximum amount of deformation is reached. When the shear rate reaches above 10⁶s^-1, in addition to the temporary viscosity loss, there is also an irreversible viscosity loss caused by polymer chain scission, which is called permanent viscosity loss. The permanent shear stability index (Permanent Shear Stability Index, PSSI) represents. Chain scission of the polymer generally occurs in the middle of the chain, where the carbon-carbon bond is split to generate two smaller molecules.

The larger the molecular weight of the polymer, the easier it is to deform and mechanically degrade. However, when the relative molecular weight of the polymer is small enough, even permanent viscosity loss will not occur, and it is usually not easy to cause further degradation, so the degradation process is self-limiting.

The shear rate of each part of the engine is different, and the Temporary Viscosity Loss is also different. The gear, bearing, piston, and cylinder wall are the largest, and the temporary viscosity loss is also large, so it is easy to cause wear and abrasion in these parts

 

Shear Rate in Gasoline Engine Oil

 

The viscosity loss is measured by Kinematic Viscosity (KV) before and after shear. Permanent shear stability is more commonly defined by the Permanent Shear Stability Index (PSSI) or simply SSI, according to ASTM D6022 as follows:

 

PSSI=SSI=（V₀-V_S）/(V₀-V_b)*100

 

V_o = viscosity of unsheared oil

V_s = viscosity of sheared oil

V_b = viscosity of the base fluid (without polymer)

 

There are three commonly used methods to measure shear stability.

 

● Diesel nozzle method

● Ultrasonic method

● L-38 method (the degree of shearing is measured after the L-38 engine runs for 10 hours)

 

The harshness of the three methods is very different. The specifications of internal combustion engine oil in the United States, Europe, and China use the L-38 method, the diesel nozzle method, and the ultrasonic method respectively. All three methods require that the viscosity after shearing still remains within the viscosity range specified for this grade of oil.

 

Comparison of SSI of ethylene-propylene copolymer (OCP) VII under different test methods.

 

 

The above table shows that the SSI value of ethylene-propylene copolymers commonly used in the United States and China is about 50% by the Diesel Injector, about 40% by Sonic Shear, and about 20% in the L-38 Engine Test.

The SSI value of ethylene-propylene copolymers commonly used in accordance with European specifications is about 30% by Diesel Injector method, about 22% by Sonic Shear, and about 12% by L-38 Engine Test.

Please Note that the SSI value is dependent on the particular shearing device used to measure shear stability.

 

Requirements for Shear Stability of various lubricating oils:

 

Gasoline Engine Oil  <  Diesel Engine Oil  <   Automatic Transmission Fluid (ATF)  <   Hydraulic Oil  <   Gear Oil.

 

Various Lubricant oil products have corresponding Shear Stability test methods to meet their requirements. Test methods are:

Crankcase Lubricant: Sonic Shear method (both 5min and 10min), 3200km (2000 miles) Road Shear test, L-38 method, Diesel Injector method, Peugeot 204 Test.

Automatic transmission fluid and hydraulic oil: Dexron ATF, HETC T-13, 10,000 cycles, Chrysler Power Steering Pump Test, 4h×93.3°C (test conditions include 4137kPa×1700r /min and 2758kPa×1800r/min).

Gear oil: A. L. I. Variable Seveity Scoring Test-Shear Stability Determination (A. L. I Variable Seveity Scoring Test-Shear Stability Determination Test) 93.3℃×120h.

 

SSI determined by various shear test methods

 

① ASTM D-2603, Min 5min.

② DIN-51382: Diesel Injector: 10W-30/10W-40.

③ Dexron ATF, HETC T-13, 10000cycle.

④ Chrysler Power Steering Pump Test, 4h×93. 3℃, Load 2758kPa, 1800r/min。

⑤ A. L. I. Variable Seveity Scoring Test-Shear Stability Determination, 93. 3℃×120h。

⑥ Acryloid 702: PMA with pour point depressing effect, Suitable for gasoline engine oil.

⑦ Acryloid 955: MA with pour point depressing and dispersing effects, suitable for gasoline and diesel engine oil

⑧ Acryloid 1017: PMA, suitable for automatic transmission fluid, hydraulic oil and multi-grade gear oil.

 

KRL Shear Stability Test Method has been recognized in the lubricant industry as an accurate shear test method for predicting the viscosity loss of lubricating oil under actual operating conditions and has become an important evaluation tool in the development of modern lubricating oil products.