Shear stability index (SSI) is an important performance of Viscosity Index Improver. It directly affects the stability of multi-grade engine oil viscosity. Shear stability is related to the relative molecular mass of the viscosity index improver, the relative molecular mass fraction and the polymer structure, but is mainly determined by the relative molecular mass. The greater the relative molecular mass of the same type of viscosity index improver, the worse the shear stability.
There are several ways to improve the shear stability:
One is to reduce the relative molecular mass of the viscosity index improver and narrow the relative molecular mass distribution, and the other is to improve the stability of the polymer structure so that the polymer maintains a higher thickening ability. At the same time, it has good shear stability.
Shear stability test:
During the use of the oil with viscosity index improver, due to the effect of mechanical shear, the high molecular polymer in the oil is sheared, which reduces the viscosity of the oil and affects normal lubrication. Therefore, shear stability is a special physical and chemical property that must be tested for this type of oil. There are many methods for measuring shear stability, including ultrasonic testing, nozzle shearing, Wicks pump shearing, and FZG gear shearing. These methods are ultimately used to determine the viscosity drop rate of oil. Shear stability measure:
Add the viscosity index improver to the Base Oil to prepare 15W/40 CF-4 oil. Suppose the shear temperature is t, and V0 and V are the viscosity of the oil before and after shearing. Then the shear stability index SSI of the viscosity index improver is: SSI=(V0-V)/V0 For ultrasonic shear, t=40°C, SSI stands for permanent viscosity loss.
For high temperature and high shear, t=150℃, SSI represents the sum of instantaneous and permanent viscosity loss. The lower the SSI, the lower the rate of decrease of the shear viscosity of the oil, that is, it is not easy to be sheared and the shear stability is good. |
| Shear Stability Index(SSI) |
|
| Diesel Nozzle | Sonic | L-38 |
| OCP meet China, US standard | 48.5 | 38 | 20-22 |
| OCP meet Europe standard | 28.3 | 21.7 | 12 |
Note:When discuss SSI of ethylene-propylene copolymer, it must be compared under the same evaluation method.
It must be emphasized: SSI is not as low as possible. If the SSI is low, the thickening ability of the viscosity index improver must be poor. To achieve the same thickening effect, a larger dose of VII must be added. The increase in the amount of VII will affect the detergent and dispersing performance of the engine oil. So SSI shoudl be appropriate but not too low.
The shear of diesel engine oil is relatively harsh, the SSI of the viscous agent can be relatively low, and the shear of gasoline engine oil is not too harsh, and the SSI can be selected relatively high. In North America, it is common to use SSI 35-50 for gasoline engine oil.
SSI is important, but HTHS is more important. A low SSI may not necessarily lead to a high HTHS.
