![]() ![]() 309 Under the same shear rate their higher viscosity 310 is the reason why star polymers are used as an improved VI in the multigrade lubricating oils. Toward these lines star polymers (e.g., star polymethacrylates, hydrogenated star PIs) are perfectly suited for VI improvers, as they have higher shear stability than the corresponding linear ones. The lower the value of SSI, the better or more resistant is the VI improver to mechanical stress or shear. Shear stability of VI (or VM) is evaluated by determining the shear stability index (SSI), which is a measure of loss of viscosity due to shearing. Lower-molecular-weight polymers are more shear resistant but do not improve viscosity as effectively as at higher temperatures and, therefore, must be used in larger quantities. Higher-molecular-weight polymers make better thickeners but tend to have less resistance to mechanical shear. ![]() As the additive (VI improver) is repeatedly sheared in the engine, it loses its ability to act as a more viscous fluid at higher temperatures, due to chain scission. The primary disadvantage of the polymers as VI is their sensitivity (chain scission) to mechanical shearing. ![]()
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