These techniques include small-angle neutron scattering (SANS), molecular dynamics simulations, and dynamic light scattering (DLS). Many techniques have been used to study the coil size directly in dilute polymer solutions as a function of temperature in hydrocarbon solvents. While there is a general paucity of direct evidence, several have proposed mechanisms such as solvent-polymer interactions and even polymer-polymer interactions or associative thickeners. Mechanistic details regarding how the polymer coil influences viscosity and improvement in VI are still being sought. Coil expansion results in slower diffusion and causes increased surface interaction with the surrounding media, therefore increasing the viscosity of the whole formulation due to known mechanisms like drag. The most well-known mechanism to improve VI is by coil expansion with increasing temperatures as mentioned with PAMAs above, first introduced by Selby in 1958. Previous studies with OCPs have shown that molecular weight and branching influence the resulting VI of the polymer-polymer interactions or associative lubricant. Since the polymeric viscosity modifiers are more costly than the base oils, it is desirable to use the least amount of additives for the targeted viscosity modification. Thickening efficiency is the enhancement in viscosity per unit of polymer in addition. OCPs are the most cost effective among the three and are known to have great thickening efficiency. PAMAs are the only known modifiers to increase in coil size with temperature, thus dramatically improving the VI. HSD, with a starlike architecture, exhibits high shear tolerance while providing an increase in VI. Each has its own advantages as viscosity modifiers in lubricants. The three main categories of viscosity modifiers that increase the VI are hydrogenated polystyrene-diene (HSD) copolymers, poly(alkylmethacrylates) (PAMAs), and olefin copolymers (OCPs). The use of VI as a measure to represent the viscosity vs temperature relationship has been questioned by several researchers due to the skew toward higher viscosity oils having higher VI values compared to lower viscosity oils. Viscosity modifiers that improve VI can allow an oil to maintain adequate viscosity at high temperatures without making the oil too viscous during cold starts. A high VI is necessary in a car engine because the viscosity of the oil has a specific requirement to quickly lubricate the engine upon starting as well as maintain a certain viscosity at operating temperature to continue to lubricate the engine effectively while in use. A high VI number describes an oil that has a smaller viscosity dependence with temperature. It is used to describe the viscosity dependence of the oil with temperature change. The VI is a unitless number that is calculated from the ratio of the kinematic viscosity of the oil at 40 and 100 ☌. Viscosity modifiers in multigrade engine oils improve the viscosity index (VI). Viscosity modifiers are a class of materials used to increase the viscosity of a lubricant.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |