Viscosity Index Improver Type, Variety And Development Trend
The viscosity index improver is generally an oil-soluble chain polymer compound. Polymethacrylate (PMA) and polyisobutylene (PIB) have been widely used in the early stage, and ethylene-propylene copolymer (OCP) and styrene-isoprene copolymer (HSD) have been formulated in recent years.
Polyisobutylene has oil solubility, thermal stability, good mechanical shear resistance, but low temperature and viscosity-increasing ability. It is not possible to formulate multi-stage internal combustion engine oil with lower viscosity grade and larger span. Polyisobutylene used in multi-stage engine oils has a relative molecular mass of about 50,000, while low isomolecular weight (about 10,000) polyisobutylene has excellent shear stability and can be used in hydraulic oils and multi-stage gear oils. Polyisobutylene has good shear stability and thermal oxidation stability, but its low temperature and viscosity-increasing ability are poor. It is not possible to formulate multi-grade engine oil with low viscosity (W30 and below) and large span. This is because the PIB polymer molecular chain has many methyl side chains, so it is relatively rigid, and its viscosity increases rapidly at low temperatures, so the low temperature performance is not good.
The shear stability index of PIB is still good, but the low temperature performance is the worst, and it has been gradually eliminated after the 1980s. The amount is small.
Alkyl methacrylate monomers having different carbon numbers are prepared by solution polymerization in the presence of an initiator and a relative molecular mass mediator. If the alkyl carbon chain is long enough, it can increase the depressing effect, and the copolymerization with the nitrogen-containing polar monomer has a certain dispersion. PMA is one of the earlier and more widely used varieties. PMA has a particularly good low temperature performance, good oil viscosity index improvement, good oxidation stability, but thickening ability, thermal stability and low temperature mechanical shear performance. . As the viscosity index improver of the internal combustion engine oil, the low temperature performance requirement can be satisfactorily satisfied, but the thickening ability is poor and the addition amount is large, thereby affecting the detergency of the internal combustion engine oil. PMA is best suited for the preparation of advanced automotive oils, CNC hydraulic fluids and automatic transmission fluids.
Ethylene-propylene copolymer (OCP) has the characteristics of better viscosity-increasing ability and shear stability, good high-temperature performance and good on-site performance, and the price is moderate. It is a variety that is used more in the world. OCP is widely used in engine oils, especially diesel engine oils, accounting for more than 80% of the viscosity of engine oil viscosity modifiers. However, its low temperature performance is somewhat poor. If it is configured with a low viscosity multistage oil, it is better to combine it with an ester type pour point depressant to improve its low temperature performance.
Hydrogenated styrene-isoprene copolymer (HSD)
HSD is a copolymer of styrene and isoprene or butadiene which is then hydrogenated to saturate the remaining double bonds in the diene monomer to enhance its thermal oxidation stability. The relative molecular mass of HSD is generally about 50W, and its thickening ability and shear stability are good. It can meet the requirements of multi-stage internal combustion engine oil, especially for the preparation of long-span multi-stage internal combustion engine oil (10W-50, 15W-50 and 20W-50). Due to the good thickening ability and shear stability of HSD, its use is increasing.