Ethylene-propylene copolymers: influence on the viscosity index
Olefin copolymers (the class covers ethylene, propylene, butylene, other unsaturated aromatic hydrocarbons) are extensively used as viscosity modifiers. Such additives are able to compensate for the dependence of the fluid lubrication on temperature.
The viscosity of mineral oils is more temperature dependent than synthetic oils. The reason is the difference in the size of the molecules. The viscosity increase is facilitated by the presence of large molecular structures in the liquid. But with increasing temperature, such agglomerates break up into components. The resulting reduction in the proportion of large molecules leads to an increase in the fluidity of highly heated lubricants.
Polymer molecules react differently to temperature changes. Ethylene-propylene copolymers are in a twisted state at low temperatures without affecting fluidity. Heating the oil during engine operation leads to straightening of the polymer chain. The resulting change in the linear scale of the molecule, namely a significant increase in size, creates obstacles inside the liquid. The viscosity rises. In this way, the copolymers compensate for the natural increase in fluidity of lubricants during heating. In modern production, high molecular weight polymers are available in several variations – from solution to solid. The other useful properties of ethylene-propylene copolymers in regard to motor oils are as listed below:
- exceptional resistance to oxidation;
- resistance to most aggressive environments;
- excellent dielectric properties;
- high long-term heat resistance.
Additionally, the olefin copolymers exhibit moderate shear stability. This means that high molecular weight chains are not so often broken into smaller fragments. Depressant additives based on ethylene and propylene copolymers are able to withstand temperature effects without significant deformations (ruptures) for a certain time. Therefore, polymeric viscosity modifiers are effective in moderately loaded usage where high shear is not present. An increase in both parameters (load, shear rate) leads to the aforementioned rupture of long molecules into small fragments. This is an irreversible reaction which affects the gradual decrease in the effectiveness of the additive during exploiting.
This behavior of copolymers has led to the development of new technologies. Modern engine oils with a high viscosity index at the same time are distinguished by the stability of their properties over long periods of operation in severe working conditions. The effect can be achieved both by adding polymeric additives and by modifying base oil molecules. One of the technologies, hydrocracking, consists in hydrocatalytic processing of raw materials. This is a technology where the process of removing unwanted elements is replaced by their conversion into hydrocarbons of the required structure.
Produced oils are characterized by improved linear configuration and degree of uniformity along the length. Liquids have an increased viscosity index, at the same time they remain resistant to mechanical degradation.
The application of ethylene-propylene rubbers in motor oils
Ethylene propylene copolymers (EPM) are used in two ways – thickening (viscosity) additives and a component in the production of synthetic hydrocarbon oils. The first application is based on the modification of the spatial structure of the polymer chain with a change in temperature. EPM additives increase the viscosity of the oil when heated and have an almost neutral effect at low temperatures. The basis of the second use is the reaction of destruction of olefin copolymers at high temperatures (390 – 430 °C) and pressure (180 – 220 atm.).
Obtaining synthetic motor oils
The use of EPM for hydrocracking started with the study of a completely opposite problem – the stabilization of polymers. This is an important thing for the use of ethylene-propylene rubbers as viscosity modifiers. The emerging interest in thermal degradation products made it possible to reveal a number of interesting facts. It turns out that heating leads to a change in the molecular composition and mass of EPM. Targeted destruction of copolymers leads to the formation of products with a lower molecular weight. Today copolymers play the role of depressants in motor oils and other petroleum products.
The reaction of thermal destruction of EPM, carried out with the usage of a catalyst, leads to the formation of the C18 – C25 fraction. The results of the modification are listed below:
- viscosity index rises to 95;
- the flammability point reaches 210°С;
- pour point drops to -52 °C.
The resulting synthetic product is a fire and cold resistant engine oil with a high viscosity index. The latter indicator means that the fluid lubrication depends weakly on temperature.
Previously, the procedure for obtaining frost-resistant high-index synthetic oils was characterized by high cost. Modern hydrocracking reactions make it possible to produce relatively inexpensive products on an industrial scale.
The second area of ethylene-propylene rubbers application is thickening additives. In this direction an important thing is not a destruction, but the stability of EPM polymer structure. At room temperature and below, engine oil has sufficient viscosity. EPM molecules are present inside the liquid in a twisted form and therefore do not have a significant effect on fluidity.
An increase in temperature leads to a decrease in the oil viscosity. Simultaneously, chains of ethylene-propylene copolymers start untwisting. Both the linear size of the molecules and the internal obstacles to fluid flow increase. As the result of this process a decrease in the oil viscosity is compensated by polymer additives.
EPM stability is important in relation to shear deformation. The impact of mechanical loads on the polymer at high temperatures leads to three types of deformation – uniaxial shear, tension and compression. For motor oils, the most important is the first type. Constant high mechanical loads are accompanied by an increase in shear stress. This leads to an increase in highly elastic deformation and partial destruction of intermolecular bonds. As the result of the process, the length of the polymer chains is reduced, which affects the ability to increase viscosity at high temperatures.
This feature limits the use of EPM in lubricants. Viscosity modifiers are effective as thickeners in motor oils operating under moderate loads. For high levels of stress, as discussed earlier, the use of synthetic fluids is recommended. It is noteworthy that these oils are also produced with the usage of ethylene-propylene rubbers.
Scope of ethylene-propylene copolymers
The application of ethylene propylene copolymers (EPM) covers different areas – from the automotive industry to waterproofing and sealing materials. Alternatively, the properties of polymer chains are heavily exploited in fluid lubricants. EPM is used for producing synthetic oils and improving the viscosity of these petroleum products.
The first direction of using EPM is based on the ability of copolymers to change their spatial structure depending on temperature. Under standard conditions, the polymer chains are folded and the sizes of the intrinsic molecules of oil and EPM are comparable. The result is that there is practically no effect of the additive on viscosity.
Heating of engine oil during engine operation leads to a change in spatial orientation. Macromolecules of ethylene-propylene copolymers unfold. The appearance of long chains of EPM in the liquid creates obstacles to internal movement. The liquid viscosity increases. This compensates for the decrease in the oil’s own viscosity with increasing temperature. Therefore, thickeners based on olefin copolymers have received an alternative name. They are called viscosity modifiers.
The second application of ethylene-propylene rubbers in engine oils is the synthesis of lubricants with improved physical characteristics. In this direction, the instability of ethylene-propylene copolymers at high temperatures and pressures is used. Controlled destruction of macromolecules leads to the formation of compounds that improve three indicators:
- viscosity index (reaches 95) – displays the degree of change in fluidity with increasing temperature, the ideal value is 100;
- threshold for solidification (reduced to -52 °C) – the value at which engine oil loses its mobility;
- flash point (increases to 210 °C) – the amount of heating when a volume of vapor is released that can lead to ignition in a mixture with air.
The improved EPM-based hydrocracking technology makes it possible to synthesize relatively inexpensive fire and frost-resistant motor oils with a high viscosity index.