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Rheological investigation of petrochemicals

Petrochemicals are chemicals derived from crude oil or natural gas. They form the basis for many everyday products, including fuels and oils for cars, plastics, pesticides and fertilizers, paints, detergents, cosmetics, candles, and much more. In fact, it would be easier to list items which contain no petrochemicals than to provide an extensive list of those that do. To name just two more examples, petrochemicals are the basis for fleece jackets and fabric softeners for laundry.

Rheological behavior of petrochemicals

As far as crude oil is concerned, the existence of paraffins and asphaltenes in oil reservoirs and pipelines can create problems in production, transportation, and the processing industries. Solid precipitation can cause depositions which may lead to plugging of the facilities. The buildup of solid components may also lead to pumping problems.

Crude oils contain a variety of light and heavy hydrocarbons. At temperatures above 60 °C the lighter components keep most of the heavier ones in solution, resulting in flow behavior showing comparatively low viscosity values. However, with decreasing temperature, the solubility of heavy components is reduced, which might cause solid precipitation. This effect is called “wax precipitation”, which can change the crude oil from a Newtonian fluid to a yield stress substance which may in turn increase the danger of plug formation in a pipeline. Wax precipitation in crude oils depends on both the composition of the oil dispersion and the environmental conditions such as temperature and pressure. Tests at high temperature and pressure in a rheometer combined with a pressure cell provide information, for example, about the effectivity of plug inhibitors under transport and production conditions.


When we talk about fuels, we commonly mean liquid fuels like petroleum, diesel, petrol, kerosene, etc. Other types of fuel include solid fuels (coal, wood, dung, etc.) and gaseous fuels (natural gases such as propane, coal gas, water gas, etc.). As far as liquid fuels are concerned, one of the most important factors influencing their consistency, besides pressure, is temperature. Diesel and petrol, for example, are exposed to a wide range of temperatures, depending on the climatic conditions. In order to remain liquid, even at very low temperatures, their freezing point must be lower than the service temperature or rather the ambient temperature. It can be generally stated that petrol has a lower freezing point than diesel fuel. In order to depress wax precipitation, additives are frequently used to improve the fluidity of fuels even at low temperatures.

Rheological tests on fuels

During cooling there are three points to describe the change of a fuel from a liquid to a solid state at low temperatures. It starts with the cloud point at which the fuel begins to become cloudy due to the initiating crystallization of waxes and paraffins. The pour point describes the viscosity value just before the fuel starts to become solid and finally reaches its freezing point. In other words, the pour point is the point at which the fuel still shows flow characteristics. This point affects the transportation of fuel in pipelines, for example, as well as usage in cars. One method for determining this pour point is a rotational test at a constant shear rate in a rotational rheometer. In a rotational test with decreasing temperatures, either the turning point or the bend where the viscosity curve finally starts to flatten out can be determined as the pour point. Both points can be calculated with an analysis program. Therefore, it is important to specify which method is used to determine the pour point. The pour point of diesel fuels can actually be influenced by adding wax modifiers which polarize the wax molecules so that they do not form larger crystals during cooling.

This test requires a rheometer equipped with a Peltier temperature control concentric cylinder system.

This is just one of the rheological investigations typically used in the automotive industry.

Lubricant additives

The basic purpose of a lubricant is to reduce friction and wear between two surfaces in relative motion by forming a load-bearing fluid film. A lubricant in an automotive engine, however, has to meet even higher expectations – like maintaining stability at high temperatures, preventing oxidation and corrosion of the metal parts, providing effective sealing, etc. Therefore, several additives such as viscosity modifiers, anti-wear agents, extreme pressure additives, anti-oxidants, etc. have to be added to the base oil.

Viscosity modifiers (polymeric structures) are added to a lubricant to minimize changes in viscosity at changing temperatures. Under ideal conditions a lubricant has to be viscous enough to form a load-bearing fluid film which separates the mating surfaces it lubricates. However, due to improvements in efficiency (changes in formulation) this film is becoming thinner and might not always be capable of keeping the surfaces apart under unexpected conditions like a sudden increase of pressure or a start/stop situation, for example. Thus, further additives like anti-wear agents and extreme-pressure additives are added in order to facilitate the formation of sacrificial surface films on the moving metal parts. 

Tribological tests on lubricant additives

The type and amount of additives added to a lubricant depend on the type of engine and the field of application. Developing customized additives and the process of combining them and the corresponding interaction processes are very challenging tasks. One way to face this challenge is with tribological measurements. These measurements provide information on the entire system, including the mating surfaces, the lubricant, and the surrounding conditions. The performance of lubricants can be measured at different contact pressures, sliding speeds, temperatures, and relative humidity. In this way the formulation can be modified until it is suitable for its specific purpose.

This test requires a rheometer/tribometer equipped with a ball-on-three-plates setup.

Lubrication greases

Hardly any mechanical construction or engine is able to run smoothly without lubrication oil or lube grease to prevent damages or breakdowns and to reduce maintenance costs. Lubricating greases are used in gears, bearings, chains, guides, and much more. Which grease is selected to increase the efficiency of a system depends on various factors, like expected lifetime or the environmental conditions. These factors depend on the inherent properties of the grease, for example on the thixotropy, density, oxidation stability, and rust protection, and on some specific tribological parameters like extreme pressure properties, loadability, etc. The pour point of the base oil and its texture are also of great importance.

There are, of course, various other parameters such as resistance towards corrosion, long-term stability, friction behavior, etc. which need to be characterized individually. The selection criteria for lubricating greases are strict and high-level measuring techniques are required for the characterization of the greases.

Rheological tests on lubrication greases

It is very important to measure the rheological behavior of a lubrication grease within a wide temperature range to show under which environmental conditions it can be used, for example in the automotive industry. Car manufacturers require greases that can also be used at temperatures as low as -40 °C. Therefore, it is highly desirable to have an instrument and measuring methods to investigate the viscoealstic behavior of greases over a very wide temperature range. This can be performed with an oscillatory rheometer equipped with a temperature control unit. A typical test, for example, is an amplitude sweep with controlled shear strain, also called a strain sweep, performed at different temperatures.

This test requires a rheometer equipped with a Peltier temperature control system.

This is just one of the rheological investigations typically used in the automotive industry.

Tribological tests on lubricant greases

While rheology describes the properties of a material, tribology describes the properties of a system which in this case includes a body, a counter-body, and a lubricant in between. With tribological measurements, the break-away force of greases in a particular tribological system can be determined as a property, for example. The break-away force is the force required to overcome the static frictional resistance of the tribological system and set it into macroscopic motion. The value of the coefficient of friction right before the onset of macroscopic motion is known as the limiting friction. Typical applications in which this parameter could be of great significance are seat regulators, sliding guides, doors, locks, fishing gears, etc. While in most cases a low break-away force is desired, it must also be noted that a certain amount of resistance is still required to inhibit involuntary movements. Accurate determination of the break-away force requires highly precise control and measurement of forces and can be done with a rheometer equipped with tribological accessories, optimized for lubricant characterization.

This test requires a  rheometer/tribometer equipped with a ball-on-three-plates setup.

This is just one of the rheological investigations typically used in the automotive industry.

Mineral oils

Crude oil, as a mixture of components, is the basis for a wide range of products such as solvents, mineral oils, lubricants, adhesives, resins, detergents, polymers (plastics), and elastomers.

Mineral oils can be found all over the world, in the form of fuel oils and petrols but also as a raw material for many products of the chemical industry. Synthesized and refined mineral oils are especially widespread in the automotive industry as lubrication oils, for example, for an optimized performance of motors and gears. These oils should resist various environmental conditions during their lifetime: from the cold start of the motor to high temperatures and high pressures under working conditions. The task for useful oils is therefore to bear all these challenging conditions without losing the required properties.

Rheological tests on mineral oils

The influence of temperature on the flow behavior of mineral oil is among the most important data which can be determined with a rheometer. Typically pure mineral oils without polymer additives show an ideally viscous/Newtonian flow behavior.

This test requires a rheometer equipped with a Peltier temperature control concentric cylinder system.