Pour point and solidifying point
As an indication for low-temperature use, the data sheets of oil manufacturers state the pour point or solidifying point. Should the oil not be used below this temperature?
The pour point and solidifying point are characteristic values for the physical properties of a lubricant at low temperatures. However, they do not say anything about its actual behaviour at low temperatures. These two terms are often used synonymously in common language. Yet, they are not identical. The pour point is the lowest temperature at which the oil still fl ows. The solidifying point is the temperature at which the oil “solidifi es”. It is the point where the cooled oil no longer fl ows under the infl uence of gravity. The solidifying point is usually 3 to 5°C lower than the pour point.
“Solidifi cation” of an oil is caused by the crystallisation of the paraffi ns in the base oil. If the crystals join to form a network at low temperatures, the oil takes on a waxy, solid consistency. Naturally, the lubricating properties of an oil change when it solidifi es. In particular, the oil supply fails because it can no longer be pumped. However, the fi rst signs of crystallisation can already be seen before it reaches the solidifying point and actually solidifi es with the consistency of Palmin or lard. The paraffi n fl akes make the oil milky or cloudy. The temperature at which the bottom of the container can no longer be seen is defi ned as the cloud point. The cloud point and the pour point are determined in the laboratory by observing the oil while it is cooled slowly. This measurement is performed by an automated device at OELCHECK.
Only limited conclusions regarding the actual behaviour of the oil at low temperatures can be deduced from these characteristics. Consequently, motor vehicle manufacturers do not specify the solidifying point, but instead measure the actual viscosity at certain temperatures below freezing. For instance, with a multigrade motor oil rating such as SAE 10W-40, the winter rating indicates that the viscosity of the oil should not be less than 7,000 mPa•s at -25°C. For the winter grades (0W to 25W), the Society of Automotive Engineers (SAE) also specifi es the minimum temperature at which the oil can still be circulated with a pump. This is primarily important for cold starts in the winter, since the oil must be pumped quickly to the lubrication points immediately after the engine starts. Some engine and gearbox manufacturers even have their own low-temperature tests because they do not see any relationship between the pour point and actual practice. Precisely defi ned limits are often not available for industrial lubricants. The pour point is often mistakenly regarded as the limit. Hydraulic system manufacturers recommend that as much as possible, the viscosity of the fl uid should not be more than 1,000 mm²/s for low-temperature starts. Start viscosities above 100,000 mm²/s can cause problems with gearboxes and may make preheating of the oil necessary.
OELCHECK can determine whether an oil actually has the necessary viscosity for the intended application at a specifi c temperature by measuring its viscosity/temperature profi le in the laboratory.
If an oil reaches a temperature below the cloud point or the pour point while in storage, in most cases this does not cause any problems if the oil is subsequently warmed up slowly. However, this does not apply to frost-sensitive products containing water and products that can be mixed with water, such as emulsions for machining metals or HFC hydraulic fluids. To be on the safe side, you should not store lubricants outdoors or expose them to extreme temperature variations. The container “breathes” with temperature variations, which can allow moisture to form in the container and contaminate the oil.
ÖlChecker Winter 2009, page 8