What is the environmental factor that has the most effect on the chemical stability of lubricants? why?

The environmental factor that has the most effect on the chemical stability of lubricants is the temperature.

Explanation:

How to affect the temperature to lubricants:

Among the main physical characteristics of lubricants that are affected by temperature are viscosity, viscosity index, pour point and base oil.

It is said that the viscosity of an oil should be the most important consideration when selecting a lubricant. The viscosity of an oil is its ability to flow or its internal resistance to flow.  

For example, when an oil film is formed between a bearing and a shaft, some of the oil molecules are attracted to the surface of the shaft, while other oil molecules are attracted to the surface of the bearing. This is known as the cutting rate (effort) and is directly affected by the viscosity and operating temperature of the oil. A multigrade oil (with viscosity index improver) with a lower (thinner) viscosity will generally have a potentially higher cutting rate, while a monograde oil (without viscosity index improver) will generally have a potentially lower cutting rate.

Since the oil with a lower viscosity and potential higher cutting rate must maintain a suitable oil film, it is quite evident that as the temperature rises, the oil film may fail and metal-metal contact may occur. If the viscosity of the oil is too high and with a low potential cut-off rate, its internal resistance to flow will dramatically increase the temperature, causing an overheating condition, which can also cause a failure of the oil film and promote its oxidation. Therefore, it is critical that the oils are selected always taking into account the operating temperature of the machine.

The most common term for defining viscosity is kinematic viscosity, which is measured in centiStokes (cSt), either at 40 ° C or 100 ° C. These data are always in the specification sheets of the oil manufacturers.

The pour point of an oil is defined as the lowest temperature at which a lubricant will flow. It is erroneously used frequently as the criterion of oil viscosity selection.

For example, let's say an oil has a pour point of -30 ° C. Most people assume that the oil will flow to the bearings of the machinery even when the ambient temperature is -30 ° C. This is a fallacy. In the best case, this oil with a pour point of -30 ° C and operating at an ambient temperature of -30 ° C, will simply limit itself to beating in the oil pump until this agitation causes an increase in the oil temperature This in turn allows the viscosity of the oil to decrease sufficiently so that it begins to flow slowly through the lubrication ducts to the lubricated components.

Frequently, this process takes 5 to 10 minutes or more, during which serious damage to various components can occur, because the oil is actually too viscous to flow. Do not select lubricants based solely on their pour point.

The viscosity index (IV) of an oil is the term used to express the "resistance to change the viscosity of the oil as the temperature changes." For example, an oil that thins (reduces viscosity) significantly as its temperature rises, is said to have a low IV. An oil whose viscosity does not change significantly when heated, is said to have a high IV.

This temperature / viscosity ratio is the most critical and important consideration when selecting oils that will operate at drastically changing temperatures. The viscosity index is of particular importance in the selection of oils for the crude winters of North America and in the mountainous regions of South America and in operations in the Arctic and Antarctic.

Most industrial mineral lubricating oils that could be used in a manufacturing or production plant with controlled temperatures only need to have IVs of 55 to 100. However, in a manufacturing or mining plant in the icy South American mountains, it may be necessary use oils with IVs up to 175. The viscosity index specification is not always available on the specification sheets of the lubricant manufacturers, but they should be.

The basic oil must also be taken into consideration in the selection of lubricants. Mineral (non-synthetic) basic oils can be of various types, depending on their molecular and chemical structure. The basic oils can be paraffinic, naphthenic or aromatic, and the selection process must take this into account.

For example, naphthenic basic oils have low natural IVs and can be used in machines where extreme temperatures do not affect their operation. On the other hand, paraffinic basic oils have natural IVs that are considerably higher than naphthenic ones, making them the best option to manufacture lubricants to be used in outdoor applications. Many of the mineral basic oils that are produced in the oil fields of North America are of the paraffinic type, while in South America they are of the naphthenic type.