Introduction:
Selecting the right base oil is crucial in formulating lubricants and greases. Several factors play an important role in the selection criteria. Specific recommendations regarding the base oil choice can be provided, but still, those recommendations can be completely different from the real environmental conditions. Lubricant manufacturers and users must develop a deep understanding of the application before selecting the base oil. Although additives are being used to enhance the performance of lubricants, the base oils’ performance properties are often undermined or ignored entirely.
Lubricants and Greases are composed of base oils, thickeners (in greases) & additives. The base oils are a major part of these systems, occupying more than 80% composition. It is now well researched and concluded that though additives enhance many properties of base oils such as Extreme Pressure, Antiwear, and Oxidation Stability, some important properties such as Thermal Stability, Hydrolytic Stability, and Seal Compatibility are inherent properties of base oils that are not touched by additives in any means. Further, the response and solubility of additives are heavily dependent on the base oil chemistry and viscosity. In this paper, the technical team from
Molygraph Lubricants explains the properties of different base oils. According to the American Petroleum Institute (API), the base oil is classified into five groups. The first three groups (Group I, II, and III) are refined from petroleum crude oil. Group IV base oils are full synthetic (Polyalphaolefin) oils. Group V is for all other base oils not included in Groups I through IV.
Mineral Oils
Group-I base oils are manufactured by the solvent extraction refining process. It comprises three primary molecule types: paraffinic, naphthenic, and aromatic, along with varieties of sulfurnitrogen-based compounds. It contains large amounts of unsaturated molecules than any other base oil type. These unsaturated molecules are responsible for accelerating the aging and degradation of finished lubricant products. Therefore, Group I mineral oils are regarded as poor in oxidative and thermal stability. However, it is a result of these unsaturated compounds due to which Group I base oils show the highest degree of additive solvency than any other base oils. Most lubricant formulators avoid using Group I Mineral oil for formulating lubricants requiring longer drain or re-lubrication intervals and/ or higher operating temperatures. However, for formulations where a high amount of polar additives are required, such as Rust Preventives or Neat Metal working fluids, Group I base oils are preferred. Group II and Group III are preferred for Engine oils, Hydraulic oils, compressor oils, Turbine oils, Greases, and are becoming dominant over the Group I base oil. Group-II base oils are manufactured similar to Group-I base oil but with a hydrocracked process. In this process, hydrogen reacts with aromatic and naphthenic molecules, and the amount of unsaturated molecules is less than the Group -I. Group III base oils follow the same hydrogen processing path as the GIIs, except they are more severely treated (higher pressure, higher temperature, longer process times), reducing unsaturated compounds nearly to zero. The hydrocracking and hydroprocessing add oxidation and thermal stability to Group II and Group III-based Mineral oils making them preferable for most Automotive and Industrial Lubricants and Greases.
Poly Alpha Olefins
Group-IV base oil consists of only Polyalphaolefins (also known as PAO). These are synthetically made by polymerizing α-olefins. The process ensures no unsaturation and aromatics in the yielded base oil, imparting excellent oxidation and thermal stability. However, this also results in poor additive response and seal compatibility. To take care of these issues, Esters are added to a certain concentration into the PAOs. PAOs are hydrolytically very stable due to the absence of a chemical functional group that could hydrolyze in the presence of water and elevated temperatures.
Esters
Ester is an example of group V base oils. Esters are synthesized by reacting Acid and Alcohol. Complex esters can be synthesized by adding some specialized production techniques. Esters have high polarity, which enhances additive solubility and seals compatibility in PAO-based lubricants and greases caused by a low polar nature of PAOs. Often, esters are added as co-base oils with PAOs or Mineral oils. However, some applications demand Esters be used as the only base oil in the formulation, such as Refrigeration compressor oils, High-temperature Chain oils used in Paint curing ovens, and Textile Stenters due to their high flashpoints, low pour points, lubricity, and low evaporation rates. Esters are generally hydrolytically unstable due to –COOR functional group which hydrolyzes quickly. However, some esters are made through advanced processes that improve hydrolytic stability.
Polyalkylene Glycols
Polyalkylene Glycols are synthetic base oils included in the group V category by API. There are three types of PAGs available commercially: water-soluble, water-insoluble, and Oil Soluble. Machinery that are using petroleum lubricants have wear grooves with carbonaceous material. Good solvency tendency of PAG helps to remove all the carbonaceous and thus revealing the wear scars which were earlier invisible. PAGs are hygroscopic but the water absorbed by the PAG is not free but is instead bound to the backbone of PAG. Thus, neither the corrosion nor the ice-crystal formation has been a problem when lubricating with PAG based lubricants.
Silicone Oil
Silicone base oils are linear polysiloxane compounds. They are chemically inert, thermo oxidatively stable and hydrolytically stable. Due to these properties silicone oil is used as a sealant. They have extremely high shear resistance compared to mineral oil.
Perfluoroalkylpolyethers (PFPE)
PFPE base oil are used as lubricants in oxygen services because they are chemically inert, thermo oxidatively stable and non-flammable. They can be used as high temperature greases because of their ability to withstand constant temperature upto 288°C.They are mainly used in vacuum pumps.
Conclusion:
Knowing the characteristics of the base oil will help to choose the right base oil according to the application. Each base oil type serves different properties and advantages. Therefore, in some application combinations of base oils are preferred.
This content was first published on the Molygraph website.
