Lubricant additives were developed to enhance the existing characteristics of the base oil(s) a lubricant is formulated with, to reduce the deficiencies of the base oils(s) or impart new performance characteristics. Engine oils were the first lubricants to be formulated with additives. They have been and still are the largest market segment for lubrication. So, it is no surprise that most of the research and development efforts have been placed on engine oil enhancement.
In 1911, the American Society of Automotive Engineers (SAE) established the oil classification system. This was related only to oil viscosity and not performance. Until the 1930s, engine oils did not contain any additives. They were only base oils. Prior to the introduction of additive chemistry, the oil drain intervals were 750 miles. Due to increasing consumer demands and economic pressures, internal combustion engines became more sophisticated. Engine oils were becoming increasingly stressed and challenges on their performance reserves gave rise to a need for additives.
The first oil additive developed was the pourpoint depressant. These acrylate polymers were developed in the mid-1930s. Anti-wear additives such as zincdithiophosphate were introduced in the early 1940s followed by corrosion inhibitors and then sulfonate detergents. The sulfonate detergents were found to provide acid neutralization as well as oxidation inhabitation as well as rust and corrosion inhabitation.
In 1932, the American Petroleum Institute (API) established a specification system for engine oil performance classification. This is an important consideration because it is the only system by which a lubricant can be deemed compatible with another from a different manufacturer without the need to test compatibility. As long as the oils are of the same viscosity grade and have the same API classification and SAE viscosity, the oils are compatible; the user can mix oils if need be. This is not the case for other lubricants.
When mixing different lubricants, an adverse reaction may occur between two oils at certain working conditions in a system. This is considered ‘lubricant incompatibility’. Most often the cause of incompatibility is the neutralization of an acidic additive in one oil by an alkaline additive in the other oil. The result is that the additives react with each other instead the metal surface, particle or free radicals in the oil. The newly formed compound become ineffective and precipitate (drop out). Most all additives are polar which is what drives this reaction. This is by design. The polarity affords surface reaction as well as contamination reactions all that benefit the asset. During the reaction of incompatibility, often a soap forms that can precipitate a grease-like gel that interferes with lubrication and oil flow.
However, mixed oils may not always lead to incompatibility issues. They can exist without precipitation or reaction in an operating system for an indefinite period until water is introduced. Water can quickly lead to a reaction between the polar additives. Iron and copper found on the molecular level can act as catalysts in these reactions. Incompatibility reactions are not reversible. Removing water by drying the system and the oil does not remove the formed gel or eliminate the soap.
Typically, acidic additives can be found in gear, hydraulic and some circulating oils. Alkaline based additives are used in engine oils. There are some additives that are neither acidic nor basic but neutral, these types of additives are used in compressors and refrigeration oils. Additives that are acidic are identified as being strong acids and will react faster than acids that are formed during the initiation stage of oxidation which are typically carboxylic acids or nitric acids and are weak acids due to the limited number to protons donated. Weak acids react slower than strong acids. This is the reason why oils that have incompatible additive chemistry react so fast. Additives are not the only culprit. Propylene glycols, polyglycols, phosphate esters, polyol esters base oils have fair to poor compatibility with mineral oil-based lubricants. While these oils may not for solid substances, they may form a sludge. Many will not mix with the mineral based lubricants.
ALS can help you determine if your oils are compatible or not. Contact ALS Tribology to learn more.
Michael D. Holloway, MLE, CRL, LLA I & II, MLT I & II, MLA I-III, OMA 1, CLS
Principle Consultant, Certified Reliability Leader