Sat at the Top Table
2019 is the International Year of the Periodic Table, celebrating the grid that defines the relationships between the chemical elements and allowed chemists to predict the properties of several of them before they were even discovered.
About 20 percent of the 90 naturally occurring elements on the table find their way into mainstream lubricants, greases and metalworking fluids. Here is an explanation of where.
The simplest lubricant additive is graphite, a crystalline form of carbon. But adding a second element to carbon increases the number of chemical compounds dramatically.
Carbon and hydrogen are the elements most commonly found together in lubricants. The vast majority of base oils are hydrocarbon (hydrogen and carbon only), whether naphthenic or paraffinic. Polyalphaolefins, polybutenes and almost all viscosity modifiers, apart from polymethacrylates, used in automotive crankcase lubricants are also made up of just carbon and hydrogen.
The other two elements commonly found in base oils are nitrogen and sulfur. Nitrogen in combination with carbon and hydrogen is found in aminic antioxidants, many dispersants and triazole corrosion inhibitors. Sulfur in combination with carbon and hydrogen is found in sulfurized olefin and polysulfide extreme pressure and antiwear additives.
Add oxygen to carbon and hydrogen, and you can make polymethacrylate pour point depressants or viscosity modifiers, polyalkylene glycol or ester base fluids and hindered phenol antioxidants. Carbon, hydrogen, nitrogen and oxygen are found together in polyurea thickeners for greases, alkanolamines for metalworking, amide friction modifiers and some dispersants for crankcase lubricants. Carbon, hydrogen, sulfur and oxygen are the sole constituents of sulfurized esters or fatty acids. Add nitrogen, and you can make ashless dithiacarbamates; take away oxygen, and you can make thiadiazole metal deactivators.
One more element that figures in additives, usually thought of as ashless, is phosphorus. Phosphorus is found with carbon, hydrogen and oxygen in phosphate EP additives and fire-resistant hydraulic fluids, phosphite EP additives and antioxidants or phosphonate detergents. Add sulfur for thiosulfate EP additives and nitrogen for amine phosphate antiwear additives.
The most common metals in lubricants are calcium and zinc. They are found in almost all crankcase packages as the metal components of detergents (calcium) or antiwear (zinc) additives. One of the more common detergents is often described as overbased calcium sulfonate. Sodium sulfonates are found as emulsifiers and corrosion inhibitors in metalworking fluids and also (same name, subtly different chemistry) in thickeners for greases. Zinc is often an abbreviation of zinc dialkyldithiophosphate, but zinc oxide and zinc naphthenates are used in grease manufacturing.
Recently, magnesium-based detergents have seen a renaissance as part of the solution to low-speed pre-ignition in modern passenger car engines.
In liquid lubes, boron is found as an additional element in dispersants, providing some antiwear and possibly corrosion protection. Boric acid was widely used in metalworking fluids to provide similar protection, often in combination with amines, plus it acted as a biocide, but legislation has limited these uses. Boric acid is still used as a complexing agent in grease manufacturing, where the boron is retained in the grease thickener structure. Boron nitride, or boron and nitrogen, is an excellent solid lubricant.
Chlorine, like boron, has seen declining use due to health and safety concerns from original equipment manufacturers in crankcase lubricants and, due to legislation, in metalworking fluids and industrial oils. Fluorine, part of the same group as chlorine, is found in polytetrafluoroethylene or polyfluoroether thickener/extenders used in some greases. Silicon is found in many lubricants as part of silicone antifoam additives, but longer silicone molecules are in greases. Silicon and oxygen are also found in bentonite clays, used as grease thickeners, alongside aluminum, calcium and sodium. The functionalization of clays to make a grease thickener often involves an ammonium salt, thereby including nitrogen.
The most widely used metal for grease thickeners is lithium as hydroxystearate (carbon, hydrogen, oxygen). Aluminum and calcium soap-thickened greases are the other important volumes. Aluminum soaps are often used in greases for incidental food contact. Both lithium and aluminum – along with magnesium, sodium, silicon and oxygen – are found in hectorite clay, another grease thickener.
Molybdenum and tungsten are two of the most prominent metals used in solid lubricants as their respective sulfides. Both are available as dithiacarbamates and other oil-soluble materials containing carbon, hydrogen, nitrogen and sulfur, or dithiophosphates, which contain phosphorus rather than nitrogen. Tungsten is the heaviest element (mass per atom) used in lubricants, just ahead of barium, found in sulfonates and naphthenates, now that lead is no longer used.
Does the structure of the periodic table help us to identify those elements used in many mainstream lubricants? It does, but with some qualification. Fifteen of the lightest 20 elements are used in lubricants. These exclude the unreactive noble gases helium, neon and argon, and the two metals beryllium and potassium, which are both relatively scarce compared with their fellow group members with similar chemistry: magnesium and sodium. Molybdenum and tungsten are in the same group and occur in nature as layered sulfides, which are solid lubricants. Most other additives containing molybdenum and tungsten also contain sulfur, making a chemical nod toward these slippery solids.
This leaves zinc and barium as the two elements where the periodic table does not work well as a predictive tool. This may be because zincs nearest neighbor, cadmium, is highly toxic, and that barium chemistry was developed while demand for its nearest chemical analogue, strontium, was very high 70 years ago for use in TV and computer screens.
These exceptions to the periodic tables predictive power help us to understand that in business – whether consumer electronics or lubricants production – it may not be the properties of a material you are using that are the most important factor. Legislation (controlling toxicity) or market forces (as with strontium versus barium) could influence your market share in the lubricants industry.ο