Since the 1950s, the most common choice of engine coolant has been a 50:50 mixture of ethylene glycol and water, with the ethylene glycol containing selected corrosion inhibitors and other additives such as nitrates, silicates, borates, phosphates, tolyltriazole (TTZ), mercaptobenzothiazole (MET), antifoam, molybdates, silicones, dye, surfactant, and alkalinity builder. Manufacturers in Europe and Asia use a different corrosion inhibition technology than those used in North America.
The function of the engine coolant is simple, to remove the heat from the engine and to protect the engine from corrosion and pitting of the metal surfaces. As the coolant accumulates miles or hours in a vehicle cooling system, it develops many different types of problems. With time, the inhibitors deplete, the ethylene glycol degrades, the pH of the coolant solution decreases, and corrosion starts to take place. Silicates can combine with excessive levels of magnesium or calcium, thus losing their ability to protect aluminum; MBT is easily oxidized and results in loss of copper protection; and phosphates can combine with calcium and magnesium to form an insoluble sediment.
A consequence of this chemistry is that the coolant that comes out of an automobile after time and use can be significantly different than what went into it. With the increased emphasis placed on proper waste disposal and recycling of used coolant, it is important to understand what is coming out of the car.
During 1989 and 1990, a statistically valid survey and characterization of used engine coolant was taken from commercial automotive service centers. The purpose of the survey was to characterize the condition of a typical used engine coolant. The samples were evaluated for suspended matter, and analyzed for pH, reserve alkalinity (RA), percent ethylene glycol (EG), diethylene glycol (DEG), propylene glycol (PG); the presence of degradation acids (acetate, glycolate, formate, and ethylenediaminetetraacetate [EDTA]); the level of inhibitors (nitrites, nitrates, TTZ, MET, benzotriazole [BZT], benzoate), oil, chloride, fluoride, sulfate concentrations, and 14 other elements (Al, B, Ca, Cu, Fe, K, Mg, Mo, Na, P, Pb, Si, Sn, Zn) in their soluble and insoluble forms. The samples were also tested following U.S. Environmental Protection Agency (EPA) guidelines to determine if the engine coolant would be considered a hazardous waste.
This paper presents the findings of this survey and shows the variation in the condition of engine coolant taken from automobiles at the time the average consumer decides to change the fluid.