Additives for antifreezes, their properties and purposes
Antifreeze is considered to be the most common name of automotive fluids which do not freeze at temperatures below 0°C. These fluids are used for engine cooling and as anti-icing protective coatings. All basic fluids contain additives, i.e. agents, added in small amounts to improve the operational properties of fuel, lubricants and other substances.
All antifreezes contain anticorrosive additives. They inhibit the corrosion effect of ethylene glycol on many metals. Anti-foaming agents are also added to the composition to prevent foaming of antifreeze.
Traditional coolants, such as Tosol, contain nitrites, silicates and borates to prevent the corrosion of metals. To protect aluminum parts, silicate-free inhibitors are widely used. Sodium mercaptobenzothiazole is often used to protect copper and brass parts. Borates or phosphates, added to the antifreeze composition create the desired PH-environment. Amines and some other agents are added as vapor-phase anticorrosive inhibitors.
To protect aluminum alloys, sodium metasilicate is added; this is one of the most effective inhibitors. Unfortunately, it is not suitable for modern engines because they operate at much higher temperatures and the density of heat flows. Silicate stabilizers cannot completely prevent radiator clogging because silicate does not fully eliminate the reason of gel formation, so it is no wonder that the domestic coolant producers do not use silicate stabilizers.
Traditional recipes of antifreezes include nitrites as inhibitors of cavitation-corrosion. One of their disadvantages is that these salts (nitrites) interreact with amines and form carcinogenic substances. Another drawback is the rapid consumption of the inhibitor. Once performed its primary function, it immediately becomes ineffective, that is, shifts to the inactive state.
Better solution to the problem of metal corrosion is additives featuring carboxylates. The agents include a composition of monoacids and dicarboxylic salts. These agents are used in antifreeze with copper corrosion inhibitor, antifoaming agent and dyes. Such fluids contain neither of the following: borates, silicates, amines, nitrates, phosphates or nitrites.
Additives of the new generation are more advanced, compared to traditional ones, because they form a thinner protective layer on the inner surface of the cooling system. Modern coolants can sustain a mileage of more than 150 thousand miles (for cars) and more than 400 miles (for trucks) because of these additives.
Now it’s time to make a conclusion. Carboxylate-based antifreeze and its silicate analogue can be produced with ethylene glycol, or seldom, with propylene glycol. However, it’s worth remembering that silicate antifreeze can cause scale deposition, which deteriorates heat transfer in the cooling system. Carboxylate antifreeze can localize corrosion at the points of occurrence.
Antifreeze additives, produced with OOAT technology (based on carboxylic acids) contain tolyltriazole for the protection of copper and brass (as well as other soft metals), but such additives do not work for as long as carboxylate additives.
It is worth mentioning, that carboxylate additives can break down into harmful or useless chemicals under certain conditions. When mixed with traditional inhibitors (by mistake), they completely lose their anticorrosive properties.