Coolant chemistry, simply
A coolant contains two things: a base fluid, almost always ethylene glycol, which sets the freeze and boil points, and an inhibitor package, which protects the metals from corrosion. The base is nearly always the same, and the entire difference lies in the inhibitors.
Three generations
IAT, the inorganic old green. Uses silicates and phosphates. Protects quickly but depletes quickly, typically in two years or 50,000 km. It is a product of the cast-iron engine era.
OAT, organic acid, long life. Uses organic acids that act slowly but last a long time, five years and more. Some contain 2-EHA.
HOAT, the hybrid. Combines both: the longevity of organic acids plus a fast-acting additive. Zerex Asian Blue is a HOAT, organic acids plus phosphate.
Phosphate or silicate, a geographical split
Both solve the same problem, protecting aluminium quickly, but with different side effects.
The Japanese school chose phosphate. Phosphate covers aluminium rapidly and, unlike silicate, does not drop out as a gel. Its weak point is that in very hard water it can react with calcium, which is why modern formulations address this deliberately.
The European school chose silicate. Silicate acts even faster, but over time it precipitates and works like an abrasive against the water-pump seal.
This is why it matters to put a fluid built on Japanese logic into a Japanese car, rather than whatever happens to be available.
Why 2-EHA is a subject of its own
2-ethylhexanoic acid, or 2-EHA, serves as an inhibitor in some OAT coolants. The problem is that it is simultaneously a plasticiser, a chemical whose function is to soften plastics and rubber.
In a cooling system containing metres of plastic pipe, rubber hose and gaskets, that is an unwelcome property. This is why many Asian manufacturers require 2-EHA-free formulations.
The Zerex Asian Blue data sheet states directly that the product contains no 2-EHA. The same applies to silicates, borates, amines and nitrites.