I found some info 
To prevent internal engine corrosion creating sludge that blocks water galleries, coolant also contains corrosion inhibitors.
In normal Mono Ethylene Glycol (MEG) coolant, the inhibitors last two to three years before they are too degraded to work and internal engine corrosion begins to set in.
About five years ago, however, a different type of coolant (Mono Propylene Glycol, MPG) became widely available.
The brand name for the additive package is Trigard, and corrosion inhibitors last at least four years. The coolant is best bought pre-mixed with purified water and, because it is environmentally friendly, old coolant can safely be flushed down a drain.
Brand names for pre-mixed MPG Trigard coolants include Comma Coldstream, Esso Ready Mixed, Quantum (VW/Audi) Ready Mixed Coolant, Bluecol Protex Anti Freeze, Unipart Super Plus Anti Freeze, Batoyle Masterfrost and Silkolene Iceguard. Forlife pre-mix from Toyota dealers (£13.61 + VAT for five litres)
So basically it lasts longer and is environmentally friendly when spent
