Glycols are organic compounds. They are clear, colorless, nearly odorless liquids. They are heavier and more viscous than water at all temperatures and have much higher boiling points than water does. They are completely soluble in water. Propylene glycol is the only low toxicity glycol, making it the glycol of choice in applications in which toxicity is an issue.
Automotive antifreeze is one such application.
Propylene glycol also is safer for bearings in the event of engine oil contamination than is ethylene glycol. Dow Chemical reports that bearings exposed to oil with 8 percent propylene glycol show no excessive wear while those exposed to oil with 1 percent ethylene glycol fail rapidly. It's a universal formulation that may be used in diesel or gasoline fueled passenger cars and commercial trucks. It's dyed purple to make it easy to distinguish from most ethylene glycol products, which are dyed green. Propylene glycol cannot be tested with the regular antifreeze testers used for ethylene glycol based coolants.