ZDDP in motor oil

In the "What's That Stuff?" on motor oil, Steve Ritter mentions zinc dialkyldithiophosphates (ZDDP) as a key antiwear agent (C&EN, March 13, page 38). It has been used in motor oils for more than 60 years.

ZDDP operates by forming a wear-protecting phosphate glass on engine surfaces in an operating engine. However, this same glass damages the engine's catalytic converter, which is used to reduce carbon monoxide and hydrocarbon emissions that exit from the tailpipe of an operating engine. ZDDP volatilizes, enters the exhaust stream and coats the converter's catalytic elements with the phosphate glass, preventing the exhaust from contacting the catalyst. The result is a shortened catalyst life.

This is particularly important because over the past two decades, the Environmental Protection Agency has steadily increased the amount of driving miles required to warranty the catalytic converter from 50,000 miles to 120,000 miles or 10 years. Over the same period, EPA has also reduced the acceptable levels of carbon monoxide and hydrocarbon emissions that can be emitted from a car's tailpipe. These factors have forced the automotive industry to develop motor oils with ever-decreasing concentrations of ZDDP while attempting to protect the engine from wear.

Philip E. Rakita
Philadelphia