Zinc dialkyl dithiophosphates

Zinc dialkyl dithiophosphates (ZDDPs) are very effective antiwear agents that are used as machine lubricants (e.g., greases, hydraulic oils, and motor oils), including applications in the automotive industry.

According to a 2004 article by Hugh Spikes at Imperial College London, ZDDPs were developed in the 1940s. Today, they are still by far the most prevalent antiwear additive in heavy equipment and off-road vehicle engine oils. They are not used in passenger vehicles because the phosphorus in ZDDPs prematurely disables catalytic converters in exhaust systems.

On paper, ZDDPs look like zinc salts, but their properties much more closely resemble those of organic compounds. As an example, they are soluble in nonpolar organic solvents, including oils, and are insoluble in water.

Zinc dibutyl dithiophosphate, shown in the images and information tables, is a typical ZDDP. But commercial ZDDPs almost always contain a mixtures of alkyl groups, ranging from C₄ to C₁₂. The specific mixture used depends on the type of lubricant and metallurgy involved. Shorter-chain alkyl groups, however, are the least thermally stable; they thus make protective films more easily.

Currently, concerns over ZDDPs’ environmental effects are driving efforts to curtail or eliminate their use. But no one has found a substitute that performs quite as well; and how harmful ZDDPs actually are at the level of usage is the topic of some rather heated discussions among lubricant formulators, vintage car enthusiasts, and environmental advocacy groups.

[Many thanks to Nancy McGuire for assistance with this article—ED].