Knowledge

Zinc dialkyldithiophosphates (ZDDPs) have been widely used for over 50 years in the lubricant industry. This multifunctional additive plays a pivotal role in a variety of applications, including engine oils, hydraulic fluids, gear oils, greases, and transmission fluids. Its effectiveness lies in its ability to function as an excellent antiwear agent, a mild extreme-pressure (EP) additive, and a reliable oxidation and corrosion inhibitor, offering all of these benefits at a relatively low cost compared to alternative chemistries available in the market.

ZDDP was first developed in December 1944 by Herbert C. Freuler of Union Oil Company. The compound’s multifunctional performance was evident early on, as it demonstrated the ability to improve both oxidation stability and corrosion resistance of lubricants at treatment levels as low as 0.1–1.0%.

The original synthesis of ZDDP involved the reaction between alcohol and phosphorus pentasulfide (P₂S₅), forming dialkyldithiophosphoric acid. This intermediate was then neutralized with zinc oxide to produce the final ZDDP compound, as shown in the following reactions:

                                                S

4 ROH + P₂S₅ ®  2(RO)₂ P SH + H₂S

                                                        S

• 4 ROH + P₂S₅ ®     2(RO)₂ P SH + H₂S

These reactions are challenging and are still used today in additive manufacturing. Two reasons make this reaction a challenge and a dangerous reaction. The first is P2S5 as a very flammable solid, and produces H2S as a byproduct of the final reaction as a highly toxic gas.

ANTIWEAR AND EXTREME-PRESSURE FILM FORMATION

One of the primary functions of ZDDP (zinc dialkyldithiophosphate) in lubricants is to provide antiwear protection. This role is essential for reducing friction between internal metal surfaces in engines and other machinery that rely on oil-based lubrication.

ZDDP molecules react with oxidized metal surfaces to form a protective film. The formation and thickness of this film are influenced by temperature. At lower temperatures, ZDDP more readily reacts with metal oxides to form this layer.

This protective film prevents direct metal-to-metal contact, thereby reducing wear. Furthermore, the film remains adhered to metal surfaces even after prolonged engine shutdown, offering continued protection during start-up.

ZDDP Use Challenges in Modern Engine Oil Formulations

Zinc dialkyldithiophosphates (ZDDPs) continue to be widely used in engine oil formulations due to their excellent antiwear and antioxidant properties. In modern formulations, however, molybdenum-based compounds are often added as friction modifiers to improve fuel efficiency.  

Research has shown that ZDDP and molybdenum compounds compete for adsorption sites on metal oxide surfaces, both attempting to form protective boundary films.

As a result, the presence of both ZDDP and molybdenum in the same formulation may not yield a synergistic effect. On the contrary, this competition can reduce the effectiveness of both additives in minimizing friction and wear, limiting the performance benefits expected from either component.

In recent years, additive formulators have also encountered new regulatory constraints on ZDDP usage. Specifically, standards such as those established by the International Lubricants Standardization and Approval Committee (ILSAC) have imposed phosphorus limits in order to protect emissions control systems. ILSAC’s GF-3 TO GF-6 specifications introduced a phosphorus cap of 0.1%, and the GF-4 specification reduced it even further. The maximum amount for Phosphorus for GF-6 and API SN, and SP is a maximum of 0.08%. Since phosphorus is a key component of ZDDP, these limits directly restrict its concentration in engine oils.

Currently, ZDDP treatment levels in organic base oils are typically limited to approximately 0.5% to 1.5%, depending on the alkyl chain structure. The primary challenge now facing engine oil formulators is meeting stringent ILSAC performance requirements—including wear protection, oxidation stability, and fuel economy—while keeping ZDDP content within regulatory limits.

Impact of Phosphorus and MoDTC in Low-Phosphorus Engine Oils

Phosphorus levels in engine oils are deliberately kept low due to their potential to poison emission control catalysts. The primary source of phosphorus in lubricants is zinc dialkyldithiophosphate (ZnDTP), a critical additive known for its antiwear and antioxidant properties. However, the reduction of ZnDTP in engine oil formulations significantly affects these protective functions.

Fuel efficiency is another growing priority in modern vehicle design. Molybdenum dithiocarbamate (MoDTC) is widely recognized for its ability to reduce friction and thereby improve fuel economy. In a series of engine and bench tests, oils formulated with MoDTC and varying phosphorus levels (from 0.00% to 0.08%) were evaluated for antiwear performance, oxidation stability, and friction durability.

The results indicated that oils containing more than 0.02% phosphorus were capable of meeting the ILSAC GF-4 performance requirements when the additive formulation was optimized. In these cases, MoDTC was able to effectively compensate for the reduced ZnDTP content. However, formulations with 0.00% phosphorus required additional additive components to meet the necessary performance criteria, particularly in terms of wear protection and oxidation resistance.