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Zinc Dialkyldithiophosphates Synthesis Essay

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You may have heard of the lubricant additive zinc dialkyldithiophosphate (ZDDP), but did you know its unique antiwear applications were discovered serendipitously?1

ZDDP: A historical perspective

ZDDP was first introduced as a lubricant additive in 1941, when three different companies—Lubrizol, American Cyanimid, and Union Oil Co.—filed patents with claims of corrosion and oxidation inhibitors in engine oils.2 It was likely that ZDDPs were first used in engine lubricants in the late 1930s.3

In the late 1940s, V8 engines with overhead valves and increased compression ratios were introduced. This design increased stresses on valve train components, and cam and follower wear. As engine lubricants containing ZDDP were already in use, it was observed in the 1950s that those containing ZDDP provided antiwear properties. The automobile industry quickly adopted the use of ZDDP in high-quality motor oils, and now employ their use in practically all current engine oils.3

What are zinc dialkyldithiophosphates?

 

ZDDPs are a family of uncharged compounds composed of zinc, phosphorus and sulfur traces that are, as mentioned above, primarily used as antiwear additives in lubricants, such as greases, gear oils, and motor oils.4

Zinc is bound to the anion of dialkyldithiophosphoric acid (e.g. ammonium diethyl dithiophosphate).3 The compound is also known as ZnDTP and ZDP.4

ZDDP characteristics

Spikes argues that ZDDPs are “the most successful lubricant additive ever invented,” as no other cost-effective compound has shown comparable antiwear performance in engine oils.3 The properties can vary depending on heat and the ratio of zinc oxide to the dialkyl dithiophosphoric acid, creating acidic, neutral and basic ZDDPs, each with a different reactivity.4 Here are a few applications:

  • Forms protective chemical films (tribofilms) on metal surfaces, preventing corrosive damage to valve train wear and bearing3,5,6
  • Antioxidant properties reduce the formation of sludge and soot deposits on engine components6
    • The tribofilm is a polyphosphate glass that can digest oxides5
  • Low vapor pressures and fugacity
  • Inhibits the onset of scuffing3

These varied characteristics have led to a vast amount of research investigating their complex role as antioxidants, and anticorrosive and antiwear agents.3 For example, research has indicated that while ZDDPs form thick reaction films, this formation does not require solid–solid rubbing contact but is driven simply by applied shear stress, in accord with a stress-promoted thermal activation model.7

The future of ZDDPs

Regulators have expressed concern over health effects related to the use of ZDDPs. While zinc dialkyldithiophosphates produce low acute toxicity in some animals, they have proven to be toxic in aquatic wildlife with long-lasting effects. They are also not readily biodegradable,6 confirming environmental concerns.

The presence of sulphur and phosphorus oxides and metal salts in engine exhaust harms engine exhaust aftertreatment devices, and ZDDP contains all three of these elements.8

Future limits of phosphorus and Sulphur in engine oils may eventually lead to the discontinued use of ZDDP. Research continues on the additive, and may one day produce alternatives with lower volatility yet comparable performance.3

References:

  1. Tribology and Lubrication Technology: Seren-ZDDP
  2. Lubricants2013, 1(4), 132-148; doi:3390/lubricants1040132
  3. Spikes, H. The History and Mechanisms of ZDDP. Tribology Letters (2004) 17: 469. https://doi.org/10.1023/B:TRIL.0000044495.26882.b5
  4. Wikipedia: Zinc dithiophosphate
  5. Varlot, K., Kasrai, M., Martin, J. et al. Antiwear film formation of neutral and basic ZDDP: influence of the reaction temperature and of the concentration. Tribology Letters (2000) 8: 9. https://doi.org/10.1023/A:1019162529554
  6. Lubrizol: Zinc Dialkyl Dithiophosphate Lubricant Additives
  7. Zhang, J. & Spikes, H. On the Mechanism of ZDDP Antiwear Film Formation. Tribol Lett (2016) 63: 24. https://doi.org/10.1007/s11249-016-0706-7
  8. Spikes, H. A. (2008), Beyond ZDDP. Lubrication Science, 20: 77–78. doi:10.1002/ls.60

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Filed Under: Lubricants Tagged With: material overview

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