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Zinc dithiophosphates are a family of coordination compounds that feature zinc bound to the anion of dithiophosphoric acid. These uncharged compounds are not salts. They are soluble in nonpolar solvents, and the longer chain derivatives easily dissolve in mineral and synthetic oils used as lubricants. They come under CAS number . In aftermarket oil additives, the percentage of ZDDP ranges approximately between 2-15%.1
The alkyl groups can be branched and linear alkanes between 1-14 carbons length, 2-butyl, pentyl, hexyl, 1,3-dimethylbutyl, heptyl, octyl, isooctyl (2-ethylhexyl), 6-methylheptyl, 1-methylpropyl, dodecylphenyl, and others. A mix of zinc dialkyl(C3-C6)dithiophosphates come under CAS number . List of other examples with their CAS numbers is here.
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Coordination chemistry
These species are produced in a two steps. First phosphorus pentasulfide is treated with suitable alcohols to give the dithiophosphoric acid. A wide variety of alcohols can be employed, which allows the lipophilicity of the final zinc product to be fine-tuned. The resulting dithiophosphate is then neutralized by adding zinc oxide:
- 2 (RO)2PS2H + ZnO → Zn[(S2P(OR)22 + H2O
In Zn[(S2P(OR)22, the zinc is tetrahedral. This monomeric compound also exists in equilibrium with dimers and oligomers caused by opening of the four-membered ZnS2P ring. Partial hydrolysis gives the cluster Zn4O[(S2P(OR)26, which adopts the structure seen for basic zinc acetate.
Applications
The main use of ZDDP is in anti-wear additives to lubricants such as greases, gear oils, and motor oils, which contain about 1%. For applications in oils for gasoline engines, zinc and phosphorus emissions could damage catalytic converters. The same compounds serve also as corrosion inhibitors and antioxidants.
Naming
These compounds are widely used and correspondingly have numerous names, including ZDDP, ZnDTP, and ZDP.
References
- ^ Allyson M. Barnes, Keith D. Bartle and Vincent R. A. Thibo “A review of zinc dialkyldithiophosphates (ZDDPS): characterisation and role in the lubricating oil” Tribology International 2001, Pages 389-395. doi:10.1016/S0301-679X(01)00028-7.
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