2)2L.svg)
Zinc dialkyldithiophosphates (often referred to asZDDP) are a family ofcoordination compounds classified as members oftransition metal dithiophosphate complexes. These compounds were introduced in the 1940s asoil additives. They are uncharged compounds, not salts. They are soluble innonpolar solvents, and the longer-chain derivatives easily dissolve inmineral andsynthetic oils used aslubricants. They come underCAS number68649-42-3 . In aftermarket oil additives, the percentage of ZDDP ranges approximately between 2 and 15%.[2] Zinc dithiophosphates have many names, including ZDDP, ZnDTP, and ZDP.
| R group | Compound | Registry number | melting point (°C) |
|---|---|---|---|
| CH3 | Zn[S2P(OMe)2]2 | 25502-69-6 | - |
| C2H5 | Zn[S2P(OEt)2]2 | 7268-60-2 | - |
| CH(CH3)2 | Zn[S2P(OC−i−Pr)2]2 | 2929-95-5 | 131 |
| C4H9 | Zn[S2P(OBu)2]2 | 6990-43-8 | 36.8 |
| CH2CH(CH3)2 | Zn[S2P(OCH2CHMe2)2]2 | 4563-55-7 | 107 |
The main application of ZDDPs are asanti-wear additives inlubricants includinggreases,hydraulic oils, andmotor oils. ZDDPs also act ascorrosion inhibitors andantioxidants. Concentrations in lubricants range from 600 ppm for modern, energy-conserving low-viscosity oils to 3000 ppm in some racing oils.[3]
It has been reported that zinc and phosphorus emissions may damagecatalytic converters and standard formulations of lubricating oils for gasoline engines now have reduced amounts of the additive due to theAPI limiting the concentration of this additive in new API SM and SN oils; however, this affects only 20- and 30-grade "ILSAC" oils. Grades 40 and higher have no regulation regarding the concentration of ZDDP, except for diesel oils meeting the API CJ-4 specification which have had the level of zddp reduced slightly, although most diesel Heavy-Duty Engine oils still have a higher concentration of this additive.[4] Crankcase oils with reduced ZDDP have been cited as causing damage to, or failure of, classic/collector car flat-tappet camshafts and lifters which undergo very high boundary layer pressures and/or shear forces at their contact faces, and in other regions such as main bearings, and piston rings and pins. Rollercamshafts/followers are more commonly used to reduce camshaft lobe friction in modern engines.[5] There are additives, such asSTP Oil Treatment, and some racing oils such as PurOl, PennGrade 1, and Valvoline VR-1, Kixx Hydraulic Oil which are available in the retail market with the necessary amount of ZDDP for engines using increased valvespring pressures.
Various mechanisms have been proposed for how ZDDP forms protectivetribofilms on solid surfaces.[6] In-situatomic-force microscopy (AFM) experiments show that the growth of ZDDP tribofilms increases exponentially with both the applied pressure and temperature, consistent with a stress-promoted thermal activation reaction rate model.[7] Subsequently, experiments with negligible solid-solid contact demonstrated that film formation rate depends on the appliedshear stress.[8]
With the formula Zn[(S2P(OR)2]2, zinc dithiophosphate features diverse R groups. Typically, R is a branched or linear alkyl between 1-14 carbons in length. Examples include 2-butyl, pentyl, hexyl, 1,3-dimethylbutyl, heptyl, octyl, isooctyl (2-ethylhexyl), 6-methylheptyl, 1-methylpropyl, dodecylphenyl, and others. A list of examples with their CAS numbers ishere.
Zinc dithiophosphate are often produced in two steps. Firstphosphorus pentasulfide is heated with suitablealcohols (ROH) to give thedithiophosphoric acid. A wide variety of alcohols can be employed, which allows the lipophilicity of the final zinc product to be fine tuned. The resulting dithiophosphoric acid is then neutralized, e.g., with ammonia or by addingzinc oxide:[9][10]
MonomericZn[(S2P(OR)2]2 appear not to exist. Instead, these complexes exist as dimers in solution or polymers in the solid form.[9] The dissociation constant for the dimers at room temperature is 10−2 M[11]
The dimers dissociate in the donor solvents (ethanol) or upon treatment withLewis bases, forming adducts:
The polymers [Zn[(S2P(OR)2]2]n (n > 1) have also been characterized. For example, zinc diethyldithiophosphate, Zn[(S2P(OEt)2]2, crystallizes as a polymers consisting of linear chains.[12] Reaction of Zn[(S2P(OR)2]2 with additional zinc oxide gives rise to the oxygen-centered cluster, Zn4O[(S2P(OR)2]6, which adopts the structure seen forbasic zinc acetate.[9]
