Zinc dithiophosphate

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Structure of a monomeric zinc dialkyldithiophosphate Zn(dtp)2.png
Structure of a monomeric zinc dialkyldithiophosphate

Zinc dialkyldithiophosphates (often referred to as ZDDP) are a family of coordination compounds developed in the 1940s that feature zinc bound to the anion of a dialkyldithiophosphoric salt (e.g., ammonium diethyl dithiophosphate). [1] 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 68649-42-3 . 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.

Contents

Applications

The main application of ZDDPs are as anti-wear additives in lubricants including greases, hydraulic oils, and motor oils. ZDDPs also act as corrosion inhibitors and antioxidants. Concentrations in lubricants range from 600 ppm for modern, energy-conserving low-viscosity oils to 2000 ppm in some racing oils.

It has been reported that zinc and phosphorus emissions may damage catalytic converters and standard formulations of lubricating oils for gasoline engines now have reduced amounts of the additive due to the API 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. [3] 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. Roller camshafts/followers are more commonly used to reduce camshaft lobe friction in modern engines. [4] There are additives, such as STP 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 valve spring pressures.

Tribofilm formation mechanism

Various mechanisms have been proposed for how ZDDP forms protective tribofilms on solid surfaces. [1] In-situ atomic-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. [5] Subsequently, experiments with negligible solid-solid contact demonstrated that film formation rate depends on the applied shear stress. [6]

Synthesis and structure

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 mix of zinc dialkyl(C3-C6)dithiophosphates come under CAS number 84605-29-8 . A list of other examples with their CAS numbers is here.

Zinc dithiophosphate are produced in two steps. First phosphorus pentasulfide is treated with suitable alcohols (ROH) 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: [7] [8]

P2S5 + 4 ROH → 2 (RO)2PS2H + H2S
2 (RO)2PS2H + ZnO → Zn[(S2P(OR)2]2 + H2O

Structural chemistry

In Zn[(S2P(OR)2]2, the zinc has tetrahedral geometry. This monomeric compound Zn[(S2P(OR)2]2 exists in equilibrium with dimers, oligomers, and polymers [Zn[(S2P(OR)2]2]n (n > 1). [7] For example, zinc diethyldithiophosphate, Zn[(S2P(OEt)2]2, crystallizes as a polymeric solid consisting of linear chains. [9] 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 for basic zinc acetate. [7]

Zinc-diethyldithiophosphate-chain-from-xtal-3D-balls.png

See also

Related Research Articles

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<span class="mw-page-title-main">Motor oil</span> Lubricant used for lubrication of internal combustion engines

Motor oil, engine oil, or engine lubricant is any one of various substances used for the lubrication of internal combustion engines. They typically consist of base oils enhanced with various additives, particularly antiwear additives, detergents, dispersants, and, for multi-grade oils, viscosity index improvers. The main function of motor oil is to reduce friction and wear on moving parts and to clean the engine from sludge and varnish (detergents). It also neutralizes acids that originate from fuel and from oxidation of the lubricant (detergents), improves the sealing of piston rings, and cools the engine by carrying heat away from moving parts.

<span class="mw-page-title-main">Lubrication</span> The presence of a material to reduce friction between two surfaces.

Lubrication is the process or technique of using a lubricant to reduce friction and wear and tear in a contact between two surfaces. The study of lubrication is a discipline in the field of tribology.

Tribology is the science and engineering of understanding friction, lubrication and wear phenomena for interacting surfaces in relative motion. It is highly interdisciplinary, drawing on many academic fields, including physics, chemistry, materials science, mathematics, biology and engineering. The fundamental objects of study in tribology are tribosystems, which are physical systems of contacting surfaces. Subfields of tribology include biotribology, nanotribology and space tribology. It is also related to other areas such as the coupling of corrosion and tribology in tribocorrosion and the contact mechanics of how surfaces in contact deform. Approximately 20% of the total energy expenditure of the world is due to the impact of friction and wear in the transportation, manufacturing, power generation, and residential sectors.

<span class="mw-page-title-main">Synthetic oil</span> Lubricant consisting of artificially made chemical compounds

Synthetic oil is a lubricant consisting of chemical compounds that are artificially modified or synthesised. Synthetic lubricants can be manufactured using chemically modified petroleum components rather than whole crude oil, but can also be synthesized from other raw materials. The base material, however, is still overwhelmingly crude oil that is distilled and then modified physically and chemically. The actual synthesis process and composition of additives is generally a commercial trade secret and will vary among producers.

<span class="mw-page-title-main">Gear oil</span> Lubricant used in vehicles and machinery

Gear oil is a lubricant made specifically for transmissions, transfer cases, and differentials in automobiles, trucks, and other machinery. It has high viscosity and usually contains organosulfur compounds. Some modern automatic transaxles do not use a heavy oil at all but lubricate with the lower viscosity hydraulic fluid, which is available at pressure within the automatic transmission. Gear oils account for about 20% of the lubricant market.

<span class="mw-page-title-main">Amsoil</span> American multi-level marketing company

AMSOIL Inc. is an American corporation based in Superior, Wisconsin that primarily formulates and packages synthetic lubricants, fuel additives, and filters. Company founder Albert J. Amatuzio developed several synthetic motor oil formulations throughout the mid-to-late 1960s. He was commercially selling synthetic motor oil by 1968. In 1972, AMSOIL 10W-40 Synthetic Motor Oil became the world's first synthetic motor oil to meet American Petroleum Institute requirements, which prompted the company to adopt "The First in Synthetics®" as its tagline. The company introduced several other synthetic lubricants that represented industry firsts throughout the 1970s and early 1980s. It distributes products in North America via a network of independent dealers paid commissions on sales.

Silicone grease, sometimes called dielectric grease, is a waterproof grease made by combining a silicone oil with a thickener. Most commonly, the silicone oil is polydimethylsiloxane (PDMS) and the thickener is amorphous fumed silica. Using this formulation, silicone grease is a translucent white viscous paste, with exact properties dependent on the type and proportion of the components. More specialized silicone greases are made from fluorinated silicones or, for low-temperature applications, PDMS containing some phenyl substituents in place of methyl groups. Other thickeners may be used, including stearates and powdered polytetrafluorethylene (PTFE). Greases formulated from silicone oils with silica thickener are sometimes referred to as silicone paste to distinguish them from silicone grease made with silicone oil and a soap thickener.

Grease is a solid or semisolid lubricant formed as a dispersion of thickening agents in a liquid lubricant. Grease generally consists of a soap emulsified with mineral or vegetable oil.

AW additives, or antiwear additives, are additives for lubricants to prevent metal-to-metal contact between parts of gears.

<span class="mw-page-title-main">Extreme pressure additive</span>

Extreme pressure additives, or EP additives, are additives for lubricants with a role to decrease wear of the parts of the gears exposed to very high pressures. They are also added to cutting fluids for machining of metals.

Friction modifiers are added to lubricants in order to reduce friction and wear in machine components. They are particularly important in the boundary lubrication regime, where they can prevent solid surfaces from coming into direct contact, substantially reducing friction and wear.

<span class="mw-page-title-main">Shell Rotella</span> Brand of heavy-duty engine lubricant

Shell Rotella is a line of heavy-duty engine lubrication products produced by Shell plc. The line includes engine oils, gear oils and coolants. The oil carries both the American Petroleum Institute (API) diesel "C" rating as well as the API gasoline engine "S" rating. Ratings differ based on the oil. Rotella oils, like the T3 15W-40, meet both the API CJ-4 and SM specifications, and may be used in both gasoline and diesel engines. However, it is formulated specifically for vehicles without catalytic converters, containing phosphorus levels beyond the 600–800 ppm range. Therefore, Rotella is not recommended for gasoline vehicles with catalytic converters due to the higher risk of damaging these emission controls. Newer formulations of Rotella T6 however are API SM rated as safe for pre-2011 gasoline vehicles.

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<span class="mw-page-title-main">Diethyl dithiophosphoric acid</span> Chemical compound

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Hugh Alexander Spikes is a British mechanical engineer. He is emeritus professor of tribology at Imperial College London. He is the former head of the Tribology Group at Imperial College. Tribology is the science and engineering of friction, lubrication and wear.

Tribofilms are films that form on tribologically stressed surfaces. Tribofilms are mostly solid surface films that result from a chemical reaction of lubricant components or tribological surfaces.

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Extreme tribology refers to tribological situations under extreme operating conditions which can be related to high loads and/or temperatures, or severe environments. Also, they may be related to high transitory contact conditions, or to situations with near-impossible monitoring and maintenance opportunities. In general, extreme conditions can typically be categorized as involving abnormally high or excessive exposure to e.g. cold, heat, pressure, vacuum, voltage, corrosive chemicals, vibration, or dust. The extreme conditions should include any device or system requiring a lubricant operating under any of the following conditions:

References

  1. 1 2 Spikes, H. (2004-10-01). "The History and Mechanisms of ZDDP". Tribology Letters. 17 (3): 469–489. doi:10.1023/B:TRIL.0000044495.26882.b5. ISSN   1023-8883. S2CID   7163944.
  2. ↑ 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, pp. 389–395. doi : 10.1016/S0301-679X(01)00028-7.
  3. ↑ "ZDDP Engine Oil – The Zinc Factor". Mustang Monthly. Retrieved 2009-09-19.
  4. ↑ McGean, Terry (1 March 2004). "Roller Camshafts – Roll With It". www.hotrod.com. Retrieved 26 January 2016.
  5. ↑ Gosvami, N. N.; Bares, J. A.; Mangolini, F.; Konicek, A. R.; Yablon, D. G.; Carpick, R. W. (2015-04-03). "Mechanisms of antiwear tribofilm growth revealed in situ by single-asperity sliding contacts" (PDF). Science. 348 (6230): 102–106. Bibcode:2015Sci...348..102G. doi: 10.1126/science.1258788 . ISSN   0036-8075. PMID   25765069.
  6. ↑ Zhang, Jie; Spikes, Hugh (2016-08-01). "On the Mechanism of ZDDP Antiwear Film Formation". Tribology Letters. 63 (2): 24. doi: 10.1007/s11249-016-0706-7 . hdl: 10044/1/33668 . ISSN   1023-8883.
  7. 1 2 3 D. Johnson and J. Hils (2013). "Phosphate Esters, Thiophosphate Esters and Metal Thiophosphates as Lubricant Additives". Lubricants. 1 (4): 132–148. doi: 10.3390/lubricants1040132 .
  8. ↑ Randolph A. McDonald (2003). "Zinc Dithiophosphates" (Google Books excerpt). In Leslie R. Rudnick (ed.). Lubricant Additives: Chemistry and Applications. CRC Press. ISBN   9780824747404.
  9. ↑ T. Ito; T. Igarashi; H. Hagihara (1969). "The crystal structure of metal diethyldithiophosphates. I. Zinc diethyldithiophosphate" (PDF). Acta Crystallogr. B. 25 (11): 2303–2309. doi:10.1107/S0567740869005619.
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