4.6. Organo-zinc compounds (Zn dithiophosphate ZDTP/ZDP)

ZDTP/ZDP work as antioxidant and anti-wear agents protecting metals against corrosion in the lubricants formula. Therefore, they are considered multifunctional additives in engine oils and hydraulic fluids. The alcoholic group used in the structure of the compound is an important factor on the performance of the product, i.e. primary and secondary ZDTPs (with aliphatic alcohols) provide better results in terms of oxidation inhibition and wear protection compared to aryl ZDTP. The overall performance of ZDTPs would be affected by the presence of other additives in the formulation of the lubricating product [31–33].

In a typical antioxidant mechanism activity of ZDTP, an acid-catalyzed ionic decomposition of hydroperoxide may occur. First, ZDTP and hydroperoxide can form a basic ZDTP and then through some sequential reactions, hydroperoxides are decomposed. A typical example of the reaction between ZDTP and hydroperoxides outlined in [1] Figures 14 and 15:

ZDTP may also directly react with peroxy radicals leading to active inhibitors as shown below:

Figure 14. A typical reaction of ZNTP with hydroperoxides.

Figure 15. Reaction mechanism of ZDTP with peroxy radical.

The final radical (RO)2PS2 . also has the potential of reacting with hydroperoxide and generating hydrogenated acid form of this compound with the functionality of an inhibitor.

## 4.7. Organo-copper compounds

4.5. Sulfur-phosphorus compounds

34 Lubrication - Tribology, Lubricants and Additives

lubricant industry for several years [2].

4.6. Organo-zinc compounds (Zn dithiophosphate ZDTP/ZDP)

of other additives in the formulation of the lubricating product [31–33].

reaction between ZDTP and hydroperoxides outlined in [1] Figures 14 and 15:

Antioxidants with both sulfur and phosphorus elements are more efficient and effective than those with either sulfur or phosphorus. Metal dialkyldithiophosphates are a group of antioxidants in this class that have been widely used and have been synthesized by the reaction between phosphorus pentasulfide and alcohols (such as aliphatic, cyclic and phenolic, lauryl, octyl, methyl cyclohexyl, etc.) to produce dithio-phosphoric acids followed by a neutralization process using a metal compounds (such as zinc, barium, calcium and molybdenum compounds or oxides). Zinc dialkyldithiophosphate (ZDDP) is one of the well known compounds in this group that have been used as an effective antioxidant and anti-wear component in the

Figure 13. Reactions of phosphite (possessing phenoxy) with alkoxy and peroxy radicals [resketched from 2].

ZDTP/ZDP work as antioxidant and anti-wear agents protecting metals against corrosion in the lubricants formula. Therefore, they are considered multifunctional additives in engine oils and hydraulic fluids. The alcoholic group used in the structure of the compound is an important factor on the performance of the product, i.e. primary and secondary ZDTPs (with aliphatic alcohols) provide better results in terms of oxidation inhibition and wear protection compared to aryl ZDTP. The overall performance of ZDTPs would be affected by the presence

In a typical antioxidant mechanism activity of ZDTP, an acid-catalyzed ionic decomposition of hydroperoxide may occur. First, ZDTP and hydroperoxide can form a basic ZDTP and then through some sequential reactions, hydroperoxides are decomposed. A typical example of the

ZDTP may also directly react with peroxy radicals leading to active inhibitors as shown below:

Copper as a transition metal has been considered an oxidation promoter which may cause damage in the lubricant or lubricating systems; however, copper salts that are soluble in oil are reported to function as antioxidants [34, 35]. Limitation of loading copper within 100 to 200 ppm to obtain the optimal control of oxidation and wear is a drawback for copper-based antioxidants. Over this range, the performance of the anti-wear components in the lubricants would drop due to the reverse impact of copper. Organo-copper antioxidants are effective in ester and mineral oil lubricants at temperatures below 250C.

Organo-copper compounds including copper naphthenates, oleates, stearates, and polyisobutylene succinic anhydrides have been reported to be synergistic with multi-ring aromatic compounds in controlling high-temperature deposit formation in synthetic base stocks [2].

According to some other studies, inclusion of oil-soluble compounds of copper in the range of 5 to 500 ppm resulted in improved performance of the automotive crankcase lubricants in terms of anti-wear, antioxidant performance and corrosion resistance [36].

### 4.8. Organo-molybdenum compounds

Molybdenum dithiocarbamate has been reported to function as an antioxidant and anti-wear component in the lubricants. However, it would lose its protective properties by time due to dropping its concentration below the critical level of activity [37]. The synergistic application of Molybdenum dialkyldithiocarbamate (MoDDC) with arylamines was tested to improve the durability and low friction performance of MoDDC over time. The DSC (differential scanning calorimetric) results have indicated that the oxidation and induction temperatures for a polyα-olefin (PAO) lubricant would increase by the addition of MoDDC to the formulation. Also, MoDDC would have an antioxidative synergism with alkylated diphenylamine antioxidants (arylamines) such as octyl- and butyl-containing diphenylamine compounds.
