Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE 1. Introduction
Use of anti-friction, antiwear and extreme pressure additives in lubricating oils and greases is a common practice. These antiwear and extreme pressure additives play an important role in forming surface protective films either by adsorption or reaction. Such surface films can prevent direct metal to metal contact, thus provide metal surface protection against wear and weld between metal surfaces in relative motion under high load conditions. 2,5-dimercapto-1,3,4–thiadiazole (DMTD) derivatives are generally used in lubricants as metal passivating agents. Such additives also have antioxidant and antiwear properties, and are used in a wide range of lubricant applications. For example, as an additive for engine oil, gasoline engine oil oxidation stability can be significantly improved. In antiwear hydraulic oils, it can provide good anti-corrosive effect on copper and assist in improving hydrolytic stability [1]. Due to the high sulfur content of DMTD, their derivatives have potential as an extreme pressure additive. Previous studies showed DMTD dimer and DMTD dimer polyether complex, a new type high performance extreme pressure additive for greases, have excellent extreme pressure properties. The structure of DMTD provides its multifunctional properties which allows it to perform as an extreme pressure agent, metal deactivator, metal passivator/antioxidant. As such, this type of additive can overcome the limitations in reduced oxidation resistance and corrosion of copper that are inherent problems with other traditional EP additives [2-8]. DMTD dimer has following chemical structure:
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Four-ball weld point results of DMTD dimer in different base greases are listed in Table 1 [2]. As can be seen from Table 1, DMTD dimer exhibits good extreme pressure properties.
Table 1. Extreme pressure performance of different greases with 3.0% DMTD dimer.
DMTD dimer can be dissolved in a polyalkylene glycol (PAG) solution to obtaining a biodegradable liquid product of DMTD/ PAG complex. The product has excellent Timken extreme pressure properties and good Four-ball extreme pressure performance [8, 9]. Instead of bidentate adsorption of DMTD dimer, DMTD/PAG complex forms multidentate adsorption on the surface, the strongest adsorption possible [9]. DMTD/PAG complex adsorption on metal surface is shown in Figure 2.
This additive in its physical form is a solid/powder and has been shown to exhibit excellent extreme pressure performance in four-ball EP test [2, 9]. In four-ball EP test, most base greases will have four-ball weld point between 126 kgf and 160 kgf. When 2.0% DMTD dimer is added to these base greases, four- ball weld point can be increased to between 250 and 400 kgf. When DMTD dimer treat rate is increased to 3.0%, four-ball weld point can reach between 620 kgf.. and up to 800 kgf or above [9]. Extreme pressure performance of DMTD dimer comes from its bidentate adsorption on the metal surface [9]. Figure 1 is a diagram of this bidentate adsorption, i.e. the two ring structure of DMTD oriented parallel to the metal surface to form a bidentate complex structure, and if only one ring is desorbed from the surface, the molecule is still attached to the surface by the other ring.
Figure 2. adsorption of DMTD dimer / butoxy triethylene glycol complex on metal surface.
Excellent EP performance supports the theory of such strong surface adsorption. Table 2 lists DMTD/PAG complex EP performance in various base greases [8, 9]. As can be seen from Table 2, this additive at low treat rates of 2% can provide a Timken OK load of up to 80 pounds and four-ball weld point up to 400 kgf in some base greases. To achieve this level of extreme pressure performance, normally heavy metals-containing EP additive is needed at 5% treat rate.
Figure 1. DMTD dimer adsorption on metal surface.
Table 2. Extreme pressure properties of DMTD/PAG complex in different base greases.
LUBE MAGAZINE NO.126 APRIL 2015 33
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