5. Antioxidants/additives synergism and antagonism

Synergistic mixtures of antioxidants from different groups or classes are generally applied in the formulation of commercial lubricants to provide better stability toward oxidation. In lubricating systems that involve a synergistic mechanism, free radical scavengers are the major antioxidant component, while the hydroperoxide decomposers function as auxiliary components for the free radicals [38]. It was reported that the oil-soluble organic molybdenum (organic molybdenum complex (MC)) and arylamine antioxidant (dioctyldiphenylamine (DO-DPA)) would have an excellent antioxidant synergism in an oil system (poly-alphaolefin synthetic lubricant (PAO)) as shown in the DSC thermogram (Figure 16). The OIT was increased from 221.4C with DODPA as the sole antioxidant to 229.7C with the combination of DODPA and MC as the synergistic antioxidants. Also, a lower acid generation and lower kinematic viscosities were observed after oxidation-corrosion test for the lubricant with antioxidant, especially in the lubricant with both DODPA and MC due to their synergistic impact (Table 3). As a consequence, a lower deposit occurs when both antioxidants are in the system (Figure 17) compared with the lubricants that contain DODPA solely.

In another study by Hu et al. [39] where a molybdate ester (ME) and dioctyldiphenylamine (DO-DPA) were used as antioxidants, a synergistic effect was reported between the two antioxidants.

Synergistic behavior of sulfonated calcium carbonate and an ashless antioxidant (N-phenyl-αnaphthylamine (T531)) in hydrogenated oil was investigated and the results suggested that anti-wear and antioxidant effect were synergistically improved in the system [40].

In contrast to synergism, the presence of mixtures of antioxidants with other additives may have antagonistic behavior. Therefore, the combinations of the antioxidants along with other lube additives in a lubricating fluid need to be optimized to prohibit or minimize antagonism. It has been shown that anti-wear and antioxidant properties of ZDDPs have been adversely affected by some additives such as detergents and dispersants (sulfonates, phenates and salicylates). This undesired effect can be due to the competition of detergents and dispersants with ZDDPs by surface adsorption or restricted interaction of this component with the metal

Figure 17. Deposit formation in PAO in the presence of DODPA with or without MC [38]. Reprinted by permission of the

Triglycerides from plant sources have been used as biolubricants with limited applications due to their low thermal and oxidative stability. As well, triglycerides have low volatility, high lubricity, low toxicity and good viscosity-temperature properties as their key advantages. Therefore, different antioxidants such as tocopherols, propyl gallate (PG), ascorbyl palmitate (AP), and some synthetic antioxidants (butylated hydroxyanisole (BHA), butylated hydroxy-

phenol) (MBP)) have been used to improve their resistance to the oxidative agents. In recent years, a great number efforts have been made to develop sustainable bio-lubricants and additives wit the preference of non-toxicity, multi-functionality and compatibility to the present systems. Cellulose fatty esters have been developed as the lubricant additive, mainly for antioxidant applications. It has been modified to cellulose ferulate, cellulose lipoate and α-

In a study by Singh et al. [44], cellulose laurate was synthesized for use as an effective biolubricant. The results indicated that the lubrication performance would increase with the degree of substitution (DS) in cellulose molecule and by increasing the concentration of cellu-


Antioxidants Classification and Applications in Lubricants

http://dx.doi.org/10.5772/intechopen.72621

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or the fluid phase [41].

6. Bio-based antioxidants and lubricants

Society of Tribologists and Lubrication Engineers, www.stle.org.

tocopherulate for antioxidant functionality [42, 43].

toluene (BHT), mono-tert-butylhydroquinone (TBHQ), or 4,4<sup>0</sup>

lose laurate in the base oil which was n-butyl palmitate/stearate.

Figure 16. DSC thermograms of PAO oxidation in the presence of DODPA with or without MC [38]. Reprinted by permission of the Society of Tribologists and Lubrication Engineers, www.stle.org.


Table 3. The change in total acid number and viscosity after 24 h in oxidation-corrosion test [38].

Figure 17. Deposit formation in PAO in the presence of DODPA with or without MC [38]. Reprinted by permission of the Society of Tribologists and Lubrication Engineers, www.stle.org.

In contrast to synergism, the presence of mixtures of antioxidants with other additives may have antagonistic behavior. Therefore, the combinations of the antioxidants along with other lube additives in a lubricating fluid need to be optimized to prohibit or minimize antagonism. It has been shown that anti-wear and antioxidant properties of ZDDPs have been adversely affected by some additives such as detergents and dispersants (sulfonates, phenates and salicylates). This undesired effect can be due to the competition of detergents and dispersants with ZDDPs by surface adsorption or restricted interaction of this component with the metal or the fluid phase [41].
