4. Activity and classification of antioxidants

Antioxidants are a series of compounds with the capability of controlling oxidation, and consequently preventing oil from breakdown and thickening (increasing viscosity), and helping better performance and longer life of an engine. Natural antioxidants are the chemical compounds that originally present in the mineral oil known as polycycloaromatics and sulfur and nitrogen heterocyclics, or with bio-oil, triglycerides and in biological systems known as tocopherol, astaxanthin, zeaxanthin, lutein, flavonoids, lycopene, etc. In the mineral oil refining process, severe conditions applied in the process strips the base oil of its natural antioxidants [1, 2]. Therefore, the lack of these important group of chemicals should be compensated by supplementation of the base oil using appropriate groups of additives. Three types of antioxidants are generally available, namely, radical scavengers (primary antioxidants), peroxide decomposers (secondary antioxidants), and metal passivators/deactivators [15].

Radical scavengers such as phenolic antioxidants, aromatic amines, and sulfur and phosphorus compounds that stop chain propagation by blocking or reacting with free radicals generated in the initiation stage of oxidation. Blocking of the radicals by the scavengers occurs through donation of hydrogen atoms that react with alkyl or peroxy radicals, leading to the formation of quinones or quinine imines [1, 2, 15].

Peroxide decomposers such as organosulfur (e.g. dialkyl sulfides and dithiocarbamates) and organophosphorus (e.g. triaryl phosphites and trialkyl phosphites) compounds have the conversion potential of hydroperoxides to non-radical derivatives such as alcohols [1, 2, 15].

Metal deactivators such as benzotriazole and N-salicylidene ethylamine acting as surface filmforming compounds or stable complex-forming agents (chelating agent) function by reducing catalytic effect of metal ions on oxidation. Chelating agents function by trapping metal ions in their structure in the form of stable complexes to reduce the catalytic oxidation activity of the metal ions. Film-forming agents by covering the surface of the metals do not let them enter into the oil phase, and/or these agents may restrict the access of the corrosive species into the metal surface resulting in a reduced corrosive impact of the corrosive agents [1, 2, 15].

Due to the synergistic effect of the antioxidants, combinations of different types of antioxidants are used in lubricant formulations. This synergistic impact of the antioxidants has been proven in several research studies. For example, the results from a study by Davis and Thompson (1996) indicated that alkali metal carboxylic acids and substituted phenols would work as synergists for arylamine antioxidants in ester-based synthetics lubricants. Their results showed that the oil was stable and sludge free when tested at high temperatures at lab scale [16]. In another study by Sharma et al. [17], a synergistic effect was reported where zinc dialkyledithiocarbamate antioxidant was used with an anti-wear additive namely antimony dithiocarbamates in a soybean oil-based lubricant using a pressure differential scanning calorimetry (PDSC) and a rotary bomb oxidation test (RBOT) [17].

Different classifications are available for antioxidants. Based on the source, they can be classified as: (a) natural antioxidants, and (b) synthetic antioxidants. Based on the solubility, they are classified as: (a) oil-soluble antioxidants, and (b) water soluble antioxidants. Based on the mechanism of action: (a) primary antioxidants (radical scavengers), (b) secondary antioxidants (Peroxide decomposers), and (c) metal deactivators. Oil-soluble organic antioxidants are an important group for (hydrocarbon) lubricating oils that can be categorized as discussed in the following subsections.
