2. Mechanism of lubricants oxidation

Oxidation is a multi-step process mainly consisting of three stages: (a) initiation; (b) chain propagation; and (c) termination. In the initiation stage, an external factor (oxidizing agent) causes generation of a free organic radical (R�) or an unpaired electron as part of the lubricant (RH) indicated below:

$$\mathbf{RH} \to \mathbf{R} \cdot + \mathbf{H} \cdot \tag{1}$$

rusting. The lubricating properties of such systems can be affected by increasing the viscosity. Oxidation stability testing is essential, as follows: i) development of new products, ii) evaluation of potential new additives and iii) assessment of storage stability [3]. For the purpose of enabling development of valuable new products, it is important to assess the performance of various antioxidants in a lubricant to determine the required treatment-rate

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ASTM D-6186 is a standard method used to measure the performance of an antioxidant in a lubricating substance. In addition, pressurized differential scanning calorimetry (PDSC) is a suitable tool for measuring the oxidative stability. Accordingly, PDSC provides estimates of oxidative stability by detecting exothermic release of heat identified as auto-oxidation. Auto-oxidation is a process by which the antioxidant capacity of the lubricating system goes into the oxidative chain reaction when the effective ingredients are consumed. For example, the effectiveness of two antioxidants was compared using the PDSC test where each antioxidant was added at 2% level to treat the base oil. The results of oxidation induction time (OIT) for both samples heated at 135 for 7 days are shown in Figure 1. Many researchers rely on high-pressure differential scanning calorimetry (HPDSC) as an appropriate tool, especially for small samples, where bulk solution effects are minimized and it is facile to detect the interchange of the sample with its atmospheric oxygen. One of the main advantages of this tool is

Oxygen pressure vessel method or ASTM D942 (OPVOT) test performance can determine various antioxidants optimum treatment rate to make the most cost-effective formulation in a very short period of time [4]. Lubricants are formulated from a range of base fluids, either mineral or synthetic oils, in which chemical additives are dissolved. The base oil formulation and the nature of the chemical additives will affect on the physical and chemical properties of the lubricant [1]. The life span of the product can be changed using chemical additives for development of lubricants for specific applications. Gas chromatography (GC) and ESI are two significant methods which are used for the analysis of additives related to the additive age,

Figure 1. An alternative antioxidant known as DT-mPM displays a greater performance than a commercial antioxidant

using PDSC study; AO: alpha-olefin [4].

the repeatability of the test procedure with a reasonable reaction time [4–6].

and the cost [4].

In the propagation stage, the free radical released during initiation stage is a highly reactive species with the potential of reacting with oxygen to form a peroxide radical. The peroxide radical is another reactive component with the potential of reacting with the lubricant or other components in the lubricant that can result in further decomposition of the lubricant and its components as follows:

$$\text{R. R.} + \text{O}\_2 \rightarrow \text{ROO} \cdot \text{O} \tag{2}$$

$$\text{ROO} \cdot + \text{RH} \rightarrow \text{ROOH} + \text{R} \cdot \tag{3}$$

Branching occurs as:

$$\cdot \text{ROOH} \rightarrow \text{RO} \cdot + \cdot \text{OH} \tag{4}$$

$$\text{RO} \cdot + \text{RH} + \text{O}\_2 \rightarrow \text{ROH} + \text{ROO} \cdot \tag{5}$$

$$\cdot\text{OH} + \text{RH} + \text{O}\_2 \rightarrow \text{H}\_2\text{O} + \text{ROO}\cdot\text{O}\tag{6}$$

In the termination stage, the radical species generated during initial and propagation stages of oxidation would combine and form a stable organic compound and the free radicals are removed from the lubricating agent. The termination stage can be effective in attenuation or ending the oxidation process if no more radicals are generated during the initiation stage.

$$\mathbf{R} \cdot \mathbf{R} + \mathbf{R} \cdot \rightarrow \mathbf{R} - \mathbf{R} \tag{7}$$

$$\text{R.} + \text{ROO.} \to \text{ROOR} \tag{8}$$

Altogether, two types of products can be produced during oxidation, namely oil soluble products (such as peroxides, alcohols, acids, esters, aldehydes, and ketones), and oil insoluble products with high molecular weight.

### 3. Measuring oxidative resistance

The chemical and physical properties of materials can be altered by oxidation. For instance, an increase in the acidity of the samples containing fats and oils can result in corrosion and rusting. The lubricating properties of such systems can be affected by increasing the viscosity. Oxidation stability testing is essential, as follows: i) development of new products, ii) evaluation of potential new additives and iii) assessment of storage stability [3]. For the purpose of enabling development of valuable new products, it is important to assess the performance of various antioxidants in a lubricant to determine the required treatment-rate and the cost [4].

Antioxidants are a group of additives that have the potential of prohibiting oxidation of base oil

Oxidation is a multi-step process mainly consisting of three stages: (a) initiation; (b) chain propagation; and (c) termination. In the initiation stage, an external factor (oxidizing agent) causes generation of a free organic radical (R�) or an unpaired electron as part of the lubricant

In the propagation stage, the free radical released during initiation stage is a highly reactive species with the potential of reacting with oxygen to form a peroxide radical. The peroxide radical is another reactive component with the potential of reacting with the lubricant or other components in the lubricant that can result in further decomposition of the lubricant and its

In the termination stage, the radical species generated during initial and propagation stages of oxidation would combine and form a stable organic compound and the free radicals are removed from the lubricating agent. The termination stage can be effective in attenuation or ending the oxidation process if no more radicals are generated during the initiation stage.

Altogether, two types of products can be produced during oxidation, namely oil soluble products (such as peroxides, alcohols, acids, esters, aldehydes, and ketones), and oil insoluble

The chemical and physical properties of materials can be altered by oxidation. For instance, an increase in the acidity of the samples containing fats and oils can result in corrosion and

RH ! R� þ H� (1)

R: þ O2 ! ROO� (2)

ROO� þ RH ! ROOH þ R� (3)

ROOH ! RO�þ�OH (4)

R: þ R: ! R � R (7)

R: þ ROO: ! ROOR (8)

RO� þ RH þ O2 ! ROH þ ROO� (5)

�OH þ RH þ O2 ! H2O þ ROO� (6)

in the lubricants and the inhibition of oil breakdown and thickening [1, 2].

2. Mechanism of lubricants oxidation

24 Lubrication - Tribology, Lubricants and Additives

(RH) indicated below:

components as follows:

Branching occurs as:

products with high molecular weight.

3. Measuring oxidative resistance

ASTM D-6186 is a standard method used to measure the performance of an antioxidant in a lubricating substance. In addition, pressurized differential scanning calorimetry (PDSC) is a suitable tool for measuring the oxidative stability. Accordingly, PDSC provides estimates of oxidative stability by detecting exothermic release of heat identified as auto-oxidation. Auto-oxidation is a process by which the antioxidant capacity of the lubricating system goes into the oxidative chain reaction when the effective ingredients are consumed. For example, the effectiveness of two antioxidants was compared using the PDSC test where each antioxidant was added at 2% level to treat the base oil. The results of oxidation induction time (OIT) for both samples heated at 135 for 7 days are shown in Figure 1. Many researchers rely on high-pressure differential scanning calorimetry (HPDSC) as an appropriate tool, especially for small samples, where bulk solution effects are minimized and it is facile to detect the interchange of the sample with its atmospheric oxygen. One of the main advantages of this tool is the repeatability of the test procedure with a reasonable reaction time [4–6].

Oxygen pressure vessel method or ASTM D942 (OPVOT) test performance can determine various antioxidants optimum treatment rate to make the most cost-effective formulation in a very short period of time [4]. Lubricants are formulated from a range of base fluids, either mineral or synthetic oils, in which chemical additives are dissolved. The base oil formulation and the nature of the chemical additives will affect on the physical and chemical properties of the lubricant [1]. The life span of the product can be changed using chemical additives for development of lubricants for specific applications. Gas chromatography (GC) and ESI are two significant methods which are used for the analysis of additives related to the additive age,

Figure 1. An alternative antioxidant known as DT-mPM displays a greater performance than a commercial antioxidant using PDSC study; AO: alpha-olefin [4].

composition and degradation of the lubricant [7–10]. Antioxidants are among the most important group of additives, that are normally composed of sterically hindered phenols or aromatic amines [1, 2]. The presence of oxygen and high temperatures within a tribological environment can be important factors for rapid oxidation of lubricants. The quantitative analysis of lubricant antioxidant additives in complex and native base oil matrices has been studied using ESI-MS and MALDI-MS [11, 12]. The rotating pressure vessel oxidation test (ASTM-D2272) is the most common method which can measure the RUL (Remaining Useful Life) of the oil's ability to resist oxidation.

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

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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

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

Hindered phenols are a group of (primary) antioxidants that function by scavenging mechanism through hydrogen donation in which the target molecules are peroxy radical intermediates. They are active over a wide range of temperature and they can provide a long-term stability of the lubricant with minimizing viscosity change and discoloration. Synergistic effect may result using a combination of hindered phenols and secondary antioxidants such as thioethers and phosphites. The sterically hindered phenols (I) with 2 and 6 positions on the ring substituted by tertiary alkyl groups (such as butyl) are very active antioxidants reacting with the peroxy radical intermediates (Figure 2). The product of the first reaction (II) is also reactive functioning as the

The maximal activity of hindered phenolic antioxidants is attainable when both 2 and 6 positions of the aromatic ring are occupied by tertiary butyl groups; with one substituent replaced by methyl instead of tertiary butyl, the relative antioxidant activity may drop by

This class of antioxidants is more active than the hindered phenols and are available in a wide range of molecular weights and forms. However, aromatic amines contribute more in discoloring

surface resulting in a reduced corrosive impact of the corrosive agents [1, 2, 15].

test (RBOT) [17].

following subsections.

4.1. Hindered phenolic compounds

scavenger of the peroxy radicals [15, 18].

37.5% as shown in Table 1 [18].

4.2. Aromatic amine compounds

Antioxidants are one of the most suitable additives to extend the lifetime of lubricants. Furthermore, antioxidants prevent the oxidative degradation of the lubricant oil thickening and the formation of sludge. Aromatic amines (e.g. dialkylated diphenylamine) and sterically hindered phenols (e.g. 2,6-di-tert-butylphenol) are two common antioxidants which are useful in lubricants stabilization to gain synergistic effects [3, 13]. A better understanding of the chemistry of antioxidants and their degradation mechanisms at the molecular level is crucial for developing more efficient lubricants. Lubricants based on mineral oils, are very complex mixtures; therefore, an analytical method with high sensitivity and selectivity to separate the components, and to characterize and quantify antioxidants and their degradation products has been established [14].
