Oxidative Degradation and Antioxidant of Lubricating Oil

Oxidation is an important reason leading to quality deterioration, consumption increase and service life shortening of lubricating oil. Due to the increasingly harsh operating conditions of lubricating oil, lubricating oil is required to have good high temperature oxidation resistance. Lubricating oil in the process of use is inevitable to contact with metal, by the action of light, heat and oxygen, and oxidation degradation. Oil oxidation generates peroxides, alcohols, aldehydes, acids, esters and other substances, which can be further condensed to form macromolecular compounds, thus increasing the viscosity of oil. At the same time, some of the generated macromolecular compounds insoluble in oil are deposited on the friction surface as paint film, thus leading to the formation of carbon deposition. The generated organic acid compounds will also cause corrosion of the metal, thereby increasing the wear and damage the equipment. These harmful substances will damage some special properties of lubricating oil and reduce its service life.

The autooxidation mechanism of lubricating oil involves free radical chain reaction, which consists of four steps: chain initiation, chain growth, chain branching and chain termination.

Chain initiation

The initiating reaction is characterized by the formation of an alkyl radical (RO·). There are two forms, one is when you take hydrogen and break the C-H bond, and the other is when you dissociate the C-C bond. When hydrocarbons come into contact with oxygen and are subjected to energy, including heat, ultraviolet light and mechanical shear, they are prone to free radical initiation reactions. The A-position hydrogen and aromatic ring containing tertiary ammonia, double bonds are *** susceptible to oxidation.

Under normal conditions, the chain initiation rate is very slow, but it is greatly accelerated by increasing temperature and the presence of transition metal ions (copper, iron, nickel, vanadium, manganese, cobalt).

Chain growth

The first step in chain propagation is the irreversible reaction of an alkyl radical with oxygen to form an alkyl peroxyl radical (ROO·). This reaction is extremely fast, and the rate depends on the radical substituents. Once formed, the peroxyl radical can easily snatch a hydrogen from another hydrocarbon molecule to form a hydroperoxide and a new alkyl radical (RO·). Based on this mechanism, for each alkyl radical formed, a large number of hydrocarbon compounds may be oxidized to hydroperoxides.

Chain branched

Form free radicals

Aldehydes and ketones are formed

At the beginning of chain branching, the hydroperoxides decompose into an alkoxy radical (RO·) and a hydroxyl radical (HO·). This reaction has a high activation energy, which is only apparent at temperature > 150℃. Metal ion catalysis can accelerate the process. The resulting radical may react as follows :(a) the alkoxy radical snatches a hydrogen atom from another alkane molecule to form a molecule of alcohol and a molecule of alkyl radical; (b) The hydroxyl radical snatches a hydrogen atom from a molecule of hydrocarbon, yielding a molecule of water and a new alkyl radical; (c) Decomposition of alkoxy radical into a molecular aldehyde and an alkyl radical; Oxidation is an important reason leading to quality deterioration, consumption increase and service life shortening of lubricating oil. Due to the increasingly harsh operating conditions of lubricating oil, lubricating oil is required to have good high temperature oxidation resistance. Lubricating oil in the process of use is inevitable to contact with metal, by the action of light, heat and oxygen, and oxidation degradation. Oil oxidation generates peroxides, alcohols, aldehydes, acids, esters and other substances, which can be further condensed to form macromolecular compounds, thus increasing the viscosity of oil. At the same time, some of the generated macromolecular compounds insoluble in oil are deposited on the friction surface as paint film, thus leading to the formation of carbon deposition. The generated organic acid compounds will also cause corrosion of the metal, thereby increasing the wear and damage the equipment. These harmful substances will damage some special properties of lubricating oil and reduce its service life.

The autooxidation mechanism of lubricating oil involves free radical chain reaction, which consists of four steps: chain initiation, chain growth, chain branching and chain termination.

Chain initiation

The initiating reaction is characterized by the formation of an alkyl radical (RO·). There are two forms, one is when you take hydrogen and break the C-H bond, and the other is when you dissociate the C-C bond. When hydrocarbons come into contact with oxygen and are subjected to energy, including heat, ultraviolet light and mechanical shear, they are prone to free radical initiation reactions. The A-position hydrogen and aromatic ring containing tertiary ammonia, double bonds are *** susceptible to oxidation.

Under normal conditions, the chain initiation rate is very slow, but it is greatly accelerated by increasing temperature and the presence of transition metal ions (copper, iron, nickel, vanadium, manganese, cobalt).

Chain growth

The first step in chain propagation is the irreversible reaction of an alkyl radical with oxygen to form an alkyl peroxyl radical (ROO·). This reaction is extremely fast, and the rate depends on the radical substituents. Once formed, the peroxyl radical can easily snatch a hydrogen from another hydrocarbon molecule to form a hydroperoxide and a new alkyl radical (RO·). Based on this mechanism, for each alkyl radical formed, a large number of hydrocarbon compounds may be oxidized to hydroperoxides.

Chain branched

Form free radicals

Aldehydes and ketones are formed

At the beginning of chain branching, the hydroperoxides decompose into an alkoxy radical (RO·) and a hydroxyl radical (HO·). This reaction has a high activation energy, which is only apparent at temperature > 150℃. Metal ion catalysis can accelerate the process. The resulting radical may react as follows :(a) the alkoxy radical snatches a hydrogen atom from another alkane molecule to form a molecule of alcohol and a molecule of alkyl radical; (b) The hydroxyl radical snatches a hydrogen atom from a molecule of hydrocarbon, yielding a molecule of water and a new alkyl radical; (c) Decomposition of alkoxy radical into a molecular aldehyde and an alkyl radical; (d) The alkoxy radical may also decompose into a molecule of ketone and an alkyl radical.

Under high temperature oxidation conditions, aldehydes and ketones can further react to form acids and high molecular weight compounds, which will make the lubricating oil sticky and subsequently form sludge and precipitation.

Chain termination

The formation of high molecular weight hydrocarbon compounds during oxidation increases the viscosity of oil products. When oil viscosity increases to a certain extent, oxygen diffusion in oil decreases significantly, and chain termination reaction will dominate. Although an alkyl radical can combine with an alkyl peroxyl radical to form a peroxide, this peroxide is unstable and easily breaks to produce more alkoxyl radicals.

Catalytic oxidation of metals

Metal ions can catalyze the radical chain initiation step through the REDOX mechanism. The activation energy required by this mechanism is reduced, so the initiation and branching steps can be carried out at lower temperatures.

High temperature degradation process of lubricating oil

Lubricating oils are oxidized at low temperatures to produce peroxides, alcohols, aldehydes, ketones and water. When the temperature is higher than 120℃, peroxides and hydroperoxides are further oxidized to carboxylic acids.

As the high temperature oxidation degradation reaction continues, acid or base will catalyze the hydroxyl aldehyde reaction and form the precursor of oil sludge. Further reaction to obtain high molecular weight products, these products will increase the viscosity of oil, *** can eventually combine with each other to form oil insoluble polymerization products, which are shown as sludge precipitation in oil or varnish precipitation on metal surface under oxidation conditions.

Antioxidant principle of phenolic antioxidants

BHT is a representative blocked milk antioxidant. Under the condition of sufficient oxygen, the probability of reaction between BHT and alkyl radical is very low. In the process of oxidation, more alkyl radical is converted into alkyl peroxyl radical, and BHT reacts with alkyl peroxyl radical by providing hydrogen atom. In this reaction, the peroxyl radical forms hydroperoxides, while the di-tert-butylcresol is converted to the phenoxyl radical (a stable structure with steric hindrance and resonance). The two tert-butyl groups in the blocked phenol ortho position effectively prevent the attack of phenoxy radical on other hydrocarbon compounds. The resonance structure of cyclohexenone radical can be further combined with the second alkyl peroxyl radical to generate alkyl peroxides. This substance is relatively stable at a temperature below 120℃ without resonance changes. Phenoxy radical can achieve chain termination by mutual combination. One phenoxy radical contributes hydrogen atoms to another phenoxy radical, resulting in a regenerated bistert-butylcresol and a methylene cyclohexenone.

Under the condition of high temperature oxidation, the cyclohexenone alkyl peroxide obtained above is no longer stable. It will decompose into an alkoxy radical, an alkyl radical and 2, 6-di-tert-butyl 1, 4-benzoquinone.

The antioxidant principle of amine antioxidants

Antioxidant mechanism at low temperature (<120℃)

Since imine radical attacks alkyl peroxyl radical to form nitryl radical and alkoxy radical, nitryl radical stabilizes through three possible resonance structures. Next, a THIRD ALKYl PEROXYl radical REACTS WITH THE NITrYL RADICAL TO FORM A COMPLEX NITrYL PEROXIDE, which CAN further eliminate an ether molecule to form NITryl cyclohexanedione. The next reaction consists of a fourth alkyl peroxyl radical and nitryl cyclohexanedione to form nitryl cyclohexanEDIone peroxide, followed by a decomposition reaction to form 1, 4-benzoquinone and alkylnitrobenzene. Thus, at low temperatures, ADPA can theoretically terminate up to twice as many alkyl peroxyl radicals as hindered phenols alone.

Antioxidant Mechanism at high temperature (>120℃)

The termination of the reaction chain can be the thermal decomposition of the generated alkoxy intermediate into ketone and the re-formation of ADPA, or the regeneration of the nitryl radical through the reaction between the hydroxyl dianiline intermediate and the alkyl peroxyl radical. Therefore, at higher temperatures, a molecule of ADPA can catalyze the removal of a large number of nitryl radicals before they are destroyed.

Under high temperature oxidation conditions, aldehydes and ketones can further react to form acids and high molecular weight compounds, which will make the lubricating oil sticky and subsequently form sludge and precipitation.

Chain termination

The formation of high molecular weight hydrocarbon compounds during oxidation increases the viscosity of oil products. When oil viscosity increases to a certain extent, oxygen diffusion in oil decreases significantly, and chain termination reaction will dominate. Although an alkyl radical can combine with an alkyl peroxyl radical to form a peroxide, this peroxide is unstable and easily breaks to produce more alkoxyl radicals.

Catalytic oxidation of metals

Metal ions can catalyze the radical chain initiation step through the REDOX mechanism. The activation energy required by this mechanism is reduced, so the initiation and branching steps can be carried out at lower temperatures.

High temperature degradation process of lubricating oil

Lubricating oils are oxidized at low temperatures to produce peroxides, alcohols, aldehydes, ketones and water. When the temperature is higher than 120℃, peroxides and hydroperoxides are further oxidized to carboxylic acids.

As the high temperature oxidation degradation reaction continues, acid or base will catalyze the hydroxyl aldehyde reaction and form the precursor of oil sludge. Further reaction to obtain high molecular weight products, these products will increase the viscosity of oil, *** can eventually combine with each other to form oil insoluble polymerization products, which are shown as sludge precipitation in oil or varnish precipitation on metal surface under oxidation conditions.

Antioxidant principle of phenolic antioxidants

BHT is a representative blocked milk antioxidant. Under the condition of sufficient oxygen, the probability of reaction between BHT and alkyl radical is very low. In the process of oxidation, more alkyl radical is converted into alkyl peroxyl radical, and BHT reacts with alkyl peroxyl radical by providing hydrogen atom. In this reaction, the peroxyl radical forms hydroperoxides, while the di-tert-butylcresol is converted to the phenoxyl radical (a stable structure with steric hindrance and resonance). The two tert-butyl groups in the blocked phenol ortho position effectively prevent the attack of phenoxy radical on other hydrocarbon compounds. The resonance structure of cyclohexenone radical can be further combined with the second alkyl peroxyl radical to generate alkyl peroxides. This substance is relatively stable at a temperature below 120℃ without resonance changes. Phenoxy radical can achieve chain termination by mutual combination. One phenoxy radical contributes hydrogen atoms to another phenoxy radical, resulting in a regenerated bistert-butylcresol and a methylene cyclohexenone.

Under the condition of high temperature oxidation, the cyclohexenone alkyl peroxide obtained above is no longer stable. It will decompose into an alkoxy radical, an alkyl radical and 2, 6-di-tert-butyl 1, 4-benzoquinone.

The antioxidant principle of amine antioxidants

Antioxidant mechanism at low temperature (<120℃)

Since imine radical attacks alkyl peroxyl radical to form nitryl radical and alkoxy radical, nitryl radical stabilizes through three possible resonance structures. Next, a THIRD ALKYl PEROXYl radical REACTS WITH THE NITrYL RADICAL TO FORM A COMPLEX NITrYL PEROXIDE, which CAN further eliminate an ether molecule to form NITryl cyclohexanedione. The next reaction consists of a fourth alkyl peroxyl radical and nitryl cyclohexanedione to form nitryl cyclohexanEDIone peroxide, followed by a decomposition reaction to form 1, 4-benzoquinone and alkylnitrobenzene. Thus, at low temperatures, ADPA can theoretically terminate up to twice as many alkyl peroxyl radicals as hindered phenols alone.

Antioxidant Mechanism at high temperature (>120℃)

The termination of the reaction chain can be the thermal decomposition of the generated alkoxy intermediate into ketone and the re-formation of ADPA, or the regeneration of the nitryl radical through the reaction between the hydroxyl dianiline intermediate and the alkyl peroxyl radical. Therefore, at higher temperatures, a molecule of ADPA can catalyze the removal of a large number of nitryl radicals before they are destroyed.