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Oxidative Degradation

Oxidative Degradation

Reactions of atmospheric oxygen with lubricants under varying conditions of temperature and oxygen pressure are undesirable processes. Such reactions lead to deterioration of lubricants. Thermally induced hydrocarbon oxidation is a self-accelerating autoxidation process and is divided into:

A. Oxidation of hydrocarbon at low temp. (30 – 120°C):

The oxidation  is consisting of four distinct stages:

1. Initiation of the Radical Chain Reaction:

Under normal conditions hydrocarbon converted to alkyl free radical and hydroperoxide ion.

2. Propagation of the Radical Chain Reaction:

a. Once an alkyl radical has been formed, this reacts irreversibly with oxygen to form an alkyl peroxy radical, which is extremely fast, and is independent of temperature:

b. Hydrogen abstraction by a peroxy radical from another hydrocarbon, which leads to a hydroperoxide and an alkyl radical:

The alkyl radical can again react with oxygen, as step (a).

The peroxy radical may undergo hydrogen abstraction via interamolecular propagation:

The alkyl-hydroperoxide radical undergo a propagation steps (a, b):

Again the hydroperoxide-peroxy radical may undergo hydrogen abstraction via interamolecular propagation:

The alkyl dihydroperoxide radical undergo a propagation steps (a, b):

3. Chain Branching:

a. At low concentrations; hydroperoxides may be cleaved homolytically to yield an alkoxy and a hydroxy radical:

Once formed, hydroxy and especially primary alkoxy radicals are so active that they abstract hydrogen atoms in non-selective reactions:

b. At high concentrations; hydroperoxides may react via a bimolecular mechanismhydroperoxides may be cleaved homolytically to yield an alkoxy and a hydroxy radical:

4. Termination of the radical chain Reaction:

Termination may be effected by the combination of radical species such as peroxy radicals to yield ketones and alcohols:

If the oxygen concentration in the bulk liquid phase is limited two additional ways of radical recombination result:

The oxidation products:

Alkylhydroperoxide (ROOH)

Dialkylperoxides (ROOR)

Ketones (RR1C=O)

Alcohols (ROH)

Aldehydes (RCHO)

Diketones RCO(CH2)xCOR1

Ketoaldehydes RCO(CH2)xCHO

Hydroxyketones RCH(OH)−(CH2)xCOR1

B. Oxidation of hydrocarbon at high temp. > 120°:

Above 120°C the degradation process can be divided into a primary and a secondary oxidation phases.

1. Primary Oxidation Phase:

Initiation and propagation of the radicals’ chain reaction are the same as discussed under low-temperature conditions.

Acids are formed by the following two reactions:

When the rate of oxidation becomes limited by diffusion, ethers are formed:

2. Secondary Oxidation Phase:

1st Step: aldehydes or ketones formed in the primary oxidation phase combine via an acid- or base-catalysed aldol condensation to form α,β-unsaturated aldehydes or ketones:

Further aldol condensations lead to high molecular weight but still oil-soluble polycondensation products (molecular weight about 2000 amu).

2nd Step: co-polymerization of two different polycondenzation species, This leads to sludge and deposit formation as well as to additional oil soluble high molecular weight products which contribute to the viscosity increase:

NOTE

Under high-temperature conditions there is always the possibility of thermal cleavage of a hydrocarbon chain, especially when the availability of oxygen is limited, leads to unsaturated molecules with lower molecular weight and higher volatility.

The model for high-temperature oxidation

Chemist

Abdelrhman Sabry

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