Vegetable oils oxidise at a fast rate as well because of the presence of unsaturation. New synthetic fully-saturated biobased fluids come into force through recent developments.
Additives
Although base oil quality has considerable impact on formulated oil’s performance, additives are able to improve several existing properties of base oils as well as to give some new.
The following properties can be improved by chemical additives: Viscosity VI
Pour point
Rheological properties at low and high temperatures Friction properties Detergency Dispersancy
Oxidation stability Anti-wear EP
Load-carrying capacity Foaming Water separation characteristics Rusting and corrosion
Industrial oils generally do not need detergents and dispersants, since these do not come in contact with the fuel combustion products, but need anti-rust, anti-foam, and oxidation inhibitor along with anti-wear and EP additives (wherever these properties are required). The nature of these additives and dosage vary according to the equipment requirement and need to be optimised in every grade. Each lubricant specification has been carefully defined to meet these requirements. This is only possible through the use of suitable chemical additives. A careful selection of the additive combination is therefore necessary in a fully formulated lubricant. Additives exhibit both synergistic and antagonistic effects when used in combination, and this requires careful selection and evaluation. The dosage of each additive in a particular product is of great importance, since this decides the cost and performance of the product.
MECHANISM OF LUBRICANT OXIDATION & DEGRADATION The process of oxidation proceeds in three stages: initiation, propagation, and termination. During the initiation stage, oxygen reacts with the lubricant to form alkyl radicals. During the propagation stage, these radicals react with oxygen and the lubricant to form peroxy radicals and hydroperoxides. As indicated by the oxygen uptake, hydroperoxides are accumulated during the induction period, after which the auto-acceleration of oxidation occurs. Hydroperoxides, either thermally or in the presence of metal, decompose to a variety of additional radicals and oxygen-containing compounds. The oxygen-containing compounds include alcohols, aldehydes, ketones, and carboxylic acids.
From these compounds, polymers, metal carboxylates can be formed; the latter can increase the rate of oxidation due to their catalytic effect.
Some metal salts, at low concentration, can act as oxidation inhibitors; this is the case for some copper salts.
From oxidation of lubricants and fuels, ASTM [2]
Some parameters affect the oxidation process profoundly: • The temperature which is supposed to double the rate of oxidation every ten degree centigrade rise.
• The wear metals • The presence of water
If oxidation is not controlled, lubricant decomposition will lead to oil thickening, sludge formation, and the formation of varnish, resin, deposits, and corrosive acids.
OVERVIEW OF INDUSTRIAL OILS OXIDATION AND THERMAL STABILITY TESTS
General overview
Numbers and various methods are used to evaluate thermal stability and anti-oxidant properties; some are specific for certain types of lubricants. Nevertheless, all these methods attempt to simulate the oxidation phenomena at various operating conditions and in various mechanical components. They are more or less based on the same principle. The oil ageing depends upon : • The thermal stress: temperature and temperature cycle • The presence of air or oxygen at a specific flow rate or through a static pressure
• The presence of metal catalysts: this may be massive metals or organometallic compounds • The presence of water
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LUBE MAGAZINE NO.140 AUGUST 2017
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