(5) Additives

-- Chemical compounds added to the base stock of a lubricating oil to:

a) Impart new properties to the lube

b) Enhance properties already present in the lube

c) Reduce undesirable changes in the lube during service

Additive packages are developed through extensive experiments and field testing. Engine oils generally contain about 20% additives.

Good additive developments have significantly enhanced the life and performance of modern engines.

 

 

Major IC Engine Additives

• VI improvers

• Oxidation inhibitors

• Antiwear agents

• Detergents and dispersants

• Corrosion inhibitors

• Pour-points depressants

• Defoamants

VI improvers

• Long-chain, high-molecular weight polymers added to oil basestock to reduce its viscosity sensitivity to temperature

• They adopt a coil form when cold and tend to detangle when hot, producing a differential thickening with temperature

• Possible breakdowns: – Temporary breakdown – Polymer chains align with direction of

shear (shear thinning)

– Permanent breakdown – Polymers are ruptured by heavy stress to low-molecular-weight materials

Oxidation inhibitors

• Oil oxidation leads to viscosity increase and surface deposits

• Oxidation inhibitors are materials that reduce the rate of lubricant oxidation by:

a) Reacting with oxidation initiators to form inactive compounds

(modest T)

b) Decomposing oxidized materials to form less active compounds

c) Reducing catalytic effects of the metals surfaces that promote oil

oxidation (high T)

ZDDP used for b) and c).

Antiwear agents

• Friction modifier (modest working conditions)

– Materials with big polar ends such as fatty acids that strongly adsorb to the metal surfaces

• EP additives (severe working conditions)

– Materials such as sulfur, chlorine or phosphorus that react with the metal surfaces to form reaction films (ZDDP used for it)

Corrosion inhibitors

• Materials that prevent engine components corrosion by:

– Forming a protective film on the metal surfaces to prevent corrosion materials from attacking (ZDDP used for this)

– Neutralizing acids formed in combustion process of the engine

Detergents and dispersants

• Materials that delay and reduce deposit formation on metal surfaces such as oxidized oil molecules

• Detergents mainly neutralize deposit precursors formed under high T

• Dispersants mainly disperse and suspend potential sludge forming materials under low T

Pour-point depressants

• Pour point is the temperature below which the lube oil stops to flow freely

• The waxes in a mineral oil tend to crystalize as temperature is reduced. Wax is the main reason for a high pour point of the oil

• PPDs are high-molecular-weight polymers that adsorb on the wax crystals to delay wax crystallization

Defoamants

• Foaming is a form of massive air bubbles in the lube oil resulting from excessive churning by moving machine parts.

• Defoamants are silicon polymers that attach themselves to the foaming bubbles to create weak points for rupture.