Comprehensive Guide to Lubricants & Additives | Functions, Types & Applications

Comprehensive Guide to Lubricants and Their Additives

This blog covers everything about lubricants, their classification, the role of additives, and detailed tables on various additives used in lubrication oils. It aims to provide an educational resource with practical insights for engineering students and professionals.

Table of Contents

• Introduction to Lubricants
• Classification of Lubricants
• Lubricating Oils
• Lubricant Additives
• Lubricant Protective Additives
• Surface Protective Additives
• Performance Additives
• Greases or Semi-Solid Lubricants
• Solid Lubricants
• Synthetic Lubricants
• Conclusion

Introduction to Lubricants

Lubricants are materials that reduce friction, wear, and heat generation between moving surfaces. They provide a continuous fluid film to separate moving, sliding, or rolling surfaces and prevent corrosion. Lubricants include oils, greases, and solid lubricants, each suited to different applications.

Classification of Lubricants

Based on their physical state, lubricants are classified as:

• Lubricating Oils (liquid lubricants)
• Greases (semi-solid lubricants)
• Solid Lubricants

Lubricating Oils

Lubricating oils are further classified into:

• Animal and Vegetable Oils
• Mineral or Petroleum Oils
• Blended Oils

Characteristics of good lubricating oils: High boiling point, low freezing point, adequate viscosity, non-corrosive properties, and stability against oxidation and heat.

Functions: Provide lubrication films to minimize friction and wear, act as cooling and sealing agents, and prevent corrosion.

Animal and Vegetable Oils

These are extracted from natural sources like animal fats or seeds. They have good oiliness and stick well to machine parts but have limited uses due to high cost and poor oxidation resistance.

Mineral or Petroleum Oils

These are derived from petroleum distillation and have hydrocarbon chains. They are cheaper and have good service stability but possess less oiliness than vegetable oils.

Blended Oils

These are mixtures of different oils and may include additives to improve performance and address specific needs like oxidation resistance or viscosity adjustment.

Lubricant Additives

An additive is a material added to lubricating oils to impart new or improved properties such as corrosion protection, foam reduction, or viscosity modification. Additives can be chemically active or inert. They are broadly categorized into three types:

• Lubricant Protective Additives
• Surface Protective Additives
• Performance Additives

Lubricant Protective Additives

Additive Type	Purpose	Functions	Typical Examples
Antioxidant	Retard oxidative decomposition	Terminate free radical chain reactions and decompose peroxides	Aromatic amines, hindered phenols
Metal Deactivator	Decrease catalytic effect of metals on oxidation rate	Complex with metal ions to form inactive layers on metal surfaces	Amines, sulfides, phosphites
Antifoamant	Prevent persistent foam formation	Speed collapse of foam by reducing surface tension	Silicon polymers

Surface Protective Additives

Additive Type	Purpose	Functions	Typical Examples
Rust and Corrosion Inhibitor	Prevent rusting and corrosion of metal parts	Neutralize corrosive acids and adsorb polar constituents to form protective films	Metal phenolates, basic metal sulfonates, fatty acids, amines
Anti-wear and EP Agent	Reduce friction, wear, and prevent scoring and seizure	Form protective films on metal surfaces with lower shear strength than metal	Zinc dithiophosphates, organic phosphates, sulfurized fats
Friction Modifier	Change coefficient of friction	Preferential adsorption of surface active materials	High molecular weight organic phosphorus and phosphoric acid esters, organic fatty acids, amines
Detergent	Keep surfaces deposits free	Neutralizes sludge and varnish precursors to keep them soluble	Magnesium phenolates, phosphates, sulfonates
Dispersant	Keep insoluble contaminants dispersed in the lubricant	Prevent agglomeration of contaminants through polar attraction to dispersant molecules	Alkylsuccinimides, polymeric alkylthiophosphonates

Performance Additives

Additive Type	Purpose	Functions	Typical Examples
Viscosity Modifier	Reduce rate of viscosity change with temperature	Polymer expands with increasing temperature to counteract oil thinning	Polymers and copolymers of olefins, alkylated styrenes, methacrylates, butadiene
Pour Point Depressant	Enable lubricant flow at low temperatures	Reduce interlocking by modifying wax crystal formation	Polymethacrylates, phenolic polymers, alkylated naphthalene
Seal Swell Agent	Cause swelling of elastomers by chemical reaction		Aromatic hydrocarbons and organic phosphates

Greases or Semi-Solid Lubricants

Greases are semi-solid lubricants formed by combining lubricating oils with thickening agents such as soaps. The thickener traps oil and enables the grease to cling to moving parts while improving heat resistance.

Unlike lubricating oils, greases flow only under pressure (non-Newtonian fluids). They are commonly used where oil cannot remain in place due to load, speed, or conditions such as bearings, railboxes, and sealed applications.

Common greases include:

• Soda-based greases (sodium soap thickener, water soluble, up to 175°C)
• Lithium-based greases (lithium soap thickener, water resistant, good temperature stability up to 150°C)
• Calcium-based greases (calcium soap thickener, water resistant, common and cheap, temperature up to 80°C)
• Axle greases (resin based, cheap, resistant to water, good for low-speed/high-load)

Solid Lubricants

Solid lubricants reduce friction by separating moving surfaces under boundary lubrication. Used as powders or mixed with oils/water, they form films with very low friction coefficients.

Common solid lubricants:

• Graphite: Flat carbon layers held by weak forces, low friction coefficient, useful over wide temperatures and conditions (powder or dispersion).
• Molybdenum Disulphide (MoS₂): Sandwich-like structure with low shear strength allowing sliding. Used in space vehicles and high-temp applications.

Synthetic Lubricants

Synthetic lubricants are manufactured chemically or polymerized to produce tailor-made properties like high-temperature stability, extended service life, and resistance in reactive environments. They are not produced during normal petroleum refining.

Below is a summary table of relative properties for common synthetic lubricants:

Synthetic Lubricant	Low Temp Properties	High Temp Stability	Viscosity Index	Lubricity	Volatility	Fire Resistance	Hydrolytic Stability
Polyalphaolefins (PAO)	Good	Good	Good	Good	Good	Poor	Excellent
Diesters	Excellent	Excellent	Variable	Good	Average	Poor	Fair
Linear alkyl benzenes (LAB)	Good	Fair	Poor	Good	Average	Poor	Good
Polyalkylene Glycols (PAGs)	Good	Good	Excellent	Good	Average	Excellent	Fair
Phosphate esters	Variable	Excellent	Poor	Good	Average	Excellent	Fair
Silicones	Excellent	Excellent	Excellent	Variable	Good	Excellent	Fair
Fluorinated Lubes	Fair	Excellent	Excellent	Variable	Average	Excellent	Fair

Conclusion

Lubricants play a crucial role in mechanical systems by reducing friction and wear, thus enhancing efficiency and longevity. Various types of lubricants exist, each suitable for specific applications, temperatures, and operating conditions. Additives greatly improve the performance, stability, and protective properties of lubricating oils. Understanding these fundamentals helps in selecting the right lubricant for any engineering application.