1. Overview

Metal working fluids (MWFs) are essential auxiliary materials in mechanical machining processes. Their quality, formulation, and performance directly impact the machining precision, production efficiency, energy consumption, material usage, and workplace environment.

Metal working fluids account for about 20% of total lubricant consumption, which is why both domestic and international industries place great emphasis on their development. Metalworking can be broadly categorized into two types: metal forming and metal cutting operations.

Cutting fluids are primarily used to:

Improve surface finish of machined parts,Extend tool life,Remove chips and heat, and

Increase overall machining productivity.

Their main functions include:

Lubrication、Cooling、Rust prevention、Cleaning.

During metal cutting, the workpiece undergoes deformation due to high pressure and friction between the tool and the workpiece. This generates cutting forces and heat, significantly affecting tool life, machining efficiency, and part quality.

To reduce cutting forces and temperatures, improvements can be made by modifying tool geometry or using better tool materials. However, the most common and cost-effective method is to use the appropriate metalworking fluid.

Classification of Metal Working Fluids by Form:

Straight Oil (Neat Oil)

Base oils: mineral oil, synthetic oil, polyolefins, alkylbenzene esters

Additives: oiliness agents, extreme pressure (EP) additives, rust inhibitors, antioxidants, anti-mist agents

Characteristics: Excellent lubrication, but poor cooling performance

General-purpose cutting oils: mineral oil + rust inhibitor + antioxidant + vegetable/animal oil — used for non-ferrous metals and applications requiring high surface finish

Extreme pressure cutting oils: contain sulfur, phosphorus, and chlorine compounds — used for heavy-duty machining of hard metals and low-speed, high-finish applications

Soluble Oil (Emulsifiable Oil)

Base oils: mineral oil, synthetic oil

Additives: lubricity agents, EP additives, rust inhibitors, emulsifiers, coupling agents, biocides, anti-mist agents

Usage: diluted with water (typically 20:1 ratio), forming an opaque emulsion

Properties: Combines the lubrication of oil and the cooling of water

Widely used for both ferrous and non-ferrous metals

pH typically maintained between 8–9

Suitable for general machining like turning, drilling, and grinding

Semi-Synthetic Fluids

Composition: 10–30% oil, 10–20% emulsifier, 10–20% other additives

Diluted form: transparent or semi-transparent due to small oil droplets (0.06–1.0 µm)

Performance: Lower lubricity than soluble oils but better cooling and cleaning properties

Ideal for high-speed cutting and grinding

Synthetic Fluids (Chemical Fluids)

Composition: Water-based, no oil content; ~50% water-soluble additives

Usage: diluted to 2–10% aqueous solution

Advantages: Excellent cooling and cleaning performance

Can replace emulsions and semi-synthetics in most applications

Easy to filter, long-lasting, and resistant to bacterial degradation

2. The World of Additives

Boundary Lubricity Additives / Oiliness Agents

These additives improve lubrication by adsorbing onto metal surfaces and forming a protective film that reduces direct metal-to-metal contact.

Typical structures: polar head (adsorption site) and hydrocarbon tail (oil/water compatible)

Examples: animal fats, vegetable oils, fatty acids, higher alcohols, synthetic esters

Extreme Pressure (EP) Additives

Unlike boundary lubricants that merely adsorb, EP additives chemically react with metal surfaces under high stress to form metal salts or protective films that act as physical barriers.

These reduce friction, wear, and damage

Common compounds:

Chlorinated paraffins、Chlorinated fats、Sulfurized fats、Sulfurized olefins、Phosphosulfurized esters、Superalkaline sulfonates (PEP)

Alkaline Reserve Additives / pH Buffers

These maintain fluid stability by neutralizing acidic contaminants and maintaining an optimal pH range to prevent corrosion.

Also function as emulsifiers

Examples:

Alkanolamines、Monoethanolamine、Triethanolamine、Aminomethylpropanol、2-(Aminoethoxy) ethanol、Sulfonates

Metal Deactivators

Prevent corrosion of non-ferrous metals (e.g., copper) by forming a protective layer on the metal surface.

Examples:

Mercaptobenzothiazole、Toluyltriazole、Benzotriazole

Emulsifiers

Reduce interfacial tension between dispersed phases and continuous media, enabling stable emulsification.

They orient at the interface to form a protective membrane, preventing coalescence

Common emulsifiers:

Sodium petroleum sulfonate、Fatty acid soaps、Hydroxycarboxylates、Amides and epoxides

Alkanolammonium salts of fatty acids

Corrosion Inhibitors (Rust Preventatives)

Polar molecules with one hydrophilic end and one hydrophobic end. They form tight molecular layers on metal surfaces to block corrosive agents.

Examples:

Sulfonates、Carboxylates、Naphthenates、Phosphates、Alkanolamine salts、Borates、Molybdates

Polyol esters、Amines

Coupling Agents

Increase soap solubility in oil to stabilize water-dilutable metalworking fluids and prevent phase separation.

Examples:

Propylene glycol、Ethylene glycol ethers、Nonionic epoxy compounds

Chelating Agents / Water Softeners

Bind calcium and magnesium ions to prevent destabilization of emulsions caused by hard water.

Prevent reactions between cations and anionic emulsifiers

Example: Ethylenediaminetetraacetic acid (EDTA)

Anti-Mist Additives

Reduce airborne mist generated during machining by increasing fluid viscosity and suppressing droplet formation.

Polymers increase elongational viscosity, reducing filament breakage and aerosol generation

Oil-based

anti-mists: ethylene-propylene copolymers, polyisobutylene (MW: 1,000,000–2,000,000)

Water-based anti-mists: polyethylene oxide

Biocides / Microbicides

Inhibit microbial growth (bacteria, fungi, yeast) in water-based fluids.

Microbial activity causes:

Odors、Corrosion、Reduced fluid performance、Clogged pipelines、Shortened fluid lifespan

Pure oil fluids are more stable due to lack of water, while water-based fluids (emulsions, semi-synthetics, synthetics) are prone to microbial attack due to presence of nutrients.

Effective biocides:

Formaldehyde-releasing compounds

Triazines (e.g., Hexahydro-1,3,5-tris(hydroxyethyl)-s-triazine)

Phenolic compounds

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