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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