A synthetic cutting fluid is a water-mix metalworking fluid with no petroleum oil, diluted with 10 to 40 parts of water, about 2.4 to 9.1 % concentrate by volume (NIOSH 98-116, 1998). "Oil-free" here means no petroleum oil: a synthetic emulsion still carries 40 to 80 wt % synthetic lubricant in the concentrate (OECD ESD No. 28, 2011).
Synthetics are the oil-free branch of the water-mix metalworking fluids. OSHA rates them as the cleanest fluids with the best heat reduction, excellent rust control and longer sump life, while one supplier's cutting fluids manual rates them lowest of three fluid families for lubrication.
Chemical treatment and ultrafiltration remove only 0 to 5 % of the chemical oxygen demand of a spent synthetic (UKLA/Pera disposal guide). The five key figures are listed below.
- Petroleum oil in the concentrate: none (NIOSH 98-116, 1998)
- In-use dilution: 1 part concentrate to 10 to 40 parts water, about 2.4 to 9.1 % by volume (NIOSH 98-116, 1998)
- ISO 6743-7:1986 categories: MAG and MAH, as quoted by secondary sources
- Concentration check: refractometer Brix reading x the product's factor from its data sheet (UKLA/HSE guide v2.5, February 2025)
- OSHA PEL for a fluid without mineral oil: particulates not otherwise regulated, 15 mg/m³ total and 5 mg/m³ respirable, 8-h TWA (29 CFR 1910.1000 Table Z-1)
Standards, regulations and exposure limits are checked against their issuing bodies, product values against the named data sheet, and every figure carries its source and date; see our methodology and fact-checking.
What Is a Synthetic Cutting Fluid?#
A synthetic cutting fluid is a concentrate of water-soluble or water-dispersible chemicals with no petroleum oil, while a soluble oil carries 30 to 85 % and a semi-synthetic 5 to 30 % severely refined base oil (NIOSH 98-116, 1998). The UKLA/Pera disposal guide sets mineral oil in the concentrate at 55 to 80 % for macro-emulsions, 10 to 55 % for semi-synthetics and none for synthetics.
How NIOSH and ISO 6743-7 classify synthetics#
ISO 6743-7:1986 places synthetics in two aqueous categories as quoted by secondary sources: MAG, a transparent solution with anticorrosion properties, and MAH, MAG plus friction-reducing or extreme pressure properties.
The codes come from the ISO 6743-7 classification of metalworking fluids, family M, whose edition 1 dates from November 1986, was last confirmed in 2020 and has a 2025 review result that is pending.
Secondary sources describe 17 categories, 8 neat (MHA to MHH) and 9 aqueous (MAA to MAI); the wording is quoted, not reproduced from the ISO text.
Solution synthetics and synthetic emulsions#
The OECD emission scenario document separates solution synthetics, with lubricity agents below 25 wt % of the concentrate, from synthetic emulsions, which carry 40 to 80 wt % synthetic lubricant. OECD ESD No. 28 (2011, published 2014) compiled the shares from industry and literature data, and the table compares them as wt % of the undiluted concentrate.
| Component | Solution synthetic (wt % of undiluted concentrate) | Synthetic emulsion (wt % of undiluted concentrate) | Source |
|---|---|---|---|
| Synthetic lubricant (lubricity agents) | below 25 | 40 to 80 | OECD ESD No. 28 (2011) |
| Emulsifiers | not listed | below 30 | OECD ESD No. 28 (2011) |
| EP additives | below 5 | below 45 | OECD ESD No. 28 (2011) |
| Corrosion inhibitors | below 10 | below 10 | OECD ESD No. 28 (2011) |
| Oiliness agents | not listed | below 10 | OECD ESD No. 28 (2011) |
| Fatty acid | below 5 | not listed | OECD ESD No. 28 (2011) |
| Coupling agents | not listed | below 5 | OECD ESD No. 28 (2011) |
| Biocides | below 2 | below 2 | OECD ESD No. 28 (2011) |
| Water | below 60 | below 5 (from neutralization) | OECD ESD No. 28 (2011) |
| Mineral (petroleum) oil | none | none | NIOSH 98-116 (1998); UKLA/Pera typology |
Shares are upper limits or ranges in percent by weight of the undiluted concentrate, not in-use concentrations.
A synthetic emulsion disperses synthetic lubricant, not the mineral oil of emulsifiable cutting fluids, with emulsifiers at below 30 wt % of the concentrate.
What Additives Are in a Synthetic Coolant Concentrate?#
A synthetic coolant concentrate combines up to ten additive functions in water, listed in Appendix 2, Table 4 of the OSHA best practices manual, from synthetic lubricants to triazole metal passivators. An additive works at a specific treat rate, from below 1 % to 35 % of the formulation (STLE TLT, September 2006).
The additive package, function by function#
Rates in the table below are percent of the concentrate unless stated as ppm in the use dilution, and the coupler and chelator rows have no verified treat rate. The table lists each function with its OSHA examples and its published rate.
| Function | Examples (OSHA Table 4) | Published rate with basis | Source | Page |
|---|---|---|---|---|
| Synthetic lubricant | EO-PO polymers (polyalkylene glycols), amides, organic esters | below 25 % of concentrate (lubricity agents, solution synthetic) | OSHA Table 4; OECD ESD | lubricity additives; polyalkylene glycols |
| Alkanolamine | MEA, TEA | MEA 1 to 5 %, TEA 1 to 5 % of concentrate (review of 209 safety data sheets, Gerulova et al. 2017); 6 to 39 % in 15 of 17 concentrates measured (Henriks-Eckerman et al. 2008) | Gerulova et al.; Henriks-Eckerman et al. | alkanolamines; monoethanolamine; triethanolamine |
| Plasticizer (coupler) | glycol ether | no verified treat rate | OSHA Table 4 | couplers |
| Biocide | triazine, oxazolidine | below 2 % of concentrate | OSHA Table 4; OECD ESD | biocides; triazine; oxazolidine |
| Defoamer | long-chain fatty alcohol | no in-product rate; 0.1 % of concentrate used as a screening level in one published test (STLE TLT, March 2017) | OSHA Table 4; STLE TLT | defoamers |
| Corrosion inhibitor | amine salts of carboxylic acids; amine salt of boric acid | below 10 % of concentrate; 15 wt % rust inhibitor package in one supplier's synthetic formulation, tested at 3 % dilution; boric acid 1 to 5 % of concentrate (Gerulova et al. 2017) | OSHA Table 4; OECD ESD; one supplier's formulation note | corrosion inhibitors; boric acid |
| Chelator | EDTA | no verified treat rate | OSHA Table 4 | chelating agents |
| Metal passivator | triazole | 50 to 1,000 ppm in the use dilution, commonly 250 to 500 ppm (one patent, US 8,722,592 B2) | OSHA Table 4; one patent | tolyltriazole; benzotriazole |
| EP additive (OECD only) | not in OSHA Table 4 | below 5 % of concentrate (solution synthetic) | OECD ESD | extreme pressure additives |
| Fatty acid (OECD only) | not in OSHA Table 4 | below 5 % of concentrate | OECD ESD | fatty acids |
| Odorant and dye | not specified | no rate; dyes show the product in a clear fluid and can stain skin and paint (STLE TLT, September 2006) | OSHA Table 4; STLE TLT | none |
Functions and examples follow OSHA Appendix 2, Table 4; every class is described under metalworking fluid additives.
The same review of 209 sheets gives AMP 1 to 10 % of concentrate, a range that does not apply to MEA or TEA.
Polyalkylene glycols and other synthetic lubricants#
Polyalkylene glycols, the EO-PO polymers OSHA lists as synthetic lubricants, dissolve in water at 50 to 75 wt % oxyethylene and are insoluble at 100 wt % oxypropylene. One supplier's statement puts their cloud point at 30 to 50 °C (86 to 122 °F) in fully synthetic high-lubricity formulations, above which the polymer comes out of solution.
Water-soluble polyalkylene glycols are not compatible with mineral oils and form a gelatinous mixture. Grades, CAS numbers and hazards are listed under polyalkylene glycols in metalworking fluids.
How Does a Synthetic Fluid Cool, Clean and Protect?#
A synthetic fluid cools and cleans better than oil-containing types and lubricates less: OSHA rates synthetics as the cleanest fluids with the best heat reduction and excellent rust control.
The table compares five properties by fluid type on one supplier's relative scale.
| Property | Straight oil | Emulsion | Synthetic | Source |
|---|---|---|---|---|
| Lubrication | ++ | + | - | one supplier's cutting fluids manual |
| Cooling | - | + | ++ | same manual |
| Cleaning | - | + | ++ | same manual |
| Corrosion protection | ++ | + | - | same manual; OSHA describes synthetics as giving excellent rust control |
| Foam potential | not rated | higher with more oil | lowest | trade-magazine article quoting one supplier |
Ratings are one supplier's relative scale (++ best, - weakest); OSHA rates synthetic rust control as excellent, so check the data sheet for the alloy in use.
The corrosion rows disagree: the supplier's table gives synthetics the lowest rating, and OSHA an excellent one. In the foam row, more oil means more foam potential, and synthetics foam least at the price of lower lubricity.
Where Do Synthetic Cutting Fluids Work Best?#
Synthetic cutting fluids work best where cooling and cleanliness outweigh lubrication, because one supplier's manual ties fast cutting to cooling and rates synthetics highest for cooling.
Grinding: heat removal and a clean wheel#
Grinding suits a synthetic because OSHA credits it with the best heat reduction and one supplier's manual rates it highest for cooling and cleaning, while the OECD default grinding concentration is 3 %. Both OECD defaults are in-use percent by volume of concentrate, not supplier recommendations.
A 2008 trade article gives a heat partition for a conventional wheel (M-50 at 20 mm³/mm/s): workpiece 72 %, fluid 10 %, chip 10 %, wheel 8 %, and with a CBN wheel in the cited test workpiece 45 % and fluid 5 %. Heat damage includes white layer, cracking and, in tungsten-carbide grinding, cobalt leaching. Wheel types, flow and nozzle design for synthetic and other grinding fluids are on the grinding page.
Cast iron and carbide: fines and cobalt leaching#
Cast iron and copper alloys form small particles that dirty machines, so one supplier's manual sets higher demands on fluid cleaning, which plays to the synthetic cleaning rating. A trade article gives about 48 h of in-process protection at proper concentration, notes that stacked cast iron may rust within hours, and advises fiber spacers, not cardboard.
Carbide adds a second issue. A published study found that cobalt leaching increases with time and with triethanolamine concentration, through a TEA-cobalt coordination compound, and synthetics carry MEA and TEA in the package OSHA lists.
One patent (US 8,722,592 B2) states that triazoles reduce cobalt leaching from metalworking tools. Cobalt limits and coolant choice for grinding cemented carbide are covered separately.
Where Do Synthetic Fluids Fall Short?#
Synthetic fluids fall short where an operation needs boundary or extreme-pressure lubrication, because one supplier's manual rates synthetics lowest for lubrication. The same manual reserves neat oils for grinding hard metals, deep-hole drilling, threading difficult materials, gear cutting and rotary table machines.
The four trade-offs are listed below.
- Lubricity: rated lowest of the three fluid families in one supplier's manual
- Corrosion protection: rated lowest in that manual, while OSHA describes excellent rust control
- Concentration reading: refractometer factors up to about 3.4 for some synthetics in one supplier's blog
- Waste treatment: chemical treatment and ultrafiltration remove only 0 to 5 % of COD (UKLA/Pera)
The OECD composition lists EP additives below 5 wt % of a solution synthetic concentrate against below 45 wt % of a synthetic emulsion concentrate.
Threading difficult materials calls for tapping and threading fluids with EP chemistry.
How Are Synthetic Coolants Mixed and Run?#
Synthetic coolants run at 2.4 to 9.1 % concentrate by volume in water under the NIOSH dilution range, mixed by adding concentrate to water and checked with the product's own refractometer factor. The mixing order is concentrate into water, never water into concentrate (UKLA/HSE guide v2.5, February 2025).
Concentration and dilution ratio#
The data sheet sets the concentration; published ranges run from about 2.4 to 9.1 % (NIOSH) to OECD defaults of 3 % for grinding and 5 % for machining. The table compares the sources as percent by volume of concentrate in the diluted fluid, with the ratio each percentage equals (C % = 1 part in 100/C total parts).
| Source | In-use concentration (% by volume of concentrate in the diluted fluid) | Ratio |
|---|---|---|
| NIOSH 98-116 (1998), synthetics | about 2.4 to 9.1 % | 1 part concentrate to 10 to 40 parts water |
| OECD ESD default, grinding (all fluid types) | 3 % | 1:33.3 total parts, 32.3:1 water to concentrate |
| OECD ESD default, machining (all fluid types) | 5 % | 1:20 total parts, 19:1 water to concentrate |
| STLE webinar (April 2022), water-mix fluids in general | 5 to 10 % | 90 to 95 % water |
Any ratio, new charge or top-up is computed in the coolant concentration calculator.
Checking concentration on a synthetic#
A refractometer reading becomes concentration only after multiplication by the product's factor, which one supplier's blog puts at about 0.9 for emulsions and up to about 3.4 for some synthetics. The instrument is zeroed on the same water used for mixing, at about 20 °C (68 °F) (UKLA/HSE guide v2.5, February 2025).
Titration is the cross-check: one supplier's kit method titrates alkalinity with 1.0 N sulfuric acid and multiplies the drops by the product's titration factor, and names it best for synthetics and semi-synthetics. Zeroing, reading and correcting a sump are covered under coolant concentration.
What Changes in Sump Management With a Synthetic?#
A synthetic sump is checked like any water-mix sump, concentration and pH at least weekly under UK HSE guidance MW5 (03/21), but its additive package, foam behavior and waste treatment differ from oil-containing fluids. OSHA credits synthetics with longer sump life.
Bacteria, foam and pH in a synthetic sump#
Bacteria grow in a synthetic sump too: an MEA/TEA synthetic with 1,000 ppm triazine failed, above 10⁵ CFU/mL for two consecutive weeks, after 4 weeks (STLE TLT, August 2013). Partial amine replacement extended control to 12 weeks or more. HSE dip-slide bands read below 10⁴ CFU/mL as good control and 10⁶ or more as poor, with dip slides weekly.
One supplier's bulletin reports that fresh clean coolants foam most and aged contaminated fluids less, and UK guidance gives mix water of 80 to 200 ppm as CaCO3, formulation-dependent, with soft water foaming. Dip slides, odor and biocide choices are on bacteria, fungi and odor in coolant.
Disposal: why synthetic solutions resist ultrafiltration#
Synthetic solutions pass through ultrafiltration and chemical treatment with only 0 to 5 % COD reduction, so evaporation (90 to 100 %) and biological treatment (60 to 80 %) carry the load. The table lists percent of chemical oxygen demand removed by route.
| Route | Macro-emulsion | Semi-synthetic | Synthetic |
|---|---|---|---|
| Chemical treatment | 90 to 100 % | 90 to 100 % | 0 to 5 % |
| Evaporation | 90 to 100 % | 90 to 100 % | 90 to 100 % |
| Ultrafiltration | 90 to 100 % | 80 to 90 % | 0 to 5 % |
| Biological treatment | 65 to 100 % | 70 to 100 % | 60 to 80 % |
Values are percent of chemical oxygen demand removed, from the UKLA/Pera disposal guide, Table 4; rules and waste codes are on disposal of waste metalworking fluids.
The same guide names ultrafiltration as suited to macro-emulsions and semi-synthetics and unsuitable for synthetics, with a membrane life of 1.5 to 3 years. The EU List of Waste names 12 01 08* and 12 01 09* for machining emulsions and solutions and 12 01 10* for synthetic machining oils (Decision 2000/532/EC as amended by Decision 2014/955/EU, from 2015-06-01).
Health and Regulatory Notes for Synthetic Cutting Fluids#
NIOSH found the strongest asthma evidence for synthetic metalworking fluid aerosol, with one study reporting an adjusted risk about three times the reference (NIOSH 98-116, 1998). The NIOSH survey of 79 shops (Piacitelli 2001, via the OECD document) measured extractable mist as geometric mean and 90th percentile: synthetic 0.24 and 1.10 mg/m³, straight oil 0.39 and 1.42 mg/m³.
OSHA has no metalworking-fluid-specific PEL. For a fluid without mineral oil the particulates-not-otherwise-regulated values apply: 15 mg/m³ total and 5 mg/m³ respirable, 8-h TWA (29 CFR 1910.1000 Table Z-1). NIOSH, ACGIH, HSE and EU values are compared under metalworking fluid exposure limits.
Five groups of package components carry regulatory flags, listed in the table with their CLP and BPR status. Symptoms go to an occupational physician.
| Component | Role in the package | EU CLP classification | Other status |
|---|---|---|---|
| Boric acid (CAS 10043-35-3) | amine borate corrosion inhibitor | Harmonized Annex VI index 005-007-00-2: Repr. 1B H360FD (ATP17); Note 11 sums boron compounds at 0.3 % or more in the mixture, in application since 2025-02-01 (ATP20) | EU BPR: approved only for PT8, expiry postponed to 2029-02-28 (Implementing Decision (EU) 2026/1128); no PT6, PT11 or PT13 approval |
| Amine borates, MEA-boric acid reaction products (boric acid) | corrosion inhibitor | notified Repr. 1B H360FD by REACH registrants, not harmonized | none captured |
| HHT triazine (CAS 4719-04-4), see triazine biocides | biocide | Harmonized Annex VI index 613-114-00-6 (original Annex VI list): Acute Tox. 4 H302, Skin Sens. 1 H317 | EU BPR: under review for PT13, no decision as of 2026-10-27 |
| Oxazolidine biocides, see formaldehyde-releasing biocides | biocide, formaldehyde releaser | see the linked page and the product SDS | none stated here |
| Benzotriazole (CAS 95-14-7) and tolyltriazole (CAS 29385-43-1), see tolyltriazole | metal passivator | Harmonized Annex VI 613-350-00-X and 613-351-00-5: Aquatic Chronic 2 H411, added by ATP22, applicable from 2026-05-01 | not biocidal actives |
Synthetic Cutting Fluid Grades and Sourcing#
A synthetic cutting fluid is bought by specification, not by brand: the request states the ISO 6743-7 category claimed, operations, materials, concentration range, refractometer factor and any boron, biocide or amine limits. The six specification items are listed below.
- ISO 6743-7 category claimed: MAG or MAH, as quoted
- Operations and workpiece materials, including carbide grinding
- Concentration range per operation and the refractometer and titration factors from the data sheet
- Boron content, given the Note 11 sum rule for boron compounds
- Biocide actives and their EU BPR PT13 status
- Mix-water hardness range from the data sheet; UK guidance gives 80 to 200 ppm as CaCO3, depending on the formulation
Products are not one-to-one replacements: one supplier's bulletin advises comparing safety data sheets and running laboratory and field tests before a switch. How to compare quotes is explained under metalworking fluid suppliers. Our worked example, not a 2026 price: a concentrate at USD 10/L used at 5 % costs USD 0.50/L (about USD 1.89/US gal) of working fluid, and at 3 % USD 0.30/L (about USD 1.14/US gal).
How Do Synthetics Relate to the Other Fluid Types?#
Synthetics are one of the four fluid categories in the OSHA manual, next to straight oils, soluble oils and semi-synthetics, and the only water-mix category with no petroleum oil in the concentrate (NIOSH 98-116). Straight oils are not designed to be diluted with water, while the other three are water-mix fluids (NIOSH 98-116, 1998).
Is a Synthetic the Same as a Semi-Synthetic?#
No: a semi-synthetic concentrate holds 5 to 30 % severely refined base oil (NIOSH 98-116, 1998), a synthetic none. Both are diluted with water. Oil content and microemulsions are explained under semi-synthetic metalworking fluids.
Can a Synthetic Run in a Machine Built for Straight Oil?#
Not always: OSHA ties straight oils to older machines built for them, and one supplier's bulletin limits gear and vane pumps to straight oils. OSHA adds that straight oils may require fire protection.
Synthetic Cutting Fluid FAQ#
These five answers cover the reader questions on synthetic coolant.
What is synthetic coolant?#
Synthetic coolant is a water-mix fluid with no petroleum oil (NIOSH 98-116, 1998), used at 1 part concentrate to 10 to 40 parts water.
Is synthetic cutting oil the same as synthetic coolant?#
No: an oil made from synthetic base stocks and used without water is a straight oil under the NIOSH definition, while synthetic coolant is water-mix.
Neat synthetic cutting oils belong with the straight oils.
Can synthetic coolant be used on cast iron?#
Yes, with the cleaning demand from cast iron fines in mind and the rust timing of stacked parts. One trade article gives about 48 h of protection at proper concentration and advises fiber spacers, not cardboard.
Is synthetic coolant mist harmful?#
Synthetic coolant mist carries the strongest NIOSH evidence for asthma among the fluid types, so exposure is kept low and symptoms go to an occupational physician.
What is synthetic coolant oil?#
"Synthetic coolant oil" is a search name for a water-mix synthetic with no petroleum oil, which can still carry 40 to 80 wt % synthetic lubricant in a synthetic emulsion concentrate.
Oil content, life and cost side by side are under soluble oil vs semi-synthetic vs synthetic coolant.