A semi-synthetic metalworking fluid is a water-miscible concentrate that contains 5 to 30 % severely refined base oil and 30 to 50 % water, which shops dilute with 10 to 40 parts water per part of concentrate into a translucent cutting fluid (NIOSH 98-116, 1998). The concentrate itself is transparent, and its higher share of emulsifiers holds the small oil share in the water.
OSHA groups metalworking fluids into straight oils, soluble oils, semi-synthetics and synthetics, and the semi-synthetic is the type with good lubrication, heat reduction and rust control, a longer sump life than soluble oils and a cleaner machine. The mixed fluid holds 5 % or less oil (OSHA Best Practices Manual), and sources differ on the oil share: the OECD Emission Scenario Document No. 28 (2011) gives up to 40 % mineral oil.
The NIOSH recommended exposure limit of 0.4 mg/m³ thoracic particulate mass applies to the mist, and the product's own refractometer factor turns a Brix reading into concentration.
- Oil in the concentrate: 5 to 30 % severely refined base oil, 30 to 50 % water (NIOSH 98-116, 1998)
- Dilution: 1 part concentrate to 10 to 40 parts water, about 2.4 to 9.1 % by volume (NIOSH)
- Oil in the mixed fluid: 5 % or less (OSHA)
- ISO 6743-7 classes as quoted: MAE and MAF, microemulsions
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 Semi-Synthetic Metalworking Fluid?#
A semi-synthetic metalworking fluid is a concentrate that combines a small share of refined oil with a large share of emulsifiers and water, and its dilution is transparent to milky rather than opaque white.
Calculated from the NIOSH ranges, 5 % oil at 1:40 (1/41) leaves 0.12 % oil in the mixed fluid, and 30 % oil at 1:10 (1/11) leaves 2.7 %, which is consistent with the OSHA figure of 5 % or less.
ISO 6743-7 Classes MAE and MAF#
ISO 6743-7:1986, the classification of metalworking fluids (Family M), places semi-synthetics in the microemulsion classes MAE and MAF, as quoted by secondary sources of the standard. MAE carries anticorrosion properties, and MAF adds friction-reducing and/or extreme-pressure properties.
The standard lists 17 categories as quoted, 8 neat (MHA to MHH) and 9 aqueous (MAA to MAI), and its status is last confirmed 2020 with the 2025 review result pending. The full category list of ISO 6743-7 is set out on its own page.
Why a Semi-Synthetic Dilution Looks Translucent#
A semi-synthetic dilution looks transparent to milky because the concentrate holds a higher share of emulsifiers than a soluble oil and only 5 to 30 % oil, which the emulsifiers disperse in the water.
What Is in a Semi-Synthetic Concentrate?#
A semi-synthetic concentrate holds up to eight ingredient groups, from up to 40 % mineral oil to up to 2 % biocides, in percent by weight of the undiluted concentrate (OECD Emission Scenario Document No. 28, 2011). The table below lists each group with its upper limit and role.
| Ingredient group | Upper limit (wt % of undiluted concentrate) | Role | Additive class |
|---|---|---|---|
| Mineral oil (part may be synthetic lubricant) | up to 40 | carrier and lubricity | base oils for metalworking fluids |
| Emulsifiers | up to 35 | disperse the oil as a microemulsion | emulsifiers and couplers |
| Oiliness agents | up to 20 | boundary lubricity | lubricity and boundary additives |
| EP additives | up to 10 | load carrying under severe contact | extreme pressure (EP) additives |
| Coupling agents | up to 8 | stabilize the concentrate | covered on the emulsifiers page |
| Corrosion inhibitors | up to 5 | ferrous and yellow-metal protection | corrosion inhibitors in metalworking fluids |
| Biocides | up to 2 | preserve the concentrate and dilution | biocides and fungicides for metalworking fluids |
| Water | up to 60 | carrier | none |
Upper limits in percent by weight of the undiluted concentrate, as compiled in the OECD Emission Scenario Document No. 28 on metalworking fluids; a product never contains every maximum at once.
Oil Content of Semi-Synthetic Concentrates by Source#
Oil content of a semi-synthetic concentrate ranges from 5 to 30 % in the NIOSH and OSHA definitions to 10 to 55 % in the UK typology, because each source defines the type differently. The table below compares six sources by oil share, basis and water share.
| Source | Oil in the concentrate | Basis | Water in the concentrate |
|---|---|---|---|
| NIOSH 98-116 (1998) | 5 to 30 % severely refined base oil | % of concentrate | 30 to 50 % |
| OSHA best practices manual | 5 to 30 %; 5 % or less oil in the mixed fluid | % by volume in the concentrate | not stated |
| CCOHS OSH Answers | 5 to 30 % petroleum oil | % of concentrate | 30 to 50 % |
| OECD ESD No. 28 (2011, published 2014) | up to 40 % mineral oil, part may be synthetic lubricant | wt % of undiluted concentrate | up to 60 % |
| UKLA/Pera disposal guide (UK typology) | 10 to 55 % mineral oil | % in concentrate | not stated |
| STLE webinar (April 2022) | 5 to 50 % hydrocarbon oil | % in concentrate | not stated |
Two oil shares compare only when basis and source match. This page uses the NIOSH range of 5 to 30 % as the definition.
Additive Package and Treat Rates#
A semi-synthetic additive package follows OSHA Appendix 2 Table 3, and the alkalinity and corrosion components run at 1 to 5 % in the concentrate for MEA, TEA and boric acid and 1 to 10 % for AMP (Gerulova et al., 2017). Of the metalworking fluid additives that OSHA lists for the type, the table below shows each function with its substance, treat rate and hazard flag.
| Function | Substance | Treat rate and basis | Source | Regulatory flag | Page |
|---|---|---|---|---|---|
| Oiliness agent | polyol ester | not sourced | OSHA Appendix 2 | none recorded | trimethylolpropane esters |
| Alkalinity, neutralization | monoethanolamine (MEA) | 1 to 5 % in concentrate | 209-SDS review, Gerulova et al. (2017) | Annex VI 603-030-00-8, original Annex VI list: H302, H312, H314, H332 | monoethanolamine |
| Alkalinity, neutralization | triethanolamine (TEA) | 1 to 5 % in concentrate | same review | no Annex VI entry; see the product SDS | triethanolamine |
| Alkalinity | aminomethylpropanol (AMP) | 1 to 10 % in concentrate | same review | see the product SDS | alkanolamines in metalworking fluids |
| Alkalinity | monoisopropanolamine (MIPA); dicyclohexylamine (DCHA) | 1 to 5 % in concentrate each | same review | see the product SDS | covered on the alkanolamines page |
| Alkalinity (historical) | diethanolamine (DEA) | 5 to 10 % in concentrate, historical | same review | Annex VI 603-071-00-1, original Annex VI list: H302, H315, H318, H373 | none |
| Corrosion inhibitor | boric acid and amine borates | 1 to 5 % boric acid in concentrate | same review | Annex VI 005-007-00-2: Repr. 1B H360FD; Note 11 at 0.3 % (ATP20) | boric acid and boron compounds |
| Corrosion inhibitor | water-soluble rust inhibitor with sebacic acid | 6 wt % of concentrate, about 1.8 % sebacic acid | one supplier's formulation guide | see the product SDS | none |
| Metal passivator | triazole (tolyltriazole, benzotriazole) | not sourced | OSHA Appendix 2 | see the product SDS | tolyltriazole |
| Chelator | EDTA | not sourced | OSHA Appendix 2 | Annex VI 607-429-00-8 (ATP01): H319; tetrasodium EDTA 607-428-00-2 (ATP01): H302, H318 | chelating agents |
| Biocide | triazine (HHT) | not sourced | OSHA Appendix 2 | Annex VI 613-114-00-6, original Annex VI list: Acute Tox. 4 H302, Skin Sens. 1 H317; see formaldehyde-releasing biocides and biocide approval for metalworking fluids | triazine biocides |
| Biocide | oxazolidine | not sourced | OSHA Appendix 2 | see the product SDS | oxazolidine biocides |
| Anti-mist | polyethylene oxide | about 250 ppm in the use dilution | one supplier's data | see the product SDS | anti-mist additives |
| Emulsifier, coupler | fatty amides, fatty acid salts, nonionics; fatty alcohol | not sourced | OSHA Appendix 2 | see the product SDS | none |
Treat rates are percent of the concentrate unless marked ppm in the use dilution; "not sourced" means no verified value for semi-synthetics.
Each class in the table is explained under metalworking fluid additives.
How Does a Semi-Synthetic Fluid Cool, Lubricate and Protect?#
A semi-synthetic fluid cools and lubricates the cut, protects part and machine tool against corrosion and removes chips, and OSHA rates the type for good lubrication, heat reduction and rust control.
OSHA names a polyol ester as the oiliness agent of the type. A water-soluble sulfurized EP additive needs severity to work: one trade article reports that it may not activate in light duty at use dilution.
Which Operations and Materials Suit Semi-Synthetic Fluids?#
A 209-SDS review (Gerulova et al., 2017) notes general machining and grinding as the operations of semi-synthetic fluids, and OSHA assigns slow, high-contact cutting to straight oils.
Operations: Machining, Grinding and Forming#
In machining and grinding a semi-synthetic runs at the OECD default dilutions of 5 % for machining and 3 % for grinding, against 10 % for pressure-impact deformation (OECD Emission Scenario Document No. 28, 2011). ASTM D8288-25 compares straight, emulsifiable, semi-synthetic and synthetic fluids by tapping torque, and ASTM D5619 was withdrawn in 2016 with no formal replacement.
One trade article reports water-soluble sulfurized EP additives effective in severe semi-synthetic drawing and stamping. Wheel, jet pressure and flow are covered under grinding fluids.
Materials: Aluminum, Copper Alloys, Cast Iron and Carbide#
On cast iron and steel a semi-synthetic depends on its corrosion inhibitor package and on the cleaning of fines, because cast iron and copper alloys form small particles that dirty machines (one fluid maker's manual). One supplier's formulation guide gives a semi-synthetic 6 wt % of a water-soluble rust inhibitor in the concentrate, of which 30 % is sebacic acid, and reports that the fluid passed a cast-iron chip test after four microbial challenges when neutralized with MIPA.
STLE (September 2006) lists mercaptobenzothiazole, tolyltriazole and benzotriazole as agents that prevent staining of copper and brass. Water-based fluids also dissolve the cobalt binder of carbide through chelating groups and amines such as triethanolamine, and dissolved Co(II) is a skin and respiratory sensitizer.
On aluminum, MEA and MIPA are avoided as the main base because of staining, and TEA is kinder to aluminum and copper than MEA (Gerulova et al., 2017). Alloy-safe chemistry is set out in cutting fluid for aluminum.
Where Semi-Synthetics Fall Short#
Semi-synthetics fall short in the processes OSHA assigns to straight oils: slow cutting speeds, high metal-to-metal contact and older machines built for straight oil.
One tapping-fluid maker's blog (June 2026) prefers neat oils with EP additives for threading, tapping, broaching and deep drilling of stainless steel. Rust on iron alloys is possible from as low as 120 ppm chloride in the mix water (STLE webinar, 9 November 2022), a figure that is not a regulatory limit.
How Are Semi-Synthetic Fluids Diluted and Mixed?#
Semi-synthetic concentrates are diluted 1 part to 10 to 40 parts water, about 2.4 to 9.1 % by volume, and always mixed by adding the concentrate to the water, never the water to the concentrate. Water poured into concentrate can form an inverse emulsion (UKLA/HSE Good Practice Guide v2.5, February 2025).
Dilution Ranges for Semi-Synthetic Concentrates#
The product data sheet sets the dilution, and the NIOSH, STLE and OECD values frame it between 2.4 and 10 % concentrate by volume of the use dilution. The table below compares four sources.
| Source | Range | Percent by volume of concentrate in the use dilution |
|---|---|---|
| NIOSH 98-116 (1998), semi-synthetics | 1 part concentrate to 10 to 40 parts water | about 2.4 to 9.1 % (calculated) |
| STLE webinar (April 2022), water-mix fluids in general | 5 to 10 % concentrate in 90 to 95 % water | 5 to 10 % |
| OECD ESD No. 28 default, machining | not applicable | 5 % |
| OECD ESD No. 28 default, grinding | not applicable | 3 % |
Mix-water hardness of 80 to 200 ppm as CaCO3 is formulation-dependent, and soft water foams (UKLA/HSE Good Practice Guide v2.5, February 2025).
Measuring Concentration in a Semi-Synthetic Sump#
Concentration in a semi-synthetic sump equals the Brix reading multiplied by the product's refractometer factor, zeroed on the mix water (UKLA/HSE Good Practice Guide v2.5, February 2025). One supplier's 2025 product data lists a factor of 1.5 for two semi-synthetic products and 1.3 for a third, so the data-sheet factor of the product in the sump is the one to use.
Alkalinity titration with 1.0 N sulfuric acid, drops multiplied by the product's titration factor, suits synthetics and semi-synthetics (one supplier's technical bulletin). The acid split takes 6 to 18 h and serves soluble oils and high-oil semi-synthetics only. Check frequency and correction steps are on coolant concentration.
How Do You Maintain a Semi-Synthetic Fluid?#
A semi-synthetic fluid stays in service when concentration, microbes, foam and tramp oil are checked and corrected, and the UKLA/Pera disposal guide gives a service life of 9 to 12 months for water-mix fluids.
Sump Life, Microbes and Odor#
Semi-synthetics run a longer sump life than soluble oils (OSHA), but glycol-containing semi-synthetics are more prone to fungal than bacterial attack (Ctgb, Netherlands, PT13 efficacy guidance).
HSE rates below 10^4 CFU/mL as good and 10^6 or more as poor, calling for immediate action. Dip slides, odor causes and biocide use are under bacteria, fungi and odor in coolant.
Foam, Residue and Tramp Oil#
Semi-synthetics foam less than soluble oils and more than synthetics, according to one fluid maker quoted in a trade article, and foam rises in soft, deionized or reverse-osmosis water. High concentration adds dermatitis risk, foaming and heavy residues (OSHA). Tramp oil stays below 2 % by volume, checked at least weekly (HSE MW5, 03/21).
Disposal and Treatment Routes#
Spent semi-synthetic fluid carries EU waste code 12 01 09* when halogen-free and 12 01 08* with halogens (Decision 2000/532/EC as amended by Decision 2014/955/EU, applicable from 2015-06-01). The table below compares chemical oxygen demand (COD) reduction by treatment route for three fluid types.
| Route | Macro-emulsion (soluble oil) | 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 | 65 to 100 % | 70 to 100 % | 60 to 80 % |
COD reduction by treatment route (UKLA/Pera disposal guide, Table 4).
Ultrafiltration suits macro-emulsions and semi-synthetics, not synthetics, and its membranes last 1.5 to 3 years (UKLA/Pera). Waste codes and permits are on disposal of waste metalworking fluids.
What Health and Regulatory Rules Apply to Semi-Synthetic Fluids?#
No exposure limit is written for semi-synthetic fluids as a type: the NIOSH limit for metalworking fluid aerosol and the OSHA limits for oil mist apply, and CLP rules reach boric acid and formaldehyde releasers in the concentrate.
Mist Exposure and Limits#
The NIOSH recommended exposure limit for metalworking fluid aerosol is 0.4 mg/m³ thoracic particulate mass (about 0.5 mg/m³ total), as a TWA for up to 10 h a day in a 40-h week (NIOSH Criteria Document 98-102, January 1998). OSHA has no metalworking-fluid PEL and applies 5 mg/m³ for mineral oil mist or 15 mg/m³ for total particulates not otherwise regulated, both as 8-h TWA (29 CFR 1910.1000 Table Z-1).
In a NIOSH survey of 79 shops, the geometric mean extractable mist was 0.20 mg/m³ for semi-synthetics, with a 90th percentile of 0.88 mg/m³, against 0.19 and 0.87 for soluble oils, 0.24 and 1.10 for synthetics and 0.39 and 1.42 for straight oils (Piacitelli, 2001 and 2006, as cited by OECD). Extractable mist is not the thoracic fraction of the limit. Limits by jurisdiction and fraction are on metalworking fluid exposure limits.
Skin and Lung Effects#
Exposure to water-mix fluids such as semi-synthetics can cause dermatitis, occupational asthma, hypersensitivity pneumonitis, bronchitis and upper respiratory irritation (HSE). NIOSH (1998, as cited by OSHA) puts 14 to 67 % of exposed workers at risk of dermatitis.
A 2001 outbreak in Ohio, reported in the CDC's MMWR, involved a semi-synthetic fluid with microbial counts of up to 10^6 CFU/mL. Of 400 workers, 107 were placed on respiratory restriction, and 2 of 5 personal aerosol samples (below 0.1 to 0.9 mg/m³) exceeded the NIOSH limit.
An occupational physician is the referral for symptoms. Symptoms and case reports are under respiratory disease from metalworking fluids.
Additive Rules That Reach the Concentrate#
Boric acid carries the harmonized CLP classification Repr. 1B H360FD (Annex VI index 005-007-00-2), and under Note 11, in application since 2025-02-01 (ATP20), a mixture is classified when its reproductive-toxic boron compounds sum to 0.3 % or more.
HHT, the triazine biocide, is classified Acute Tox. 4 H302 and Skin Sens. 1 H317 (Annex VI index 613-114-00-6, original Annex VI list), not Carc. 1B. EU biocidal product type 13 covers working or cutting fluid preservatives (Regulation (EU) 528/2012, Annex V). A UK survey of in-use water-mix fluids found a median of 0.4 µg/mL of NDELA (N-nitrosodiethanolamine).
How to Source Semi-Synthetic Grades#
Source a semi-synthetic grade by specification, not by brand: state the operations and metals, the system volume, the mix water, the chemistries to exclude and the ISO 6743-7 class, and compare quotes by cost in use per liter of working fluid. Our checklist of seven specification items is listed below.
- Operations and severity: cuts, tooling, finish and tolerance targets
- Metals: alloys and their share of work, staining limits for aluminum and yellow metals
- System: individual sumps or central system, volume in L (US gal), filtration rating, high-pressure coolant in bar (psi)
- Mix water: hardness as CaCO3, chloride, sulfate, conductivity
- Excluded chemistries: chlorine, boron, secondary amines, formaldehyde releasers, nitrite
- Data sheet items: ISO 6743-7 class, refractometer factor, dilution range
- Commercials: annual volume, drum 208 L (55 US gal) or IBC 1,000 L (264 US gal), price per L of concentrate and cost in use per L of working fluid
How quotes compare is set out under metalworking fluid suppliers. Cost in use is the concentrate price multiplied by the concentration: in our worked example (October 2026), a concentrate at USD 10/L used at 5 % costs USD 0.50 per liter of working fluid (about USD 1.89 per US gal).
No ISO or ASTM document defines a metalworking-fluid request for quotation, so the specification carries the scope.
Where Semi-Synthetics Sit Among Soluble Oils, Synthetics and Straight Oils#
Semi-synthetics sit between soluble oils, with 30 to 85 % severely refined base oil in the concentrate, and synthetics, with no petroleum oil, in the NIOSH grouping of water-miscible metalworking fluids (NIOSH 98-116, 1998). Straight oils are not diluted with water.
The side-by-side verdict is in soluble oil vs semi-synthetic vs synthetic coolant. The five sibling pages are listed below.
- Soluble oils: 30 to 85 % severely refined base oil in the concentrate, milky emulsion
- Synthetic cutting fluids: no petroleum oil, water-soluble or water-dispersible
- Cutting oils (straight and neat oils): not diluted with water
- Vegetable and ester-based cutting fluids: ester and vegetable bases
- How to choose a metalworking fluid: choice by operation, material, machine and water
Semi-Synthetic Coolant FAQ#
These five answers cover the definition, oil content, mix ratio, limits and microemulsion class of semi-synthetic coolant.
What is a semi-synthetic coolant?#
A semi-synthetic coolant is a concentrate with 5 to 30 % severely refined base oil and 30 to 50 % water, a higher share of emulsifiers and a transparent look (NIOSH 98-116, 1998).
How much oil is in a semi-synthetic cutting fluid?#
A semi-synthetic concentrate holds 5 to 30 % oil by the NIOSH and OSHA definitions, and the mixed fluid holds 5 % or less (OSHA).
What is the mix ratio for a semi-synthetic coolant?#
The product data sheet sets the mix ratio, and NIOSH gives 1 part concentrate to 10 to 40 parts water, about 2.4 to 9.1 % by volume.
What are the disadvantages of semi-synthetic cutting fluid?#
Semi-synthetic cutting fluid falls short in slow, high-contact cuts (OSHA), foams more than synthetics (one fluid maker quoted in a trade article) and, in glycol-containing grades, is more prone to fungal attack (Ctgb).
Is a semi-synthetic coolant a microemulsion?#
Yes: the ISO 6743-7 classes MAE and MAF, as quoted by secondary sources, are microemulsions, and OSHA describes the appearance as transparent to milky.