Metalworking fluid additives are components blended into a base oil or water-miscible concentrate, most at levels from below 1 % to about 35 % of the formulation (STLE, 2006); this reference groups them into 11 classes in 4 functional groups. Which additive does which job, and at what level?
Additives exist because cutting fluids do several jobs at once: the cutting fluids (metalworking fluids) they go into cool, lubricate and protect the cut.
EP additives reach below 45 wt % of a synthetic-emulsion concentrate (OECD ESD No. 28, 2011), and formaldehyde releasers, boric acid and chlorinated paraffins carry dated EU rules.
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 Are Metalworking Fluid Additives?#
A metalworking fluid additive is a chemical component or blend added at a specific treat rate to provide one or more functions, and it dissolves in mineral oil, water or both (Gresham, STLE TLT, September 2006). Published class lists differ: STLE named 12 classes in 2006 and 13 in 2017, and the German DGUV Regel 109-003 lists 11 additive groups, including substances added during use.
The three lists diverge on dyes, wetting agents and solubilizers, so this independent reference on metalworking fluids and their additives groups them into 11 classes and names base oils as the carrier fluid.
What is cutting fluid made of?#
Cutting fluid is made of a carrier, base oil or a water-miscible concentrate, plus additives that lubricate, protect and control the fluid (OSHA best-practices manual, Appendix 2). The two forms are listed below.
- Straight oil: base oil as the carrier fluid, 60 to 90 wt % of the concentrate (OECD ESD No. 28, 2011), plus oiliness, EP, antioxidant, passivator, anti-mist and other additives.
- Water-miscible fluid: a concentrate of base oil or synthetic lubricant, emulsifiers and additives, diluted in water before use.
CNC coolant is the diluted form: water makes up 85 to 95 % of an in-use water-miscible fluid (a 2025 review in the journal Life), and the OECD default in-use dilution is 5 % concentrate in water for machining and 3 % for grinding.
How much of a concentrate is additive?#
Most single additives sit at below 1 % to about 35 % of the formulation (STLE 2006), but that is not a ceiling. Antioxidants, dyes and odorants stay below 1 wt % each of a straight-oil concentrate, while OECD lists EP additives in a synthetic-emulsion concentrate at below 45 wt %. The share of one class depends on the fluid type: EP additives fall to below 5 wt % of a solution-synthetic concentrate.
The 11 Additive Classes in 4 Functional Groups#
This reference groups metalworking fluid additives into 11 classes in 4 functional groups: the base fluid and emulsion system, film strength at the cut, metal and fluid protection, and sump control. The diagram above and the table below follow the same order.
1. Base fluid and emulsion system (2 classes)#
The base fluid and emulsion system covers 2 classes: base oils, the carrier fluid the additives are carried in, and emulsifiers with couplers, which hold oil in water.
Base oils, such as naphthenic and paraffinic mineral oils, esters and glycols, make up 60 to 90 wt % of a straight-oil concentrate, 40 to 85 wt % of a soluble oil and below 40 wt % of a semi-synthetic (OECD ESD No. 28, 2011).
Emulsifiers, such as petroleum sulfonates, fatty acid salts and nonionic surfactants, stay below 30 wt % of a soluble-oil and below 35 wt % of a semi-synthetic concentrate. Couplers, such as propylene glycol and glycol ethers, stabilize the emulsion at below 5 and below 8 wt % of the same two concentrates.
2. Film strength at the cut (2 classes)#
Film strength at the cut comes from 2 classes: lubricity (boundary) additives that adsorb on the metal and EP additives that react with it.
Lubricity additives, such as polyol esters, lard oil, canola oil and fatty acids, adsorb with a polar head on the metal and work from ambient temperature to 200 °C (392 °F) (Canter, STLE TLT, 2007). OECD lists them at below 40 wt % of a straight oil.
EP additives, such as chlorinated paraffins, sulfurized fats and olefins, phosphorus esters and overbased sulfonates, form a metal salt layer. Chlorinated EP activates at about 180 to 420 °C (356 to 788 °F), sulfur EP works up to about 1,000 °C (1,832 °F), and phosphorus sits between the two (Canter, 2007). As a cutting oil additive, EP chemistry stays below 15 wt % of a straight-oil concentrate.
3. Metal and fluid protection (3 classes)#
Metal and fluid protection covers 3 classes: corrosion inhibitors, alkanolamines and antioxidants.
Anti-corrosion additives protect ferrous parts from rust and copper alloys from staining: sulfonates and fatty acid soaps protect steel, while metal deactivators, such as tolyltriazole, benzotriazole and mercaptobenzothiazole, prevent copper and brass staining and reduce galvanic corrosion; one patent (US8722592B2) doses triazoles at 50 to 1,000 ppm in the diluted fluid.
Alkanolamines, such as monoethanolamine (MEA), triethanolamine (TEA), aminomethylpropanol (AMP) and monoisopropanolamine (MIPA), set the pH and supply reserve alkalinity. A review of 209 safety data sheets (Gerulova et al., 2017) found MEA and TEA at 1 to 5 % and AMP at 1 to 10 % of the concentrate, and in-use pH sits at 8.0 to 9.5 (Lotierzo et al., 2016).
Antioxidants, such as alkylated phenols and aminic types, scavenge the radicals behind thermal breakdown at below 1 wt % of a straight-oil concentrate.
4. Sump control (4 classes)#
Sump control covers 4 classes that keep a diluted coolant usable in the machine: biocides, defoamers, chelating agents and mist suppressants.
Biocides, such as triazine, oxazolidine, benzisothiazolinone (BIT) and iodopropynyl butylcarbamate (IPBC), control bacteria and fungi at below 2 wt % of a water-miscible concentrate (OECD ESD No. 28, 2011).
Defoamers, such as silicone (polydimethylsiloxane), polyethers and long-chain fatty alcohols, break foam in the CNC sump as a coolant additive at below 1 wt % of a water-based concentrate.
Chelating agents, such as EDTA, MGDA and GLDA, bind hardness ions from the mix water; STLE (2006) calls them water conditioners.
Mist suppressants, such as polyisobutylene in oil and polyethylene oxide in water-based fluids, enlarge droplets so that mist settles. Polyisobutylene at 20 to 100 ppm in a straight oil cuts mist by 70 to 90 % in one plant study (SAE 950245, 1995).
Complete Table of Metalworking Fluid Additive Classes and Treat Rates#
The table lists all 11 classes of metalworking fluid additives with their function, treat rate and basis, example substances, the fluids that use them and the main regulatory flag.
| # | Class | What it does | Treat rate and basis | Example substances | Main fluid types | Main regulatory or hazard flag |
|---|---|---|---|---|---|---|
| 1 | base oils (carrier fluid) | carrier fluid the additives are carried in; lubricates | 60 to 90 wt % of straight-oil concentrate; 40 to 85 soluble oil; below 40 semi-synthetic | naphthenic and paraffinic mineral oils, esters, glycols | straight, soluble, semi-synthetic | hazard line depends on the grade: see the product SDS; untreated and mildly treated mineral oils: California Prop 65, cancer, listed 1987-02-27 |
| 2 | emulsifiers and couplers | disperse oil in water; couplers stabilize | emulsifiers below 30 wt % (soluble oil), below 35 (semi-synthetic) of concentrate; coupling agents below 5 (soluble oil), below 8 (semi-synthetic) | petroleum sulfonates, fatty acid salts, nonionics; propylene glycol, glycol ethers | soluble, semi-synthetic | nonylphenol ethoxylates: REACH Annex XVII entry 46, not used at 0.1 % or more for metal working except controlled closed systems |
| 3 | lubricity additives | adsorbed boundary film, ambient to 200 °C (392 °F) | below 40 wt % (straight), below 20 (semi-synthetic), below 25 (solution synthetic) of concentrate | polyol esters, lard oil, canola oil, fatty acids | all | no class-wide flag |
| 4 | EP additives | react with metal to form a salt layer | below 15 wt % (straight), below 30 (soluble), below 45 (synthetic emulsion) of concentrate | chlorinated paraffins, sulfurized fats and olefins, phosphorus esters, overbased sulfonates | straight, soluble | SCCP: EU POPs Reg. (EU) 2019/1021 Annex I Part A; MCCP: derogation 3(a) for heavy-duty MWF with collection systems until 2036-12-31 (C(2026) 6262, adopted by the Commission on 11 Sep 2026, scheduled to apply from 16 Dec 2026) |
| 5 | corrosion inhibitors | protect ferrous and yellow metals | below 10 wt % (soluble, synthetic), below 5 (semi-synthetic) of concentrate; triazoles 50 to 1,000 ppm in the diluted fluid in one patent (US8722592B2) | calcium and sodium sulfonates, fatty acid soaps, tolyltriazole, benzotriazole | all | benzotriazole (613-350-00-X) and tolyltriazole (613-351-00-5): CLP Annex VI Aquatic Chronic 2 H411 (ATP22, Del. Reg. (EU) 2024/2564, since 2026-05-01) |
| 6 | alkanolamines | set pH (8.0 to 9.5 in use, Lotierzo et al. 2016), reserve alkalinity | MEA and TEA 1 to 5 %, AMP 1 to 10 %, MIPA 1 to 5 % of concentrate (Gerulova et al. 2017) | MEA, TEA, AMP, MIPA | water-miscible | diethanolamine (DEA): California Prop 65, cancer, listed 2012-06-22 |
| 7 | antioxidants | stop thermal and oxidative breakdown | below 1 wt % of straight-oil concentrate | phenolic types (alkylated phenols) and aminic types | straight, ester, rolling oils | no class-wide flag |
| 8 | biocides | control bacteria and fungi | below 2 wt % of concentrate | triazine, oxazolidine, BIT, IPBC, biphenyl-2-ol | water-miscible | six actives approved for PT13 on 27 Oct 2026; C(M)IT/MIT and biphenyl-2-ol approvals run to 30 Jun 2027; MBM (607-721-00-5), MBO (612-290-00-1), HPT (612-291-00-7): Carc. 1B H350, Muta. 2 H341, ATP10 Reg. (EU) 2017/776 |
| 9 | defoamers | break foam | below 1 wt % of water-based concentrate | silicone (PDMS), polyethers, long-chain fatty alcohols | water-miscible | D4, D5, D6 residues in silicone defoamers: REACH Annex XVII entry 70 (Reg. (EU) 2024/1328), below 0.1 % each in mixtures after 2026-06-06 |
| 10 | chelating agents | bind hardness ions | no published rate in the sources used here | EDTA, MGDA, GLDA | water-miscible | no class-wide flag |
| 11 | anti-mist additives | enlarge droplets so mist settles | below 1 wt % of straight-oil concentrate; polyisobutylene 20 to 100 ppm in use (SAE 950245, 1995) | polyisobutylene, ethylene-propylene copolymers, polyethylene oxide | straight; PEO in water-miscible | no class-wide flag |
Ranges are upper limits or ranges from OECD ESD No. 28 (2011) unless a row says otherwise; wt % of the undiluted concentrate.
Forming lubricants carry far higher EP levels than the OECD straight-oil range, and chlorinated paraffins are the documented case. Each substance in the table has an entry in the A to Z of metalworking fluid additive substances with its CAS number and hazard line.
Typical Additive Package by Fluid Type#
Each fluid type carries a different additive package: a straight oil is 60 to 90 wt % mineral oil, while a soluble oil holds below 30 wt % emulsifiers and below 30 wt % EP additives (OECD ESD No. 28, 2011). The table compares five fluid types in wt % of the undiluted concentrate.
| Component | Straight oil | Soluble oil | Semi-synthetic | Synthetic emulsion | Solution synthetic |
|---|---|---|---|---|---|
| Mineral oil or synthetic lubricant | 60 to 90 | 40 to 85 | below 40 | synthetic lube 40 to 80 | n.a. |
| Emulsifiers | n.a. | below 30 | below 35 | below 30 | n.a. |
| Lubricity or oiliness agents | below 40 | below 10 | below 20 | below 10 | below 25 |
| EP additives | below 15 | below 30 | below 10 | below 45 | below 5 |
| Corrosion inhibitors | n.a. | below 10 | below 5 | below 10 | below 10 |
| Coupling agents | n.a. | below 5 | below 8 | below 5 | n.a. |
| Fatty acid | n.a. | n.a. | n.a. | n.a. | below 5 |
| Biocides | n.a. | below 2 | below 2 | below 2 | below 2 |
| Water | n.a. | below 5 | below 60 | below 5 | below 60 |
| Antioxidants, anti-mist agents, metal passivators, dyes, odorants | each below 1 | n.a. | n.a. | n.a. | n.a. |
n.a. = not listed in the source (OECD Emission Scenario Document No. 28, 2011).
Water-miscible fluids account for about 80 % of the metalworking fluid market, according to one trade-press market report (2026).
How Are Additive Treat Rates Expressed? Concentrate, Use Dilution and ppm#
Additive treat rates are stated on three bases: percent of the concentrate, percent of concentrate by volume in the diluted fluid, or parts per million in the diluted fluid, and a rate without its basis cannot be compared. The three bases are listed below.
- Percent of the concentrate: the formulator's treat rate.
- Percent by volume in the diluted fluid: the in-use concentration.
- Parts per million in the diluted fluid: additions to the running fluid.
Boric acid at 1 to 5 % of the concentrate (Gerulova et al., 2017), diluted to 5 %, gives 0.05 to 0.25 % in the diluted fluid; 1 % equals 10,000 ppm, so 20 to 100 ppm equals 0.002 to 0.01 %. The in-use concentration comes from the °Brix reading times the product's own refractometer factor, zeroed on the mix water at about 20 °C (68 °F) (UKLA and HSE good practice guide, v2.5, February 2025).
STLE (2006) warns that arbitrary tankside additions carry substantial risk unless supervised by a chemist familiar with the original formulation, and the German DGUV Regel 109-003 requires hazardous in-use additions to be documented.
Which Metalworking Fluid Additives Are Restricted?#
Three additive groups carry the EU classifications and listings that restrict them in metalworking fluids: formaldehyde-releasing biocides (Carc. 1B), boron compounds such as boric acid (Repr. 1B) and chlorinated paraffin EP additives (EU POPs Regulation).
The formaldehyde releasers MBM (index 607-721-00-5), MBO (612-290-00-1) and HPT (612-291-00-7) are classified Carc. 1B H350 and Muta. 2 H341 in CLP Annex VI by ATP10 (Reg. (EU) 2017/776) since 2018-12-01. Boric acid is listed in Annex VI under 005-007-00-2 as Repr. 1B H360FD, with Note 11 in application since 2025-02-01 (ATP20), and it has been on the REACH Candidate List since 2010-06-18.
The EU counts six actives approved for PT13 on 27 Oct 2026 (BIT, MIT, C(M)IT/MIT, IPBC, biphenyl-2-ol, chlorocresol); C(M)IT/MIT and biphenyl-2-ol approvals run to 30 Jun 2027. C(M)IT/MIT labels and SDS must state no use in metalworking fluids at a concentration triggering skin-sensitizer classification, unless it can be demonstrated that risks can be reduced to an acceptable level by other means (Impl. Reg. (EU) 2016/131).
SCCP is prohibited under EU POPs Regulation (EU) 2019/1021, Annex I Part A. Commission Delegated Regulation C(2026) 6262 on MCCP, adopted by the Commission on 11 Sep 2026 and scheduled to apply from 16 Dec 2026, provides derogation 3(a) for EP use in heavy-duty metalworking fluids with collection systems until 2036-12-31. Nonylphenol ethoxylates are not used at 0.1 % or more for metal working except in controlled closed systems (REACH Annex XVII entry 46), and D4, D5 and D6 are not placed on the market in mixtures at 0.1 % or more each after 2026-06-06 (entry 70, Reg. (EU) 2024/1328). The metalworking fluid regulations hub lists each instrument.
How Is an Additive Package Put Together?#
A metalworking fluid formulation starts from four inputs, the fluid type, the machining or forming operation, the workpiece metal and the mix water, and each input narrows the additive classes the package needs. OECD lists no emulsifiers or biocides in a straight oil, while every water-miscible type carries biocides below 2 wt % of the concentrate.
Additives interact: chlorine and sulfur EP additives show a well-documented synergy (Canter, 2007). In one test reported by Brutto (STLE TLT, August 2013), an MEA and TEA synthetic with 1,000 ppm triazine failed after 4 weeks, and partial amine replacement extended control to 12 weeks or longer.
STLE (2017) counts at least five additive types that need new options, and a fleet fluid change costs 3 to 4 % of a machine shop's annual production schedule (Miles Free, PMPA, in STLE 2017). Which operations need EP chemistry is set out in the guide to cutting fluids by machining operation.
What Happens to Additives Once the Fluid Is in Use?#
Additives in a diluted metalworking fluid are consumed by work, drag-out, microbes and water, so the package that leaves the drum is not the package in the sump a month later.
Additive depletion and top-up#
Additives in the sump are tracked through concentration and pH, with in-use pH at 8.0 to 9.5, the range Lotierzo et al. (2016) describe as typical. German IFA guidance lists falling reserve alkalinity, a marked pH drop, rising nitrite and emulsion instability as signs of microbial activity.
In coolant management, top-ups follow the refractometer reading times the product's own factor, and a coolant management routine checks concentration and pH before any additive is added.
Dyes, odorants and other minor components#
Dyes and odorants are minor components, each below 1 wt % of a straight-oil concentrate. Dyes show product presence in a clear fluid but can stain skin and paint (STLE 2006).
Additives and operator health#
Additives affect operator health through skin contact and mist, and the additive choice changes both: MBO (612-290-00-1) and HPT (612-291-00-7) carry Skin Sens. 1A H317 in Annex VI.
NIOSH sets a recommended exposure limit for metalworking fluid aerosol of 0.4 mg/m³ thoracic particulate mass (about 0.5 mg/m³ total particulate mass) as a TWA for up to 10 h/day in a 40-h week (Criteria Document 98-102, January 1998), not an OSHA limit. Skin, lungs and exposure control are covered in the hub on metalworking fluid health hazards.
FAQ#
How many additives does a cutting fluid contain?#
OSHA's best-practices manual lists 9 additive functions for straight oils and 10 for soluble oils. Soluble oils add emulsifiers, alkanolamines, biocides, coupling agents and defoamers.
Does every cutting fluid need EP additives?#
Not every formulation carries the same EP level: OECD lists EP additives at below 5 wt % of a solution-synthetic concentrate and below 45 wt % of a synthetic-emulsion concentrate. EP chemistry takes over where lubricity additives, active up to 200 °C (392 °F), no longer hold (Canter, 2007).