Extreme Pressure (EP) Additives in Metalworking Fluids: Chlorine, Sulfur and Phosphorus Chemistry

Extreme pressure (EP) additives are chlorine, sulfur and phosphorus compounds, plus overbased calcium sulfonates, that react with the metal surface to form a sacrificial salt layer where the oil film fails, preventing welding at the tool and die contact. Heavy-duty metalworking fluids for tapping, broaching and forming depend on them.

The four groups activate in sequence. Chlorinated paraffins work from about 180 to 420 °C (356 to 788 °F), phosphorus compounds above that range, sulfur carriers up to about 1,000 °C (1,832 °F), and overbased sulfonates lay down a film below 500 °C (932 °F) without heat activation (Lubrizol chart in Canter, STLE TLT, 2007).

Short-chain chlorinated paraffins (SCCP) are prohibited in the EU, and medium-chain chlorinated paraffins (MCCP) carry a Stockholm metalworking exemption that may be available until 2036. Published levels run from 3 % of one phosphate ester in a soluble-oil concentrate to about 80 % MCCP in one forming oil.

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 Extreme Pressure Additives in Metalworking Fluids?#

An extreme pressure additive is a metalworking fluid component that reacts chemically with the metal surface to form a metal salt layer, instead of only adsorbing on it like a boundary lubricity additive (Gresham, STLE TLT, September 2006). That reaction makes EP chemistry one class of metalworking fluid additives, the class that carries the load once the adsorbed film is gone.

Canter (STLE TLT, 2007) names four EP classes: chlorinated, phosphorus, sulfur and overbased sulfonate compounds; this reference splits sulfur into two rows. Metalworking fluid additive demand was about 575 kt in 2012, and corrosion inhibitors, emulsifiers, friction modifiers and EP additives together made up more than 80 % of it (Kline, reported in STLE TLT, March 2014).

EP Additives vs Antiwear and Boundary Lubricity Additives#

Boundary lubricity additives such as lard oil and canola oil adsorb on the metal through a polar group and work from ambient to about 200 °C (392 °F); EP additives react chemically and activate higher on the same chart. The adsorbed molecule has a polar head on the metal and a tail compatible with oil or water (Gresham, STLE TLT, 2006).

Phosphate esters cross the line: Canter (STLE TLT, 2007) lists their functions as EP, anti-wear, emulsifier and corrosion inhibitor. The two properties have separate tests. ASTM D4172-25 evaluates preliminary anti-wear properties in sliding contact, while ASTM D2783-25 measures the load-wear index and weld point of EP fluids.

How Do EP Additives Work at the Tool and Die Contact?#

EP additives react with the hot metal surface to form low-shear films: iron chloride from chlorinated paraffins, iron sulfide from sulfur carriers and a phosphate or phosphide film from phosphorus compounds. Overbased calcium sulfonates work differently: their colloidal calcium carbonate deposits a barrier film on iron without temperature activation (Canter, STLE TLT, 2007). Chlorinated paraffins first adsorb on ferrous surfaces, and their EP strength rises with chlorine content (STLE TLT, November 2022).

The hottest contacts narrow the choice. The Stockholm review committee (POPRC.19/9/Add.1) places the heavy-duty cases in which MCCP is hard to replace at contact temperatures of 600 to 1,000 °C (1,112 to 1,832 °F).

At What Temperature Does Each EP Chemistry Activate?#

The Lubrizol chart reproduced by Canter (STLE TLT, 2007) ranks activation from boundary additives through chlorine and phosphorus to sulfur, and places overbased sulfonates outside the temperature sequence. The table below lists each range with its source.

Chlorine sets its own upper limit. Chlorinated paraffins release significant HCl above 300 °C (572 °F), and at high speed, high temperature or with water that acid corrodes tools and workpieces (STLE TLT, November 2022).

Why Formulators Combine Chlorine, Sulfur and Phosphorus#

Chlorine plus sulfur is the best-documented EP synergy, and sulfur, phosphorus and overbased sulfonates also work together (Canter, STLE TLT, 2007). In a LANXESS thread-forming test on stainless steel (STLE TLT, November 2022), 5 % sulfurized olefin plus 5 % overbased calcium sulfonate in an ISO VG 46 Group I oil gave the lowest torque, below 10 % MCCP and below 10 % of either component alone.

Which EP Additives Are Used in Metalworking Fluids?#

This reference groups the EP substances used in metalworking fluids into five rows: chlorinated paraffins, sulfurized fats and esters, sulfurized olefins and polysulfides, phosphate esters and amine phosphates, and overbased calcium sulfonates. The OSHA Metalworking Fluids Best Practices Manual names sulfurized fatty materials, chlorinated paraffins and phosphorus derivatives as EP agents. The table below gives treat rates with basis and source per row.

A supplier needs five data points to quote an EP additive, listed below.

• EP chemistry and CAS number
• total and active sulfur or chlorine content in % by weight
• fluid type and target treat rate with its basis (% of the concentrate or of the straight oil)
• operation and workpiece metals, including copper alloys, titanium or nickel alloys
• regulatory constraint: summed C14-17 content on the EU SDS once C(2026) 6262 applies, or zero-chlorine and zero-sulfur part specifications

Chlorinated Paraffins: The Chlorine Carriers#

EP strength of chlorinated paraffins rises with chlorine content, and published levels run from 10 % MCCP in a thread-forming oil (STLE TLT, November 2022) to about 80 % MCCP in the forming oil Iloform TDN 81 (Danish EPA, 2005). The 2022 series also used 15 % MCCP in a milling oil, without stating weight or volume.

Chain length decides the legal status: SCCP (C10-13, CAS 85535-84-8) is prohibited in the EU, MCCP (C14-17, CAS 85535-85-9) carries the Stockholm metalworking exemption, and long-chain chlorinated paraffins (LCCP, CAS 63449-39-8) are not listed as persistent organic pollutants.

Sulfurized Fats, Esters and Olefins: Active and Inactive Sulfur#

Sulfur carriers split into sulfurized fats and esters and sulfurized olefins and polysulfides; active sulfur is free or weakly bonded and reacts with copper, while inactive sulfur is strongly bonded (STLE TLT, April 2019).

LANXESS data sheets show the spread: Additin RC 2540, a sulfurized olefin, carries 40 % total and 36 % active sulfur, RC 2526 carries 26 % and 15 %, and the sulfurized triglyceride RC 2410 carries 10 % total and about 1 % active sulfur. Sulfur content sets the dose: 0.25 % by weight of a 44 % sulfur polysulfide delivers the same sulfur as 1 % by weight of an 11 % sulfur fatty ester (STLE TLT, April 2019).

Pentasulfides release active sulfur faster and at lower temperature than trisulfides, across the 60 to 250 °C (140 to 482 °F) range studied, and suit low-speed, high-torque work (Court, Arkema, in Canter, STLE TLT, 2007). Sulfurized isobutylene is ruled out of open metalworking systems by its odor (STLE TLT, April 2019).

Phosphate Esters and Amine Phosphates#

Phosphate esters form an oil-insoluble phosphate or phosphide film above the chlorine range (Canter, STLE TLT, 2007). One alkyl ether phosphate ester, Lanphos PE74 from Lankem, runs at 3 to 10 % of the concentrate in mineral- or vegetable-oil soluble oils and semi-synthetics after neutralization with alkanolamines, according to its data sheet.

Phosphate esters serve aluminum machining for lubricity and stain inhibition (Canter, STLE TLT, March 2014). Phosphorus can promote microbial growth and foam, so it is avoided in bioresistant fluids and for magnesium (Canter, 2014; STLE TLT, March 2021). Zinc dialkyldithiophosphate (ZDDP) can leave burn residue at high temperature (POPRC.19/9/Add.1, paragraph 148).

Overbased Calcium Sulfonates: Passive EP#

Overbased calcium sulfonates deposit a carbonate barrier film below 500 °C (932 °F) without temperature activation, and EP grades carry a total base number (TBN) of 300 to 400 mg KOH/g (Canter, STLE TLT, 2007). Alone at 10 % in a LANXESS thread-forming oil (STLE TLT, November 2022), the sulfonate gave the highest torque of the four oils. In forming lubricants, the calcium-based passive EP works on ferrous and non-ferrous alloys (STLE TLT, March 2021).

How Much EP Additive Does Each Fluid Type Contain?#

EP additives make up less than 15 % of a straight oil and less than 30 % of a conventional soluble oil concentrate, by weight of the undiluted concentrate (OECD ESD No. 28, 2011). The same compilation gives less than 10 % for semi-synthetic, less than 45 % for synthetic emulsion and less than 5 % for solution synthetic concentrates. The table below adds the OSHA manual's EP agents and the categories declaring EP, as quoted by suppliers from ISO 6743-7:1986.

Straight Oils and Forming Oils#

Straight oils carry EP agents undiluted: Table 1 of the OSHA Metalworking Fluids Best Practices Manual lists sulfurized fatty materials and chlorinated paraffins as their EP agents. For EP agents carried undiluted in straight oils, the level in the oil is the in-use level.

Forming oils carry the highest published levels: 40 % MCCP at 50 % chlorine in a fine-blanking test oil (STLE TLT, November 2022) and about 80 % MCCP in Iloform TDN 81 (Danish EPA, 2005). One supplier's guide (Fuchs) states that cutting oils for tougher machining of steel, aluminum and titanium almost always contain sulfur, phosphorus or chlorine EP additives.

Soluble Oils, Semi-Synthetics and Synthetics#

Water-soluble sulfurized additives may not activate in light-duty work at use dilution, but they are effective in severe semi-synthetic drawing and stamping (Canter, STLE TLT, 2007). Emulsifiable sulfur grades for soluble oils exist, such as Additin RC 2411 and RC 2526 (LANXESS), without a published treat rate.

Phosphate esters are the water-side EP chemistry with a published rate. Neutralized phosphate esters in soluble oils and semi-synthetics run at 3 to 10 % of the concentrate in one ester's data sheet (Lanphos PE74, Lankem).

How Do EP Additives Perform, and Where Do They Fail?#

In tests published in STLE TLT (November 2022), 5 % of a sulfur carrier outlasted a 15 % MCCP milling oil at higher cutting speed, while none of four chlorine-free lubricants passed a Danish EPA stainless forming trial (2005). The table below collects the comparative results with their sources.

The evidence points both ways. An industry presentation by the Chlorinated Paraffins Industry Association (2024) reports one A286 deep-draw shop case in which a 70 % CP oil reached 7,000 parts per die sharpening, against 7 for the best of five chlorine-free oils. In the 2022 strip-drawing test, two sulfur carriers at 30 % gave friction coefficients of 0.09 and 0.08 against 0.14 for MCCP.

Copper Staining From Active Sulfur#

Active sulfur reacts with copper and stains brass, bronze and other yellow metals, and ASTM D130 (current edition D130-26) screens sulfur additives for it.

ASTM D1662-24 measures active sulfur in cutting oils as the sulfur that reacts with copper powder at 150 °C (302 °F), with no established correlation to field performance. One deactivated polysulfide (Additin RC 2541) keeps 35 % active sulfur without copper staining. A thiolated polyglycerol ester rated 1b in the D130 test against 4b for a commercial sulfurized additive (STLE TLT, March 2021).

Metals and Parts That Restrict Chlorine or Sulfur#

Five metals and part groups restrict chlorine, sulfur or phosphorus EP chemistry, listed below with their sources.

• titanium, where chlorinated paraffins are avoided because of stress-corrosion cracking concerns (Canter, 2007); see cutting fluid for titanium
• nickel alloys, restricted with titanium by OEM approvals (Canter, 2007)
• aluminum, where most machining has moved to non-chlorinated chemistries (Canter, STLE TLT, March 2014); see cutting fluid for aluminum
• magnesium, where phosphorus additives are avoided (Canter, 2014; STLE TLT, March 2021)
• medical parts, whose specifications can require zero chlorine and zero sulfur (Pearce, industry presentation by the Chlorinated Paraffins Industry Association, 2024)

The same presentation lists shop downsides of chlorinated paraffins: disposal, staining, weldability, cleanability, removal before heat treatment and alpha case on titanium. Stainless steel has its own comparison of chlorine-free options for stainless steel.

How Are EP Additives Tested?#

EP additives are ranked by load-carrying tests, the four-ball EP test (ASTM D2783-25) and the Falex pin and vee block test (ASTM D3233-24), and whole metalworking fluids by the tapping torque test (ASTM D8288-25). The table below lists each method with its current edition.

Expert views in STLE TLT (March 2018) hold that the four-ball weld point is repeatable but more meaningful for lubricants than for metalworking fluids, and that it suits EP differentiation of straight oils. ASTM D5619 was withdrawn in 2016 with no formal replacement; D8288-25 is the current tapping torque method.

What Rules Apply to EP Additives?#

Chlorinated paraffins carry the regulatory weight of the EP class: SCCP and MCCP have harmonized CLP classifications and REACH Candidate List entries, while the sulfur, phosphorus and sulfonate EP substances have no Annex VI entry. SCCP is classified Carc. 2 H351, Aquatic Acute 1 H400 and Aquatic Chronic 1 H410 under Annex VI index 602-080-00-8, and MCCP Lact. H362, H400 and H410 under index 602-095-00-X, both from ATP01, in application since 2010-12-01, as recorded in ECHA's harmonized classification for MCCP. The matrix below lists the captured status per substance; an empty cell means no status is recorded.

In the United States, ten significant new use rules at 40 CFR 721.11068 to 721.11077 (84 FR 49050, 18 Sep 2019) each permit "additive in lubricants including metalworking fluids". The OSHA Metalworking Fluids Best Practices Manual, guidance rather than a standard, advises avoiding potentially carcinogenic components such as chlorinated paraffins.

Chlorinated Paraffins Under the Stockholm Convention and EU POPs Regulation#

SCCP is listed in Annex A of the Stockholm Convention by decision SC-8/11 (2017) and prohibited in the EU under Regulation (EU) 2019/1021 Annex I Part A, with no metalworking exemption. MCCP was added by decision SC-12/10 (2025), whose specific exemption, which may be available until 2036, covers extreme temperature and pressure additives in metalworking fluids for heavy-duty processes in professional or industrial settings with collection systems.

In the EU, MCCP follows by Commission Delegated Regulation C(2026) 6262, adopted by the Commission on 11 Sep 2026, scheduled to apply from 16 Dec 2026. The act provides a trace limit of 0.1 % w/w (1,000 mg/kg) of summed defined C14-17 congeners, and its derogation 3(a) covers MCCP as an EP additive in heavy-duty metalworking fluids with collection systems until 2036-12-31.

Are Chlorinated EP Additives Banned in the EU?#

Partly: SCCP is already prohibited, while MCCP is not yet, because Commission Delegated Regulation C(2026) 6262 was adopted by the Commission on 11 Sep 2026 and is scheduled to apply from 16 Dec 2026. From that date the act will prohibit MCCP above a 0.1 % w/w trace limit, with derogation 3(a) for heavy-duty metalworking fluids until 2036-12-31; LCCP is not listed.

Do EP Additives Have Their Own Exposure Limits?#

No EP-specific limit applies: no chlorinated-paraffin limit is listed in OSHA Table Z-1, the NIOSH REL list or HSE EH40, and the general metalworking fluid limits cover EP-containing aerosols. The NIOSH REL for metalworking fluid aerosol is 0.4 mg/m³ thoracic particulate mass (about 0.5 mg/m³ total particulate mass), TWA for up to 10 h/day in a 40-h week (NIOSH Criteria Document 98-102, 1998). Values by jurisdiction are collected under metalworking fluid exposure limits.

Chlorine-Free Cutting Fluids: What Replaces Chlorinated Paraffins?#

A chlorine-free cutting fluid replaces chlorinated paraffins with sulfur carriers, phosphate esters, overbased calcium sulfonates, polymeric esters or water-based PAG forming lubricants, the alternatives documented by the Stockholm review committee (POPRC.19/9/Add.1). The committee's eight alternatives are listed below.

• sulfurized olefins and polysulfides
• sulfurized fats and esters, including sulfurized and polymerized triglycerides
• overbased calcium sulfonates used with sulfur carriers
• phosphate acid esters and amine phosphates
• polymeric and complex esters
• thiolated polyglycerol esters (mild EP)
• PAG and polymer high-solids water-based forming lubricants
• non-reactive solids such as graphite, MoS2 and calcium carbonate

The Stockholm BAT/BEP guidance (version 06/2026) advises against switching to LCCP or other chlorinated or fluorinated substitutes and names polyalkylene glycols and amine phosphates for metal forming.

Sulfur carriers are the alternative tested head to head against MCCP in the 2022 STLE TLT series. Those chlorinated and sulfurized results are weighed in chlorinated vs sulfurized EP additives.

In a vendor case reported by The Fabricator (6 Aug 2020), a chlorine-free soluble oil at a 4:1 dilution ratio matched a chlorinated straight oil on a 4 mm (0.16 in) steel hub, with less galling.

As low-molecular-weight chlorinated paraffins were restricted, formulators moved to medium- and long-chain grades at higher loading and to sulfur and phosphorus compounds (Kline, STLE TLT, March 2014). The Danish EPA trial (2005) remains the counter-case.

Where Do EP Additives Sit Among Lubricity and Other Metalworking Fluid Additives?#

EP additives take over where lubricity and boundary additives stop, above about 200 °C (392 °F) at the contact, and this reference groups the remaining metalworking fluid additives by function into their own classes. Below 200 °C the work belongs to lubricity and boundary additives, such as esters, fatty acids and lard oil. Base fluids such as mineral oils and water are carriers of these additives, not additives.

EP Additives: Frequently Asked Questions#

The four answers below cover uses, sulfurized cutting oil, water-based coolants and disposal.

What Are Extreme Pressure Additives Used For?#

EP additives prevent welding at the contact in heavy-duty cutting and forming, such as tapping, broaching, deep drawing, fine blanking and stamping (POPRC.19/9/Add.1). Straight oils and forming oils carry the highest published levels.

What Is Sulfurized Cutting Oil?#

Sulfurized cutting oil is a straight oil carrying sulfurized fatty materials or other sulfur carriers as its EP agent. Active sulfur grades darken copper alloys, so inactive grades such as Additin RC 2410 (LANXESS), at about 1 % active sulfur, are made for yellow metals.

Do EP Additives Work in Water-Based Coolants?#

Yes, at lower levels: semi-synthetic concentrates carry less than 10 % and solution synthetics less than 5 % EP additives by weight (OECD ESD No. 28, 2011). Water-soluble sulfurized additives may not activate in light-duty work at use dilution (Canter, 2007).

Does a Chlorinated EP Additive Change How Spent Fluid Is Disposed Of?#

Yes in the US: used oil above 1,000 ppm total halogens is presumed hazardous waste under 40 CFR 279.10(b)(1)(ii), a rebuttable presumption with an exception for chlorinated-paraffin metalworking fluid reclaimed under a 279.24(c) tolling arrangement. In the EU, halogenated spent fluids take 12 01 06* (mineral machining oils) or 12 01 08* (emulsions and solutions). Treatment routes are set out under disposal of waste metalworking fluids.