EP-Additives Extreme pressure (EP) additives are critical components in modern metalworking fluids, designed to protect tools and workpieces under the most demanding conditions. These additives play a vital role in machining operations such as cutting, drilling, milling, and forming, where high loads, temperatures, and friction are common. Without EP additives, tool wear would increase significantly, surface finishes would degrade, and productivity would suffer. EP additives are chemical compounds formulated to react with metal surfaces under high pressure and temperature conditions. Their primary function is to form a protective film on metal surfaces, preventing direct metal-to-metal contact. This film reduces friction and wear, especially in boundary lubrication regimes where the lubricant film is too thin to fully separate the surfaces. Common EP additives include chlorinated paraffins (CLP) and sulfur based compounds (sulfur carriers). Phosphorus-based compounds are also often referred to as EP additives, but mainly work as antiwear additives. Chlorinated paraffins CLPs are saturated, polychlorinated hydrocarbons with a chain length of approximately 10 to 30 carbon atoms and a chlorine content of about 35 to 70 %. For nearly 100 years, chlorinated paraffins have been used as EP additives in metalworking fluids to reduce or prevent adhesive wear, especially during metal processing. Chlorinated paraffins are classified according to the number of carbon atoms in their molecules: short-chain (SCCPs, C 10 -C 13 ), medium-chain (MCCPs, C 14 -C 17 ), long-chain (LCCPs, C 18 -C 20 ), and very long-chain (VCCPs, C> 20 ). Chlorinated paraffins are poorly biodegradable; SCCPs and MCCPs are acutely and chronically very toxic to aquatic organisms, bioaccumulative, and persistent in the environment. They accumulate in the food chain of humans and animals and can now be detected in water, soil, sewage sludge, as well as in many living organisms, in human fatty tissue, and breast milk. Since SCCPs are also potentially carcinogenic and toxic to reproduction, their use in metalworking fluids has been banned in the EU since 2002. Current efforts, such as those by ECHA and the UN Stockholm Convention, aim to heavily restrict the use of medium-chain chlorinated paraffins as well. There are also intentions to ban or restrict the use of MCCPs by e.g. the South Korean and Japanese authorities and by Health Canada. In the longer term, such measures are also expected for long-chain chlorinated paraffins. A corresponding assessment of very longchain chlorinated paraffins is not foreseeable at this time. However, due to their very high viscosity and limited solubility in base oils, these compounds are only conditionally suitable for use as additives in metalworking fluids. In many negative substance lists used in the manufacturing industry, chlorinated paraffins are only permitted in metalworking fluids when technically necessary (e.g., in stainless steel forming). Science and Research 35 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0017 EP Additives with Enhanced Sustainability for Water-miscible Metalworking Fluids Wilhelm Rehbein, Isabell Lange, Kevin DiNicola, Salvatore Rea, John Williams* Presented at GfT Conference 2025 Extreme pressure additives are essential components for many water-borne metalworking fluids. They are used to prevent adhesive wear and cold welding by generating protective layers on metal surfaces under boundary lubrication conditions in heavy duty cutting and forming processes. Sulfur carriers are high-performing EP-additives that can support the metalworking industry on their way to a more sustainable future. Keywords Extreme Pressure additives, Sulfur carriers, Chlorinated paraffins, Water-miscible metalworking fluids, Sustainability, Wear prevention Abstract * Dipl.-Ing. Wilhelm Rehbein Isabell Lange LANXESS Deutschland GmbH, Mannheim, Germany Salvatore Rea Kevin DiNicola John Williams LANXESS Corporation, Naugatuck, CT, USA tain between 1 and 5 sulfur atoms, which form a bridge connecting the hydrocarbon or ester structures of the molecules. Similar to chlorinated paraffins, they form a protective layer of metal sulfides that prevents cold welding between the tool and the workpiece. Due to their higher polarity compared to sulfurized hydrocarbons, sulfurized esters can also reduce friction even under low mechanical or thermal stress. Sulfur carriers are available with various molecular structures, differing in polarity and activity, allowing them to be precisely tailored to the requirements of specific metalworking processes. Their performance spans a wide temperature range - from slow machining operations to high-speed processing. Their effectiveness can be further enhanced through synergistic combinations with overbased calcium sulfonates, polycarboxylates, or other polar compounds. In machining processes, such as deep hole drilling, sulfur carriers prevent the formation of long chips that could damage or block the tool. This is achieved by forming metal sulfide layers on the chip surfaces, which make the chips more brittle and cause them to break more easily. Modern sulfur carriers are light in color, have low odor, and are compatible with most additives used in lubricants. No special measures are required for their disposal. In terms of human and environmental toxicity, they are significantly more favorable than chlorinated paraffins. Unlike chlorinated paraffins, sulfur carriers are not subject to labeling requirements under GHS or hazardous substance regulations. Many sulfur carriers are more than 50 % derived from renewable raw materials, such as vegetable oils or vegetable oil methyl esters. Lots of them are even suitable for Science and Research 36 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0017 Chlorinated paraffins in metalworking processes CLPs are used in metalworking coolants as extreme pressure (EP) additives, meaning they prevent adhesive tool wear that occurs under high mechanical pressure. In the first step, the polar chlorinated paraffin molecules adhere to the metal surface, forming a friction-reducing film. In a second step, under sufficient mechanical or thermal stress, a chemical reaction between the chlorine atoms and the metal surface forms a very thin layer of metal chlorides (see Figure 1). Due to its salt-like structure, this layer is significantly less shear-resistant than the metal itself. As a result, cold welding or adhesion of the workpiece material to the tool is prevented. This would otherwise lead to the breaking out of larger particles from the tool surface. In metalworking, chlorinated paraffins are highly effective at low machining speeds. At higher machining speeds, which usually also lead to increased temperatures at the tool and workpiece, they decompose and form hydrogen chloride, which causes increased tool wear. In the presence of water or high humidity, there is a risk of hydrolysis. The hydrochloric acid formed in this process corrodes tools, workpieces, and the entire machine tool. Compared to other EP additives, the price of chlorinated paraffins is relatively low. However, disposal costs are high in many regions, as they must be incinerated in special high-temperature combustion facilities to prevent the formation of toxic dioxins. Sulfur carriers As an alternative to chlorinated paraffins, sulfurized hydrocarbons and sulfurized synthetic or natural esters are used as EP additives. These so-called sulfur carriers con- Figure 1: Formation of adsorption and reaction layers by EP-additives use in lubricants that meet the requirements of the EU Ecolabel and the US Vessel Incidental Discharge Act. Sulfur carriers can be easily emulsified and are suitable components for soluble oils and semisynthetic metalworking fluids. Some sulfur carriers are even water-soluble and work as excellent EP additives for synthetic cutting and forming fluids. Tribological test results All sulfur carriers in the following tests can be certified as: ▪ based on renewable raw materials for more than 50 % ▪ having a low impact on the environment ▪ not labelled as hazardous to humans or to the environment ▪ meeting the requirements of the LuSC list for environmentally acceptable lubricants To evaluate the performance of the sulfur carriers with improved sustainability in comparison with chlorinated paraffins in emulsifiable metalworking fluids, tribological tests were conducted under laboratory conditions. Pin & Vee tests The Pin & Vee Test is a widely recognized laboratory method used to evaluate the extreme pressure (EP) performance of metalworking lubricants. It simulates the conditions under which lubricants need to prevent wear, seizure, and cold welding between metal surfaces under high load and low speed which are typical in machining operations such as drilling, milling, and turning. The test apparatus consists of two stationary V-shaped blocks (the “Vees”) and a rotating cylindrical pin. The Vee block is typically made of hardened steel and features two angled surfaces that cradle the pin. The pin, also made of steel, rotates under a controlled load and speed while being pressed into the Vee block. The lubricant under test is applied to the contact area between the pin and the Vee block. During the test, a progressively increasing load is applied to the Vees until a failure condition is reached, usually indicated by a sudden rise in friction, temperature, or visible damage to the metal surfaces. Pin & Vee test results of metalworking fluid formulations can be easily transferred to the performance of the formulations in metalworking processes such as tread cutting or thread forming. When running the tests with different metalworking fluid emulsions, the following parameters were used: ▪ All tests were done with emulsions of soluble oil concentrates, containing 40 % mineral oil and 10 % EPadditive. The soluble oils were diluted by 2 % in deionized water. ▪ The test Pins were made of SAE 3135 carbon steel, the Vee blocks are made of AISI 1137 carbon steel ▪ The tests were carried out acc. to ASTM D3233A with a speed of rotation of 290 +/ - 10 rpm and a temperature at test start of 22 °C. After a run-in phase of 5 minutes at 300 lbs, the load was continuously increased from 300 to 4,500 lbs. The EP-performance of three different sulfur carriers and one medium chain chlorinated paraffin was compared in the Pin & Vee test by using these parameters (Table 1). When comparing the different sulfur carriers by Pin & Vee test (Figure 2), the highest load of > 4,500 lbs was achieved by the formulation containing 10 % inactive sulfurized triglyceride. This result shows the excellent friction reducing properties of this additive. With 10 % active sulfurized ester, the torque curve over time was slightly lower but the test stopped due to the break of the Pin at approximately 4,200 lbs. The reason Science and Research 37 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0017 EP additive Chemical structure Sulfur resp. chlorine content Kin. Viscosity at 40°C Content of sustainable raw material MCCP Medium chain chlorinated paraffin, C 14 -C 17 50% Cl 180 mm²/ s 0% Active sulf. ester Sulfurized methyl ester 17% S (8% active S) 55 mm²/ s > 80% Inactive sulf. tg Sulfurized vegetable triglyceride 9.5% S (<1% active S) 230 mm²/ s > 80% Active sulf. tg Sulfurized vegetable triglyceride 18% S (9% active S) 220 mm²/ s > 80% Table 1: properties of the EP-additives tested by Pin & Vee and four-ball tests Science and Research 38 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0017 Figure 3: Pin & Vee test results of soluble oil formulations diluted to 2 % with deionized water. The formulations contain no EP-additive respectively a medium chain chlorinated paraffin or an active sulfurized triglyceride. Figure 2: Pin & Vee test results of 3 different sulfur carriers. The dashed lines show the applied loads, the solid lines the resulting torques. for this effect is that a protective layer was formed already at lower mechanical load and temperature due to the higher activity of this sulfur carrier. However, because of the lower viscosity and polarity of this sulfur carrier, the protective layer was less stable and broke down at the maximum load of 4,200 lbs. The small torque peak at the maximum load indicates the collapse of the protective layer, following by a direct metal-to-metal contact and cold welding. The test result with the 10 % active sulfurized triglyceride is even slightly lower with a maximum achievable load of approximately 4,100 lbs. The lower maximum achievable load is probably caused by a kind of competitive situation between the protective layer which is formed by the active sulfur, and the physical adsorption layer formed by the polar centers of the sulfurized triglyceride molecule. Compared with the test result of the diluted soluble oil formulation containing 10 % of the MCCP or with the maximum achievable torque of the diluted soluble oil which does not contain any EP-additive, even the active sulfurized triglyceride showed a significantly better performance (Figure 3). Four-Ball tests In addition to the Pin & Vee tests, Four-Ball tests were carried out to evaluate the capability of the EP-additives to prevent abrasive and adhesive wear under high load and high speed conditions. The test results are transferable to real cutting processes with rotating tools or work pieces, e.g. turning or milling processes. The ASTM D2783 Four-Ball extreme pressure test evaluates the EP properties of lubricating fluids. It helps differentiate between fluids with low, medium, and high EP performance, which is crucial for applications involving high-load conditions. In this test, three steel balls are clamped together in a cradle, and a fourth ball is rotated against them under increasing loads. ▪ In this test, the speed of rotation is 1800 rpm. The tests starts at ambient temperature (approx.70 °F / 21 °C) and runs for 10 seconds. Additional to the weld load, which is the load at which the rotating ball welds to the stationary balls, there are two more parameters: ▪ The last non-seizure load gives an indication about the stability of the protective layer, formed by the lubricant additives, at increasing loads. It is the highest load at which no seizure (metal-to-metal contact without lubrication) occurs during the test. The higher the last non-seizure load, the better the lubricant is suited for high-pressure applications ▪ The load-wear index (LWI) is a measure of a lubricant’s ability to withstand extreme pressure conditions. It indicates how well a lubricant can prevent wear as the load increases. The LWI is calculated from test results at various load levels where wear occurs. It’s a weighted average of the loads at which wear is observed. A higher LWI value means better lubricating performance and wear protection. The ASTM D4172 Four-Ball wear test is used to assess the wear preventive characteristics of lubricating fluids Science and Research 39 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0017 Figure 4: Four-Ball test results of soluble oil formulations containing no respectively different EP-additives according to ASTM D2783 and ASTM D4172. major concerns when using sulfurized additives in water miscible metal working fluids are: ▪ The release of H 2 S due to a decomposition of the sulfur chain. ▪ An increase of the acidity of the water phase due to decomposition of the sulfur carrier, either at the sulfur chain or at the ester group. The hydrolytic stability of the sulfur carriers was tested in a modified ASTM D2619 Beverage Bottle test (Figure 5). To simulate the chemical conditions of a waterborne metalworking fluid, the pH of the water phase was adjusted to a value of 9.0. In this test, 75 mL of a blend of 20 % sulfur carrier and 80 % mineral oil and 25 mL of water, adjusted to pH 9.0 Science and Research 40 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0017 under sliding contact conditions and indicates the lubricants ability to prevent abrasive wear. Similar to D2783, three stationary steel balls are clamped together, and a fourth ball is rotated against them. ▪ The parameters in these tests are a speed of rotation of 1200 rpm, a starting temperature of 86 °F (30 °C), a test duration of 60 minutes and a constant load of 20 kg. Again the different EP-additives were tested with 10 % concentration in soluble oil formulations which were diluted to 10 % with deionized water. The results of the Four-Ball EP test (Figure 4) impressively show that sulfur carriers are significantly more effective in preventing adhesive wear when compared to medium chain chlorinated paraffins. The weld load of the inactive sulfurized triglyceride is on the same level as the MCCP, the weld loads of the more active sulfurized ester and sulfurized triglyceride are higher. Even more, the last non-seizure load and the load-wear-index, showing the stability of the protective layer and the overall resistance to adhesive wear, are much higher for the sulfur carriers. The same trend is visible in the ASTM D4172 wear tests. Without any EP-additive, it was not possible to run the test due to strong friction vibrations. With the different sulfur carriers, the Four-Ball wear is almost on the same level and approximately 30 % lower than with the chlorinated paraffin. Hydrolytic stability For water-miscible metalworking fluids, the hydrolytic stability of their components plays a crucial role. The Figure 5: Beverage Bottle test, used for the evaluation of the hydrolytic stability of the sulfur carriers Table 2: test results of the beverage bottle test of the sulfur carriers. The results outlined in red are the most important ones. were filled in a glass bottle with a volume of approx.. 207 mL (7 fl.oz). The bottle is sealed and placed in a rotating oven which is heated up to 93 °C (200 °F). After 48 hours of continuous rotation, the liquids are filled in a separatory funnel and the acid number of both the oil and water phases are measured (Table 2). After 48 hours in the Beverage Bottle tests, all three sulfur carriers show no change of the acidity of the water phase or oil phase and also almost no release of H 2 S. This is proof that sulfur carriers are stable even under the conditions in a water-mixed cooling lubricant and are not prone to hydrolysis. Summary The test results demonstrate that it is possible to formulate water-miscible metalworking fluids with excellent performance by using EP additives based on sulfurized renewable raw materials. All sulfur carriers in these tests can be certified as: ▪ Based on renewable raw materials for more than 50 %. ▪ Having a low impact on the environment. ▪ Are not labelled as hazardous to humans or to the environment. ▪ Meeting the requirements of the LuSC list for environmentally acceptable lubricants. The Pin & Vee test results show that sulfur carriers with improved sustainability have a positive influence on energy consumption and tool life in cutting and forming processes when used as EP additives in metalworking fluid emulsions. Higher Four-Ball weld loads, last non-seizure loads and load-wear-indices and lower Four-Ball wear test results indicate that these sulfur carriers can effectively prevent adhesive and abrasive wear in water-borne metalworking fluids and exceed the performance of conventional additives like chlorinated paraffins. Science and Research 41 Tribologie + Schmierungstechnik · volume 72 · issue 3-4/ 2025 DOI 10.24053/ TuS-2025-0017