EP vs Anti-Wear Additives: Activation Kinetics, ASTM Test Decoding, and Formulation Trade-Offs on a Data Sheet

The Real Boundary Line: Stribeck Lambda (Λ) Regimes, Stress-Augmented Kinetics, and Flash-Temperature Activation

Along the Stribeck curve defined by the ratio of film thickness Λ = h_min/σ^*, hydrodynamic lubrication occurs when Λ ≥ 3. Under this condition, the pressure of the bulk oil carries the entire load. Because the metal surfaces do not touch, both anti-wear and extreme-pressure chemistries stay dormant inside the fluid.

When the oil film thins into the mixed regime between Λ = 0.5 and 3, contact pressures stay below 1.5 GPa. At the same time, bulk temperatures are between 80–180°C. In this window, an anti-wear additive reacts on the iron surface to build a compliant glass tribofilm of polyphosphate. This resilient film can cushion colliding micro-asperities and drops the dimensionless Archard wear coefficient K from 10⁻³ down to 10⁻⁷.

If the film collapses below Λ = 0.5 and pressures exceed 1.5 GPa, severe shear will tear away the phosphate glass. According to the Blok equation Δ T_flash ∝ μ W |u₁ – u₂|, colliding asperity tips experience heat spikes above 220–800°C within milliseconds. Those thermal spikes will trigger an extreme-pressure additive to cleave on the spot. The released sulfur reacts with bare iron to form a low-shear layer of iron sulfide (FeS) that can stop seizure and welding.

Two common myths claim that AW additives do not depend on temperature and that EP additives work only through physical adsorption. High-temperature atomic force microscopy disproves both ideas by showing that the growth of a ZDDP film follows the Bell/Eyring rate law Γ = Γ₀ exp[-(E_a – σΔ V^*)/k_B T]. Pure thermal breakdown of ZDDP in oil needs temperatures above 130–150°C, but a 1 GPa shear stress lowers the barrier so films can nucleate at 25–80°C. By contrast, sulfur-phosphorus EP agents must react chemically above 200°C. Overbased calcium sulfonate complex greases are the only non-reactive exception because they deposit physical wafers of CaCO₃ calcite.

When arranged from the lowest to the highest activation temperature, the four boundary additive classes work in separate kinetic windows:

1. Polar Friction Modifiers: Physisorb or weakly chemisorb between 25–100°C to reduce boundary friction coefficients to 0.05–0.08, then desorb above 120°C.
2. Organophosphorus AW Agents: Nucleate polyphosphate glass between 80–180°C via stress-assisted thermal decomposition, ordered in reactivity from secondary ZDDP to primary ZDDP and aryl phosphates above 200°C.
3. Chlorinated Paraffins: Cleave between 180–240°C to form FeCl₂ films with a 680°C melting point, now restricted under the Stockholm POPs Convention and EU REACH SVHC regulations.
4. Organosulfur EP Carriers: Active polysulfides with sulfur rank ≥ 3 and 140 kJ/mol activation energy cleave at 140–220°C, whereas inactive mono- and disulfides with 260 kJ/mol activation energy require 250–600°C flash temperatures.

The Chemistry Split: Stratified Phosphorus Polyphosphate Glass vs. Sacrificial Iron Sulfide Layers

Phosphorus-Dominated Anti-Wear Chemistries: ZDDP, Amine Phosphates, and Ashless Phosphorothioates

ZDDP with CAS 68649-42-3 is made by reacting primary or secondary alcohols with P₂S₅ and neutralizing the acid with ZnO. On rubbed steel, it builds a layered, self-limiting pad-like tribofilm that is 20–150 nm thick. The bottom 5–15 nm base of this film consists of iron and zinc sulfides FeS/ZnS mixed with Fe₃O₄ oxide on the metal. Above that base sits a 30–100 nm body of zinc and iron orthophosphate and pyrophosphate glass. This glass has a hardness of 4–6 GPa and an elastic modulus of 80–120 GPa, while a 5–20 nm polymeric polyphosphate layer cushions the top.

The structure of the alkyl chain controls how fast the additive breaks down and how long it can endure heat. Secondary alkyl ZDDP goes through rapid β-hydrogen cis-elimination when flash temperatures are lower, which gives fast protection during break-in. Primary alkyl ZDDP reacts through a slower Sₙ2 transfer of the alkyl group. As a result, it is stable up to 200–210°C and can resist water hydrolysis under ASTM D2619. Because of this trade-off, blenders often use primary-secondary alkyl ZDDP so the oil can form a film quickly during a cold start and still survive high sump temperatures.

Some industrial lubricants cannot use zinc or exceed the sulfated ash limits of ASTM D874. These applications include environmentally acceptable lubricants, oils for rotary screw compressors, and industrial gear oils that must resist micropitting. In these blends, formulators can use neutralized amine phosphates that have 4.5–6.0 wt% P and 3.5–5.0 wt% N. These ashless salts adsorb onto the steel and do not leave metallic soap sludge. When temperatures climb higher, triphenyl phosphorothioate (TPPT) with CAS 597-82-0, 9.0 wt% P, and 9.3 wt% S has no aliphatic β-hydrogens. Because of that aromatic ring, it will resist thermal breakdown above 280°C in synthetic PAO and ester fluids.

When strict phosphorus limits or ashless requirements govern a formula, dithiocarbamate chemistry is another practical path. Blending with methylenebis(dibutyldithiocarbamate) at CAS 10254-57-6, which has 29 wt% S and 6 wt% N, gives the oil both anti-wear protection and moderate extreme-pressure capacity. At the same time, it acts as a high-temperature decomposer of peroxides and leaves zero metallic ash.

Sulfur-Dominated Extreme-Pressure Carriers and Inorganic Borate Dispersions

When severe shock loads break through the phosphate glass, active chains of polysulfide with 3 to 6 sulfur atoms take over the protection of the metal. In sulfurized isobutylene (SIB) with CAS 68511-50-2 and 43–48 wt% total sulfur, the weak sulfur-sulfur bonds will split under heat to release thiyl radicals. Those radicals react with fresh iron to build a 100–300 nm FeS/FeS₂ layer. Iron(II) sulfide melts at 1,190°C and shears at 20–40 MPa, whereas bare steel needs over 400 MPa to shear. Because the sulfide layer is so easy to shear, colliding asperities can slide smoothly along the sacrificial plane instead of welding together.

Many metalworking fluids and industrial gear oils need a lighter color, a milder odor, and a controlled release of active sulfur. For these blends, formulators can choose a sulfurized olefin extreme-pressure additive made from C₁₄–C₁₈ linear α-olefins that has 38.5 wt% total sulfur. This carrier responds quickly at the boundary without releasing the sharp smell of mercaptan that comes from lighter cuts of isobutylene.

If a machine has yellow metals such as brass, bronze, or copper, or if a metalworking process cannot tolerate dark staining, active sulfur will corrode the surface. In those cases, blenders can pair inactive sulfur carriers with inorganic borates to stop copper attack. A polar low-activity sulfurized triglyceride with 10–12 wt% total sulfur and ≤ 1.0 wt% active sulfur at 150°C will lubricate the contact without staining yellow metals. An amorphous dispersion of potassium triborate with 2.5 wt% boron works in the same way. It can reach a 400 kgf weld load in the four-ball test and still keep a Class 1a rating in both oil and grease copper strip tests.

The polarity of the base oil also changes how well these additives can reach the metal surface and build a film. Non-polar API Group II/III hydrocracked base oils and Group IV PAO can extend the oxidation life of ZDDP by 50%. However, pure PAO needs 5–15 wt% of a synthetic ester or alkylated naphthalene co-solvent. Without that co-solvent, polar amine phosphates or tricresyl phosphate may drop out of solution. On the other hand, highly polar Group V synthetic esters and polyalkylene glycols will compete directly with ZDDP for the same iron adsorption sites.

How to Measure EP vs. Anti-Wear: Decoding ASTM, ISO, DIN, and FVA Bench Tests

In industrial and automotive lubricants, the “EP” label on enclosed gear oils and hypoid axle fluids means the oil is fortified against seizure. Rust-and-oxidation oils and standard hydraulic fluids do not carry this level of protection. To qualify as an extreme-pressure lubricant, a finished oil must pass FZG gear scuffing at failure load stage ≥ 12. It also needs a four-ball weld load ≥ 250 kgf (and ≥ 400 kgf for API GL-5 axle oils) along with a Timken OK load ≥ 60 lbs. The table below summarizes the 11 international bench tests and pass limits:

Reading Real TDS Numbers: Cross-Chemistry Specification Matrix & Three Data-Sheet Traps

A side-by-side look at Certificate of Analysis and Technical Data Sheet values across nine commercial additives shows a clear pattern. The level of 150°C active sulfur drives the trade-off between moderate-load wear and the weld point. Unless noted otherwise, the bench figures in the table below come from blends in API Group I/II mineral base oils at standard treat rates:

Trap 1: Test Geometry Non-Comparability and the Tribocorrosive Wear Scar Paradox

Buyers often mix up the numbers from four-ball wear tests and four-ball extreme-pressure tests when they read a supplier data sheet. Four-ball wear testing for oils (ASTM D4172) and greases measures the diameter of the wear scar. The rig runs under a steady 40 kgf load at 1,200 rpm and 75°C for 1 hour. By contrast, four-ball EP testing for oils (ASTM D2783) and greases runs 10-second step loads at 1,760 rpm until the steel balls weld together.

If a blender uses an active sulfur EP carrier by itself, it will actually make moderate-load wear worse through chemical-mechanical polishing corrosion. At 5 wt% in an ISO VG 220 base oil, sulfurized isobutylene can reach an 800 kgf four-ball weld load. Yet its 1-hour wear scar grows to 0.80 mm because the active sulfur keeps turning the iron into soft FeS with a 30 MPa shear strength that is wiped away. When the blender pairs SIB with primary alkyl ZDDP or an ashless amine phosphate, the phosphorus additive builds a 5 GPa polyphosphate pre-film. That hard film shields the steel and keeps the wear scar ≤ 0.38 mm.

Trap 2: Active Sulfur vs. Total Sulfur and Passivator Masking

The total sulfur test under ASTM D1552 counts every sulfur atom in the molecule. By contrast, the 150°C active sulfur test under ASTM D1662 measures only the weak polysulfide chains that split and react with metal. For instance, an inactive sulfurized triglyceride has 11 wt% total sulfur, yet its active sulfur is ≤ 1 wt%, so it passes the 3-hour copper test at Class 1a. On the other hand, an active sulfurized α-olefin has 38.5 wt% total sulfur and 38.0 wt% active sulfur, which pushes its unpassivated copper rating to Class 3b. A full comparison against high-pressure SIB with 45 wt% total sulfur and 25 wt% active sulfur is available in our sulfurized isobutylene vs. sulfurized olefin specification comparison.

In a liquid gear oil, adding 0.05–0.15 wt% of a dimercaptothiadiazole (DMTD) metal deactivator or a benzotriazole/tolyltriazole passivator can hide active sulfur. That passivator film holds up during a short 3-hour ASTM D130 oil copper strip test. However, the film will break down once thermal oxidation consumes the passivator in service. The same failure shows up in lubricating greases during the 24-hour ASTM D4048 copper strip test. In a grease, the soap fibers of the thickener block liquid passivators from diffusing to the copper strip, so unpassivated polysulfides can form black Cu₂S corrosion.

Trap 3: Crankcase Phosphorus Budget Mathematics and Low-SAPS Limits

Industrial EP top-treats must never go into automotive crankcase engine oils. Active sulfur and chlorine will corrode copper-lead connecting-rod bearings in the ASTM D6594 high-temperature corrosion test. They also strip away the ZDDP film and poison three-way catalysts and diesel particulate filters. Current API SP and ILSAC GF-6/GF-7 passenger car engine oil rules cap phosphorus in the finished oil at 0.06–0.08 mass% (600–800 ppm). They also require ≥ 81% Sequence IIIH phosphorus retention under ASTM D8111 and ≤ 0.8 wt% sulfated ash in ACEA C-series low-SAPS oils:

A 1.0 wt% dose of Primary ZDDP with 7.20 wt% phosphorus already puts 720 ppm of phosphorus into the oil. Because of this ceiling, formulators must balance cold-start valvetrain protection against phosphorus volatility. This kinetic trade-off is detailed in our guide to primary vs. secondary alkyl ZDDP kinetics under ILSAC GF-6/GF-7. To stay within the limit, blenders cap mixed ZDDP at 0.65–0.90 wt% and add 0.3–0.6 wt% of molybdenum dithiocarbamate (MoDTC) or ashless dithiocarbamate (ADTC). That combination grows MoS₂ nanosheets inside the phosphate glass, cuts boundary friction by 30%, and passes Sequence IVB valvetrain wear under ASTM D8350 at ≤ 2.7 mm³.

Application-Specific Formulation Architecture & Five Shop-Floor Troubleshooting Cases

Industrial Hydraulic Fluids and Rotary Screw Compressor Oils

The metallurgy of the pump decides which additive a blender can use in industrial hydraulic oil formulations that meet Parker Denison HF-0 and German HLP standards. Steel-on-steel rotary vane pumps such as the Eaton Vickers 35VQ25A run at 210 bar. They rely on 0.4–0.6 wt% primary ZDDP to stop scuffing between the ring and the vanes. High-pressure axial piston pumps such as the Bosch Rexroth RDE 90235 at 350–450 bar and the Denison T6H20C have bronze slippers and silver-plated barrels. Those yellow-metal parts will corrode if they are exposed to active sulfur or unstable ZDDP. Cross-mixing a basic ZDDP and calcium-sulfonate hydraulic oil with an acidic ashless amine-phosphate fluid can also form ionic gel complexes. Those gels will plug 3–5 μm servo-valve filters in the ISO 13357-2 wet filterability test.

In rotary screw compressor oils, formulators leave out zinc additives so that hot discharge valves do not build up coking deposits. In one shop-floor case on an API Group II/III 150N compressor oil, the blender needed a non-seizure load PB ≥ 100 kgf and a wear scar ≤ 0.30 mm. At the same time, the oil had to keep a rotary pressure vessel oxidation life > 2,000 min under ASTM D2272. After testing ashless options against standard ZDDP and anti-wear additive components, the plant paired 0.30 wt% of an ashless amine phosphate with 200 ppm of an oil-soluble organomolybdenum complex. This blend held the wear scar at 0.30 mm and pushed PB ≥ 100 kgf through MoS₂ reinforcement without shortening oxidation life or darkening the oil.

Because polar amine phosphates and rust inhibitors are active at the oil-water interface, they narrow the water-separation window and make the oil sensitive to demulsifier over-dosing. In a related case, a PAO and synthetic ester hydraulic transmission fluid with a 7.8 wt% additive package failed ASTM D1401 water separation. When a technician dropped 1 undiluted drop of polyether demulsifier into 300 g of oil, that single drop flipped the interfacial tension and locked the oil into a permanent emulsion. To prevent this mistake in production, the plant set a strict shop-floor rule. The operator must pre-dilute 30 ppm of active polyether demulsifier in 1 kg of base oil mother liquor before it is sprayed into the batch.

Automotive Hypoid Axles, Industrial Gearboxes, and EV e-Axle Fluids

Gear oil formulations divide across extreme-pressure additive components based on sliding speed and whether yellow metals are inside the gearbox. Automotive hypoid axles run under API GL-5 and SAE J2360 at 3.0 GPa contact stress. They need 5.0–12.0 wt% of a sulfur-phosphorus package that has 2.5–3.5 wt% SIB and amine phosphates. That high dosage is required to pass CRC L-42 high-speed shock scoring, CRC L-37 high-torque wear, and a weld load ≥ 400 kgf in the four-ball test. However, if that same GL-5 axle oil is poured into an API GL-4 manual transmission, the active sulfur will form a brittle Cu₂S layer on the brass. Because that sulfide film is softer than the brass beneath it, the synchronizer rings will spall within 5,000 km.

By contrast, industrial enclosed gear drives that meet Siemens Flender Rev. 16 and DIN 51517-3 CLP rules use 1.2–2.5 wt% of an ashless sulfur-phosphorus package. This lower treat rate passes FZG scuffing stage ≥ 12, micropitting GFT-High ≥ 10, water separation ≤ 15 min, and 3-hour copper corrosion ≤ Class 1b. If the gearbox is a steel-on-bronze worm drive, the blender must switch to polyalkylene glycols or 5 wt% of an inactive sulfurized triglyceride to avoid wearing down the bronze wheel. Meanwhile, integrated electric-vehicle e-Axle fluids pair low-activity sulfur or ADTC with liquid triazole passivators so that conductive Cu₂S deposits cannot form and short out the copper motor windings.

Lubricating Greases: Solving Copper Corrosion and Thickener Shear Stability

Industrial extreme-pressure greases that meet DIN 51825 KP2K-20 and ISO 6743-9 standards operate below Λ = 1.0 and need strong anti-weld protection. An unfortified lithium 12-hydroxystearate base grease reaches only a 126 kgf four-ball weld point and less than 15 lbs in the Timken OK load test. To raise the four-ball weld load to 250–800 kgf, formulators blend in 2.5–4.0 wt% of a sulfur-phosphorus-zinc package, ADTC, or molybdenum disulfide. Another option is to use an overbased calcium sulfonate complex thickener that carries a native 500 kgf weld point.

In a shop-floor case on a white lithium grease, the formulator ran into a three-way conflict between the wear scar, the weld point, and the 24-hour copper corrosion test. Standard Primary ZDDP failed the 24-hour copper strip test, but cutting the ZDDP dose let the wear scar grow past 0.60 mm. Adding liquid DMTD or TTZ passivators also failed because they could not diffuse through the stiff soap fibers. Follow-up trials with dibutyl phosphite at 15 wt% P, acidic thiophosphate amine salts, and solid white EP agents all missed either the wear limit or the weld target. Switching to an amorphous potassium borate dispersion solved the problem. It delivered a 400 kgf weld load in the four-ball test, a low wear scar, and a Class 1a rating in the 24-hour copper test.

A second grease quality-control case cleared up supplier confusion around CAS 68649-42-3, which is Primary ZDDP supplied as 90% active zinc salt in 10% diluent oil. Lab checks confirmed that molybdenum dithiocarbamate (MoDTC) is a friction modifier and hindered phenols are antioxidants. Neither additive can replace a DMTD copper passivator in a 24-hour copper corrosion test for grease. In high-shear lithium and polyurea greases, ionic ZDDP can also break the hydrogen bonds between soap fibers and soften the worked cone penetration under ASTM D217. Switching to ashless aminothiophosphates and ADTC avoids that fiber breakdown and keeps the thickener stable under shear.

High-Speed Textile Spindle Oils and Bearing Superfinishing Fluids

In a failure case on a 20,000 rpm textile spindle oil, the plant used an ISO VG 10 fluid with a viscosity of 12 mm²/s at 40°C under ASTM D445. That original blend seized on the four-ball rig and left a 0.80 mm wear scar against a ≤ 0.40 mm customer limit. The formula had 0.25 wt% Secondary ZDDP, 0.10 wt% SIB, and 0.20 wt% of an oil-soluble benzotriazole amine salt. Lab tests revealed three mistakes working together. First, 0.25 wt% ZDDP was too weak to hold a phosphate film under high shear. Second, 0.10 wt% SIB caused mercaptan odor complaints in the shop without stopping wear, while 0.20 wt% of the polar benzotriazole amine salt crowded ZDDP off the iron surface.

To stop both the surface competition and the odor, the plant removed SIB from the formula. In its place, the blender put in 0.50 wt% of an ashless neutral/tricresyl phosphate ester that has 7 wt% phosphorus. This ester is rated for FZG stage ≥ 12 and a weld load ≥ 2,450 N in the four-ball test. At the same time, the blender doubled Secondary ZDDP to 0.50 wt% for fast film growth and cut the benzotriazole amine salt down to 0.05 wt%. Keeping low-odor sulfurized α-olefin only for shock-load grades brought the wear scar back within the ≤ 0.40 mm target.

In a fourth shop-floor case on a bearing steel superfinishing and honing oil that needed ≤ 15 min water separation, pilot runs proved out a four-part sulfur-phosphorus-zinc formula. The blend used 10 wt% of an inactive sulfurized fatty acid ester with a viscosity of 35 mm²/s at 100°C. That ester gave a non-seizure load PB ≥ 784 N and Class 1b copper protection, and it was paired with 3 wt% of an ashless amine phosphate ester at 20 mg KOH/g acid number and FZG ≥ 12. Adding 1.5 wt% primary-secondary ZDDP with an open-cup flash point ≥ 180°C strengthened the phosphate film. Finally, 1.0 wt% of a polyisobutylene anti-mist polymer and a pre-diluted polyether demulsifier kept water separation under 15 minutes.

Concentrated sulfur-phosphorus packages and low-viscosity superfinishing fluids have a high specific gravity, a low surface tension, and strong capillary creep. Because of these traits, thin 10–20 kg tinplate pails began to leak along their welded seams when stacked in ocean containers. To stop the leaks, the plant switched to thick-walled fluorinated high-density polyethylene jerricans for trial samples and palletized 200 kg UN-certified steel drums for bulk export orders.

Can AW and EP Additives Replace Each Other? Synergy, Surface Competition, and Storage Boundaries

Anti-wear and extreme-pressure additives work as partners rather than substitutes for one another. In a well-balanced sulfur-phosphorus package, the organophosphorus AW agent and the organosulfur EP carrier react together to build a ductile mixed iron thiophosphate and iron sulfide FeSₓPᵧO₂ eutectic film. That combined film handles both steady wear at 80–180°C and sudden shock loads above 250°C. A pre-formed ZDDP polyphosphate film also passivates steel and copper surfaces. This pre-film softens the attack of active sulfur in 3-hour copper strip tests and improves a Class 4c blend to 3b or a Class 3a blend to 1b.

However, putting in too much of a polar additive will set off fierce competition for the metal surface. If a blender over-doses polar amine phosphates, sulfonate or carboxylic acid rust inhibitors, or ZDDP, those polar molecules will crowd non-polar SIB out of the iron sites. That surface crowding leads to high-speed hypoid gear scuffing in the CRC L-42 bench test. In the warehouse and the blending kettle, ZDDP must stay below 50–60°C and steam coils must stay below 85°C. If the drum gets too hot, the additive will break down into toxic H₂S gas, foul mercaptans, and sticky zinc pyrophosphate sludge. Amine phosphates must also stay sealed in dry drums so moisture cannot hydrolyze the ester and raise the ASTM D664 acid number.

RFQ Technical Checklist

To select an anti-wear or extreme-pressure additive or to set up a custom sulfur-phosphorus ratio, send the four operating details below to the CheMost formulation engineering team. With these baseline numbers in hand, the engineering team can match the right COA and TDS, verify 150°C active sulfur, and arrange third-party SGS or Intertek inspection before shipment.

Frequently Asked Questions (FAQ)

Are EP and anti-wear additives the same thing?

No, anti-wear and extreme-pressure additives work in different contact-pressure and flash-temperature zones. Organophosphorus AW additives such as ZDDP and amine phosphates build a 20–150 nm polyphosphate glass film when contact stress stays below 1.5 GPa at 80–180°C. That glass film keeps the wear scar small during normal operation. By contrast, organosulfur EP carriers split apart during heavy shock loads above 1.5 GPa and 220–800°C flash temperatures. They react with iron to form a sacrificial FeS layer that raises the weld load in the four-ball test and stops metal seizure.

Can I use an EP additive instead of an anti-wear additive in hydraulic oil?

No, active sulfur EP additives must never take the place of anti-wear agents in industrial hydraulic fluids. During steady moderate-load pump operation, active sulfur causes rapid chemical-mechanical polishing wear. It also eats away at the bronze slippers and silver-plated barrels inside axial piston pumps. Over time, this reaction ruins hydrolytic stability and creates corrosion particles that can plug 3–5 μm servo-valve filters.

Is ZDDP the same as zinc?

No, ZDDP is not metallic zinc dust or elemental zinc. As explained in our guide on zinc dialkyldithiophosphate (ZDDP) fundamentals, ZDDP is an oil-soluble organometallic compound with CAS 68649-42-3 where zinc is bonded to dialkyldithiophosphate groups. The protective polyphosphate glass film comes from its 7–8 wt% phosphorus and 14–16 wt% sulfur reacting on the iron surface rather than from metallic zinc by itself.

What test numbers should I ask for when buying an EP or anti-wear additive?

Buyers should ask the supplier for a complete set of elemental and bench test results on both the COA and the TDS. The key numbers to check are the mass percentages of P, S, Zn, and N, along with the active sulfur content at 150°C. It is also important to check the copper strip corrosion ratings in both the 3-hour oil test and the 24-hour grease test. Formulators should verify the wear scar diameter, the weld load and load-wear index in the four-ball test, and the FZG gear scuffing and micropitting stages at the planned treat rate.

Do greases use the same EP additives as gear oils?

Sulfur-phosphorus-zinc packages and sulfurized olefins are used in both products, but lubricating greases have stricter limits on odor and how liquid additives move through the soap network. Open-gear and bearing greases usually replace smelly SIB with low-odor sulfurized α-olefins, ashless dithiocarbamates, or potassium borate dispersions. Greases also cannot rely on liquid DMTD or TTZ copper passivators alone to pass the 24-hour copper strip test. That is because the thickener soap fibers block liquid passivators from reaching the metal surface.

What drives the cost difference between EP and anti-wear additives?

The price per kilogram of an additive and its true cost per metric ton of finished lubricant often move in opposite directions. Ashless triphenyl phosphorothioate, neutralized amine phosphates, and sub-micron potassium borate dispersions cost much more per kilogram than commodity sulfurized isobutylene or primary ZDDP. However, the cost in the finished oil balances out. Ashless AW agents are dosed at only 0.15–0.50 wt%, whereas bulk sulfur EP carriers need a 1.0–5.0 wt% treat rate.