Sulfurized isobutylene EP additive — the product family commonly referenced in China as T321 — is one of the most effective extreme-pressure chemistries available for heavy-duty metalworking. When a tap, broach, or gear-cutting tool meets a workpiece under high load and high temperature, ordinary lubricating films fail in milliseconds. The sulphur in T321 does not wait for that failure: it reacts with the freshly cut metal surface to form an inorganic sulphide film that prevents welding and seizure. This article explains how sulfurized isobutylene works, where it outperforms other EP additives, and what to check when specifying it.
What Is Sulfurized Isobutylene (T321)?
Sulfurized isobutylene is a sulphurised olefin made by reacting isobutylene oligomers with elemental sulphur under controlled conditions. The result is an oil-soluble, high-sulphur-content additive — commercial grades typically carry 40% or more total sulphur, with a significant share present as active sulphur capable of reacting with metal surfaces at elevated temperature.
Its importance in modern metalworking fluids has grown sharply. For decades, chlorinated paraffins were the default heavy-duty EP additives, but their environmental and disposal problems have pushed formulators toward sulphur-based alternatives. Sulfurized isobutylene delivers similar load-carrying performance without chlorine, which is why it now anchors most high-performance cutting and forming oil packages.
How the Sulphur Film Works
Extreme-pressure protection is fundamentally different from anti-wear protection. Anti-wear additives such as ZDDP form protective films at moderate temperatures through milder chemistry. EP additives must handle the far harsher regime where the oil film has already collapsed — temperatures at the tool-chip interface can exceed several hundred degrees, and local pressures are extreme.
Under those conditions, the active sulphur in T321 decomposes and reacts with the iron surface to form a layer of iron sulphide. Iron sulphide has a lower shear strength than the steel itself, so it acts as a solid lubricant: instead of the tool and workpiece welding together, the sulphide layer shears preferentially, allowing the cut to proceed. The film is consumed and re-formed continuously, which is why a continuous supply of active sulphur matters — and why treat rates and replenishment are set carefully in circulating systems.
Active Sulphur vs Inactive Sulphur
Not all sulphur in a sulfurized isobutylene EP additive behaves the same way. The distinction between active and inactive sulphur is central to formulation:
- Active sulphur reacts with metal at the temperatures reached during heavy cutting, providing EP performance — but it can also stain yellow metals such as copper and brass at lower temperatures.
- Inactive (or “passive”) sulphur is bound more strongly in the molecule and only becomes reactive at much higher temperatures, giving milder EP behaviour with less risk of copper corrosion.
Formulators balance the two: a high active-sulphur grade for tough operations on ferrous metals, and a lower-activity grade where copper-bearing alloys are present in the system. The ASTM D130 copper strip test and the four-ball weld load test are the two specification checks that quantify this balance, alongside total sulphur content.
Where Sulfurized Isobutylene Shines
Sulfurized isobutylene EP additive is the first choice for the most demanding metalworking operations:
- Tapping, threading, and broaching — operations with poor chip clearance where seizure risk is highest.
- Gear cutting and hobbing — where the tool form must be preserved over long runs.
- Deep-hole drilling and reaming — high heat generation, continuous film replenishment required.
- Stamping and severe forming — where galling on die surfaces must be prevented.
It also performs in gear oils, often blended with phosphorus and other sulphur carriers, though gear-oil formulations typically favour lower-activity sulphur sources to protect synchronisers and yellow-metal components.
Copper Corrosion, Compatibility, and Formulation Balance
The main constraint on T321 use is yellow-metal compatibility. In neat cutting oils, operators must know whether the sump, pump, or any component contains copper alloys; if it does, the active-sulphur content must be limited or a copper passivator added. The same logic applies to the oil itself — high-sulphur grades can darken oils over time and may interact with other additives, so stability testing in the final formulation is essential.
Practical specification checks when purchasing sulfurized isobutylene EP additive include: total sulphur content, active sulphur level or copper-strip corrosion rating, viscosity at 40°C, acid number, and weld load from the four-ball test. Reputable suppliers state these values on the certificate of analysis and keep batch-to-batch sulphur content tight — drift in sulphur content translates directly into drift in field performance.
Conclusion
Sulfurized isobutylene EP additive (T321) remains the workhorse of chlorine-free extreme-pressure chemistry: a high-sulphur, oil-soluble additive that forms sacrificial iron sulphide films at the tool-workpiece interface, protecting taps, broaches, and gear tools through the most severe operations. The active-versus-inactive sulphur balance determines where each grade fits, and copper-strip and four-ball tests turn that balance into numbers a buyer can verify. For a broader view of metalworking oil formulations and cutting-oil additive systems, see our guides on cutting oil additives and additives for metalworking lubricants.