Sulfurized Isobutylene 101: Things You Should Know

In the gear lubricant formulation, sulfurized isobutylene (SIB) is almost a "standard component". But why do some SIBs have a strong odor, while others have almost no odor? Is it a safe chemical? Here, we talked about some key points about this typical EP additive - sulfurized isobutylene.

 

1. What Is Sulfurized Isobutylene (SIB)?

 

Sulfurized isobutylene (SIB), also known as T321 (Chinese Lube Additive Product Code), is an organic sulfur-based EP/anti-wear additive. Its main component is di-tert-butyl polysulfide, with a simplified structure :

 

R-S_x-R

 

(R = tert-butyl, x = 1–6).

 

 

SIB appears as a light yellow to dark orange transparent liquid with a mild odor. Density ranges from 1.0 to 1.1 g/cm³. It offers excellent solubility in mineral oils, synthetic hydrocarbons, and ester base oils.

 

Sulfur content, typically between 40% and 46%, is a critical performance indicator. While higher sulfur levels generally enhance EP performance, excessive sulfur can compromise oil solubility and increase odor.

 

Common Trade Names of Sulfurized Isobutylene (SIB)

Chinese Product Code: T321

International equivalents:

• Ang 33
• Hit 312
• LZ6901A

 

 

2. How Does SIB (as an extreme pressure EP Additive) Prevent Metal Abrasion?

There are 3 classic operating conditions in the lubrication system: fluid lubrication, mixed lubrication, and boundary lubrication.

 

Under normal operating conditions, the oil film is sufficiently thick, preventing direct metal contact. However, under high-load, low-speed, or impact load conditions - such as at the moment of gear meshing, when the oil film breaks down, and metal comes into direct friction, extreme pressure additives come into play.

 

 

The EP protection of SIB is divided into 3 levels:

 

• Under low loads, molecules adsorb onto the metal surface, forming a physical protective layer that reduces minor friction.
• Under moderate loads, sulfur atoms undergo chemical reactions with the iron surface, forming a protective layer of iron sulfide (FeS), which has a moderate hardness, isolating the metal and preventing it from fragmenting under impact.
• Under high-load impact, the protective film is repeatedly destroyed and regenerated - this dynamic process maintains continuous extreme pressure protection.

 

3. What are the main applications of sulfurized isobutylene?

 

Vehicle gear oil GL-5 is the most typical application for sulfurized isobutylene.

The GL-5 specification (MIL-PRF-2105E) is currently one of the most stringent vehicle gear oil standards globally, requiring bench tests to be passed under both high-speed impact and low-speed high-torque extreme conditions. The treat rate of T321 in the GL-5 formula is generally between 0.5% and 1.5%, and the specific dosage depends on the base oil type and the formula's compatibility with other additives.

 

Sulfurized isobutylene (SIB) works synergistically with ZDDP (zinc dialkyldithiophosphate): SIB delivers extreme pressure performance, while ZDDP provides anti-wear protection. The two complement each other, and this combination is a typical EP gear oil formulation.

 

Industrial gear oil is another important application area.

Closed industrial gearboxes with a viscosity range of ISO VG 220~680 typically require evaluation according to AGMA 9005 or American Steel 224 specifications. These specifications have clear requirements for extreme pressure and anti-wear performance, and SIB is one of the common solutions.

 

Application of SIB in lubricating grease.

SIB is also used in grease, e.g., composite lithium grease, composite calcium sulfonate grease, polyurea grease, at a dosage of 1%~2%. It is normally used in combination with organic molybdenum or phosphate esters.

 

4. Why does the SIB have a strong odor?

 

The odor of sulfurized isobutylene has always been a drawback. That smell of SIB compounds can make an unpleasant experience, especially when handling the lubricants with SIB as a main component in an enclosed room.

 

Years ago, some traditional lubricant professionals in the industry would judge the quality of a gear oil simply by smelling it - the stronger the odor, the more "authentic" they thought the gear oil additive was.

With growing awareness of environmental and health concerns, low-odor SIB naturally becomes more desirable in the lubricant market.

So where does the unpleasant odor actually come from?

It mainly comes from by-products during the production of SIB - specifically, mercaptans and thiones.

Actually, The core component of SIB - di-tert-butyl polysulfide (R-S_x-R) - does not have a strong odor due to its high molecular weight (low volatility). However, mercaptans and thiones are highly volatile small-molecule chemicals. Even at very low concentrations in the SIB product, they produce a strong unpleasant odor.

Currently, high-pressure method is the the mainstream production process for SIB in China. This process achieves a higher production rate, and as the same time, by-products are fewer. Combination of a deodorization step, the unpleased smell chemical - mercaptan (mainly tert-butyl mercaptan) is reduced a lot, and we have a low- odor sulfurized Isobutylene.

 

5. Is it sulfurized isobutylene toxic?

 

Paracelsus said: "Alle Dinge sind Gift, und nichts ist ohne Gift; allein die Dosis macht, dass ein Ding kein Gift ist."

In English "All things are poison, and nothing is without poison; only the dose makes a thing not a poison."

 

Many people feel uneasy when they hear "sulfide chemicals". When they smell the unpleasant odor of sulfurized isobutylene, and it's easy to assume SIB must be toxic. But what do the facts say?

 

The LD ₅₀ of sulfurized isobutylene (SIB) is about 5980 mg/kg, which is at the low toxicity level and is actually safer than the salt (NaCl) we eat every day.

 

Substance	LD₅₀ (Oral, Rat, mg/kg)
Sulfurized Isobutylene (SIB)	~5,980
Table Salt (Sodium Chloride)	~3,000
Chlorinated Paraffins	1,000–2,000

 

6. What is the quality level of China-made sulfurized isobutylene today?

 

China's sulfurized isobutylene (SIB) industry has made significant progress in recent years.

Traditional SIB production processes (chlorine + sulfur route) produced products with high impurities and strong odor. However, with process improvements - especially the high-pressure process, China-made SIB has achieved purity, stability, and odor control levels close to those of major international brands. T321 is now being used in applications such as wind turbine gear oils and other industiral lubricants.

 

However, product performance variations still exist among suppliers. When selecting a SIB, it is recommended to evaluate:

• Test reports
• Batch-to-batch stability data

Using these factors together will help you make a more informed decision.

 

Chours' sulfurized isobutylene (SIB), known for its stable quality and excellent extreme pressure performance, is exported in large volumes to high-end lubricant markets including the United States, where it has earned wide customer recognition. Get in touch.