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T321 Sulfurized Isobutylene
CAS No: 68511-50-2
Description：
Factory-direct SIB/T321 extreme pressure additive. 45% sulfur, chlorine-free. CAS 68511-50-2. Weld load 650 kg. For gear oils and metalworking fluids. Request TDS.
Category：
Extreme Pressure (EP) Additive
Product Details
Quick Specs
| Appearance | Light yellow liquid |
| CAS Number | 68511-50-2 |
| Sulfur Content | 45% |
| Weld Load (PD) | 650 kg |
| Density at 20°C | 1005 kg/m³ |
| Viscosity at 40°C | 3.5 mm²/s |
| Flash Point | 100 °C |
| Copper Corrosion (100°C, 3h) | 2e (ASTM D130) |
| Chlorine Content | <30 ppm (chlorine-free process) |
| Packaging | 200 kg metal drum / 1000 kg IBC tote |
| MOQ | 1 drum / 1 IBC |
| Lead Time | 7–15 days after order confirmation |
What is Sulfurized Isobutylene?
Sulfurized isobutylene — SIB, or T321 — is the standard extreme pressure additive for automotive and industrial gear oils. 45% sulfur. Zero chlorine. It's held that position since the 1940s. For a quick overview, see what is sulfurized isobutylene.
The molecule is a dialkyl polysulfide: isobutylene units linked by di-, tri-, and tetrasulfide bridges. Short bridges release sulfur slowly for long-term wear control. Longer bridges break down fast when gear mesh temperature spikes. The distribution between these bridge lengths — not just total sulfur — determines behavior in service.
CheMost produces SIB via high-pressure H2S synthesis — elemental sulfur and isobutylene at 50–60 bar with H2S as the reducing agent. No chlorine in the process at any stage. The result is a light yellow, oxidatively stable liquid. Dark sulfurized fats — cheaper, uncontrolled, still sold as alternatives — cannot match this on stability or emissions.
Sulfurized Isobutylene Structure & Chemistry
SIB is nonpolar. That single word explains most of how it behaves in a formulation.
The isobutylene backbone carries no oxygen, no ester groups, no reactive functionality beyond the sulfur bridges. This creates a defining trade-off: unlimited oil solubility, zero lubricity contribution. SIB dissolves cleanly in Group I, II, and III base stocks above 5% treat rate — no haze, no precipitate. But under low-load conditions it contributes nothing to boundary friction. Gear oil formulators pair SIB with fatty esters or sulfurized triglycerides: SIB handles the EP, the co-additive handles the lubricity. Each does what the other can't.
Active vs. Inactive Sulfur — know the difference:
- Active sulfur (ASTM D1662) reacts at low temperature. Fast EP film formation. Stains copper. Pentasulfides hit 90% active.
- Inactive sulfur needs high temperature to decompose. Safe for yellow metals. Triglyceride carriers sit at <5% active.
- SIB lands in the middle: 45% total sulfur, ~25–35% active. Fast EP response without the uncontrolled copper staining. This is the engineered center ground for gear oils.
Technical Specifications
| Property | Unit | Typical Value | Test Method |
|---|---|---|---|
| Appearance | — | Light yellow liquid | Visual |
| Density at 20°C | kg/m³ | 1005 | ASTM D4052 |
| Viscosity at 40°C | mm²/s | 3.5 | ASTM D445 |
| Flash Point | °C | 100 | ASTM D93 |
| Sulfur Content | wt% | 45 | ASTM D4951 |
| Copper Corrosion (100°C, 3h) | rating | 2e | ASTM D130 |
| Weld Load (PD) | kg | 650 | ASTM D2596 |
| Recommended Treat Rate | % | 1.0–5.0 | — |
| Chlorine Content | ppm | <30 (chlorine-free process) | — |
* All values are typical data from published CheMost specifications. Batch-specific COA available on request.
SIB Extreme Pressure Additive Features
45% Sulfur Content
Highest sulfur loading of any commercial extreme pressure additive class. A 3% treat rate delivers 1.35% sulfur to the finished oil — enough to prevent welding under hypoid gear contact pressures.
Chlorine-Free by Process Design
No chlorine at any stage: no S2Cl2, no chloride residues, no HCl formation. Meets the <30 ppm automotive gear oil spec without post-reaction washing. This is process chemistry, not a marketing claim.
3.5 mm²/s at 40°C
Pours at ambient temperature. No heated storage, no pre-warming before blending. For plants running 50 drums a day, that's real time saved.
Light Color = Quality Signal
Saturated structure from high-pressure synthesis: zero residual double bonds, no chromophoric thioketones, no continued polymerization in storage. Dark sulfurized fats look that way for a reason — uncontrolled side reactions.
2e Copper, Balanced
Safe for yellow metal components at standard treat rates. For complete passivity, 1–2% ZnDDP co-additive suppresses copper corrosion through a documented synergistic mechanism.
PEP Synergy with Overbased Sulfonates
Active sulfur + overbased Ca/Na sulfonates = load-carrying beyond what either additive achieves alone. First documented in the 1980s, now standard practice replacing chlorinated paraffins in heavy-duty metalworking.
Sulfurized Isobutylene Synthesis
There are two ways to make sulfurized olefins. One is cheap and dirty. One works. CheMost runs the one that works, at our Jinzhou plant with 20+ reactors, 20,000 tons annual capacity, and 70+ QC checks per batch.
Stage 1 — High-Pressure H2S Synthesis
Isobutylene + elemental sulfur are charged into a high-pressure reactor with H2S gas at 50–60 bar, 120–170°C, catalyzed by organic amines. Under these conditions, H2S adds cleanly across the isobutylene double bonds to form mercaptan intermediates. The mercaptans react with sulfur in a redox reaction, building di-, tri-, and tetrasulfide bridges while regenerating H2S — a near-closed loop. No oxidative attack on vinylic C–H bonds. No residual unsaturation. No thioketone formation.
The Difference: High-Pressure vs. Atmospheric "Black" Sulfurization
Atmospheric sulfurization (still used for cheap dark sulfurized fats) heats sulfur flower with olefins without H2S. Sulfur oxidatively attacks vinylic C–H bonds — uncontrolled. The product is black, smelly, with residual double bonds that keep polymerizing in storage. High-pressure H2S process flips the chemistry: H2S acts as reducing agent, adds cleanly to double bonds, produces a saturated, light-colored, stable product. Higher equipment cost, much better result.
Stage 2 — Finishing & Quality Control
Residual H2S and light ends are vacuum-stripped. Product is filtered and packaged. Every batch runs through 70+ QC checks: sulfur content by ASTM D4951, viscosity by D445, flash point by D93, copper corrosion by D130, weld load by D2596. 20+ pieces of testing equipment. Lot-to-lot consistency is the metric that matters.
Applications
Automotive Gear Oils
The reason SIB exists. Hypoid and spiral bevel gear lubricants have relied on it since the 1940s. Treat rate: 2–5% in finished oil, delivered as part of a complete gear oil package at 5–12% total. The moderate active sulfur fraction balances immediate EP protection during run-in with long-term bearing wear control.
Industrial Gear Oils
Treat rate: 1–3%. Chlorine-free chemistry is critical here — vented industrial sumps circulate HCl from chlorine-containing EP additives through the entire system. For open gears on mining mills and kilns, combine SIB with overbased calcium sulfonate for PEP synergy.
Metalworking Fluids
Treat rate: 1–5% in neat oil. Broaching, gear hobbing, deep drawing stainless — operations where built-up edge kills tool life. One caveat: machining brass or copper? Use a metal deactivator or switch to an inactive sulfur carrier.
Greases
Treat rate: 1–4% in enclosed systems — CV joints, wheel bearings, sealed gearboxes. SIB + MoDTC is the proven combination: molybdenum provides the MoS2 film, SIB supplies reactive sulfur to keep it regenerating under load.
Gear Oil Additive Packages
SIB is typically delivered to formulators as part of a complete gear oil additive package rather than as a standalone component. The package handles everything — EP, AW, corrosion inhibition, foam control — at 5–12% total treat rate. CheMost supplies both the single component (SIB/T321 neat) and fully formulated gear oil packages built around it.
Compatibility & Blending
Base Oils
Group I, II, III mineral oils; PAO; synthetic esters. Unlimited solubility at all practical treat rates. No haze, no precipitation.
Strong synergy. 1–2% ZnDDP co-additive suppresses copper corrosion and improves odor. Active sulfur carriers shift from 4c to 3b in ASTM D130 with 1.5% ZnDDP.
Overbased Sulfonates
PEP synergy — enhanced EP in metalworking and open gear applications. Active sulfur + overbased Ca/Na sulfonate outperforms either alone.
Blend both for tailored EP/copper profiles. SIB (C4, 45% S) for maximum EP. T2040 (longer chain) for lubricity and low-temp. Many formulators use both.
Fatty Esters / Triglycerides
SIB handles EP. Esters handle boundary friction. Together they outperform either alone. Standard pairing in gear oil and metalworking formulations.
Not Compatible With
Strong acids and bases. Direct contact with yellow metals (copper, brass, bronze) without passivation. Use ZnDDP or benzotriazole deactivator.
Active vs. Inactive Sulfur — Why Your Copper Strip Test Matters
Here's something the sulfur content number alone won't tell you.
High Activity = Fast EP, Copper Risk
Dialkyl pentasulfides at 90% active sulfur form EP films instantly. Great for total-loss metalworking where tool protection is everything and the fluid gets changed every shift. Put this in a gearbox and the bronze bushings dissolve.
Low Activity = Copper Safe, Slower EP
Sulfurized triglycerides at <5% active sulfur are safe for yellow metals at any treat rate. But the four-ball weld load is half what SIB delivers. Fine for moderate-duty hydraulics, not enough for hypoid gears.
SIB splits the difference. 45% total sulfur, ~25–35% active. Fast enough to prevent gear scoring under shock loads. Controlled enough to pass ASTM D130 at standard treat rates. A gearbox contains steel gears and, often, a bronze bushing in the same oil bath. A lubricant that protects the gears while dissolving the bushing is a warranty claim, not a working fluid. SIB's engineered center-ground activity profile is why it's still the default EP additive for filled-for-life gearboxes after 80 years.
Synonyms & Regulatory Information
| Common Name | Sulfurized Isobutylene (SIB) |
| Grade Code | T321 |
| CAS Number | 68511-50-2 |
| HS Code | 3811.90.00 |
| Synonyms | Sulfurized isobutene, Diisobutylene polysulfide, Sulfurized C4 olefin, SIB EP additive, T321 |
| REACH | Registered |
Documentation
| Document | Description | Download |
|---|---|---|
| Technical Data Sheet (TDS) | Complete physical and chemical specifications | Download PDF |
| Material Safety Data Sheet (MSDS) | Safety, handling, storage, and regulatory classification | Download PDF |
| Certificate of Analysis (COA) | Batch-specific quality test results — issued per production lot | Request via Email |
Need TDS/MSDS for a specific shipment? Mention your order number — documents will be sent within 6 hours.
Frequently Asked Questions
Ready to Evaluate SIB for Your Gear Oil Formulation?
We ship free 1 kg samples with TDS and MSDS within 3 working days. Tell us your base oil type and whether copper passivity or maximum EP is your priority.
Request Free Sample & TDS ⟶Last updated: May 6, 2026
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