Insoluble sulfur: why, how, how much?
How insoluble sulfur prevents blooming, IS-60/IS-90 and OT20 types, mixing temperature, storage and sulfur equivalent calculation.
The problem: sulfur blooming
Normal (rhombic) sulfur dissolves in rubber in limited amounts. More dissolves at mixing temperature; when the compound cools, the excess crystallizes and comes to the surface. This is called blooming.
In tire building, semi-finished plies are stuck to each other. Sulfur crystals on the surface impair this adhesion (tack) and can lead to separation between plies.
The solution: polymeric sulfur
Insoluble sulfur is a polymeric form in which sulfur atoms make long chains, and it does not dissolve in rubber. It stays dispersed in the compound as fine particles and does not come to the surface on cooling.
At vulcanization temperature, polymeric sulfur converts to normal sulfur and reacts in the usual way. There is no difference in the cured product; the difference is during processing.
Types
The number in the type name shows the percentage of insoluble sulfur in the total sulfur: IS-90 contains at least 90%. Oil-treated types (OT, 'oil treated') reduce dust and ease dispersion into the compound; OT20 contains 20% oil.
High thermal stability (HS) types are more resistant to conversion into normal sulfur during mixing.
Mixing and storage
Insoluble sulfur reverts to normal sulfur over time with heat. For this reason it is added in the final mixing stage and at low temperature (below about 100 °C).
Cool storage and stock rotation are important. Amines and basic substances can speed up the conversion; raw materials should be kept apart.
Dosage calculation
The sulfur amount in the compound formulation is usually written as pure sulfur. If an oil-treated type is used, the product amount is increased accordingly: for OT20, 80% of the product is sulfur.
Example: if the compound formulation calls for 2.0 phr sulfur, add 2.0 / 0.80 = 2.5 phr of OT20 type.
| Type | Insoluble sulfur (in total sulfur) | Oil | Sulfur content (in product) |
|---|---|---|---|
| IS-90 | ≥ 90% | None | ~100% |
| IS-90 OT20 | ≥ 90% | 20% | ~80% |
| IS-60 | ≥ 60% | None | ~100% |
Substances in this guide
CAS 9035-99-8CAS 95-31-8CAS 95-33-0CAS 4979-32-2