Sulphur Preparations based on insoluble sulphur

Insoluble Sulphur

Insoluble sulphur is primarily used in the rubber industry. Its advantage over ground natural sulphur is that it does not cause bloom, and that it does not dissolve and migrate in rubber. The S8 ring molecules present in soluble sulphur at room temperature open at temperatures above 158 °C, allowing chains of sulphur to combine with one another to form polymeric sulphur. Insoluble sulphur is produced by quenching the polymeric melt at low temperatures, preventing the reversion into S8 rings, leaving the polymer structure meta-stable at roomtemperature.

Polymeric sulphur is insoluble in organic solvents and elastomers. In a rubber compound it is suspended, similar to inert filler, though exhibiting thermoplastic behaviour, softening to a degree at processing temperature.

Most insoluble sulphurs commercially available contains over 90 % polymeric sulphur and less than 10 % soluble sulphur. These products often have very fine particles that tend to agglomerate under the influence of a static charge, which are in turn difficult to disperse in rubber. That insoluble sulphur is more likely to become electrically charged than soluble sulphur is well illustrated when insoluble sulphur is added to a gum stock on a two roll mill and sulphur particles shoot up above the nip.

Following Struktol products are sulphur preparations based on insoluble sulphur:

  • Struktol® SU 50
  • Struktol® SU 109
  • Struktol® SU 135
  • Struktol® SU 135 GR 75

For this reason, maintaining a fine particle size is imperative for a uniform distribution of polymeric sulphur as an insoluble compounding material, though this makes it more difficult to achieve excellent dispersion in rubber. These very problems, i.e. the build-up of static charge, dispersion difficulties and explosion risks (since polymeric sulphur is an extremely dusty material) have lead to our development of preparations that address these concerns and that work in concert with very high efficiency dispersing agents.

It is often possible to work with a mixture of soluble and insoluble sulphur in order to take advantage of the limited solubility of sulphur in a rubber compound. For example, Struktol SU 135 is a 1:1 combination of both modifications and offers the industry's best compromise between bloom, dispersability and cost for the use in technical rubber articles.