Abstract
It is widely acknowledged that waste sulfur generated from the petroleum industry creates huge storage and ecological problems. Therefore, the various methods of utilization are becoming increasingly attractive research topics worldwide. The thermal ability of elemental sulfur to homolytic cleavage of S8 rings enables its free radical copolymerization with unsaturated organic species and the obtaining of chemically stable polymeric materials. Here we report a novel possibility to use sulfur/organic copolymers obtained via “inverse vulcanization” as curatives for rubber. For this purpose, several various sulfur/organic copolymers were synthesized and analyzed from the point of view of their performance as rubber crosslinking agents. Solvent extraction was used to purify sulfur/organic copolymers from unreacted (elemental) sulfur. Thermal properties of the prepared copolymers were characterized by thermogravimetric analysis and differential scanning calorimetry (TGA–DSC). Crosslink density and structure of cured elastomers was studied by equilibrium swelling, thiol-amine analysis and freezing point depression. Mechanical properties of the vulcanizates were determined under static and dynamic conditions (DMA—dynamic mechanical analysis). It is proved that the utilization of sulfur/organic copolymers as curatives enables an effective crosslinking process of rubbers. Taking into account the results of a crosslink density analysis and mechanical properties of the vulcanizates cured with purified copolymers, it is evident that relatively long copolymer macromolecules are also involved in the formation of chemical bonds between unsaturated rubber macromolecules.
Highlights
Elemental sulfur occurs in nature in a pure form or in a form of various sulfur-containing minerals, but it is a by-product from petroleum industry, mainly from hydro-desulfurization process
After solvent extraction of unreacted sulfur in the form of S8 the remained amorphous polymeric phases can be used as curatives for rubber
The comparison of crosslinking with sulfur/organic copolymers before and after the purification procedure taking into account crosslink density values, provided an unambiguous information about participation of sulfur/organic macromolecules in the rubber curing process
Summary
Elemental sulfur occurs in nature in a pure (native) form or in a form of various sulfur-containing minerals (sulfides or sulfates), but it is a by-product from petroleum industry, mainly from hydro-desulfurization process. Enormous amounts of sulfur are generated annually as waste. The necessity for sulfur removal in petroleum refining processes is strictly related to rigorous requirements for sulfur content in diesel fuels (even up to 5 ppm) [1,2]. A very promising and environmentally beneficial approach to utilize waste sulfur is associated with its thermal behavior: the ability to transformation into chemically stable polymeric materials [2]. The most thermodynamically stable form of sulfur under ambient conditions is orthorhombic sulfur (Sα ), present in the form of eight-membered rings (S8 or cycloocta-S).
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