Wednesday, 2 July 2014

Sulphur - part of Itac's cornerstone technology

Elizabeth Henderson
Product Development Manager
Sulphur is this month’s topic.  It occurs naturally as the yellow element and is widely used in the chemical industry. Sulphuric acid consumption per capita is used as an index of industrialisation in the same way as titanium dioxide consumption.  Like carbon, discussed in Itac’s first technical blog, sulphur demonstrates the property of allotropy with dozens of stable crystal forms, but unlike carbon these different structures do not influence the way sulphur is used by us. Here at Itac sulphur is part of our cornerstone technology – rubber processing.  Organic sulphur-containing compounds such as diphenylthiourea or (straight-chain example) tetra methyl thiuram disulphide are mixed with the rubber in the milling stage, as an accelerator for elemental sulphur which is also in the mix.  As Itac’s focus is on putting rubber into solution, the mix has to be processed carefully at controlled temperatures or the hydrocarbon-sulphur bonds become too numerous and the rubber will not dissolve. This phenomenon is known as ‘scorching’. Rubber compounding does not rely solely on sulphur – the pre-milling stage incorporates pigments, fillers, oils, stearic acid and zinc compounds. As discussed in an earlier blog, zinc plays a vital role in this process. The formulation determines the final mechanical and chemical properties of the finished rubber article – for example, incorporating carbon black at this stage hardens and strengthens the finished article.
Sulphur also makes a valuable contribution in our coloured materials. We use ultramarine powder to give a blue colour to some products. Although the original ultramarine pigment was powdered lapis lazuli, modern ultramarine is made by heating powdered sulphur, sodium sulphate and sodium carbonate in the presence of iron-free clay and a reducing agent such as coal or pitch. The beautiful blue colour arises because of the S8 groups caught in the aluminosilicate cage.
An area of adhesive technology exploiting sulphur’s chemistry is in crosslinking epoxies. Although the epoxy link will cross-link with amine, this reaction generally requires high temperature for initiation. A mercaptan group in the mix catalyses the reaction by reacting with the amine to form a mercaptide ion, which readily opens the epoxy link. The low activation energy for this process means the reaction can occur at room temperature, allowing epoxy technology to be used on heat-sensitive substrates.

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