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Elizabeth Henderson
Product Development Manager
ITAC Ltd
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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.
