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Elizabeth Henderson
Product Development Manager ITAC Ltd |
Tin is one of the elements which have been in use longer
than we know. At Itac it plays a key
behind-the-scenes role, as dibutyl tin dilaurate is a catalyst in the
manufacture of polyurethanes. We use these polyurethanes as components of our
formulated coatings and adhesives. We also use dibutyl tin dilaurate in some of
our formulations as a curing agent, but environmental concerns about the use of
tin salts mean that we are phasing it out wherever possible. A demonstration of
the powerful life-system effects of tin compounds is the sex-change effect of
tri butyl tin (TBT) on marine fauna in coastal waters. TBT is an extremely
effective biocide and was applied to boats to prevent the accumulation of
plants and animals on the bottom. Leaching of TBT from the film, and overspray
and waste from the coating processes meant that there was sufficient in the sea
to transform female dog whelks into males. Its use has now been superseded in
this application by copper compounds.
Tin was a vital component of early antibiotics for human use
– ‘Stannoxyl’ was formulated using metallic tin and tin oxide, and was used in
experiments on lung infections by being given as pills. It was later used in an
ointment for acne and boils.
The primary ore of tin is cassiterite, whose name is derived
from the Greek word for tin. It forms tetragonal crystals, space group P42/mnm.
The material is a member of the
rutile group, which is named after the titanium dioxide discussed in April 2014.
It is very hard (hardness 7, cf agate
7, diamond 10) and thus survives in alluvial placer deposits, sometimes in
large enough quantities to be commercially exploitable. Generally the
appearance of the crystals is too poor to allow it to be cut into jewellery,
but some material is mined for this purpose in Bolivia.
The process developed at Pilkingtons for making large sheets
of glass for buildings and other applications in the twentieth century is
completely dependent on tin’s physical characteristics. Tin melts at the
relatively low temperature of 232°C, and a bath of molten tin forms a perfectly
level surface onto which fluid glass can be poured. The glass (introduced at
1100°C) flows over the surface of the tin to form a uniform sheet leaves the
float bath at 600°C.
‘Tin’ cans have
always been made out of steel with a layer of tin on the outside, to prevent
corrosion, and it was widely used in cookware and cutlery because of its low
density and easy workability.
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