The many uses of tin

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 Its use has now been superseded in this application by copper compounds.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.

The importance of Magnesium

Elizabeth Henderson
Product Development Manager
ITAC Ltd

Magnesium plays a significant role in Itac’s processing of solid polymers. We regularly chip materials such as Neoprene and natural rubber before dissolving them, but unless the chips are coated with powder they will re-agglomerate. Magnesium stearate is sometimes used for this, whereas magnesium oxide is more reactive. It is an effective curing agent and acid scavenger in halogen-containing elastomers. Its mopping-up of excess acid prevents pre-vulcanisation of the rubber in our solutions and helps prevent ‘scorching’ during processing. Magnesium oxide occurs naturally and is known as ‘Magnesia’ as this province of mainland Greece was an early source. It is mined as periclase, crystal system cubic 4/m 3¯ 2/m. The element itself was initially identified by Sir Humphrey Davy.

Magnesium’s most important job on earth is to work in the chlorophyll molecule, capturing energy from the sun. Chlorophyll molecules are porphyrins, all of which have four nitrogen atoms providing a site for a metal ion in the centre. In particular, chorophyll has magnesium in this site, and the molecule absorbs red and blue light for synthesis of sugars from water and carbon dioxide. Without this chemistry, there would be no mechanism for plants to convert energy from the sun into food and chemical fuels.

Most applications of magnesium focus on its physical properties – because it has a low density it is incorporated in casting alloys for racing engines and other applications where weight reduction is crucial, but this can be a source of problems as magnesium metal is highly flammable. Although pieces of the metal are normally covered by a thin layer of inert oxide, if a flame is held to them they burn with a bright white flame. This property means magnesium is a regular constituent of fireworks and safety flares, and was a component of early flash powders for studio photography. It was also built into early flash bulbs, where the foil was ignited electronically to give a bright white flash without releasing any smoke.

Magnesium plays a major role in synthesis of organic compounds, as a component of Grignard reagents. These highly reactive species react with carbonyl groups in molecules such as aldehydes and ketones to form of new carbon-carbon bonds. Grignard reagents can also be used in reactions with groups as diverse as epoxies and nitriles to alkylate a target carbon atom. They have wide applications in the synthesis of molecules for drug intermediates.

Barium enhances the properties of adhesives

Elizabeth Henderson
Product Development Manager
ITAC Ltd

Snow-white barium sulphate is the barium compound which brings this element to the world of adhesives and surface coatings. The material is sometimes referred to as ‘Blanc fixe’, or ‘permanent white’.  Barium itself is next door but one to calcium, which was the subject of the Itac blog at the beginning of 2014, and its chemistry is similar. The principal natural source of barium is barium sulphate which is mined in China, India and Morocco. Barium sulphate occurs naturally in orthorhombic crystals with the structure 2_m2_m2_m and its pure whiteness and high specific gravity (4.5) are the properties which we exploit in our formulations at Itac – we use it to whiten our coatings, and the low volume it occupies in the finished film means we can incorporate a high percentage by weight.  This high proportion can be adjusted to control the rheology of the materials we supply. Inclusion of barium sulphate affects the appearance of the finished film, and its physical properties such as sandability and hardness. We have also used barium sulphate to enhance the adhesive properties of adhesives for carpets based on natural starch.

The volume of barium sulphate used for coatings is considerable, but most of the world’s production is used in the manufacture of drilling fluids. These materials also exploit the high density of barium sulphate, and its insolubility in water. The drilling fluids are slurries which transmit the drill pressure precisely into a cavity as it forms, as well as keeping the drill bit cool. A further exploitation of barium sulphate’s insolubility in water is its use as a contrast agent for clinical X-rays. Although barium salts are poisonous, they cannot be absorbed from an aqueous suspension. The strong scattering from barium sulphate in someone’s guts allows any structural problems to show clearly.

 As well as being used in our functional coatings barium sulphate plays a vital rôle in colour management – it is used to whiten the inside of the sample chamber of some colour measurement machines, ensuring that the source light falling on the sample is as white as possible and thus the measurements as accurate as possible.

Barium makes its contribution to æsthetics in the world of colour – the brilliant green in last month’s fireworks came from barium in the formulations. It is also a vital element in two of the pigments used on the Chinese terracotta soldiers, which were painted with BaCuSi₄O₁₀ (Han blue) and the less stable but more beautiful BaCuSi₂O₆ Han purple. A rare fluorescent blue gemstone is found in California and is its official state gem – benitoite is barium titanium silicate.

Itac uses Antimony

Elizabeth Henderson
Product Development Manager
ITAC Ltd

This month we stay in Group 5 of the periodic table, skipping down from phosphorus over arsenic and landing on antimony. The first known application of antimony in the coatings industry was as make-up. Antimony’s naturally occurring compound black stibnite (Sb₂S₃) was used as eyeliner by the vain in ancient Egypt, but as antimony is poisonous its use has been discontinued. ‘Tartar emetic’ (antimony potassium tartrate) was formerly used as an anti-helminthic, which acted by poisoning intestinal worms. Antimony metal exists in only one crystal form viz trigonal crystals. Antimony is found as the metal in Finland, but most of the world’s current supply is mined as stibnite in China.

Itac uses antimony by incorporating antimony trioxide, Sb₂O₃, in the fire resistant coatings we make for textiles and films.  It acts in synergy with chlorine-containing organic compounds to form free radicals in the flames, which quench combustion. In addition to this, the antimony promotes the formation of a carbon-based char on the surface of the burning material, which prevents continuing vapourisation of the fuel. Antimony oxide can also be incorporated in plastics to improve their fire performance.

Antimony played a big part in the advance of printing. In common with cast iron and water, the liquid form is denser than the solid at temperatures immediately above the freezing point. This implies that when poured into a mould it will expand into the crevices as it sets, forming a perfect cast of the void. Gutenberg exploited this property of antimony and developed an alloy of tin, lead and antimony which had the ideal hardness, smoothness and sharpness of edge for making type for printing presses. A similar material was formerly used for making typewriter keys.

Because of its nature as a semiconductor, antimony has many more modern applications than in simple pesticides and letterpress. It can be used as a Hall Effect sensor for measuring electric current and magnetic fields. It is also used to strengthen lead electrodes in vehicle batteries – pure lead is very soft and would not withstand vibration in an engine cavity without help. A major source of antimony for industrial applications is recycled vehicle batteries. An alloy of antimony with germanium and tellurium (Ge₂Sb₂Te₅) has recently been patented for use in a nanodimensional flexible screen which can display an image less than a tenth of a millimetre in diameter.

Why we use Phosphorus in our fire-resistant coatings

Elizabeth Henderson
Product Development Manager
ITAC Ltd

Phosphorus plays a dual rôle in the context of fire – it is used in matches, and at Itac we incorporate it in our fire-resistant coatings. The element was first isolated from animal urine, and this gives us a clue as to its ubiquity. Phosphorus is a key component of all living organisms, found in nucleic acids and playing a major part in cell biochemistry. The element itself is comparable to its neighbours in the periodic table, carbon and sulphur, in that it exists in more than one crystal form. White phosphorous is a tetrahedron of atoms in either a body-centred cubic (α-form) or triclinic (β-form) array. Both these forms will gradually decompose to amorphous red phosphorus with time. White phosphorus is very reactive and must be stored in water to prevent it bursting into flame

Itac’s inclusion of organic phosphate esters in fire-resistant coatings is effective because at high temperatures the organic part of the material burns away and when the residual phosphorus is further heated it will form a polymeric form of phosphoric acid. This acid causes a char layer, which shields the remaining material from oxygen, in that way preventing the formation of flammable gases.  Organic phosphate esters are liquid – this means they are readily incorporated in our mixes and compatible with the organic solvents we use.  They have the advantage of being halogen-free, so no volatile acids are formed as by-products in a fire.

 The only source of phosphorus for the modern chemical industry is phosphate rock, and large deposits of phosphate from igneous rock are found in Canada, Russia, and South Africa. ‘Coprolites’ discovered in 1842 in Suffolk were formerly mined for use in fertiliser due to their high phosphate content. They are fossilised animal dung and for some time they were a major raw material for fertilisers but their use diminished towards the end of the 19^th century.

In addition to the uses we make of phosphorus at Itac and their application in fertilisers, its compounds are very effective surfactants. Phosphate end-groups on polymer chains allow a molecule to have an affinity for both hydrocarbons and polar surfaces, allowing their use as wetting agents for materials such as pigments. Phosphorus is also an important component of phosphor bronze – an alloy of copper, tin and phosphorus which has excellent mechanical and workability properties.