Lead – too toxic for modern coatings

Peter Grant
Technical Manager
Itac Limited

Lead is part of the coating industry’s long past, and is chiefly obtained from the natural mineral galena (lead sulphide).  This mineral is found in locations all over the world, and often contains up to 2% silver sulphide which can be of more commercial value than the lead sulphide. Galena crystallises in cubic crystals, space group Fm3m, and enjoys the huge specific gravity of 7.2 – 7.6. In common with stibnite, galena was used as make-up and was the original kohl eyeliner used in ancient times. Metallic lead has been in use for many purposes since before records began, and examples of applications include the formation of water pipes and gutters from sheets of metal by the Romans. It is still used in building applications, for example to make flashings around chimneys and as a waterproof layer in roofs. It has limited reactivity in air and is reasonably resistant to acid rain, and these properties in combination with its proverbially high density make for a long-lived unshiftable roof.  Lead’s flexibility and malleability allowed the manufacture of detailed articles ranging in size and complexity from holders for small glass window panes to life-size human statues for tombs and monuments.

Lead and its compounds are poisonous, and their use has tapered to practically nothing in the coating industry. Previously, lead-based colours were a glowing presence in the painter’s pallete. Red lead was the characteristic colour used for early sports cars and GPO vans – the pigment is also known as minium, and it is lead II/IV oxide. It was made by calcining litharge (natural lead oxide) in air. Basic lead (II) chromate was an orange pigment which was ground-up naturally-occurring crocoite. This mineral is highly prized for its wonderful colour, and was rare until substantial deposits were found in Tasmania. Both these pigments have been identified in red ornaments on ancient manuscripts. Lead (II) antimonite (Naples yellow) has a greenish shade and has been identified on pottery artefacts from ancient Egypt. Other yellow pigments include chrome yellow (lead (II) chromate), and lead-tin yellow type II. This interesting material is a fusion of lead, tin and quartz.  The pigment known as lemon yellow is a mixture of lead chromate and lead sulphate. The most common, cheap and widely used of all the lead pigments used to be lead white, which was lead carbonate. It was used as an adhesion primer and corrosion inhibitor as well as for its colour. It was also first-class as a preventer of mildew which is no surprise as all these lead compounds are very poisonous. Their wonderful colours could be interpreted as a warning, just like the colours on tropical frogs.

Like many manufacturers within the paint and surface coatings industries, Itac has superseded lead white as a pigment by utilising titanium dioxide, for pigmentation. Titanium dioxide has many similar characteristics and is so safe that it can also be used in food colourings and sunscreen. 

Itac use Copper for colour and catalysis

Elizabeth Henderson
Product Development Manager
ITAC Ltd

Copper and Itac overlap in the worlds of colour and catalysis. Copper phthalocyanine is the starting point for all phthalocyanine pigments, as copper bonds readily with the nitrogen atoms in phthalonitrile (1,2 dicyanobenzene) to form a stable complex. Treatment with acid removes the copper from the centre of the molecule, leaving the phthalocyanine copper-free. This can subsequently be reprocessed (for instance by recrystallization from sulphuric acid) to refine its colour and dispersion properties. Itac uses dispersions of phthalocyanine pigments to provide strong blues and greens for our coatings products.

Just as copper is a catalyst in the production of these colours (the metal used initially is recoverable from the process), copper saccharinate can be used as a co-catalyst for the polymerisation of methyl methacrylate to form structural acrylic adhesives. In the presence of cumene hydroperoxide and N,N-dimethyl-p-toluidine, the Cu(II) is reduced to Cu(I). This free Cu(I) complexes with N,N-dimethyl-p-toluidine to form a complexed ion which is a strong reductant for cumene hydroperoxide, whose decomposition is further catalysed by saccharine. The copper saccharinate thus initiates the creation of free radicals from cumene hydroperoxide which in turn initiate polymerisation of methyl methacrylate.

As well as providing useful copper compounds, copper metal itself has been mined and refined for millennia. The Manchester Museum is home to the Alderley Edge shovel which was found in the exhausted copper mines, along with various stone tools, in the nineteenth century. Radiocarbon dating has shown that it was made around 1750 BC. Malachite is the green copper ore found at Alderley Edge – it is copper carbonate, with the characteristic brittleness of ionic solids. It crystallises in space group P2₁/a, and as well as being found in Cheshire, there are major deposits in the Ural Mountains, southern Africa and Latin America. Cornwall’s tin mines also yielded copper as a by-product.

Copper’s excellent electrical conductivity and malleability have been exploited by its use for power and communications delivery systems. Until the advent of fibre optic cable and wireless transmission (the clue’s in the name) all ‘instant’ communication was made using electrical impulses sent down copper wires. Copper is still a component of modern under-sea cables.

Copper has made another contribution in the maritime world, by its use as an outer shell for the hulls of timber ships. It is impermeable to teredo worms, resistant to corrosion by sea water, and its chemistry limits the growth of marine life on the vessel.

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.