Wednesday, 26 August 2015

Lead – too toxic for modern coatings

Technical Manager
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.