Silver - more valuable than you think

Elizabeth Henderson
Product Development Manager
ITAC Ltd

Silver was formerly seen every day in our pockets – although it has been superseded in British coins it still works in many other contexts. Its ductility and malleability means it has been used since ancient times to make jewellery, ornaments and luxury items. At Itac we exploit its properties as a biocide – silver ions when in contact with bacterial DNA prevent its replication and appear to do this by interrupting the S-S bonds in the molecule. We can incorporate small amounts of silver containing compounds in coatings to exploit this effect. Silver nitrate in a block or as a solution was applied to skin infections to kill the bacteria in the nineteenth century, but in the twenty-first silver nanoparticles have been developed for use in textile medical dressings.

In other parts of the chemical industry, silver has been used as a catalyst for production of ethylene oxide and formaldehyde, particularly for ethylene oxide which is used as a building block for polyesters (step to polyurethanes). Itac uses these catalysts indirectly as we use a number of polyester PUs in our products for textiles.

The photosensitivity of silver was exploited to make pictures from early developments in the 1830s until the present day, although its use diminishes as digital photography improves. Colourless silver ions are reduced to black particles of silver metal by visible light, and will form a ‘shadow’ of a pattern placed between the silver ions and a light source. Over this time, the technology for using silver was refined from silver nitrate solution on a glass plate to emulsions of silver nitrate in gelatine on a flexible film.

Silver-containing materials also play a major role in everyday electrical items due to its excellent conductivity. Inks formulated with silver are used to produce printed circuit boards and other items such as contact films beneath computer keyboards. The heating elements on car rear windows are made of silver-based ink to conduct both electricity and heat across the glass.

Sulphur - part of Itac's cornerstone technology

Elizabeth Henderson
Product Development Manager
ITAC Ltd

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.

Take a walk on the white side - Titanium dioxide

Elizabeth Henderson
Product Development Manager
ITAC Ltd

This month’s blog discusses the use Itac makes of titanium dioxide.  This material is seen everywhere and never looked at – our principal use for it is as a whitener in our paints and adhesives, but it is also used in paper, fibres, food, and cosmetics. It plays an important role in u-PVC window frames, as it stabilises the polymer mix against UV light. The mineral occurs naturally as ilmenite, and is also seen as rutile inclusions (brown needle-like strands) in gemstones such as rubies and sapphires. Its desirability as a white pigment is based on its high refractive index (rutile 2.6 cf talc 1.6) and its chemical inertness which makes it stable in formulations and biologically acceptable (compare this with one of its predecessors as white pigment, lead oxide). Production and purification of pigment grades is demanding of energy and requires very aggressive chemicals – the two routes normally used require the use of either sulphuric acid or chlorine gas. A nation’s consumption of TiO₂ per capita is used as an index of industrialisation, as increasing urbanisation needs more surface coatings and paper products.

The name ‘titanium’ was originally chosen for the metal on account of its high strength/density ratio, which means it can be used for aircraft components and other small-volume applications such as artificial human joints and posts in teeth. The oxide’s good colour strength in paints means that relatively low quantities can be used in pale colours such as magnolia and grey, where iron oxides and carbon black will contribute to the opacity.

Titanium compounds also play major roles in catalytic processes. Nanoparticle TiO₂ when exposed to UV light, reacts with water to generate protons which will decompose any organic material they contact. Such particles can be sintered into the surface of glass, making it self-cleaning. There are limitations with incorporating them into paints because they destroy the organic binders, but silicone binders may emerge in future which it will be possible to use. Polypropylene was first discovered in the lab thanks to someone investigating Ziegler-Natta catalysts, based on mixed oxidation state compounds of titanium with chlorine, and finding a strange white material in reaction products. Ziegler-Natta catalysts have been used for this application ever since.

For all the diversity and importance of the applications of titanium and its compounds, the most striking use of titanium metal is at the Guggenheim art museum in Bilbao, which is covered in glittering plates of titanium metal – clean, beautiful and strong.

Iron produces a spectrum of colour

Elizabeth Henderson
Product Development Manager
ITAC Ltd

This month’s blog discusses an element used by Itac only as a compound – iron. But as the mainstay of the construction industry and one of the commonest elements of our planet, it plays a major role in our business. We use coloured iron compounds in our products - iron’s oxides are familiar as rust, and they are refined to produce stable pigments of either red or yellow colour. They have excellent resistance to sunlight as well as good chemical stability. With good hiding power and colour strength they make a valuable contribution to the appearance of a painted item.

 In addition to its place in the red and yellow part of the spectrum, iron is the key component of Prussian Blue pigment, which is iron(III) ferrocyanide. The yellow colour of FeO arises from iron in oxidation state 2, the red in Fe₂O₃ from iron in oxidation state 3. The characteristic blue of iron(III) ferrocyanide arises from the presence of iron in both oxidation states in the compound.  These three colours all from the same element are demonstrated in the colours found in natural mineral Tiger’s Eye. This material has a characteristic fibrous appearance associated with its source crocidolite which has been replaced by silica, and the three naturally occurring colours (yellow-brown, red and blue) arise because of contamination by iron in various oxidation states.

Itac has recently bought Delvemade Ltd, and we now supply paints for roofing, designed to prevent iron doing what it likes to do most of all – oxidise to rust. ‘Delcote’ coatings are applied to corrugated iron which has been factory finished in a coil-coating process but needs refreshing because of weathering and exposure to pollutants. The paint has to bond successfully to the original finish and extend the life expectancy of the building as well as improving its appearance.

The magnetic properties of iron are also exploited in specialised adhesives and coatings. Magnetite (mixed oxidation-state iron oxide) is incorporated in specialist inks (seen on the bottom of cheques) which can be read by magnetic readers. High-spec adhesives have been devised incorporating very fine iron dust, which heats up very quickly in a magnetic field and initiates the thermal cure of the adhesive medium very quickly.

Itac use chlorine as a compound

Elizabeth Henderson
Product Development Manager
ITAC Ltd

This month’s blog discusses an element used by Itac only as a compound – chlorine. Every day we all encounter sodium chloride as crystalline table salt, scattered on crisps and peanuts. Salt played a major role in the development of the science of crystallography, as its structure was the first to be determined by the scattering of X-rays. Understanding of the chemistry of the Earth’s atmosphere has increased dramatically during recent decades and many chlorine compounds formerly used in industrial applications have been identified as causes of harm to the ozone layer and upper atmosphere. But the unique properties of chlorine mean it still plays a role in our specialised materials – we use modern control methods to minimise escapes of volatile chemicals and whenever possible we replace chlorine compounds with less harmful products.

Gaseous chlorine readily forms chlorine radicals in the presence of UV light, and this very reactive species plays a major part in the flame retardant products we make at Itac. In a fire there’s very little UV light and gaseous chlorine is hard to use efficiently, so we use chlorine-containing polymers to deliver chlorine radicals into the flames. The polymers contain carbon-chlorine bonds which are broken to release the radicals into the fire gases. These gases are a mixture containing hydrocarbons which react with the chlorine radicals to form hydrogen chloride gas. This key species reacts with hydroxyl radicals also in the fire gases to give water and regenerate the chlorine radical. The water is a stable material so hydroxyl radicals cease to be active, and the chlorine radicals are regenerated to carry on working.

At Itac most of the flame retardant coatings we produce are for textile applications, and we introduce the chlorine compounds used along with other flame retardant technologies, either by using a chlorinated polymer (eg Neoprene) for the binder system or by incorporating a halogenated oil into a more conventional polymer mix. Use of a conventional hydrocarbon means a plentiful supply of fire gases to react with the chlorine radicals. The fibre used for the textile has a powerful effect on the combustion behaviour, and the classification of the fire performance of the finished textile depends on both the fabric and the coating.