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.

How we use Calcium

Elizabeth Henderson
Product Development Manager
ITAC Ltd

This month we’re going to consider calcium - far away to the left on the periodic table from the materials we’ve looked at so far. Calcium exists very widely in nature, as naturally occurring minerals as disparate as marble and chalk. Crystalline mineral Blue John (fluorspar, CaF₂) is exploited to make jewellery and ornaments and calcite (CaCO₃) is also sometimes found in spectacular natural crystals. However, the materials used by Itac tend to be white powders which have been mined and refined for industrial applications. Calcium’s chemistry is hardly exploited at all in Itac’s systems, but its compounds play a valuable role as fillers and are used in large quantities. Itac’s products are principally solvent-borne, which means that calcium compounds do not dissolve in them at all. In spite of this the particles’ large surface areas and the low surface tension of the solvent mix (compared to water) means that they wet out readily and form stable suspensions.

Calcium carbonate is widely used in Itac’s formulations for building products. Ultrafine calcium carbonate has been shown to improve the tear strength, tensile strength, and modulus of natural rubber latex film – it gives similar improvements in properties to Itac’s gutter paint films. The powder is easily incorporated and does not require remixing when the material has been stored. Calcium carbonate also plays a big part in construction adhesive formulation, by contributing to the rheology of the mix. It raises the viscosity, but the shear-sensitivity of the mix is limited so the product dispenses readily but smoothly and a bead of sealant does not flow out of shape after delivery.

An additional property of calcium materials which could be of interest to Itac is their flame-retardant nature. Calcium sulphate (gypsum) is used to make building products which are intrinsically flame retardant, and we have done some work here to try and use calcium compounds to form a coating which when subjected to high temperatures forms a ceramic-type matrix which would be a non-combustible shield impenetrable to smoke. So far we haven’t succeeded in this, but the idea is valid in the context of calcium chemistry.

Zinc and its compounds

Elizabeth Henderson
Product Development Manager
ITAC Ltd

The Itac technical blog is sticking to elements close together in the periodic table, by taking a step down and to the left of aluminium to land on zinc. Zinc occurs naturally as zinc sulphide and also in much smaller quantities as crystalline zinc oxide called Zincite.  Zincite has also been reclaimed from zinc smelters’ chimney deposits.  Zinc has been in use as a metal for centuries, and is combined with copper to make brass. The principal modern uses of zinc are in batteries and corrosion protection, but Itac exploits chemical properties of its compounds rather than the metal itself.

Natural rubber is polyisoprene, and its properties need modifying for it to be hard, flexible and sufficiently weather resistant for industrial applications. In particular, the vulcanisation of rubber is achieved by milling sulphur into the hydrocarbon chains. These chains are strengthened by the presence of carbon-sulphur bonds in the matrix. The bonds cannot be formed without the presence of catalytic quantities of zinc oxide being included in the milling. Zinc reacts readily with sulphur, forming a labile compound which makes the sulphur available for incorporation in the hydrocarbon chain. The amount of zinc oxide required is typically 5 parts per hundred parts of rubber.

As well as using zinc oxide as a catalyst in these reactions, Itac exploits its bacteriacidal properties in adhesives for medical applications. Zinc oxide acts on bacterial cell surfaces causing the cells to leak, and as the zinc oxide leaks into the cell it causes ‘oxidative stress’, which is an imbalance between anti-oxidants and pro-oxidants in the cell. This inhibits cell growth and eventually causes death of the cells.

Itac also uses zinc pyrithione as an additive in coatings for food-bearing conveyor belts. Its presence inhibits the growth of algae and fungi in the polyurethane matrix coating the belts. Exposure of yeast cells to zinc pyrithione renders them unable to prevent the uptake of toxic copper from their surroundings. Copper is incorporated by the yeast cells as copper pyrithione, and this compound targets iron-sulphur proteins in the cells, killing them.

Aluminium - such a versatile metal

Elizabeth Henderson
Product Development Manager
ITAC Ltd

At Itac Ltd we use some materials which have been part of the chemical industry for centuries (e.g. lanolin and talc), as well as more up-to-date products such as silicones. Following the blogs on carbon and silicon we take a step to the left to discuss aluminium, which is both ancient and modern. Compounds of aluminium occur naturally as bauxite and alum, as well as rubies and sapphires, but production of the metal (as used for wrapping turkeys) was not achieved until the 19^th century. The process is electrolytic and requires a lot of current at high voltage – the work is done in places such as Brazil, Canada and Norway where hydroelectric power is readily available.

In Itac’s materials we use aluminium metal as a straightforward heat reflector when incorporated in coatings for fire curtains. The metal is supplied as a paste of small flakes which can readily be stirred into solvent borne mixtures.  When the solvent is lost during drying, the aluminium flakes tend to remain on the surface of the coating and act as mirrors.  Aluminium flakes are also used as a pigment to give a metallic finish to some of the paints we make for the building industry. As far as compounds are concerned, aluminium trihydrate is in some of our flame retardant coatings. It decomposes when heated and releases water, which cools the fire and dilutes the fire gases. Its decomposition also consumes energy, contributing to the cooling of the mix. This material has to occupy a high volume to be effective in the coating film, but a benefit of this is the minimisation of combustible binders. Aluminium silicate is also a component of some of our flame retardant coatings, and this is thought to act by partially fusing into a ceramic layer, preventing smoke permeating the substrate.

The chemical properties of aluminium are also exploited in our adhesives products.  Aluminium acetylacetonate is added to our acrylic adhesives, and causes them to crosslink when solvent is driven off a coated surface. Due to this crosslinking, the adhesive film has less flow and will be resistant to removal by solvents after drying.

Another aspect of aluminium chemistry exploited in the coatings industry is the use of salts of aluminium with medium-chain carboxylic acids, and aluminium chelates to form gels in ink varnishes. The acid and hydroxyl groups in the varnish can both react with the aluminium forming a rheologically stable gel suitable for use in offset lithographic inks.