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.

Silicon compounds in Itac's products

Elizabeth Henderson
Product Development Manager
ITAC Ltd

Itac’s last technical blog discussed applications of elemental carbon in our coatings. Moving a step down the periodic table to silicon, this article looks at the uses of some of its compounds in our products. As an element it is a lightweight solid with a shiny appearance, but it occurs naturally as compounds with oxygen. These are generally crystalline materials (e.g. amethyst, quartz, sand). The raw material for silicon compounds is readily available and cheap but a great deal of energy is required to reduce sand to silicon, which is the starting point for high-spec silicon-based materials.

Fumed and precipitated silica powders are made by two different processes. Fumed silica is made by burning tetrachlorosilane in air, so the silicon dioxide forms in the combustion chamber like flakes of soot forming above a coal fire.

Precipitated silica is formed by treating a basic solution of sodium silicate with an acid such as concentrated sulphuric acid. The reaction produces a fine suspension of silica in the aqueous medium, which can be separated by filtration and dried.  Both these processes yield feather-like particles, that is to say particles with very low bulk density and very high surface area to volume ratios. Surface areas of silicas can be as high as 600m²g^-1. These physical characteristics allow silica to be used in a pigment dispersion to keep the pigment in suspension, and if fumed or precipitated silica is stirred into a mixture of other powders such as organic pigments, it ‘floats’ to the top of the mixture.

We routinely use fumed silica to increase the viscosity of polymer solutions. Even when thoroughly mixed into the solvent, the particles hang together and provide resistance to flow in the liquid. This structure is also effective in keeping high-density pigments such as antimony trioxide in suspension. 

The chemically inert silica can also be modified to change its behaviour in various media. For instance, it can be treated with wax to make it hydrophobic. This material is very effective as a matting agent, as it will lie on the surface of a solvent-based paint film as it dries, and the rough texture disrupts the reflection of light from the outer layer. A different coating will make silica hydrophilic, allowing incorporation in water-borne coatings to achieve similar effects.

Graphite adds special properties to Itac coatings

Elizabeth Henderson
Product Development Manager
ITAC Ltd

Itac Ltd makes use of two allotropes of carbon to endow materials with specific properties. We haven’t gone to the lengths of using diamonds to get the sparkle we need for coatings for buildings, but we use graphite (as seen in pencils) and amorphous carbon (as seen on a smoky barbeque) to get special properties for our coatings. In particular, amorphous carbon is used for our conductive coatings for textiles.

We exploit the crystalline character of acetylene black to get an effective conducting property at low pigment loading. The process used to incorporate this in our coatings is a straightforward mix into solvent borne polymer solution, using an enclosed mixer with a central horizontal shaft. The main problems to overcome are getting the powder to wet out effectively, and coping with the dust.

Particle size is not critical for our applications, and as long as the particles are wet they will allow the coating to conduct electricity when dry. We can incorporate amorphous carbon into various polymer types such as polyurethanes or acrylics, to allow us to supply a coating fit for our customers’ substrates. Application of these materials is very straightforward.

Graphite is also used, in our flame retardant intumescent coatings. The structure of the allotrope and the processing of the graphite are crucial to the performance of our finished products. Graphite has a lamellar structure and naturally forms smooth shiny flakes, and the particles have to be incorporated in our product in such a way that they are the right size to be applied. For instance, if a customer wishes to spray a coating the flakes must be small enough to go through the spray nozzle.

For a product to be applied evenly onto cloth by a knife-over-roller process, the flakes must not form lumps like piles of coins. The mixing process must be appropriate to get the right coating properties. For small flakes, the graphite needs to be milled and banded. The high shear rate of this process breaks down the particles. For a spreadable product the mixture needs a Z-blade action in a high-viscosity medium (about 10 000 poise).

We generally use graphite in combination with other flame retardant technologies such as aluminium trihydrate, antimony trioxide and chlorinated hydrocarbons. The final formulation is dictated by the substrate to be coated and the fire performance required from the finished article.