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.