Wednesday, 18 December 2013

Aluminium - such a versatile metal

Elizabeth Henderson
Product Development Manager
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 19th 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.