Thursday, 28 November 2013

Graphite adds special properties to Itac coatings

Elizabeth Henderson
Product Development Manager
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.

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