The carbon, which is described in the journal Adsorption Science & Technology, is a spherical high performance adsorbent made by Blücher. This synthetic carbon with an internal nanostructure that has been optimised to produce a highly efficient material for the selective filtration of volatile cigarette smoke toxicants. The carbon used in one cigarette filter has an internal volume equivalent to one drop of water and a huge internal surface area equivalent to 1/3rd the area of a tennis court.
This research is part of an effort by scientists at British American Tobacco to develop innovative technologies to reduce toxicant levels in smoke. This is an essential part of a research programme that seeks to reduce toxicant exposure in smokers and then determine whether this has any impact on the risk of developing smoking-related diseases. Standard cigarette filters are made from cellulose acetate, a cotton-like material derived from wood cellulose. Carbon (often referred to as charcoal) can be added to improve the filter’s ability to reduce certain volatile smoke constituents. The carbon typically used is derived from coconut shells.
The new carbon is derived from a polymer and has a bimodal pore size distribution with micropores (<2 nm in diameter) to trap the smoke toxicants and larger (3-80 nm pore diameter) ‘transport’ pores that are especially useful when the flow rate is high (such as those encountered during smoking).
‘The smoke moves through the filter at speed,’ explains Peter Branton, a senior scientist at British American Tobacco. ‘Without the larger transport pores to guide the toxicant molecules into the smaller pores, getting them in there would be a bit like trying to park a car at right angles whilst driving at speed,’ he says.’
Tests using 60mg (equivalent to 1/70th of a teaspoon of sugar) of carbon in the cigarette filter can improve the filtration efficiency of certain smoke toxicants by as much as 60% compared to the existing coconut-shell derived carbon used. Even under intensive machine smoking, improvements are observed, for example a 50% reduction in benzene and a 15% reduction in formaldehyde levels.
Chris Proctor, Chief Scientific Officer at British American Tobacco, cautions, however, that: ‘Even if you can reduce the levels of a significant number of toxicants in tobacco smoke, there is no guarantee that this will result in reduced exposure in people or result in a reduction in health risks.’ This is why scientists at British American Tobacco are testing prototype cigarettes made using this highly activated carbon in clinical studies in Germany to determine whether reductions in toxicant levels seen in smoking machine tests translate to reduced exposure levels in smokers.
Traditional activated carbon is produced by charring coconut shell at 300-500oC and then activating it in a rotary kiln at 900-950ºC using steam. The resulting carbon is cooled, ground and sieved, resulting in irregular shaped carbon granules of a specified size range. The synthetic activated carbon is produced with indirect heated rotary kilns, under reduced pressure in an inert atmosphere. The polymer feedstock is firstly thermally stabilised using an excess of oleum, after which it is slowly heated to 500°C. The material is then further heated to 900 – 1000°C for activation in steam followed by activation in CO2 which results in the production of uniform, spherical carbon beads containing both micropores and mesopores.