In the 1960s it was also found that cellulose acetate filters using the compound triacetin to strengthen the filter were associated with reductions of up to 80% of volatile phenols.
Cigarettes with charcoal (activated carbon) filters - known for their selective reduction of many vapour phase smoke toxicants - are popular in some countries, such as Japan, South Korea, Venezuela, Hungary, Romania and Russia.
We are currently researching both conventional and novel filter designs.
Most commercial cigarette filters are made from cellulose acetate (CA) fibre. The amount of fibre in the filter rod - associated with a parameter called ‘tow weight’ - determines its density. Firmness can be controlled by adding a small amount of plasticizer, for example glycerol triacetate, usually referred to as triacetin.
Routinely measured filter parameters include pressure drop and ‘tar’ (or NFDPM, nicotine-free dry particulate matter) filtration efficiency.
Pressure drop - or draw resistance - determines the amount of suction a smoker needs to apply to the cigarette to draw smoke through the filter. The pressure drop of the filter plus that of the tobacco column must be within a range acceptable to consumers - typically approximately 100mm water gauge (WG).
Filtration efficiency refers to the proportion of the material retained by the filter compared with that entering it. The filtration efficiency is affected by the smoke flow velocity and is typically determined using a standard machine-smoking regime.
Pressure drop and filtration efficiency are related parameters.
The main purpose of a cigarette filter is to reduce the particulate smoke yield - achieved by mechanical filtration of aerosol particles.
Removal of specific constituents of the particulate phase is generally in proportion to the ‘tar’ filtration efficiency. Gas phase smoke passes through cellulose acetate filters largely unaffected. Some semi-volatile constituents, e.g. phenolic species can be selectively retained depending on the type and the levels of plasticizer used.
Filter parameters affect ISO ‘tar’ removal efficiency.
The cube-plot below shows NFDPM filtration efficiency in percentages.
Cellulose acetate is also selective for phenol removal. The effect of triacetin, filter length and tow weight on the percentage phenol filtration efficiency is shown in the cube-plot below.
Ventilation holes may be introduced along the filter to further dilute the smoke during puffing. Some of our research suggests that the aerosol particle size distribution may vary modestly with level of filter ventilation.
The graph below shows the relationship between smoke aerosol particle diameter and filter ventilation.
To understand the behaviour of the cellulose acetate filter, the physics of aerosol filtration by fibrous materials needs to be examined.
This normally involves a flow field either parallel or perpendicular to fibres of circular cross-section. Cellulose acetate fibres in a cigarette filter can be considered as lying approximately parallel to the flow of smoke aerosol.
By using mono-, poly-dispersed and cigarette smoke aerosols at different velocities, the validity of some well-known filtration mechanisms can be analysed.
One such mechanism was described by Stechkina, Fuchs and Kirsch. This is commonly referred to as the mechanical filtration mechanism. The S-F-K filtration mechanism considers the following three individual contributions:
Assuming that gravitational settling and electrostatic effects are negligible, a staggered array of cylindrical fibres was used as the model filter and the flow field around the fibres was solved to obtain the single fibre efficiency, written as:
The total single fibre efficiency, η, is the sum of the individual efficiencies for each mechanism:
Each of the above terms can be calculated. All four mechanisms are relevant to different extents within the range of flow velocity typically experienced by cigarette filters - approximately 0.2 to 0.5 m s-1 .
Interception and impaction become more significant at higher flow velocities. Using a modelling approach involving aerosols of different size distributions, we found that the S-F-K mechanism can reasonably predict the efficiency of cigarette filters over a wide range of velocities - 0.1 to 3.0 m s-1 - for both ventilated and unventilated cigarettes.
Vegetable-based activated carbon - for example from coconut shell - is now used in filters in an increasing number of cigarettes.
Below are scanning electron micrographs (SEMs) of two types of granular carbon.
During a 2 second puff of 35ml volume, the gas flow passing through a cigarette filter reaches approximately 1 litre per minute.
The residence time for a standard 27mm length filter is therefore typically of the order of milliseconds. In other words, the adsorption is kinetically-controlled. Adsorption by activated carbon is typically physisorption. The pore size and volume is important for maximising adsorption.
The International Union of Pure and Applied Chemistry (IUPAC) defines micropore, mesopore and macropore as follows:
The schematic below is a description of the different types of pores sizes.
Initial screening of the performance of different filter additives, e.g. carbons, in adsorbing smoke constituents is conducted at BAT as follows:
60mg of the additive is weighed into the cavity of a reference cigarette filter (the cigarette having a Virginia style tobacco blend, circumference of 24.45mm, tobacco rod length of 56mm, filter length of 27mm with a 4mm cavity positioned 13mm from the mouth end). Cigarettes are conditioned at 22°C and 60% relative humidity (RH) for 3 weeks and then smoked under ISO smoking conditions. A cigarette with an empty cavity of similar volume is employed as a control. The percentage reductions of the various smoke components are calculated normalised to unit tar and relative to the cigarette containing no additive in the filter. Where yields are below the limit of quantification (LOQ), the LOQ values are used to calculate the percentage reductions. This method serves as a useful technique for the screening of filter additive activity for the removal of smoke constituents.
Vapour phase analytes sometimes measured are selected aromatic volatiles (using an Automated Thermal Desorption/Gas Chromatography-Mass Spectrometry technique, or other analytical techniques), hydrogen cyanide (via a continuous flow analysis technique) and selected carbonyls (via a High Performance Liquid Chromatography technique).
The effect of micropore volume on smoke constituent yields is shown below using coconut based carbon that has been steam activated for periods of one day (yielding a 50-60% CTC active carbon) and two days (yielding a 90-100% CTC active carbon) [ 5 ]. In general, the longer the activation time, the more microporous the carbon will be.
The ISO mainstream machine smoke yields: using 60mg of activated carbon per cigarette filter in this reference cigarette using the procedure described above are tabulated below [ 5 ].
|Smoke Yields (% Reductions)|
|Filter additive||No additive||>50-60% |
|Acetaldehyde (μg/cig)||506||287 (41)||206 (56)|
|Acetone (μg/cig)||263||107 (58)||47 (81)|
|Acrolein (μg/cig)||57||18 (67)||6.3 (88)|
|Butyraldehyde (μg/cig)||33||13 (59)||4.4 (86)|
|Crotonaldehyde (μg/cig)||17||4.7 (71)||1.8 (89)|
|Formaldehyde (μg/cig)||50||22 (55)||18 (62)|
|Methyl Ethyl Ketone (μg/cig)||64||23 (63)||9.2 (85)|
|Propionaldehyde (μg/cig)||44||17 (60)||6.9 (83)|
|HCN (μg/cig)||113||44 (60)||31 (71)|
|Pyridine (μg/cig)||7.9||2.7 (65)||1.0 (86)|
|Styrene (μg/cig)||10||2.5 (74)||0.7 (93)|
|1,3-butadiene (μg/cig)||35||22 (35)||8.5 (74)|
|Isoprene (μg/cig)||304||130 (56)||LOQ (92)|
|Acrylonitrile (μg/cig)||8.6||3.0 (64)||LOQ (90)|
|Benzene (μg/cig)||45||16 (63)||LOQ (94)|
|Toluene (μg/cig)||70||25 (63)||LOQ (92)|
Reductions based on limit of quantitation (LOQ) values.
As seen, both activated carbon samples reduce the smoke vapour constituents measured (the higher the activity, the greater the reduction). It is extremely likely that other vapours not measured here will also be adsorbed by carbon and this difference in smoke chemistry using carbon could cause a change in the taste.
Activated carbon will not selectively reduce smoke compounds present in the particulate phase, for example benzo(a)pyrene and Tobacco Specific Nitrosamines (TSNAs). Nor will it reduce the permanent gases carbon monoxide or nitric oxide, due to their extremely high vapour pressures at room temperature.
The most effective ‘general’ vapour adsorbent found to date is activated carbon.