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Synthetic carbon

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Synthetic carbon

In collaboration with an external research partner, we have developed a new polymer-derived (synthetic) carbon with enhanced filtration performance. In some instances, this carbon is approximately twice as effective at removing a number of volatile smoke toxicants compared to conventional coconut shell carbon.

  • The associated peer-reviewed manuscript is freely available to download via Adsorption Science and Technology's open access resource - link Opens new window

Active carbons are effective adsorbents for many volatile toxic compounds that are encountered in every day situations including domestic and industrial settings. The use of active carbons in modern cigarette filters has been well documented[1 - 4].

The challenges faced by using active carbon as a cigarette filter material are considerable. To begin with, the carbon has to filter a broad range of organic and inorganic species (for example carbonyls, aromatics, metal oxides) at varying concentrations in a humid smoke stream. For activated carbon to be effective, very specific properties are required, manly due to the fact that only  small quantities of carbon that can be incorporated into a cigarette filter and the contact time with the smoke stream is very short (millionths of a second)[5].

Most active carbons have a significant volume of micropores that are capable of adsorbing a variety of vapour species[6 - 7]. However, under dynamic flow conditions, diffusion within these micropores can be slow; it is therefore beneficial to include additional wider transport pores to maximise the adsorption. Moreover, the hydrophilic character of active carbons is widely recognised as a major obstacle for use in many applications[8]. In humid conditions the adsorption of organic species is hindered by the competitive and rapid adsorption of water[9]. This ultimately fills and blocks access to pores, which are then unavailable to adsorb the target species[10].

When using active carbons for a specific application, there are twon main factors to consider:-

  • Pore size distribution and volume – pore characteristics.
  • Surface chemistry.

Ideally, when using active carbon  in a cigarette filter it should have sufficient microporosity to allow adsorption of the various vapour species found in cigarette smoke, plus wider transport pores to allow easy access to these micropores and a slightly hydrophobic surface to avoid pores becoming blocked by water vapour. This combination of characteristics is important to maximise filtration efficiency in a cigarette filter.

Traditional coconut charcoal filtered cigarettes are produced as follows: the coconut shell raw material is charred at 300ºC -500ºC and then activated in a kiln at 900ºC -950ºC using steam. The resulting carbon is cooled, ground and sieved, resulting in irregular shaped carbon granules within a specified size range.

Polymer-derived carbon was produced by a process described by Von Blücher and De Ruiter 2004[11], Von Blücher et el 2006[12], and Böhringer and Fichtner 2008[13]. The polymer-derived active carbon is produced using a batch process with indirect heated rotary kilns, under reduced pressure in an inert atmosphere.  After preparation of the spherical polymer feed stock, the material is thermally stabilised and slowly heated to 500°C, resulting in the carbonisation of the polymer and the release of predominantly SO2  and H2O.  The resulting carbon has an initial pore system that is not accessible for typical adsorptives. To create a porous system capable for adsorption, thesarah material is further heated to 900°C – 1000°C for activation with oxidising agents (steam).  This establishes a pore system predominantly composed of micropores with pore sizes between 0.7 and 3 nm.  Subsequent activation with CO2 leads to the formation of predominantly larger mesopores in the range of 3nm to 80nm.  Combining the steam and CO2 activation steps offers a flexible strategy for producing the desired pore characteristics.

Activated carbons have been characterised using adsorption principles, equilibrium isotherm data and dynamic breakthrough times and volumes, and this approach has helped identify key criteria for dynamic toxicant adsorption and gives a basis for which activated carbons may be selected for use in cigarette filter and other filter applications. Experimentally, using 60mg of additive in a cavity filter design of a reference cigarette and smoking under ISO conditions, revealed that the polymer-derived carbon was roughly twice as effective at removing the majority of volatile cigarette smoke toxicants measured compared to coconut shell-derived carbon[5] (traditionally found in cigarette filters).

The table below shows percentage reductions for coconut carbon and polymer carbon compared to a control cigarette under ISO smoking conditions[5]. A similar comparison can be seen for Canadian intense smoking regimes in the manuscript[5]. For all smoke chemicals below, polymer-derived carbon is as or more effective than traditional coconut carbon. 

Smoke Analyte / Characteristics

 

Yields of coconut carbon (60mg)

Yileds of polymer carbon (60mg)

% Reductions coconut carbon

% Reductions polymer carbon

Cigarette puff No.

7.1

6.8

7.1

~

Water*

3.1

2.3

1.7

~

Nicotine free dry particulate matter*

11.8

10.3

10.0

CO*

11.4

11.5

11.5

~

Nicotine*

0.94

0.85

0.83

~

~

 Acetaldehyde**

584

384

289

34

51

Acetone**

281

155

40

45

86

Acrolein**

78.4

39

11.9

50

85

Butyraldehyde**

38.9

20.5

4.5

47

88

Crotonaldehyde**

23.9

10.2

2.2

57

91

Formaldehyde**

59.3

35.5

27

40

54

 2-Butanone**

 69.7

34.5

4.5

51

94

 Propionaldehyde**

49.4

27.3

8.2

45

83

 HCN**

 118.6

66.1

54.9

44

54

 1, 3-butadiene**

 20

16

3.4

20

83

 Acrylonitrile**

8.7

4.8

1.4

45

84

 Benzene**

32

18.2

1.4

45

84

Isoprene**

199

117

21

41

89

Toluene**

45.7

29.5

18.1

35

60

  * mg/cigarette
** (µg/cigarette)

Cigarette smoke contains other toxicants that are not associated with vapour phase, such as aerosol droplets. These species will not be adsorbed by the active carbon and reductions of these toxicants have to be achieved using other technologies.

Clinical Studies

About our Clinical Studies: This year we hope to report the results of our first clinical study of prototype reduced toxicant products. This study involves testing prototype cigarettes modified to produce reduced levels of specific toxicants compared with conventional cigarettes. The aim is to determine whether smokers who switch to the prototype products have lower levels of markers of toxicant exposure in their biological fluids than the smokers who continue with their regular cigarettes. Non-smokers act as a control for the markers of toxicant exposure.

Study Title: “To compare the exposure levels of selected smoke constituents as determined by biomarkers of exposure, filter analysis, sensory perception and other parameters when smokers using commercial cigarettes are switched to novel cigarettes” - ISRCTN2157335 Opens new window


  1. Mola, M., Hallum, M., Branton, P. (2008) The characterisation and evaluation of activated carbon in a cigarette filter. Adsorption. 14 (2-3): 335-341. Abstract: The characteristics and evaluation... Opens new window
  2. Branton, P., Lu, A. Schuth, F. (2009) Effect of carbon pore size and distribution on gas adsorption in a cigarette filter. Carbon. 47 (4) 1005-1011. Abstract: The effects of carbon pore size... Opens new window
  3. Branton, P., Bradley, R. (2010) Activated Carbons for the Adsorption of Vapours from Cigarette Smoke. Adsorption Science and technology. 28 (1) 3-21. Abstract: Activated carbons... Opens new window
  4. Branton, P., Bradley, R. (2011) Effects of active carbon pore size distributions on adsorption of toxic organic compounds. Adsorption. 17 293-301. Full text: Effects of active carbon pore sizes... Opens new window
  5. Branton, P., McAdam, K., Duke, M., Liu, C., Curle, M., Mola, M., Proctor, C., Bradley, H. (2011) Use of classical adsorption theory to understand the dynamic filtration of volatile toxicants in cigarette smoke by active carbons. Adsorption Science & Technology. 29, 117-138. Full text:  Use of classical adsorption theory. Opens new window
  6. Stoeckli, H. F. (1974) Helvetica Chemica Acta, 57,7.
  7. Everett, D. H., and Powl, J, C. (1976) J. Chem. Soc., Faraday Trans., 72, 619.
  8. Barton, S., Evans, M. J. B. and Harrison B H. (1973) J. Colloid Interface Sci., 45, 542.
  9. Dubinin, M. M. and Serpinsky, V.V. (1981) Carbon, 19(5): 402.
  10. Adams, L.B., Hall, C.R., Holmes, R.J. and Newton, R.A. (1988) Carbon, 26, 451.
  11. Von Blücher, H. and De Ruiter, E. (2004) Patent No. 2004/0038802.
  12. Von Blücher, H., Böhringer, B. and Giebelhausen, J-M. (2006) Patent No. EP1918022.
  13. Böhringer, B. and Fichtner, S. (2008), Patent No. WO002008110233.
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