The maximum theoretical carbon yield for the carbonisation of viscose fibres, which consist of the renewable resource cellulose, is 44%. In reality the carbon yield is just in the range of 20%. By chemical activation of viscose fibres, carbon yields of more than 30% were achieved along with a reduction of undesired tar formation by 75%. A further increase in carbon yield was realised by supplemental incorporation of carbon black into the cellulose matrix of the viscose fibres. The chemical activation did not change the state of chemical bonding of the carbon atoms and the supramolecular structure within the carbonised fibres. Also the fibre morphology remained unchanged in terms of shape of cross section and surface structure. However, activated viscose fibres delivered carbon fibres with a much higher specific surface area. Carbon fibres from activated viscose fibres provide a similar or even improved application potential as carbon fibres from standard viscose fibres.
Carbon fibres are accessible from several raw materials. The carbonisation of the cellulosic viscose fibres is a pyrolytic process in which, theoretically, only water is demerged and pure carbon is formed, while with increasing temperature, the carbon structure develops towards graphite structure (Figure 1). Thus, the maximum theoretical carbon yield is 44%. However, in reality the carbon yield is much lower, since carbon containing molecules are formed as volatile side products: carbon monoxide and dioxide, hydrocarbons and oxygen containing organic compounds, of which levoglucosan is probably the most famous one. Further dehydration of this intermediate leads to formation of tar which condenses in the carbonisation equipment and causes undesirable impurities.
This article appeared in the May–June 2019 issue of Reinforced Plastics.
