Polyacrylonitrile (PAN): how is it made?
Toray carbon fibre is made using polyacrylonitrile, more commonly known as PAN. The French site in Lacq, Toray Carbon Fibers Europe, is adding a new production facility for this precursor. But how is it made? While the PAN manufacturing process is well established, and used by all manufacturers of the afore-mentioned precursor, Toray Carbon Fibers keeps some of the details of its own manufacturing process, which makes its PAN an exceptionally high-quality precursor, a secret. The PAN produced at the Lacq site in 2014 shall be used to meet European demand and will complement Toray’s current supplies to Japan and the United States.
There are four essential stages in the production of PAN using precious acrylonitrile monomers: polymerisation, spinning/coagulation, stretching and winding. The final quality of the PAN precursor depends on these stages being carried out accurately and therefore also affects the quality of the carbon fibres used in numerous applications.
First stage: polymerisation
This stage is fundamental to the rest of the process and consists of obtaining a polymer solution made up of monomer (acrylate). Polymerised in a solvent, a catalyst and some additives, the proportion of this monomer varies from one manufacturer to another, and impacts on the stability of the precursor in the oxidation stage. Carefully chosen to optimize stabilization, they also stop the filaments from binding together. To this end, the latter are generally covered with a protective agent (silicon product) which prevents the filaments from binding together and also limits the exothermic effect in the oxidation mechanism during the heat treatment, leading to the creation of carbon fibres.
Spinning and coagulation
This new stage depends on the PAN solvent and the coagulant solution chosen by each manufacturer. These choices, linked to the processes, are important as they determine the PAN’s purity, which is directly linked to its thermal stability and to the mechanical properties of the final carbon fibres. The shape and diameter of the monofilaments depend on the shape and diameter of the chain used as well as the combination of solvent/coagulant solution. Different degrees of surface regularity can be achieved depending on the solvent and coagulant solution used. The nature of the metallic impurities present in the carbon fibre depends on the impurities present in the PAN precursor and on the salts used in the manufacture of the precursor.
Washing, drying and stretching the monofilaments follow the spinning and coagulation stage. In order to give them the intended axial orientation, the stretching operation can lengthen the monofilaments more than 500% and includes heat treatment. This is done by passing them over heated rolls or through steam, and before the final finish they go through a covering treatment.
In groups of 1,000 to 24,000, the monofilaments, each with a diameter of 10 to 20µm form the PAN precursors, ready to be wound and then moved to the carbon fibre production facilities. For some textile applications (for example the manufacture of jumpers, covers, etc) the PAN precursors can be made up of between 48,000 and 320,000 filaments.
The new facilities will ensure that within two years the French PAN precursor production site will no longer have to rely on importing the raw material needed for carbon fibre production.