Structurally, carbon fibers contain a blend of amorphous carbon and graphitic carbon , high tensile modulus results from the graphitic form, in which the carbon atoms are arranged in a crystallographic structure of parallel planes or layers
Carbon fibers are manufactured from two types of precursors (starting material) namely,1) Textile precursor Polyacrylonitrile (PAN)2) Pitch
The molecular structure of PAN contains highly polar CN groups that are randomly arranged on either side of chain,The filaments are wet spun from a solution of PAN and stretched at an elevated temperature during which the polymer chains are aligned in the filament directionThe stretched elements are then heated in air at 200C – 300C for a few hours ,during this stage the CN groups located on the same side of the original chain combine to form a more stable and rigid ladder structure and some of the CH2 groups are oxidizedNext step PAN filaments are carbonized by heating them at a controlled rate at 1000C -2000C in an inert atmosphere.
Tension is maintained on the filaments to prevent shrinking as well as to improve molecular orientationWith the elimination of oxygen and nitrogen atoms ,the filaments now contain mostly carbon atoms ,arranged in aromatic ring patterns in parallel planesCarbonized filaments are subsequently heat treated at or above 2000C their structure becomes more ordered and turns toward graphitic form with increasing heat treatment temperature.
POLYACRYLONITRILE (PAN) Wet spinning and stretching PAN FILAMENT Carbonization Heating and stretching at 1000C -2000C in an inert atmosphere for 30 min RELATIVELY LOW MODULUS (BETWEEN 200 & 300 GPa) HIGH STRENGTH CARBON FIBERS Heating above 2000C with or without stretchingGraphitization WITH OUT STRETCHING :RELATIVELY HIGH MODULUS (BETWEEN 500 & 600 GPa )CARBON FIBERS WITH STRETCHING :CARBON FIBERS WITH IMPROVED STRENGHT
Pitch is a byproduct of petroleum refining or coal cockingCarbon atoms in pitch are arranged in low molecular weight aromatic ring patterns ,Heating to temperature above 300C polymerizes these molecule into long , two dimensional sheet like structures .The highly viscous state of pitch at this stage is referred to as mesophasePitch filaments are produced by melt spinning mesophase passing through a spinneret die , the mesophase pitch molecules become aligned in the filament direction
The filaments are cooled to freeze the molecular orientation and subsequently heated between 200C and 300C in an oxygen containing atmosphere to stabilize them and make make them infusibleNext step filaments are carbonized at temperatures around 2000C.The rest of the process of transforming the structure to graphitic form similar to that followed for PAN precrsors.
PITCH (ISOTROPIC) Heat treatment at 300C-500C MESOPHASE PITCH (ANISOTROPIC) Melt spinning & drawing followed by heat stabilization at 200C-300 PITCH FILAMENT Heating and stretching at 1000C -2000C in an inert atmosphereCarbonization for 30 min RELATIVELY LOW MODULUS (BETWEEN 200 & 300 GPa) HIGH STRENGTH CARBON FIBERS Graphitization Heating above 2000C with or without stretching WITH OUT STRETCHING :RELATIVELY HIGH MODULUS (BETWEEN 500 & 600 GPa )CARBON FIBERS WITH STRETCHING :CARBON FIBERS WITH IMPROVED STRENGHT