Hot Stretching during Processing of Carbon Fiber

As mentioned before, the early fiber made from a cellulosic precursor had a low modulus and manufacturers had to resort to hot stretching to orient the fine structure of the fiber and obtain a significant improvement of modulus, albeit at a cost. This approach is not specific to carbon fiber and stretching is widely used to improve properties of many manmade fibers e.g. viscose rayon. 

Stretching cannot be undertaken in the pyrolysis stage due to the low strength of the fiber. 

The modulus of the fibers is dependent on the orientation of the individual graphite planes and the tensile strength is dependent on gage length, but does increase as the modulus increases. High modulus carbon fibers are practically 100% carbon and Bacon and Schalamon [36] report that the structure is of small ribbon-like graphite layers, preferentially 

Spry [39] claims that treating a rayon yarn previously carbonized at 1200°C followed by treatment at 2800°C with stretching at this latter temperature, produeed a six-fold inerease in modulus and a three-and-a-half-fold increase in strength. 

For many applications where the product is to be used as a packing material, it is necessary to apply a size to aid in braiding the product and ensure that the packing material beds down well. These sizes are specific to a given application and can vary from PTFE, polyvinyl pyrrolidone 

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