Thermal Degradation of Polymers to Polymeric Carbon—An Approach to the Synthesis of New Materials

Abstract

Dedicated to Professor Matthias Seefeider on the occasion of his 60th birthdayOne hundred years ago, Edison succeeded in preparing carbon fibers for his incandescent lamp bulb by thermal decomposition of natural polymeric fibers. Ten years ago, progress reports about "Novel Forms of Carbon" predicted outstanding properties and promising new applications for the carbonization products of synthetic polymers. Research and development in this field have been promoted by the problems of conventional technology (shortages of raw material and energy, pollution problems). Polymeric carbon materials-prepared by thermal degradation of synthetic polymers-exhibit a special ribbon-like microstructure. They will provide the chemist with many challenges. IntroductionThe thermal degradation of polymeric, organic natural substances, which is one of the oldest technologies of mankind, results in residues with high carbon content. They have a porous structure and low strength, because the cellular structure of the natural precursor material is preserved after carbonization. Moreover, the fibrilous structure of fibrous natural precursor materials is also found in the carbon residues. The mass loss due to elimination of volatile by-products during pyrolysis causes additional porosity and, above all, isotropic shrinkage. Only low carbon yields (< 30%) are obtained if no special chemical pretreatment of the precursor is performed before the carbonization. An essential condition for making carbon fibers from polymers is carbonization as a solid without softening or melting. This was realized in case of the carbon fiber in Edison's incandescent light bulbs and also in case of the first high strength carbon fibers produced from rayon in the 1960'~['.~]. The revolutionary increase of strength and stiffness of the carbon fibers in the last ten years has not been accomplished merely by chemical means, i. e. by using fully synthetic polymer fibers, but by way of morphological changes in the initially formed porous carbon material. These morphological changes can be induced by hot stretching or hot working, at temperatures above 2600 "C (cf.l31), as in the hot working of glasses or metals.The hot working of carbon requires similar techniques to the hot formation of metals, the drawing of molten glass, or methods of shaping for thermoplastics, but at much higher temperatures. In the wake of this success of hot-stretched carbon fibers not only were prospects for the applications of carbon fibers discussed, but chemical research was also started on the pyrolysis mechanism of polymers[41. It was expected that the properties important for the technical appli- 375 cation can be improved by using suitable synthetic polymers instead of rayon as precursor material. The chemist may be surprised that a material with improved properties should result on decomposition and destruction of the polymeric structure. It is therefore appropriate to consider the outstanding properties of carbon. Pure carbon is resistant to attack by weather, water, fungicides, and...

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