Pressure Carbonization of Petroleum Pitches

13 Pressure Carbonization of Petroleum Pitches

Downloaded by UNIV OF ROCHESTER on January 17, 2018 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0021.ch013

R. L. BURNS and J. L. COOK Fiber Materials, Inc., Biddeford, Maine 04005

Densification of graphite and woven fibrous composites is accomplished by the impregnation of the open porosity with a liquid carbon precursor and the conversion or carbonization in situ to a solid carbon and the transformation to graphite by heat treatment. Conventionally, the densification process is conducted at atmospheric pressure and must be repeated several times to reduce the porosity to an acceptable level. The multiplicity of cycles and the ensuing long process times are the result of the following characteristics: 1) low-carbon yield from the carbonbearing liquid impregnant, 2) premature flow of the impregnant from the pores of an impregnated body during carbonization, 3) lack of sufficient impregnating pressure to thoroughly and uniformly impregnate a composite, 4) large volumetric shrinkage of the impregnant, 5) slow carbonization cycles required to obtain isothermal heating, and to minimize the effects of gas evolution from the carbonizing impregnant. As the result of these limiting characteristics, a technique was developed by Cook and Lambdin(1) at the Oak Ridge Y-12 Plant, Oak Ridge, TN, which utilized an isostatic pressure to effectively impregnate carbon composites continuously during the melting and heating phase of the carbonization cycle. This work showed that carbon or graphite fibrous composites with a pitch binder could be successfully compacted and carbonized in a sealed can with 95% conversion of the carbon precursor binder to solid carbon. Further work both at Oak Ridge and Fiber Materials showed that the pressure carbonization technique with its characteristic high carbon yields was advantageous to the densification of woven carbon and graphite structures. In related work, Price and Yates(2) baked graphite precursor composites to 800°C under pneumatic pressure at a temperature increase of 6°C per hour. This gave increased densities, reduced variability between samples, and permitted baking rates which, under normal conditions, would induce unacceptably high weight losses. Others(3) have used Isostatic mechanical pressures up to 2200 psi by means of an impermeable, deformable sample container. 139

Deviney and O'Grady; Petroleum Derived Carbons ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

P E T R O L E U M DERIVED CARBONS


140

Equal gas pressure was maintained and controlled inside the container by a separate connection until the desired process temperature (590°C) was reached. The Internal pressure was then released which permitted compaction of the preformed specimen. Analagous processing(4) reveals the confining of a molded composite of carbon in a substantially pressure-tight container in which gaseous pressures above atmospheric can develop during baking or by increasing the temperature. Recently, the studies of Dr. E. Fitzer and co-workers(5) in Germany have confirmed the u t i l i t y of the pressure-carbonization technique. Also, they have shown that the extreme pressures (15,000 psi) utilized in the early Oak Ridge studies were not required to obtain the Increased carbon y i e l d . This paper discusses the effect of pressure, from 500 to 15,000 psi on densification rate, microstructure, and coking yield for petroleum pitches. Microscopy is used to show the development of a carbon from the liquid precursor. Description of Pressure Carbonization Pressure-impregnation-carbonization (PIC) of pitch materials as a technique for the densification of carbon/carbon composites has received only a limited amount of attention. The PIC technique being employed by Fiber Materials is based on results of previous works(1,5) and the confirming data presented in Table I. Table I Effects Of Pressure On The Densification Rate For Pitch Impregnated C/C Composites Pressure (psi) Atm. 1,000 7,500 15,000

Coke Yield 51% 81% 88% 89% 90%

Density (g/cc) Initial Final 1.62 1.51 1.59 1.71 1.66

1.65 1.58 1.71 1.80 1.78

A schematic of the densification process is shown in Figure 1. A composite to be densified is i n i t i a l l y vacuum-pressure impregnated using conventional methods and carbonized while submerged in the pitch impregnant. This i n i t i a l cycle is not performed under pressure to prevent damage or distortion to the composite. When the part has completed this treatment, i t has the r i g i d i t y and strength to be processed through a PIC cycle. The PIC cycle Involves a stainless steel container holding the


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BURNS AND COOK

Carbonization of Petroleum Pitches

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Carbonization of Petroleum Pitches

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PETROLEUM DERIVED CARBONS


Literature Cited (1) Cook, J.L. and Lambdin, F., "Fabrication of Discontinuous, High-Fiber Content, Isotropic Carbon/Carbon Composites", Report No. Y-1784, Oak Ridge Y-12 Plant, Oak Ridge, TN (1971). (2) Price, M.S.T. and Yates, F.W.; Conference of Industrial Carbon and Graphite, pp 111-125; Society of Chemical Industry, London, England (1958) (3) Pressure Baking, USAF Contract Number AF33 (657) 11738 with Great Lakes Research Corporation; Elizabethton, TN (1964) (4) "Improvements in or Relating to Production of Carbon Masses", United Kingdom Patent 759, 160. (5) Dr. E. Fitzer and B. Terwiesch, Carbon Vol. 11, pp 570-574 (1973).


Pressure Carbonization of Petroleum Pitches

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