After curing, phenol-formaldehyde resins were postcured at 230°C in air for 32 h and then carbonized and graphitized from 300 to 2400°C. Thermal fragmentation and condensation of the polymer structure occurred above 300°C. The crystal size of the cured phenolic resins decreased with the temperature increase. Above 600°C the original resin structures disappeared completely. Below 1000°C the stack size (L c ) and crystal size (L a ) were small. Above 1000°C the L c increased with the increasing treatment temperature. The carbonized and graphitized resins were characterized using Raman spectroscopy. Below 400°C there were no carbon structures in the Raman spectra analysis. Above 500°C the G and D bands appeared. The frequency of the G band of all carbonized and graphitized samples shifted to 1600 cm Ϫ1 from the 1582 cm Ϫ1 of graphite. The D band shifted to 1330 cm Ϫ1 from the 1357 cm Ϫ1 of the imperfect carbon. The carbonized and graphitized phenolic resins could not be considered as truly glassy or amorphous carbon materials because they had some degree of order in the basal plane. However, the crystal size was very small even at 2400°C.
SYNOPSISModification of polyacrylonitrile (PAN) fibers with potassium permanganate has reduced the time required for stabilization and also improved the mechanical properties of the resulting carbon fibers. In this study, the effect of modification on the physical properties, microstructure, and elemental composition of fibers during the carbonization process was examined for the first time. The resulting carbon fibers developed from modified PAN fibers had a higher density, a greater stacking size ( L c ) , and a higher preferred orientation than those developed from unmodified PAN fibers. The carbon fibers developed from the modified PAN fibers also showed an improvement in tensile strength from 20 to 40%. These fibers showed a radial structure in the fracture surface and were somewhat different structurally in the cross section than were the carbon fibers developed from the original PAN fibers. A model for the structure of both carbon fibers is presented. The relationship between the formation of closed pores from open pores and the variety of cumulative pore area during the heat-treatment stage is also discussed.
SynopsisPolyacrylonitrile (PAN) fibers pretreated with potassium permanganate have reduced the activation energy of cyclization and the oxidation time, and also improved the properties of the resultant carbon fibers. The activation energy of cyclization is reduced to 24 5 1 kcal/mol from 30 3 kcal/mol and the tensile strength of the carbon fibers increases by about 20-40%. The method of measuring the "aromatization index" (AI) value is modified and is recommended in checking the oxidation process. All fully stabilized PAN fibers have almost the same A1 value. However, the oxidation time is decreased by 30 min when oxidation temperature is raised by looc. INTRODUCTIONPolyacrylonitrile (PAN) fiber has been used as the precursor for making high-performance carbon fibers. An important step in preparing carbon fiber from the PAN fiber is to heat the precursor from 200 to 300°C in the air. Morita et a1.l and Raskovic and Marinkovic2 stated that treating PAN fiber by sulfur dioxide gas instead of air oxidation, is a better method of stabilizing fiber structure, and results in more economical production of high-performance carbon fibers.It has been established that heat treatment of PAN fiber under a tension load improves fiber quality.3* However, recent trends in developing highperformance carbon fibers from PAN is through chemical modification of the precursor with certain additives before oxidation. Mathur et al.5 and Bahl et a1.6 have pretreated PAN fiber with CuCl to make high-performance carbon fibers. Our previous study7 presented the preliminary results of the process taking place during the pretreatment of PAN fibers with potassium permanganate. This study offers further details of our investigations, the microstructure, physical, and mechanical properties of the oxidized fibers. EXPERIMENTALA special grade acrylic fiber, Courtelle fiber (Courtaulds Limited, Coventry, UK), containing 6% methyl acrylate and 1% (itaconic) acid copolymer was used in our work. Each single tow of the fiber contains 6000 strands of 1.1 denier monofilament. A fiber-wound frame was pretreated by immersing it in a 7% potassium permanganate solution at 85°C for a few minutes, then washed with distilled water and dried to a constant weight in an oven. The color changed from clear white to brown after pretreatment. The manganese content in the prepared carbon fibers was determined by atomic emission spectrometer analysis and found to be 25 ppm for the untreated, and 1630 Oxidized PAN fibers and the pretreated precursor were heat-treated in a constant temperature zone furnace under constant load (0.1 g/denier) a t 230, 250, and 270°C from 1 to 10 h, respectively, in a purified air atmosphere. The oxidized fiber tows were carbonized to 1300°C in a ceramic reaction tube under oxygen-free Ar atmosphere. A heating rate of 250°C/h was carried out up to lOOO"C, then continued a t 6OoC/h until reaching 1300"C, then the specimens were cooled down immediately.Mechanical properties of PAN and stabilized fibers were measured by Instron 1122 tensile-tes...
SYNOPSISA continuous stabilization and carbonization process was used to prepare polyacrylonitrile (PAN) -based carbon fibers. The stepwise stabilization of PAN fibers was tried at various temperatures. The effect of stepwise stabilization on the physical properties and microstructure of the final carbon fibers is reported in this article. The fixed temperature in stepwise stabilization is kept below the fusion temperature of PAN precursors to avoid overstabilization of the fibers. The optimum stepwise stabilization process not only increases the amount of ladder polymer in stabilized fiber but also improves the physical and mechanical properties of the resultant carbon fibers. The formation of closed pores from open pores in carbon fiber occurs at llOO°C, but the formation of closed pores occurs at 200°C lower for carbon fiber developed from overstabilized fiber. The effect of continuous stepwise stabilization on the properties of resulting stabilized fibers and the variation in physical properties, element composition, and microstructure of carbon fibers during the carbonization process are also reported in this article.
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