The conversion of poly (hexamethylene adipamide) or polyamide 66 precursor fiber to carbon fibers was accomplished through thermal stabilization and carbonization processes. Thermal stabilization was conducted of cupric chloride (CuCl2)–ethanol-impregnated polyamide 66 (PA66) fibers in the air. To determine the influence of heating rate on the fiber structure and properties of the resultant carbon fibers, carbonization experiments were performed at selected temperatures of 500, 700, 900, and 1100°C using 2.5 and 5 °C/min heating rates with no dwelling. The results conclusively revealed that the volume density and tensile properties of the PA66 fiber were higher at 2.5 °C/min heating rate. After fixing the heating rate as 2.5°C/min, further carbonization experiments were conducted at temperatures from 500 to 1100°C, using increments of 100°C with no dwelling time. Linear density, volume density, fiber diameter, carbon yield, elemental composition, tensile, and electrical properties exhibited a strong dependence on the carbonization temperature. After taking into account the effects of structural defects (i.e., microvoids), tensile strength, and tensile modulus of the carbon fibers increased to 794 MPa and 92.4 GPa, respectively, when carbonized at 1100°C. X-ray diffraction analysis of the carbon fibers further revealed the existence of a greatly disordered (i.e., amorphous) structure, which developed during the carbonization process. FT-IR analysis confirmed the formation of highly aromatic carbon clusters at temperatures of 500°C and higher. The outcomes of electrical conductivity in this study confirm that the PA66 precursor was converted into a semi-conducting state once it was carbonized.
Thermal‐oxidative stabilization of the polyacrylonitrile (PAN) precursor was performed employing a multi‐step heat treatment strategy in an air circulating furnace. In this approach, the applied temperature was gradually increased from 200°C to 250°C employing several stages for different stabilization durations. Fifteen percent guanidine carbonate (GC) was found as optimum to incorporate with the PAN precursor fibers to accelerate the thermal‐oxidative stabilization process. Characterization techniques, including X‐ray diffraction (XRD), Fourier transform infrared spectroscopy (FT‐IR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), tensile strength, volume density, linear density, fiber thickness, and burning test have been performed to monitor the changes in PAN structure. Test results of the stabilized samples were compared with the reference sample results to demonstrate the accelerating effect of GC integration. Findings showed that GC pretreatment enhanced and accelerated the cyclization of nitrile groups in the PAN polymer structure and allowed the quicker formation of a thermally stable structure. The analysis of the experimental results revealed that GC integration and employing the multi‐step heat treatment strategy helps greatly to cut the overall PAN‐based carbon fiber production cost.
The structure and effects of thermally stabilized PAN original fibers were characterized utilizing a mixture of volume density, color change observations, flame tests, X-ray diffraction (XRD), infrared spectroscopy (FT-IR), and thermogravimetric analysis (TGA) measurements. The results obtained from the analysis of XRD work showed the conversion of the original molecular structure from a highly laterally ordered condition to a disordered amorphous structure. The experimental results acquired from FT-IR analysis indicated rapid and concurrent aromatization and dehydrogenation reactions assisted by the formation of oxygen-containing functional groups. TGA analysis showed a carbon yield of 72% at 1000 °C. The application and use of NH4Br pretreatment are expected to increase the productivity of carbon fiber processing at lowered cost by significantly reducing the processing time necessary for the successful completion of thermal stabilization reactions.
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