No abstract

The effect of carbonization temperature on the structure and properties of poly(p-phenylene terephthalamide)-based carbon fibers from commercially available Twaron ® (PPTA) presursor is reported. Turbostratic PPTA-based carbon fibers were produced using a single step procedure in an inert atmosphere at temperatures ranging from 600 to 1100 o C. In the present study, fiber diameter, mass yield, density, elemental analysis, X-ray diffraction, Raman spectroscopy, tensile testing and electrical conductivity measurements were performed and evaluated to follow and monitor the properties and structural transformations of carbon fibers with rising temperature. The increase of heat-treatment temperature to 1100 o C decreased the interlayer d-spacing (d 0 0 2 ) and increased the in-plane size (L a ) and thickness (L c ) of the graphene layers. The intensity ratios of D to G bands in the Raman spectra increased with rising temperature, suggesting, in agreement with the X-ray diffraction measurements, that the in-plane size (L a ) of the graphene planes increased with temperature. The density, carbon content, C/H ratio, apparent crystallite size (L a and L c ), electrical conductivity and tensile properties of the resultant carbon fibers were enhanced with rising temperature. It has been shown that the gage length of the carbon fibers tested has a significant effect on the tensile strength obtained. After taking into account the effects of gage length and porosity dependence, the carbonization of PPTA precursor fibers prepared at 1100 o C gave a tensile strength of 191 MPa and a tensile modulus of 83 GPa, respectively.

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.