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    A Study on Structural Characterization of Thermally Stabilized PAN Precursor Fibers Impregnated with Ammonium Bromide before Carbonization Stage
    (Korean Fiber Soc, 2022) Demirel, Tuba; Rahman, Md. Mahbubor; Tuncel, Kemal Sahin; Karacan, Ismail
    Thermal-oxidative stabilization of polyacrylonitrile (PAN) fiber pretreated with ammonium bromide (NH4Br) was performed in the air atmosphere at temperatures between 200 and 250 degrees C for periods of 5 to 75 min in a multistep approach. The study demonstrates that the NH4Br incorporation is highly effective in accelerating nitrile group cyclization by reducing the time required to form a thermally stable structure. After 60 min of the multistep stabilization, NH4Br incorporated and stabilized PAN was entirely thermally stable, infusible, and non-burning. XRD analysis showed the conversion of the pristine PAN molecular structure from a laterally ordered condition to a very disordered amorphous structure by crosslinking and aromatization process. Infrared analysis indicated rapid and concurrent aromatization and dehydrogenation reactions assisted by the formation of oxygen-containing functional groups. With the progression of the stabilization period, TGA thermograms revealed a comparative increase in thermal stability, as directed by the continuous rise of carbon yield. By decreasing the required time for the stabilization process of PAN fiber, the use of NH4Br impregnation is expected to enhance carbon fiber productivity at a reduced cost considerably.
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    Accelerated multistep thermal stabilization of polyacrylonitrile fibers using an ethylenediamine pretreatment
    (Springer, 2024) Tuncel, Kemal Sahin; Demirel, Tuba; Karacan, Ismail
    The polyacrylonitrile multifilament yarn underwent a multistep heat treatment process including stabilization times ranging from 5 to 75 min following impregnation with a 30% ethylenediamine (EDA) aqueous solution. A series of measurements were employed to determine the structure and properties of thermally stabilized PAN samples. These included fiber thickness, fiber density, flame testing, tensile testing, X-ray diffraction, thermal analysis (differential scanning calorimetry and thermogravimetric analysis), and infrared spectroscopy. The results from XRD and IR spectroscopy indicated that the rate of aromatization reactions increased with longer stabilization times. A detailed examination of the XRD curves obtained through curve-fitting procedures suggested a rapid transformation of the original structure into a disordered amorphous phase containing pre-graphitic domains, evidenced by a gradual reduction in the degree of apparent crystallinity of the original PAN sample. The integration of EDA before the thermal stabilization stage significantly reduced the cyclization time of nitrile groups in the PAN polymer structure, thereby accelerating the stabilization reactions. This chemical pretreatment also improved the thermal stability of the samples by promoting oxidative cross-linking of the PAN polymer chains. After a 75 min multistep stabilization, the carbon yield at 1000 degrees C was 70.5%. Conversion index values, calculated using IR, XRD, and DSC methods, were 98.3%, 94.8%, and 89.5% respectively for the 75 min sample. These findings highlight the importance of EDA in accelerating the formation of an aromatic structure, which is critical for withstanding the high temperatures of subsequent carbonization stages.
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    An Evaluation of the Beneficial Effects of Polyamide 6's Thermal Stabilization by Ferric Chloride Complexation as a Novel Carbon Fiber Precursor
    (Korean Fiber Soc, 2024) Demirel, Tuba; Tuncel, Kemal Sahin; Karacan, Ismail
    This study investigated the impact of stabilization time on pretreated polyamide 6 (PA6) fibers using various analytical techniques, including thermal analysis (TGA and DSC), infrared (IR) spectroscopy, X-ray diffraction (XRD), tensile testing, and density measurements. The two-step atmospheric air-based stabilization process for pretreated PA6 multifilament bundles involved initial thermal stabilization at 170 degrees C in an air atmosphere after ferric chloride impregnation, followed by a second step of thermal stabilization at 245 degrees C. Ferric chloride impregnation followed by thermal stabilization in an air atmosphere resulted in crucial structural transformations. The density values of the samples increased following thermal stabilization, accompanied by a decrease in tensile values. Ferric chloride pretreated and thermally stabilized PA6 fibers were found to be fully stabilized after 120 min of stabilization before the carbonization stage. The findings obtained from the DSC, XRD, and IR spectroscopy methods indicated the occurrence of disordering phases due to the scission of hydrogen bonds. The TGA findings showed significant increases in carbon yield percentages at 500 degrees C and 850 degrees C, reaching 71.4% and 63.5%, respectively, for the sample heat treated at 245 degrees C for 120 min. The addition of ferric chloride is expected to potentially reduce processing costs for final carbon fiber production by decreasing the time required for the thermal stabilization of PA6.
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    The beneficial effect of eco-friendly chemical impregnation on the thermal stabilization process of poly(hexamethylene adipamide) multifilament
    (Elsevier, 2022) Rahman, Md Mahbubor; Demirel, Tuba; Tuncel, Kemal Sahin; Karacan, Ismail
    The influence of the impregnation of phosphoric acid, boric acid, and urea (in brief PBU) as eco-friendly chemicals and the thermal-oxidative stabilization (TOS) on the properties of poly(hexamethylene adipamide) or polyamide 66 multifilament were studied at temperatures up to 245 degrees C for different stabilization periods. PBU impregnation followed by a two-step TOS process in an air environment leads to major changes in the structural, physical, and mechanical properties of the polyamide 66 (PA66) multifilaments. In this study, X-ray diffraction (XRD), infrared (IR) spectroscopy techniques, thermogravimetric analysis (TGA), and optical microscopy were employed to perform structural characterization of the original, and PBU impregnated and thermally stabilized samples. Characterization of physical changes was accomplished by observing fiber burning behavior, change of color, linear density, fiber density, fiber thickness, and mechanical properties for the different stabilization periods. The outcomes showed that PBU pretreatment enhanced the thermal stability of PA66 multifilaments before the carbonization stage. Continuous loss of crystalline structure was observed from the investigation of the equatorial X-ray diffraction results. This loss is caused by disordering methods due to the breakage of hydrogen bonding (H-bonding) of the PA66 chains. The TGA thermograms revealed a comparative enhancement in thermal stability of the samples by the increased char yield percentage with an increasing stabilization period. The PBU impregnated PA66 multifilaments oxidized at 245 degrees C for 150 min showed the highest value of carbon yield of 47% at 1000 degrees C. Infrared analysis showed the steady loss of intermolecular and intramolecular H-bonding due to the concurrently occurred dehydration and dehydrogenation reactions. (C) 2022 Published by Elsevier B.V.
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    The effect of the ammonium persulfate and a multi-step annealing approach during thermal stabilization of polyacrylonitrile multifilament prior to carbonization
    (Springer, 2021) Rahman, Md Mahbubor; Demirel, Tuba; Tuncel, Kemal Sahin; Karacan, Ismail
    Thermal stabilization of polyacrylonitrile (PAN) multifilaments was performed using a multi-step annealing approach in an air environment. Ammonium persulfate (APS) pretreatment of PAN multifilaments was performed prior to the thermal stabilization process. APS pretreatment of PAN resulted in the sulfation of polymer chains. The results suggest that APS pretreatment accelerated the thermal stabilization reactions of the PAN multifilaments. Thermal treatment of APS-treated PAN multifilaments resulted in enhanced thermal stability. It seems that APS promoted the oxidative cross-linking of polymer chains. The structure and properties of thermally stabilized samples were determined employing a set of measurements, such as fiber thickness, linear density, volume density, flame test, mechanical properties, X-ray diffraction (XRD), thermogravimetric analysis (TGA), infrared (IR) spectroscopy, and scanning electron microscopy (SEM). Analysis of the X-ray diffraction and FT-IR spectroscopy results suggested rapid aromatization reactions with increasing stabilization times. Detailed analysis of X-ray diffraction curves obtained after a curve-fitting procedure suggested the rapid transformation of the original structure to a totally disordered amorphous phase containing pre-graphitic domains as shown by the gradual reduction in the degree of apparent crystallinity of the original PAN multifilament. The experimental results suggest that ammonium persulfate was very effective and efficient in promoting stabilization reactions. The use of APS is expected to reduce the overall manufacturing cost of the final carbon fiber processing by reducing the time needed for the thermal stabilization step. [GRAPHICS] .
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    The impact of guanidine carbonate incorporation on the molecular structure of polyacrylonitrile precursor fiber stabilized by a multistep heat treatment strategy
    (Wiley, 2022) Tuncel, Kemal Sahin; Rahman, Md Mahbubor; Demirel, Tuba; Karacan, Ismail
    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 degrees C to 250 degrees 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.