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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.