Investigation of structural transformations taking place during oxidative stabilization of viscose rayon precursor fibers prior to carbonization and activation

An investigation was carried out on the physical and structural transformations occurring during oxidative stabilization of viscose rayon precursor fibers impregnated with diammonium hydrogen phosphate (DAP). Structural characterization was performed using optical microscopy, X-ray diffraction, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and infrared (IR) spectroscopy techniques. The results indicated that the use of DAP impregnation improved the thermal stability of viscose rayon fibers prior to carbonization and activation steps. Analysis of the equatorial X-ray diffraction traces demonstrated gradual loss of crystalline structure due to the disordering processes caused by the interruption of intermolecular hydrogen bonds. A novel X-ray stabilization index for the evaluation of oxidative stabilization was devised specifically for the oxidized viscose rayon fibers. The results obtained from DSC and TGA measurements demonstrated that there was an improvement in the thermal stability due to the formation of an increasing amount of ladder-like structures containing aromatic entities with progressing temperature. The results obtained from the analysis of IR spectra showed the gradual and continuous loss of intramolecular and intermolecular hydrogen bonds associated with dehydrogenation and dehydration reactions. IR spectra demonstrated the loss of crystallinity with progressing temperature confirming the results obtained from X-ray diffraction measurements. IR spectra also demonstrated the formation of double bonded C= C bonds attributed to the formation of a crosslinked ladder-like structure. DSC and TGA measurements show that the sample oxidized at 250 C possesses high thermal stability which can be used for the subsequent carbonization and activation steps. (C) 2013 Elsevier B.V. All rights reserved

Structure and properties of oxidatively stabilized viscose rayon fibers impregnated with boric acid and phosphoric acid prior to carbonization and activation steps

The role of boric acid-phosphoric acid (BA-PA) impregnation and oxidation on the structure and properties of viscose rayon fibers was examined in air at temperatures ranging from 150 to 250 A degrees C. The results obtained from the measurements of fiber thickness, linear density, X-ray diffraction, thermal analysis (DSC and TGA), and infrared spectroscopy demonstrated that oxidation temperature had a significant influence on the structure and properties of oxidized viscose rayon fibers. Physical transformations were characterized by fiber thickness and linear density values together with color variations and improved burning behavior with progressing oxidation temperature. The DSC analysis showed that BA-PA impregnation enhanced thermal stability and prevented the evolution of volatile by-products by blocking the primary hydroxyl groups. TGA thermograms revealed an enhancement in the char yields. X-ray diffraction analysis showed the loss of cellulose II crystalline structure caused by the decrystallization process initiated by the gradual loss of intermolecular hydrogen bonds. Analysis of IR spectra revealed gradual and continuous loss of intramolecular and intermolecular hydrogen bonding as part of the simultaneously occurring dehydrogenation and dehydration reactions. Analysis of IR data also demonstrated the disturbance of the cellulose II crystalline structure with increasing oxidation temperature in agreement with the results obtained from X-ray diffraction measurements. The formation of C=C bonds attributed to the crosslinked ladder-like structure was also confirmed by the IR spectra.The role of boric acid–phosphoric acid (BA–PA) impregnation and oxidation on the structure and properties of viscose rayon fibers was examined in air at temperatures ranging from 150 to 250 C. The results obtained from the measurements of fiber thickness, lineardensity, X-ray diffraction, thermal analysis (DSC and TGA), and infrared spectroscopy demonstrated that oxidation temperature had a significant influence on the structure and properties of oxidized viscose rayon fibers. Physical transformations were characterized by fiber thickness and linear density values together with color variations and improved burning behavior with progressing oxidation temperature. The DSC analysis showed that BA–PA impregnation enhanced thermal stability and prevented the evolution of volatile by-products by blocking the primary hydroxyl groups. TGA thermograms revealed an enhancement in the char yields. X-ray diffraction analysis showed the loss of cellulose II crystalline structure caused by the decrystallization process initiated by the gradual lossof intermolecular hydrogen bonds. Analysis of IR spectra revealed gradual and continuous loss of intramolecular and intermolecular hydrogen bonding as part of the simultaneouslyoccurring dehydrogenation and dehydration reactions.Analysis of IR data also demonstrated the disturbance of the cellulose II crystalline structure with increasing oxidation temperature in agreement with the results obtained from X-ray diffraction measurements. The formation of C=C bonds attributed to the crosslinked ladder-like structure was also confirmed by the IR spectra