Thermal stabilization of poly(hexamethylene adipamide) (polyamide 66, PA66) fibers in the presence of ferric chloride was performed in air at 215 degrees C for stabilization times ranging from 15 min to 6 h. The presence of ferric ions in the PA66 structure enhanced thermal stabilization reactions. Optical microscopy, density, elemental analysis, X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and infrared (IR) spectroscopy measurements were used to examine and characterize the structural transformations occurring during the thermal stabilization. Ferric chloride (FeCl3) pretreatment followed by thermal stabilization in an air atmosphere resulted in major physical and structural transformations. Physical transformations were revealed as a reduction in fiber diameter and an increase in density values together with major color changes. Ferric chloride pretreated and thermally stabilized PA66 fibers were found to be fully stabilized after 6 h of stabilization and acquired infusible and nonburning characteristics prior to the carbonization stage. Elemental analysis showed a reduction in carbon, nitrogen and hydrogen contents. The DSC and TGA measurements demonstrated that there was an improvement in the thermal stability. The TGA thermograms showed a relative improvement in thermal stability as indicated by increasing char yield with progressing time. The char yield reached a maximum value of 40% at 1000 degrees C for the ferric chloride pretreated PA66 fibers stabilized for 6 h at 215 degrees C. Experimental results obtained from the DSC, X-ray diffraction and infrared spectroscopy methods indicated gradual and continuous disordering reactions as a result of the scission of the hydrogen bonds with progressing time. The IR measurements also indicated the formation of the carbonyl (C=O) group as a result of oxidative crosslinking reactions and the occurrence of dehydrogenation reactions caused by the loss of hydrogens from the methylene (CH2) groups in agreement with the elemental analysis. Sample preparation appeared to play an important role in the formation of ferric ion coordination bonds with the carbonyl oxygen atoms. The unconstrained stabilization method caused an enhancement of the complexation of ferric ions with the amide groups. A new method of evaluating the X-ray stabilization index was devised specifically for the thermally stabilized PA66 fiber. (C) 2013 Elsevier Ltd. All rights reserved.