The beneficial effect of eco-friendly chemical impregnation on the thermal stabilization process of poly(hexamethylene adipamide) multifilament

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 °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 °C for 150 min showed the highest value of carbon yield of 47% at 1000 °C. Infrared analysis showed the steady loss of intermolecular and intramolecular H-bonding due to the concurrently occurred dehydration and dehydrogenation reactions.