Structural, Adsorption, and Mass-Transfer Properties of Polycaproamide

Abstract

Results from a comprehensive investigation of the structural and adsorption properties of fibrous polycaproamide were presented. An analysis of them allowed polycaproamide to be assessed as a desiccant. Mechanistic features of the interaction of polycaproamide with water were examined. The adsorption required to design a drying process was determined. The mass-transfer coefficient was analyzed as a function of moisture content and drying temperature.Drying of polycaproamide (PCA) granules after extraction from them of low-molecular-mass compounds by water is an important stage in the industrial manufacturing of polyamide fiber. PCA must be highly dried (to C = 0.05%) because a moisture content above 0.1% promotes polymer destruction on the melt grid, decreases its degree of polymerization, and increases fiber breakage.The important characteristics of a desiccant are its structural, adsorption, and mass-transfer properties, which must be known in order to select a drying method and regime [1][2][3].The structure of PCA samples was investigated using three independent methods in order to obtain the most complete representation of it and to compare the results. Mercury porosimetry and electron microscopy studied the porosity, which was secondary for this desiccant. The finer structure was investigated by adsorption, which enabled structural features of the dried material to be evaluated. Adsorption is known to measure pore sizes in the range from 2⋅10 -9 to 5⋅10 -8 m; mercury porosimetry, from 10 -6 to 10 -9 m [2]. Figure 1 shows integral and differential characteristics of the PCA secondary porosity that were obtained using pressurized mercury [4] considering its compressibility. The PCA pore volume determined by mercury porosimetry was practically the same as the total pore volume calculated from the difference of inverse quantities, i.e., the apparent (1120 kg/m 3 , Hg) and true (1150 kg/m 3 , benzene) densities, and was 0.022 cm 3 /g. The predominant pore radius R was 1,000 Å. Such pores are filled only by direct contact with the liquid and not during adsorption. They were probably present in PCA not because of the supramolecular structure but because of mechanical defects during its production.Photographs of a fracture of a PCA chip (Fig. 2) were obtained using a Stereo scan electron microscope at magnification 2000× and 1000× and confirmed that the material had large pores.The adsorption isotherm of water vapor by PCA (Fig. 3) was taken in an evacuated sorption apparatus with a MacBain-Bakr balance.The initial part of the adsorption isotherm (for P/P s up to 0.3 and material moisture content <2%) was approximated satisfactorily by Henry's law:where a is the adsorption; Γ, Henry's coefficient; and P/P s , the air relative humidity.