Franz-Josef Wortmann

Temperature dependence of the water-sorption isotherms of wool

Against the background of various other theories, the well-established DArcy/Watt model is applied to sorption data of wool between 20 and 100°C to determine its suitability to describe the isotherms and to systematize their temperature dependence. The model contains three components that represent two types of primary adsorption processes (Langmuir- and Henry-type adsorption) and a third one, describing multilayer formation of water molecules. Sorption isotherm data, as taken from the literature, could, in all cases, be fitted extremely well by the model. The temperature dependence of the five parameters of the model, related to the continuous decrease of water regain with increasing temperature for all humidities, reveals a number of inconsistencies. Probably the most important of these is that the Langmuir capacity constant apparently becomes zero at the glass transition temperature of dry wool. This is at variance with the idea of specific molecular sites of water sorption, which is inherent to the model. Other inconsistencies relate to the small vant Hoff enthalpies and possible compensation effects for various parameters. These observations indicate that the DArcy/Watt theory, despite its physicochemical plausibility and empirical success, overinterprets the complexity of the mechanisms underlying the sorption behavior of wool and other α-keratin fibers.

Keratin made micro-tubes: The paradoxical thermal behavior of cortex and cuticle.

Keratin micro-tubes were obtained by heating medullated keratin fibres to temperatures above 230°C under nitrogen atmosphere, when, as documented by microscopy, the cortex (the core of the fibre) melts from the medulla outwards, followed by pyrolysis of the material through the remaining solid cuticle (shell) layer. The resulted hollow tubes from fibres void of cortical material keep the external cuticle structure, as shown by AFM investigation, and the moisture sorption properties of the initial keratin fibre. Despite similar amino-acid compositions of cuticle and cortex the two morphological components differ significantly in their thermal behaviour, which appears to be a cortex-cuticle thermal stability paradox.

The desorption of moisture from wool

We have carried out a DSC investigation under nitrogen flow of moisture desorption from wool equilibrated with various amounts of water. The recorded data have been used to calculate the kinetic parameters of the desorption process assuming a single process and within the framework of the reaction order. We found that while the reaction order and the peak temperature change with the equilibrium water content (e.w.c.) which wool fibre had at the beginning of the desorption experiment, the activation energy and the pre-exponential factor remain fairly constant. The overall reaction rate shows that, due to the change of the kinetics of wool-water system with the change of the initial moisture content, higher the initial water content of the fibre is, faster the desorption occurs. The results point out clearly that any study of moisture desorption from keratins should specify the initial value of the equilibrium water content.