1. Characterising methods Wetting and penetration behaviour
A new concept to understand the liquid penetration into polyester fabrics
Alfredo Calvimontes1, Victoria Dutschk1, Gerd Heinrich1, Chokri Cherif2
1Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Straße 6, Dresden
2Institut für Textil- und Bekleidungstechnik, Technische Universität Dresden, Dresden
Abstract
Previous work has shown that there are significant differences between the soiling behaviour and cleanability of polyester textile materials with different topographic structures despite the similarity of their chemical nature. Toward a better mechanistic understanding soil removal effects as well as a quantitative correlations between them and topographical non-identity of fabrics, previous roughness and dynamic wetting results, were analysed in respect of liquid transport driven into a porous system by capillary forces. In general, wetting of a fibrous assembly, such as a fabric, is a complex process. Particularly, capillary flow is not determined by a constant advancing contact angle, as frequently assumed, but it depends on a dynamic contact angle corresponding to the instantaneous velocity of the moving meniscus.
Materials
In this study, polyester fabrics with three different basic types of weave – plain, twill and Panama – were manufactured at the ITB Dresden.
Characterising methods
Surface topography of fabrics was obtained using an imaging optical sensor MicroGlider (FRT, Germany), operating on the principle of chromatic aberration.
Dynamic wetting measurements were performed with an automatic absorption and contact angle tester FibroDAT 1122HS (Fibro System, Sweden) in a temperature and humidity controlled laboratory maintained 23  1 °C and 50  4%, respectively.
Cleanability of fabrics, stained with a carbon black-paraffin oil mixture in the ratio of 98:2, was characterised through digitalising their intensity images; the soil-additional-density (SAD) was obtained using a scanner.
Results
Wetting and penetration behaviour
On the basis of macroscopic water drop base changes, the wetting behaviour of the water drop can be divided into three different regimes:
dynamic wetting (spreading), quasi-static wetting and penetration.
Wetting dynamics
Mesomorphology of three different basic types of weave
Plain weave: an increase of the waviness depth causes a decrease of the spreading rate; warp threads (‘hills’) slow down the liquid motion.
Twill weave: an increase of the waviness depth causes formation of deep and long domains of weft threads (‘canals’) with small ‘islands’. As a consequence, an increase of the spreading rate is observed.
Panama weave: an increase of the waviness depth causes formation of long and und quasi-endless (without ‘islands’) deep domains (‘canals’). Consequently, the waviness depth and spreading rate are proportional to each other.
Comparison of fabric surface topography between three different types of weave demonstrates that mesomorphology controls the spreading rate of a liquid drop on textile structures.
Quasi-static wetting
On a small scale, below 500 µm, topography measurements provide important information about changes in textile microstructures.
Note, a study of micro-roughness of weft and warp treads have to be carried out on a micro-scale separately. From this point of view, the behaviour of a liquid drop on a fabric surface during the wetting or spreading process can be explained. As a textile surface consists of parallel horizontal und vertical capillaries, the surface micro-roughness controls the wetting behaviour of a liquid.
There is a limiting value of micro-roughness (about 20 µm), from which the wetting behaviour drastically changes from hydrophobic to hydrophilic. This phenomenon can be explained using approaches according to Wenzel and Cassie: the little a number of contact points with the liquid drop (small solid fraction), the hydrophobic the surface. For a solid surface it means, that the rougher the surface, the more single filaments exist, therefore, the smaller solid fraction. However, a textile surface will be better wettable having a certain roughness value, because the capillarity will increased with increasing roughness.
(b)
(a)
Micro-morphology (micro-roughness) controls the wetting behaviour of a liquid drop on fabric surfaces: (a) warp thread, (b) weft thread.