Reactivity and electrokinetical properties of different types of regenerated cellulose fibres

Abstract

Comparative investigations between new lyocell, conventional viscose and modal fibres were made in order to explain the reasons for the differences in the reactivity and electrokinetic properties of these fibres. Lyocell fibres are a new generation of regenerated cellulose fibres produced using environmentally friendly process, which account for the main differences in the fine structure of each fibre despite having the same chemical composition. Analyses of the structural characteristics of lyocell fibres and their comparison with viscose and modal fibres were performed. The molecular (density ρ, degree of polymerisation DP, molecular mass M) and the fine structure (crystallinity index CrI, molecular orientation f_Δn, void volume V_p) of cellulose fibres were investigated. Differences in the molecular and fine structure between various types of regenerated cellulose fibres cause different reactivity, sorption and electrokinetic properties. Differences in the reactivity of investigated regenerated cellulose fibres were analysed using various methods for determining water adsorption (moisture sorption, water retention, swelling in water) and adsorption of different types of surfactants (spectroscopic determination of the surfactant adsorption). The accessibility of free adsorption places in the less ordered regions of cellulose fibres was investigated by determining of electrokinetic properties. The zeta potential (ζ) was determined using streaming potential measurements as a function of the pH and structure/concentration of surfactants in the liquid phase. The structure analyses show that the new lyocell fibres have a greater degree of crystallinity and molecular orientation, and the voids structure is similar to that of viscose fibres (T. Kreze, Dissertation, University of Maribor, 1999). The adsorption phenomena in the aqueous medium and the reactivity of fibres depend, with the exception of the portion and orientation of amorphous regions, predominantly on the void system i.e. diameter, volume and inner surface of voids. The voids of modal fibres are the smallest. The voids of lyocell fibres are similar than those of viscose fibres, and so are their swelling and adsorption properties in aqueous medium (T. Kreze, S. Strnad, K. Stana-Kleinschek, V. Ribitsch, Mater. Res. Innovations, 4(2/3) (2001) 107–114). There exists an excellent correlation between structure, adsorption and electrokinetic properties.

Introduction

Different production processes for conventional cellulose fibres (viscose, modal) and new cellulose fibres (lyocell) cause differences in the structure and in the reactivity—adsorption character of the fibres despite having the same chemical composition [3], [4], [5], [6], [7], [8], [9]. Regenerated fibres have a crystalline/amorphous microfibrillar structure (two-phase model), independent of the transformation process of cellulose into the solution and further fibre forming processes. The fibres consist of elementary fibrils, which consist of a succession of crystallites and intermediate less-ordered amorphous regions [10], [11]. The crystallites (A) are characterized by their size and their orientation. Less-ordered amorphous regions (B) connect successive crystallites length-wise, they are characterized by their size, density, and orientation. Lateral tie molecules-region (C) connect laterally adjacent amorphous regions. The cluster formations (D) are regions where crystallites are fused to large aggregates and region (E) represents the voids (Fig. 1) [12], [13]. The amorphous regions and inner surface area of voids is a decisive factor with regard to accessibility, reactivity and adsorption properties of fibres.

Analyses of the structure characteristics of regenerated fibres were performed. The molecular (density ρ, degree of polymerisation DP, molecular mass M) and supermolecular structure (crystallinity index CrI, molecular orientation f_Δn) of the cellulose fibres were investigated. In addition the amorphous regions and void fraction (void volume V_p) which significantly influence the adsorption properties of fibres (moisture adsorption, water retention, swelling in water, adsorption of different types of surfactants) were determined.

We tried to determine the correlation between structural differences and the adsorption abilities of fibres by determining the electrokinetic character of the fibres. These properties are especially important since they very often reflect the interaction ability of polymers with the ingredients of the liquid phase such as several kinds of ions, specific enzymes, surfactants, and dyes. It was therefore of interest to investigate the relationship between the adsorption abilities—the reactivity of different types of the regenerated fibres determined using zeta potential measurements (zeta potential=function of the pH and surfactant concentration), spectroscopy (adsorption of different types of surfactants) and differences in the fibre structure.

A physicochemical peculiarity of cellulose, a strong sorption power and high sorption capacity, gives the fibres a pronounced hydrophilic character. The accessibility in the water-swollen state is of even greater importance than the accessibility in the dry or conditioned state for many industrially performed processes on cellulose fibre [10], [11]. Fibre sorption properties are influenced by the molecular (chemical structure, molecular mass, number of functional groups) as well as the supermolecular structure of fibre (molecular orientation, degree of crystallinity, crystallite dimensions, portion of amorphous regions, size and shape of voids, etc.). A significant influence on the adsorption properties of fibres is the amount of accessible hydroxyl and carboxyl groups and the portion of amorphous regions where the adsorption processes take place. Namely, neither the water nor the aqueous solutions of electrolyte, dyes or surfactants can penetrate into the crystalline regions of the fibre. Cellulose-water interaction can best be understood as a competition of hydrogen-bond formation between hydroxyl groups of the polymer and those of the polymer and a water molecule or water cluster [11]. The water penetrates inside the fibre in the form of vapour or liquid water. It breaks down the secondary interactions between cellulose macromolecules and is adsorbed into the fibre by hydrogen bonds, which causes a swelling of the fibres. The amount of water vapour adsorbed at certain temperature and the humidity is one of the important criteria for describing the adsorption properties of regenerated cellulose. The water retention value obtained by measuring the amount of liquid water retained by the swollen fibre under defined conditions can also be considered as a valuable accessibility criterion for cellulose fibres [14], [15].

Currently some new alternative methods for the characterizing the hydrophilicity of a fibre surface are being applied. One of them is a determination of the electrokinetic character of the fibre samples [16]. It can be assumed that surface potential is mainly responsible for different kinds of solid–liquid interactions. Since the surface potential cannot be measured directly, use is generally made of the experimentally accessible zeta potential (ζ), which refers to the potential at some idealized plane of shear between the solid and liquid phases when any relative motion is induced between them. The general features and properties of the electric double layer at the solid/liquid boundary in an aqueous medium are well known [17]. The cellulose fibres are negatively charged due to the presence of carboxyl and hydroxyl-groups. The adsorption of water or electrolyte solutions causes an interfibrillar swelling of the surface layers and so the size of the active surface is increased, but the nature of the dissociable groups should not change. This swelling itself causes a reduction of the ζ, because of the shift of the shear plane into the liquid phase (Fig. 2).

In general, little written data is available discussing the influence of zeta potential (ζ) on the interaction between fibres and surfactants or dyes [18], [19], [20], [21], [22], [23]. The interaction of the solid with components of the solvent (adsorption, desorption) depends, among other things, on the sign of charge of both components. Adsorption of species with the same charge as the solid phase causes an increase of the zeta potential whereas species of an opposite charge decrease the ζ, leading finally to a charge reversal at sufficient concentration. Surfactants (both ionic and non-ionic) are used at the solid–liquid and liquid–liquid interface in order to control the surface charge and/or the hydrophobic hydrophilic character of the surface.

According to Fuerstenau [17], we have to distinguish between the behaviour on hydrophobic and hydrophilic surfaces. On hydrophobic surfaces, the surfactant displaces water from the solid surface so it tends, initially, to adsorb parallel to the surface, gaining energy from the hydrophobic interaction between the long hydrocarbon chain and the underlying surface. On the other hand, the surfactant cannot easily displace water from a hydrophilic surface. Adsorption in that case occurs mainly by electrostatic interactions between the surfactant head group and the surface groups. If the surface is sufficiently charged, it can adsorb enough surfactant to permit lateral interaction to occur, and hemimicelle formation is again possible, with a consequent reversal of the zeta potential in many cases. The adsorption of (for example) a cationic surfactant on an anionic fibre causes a decrease in the negative zeta potential. At a critical concentration of surfactant the iso-electric point is reached (zeta potential=0). With the addition of more cationic surfactant the ζ and hence the sign of the surface charge of the fibres is changed into a positive one. This surfactant concentration is called the charge reversal concentration (CRC). The adsorption of surfactants with different molecular mass on the charged solids can also be used to characterize those surfaces which do not show differences in chemical nature but rather in the amount and accessibility of dissociable groups or adsorption sites. So the adsorption of surfactants (cationic) was used to study the differences in the accessibility of dissociated groups in or at the fibres’ void system.