Wetting of fibres

Although the pressure is constant, the droplets are not spherical. In order to produce a constant curvature surface (Laplace s condition) joining up with the thin cylinder, the droplet must assume a wavelike shape. Indeed, at the join with the fibre, there must be a negative radius of curvature of the order of b to compensate for the positive curvature imposed there by the fibre. This point of inflection, and the wavelike shape, make direct measurement of the contact angle e rather difficult. The best way to determine Oe is to deposit droplets of known volume, measure their geometrical dimensions (1) and then use the tables of 6e values established by Carroll (at Unilever). 

We may wonder what would happen if we deposited a liquid sheath onto a fibre, as shown in Fig. 1.15a. A cylinder of liquid is by its very nature unstable, because the surface area of a cylinder is greater than that of a string of droplets, as soon as the radius of these droplets becomes of the same order of magnitude as the radius of the cylinder. 

Exercise. A cylinder of liquid of length L = 1 m and radius b is broken into n droplets of radius 7 . Calculate n and TZn b) such that the area of the droplets is just less than that of the cylinder. 

This instability, which can be observed by slowly turning on a tap, was studied by J. Plateau, who noticed that it is governed by surface energy considerations, and by Lord Rayleigh, who explained the regularity of the string of droplets. On a fibre, a liquid sheath will thus spontaneously separate out into droplets. A wavelike deformation of wavelength A leads to a reduction in surface area as soon as A 2TT b + e). It is amplified and the droplet size corresponds to the fastest mode 

Following the primary instability, a liquid sheath remains, but it too is unstable and the final image is often an alternation of large and small droplets. However, the period is imposed by Am- 

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Industrial Applications. The wetting of fibres is an essential step in the production of synthetic textile filaments, such as polyamides and polyesters. 

Cellulosic fibres used for reinforcement in nonpolar thermoplastics, such as PP, have to be modified because effective wetting of fibres and strong interfacial adhesion are required to obtain composites with optimised mechanical properties [31, 32]. Several methods for improvement in the adhesion between polymer and cellulosic fibres have been developed. 

Modified PAN fibres have been obtained from copolymers containing up to 15% or ISP units using the wet spinning process30. Some properties of modified fibres are presented in Table 1. For comparison are also given the properties of fibres obtained from copolymers additionally crosslinked with conventional crosslinking agents used in the vulcanization of nitrile rubbers. 

As will be shown in this report, polymer fibres gain additional strength by an increase of the molecular weight and by a more contracted orientation distribution, i.e. a higher modulus. For the wet-spun fibres, a strength increase can be achieved by improvement of the coagulation process, which makes for a more uniform structure and chain orientation in the cross section of the fibre, and by a reduction of the amount of impurities. 

The surface area of fibres increases during refining. However, the definition of surface area for cellulose is not straightforward and it is necessary to define it a little more precisely. It can be measured when the pulp is in either the wet or the dry state and very different... 

A low consistency suspension of fibres, pigments and chemical additives (thin stock) flows on to the moving wire mesh filtration medium of the paper machine where the wet web is formed. The water which drains away in this process (white water) is then recycled as far as it is practical to do so and is used to dilute the incoming high consistency suspension (thick stock). Retention can be considered either in overall terms or in terms of a single pass of the thin stock across the machine wire (first pass retention). For efficient operation, paper makers aim to achieve as high a first pass retention as possible, which they do by the use of retention aids. This reduces material losses and also assists in minimising the level of suspended solids in the effluent. 

Notes on the reagents. The permanganate reagent oxidizes and dissolves the lignin and any tannins. The 2-methylpropan-2-ol enhances the wetting of the ADF fibres by the permanganate. The acetic acid in the buffer solution is to neutralize the alkali formed in the oxidation reaction ... 

Both 2 1 chromium and cobalt complex azo dyestuffs have little or no affinity for cellulosic fibres and until the early 1960s their use was restricted to wool and nylon. With the introduction by ICI of their Procion range of fibre-reactive dyes, however, their use was extended to cellulosic fibres on which they give prints having excellent fastness to light and wet treatments. Before that time the development of metal complex dyes for cellulose had followed a similar pattern to that of the development of such dyes for wool but, in this case, the most important metal was copper. Early work in this field has been reviewed by several authors.1 The after-treatment of dyeings on cotton obtained from dyestuffs such as (11) with copper salts was used for many years to improve fastness... 

The fibres thus obtained, after removal of the knots and pieces not well defibred, are grouped so as to make a roughly circular tuft about 1 cm. in diameter and 1 mm. thick. This tuft is placed on the surface of distilled water contained in a weighing bottle and observed during a period of a few minutes. With raw products, the tuft of fibres does not become wetted and remains floating even when the liquid is agitated with a bleached product, on the other hand, the tuft is wetted more or less rapidly and, when the liquid is shaken, falls slowly to the bottom. 

For many applications the wetting behavior of a network of fibres is important. An example is the water repellent ability of clothing. As a simple model we consider a bundle of parallel cylinders which are separated by a certain spacing. This spacing is assumed to be small compared to the capillary constant, so that the shape of the liquid surface is assumed to be determined only by the Laplace equation. Practically, this leads to cylindrical liquid surfaces. 

The viscose process has some variants depending on the quality of the cellulose and the composition of the regenerating bath, special high added value products can be obtained so-called modal-polynosic fibres, or modal-high wet modulus fibres, for instance. 

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