Modern Textile Technology By J.B.Rattan

CONTENTS

One
Technological Analysis of Spinning
In the beginning of the history of spinning, progress in spinning technique was mainly made by accumulating empirical facts; that is to say, by repeating a set of procedures such as setting a spinning, condition and measuring the resultant properties and structures of the spun fibers. In melt spinning, we can predict the diameter and temperature and the tension in a running filament if the spinning conditions and the rheological properties of a polymer used in the spinning process are given; the predicted values are, of course, in good agreement with the experimental results. Such a prediction, however, can be made only when no significant crystallisation occurs during the spinning process.

In the theory of Kase and Matsuo, mean values of temperature and of stress over the whole area of a transverse section of the filament were used. In the process of high speed spinning, however, the variables such as temperature, stress, orientation and crystallinity must be expressed as functions of the radial distance from the central axis of the filament as well as the distance from the spinneret. For example, consideration of the radial distribution of these variables is inevitable in discllssing inhomogenous structures such as skin-core structure. In order to understand the spinning process, it is indispensable for us to know how structure will be formed during the process as weB as to carry out the detailed technological analysis of the process.

Structural formation during spinning The structural changes in the spinning process, crystallisation, gelation, and phase separation, are discussed here. Crystallisation arising from the state of molecular orientation in a polymer solution, melt or amorphous solid is termed ‘oriented crystallisation’. Crystallisation during spinning is a typical example of oriented crystallisation.

Structure formation in oriented crystallisation is of great interest because it is a phenomenon reflecting the nature of the macromolecule. Since the 1960s, studies on oriented crystallisation have been carried out extensively and the publications so far are too many to mention. Of all these studies, that of flow-induced crystallisation of polyethylene in particular attracted researchers’ attention. When polyethylene is crystallised from solution by stirring the solution, what is called the shish-kebab structure is formed. The following are general features of oriented crystallisation:

The morphology of the crystallized materials changes according to the degree of molecular orientation. With increasing degree of molecular orientation, the temperature which gives the maximum rate of crystallisation goes up and, in some cases, the maximum rate itself also increases by several orders of magnitude. The mechanism of oriented crystallisation may be very different from that of non-oriented crystallisation.

Two
Applications of Textile Mechanics
A somnolent drunkard, breathing stertorously in the gutter on a Saturday night, is in a blissful state of equilibrium. A similar argument holds true when an object is undergoing uniform motion, either in linear or in circular fashion. All the visible evidence indicates that the uniform motion will continue. As far as we can tell by direct observation, the moon will carryon revolving around the earth in a perfectly satisfactory manner, to the delight of lovers and poets, for the foreseeable future, and the dire predictions of astronomers can be regarded as irrelevant nonsense in the context of one night, one year or even one lifetime.

In both these cases, then, the object is in equilibrium with its surroundings. This eqUilibrium may be maintained is by the complete absence of any force acting on the body. We have already seen that a body remains at rest, or continues to move uniformly in a straight line, so long as no external force acts on it. In practice, however, it is virtually impossible to visualize any object existing in complete isolation from any force whatsoever unless one enters the realms of metaphysics or leaves the physical universe so far behind that gravitational attraction no longer exists even to the most minute degree.

Nevertheless, equilibrium can easily be achieved in the everyday experience of mere mortals, so we must seek a more realistic way of defining its existence. Consider a particular single fibre. in a roving can, awaiting spinning. Let us assume that, from its rest position, it is pulled uniformly into the drawframe and. after passing through one drafting zone. becomes part of the fly. drifting gently down until it settles on the ground. From the mechanical point of view, the fibre is initially at rest, is suddenly accelerated to a specific speed and moves at this speed towards the frame. It is then subjected to an acceleration in the drafting process and, before it can achieve uniform motion again, is thrown or blown, with a sideways’ acceleration, out of the fibre bundle. It now undergoes an acceleration vertically downwards under the influence of gravity until it finally returns to the rest state once again.

We can examine. in a somewhat simplified way, the forces acting on the fibre throughout the changes. In rest positions, the weight of the fibre, the force produced by the effect of gravity on its mass, is acting vertically downwards and an equal but opposite force. the normal reaction at the surface (or other fibrous material) on which it rests, is acting on the fibre to prevent it from falling under the gravitational attraction. The same two forces exist when the fibre is moving at uniform speed in the roving.

In addition, the frictional forces. which maintain the roving in a coherent form must be present but. since the fibre is not accelerating. we deduce that any force of this nature must be balanced by an equal but opposite force in some way. This fact leads us to a preliminary definition of equilibrium as a slate in which there is no net force acting on the body; that is, all the external forces acting on the body must be able to give a resultant of zero. It is then easy to see why. in the remaining parts of its motion, our fibre is not in equilibrium. By the very nature of the drawing operation. a fibre must be accelerated forwards by forces originating with the pull of the roller nip and transmitted gradually by inter-fibre frictional contact throughout the drafting zone.