ELASTIC PROPERTIES

The textile fibres should have not only strength but also elastic property i.e. extensibility. These two mechanical properties should always be considered together which measure the toughness of the fibre.

Extensibility: As we know, the fibres which possess a well orientated structure in more crystalline are generally less extensible than those which are orientated at random or possess folded chains which can uncoil.

In this way it is possible to deduce from extension at break the nature of forces which resist deformation the highly orientated structures tend to throw the strain on the main chemical bonds almost immediately. The lack of extensibility results the fibre brittle which have very limited application.

In addition to extension at break, it is important to give consideration to the stress-strain modulus in this connection. 

Figure 1: Elementary load-extension diagram.

In this diagram, the small bend in the curve which is observed at low stress is due to the straightening of small convolutions or kinks in the fibre, the dotted line shows the true course of the load-extension curve, for these low stresses. The next part of the curve is usually straight and indicates that the strain varies with the stress; in this part Hooke’s Law applies. As the stress is proportional to the strain, it is possible to meaner the Young’s Modulus in this region; this is the ratio of stress to strain and may be regarded as the load necessary to extend the fibre a certain amount.

The final part of the curve indicates a sudden increase in extension for a small increase in load; the fibre has ceased to be elastic and is yielding to the stress. This point is indicated by X, and is generally called the yield point; it is actually a region rather than a point. Beyond the yield point the fibre is no longer elastic, but exhibits plastic flow, and recovery after the relief of stress is incomplete in this region.

Considering the cellulosic fibres, the native fibres such as cotton and ramie are not extensible i.e. they resist deformation. Acetate region is a relatively weak fibre but it is quite extensible, possesses considerable stretch. Wool is amongst the weakest fibres and cotton among the strongest; but wool is extensible, but cotton is not. The wide generation those strong fibres are not highly extensible except to silk and nylon. Young’s Modulus of elasticity (a measure of stretch ability) shows that the native cellulose fibres are the least extensible and the native protein fibres the most extensible.

Elastic Recovery: Although the relation of stress to strain is of considerable importance in assessing the merits of textile fibres, it is equally important the power of recovery from strain. This power of recovery from deformation may be defined as elasticity. The elastic power recovery determination is not so easy. One of the chief difficulties is that the recovery is not immediate, but there is an elastic after effect. Hence time factor must be take into account. Another difficulty is that fibres are not solid throughout, and moreover, their sectional area varies along their length.

The extension of a fibre comprises two components an elastic extension which may be recoverable; and a plastic extension is not recoverable. The plastic extension is due to ‘flow’ whereby the extended molecules slip on one another and do not return when force is removed. Elastic recovery is the ratio of the elastic extension to the total extension. Resilience factor is the ratio of energy required to extend a fibre on loading and the energy returned on unloading.

The elastic recovery of fibres from extension comes into 3 (three) classes. The highest recovery is seen in nylon and wool, whereas the lowest is given by the native cellulose fibres. The intermediate position is occupied by silk, acetate rayon, and viscose rayon. The elastic recovery is mainly due to two main factors (a) entropy effects i.e. thermo-elasticity and (b) the mutual attraction of certain groups in the main chains producing an entanglement.