PHYSICAL PROPERTIES

The fibrous materials consist of compounds of very high molecular weight and existing in the form of long chain-molecules which are more or less parallel. Some of the chief physical properties which have been observed in the successive natural, regenerated and synthetic fibres are discussed below. Successful textile fibres must possess these properties in greater or less degree, but in addition they have to possess a certain ratio of length to width in order that they may be fibres and suitable for manufacture into fabrics.

Strength: The tensile strength is described as lb/sq inch, or Kg/sq meter. Strength may be described as tenacity in terms of gm/denier. The denier is a unit of weight which is utilized with silk and rayon filaments; a denier corresponds to 0.05 gm. The denier of rayon yarn is the weight in gm of 9000 meters. Tenacity is the breaking force in terms of the yarn number, whereas tensile strength is the breaking force in terms of the unit of area.

Value for tensile strength are directly comparable, but the tenacities are not, because in the tenacity is included the density of the material. Hence, two fibres may have the same tenacity but possess different tensile strengths, because their cross-sectional areas may differ. The breaking bond has little significance without reference to the cross-sectional area. The well-orientated (high degree of crystallinity) filaments have high tensile strength, whereas poorly orientated fibres (where amorphous region is prominent) have low tensile strength.  

Where the orientation of chain molecules is of a high order, the secondary forces, although small, are considerable in the mass, and breakage takes place by fracture rather than slippage. With poorly orientated fibres, the stretching brings about a slippage of the chain-molecules. Whose intermolecular forces are of a relatively low order. The slipping will also tend to occur more readily with sort molecular chains. Assuming that strength is due to chain length and orientation, it follows that above a certain limit, the breakage will be due to fracture of the chains and not to slippage.

The breaking load is a very important factor. Where folded molecular chains are encountered, as in wool, a high extension may be expected because the chains will unfold to a considerable extent before breakage occurs, but with straight chains, extension is determined by the arrangement of the crystallites. With a high degree of orientation, the crystallites will only slip on each other to a slight extent until alignment takes place and the fibre breaks with a low extension of 2%; the breaking load is high because the fibre-bundle must be broken more or less as one unit. With low degree of orientation the breaking load is low, but the extension at break is high; the effect of tension is to bring about extension and orientation, the filament or fibre becoming thinner in the process, so that the load is applied to a smaller unit area.