ELASTICITY OF WOOL

Waipa Post, Volume 52, Issue 3703, 8 January 1936, Page 7

ELASTICITY OF WOOL

HOW ITS MOLECULES DIFFER FROM COTTON AND SILK. Wli» a previous article I explained w hy there is no possible chance at the moment of making wool by chemical means, either in the laboratory or on a commercial scale (writes S.W.P. in the Melbourne Age). I also described the way in which artificial silk is being used together with wool in the production of mixed fabrics, which go by the name of “ synthetic wool” a name which, to say the least, is very misleading. One of the shortcomings of this f substitute, or rather of the artificial silk contained in it, is a lack of natural elasticity. Now it happens that elasticity is, perhaps, the outstanding virtue of wool, without which any wool substitute must always remain at a disadvantage. In recent years it has become increasingly plain that the internal structure of the molecule is responsible for certain physical properties, of which elasticity is one. This is particularly so when the molecules are large, approaching colloidal, as they are in the case of wool and cotton and silk. DIFFICULT TO INVESTIGATE. Such molecules are very difficult to investigate, but recent crystallographic work, combined with X-ray and general physico-chemical evidence, now gives us an insight into the internal structure of these particular molecules, thus serving to make clear why wool fibre on the one hand is elastic, while cotton and silk on the other are practically inelastic. An understanding of the actual ' source of elasticity somewhat clarifies the problem of producing an artificial thread which will possess this important property, although it does not necessarily bring the solution of the problem any nearer. Rather does it serve to make the difficulties more obvious, from some points of view almost forbidding. However, this is the problem, more than any other, which lies at the root of “ synthetic w’ool ” developments. If we would look further into it, we must be prepared to peer into the heart of the molecules themselves. Thanks to modern developments, this we are now able to do. As cotton and artificial silk are more or less pure forms of cellulose.

their molecules will necessarily be much alike. We can form a useful picture of a cellulose molecule, either of cotton or rayon, by thinking of it as a long chain consisting of something like 100 links or more, where each link is ring-shaped and composed of a molecule of glucose, or, rather, a glucose residue. In the natural cotton fibre, as also in the present-day artificial thread, these chains are by no means mixed together in horrible disorder, nor are they neatly coiled like anchor chains; they are for the most part lying side by side, fully extended, in the direction of the thread. SEEN BY X-iRAYS. Man has never been able to see this with his own eyes; he has adopted for the purpose those super-sensitive, scientific eyes known as X-rays. With their help he sees that the cellulose molecules are not only fully extended but they are gathered together into I bundles, which are also lying in orderly arrangement in the direction of the fibre. These bundles, or groups of molecules, show various colloid properties. At the same time their X-ray pattern indicates that they are crytsalline. As their crystal form is not well developed, one temporises for the time being, and calls them, not crystals, but crystallites. p It is interesting to compare the way in which the crystallites are linked together to form a single fibre, with the m ann er in which man links the fibres together to form a textile yam. There is, in fact, a striking analogy. The yam is made up of innumerable fibres all twisted and twined together in the direction of the length of the thread, the twistings and twinings giving it the/necessary strength. Each fibre, on the other hand, is made up of innumerable crystallites or bundles of molecules lined up in the direction of the fibre. But they are net twisted and twined together in man’s crude way—Nature does it much better than that. They are stuck strongly together by residual valence forces in a way that man cannot imitate, or, in fact, wholly understand. LOW ELASTIC LIMIT. When a single fibre of cotton, or rayon, is stretched, it is found to be inelastic, the elastic limit being very low, something like 2 per cent. If stretched beyond this limit it never comes back again, and yet the X-ray pattern, which, like all X-ray patterns, is due to the internal arrangements within the crystallites, shows practically no alteration. From this we conclude that the crystallites themselves, and also the molecules of which they are composed, suffer no alterations; the stretching sinqj.y causes them to slip past one another to take up new “ permanent ” positions; the fibre has a new “permanent set.” Both cotton and rayon behave in this way, their lack of elast ticity being due to a lack of elasticity * within the crystallite. Natural silk, although an entirely

different chemical from cellulose, being protein in nature, behaves in a similar way. The molecules are fully extended and inelastic, and the X-ray pattern shows no alteration on stretching.

But with wool it is a different matter altogether. The unstretched fibre shows a normal X-ray pattern, roughly comparable with that of silk or cellulose, indicating that the wool cules are once again collected together as crystallites. But as soon as the wool is stretched an entirely new pattern appears, which reverts to the original when the wool is allowed to contract. Now, a new pattern means that there has been a change within the crystallites, and, seeing that the pattern reverts to the old, his catn mean only hat the molecules themselves revert to tehir old forms as soon as the stretching force is removed. REVERSIBLE CHONGE. From which we conclude that the elastic properties of wool are due to a reversible change, which proceeds within the molecule itself. The molecules may lie side by side as they do i ncellulose and silk, but they are not fully extended and become so only when the fibre is stretched to the full elastic limit. There is other evidence to support this view. For example, a comparison of the lengths of the amino-acid residues that go to make up the molecules of wool and silk shows that they are always the shorter in wool, and cover the same ground a.! silk only when the wool is fully stretched. It is therefore concluded that each wool molecule has a zig-zag structure, a sort of concertina structure, if you like, which can be pulled out, and which snaps back on being released. These are the interesting conclusions to which the modem work has led. They make clear the tremendous problem involved in producing an elastic thread from an inelastic molecule like cellulose. How is it going to be done ? Ido not know. The cellulose molecule will have to be modified either before or after spinning into rayon. It has been done to a certain extent in the production of “ creaseless cotton,” but the efforts will have to go much further. It is possible that developments will take place in a different direction altogether, but this is the one which, if successful, would offer the most serious challenge to the wool industry.