What fiber is wool made of

Wool: chemistry and construction


Lecture by Susanne D thorn as part of the "Exercises in Lectures with Demonstrations - Organic Chemistry", SS 2010


Structure:


Introduction


Fig. 1: Sheep [9]

Today, wool only accounts for around four percent of world production of textile fibers. Nevertheless, almost everyone has woolen clothing, such as the knitted socks that you get from your grandmother. Most of the time, however, these end up unworn in the wardrobe. They are thought to be old-fashioned, ugly, or scratchy. But this behavior is not justified due to the many positive physiological properties of clothing.


1 wool is water-repellent

The wool fiber consists of a sheath (cuticula) and a core (cortex). The coat is made up of three different layers. The outermost of these layers (= epicuticle) contains the F-layerwhich consists of lipids and thus hydrophobic, so it is water-repellent.

This image has been removed for copyright reasons.
Fig. 2: Structure of the cuticle [6]


2 Wool is hygroscopic

97% of wool is made of Proteins. Proteins are amino acids linked by peptide bonds. The polar carbonyl and polar amino groups of the proteins can get over water Hydrogen bonds tie. Up to 33% of the wool's own weight can absorb water without the fiber feeling damp or clammy. This means that 1 kg of wool can absorb up to 0.33 l of water and still feels dry on the outside. One speaks of hygroscopic behavior.


Fig. 3: Formation of a dipeptide


3 Wool has an insulating effect

A single wool fiber consists of many dimers, which are put together over several intermediate steps in the crosslinking to form macrofibrils. The macrofibrils themselves are again a sub-unit of the core of the fiber. This multi-component structure in layers leads to that many Cavities are present in the fiber. In woolen clothing, the air in the cavities is warmed by your own body heat. So you have a kind of warm air cushion around your body, which leads to good insulation behavior.


Fig. 4: Structure of a wool fiber [6]


4 Wool is stretchy and elastic

The macrofibril is the main component of the wool fiber and therefore the carrier of the mechanical properties. On the one hand, it consists of sulfur-rich proteins without a helical structure, the so-called keratin-associated proteins (KAP). Above all, they form stable ones Disulfide bridges which contribute to elasticity and tear resistance. On the other hand, it consists of low-sulfur proteins with an alpha-helical secondary structure, the so-called keratins. These form intermolecularHydrogen bonds which lead to a good flexibility of the fiber. In addition, the alpha-helical structure is transformed into the elongated beta-sheet structure through stretching. The entire fiber lengthens by up to 25%. This transformation is responsible for the stretchability of the fiber. From an elongation of 48%, however, fiber breakage occurs due to the cleavage of disulphide bonds.

Salt bridges, hydrogen bonds, disulfide bonds and van der Waals forces exist between the dimers. These interactions are also a reason for the high stability of the fiber.


5 Wool is easy to dye

This property is shown by an experiment in which the dyeability of wool is compared with that of cotton.

Material:Wool and cotton thread
Chemicals:cherry-red coloring (E 104, E 110, E 124, E 132), tartaric acid
Implementation:Mix a few drops of the cherry-red dye with hot water and add a little tartaric acid. Then you add a wool and a cotton thread. After a few minutes, the dyed threads are washed in water.
Result:The wool is dyed more intensely than the cotton. The dye used here consists of anionic dyes and is bound to the protonated amino group of the fiber via an ion bond.

Summary

Wool is:

  • water-repellent (hydrophobic F-Layer)

  • hygroscopic (formation of hydrogen bonds)

  • stretchable and elastic (various inter- and intramolecular interactions)

  • insulating (layered structure with many cavities)

  • easy to color (protonated amino group)

One of the reasons why wool now only accounts for four percent of the world’s production of textile fibers could be that great care must be taken when washing in hot water and drying, and that some rules must be observed to prevent loss of shape avoid.


6 literature

  1. H. Zahn, F.-J. Wortmann, H. Hoecker, Chem. Our time1997, 31, 280-289.
  2. K. Vollhardt, N. Schore, Organic chemistry, 4th edition, WILEY-VCH, Weinheim 2007.
  3. http://www.filzlexikon.de/found-wolle.html?fillex/wolle/chemie/wollprotein.html, April 9th, 2010.
  4. http://www.filzrausch.de/info/found_projekt_n1.html?projekt/nelli/2-1-3.html, April 9th, 2010.
  5. http://www.thomasmusolf.de/fuer_schueler_und_eltern/Chemie/Klasse%2010%20Kohlenstoffchemie/V%20Peptide/Wolle.htm, 04/11/2010.
  6. https://de.wikipedia.org/wiki/Haar#/media/File:Hierarchical_structure_of_hair_in_the_cortex_and_cuticle.png License: CC BY 4.0 03/30/2017
  7. http://www.umweltlexikon-online.de/fp/archiv/RUBlandwirtsrohstoffe/Schafwolle.php, April 16, 2010.
  8. http://www.schule-bw.de/unterricht/faecher/nwt/unterrichteinheitheiten/einrichtungen/baeren/seite3-1.html, April 26th, 2010.
  9. https://pixabay.com/de/schafe-essen-gras-weiden-lamm-161389/ License: CC0 Public Domain (03/30/2017)

E-Mail: Walter.Wagner t uni-bayreuth.de, as of March 30, 2017