Wool is an animal fiber that forms a protective layer of sheep. It mainly consists of a protein called keratin, insoluble in water. Any wool fiber is substantially characterized by three components:
- Cuticle that forms the characteristic flakes visible under a microscope;
- Cortex constituted by prolonged cells, parallel to the fiber’s axis;
- Core (absent from subtle fibers) constituted by empty cell walls reticulum, full of air.
From chemical point of view, as we said earlier, wool is composed of keratin. It is a polypeptide string (consisting of many amino acids such as cysteine) with a three-dimensional helix shaped structure. Single helixes link through some weak interactions and become more and more complex structures until they form a fiber as we know it.
Nevertheless, freshly shorn raw wool contains many other substances such as wax, sand, dirt and vegetable substances. This is why it needs to be cleaned and treated: washing, fulling (a controlled felting process that gives the woolen cloth resistance and strength), dyeing, fixing and drying are the most damaging steps in wool processing.
Why wool?
The physical and chemical structure of wool together with its processing stages give this fiber various characteristics that made it widely used in textile industry. Its main features are:
- Elasticity and tear resistance
The polypeptide strings, thanks to their helix structure, behave like springs, therefore the fibers can be extended by up to 50% more than their original length when they are wet, and up to 30% when they are dry. Wool fiber flexibility increases its durability. A wool fiber can be folded up to 20.000 times without breaking comparing to 3.000 times in case of cotton and 2.000 times in case of silk.
- Fiber hygroscopy
Wool hygroscopy that is the ability to absorb water molecules from the surrounding environment makes wool a temperature regulator, that protects the body in both cold and hot conditions. It can absorb humidity up to 33% of its weight without seeming wet.
- Easy dye
Wool absorbs many colors in a uniform and direct way. It reacts with both acid and basic dyes thanks to its chemical structure. The color penetrates into the core where a chemical reaction takes place that binds the color permanently, unless it is exposed to prolonged and extreme fading conditions.
- Flame resistance
When wool is burned, it gives out that characteristic smell similar to burned feathers or burned hair. When the flame is removed, the wool stops burning (it is self-extinguishing). Therefore, wool is relatively flame resistant also thanks to the fact that it can absorb water.
http://www.wool.ca/uploads/files/PDF/wool-fact-sheets-charcteristics.pdf
Felting and dimension variations
Felting is a process of compacting and tangling woolen fibers which takes place when wool is placed and shaken in water; it is used in industrial felts production, unwoven woolen cloths and in finishing processes called fulling in order to give soft and uniform finishing to certain types of woolen cloths.
Fiber friction characteristics are the basis for the felting procedure. Friction happens thanks to the presence of flakes on the fiber surface that hook up and tangle together. The process is irreversible.
As said earlier, felting takes place in water and it is made easier by using lubricants such as soap or by increasing the solution viscosity. Moreover, it can vary with PH and the temperature conditions.
After the mechanical action while washing in water, the fibers hook up with each other provoking the shrinkage of the woolen garment and making it impossible to be undone. The process is irreversible and the garment won’t come back to its original size after the treatment. For this reason, once the woolen cloth is finished and transformed into garments, felting is to be avoided. In the past, such result could be obtained exclusively by keeping the agents’ level low so that the fibers would not tangle, or by dry cleaning (in an almost complete absence of water).
Nowadays, there are numerous anti-shrinkage treatments that reduce or prevent from felting:
- Flakes removal from the fiber: not much used technique because really damaging;
- Polymers deposit on fabrics: thanks to this technique, the fibers glue together every time they get in contact which prevents from tangling;
- Resin coverage on fibers after deteriorating action: in this way the presence of polymers reduces the friction, consequently the fibers won’t tangle;
- Chemical treatment of cuticle: the most common agent used to degrade the keratin of the cuticle and make it unshrinkable is chlorine. It can be used in water solution (acid, neutral or alkaline) or as gas. If it is used in water, it is necessary to carefully prevent the most internal fiber layers from deterioration, therefore either expose the wool to it for a short time only or add the fiber swelling repressing agents so that the chlorine penetration is slowed down.
By Ecochem s.r.l. and Ritex Textiles Reasearch and Testing
DETERGO MAGAZINE
MAY 2016
