What is Wool Fibre? and It's Uniqueness
Wool fibre has a special place among all the natural fibres known to humankind in terms of its warmth, elasticity, and luxurious comfort properties especially during winter seasons. Wool, an offspring of sheep, has long been a beloved item of centuries-old handcrafted shawls in Kashmir and elegant merino suits of the current day fashion. What is really astonishing about wool is not only its feel but a complicated structure and chemical behavior that enables it to react differently to processing compared to plant fibres such as cotton or jute.
Wool is a protein fibre, unlike cellulose fibres, it is mainly composed of keratin. This gives it a unique chemistry, which defines its reactions with dyes, heat and chemicals. All wool fibres contain miniature scales that render the material soft, yet hard, elastic yet easy to a felting-action, a two-sidedness that requires special caution when processing the raw material in wet conditions.
Learning about the physical and chemical characteristics of wool is beneficial in assisting the textile experts and students make the appropriate selection of the treatments and finishes. It is also very important in spinning, weaving, dyeing and finishing processes of fabric manufacturing. In the case of industries, this knowledge guarantees quality and durability of products to customers and quality.
This posting will be discussing the composition, structure, physical and chemical properties, most common wet processing treatment of wool and it's uses. Knowing about the details of these fibres makes a textile student or researcher strong in that field whenever there's need to use this fibre.
Origin and Composition of Wool
Wool is a natural protein fibre made by the fleece of a sheep and in certain instances, it can also be made of similar animals such as goats (cashmere, mohair), camels, alpacas and even rabbits. But in the textile industry when we talk of wool, we normally mean sheep wool.
Structure of Wool Fibre
The wool fibres are living biological materials that have a three-layered structure that is unique to each fibre:
Cuticle - The outer most layer which is composed of overlapping scales like roof tiles. It is these scales that are the cause of lustre and felting behaviour in the fibre. Exposure to moisture, heat and friction interlocks the scales leading to shrinkage and felting.
Cortex - This is the bulk of the fibre and is made up of spindle shaped cortical cells which provide the resistance to weight, elasticity and crimp to wool. There are two kinds of cells (orthocortex and paracortex) which are differentially curled to form the typical crimp.
Medulla - This is only present in rough wools. It is an empty area that assists in lightening and warming the fibre since there is trapping of air.
The scaly surface and internal crimp are the reason why wool is so easy to spin and provide it with great resilience and warmth.
The chemical composition of the wool is summarized as follows:
Wool is a protein fibre mostly composed of the same keratin that is present in human hair and nails. Its approximate chemical formula is:
- Carbon (C): 50%
- Hydrogen (H): 7%
- Nitrogen (N): 17%
- Oxygen (O): 21%
- Sulphur (S): 3-4%
Protein fibres such as wool and silk are found to contain the element of sulphur. It creates disulphide bonds (-S-S-) in the molecule of keratin which makes wool elastic, strong and resistant to chemicals.
Amino acid chain of wool has functional groups such as, -NH2, -COOH, -OH, and hence amphoteric in nature, that is, capable of reacting both with the acids and alkalis; which is highly beneficial when dyeing and finishing.
Physical Characteristics of Wool Fibre
Wool is one of the most demanded natural textile fibres due to its physical characteristics which include the ability to be used in clothing, carpets and industrial applications. Their origins are largely due to its crimped form, scaly surface and distinctive protein composition.
Appearance and Lustre
Wool itself is naturally shiny and the scale structure and fineness of the fibre determines how shinny it is.
- Fine Merino wool is duller as the scales are closely packed such that they reflect light.
- The coarse wools, however, contain less scales to the inch, and are more lustrous.
This range of lustre also assists the manufacturers to choose the appropriate wool to be used in the various applications like fine wool being used in clothing and coarse wool being used as upholstery or carpet.
Fineness and Diameter
The diameter of wool fibres normally measures between 17 and 40 micrometers.
- Fine wool (17-25 um): It is applied in high quality garments and suiting cloth.
- Medium wool (25-35 um): Fleece sweaters and blankets.
- Rough wool (35-40 um): It is used in carpets and heavy fabrics.
The finer the wool the silkier the feel and the more expensive.
Length and Crimp
The staple length can range between 25mm (short merino) to 400mm (Lincoln wool ).
Crimp The natural waviness ranges between 1 and 30 crimps per centimeter.
- The more the crimp, the more the elasticity, softness and bulk.
- Lesser crimp = stiffer handle and reduced flexibility.
Crimp assists in trapping air that makes wool a natural thermal insulator and keeps the wearer warm during the winter seasons and cool during the summer seasons.
Strength and Elasticity
Wool is medium in strength and its tenacity is 1-1.7 g/denier when it is dry. But when wet it loses approximately a quarter of its strength as water dissolves the hydrogen bonds between fibre.
Wool is resistant to wrinkles because of its high elasticity which enables it to be stretched up to 25-30 percent and back to its original form. This is elastic because of the helical configuration of keratin molecules which are similar to small springs.
Moisture regain and Absorbency
Wool is a good moisture absorber - it can absorb as much as 30 percent of its weight in water without retaining any of the moisture.
- This regain is high and it is what makes wool fabric comfortable in a humid as well as cold environment.
- It also helps to enhance good dye uptake so that it can take up bright and uniform shades.
The hydrophilic groups (-NH2 and -COOH) are internal, and the outer cuticle is hydrophobic, that is, it is not saturated with water.
Thermal Behavior
Wool is not a good conductor of heat and this is why it keeps one warm very well. The air that is trapped in its crimped structure is an insulating layer.
It starts to turn yellow at about 100degC and breaks down at a temperature over 150degC producing a burnt hair smell.
It is naturally flame retardant unlike synthetics as it does not melt and chars gradually.
Density and Handle
Wool weighs approximately 1.30 g/cm3 that is less than cotton and hence it has a soft and voluminous feel.
It is soft to touch, easy to customize because of its natural elasticity and springiness.
Felting Behavior
The peculiarity of wool is felting, which is problematic in some cases.
The scales of the fibre surface interlock permanently when the fibre is subjected to heat, moisture and mechanical action and consequently shrink.
This is the reason why raw wool clothes tend to shrink when washed.
Nevertheless, the property is also used to make felt fabrics at an advantage.
The Wool Fibre Chemical Properties
Wool is composed of a complex protein that is mainly composed of sulphur containing amino acids like cystine. This structure causes wool to be quite a chemical deviation of plant fibres such as cotton. Knowledge of these chemical properties gives a clue about how to process wool safely during scouring, dyeing and finishing.
Action of Acids
Wool is resistant to slight acids but is destroyed by intense acids in the presence of heat.
- Dilute acids (such as acetic acid, H2SO4 at low concentration): Do not do much harm, and are even applied to neutralize alkaline residues after scouring.
- Strong acids: Hydrolyse estalon-keratin peptides, and make the tissue weaker and disintegrated.
- Example: Wool will be fully dissolved in strong sulphuric acid to produce ammonium sulphate, and other degraded products.
Therefore, the cold nature of acid treatments in the finishing of wool is typically maintained and diluted to prevent damage of the fibres.
Action of Alkalis
Wool is extremely alkaline-sensitive. When used continuously even mild alkalis such as sodium carbonate may result in weakening of fibre.
- Strong alkalis (e.g., caustic soda): Cleave peptides and disulphide cysts and result in fibre swelling, felting, and eventual dissolution.
- Mild alkalis (such as soap solutions): Do not tolerate at any temperature below 50degC.
It is the reason that alkaline scouring agents applied to cotton can not be applied to wool but rather mild detergents and neutral soaps are used.
Action of Oxidising Agents
The Sulphur bonds of the keratin can be modified by the oxidizing agents to cause chemical changes in the fibre.
- Hydrogen peroxide is a mild oxidizer which can be used safely in wool bleaching as it causes minimum destruction in case it is effectively controlled.
- When strong oxidizers (such as sodium hypochlorite) break the disulphide bonds (linkages) the wool becomes rough, weak, and yellow.
Controlled oxidation is however can be applied to advantage- such as the chlorination of wool minimizes felting by partially dissolving the scale structure (used in superwash wool).
Action of Reducing Agents
Oxidation damage can be partially undone by the addition of reducing agents, including sodium bisulphite or sodium hydrosulphite and is occasionally employed in dyeing to restore fibre softness.
The fibre however can be weakened by overuse.
Action of Organic Solvents
Most organic solvents do not dissolve wool, although with exposure this could be lost and become hard.
- Wool fabrics can be dry-cleaned using solvents such as perchloroethylene which is normally applied during the maintenance of garments.
- Alcohols and hydrocarbons are unable to dissolve keratin though they can strip off lanolin (natural grease).
Action of Heat
In moderately heated form, moisture is expelled by wool and makes it more elastic.
It turns yellow at 100degC, and at 150degC it decomposes slowly and with an unpleasant smell, by sulphur compounds.
Wool being self-extinguishing to flame since it does not melt, is an important feature in protective textiles.
Action of Light
When subjected to prolonged sunlight exposure or UV rays, the keratin protein may destroy the strength and become yellow.
As a measure to mitigate photo degradation, wool has been impregnated with UV absorbers or dyed in darker colors which prevents entry of light.
Reaction with Dyes
Due to its amino and carboxyl group, wool is amphoteric, which implies that it can be reacted with both acidic and basic substances.
This gives it a great affinity to acid dyes, which are ionically bonded to amino groups of keratin.
Other compatible classes of dyes are:
- Acid dyes - vivid colors, and excellent levelness.
- Milling dyes - better wet fastness.
- Metal-complex dyes - high light and wash fastness.
The dyeing is usually done at pH level of 4.5-5.5 at controlled temperature to avoid damage to the fibres.
Wool Wet Processing Treatments
Wool is a delicate fabric that needs special treatment in wet processing due to its sensitivity to heat, friction, and alkali due to its scales being covered with protein-based structure. Wool can felt or shrink easily when treated roughly unlike cotton or synthetic fibres. Therefore, each of the steps, such as scouring up to finishing is under the controlled conditions to maintain the natural properties of the fibre.
Scouring of Wool
The wet process that is most significant and important on raw wool is scouring. The purpose is to eliminate grease (lanolin), dirt, salt of sweats and other contaminants that constitute 30-40 percent of the weight of the raw fleece.
Purpose:
- Eliminate natural and acquired impurities.
- Enhance properties of dyeing and finishing.
- Stop irregular colouration and smell development.
Process Details:
Wool scouring is commonly done when a type of detergent or soap solution of a known level of alkalinity in a series of bowls.
- Temperature: 45-55 deg C (greater heat can destroy wool).
- pH: 8-9 (mildly alkaline).
- Reagents Used: Non ionic or anionic detergents, sodium carbonate (in minute quantities), ammonia, or synthetic wetting agents.
Key Note:
The peptide bonds of wool can be hydrolysed by over-scouring or using harsh alkali, and this lowers strength, and results in yellowing.
Carbonizing of Wool
Small vegetable impurities such as burrs and seeds are still left in wool after scouring. These are eliminated by the process of carbonising where a mild acid is used to eliminate the cellulose matter but leave the protein fibre intact.
Steps:
- Dilute Sulphuric acid (H 2 SO 4) is impregnated into the fabric or yarn.
- The plant impurities were dried at about 95-110degC.
- The result is the carbonised matter, which is crushed and beaten off, either by beating or by mechanical violence.
- The textile is neutralised by using sodium carbonate and thoroughly rinsed.
Chemical Reaction:
The acid removes moisture in cellulose making it a brittle carbon-like material that can be easily extractable.
Precautions:
- Acid concentration and temperature should be taken care of.
- Too much acid or too much heat may destroy the keratin and decrease the tensile strength of wool.
Bleaching of Wool
Wool bleaching is only carried out when the white or very light shade is desired. Wool is naturally off-white or cream, as a result of pigments and impurities, and therefore bleaching is applied to allow it to be pastel or brilliantly dyed.
Bleaching Agents Used:
- Hydrogen peroxide (H2O2): The most widespread and harmless.
- Sodium perborate: Employed under mild conditions.
- Sulphurous acid: Temporarily whitens (not long lasting).
Typical Conditions:
- pH: peroxide bleaching (regulated by ammonia or sodium silicate), 8-9.
- Temperature: 50-60degC.
Precautions:
- Do not use the chlorine based bleaches; they destroy the protein.
- Peroxide decomposition is checked by adding stabilizers.
Result:
even, white wool, with little sacrifice of strength and the greatest brightness.
Dyeing of Wool
Wool is among the simplest natural fibre to dye due to its amide and amino groups which have strong bond with most classes of dye. The type of dye used is based on the required level of fastness, brightness, and mode of application.
Common Dyes for Wool:
- Acid dyes: The most common dyes; they give bright colors and have excellent wash fastness.
- Metal-complex dyes: Metal-complex dyes offer great light and wet fixation; apply to industrial or uniform fabrics.
- Reactive dyes: These are utilized with mild conditions to provide high fixation.
- Basic dyes and sulphur: These are employed with certain effects.
Typical Dyeing Conditions:
- Temperature: 80-98degC.
- pH: 4-5 (acidic medium, with either acetic or formic acid).
- Time: 45-60 minutes, shade depth.
After-Treatments:
- Washing and rinsing in soap are necessary to fix any unbound dye and have a smooth and even shade.
- Wool Treatments and Finishing.
- The dyeing of wool fabrics is followed by finishing which enhances the texture, stability and appearance of the fabrics.
Common Wool Finishes:
- Milling (Fulling): To fel and make better cover, body and warmth.
- Decatising: High pressure steam treatment to stabilise size and enhance handle.
- Anti-felting/ shrink-resist finish: Finished with chlorination or polymer (e.g., Hercosett treatment).
- Softening: Luxurious feel with silicone or lanolin based softeners.
- Crabbing: Thermal changing procedure to eliminate wrinkles and distortions.
Anti-felting Finishes - One of the Innovations:
Anti-felting treatments are essential as the fibres of wool overlap with each other giving rise to felting.
The most frequent is the process of Hercosett:
- To smoothen surface scales, a layer of resin is applied over them before they are partially chlorinated.
- This causes wool to be machine washable and dimensionally stable.
Summary Insight
Wool wet processing is a fine art of chemistry and handling. Each step, such as scouring, finishing, is done to maintain the integrity of fibres and enhance performance and appearance.
If handled properly:
- Wool does not lose its natural elasticity, warmth and lustre.
- It can easily be dyed, maintained in the form, and touchable.
- Contemporary coats guarantee that wool clothes are easy-to-clean and do not shrink.
Uses of Wool Fibre
Wool is one of the oldest and most versatile natural fibre and therefore it is still very powerful in the world textile industry. Its outstanding mix of comfort, warmth, and beauty allows using it in a vast variety of applications - both as a traditional cloth and technical fabrics.
Apparel and Fashion Uses
- Winter Clothing: The most widely used wool products are in the form of sweaters, coats, jackets, suits, and scarfs. Its natural insulation traps air thus it gives excellent warmth even in extreme cold.
- Formal Wear and Suits: Luxury, breathable, wrinkle-resistant Suiting This fabric is made with fine Merino wool.
- Sportswear: Wool is a trekking and athletic wear fabric and especially in cold areas, the clothes will be wet and the body will be dry hence it absorbs moisture.
- Knitwear and Hosiery: Delicate and fine wool are twisted into a or fine yarn to form comfortable innerwear and high quality knit products.
Interior and Home Furnishing
- Carpets and Rugs: Wool carpets have a good resistance and sound absorbency and they are aesthetically pleasing.
- Blankets and Throws: Wool is a warm and soft material, so it can be used as blankets in all conditions.
- Upholstery Wool blends are durable and fireproof in furniture cloth and interior panels.
- Curtains and Drapes: They are applied due to their natural flame retardancy and luxurious feel.
Industrial and TechnicalTextiles
- Insulation Materials: Wool is eco-friendly as it has thermal and acoustic advantages, which qualify it as an insulation panel in any building.
- Filtration Fabrics: It has crimp and surface properties that enable it to be used in the filtration of dust and air in industries.
- Protective Clothing: Wool has high temperature of ignition and self-extinguishing characteristic that can be used in flame resistant uniform.
- Composites: Synthetics are blended with composites to produce lightweight, high-strength composite used in transport and packaging.
Medical and Personal Care Applications
- Bandages and Dressings: Wool is a natural absorbent and hypoallergenic textile that can be used in medicine and orthopedic textile industry.
- Baby Products: Fine Merino wool is becoming a popular choice of baby clothing because of its softness and breathability.
Craft and Cultural Uses
The wool in India and most other countries has high cultural value. Shawls, carpets, felted work of some parts of Kashmir, Himachal and Rajasthan are handwoven by the tradition and livelihood of many rural groups.
Frequently Asked Questions (FAQ)
What is the difference between wool and cotton?
Wool contains protein based fibre (keratin) when compared to cotton which is cellulosic. Wool is warmer and more elastic as compared to cotton which is cooler and more breathable.
What is the reason that wool shrinks in hot water?
Because the wool fibres overlap in scale structure, friction and heat make wool fibres interlock, thus shrink.
How does wool become machine-washable?
By using anti-felting or shrink-resist finishes such as the Hercosett process, in which the fibre scales undergo a process of being smoothed and coated with resin.
Can wool be dyed easily?
Yes. Amino and carboxyl groups present in wool also react well with acid and reactive dyes producing bright, long lasting colours.
Which are the principal drawbacks of wool?
It is easily attacked by the moth, may be shrunk when not washed correctly, and itchy in rough grades.
Is wool friendly to the environment?
Yes. Wool is renewable, biodegradable and long running. Nevertheless, to be really sustainable, ethical sourcing and eco-friendly processing are of value.