Saturday, September 7, 2013

Protein Fibers – Wool[1-2]
Art Resource

Marie-Therese Wisniowski

Preamble
This is the nineteenth blog in the "Art Resource" series, specifically aimed to construct an appropriate knowledge base in order to develop an artistic voice in ArtCloth.

Other posts in this series are:
Glossary of Terms
Units Used In Dyeing And Printing Of Fabrics
Occupational, Health & Safety
A Brief History Of Color
The Nature Of Color
Psychology of Color
Color Schemes
The Naming Of Colors
The Munsell Color Classification System
Methuen Color Index And Classification System
The CIE System
Pantone - A Modern Color Classification System
Optical Properties Of Fiber Materials
General Properties of Fiber Polymers and Fibers - Part I
General Properties of Fiber Polymers and Fibers - Part II
General Properties of Fiber Polymers and Fibers - Part III
General Properties of Fiber Polymers and Fibers - Part IV
General Properties of Fiber Polymers and Fibers - Part V
Protein Fibers - Speciality Hair Fibers
Protein Fibers - Silk
Protein Fibers - Wool versus Silk
Timelines of Fabrics, Dyes and Other Stuff
Cellulosic Fibers (Natural) - Cotton
Cellulosic Fibers (Natural) - Linen
Other Natural Cellulosic Fibers
General Overview of Man-Made Fibers
Man-Made Cellulosic Fibers - Viscose
Man-Made Cellulosic Fibers - Esters
Man-Made Synthetic Fibers - Nylon
Man-Made Synthetic Fibers - Polyester
Man-Made Synthetic Fibers - Acrylic and Modacrylic
Man-Made Synthetic Fibers - Olefins
Man-Made Synthetic Fibers - Elastomers
Man-Made Synthetic Fibers - Mineral Fibers
Man Made Fibers - Other Textile Fibers
Fiber Blends
From Fiber To Yarn: Overview - Part I
From Fiber to Yarn: Overview - Part II
Melt-Spun Fibers
Characteristics of Filament Yarn
Yarn Classification
Direct Spun Yarns
Textured Filament Yarns
Fabric Construction - Felt
Fabric Construction - Nonwoven Fabrics
A Fashion Data Base
Fiber Construction - Leather
Fabric Construction - Films
Glossary of Colors, Dyes, Inks, Pigments and Resins
Fabric Construction – Foams and Poromeric Material
Knitting
Hosiery
Glossary of Fabrics, Fibers, Finishes, Garments and Yarns
Weaving and the Loom
Similarities and Differences in Woven Fabrics
The Three Basic Weaves - Plain Weave (Part I)
The Three Basic Weaves - Plain Weave (Part II)
The Three Basic Weaves - Twill Weave
The Three Basic Weaves - Satin Weave
Figured Weaves - Leno Weave
Figured Weaves – Piqué Weave
Figured Fabrics
Glossary of Art, Artists, Art Motifs and Art Movements

The Glossary of Terms, Timelines of Fabrics, Dyes and Other Stuff, A Fashion Data Base, Glossary of Colors, Dyes, Inks, Pigments and Resins, Glossary of Fabrics, Fibers, Finishes, Garments and Yarns and Glossary of Art, Artists, Art Motifs and Art Movements have been updated in order to better inform your art practice.

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Introduction
In order to understand dyeing textile materials, we need to have some understanding of the properties of fibers that make up the yarns that are the basics of the fabric we wish to dye. The next few Art Resource blogs will center on various fibers. It will not comprehensively cover all fibers, but rather will cover some fibers in each category. We shall first begin with the protein fibers.

Textile fibers composed of protein are natural fibers: wool, silk, mohair, alpaca, angora, cashmere etc. Today the spotlight will be on wool.

The word "wool" was "wull" in old English, "wullo" in Teutonic, and "wlna" in pre-Teutonic days. Wool is the fiber from the fleece of domesticated sheep. It is a natural, protein, multi-cellular, staple fiber. The fiber density of wool is 1.31 g/cm3, which tends to make wool a medium weight fiber.

Man driving goats and sheep at Balawat gates. British Museum.
Balawat Gates in Ancient Assyrian Walls and Gates.

The polymer that constitutes wool is a linear, keratin polymer with some very short side groups that normally has a helical configuration. The wool polymer is composed of twenty amino acids, and it is about 140 nm (140 x 10-9 meters) long and 1 nm thick. There are a number of bonding mechanisms that hold proto fibrils of wool together, ranging from disulfide bonds, salt linkages, hydrogen bonding and much weaker van der Waals forces.


The Macro Structure of Wool
The wool fiber is a crimped, fine to thick, regular fiber. Fine wools may have as many as 10 crimps per cm, whilst coarse wools have less than 4 crimps per 10 cm. As the diameter of wool fibers increases, the number of crimps per unit length decreases. The number of crimps per unit length may be taken as an indication of the wool fiber diameter or wool fiber fineness.

A wool fiber showing its spiralling crimp (i.e. spiraling waves).
Note: The para cortex always appears on the inside of the spiral and so the ortho-cortex absorbs more dye than the para-cortex.
Courtesy of reference[1].

The crimp configuration prevents wool fibers from aligning themselves too closely when being spun into yarn. As a result, it is possible for a woolen textile to have air spaces occupying about two thirds of its volume. It is the high ratio of the air spaces that promotes wool to be warm.

The very good resilience of wool fabrics is partly attributable to the elastic nature of the wool fiber and mainly due to its crimp. It makes the fiber “springy” and enables the fiber to return to its original crimped configuration.

The length of wool fiber ranges from 5 cm - for the finest wool - to 35 cm for the longest and coarsest wools. For textiles wool fibers that are preferred range from 5 to 12 cm in length. Such fiber lengths permit the most versatile and economical yarn manufacture. Wools vary greatly in their fiber diameter ranging from 14 micron (14 x 10-6 meters) to 45 micron for the coarsest wool.

Image of the finest wool.
Note: These fine fibers produce light-weight and pleasant handling fabrics.

Fiber length to breadth ratio can be critical with wool, since the short, coarse fibers spin into less attractive yarns than do those of fine wool. In general, fiber length-to-breadth ratio varies from 2500:1 for finer shorter wools to about 75:1 for coarser, longer wools.

Wool fibers may vary from off-white to a light cream in color, the variation is due to the disulfide bonds, which act as the colorants (or chromophores). As a result, the reflected light from the fleece gives an off-white or yellow appearance. A darker appearance may be due to polymer degradation on the surface of the fiber, although colored fleeces are becoming more popular of late.

Colored fleece.

The coarser, longer wool fibers have less crimp and are more linear enabling them to reflect incident light more evenly and so resulting in a subdued luster, which is not significant when compared to the luster of man-made fibers.


Felting of Wool
Felting of wool is the irreversible shrinkage of the length, breadth and/or thickness of the material. It is achieved by subjecting the wool textile material to agitation in an aqueous solution. Whilst it is often an intended process, nevertheless, when it is not, the tendency for wool to felt is a disadvantage of untreated woolen articles of clothing that require frequent laundering.

When a wet, untreated wool textile material is agitated, as during laundering, the wool fibers tend to move in a rootward direction. In actual fact, the root end of the fiber curls upon itself, causing a mass of entangled fibers.

A simplified representation of the wool felting process.

At step 1, the fiber is in its original position. At steps 2-6, the fiber curls at the root end, drawing up the tip end. Note that the fiber does not move much outside of its distance.
Courtesy of reference[1].

Felting of wool is significantly enhanced by heat, acid or alkali. Heat will make the wet fiber more elastic and plastic, easier and more likely to move, and to distort and entangle itself with other fibers. Heat will also cause the fiber to swell more and this effect is enhanced in acidic or alkali conditions. Increased swelling results in more inter-fiber contact and increased inter-fiber friction.


Physical Properties of Wool
Tenacity of Wool
Wool is a comparatively weak fiber. The low tensile strength of wool is due to the relatively few hydrogen bonds that are formed. The lack of strength is somewhat countered by the alpha/beta-keratin (anti- and clockwise coiled) configurations, which wool polymers are able to assume.

When wool absorbs water, the water molecules eventually saturate the spaces between the wool polymers and gradually force a sufficient number of polymers apart and in doing so, break hydrogen bonds. In addition, the water molecules hydrolyze many salt linkages in the amorphous regions of the wool fiber. The breakage of the hydrogen bonds and the hydrolysis of the salt bridges, both of which perform as inter polymer forces of attraction, act in concert and result in a loss in tenacity of a wet wool textile material.

Wool has a very good elastic recovery and excellent resilience. The ability of wool fibers to recover when being stretched or compressed is partly due to its crimp configuration and partly due to the alpha-keratin configuration of the wool polymers. When stretched, flexed or compressed, the wool fibers will return to their coiled configuration. Nevertheless, repeated stretching may permanently deform wool fibers.

The medium to soft handle of wool fibers may be partly attributed to their crimp and partly due to their amorphous nature of the wool polymer as well as to the coil configuration of its polymers. The amorphous regions contain voids into which wool polymers may be crushed when pressure is brought to bear on the wool fibers. Similarly, the coiled configurations of the polymers also give sway under pressure. The ability to give or yield under pressure is responsible for wool to possess a soft handle.

Hygroscopic Nature of Wool
The very absorbent nature of wool is due to the polarity of the wool polymer peptide groups, the salt linkages and the amorphous nature of the wool polymer system. The peptide groups and salt linkages attract water molecules, which readily enter the amorphous region of the wool fiber.

In relatively dry weather, wool may develop static electricity. This is because water molecules in the wool fiber have evaporated, and so the water molecules are depleted and not enough are left to dissipate the build up of static electricity.

In damp and humid conditions, wool textile materials distort more easily due to the polymer system of the wool fibers attracting more water molecules.

Wool fibers resist wetting and yet they can absorb water up to 40% of their mass without feeling damp.
Courtesy reference[2].

Heat of Wetting Wool
Wool gives off a small steady amount of heat whilst absorbing water. This is known as heat of wetting. This is due to an energy release because of the collision between the water molecules and the polar groups of the wool fiber. Hence, as wool textile materials absorb moisture, the wearer feels a slight warmer environment, and so wool fibers have a much less chilling effect on the skin when compared to other textile materials under similar conditions. This heat liberation due to these collisions will continue until the wool polymer system becomes saturated with water molecules.

Thermal Properties of Wool
Wool smoulders rather than burns. This seems to be due to water molecules held by hydrogen bonds on the keratin polymers. Therefore if wool is exposed to a naked flame much of the heat is dissipated by evaporating the water molecules held in the polymer system of the fiber, causing it to smoulder.

Setting of Wool
Wool can be temporarily set by inducing, via drying, a wet wool in a chosen position. This is thought to be due to bonds being broken by the wetting process and on drying (water evaporating from the polymer fiber system) inducing a different bond configuration in the polymer fiber system. On further wetting, the original chosen position can once again be altered.

A relatively permanent setting may be achieved by prolonged steaming. This occurs because the old disulfide bond linkages are semi-permanently disrupted into “new” disulfide linkages, which retains the memory of the “new” setting. However, on considerable further steaming the new setting can once again be disrupted and altered.

Chemical setting is achieved by treating the wool textile material with reducing agents such as sodium bisulfite etc. These reducing agents break the disulfide bonds. The material is then manipulated into the desired configuration (e.g. folded in pleats etc.) Stream pressing of the wool textile material in the new configuration allows disulfide to reoxidize and therefore to reform and reconfigure to the new, desired setting of the woolen textile. The cross links of the disulfide bond can be thought of as cross-links in a scaffold and so provide a very strong structure to ensure that the wool fibers adhere to its new setting. This is the only permanent way to set for crease lines, folded pleats etc.


Chemical Properties of Wool
Effects of Acids on Wool
Wool is more resistant to acids than it is to alkalis. Acids hydrolyze the peptide groups but leave the disulfide bonds intact. Although the wool polymer system is weakened by acids and is vulnerable to further degradation, it does not dissolve. Therefore, it is essential that wool be neutralized after any acid treatment.

Effect of Alkalis on Wool
Wool dissolves in alkaline solution, since the solution hydrolyzes all the bonding mechanisms in wool (hydrogen bond, salt linkages, disulfide bonds) and so causes the wool polymers to separate from each other. Prolonged exposure to alkalis even causes the hydrolysis of the peptide bonds and so completely breaks down all the wool polymer fiber system into fragments that are effectively dissolved.

Effect on Bleaches on Wool
An effective method of bleaching wool is to use a reducing bleach followed by an oxidizing bleach. The reducing bleach (e.g. acidified sodium sulfite) coverts the discoloration on the fiber surface to colorless compounds. Applying an oxidizing bleach (e.g. hydrogen peroxide) after the reducing bleach coverts the colorless compounds to water-soluble compounds, that can then be washed and rinsed off. Exposing bleached wool to the oxygen in the atmosphere means that it will revert back to its off white color and then eventually to a yellow color.

Effect of Sunlight and Weather on Wool
Exposure to sunlight and weather tends to yellow or dull color wool textiles. The UV rays of the sun causes the peptide and disulfide to break, resulting in surface degradation of the wool fiber system. The degradation products cause the surface of the wool not only to absorb more sunlight (compounding the problem) but also to scatter the incident light to a greater extent, thereby giving the surface a duller appearance. Prolonged exposure just amplifies these processes.

Color Fastness of Wool
Wool is considered an easy fabric to dye, since its fibers are readily dyed by most classes of dyes (e.g. acid, chrome or mordant, premetallized and reactive dyes). The ease at which the wool polymer system takes up these dyes is due to the polarity of the protein polymer and its amorphous nature. The polarity will readily attract any polar dye molecules and draw them into the polymer system.

As with all textile fibers, the dye molecules can only enter the amorphous region of the polymer system, since the spaces in crystalline regions are too small to allow entry to relatively large and bulky dye molecules.

Different regions of the wool polymer system.
Note: (i) The crystalline regions spaces are small compared to the amorphous regions, with the former being too small for large dye molecules or complexes, whereas the latter have much larger spaces to accommodate large dye molecules or complexes; (ii) The importance of the disulfide links (S-S) to maintain the structural integrity of the wool fiber polymer system.
Courtesy of reference[2].

Acid Dyes
Acid dyes are so called because they are normally applied from an acidic dye liquor. The excellence light-fastness of acid dyed and printed woolen textiles is due to stability of the acid dye chromophores in the presence of sunlight.

In general, acid dye and printed woolen textiles have only fair wash fastness, since these dye molecules are attached to the wool polymers by van der Waals forces, which are weak and so can be easily broken by water molecules, thereby rinsing out the dye during laundering.

Mordant or Chrome Dyes and Premetallized Dyes.
Premetallized dyes have largely replaced mordant or chrome dyes. Nevertheless, both classes of dyes have similar fastness properties.

The very good wash fastness of mordant and premetallized dyes is due to the stability that the metal atom delivers to the dyed molecules that are attached to it. For example, if using chrome, six dye molecules are attached to the chrome atom, yielding a three-dimensional octahedral structure in which six spokes are attached to each dye molecule at each apex and with the chrome atom being the center of the shape.

Shape of metal atom and dye molecule complex.
Note: (i) M is the metal (such a chrome) and L is the dye molecule; (ii) The metal-dye complex is much larger than the individual dye molecules and so find it difficult to be rinsed out of the fabric. Moreover, the metal atom gives the complex greater structural integrity and greater resistance to photochemical damage by UV light.

This extra stability of the combined metal-dye configuration ensures that the dyed or printed woolen fabrics are resistant to chemical degrading agents, such as alkaline laundry liquors. Furthermore, the relatively large size of the metal-dye complex that is formed within the amorphous region of the polymer system becomes trapped in that region since all exit channels from this region are now smaller than the metal-dye complex.

The good light fastness properties of acid dyed and printed woolen textiles is due to the electronic properties of the metal atom that is attached to the dye molecules being able to withstand photochemical degradation caused by UV light.

Reactive Dyes
These dye molecules react with the wool polymer to form covalent bonds, which are extremely strong in nature (e.g. much stronger than hydrogen bonds or van der Waals forces). Reactive dyed and printed wool fabrics therefore possess very good light and wash fastness, since it takes a significant amount of energy to sever these bonds.


References:
[1] E.P.G. Gohl and L.D. Vilensky, Textile Science, Longman Cheshire, Melbourne (1989).

[2] A Fritz and J. Cant, Consumer Textiles, Oxford University Press, Melbourne (1986).

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