Saturday, February 1, 2014

Cellulosic Fibers (Natural) – Cotton[1-3]
Art Resource

Marie-Therese Wisniowski

Preamble
This is the twenty-fourth post 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 - Wool
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
Fabric 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
Crêpe Fabrics
Crêpe Effect Fabrics
Pile Fabrics - General
Woven Pile Fabrics
Chenille Yarn and Tufted Pile Fabrics
Knit-Pile Fabrics
Flocked Pile Fabrics and Other Pile Construction Processes

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 fabrics we wish to dye. The next few Art Resource blogs will center on cellulosic fibers.

Textile fibers composed of pure cellulose are:
(i) Natural cellulosic fibers, namely, abaca, coir, cotton, flax, hemp, henequen, jute, kenaf, sisal etc.
(ii) Man-made cellulosic fibers: cuprammonium, polynosic and viscose etc.

Today the spotlight will be on cotton – a natural cellulosic fiber.

Mature Cotton Boll.


History of Cotton
No one knows exactly how old cotton is. Scientists searching caves in Mexico found bits of cotton bolls and pieces of cotton cloth that proved to be at least 7,000 years old. They also found that the cotton itself was much like that grown in America today.

In the Indus River Valley in Pakistan, cotton was being grown, spun and woven into cloth 3,000 years BC. At about the same time, natives of Egypt’s Nile valley were making and wearing cotton clothing. Cotton was considered to be a "tree wool" in the early ages of mankind. It was thought that cotton was a type of "lamb" that grew of shrubs and bent down in the wind to graze on land. Alexander the Great was given credit for bringing the first cotton to Europe and North Africa from India, where it was grown and spun on looms for at least 2,000 years. Today some of the best cotton is still grown in the Nile valley of Egypt.

Cotton Plant.

When the Spaniards came to America, they found the Pima Indians in the South West of the USA were growing cotton. After the American revolution, the South Eastern States of the USA became an important cotton producing area.

The most tedious chore in preparing cotton for yarn was removing the seeds from the cotton fibers. After Eli Whitney invented his cotton engine (or "gin") to do this job in 1793, it became possible to produce cotton on a commercial scale.

Eli Whitney's Cotton Gin.

The word "cotton" is derived from the Arabic language and depending on the dialect was pronounced kutan, qutn, qutun etc. As cotton fiber is derived from a plant it is classified as a natural, cellulose, seed, mono-cellular staple fiber. The fiber density is 1.52 g/cm3, which makes cotton a rather heavy weight fiber.

Cross-Section of a Mature Cotton Boll.
Note: Hand holding a sectioned mature boll (seed capsule) from a cotton plant (Gossypium sp.) Cotton is grown for the white fibers (lint) seen in this boll which are used to make cloth.

There are many varieties of cotton, each serving a different purpose. Pima, Supima and Egyptian are the name of a long staple fiber. This means that the fibers are longer and finer than usual. They average between 1.25" (3.2 cm) and 2" (5.1 cm) in length. The most common variety, Upland, which is used in most cotton articles, averages 0.5" (1.25 cm) to less than 1" (2.54 cm) in length.


Structure of Cotton Fiber
The cotton polymer is a linear, celloluse polymer. The repeating unit in the polymer is cellobiose, which consists of two glucose (or sugar) units. The cotton polymer consists of 5,000 of these units. It is a very long, linear polymer, about 5,000 nm in length and 0.8 nm thick.

The most important chemical groups are the hydroxyl group (-OH) and the methylol group (-CH2OH). Their polarity gives rise to hydrogen bonds between the OH groups of adjacent cotton polymers, yielding a structural integrity to the cotton polymer system. It should be also noted that van der Waals forces are also present, but these forces are much weaker than hydrogen bonds.

Shape of Cotton Cellulose.
Note: Cotton cellulose is like a ribbon – long, thin and flat and so the cellulose structure neatly packs into an organized crystalline system.
Courtesy reference[1].


Microstructure of Cotton Fiber.

Cotton fibers are amongst the finest in common use, with a fiber diameter ranging from 11 to 22 microns. Compared to wool, the cotton fiber diameter is not considered as critical as its length. The fiber length to breadth ratio ranges from 6,000:1 for the longest and best fibers, to 350:1 for the shortest and coarsest cotton types. The greater this ratio the more readily can cotton fibers be spun into a yarn.

Electronically Scanned (Electron Micrograph) Cotton Fibers.

Cotton fibers vary in color from white to light tan, depending on its type, environment, soil and climatic conditions under which it is grown. These factors influence the amount of protein and minerals, which occur in the fiber and hence determine its natural color.


Macro Structure of Cotton
Cotton is a crystalline fiber, not too dissimilar to silk, in that 65 to 75% is crystalline and 35-30% is amorphous. As a result the cotton polymers are well orientated and the polymeric units are not further than 0.5 nm apart in the crystalline regions, since this is the maximum distance that hydrogen bonding can take place.

The cellobiose units have a hexagonal shape (see above) and are connected via oxygen linkages. They can be thought of having the same structure as chicken wire. The crystalline regions are therefore the well-ordered lines and rows of hexagonal holes of the wire netting. The amorphous regions are a disarrangement of these orderly lines and rows of hexagons.

Cross-Section of Cotton Fiber.
Note: The cross-section of cotton has a kidney like shape.
Courtesy reference[1].


Physical Properties
Burning Behavior
Cotton burns like paper, which is also cellulosic. It will continue to burn when the source of the fire is removed. The odor is that of burning paper; there is a soft grey ash and an afterglow.

Dimensional Stability
Shrinkage can be expected of all cotton cloth unless subjected to shrinkage control processes.

Elastic-Plastic Nature
The cotton fiber is relatively inelastic, due to the crystalline structure of the cotton polymer system and for this reason, cotton fabrics tend to wrinkle and crease easily. Only under considerable strain will cotton polymers slide pass each other, thereby causing permanent deformation. Usually, the cotton polymers are prevented from doing so by their extreme length and countless hydrogen bonds, which tend to bind them within their polymer system. Bending or crushing of cotton fabric places considerable strain on the fibers’ polymer system and so it will cause polymer fracture since the crystalline nature of the cotton polymer system makes it difficult for the cotton polymer to be displaced by crushing or bending. Such weakening of the polymer system, and therefore fiber structure, causes cotton fabrics to readily crease and wrinkle.

Hygroscopic Nature
Cotton fibers are very absorbent owing to their polar –OH groups contained in its polymer structure that will attract the polar water molecules. However, water can only enter into the amorphous region of the cotton polymer system, as the inter-polymer spaces in the crystalline region are far too small for the water molecule to penetrate. Aqueous swelling of the cotton fiber is due to a separation or forcing apart of polymers by water molecules in the amorphous regions only.

Water Absorption in Amorphous Region of the Cotton Polymer System.
Note: The only region water can go is into the voids of the amorphous region of the cotton polymer system, thereby being able to hydrogen bond with the hydroxyl groups of the cellulose.
Courtesy reference[1].

The general crispness of dry cotton fabrics is attributed to the rapidity with which the fibers can absorb moisture from the skin. This rapid absorption imparts a sensation of dryness, which in association with the fibers’ inelasticity or stiffness creates a sensation of crispness.

The hygroscopic nature of cotton fabrics generally prevents it from developing static electricity. The polarity of the water molecules, attracted to the hydroxyl groups on the cotton polymers, dissipates any static charge.

Microscopic Appearance
Each fiber has a natural twist. Short fibers can make strong yarns as the fibers tend to adhere together.

Natural Body
Cotton is limp unless specially treated.

Resiliency and Elasticity
Fabrics of cotton will wrinkle easily and need ironing after wear and laundering, unless treated with a special finish.

Static Electricity
Free from static electricity problems, cotton fabrics will not cling in cold, dry weather. Cotton cloths are safe for use in operating rooms and near oxygen tents as they do not generate sparks.

Susceptibility to Moths and Mildew
Not affected by moths. Mildew will grow on cotton fabric if left moist in a warm place for a long time.

Tenacity (Strength)
The strength of cotton fibers is attributed to the good alignment of its long polymers (65 - 70% being crystalline), the countless, regular hydrogen bonds that hold adjacent polymers together and the spiralling fibrils in the primary and secondary cell walls of the fibers (see above).

It is one of the few fibers that actually becomes stronger the wetter it is. This is because the water molecules (that have infused in the amorphous region of the cotton polymer system) temporarily improve the polymer alignment in this region, due to the additional hydrogen bond formations, resulting in a 5% increase in fiber tenacity.

Cross-Section of Cotton Fiber on Water Absorption.
Note: Water absorption causes greater alignment of the crystalline regions in the micorfibrils.
Courtesy of reference[1].

Thermal Properties
Cotton fibers have the ability to conduct heat, minimizing destruction caused by heat accumulation. Thus they can readily withstand hot ironing temperatures. The crystalline structure of the cotton polymer system implies that hot iron temperatures will vibrate the polymers and so rupture many hydrogen bonds, but in doing so many other hydrogen bonds will be formed as the polymers get to within 0.5 nm of each other. Thus as many bonds are broken as are formed, maintaining the structural integrity of the cotton polymer system under an applied heat stress.

Excessive application of heat causes the cotton fiber to char and burn, without any prior melting. This implies that cotton fabrics are not thermoplastic, which is attributable to the extremely long fiber polymers and the countless hydrogen bonds they form. These prevent the polymers from assuming new positions when heat is applied, as would be the case with shorter length polymers of thermoplastic fibers. When excessive heat is applied, cotton polymers violently vibrate destroying all the hydrogen bonds, and not allowing others to form. Hence the integrity of the structure of the cotton polymer system is destroyed and moreover, eventually results in violent chemical reactions, which is the hallmark of fiber combustion.

Washability
Fabrics of cotton can easily be washed in hot water with strong soaps. Bleach may be used on cloth that has not been resin treated. A hot iron can be used, but a very hot iron may scorch the fiber.

Other Properties
Cotton is weakened and will eventually disintegrate if exposed to strong sunlight. Perspiration and anti-perspirants can damage cotton, especially in the presence of heat. For this reason it is not wise to press a soiled garment.


Chemical Properties
Effect Of Acids
Cotton fibers are weakened and destroyed by acids, since acids hydrolyze cotton polymer at the glucoside oxygen atom (O), which links the two glucose units together to form the cellobiose unit. Mineral or inorganic acids (such as hydrogen chloride) will hydrolyze the cotton polymer more rapidly than the weaker organic acids (such as citric acid).

Effect Of Alkalis
Cotton fibers are resistant to alkalis and so are relatively unaffected by normal laundering. The resistance is attributed to the lack of attraction between the cotton polymers and alkalis. Mercerising without tension, or slack mercerising, causes cotton fiber to swell; that is, an increase in thickness and a contraction in length. The swelling is due to alkalis molecules or their radicals, entering the amorphous region of the cotton polymer system. In doing so they force the cotton polymers further apart causing swelling. Swelling creates greater inter-polymer spaces, permitting poorly aligned polymers to orient themselves more satisfactorily and so create additional hydrogen bonds. The latter explains the increase in fiber strength on mercerising.

Mercerising under tension, which can only be carried out on the cotton yarn or fabric, causes some fiber swelling or fiber contraction. The fiber emerges with increased tenacity and with a distinct, though subdued luster. Tensioning the cotton yarn or fabric in an aqueous alkali liquor assists the fiber molecules to align themselves further, leading to an increase in hydrogen bond formation and thus to an increase in tenacity. Mercerising under tension also causes the fiber surface to become smooth and more regular, thereby enabling it to reflect incident light more evenly. This is responsible for the subdued luster that is associated with tension mercerised cotton textile materials.

Either type of mercerising swells the fibers sufficiently to alter their normal kidney-shaped cross-section to a circular one. Hence mercerised cotton fibers dye and print a deeper hue; that is, a hue with more chroma compared with the equivalent unmercerised cotton fibers when using the same quantity of dye.

Effect Of Bleaches
Most common bleaches used on cotton fabrics are sodium hypochlorite (NaOCl) and sodium perborate (Na2BO2H2O2.3H20). The former is a yellowish liquid smelling of chlorine (Cl), whereas the latter is a white powder commonly available in most domestic laundry detergents. Sodium hypochlorite bleaches cotton at room temperature, whereas sodium perborate is more effective when the laundry solution exceeds 500C.

These two bleaches are oxidizing bleaches, which is the class of bleaches used most frequently on cotton textile materials. They bleach most effectively in alkaline conditions to which cotton textile materials are resistant.

These bleaches liberate oxygen, which actual does the bleaching. In general it is thought that the liberated oxygen forms water-soluble compounds with the fiber surface contaminants, and these water-soluble compounds can then be rinsed from the surface of the textile material.

Careful bleaching leaves the fiber polymer system largely intact and in fact, retards further chemical attack of the bleaches to the fiber surface.

Effect Of Sunlight And Weather
Atmospheric moisture (humidity) significantly contributes to the breakdown of the polymers on the surface of the cotton fibers via hydrolytic reactions. Initially the polymer hydrolysis is noticed as a slight discoloration, which accelerates due to the accumulation of hydrolytic products, which further assists in the breakdown of the fiber, thereby destroying the structure of the cotton polymer system.

In general, air pollutants are acidic and may rapidly breakdown, via acid hydrolysis, the cotton polymer system.

Fading of colored cotton fabrics is partly due to the breakdown of the fiber molecule in the fibers’ polymer system.

Color-Fastness
Cotton is considered a relatively easy fiber to dye and print, with azoic, direct, reactive, sulfur and vat dyes being used. The ease in which cotton takes up dyes, and other coloring matter, is due to the polarity of its polymers and its polymer system. Its polarity readily attracts any polar dye molecule into the amorphous region of its polymer system. It should be noted that the crystalline region of the cotton polymer system is not spacious enough to house dye molecules. In fact any dye molecules, which can be dispersed in water will be absorbed by the cotton polymer system.

Azoic Dyes
Azoic dyeing or printing occurs when two relatively small, water-soluble molecules are made to react, within the amorphous region of the polymer system, to form comparatively much larger, water-insoluble azoic dye molecules.

The very good to excellent light-fastness of azoic dyed and printed cotton fabrics is due to the resistance of the azoic dye molecules to photochemical degradation of UV light.

The very good to excellent wash-fastness of azoic dyed and printed cotton fabrics is due to the relatively large azoic dyed molecules unable to exit because of the much smaller exit gaps in the amorphous region of the cotton polymer system. The azoic dyes are attracted to the fiber polymer via van der Waals forces and as these forces are weak, it is the water insolubility and the relatively large size of the dye molecules and entanglement in the amorphous region of the cotton polymer system, which is responsible for the very good to excellent wash-fastness properties.

Direct Dyes
The attraction between fiber molecules and dye molecules is called substantivity. Since the substantivity of direct dyes for cotton is very great, they have also been given the name of the cotton colors.

Direct dyed and printed cotton fabrics have only moderate light-fastness due to the direct dye molecules being affected by photochemical and atmospheric degradation, the latter is due to air pollutants.

The poor wash-fastness of direct dyed and printed fabrics is attributed to the good water solubility of direct dye molecules. As the direct dye molecules are only attached to the cotton polymer via hydrogen bonding and weak van der Waals forces, an aqueous solution will break these forces of attraction, since the attraction of water molecules to a direct dye molecule exceeds the attraction between the direct dye molecule and the cotton polymer. The cotton fiber will swell in water, enabling some of the direct dye molecules to be removed from the amorphous region of the cotton polymer system. Hence increasing the molecular size of direct dyes can mitigate this process somewhat, because even if it is swollen, very large direct dye molecules cannot find large enough spaces to escape the amorphous region of the cotton polymer system.

Reactive Dyes
As their name applies, reactive dyes react chemically with the hydroxyl groups of the fiber polymer to form strong covalent bonds, which require large inputs of energy to sever the bonds. Hence reactive dyed and printed fabrics are inert to most degrading agents, and so are also resistant to photochemical and environmental degradation; that is, good light-fastness and good wash-fastness properties.

On the other hand, chlorine bleaches and chlorinated water, such as seawater and water in swimming pools, degrade some reactive dyes. The presence of chlorine, the ions of which are very electronegative, disrupts the covalent bonds and therefore assist in the de-coloration of the cotton fabric. Note: Only some and certainly not most, reactive dyes are chlorine sensitive.

Sulfur Dyes
Sulfur dye cotton fabrics have excellent wash-fastness since the relatively large sulfur dye molecules become tapped and entangled in the amorphous region of the cotton polymer system. These dye molecules are also insoluble in water and so this property further assists the wash-fastness of sulfur dyed and printed cotton fabrics.

The sulfur dyed molecules are held in the amorphous region of the cotton polymer system by van der Waals forces, but as these forces are extremely weak, their presence would not account for excellent wash-fastness properties of sulfur dyed and printed cotton fabrics.

Sulfur dye and printed cotton fabrics have only fair light-fastness, which is attributable to the lack of resistance of sulfur dye molecules to photochemical degradation. An “after treatment” of sulfur dyed and printed cotton fabric can improve their light-fastness.

Vat Dyes
Vat dyed and printed cotton fabrics have excellent wash-fastness, because the vat dyes are large molecules, which become trapped and entangled with the amorphous region of the cotton polymer system. This process together with vat dye molecules being water insoluble make it next to impossible for water molecules to remove the vat dyes from the amorphous regions of the cotton polymer system. The vat dyes being non-polar can only form van der Waals interactions with the cotton polymer molecules and so contribute little to their wash-fastness.

Generally, vat dyed and printed cotton fabrics have excellent light-fastness due to the chemical composition of the vat dyes that make them resistant to photochemical and atmospheric degradation. However, there are some exceptions to this observation, (e.g. indigo).


References:
[1] A Fritz and J. Cant, Consumer Textiles, Oxford University Press, Melbourne (1986).
[2] E.P.G. Gohl and L.D. Vilensky, Textile Science, Longman Cheshire, Melbourne (1989).
[3] E.J. Gawne, Fabrics for Clothing, Chas. Bennett Co. Inc., Illinois (1973).

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