Saturday, November 2, 2013

Protein Fibers – Silk[1-2]
Art Resource

Marie-Therese Wisniowski

Preamble
This is the twenty-first 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 - 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 fabrics we wish to dye. Today our spotlight will be on silk.

Cocoon of a silk worm.

In Old English, silk was sioloc. The name was thought to have originated from the Greek word seres, meaning the people from Eastern Asia, namely Chinese. China was where the Europeans first sourced silk.

Silk is a natural, protein filament. Its filament density is 1.34 g/cm3, which makes it a medium weight fiber, not too dissimilar in density to wool fiber. However, very lightweight silk materials may be manufactured from silk filaments.

Pure silk crepe de chine.

The silk polymer is a linear, fibrion polymer. It differs from the wool polymers as follows:
(i) Silk is composed of sixteen amino acids compared to the twenty amino acids of wool. Three of these amino acids of silk (alanine, glycine and serine) make up 80% of the silk polymer composition;
(ii) None of the silk amino acids contain sulfur and so disulfide bonds (S-S) do not exist in its structure;
(iii) The silk polymers only occur in the beta configuration, whereas wool has both alpha and beta configurations.

The silk polymer is about as long (140 nm), or slightly longer than the wool polymer, and about 0.9 nm thick. The important chemical groups in the silk polymer gives rise to hydrogen bonding (peptide groups) and salt bridges (amine and carboxyl groups).

Banana silk yarns


Macro Structure of Silk
The polymer system of silk is considered being composed of layers of folded, linear polymers as shown in the figure below. Such a polymeric structure explains why the estimated crystalline region of the silk polymer system corresponds to about 65 - 70% crystalline and 35 - 30% amorphous regions. The major force of attraction between the silk polymers is due to hydrogen bonding. Hydrogen bonding is only effective across a distance less than 0.5 nm, and so the fibroin polymers must lie closer than this distance, which is consistent with the highly ordered nature of the silk polymer system.

The arrangement of folded beta-configuration of the linear fibroin polymers.
Note: This explains the very crystalline nature of the silk polymer system.
Courtesy reference [2].


Physical Properties
Tenacity
The silk filament is strong due to its linear, beta configured polymers and its very crystalline nature (see figure above). These two factors allow for many hydrogen bonds to be formed in a more regular manner.

If silk is wet, it loses strength due to the water hydrolyzing a significant number of hydrogen bonds and so in the process weakening the silk fiber.

Elastic-Plastic Nature
Silk is considered to be more plastic than elastic, since its very crystalline nature of the polymer system does not permit the amount of polymer movement, which would occur in polymer systems that possess a large amorphous region. Hence if the silk fabrics are stretched excessively, the silk polymers, which are already in a stretch state due to its beta configuration, will slide past each other, thereby rupturing a significant number of hydrogen bonds. This means that polymers do not return back to their original position, but rather remain in their new position. This disorganizes the polymer system of silk, which translates into a distortion and wrinkling or creasing of the silk fabrics.

The handle of silk is medium, as its very crystalline nature imparts a certain amount of stiffness to the filaments. This is often at odds with its touch - as soft - which is due to the smooth, even and regular surface structure of the silk filaments.

Silk appears soft to touch.

Hygroscopic Nature
As silk is a very crystalline polymer system it is less absorbent than wool, since its crystalline nature allows fewer water molecules to enter, unlike the large amorphous region of wool. The other hygroscopic properties of silk are similar to wool.

Thermal Properties
Silk is more sensitive to heat than is wool. This is due to the lack of any covalent cross links in the silk polymer system compared with the disulfide bonds of wool. All the bonding mechanisms in the silk polymer system that holds the fabric together tend to break down once the temperature exceeds 100oC.

In the case of discoloration due to heat, scorching or burning, reference should be made to the comments made in the case of wool, since these two fibers are rather similar with respect to these properties.


Chemical Properties
Effect of Acids
Silk is degraded more readily by acids than wool. This is because silk does not possess, like wool, the disulfide covalent bonds, which are cross linked between silk polymers. Thus perspiration, which is acidic, will cause immediate breakdown of the silk polymer system and so causes a distinct weakening of the silk fabric.

Effect of Alkalis
Alkali solutions cause the silk polymer to swell due to the partial separation of the silk polymers by the alkali molecules. Salt bridges, hydrogen bonds and van der Waals forces hold the polymer system of silk together. All of these inter-polymer linkages are hydrolyzed by the alkali, resulting in the dissolution of the silk filament. Initially the dissolution results from polymer separation but after prolonged alkali action peptide bond hydrolysis occurs, resulting in the complete destruction of the silk polymer.

The yellowing of white or the dulling of the colored silk textile on laundering is due to a filament surface rearrangement of the polymers, as well as silk polymer degradation. Both of these processes affect light reflection and result in yellowing or dulling of the fabric.

Effect of Bleaches
An effective method of bleaching silk (like 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. See wool for further comments, since they also apply to silk.

Effect of Sunlight and Weathering
The resistance of silk to the environment is not as good as in the case of wool due to the lack of covalent cross-links in the polymer system of silk. For further explanation with respect to the effects of sunlight and weather on silk, see the corresponding section on wool.

Color-Fastness
The explanations and descriptions of dyeing and printing with respect to color-fastness of wool also apply to silk, with the proviso that the luster of silk will cause its dyed and printed silk fabrics to appear much lighter in color than the equivalent dyed or printed wool fabric.


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).

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