Saturday, December 7, 2013

Protein Fibers – Wool versus Silk[1]
Art Resource

Marie-Therese Wisniowski

Preamble
This is the twenty-second 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
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
All protein fibers contain the chemical elements of carbon (C), hydrogen (H) and nitrogen (N). Wool also contains sulfur (S), which imparts some properties that are different from those of silk. Natural protein fibers are products of animal growth. Man-made protein fibers (regenerated) are made by dissolving and re-solidifying protein from animals and plants.

Long design fashion jacquard wool sweater dress.

The protein of a wool fiber is called keratin and that of a silk fiber is fibroin. The basic units of the protein molecule of both fibers are amino acids, joined together in linear polypeptide chains.

Above: A linear polypeptide chain that makes up the backbone of keratin and fibroin proteins. The differences between the wool and silk proteins is not highlighted here.

Proteins are amphoteric; that is, they have both acidic and basic properties, although the basic nature is more dominant. They are susceptible to attack by light, bleaches and alkali. The backbone structure of proteins (structure above) yield a somewhat folded structure (see below).

The folded structure of a silk fibroin is more evident in a three-dimensional depiction of the protein (see above). The folded structure contributes to the flexibility, resiliency and elasticity of protein fibers.

Silk fibers are continuous (filaments) and can be a few thousand meters in length, while wool fibers are shorter (staple) and have a of maximum length of ca. 13 cm. The two halves of wool fiber are slightly different in their chemistry and so absorb water differently. This accounts for the bending and crimping of wool - a feature that is copied in most synthetic fibers.


Properties Common To All Protein Fibers
Whilst the detail of the protein structure varies from fiber to fiber, they do share a number of properties due to their similar structures. This has important bearing on the attractiveness of the fiber to consumers at large.

Resiliency
Due to their resiliency, protein fibers generally resist wrinkling. Wrinkles hang out during wearing. Fabrics tend to hold their shape. One measure of the resilient nature of a fiber is to test the fiber resistance against breakage. If we invoke standard conditions such as 65% relative humidity at 70oF we can stretch the fiber until it breaks. The greater the fiber can be stretched the more resilient it is. Wool fiber can be stretched to 25-35% of its length when dry and 25-50% of its length when wet before breaking. Silk in dry conditions can only be stretched to 20% of its length. On the other hand, a non-protein finer such as cotton in dry conditions can only be stretched 3-7% of its length.

Hygroscopic
This property imparts a comfortable wear in a cool damp climate. One measure of the hygroscopic nature of a fiber is its absorbency. Wool under standard conditions has an absorbency of 16%, and silk 10%, whereas a non-protein fiber such as cotton only 7%.

Weaker when Wet
Protein fibers must be handled carefully during washing. Wool loses about 40% of its strength and silk loses about 15%. The fiber strength or tenacity of wool when dry is 1.7 - 1.0 grams per denier and when wet is 1.63 – 0.76 grams per denier.

Specific Gravity
The density and specific gravity of wool and silk are about the same: 1.32 and 1.30 grams per centimeter respectively, whereas that of non-protein fibers such as cotton and flax is 1.54 and 1.50 grams per centimeter respectively. Hence wool and silk feel lighter than the cellulosic fibers of the same thickness.

Harmed by Alkali
Protein fibers are amenable to neutral or slightly alkaline soap or detergent. However, they are fragile to more stronger alkali products. For example, even perspiration weakens protein fibers such as silk.

Harmed by Oxidizing Agents
Chlorine bleaches damage protein fibers and so they should not be used. Sunlight causes white protein fabrics to turn yellowish.

Harmed by Dry Heat
Wool becomes harsh and brittle and scorches easily with dry heat. Hence steam is used when ironing! White silk and wool turn yellow under these conditions. The safe ironing temperature of wool is 300 oF.

Flame Resistant
Both wool and silk do not burn readily and are self-extinguishing. They have the odor of burning hair, and form a crushable ash. Wool decomposes between 266 – 400 oF.


Differences between Wool and Silk
There are significant differences between wool and silk fibers.

Wool fibers are staple only, whereas silk is a filament and staple fiber. A filament is a fiber of indefinite length, whereas a staple is a fiber of short length. For example, Merino wool is typically 3 – 5 inches in length and is very fine (between 12 and 24 microns in diameter).

Close-up view of a silk fiber taken with a scanning electron microscope.
Image courtesy of N. Huby.

Wool possess a cellular structure, whereas the structure of silk is that of a sold fiber.

Schematic diagram of the wool fiber.

Wool has an elliptical cross-section (see above), whereas silk has a triangular cross-section.

A cross section of silk fiber magnified 1300 times.

Both wool and silk have a molecular crimp, which may not translate at the fiber level. For example, wool has a three-dimensional fiber crimp, whereas silk has no fiber crimp. Note: Crimp is the waviness of a fiber.

Three-dimensional crimp of the wool fibre.

Wool fiber has a bi-component structure (i.e. double molecular chain structure) whereas silk possesses a single component structure (i.e. single molecular chain structure).

Wool has cross-links (i.e. links between strands of proteins) whereas silk has no cross-links.

Disulfide cross linkage in wool.


Reference:
[1] N. Hollen and J. Saddler, Textiles, 3rd Edition, Collier-Macmillan Ltd, London (1968).

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