unit 7-silk.pdf

1Unit 7Silk

Politecnico of Turin www.polito.it

Course of Textile Fibres and Technology

Claudio Tonin [email protected] 2013/14

venerd 9 maggio 2014

1Unit 7Silk

CNR ISMAC Institute for Macromolecular Studies Biella www.bi.ismac.cnr.it

Politecnico of Turin www.polito.it

Course of Textile Fibres and Technology

Claudio Tonin [email protected] 2013/14


Silk is a natural protein fiber (namely the fibrous protein called fibroin), some forms of which can be woven into luxury textiles.

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Silk fabric was first developed in ancient China, possibly as early as 6000 BC and definitely by 3000 BC. Legend gives credit to a Chinese empress, Xi Ling-Shi (Hsi-Ling-Shih, Lei-tzu),wife of the mythical Yellow Emperor. She is credited with the introduction of silkworm rearing and the invention of the loom.

Half a silkworm cocoon unearthed in 1927 from the loess soil astride the Yellow River in Shanxi Province, in northern China, has been dated between 2600 and 2300 BC.Silks were originally reserved for the Kings of China for their own use and gifts to others, but spread gradually through Chinese culture both geographically and socially, and then to many regions of Asia.

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Silk rapidly became a popular luxury fabric in the many areas accessible to Chinese merchants because of its texture and luster. Silk was in great demand, and became a staple of pre-industrial international trade.


The first evidence of the silk trade is the finding of silk in the hair of an Egyptian mummy of the 21st dynasty, 1070 BC. Ultimately the silk trade reached as far as the Indian subcontinent, the Middle East, Europe, and North Africa.

This trade was so extensive that the major set of trade routes between Europe and Asia has become known as the Silk Road.

The Emperors of China strove to keep knowledge of sericulture secret to maintain the Chinese monopoly.

Nonetheless sericulture reached Korea around 200 BC and by AD 300 the practice had been established in India.

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Venetian merchants traded extensively in silk and encouraged silk growers to settle in Italy. By the 13th century, Italian silk was a significant source of trade. Since that period, the silk worked in the province of Como has been the most valuable silk in the world. Italian silk was so popular in Europe during the Italian Renaissance, that Francis I (14th century) invited Italian silk makers to France to create a French silk industry, especially in Lyon.

Silk was expensive in Europe and used only by the rich, and textile industry began to look for substitutes, which led to the invention and development of man made (first artificial, then synthetic) fibres.

In the late 19th century, China, Japan, and Italy still were the major producers of silk.

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Nevertheless, the silk processing industry still remains important in Italy for high fashion apparel.

Testo

The cultivation of silk is called sericulture. Over 30 countries produce silk, and the major ones are China, India, Japan and, more recently, Brazil.

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The world annual production of silk is currently about 125 000 t.(source: Indian Central Silk Board, www.indiansilk.kar.nic.in)

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Silk todayWorld silk production has approximately doubled during the last 30 years in spite of man-made fibers replacing silk for some uses. China and Japan during this period have been the two main producers, together manufacturing more than 50% of the world production each year. During the late 1970's China, the country that first developed sericulture thousands years ago dramatically increased its silk production and has again become the world's leading producer of silk.

Testo

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There are many indigenous varieties of wild silk moths found in a number of different countries.

Mulberry from the silkworm Bombyx mori, Tasar (Tussah) generated by the silkworm Antheraea mylitta,

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Four of the most important domesticated silk worms, together with their adult moth forms, Meyers Konversations-Lexikon

Mulberry: The bulk of the commercial silk produced in the world comes from this variety and often silk generally refers to mulberry silk. Mulberry silk comes from the silkworm, Bombyx mori L. which solely feeds on the leaves of mulberry plant. These silkworms are completely domesticated and reared indoors.


The Silk moths lifecycle

Silk moth lays eggs on specially prepared paper.

It lays 500 or more eggs in four to six days and dies soon after. The eggs are like pinpoints one hundred of them weigh only one gram.

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The eggs must be kept at 65 degrees F, increasing gradually to 77 degrees at which point they hatch.

From one ounce of eggs come about 30,000 worms which eat a ton of mulberry leaves and produce twelve pounds of raw silk.

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After the eggs hatch, the baby worms feed day and night every half hour on fresh, hand-picked and chopped mulberry leaves until they are very fat.

From the day it hatches to the time it stops feeding,

(a period of 25 to 30 days),

the silkworm gains in weight about 10,000 times.

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White Mulberry (Morus alba) leaves are the preferred feedstock for silkworms.

Cultivation of White Mulberry for silkworms began over 4,000 years ago in China, with 6,260 km currently devoted to the species in China.

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The caterpillars (silkworms) are fed fresh mulberry leaves.

During this time they eat some 22g of leaves each and convert more than 70% of its intake of nitrogenous material into silk.

To produce 1kg of raw silk about 6.25 kg of fresh cocoons are necessary, while for raising 6.25 kg of cocoons about 104 kg of mulberry leaves containing 26 kg of solid matter are required.

Today, artificial fodders are investigated to feed silkworms.

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After about 35 days and 4 moltings (change of the skin), the caterpillars are ready to begin spinning a cocoon; this is a stage of the process called metamorphosis.

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When it is time to build their cocoons, the worms produce a jelly-like substance in their silk glands, which hardens when it comes into contact with air. Silkworms spend three or four days spinning a cocoon around themselves until they look like puffy, white balls.

A straw frame is placed over the tray of caterpillars, and each caterpillar begins spinning a cocoon by moving its head in a "figure 8" pattern.

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Caterpillar envelops itself into the cocoons.

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A cocooning frame.

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Cocoon rearing in a sericulture farm.

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The spinning apparatus (spinneret) of caterpillars spin twin silk fibroin filaments enveloped by a non fibrous (gum) protein called sericin.

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Liquid silk fibroin is coated by sericin, a water-soluble protective gum, and solidifies on contact with the air.

For textile uses, normally sericin is totally or partially eliminated from fibroin (the fibrous protein) by a finishing process called degumming.

Sericin

Silk fibroin

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After eight or nine days in a warm, dry place the cocoons are ready to be unwound. First they are steamed or baked to kill the worms, or pupas.

Some caterpillars are allowed to metamorphose into moths to breed the next generation of caterpillars.

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Moths are going to start for a new generation.

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But commercially reared silkworm pupae are killed by dipping them in boiling water before the adult moths emerge, allowing the whole cocoon to be unraveled as one continuous thread.

(On average 75% of the fresh green cocoon, by weight is chrysalis).

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The cocoons are then dipped into hot water to loosen the tightly woven filaments. These filaments are unwound onto a spool.

This permit a much stronger cloth to be woven from the continuous silk filaments.

The outer layer of floss and the inner layer are collected as silk waste and used in spun silk manufacture.

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Silk fibroin is made up of the amino acids listed in the table;its native secondary structure is Beta pleated sheets.

The high proportion (>70%) of glycine, alanine and serine, which are small, allows tight packing and the fibers are strong and resistant to stretching.

The isoelectric point of fibroin is (pI) pH = 3.2; the Molecular Weigth is up to 300 KDa.

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H2N-C-COOH H2N-C-COOH H2N-C-COOH

H|

H|

H|

|H

| CH3

| CH2

| OH

Glycine Alanine Serine


Interchain H-bonds are formed while side chains are above and below the plane of the H-bond network.

The sheet (also -pleated sheet) is the second form of regular secondary structure in proteins consisting of beta strands connected laterally by hydrogen bonds, forming a generally twisted, pleated sheet (the most common form of regular secondary structure in proteins is the alpha helix already seen in wool). A beta strand (also -strand) is a stretch of amino acids whose peptide backbones are almost fully extended.

The tensile strength is due to covalent peptide bonds. Since the protein forms a Beta sheet, when stretched the force is applied to these strong bonds and they do not extend so much.

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Mechanical properties:Tensile

Elongation (%)

Tenacity(cN/dTex)

Flax

Cotton

Silk Polypropilene S

Polyamide

Polyester

Acrylic Polypropilene

Wool


Silk

Origin: secretion of moth larvae (bombix mori, .)

Chemical and macromolecular structure:

protein (fibroin)

MW up to 300.000

isoelectric pH (pI): 3.2

Supramolecular structure:

-sheet, crystalline (small lateral groups)

Morphology structure:

non cellular, with fibrillar structure.

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Silk has a smooth, soft texture that is not slippery, unlike many synthetic fibers. Its tenacity is 4.5 g/den when dry and 2.8-4.0 g/den when wet.

It is very fine (~10 m) and it is the only natural continuous filament, 2-3000 m long.

Silk is one of the strongest natural fibers but loses up to 20% of its strength when wet. It has a good moisture regain of 11%.

Its elasticity is moderate to poor: if elongated even a small amount it remains stretched.

It can be weakened if exposed to too much sunlight. It may also be attacked by insects, especially if left dirty.

Silk tends to fibrillate under mechanical stress when wet.

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The shimmering appearance for which silk is prized comes from the fibers' triangular prism-like cross section with rounded corners.This reflects light at many different angles, giving silk a natural shine.


Main chemical & physical properties of silk

Heat: doesnt melt; turns yellow at T > 120C; decomposes at T > 170C.

Flame: burns difficultly and self-extinguishes (LOI > 21%). Water: hydrophilic (Regain 11%). Acids: resists to most diluted mineral acids because of its isoelectric point at acidic pH, but will dissolve in sulfuric acid. Concentrated acids (hot and cold) degrade the fibre (hydrolisis of the amide bonds). Nitric acid gives the xantoprotin reaction (because of the high amount of tyrosin 11%) Bases: soluble in boiling caustic alkali. It is less sensible than wool to bases, because of the absence of cystine.

Solvents: resists to organic solvents used in dry cleaning.

Oxidants: chlorine, bromine, hypochlorite cause yellowing. Bleaching is carried out with hydrogen peroxide.

Reductants: resists to reductants.

Dyeing: easy dyed with many dyes (acids, metal complex, reactive,....).

Sun eposure: turns yellow by light exposure, because of the high amount of tyrosin, which is very reactive with Nitrogen oxides (NO, NO2) that easy nitrate the benzene ring.

Biodegradation: quite resistant to moulds and bacteria.33venerd 9 maggio 2014

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Tyrosine


Uses & behaviour of silk-based textiles

Silk has a fairly high natural resistance to creasing. Attempts to improve on this and also to apply easy-care treatments have not been very successful, as they impair the unique handle, or feel, of silk which is one of its most valuable properties.

Silks good absorbency makes it comfortable to wear in warm weather and while active. Its low conductivity keeps warm air close to the skin during cold weather.Silk has good elasticity and resilience (elastic recovery), excellent hand, quite good resistance to abrasion and pilling, good tensile properties (tenacity, elongation).

Maintenance is quite difficult bacause the fibre is delicate, but stability to domestic washing is good (it doesnt shrink). Silk is sensible to chemicals contained in wet processing (pH, enzymes, surfactants, It has good color fastness (wet & light) with many dyes (acid, pre-metallized, reactive,....) and it is bleached with oxidants (no hypochlorite) and fluorescent dyes

It is often used for clothing such as shirts, blouses, formal dresses, high fashion clothes, ties, negligees, pyjamas, robes, skirtsuits, sun dresses and underwear.Silk's elegant, soft luster and beautiful drape makes it perfect for many furnishing applications. It is used for upholstery, wall coverings, window treatments (if blended with another fiber), rugs, bedding and wall hangings.

While on the decline now, due to artificial fibers, silk has had many uses in the past: parachutes, bicycle tires. A special manufacturing process makes it suitable as non-absorbable surgical sutures.

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Wild silk

"Wild silks" are produced by caterpillars other than the mulberry silkworm and cannot be artificially cultivated.

The rearings are conducted in nature on the trees in the open.

A variety of wild silks have been known and used in China, South Asia, and Europe since early times, but the scale of production was always far smaller than that of cultivated silks.

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Wild silks differ from the domesticated varieties in color and texture.


Tussah silk butterflyTussah, a kind of wild silkworm, lives and multiplies wherever oak trees or leaves of the Mongolianoak, rubber trees or Chinese toon trees are available. There is one main type of tussah silkworm, Antheraea pernyi, and one general type of oak tree onwhich it feeds, Quercus serrata.

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Tussah silk-worm


Wild tussah silk cocoons

The Tussah chrysalis and cocoon are larger than those of the mulberry silkworm, but the cocoon shell is thinner, has a lower layer ratio and yields less silk. The Tussah cocoon is reddish-brown in colour, while the mulberry cocoon is white.

Cocoons gathered in the wild usually have been damaged by the emerging moth before the cocoons are gathered, so the silk thread that makes up the cocoon has been torn into shorter lengths.

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Tussah (Tasar) is copperish colour, coarse silk mainly used for furnishings and interiors. It is less lustrous than mulberry silk, but has its own feel and appeal.


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