SILK(Part-2)
#Chemical composition of silk
◆Like wool keratin, silk is formed from amino acid. In its chemical composition, silk fibroin is much simpler than wool (keratin) fibre.
◆Silk fibre, chemically a polypeptide, is built up from mainly four amino acids-
glycine (38-41 %),
alanine (30-33%),
serine (12-16 %)
and tyrosine (11-12%).
glycine (38-41 %),
alanine (30-33%),
serine (12-16 %)
and tyrosine (11-12%).
#Different types of silk
Several species produce proteinous fibre. These can be categorised into two types-(a) Cultivated silk e.g mulberry silk
(b) Wild silk e.g. Tussar silk, muga silk, ERI silk
(i)Mulberry silk:-
◆As name indicates these are produced by worms which feed on mulberry leaves.
◆The most important one is Bombyx mori. These are used in sericulture and can easily be domesticated.
◆Gorgeous and fabulous silk materials are produced out of this variety.
◆In India it is produced in West Bengal, Mysore, Tamil Nadu, Kashmir, Assam and Punjab.
(ii) Tussar silk:-
◆Tusser silk is produced by species Antheraea mylitta In India, by Antheraea paenyi in China and byAntheraea yamamai in Japan. Also known by several name such as Tisare, Tushar and Tussah.
◆This species yields two crops of cocoons in a year.These are difficult to domesticate and lead an outdoor life and are found in forests upto an altitude of 5000feet.
◆The worm feeds on a variety of leaves like oak leaves, caster plant leaves, cherry leaves etc.
◆Tussar worm do not split the silk filaments as a result continuous filaments are achieved.
◆In India it is found in Chota Nagpur, Orissa, West Bengal and Kashmir accounting over 90 per cent of the total production.
◆Clothings made out this variety have a 'Dry Look'. They have the subdued brilliance of morning sun.
(iii) Muga silk:-
◆It is produced only in North-eastern region of India (Assam).
◆It is obtained from the species Antheraea assama. The worm feeds on sun and solar leaves and produces golden yellow silk.
(iv) ERI silk:-
◆It is produced from species 'Attacus racini' or Philosamia cynthia' in Assam, part of China or South East Asia.
◆The Eri culture extends up to an altitude of 1500 meters in the hills and is practised in temperatures varying from 12°C in winter to 37°C in summer with a humidity of 80 to 100 %. Eri worms are reared indoors.
#Comparison between wild silk and cultivated silk:-
1. Wild silk is more durable owing to greater thickness of threads, though cultivated silk threads are stronger in finished fabric.
2. More cheaper and marked by high productivity.
3. Wild silk worm do not need any care and attention unlike cultivated one.
4. Yield several crops per year whereas cultivated one gives only one.
5. Cultivated variety feeds only on mulberry leaves whereas wild one feed on several ones like caster plant, oak leaves, cherry leaves etc.
#Properties of silk
(A) Physical properties
(i) Microscopic appearance:-
◆The irregular, random, longitudinal stratitions are seen on raw silk.
◆ Consists of double fibre strand.
◆ In cross-section it is triangular.
(¡¡) Length and diameter:-
◆Length averages upto 1300 mm and thickness is about 0.00054-0.0018 inch.
◆Length averages upto 1300 mm and thickness is about 0.00054-0.0018 inch.
(iii) Colour and lustre:-
◆It is yellowish in colour and having bright lustre.
◆It is yellowish in colour and having bright lustre.
(iv) Strength:-
◆ Exceptionally strong having tensile strength of 2.5-5.0 gm per denier.
◆ Exceptionally strong having tensile strength of 2.5-5.0 gm per denier.
(v)Moisture regain:-
It has a moisture regain of 11%, less absorbant than wool.
It has a moisture regain of 11%, less absorbant than wool.
(vi) Specific gravity:-
1.3 g/cc in raw state, 1.25 g/cc in degummed state.
1.3 g/cc in raw state, 1.25 g/cc in degummed state.
(vii) Elongation:-10-25 % of length.
(B) Chemical properties
(i)Effect of light:- gets weakened.
(ii)Effect of heat: begins to yellow. Above 330 °F disintegrates.
(iii) Effect of water:- Temporary loss of strength upto 10-25%.
(iv) Effect of acids:-
◆Mineral acid (Sulphuric acid, hydrochloric acid): Acid dyes do not injure even at boil, concentrated
acids destroy the silk.
◆Mineral acid (Sulphuric acid, hydrochloric acid): Acid dyes do not injure even at boil, concentrated
acids destroy the silk.
◆Volatile organic acids (Formic acid and acetic acid): No damnage unless heated.
◆Non volatile organic acids (oxalic acid, tartaric acid and citric acid): Must be washed out quickly to prevent weakening.
(v) Effect of alkalies:-
◆Strong alkalies (soda ash and caustic soda): Gets damaged.
◆Strong alkalies (soda ash and caustic soda): Gets damaged.
◆Weak alkalies (Borax, sodium silicate, soap): Alkalies weaken silk, soaps must be avoided in washing.
(vi) Effect of oxidising agents (H202 and KMnO4):-No damage if after treated with reducing agent and rinsed.
(vii) Affinity for dyestuffs:- Good affinity for acid dyes.
(viii) Effect of mildew:- Silk is slightly attacked by moth and mildew.
Q1.What is Thrown silk?
◆Reeled silk is unsuitable for weaving or knitting and is transformed into yarn by process called 'silk throwing.
◆In this proper number of reeled threads are taken together and twisted to obtain yarn suitable for weaving or knitting.
Q2.What is Spun silk?
◆Broken silk filaments unsuitable for reeling constitute silk-waste which on weight basis nearly equal the quantity of silk collected in the continuous filament form.
◆The waste silk is first cleaned and degummed and after degumming, the broken, short filament pieces are subjected to processes similar to those used for short staple fibres like cotton and wool.
◆The waste fibres are opened and loosened into a silver in a machine.
◆The silver is then combed, sorted into different groups on length basis and then drawn into rovings to be subsequently spun by twisting, so that the short-fibres hold tightly together to form the yarn.
◆Such silk yarns commonly called 'spun silk are used for making woven dress materials, velvet's and pile fabrics , scarves, ties, etc.
Q3.What is Degummed silk?
◆The natural gum or the cementing agent serecin is left on the silk during reeling, throwing or weaving.
◆It has the capacity to protect the fibres from mechanical damage. It is usually removed by boiling the raw yarns or fabrics in soap water.
◆After degumming, the yarn or fabric feels softer and lighter and looks much lustrous too.
◆The degumming process may lead to a loss of one third of the weight of the yarn or fabric.
◆Raw silk with the gum still on the filaments is called 'hard silk'. Degummed silk is called 'soft silk'.
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