Various parts of a silk fiber

1. COMPOSITION AND
STRUCTURE OF SILK
FIBRE
Dr. Kuntamalla Sujatha, Madichetty Sai Abhishek,
Assistant Professor, Manager, BDM.
Sericulture Dept, Hyderabad.
Kakatiya University, Hanamkonda,
Warangal.
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Dr. Kuntamalla Sujatha & M. Sai Abhishek

2. The raw silk is composed of two filaments composed of 16 various amino acids
which are produced by fibroin that is gummed by sericin and different types of
mineral salt, fat and wax. The shimmering appearance of silk is due to the
triangular prism like structure of the silk fibre. The silk fibre is triangular /
smooth and is reason for the silk fibre to be lustrous. The diameter of single
mulberry silk is 15 – 25 µm while that of wild silk is 65 µm. The total length of
univoltine and bivoltine varies from 1000 – 1400 meters of which 75 – 80% is
reelable while multivoltine are 350 – 600 meters. Insoluble form of fibroin the
molecules occur in the ∞ form with inter molecular hydrogen bonded helical
structure.
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Dr. Kuntamalla Sujatha & M. Sai Abhishek

3. Fibroin molecules are polypeptide chains of considerable length. Extensibility of fibroin
in other words the strength and resistance are attributed to the strong inter chain forces.
The low proline / hydroxyprotein content and absence of cysteine distinguish the fibroins
from collagen and keratin.
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Dr. Kuntamalla Sujatha & M. Sai Abhishek

4. Silk fibres are strong, extensible and mechanically compressible which are natural protein
fibre woven into textiles. The protein fiber of silk is composed mainly of fibroin and
sericin, fibroin being the structural center of the silk and sericin being the sticky material
produced by insects, surrounding it to form the cocoons. Cocoons are protective sheath
produced by silkworms to protect itself from adverse climatic conditions. Fibroin is largely
made of Gly-Ser – Gly-Ala – Gla-Ala and forms beta pleated sheets p. keratin
Fibroin – 75%
Sericin – 22.5%
Fat & Wax – 1.5%
Ash of silk fibroin – 0.5%
Mineral salts – 0.5%
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Dr. Kuntamalla Sujatha & M. Sai Abhishek

5. Chemical structure of silk composition of fibres
Insoluble form of fibroin, molecules occur in the form with inter molecularly
hydrogen bonded helical structure.
Glycine, alanine and serine are three simple amino acids which constitute major part of
the fibroin occurs as long sequences in B.mori fibroin whereas there are evidences of
occurrence of some polyalanine sequences in tasar fibres.
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Dr. Kuntamalla Sujatha & M. Sai Abhishek

7. Molecular packing of Filaments
•The silk fibres appear as solid opaque rods under microscope and break down into fibrils.
•The silk fibre is not a completely homogenous solid but composed of fibrillar region held
by protein.
Cross section of filaments of mulberry silkworm is like a swollen triangle about 10 µ
across, with filaments appearing as solid rod without markings. The diameter of silk varies
from 15 – 25 µ in B.mori while its 65 µm for bave of tasar silk which varies even within
the cocoon layers.
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8. Mechanical properties
Silk is unique because of its strength and breaking extension withstanding repetitive bending
cycles a number of times before rupturing hence highly needed in textile material
manufacturing. There is a relationship between fibre fineness and recovery making the fibre
more fine under given stress conditions. When exposed to UV light tyrosine, threonine and
leucine are affected readily. Yellowing of silk fibre is due to decomposition of tryptophan.
Due to presence of tyrosine if coated with 1% sodium sulphate, there is a decrease in
yellowing. The silk is intermediate between wool and polyester when a comparison is drawn
in relation with bending recovery and coercive properties.
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Dr. Kuntamalla Sujatha & M. Sai Abhishek