The Pharma Innovation Journal 2022; SP-11(3): 732-738

ISSN (E): 2277- 7695

ISSN (P): 2349-8242 Physical and chemical properties of Indian silk fibres
NAAS Rating: 5.23
TPI 2022; SP-11(3): 732-738
© 2022 TPI Shilpa Saikia and Monimala Saikia
www.thepharmajournal.com
Received: 10-01-2022
Accepted: 12-02-2022 Abstract
Silk is the naturally occurring filament fibre which is an opulent gift of mother nature to mankind. Its
Shilpa Saikia unique qualities and exceptional glory make it distinct from other natural and man-made fibres. It is the
Sericulture Facilitator, only fibre which has variety of uses such as suiting, sarees, neckties, shawls, scarves, gowns, upholstery,
North East Region Textile carpets, parachutes, gas mantles, surgery and fishery. The knowledge of fibre properties is important as it
Promotion Scheme, Directorate determines the nature of resultant product. It can be classified as physical, mechanical, chemical,
of Sericulture, Govt. of Assam, electrical and biological. Silk can be classified as mulberry and non-mulberry out of which four are
Guwahati, Assam, India commercially exploited viz., mulberry, tasar, eri and muga. It is composed of different amino acids and
exhibit amphoteric nature and hence can be readily dyed with all class of dying agents. It is widely
Monimala Saikia
Assistant Professor,
accepted for excellent strength as well as elastic recovery, sheen, drapability, pliability, high moisture
Department of Sericulture, regain capacity and resiliency which make it suitable for use as high-end textile apparel.
Assam Agricultural University,
Jorhat, Assam, India Keywords: Eri, muga, drapability, pliability, sheen, upholstery

1. Introduction
The history of fibre is as old as human civilization. A fibre is a unit of matter which is usually
at least 100 times longer than its thickness (Gohl and Vilensky, 1987) [17]. The basic entity,
either natural or manufactured, which is twisted into yarns and then used in the production of a
fabric (Anon., 2019). For many thousand years, the usage of fibre was limited by natural fibres
such as flax, cotton, silk, wool etc. Silk is rightly called the ‘queen of textiles’ for its lustre,
sensuousness and glamour. Silk’s natural beauty and properties of comfort in warm weather
and warmth during colder months have made it use in high-fashion clothing (Reddy, 2009) [39].
Silk fibres have some outstanding properties that rival the most advanced synthetic polymers;
however, the production does not require harsh processing conditions.
Silk is one of the oldest natural fibres utilized by man since time immemorial. The
composition, structure and properties of silk vary depending on their specific source and
function (Altman et al., 2003; Craig et al., 1999) [1, 13]. This filament is well known for its
sheen, lustre, water absorbency, dyeing affinity, thermal tolerances along with insulation
properties (Sheikh et al., 2016). In relation to fibre length, silk fibres are extremely long
continuous filaments. It is the lightest among all-natural fibres and display and unusual
combination of strength and toughness. Some of the textile properties such as finesse, strength,
elasticity, dyebility, softness, flexibility, gloss, elegance made it suitable to use in the textile
Industry (Khan et al., 2010; Ude et al., 2014) [24, 56]. The silk is mechanically robust
biomaterial with environmental stability, biocompatibility and biodegradability and also offers
wide range of practical properties for biomedical applications (Reddy and Prasad, 2011) [40].
Silkworms secrete silk fibres to build protective shell known as cocoons during the end of
larval stage in their life cycle.
There are four types of natural silk which are commercially known and produced viz.,
mulberry, tasar, muga and eri silk produced by the silkworm Bombyx mori, Antheraea mylitta,
Antheraea assamensis and Samia ricini, respectively. The tasar, muga and eri are non-
mulberry silks also known as wild silk or Vanya silk in India. India enjoys the sole monopoly
in the world for the fabulously famed golden yellow coloured muga silk which is produced
only in the state of Assam and adjoining hills (Chowdhury, 1992).

1.1 Classification of fibres

Corresponding Author On the basis of origin, the fibres may be divided in two categories: Natural and man-made
Shilpa Saikia
Sericulture Facilitator,
(Singh, 2004) [47]. Natural fibres may be further classified into the following three groups:
North East Region Textile 1. Vegetable Fibres: It includes cotton, linen, jute, hemp, sisal, kapok, ramie, coir and pina.
Promotion Scheme, Directorate 2. Animal Fibres: It includes wool, silk and hair.
of Sericulture, Govt. of Assam, 3. Mineral Fibres: Asbestos is the only naturally occurring mineral fibre.
Guwahati, Assam, India
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1.1.1 Manmade Fibres are further classified into the contains fibroin protein (Padaki et al., 2014).
following groups
1. Cellulosic Fibres: It includes rayon, acetate and triacetate. 1.4 Composition of silk
2. Non-cellulosic polymers: It includes nylon, aramid, The silk is almost a pure protein composed of two types of
polyester, acrylic, modacrylic, spandex, olefin, vinyon, proteins viz., fibroin (70-80%) and sericin (20-30%) and other
saran, novoloid, polycarbonate, alginate, anidex, graft etc. components such as carbohydrates, waxes and ash (1-2%)
3. Protein Fibre: Azlon is the only artificially manufactured (Takasu et al., 2002). Sericin (C15H25N5O8) comprises of 18
protein fibre. amino acids, with strong polar side groups and high content of
4. Rubber: It includes synthetic rubber. serine, aspartic acid and glycine resulting in hydrophilic
5. Metallic Fibre: It includes metal made of aluminium, protein (Kunz et al., 2016) [27]. The silk fibre after degumming
silver etc. contains fibroin protein which is composed of about 20 amino
6. Mineral Fibre: It includes glass, ceramic and graphite. acids (Gupta et al., 2000) [20]. Fibroin (C15H23N5O6) is made
up of two components viz., crystalline and amorphous. In
1.2 Microstructure and appearance crystalline component, amino acids are present in a definite
Silk comprises of two important proteins, fibroin (the silk manner with a definite space and the glycine residues repeats
filament) and sericin (the gum). Each raw silk strand in the with other amino acids in the ratio of 3:2:1 (Strydom et al.,
cocoon is known as “bave”; it is composed of the fibroin 1977) [51]. Japanese scientists have revealed that fibroin is
filaments called “brin” that are held together by sericin gum. composed of major homogenous part called heavy chain or H-
Microfibrils are minute protein strands composed of ordered chain (350KD) and minor heterogenous part called light chain
amino acid chains. The microfibrils are held together in or L-chain (25 KD) connected to each other by disulphide
bundles and several such bundles constitute a single fibroin. linkages (Shimura, 1988 and Gopinathan, 1992) [46, 18].
The cross-sections of mulberry silk fibres are more or less In mulberry silk, glycine, alanine, and serine together
triangular whereas non-mulberry fibres are near rectangular. constitute about 82%, of which serine constitute 10%.
Striations are observed on the surfaces of muga and tasar silk Tyrosine and valine may be considered next to these at about
fibres (Gupta et al., 2000) [20]. 5.5 and 2.5%, respectively. The dominance of acidic amino
groups (i.e., aspartic and glutamic acids) in the mulberry
1.3 Composition of silk cocoon shells variety is more than that of the basic amino acids. The ratio of
Silkworm cocoons are composed of proteins, which account basic to acidic amino acids is 0.65 for the bivoltine and 0.75
for more than 95% of its content; other impurities, such as for the crossbreed variety. Similarly, the ratio of bulky to non-
waxes, mineral salts, and ash, constitute about 4-5%. The wild bulky amino acids in these two varieties is ranged from 0.17–
silk cocoon shell has a lower sericin content and higher levels 0.18. The ratio of hydrophilic to hydrophobic groups in both
of wax, minerals wax, ash and other impurities. The raw silk varieties suggests that the fibres are basically hydrophilic in
fibre extracted from silk cocoon is subjected to degumming character, although the crossbreed variety has a slightly
process to remove sericin from it. After degumming silk fibre higher value at 0.29 (Sen and Babu, 2004) [42, 43].

Table 1: Amino acid composition of different varieties of silk fibres

Amino acid composition (mol %)
Amino acid Mulberry(bi) Mulberry (cross) Tasar Muga Eri
Aspartic acid 1.64 1.49 6.12 4.97 3.89
Glutamic acid 1.77 1.53 1.27 1.36 1.31
Serine 10.38 10.85 9.87 9.11 8.89
Glycine 43.45 43.73 27.65 28.41 29.35
Histidine 0.13 0.15 0.78 0.72 0.75
Arginine 1.13 1.16 4.99 4.72 4.12
Threonine 0.92 0.76 0.26 0.21 0.18
Alanine 27.56 28.36 34.12 34.72 36.33
Proline 0.79 0.76 2,21 2.18 2.07
Tyrosine 5.58 5.76 6.82 5.12 5.84
Valine 2.37 2.89 1.72 1.5 1.32
Methionine 0.19 0.11 0.28 0.32 0.34
Cystine 0.13 0.12 0.15 0.12 0.11
Isoleucine 0.75 0.78 0.61 0.51 0.45
Leucine 0.73 0.75 0.78 0.71 0.69
Phenylalanine 0.14 0.18 0.34 0.28 0.23
Tryptophan 0.73 0.75 1.26 2.18 1.68
Lysine 0.23 0.25 0.17 0.24 0.23
(Sen and Babu, 2004) [42, 43]

2. Physical properties of silk fibres are more preferred because they can be dyed or printed
2.1 Colour with any hue of colour.
Mulberry silk fibre is white or yellow or light green. Muga
silk is golden yellow, tropical tasar is copperish yellow or 2.2 Lustre
fawn coloured, oak tasar is yellow grey and eri silk is white, It is a subjective measure of the reflection of incident light
dull white (Krishnaswami et al., 1988). White or colourless from a fibre, filament or textile material. A uniform cross-
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sectional area of the silk fibre coupled with filament form bonded β sheet based on the characterization of B. mori
imparts high lustre to the mulberry, muga and tasar silk yarns fibroin (Marsh, 1955) [32]. Physical parameters such as
due to even reflection of incident light. Eri silk is also lustrous crystallinity, crystallite size, crystallite orientation is
due to its higher translucency with a uniform cross-sectional determined through X-ray diffraction. Crystalline regions in
area. As per birefringence data, the molecular arrangement in the mulberry silk yield 48%, tasar silk about 39%, while muga
mulberry silk is highly oriented whereas three non-mulberry at about 40% and eri about 36%. It is observed from X-Ray
silk fibres have lower orientation with no significant variation diffractograms that mulberry silk varieties have similar
(Gupta et al., 2000) [20]. Birefringence is the difference distinct peaks, with better-ordered regions in comparison with
between the refractive indices of fibre in two directions i.e., the double peaks and distributed intensities as in non-
along fibre axis (n║) and perpendicular to the fibre axis (n┴). mulberry silks (Sen and Babu, 2004) [42, 43].

2.3 Fibre length 2.6.1 Crystallinity and Crystallite size

Silk filaments are extremely long continuous filaments. Crystallinity is the indication of crystalline region in polymer
Longer fibres are easier to process and more even yarns with with respect to polymer content. Silk is a semi-crystalline
high strength can be produced from them. material. Earlier studies by X-ray diffraction analysis
indicated 62%-65% crystallinity in cocoon silk fibroin from
Silk Total Filament Length (m) the silkworm, 50%-63% in wild-type silkworm cocoons, and
Mulberry (Uni) 800-1500 lesser amounts in spider silk (Warwicker, 1956). According to
Mulberry (Bi) 1000-1500 Bhat and Nadiger (1980) [6], non-mulberry silk varieties show
Mulberry (Multi) 300-700 large crystallite size than mulberry silks attributed to higher
Multi-bi hybrids 700-900 alanine content (Ala-Ala links) in crystalline region. Three
New hybrids 1000-1200 non-mulberry silks show large crystallite size and low
Tasar 650-1300 crystallinity than mulberry silks which indicates high
Oak Tasar 650-750
elongation. The presence of amino acids with bulky side
Muga 300-500
groups offers lower crystallinity in non-mulberry silks with
(Srivastav and Thangavelu, 2005) [49, 50]
increase in crystallinity and crystallite size from outer layers
to inner layers (Sen and Babu, 2004) [42, 43].
2.4 Fibre fineness
The size of the bave, raw silk is expressed in terms of denier,
which refers to the weight of a length of 450m in 0.05gm 2.6.2 Moisture regain
Silk is a very hygroscopic textile fibre. Mulberry raw silk has
units or 9000 m weighs one gram. Filament denier is not
uniform throughout the reelable length of filament. It plays an a moisture regain of 11%, which reduces to about 9% after
important role in determining the quality of resultant yarn and degumming (at standard atmospheric condition, 27 ºC and
65% RH) (Lee, 1999) [28]. Tasar silk shows highest moisture
hence of the resultant fabrics. The lower the denier, the finer
the silk and vice-versa. Mulberry silk is the finest, followed regain value followed by eri and muga for outer layers. The
by eri, tasar and muga (Gupta et al., 2000) [20]. Non-breakable higher moisture regain of non-mulberry silk suggests that they
may consist of higher ratio of hydrophilic and hydrophobic
filament length (NBFL) is the length of silk filament that is
present continuously in the cocoon. amino acid residues. Moisture regain of inner layers is less
compared to outer layers because of compactness (Sen and
Table 2: Fibre fineness and non-breakable filament length of silk Babu, 2004) [42, 43]. Silk fibres can absorb upto 30% moisture
fibres from the air without feeling damp and the fibres display 69J/g
of heat of wetting from 0 ºC which indicate that coupled with
Type of silk Fibre fineness(den) NBFL(m) high regain of about 10%, it offers sufficient time for the
Mulberry (Bi) 2-3 700-800 wearer to acclimatize to the change in weather (Gohl and
Mulberry (multi) 2-3 400-600
Vilensky, 1987) [17]. Silk fibres swell about 30% of their
Tasar 8-12 100-250
volume under wet conditions: because of this, silk textile
Oak Tasar 3-5 300-450
Muga 4-7 150-250
materials have a lower dimensional stability compared to
Eri 3-4 0.05-2.0 other natural fibres. Due to this swelling action, silk fibres
(Padaki et al., 2015) [37] display partial loss of strength under wet condition.

2.5 Linear density 3. Mechanical properties

Linear density is the amount of mass per unit length. The 3.1 Tenacity and elongation
mulberry silk has relatively higher degree of order and more The average tenacity of mulberry (bivoltine) and mulberry
compact molecular packaging than non-mulberry varieties crossbreed was 3.75 g/d and 3.85 g/d respectively, while
because of high glycine content and lower long chain/short tenacity of tasar was highest i.e., 4.5 g/d among the three non-
chain ratio. The mulberry varieties do not exhibit pores or mulberry silks (Sen and Babu, 2004) [42, 43]. The tenacity of
voids in their cross-sections whereas tasar, muga and eri silk muga ranged between 3.2 and 4.95 g/d (Tsukada et al., 1994).
show the presence of voids. Among the three non-mulberry Eri silk showed lowest tenacity value, ranging between 2.3
silks, muga silk exhibits the highest density (Gupta et al., and 4.0 g/d (Iijuka and Itoh, 1997) [21]. The variation in
2000) [20]. mulberry is not all that high, making eri the most non-uniform
in terms of tenacity along the fibre. Wild silk possesses better
2.6 X-Ray Diffraction elongation compared to mulberry silk. The mechanical
Silk fibroin have been studied extensively on X- ray properties of silk change substantially along the length and
diffraction of varieties of silk. The crystalline structure of the follow a definite trend.
silk was first described in 1950s as an anti-parallel, hydrogen
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3.2 Elastic-plastic nature and, on release of pressure, will tend to return to its original
Silk is considered to be more plastic than elastic because of its shape. This quality causes the fabric to be wrinkle-resistant.
very crystalline polymer system. Silk fibre may be stretched Silk fibres retain their shape and have moderate resistance to
from 1/7 to 1/5 of its original length before breaking and wrinkling. Fabrics that contain a large percentage of
returns to its original size with losing little elasticity (Singh, weighting or made from short-staple spun silk have less
2004) [47]. Stretching disorganizes the polymer system of silk resilience (Singh, 2004) [47].
which is seen as a distortion and wrinkling or creasing of the
silk textile material (Gohl and Vilensky, 1987) [17]. There is a 3.7 Drapability
slow elastic recovery or creep after extension, but the silk The drapability of a fabric, or its ability to hang and fall into
does not regain its original length (Cook, 1984). graceful shape and folds. Silk has excellent drapability
because of its elastic and resilient properties (Singh, 2004) [47].
3.3 Breaking extension
Elongation is defined as the length to which a fibre may be 4. Dielectric and frictional properties
stretched before breaking. Elongation-at-break showed a Silk fibres are insulators for electrical conduction. Therefore,
higher value for all the non-mulberry silks compared to under the action of friction, static electric charges tend to
mulberry varieties. The values range between 31% and 35% develop in the fibres. The moisture regain dissipates the static
for tasar, 34%-35% for muga and 29%-34% for eri silks, charges effectively; however, under low humidity conditions,
respectively. The elongation values for mulberry varieties static charges pose problems for silk fibre handling. Like most
ranged between 19% and 24% (Babu, 2020) [4]. The relatively textile fibres, silk fibres get positive static charges.
high degree of extension in the case of non-mulberry silks The insulation resistance and dielectric strength of silk fibres
may be attributed to the following: (1) All the non-mulberry give an indication of their dielectric constant, current leakages
silks contain more amino acid residues with bulky side groups at certain voltages, moisture content, and stability under
than the mulberry silk varieties (Iizuka, 1985) [23]. This electric fields. Electrical and dielectric properties have gained
enables molecular chains in non-crystalline regions in the importance with applications such as moisture measurement,
fibre structure to slip easily when stretched and thus show evenness measurement, and the use of silk fibres in the form
higher elongation at break. (2) Unfolding of the long fibroin of fibre reinforced composites as insulating materials for
chains in the amorphous regions is a result of either less special applications (Padaki et al., 2015) [37]. The electrical
orientation or less crystallinity (Iizuka, 1997) [22]. resistance (Rs, Ώ kg/m2) of silk fibres is 9.8(log Rs value),
which is much higher than cotton (about 7), wool (8.4), and
3.4 Initial modulus polyester (8) fibres at 65% RH (Morton and Hearle, 2008) [33].
Another important tensile parameter is the initial modulus. The electrical resistance decreases with rise in humidity and
The initial modulus values for different varieties changes temperature.
along the filament length. The initial modulus values follow a The force that holds together the fibres in a spun yarn and the
similar trend as that of tenacity, showing higher values for the interlacing threads in a fabric is called as frictional force. If
inner layers. The values ranged between 53.9 and 136.8 g/d the friction is too low, the yarn strength will fall and
for mulberry (bivoltine), 83.8 and 129.8 g/d for mulberry dimensional stability of cloth will be reduced. Textile fibres
(crossbreed), 61.3 and 100.7 g/d for muga, 61.4 and 107 g/d have coefficient of friction (µ) values ranging from 0.1 to 0.8,
for tasar, and 71.1 and 107 g/d for eri silk. This definitely the lower value indicating higher frictional resistance (Morton
indicates an increase in orientation in both the crystalline and and Hearle., 2008) [33]. Typical µ values for silk fibre to fibre
the amorphous regions, as one moves from the outer to the friction is 0.26 for crossed fibres and 0.53 for parallel fibres.
inner layers (Sen and Babu, 2004) [42, 43]. The higher value is because of the smooth fibrous surface
coupled with crystalline regions in the fibre while this fibrillar
3.5 Stress-strain curve nature makes the silk fibres a poor resistant to abrasive wear
Silk is a visco-elastic material and exhibits the phenomenon (Padaki et al., 2015) [37].
of stress-strain. Creep and stress behaviour of silk has been
reported by Das (1996) [15]. The extension, secondary creep is 5. Thermal properties
higher for tasar silk compared to those for mulberry silk and Silk fibre is thermally stable below 100 ˚C. High degree of
stress relaxation was also found to be more in non-mulberry molecular orientation of silk fibroin helps the thermal stability
silks. Non mulberry silk exhibits a high initial resistance of the silk fibre. Yellowing begins to occur in silk fibres at
followed by substantial yielding. The reasons are (1) fewer 110 ˚C after 15 minutes of exposure (Lee, 1999) [28]. From the
crystalline regions and (2) imperfect crystallites or peaks of Differential Scanning Calorimetry (DSC) curves of
entanglements, which initially offer resistance but later give silk, Nakamura et al. (1994) predicted that the glass transition
way and allow easy deformation in the amorphous regions temperature of silk is about 175 ˚C and silk fibre degradation
until strain hardening occur (Sen and Babu, 2004) [42, 43]. begins at 280 ˚C with an initial weight loss starting at about
250 ˚C. When silk fibre is subjected to heat, no significant
3.5.1 Cohesion and abrasion changes occurs in the crystalline structure but the amorphous
Cohesion implies the force with which the composite region becomes highly oriented (Tsukada., 1992) [55].
filaments cohere to gather inside a strand. The measurement is Silk is a good insulator of heat among the textile fibres; the
done by degree of agglutination of filaments and sericin plays specific heat of dry silk fibre is 1.38j/gK (Morton and Hearle,
the role in it (Manna et al., 1989) [31]. Abrasion is the rubbing 2008) [33]. The thermal conductivity of mulberry silk fibre in
or scrapping off the surface of a thread. Fibre crimp increases longitudinal and transverse direction is 1.49 and 0.119
the cohesiveness, resilience and resistance to abrasion. W/(mK), which indicates high orientation of fibroin
molecules along the direction of fibre. Due to lower thermal
3.6 Resiliency
conductivity and high moisture regain of silk fibres, the
Resilience means that the fibre can be compressed or crushed
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comfort level of wearing silken items is decreased in hot and treatment with borax. If raw silk is steeped in lime water, the
humid areas. The heat of setting of twisted silk yarns is also fibre swells and sericin becomes soft. Continued treatment
carried out by utilizing the thermal behaviour of silk fibres, with lime water cause brittleness in silk (Srivastav and
Steam heating of the twisted silk yarn increases the moisture Thangavelu, 2005) [49, 50].
level and temperature in the silk fibres (Padaki et al., 2015)
[37].
6.3 Action of metallic salts
Silk has great affinity for metallic salts. This characteristic is
6. Chemical properties employed for the process of silk weighting, to improve
Silk is a protein fibre; it is composed of different amino acids. draping properties. Stannic chloride is used for weighting
The properties of proteins depend primarily on the properties unless the material is to be dyed black. Black silk is often
of the reactive groups of their constituent amino acids dyed and weighted by using log-wood and iron salt. A
combined with the properties associated with the size of the moderately weighted silk contains 25-50% of salt (Cook,
protein. Amino acids are bifunctional, i.e., they contain both 1984).
acidic carboxyl (coo-) and amino group (NH3+) thereby
acting as Zwitterion. 6.4 Effect of organic solvents
6.4.1 Effect of different organic solvents on mulberry silk:
6.1 Action of water While treatment of mulberry silk with ethanol, ethyl acetate,
Water does not permanently affect silk fibre. It decreases methanol there is change in the secondary structures of silk
about 20% in strength when wet, but regains the original fibroin from random coil to high strength β sheet structure and
strength upon drying (Lee, 1999) [28]. The amorphous regions thermal stability also increases (Prasong et al, 2010) [35].
of silk are reported to be more accessible to deterioration by Methanol is the best solvent to induce strength and thermal
water (Crighton, 1993) [14]. When steeped in warm water, the properties (Nuanchai et al., 2010) [35].
fibre swells but does not dissolve. The water use for reeling
should be clean to prevent the dissolved substances present in 6.4.2 Treatment of muga silk with organic acids: Modifies
it. the fibre chemically and increases the tensile strength.
Treatment with Methanol and phenol shows higher tensile
6.1.1 Action of acids strength (Talukdar et al., 2011) [53].
Concentrated hydrochloric acid dissolves the silk in one or
two minutes. Non-mulberry silks are less affected by the 6.5 Action of dyes
action of acids than mulberry silk (Krishnaswami et al., Dye-stuffs are absorbed by silk more readily and at lower
1988). When silk is treated with dilute hydrochloric acid, it temperature than any other normal fibre (Chowdhury, 1992).
dissolves in 1-2 minutes without losing its strength. If silk is Silk was dyed with vegetable dyes extracted from stems,
treated with strong sulphuric acid for few minutes, then rinsed roots, stalks, foliage, barks etc of different plants in older
and neutralized, contraction of fibres occurs from 30-50% in times (Das, 1992 and Chowdhury, 1992) [16]. The selection of
length and losses its lustre. This property is utilized to impart dye is very important as dyeing increases the value of silk
creep effects (Srivastav and Thangavelu, 2005) [49, 50]. Action (Chowdhury, 1982) [10]. Acid dyes produce brilliant shades
of nitric acid produces a bright yellow colour on silk which with good light fastness but poor to moderate wash fastness
can be removed by treatment with stannous chloride. Formic which can be improved by treatment with cationic dye fixing
acid and acetic acid have no injurious effect on s silk. agent at 40-50 ˚C for 20 min (Chakraborty, 2010). Dyeing of
Exposure to weak acids results in the ‘scroop effect’, which is silk fabric with acid dyes improves good colour fastness and
a famous finishing treatment that produces a crackling noise mechanical properties like count, cloth thickness and cloth
when such fibres are rubbed together (Sonwalkar, 1993) [48]. weight (Mahale and Naikwadi, 2019) [30]. Reactive dyes have
Shell weight loss and silk filament recovery is influenced on gained importance on account of brilliant colours, fastness
cooking the tasar cocoons with tartaric acid. Mechanical properties and comparatively low cost. Treatment of muga
properties do not change with the process parameters silk fabric with acid and reactive dye results in decreased
(Gulrajani, 1988). Muga silk could be chemically modified fabric weight, tensile strength, bending length but increased
and tensile properties can be changed. Methanol and phenol crease recovery of muga silk fabric (Arora, 2016) [3].
treated fibre show higher tensile strength whereas
Formaldehyde degrades the molecular weight and hydrogen 6.5.1 Dyeing of silk with natural dyes
bonding (Talukdar et al., 2011) [53]. Mulberry silk yarn dyed with mixture of bark of monkey jack
and henna leaves produce different shades of brown colour,
6.2 Action of alkali beige as well as green colour with satisfactory colour fastness
Silk fibres have low resistance to alkali and are easily properties and more denier (Boruah, 2016) [7]. The dye
damaged by exposure to weak alkali at elevated temperatures. extracted from fruits of Ficus racemose have good affinity
Alkali conditions hydrolyze the polypeptide bonds of fibroin towards silk even without the use of mordants (Sudhakar and
from its molecular chain ends, thus degrading silk fibre Ninge, 2011) [52]. Camellia sinensis (L.) kuntze (tea leaves),
rapidly. Alkali solutions cause the silk to swell, due to the Allium cepa L. (onion peel), Laccifer lacca Kerr (lac insect)
partial separation of the silk polymers by the molecules of and iron ore were used as natural dyes in presence of different
alkali. Salt linkages, hydrogen bonds, vander walls force are mordanting agents for dyeing of eri silk yarns. It improved the
all hydrolysed by alkali so dissolution occurs readily (Gohl colour fastness to washing and light considerably which will
and Vilensky, 2005). Silk fibre dissolves on treatment with motivate the weavers to use natural dyes with eco-friendly
strong hot alkalies such as caustic soda or potash. Ammonia character (Banerjee et al., 2017).
and alkaline soaps dissolve only the sericin layer of silk but
have no effect on fibroin. Sericin or fibroin has no effect on

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6.6 Action with bleaching agents 11. References

Degumming followed by sequential oxidative and reductive 1. Altman GH, Diaz F, Jakuba C, Calabro T, Horan RI,
bleaching on mulberry and tasar silk fabrics decreases fabric Chen J, et al. Silk based biomaterials. Biomaterials.
weight, thickness, tenacity, crease recovery, bending length 2003;24:401-416.
whereas air permeability increases (Sharma et al., 1999) [44]. 2. Anonymous. Garment and fabric terminology: A Primer.
Hydrogen peroxide, performic acid, peracetic acid and Retrieved on 28th January, 2022. From
potassium permanganate are some of the oxidising agents use https://blog.unitexdirect.com/garment-and-fabric-
for bleaching of silk. The reducing agents used are sulphur terminology-a-primer/.2009.
dioxide and sodium hydrosulphite. Treatment of natural 3. Arora S. Effect of printing on physical properties of
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