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The Journal of The Textile Institute
ISSN: 0040-5000 (Print) 1754-2340 (Online) Journal homepage: http://www.tandfonline.com/loi/tjti20
Comparative study of degumming of silk varieties
by different techniques
Shweta K. Vyas & Sanjeev R. Shukla
To cite this article: Shweta K. Vyas & Sanjeev R. Shukla (2015): Comparative study of
degumming of silk varieties by different techniques, The Journal of The Textile Institute, DOI:
10.1080/00405000.2015.1020670
To link to this article: http://dx.doi.org/10.1080/00405000.2015.1020670
Published online: 16 Mar 2015.
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Comparative study of degumming of silk varieties by different techniques
Shweta K. Vyas and Sanjeev R. Shukla*
Department of Fibres & Textile Processing Technology, Institute of Chemical Technology (University under Section 3 of UGC Act
1956), Mumbai, India
(Received 15 September 2014; accepted 26 January 2015)
Different varieties of silk differ from each other in their chemical composition as well as the physical, chemical and
mechanical properties. Silk degumming is a primary important process, which needs to be carried out uniformly to a
sufficient level to get good quality in further processes such as dyeing. In the present study, we have compared the
efficiencies of five different degumming treatments for eri, tasar and mulberry silk fabrics using conventional, ultrasonic
and microwave techniques. Among the various treatments, soda ash plus hydrogen peroxide and enzyme (papain)
degumming recipies gave the best results in terms of weight loss and absorbency. As compared to the conventional
technique, the treatments under ultrasonication and microwave enhanced the efficiency. The microwave technique was
found to be superior, with the advantage of drastic reduction in treatment time, better retention of tensile strength and
smooth appearance of silk. The dye uptake also increased after degumming. The molecular conformation of the different
silk varieties, as estimated by FTIR spectroscopy, and the crystalline structure, as elucidated by X-ray diffraction,
remained unchanged. SEM showed distinct morphological changes on degumming.
Keywords: silk varieties; degumming processes; ultrasonication; microwave; FTIR; XRD; SEM
Introduction
Silk is a unique natural protein filament fibre produced
by sericigenous insects. It is one of the world’s most
exclusive and luxurious fibres. Silk is categorized into
mulberry or domesticated silk and non-mulberry or wild
silk like muga, eri and tasar. Eri silk, also known as
‘Ahimsa Silk’in India and produced by the worm
Philosamia ricini, is a secretion of a lepidopteran insect
Samia ricini and it is available in the north-east region
of India, particularly Assam. Tasar silk is produced by
the worm Antheraea mylitta, whereas mulberry silk is
produced by the worm Bombyx Mori and it is available
in Andhra Pradesh, Karnataka, Tamil Nadu and West
Bengal (Kariyappa, 2008). Although labelled as ‘silk’,
their characteristic properties differ widely, and hence a
comparison of their processing is necessary for obtaining
desired optimum performance.
The silk fibre is composed of two cores of fibroin
surrounded by a layer of sericin in a structure known as
a bave (each individual fibroin core is known as a brin)
(Pérez-Rigueiro, Viney, Lorca, & Elices, 2000). Fibroin
is the structural protein of silk fibre, whereas sericin is
the water-soluble proteinaceous glue that serves to bond
the fibres together. The majority of fibroin’s composition
contains amino acids with bulkier side chains. The
sericin content of mulberry is 20–30%, tasar of 7–15%
and that of eri silk is as low as 7–12%, since most of it
gets removed during the cooking of cocoons (Srisuwan,
Narkkong, & Srihanam, 2009). The composition of the
fibroin is glycine, alanine and serine; and that of sericin
is serine, aspartic acid and glycine, with threonine
present in significant amounts.
Degumming and bleaching are the important
treatments of silk processing. The presence of gum
sericin makes the silk harsh and stiff, and masks its
natural lustre. Degumming is a process of cleavage of
peptide bonds of sericin by hydrolysis or enzyme action
(Chopra, Chattopadhyay, & Gulrajani, 1996). The
recommended standard method of degumming is based
on Marseilles soap, which is prepared from olive oil.
However, degumming of silk has been carried out
conventionally by soap–soda ash method and it is
considered the best, although it has the drawback of
some fibre degradation leading to dull appearance and
loss in tensile strength. Furthermore, studies have been
reported on the removal of sericin by different types of
enzymes (Freddi, Mossotti, & Innocenti, 2003; Gulrajani,
Gupta, Gupta, & Suri, 1996). Organic acids have been
attempted as alternative agents due to their milder action
to overcome the problems present in traditional
degumming process. Degumming of silk fabric with
tartaric acid showed excellent performance, in terms of
silk sericin-removal efficiency (Freddi, Allara, &
Candiani, 1996; Khan et al., 2010).
The work reported here aims to compare the
efficiencies of degumming, bleaching and dyeing
*Corresponding author. Email: srshukla19@gmail.com
© 2015 The Textile Institute
The Journal of The Textile Institute, 2015
http://dx.doi.org/10.1080/00405000.2015.1020670
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processes for eri, mulberry and tasar silk using
conventional, ultrasonic and microwave techniques. The
characterizations done for differentiating the effect of
treatment on physical and structural properties of eri,
mulberry and tasar silk were FTIR, XRD and scanning
electron microscope (SEM).
Experimental
Materials
The plain woven tasar and eri silk fabric varieties were
procured from Jharcraft Silk Textiles and Handicraft
Development Corp. Ltd., Mumbai. Tasar silk fabric with
80 × 68 yarns/cm single yarn in warp and weft (GSM of
40 g/m
2
) and eri silk fabric with 86 × 52 yarns/cm,
twisted ply yarn in warp and weft (GSM of 86 g/m
2
).
Plain woven mulberry silk fabric was procured from
Central Silk Board, Bangalore, with 100 × 88 yarns/cm,
single yarn in warp and weft (GSM of 40 g/m
2
). All the
chemicals were of Laboratory Reagent grade. The
enzyme, namely Papain, was supplied by Rossari
Biotech Pvt. Ltd., India. The dye C.I. Acid Blue 40 was
obtained from DyStar Pvt. Ltd., Mumbai.
Methods
Silk fabric was cut into 15 × 15 cm size sample and
conditioned at 65% RH and 27 °C for 24 h. Degumming
was carried out at a liquor ratio 40 using conventional,
ultrasound and microwave techniques. The conventional
degumming was carried out at 90 °C for 2 h using
different recipes, except for enzymatic degumming,
which was carried out at 60 °C for 2 h (Table 1).
Degumming under ultrasonication (Ultrasonic bath with
temperature control; Dakshin Ultrasonicators, Mumbai;
Model -6L, frequency 40 kHz and output 220 W) was
carried out at 60 °C for 30 min, whereas under
microwave (Domestic microwave, model CE305CF,
Samsung, India), it was carried out for 5 min with the
following set-up: power consumption 850 W and
operating frequency 2450 MHz. at 50% power. All the
degummed samples were washed thoroughly, dried,
conditioned and weighed prior to testing.
Dyeing of silk fabrics
An acid dye C.I. Acid Blue 40 was dissolved in water.
The dye concentration was 1% owf and the material-to-
liquor ratio was 1:40. Acetic acid was used to adjust the
pH of dye bath to about 3.5. The temperature for dyeing
was raised from 30 to 80 °C and maintained at 80 °C for
45 min. After dyeing, the samples were taken out from
the dye bath, washed thoroughly, dried, conditioned and
weighed prior to testing.
Measurements
Weight loss
Degumming ratio, which corresponds to the amount of
sericin removed, was calculated from the weight loss of
silk before and after the degumming treatment.
Weight lossð%Þ¼W1W2
W1
100;(1)
where W
1
and W
2
are the weights of the sample before
and after degumming in grams, respectively.
Wettability
The water drop absorbency of the degummed samples
was determined using AATCC Test Method 39–1980
(evaluation of wettability). The time lapsed (in sec)
between the contact of the water drop with the fabric
and the disappearance of the water drop into the fabric
was counted as the wetting time. The time of drop
disappearance was averaged out from measurements at
different points of the fabric sample.
Tensile strength
The tensile strength and elongation were measured on
Universal Tensile Tester (Tinius Olsen, H5K-S UTM,
USA) using ASTM D 5035 (Strip method) at a gauge
length of 5 cm and a strain rate of 1 cm/min.
Whiteness index
The Premier Colorscan, SpectraScan 1500, India, was
used for measuring Hunter’s Whiteness index.
Dye uptake
Dye uptake was estimated in terms of exhaustion of dye
bath using a Perkin–Elmer UV–Vis spectrometer as
absorbance at the maximum wavelength (λ
max
) 286 nm
for the dye.
Table 1. Various degumming treatment agents and their
coding.
S.
No. Treatment agent
Treatment
code
1 Nil A
2 Hot water B
3 5 g/l Soda ash (pH 10.5) C
4 1 g/l Nonionic detergent (pH 7.5) D
5 1 g/l Soda ash + 2 g/l hydrogen peroxide
(50%) (pH 9.6)
E
6 5 g/l Enzyme (papain) (pH 6.8) F
2S.K. Vyas and S.R. Shukla
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Dye uptakeð%Þ¼A0At
A0
100;(2)
where A
0
and A
t
are the absorbance values before and
after the dyeing, respectively.
Infrared spectra
The silk samples were evaluated with FTIR spectroscopy
(Shimadzu FTIR-8400S, Japan) by the ATR method in
the region of 4000–400 cm
−1
.
XRD
Wide-angle X-ray diffraction (WAXD) profile was
obtained by PAN-alytical, X’Pert HighScore, the
Netherlands, diffractometer. Diffraction curves were
recorded using Cu–(Kα) radiation (λ= 1.54 Å) generated
at 40 kV and 150 mA at a scanning rate of 1.5°/min.
Silk fabrics were cut into powder form and pressed into
a die press in the form of a pellet. The pellet was put in
the X-ray diffractometer and X-ray spectra were
recorded at a scan step size of 0.1°.
Scanning electron microscope
The morphology of treated silk samples was examined
with Philips XL 30 SEM at 12 kV of acceleration
voltage. Before placing the samples in the SEM
chamber, the samples were mounted onto an aluminium
stud and sputter-coated with gold/palladium for 180 s
(E-1010 ION SPUTTER) to prevent charging.
Results and discussion
Degumming
Degumming is an important step for silk to achieve
good and uniform absorbency. Various processes are
Table 2. Effect of various degumming treatments on physical properties of eri silk with different techniques.
Treatment
Weight loss (%) Absorbency (Sec) Tensile strength (KgF) Elongation (%)
CUM C UM C U M C U M
A–––340 ––19.90 ––46 ––
B 4.00 4.24 4.11 9.00 7.00 4.00 16.50 17.87 18.98 42.33 45.30 45.68
C 5.90 5.95 5.85 2.33 1.75 1.33 12.60 16.03 14.67 44.24 48.60 43.60
D 3.58 3.14 4.04 3.44 1.33 2.66 15.85 16.42 17.36 43.27 45.80 44.35
E 4.90 5.94 7.29 1.43 1.40 0.24 14.87 16.67 15.02 43.38 44.58 44.00
F 3.73 4.48 6.64 2.34 2.67 1.00 19.81 18.09 15.58 44.43 44.00 42.15
Table 3. Effect of various degumming treatments on physical properties of tasar silk with different techniques.
Treatment
Weight loss (%) Absorbency (Sec) Tensile strength (KgF) Elongation (%)
CUM C UM C U M C U M
A–– –450 ––12.47 ––42.67 ––
B 2.12 5.38 6.66 8.55 9.00 8.00 10.45 10.91 11.45 34.40 39.20 39.20
C 5.60 6.97 7.60 1.22 5.00 1.00 8.13 8.64 8.16 28.56 40.96 42.60
D 2.73 5.82 7.44 2.00 5.00 1.00 7.68 10.57 10.50 33.28 34.60 41.60
E 3.43 7.24 11.82 1.00 2.00 1.00 6.02 10.24 11.80 34.40 39.60 37.60
F 2.77 6.19 8.24 1.15 3.50 1.00 7.59 11.92 8.14 34.40 35.75 38.56
Table 4. Effect of various degumming treatments on physical properties of mulberry silk with different techniques.
Treatment
Weight loss (%) Absorbency (Sec) Tensile strength (KgF) Elongation (%)
CUMC UMCUMCUM
A–––430 ––22.16 ––43.84 ––
B 3.12 4.00 2.89 18.00 18.00 20.00 14.62 15.23 16.56 33.25 34.78 42.56
C 5.60 6.00 5.40 4.45 3.56 4.89 12.22 13.25 15.97 36.39 36.15 41.36
D 2.73 4.23 5.26 7.65 4.78 2.00 12.27 14.11 16.97 33.26 33.00 40.36
E 3.43 5.89 7.89 3.00 3.21 4.00 12.75 13.87 16.66 37.51 38.56 43.04
F 2.77 3.78 5.19 9.00 7.65 9.00 14.58 15.23 14.52 41.95 42.12 39.20
The Journal of The Textile Institute 3
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known to provide different levels of degumming.
However, a comparative evaluation, in particular with eri
silk, is missing. Eri silk is different than other non-
mulberry and mulberry silks, since a major part of
degumming occurs at the cocoon stage itself, leaving
little sericin left to be removed later.
Tables 2–4give the results of different treatments in
terms of weight loss, absorbency, tensile strength and
elongation for the three silk varieties degummed with
conventional (C), ultrasonic (U) and microwave (M)
techniques (Figure 1).
It may be observed that for all three silk varieties,
soda ash plus hydrogen peroxide gave maximum weight
loss due to removal of sericin (treatment E). The
advantages of the process are saving of time and heat
energy as compared with other degumming treatments
with individual agents. Further, advantage was accrued
in term of improved whiteness (Table 5). Also, under
ultrasonication as well as microwave techniques,
substantially large amount of sericin was removed as
compared to conventional technique with treatment C, D,
E and F being higher under microwave and that too in
only 5 min. The trend was similar for all the three silk
varieties eri, tasar and mulberry, although weight loss
levels varied due to different sericin contents of each.
Degumming caused reduction in the time taken for
absorption of a water drop in all the techniques
employed and was dependent on weight loss, although
no linear relation could be established.
Tables 2–4also show that tensile strength by
conventional degumming decreased from 19.90 to 15.85
KgF for eri, from 12.47 to 7.68 KgF for tasar and from
22.16 to 12.27 KgF for mulberry silk due to removal of
sericin which provides strength to the silk filament. Loss
of strength was however, found to be less in ultrasound
and microwave treatments. This may be attributed to the
fact that sonication does not distort hydrogen bond order
as well as β-chain structure in the polymer chain of the
silk. (Mahmoodi, Firoozmehr, & Rahimi, 2010a;
Mahmoodi, Firoozmehr, Moghimi, & Rahimi, 2010b).
The elongation at break was found to decrease with
an increase in the weight loss in all the degumming
techniques and agents followed. This may be attributed
to the removal of sericin, however little, acting as a
Figure 1. Effect of treatment E on physical properties of different silk varieties by various techniques: (a) weight loss,
(b) absorbency, (c) tensile strength and (d) elongation.
4S.K. Vyas and S.R. Shukla
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