Physical properties and dyeability of silk fibers degummed with citric acid
Md. Majibur Rahman Khan
a
, Masuhiro Tsukada
b,*
, Yasuo Gotoh
b
, Hideaki Morikawa
b
, Giuliano Freddi
c
,
Hideki Shiozaki
d
a
Department of Biosystems Engineering, University of Manitoba, Winnipeg, Manitoba, Canada R3T 5V6
b
Faculty of Textile Science and Technology, Shinshu University, Tokida 3-15-1, Ueda, Nagano 386-856, Japan
c
Stazione Sperimentale per la Seta, Via G. Colomob 83, 20133 Milano, Italy
d
Research Institute for Industrial Art of Kanagawa, Odawara City, Kanagawa 250-0055, Japan
article info
Article history:
Received 11 August 2009
Received in revised form 29 April 2010
Accepted 31 May 2010
Available online 2 July 2010
Keywords:
Biopolymer silk
Citric acid
Degumming ratio
Structure and properties
Dyeability
abstract
Silk fibers from Bombyx mori silkworm was degummed with different concentration of citric acid, and the
physical properties and fine structure were investigated to elucidate the effects of citric acid treatment.
The silk sericin removal percentage was almost 100% after degumming with 30% citric acid which
resulted in a total weight loss of 25.4% in the silk fibers. The surface morphology of silk fiber degummed
with citric acid was very smooth and fine, showed perfect degumming like traditional soap-alkali
method. The tensile strength of silk fiber was increased after degumming with citric acid (507 MPa),
where as the traditional soap-alkali method causes to decrease the strength about half of the control silk
fiber (250 MPa). The molecular conformation estimated by Fourier transform infrared spectroscopy and
the crystalline structure evaluated from X-ray diffraction curve stayed unchanged regardless of the
degumming with citric acid and soap. The dye uptake percentage of silk fiber degummed with citric acid
decreased slightly, about 4.2%. On the other hand, the dye uptake percentage of silk degummed with soap
was higher which indicates the disordering of the molecular orientation of the laterally ordered structure,
accompanied with the partial hydrolysis of silk fibroin molecules by the alkali action of soap. The thermal
properties were greatly enhanced by soap and citric acid degumming agents. Dynamic mechanical ther-
mal analysis showed silk degummed with citric acid is more stable in higher temperature than that of
soap. With heating at above 300 °C, the silk degummed with citric acid shows an increase in storage mod-
ulus and an onset of tan dpeaks at 325 °C and the melt flow of the sample was inhibited. The degumming
of silk fibers with citric acid is safe and the results obtained are quite promising as a basis for possible
future industrial application.
Ó2010 Elsevier Ltd. All rights reserved.
1. Introduction
Silk fiber is one of the most familiar, as well as being a very use-
ful biopolymer and is universally acclaimed for most of the desir-
able properties of textile fiber: fineness, strength, elasticity,
dyeability, softness, flexibility, smooth feeling, luster, elegance,
grace and high rating (Trotman, 1975; Shinohara, 2000). It is the
only commercially available natural fiber in continuous filament
form, produced by the larva of some insects, especially silkworms.
Several species of silk spinning insects exist in nature. However,
silk thread spun by the larvae of silkworm, Bombyx mori (B. mori)
is of practical importance as a source of textile grade fibers.
The silkworm cocoon silk fiber 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) (Pe’rez-
Rigueiro et al., 2000). Fibroin is the structural protein of silk fiber,
whereas sericin is the water soluble proteinaceous glue that serves
to bond the fibers together. The majority of fibroin’s composition is
highly periodic, with simple repeating sections broken by more
complex regions containing amino acids with bulkier side chains.
The highly repetitive sections are composed of glycine (45%), ala-
nine (30%), and serine (12%) in a roughly 3:2:1 ratio and dominated
by [Gly-Ala-Gly-Ala-Gly-Ser]
n
sequences. Fibroin is known to form
mainly three kinds of conformations: silk I with a helical confor-
mation, silk II with an antiparallel b-sheet, and a random coil with-
out definite orders. Sericin contains glycine, serine, and aspartic
acid totaling over 60% of the overall composition (Zhang et al.,
2002; Kaplan et al., 1997; Lotz and Colonna Cesari, 1979).
Silk processing from cocoons to the finished clothing materials
consists of a series of steps which include: reeling, weaving,
degumming, dyeing or printing, and finishing (Zahn, 1993). Silk
sericins play important role in the silk reeling, finishing, and
weaving process. When silk fiber is finally used as textile products,
0960-8524/$ - see front matter Ó2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.biortech.2010.05.100
*Corresponding author. Tel.: +81 268 21 5355; fax: +81 268 21 5388.
E-mail addresses: tsukada@shinshu-u.ac.jp,majib@hotmail.com (M. Tsukada).
Bioresource Technology 101 (2010) 8439–8445
Contents lists available at ScienceDirect
Bioresource Technology
journal homepage: www.elsevier.com/locate/biortech
sericin is almost removed from raw silk fibers by alkaline treat-
ment. Degumming is the procedure to remove silk sericin using al-
kali solution and/or enzyme solution. After removing sericin, the
luster of pearls appears on the surface of silk fibroin fibers and
good brightness and excellent elasticity to the silk fibers. Addition-
ally the hand of silk fabric is primarily depends on the quantity of
sericin remained on the silk fibroin. Properties of silk fiber, such as
handling, luster, and rubbing behavior, is thus greatly influenced
by the residual sericin. The degumming process is thus very
important.
Traditionally, degumming of silk has been carried out by soap–
soda ash method and it is considered as the best sericin removal
method of silk (Kato, 1968). Recently, soap is replaced by synthetic
detergents in continuous degumming systems, because it cannot
compensate the acidity of sericin hydrolysis products accumulat-
ing in the bath, thus limiting the use of the degumming bath for
weekly degumming cycles (Svilokos Bianchi and Colonna, 1992).
In recent years, various studies have been dealt with the removal
of sericin by using different types of enzymes as degumming
agents (Freddi et al., 2003; Gulrajani et al., 1996). Enzyme de-
gummed silk fabric displayed a higher degree of surface whiteness,
but higher shear and bending rigidity, lower fullness, and softness
of handle than soap and alkali degummed fabric, owing to residual
sericin remaining at the cross over points between warp and weft
yarns (Chopra et al., 1996). It has been thus interesting from basis
of industrial aspect to note the importance of recovering sericin
from the degumming baths should not be overlooked, with notice-
able advantages from the environmental point of view, due to the
reduction of the pollution load. Organic acids would be alternative
agents to overcome the problems present in traditional soap/en-
zyme degumming process.
Many researches have been performed on degumming and fin-
ishing of silk fiber using acid agents for enhancing the physical
properties of silk (Freddi et al., 1996; Gulrajani and Chatterjee,
1992; Gulrajani et al., 1992). It has been pointed out that the action
of organic acids is generally milder and less aggressive than the ac-
tion of alkali. Freddi et al. (1996) studied on the degumming of silk
fabrics with tartaric acid and showed the excellent performances of
tartaric acid, both in terms of silk sericin removal efficiency and of
intrinsic physico-mechanical characteristics of silk fibers. The de-
gummed silk fabric with tartaric acid exhibited a good luster and
a ‘scroopier’ handle in compared with soap degummed fabric. They
also demonstrated that dyeability with acid dyes and comfort
properties (such as wicking, wettability, water retention and per-
meability) are also enhanced and concluded that the acid degum-
ming process shows potential for possible industrial application.
Citric acid is the main organic acid produced today by fermen-
tation. Citric acid product is mainly used in the food, beverage,
pharmaceutical and cosmetic industry owing to its general recog-
nition as safe having pleasant taste, high water solubility and che-
lating and buffering properties (Berovic and Legisa, 2007).
Recently, citric acid has been used in textile finishing treatment
as a chemical fixing agent to increase textile properties, such as
anti-odor, low-shrinkage capacities and wrinkle resistance, as well
as antimicrobial properties (Yang, 1993a,b). Researches on the fin-
ishing treatment on silk using citric acid have also been performed
(Yang and Li, 1993). Both the dry and wet resiliency of finished silk
was remarkably increased with citric acid treatment. However, un-
til recently, there has been no investigation on structure and phys-
ical properties and as well as, effects of citric acid on silk
degumming process.
The aim of this study was to investigate the effects of citric acid
treatment on physical properties of silk fibers. Here we reported
sericin removal efficiency, fiber morphology, dyeability, thermal
and mechanical properties of cocoon silk degummed with citric
acid and compared with those of soap degummed silk fibers.
2. Methods
2.1. Materials
Raw silk fibers of 27 denier were obtained after reeling of
cocoon threads of a commercial silkworm variety of B. mori.To
remove sericin, raw silk fiber was degummed with different
degumming chemical agents, i.e. citric acid, soap.
2.2. Degumming with citric acid
Silk fibers were degummed in the degumming bath containing
citric acid (Wako Pure Chemicals Ltd.) solutions at the concentra-
tion of 15% and 30%, respectively, and Noigen HC nonionic deter-
gent (0.2%) at 98 °C for 30 min. The material-to-liquor ratio was
1:20.
2.3. Traditional degumming using soap–soda ash method
Silk fiber was degummed at 98 °C for 30 min in the degumming
solution containing 15% marseilles soap (Mira Lanza Inc.), 1.5% so-
dium carbonate, 0.05% Noigen HC nonionic detergent and 0.05%
EDTA, metal block masking agent. Material-to-liquor ratio 1:30
was kept constant. After degumming, sample was treated at
98 °C for 5 min with 0.2% sodium carbonate solution to remove
soap which remains on the surface of the sample. Then, degummed
samples were washed with cold and warm water and finally dried
immediately at 80 °C for 1 h, and then kept at room temperature
for 48 h.
For different measurements and analysis, raw silk fiber was re-
ferred as untreated control sample and ‘Blank’ sample is the silk fi-
ber treated in water containing without soap and citric acid at
98 °C for 30 min.
2.4. Determination of degumming ratio
Degumming ratio, D
r
which correspond to the amount of sericin
removed by different degumming treatment, was calculated from
the weight loss of silk fiber before and after degumming treatment
using the following equation:
D
r
¼W
b
W
a
W
a
100
where W
a
and W
b
are the weights of the dried fiber before and after
degumming, respectively.
2.5. Dyeing of silk fibers
Dyeing tests were carried out with acid dyeing stuffs of Acid
Red 6 (Suminol Fast Red B conc) and Acid Blue (Kayanol Milling
Blue BW). The general idea of dyeing experiments was shown
in Table 1. These dyeing stuffs are purchased from Sumitomo
Chemicals Co., Ltd. The dye concentration was 1% owf (on the ba-
sis of fiber weight) and the material-to-liquor ratio was 1:50. For
Acid Red, 2% owf acetic acid was used to adjusting pH at 3.78 of
dyeing bath. For Acid Blue 138 dyeing, 1% owf dyeing stuff, 5%
owf sodium sulfate contained in the dyeing bath. Initial pH of
dyeing bath was adjusted to 5.89. The temperature for Acid Red
6 dyeing was increased from 25 to 60 °C over 30 min and main-
tained at 60 °C for 40 min, while that temperature was increased
from 25 to 80 °C over 60 min and maintained at 80 °C for 60 min
for Acid Blue dyeing. After dyeing the samples were taken out of
the dyeing bath, after the temperature was allowed to slowly de-
crease until 50 °C. After all dye tests, the fibers were thoroughly
rinsed with water.
8440 M.M.R. Khan et al. / Bioresource Technology 101 (2010) 8439–8445
Dye uptake was measured by spectrophotometer using a
Perkin–Elmer spectrometer and assessed in terms of standard K/S
values. Dyed fibers were evaluated for hue and depth of shade,
using L,a,breadings generated by color difference meter. The
Hunter L,a,bsystem is a uniform color scale based on the oppo-
nent-colors theory of vision (Hunter and Harold, 1987). According
to Hunter color scale theory, positive ‘a’ is red, negative ‘a’ is green,
positive ‘b’ is yellow and negative ‘b’ is blue.
2.6. Measurements
Wide-angle X-ray diffraction (WAXD) profile was obtained by a
Rigaku Rotorflex RU-200B diffractometer using Ni-filtered Cu K
a
radiation generated at 40 kV and 150 mA.
Fourier transform infrared (FT-IR) spectroscopy was measured
with a Shimadzu FT-IR-8400S infrared spectrometer by the ATR
method in the region of 4000–400 cm
1
at room temperature.
Dynamic mechanical thermal analysis (DMTA) was measured
from room temperature to 350 °C in air by using an ITK Co. DVA-
225 at the stretching mode of 10 Hz and a heating rate of
10 °C min
1
.
Differential scanning calorimetry (DSC) measurement was per-
formed by a Rigaku Denki Co., Ltd. instrumental (model DSC-8230)
at a heating rate of 10 °C min
1
under N
2
gas atmosphere from
room temperature to 350 °C. All the DSC curves were related to
the first scan.
A Rigaku Denki model CN-8361 apparatus for thermomechani-
cal analysis (TMA) was used to detect the thermal expansion and
contraction properties in the course of the heating process. The
temperature range studied was from 20 to 350 °C. The heating rate
was 10 °C min
1
, and sweep dry N
2
gas provided the inert atmo-
sphere. TMA full scale was ±500
l
m. The initial load applied to
the sample was 1 g. All the measurements were repeated for
reproducibility.
The morphologies of the fibers were examined with a Hitachi
S-2380N scanning electron microscopy (SEM) at 15 kV of accelera-
tion voltage. Before placing the samples in the SEM chamber, the
samples were mounted onto an aluminum stud and sputter-coated
with gold/palladium for 180 s (E-1010 ION SPUTTER, Hitachi,
Japan) to prevent charging.
The tensile properties were measured with a Tensilon Model
UTM-II-20, (Orientec Co.), Japan using standard technique at
22 °C and 65% RH at a gauge length of 40 mm and strain rate of
100% min
1
. Each value reported is the average of 20
measurements.
3. Results and discussion
3.1. Degumming ratio
Table 2 shows the results of citric acid degumming tests on silk
fibers and compared with blank and traditional soap degumming
method. The degumming ratio of silk fiber treated with hot water,
soap solution, 15% and 30% citric acid solution were 6.5%, 25.3%,
22.0% and 25.4%, respectively. The sericin removal efficiency of
degumming process using 30% citric acid was as similar as soap
degumming.
The effects of the different concentrations of citric acid on silk
degumming method were evaluated. Fig. 1 shows the denier (size),
degumming ratio and amount of residual sericin for the silk fibers
treated with different concentration of citric acid at 98 °C for
30 min. Initially, the degumming ratio of silk fibers increased grad-
ually with raising the concentration of citric acid and then the va-
lue attained to the saturation level, at about 25%, after degummed
with above 20% citric acid. The amount of residual sericin which
remains on the raw silk fiber decreased with increasing citric acid
concentration. The size of raw silk decreased gradually with
increasing the citric acid concentration. Becker et al. (1995) con-
firmed that sericin content in B. mori cocoons varies from 19% to
28% and reported that it gradually declines during cocoon spin-
ning. From the obtained results we can say that almost all of the
sericin was removed after degumming with 25% citric acid.
3.2. Tensile properties
Tensile properties are the most important factors for evaluating
the performance of fibers for proper applications. The ultimate
Table 1
Dyeing behaviors of degummed silk fibers.
Sample Dye uptake (%) aValue
*
(a) Dyeing condition: Acid Red 6 (C.I. 14680, Suminol Fast Red B conc, Labeling
type dyestuff), 1% owf dyestuff, 2% acetic acid, initial pH of dyeing bath is
3.78
Raw silk 98.2 58.09
Silk degummed with hot water 97.1 59.71
Silk degummed with 15% soap 96.7 63.05
Silk degummed with 15% citric acid 94.0 62.10
Silk degummed with 30% citric acid 93.1 60.97
Sample Dye uptake (%) Final pH bValue
**
(b) Dyeing condition: Acid Blue 138 (C.I. 62075, Kayanol Milling Blue BW),
Milling type, 1% owf dyestuff, 5% sodium sulfate, initial pH of dyeing bath is
5.89
Raw silk 99.3 6.85 40.45
Silk degummed with hot water 98.2 7.11 39.09
Silk degummed with 15% soap 97.5 9.59 40.30
Silk degummed with 15% citric acid 90.3 7.04 38.68
Silk degummed with 30% citric acid 96.0 7.04 40.24
*
avalue is the color index. The positive value of ‘a’ indicates the red color.
**
bvalue is the color index. The negative value of ‘b’ indicates the blue color.
Table 2
Sericin removal percentage and tensile properties of silk fibers.
Sample Degumming
ratio (%)
Initial
modulus
(GPa)
Ultimate
tensile
strength (MPa)
Strain
(%)
Raw silk – 8.3 422 13.6
Silk degummed with
hot water
6.5 7.9 394 13.0
Silk degummed with
15% soap
25.3 7.2 250 18.1
Silk degummed with
15% citric acid
22.0 7.4 507 20.0
Silk degummed with
30% citric acid
25.4 6.9 423 17.4
0
5
10
15
20
25
30
0
20
40
60
80
100
120
140
0 5 10 15 20 25
de
g
ummin
g
ratio (%)
denier (d)
acid concentration (%)
Fig. 1. Yarn size, degumming ratio and residual sericin percentage of silk fibers
degummed with different concentration of citric acid; (a) degumming ratio (%), (b)
residual sericin (%), and (c) fiber fineness in denier.
M.M.R. Khan et al. / Bioresource Technology 101 (2010) 8439–8445 8441
tensile strength, initial modulus and strain % of silk fibers de-
gummed with different chemical agents are presented in Table 2.
The ultimate tensile strength and strain % of control raw silk fiber
was 422 MPa and 13.6%, respectively. After soap degumming, the
tensile strength was decreased at about 250 MPa, indicating partial
harmful damages of silk molecules after soap-alkali degumming. It
is interesting to note that the strength was remained similar as raw
silk or increased in citric acid degumming, 507 MPa for 15% citric
acid and 423 MPa for 30% citric acid, respectively. The trend of
these strength results followed the recent cited report of silk
degummed with tartaric acid (Freddi et al., 1996). The strain % of
silk fibers was noticeably increased for both soap and acid degum-
ming method. There was not so much influence on strength and
elongation in blank samples. The initial modulus of untreated con-
trol silk fiber was 8.3 GPa and this value decreased after degum-
ming with citric acid and soap solutions, suggesting the silk
fibers become soft and stretchable after degumming treatment.
3.3. Surface morphology
It is interesting to investigate the surface morphology of de-
gummed silk fibers using different kinds of chemical agents by
SEM. The SEM micrographs are illustrated in Fig. 2. In the control
silk fiber micrographs (Fig. 2a), sericin appears as a partially non-
uniform coating on the surface of the fibers and various granular
deposits are visible in the interstices between filaments. Fiber sur-
face of blank sample is rough with granular deposits as in the con-
trol sample (Fig. 2b). In some parts of the yarn, sericin layers break
away from the main fiber axis, leaving the clean fibroin fiber sur-
face to appear. However, the micrographs of samples degummed
by citric acid and soap solutions show perfect degumming and
no sign of destruction and damage to the surface of the silk fibers
(Fig. 2c and d). The fiber surface is highly smooth, showing only
very fine longitudinal striation attributable to the fibrillar structure
of the truly degummed silk fibers (Arai et al., 2004).
3.4. Structural characteristics
In an attempt to examine whether the acid degumming induced
some structural changes, we investigate the changes in secondary
structure of silk fibers degummed with different chemical agents.
Fig. 3 shows FT-IR spectra of silk fibers degummed with different
chemical agents. All silk fibers are characterized by the absorption
bands at 1655 cm
1
(amide I), assigned to random coil conforma-
tion, 1630 cm
1
(amide I) and 1530 cm
1
(amide II), attributed to
the b-sheet structure (Shao et al., 2005; Bhat and Ahirrao, 1983;
Mathur et al., 1997). The above data show the similar FT-IR spectra
imply that the molecular conformation of silk fibers does not
change even after different treatments of degumming and they as-
sume a b-structure and random coil conformation.
Wide-angle X-ray diffraction profiles were measured of control
and different degummed silk fibers to elucidate the changes in
crystalline structure in more details (Fig. 4). The diffraction curves
of all silk fibers exhibit the typical pattern of a silk II crystal with
high crystallinity and show a series of diffraction peaks at 21.1°
and 29.9°, corresponding to crystalline spacing of 4.41 and
2.98 Å, respectively (Freddi et al., 1995). The above results demon-
strate that the crystalline structure of raw silk fiber with oriented b
crystals remained essentially unaffected by the degumming pro-
cess using soap and/or citric acid.
Fig. 2. SEM micrographs of silk fibers degummed with different chemical agents. (a) Raw silk; (b) degummed by hot water (blank); (c) degummed by 15% soap; and
(d) degummed by 15% citric acid.
8442 M.M.R. Khan et al. / Bioresource Technology 101 (2010) 8439–8445
3.5. Thermal properties
The thermal behavior of silk fibers degummed with different
kinds of degumming agents was evaluated on the basis of DSC
and TMA measurements. Fig. 5 shows the DSC thermograms of
control and different degummed silk fibers. The DSC curve of con-
trol silk fiber is characterized by a single predominant endothermic
transition at 316 °C, which is started at around 270 °C, attributed
to the thermal decomposition of the silk fibroin with orientated
b’ configuration (Ishikawa et al., 1972), remained unchanged
regardless of the blank, acid and alkali degummed fibers. However,
the endothermic peak of silk fiber degummed with soap became
broader and shifted slightly higher temperature, 324 °C. We calcu-
lated the enthalpy of fusion (
D
H
f
) from the DSC data which indi-
cates how much energy is needed to melt accompanying thermal
decomposition for the silk threads from their crystalline state.
The results showed that the value of
D
H
f
was 261, 284, 258 and
297 J/g for control, 15% citric acid, 30% citric acid and 15% soap de-
gummed silk, respectively. The influence of citric acid degumming
on the expansion and contraction properties of silk fibers was stud-
ied by TMA measurements (Fig. 6). The control raw silk fibers dem-
onstrated a little contraction of about 1.0% from room temperature
to 200 °C, which may be attributed to the evaporation of humidity
absorbed by the specimen. Then the length of the specimen was re-
tained unchanged to 280 °C and then started to extend rapidly at a
constant rate above 280 °C upward. The thermal behaviors of raw
silk fiber observed above 280 °C are due to the breaking and
reforming of the intermolecular hydrogen bonds and to the partial
thermal decomposition. The position of the final extension is in
accordance with the decomposition temperature of the fiber with
bmolecular configuration. It is interesting to note that the thermal
properties elucidated from the TMA thermograms strongly support
the results of DSC measurement. The starting onset position of ra-
pid extension of silk fiber at around 280 °C corresponds to the ini-
tial beginning temperature of endothermic peak. Silk fiber treated
with soap and citric acid 15% showed a prominent two-step con-
traction in the temperature range from 25 to 180 °C. The first step
appeared in the range from 25 to about 125 °C and could be related
to the evaporation of the water absorbed by the fiber. The second
step started at above 200 °C, accompanied by the abrupt change
of slope of the TMA curve, and attained a maximum at
280–300 °C. The final abrupt extension for the all silk fibers
occurred at about 310 °C.
3.6. Dynamic mechanical behavior
DMTA is known as a very sensitive method in detecting the fine
structure of silk molecules. We studied the viscoelastic properties
1800 1600 1400 1200 1000
Absorbance / arb. unit
Wavenumber / cm-1
(a)
(b)
(c)
(d)
(e)
Fig. 3. FT-IR-ATR spectra of silk fibers degummed with different chemical agents.
(a) Raw silk; (b) degummed by hot water (blank); (c) degummed by 15% soap; (d)
degummed by 15% citric acid, and (e) degummed by 30% citric acid.
10 20 30 40 50
0
1000
2000
3000
4000
5000
(a) Raw silk
(b) Silk_water
(c) Silk_15wt% soap
(d) Silk_15wt% citric acid
(e) Silk_30wt% citric acid
Intensity
Diffraction angle 2θ / o
Fig. 4. Equatorial WAXD profiles of silk fibers degummed with different chemical
agents.
200 250 300 350
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
(a) Raw silk
(b) Silk_water
(c) Silk_15wt% soap
(d) Silk_15wt% citric acid
(e) Silk_30wt% citric acid
Endo Heat Flow (w/g) Exo
Temperature / oC
Fig. 5. DSC curves of silk fibers degummed with different chemical agents.
050100150200250300350
-1.5
-1.0
-0.5
0.0
0.5
1.0
(a) Raw silk
(b) Silk_15wt% soap
(c) Silk_water
(d) Silk_15wt% citric acid
(e) Silk_30wt% citric acid
TMA / %
Tem
p
erature / oC
Fig. 6. TMA curves of silk fibers degummed with different chemical agents.
M.M.R. Khan et al. / Bioresource Technology 101 (2010) 8439–8445 8443