Indian Journal of Fibre & Textile Research
Vol. 34, March 2009, pp. 31-35
Evaluation of silk yarn cohesion based on peeling force
Subrata Dasa & Anindya Ghoshb
Government College of Engineering and Textile Technology, Berhampore 742 101, India
Received 2 April 2008; revised received and accepted 13 June 2008
The peeling force required to separate out the filaments has been evaluated to measure the cohesion of silk yarn. Such
force has been correlated well with the values obtained on conventionally used Duplan cohesion tester. The results show that
the mulberry yarns are more cohesive than tussah yarns. The effect of sericin content on cohesion has also been investigated.
Higher percentage of residual sericin imparts more peeling force. Even at low level of sericin content, the peeling force
determined on Instron can be able to detect the change in cohesion values.
Keywords: Cohesion, Mulberry, Peeling force, Sericin, Silk, Tussah
1 Introduction
The sericin protein envelops the fibroin fibre with
successive sticky layers which ensures the cohesion
by gluing silk filaments together.1,2 Sericin of different
types of cocoon shells has been the subject of many
investigations.3-8 Cocoon filaments are bonded
together during the process of silk reeling. The degree
of agglutination of such bonding of cocoon filaments
forming raw silk thread plays an important role in
processing of silk yarns. It is a highly desirable
feature in silk yarn to withstand stress and strain in
weaving. Obviously, higher cohesion lessens the end
down rate during weaving, by way of the reduction of
fraying and entanglements. Evaluation of silk yarn
cohesion is not only important from the viewpoint of
quality control, but also it help to improve the
processing performance of yarns. In order to improve
silk yarn cohesion, some of the factors, such as
temperature, amount of reeling tension, sufficient
croissure and good drying, of raw silk need to be
carefully considered.9 Cohesion of mulberry silk
filaments was studied by Manna et al.10, using
conventional cohesion tester.
Duplan cohesion tester has been used for evaluation
of silk yarn cohesion. It works on the principle of
applying strokes on a bed of silk threads held under
______________________
Present address: Central Silk Technological Research Institute,
Bangalore 560 068, India
a
b
To whom all the correspondence should be addressed.
E-mail: [email protected]
tension till fraying appears (i.e. open places of 6 mm
or more) at 10 different locations.11 In general,
machine is stopped after every 10 strokes and every
single yarn is inspected very carefully to see if there
are any fraying or open places having a dimension of
6 mm or more. If the number of these open places is
less than 10, then another set of 10 strokes is applied
and so on. The method has certain inherent
shortcomings which may be read as follows. First of
all, manual counting of the number and size of open
places is not only monotonous but also laborious too
and hence may lead to erroneous results. Moreover,
since the strokes are administered 10 at a time,
invariably the numbers of frayed portions are more
than 10, when the test is stopped. Hence, end point
determination is ambiguous in nature. In addition, the
yarn is considered to have been frayed at a particular
location if the opening is 6 mm or more in length. In
actual practice, the length is not measured. The result,
therefore, depends more on individual judgement of
the person performing the test and therefore liable to
be subjective.
In view of the supra, there appears a need for
developing an alternate method which is free not only
from subjective errors, but also much more sensitive
and reliable for scientific investigations. Since sericin
acts as a binder and holds the filaments together, the
force of attachment between filaments can be a
criterion to judge cohesion. The present investigation
is endeavored to determine the peeling force required
to separate out the filaments of different silk yarns.
INDIAN J. FIBRE TEXT. RES., MARCH 2009
32
2 Materials and Methods
2.1 Materials
Indian tussah and mulberry silk yarns, collected
from Central Silk Board, India, were used for the
experiments. The specifications of different varieties
of silk yarns are given in Table 1.
2.2 Methods
The experimental samples were conditioned in a
standard atmosphere of 65 ± 2% RH and 27 ± 2°C
temperature for 24 h.
2.2.1 Determination of Cohesion on Duplan Cohesion Tester
Cohesion of experimental samples was determined
on Duplan cohesion tester as per the standard
procedure.11 The silk yarn was passed over a set of 10
hooks on each side of the frame under a uniform
constant total tension of 180 cN in such a way that the
thread is subjected to strokes at 20 different places
simultaneously at a rate of 140 strokes/min. The
number of strokes needed to produce 10 open places
(having at least 6 mm dimension) was recorded by
repeatedly stopping the instrument after each stroke.
At least 30 observations were taken for each sample
and a total length of 3.6 m was tested for each
observation.
2.2.2 Determination of Peeling Force
Since filaments in a silk yarn adhere to each other
due to the presence of sericin on their surfaces, a true
estimate of cohesive force will be available if the
filaments are peeled out, i.e. separated from each
other. The force required to peel the filaments can be
taken as a reliable estimate of cohesion. To
accomplish this, a section of silk yarn was randomly
selected and cut by scissors. Filaments from one of its
end were separated into two equal parts. Each part
was then gripped in the jaws of the Instron tensile
tester (4201) set at a distance of 1 cm and a crosshead speed of 50 cm/min was used to peel the yarn.
Table 1—Specifications of different varieties of silk yarns
Type of yarn
Yarn size
den
Filaments per
cross-section
Possible number
of broken
contact points
Tussah A
Tussah B
Tussah C
Mulberry D
(Multivoltine)
Mulberry E
(Bivoltine)
91
52
78
19
14
24
28
14
7, 9
9, 11
10, 11
7, 9
20
14
7, 9
The force required to peel off 5 cm of yarn was
recorded. At least 30 observations were taken for each
variety of silk yarn and the average value of the
peeling force was calculated.
2.2.3 Determination of Residual Sericin Content
Cohesion in the silk yarn is mainly attributed to the
residual sericin. To estimate the content of sericin,
leas of silk yarn were first weighed (W1) and
degummed in a solution of 25% Marseilles soap for
90 min at boil, keeping the liquor ratio at 50:1. Then
the leas were taken out, thoroughly washed and dried.
The weights (W2) of the degummed leas were
recorded again after conditioning for 24 h in standard
atmosphere. The residual sericin content was
calculated using the following equation:
Residual sericin content (%) =
W1 - W2
× 100
W1
To study the relationship between sericin content
and peeling force, tussah yarn (type C) was selected.
It contained maximum sericin (7.8%) among all the
tussah silk yarns. After degumming of silk, the
duration of treatment was varied from 5 min to
125 min to produce samples left with different
amount of residual sericin. The yarn samples were
subsequently tested on the Instron tensile tester to
evaluate the peeling force.
3 Results and Discussion
The primary source of cohesion is the presence of
residual sericin. Besides sericin, the cohesion would
also depend upon its nature and total available surface
area of filaments mutually in contact. The available
surface area, in turn, is decided by the number of
filaments in yarn cross-section and its fineness and
cross-sectional shape. Finer and more filaments in a
yarn of given count would enhance greater surface
area of contact and cohesion. The actual area of
contact will however depend upon cross-sectional
shape of filaments too. Assuming filaments to be
randomly distributed in the yarn cross-section, it can
be said that the more its cross-sectional shape deviates
from circularity, the less would be the possible
contact areas. This is so because filaments with noncircular cross-section will hinder close packing.
Further, when filaments from a yarn are peeled out
into two equal parts, group-wise filament separation
takes place keeping mutual contact between filaments
within a group intact. Therefore, all the contact points
DAS & GHOSH: EVALUATION OF SILK YARN COHESION BASED ON PEELING FORCE
between filaments are not likely to break. In the
present case, since residual sericin content, yarn and
filament fineness are different for all the yarns used in
this study, a corresponding difference in cohesion and
peeling force values would be obvious. Therefore, a
strict comparison between peeling force values of the
yarns are not possible. For true comparison, peeling
force values are to be normalized by finding out
peeling force per unit surface area or peeling force per
broken contact point.
Assuming circular cross-section, the surface area of
a filament is proportional to the (N)1/2, where N is the
filament denier. Hence, it can be shown that:
PS ∞
P
nb N D
33
Table 2—Cohesion and peeling force for various types of tussah and
mulberry silk yarns
Residual
sericin
%
Cohesion
strokes
(Duplan)
P
cN
P1
cN.den-1/2
P2a
cN
Tussah A
3.14
0.00
0.46
0.018
0.058
Tussah B
5.02
0.00
0.84
0.033
0.084
Tussah C
7.78
21.00
4.80
0.145
0.457
Mulberry D
20.70
31.00
4.22
0.364
0.527
Mulberry E
24.70
43.33
5.12
0.432
0.64
Type of
yarn
a
The average value of nc was considered.
…(1)
where Ps is the peeling force per unit surface area; P,
the absolute peeling force; nb, the number of
filaments; and ND, the denier of each filament. Thus,
the normalized value of peeling force based on the
surface area (P1) was calculated from the following
equation:
P1 =
P
nb ND
…(2)
In extreme situation of packed structure, a
hexagonal close packed structure of filaments as
postulated by Schwarz12 for circular fibres would be
expected as shown in Fig. 1, where each circle
represents a filament. Since the separation may take
place along any arbitrary line in the cross-sectional
plane of yarn as shown in Fig. 1, the possible number
of broken contact points may vary (Table 1). The
peeling force based on the broken contact points (P2)
was calculated from the following equation:
P2 =
P
nc
…(3)
where nc is the number of broken contact points.
Cohesion data as obtained on Duplan cohesion
tester and Instron tensile tester are presented in
Table 2. The results from Duplan cohesion tester
demonstrate that the mulberry type E shows the
highest value of cohesion followed by mulberry D
and tussah type C. No cohesion is observed for type A
Fig. 1—Possible number of broken contact points for hexagonal
closed packed structure [(a)—14 filaments in cross-section with
7 broken contact points , (b)—14 filaments in cross-section with
9 broken contact points, (c)—24 filaments in cross-section with
9 broken contact points, (d)—24 filaments in cross-section with
11 broken contact points, (e)—28 filaments in cross-section with
10 broken contact points, and (f)—28 filaments in cross-section
with 11 broken contact points]
and type B tussah yarns. The peeling force,
determined on Instron also shows a highest value for
mulberry type E followed by tussah type C, mulberry
type D and tussah types B and A. Peeling force
therefore can be discriminated between tussah types A
34
INDIAN J. FIBRE TEXT. RES., MARCH 2009
Fig. 3—Effect of residual sericin on peeling force
Fig. 2—Correlation between cohesion strokes and peeling force
for tussah and mulberry silk yarns
and B, which Duplan cohesion tester fails to do. Low
peeling force values are observed for type A and type
B tussah yarns. A comparison in terms of normalized
peeling force shows that the cohesion increases in the
order: type A < type B < type C < type D < type E.
The order between type C and type D changes in
comparison to the order obtained based on absolute
peeling force and matches with that obtained by
cohesion strokes. The grading in terms of normalized
cohesion therefore remains identical to that obtained
by cohesion strokes but becomes more sensitive. A
high correlation coefficient between cohesion
measured on Duplan and normalized peeling force
based on surface area as well as number of broken
contact points (Fig. 2) also substantiate this fact.
In general, tussah yarns are less cohesive than
mulberry yarns. One can further notice (Table 2) that
mulberry variety of type E which contains highest
residual sericin also exhibits highest cohesion and
peeling force. A high level of sericin content and finer
filament denier may be ascribed to this. Amongst
tussah, type C shows highest values of cohesion and
contains maximum amount of sericin as well. Besides,
it contains maximum number of filaments. Cohesion
strokes are almost zero for residual sericin level of 5%
or below. Peeling force however shows a reasonable
value, thus indicating its sensitiveness to the low level
of sericin. It appears therefore that residual sericin and
characteristics of tussah variety affect cohesion
values.
To determine the influence of only residual sericin
on peeling force, the level of residual sericin in type C
tussah yarn was varied through degumming for
different duration of time. A plot between peeling
force and residual sericin as depicted in Fig. 3 clearly
brings out the fact that the peeling force based on the
number of broken contact points increases with the
increase in residual sericin content, initially slowly
but becomes too rapid beyond 5% level of sericin.
4 Conclusions
A strong correlation exists between normalized
peeling force and cohesion strokes determined on
Duplan cohesion tester. Peeling force increases
rapidly with sericin especially at higher residual
sericin content. The peeling force determined on
Instron is much more sensitive to the change in
cohesion values especially when the sericin content is
low. The mulberry yarns are more cohesive than
tussah yarns.
Industrial Importance: The proposed method of
determining the silk yarn cohesion based on the
peeling force is more accurate and reliable and it
could substitute the need of Duplan cohesion tester in
silk industry.
DAS & GHOSH: EVALUATION OF SILK YARN COHESION BASED ON PEELING FORCE
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