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CHAPTER 4
STUDIES ON THE RELATIONSHIP BETWEEN
DRAPE AND MECHANICAL PROPERTIES
OF GOAT SUEDE LEATHERS
4.1 INTRODUCTION
Goat leathers account for about 8-10% of the total world leather
production (John 1997). The application of goat leathers includes clothing,
fancy goods, shoe uppers, gloves and lining leather. In raw goat skin,
papillary layer represents about 30-40% and the reticular layer about 40-50%
of the total thickness of 1-2 mm (John 1997). Both layers are tightly
connected with each other such that loose grain, as found frequently in sheep
skin is less often observed. Average size of the goat skins is about 0.5-0.9 m2.
Goat skins are generally processed in to grain or suede finish for apparel
application. Goat suede leather has a nap or velvet effect on the flesh side and
the grain layer is completely removed. Full-grain leather is made from the
outer side (grain) of an animal skin whereas suede leather uses the inner side
(flesh). Goat suede leather is the thinnest among the apparel leathers. In recent
years, there has been a renewed interest in investigating the aesthetic behavior
of clothing materials due to the developments in objective evaluation
techniques (Kenkare and May-Plumlee 2005). In this study, drape properties
namely drape coefficient and number of drape nodes and other related
mechanical properties such as softness, flexural rigidity, formability, initial
tensile modulus, weight and thickness of goat suede leathers were quantified
and correlated.
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4.2 MATERIALS
Goat suede leathers of Indian origin (commercially available, top
grade) were procured from four different firms. Four leathers were chosen
from each firm with fairly uniform thickness and size (area around 0.5 m2).
Four circular samples were cut from each leather and the samples were
designated as SL (shoulder left), SR (shoulder right), BL (butt left) and BR
(butt right) based on location. These circular samples were used to measure
drape coefficient, number of nodes, softness, thickness and weight. From
these circular samples, rectangular samples were cut along, across and bias
backbone directions as shown in Figure 4.1 for measurement of flexural
rigidity and tensile modulus. Bias backbone samples were cut in order to
understand the tensile and bending behavior in 45° to backbone line. The
tensile strength, % elongation and tear strength values of the goat suede
leathers from four different firms are given in Table 4.1.
Figure 4.1 Schematic of location of the circular sample at butt right
position and the rectangular samples inside the circular
sample in goat suede leather
L1
X2
X1
L2
C1
X4
X3
C2
Back
bone
Lin
e
Circular sample at Butt Right
Legend:L1 and L2 Samples along backboneC1 and C2 Samples across backboneX1 to X4 Samples bias backbone
76
Table 4.1 Tensile and tear properties of goat suede leathers from
different sources
Tensile strength(MPa)
Elongation(%)
Tear strength(N)
Firm 1Leather 1 19.73 ± 3.03 45.16 ± 2.37 24.42 ± 6.04
2 21.06 ± 3.45 48.93 ± 4.53 22.91 ± 3.763 25.42 ± 3.51 46.24 ± 5.15 29.07 ± 2.874 25.72 ± 2.49 46.14 ± 4.95 29.00 ± 8.51
Firm 2Leather 5 19.15 ± 2.52 46.73 ± 4.20 23.22 ± 4.94
6 20.55 ± 3.72 49.06 ± 7.04 20.06 ± 1.567 20.69 ± 2.16 46.38 ± 7.12 24.42 ± 3.208 21.90 ± 2.56 46.92 ± 5.53 23.48 ± 2.44
Firm 3Leather 9 20.88 ± 3.76 43.15 ± 5.90 18.30 ± 1.40
10 19.62 ± 2.81 46.29 ± 8.69 21.62 ± 1.1011 19.77 ± 3.25 47.71 ± 3.71 22.38 ± 5.6012 20.93 ± 2.04 44.19 ± 5.32 18.79 ± 2.91
Firm 4Leather 13 18.03 ± 2.84 50.19 ± 6.40 27.84 ± 2.65
14 28.54 ± 7.15 56.05 ± 5.65 37.06 ± 2.7415 34.51 ± 5.31 50.53 ± 5.49 26.20 ± 5.4016 30.00 ± 3.95 57.81 ± 7.68 36.22 ± 4.07
The values are average of sixteen samples along with standard deviation
4.3 MEASUREMENT AND CALCULATION
Measurement and calculation of drape coefficient, number of nodes
and other mechanical properties were carried out as explained in section 3.2.2
of Chapter 3. Softness was determined following IUP36 (2000) test method
using ST300 digital leather softness tester and the size of the reducing ring
used in the softness tester was 20 mm. All the samples were conditioned at a
77
temperature of 20±2°C and relative humidity 65±5% for 48 h immediately
before its use in an experiment.
4.4 RESULTS AND DISCUSSION
4.4.1 Drape Parameters
Drape parameters of circular goat suede leather samples were
measured with grain as well as flesh side up. To find out the correlation, mean
drape coefficient values of grain side up samples were plotted against flesh
side up samples from individual leathers as shown in Figure 4.2. The
correlation between the two values seems to be linear, as indicated by the
correlation coefficient value of 0.98. Hence, the mean value of both
measurements was used for further comparison. Similarly, the number of
nodes of grain side up samples was plotted against flesh side up samples as
shown in Figure 4.3. From the plot, it is evident that the correlation between
the two values is good (R = 0.87). Hence, the mean value of both
measurements was calculated and used for further analysis.
15 20 25 30 35 40 45 5015
20
25
30
35
40
45
50
55
Mea
n dr
ape
coef
ficie
nt, f
lesh
sid
e up
(%)
Mean drape coefficient, grain side up (%)
Y = 1.11X - 3.74 R = 0.98
Figure 4.2 Mean drape coefficient of flesh side up samples versus grain
side up samples of individual goat suede leathers
78
5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
Mea
n nu
mbe
r of n
odes
, fle
sh s
ide
up
Mean number of nodes, grain side up
Y = 0.83 X + 1.00 R2 = 0.87
Figure 4.3 Mean number of nodes of flesh side up samples versus grain
side up samples of individual goat suede leathers
The mean values of drape coefficient and number of nodes formed
for circular samples from goat suede leathers procured from four differentfirms are given in Table 4.2 along with standard deviation. The leathers are
numbered based on the increasing order of their apparent density for eachfirm. Mean drape coefficient value varies from 19.8 to 50.4% for individual
goat suede leathers. The variation can be attributed to the differences in the
processes as well as mechanical operations adopted by various firms. It isevident that the observed drape coefficient values for goat suede leathers are
lower than the sheep nappa leathers meant for garment application. This may
be due to the low thickness, low weight per unit area and high softness of goat
suede leathers compared to sheep nappa leathers. The mean of number of
nodes formed varies between 5.5 and 8.5. The mean values of drapecoefficient were plotted against mean number of nodes in order to find the
interdependence as shown in Figure 4.4. The value of R suggests that there isa good correlation between the two values with negative slope. The drape
coefficient values decrease with the increase in number of nodes showing the
inverse relationship between the two. In other words, lesser value of drapecoefficient and higher value of number of nodes indicate better drape ability.
79
Table 4.2 Drape parameters of goat suede leathers from different sources
Drape coefficient (%) Number of nodes*
Firm 1Leather 1 35.3 ± 3.6 6.5 ± 0.5
2 33.3 ± 3.5 6.5 ± 0.53 38.1 ± 7.7 6.0 ± 1.04 43.6 ± 3.1 5.5 ± 0.0
Firm 2Leather 5 20.5 ± 1.6 8.0 ± 1.0
6 20.4 ± 1.6 7.0 ± 0.57 19.8 ± 1.3 8.5 ± 0.58 22.5 ± 3.2 8.0 ± 1.0
Firm 3Leather 9 24.4 ± 1.6 7.0 ± 0.5
10 28.4 ± 0.8 7.0 ± 0.511 24.4 ± 0.8 7.5 ± 0.512 25.5 ± 3.1 7.0 ± 0.5
Firm 4Leather 13 50.4 ± 6.0 5.5 ± 0.0
14 43.4 ± 7.9 5.5 ± 0.515 33.9 ± 3.1 6.5 ± 0.516 33.9 ± 3.1 6.5 ± 0.5
The values are average of four samples along with standard deviation*Number of node values is round off to nearest 0.5
Drape coefficient and number of nodes of each circular goat suede
sample from different locations were plotted as radar chart as shown in
Figures 4.5 and 4.6. From the figures it is observed that, even though the
variation in drape coefficient is more between leathers from different firms,
the variation is less between different positions in any particular leather. The
standard deviation in drape coefficient between different locations such as SL,
SR, BL and BR is less than 3.7 (except in three leathers). The variation in
number of nodes is also not significant between different locations.
80
5.5 6.0 6.5 7.0 7.5 8.0 8.515
20
25
30
35
40
45
50
55
Mea
n dr
ape
coef
ficie
nt (%
)
Mean number of nodes
Y = -9.50 X + 95.56 R = - 0.92
Figure 4.4 Mean drape coefficient versus mean number of nodes of
individual goat suede leathers
Figure 4.5 Graph of drape coefficient of circular goat suede samples
based on location
1 to 16: Leathers0 to 60: Drape coefficient
81
Figure 4.6 Graph of number of nodes of circular goat suede samples
based on location
4.4.2 Softness Versus Drape Parameters
The mean value of softness of goat suede leathers from different
firms are given in Table 4.3 along with standard deviation. Mean softness
values were shown to be from 5.22 to 6.39 mm for individual goat suede
leathers. It has been reported that highly soft leathers meant for different
applications possess softness values in the order of 4 to 8 (Landmann et al
1994). It is seen that the softness values of the goat suede leathers procured
from different firms are higher than that of the values reported previously for
sheep nappa leathers meant for garment application. The variation of softness
values of leathers between different firms may be attributed to different
processing conditions.
1 to 16: Leathers0 to 10: Nodes
82
Table 4.3 Softness, thickness and weight of goat suede leathers from
different sources
Softness (mm) Thickness (mm) Weight (g/dm2)Firm 1
Leather 1 6.02 ± 0.10 0.63 ± 0.01 3.73 ± 0.092 5.62 ± 0.38 0.66 ± 0.02 3.97 ± 0.233 5.64 ± 0.32 0.70 ± 0.02 4.21 ± 0.164 5.32 ± 0.19 0.68 ± 0.01 4.08 ± 0.10
Firm 2Leather 5 6.32 ± 0.11 0.56 ± 0.02 2.89 ± 0.12
6 6.33 ± 0.05 0.53 ± 0.03 2.80 ± 0.137 6.39 ± 0.22 0.55 ± 0.01 2.97 ± 0.058 5.61 ± 0.19 0.52 ± 0.04 2.84 ± 0.25
Firm 3Leather 9 6.22 ± 0.23 0.58 ± 0.02 2.94 ± 0.12
10 5.75 ± 0.15 0.63 ± 0.01 3.31 ± 0.1111 6.05 ± 0.13 0.60 ± 0.02 3.17 ± 0.0912 6.13 ± 0.21 0.58 ± 0.02 3.07 ± 0.12
Firm 4Leather 13 5.22 ± 0.16 0.63 ± 0.02 3.24 ± 0.19
14 5.75 ± 0.30 0.74 ± 0.01 3.86 ± 0.0315 6.04 ± 0.16 0.57 ± 0.03 3.21 ± 0.1716 6.15 ± 0.11 0.68 ± 0.02 3.91 ± 0.21
The values are average of four samples along with standard deviation
The mean drape coefficient values of the individual leathers have
been plotted against the mean softness values as shown in Figure 4.7. It is
observed that the drape coefficient values are inversely related to softness and
exhibit a negative slope. The drape coefficient values decrease with the
increase in softness values. The value of correlation coefficient (R = –0.77)
shows a fairly good correlation between the drape coefficient and softness of
goat suede leathers. The mean of number of nodes of individual leathers has
been plotted against the mean softness values as shown in Figure 4.8. It is
83
seen that the number of drape nodes formed increases with increase in
softness with a positive slope.
5.2 5.4 5.6 5.8 6.0 6.2 6.415
20
25
30
35
40
45
50
55
Mea
n dr
ape
coef
ficie
nt (%
)
Mean softness (mm)
Y = -20.10 X + 149.90 R = -0.77
Figure 4.7 Mean softness versus mean drape coefficient of individual
goat suede leathers
5.2 5.4 5.6 5.8 6.0 6.2 6.4
5.5
6.0
6.5
7.0
7.5
8.0
8.5
Mea
n nu
mbe
r of n
odes
Mean softness (mm)
Y = 1.66 X - 3.03 R = 0.66
Figure 4.8 Mean softness versus mean number of nodes of individual
goat suede leathers
84
4.4.3 Thickness Versus Drape Parameters
The thickness of circular samples was measured at four different
positions equidistant from the center and circumference of the sample and
averaged. The mean values of the thickness of circular leather samples from
different firms are shown in Table 4.3. The thickness of leather samples varies
from 0.52 to 0.74 mm for individual goat suede leathers from different firms.
The thickness of goat suede leathers is comparable to that of sheep nappa and
cow nappa leathers used in the previous study. To find out the relationship
between the DC and thickness, the mean thickness values of individual
leathers have been plotted against mean drape coefficient as shown in Figure
4.9. It is observed that the drape coefficient increases with the increase in
thickness as evidenced from fairly good correlation coefficient value.
It is interesting to note that the thickness and drape coefficient have a
positive linear relationship for goat suede leather, whereas for cow nappa
leather it is inversely related. This may be due to the uniformity in structure
over the entire cross section of goat suede leather. The cross section of goat
suede (0.6 mm thick) leather captured using scanning electron microscope
with 300 X magnification is shown in Figure 4.10 (Krishnaraj 2002). In goat
suede leather grain layer is more or less removed and the entire cross section
is made up of more spongy, porous, corium proper layer whereas in cow
nappa leather it is made up of both densely packed, grain and corium proper
layers. So, any increase in thickness of goat suede leather is associated with
increase of same kind of porous, corium proper layer, which does not change
the uniformity of the cross section and hence lead to increase in drape
coefficient.
85
0.50 0.55 0.60 0.65 0.70 0.7515
20
25
30
35
40
45
50
55
Mea
n dr
ape
coef
ficie
nt (%
)
Mean thickness (mm)
Y = 113.05 X - 38.41 R = 0.77
Figure 4.9 Mean thickness versus mean drape coefficient of individual
goat suede leathers
Figure 4.10 Cross section of goat suede leather (300X magnification)
Grain side
Flesh side
86
The mean number of nodes formed has been plotted against mean
thickness of individual leathers as shown in Figure 4.11. Inverse relationship
was observed between the number of nodes and the thickness and the
correlation coefficient is –0.78. It can be attributed that thinner goat suede
leathers have better drape ability and vice versa. These results are in
agreement with the observation made in drape parameter study.
0.50 0.55 0.60 0.65 0.70 0.75
5.5
6.0
6.5
7.0
7.5
8.0
8.5
Mea
n nu
mbe
r of n
odes
Mean thickness (mm)
Y = - 11.12 X + 13.62 R = - 0.78
Figure 4.11 Mean thickness versus mean number of nodes of individual
goat suede leathers
4.4.4 Weight Versus Drape Parameters
The weight of each circular sample was measured and the weight
per unit area of the samples was calculated. From the calculated values,
leather wise average of the samples was obtained and the values are shown in
Table 4.3. The mean weight per unit area of goat suede samples was found to
be between 2.80 and 4.21 g/dm2 for individual leathers from different firms.
The mean weight per unit area of individual leathers has been plotted against
87
mean drape coefficient values as shown in Figure 4.12. As in the case of
thickness, the value of drape coefficient increases with the increase in weight
per unit area of goat suede leathers. Also the value of correlation coefficient
(R = 0.71) suggests fairly good correlation between the two values.
Figure 4.13 shows the plot of mean weight per unit area of individual leathers
and the mean number of nodes generated during drape measurement. As
expected, the plot shows an inverse relation between the weight and number
of nodes. The correlation coefficient value is similar to that of mean drape
coefficient versus mean weight, however with a negative slope. Increasing
weight of leather indicates increase in drape coefficient and decrease in
number of nodes, leading to poor drape ability. Hence, light goat suede
leathers have better drape ability. These results are in good agreement with
that of thickness and drape parameter study.
2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.415
20
25
30
35
40
45
50
55
Mea
n dr
ape
coef
ficie
nt (%
)
Mean weight per unit area (g/dm2)
Y= 13.58 X - 14.89 R = 0.71
Figure 4.12 Mean weight versus mean drape coefficient of individual
goat suede leathers
88
2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4
5.5
6.0
6.5
7.0
7.5
8.0
8.5
Mea
n nu
mbe
r of n
odes
Mean weight per unit area (g/dm2)
Y = - 1.33 X + 11.29 R = - 0.71
Figure 4.13 Mean weight versus mean number of nodes of individual
goat suede leathers
4.4.5 Flexural Rigidity Versus Drape Parameters
Flexural rigidity was calculated from the bending length measured
using stiffness tester (IS 6490 1971). Higher values of flexural rigidity
indicate more resistance to flexing, while lower values indicate easier flexing
and hence better drape ability. Flexural rigidity of goat suede leathers was
measured along, across and bias backbone directions and the mean values are
given in Table 4.4 along with standard deviation. The mean value of flexural
rigidity in all the three directions varies from 11.3 to 60.8 mNmm among
individual leathers from different firms. Although the flexural rigidity values
are varying significantly for leathers from different firms, such variations are
negligible within individual leathers for different directions. In other words,
there is no significant difference in flexural rigidity values measured bias
backbone compared to that of along and across backbone directions in most of
the leathers.
89
Table 4.4 Flexural rigidity of goat suede leathers from different sources
Flexural rigidity (mNmm)Along
backbone*Across
backbone*Bias
backbone#
Firm 1Leather 1 40.4 ± 10.2 28.1 ± 7.0 35.6 ± 5.8
2 37.8 ± 11.7 28.4 ± 4.9 34.4 ± 8.63 53.1 ± 11.6 41.0 ± 10.3 50.1 ± 12.34 44.5 ± 6.7 43.9 ± 6.2 49.5 ± 4.8
Firm 2Leather 5 13.6 ± 1.2 12.1 ± 1.6 14.1 ± 1.1
6 12.7 ± 1.7 11.3 ± 1.0 13.7 ± 1.87 13.3 ± 1.3 11.9 ± 1.2 13.5 ± 0.88 14.6 ± 1.7 14.9 ± 3.8 16.5 ± 4.6
Firm 3Leather 9 17.8 ± 4.3 14.9 ± 2.2 16.1 ± 2.5
10 23.1 ± 1.9 23.4 ± 3.7 26.8 ± 2.811 17.9 ± 3.1 16.9 ± 1.0 18.7 ± 2.112 19.2 ± 1.2 18.0 ± 3.8 20.5 ± 4.1
Firm 4Leather 13 44.9 ± 9.8 60.8 ± 10.4 56.6 ± 11.8
14 57.9 ± 11.6 49.0 ± 12.9 53.6 ± 11.015 28.2 ± 5.1 35.0 ± 6.5 29.7 ± 4.016 39.0 ± 3.9 36.8 ± 8.8 39.0 ± 6.7
* The values are average of eight samples along with standard deviation# The values are average of sixteen samples along with standard deviation
Mean flexural rigidity of individual leathers measured along, across
and bias backbone directions of leathers have been plotted against mean drape
coefficient values of corresponding leathers as shown in Figures 4.14, 4.15
and 4.16, respectively. The correlation between mean flexural rigidity and
mean drape coefficient is very good in all the three backbone directions. The
plots of mean flexural rigidity across as well as bias backbone directions
90
versus mean drape coefficient show the highest correlation (R = 0.98). It is
seen that, as the mean flexural rigidity values increase the mean drape
coefficient values also increase in all three directions. Hence, the drape ability
of goat suede leathers decreases with the increase in flexural rigidity.
10 20 30 40 50 6015
20
25
30
35
40
45
50
55
Mea
n dr
ape
coef
ficie
nt (%
)
Mean flexural rigidity, along backbone (mNmm)
Y = 0.56 X +14.43 R = 0.91
Figure 4.14 Mean drape coefficient versus mean flexural rigidity ofindividual goat suede leathers along backbone
10 20 30 40 50 6015
20
25
30
35
40
45
50
55
Mea
n dr
ape
coef
ficie
nt (%
)
Mean flexural rigidity, across backbone (mNmm)
Y = 0.61 X + 14.11 R = 0.98
Figure 4.15 Mean drape coefficient versus mean flexural rigidity ofindividual goat suede leathers across backbone
91
10 20 30 40 50 6015
20
25
30
35
40
45
50
55
Mea
n dr
ape
coef
ficie
nt (%
)
Mean flexural rigidity, bias backbone (mNmm)
Y = 0.59 X + 12.97 R = 0.98
Figure 4.16 Mean drape coefficient versus mean flexural rigidity ofindividual goat suede leathers bias backbone
Similarly, the mean flexural rigidity values measured along, acrossand bias backbone directions have been plotted against the mean number ofnodes of corresponding leathers as shown in Figures 4.17, 4.18 and 4.19,respectively.
10 20 30 40 50 60
5.5
6.0
6.5
7.0
7.5
8.0
8.5
Mea
n nu
mbe
r of n
odes
Mean flexural rigidity, along backbone (mNmm)
Y = -0.05 X + 8.34 R = -0.88
Figure 4.17 Mean number of drape nodes versus mean flexural rigidityof individual goat suede leathers along backbone
92
From the plots, it is evident that the number of nodes has inverserelation with the flexural rigidity for goat suede leathers. Fairly highcorrelation coefficients around –0.9 in all the three directions indicate a closelinear relationship between mean flexural rigidity and the mean number ofdrape nodes.
10 20 30 40 50 60
5.5
6.0
6.5
7.0
7.5
8.0
8.5
Mea
n nu
mbe
r of n
odes
Mean flexural rigidity, across backbone (mNmm)
Y = -0.05 X + 8.28 R = -0.89
Figure 4.18 Mean number of drape nodes versus mean flexural rigidityof individual goat suede leathers across backbone
10 20 30 40 50 60
5.5
6.0
6.5
7.0
7.5
8.0
8.5
Mea
n nu
mbe
r of n
odes
Mean flexural rigidity, bias backbone (mNmm)
Y = -0.05 X + 8.42R = -0.91
Figure 4.19 Mean number of drape nodes versus mean flexural rigidityof individual goat suede leathers bias backbone
93
4.4.6 Formability Versus Drape Parameters
Formability was calculated for goat suede leather samples cut
along, across and bias backbone directions of the leather and the mean values
are given in Table 4.5 along with standard deviation. The mean formability of
goat suede leathers varies between 0.003 and 0.017 mm2 for all backbone
directions of leathers from different firms. It is seen that the mean formability
values along backbone are similar or higher than that of across backbone
direction.
Table 4.5 Formability of goat suede leathers from different sources
Formability (mm2)Along
backbone*Across
backbone*Bias
Backbone#
Firm 1Leather 1 0.012 ± 0.001 0.007 ± 0.003 0.010 ± 0.001
2 0.011 ± 0.002 0.006 ± 0.001 0.007 ± 0.0023 0.011 ± 0.004 0.008 ± 0.003 0.011 ± 0.0044 0.011 ± 0.004 0.008 ± 0.001 0.010 ± 0.002
Firm 2Leather 5 0.004 ± 0.000 0.003 ± 0.000 0.004 ± 0.000
6 0.003 ± 0.001 0.003 ± 0.001 0.004 ± 0.0017 0.004 ± 0.000 0.004 ± 0.001 0.004 ± 0.0008 0.003 ± 0.000 0.003 ± 0.000 0.003 ± 0.000
Firm 3Leather 9 0.003 ± 0.001 0.003 ± 0.001 0.004 ± 0.001
10 0.005 ± 0.001 0.005 ± 0.002 0.006 ± 0.00111 0.005 ± 0.001 0.004 ± 0.001 0.004 ± 0.00112 0.004 ± 0.001 0.003 ± 0.000 0.005 ± 0.001
Firm 4Leather 13 0.011 ± 0.003 0.011 ± 0.001 0.010 ± 0.002
14 0.017 ± 0.004 0.011 ± 0.003 0.015 ± 0.00115 0.007 ± 0.002 0.006 ± 0.002 0.008 ± 0.00216 0.013 ± 0.005 0.010 ± 0.002 0.012 ± 0.003
* The values are average of eight samples along with standard deviation# The values are average of sixteen samples along with standard deviation
94
Mean formability values calculated for samples cut along, across
and bias backbone directions have been plotted against mean drape coefficient
values of the individual leathers, as shown in Figures 4.20, 4.21 and 4.22,
respectively. A very good relation between formability and drape coefficient
is seen when fit linearly for all three directional samples. The plot of
formability across backbone and drape coefficient recorded the highest
correlation (R = 0.92). Form the linear relationship, it is evident that the
increase in formability relates to increase in the drape coefficient of goat
suede leathers. In other words lower formability leathers tend to drape better.
Such trend has already been reported for textile fabrics (Griffiths and Kulke
2002).
0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.01815
20
25
30
35
40
45
50
55
Mea
n dr
ape
coef
ficie
nt (%
)
Mean formability, along backbone (mm2)
Y = 1837.42 X + 16.83 R = 0.86
Figure 4.20 Mean drape coefficient versus mean formability of
individual goat suede leathers along backbone
95
0.002 0.004 0.006 0.008 0.010 0.01215
20
25
30
35
40
45
50
55
Mea
n dr
ape
coef
ficie
nt (%
)
Mean formability, across backbone (mm2)
Y =3026.17 X + 13.32 R = 0.92
Figure 4.21 Mean drape coefficient versus mean formability of
individual goat suede leathers across backbone
0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.01615
20
25
30
35
40
45
50
55
Mea
n dr
ape
coef
ficie
nt (%
)
Mean formability, bias backbone (mm2)
Y = 2248.70 X + 14.67 R = 0.88
Figure 4.22 Mean drape coefficient versus mean formability of
individual goat suede leathers bias backbone
The mean formability of leathers measured along, across and bias
backbone directions have been plotted against the mean number of drape
nodes as shown in Figures 4.23, 4.24, and 4.25, respectively. The correlation
coefficients are more than –0.80, which indicates that the correlation between
96
the mean formability and the number of nodes is reasonably good and
inversely proportional.
0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018
5.5
6.0
6.5
7.0
7.5
8.0
8.5
Mea
n nu
mbe
r of n
odes
Mean formability, along backbone (mm2)
Y = -169.52 X + 8.10 R = -0.82
Figure 4.23 Mean number of nodes versus mean formability of
individual goat suede leathers along backbone
0.002 0.004 0.006 0.008 0.010 0.012
5.5
6.0
6.5
7.0
7.5
8.0
8.5
Mea
n nu
mbe
r of n
odes
Mean formability, across backbone (mm2)
Y = -265.80 X + 8.34 R = -0.83
Figure 4.24 Mean number of nodes versus mean formability of
individual goat suede leathers across backbone
97
0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016
5.5
6.0
6.5
7.0
7.5
8.0
8.5
Mea
n nu
mbe
r of n
odes
Mean formability, bias backbone (mm2)
Y = -212.34 X + 8.33 R = -0.85
Figure 4.25 Mean number of nodes versus mean formability of
individual goat suede leathers bias backbone
4.4.7 Initial Tensile Modulus Versus Drape Parameters
Initial tensile modulus was measured for samples cut along, across
and bias backbone directions of the goat suede leathers and the mean values
are given in Table 4.6 along with standard deviation for different firms. The
mean values of the initial tensile modulus in all the three directions vary from
3.29 to 6.43 N/mm for individual goat suede leathers from different firms.
The mean initial tensile modulus across backbone direction is higher than that
of along and bias backbone directions for most of the leathers. Mean initial
tensile modulus values of samples cut along, across and bias backbone
directions have been plotted against mean drape coefficient values of
individual leathers, as shown in Figures 4.26, 4.27 and 4.28, respectively. It is
seen that the influence of initial tensile modulus on drape coefficient is not
substantial since there is no significant correlation coefficient observed
between the two properties in all the three directions. It is also observed that
the correlation between the mean initial tensile modulus and the mean number
98
of drape nodes is not significant (plots not shown). It is observed that the
correlation between initial tensile modulus and drape coefficient is poor in
goat suede leathers, whereas the correlation between the two properties is
fairly good for nappa leathers (sheep and cow). This may be attributed to the
removal of grain layer in suede leathers as against the presence of grain layer
in nappa leathers. In general the epidermis and the top layer of dermis (grain)
contribute more towards tensile strength and initial tensile modulus compared
to the bottom layer.
Table 4.6 Initial tensile modulus of goat suede leathers from different
sources
Initial tensile modulus (N/mm)Along backbone* Across backbone* Bias backbone#
Firm 1Leather 1 3.35 ± 0.57 3.92 ± 0.61 3.72 ± 0.77
2 3.52 ± 0.86 5.05 ± 1.03 4.67 ± 0.683 4.86 ± 0.38 5.61 ± 0.99 4.53 ± 0.584 4.21 ± 0.97 5.51 ± 0.35 4.98 ± 1.01
Firm 2Leather 5 3.87 ± 0.42 3.74 ± 0.69 3.86 ± 0.54
6 3.96 ± 0.95 4.39 ± 2.38 3.85 ± 0.367 3.32 ± 0.18 3.51 ± 0.86 3.70 ± 0.278 4.93 ± 0.42 5.87 ± 1.44 5.67 ± 1.39
Firm 3Leather 9 5.46 ± 1.87 4.68 ± 1.45 3.85 ± 1.06
10 4.70 ± 0.88 5.17 ± 1.72 4.85 ± 0.8011 3.95 ± 0.48 4.19 ± 1.35 4.39 ± 0.9312 4.64 ± 0.92 5.53 ± 1.60 4.04 ± 0.75
Firm 4Leather 13 4.08 ± 0.49 5.55 ± 1.26 5.39 ± 0.38
14 3.53 ± 0.44 4.66 ± 0.80 3.59 ± 0.5015 4.09 ± 1.38 6.43 ± 2.11 3.71 ± 0.6616 3.38 ± 1.19 3.70 ± 0.11 3.29 ± 0.56
* The values are average of eight samples along with standard deviation# The values are average of sixteen samples along with standard deviation
99
3.0 3.5 4.0 4.5 5.0 5.515
20
25
30
35
40
45
50
55
Mea
n dr
ape
coef
ficie
nt (%
)
Mean initial tensile modulus, along backbone (N/mm)
Y = -2.35 X + 40.79 R = -0.16
Figure 4.26 Mean drape coefficient versus mean initial tensile modulus
of individual goat suede leathers along backbone
3.5 4.0 4.5 5.0 5.5 6.0 6.515
20
25
30
35
40
45
50
55
Mea
n dr
ape
coef
ficie
nt (%
)
Mean initial tensile modulus, across backbone (N/mm)
Y = 4.00 X + 11.74 R = 0.37
Figure 4.27 Mean drape coefficient versus mean initial tensile modulus
of individual goat suede leathers across backbone
100
3.0 3.5 4.0 4.5 5.0 5.5 6.015
20
25
30
35
40
45
50
55
Mea
n dr
ape
coef
ficie
nt (%
)
Mean initial tensile modulus, bias backbone (N/mm)
Y = 3.23 X + 17.38 R = 0.24
Figure 4.28 Mean drape coefficient versus mean initial tensile modulus
of individual goat suede leathers bias backbone
4.5 COMPARISON OF CORRELATION COEFFICIENTS FOR
SHEEP NAPPA, COW NAPPA AND GOAT SUEDE
LEATHERS
The correlation coefficient values of the various plots between
drape coefficient and mechanical properties of sheep nappa, cow nappa and
goat suede leathers are shown in Table 4.7. It is observed that the flexural
rigidity is directly related to drape coefficient for all the three leather types in
all the three directions as seen from the values of the correlation coefficients.
In most of the cases the correlation coefficient is around 0.9. With respect to
the correlation between flexural rigidity and drape coefficient the various
apparel leathers is in the order of goat suede > sheep nappa > cow nappa. The
correlation between weight of the leather and drape coefficient is consistently
high for all the three leathers and ranges around 0.8. The above observation
indicates that the weight of the leather is one of the influencing parameter on
drape coefficient. In the case of weight versus drape coefficient the various
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apparel leathers can be arranged as sheep nappa > cow nappa > goat suede.
Next to weight, the correlation coefficient of softness versus drape coefficient
ranges around 0.7 and it is also consistent in its behavior for all the three
leather types. Regarding the correlation between softness and drape
coefficient the various apparel leathers is in the order of cow nappa > goat
suede > sheep nappa. It is interesting to note that different type of leather is
preferred over the other considering the mechanical properties namely
flexural rigidity, weight and softness with respect to drape coefficient.
Table 4.7 Correlation coefficients for the plots of drape coefficient
versus various mechanical properties for apparel leathers
Correlation Coefficient (R)Correlation plot Sheep
nappaCow
nappaGoatsuede
DC Vs Softness -0.68 -0.94 -0.77
DC Vs Flexural rigidity (Along backbone) (Across backbone) (Bias backbone)
0.880.91
--
0.780.800.79
0.910.980.98
DC Vs Thickness 0.44 -0.75 0.77
DC Vs Weight 0.83 -0.80 0.71
DC Vs Initial Tensile modulus (Along backbone) (Across backbone) (Bias backbone)
0.340.82
--
0.950.890.90
0.160.370.24
DC Vs Formability (Along backbone) (Across backbone) (Bias backbone)
0.440.34
--
-0.51-0.58-0.59
0.860.920.88
102
The correlation coefficient between thickness and drape coefficient
is good for cow nappa and goat suede but not in the case of sheep nappa. For
cow nappa and goat suede, the correlation coefficient between thickness and
DC is almost same but inverse relation is observed in the case of cow nappa.
More detailed studies are needed to analyse the drape behavior of sheep nappa
with respect to thickness. The value of formability is much lesser for goat
suede leathers compared to sheep and cow nappa leathers. On the other hand,
the formability of goat suede leathers had high correlation with drape
coefficient whereas sheep and cow nappa leathers did not show such
correlation. This observation with respect to apparel leathers are in agreement
with the observation for textile materials. Although higher formability
materials lead to low seam puckering and better sewability, such materials
possess poor drape ability (De Boos and Roczniok 1996, Griffiths and Kulke
2002).
Cow nappa leathers show good correlation for the plot of initial
tensile modulus versus drape coefficient for all the three back bone directions,
whereas the correlation is significant only across backbone for sheep nappa
leather. Only in the case of initial tensile modulus, the directional variation in
samples seems to influence the correlation coefficient. Interestingly, goat
suede leather which had high correlation with all the other mechanical
properties does not have any correlation with respect to initial tensile modulus
versus drape coefficient. This behavior may be attributed to the absence of
grain layer in suede leather as against its presence in nappa leathers as
discussed earlier in this Chapter. The correlation coefficient values for bias
backbone samples are almost similar to that of along and across backbone
samples for cow nappa and goat suede leathers.
103
4.6 MULTIPLE LINEAR REGRESSION OF DRAPE
COEFFICIENT WITH MECHANICAL PROPERTIES FOR
SHEEP NAPPA, COW NAPPA AND GOAT SUEDE
LEATHERS
All along this thesis, the correlation of drape coefficient with
individual mechanical properties were analysed using simple linear
regression. To find out the combined effect of different mechanical properties
on drape coefficient, it was decided to use multiple linear regression analysis.
Multiple linear regressions are extensions of simple linear regression with
more than one independent variable. Using Origin 7.0 software, multiple
linear regression is carried out and the regression equations were derived
keeping drape coefficient as dependent variable and other mechanical
parameters as independent variables. The regression equation and the
Coefficient of Determination (COD), (R2), for different apparel leathers are
given in Table 4.8. For this analysis, the mean values of along, across and bias
backbone of flexural rigidity, initial tensile modulus and formability values
were used.
Table 4.8 Regression equations and COD values for the multiple linear
regression of drape coefficient with various mechanical
properties for apparel leathers
Regression equation COD (R2)DC Sheep nappa = -5.56 + 5.18 S – 0.04 FR + 2.83 W -60.82 T
+ 14.57 ITM + 733 F0.96
DC Cow nappa = 140.23 + 5.65 S + 0.22 FR + 3.36 W - 68.53 T - 3.88 ITM - 2650 F
0.99
DC Goat suede = 59.59 - 4.18 S + 0.66 FR - 1.83W -22.74 T - 0.77 ITM + 71 F
0.98
S: Softness; FR: Flexural Rigidity; W: Weight; T: Thickness; ITM: Initial Tensile Modulus;
F: Formability
104
It is interesting to note that the coefficient of determination values
are above 0.96 for all the three leather types, in fact its value is 0.99 for cow
nappa leathers. The COD value is related to residual variance. The smaller the
variability of the residual values around the regression line relative to the
overall variability, the better is COD value. In other words, COD close to 1.0
indicates that we have accounted for almost all of the variability with the
variables specified in the model. From the values of COD obtained in the
multiple linear regression, it can be determined that all the selected
mechanical properties in combination are related to the drape coefficient even
though some of the mechanical properties do not correlate well with DC in
the individual correlation.
4.7 CONCLUSIONS
Drape parameters such as drape coefficient and number of nodes
were measured for goat suede leathers and correlated with some relevant
mechanical properties related to garment construction. It is found that the
drape coefficient and the number of nodes have inverse relation. In other
words, goat suede leathers having less value of drape coefficient will produce
more number of nodes and possess better drape ability. From this study, it is
observed that flexural rigidity and formability have very good correlation (R
0.8) with drape coefficient as well as number of nodes in all the three
backbone directions. Other properties such as softness, thickness and weight
have fairly good correlation (R 0.7) with both the drape parameters. Overall
all the selected mechanical properties showed good correlation with drape
coefficient and number of drape nodes, except initial tensile modulus for goat
suede leather. When compared with all types of apparel leathers namely
sheep nappa, cow nappa and goat suede, it can be concluded that the flexural
rigidity, weight and softness were most significantly related to drape
coefficient. One of the key findings is that the goat suede leathers possess
105
significantly better drape ability compared to sheep nappa and cow nappa
leathers used for apparel application. This would facilitate in the selection of
leathers for apparel requiring more fall, flexibility and textile like clothing. It
is shown that all the selected mechanical properties in combination are
significantly related to drape coefficient using multiple linear regression
analysis.
