Extrusion and Mechanical Characterization
of PVC-Leather Fiber Composites
TOMk3 J. MADERA-SANTANA’, ALBERT0 CAMPOS TORRES2,
and ALFRED0 M k Q U E Z LUCERO”
'Centre de Investigdn Cientifica de Yuc&
A. P. 87,Cordemex, C.P. 97310
Merida, Yucath, Mexico
2Escuela Militar de Ingenieros
CaLZada Mtkico-TaCuba
C.P. 1 1401,M&xico, D.F.
Every year great quantities of chrome tanned leather wastes produced by the
footwear and clothing industries are buried or burned. These practices produce
several contaminants that are released into the environment. An alternative to disposing of these wastes is to reuse them. In this work, a method to use these wastes
as filler in a polymer matrix is proposed. Also, a technique for processing the composite obtained by continuous extrusion is demonstrated. To evaluate this technique, a series of PVC-leather fiber composites were prepared and extruded
through a flat die to produce sheets. The process produced a leather-like sheet that
could be used in several applications. The influence of the filler content on the
processability and the final properties of the composite sheets were evaluated. The
tests revealed that the sheets are flexible and exhibit s d c i e n t water absorption to
be suitable for several applications in the footwear and clothing industry. Finally,
the tests show that this composite can be formulated and processed at high productivity levels and at a low cost.
XNTRODUCTION
T
housands of tonnes of chrome tanned leather
wastes are generated every month by the shoe
and clothing industries. Currently, in several countries, these kinds of waste are burned, or, even worse,
are buried in suburban fields. These practices produce several problems as,for instance, the production
of toxic chromium compounds that are dissolved in
water and filtered to the ground table waters. Nonetheless it is important to stress that instead of burying or incinerating the wastes it is feasible to use
them as fillers in several types of polymeric composites. Various methods using leather fibers as fillers in
thermoplastics or rubbers have been reported in the
literature (1-8). These methods permit the production
of a ‘semi-synthetic leather” that is useful in several
applications (for example in the footwear industry).
Indeed, currently some commercially available “recycled leathers” are being produced from leather wastes.
However, they are fabricated using low speed or batch
T o whom correspondenceshould be addrrssed.
POLYMER COMPOSITES, AUGUST 1998, Vol. 19, No. 4
processing methods such as compression molding or
calendering. As a result, the cost of these materials is
too high and their properties too poor to be competitive in the market. An interesting alternative is to produce flexible and resistant composites using high
speed processing techniques such as extrusion. In
this work, leather waste was ground to obtain bundles of short leather-fibers. These bundles were incorporated into a highly plasticized poly(viny1 chloride)
(pPVC) matrix at 180°C. The composite obtained was
processed by extrusion in order to create sheets. The
influence of the filler content on the processability and
the final properties of the composite sheets was evaluated in order to assess their suitability for eventual
application in the footwear and clothing industries.
ExPERlMEnlTAL
Materialm
F‘1yr kilograms of leather burrs obtained during the
processing of chrome tanned leathers were kindly furnished by the Fabrica de Vestimenta y Equipo (FAVESEDENA) of Mexico City. After washing and drymg the
waste was grounded in a Paganini-Diconet Miller,
431
Tomas J. Madera-Santana, Albert0 Campos Torres, and Alfred0 Marquez Lucero
Model 1520, in order to get micro-bundles of leather
fibers. These micro-bundles were sieved using a 2 mm
mesh. The aspect ratio distribution of the fibers within the bundles was evaluated using an optical ICM
Zeiss microscope. From this data two aspect ratio averages were evaluated. The number average aspect
ratio is expressed as:
yd),
(WrI = p"
(1)
and the weight average aspect ratio is expressed as:
?M/&
(W)W
=
7
Tests
Values
Humidity (wW0)
Greases and oils (wtYo)
Ash (wP/o)
Chrome oxide (wW0)
pH in water extract
Nitrogen (wW0)
Protein (wtYo)
Descomposition temp. ("C)
Diameter average (pm)
Length average (pm)
7.92 2 0.22
1.97 2 0.36
12.86 2 0.20
3.41 t 0.10
4.15 5 0.20
9.71 2 1.41
54.58 5 3.80
323.0 2 10.0
4.52 2 0.03
258.5 2 2.50
(2)
when n, is the number of fibers with a n aspect ratio
Wdi.
Both statistical parameters were calculated from the
aspect ratio distribution of a sample of 200 fibers. It
must be mentioned that the weight average ( l / d wis
commonly accepted as the parameter that best describes the aspect ratio influence on several phenomena because of its correlation with fiber volume fraction
through the accumulated fiber aspect ratio term (9).
Chemical Characterization of Leather Waste
The leather fibers were characterized following procedures established by the American Society for Testing and Materials (ASTM) (10, 11).The moisture content and the amount of hexane-soluble lubricant
(grease and oil) were determined according to ASTM
379 and 3495-83, respectively. The ash content was
evaluated following ASTM 2617-69. The degree of tannage of the leather samples was estimated by measuring the chromic oxide content (in general, properly
tanned leather should contain 3 to 5 wtYo). Using an
acid digestion method following ASTM 2807, the
leather stability was evaluated by measuring the pH
value of its aqueous extract. The value measured
(4.15 ? 0.2) was well within the limits of 3 to 5 commonly considered as t h e boundary condition for
tanned leather to be stable and un-degraded (11).
Likewise, no damage by micro-organism decomposition was observed in the wastes and the protein substance content in the leather was evaluated according
to ASTM 2868. This method uses the nitrogen percentage in the leather samples as a measure of the
protein substance proportion. The nitrogen content
was measured by digestion of the samples with acid
in the presence of a catalyst to convert the nitrogen to
ammonium ions. The quantity of ammonia in the
boric acid solution was determined by back titration
with standardized acid. The results of all the above
test are shown in Table 1 .
A plasticized poly(viny1 chloride) (pPVC), commercially available as Iztablend 143-9C-00 (supplied by
Polimeros de Mckico S.A.), was used as the composite
matrix. This resin contained 45.8 wt% plasticizer
(dioctyl phthalate) and 0.5 wt% of commercial stabilizers and antioxidants. The average molecular weight
(Mw)of the pristine WC was 86,880. A processing
432
Table 1. Physico-Chemical Characterization
of Leather Waste.
range between 140 and 180°C was recommended by
the supplier, and in order to produce low polymer viscosity during the extrusion tests a processing temperature of 180°Cwas chosen.
To evaluate the thermal stability of the leather
fibers during processing they were submitted to a
thermogravimetric testing using a Perkin-Elmer
Thermal Analyzer Model TGS-2, under an inert a t mosphere, temperature range from 60 to 600°C. and
at a heating rate of 10°C/min. The thermogravimetric
results (see Fig. 1 ) confirm that leather fibers contain
a high percentage of moisture, which leads to a significant weight loss at relative low temperatures (T <
125°C).indicating that short leather fibers should be
dried prior to processing (12).Furthermore, a moderate but constant mass loss takes place in the range
150"C-3OO0C.This is mainly due to the removal of the
crystallized water and some volatile components such
as oils and greases of low molecular weight from the
fibers (8, 13). A considerable mass loss is observed
from 300 to 600°C as a result of extensive protein
degradation and calcination.
It is observed that the pWC matrix is stable up to
I00 -
80
-
60
-
1
40 -
20
, , -,
"m
e;eL
- - - pPVCmatrix
I
, , , ,
,
, , , ,
,,
m, I
,
, ,
0
0
100
200
300
400
500
600
700
Temperature, OC
Fig. 1. ?hermogram of short leatherjiber and pPVC m a b i ~ ~
POLYMER COMPOSITES, AUGUST 1998, Vol. 19, No. 4
Extrusion and Mechanical Characterization
Table 2. Temperature Profile During the Pelletization
Process in a Single Screw Extruder.
Barrel
Zone 1 (“C)
155
Die
Zone 3 (“C)
165
Zone 2 (“C)
160
Zone 4 (“C)
170
Table 3. Temperature Profile Used During the Sheet
Extrusion Process.
Z1
22
23
(“C) (“C) (“C)
150
155
165
24
(“C)
170
25
Z6
27
Z8
Z9
(“C) (“C) (“C) (“C) (“C)
175
180
185
180
180
230°C. after which it suffers substantial decomposition. In line with the previous results, once that moisture has been released, the composites (leather fibers
in pWC) are reasonably stable below 200°C. Therefore, minimal thermal degradation of both components after processing may be expected.
c-
-on
and Extnwion
All the short leather fibers were dried in a recirculating convection oven at 105°C for 24 h to remove superficial moisture. After drymg, systematically varied
formulations of pWC composites containing 10, 20,
30, 40, 50, and 60 wtYo of leather fibers were preblended (to ensure homogeneity) and then compounded using a single screw extruder (Nieto Model HD125,
31 mm diameter and L/D = 2 1/ 1).The three heating
zones on the barrel and one on the die were activated
to produce a linear temperature profile as reported in
Table 2. After running the extruder, the melted pWC-
Chrome tanned
leather wastes
leather fiber compound was passed through a water
bath and then chipped into pellet form using a
Brabender Lab pelletizer Model 10-1272. As in the
first extrusion step, each pelletized formulation was
dried for 8 h at 105°C prior to use.
In order to obtain sheets, a second extrusion step
was performed in a semi-industrial single screw extruder (Nieto Model HD60/24, 60 mm diameter and
L/D = 24/1). The nine heating zones of the barrel
were activated to produce the temperature profile
shown in Table 3. The die temperature and the screw
speed were 180 and 40 rpm, respectively, and a slit
die of 600 mm width was adapted. The gap between
the die lips was 1.5 mm. Subsequently, the extruded
sheets were cooled in a calender using three 45 mm
diameter rollers. A flow chart of the entire preparation
process is shown in Rg. 2. Using the described procedure 20 kg of each formulation was processed.
From known extrusion residence times and corresponding material weights, the output rates of the operation were determined. Imperfect edges of samples
were trimmed using a cutter positioned between the
two rollers and horizontally disposed with a span of
50 cm. The sheets were wound and unwound between
these rollers to check the finishing quality as well as
its length. The final width of the sheets were measured and registered for each formulation and the average thickness of sheets measured by taking ten
readings between the central zone and the edges in
each case.
-
compo.itachprocterirption
A series of specimens for mechanical testing were
cut at three different orientations (0”.
45” and 90”)to
Grinding
and drying
Adition of
pPVC
Pelletization
Sheet extrusion
and calendering
Useful width
Calibration
process
Mechanical
evaluation
m.2. schematicprocedure for the composite prepamtion process.
POLYMER COMPOSITES, AUGUST 1998, Vol. 19, No. 4
433
Tom6.s J . M a d e r a - S a n t a n a , Albert0 Campos Torres, and Alfedo M a r q u e 2 Lucero
the extrusion flow direction. An automatic hollow die
punch (CEAST Model 6050) was used to produce tensile specimens following ASTM D-638-82 (Type N)
Method. The tensile properties were measured using
an universal tensile machine (Instron Model 1125).
The crosshead speed used was 10 mm/min and a
minumum of six samples tested for each composite
formulation. The data produced was processed using
Instron Series M software and the fracture surfaces of
the tensile specimens micro-photographed using a
scanning electron microscope JEOL JSM-5400 LV.
Water absorption tests were performed on 1.0 X 4.0
cm samples cut from each composite without the superficial layers that were removed by sanding. The tests
involved submerging the samples in distilled water for
periods of 8 and 24 h. The static absorption of water
was determined gravimetrically following the ASTM
D- 1815-70method and the density of each composition
was determined from ASTM D-792. This enabled the
fiber volume fraction of each sample to be calculated.
RESULTS
Leather fibers are mainly composed of collagen.
Collagen itself is made up of three polypeptides of approximately equal molecular weight arranged as a
triple helix. These molecules form into protofibrils that
further aggregate to produce leather fibers. The fibers
are grouped in bundles that constitute a three-dimensional "woven fabric" (14).m e 3 shows some m i cal fiber bundles obtained after grinding. These bundles typically are formed by 10 to 50 fibers of similar
aspect ratio slightly intermeshing among themselves.
The aspect ratio distribution of the fibers contained in
the bundles is shown in Fig. 4.mom this distribution
the aspect ratio averages by number (Z/dj, = 45.1 and
by weight ( l / d j W = 66.1 were calculated. The corresponding polydispersity index was - 1.46.
It is important to indicate that many of the following
results are expressed as relative magnitudes for each
formulation experimented. All the relative magnitudes
(M,) used are defined as the quotient between the
value corresponding to the composite (M,)and this
corresponding to the pristine matrix (M0):
b
Fig. 3. Micrographs of short ieatherjibers.
40
35
i
Leather fibers
M, = MJMo
Also, it is important to mention that formulations
with a fiber content > 40 wtYo were difficult to process
and are not reported. m
e 5 shows the relative output rate of extruded sheet as a function of the composite fiber content. An important dropoff in output
rate is observed at high fiber contents. The evolution
of relative useful width and thickness of extruded
sheets is plotted as a function of the fiber content in
Fig. 6. It can be seen that an increase in the fiber content reduces the useful width of the composite sheet
by up to %Yo, while the thickness increases by 40%.
Concerning the mechanical properties of the composites, Figs. 7 a - b show the fracture surfaces often-
434
LL
15
10
5
0
0
50
100
150
200
250
300
350
Aspect ratio ( / I d 1
Fig. 4. Aspect ratio R / d l disirhution of short e m s .
POLYMER COMPOSITES, AUGUST 1998, Vol. 19, No. 4
Extrusion and Mechanical Characterization
\
d
0.8 -
6
U
e
5 0.6
n
C
a
0
w
2
Y
-m
04-
2
Q, (pPVC) = 375.0 2 5.0g h i n
1
o'2
0.0
10
0
40
30
20
Fiber content, wt %
Q. 5. Relative extrusion output rate as afunction of the
lee*
cuntent.
w,,(pPVC)=
%- 1.00 -
3
a
5
a
0.95
-:
f
;D
- 1.3 2
E.
m
-
0.90-
"
1.2
g
X
-a
0.85
p:
0.80 -
c
582.0+0.5mm
b
8
-
- 1.1
a3
.-
Q. 7. Micrographs of compositefmcture surfaces.
+ Relative useful mdth, w,
&
1.2
Relative thtdtness. 1,
li
0 70
0
10
20
30
40
Fiber content, wt %
Q. 6. Average values of relatiw usefur width and thickness
of ewbuded sheets.
d
i%\
Go(pPVC)= 5.81 MPa
4
cn
sile samples with 20 wto/o leather fiber at 0" and 90".
respectively. In these surfaces preferential fiber orientations are clearly absent. This may be due to the
ability of the flexible fibers to bend and coil during
flow thus avoiding alignment (15).Furthermore, visual surface inspection of the sheets indicate that the
fibers are dispersed in the composite up to a fiber
content of 30 W w o . At this leather composition a significant number of fiber bundles were noticeable to
the naked eye. These bundles could have emerged
from the original fiber bundles that were not dispersed by the shear stress flow during the processing
or h m the relatively long flexible leather fibers in the
molten polymer.
POLYMER COMPOSl7ES,AUGUST 1998, Vol. 19, No. 4
oo
-.-45"
C
0.2 -
435
Tonuis J . Madera-Santana, Albert0 Campos Torres, and Alfred0 Marquez Lucero
15
I
Orientation of tensile test
+ 0"
--c 45"
4 90"
ru-
-aa
In
TI
0
E
.0
C
-m
In
0)
.->
-m
d
Q)
CI
0 1 ,
0
I
I
I
I
10
20
30
40
0
1
0
elastic modulus values at 0
'
. 45" and 90"to
the exbusionjbw direction
Futhermore, Rg.8 illustrates the variation in tensile
strength as a function of the fiber content. The presence of leather fibers in the pWC matrix produces an
important drop in tensile strength until a minimum
value is reached a t a fiber content of 20 wt%.
However, a slight increase of this parameter with the
fiber content is observed at higher compositions. Also,
although tensile strength was measured at three different orientations to the extrusion flow (direction OD),
no anisotropy was observed. This behavior was expected because the leather fibers did not show any
tendency towards preferred orientations.
Elastic modulus is shown to be strongly influenced
by fiber content, especially at compositions > 30 Wh
(Rg.9). Moreover, it is noted that there are few anisotropic effects.
Finally, Flg. 10 illustrates how water absorption increases with fiber content. Assuming for the present
purposes that pWC is a hydrophobic matrix and the
leather fibers are hydrophobic, we noted that the
water absorption of the sheets with 30 and 40 wtYo of
leather fibers is greater than the standard required by
the footwear industry (30%) and hence could have
commercial significance.
DISCUSSION
Flow properties of pWC-short leather fiber suspensions, in similar formulations and processing temperature as those used in this work, have been reported
previously (151.It has been shown that the addition of
short leather fibers into pWC matrices produces an
increase in viscometry-torque values as illustrated in
liyg. 1 1 . This is due to viscosity increases in the system caused by interaction between the fibers (Rg.12).
However, suspensions with fiber content between 0.45
436
30
40
20
Fiber content, wt K
Fiber content, wt %
Q. 9. Re-
~
10
Q. 10. Water absorption of pPVC-leatherj b r composites
a m 8 and 24 h of immersion.
/?
+
A
+
3.15radls
5.25radls
0 '
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Fiber content, wt %
Q. 1 1 . Torque ualues of pPVC-&&Jiber
composites measured by torquerheometry, a)er M t v q u Q et aL (15).
and 0.55 v01Yo show a reduction in torque and viscosity values. This is related to the appearance of voids
and fissures in the material when removed from the
torque-rheometer. From previous observations, the
critical pigment volume concentration (CPVC) described by Patton (16)was found to be exceeded. In
other words, at this concentration maximum effective
fiber packing is achieved and the addition of further
fibers to the pWC no longer produces a homogeneous
composite, but leads to the formation of voids and
discontinuities in the matrix. These discontinuities
POLYMER COMPOSITES, AUGUST 1998, Vol. 19, No. 4
Extrusion and Mechanical Characterization
qa(pPVC,Iradk) = 1.077 5 0.003KPa.s
0
-+
1
2.10ndls
v
+
F
.-dcn
rp
4.20rad/s
5.25 rad/s
+ I
4 -
l
1
0.6
D
.-
0
3 -
Q
.->
-m
c)
P
I
T=18OoC
J
0
.-cn>
Relative consistency index, m,
Power law index, n
- 0.4
s-.
J
n
2
~
0.0
I
I
I
I
0.2
0.4
0.6
0.8
0.0
Fiber content, wt O
h
Fig. 12. Relatiue viscosity values of pWC-katkjiber composites measured at 180°C,a&r M&rquez et al. (1 5).
make further production of continuous sheets by extrusion impossible.
The relationship between the viscosity (q) and the
shear rate (i.) of the pWC-leather fiber suspensions
(the suspension forms when the pWC is melted at
180°C),may be fairly well described by a power law
equation:
(3)
where rn and n are the consistency and power law indexes respectively. Figure 13 shows the values of
these indices as a function of the fiber content. The
suspension behavior becomes more viscous as the
fiber content was increased (17). Evidently, this
change in the rheological properties of the suspension
produces a significant modification of the flow profile
in the extrusion die. The flow rate in this zone is susceptible to change in the flow distribution of the suspension over the width of the die before it reaches the
final lip for thickness adjustment. Any slight change
in the rheology of the suspension leads to instability,
which may produce irregularities observed in the edge
zone. It also explains why these irregulanties increase
with fiber content.
The fact that the sheet thickness increases with
fiber content is mainly due to calendering. Indeed, the
polymer matrix is more easily compressed than the
filled composite and some factors such as plastic recovery, swelling and relaxation of both components
will also have an important influence on this phenomenon.
Concerning the mechanical properties of the sheets,
these are affected by several parameters particularly
fiber characteristics such as length, orientation, dispersion, geometry and degree of interfacial adhesion
POLYMER COMPOSTTES, AUGUST 1998, Vol. 19, No. 4
8a
0.0
0.2
0.3
0.4
0.5
0.2
X
;
0.6
Fiber Content, wt%
Fig. 13. Power law index values of pWC-leatherjibercomposites measured at 180°C *
a
Mcjrquez et al. (15).
to the matrix (18-23). A number of authors claim that
the main factors controlling the properties of a fiberfilled composite are the critical length of the fiber and
interfacial shear strength between fiber and matrix.
The critical length of the fiber (fc)in composites is a
parameter that determines the amount of stress
transferred to the fiber. In a study carried out by
Termonia (24), the effect of fiber characteristics on the
mechanical properties of short fiber-reinforced composites has revealed that the influence of fiber orientation on elastic modulus and tensile strength of the
composite is weak, but an optimum fiber aspect ratio
is essential for effective reinforcement. A micro-failure
mechanism that originates at the fiber ends and propagates along the fiber-matrix interface without the
fiber breaking is reported as being common.
On the other hand, interfacial shear strength effects
produced on the surface of the fiber are said to be due
to the 'shear lag" between fiber and matrix during
composite failure. Monette (25) reports that the critical aspect ratio is related to interfacial shear strength
and fiber strength only and not to matrix properties.
Thus it is also essential to improve the interfacial
shear strength to obtain a stronger composite.
In the case of the current composites, micro-photographs of the fracture surfaces of the samples (Figs.
7a and 7b)reveal that the leather fibers are well distributed over the surface as well as a poor adhesion of
leather fibers to the pWC matrix. Indeed, fibers that
have been pulled from the matrix do not show any
signs of polymer residues on their surfaces. This lack
of adhesion is mainly due to chemical and physical
differences between leather fibers (made up of collagen macromolecules)and thermoplastic. Also. it is aPparent that the fiber critical aspect ratio needed to im437
Tomcls J . Madera-Santana. Albedo Campos Torres, and Alfiedo Marquez Lucero
prove mechanical properties is not reached in the current case. This leads to a fall in tensile strength as the
fiber content is increased. However, it is observed that
there is a composition level, beginning a t 20 wt%.
where the tensile strength increases with the fiber
content. At this composition, bundles and agglomerates of leather fibers begin to appear. The addition of
further leather fiber contributes to a n increase in the
number and size of agglomerates, but not the number
of isolated fibers. This means that the number of microfractures originated in such fibers remains constant, leading to a net improvement of tensile properties a s the nonfissure-producing reinforcement
volume fraction is increased. This is a kind of compound that "benefits" from having an increase in the
number of fibers not leading to fissure. This result
produces a positive balance and a consequential increase in tensile strength.
To increase the tensile strength of the composites
described it has been found there are two alternatives:
a) to increase the fiber length and b) to increase the
compatibility of fibers with the matrix. The first factor
can be improved by a method patented by Picagli et
aL (26).where leather wastes are submitted to a wet
fibrillation process. In this process, leather is stirred
in water to a concentration of 2-15 wt%. Then the
slurry is passed into a fibrillating apparatus in which
it is subjected to intense and prolonged rubbing or
shearing actions. The wet fibrillation is continued
until the slurry reaches a 'Canadian Standard Freeness" of about + 100 d to -800 d,with the time required to finish the process being - 2 hours. This
produces a material with unbroken fibrils capable of
being transformed into leather-like composites.
Concerning the need to improve compatibility between matrix and reinforcement, various alternative
treatments for natural fibers (for use with thermoplastics) have been carried out. In the case of thermoplastics filled with cellulosic fibers. two main methods
have been used. The first is to apply coupling agents
such a s silane. titanate. or zirconate derivatives (27.
28). The second is the superficial modification of the
fiber by polymer chain grafting. This last method may
be particularly useful for leather fiber-thermoplastic
compatibilization. Polymer grafting directly on to
leather fibers has been studied by some authors, and
the use of this kind of filler may be used to reinforce
thermoplastics (5-7).
CONCLUSIONS
The current work demonstrates the feasibility of
producing low cost "leather-like" composites (with a
leather fiber content as high as 40 wtoh)a t high production rates. The production methods described
could help eliminate the major pollution environmental problems associate with leather wastes. An increase in elastic modulus with fiber content was found
for all the composites formulations investigated. In
most cases, a decrease in tensile strength with the
438
fiber content was observed, reaching a minimum value
at 20 wtvo of reinforcement (at higher leather contents
a slight increase in strength was measured). In all
cases, the tensile and moisture absorption properties
of the composites were found to be high enough for
use in industries such as shoe manufacturing.
ACKN0WLEDGHE"S
The authors are grateful to the Consejo Nacional de
Ciencia y Tecnologia (CONACYT) Mexico, for financial
support of this research. The authors would also like
to give special thanks to Dr. Melvin 0. W. Richardson
from Loughborough University a n d to Dr. Flavio
Vazquez from Universidad Autonoma del Estado de
Mexico for their valuable personal communications
and discussions. Also we wish to thank MC. Silvia M.
Arce of the Universidad de Guadalajara and Jafet
Quijano of the Centro de Investigacion Cientifica de
Yucakin for their valuable help in the SEM microphotographs and performing calculations for this work.
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