Effect of Compatibilizer and Bamboo Fibe
EFFECT OF COMPATIBILIZER AND BAMBOO FIBER CONTENT ON
THE MECHANICAL PROPERTIES OF PP-G-MA COMPATIBILIZED
POLYPROPYLENE/BAMBOO FIBER COMPOSITES
B. C. Bonse1*, M. C. S. Mamede1, R. A. da Costa1 and S. H. P. Bettini2
1
Department of Metallurgical and Materials Engineering, Centro Universitário da FEI, São Bernardo do Campo, SP,
Brazil – [email protected]; [email protected]; [email protected]
2
Department of Materials Engineering,Universidade Federal de São Carlos, São Carlos, SP, Brazil –
[email protected]
An investigation was carried out regarding the effect of compatibilizer (maleic anhydride grafted PP) and bamboo fiber
content on the mechanical properties of polypropylene/bamboo fiber composites according to a two-level factorial
central composite design. Composites were prepared in a co-rotating twin-screw extruder and specimens were injection
molded. Effect of the variables on mechanical properties has been assessed through flexural modulus and strength,
tensile strength, elongation at break and impact strength. With the exception of impact strength, all properties assessed
showed to be significantly affected by the variable bamboo fiber content: positively in the case of tensile strength,
flexural strength and modulus; and negatively in the case of elongation and energy at break. Compatibilizer content
showed significant positive effect on all properties, except impact strength and flexural modulus. The only variable that
seemed to affect impact strength was the interaction between bamboo fiber and compatibilizer content. This interaction
effect also seemed to be significant for the tensile, but not for the flexural properties. Scanning electron microscopy
revealed the adhesion achieved between the nonpolar hydrophobic polymer matrix and the polar hydrophilic bamboo
fibers when using the compatibilizer PPgMA.
Introduction
Currently, great efforts are being made to create
sustainable eco-efficient practices and products. Within
this context biofiber reinforced polymer composites are
increasingly gaining attention as viable alternative to
synthetic fiber reinforced composites, especially glassbased ones (Coutinho and Costa, 1999; Saheb and Jog,
1999; Bettini et al., 2008). In certain composite
applications biofibers have shown to be competitive in
relation to glass fiber (Mohanty et al., 2005). In
addition to being renewable and biodegradable,
advantages of biofibers over synthetic ones include low
cost, light weight, low abrasiveness and excellent
strength to weight ratio (Sanadi et al., 1994).
However, there are some limitations in using biofibers
as reinforcement in polymers. These include
processing temperature, which should not exceed the
degradation temperature of these fibers of around 200
°C, and high moisture absorption, which may impair
mechanical properties as well as facilitate fungus
growth (Kalia et al., 2009). One of the plastics that can
be easily processed at temperatures below 200 °C is
polypropylene (PP). Compared to the most commonly
used biofibers (wood, jute, coir, sisal, banana etc)
bamboo exhibits low density and high mechanical
strength. In this investigation moso bamboo
(Phyllostachys Eduli) was used as reinforcement in PP.
However, polarity difference between hydrophobic PP
and hydrophilic bamboo results in incompatible
composites, which are not useful in load-bearing
applications.
To
overcome
this
problem
compatibilizers are required, such as maleic anhydride
grafted PP (PPgMA), which reduce interfacial stresses
and improve adhesion between the polymer and fiber
(Sanadi et al., 1994; Keener et al., 2003; Karmarkar et
al., 2007).
In this investigation the effect of compatibilizer
PPgMA and bamboo fiber content on the mechanical
properties of polypropylene/bamboo fiber composites
were assessed according to a two-level factorial central
composite design.
Experimental
Materials
KM6100 polypropylene pellets were donated by
Quattor (Mauá, Brazil) with MFI = 3.1 g/10min
(230°C/2.16 kg). Compatibilizer used was Polybond
3200 maleic anhydride grafted PP (PPgMA) with MFI
= 110 g/10 min (190 C/2.16 kg) purchased from
Crompton-Uniroyal Chemical (São Paulo, Brazil).
Bamboo fibers used were waste collected from a small
bamboo laminating mill. This usually disposed of
waste is generated during cutting, milling and finishing
operations.
Proceedings of the Polymer Processing Society 26th Annual Meeting ~ PPS-26 ~ July 4-8, 2010 Banff (Canada)
Methods
The bamboo waste fibers were ground in a MAK-250
grinder from Kie Máquinas e Plásticos (Louveira/SP),
using a 3-mm screen. Grinding required pre-drying the
fibers for 4 hours at 105°C.
PP/bamboo fiber composites were prepared according
to a central composite experimental design, shown in
Table 1, based on previous investigation by Bettini et
al. (2010) with coir fibers. Their findings indicated that
the lowest PPgMA content used, i.e. 4wt%, was
sufficient for composite compatibilization. Hence in
the present investigation it was decided to use 4wt% as
the highest level in the experimental design. Reference
formulations were also prepared (Table 2). Ground
fibers were dried for 12 hours at 105°C, as moisture
content drying oven test results indicated that after this
period moisture content remained stable (not shown).
PP pellets, compatibilizer PPgMA and ground bamboo
fibers were then pre-mixed in a Mecanoplast ML40
intensive mixer (Rio Claro/SP) for one minute and
subsequently extruded in a Thermo Scientific HAAKE
PolyLab OS RheoDrive co-rotating twin-screw
extruder. Temperature profile used was: 170°C, 175°C,
180°C, 180°C, 190°C and 185°C. Rotor frequency was
250 rpm. Extruded strand was air-cooled by means of a
fan, pelletized and stored for subsequent injection
molding of tensile, bending and impact test specimens.
Prior to injection molding pelletized composites were
dried for 12 hours at 105°C. Injection molding was
performed in an HM60/350 Battenfeld machine at the
following conditions. Injection pressure 700 bar;
holding pressure 560 bar; feed and compression section
temperature 193°C; metering section and nozzle
temperature 198°C; tool temperature 85°C; holding
time 10 s; cooling time 20 s.
The injected specimens were submitted to tensile and
flexure tests in an Instron 5565 Universal Testing
Machine at test speeds of 5 mm/min and 1.3 mm/min,
according to ASTM D638 and D790, respectively.
Impact tests were carried out in an analogical VEB
Werkstoff Prüfmaschinen Leipzig Charpy impact tester
in accordance with ASTM D 6110.
Fractured surfaces of the tensile tests were gold-coated
and analyzed in a JEOL JSM-T330A scanning electron
microscope, SEM.
Table 1 – Central Composite Design
codified
decodified
variables
variables
Assays
BF PPgMA
x
y
(%)
(%)
1
-1 -1
20
1
2
1
-1
40
1
3
-1
1
20
4
4
1
1
40
4
5
0
0
30
2.5
6
0
0
30
2.5
7
0
0
30
2.5
8
-√2 0 15.86
2.5
9
+√2 0 44.14
2.5
10
0 -√2
30
0.38
11
0 +√2
30
4.6
Table 2 – Reference Formulations
BF PPgMA
Assay x
y
(%)
(%)
12
13
-1
20
14
0
30
15
1
40
16
4.6
Results and Discussion
Analysis of PP/bamboo fiber composites
Results of the mechanical tests are listed in Table 3.
Table 3 – Results of the tensile, impact and flexural
tests: average of ten specimens, except for IS (five
specimens)
RT
Enbreak
IS
FM
break
flex
Assay
(MPa) (%) (J/mm2) (kJ/m2) (MPa) (MPa)
1 35.8 6.5
0.10
3.54
2931 45.6
2 38.2 2.6
0.04
4.63
4031 57.7
3 38.2 12.2
0.21
4.47
2761 44.0
4 45.8 4.9
0.10
5.53
3853 57.1
5 40.6 6.6
0.12
5.48
2983 46.5
6 41.1 6.1
0.11
4.56
3208 49.1
7 40.7 7.0
0.13
5.87
3010 46.6
8 38.0 11.0
0.19
4.35
2434 39.3
9 45.8 3.7
0.07
4.88
4369 62.8
10 34.1 4.0
0.06
5.22
3288 47.8
11 41.1 7.9
0.14
4.16
3383 52.3
12 34.1 >500
4.17
1802 31.4
13 32.6 9.6
0.14
5.15
2692 41.3
14 30.9 3.8
0.05
5.14
3148 43.8
15 27.6 2.5
0.03
4.45
3998 48.3
16 34.4 >500
4.40
1725 30.4
TS: tensile strength; break: strain at break; Enbreak:
energy at break (J/mm2); IS: impact strength; FM:
flexural modulus at 0.3% strain; flex: flexural stress at
2% strain
Effect of BF incorporation with no compatibilizer can
be assessed by analyzing assays 12 to 15. Tensile
Proceedings of the Polymer Processing Society 26th Annual Meeting ~ PPS-26 ~ July 4-8, 2010 Banff (Canada)
strength decreases as BF content increases (from 34.1
to 27.6 MPa, as %BF increases from 0 to 40%). A
drastic decrease in strain at break is seen when 20wt%
BF is introduced in PP, from higher than 500% to
9.61%. Increasing BF content reduces elongation
further down to 2.55% at 40wt% BF. These results can
be explained by the lack of adhesion at the interface
between the polar hydrophilic fibers and the nonpolar
hydrophobic PP. Thus, during tensile loading stress
transfer is impaired and the fibers act as stress
concentrators, resulting in composite embrittlement.
Lack of adhesion in composites without compatibilizer
is evidenced in the SEM micrograph of Figure 1a,
whereas in Figure 1b the fibers in the composite
containing compatibilizer are seen to be well adhered
to the matrix. Composite embrittlement with increasing
fiber content without compatibilizer is also observed
by the reduction in the values of energy at break (area
under the stress-strain curve) and impact strength. The
findings regarding impact strength are in conflict with
those of Bettini et al. (2010) who observed an increase
in this property at increasing coir contents above 20
wt%. However, it should be mentioned that depending
on composite nature (different fibers with different
stiffness) and type of impact test, apparent impact
strength may either increase or decrease (Nielsen,
1994). With regard to flexural tests of the
uncompatibilized composites, both flexural modulus
and strength increase markedly with increasing BF
content (assays 12 to 15, from 0 to 40wt% BF), due to
the high stiffness of this fiber in relation to the PP
matrix.
Figure 1a – SEM micrograph of tensile fractured
surface of 20wt %BF composite without PPgMA. Note
the gap between fibers and PP matrix
Figure 1b – SEM micrograph of tensile fractured
surface of 20wt% BF composite with 4wt% PPgMA.
No gaps between fibers and PP matrix
Preliminary analysis of the compatibilized composites
containing lowest and highest levels of BF and PPgMA
(assays 1 to 4) shows that increasing BF content (at the
same level of PPgMA) increases tensile, impact and
flexural strength, and reduces strain and energy at
break. Moreover, increase in PPgMA content (at the
same level of BF content) increases all assessed
properties, but flexural strength, which decreases. It
should be mentioned that comparing tensile strength of
40 wt% compatibilized with uncompatibilized
composite (assay 4 vs. 15) shows an increase in tensile
strength of 66% and with pure PP (assay 4 vs. 12) an
increase of 34%. These values are similar to those
obtained by Bettini et al. (2010) with the same amounts
of coir fiber and compatibilizer (63% and 35%),
despite the higher reported tensile strength and
modulus of bamboo fiber in relation to coir (Suddell
and Evans et al., 2005). When the same analysis is
applied to flexural modulus the compatibilized BF
composite shows an increase of 114% in relation to
pure PP, against 78% obtained by Bettini et al. Thus
fiber stiffness seems to have a stronger effect on
composite flexural properties than on tensile
properties.
For more in-depth assessment of the effect of BF and
PPgMA content - and possible interactions - on the
mechanical properties of the composites, multiple
regression analysis was performed by means of
software Statistica® on all data used to generate Table
3. The mechanical properties could be fitted by a
second order model, at significance level of 5%. Table
4 contains the coefficients of the fitted polynomial
equations: a0+a1x+a2y+a3xy+a4x2+a5y2, where x and y
are the codified variables referent to BF and PPgMA
content, respectively.
Proceedings of the Polymer Processing Society 26th Annual Meeting ~ PPS-26 ~ July 4-8, 2010 Banff (Canada)
2
The encountered R values indicate that, except for
impact strength, the model fits the data reasonably
well. The coefficients reveal whether an independent
variable (%BF, %PPgMA, %BF.%PPgMA, %BF2 and
%PPgMA2) has a positive or negative effect on the
dependent variable (mechanical property). The a1
values thus show that BF content has a positive effect
on tensile strength, flexural modulus and strength and
negative effect on elongation at break and energy to
break (area under the stress-strain curve). The a2 values
show that PPgMA content has a positive effect on all
properties shown in Table 4, except for flexural
modulus, which is not significantly affected by PPgMA
content. The a3 values show that, with the exception of
the flexural properties, all other properties are affected
by the interaction between BF and PPgMA content.
Coefficients a4 and a5 refer to the quadratic effects of
BF and PPgMA content, respectively.
The above polynomial equations allow construction of
3D contour plots showing the response of the
mechanical property as a function of BF and PPgMA
content. As example are shown 3D contour plots of a
tensile and flexural property in Figures 2 and 3. Raw
data are also shown in these plots as dots.
3D C ontour P lot of T ensile S trength (M P a) against % B F a
% P P gM A
5 .0 0 %
4 .5 0 %
4 .0 0 %
% PPgM A
3 .5 0 %
3 .0 0 %
and intermediate PPgMA contents are required. At
high BF content increase in %PPgMA seems to be
effective only at low concentrations of this
compatibilizer. At low BF content increasing PPgMA
content does not lead to high increase in tensile
strength.
3 D C o n to u r P lo t o f F lex u ral M o d u lu s @ 0 ,3 % (M P a) ag ain st %
B F an d % P P g M A
5 .0 0 %
4 .5 0 %
4 .0 0 %
3 .5 0 %
% PPgM A
Table 4 – coefficients of the polynomial equations
obtained from the second order model fit
linear terms
quadratic terms
a0
a1
a2
a3
a4
a5
TS 40.81 2.594 2.462 1.248 0.5052 -1.6623
1.717 -0.830 0.3450 -0.3516
break 6.57 -2.669
Enbr 0.1177 -0.0447 0.03699 -0.0130
-0.00262
FM 3066.8 616.5
--- 173.684 140.659
-1.9860 1.4585
flex 47.39 7.306 0.5139
IS
0.00194
R2 of fits: 0.97, 0.87, 0.94, 0.95, 0.93 and 0.11
respectively.
3 .0 0 %
2 .5 0 %
2 .0 0 %
Flexural M odulus
1 .5 0 %
(M Pa)
1 .0 0 %
> 5000
< 5000
0 .5 0 %
< 4500
0 .0 0 %
1 0 .0 0 %
< 4000
2 0 .0 0 %
1 5 .0 0 %
3 0 .0 0 %
2 5 .0 0 %
4 0 .0 0 %
3 5 .0 0 %
5 0 .0 0 %
4 5 .0 0 %
< 3500
< 3000
% BF
Figure 3 – 3D contour plot of flexural modulus against
BF and PPgMA content.
From Figure 3 increasing BF content is seen to always
increase flexural modulus, regardless of PPgMA
content. Thus, compatibilizer content has no effect on
flexural modulus as already observed when analyzing
the coefficients of the polynomial equation obtained
from the second order model fit.
Conclusions
Investigation of the effect of bamboo fiber and
compatibilizer content on mechanical properties of
PP/bamboo fiber composites showed that bamboo fiber
content has a positive effect on tensile strength,
flexural modulus and strength, and negative effect on
elongation at break and energy to break (area under the
stress-strain curve). PPgMA content showed a positive
effect on all assessed properties, except for flexural
modulus and impact strength, which showed to be
unaffected by this dependent variable. With the
exception of the flexural properties, all other properties
showed to be affected by the interaction between
bamboo fiber and PPgMA content.
2 .5 0 %
2 .0 0 %
Tens ile Strength
1 .5 0 %
(M Pa)
1 .0 0 %
PPgMA concentrations between 2 and 4wt% seem to
be sufficient for composite compatibilization.
> 50
< 50
0 .5 0 %
< 46
0 .0 0 %
1 0 .0 0 %
< 42
2 0 .0 0 %
1 5 .0 0 %
3 0 .0 0 %
2 5 .0 0 %
4 0 .0 0 %
3 5 .0 0 %
5 0 .0 0 %
4 5 .0 0 %
< 38
< 34
% BF
Figure 2 – 3D contour plot of tensile strength against
BF and PPgMA content.
It can be seen from Figure 2 that, at the investigated
levels of BF and PPgMA, to achieve high tensile
strength relatively high BF contents (around 40wt %)
Bamboo fibers can be successfully used as
reinforcement in polymer, with the aid of
compatibilizer, considering mechanical properties,
processing and environmental aspects.
Acknowledgements
Acknowledgements are due to Quattor and tiva-design
in bamboo (www.tivadesign.com.br) for donating the
polypropylene and bamboo waste fibers, respectively.
Proceedings of the Polymer Processing Society 26th Annual Meeting ~ PPS-26 ~ July 4-8, 2010 Banff (Canada)
References
Bettini S. H. P., Uliana A. T., Holzschuh D. J., “Effect
of Process Parameters and Composition on
Mechanical, Thermal and Morphological Properties of
Polypropylene/Sawdust Composites”, Appl.Polym.
Sci., 108, 2233-2241 (2008).
Bettini S. H. P., Antunes M. C., Bicudo A. B. L. C.,
Augusto I. S., Antunes L. A., Morassi P. L., Condotta,
R., Bonse B. C., “Investigation on the Use of Coir
Fiber as Alternative Reinforcement in Polypropylene”,
Journal of Applied Polymer Science (2010)
Coutinho F. M. B., Costa T. H. S., “Performance of
polypropylene-wood fiber composites”, Polym.
.Test.,18,581-587 (1999)
Kalia S., Kaith B.S., Kaur I., “Preteatments of natural
fibers and their application as reinforcing material in
polymer composites – A review”, Polymer Eng. and
Science, 49, 1253-1272 (2009)
Karmarmar A., Chauhan S. S., Modak J. M., Chanda
M., “Mechanical properties of wood-fiber reinforced
polypropylene composites: Effect of a novel
compatibilizer with isocyanate functional group”
Composites Parte A, 38, 227-233 (2007)
Keener T. J., Stuart R. K., Brown T. K., “Maleated
coupling agents for natural fibre composites”,
Composites Part A, 35, 357-362 (2004)
Mohanty A. K., Natural Fibers, Biopolymers and
Biocomposites, CRC Publishers, U.S. (2005)
Nielsen L. E, Landel R. F.: Mechanical Properties of
Polymers and Composites, 2nd Edition, Marcel Dekker
Inc, New York (1994)
Saheb D. N., Jog J. P., “Natural Fiber Polymer
Composites: A Review”, Adv. Polymer Technology,
18, 351-363 (1999)
Sanadi A. R., Caulfield D. F., Rowell R. M.,
“Reinforcing Polypropylene With Natural Fibers”,
Plastics Eng., 50, 27-28 (1994)
Suddell, B. C., Evans, W. J., “Chapter 7 Natural Fiber
Composites in Automotive Applications”, in Natural
Fibers, Biopolymers and Biocomposites, Mohanty et
al. (Ed.), Taylor &Francis Group, Florida p. 237–297
(2005).
Proceedings of the Polymer Processing Society 26th Annual Meeting ~ PPS-26 ~ July 4-8, 2010 Banff (Canada)
THE MECHANICAL PROPERTIES OF PP-G-MA COMPATIBILIZED
POLYPROPYLENE/BAMBOO FIBER COMPOSITES
B. C. Bonse1*, M. C. S. Mamede1, R. A. da Costa1 and S. H. P. Bettini2
1
Department of Metallurgical and Materials Engineering, Centro Universitário da FEI, São Bernardo do Campo, SP,
Brazil – [email protected]; [email protected]; [email protected]
2
Department of Materials Engineering,Universidade Federal de São Carlos, São Carlos, SP, Brazil –
[email protected]
An investigation was carried out regarding the effect of compatibilizer (maleic anhydride grafted PP) and bamboo fiber
content on the mechanical properties of polypropylene/bamboo fiber composites according to a two-level factorial
central composite design. Composites were prepared in a co-rotating twin-screw extruder and specimens were injection
molded. Effect of the variables on mechanical properties has been assessed through flexural modulus and strength,
tensile strength, elongation at break and impact strength. With the exception of impact strength, all properties assessed
showed to be significantly affected by the variable bamboo fiber content: positively in the case of tensile strength,
flexural strength and modulus; and negatively in the case of elongation and energy at break. Compatibilizer content
showed significant positive effect on all properties, except impact strength and flexural modulus. The only variable that
seemed to affect impact strength was the interaction between bamboo fiber and compatibilizer content. This interaction
effect also seemed to be significant for the tensile, but not for the flexural properties. Scanning electron microscopy
revealed the adhesion achieved between the nonpolar hydrophobic polymer matrix and the polar hydrophilic bamboo
fibers when using the compatibilizer PPgMA.
Introduction
Currently, great efforts are being made to create
sustainable eco-efficient practices and products. Within
this context biofiber reinforced polymer composites are
increasingly gaining attention as viable alternative to
synthetic fiber reinforced composites, especially glassbased ones (Coutinho and Costa, 1999; Saheb and Jog,
1999; Bettini et al., 2008). In certain composite
applications biofibers have shown to be competitive in
relation to glass fiber (Mohanty et al., 2005). In
addition to being renewable and biodegradable,
advantages of biofibers over synthetic ones include low
cost, light weight, low abrasiveness and excellent
strength to weight ratio (Sanadi et al., 1994).
However, there are some limitations in using biofibers
as reinforcement in polymers. These include
processing temperature, which should not exceed the
degradation temperature of these fibers of around 200
°C, and high moisture absorption, which may impair
mechanical properties as well as facilitate fungus
growth (Kalia et al., 2009). One of the plastics that can
be easily processed at temperatures below 200 °C is
polypropylene (PP). Compared to the most commonly
used biofibers (wood, jute, coir, sisal, banana etc)
bamboo exhibits low density and high mechanical
strength. In this investigation moso bamboo
(Phyllostachys Eduli) was used as reinforcement in PP.
However, polarity difference between hydrophobic PP
and hydrophilic bamboo results in incompatible
composites, which are not useful in load-bearing
applications.
To
overcome
this
problem
compatibilizers are required, such as maleic anhydride
grafted PP (PPgMA), which reduce interfacial stresses
and improve adhesion between the polymer and fiber
(Sanadi et al., 1994; Keener et al., 2003; Karmarkar et
al., 2007).
In this investigation the effect of compatibilizer
PPgMA and bamboo fiber content on the mechanical
properties of polypropylene/bamboo fiber composites
were assessed according to a two-level factorial central
composite design.
Experimental
Materials
KM6100 polypropylene pellets were donated by
Quattor (Mauá, Brazil) with MFI = 3.1 g/10min
(230°C/2.16 kg). Compatibilizer used was Polybond
3200 maleic anhydride grafted PP (PPgMA) with MFI
= 110 g/10 min (190 C/2.16 kg) purchased from
Crompton-Uniroyal Chemical (São Paulo, Brazil).
Bamboo fibers used were waste collected from a small
bamboo laminating mill. This usually disposed of
waste is generated during cutting, milling and finishing
operations.
Proceedings of the Polymer Processing Society 26th Annual Meeting ~ PPS-26 ~ July 4-8, 2010 Banff (Canada)
Methods
The bamboo waste fibers were ground in a MAK-250
grinder from Kie Máquinas e Plásticos (Louveira/SP),
using a 3-mm screen. Grinding required pre-drying the
fibers for 4 hours at 105°C.
PP/bamboo fiber composites were prepared according
to a central composite experimental design, shown in
Table 1, based on previous investigation by Bettini et
al. (2010) with coir fibers. Their findings indicated that
the lowest PPgMA content used, i.e. 4wt%, was
sufficient for composite compatibilization. Hence in
the present investigation it was decided to use 4wt% as
the highest level in the experimental design. Reference
formulations were also prepared (Table 2). Ground
fibers were dried for 12 hours at 105°C, as moisture
content drying oven test results indicated that after this
period moisture content remained stable (not shown).
PP pellets, compatibilizer PPgMA and ground bamboo
fibers were then pre-mixed in a Mecanoplast ML40
intensive mixer (Rio Claro/SP) for one minute and
subsequently extruded in a Thermo Scientific HAAKE
PolyLab OS RheoDrive co-rotating twin-screw
extruder. Temperature profile used was: 170°C, 175°C,
180°C, 180°C, 190°C and 185°C. Rotor frequency was
250 rpm. Extruded strand was air-cooled by means of a
fan, pelletized and stored for subsequent injection
molding of tensile, bending and impact test specimens.
Prior to injection molding pelletized composites were
dried for 12 hours at 105°C. Injection molding was
performed in an HM60/350 Battenfeld machine at the
following conditions. Injection pressure 700 bar;
holding pressure 560 bar; feed and compression section
temperature 193°C; metering section and nozzle
temperature 198°C; tool temperature 85°C; holding
time 10 s; cooling time 20 s.
The injected specimens were submitted to tensile and
flexure tests in an Instron 5565 Universal Testing
Machine at test speeds of 5 mm/min and 1.3 mm/min,
according to ASTM D638 and D790, respectively.
Impact tests were carried out in an analogical VEB
Werkstoff Prüfmaschinen Leipzig Charpy impact tester
in accordance with ASTM D 6110.
Fractured surfaces of the tensile tests were gold-coated
and analyzed in a JEOL JSM-T330A scanning electron
microscope, SEM.
Table 1 – Central Composite Design
codified
decodified
variables
variables
Assays
BF PPgMA
x
y
(%)
(%)
1
-1 -1
20
1
2
1
-1
40
1
3
-1
1
20
4
4
1
1
40
4
5
0
0
30
2.5
6
0
0
30
2.5
7
0
0
30
2.5
8
-√2 0 15.86
2.5
9
+√2 0 44.14
2.5
10
0 -√2
30
0.38
11
0 +√2
30
4.6
Table 2 – Reference Formulations
BF PPgMA
Assay x
y
(%)
(%)
12
13
-1
20
14
0
30
15
1
40
16
4.6
Results and Discussion
Analysis of PP/bamboo fiber composites
Results of the mechanical tests are listed in Table 3.
Table 3 – Results of the tensile, impact and flexural
tests: average of ten specimens, except for IS (five
specimens)
RT
Enbreak
IS
FM
break
flex
Assay
(MPa) (%) (J/mm2) (kJ/m2) (MPa) (MPa)
1 35.8 6.5
0.10
3.54
2931 45.6
2 38.2 2.6
0.04
4.63
4031 57.7
3 38.2 12.2
0.21
4.47
2761 44.0
4 45.8 4.9
0.10
5.53
3853 57.1
5 40.6 6.6
0.12
5.48
2983 46.5
6 41.1 6.1
0.11
4.56
3208 49.1
7 40.7 7.0
0.13
5.87
3010 46.6
8 38.0 11.0
0.19
4.35
2434 39.3
9 45.8 3.7
0.07
4.88
4369 62.8
10 34.1 4.0
0.06
5.22
3288 47.8
11 41.1 7.9
0.14
4.16
3383 52.3
12 34.1 >500
4.17
1802 31.4
13 32.6 9.6
0.14
5.15
2692 41.3
14 30.9 3.8
0.05
5.14
3148 43.8
15 27.6 2.5
0.03
4.45
3998 48.3
16 34.4 >500
4.40
1725 30.4
TS: tensile strength; break: strain at break; Enbreak:
energy at break (J/mm2); IS: impact strength; FM:
flexural modulus at 0.3% strain; flex: flexural stress at
2% strain
Effect of BF incorporation with no compatibilizer can
be assessed by analyzing assays 12 to 15. Tensile
Proceedings of the Polymer Processing Society 26th Annual Meeting ~ PPS-26 ~ July 4-8, 2010 Banff (Canada)
strength decreases as BF content increases (from 34.1
to 27.6 MPa, as %BF increases from 0 to 40%). A
drastic decrease in strain at break is seen when 20wt%
BF is introduced in PP, from higher than 500% to
9.61%. Increasing BF content reduces elongation
further down to 2.55% at 40wt% BF. These results can
be explained by the lack of adhesion at the interface
between the polar hydrophilic fibers and the nonpolar
hydrophobic PP. Thus, during tensile loading stress
transfer is impaired and the fibers act as stress
concentrators, resulting in composite embrittlement.
Lack of adhesion in composites without compatibilizer
is evidenced in the SEM micrograph of Figure 1a,
whereas in Figure 1b the fibers in the composite
containing compatibilizer are seen to be well adhered
to the matrix. Composite embrittlement with increasing
fiber content without compatibilizer is also observed
by the reduction in the values of energy at break (area
under the stress-strain curve) and impact strength. The
findings regarding impact strength are in conflict with
those of Bettini et al. (2010) who observed an increase
in this property at increasing coir contents above 20
wt%. However, it should be mentioned that depending
on composite nature (different fibers with different
stiffness) and type of impact test, apparent impact
strength may either increase or decrease (Nielsen,
1994). With regard to flexural tests of the
uncompatibilized composites, both flexural modulus
and strength increase markedly with increasing BF
content (assays 12 to 15, from 0 to 40wt% BF), due to
the high stiffness of this fiber in relation to the PP
matrix.
Figure 1a – SEM micrograph of tensile fractured
surface of 20wt %BF composite without PPgMA. Note
the gap between fibers and PP matrix
Figure 1b – SEM micrograph of tensile fractured
surface of 20wt% BF composite with 4wt% PPgMA.
No gaps between fibers and PP matrix
Preliminary analysis of the compatibilized composites
containing lowest and highest levels of BF and PPgMA
(assays 1 to 4) shows that increasing BF content (at the
same level of PPgMA) increases tensile, impact and
flexural strength, and reduces strain and energy at
break. Moreover, increase in PPgMA content (at the
same level of BF content) increases all assessed
properties, but flexural strength, which decreases. It
should be mentioned that comparing tensile strength of
40 wt% compatibilized with uncompatibilized
composite (assay 4 vs. 15) shows an increase in tensile
strength of 66% and with pure PP (assay 4 vs. 12) an
increase of 34%. These values are similar to those
obtained by Bettini et al. (2010) with the same amounts
of coir fiber and compatibilizer (63% and 35%),
despite the higher reported tensile strength and
modulus of bamboo fiber in relation to coir (Suddell
and Evans et al., 2005). When the same analysis is
applied to flexural modulus the compatibilized BF
composite shows an increase of 114% in relation to
pure PP, against 78% obtained by Bettini et al. Thus
fiber stiffness seems to have a stronger effect on
composite flexural properties than on tensile
properties.
For more in-depth assessment of the effect of BF and
PPgMA content - and possible interactions - on the
mechanical properties of the composites, multiple
regression analysis was performed by means of
software Statistica® on all data used to generate Table
3. The mechanical properties could be fitted by a
second order model, at significance level of 5%. Table
4 contains the coefficients of the fitted polynomial
equations: a0+a1x+a2y+a3xy+a4x2+a5y2, where x and y
are the codified variables referent to BF and PPgMA
content, respectively.
Proceedings of the Polymer Processing Society 26th Annual Meeting ~ PPS-26 ~ July 4-8, 2010 Banff (Canada)
2
The encountered R values indicate that, except for
impact strength, the model fits the data reasonably
well. The coefficients reveal whether an independent
variable (%BF, %PPgMA, %BF.%PPgMA, %BF2 and
%PPgMA2) has a positive or negative effect on the
dependent variable (mechanical property). The a1
values thus show that BF content has a positive effect
on tensile strength, flexural modulus and strength and
negative effect on elongation at break and energy to
break (area under the stress-strain curve). The a2 values
show that PPgMA content has a positive effect on all
properties shown in Table 4, except for flexural
modulus, which is not significantly affected by PPgMA
content. The a3 values show that, with the exception of
the flexural properties, all other properties are affected
by the interaction between BF and PPgMA content.
Coefficients a4 and a5 refer to the quadratic effects of
BF and PPgMA content, respectively.
The above polynomial equations allow construction of
3D contour plots showing the response of the
mechanical property as a function of BF and PPgMA
content. As example are shown 3D contour plots of a
tensile and flexural property in Figures 2 and 3. Raw
data are also shown in these plots as dots.
3D C ontour P lot of T ensile S trength (M P a) against % B F a
% P P gM A
5 .0 0 %
4 .5 0 %
4 .0 0 %
% PPgM A
3 .5 0 %
3 .0 0 %
and intermediate PPgMA contents are required. At
high BF content increase in %PPgMA seems to be
effective only at low concentrations of this
compatibilizer. At low BF content increasing PPgMA
content does not lead to high increase in tensile
strength.
3 D C o n to u r P lo t o f F lex u ral M o d u lu s @ 0 ,3 % (M P a) ag ain st %
B F an d % P P g M A
5 .0 0 %
4 .5 0 %
4 .0 0 %
3 .5 0 %
% PPgM A
Table 4 – coefficients of the polynomial equations
obtained from the second order model fit
linear terms
quadratic terms
a0
a1
a2
a3
a4
a5
TS 40.81 2.594 2.462 1.248 0.5052 -1.6623
1.717 -0.830 0.3450 -0.3516
break 6.57 -2.669
Enbr 0.1177 -0.0447 0.03699 -0.0130
-0.00262
FM 3066.8 616.5
--- 173.684 140.659
-1.9860 1.4585
flex 47.39 7.306 0.5139
IS
0.00194
R2 of fits: 0.97, 0.87, 0.94, 0.95, 0.93 and 0.11
respectively.
3 .0 0 %
2 .5 0 %
2 .0 0 %
Flexural M odulus
1 .5 0 %
(M Pa)
1 .0 0 %
> 5000
< 5000
0 .5 0 %
< 4500
0 .0 0 %
1 0 .0 0 %
< 4000
2 0 .0 0 %
1 5 .0 0 %
3 0 .0 0 %
2 5 .0 0 %
4 0 .0 0 %
3 5 .0 0 %
5 0 .0 0 %
4 5 .0 0 %
< 3500
< 3000
% BF
Figure 3 – 3D contour plot of flexural modulus against
BF and PPgMA content.
From Figure 3 increasing BF content is seen to always
increase flexural modulus, regardless of PPgMA
content. Thus, compatibilizer content has no effect on
flexural modulus as already observed when analyzing
the coefficients of the polynomial equation obtained
from the second order model fit.
Conclusions
Investigation of the effect of bamboo fiber and
compatibilizer content on mechanical properties of
PP/bamboo fiber composites showed that bamboo fiber
content has a positive effect on tensile strength,
flexural modulus and strength, and negative effect on
elongation at break and energy to break (area under the
stress-strain curve). PPgMA content showed a positive
effect on all assessed properties, except for flexural
modulus and impact strength, which showed to be
unaffected by this dependent variable. With the
exception of the flexural properties, all other properties
showed to be affected by the interaction between
bamboo fiber and PPgMA content.
2 .5 0 %
2 .0 0 %
Tens ile Strength
1 .5 0 %
(M Pa)
1 .0 0 %
PPgMA concentrations between 2 and 4wt% seem to
be sufficient for composite compatibilization.
> 50
< 50
0 .5 0 %
< 46
0 .0 0 %
1 0 .0 0 %
< 42
2 0 .0 0 %
1 5 .0 0 %
3 0 .0 0 %
2 5 .0 0 %
4 0 .0 0 %
3 5 .0 0 %
5 0 .0 0 %
4 5 .0 0 %
< 38
< 34
% BF
Figure 2 – 3D contour plot of tensile strength against
BF and PPgMA content.
It can be seen from Figure 2 that, at the investigated
levels of BF and PPgMA, to achieve high tensile
strength relatively high BF contents (around 40wt %)
Bamboo fibers can be successfully used as
reinforcement in polymer, with the aid of
compatibilizer, considering mechanical properties,
processing and environmental aspects.
Acknowledgements
Acknowledgements are due to Quattor and tiva-design
in bamboo (www.tivadesign.com.br) for donating the
polypropylene and bamboo waste fibers, respectively.
Proceedings of the Polymer Processing Society 26th Annual Meeting ~ PPS-26 ~ July 4-8, 2010 Banff (Canada)
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Proceedings of the Polymer Processing Society 26th Annual Meeting ~ PPS-26 ~ July 4-8, 2010 Banff (Canada)