Article
Influence of fiber hybridization on the
dynamic mechanical properties of glass/
ramie fiber-reinforced polyester
composites
Journal of Reinforced Plastics
and Composites
31(23) 1652–1661
! The Author(s) 2012
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DOI: 10.1177/0731684412459982
jrp.sagepub.com
Daiane Romanzini1, Heitor L Ornaghi Jr2, Sandro C Amico2 and
Ademir J Zattera1
Abstract
This study aims to evaluate the influence of fiber hybridization and frequency on the dynamic mechanical properties of
ramie/glass hybrid fiber-reinforced polyester composites. The storage modulus (E0 ), loss modulus (E00 ) and damping
behavior (tan d) were evaluated as a function of different relative glass/ramie fiber volume ratios. For the storage
modulus, the importance of the reinforcement effect above Tg was revealed. Also, the peak height, peak width at
half-height and relaxation area were investigated for the loss modulus and tan d curves, showing the influence of a
shoulder below Tg for each case. Finally, the tan d peak shifts to higher temperatures by increasing the frequency. Higher
activation energy was observed for 75% glass fiber-containing composites.
Keywords
Natural fibers, glass fibers, hybrid composites, dynamic mechanical analysis
Introduction
Ramie is a plant of the Urticaceae family, derived from
the bast of Boehmeria nivea. The fibers are obtained
from the outer part of the stem and have been used
for centuries as a textile fiber in China, Japan and
Malay Peninsula.1 Margemi et al.2 reported that the
exceptional tensile strength of the ramie fiber has motivated its application in composites. The study of the
incorporation of ramie fiber in a thermosetting matrix
using dynamic mechanical analysis revealed an increase
in the storage modulus or stiffness of the composite as
the fiber is incorporated.
The behavior of natural fiber-reinforced polymer
composites might be improved by the incorporation
of glass fiber. Thus, the incorporation of two or more
fibers into a matrix has led to development of hybrid
composites. As a consequence, the drawbacks of the
natural fiber can be obviated by the incorporation of
a synthetic fiber.3 Both the advantages and disadvantages of natural and synthetic fiber and the mechanical
properties of hybrid composites were reported by several researchers.3,4
Dynamical mechanical analysis (DMA) is an
important technique to study the mechanical behavior
of polymer composite materials. The analysis provides
to calculate the effectiveness of reinforcement on the
moduli of the composites, whose value is an indicative
of maximum stress-transfer between the fiber and
matrix.5 Moreover, as a result of the molecular
motion involving the chain segments, DMA allows
for the evaluation of relaxation processes, such as the
a-relaxation.
1
Programa de Pós-Graduação em Engenharia de Processos e Tecnologias
(PGEPROTEC)/CCET, Universidade de Caxias do Sul, Brasil
2
Programa de Pós-Graduação em Engenharia de Minas, Metalúrgica e de
Materiais, Universidade Federal do Rio Grande do Sul, Brasil
Corresponding author:
Ademir J Zattera, Universidade de Caxias do Sul, Bloco V, Rua Francisco
Getúlio Vargas, 1130 - Bairro Petropólis, CEP: 95070-560, Caxias do Sul,
RS, Brasil.
Email: [email protected]
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Romanzini et al.
1653
Another important aim reported by researches6–8
is the possibility of the estimation of the apparent activation energy (Ea) required to initiate a reaction or
transition. Also, due to the micromechanical transitions that occur at the fiber–matrix interphase8–10 a
second peak for loss modulus was reported at low
temperatures.
The studies reported are generally directed to the
influence of embedded fiber content on the dynamic
mechanical properties,5,6,8,9 but there are few studies
related to the hybridization of glass and natural
fibers.7,10 For instance, Ornaghi et al.7 studied the performance of curaua/glass hybrid composites on the
dynamic mechanical analysis. The results showed that
the glass transition temperature and the activation
energy were higher as a result of glass fiber incorporation. Also, the peak height and the peak width at halfheight were maximized for glass fiber composites.
The present investigation concentrates on the evaluation of the dynamic mechanical properties of ramie/
glass hybrid polyester composites including storage
modulus (E0 ), loss modulus (E00 ) and damping behavior
(tan d). The effect of frequency is reported, and the
activation energy at the glass transition temperature
was calculated for all glass/ramie fiber contents.
Experimental
Materials
Ramie roving was purchased from Sisalsul Fibras
Naturais (SP, Brazil), and glass fiber roving from
Vetrotex (SP, Brazil). Modified unsaturated polyester
resin UCEFLEX UC 5530 -M was supplied by
Elekeiroz S.A (SP, Brazil). Mold-releasing agent poly
(vinyl alcohol) (PVA), curing agent methyl ethyl ketone
peroxide in diisobutyl phthalate (BUTANOX LPT)
and the curing promoter dimethylaniline (DMA) were
purchased from Disfibra (RS, Brazil).
Preparation of composites
Before molding, the mat was pressed under the following conditions: 10 min, 49 kN and 80 C aiming to
remove the moisture that may still be present on the
fiber surface. The polyester resin used was mixed with
0.5 wt% Butanox LPT and 0.1 wt% DMA, respectively. The process parameters used in the resin transfer
molding (RTM) were mold temperature between 20 C
and 25 C and positive pressure of 0.5 bar. The curing
was performed at 25 C for 24 h, followed by a first
post-curing at 80 C for 6 h and a second post-curing
at 120 C for 2 h. The first post-curing ensures that no
further chemical modification will occur while the
second post-curing eliminates mechanical stress.11,12
Characterization
The surface cross-section of the hybrid composites
(cryogenically fractured samples) was examined using
a Scanning Electron Microscope (SEM - JEOL JSM6060). All specimens were sputtered with a layer of
gold prior to SEM observations. Samples were ovendried at 70 C with air circulation for 24 h. After that,
the composites were mounted on aluminum holders
using double-sided electrically conducting carbon adhesive tabs prior to the analysis.
Dynamic mechanical properties were assessed using
a Dynamic Mechanical Analyzer DMA 2980.
Rectangular specimens of 60 mm 10 mm 4.5 mm
size were used. The analyses were performed under a
dual cantilever mode (oscillation amplitude: 15 mm) at
0.1, 1, 10 and 100 Hz frequencies. The analysis of effect
of hybridization was carried out at 1 Hz frequency. The
samples were heated from room temperature up to
180 C at a 3 C/min heating rate. The effectiveness coefficient C is the ratio between the composite storage
modulus (E0 ) in the glassy and rubbery regions in relation to the neat resin8 and can be calculated using
0
0
Eg =Er composite
C¼
E0g =E0r re sin
Mat manufacturing
Glass and ramie fibers were chopped into 45 mm length.
The natural fiber was washed in distilled water (20–
25 C) for 50 min and then oven-dried at 105 C with
air circulation for 60 min. After that, the fibers were
mixed and manually arranged in a pre-mold of same
shape of the mold in order to produce a hybrid mat.
The overall fiber loading was maintained constant (21
vol.%) and the following relative volume fractions of
glass fiber (GF) and ramie fiber (RF) were used: 0:100,
25:75, 50:50, 75:25. For example, 25% glass fiber and
75% ramie fiber loading composite was designated
as 25:75.
0
ð1Þ
0
Where Eg and Er are the storage modulus values in the
glassy and rubbery regions, respectively.
According to Correa et al.,13 the adhesion efficiency
may be determined by the adhesion factor A calculated
as follows
A¼
1 tan c
1
1 f tan p
ð2Þ
The adhesion factor A was calculated in terms of the
relative damping of the composite (tan dc) and the
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Journal of Reinforced Plastics and Composites 31(23)
polymer (tan dp) and the volume fraction of the
reinforcement (Tf) at the glass transition temperature.
Results and discussion
Effect of hybridization – Storage modulus
The effect of hybridization on the storage modulus of
polyester and composites is shown in Figure 1. In
accordance with DMA analysis, the storage modulus
is the contribution of the elastic component and it is
indicative of the materials ability to retain the stored
energy.14 In the glassy region (highlighted), the modulus is determined primarily by the strength of the intermolecular forces and also by the way in which the
polymer chains are packed.8 An improvement in the
modulus can be noted resulting from the incorporation
of reinforcement, this being attributed to the stiffness
imposed by the fillers. In addition, the modulus
increases by glass fiber incorporation. This can be
attributed both to the higher glass fiber modulus15
and the stronger adhesion to the polymer chains at
the interface.7
There is a decrease in stiffness as the temperature
increases, showing a sharp fall in E0 when passing
through the glass transition temperature (Tg). For the
composites, the drop is influenced by the reinforcing
effect of glass/ramie fibers in the polymer matrix (stiffness and fiber/matrix interaction).5 In general, the
decreasing in the E0 values can be explained based on
the microbrownian motion of the polymer chains as the
polymer approached the Tg. Micro-Brownian movements are related to the cooperative short-range diffusional motion of the main chain segments and to their
relaxation stress.7
From Figure 1 it is clear that the fall in E0 is smaller
for the composites in comparison with the neat resin.
The authors related that it can be attributed to the
combination of the hydrodynamic effects of the fibers
embedded in a viscoelastic medium and to the mechanical restraint introduced by the reinforcement.8 Thus,
the effect of reinforcement is higher on the modulus
above Tg than below it, even for those composites containing ramie fiber only.
On the basis of studies reported earlier in the literature5,8,9,16 it was possible to evaluate the extent of the
reinforcing effect by measuring the effectiveness coefficient C (Table 1).
To assess the effect of reinforcement with increasing
temperature the coefficient C was calculated at
four different temperatures (in the rubbery region).
Table 1. Effectiveness coefficient C for different glass/ramie
composites
Sample
C (40–100 C)
C (40–120 C)
C (40–140 C)
C (40–160 C)
0:100
0.50
0.18
0.11
0.09
25:75
0.62
0.23
0.11
0.09
50:50
0.64
0.23
0.12
0.10
75:25
0.53
0.26
0.14
0.11
Figure 1. Storage modulus (E0 ) for different relative volume fraction of fiber (GF:RF).
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Romanzini et al.
1655
For example, C (40–160 C) means that 40 C and 160 C
are the temperatures in the glassy and in the rubbery
region, respectively. Maximum effectiveness occurs
when the maximum stress-transfer between fiber and
matrix takes place and lower C values indicate better
effectiveness.5
According to Table 1 the lowest C values occur for
the highest temperatures, at 160 C (elastomeric region),
which is expected. This confirms that the reinforcement
effect on the modulus is higher above Tg than below it.
Also, lower C values were expected for higher glass
fiber content.16 In this work, the values of C (40–
160 C) are similar, and tend to decrease as the natural
fiber content increases, indicating good tension transfer
to the fiber/matrix at higher temperatures, especially for
those samples containing 100% natural fiber. Figure 2
shows the cryogenic fracture surface of the 25:75 and
75:25 composites. Analyzing the ramie/matrix
(Figure 2(a)) and glass/matrix (Figure 2(b)) interfaces
of the composite 25:75 and comparing with the glass/
matrix interface of the composite 75:25 (Figure 2(c)), it
was possible to verify that by increasing the volume
fraction of glass fiber, agglomeration of fibers takes
place which decreases the effective fiber/matrix stress
transfer. This behavior was also observed in studies
on pineapple leaf and glass fiber.9
Effect of hybridization – Loss modulus
Figure 3 shows the loss modulus (E00 ) as a function of
temperature for different relative volume ratios between
glass and ramie fibers (GF:RF). In DMA analysis, the
loss modulus is the contribution of the viscous
component and is indicative of the energy dissipated
by the system.14
The loss modulus (E00 ) reaches a maximum (some
authors suggest this maximum as the glass transition
temperature Tg) and decreases as the temperature
increases. This behavior results from the polymer
chains free movement at higher temperatures.17 In
other words, the rapid rise in loss modulus in a
system indicates an increase in the polymer structural
mobility due to a relaxation process that allows greater
amounts of motion along the chain than is not possible
below the glass transition temperature.10 Also, it is
important to emphasize that the maximum peak of
the curve E00 versus temperature corresponds to the
situation of maximum dissipation of mechanical
energy, which in the region of glass transition is associated with the shifting from the glassy to the elastic
state.14
As glass is incorporated in the composites higher loss
modulus peaks are obtained. This occurs probably by
the inhibition of the relaxation process in composites
due to the increased number of chain segments and
higher free volume resulting from fiber addition.6 The
increased modulus of composite (75:25), as compared
with the neat resin, can be explained by the higher
internal friction that improves the energy dissipation.6
So, it can be concluded that the peak height is an indicative of the energy dissipated in a system. When this
concept is applied to the composites, there is an
increase in peak height resulting from the rise in the
glass fiber volume fraction.
The loss modulus peak height can be associated with
the impact strength, which increases with the glass fiber
Figure 2. Micrograph of (a) ramie/polyester interface and (b) glass/polyester interface of the 25:75 composite and the (c) glass/
polyester interface of the 75:25 composite.
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Journal of Reinforced Plastics and Composites 31(23)
incorporation.7 Similar studies with ramie and glass
fibers composites in different ratios were performed
by Romanzini et al.18 According to the authors, the
increase in the impact strength with the glass fiber content can be attributed to the higher energy dissipation
at the glass/matrix interface in order to detach the fibers
from the matrix. For example, the impact strength of
the neat resin was found to be 48.6 6 MPa. By the
incorporation of ramie fiber (0:100 – 45 mm fiber
length), the impact strength increases by approximately
309%. Moreover, the impact strength of the (50:50)
composite increased 80.2% in comparison to the
(25:75) composite. For the (75:25) composite, there
was a rise of 142.6% in comparison to the (25:75)
composite.
Also, Figure 3 shows a small hump (shoulder) due to
the additional relaxation,9 observed at a temperature
lower than Tg for all hybrid composites. The shoulder
is more evident for composites with higher natural fiber
content (0:100, 25:75 and 50:50). The additional relaxation can be related to the micromechanical transition
arising from the immobilized polymer layer, which acts
as interlayer,8 and to the effect of the presence of a
strong interphase in the composites relaxation.9 In addition, the curves are also found to be flattened by the
glass/ramie fiber incorporation.10 However, in the composite (75:25), the second peak is not evident. Thus, the
curves are not to be flattened, and consequently, the
peak height is higher than in other composites, as
shown in Table 2.
In addition, Table 2 presents the peak width at halfheight and the relaxation area for the composites
and for the neat resin. Due to the wider distribution
of relaxation times both peak width and the relaxation
area increase with glass fiber incorporation. According
to Pistor et al.,19 the peak width at half-height varies
according to the homogeneity of the system. A broader
distribution reflects thus deeper differences in chain segments relaxation times.
Effect of hybridization – tan The ratio of the loss modulus to the storage modulus
provides the mechanical loss factor or damping (tan d).
The tan d parameter confers the balance between the
elastic and the viscous phase in a polymer structure.6
Table 2. Peak height, peak width at half-height and relaxation
area (E00 )
Sample
Peak
height
Peak width at
half-height
Relaxation
area
Resin
0:100
25:75
50:50
75:25
243
299
324
340
434
4058
5818
5929
6028
7429
8095
11,544
11,834
12,093
14,846
Figure 3. Loss modulus (E00 ) for different relative volume fraction of fiber (GF:RF).
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Romanzini et al.
1657
Also, the tan d peak is associated with the partial
motion of a polymer structure, in which small groups
and chain segments initially ‘frozen’ start to move in a
cooperative way. This is indicative of the glass transition temperature, where the material changes from a
rigid to a more elastic state.8
Figure 4 shows tan d as a function of temperature
for different relative volume fraction of glass and ramie
fibers (GF:RF). In polymeric composites, tan d is
affected by reinforcement incorporation. Lower values
for the tan d peak are found for the composites, as
compared with the neat resin. This can be justified by
the restriction of polymer molecules motions resulting
from the incorporation of a rigid fiber.6
Also, as the energy dissipation will occur at the fiber/
matrix interface, a stronger interface is characterized by
lower energy dissipation. Thus, lower values for tan d
peak also indicate better interfacial adhesion.8
According to this concept, one would expect lower
peaks for the higher glass fiber content composites
(resulting from better interfacial adhesion). However,
the highest energy dissipation values (peak height)
and largest relaxation times distribution (peak width
at half-height and relaxation area) were found for the
composites containing 75:25 (GF:RF), as shown
in Table 3.
Similar results were found by Ornaghi et al.7 studying hybrid composites containing glass and curaua
fibers and by Devi et al.9 studying pineapple leaf and
glass fibers. According to the authors, at higher glass
fiber fractions, agglomeration takes place resulting in
incompatibility between the fiber and the matrix. A
schematic representation showing the distribution of
glass and ramie fibers in different fractions is presented
in Figure 5. The micrograph of the composites with
25:75 (Figure 5(a)) and the agglomeration in the 75:25
composite (Figure 5(b)) are also shown.
So, strong interaction of fiber and matrix occurs at
higher ramie fibers fractions (Figure 5(a)), which
reduced the mobility of molecular chains at the interface and the tan d peak is reduced accordingly. This
corresponds with the adhesion factor A values
(Table 3), in which the composites with lower ramie
fiber fraction show better fiber-matrix adhesion (low
values of the adhesion factor A suggest improved interactions at the fiber/matrix interface13).
In addition, according to Table 3, the (0:100) composite showed the highest Tg (116 C). Jawaid et al.4
Table 3. Parameters obtained from tan d spectra
Peak Peak width at Relaxation Adhesion
Sample height half-height
area
factor A Tg ( C)
Resin
0:100
25:75
50:50
75:25
0.58
0.32
0.33
0.34
0.38
10.3
5.23
5.55
5.96
6.29
Figure 4. Tan d for different relative volume fraction of fiber (GF:RF).
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20.62
10.43
11.09
11.90
12.53
–
0.306
0.268
0.245
0.170
115
116
112
112
113
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Journal of Reinforced Plastics and Composites 31(23)
also found that Tg values were lower for hybrid composites than those of natural fiber composites and the
neat resin. According to the authors, hybrid composites
show lower Tg values due to the decreasing of the fiber/
matrix interaction which shifts the relaxation temperature towards lower temperatures. Besides, a comparison among hybrid composites shows no significant
influence of the various ramie/glass fractions on the
Tg values (from 112 C to 113 C).
Effect of frequency
The viscoelastic properties of the studied material are
affected by time, temperature and frequency, thus influencing the storage modulus, loss modulus and tan d
curves of the composites. Figure 6 shows the variation
of storage modulus for a range of (a) temperature and
(b) frequency for (25:75) composites. There is an
increase in the storage modulus as the frequency
increases and the temperature decreases. This behavior
was already reported by Abraham et al.,20 studying
polypropylene composites laminates.
If a material is subjected to a constant stress, their
elastic modulus will decrease over a period of time, due
the molecular rearrangement in an attempt to minimize
the localized stresses. So, the modulus measurements
performed over a short time (high frequency) result in
higher values.8
Figure 7 shows the effect of frequency on tan d
curves for the composite (25:75). The general behavior
of the curves is repeated for the hybrid composites studied. Increasing the frequency, the tan d peak also shifts
to higher temperatures. This occurs due to the fact
that, at higher frequencies (short time) there is a
decreasing in rotational and translational motions of
Figure 5. Schematic representation showing the distribution of glass and ramie fibers in different fractions and the micrograph of the
composites with (a) 25:75 and (b) 75:25.
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Romanzini et al.
1659
Figure 6. Storage modulus vs (a) temperature and (b) frequency of (25:75) composites.
Figure 7. Effect of frequency on the tan d curve of (25:75) composites.
the molecular chains, which require more energy to
start the cooperative movement.
Furthermore, a second peak can be observed on tan
d curves for the composite with 25% natural fiber,
which appears at all frequencies. The behavior of the
tan d curves is influenced by this additional relaxation
that is most noticeable on loss modulus curves
(Figure 8). Upon scrutinizing the effect of frequency
on loss modulus curves for (25:75) and (75:25) composites, it is evident that the shape of the loss curve is
affected by the incorporation of different fractions of
glass and ramie fibers.
Thus, this behavior is characterized by micromechanical transitions at the interface (discussed in the 3.2
item), that is further evidenced in composites containing higher fractions of natural fibers, that is an indicative of a strong interface in the composite.9
Also, the calculation of the activation energy (Ea)
was performed on the basis of the effect of the frequency on the glass transition temperature of the
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Journal of Reinforced Plastics and Composites 31(23)
Figure 8. Effect of frequency on the loss modulus curve of (a) (25:75) and (b) (75:25) composites.
Table 4. Activation energy (Ea) for different
glass/ramie composites
Sample
Ea (kJ/mol)
R2
0:100
25:75
50:50
75:25
354
414
419
421
0.994
0.996
0.992
0.995
composites (Tg shifts to higher temperatures). The activation energy for the relaxation process in the glass
transition region (a-transition) can be explained by
the Arrhenius relationship, as already reported in the
literatures.6–8. Table 4 shows the activation energy (Ea),
the determination coefficient (R2) of the fitting curves
used.
The increase in activation energy resulting from glass
fiber incorporation is due to the rigidity imposed by
the glass fiber.6 Upon increasing the reinforcement,
more energy is required to promote the initial movement of some molecular segments in the polymer
backbone.16
An improvement in the storage modulus resulting from
reinforcement incorporation is due to the stiffness
imposed by the reinforcement (in the glassy region).
Furthermore, it was confirmed that the effect of
reinforcement on the modulus is higher above Tg than
below it. Also, scrutinizing the loss modulus and tan d
curves, higher energy dissipation (peak height) and
larger relaxation times distribution (peak width at
half-height and relaxation area) were found for the
composites (75:25), showing the influence of a second
peak below Tg. And at higher glass fiber fractions,
agglomeration takes place resulting in incompatibility
between the fiber and the matrix, the fiber/matrix interactions are improved with the increase of ramie fiber
fraction. So, to evaluate the composites properties (e.g.
storage modulus), it is more important to evaluate the
fiber properties than the interface. Finally, the tan d
peak was shifted to higher temperatures with the
increase in frequency and the activation energy was
higher for the 75% glass fiber composite.
Funding
This research received no specific grant from any funding
agency in the public, commercial, or not-for-profit sectors.
Acknowledgements
Conclusion
This study concentrates on evaluating the influence of
hybridization (glass/ramie fiber) and frequency on the
dynamic mechanical properties of polyester composites.
The authors wish to thank CNPQ and CAPES for the financial support, LPOL for providing the experimental testing
and particularly Elekeiroz S.A for providing the polyester
resin. Authors are also indebted to the PGEPROTEC
(UCS) and PPGEM (UFRGS) postgraduate programs.
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Romanzini et al.
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