Graft copolymer EVA-g-MA was successfully produced by melt mixing with the highest
content MA grafted on EVA reaching 1.16 wt.%. The presence of MA improved the interaction
between EVA-g-MA and BF, thus, enhanced tensile properties of EVA-g-MA/BF composites.
The addition of EVA-g-MA into EVA/BF and PP/BF composites formed the good adhesion at
the interface of polymer matrix and BF filler and helped to significantly improve the tensile
properties of these composites. These results proved that EVA-g-MA could be used as a
compatibilizer for WPCs production.
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Vietnam Journal of Science and Technology 56 (3B) (2018) 199-208
SYNTHESIS OF EVA-g-MA AND ITS EFFECTS ON TENSILE
PROPERTIES AND MORPHOLOGY OF ETHYLENE VINYL
ACETATE COPOLYMER/BAMBOO FLOUR AND
POLYPROPYLENE/ BAMBOO FLOUR COMPOSITES
Mai Duc Huynh, Do Van Cong, Do Quang Tham, Tran Huu Trung, Nguyen Vu Giang
*
Institute for Tropical Technology, VAST, 18 Hoang Quoc Viet, Cau Giay, Ha Noi
*
Email: nvgiang@itt.vast.vn; vugiang.lit@gmail.com
Received: 17 July 2018; Accepted for publication: 7 September 2018
ABSTRACT
Thermoplastic composites reinforced with natural fillers (WPCs) have been being attracted
many attentions in recent years. The biggest challenges for WPCs production have to face is the
poor adhesion and interaction between the components in the composites. This study
investigates the technique to overcome these challenges without the treatment and surface
modification of natural additive fillers. The obtained experimental results showed that using
maleic anhydride grafted ethylene vinyl acetate copolymer (EVA-g-MA) with 1.16 wt.% of MA
could enhance the adhesion and interaction of bamboo flour (BF) with EVA and polypropylene
(PP) matrices. The improvements of those properties were reflected in tensile properties and
mophological structure of EVA/BF composite and PP/BF composite with the presence of EVA-
g-MA. Tensile strength of EVA/BF and PP/BF composites at the same 50 wt% of BF, increased
by 67 % and 12 %, respectively when EVA-g-MA was introduced.
Keywords: compatibiliser, wood plastic composites, ethylene vinyl acetate, polypropylene,
bamboo flour.
1. INTRODUCTION
Thermoplastic composites reinforced with natural fillers have been gaining many attentions
in recent years. These materials have outstanding properties such as environment-friendly
behaviours, low density, easy to recycle [1-2]. Consequently, they can be widely applied for
decking, siding, indoor building material and so on [3]. However, the use of wood to prepare
these composites faces the environmental problem associated with the exploitation and
protection of forest. Recently, wood flour has been gradually replacing by bamboo flour in wood
plastic composites (WPCs). Bamboo is well known as one of the fastest growing trees in the
world with a maturity cycle of 3-4 years [4-5]. Besides, bamboo has high mechanical properties
in comparison with the other fibers. Therefore, bamboo flour (BF) is considered as one of the
best reinforcing fillers for WPCs.
Mai Duc Huynh, Do Van Cong, Do Quang Tham, Tran Huu Trung, Nguyen Vu Giang
200
The biggest challenges for above WPCs production has to face is the quite poor adhesion
between bamboo flour and plastic matrix. These issues are often solved by treating BF with
alkaline solution or modifying its surface with surfactants in order to improve the compatibility
and adhesion between plastic matrices and BF. Sukmawan et al. used BF which were treated
with alkaline solution to enhance adhesion with poly (latic acid) (PLA) [6]. In another work, the
mechanical properties of PLA/BF composite were improved by using lysine-diisocyanate (LDI)
as a coupling agent to modify the surface of BF [7]. However, the use of solvents in the surface
modification of BF is difficult to apply for WPCs manufacture in the industrial scale due to high
cost and environmental pollution by solvent release. Therefore, the usage of compatibilizers for
manufacturing WPC can help to reduce the demand for solvents and environmental impacts
associated with chemical bamboo treatment. Sanjay K. Chattopadhyay et al. utilized
polypropylene grafted maleic anhydride (PP-g-MA) as a compatibilizer for
polypropylene/bamboo flour composite to improve the adhesion between filler and matrix [8]. In
Viet Nam, Bui Chuong and co-workers also indicated that the application of PP-g-MA or maleic
anhydride grafted polyethylene copolymer (PE-g-MA) could improve the mechanical properties
of WPCs obtained from PP and PE with natural fillers such as bamboo, jute, pineapple, etc.[9-
10].
In general, compatible materials such as PE-g-MA or PP-g-MA are commonly used for
composites based on polyolefin (PP, PE) resins, however the application of graft-copolymers
from ethylene vinyl acetate copolymer (EVA) for WPCs production have not much been
mentioned. This study focused on the synthesis of EVA-g-MA graft copolymers and
investigated the interaction and adhesion between EVA-g-MA and BF as well as their effects on
the tensile characteristics and morphologies of EVA/BF and PP/BF composites.
2. MATERIALS AND METHODS
2.1. Materials
Ethylene vinyl acetate copolymer (EVA) containing 18 wt.% of vinyl acetate and density of
0.93 g/cm
3
was purchased from Honam company (South Korea). Maleic anhydride (MA), purity
99 % and dicumyl peroxide (DCP) were purchased from Merck (Germany). The BF (moisture
content < 8 %) of Dendrocalamus barbatus (North Vietnam) with diameter of 150 µm was
provided by VNDD Ltd. (Vietnam). Acetone, purity 99.7 % and xylene, purity 99.7 % were
supplied by Duc Giang chemical company (Vietnam).
2.2. Sample preparation
2.2.1. Synthesis of EVA-g-MA graft copolymers
The EVA-g-MA graft copolymers were synthesized by melt mixing method in an internal
mixer (Haake Rheomix 610). The processing temperature, rotor speed, and time mixing were
held at the same parameters for all samples. Predetermined amounts of EVA, DCP and MA (as
indicated in Table 1) were charged into the mixing chamber which was preheated to constant
temperature of 150 °C, the rotor speed was set at 50 rpm. After 5 minutes of mixing, graft
copolymers were taken out of the chamber, quickly cut into pieces and cooled down to room
temperature.
Synthesis of EVAgAM and effect its on tensile properties and morphology of EVA/BF
201
Table 1. Weights of components for EVA-g-MA preparation.
Sample code EVA (g) AM (g) DCP (g)
EVAg1 45 0.45 0.045
EVAg2 45 0.90 0.090
EVAg3 45 1.35 0.135
EVAg4 45 1.80 0.180
2.2.2. Preparation of EVA-g-MA/bamboo flour composites
EVA-g-MA/BF composites were prepared by mixing different EVA-g-MA copolymers
with BF by melt mixing in the Haake Rheomix 610 at 150
o
C and rotors speed of 50 rpm for 5
minutes. The ratio weight of EVA-g-MA/BF was kept constant of 80/20 for all samples, as
shown in Table 2. After mixing, the molten products were taken out of the chamber and hot
pressed at 170 °C with the pressure of 5 MPa into 2-mm thickness sheets. The samples were
then cooled down and stored at room temperature for at least 24 hours before characterization.
Table 2. EVA-g-AM/BF composites (wt./wt. of 80/20) with different graft copolymers.
Samples code EVAgMA copolymer /weight (g) BF (g)
EVAg1BF EVAg1 /39.74 9.93
EVAg2BF EVAg2 /39.74 9.93
EVAg3BF EVAg3 /39.74 9.93
EVAg4BF EVAg4 /39.74 9.93
2.2.3. Preparation of EVA/BF composite and PP/BF composite
Table 3. The composition of EVA/BF composites and PP/BF composites.
Samples code EVA (wt.%) PP (wt.%) EVAgMA (G) (wt.%) BF (wt.%)
EVA/G/BF30 66 - 4 30
EVA/G/BF40 56 - 4 40
EVA/G/BF50 46 - 4 50
EVA/G/BF60 36 - 4 60
EVABF30 70 - - 30
EVABF40 60 - - 40
EVABF50 50 - - 50
EVABF60 40 - - 60
PP/G/BF30 - 66 4 30
PP/G/BF40 - 56 4 40
PP/G/BF50 - 46 4 50
PP/G/BF60 - 36 4 60
PPBF30 - 70 - 30
PPBF40 - 60 - 40
PPBF50 - 50 - 50
PPBF60 - 40 - 60
Mai Duc Huynh, Do Van Cong, Do Quang Tham, Tran Huu Trung, Nguyen Vu Giang
202
EVA/BF and PP/BF composites with different BF contents (30, 40, 50 and 60 wt.%), using
4.0 wt% EVA-g-MA (G) as a compatibiliser were produced in melting state in a internal Haake
mixer at rotor speed of 50 rpm for 5 minutes. The processing temperatures for composites
production were 150
o
C and 175
o
C respective to EVA/BF and PP/BF composites. The
composition of these composites were listed in Table 3.
2.3. Characterization methods
2.3.1. Determination of MA grafted content in EVAgMA by chemical titration
Removal process of residue MA from EVA-g-MA: A certain amount of EVAgMA
copolymer was dissolved in xylene at concentration of 5 wt.% under magnetic stirring at 60 °C
for 2 hours. After homogeneuous solution was obtained, residue MA was removed from EVA-g-
MA by precipitating the solution into excess 20-fold volume of cold acetone, the precipitant was
collected and then washed 3 times with acetone, and finally dried in vacuum at 50°C for 24
hours to obtain precipitated EVA-g-MA.
The MA grafted content in EVA-g-MA copolymers was determined by acid-base titration
method, as follows: Complete dissolution of 0.3g precipitated EVAgMA in 20 mL xylene at
60 °C for 2 hours, then two drops of 1wt.% phenolphthalein/ethanol solution as an indicator
were added into the solution. An excess volume of 0.1N KOH/ethanol (VKOH) was dropwise into
the solution to get stable pink color for 30 min under continuous stirring. The unreacted KOH
was neutralise by adding dropwise of 0.1N HCl/ethanol solution (VHCl) into reacting solution
until pink color disappeared.
The graft content can be calculated according to the formula:
.
2.3.2. Fourrier transform infrared (FTIR) spectroscopy
FTIR spectra were recorded using a NEXUS 670 (United States) at the Institute for
Tropical Technology with the resolution of 4 cm
-1
and 32 scans in the wave number range of
4000
- 400 cm
-1
. For FTIR sampling, graft copolymers EVAgMA were dissolved in xylene and
casted into film with thickness about 20 µm. FTIR of BF was carried out by mixing BF with
KBr and pressed into a small disc. Whereas, EVAg3BF was analyzed by using attenuated total
reflectance Fourier transform infrared spectroscopy (ATR-FTIR).
2.3.3. Mechanical properties
Mechanical properties: tensile strength, elongation at break of the composites were
measured by using Zwick Z2.5 instrument (Germany) according to DIN 53503, with cross-head
speed of 50 mm/min at room temperature. Each sample was measured three times to obtain
average results.
3. RESULTS AND DISCUSSION
3.1. The formation of EVA-g-MA graft copolymers
Synthesis of EVAgAM and effect its on tensile properties and morphology of EVA/BF
203
Figure 1 shows FTIR spectra of neat EVA and precipitated EVA-g-MA graft copolymers
(EVAg1, 2, 3, 4) which were respectively modified with 1, 2, 3, and 4 wt.% of MA. In FTIR
spectrum of neat EVA, C-H stretching vibrations (ν) can be observed in the 3000 - 2800 cm-1
range. The peaks at 1739, 1467 and 1371, 1242 cm
-1
are assigned to C=O stretching, CH2 in-
plane bending (δip), CH3 bending (δ), C-O stretching, respectively (abbreviated as: ν(C=O),
δip(CH2), δ(CH3), ν(C-O) [11].
Figure 1. (a) FTIR spectra of original EVA and EVAgAM with different AM content;
(b) Zoomed FTIR spectra in region from 1500-2000 cm
-1
.
In FTIR spectra of EVAg2, EVAg3 and EVAg4 graft copolymer samples, there were new
peaks at 1856 cm
-1
and the shoulders at 1780 cm
-1
which were attributed to symmetrical and
asymmetrical stretching vibrations of C=O of cyclic anhydride when MA was grafted in EVA.
This can be clearly observed on the zoomed spectra at the region of 2000 – 1500 cm-1 (Fig. 1b).
It is worth to mention that MA monomers were grafted on EVA macromolecules. The above
peaks was not observed in spectrum of EVAg1 copolymer, it may be due to amount of MA
grafted on EVA was too small [12, 13].
Table 4. MA grafted content (wt.%) in EVAgAM.
Sample EVAg1 EVAg2 EVAg3 EVAg4
MA grafted
content (wt.%)
0.28 0.75 1.16 0.98
In addition, it is noticably that peak intensity of C=O of cyclic anhydride rised as grafted
MA content increased. This result is consistent with MA grafted content in EVAgMA by acid-
base titration and is presented in Table 4. The content of grafted MA strongly increases with the
MA amount introduced into EVA and reaches to the highest value of 1.16 % for EVAg3.
However, a slight decrease in the MA grafted content was seen for EVAg4. The decrease is
probadly due to the competition between grafting and self-polymerizing reactions, which
reduced the grafting efficiency as AM content added into EVA is too much [14].
3.2. Investigation of the interaction between EVA-g-AM and BF
Figure 2 (a) and (b) exhibit FTIR spectra of BF and EVAg3BF. Figure 2 (a) is FTIR
spectrum of BF showing the peaks at 3420, 1626 cm
-1
, which can be attributed to the absorption
Mai Duc Huynh, Do Van Cong, Do Quang Tham, Tran Huu Trung, Nguyen Vu Giang
204
bands of OH group. The peaks at 2901, 1426, 1384 cm
-1
and 717 are characteristic of methylene
groups in WF. Besides, the vibration of C-O bonds in WF is exhibited at 1053 cm
-1
.
Figure 2. FTIR spectra of BF(a) and EVAg3BF composite (b).
Figure 2 (b) shows some peaks assigned for both BF and EVAg3. In comparison with
EVAg3 (as shown in Figure 1), the C-O stretching shifted from 1242 cm
-1
to lower wave number
at 1237 cm
-1
, it may be due to interaction between EVA and BF which is similar to the results
obtained when adding PP-g-AM into PP/BF composites [8]. The interaction between EVA-g-
MA with BF includes hydrogen bond between hydroxyl groups on BF surface with C-O-C or
carboxyl groups in EVA-g-MA. Besides, during the melt mixing EVA-g-MA and BF, the
esterization reaction between hydroxyl on BF surface and anhydride groups in EVA-g-MA may
occur as followed [8]:
To prove the role of EVA-g-AM in the improvement in the interactions of EVA with BF,
the tensile properties of EVA/BF and EVA-g-MA/BF composites were tested and shown in
Table 5. The tensile strength of original EVA is about 14.2 MPa. With the same addition of 20
wt.% BF, tensile strength and elongation at break of both EVA/BF and EVA-g-MA/BF
composites decreased. The elongation at break of all composites reduced more than half
compared to EVA. However, elongation at break of EVA-g-MA/BF decreased more
significantly than that of EVA/BF composites and the reduction increases with the increasing
content of MA grafted on EVA. The decrease in tensile strength of EVA/BF composites
occurred more sharply, falled down to 6.06 MPa in the comparison with EVA-g-MA/BF
composites. Tensile strength of the composites using grafted EVA even increases with the rise of
MA grafted content and reaches up to 9.07 MPa for EVAg3BF composites, 49.7 % higher than
Synthesis of EVAgAM and effect its on tensile properties and morphology of EVA/BF
205
that of EVA/BF composites. This increase can be explained by the presence of MA that
improves the interaction and adhesion between EVA matrix and BF filler as mentioned above.
The highest increase in tensile strength seen in EVAg3BF can be attributed to the greatest
improvement of the interaction and adhesion between EVA matrix and BF as the MA content
grafted on EVA reached the maximum value, which is confirmed in above section. Obviously,
thanks to the grafting MA on EVA molecules, the interaction and adhesion between EVA and
BF is enhanced, thus the mechanical properties of the obtained composites are improved. This
suggests that EVAgMA can be used as a compatibilizer for the olefin composites reinforced
with natural additive filler such as BF.
Table 5.Tensile strength and elongation at break of EVA-g-AM/BF (80/20) composites.
Sample
Tensile characteristics
Tensile strength (MPa) Elongation at break (%)
EVA 14.20 640
EVABF 6.06 330
EVAg1BF 6.60 291
EVAg2BF 7.55 288
EVAg3BF 9.07 266
EVAg4BF 9.00 256
3.3. Effect of EVA-g-AM on tensile properties and morphologies of EVA/BF and PP/BF
composites
Table 6. The effect of EVA-g-MA on tensile characteristics of EVA/BF and PP/BF composites.
BF
content
(wt.%)
Tensile characteristics
Tensile strength (MPa) Elongation at break (%)
PP/BF PP/G/BF EVA/BF EVA/G/BF PP/BF PP/G/BF EVA/BF EVA/G/BF
0 28.02 28.02 14.20 14.20 145.12 145.12 640 640
30 22.63 23.25 5.93 9.31 4.43 4.02 332 291
40 16.69 18.71 6.31 11.00 3.43 2.30 230 200
50 15.01 17.80 7.42 12.32 1.80 1.78 47 30
60 15.24 17.06 7.05 11.24 1.76 1.62 11 18
To evaluate the effectiveness of mechanical improvement for WPC material, EVAg3 (G)
was used as compatibilizer for preparing EVA/BF and PP/BF composites with various BF
contents. Tensile strength and elongation at break of EVA/BF and PP/BF composites are shown
in Table 6. In term of elongation at breaks, there is a dramatic decrease for all samples as BF
Mai Duc Huynh, Do Van Cong, Do Quang Tham, Tran Huu Trung, Nguyen Vu Giang
206
content increases. Particularly, elongation at break of PP/BF and PP/G/BF composites are not
much different and these values are lower than 5 % for all PP/BF composites.
For EVA/BF composites, the elongation at break of EVA/G/BF composites are lower than
that of EVABF composites. This decrease can be explained by the presence of EVAg3 (G)
which helps to improve the adhesion of matrix and filler and that might restrict EVA molecular
mobility under the impact of tensile stress.
Figure 3. The SEM fractured surface images of (a) EVA/BF composite; (b) EVAg/BF composite;
(c) PP/BF composite and (d) PP/G/BF with same 50 wt.% of BF content.
With BF addition, the tensile strength of PP/BF composite showed a gradual downtrend
from 28.01 MPa for neat PP to 15.24 MPa for composites filled 60 wt.% of BF. This may be due
to the difference in hydrophobicity of BF and PP resin, leading to the poor interaction and
adhesion of BF with PP matrix. However, a much less decrease in tensile strength was also seen
for the PP/G/BF composites containing 4 wt.% EVAg3 . At 60 wt.% of BF content, the tensile
strength of PP/G/BF composite was about 17.06 MPa and increase by 12 % in comparison to the
PP/BF composite. For EVA/BF composites, the introduction of BF reduced the tensile strength
of EVA. However, there is a slight increase in tensile strength for EVA/BF composites with the
BF content. At 50 wt% of BF content, the tensile strength of the composites reached a peak of
7.42 MPa and then slowly decreased to 7.05 MPa as BF content ascended 60 wt.%. In the case
of EVA/G/BF composites, tensile strength has been strongly improved thanks to the presence of
EVA3. Similarly, EVA/G/BF sample reaches to the highest value of 12.32 MPa, about 1.67
times higher than EVA/BF composites at the same 50 wt.% of BF. This means that there is a
good adhesion between the matrix and the filler at the interfacial phase, resulting in the higher
tensile strength for EVA/G/BF samples. EVA3 plays a role of a compatibilizer with higher
performance for EVA/BF composites than PP/BF composite. It is clearly understood due to
higher polarity of EVA in comparison with PP.
Figure 3 shows SEM fractured surface images of EVA/BF composites and PP/BF
composites. In Figure 3(a) and (b), there is no void appeared between EVA resin and BF. This
means that the adhesion between the matrix and the fillers is relative good. It is worth noting that
Synthesis of EVAgAM and effect its on tensile properties and morphology of EVA/BF
207
the same morphologies were observed for the EVA/BF composite without the compatibilizer.
This can be explained that interaction between the polar functional groups of EVA and hydroxyl
groups on BF surface is sufficient to create the interfacial adhesion and to form the continuous
structure at the interphase of EVA and BF.
In Figure 3 (c), the gaps at interface of PP matrix and BF are shown, this suggests that the
adhesion and interaction between PP matrix and BF is quite poor. For the PP/G/BF composite,
these gaps disappeared (Fig.3.d). This indicates an improvement of the compatibility between PP
and BF due to the addition of EVA-g-MA.
4. CONCLUSIONS
Graft copolymer EVA-g-MA was successfully produced by melt mixing with the highest
content MA grafted on EVA reaching 1.16 wt.%. The presence of MA improved the interaction
between EVA-g-MA and BF, thus, enhanced tensile properties of EVA-g-MA/BF composites.
The addition of EVA-g-MA into EVA/BF and PP/BF composites formed the good adhesion at
the interface of polymer matrix and BF filler and helped to significantly improve the tensile
properties of these composites. These results proved that EVA-g-MA could be used as a
compatibilizer for WPCs production.
Acknowledgements. We would like to thank Vietnam Academy of Science and Technology (VAST,
TFR01.03/18-19) for financial support to this research.
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