Gypsum filler was successfully modified by EBS using melt-mixing method at 155 oC for 30
minutes. Stretching vibration of H-O-H shifted from 1,623 cm-1 to 1,629 cm-1 could be assigned
to the hydrogen bonds between –OH groups of gypsum and amide groups of EBS. By adding 5
wt.% APP and 3 wt% ZB into HDPE/EVA/Gyp composite, the material reached V2 grade of
flame resistance according to UL- 94 standard. While t1 of both EGS composite and OGS
composite are same, t2 of EGS composite is shorter than that of OGS. The tensile strength and
elongation at break of composite containing EGS are higher than that of composite containing
OGS proved the EBS modification effect on the gypsum.
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Vietnam Journal of Science and Technology 56 (3B) (2018) 87-95
MECHANICAL PROPERTIES AND FLAME RESISTANCE OF
COMPOSITE BASED ON HIGH DENSITY
POLYETHYLENE/ETHYLENE VINYL ACETATE BLEND AND
NOVEL ORGANICALLY MODIFIED WASTE GYPSUM
Tran Huu Trung, Mai Duc Huynh, Do Van Cong, 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: 30 July 2018; Accepted for publication: 9 September 2018
ABSTRACT
In this study, waste gypsum (WGS) was used as the filler in blend of high density
polyethylene (HDPE) and ethylene vinyl acetate copolymer (EVA) in order to improve the flame
resistance of this composite. After modification by ethylene bis stearamide (EBS), modified
gypsum (EGS) at various contents was dispersed into HDPE/EVA (95/5 wt.%) blend by melt-
mixing method to obtain EGS/HDPE/EVA composites. The IR analysis of the EGS showed the
interaction between EBS and gypsum. The difference in morphology of HDPE/EVA/EGS and
HDPE/EVA/OGS composites was also observed in SEM images. By adding EGS and amounts
of flame retardant additives such as ammonia polyphosphate and zinc borate, the thermal
stability and the flame resistance of HDPE/EVA/EGS composites have improved and classified
at V2 grade of UL94 standard. Moreover, the second self extinguishing time (t2) of EGS
composite is shorter than that of unmodified gypsum (OGS) composite because of the quality
char forming after the first burning and it plays a role as thermal barriers to protect material from
heat of the second ignition. About mechanical properties, both tensile strength and elongation at
break of the composite decreased with increasing the gypsum content and flame retardant
additives because of filler aggregation.
Keywords: waste gypsum, HDPE, EVA, flame resistance, ethylene bis stearamide.
1. INTRODUCTION
Waste gypsum (WGS) has been known as a by-product of phosphate fertilizer production
process. Normally, WGS is stored in landfills without treatments and becomes huge pollution
sources [1]. To diminish this pollution, an amount of WGS has utilized as building materials
such as bricks, cement, plasterboard or filler to amend the physico-chemical properties of soils
[2, 3]. Besides, by owning many advantages such as small particle size, thermal stability,
softness, light weight, low cost, etc. [4], WGS has attracted researchers to prepare composite
with some kind of matrix materials such as thermoplastic, epoxy resin, etc. [5-8]. Yordan D.
Denev et al. have introduced WGS into polyethylene at melting state at different concentration
Tran Huu Trung, Mai Duc Huynh, Do Van Cong, Nguyen Vu Giang
88
ranging from 10 to 60 wt.% to obtain the composite owning high tensile strength and modulus
strength [7]. Khuong V. H. also used sodium dodecyl sulfate modified WGS as the filler in
HDPE/EVA blend to improve the flame resistance of the composite [8]. However, the
compatibility between WGS and polymers is quite poor due to the difference in chemical nature
such as polarity, surface activation energy etc. Hence, the organic modification of WGS is
necessary to overcome this problem. Some surfactants, i.e stearic acid (SA) and sodium dodecyl
sulfate (SDS) have been commonly used as organic modifiers for WGS [6, 8]. SA has been
proved as one of the most popular organic substances for the modification of the additive fillers
with low melting point around 70
o
C. Meanwhile, SDS has the melting temperature much higher
than that of polyolefins, thus it causes the inconvenience during material processing, such as
polymer decomposition or low modification efficiency. EBS has melting point around 146
o
C
that near to the melt temperature of polyolefins. Thus, it expected to improve the interaction of
EBS-modified WGS and polymer matrix in composite prepared by melt-mixing method.
Nevertheless, using EBS as a surface modifier for WGS has been received little attention until
now.
High density polyethylene (HDPE) is a widely used thermoplastic in industry and life
because of its high mechanical properties, good corrosion resistance, excellent chemical
resistance as well as easy processibility. By adding ethylene vinyl acetate copolymer (EVA) into
HDPE, the obtained blend owns high stress crack resistance and good impact strength that can
be applied in the interior decoration [9]. However, as nature of thermoplastic, both HDPE and
EVA have low flame resistance that limits the application of these blends. In this work, the
composites of EBS modified gypsum (EGS) and HDPE/EVA blend in the presence of
ammonium polyphosphate (APP) and zinc borate (ZB) as flame retardant additives were
prepared. The effects of the gypsum content with and without the modification by EBS on the
morphology structure, thermal properties, flame resistance and mechanical properties of
obtained composites were investigated.
2. MATERIALS AND METHODS
2.1. Materials
Waste gypsum is a by-product of DAP-Vinachem Limited Company (Vietnam) with the
particle size range in 20 – 30 μm. Ethylene bis stearamide (EBS), amonium polyphosphate
(APP) and zinc borate (ZB) are high purity commercial products provided by Guangdong Sci-
Tech Co.ltd (China). High density polyethylene (HDPE), 20 g/10 min. of MFI (190
o
C/2.16 kg)
and 0.956 g/cm
3
of density, was purchased from ExxonMobil Company (Saudi Arabia).
Ethylene vinyl acetate copolymer (EVA) containing 18 wt.% of vinyl acetate, was purchased
from Honam Petrochemical Corporation (South Korea).
2.2. Preparation of HDPE/EVA/EGS composite
WGS was neutralized to pH 7 by NaOH 5 wt.% solution and washed by water three times
before drying at 110
o
C for 3 hours and grinding to obtain unmodified WGS (OGS). The
modification of gypsum was performed by melt-mixing OGS with EBS at 155
o
C for 30 minutes
by using low speed agitator. EBS contents for modifying WGS are 3, 4, and 5 wt.%.
In this study, HDPE and EVA was blended together at ratio 95/5 wt.%. HDPE/EVA/EGS
composites containing 0, 2, 5, 7, 10, and 15 wt.% of EGS were prepared by melt mixing method
Mechanical properties and flame resistance of composite based on high density
89
in a Haake internal mixer at 175
o
C for 5 minutes. After that, the composites were hot-pressed in
melting state at 175
o
C to form the sheets with the thickness of 1.5 - 2.0 mm. The samples were
maintained at room temperature for 24 hours before running property measurements.
In order to improve the flame resistance of HDPE/EVA/EGS composites, a mixture of
flame retardant additives including APP and ZB was introduced into composites at melt-mixing
state [10, 11]. By measuring the burning rate of composites consisting APP and ZB at various
contents, the composites containing 5 wt.% APP and 3 wt.% ZB showed the best flame
resistance and these optimum contents was kept for all works in this study.
2.3. Characterizations
The tensile strength and elongation at break results were obtained as average value by
measuring each sample with five times at a crosshead speed of 50 mm.min
-1
in Zwick Tensiler
2.5 (Germany) according to ASTM D 638 standard. The Fourier transform infrared spectroscopy
(FTIR, Nexus, USA) was carried out at conditions: resolution 16 cm
-1
, each sample was scanned
32 times in wave number range from 4000 cm
-1
to 400 cm
-1
. Before FTIR analysis, samples were
extracted in ethanol solution by using soxhlet equipment for 2 hours to remove the excess EBS
content. Field emission scanning electronic microscope (FESEM, model S-4500, Hitachi, Japan)
was employed to examine the fracture surface of samples and also to investigate the dispersion
of gypsum particles in polymer matrix. Burning experiments of the composites were carried out
under UL94 standard in Institute for Tropical Technology in order to measure the burning time
and burning rate of samples [12].
3. RESULTS AND DISCUSSION
3.1. Fourier transform infrared analysis
Figure 1. FT-IR spectra of gypsum before (OGS) and after EBS modification (EGS 4 %).
Figure 1 shows FT-IR spectra of EBS, OGS and 4 % EBS modified waste gypsum (EGS
4 %). The peaks located at 3300-3600 cm
-1
were contributed to the vibration of water-OH in
gypsum. Meanwhile, the peak at 1121 cm
-1
is charateristic of covalence bond of SO4
2-
of
Tran Huu Trung, Mai Duc Huynh, Do Van Cong, Nguyen Vu Giang
90
gypsum. Besides, douple peaks at 1687 cm
-1
and 1623 cm
-1
might contribute to stretching
vibration of H-O in gypsum. After EBS modification, the appearance of new peaks at 2919 cm
-1
and 2861 cm
-1
are symmetric vibration of -CH- of hydrocarbon chain of EBS. Moreover, peak at
3608 cm
-1
was contributed to N-H bond and peak at 1008 cm-
1
characterized by -C-O linkage in
EBS. These evidences show that EBS was successfully attached onto the surface of gypsum.
Especially, there is a shift of stretching vibration of H-O groups from 1623 cm
-1
to 1629 cm
-1
, it
might be due to the interaction between gypsum and EBS. This interaction is mainly hydrogen
bonds between –OH groups in gypsum and amide groups of EBS (1638 cm-1) attached on EGS
surface.
3.2. Field emission scanning electronic microscope
Figure 2. SEM images of (a)HDPE/EVA/OGS composite, (b) HDPE/EVA/EGS composite and
(c) HDPE/EVA/EGS/APP-ZB at 7 wt.% EGS.
Figure 2 presents FE-SEM fracture surface images of HDPE/EVA/OGS composite (a) and
HDPE/EVA/EGS composites with (c) and without (b) flame retardant additives. Without
modifying by EBS, gypsum particles dispersed unevenly in HDPE/EVA blend matrix and
aggregated to clusters with about 2-6 µm in size that separated with polymer matrix at the
interface (Fig. 2a). This illustrates that the unmodified gypsum particles are not compatible with
the polymer matrix and the poor adhesion between them. After modifying by EBS, gypsum
particles dispersed finely and more homogeneous in the blend matrix, the agglomeration of these
particles is less to occur (Fig. 2b). In addition, a very thin film seemly appeared surrounding the
gypsum particles and blend matrix connecting these two phases together and the phase
separation is not existed. It is clear that the modification by EBS improved the dispersion and
adhesion of filler particles in polymer matrix. When the flame retardant additives APP and ZB
Mechanical properties and flame resistance of composite based on high density
91
were introduced into the composites, the gypsum particles and additive particles poorly
dispersed widely in the blend matrix, but tend to aggregate together to form some clusters due
to the great affinity between them. It is interesting that the quite good adhesion between these
dispersed particles and polymer matrix is still observed (Fig. 2c). This confirms again that the
surface modification by EBS has improved significantly on the adhesion between the
components in the composites.
3.3. Thermal gravimetric analysis
Figure 3. Thermal gravimetric curves of composite samples at 7 wt.% gypsum.
To determine the effect of gypsum and flame retardant additives on the thermal property of
composites, these samples were analyzed by TGA from room temperature to 600
o
C in nitrogen
environment as shown in Fig. 3.
There is a slight weight loss (10 wt.%) at the temperature up to 400
o
C which is ascribed to
the evaporation of absorbed water in gypsum and partly decomposition HDPE/EVA blend and
HDPE/EVA/EGS composites. The decomposition process of HDPE and EVA chains mainly
occurred from 400 to 500
o
C. Thus, the samples with higher EVA and HDPE ratios resulted in
high weight loss. The total weight losses at 500
o
C were recorded about 91.05 wt.% and 84.26
wt.% for HDPE/EVA blend and HDPE/EVA/EGS composites, respectively. Whereas, the
HDPE/EVA/EGS/APP-ZB and HDPE/EVA/OGS/APP-ZB samples were lower at about 81.52%
lower and 80.23 wt.%, respectively. In presence of flame retardant additives, the composites
have higher thermal stability by showing the smaller weight loss than materials without APP and
ZB. This different weight loss was observed clearly in temperature range from 440 to 480
o
C.
Finally, the complete degradation of HDPE and EVA chains was recognized at the last stage
between 500
o
C and 600
o
C, while gypsum was not affected by heat and remained at the end of
analysis.
3.4. Flame resistance and self-extinguishing time
Tran Huu Trung, Mai Duc Huynh, Do Van Cong, Nguyen Vu Giang
92
In order to evaluate the flame resistance of HDPE/EVA/WGS composites, the samples
were burned vertically to measure the burning time according to UL-94 standard. Figure 4
presents the burning time of the composites at various OGS and EGS contents. When the content
of gypsum varied from 0 to 15 wt.%, the burning time of composite samples increased. It can be
explained by the role of gypsum particles as thermal absorption sites in the composites. The
more gypsum particles in composite, the more thermal absorption sites which improves thermal
barriers and slow down the burning process [6]. However, without EBS modification, OGS
could not dispersed well in HDPE/EVA matrix resulting the defects in composite. Those defects
might contain oxygen in air that promote the burning process. That explained the burning time
of OGS composites is shorter than EGS composites at same gypsum content.
Figure 4. Effect of gypsum content on burning time of HDPE/EVA/EGS/APP-ZB and
HDPE/EVA/OGS/APP-ZB composites.
To improve the flame resistance ability, APP and ZB were added in melting composite in
preparation process. By adding a mixture of 5 wt.% APP and 3wt. % ZB, the composite became
self-extinguished. To measure the self-extinguishing time of the material, the samples were
burned horizontally according to UL-94 standard. During burning process, APP will be degraded
to form phosphorus acid derivatives which suspend the burning process of polymer [10, 11]. On
the other hand, ZB is known as a flame retardant and smoke suppressant. The combination of
ZB with APP could enhance both char formation and char quality that improving of flame
retardancy of material [10].
Self-extinguishing time (t) of material is the time that material extinguished itself after
removal of flame source, the short t means the materials is good resistance to flame. As defined
by UL-94 standard, t is the sum of t1 and t2 of five pieces of sample, in which t1 and t2 are
respective to the self-extinguishing time of the first and second ignition. By adding APP and ZB,
composites using OGS and EGS became self-extinguished and achieved V2 grade of UL-94
standard. In Figure 5, t1 of OGS composites are not different from that of EGS composites at the
same gypsum contents. However, at the second burning, t2 of EGS composite is shorter than
that of OGS composite. This phenomenon can be explained that the char formation of EGS
composite might be more quality than that of OGS composite, thus, it works as thermal barriers
to protect material from heating of flame source.
Mechanical properties and flame resistance of composite based on high density
93
Figure 5. Self-extinguishing time t1 and t2 of composites with various EGS and OGS contents.
3.5, Mechanical properties
Figure 6. Influence gypsum contents and flame retardant additives on elongation at break and tensile
strength of composites.
Figure 6 presents the influence of gypsum contents and flame retardant additives on tensile
strength and elongation at break of HDPE/EVA/WGS composites. It is seen that both tensile
strength and elongation at break of all composites decreased with the raise of gypsum content.
The graphs showing the dependence of tensile strength and elongation at break of
HDPE/EVA/EGS composites on the gypsum content lied above in comparison to those of two
other composites. It means that at the same gypsum content, tensile strength and elongation at
break of HDPE/EVA/EGS composites are the highest among three composites. When adding 2
and 5 wt.% gypsum, tensile strength and elongation at break of HDPE/EVA/EGS composites
decreased gradually whereas their reduction in the other composites happened intensively. The
incorporation of 5 wt.% EGS only reduced 26.5 % tensile strength (from 39.2 MPa down to 28.8
MPa) and 17 % elongation (from 650 % down to 540 %) of HDPE/EVA blend. While these
declines reached to 54 % and 57 % respective to tensile strength and elongation at break for
HDPE/EVA/OGS composites with the same gypsum content. Obviously that the modification of
WGS by EBS improved the interaction and adhesion of gypsum particles with HDPE/EVA
blend matrix, therefore enhanced the mechanical properties of the obtained composites. The
Tran Huu Trung, Mai Duc Huynh, Do Van Cong, Nguyen Vu Giang
94
addition of the flame retardants as APP and ZB resulted good resistance to flame for both
HDPE/EVA/EGS and HDPE/EVA/OGS composites as mentioned in the above section.
Unexpectedly, the presence of APP and ZB caused the decrease in mechanical properties of the
composites. Tensile strength and elongation at break of HDPE/EVA/EGS composites declined
relative quickly with the introduction of APP and ZB. However, these parameter values are still
higher than those of HDPE/EVA/OGS composites with the same filler content. In other words,
the modification of gypsum by EBS helps to lower the reduction in mechanical properties of the
HDPE/EVA/EGS/APP-ZB composites. The improvement of the adhesion between the additive
filler particles (including gypsum and flame retardant filler particles) with HDPE/EVA blend
matrix after the modification with EBS but the aggregation caused by their great affinity is the
reason explain for this phenomenon. The aggregation of gypsum and APP, ZB particles as
observed in Fig.2 leads to the void formation and interrupts the continuity of the matrix phase,
hence caused the decrease in tensile strength and elongation at break of the composites.
4. CONCLUSIONS
Gypsum filler was successfully modified by EBS using melt-mixing method at 155
o
C for 30
minutes. Stretching vibration of H-O-H shifted from 1,623 cm
-1
to 1,629 cm
-1
could be assigned
to the hydrogen bonds between –OH groups of gypsum and amide groups of EBS. By adding 5
wt.% APP and 3 wt% ZB into HDPE/EVA/Gyp composite, the material reached V2 grade of
flame resistance according to UL- 94 standard. While t1 of both EGS composite and OGS
composite are same, t2 of EGS composite is shorter than that of OGS. The tensile strength and
elongation at break of composite containing EGS are higher than that of composite containing
OGS proved the EBS modification effect on the gypsum.
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