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Cent. Eur. J. Chem. • 11(6) • 2013 • 953-959
DOI: 10.2478/s11532-013-0227-2
Central European Journal of Chemistry
Preparation of low initial expansion
temperature expandable graphite
and its flame retardancy for LLDPE
Research Article
Xiuyan Pang*, Yu Tian, Mingwei Duan, Meng Zhai
College of Chemistry and Environmental Science,
Hebei University, Baoding 071002, People’ Republic of China
Received 25 October 2012; Accepted 23 January 2013
Abstract: To get expandable graphite (EG) flame retardant for Liner Low-Density Polyethylene (LLDPE) with low initial expansion temperature and
high dilatability, the effects of various factors on dilatability were investigated including the dosages of oxidant KMnO4, intercalating
reagent H2SO4, assistant intercalating reagent acetic acid (HAc) and reaction temperature. Feasible conditions were obtained according to
the results of L9 (34) experiments and single factor experiments. EG with an initial expansion temperature of 160°C and expansion volume
of 460 mL g-1 could be prepared according to the mass ratio of material graphite C : KMnO4 : 100% H2SO4 : HAc = 1.0 : 0.4 : 5.0 : 1.0
(H2SO4 should be diluted to the mass concentration of 75% before the intercalation reaction); the reaction time was 1.0 hour at 25°C.
It was found that reaction temperature and H2SO4 dosage were the most important influence factors for dilatability. The limiting oxygen
index could be improved to 28.1% by adding 30% of the prepared EG to LLDPE, and the synergistic anti-flame capability of 20% EG with
10% Ammonium polyphosphate (APP) (I) can reach to 33.9%. According to thermal gravimetric and differential thermal analysis results,
70% LLDPE /10% APP (I) /20% EG synergistic anti-flame system shows higher residual carbon and thermal stability.
Keywords: Expandable graphite • Acetic acid • Initial expansion temperature • Expansion volume • Flame retardant
© Versita Sp. z o.o.
1. Introduction
Graphite is a kind of crystal compound with layer
structure. Expandable graphite can be prepared when
non-carbonaceous reactants are inserted into graphite
layers through chemical or electrochemical reaction.
Expandable graphite has many favourable properties:
it can be used as a catalyst [1]; and porous expanded
graphite can be prepared with expandable graphite
expanded instantly at high temperature. Expanded
graphite is a kind of adsorption material for heavy oil and
dyes wastewater [2-5]. In addtion, expandable graphite
is a good intumescent type flame-retardant for its good
capability of halogen-free, non-dropping, low-smoke and
low pollution potential [6,7]. When exposed to flame, the
expandable graphite will form a swollen multicellular char,
which can protect materials from the heat of combustion,
limit the access of oxygen to the polymer, and reduce
the production of smoke. Simultaneously, expandable
graphite absorbs huge heat during the process of instant
expansion, which can decrease the burning temperature.
When it gets oxidized on reaction with H2SO4 at high
temperature as shown in Eq. 1, the releasing gases can
reduce the concentration of oxygen.
C + 2 H 2 SO 4 → CO 2 ↑ +2 SO 2 ↑ +2 H 2O ↑
(1)
Due to its outstanding anti-flame capability,
expandable graphite has been used widely in the flame
retardance of polyolefin blends [8,9], high-density PUF,
polyurethane-poly-isocyanurate foam [10-16] and
poly(methyl methacrylate) [17]. In these reports, most
of the expandable graphites were purchasable goods,
and the research was mainly focused on the influences
caused by its size or dosage on flame retardance [18].
As a flame retardant, the parameter of initial
expansion temperature of expansible graphite is
important. According to initial expansion temperature (T0),
expandable graphite is classified into three kinds [19]: low
initial expansion temperature expandable graphite (T0 is
* E-mail: [email protected]
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Preparation of low initial expansion temperature
expandable graphite and its flame retardancy for LLDPE
Table 1. Influence factors and experiment dosages of reactants a.
Levels of factors
1
a
KMnO4 (g)
100% H2SO4 (g)
HAc (g)
T (°C)
0.2
3.0
1.0
25
2
0.4
4.0
2.0
40
3
0.6
5.0
3.0
55
Based on 1.0 g material graphite
between 80-150°C), middle initial expansion temperature
expandable graphite (T0 is between 180-240°C) and
high initial expansion temperature expandable graphite
(T0 is between 250-300°C).
It is well known that the dilatability of expandable
graphite is affected by the dosages of oxidation reagent,
intercalation reagent, ancillary intercalation reagent and
reaction temperature. With KMnO4, HNO3 and HBrO3
used as intercalation system, a kind of expandable
graphite with an initial expansion temperature of 130°C
and expansion volume (EV) of 350 mL g-1 was prepared
[20]. Expandable graphite holding an initial expansion
temperature of 310°C and EV of 270 mL g-1 could be
prepared with 85% H2SO4, KMnO4 and FeSO4 [21].
Linear
Low-Density
Polyethylene
(LLDPE)
possesses low machining temperature (less than
140°C) and it is very flammable. In this research, in
order to prepare an expansible graphite (EG) with low
initial expansion temperature, high dilatability and fit for
LLDPE flame retardancy, KMnO4, H2SO4 and HAc were
used as oxidation reagents, intercalating reagent and
assistant intercalating reagent, respectively, which is
helpful to change product initial expansion temperature
and improve its dilatability [22]. The dosages of KMnO4,
H2SO4, HAc and reaction temperature were optimized
in the intercalation reaction of material graphite. The
method to preparing EG fitting for LLDPE was founded,
and its anti-flame properties were determined. Thermal
gravimetric TG and differential thermal analysis DTA
were performed to illuminate its anti-flame mechanism.
2. Experimental procedure
2.1. Instruments and materials
SX3-4-13 Muffle furnace (Tientsin, precision of
temperature ± 0.1%-0.4%°C); 101-3 Oven (Shanghai,
precision of temperature ± 2°C); Muller (Jiangsu);
Instrument of limiting oxygen index (LOI) (Chengde);
KYKY-2800B scanning electron microscope SEM
(Beijing); Y-4Q X-ray diffractometer XRD (Dandong,
China); NETZSCH TG209 F3 Thermal gravimetric and
Mass Spectrum TG/MS (Germany); STA 449 C Thermal
gravimetric TG and Differential Thermal Analysis DTA
(Germany) were used in this experiment.
Material graphite (C, 5092) was provided by
Action Carbon CO. LTD, Baoding, China. HAc, H2SO4
(98%), KMnO4 are all analytical reagents. Ammonium
polyphosphate APP (I) was purchased from Sichuan,
China. LLDPE 7540 was purchased from Daqing,
China.
2.2. Preparation of the EG
In the intercalation reaction of material graphite,
dosages of KMnO4, H2SO4 and HAc were optimized
through L9 (34) orthogonal tests and the masses used
in the experiments are listed in Table 1. The reactants
were quantified according to a definite mass ratio of
material graphite C : H2SO4 (100%) : KMnO4 : HAc, and
the H2SO4 used was diluted to a mass concentration of
75%. Then, under a definite reaction temperature, the
quantified C was mixed with H2SO4 (75%) and KMnO4
in a 250 mL beaker, and HAc was intermittently instilled
under stirring. After 1.0 hour the product was washed
with de-ionized water and dipped in water for 2.0 hours
until the pH of the waste water reached 6.0-7.0; then
filtered and dried at 60-80°C for about 6.0 hours. EG
was obtained.
2.3. Character of EG
2.3.1. Detection of initial expansion temperature
0.30 g of the prepared EG was spread on an evaporating
dish and heated in an oven for some time, then the
sample was removed and its volume was measured.
When the detected volume of EG was 1.5 times of its
initial volume, the oven temperature was defined as the
initial expansion temperature T0.
2.3.2. Detection of EV
Ten EG samples with a respective mass of 0.30 g
were prepared, and they were heated at different
temperatures in the range of 100-1000°C, then EV
was detected. Expansion times corresponding to the
different expansion temperature are listed in Table 2,
and the higher the expansion temperature was, the
shorter expansion time was observed.
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Table 2. Expansion times used corresponding to different expansion temperature.
Expansion
temperature (°C)
100
200
300
400
500
600
700
800
900
1000
Expansion times
30 min
30 min
5 min
1 min
12S
12S
12S
5S
5S
5S
2.3.3. XRD and TG/MS analysis of EG
XRD analysis was performed with a Y-4Q X-ray
spectrometer using the Cu Kα 1, 2 radiation in the range
of 5° < 2θ< 70°. TG-MS analysis of the prepared EG
was carried out under the condition of N2 ambience and
a heating rate of 20.0 K min-1.
2.3.4. Measurement of EG anti-flame properties
A certain amount of EG was added into melting LLDPE
at less than 120°C, the mixtures were pressed at 140°C
and 10MPa, and then chopped in sliver. The slivers
were used to measure LOI according to Standard of GB/
T2406-1993 with oxygen index instrument.
2.3.5. TG and DTA analysis
TG analysis condition: N2 flux was controlled as
25 mL min-1, and about 10.0 mg sample was laid in a
porcelain crucible, then it was heated to 900°C at a
heating rate of 10 K min-1. Changes of sample weight
with temperature were recorded.
DTA was carried under N2 ambience with a flux of
25 mL min-1. Al2O3 was used as reference compound,
and a heating rate of 10 K min-1 was used.
3. Results and discussion
3.1. Results of the orthogonal test to prepare
EG
In the oxidation of KMnO4, the EG initial expansion
temperature and EV can be influenced by intercalation
of H2SO4 and HAc into graphene planes. So orthogonal
tests were arranged according to Table 2, and the
experiment results are listed in Table 3.
Range analysis shows that reaction temperature
is the most influential factor; the second factor is the
dosage of KMnO4. In the range of the test levels, it is can
be seen that the initial expansion temperature decreases
with the increase of KMnO4 and H2SO4 dosages, but it
increases with the increase of reaction temperature and
HAc dosage.
3.2. Influence of H2SO4 dosages on EG initial
expansion temperature and EV
In order to optimize the condition of H2SO4 dosage,
single-factor experiments were carried out by changing
H2SO4 dosage in the range of 4.5~7.0 g g-1. Experiments
were carried out under the constant mass ratio C:
KMnO4: HAc of 1.0 : 0.4 : 1.0 according to orthogonal
test results; the reactions lasted 1.0 hour at 25°C, and
H2SO4 should be diluted to the mass concentration
of 75%. Results listed in Table 4 show that the initial
expansion temperature and EV of EG are related to the
dosage of H2SO4, and the best mass ratio of H2SO4 to C
should be 5.0 g g-1. Under this ratio, the initial expansion
temperature and EV can reach 160°C and 460 mL g-1,
respectively.
3.3. Influence of reaction temperature on EG
initial expansion temperature and EV
Single-factor experiments of reaction temperature were
carried out under the mass ratio C : KMnO4 : 100%
H2SO4 : HAc of 1.0 : 0.4 : 5.0 : 1.0, the reactions lasted
1.0 hour at the controlled temperature, and H2SO4 was
diluted to a mass concentration of 75%. As shown in
Table 5, when the reaction temperature is controlled
at 25°C, EG with an initial expansion temperature of
160°C and an EV of 460 mL g-1 is obtained. Too low or
too high reaction temperature can lead to the increase
of initial expansion temperature and decrease of EG
dilatability. When the temperature is lower than 25°C, the
intercalation reaction of H2SO4 and HAc is scant. While,
higher temperatures cause the excessive oxidation of
EG. Therefore, the feasible reaction temperature is set
as 25°C.
3.4. Influence of HAc dosage on EG initial
expansion temperature and EV
Under the constant mass ratio C : KMnO4 : 100% H2SO4
of 1.0 : 0.4 : 5.0, the dosage of HAc is changed in the
range of 0.2-1.0 g g-1 with the reaction lasting 1.0 hour
at 25°C. It is found that EG with an initial expansion
temperature of 160°C and EV of 460 mL g-1 can be gained
at the HAc dosage of 0.2 g g-1. Too high HAc dosage
may lead to the relative insufficient KMnO4 dosage, the
deficient oxidation and intercalation. On the contrary,
too low HAc dosage may lead to the relative excess of
KMnO4 dosage and excessive oxidation of EG. All these
can lead to the increase of initial expansion temperature
and decrease of EV.
3.5. Feasible condition of EG preparation
According to the results of orthogonal tests and singlefactor experiments, conditions of preparing EG fitting
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Preparation of low initial expansion temperature
expandable graphite and its flame retardancy for LLDPE
Table 3. L9 (34) orthogonal test results of EG initial expansion temperature b.
Factors
Samples
b
KMnO4
(g)
H2SO4
(g)
HAc
(g)
Reaction temperature (°C)
Initial expansion
temperature (°C)
1
0.2
3.0
2.0
55
235
2
0.4
3.0
1.0
25
175
3
0.6
3.0
3.0
40
200
4
0.2
4.0
1.0
40
210
5
0.4
4.0
3.0
55
220
6
0.6
4.0
2.0
25
175
7
0.2
5.0
3.0
25
187
8
0.4
5.0
2.0
40
170
9
0.6
5.0
1.0
55
177
Sumlevel 1
632
610
562
537
Sumlevel 2
565
605
580
580
Sumlevel 3
552
534
607
632
R
82
78
47
93
1749
Based on 1.0 g material graphite
Table 4. Influence of H2SO4 dosage on EG initial expansion temperature and EV c.
c
Dosage of 100% H2SO4 (g g-1)
4.5
5.0
5.5
6.0
7.0
EV (mL g-1)
420
460
440
433
416
Initial expansion temperature (°C)
180
160
180
170
170
Based on 1.0 g material graphite
Table 5.
Influence of reaction temperature on initial expansion temperature and EV.
Temperature (°C)
0
15
25
40
EV (mL g-1)
200
400
460
380
Initial expansion temperature (°C)
240
195
160
160
for retardancy of LLDPE can be set as: mass ratio C :
KMnO4 : 100% H2SO4 : HAc are controlled as 1.0 : 0.4 :
5.0 : 1.0; the reaction lasts 1.0 hour at 25°C, and H2SO4
is diluted to a mass concentration of 75%, then EG with
an initial expansion temperature of 160°C and EV of
460 mL g-1 can be obtained. The expansion temperature
curve of the EG is shown in Fig. 1. When the expansion
temperature is between 600-900°C, EV of EG is all
above 400 mL g-1.
3.6. Preparation of EG1 with no HAc intercalation
Compared with EG, EG1 was prepared under the
mass ratio C : KMnO4 : 100% H2SO4 of 1.0 : 0.4 : 5.0;
the reaction lasted 1.0 hour at 25°C, and H2SO4 was
diluted to a mass concentration of 75%. The initial
expansion temperature and EV of EG1 was detected
as 170°C and 390 mL g-1, respectively. So that, HAc
can affect EG initial expansion temperature and
dilatability.
3.7. XRD analysis of EG
XRD analysis for material graphite, EG were performed.
As shown in Fig. 2a, the two peaks with the interplanar
crystal spacing of 3.34 Å and 1.67 Å corresponding to
diffraction angle of 26.6°, 54.8° are the characteristic
spectrum of material graphite. While, as shown in
Fig. 2b, the characteristic peak of EG transfers to a small
angle of 26.2°, it corresponds to a big interplanar crystal
spacing of 3.40 Å due to intercalation in graphene planes.
Material graphite is a crystal compound with graphene
planes structure by Van Der Waals force. Under the
oxidation of KMnO4 and H2SO4, the plane repellency will
increase due to the formation of carbon cations through
losing the plane electrons. Then the non-carbonaceous
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Table 6. LOI Results of samples adding different flame retardants.
LLDPE%
EG%
APP (I) %
LOI%
0
0
17.5
70
0
30
20.8
70
10
20
31.7
70
20
10
33.9
70
30
0
28.2
100
reactants can easily insert into the planes, leading to
the increase of interplanar crystal spacing, which allows
graphite to absorb heat, release volatile gas and expand
under the condition of high heat. This makes it suitable
as a flame retardant.
Figure 1. Expansion temperature curve of EG (EV is the volume of
1.0 g EG after expanding at different oven temperature).
3.8. TG/MS analysis of EG
Via chemical or electrochemical oxidation, noncarbonaceous reactants can insert into graphene planes.
Under high temperature conditions, the intercalating
substances can decompose and give out volatile gases
which lead to instant expansion of graphite along C axis
and give birth to graphite worm. Under the condition
of N2 ambience and a calefactive ratio of 20.0 K min-1,
TG/MS analysis of the prepared EG was carried
out. As shown in Fig. 3, it can be seen that when the
temperature is less than 200°C, the decrease of weight
is very small, and only smaller intercalating substances
decompose. When the temperature is above 300°C, EG
weight decreases rapidly, and its expansion capability
rises with the increase of temperature. Additionally, SO2
can be detected at the temperature of 400°C and 700°C,
which indicates that SO2 plays a more important role for
getting EG with high EV.
(a) Material graphite
3.9. Detection of EG anti-flame capability
The processing temperature of LLDPE is lower than
140°C, so the prepared EG can be used as flame
retardant. Flame retardants were added to LLDPE
according to the proportion listed in Table 6. The results
show that addition of 30% EG can make LOI improve up
to 28.2%. But the addition of the same amount of EG1
or purchasing EG to LLDPE can only make LOI reach
24% and 23% [23], respectively. LOI is only 21% under
the condition of single 30% APP (I) as flame retardant,
but it can be improved to 33.9% when 20% EG together
with 10% APP (I) are applied as flame retardant. After
combustion, EG changes to graphite worm on the
surface of LLDPE (Fig. 4), and a heat insulation layer
is formed by the swollen multicellular char. During the
expansion of EG, the existence and decomposition of
(b) EG
Figure 2.
XRD analysis of material graphite and the EG (Cu Kα 1, 2
radiation in the range 5° < 2θ< 70°).
APP (II) might cause compact coking charred layers.
This char structure can limit heat and mass transfer,
which is helpful to retard combustion [18,23].
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Preparation of low initial expansion temperature
expandable graphite and its flame retardancy for LLDPE
Figure 3. TG/MS analysis of EG (Under the condition of N2 ambience and a calefactive ratio of 20.0 K min-1).
samples. Mass loss of these anti-flame systems
between 100-400°C is less than 5%, which is caused by
incomplete decomposition of APP (I) and EG expansion.
The loss of weight is about 13% when temperature
increases to 450°C. Additionally, 60% weight loss occurs
around 450-520°C, which is in a good agreement with the
high dilatability of EG under this temperature (as shown
Fig. 1). Compare LLDPE retardance sample of 70%
LLDPE/10% APP (I) /20% EG with 70% LLDPE/30%
EG, the former gives higher residual carbon of 25.9%
than the latter, which testifies the synergistic anti-flame
of EG with APP (I) [18,23].
Figure 4. SEM
analysis of the burn-out LLDPE with 20% EG and
10% APP (I) as flame retardants.
Figure 5.
TG analysis of LLDPE retardance samples 1. 70%
LLDPE/10% APP/20% EG; mass percent of LLDPE, APP
(I) and the EG is 70%, 10% and 20%, respectively 2. 70%
LLDPE/30% EG; mass percent of LLDPE and the EG is
70% and 30%, respectively.
3.10. TG analysis of retardance LLDPE samples
Fig. 5 shows the TG analysis results of 70% LLDPE/10%
APP (I) /20% EG and 70% LLDPE/30% EG retardance
3.11. DTA analysis
It is well known that huge heat will be consumed during
the decomposition of APP (I) and expansion of EG,
which results in the decrease of material temperature.
As shown in Fig. 6, the peaks corresponding to 111°C
are melting decalescence peak of LLDPE. Peaks
corresponding to 483.5°C and 481°C, should be two
strong decalescence peaks corresponding to EG
expansion containing in LLDPE retardance samples of
70% LLDPE/10% APP/20% EG and 70% LLDPE/30%
EG, respectively. The higher decomposing temperature
of 483.5°C than 481°C shows that the addition of APP
will form compact multicellular char, and improve thermal
stability of 70% LLDPE/10% APP/20% EG retardance
system.
3.12. Analysis of anti-flame mechanism
According to the results of TG and DTA, the anti-flame
mechanism of EG was proposed as follows: when
touched with flame, the structure of EG will change into a
swollen multicellular char, which limits the transfer of heat
and mass, as well as oxygen diffusion. Instantaneous
expansion of the EG can absorb huge heat and releases
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Figure 6.
DTA analysis of LLDPE retardance samples 1. 70%
LLDPE/10% APP/20% EG; mass percent of LLDPE, APP
(I) and the EG is 70%, 10% and 20%, respectively 2. 70%
LLDPE/30% EG; mass percent of LLDPE and the EG is
70% and 30%, respectively.
SO2, CO2, H2O; and the coexistence of EG and APP (I)
can give more residual carbon and improve thermal
stability of the LLDPE retardance system.
4. Conclusions
In this work, EG with different initial expansion
temperature and EV can be prepared through adjusting
dosages of KMnO4, H2SO4 and HAc. The preparing
condition of EG with low initial expansion temperature
and high EV is: C : KMnO4 :100% H2SO4 : HAc = 1.0
: 0.4 : 5.0 : 1.0 (mass ratio); the reaction lasts 1.0 h
at 25°C, and H2SO4 needs to be diluted to a mass
concentration of 75%. Under this condition, EG with
an initial expansion temperature of 160°C and EV of
460 mL g-1 can be prepared.
Reaction temperature has important influence on
EG dilatability. It can affect reaction environment and
reactant activity; affect open degree of graphene planes;
and influence the inserting dosages of HAc and H2SO4;
then influence the expansion capability of EG.
The excellent anti-flame capability of EG is
contributed by the ability of absorbing huge heat and
the formation of swollen multicellular char, which acts as
physical barrier for heat and mass transfer. It is can be
seen that addition of 30% EG to LLDPE can improve LOI
up to 28.2% from 21%. Moreover, addition of 20% EG to
LLDPE together with 10% APP (I) leads to a higher LOI
of 33.9% because of the synergistic flame retardancy
between EG and APP (I).
Acknowledgements
This study is supported by Undergraduate Scientific
and Technological Creation Project (STCP) of Hebei
University (2011075) and Doctor Foundation of Hebei
province Education Office (China, No.B2004402). We
gratefully acknowledge their support during the study.
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