Polymer Degradation and Stability 94 (2009) 57–60
Contents lists available at ScienceDirect
Polymer Degradation and Stability
journal homepage: www.elsevier.com/locate/polydegstab
Characterization of poly(ethylene-co-vinyl acetate) (EVA) filled with low grade
magnesium hydroxide
A.I. Fernández a, *, L. Haurie b, J. Formosa a, J.M. Chimenos a, M. Antunes c, J.I. Velasco c
a
Department of Materials Science and Metallurgical Engineering, Universitat de Barcelona, c/Martı́ i Franquès 1, 08028 Barcelona, Spain
EPSEB, Edifici P Lab. Materials, Universitat Politècnica de Catalunya, Av. Dr. Marañon, 44-50, 08028 Barcelona, Spain
c
Centre Català del Plàstic, Universitat Politècnica de Catalunya, c/Colom 114, 08222 Terrassa (Barcelona), Spain
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 August 2008
Received in revised form
25 September 2008
Accepted 7 October 2008
Available online 21 October 2008
Low-grade magnesium hydroxide (LG-MH) is a solid by-product that undergoes an endothermic
decomposition in the temperature range of 300–750 C. Due to its thermal behaviour and its lower cost
relative to pure Mg(OH)2, it was studied as a non-halogenated flame retardant filler in a 28% vinyl acetate
(VA) content poly(ethylene-co-vinyl acetate) matrix. The solid was characterized by XRF and the crystalline phases determined by XRD, composed predominantly of Mg(OH)2 and calcium and magnesium
carbonates. Particle size reduction was performed by both mechanical as well as air jet milling in order to
optimize the particle size distribution.
Composites with different filler concentrations were prepared to evaluate the mechanical properties and
flame retardancy by means of limiting oxygen index tests. LOI was also determined in specimens filled
with commercial flame-retardants to analyse the effectiveness of this solid.
Ó 2008 Published by Elsevier Ltd.
Keywords:
EVA
Low-grade magnesium hydroxide
Flame retardancy
1. Introduction
The tightening of the current regulations, and the increase in the
general awareness about the risks related to materials flammability,
force us to seek alternatives to ensure the benefits of numerous
materials present in everyday life, that so far did not require
a special consideration on these properties [1]. In this context, an
alternative is the use of a low-grade magnesium hydroxide, which
means adding value to an industrial by-product resulting from the
calcination of natural magnesite. The flue dust collected in the
cyclones and fabric filters is stockpiled, tempered with water and
then weathered for a long period, resulting in the hydration of
magnesium oxide and the carbonation of lime.
Because of its endothermic decomposition between 300 and
750 C, and its lower cost relative to pure magnesium hydroxide, it
was deemed of interest to assess the ability of the LG-MH to act as
low cost flame retardant [2], including it in the group of halogen
free flame-retardants extensively studied in order to reduce smoke
toxicity [3]. This filler may be suitable for materials commonly used
without flame-retardants in their formulation for costs reasons.
The high filler content needed to reach adequate fire retardancy
properties makes it necessary to optimize the solid characteristics
to improve the processing and the final properties of the composite.
* Corresponding author.
E-mail address: ana_inesfernandez@ub.edu (A.I. Fernández).
0141-3910/$ – see front matter Ó 2008 Published by Elsevier Ltd.
doi:10.1016/j.polymdegradstab.2008.10.008
Among these characteristics, particle size reduction and good
homogenization of the mineral filler are considered as critical [4,5]
to achieve suitable plastic formulations. Huang et al. [6] studied the
effect of particle size on flame-retardancy in the system EVA–
Mg(OH)2 and conclude that the good distributive dispersion and
the small particle size tend to result in better flame-retardancy.
In this study, two milling alternatives were considered and the
solid resulting with the smallest particle size was the filler used for
compounding.
A series of composites were prepared using an EVA matrix and
different concentrations of LG-MH to evaluate the mechanical
properties. LOI tests were performed to evaluate the flame behaviour and the results compared with those obtained with commercial non-halogenated flame-retardants [7].
2. Experimental
2.1. Materials
The polymer used in this study was a poly(ethylene-co-vinyl
acetate), EVA Escorene Ultra UL 00728 from ExxonMobil Chemical,
and with a vinyl acetate content of 28%. Commercial non-halogenated flame-retardants used for LOI comparison were Magnifin
(MH) H5KV supplied by Martinswerk and natural hydromagnesite–
huntite Ultracarb C5-10 (U) produced by Minelco. The low-grade
magnesium hydroxide LG-MH (EnvimagÒ) was supplied by Magnesitas Navarras S.A.
58
A.I. Fernández et al. / Polymer Degradation and Stability 94 (2009) 57–60
2.2. LG-MH characterization
2.2.1. Particle size reduction
Mechanical milling of the LG-MH was performed using a Retsch
ring mill for 5 min and 1400 rpm. Air jet milling of LG-MH was
performed on a 100AFG jet mill with a collector speed of
12.000 rpm and air pressure of 500 kPa. Particle size distribution
before and after the size reduction processes was determined by
light scattering with a Beckman Coulter LS13 320 apparatus.
2.2.2. Physicochemical characterization
After the particle size reduction by air jet milling, the resulting
solid was analysed by X-ray fluorescence (XRF) using a Philips
PW2400 X-ray sequential spectrophotometer to elucidate major
and minor components. X-ray diffraction was performed in
a Bragg–Brentano Siemens D-500 powder diffractometer with Cu
Ka radiation to get information about the crystalline phases.
Density was measured with a helium pycnometer and the specific
surface area by the BET single point method with a Micromeritics
porosimeter.
2.2.3. Thermal analysis
A TGDTA92 Setaram device was employed to evaluate the LGMH thermal decomposition by performing thermogravimetric
(TGA) and differential thermal analysis (DTA) in air flow at a heating
rate of 10 C/min from 30 to 1000 C. Differential scanning calorimetry (DSC) was performed in air atmosphere using a Mettler
Toledo DSC-822e/400 calorimeter to determine the heat associated
to the thermal decomposition in the range from 30 to 600 C.
Fig. 1. Volumetric distribution of LG-MH without particle size reduction, after air jet
milling and mechanical milling processes.
specimens in an oxygen–nitrogen atmosphere, was measured in
accordance with ISO 4589 standard.
3. Results and discussion
3.1. Particle size reduction
LG-MH was first dried at 100 C for 24 h before compounding.
Composites were prepared in a single stage using a Collin Kneter
25X36D twin-screw extruder with an L/D of 36, after which they
were water-cooled and pelletized. The pelletized material was then
compression-moulded in a hot-plate press (IQAP-LAP PL-15) at
160 C to obtain 1 mm thick sheets. The samples and compositions
prepared are listed in Table 1, expressing the filler content as phr
(parts per hundred of resin). Table 1 also shows samples density
and respective filler concentration determined by ashing (UNE-EN
ISO 3451-1).
It has been largely described that the presence of aggregates
and/or a wide particle size distribution can result in a poor
homogenization of the filler in the polymer matrix, ultimately
affecting the composite mechanical properties. The LG-MH presents an extremely wide particle size distribution with an important
amount of particles above 200 mm, making absolutely necessary
a reduction of particle size. Fig. 1 shows the volumetric particle size
distribution of LG-MH before milling and the two distributions
after both mechanical and air jet milling. The last of the two
procedures was preferred as the whole sample showed a particle
size diameter below 10 mm, while the mechanical milling lead to
a bimodal distribution with particle sizes over 40 mm.
Magnifin (MH) and Ultracarb (U) fillers used for comparison
have both particle sizes lower than 6 mm. The d10 and d90 values for
the MH filler are 0.7 and 4.4 mm respectively. In the case of U filler
these values are 0.2 and 5.6 mm.
2.4. Composites characterization
3.2. Physicochemical characterization
2.4.1. Mechanical properties
Tensile properties were measured using a universal testing
machine Galdabini sun 2500 with a constant displacement speed of
10 mm/min. The composites stiffness was evaluated using the
secant elasticity modulus at 1% of strain (E1%), yield strength sy, and
maximum elongation 3max. The fracture surface was observed by
scanning electronic microscopy (SEM).
The diffraction pattern in Fig. 2 shows Mg(OH)2 as the major
phase and unburned magnesite as a minor one, as well as dolomite
and quartz that occur in natural magnesite. Calcium carbonate
originated during the tempering and weathering of the cyclone flue
2.4.2. Flame testing
Limiting oxygen index LOI which corresponds to the minimum
percentage of oxygen needed for the combustion of 80 10 1 mm
Table 1
Samples codes, composition and density.
Sample
EVA
EVA41
EVA46
EVA51
EVA60
* Mg(OH)2
*
*
• MgCO3
♦ MgCO3·CaCO3
º CaCO3
§ SiO
2
I (a.u.)
2.3. Polymer compounding
* •
•
♦
Density (g/cm3)
Composition (phr)
EVA
LG-MH
100
58.9
54.5
49.0
39.8
0
41.1
45.5
51.0
60.2
•
º
§
º
0.952
1.11
1.22
1.23
1.38
10
15
20
25
30
♦ º
º ♦
35
40
*
*
45
50
2θ
Fig. 2. X-ray pattern of LG-MH.
55
60
*
65
70
A.I. Fernández et al. / Polymer Degradation and Stability 94 (2009) 57–60
59
8
Table 2
LG-MH composition determined by XRF.
Composition
Percentage (%)
MgO
CaO
SiO2
Fe2O3
Al2O3
MnO
TiO2
P2O5
49.57
7.98
3.24
2.14
0.41
0.26
0.02
0.08
EVA
EVA41
EVA46
EVA51
EVA60
7
Stress (MPa)
6
5
4
3
2
1
dust is also present in a lower extent. The chemical composition
obtained from X-ray fluorescence is given in the form of oxides in
Table 2, corroborating the crystalline phases identified by X-ray
diffraction. Metal impurities such as iron, aluminium, manganese,
titanium and phosphorous also occur in the natural magnesite. The
measured density and specific BET surface area of the LG-MH after
the air jet milling are respectively of 2.44 g/cm3 and 30.0 m2/g.
3.3. Thermal analysis
The thermogravimetric analysis of the LG-MH performed in air
atmosphere is shown in Fig. 3. The weight loss below 200 C
corresponds to the equilibrium moisture content of the filler. The
first step corresponds to the thermal breakdown of magnesium
hydroxide with water release in the range of 320–480 C, followed
by the decarbonation of magnesite and dolomite until 750 C. The
total weight loss at 1000 C is of 34%. However, to fulfill polymer
flame retardant applications, we should focus on the weight loss
between 200 and 450 C corresponding to magnesium hydroxide
decomposition, which has an associated heat measured with DSC of
485.2 J g 1. The differential thermal analysis (DTA) also shown in
Fig. 3 corroborates the endothermic decomposition corresponding
to dehydroxylation around 375 C. Subsequent endothermic peaks
between 480 and 750 C correspond to the decomposition of
carbonates. Commercial pure magnesium hydroxide (MagnifinÒ)
has an associated decomposition heat of 1371 J g 1, corresponding
to a one-step decomposition with a weight loss of 29% at 340 C.
The other commercial flame retardant used in this study is a natural
basic magnesium carbonate associated with huntite (UltracarbÒ). It
shows an endothermic decomposition in several stages with
a weight loss of 42% from 200 to 600 C.
3.4. Polymer compounding
Due to the impurities content, LG-MH is a grey coloured solid
and therefore the composites prepared with this filler also present
0,2
0
TGA
DTA
-5
0
-0,2
-15
-0,4
-20
-25
-0,6
-30
-0,8
-35
-1
-40
-45
-50
50
150
250
350
450
550
650
750
850
Temperature (ºC)
Fig. 3. TGA and DTA curve in air, 10 C/min.
950
ΔT
Weight loss (%)
-10
0
0
10 20 30 40 50 60 70 80 90 100 110 120 130 140
Strain (%)
Fig. 4. Stress–strain curves of studied composites.
a dark grey final colour. The samples for both mechanical testing as
well as for LOI measurement were obtained from the 1 mm thick
sheets prepared for each composition.
3.4.1. Mechanical properties
The tensile stress–strain curves of the different composites are
depicted in Fig. 4. Maximum elongation is always over 200% except
for EVA60 that, even keeping an important ductility, is significantly
lower (60%). This fact is mainly due to poor compatibility between
filler and polymer matrix, which probably could be improved by
the use of filler surface treatment. On the other hand, this sample
presents the highest yield strength. The elastic modulus increases
with the filler content from 19 MPa for pure EVA up to 122 MPa for
EVA60. Table 3 summarizes these results.
Fig. 5 shows the fracture surface images of samples EVA46 and
EVA60 obtained by SEM. Plastic deformation of the polymer matrix
and isolated filler particles with poor adhesion are observed. This
lack of adhesion is directly related with the decrease of elongation
for the sample with higher filler content. Differences on the plastic
deformation and on the amount of filler particles are observed
between both samples. Sample EVA46 shows an important plastic
deformation while the amount of filler particles is higher for
sample EVA60. Even though the plastic deformation of this sample
is lower than the rest of samples it seems sufficient for some
applications like some building materials.
3.4.2. Flame testing
Table 3 also includes LOI values. Unfilled EVA shows the lowest
LOI value (21.0), while increasing the filler content from 41 to 51%
only increases the LOI value up to 22.9. However, this parameter
improves significantly, up to 28.0, when the filler content increases
up to a concentration of 60%. In order to compare this flameretardancy behaviour with some commercially available flameretardants, EVA composites having a 60 phr of filler were prepared.
The LOI values of composites of MagnifinÒ H5 KV and UltracarbÒ
were 39 and 34, respectively.
Taking into account that, in the range from 200 to 500 C only
the 15% of the LG-MH is acting as an effective endothermic flame
Table 3
Mechanical properties and LOI values.
-1,2
Sample
LOI
E1%/MPa
sy/MPa
Elongation/%
-1,4
EVA
EVA41
EVA46
EVA51
EVA60
21.0
22.7
22.7
22.9
28.0
19 2
30 3
44 4
54 6
122 13
3.0 0.2
4.5 0.1
5.2 0.3
5.4 0.2
6.2 0.3
>200
>200
>200
>200
60
60
A.I. Fernández et al. / Polymer Degradation and Stability 94 (2009) 57–60
retardant, it is reasonable to assume the lower LOI value for EVA60
when compared to those obtained using commercial flame retardants. Nonetheless, this filler is presented as an economic alternative suitable for applications that do not include flameretardants within the formulation by cost reasons.
4. Conclusions
LG-MH has a chemical composition based on magnesium
hydroxide and magnesium and calcium carbonates showing an
endothermic decomposition between 200 and 720 C.
From the tensile tests it is concluded that a compatibilising
agent or filler surface treatment should be used to improve the
mechanical properties of highly loaded composites.
Flame testing of the prepared EVA matrix composites reveals
that filler contents above 50 phr are required to achieve
significant results.
For a filler content of 60 phr LOI, values of LG-MH composites
are lower than those obtained using commercial flame retardants, in accordance with the effective Mg(OH)2 content.
LG-MH could be used as a low cost flame retardant filler for
applications where the composite does not already include
flame-retardants in its formulation.
Acknowledgements
The authors would like to thank Magnesitas Navarras S.A. for
supporting and financing this research project.
References
Fig. 5. (a) Fracture surface SEM image of sample EVA46. (b) Fracture surface SEM
image of sample EVA60.
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