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EPDM formulations for electric wires and cables
Article · January 2001
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ELASTOMERE UND KUNSTSTOFFE
ELASTOMERS AND PLASTICS
EPDM compositions aluminum hydroxide mechanical properties electrical properties
EPDM formulations were developed
with both carbon black and aluminum hydroxide as fillers. Mechanical
and electrical properties were evaluated with the purpose to find out
those compositions which would
meet the Brazilian Association for
Technical Standards, ABNT, requirements as for their use in wires and
cables. As expected carbon black
has a negative effect and its presence should be restricted to less
than 22 phr if these materials are to
be used in the range of medium
voltages (up to 15 kV).
EPDM Rezepturen fuÈr ElektrodraÈhte und Kabel
EPDM Rezepturen Aluminium Hydroxid mechanische Eigenschaften elektrische Eigenschaften
EPDM Rezepturen mit Ruû und Aluminium Hydroxid als FuÈllstoffe wurden mit dem Ziel entwickelt, die
mechanischen und elektrischen Eigenschaften den Anforderungen der
Brazilianischen Vereinigung fuÈr
Technische Normen (ABTN) anzupassen und eine Verwendung bei
der Isolation vor DraÈhten und Kabeln zu finden. Wie erwartet hat Ruû
einen negativen Effekt. Daher sollte
die Ruûkonzentration auf 22 phr beschraÈnkt werden, wenn diese Materialien im mittleren Spannungsbereich (bis zu 15 kV) eingesetzt werden.
56
EPDM Formulations for Electric
Wires and Cables
C. Canaud, M. Antonio Sens, L. L. Yuan Visconte,
R. C. Reis Nunes, Rio de Janeiro (Brazil)
Ethylene-propylene-diene copolymers
(EPDM) constitute a class of terpolymers
whose rheological and physical properties like flow patterns or the degree of
crystallinity, respectively, are strongly affected by the polymer composition, that
is, the relative amounts of the comonomers involved [1].
Due to the saturated chain backbone,
EPDM can be employed in different formulations, each one adequate to a specific application, in which characteristics
such as resistances to ozone, heat,
moisture and intemperism, flexibility to
low temperatures, wide range of tensile
strength and excellent electrical insulation properties are required [1 ± 3]. These
properties, being particularly important in
the electrical sector, have stimulated an
increasing utilization of EPDM-based
compositions in medium and high voltage insulating cables and wires, in cables
electric terminals and in polymeric insulators [4 ± 7]. In the past this market has
been supplied mostly by PVC, mainly because of its low flammability, low cost,
good performance and processability.
However PVC has been considered a
matter of concern by the ambientalists
due to the presence in its compositions
of chlorine and heavy metal based stabilizers which are liberated or can liberate
toxic gases during combustion [8].
Nevertheless, the addition of aluminum
hydroxide has rendered these effects
weaker.
Despite these steps forward concerning the use of PVC, EPDM rubbers have
increased their participation in the market
of wires and cables since they present
several advantages, compared to PVC.
Among them a low sensitivity to UV radiation, low toxicity and corrosion power and
a remarkable chemical resistance have to
be mentioned. Yet, EPDM is highly flammable and needs to be protected with
high percentages of a flame retarder. In
general, amounts as high as 100 phr
are necessary to impart EPDM a good
level of flame retardancy. So, although relatively expensive when compared to
other rubbers, EPDM is able to accept
high amounts of fillers which, at the
end, helps to achieve cheaper final products. Thus the combination of characteristics such as processability, low
cost, low weight and excellent insulating
properties has turned polymeric materials
into those the most used in electrical applications [9].
Wire and cables insulation is expected
to avoid the contact among the conducting elements, where electricity goes
through, from other elements, providing
the conductors also with an appropriate
mechanical support. The insulator is applied on the conductor as thin layers and
one must point out that these small
amounts employed are submitted to elevated electrical demands and thus these
materials have to be looked at with special care as to their mechanical properties.
The heat overload in the conductors
resulting, for example, from imperfect
connections, is a potential source of ignition, which may put the whole electrical
installation in danger, since the structural
element present in the cable coating, that
is, the polymer, is a highly inflammable
material. Thus, the use of polymeric materials in electrical applications requires a
special precaution with regard to fire possibility. Both material and the shape of the
electrical appliances must be carefuly selected as to guarantee that they will not
KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 1-2/2001
EPDM Formulations for Electric Wires and Cables
Table 1. Formulations used in EPDM compositionsa
Ingredient
EPDM
Carbon black ± N330
ATHb
Zinc oxide
Stearic acid
Sulfur
Paraffinic oil
MBTc
TMTDd
0
1
2
3
4
100
0
0
5
1
1.5
25
0.5
1.0
100
22
150
5
1
1.5
25
0.5
1.0
100
15
160
5
1
1.5
25
0.5
1.0
100
7.5
170
5
1
1.5
25
0.5
1.0
100
0
180
5
1
1.5
25
0.5
1.0
a
In phr
Aluminum hydroxide; 2 types were used: with and without surface treatment
c
Mercaptobemzothiazole
d
Tetramethylthiuram dissulfide
b
burst into fire or facilitate fire propagation
when in operation conditions, due to material failure or exposition to an external
flame [10 ± 13].
Amongst flame retarders, aluminum
hydroxide (ATH) is the most used presently. The main reason for the increasing
usage of this material can be credited to
its cost-benefit relationship [14, 15]. The
main advantages of ATH over other flame
retaders are the low cost and reduced
toxicity, and the fact that ATH can act simultaneously as both flame retardant and
smoke suppressor agents. It does not liberate toxic or corrosive substances in
high temperature conditions [14,16 ± 18].
As a semi-reinforcing filler, ATH does
not significantly increase mechanical resistance when incorporated in a polymeric material and, thus, a few superficial
treatments with silanes and titanates
have been carried out with the purpose
to improve the reinforcing character of
ATH [14].
In this work, EPDM formulations were
developed by incorporating carbon black
(N330) and ATH as fillers, in different
amounts. Mechanical as well as electrical
tests were performed to determine the
applicability of these compositions as insulating coatings for electrical wires and
cables.
measurements were carried out on a
Waynekerr Precision Inductance Analyser 3245, using a Tettex AG guard ring
capacitor. The dielectric resistance was
determined by using a AC Dielectric
Test Set model 7100-10, as the voltage
source, and a Peak Voltmeter VAH 792
to measure the breakdown voltage. All
experiments followed ASTM methods
and the results were compared to pure
gum.
Results and discussion
EPDM formulations were obtained by
partially replacing carbon black by ATH,
superficially trated or not with silane,
with the purpose of determining the effect
of this substitution on mechanical and
electrical properties. Since these compositions were thought to be used in electrical applications, they had to be adjusted
as to fulfill the Brazilian Association for
Technical Standards (ABNT) together
with the requirement to present characteristics of flame retardancy. According
to the literature, anti-flame characteristics
are met when ATH is incorporated in
quantities in the range of 150 to
Table 2. Stress and strain at break for EPDM compositions
Experimental
The recipes used to prepare EPDM formulations are presented in Table 1.
The compositions were prepared in a
roll mill at 30 8C and vulcanized in a hot
press at 160 8C. Volumetric and superficial resistivities were determined on a
610 C Solid State Electrometer. The relative permissivity and the dissipation factor
180 phr [19, 20]. Thus, in this work the
compositions studied had a minimum
of 150 and a maximum of 180 phr of
ATH. As the fillers used, N330 and
ATH, have different densities, the
amounts of each filler in the mixes were
calculated on a volume basis, with the
precaution to keep constant the total volume of fillers at 74:4 cm3 .
Table 2 presents the values of stress
and strain at break for the compositions
investigated.
Comparing to pure gum, the semi-reinforcing character of ATH can be observed since the presence of this filler improves the stress at break. However, this
improvement is more significative when
N330 is added in larger quantities. Apparently, the surface treatment had a positive
effect only in the composition having ATH
as the only filler.
According to ABNT, the compositions
150/22, 160/15, 160/15* and 170/7.5
meet the requirements of stress at break
for materials to be applied in electrical
wire and cables. Considering strain at
break, all compositions could be used.
From the values of modulus, the formulations were all superior to pure
gum, specially those in which ATH had
been treated with silane (Table 3). However, for each type of ATH, the variation
in the amount of filler did not have
much influence on this property.
Hardness is a measure of the resistance of a material to local deformations
[21 ± 23] and depends, among other factors, on the amount of filler, the number of
crosslinks and may also be related to
modulus. As already observed for this
last property, hardness is positively influenced by filler incorporation, as shown in
Table 3, since all compositions presented
hadness values higher than pure gum.
This property, however, was not affected
*
ATH/N330 (phr)
Stress at break (MPa)
Strain at break (%)
0/0
150/22*
150/22
160/15*
160/15
170/7.5*
170/7.5
180/0*
180/0
ABNT requirements
1.65
9.1
11.4
11.2
13.8
8.7
10.8
8.0
6.6
> 10
178
329
417
360
404
424
503
393
427
> 300
ATH with silane
KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 1-2/2001
57
EPDM Formulations for Electric Wires and Cables
Table 3. Modulus at 100 %, hardness and tear strength of EPDM compositions
*
ATH/N330 phr
Modulus at 100 %
MPa
Hardness Shore A
Tear strength
kN/m
0/0
150/22
150/22*
160/15
160/15*
170/7.5
170/7.5*
180/0
180/0*
1.3
2.9
4.8
4.4
5.2
3.6
5.1
3.7
5.3
45
71.5
73
73.5
73
71.5
72.5
71
72
6.32
31.09
38.16
29.16
35.83
23.92
35.17
22.14
30.07
ATH treated with silane
neither by the amount nor the type of
ATH.
In applying polymeric materials as insulating coatings for electrical wires and
cables, tear resistance is a very important
property. Low tear resistances may increase the chances of microfissures
which are responsable for the appearing
of electric discharges and, in consequence, of electric ruptures and insulation loss [24]. The behavior of tear resistance for the compositions investigated
can also be seen in Table 3.
From this Table, all compositions presented tear resistance values superior
to pure gum for both types of ATH, but
even higher for the silane-treated series.
Large amounts of ATH were, nevertheless, deleterious to this property. Thus,
in the search for better mechanical properties, higher incorporations of carbon
black were tried by preparing compositions with filler ratios equal to 120/45
and 140/30, but as will be seen later,
these formulations were not suitable for
electric applications.
The electrical properties evaluated in
this work were superficial and volumetric
resistivities, dielectric constant, dielectric
rigidity and dissipation factor, which are
the most important properties in qualifying a material as an electric insulator
[25, 26].
Electrical properties can be divided in
two groups. In general, in the first one
the resistance of a polymer to a low intensity electric field is evaluated and in this
group properties like dielectric constant
and dissipation factor are included. The
second group is constituted by those
properties which are important in more intense electric fields, such as electric discharge and rupture [9, 27, 28].
The dielectric rigidity of an insulating
material can be defined as the maximum
58
tension necessary to produce a dielectric
rupture. It is expressed in volts divided by
the material thickness [9, 26, 29] and data
for this property are shown in Table 4.
From the analysis of the Table, a drastic
reduction in the dielectric rigidity, which is
related to the application of elevated electric fields, is observed for the composition
120/45. The others, excepting composition 150/22, had an increase in the rigidity
as more ATH was incorporated. This can
be due to differences in shape presented
by the different fillers. Carbon black particles are agglomerated and, although the
average size of its particles (3ÿ10 m) is
smaller than ATH particles (10ÿ6 m) the
tendency for agglomeration results in larger volumes containing occluded voids.
These voids can promote current escapes, exposing the installation to electric discharges and leading to a rapid perforation of the polymeric composition.
Data also show a slightly superior behaviour for the compositions with non-treated ATH, since the treatment imparts
an induced polarity to this filler.
Values of superficial and volumetric resistivities are also presented in Table 4.
Electric resistivity measures how resistant a material is to the the passage of
current through its surface or its volume,
thus avoiding the escaping currents, responsible for its heating and capacity
loss as insulator, for a given application.
In this regard, resistivity data must be
as high as possible.
Minimum values are found for the composition 120/45 and these values approach that for pure gum, as the amount
of N330 is reduced. It can also be observed that the silane treatment did not
bring any significant effect. Probably,
the expected positive effects resulting
of a better matrix-filler adhesion, caused
by the surface treatment, have been
counterbalanced by the presence of the
polar groups in the coupling agent,
thus leading to a reduction in the volumetric resistivity. The superficial resistivity,
for its turn, takes into account factors as
impurities and moisture.
The dielectric constant, also shown in
Table 4, measures the ability a material
has to store charge [30, 31] and the absolute values for this property should be as
low as possible for a material be considered as an insulator. Again, the values of
dielectric constant approach the one obtained for pure gum, as the amount of
ATH increases. The effect of the silane
treatment is not significative for the
same reasons as discussed before.
The dissipation factor is said to be the
ratio between the current lost by the insulator (escape current) and the amount of
charge that crosses the conductor, when
a tension is applied, this quantity being
expressed in percentage. The dissipation
factor is then a measure of the dielectric
loss that occurs due to the dielectric re-
Table 4. Dielectric properties of EPDM compositions
*
ATH/N330 phr
Dielectric rigidity
kV/mm
Log Superficial
resistivity X
Log Volumetric
resistivity Xm
Log Dielectric
constant
0/0
120/45
120/45*
140/30
140/30*
150/22
150/22*
160/15
160/15*
170/7.5
170/7.5*
180/0
180/0*
27.4
1.2
1.8
9.6
11.0
25.8
24.7
21.8
20.5
26.6
25.2
32.9
32.7
16.19
7.65
8.00
15.41
15.17
14.18
15.35
14.37
14.44
15.80
16.04
15.84
15.63
16.52
7.94
7.93
14.71
14.67
14.80
14.76
14.81
14.76
15.09
14.91
14.85
14.73
0.37
2.74
2.78
0.94
0.95
0.59
0.60
0.65
0.66
0.59
0.60
0.54
0.57
ATH treated with silane
KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 1-2/2001
EPDM Formulations for Electric Wires and Cables
Figure 1. Influence of filler composition on dissipation factor
laxation, in consequence of a delay in the
dipole orientation with the applied field.
The variation of the dissipation factor
with frequency is due to a reduction of
the dielectric constant as the frequency
increases and is directly related to a higher or lower easiness the dipoles have to
align themselves with the alternate field
[32]. So, larger molecules or agglomerates experience more difficulty to orient
themselves in the alternate field frequency and become out of phase in relation to it, causing a reduction in polarization and in dielectric constant.
From Figure 1, the most intense dissipation is observed when 45 phr of N330
are used, due to the conducting characteristics of this filler, and decreasing values are found as the amount of N330
is also decreased. Slightly superior values
Figure 2. Variation of dissipation factor with frequency in EPDM
compositions containing silane treated ATH
are also observed for the compositions
with silane treated ATH.
In order to verify the possibility of moisture absorption, which in big amounts
would rise questions concerning the validity of the aforementioned electric analysis, the dissipation factor was measured
at different frequencies. A crescent linear
relationship would be an indication of
moisture absorption.
Figures 2 and 3 present the dissipation
factor behavior as a function of frequency
for the different compositions and show a
small variation of this property as frequency increases, followed by a tendency to level off, suggesting that there
was not enough moisture absorption as
to disguise the results obtained.
From the results, it can be concluded
that, due to its conductive nature, the
Figure 3. Variation
of dissipation factor with frequency
in EPDM compositions containing
untreated ATH
KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 1-2/2001
presence of N330 leads to a reduction in
the dielectric properties (for use as insulators), when the formulations are compared to pure gum, and that this effect
is more drastic when this filler is present
in higher amounts. This can be related to
the formation of a conductive filler network above a percolation limit [33].
According to ABNT, to be useful as
electric insulators, the material must
have the following characteristics:
* A dielectric constant varying between
2.0 and 4.0, at room temperature
and 1 kHz frequency;
* Concerning the dielectric rigidity, the
material must resist to 22 kV tension,
to be used in voltages up to 15 kV,
and to 30 kV, to be used in voltages
up to 20 kV;
* The loss factor must not exceed 2 %,
at room temperature.
Thus, analysing the results and considering only the electric aspects, the compositions 150/22 and 170/7.5 are suitable for application in medium voltages
(up to 15 kV) and 180/0 for high voltages
(up to 20 kV) wires and cables. However,
when mechanical resistances are also
considered, only compositions 150/22
and 170/7.5, both containing untreated
ATH, meet the ABNT requirements.
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59
EPDM Formulations for Electric Wires and Cables
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The authors
Cristine Canaud concluded her M. Sc. Thesis on this
subject. Marcio Antonio Sens, Materials Department,
Centro de Pesquisas de Energia EleÂtrica ± CEPEL,
Cidade UniversitaÂria. Regina CeÂlia Reis Nunes and
Leila LeÂa Yuan Visconte are Associate Professors
at the Instituto de MacromoleÂculas Professora Eloisa
Mano, Universidade Federal do Rio de Janeiro
Corresponding author
Regina CeÂlia Reis Nunes
Centro de Tecnnologia
Bloco I
Cidade UniversitaÂria
Ilha do FundaÄo
C.P. 68525
CEP 21945-970 Rio de Janeiro
RJ, Brasil
KGK Kautschuk Gummi Kunststoffe 54. Jahrgang, Nr. 1-2/2001
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