ROHSTOFFE UND ANWENDUNGEN
RAW MATERIALS AND APPLICATIONS
Aluminium powder Bonding agents Nitrile rubber Thermal conductivity
Effects of bonding agents like hexamethylenetetramine-resorcinol system, bis[3-(triethoxysilyl)propyl] tetrasulphide and toluene diisocyanate on
the properties of aluminium powder
filled nitrile rubber composites were
investigated. Shore A hardness, tear
strength and tensile properties were
increased by the use of bonding
agents. Incorporation of aluminium
powder increased the thermal conductivity and the resistance towards
thermal ageing. Presence of bonding
agent decreased the equilibrium
swelling due to the improved adhesion
between aluminium powder and nitrile rubber.
Mit Aluminiumpulver gefüllte
Nitrilkautschuk-Verbundwerkstoffe: Der Effekt von
Haftmitteln
Aluminiumpulver Haftmittel Nitrilkautschuk Wärmeleitfähigkeit
Es wurden Auswirkungen von Haftmitteln wie Hexamethylentetraminreosrcinol, Bis[2-(triethoxysilylpropyl]-tetrasulfid und Tolyldiisocyanat
auf die Eigenschaften von mit Aluminiumpulver gefülltem Nitrilkautschuk untersucht. Durch den Einsatz
von Haftmitteln wurden die Härte
Shore A, der Reißwiderstand und die
Festigkeitseigenschaften erhöht.
Die Wärmeleitfähigkeit und die
Wärmealterungsbeständigkeit wurden durch das Aluminiumpulver erhöht. Durch die verbesserte Adhäsion zwischen Aluminiumpulver und
Nitrilkautschuk wurde der Gleichgewichtsquellengrad herabgesetzt.
Aluminium Powder Filled Nitrile
Rubber Composites:
Effect of Bonding Agents
The horizon of application of polymers has
been widened with the recent discovery of
metal-filled polymers in which the inherent
thermal and electrical characteristics of polymers have been substantially modified.
Metal powder incorporated polymer composites have applications like heat conduction, electrical heating, and discharging
static electricity. These composites have
the advantage of high corrosion resistance, lower specific weight, great accessibility and ease of processing. The higher thermal conductivity imposed by metal powders in rubber composites is useful in the
vulcanization of thick articles [1]. This helps
to reduce the additional vulcanization time
needed for the curing of thick rubber product and imparts uniform curing throughout the material, which offers longer service life to the product. Special conductive
blacks can be used in significant amount to
produce conductive rubbers [2 – 5]. Conductivity of such composites is found to
be less than that of metal powder incorporated composites. Numerous articles related to the properties of metal powder-polymer composites have appeared in the literature [6 – 10].
Good rubber-filler interaction is necessary
for obtaining good physical and mechanical properties. In particulate filled systems
an improved interaction of the filler with
the matrix can be achieved through the
use of bonding/coupling agents [11, 12].
The use of resorcinol-silica-hexamethylene
tetramine as a bonding agent to improve
the adhesion between natural rubber
and aluminium powder along with various
vulcanization systems was reported by the
authors [13]. It is found that this bonding
system is most effective in conventional
vulcanization system due to the high level
of sulphur to accelerator ratio. Maity and
Ghosh[6] used titanate coupling agent
for the surface treatment of silver powder
in polypropylene. Modifications of the polymer also enhance the rubber-filler interaction. Introducing a functional group
can improve the adhesion between rubber
and filler by enhancing the surface interaction between the phases. Bosscott and
Lebrle [14] carried out partial epoxidation
KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 12/2004
of natural rubber in order to assess its effect on rubber-to-brass adhesion with out
sacrificing the desirable physical properties
of the rubber.
Effects of various bonding/coupling agents
like hexamethylenetetramine- resorcinol
(HR), bis[3-(triethoxysilyl) propyl] tetrasulphide (Si-69) and toluene diisocyanate
(TDI) on NBR- aluminium powder composites are reported in this article.
Experimental
Nitrile rubber used was Europrene N 3945.
Hexamethylenetetramine (hexa), resorcinol, bis[3-(triethoxysilyl)propyl] tetrasulphide and toluene diisocyanate were of laboratory reagent grade. Aluminium powder
was obtained from M/s Kosla Metal Powder Co. Pvt. Ltd, India. It has a specific gravity of 2.69 and particle size 127 to 200
nm. Other ingredients and fillers were of
commercial grade.
The base formulations used are given in
Tab. 1. The dosages of hexa and resorcinol
were in the ratio 1:2 throughout this study.
At higher loadings of aluminium powder,
the concentration of bonding agent varied
as the multiples of the ratio of filler to bonding agent used in the base formulation.
While plotting the figures, in the case of
HR-system we have taken the amount of
resorcinol on the abscissa, where as the
hexa varies according to the ratio. The
composites were prepared in a two-roll
mill (150 300 mm). The compounds
were cured upto their optimum cure
time at 150 8C. The mechanical properties
were tested according to the respective
ASTM procedures. Thermal conductivity
V. S. Vinod, S. Varghese, and
B. Kuriakose, Kottayam, Kerala (India)
Corresponding author:
Dr. Siby Varghese
Rubber Research
Institute of India
Kottayam, Kerala,
686 009, India
Tel.: +91/481/235 3311
Fax: +91/481/235 3327
641
Results and discussions
Tab. 1. Base formulations
Ingradients
GUM
WoB
HR
Si-69
TDI
Nitrile rubber
Stearic acid
Zinc oxide
TDQ
Aluminium powder
Hexamethylene tetramine
Resorcinol
Si-69
Toluene diisocyanate
CBS
Sulphur
100
1.5
5.0
1.0
–
–
–
–
–
1.0
1.5
100
1.5
5.0
1.0
10
–
–
–
–
1.0
1.5
100
1.5
5.0
1.0
10
0.5
1.0
–
–
1.0
1.5
100
1.5
5.0
1.0
10
–
–
1.0
–
1.0
1.5
100
1.5
5.0
1.0
10
–
–
–
1.0
1.0
1.5
TDQ – 2,2,4- trimethyl- 1,2- dihydro quinoline
Si-69 – bis[3-(triethoxysilyl) propyl] tetrasulphide
CBS – N-cyclohexyl benzothiazyl sulphenamide
Tab. 2. Properties of NBR-aluminium powder composites
Sample
Gum
10 Phr
20 Phr
30 Phr
40 Phr
40 Phr
40 Phr
40 Phr
Al
Al
Al
Al
Al+HR system
Al+Si-69
Al+TDI
Thermal conductivity, LOI, n
W/mK
Retention in tensile
strength, %
0.209
0.350
0.454
0.515
0.561
0.545
0.558
0.553
84
111
113
113
108
105
100
98
was measured using quick thermal conductivity meter, “Kemtherm” QTM D-3
(Kyoto Electronics, Japan). Flammability
was tested by SR-FTA Flammability Tester.
“Zwick” Universal Testing Machine (model
1474) was used at 500 mm/min to determine the tensile properties (ASTM D-41280). For swelling studies, the samples were
cut circularly by means of a sharp edged
circular die. The initial weight of the sample was taken and immersed in toluene at
27 8C. After attaining the equilibrium swel-
18.0
18.3
18.7
19.8
20.6
20.0
19.6
20.4
ling (at equilibrium swelling, weight of the
sample does not change with time) the
sample was taken out and the swollen
weight was noted after the wet surface
was dried using a blotting paper. It is
then expressed as number of moles of solvent absorbed by 100 g of the polymer.
The ageing resistance was determined by
keeping the tensile test pieces at 70 8C
for 7 days. The percentage retention in
tensile strength is calculated to assess
the ageing resistance.
Fig. 1. Hardness of NBR-composites as a function of (a) the amount of
bonding agent, at 10 phr Al powder (b) the amount of aluminium powder with and without bonding agents
642
Thermal conductivity of aluminium powder incorporated nitrile rubber compounds
are given in Tab. 2. As the loading of aluminium powder increased, the thermal
conductivity also increased. Bonding
agents, like hexamethylene tetramine-resorcinol system (HR- system), bis[3-(triethoxysilyl) propyl] tetrasulphide (Si-69) and toluene diisocynate (TDI) have only a slight
effect on thermal conductivity. Resistance
to flammability, measured as limiting oxygen index (LOI), are shown in Tab. 2. Limiting oxygen index (n) is defined as the volume fraction of oxygen in an oxygen-nitrogen atmosphere that will just support
steady candle like burning of a material.
From the Table, it is clear that, as the loading of aluminium powder increased the
LOI of NBR-composites. The bonding/
coupling agents slightly decreased the
LOI and the maximum decrease was found
with Si-69.
Shore A hardness values of the composites
are shown in Figs. 1a and 1b. At a given
loading (Fig. 1a) as the concentration of
bonding agent increased the hardness increased. The maximum increase was observed with HR-system followed by Si-69
(Fig. 1b). On increasing the loading of aluminium powder, sharp increase in Shore A
hardness was observed. The bonding/
coupling agents such as, hexa-resorcinol
system, Si-69 and TDI further increased
the hardness values.
Tear strength for aluminium powder filled
NBR-composites are shown in Figs. 2a and
2b. The bonding/coupling agents increa-
Fig. 2. Tear strength of NBR-composites as a function of (a) the amount
of bonding agent, at 10 phr Al powder (b) the amount of aluminium
powder with and without bonding agents
KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 12/2004
Fig. 3. Equilibrium swelling in toluene of NBR-composites as a function
of (a) the amount of bonding agent, at 10 phr Al powder (b) the amount
of aluminium powder with and without bonding agents
sed the tear strength of these composites.
The ability of various bonding systems to
improve the tear strength of aluminium
powder filled nitrile rubber compounds
are in the order, HR system>Si-69>TDI>WoB. The higher value with HR-system
may be due to the improved adhesion between NBR and aluminium powder and
also due to the additional crosslinks formed in presence of hexamethylene tetramine.
Equilibrium swelling values of aluminium
powder filled NBR- composites are presented in Figs. 3a and 3b. The swelling was
conducted in toluene at 27 8C, and the
maximum uptake of the solvent by the
composite was expressed as moles of solvent sorbed by 100 g of composite. Equilibrium swelling of a composite was affected by many factors such as structure of
the polymer, type of cross linking, cross
link density, penetrant size, temperature,
presence of fillers etc [15]. The presence
of active filler reduces the equilibrium
swelling [16] and the interaction between
rubber and the filler has a clear role in the
equilibrium swelling of the composites.
Equilibrium swelling can be taken as a
measure of the adhesion between filler
and the rubber (Fig. 3a). On increasing
the loading of aluminium powder
(Fig. 3b) the equilibrium swelling was decreased. This is due to the filler effect, since, the fillers are not assumed to absorb
solvents. At higher loadings also the bonding agents decreased the equilibrium
swelling values. This is due to the improved
adhesion between aluminium powder and
nitrile rubber in presence of bonding
agents.
Fig. 4. Tensile strength of NBR-composites as a function of (a) the
amount of bonding agent, at 10 phr Al powder (b) the amount of aluminium powder with and without bonding agents
The improved adhesion with HR system is
due to the formation of a resin by the condensation reaction of hexamethylene tetramine and resorcinol. It has the following
structure. (Scheme 1)
When the constituents are intimately mixed with rubber, a resin is formed during
vulcanisation, which increases the bonding
between the constituents [17]. This makes
great improvements in bonds between nitrile rubber and aluminium powder. The
coupling mechanism with silanes involves
two-fold reaction with both the organic
polymer and the mineral substrate. The organo functional silane must be compatible
with the organic phase so that the silane
KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 12/2004
becomes part of the polymer. The silane
by co-reacting with the polymers modifies
the polymer morphology at the interface
to improve stress transfer. The silane coupling agent, bis [3-(triethoxy silyl) propyl] tetrasulphide has the following structure.
(Sheme 2)
The silane triol formed by hydrolysis of
trialkoxy silane coupling agent has unique
bonding capability with mineral surfaces
[11]. This makes great improvements in adhesion between nitrile rubber and aluminium powder. Isocyanate are used in elastomeric compounds to improve the bonding
[18]. Adhesion of isocyanate to rubbers initiates a chemical reaction, which might
Scheme 1
Scheme 2
643
Fig. 5. Modulus (300 %) of NBR-composites as a function of (a) the
amount of bonding agent, at 10 phr Al powder (b) the amount of aluminium powder with and without bonding agents
feasibly account for the bond. It is suggested that the isocyanate itself unites with
hydrated oxide layers on the surface of
the metal.
Figs. 4a and 4b show the tensile strength
of aluminium powder filled NBR-composites. At a given loading the use of bonding
agents increased the tensile strength. Similarly as the loading of aluminium powder
increased the tensile strength also increased. The same trend was reflected in the
modulus at 300 % elongation values (Figs. 5a and 5b). In all the cases modulus
increased by the se of bonding agents.
The increase in modulus and tensile
strength followed the order, HR system>Si-69>TDI>WoB. Elongations at break values of the composites are presented in Figs. 6a and 6b. Presence of bonding agents
decreased the elongation at break. At a
particular loading (Fig. 6a) as the concentration of bonding agent increased the
elongation at break decreased gradually.
The increased adhesion in presence of
bonding agent restricts the polymer chain
movements, which result in decreased
elongation. On increasing the loading of
aluminium powder this effect is much pronounced (Fig. 6b). The maximum decrease
was observed when hexamethylene tetramine-resorcinol system was used as the
bonding agent.
Percentage retention of tensile strength of
nitrile rubber composites containing aluminium powder with and without bonding
agents are given in Tab. 2. These composites were aged for 7 days at 70 8C. The gum
compound retained only 84 % of its origi-
644
Fig. 6. Elongation at break of NBR-composites as a function of (a) the
amount of bonding agent, at 10 phr Al powder (b) the amount of aluminium powder with and without bonding agents
nal tensile strength after ageing. Whereas
the aluminium powder filled NBR composites without bonding agent, showed an
increase in tensile strength (more than
100 % retention) after 7 days at 70 8C.
Ageing at elevated temperature leads to
two competing reactions in rubber composites; continued crosslinking of polymer
chains and scission due to polymer degradation. In gum vulcanizates polymer degradation is the main reaction. In the aluminium powder incorporated NBR-composites the chain crosslinking is predominant
than chain degradation after.
Conclusions
A marked increase in thermal conductivity
was obtained with incorporation of aluminium powder in NBR-compounds. These
composites had higher limiting oxygen index values than the gum vulcanizate. The
bonding agents like hexamethylene tetramine-resorcinol system, bis[3-(triethoxysilyl) propyl] tetrasulphide, and toluene diisocyanate increased the ShoreA hardness,
300 % modulus, tensile strength, tear
strength etc. and the increase was found
in the order, HR-system>Si-69>TDI. At a given loading of aluminium powder these
properties increased gradually as the concentration of bonding agent increased.
The equilibrium swelling in toluene decreased as the bonding agent concentration increased at a given loading, and the same
pattern was observed at higher loadings
of aluminium powder. These results suggested an improved aluminium powder-ni-
trile rubber interaction/adhesion in presence of bonding agents. The aluminium
powder filled NBR- composites showed
better resistance towards oxidative ageing.
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KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 12/2004