Zinc Oxide Nanoparticles as an Activator for Natural Rubber Latex
Susith Fernando1 , Nadeesh Madusanka2 , Nilwala Kottegoda*1, 2 , U. N. Ratnayake 3
1. Department of Chemistry, University of Sri Jayewardenepura, Gangodawila, Nugegoda, Sri Lank a.
2. Sri Lanka Institute of Nanotechnology, Biyagama Export Processing Zone, Biyagama, Sri Lanka .
3. Rubber Research Institute, Thelawala Road, Ratmalana, Sri Lanka .
Telephone: 0094 773830313, e-mail address: [email protected]
ABSTRACT:
This work focuses on the study the effect of ZnO nanoparticles on the vulcanizate properties of natural rubber (NR)
latex films. Zinc oxide nanoparticles were synthesized by a simple and an efficient wet chemical method. The
Powder X-ray Diffraction (PXRD) characterization confirmed the successful synthesis of pure ZnO and the average
particle size was found to be 50 – 100 nm as indicated by particle size analysis. ZnO nanoparticles thus prepared
were surface modified with oleic acid as a capping agent and the modification was carried out under ultrasound
sonication in order to provide mechanical forces to separate nanoparticles leading to an efficient surface reaction.
The existence of organic layer was confirmed by the Fourier Transform Infrared (FT-IR) spectra.
The oleic acid modified ZnO nanoparticles were then intro duced into natural rubber centrifuged latex. The
mechanical properties and swelling characteristics of the films were compared with those prepared using
conventional ZnO particles (particle size ~ 800 nm and concentration 0.25 phr). The experiment was carried out
with varying particle sizes of synthesized ZnO particles (30 nm, 50 nm, 500 nm) and by adding varying
concentrations of ZnO nanoparticles with an average particle size of 30 nm (0.25 – 0.06 phr). A significant
improvement in strength characteristics was observed for the films prepared with 0.25 and 0.125 phr of oleic acid
modified ZnO nanoparticles whilst those of the films prepared using 0.06 phr were almost comparable with that of
the conventional films. The swelling characteristics in toluene were significantly reduced in the films prepared with
ZnO nanoparticles corroborating the improved cross linking density as observed from the improvement of
reinforcement of NR latex vulcanizates.
It can be thus concluded that the increase in the available surface area of ZnO nanoparticles increases the efficiency
of vulcanization, while modification with oleic acid increases the compatibility between rubber matrix and the
inorganic fillers which led to an increase in the degree of cross linking. The study therefore significantly contributes
to reduce the ZnO concentration in latex compound formulation in the latex industry.
Introduction
Zinc oxide is one of the basic components of rubber
compounds which act as an activator for rubber
vulcanization process with sulphur. However, since
2004, the European Union has classified zinc oxide
(ZnO) as dangerous for the environment and has
legislated that its application in rubber products be
reduced and controlled. [1]
According to Beniska and Dogadkin, ZnO promotes
the initial response by activating the vulcanization
reaction. Moore et al. showed that ZnO plays the role
of a catalyst, and Barton et al. argued that ZnO has an
influence on the degree of cross linking of natural
rubber (NR) and improves the heat resistance of the
vulcanizate. [2].
There are several different methods available in the
literature to synthesize ZnO nanoparticles such as
high temperature solid–vapor deposition, solution
phase methods etc. [3] In the present study, a simple
and an efficient wet chemical method based on solgel processing was used.
Hence, the aim of the present study is to investigate
the effect of ZnO nanoparticles as cure activator in
natural rubber (NR) centrifuged latex. NR latex finds
use in industrial and medical gloves, condoms,
balloons etc.
Experimental
Synthesis of zinc oxide nanoparticles
Method
NaOH solution of 0.90 M was heated at 55 ºC. The
Zn(NO3 )2 solution (0.45 M) was added drop wise (1
drop per 2 seconds) to the heated solution of NaOH
under a stir rate of 500 rpm in a magnetic stirrer. The
stirring was continued for 2 hours. The precipitated
ZnO nanoparticles was washed with deionized water
and ethanol, and then dried in air atmosphere at 60
ºC. The above reaction was repeated with different
rate of additions and stirring rates (see Table 1).
T ABLE I
DESIGNATION FOR ZINC OXIDE
Addition rate
ZnO’ N25
ZnO’ N55
ZnO’ N57
1 drop per 2 seconds
1 drop per 5 seconds
1 drop per 5 seconds
S tir rate /
rpm
500
500
700
Surface modification of ZnO nanoparticles
Oleic acid (1.5 cm3 ) was dissolved in 1% sodium
dodecyl sulphate (50 cm3 ) in a flask to form the
solution. Then 1 g of above prepared nano ZnO was
added into this solution. The mixture was
ultrasonicated for 30 minutes at 50 KHz. The
particles were collected by centrifugal separation and
washed three times with toluene then dried in air
atmosphere at 50 °C.
Compounding latex
T ABLE II
FORMULATION [4] AND COMPOUND DESIGNATION FOR NR LATEX
All weights are parts per 100 grams of rubber
Compound
designation
Formulation
60%
NR
10%
KOH
Potassium
20%
Laurate
50%
Sulphur
50%
ZDEC
Commercial
50%
ZnO
10%
ZnO’ N25
Phenolic
50% Antioxidant
S ample
A
S ample
B
S ample
C
S ample
D
100
0.3
100
0.3
100
0.3
100
0.3
0.2
0.5
0.75
0.2
0.5
0.75
0.2
0.5
0.75
0.2
0.5
0.75
0.25
0.25
0.125
0.0625
0.5
0.5
0.5
0.5
The compounds were mixed as per formulations
given in Table II. The same compounding procedure
was carried out with ZnO’ N55 and ZnO’ N57 that
were synthesized using different experimental
conditions. All ingredients were mixed with constant
stirring. Then all compound samples were kept for a
maturation period of 24 hours.
Preparation of cast films
NR latex films were casted using glass plates and
then the cast films were dried under normal
atmospheric conditions . After drying, the cast films
were vulcanized at 120 °C for 20 minutes.
Characterization of cast films
Tensile strength
Tensile tests were carried out in accordance with
ASTM D412, Standard Test Methods for Vulcanized
Rubber
Swelling Studies
Swelling tests were performed to evaluate the degree
of cross linking of the vulcanizates. Three test pieces
with a diameter of 2 cm were left in toluene for 72
hours at 30 °C. The swelling ratio was calculated by
measurement of the weight before swelling (M 1 ) and
the final weight (M 2 ) and with the following
equation:
( )
(
)
Results and Discussion
Characterization of ZnO nanoparticles
Figure 1(a) shows the FTIR spectrum obtained from
commercial ZnO. Weak absorption bands near 3446
cm−1 represent O-H stretching vibrations of hydroxyl
groups. In the FTIR spectrum of ZnO nanoparticles
which is represented in Figure 1(b), the O-H
stretching mode vibration has shifted to a lower
frequency of 3403 cm−1 . This strong band at 3403
cm−1 can be clearly assigned to the hydroxyl species
(HO–ZnO), which is formed via dissociative
adsorption of water on oxygen vacancy sites of ZnO
nanoparticles, which indicates a strong interaction of
water with ZnO nanoparticles in contrast to
conventional ZnO; thus confirming the presence of
larger amount of O-H groups compared to macrosize
ZnO particles which have lower number of surface
O-H groups. In addition, the presence of molecularly
chemisorbed water (monolayer) on ZnO nanoparticle
surfaces is confirmed by the observation of the
scissoring mode at 1644 cm-1 , which appears as a
very weak band in the FTIR spectrum of
conventional ZnO. In conventional ZnO the Zn -O
stretching is attributed to peaks appearing at 877 and
692 cm-1 . The peak at 692 cm-1 has shifted to a lower
frequency of 674 cm-1 in the FTIR spectrum of ZnO
nanoparticles, due to the hindrance of ZnO stretching
vibration by molecularly chemisorbed water
monolayer. Figure 1(c) shows the FTIR spectrum of
oleic acid modified ZnO nano particles. The O-H
stretching mode vibration has further shifted to a
lower frequency of 3376 cm−1 , which indicates that
the O-H stretching vibration is hindered and is
evidence for the presence of oleic acid molecules
around the ZnO nanoparticle. The Zn-O stretching
vibration has also shifted to a lower frequency of 669
cm-1 and 834 cm-1 , which indicates that the Zn-O
stretching vibration is hindered and further supports
the evidence for the presence of oleic acid molecules
around the ZnO.
In the FTIR spectrum of pure oleic acid there are two
sharp bands at 2924 and 2854 cm-1 which can be
attributed to the asymmetric and symmetric
stretching vibrations of CH2 groups, respectively. The
intense peak at 1710 cm-1 is derived from the
existence of the C=O stretching vibration and the
band at 1285 cm-1 exhibits the presence of the C-O
stretching vibration. The O-H in-plane and out-ofplane bands appear at 1462 and 937 cm-1 ,
respectively.
In comparison with the FTIR spectrum obtained from
ZnO nanoparticles modified with oleic acid (fig. 1
C), the asymmetric and symmetric stretching
vibrations of CH2 shifted to 2923 and 2850 cm-1 ,
respectively. The surfactant molecules in the
adsorbed state were subjected to the field of the solid
surface. As a result, the characteristic bands shifted to
a lower frequency region which indicated that the
hydrocarbon chains in the monolayer surrounding the
nanoparticles were in a closed-packed, crystalline
state. The C=O stretch band of the carboxyl group,
which is present at 1710 cm-1 in the IR spectrum of
pure liquid oleic acid, is absent in the spectrum of the
modified ZnO nanoparticles. Instead there appeared
two new bands at 1583 cm-1 and 1389 cm-1 , which
was characteristic of the asymmetric (COO) and the
symmetric (COO) stretching vibrations, respectively.
This reveals that oleic acid is chemisorbed as a
carboxylate onto the ZnO nanoparticles. [5]
Combined with previous studies of carboxylates, the
interaction between the carboxylate head and the
metal atom can be categorized as four types:
monodentate,
bridging
(bidentate),
chelating
(bidentate), and ionic. [6] The wave number
separation D between the symmetric (COO–) and
asymmetric (COO–) IR bands can be used to
distinguish the type of the interaction between the
carboxylate head and the metal atom. The largest D
(200–320 cm-1 ) was corresponding to the
monodentate interaction and the smallest D (<110
cm-1 ) was for the chelating bidentate. The medium
range D (140– 190 cm-1 ) was for the bridging
bidentate. [6] In this work, the D (1583-1389 = 194
cm-1 ) was ascribed to bridging bidentate interaction.
Fig. 1 FT IR spectra of (a) conventional ZnO, (b) nano ZnO, and
(c) oleic acid modified
The PXRD pattern of the ZnO nanoparticles is
identical to the hexagonal phase with Wurtzite
structure. [6] The peaks at diffraction angles (2θ) of
31.37, 34.02, 35.86, 47.16, 56.25, 62.54, 67.63 and
68.79 correspond to the reflection from: 100, 002,
101,102, 110, 103, 200 and 112 crystal planes,
respectively.
The particle size distribution of ZnO produced using
an addition rate of 1 drop per 2 seconds and a stir rate
of 500 rpm is illustrated in figure 2. Only one
distribution peak is observed. The average diameter
of particles is 530 nm. As seen from the particle size
distribution results particle agglomeration has
occurred. Agglomeration was possibly due to fast
addition rate of zinc nitrate solution during the
synthesis. Addition rate of the reactants are important
to control the size of ZnO nanocrystals. The
increased extent of nucleation at high feed addition
rates produces higher particle sizes .
Fig. 2 Particle size distributions of ZnO’ N25
The particle size distribution of ZnO produced using
an addition rate of 1 drop per 5 seconds and a stir rate
of 500 rpm is illustrated in figure 3. There are three
distributions, and therefore, there are three average
particle sizes. The average particle size of the entire
sample is 45 nm.
ZnO (45
nm)
ZnO (38
nm)
Fig. 3 Particle size distributions of ZnO’ N55
The particle size distribution of ZnO produced using
an addition rate of 1 drop per 5 seconds and a stir rate
of 700 rpm is illustrated in figure 4. There are three
distributions and therefore there are three average
particle sizes. The average particle size of the entire
sample is 38 nm.
0.25
0.56
3.84
584
0.125
0.55
3.85
597
0.0625
0.54
4.13
671
0.25
0.56
3.60
539
0.125
0.55
3.60
557
0.0625
0.42
3.13
655
At higher swelling ratios, the solvent penetrated into
the rubber matrix more easily, which means that the
degree of cross linking was low. ZnO act as an
activator which reduces the activation energy barrier
for the vulcanization reaction. The swelling ratio has
decreased in NR latex compounds where,
conventional ZnO was replaced with similar
quantities of oleic acid modified ZnO nanoparticles,
which correlates to an increase in cross linking
density. With decreasing concentration of oleic acid
modified ZnO nanoparticles in the NR latex
compounds, the swelling ratio has increased, which
correlates to a decrease in cross linking density. The
increase in surface area of nanoparticles with the
reduction in size have increased the active sites on
ZnO; thus reducing the activation energy barrier for
the vulcanization reaction and an increase in cross
linking density.
Mechanical properties
Tensile strength results are tabulated in Table IV with
the ZnO amount varied between 0.25-0.0625 phr.
T ABLE IV
T ENSILE P ROP ERTIES OF NR VULCANIZATES
Fig. 4 Particle size distributions of ZnO’ N57
Characterization of cast films
The variations of swelling ratio with different particle
sizes and different levels of ZnO are listed in table
III.
T ABLE III
Type of ZnO
phr
Tensile strength (M Pa)
Commercial ZnO
0.25
14.37 ± 0.29
0.25
13.25 ± 0.32
0.125
12.25 ± 0.44
0.0625
9.80 ± 1.12
0.25
17.41 ± 0.82
0.125
12.60 ± 0.09
0.0625
10.99 ± 1.14
0.25
17.64 ± 0.08
0.125
14.63 ± 0.99
0.0625
13.80 ± 0.60
ZnO (530 nm)
SWELLING RATIO FOR NATURAL RUBBER VULCANIZATES
Initial
Type of
ZnO
phr
Weight/
g
After
swelling
Swellin
mean
g Ratio
ZnO (45 nm)
weight/g
Commercia
l ZnO
0.25
0.51
3.61
610
0.25
0.47
3.29
606
0.125
0.47
3.51
648
0.0625
0.36
2.85
684
ZnO (38 nm)
ZnO (530
nm)
Tensile strength of 14.37 MPa was observed with
0.25 phr of conventional ZnO. At the particle size of
38 nm, an excellent tensile strength of 13.80 MPa
was observed at 1/4th (0.0625 phr) quantity and a
remarkable tensile strength of 14.63Mpa was
observed at 1/2th (0.125 phr) quantity. The value is
very significant when compared to those observed
when 0.25 phr of commercial ZnO.
Surface to volume ratio increases with the reduction
of particle size to nanoscale. As surface to volume
ratio increases and, as result, a greater amount of a
substance comes in contact with surrounding
material. This results in better catalysis, since a
greater proportion of the material is exposed for
potential reaction.
The ease of incorporation of the zinc oxide into
rubber compound can be improved by coating the
particles with oleic acid. The acid binds to the zinc
oxide surface and the resulting hydrocarbon chain is
more compatible with the rubber compound.
Conclusions
ZnO nanoparticles with an average particle size of 50
nm were successfully synthesized. Agglomeration of
the ZnO nanoparticles was prevented by the surface
modification with oleic acid. FT-IR results confirm
that an organic layer exists and the linkage between
inorganic nuclei and organic layer is a chemical
bond.
The effects of modified nano ZnO particles on the
mechanical properties of natural rubber latex
compounds were evaluated. In particular, only about
50 % ZnO nanoparticles, in comparison to
commercial ZnO, were sufficient to obtain similar
mechanical properties due to the high surface area
available and the compatibility with the matrix
leading to an increase in the degree of cross linking
of the vulcanizates.
With a reduction in size therefore the increase in the
surface area of the ZnO nanoparticles, they
effectively formed the complex with the accelerator,
sulfur. Further the hydrophobic long chain in the
oleic acid acted as the bridge to compatebilize the
ZnO nanoparticles with rubber matrix. The increase
in the available surface area of ZnO nanoparticles
and the compatibility between the organics and
inorganics led to an increase in the degree of cross
linking of the vulcanizates. Therefore, oleic acid
modified ZnO nanoparticles can be used to reduce
the amount of ZnO in conventional formulations.
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International Conference on Advanced Materials, Science and Engineering, July 01-04, 2012, Colombo, Sri Lanka