SURFACE MODIFICATIONS AND ADHESION OF VULCANIZED RUBBER
CONTAINING AN EXCESS OF PARAFFIN WAX TREATED WITH 2 wt%
TRICHLOROISOCYANURIC SOLUTION AT DIFFERENT TEMPERATURE
Andrés Yáñez-Pacios, Jose Miguel Martín-Martínez
Adhesion and Adhesives Laboratory, University of Alicante, Alicante, Spain
andresjesus.yanez@ua.es
Introduction
Sulfur vulcanized rubbers are commonly used
in the manufacturing of footwear soles, tires and
belts for construction. These rubbers contain
different additives (vulcanization agent and
accelerator, fillers, plasticizers, processing aid
(stearic acid+ zinc oxide) and antiozonants (mainly
paraffin wax) in their formulation. Adhesion of
vulcanized rubber is compromised because of its
low surface energy and the existence of a physical
barrier of paraffin wax antiozonant of about 2
microns thick produced by migration from the bulk
to the surface during and after vulcanization1, 2. The
wax antiozonant layer on the rubber surface should
be removed to improve adhesion and roughness and
surface polarity should be created by surface
treatments.
Different surface treatments for vulcanized
rubber
have
been
proposed3-7
including
halogenation with trichloroisocyanuric acid (TCI)
solutions, plasma torch, UV radiation, corona
discharge, and low pressure plasmas. The
effectiveness of these surface treatments varies
depending on the rubber composition, particularly
the content of paraffin wax. Paraffin wax migration
is controlled by temperature8, being inhibited for
temperatures higher than 55ºC. Therefore, the
halogenation treatment combined with temperature
may favor its effectiveness in vulcanized rubber. To
the best of our knowledge, this study has not been
carried out yet. In this work a difficult to bond
vulcanized rubber (L2 rubber) was treated with TCI
solutions in MEK using different temperatures (25,
45, 80 and 120ºC). The surface modifications were
studied by water contact angle measurements,
ATR-IR and XPS spectroscopy, and scanning
electron microscopy (SEM). Adhesion was
obtained from T-peel tests of treated L2
rubber/polyurethane adhesive/leather joints.
Table 1. Typical composition of L2 rubber (phr:
parts per 100 parts of rubber).
Ingredient
Percentage
(phr)
Polyisoprene
100
Silica
20
Sulfur
1.5
Poly (ethylene glycol) (Mw=6000)
1.1
Zinc oxide
1.5
Stearic acid
1.5
Phenolic antioxidant
0.5
Microcrystalline paraffin wax
3.0
Benzothiazyl disulfide
1
Tetramethyl tiuram disulphide
0.5
Naphtenic and paraffinic oil
5-30
Surface treatment
Halogenation. The chlorinating agent was a
solution of 2 wt% trichloroisocyanuric acid (TCI)
in methyl ethyl ketone (MEK) – 2 wt% TCI/MEK.
The chemical structure of TCI is given in Figure 1.
The just prepared solution was brushed onto the
rubber surface, making three consecutive passes
and letting the reaction proceed along for 2 hours at
different temperature. Before halogenation, some of
the as-received L2 rubber was solvent wiped with
MEK to remove the excess of antiozonant waxes on
the rubber surface.
Cl
O
Materials
Sulphur vulcanized rubber (named as L2)
provided by Cauchos Arnedo (Quel, La Rioja,
Spain) was used; the dimensions of the samples
were 150x30x4mm. The typical formulation of this
rubber is given in Table 1 and intentionally it was
formulated containing an excess of antiozonant
paraffin wax.
C
N
Cl
Experimental
N
C
O
N
C
Cl
O
Figure
1:
Chemical
trichloroisocyanuric acid (TCI).
structure
of
Experimental techniques
Contact angle measurements. The wettability
and surface energy of the as-received and treated
L2 rubber surfaces was evaluated from contact
angle measurements at 25 ºC using a Ramé-Hart
ATR-IR spectroscopy. Bruker Alpha (Bruker
Optiks, Etlinger, Germany) spectrometer was used
to obtain the IR spectra of the as-received and
treated L2 rubber surfaces. The attenuated total
multiple reflection technique (ATR) was used to
analyze the chemical modifications produced in
about 1 m depth of the rubber surface. A Ge prism
was used. 60 scans with a resolution of 4 cm-1 were
obtained and averaged. The incident angle of the IR
radiation was 45º.
rubber. Whereas, chlorination at 45ºC enhanced the
effects of chlorination (more intense band at 1710
cm-1) an important migration of wax is produced
(bands at 720-730, 2841 and 2910 cm-1).
Chlorination at 80ºC produces the most efficient
chemical modification and noticeable removal of
antiozonant. If MEK wiping is carried out before
chlorination treatment, the effect of the temperature
is minor and good performance is obtained at 25ºC
without migration of antiozonants.
1.0
0.9
0.8
120ºC
0.7
Absorbance (a.u.)
100 goniometer (Ramé-Hart, Netcong, NJ, USA).
Drops (4 l) of bi-distilled deionized water or
diiodomethane were placed on the treated rubber
and the contact angle values were obtained after 5
minutes. Surface energy was obtained by using the
Owens-Wendt approach.
0.6
80ºC
0.5
0.4
25ºC
X-ray Photoelectron Spectroscopy (XPS).
Chemical modifications produced on the outermost
surface layer (about 2 nm) on the as-received and
the treated L2 rubber were analyzed in K-Alpha
spectrometer (Thermo Scientific, West Palm Beach,
USA) using a Al-K X-ray source (1253.6 eV)
operating at 15 keV and 300 W. The take-off angle
was 45º. For each sample, a survey scan
encompassing the region 0-1200 eV was first
obtained. High resolution spectra of all photopeaks
were obtained in a 20 eV range.
Scanning Electron Microscopy (SEM). The
topology of the as-received and treated L2 rubber
surfaces were analyzed using JEOL JSM-840 SEM
system. The samples were coated with gold to
obtain enough contrast in the SEM micrographs and
the energy of the electron beam was 15 kV.
Results and Discussion
1.0
0.3
0.2
45ºC
0.1
0.0
4000
3000
2000
Wavenumber (cm -1)
1000
Figure 2b: ATR-IR spectra of the MEK wiped+2
wt% TCI/MEK treated L2 rubber.
Table 2 shows the chemical composition of the
outermost surface of treated L2 rubber. Higher level
of oxidation and chlorination are produced in the
MEK wiped and chlorinated rubber at 80 ºC.
Table 2.Chemical composition of as-received and
surface chlorinated L2 rubber.
C
O
Cl
Si
Treatment
(at%) (at%) (at%) (at%)
84
12
0
3
As received
95
3
0
2
TCI
89
5
3
3
MEK+TCI
90
4
1
5
TCI (80ºC)
83
6
8
3
MEK+TCI(80ºC)
0.9
45ºC
0.8
Absorbance (a.u.)
0.7
0.6
0.5
80ºC
0.4
120ºC
0.3
Figure 3 shows higher wettability in the
chlorinated rubber at 80 ºC and lower water contact
angle value in the MEK wiped and chlorinated
rubber. Always contact angle values higher that 85
degrees are obtained.
Water contact angles
0.2
0.1
25ºC
115
3000
2000
Wavenumber (cm -1)
1000
Figure 2a: ATR-IR spectra of the as-received+2
wt% TCI/MEK treated L2 rubber.
Figure 2a shows the ATR-IR spectra of the asreceived and chlorinated L2 rubber at different
temperature (25 to 120 ºC), and Figure 2b shows
the spectra for the MEK wiped and chlorinated
samples. Figure 2a shows more noticeable
influence of the temperature in the chemical
modifications produced by chlorination of L2
Contact angle (º)
0.0
4000
120
25ºC
110
AR
105
MEK
100
95
90
85
80
0
20
40
60
80
100
120
140
Temperature (ºC)
Figure 3. Water contact angle values (25ºC) of the
TCI treated L2 rubber as a function of the
chlorination temperature.
Figure 4 shows that the surface energy of L2
rubber increases after chlorination and it is not
depending on the chlorination temperature; the
dispersive component of the surface energy is
dominant and the highest polar contribution is
obtained by chlorination at 80ºC .
Table 3. Adhesion strength and locus of failure of
the as-received and 2 wt% TCI/MEK treated L2
rubber/polyurethane adhesive joints. Locus of
failure : A: Adhesion failure; C: Cohesion failure in
the rubber and D: Cohesion failure in the leather.
Total Surface Energy (mJ/m 2)
MEK wiped TCI Surface energy
Total
50
45
40
35
30
25
20
15
10
5
0
0
No
TCI
Dispersive
Polar
20
40
60
80
100
120
140
TCI treatment temperature (ºC)
Figure 4. Surface energy and its components of
MEK wiped+2wt% TCI/MEK treated L2 rubber.
Figure 5 shows the existence of the antiozonant
wax layer on the as-received L2 rubber surface.
Chlorination removes this layer, created roughness
and prismatic crystals of isocyanuric acid are
deposited. The increase in chlorination temperature
causes greater roughness and higher isocyanuric
acid crystal deposition, more noticeably if the L2
rubber is MEK wiped before chlorination.
a)
10μm
b)
10μm
d)
10μm
AR + TCI
AR + TCI + T45
AR + TCI + T80
T-peel
strength
(kN/m)
4.7 ± 0.2
5.1 ± 1.5
3.5 ± 0.1
MEK + TCI
MEK + TCI + T45
MEK + TCI + T80
12.9 ± 1.0
11.5 ± 1.1
10.3 ± 0.9
Treatment
c)
10μm
e)
10μm
Figure 5. SEM micrographs of a) as received L2
rubber; b) TCI at 80ºC; c) TCI at 120ºC;
d)MEK+TCI at 80ºC ; e) MEK+TCI at 120ºC.
Table 3 shows the peel strength values and loci
of failure of the joints. MEK wiping before
chlorination produces higher peel strength values,
and chlorination temperature has not an influence
on the adhesion of L2 rubber. Therefore, the
incidence of antiozonant on the L2 rubber surface is
the limiting factor in its adhesion.
Locus
of failure
A, D
A, C, D
A, D
C, D
C, D
A, D
Conclusions
Application of temperature during chlorination
of high antiozonant content rubber does not affect
the adhesion strength of the joints. However,
changes in wettability, surface chemistry and
roughness are produced, all being irrelevant for
improving adhesion.
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