Investigation of reaction rate of bis(triethoxysilylpropyl)tetrasulphide in silica-filled compound using pyrolysis-gas chromatography/mass spectrometry

Article abstract of DOI:10.14233/ajchem.2013.f26

Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) was used to examine the reaction rate of bis(triethoxysilylpropyl) tetrasulphide in silica-filled rubber. The major pyrolysis product of bis(triethoxysilylpropyl) tetrasulphide, which is used as a silane coupling agent, was found to be an allyltriethoxysilane. Allyltriethoxysilane was also detected in the uncured silica-filled styrenebutadiene rubber compound with bis(triethoxysilylpropyl) tetrasulphide. To investigate the silica/silane reaction rate, the allytriethoxysilane content was used as an indicator and quantified by the relative peak area ratio of allytriethoxysilane/styrene. Styrene is a pyrolysis product of styrene-butadiene rubber. The results revealed an increase in reaction rate with increasing allytriethoxysilane content. Overall, pyrolysisgas chromatography/mass spectrometry can be used to estimate the reaction rate of the silica/silane system.

Full text of DOI:10.14233/ajchem.2013.f26

Asian Journal of Chemistry; Vol. 25, No. 9 (2013), 5245-5250
http://dx.doi.org/10.14233/ajchem.2013.F26
Investigation of Reaction Rate of Bis(triethoxysilylpropyl)tetrasulphide in Silica-Filled
Compound Using Pyrolysis-Gas Chromatography/Mass Spectrometry†
*
SUNG-HO HA , SUNG-WOO KIM and HO-KYUN JEONG
NEXEN Tyre Corporation R&D Center, Kyungnam 626-230, Republic of Korea
*
Corresponding author: Fax: +82 55 3705366; Tel: +82 55 3704871; E-mail: saintsky@nexentire.co.kr
AJC-13181
Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) was used to examine the reaction rate of bis(triethoxy-
silylpropyl)tetrasulphide in silica-filled rubber. The major pyrolysis product of bis(triethoxysilylpropyl)tetrasulphide, which is used as a
silane coupling agent, was found to be an allyltriethoxysilane. Allyltriethoxysilane was also detected in the uncured silica-filled styrene-
butadiene rubber compound with bis(triethoxysilylpropyl)tetrasulphide. To investigate the silica/silane reaction rate, the allytriethoxysilane
content was used as an indicator and quantified by the relative peak area ratio of allytriethoxysilane/styrene. Styrene is a pyrolysis product
of styrene-butadiene rubber. The results revealed an increase in reaction rate with increasing allytriethoxysilane content. Overall, pyrolysis-
gas chromatography/mass spectrometry can be used to estimate the reaction rate of the silica/silane system.
Key Words: Silane coupling agent, Bis(triethoxysilylpropyl)tetrasulphide, Silica, Pyrolysis-gas chromatography/mass spectrometry.
to ensure the reaction between the coupling agent and silica,
whereas the coupling agent begins to react with the rubber or
donate sulfur at temperatures >160 ºC. Hunsche and Goerl
INTRODUCTION
Silica is an important reinforcing agent (filler) in a rubber
1
-5
compound together with carbon black . On the other hand,
silica-filled compounds can cause poor dispersion because of
the strong filler-filler interaction by hydrogen bonding between
the hydroxyl groups on the surface of silica. In addition, the
formation of chemical bonds between hydrophilic silica and
18,19
et al.
reported a horizontal reaction model consisting of
two main reactions known as primary and secondary reactions
in the silica/silane system using CP/MAS solid state NMR
spectroscopy. Therefore, rubber manufacturers need to control
the chemical reaction during the mixing process to take
advantage of the silica/silane system.
6
,7
hydrophobic rubber are difficult . These phenomena are
improved by the addition of organosilane as a silane coupling
Pyrolysis-gas chromatography/mass spectrometry
Py-GC/MS) has been used for several decades to analyze
8,9
agent in rubber compound . The use of silica in combination
with a silane coupling agent, which is often called the silica/
silane system, in the tyre industry improved the tyre perfor-
(
20-23
polymer and polymer additives . One of the advantages of
this technique is that all polymer and additive compositions
can be investigated without a pretreatment, providing important
1
0-12
mance such as the wet traction and rolling resistance, etc.
3,13-16
The silica/saline system has been studied extensively
.
.
24
compositional and structural information .
Each of the active groups of the silane coupling agent is
bonded between rubber and silica under certain conditions.
Bis(triethoxysilylpropyl)tetrasulphide (TESPT) is used widely
as a silane coupling agent in the tyre industry. The triethoxysilyl
groups of TESPT react with the silanol groups on the silica
surface during mixing, and the sulfur of TESPT reacts with
This study investigated the reaction rate of TESPT in a
silica-filled compound using pyrolysis-GC/MS. To establish
the validity of pyrolysis-GC/MS, uncured silica-filled styrene-
butadiene rubber (SBR) compounds were investigated under
a range of compounding conditions.
17
EXPERIMENTAL
the rubber chain during curing. Reuvekamp et al. examined
the effects of time and temperature on the reaction of TESPT
silane coupling agent during mixing. As discussed by
Silica-filled SBR compounds were prepared. The filled
compounds were made from SBR, silica, silane coupling agent,
cure activator and curatives. SBR 1783 with a styrene content
1
7
Reuvekamp et al. , a temperature of at least 130 ºC is necessary
†Presented to the International Rubber Conference (IRC-2012), May 21-24, 2012, Jeju, Republic of Korea

5
246 Ha et al.
Asian J. Chem.
2
5
of 23.5 wt.% was used as the SBR. Ultrasil 7000GR was em-
ployed as the silica compound with a content of 50 phr in the
compound. Bis(triethoxysilylpropyl)tetrasulphide was used as
the silane coupling agent. Bis(triethoxysilylpropyl)tetrasulphide
with a 0.0-14.0 wt.% silica content was then added. Zinc oxide
and stearic acid were used as the cure activator. Sulfur, N-
cyclohexyl-2-benzothiazole sulfenamide (CBS) and diphenyl
guanidine (DPG) were used as the vulcanizing agent and
accelerator, respectively. The formulation is listed in Table-1.
apparent crosslink density . The experiment was carried out
three times and averaged.
Pyrolysis-gas chromatography/mass spectrometry (Py-
GC/MS): The pyrolysis-GC/MS experiments were carried out
on a JCI-22 Curie point pyrolyzer (JAI Co.) coupled to an
Agilent 7890 gas chromatograph equipped with an Agilent
5975C series GC/MSD. Pyrolysis was carried out at 590 ºC
for 5 s. The GC column was a DB-5MS fused silica capillary
column and the oven temperature was held initially at 50 ºC for
2
min, then increased to 60 ºC at 5 ºC/min, to 120 ºC at 10 ºC/min
TABLE-1
FORMULATION OF THE FILLED SBR COMPOUND
and to 180 ºC at 30 ºC/min. The mass spectra were recorded at
an electron impact ionization energy of 70 eV. Table-2 lists
the instrument conditions. The pyrolysis-GC/MS experiment was
carried out three times and the mean was used for further analysis.
Component
phr
a
SBR 1783
137.5
50.0
Various
2.0
Silica
TESPT
TABLE-2
PYROLYSIS-GC/MS INSTRUMENT CONDITIONS
Stearic acid
ZnO
3.0
Pyrolyzer conditions
Sample loading
Pyrolysis temperature
Pyrolysis time
Sulfur
CBS
DPG
1.5
About 0.5 mg
590 ºC
5s
1.5
1.0
a
SBR 1783: 27.3 % oil-extended styrene-butadiene rubber.
Gas chromatograph conditions
Column
J&W DB-5, 30m × 0.25mm × 0.25 µm
280 ºC
Helium
Split 50:1
50 ºC (hold 2min), 60 ºC at 5 ºC/min,
Compounding: To measure the reaction rate of silica and
the silane coupling agent, compounding was performed under
a range of mixing conditions. Mixing was performed in a
Banbury type mixer. The master batch (MB) and final mixing
Inlet temperature
Carrier gas
Injection mode
Oven
1
20 ºC at 10 ºC/min, and 180 ºC at
30 ºC/min
Mass spectrometer conditions
(
FM) stages were carried out at a rotor speed of 76 and 50
rpm, respectively. The initial temperatures of the mixer were
10 ºC and 70 ºC for the master batch and final mixing steps,
respectively. The mixing times of master batch ranged from
.0 to 420 seconds. The dumping temperatures of master batch
Mass temp
Transfer line
MS
230 ºC (Source), 150 ºC (Quad)
250 ºC
Electron ionization, SIM and Scan mode
1
0
were 130, 150, 170 and 190 ºC, respectively. The vulcanizates
were prepared by curing at 165 ºC for 20 min.
RESULTS AND DISCUSSION
Bound rubber: The bound rubber contents (BdR%) were
determined by extraction with toluene at room temperature
for 7 days using the uncured SBR-filled compound. The sample
weight for the extracting experiment was ca. 0.2 g. The weights
of the sample before and after extraction were measured and
the bound rubber contents were calculated as
Pyrolysis-GC/MS analysis was performed to identify a
characteristic peak of TESPT. Fig. 1(a) presents the pyrogram
of TESPT. Bis(triethoxysilylpropyl)tetrasulphide showed a
major pyrolysis product at 10.55 min. This was assigned to
allyltriethoxysilane (ATES) using the NIST mass library
program. Allyltriethoxysilane was detected because the
triethoxy groups of TESPT do not react with the silica surface.
Therefore, ATES was regarded as characteristic pyrolyzate of
unreacted TESPT. For more accurate identification, an
allyltriethoxysilane agent (Aldrich) was analyzed by GC/MS
without a pyrolysis process (shown in Fig. 1(b)). The main
peaks of TESPT and ATES appeared at the same retention
time and had an identical mass spectrum. Therefore, the charac-
teristic peak of TESPT was identified asATES. The formation
process ofATES was expected due to cleavage of the relatively
weak C-S bonds and/or the S-S bond dissociation energy of
TESPT during the pyrolysis process (Fig. 2).
BdR (%) = (Wfg-W × m
where Wfg is the weight of the filler and gel, W is the weight
of the specimen, and m is the phr of the filler and inorganic
ingredient in the compound. m is the phr of the polymer in
the compound and m is the total phr of the compound. The
f t p t
/m )/(W × m /m ) × 100
f
p
t
experiment was carried out three times and the mean was used
for further analysis.
Crosslink density: The crosslink densities of the samples
(cured SBR-filled compounds) were measured using the swell-
ing method. The organic additives in the samples were removed
by extraction with tetrahydrofuran (THF) and n-hexane for 2
and 1 day, respectively, followed by drying in a vacuum oven
for 1 day.After the weights of the dried samples were measured,
they were swollen in toluene for 2 days. The weights of the
swollen samples were then measured. The swelling ratio (Q)
was calculated using the equation:
Fig. 3 shows the pyrograms of each uncured silica filled
SBR compound with/without TESPT. Py-GC/MS was perfor-
med in selected ion monitoring (SIM) mode to detect the peaks
of only SBR and TESPT. The major pyrolysis products of SBR
obtained from SIM mode were butadiene (2.72 min), C7-
species (5.65 min), 4-vinylcyclohexene (6.98 min) and styrene
Q = (W
are the weights of the swollen and dried
samples. The reciprocal swelling ratio (1/Q) was used as the
s u u
-W )/W
2
6
(8.12 min) . The characteristic peak (above-mentioned
ATES) of TESPT at 10.55 min appeared in the pyrogram of
s u
where W and W

Vol. 25, No. 9 (2013)
Investigation of Reaction Rate of Bis(triethoxysilylpropyl)tetrasulphide in Silica-Filled Compound 5247
O
O
S4
Si
O
O
Silica
O
Si
O
O
Si
O
Silica
O
Sx
Rubber
O
Si
O
O
Rubber
Sx
O
O
Si
O
Si
O
O
Silica
Silica
O
S4
O
Si
O
O
O
S4
Si
O
O
(
a)
(b)
Fig. 4. Expected TESPT bonding structure for the formation of ATES (a)
impracticable bonding structure (b) practicable bonding structure
To prove this, the uncured silica-filled SBR compounds
under different mixing conditions were investigated by
pyrolysis-GC/MS.A comparison of the peak area of SBR units
revealed the styrene unit to have the lowest variation of all
SBR units under different mixing conditions. This suggests
that the styrene unit is almost unaffected by the mixing condi-
tion or additives. Table-3 shows the variations of the peak area
of the SBR unit measured by Py-GC/MS. The relative peak
area ofATES and styrene in the uncured compounds was used
to calculate theATES content. TheATES content was calculated
using the following equation:
Fig. 1. Pyrolysis-GC/MS chromatogram and mass spectrum of TESPT (a)
and ATES (b)
O
O
S
S
O
S
S
O
Si
Si
O
O
O
Si
O
Pyrolytic process
O
Fig. 2. Formation process of ATES from TESPT by pyrolysis-GC/MS
C
ATES = PATES/PStyrene × 100
where CATES is the relativeATES content, PATES is the peak area
of ATES and PStyrene is the peak area of styrene. A high CATES
value indicates a low reaction rate of TESPT with the silica
surface.
TABLE-3
CHANGE IN PEAK AREA OF THE SBR
UNIT MEASURED BY Py-GC/MS
TESPaT
Butadiene
C7-
species
4-Vinyl
cyclohexene
Styrene
(%)
Nor. area (%)
16.2
0
.0
.0
.0
.0
19.9
19.3
18.8
18.3
16.5
18.0
9.6
9.5
9.0
8.9
8.5
8.2
54.2
53.8
54.2
55.0
54.5
55.7
2
5
8
17.4
18.0
17.8
1
1.0
4.0
20.5
1
18.2
Variation (%)
0.0
0
.0
.0
.0
.0
0.0
-3.1
-5.8
-8.1
-17.0
-9.7
0.0
-1.8
0.0
-0.8
-0.1
1.5
2
7.4
5
8
11.1
-6.4
Fig. 3. Pyrolysis-GC/MS pyrograms of the uncured silica-filled SBR
compound with TESPT (a) and without TESPT (b)
9.5
-7.6
1
1.0
4.0
26.0
-11.8
-15.4
0.5
1
12.0
2.7
a
an uncured compound with TESPT [Fig. 3(a)]. On the other
hand, ATES did not appear in the pyrogram of the uncured
compound without TESPT (Fig. 3(b)). This indicates the pres-
ence of residual ethoxy groups of TESPT that did not react
with the silica surface in uncured silica-filled compound with
TESPT. The reaction rate of TESPT in the uncured compound
can be measured by pyrolysis-GC/MS. Fig. 4 shows the TESPT
bonding structure that can formATES in silica-filled SBR com-
pound. If there are many reactions between silica and TESPT,
ATES should barely be detected by pyrolysis-GC/MS. There-
fore, the reaction rate of TESPT and the amount ofATES might
be inversely proportion to each other.
%
for TESPT on 100 parts silica filler.
Fig. 5 shows the results of pyrolysis-GC/MS analysis of
the uncured silica-filled SBR compounds according to the
TESPT concentration (0.0-14.0 %, on 100 parts silica). The
ATES content increased with increasing TESPT concentration.
On the other hand, the ATES content decreased slightly to a
TESPT concentration of ca. 6 % and then increased rapidly.
This suggests that there is an optimal concentration on silica/
TESPT reaction and TESPT does not react with silica after
reaching the optimal concentration. A similar result was
19
reported by Hunsche et al. using CP/MAS NMR.

5
248 Ha et al.
Asian J. Chem.
1
1
1
4
2
0
8
6
4
2
0
10
9
8
7
6
5
4
3
2
1
0
0
50
100
150
200
250
300
350
400
450
0
2
4
6
8
10
12
14
Mixing time (s)
TESPT concentration (%)
Fig. 7. ATES content of uncured SBR-filled compounds as a function of
the mixing times
Fig. 5. ATES content of the uncured SBR-filled compounds as a function
of the TESPT concentrations
Fig. 8 shows the effect of the storage time on the silica/
TESPT reaction in uncured silica filled SBR compounds. Two
samples were stored at room temperature for 7 and 30 days in
open containers, respectively. These samples were analyzed
three times. The samples stored for 30 days decreased by more
than 50 % compared to those for 7 days. This shows that the
reaction between TESPT and silica in an uncured silica filled
SBR compound continues at room temperature during the
Fig. 6 shows the results of pyrolysis-GC/MS analysis of
uncured, silica-filled SBR compounds according to the changes
in silica surface area. The TESPT concentration was 8 % per
1
00 parts silica, and the BET (Brunauer-Emmett-Teller) surface
2
area of silica was 115, 175 and 235 m /g, respectively. The
ATES content decreased with increasing silica surface area.
In general, the silanol content of precipitated silica, which is
used mainly in the tyre industry, is ca. 8-9 hydroxyl groups
19
storage time, as suggested by Hunsche et al. . Hunsche et al.
reported that the ethoxy group of TESPT is removed continu-
ously by hydrolysis with moisture in an ambient atmosphere.
2
2 3
per nm (OH/nm ) . The increase in surface area can lead to
an increase in hydroxyl groups. Therefore, the reaction site
between TESPT and the silica surface is increased and the
ethoxy groups of TESPT are decreased. The silica/TESPT
reaction rate increased with increasing silica surface area.
2
1
1
1
1
1
0
8
6
4
2
0
8
6
4
2
0
70
6
0
1
1
1
4
2
0
8
6
4
2
0
50
40
30
20
1
0
0
1
2
3
Sample (#)
Fig. 8. ATES content of uncured SBR-filled compounds as a function of
the storage time
1
15
175
Surface area (m /g
235
2
Finally, the effect of the dumping temperature (130, 150,
70 and 190 ºC) with a constant mixing time at the master
1
Fig. 6. ATES content of uncured SBR-filled compounds as a function of
the silica surface area
batch step on silica/TESPT reaction was studied. The TESPT
concentrations of each compound were 5, 8 and 11 % on silica
100 parts, respectively. Figs. 9 and 10 show the crosslink
density and bound rubber contents according to the TESPT
concentration and dumping temperature, respectively. When
the TESPT concentration is equal, the crosslink density shows
similar results regardless of the dumping temperature. On the
other hand, the bound rubber contents increase with increasing
TESPT concentration and dumping temperature. The bound
rubber was increased not only by the silica/silane reaction but
also by the curing process. The bound rubber contents increased
because TESPT reacted as a sulfur donor with rubber at tempe-
Fig. 7 shows the results of the uncured silica filled SBR
compounds at different mixing times in the master batch step.
In this experiment, the mixing time ranged from 0.0 to 420 s
in the master batch step. The mixing temperature remained
constant at 145 ºC. The ATES content decreased with increasing
mixing time.At a mixing time > 300 s, however, the downward
trend was gentle and the ATES content reached a constant
value. This suggests that most of the silica/TESPT reaction is
in an equilibrium state after a certain mixing time. Therefore,
the silica/TESPT reaction requires a sufficient mixing time to
obtain a proper reaction rate.
17
ratures >160 ºC . Therefore, the mixing temperature has a

Vol. 25, No. 9 (2013)
Investigation of Reaction Rate of Bis(triethoxysilylpropyl)tetrasulphide in Silica-Filled Compound 5249
Fig. 9. Crosslink density of the uncured silica-filled SBR compounds as a
function of the master batch (MB) dumping temperature
Fig. 11. ATES content of the uncured compound and bound rubber as a
function of the master batch (MB) dumping temperature
Fig. 10. Bound rubber content of the uncured silica-filled SBR compounds
as a function of the master batch (MB) dumping temperature
Fig. 12. ATES content of the extracted polymer during the bound rubber
experiment
significant effect on the bound rubber formation in a silica-
filled compound. Allyltriethoxysilane should be detected in
abundance at relatively low mixing temperatures and bound
rubber contents. Fig. 11 shows the pyrolysis-GC/MS result of
the bound rubber (BdR) and uncured compound.As expected,
the ATES content at low temperatures was higher than that at
high temperatures in both the uncured compound and bound
rubber. TheATES contents of bound rubber showed a decreasing
trend against the uncured compound because most of the
TESPT reacted to produce bound rubber. The unbound polymer
extracted by toluene in the bound rubber experiment was
analyzed. Fig. 12 shows result of pyrolysis-GC/MS of the
unbound polymer obtained as bound rubber experiment. The
ATES was also detected in the unbound polymer and the
resulting trend was similar to that of the uncured compound.
This suggests that the TESPT, which did not react with the
Allyltriethoxysilane was produced by the pyrolysis of TESPT,
and was detected in the uncured compound. The relativeATES
content was used to determine the reaction rate. The uncured
compound under various mixing conditions was used to
examine the effects of the TESPT concentration, mixing time,
temperature, storage time and silica surface area. With increasing
TESPT concentration in the compound, the ATES content
increased slightly and then increased rapidly at some point.
The ATES content decreased with increasing mixing time,
temperature, storage time and silica surface area. In general,
the reaction rate between TESPT and silica increases with
increasing mixing time, temperature, storage time and silica
surface area. The reaction rate of silica/silane and the ATES
contents are inversely proportional to each other. Overall, by
analyzing the ATES contents, it will be possible to examine
the optimal mixing conditions for the silica-filled compound,
as suggested by NMR or by measuring the physical properties.
27
silica surface and/or formed a pendent group bonded to a
rubber chain, can be extracted in the uncured compound.
Because TESPT has fewer reactions with the silica surface
and/or rubber chain at relatively low temperatures, the ATES
content increases with increasing dumping temperature at the
master batch step.
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