Crack healing and reclaiming of vulcanized rubber by triggering the rearrangement of inherent sulfur crosslinked networks

Article abstract of DOI:10.1039/c5gc00754b

CuCl₂ has been shown to effectively catalyze reshuffling of the inherent sulfur crosslinked networks of vulcanized rubber. Once activated, CuCl₂-based complex catalysis enables disulfide metathesis through circulated crossover reactions among disulfide and polysulfide bonds without forming radicals and ionic intermediates. By taking advantage of this mechanism, the model material of this study, vulcanized polybutadiene rubber, acquires thermal remendability as characterized by repeated restoration of mechanical properties. Moreover, it can be reprocessed like thermoplastics. The compositions and fabrication of the model material simulate those of industrial vulcanized rubber, so as to facilitate formulation optimization under the circumstances close to the actual situation for possible future practical applications. It is hoped that the results of the present preliminary exploration would provide the basis for extending the service life and developing new recycling techniques of vulcanized rubber, which is produced, used and scrapped in large quantities every day.

Full text of DOI:10.1039/c5gc00754b

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ARTICLE
Crack healing and reclaiming of vulcanized rubber by triggering
rearrangement of inherent sulfur crosslinked networks
H. P. Xiang,a H. J. Qian,b Z. Y. Lu,b M. Z. Rong*a and M. Q. Zhang*a
Received 00th January 20xx,
Accepted 00th January 20xx
CuCl2 is shown to effectively catalyze reshuffling of the inherent sulfur crosslinked networks of vulcanized rubber. Once
activated, CuCl2-based complex catalysis enables disulfide metathesis through circulated crossover reaction among
disulfide and polysulfide bonds without forming radicals and ionic intermediates. By taking advantage of this mechanism,
the model material of this study, vulcanized polybutadiene rubber, acquires thermal remendability as characterized by
repeated restoration of mechanical properties. Moreover, it can be reprocessed like thermoplastics. The compositions and
fabrication of the model material simulate those of industrial vulcanized rubber, so as to facilitate formulation
optimization under the circumstances close to actual situation for possible future application in practice. It is hoped that
the results of the present preliminary exploration would provide basis for extending service life and developing new
recycling technique of vulcanized rubber, which is produced, used and scraped in large quantity every day.
DOI: 10.1039/x0xx00000x
www.rsc.org/
have not yet dealt with the widely used vulcanized rubber.
Most studies are focused on tailor-made elastomers and
elastomer blends, such as polydimethylsiloxane (PDMS)
Introduction
Vulcanized rubber developed in the 19th century is one of the
largest utilized polymers in the world. It has been applied to
various industrial products like tyres, conveyor belts, shoe
soles, hoses, seals, gaskets, etc. The large quantities of rubber
consumption, however, bring about heavy burden to the
environment because of the limited disposals of scraps, which
contain nonreversible permanent crosslinked networks. To
solve the problem, waste rubbers used to be reclaimed by
physical, chemical and biological degradation to covert the
insoluble and infusible thermoset into small molecular weight
fragments with commercial value, or simply burned and
buried.1,2
containing encapsulated healing agent,3 thermoreversible
rubber from supramolecular interaction,4,5 thiol-functionalized
silicone elastomer crosslinked by silver nanoparticles,6 nitrile
rubber/hyperbranched
polyethylenimines
compound,7
epoxidized natural rubber and its blend with ionomer,8,9
alkoxyamine crosslinked polyurethane elastomer10 and epoxy
elastomer,11 polybutadiene with Grubbs’ second-generation Ru
metathesis catalyst,12 living crosslinked PDMS,13 crosslinked
poly (urethane-urea),14 and multiphase supramolecular
thermoplastic elastomer.15
Considering that vulcanized rubber made of natural rubber
or synthetic rubbers possesses optimized structures and
properties after years of development, addition of healing
microcapsules into the material or introduction of specific
reversible bonds into rubber molecules would not be the most
suitable way to impart it with self-healability. In fact, rubber is
vulcanized mostly by sulfur or sulfur-donor, resulting in
crosslinked networks with tremendous monosulfidic, disulfide
and polysulfide linkages. (Note: According to the
accelerator/sulfur ratio, commercial sulphur vulcanization
systems are usually classified into: conventional (0.1~0.6),
Unlike the above methods based on rupture of
intermolecular links, the concept of self-healing arising in
recent years offers a solution from another angle. That is,
extension of service life and reduction of waste of rubber
products by re-bonding the damaged materials. Nevertheless,
literature survey indicates that the researches in this aspect
a.Key Laboratory for Polymeric Composite and Functional Materials of Ministry of
Education, GD HPPC Lab, School of Chemistry and Chemical Engineering, Sun Yat-
sen University, Guangzhou 510275, P. R. China. E-mail: cesrmz@mail.sysu.edu.cn
and ceszmq@mail.sysu.edu.cn..
semi-efficient
(0.7~2.5),
and
efficient
vulcanization
(2.5~12).16,17 The produced monosulfide, disulfide and
polysulfide crosslinks in the total number of crosslinks amount
to 0~5, 30~40 and 60~70 % for conventional vulcanization,
0~20, 50~70 and 10~30 % for semi-efficient vulcanization, and
b.State Key Laboratory of Theoretical and Computational Chemistry, Institute of
Theoretical Chemistry, Jilin University, Changchun 130023, China
†Electronic Supplementary Informaꢀon (ESI) available: [Determinaꢀon of Keq, Ea
and concentration of sulfur crosslinks. Fig. S1, rheographs. Fig. S2, SEM and EDS
images. Fig. S3 and S4, HLPC analyses and mass spectra. Fig. S5, S13, S14, S16 and
S18, HPLC analyses. Fig. S6, time dependences of amount of metathesis products.
Tables S1-S4, influential factors of Keq. Fig. S7 and S8, conversion versus time. Fig.
S9 and S12, GC-MS analyses. Fig. S10 and S11 mass spectra. Fig. S15, HLPC analyses
and ESR spectra. Fig. S17, TGA curves. Fig. S19, non-contact regions. Fig. S20 and
S21, EDS images. Fig. S22, tensile stress-strain curves.]. See
40~50, 35~50 and 5~20
% for efficient vulcanization,
respectively.16~22) Disulfide bonds themselves have enabled
self-healing of polymers through exchange or metathesis
reaction by using their dynamic reversibility.23~29 If disulfide
DOI: 10.1039/x0xx00000x
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exchange or metathesis among different disulfide and form complex with CuCl2 in the vulcanized rubber. For
polysulfide linkages can also be triggered in sulfur crosslinked achieving the targets, in reality, part of the organosilane is
networks of vulcanized rubber by tiny amount of catalyst, the grafted onto nano-silica in advance, while the rest is directly
rubber would acquire self-healability accordingly, while compounded with all ingredients. Although incorporation of
keeping its intrinsic quality and original production process. the organosilane might somewhat weaken mechanical
Moreover, reshuffling of macromolecular networks due to strength of the model rubber, we prefer to conduct proof-of-
dynamic exchange of disulfide and polysulfide bonds would concept study at first using the current recipe before working
allow vulcanized rubber to be reprocessed without needs of out a solution for real application.
traditional practice of decrosslinking.2 The key issue lies in
finding out proper trigger mechanism and catalyst.
Experimental
To activate the desired reversible reaction of disulfide and
polysulfide linkages in vulcanized rubber, heating, rather than
light, pH, etc., should be used as a means of stimulus in view of
practicality and work efficiency. Additionally, the matching
catalyst had better to be not only thermosensitive but also
thermotolerant because of the following reasons. (i) Rubber
vulcanization belongs to a typical thermal processing, which
requires all the ingredients to be robust enough to survive the
manufacturing. (ii) Healing (or reclaiming) temperature higher
than the operating temperature of rubber products is
necessary for ensuring stableness of structure and properties
of the latter during service. Room temperature reshuffling of
crosslinked networks26,28,30 that leads to obvious creep of the
material at ambient condition is not encouraged in the present
case. According to these criteria, thiol,25 phosphines26,31 and
rhodium,32 which have successfully assisted or catalyzed
thermal exchange or metathesis of disulfide, are no longer
qualified. Thiol is easy to be oxidized in air. For example, thiol-
disulfide exchange reaction was hindered at temperature
above 70 °C as a result of accelerated oxidation of free thiol
groups.29 Phosphine catalyst is vulnerable to oxygen especially
at elevated temperature.26,31 As for rhodium catalyst, the high
cost restricts its large scale application.
Materials
Polybutadiene rubber (trade name: BR 9000) was purchased
from Sinopec Group, China. Chloroprene rubber (trade name:
CR121) was supplied by Changhui Chemical Co. Ltd., China.
Poly (1,2-dihydro-2,2,4-trimethyl-quinoline) (trade name: RD)
was purchased from Lanzhou Chemical Industry Co., China.
Tris(2,4-di-tert-butylphenyl) phosphate (trade name: 168) was
supplied
by
Tokyo
Chemicals
Industry,
(DPTT),
Japan.
bis(3-
Dipentamethylenethiuram
tetrasulfide
triethoxysilylpropyl)tetrasulfide (trade name: Si-69), 2,2
′-
hydroxy ethyldisulfide (HEDS), diethyl disulfide (DEDS), dibutyl
disulfide (DBDS), diallyl disulfide (DADS), dipropyl disulfide
(DPDS), dimethyl trisulfide (DMTS), dicumyl peroxide (DCP),
stearic acid, ZnO, sulfur and CuCl2 were provided by Aldrich. All
chemicals were of analytical grade reagents and used as
received. Scheme 1 shows the structures of the substances
involved in this study that contain disulfide and polysulfide
bonds. Silica nanoparticles (Aerosol A380) with average
primary particle size of 7 nm and specific surface area of 380
m2 g-1 was supplied by Degussa. High abrasion furnace carbon
black (N330) with average primary particle size of 35 nm was
supplied by Cabot Corporation.
Sx
Here, for the first time we report CuCl2 as a heat resisting,
oxygen insensitive and cost effective catalyst to trigger
disulfide metathesis in vulcanized rubber. It was discovered by
contrastive analysis of periodic table of elements and probe
trial. CuCl2 used to serve as catalyst for radical addition
reaction, chlorination and oxidation, additive for
electroplating, photoengraving and feed, and colorant for
glass, ceramics, etc. To the best of our knowledge, it has not
been applied in the manner in which we employ it. To verify
the feasibility of our design, CuCl2-loaded polybutadiene
rubber is prepared and characterized in the following.
n
S
x=2-6, n=80-100)
n
x
Cl
Cl
Chloroprene rubber
HEDS: R1=R2= -C2H4OH
DPDS: R1=R2= -C3H7
DEDS: R1=R2= -C2H5
DBDS: R1=R2= -C4H9
S
S
R1
R2
S
S
DMTS:
DADS:
S
S
S
Since this work is a trial towards development of a
technology for possible future application, the basic formula of
the model material of this study simulates that for tyres (Table
O
O
1), which contains not only polybutadiene rubber as the base
Si
S
S
O
S
S
Si
O
material but also other components like carbon black, nano-
silica, sulfur, stearic acid, anti-ager, etc. Nevertheless, fractions
of the components of our model material differ from those of
the industrial product. To increase concentrations of disulfide
and polysulfide bonds and mobility of CuCl2 in the sulfur
crosslinked network, bis(3-triethoxysilylpropyl) tetrasulfide, a
polysulfide organosilane coupling agent and crosslinking agent
for rubber industry, is included. It is expected (i) to take part in
the crosslinking reaction of polybutadiene rubber and (ii) to
O
O
Bis(3-triethoxysilylpropyl)tetrasulfide (Si-69)
Scheme 1 Structures of the disulfide and polysulfide-containing compounds used
in this study.
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Compounding, vulcanization and molding
minimum torque, is estimated to be 6.98 min. Based on this
measurement, the rubber VR-SH was cured at 150 °C for 6.98
min under 20 MPa, producing 2 mm thick sheet for the
subsequent characterization.
According to the recipe listed in Table 1, SiO2 nanoparticles
(100 g) were firstly mixed with toluene (1000 ml) under
ultrasonication. Then, Si-69 (20 ml) was added to the SiO2 sol
with mechanical stirring and reflux for 5 h. Finally, the SiO2
grafted by Si-69 (SiO2-g-Si-69) was centrifuged, ultrasonically
washed with cyclohexane for three times, and freeze dried for
24 h. The percent grafting was about 10 %, as determined by
thermogravimetric analysis (TGA).
To understand the dispersion status of CuCl2 catalyst,
scanning electron microscopic (SEM) micrograph of a model
system consisting of only CuCl2 and polybutadiene rubber, and
energy dispersive spectroscopy (EDS) image of VR-SH were
collected. The observation by means of EDS is conducted
because CuCl2 is not the only particulate filler in VR-SH.
Mapping of copper is more suitable for identification of CuCl2.
The model rubber of this work was fabricated as follows.
Firstly, polybutadiene rubber was masticated in a two-roll mill
at room temperature. Next, nano-silica (or Si-69 grafted nano-
silica) and carbon black were mixed with the matrix rubber.
Afterwards, Si-69, CuCl2, ZnO, stearic acid, anti-ager RD, and
antioxidant 168 were added. Finally, sulphur and accelerator
DPTT were incorporated to the premixed compounds. All the
raw materials were poured evenly along the nip with
alternating cuts. After uniformly compounding, ten end-roll
passes were made before sheeting off. The whole process took
about 10 min.
As shown in the SEM photo (ESI,† Fig. S2a), the CuCl2 particles
are well dispersed in polybutadiene rubber except for tiny
amounts of small aggregates even when the particles content
is as high as 1 phr, which is 10 times higher than that in VR-SH
(
Table 1). Accordingly, the average particle size is estimated to
be about 3 m. The well dispersion of CuCl2 particles is
evidenced by the EDS image (ESI, Fig. S2b), which shows that
ꢀ
†
the fillers are separated from each other in the rubber. In fact,
well distribution of CuCl2 is important for the proposed
disulfide metathesis. If CuCl2 particles were severely
agglomerated, the exchange reaction of S-S bonds could not
efficiently proceed and hence the crack healing and reclaiming
would be hindered.
To determine the optimal curing conditions, rheograph of
the above compounds (i.e. the model material of this study,
named VR-SH) was recorded at 150 °C by a moving die
rheometer (UR-2010SD-A, U-CAN Dynatex Inc.). It is seen from
For comparison, references control 1 and control 2 (Table
(ESI,† Fig. S1) that the torque drastically increases at the
1
) were prepared by the same procedures, except that the
beginning and then levels off demonstrating the formation of
crosslinked structure throughout the material. The
characteristic t90, the optimum vulcanization time required for
reaching specific torque value which is the sum of minimum
torque and 90 % of the differentials between maximum and
curing time of control 1 was 9.70 min and that of control 2 was
1.95 min as suggested by their t90
(ESI,†
Fig. S1).
Table 1 Compositions of the rubbers studied in this work
Rubbers
Polybutadiene Sulfur
rubber
CuCl2
SiO2
Si-69 d)
SiO2-g-Si-69e) Carbon ZnO Stearic acid
black
RD f)
168 g)
DPTT h)
DCP i)
VR-SHa)
Control 1b)
Control 2c)
100
100
100
1
1
0
0.1
0
0
0
10
10
0
20
20
0
5
5
5
1
1
0
0.5
0.5
0
1
1
1
1
1
1
1
1
0
0
0
0.1
20
1.5
a) Name of the model material of this study; b) Reference material containing the same components as VR-SH except that CuCl2 is excluded; c) Reference material
containing the same components as VR-SH except that peroxide DCP, rather than sulfur, serves as the crosslinking agent; Control 1 and control 2 are used to evidence
that both reversible sulfur crosslinks and CuCl2 are necessary for healing and reclaiming of vulcanized rubber. Control 1 contains reversible sulfur crosslinks without
CuCl2, while control 2 contains CuCl2 without reversible sulfur crosslinks. d) Trade name of bis(3-triethoxysilylpropyl)tetrasulfide; e) SiO2 nanoparticles grafted by Si-69;
f) Trade name of anti-ager poly(1,2-dihydro-2,2,4-trimethyl-quinoline); g) Trade name of antioxidant tris(2,4-di-tert-butylphenyl)phosphate; h)
Dipentamethylenethiuram tetrasulfide serving as accelerator; i) Dicumyl peroxide.
Model disulfide metathesis experiments
on an Agilent 1100 using a mobile phase of CH3CN/H2O = 3/1
(v/v) at a flow rate of 1 mL min-1 with UV detector at 254 nm.
Gas chromatography-mass spectroscopy (GC-MS) analyses
were performed on a Thermo DSQ-EI mass spectrometer. The
GC column oven was heated to 290 °C by 20 °C min-1, followed
by a hold at this temperature for 10 min. The disulfide
metathesis in macromolecules was characterized by gel
permeation chromatography (GPC) on a Waters 1515 with THF
as the eluent and polystyrene as the calibration standard.
DEDS (0.24 g, 2.0 mmol), HEDS (0.31 g, 2.0 mmol), CuCl2 (2.5
mg, 18.6
ꢀmol) and acetonitrile (10 mL) were mixed by
magnetic stirring at 25 °C. Samples were extracted at different
reaction times to estimate the extent of disulfide metathesis.
The effects of solvent and structures of disulfide compounds
were studied following the same procedure as above.
Characterization
Ultraviolet-visible (UV-Vis) spectra were collected by
a
The progress of disulfide metathesis in small molecules was
monitored by high-performance liquid chromatography (HPLC)
Shimadzu UV-300 spectrophotometer. Fluorescence spectra
were obtained from a Varian Cary Eclipse with excitation
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wavelength of 343 nm. Electron spin resonance (ESR) spectra
were recorded on a Bruker A300-ESR spectrometer with 100
kHz modulation frequency. TGA was used to assess thermal
stability of the rubbers from 40 to 800 °C with a ramp of 10 °C
min-1 under N2 atmosphere on a TA Q50. Stress relaxation was
measured on a MetraviB DMA-25N using tensile mode with a
deformation of 20 %. Morphological observation and energy
dispersive spectroscopy (EDS) analysis were conducted by a
Quanta 400 FEG field emission scanning electron microscope
(SEM). Prior to the experiment, the sample surface was coated
by gold/palladium sputtering.
S
C2H5
HOH4C2
S
HEED
S
C2H5
H5C2
S
S
C2H4OH
HEDS
HOH4C2
S
DEDS
60 min
5 min
2 min
0 min
0
1
2
3
4
5
6
7
8
To evaluate healing ability, tensile tests were conducted on
dumbbell-shaped specimens (total length: 50 mm, thickness: 2
mm, width of parallel part: 4 mm) according to ASTM D412
with a SANS CMT 6000 universal tester at a crosshead speed of
50 mm min-1 at 25 °C. When the specimen was broken apart,
the two parts were placed back in contact. After healing at 110
°C for a period of time, the healed specimen was tested to
failure again. Healing efficiency was calculated from the ratio
of tensile strength of healed specimen to that of the virgin
one.
Retention time [min]
Fig. 1 HLPC analysis of equimolar mixture of HEDS and DEDS as a function of time (25
°C, acetonitrile, 0.5 mol% CuCl2).
The effect of CuCl2 dosage is examined (ESI,† Fig. S6a). It
seems there is a critical threshold for the catalyst. In the case
of 0.05 mol% CuCl2, for example, the reaction does not
equilibrate within 4 h and the conversion is only ~10 %. When
the dosage of CuCl2 is raised to 0.25 mol%, however, the
reaction manages to reach equilibrium after 2 h. A much faster
equilibrium can be achieved due to further increase of CuCl2
dosage to 0.5 mol%. Therefore, the minimum CuCl2 dosage
used for the rubber system should be higher than 0.25 mol%.
Reclaiming of the rubbers was conducted by breaking the
specimens into small particles through freeze grinding and
then compression molding of the rubber particles under 10
MPa at 110 °C for 3 h.
ESI,
† Fig. S6b studies the effect of initial molar ratio of
reactants on the speed of disulfide metathesis. Because the
amount of CuCl2 has exceeded the above lower limit, all the
reactions are allowed to reach equilibrium within short time.
Quantification of the above reactions offers equilibrium
constant, Keq, which only varies with reaction temperature and
is not affected by initial molar ratio of reactants and catalyst
RESULTS AND DISCUSSION
So far, there is no report on the ability of CuCl2 for catalyzing
disulfide metathesis. We have to conduct a series of model
tests with small molecules first. The high-performance liquid
chromatography (HPLC) spectra (Fig. 1) indicate that after the
addition of CuCl2 into the equimolar mixtures of two disulfide
compounds (2,2’-hydroxy ethyldisulfide (HEDS) and diethyl
disulfide (DEDS)) in acetonitrile, a new peak appears, whose
area increases with time and reaches the equilibrium within 60
min. Meanwhile, the peak areas of HEDS and DEDS decrease.
The steady molar proportions of HEDS, DEDS and the reaction
dose (ESI,† Table S1-S4). It is thus known that the disulfide
metathesis catalyzed by CuCl2 belongs to dynamic reversible
reaction. Besides, activation energy of disulfide metathesis in
acetonitrile is calculated to be 22.3 kJ/mol, and that in n-
heptane is 172.3 kJ/mol (ESI,† Fig. S7 and S8). Clearly, more
energy needs to be input for the reaction in non-polar
environment.
product are 24:26:50 (ESI,† Fig. S3a), which coincide with the
theoretical values supposing the reaction product is
hydroxyethyl ethyl disulfide (HEED) according to the
To further understand the effect of structures of disulfide
molecules on disulfide metathesis, equimolar mixtures of
DPDS/diallyl disulfide (DADS) and dibutyl disulfide (DBDS)/
dimethyl trisulfide (DMTS) pairs as well as DMTS alone were
tested with CuCl2 as catalyst, respectively. Allyl disulfide bridge
is a common constituent of sulfur cosslinkages in vulcanized
rubber1,16~21 while DMTS simulates polysulfidic bonds. The
experiments are expected to get close to the actual situation in
vulcanized rubber by choosing these reagents. The mass
spectra of the metathesis products indicate that allyl propyl
assignment of mass spectrum (ESI,
† Fig. S3b). Evidently,
disulfide exchange reaction can be effectively catalyzed by
CuCl2 without side-reaction. When the polar acetonitrile is
replaced by non-polar n-heptane as solvent, moreover,
disulfide metathesis between dipropyl disulfide (DPDS) and
DEDS also takes place in the presence of CuCl2, despite that
reaction temperature has to be higher than 30 °C and longer
time is needed to attain equilibrium (ESI,† Fig. S4). It means
that the mechanism of ionic exchange catalyzed by
phosphine,26,31 which has apparent solvent effect, can be
excluded from disulfide metathesis under catalysis of CuCl2.
What is more, we also inspected the case in the absence of any
catalyst and found that disulfide metathesis did not occur
between aliphatic disulfide compounds after long time
disulfide is generated by DPDS/DADS (ESI,
methyl butyl disulfide and methyl butyl trisulfide are created
by DBDS/DMTS (ESI, Fig. S10). Moreover, there are also
† Fig. S9), and
†
metathesis products (i.e. dimethyl disulfide and dimethyl
tetrasulfide) in the system of DMTS and CuCl2, which excludes
any disulfide at the beginning (ESI,† Fig. S11). Clearly, disulfide
regardless of solvent polarity and temperature (ESI,† Fig. S5).
metathesis is allowed to proceed among not only disulfides
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3.0x105
5
but also polysulfide molecules, probably because of similar
bonding energies of S-S and -Sx- (x 3) bonds (273 and 256
kJ/mol, respectively22). These outcomes suggest that CuCl2
should be applicable to sulfur crosslinks of vulcanized rubber
that contain both disulfide and polysulfide bonds.
Mw
≥
2.5x105
2.0x105
1.5x105
1.0x105
Mn
Mw/Mn
4
3
In fact, CuCl2 can ceaselessly catalyze disulfide metathesis.
When the third disulfide DBDS was added into the equilibrated
DEDS/DPDS system with ethyl propyl disulfide (EPDS) as the
metathesis product, two new products (i.e. ethyl butyl
disulfide (EBDS) and propyl butyl disulfide (PBDS)) are yielded
2
1
0
5.0x104
0.0
and a new equilibrium is reached again (ESI,
† Fig. S12).
Additionally, the high catalytic activity of CuCl2 is also reflected
by the control test with triphenylphosphine (PPh3). It can be
0
2
4
6
8
10
12
Reaction time [h]
seen from ESI,† Fig. S13 that disulfide metathesis is allowed to
proceed with the assistance of PPh3 at 100 °C, but the
conversion efficiency is too low to have use value.
Fig. 2 Time dependences of number average molecular weight, Mn, weight average
molecular weight, MW, and molecular weight distribution, Mw/Mn, of chloroprene
In the formula of the rubber to be studied (Table 1), there rubber in the solution of tetrahydrofuran (3 mg/ml) at 25 °C (CuCl2: 0.5 mol%).
are quite a few chemical additives having different functions.
They might deactivate CuCl2 and disable disulfide metathesis.
ESI, Fig. S14 checks the influence of two representative
The above results and discussion have revealed that
disulfide metathesis is enabled by CuCl2, but the underlying
mechanism is still unknown. The UV-Vis spectrum in Fig. 3a
shows that CuCl2 in acetonitrile has two absorption bands. The
strong one at 309 nm results from the absorption of CuCl2,
while the weak one at 460 nm is ascribed to the complexation
effect between CuCl2 and acetonitrile, i.e. Cu(MeCN)Cl2.34,35
Once disulfides are added, the strong peak at 309 nm
disappears, and the peak height at 460 nm obviously
decreases. In contrast, another strong peak due to absorption
of disulfide bonds is observed at 248 nm, which is intensified
with a rise in CuCl2 concentration. Similarly, electron spin
resonance (ESR) spectra indicate that the g value of CuCl2 in
acetonitrile changes from 2.12786 (317 mT) to 2.0616 (327
mT) when equimolar HEDS and DEDS are incorporated (Fig.
3b), implying variation in the nature of the related chemical
bonds.36 On the other hand, earlier study of fluorescence of
disulfides demonstrated that the formation of complex
between Cu2+ and disulfides led to quenching effect.37,38 The
conclusion agrees with our own observation. Fig. 3c exhibits
that the fluorescence intensity of disulfides decreases with the
concentration of CuCl2. The fluorescence is completely
quenched when the concentration of CuCl2 is higher than 1
mol%. All these results manifest that there is complex effect
between CuCl2 and disulfides.
†
additives, anti-ager and antioxidant, on disulfide metathesis
catalyzed by CuCl2. The dosages of both anti-ager and
antioxidant are intentionally much higher than that of CuCl2,
while different pairs of disulfides are employed to investigate
the possible structural effect of disulfides. Anyhow, the
conversion percentage of the reaction approaches 45 % within
2 h for the tests. Hence, the catalytic activity of CuCl2 should
be valid for disulfide metathesis in vulcanized rubbers.
In addition to the low-molecular-weight disulfides, linear
macromolecule carrying disulfide and polysulfide bonds on the
backbone represented by chloroprene rubber (Scheme 1) was
also employed as a model compound to examine the catalytic
ability of CuCl2 for disulfide metathesis. The molecular weight
variation of chloroprene rubber in tetrahydrofuran solution
with CuCl2 shows that both number average and weight
average molecular weights decrease with time, while the
molecular weight distribution increases (Fig. 2). It means that
disulfide metathesis have proceeded among the main chains of
chloroprene rubber. The variation tendency resembles that
reported in reference,33 reflecting the random nature of the
metathesis in macromolecules with multiple reversible bonds.
As it turns out, disulfide metathesis could occur in disulfide
and polysulfide containing macromolecules with the assistance
of CuCl2.
4
(a)
CuCl2
HEDS:DEDS = 1:1
HEDS:DEDS = 1:1, 0.5 mol% CuCl2
HEDS:DEDS = 1:1, 1.0 mol% CuCl2
HEDS:DEDS = 1:1, 2.0 mol% CuCl2
3
2
1
0
200
300
400
500
600
700
800
Wavelength [nm]
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Journal Name
Another possible mechanism is related to formation of
ionic intermediates for phosphines catalyzed disulfide
metathesis.26,31 However, the above experiment has shown
(b)
g = 2.06160
that CuCl2 works in non-polar solvent (ESI,
†
Fig. S4), which
Fig.
HEDS:DEDS = 1:1
CuCl2
disagrees with the solvent effect of phosphines.31 ESI,
†
HEDS:DEDS = 1:1,
0.5 mol% CuCl2
S16a depicts a group of model tests by introducing diverse
inhibitors (i.e. benzoquinone, ethyl acetate, HCl and H2O) to
the disulfide metathesis with CuCl2. The conversion rates of
the systems containing inhibitors after 30 min are almost the
same as that of the system without any inhibitor. Supposing
there were mercaptides (RS-) or sulfenium cations (RS+) in the
reaction process, the inhibitors would interact with the ions
and the conversion rate would be decreased. The truth is the
opposite, meaning that the reaction has nothing to do with
ions.
g = 2.12786
0
100
200
300
400
500
600
700
Field [mT]
500
400
300
200
100
0
(c)
On the basis of above results and discussions, it is deduced
that neither radicals nor ions are involved in the disulfide
metathesis catalyzed by CuCl2, which differs from the cases
with light- and phosphines-catalysis.26,31,33,39
HEDS:DEDS = 1:1
HEDS:DEDS = 1:1, 0.5 mol% CuCl2
HEDS:DEDS = 1:1, 1.0 mol% CuCl2
HEDS:DEDS = 1:1, 2.0 mol% CuCl2
We found that CuCl can also catalyze disulfide metathesis
in the case of higher dosage and reaction temperature, and
longer reaction time (ESI,
† Fig. S16b). Despite the poorer
catalytic activity than CuCl2, CuCl takes effect depending on its
reducibility,2 while CuCl2 is often used as oxidant due to the
high valence state. The conflicting information manifests that
the CuCl2 catalyzed disulfide metathesis should not be a redox
process.
200
300
400
500
600
700
800
Wavelength [nm]
Fig. 4a gives quantum chemical study of CuCl2 catalyzed
disulfide metathesis in acetonitrile. All structures are
optimized using B3LYP method with the 6-31G (d) basis set for
C, H, O, S atoms and SDD basis set for Cu element. PCM
solvation model in acetonitrile is performed on the basis of
optimized equilibrium geometries by Gaussian Package.
Vibrational analysis is performed at the same level of theory.
The results indicate that there is complex effect between
disulfides and CuCl2, which brings forth the reaction
intermediates, IM1. Then, IM1 reaches the transient state,
TS1, when the potential energy overcomes the energy barrier,
which is the rate-determining step.31 In the course of
transformation, bond lengths of S-S and Cu-Cl also change.
Finally, two different sulfur atoms of disulfides in the transient
state are split and recombined into a metathesis product and
CuCl2-disulfide complex.
Fig. 3 (a) UV-Vis, (b) ESR, and (c) fluorescence spectra (excitation wavelength: 343 nm)
of disulfides and CuCl2 in acetonitrile at 25 °C.
Amamoto et al. suggested that free radicals are involved in
reshuffling of thiuram disulfide moieties under visible light.39
To see whether the present disulfide metathesis is driven by a
similar mechanism, TEMPO was included to capture the
possible radicals. As exhibited by ESI,
† Fig. S15a, the
conversion of the disulfide metathesis increases with time like
the case without TEMPO (Fig. 1), but the reaction rate is
slowed down after the addition of TEMPO. It might thus be
thought that radicals would have been created, which were
caught by TEMPO leading to retardation of the reaction.
However, the signal intensity of the TEMPO radicals in the
reaction system is the same as that in the system without any
disulfide under the same conditions (ESI,† Fig. S15b and S15c).
According to the results of both experiments and
theoretical prediction, the mechanism involved in disulfide
That is, no TEMPO is consumed during the disulfide
metathesis. The reduction in reaction rate should result from
metathesis utilizing CuCl2 as catalyst is depicted in Fig. 4b
.
decay of catalytic activity of CuCl2. ESI,
†
Fig. S15d shows a
Firstly, disulfides and CuCl2 interact with each other as soon as
they are mixed together due to the lone pair electrons of sulfur
atoms in disulfides and the vacant 3d orbit of Cu(II).34 This
leads to the formation of an intermediate. Then, spatial
conformation of the complex intermediate is optimized
through adjusting bond angle and bond length between
disulfide and CuCl2. That is, the system enters the transient
state. At last, rearrangement and localization of electrons of
disulfide bonds make the latter fracture. Recombination of the
two different sulfur atoms offers the metathesis product,
while CuCl2-disulfide complex is remained. The newly
reaction similar to that recorded in ESI,
†
Fig. S15a except for
the feeding sequence. Here no metathesis product is detected
only because CuCl2 and TEMPO are mixed for a while in
advance. Clearly, the complex effect between CuCl2 and
TEMPO restricts the complexation between CuCl2 and
disulfide. Besides, if radicals were present in the CuCl2
catalyzed disulfide metathesis, the S-based radicals should
have response signals at 350 and 370 mT (g = 1.925 and 2.004)
in acetonitrile.39 This is not true for our case as there are no
homologous peaks in Fig. 3b
.
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generated CuCl2-disulfide complex would rapidly interact with the references (i.e. control 1 and control 2) are close to each
another disulfide, repeating the above processes.
other. Only control 2 possesses slightly higher onset
temperature because of the higher C-C bond energy than S-S.22
It is thus known that the rubbers have acquired good
thermostability, which is not affected by the addition of CuCl2.
Besides, because (i) the dosage of CuCl2 is low, (ii) the
complexation between CuCl2 and disulfide bonds weakens the
ability of CuCl2 to induce ageing of the rubbers, and (iii) anti-
ager is added, thermal ageing tests at 110 °C indicated that the
ageing resistance of the rubbers was not deteriorated in the
presence of CuCl2.
Fig. 5a plots stress relaxation of VR-SH measured at various
temperatures. When temperature is lower than 90 °C, VR-SH
behaves like conventional polymers. The significant reduction
in stress at the beginning stage due to viscoelastic feature is
gradually replaced by a steady plateau. In the case of higher
temperature, however, the scenario is quite different. At 110
°C, for example, the stress continues to decrease at slower
rate for a long period of time after the initial drastic decline
and eventually approaches zero (Fig. 5a). It means that
multiple stress relaxation mechanisms are involved.25 In
contrast, the curves of control 1 and control 2 (Fig. 5b), in
which disulfide metathesis is disabled (Table 1), resemble that
of VR-SH recorded at 30 °C. Naturally, this relaxation manner
of VR-SH is associated with disulfide metathesis as it would
lead to rearrangement of the sulfur crosslinked networks once
triggered, during which stress relief occurs. Our previous study
shows that the activation energy of CuCl2 catalyzed disulfide
metathesis of small molecules in non-polar solvent is higher
than that in polar solvent. Here the main component of VR-SH,
polybutadiene rubber, is also non-polar, so that disulfide
metathesis takes effect at elevated temperature instead of
room temperature.
(a)
(b)
Fig. 4 (a) Quantum chemical study of CuCl2 catalyzed disulfide metathesis in
acetonitrile. Energies are given in kcal/mol; (b) Mechanism of CuCl2 catalyzed
metathesis between low molecular weight disulfides.
From
a practical perspective, as mentioned in the
As the model experiments and mechanism investigation
have revealed the capability of CuCl2, we start with the work of
the model material of this study, VR-SH, whose compositions
and curing procedures are carefully described in Table 1 and
Experimental section, respectively. The CuCl2 dosage in VR-SH
was determined in consideration of speed of disulfide
metathesis and ageing resistance of the rubber material. The
above model experiments have shown that disulfide
metathesis can quickly equilibrate with 0.5 mol% CuCl2.
However, if the CuCl2 dosage exceeded 1 mol%, thermal
ageing property of VR-SH would be significantly deteriorated.
Therefore, the ideal CuCl2 content in VR-SH was chosen as 0.8
mol% (i.e., 0.1 phr as shown in Table 1) to have balanced
performance. Firstly, some fundamental properties of the
materials are characterized. The percentages of disulfide and
polysulfide crosslinks in the total number of crosslinks in VR-SH
are estimated to be 62 and 31 %, respectively (details of the
determination refer to Supporting Information), while the
same parameters of the reference material, control 1, are 54
and 42 %. It means that sufficient amount of reversible sulfur
crosslinkages are present in the rubbers like the above-
mentioned commercial versions. The thermal degradation
Introduction, high temperature reshuffling of crosslinked
networks of vulcanized rubber is favorable. In this way the
crack healing process would not interfere with service stability
of rubber products at relatively lower temperature. That is,
thermal stability of the material structure is preserved while
allowing for reversibility.40 In normal driving condition, for
example, tyre temperature ranges from 30 to 100 °C.41,42
Compared with this, CuCl2 seems to be quite suitable for
vulcanized rubber as disulfide metathesis induced stress
relaxation is obvious only above 110 °C (Fig. 5a). In this
context, 110 °C is selected as the healing (or reclaiming)
temperature hereinafter.
Such a criterion based on
accelerated exchange reaction of reversible covalent bonds
was also adopted in recycling of polyimine film.43
curves in ESI,† Fig. S17 exhibit that the initial weight loss
temperatures and 50 % weight loss temperatures of VR-SH and
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0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
30oC
70oC
3.5
3.0
2.5
90oC
Virgin
12 h
9 h
6 h
3 h
100oC
110oC
120oC
2.0
1.5
1.0
0
100
200
300
400
500
600
700
0.5
0.0
Time [min]
0
50
100
150
200
250
300
350
400
(a)
Strain [%]
0.8
(a)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Control 1
Control 2
VR-SH
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Virgin
1st
2nd
3rd
0
100
200
300
400
500
600
700
Time [min]
0
50
100
150
200
250
300
350
400
(b)
Strain [%]
Fig. 5 Stress relaxation curves of (a) VR-SH measured at various temperatures, and (b)
VR-SH, control 1 and control 2 measured at 110 °C.
(b)
Fig. 6a shows typical tensile stress-strain curves of virgin
and healed VR-SH specimens. Healing can indeed proceed as
characterized by the fact that the healing efficiency increases
with increasing healing time. About 75 % tensile strength can
be restored after 12 h. Furthermore, healing of the same place
can be repeated (Fig. 6b). To check the contribution of
hydrogen bonding, surfaces of tensile-fractured specimens
were not immediately recombined but exposed to air for 12 h
prior to recombination to eliminate the possible hydrogen
bond interaction. Accordingly, the healing efficiency
approaches 72 % for healing time of 12 h (Fig. 6c). As a matter
of fact, the non-polar polybutadiene rubber matrix should not
possess any hydrogen bond. The result of Fig. 6c instead
demonstrates the oxygen insensitivity of the catalytic activity
of CuCl2. Comparatively, only ~12 % healing efficiency is
measured for control 1 and control 2 under the same
condition (Fig. 6d) as a result of intermolecular entanglements
of dangling chains across the fractured surfaces.44~46 It can thus
be concluded that disulfide metathesis works in healing of the
vulcanized rubber by triggering rearrangement of inherent
sulfur crosslinked networks under the catalysis of CuCl2.
3.5
Virgin
0 h
12 h
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
50
100
150
200
250
300
350
400
450
Strain [%]
(c)
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4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
reaction due to lack of catalyst to form complex with them, a
prerequisite of the reaction (Fig. 4).
Control 1 (virgin)
Control 1 (healed)
Control 2 (virgin)
Control 2 (healed)
(a)
(b)
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Virgin
60 mesh
50 mesh
30 mesh
0.0
0
50 100 150 200 250 300 350 400 450 500 550
Strain [%]
0
50
100
150
200
250
300
350
400
Strain [%]
(d)
Fig. 6 (a, b, c) Typical tensile stress-strain curves of virgin and healed VR-SH specimens.
(a) Effect of healing time. (b) Effect of repeated healing (healing time: 12 h). Because
the tensile strength of the virgin specimen cannot be fully restored, the reconnected
interface became the weakest part, and had to break again during the subsequent
tensile tests. This ensured that the second and third tensile failures happened to the
same healed portion. (c) Effect of waiting time (which is the time for the broken
specimens to wait for being reconnected in air at 25 °C; healing time: 12 h). (d) Typical
tensile stress-strain curves of virgin and healed control 1 and control 2 specimens
(healing time: 12 h). Healing temperature for all the healed specimens in this figure:
110 °C.
(c)
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Virgin
1st
2nd
0
50
100
150
200
250
300
350
400
During the high temperature vulcanization of the rubber,
the catalyst CuCl2 might be turned into Cu2S, CuS, Cu2O, and
CuO as a result of reaction with the surrounding elements. If
so, CuCl2 would lose its ability to catalyze disulfide metathesis,
as no metathesis product is generated in the presence of these
Strain [%]
(d)
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Control 1
Control 1 (reprocessed)
Control 2
Control 2 (reprocessed)
chemicals (ESI,† Fig. S18). The results manifest that CuCl2 must
have kept intact in the vulcanized rubber, otherwise no healing
behavior would be observed.
It is worth noting that the mechanical property of VR-SH
cannot be fully restored (Fig. 6a), and the healing efficiency
declines with number of healing (Fig. 6b). Misalignment of the
fractured surfaces should be partly responsible for the
phenomenon as it leads to pseudo contacts during healing.
The non-contact zones make up 6~10 % of total area of the
0
50 100 150 200 250 300 350 400 450 500 550
Strain [%]
Fig. 7 (a) Reclaiming of VR-SH. Sheeted rubber was cut, cryogenically ground and
fractured surface (ESI,
†
Fig. S19). It means that the measured compression molded. The attached scale bars represent 10 mm in length. (b~d) Typical
tensile stress-strain curves of virgin and reclaimed rubbers. (b) Effect of particle size of
healing efficiency should have been higher, if the non-contact
regions were intimately contacted with one another arousing
additional disulfide metathesis across the interface. Another
reason for the suboptimal healing effect lies in the distribution
of CuCl2. Although the catalyst particles are well dispersed in
VR-SH. (c) Effect of repeated reclaiming of VR-SH (particle size: 60 mesh). (d) Control 1
and control 2 (particle size: 60 mesh). To categorize the size of the powdered rubber
for reclaiming, the latter was run through sieves with different mesh sizes. The size of
the screened particles is characterized by the corresponding mesh size of the sieve. The
larger the mesh number, the smaller the particle size of the powder.
the matrix (ESI,† Fig. S2), it is hard to distribute CuCl2 on
molecular level nearby each disulfide (or polysulfide) bond (ESI,
Despite all this, VR-SH proves to acquire remendability due
to reconstruction of sulfur crosslinked networks in terms of
disulfide metathesis in damaged zones. Taking advantage of
the same mechanism, reclaiming of the rubber should also be
possible, as discussed in the Introduction. Accordingly, VR-SH
was frozen by liquid nitrogen, ground into small particles and
then hot pressed at 110 °C for 3 h (Fig. 7a). Fig. 7b shows
typical tensile stress-strain curves of the reclaimed VR-SH in
comparison with that of virgin VR-SH. Evidently, the ground
rubber powders can stick to each other under the reclaiming
conditions, reforming a compact bulk material with load
†
Fig. S20), so that no disulfide metathesis can be triggered in
the regions short of CuCl2 during healing. Although fluidic
polysulfide coupling agent Si-69 is introduced to greatly
improve mobility (i.e. dispersion) of CuCl2 through the
complexation between Si-69 and CuCl2 in the course of
compounding (ESI,† Fig. S21), which helps to raise the healing
efficiency (ESI, † Fig. S22), CuCl2 is not homogeneously
distributed in the matrix after all. There are always disulfide
and polysulfide bonds that fail to take part in the metathesis
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bearing capability as their ancestor. Careful survey of Fig. 7b stable as the version excluding CuCl2 without fear of the
indicates that tensile strength and elongation to break of the influence of disulfide metathesis.
reclaimed VR-SH increase with decreasing rubber particles
Further efforts should be made to develop next generation
size. In general, smaller rubber particles mean larger specific catalyst, which possesses similar activity like CuCl2 but much
surface area and higher surface concentration of exposed higher solubility in rubber matrix. As a result, as many as
disulfide and polysulfide bonds. Therefore, when smaller possible disulfide and polysulfide bonds would be involved in
rubber particles are closely packed, more disulfide and metathesis, leading to higher healing efficiency. Meanwhile,
polysulfide bonds are involved in metathesis. The we will try to reduce the content of organosilane and increase
reconstructed sulfur crosslinked networks become stronger. the content of carbon black, so that the material would be
Although the rubber particles size cannot be further reduced closer to actual industrial rubber.
due to limit of the grinding machine in our lab, the trend
revealed by Fig. 7b is straightforward. Additionally, Fig. 7c
Acknowledgements
shows that VR-SH can be repeatedly reclaimed owing to the
dynamic reversible characteristics of disulfide metathesis. On
the other hand, the above-mentioned uneven distribution of
CuCl2 catalyst should also partly account for the poorer
mechanical properties of the reclaimed VR-SH than the original
version.
The authors thank the support of the Natural Science
Foundation of China (Grants: 51273214 and 51333008), the
Natural Science Foundation of Guangdong (Grants:
2010B010800021 and S2013020013029), the Science and
Technology Program of Guangzhou (Grant: 2014J4100121),
and the Basic Scientific Research Foundation in Colleges and
Universities of Ministry of Education of China (Grant: 12lgjc08).
In the meantime, control 1 and control 2 were also ground
and tested under the same conditions. Because of lack of
disulfide metathesis, the reclaimed materials possess very
poor mechanical strength (Fig. 7d) like the cases in Fig. 6d
.
Unlike devulcanization, a conventional technique of rubber
recycling that cleaves sulfur crosslinks of vulcanized rubber,
here the reclaiming is based on reshuffling of the crosslinked
network through disulfide metathesis. It does not affect the
crosslinking density and can maintain strength of the rubber in
principle. For example, the percentages of disulfide and
polysulfide crosslinks in the total number of crosslinks in
reclaimed VR-SH (from the particles of 60 mesh) are 60 and 28
%, respectively, almost identical to the data of the above-
mentioned virgin material.
Notes and references
1
2
3
4
5
6
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A. C. Schüssele, F. Nübling, Y. Thomann, O. Carstensen, G.
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This study demonstrates the possibility of healing and
reclaiming of vulcanized rubber by triggering rearrangement of
the inherent sulfur crosslinked networks. The key issue lies in
the application of a novel catalyst, CuCl2, which arouses
successive disulfide metathesis among various disulfidic and
polysulfidic crosslinkkages via CuCl2-based complex catalysis.
,
,
2024-2027.
14 H. Z. Ying, Y. F. Zhang and J. J. Cheng, Nat. Commun., 2014,
5
,
Although
disulfide
metathesis
has
been
known
3218-3227.
15 Y. Chen, A. M. Kushner, G. A. Williams and Z. B. Guan, Nat.
Chem., 2012, , 467-472.
elsewhere,23,26,28,30~33 the CuCl2 aided version has not yet been
reported. CuCl2 is robust enough to survive the rubber
vulcanization, and can take effect in multi-component
material, being not disturbed by neighboring additives. Either
properties restoration of damaged rubber or rubber recycling
is allowed to proceed in air at 110 °C, which is higher than
working temperatures of many rubber products. Therefore,
under conventional conditions the sulfur crosslinks are as
4
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Green Chemistry
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DOI: 10.1039/C5GC00754B
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Vulcanized rubber made from an industrial formula can be healed and reclaimed by activating rearrangement of the inherent
sulfur crosslinked networks under the catalysis of CuCl2.
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