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Balancing wear and traction with lithium catalyzed polymers.

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Die Angewandte Makronkolekulare Cheniie 29/30 (1973) 291-305 ( N r . 367)
From the Firestone Central Research Laboratories, the Firestone Tire & Rubber Co.,
Akron. Ohio 44317 USA
Balancing Wear and Traction with Lithium
Catalyzed Polymers
By A. E. OBERSTER*,T. C. BOUTON,
and J. K. VALAITIS
(Eingegangen am 6. Marz 1972)
SUMMARY:
This paper is a report of methods of balancing wear and traction in passenger
tire treads with lithium catalyzed polymers. The results demonstrate the versatility
of the alkyllithium system for preparing a wide variety of polymers and copolymers
with differing wear and traction properties in passenger tire treads. In addition, the
use of this system for preparing polymers with improved processing and green
strength is briefly discussed.
ZUSAMMENFASSUNG :
Es wird iiber den Ausgleich von VerschleiB und Rutschverhalten bei Personenwagenreifen mit Lithium-katalysierten Polymeren berichtet. Die Ergebnisse zeigen
die Vielfaltigkeit von Alkyllithium-Systemen zur Herstellung einer breiten Palette
von Polymeren und Copolymeren mit verschiedenen VerschleiB- und Abriebeigenschaften. Ferner wird die Anwendung dieses Systems fiir die Herstellung von
Polymeren mit verbesserter Verarbeitbarkeit und Festigkeit diskutiert.
In a previous paperl, we demonstrated the versatility of the alkyllithium
system for preparing butadienelstyrene copolymers with a wide variety of
propert,ies and applications. I n addition, we demonstrated a relationship
between tread wear and traction properties in passenger tires constructed from
butadiene/styrene copolymers with varying styrene content and varying
microstructure. We showed t h a t as the styrene content or vinyl content was
increased in the polymers, a corresponding increase in T, (glass transition
temperature) resulted. Also, the compounded stocks from these polymers
exhibit,ed an increase in the YMI** with increasing vinyl or styrene content.
The wear resistance properties of tires were shown t o decrease with increasing
*
**
Paper presented a t the meeting of the GDCh-Fachgruppe “Makromolekulare Chemie” in Bad Nauheim, Germany, March 21, 1972.
Temperature where YOUNG’S
Modulus Index reaches 10000 lbs./sq.in. - ASTM
797-64.
291
A. E. OBERSTER,T. C. BOUTON,
and J. K . VALAITIS
vinyl content or with styrene content whereas the wet traction properties were
shown t o increase with increasing vinyl or styrene content. Similar results on
butadienelstyrene copolymers have been reported recently by other workers2.
The versatility of this alkyllithium system is not restricted to the butadienel
styrene copolymers but can be extended also to homopolymers as well. It is
not restricted merely to changes in microstructure but can be utilized to
prepare polymers with variations in molecular weight distribution, branching,
and functional groups. I n copolymers, variations in the quantity of blocks,
size of blocks and position of blocks are also possible. These variations in the
polymer structure can result in a number of changes in polymer properties, e.g.,
tread wear, wet traction, processing and green strength. Green strength is the
strength of the uncured compounded stock. We now wish to report further
studies on alkyllithium polymers with particular emphasis on tread wear and
traction properties. Polymers with increased green strength and improved processing will also be discussed.
The addition of polar modifiers such as ethers, amines, etc. to an alkyllithium
system for the polymerization of 1,3-butadiene in hydrocarbon solvent is
known to cause substantial changes in the microstructure of the resulting
polybutadienes3. Modified alkyllithium systems thus enable one to prepare a
large number of polymers of identical composition but differing in microstructure and T,. The microstructure is determined by the modifier type, modifiercatalyst ratio and concentration, and the polymerization temperature.
I n Table 1, we show the effect of different types of modifiers on the microstructure of the polybutadiene produced. The effectiveness of these modifiers
can be seen to vary widely. Under the conditions studied, anisole is essentially
inactive, diethyl ether and triethyl amine are of a low order of effectiveness,
Table 1. Effect of Polar Modifiers on Microstructure of Polybutadienes.
Modifier
Diethyl ether
Tetrahydrofuran (THF)
Diglyme
Anisole
Triethylamine
1,4-Diazabicyclooctane(DABCO)
N, N, N',N'-Tetramethylethylenediamine
(TMEDA)
~~~
Molar Ratio
Modifier/BuLi
96:l
5:l
0.l:l
120: 1
270: 1
26
25
23.8
10.5
3:l
33
41 *
0.6 : 1
-
47
11
~
* All polymerizations were carried out at 5OoC except this run which was made at
30 "C.
292
Lithium Catalyzed Polymers
and diethylene glycol dimethyl ether (diglyme) and N,N,N',N'-tetramethylethylenediamine (TMEDA) are of a high order of activity. Tetrahydrofuran
(THF) and 1,4-diazabicyclooctane (DABCO) are intermediate in effectiveness.
I n Table 2, the effect of modifier-alkyllithium ratio and temperature on the
microstructure of the polybutadienes is shown. As the modifier-alkyllithium
ratio increases, there is an increase in the 1,2-content of the polymer. As the
polymerization temperature increases, there is a decrease in the effect of the
modifier.
Modifier
Polymerization Temperature
Molar Ratio
Modifier/BuLi
THF
5:l
45: 1
85:l
0.1 : 1
0.45 : 1
0.8: 1
0.06: 1
0.6 : 1
1.14:l
Diglyme
TMEDA
30 "C
50 "C
70 "C
44
69
73
51
77
77
26
57
76
25
41
49
24
56
64
14
47
61
20
39
46
14
28
40
13
31
46
The observed results can be explained simply on the basis of the formation
of complexes of the butyllithium or polymerlithium with the modifier. Such
complexes involving coordination of the modifier with lithium, shown in (l),
have been proposed by other workers and have been isolated in certain
cases4-6 :
6 0 nr,
R-Li
/R
:O
(1)
t-
\R
Coordination of a modifier with an active lithium compound is accompanied
by an increase in the ionic character of the C-Li bonding. It is well known that
an increase in such ionic character in butadiene polymerization results in
increased 1,2-microstructure of the polymer. Hence, the addition of ethers,
amines, etc., t o such systems is accompanied by an increase in the ionicity of
the system and, in turn, an increased vinyl content of the resulting polymers.
The greater effectiveness of bidentate ligands such as TMEDA or diglyme
suggests that there is coordination of both ligands as shown in (2) with enhanced effect on ionicity.
293
A. E. OBERSTER,T. C. BOUTON,
and J. K. VALAITIS
The lesser effectiveness of triethylamine and diethyl ether can be ascribed
to steric factors. When the R groups are tied back as in T H F or DABCO, the
effectiveness of the modifier is increased. With anisole, which we consider
inactive as a modifier, there is a serious steric problem plus the fact that the
phenyl group may act as a “sink” for the electrons on oxygen, making them
less available for coordination with the lithium.
The effect of temperature is consistent with the idea of complex formation
between alkyllithium and modifier. One can readily visualize an equilibrium
between complexed and non-complexed alkyllithium which would be influenced by temperature as shown in (3).
6 0 69
R-Li
+--
d R-Li
:O/R -.--
\R
+
:O/R
(3)
\R
However, other factors which may affect microstructure are the dielectric
constant of the medium, the coordinating ability of the modifier, the true
molar concentration of the Li compound and modifier, the steric and inductive
effects of the modifier, and the temperature dependence of each of these
factors. The derivation of a suitable mathematical expression for evaluating
the necessary constants would be a long and arduous task. A simpler correlation for this relationship was desired since our primary objective was to
correlate only tire performance with microstructure.
It is often possible to obtain excellent correlations for such complex situations through the use of designed experiments. From the experimental design
the coefficient of a predictor or response equation can be determined. This
response equation then can be used to predict the response of the system to
any level of the factors involved. For this study a two factor, three level, nine
point design was used for each modifier. The factors considered were polymerization temperature and modifier-alkyllithium ratio. The design required nine
polymerizations (three temperatures a t three modifier levels) for each modifier.
The data from the nine experiments* were used t o determine the coefficients for a second order predictor equation via regression analysis. This was
accomplished through the use of a digital computer.
* These experiments were
made with a 20 wt. yo butadiene solution in hexane
(mixed isomers) and an effective concentration of n-BuLi of 1.0 mM phm.
(mM phm = milimoles per hundred grams monomer).
294
Lithium Catalyzed Polymers
The second order equation used was of the form
+
+
+
+
Y = Ao $- AlXl
AZXZ A1lXlz A Z Z X Z ~AlzXlXz
(4)
where Y is the vinyl content,
XI is the temperature,
X2 is the modifier/alkyllithium ratio.
This type of equation was selected since it allows curvature in the response
(X12 and X22 terms) and accounts for interaction of the factors (X1Xz term).
Once these coefficients were determined, contour plots were generated by
the computer for each modifier. Since the data are forced to fit the predictor
equation, extrapolation outside the experimental range was avoided.
The contour plots for TMEDA as the modifier are shown in Fig. 1, and
similar plots for diglyme are shown in Fig. 2. I n Fig. 2, the contour lines A
thru K represent 7 , 1,2-microstructure varying from 14 to approximately
SOYo. From these graphs, one can easily define operating conditions for making
a polybutadiene of a given vinyl content. Hence, when one desires t o prepare a
polybutadiene of approximately 47 o/o 1,2-microstructure, reference t o line F
shows the range of temperature and modifier level that may be used. Thus a t
3 0 T , one would use a modifier to n-BuLi ratio of 0.09 and a t 50°C, a ratio of
0.22. A t 70°C with this modifier, it would not be possible to obtain this desired
microstructure.
En
0'
10
I
I
30
50
70
I
90
Temp. ("C)
Fig. 1. TMEDA/n-BuLi Ratio and Temperature Effect on Polybutadiene 1,2-
Microstructure.
295
A. E. OBERSTER,T.C.BOUTON,
and J. K. VALaITIS
Temp. ( " C )
Fig. 2. Diglyme/n-BuLi .Ratio and Temperature Effect on Polybutadiene 1,2Microstructure.
The contour plots for T H F as the modifier are shown in Fig. 3 and appear
to be somewhat different from those of TMEDA and diglyme. The range of
concentrations studied was much wider than for TMEDA and diglyme. Fig. 1
and 2 represent the contour plots for the more effective modifiers while Fig. 3
represents the contour plot for a less effective modifier. The odd shape of the
contours results from the fact that a t the boundaries chosen, 45 and 85 mM
phm, the effect of temperature is slight. Hence a t 85 mM phm, between 50 and
70°C, the vinyl content remains in the F region (48y0),while a t 45 mM phm,
between 50 and 70°C, the vinyl content remains between D and E (38.542.6%). A study of the more effective modifiers over a wide range of concentrations would be expected to produce similar contours.
For our study of the wear and traction properties of tires having treads made
from polybutadienes of varying 1,2-microstructure as described above, we
chose to use the polymers listed in Table 3. Polymer A contains about 11%
vinyl structure and was prepared using n-BuLi as initiator and with no modifier present. Polymers B, C and D contain 33, 47 and 55% 1,2-microstructure,
respectively, and were prepared using a n-BuLi initiator modified with diglyme.
Polymer E is a polybutadiene containing 45% 1,2-microstructure and was
prepared with a modified sodium catalyst. The latter catalyst system provides
296
Lithium Catalyzed Polymers
polymer with a much broader molecular weight distribution (MWD) than the
alkyllithium modified catalyst system affords and was included to learn
whether there might be an effect of molecular weight distribution.
Temp. ("C)
Fig. 3.
Table 3.
THF/n-BuLi Ratio and Temperature Effect on Polybutadiene 1,2-Microstructure.
Polymer Properties.
Polymer
Initiator System
Coupling System
ML/4/10O0C
T o "C
DSV, dl./g
Gel
yo 1.2 by Infrared
MWD
/
A
n-Buli
-
45
-94
2.4
0
11
Narrow
\
B
I
C
I
D
1
E
n-BuLi/ n-BuLi/ n-BuLi/ n - B a a /
diglyme diglyme diglyme KOtBu
Sic14
Sic14
100
75
83
136
-7 7
-68
-62
-65
3.02
2.92
2.42
3.78
0
0
0
0
33.1
46.9
55.2
44.5
Broad
Broad
Narrow
Very
broad
The alkyllithium system generally leads to polymers having a linear structure with narrow molecular weight distribution. This narrow molecular weight
297
-4.
E. OBERSTER,T. C. BOUTON,
and J. K. VALAITIS
distribution and linear structure results in poor processing qualities in tire
rubber applications. As a result, these polymers are generally used in blends
with other polymers such as emulsion SBR in order to attain satisfactory
processing. There are modifications which can be utilized in the alkyllithium
system which can result in modifications in the structure to give a processible
tire rubber.
Methods of modifying the system include coupling reactions11 with reagents
such as Sic149 and RCHC12 which increase the molecular weight or give
branched structures leading to improved processing. The two part catalyst
systemlo consisting of a FRIEDEL-CRAFTS
catalyst plus a cocatalyst such as
Tic14 can also be used to crosslink the polymer resulting in a high molecular
weight branched structure having improved processing characteristics.
The polar modified butyllithium system also leads to linear, narrow molecular weight polymers with poor processing characteristics. I n order to obtain
polymers with high molecular weight for improved processing, polymers B
and C were prepared a t a high initiator level and then coupled with SiC14. Of
course, it is also possible to get high molecular weight polymer without
coupling if careful purification procedures of the polymerization system are
used (polymer D).
These polymers were compounded in a passenger tire tread stock recipe;
the laboratory evaluation is shown in Table 4. The stocks were compounded
with an HS HAF black (70 phr) and oil (48 phr). The processing characteristics
of all these stocks were satisfactory from the standpoint that we were able to
build tires on small scale but would not be sufficient for plant production of
Table 4. Tread Stock Recipe, Laboratory Evaluation.
Po1y mer
yo 1.2
I
A
l
B
11
33
65
162
560
57
84
164
490
61
142
-72
139
-55
I
C
I
47
n
l
E
55
45
58
160
600
57
86
179
460
59
136
-45
122
-55
109.7
103.3
100
96.7
Stress-Strain Properties
Cure 23 Min. at 300°F
300% Modulus (kglcrnz)
Tensile Strength (kg/cmZ)
Ultimate Elongation (yo)
Shore A Hardness
Processing
Running Temperature ("C)
YOUNG'S
Modulus Index ("C)
58
157
590
58
satisfactory
143
-49
STANLEY -LONDON'
Wet Skid Resistance
0.4 Coeff. Friction
0.6 Coeff. Friction
298
85.7
86.8
100
95
104.7
100
Lithium Catalyzed Polymers
tires. The YMI values shown indicate that with increasing vinyl content, the
YMI increases. As the YMI and vinyl contents increase, there is a corresponding
increase in wet traction as indicated by the results of the sTANLEY-LONDON7
wet skid resistance test (British Portable Skid Tester).
Passenger tires were constructed from stocks containing these polymers and
tested for traction as well as for wear resistance. All traction tests in this report
are wet braking traction using a locked front brake test on a wet asphalt surface with a coefficient of friction of 0.4. The wear tests were carried out using
4-way tread sections in bias-belted tires, the cars were run on a highway
Wear
Index *
yo 1.2
Polymer
Braking**
Traction Index
Diene/SBR 40/60
100
100
Emulsion SBR
90
107
A
11
130
85
B
33
100
96
47
91
100
C
55
I1
103
D
E
45
88
102
Wear Index - The Diene/SBR 40/60 Control is set at 100 and index values are
based on this.
** Braking Traction Index - The Diene/SBR 40/60 Control is set at 100 and index
values are based on this.
*
85 120
Emulsion SBR
Diene/SBR 40/60
o A 11 %1.2
A
.U
c
-3
0
0
c
.-
*
110-
B 33% 1.2
47 %1.2
rn D 55% 1.2
P
.x
m'
A C
100-
90 -
I
Fig. 4.
,
!
I
I
Wear Traction Relationship for Tire Treads Containing Polybutadienes
with Varying 1,2-Microstructure.
299
A. E. OBERSTER,T. C. BOUTON,
and J. K . VALAITIS
course in Texas on a rough aggregate asphalt surface with many sharp turns.
The cars were run a t 65-75 miles per hour, the tires were rotated every 1000 miles and tested t o 12000 miles. The average temperature a t the course is 26°C.
G
2
>-
Polybutadienes with
0
-70 -
\
-65 -60 -
11 'lo1.2
B 33% 1.2
a C 47% 1.2
D 55 Yo 1.2
oA
D
-55 -50
-
-45 -
-40-
gb
-3540 85
45
160 165
1;O
1;5
Braking traction index
Fig. 5. Relationship of YMI and Braking Traction for Stocks Containing Polybutadienes with Varying 1,2-Microstructure.
-45
Polybutadienes with
o A 11 'lo1.2
o B 33 '10 1.2
c 47 % 1.2
1
-40
-351
70
2
I
80
90
I
100
t
110
120
130
Wear index
Fig. 6. Relationship of YMI and Wear for Stocks Containing Polybutadienes
with Varying 1,2-Microstructue.
300
Lithium Catalyzed Polymers
The results are shown in Table 5. The tests show that braking traction is
improved with an increase in vinyl content. As the vinyl content increases,
there is a corresponding decrease in the wear.
I n Fig. 4, we see a comparison of the wear index with braking traction
index on tires prepared from stocks containing this series of polymers. We see
a good correlation and an inverse relationship of tire wear and traction properties.
I n Fig. 5, we have plotted the YMI versus braking traction index for these
stocks and note that the traction increases as the YMI increases. I n Fig. 6,
Table 6. Tread Stock Recipe, Laboratory Evaluation.
I
Polymer
Emuls.
SBR
yo Styrene
23.5
Stress-Strain Properties
Cure 23 Min. at 300°F
800% Modulus (kglcmz)
70
Tensile Strength (kglcmz)
193
Ultimate Elongation (yo)
600
Shore A Hardness
54
Processing
Running Temperature ("C)
143
Y ~ ~ ~ ~ ' ~ M o d u l u s("C)
I n d e x -40
sTANLEY-LONDON7
Wet Skid Resistance
0.4 Coeff. Friction
110.2
1
Polar
Solution Bd/Sty
Modif.
18
23
28
32
18
74
197
590
56
70
193
650
48
I1
197
620
59
79
202
630
57
60
176
660
54
138
-54
146
-44
all good
140
-37
138
-38
138
-43
100.0
102.4
105.1
112.2
107.7
X
Emulsion SBR
Sol'n SBR 18'lo Sty.
n Sol'n SBR 23 'lo Sty.
Sol'n SBR 28% Sty.
A Sol'n SBR 32/'
Sty.
w '
1
8 Sty/0d-39'lo 1.2
o
-a0
l
.._
s 120e
c
u
m
c
I
c
m
.-
%
m'
\;O1
'l00
90
A
I
I
I
I
I
30 1
A. E. OBERSTER,
T. C. BOUTON,
and J. K. VALAITIS
we have plotted YMI versus wear resistance and note that as YMI increases
there is a corresponding decrease in wear.
I n Table 6, we show the tread stock laboratory evaluation of the series of
butadienelstyrene copolymers reported in our earlier publicationl. We can
note that the processing rating for these stocks was good. I n Fig. I, the wear
traction relationship for this series of polymers is shown and we note this same
inverse relationship of wear and traction.
I n Fig. 8, we show the wear traction relationship for the various polybutadiene polymers and the butadienelstyrene copolymers. As can be noted,
all the polymers fall in a region approaching this straight line relationship.
z
U
c
.-
120
-
0
c
._
% 110
Lc
m
c
._
x
E
0 DieneISBR 40160
a Emulsion SBR
0 A 11 '
Io
1.2
B 33 '10 1.2
A c 47 % 1.2
D 55 % 1.2
o E 45 % 1.2
v 18 o/o Sty/ Bd Copoly,
o 23.9 'lo
V 28.4%
4 32.6%
o 18 'loSty/ Bd 39% 1.2
'
l
o
-
om
1oc
9c
'5
85
,
JOJ
95
105
115
125
135
8C
Wear index
Fig. 8.
Wear Traction Relationship for Tire Treads Containing Various Polybutadiene and Styrene/Butadiene Polymers.
The implications of these results are that wear and traction properties are
related t o the T, of the polymer and t o the YMI of the compounded stocks.
With increasing T, of the polymers, whether by increasing vinyl content or by
introducing styrene in a copolymer, there is an accompanying increase in wet
traction and a corresponding increase in rate of wear. We are thus faced with
a dilemma and must choose a compromise which can provide acceptable
traction and wear resistance.
Lithium Catalyzed Polymers
Based on our technology of the alkyllithium polymerization system, we feel
that the best compromise for acceptable traction and wear is provided with
the butadienelstyrene copolymers. With these copolymer systems, we have
achieved an acceptable wear-traction relationship and, in addition, we have
been able to achieve excellent processing tread rubbers. The polybutadienes
of varying microstructure can satisfy the wear-traction relationship needed,
but further research and technology will be necessary to achieve rubbers with
comparable processing characteristics.
I n addition to their use in tread rubbers, s-ynthetic polymers based on the
alkyllithium solution polymerization system are finding wider use in body
stock applications. Their main disadvantage thus far has been their inadequacy in building tack and green strength. Blends with natural rubber and
tackifying cements have provided the necessary tack and green strength for
limited use of these synthetic polymers in body stock applications. However,
it would be advantageous to be able to use an all synthetic body stock rubber.
c;,
Y
1O.(
56
Fig. 9.
Instron Green Stress-Strain Test, pulled at 1.27 m/min.
303
A. E. OBERSTER,
T. C. BOUTON,
and J . K. VALAITIS
Using the alkyllithium catalyst system, we have been able to produce
butadienelstyrene copolymers with excellent green strength properties which
are suitable for use in body stock applications. I n Fig. 9, we show a green
stress-strain curve for one of these polymers. This solution polymer is a 60140
butadienelstyrene copolymer in which part of the styrene ( 6 8 % ) is present in
the form of a block. This polymer has broad molecular weight distribution
and shows excellent green strength in a 1 0 0 ~body
o
stock compound. As noted
in Fig. 9, the stock gives excellent green strength (11.2 kg versus 1.7 kg for a
natural rubberlsynthetic rubber stock a t peak). It is also important to note
that the peak pounds for the synthetic stock occurs a t 630% elongation,
whereas, the peak for the natural rubberlsynthetic stock occurs a t 0% elongation. We have constructed and tested tires using this polymer in the body
stock application. The tires performed well surpassing DOT* requirements
and are equivalent to production tires in all respects.
This application further demonstrates the wide utility for the alkyllithium
catalyst system. We do not feel that this green strength can be achieved with
polybutadiene but is readily achieved in the butadienelstyrene copolymer
systems.
I n summary, we have shown again the correlation between wear traction
properties and have shown the versatility of the alkyllithium initiator system
for preparing a variety of polymers. We have shown the usefulness of this
system not only in preparing suitable tread rubbers with good processing
qualities but have also shown its usefulness in preparing polymers with ex.
cellent green strength in body stock applications.
The authors wish to thank The Firestone Tire & Rubber Co. for permission
t o publish this work.
1
2
3
4
5
6
7
8
9
*
A. E. OBERSTERand R. L. BEBB,Angew. Makromol. Chem. 16/17 (1971) 297
R. N. KIENLE,E. S. DIZOV,T. J. BRETT,C. F. ECKERT,
Rubber Chem. Technol.
44 (1971) 996
T. A. ANTKOWIAK,
A. E. OBERSTER,
A. F. HALASA,
and D. P. TATE,J. Polym.
Sci. A 1 (1972) 1319, and references cited therein
F. J. WELCH,
J. Amer. Chem. SOC.82 (1960) 6002
A. A. KOROTKOV,
S. P. MITSENGENDLER,
and K. M. ALEYEV,
Pol. Sci. U.S.S.R.3
(1962) 487
C. SCRETTAS
and J. EASTHAM,
J. Amer. Chem. SOC.87 (1965) 3276
A. C. BASSI,Rubber Chem. Technol. 38 (1965) 112
W. E. CLAXTON,
Rubber World 159 (3) (1968) 43
R. P. ZELINSKI and C. F. WOFFORD,
J. Polymer Sci. A 1 (1965) 93; Rubber
Chem. Technol. 38 (1965) 871
U. S. Government Department of Transportation Endurance Test.
304
Lithium Catalyzed Polymers
10
E. F. ENOEL,J. SCHAFER
and K. M. KIEPERT,Rubber Age (New York) 96 (3)
11
U. S. Patent 3 639 367 (1972), Firestone Tire & Rubber Co., Inv. : A. F. HALASA
(1964) 410
305
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