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Effect of the filler characteristics on the miscibility of styrene-butadiene rubber and nitrile-butadiene rubber blends.

INTRODUCTION

Polymeric blends are a new kind of materials prepared by mixing two or more components with different characteristics in order to increase the resistance to chemicals and the mechanical and thermal properties by synergetic effects. Because polymers with different polarity such as styrene-butadiene rubber (SBR) and nitrile-butadiene rubber (NBR) are incompatible, fillers with specific physical and chemical properties which reduce the interfacial tension and the domain size can be used as compatibilizers in order to obtained miscible blends (1). The ability of a filler to act as a compatibilizer is due to the strong adsorption of polymer on the filler's surface (2), (3), decreasing its mobility and thus preventing phase separation. Lipatov provides a thorough discussion of the therc considerations related to the increase in the system stability by introducing active fillers (4).

NBR is a polar elastomer, which has high resistance to non-polar substances such as hydrocarbons and oils; it also has a high modulus and abrasion resistance, but it is expensive. On the other hand, SBR is less expensive, has higher resilience and it is resistant to aqueous solutions, thus being more appropriate for many applications.

Fillers with small particles and with high aspect ratio such as carbon nanotubes, silica nanoparticles, and layer silicates show a better dispersion and significant increase in mechanical properties, even at low loadings (5). The chemical surface of the filler determines the type of interaction between polymer and filler. It is possible to promote van der Waals, polar (hydrogen bonding, electron donor-acceptor, and acid-base) or covalent interactions by grafting functional groups onto the filler, which are able to interact with the polymers and also to reduce the agglomeration caused by filler-filler interactions (6).

Precipitated silicas have been extensively used as fillers; however, they present very poor performance compared to carbon black, due to the fact that they do not interact with elastomers of low polarity such as butadiene rubber and styrene-co-butadiene rubber because they are relatively polar and have a low surface area. The use of siliceous materials such as mesoporous silicas offers a prominent alternative, since, besides their high surface area (higher than 600 [m.sup.2]/g), they have lower polarity because of their lower silanos content and organized mesoporous structure where the polymer chains can be trapped (7).

In this work, we study the miscibility of SBR + NBR blends reinforced with mesoporous silica. The polymer-filler interactions are determined by the amount of rubber bound, which is a measure of the insoluble polymer in a good solvent (8), by changes in [T.sub.g] and by mechanical measurements. The composition of the bound rubber is analyzed to determine which polymer presents stronger interactions with the filler. The results are compared with those obtained by using commercial precipitated silica.

EXPERIMENTAL SECTION

Reagents

Sodium silicate solution (technical grade, weight % Si[O.sub.2]: 27, [Na.sub.2]O: 11.9), Sodium hydroxide (analytical grade), zinc oxide, and stearic acid (technical grade) were supplied by Aldrich. Hydrochloric acid (analytical grade) was from Merck. Cetyltrimethylammonium bromide (CTMABr), from Across was used as a cationic surfactant. Precipitated silica Rubbersil RS-200 was from Degussa. SBR 1502 with 23.55 wt% styrene ([M.sub.w] = 3.26 X [10.sup.5] Da, PD = 3.4, Polysar), NBR with 33 wt% Acrylonitrile ([M.sub.w] = 2.45 X [10.sup.5] Da, PD = 3.5, Krynac 3345, Bayer), and vulcanization additives, 2,2,4-trimethyl-1, 2-hydroquinoline (agerite resin D), zinc diethyl dithiocarbamate (ZDEC), and N-cyclohexylbenzothiazole-2-sulphenamide commercial grade were supplied by Monsanto.

Mesoporous Silica Preparation

The mesoporous silica MCM-41 was synthesized as reported elsewhere (9). The reagent molar ratio was 1Si[O.sub.2], 4.32CTABr, 5.95Na[O.sub.2], 6.1HCl, 343.2[H.sub.2]O. In a typical synthesis 9.6 g of CTAMBr were dissolved in 141.6 mL of NaOH 0.60 M, followed by the slow addition of 24 g of sodium silicate. A clear solution was obtained and kept under stirring for 30 min, then 96 mL of HCI 2M were added quickly. The reaction gel was kept for 24 h under stirring at room temperature, and then the silica was recovered by filtration, washed, and dried at 60[degrees]C. Finally, it was calcined at 540[degrees]C. for 3 h to eliminate the surfactant and to generate the porosity.

Blend Preparation

The SBR + NBR blends were mixed together on a two-roll open mill at room temperature and then the filler and the vulcanization additives were incorporated in the following order: the filler was added first, second the zinc oxide (ZnO) as sulfur activator, stearic acid as surfactant to avoid chemisorption of the ZnO, and the 2,2,4-tri-methyl- 1,2-hydroquinoline (agerite resin D) as antioxidant and finally 1.5 phr of sulfur and the zinc diethyl dithiocarbamate (ZDEC) and N-cyclohexylbenzothiazole-2-sulphenamide (CBS) as vulcanization agents.

The samples are named as follows: "S #_# M," where the first and second number stands for SBR and silica content, respectively, and M means blend with mesoporous silica.

Unvulcanized blends, without any additive, were annealed at 150[degrees]C under pressure (3000 psi) during 15 min and thin films were obtained for the analyses.

Vulcanization. Table 1 shows the composition of the vulcanized samples. The samples were vulcanized at 150[degrees]C and 3000 psi. The vulcanization times were measured by DSC.
TABLE 1. Blend composition in phr.

 S0 S25 S50 S75 S100

SBR 0 25 50 75 100
NBR 100 75 50 75 0
Silica 60 60 60 60 60
Zinc oxide 5 5 5 5 5
Stearic acid 1 1 1 1 1
Agerite Resin D (a) 2 2 2 2 2
CBS (b) 2 2 2 2 2
ZDEC (c) 0.8 0.8 0.8 0.8 0.8

(a) 2,2,4-trimethyl-1,2-hydroquinoline.
(b) N-cyclohexylbenzothiazole-2-sulphenamide.
(c) Zinc diethyl dithiocarbamate.


Material Characterization

The pore size distribution, surface area, and pore volume of the fillers were determined by nitrogen adsorption isotherms by using an ASAP 2010 Micromeritis equipment.

Scanning Electron Microscopy (SEM). The silica powder and the polymer films were directly analyzed in a JEOL JSM 5800 using the back scattered electron mode.

X-ray Diffractometry. X-ray diffractometry results were obtained with a Rigaku Miniflex X-ray spectrometer.

Thermogravimetric Analyses (TGA). All the analyses were done under air atmosphere, heating from room temperature to 800[degrees]C at 10[degrees]C/min in a Q500 TA Instruments thermogravimetric analyzer.

Bound Rubber Determination. Approximately 200 mg of small pieces of blend film was introduced into a stainless steel mesh and immersed in 30 mL of toluene during 5 days. Thereafter the material was washed thoroughly with acetone to remove toluene residuals and dried at 60[degrees]C by 24 h.

The bound rubber content in wt% was calculated as follows:

%BR = %[R.sub.nonsoluble-fraction]/%[Silica.sub.nonsoluble-fraction]/%[R.sub.in-blend]/%[Silica.sub.in-blend] (1)

where %[R.sub.nonsoluble-fraction] and %[Silica.sub.nonsoluble-fraction] are the rubber and residual silica wt%, respectively, in the extracted samples obtained by TGA. During milling, some rubber and silica can be lost, therefore the actual rubber and silica wt%, %[R.sub.in-blend] and %[Silica.sub.in-blend], in the initial blend, are also determined by TGA.

SBR and NBR composition of the bound rubber: Standards with different SBR + NBR ratios were prepared and the infrared spectra were collected in a Perkin-Elmer Spectrometer, model Spectrum One. The NBR percentage in the extracted fraction was calculated from a calibration curve prepared using the absorbance at 2236 [cm.sup.-1] corresponding to the stretching vibration of C [equivalent to] N of the nitrile rubber. The NBR wt% in BR is calculated as shown in Eq. 2, where A is de absorbance at 2236 [cm.sup.-1].

%NBR = %NB[R.sub.in-blend] - [(100 - %BR) x (A/0.607)]/%BR (2)

The SBR wt% was calculated by subtracting the NBR wt% from the total bound rubber percentage.

Gel Permeation Chromatography Analysis. A calibration plot using polystyrene standards was made with a HPLC Waters chromatograph equipped with a refraction index detector and a Styragel column HT6E. The molecular weight of the initial SBR and NBR samples and the extracted and nonextracted rubber from the blends (SO_60M and S100_60M) dissolved in THF was determined.

Modulated Temperature Differential Scanning Calorimetry (MTDSC). The samples were analyzed in a Thermal Analyzer Q100 system from TA instruments. Approximately 12 mg of pulverized filled blend without vulcanization additives were annealed at 150[degrees]C for 15 min in a compression molding machine at 20.7 MPa. First the thermal history was erased by heating at 30[degrees]C/min up to 150[degrees]C, and then the samples were cooled to--80[degree]C at 30[degrees]C/min. The thermograms were collected at 3[degrees]C/min and the temperature was modulated at [+ or -] 1.00[degrees]C every 60 s.

Tensile Test. V-type test probes of vulcanized blends were analyzed in a tensile machine MTS TEST/5, according to the ASTM D638 procedure, strain speed 50 mm/min, at room temperature 25[degrees]C.

RESULTS AND DISCUSSION

Characterization of Silica

According to Fig. 1, the mesoporous MCM-41 silica synthesized is composed of small ~ 100 nm size spheroid particles and few agglomerates. The precipitated silica presents loose nonregular shape agglomerates.

[FIGURE 1 OMITTED]

The nitrogen adsorption isotherms are shown in Fig. 2. The mesoporous MCM-41 silica presents a type IV isotherm, which is characteristic for mesoporous materials (9). Precipitated silica presents a type II isotherm, which is typical of nonporous materials.

[FIGURE 2 OMITTED]

The mesoporous silica exhibits a BET surface area, average pore diameter, and pore volume of 843.5 [m.sup.2]/g, 2.7 nm, 0.54 [cm.sup.3]/g, respectively. The precipitated silica has a BET surface area of 170.5 [m.sup.2]/g and smaller pore volume.

Figure 3 shows the X-ray diffractogram for the mesoporous silica, which exhibits an organized hexagonal pore array corresponding to MCM-41 type material in agreement with that reported by Beck et al. (10).

[FIGURE 3 OMITTED]

Characterization of Blends

SEM Analysis. The silica shows better dispersion in NBR than in SBR. For all SBR + NBR blends, the silica dispersion is better when the NBR content increases. The silica does not disperse in matrices of low polarity like SBR, but the more polar nitrile groups of NBR are able to interact with the silanol groups of silica, probably through hydrogen bonds and acid-base interactions, whereby NBR wraps around the silica thus diminishing its surface tension and improving its dispersion in SBR. Figure 4 shows the SEM images for a blend constituting only of SBR and a blend containing NBR 25 phr, both filled with 60 phr of mesoporous silica, indicating the dispersion effect caused by the presence of NBR rubber.

[FIGURE 4 OMITTED]

Composition of Bound Rubber. Polymer chains, interacting directly with the filler, are insoluble in a good solvent. It is important to know the composition of the bound rubber since it allows determining which polymer presents stronger interactions with the filler. Table 2 shows the results obtained for composition as explained in the experimental section. Figure 5 shows as illustration the intensity of the infrared bands due to the nitrile absorption in the extracted rubber for the samples S75_60M and S50_60M.

[FIGURE 5 OMITTED]
TABLE 2. Bound rubber content in the blends.

Sample %BR (a) %NBR (b)

S0_65 24
S0_60M 27
S25_60 20 82
S25_60M 22 86
S50_60 18 66
S50_60M 20 69
S75_60 20 32
S75_60M 22 35
S100_60 19
S100_60M 24

(a) Bound rubber content.
(b) NBR content in bound rubber.


It can be concluded for all blends, that the relative wt% of NBR with respect to wt% of SBR is higher in the bound rubber than in the initial nonextracted blend. The is due to the stronger interaction between the--C [equivalent to] N of NBR and silanol groups, which favors the wrapping of filler particles by NBR chains. According to Lipatov et al. (4), in our case, there is a competition between the two polymers for the filler surface, but rather the NBR-silica shows better interaction and it is the most stable system from the thermodynamic point of view, and thus more negative free energy change is expected for it.

Bound rubber is higher for blends prepared with mesoporous silicas than for those prepared with mesoporous silicas than for those prepared with precipitated silica, indicating better rubber-silica interactions due to the highest surface area as well as penetration of the polymer chains into the mesopores.

Molecular Weight Determination. Table 3 shows the molecular weight for the initial SBR, initial NBR and for the extracted and nonextracted rubber (bound rubber) present in SBR + NBR + mesoporous silica blends.
TABLE 3. Molecular weight for the polymers and its different fractions.

Sample [M.sub.w] [M.sub.w]/[M.sub.n]
 (X [10.sup.5] Da)

Pristine NBR 2.47 3.5
S0_60M extracted rubber 2.37 3.0
S 0_60M bound rubber 2.32 3.1
Pristine SBR 3.26 3.4
S 100_60M extracted rubber 3.39 2.9
S 100_60M bound rubber 1.29 3.4


For the NBR-mesoporous silica blends, there is no significant difference in the molecular weight of the extracted rubber, the bound rubber, and the initial NBR. This result leads us to conclude that every NBR chain interacts in the same way with silica, independently of its size. This means that the forces of enthalpy are able to overcome the entropy barrier imposed by the conformational changes generated by the interaction between polymeric chains silica surface.

For an SBR-mesoporous silica blend (S100_60M), the extracted rubber is composed of the largest chains and the bound rubber of the shortest ones. Short chains can easily change their conformation, but in the case of large polymer chains, the enthalpy forces are not sufficient to overcome the energy barrier required to rearrange them when they are anchored to the solid.

Modulated Temperature Differential Scanning Calorimetry

Blend Characterization. The glass transition temperature ([T.sub.g]) of the blends depends on the filler-polymer interaction and the cohesive forces between the polymer chains. High intermolecular polymer-polymer interaction provides a high immobility of the bulk polymer generating, as a consequence, a decrease in the mobility of the unbound chains and therefore a large change in the glass transition temperature. This is not the case for elastomers.

The results obtained for unfilled and filled rubber blends are shown in Table 4. A reinforced polymer blend has a thermal behavior different from an unfilled blend. A slight increase in [T.sub.g], for both SBR and NBR can be observed, in almost all blends, corresponding to these unbound polymer phases, which indicate weak polymer-polymer cohesive interactions such as van der Waals forces (11). On the other hand, the bound polymer chains do not present [T.sub.g] signals.
TABLE 4. Glass transition temperatures determined by DSC.

Sample [T.sub.g] SBR ([degrees]C) [T.sub.g] NBR ([degrees]C)

S0 - 34
S0_60 - 33
S0_60M - 33
S25_0 - 55 - 33
S25_60 - 55 - 33
S25-60M - 55 - 33
S50 - 55 - 33
S50_60 - 54 - 32
S50_60M - 54 - 33
S75_0 - 55 - 33
S75_60 - 54 - 32
S75-60M - 54
S100_0 - 56
S100_60 - 54
S100_60 - 54


Chains bonded in a supramolecular structure do not present glass transition at the same temperature as free chains (4). This is the case for NBR in the blend S75_60M, for which its [T.sub.g] disappears because of its high interaction with the filler, which is corroborated by its high content in bound rubber and its significant participation in the interphase. The blend S25_60M, although it showed higher bound rubber content, presents glass transition for SBR since it is not completely involved in the interphase (9.7 wt% of SBR is free according with Table 5).
TABLE 5. Interphase composition for the filled blends.

 Polymer at interphase Polymer at filler interphase

Sample F % Total % SBR % NBR % BR %SBR % NBR

S25_0 1.1 0 0 0 0 0 0
S25_60 0.8 16.3 0.8 15.3 20.2 3.6 16.6
S25-60M 0.6 37.1 7.3 30.3 22.3 3.1 19.2
S50 1.1 0 0 0 0 0 0
S50_60 0.7 32.4 17.9 14.7 18.1 5.6 12.5
S50_60M 0.6 41.6 14.6 26.5 20.3 6.9 13.3
S75_0 1.0 0 0 0 0 0. 0
S75_60 0.8 23.2 22.9 1.2 20.3 13.7 6.6
S75-60M 0.5 49.5 22.0 25.0 22.2 14.3 7.9

 Interphase SBR + NBR

Sample % SBR % NBR

S25_0 0 0
S25_60 -2.8 -1.3
S25-60M 4.2 11.1
S50 0 0
S50_60 12.3 2.2
S50_60M 7.7 13.2
S75_0 0 0
S75_60 9.2 -5.4
S75-60M 7.7 17.1


Figure 6 shows the reversible thermal behavior of bound rubber for blend S75_60M, where no transitions are observed in the temperature range where free polymer presents them. However, after silica removal, the polymers recover their mobility and glass and melting transitions are observed at the same temperature as nonfilled polymer.

[FIGURE 6 OMITTED]

Miscibility of Rubber Blends. Hourston offers a complete discussion on the use of MTDSC for the characterization of immiscible polymer blends. The change of [C.sub.p] at each [T.sub.g] may be related to the amount of polymer able to suffer that transition (12), (13). Figure 7B shows the [C.sub.p] derivative with respect to temperature, from which [DELTA][C.sub.p] for each polymer is calculated by integration of its corresponding peak. For an immiscible blend, the total change in heat capacity can be obtained as the algebraic sum of the heat capacity changes for each polymer at its respective [T.sub.g] as follows (14):

[FIGURE 7 OMITTED]

[DELTA][C.sub.P] = [w.sub.1][DELTA][C.sub.P1] + [w.sub.2][DELTA][C.sub.P2] (3)

where w is the weight fraction and subscripts 1 and 2 refer to each component.

When the system is partly miscible, we have:

[DELTA][C.sub.P] = [w.sub.1][DELTA][C.sub.P1] + [w.sub.2][DELTA][C.sub.P2] + [DELTA][C.sub.Pint]. (4)

The content of each component at the interphase w is calculated as:

[w.sub.i-interphase] = [w.sub.i] - [DELTA][C.sub.Piblend]/[DELTA][C.sub.Pi], (5)

where [DELTA][C.sub.Piblend] and [DELTA][C.sub.pi] refer to the component i in the blend and before mixing, respectively.

The weight fraction F of nonengaged polymer at any interphase, which can be calculated as shown below, is associated with the miscibility. For instance, when F is 1, no interface exists.

F = [([DELTA][C.sub.P1] + [DELTA][C.sub.p2])/([w.sub.1][DELTA][C.sub.P1blend] + [w.sub.2][DELTA][C.sub.P2blend])]. (6)

In Table 5, it is observed that for all unfilled rubber blends, the F values are ~ 1, which indicates that no interphases are present. For the filled blends, F is smaller than 1, therefore some polymer fraction is present at the interphase. To make the calculation of F, the [C.sub.p] is normalized with respect to the polymer weight in the blends.

In the filled rubber blends two interphases are identified: one of polymer-filler (bound rubber) and the other of SBR + NBR. In these interphases, the total polymer is calculated as (1 - F) X 100 and the amount of each rubber by using Eq. 4. The content of each polymer in the bound rubber is calculated by using the data in Table 1 and the following equation:

%[NBR.sub.at-filler] = %[[[NBR.sub.BR] x %BR]/100] and %[SBR.sub.at-filler] = [[(100 - %[NBR.sub.BR]) x %BR]/100], (7)

%[NBR.sub.at filler] and %[SBR.sub.at filler] are the wt% of each rubber at the interphase with the filler, %BR and %[NBR.sub.BR] are the percent of bound rubber and the percent of NBR in the bound rubber, respectively.

Finally, the composition of the interphase SBR + NBR is calculated considering the difference between the amounts of each rubber in the total polymer in the interphases and in the bound rubber.

The blends reinforced with mesoporous silica show the highest rubber content at the SBR + NBR interphase, indicating that the high surface area of the filler plays an important role in improving the system compatibility. According to this result, it can be deduced that the interphase SBR + NBR is preferentially located around the bound rubber that covers the filler particles, and therefore filler with high surface area promotes interphase stability by decreasing the mobility of the rubber chains preventing their phase separation.

Figure 7 shows the reversible heat flow signal as a function of temperature for the blends containing 50 phr of each polymer. For the unfilled blend and for the blend filled with silica RS-200 an endothermic transition was observed. This corresponds to the melting of the crystalline fraction of the butadiene component in both elastomers. The nonreversible heat flow (not shown) also exhibits an exothermic peak around 0[degrees]C, which is due to the butadiene crystallization; but for the blend prepared with mesoporous silica, this phenomenon is not detected, since the polymer chains can penetrate into the silica pores, avoiding their crystallization, as discussed elsewhere (15).

The NBR has a higher tendency to be incorporated in the bound rubber due to the strong interaction between its polar segments having--C[equivalent to] groups and the silica. The presence of SBR in the SBR + NBR interphase is due to the interaction of its chains with the nonpolar tails of the NBR chains. Figure 8 illustrates the possible interphases found in SBR + NBR blends reinforced with mesoporous silica.

[FIGURE 8 OMITTED]

The MTDSC results shown in Table 5 are in concordance with the values obtained by infrared for the bound rubber. The negative polymer amounts at the SBR + NBR interphase for the S25_60 and S75_60 blends, indicate that this method is less sensitive to low amounts of interphase as is the case for these blends prepared with precipitated silica.

Mechanical Properties. According to Table 6 the filled SBR + NBR blends have higher strength at break than each filled elastomer independently, indicating that the compatibility between the polymers is improved with the filler content, producing a synergy of this property. Also it is seen in this table that those blends prepared with mesoporous silica exhibit the highest values.
TABLE 6. Mechanical properties of the blends.

 Strength at break (MPa) %Strain at break

Sample Value [SIGMA] Value [SIGMA]

S0_60 1.29 0.08 535 30
So_60M 1.56 0.07 585 9
S25_60 1.41 0.04 595 14
S25_60M 1.66 0.02 557 31
S50_60 1.35 0.09 567 40
S50-60M 1.52 0.09 462 28
S75_60 1.34 0.11 255 29
S75_60M 1.64 0.10 244 16
S100_60 1.22 0.11 472 18
S100_60M 1.47 0.08 597 12


CONCLUSIONS

The NBR presents better interaction with silica than does SBR, due to the high chemical affinity between the nitrile and silanol groups. The silica is better dispersed in the blends containing NBR than in SBR alone. The interaction of SBR with silica fillers is given by the penetration effect of short chains inside of their pores.

A greater improvement in the miscibility of SBR + NBR blends is attained by reinforcement with mesoporous silica than with precipitated silica. This is due to the larger surface area and organized porous structure of the former, which allows a better filler + polymer interaction and a higher strength at break for the silica + rubber blends.

ACKNOWLEDGMENTS

We are grateful to the Colciencias (Colombia) to the program "Apoyo a la comunidad cientifica Nacional a traves de las becas para estudios doctorales".

REFERENCES

(1.) N.A. Darwish, A.B. Shehata, A. EI-Megeed, S.F. Halim, and A. Mounir, Polym. Plast. Technol. Eng., 44, 1297 (2005).

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(10.) J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, and J.L. Schlenker, J. Am. Chem. Soc., 114, 10834 (1992).

(11.) Y.S. Lipatov, Polymer Reinforcement, Toronto, Canada (1995).

(12.) D.J. Hourston and M. Song, J. Appl. Polym. Sci., 76, 1791 (2000).

(13.) D.J. Hourston, M. Song, A. Hammiche, H.M. Pollock, and M. Reading, Polymer, 38, 1 (1997).

(14.) W. Brostow, B. Bujard, P. Cassidy, H. Hagg, and P. Montemartini, Mater. Res. Innovations, 6, 7 (2002).

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Correspondence to: B.L. Lopez: e-mail: blopez@udea.edu.co

Contract grant sponsor: University of Antioquia.

DOI 10.1002/pen.21126

Published online in Wiley InterScience (www.interscience.wiley.com). [C] 2008 Society of Plastics Engineers

Leon D. Perez, Ligia Sierra, Betty L. Lopez

Grupo Ciencia de los Materiales, Universidad de Antioquia, Sede de Investigation Universitaria Medellin, Colombia
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Date:Oct 1, 2008
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