Rm

TD

ARRAA

KBNSM

1

bchf[pSefipsd

hihtttBom

0h

Applied Surface Science 280 (2013) 888– 897

Contents lists available at SciVerse ScienceDirect

Applied Surface Science

jou rn al h omepa g e: www.elsev ier .com/ locate /apsusc

eactivity of sulfide-containing silane toward boehmite and in situodified rubber/boehmite composites by the silane

engfei Lin, Lixin Zhu ∗, Weiwei Chen, Siwu Wu, Baochun Guo ∗, Demin Jiaepartment of Polymer Materials and Engineering, South China University of Technology, Guangzhou 510640, PR China

a r t i c l e i n f o

rticle history:eceived 7 January 2013eceived in revised form 19 May 2013ccepted 19 May 2013vailable online 25 May 2013

a b s t r a c t

The silanization reaction between boehmite (BM) nanoplatelets and bis-[3-(triethoxysilyl)-propyl]-tetrasulfide (TESPT) was characterized in detail. Via such modification process, the grafted sulfide moietieson the BM endow reactivity toward rubber and substantially improved hydrophobicity for BM. Accord-ingly, TESPT was employed as in situ modifier for the nitrile rubber (NBR)/BM compounds to improve the

eywords:oehmiteitrile rubberilaneechanical property

mechanical properties of the reinforced vulcanizates. The effects of BM content and in situ modification onthe mechanical properties, curing characteristics and morphology were investigated. BM was found to beeffective in improving the mechanical performance of NBR vulcanizates. The NBR/BM composites couldbe further strengthened by the incorporation of TESPT. The interfacial adhesion of NBR/BM compositeswas obviously improved by the addition of TESPT. The substantially improved mechanical performancewas correlated to the interfacial reaction and the improved dispersion of BM in rubber matrix.

. Introduction

During the last two decades, rubber nanocomposites reinforcedy light-colored nanosized inorganics, such as silica [1–3], calciumarbonate [4], montmorillonite [5,6], halloysite nanotubes [7–9]ave drawn extensive interests both academically and industrially

or their unusual performance, such as higher reinforcing efficiency10], reduced loss resistance in tread application [11], decreased gasermeability [12] and anisotropic mechanical performance [13,14].uch outcomes derived not only from the effects of size and geom-try, but also from the unique surface properties of the nanosizedllers. Therefore, the further exploitation of the rubber nanocom-osites consisting the reinforcement with unique geometric andurface characteristics for the tailorable interfacial features andesired performance is still of great importance.

Boehmite (BM), with the ideal chemical formula of �-AlO(OH),as been reported since 1925 [15]. BM, as described in Fig. 1,

s composed of Al O double layers which are interconnected byydrogen bonds between the hydroxyl groups. The unit of struc-ure of boehmite is orthorhombic end-centered on (1 0 0) and hashe axes a = 3.69 A, b = 12.14 A and c = 2.86 A [16]. BM has been usedo synthesize the carboxylate–alumoxane nanostructures [17,18].

esides, BM can be used as absorbent [19], filler in membrane [20],ptical materials [21], coatings [22] and composite reinforcementaterial in ceramics [23] and so on.

∗ Corresponding authors.E-mail addresses: pszlx@scut.edu.cn (L. Zhu), psbcguo@scut.edu.cn (B. Guo).

169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2013.05.084

© 2013 Elsevier B.V. All rights reserved.

Similar to the layered clay, most of commercialized BM is alsonanoplatelet-like. This kind of 2-D inorganics have attracted con-siderable interest in fabrication of polymer nanocomposites withremarkably improved mechanical strength [24], flame retardancy[25] or changed crystallization behavior [26]. For instance, Flor-janczyk et al. examined the modification of BM with phosphoricacid diester and the reinforcement of carboxylated styrene-butadiene latex with the modified BM [27]. Mulhaupt et al.prepared polyethylene/BM composites by in situ polymerizationand discovered that organoboehmite have a positive effect onincreasing the catalyst activity of metallocenes [28]. Siengchin et al.demonstrated that an additional incorporation of BM in 2.5 wt%could enhance the stiffness and strength of the PA 6/HNBR blend[29]. In our previous work [30], methacrylic acid (MAA) was inves-tigated as a novel interfacial modifier for SBR/BM composites curedby peroxide. The reaction mechanism for the modification wasrevealed to be the BM/MAA coordination and MAA/rubber graft-ing reaction. The substantiated BM/MAA coordination can improvethe dispersion of BM in rubber matrix. It is illustrated that theultimate mechanical performance and interfacial interaction aresignificantly enhanced by the in situ modification.

In sulfur-cured rubber/silica compounds, the sulfide-containingsilane, for instance, bis-(�-triethoxysilylpropyl)-tetrasulfide(TESPT), is generally utilized to improve the overall performance ofthe reinforced rubber. The tetrasulfide groups on TESPT could react

with the rubber in the presence of accelerators at elevated tem-peratures. During the compounding, the ethoxy groups on TESPTcould simultaneously react with the surface hydroxyls on silica,via a silanization reaction. By using TESPT in a silica-filled rubber

T. Lin et al. / Applied Surface Science 280 (2013) 888– 897 889

rhom

cwtteAbssd

i[Biotrptom

2

2

JdlatorTa

TC

Fig. 1. The structure of boehmite: arrangement of ortho

ompounds, the mechanical properties and abrasion resistanceere found substantially improved [31–34]. Seo et al. [35] studied

he networked silica prepared using TESPT as a replacement forhe conventional silica-reinforced system and found that couldffectively reinforce the physical properties of SBR compounds.nsarifar et al. [36] reduced excessive use of the chemical curativesy optimizing the chemical bonding between rubber blends andilica nanofiller via TESPT. Jiang et al. [37] used TESPT to modify theurface of MWCNTs and confirmed they reacted with the activeouble bonds of NR to form a strong interface.

So far, the related research of TESPT toward BM is still lack-ng. Due to the potential reactivity of silanes toward aluminols38,39], it is expected that TESPT could also be used to modifyM which possesses abundant aluminols on the surface. Accord-

ngly, the authors aimed to explore the potential of the reactivityf TESPT toward BM and the effectiveness of such kind of silane onhe performance of rubber/BM compounds. In the present work, theeaction between BM and TESPT was first examined. NBR/BM com-osites were then prepared by direct blending. TESPT was addedo in situ modify BM during this process. The effects of the additionf TESPT on the mechanical properties, curing characteristics andorphology properties of NBR/BM composites were investigated.

. Experimental

.1. Raw materials

NBR, with 40% content of acrylonitrile, was manufactured byilin Chemical Co. Ltd., Jilin, China. BM was purchased from Shan-ong Aluminum Co. Ltd., Zibo, China. The content of Al2O3 was no

ess than 80%, the bulk density was 0.4–0.6 g/mL. The BET surfacerea was determined to be 142.8 m2/g. The average aggregated par-icle size of the BM was about 1–20 �m. The single nanoplatelet

f BM possesses thickness of about 3 nm and in-plane size in theange of 200–400 nm. Zinc oxide (ZnO) was analytical grade fromianjin Fuchen Chemical Reagents, Tianjin, China. TESPT and otherdditives were industrial grade and used as received.

able 1omposition of NBR/BM rubber composites (phr).

Code NBR BN-10 BN-10-Si BN-20

NBR 100 100 100 100

ZnO 5 5 5 5

Sta 2 2 2 2

MBa 1 1 1 1

4010 NAb 1 1 1 1

DMc 0.5 0.5 0.5 0.5

CZd 1.5 1.5 1.5 1.5

Sulfur 1.5 1.5 1.5 1.5

BM 0 10 10 20

TESPT 0 0 0.75 0

a MB: 2-mercaptobenzimidazole zinc.b 4010 NA: N-isopropyl-N′-phenyl-p-phenylenedi-amine.c DM: 2,2′-dibenzothiazole disulfide.d CZ: N-cyclohexyl-benzothiazole-2-sulfenamide.

bic boehmite (left) and the layer type structure (right).

2.2. Preparation of TESPT modified BM (Si–BM)

To study the modification effect of TESPT on BM, a model com-pound of TESPT modified BM (Si–BM) was synthesized accordingto the below procedure. BM (5 g) was dispersed in toluene (500 mL)by vigorous stirring. TESPT (20 g) was then slowly added at roomtemperature. The weight ratio between TESPT and BM is 4:1. Themixture was heated to 80 ◦C under stirring and the reaction waslasted for 12 h. After this process, the mixture was filtered andwashed by centrifugation with toluene and ethanol repeatedly. Theobtained Si–BM was dried at 80 ◦C for 2 days.

2.3. Preparation of NBR/BM composites

NBR and additives were mixed with a HAAKE Polylab OS at therotating speed of 30 rpm with the beginning temperature of 40 ◦C.The compositions of the rubber compounds are listed in Table 1.The content of BM is variable and the weight ratio between TESPTand BM is fixed as 7.5:100. The sample code of BN-x symbolizesNBR/BM compounds with x phr of BM. The sample code of BN-x-Sirepresents NBR/BM compounds modified by TESPT with x phr ofBM. The internally mixed compounds were press-cured into 1 mmthickness sheets at 160 ◦C × Tc90.

2.4. Characterizations

The hydrophobicity of the modified BM was characterized bythe extraction experiment. 0.1 g BM and 0.1 g Si–BM were sepa-rately added to two glass bottles. The bottles are filled with 5 mLtoluene and followed by bath sonication for 10 min. After furtherfilling 5 mL deionized water, the bottles are subjected to furtherbath sonication for 10 min. After standing for a while, the observa-tions on the retention of both samples in solvent were captured.TGA studies were performed with a TA Q5000 thermogravimetric

analyzer (TA instruments, USA). The samples were scanned from30 to 650 ◦C at a heating rate of 10 ◦C/min under nitrogen atmo-sphere. FTIR spectra of BM and Si–BM were collected on a Bio-Rad165 spectrophotometer (Bio-Rad Laboratories, Hercules, USA) with

BN-20-Si BN-40 BN-40-Si BN-60 BN-60-Si

100 100 100 100 1005 5 5 5 52 2 2 2 21 1 1 1 11 1 1 1 10.5 0.5 0.5 0.5 0.51.5 1.5 1.5 1.5 1.51.5 1.5 1.5 1.5 1.5

20 40 40 60 601.5 0 3 0 4.5

890 T. Lin et al. / Applied Surface Science 280 (2013) 888– 897

Al

Al

Al

Al

Al

Al

Al

Al

Al

HO

O

O OH

O

O OH

O

OHO

HO

O

O

O

OH

OH

HO

OHHO

HO

HO OH

+

Si

OC2H5

(CH2)3

C2H5O

OC2H5

Si

OC2H5

(CH2)3C2H5O

OC2H5S4

Al

Al

Al

Al

Al

Al

Al

Al

Al

HO

O

O O

O

O O

O

OHO

HO

O

O

O

O

O

HO

OHHO

HO

HO OH

Si

OC2H5

O

(CH2)3 S4

Si

OC2H5

(CH2)3

S4

Si

O

(CH2)3

Si

O

(CH2)3

S4

I. hydrolysis

II. condensation

dification reaction of boehmite with TESPT.

trAmwaf2

mdieb((TisbS

3

3

ctsno

prodcphcB

S

Fig. 3. Comparison of the extraction of BM and Si–BM in toluene (upper)/water(bottom) solvents. Left: BM; right: Si–BM.

Boehmite TESPT

Fig. 2. Schematic representation of mo

he samples in KBr pellets. XPS spectra of BM and Si–BM wereecorded by using an X-ray Photoelectron Spectrometer (Kratosxis Ultra DLD; Kratos Analytical, Eppstein, Germany) with an alu-inum (mono) Ka source (1486.6 eV). The aluminum Ka sourceas operated at 15 kV and 10 mA, respectively. For both samples,

high-resolution survey (pass energy is equal to 48 eV) was per-ormed at spectral regions. The data was calibrated with C 1s at84.6 eV.

The curing characteristics of the SBR compounds were deter-ined at 170 ◦C by U-CAN UR-2030 vulcameter, Taiwan. Crosslink

ensities were determined by the equilibrium swelling methodn toluene and calculated according to the classic Flory–Rehnerquation [40]. The cryogenically fractured surfaces of the rub-er composites were observed with SEM machine of EVO 18Germany). TEM observations for the ultramicrotommed samplesLeica EM UC6; Leica, Wetzlar, Germany) were done by Philipsecnai 12 TEM machine (Eindhoven, Netherlands) at an accelerat-ng voltage of 30 kV. Tensile tests were performed following ISOtandard 37-2005. Tensile strength, modulus and elongation atreak were measured using U-CAN UT-2060 (Taiwan) instrument.hore A hardness was measured according to ISO standard 48:1994.

. Results and discussion

.1. Reactions between BM and TESPT

In the present study, it is believed that the surface aluminolsould also react with the silane via silanization process similaro the reaction between silanol and the silane. Considering theelf-condensation of TESPT, the silanization between the alumi-ols of BM and ethoxy groups of TESPT, the modification reactionf boehmite with TESPT may schematically depict in Fig. 2.

To test the effectiveness of the preparation of Si–BM, the dis-ersibility of BM and Si–BM in solvents was compared and theesult is presented in Fig. 3. It is well known that BM is a kindf hydrophilic inorganic filler. One can distinctly observe that BMisperses in bottom aqueous phase. However, after the modifi-ation by TESPT, Si–BM is found to be located in upper toluenehase steadily. It seems that TESPT has reacted with BM and theydrophobic organic moieties has successfully been grafted or

oated on the surface of BM. As a result, the surface properties ofM changed significantly.

Fig. 4 depicts the TGA curves and the appearance of BM andi–BM powder before and after the TGA running. BM exhibits two

Fig. 4. TGA curves and the appearance of BM and Si–BM before and after the heatrunning.

T. Lin et al. / Applied Surface Science 280 (2013) 888– 897 891

3500 3000 2500 2000 1500 1000 500

Tra

ns

mit

tan

ce

(a

.u.)

-1

Si-BM

BM

2927

1243

1070

478617

740

1637

3097

3325

2856

d(w2ihtolhia3dthBcfttIotctis

a6Ao7vr2itf

dsmtsA

Table 2Calculated relative intensities of the characteristic peaks of BM and Si–BM.

Peaks BM Si–BM

−1

at 531.55 eV), and relatively decreasing in the peak densities forAl OH (74.66 eV for Al 2p and 532.27 eV for O 1s) convincinglydemonstrate the silanization between BM and TESPT.

Wavenumber (cm )

Fig. 5. Comparison of FTIR spectra of BM and Si–BM.

istinct mass loss regions. The weight loss of BM below 160 ◦Cabout 10 wt%) is due to the release of the physically absorbedater and chemically bonded water in BM. The weight loss between

50 and 600 ◦C (about 20 wt%) is linked to the dehydration of BMnto Al2O3. The total weight loss of BM is about 30%. On the otherand, the TGA curve of Si–BM is quite different from that of BM. Athe temperature below 200 ◦C, less water release (less than 5 wt%)wing to the improved hydrophobicity of Si–BM. Si–BM begins toose weight at around 300 ◦C and shows a weight decrease withigher slope from 250 to 600 ◦C, which is attributed to the elim-

nation of the grafted silane moiety on the surface of BM sheetsnd the dehydration of BM. The total weight loss of Si–BM is about3 wt%. The minimal weight decrease above 500 ◦C in Si–BM is stillue to the dehydration among some aluminols in BM crystals ashe organic moieties on Si–BM are definitely carbonized at suchigh temperature. Such observation is still found in the pristineM. Besides, as shown in Fig. 4, the appearance of BM does nothange much after the heating, whereas the color of Si–BM changesrom light yellow to deep brown. As mentioned in the experimen-al part, Si–BM was filtered and washed by centrifugation witholuene and ethanol repeatedly after the modification procedure.n this case, as the ungrafted excessive TESPT have been washedff, only the grafted silane moiety could retain on the surface dueo the potential interaction between TESPT and BM. This obviouslyhanged appearance of Si–BM sample could only be associated withhe carbonization of organic moieties on the surface of BM, provid-ng additional indication for the possible grafting of TESPT on BMurface.

Fig. 5 represents the comparison of the FTIR spectra of BMnd Si–BM. Both BM and Si–BM exhibit peaks around 478 and17 cm−1, characterizing the stretching vibrations in the distortedlO6 octahedron [30]. The characteristic peaks of aluminol groupsf BM (�as at 3325 cm−1, �s at 3097 cm−1, �s at 1070 cm−1 and �as

40 cm−1) are also observed in both samples [41,42]. Such obser-ation indicates that not all the aluminols are consumed by theeaction between BM and the silane. In addition, the peaks around927 cm−1, 2856 cm−1 and 1243 cm−1, characterizing C H stretch-

ng and deformation vibrations respectively [43], are also found inhe spectrum of Si–BM. This could be attributed to the methylenerom the grafted TESPT on BM.

During the silanization between BM and TESPT, besides the con-ensation between aluminol and ethoxy, the silane may also beubjected to self-condensation. Consequently, the grafted organic

oieties may simultaneously contain Si O Al and Si O Si struc-

ures. However, it is difficult to distinguish the asymmetrictretching of the Si O Si from the vibration stretching of thel OH in the spectra as both are located around 1100–1000 cm−1

1075 cm 0.645 0.7813325 cm−1 0.912 0.820

[44]. Therefore the relative decrease in the peak intensity of alu-minol is selected for further characterize the reaction between BMand TESPT. The stretching vibrations of AlO6 octahedron around478 cm−1 are chosen as internal standard peak, the value of therelative intensity of the characteristic peak at 1070 cm−1 and3325 cm−1 of both samples is calculated and presents in Table 2. Therelative intensity of Al OH at 3325 cm−1 is weakened from 0.912to 0.820, indicating a consumption of hydroxyl groups after mod-ified with TESPT. Meanwhile, the relative intensity at 1075 cm−1

increases from 0.645 to 0.781, which is attributed to the overlayingof the Si O Si structure.

XPS is employed to further substantiate the reaction betweenBM and TESPT. The XPS spectra of BM and Si–BM are compared inFig. 6. As shown in Fig. 6, the Al 2p spectrum shows the presence oftwo different aluminum species in BM sample, which peaks are at74.72 eV and 73.98 eV. These two peaks are assigned to Al OH andAl O Al bonded species respectively [45]. However, after modifiedwith TESPT, the Al 2p signal in Si–BM could be deconvoluted intothree kinds of Al environment. These three kinds of Al 2p in Si–BMare located at 74.66, 74.09, and 73.52 eV, which are assigned toAl OH, Al O Al and Al O Si bonded species respectively.

Some arguments have pointed out that the discrepancy amongthe chemical shifts in the Al 2p binding energies for the oxides,hydroxides and oxohydroxides are generally too slight to deter-mine precisely in general XPS equipment [46,47]. Therefore, theO 1s XPS survey is also studied for scrupulous verification. As shownin Fig. 7, three O 1s transitions are observed at 530.97, 532.10 and533.10 eV for BM, associated with Al O Al, Al O H and H O H,respectively [46]. While for Si–BM, two more kinds of oxygenspecies are observed at 531.55 and 530.15 eV, which are assignedto Al O Si and Si O Si, respectively. The newly emerging Al 2penvironment (Al O Si at 73.52 eV), O 1s environment (Al O Si

Fig. 6. Al 2p XPS survey of BM and Si–BM.

892 T. Lin et al. / Applied Surface Science 280 (2013) 888– 897

536 534 532 530 528

536 534 532 530 528

BM

Inte

nsit

y(a

.u.)

533.10 eV

532.27 eV531.55 eV

530.82 eV

530.15 eV

530.97 eV533.10 eV

532.10 eV

Si-BM

3c

irTAsi

oiatiwNisapstaofTi

TV

0 10 20 30 40 50 602.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

Cro

ss

lin

k d

en

sit

y (m

ol.

cm

-3.1

0-4

)

BM content (phr)

NBR/BM composites

NBR/BM composites with TESPT

Binding energy (eV)

Fig. 7. O 1s XPS survey of BM and Si–BM.

.2. Curing behavior and thermodynamics behavior in NBR/BMomposites with and without TESPT

Based on the illustrated reaction between BM and TESPT, its believed that TESPT could be used as in situ modifier for BM-einforced rubber composites. The silanization reaction betweenESPT and BM may also undergo during the vulcanization process.ccordingly, the grafted sulfides may be further decomposed intoulfide radical to participate in the vulcanization to yield covalentnterface between BM and the rubber.

The curing behavior is first examined. The curing characteristicsf NBR/BM composites with and without TESPT are summarizedn Table 3. The scorch time (Tc10) of NBR/BM composites gradu-lly decrease with the increase of BM content. BM could acceleratehe decomposition of sulfonamide type accelerators for the basic-ty surface aluminols of BM and the release of physically absorbed

ater or chemically bonded water during heating, the Tc10 ofBR/BM composites is hence found to decease [48]. With the

ncreasing BM content, the adsorption of curing additives on theurface of BM may lead to the increase in vulcanization time (Tc90)ccordingly. With the addition of TESPT, Tc10 for the modified com-ound is always longer than that for the unmodified one at theame filler content. Due to the decreased basicity of the surface ofhe filler, the accelerating effect caused by aluminols is alleviatednd consequently Tc10 is increased compared with the unmodified

ne. Actually the introduction of TESPT increases the effective sul-ur content in the system. Consequently, Tc90 is also found to rise.his may lead to the increase in crosslink density which will bellustrated later. It can also be seen that, both in the modified and

able 3ulcanization properties of NBR/BM composites with and without TESPT.

BM (phr) Tc10 (min:s) Tc90 (min:s) ML (dN × m) MH (dN × m)

NBR/BM composites0 1:49 9:09 1.4 13.910 2:34 12:56 1.4 15.520 1:44 12:07 1.6 18.240 1:02 14:18 2.9 22.160 0:55 15:36 4.8 26.6

NBR/BM/TESPT composites10 2:52 17:37 1.3 15.820 2:14 16:56 1.5 19.940 1:25 16:59 2.6 27.860 1:04 17:14 3.1 39.2

Fig. 8. Effects of BM amount on crosslink density of NBR/BM composites with andwithout TESPT.

unmodified composites, with the increasing of filler loading, themaximum torque (MH) and minimum torque (ML) of all samplesconsistently go up due to the hydrodynamic effect. Furthermore,at the same filler loading, MH of the modified vulcanizate is alwayshigher than that of the unmodified one. Besides, ML of all the sam-ples modified with TESPT is lower than that of without TESPT.The increased MH is attributed to the better dispersion of BM,the improved interfacial interaction and the additionally increasedcrosslinking density. On the one hand, TESPT can anchor onto thesurface of BM and improve the compatibility of BM with rubbermatrix, which facilitate the dispersion of BM and hence increasethe rubber–BM interfacial area. On the other hand, the reactive sites(radicals) on the rubber occurring during the mastication processmay form a chemical linkage (C S linkages) with the tetrasulfanegroups of TESPT, which consequently strengthen the rubber–BMinterfacial bonding and increase the torque of the vulcanizates.The slightly decreased ML of the modified compound comparingwith the unmodified one provides implication for the better fillerdispersion in the modified compound. The aggregation or agglom-eration of filler will result in entrapment of rubber in the aggregatesor agglomerates and will consequently increase the effective fillervolume in the rubber [49]. Therefore the effective filler volume isconsidered to be more accurate than actual filler volume to affectthe reinforcement of the filled rubber. In the present study, theintroduction of TESPT leads to better dispersion of BM in the com-pounds and slightly lower effective filler volume. As a consequence,the modified compound possesses slightly lower ML value.

Fig. 8 represents the effects of TESPT and BM content on thecrosslink density of NBR/BM composites. In NBR/BM system, thecrosslink density increases slightly and the value of all the samplesis below 3 × 10−4 mol/cm3. While in NBR/BM/TESPT system, thecrosslink density grows remarkably with the addition of TESPT. Forexample, with the content of 60 phr of BM, the crosslink density ofNBR/BM/TESPT composite is as high as 5.2 × 10−4 mol/cm3. As men-tioned above, the dispersion of BM and the rubber–BM interfacialbonding climbs up by the anchoring of TESPT onto the surface ofBM. Further reaction between the tetrasulfane groups of TESPT withthe radicals of rubber matrix substantially increases the crosslinkdensity of NBR/BM composites. Actually the effective sulfur contentincreases by introducing TESPT in the compound. Together with thesubstantial increase in interfacial crosslinking, the crosslink densityis found to be remarkably increased by the incorporation of TESPT.

Fig. 9 depicts the TGA and DTG curves of NBR/BM compositeswith and without TESPT. It is observed that the TGA curves andthe DTG peak values of NBR shift to higher temperature with the

T. Lin et al. / Applied Surface Science 280 (2013) 888– 897 893

300 400 5000

20

40

60

80

100

Temperature (oC)

We

igh

t (%

)

NBR

BN-20

BN-20-Si

BN-40

BN-40-Si

BN-60

BN-60-Si

300 400 500

0.0

0.5

1.0

1.5

Deri

vati

ve w

eig

ht

(%/

o C)

Temperature (oC)

NBR

BN-20

BN-20-Si

BN-40

BN-40-Si

BN-60

BN-60-Si

(a) (b)

BR/BM

aipoNtbrgssttfIs

3N

SoBhwss(toTIttTttsat

Tt2

Fig. 9. TGA (a) and DTG (b) curves of N

ddition of BM, showing that BM platelets play a positive role inmproving the thermal stability of NBR rubber. Besides, the DTGeaks of NBR/BM composites gradually decline with the increasef BM, indicating the decrease in thermal decomposition rate ofBR/BM composites. The improved resistance to thermal degrada-

ion by the incorporation of BM platelets could be ascribed to thearrier effects to heat and mass transfer, which have been widelyeported in the other polymer composites with platelet-like inor-anics such as montmorillonite [50,51]. However, judging from thelightly decreased peak temperature in DTG curves, the thermaltability of NBR/BM composites with TESPT are slightly lower thanhe unmodified counterpart. Considering the thermal decomposi-ion of TESPT may take place at about 250 ◦C (as we have discussedor Fig. 4), the decomposition of TESPT may take place before 460 ◦C.t may be the main reason to explain the slightly lowered thermaltability of NBR/BM composites with TESPT.

.3. Effects of TESPT on morphology and mechanical properties ofBR/BM composites

The effect of TESPT on the dispersion of BM is investigated byEM and the results are shown in Fig. 10. It clearly shows that with-ut TESPT, most BM particles have a tendency to form aggregation.y the increase of the content of BM, the BM particles become moreeterogeneous and the particle size observably increases. However,ith the incorporation of TESPT, one can observe that the disper-

ion of BM becomes uniform in the rubber matrix and the particleize remarkably decreases. For instance, as shown in Fig. 10(g) andh), when the concentration of BM increases to 60 phr, the BM par-icle aggregates are apparently observed. However, the aggregationf the BM particle is definitely alleviated by the addition of TESPT.ESPT shows positive effect in dispersing BM in the rubber matrix.n addition, in the unmodified systems, more exposed particles onhe fractured surface are observed. It is believed to be evident forhe worse interfacial interaction comparing with the system withESPT. The significantly improved dispersion may be attributed tohe increased interfacial bonding which is originated from the reac-ion between the ethoxy groups with the aluminol groups on theurface of BM. Therefore, the number of the aluminol groups dropnd the inorganic become more difficult to re-aggregate because ofhe improved compatibility between the filler and rubber matrix.

The TEM photographs of NBR/BM composites with and withoutESPT are compared in Fig. 11. One can readily observe that most ofhe BM particles aggregate into agglomerates with diameter about00–400 nm without TESPT. With the incorporation of TESPT, one

composites with and without TESPT.

can find the dispersion of BM increases remarkably and the agglom-erate size becomes much smaller. Most of the BM is separately anduniformly dispersed as thin layers with thickness down to 3–5 nm.The significantly improved dispersion by TESPT provides additionalevidence of the increased interfacial interactions between NBR andBM.

Table 4 shows the effect of BM and TESPT on the mechani-cal properties of NBR vulcanizates. The tensile strength and tearstrength increase with increasing BM loading. The permanent setsalso increase by the addition of BM. Furthermore, the overallmechanical properties significantly increase with the incorpora-tion of TESPT. Compared with NBR/BM composites in the same 60phr of BM, the tensile strength and tear strength of NBR/BM/TESPTcomposites increase from 12.4 to 17.2 MPa, 37.7–54.1 kN m−1,respectively. Both the elongation at break and permanent setdecrease accordingly. The increase in elongation at break alongthe increasing of BM content should be interpreted by the limitedinterfacial interactions and the adsorption of the curatives onthe BM particles. Concerning the decrease of elongation at breakin the NBR/Si–BM composites, it should be due to the remark-ably increased crosslink density with increasing filler content (andsimultaneously increased TESPT content). All the substantiallyimproved mechanical performance is related to the improved dis-persion of BM in rubber matrix and the strengthened rubber–BMinterfacial bonding with the present of TESPT.

Mooney–Rivlin equation [52,53], as written below, could beemployed to evaluate the structure–property relationship by ana-lyzing the tensile stress–strain curves.

�∗ = �

2(� − �−2)= C1 + C2 × 1

�(1)

where � is the stress applied, � is the extension ratio and C1 and C2are constants which are independent of �. It has been suggested thatthe Mooney–Rivlin constants C1 and C2 are associated with the net-work structure and the flexibility of the network, respectively [9].Bokobza et al. [54] reported that the upturn of the Mooney–Rivlinplot indicates the limited extensibility of the chains and will affectthe elastic properties of filled networks. The upturn in the modu-lus is observed if a strong interaction between polymer and fillersexists [55].

As illustrated in Fig. 12, the �* of the modified compounds are

significantly higher than that of the unmodified one. In the region ofsmall extension ratio (�−1 > 0.65), �* lower sharply during stretch-ing, which can be attributed to the Payne effect [56]. An upturn of �*is observed at a high extension ratio, which is ascribed to the finite

894 T. Lin et al. / Applied Surface Science 280 (2013) 888– 897

Fig. 10. SEM photos of NBR/BM composites with and without TESPT. (a) 10 phr, (c) 20 phr, (e) 40 phr, (g) 60 phr BM without TESPT; (b) 10 phr, (d) 20 phr, (f) 40 phr, (h) 60phr BM with TESPT, respectively.

T. Lin et al. / Applied Surface Science 280 (2013) 888– 897 895

T (a)

etlT

TM

Fig. 11. TEM photos of NBR/BM composites with and without TESP

xtensibility of polymer chains bridging neighboring filler [55]. Forhe TESPT-modified composites, the upturn point appears at muchower extension ratio, compared with that of the unmodified one.he value of the �−1 at which the upturn occurs decreases with the

able 4echanical properties of NBR/BM composites with and without TESPT.

BM (phr) Elongation atbreak (%)

Tensilestrength (MPa)

NBR/BM composites0 612.0 3.7

10 751.0 8.9

20 786.5 8.7

40 812.2 10.4

60 818.5 12.4

NBR/BM/TESPT composites10 774.2 7.9

20 767.0 9.1

40 738.1 14.9

60 634.7 17.2

and (b) 40 phr BM; (c) and (d) 40 phr BM with TESPT, respectively.

increasing filler loading. This is because the chain connecting BMexperiences different overstrains. The additional TESPT providedthe extra chemical cross-linking points, thereby the polymer chainsare easily extended under smaller extension.

Tear strength(kN m−1)

Shore Ahardness

Permanentset (%)

12.4 48 516.8 52 1516.2 54 1627.9 62 3137.7 70 43

15.3 52 1721.6 55 1741.8 64 2054.1 72 27

896 T. Lin et al. / Applied Surface Scie

0 200 400 600 8000

4

8

12

16

BN-40-Si

BN60

BN40

Str

es

s (

MP

a)

Strain (%)

BN-60-Si

(a)

0.2 0.4 0.6 0.8 1.0

0.5

1.0

1.5

2.0

Upturn Poin tsBN-60-Si

BN-40-Si

BN60

BN40

Red

uced

Str

ess

(σ*)

λ-1

Payne Effect

(b)

Fc

4

ta(oTictbtpTiistuiabBi

[

[

[

[

[

[

[

[

[

[

[

membranes, Journal of Membrane Science 401–402 (2012) 132–143.[21] D. Mishra, S. Anand, R.K. Panda, R.P. Das, Hydrothermal preparation and char-

ig. 12. Tensile stress–strain curves (a) and plots of �* versus �−1 (b) for NBR/BMomposites with and without TESPT.

. Conclusions

Boehmite (BM) could be chemically modified by the silaniza-ion process between the aluminols of BM and ethoxy groups of

sulfur containing silane, bis-[3-(ethoxysilyl)-propyl] tetrasulfideTESPT). The silanization is accompanied by the self-condensationf the silane. Such chemical reaction is verified by FTIR and XPS.hrough such modification, the hydrophobicity of BM is largelymproved and believed to be reactive in rubber compounds byleavage of the grafted tetrasulfide. Accordingly, TESPT was utilizedo in situ modify the compounds of nitrile rubber (NBR) reinforcedy BM. TESPT improves the crosslink density largely and delayshe vulcanization process. According to TGA and DTG studies, BMlatelets could improve the thermal stability of NBR rubber whileESPT slightly decline the thermal stability of NBR/BM compos-tes. SEM and TEM results show the dispersion of BM in the rubbermproves remarkably and the agglomerate size becomes muchmaller by the addition of TESPT. The mechanical properties ofhe modified composites are substantially higher than those of thenmodified ones when the filler content is higher than 20 phr. For

nstance, compared with the unmodified one, the tensile strengthnd tear strength of the modified composite are further increased

y 28% and 44% with the same BM loading in 60 phr, respectively.oth the elongation at break and permanent set cut down accord-

ngly. All the substantially improved mechanical performance is

[

nce 280 (2013) 888– 897

related to the improved dispersion of BM in rubber matrix and thestrengthened rubber–BM interfacial bonding with the present ofTESPT.

Acknowledgements

The authors appreciate the financial supports from the NationalNatural Science Foundation of China (51222301, 50933001 andU1134005), New Century Excellent Talents in University (NCET-10-0393), and Fundamental Research Funds for the CentralUniversities (2012ZG0002).

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