Master curve of filler localization in rubber blends at ...

Master curve of filler localization in rubber blends at an equilibrium state

H. H. Le, K. Osswald, S. Ilisch, X. T. Hoang, G. Heinrich & H.J. Radusch Journal of Materials Science Full Set - Includes `Journal of Materials Science Letters' ISSN 0022-2461 Volume 47 Number 10 J Mater Sci (2012) 47:4270-4281 DOI 10.1007/s10853-012-6277-6

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Author's personal copy J Mater Sci (2012) 47:4270–4281 DOI 10.1007/s10853-012-6277-6

Master curve of filler localization in rubber blends at an equilibrium state H. H. Le • K. Osswald • S. Ilisch • X. T. Hoang G. Heinrich • H.-J. Radusch

•

Received: 7 September 2011 / Accepted: 16 January 2012 / Published online: 8 February 2012 Ó Springer Science+Business Media, LLC 2012

Abstract In this study, the phase-specific localization of filler in NBR/NR blends was characterized by means of the selective extraction method and wetting concept. A strong dependence of silica localization on the filler loading was found. A model based on thermodynamic data was proposed for a quantitative prediction of filler localization in rubber blends. The filler localization can be described by a master curve demonstrating a characteristic behavior in dependence on the filler surface tension data of blend components and filler. The effect of filler loading on the silica localization is sufficiently explained by this model by taking into consideration the deactivation of the silanol groups on the silica surface by adsorbed curing additives. Using the master curve, the surface tension of filler affected by curing additives and silane addition can be estimated that may be useful for evaluation and comparison H. H. Le (&) H.-J. Radusch Center of Engineering Sciences, Martin Luther University Halle-Wittenberg, 06099 Halle (Saale), Germany e-mail: [email protected] K. Osswald University of Applied Sciences, 06217 Merseburg, Germany S. Ilisch Styron Deutschland GmbH, 06258 Schkopau, Germany X. T. Hoang University of Technology, National University HCM, Ho Chi Minh City, Vietnam G. Heinrich Leibniz Institute of Polymer Research (IPF) Dresden, 01069 Dresden, Germany G. Heinrich Institute of Material Sciences, Technical University Dresden, 01069 Dresden, Germany

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of the effect of different coupling agents. Surface tension values of different fillers were estimated by means of the master curve and they lie in the same order compared to those reported in literature. A potential transfer of filler within a rubber blend can be also quantitatively predicted.

Introduction For rubber compounds and blends, carbon black (CB) and silica are the most widely used fillers in the rubber industry to improve the static and dynamic mechanical properties, like modulus, tear strength, abrasion, and fatigue resistance [1–4]. In the past several studies [5–29] were conducted to characterize the phase selective filler localization and related mechanical properties of filled rubber blends. Sirca [7, 8] studied the effect of heterogeneous filler localization on the properties of a polybutadiene rubber (BR)/styrene butadiene rubber (SBR) blend and a BR/natural rubber (NR) blend and BR/polyisoprene rubber (IR) blend by means of nuclear magnetic resonance (NMR). Varying the sequence of filler addition caused a change in filler localization. They found that superior hysteresis properties of the blend were obtained when most of the filler was in the BR phase. Massie et al. [14] studied the localization of filler in NR/BR blends and found that N550 black has no preference for either the NR or the BR. It was also found that the cut-growth resistance of the rubber blends in which filler is mainly in the BR phase is poorer than that of the blend with evenly distributed black. Hess [5, 6], Walter [16], and Lee [17] used transmission electron microscopy (TEM) and pyrolysis gas chromatography for study of filler localization in many NR/synthetic rubber blends including NR/chloroprene rubber (CR), NR/nitrile butadiene rubber (NBR), and NR/SBR and BR/ethylene propylene diene

Author's personal copy J Mater Sci (2012) 47:4270–4281

rubber (EPDM) blend and they found that CB preferentially resided in the synthetic rubber phases. Recently, Jeon [18], Herrmann [19], and Tsou [20] studied the localization of fillers in NR/BR blends by use of atomic force microscopy (AFM) and found that CB resides predominantly in the BR phase, whereas silica mainly exists in the NR phase. Maiti et al. [22] characterized the localization of CB and silica in NR/epoxidized natural rubber (ENR) blends using a dynamic mechanical thermal analysis (DMA) technique. They found that silica migrated preferentially to the ENR phase. It was believed that the reasons for the preferential migration of silica to the ENR phase included the low viscosity of the ENR and a physical interaction between the epoxide groups of the ENR and the silanol group of the silica. Cotton [23] and Woolard [24] applied the thermogravimetric analysis (TGA) and several research groups [12, 25–28] used DMA technique for the determination of the phase-specific localization of CB, silica and nanoclay in different rubber blends. In most of the studies mentioned above, the preferred localization of the filler is actually governed by thermodynamics as soon as rheological effects do not play a significant role. Sumita et al. [30, 31] introduced a model for calculation of the wettability parameter xS as a function of the interfacial tension between blend components. If xS lies within the range -1 and ?1, a localization of filler at the interphase is expected. If xS is outside of this range, filler will be localized only in one blend phase. This parameter was used by several authors [32–39] for a qualitative prediction of the filler localization in their blends. If the surface tension of filler is changed by a filler treatment a filler migration within the blends can take place. Using TEM, Elias et al. [34] and Ziegler et al. [40] observed a migration of silica from the thermodynamically less favorable phase toward the more favorable phase in immiscible polymer blends. In immiscible polypropylene (PP)/polyethylene-co-vinyl acetate (EVA) blend hydrophilic silica was found to migrate from the PP matrix to the dispersed EVA domains and remained confined inside them [34]. The same silica with a hydrophobic surface treatment moved and accumulated to the blend interface and in PP. Untreated silica with hydrophilic nature was found to be localized mainly in the polar NBR phase of a BR/NBR blend [40]. After modification with silane the hydrophobic silica migrated into the BR phase. Concerning the phase selective filler localization in rubber blends, recently, we also developed a method based on the online measured electrical conductance for characterization of the localization of CB and nanoclay in rubber blends directly during the processing [41, 42]. Another method based on the wetting behavior of the filler by the blends components has been also developed for determination of the kinetics of CB and silica in highly filled

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binary and ternary rubber blends [43–46]. In this article, we propose a model for a quantitative prediction of the filler localization in rubber blends at a thermodynamic equilibrium state. Using this model, the physical background of the filler localization under influence of the addition of curing additives and coupling agent will be discussed.

Model for prediction of filler localization in rubber blends at an equilibrium state The thermodynamic criterion for miscibility of polymer blends is a negative Gibbs free energy of mixing. The Gibbs free energy of mixing is given by the following equation: DGm ¼ DHm TDSm ;

ð1Þ

where DHm is the change in enthalpy of mixing, and DSm the change in entropy of mixing. T is the absolute temperature. The change in enthalpy during mixing is the result of dipole interactions, van der Waals interactions, acid–base interactions, Coulombic interactions, and the interaction energy between the different components arising from hydrogen bonds. Due to the constraints of segmental mobility of polymer chains the change in entropy is usually too small to compensate the change of mixing enthalpy. Thus, the Gibbs free energy of mixing is nearly similar to the change in enthalpy of mixing: DGm DHm :

ð2Þ

Mixing a phase F into a phase A, the relationship between the change in enthalpy of mixing of the phases A and F: DHAF m , and the interfacial tension cAF between two phases can be described according to Hildebrand and Scott by Eq. 3 [47]: DHmAF ¼ KðcAF Þ2 VAF UA UA F;

ð3Þ

where VAF is the average molar volume of the two components, UA and UA F are the volume fractions of the components A and F, respectively. K is a constant and gets a value of 1 according to Scatchard [48, 49]. Accordingly, for the system BF consisting of the components B and F the analogous relationship to Eq. 3 can be found: DHmBF ¼ KðcBF Þ2 VBF UB UBF ;

ð4Þ

where UB and UBF are the volume fractions of the components B and F, respectively. cBF is the interfacial tension between B and F. From these equations, it can be seen that blends with high interfacial tension cAF or cBF require more energy for dispersion. If the interfacial tension is small enough, a potential for molecular miscibility might exist [50].

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In close systems like AF and BF, cAF, VAF, UA and B uA F of AF and cBF, VBF, UB and uF of BF are constant, AF BF therefore DHm and DHm remain unchanged. If mixing the system AF with BF, we obtain a ternary system AFB consisting of two open systems AF and BF. We consider them as open systems because at the interphase the filler F can migrate between AF and BF. That means UA B and uA F as well as UB and uF will be changed to miniAF mize the values of DGm and DGBF m . The filler transfer process goes on until the thermodynamic equilibrium state is reached. A thermodynamic equilibrium state is reached if BF DGAF m ¼ DGm ;

ð5Þ

or DHmAF ¼ DHmBF :

ð6Þ

Setting Eqs. 3 and 4 into Eq. 6, the equilibrium state can be described by Eq. 7: 2 B ðcAF Þ2 VAF UA UA F ¼ ðcBF Þ VBF UB UF ;

U*A

ð7Þ

U*B

where and are the volume fractions of the phases A and B, respectively, and UA* and UB* F F are the volume fractions of the component F in the phases A and B, respectively, at the equilibrium state, V*AF and V*BF are the average molar volume of the two components, respectively, at the equilibrium state. From Eq. 7 we can get Eq. 8: UB VAF UA cAF 2 F ¼ : ð8Þ U VBF UA B cBF F For a simple case with the same volume of phase B and A the ratio V*AFU*A/V*BFU*B is considered as 1. Equation 8 can be written as follows: 2 UB uB c F F ¼ A ¼ AF : ð9Þ A cBF u UF F B uA F and uF are the weight fractions of the component F in the phases A and B, respectively. Using the Girifalco– Good equation [51] describing the relationship between surface tension and interfacial tension Eq. 9 can be written as follows:

uBF ¼ uA F

pffiffiffiffiffiffiffiffiffi c A þ cF 2 cA cF 2 pffiffiffiffiffiffiffiffiffi : cB þ cF 2 cB cF

ð10Þ

cA, cB, and cF are the surface tension values of the B phases A, B and F, respectively. Setting uA F = 1 - uF into B Eq. 10 the weight fraction uF can be calculated using Eq. 11. x uBF ¼ ð11Þ xþ1

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with pffiffiffiffiffiffiffiffiffi cA þ c F 2 cA c F 2 x¼ pffiffiffiffiffiffiffiffiffi : cB þ c F 2 cB cF

ð12Þ

Materials and experimental The polymers were acrylonitrile butadiene rubber (NBR) Perbunan 3445F (Lanxess) with a nitrile content of 34% and natural rubber (NR) SMR 10 (Standard Malaysian Rubber). The used silica was Ultrasil 7000GR (Evonik) with the specific surface area CTAB of 160 m2/g and BET of 170 m2/g. Stearic acid, zinc oxide (ZnO), N-cyclohexylbenzothiazole-2-sulfenamide (CBS), and sulfur were used as curing additives. The surface tension and Mooney viscosity values of the used materials are given in Table 1. Mixing experiments were performed by an internal mixer Plasticorder PL 2000 (Brabender). A rotor speed of 50 rpm and starting temperature of 50 °C were used for all tests. A fill factor of 0.7 was chosen for low-filled blends, while it was reduced to 0.6 for highly filled blends to keep the mass temperature below 100 °C. Two series of silicafilled 50/50 NBR/NR blends, without and with curing additives, were prepared according to Tables 2 and 3 by variation of mixing time and silica loading. The localization of different fillers in NBR/NR blends prepared according to Table 4 was also investigated. Different fillers like organoclay Nanofil 5 (Southern Clay Products), carbon black (CB) N990 and graphitized N234 (3000 °C, 10 h) (Evonik), carbon nanotubes (CNT) Baytubes 150 HP (Bayer MaterialScience), layered double hydroxide (LDH) SorbacidÒ911 (Su¨d-Chemie AG) as well as silica Ultrasil VN-3 (Evonik) were used. A NBR/NR blend filled with dried silica was also prepared. For this blend Ultrasil 7000GR was dried in an oven at 100 °C for 2 h to remove moisture from the filler before mixing. Table 1 Surface tension and Mooney viscosity of materials used Materials

Surface tension (mN/m)

Mooney viscosity MU [(ML 1?4) 100 °C]

NBR

27.2 [52]

45

NR

22 [52]

49

Ultrasil 7000GR

73 ± 7 [53]

Table 2 Formulation and mixing conditions for preparation of filled NBR/NR blends without any curing additives Mixing time (min) 0 5

Ingredient

Loading (phr)

NBR

50

NR

50

7000GR

10

Stopped and dumped at 7, 9, 12, and 15 min


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Table 3 Formulation and mixing conditions for preparation of NBR/ NR blends filled with different silica loading (with a constant loading of curing additives) Mixing time (min)

Ingredient

Loading (phr)

0

NBR

50

NR

50

Stearic acid

2

ZnO

3

CBS

2

Sulfur

1.5

5

7000GR

(Stearic acid ? ZnO ? CBS)/7000GR ra

5 or

7/5

7.5 or

7/7.5

10 or

7/10

20 or

7/20

50 (highly filled)

7/50

Stopped and dumped at 15 min a

r is the mass ratio of additives consisting of stearic acid, ZnO, and CBS to silica

Table 4 Formulation and mixing conditions for preparation of NBR/ NR blends filled with different filler (without any curing additive) Mixing time (min)

Ingredient

Loading (phr)

0

NBR

50

NR

50

Fillera

10

5 Stopped and dumped at 15 min a

Filler was Ultrasil 7000GR dried, Nanofil 5, CB N990, CB N234, CNT, Ultrasil VN-3, LDH, respectively

Table 5 Formulation and mixing conditions for preparation of silica filled NBR/NR blends with different blend ratios for determination of the calibration curve Mixing time (min) 0 5

Ingredient

Loading (phr)

NBR

100/80/60/50/40/20/0

NR

0/20/40/50/60/80/100

7000GR

50

Stopped and dumped at 25 min

To use the wetting concept a calibration curve is to be created by preparation NBR/NR blends with different blend ratios according to Table 5. The 100/0 NBR/NR and 0/100 NBR/NR compounds were extracted by toluene for determination of the rubber-layer LNBR and LNR P P , respectively. Quantification of the filler localization in low-filled NBR/NR blends For blends with filler loading below 20 phr extraction experiments were carried out to determine the filler loading

in each phase of the blends after selective extraction of one blend phase from the blend by use of a suitable solvent. For the investigation 0.2 g of each uncured mixture was stored for 24 h in 100 mL cyclohexane at 70 °C. The solution containing the soluble NR phase and silica in it was casted from the flask and the insoluble-filled NBR phase was taken out and dried up at 70 °C to a constant mass. The thermogravimetric analysis of the insoluble NBR phase was carried out by a thermobalance TGA/SDTA 851 (Mettler Toledo). The samples were heated up to 600 °C with a heating rate of 20 K/min in nitrogen atmosphere and then heated up to 800 °C in oxygen atmosphere. The remaining mass determined at 700 °C is silica and it is used for calculation of the silica loading localized in each blend phase by taking into consideration the concentration of curing additives and blend ratio. For CB or CNT-filled blends the remaining mass determined at 550 °C is the filler mass. For organoclay or LDH-filled blends the remaining mass at 700 °C is the inorganic part of the filler. By calculation of the filler localization in these blends the fraction of the organic part of fillers was taken into account. Quantification of the filler localization in highly filled NBR/NR blends For rubber blends containing more than 40 phr filler the silica localization in both phases can be quantified according to the wetting concept described in details in our previous studies [43–46]. For the investigation of the rubber-filler gel 0.2 g of each uncured mixture was stored for 7 days in 100 mL toluene at room temperature. After 4 days the solvent was completely renewed. The solution was casted from the flask and the rubber-filler gel was taken out and dried up to a constant mass. The analysis of the rubber-filler gel of the highly filled blends for determination of the rubber-layer LB(NBR) of the NBR phase and LB(NR) of the NR phase in the gel was carried out by use of a Fourier transformed infrared (FTIR) spectrometer S2000 (Perkin Elmer) equipped with a diamond single Golden Gate ATR cell (Specac). Using Eq. 13 for compounds prepared according to Table 5 the rubber-layer at the end of wetting process LNBR and LNR of NBR and NR were P P determined. m2 m1 cS L¼ : ð13Þ m2 The mass m1 corresponds to the rubber compound before extracting; it is the sum of the mass of the insoluble rubber, the mass of the soluble rubber and of silica. m2 is the mass of the rubber-filler gel, which is the sum of the insoluble rubber and the mass of silica. cS is the mass concentration of silica in the mixture.

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For creation of the calibration curve, blends with different NBR/NR ratios prepared according to Table 5 were investigated by FTIR according to the procedure described in our previous study [45]. The peaks of the NBR phase at 2237 cm-1 and of the NR phase at 1378 cm-1 were taken for calculation of the ratio of the surface under peak ANBR/ ANR. The correlation between the ANBR/ANR ratio and the given NBR/NR ratio is described by a straight line with a slope fNBR/NR = 0.89. The ratio LB(NBR)/LB(NR) in the rubber-filler gel can be determined using Eq. 14. LBðNBRÞ ðtÞ 1 ANBR ðtÞ : NR ¼ BðNRÞ fNBR=NR A ðtÞ L ðtÞ

ð14Þ

The filler fractions uNBR and uNR in 50/50 NBR/NR F F blends can be calculated using Eq. 15. uNBR ðtÞ LNR LBðNBRÞ ðtÞ F P ¼ NBR BðNRÞ : NR uF ðtÞ LP L ðtÞ

ð15Þ

Atomic force microscopy Morphological investigations were carried out by an atomic force microscope Q-Scope 250 (Quesant), operated in intermittent mode with a scan-head of 40 lm. Samples were produced by cutting in a cryo-chamber CN 30 of a rotary microtom HM 360 (Microm) with a diamond knife at -100 °C. Transmission electron microscopy Microstructure was examined using a transmission electron microscope JEM-2100 F (Jeol). Ultrathin sections of each sample (ca. 100 nm) were prepared at -100 °C from a bulk specimen using an ultramicrotome Ultracut R (Leica) with cryo-system EM FCS (Leica).

Results and discussion Prediction of the filler localization in NBR/NR blends at an equilibrium state At the equilibrium, the preferential location of the silica particles is predicted by the wettability parameter xS calculated using Eq. 16 [30, 31]: c cAF xS ¼ BF ; ð16Þ cAB where cAB is the interfacial tension between A and B. If A is NBR, B is NR, and F is silica and using the data listed in Table 1 a wettability parameter xS = -12.3 was determined. Accordingly, a complete localization of silica in the NBR phase, i.e., a weight fraction of silica in NR phase uNR F = 0 is predicted for the investigated blends.

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This prediction may be supported by the fact that the bonding of silica to NBR is more favorable than to NR. According to the study of Kralevich and Koening [54] van der Waal forces are responsible for the formation of the bound rubber in a non-polar NR. Ono et al. [55, 56] applied high-resolution solid-state NMR for NR/silica composites and found no evidence for direct coupling between silanol groups and NR molecules at low processing temperature. Ziegler et al. [40] detected hydrogen bondings between silanol groups of silica and C:N groups of NBR by means of infrared spectroscopy (FTIR). Wang et al. [57] investigated the adsorption of model substances to the silica surface using inversed gas chromatography. They found that the interaction of model substances to silica changes according to the order: NBR [ SBR [ NR [ BR [ EPR [ IIR. Using our model proposed in this study the filler localization in 50/50 NBR/NR blends can be calculated in dependence on the surface tension of the filler according to Eqs. 10–12 and presented in Fig. 1. The dependence of filler fraction in the NR phase uNR F on the surface tension c shows different behavior in three ranges I, II, and III. In the range I, at low filler surface tension far away from cNR an even localization of filler is nearly received because filler shows similar bad affinity to both blend phases. With increasing cF the filler fraction in the NR phase uNR F increases and nearly reaches the value of 1.0 when cF = cNR. In the range II, passing cNR the filler fraction uNR F decreases because the affinity of the filler to NR becomes worse and to NBR better. When cF = cNBR, a complete localization of filler in the NBR phase is nearly obtained. In the range III, with increasing cF the filler loading uNR F increases and approaches the value 0.5 (even localization) at a high filler surface tension far away from that of both blend phases. From the master curve presented above it is obvious to recognize some main features. First, a nearly even localization of filler can be received when the filler surface tension is far away from those of both blend phases (similar bad affinity of filler to both rubber phases), or lies in between them (similar good affinity of filler to both rubber phases). Second, a very strong dependence of the filler localization on the filler surface tension is obtained in the range II. A small change of filler surface tension in this range can lead to an extremely large change in filler localization. Third, according to the proposed model a localization of the filler at the interphase is not a thermodynamic equilibrium state. It is rather a result of an interplay between thermodynamic driving forces and rheological effects [34, 58–60]. Elias [34] stated that the presence of a large proportion of silica near the EVA/PP interface is due to the short mixing time. In other words, the nanoparticles do not have enough time to reach their preferred phase by Brownian motion. Furthermore, the


Fig. 1 Master curve presenting the filler fraction in the NR phase uNR F in dependence on the surface tension of filler

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Fig. 2 Kinetics of silica localization in 50/50 NBR/NR blends filled with 10 phr silica, without and with curing additives according to Tables 2 and 3

formation of a packed layer at the interface by capillary interaction cannot be a relevant process for such high viscosity emulsions [11]. Based on the surface tension data of silica given in Table 1, i.e., cF = 73 mN/m, a weight fraction of silica in NR phase uNR F = 0.34 was predicted by fitting cF = 73 mN/m to the master curve as shown by two arrows presented in Fig. 1. Kinetics of silica localization in NBR/NR blends without and with curing additives The silica fraction in NR phase uNR of NBR/NR blends F prepared without and with curing additives according to Tables 2 and 3 is presented in Fig. 2 in dependence on the mixing time. In blends without curing additives uNR F increases with mixing time and after 10 min it reaches a level-off value. At this equilibrium state a value of 0.37 was determined for uNR F , which corresponds very well with the value of 0.34 determined from Fig. 1. In NBR/NR blends with curing additives (r = 7/10) no silica was found in the NR phase for the whole mixing times. Effect of silica/curing additives ratio r on the filler localization To understand the effect of curing additives on the localization behavior of silica, NBR/NR blends filled with different silica loading were prepared by keeping constant the loading of curing additives according to Table 3. The silica fraction in NR phase uNR of low-filled blends (silica F loading up to 20 phr) was experimentally determined by means of the selective extraction method and of highly

Fig. 3 Silica fraction in NR phase uNR in dependence on the ratio F r and silica loading

filled blends (50 phr silica) using the wetting concept. In Fig. 3, the silica fraction in NR phase uNR F is presented in dependence on the ratio r and silica loading. Without curing additives uNR F = 0.37 was determined as discussed above. With increasing ratio r, i.e., with decreasing silica loading the silica fraction uNR F decreases and reaches zero at r = 7/10. Passing this value the silica fraction uNR F strongly increases and reaches a value of 0.5 when r = 7/7.5 and 1.0 when r = 7/5. The morphological investigation of NBR/NR filled with different silica loading was carried out by AFM and TEM. The images of blends with different ratio r are presented in Fig. 4 and that support very well the silica localization determined by the selective extraction experiments shown in Fig. 3.

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Fig. 4 TEM and AFM images of blends with different ratio r

The change of filler localization behavior in rubber blends by variation of filler loading has been also frequently reported in literature [22, 27, 29]. Phewphong [27] observed a significant influence of the silica loading on the filler localization in chlorinated polyethylene (CPE)/NR blends. He stated that the counter-balancing effects of relatively low viscosity of the NR phase and strong silica– CPE interaction, which are changed with increasing silica loading, are responsible for the change of the silica localization. Maiti et al. [22] found when the silica content was increased from 10 to 40 phr in NR/ENR blends, the weight fraction of silica in the ENR phase decreased. In that case, the authors explained that at the lower levels of filler loading, ENR accumulated more silica than NR. As the filler loading increased, ENR gradually became saturated and silica slowly migrated to the NR phase. It is obvious to recognize that the wettability parameter xS could not used for explanation of the strong dependence of filler localization on filler loading as presented in Fig. 3 and in the studies [22, 27, 29] mentioned above. We believe, this behavior is related to the fact that the adsorption of curing additives like stearic acid, ZnO, and CBS on the surface of silica will make it more hydrophobic. The adsorption of them and its impact on the scorch time and reduction of the crosslink density in silica-filled rubber compounds have been frequently characterized [61]. Regarding the effect on the filler localization with increasing silica loading by keeping constant the additive concentration, the surface of silica will be differently modified that leads to change of the affinity of silica to the blend phases and consequently to change of silica Fig. 5 Silica fraction in NR phase uNR F in dependence on the filler surface tension (a), filler surface tension in dependence on the ratio r and silica loading (b)

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localization behavior. The use of stearic acid as a modifier for silica and other fillers like CaCO3 and Mg(OH)2 was reported [62–66]. The authors found that the presence of adsorbed stearic acid on the filler surface reduces the hydrophilicity of silica surface and enhances the compatibility between filler and matrix that leads to an improvement of filler dispersion and related mechanical performance of composites. Kosmalska [67] also investigated the adsorption of DPG, ZnO, and sulfur on the silica surface and reported that the bonding of DPG/ZnO and ZnO to silica causes a reduction of surface energy of silica from 66 to 28.75 mN/m and 35.49 mN/m, respectively. The similar effect of ZnO on the surface tension of silica was also found by Laning [68] and Reuvekamp [69]. Quantification of the filler surface tension changed during the mixing process The silica surface tension, which was changed by adsorption of curing additives, can be determined by fitting the values of uNR F observed from Fig. 3 to the master curve as shown in Fig. 5a. In Fig. 5b, the observed surface tension data is presented in dependence on the ratio r. With increasing ratio r the silica surface tension decays first strongly and then slowly approaches a level-off value when silica surface is saturated. The polar part of additives react with OH groups of silica and the non-polar part will shield the silica surface and determine the hydrophobicity of silica. Thus, the surface tension of saturated silica agglomerates is csat, which is considered as the surface tension of the non-polar part of additives. The decrease of cF with r


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follows an exponential decay function and can be empirically described by Eq. 17: r

cF ðrÞ ¼ csat þ ðcF csat Þ ea ;

ð17Þ

where a is a factor describing the effectiveness of additives with respect to the reduction of the filler surface tension. Fitting Eq. 17 to the data presented in Fig. 5b by setting surface tension value of silica cF = 73 mN/m for blends without additives (r = 0), we got Eq. 18: r

cF ðrÞ ¼ 22:4 þ 50:6 e0:28 : ð18Þ If silica surface is fully covered by additives a value of 22.4 mN/m is observed for csat. Investigation of the effect of each additive on the silica surface tension was not conducted in the frame of this study. Taking into consideration the change of silica surface tension as shown in Fig. 5b the wettability parameter xS can be correspondingly recalculated and presented in dependence on the ratio r in Fig. 6. When the value of r is below 0.7, the wettability parameter is still smaller than -1 and it indicates a complete silica localization in the NBR phase. Above 0.7 a localization of silica at the interphase is expected. However, comparing the prediction based on xS with the experimentally determined silica localization presented in Fig. 3 it is clear to see the failure of Eq. 16. Quantification of the effect of silane on the silica localization Surface energies of silica have a low dispersive component and a high specific component when compared to CB with equivalent surface area and structure [58]. The low dispersive component of the surface free energy of silica is related to weaker polymer–filler interaction and the higher

specific component results in strong filler–filler interaction. The surface characteristics of silica can be changed by surface modification, for example, by chemical modification of silica with so-called coupling systems such as a polyfunctional organosilane. The specific component of the surface free energy (csp s ) is significantly reduced, leading to improved interaction between silica and rubber for improved compatibility [58]. A reduction of filler–filler interaction results in better dispersion and reduced viscosity. For the quantification of the effect of silane on the silica localization in rubber blends we used the data presented in the study of Ziegler et al. [40]. Silica 7000GR was pretreated by silane before mixing into 50/50 NBR/BR blends. Three types of silane used were triethoxy-propylsilane (Si2O3), triethoxy-octylsilane (Si2O8), and bis-triethoxypropyl disulfide (Si75). After preparation of blends the silica localization was determined by means of a method based on DMA data. The sample name, the filler treated and the silica fraction in the BR phase uBR F are given in Table 6. The effect of silane treatment on the silica localization can be quantified by fitting the filler fraction in BR phase uBR F to the master curve as shown in Fig. 7. It is clearly seen that the surface tension of silica decreases with silica treatment differently depending on the silane type and amount. The surface tension of the modified silica is determined from Fig. 7 and presented in Table 6. By a moderate silane treatment, when silica surface tension is decreased but still higher than that of the NBR phase, a preferred localization of silica in the NBR phase is expected. By a strong treatment, the silica surface tension falls below that of the NBR phase. Silica transfers strongly from the NBR phase to the NR phase, when silica surface tension decreases slightly from 24.7 mN/m (blend B) to 24.1 mN/m (blend D and E). As discussed above, it is difficult to control the filler localization when the filler Table 6 Silica treated with silane and its localization in NBR/BR blends

Fig. 6 Wettability parameter xS in dependence on the ratio r and silica loading

Blend

Treated fillera

A

7000GR ? additives

0.27

57

B

7000GR ? additives ? (2/3) Si2O3

0.38

24.7

C

7000GR ? additives ? Si2O3

0.57

24.4

D

7000GR ? additives ? Si75

0.72

24.2

E

7000GR ? additives ? Si2O8

0.74

24.1

a

Data obtained from [40]

b

Data obtained from Fig. 6

Silica fraction in BR phasea uBR F

Silica surface tensionb cF (mN/m)

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J Mater Sci (2012) 47:4270–4281

Fig. 7 Filler fraction in BR phase uBR of NBR/BR blends in F dependence on the silane treatment of silica (localization data obtained from the study of Ziegler et al. [40], cBR = 22.1 mN/m [52]

surface tension varied in the range within those of both blend phases. In a study made by Castellano [70], the influence of the chemical structure of a triethoxysilane (TES), octadecyltriethoxysilane (ODTES), and bistriethoxysilylpropyltetrasulfane (TESPT) was investigated by inverse gas chromatography (IGC) at infinite dilution. Thermodynamic results indicate a higher polarity of the silica surface modified with TES as compared to that of the unmodified silica due to new OH groups deriving from the hydrolysis of ethoxy groups of the silane. A grafting degree with 4.6 wt% of ODTES is enough to obtain a silica surface tension of 38.6 mN/m, similar to those of hexadecanol-modified silica (34 mN/m) [71] and of polyethylene (24–42 mN/m) [72, 73]. The corresponding value for TESPT (Si69) modified silica is 58 mN/m; this suggests that the long alkyl chains of ODTES may form a shielding layer, leading to a lowpolarity surface.

Fig. 8 Filler fraction in NR phase uNR F of NBR/NR blends filled with different fillers in dependence on the filler surface tension

105 mN/m, while Nanofil 5 shows its hydrophobicity with a surface tension of 25.2 mN/m far away from that of unmodified Na?-clay after cation exchange process with surfactant. Surface tension of CB N990 and N234 as well as CNT nearly lies in the same range from 25 to 33 mN/m. The values of filler surface tension determined from Fig. 8 are presented in Fig. 9 in comparison with those obtained from literature. Authors and references for the obtained surface tension values are given in Table 7. The values determined from this study lie clearly in the same order with those reported in literature as shown in Fig. 9. Depending on the methods used and corresponding sample preparation procedure different values were obtained for the same investigated filler. For instance, Stoeckelhuber et al. [52] determined the surface tension values of a series of fillers and rubber polymers by fitting of Fowkes’ equation out of the advancing, receding and the

Localization of different fillers in NBR/NR blends According to Table 4 NBR/NR blends were prepared with 10 phr of different fillers without any additives. The silica localization determined by means of the selective extraction method is fitted to the master curve as seen in Fig. 8 to obtain the filler surface tension. After drying the surface tension of silica 7000GR decreases from 73 to 59 mN/m that results in a decrease of uNR F from 0.37 to 0.34. Park et al. [74] also found that after thermal treatment moisture was removed and the condensation of surface hydroxyls took place to form siloxane bonds. The elimination of active sites due to condensation of surface hydroxyls produces an important decreasing of the polar component (csp s ) that makes silica more hydrophobic in nature. The LDH shows a high surface tension of

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Fig. 9 Surface tension of different fillers determined from Fig. 7 in comparison to those obtained from literature

Author's personal copy J Mater Sci (2012) 47:4270–4281 Table 7 References for surface tension of different fillers related to Fig. 8 Column

Author and reference

d

Stockelhuber [52], method 1

e


f


g

Determined from the proposed model

h

Park [74]

i

Khayeta [75] Jo¨nsson [53]

k l

4279

circle formed by the arc of the liquid interface contacting the plate to aid placement of the tangent. The optical cell, a Wilhelmy plate, a motorized sample holder and a Cahn 2000 recording microbalance were used for the instrumental method III. The contact angle measured at the chloroform/water interface using method I was 51.8 ± 8.23. The same material measured using method II was 44.9 ± 11.8 and using method III it was 38.9 ± 0.00. Thus, an uncertainty in determination of data of surface tension of blend components can be an obstacle for a feasible prediction procedure.

Elias [34]

Transfer of filler from a thermodynamic nonequilibrium to an equilibrium state mean values of the contact angles, measured by the Wilhelmy method. The obtained values are different from each other as seen in Fig. 9. Milmana et al. [76] compared the accuracy and precision of interfacial contact angle measurements estimated by three methods. Method I involved subjective determination of the position of the tangent. Method II employed an additional step of completing the

Fig. 10 Nanoclay transfer from a non-equilibrium to equilibrium state (data obtained from Ref. [42])

If filler was pre-distributed too much in one phase of a binary rubber blend, i.e., the filled blend is in a thermodynamic non-equilibrium state, a transfer process of the filler will take place within two blend phases during mixing until the equilibrium state is reached. As an example, a transfer of nanoclay in 50/50 NBR/NR blends was observed in our previous study [42]. Nanoclay was first premixed in the NR phase and then in the subsequent mixing process the nanoclay-filled NR mixture was mixed with fresh NBR. Along the mixing time samples were taken out for investigation of filler localization by means of AFM and selective extraction. Using the master curve of localization we can determine whether a filler transfer process in a certain blends takes place and when it stops. The filler fraction in the NR phase uNR determined F experimentally from this study [42] is presented in Fig. 10 with respect to the master curve of filler localization. At beginning of the blending of nanoclay-filled NR masterbatch with the fresh NBR phase the filler fraction uNR is F equal 1. According to the master curve, a filler fraction uNR F = 0.25 was predicted for an equilibrium state. As a result, a transfer process of nanoclay from the NR phase to NBR phase took place within a mixing period of 40 min until the equilibrium value of uNR F is reached. AFM images presented in Fig. 11 show clearly the transfer of nanoclay particle (black domains) from the NR to NBR phase in dependence on mixing time.

Fig. 11 AFM images presenting nanoclay transfer from the NR to NBR phase in dependence on the mixing time

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Conclusions A model based on thermodynamic data was proposed for a quantitative prediction of filler localization in NBR/NR blends. Using this model, a master curve presenting a characteristic dependence of filler localization on the surface tension values of filler and blend components was created. The obtained effect of filler loading on the silica localization is sufficiently explained by this model by taking into consideration the deactivation of the silanol groups on the silica surface by adsorbed curing additives. Using the master curve the surface tension of filler affected by addition of curing additives and silane can be estimated, that may be useful for evaluation and comparison of the effect of different coupling agents. Surface tension values of different fillers were also estimated by means of the master curve and they lie in the same order compared to those reported in literature that supports the reliability of the proposed method. A potential transfer of filler within a rubber blends can be quantitatively predicted. Acknowledgements The authors wish to thank the German Research Foundation (DFG) for the financial support of this study and Prof. W. Focke (University of Pretoria, South Africa) for TEM images.

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