Bentonite as a reinforcing and compatibilizing filler for natural rubber

Bentonite as a Reinforcing and Compatibilizing Filler for Natural Rubber and Polystyrene Blends in Latex Stage

Jareerat Ruamcharoen,1 Tanakorn Ratana,2 Polphat Ruamcharoen3 1 Department of Science, Faculty of Science and Technology, Prince of Songkla University, Muang, Pattani, 94000 Thailand 2

Department of Industrial Chemistry, Faculty of Applied Science, King Moungkut Institute of Technology North Bangkok, Bang Sue, Bangkok, 10600 Thailand

3

Rubber and Polymer Technology Program, Faculty of Science and Technology, Songkhla Rajabhat University, Songkhla, 90000 Thailand

Bentonite clay was used as a reinforcing and compatibilizing filler for natural rubber/polystyrene (NR/PS) blend via latex blending process. The reinforcing and compatibilizing performance of bentonite clay in the NR/PS blends were evaluated. The improvement of the mechanical properties of NR/PS blends with the weight ratios of 90/10, 80/20, and 70/30 was found with the addition of 3 and 5 parts per hundred rubber (phr) clay. The characterization by using Fourier transform infrared spectroscopy and X-ray diffraction (XRD) gave the evidence that the silicate layer was intercalated by NR and PS molecular chains. The morphology of tensile fracture surface by scanning electron microscope showed the separated phase boundaries of PS and NR blend and gradual disappearance with the bentonite content. This could be implied that the bentonite contributes to the compatibilization between PS and NR. The compatibilization action of the bentonite clay was also reflected by the shift of glass transition temperature (Tg) of NR to higher temperatures than those of the blends. These results suggested that the tensile and tear properties of the blends were controlled by compatibility between NR and PS. The most enhanced properties of blends were found with the addition of 3 phr bentonite clay. POLYM. ENG. SCI., C 2013 Society of Plastics Engineers 54:1436–1443, 2014. V

INTRODUCTION Natural rubber (NR) is a polymeric material that naturally occurs as a milky colloidal or latex and possesses Correspondence to: Jareerat Ruamcharoen; e-mail: [email protected]. psu.ac.th Contract grant sponsor: Thailand Research Fund (TRF). DOI 10.1002/pen.23665 Published online in Wiley Online Library (wileyonlinelibrary.com). C 2013 Society of Plastics Engineers V

many interesting physical properties owing to its ability to crystallize under strain, high tensile strength, resilience, excellent flexibility, and resistance to impact and tear [1–3]. However, NR exhibits low tensile modulus, thermal, and oxidation stabilities and a high dependence of dynamic properties on temperature because of damping derived from the high glass transition temperature. Therefore, attempts to overcome these limitations have been done by several methods i.e. vulcanization process development, addition of filler, chemical modification, and blending with other polymers [4–9]. Blending of polymers has been an active field of research as it leads to new polymeric materials having a better balance of properties than obtaining with a neat polymer. An elastomer is generally mixed to another one for three main purposes: to improve the properties of the original material, to improve processability and/or to be lower cost. The resulting blend will be composed of distinct high molecular weight polymers with different chemical structures and will, in consequence, be partially or completely incompatible [10]. Recently, clays such as kaolinite, montmorillonite, bentonite etc., have been extensively used as fillers for polymers in many years. The main objectives are the modification of certain characteristics of polymers such as stiffness, electrical insulation, thermal stability, barrier properties, etc [11–15]. Several researches had reported the study of synthetic and NR/organoclay composites crosslinked by sulfur or peroxide system [15–20]. Arroyo et al. [17] revealed that the blend of 10 parts per hundred rubber (phr) modified montmorillonite with NR gave the similar mechanical properties to the rubber compound with 40 phr of carbon back. Varghese and Karger-Kocsis POLYMER ENGINEERING AND SCIENCE—2014

[18] studied NR/clay composites, which layered silicate as a reinforcing agent. Layered silicate clays filled NR vulcanizates exhibited a great increase in modulus and tensile strength especially in the case of fluorohectorite. Teh et al. [20] have already reported the most significant advances happening in the efficiency of epoxidized natural rubber (ENR) as a compatibilizer for NR/organoclay nanocomposites. This was reflected in the improvement of mechanical and physical properties of nanocomposites. In addition, it has been known that the incorporation of fillers into a binary polymer immiscible blend can affect the phase morphology due to the interaction of the individual components of the blend with the solid surface. Recently, trends of reports have mentioned the utilization of inorganic fillers for the purpose of reinforcement and compatibilization [21–27]. An organically modified clay was applied to use as an emulsifier for the immiscible polystyrene (PS)/poly(ethyl methacrylate) pair of polymer [21]. The size of the PS domains in the blends decreased with the increase in the amount of organoclay. The use of montmorillonite as a reinforcing and compatibilizing filler for acrylonitrile-butadiene rubber/styrene-butadiene rubber blend has been demonstrated [23]. The nanocomposites based on NR/ENR blends have already mentioned [24]. It has been deduced that the properties of the compounds strongly depend on the extent of the silicate nanolayer dispersion as well as on the organoclay type and elastomer compatibility. Moreover, it was found the decrease in the size of the dispersed ENR phase droplets. This suggested that the organoclays might behave as compatibilizers, which reduce the interfacial tension between both polymer phases. Polymer/clay nanocomposites can be carried out by several methods, including in situ polymerization accompanied by intercalation, intercalation from solution or melts or even emulsion. Comparing with other methods, latex stage is a promising method for industrialization due to the simplicity of preparation, low cost and superior performance [28]. Surprisingly, this method makes dispersion better of clay in latex matrix. A few works have been done on clay reinforced and compatibilized in the vulcanized latex blend. Therefore, this work focused on the improvement of physical and mechanical properties of NR by blending with PS via latex compounding. The bentonite clay was expected to play an important role in reinforcing and compatibilizing filler. EXPERIMENTAL Materials The bentonite clay with a cationic exchange capacity of 93 mequiv/100 g was obtained from Aumarin Clay Factory, (Thailand). NR used in the present study was commercial high ammonia NR latex which purchased from Chalong Latex Industry (Thailand). The PS latex containing 30% total solid content was synthesized by DOI 10.1002/pen

TABLE 1.

Formulation of NR and NR/PS latex blending.

Ingredients 30% 20% 20% 50% 50% 50%

Dry content (parts by weight)

NR latex PS latex Bentonite dispersion Sulfur dispersion ZDC dispersiona Zinc oxide

100 0 1.50 1.00 1.8

90 10 0, 3, 1.35 0.90 1.62

80 20 and 5 1.20 0.80 1.44

70 30 1.05 0.70 1.26

a

Zinc diethyl dithiocarbamate (ZDC).

emulsion polymerization as described in polymer synthesis and characterization [29]. The viscosity average molecular weights of NR and PS were 7.79 3 105 and 2.23 3 105, and Tg of NR and PS investigated by DSC were 264.0 and 105.3 C, respectively. Blend Preparation The prevulcanized NR latex was mixed with the aqueous suspension of bentonite clay (B) with 3 and 5 phr and other ingredients listed in Table 1 under slow speed stirring. It should be noted that the bentonite content which was more than 5 phr gave the coagulation of NR/ PS blends. The NR/PS/B latex was stirred until a homogeneous mixture obtained. The compounded NR/PS/B latex was then matured making films to uniform properties. The dirt and coarse particles were removed by filtering through a sieve. The latex compound was cast in a mold built of glass plates (dimensions: 150 3 150 3 2 mm3) and then vulcanized at 70 C for 6 h. Fully vulcanized blend samples were then cooled and packed in sealed polyethylene bags for testing. Latex blends with various NR/PS ratios (i.e. 90/10, 80/20, and 70/30) with and without bentonite were produced in a similar way as described above. Mechanical Properties Tensile tests, to investigate the ultimate properties (strength, elongation), along with the moduli at selected elongation were performed at room temperature on dumbbell shaped specimens according to ASTM D412 on a Hounsfield universal testing machine at a crosshead speed of 500 mm/min. The tear strength was measured according to ASTM D624 by using crescent-shaped specimens at a crosshead speed of 500 mm/min. At least five specimens of each sample were tested and the average of the values was taken. Swelling Measurement The swelling behavior of samples was determined according to ASTM D471 and reported in terms of changes in weight after immersion in toluene for 48 h. POLYMER ENGINEERING AND SCIENCE—2014 1437

sample of 0.01 g was put into an aluminum pan and sealed properly with an aluminum cover. The temperature was raised from 2100 to 150 C at the scanning rate of 20 C/min then cooled down at the rate of 10 C/min to 2100 C and heated up again to 150 C at the scanning rate of 10 C/min. The glass transition temperatures of specimens symbolized as Tg, were determined. The storage modulus (Eo), loss modulus (E0 ), and the mechanical loss factor (tand 5 E0 /Eo) as a function of temperature (T), were assessed by dynamic mechanical thermal analysis (Rheometric Scientific DMTA V). DMTA spectra were taken in tension mode at 10 Hz frequency in a temperature range (T 5 2100 to 1100 C). FIG. 1. Stress-strain curves of NR, NR80/PS20, and NR80/PS20/B3.

The blend samples were removed from the test bottles, the adhering solvent was cleaned from the surface, and the blend samples were immediately weighed. The swollen weights of the samples were recorded for the determination of the percent swelling. Organoclay Characterization and Morphology Study To obtain information of the interaction between bentonite clay and blend samples, Fourier transform infrared spectroscopy was also conducted in a Nicolet Avatar TM 360 equipment. Thin film of NR/PS blends was taken at 16 scans in the range of 4000–400 cm21 with 4 cm21 resolution of spectra. The bentonite clay was dispersed in a dry KBr powder and then the mixture was ground into fine particles. Thin KBr pellet was prepared by pressing the mixture and the spectrum was recorded in transmittance procedure. To investigate the role of bentonite clay in the NR/PS blends, the techniques of X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed. XRD was used to characterize the nature and extent of clay dispersion in filled blend samples. XRD patterns were obtained on a Philips diffractometer using Cu-Ka radiation with wavelength equal to 0.154 nm. Typical power settings were 30 mA and 40 kV. The samples were scanned in the step mode at a scanning rate of 0.5 degree/min from 2h 5 2 210 . Bragg’s law was used to compute the basal spacing (d) of the bentonite clay. The fracture surface of the blend samples was studied in Leo1455 VP scanning electron microscopy. The specimens were coated with Au-Pd in a sputter to prevent electrostatic charging during observation. SEM measurement was performed with applying an acceleration voltage of 10 keV. Thermal Analysis Thermal analysis was carried out with a differential scanning calorimeter (Perkin-Elmer DSC 7). The blend 1438 POLYMER ENGINEERING AND SCIENCE—2014

RESULTS AND DISCUSSION Mechanical Properties The mechanical properties studied in this report are tensile and tear properties. The tensile properties are given in terms of the moduli at different strains (100, 300, and 500%) and the tensile strength. The stress-strain curves of the NR, NR80/PS20, and NR80/PS20/B3 are presented in Fig. 1. It can be clearly seen that the stresses at all strains are highest for the NR/PS blend with the addition of 3 phr bentonite clay (NR80/PS20/B3). The high stresses may be due to the reinforcement of silicate layers of the bentonite clay. This could be discussed later in the next section. Considering the moduli of NR and NR/PS blends (90/ 10, 80/20, and 70/30) with and without addition of 3 and 5 phr bentonite clay in Fig. 2a–c, the moduli of the blends increase when the PS content increases. This is possibly due to the thermoplastic nature (high modulus of PS) in the NR/PS blend [9]. As expected, the silicate reinforced systems prepared by mixing bentonite clay in NR/ PS latex show a superior modulus relative to ordinary rubber and NR/PS blends. It is obvious that even with the addition of bentonite loading (3 and 5 phr), the tensile moduli increase considerably. The effect of the bentonite clay on the stiffness of the NR/PS blend is more evident when measured at higher strain. The moduli at 100, 300, and 500% of the NR70/PS30 containing 3 phr of clay are 1.52, 4.72, and 9.70 MPa, respectively. This corresponds to 153, 275, and 330% higher than that of the NR. The optimum tensile strength of the blends is achieved at 3 phr of bentonite clay. This result indicates that the dispersion of bentonite clay determines an efficient reinforcement effecting of these inorganic fillers even at low content (3 phr), which leads to improve stiffness and strength. The improved mechanical properties can be explained by the interactions at the phase boundaries upon incorporating the filler, which plays an influential role in causing compatibilization at a molecular level [17]. It has already been demonstrated that the tear behavior of NR/PS blend can be significantly improved by DOI 10.1002/pen

FIG. 2. Tensile properties of NR, NR/PS, and NR/PS/B: (a) 100% modulus, (b) 300% modulus, (c) 500% modulus, and (d) tensile strength.

bentonite clay as illustrated in Fig. 3. Increment of above 200% in the tear strength of NR/PS blends with the addition of 3 phr of bentonite clay is obtained. It could be due to the reinforcing and compatibilizing effect on the blends. The tear strength decreases with the increasing of bentonite clay up to 5 phr. The decrease of mechanical properties of the blends with 5 phr of bentonite clay is due to the agglomeration of bentonite clay. The agglomerated particle is the weak area under the applied load. Mohan et al. [30] reported that the tear strength was found to be increased with 2% wt nanoclay and then decreased at 3% wt nano-

clay content reinforced in NR-styrene butadiene rubber. This result also suggested that the agglomeration of clay is an inevitable phenomenon in the composites at higher organoclay content and causes reduced properties irrespective of processing conditions. This will be discussed later in the morphological study section.

FIG. 3. Tear strength of NR, NR/PS, and NR/PS/B.

FIG. 4. Swelling behavior of NR, NR/PS, and NR/PS/B.

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Swelling Behavior It can also be observed in Fig. 4 that incorporating the filler into the blend causes a decrease in the equilibrium

POLYMER ENGINEERING AND SCIENCE—2014 1439

FIG. 5. FTIR spectra of (a) bentonite, NR80/PS20, NR80/PS20/B3, and NR80/PS20/B5 and (b) bentonite, NR70/PS30, NR70/PS30/B3, and NR70/PS30/B5.

swelling in toluene. This proves that the bentonite helps increase the polarity of the overall system due to the presence of hydroxyl groups on its surface. This leads to decrease the interaction with toluene. Additional evidence for this explanation is the reincrease in the equilibrium swelling in toluene with 5 phr loading of clay. This may be due to the agglomeration of the bentonite filler.

vibration at 470 cm21 shifts to 460 and 462 cm21, respectively, due to the fact that the polymer is capable of intercalating [32]. By observing the infrared spectra NR70/PS30 and composites (Fig. 5b), it should be noted that the absorption bands of clay in the NR70/PS30 samples also shift to lower wavenumber. This is similar to the NR80/PS20 blend ratio. This means that there is a rather good intercalation of silicate layer in the blend. The evidence of intercalation of both NR and NR/PS into the silicate layers of bentonite clay is also confirmed by XRD as shown in Fig. 6. The bentonite (B) spectrum exhibits a broad peak at higher angles (2h 5 6.2 ), which corresponds to the interlayer spacing (1.43 nm) of the pristine clay [24]. The interlayer spacing of bentonite clay increases when clay is introduced in the NR and NR/PS blends. The XRD spectrum of NR composite containing 3 phr of clay (NR/B3) shows two characteristic peaks 2h equal to 4.5 and 2.8 , which correspond to the interlayer distances of 1.96 and 3.27 nm, respectively. This suggests that NR chains intercalated into the clay with the gallery height of 3.27 nm. It is important to mention that nanocomposites with an intercalated structure have also been obtained with NR/PS blends containing 3 phr of clay. The result is shown that the interlayer distance of the layered silicate of bentonite increases from 1.43 to 1.84 nm and 3.68 nm, respectively, corresponding to NR90/PS10 blend. The typical characteristics of NR80/PS20 and NR70/PS30 adding 3 phr of clay is similar to NR90/PS10 with amount of clay 3 phr. The XRD patterns are characterized by the absence of the 001 diffraction peak, providing a strong evidence of the insertion of the polymer chain into the silicate galleries [18, 30]. Figure 7 shows SEM micrographs of the tensile failure surface of the NR, NR/PS blends introduced bentonite clay, respectively. Considering the tensile mechanical data in Fig. 2 and having a look at the fracture surfaces in Fig. 7, it seems that the rougher the fracture surface is

Dispersion Characterization Interesting information of molecular structure can be derived from the Fourier transform infrared spectroscopy (FTIR) analysis. A few attempts to characterize polymer/ clay composites by using FTIR spectroscopy have already been reported [31, 32]. Difference between the spectra of unfilled and filled NR/PS blends are seen in Fig. 5. In Fig. 5, the IR spectrum of bentonite clay reveals mainly two bands corresponding to the SiAO stretching vibration at the 1049 cm21, and the SiAO bending vibration at 470 cm21 [33]. As presented in IR spectra of NR/PS containing bentonite clay, the SiAO stretching vibration, at 1049 cm21 in the case of the clay system, shifts to 1033 and 1039 cm21, respectively. Moreover, the SiAO bending 1440 POLYMER ENGINEERING AND SCIENCE—2014

FIG. 6. XRD patterns of bentonite, NR/B3, NR90/PS10/B3, NR80/ PS20/B3, and NR70/PS30/B3.

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FIG. 7. SEM micrographs of (a) NR, (b) NR/B3, (c) NR/B5, (d) NR80/PS20, (e) NR80/PS20/B3, and (f) NR80/PS20/B5.

FIG. 8. DSC thermograms of (a) NR, NR90/PS10, NR90/PS10/B3, and NR90/PS10/B5, (b) NR, NR80/ PS20, NR80/PS20/B3, and NR80/PS20/B5, and (c) NR, NR70/PS30, NR70/PS30/B3, and NR70/PS30/B5.

DOI 10.1002/pen


agglomeration of bentonite clay particles can also be seen in Fig. 7c and f. The agglomerating domains of bentonite clay are highlight in white circle. This agglomeration of particles may reduce the tensile and tear strength of polymer blend nanocomposites [30]. Thermal Analysis

FIG. 9. Effect of bentonite clay on the storage modulus (a), and tan d (b) versus temperature for NR80/PS20 blend.

the better the mechanical properties of the related nanocomposite are. The fracture surface of NR in Fig. 7a is fairly smooth. From SEM micrographs shown in Fig. 7b and c, the NR containing 3 and 5 phr of bentonite clay displays very coarse phase morphology. It is well known that the physico-mechanical properties of the original components NR and PS are very weak in the absence of a compatibilizer or reinforcing filler, and consequently the physicomechanical properties of their blends are also inferior and this originates principally from the incompatibility of both components with each other. A smooth fracture surface usually indicates for low compatibility accompanied with premature, rather brittle type fracture [32]. This is the case for the uncompatibilized NR/PS blend (Fig. 7d). Figure 7e and f substantiates that the incorporation of 3 and 5 phr bentonite clay strongly affects the morphology and thus also the fracture behavior. The separated phase boundaries of PS and NR blend are clearly seen and gradually disappear with the bentonite content. This suggests that the organoclay may act as an interfacial agent, reducing the interfacial tension with a concomitant breakup of the PS phases and, in consequence, a reduction in the particle size [22, 27, 34]. It could be implied that the bentonite clay contributes to the compatibilization between PS and NR. However, some 1442 POLYMER ENGINEERING AND SCIENCE—2014

To further investigate the interfacial interactions in NR/PS blends, DSC measurements of NR were performed, and thermograms at Tgs are shown in Fig. 8. The glass transition temperatures of vulcanized NR and NR90/ PS10 blend are 264.0 and 263.8 C, respectively. After loading of bentonite clay in NR/PS blends, the glass transition temperatures of NR in NR/PS blends unfilled and filled bentonite clay of 3 phr, increase to the higher temperature, that is, 262.5 C. However, the increase of bentonite clay content from 3 to 5 phr results in the lower Tg. The similar results are obtained with other blend compositions as seen in Fig. 8b and c. This result indicates that bentonite clay in the blends acts as a compatibilizer. The similar results have been reported in other studies [21]. In addition, it is noticed that the glass transition regions of NR in the blends are wider than that of neat NR. The result can be ascribed to the increase in the amount of interaction between the interface of NR and PS due to increasing bentonite clay content. However, the blends do not show any Tgs of PS in the temperature range of 90–110 C. This phenomenon is not expected because of phase separation observed in the above SEM photographs [35, 36]. The dynamic storage modulus (Eo) and mechanical loss factor (tan d) versus temperature traces for the NR80/PS20 blend filled bentonite clay are shown in Fig. 9. The Eo above the glass transition temperature (Tg) is the highest for the blend with 3 phr bentonite clay (Fig. 9a). A secondary peak is observed for the NR80/PS20 with bentonite clay (see Fig. 9b). This might be attributed to Tgs of PS. These Tgs tend to decrease with bentonite content. Hence, it could be suggested that the bentonite clay is a compatibilizer of PS/NR blends. CONCLUSIONS On the basis of results and discussion previously, the following conclusion may be drawn. The blends of NR/ PS with bentonite clay as a reinforcing and compatibilizing filler can be performed in latex blending process. Tensile and tear properties of NR/PS blends with the addition of 3 phr are improved comparing with unfilled bentonite clay. In addition, the resistance to swelling in toluene becomes higher. The FTIR and XRD characterization reveals the evidence that the layered silicate is intercalated by NR and PS. According to SEM micrographs, it was indicated that bentonite clay with 3 phr helps improve the interfacial adhesion. The compatibilization action of the bentonite clay was also reflected by the DOI 10.1002/pen

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