Natural Rubber Blended with Polystyrene Nanoparticles Prepared by Differential Microemulsion Polymerization

Natural Rubber Blended with Polystyrene Nanoparticles Prepared by Differential Microemulsion Polymerization SAOWAROJ CHUAYJULJIT 1,2, * AND ANYAPORN BOONMAHITHISUD 1,2 1 Department of Materials Science, Faculty of Science Chulalongkorn University, Bangkok 10330, Thailand 2 National Center of Excellence for Petroleum, Petrochemicals and Advanced Materials, Chulalongkorn University Bangkok 10330, Thailand ABSTRACT: Blends of natural rubber (NR) and polystyrene (PS) nanoparticles at 3 9 phr by latex compounding were investigated for their tensile properties, dynamic mechanical behaviors, and flammability. The nanolatex of PS was synthesized via differential microemulsion polymerization. The properties of NR were improved as a result of the incorporation of PS nanoparticles at 3 phr for tensile properties except the % elongation at break, and up to 9 phr for flammability. The results from dynamic mechanical analyzer showed that the elastic properties of NR near the glass transition temperature increased when the amount of PS nanoparticles was over 3 phr. KEY WORDS: natural rubber, PS nanoparticles, differential microemulsion polymerization, tensile properties, dynamic mechanical behaviors. INTRODUCTION NATURAL RUBBER (NR) is a renewable elastic hydrocarbon polymer that naturally occurs as a milky colloidal suspension or latex and *Author to whom correspondence should be addressed. E-mail: saowaroj.c@chula.ac.th JOURNAL OF ELASTOMERS AND PLASTICS Vol. 42 July 2010 375 0095-2443/10/04 0375 13 $10.00/0 DOI: 10.1177/0095244310371360 ß The Author(s), 2010. Reprints and permissions: http://www.sagepub.co.uk/journalspermissions.nav

376 S. CHUAYJULJIT AND A. BOONMAHITHISUD possesses many interesting properties including low cost, low heat build up, high resilience, and good formability [1]. In commercial scale production, the addition of reinforcing fillers into the NR is required to obtain the appropriate properties for a variety of applications [2]. Traditional reinforcing fillers, for example, carbon black, ultra-fine calcium carbonate, montmorillonite, and silica have been used successfully in dry NR, but they are not so effective for NR latex [3,4]. Consequently, a number of attempts have been explored in order to enhance the mechanical properties of the products processed from NR latex. In recent years, a great deal of research has been devoted to nanostructured materials. Nanosized polymer particles (especially with diameters 550 nm) are speculated to be candidates for various applications such as medical, semiconductor, catalyst, and nanocomposites [5,6]. Microemulsion polymerization has been known as an effective process that can produce polymer nanolatex particles with diameters as small as 10 50 nm [7]. However, microemulsion polymerization receives little attention for preparing polymer nanoparticles because this process requires a large amount of surfactant and produces only a relatively low polymer content (510 wt%). This drawback has hindered the microemulsion process from scaling up to industrial level due to the high cost of surfactant and the post-treatment required for removing the surfactant after polymerization [5,7,8]. Differential microemulsion polymerization of styrene has been developed as a new technique for producing polystyrene (PS) nanosize particles of less than 50 nm by using a much smaller amount of surfactant than that used in the microemulsion polymerization [9]. In this process, the styrene monomer is gradually fed dropwise into the polymerizing system. PS nanoparticles of less than 20 nm diameter have been synthesized by differential microemulsion process involving the use of small amount of poly(methyl methacrylate) as the seeds, and sodium dodecyl sulfate (SDS) and ammonium persulfate as the surfactant and initiator, respectively [10]. The introduction of polymeric nanoparticles at a very low loading ratio into NR is expected to improve the application properties of the rubber products as effective as the inorganic nanofillers. Among the polymeric nanoparticles, PS has received considerable attention for developing polymer nanotechnology [10]. To the best of our knowledge, the properties of NR blended with PS nanoparticles synthesized by differential microemulsion polymerization have not been reported so far. This present work aims to prepare PS nanolatex particles by differential microemulsion polymerization and to subsequently blend them with NR in the latex form. It was expected that a small amount of PS

Natural Rubber Blended with Polystyrene Nanoparticles 377 nanoparticles would improve the application properties of the resulting rubber products. The mechanical performance was evaluated by means of tensile testing and dynamic mechanical analysis. The flammability and morphology of the specimens were also observed. EXPERIMENTAL Materials Natural rubber latex (60 wt% dry rubber content) was compounded with aqueous solutions of potassium oleate (20 wt%) and potassium hydroxide (10 wt%), and aqueous dispersions of sulfur (50 wt%), zinc oxide (50 wt%), zinc diethyl dithiocarbamate (50 wt%), CPL antioxidant (50 wt%), and calcium carbonate (70 wt%), as shown in Table 1. Styrene monomer, SDS and 2,2 0 -azobisisobutyronitrile (AIBN), were used in the polymerization system. All materials were used as received without further purification. Polymerization and Characterization of PS Nanolatex The nanolatex of PS was synthesized by differential microemulsion polymerization using SDS and AIBN as the surfactant and initiator, respectively. SDS (8 g) and AIBN (0.12 g) were mixed in a 500 ml Pyrex glass reactor, which was equipped with a reflux condenser, a N 2 gas inlet, and a dropping funnel for monomer feeding. Distilled water (60 ml) was thereafter added and the system was heated up to 708C with stirring at 250 rpm using a magnetic stirrer under an atmosphere of N 2 gas. After the temperature was raised to 708C, the styrene Table 1. Formulation of the rubber compound (based on dry weight). Ingredient Amount (phr) Natural rubber 100 Potassium oleate 0.2 Potassium hydroxide 0.5 Sulfur 1.5 Zinc diethyl dithiocarbamate 1.0 Antioxidant (CPL) 1.0 Zinc oxide 1.0 Calcium carbonate 30

378 S. CHUAYJULJIT AND A. BOONMAHITHISUD monomer was fed very slowly, in a dropwise manner, over a period of 1.5 h. The reaction system was then maintained at 708C with constant agitation for an additional hour before a water cooling operation was applied. Part of the latex was precipitated with methanol, washed with distilled water, and dried at 608C for 12 h. The dried product was further used for characterizations. The resultant solid content (% solid) was determined by calculating from the following equation: % solid ¼ W 1 =W 2 100 where W 1 and W 2 are the weights of dried nanoparticles and nanolatex, respectively. The number-average diameter (D n ) and morphology of the prepared nanoparticles were investigated using a dynamic light scattering analyzer (DLS, Nano-series ZX) and transmission electron microscope (TEM), respectively. TEM was carried out using a Jeol JEM-2100 at an accelerating voltage of 80 kv. Preparation of NR PS Blends Polymer blend sheets were prepared by mixing NR latex with the nanolatex of PS at 3, 5, 7, and 9 phr (based on dry weight) using a mechanical stirrer at 150 rpm for 3 h. The homogeneous latex was then cast into a sheet on a glass mold (20 20 0.15 cm 3 ), allowed to air-dry at room temperature for 24 h, and subsequently cured at 1108C for 3 h. Determination of Tensile Properties The tensile strength, modulus at 300% strain, and elongation at break of the cured samples were measured according to ASTM D-412, using dumbbell-shaped specimens. Five specimens from each blend were tested by the use of an Instron Testing Machine Series IX-1011, equipped with a load cell of 1 kn, working under the test speed of 500 mm/min. Determination of Dynamic Mechanical Properties Dynamic mechanical analysis (DMA) was carried out with a Mettler Toledo DMA/SDTA 861 e instrument under shear mode with the following parameters: frequency ¼ 1 Hz; scan rate ¼ 3.0 K/min; temperature range ¼ 1008C to 1508C.

Morphology The morphology of the NR PS blends was observed using a scanning electron microscope (Jeol JSM-6400 SEM) at an accelerating voltage of 15 kv and a magnification of 500. Samples were fractured in liquid nitrogen where fractured surfaces were sputter coated with a thin layer of gold prior to SEM examination. Flammability Natural Rubber Blended with Polystyrene Nanoparticles 379 The limiting oxygen index (LOI) of the samples was measured using a Limiting Oxygen Indexer (Stanton Redcroft) on sheets (140 52 1.5 mm 3 ) according to the standard oxygen index test ASTM D-2863-91. The test was carried out under controlled nitrogen oxygen mixed environments, where the mixture of nitrogen and oxygen was allowed to pass through the sample burning in a carefully controlled rate. The amount of oxygen in the gas mixture that was just sufficient to keep the sample burning was taken for calculating the LOI value from the following equation: LOI ¼ O 2 N 2 þ O 2 100 where O 2 and N 2 are the volume concentration of O 2 and N 2, respectively. The flame spread rate of the specimens was measured using an Altas 458 automatic flammability tester according to ASTM 1230. RESULTS AND DISCUSSION Characterization of PS Nanolatex Particles The obtained nanolatex had a calculated solid content of about 22%. The number-average diameter (D n ) of the nanoparticles was determined to be about 36 nm from the particle size analyzer. Figure 1 displays representative TEM image of spherical nanoclusters of PS particles. Tensile Properties The tensile properties of the specimens, namely the changes in tensile strength, modulus at 300% strain, and elongation at break as a function of the PS nanoparticle concentration in the NR matrix, were determined and are summarized in Figures 2 4.

380 S. CHUAYJULJIT AND A. BOONMAHITHISUD FIGURE 1. Representative TEM image of PS nanoparticles synthesized by differential microemulsion polymerization. 20 Tensile strength (MPa) 18 16 14 12 10 8 6 NR pure NR/PS 4 2 0 3 5 7 9 PS nanoparticle content (phr) FIGURE 2. Tensile strength of NR and NR PS blends.

Natural Rubber Blended with Polystyrene Nanoparticles 381 3 Modulus at 300% strain (MPa) 2.5 2 1.5 1 0.5 NR pure NR/PS 0 3 5 7 PS nanoparticle content (phr) 9 FIGURE 3. Modulus at 300% strain of NR and NR PS blends. 900 Elongation at break (%) 800 700 600 500 400 300 200 NR pure NR/PS 100 0 3 5 7 PS nanoparticle content (phr) 9 FIGURE 4. % Elongation at break of NR and NR PS blends. The tensile strength of the unfilled-nr sample was about 12.5 MPa. When the PS nanoparticles were added into NR at 3 phr, a significantly greater (and maximal) tensile strength of about 17 MPa was observed, which represents a 36% increase in the tensile strength compared to the neat NR (Figure 2). However, the reinforcing effect provided by the PS nanoparticles decreased as they increased to above 3 phr

382 S. CHUAYJULJIT AND A. BOONMAHITHISUD (i.e., at 5, 7, and 9 phr), and resulted in a lower tensile strength than that of the neat NR for all three higher PS levels at 12 MPa down to 11 MPa for PS inclusion levels of 5 9 phr. This is because a higher PS nanoparticle content results in their aggregation and poor dispersion. The increase in the tensile strength with the inclusion of PS at 3 phr may thus reflect that the nanoparticles are homogeneously distributed throughout the NR matrix, whereas the decrease in the tensile strength can be attributed to the heterogeneous dispersion of aggregated PS nanoparticles in the NR matrix. The modulus of all four PS NR polymer blends was just under twofold higher than that of the neat NR, and did not change significantly with increasing PS nanoparticle concentrations within the tested range (3 9 phr) (Figure 3). The increase in the modulus may be due to the rigidity of the spherical nanoclusters of PS that restrict the NR main chains movement. The inclusion of PS nanoparticles into NR reduced the elongation at break level for all four composite levels to a lower level (13 20%) than that of NR (Figure 4). Indeed, a numerically slight, although not statistically significant, dose-dependent decrease in the elongation at break was noted with increasing PS ratios (Figure 4). This is an expected result since it is well known that the addition of stiff particles can reduce the elongation at break of the matrix [11]. However, the elongation at break of NR (757%) is largely retained due to the very low loading of PS nanoparticles, giving an elongation at break range of between 614% and 663%. Dynamic Mechanical Properties The dynamic mechanical properties of the NR and the four NR PS blends, in terms of the storage modulus (M 0 ) and loss tangent (tan ), also revealed clear differences between the NR and the NR PS composites. The effects of temperature and the composition on M 0 of the samples are shown in Figure 5(a), where the storage modulus of NR and NR filled with PS nanoparticles at 3 phr are very similar (due to the very low concentration of the PS nanoparticles) and show a rapidly drop in modulus within the range of 708Cto 508C, and then flatten out. As the PS loading ratio is increased from 3 to 9 phr, the modulus values of the NR PS blends markedly decreased, which indicated that the samples have some elastic responses near their glass transition temperatures (T g ). The decrease in modulus may presumably be caused by the semi-interpenetrating (semi-ipn) nanostructure. In this case, PS nanoparticles were entrapped in the NR networks. Thus, the

Natural Rubber Blended with Polystyrene Nanoparticles 383 (a) MPa Storage modulus (MPa) 1400 1200 1000 800 600 400 NR pure NR_3PS NR_5PS NR_7PS NR_9PS 200 0 100 80 60 40 20 0 20 40 60 80 100 120 140 C Temperature ( C) (b) Tan δ (MPa) 1.4 1.2 1.0 0.8 0.6 NR pure NR_3PS NR_5PS NR_7PS NR_9PS 0.4 0.2 0.0 100 80 60 40 20 0 20 40 60 80 100 120 140 C Temperature ( C) FIGURE 5. Effects of temperature on (a) the storage modulus (M 0 ) and (b) the loss tangent (tan ) of the NR and NR PS blends.

384 S. CHUAYJULJIT AND A. BOONMAHITHISUD agglomerated PS nanoparticles in the NR network provided an increase in the free volume and also in the mobility of NR molecules. The variation in the loss tangent of NR and NR PS nanoparticle blends as a function of temperature is summarized in Figure 5(b), where the glass transition temperatures of the neat NR and PS in the blends can be determined from the peaks in the tan curves. The curve exhibited two transitions, corresponding to the NR and PS phases, and indicated the incompatibility of the two components. The neat NR exhibits a T g at 508C and there is no significant change in the T g of NR in the blends. The -transition or T g of PS in the blends occurs at about 1108C, while the ß-transition, which is attributed to the local segmental motions of the main chain, is in the range of 15 208C. The height of the tan curve for the blend of 100NR/3PS is close to the pure NR, due to the very low content of the PS nanoparticles. However, the addition of the PS nanofillers at over 3 phr (i.e., 5, 7, and 9 phr) resulted in remarkably lower height of the respective tan curves, which implies that the elastic properties of the rubber molecules near the glass transition temperature increased with the addition of the PS nanolatex, presumably caused by the semi-ipn nanostructure as mentioned earlier. Morphology SEM micrographs of the cryofractured surfaces of the neat NR and NR PS blends revealed that the neat NR and the 100NR/3PS blend show a smooth profile (Figure 6(a) and (b)), typically indicating a brittle fracture. Whereas, in some contrast, the three higher PS ratio NR PS blends show low ridges and shallow grooves (Figure 6(c) (e)), characterizing a ductile behavior induced by the semi-ipn nanostructure. This is in good agreement with the results obtained from the DMA analyzer. Flammability The oxygen index test is a general method used for evaluating the flammability of materials. The LOI values obtained for the neat NR and the four NR PS blends revealed that, compared to the neat NR, the blends showed a slight (3.6 9.1%) and dose-dependent increase in the LOI value (Table 2). This indicated that the blends confer a small improvement in the flammability, but not enough to achieve flame retardancy, due to the very low loading of PS nanoparticles in the NR matrix. Moreover, the addition of PS nanoparticles into the NR matrix showed a dose-dependent reduction (3.2 22%) reduction in the flame

Natural Rubber Blended with Polystyrene Nanoparticles 385 (a) (b) (c) (d) (e) FIGURE 6. SEM micrographs (all 500 magnification) of: (a) neat NR and blends of (b) 100NR/3PS, (c) 100NR/5PS, (d) 100NR/7PS, and (e) 100NR/9PS. Table 2. LOI values and flame spread rate of NR and NR PS blends. Composition LOI Flame spread rate (cm/s) NR 16.9 0.157 100NR/3PS 17.5 0.152 100NR/5PS 17.9 0.136 100NR/7PS 18.2 0.128 100NR/9PS 18.6 0.123 spread rate of the specimens (Table 2). This is because the PS nanoparticles tend to accumulate near the sample surface and consequently form a charred layer, which acts as a heat insulation barrier. CONCLUSION The tensile properties, dynamic mechanical properties, and flammability of NR affected by the addition of PS nanoparticles were studied. The PS nanoparticles used in this work were synthesized by differential microemulsion polymerization using SDS and AIBN as the surfactant and initiator, respectively. The blends of NR and PS nanoparticles were

386 S. CHUAYJULJIT AND A. BOONMAHITHISUD successfully prepared by latex compounding. The incorporation of an appropriate amount of PS nanoparticles apparently improved the tensile strength (3 phr only), modulus at 300% strain (all, but 3 phr optimal), and flammability (all, but 9 phr optimal) of the NR, whereas the elongation at break deteriorated with the incorporation of PS nanoparticles. The results from SEM micrographs and dynamic mechanical tests showed that the elasticity of rubber molecules near the glass transition temperature increased with the addition of PS nanoparticles, which may be caused by the semi-ipn nanostructure, observed when the PS concentration was over 3 phr. ACKNOWLEDGMENTS The authors gratefully acknowledge the Faculty of Science, the National Center of Excellence for Petroleum, Petrochemicals and Advanced Materials, Chulalongkorn University and the Thailand Research Fund for financial, material, and instrument support. We would also like to thank the PCU, Faculty of Science, Chulalongkorn University for the manuscript correction and suggestion. REFERENCES 1. Derouet, D., Intharapat, P., Tran, Q.N. and Gohier, F. (2008). Graft Copolymers of Natural Rubber and Poly(dimethyl(acryloyloxymethyl) phosphonate) (NR-g-PDMAMP) or Poly(Dimethyl(Methacryloyloxyethyl) Phosphonate) (NR-g-PDMMEP) from Photopolymerization in Latex Medium, Eur. Polym. J., 45: 820 836. 2. Jincheng, W., Yuehui, C. and Jihu, W. (2005). Novel Reinforcing Filler: Application to Natural Rubber (NR) System, J. Elast. Plast., 37: 169 180. 3. Peng, Z., Kong, L.X., Li, S.D., Chen, Y. and Huang, M.F. (2007). Self-assembled Natural Rubber/Silica Nanocomposites: Its Preparation and Characterization, Compos. Sci. Technol., 67: 3130 3139. 4. Cai, H.H., Li, S.D., Tian, G.R., Wang, H.B. and Wang, J.H. (2003). Reinforcement of Natural Rubber Latex Film by Ultrafine Calcium Carbonate, J. Appl. Polym. Sci., 87: 982 985. 5. He, G., Pan, Q. and Rempel, G.L. (2003). Synthesis of Poly(methyl methacrylate) Nanosize Particles by Differential Microemulsion Polymerization, Macromol. Rapid Commun., 24: 585 588. 6. Zhang, C., Wang, Q., Xia, H. and Qiu, G. (2002). Ultrasonically Induced Microemulsion Polymerization, Eur. Polym. J., 38: 1769 1776. 7. Ming, W., Zhao, Y., Cui, J., Fu, S. and Jones, F.N. (1999). Formation of Irreversible Nearly Transparent Physical Polymeric Hydrogels During a Modified Microemulsion Polymerization, Macromolecules, 32: 528 530.

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