Preparation and Properties of Chloroprene Rubber (CR)/Clay

Preparation and Properties of Chloroprene Rubber (CR)/Clay Nanocomposites Yao-Yi Cheng*, Ynh-Yue Yen, Peng-Hsiang Kao, Norman Lu and Hsin-TaWang Institute of Organic and Polymeric Materials, National Taipei University of Technology, Taiwan, R.O.C. Received: 8 October 2008, Accepted: 6 August 2009 SUMMARY Chloroprene rubber (CR)/organic modifier/sodium montmorillonite (NA-MMT) nanocomposites were prepared by melt intercalation. The morphology of the nanocomposites was studied using X-ray diffraction (XRD) and transmission electron microscopy (TEM). TEM results showed that the silicate layers of the clay were dispersed in the CR matrix on the nano-scale by adding the organic modifier. X-ray diffraction also indicated that the silicate layers of clay were exfoliated. The mechanical and thermal properties of the nanocomposites could be therefore improved by adding clay and organic modifier, as the silicate layer of the clay was uniformly dispersed. Dynamic mechanical analysis was further performed to investigate the thermomechanical properties of CR clay nanocomposites containing various amounts of clay. The glass-transition temperature of CR clay nanocomposites increased with increasing amount of clay and organic modifier. 1. INTRODUCTION Polymer/clay nanocomposites have attracted great attention recently because of their unusual properties 1-3. Compared to the conventional composites, these nanocomposites exhibit improved mechanical properties and enhanced thermal stability 4. To facilitate the silicate layers interaction with a polymer, clay can be modified with an alkylammonium salt by a cation exchange reaction, and thus the hydrophilic clay surfaces become organophilic 5. The modified clay has been used to make nanocomposites with many polymers such as EPDM 6 and polypropylene (PP) 7. In the previous studies, there have been two distinct nanostructures identified with polymer/clay nanocomposites: intercalated and exfoliated 8. Intercalated nanocomposites have polymer chains inserted between silica layers of clay to generate ordered multi-layered structures. Exfoliated nanocomposites have exfoliated silica layers dispersed in the polymer matrix. The formations of these structures are not only determined by the properties of polymer and clay, but also by the preparation process. For melt blending, if the polarity of polymer chains is higher, polymer/clay nanocomposites with intercalated or exfoliated structure can be developed more easily. Ergungor et al. 9 have investigated the effect of melting temperature on the phase behaviour in Nylon 6/clay nanocomposites. Recently, EPDM/ clay nanocomposites have been prepared by mixing EPDM and organoclay during the vulcanization process 10. This paper aims at the preparation of CR/clay nanocomposites directly using natural (Na + -base) clay and organic modifier via vulcanization process. 2. EXPERIMENTAL 2.1 Materials Commercial sodium montmorillonite (Na-MMT) produced by Southern Clay Products Inc. (USA), with a cation exchange capacity (CEC) value of about 98 mmol/100 g was used as received. Chloroprene rubber (CR) was supplied by Du Pont Corporation (USA). NA-22 used as vulcanization accelerator was supplied by S.N.A. Chemical Co. (TAIWAN). MgO and ZnO were supplied by H.U.S Co. (TAIWAN). Organic modifier, RF-751, was supplied by SINO-Japan Co. of molecular weights 405.05 g/mol. RF- 751, an ammonium salt with alkyl side group, can exchange with Na cations in clay and therefore promote mixing of CR and clay. The chemical formula of RF-751 is RN + (CH 2 CH 2 OH) (CH - 2 2 CH=CH 2 ), where R represents alkyl group (C8: 7 mole%, C10: 10 mole%, C12: 51 mole% C14: 19 mole%, C16: 8 mole%, C18: 2 mole%, and others: 6 mole%). Smithers Rapra Technology, 2010 17

Yao-Yi Cheng, Ynh-Yue Yen, Peng-Hsiang Kao, Norman Lu and Hsin-TaWang 2.2 Preparation of the CR Clay Nanocomposite Chloroprene rubber, organic modifier (RF-751), and NA-MMT were meltmixed in a Brabender (GmbH & Co. KG Brabender Plastograph internal mixer PL2000 W50) at 70 C with rotor speed 30 rpm for 10 minutes (step 1). ZnO, MgO, and NA-22 were then added into the Brabender to mix at 90 C for another 10 minutes (step 2). The resulting mixture was sheeted on a two-roll mill at room temperature for 20 minutes (step 3), and compressionmoulder was at 175 C for the optimum cure time of 15 minutes to yield crosslinked rubber (step 4). The compositions of prepared CR-clay nanocomposites are summarized in Table 1. The amounts of RF751 in composites CR048, CR10 and CR18 were 0.48, 1 and 1.8 phr, respectively. 2.3 Characterization Small angle X-ray scattering (SAXS) measurement was conducted using an X-ray diffractometer (Osmic, USA). The scanning range was 0.22 to 8 degrees with scanning Table 1. Composition of CR/clay nanocomposites Composition of the CR/clay nanocomposites (phr) Samples CR MgO ZnO NA-22 RF-751 Na-MMT CR-PC 100 4 2 2 0 0 CR-PC -1 100 4 2 2 0 1 CR -PC-3 100 4 2 2 0 3 CR -PC-6 100 4 2 2 0 6 CR -PC-10 100 4 2 2 0 10 CR-048-1 100 4 2 2 0.48 1 CR-048-3 100 4 2 2 1.44 3 CR-048-6 100 4 2 2 2.88 6 CR-048-10 100 4 2 2 4.8 10 CR-10-1 100 4 2 2 1 1 CR-10-3 100 4 2 2 3 3 CR-10-6 100 4 2 2 6 6 CR-10-10 100 4 2 2 10 10 CR-18-1 100 4 2 2 1.8 1 CR-18-3 100 4 2 2 5.4 3 CR-18-6 100 4 2 2 10.8 6 CR-18-10 100 4 2 2 18 10 Figure 1. XRD patterns of CR/ Na-MMT (6 phr) composites: (a) after mixing in the Brabender, (b) after mixing with MgO, ZnO, and Na-22 in the Brabender, (c) after being compressed into sheets on a two-roll mill, (d) after being cured at 175 C rate of 2 /min. The samples for transmission electron microscopy (TEM) were cut into 50~100 nm thick sections. The microstructure image of the nanocomposites was obtained with a JEOL 2000 EX HRTEM. Thermogravimetric analysis (TGA) was conducted using a Perkin-Elmer TGA instrument under N 2 atmosphere with the temperature from room temperature increasing to 600 C, with a heating rate of 10 C/min. Dynamic mechanical analysis (DMA) was carried out with a Perkin-Elmer DMA-7 in three-point bending mode. The frequency was fixed to 1 Hz and the heating rate was 5 C/min. The temperature range was from -40 to 40 C. Tensile testing was performed at room temperature and at an extension rate of 500 mm/min according to ASTM D-412. 3. RESULTS AND DISCUSSION Figure 1 shows the results of smallangle powder XRD patterns for a series of CR/Na-MMT nanocomposite. The d-spacing was calculated from peak positions using Bragg s law: λ = 2dsinθ, where λ is the X-ray wavelength (1.5418Å). The diffraction peaks in the range 2θ = 0.22 8 were recorded. The d-spacing of vacuumdried pristine MMT clay was 1.2 nm (2θ = 7 ). As shown in Figure 1, for CR/ Na-MMT nanocomposites prepared directly using natural (Na + -base) clay (without organic modifier), there is an obvious shift of the diffraction peak 18

towards lower angle from that of the corresponding MMT. The d-spacing of the CR/clay nanocomposite directly using natural (Na + -base) clay increased right after step 1. This implies that the interlayer spacing of the clay became large enough for penetration of CR macromolecules. The CR/ clay nanocomposites became more exfoliated from step 1 to step 4. As shown in Figure 2, for CR/Na- MMT nanocomposites using natural (Na + -base) clay with 1 phr organic modifier, there is more shifting of the diffraction peak towards lower angle compared with CR/Na-MMT nanocomposites without organic modifier. The d-spacing of the CR/clay nanocomposite directly using natural (Na + -base) clay was increased further from 3.7 nm to 4.4 nm after step 1. The CR/clay nanocomposites with organic modifier also became more exfoliated from step 1 to step 4. As the results show, the excess amount of organic modifier does make the CR clay nanocomposites more exfoliated. As shown in Figure 3, there is more shifting of the diffraction peak towards lower angle as the amount of organic modifier increases from 0.48 phr to 1.8 phr. Figure 2. XRD patterns of CR/organic modifier (1 phr) / MMT (6 phr) composites: (a) after mixing in the Brabender, (b) after mixing with organic modifier in the Brabender, (c) after compression into sheets on a two-roll mill, (d) after being cured at 175 C Figure 3. XRD patterns of CR/Na-MMT(6phr) nanocomposites with various amounts of organic modifier: (a) none (b) 0.48 phr (c) 1 phr (d) 1.8 phr Direct evidence of nanometre-scale dispersion of intercalated MMT can be found by TEM. Figure 4 shows a TEM micrograph of CR/clay with or without organic modifiers. Besides, the light regions represent CR and the dark lines correspond to the silicate layers of clay. The individual exfoliated silicate layers can be easily observed in the CR/clay matrix with 1.8 phr organic modifier, which cannot be found in the CR/clay matrix without organic modifiers. The decomposition temperature of the hybrid increases with the increase of clay content, since the clay exhibits good thermal stability. Figure 5 illustrates the TGA curves of these CR/ clay nanocomposites. The results are summarized in Table 2. The thermal decomposition temperature (Td) Table 2. Decomposition temperature of CR/clay nanocomposites Temperature ( C) of 20% Weight Loss Clay content 1 phr 3 phr 6 phr 10 phr CR-PC 284 288 294 283 CR-048 299 301 303 304 CR-10 293 303 305 306 CR-18 302 305 314 308 19

Yao-Yi Cheng, Ynh-Yue Yen, Peng-Hsiang Kao, Norman Lu and Hsin-TaWang Figure 4. TEM photographs of CR/organic modifier/mmt (6 phr) composites: (a) pure clay (b) 1.8% organic modifier corresponding to 20% weight loss of the CR nanocomposite with 6 phr clay can be increased up to 314 C with an amount of organic modifier up to 1.8 phr. The excellent thermal properties could be considered to result from the more uniformly dispersed silicate layers with an increased amount of organic modifier. Figure 6 and Figure 7 show the tensile strength and elongation of these CR/ clay nanocomposites, which increased with an increase of the amount of organic modifier and clay, acting together as a good reinforcing filler. The excellent mechanical properties of the CR/clay nanocomposites with up to 6 phr clay could again be associated with the uniformly dispersed silicate layers with the increased amount of organic modifiers. The slight drop of mechanical properties can that be observed when the amount of clay reaches 10 phr is considered to result from exceeding the maximum compatibility of clay with the CR matrix. Figure 5. TGA curves of the CR/MMT (6phr) nanocomposites containing various amounts of organic modifier: (a) none (b) 0.48 phr (c) 1 phr (d) 1.8 phr Finally, Table 3 shows the effect of the clay as fillers on the Tg of CR/clay nanocomposite obtained from DMA (the peak temperature of tan δ). Without organic modifiers, an increase of clay content will result in a slight shifting in Tg toward to a higher temperature (from 23.0 to 18.9 C). With as little as 0.48 phr of organic modifier, the Tg can be much increased, which is ascribed to the strong interaction between the rubber matrix, organic modifier, and Na-MMT. However, it is considered that the Tg can be slightly decreased by adding 1.8 phr of organic modifier as plasticizer. CONCLUSIONS CR/organic modifier/na-mmt nanocomposites were prepared successfully by melt intercalation. The XRD and TEM analysis proved that the nanocomposites were exfoliated. The mechanical and thermal properties of the nanocomposites were improved 20

Figure 6. The tensile strength of CR/Clay nanocomposites by adding clay and organic modifier, when the silicate layer of the clay was uniformly dispersed. The incorporation of clay also dramatically increased the value of Tg, which demonstrated the reinforcing effect of dispersed clay with organic modifier on the CR matrix. REFERENCES Figure 7. The elongation at break of CR/Clay nanocomposites 1. Pysklo L., K. Nicinski, M. Piaskiewicz, M. Bereza, and W. Lojkowski, Elastomery 11(1), (2007) 10. 2. Xiao C.-B., Z.-P. Zhang, J.-H. Liu, R. Li, and Z.-P. Zhang, Key Engineering Materials 334, (2007) 865. 3. Zhang Z.-P., J.-H. Liu, L.-B. Liao, C.-B. Xiao, R. Li, and Z.-P. Zhang, Key Engineering Materials 334, (2007) 833. 4. Hwang W.-G. and K.-H. Wei, Polym. Eng. Sci. 44, (2004) 2117. 5. Alexandre M and P. Dubois, Mater. Sci. Eng. 28, (2000) 1. 6. Wu Y.-P., Y. Ma, Y.-Q. Wang, and L.- Q. Zhang, Macromol. Mat. Eng. 289, (2004) 890. 7. Kato M., A. Usuki, and A. Okada, J. Appl. Polym. Sci., 66, (1997) 1781. 8. Kim D. S., and Lee, K. M. J. Appl. Polym. Sci., 90, (2003) 2629. 9. Ergungor Z., M. Cakmak, M., and C. Batur, Macromol. Symp., 185, (2002) 259. 10. Usuki A., A. Tukigase, and M. Kato, Polymer, 43, (2002) 2185. Table 3. Glass transition temperature of CR/clay nanocomposites Tg ( C) Clay content 1 phr 3 phr 6 phr 10 phr CR-PC -23. -19.7-20.2-18.9 CR-048-10.4-4.5-2.8-1.4 CR-10-7.6-6.4-1.8-3.8 CR-18-5.2-3.1-6.5 1.3 21