Natural Rubber Nanocomposite Reinforced With Nano Silica*
Ying Chen,1 Zheng Peng,1,2 Ling Xue Kong,2 Mao Fang Huang,1 Pu Wang Li1,2 1 Chinese Agricultural Ministry Key Laboratory of Natural Rubber Processing, Agricultural Product Processing Research Institute at Chinese Academy of Tropical Agricultural Sciences, Zhanjiang 524001, China 2
Center for Material and Fiber Innovation, Deakin University, Waurn Ponds Campus, Vic 3217, Australia
Inorganic nano fillers have demonstrated great potential to enhance the properties of natural rubber (NR). The present article reports the successful development of a NR nanocomposite reinforced with nano silica (SiO2). Its dynamic mechanical properties, thermal aging resistance, and morphology are investigated. The results show that the SiO2 nanoparticles are homogenously distributed throughout the NR matrix in a form of spherical nano-cluster with an average size of 80 nm when the SiO2 content is 4 wt%. With the introduction of SiO2, the thermal resistance and the storage modulus of NR host significantly increase, and the activation energy of relaxation of the nanocomposite, compared to the raw NR, increases from 90.1 to 125.8 kJ/mol. POLYM. ENG. SCI., 48:1674–1677, 2008. ª 2008 Society of Plastics Engineers
INTRODUCTION Natural rubber (NR) with excellent chemical and physical properties has been widely used in various areas [1– 3]. The raw NR is usually reinforced with antiageing agents [4, 5] and inorganic fillers [6, 7] before it is manufactured to products, as its poor ageing resistance and mechanical properties generally could not meet the requirements of applications. However, the introduction of antiageing agents and inorganic fillers may bring some hygienic or environmental pollution issues. Therefore, it is essential to exploit new approaches to reinforce raw NR. Introduction of inorganic nano-fillers into NR matrix to overcome the aforementioned disadvantages has recently attracted enormous interests [8, 9]. Particularly, fumed *This paper was presented at the 2007 International Conference on Parallel Processing (ICPP-07). Correspondence to: Zheng Peng; e-mail:
[email protected] Contract grant sponsor: the International Cooperation Project Foundation of Chinese Agricultural Ministry Key Laboratory of Natural Rubber Processing; contract grant numbers: 706068, 706069; Contract grant sponsor: The Natural Science Foundation of China; contract grant number: 50763006. DOI 10.1002/pen.20997 Published online in Wiley InterScience (www.interscience.wiley.com). C 2008 Society of Plastics Engineers V
SiO2 nanoparticles have been extensively used to prepare polymer/silica nanocomposites via melt compounding [10] and other physical blending [11]. However, silica has a number of hydroxyl groups on the surface, which results in the strong filler-filler or particle–particle interactions and adsorption of polar materials by hydrogen bonding. SiO2, therefore, has strong self-aggregation nature. In such conditions, the fumed SiO2 nanoparticles trend to form loosely agglomerates that are dispersed with an average size in the range 300–400 nm, and these aggregated particles cannot be broken down by the shear forces during melt compounding [10]. In our previous work, a selfassembly nanocomposite process was developed to prepare a bulk polyvinyl alcohol/silica (PVA/SiO2) nanocomposites [12, 13]. We found that the strong self-aggregation of SiO2 nanoparticles was greatly restricted and the chemical and physical properties of nanocomposites, compared to the polymer host, were significantly enhanced. We recently extended this self-assembly process incorporating latex compounding technique to prepare NR/SiO2 nanocomposites [14]. In the present article, we will investigate the impact of SiO2 on the thermal resistance, dynamic mechanical properties, and morphology of NR matrix. EXPERIMENTAL Materials Natural rubber latex (NRL) with a total solid content of 62% was purchased from Shenli Rubber Plantation Zhanjiang, China. Silica nanoparticles (average diameter: 14 nm) and poly (diallyldimethylammonium chloride) (PDDA) (mol wt ca. 100,000–200,000) were brought from Sigma-Aldrich.
Preparation of Nanocomposite The preparation method was reported in our previous work [12]. It is a process combining latex compounding POLYMER ENGINEERING AND SCIENCE—-2008
FIG. 3. TG/DTG curves of NR and NR/SiO2 nanocomposite. FIG. 1. SEM micrograph of NR/SiO2 nanocomposite.
and self-assembly techniques. The nanocomposite in the current work contains 4 wt% silica. Characterizations SEM micrographs were taken with a Philips XL 30 FEG-SEW instrument at an acceleration voltage of 10 kV. TEM observation was done on a JEM-100CXII instrument with an accelerating voltage of 100 kV. A Perkin Elmer TGA-7 thermogravimetric analyser was used for thermal decomposition measurement. In nitrogen, the measurement of the films was carried out from 100 to 6008C at a heating rate of 208C/min. DMA was conducted on a NETZSCH DMA 242C instrument in tension model under the following conditions: frequency ¼ 1, 2.5, 5, and 10 Hz, respectively; dynamic force ¼ 2.5 N; static force ¼ 0.5 N; temperature scanning rate ¼ 58C/min. RESULTS AND DISCUSSION Morphology The SiO2 nanoparticles are homogenously distributed in NR matrix as nano-clusters with an average size
around 80 nm (see Fig. 1), which indicates that the heavy aggregation of SiO2 nanoparticles caused by strong particle–particle interaction has been greatly suppressed and only primary aggregations involves during preparation. Another illustrative evidence is given by the TEM micrograph, where the uniformly dispersed spherical nano-clusters with the same diameter (presented as dark circle pies) can be clearly observed (see Fig. 2). These primary aggregations are not likely caused by the strong particle–particle interaction, but the adsorption between polymer molecular chain and SiO2 nanoparticles during the assembly steps as discussed in our previous work [14].
Thermal Ageing Resistance Figure 3 is the TG/DTG curves of the raw NR and nanocomposites in nitrogen. The thermal decomposition for the raw NR and nanocomposite is similar, as their TG curves contain only one obvious turn and one corresponding peak in their DTG curves. It suggests that the thermal decomposition for NR and nanocomposite is one-step decomposition, which may be primarily initiated by thermal scissions of CC chain bonds accompanying a transfer of hydrogen at the site of scission. Various degradation temperatures such as initial (T0), peak (Tp), and final (Tf) degradation temperature can be calculated with a bi-tangent method from the TG/DTG curves (see Fig. 3). The temperature range of the thermal degradation is expressed as DT ¼ Tf 2 T0. During thermal decomposition, T0, Tp, and Tf of the nanocomposite increase 10.48C, 10.38C, and 12.28C compared to those of the raw NR, respectively (Table 1). As these increases occur under temperature scanning model, they may be much more significant when the nanocomposites are naturally decomposed. The increase in decomposition temperTABLE 1. Characteristic temperatures of thermal decomposition for both raw NR and NR/SiO2 nanocomposite. Sample
FIG. 2. TEM micrograph of NR/SiO2 nanocomposite.
DOI 10.1002/pen
Raw NR Nanocomposite
T0 (8C)
Tp (8C)
Tf (8C)
DT (8C)
385.0 395.4
409.9 420.2
435.4 448.6
50.4 53.2
POLYMER ENGINEERING AND SCIENCE—-2008 1675
FIG. 4. DMA curves of NR and NR/SiO2 nanocomposite.
atures indicates that the thermal ageing resistance of the NR/SiO2 nanocomposite has been markedly improved, which is confirmed with the increase in DT. With the introduction of SiO2 nanoparticles, the decomposition process of NR is delayed and therefore the DT of nanocomposite increases 2.88C over that of the raw NR. The SiO2 and NR molecular chains strongly interact through various effects such as the branching effect, nucleation effect, size effect, and surface effect. The diffusion of decomposition products from the bulk polymer to gas phase is therefore slowed down. Another reason for the improvement in ageing resistance of the nanocomposite is that SiO2 will migrate to the surface of the composites at elevated temperatures because of its relatively low surface potential energy. This migration results in the formation of a SiO2/NR char, which acts as a heating barrier to protect the NR inside. Similar result was found in Gilman et al., [15] and Vyazovkin’s et al.’s [16] work, where a clay/polymer char greatly enhances the thermal resistance of the host polymers.
Dynamic Mechanical Properties Figure 4 depicts the temperature dependence storage modulus (E0 ) and loss tangent (tan d) of the raw NR and nanocomposite under the scan frequency of 5 Hz. The E0 of the raw NR is around 4200 MPa at 21008C, while it significantly increases to 7600 MPa when 4 wt% silica nanoparticles are introduced into the NR matrix. Because of the rigidity of SiO2, the nanocomposite becomes more rigid. From the DMA curve, an obvious peak corresponding to the grass transition can be clearly observed. The glass transition temperature (Tg) can be taken at the peak of tan d (see Fig. 4). The Tg of raw NR is 262.08C, while that of the nanocomposite is 252.58C. The remarkable increase in Tg is largely due to the restriction of the NR molecular chain movement, namely, the SiO2 is not just physically blended with NR, but strongly interacts with NR molecular chains. However, the increase in Tg may cause some disadvantages for NR including its applied properties at low temperatures. 1676 POLYMER ENGINEERING AND SCIENCE—-2008
Another disadvantage introduced by SiO2 is the increase in heat accumulation, as the maximum tan d of the nanocomposite is higher than that of the raw NR (see Fig. 4). In other words, the ability for absorbing energy of the nanocomposite should be stronger than that of the raw NR under an acute strain environment, particularly for tyre applications. The accumulation of absorbed energy will cause thermal degradation and further reduce the mechanical properties of materials. Therefore, it is crucial to further study how to improve the low temperature properties and heat accumulation of the nanocomposite. To investigate the activation energy (Ea), the dynamic mechanical analysis of the raw NR and nanocomposites were studied under multifrequencies of 10, 5, 2.5, and 1 Hz. The glass transition temperature shifts to higher temperature when the applied frequency is increased. The change of the Tg with the applied frequency is in accordance with the Arrhenius equation: f ¼ f0 exp
Ea RT
(1)
or ln f ¼ ln f0
Ea RT
(2)
where f is the applied frequency, T is the absolute glass transition temperature, R is the universal gas constant and, Ea is the activation energy. By plotting the Tg versus logarithmic frequency, a line can be obtained (see Fig. 5). From its slope, the activation energy can be obtained. The Ea of the raw NR is 90.1 kJ/mol. With the addition of the SiO2, the Ea of the nanocomposite increases to 125.8 kJ/ mol. When SiO2 nanoparticles are introduced into NR matrix, the SiO2 will strongly interact with NR molecular chains and the movements of the NR molecular segments is restricted. Therefore, the relaxation of the nanocomposite, compared to the raw NR, requires more Ea.
FIG. 5. Relationship between logarithmic frequency and reciprocal glass transition temperatures.
DOI 10.1002/pen
CONCLUSIONS The self-assembly nanocomposite process has been successfully used to prepare a NR/silica nanocomposite. The SiO2 nanoparticles are homogenously distributed in NR matrix as nano-cluster with an average size of 80 nm. In comparison with pure NR, thermal ageing resistance of developed nanocomposite has been significantly improved and various degradation temperatures obtained from thermogravimetric analysis increase up to 12.28C. With the introduction of SiO2, the storage module and relaxation activation energy of NR significantly increases, while its glass transition temperature greatly decreases. REFERENCES 1. H. Angellier, S. Molina-Boisseau, and A. Dufresne, Macromolecules, 38, 9161 (2005). 2. S. Sato, Y. Honda, M. Kuwahara, H. Kishimoto, N. Yagi, K. Muraoka, and T. Watanabe, Biomacromolecules, 5, 511 (2004). 3. K. Sanguansap, T. Suteewong, P. Saendee, U. Buranabunya, and P. Tangboriboonrat, Polymer, 46, 1373 (2005). 4. J.K. Kurian, N.R. Peethambaran, K.C. Mary, and B. Kuriakose, J. Appl. Polym. Sci., 78, 304 (2000).
DOI 10.1002/pen
5. L. Abad, L. Relleve, C. Aranilla, A. Aliganga, C.S. Diego, and A.D. Rosa, Polym. Degrad. Stab., 76, 275 (2002). 6. H.H. Cai, S.D. Li, T.G. Rian, H.B. Wang, and J.H. Wang, J. Appl. Polym. Sci., 87, 982 (2003). 7. L. Jose and R. Joseph, Kaut. Gummi. Kunstst., 46, 220 (1993). 8. P.L. Teh, Z.A.M. Ishak, A.S. Hashim, J. Karger-Kocsis, and U.S. Ishiaku, Eur. Polym. J., 40, 2513 (2004). 9. S. Varghese and J.J. Karger-Kocsis, Appl. Polym. Sci., 91, 813 (2004). 10. S.H. Kim, S.H. Ahn, and T. Hirai. Polymer, 44, 5625 (2003). 11. T.C. Merkel, Science, 296, 519 (2002). 12. Z. Peng, L.X. Kong, and S.D. Li, Polymer, 46, 1949 (2005). 13. Z. Peng, L.X. Kong, and S.D. Li, J. Appl. Polym. Sci., 96 (4), 1436 (2005). 14. S.D. Li, Z. Peng, L.X. Kong, and J.P. Zhong, J. Nanosci. Nanotechnol., 6, 541 (2006). 15. J.W. Gilman, C.L. Jackson, A.B. Morgan, R. Harris, E. Manias, E.P. Giannelis, M. Wuthenow, D. Hilton, and S.H. Phillips, Chem. Mater., 12, 1866 (2000). 16. S. Vyazovkin, I. Dranca, X.W. Fan, and R. Advincula, Macromol. Rapid. Commun., 25, 498 (2004).
POLYMER ENGINEERING AND SCIENCE—-2008 1677
