Morphology And Characterization Of Clay-reinforced Epdm Nano Composites
This document was uploaded by user and they confirmed that they have the permission to share it. If you are author or own the copyright of this book, please report to us by using this DMCA report form. Report DMCA
Overview
Download & View Morphology And Characterization Of Clay-reinforced Epdm Nano Composites as PDF for free.
More details
- Words: 3,077
- Pages: 10
Morphology and Characterization of Clay-reinforced EPDM Nanocomposites SEYED JAVAD AHMADI, YU DONG HUANG* AND WEI LI Department of Applied Chemistry, Faculty of Science Polymer Materials and Engineering Division Harbin Institute of Technology Harbin 150001, People’s Republic of China (Received January 12, 2003) (Accepted July 17, 2004)
ABSTRACT: The clay-reinforced ethylene propylene diene terpolymer (EPDM) nanocomposites with organo-montmorillonite (OMMT) and EPDM–clay conventional composites with pristine MMT are compared in terms of their morphology, mechanical properties, and solvent resistance. The maleic anhydride grafted EPDM (MAH-g-EPDM) is used as the compatiblizer for preparing clay-reinforced EPDM nanocomposites via a melt intercalation process. The formation of exfoliated nanocomposites is confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Furthermore, the morphological change during the mixing times of vulcanization process through exerting shearing force on clay particles is discussed. The mechanical properties and solvent resistance of clay-reinforced EPDM nanocomposites are examined as a function of the organoclay content in the matrix of the polymer, the result shows remarkable improvement relative to that of conventional composites. KEY WORDS: clay-reinforced EPDM nanocomposites, exfoliation, organoclay, melt intercalation, morphology.
INTRODUCTION fillers to a polymer matrix has been demonstrated to be an effective method to achieve reinforcement of the polymer hybrid materials consisting of organic and inorganic components. Nanocomposites are formed when phase mixing occurs on a nanometer length scale. Due to the improved phase morphology and interfacial properties, nanocomposites exhibit mechanical, thermal, barrier, and chemical properties superior to conventional composites [1–7]. Montmorillonite is the most commonly used clay to prepare nanocomposites, but the lack of affinity between hydrophilic layered silicates and hydrophobic polymer makes them difficult to be miscible
A
DDITION OF INORGANIC
*Author to whom correspondence should be addressed. E-mail: [email protected]
Journal of COMPOSITE MATERIALS, Vol. 39, No. 8/2005 0021-9983/05/08 0745–10 $10.00/0 DOI: 10.1177/0021998305048154 ß 2005 Sage Publications
745
746
S. J. AHMADI
ET AL.
at the nanoscale level. Thus, chemical modification of layered silicates is important in the preparation of nanocomposites. This can be done by an ion-exchange reaction through ions that already exist in clay such as Kþ, Naþ, Caþ, Mgþ, and organic cations such as alkyl ammonium ions to form the organosilicate [8]. The organosilicate can be broken down into their nanoscale building blocks and uniformly dispersed in the polymer matrix. When the clay platelets are thoroughly dispersed in the polymer matrix, the nanocomposites are ‘exfoliated.’ EPDM is an unsaturated polyolefin rubber with a wide range of applications. However, it is incompatible with polar organophilic clay to prepare products having desired properties because it does not include any polar group in its backbone. Recently, Kato et al. [9–12] have reported a new approach to prepare nonpolar polymer clay hybrids by using a functional oligomer. Till now, there are few studies on the formation, morphology, and properties of EPDM–clay nanocomposites. Usuki et al. [13] and Chang et al. [14] have reported the preparation and mechanical properties of EPDM–clay nanocomposites, prepared via the vulcanization process by using some special vulcanization accelerators and the melt compounding process with a liquid low molecular weight EPDM, respectively. In this paper, we compare the clay-reinforced EPDM nanocomposites with EPDM– clay conventional composites prepared through the melt blending method by using maleic anhydride grafted EPDM (MAH-g-EPDM) as a compatibilizer, in terms of their morphology, mechanical properties, and solvent resistance. The morphological change caused by shearing force during the mixing times of vulcanization process is also discussed.
EXPERIMENTAL Materials Pure sodium montmorillonite (Kunipia-F) with a cation-exchange capacity (CEC) of 119 meq/100 g was supplied by Kunimine Mining Ind. Co. (Tokyo, Japan). Octadecylamine purchased from Fluka was used as the organic modifier for MMT. MAH-g-EPDM (0.8 wt%) oligomer from Huzhou Genius Engineering Plastics Co. Ltd (Huzhou, China) was used. The EPDM (J-3062E) ENB type was obtained from Jilin Chemical Ind. Co., Ltd (Jilin, China). All chemicals were used without further purification.
Preparation of Organophilic Clay and Clay-reinforced EPDM Nanocomposites The organophilic clay was prepared via ion-exchange reaction in water by using octadecylamine as the reference [11]. In order to prepare clay-reinforced EPDM nanocomposites, different amounts of organoclay powder were premixed by shaking with 100 phr (part per hundred of rubber by weight) of EPDM and 20 phr of MAH-g-EPDM in a bag. This mixture was melt blended together in a twin-screw blender, model RM-200, with a chamber size of about 40 cm3 at 150 C. The rotational speed of the screw was 90 rpm and mixing time was 15 min for all cases. To compare the properties of nanocomposites with that of the conventional composites, the EPDM–clay composites with pristine MMT were prepared under the above-mentioned conditions. Then, the EPDM hybrids (100 phr) were sequentially mixed with zinc oxide (5 phr), stearic acid (1 phr),
Morphology and Characterization of EPDM Nanocomposites
747
vulcanization accelerator [M (2-Mercapto benzothiazole, 0.5 phr) and TMTD (tetramethyl thiuram disulfide, 1.5 phr)] and sulfur (1.5 phr) by using a roll mill, model SK-160B, at a temperature about 60 C. The vulcanization was carried out in standard hot press at 150 C for 30 min to yield rubber sheets (340 150 2 mm3). To study the effect of MAH-g-EPDM as a compatibilizer, the composites of EPDM and organoclay without MAH-g-EPDM were also prepared.
Measurement The Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet spectrophotometer model Nexus-670 in the range of 400–4000 cm1. The KBr discs were used for powder samples. Dispersibility of the organoclay in the polymer matrix was evaluated by XRD and TEM. XRD patterns were recorded by a Phillips X’Pert X-ray generator with Cu K radiation at 40 kV and 40 mA. The diffractograms were scanned in 2 ranges from 1 to 9 at a rate of 2.4 /min. The basal spacing of the silicate layers, d, was calculated according to Bragg’s equation, ¼ 2dsin. A transmission electron micrograph was obtained with a transmission electron microscope (H-800, Hitachi Co.) using an acceleration voltage of 200 kV. The tensile measuring test was carried out with universal tensile tester (Model DSC-5000, Shimadzu Co.) at 25 C with a head speed of 500 mm/min. The shore hardness was measured by using a Shore-A hardness instrument (LX-A, Shanghai Liuling Instrument Factory, China). All measurements were taken three times and the result values were averaged. The solvent resistance of EPDM–clay hybrids was measured as follows: The samples with similar shape (20 10 2 mm3) were entirely immersed in a container of pure xylene (xylene is a good solvent for EPDM) maintained at about 25 C for 24 h. The percentage of the change in mass was calculated by using the formula: Change in mass% ¼
M M0 100 M0
where, M0 represents the dried weight of the specimen and M the weight of the specimen after immersion.
RESULTS AND DISCUSSION Morphology of Clay-reinforced EPDM Nanocomposites IR, XRD, and TEM were used to investigate the morphology of clay-reinforced EPDM nanocomposites. Figure 1 is the IR spectra of pristine MMT and organo-MMT. The absorption band at 1090 cm1 was characteristic of Naþ-MMT (Figure 1(a)). After the treatment, organo-MMT exhibited the characteristic bands of C–H stretching at 2921 cm1 and 2850 cm1 as well as the peak at 1469 and 724 cm1 related to CH2 (Figure 1(b)). So it can be concluded that the alkyl ammonium has been exchanged with cations of the MMT interlayer. Further evidence of ion-exchange reaction and intercalation of alkyl ammonium chains between the nanolayers was supported by XRD patterns. The X-ray diffraction patterns are shown in Figure 2. The peak of organo-MMT
748
S. J. AHMADI
ET AL.
Figure 1. IR spectra of MMT and OMMT.
Figure 2. XRD patterns of (a) pure clay (MMT); (b) organoclay (OMMT); (c) EPDM þ MMT; and (d) OMMT þ MAH-g-EPDM.
(Figure 2(b)) is shifted to lower angles compared with Naþ-MMT (Figure 2(a)). Thus, the interlayer distance of Naþ-MMT was extended after organic modification with the octadecylamine. This clearly indicates the intercalation of alkyl ammonium chains between the silicate layers. Figure 2(c) shows the XRD pattern of melt mixing of organoclay with EPDM (EPDM– OMMT). It is clear that, the d-spacing of EPDM–OMMT is nearly the same as that of OMMT (Figure 2(b)). It indicates that the EPDM–OMMT has deintercalated the morphology, which may result from immiscibility between EPDM and organoclay because EPDM does not include any polar group in its backbone. Figure 2(d) shows the XRD patterns of composites of MAH-g-EPDM with organoMMT (MAH-g-EPDM–OMMT). After melt compounding, in the case of MAH-gEPDM–OMMT, the characteristic peak is widening and shifting to angles smaller than that of organoclay. This indicates that the molecules of MAH-g-EPDM are intercalated into the layered silicate of clay, expanding the basal spacing of organoclay. The strong hydrogen bonding between the maleic anhydride group and the oxygen group of the
Morphology and Characterization of EPDM Nanocomposites
749
Figure 3. XRD patterns of EPDM–organoclay nanocomposites with different clay content: (a) before and (b) after vulcanization process.
silicate as well as the shear force exerted on OMMT during melting compound can produce the driving force necessary for the intercalation [9]. The interlayer spacing of the clay as well as its compatibility with polymer are increased, so the clay galleries could easily be intercalated with the EPDM. The XRD results of EPDM–clay nanocomposites with different clay contents, (a) before and (b) after vulcanization process, are shown in Figure 3. Before the vulcanization process for some nanocomposites with low clay content (2–10 phr), an exfoliated structure was obtained as verified by the absence of any distinct diffraction peak, as well as by TEM observations. For some composites with high clay content (15 phr), there is a weak peak around 2.5 , suggesting that some portion of the organoclay is aggregated, but it can be seen that after the vulcanization process, the weak peak of the high clay content has disappeared, which means the dispersion of organo-MMT in the EPDM matrix is significantly improved and the basal spacing of clay layers has increased. A homogenized dispersion of MMT in the EPDM matrix was obtained by applying high shear stress during compounding by roll mill, which causes the agglomerates of organoclay to become smaller and the intercalated structure to be transferred into an exfoliated structure. This is caused by the shear-induced diffusion of polymer chains into the agglomerates and the diffusion of polymer chains within the silicate galleries. In general, the organophilic modification is accompanied by increasing interlayer distances. Then, the energy gained through favorable interaction between the compatiblizer (MAH-g-EPDM) and the organo-silicate layer, while exerting a shearing force, leads to obtaining exfoliation structure. It is to be emphasized that the coexistence of the three parameters, clay modification, use of a suitable compatiblizer, and exerting a shearing force were necessary for achieving a high degree of exfoliation structure in EPDM–clay nanocomposites. Figure 4 shows the XRD patterns of a vulcanized EPDM–clay conventional composite. It contains two peaks at 7.1 corresponding to a basal spacing of 1.24 nm and 6.6 corresponding to a basal spacing of 1.33 nm. The peak at 7.1 (1.24 nm) is related to silicate layers of MMT in EPDM clay composites (Figure 2(a)) and indicates that the gallery distance of clay is not changed. Thus, the EPDM chains does not intercalate into the gallery of MMT. The second peak that appears at 2 ¼ 6.6 (1.33 nm) which can also be seen in the vulcanized EPDM and vulcanized mixture of EPDM þ MAH-g-EPDM (Figure 5), maybe caused by a high content of ethylene in the EPDM that we used in this
750
S. J. AHMADI
ET AL.
Figure 4. XRD patterns of vulcanized EPDM–clay composites with different clay content.
Figure 5. XRD patterns of: (a) vulcanized EPDM and (b) vulcanized EPDM þ MAH-g-EPDM.
study (about 68.5–74.5%). However, in the EPDM nanocomposites, this peak does not exist (Figure 3). The strong interaction between the organoclay and the molecules of polymer can change the morphology of nanocomposites. Transmission electron microscopy was also used to visually evaluate the morphology and dispersion of organoclay in the polymer matrix. Figure 6(a) determines that every layer of silicate is well dispersed in the EPDM matrix and a high degree of exfoliation of nanometer-size clay particles is obtained; the dark lines represent a cross section of the clay layers. But in Figure 6(b), for EPDM composites prepared by pristine MMT, the agglomerated particles are detected. From the above result, it can be concluded that the modification of MMT with an organic modifier and MAH-g-EPDM as compatiblizer, as well as the high shear stress are all important factors influencing the morphology development of EPDM–organoclay nanocomposites.
Influence of Clay Dispersion on Mechanical Properties The mechanical properties tests were performed on EPDM–clay hybrids under different MMT loadings and the results are shown in Figures 7–9. Figure 7 shows that below 5 phr OMMT, the tensile strength increases with increasing clay content but decreases a little bit above 5 phr. The EPDM–organoclay nanocomposites have higher values of tensile strength when compared to the macrocomposites with the pristine clay and unfilled EPDM.
Morphology and Characterization of EPDM Nanocomposites
751
Figure 6. Transmission electron micrographs of: (a) EPDM–organoclay nanocomposites and (b) EPDM–clay composites.
For example, the tensile strength of nanocomposites with 5 phr organoclay is about 175% higher than that of the conventional composites with the same value of clay content and also 158% bigger than that of unfilled EPDM. The tensile modulus of EPDM nanocomposites also is higher than that of EPDM composites and unfilled EPDM (Figure 8). For instance, by the addition of 15 phr clay content, the tensile modulus of nanocomposites is increased to about 60% higher than that of EPDM macrocomposites and 97% higher than that of EPDM without clay. The enhancement in tensile strength and tensile modulus is directly attributed to the dispersion of nanosilicate layers in the EPDM matrix and strong interaction between EPDM and organoclay. The lower tensile strength above 5 phr OMMT can be attributed to the inevitable aggregation of the silicate layers in high clay content. A similar result is observed for the shore hardness of EPDM–clay hybrids (Figure 9). The shore hardness of EPDM–organoclay nanocomposites increases with increase in clay content. Improvement in hardness with increase in clay content for EPDM
752
S. J. AHMADI
ET AL.
Figure 7. Effect of filler loading on tensile strength of: (a) EPDM–organoclay nanocomposites and (b) EPDM– clay composites.
Figure 8. Effect of filler loading on tensile modulus of: (a) EPDM–organoclay nanocomposites and (b) EPDM– clay composites.
Figure 9. Effect of filler loading on shore hardness for: (a) EPDM–organoclay nanocomposites, and (b) EPDM–clay composites.
Morphology and Characterization of EPDM Nanocomposites
753
Figure 10. Solvent resistance of unfilled-EPDM, EPDM–organoclay nanocomposites, and EPDM–clay composites with different clay content in pure xylene after 24 h at 25 C.
nanocomposites is observed to be greater than that of the conventional EPDM. The uniformly distributed exfoliation platelets improved the stiffness of EPDM. The presence of these stiff clay platelets and entanglement of polymer chains made the EPDM nanocomposites harder.
Solvent Resistance Property of Clay-reinforced EPDM Nanocomposites Figure 10 shows the results of the solvent uptake measurements of unfilled-EPDM, EPDM–organoclay nanocomposites, and EPDM–clay composites. The nanocomposites exhibited not only superior mechanical properties but also exceptional solvent resistance; the solvent uptake ratio of the EPDM nanocomposites was lower than that of the unfilledEPDM and the conventional EPDM composites. For example, the solvent uptake ratio was decreased from 358.8% for unfilled-EPDM and 351.2% for EPDM composites (clay content of 5 phr clay) to 180.6% for EPDM nanocomposites (clay content same as that of EPDM composites). This phenomenon can be explained by the fact that the large aspect ratio of organo-MMT layers possesses excellent barrier properties, and especially the exfoliated structure of MMT layers can maximize the available surface area of the reinforcing phase [15].
CONCLUSIONS EPDM nanocomposites and EPDM conventional composites were prepared through the melt blending method. Their morphologies were demonstrated by XRD, IR and TEM, the results show that the silicate layers of nanocomposites were exfoliated and dispersed uniformly in the polymer matrix. The nanocomposites had superior mechanical properties and solvent resistance when compared to that of the conventional composites. These enhancements are attributed to the more uniformly dispersed nanoparticles of organoclay in the polymer matrix.
754
S. J. AHMADI
ET AL.
ACKNOWLEDGMENTS The authors wish to thank the Harbin University of Science and Technology for providing the twin-screw blender instrument and also to thank Heilongjiang Plastics Engineering Institute of Technology for their technical help in the tensile properties measurements.
REFERENCES 1. Giannelis, E.P. (1996). Polymer Layered Silicate Nanocomposites, J. Adv. Mater., 8: 29–35. 2. Lan, T. and Pinnavaia, T.J. (1994). Clay-Reinforced Epoxy Nanocomposites, J. Chem. Mater., 6: 2216–2219. 3. Uhl, F.M. and Wilkie, C.A. (2002). Polystyrene/Graphite Nanocomposites: Effect on Thermal Stability, Polymer Degradation and Stability, 76(1): 111–122. 4. Agag, T., Koga, T. and Takeichi, T. (2001). Studies on Thermal and Mechanical Properties of Polyimide-Clay Nanocomposites, Polymer, 42(8): 3399–3408. 5. Messersmith, P.B. and Giannelis, E.P. (1993). Polymer-Layered Silicate Nanocomposites: In-situ Intercalative Polymerization of "-Caprolactone in Layered Silicates, Chem. Mater., 5(8): 1064–1066. 6. Kojima, Y., Usuki, A., Kawasumi, M., Okada, A., Fujushima, A., Kurauchi, T. and Kamigaito, O. (1993). Mechanical Properties of Nylon-6-Clay Hybrid, J. Mater. Res., 8(5): 1185–1189. 7. Usuki, A., Kojima, Y., Kawasumi, M., Okada, A., Fujushima, A., Kurauchi, T. and Kamigaito, O. (1993). Synthesis of Nylon-6-Clay Hybrid, J. Mater. Res., 8(5): 1179–1183. 8. Whittingham, M.S. and Jacobson, A.E. (1982). Intercalation Chemistry, Academic Press, New York. 9. Kawasumi, M., Hasegawa, N., Kato, M., Usuki, A. and Okada, A. (1997). Preparation and Mechanical Properties of Polypropylene-Clay Hybrids, Macromolecules, 30(20): 6333–6338. 10. Kato, M., Usukai, A. and Okada, A. (1997). Synthesis of Polypropylene Oligomer-Clay Intercalation Compounds, Journal of Applied Polymer Science, 66(9): 1781–1785. 11. Hasegawa, N., Kawasumi, M., Kato, M., Usuki, A. and Okada, A. (1998). Preparation and Mechanical Properties of Polypropylene-Clay Hybrids Using a Maleic Anhydride Modified Polypropylene Oligomer, Journal of Applied Polymer Science, 67(1): 87–92. 12. Hasegawa, N., Okamoto, H., Kato, M. and Usuki, A. (2000). Preparation and Mechanical Properties of Polypropylene-Clay Hybrids Based on Modified Polypropylene and Organophilic Clay, Journal of Applied Polymer Science, 78(11): 1918–1922. 13. Usukai, A., Tukigase, A. and Kato, M. (2002). Preparation and Properties of EPDM-Clay Hybrids, Polymer, 43(8): 2185–2189. 14. Chang, Y.W., Yang, Y., Ryu, S. and Nah, C. (2002). Preparation and Properties of EPDM/ Organo-Montmorillonite Hybrid Nanocomposites, Polymer International, 51(4): 319–324. 15. Pinnavaia, T.J. and Beall, G.W. (2000). Nanocomposite Technology for Enhancing the Gas Barrier of Polyethlene Terephthalate, In: Polymer-Clay Nanocomposites, pp. 207–211, John Wiley & Sons Ltd, England.
ABSTRACT: The clay-reinforced ethylene propylene diene terpolymer (EPDM) nanocomposites with organo-montmorillonite (OMMT) and EPDM–clay conventional composites with pristine MMT are compared in terms of their morphology, mechanical properties, and solvent resistance. The maleic anhydride grafted EPDM (MAH-g-EPDM) is used as the compatiblizer for preparing clay-reinforced EPDM nanocomposites via a melt intercalation process. The formation of exfoliated nanocomposites is confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Furthermore, the morphological change during the mixing times of vulcanization process through exerting shearing force on clay particles is discussed. The mechanical properties and solvent resistance of clay-reinforced EPDM nanocomposites are examined as a function of the organoclay content in the matrix of the polymer, the result shows remarkable improvement relative to that of conventional composites. KEY WORDS: clay-reinforced EPDM nanocomposites, exfoliation, organoclay, melt intercalation, morphology.
INTRODUCTION fillers to a polymer matrix has been demonstrated to be an effective method to achieve reinforcement of the polymer hybrid materials consisting of organic and inorganic components. Nanocomposites are formed when phase mixing occurs on a nanometer length scale. Due to the improved phase morphology and interfacial properties, nanocomposites exhibit mechanical, thermal, barrier, and chemical properties superior to conventional composites [1–7]. Montmorillonite is the most commonly used clay to prepare nanocomposites, but the lack of affinity between hydrophilic layered silicates and hydrophobic polymer makes them difficult to be miscible
A
DDITION OF INORGANIC
*Author to whom correspondence should be addressed. E-mail: [email protected]
Journal of COMPOSITE MATERIALS, Vol. 39, No. 8/2005 0021-9983/05/08 0745–10 $10.00/0 DOI: 10.1177/0021998305048154 ß 2005 Sage Publications
745
746
S. J. AHMADI
ET AL.
at the nanoscale level. Thus, chemical modification of layered silicates is important in the preparation of nanocomposites. This can be done by an ion-exchange reaction through ions that already exist in clay such as Kþ, Naþ, Caþ, Mgþ, and organic cations such as alkyl ammonium ions to form the organosilicate [8]. The organosilicate can be broken down into their nanoscale building blocks and uniformly dispersed in the polymer matrix. When the clay platelets are thoroughly dispersed in the polymer matrix, the nanocomposites are ‘exfoliated.’ EPDM is an unsaturated polyolefin rubber with a wide range of applications. However, it is incompatible with polar organophilic clay to prepare products having desired properties because it does not include any polar group in its backbone. Recently, Kato et al. [9–12] have reported a new approach to prepare nonpolar polymer clay hybrids by using a functional oligomer. Till now, there are few studies on the formation, morphology, and properties of EPDM–clay nanocomposites. Usuki et al. [13] and Chang et al. [14] have reported the preparation and mechanical properties of EPDM–clay nanocomposites, prepared via the vulcanization process by using some special vulcanization accelerators and the melt compounding process with a liquid low molecular weight EPDM, respectively. In this paper, we compare the clay-reinforced EPDM nanocomposites with EPDM– clay conventional composites prepared through the melt blending method by using maleic anhydride grafted EPDM (MAH-g-EPDM) as a compatibilizer, in terms of their morphology, mechanical properties, and solvent resistance. The morphological change caused by shearing force during the mixing times of vulcanization process is also discussed.
EXPERIMENTAL Materials Pure sodium montmorillonite (Kunipia-F) with a cation-exchange capacity (CEC) of 119 meq/100 g was supplied by Kunimine Mining Ind. Co. (Tokyo, Japan). Octadecylamine purchased from Fluka was used as the organic modifier for MMT. MAH-g-EPDM (0.8 wt%) oligomer from Huzhou Genius Engineering Plastics Co. Ltd (Huzhou, China) was used. The EPDM (J-3062E) ENB type was obtained from Jilin Chemical Ind. Co., Ltd (Jilin, China). All chemicals were used without further purification.
Preparation of Organophilic Clay and Clay-reinforced EPDM Nanocomposites The organophilic clay was prepared via ion-exchange reaction in water by using octadecylamine as the reference [11]. In order to prepare clay-reinforced EPDM nanocomposites, different amounts of organoclay powder were premixed by shaking with 100 phr (part per hundred of rubber by weight) of EPDM and 20 phr of MAH-g-EPDM in a bag. This mixture was melt blended together in a twin-screw blender, model RM-200, with a chamber size of about 40 cm3 at 150 C. The rotational speed of the screw was 90 rpm and mixing time was 15 min for all cases. To compare the properties of nanocomposites with that of the conventional composites, the EPDM–clay composites with pristine MMT were prepared under the above-mentioned conditions. Then, the EPDM hybrids (100 phr) were sequentially mixed with zinc oxide (5 phr), stearic acid (1 phr),
Morphology and Characterization of EPDM Nanocomposites
747
vulcanization accelerator [M (2-Mercapto benzothiazole, 0.5 phr) and TMTD (tetramethyl thiuram disulfide, 1.5 phr)] and sulfur (1.5 phr) by using a roll mill, model SK-160B, at a temperature about 60 C. The vulcanization was carried out in standard hot press at 150 C for 30 min to yield rubber sheets (340 150 2 mm3). To study the effect of MAH-g-EPDM as a compatibilizer, the composites of EPDM and organoclay without MAH-g-EPDM were also prepared.
Measurement The Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet spectrophotometer model Nexus-670 in the range of 400–4000 cm1. The KBr discs were used for powder samples. Dispersibility of the organoclay in the polymer matrix was evaluated by XRD and TEM. XRD patterns were recorded by a Phillips X’Pert X-ray generator with Cu K radiation at 40 kV and 40 mA. The diffractograms were scanned in 2 ranges from 1 to 9 at a rate of 2.4 /min. The basal spacing of the silicate layers, d, was calculated according to Bragg’s equation, ¼ 2dsin. A transmission electron micrograph was obtained with a transmission electron microscope (H-800, Hitachi Co.) using an acceleration voltage of 200 kV. The tensile measuring test was carried out with universal tensile tester (Model DSC-5000, Shimadzu Co.) at 25 C with a head speed of 500 mm/min. The shore hardness was measured by using a Shore-A hardness instrument (LX-A, Shanghai Liuling Instrument Factory, China). All measurements were taken three times and the result values were averaged. The solvent resistance of EPDM–clay hybrids was measured as follows: The samples with similar shape (20 10 2 mm3) were entirely immersed in a container of pure xylene (xylene is a good solvent for EPDM) maintained at about 25 C for 24 h. The percentage of the change in mass was calculated by using the formula: Change in mass% ¼
M M0 100 M0
where, M0 represents the dried weight of the specimen and M the weight of the specimen after immersion.
RESULTS AND DISCUSSION Morphology of Clay-reinforced EPDM Nanocomposites IR, XRD, and TEM were used to investigate the morphology of clay-reinforced EPDM nanocomposites. Figure 1 is the IR spectra of pristine MMT and organo-MMT. The absorption band at 1090 cm1 was characteristic of Naþ-MMT (Figure 1(a)). After the treatment, organo-MMT exhibited the characteristic bands of C–H stretching at 2921 cm1 and 2850 cm1 as well as the peak at 1469 and 724 cm1 related to CH2 (Figure 1(b)). So it can be concluded that the alkyl ammonium has been exchanged with cations of the MMT interlayer. Further evidence of ion-exchange reaction and intercalation of alkyl ammonium chains between the nanolayers was supported by XRD patterns. The X-ray diffraction patterns are shown in Figure 2. The peak of organo-MMT
748
S. J. AHMADI
ET AL.
Figure 1. IR spectra of MMT and OMMT.
Figure 2. XRD patterns of (a) pure clay (MMT); (b) organoclay (OMMT); (c) EPDM þ MMT; and (d) OMMT þ MAH-g-EPDM.
(Figure 2(b)) is shifted to lower angles compared with Naþ-MMT (Figure 2(a)). Thus, the interlayer distance of Naþ-MMT was extended after organic modification with the octadecylamine. This clearly indicates the intercalation of alkyl ammonium chains between the silicate layers. Figure 2(c) shows the XRD pattern of melt mixing of organoclay with EPDM (EPDM– OMMT). It is clear that, the d-spacing of EPDM–OMMT is nearly the same as that of OMMT (Figure 2(b)). It indicates that the EPDM–OMMT has deintercalated the morphology, which may result from immiscibility between EPDM and organoclay because EPDM does not include any polar group in its backbone. Figure 2(d) shows the XRD patterns of composites of MAH-g-EPDM with organoMMT (MAH-g-EPDM–OMMT). After melt compounding, in the case of MAH-gEPDM–OMMT, the characteristic peak is widening and shifting to angles smaller than that of organoclay. This indicates that the molecules of MAH-g-EPDM are intercalated into the layered silicate of clay, expanding the basal spacing of organoclay. The strong hydrogen bonding between the maleic anhydride group and the oxygen group of the
Morphology and Characterization of EPDM Nanocomposites
749
Figure 3. XRD patterns of EPDM–organoclay nanocomposites with different clay content: (a) before and (b) after vulcanization process.
silicate as well as the shear force exerted on OMMT during melting compound can produce the driving force necessary for the intercalation [9]. The interlayer spacing of the clay as well as its compatibility with polymer are increased, so the clay galleries could easily be intercalated with the EPDM. The XRD results of EPDM–clay nanocomposites with different clay contents, (a) before and (b) after vulcanization process, are shown in Figure 3. Before the vulcanization process for some nanocomposites with low clay content (2–10 phr), an exfoliated structure was obtained as verified by the absence of any distinct diffraction peak, as well as by TEM observations. For some composites with high clay content (15 phr), there is a weak peak around 2.5 , suggesting that some portion of the organoclay is aggregated, but it can be seen that after the vulcanization process, the weak peak of the high clay content has disappeared, which means the dispersion of organo-MMT in the EPDM matrix is significantly improved and the basal spacing of clay layers has increased. A homogenized dispersion of MMT in the EPDM matrix was obtained by applying high shear stress during compounding by roll mill, which causes the agglomerates of organoclay to become smaller and the intercalated structure to be transferred into an exfoliated structure. This is caused by the shear-induced diffusion of polymer chains into the agglomerates and the diffusion of polymer chains within the silicate galleries. In general, the organophilic modification is accompanied by increasing interlayer distances. Then, the energy gained through favorable interaction between the compatiblizer (MAH-g-EPDM) and the organo-silicate layer, while exerting a shearing force, leads to obtaining exfoliation structure. It is to be emphasized that the coexistence of the three parameters, clay modification, use of a suitable compatiblizer, and exerting a shearing force were necessary for achieving a high degree of exfoliation structure in EPDM–clay nanocomposites. Figure 4 shows the XRD patterns of a vulcanized EPDM–clay conventional composite. It contains two peaks at 7.1 corresponding to a basal spacing of 1.24 nm and 6.6 corresponding to a basal spacing of 1.33 nm. The peak at 7.1 (1.24 nm) is related to silicate layers of MMT in EPDM clay composites (Figure 2(a)) and indicates that the gallery distance of clay is not changed. Thus, the EPDM chains does not intercalate into the gallery of MMT. The second peak that appears at 2 ¼ 6.6 (1.33 nm) which can also be seen in the vulcanized EPDM and vulcanized mixture of EPDM þ MAH-g-EPDM (Figure 5), maybe caused by a high content of ethylene in the EPDM that we used in this
750
S. J. AHMADI
ET AL.
Figure 4. XRD patterns of vulcanized EPDM–clay composites with different clay content.
Figure 5. XRD patterns of: (a) vulcanized EPDM and (b) vulcanized EPDM þ MAH-g-EPDM.
study (about 68.5–74.5%). However, in the EPDM nanocomposites, this peak does not exist (Figure 3). The strong interaction between the organoclay and the molecules of polymer can change the morphology of nanocomposites. Transmission electron microscopy was also used to visually evaluate the morphology and dispersion of organoclay in the polymer matrix. Figure 6(a) determines that every layer of silicate is well dispersed in the EPDM matrix and a high degree of exfoliation of nanometer-size clay particles is obtained; the dark lines represent a cross section of the clay layers. But in Figure 6(b), for EPDM composites prepared by pristine MMT, the agglomerated particles are detected. From the above result, it can be concluded that the modification of MMT with an organic modifier and MAH-g-EPDM as compatiblizer, as well as the high shear stress are all important factors influencing the morphology development of EPDM–organoclay nanocomposites.
Influence of Clay Dispersion on Mechanical Properties The mechanical properties tests were performed on EPDM–clay hybrids under different MMT loadings and the results are shown in Figures 7–9. Figure 7 shows that below 5 phr OMMT, the tensile strength increases with increasing clay content but decreases a little bit above 5 phr. The EPDM–organoclay nanocomposites have higher values of tensile strength when compared to the macrocomposites with the pristine clay and unfilled EPDM.
Morphology and Characterization of EPDM Nanocomposites
751
Figure 6. Transmission electron micrographs of: (a) EPDM–organoclay nanocomposites and (b) EPDM–clay composites.
For example, the tensile strength of nanocomposites with 5 phr organoclay is about 175% higher than that of the conventional composites with the same value of clay content and also 158% bigger than that of unfilled EPDM. The tensile modulus of EPDM nanocomposites also is higher than that of EPDM composites and unfilled EPDM (Figure 8). For instance, by the addition of 15 phr clay content, the tensile modulus of nanocomposites is increased to about 60% higher than that of EPDM macrocomposites and 97% higher than that of EPDM without clay. The enhancement in tensile strength and tensile modulus is directly attributed to the dispersion of nanosilicate layers in the EPDM matrix and strong interaction between EPDM and organoclay. The lower tensile strength above 5 phr OMMT can be attributed to the inevitable aggregation of the silicate layers in high clay content. A similar result is observed for the shore hardness of EPDM–clay hybrids (Figure 9). The shore hardness of EPDM–organoclay nanocomposites increases with increase in clay content. Improvement in hardness with increase in clay content for EPDM
752
S. J. AHMADI
ET AL.
Figure 7. Effect of filler loading on tensile strength of: (a) EPDM–organoclay nanocomposites and (b) EPDM– clay composites.
Figure 8. Effect of filler loading on tensile modulus of: (a) EPDM–organoclay nanocomposites and (b) EPDM– clay composites.
Figure 9. Effect of filler loading on shore hardness for: (a) EPDM–organoclay nanocomposites, and (b) EPDM–clay composites.
Morphology and Characterization of EPDM Nanocomposites
753
Figure 10. Solvent resistance of unfilled-EPDM, EPDM–organoclay nanocomposites, and EPDM–clay composites with different clay content in pure xylene after 24 h at 25 C.
nanocomposites is observed to be greater than that of the conventional EPDM. The uniformly distributed exfoliation platelets improved the stiffness of EPDM. The presence of these stiff clay platelets and entanglement of polymer chains made the EPDM nanocomposites harder.
Solvent Resistance Property of Clay-reinforced EPDM Nanocomposites Figure 10 shows the results of the solvent uptake measurements of unfilled-EPDM, EPDM–organoclay nanocomposites, and EPDM–clay composites. The nanocomposites exhibited not only superior mechanical properties but also exceptional solvent resistance; the solvent uptake ratio of the EPDM nanocomposites was lower than that of the unfilledEPDM and the conventional EPDM composites. For example, the solvent uptake ratio was decreased from 358.8% for unfilled-EPDM and 351.2% for EPDM composites (clay content of 5 phr clay) to 180.6% for EPDM nanocomposites (clay content same as that of EPDM composites). This phenomenon can be explained by the fact that the large aspect ratio of organo-MMT layers possesses excellent barrier properties, and especially the exfoliated structure of MMT layers can maximize the available surface area of the reinforcing phase [15].
CONCLUSIONS EPDM nanocomposites and EPDM conventional composites were prepared through the melt blending method. Their morphologies were demonstrated by XRD, IR and TEM, the results show that the silicate layers of nanocomposites were exfoliated and dispersed uniformly in the polymer matrix. The nanocomposites had superior mechanical properties and solvent resistance when compared to that of the conventional composites. These enhancements are attributed to the more uniformly dispersed nanoparticles of organoclay in the polymer matrix.
754
S. J. AHMADI
ET AL.
ACKNOWLEDGMENTS The authors wish to thank the Harbin University of Science and Technology for providing the twin-screw blender instrument and also to thank Heilongjiang Plastics Engineering Institute of Technology for their technical help in the tensile properties measurements.
REFERENCES 1. Giannelis, E.P. (1996). Polymer Layered Silicate Nanocomposites, J. Adv. Mater., 8: 29–35. 2. Lan, T. and Pinnavaia, T.J. (1994). Clay-Reinforced Epoxy Nanocomposites, J. Chem. Mater., 6: 2216–2219. 3. Uhl, F.M. and Wilkie, C.A. (2002). Polystyrene/Graphite Nanocomposites: Effect on Thermal Stability, Polymer Degradation and Stability, 76(1): 111–122. 4. Agag, T., Koga, T. and Takeichi, T. (2001). Studies on Thermal and Mechanical Properties of Polyimide-Clay Nanocomposites, Polymer, 42(8): 3399–3408. 5. Messersmith, P.B. and Giannelis, E.P. (1993). Polymer-Layered Silicate Nanocomposites: In-situ Intercalative Polymerization of "-Caprolactone in Layered Silicates, Chem. Mater., 5(8): 1064–1066. 6. Kojima, Y., Usuki, A., Kawasumi, M., Okada, A., Fujushima, A., Kurauchi, T. and Kamigaito, O. (1993). Mechanical Properties of Nylon-6-Clay Hybrid, J. Mater. Res., 8(5): 1185–1189. 7. Usuki, A., Kojima, Y., Kawasumi, M., Okada, A., Fujushima, A., Kurauchi, T. and Kamigaito, O. (1993). Synthesis of Nylon-6-Clay Hybrid, J. Mater. Res., 8(5): 1179–1183. 8. Whittingham, M.S. and Jacobson, A.E. (1982). Intercalation Chemistry, Academic Press, New York. 9. Kawasumi, M., Hasegawa, N., Kato, M., Usuki, A. and Okada, A. (1997). Preparation and Mechanical Properties of Polypropylene-Clay Hybrids, Macromolecules, 30(20): 6333–6338. 10. Kato, M., Usukai, A. and Okada, A. (1997). Synthesis of Polypropylene Oligomer-Clay Intercalation Compounds, Journal of Applied Polymer Science, 66(9): 1781–1785. 11. Hasegawa, N., Kawasumi, M., Kato, M., Usuki, A. and Okada, A. (1998). Preparation and Mechanical Properties of Polypropylene-Clay Hybrids Using a Maleic Anhydride Modified Polypropylene Oligomer, Journal of Applied Polymer Science, 67(1): 87–92. 12. Hasegawa, N., Okamoto, H., Kato, M. and Usuki, A. (2000). Preparation and Mechanical Properties of Polypropylene-Clay Hybrids Based on Modified Polypropylene and Organophilic Clay, Journal of Applied Polymer Science, 78(11): 1918–1922. 13. Usukai, A., Tukigase, A. and Kato, M. (2002). Preparation and Properties of EPDM-Clay Hybrids, Polymer, 43(8): 2185–2189. 14. Chang, Y.W., Yang, Y., Ryu, S. and Nah, C. (2002). Preparation and Properties of EPDM/ Organo-Montmorillonite Hybrid Nanocomposites, Polymer International, 51(4): 319–324. 15. Pinnavaia, T.J. and Beall, G.W. (2000). Nanocomposite Technology for Enhancing the Gas Barrier of Polyethlene Terephthalate, In: Polymer-Clay Nanocomposites, pp. 207–211, John Wiley & Sons Ltd, England.
