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Effect of dispersion of sepiolite in sepiolite-NBR composite on the tensile strength
Composites: Part B 44 (2013) 260–265
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Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Effect of dispersion of sepiolite in sepiolite-NBR composite on the tensile strength T. Takei a,⇑, R. Oda a, A. Miura a, N. Kumada a, N. Kinomura a, R. Ohki b, H. Koshiyama b a b
Center for Crystal Science and Technology, University of Yamanashi, 7-32 Miyamae, Kofu, Yamanashi 4008511, Japan Asktechnica Co. Ltd., 1488 Ichikawadaimon, Ichikawamisato, Nishiyatsushiro, Yamanashi 4093601, Japan
a r t i c l e
i n f o
Article history: Received 28 December 2011 Received in revised form 13 April 2012 Accepted 22 May 2012 Available online 1 June 2012 Keywords: A. Fibers B. Strength D. Electron microscopy E. Isostatic processing
a b s t r a c t The effect of dispersion of sepiolite in a sepiolite-NBR composite on the tensile strength of the composite was examined. Fibrous sepiolite was ultrasonically treated in distilled water with various types of dispersant. The ultrasonicated sepiolite was mixed with NBR latex to form a sepiolite-NBR complex. The tensile strength of the complex was examined to evaluate the degree of dispersion of sepiolite. The tensile strength results for the composites confirmed that a lower ultrasonication frequency produced a high strength in the composite. The tensile strength actually increased from 3 to 8 MPa as a result of ultrasonication at 28 kHz. To improve and stabilize the dispersion state, five kinds of dispersant were used. The addition of two dispersants in particular, ammonium polycarboxylate and amino alcohol polyphosphate, resulted in an increase of the strength from 8 to 15–16 MPa. When the composites were heated at 450 °C, the strengths of the composites produced with and without the dispersant were approximately 1.4–1.7 and 0.85 MPa, respectively. Thus, the dispersant still affected the strength of the heated samples. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Sepiolite (Mg8(OH)4(Si12O30)nH2O) is a strong candidate for a post-asbestos material, because it has a physically flexible fibrous shape. Sepiolite has a 2:1 layered structure composed of a brucite layer sandwiched by two tetrahedral (Si2O5)2 layers, and periodic dissociation forms micropore channels of 1.4 nm in width [1,2]. These micropore channels usually incorporate H2O molecules, and the pores sometimes serve as adsorption sites after dehydration. It has been reported that the sepiolite phase releases the zeolitic H2O at around 120 °C without collapse of the channel structure, changes topotactically at 350 °C, and consequently forms the anhydrate at 500 °C [2–6]. Thus, sepiolite has a relatively high thermal stability. Most mineral sepiolite is produced in China, Spain, Turkey, and the USA. Since Spanish, Turkish, and US sepiolites have bulky or short-fibrous particle shapes, such sepiolites are limited in their potential uses. On the other hand, Chinese sepiolite has fibrous particle shapes. The shape is a strong advantage for use in composites applicable to structural, heat-insulating, and adsorption materials and so on, as follows [7–11]. A structural composite consisting of sepiolite, kaolinite, and nitrile butadiene rubber (NBR) was studied by Ohki et al. [7]. Fuente et al. reported on a cement composite in which sepiolite was used as an additive [8]. As for heatinsulating materials, composites including sepiolite in an inorganic matrix (alumina, muscovite, and glass fiber) were reported by ⇑ Corresponding author. Tel.: +81 55 220 8616; fax: +81 55 254 3035. E-mail address: [email protected] (T. Takei). 1359-8368/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2012.05.034
Noda et al. [9], and composites including sepiolite in an organic matrix (epoxy resin) were reported by Nohales et al. [10]. Konta summarized many applications of clay minerals including sepiolite [11]. Formation of cordierite from sepiolite will enhance the thermal durability of the composite for use as heat-insulating material, because cordierite has a very low thermal expansion coefficient. Noda et al. also reported that the cordierite phase can be formed by addition of an alumina component [12]. In general, even long-fibrous mineral sepiolites (most of those made in China) form a bundled texture. Therefore, fibrillation of the bundle may be necessary for use of the sepiolite. However, control of the degree of fibrillation is very difficult, because the degree of fibrillation is very difficult to measure. Ultrasonication is a strong candidate process for dispersion of inorganic materials. The relationship between the frequency and the radius of the cavities produced by sonication can be expressed as follows:
f r ¼ 3:26;
ð1Þ
where f and r are the frequency (Hz) and radius (m) of the cavities [13]. Thus, a lower frequency is better at dispersing agglomerates, because a low frequency produces large cavities. However, agglomerates composed of very small grains (e.g., several nanometers in size) can only be dispersed using small cavities. In this case, a high frequency is necessary for dispersion. For fibrous sepiolite, it is necessary to examine whether ultrasonication can promote fibrillation or dispersion. Therefore, we examine the fibrillation and dispersion of Chinese sepiolite by ultrasonication, as well as the effect of the degree of dispersion on the physical strength of a composite with NBR.
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2. Experiments 2.1. Preparation of composite Sepiolite was added to distilled water to a concentration of 17.5 g/L, and five kinds of dispersants were added into sample suspensions to a concentration of 4 mL/L. The dispersants used were as follows: 40% ammonium polynaphthalene sulfonate (PWA-40 K), 40% ammonium polycarboxylate (SN dispersant 5468), 50% amino alcohol polyphosphate (SN dispersant 2060), 50% dioctyl sulfosuccinate (Nopcowet 50), and 93% nonionic dispersant (SN wet 126). Each percentage means concentration of as-received dispersant itself. All dispersants used in this paper were provided by San Nopco Limited. These dispersants were designated as 40 K, 5468, 2060, N50, and SN126, respectively. The dispersants can be divided into two groups: 40 K, 5468, and 2060 are polymers, and N50 and SN126 are nonpolymer dispersants. After the dispersant was added, the suspension was ultrasonicated (VS-100III, As One Corp.) for 0, 30, 60, 90, and 120 min to disperse the sepiolite bundle. Ultrasonication frequencies of 28, 45, and 100 kHz were used. NBR latex was added to the ultrasonicated suspension to a concentration of 7.5 g/L, and the suspension was stirred for 5 min. After that, aluminum sulfate aqueous solution was added into the suspension as a coagulation agent to a concentration of 50 mmol/L. Some sample suspensions were prone to foaming during ultrasonication, so two defoaming methods were carried out: addition of a 0.2 mL/L emulsion-type silicone antifoamer (TSA732A, Momentive Performance Materials Inc.) or use of a defoaming device (UM-117, Japan Unix Co., Ltd.). The defoaming device defoams the solution by using the centrifugal force produced by a planetary mixing action. In this experiment without silicone antifoamer, the suspension in the pot was revolved at 1500 rpm for 3 min with rotation at 300 rpm. The resultant defoamed suspensions (defoamed by addition of the silicone antifoamer and by the defoaming device) was then treated by suction filtration for 30 min, dried at 50 °C overnight, and pressed at 10 MPa for 30 min. The resulting sample plate was stamped out with a die whose size is consistent with the type 1 dumbbell (120 mm 25 mm) of JISK-6251. 2.2. Characterization The texture of the obtained composite was observed by field emission scanning electron microscopy (FE-SEM; JSM-6500F, Jeol) and optical microscopy (VHX-200, Keyence Corp.) to examine the degree of dispersion. The tensile strength of the composite was also measured to investigate the effects of the degree of dispersion, ultrasonication, and defoaming. The prepared test piece was used as a tensile specimen in a tensile tester (TENSILON RTF-1325, A&D Company, Limited) with a tensile rate of 300 mm/min.
Fig. 1. Tensile strength of sepiolite-NBR composites versus ultrasonication period.
of many unbroken cavities. Thus, ultrasonication at a low frequency results in high dispersion of the sepiolite to yield a high tensile strength for 30 min of ultrasonication, although the generation of large bubbles may reduce the strength of the composite for longer ultrasonication periods. To decrease the number of these bubbles, defoaming treatments were carried out, as mentioned in the experimental section. Fig. 2 shows the changes in tensile strength of the composite produced by different defoaming treatments as functions the ultrasonication period at 28 kHz. The strengths of the two composites increase up to 60 minutes of ultrasonication, then reach a plateau for longer periods. These results confirm that both defoaming treatments worked very well at decreasing the size and number of unwanted bubbles. Between the two defoaming methods, the strength of the composite produced using the defoaming device is larger than the strength of the composite produced using silicone antifoamer. This difference could exist because the antifoamer prevented direct contact of the sepiolite with the NBR. Consequently, sepiolite should be ultrasonicated at 28 kHz, and the suspension must be treated by a defoaming device for all samples.
3. Results and discussion 3.1. Effect of frequency on ultrasonication and defoaming treatments Fig. 1 shows the relationships between the ultrasonication period and the tensile strength of the sepiolite composite with NBR. All samples show increases in tensile strength up to 30 min, followed by decreases for longer periods. The maximum tensile strengths are approximately 5.6, 4.7, and 3.6 MPa for the composites treated with 28, 45, and 100 kHz ultrasonication. This trend results from the cavities and bubbles generated within the suspension by ultrasonication. As shown in Eq. (1), a lower frequency produces larger cavities with high energies. However, the lower frequency tends to result in larger bubbles through the coalescence
Fig. 2. Tensile strength of sepiolite-NBR composites versus ultrasonication period for different defoaming treatments.
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3.2. Effect of dispersant Dispersants are often effective for dispersion of inorganic particles. In this study, five different dispersants were used for sepiolite dispersion. Fig. 3 shows the stress–strain curves of the composites produced without ultrasonication and with ultrasonication for 120 min using five dispersants. The composites produced without ultrasonication have the smallest tensile strengths, approximately 3 MPa. On the other hand, the composites produced with 120 min of ultrasonication have strengths higher than 3 MPa. The tensile strengths of the composites ultrasonicated with 40 K, 5468, and 2060 clearly increased, although the composites ultrasonicated with N50 and SN126 had reduced tensile strengths. The slopes of these curves, which reflect the Young’s modulus, are also larger for the composites produced with 40 K, 5468, and 2060 than for the others. These strengths probably result from the type of molecule in the dispersant. The 40 K, 5468, and 2060 dispersants are polymers with larger molecular weights. On the other hand, the composites produced with the nonpolymer dispersants, N50 and SN126, have lower strengths than the composite produced without dispersant. Thus, the polymer dispersants better facilitate the dispersion of sepiolite by ultrasonication than the nonpolymer dispersants. The nonpolymer dispersants may prevent contact between the sepiolite and the NBR, as was the case for the silicone antifoamer. Fig. 4 shows the relationship between the ultrasonication period and the tensile strength. The general trends are higher tensile strengths for the composites produced using polymer dispersants and lower strengths for those produced with nonpolymer dispersants, as compared to the strength of the polymer produced without dispersant. For the composites produced with 5468 and 2060 in particular among the polymer-type dispersants, the strength increased linearly with ultrasonication period and eventually reached approximately 16 MPa at 120 min. This maximum value is double that of the composite produced without dispersant. These trends imply that dispersion of the sepiolite with a polymer dispersant may have reached a stage that is unachievable without any dispersants, since no plateau of the tensile strength was observed. Fig. 5 shows FE-SEM micrographs of the composites ultrasonicated for 120 min without and with each dispersant. Additional expanded micrographs are also shown at the upper right within each micrograph. These micrographs confirm that the sepiolite
Fig. 3. Stress–strain curves of the defoamed sepiolite-NBR composites produced using an ultrasonication period of 120 min with various dispersants.
Fig. 4. Relationship between ultrasonication period and tensile strength of the defoamed sepiolite-NBR composites.
was well dispersed by the ultrasonication with 40 K, 5468, and 2060 polymer dispersants. For the composites produced with the nonpolymer dispersants, N50 and SN126, the sepiolite fiber seems to be wetted by these dispersants, which behave like grease, especially as shown in the expanded micrographs. 3.3. Strength change due to heating at 450 °C Markovic et al. have reported that NBR decomposes thermally at 400-450 °C [14]. The NBR agglomerate in this study must also be decomposed by increasing temperatures. Therefore, thermal treatment will result in a decrease in strength of the composite. In this case, sepiolite dispersion becomes important in order to avoid reduction of the strength as much as possible. Fig. 6 shows the stress–strain curves of the composite heated at 450 °C for 24 h. In all samples, the strengths were reduced to around 0.7– 1.7 MPa by heating at 450 °C. However, the composites produced with 2060 and 5468 show relatively large tensile strengths, approximately 1.4 and 1.7 MPa, compared to the others, even after the heat treatment at 450 °C. The heated composite sample produced without dispersant shows a strength of approximately 0.85 MPa. These results indicate that the strengths of all samples were rapidly reduced by the heat treatment. However, the composites ultrasonicated with 2060 and 5468 still have a slight advantage in strength. Fig. 7 shows FE-SEM micrographs of the composites made by ultrasonication for 120 min with the various dispersants and then heated at 450 °C. These micrographs show that the fibrous texture of the sepiolite was maintained after the heat treatment at 450 °C in all composites, despite the drastic decrease in strength. In micrograph (b), a relatively large tumor of approximately 10 lm can be observed. A few such tumors can be observed in all samples. The tumor may form during the heating at 450 °C as a result, at least partially, of cracks several tens of micrometers in size. The mass ratio of sepiolite to NBR in the composite is 70:30. Since the densities of sepiolite and NBR are around 2.1 and 1.0 g/cm3, respectively, the volume ratio is approximately 1:1. The NBR will decompose at 400-450 °C, as mentioned above. On the basis of these properties, the composite probably consists of a heated sepiolite fiber skeleton and no NBR after heating. Such a skeleton will exhibit the tensile strength of the heated sepiolite sheet itself. Therefore, thermal changes of the sepiolite are considered. Fig. 8 shows hightemperature XRD patterns of sepiolite up to 800 °C. These patterns
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Fig. 5. FE-SEM photographs of the defoamed sepiolite-NBR composites.
of the composite and the strength of NBR. When the pull-out effect is negligible, the Young’s modulus of a composite containing a planar-oriented fiber can be expressed by Krenchel’s equation, as follows:
Ec ¼ Ef V f f þ Em ð1 V f Þ;
ð2Þ
confirm that a calcite phase is included in Chinese sepiolite. A change of the sepiolite phase to sepiolite anhydrate [2–6] was difficult to recognize. However, certain structural changes can be observed, as follows. The 0 0 2 and 0 1 1 diffraction lines at 2h values of 6.5 and 7.4° in the as-received sepiolite vanished above 300 °C, although the 0 4 0 and 2 1 0 diffraction lines at 26.6 and 34.6° were preserved up to around 600 °C. The retention of the fibrous shape of sepiolite after heat treatment at 450 °C could possibly be owed to the stability of the short-range ordered structure. The strength of the composite heated at 450 °C, approximately 0.85 MPa, may be smaller than that of pure sepiolite because of the partially collapsed channel structure.
where Ec, Ef, and Em are the Young’s moduli of the composite, fiber, and matrix, respectively, Vf is the volume fraction of the fibers, and f is the effectiveness factor of fiber. In this study, f can be regarded to be 3/8 [16], and Vf is approximately 0.5, as mentioned above. The Young’s modulus in this equation can be replaced with tensile stress at fixed strain, assuming a linear increase of the stress–strain curve. For instance, at a strain of 3.5%, if the tensile stresses of pure sepiolite and NBR are assumed to be x and 0.0 MPa, respectively, the tensile stress of the composite can be estimated at 0.2x MPa by Krenchel’s equation. Here, the tensile stress of NBR (0.0 MPa) at 3.5% strain is estimated from the tensile stress of 2.7 MPa at 420% strain given in the literature [15]. Since the tensile strength of the composite produced without ultrasonication is approximately 3 MPa, as shown in Figs. 1 and 3, the tensile strength of the pure sepiolite x can be estimated to be approximately 15 MPa. This estimated strength is larger than that of the piece heated at 450 °C. The stresses at 3.5% strain of the composites ultrasonicated for 120 min without dispersant and with dispersants 2060 and 5468 are approximately 7, 8, and 16 MPa, respectively. In the case of the composite produced with 5468 especially, the stress is similar to the estimated stress (15 MPa) of the pure sepiolite piece. Since the mean tensile stress weighted by Vf between pure sepiolite and NBR is around 7–8 MPa, the stress of the composite produced with 5468 is two times as large as the mean stress. Such a particularly large strength due to enhancement of the pull-out effect is a strong advantage of the dispersion of sepiolite by ultrasonication with dispersant in combination with a defoaming process in this study.
3.4. Estimation of the strength of sepiolite
4. Conclusion
The tensile strength of NBR is reported to be around 2.7 MPa at 420% strain [15]. On the other hand, the tensile strength of sepiolite has not been reported because of the difficulty of preparing sample pieces composed of only sepiolite. Therefore, the tensile strength of a pure sepiolite sheet is estimated from the strength
Sepiolite dispersion was carried out by ultrasonication, defoaming, and addition of dispersant, and the effect of the degree of the dispersion on the tensile strength of the dispersion-treated sepiolite-NBR composite was examined. The following results were obtained.
Fig. 6. Stress–strain curves of the defoamed sepiolite-NBR composites heated at 450 °C.
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Fig. 7. FE-SEM photographs of the defoamed sepiolite-NBR composites heated at 450 °C.
dispersant, which had a tensile strength of approximately 0.85 MPa. These results confirmed that the composites ultrasonicated with ammonium polycarboxylate and amino alcohol polyphosphate still had a slight advantage in strength.
Acknowledgements This research was partially supported by a New Energy and Industrial Technology Development Organization (NEDO) 2010 Grant (No. 0730001). The five types of dispersant were provided for free by San Nopco Limited. References
Fig. 8. High-temperature XRD patterns of the sepiolite.
1. Ultrasonication clearly enhanced the tensile strength of the sepiolite-NBR composite. The optimum ultrasonication frequency was 28 kHz. 2. The tensile strength of the sepiolite-NBR composite increased up to around 60 min of ultrasonication and reached a plateau for longer ultrasonication times. 3. Addition of dispersants, especially ammonium polycarboxylate and amino alcohol polyphosphate, resulted in an extraordinary increase of the tensile strength from around 8 to 15–16 MPa. 4. After heat treatment at 450 °C, the composites produced with ammonium polycarboxylate and amino alcohol polyphosphate showed relatively larger tensile strengths, approximately 1.4 and 1.7 MPa, than the heated composite produced without
[1] Sánchez del Río M, Suárez M, García Romero E, Alianelli L, Felici R, Martinetto P, et al. Mg K-edge XANES of sepiolite and palygorskite. Nucl Instrum Method Phys Res B 2005;238:55–60. [2] Preisinger A. X-ray study of the structure of sepiolite. In: Swineford A, editor. Proceedings of the sixth national conference on clays and clay minerals. Berkeley, August 1957. p. 61–7. [3] Weir MR, Kuang W, Facey FA, Detellier C. Solid-state nuclear magnetic resonance study of sepiolite and partially dehydrated sepiolite. Clay Clay Miner 2002;50(2):240–7. [4] Yener N, Önal M, Üstünısßık G, Sarıkaya Y. Thermal behavior of a mineral mixture of sepiolite and dolomite. J Therm Anal Calorim 2007;88(3):813–7. [5] Frost RL, Kristóf J, Horváth E. Controlled rate thermal analysis of sepiolite. J Therm Anal Calorim 2009;98(3):749–55. [6] Alver BE, Sakici M, Yörükog˘ullari E, Yılmaz Y, Güven M. Thermal behavior and water adsorption of natural and modified sepiolite having dolomite from turkey. J Therm Anal Calorim 2008;94(3):835–40. [7] Ohki R, Takei T, Yonesaki Y, Kumada N, Kinomura N, Kobayashi M. Highly-filled composite with nonasbestine inorganic fibers. J Soc Inorg Mater Jpn 2010;17:43–8. [8] Fuente E, Jarabo R, Moral A, Blanco Á, Izquierdo L, Negro C. Effect of sepiolite on retention and drainage of suspensions of fiber-reinforced cement. Constr Build Mater 2010;24:2117–23. [9] Noda H, Matsubayashi K, Takei T, Yonesaki Y, Kumada N, Kinomura N. Thermal behavior of sepiolite composites. J Soc Inorg Mater Jpn 2009;16: 15–22. [10] Nohales A, Solar L, Porcar I, Vallo CI, Gómez CM. Morphology, flexural, and thermal properties of sepiolite modified epoxy resins with different curing agents. Euro Polym J 2006;42:3093–101. [11] Konta J. Clay and man: clay raw materials in the service of man. Appl Clay Sci 1995;10:275–335. [12] Noda H, Miyagawa K, Kobayashi M, Horiguchi H, Ozawa K, Kumada N. Preparation of cordierite from fibrous sepiolite. J Ceram Soc Jpn 2009;117(11):1236–9. [13] Roy RA. Sonochemistry and sonoluminescence. In: Crum LA, Mason TJ, Reisse JL, Suslick KS, editors. Cavitation sonophysics. NATO ASI series C:
T. Takei et al. / Composites: Part B 44 (2013) 260–265 mathematical and physical sciences, vol. 524. Netherlands: Kluwer Academic Publishers. p. 5–38. [14] Markovic G, Radovanovic´ B, Vodnik V, Marinovic´-Cincovic´ M, BudinskiSimendic´ J. Investigation of chemical interactions between CSM and NBR. Kautsch Gummi Kunstst KGK 2009;62(3):103–7.
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[15] Ichazo MN, Hernández M, Albano C, González J, Sousa WD. NBR/bentonite composites: treatment of the filler on rheometric and tensile properties of NBR/bentonite vulcanizates. In: Proceedings of 17th international conference on composite materials (ICCM-17). Edinburgh, July 2009. IE4-3. [16] Krenchel H. Fibre reinforcement. Copenhagen: Akademisk Forlag; 1964.
Contents lists available at SciVerse ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Effect of dispersion of sepiolite in sepiolite-NBR composite on the tensile strength T. Takei a,⇑, R. Oda a, A. Miura a, N. Kumada a, N. Kinomura a, R. Ohki b, H. Koshiyama b a b
Center for Crystal Science and Technology, University of Yamanashi, 7-32 Miyamae, Kofu, Yamanashi 4008511, Japan Asktechnica Co. Ltd., 1488 Ichikawadaimon, Ichikawamisato, Nishiyatsushiro, Yamanashi 4093601, Japan
a r t i c l e
i n f o
Article history: Received 28 December 2011 Received in revised form 13 April 2012 Accepted 22 May 2012 Available online 1 June 2012 Keywords: A. Fibers B. Strength D. Electron microscopy E. Isostatic processing
a b s t r a c t The effect of dispersion of sepiolite in a sepiolite-NBR composite on the tensile strength of the composite was examined. Fibrous sepiolite was ultrasonically treated in distilled water with various types of dispersant. The ultrasonicated sepiolite was mixed with NBR latex to form a sepiolite-NBR complex. The tensile strength of the complex was examined to evaluate the degree of dispersion of sepiolite. The tensile strength results for the composites confirmed that a lower ultrasonication frequency produced a high strength in the composite. The tensile strength actually increased from 3 to 8 MPa as a result of ultrasonication at 28 kHz. To improve and stabilize the dispersion state, five kinds of dispersant were used. The addition of two dispersants in particular, ammonium polycarboxylate and amino alcohol polyphosphate, resulted in an increase of the strength from 8 to 15–16 MPa. When the composites were heated at 450 °C, the strengths of the composites produced with and without the dispersant were approximately 1.4–1.7 and 0.85 MPa, respectively. Thus, the dispersant still affected the strength of the heated samples. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Sepiolite (Mg8(OH)4(Si12O30)nH2O) is a strong candidate for a post-asbestos material, because it has a physically flexible fibrous shape. Sepiolite has a 2:1 layered structure composed of a brucite layer sandwiched by two tetrahedral (Si2O5)2 layers, and periodic dissociation forms micropore channels of 1.4 nm in width [1,2]. These micropore channels usually incorporate H2O molecules, and the pores sometimes serve as adsorption sites after dehydration. It has been reported that the sepiolite phase releases the zeolitic H2O at around 120 °C without collapse of the channel structure, changes topotactically at 350 °C, and consequently forms the anhydrate at 500 °C [2–6]. Thus, sepiolite has a relatively high thermal stability. Most mineral sepiolite is produced in China, Spain, Turkey, and the USA. Since Spanish, Turkish, and US sepiolites have bulky or short-fibrous particle shapes, such sepiolites are limited in their potential uses. On the other hand, Chinese sepiolite has fibrous particle shapes. The shape is a strong advantage for use in composites applicable to structural, heat-insulating, and adsorption materials and so on, as follows [7–11]. A structural composite consisting of sepiolite, kaolinite, and nitrile butadiene rubber (NBR) was studied by Ohki et al. [7]. Fuente et al. reported on a cement composite in which sepiolite was used as an additive [8]. As for heatinsulating materials, composites including sepiolite in an inorganic matrix (alumina, muscovite, and glass fiber) were reported by ⇑ Corresponding author. Tel.: +81 55 220 8616; fax: +81 55 254 3035. E-mail address: [email protected] (T. Takei). 1359-8368/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2012.05.034
Noda et al. [9], and composites including sepiolite in an organic matrix (epoxy resin) were reported by Nohales et al. [10]. Konta summarized many applications of clay minerals including sepiolite [11]. Formation of cordierite from sepiolite will enhance the thermal durability of the composite for use as heat-insulating material, because cordierite has a very low thermal expansion coefficient. Noda et al. also reported that the cordierite phase can be formed by addition of an alumina component [12]. In general, even long-fibrous mineral sepiolites (most of those made in China) form a bundled texture. Therefore, fibrillation of the bundle may be necessary for use of the sepiolite. However, control of the degree of fibrillation is very difficult, because the degree of fibrillation is very difficult to measure. Ultrasonication is a strong candidate process for dispersion of inorganic materials. The relationship between the frequency and the radius of the cavities produced by sonication can be expressed as follows:
f r ¼ 3:26;
ð1Þ
where f and r are the frequency (Hz) and radius (m) of the cavities [13]. Thus, a lower frequency is better at dispersing agglomerates, because a low frequency produces large cavities. However, agglomerates composed of very small grains (e.g., several nanometers in size) can only be dispersed using small cavities. In this case, a high frequency is necessary for dispersion. For fibrous sepiolite, it is necessary to examine whether ultrasonication can promote fibrillation or dispersion. Therefore, we examine the fibrillation and dispersion of Chinese sepiolite by ultrasonication, as well as the effect of the degree of dispersion on the physical strength of a composite with NBR.
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2. Experiments 2.1. Preparation of composite Sepiolite was added to distilled water to a concentration of 17.5 g/L, and five kinds of dispersants were added into sample suspensions to a concentration of 4 mL/L. The dispersants used were as follows: 40% ammonium polynaphthalene sulfonate (PWA-40 K), 40% ammonium polycarboxylate (SN dispersant 5468), 50% amino alcohol polyphosphate (SN dispersant 2060), 50% dioctyl sulfosuccinate (Nopcowet 50), and 93% nonionic dispersant (SN wet 126). Each percentage means concentration of as-received dispersant itself. All dispersants used in this paper were provided by San Nopco Limited. These dispersants were designated as 40 K, 5468, 2060, N50, and SN126, respectively. The dispersants can be divided into two groups: 40 K, 5468, and 2060 are polymers, and N50 and SN126 are nonpolymer dispersants. After the dispersant was added, the suspension was ultrasonicated (VS-100III, As One Corp.) for 0, 30, 60, 90, and 120 min to disperse the sepiolite bundle. Ultrasonication frequencies of 28, 45, and 100 kHz were used. NBR latex was added to the ultrasonicated suspension to a concentration of 7.5 g/L, and the suspension was stirred for 5 min. After that, aluminum sulfate aqueous solution was added into the suspension as a coagulation agent to a concentration of 50 mmol/L. Some sample suspensions were prone to foaming during ultrasonication, so two defoaming methods were carried out: addition of a 0.2 mL/L emulsion-type silicone antifoamer (TSA732A, Momentive Performance Materials Inc.) or use of a defoaming device (UM-117, Japan Unix Co., Ltd.). The defoaming device defoams the solution by using the centrifugal force produced by a planetary mixing action. In this experiment without silicone antifoamer, the suspension in the pot was revolved at 1500 rpm for 3 min with rotation at 300 rpm. The resultant defoamed suspensions (defoamed by addition of the silicone antifoamer and by the defoaming device) was then treated by suction filtration for 30 min, dried at 50 °C overnight, and pressed at 10 MPa for 30 min. The resulting sample plate was stamped out with a die whose size is consistent with the type 1 dumbbell (120 mm 25 mm) of JISK-6251. 2.2. Characterization The texture of the obtained composite was observed by field emission scanning electron microscopy (FE-SEM; JSM-6500F, Jeol) and optical microscopy (VHX-200, Keyence Corp.) to examine the degree of dispersion. The tensile strength of the composite was also measured to investigate the effects of the degree of dispersion, ultrasonication, and defoaming. The prepared test piece was used as a tensile specimen in a tensile tester (TENSILON RTF-1325, A&D Company, Limited) with a tensile rate of 300 mm/min.
Fig. 1. Tensile strength of sepiolite-NBR composites versus ultrasonication period.
of many unbroken cavities. Thus, ultrasonication at a low frequency results in high dispersion of the sepiolite to yield a high tensile strength for 30 min of ultrasonication, although the generation of large bubbles may reduce the strength of the composite for longer ultrasonication periods. To decrease the number of these bubbles, defoaming treatments were carried out, as mentioned in the experimental section. Fig. 2 shows the changes in tensile strength of the composite produced by different defoaming treatments as functions the ultrasonication period at 28 kHz. The strengths of the two composites increase up to 60 minutes of ultrasonication, then reach a plateau for longer periods. These results confirm that both defoaming treatments worked very well at decreasing the size and number of unwanted bubbles. Between the two defoaming methods, the strength of the composite produced using the defoaming device is larger than the strength of the composite produced using silicone antifoamer. This difference could exist because the antifoamer prevented direct contact of the sepiolite with the NBR. Consequently, sepiolite should be ultrasonicated at 28 kHz, and the suspension must be treated by a defoaming device for all samples.
3. Results and discussion 3.1. Effect of frequency on ultrasonication and defoaming treatments Fig. 1 shows the relationships between the ultrasonication period and the tensile strength of the sepiolite composite with NBR. All samples show increases in tensile strength up to 30 min, followed by decreases for longer periods. The maximum tensile strengths are approximately 5.6, 4.7, and 3.6 MPa for the composites treated with 28, 45, and 100 kHz ultrasonication. This trend results from the cavities and bubbles generated within the suspension by ultrasonication. As shown in Eq. (1), a lower frequency produces larger cavities with high energies. However, the lower frequency tends to result in larger bubbles through the coalescence
Fig. 2. Tensile strength of sepiolite-NBR composites versus ultrasonication period for different defoaming treatments.
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3.2. Effect of dispersant Dispersants are often effective for dispersion of inorganic particles. In this study, five different dispersants were used for sepiolite dispersion. Fig. 3 shows the stress–strain curves of the composites produced without ultrasonication and with ultrasonication for 120 min using five dispersants. The composites produced without ultrasonication have the smallest tensile strengths, approximately 3 MPa. On the other hand, the composites produced with 120 min of ultrasonication have strengths higher than 3 MPa. The tensile strengths of the composites ultrasonicated with 40 K, 5468, and 2060 clearly increased, although the composites ultrasonicated with N50 and SN126 had reduced tensile strengths. The slopes of these curves, which reflect the Young’s modulus, are also larger for the composites produced with 40 K, 5468, and 2060 than for the others. These strengths probably result from the type of molecule in the dispersant. The 40 K, 5468, and 2060 dispersants are polymers with larger molecular weights. On the other hand, the composites produced with the nonpolymer dispersants, N50 and SN126, have lower strengths than the composite produced without dispersant. Thus, the polymer dispersants better facilitate the dispersion of sepiolite by ultrasonication than the nonpolymer dispersants. The nonpolymer dispersants may prevent contact between the sepiolite and the NBR, as was the case for the silicone antifoamer. Fig. 4 shows the relationship between the ultrasonication period and the tensile strength. The general trends are higher tensile strengths for the composites produced using polymer dispersants and lower strengths for those produced with nonpolymer dispersants, as compared to the strength of the polymer produced without dispersant. For the composites produced with 5468 and 2060 in particular among the polymer-type dispersants, the strength increased linearly with ultrasonication period and eventually reached approximately 16 MPa at 120 min. This maximum value is double that of the composite produced without dispersant. These trends imply that dispersion of the sepiolite with a polymer dispersant may have reached a stage that is unachievable without any dispersants, since no plateau of the tensile strength was observed. Fig. 5 shows FE-SEM micrographs of the composites ultrasonicated for 120 min without and with each dispersant. Additional expanded micrographs are also shown at the upper right within each micrograph. These micrographs confirm that the sepiolite
Fig. 3. Stress–strain curves of the defoamed sepiolite-NBR composites produced using an ultrasonication period of 120 min with various dispersants.
Fig. 4. Relationship between ultrasonication period and tensile strength of the defoamed sepiolite-NBR composites.
was well dispersed by the ultrasonication with 40 K, 5468, and 2060 polymer dispersants. For the composites produced with the nonpolymer dispersants, N50 and SN126, the sepiolite fiber seems to be wetted by these dispersants, which behave like grease, especially as shown in the expanded micrographs. 3.3. Strength change due to heating at 450 °C Markovic et al. have reported that NBR decomposes thermally at 400-450 °C [14]. The NBR agglomerate in this study must also be decomposed by increasing temperatures. Therefore, thermal treatment will result in a decrease in strength of the composite. In this case, sepiolite dispersion becomes important in order to avoid reduction of the strength as much as possible. Fig. 6 shows the stress–strain curves of the composite heated at 450 °C for 24 h. In all samples, the strengths were reduced to around 0.7– 1.7 MPa by heating at 450 °C. However, the composites produced with 2060 and 5468 show relatively large tensile strengths, approximately 1.4 and 1.7 MPa, compared to the others, even after the heat treatment at 450 °C. The heated composite sample produced without dispersant shows a strength of approximately 0.85 MPa. These results indicate that the strengths of all samples were rapidly reduced by the heat treatment. However, the composites ultrasonicated with 2060 and 5468 still have a slight advantage in strength. Fig. 7 shows FE-SEM micrographs of the composites made by ultrasonication for 120 min with the various dispersants and then heated at 450 °C. These micrographs show that the fibrous texture of the sepiolite was maintained after the heat treatment at 450 °C in all composites, despite the drastic decrease in strength. In micrograph (b), a relatively large tumor of approximately 10 lm can be observed. A few such tumors can be observed in all samples. The tumor may form during the heating at 450 °C as a result, at least partially, of cracks several tens of micrometers in size. The mass ratio of sepiolite to NBR in the composite is 70:30. Since the densities of sepiolite and NBR are around 2.1 and 1.0 g/cm3, respectively, the volume ratio is approximately 1:1. The NBR will decompose at 400-450 °C, as mentioned above. On the basis of these properties, the composite probably consists of a heated sepiolite fiber skeleton and no NBR after heating. Such a skeleton will exhibit the tensile strength of the heated sepiolite sheet itself. Therefore, thermal changes of the sepiolite are considered. Fig. 8 shows hightemperature XRD patterns of sepiolite up to 800 °C. These patterns
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Fig. 5. FE-SEM photographs of the defoamed sepiolite-NBR composites.
of the composite and the strength of NBR. When the pull-out effect is negligible, the Young’s modulus of a composite containing a planar-oriented fiber can be expressed by Krenchel’s equation, as follows:
Ec ¼ Ef V f f þ Em ð1 V f Þ;
ð2Þ
confirm that a calcite phase is included in Chinese sepiolite. A change of the sepiolite phase to sepiolite anhydrate [2–6] was difficult to recognize. However, certain structural changes can be observed, as follows. The 0 0 2 and 0 1 1 diffraction lines at 2h values of 6.5 and 7.4° in the as-received sepiolite vanished above 300 °C, although the 0 4 0 and 2 1 0 diffraction lines at 26.6 and 34.6° were preserved up to around 600 °C. The retention of the fibrous shape of sepiolite after heat treatment at 450 °C could possibly be owed to the stability of the short-range ordered structure. The strength of the composite heated at 450 °C, approximately 0.85 MPa, may be smaller than that of pure sepiolite because of the partially collapsed channel structure.
where Ec, Ef, and Em are the Young’s moduli of the composite, fiber, and matrix, respectively, Vf is the volume fraction of the fibers, and f is the effectiveness factor of fiber. In this study, f can be regarded to be 3/8 [16], and Vf is approximately 0.5, as mentioned above. The Young’s modulus in this equation can be replaced with tensile stress at fixed strain, assuming a linear increase of the stress–strain curve. For instance, at a strain of 3.5%, if the tensile stresses of pure sepiolite and NBR are assumed to be x and 0.0 MPa, respectively, the tensile stress of the composite can be estimated at 0.2x MPa by Krenchel’s equation. Here, the tensile stress of NBR (0.0 MPa) at 3.5% strain is estimated from the tensile stress of 2.7 MPa at 420% strain given in the literature [15]. Since the tensile strength of the composite produced without ultrasonication is approximately 3 MPa, as shown in Figs. 1 and 3, the tensile strength of the pure sepiolite x can be estimated to be approximately 15 MPa. This estimated strength is larger than that of the piece heated at 450 °C. The stresses at 3.5% strain of the composites ultrasonicated for 120 min without dispersant and with dispersants 2060 and 5468 are approximately 7, 8, and 16 MPa, respectively. In the case of the composite produced with 5468 especially, the stress is similar to the estimated stress (15 MPa) of the pure sepiolite piece. Since the mean tensile stress weighted by Vf between pure sepiolite and NBR is around 7–8 MPa, the stress of the composite produced with 5468 is two times as large as the mean stress. Such a particularly large strength due to enhancement of the pull-out effect is a strong advantage of the dispersion of sepiolite by ultrasonication with dispersant in combination with a defoaming process in this study.
3.4. Estimation of the strength of sepiolite
4. Conclusion
The tensile strength of NBR is reported to be around 2.7 MPa at 420% strain [15]. On the other hand, the tensile strength of sepiolite has not been reported because of the difficulty of preparing sample pieces composed of only sepiolite. Therefore, the tensile strength of a pure sepiolite sheet is estimated from the strength
Sepiolite dispersion was carried out by ultrasonication, defoaming, and addition of dispersant, and the effect of the degree of the dispersion on the tensile strength of the dispersion-treated sepiolite-NBR composite was examined. The following results were obtained.
Fig. 6. Stress–strain curves of the defoamed sepiolite-NBR composites heated at 450 °C.
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Fig. 7. FE-SEM photographs of the defoamed sepiolite-NBR composites heated at 450 °C.
dispersant, which had a tensile strength of approximately 0.85 MPa. These results confirmed that the composites ultrasonicated with ammonium polycarboxylate and amino alcohol polyphosphate still had a slight advantage in strength.
Acknowledgements This research was partially supported by a New Energy and Industrial Technology Development Organization (NEDO) 2010 Grant (No. 0730001). The five types of dispersant were provided for free by San Nopco Limited. References
Fig. 8. High-temperature XRD patterns of the sepiolite.
1. Ultrasonication clearly enhanced the tensile strength of the sepiolite-NBR composite. The optimum ultrasonication frequency was 28 kHz. 2. The tensile strength of the sepiolite-NBR composite increased up to around 60 min of ultrasonication and reached a plateau for longer ultrasonication times. 3. Addition of dispersants, especially ammonium polycarboxylate and amino alcohol polyphosphate, resulted in an extraordinary increase of the tensile strength from around 8 to 15–16 MPa. 4. After heat treatment at 450 °C, the composites produced with ammonium polycarboxylate and amino alcohol polyphosphate showed relatively larger tensile strengths, approximately 1.4 and 1.7 MPa, than the heated composite produced without
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