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MAS NMR study of surface-modified calcined kaolin
Applied Clay Science 19 Ž2001. 89–94 www.elsevier.nlrlocaterclay
MAS NMR study of surface-modified calcined kaolin Qinfu Liu a,) , D.A. Spears b, Qinpu Liu c a
China UniÕersity of Mining and Technology, Beijing 100083, People’s Republic of China b Centre for Analytical Sciences, Sheffield UniÕersity, Sheffield, UK c Xuchang Teacher’s College, Xuchang 461000, People’s Republic of China
Received 5 April 2000; received in revised form 20 December 2000; received in revised form 12 March 2001; accepted 14 March 2001
Abstract Calcined kaolins, modified by a silane coupling agent, were analyzed by magic angle spinning nuclear magnetic resonance ŽMAS NMR. and compared to unmodified samples. The results show no chemical shift for 29 Si, whereas there is a marked change for 27Al. The chemical shifts of 5.44 and 65.69 ppm of 27Al in unmodified samples are respectively moved to 3.8–4.4 and 54.6–59.9 ppm after modification. This is attributed to changes in the chemical environment around the surface Al ions on the calcined kaolin resulting from chemical bonding of the silane coupling agent molecules to the Al. The results demonstrate that MAS NMR is a useful technique for characterizing the surface modification mechanisms of minerals. q 2001 Elsevier Science B.V. All rights reserved. Keywords: MAS NMR; Surface modification; Silane coupling agent; Calcined kaolin
1. Introduction Kaolinite-rich rocks are very abundant in the Permo-Carboniferous coal-bearing strata of North China and are widely used. Some deposits have high carbon contents and are hard; consequentially the rocks are often finely ground and calcined at 9508C. At this temperature kaolinite is transformed to metakaolinite. The thermal transformation of kaolinite has been investigated by Brown et al. Ž1985., Bulens et al. Ž1978., Chakrabory and Ghose Ž1978., Lambert et al. Ž1989., Rocha and Klinowski Ž1990., Sanz et al. Ž1988. and others. It is generally agreed that kaolinite forms metakaolinite by heating from 5508C to 9508C, and mullite mainly forms above
)
Corresponding author. Fax: q86-10-62325016. E-mail address: [email protected] ŽQ. Liu..
12008C. The calcined kaolin is often used as filler in rubber, plastics and other polymers. The surfaces of clays are generally hydrophilic, which impedes rapid wetting and dispersion in the organic phase. One very successful method of modifying clay surfaces to achieve strong polymer–filler interaction is to react the minerals with a variety of organosilanes ŽRehner et al., 1960; Libby et al., 1967; Ranney et al., 1973; Hawthorne and Solomon, 1974; Dannenberg, 1975; Helmer et al., 1976; Ishida and Koenig, 1980; Ishida and Miller, 1985; Dai and Huang, 1999.. According to some authors, the hydroxyls on the surfaces of the kaolinite particles play an important role in linking organosilane molecules to the particles ŽHelmer et al., 1976; Dannenberg, 1975; Dai and Huang, 1999; Theng, 1979.. The coupling agent is thought to be attached to clay particles by chemical bonding and adsorption to form a monomolecular layer or oligomer film on the
0169-1317r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 1 7 Ž 0 1 . 0 0 0 5 7 - 6
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clay mineral surfaces ŽDai and Huang, 1999; Ishida and Koenig, 1980; Ishida and Miller, 1985.. However, the adsorbed organic layer is often displaced by solvents or during compounding ŽDannenberg, 1975; Helmer et al., 1976.. Chemical bonding gives rise to a solid linkage between the coupling agent and clay particles thus improving the properties of the polymer products. Infrared ŽIR. spectrum measurement based on the absorption band changes of functional groups in minerals has been used by Dai and Huang Ž1999., Ishida and Koenig Ž1980., and Ishida and Miller Ž1985., to determine if there is chemical bonding. In some cases, changes of absorption bands are too weak to be detected by IR measurement. The present authors believe that magic angle spinning nuclear magnetic resonance ŽMAS NMR. is a useful technique for studying the surface modification of minerals. According to Rocha and Klinowski Ž1990., MAS NMR is a sensitive method for investigating atomic environments in a variety of solids, including minerals. If the ions on the surface of minerals are grafted or linked with the coupling agent molecules this will result in changes in the chemical environment around the ions that are potentially detectable by MAS NMR and would be manifested as chemical shifts of atoms. The chemical environment around the ions is related to the nature of the bonding and the structure of the adjacent atomic groups. This interdependence of chemical shift and chemical environment is a consequence of local magnetic fields induced by electrons surrounding the nucleus and adjacent nuclei. Nuclei having different chemical environments give rise to different resonance signals. For example, Si linked via oxygens to three other Si atoms with no nearest Al neighbors has a chemical shift of y92 to y98 ppm in the 29 Si MAS NMR spectrum, whereas Si with one nearest Al neighbor is y85 to y90 ppm ŽKirkpatrick, 1988.. Shielding effects increase with the increasing electron cloud density around the nuclei and the resonance signal shifts to a higher field with a decrease in the chemical shift value. Solid-state MAS NMR has proved to be sensitive to the short range ordering of minerals and to be a useful tool for studying the properties of surface adsorbed organic species, intercalation complexes and their reactions with clay minerals ŽGoodman and Chudek, 1994; Schroeder and Pruett, 1996; Schroeder
et al., 1998.. Thompon Ž1985. has reported that increased shielding by the organic molecule in organic intercalates of kaolinite causes the 29 Si resonance to be shifted to a higher field than in untreated kaolinite. A similar shift was seen by Sugahara et al. Ž1989. in intercalates of kaolinite with pyridine and monosubstituted pyridine derivatives. The objective of this paper is to demonstrate that MAS NMR can be used to investigate the nature of the bonding between coupling agent and metakaolinite. There are currently relatively few reports of this technique having been used to study mineral surface modifications.
2. Experimental materials and methods The raw material used in this work for making calcined kaolin is tonstein, which is a kaolinitic claystone of volcanic origin found as partings in coal seams of Permo-Carboniferous strata in Datong coal mines from Shanxi province in North China. The beds of tonstein are ; 0.5 m thick and are widespread in the coal-bearing strata of the Datong coalfield. The kaolinite content in the rocks is up to 95% and the quality is very good for industrial use. The rocks were ground to 1250 mesh with an airflow-crushing mill and then the kaolin powders were heated to 9508C in an oven. The brightness of calcined kaolin is up to 90%. The calcined kaolin was modified by silane coupling agent under specific conditions. The silane coupling agent is KH550, manufactured by Shuguang Chemical Company of Nanjing in China and has a formula of NH 2 C 3 H 6 SiŽOC 2 H 5 . 3 . In order to explore the surface reactions, the modified calcined kaolin was analyzed by MAS NMR and compared with untreated calcined kaolinite. Solid-state NMR spectra were collected on a Bruker AM300 spectrometer at the NMR laboratory, Beijing Petroleum Chemical Industrial Research Institute, Beijing, P.R. China. 27Al spectra were obtained at 78.2 MHz Ž7 T. using AlŽH 2 O. 63q as an external standard and with a 0.1-s pulse delay. 29 Si spectra were obtained at 59.6 MHz Ž7 T. using TMS as an external standard and with a 5-s pulse delay. The spinning speed was 3 kHz.
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3. Results and discussion 3.1. MAS NMR spectra of unmodified kaolinite and metakaolinite (calcined kaolin) Kaolinite is a hydrous layer silicate clay mineral. The structural unit of kaolinite consists of a Si`O tetrahedral sheet and an Al`OŽOH. octahedral sheet. Kaolinite is transformed to metakaolinite as the structural water is driven off at temperatures from 550–9508C. The 29 Si and 27Al MAS NMR spectra of the kaolinite and metakaolinite are given in Fig. 1. The 29 Si MAS NMR spectrum of kaolinite consists of a single resonance at y91 ppm with full width at half maximum ŽFWHM. of 4 ppm, characteristic of layer silicates and assigned to Si linked via oxygens to three other Si atoms ŽRocha and Klinowski, 1990; Guo et al., 1997.. The chemical shift of 29 Si for metakaolinite Žcalcined kaolin at heating temperature of 9508C, unmodified. is changed to y106 ppm with FWHM of 15 ppm, which is characteristic of Si linked to four other Si atoms in silica polymorphs ŽSmith and Blackwell, 1983. and indicates the presence of amorphous silica ŽRocha and Klinowski, 1990. ŽFig. 1b.. The 27Al MAS NMR spectrum of
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kaolinite ŽFig. 1a. contains one peak at y2 ppm ŽFWHMs 16.6 ppm. that is assigned to 6-coordinated Al. After calcining at 9508C, metakaolinite exhibits two other resonances at 5.4 ppm, assigned to 6-coordinated Al and another at 65.7 ppm with a FWHM of 156 ppm attributed to 4-coordinated Al ŽRocha and Klinowski, 1990; Guo et al., 1997.. The broader and asymmetrical peak shapes of metakaolinite show its disordered structure. 3.2. MAS NMR spectra for modified calcined kaolin Fig. 1c, d and e shows the MAS NMR spectra of Si and 27Al for modified calcined kaolin. The comparative study shows that the chemical shift of y106 ppm of 29 Si is unchanged between modified and unmodified samples, but the chemical shifts of 27Al are significantly different. The chemical shifts of 5.44 and 65.69 ppm of 27Al in the unmodified samples are shifted to 3.8–4.4 and 54.6–59.9 ppm, respectively, after modification. This shows that the chemical environment around Al on the surface of calcined kaolinite is significantly altered by the addition of the silane coupling agent molecules, but there is insufficient evidence to say categorically whether or not this also applies to the Si ions. 29
3.3. Comparison of MAS NMR with IR analysis
Fig. 1. Comparison of MAS NMR Spectra. Ža. Kaolinite Žuncalcined, unmodified.; Žb. metakaolinite Žcalcined at 9508C, unmodified.; Žc,d,e. modified calcined kaolin. SSB denotes spinning sidebands.
Infrared ŽIR. spectrum measurements have been used to study surface modifications ŽIshida and Koenig, 1980; Dai and Huang, 1999.. Molecules of the coupling agent chemical bonded to the clay surfaces will change the IR spectra, whereas molecules coating or adsorbed to the surfaces will have no effect on the IR vibrations of various groups in the clay. Dai and Huang Ž1999. observed changes in the Al`OH vibration band and the peaks of the `CH 3 groups at 2923 and 2852 cmy1 in modified illite, but little change in the Si`O vibration. The authors of this paper also made IR measurements for modified and unmodified calcined kaolin ŽFig. 2.. The 1102 and 480 cmy1 peaks for the Si`O bond show little or no change when the unmodified and modified samples are compared, but the peak for the Al`O bond in the unmodified sample changes from 819 to 804 cmy1 after modification. In addition, the 2920, 2852 and 1407 cmy1 peaks attributable to `CH 3 or `CH 2 group vibrations appear in the IR spectra of the modified sam-
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Fig. 2. IR spectra of unmodified Ža. and modified Žb. calcined kaolin.
ples. Although our results are similar to those of Dai and Huang Ž1999., the present authors believe that the appearance of `CH 2 or `CH 3 peaks in the IR spectra of the modified samples only shows that the coupling agent is present and does not prove that there is bonding to the surface of the minerals. The appearance of `CH 2 or `CH 3 peaks in the IR spectra of the modified sample is not thought to be the best evidence on which to base an explanation for the chemical bonding mechanism. The results from IR analysis are consistent with those from MAS NMR. Both show that the Al`O group on the surface of the mineral reacts with the silane coupling agent and no reaction was detected for the Si`O group. The authors therefore suggest that the surface modification be mainly completed by linking silane coupling agent molecules to Al`O groups on the surface of calcined kaolin. At this stage, the evidence relating to the Si`O groups is insufficient to establish any linkage.
cross-linked rubber–silane–clay network system greatly improves abrasion resistance, tensile strength and other properties compared with the unmodified filler–rubber system. Heating the kaolinite results in a loss of hydroxyls. Subsequent exposure to the air results in water being adsorbed and a layer of hydroxyls formed on the surface of metakaolinite. A low, broad peak at 3400–3500 cmy1 detected in the IR spectra of modified samples of uncalcined and calcined kaolin ŽFig. 3. is attributed to the absorption of associated hydroxyls formed by the coupling agent molecules on the surface of the mineral. Based on our experience the modification mechanism may be explained as follows. Ž1. Silane hydrolysis
where ORX denotes an alkoxyl group such as `OC 2 H 5 , `OCH 3 , etc., and R represents a group that can be combined with polymers, such as `CH 2 NH 2 , etc. Ž2. Reaction of hydrolyzed silane with Al`OH on the surface of metakaolinite
3.4. Surface modification mechanism for calcined kaolin The surface modification mechanism of noncalcined kaolin has been reported by a number of authors ŽDannenberg, 1975; Helmer et al., 1976; Theng, 1979; Zheng, 1995.. It is generally considered that the reaction has two stages. The silane-coupling agent is first hydrolyzed by reaction with water and then the hydrolysed silanol group reacts with the surface hydroxyl groups on the kaolinite. The unattached end of the silane molecule is free to react with the rubber during vulcanisation. The resulting
Fig. 3. IR spectra of associated hydroxyls for modified samples. Ža,b. Uncalcined kaolin, modified; Žc,d. calcined kaolin, modified; Že. Calcined kaolin, unmodified. The absorption peaks of 3621– 3695 cmy1 are assigned to the hydroxyls in the kaolinite structure, which disappear in the calcined samples. The peaks of 3424, 3447 and 3460 cmy1 are assigned to the associated hydroxyls formed by coupling agent molecules in the surface of minerals.
Q. Liu et al.r Applied Clay Science 19 (2001) 89–94
The formation of coordinated bonds between oxygen and aluminum atoms makes the electron cloud around the Al atoms on the surface of the modified metakaolinite denser than that on unmodified metakaolinite. The shielding effect increases with increasing electron cloud density around the nuclei. More negative and less positive chemical shifts correspond to larger shieldings ŽKirkpatrick, 1988. and thus the resonance signals of 27Al in MAS NMR spectra of modified samples are shifted to a high field and the chemical shift decreases. This may be a reasonable explanation for the observed changes in the 27Al chemical shift. When modified calcined kaolin is used in rubber, the organophilic group ŽR-. will react with the large molecules of rubber thus enhancing their compatibility with and dispersion in a polymer matrix leading to an improvement in the physico-mechanical properties of the rubber. 4. Conclusion Changes in the chemical environment around the Al ions, detected in MAS NMR spectra of modified calcined kaolin samples, indicate that the modification of calcined kaolin is mainly caused by a linkage between the silane coupling agent molecules and the Al on the surface of the metakaolinite. This was confirmed by IR analysis but neither MAS NMR nor IR could detect any changes relating to the Si ions on the surface of calcined kaolinite. References Brown, I.W.M., Mackenzie, K.J.D., Bowden, M.E. et al., 1985. Outstanding problems in the kaolinite–mullite reaction sequence investigated by 29 Si and 27Al solid-state nuclear magnetic resonance: II. High-temperature transformation of metakaolinite. J. Am. Ceram. Soc. 68 Ž6., 298–301. Bulens, M., Leonard, A., Delman, B., 1978. Spectroscopic investigations of the kaolinite–mullite reaction sequence. J. Am. Ceram. Soc. 61 Ž1–2., 81–84.
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Chakrabory, A.K., Ghose, D.K., 1978. Reexamination of the kaolinite-to-mullite reaction series. J. Am. Ceram. Soc. 61 Ž3–4., 170–175. Dai, J.C., Huang, J.T., 1999. Surface modification of clays and clay–rubber composite. Appl. Clay Sci. 15 Ž1–2., 51–65. Dannenberg, E.M., 1975. The effects of surface chemical interactions on the properties of filler-reinforced rubbers. Rubber Chem. Technol. 48, 410–444. Goodman, B.A., Chudek, J.A., 1994. Nuclear magnetic resonance spectroscopy. In: Wilson, M.J. ŽEd.., Clay Mineralogy: Spectroscopic and Chemical Determinative Methods. Chapman & Hall, London, pp. 120–170. Guo, J.G., He, H.P., Wang, F.Y., 1997. Kaolinite–mullite reaction series: 27Al and 29 Si MAS NMR study. Acta Mineral. Sin. 16 Ž2., 250–259, in Chinese, with English abstract. Hawthorne, D.G., Solomon, D.H., 1974. Reinforcement of polyethylene by surface-modified kaolins. J. Macromol. Sci. Chem. 8, 659–671. Helmer, B.M., Prescott, P.I., Sennett, P., 1976. Surface-modified kaolin in plastic. 31st Annual Technical Conference, Reinforced PlasticrComposites Institute, The Society of the Plastic Industry, pp. 1–4, Section 8-G. Ishida, H., Koenig, J., 1980. A Fourier-transform infrared spectroscopic study of the hydrolytic stability of silane coupling agents on e-glass fibers. J. Polym. Sci., Polym. Phys. Ed. 18, 1931–1943. Ishida, H., Miller, J.D., 1985. Cyclization of methacrylate-functional silane on particulate clay. J. Polym. Sci. 23, 2227–2242. Kirkpatrick, R.J., 1988. MAS NMR spectroscopy of minerals and glasses. In: Hawthorne, F. ŽEd.., Spectroscopic Methods in Mineralogy and Geology. Rev. Mineral., vol. 18, Mineralogical Society of America, pp. 341–404. Lambert, J.F., Millman, W.S., Fripiat, J.J., 1989. Revisiting kaolinite dehydroxylation: a 29 Si and 27Al MAS NMR study. J. Am. Ceram. Soc. 111 Ž10., 3517–3552. Libby, P.W., Iannicelli, J., McGill, C.R., 1967. Elastomer reinforcement with amino silane grafted kaolin. American Chemistry Society, Division of Rubber Chemistry, Spring Meeting, 18 pp. Ranney, M., Cameron, G.M., Lipinski, B.W., 1973. Improvement of the static and dynamic properties of filled elastomers by silane coupling agents. Kautsch. Gummi Kunstst. 26, 409–416. Rehner, J., Wiese, H.K., Gessler, A.M., 1960. Cyclodienylhalosilane-treated mineral pigments. United States Patent 2,928,802 Žto Esso Research and Engineering.. Rocha, J., Klinowski, J., 1990. 29 Si and 27Al magic-angle-spinning NMR studies of the thermal transformation of kaolinite. Phys. Chem. Miner. 17, 179–186. Sanz, J., Madani, A., Serratosa, J.M., 1988. Aluminum-27 and silicon-29 magic angle spinning nuclear magnetic resonance
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study of the kaolinite–mullite transformation. J. Am. Ceram. Soc. 71 Ž10., c418–c421. Schroeder, P.A., Pruett, R.J., 1996. Iron ordering in kaolinites: insights from 29 Si and 27Al NMR spectroscopy. Am. Mineral. 81, 26–38. Schroeder, P.A., Pruett, R.J., Hurst, V.J., 1998. Effects of secondary iron phases on kaolinite 27Al MAS NMR spectra. Clays Clay Miner. 46 Ž4., 429–435. Smith, J.V., Blackwell, C.S., 1983. Nuclear magnetic resonance of silica polymorphs. Nature 303, 223–225. Sugahara, Y., Satokawa, S., Yoshioka, K.-I., 1989. Kaolinite–
pyridine intercalation compound derived from hydrated kaolinite. Clays Clay Mineral. 37 Ž2., 143–150. Theng, B.K.G., 1979. Formation and Properties of Clay–Polymer Complexes. Elsevier, Amsterdam, pp. 133–154. Thompon, J.G., 1985. Interpretation of solid state 13 C and 29 Si nuclear magnetic resonance spectra of kaolinite intercalates. Clay Clay Miner. 33 Ž3., 173–180. Zheng, S.L., 1995. Fenti Biaomian Gaixing. wSurface Modification of Mineral Powderx China Construction Material Publishing, Beijing, pp. 10–80, in Chinese.
MAS NMR study of surface-modified calcined kaolin Qinfu Liu a,) , D.A. Spears b, Qinpu Liu c a
China UniÕersity of Mining and Technology, Beijing 100083, People’s Republic of China b Centre for Analytical Sciences, Sheffield UniÕersity, Sheffield, UK c Xuchang Teacher’s College, Xuchang 461000, People’s Republic of China
Received 5 April 2000; received in revised form 20 December 2000; received in revised form 12 March 2001; accepted 14 March 2001
Abstract Calcined kaolins, modified by a silane coupling agent, were analyzed by magic angle spinning nuclear magnetic resonance ŽMAS NMR. and compared to unmodified samples. The results show no chemical shift for 29 Si, whereas there is a marked change for 27Al. The chemical shifts of 5.44 and 65.69 ppm of 27Al in unmodified samples are respectively moved to 3.8–4.4 and 54.6–59.9 ppm after modification. This is attributed to changes in the chemical environment around the surface Al ions on the calcined kaolin resulting from chemical bonding of the silane coupling agent molecules to the Al. The results demonstrate that MAS NMR is a useful technique for characterizing the surface modification mechanisms of minerals. q 2001 Elsevier Science B.V. All rights reserved. Keywords: MAS NMR; Surface modification; Silane coupling agent; Calcined kaolin
1. Introduction Kaolinite-rich rocks are very abundant in the Permo-Carboniferous coal-bearing strata of North China and are widely used. Some deposits have high carbon contents and are hard; consequentially the rocks are often finely ground and calcined at 9508C. At this temperature kaolinite is transformed to metakaolinite. The thermal transformation of kaolinite has been investigated by Brown et al. Ž1985., Bulens et al. Ž1978., Chakrabory and Ghose Ž1978., Lambert et al. Ž1989., Rocha and Klinowski Ž1990., Sanz et al. Ž1988. and others. It is generally agreed that kaolinite forms metakaolinite by heating from 5508C to 9508C, and mullite mainly forms above
)
Corresponding author. Fax: q86-10-62325016. E-mail address: [email protected] ŽQ. Liu..
12008C. The calcined kaolin is often used as filler in rubber, plastics and other polymers. The surfaces of clays are generally hydrophilic, which impedes rapid wetting and dispersion in the organic phase. One very successful method of modifying clay surfaces to achieve strong polymer–filler interaction is to react the minerals with a variety of organosilanes ŽRehner et al., 1960; Libby et al., 1967; Ranney et al., 1973; Hawthorne and Solomon, 1974; Dannenberg, 1975; Helmer et al., 1976; Ishida and Koenig, 1980; Ishida and Miller, 1985; Dai and Huang, 1999.. According to some authors, the hydroxyls on the surfaces of the kaolinite particles play an important role in linking organosilane molecules to the particles ŽHelmer et al., 1976; Dannenberg, 1975; Dai and Huang, 1999; Theng, 1979.. The coupling agent is thought to be attached to clay particles by chemical bonding and adsorption to form a monomolecular layer or oligomer film on the
0169-1317r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 1 7 Ž 0 1 . 0 0 0 5 7 - 6
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Q. Liu et al.r Applied Clay Science 19 (2001) 89–94
clay mineral surfaces ŽDai and Huang, 1999; Ishida and Koenig, 1980; Ishida and Miller, 1985.. However, the adsorbed organic layer is often displaced by solvents or during compounding ŽDannenberg, 1975; Helmer et al., 1976.. Chemical bonding gives rise to a solid linkage between the coupling agent and clay particles thus improving the properties of the polymer products. Infrared ŽIR. spectrum measurement based on the absorption band changes of functional groups in minerals has been used by Dai and Huang Ž1999., Ishida and Koenig Ž1980., and Ishida and Miller Ž1985., to determine if there is chemical bonding. In some cases, changes of absorption bands are too weak to be detected by IR measurement. The present authors believe that magic angle spinning nuclear magnetic resonance ŽMAS NMR. is a useful technique for studying the surface modification of minerals. According to Rocha and Klinowski Ž1990., MAS NMR is a sensitive method for investigating atomic environments in a variety of solids, including minerals. If the ions on the surface of minerals are grafted or linked with the coupling agent molecules this will result in changes in the chemical environment around the ions that are potentially detectable by MAS NMR and would be manifested as chemical shifts of atoms. The chemical environment around the ions is related to the nature of the bonding and the structure of the adjacent atomic groups. This interdependence of chemical shift and chemical environment is a consequence of local magnetic fields induced by electrons surrounding the nucleus and adjacent nuclei. Nuclei having different chemical environments give rise to different resonance signals. For example, Si linked via oxygens to three other Si atoms with no nearest Al neighbors has a chemical shift of y92 to y98 ppm in the 29 Si MAS NMR spectrum, whereas Si with one nearest Al neighbor is y85 to y90 ppm ŽKirkpatrick, 1988.. Shielding effects increase with the increasing electron cloud density around the nuclei and the resonance signal shifts to a higher field with a decrease in the chemical shift value. Solid-state MAS NMR has proved to be sensitive to the short range ordering of minerals and to be a useful tool for studying the properties of surface adsorbed organic species, intercalation complexes and their reactions with clay minerals ŽGoodman and Chudek, 1994; Schroeder and Pruett, 1996; Schroeder
et al., 1998.. Thompon Ž1985. has reported that increased shielding by the organic molecule in organic intercalates of kaolinite causes the 29 Si resonance to be shifted to a higher field than in untreated kaolinite. A similar shift was seen by Sugahara et al. Ž1989. in intercalates of kaolinite with pyridine and monosubstituted pyridine derivatives. The objective of this paper is to demonstrate that MAS NMR can be used to investigate the nature of the bonding between coupling agent and metakaolinite. There are currently relatively few reports of this technique having been used to study mineral surface modifications.
2. Experimental materials and methods The raw material used in this work for making calcined kaolin is tonstein, which is a kaolinitic claystone of volcanic origin found as partings in coal seams of Permo-Carboniferous strata in Datong coal mines from Shanxi province in North China. The beds of tonstein are ; 0.5 m thick and are widespread in the coal-bearing strata of the Datong coalfield. The kaolinite content in the rocks is up to 95% and the quality is very good for industrial use. The rocks were ground to 1250 mesh with an airflow-crushing mill and then the kaolin powders were heated to 9508C in an oven. The brightness of calcined kaolin is up to 90%. The calcined kaolin was modified by silane coupling agent under specific conditions. The silane coupling agent is KH550, manufactured by Shuguang Chemical Company of Nanjing in China and has a formula of NH 2 C 3 H 6 SiŽOC 2 H 5 . 3 . In order to explore the surface reactions, the modified calcined kaolin was analyzed by MAS NMR and compared with untreated calcined kaolinite. Solid-state NMR spectra were collected on a Bruker AM300 spectrometer at the NMR laboratory, Beijing Petroleum Chemical Industrial Research Institute, Beijing, P.R. China. 27Al spectra were obtained at 78.2 MHz Ž7 T. using AlŽH 2 O. 63q as an external standard and with a 0.1-s pulse delay. 29 Si spectra were obtained at 59.6 MHz Ž7 T. using TMS as an external standard and with a 5-s pulse delay. The spinning speed was 3 kHz.
Q. Liu et al.r Applied Clay Science 19 (2001) 89–94
3. Results and discussion 3.1. MAS NMR spectra of unmodified kaolinite and metakaolinite (calcined kaolin) Kaolinite is a hydrous layer silicate clay mineral. The structural unit of kaolinite consists of a Si`O tetrahedral sheet and an Al`OŽOH. octahedral sheet. Kaolinite is transformed to metakaolinite as the structural water is driven off at temperatures from 550–9508C. The 29 Si and 27Al MAS NMR spectra of the kaolinite and metakaolinite are given in Fig. 1. The 29 Si MAS NMR spectrum of kaolinite consists of a single resonance at y91 ppm with full width at half maximum ŽFWHM. of 4 ppm, characteristic of layer silicates and assigned to Si linked via oxygens to three other Si atoms ŽRocha and Klinowski, 1990; Guo et al., 1997.. The chemical shift of 29 Si for metakaolinite Žcalcined kaolin at heating temperature of 9508C, unmodified. is changed to y106 ppm with FWHM of 15 ppm, which is characteristic of Si linked to four other Si atoms in silica polymorphs ŽSmith and Blackwell, 1983. and indicates the presence of amorphous silica ŽRocha and Klinowski, 1990. ŽFig. 1b.. The 27Al MAS NMR spectrum of
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kaolinite ŽFig. 1a. contains one peak at y2 ppm ŽFWHMs 16.6 ppm. that is assigned to 6-coordinated Al. After calcining at 9508C, metakaolinite exhibits two other resonances at 5.4 ppm, assigned to 6-coordinated Al and another at 65.7 ppm with a FWHM of 156 ppm attributed to 4-coordinated Al ŽRocha and Klinowski, 1990; Guo et al., 1997.. The broader and asymmetrical peak shapes of metakaolinite show its disordered structure. 3.2. MAS NMR spectra for modified calcined kaolin Fig. 1c, d and e shows the MAS NMR spectra of Si and 27Al for modified calcined kaolin. The comparative study shows that the chemical shift of y106 ppm of 29 Si is unchanged between modified and unmodified samples, but the chemical shifts of 27Al are significantly different. The chemical shifts of 5.44 and 65.69 ppm of 27Al in the unmodified samples are shifted to 3.8–4.4 and 54.6–59.9 ppm, respectively, after modification. This shows that the chemical environment around Al on the surface of calcined kaolinite is significantly altered by the addition of the silane coupling agent molecules, but there is insufficient evidence to say categorically whether or not this also applies to the Si ions. 29
3.3. Comparison of MAS NMR with IR analysis
Fig. 1. Comparison of MAS NMR Spectra. Ža. Kaolinite Žuncalcined, unmodified.; Žb. metakaolinite Žcalcined at 9508C, unmodified.; Žc,d,e. modified calcined kaolin. SSB denotes spinning sidebands.
Infrared ŽIR. spectrum measurements have been used to study surface modifications ŽIshida and Koenig, 1980; Dai and Huang, 1999.. Molecules of the coupling agent chemical bonded to the clay surfaces will change the IR spectra, whereas molecules coating or adsorbed to the surfaces will have no effect on the IR vibrations of various groups in the clay. Dai and Huang Ž1999. observed changes in the Al`OH vibration band and the peaks of the `CH 3 groups at 2923 and 2852 cmy1 in modified illite, but little change in the Si`O vibration. The authors of this paper also made IR measurements for modified and unmodified calcined kaolin ŽFig. 2.. The 1102 and 480 cmy1 peaks for the Si`O bond show little or no change when the unmodified and modified samples are compared, but the peak for the Al`O bond in the unmodified sample changes from 819 to 804 cmy1 after modification. In addition, the 2920, 2852 and 1407 cmy1 peaks attributable to `CH 3 or `CH 2 group vibrations appear in the IR spectra of the modified sam-
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Fig. 2. IR spectra of unmodified Ža. and modified Žb. calcined kaolin.
ples. Although our results are similar to those of Dai and Huang Ž1999., the present authors believe that the appearance of `CH 2 or `CH 3 peaks in the IR spectra of the modified samples only shows that the coupling agent is present and does not prove that there is bonding to the surface of the minerals. The appearance of `CH 2 or `CH 3 peaks in the IR spectra of the modified sample is not thought to be the best evidence on which to base an explanation for the chemical bonding mechanism. The results from IR analysis are consistent with those from MAS NMR. Both show that the Al`O group on the surface of the mineral reacts with the silane coupling agent and no reaction was detected for the Si`O group. The authors therefore suggest that the surface modification be mainly completed by linking silane coupling agent molecules to Al`O groups on the surface of calcined kaolin. At this stage, the evidence relating to the Si`O groups is insufficient to establish any linkage.
cross-linked rubber–silane–clay network system greatly improves abrasion resistance, tensile strength and other properties compared with the unmodified filler–rubber system. Heating the kaolinite results in a loss of hydroxyls. Subsequent exposure to the air results in water being adsorbed and a layer of hydroxyls formed on the surface of metakaolinite. A low, broad peak at 3400–3500 cmy1 detected in the IR spectra of modified samples of uncalcined and calcined kaolin ŽFig. 3. is attributed to the absorption of associated hydroxyls formed by the coupling agent molecules on the surface of the mineral. Based on our experience the modification mechanism may be explained as follows. Ž1. Silane hydrolysis
where ORX denotes an alkoxyl group such as `OC 2 H 5 , `OCH 3 , etc., and R represents a group that can be combined with polymers, such as `CH 2 NH 2 , etc. Ž2. Reaction of hydrolyzed silane with Al`OH on the surface of metakaolinite
3.4. Surface modification mechanism for calcined kaolin The surface modification mechanism of noncalcined kaolin has been reported by a number of authors ŽDannenberg, 1975; Helmer et al., 1976; Theng, 1979; Zheng, 1995.. It is generally considered that the reaction has two stages. The silane-coupling agent is first hydrolyzed by reaction with water and then the hydrolysed silanol group reacts with the surface hydroxyl groups on the kaolinite. The unattached end of the silane molecule is free to react with the rubber during vulcanisation. The resulting
Fig. 3. IR spectra of associated hydroxyls for modified samples. Ža,b. Uncalcined kaolin, modified; Žc,d. calcined kaolin, modified; Že. Calcined kaolin, unmodified. The absorption peaks of 3621– 3695 cmy1 are assigned to the hydroxyls in the kaolinite structure, which disappear in the calcined samples. The peaks of 3424, 3447 and 3460 cmy1 are assigned to the associated hydroxyls formed by coupling agent molecules in the surface of minerals.
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The formation of coordinated bonds between oxygen and aluminum atoms makes the electron cloud around the Al atoms on the surface of the modified metakaolinite denser than that on unmodified metakaolinite. The shielding effect increases with increasing electron cloud density around the nuclei. More negative and less positive chemical shifts correspond to larger shieldings ŽKirkpatrick, 1988. and thus the resonance signals of 27Al in MAS NMR spectra of modified samples are shifted to a high field and the chemical shift decreases. This may be a reasonable explanation for the observed changes in the 27Al chemical shift. When modified calcined kaolin is used in rubber, the organophilic group ŽR-. will react with the large molecules of rubber thus enhancing their compatibility with and dispersion in a polymer matrix leading to an improvement in the physico-mechanical properties of the rubber. 4. Conclusion Changes in the chemical environment around the Al ions, detected in MAS NMR spectra of modified calcined kaolin samples, indicate that the modification of calcined kaolin is mainly caused by a linkage between the silane coupling agent molecules and the Al on the surface of the metakaolinite. This was confirmed by IR analysis but neither MAS NMR nor IR could detect any changes relating to the Si ions on the surface of calcined kaolinite. References Brown, I.W.M., Mackenzie, K.J.D., Bowden, M.E. et al., 1985. Outstanding problems in the kaolinite–mullite reaction sequence investigated by 29 Si and 27Al solid-state nuclear magnetic resonance: II. High-temperature transformation of metakaolinite. J. Am. Ceram. Soc. 68 Ž6., 298–301. Bulens, M., Leonard, A., Delman, B., 1978. Spectroscopic investigations of the kaolinite–mullite reaction sequence. J. Am. Ceram. Soc. 61 Ž1–2., 81–84.
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Chakrabory, A.K., Ghose, D.K., 1978. Reexamination of the kaolinite-to-mullite reaction series. J. Am. Ceram. Soc. 61 Ž3–4., 170–175. Dai, J.C., Huang, J.T., 1999. Surface modification of clays and clay–rubber composite. Appl. Clay Sci. 15 Ž1–2., 51–65. Dannenberg, E.M., 1975. The effects of surface chemical interactions on the properties of filler-reinforced rubbers. Rubber Chem. Technol. 48, 410–444. Goodman, B.A., Chudek, J.A., 1994. Nuclear magnetic resonance spectroscopy. In: Wilson, M.J. ŽEd.., Clay Mineralogy: Spectroscopic and Chemical Determinative Methods. Chapman & Hall, London, pp. 120–170. Guo, J.G., He, H.P., Wang, F.Y., 1997. Kaolinite–mullite reaction series: 27Al and 29 Si MAS NMR study. Acta Mineral. Sin. 16 Ž2., 250–259, in Chinese, with English abstract. Hawthorne, D.G., Solomon, D.H., 1974. Reinforcement of polyethylene by surface-modified kaolins. J. Macromol. Sci. Chem. 8, 659–671. Helmer, B.M., Prescott, P.I., Sennett, P., 1976. Surface-modified kaolin in plastic. 31st Annual Technical Conference, Reinforced PlasticrComposites Institute, The Society of the Plastic Industry, pp. 1–4, Section 8-G. Ishida, H., Koenig, J., 1980. A Fourier-transform infrared spectroscopic study of the hydrolytic stability of silane coupling agents on e-glass fibers. J. Polym. Sci., Polym. Phys. Ed. 18, 1931–1943. Ishida, H., Miller, J.D., 1985. Cyclization of methacrylate-functional silane on particulate clay. J. Polym. Sci. 23, 2227–2242. Kirkpatrick, R.J., 1988. MAS NMR spectroscopy of minerals and glasses. In: Hawthorne, F. ŽEd.., Spectroscopic Methods in Mineralogy and Geology. Rev. Mineral., vol. 18, Mineralogical Society of America, pp. 341–404. Lambert, J.F., Millman, W.S., Fripiat, J.J., 1989. Revisiting kaolinite dehydroxylation: a 29 Si and 27Al MAS NMR study. J. Am. Ceram. Soc. 111 Ž10., 3517–3552. Libby, P.W., Iannicelli, J., McGill, C.R., 1967. Elastomer reinforcement with amino silane grafted kaolin. American Chemistry Society, Division of Rubber Chemistry, Spring Meeting, 18 pp. Ranney, M., Cameron, G.M., Lipinski, B.W., 1973. Improvement of the static and dynamic properties of filled elastomers by silane coupling agents. Kautsch. Gummi Kunstst. 26, 409–416. Rehner, J., Wiese, H.K., Gessler, A.M., 1960. Cyclodienylhalosilane-treated mineral pigments. United States Patent 2,928,802 Žto Esso Research and Engineering.. Rocha, J., Klinowski, J., 1990. 29 Si and 27Al magic-angle-spinning NMR studies of the thermal transformation of kaolinite. Phys. Chem. Miner. 17, 179–186. Sanz, J., Madani, A., Serratosa, J.M., 1988. Aluminum-27 and silicon-29 magic angle spinning nuclear magnetic resonance
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pyridine intercalation compound derived from hydrated kaolinite. Clays Clay Mineral. 37 Ž2., 143–150. Theng, B.K.G., 1979. Formation and Properties of Clay–Polymer Complexes. Elsevier, Amsterdam, pp. 133–154. Thompon, J.G., 1985. Interpretation of solid state 13 C and 29 Si nuclear magnetic resonance spectra of kaolinite intercalates. Clay Clay Miner. 33 Ž3., 173–180. Zheng, S.L., 1995. Fenti Biaomian Gaixing. wSurface Modification of Mineral Powderx China Construction Material Publishing, Beijing, pp. 10–80, in Chinese.