Materials Chemistry and Physics 85 (2004) 410–415

Characterization of organobentonite used for polymer nanocomposites

J.Y. Lee, H.K. Lee∗Department of Chemical Engineering, Woosuk University, 490 Hujung-Li, Samrye-Eub, Wanju-Kun, Jeonbuk 565-800, South Korea

Received 13 October 2003; received in revised form 25 November 2003; accepted 29 January 2004

Abstract

Montmorillonite-rich clay was fractionated from bentonite mined from Kampo area in Korea, and it was treated with many cationicorgano-surfactant. The chemical and physical characteristics of them are investigated, and epoxy nanocomposites were also studied.To calculate the exchanged content of organo-surfactant, thermogravimetric was carried out and interlayer distance was measured bywide-angle X-ray diffractometer. The interlayer distance for MMT-III, HDA-M, ODA-M, CTMA-M, and ODTMA-M were 1.21, 1.53,1.57, 2.04, and 2.07 nm. All organobentonites were delaminated in the epoxy matrix forming the epoxy/organobentonite nanocompositeswith various contents. Tensile strength and Young’s modulus were modified by loading the organobentonite.© 2004 Elsevier B.V. All rights reserved.

Keywords: Nanocomposite; Natural bentonite; Epoxy; Organosurfactant; Layer distance

1. Introduction

Many kinds of clay minerals such as bentonite, kaoline,talc, mica, etc. have been used as inorganic fillers for theconventional polymer composites to reduce the cost or togive them special properties such as modulus, hardness,thermal stability, electrical insulation, thickening, opacityand brightness, etc.[1–4]. However, the difference of thesurface energy and the elasticity between the dissimilarmaterials induces the decrement of other properties suchas toughness, stiffness, strength, etc. Therefore, many re-searchers have investigated to overcome the demerits. Toreduce the difference of surface energy, clay minerals aretreated with coupling agents and to reduce the internalstress on the interface, they should be prepared in smallersize as far as possible, and polymer nanocomposites withnano-sized clay minerals have developed in recent.

In order to prepare nanocomposites, several approachesare used: (i) solution and drying method[5,6], (ii) monomerintercalation and polymerization method[5,7,8], (iii) poly-mer intercalation and compounding method[5,9–11]. In thesolution and drying method, organoclay is swelled in thepolymer solution and the nano-sized clay mineral is dis-persed in the polymer matrix during the drying process. Theprocedure of the monomer intercalation and polymeriza-tion method is composed of the monomer penetration intothe organoclay gallery and the delamination through the

∗ Corresponding author.E-mail address: hongkil@woosuk.ac.kr (H.K. Lee).

polymerization in the gallery. In the polymer intercalationand compounding method, the delamination of the organ-oclay is performed through polymer melt penetration intothe gallery during mechanical mixing. The polymer-claynanocomposites synthesized by the above methods are clas-sified into intercalated and exfoliated according to the stateof the multilayer of the clay[5,6–8]. In the intercalatedstate, polymer chains are inserted into the gallery of the claymineral, but it preserves the well-ordered multilayer struc-ture of silicate. On the other hand, exfoliated state showsthe fully laminated structure of the multilayers, where theinteraction between the silicate monolayers no longer exists.

Messersmith and Giannelis[12] reported that the glasstransition temperature of epoxy/clay nanocomposite washigher than that of epoxy matrix. They also showed that ata loading of only 4 vol.% of clay, the storage modulus at theglass transition region increased 58% and that at rubberyplateau region increased 450%. Mülhaupt and co-workers[13] demonstrated that toughness and stiffness of epoxyresin were modified by the addition of nano-sized mica,bentonite or hectorite. Pinnavaia and co-workers[14,15]studied the effect of organo-surfactant chain length onthe delamination of epoxy/clay nanocomposites, and theyfound that tensile strength and Young’s modulus increasedwith the increment of organo-surfactant chain length andthe loading content.

In this study, montmorillonite-rich clay was fractionatedfrom bentonite mined from Kampo area in Korea, and it wastreated with many cationic organo-surfactant. The chemicaland physical characteristics of them are investigated in order

0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.matchemphys.2004.01.032

J.Y. Lee, H.K. Lee / Materials Chemistry and Physics 85 (2004) 410–415 411

to use them as nano fillers in polymer matrix, and epoxynanocomposites were prepared.

2. Experimental

2.1. Materials

Ca-type natural bentonite denoted as NB was mined fromKampo area, Korea[16]. It was pulverized and screenedwith a sieve of 325 mesh. The 100 g of the passing partwas treated with 1 l of 0.5 M-NaCl solution at 70◦C for48 h. The swelling part was separated using separating fun-nel to remove the impurity, which had higher density thanthe swelling part. The classified montmorillonite-rich claywas washed with distilled water until no chloride was de-tected by addition of 0.1 N-AgNO3 and it was dried fully at110◦C, ground into 325 mesh under, and stored in a desic-cator. It was denoted as MMT-I. The 100 g of MMT-I wasactivated with 1 l of 0.5 M-NaCl solution at 70◦C for 48 h.The activation mechanism was explained by the cation ex-change of Ca2+ in the montmorillonite interlayer with Na+in water. The swelling part was separated, washed with dis-tilled water, dried fully at 110◦C and ground into 325 meshunder (MMT-II). The MMT-II was treated once more with1 l of 0.5 M-NaCl solution for 48 h through the same proce-dure, and it was denoted as MMT-III. It was dried at 110◦Cfor 24 h and stored at a desiccator.

NaCl was obtained from Duksan Pharmaceutical Co., Ltd.in Korea, and organo-surfactants from Aldrich Company,Inc. in USA were 1-hexadecylamine (HDA), 1-octadecyl-amine (ODA), cetyltrimethylammonium bromide (CTMA)and octadecyltriethylammonium bromide (ODTMA).

Epoxy matrix was consisted of diglycidyl ether of bisphe-nol A (DGEBA, epoxy resin), 4,4′-methylene dianiline(MDA, curing agent) and malononitrile (MN, chain exten-der) [1].

2.2. Preparation of organobentonite

In a container, 10 g of MMT-III was swelled in 200 mlof water/ethanol (4/1, v/v) solution at 70◦C for 2 h, and in

Table 1Characteristics of the organobentonites

Sample name

MMT-III HDA-M ODA-M CTMA-M ODTMA-M

Organo-surfactant Main chain carbon number 0 16 18 16 18Amine-type – Primary Primary Tertiary Tertiary

Swelling power (ml/2 g) In H2O 41.6 7.4 6.0 5.5 5.1In n-dodecane 3.8 4.7 4.8 5.0 5.5

Amount of organo-surfactant (wt.%) 0 17.5 17.9 20.6 21.2Exchanged organo-surfactant (mmol/100 g) 0 72 66 56 54Interlayer distance (nm) 1.21 1.53 1.57 2.04 2.07

another container, 20 mmol of organo-surfactant, which wasdouble amount of the CEC (=10.8 meq/10 g) of MMT-III asshown inTable 1, was poured into 100 ml of water/ethanol(1/1, v/v) solution. If amine-type organo-surfactant wasused, HCl acid with amine equivalent quantity was added.Then, the solution was heated until it became transparent.The solutions of the two containers were vigorously mixedfor 1 h with a sonicator and stirred with magnetic bar for24 h at 70◦C. The organobentonite was filtered, washedand dried at 70◦C for 48 h. It was stored at desiccator.The organobentonites treated with HDA, ODA, CTMA andODTMA were denoted as HDA-M, ODA-M, CTMA-Mand ODTMA-M, respectively.

2.3. Characterization

Cation exchange capacity (CEC) was measured by pH=7.0 ammonium acetate method[17] and specific surface areawas obtained by ethyleneglycolmonoethylether (EGME)method[18]. Swelling power and pH were measured by fol-lowing method; 2 g of the samples were poured into 100 mlof distilled water orn-dodecane in mass cylinder. After 24 h,the apparent volume of the swelling clay and pH of super-natant were measured. Moisture content was measured bydrying percent of the clay at 105◦C for 3 h. To calculate theexchanged content of organo-surfactant[13], thermogravi-metric analysis (TGA, Cahn TG-121) was carried out fromroom temperature to 900◦C at a heating rate of 10◦C min−1

in the nitrogen atmosphere of 5 ml min−1. The changeof interlayer distance was measured by wide-angle X-raydiffractometer (WAXD, XRD30, Rigaku). The X-ray beamwas nickel-filtered Cu K�1 (λ = 0.154 nm) radiation oper-ated at a tube voltage of 40 kV and tube current of 30 mA.The scanning range was 2θ = 2–20◦ with a rate of 1◦ min−1.

2.4. Preparation of nanocomposites and tensile test

Nanocomposites was prepared by the following pro-cedure: organobentonite was homogeniously mixed withepoxy resin for 1 h with a sonicator and it was well mixedwith MDA (30 phr) and MN (5 phr). The mixed sample wascured at 150◦C for 1 h after curing at 80◦C for 1.5 h.

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Tensile test for the cured epoxy nanocomposite was car-ried out according to ASTM D683. It was measured by us-ing Universal Testing Machine (SFM-10, United Co.) at across-head speed of 10 mm min−1.

3. Results and discussion

Fig. 1 shows the characteristics of the natural or cationexchanged bentonite in NaCl solution. All characteristicterms showed that grade of bentonite became better for nanomaterial, when the number of NaCl-treatment increased,and MMT-III sample showed the similar properties of theKunifia-F (Kunimine Co., Tokyo, Japan), whose montmo-rillonite content was 98–99%. It meant that the purificationand cation exchange of the natural bentonite were effec-tively carried out and MMT-III could be used for polymernanocomposites.

The montmorillonite monolayer with 1 nm thickness hasa crystal lattice with one alumina octahedral sheet sand-wiched between two silica tetrahedral sheets. If the Si4+ inthe tetrahedral sheet is partially substituted by Al3+ or ifAl3+ in the octahedral sheet is done by Mg2+ or Fe2+, the

Fig. 1. Characteristics of the natural or NaCl-treated bentonite. Funifia-F is a trade name of montmorillonite (Kunimine Co., Tokyo, Japan). Specificsurface area was calculated from EGME method[18].

shortage of positive charge within the monolayer inducesnegative charge, and it is compensated by the exchangeablecations in the interlayer gallery between two monolayers.The amount of the exchangeable cations such as Na+, K+,Ca2+ and Mg2+ expressed by cation exchange capacity is animportant factor to determine the grade of bentonite. Gener-ally, bentonite has CEC of around 80–120 meq/100 g, and inhere, as the bentonite was purified, CEC increased as shownin Fig. 1.

Specific surface area was increased with increasing purifi-cation and NaCl-treatment and that of MMT-III was higherthan that of NB by 250 m2 g−1. The montmorillonite con-tent of MMT-III was 99% and that of NB was 68%, whichwas calculated from the assumption that the specific surfacearea of pure montmorillonite was 810 m2 g−1 [18].

As the purification and Na+ exchange proceeded,swelling power increased. Especially, the values of MMT-IIand MMT-III with similar montmorillonite content werelargely different. It meant that the more Na+ exchanged inthe interlayer, the better swelling power was. The swellingphenomenon was caused by bipolar water entering to theinterlayers due to the cations in the galleries. If the Ca2+cation is main species in the galleries, the swelling power

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Fig. 2. XRD patterns for the natural bentonite or MMT-III.

was lower than that of Na+ exchanged bentonite due tothe relatively strong interaction between Ca2+ and silicatetrahedral. Moisture contents of NB, MMT-I, MMT-II andMMT-III were 8.5, 9.9, 10.1 and 10.2%, respectively, whichcould also be explained by the same comment. pH for NB,MMT-I, MMT-II and MMT-III were 6.8, 8.4, 9.3 and 9.8,respectively.

As shown inFig. 2, XRD patterns showed that the nat-ural bentonite co-mixed with quartz and feldspar[16] wasCa-type, and it was also found that the purification and cationexchange of the natural bentonite were effectively carriedout. Interlayer distance of the natural bentonite was 1.33 nm(2θ = 6.63◦), which was larger than that of MMT-III with1.21 nm (2θ = 7.33◦).

Fig. 3 shows the TG curves for MMT-III, HDA-M andCTMA-M at 10◦C min−1. On the dried MMT-III (Na-typebentonite) curve, 2.6% weight-loss was checked at the tem-perature range of 40–140◦C and 8.0% weight-loss appearedat 550–900◦C. The first was due to the evaporation of thedesorpted water molecules, which were adsorpted to thecations in the interlayer of the bentonite. The second was

Fig. 3. TG analysis of the organobentonite at 10◦C min−1.

related to the dehydration of water molecules from the crys-tal lattice with one alumina octahedral sheet sandwiched be-tween two silica tetrahedral sheets.

On the HDA-M or CTMA-M curve, the weight-loss ofwater desorption was about 0.6% and very large amountof weight-loss was shown at the temperature range of200–550◦C, which was related to the thermal decompositionof the organo-surfactant. The amount of weight-loss at above550◦C for organobentonite was larger than that of MMT-IIIand it was due to the dehydration of water molecules in thecrystal lattice and the decomposition of carbon residue fromorgano-surfactant. The weight-loss of water desorption inthe initial area for the organobentonites was far smallerthan that for MMT-III, and it was said that the interlayer ofthe bentonite was changed to organophillic atmosphere bytreating with organo-surfactant. The organophillic conditionof the organobentonite was confirmed by the steep decre-ment of swelling power in water and by the weak incrementof swelling power inn-dodecane as shown inTable 1.

The weight-loss of CTMA-M was larger than that of HDAby 3.1%, which meant that the amount of the exchangedCTMA in the montmorillonite interlayer was larger than thatof the exchanged HDA as shown inTable 1which was cal-culated from the difference between the weight-loss of theorganobentonites and that of MMT-III[13]. By the samemethod, the amounts of the exchanged ODA and ODTMAwere also listed inTable 1. As the molecular weight of theorgano-surfactants increased, the exchanged weight contentof them increased. However, if the weight unit convertedinto molar unit, the exchanged content decreased with theincrement of molecular weight of the organo-surfactants.For the same main chain carbon number of the surfactants,the exchanged contents in mole of the primary types werehigher than those of the tertiary types. These were due tothat the smaller molecular weight or the larger moleculardiameter was, the easier and the deeper diffusion of theorgano-surfactant through the interlayer was[19,20], andthis phenomenon was confirmed by the thermograms ofHDA-M and CTMA-M in Fig. 3. Thermal decompositionstarting temperature of CTMA-M treated with tertiary aminewas lower than that of HDA-M with primary amine, andthe decomposition rate of CTMA-M was steep, while thatof HDA-M was gentle. Similar tendency for ODA-M andODTMA-M was also shown.

Fig. 4 shows the X-ray diffractograms of MMT-III,HDA-M, ODA-M, CTMA-M and ODTMA-M. The peaksfor the interlayer distance (d-space) of MMT-III, HDA-M,ODA-M, CTMA-M and ODTMA-M were shown at 2θ =7.33◦, 6.05◦, 5.88◦, 4.44◦, and 4.37◦, respectively. To calcu-late the d-space, these values were introduced to the Bragg’sformula λ = 2d sinθ, whereλ is the wavelength of X-raybeam andθ the scattering angle[21]. The d-space for each2θ were 1.21, 1.53, 1.57, 2.04, and 2.07 nm, respectively,which were shown inTable 1. As the amount of exchangedorgano-surfactant increased, d-space increased, which wasdue to the organophilic surfactant of the interlayer repulsed

414 J.Y. Lee, H.K. Lee / Materials Chemistry and Physics 85 (2004) 410–415

Fig. 4. XRD patterns for MMT-III and organobentonites.

the hydrophilic surface of clay mineral, and the effect in-creased with the increasing the molecular weight of theorgano-surfactant.

To estimate the formation of nanocomposites, XRD anal-ysis was carried out for the epoxy/organobentonite compos-ites and epoxy/MMT-III composite. The peak of d-space at6.05◦ for HDA-M disappeared in the epoxy/HDA-M sys-tem, and the peaks for the other organobentonites also dis-appeared in the epoxy/organobentonite composites (Fig. 5).It was due to the delamination of the clay layers each other.However, the peak of d-space at 7.33◦ for MMT-III shiftedto 6.05◦ in the epoxy/MMT-III, that is, the d-space becamewider.

Fig. 6 shows that epoxy/HDA-M nanocomposite is for-mulated regardless of HDA-M content in the experimentalrange, and the other nanocomposites were also formed re-gardless of the organobentonite content.

Fig. 7 shows the tensile strength and Young’s modulusfor the epoxy/organobentonite composites. Tensile strengthincreases with the treatment of organo-surfactants and withthe increasing clay loading content. As the chain length oforgano-surfactant increases, the effect is much higher. This

Fig. 5. XRD patterns for epoxy/organobentonite composites.

Fig. 6. XRD patterns for epoxy composites with various HDA-M content.

is due to that the delaminated silicates with high aspectratio are rearranged in the direction of external force anddisperse the stress[14]. Young’s modulus also increaseswith the treatment of organo-surfactants and with the in-creasing clay loading content, however the longer chain

Fig. 7. Tensile strength and Young’s modulus for epoxy/organobentonitecomposites.

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length of organo-surfactant make the value decrease. Itcan be explained by the plasticity effect of the long chainorgano-surfactants[14].

4. Conclusion

Montmorillonite-rich clay with 99% purity was fraction-ated from bentonite in Korea, and it was found that the phys-ical properties such as cation exchange capacity, specificsurface area, swelling power, etc. were similar to those ofKunifia-F (Kunimine Co., Tokyo, Japan). It was found thatthe exchanged weight contents of the organo-surfactants in-creased with the increasing molecular weight of them. Forthe same main chain carbon number of the surfactants, theexchanged contents in mole of the primary types were higherthan those of the tertiary types. The interlayer distance forMMT-III, HDA-M, ODA-M, CTMA-M, and ODTMA-Mwere 1.21, 1.53, 1.57, 2.04, and 2.07 nm. All organoben-tonites were delaminated in the epoxy matrix forming theepoxy/organobentonite nanocomposites with various con-tents. Tensile strength and Young’s modulus increased withthe treatment of organo-surfactants and with the increasingclay loading content.

Acknowledgements

This paper was supported by Woosuk University.

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