Clays for polymeric nanocomposites

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Clays for polymeric nanocomposites

Utracki, Leszek A.; Broughton, Bill; Gonzalez Rojano, Norma; Hecker de

Carvalho, Laura; Achete, Carlos A.

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Clays for Polymeric Nanocomposites

Leszek A. Utracki,1Bill Broughton,2 Norma Gonza´lez-Rojano,3Laura Hecker de Carvalho,4 Carlos A. Achete5

1

National Research Council Canada, Industrial Materials Institute, 75 de Mortagne, Boucherville, QC, Canada J4B 6Y4

2

Bio, Polymeric and Composite Materials, Industry and Innovation Division, National Physical Laboratory, Materials Division, Teddington, TW11 0LW, UK

3

Centro Nacional de Metrologia (CENAM), Divisio´n de Materiales Orga´nicos, km 4,5 carretera a Los Cue´s, El Marque´s, Quere´taro, QT, C.P. 76 241, Mexico

4

Universidade Federal de Campina Grande, Centro de Ciencias e Tecnologia, Av. Aprigio Velososo 882, Bodocongo´, Campina Grande, Brazil

5

Instituto Nacional de Metrologia, Normalizac¸a˜o e Qualidade Industrial - INMETRO, Divisa˜o de Metrologia de Materiais, Av. N.S. das Grac¸as 50, Duque de Caxias, RJ, Brasil

We discuss test methods and results for determining individual clay platelets shape, size, size distribution, elemental composition, and impurities. Commercial so-dium salt varieties of natural, semisynthetic and syn-thetic clay (Cloisite1-Naþ

, Somasif ME-100, and Topy-Naþ

, respectively) were analyzed. In this international collaboration, eight laboratories on three continents carried out the work within the VAMAS TWA-33 activ-ities. There are large differences between the three nanofillers as far as: (1) the platelet orthogonal dimen-sions, (2) chemical composition, and (3) contaminants (their diversity and quantity) are concerned. Elaborate purification of natural clays leaves behind 2–5 wt% of organic and mineral impurities, whose nature, shape, size, and chemistry depend on the clay origin. These contaminants affect nanocomposite performance, thus controlling their composition and quantity is essential. The article describes the developed methods, summa-rizes the preliminary results, discusses the encoun-tered difficulties, and proposes methods for solving them. POLYM. ENG. SCI., 00:000–000, 2011.ª2011 Society of Plastics Engineers

INTRODUCTION

As the pace of nanocomposites research and applica-tion has intensified, it became apparent that the conven-tional approach to understand and optimize the polymeric

nanocomposites (PNC) performance is unsatisfactory. The misconception is that, it is simply a case of scale and that technology applicable to discontinuous reinforced compo-sites is transferable directly to PNC. However, at the nanoscale, the conventional approach used to determine structure-property relationships is unsuitable when the length-scale of the reinforcement and that of the onset of nonbulk (localized) properties coincide.

The nanosized particles are characterized by large sur-face-to-volume ratio, for example, the specific surface area of layered montmorillonite (MMT) isAsp¼750–800 m2/g,

thus about half of atoms are on the surface. These two quantities are smaller for spherical particles, for example, a 10 nm sphere with similar density hasAsp230 m2/g and

15% of surface atoms. In consequence, the layered clay platelets are capable of immobilizing large quantity of polymeric segments, thus manifold increasing effective volume fraction of the reinforcing solids. Thus, in compari-son to classical polymer matrix composites (PMCs), the fil-ler content needed for enhancing material performance of PNCs is significantly reduced to, typically, 1–5 vol%. The addition of nanoparticles has a beneficial effect on a wide suite of mechanical and physical properties (e.g., stiffness, strength, thermal stability, fire retardancy, and fracture toughness), improving functionality to levels not achieva-ble using larger scale fillers in the same quantities.

PNC performance and the functionality increases of the matrix polymer by the addition of nanoparticles depend on a number of factors including shape, size, size distribution, chemical composition, and purity of the

Correspondence to: Dr. Leszek Utracki; e-mail: leszek.utracki@nrc.ca DOI 10.1002/pen.21807

Published online in Wiley Online Library (wileyonlinelibrary.com).

V

nano-filler. These aspects are of prime interest to the Ver-sailles Project on Advanced Materials and Standards (VAMAS), Technical Work Area on Polymer nanocompo-sites (TWA-33). The relevance of the work cannot be understated particularly as the potential global market for PNCs is expected to exceed £1bn/annum by 2015, and it hinges on the availability of accurate and reliable techni-ques for measuring the above properties. Material prop-erty enhancement and their modeling depend on the avail-ability of improved measurement methods and reliable in-formation of the PNC individual components.

As this article summarizes work performed within the TWA-33, a short historical note is appropriate. In 1982, leaders of the G7 countries created the VAMAS for supporting trade through the international collaborative projects aimed at providing the technical basis for harmonized measurements, testing, specifications, and standards for advanced materials. VAMAS carries out its work within topical groups known as the Technical Working Areas (TWA’s) [see: www.vamas.org]. The TWA-33 is relatively recent, viz., www.vamas.org/twa33/ documents/2009_vamas_twa33_general.pdf. Its aim is the selection of specific test methods for PNCs, generation of round robin test results, and transmitting the technical data to the international standardization organization (ISO) for establishing new standards. The aim of Project #1 is the characterization of nanoparticles, whereas that of Projects #2 and #3 is development of test methods for the electrical, dielectric, and mechanical properties of PNCs.

This article summarizes the early results of Project #1 generated by the members listed in Table 1. The authors welcome the reader’s comments and encourage joining the organization by contacting any member listed in that Table.

EXPERIMENTAL Materials

The study used sodium salts of commercial clays: (1) natural MMT from Southern Clay Products, Cloisite1-Naþ (C–Naþ); (2) a semisynthetic fluoro-hectorite, Somasif ME-100 (ME-100) from CBC (Japan); and (3) synthetic fluoro-tetrasilicic mica from Topy Industries (Topy-Naþ

). The C–Naþ

is a Wyoming clay that contains 4–9 wt% moisture and 7 wt% loss on ignition (LOI) [1]. It is an off-white powder with the specific and bulk density q ¼ 2.86 and 0.34 g/mL, respectively. In the ‘‘as supplied’’ powder, the aggregated particle diameter ranges from about 2 to 13 lm. The X-ray diffraction (XRD) interlayer spacing is d001 ¼ 1.17 nm and the unit cell composition

is [Al3,34Mg0,66 Na0,66](Si8O20)(OH)4. The C–Na þ

plate-lets thickness is, tz ¼ 0.96 nm, the cation exchange

capacity is CEC ¼ 0.92 meq/g, and the average nominal aspect ratio isp : (diameter/thickness)  280.

Tateyama et al. patent [2] describes the synthesis of ME-100, which involves the following steps: mixing the

pow-ders of purified, natural talc with that of sodium fluoro sili-cate, Na2SiF6, and lithium fluorite, LiF, then heating it in an

electric furnace for several hours at 850–9008C. The reac-tion results in high aspect ratio lamellar phyllosilicate, with a structure similar to hectorite. The unit cell composition is (NaF)2.2(MgF2)0.1(MgO)5.4(SiO2)8. The particle size of the

agglomerated bright white powder ranges from about 5 to 7 lm, its specific surface area is 9 m2/g, the nominal as-pect ratio isp  6000, density q ¼ 2.6, CEC ¼ 1.2 meq/ g, and the interlayer spacing isd001¼0.95 nm [3, 4].

During the preliminary tests, synthetic clay was also examined. Since the company does not provide sodium salt, a small quantity of Topy-Naþ

was prepared at NRCC/IMI [5]. Topy products are high purity materials manufactured from salts and oxides at the temperature above 15008C followed by controlled crystallization. The expandable fluoro-mica is ion-exchanged with am-monium salts. Typically, its unit cell composition is NaMg2.5-Si4O10F2. The analysis gives Na ¼ 4–9, Li \

0.5, MgO ¼ 21–29, SiO2 ¼ 55–65, and F ¼ 6–15 wt%.

The aspect ratio of Topy-Naþ is p  5000, CEC ¼ 0.80 meq/g, andd001¼1.23 nm [4].

Test Procedures

The clays have been characterized for:

1. Shape, size, and size distribution of platelets; 2. Chemical composition;

3. Impurities

Shape, Size, and Size Distribution of Clay Platelets [6]. The objective of this task is determination of the shape of individual clay platelets by measuring their orthog-onal size, viz. platelet length (the longest dimension ¼ L), platelet width (perpendicular to length ¼W), and the plate-let thickness (orthogonal to the clay plateplate-let surface ¼ tz),

followed by calculation of their averages and size distribu-tions. Evidently, success of the process critically depends on the extent of clay dispersion, that is, on exfoliation. Accordingly, suspension of 0.002 g/L of Na-clay in demine-ralized water is prepared by preswelling the clay, and then dispersing the platelets in ultrasonic bath at 40 kHz and T ¼60–808C. A droplet of the resulting suspension is depos-ited on a polycarbonate (PC) membrane filter (SPI-PoreTM, pore diameter ¼ 220 nm). After drying, the membrane with clay platelets is observed in scanning (SEM), transmission (TEM), or atomic force (AFM) microscope.

For reliable average measures of the platelets (L, W, and thickness, tz), over 200 particles (from 30–40

micro-graphs) need to be analyzed. When using SEM with the field-emission gun (FEG-SEM) a drop of exfoliated clay suspension was deposited on the PC-filter, for good con-trast metalized with Pt and/or Au, and observed under low voltage (e.g., 1 kV). Next, the platelet contours were manually traced and scanned for the image analysis using

commercial software (Visilog, SigmaScan Pro, Image-Pro1-Plus, etc.).

When using AFM, a drop of exfoliated clay suspension was deposited on a hot, freshly cleaved mica flake, and scanned using a Si cantilever tip at a speed of 2–29 mm/s in a 300 kHz tapping mode. The size and size distribution was computed using the instrument software.

Chemical Composition of Clay Platelets The energy-dispersive X-ray spectroscopy (SEM-EDX) provides infor-mation on the chemical composition of a specimen. The high-energy primary electrons in SEM eject electrons from

the specimen’s atoms. Replacements of the inner shell elec-trons by those from the outer ones engender X-rays with the energies characteristic of the elements. The signal origi-nates from the few micrometers thick surface layer. As clay may have locally diverse composition, each particle should be sampled from ‡5 locations, collecting ‡30 scans for sta-tistical analysis. The instrument must be calibrated using known specimen atomic composition at the test voltage.

Few particles of clay powder on metallic support are covered with thin layer of colloidal graphite. After drying, the specimen is placed in SEM for observation of its shape and possible presence of impurities. Next, the

ele-TABLE 1. VAMAS TW-33 Project #1 participants.

Country Name Organization Address TWA-33

Brazil C. A. Achete, Instituto Nacional de caachete@inmetro.gov.br; Member B. Archanjo, Metrologia, bsarchanjo@inmetro.gov.br; Member E. Gravina, Normalizac¸a˜o e eggravina@inmetro.gov.br; Member A. Kuznetsov Qualidade industrial–INMETRO okuznetsov@inmetro.gov.br; Member Brazil L. Hecker de Carvalho Universidade Federal de Campina Grande laura@dema.ufcg.eqd.br Member Canada F. Perrin-Sarazin, Nation Research Council Canada, IMI, Florence.Perrin@cnrc-nrc.gc.ca; Member

L. A. Utracki Boucherville, QC leszek.utracki@nrc.ca Chair Italy G. Camino Politecnico di Torino, Sede di Alessandria giovanni.camino@polito.it Co-chair

Japan M. Okamoto Toyota Technological Institute, Nagoya okamoto@toyota-ti.ac.jp Current Project Leader Mexico N. Gonzalez-Rojano Centro Nacional de Metrologia, Quere´taro, ngonzale@cenam.mx Member

J. L. Cabrera Qt Member

UK W. Broughton National Physical Laboratory, Teddington bill.broughton@npl.co.uk Member South Africa L. Adlem Nation Metrology Institute, NMISA ladlem@nmisa.org Member

FIG. 1. (1a and 1b) SEM images of ME-100 exfoliated and aggregated, respectively [11]. (1c and 1d) Anal-ysis of SEM image of C–Naþ

mental analysis is carried out at accelerating voltage of 15–35 kV and about 340k magnification. The specimen spectrum is corrected for the method artifacts and carbon presence (left after graphitization) and then the statistics of chemical composition is computed.

Clay Impurities. Commercial clays contain diverse and variable impurities. For example, the recent publication reports that commercial bentonite contains 63 wt% of so-dium (MMT-Naþ

) with the reminder consisting of con-taminants, such as quartz, kaolinite, carbonates, etc. [7, 8]. As the natural mineral composition varies not only with the geographical location, but also with strata, it is important that for reproducible performance of PNC the properties of ingredients are constant. There are three types of impurities: organic (e.g., humic substances, HS), nonexpandable clays (e.g., amorphous clays, vermiculite, kaolin) and diverse particulate minerals (e.g., silica, feld-spar, gypsum, orthoclase, apatite, halite, calcite, dolomite, quartz, biotite, muscovite, chlorite, hematite) [8].

For the analysis of contaminants, 0.5 g clay was dispersed in 50 mL demineralized water [9, 10]. The suspension was centrifuged, sedimenting particulate minerals, aggregates and non-expandable clays. After drying at 608C, the sediment was analyzed by XRD. The supernatant suspension was filtered through a cellu-lose filter producing well aligned in z-axis, several mi-crometer thick-layered clay films. Dry film was sub-jected to four XRD tests at ambient temperature: (1) directly, (2) after impregnation with ethylene glycol (overnight at 608C), (3) after heating to 4008C, and af-ter heating to 5508C.

RESULTS

Size, Size Distribution and Shape of the Clay Platelets SEM Results. As an example, Fig. 1 shows three SEM micrographs at the same magnification and manually traced contours of clay platelets. The top two images dis-play an exfoliated platelet and a short stack of ME-100 reported by Carvalho et al. [11], whereas in the bottom one there are 16 distinguishable platelets of C–Naþ described by Perrin-Sarazin and Sepehr [6]. The platelets’ contours (in Fig. 1D) were used for the statistical analysis by Image-Pro1-Plus. Such micrograph image analysis (MIA) yields the number and weight averages of L, W (i.e., Ln, Lw, Wn, Ww) and their ratios [(L/W)n, (L/W)w],

listed in Table 2.

Figure 2 presents images of the three sodium clays: C–Naþ, ME-100, and Topy-Naþ. Since the micrograph pore diameter is the same (220 nm) evidently, the platelet size dramatically increases from C–Naþ

to Topy-Naþ

[6]. Table 3 summarizes the C–Naþ

and ME-100 clay pla-telets dimensions determined by the National Physical Laboratory (NPL) [12]. Figure 3 shows these data plotted in probability coordinates. Apparently, while C–Naþ has smaller clay platelets and Gaussian (normal) size distribu-tion, ME-100 has larger platelets and more complex dis-tribution, with higher than normal population of smallest and largest plates. However, as shown in Fig. 3B, the ra-tio of L/W for both clays is similar, with the mean value of (L/W)av¼1.4 6 0.4.

AFM Results. Several Project members used AFM for determining the shape and size of clay platelets. As for the SEM measurements, here also full exfoliation is essential, thus the preparation of specimens is similar, viz. clay concentration of 0.002 g/L in demineralized water prepared by ultrasonication at about 40 kHz and T ¼ 60– 808C; a drop of the suspension should be deposited on hot, freshly cleaved mica. Figure 4 displays few examples [6, 10]. The number of micrographs taken for C–Naþand ME-100 clays was 160 and 92, respectively [10]. Due to the high clay concentration, the automatic image analysis was unable to distinguish individual platelets. However, manual examination of the automatic scans yielded

cor-TABLE 2. Statistical analysis of three clays [6].

Clay Counted platelets LengthL (nm) Width,W (nm) Ratios Ln Lw Wn Ww (L/W)n (L/W)w C–Naþ 234 290 350 183 219 1.58 1.60 ME-100 304 872 1097 572 743 1.52 1.48 Topy-Naþ 447 1204 1704 761 1186 1.58 1.44 Error – 60.2 60.2

FIG. 2. Comparison of the relative platelet size of C–Naþ

(2a), ME-100 (2b), and Topy-Naþ

(2c). The SEM magnification was x40k, x20k, and x10k, respectively. Note that in all micrographs the diameter of membrane pores is 220 nm [6]. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

rect orthogonal dimensions, for example, for C–Naþ and ME-100 the clay platelet thickness (see Fig. 4), tz ¼ 1.2

and 1.0 6 0.2 nm, length: L ¼ 222 and 640 nm, respec-tively. It is noteworthy that some micrographs (e.g., see Fig. 5) evidence the presence of contaminating particles, clearly visible in the C–Naþ

.

Other Methods. Some participants also evaluated the automatic particle size analyzers, including highly advanced ones, with rather disappointing result. For example, the Horiba Partica is the second-generation Low Angle Light Scattering (LALS) instrument. It uses the Mie Scattering for measuring the particle size within the range of 0.01–3000 lm, with stability and accuracy guar-anteed to 0.6%, precision to within 0.1%. The sample-to-sample measurement time is 60 s. The system has a cen-trifugal pump and a 130 W in-line ultrasonic probe for dispersion that ‘‘allows the complete sample dispersion and analysis sequence to be handled without the need for external sample preparation’’ [13].

For testing clays, first a 5 wt% clay suspension was ball-milled then kept at 858C for 12 h. For comparison, low-concentration clay suspensions (0.5 and 0.005 wt%) were also prepared by 1 h ultrasonication and then kept for 12 h at 858C. The results for C–Naþand ME-100 are shown in Figs. 6A and B, respectively–for comparison the platelet size distribution determined by SEM with MIA is also dis-played [14]. In the case of C–Naþ, depending on concentra-tion, two or three peaks are detectable, with the first ones located not far from that of MIA, viz. Lpeak ¼209, 261,

and 350 nm for MIA, 0.05 and 5 wt% LALS. The other peaks with Lpeak ¼1.9 to 20.1 lm reflect the presence of

aggregates – their size increases with clay content.

Figure 6B shows that for ME-100, there is a significant difference in the locations of MIA and LALS peaks. The peak maximum of the former is at 0.60 lm, whereas the first peaks from LALS are located atLpeak ¼7.3, 7.3, and

11.2 lm for 0.005, 0.05, and 5 wt% clay, respectively. Interestingly, the data indicate that even at the highest clay content of 5 wt%, the ball milling eliminated the largest ME-100 aggregates, evident in the ultrasonicated samples at peak positionsLpeak ¼945 and 712 lm, for 0.005 and

0.05 wt% clay, respectively. It is noteworthy that the nom-inal maximum size of ME-100 platelets is about 6 lm [3]. Similar results were obtained by Gonzalez-Rojano and co-workers (Table 4) using the dynamic light scattering (DLS) or photon correlation spectroscopy (PCS) [10].

The lack of agreement between AFM and DLS may be related to the flow orientation of the large ME-100 clay platelets. Zanetti-Ramos et al. measured D of spherical polyurethane particles. AFM indicated presence of two generations of particles with D ¼ 127 6 26 and 218 6 36 nm, whereas DLS showed single generation with Dav

¼262 nm [15].

Chemical Composition of Clays

The chemical composition of C–Naþand ME-100 was determined following the earlier described procedure. For

TABLE 3. Statistics of 204 clay platelets dimensions of C–Naþ

and ME-100 [12].

Clay Parameter Mean Std

Dev Minimum Median Maximum C–Naþ Length (nm) 543 144 182 534 1129 Width (nm) 377 100 147 377 891 Perimeter (nm) 1786 472 569 1795 4190 Area (3 103mm2) 148 73 27 142 734 Effective D (nm) 211 50 93 213 483 Length/Width 1.48 0.38 1.00 1.42 4.14 ME-100 Length (nm) 811 244 338 783 1557 Width (nm) 589 198 209 547 1340 Perimeter (nm) 2735 813 994 2607 5640 Area (3 103_mm2₎ 340 88 55 299 1240 Effective D (nm) 317 88 132 309 628 Length/Width 1.44 0.44 1.00 1.29 4.22

FIG. 3. Probability plots for the length and width of C–Naþ

and ME-100 clay platelets (3a) and their ratios (3b) and data from [12].

example, the X-ray spectra were obtained using Oxford Instruments Inca Energy EDX system with a Si(Li) de-tector. Table 5 summarizes the results from four labora-tories, with statistics of 30 scans at NPL [16] and Poli-tecnico di Torino [17]. The composition was adjusted by eliminating data for carbon and normalizing the content to 100 wt%.

In Table 6, the converted data from Table 5 (with results reported by Sepehr et al. [18]) express the molar

composition of crystalline cells. The literature formula [4, 8] and those based on results from the four participating laboratories are shown. As evident from the statistics, in spite of the large number of spectra the statistical standard error of measurements is significant, for C–Naþand ME-100 ranging from 6 13 to 53 and from 69 to 31 in wt%, respectively. It is noteworthy that results from other labo-ratories not always fall within this margin of error.

In addition, for identification of impurities present in C–Naþ and ME-100 clays qualitative elemental analysis

FIG. 4. Example of tapping mode AFM micrograph of C–Naþ

and ME-100 clay platelet length, L and thickness,tz(nm). The ME-100 platelet in 4a hasL ¼ 1180 6 10 and tz¼1.06 6 0.20 nm [6]; these sizes

for C–Naþ

and ME-100 platelets (in 4b and 4c, respectively) are:L ¼ 222 6 15 and 640 6 10 nm; tz¼1.2

6 0.2 and 1.2 6 0.2 nm [10].

FIG. 5. AFM measurements of ME-100 clay platelets with contaminat-ing particles. Note that the measured area is 2200 3 2200 nm2_{and the}

vertical scale is 19 nm [10].

FIG. 6. The low angle light scattering (LALS) measurements of C–Naþ

(6a) and ME-100 (6b) suspensions containingw ¼ 5.0, 0.5, and 0.005 wt% clay (see text). For comparison, the SEM-MIA results are presented [14].

using SEM-EDX and X-ray fluorescence (XRF) was con-ducted. These data will be presented in the next part [10]. Impurities [10]

As displayed in Fig. 5, AFM is capable detecting the presence of particulate contaminants [10]. However, for the systematic impurity identification a more elaborate procedure (described in Test Procedures–Clay Impurities) is required. In principle, centrifugation of clay suspension separates the components into two fractions, the heavier sediments and the supernatant liquid suspension. The XRD identifies the components of solid powders. Figures 7–10 present the key results.

Figures 7 and 8 compare the XRD diffractograms of ‘‘as received’’ clays (C–Naþ and ME-100, respectively) with their sediments spectra enriched with impurities. In Figs. 7a and b, the basal interlayer spacing of MMT is evident at d001 ¼1.18 nm. However, while the

diffracto-gram Fig. 7a mainly shows the pattern of C–Naþ that in 7b is significantly more crowded. After expanding its in-tensity scale the diffractogram (not shown) displays dif-fraction peaks corresponding to MMT (now d001 ¼ 1.35)

as well as those of contaminants: quartz (dominant peak at d ¼ 0.33 nm with 0.42, 0.23, and 0.18 nm secondary

ones), cristobalite (d ¼ 0.40 nm), anorthite (d ¼ 0.62 nm), and analcime (d ¼ 0.56 nm). Spacings of all these agree with XRD database lists. The absence of vermicu-lite peak atd ¼ 0.154 nm is noteworthy.

Figure 8 provides the corresponding information to that in Fig. 7, but now for ME-100. As evident from the diffractogram 8a, ME-100 is nearly contaminants-free, with a basal plane spacing of fluoro-hectorite at d001 ¼ 1.24 nm. The XRD of powder, recovered from

the clay sediment (8b), mainly repeats the pattern of the clay with traces of such contaminants as rectorite, NaAl4(SiAl)8O20(OH)4
2H2O with d ¼ 2.40 nm, quartz

(d ¼ 0.33 nm), talc with d ¼ 0.96 nm, and anthophyllite [Mg7Si8O22(OH)2]. While the rectorite presence indicates

local overheating during ME-100 synthesis, the other min-erals probably entered the synthesis with talc [10].

The supernatant liquid after centrifugation of C–Naþ or ME-100 has formed aligned, thin film on the filter. Figures 9 and 10 display the XRD diffractograms of C–Naþ and ME-100, respectively. The four diffracto-grams in each Figure represent, respectively:

1. Dry film sample

2. Clay (as in A) but preswollen in ethylene glycol 3. Clay heated to 4008C

4. Clay heated to 5508C

It is noteworthy that spectra 9A and 10A indicate pres-ence of a single dominant mineral–in the former Figure the peak at d001 ¼1.25 nm has a shoulder at d001¼1.48

nm, whereas in the latter a single peak atd001 ¼1.24 nm

is present. Auxiliary tests identified the presence of two smectites in C–Naþ powder (9A): MMT and beidellite, and single clay in ME-100 powder (10A), the fluoro-hec-torite. Thus,d001of C–Na

þ

film expanded after treatment with ethylene glycol from the original value of 1.25 and 1.48 to 1.70 nm (9B), while that of ME-100 expanded from d001 ¼ 1.24 nm to 1.70 nm (10B). Heating these

TABLE 4. Results of the PCS clay size determination [10].

Clay Conc. (g/L) Av. Dia. (nm) Std (nm) PI (2) Std (2) 2-nd peak (lm) C–Naþ 0.002 270 30 0.305 0.05 5 C–Naþ 0.20 338 55 0.276 0.08 5 ME-100 0.002 469 66 0.504 0.08 5 ME-100 0.20 474 82 0.347 0.15 5

Conc. ¼ concentration of clay in suspension; Av. Dia ¼ average di-ameter of clay particles; Std ¼ standard deviation; PI ¼ dimensionless polydispersity index – a measure of nanoparticles size distribution; 2-nd peak ¼ size of aggregates at the peak value.

TABLE 5. Chemical composition of C–Naþ

and ME-100 [6, 10, 15, 17]. All result in weight percent.

Clay Parameter O Na Mg Al Si Fe Total

C–Naþ Mean [6] 52.40 2.51 2.65 11.78 30.66 – 100 C–Naþ Mean [10] 50.16 3.60 1.34 11.78 29.41 3.71 100 C–Naþ Mean [17] 64 6 4 3 6 0 – 11 6 4 22 6 3 – 100 C–Naþ Mean [16] 48.21 2.38 1.21 11.05 32.10 5.04 100 Std. deviation 7.38 0.31 0.26 1.43 4.30 2.63 Max. 63.57 3.01 1.76 13.70 40.14 16.63 Min. 31.20 1.79 0.82 8.69 21.92 2.47

Clay Parameter O F Na Mg Si Al Total

ME-100 Mean [6] 27.04 28.90 3.88 16.44 23.74 – 100 ME-100 Mean [10] 37.37 19.92 5.40 14.32 22.44 0.56 100 ME-100 Mean [17] 51 6 3 7 6 2 3 6 0 15 6 3 22 6 2 – 100 ME-100 Mean [15] 43.59 12.48 2.59 14.50 26.84 – 100 Std. deviation 3.77 3.92 0.49 1.47 6.00 – Max. 49.60 18.79 3.73 17.16 42.59 – Min. 34.97 4.12 1.34 11.22 17.89 –

films at 4008C caused a collapse of the interlayer spacing to the theoretical value of d001 ¼ 0.96 6 0.01 nm (the

smectite diffractograms Figs. 9C and 9D show a weak peak at 0.34 nm). Heating at 5508C produced no addi-tional changes in XRD spectra. Table 7 provides a list of 14 minerals identified by the XRD method.

To confirm the identity of contaminants in C–Naþ

and ME-100, the centrifuged sediments was subjected to quali-tative identification of chemical composition using SEM-EDX and XRF. The results may be summarized as follows:

1. In C–Naþsuspension SEM-EDX detected Na, Mg, Al, Si, O, K, Fe as major and Ca, Ti, S, as minor compo-nents.

2. In C–Naþ

suspension XRF detected Na, Al, Si, Fe, as more intensive and Ti, K, S, Mg, O as less intensive peaks.

3. In ME-100 suspension SEM-EDX detected F, Mg, Si, O, as major and Na, Al as minor components.

4. In ME-100 suspension, XRF detected Si, Mg, as more intensive and Fe, Na, Al, O as less intensive peaks.

The elements detected in both analyses are in part from the sedimented impurities identified in the XRD measurements.

DISCUSSION

Natural clays originate from the hydrothermal altera-tion of alkaline volcanic ashes and rocks of the Creta-ceous period (85–125 million years ago), deposited by winds in seas and lakes [19, 20]. The plastics industry is interested in crystalline, swellable clays (e.g., hydrous sili-cates of Al and Mg, or phyllosilisili-cates) with the specific surface area of about 750 m2/g, which makes them highly physico- and chemico-sorptive. As a consequence, clay deposits not only contain admixed impurities (e.g., quartz, sand, silt, feldspars, mica, chlorite, opal, volcanic dust, fossil fragments, heavy minerals, sulfates, sulfides, carbo-nates, zeolites, and amorphous contaminants), but also 3– 5 wt% of adsorbed organic and inorganic compounds.

TABLE 6. C–Naþ

and ME-100 molar composition from EDS [8, 10, 16, 18]. All results in weight percent.

Source C–Naþ

ME-100

Literature [8] [Al3.34Mg0.66Na0.66](Si8O20)(OH)4 (NaF)2.2(MGf2)0.1(MgO)5.4(SiO2)8

NRC [18] [Al3.2Mg0.8Na0.8](Si8O20)(OH)4 (NaF)1.6(MgF2)6.4(MgO)0(SiO2)8

CENAM [10] [Al3.3Fe0.5Mg0.4Na1.2](Si8O20)(OH)4 (NaF)2.3(MgF2)4.1(MgO)1.8(Fe2O3)0.1(SiO2)8

NPL [16] [Al2.9Fe0.6Mg0.35Na0.72](Si8O20)(OH)4 (NaF)0.94(MgF2)2.3(MgO)2.7(SiO2)8

O 62.61 6 9.5 (6 15% error) 53.98 6 4.7 (6 9% error) Na 2.14 6 0.27 (6 13% error) 2.22 6 0.43 (6 19% error) Mg 1.04 6 0.21 (6 21% error) 11.84 6 1.18 (6 10% error) Al 8.61 6 1.19 (6 13% error) – Si 23.74 6 3.26 (6 13% error) 18,94 6 4.3 (6 22% error) Fe 1.87 6 0.98 (6 52% error) – F – 13.02 6 4.02 (6 31% error)

FIG. 7. X-ray powder diffractogram of ‘‘as received’’ C–Naþ

(a) and its centrifuged sediment (b). In the latter case, the prominent diffraction peaks with spacings: d ¼ 0.33, 0.40, 0.56, and 0.62 nm, evidence the presence of quartz, cristobalite, analcime, and anorthite, respectively (see text) [10].

FIG. 8. X-ray powder diffractogram of ‘‘as received’’ of ME-100 (a) and its centrifuged sediment (b). In the latter case the diffraction peaks evidence the presence of a needle-like rectorite, NaAl4(SiAl)8

O20(OH)4
2H2O, talc, Mg3Si4O10(OH)2, (spacing 2.4 and 0.96 nm,

respectively), anthophyllite, Mg7Si8O22(OH)2, and quartz (d ¼ 0.33 nm)

Bentonite is the raw clay, usually containing 60–80 wt% of MMT. After purification and conversion to sodium-salt, the MMT-Naþ

crystalline unit cell is [8]:

Layer sandwich of two Si–O tetrahedron sheets and a central AlMg–O octahedral sheet with 0.67 negative charge per unit cell

[Al3.33Mg0.67]20.67(Si8O20)(OH)4

Aqueous interlamellar layer containing 0.67 Naþ

cations per unit cell

(nH2O)Naþ0.67

Purification of natural clays is a labor-intensive, com-plex process involving nearly 300 steps. There is exten-sive literature on the topic [8, 21–26]. In an early report on purification of Wyoming clays Earley et al. outlined laboratory purification and fractionation by progressive sedimentation and centrifugation that eventually lead to a suspension of exfoliated individual platelets, still contain-ing amorphous and crystalline impurities [27]. Roberson et al. [28] also used sequential centrifugation method to separate individual MMT platelets from microaggregates constituting about 80 wt% of clay. Interestingly, the authors suggest that the aggregates are ‘‘due to the inter-locking of flakes in microaggregates during crystal growth, which prevents their complete separation in dilute suspension.’’ The tabulated length and width of the Wyoming clay platelets was 300–350 and 200–150 nm, respectively. The clay was brownish, containing 5–7 wt% Fe2O3[28].

Because of the variability of natural clay composition and inevitable presence of contaminants, there is a grow-ing tendency for its replacement by synthetic or semisyn-thetic clays, which contain no organic and much less

inor-ganic impurities [4]. The major obstacle for this replace-ment is the limited number of manufacturers and their low production capacity.

This article considers three aspects of clays deemed important for the production of reliable and reproducible PNC: (1) size and shape of clay platelets, (2) their chemi-cal composition, and (3) presence of impurities. As evi-dent from the data presented above, there is significant di-versity of results obtained for the tested clays.

Shape and Size

Starting with the first aspect, the clay platelet shape and size was examined using three methods:

1. SEM followed by contour tracing of well-defined sin-gle platelets and then statistical evaluation of their pol-ydispersity.

2. AFM followed by an automatic or manual image analysis. 3. Analyses of flowing suspensions.

FIG. 9. X-ray powder diffractogram of minerals recovered from super-natant liquid after centrifugation of C–Naþ

suspension. The diffraction peaks evidence the presence of MMT and beidellite (d001 ¼ 1.48 and

1.25, respectively), which after ethylene glycol expand (both) to 1.70 nm. Heating the specimens to 4008C reduced d001 to 0.97 nm, with a

weak signal at 0.34 nm. At 5508C, no changes in XRD spectra were observed (see text) [10].

FIG. 10. X-ray powder diffractogram of minerals recovered from su-pernatant liquid after centrifugation of ME-100 suspension. The diffrac-tion peaks indicate the presence of fluoro-hectorite, only. Treating the specimen with ethylene glycol expanded d001 from 1.24 to 1.70 nm

while heating it to 400 or 5508C collapsed it to 0.96 nm (see text) [10].

TABLE 7. Contaminants identified in centrifiged sediments of C–Naþ

[10].

Name Group Formula

Montmorillonite Expandable semecities

NA0.3(Al, Mg)2Si4O10(OH)24H2O

Beidellite Na0.3Al2(Si, Al)4O10(OH)2
2H2O

Analcime Zeolite Na2Al2Si4O122H2O

Quartz Oxides SiO2

Cristobalite SiO2

Rutile TiO2

Anorthite Feldspar CaAl2Si2O8

Microcline KAlSi3O8

Aragonite Carbonates CaCO3

Vaterite CaCO3

Dolomite CaMg(CO3)2

Gypsum Sulphates CaSO4
2H2O

Anhydrite CaSO4

Of the three methods, SEM and AFM followed by manual identification and analysis of platelet size offer reliable approach. In the case of AFM with automatic image analysis, the computed thickness ranged from 1 to 35 nm, indicating uncritical acceptance of any particle size and shape, viz. exfoliated platelets and large stacks. Similarly, as shown in Fig. 6, LALS does not distinguish exfoliated from nonexfoliated particles. Two obvious problems of the applied procedures are the inadequate exfoliation and irreproducible sampling that neglects the effects of the time and sedimentation.

The shape of clay platelets is irregular, but the length-to-width ratio was found to be nearly the same for all clays so far tested (L/W)av¼1.4 6 0.4. While the

thick-ness of layered silicates is about tz  1 nm, the average

inscribed (or effective) diameter,Din, and the aspect ratio:

p ¼ Din/tz, range from about 25 to 6000 [18]. The natural

clay, exemplified in this study by C–Naþ, has mid-size platelets,Din211 nm (Table 3), and it follows the

nor-mal probability curve (in Fig. 3 the correlation coefficient r ¼ 0.99).

Swellable 2:1 phyllosilicates have ionically imbalanced tetrahedral (T) or octahedral (O) layers that create a need for the formation of an interlayer space, which houses the balancing ions and moisture. The hectorite crystalline structure differs from that of talc by having some divalent Mg2þatoms in O-layer replaced by monovalent ones, Liþ

or Naþ

[4]. Accordingly, the production of the semisyn-thetic ME-100 amounts to a partial replacement of Mg by Li or Na, accompanied by a partial substitution of OH groups by F [2]. The process requires heating a mixture of talc with, for example, LiF and Na2SiF6. For

the control of clay crystalline structure, the furnace tem-perature is critical, namely, the fluoro-hectorite produced at 700–7508C has too small interlayer spacing, d001 ¼

0.91 nm, while that produced at 780–9008C the desired spacing: d001 ¼ 1.61 nm [2]. However, at T [ 9508C

instead of layered hectorite a needle-like richterite, Na[NaCa][(Mg,Fe2þ)5](Si8O22)(OH,F)2, forms [29]. The

natural talc contains such impurities as, for example, Fe2O3, Al2O3, CaO, AsS2, NiS2, [30], thus its rigorous

purification is essential for high quality of the semisyn-thetic ME-100.

Determination of the ME-100 platelet dimensions is more difficult than that of C–Naþ

as this clay does not exfoliate easily. This is because of factors like: (i) large clay platelets D  6 lm, thus high probability of inter-plate crystalline welding (e.g., by insufficient substitution of Mg2þ by a monovalent ion); (ii) large platelet size result in tendency for aggregation and sedimentation of its suspensions; (iii) partial replacement of OH groups by F reduces the hydrophilic clay character. It is note-worthy that the rate of molecular diffusion (which con-trols exfoliation) into stacks of circular discs with diame-terDindecreases withD

2 in[8].

The measured platelet length, L \ 1.6 lm (Table 3, Fig. 3a), is significantly smaller than that quoted by the

manufacturer, L  6 lm. Tables 2 and 3 show similar values of the mean length, namely, Ln¼872 and 811 6

244 nm, respectively. This may suggest that the largest platelets either sedimented to the bottom of the test tube before the clay suspension was sampled or that they were fragmented during ultrasonication. Contrary to the nomi-nal dimensions, the largest platelets were found to be those of Topy-Naþ.

Achieving full exfoliation of Na-clays poses a prob-lem, smaller for C–Naþ and Topy-Naþ more serious for ME-100. In the patent literature, the first step of clay intercalation is its dispersion at concentration w  2 wt%. Thus, Na-clay is added to demineralized water at temperature of 50–808C with vigorous stirring, ultrasoni-cation or ball milling for at least 4 h. In industry, clay is considered exfoliated when at least 80% platelets are fully and randomly dispersed. The dispersed system undergoes centrifugation to remove contaminants and aggregates and then is treated with intercalating onium salts [8, 31, 32]. It is noteworthy that as Fig. 6 shows, ultrasonication is less effective in eliminating aggregates than ball milling. However, there are reports that me-chanical grinding changes the clay morphology toward amorphization [33]. By contrast, as L. Pe´rez-Maqueda et al. reported, ultrasonication of mica (20 kHz, 0.75 kW, t ¼ 10–100 h) resulted in reduction of particle size by a factor of 10, while preserving the crystalline struc-ture and yielding aggregate-free platelets with relatively narrow size distribution [34]. It may be that the stand-ard laboratory ultrasonic bath (42 kHz, 70 W output) that is used for the clay dispersion is not suitable for peeling clay platelets from the stack. Recent report of MMT sonication at 35 and 70 W for up to 60 min showed a progressive shift of the LALS bimodal signals toward a decade smaller diameters. For example, start-ing withD0 ‡ 200 nm platelets their diameter after

son-ication for 1 h was D60 ‡ 30 nm, with the lower peak

position at D  70 nm, what may indicate attrition of exfoliated platelets [35]. Cavitation during ultrasonica-tion locally produces high temperatures, for example, 5075 6 156 K [36].

The second aspect of clay suspension is its stability. Ideally, one would wish that only contaminants and aggregates formed by faulty crystallization would sedi-ment, leaving stable suspension of exfoliated clay plate-lets for the analysis. The influence of acid-base balance and electrolyte content (pH and pK) on the stability of clay suspensions has been frequently discussed, both based on mathematical model or experimental data [21, 37, 38]. The suspension stability depends on the repulsive interaction between clay double layers, controlled by the clay composition (Si–OH and Al–OH sites on the platelet faces and edges) pH and pK that may lead to edge-to-face, face-to-face or edge-to-edge association. The experi-ments indicated that for Wyoming clay the most stable system is at pH ¼ 8.5 and NaCl content about 100 mmol/ L. The presence of sodium diphosphate, Na4P2O7, also

increased stability of the MMT-Naþ

dispersions against coagulation by NaCl [21]. However, adding a salt to clay suspension may enhance platelets aggregation and (upon drying) its crystals may be taken as impurity. Thus, wash-ing the test specimen with demineralized water solves is essential.

Cadene et al. studied Wyoming bentonite, purified by dispersing it in demineralized water then centrifugation, stirring at 808C for 12 h in water at pH ¼ 5, and finally redispersed in 0.1 M NaCl solution for 12 h followed by filtration and repeated washing until total absence of Cl2 ions [39]. The authors also successfully dispersed the purified clay in NaCl solution (1 mmol/L) and measured individually selected exfoliated platelets dimensions using AFM. The reported data are: L ¼ 320–400 nm, W  250 nm (hence L/W ¼ 1.44 6 0.16) and tz ¼ 1.2 6 0.1 nm.

These values are not far from C–Naþ dimensions in Tables 2 and 3.

Dynamic light/neutron scattering could be a useful technique for measuring the particle size, However, there are several potential problems related to sample prepara-tion, clay and salt concentration, refractive index, platelet alignment in the flow direction and others, which may affect these measurements and influence the apparent size [15, 40].

Chemical Analysis of Clay Powder

The second aspect of these measurements is the chemi-cal analysis of the original powder particles. In mineral-ogy, there are several test methods with complexity increasing upon the required degree of precision, includ-ing X-ray fluorescence instruments (XRF) or the electron probe microanalyzers (EPMA). The XRF precision for major elements (excepting hard to detect Al, Mg, and Na) is about 1% [41]. The newer EPMA’s (e.g., JXA-8530F) are highly sophisticated instruments, with magnification up to x50k, beam diameter 5–10 lm and elemental sensi-tivity 0.005–0.01 wt% [42, 43]. For the elemental analysis of clays the energy dispersive X-ray spectroscopy (EDS or EDX) used in conjunction with SEM has been used frequently [44, 45]. Because of wide availability of this instrument, mainly the latter method was used in the VAMAS project.

The SEM-EDX, is a simplified version of EPMA, where the accelerated electrons from SEM gun eject the secondary electrons from the probed atoms. Compared to electron probe, SEM-EDX is easy to use, but for quantitative analysis, calibration of the instrument must be carried out (at the test voltage) using standards of known composition (similar to the test specimens). Fur-thermore, the results must be corrected for the back-ground signal. SEM-EDX is about 10 times less sensi-tive than EPMA and suffers from several well-described artifacts [46].

As evident from data in Tables 4–6, the spread of val-ues, collected in a single laboratory and the averages

reported by different laboratories, is large. As the scatter is bigger for C–Naþthan for ME-100, the local variability of C–Naþ composition may be partially responsible. However, for ME-100 the spread of values reported by different laboratories is also uncomfortable, for example, for oxygen: O ¼ 23–51 wt%; for sodium: Na ¼ 1–5; for fluor: F ¼ 5–29; hence, it significantly exceeds the reported instrument error and possible variability of clay composition. At the present stage, it is unclear whether this is a problem of calibration, uncorrected artifacts’, error in peak identification or all of these.

Clay Impurities

The last aspect of the clay analyses is the identification of impurities. Clarey et al. obtained patent for purification of natural clays involving 296 steps of grinding, dispers-ing, filterdispers-ing, centrifugdispers-ing, chemical treatment, hydrocy-cloning, etc. [21]. As the document specifies, the resulting polymer-grade clay contains \ 5 wt% (‘‘preferably less than about 2% by weight’’) of impurities. The most diffi-cult to remove are amorphous silicates, stacks of crystal-lographically welded platelets and the residual 0.3–2 wt% quartz particles with size [ 300 nm. Additionally, puri-fied clay such as C–Naþ contains about 2 wt% of mois-ture and 7 wt% of loss on ignition, LOI (comprising organics, hygroscopic, and bound H2O, carbonic acid,

etc.) [1].

The reported impurities of smectites include [22]: oxides (Fe2O3, SiO2such as quartz, cristobalite and opal),

silicates (albite, anorthite, biotite, feldspar, kaolinite, mus-covite, orthoclase, stilbite), sulfates (gypsum), carbonates (calcite, dolomite, siderite), phosphates (apatite), sulfites (pyrite), chlorides (sylvite, halite), etc. From Table 7, it is evident that in spite of complex purification most of these contaminants are present in the centrifuged sediments of C–Naþsuspensions.

The X-ray powder diffraction method was used for identification of C–Naþ impurities. However, the draw-back of this method is that if the data overlap, the struc-ture determination is problematic. The Rietveld computer program method separates the overlapping peaks, thereby allowing for accurate determination of the structure. This method (also called Quantitative Phase Analysis, QPA) is quite powerful for determining the quantities of crystal-line and amorphous components in multiphase mixtures [47, 48]. Recently, Ufer et al. examined 36 contaminated bentonite samples from 16 locations, applying the newly modified Rietveld refined program for quantitative phase analysis, using corundum as an internal standard. The method is suitable for identification and quantification of clay impurities [49]. However, the experimental error of mineral content by the QPA is still large, namely, 6 to 12% [50, 51].

As shown in Fig. 5, AFM is also helpful in analysis of clay impurities. As the lateral force microscopy (LFM) is

sensitive to differences in chemistry of the surface its use with AFM offers an alternative approach.

CONCLUSIONS

This article summarizes the preliminary results from laboratories in seven countries working on development of the test methods for standardization of testing commer-cial clays destined for the PNC. Natural, semisynthetic, and synthetic clays were studied. The results show great polydispersity of these materials (polydispersity of shape, size, degree of dispersion, impurity content, and chemical composition). During the 80 years or so, the polymer industry learned to produce a great variety of specified grades of chemically identical polymers. Similarly, there is a great (but controlled) diversity of reinforcing materi-als for the classical composites. It is expected that in the future the nanofillers will have to be similarly controlled. Development of reliable procedures and test methods is the first step in that direction.

The reported results may be summarized as:

1. The clay platelets have irregular shape characterized by three orthogonal dimensions: length (L–the longest dimension), width (W–perpendicular to L), and thick-ness,tz.

2. The number- or weight-averaged ratio: L/W ¼ 1.4 6 0.2.

3. The dry platelet thickness:tz¼1.09 6 0.09 nm.

4. The effective platelet diameter:Din(nm) ¼ 247 6 37;

428 6 97; and 658 6 66 for C–Naþ, ME-100, and Topy-Naþ

, respectively.

5. The elemental composition of C–Naþ and ME-100 (compare data in Tables 5 and 6) showed large errors within the same laboratory (from 9 to 52%), dependent on the atomic number and content. In spite of that, using the reported average values and calculating the molecular formulas (see the top four rows in Table 6) lead to consistent results. The scatter of chemical com-position in part originates in the presence of impur-ities.

6. Only one laboratory analyzed the impurities. The method applied to C–Naþ

and ME-100 lead to identifi-cation of the main clay components (MMT and beidel-lite in C–Naþ and fluoro hectorite in ME-100) and contamination of these two clays by 13 and 3 mineral, respectively.

Some laboratories encountered problems exfoliating clay (especially ME-100), and/or preventing reaggregation of clay platelets during drying on a PC-filter or mica sur-face. To prevent this, it is advisable to remove excess liq-uid and wash the sediment with demineralized water. The focus should stay on the development of better methods of clay exfoliation, possibly different ones for different type of clays. Furthermore, it is evident that human inter-vention is needed for identifying the well-defined exfoli-ated platelets, for the statistical size analysis. Thus, the original SEM/manual tracing method is worth pursuing,

but so is the AFM [7, 38]. In the latter case, the operator should select individual platelets or a computer program should reject the ones thicker than, for example, 1.9 nm. The diverse statistical methods of size analysis used in the Project also need scrutiny.

There is a significant discrepancy between expected precision of the SEM-EDX method for clay chemical analysis, namely, instrument specification of less than 0.2 % versus the errors in Tables 5 and 6 of [ 9 %; the sources of this discrepancy should be identified. However, it is noteworthy that the literature data of natural clays composition show a similar scatter.

Identification of impurities in the natural C–Naþ

and semisynthetic ME-100 clays was performed in a single laboratory, thus it should be confirmed by other partici-pants. It is also desirable that the test procedure quantifies the impurities. A large number of contaminants was expected for C–Naþ, but contamination of ME-100 by anthophyllite and gypsum was not. It is probable that small quantity of these were brought into the semi-syn-thetic clay with the key reaction ingredient, the natural talc.

Now-a-days purification of natural clays is not capable eliminating diverse impurities. Furthermore, their nature, shape, size, and chemistry depend on the source and their effects on performance are largely unknown. In conse-quence, better analytical methods should be devised to characterize these clays (or organoclays). The Rietveld method or the LFM with AFM offers good potential for analyzing the impurities.

ACKNOWLEDGMENTS

The authors acknowledge the participation of INME-TRO’s group, B. S. Archanjo, E. G. Gravina, and A. Yu. Kuznetsov, in experimental analyses and discussions. The UFCG coauthor is grateful to A. E. Zanini from UFBA for helpful analysis and comments. The CENAM coauthor is grateful to J. L. Cabrera, F. Rosas, E. Zapata, J. M. Juarez, and E. Ramirez, for the experimental analyses.

NOMENCLATURE

p ¼ Din/tz clay platelet effective aspect ratio

AFM atomic force microscope

CEC cation exchange capacity

C–Naþ Cloisite1-Naþ

d crystal spacing

d001 clay interlayer spacing

Din average inscribed or effective diameter

DLS dynamic light scattering

EDS or EDX energy dispersive X-ray spectroscopy

EPMA electron probe microanalyzer

FEG-SEM SEM with the field-emission gun

L, Ln,Lw clay platelet length, its number and

LALS low angle light scattering

LOI loss on ignition

ME-100 Somasif ME-100

MIA micrograph image analysis

MMT montmorillonite

NPL National Physical Laboratory

NRCC/IMI National Research Council Canada,

Industrial Materials Institute

PC polycarbonate

PCS photon correlation spectroscopy

PMCs polymer matrix composites

PNC polymeric nanocomposites

QPA Quantitative Phase Analysis

SEM scanning electron microscope

SEM-EDX energy-dispersive X-ray spectroscopy

t ultrasonication time

T temperature

TEM transmission electron microscope Topy-Naþ synthetic clay from Topy Industries

TWA’s Technical Working Areas

TWA-33 Technical Work Area on Polymer

Nanocomposites tz thickness of clay platelet

VAMAS Versailles Project on Advanced

Materials and Standards

w clay content (wt%)

W, Wn,Ww clay platelet width, its number and

weight-averages

XRD X-ray diffraction

XRF X-ray fluorescence

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