Resol/montmorillonite nanocomposites obtained by in situ polymerization

Resol/Montmorillonite Nanocomposites Obtainedby In Situ Polymerization

Guadalupe Rivero,1 Analıa Vazquez,2 Liliana B. Manfredi1

1Research Institute of Materials Science and Technology (INTEMA), Polymer Division,National University of Mar del Plata, Juan B. Justo 4302, Mar del Plata 7600, Argentina2Polymer and Composite Material Group (INTECIN), Facultad de Ingenierıa,Universidad Nacional de Buenos Aires, Las Heras 2214, Buenos Aires 1127 AAR, Argentina

Received 26 November 2008; accepted 5 April 2009DOI 10.1002/app.30574Published online 27 May 2009 in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Nanocomposites based on resol resin andlayered silicates were obtained by in situ polymerization.An increment in the viscosity and in the content of freeformaldehyde in the prepolymer with the addition of claywas observed. It could be attributed to the entrance ofsmall phenol molecules into the galleries modifying theconcentration of reagent enabled to react outside the claygalleries. The composites were cured by temperature and itwas observed that the type of clay affect their dispersion inthe polymer. The unmodified CloisiteV

R

Naþ montmorillon-ite (CNa) was the best dispersed in the polymeric matrix,whereas the modified montmorillonites (MMTs) showedan interlayer contraction after curing. It seems to be due tothe loss of modifiers from the clay or to the different rate ofcuring inside and outside the galleries. In addition, the

composite containing CNa presented higher crosslinkingdensity than the others with modified MMTs. More adhe-sive strength was observed in that composite as well. Itwas related with the void content and the polarity of thesamples. These results were compared with the onesobtained when the nanocomposites were synthesized bymixing the clay with the prepolymer. The nanocompositeswith the addition of CNa showed similar properties inde-pendent of the way of synthesis used. However, it seemsthat the modified clays are better dispersed by mixing withthe prepolymer than by in situ polymerization. VVC 2009 WileyPeriodicals, Inc. J Appl Polym Sci 114: 32–39, 2009

Key words: nanocomposites; clay; thermosets; structure–property relations; adhesion

INTRODUCTION

Phenolic resins are thermosetting resins used inmany different applications since 1907 when theywere first synthesized.1 The principal advantages ofthese resins are their excellent chemical and thermalresistance. The application field of different poly-mers was widened with the emergency of nanotech-nology. It was proved that the use of lowpercentages of nanoparticles as reinforcement ofpolymer matrices is an effective way to improvesome properties such as mechanical, adhesive, bar-rier, and thermal.2 Layered materials such as mont-morillonite (MMT), saponite, and mica are widelyused clay nanoreinforcements because of their avail-ability and low cost.3 To reach an improvement inthe properties of the material, the layers of the claymust be dispersed throughout the polymer matrixphase. When the layers are fully separated, the clay

is considered to be exfoliated. On the other hand,when polymer resin is inserted into the gallerybetween the adjacent layers and the interlayer gal-lery expands, it is known as intercalated composite.Surface modification of the layers should favor theentrance of the polymer into the clay galleries. Func-tionalized clays with quaternary alkylammoniumsalts containing long carbon chains and compatiblegroups with the polymer matrix were selected.4

Different synthesis methods have been proposedto obtain nanocomposites: intercalation of polymerfrom solution, in situ intercalative polymerization, ormelt intercalation. Different ways of synthesis wereused to obtain nanocomposites based on clay andphenolic resins, both novolacs and resols. It wasreported the synthesis of phenolic resins by meltintercalation,5–9 in situ polymerization,10–13 andintercalation of prepolymer.14,15 In those works, dif-ferent conditions of synthesis and curing, formalde-hyde to phenol molar ratio, type of clay, and organicmodifiers were used. It was stated that the experi-mental conditions as well as the surfactant selectedto compatibilize the clay with the polymer and theintrinsic properties of the polymer are crucial tocontrol the dispersion. The aim of this work is toobtain phenolic/MMT nanocomposites synthesizedby in situ polymerization maintaining similar

Journal of Applied Polymer Science, Vol. 114, 32–39 (2009)VVC 2009 Wiley Periodicals, Inc.

Correspondence to: L. B. Manfredi ([email protected]).

Contract grant sponsor: National Research Council ofArgentina (CONICET).

Contract grant sponsor: ANPCYT; contract grantnumber: PICT’06-1560.

conditions of synthesis and curing than those usedin a previous work.15 Although in the previouswork the resol nanocomposite was obtained byintercalation of the prepolymer, in this study, in situpolymerization was used. It will allow us to com-pare the results obtained from the characterizationof the materials and to choose the way to obtain thematerial with the best properties.

EXPERIMENTAL

Materials

Nanocomposites based on resol and montmorillonitewere obtained in situ, by adding 5% of clay duringthe polymerization reaction. The resol-type phenolicresin was prepared using a formaldehyde to phenolmolar ratio equal to 1.3 under basic conditions, asexplained in the following. Melted phenol wasmixed with the clay and then sonicated at 25�C for 1h to enhance the dispersion. The mixture was placedin a 1-L stainless steel reactor with a low velocitystirrer, thermometer, and reflux condenser. Formal-dehyde was added, the pH was kept at 9.0 with asolution of NaOH 40% (w/w), and the mixture wasallowed to react for 2 h at 90�C. Thereafter, the mix-ture was neutralized with a solution of boric aciduntil a pH value of 6.8–7.0 was reached. The pre-polymer was dehydrated under vacuum at 80�Cuntil total extraction of water.

Resol was synthesized without the addition ofclay, placing phenol and formaldehyde in the reac-tor. Then, the same procedure explained earlier wasfollowed. After synthesis, resol and nanocompositeswere kept at –18�C until they were used.

Purified montmorillonite CloisiteVR

Naþ and theorganically modified CloisiteV

R

30B and CloisiteVR

10A, supplied by Southern Clay Products (TX), wereused as polymer nanoreinforcement. The organicmodifiers and the interlayer distance of the clays areshown in Table I to account the structural modifica-tions of the functionalizations.

Thereafter, resol and composites were placed overaluminium plaques and cured in an oven maintain-

ing 1 h at the following temperatures: 50, 80, 100,110, 130, 150, and 190�C.

Method

Free formaldehyde content in the resins was meas-ured by the hydroxylamine method.16 Viscosity ofthe prepolymers was measured in a cone-plateBrookfield HBTDV viscometer at 25�C. Solid contentwas determined by curing the resol and nanocompo-sites at 135�C until constant weight. Refractive indexwas measured by means of a refractometer AbbeNAR-2T.X-ray diffraction analyses (XRD) of the composites

were performed in a Philips PW 1710 diffractometer(45 kV and 30 mA) at 2�/min, with a CuKa radiation(k ¼ 1.54 A).Fourier transform infrared (FTIR) spectra of the

cured materials were acquired in attenuated totalreflection (ATR) mode using a Matson Genesis IIspectrometer. A spectral width of 400–4000 cm�1, 16accumulations, and 2 cm�1 resolution was used inthe analyses. The spectra were normalized using theband at 1610 cm�1, corresponding to the stretchingC¼¼C of the benzene ring that was expected toremain constant in all the samples.17

Transmission electron microscopy (TEM) imagesof the samples were obtained with a JEOL 100 CX IIat 80 kV of acceleration voltage. All samples wereultramicrotomed at room temperature to give sec-tions with a nominal thickness of 100 nm.The void content of the samples were determined

from scanning electron microscopy (SEM) imagesobtained with a JEOL JSM-6460LV. Polished lateralsurfaces of the samples coated with a 300 A goldlayer were analyzed.Single Lap-Shear test (ASTM D-1002-94) was used

to measure the adhesive strength of the polymer/aluminium joint. Tests were carried out at ambienttemperature with an Instron 4467 machine at acrosshead speed of 1.25 mm/min. The aluminiumsurface was abraded using grid 500 and 1000 sand-papers and cleaned with acetone before the polymerapplication. The assemblies were joined and fastened

TABLE IOrganic modifiers and Interlayer Distance of the clays

Type of clayOrganicmodifier

Concentration oforganic modifier[meq/100 g clay]

Interlayerdistance(d001) [A]

CloisiteVR

30B (C30B) 125 18.5

CloisiteVR

10A (C10A) 90 19.2

RESOL/MONTMORILLONITE NANOCOMPOSITES 33

Journal of Applied Polymer Science DOI 10.1002/app

between two glasses, and the curing cycle wasapplied. Spacers were used to ensure the bond thick-ness of 0.60 mm.

Contact angles were measured by the sessile dropmethod in a goniometer using high purity water asreference liquid.

RESULTS AND DISCUSSION

Resol and nanocomposite prepolymers were charac-terized in the liquid state measuring the refractiveindex, viscosity, free formaldehyde, and solid con-tent (Table II). The composite with CNa containedthe highest percentage of solid content. This resultcould be related with the resin crosslinking densitystudied by FTIR and discussed later. The compositethat showed the highest crosslinked structure aftercuring should have the highest solid content. It wasobserved an increment in the viscosity and freeformaldehyde content of the resol with the additionof clay. Nanocomposite prepolymers were obtainedby first mixing the phenol with the clay, and then,formaldehyde was added. The formaldehyde to phe-nol molar ratio outside the galleries should change ifpart of the phenol is able to go into the interlayerspace of the clay then, the free formaldehyde shouldbe higher than in the polymer obtained without theaddition of clay.

On the other hand, it was expected a higher incre-ment in the viscosity of the prepolymer, where thenanoparticles were better dispersed. Then, the sam-ples containing the highest free formaldehyde con-tent showed a higher increase in the viscosity, whichshould indicate that phenol molecules have migratedinside the galleries favoring the separation of theclay layers. Despite the lower initial interlayer dis-tance of the unmodified MMT, it seems that it wasdispersed in the liquid resol as far as the organicallymodified clays. It could be due to the hydrophilicOH groups contained in the surface of the CNalayers that are compatible with the hydrophilic char-acter of species in the resol prepolymer. The disper-sion of the modified MMTs should be favored bythe organic modifiers containing OH groups and abenzene ring in the C30B and C10A, respectively. Atthe beginning, phenol was the unique molecule that

could enter into the galleries and then, when formal-dehyde was added, part of the formaldehyde couldreact with the phenol that is both outside and insidethe galleries. Thereafter, the oligomers that areformed in the medium are also able to enter in theinterlayer spacing.In a previous work,15 we synthesized resol nano-

composites by mixing the clay with the prepolymer,instead of using in situ polymerization. Only themodified MMTs were observed to be better dis-persed in the liquid prepolymer in that work. The insitu polymerization allows a better dispersion of theunmodified MMT because it seems to be easier tointroduce the phenol than the prepolymer in thelower initial interlayer spacing. The affinity of thepolymer segments for the silicate surface is a keyfactor to understand the polymer–clay interaction. Inthis case, the OH groups in the surface of theunmodified clay layers are chemically compatiblewith the phenol groups contained in the polymermatrix.X-ray diffraction analyses were performed in the

samples to study the dispersion of the different claysin the polymer. The XRD patterns are showed inFigure 1. It was observed that the nanocompositecontaining CNa was the one that showed the higherintergallery distance due to the displacement of thebasal peak of the clay from 2y ¼ 7.3 to 4.7. Thisbehavior could be attributed to the interactionbetween the AOH groups of the clay layers and thepolymer, allowing their entrance in the galleries. Onother hand, nanocomposites containing modifiedMMTs showed an interlayer contraction because thebasal peaks were displaced to higher angles com-pared with the pristine clays. The same behaviorwas observed in other nanocomposites,18–21 and itwas attributed to loss of clay modifiers from the gal-leries, degradation of surfactants, and/or to the

TABLE IICharacterization of Liquid Resol and Nanocomposites

Simple

Freeformaldehyde

[%]Viscosity[Pa s]

Solid[%]

Refractiveindex

Resol 4.88 1520 53.67 1.5775Resol þ CNa 7.80 3608 65.45 1.5755Resol þ C10A 7.14 4060 60.97 1.5705Resol þ C30B 6.46 2063 52.35 1.5705

Figure 1 XRD patterns of the resol and nanocomposites.

34 RIVERO, VAZQUEZ, AND MANFREDI


different rate of polymer curing inside and outsidethe galleries. As these processes may occur simulta-neously, it is difficult to interpret the results. It wasproposed22 that the exfoliation mechanism in epoxyresins occurred in a first step by the diffusion of theresin in the clay galleries due to the high surfaceenergy of the clay, which attracts the polar resinmolecules. Then, the reactions that involve resinpolymerization could occur inside and outside thegalleries at the same time. The cure outside the gal-leries inhibits migration of more monomers andoligomers into the galleries producing a differentcuring rate in both places. Then, both balancebetween inter- and extragallery reactions and mono-mers, oligomers and resin diffusion are the key fac-tors, which control the clay exfoliation. The resultsobtained about the dispersion of the different claysin the polymer were not similar from those observedwhen the synthesis of the nanocomposites was notmade in situ. In the previous work,15 where the pre-polymer was mixed with the clay, the modified clayswere better dispersed in the polymer than the CNa.It was attributed to the affinity between the resinand the modifiers containing compatible groupswith the chemical structure of the resol. It seemsthat the better dispersion observed in the liquid statewas kept for the CNa, but the modified MMTs con-tract during curing. It could be due to the lowercrosslinking density of the polymer network whenthe modified clays are present, observed by FTIRand explained later.

The structure of the nanocomposites is typicallyelucidated using XRD and TEM. Although XRDoffers a convenient way to determine the interlayerspacing of the silicate layers in the nanocomposite, itis not possible to obtain conclusive results from thistechnique. XRD and TEM are often combined toensure the conclusion derived from separate analy-sis. TEM allows a qualitative understanding of themicrostructure through direct visualization of thenanoreinforcement arrangement. TEM microphoto-graphs of the composites are shown in Figure 2. Itwas observed that the layers of CNa [Fig. 2(a)] werethe better dispersed than the modified clays [Fig.2(b,c)] in the polymer. The microphotograph corre-sponding to the composite containing CNa showeda regular distribution of partially aligned clay sheetsand small regions of polymeric matrix. However,both the C10A and C30B showed a poor dispersionin the polymer matrix maintaining the alignment ofthe clay sheets with no appreciable expansion of thegalleries. Additionally, an inhomogeneous clay dis-tribution with bigger sections containing only poly-meric matrix was observed in Figure 2(b,c). Theseresults were in accordance with XRD results. Also, itwas observed that some of the sheets are curveddue to their high aspect ratio. It was explained that

to take full advantage of the nanoreinforcement, theclay layers must to be oriented in the appropriatedirection and not curled or curved.21

The influence of the addition of clay in the chemi-cal structure of the resol was studied by FTIR. Nor-malized spectra of the resol and the nanocompositesare shown in Figure 3. To analyze the chemicalstructure of the polymer, bands corresponding to

Figure 2 TEM images of the nanocomposites with: (a)CNa; (b) C30B; and (c) C10A.



typical groups of the phenolic resin such as methyl-ene bridges between phenolic rings and primaryhydroxyls were studied. Those methylene bridgesare formed during the crosslinking reactions thatinvolve two methylolphenols or a methylolphenolwith a free ortho or para position in the benzene ring[Fig. 5(b)]. The methylolphenols were formed fromthe addition of formaldehyde to phenol in a firststep [Fig. 5(a)]. The addition occurs in the activatedpara and ortho position of the benzene ring.1 Thebands corresponding to the methylene bridges arethose that appeared at 1450 and 1480 cm�1, for thepara-para’ and ortho-para’ bridges, respectively.17 Therelative quantity of those bridges measured from theFTIR spectra is shown in Figure 4. It was observedthat the methylene bridges content is higher for thecomposite containing CNa, corresponding to ahigher crosslink network. The composites withmodified clays reached a lower crosslinking densitythan the resol resin and the composite with CNa.

The primary OH groups content was measured inthe peak at 1015 cm�1.23 These OH groups are fromthe methylols that are formed during the addition offormaldehyde to phenol [Fig. 5(a)], but thereafterthey are condensed forming more substituted spe-cies, water, and formaldehyde [Fig. 5(b)]. As it wasexpected, the composites showing a higher crosslink-ing density are the ones that showed the lowerquantity of free OH groups (Fig. 6). Similar resultsabout the higher crosslinking density of the compo-sites containing CNa were obtained in previousworks when resol nanocomposites were synthesizedby mixing the prepolymer with the clays and thencured by temperature15 as well as cured by the addi-tion of an acid catalyst.14 It was attributed to thepresence of Naþ ions in the medium that are able to

form intermediate chelates favoring the additionreactions between phenol and formaldehyde,increasing the activation, and the rate of polymeriza-tion.24 The lower crosslinking density with the modi-fied MMTs could be attributed to the presence of thelong carbon chains of the surfactants producingmore free volume. Because of the contractionobserved, it seems that the organic modifiers areexpulsed from the galleries and then they are able tointeract with the polymer affecting the reactions.As these materials could be used as metal coat-

ings, the strength of the adhesive joint between theresin and aluminium was measured by the SingleLap-Shear test. An increment in the shear stress ofthe nanocomposites when compared with the neatresin was observed (Table III). The best result wasobtained with the CNa reaching a 34% of incrementin the shear stress. In addition, the void content wasmeasured in the samples, as the voids area/totalarea (Table III). The composites containing lesserquantity of voids were the ones that showed the bet-ter strength of the adhesive joint between the metaland the polymer. Also, the ones that presented ahigher quantity of voids contained a bigger voids(Table III), resulting in detriment of the adhesivebehavior. The presence of voids in the polymer layeraffects the bond strength because of the lower con-tact area and the diminution in the polymer mechan-ical properties. The crack surfaces from the Lap-Shear test showed both adhesive and cohesive fail-ures. Niklaus et al.25 investigated the influence ofdifferent bonding parameters on void formation in abond. They reported an increment in the bondstrength when using void-free polymer coatings ona silicon substrate.

Figure 3 FTIR spectra for the resol and nanocomposites.

Figure 4 Relative methylene bridges (o-p0 and p-p0) in theresol and nanocomposites, measured by FTIR.



Influence of void content on the adhesive strengthwas also observed in the previous work, where thecomposites were synthesized by intercalation of the

prepolymer.15 Under both conditions, the compositescontaining the modified MMTs showed higher quan-tity of voids than the composite containing theunmodified clay. It seems that part of the bubblesthat are trapped during the crosslinking reactionscould be due to the water evolved from the clay andin a lesser quantity to the decomposition of the sur-factants. There were published several works wherethe thermal degradation of different clays was stud-ied. It was reported26 that the Naþ montmorilloniteloss mass in the temperature range between 100 and300�C due to the evolution of free water and water

Figure 6 Relative quantity of OH groups in the samples,measured by FTIR.

TABLE IIIShear Stress and Void Content Measured in the Samples

Sample

Shearstress(MPa)

Voids [%](voids area/total area)

Voidsdiametermean (nm)

Resol 2.08 � 0.15 – –Resol þ CNa 2.79 � 0.40 0.17 1.07 � 0.51Resol þ C10A 2.27 � 0.51 0.95 6.02 � 1.53Resol þ C30B 2.12 � 0.36 1.15 20.30 � 7.61

Figure 5 Reactions of addition (a) and condensation (b) between formaldehyde and phenol.



from the interlayer region and then, the dehydroxy-lation of bounded hydroxyl groups of structuralwater occurs in the temperature range 500–1000�C.However, the onset decomposition temperature ofthe organically modified silicates was approximately200�C. It has been proposed that the organic modi-fiers (quaternary ammonium salts) start decompos-ing releasing first the low molecular weightmolecules, whereas the relatively high molecularweight may still exist between the interlayers untilthe temperature is high enough to lead to their fur-ther decomposition. Cui et al.27 published a detailedstudy of the surfactant degradation analyzing theinfluence of the quantity and type of modifier on thenanocomposite thermal degradation. They con-cluded that several factors such as surfactant typeand surfactant excess in the organoclay affect thethermal stability of the organoclays. The substituentson the ammonium cations may also influence theirthermal stability. The degradation temperatureincrease with the addition of groups such as benzylto the alkylammonium salts. The resol compositesused in this work were cured at temperatures up to190�C to be sure to obtain a totally cured material.Then, at the cure temperature the degradation of thesurfactants should be low. However, the presence ofmore quantity of bubbles in the nanocomposites con-taining the OMS could be due to the water elimi-nated from the clay that was lost at lowertemperatures, as explained earlier.

Polarity also influences the shear strength. Theangles formed by a drop of water on the surface ofthe materials were measured (Fig. 7). The lowerangle was obtained with the composite containingCNa indicating their higher affinity for water, thentheir higher polarity compared with the others speci-

mens. This could be due to the OH groups presentin the surface of the clay layers. The compositeswith modified clays showed similar results amongthem, showing lesser polarity. The more polar mate-rial would have higher affinity with the polar metalsurface.

CONCLUSIONS

Nanocomposites based on resol resin and differentclays were obtained by in situ polymerization. Pre-polymers were characterized and it was observed anincrement in free formaldehyde content and in theviscosity with the addition of clay to the polymer. Itwas attributed to the entrance of part of phenol intothe clay galleries both allowing the separation of theclay sheets and changing the formaldehyde to phe-nol molar ratio inside and outside the galleries.The dispersion of montmorillonite in the cured

composites was analyzed by XRD and TEM. It wasobserved that the better dispersed in the polymerwas the unmodified clay CNa, whereas the modifiedclays showed a diminution in the width of the claygalleries compared with the pristine montmorillon-ites. This contraction was attributed to the loss ofmodifiers from the galleries or to the different rateof curing reactions inside and outside the interlayerspace. In addition, a difference in the crosslinkingdensity among the composites was observed. Thecomposite containing CNa showed the higher cross-linked network, whereas the composites with C10Aand C30B showed an even lesser crosslinked densitythan the pristine resol. It was attributed to theincreasing in the rate of polymerization when theNaþ is in the medium due to the formation of che-lates. A similar behavior was obtained in a previouswork when the nanocomposites were synthesized bymixing the clay with the prepolymer. The lessercrosslinking density observed in the nanocompositescontaining the modified clays was attributed to thepresence of the long carbon chains that incrementthe free volume of the system.An increment in the polymer/aluminium joint ad-

hesive strength was observed when clay was addedto the polymeric matrix. It was related to the voidcontent and the polarity of the samples. A highervoid content and a low polarity produced a detri-ment in the shear stress. The composites containingCNa as nanoreinforcement showed the best adhesivebehavior. Similar results were observed in a previ-ous work using another way of synthesis.As conclusion of the comparison of the two works,

it could be stated that independently of the way ofsynthesis, the type of clay added to the polymergreatly influence the chemical structure and the finalproperties of the resol resin. Particularly, similarresults were observed in the crosslinking density

Figure 7 Contact angle measured in the samples, usingwater as reference liquid.



and in the shear stress increment of the compositescontaining CNa synthesized by both methods. How-ever, a poor dispersion of the modified clays wasobserved when the composites were obtained by insitu polymerization. Then, it seems that the bestway to obtain the composites with modified clays isby mixing the MMTs with the prepolymer. How-ever, similar results were obtained when theunmodified clay was added to the polymer both bymixing the clay with the prepolymer or by in situpolymerization.

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Resol/montmorillonite nanocomposites obtained by in situ polymerization