Acrylic-based nanocomposite resins for coating applications

Acrylic-Based Nanocomposite Resins for CoatingApplications

M. L. Nobel, E. Mendes, S. J. Picken

Polymer Materials and Engineering, Faculty of Applied Sciences, Delft University of Technology, 2628 BL Delft,The Netherlands

Received 25 November 2005; accepted 23 February 2006DOI 10.1002/app.24318Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Acrylic-based nanocomposite resins havebeen investigated in view of future application in waterborne automotive coatings by the aid of postemulsificationprocesses. Mechanical, flow and leveling properties of thenanocomposite resins, containing various concentrations ofsilicate, have been investigated. The results are related tomorphological information obtained from TEM and WAXSmeasurements in the liquid suspension as well as from thecured film. At low silicate loadings, when flow propertiesare still acceptable for typical emulsification processing, astrong increase in modulus of the cured coating films isobserved because of the mainly exfoliated silicate platelets.

The rate of increase in modulus of the cured films decreasesat higher silicate loadings. Analyzing the mechanical data,using Halpin–Tsai theory indicates that this is due to lessperfect exfoliation at higher silicate loading, which is con-firmed by TEM analysis. In addition at higher silicate load-ing, the flow properties of the resin as analyzed by DMAshow solid-like behavior. This can lead to poor film forma-tion in future application. © 2007 Wiley Periodicals, Inc. J ApplPolym Sci 104: 2146–2156, 2007

Key words: acrylic resin; nanocomposite; organic layeredsilicate

INTRODUCTION

Mineral fillers, metals, and fibers have been added tothermoplastics and thermosets for many years to formcomposites.1 Compared with the unfilled polymer sys-tem, these composites have a number of improvedproperties including tensile strength, heat distortiontemperature, and modulus. More recently, advancesin synthetic techniques and the ability to readily char-acterize materials on an atomic scale has led to interestin nanometer-size particles. Polymer nanocompositescombine the two concepts of composites and nanom-eter-size particles. But the properties of a (nano)com-posite are not simply the sum of the properties of thecomponents. The composite properties are also deter-mined by the size, shape, and spatial distribution ofthe filler particles in the matrix and by the forcesacting between the components.1 It has been foundthat decreasing the average size of the fillers from afew micrometers to a few nanometers can result in ahigher gain in yield stress and modulus. The mostimportant parameter for the altered behavior of nano-composite particles compared with that of conven-tional filled materials is the dramatically increased

surface area of the filler. This surface area is a factor,since interactions between the filler and the matrix arevery important for the final composite properties.Some of the properties of nanocomposites, such asincreased tensile strength, are achieved using muchlower filler loading, thereby gaining in terms of re-duced weight and increased transparency and gloss.Other properties of nanocomposites, such as clarity orimproved barrier properties cannot be duplicated byconventionally-filled resins at any loading level.

Nanostructured composites of montmorillonitetype silicates embedded within various polymer ma-trices have been intensively studied in recent years.Technological interest in these types of layered nano-composites is spurred by their enhanced mechanicaland barrier properties, compared with that of conven-tional composites. Most of these materials are, how-ever, processed or commercialized in organic solvents,and environmental regulations in automotive produc-tion procedures places the use of solvent based (SB)lacquer systems under increased pressure. As a con-sequence, there is a growing need for less environ-mental unfriendly alternatives such as solvent freelacquer systems raising the interest, for instance, onwater borne (WB) systems. These contain substantiallyless solvent and are easy to produce. A disadvantageis that up to now they contain components that are farmore expensive than the components of SB systems.Moreover, it is harder (and therefore more expensive)to obtain the same product quality as SB lacquer sys-tems. The practical application of our research is

Correspondence to: M.L. Nobel ([email protected]).

Contract grant sponsors: Ministry of Economic Affairs,Education, Culture, and Sciences; Ministry of Housing, Spa-tial Planning, and the Environment; Senter and Novem.

Journal of Applied Polymer Science, Vol. 104, 2146–2156 (2007)© 2007 Wiley Periodicals, Inc.

aimed to develop an innovative technology for WBnanocomposite resin systems, yielding automotivecoatings with a superior mix of material and applica-tion properties.2 The desired improvements includescratch resistance, chemical degradation resistance,and improved mechanical properties, which may re-sult from the synergistic effects of the organic andinorganic components. The effects of different nano-particles on the properties of polymers vary a lotdepending on the system, and so this requires moredetailed analysis. In this article, motivated by the pos-sibility of preparing WB systems via inverse emulsifi-cation, we focus on the analysis of a high performanceorganic SB automotive resin system, that is known tobe emulsifiable in water.3 In this article, the study ofthe structure and flow properties of the oligomers/nanoparticulate mixture in organic solvent is de-scribed, as well as the relation between the obtainedcured films and their morphology and performance.

EXPERIMENTAL

Materials

The organic layered silicate (OLS) used for the prep-aration of the polyacrylate nanocomposite coatingswas supplied by Southern Clay Products (Gonzales,TX) under the trade name Cloisite 30B. It is a naturalmontmorillonite treated with an organic modifier.Montmorillonite is a clay with a low content of alu-minum, sodium, iron, and magnesium. It belongs tothe smectite family and is found in the shape of thinsheets consisting of three layers. These layers consistof octahedral and tetrahedral planes in which certainatoms can be replaced by others. Thus, the silicon oftetrahedral layers can be replaced by aluminum,whereas in the octahedral layers, aluminum atoms can

leave the network and can be replaced by variouselements such as iron or magnesium. These substitu-tion mechanisms give rise to a deficit of electroniccharge that can be compensated by ions or other in-terfoliated layers.

The ammonium cations present in Cloisite 30B(C30B) are methyl tallow bis-2-hydroxyethyl quater-nary ammonium, at a modifier concentration of 90meq/100 g. This value is related to the cationic ex-change capacity of the clay, which in this case is uti-lized to 90 cmol of charge per kg clay. The basalspacing, d, of C30B is 1.8 nm, as measured by X-raydiffraction using Bragg’s law, n� � 2dsin �, from theposition of the (001) peak. Figures 1 and 2 give thestructural information, weight loss upon heating, andthe X-ray diffraction pattern of the organoclay. Thematerial is available in the form of a fine powder. Theclay was dried for 48 h at 80°C, hereby removing 2% ofinternal moisture before processing.

A resin formulation was selected as a model systemto study the influence of OLS. BASF patent US5670,600 relates to the invention of a two componentaqueous polyurethane coating composition compris-ing a water dilutable polyacrylate resin and a polyiso-cyanate component as a crosslinking agent. The aque-ous polyacrylate resin is obtainable via a two-stepsolution polymerization. On the basis of the examplesstated in this patent, Nuplex Resins synthesized anoligomer solution in ethoxyethyl propionate. Aftercomposite preparation, these can be emulsified in anext stage via addition of water and removal of theorganic solvent. Table I provides the composition andcharacteristics of the synthesized solvent-based poly-acrylate resin used for composite preparation.

As a crosslinking agent for this system, NuplexResins provides US138BB-70, a solution of nonplasti-cized melamine resin with a very high reactivity, un-der the brand name Setamine.

Figure 1 TGA curves of organoclay Cloisite 30B. The insetshows structural information of the organomodification.

Figure 2 X-ray diffraction powder pattern of organoclayCloisite30B. The basal spacing of C30B is shown to be 1.8 nm(4.73° 2e).

ACRYLIC-BASED NANOCOMPOSITE RESINS 2147

Journal of Applied Polymer Science DOI 10.1002/app

Compounding and preparation of polyacrylatenanocomposites

The synthetic route of choice for making a nanocom-posite depends on whether the final material is re-quired as an intercalated or exfoliated system. In thecase of an intercalate, the organic component is in-serted between the layers of the clay such that theinterlayer spacing is expanded, but the layers stillhave a well-defined spacing. In an exfoliated struc-ture, the layers of the clay are completely separatedand the individual silicate sheets are distributedthroughout the organic matrix. A third alternative isdispersion of the clay powder particles (tactoids)within the polymer matrix, but this simply representsuse of the clay as a conventional filler. The correctselection of modified clay is essential to ensure effec-tive penetration of the polymer or its precursor intothe interlayer spacing of the clay so as to obtain thedesired exfoliated or intercalated product. Polymerscan be incorporated either as the polymer itself or viathe monomer, which is polymerized in situ to give thecorresponding polymer–clay nanocomposite. Poly-mers can be introduced by melt blending, for exampleextrusion, or by solution blending. Melt blending(compounding) depends on shear, to help delaminatethe clay and can be less effective than in situ polymer-ization to produce an exfoliated nanocomposite.

The polyacrylate nanocomposites are prepared asshown in Scheme 1.

C30B powder is swollen in xylene under high shear(6000 rpm). After 20 min, 1,2-propylene carbonate(PC) is added still under high shear (C30B/PC � 1:1).This polar activator weakens the van der Waals forcesthat are keeping the platelets together. As a conse-quence, within 15 min, a large viscosity increase is

observed because of the increased spacing betweenthe platelets and the occurrence of OHObridging viathe platelet edges. To promote exfoliation at this stage,15 min of simultaneous ultrasonication is applied. Dis-persion by ultrasound is a consequence of microtur-bulence caused by fluctuations of pressure and bycavitation. Cavitation bubbles expand and subse-quently implode within a very short time. The implo-sions locally result in very high pressures and temper-atures.

To block the platelet edges from OHObridging,1,1,1,3,3,3-hexamethyldisilazane (HMDS) is added at afixed proportion with the PC/clay addition: C30B/PC/HMDS � 1 : 1 : 1. As the mixture is still underhigh shear, the resin in solution is added and leftunder high shear for 30 min. The obtained compositeresins consisting of a oligomer/nanoparticle mixturein organic solvent are used for rheological and X-raydiffraction analysis. After completing this resin com-posite preparation, the crosslinker is added in a ratioof resin/crosslinker � 70 : 30. Films are cast on apolypropylene substrate with a doctor blade gap of200 �m. After curing for 1.5 h at 140°C, free coatingfilms of �150 �m can be obtained. The performanceand structure of the obtained free nanocomposite coat-ing films are analyzed by means of DMTA, X-raydiffraction, and TEM. For further research, an emul-sion in water can also be prepared, as shown inScheme 1.

Analysis techniques

Transmission electron microscopy

Transmission electron microscopy (TEM) was per-formed using a Philips CM30T electron microscope

TABLE IResin Formulation Composition and Characteristics

Explanation

CompositionMethod 2 stageSV 30 Solvent mixture of butyl acetate and ethoxyethylpropionateCardura E10 10 Glycidyl ester of versatic acidn-BMA 20 n-butyl metacrylateMMA 16 Methyl methacrylateStyrene 15.4EHA 10 Ethylhexyl acetateHEMA 22 Hydroxyethyl methacrylateAA 6.6 Acrylic acidTBPEH 6.0 t-Butyl perethylhexanoaatCharacteristicsNonvolatile (%) 73.2 Solids content in %AN (mg KOH/g) 34.8 Acid number in mg KOH/gVisc (Pa s) ntb Viscosity in Pa s at 23°C determined by PhysicaAppearance Clear Color 68 APAH determined by SBM 008F

Composition (in parts) and characteristics of the solvent based polyacrylate resin prepared according to U.S. Pat. No.5,670,600.

2148 NOBEL, MENDES, AND PICKEN


with a LaB6 filament operated at 300 kV. Sampleswere mounted on Quantifoil ® carbon polymer sup-ported on a copper grid, by placing some ultra-microtomed-slices on the grid. Four cured acrylicfilms, with varying clay content in the range of 0.5–8wt %, were studied using this technique.

Dynamic mechanical thermal analysis

Dynamic mechanical measurements (DMTA) werecarried out using a modified Rheovibron (Toyo Bald-win, type DDV-II-C) at a frequency of 11 Hz with adynamic strain of 0.03%. The temperature was variedbetween �50 and 200°C at a heating rate of 5°C/min.

To keep the sample sufficiently loaded during theexperiment (to keep the sample flat), a static force Fstais applied on the sample. As the stiffness of the samplechanges with temperature, the static force level is ad-justed at each temperature. In the glassy state and inthe glass transition temperature region, the staticstress is set at five times the dynamic stress as resultsfrom the applied dynamic strain of 0.03%.

In the rubber region, a constant static stress is ap-plied on the sample. This static stress is typically 0.4MPa.

The samples were stamped from a free standingfilm of the coating, using a special cutting deviceplaced in a press. The width and length of the clampedsamples was 3 and 30 mm, respectively.

The thickness of the sample was determined usingan inductive thickness gauge (Isoscope� MP, FischerInstrumentation). The thickness was determined on atleast five different spots on the sample and the aver-

age thickness and the standard deviation were deter-mined.

All measurements were done in tensile mode and ateach temperature the tensile storage modulus E�, thetensile loss modulus E�, and the value of the tan � (tan� � E�/E�) were determined.4

Thermogravimetric analysis

The exact amount of C30B in composite samples wasdetermined by using TGA. The samples were heatedfrom 25 to 1000°C at a rate of 50°C/min and were keptat this temperature for 30 min. Since the acrylic resinand the organic modification of C30B degraded com-pletely without leaving any residue, the remainingpart gave us the w/w concentrate of our filler. Theamount of weight loss as observed by TGA on thepure Cloisite (in the form of dried powder) was takeninto account in this measurement.

We note that the relationship between the weightfraction of the organic montmorillonite (OMM) andvolume percentage of OMM platelets was calculatedwith eq. (1):

VOMM �wOMM �OMM

1

wOMM �OMM1 � �1 � wOMM��S

�1

with �OMM � 1.98 g/cm and �sAO–xylene � 0.88 g/cm

(1)

The amount of surfactant on the MMT platelets wasdetermined using TGA, which show a residue of 67.21

Scheme 1 Scheme of nanocomposite preparation.



wt %, (weight loss by degradation of the crystal itselfwas taken into account.)

For example, compounding 5 wt %, C30B in thefinal cured film contains 0.024 volume fraction ofMMT platelets (�OMM � 1.98 g/cc and �solvent, xylene �0.88 g/cc).

DMA-rheology

The rheological experiments were conducted on a con-trolled strain Rheometrics Ares rheometer equippedwith a Couette geometry. The diameter of the bob andcup were 31.97 and 33.9 mm, respectively. The lengthof the bob was 33.35 mm. Steady shear rate and tran-sient and oscillatory measurements were carried outto study the rheological behavior, with varying con-centrations of filled acrylic polymer solutions. Be-tween successive series of measurements, the solutionwas allowed to rest 30 min. All measurements wereperformed at room temperature.

X-ray diffraction

A Bruker-Nonius D8-Discover set-up with a 2D detec-tor was used to perform the experiments. The sampleto detector distance was set to 10 cm; the incidentbeam wavelength was 1.54 Å. Special attention waspaid to sample preparation, and fluid resins weremeasured in a capillary to ensure a constant thicknessand random orientation. Cured films were prepared inthe absence of surface defects and at an overall thick-ness of 0.565 mm.

RESULTS AND DISCUSSION

Rheological properties of the resin composites bydynamic mechanical analysis

Preferably, coating systems for automotive purposesafter application provide a good chemical and physi-cal resistance as well as the smoothest possible surfacewith a certain amount of gloss. Before that result isaccomplished, different phenomena occur at severalstages of the paint application. Figure 3 gives an over-view of the shear rates of the several processes.

A good rheological behavior of a paint system de-pends on several stages. During storage of the paint,the stability is affected by settling behavior. Thebrush- or spray-ablility of the paint influences thebehavior during application. Finally, the ability toflow during film formation stage and the preventionof sagging during the curing stage will have a largeinfluence on the performance of a paint system.

Unfortunately, water borne (WB) latex coatings donot possess the same rheological properties as solventbased (SB) coatings. If not correctly formulated, a latexpaint can have inferior flow and leveling properties.

They may not give an adequate film formation, andcan be plagued by spattering during application. Therheological properties of a complete coating formula-tion do not depend only on the resin that is used as thebasis for the coating formulation. Always, fine-tuningis performed via the use of additives. At present, onlythe influence of the addition of nanoparticles on thebare resin is examined, i.e., the effect of additives isnot considered.

The rheological experiments were conducted on acontrolled strain Rheometrics Ares rheometerequipped with Couette geometry. Oscillatory mea-surements are performed to compare the rheologicalbehavior of an acrylic-based resin containing an in-creasing amount of C30B to that of the pure acrylicresin at 55% solid content. Between successive mea-surements and after loading, the material was allowedto rest for 30 min under environmentally controlledconditions to prevent solvent evaporation. A series ofdynamic strain sweeps were preformed to determinethe linear region of deformation. At a temperature of23°C and a deformation frequency of 1 rad/s, theviscoelasticity of the material becomes nonlinear atstrains above 5%.

Frequency spectra were recorded in the linear re-gion of deformation (5%strain). From this, the storage(G�) and loss (G�) moduli as a function of volumepercentage of clay were measured at different fre-quencies [Fig. 4(a,b)]. G� and G� at an oscillation fre-quency of 5 rad/s, a typical frequency associated withthe painting process, are shown in Figure 4(c) as afunction of clay content.

The curves of the storage and loss moduli increaselinearly with clay content. The features of the pureresin are the same as those found in a dilute solutionof random coils. Incorporation of clay in the polymermatrix causes the curves to shift upwards. This behav-ior is also shown in Figure 5, which describes theviscosity of the filled resins as a function of shearstress.

The increase of the storage and loss moduli asshown in Figure 4(a,b) are characteristic for the addi-

Figure 3 General viscosity versus shear stress profile oc-curring during various paint processing phenomena.



tion of a reinforcing-nanofiller to a polymer. FromFigure 5, it can be seen that the resistance to flowincreases at higher particle loadings. This build up ofa yield stress is beneficial for sagging resistance, but itwill have a negative effect on the coalescence duringfilm formation from an emulsion.

At all concentrations of the silicate clay in the resincomposition, we observe Newtonian flow. Increase ofnanoparticle loading leads to an increase in viscosity.Since the viscosity of suspensions is considerablyhigher, when compared to that of the unfilled polymermatrix, one might imagine that the suspensions havebecome highly flocculated upon addition of the C30Bparticles to the acrylic polymer matrix. However, inthat case the overall flow profile would become shearthinning by the shear-induced breakdown of floccu-lated structures which is not observed.

Another reason for the large increase in viscositycould be that the polymer–nanoparticle interactionsare somewhat enhanced by hydrogen bonding be-tween the acrylic polymer and the platelets edges,since the OH groups at the platelet edge are not com-pletely blocked upon addition of the silane.

To examine the relaxation behavior of the nanocom-posite resins, constant rate experiments were per-formed The torque from which the shear stress isdetermined is measured continuously, thus providinginformation on startup and steady state properties. Toerase the sample history before performing the exper-iments, a shear rate of 800 s�1 was applied for 300 s.

Figure 6 shows the stress build up and relaxation at

Figure 4 Storage and loss moduli of PNC resins containingincreasing Cloisite 30B content. (a) Storage moduli at 5 Hz,(b) loss moduli at 5 Hz and (c) storage and loss moduli at 5Hz as a function of volume fraction Cloisite 30B.

Figure 5 Viscosity/stress profile of acrylic PNC resins con-taining increasing C30B content.

Figure 6 Relaxation curves PNC resins at 10% strain. PNCresins containing up to 8 wt % of Cloisite 30B were pres-heared at 800 s�1 before the measurements.



10% strain for suspensions containing 0 to 8 wt % ofnanoparticles. The deformation rate was instanta-neously applied, maintained constant for 60 s, andsuddenly removed. After 60 s of 0 shear rate, theprocess is repeated in the opposite direction. The re-sulting stress curves comprise instantaneous, retardedand constant steady state stress followed by the samebut with the reverse sign.

The constant-rate stress is due to viscous flow. Theinstantaneous stress and viscous flow were observedfor all suspensions containing more than 2 wt % offiller loading and it increases linearly.

Morphological information from X-ray diffraction

In literature, X-ray powder diffraction in reflection ortransmission is extensively used to characterize thestructure of polymer nanocomposites (PNC). The ex-tensive use of powder diffraction is largely based onthe established procedures developed for the identifi-cation and structural characterization of layered sili-cate minerals.5–7 The position, full width at half-max-imum, and intensity of the (001) peak is used to char-acterize the morphology as immiscible, intercalated,or exfoliated. Nanocomposites are characterized bytheir large spacings (d001 � 2.0 nm), which necessi-tates the collection of data at scattering angles of lessthan 10°. Layer disorder, silicate volume fractions lessthan 0.1, and experimental conditions all contribute tobroadening and weakening of the peak, which intro-duces uncertainty in reliably determining layer–layercorrelations. Vaia8 stated that because of the strongdependence of experimental factors at low values of 2e giving raise to artifacts, XRD should be compli-mented by microscopic observations, especially forPNC with low volume fractions of organically-modi-fied layered silicates (OLS) that might have an exfoli-ated morphology. In practice, microscopy (i.e., trans-mission electron microscopy and atomic force micros-copy) indicate that most systems fall short of theidealized morphology, more commonly exhibiting in-termediate states or mixed structures.7

hkl Reflections in X-ray diffraction patterns

Of major interest for layered silicate-PNC is the rela-tive distance and orientation of the inorganic layers,and not so much the crystal structure of the individuallayers. The length scale of the former (1–4 nm) isapproximately an order of magnitude larger than thatof the latter (0.1–0.3 nm). Therefore, the scattering ofinterest for layered silicate-PNC characterization canbe analyzed in terms of a random collection of crys-tallites consisting of 2D layers on a one-dimensionallattice. The hk0 reflections provide an internal stan-dard for silicate concentration and, potentially, mayprovide information on the strain within the layers

resulting from layer bending, deformation, or thermalhistory that the sample may have been exposed to.9

The various clay mineral hkl reflections interfere witheach other. A thick random powder specimen and adiffractogram with high peak to background resolu-tion are needed. The peaks are weak, but can satisfac-torily be resolved from the background by step scanprocedures using long aquisition times. Forcrosslinked films using our set-up, this means a counttime of 30 min to 2 h, while the amorphous resinnanocomposites need 5–10 h of count time. Unfortu-nately, the higher order reflections that we find as(110) at 2�� 0.154 nm � 23.15, q� 0.154 nm � 1.65 arerarely discussed in the layered silicate-PNC litera-ture.5,6

Analyzing polymer layered silicate nanocompositescattering

C30B particle stacks have individual layers of �0.98nm thickness and a radius between 50 and 150 nm.Decreasing the mean stack size and increasing thefraction of individual layers broadens the basal reflec-tions. Additionally, it lowers the magnitude of I (q 30), which is proportional to the mean size of the stacks.

Completely suspended clay scatters strongly at lowq, where the scattering vector is q � 4��1 sin(�).10

Regardless of the layer composition in our samples,the structure factor increases markedly at low scatter-ing angles. As required at low angles, the intensityincreases with increasing clay content, whereas thelack of additional peaks confirms exfoliation.

The power-law dependence of I (q) at low q, smallerthan the first basal reflection, approaches a slope �2for highly dispersed thin plates. Scaling behavior inthis regime for layered stacks is substantially less (I� qm, m �2). At high q (thickness of platelets �1nm), the use of the complete expression for the formfactor of a (infinite) plate rather than the thin-plateapproximation is required to correctly interpret therelative magnitudes of the reflections.7 Experimen-tally, it is expected that polydispersity in lateral size,as well as lateral misalignment that yield an effectivestack radius, which is larger than the radius of theindividual platelets will smear the Guiner plateau thatappears at q (radius of platelets) 1 nm�1.

Atomic level details of the layer, not considered bythe uniform plate description, produce additional fea-tures that should be considered for quantitative inter-pretation of higher order reflections at qthickness � 1nm. The impact of the structure of the layer stacks (i.e.,crystallites and tactoids) on the intensity for a largerange of q emphasizes the importance of obtainingdata with a high signal-to-noise ratio at both high(qthicknes � 1) and low (qradius � 1) q. For layeredsilicates, this will conservatively range from 0.05 q 2.1 nm�1 within the range of basal reflections of



intercalated systems.11 The results shown in this arti-cle are for experimental reasons limited to the range of0.15 q 1.8 nm�1.

Morphology of the nanocomposites in the resin andcured state from X-ray diffraction

It is well known that a complete characterization ofnanocomposites not only needs accurate analysis ofthe nanophase, it also requires information on themesoscale and microscale (0.1–100 um). With all ofthese length scales being considered as factors, whichmay have been hidden before, orientational correla-tions and concentration fluctuations become apparent.Vaia10 described the swelling of a layered silicate tac-toid to an interlayer spacing greater than 10 nm, whichdoes not imply a uniform distribution of inorganiclayers on the micrometer scale. Even though XRDmeasurements may show that the interlayer spacinghas increased to a single layer level, this is insufficientto prove an exfoliated morphology. It should also bekept in mind that systems are not necessarilymonophasic. In practice, many systems appear to bemixtures of individual and stacked layers. This, com-bined with the high aspect ratio and grainy texture ofthe nanoparticles, gives rise to complex correlationswithin the systems, which cannot all be analyzed bymeans of XRD.

Figure 7(a) summarizes representative data sets forour acrylic nanocomposites resins with dispersedC30B, while Figure 7(b) comprises the data sets forcured acrylic nanocomposite films. The scatteringcurves are similar and show that no significant struc-tural changes occur for concentrations from 0.5 to 8 wt% (equal to 0.0015 volume fraction to 0.0242 volumefraction). As the concentration of C30B increases, thescattered intensity increases for all q in the resin, aswell as the cured film state. However at higher load-ings, the presence of stack interaction peaks, for ex-ample, around q � 4 nm�1 to q � 0.6 nm�1 reveals theformation of stacks, indicating that the C30B particlesare not fully exfoliated. The first characteristic distanceinside the stacks can be estimated using d [100] � 2/qleading to d [100] � 17.9 nm for C30B intercalatedacrylic resin and d [100] � 15.7 nm for crosslinked C30Bintercalated resin. In contrast to the crosslinked filmsat all silicate concentrations, the resin samples exhibitbroader, less intense (001) reflections. This is consid-ered representative of a more disordered system. Be-side the intensity difference between disordered sys-tems [noncrosslinked, Fig. 7(a)] and to some extentordered systems [crosslinked films, Fig. 7(b)], it isplausible to consider that crosslinking of the materialat high temperatures gives rise to a certain amount ofrestacking of the layers, since they are driven togetherby depletion interactions with the growing polymerchains.

These somewhat qualitative results obtained fromFigure 7 are insufficient to establish complete struc-ture–property relationships. Quantifiable data such asparticle–particle distance, relative orientation of parti-cles, fraction of individual layers, and particle numberdensity are preferred.

Analysis of the modulus of cured nanocompositeresins

The aspect ratio of the reinforcing particles in eachnanocomposite is calculated using the Halpin–Tsaimodel, by comparing the measured modulus of thenanocomposites with that of the unfilled matrix. Themodulus of coating films is strongly influenced by theamount of crosslinking, therefore extra care has beentaken to assure that the sample films were completelycured to avoid a change in the modulus of the matrixmaterial without actually changing the microstructureof the nanocomposite.

The Halpin–Tsai model12,13 can be used to provide arelatively simple model for determining the aspectratio of the reinforcing particles. Polydispersity of thereinforcing particles leads to an average particle as-pect ratio.

The Halpin–Tsai equation is shown in eq. (2)

Figure 7 X-ray diffraction patterns of PNC’s containingincreasing Cloisite 30B content. (a) Patterns of nanocompos-ite resins; and (b) patterns of nanocomposite films.



Ec � Em

Ef�1 � �f� � Em� � �f�

Ef�1 � �f� � Em� � �f�(2)

where Ec is the composite Young’s modulus, Ef thefiller modulus, Em the matrix modulus, the shapefactor, which depends on the geometry, orientationand aspect ratio of the particles, and �f the volumefraction filler. The shape factors for the tensile moduliof platelet reinforced composites are9 E11 or E22 �2/3�(w/t) (in the radial direction of the platelets), E33 � 2 (perpendicular to the platelets), and (w/t) � widthof platelet/thickness of platelet � aspect ratio.

When the aspect ratios are much smaller than theratio of filler and matrix modulus ( Ef/Em), theHalpin–Tsai model gives results close to a seriesmodel, while if the aspect ratios are much larger thanthe ratio of filler and matrix modulus (�� Ef/Em), theHalpin–Tsai model gives results close to a parallelmodel. For nanocomposites, the series model under-estimates the modulus, because the volume fraction offiller is very low, and thus yields values close to thematrix modulus. However, the parallel model overes-timates the composite modulus at low matrix moduli,since it assumes a continuous reinforcing phase. TheHalpin–Tsai model [eq. (2)] leads to results that are inbetween these two extremes, when the aspect ratio isof the same order of magnitude as the ratio of moduli( Ef/Em). The aspect ratio of the particles has astrong influence on the results and, therefore, deter-mines how close the results are to either the series orparallel model. The influence of the aspect ratio and,therefore, also of the level of exfoliation on the mod-ulus is very important. Platelets aspect ratios for lay-ered silicate nanocomposites are typically around100–200; consequently when exfoliation is not opti-mal, the resulting reduction in the effective aspectratio has a large effect on the modulus. The Halpin–Tsai model can be used to calculate the aspect ratio ofthe reinforcing particles when the modulus of thecomposite, matrix, and filler are known. The Halpin–Tsai model only gives an effective aspect ratio, becausethe particles can have different shapes, sizes, andthicknesses. The effective aspect ratio represents thebest fit with the measured moduli and can be consid-ered a useful parameter to compare different nano-composite compositions, as well as providing a rea-sonable estimate of the achieved level of exfoliation.

The reinforcement that causes increase in moduluswhen nanofillers are added to polymers are oftenassumed, in the literature, to be because of the re-duced mobility of a constrained polymer phase closeto the silicate layers. The large surface area of theexfoliated platelets is assumed to be responsible forthis constrained polymer phase with a higher modu-lus.14–16 The increased reinforcement of the wholecomposite benefits from strong ionic bonds betweenthe polymer and the silicate platelets, using more tra-

ditional composite models.17 The modulus of the com-posite is expected to be influenced by the modulus ofthe matrix, the modulus of the filler, and the shapeand orientation of the filler particles.18,19

The measured results for the tensile modulus of thenanocomposite samples at various silicate concentra-tions are shown in Figure 8(a), along with the calcu-lated aspect ratios (AR) from the HT model as shownin Figure 8(b).

Over the whole range, an increase in Youngs mod-ulus is clear. The effect is most pronounced above Tg,with the largest increase occurring at higher silicateloadings (Table II).

The measured matrix modulus of the unfilled sys-tem is used in the Halpin–Tsai model to calculate theeffective aspect ratio. The calculated effective aspectratios are determined assuming that the particles arealigned with their long axes along the test axis, whichis a reasonable assumption according to the TEM re-sults shown in the next section. It can be seen fromFigure 8(b) that the effective aspect ratio decreaseswith increasing silicate content, which can be ex-plained by a less effective exfoliation at higher silicateloadings. The reduced exfoliation at higher silicatecontent has been reported previously by several au-thors.20–22 With increasing concentration, the exfolia-tion becomes less perfect, resulting in an effective

Figure 8 (a) DTMA results in the cured film of C30B-carylate resin. (b) The expected moduli according to the HSmodel for a aspect ratio of 120.



aspect ratio that is approximately half of the highestvalue at low filler concentration. It has to be men-tioned that especially at low silicate contents, the cal-culation of the effective aspect ratio is rather sensitiveto the error in the modulus measurements.

TEM analysis of cured films

TEM analysis of the four cured nanocomposite resinswas performed. Figure 9(a–d) shows an overview ofthe obtained TEM results.

At a silicate concentration of 1 wt %, mostly indi-vidual silicate platelets can be observed with a signif-icant amount of them oriented in the longitudinaldirection. At higher silicate loadings of 2 and 5 wt %,an increase in stacking is observed, where almost allthe silicate layers are oriented in the longitudinal di-rection. In the stacks, a layer distance of �15 nm canbe found in good agreement with the XRD measure-ments discussed in the Morphology of the Nanocom-posites in the Resin and Cured State from X-ray Dif-fraction section.

At the highest silicate loading of 8 wt %, the firstpicture shows a dispersion of agglomerated platelets.This sample consists of well-dispersed agglomeratestogether with individual platelets, where the layerdistance within the stacks is �5 nm. As TEM analysisis a very visual tool, care has to be taken with theinterpretation of the results. Errors are likely to occurif the conclusions on the morphology the samples arebased on the examination of a few local regions withina sample. Especially using high aspect ratio particles,such as layered silicates, the sample preparation andprocessing history will not only result in preferentialparticle alignment but also in a skin-core structurewith different particle concentration and alignmentoccurring near the surface and in the bulk of thesample.

CONCLUSIONS

A route for the preparation of C30B filled acrylic-based nanocomposite resins is described. This route

yields nanocomposite resins in organic solvent, whichin a second stage can be emulsified to form nanocom-posite resin emulsions where the nanoparticles arelocated inside the emulsion droplets. Various experi-mental techniques were used to characterize the nano-particle resin dispersions and cured films, namely rhe-ology, X-ray diffraction, DMTA, TEM, also keeping inmind the shear rates involved in the envisaged coatingprocess.

For the dispersion route described in this manu-script, it is shown that up to 2 wt % of silicateloading mainly exfoliated platelets are present. Athigher silicate loading, stacking is observed with ad-spacing of �15 nm. DMTA measurements showthat there is a significant improvement of modulusupon addition of high aspect ratio silicate platelets,especially above Tg of the cured resin. The rate ofincrease in the modulus of the cured free standingfilms is reduced at higher silicate loading. This isconsistent with the outcome of effective aspect ratiodetermination based on the Halpin–Tsai theory. Atthe highest concentration of 8 wt % silicate, theeffective aspect ratio is only half of that determinedat the lowest concentration. It is shown that athigher silicate loading, the solid-like rheology of thematerial starts to dominate, making the samplesresistant to flow. This is a major disadvantage forprocessing of the resins and also gives rise to poorleveling during the curing process.

TEM analysis at all loadings shows a homogeneousdispersion of particles (which are in the form of sep-arate platelets at low silicate loadings and aggregatedstacks at higher silicate loadings) in the polymer ma-trix. Cross-sectional images of the cured films at allconcentrations indicate spontaneous alignment of thehigh aspect ratio plate-like C30B particles in the planeof the film

As a final remark it should be mentioned thatstrengthening of the polymer–nanoparticle interface iscritical to manage stress transfer during deformationof the final cured film. A methodology to increase thiswould involve the use of in situ compatibilization,whereby either the nanoplatelets or the matrix poly-

TABLE IIIncrease in Moduli Above and Below Tg of the Cured PNC Resins According to the DTMA Results

w% C30Bvfr

C30B

Below Tg at 24.7°C Above Tg at 154.6°C

E�Incr E�

(%) E�Incr E�

(%) E�Incr E�

(%) E� Incr E� (%)

0 0 2.38405 0.08044 0.0632 0.022180.5 0.0015 2.46826 103 0.13631 169 0.0920 144 0.03559 1601 0.0029 2.57829 108 0.14299 177 0.8901 140 0.03734 1682 0.0059 2.83809 19 0.14044 174 0.1409 221 0.04824 2174 0.0119 3.32719 139 0.16675 207 0.2466 388 0.05573 2515 0.0149 3.54071 148 0.17502 217 0.3056 481 0.06920 3116 0.0180 4.03955 169 0.15944 198 0.6058 953 0.14905 6728 0.0242 4.59760 193 0.17399 216 1.0198 1605 0.22107 996



mer would be chemically designed so as to actuallyreact with the other and form covalent bonds with itduring the crosslinking process.

Akzo Nobel Chemicals and Nuplex Resins (previouslyAkzo Nobel Resins) are kindly acknowledged for provid-ing the resins and for guidance. We acknowledge in par-ticular Dr. M. Bosma and M. van der Horst (AN-Chemi-cals) for the DTMA measurements and Dr. C. Vijverberg,Dr. F. van Wijk and Dr. A. Roelofs(Nuplex resins) for alltheir support. Dr. P.J. Kooyman of the National Center forHigh Resolution Electron Microscopy, TU Delft is ac-knowledged for the TEM analysis. B. Norder and G. deVos of Polymer and Materials Engineering, and TU Delftare acknowledged for the rheological measurements andTEM sample preparation.

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Figure 9 TEM micrographs of acrylic PNC’s containing (a)1 wt %, (b) 2 wt %, (c) 5 wt % and (d) 8 wt % Cloisite 30B.



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Acrylic-based nanocomposite resins for coating applications