Morphology and properties of waterborne adhesives made from hybrid polyacrylic/montmorillonite clay colloidal dispersions showing improved tack and shear resistance

ORIGINAL CONTRIBUTION

Morphology and properties of waterborne adhesives madefrom hybrid polyacrylic/montmorillonite clay colloidaldispersions showing improved tack and shear resistance

Audrey Bonnefond & Matej Mičušík & Maria Paulis &

Jose R. Leiza & Roberto F. A. Teixeira & Stefan A. F. Bon

Received: 23 January 2012 /Revised: 26 March 2012 /Accepted: 28 March 2012 /Published online: 1 May 2012# Springer-Verlag 2012

Abstract The morphology and adhesive properties of wa-terborne films from n-butyl acrylate/methyl methacrylate/montmorillonite clay hybrid polymer latexes which weresynthesized by miniemulsion polymerization in the presenceof a reactive organoclay ((2-methacryloylethyl) hexadecyl-dimethylammonium modified montmorillonite, CMA16)were investigated. It was found by cryo-TEM analysis thatthe hybrid dispersions were a mixture of colloidal particlescomposed of a small fraction of free montmorillonite clayplatelets, polymer latex particles, polymer particles to whichone or more clay platelets where adhered onto its surfaceand a fraction of colloidal material consisted of a clayplatelet with a polymer lob adhered to either side, in otherwords hybrid particles with a dumbbell-like morphology.The films made from these waterborne hybrid dispersions

presented a homogeneous dispersion of the clay plateletsand exfoliated morphology. The shear adhesion failuretemperature (SAFT) and shear resistance of the hybridlatex films synthesized with CMA16 were better thanthose prepared with a commercial clay (Cloisite 30B),but presented a liquid-like probe-tack performance. Whenallyl methacrylate (AMA) was added in the formulation,SAFT and shear resistance improved, but the film had a verylow energy of adhesion due to the excessively crosslinkedmatrix. In order to reduce crosslink density and thus improvethe adhesion energy, small amounts of chain transfer agent, inthis case n-dodecyl mercaptan (n-DDM), were used in theminiemulsion polymerization process. Adhesive films madefrom these waterborne hybrid dispersions showed excellentSAFT and shear resistance, and good energy of adhesion.

Keywords Waterborne acrylic/montmorillonite claynanocomposites . Miniemulsion polymerization . Adhesiveproperties

Introduction

Addition of clay to waterborne “soft” polymer latexes canlead to improved mechanical and physical properties ofadhesive films casted from these dispersions. Incorporationof the clay into the particle morphology can avoid depletionand thus clustering of the clay upon film formation, whichwould lead to increased opacity but more worryingly tocracked films and thus poor mechanical stability. In addi-tion, good adhesion of clay platelets to the polymer latexparticles can lead to enhanced tack adhesion energies of thehybrid films. For these reasons, vast efforts are undertakenin strategies to prepare the so-called hybrid polymer-clay

Electronic supplementary material The online version of this article(doi:10.1007/s00396-012-2649-3) contains supplementary material,which is available to authorized users.

A. Bonnefond :M. Mičušík :M. Paulis : J. R. Leiza (*)Institute for Polymer Materials, POLYMAT, Kimika AplikatuaSaila, University of the Basque Country UPV/EHU,Joxe Mari Korta Zentroa, Tolosa Hiribidea 72,20018 Donostia-San Sebastián, Spaine-mail: [email protected]

M. MičušíkDepartment of Composite Materials,Polymer Institute Slovak Academy of Sciences,Du´ bravska´ cesta 9,842 36 Bratislava, Slovakia

R. F. A. Teixeira : S. A. F. BonDepartment of Chemistry, University of Warwick,Coventry CV4 7AL, UKURL: www.bonlab.info

Colloid Polym Sci (2013) 291:167–180DOI 10.1007/s00396-012-2649-3

latexes. These hybrid colloidal materials are commonlysynthesized by heterogeneous polymerizations like emul-sion [1–7], miniemulsion [8–13] and suspension [14–19]polymerizations due to the ease of manipulation, the lowcost and the environmental friendliness. Key to the fabrica-tion of these nanocomposite hybrids and thus the choice ofthe clay are both the geometric characteristics (aspect ratio,size) and wettability (hydrophilicity/hydrophobicity) of theclay and the particle morphology sought (encapsulated,Pickering) [20].

The aim of this work is the production of hybridpolymer-clay dispersions made by miniemulsion polymer-ization to be used as waterborne pressure sensitive adhe-sives which show enhanced shear resistance and tackproperties. Ideally, one would like to produce hybrid poly-mer/clay latexes with different morphologies to elucidatethe morphology that offers the best end-use properties(mechanical, adhesive and permeability) of the films pro-duced from such latexes. Unfortunately, this has not beenreported yet for a single polymer system and type of clay.For instance, for montmorillonite type of clay, the produc-tion of encapsulated morphologies is elusive [13, 21, 22],and in most of the cases, the clay platelets migrated to thepolymer particle/aqueous phase interface driven by ther-modynamics (given by the interfacial tensions of the clay-water and clay-monomer) [23].

For Laponite clay (which has smaller aspect ratio thanmontmorillonite), hybrid latexes have been synthesizedwith different morphologies. Bon et al. [9, 24] reportedthe production of Pickering latexes (with very low solidscontent) through miniemulsion polymerization usingLaponite as the only stabilizing agent. Stable Pickeringlatexes using varied monomers like styrene [24] but alsolauryl (meth)acrylate, butyl (meth)acrylate, octyl acrylateand 2-ethyl hexyl acrylate [9] were synthesized. Recently,they reported that these hybrid latexes could also be madethrough Pickering emulsion polymerization [7]. Eventhough these latexes were synthesized at low to interme-diate solids content, they showed [25] that these Pickeringparticles with a soft polymer core could be used as ananocomposite filler in a standard poly(butyl acrylate-co-acrylic acid) waterborne pressure sensitive adhesive.They demonstrated that by the addition of 2.7wt% of thePickering particles, an increase of the plateau stress andan extension of the stress at failure could be obtained,which implied that the tack energy improved. Latexescontaining an equivalent amount of non-armoured poly-lauryl acrylate, Laponite clay discs or both did not lead toincreases of the same magnitude [25].

The group of Bourgeat-Lami et al. [12] reported hybridpoly(S-co-BA)/Laponite latexes with both honey-comband encapsulated morphologies using different modifica-tions of the Laponite and different polymerization

strategies. Large differences were found on the mechani-cal performance of both types of morphologies. For thehoney-comb particles, high reinforcement factors wereobserved due to the interactions between the clays, where-as lower reinforcement was observed for the samplewhere the clay platelets were encapsulated. Also, theyobserved mechanical damage in the honey-comb mor-phology films (the elastic modulus dropped after the firsttensile cycle), and they argued that this was due to stresslocalization at the Laponite edges. This was not observedfor the encapsulated morphologies.

In addition to the morphology of the hybrid latex par-ticles, another important aspect for the end-use properties ofthe films is the strength of adhesion and thus the interactionbetween the clay and the polymer matrix. It has been shownthat when the clay platelets are covalently bonded to thepolymer matrix, mechanical and adhesive properties amongothers improved [13].

For hybrid waterborne adhesives synthesized with re-active (CMA16, (2-methacryloylethyl)hexadecyldimethy-lammonium modified montmorillonite) and non-reactive(Cloisite 30B, methyl-bis-2-hydroxyethyl tallow ammoni-um modified montmorillonite) organically modified clays(see Fig. 1. for a detailed structure of the modifiers), wefound that shear resistance significantly increased whenthe reactive CMA16 modified clay was used [13]. Themorphology of the hybrid particles obtained depended onwhich type of clay was used as illustrated in Fig. 1. In thecase of Cloisite 30B, the clay was located at the interfaceof the polymer particle and the water. When CMA16 wasused, some dumbbell-like particles with clay platelets inbetween the two lobes were observed due to the bettercompatibility (with the monomer) and reactivity of themodifier.

In this work, we further explore the possibility of en-capsulating CMA16 clay platelets in the miniemulsionpolymerization of n-BA/MMA with an adhesive composi-tion by including a crosslinking agent in the polymeriza-tion process, which is allyl methacrylate, aiming atminimizing the migration of the CMA16 to the surface ofthe particles during the polymerization. In addition, insome experiments, n-dodecyl mercaptan was employed asa chain-transfer agent to control to some extent the cross-link density and to fine-tune the adhesive properties. Themorphology of the resulting hybrid particles and films wasstudied by (cryo) transmission electron microscopy. Theadhesive properties of the hybrid films were assessed bymeasuring the probe-tack energy, the resistance to shearand peel, and the shear adhesion failure temperature(SAFT). Latexes synthesized with CMA16 modified claywere compared with those synthesized with non-reactiveclay (Cloisite 30B) and waterborne films made from latex-es in the absence of clay.

168 Colloid Polym Sci (2013) 291:167–180

Experimental part

Materials

The natural clay, Cloisite-Na, was provided by Southern ClayProducts Inc. (Texas, USA). Cloisite-Na has a cationic ex-change capacity of 92.6 meq per 100 g clay, and its X-raydiffraction (XRD) analysis shows an interlayer space of 1.15nm. The monomers methyl methacrylate (MMA) and n-butylacrylate (n-BA) were purchased from Quimidroga (Spain).Allyl methacrylate (AMA, Aldrich) was used as crosslinkeragent. n-dodecyl mercaptan (n-DDM, Fluka) was used as chaintransfer agent. Dowfax 2A1 (Dow, USA) and Disponil AFX4060 (Cognis, Germany) were used as anionic and non-ionicsurfactants, respectively. Stearyl acrylate (SA, Sigma-Aldrich)was used as co-stabilizer, and sodium pyrophosphate (Na4P2O7,

Sigma-Aldrich) was used as peptizing agent. Sodium bicarbon-ate (NaHCO3, Sigma-Aldrich) was used as buffer to have abasic pH during the reaction. Azoisobutyronitrile (AIBN, AcrosOrganics) and potassium persulfate (KPS, Sigma-Aldrich) wereused as initiators. (2-Methacryloyethyl)hexadecyldimethyl am-monium bromide (MA16) was synthesized as shown elsewhere[13]. All materials were used as received.

Miniemulsion polymerization with organically modifiedclay

Preparation of Cloisite/MA16 (CMA16) organoclay

The MA16 cationic monomer (1.1 meq per gram of Cloisite-Na) was dissolved in distilled water and then continuouslyadded dropwise to the aqueous clay dispersion at 25 °C for 6h under stirring. At the end of the feeding, the modified clay(CMA16) was filtered and washed several times with dis-tilled water in order to wash the possible adsorbed MA16,

until the conductivity of the filtered solution reached theconductivity of the distilled water. Afterwards, the clay wasdried in a vacuum oven at room temperature for 24 h.

Miniemulsion polymerization

The miniemulsion polymerizations were carried out in a 500-mL jacketed reactor equipped with a reflux condenser, nitro-gen inlet, a sampling device and a stainless steel anchor stirrerrotating at 250 rpm. A typical formulation employed in thepolymerization is displayed in Table 1. The oil phase wasprepared by dissolving the co-stabilizer SA and the organicallymodified clay in the monomers (n-BA/MMA090/10wt-%).This mixture was stirred at 1,000 rpm with a magnetic stirrerovernight. The aqueous phase was prepared by dissolving theemulsifier, the pepticizer and the buffer in water. As emulsifier,Dowfax 2A1 or a combination of Dowfax 2A1 and DisponilAFX 4060 was used. Then, both phases were brought togetherand mixed for 15 min at 1,000 rpm. The dispersion wassonified using a Branson Sonifier 450 (operating at an ampli-tude≈118 μm and 80 % duty cycle) for 15 min in an ice bathand under magnetic stirring. The latexes were synthesizedbatchwise; therefore, the miniemulsion prepared as discussedpreviously was charged into the reactor, and when the temper-ature reached 70 °C, the initiator dissolved in a small amountof water (KPS) or monomer (AIBN) was added in a shot.

Characterization techniques

The organic content of the organomodified clay was studied bythermogravimetric analysis (TGA). To perform the TGA

Modifier of Cloisite 30B Modifier of CMA16

HO

N+

CH3

A HO

A: alkyl chain (14, 16 and 18C)

N+

(CH2)15

CH3

O

CH3O

CH3

H2C

Fig. 1 Structure of the modifiers used in Cloisite 30B and CMA16clays and their respective latex morphology [12]

Table 1 Formulation used in the miniemulsion polymerizations car-ried out at 30wt-% SC and 70 °C

Ingredient Amount/g Content/wbm-%

Oil phase

n-BA 81 90

MMA 9 10

SA 2.7 3

Organoclay 1.8 2

n-DDM 0.135 0.15

AMA 0.209 0.23

Aqueous phase

Water 210 233.3

Dowfax 2A1 3.996 2

Disponil AFX 4060 0–0.9 0–0.6

Total surfactant 3–3.9 2–2.6

NaHCO3 0.9 1

Na4P2O7 0.2 0.22

wbm weight based on main monomers (n-BA and MMA)

Colloid Polym Sci (2013) 291:167–180 169

analysis, the sample was heated from 0 to 800 °C with aheating rate of 10 °C/min under nitrogen atmosphere usingthe Thermogravimetric Analyzer model Q500 (TAInstruments).

Static contact angle (CA) measurements were performed onpressed organoclay discs with distilled water, using a goniom-eter OCA 20 (DataPhysics Instruments GmbH), in air undercontrolled environment (23 °C and 55 % humidity).

Polymer particle and monomer droplet size distributionswere measured by dynamic light scattering (DLS) using aZetasizer Nano Series (Malvern Instruments Ltd.). For thisanalysis, a fraction of the latex (or miniemulsion) was dilut-ed with deionized water (saturated with monomers in thecase of miniemulsion droplet size measurement). Thereported average particle size (droplet size) values representan average of two repeated measurements. The averagedroplet/particle size was used to calculate the number ofdroplets of the miniemulsion and the number of particlesof the latex, namely, Nd and Np, respectively. Note that thesevalues should be taken carefully when broad distributions,as those obtained in this work, are obtained. The monomerconversions were measured gravimetrically.

Cryo-TEM was used to determine the morphology ofthe hybrid miniemulsion droplets and latex particles. Sam-ples were prepared in a Gatan CP3 cryoplunger. Thesample (8 μL) was placed on a glow-discharged (in air)lacey carbon grid (300 mesh Cu, Agar Sc. S166-3H) andblotted for 8 s. The sample was plunged into liquid ethaneat −170 °C. The chamber had a relative humidity of 80 %.The equipment was a Jeol 2011 microscope at 200KVwith a LaB6 filament, and data collection was done on a2K Gatan Ultrascan 1000 camera. The cryoholders wereGatan 626 holders.

The morphology of the films was studied by means oftransmission electron microscopy (TEM TecnaiTM G2 20Twin device at 200 kV (FEI Electron Microscopes)). Forthe preparation of the film, a 120-μm wet film was appliedonto the backing and dried for 3 h at 23 °C and 55 % ofhumidity, followed by 15 min at 60 °C. This resulted infilms around 40–60 μm thick. The dried films were cryo-sectioned with a Reichert-Jung Ultracut E cryoultramicro-tome at −70 °C with a Diatome 45° diamond knife, and theobservations were made with a Philips CM10 transmissionelectron microscope operated at 80 kV without staining.

Wide-angle X-ray diffraction (WAXD) analyses wereperformed on a D8 Advance (Bruker) (Cu KR radiationwith λ00.154056 nm) at room temperature. The range ofthe diffraction angles was 2θ02–12° with a scanning rate of0.01°/5 s. The (001) basal spacing of the clay (d) wascalculated using the Bragg equation.

The probe tack tests were carried out on a Stable MicroSystems TA HD Plus Texture Analyser using the AveryAdhesive Tape (ADH7_P1S) and the 1'' ball probe. For the

preparation of the samples, 120-μm wet films were appliedonto clean glass slides and dried for 3 h at 23 °C and 55 % ofhumidity, followed by 15 min at 60 °C.

Shear adhesion failure temperature (SAFT) tests were car-ried out in a Binder oven (Sneep Industries) using treated PETas backing and stainless steel as substrate. For the preparationof the film, a 120-μm wet film was applied onto the backingand dried for 3 h at 23 °C and 55 % of humidity, followed by15 min at 60 °C. This resulted in films of around 50–60 μmthick. Also for the SAFT tests only, a high temperature resis-tance tape, a siliconed polyester tape (Venture tape), wasattached to the surface of the backing to reinforce it.

The shear resistance tests were also done using a Binderoven (Sneep Industries), using the same film preparation,but the tests were carried out at a constant temperature of70 °C. Peel resistance was measured at 180° angle at aspeed of 300 mm/min in the Stable Micro Systems TA HDPlus Texture Analyser. The films (prepared as discussedpreviously for shear resistance) were attached in a stainlesssteel panel. The value was the average peel force obtainedduring the peeling process. The final peel resistance valuewas calculated as the average of four measurements perlatex film.

The gel contents were measured gravimetrically after 24h of Soxhlet extraction with tetrahydrofuran (THF). The solmolecular weight distributions were measured by size exclu-sion chromatography (SEC). The equipment was calibratedwith polystyrene standards, and for sample analysis, theMark–Houwink constants of the copolymers were calculatedfrom the values for the homopolymers, taking into account thecopolymer composition. The values of the Mark–Houwink[26] constants employed were KBA ¼ 12:2� 10�3mL=g;KMMA ¼ 14:3� 10�3mL=g; aBA ¼ 0:7 and aMMA ¼ 0:71.

Results and discussion

Wettability of clay in water and monomer phase

One of the aims was to promote the formation of polymer-clay hybrid particles in miniemulsion polymerization. Wetherefore modified native Cloisite-Na montmorillonite claywith 2-methacryloxyethyl hexadecyldimethylammoniumbromide (MA16) in order to improve its wettability withthe monomer mixture and thus to promote adhesion to orencapsulation into monomer droplets upon emulsification.The in-house synthesized organomodified CMA16 claycontained 0.75 meq MA16/g clay (determined by thermog-ravimetric analysis [13]) and presented an equilibrium con-tact angle with water of 69°. Its compatibility with themonomer mixture was determined by mixing 3wbm%CMA16 in n-BA/MMA (90/10wt-%) for 3 h at 1,000 rpm

170 Colloid Polym Sci (2013) 291:167–180

and leaving at rest. A phase separation was observed onlyafter 8 days, whereas for the natural clay Na-MMT, thesedimentation was immediate, and for the Cloisite 30B,the sedimentation was observed after 48 min.

This test shows that the modified clay has better com-patibility with the monomer mixture than the commercialCloisite 30B clay, indicating good wettability. The mea-sured value of 69° for the equilibrium contact angle indi-cates poor wetting characteristics with water. However, thevalue is not high enough to speak of “super” hydrophobicclay, and according to the equilibrium morphology maps ofhybrid monomer/MMT platelets, the most probable loca-tion for the platelets is at the monomer/aqueous phaseinterface [13, 23].

Miniemulsion polymerization

In miniemulsion polymerization, the monomer droplet sizeof an oil-in-water emulsion is reduced by combining asuitable emulsifier and an efficient emulsification apparatus(sonication in this work). The Ostwald ripening of themonomer droplets is controlled by using a co-stabilizer (ahydrophobe of low molar mass). Typically, hexadecane isemployed, but in this work, stearyl acrylate was used be-cause it is covalently incorporated into the polymer. In anideal miniemulsion polymerization reaction, all monomerdroplets should yield a polymer particle, and hence, mono-mer transport through the water phase is not essential.Therefore, this polymerization technique is preferred forthe synthesis of latexes where hydrophobic material mustbe incorporated into the polymer particles [27]. In thiscontext and to incorporate the organically modifiedCMA16 clay into the polymer particles, miniemulsion po-lymerization was chosen to produce the hybrid latexes.

Table 2 summarizes the recipes for the miniemulsionpolymerizations carried out with the CMA16 clay underdifferent conditions. Two series of experiments were per-formed; in the first series, the miniemulsions were stabilizedwith a mixture of non-ionic (Disponil AFX4060) and an-ionic (Dowfax 2A1) surfactants. In the second series, onlyDowfax 2A1 was used as surfactant. The mixture of surfac-tants was chosen to increase the size of the droplets and helpin accommodating the large MMT platelets. In some mini-emulsion polymerizations, a monomer crosslinking agent,allyl methacrylate (AMA), was used. The purpose of this co-monomer was to restrict the mobility of the modified clay bycreating a polymer network. For the second series, a chaintransfer agent was used in Run 5 to tailor the crosslinkingdensity and avoid the deleterious effect of a too high degreeof crosslinking in the adhesive properties of the nanocom-posite film.

As discussed previously, Cloisite-Na clay was organical-ly modified to improve wettability with the monomer drop-lets. To study whether this was the case, we analysed ourprepared miniemulsions prior to polymerization by meansof cryo-TEM. Figure 2 displays two cryo-TEM micrographsof the hybrid monomer/clay miniemulsions before polymer-ization with the formulation of Table 1 for Run 3 (Table 2).As can be seen, a broad monomer droplet size distribution isobtained, which is consistent with the process of emulsifi-cation. It is important to note the difference in length scalebetween the clay platelets and the monomer droplets. Thediscrepancy between sizes makes it geometrically not pos-sible for fully armoured droplets to be formed, which is amarked difference from miniemulsions where smaller claydiscs such as Laponite are employed [9, 24]. However, themodified clay platelets clearly do wet the monomer droplets,as adhesion is observed. This leads to monomer dropletswith one or more clay platelets adhered to their surface, but

Table 2 Final conversion (X), average droplet size (Dd), average particle size (Dp) and the ratio of particle to droplet number (Np/Nd) for theexperiments containing CMA16 clay

Run Recipe Initiator X/% Coag./% Buffer+pept. agent Dd/nm Dp/nm Np/Nd

2wt%Dowfax/0.6wt%Disponil

1 2wt%CMA16 AIBN 100 4 NaHCO3 159 169 0.7Na4P2O7

2 2wt%CMA16 KPS 98.5 – NaHCO3 173 169 0.870.23 %mol AMA Na4P2O7

2wt%Dowfax

3 2wt%CMA16 KPS 98 – NaHCO3 137 140 0.76Na4P2O7

4 2wt%CMA16 KPS 90 0.5 NaHCO3 180 164 0.980.23 %mol AMA Na4P2O7

5 2wt%CMA16 KPS 100 2.6 NaHCO3 132 136 0.750.23 %mol AMA Na4P2O7

0.15wbm%DDM

Colloid Polym Sci (2013) 291:167–180 171

also to structures where a monomer droplet was adhered toeither side of a clay platelet, a clear precursor for a dumbbellhybrid particle morphology. The presence of individual clayplatelets not associated to monomer droplets was scarce.This result confirms the predictions of a thermodynamicequilibrium morphology map of hybrid monomer/clayplatelet nanodroplets computed using a Monte Carlo algo-rithm for a clay like CMA16, which presents reasonablewettability in the monomer mixture, but it is only slightlyhydrophobic [23].

Next, the miniemulsions were polymerized, and the dif-ference in the number of particles (Np) after and beforepolymerization (monomer droplets, Nd) was analysed toinvestigate the importance of secondary nucleation.

Table 2 shows that the ratio Np/Nd was close to 1 in all theexperiments, indicating the predominant droplet nucleation.The fact that Np/Nd was lower than 1 in all the casesindicated that additional particles were not produced bymicellar or homogeneous nucleation, but that a fraction of

the original monomer droplets did not nucleate to generate apolymer particle and acted as a monomer reservoir as inconventional emulsion polymerization. It is worth notingthat for Runs 2 and 4, the Np/Nd is lower than 1 even thoughthe particle size is smaller than the initial droplet size of theminiemulsion. The differences in the monomer and copoly-mer densities and the conversions achieved in the experi-ments explain this result.

High conversions were obtained in all of the polymer-izations, and coagulum was negligible except for Runs 1and 5, where it was also low (4 wt% and 2.5wt%, respec-tively). The particle size of the series with the mixture ofanionic and non-ionic surfactants yielded slightly largerparticle sizes than when only Dowfax2A1 was used. Un-fortunately, the size was not large enough so as to accom-modate the large platelets (100–300 nm). Attempts toincrease the size distribution of the nanodroplets at thesolids content of 30wt% and the same clay loading led tounstable latexes (coagulum was produced) afterpolymerization.

Morphology of the hybrid latex particles

Figure 3 displays cryo-TEM micrographs obtained for la-texes synthesized without crosslinking agent and with thetwo surfactant systems (Runs 1 and 3). Figure 3a displaysthe micrographs for the hybrid latex synthesized with themixture of surfactants (Run 1). Particle size distribution wasbroad, and some large particles (>400 nm) could be ob-served, in agreement with cryo-TEM analysis of the dropletsize distributions (see Fig. 2). This bimodality was alreadyreported for latexes prepared with the mixture of anionic/non-ionic surfactants using an organomodified commercialclay, Cloisite 30B [13]. In the present case, the clay plateletswere preferentially located at the aqueous phase/particleinterface, and platelets were always associated to polymerparticles, namely, platelets were not seen in the aqueousphase. Interestingly, as also observed in the cryo-TEM anal-ysis of the miniemulsion prior to polymerization, someplatelets were associated to more than one particle produc-ing dumbbell-like morphologies (see micrograph on theright). Figure 3b corresponds to latex Run 3 synthesizedwith the anionic surfactant. The particle size distributionwas also broad, but large particles (>500 nm) were notfound in the micrographs, indicating a better stabilizationof the miniemulsion droplets by only the anionic surfactant.Clay platelets were also associated to polymer particles, andaqueous phase/particle interface was the preferred location.In these micrographs, a larger amount of dumbbell-likeparticles, also present in the original miniemulsion as shownin Fig. 2, was identified.

Figure 4 displays the cryo-TEM micrographs obtainedfor latexes synthesized in the presence of the crosslinking

200 nm

200 nm

Fig. 2 Cryo-TEM micrographs of hybrid monomer/clay plateletsnanodroplets produced in miniemulsion, Run 3

172 Colloid Polym Sci (2013) 291:167–180

agent and using both Dowfax/Disponil and only Dowfax(Run 2 and Run 4). The cryo-TEM images clearly showthat platelets are visible in a large number of particles andthey are preferentially embedded between two particles,forming dumbbell-like/snowman morphologies as alsoobserved for Run 1 and Run 3 in Fig. 3. Additionalmicrographs (see supporting information) also show sin-gle spherical particles with platelets at the particle/aque-ous phase interface.

Note that dumbbell-like morphologies were not observedwhen other organically non-reactive clays were used insimilar polymerization conditions [10, 13, 28]. This is anindication of the importance of the improved wettability ofthe CMA16 clay with n-BA/MMA monomer mixture andthe reactivity of the cationic modifier employed in theCMA16 clay. The size of the platelets observed is verybroad, and platelets in the range of 100–300 nm could beidentified.

Dumbbell-like hybrid polymer particles like those shownpreviously were also reported in the starved feed semibatch

polymerization of MMAwith MMT platelets modified witha reactive (containing a double bond) silane [21].

Morphology of the cryosectioned films

Adhesive films were casted from the hybrid polymer dis-persions. Figure 5a, b displays TEM micrographs of cryo-sectioned films from the latexes synthesized withoutcrosslinking agent and with the mixture of surfactants(Run 1) and with only the anionic surfactant (Run 3),respectively. The platelets were well dispersed throughoutthe polymeric matrix and did not present honey-comb mor-phologies as those observed when the platelets were prefer-entially located at the surface of the polymer particles [13,29]. However, in certain domains, it was possible to identifyplatelets forming hexagonal type patterns that might indicatelocation of the clay at the particle surface which is inagreement with the particle morphology presented inFig. 3. In the higher magnification, exfoliated platelets canbe observed, and this was also confirmed by the WAXD

a

b

200 nm 200 nm

200 nm 200 nm

Fig. 3 Cryo-TEM micrographsof samples synthesized withCMA16, without AMA: a withthe mixture of surfactants(Run 1) and b with only theanionic surfactant (Run 3)

Colloid Polym Sci (2013) 291:167–180 173

spectra presented in Fig. 6a, where no peak could be ob-served, which means that as an average, most of the plateletswere at distances higher than 40 Å.

Figure 7 presents the TEM micrographs of the filmsprepared from the latexes synthesized with AMA (Run 2and Run 4). The lower magnification (left images) showsthat platelets were well dispersed in the copolymer matrix,but some areas with platelet aggregation (darker regions)can also be seen, especially for Run 2. The higher magnifi-cation micrographs (right images) show that individual ex-foliated platelets were also present in the polymer matrix.The XRD of Fig. 6b also presented no peaks for the twolatex films, in agreement with the TEM micrographs.

Adhesive properties

Shear adhesion failure temperature (SAFT) measurements,shear and peel resistances, and probe-tack analysis werecarried out on the nanocomposite films obtained from latex-es 1–5 and other reference latexes listed in Table 3 and 4.For the preparation of the film, a 120-μm wet film wasapplied onto the backing and dried for 3 h at 23 °C and

55 % of humidity, followed by 15 min at 60 °C. Thisresulted in films of around 40–60 μm thick.

Table 3 presents the molecular weight distributions (solmolecular weights and gel contents), SAFT and shear andpeel resistances of the latexes synthesized with the mixtureof surfactants (Run 1 and Run 2 in Table 2), two blanklatexes (without clay) synthesized without and with AMAand a latex synthesized with a commercial organically mod-ified clay Cloisite 30B.

Table 4 presents the same information for the latexessynthesized with the anionic surfactant Dowfax 2A1 (Run3, Run 4 and Run 5), three blank latexes (all of them withoutclay, one with AMA and another one with AMA and n-DDM) and a latex synthesized with the organomodifiedC30B clay. The synthetic procedure for the blank latexeswithout crosslinker and the latexes synthesized with C30Bwere presented elsewhere [13], and it is worth noting thatthe formulation presented in Table 1 was used to synthesizethem.

Table 3 shows that the gel content obtained for latexessynthesized without AMA was relatively low (< 40 %).Crosslinking did occur through intermolecular chain

a)

b)

100 nm 100 nm

100 nm 100 nm

Fig. 4 Cryo-TEM micrographsof hybrid latex particlessynthesized with CMA16 andAMA: a with the mixture ofsurfactants (Run 2) and b withonly the anionic surfactant(Run 4)

174 Colloid Polym Sci (2013) 291:167–180

transfer to polymer plus termination by combination lead-ing to polymer networks and hence to a fraction of insol-uble polymer (the so-called gel fraction). The reason forthe low value (4wt%) of gel content for the experimentwith Cloisite 30B was the fixed reaction time (1.5 h) for allthe reactions that did not allow achieving full conversionyielding a lower gel fraction. Note that the mechanism forthe production of crosslinked polymer depends on theconcentration of polymer in the particles; the lower theconcentration of polymer is, the lower is the crosslink-ing density, and the lower is the gel polymer. Theaddition of small amounts of crosslinking agent led tosignificant increase of the gel contents (up to 95 %)[30, 31]. A similar trend was observed for the latexessynthesized with the anionic surfactant (see Table 4).However, Run 3 presented a higher gel content than thecorresponding blank latex or the latex synthesized withthe commercial non-reactive clay, C30B. As discussedelsewhere [11, 13], this can be attributed to the

reactivity of the macromonomer MA16 that acts as a“crosslinker” due to its double functionality: the cationicend linking to the platelet surface and the double bondlinking to the polymer matrix. In the presence of allylmethacrylate (Run 4 and Run 5 and the blank latexes),the gel content substantially increased (up to 96 %), andit was slightly reduced when small amounts of chaintransfer agent were added to the formulation. The re-duction of the gel content was lower than that obtainedfor other authors at similar concentrations [32, 33], andit was likely due to the presence of the reactive CMA16clay adhering considerable amounts of polymer to theclay.

Table 3 shows that the SAFT obtained for the blank andC30B containing latexes was lower than that for the latexsynthesized with the reactive CMA16 clay, which reachedthe limit of the equipment. The shear values and adhesionenergy (tack) were also higher for the nanocomposites syn-thesized with CMA16. Run 2 (with CMA16 and AMA)

Fig. 5 TEM micrographs ofcryosectioned thin films fromlatexes synthesized withCMA16, without AMA: a withthe mixture of surfactants(Run 1) and b with only theanionic surfactant (Run 3)

Colloid Polym Sci (2013) 291:167–180 175

presented a high SAFT and shear resistance (higher thanRun 1), but the adhesion energy was very poor due toexcessively crosslinked matrix. This is an indication of goodcohesive strength and poor tack.

Figure 8 displays the results of the probe-tack analysesfor Run 1 and Run 2. Even though the energy of adhe-sion of Run 1 was higher than that of Run 2, thebehaviour of the product was liquid-like, and a cohesivefailure was observed. On the contrary, in the presence ofAMA, the behaviour was the one of a solid, and theadhesive detached fast from the probe without leavingany residue on it. The low gel contents and molecularweights obtained for Run 1 explained the higher energyof adhesion and the liquid-like behaviour obtained. Thehigh gel content obtained with AMA was responsible forthe solid-like behaviour and the low energy of adhesionobserved.

Table 4 presented the SAFT, shear and peel resistan-ces, and adhesion energy of the latexes synthesized withonly the anionic surfactant. Contrary to what was found

for the series with the mixture of surfactants, latex Run3 did not present noticeable differences with respect toa blank and a latex synthesized with a commercial clay(C30B). The latexes synthesized with AMA andCMA16 (Run 4) improved the cohesive strength (highershear resistances) but at the expense of reducing peelresistance and tack. In addition, no significant improve-ment with respect to a blank could be observed. Thecomparison of both series indicates that in the absenceof AMA, the presence of the non-ionic surfactant influ-enced the adhesive properties, e.g. lowering the shearresistance. In the presence of AMA, the properties arecontrolled by the highly crosslinked structure of theparticles, and the effect of the surfactant was notnoticeable.

The peel resistances of both series showed different ten-dencies too. Whereas for the series with the mixture ofsurfactants the peel resistance increased for the formulationscontaining clay (see Table 3), the opposite was found for theseries with the anionic surfactant (see Table 4). The blanklatexes (with similar sol molecular weight and gel contents)also presented substantially different peel resistances; thevalue was higher for the latex synthesized with only theanionic surfactant. Admittedly, no explanation can be of-fered for these results.

Run 5 was synthesized with only the anionic surfac-tant, and in addition to AMA, a 0.15wbm% of chaintransfer agent was included in the formulation to reducethe crosslinking density. For this latex, the cohesivestrength was not damaged, but interestingly, the peelresistance and the adhesion energy increased (and thevalue was similar with respect to a blank latex synthe-sized without CMA16 clay). The lower gel content andlower molecular weights of the sol (see Table 4) and thecovalent bond of the modified clay with the matrix wereresponsible of such enhanced behaviour. Figure 9 dis-plays the probe-tack results for Run 3, Run 4 and itscorresponding blank, and Run 5 and its correspondingblank. The most remarkable result was obtained for Run5, in the presence of CMA16 clay, AMA and DDM; themaximum nominal stress and the maximum nominalstrain were higher, and a plateau could be observed.

This result indicates that by fine-tuning the micro-structure of the hybrid particles, it is possible to developadhesives with enhanced cohesive strength and shearresistance without any noticeable deleterious effect onpeel resistance and adhesion as it was found in previousattempts to use montmorillonite clay to reinforce water-borne adhesives [18, 34–36]. A similar or even largerenhancement of the adhesive properties was alsoachieved by Wang et al. [25] when Laponite Pickeringlatexes of polylauryl acrylate where used as nanofiller ofconventional PSA poly(butyl acrylate-co-acrylic acid)

a

b

0

200

400

600

800

1000

1200

CMA16Run 1Run 3

Sig

nal

2Theta (°)

0

500

1000

1500

2000

2 3 4 5 6 7 8

2 3 4 5 6 7 8

CMA16 Run 2Run 4

Sig

nal

2Theta (°)

Fig. 6 WAXD spectra of the nanocomposite latex films synthesizedwith the mixture of surfactants or with only the anionic surfactant: aRun 1 and Run 3, and b Run 2 and Run 4

176 Colloid Polym Sci (2013) 291:167–180

latexes. The advantage of the hybrid latexes presented inthis work is that they can be synthesized in one pot,whereas in the case of Wang et al. [25], a two-pot systemwhere two different latexes must be blended is required.

Conclusions

The morphology and adhesive properties of hybrid acrylic/montmorillonite latexes synthesized by miniemulsion

Table 3 Summary of adhesive properties for latexes synthesized with 2 wt%Dowfax/0.6 wt%Disponil

Run Recipe X/% Gel/% Sol Mw *106/ g mol-1 SAFT/°C Shear/h Peel/ N cm-1 Wadh/ J m-2a

Blank 100 37 1.5 156±36 25±4 1.3±0.2 34

C30B 94 4 1.9 161±29 15±5 1.8±0.2 150

Run 1 CMA16 100 26 0.75 >200 38±8 2.9±0.4 200

Blank 100 88 1.25 >200 >200 2.1±0.2 67AMA

Run 2 CMA16 98 96 0.87 >200 >200 0.6±0.08 81AMA

aWadh: energy of adhesion,Wadh � h0R σmax

0 σ "ð Þd"; h0 is the film thickness

Fig. 7 TEM micrographs ofcryosectioned thin films fromlatexes synthesized withCMA16 and AMA: a with themixture of surfactants (Run 2)and b with only the anionicsurfactant (Run 4)

Colloid Polym Sci (2013) 291:167–180 177

polymerization using two surfactant systems were investi-gated. A reactive organomodified clay (CMA16) was used,and the adhesive properties were tuned by including in theformulation a crosslinking agent (AMA) and a chain transferagent (n-DDM).

The hybrid latex particles presented a substantial numberof dumbbell-like particles in which the clay platelets wereembedded, presenting an encapsulated morphology, andsome platelets were also located at the interface polymer/water. The addition of a crosslinker, to favour the encapsu-lation of the CMA16 clay platelets within the polymerparticles, did not noticeably change this morphology of thelatex particles.

The adhesive properties of the hybrid latex films wereanalysed by the SAFT, shear and peel resistances, and probetack tests. Although the hybrid latexes produced with the

reactive CMA16 clay were better than latexes synthesizedwith commercial organomodified clays (Cloisite 30B), theadhesive performance was very poor because liquid-likebehaviour was observed. However, the performance of thehybrid adhesives was considerably improved when the mi-crostructure (sol molecular weight and gel content) wasfine-tuned by adding AMA and n-DDM in addition to theCMA16 clay.

Thus, hybrid adhesives with excellent SAFT, shear andpeel resistances, and good adhesion energy were obtained

0

0.2

0.4

0.6

0.8

1

0 2 4 6 8 10

Run 1Run 2

Str

ess

(MP

a)

Strain

Fig. 8 Probe tack results for Run 1 and Run 2

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5 6 7

Run 3Run 4Blank AMARun 5Blank AMA + DDM

Str

ess

(MP

a)

Strain

Fig. 9 Probe tack results for Run 3, Run 4 and its correspondingblank, and Run 5 and its corresponding blank

Table 4 Summary of adhesive properties for latexes synthesized with 2 wt%Dowfax

Run Recipe X/% Gel/% Sol Mw*106/ g mol-1

SAFT/°C Shear/h Peel/ N cm-1 Wadh/J m-2a

Blank 98 39 1.5 195±5 >70b 2.4±0.4 130

C30B 94 11 1.9 152±2 52±4 1.7±0.2 148

Run 3 CMA16 96 52 0.4 >200 >70b 0.9±0.06 132

Blank 100 94.5 1.23 >200 >200 1.9±0.08 106AMA

Run 4 CMA16 90 96 0.87 >200 >200 0.8±0.1 68AMA

Blank 100 81.6 0.17 81 >200 4.6±0.3 162AMA

DDM

Run 5 CMA16 100 88.8 0.13 >200 >200 2.5±0.2 97AMA

DDM

aWadh: energy of adhesion, Wadh ¼ h0R σmax

0 σ "ð Þd"; h0 is the film thicknessb The measurement was stopped at this time

178 Colloid Polym Sci (2013) 291:167–180

with a formulation containing 2wbm% CMA16, 0.23wbm%of AMA and 0.15wbm% of n-DDM.

Acknowledgements Financial supports from the European Union(Napoleon project NMP3-CT-2005-011844) and Basque Government(GV-IT-303-10) and Ministerio de Ciencia y Innovación (CTQ 2006-03412) are gratefully acknowledged. BASF is acknowledged for fund-ing (RFAT). The sGIKer UPV/EHU for the electron microscopy facil-ities of the Gipuzkoa unit and SGI/IZO-sGIker UPV/EHU (supportedby the “National Program for the Promotion of Human Resourceswithin the National Plan of Scientific Research, Development andInnovation-Fondo Social Europeo, Gobierno Vasco and MCyT”) isalso gratefully acknowledged. Part of the equipment used in thisresearch was obtained through Birmingham Science City: InnovativeUses for Advanced Materials in the Modern World (West MidlandsCentre for Advanced Materials Project 2) with support from AdvantageWest Midlands (AWM). The authors would like to thank the ElectronMicroscopy Facility, School of Life Sciences, University of Warwick(Welcome Trust grant reference: 055663/Z/98/Z) for instrument useand technical support and especially to Ian Portman for his help in thepreparation of the samples and the cryo-TEM pictures.

References

1. Lee DC, Jang LW (1996) Preparation and characterization ofPMMA-clay hybrid composite by emulsion polymerization. JAppl Polym Sci 61:1117–1122

2. Ashby NP, Binks BP (2000) Pickering emulsions stabilized byLaponite clay particles. Phys Chem Chem Phys 2:5640–5646

3. Chern CS, Lin JJ, Lin YL, Lai SZ (2006) Kinetics of styreneemulsion polymerization in the presence of montmorillonite. EurPolym J 42:1033–1042

4. Voorn DJ, Ming W, van Herk AM (2006) Polymer-clay nano-composite latex particles by inverse Pickering emulsion polymer-ization stabilized with hydrophobic montmorillonite platelets.Macromolecules 39:2137–2143

5. Diaconu G, Paulis M, Leiza JR (2008) Towards the synthesis ofhigh solids content waterborne poly(methyl methacrylate-co-butylacrylate)/montmorillonite nanocomposites. Polymer 49(10):2444–2454

6. Bourgeat-Lami E, Guimaraes TR, Pereira AMC, Alves GM,MoreiraJC, Putaux JL, dos Santos AM (2010) High solids content, soap-free,film-forming latexes stabilized by Laponite clay platelets. MacromolRapid Commun 31:1874–1880

7. Teixeira RFA, McKenzie HS, Boyd AA, Bon SAF (2011) Picker-ing emulsion polymerization using Laponite clay as stabilizer toprepare armored “soft” polymer latexes. Macromolecules 44(18):7415–7422

8. Moraes RP, Santos AM, Oliveira PC, Souza FCT, do Amaral M,Valera TS, Demarquette NR (2006) Poly(styrene-co-butyl acrylate)_brazilian montmorillonite nanocomposites, synthesis of hybrid latexesvia miniemulsion polymerization. Macromol Symp 245:106–115

9. Bon SAF, Colver PJ (2007) Pickering miniemulsion polymeriza-tion using Laponite clay as a stabilizer. Langmuir 23:8316–8322

10. Diaconu G, Paulis M, Leiza JR (2008) High solids content water-borne acrylic/montmorillonite nanocomposites by miniemulsionpolymerization. Macromol React Eng 2:80–89

11. Diaconu G, Mičušík M, Bonnefond A, Paulis M, Leiza JR (2009)Macroinitiator and macromonomer modified montmorillonite forthe synthesis of acrylic/MMT nanocomposite latexes. Macromole-cules 42:3316–3325

12. Faucheu J, Gauthier C, Chazeau L, Cavaillé JY, Mellon V,Bourgeat-Lami E (2010) Miniemulsion polymerization for

synthesis of structured clay/polymer nanocomposites: short reviewand recent advances. Polymer 51:6–17

13. Mičušík M, Bonnefond A, Reyes Y, Bogner A, Chazeau L, PlummerCJG, Paulis M, Leiza JR (2010) Morphology of polymer/clay latexparticles synthesized by miniemulsion polymerization: modeling andexperimental results. Macromol React Eng 4:432–444

14. Wang D, Zhu J, Yao Q, Wilkie CA (2002) A comparison of variousmethods for the preparation of polystyrene and poly(methyl meth-acrylate) clay nanocomposites. Chem Mater 14:3837–3843

15. Sun Q, Schork FJ, Deng Y (2007) Water-based polymer/claynanocomposite suspension for improving water and moisture bar-rier in coating. Comp Sci Technol 67:1823–1829

16. Rana PK, Sahoo PK (2007) Synthesis and pressure sensitiveadhesive performance of poly(EHA-co-AA)/silicate nanocompo-site used in transdermal drug delivery. J Appl Polym Sci106:3915–3921

17. Yang WT, Ko TH, Wang SC, Shih PI, Chang MJ, Jiang GJ (2008)Preparation of polystyrene/clay nanocomposite by suspension andemulsion polymerization. Polym Comp 29:409–414

18. Kajtna J, Sebenik U (2009) Microsphere pressure sensitiveadhesives-acrylic polymer/montmorillonite clay nanocompositematerials. Int Adhes Adhes 29:543–550

19. Clarke N, Hutchings LR, Robinson I, Elder JA, Collins SA (2009)Suspension polymerization of poly(methyl methacrylate)/claynanocomposites. J Appl Polym Sci 113:1307–1315

20. Paulis M, Leiza JR (2009) Polymer/clay nanocomposites throughemulsion and suspension polymerization. In: Mitall V (ed) Advancesin polymer nanocomposites technology, Nova Science Publishers,pp 53–100

21. Voorn DJ, Ming W, van Herk AM (2006) Clay platelets encapsu-lated inside latex particles. Macromolecules 39:4654–4656

22. Samakande A, Sanderson RD, Hartmann PC (2008) Encapsulatedclay particles in polystyrene by RAFT mediated miniemulsionpolymerization, J. Polym Sci Part A: Polym Chem 46:7114–7126

23. Reyes Y, Paulis M, Leiza JR (2010) Modeling the equilibriummorphology of nanodroplets in the presence of nanofillers. J Col-loid Interf Sci 352:359–365

24. Cauvin S, Colver PJ, Bon SAF (2005) Pickering stabilized mini-emulsion polymerization: preparation of clay armored latexes.Macromolecules 38:7887–7889

25. Wang T, Colver PJ, Bon SAF, Keddie JL (2009) Soft polymer andnano-clay supracolloidal particles in adhesives: synergetic effectson mechanical properties. Soft Matter 5:3842–3849

26. Elizalde O, Arzamendi G, Leiza JR, Asua JM (2004) Seededsemibatch emulsion copolymerization of n-butyl acrylate andmethyl methacrylate. Ind Eng Chem Res 43:7401–7409

27. Asua JM (2002) Miniemulsion polymerization. Prog Polym Sci27:1283–1346

28. Tong Z, Deng Y (2006) Synthesis of water-based polystyrene-nanoclay composite suspension via miniemulsion polymerization.Ind Eng Chem Res 45:2641–2645

29. Negrete-Herrera N, Putaux JL, David L, De Haas F, Bourgeat-Lami E (2007) Polymer/Laponite composite latexes: particle mor-phology, film microstructure, and properties, Macromol. RapidCommun 28:1567–1573

30. Bouvier-Fontes L, Pirri R, Asua JM, Leiza JR (2005) Seededsemibatch emulsion copolymerization of BA with crosslinkers.Macromolecules 38:1164–1171

31. Agirre A, Nase J, Degrandi E, Creton C, Asua JM (2010) Mini-emulsion polymerization of 2-ethylhexyl acrylate. Polymer archi-tecture control and adhesion properties. Macromolecules 43:8924–8932

32. Plessis C, Arzamendi G, Leiza JR, Alberdi JM, Schoonbrood HAS,Charmot D, Asua JM (2001) Seeded semibatch emulsion polymeri-zation of butyl acrylate: effect of the chain transfer agent on thekinetics and structural properties. J Polym Sci 39:1106–1119

Colloid Polym Sci (2013) 291:167–180 179

33. Chauvet J, Asua JM, Leiza JR (2005) Independent control of solmolar mass and gel content in acrylate polymer/latexes. Polymer46:9555–9561

34. Li H, Yang Y, Yu Y (2004) Acrylic emulsion pressure sensitiveadhesives (PSAS) reinforced with layered silicate. J Adhes SciTechnol 18:1759–1770

35. Lofton L (2004) Clay/polymer nanocomposites for pressure sensi-tive adhesives. Adhesives Sealants Ind 11:31–36

36. Sebenik U, Krajnc M (2011) Acrylic-clay nanocomposites bysuspension and emulsion polymerization. In Mittal V (ed) Polymernanocomposites by emulsion and suspension polymerization, RSCPublishing, pp 111–123

180 Colloid Polym Sci (2013) 291:167–180