European Polymer Journal 49 (2013) 664–674

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European Polymer Journal

journal homepage: www.elsevier .com/locate /europol j

Study of multivesiculated polyester particles synthesisby double emulsion process

0014-3057/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.eurpolymj.2013.01.004

⇑ Corresponding author. Tel.: +351 225081601; fax: +351 225081449.E-mail address: fdmagalh@fe.up.pt (F.D. Magalhães).

Ângela Dias a,b,c, Joana Fidalgo a, João Machado b, Jorge Moniz c, Adélio M. Mendes a,Fernão D. Magalhães a,⇑a LEPAE, Departamento de Engenharia Química, Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugalb CIN – Corporação Industrial do Norte, S.A., Av. Dom Mendo, 831, Apartado 1008, 4471-909 Maia, Portugalc Resiquímica – Resinas Químicas, S.A., Rua Francisco Lyon de Castro, 28, 2725-397 Mem-Martins, Portugal

a r t i c l e i n f o a b s t r a c t

Article history:Received 1 October 2012Received in revised form 28 November 2012Accepted 3 January 2013Available online 11 January 2013

Keywords:Waterborne dispersionsParticle size distributionHollow particlesOpacity

This work studies the synthesis of multivesiculated particles (MVPs) from a w/o/w doubleemulsion comprising water and unsaturated polyester dissolved in styrene. These particlescan be used as opacifying agents in paint formulations, due to light refraction within theair-containing voids, after water evaporation. The effect of some key variables was ana-lyzed: time of dispersion and concentrations of added amine, polyester and protective col-loids. The final products were characterized in terms of particle size distribution, vesiclemorphology and dry film opacity. All the studied variables were shown to play a role inthe quality of vesiculation. The underlying mechanisms were proposed, allowing for a bet-ter understanding of this sparsely studied process.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction positions [4]. Larger particle sizes may be used to impart

Multivesiculated particles (MVPs) can be synthesized aswaterborne dispersions of unsaturated polyester cross-linked with styrene, each particle containing innumerouswater-filled vesicles [1]. Depending on the particle sizes,MVPs can have different applications. In a range between0.5 and 40 lm they can be used as opacifying agents inwaterborne coating formulations [1,2], for beneficialreplacement of expensive titanium dioxide pigment. Upondrying, water quickly evaporates from the internal vesicles,creating air voids. The large difference in refractive indexesbetween the entrapped air and the polymer walls originateslight scattering, contributing to the hiding power and whiteappearance of the coating. This is the same optical phenom-enon that occurs in snow and sea foam, where the opacityand whiteness arises from the interaction of light with amultiplicity of interfaces and microvoids. MVPs can alsobe used as opacifiers in paper coatings [3] and molding com-

special effects in paints (e.g. textures) [2].Methods for production of MVPs can be roughly divided

in two-step and one-step processes. The first is describedin most of the existing patents [3–10]. Aqueous phase isfirst emulsified in an unsaturated polyester–styrene solu-tion, in the presence of an amine (with at least 3 aminogroups per molecule) [8]. This water-in-oil (w/o) emulsionis then dispersed in an aqueous solution containing poly(vinyl alcohol) (PVA) and hydroxyl ethyl cellulose (HEC),forming the final water-in-oil-in-water (w/o/w) doubleemulsion. Free radical polymerization is then initiated toform cross-linked polyester particles encapsulating multi-ple water domains, which form the vesicles.

The one-step process is less described [1,2], but consistson a more straightforward approach. The unsaturatedpolyester–styrene mixture is first neutralized with anamine, and then is directly emulsified in the aqueous solu-tion of protective colloids (PVA and HEC). The diffusion ofwater into the dispersed organic phase (Fig. 1a) originatesstable droplets, forming the double emulsion (Fig. 1b). Inthe patent literature available, interpretation of this vesic-ulation process is limited to mentioning that the added

Fig. 1. Double emulsion formation, in the one-step process: (a) water diffusion from the aqueous phase (water, PVA and HEC) into the emulsified organicdroplets (polyester, styrene and amine); (b) w/o/w double emulsion; (c) stabilization of water droplets in organic phase by polyester salts.

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amine forms hydrophilic polyester salts with the polyes-ter’s carboxylic groups, which act as water emulsifiers inthe organic phase (Fig. 1c). The early work by Horie andco-workers [11] on formation of w/o emulsions in polyes-ter is cited as a reference describing this inverted emulsionmechanism.

The final curing procedure consists in the reticulation ofpolyester with styrene by free radical polymerization, andsubsequent obtention of solid vesiculated particles. Theinitiation process is based on an oxidation–reduction reac-tion, comprising cumene hydroperoxide, diethylenetria-mine and ferrous sulphate. The amine forms a complexwith iron ions, which then reacts with cumene to formperoxide radicals [12], as shown in Eq. (1), whereR = C6H5C(CH3)2 and R0 = NH2CH2NHCH2.

ROOH� þ R0NH2Fe2þ ! RO� þ R0NH� þ Fe2þ� þH2O ð1Þ

The radicals RO� initiate polymerization involving styreneand the double bonds of the unsaturated polyester. As a re-sult, polyester chains cross linked by polystyrene are ob-tained (Fig. 2).

The present work studies the influence of different fac-tors on the one-step MVPs synthesis process, namely thetime of dispersion of the organic phase in the aqueousmedium, and the concentrations of amine, polyester, andprotective colloids. A detailed analysis of the effect of thisset of factors has not been reported in the published liter-ature that deals with this process. The particles obtainedare characterized in terms of size distribution, vesicle mor-phology and dry film opacity. This last property is an indi-rect evaluation of the quality of particle vesiculation, and itis relevant in the context of use of MVPs as paint opacifyingagents. The purpose of this work is to obtain a clearerunderstanding of this complex and, to a certain extent,unfamiliar process.

2. Experimental

2.1. Materials

The unsaturated polyester (UP) used was provided byResiquímica (Mem Martins, Portugal) as 70 wt.% solutionin styrene. According to the provider, it has an acid value

in the range of 15–20 mg KOH/g and a Brookfield viscosityin the range of 1000–2000 mPa s. Styrene, poly (vinyl alco-hol) (PVA, Mw > 205000 g/mol, degree of hydroly-sis = 88%), hydroxy ethyl cellulose, ammoniacal ferroussulphate and biocide were also provided by Resiquímica.Diethylenetriamine (DETA, 98% purity confirmed by titra-tion with a hydrochloric acid aqueous solution) and cu-mene hydroperoxide (technical grade, 80%) were suppliedby Sigma–Aldrich and used as received.

2.2. Multivesiculated particles production

The single-step double emulsification formulation de-scribed in literature [1,2] was adopted as the reference.DETA was added to a solution of UP in styrene and thismixture was gradually added to an aqueous solution, con-taining PVA, HEC, and DETA, in a jacketed glass reactor(100 ml), under mechanical agitation (600 rpm with a40 mm impeller). After adding all organic phase, stirringwas maintained for a predefined dispersion time. Finally,additional water and the curing system (cumene hydroper-oxide and ferrous sulphate) were added to the reactor. Thecure stage lasted 5 h at room temperature. The tempera-ture in the liquid medium was kept homogenous usingan anchor stirrer at low rotation speed. At the end of thistime, biocide was added and the dispersion was stirredto ensure homogeneity.

The standard formulation described in the literature in-cludes a small amount of titanium dioxide to provide wetfilm opacity (before water evaporation from the vesicles).After confirming in a set of tests that addition of this pig-ment did not affect the final dry film opacity, particle size,pH and whitening index, it was excluded from theformulation.

2.3. Multivesiculated particles characterization

2.3.1. Brookfield viscosity measurementsThe viscosity of the polyester phase was determined

using a Brookfield LVDV-III Ultra rheometer, at 20 �C. Allthe measurements were made using the same spindle(LV-2) at the same rotational speed (20 rpm). The viscosityof the final MVPs dispersions was determined under the

Fig. 2. Simplified scheme of cure process.

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same conditions. However, the spindle type was chosendepending on measured torque percentage, which shouldpreferentially be close to 50%. All readings were taken after20 s stabilization time.

2.3.2. Contrast ratio measurementsOpacity is the ability of a film to optically obliterate the

substrate where it is applied. One method for evaluatingthis property, commonly used by paint industry, is knownas contrast ratio (CR). It gives a useful qualitative evalua-tion of the quality of vesiculation. In this method, a filmwith a controlled wet thickness (100 lm) is applied on aLeneta 2A opacity chart fixed on a vacuum table, coveringthe black and white background portions of the chart. Thisfilm is left to dry for 24 h. The reflectance on the black andwhite portions is then measured using a GretagMacbethColoreye 3100 spectrophotometer, at wavelengths be-tween 400 and 700 nm. The equipment software computesthe ratio between the two reflectance values, which corre-sponds to the CR value. A completely opaque film wouldhave a CR of 100%, meaning that the substrate is fully hid-den by the coating.

2.3.3. Particle size distributionThe particle size distribution of the waterborne MVPs

dispersions was measured on a Beckman Coulter LS230light scattering system. Dispersions of cured MVPs werediluted and sonicated for 20 min to eliminate agglomera-tion. Measurements with uncured dispersions were

performed without sonication. Special care was taken toperform the analysis immediately after removal from thereactor.

2.3.4. Scanning electron microscopyParticle morphology and internal vesiculation were ob-

served by scanning electron microscopy (SEM), using a JEOLJSM-6301F, Oxford INCA Energy 350 equipment. ThickMVPs films were applied on 1 cm2 glass slabs, dried for24 h and then fractured in liquid nitrogen. This resulted infracture a fraction of the MVPs present, allowing for obser-vation of the internal vesiculation structure. Before beinganalyzed, samples were sputtered with gold/platinum usinga K575X Sputter Coater by Quorum Technologies.

2.3.5. Optical microscopyThe size and morphology of polyester droplets along

emulsification time was analyzed by optical microscopyusing an Olympus IX 51 inverted optical microscope. Thesamples were previously diluted in water to allow betterdroplet individualization.

3. Results and discussion

3.1. Reference formulation

MVPs produced according to the reference formulationhave a solids content of 24 wt.%. The aqueous dispersionspresent low syneresis and no sediment after 1 month

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storage at room temperature. The average dry film contrastratio (CR) is 89%. Fig. 3 is a representative SEM image of adry film fractured under liquid nitrogen, showing uni-formly vesiculated particles, with well-defined sphericalalveoli with thin polymer walls, mostly sized between0.15 and 1.2 lm. Particle size distribution (PSD) measure-ments showed an average particle size of about 10 lm.Some small holes are seen on the external particle surfaces,corresponding to collapse of the thin walls, probably dur-ing drying.

3.2. Effect of dispersion time

In the reference synthesis procedure, after completingaddition of the organic phase to the aqueous phase, the dis-persion is left under mechanical stirring for 20 min. In or-der to evaluate the influence of this dispersion time on theresulting organic droplets, samples were collected afterdispersion times of 0 min, 5 min, 20 min, 1 h and 5 h, andanalyzed by optical microscopy (Fig. 4).

Right after complete addition of the organic phase(Fig. 4a), a wide variety of droplet sizes is visible (from about1 lm to several tens of lm). However, as the dispersion timeprogressed, the droplet sizes decreased and became morehomogeneous (Fig. 4b). The image corresponding to the dis-persion time of 1 h is not shown since it is qualitatively sim-ilar to the one for 20 min (Fig. 4c). It is interesting to notethat the internal morphology of the droplets is apparent inthese images. Fig. 4a already shows the presence of awater-in-oil emulsion within the organic phase.

The droplet size distributions measured for the col-lected samples are shown in Fig. 5. In agreement withthe optical microscopy results, there is a significant declinein particle sizes during the first 5 min of agitation, togetherwith a gradual evolution towards a narrower distribution –between about 2 and 12 lm, attained after 20 min of dis-persion. For longer times, the distribution does not changevery significantly.

The initially broad particle size distribution is due to thepresence of droplets with different ages in the liquid

Fig. 3. SEM image of a MVPs film fractured in liquid nitrogen (magni-fication 6000�) produced under reference conditions.

medium. Droplets formed at the beginning of organicphase addition have been sheared to smaller sizes, whilethe last droplets added are still large. As mechanical agita-tion proceeds, all droplets evolve towards more uniformsizes.

The collected samples were cured in order to obtain a sus-pension of solid particles. Note that it was previously con-firmed that the droplet and particle size distributions areessentially the same before and after cure. Table 1 shows theaverage particle sizes, Brookfield viscosities, and dry film con-trast ratios measured for the cured dispersions. Fig. 6 showsrepresentative SEM images of the corresponding fractureddry films, showing the internal particle structures. Apparentlywell vesiculated particles are seen for dispersion times up to1 h (Fig. 6a–c). For 5 h, however, the presence of monovesicu-lated particles was observed (Fig. 6d).

From Table 1, the best opacity result (CR = 89%) was ob-tained for 20 min dispersion time, which corresponds tothe reference condition. A value almost as high was ob-tained for 5 min dispersion, indicating that internal vesicu-lation was already well established at this time, asconfirmed when comparing figures Fig. 6a and b. The opac-ity decreased for MVPs cured after 1 h dispersion and be-came quite low after 5 h, indicating that the quality ofvesiculation was negatively affected. After this long disper-sion time, many water-in-oil droplets end up coalescinginto single domains, due to deficient long-time stability,originating monovesiculated particles like the one seen inFig. 6d. The presence of small bright droplets in the liquidphase after 5 h dispersion, shown in Fig. 4d, was also anindication of the worsening in vesiculation quality.

Average vesicle sizes measurements were based on sev-eral SEM images taken for the samples collected up to 1 hdispersion time. These are shown in Fig. 7. Despite thelarge scattering of the data, there seems to be a tendencyfor increasing vesicle size as time progresses, which isprobably associated to progressive water intake into ini-tially formed vesicles.

The decrease in viscosity of the cured MVPs dispersionalong dispersion time (Table 1) may be due to decreasingconcentration of dissolved protective colloids in the con-tinuous phase. The observed lowering in droplet size withdispersion time implies higher interfacial area and hencehigher colloid adsorption.

In summary, during the dispersion time shear forces in-duced by mechanical agitation cause rupture of larger or-ganic droplets, and the system evolves towards anequilibrium size distribution. Formation of water dropletswithin the organic phase occurs very rapidly – in the firstminutes of dispersion these are already visible by opticalmicroscopy. However, some water intake seems to proceedalong dispersion time, increasing vesicle sizes. The water-in-oil droplets are not stable for long dispersion times,and end up coalescing, significantly decreasing the numberof vesicles per particle.

3.3. Effect of amine addition

A key component in the synthesis of MVPs is the amine(diethylenetriamine – DETA) added to the organic phase.Fig. 8a and b show optical microscopy images of organic

Fig. 4. Optical microscopy images of organic phase emulsions in aqueous medium (magnification 1000�), produced at different dispersion times: (a) 0 min;(b) 5 min; (c) 20 min; (d) 5 h. Reference dispersion time corresponds to 20 min.

Fig. 5. Variation of organic droplet size distribution with dispersion time.Reference dispersion time corresponds to 20 min.

Table 1Influence of dispersion time on average particle size (lm), contrast ratio, CR(%) and Brookfield viscosity (cP) at 20 �C, of cured MVPs.

Dispersiontime

Average particlesize (lm)

Brookfield viscosity,20 �C (cP)

CR(%)

0 min 48.4 a b

5 min 16.4 3730 8620 min 10.2 1580 891 h 7.2 535 815 h 6.2 137 49

a Phase separation occurred after the curing time.b An uniform dry film could not be formed for measurement of contrast

ratio.

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droplets produced with and without amine addition,respectively. In the second case, not only vesiculation de-creased significantly but also much larger droplets wereformed. In addition, complete phase separation occurredsoon after stopping agitation, indicating that the oil-in-water dispersion became highly unstable.

The hydrophilic acid–base ion-pairs formed in the reac-tion between the polyester’s carboxyl end-groups and DE-TA’s amino groups are considered to be responsible for theformation of the water-filled vesicles within the organicphase [11]. The amine is therefore essential for formationof the water-in-oil-in-water double emulsion, where waterdroplets are stabilized within the dispersed organic phaseby polyester chains terminated by carboxylate anions inthe water/oil interface. The presence of a few vesicles inFig. 8b, where no amine was added to the organic phase,can be justified by the addition of a small amount of DETAto the aqueous phase, used as part of the redox curingsystem.

The observation that removal of DETA originates anunstable organic phase dispersion, despite the presenceof protective colloids in solution, was somewhat surpris-ing. This indicates that the amine also plays a role in theoil-in-water stabilization mechanism. This can be under-stood considering that a fraction of the neutralized car-boxyl end groups are positioned at the external surface ofthe organic droplets, creating a dense water solvation layersurrounding the hydrophilic ion-pairs. This provides an oil-in-water stabilization mechanism similar to the existing inaqueous polyurethane dispersions (PUDs), where the poly-mer chains contain amine-neutralized pendant carboxylgroups [13–15].

Fig. 6. SEM images of MVPs films fractured in liquid nitrogen (magnification 6000�), produced at different dispersion times: (a) 5 min (b) 20 min, (c) 1 h (d)5 h. Reference dispersion time corresponds to 20 min.

Fig. 7. Influence of dispersion time on average vesicle size of MVPs. Theerror bars represent standard deviations based on at least 100 sizemeasurements.

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The influence of DETA on particle size, vesicle morphol-ogy and dry film opacity was studied in more detail. Theamount of amine added is represented here in terms ofstoichiometric degree of neutralization (n), defined as themolar ratio of basic groups in the amine to acid groups inthe polyester. Note that DETA has two basic primary aminogroups (–NH2) and the secondary amino group (–NH) hasno basic character. The relation between dry film opacity

and average particle size, for different degrees of neutral-ization (from 0.5 to 2.1), is shown in Fig. 9. The referenceconditions correspond to n = 1.2.

A significant increase in film opacity was observed untiln = 1.2. For this lower range of neutralization values, add-ing more amine increases the concentration of hydrophilicacid–base ion pairs improving water retention in the or-ganic phase. This promotes vesiculation and hence higheropacity. Note that at equimolar neutralization (n = 1) notall acid end groups are yet deprotonated, and thus additionof excess amine further improves internal vesiculation, un-til n = 1.2. Past this point, film opacity increased moregradually. Particle sizes decreased continuously withamine addition, but a significant drop was observed aftern = 1.2. This may be an indication that after neutralizingthe majority of carboxyl groups, additional amine migratesto the droplet’s external interface, promoting additionalstabilization of the organic phase dispersion and hencecontributing to reduce droplet sizes. This may involveinteraction with the protective colloids (PVA and HEC) ad-sorbed at the surface. Therefore, it may be suggested thatfor degrees of neutralization above 1.2 the film opacitydoes not increase due to improved particle vesiculation,but to the lowering in particle sizes, which may originatemore effective packing and light scattering within the dryfilm.

Fig. 8. Optical microscopy of organic phase emulsions in aqueousmedium (magnification 1000�) produced with: (a) 1 wt.% DETA addedto the organic phase (reference condition); (b) no amine.

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Differences in internal particle structure were followedby SEM imaging. Two representative examples are shownin Fig. 10. For the lower degree of neutralization tested(n = 0.5), Fig. 10a shows a fractured particle that is anexample of deficient vesiculation: one single large vesiclein the center and a thick polymer wall with a few voids.For a degree of neutralization of 2 (Fig. 10b), one observesthe same type of vesiculation as for the reference condi-tions: small vesicles separated by thin walls, uniformly dis-tributed within all particles.

Fig. 9. Relation between polyester neutralization with DETA and contrastratio of MVPs films (%), and average particle size (lm).

3.4. Effect of polyester concentration

Different concentrations of polyester in the organicphase were tested, in the range between 30 wt.% and57 wt.% (g of polyester/g of organic phase � 100). Thesewere obtained by adding different amounts of styrene tothe polyester solution. The total amount of organic phasedispersed in the aqueous medium was maintained con-stant to ensure the same final solids content. The referencepolyester concentration corresponded to 48 wt.%. The neu-tralization index was maintained constant, equal to thereference value (n = 1.2).

The results are grouped into two concentration ranges:values above and below 40 wt.%. As shown in Fig. 11,increasing the concentration of polyester between 40 and57 wt.% increased the average particle size from 6 to20 lm. This can be attributed to the corresponding seven-fold increase in organic phase viscosity, as shown in Fig. 12.Higher viscosity of the dispersed phase implies more diffi-cult droplet break-up.

The influence of polyester concentration on the dry filmopacity and its relation with the average particle sizes arepresented in Fig. 13. Initially, contrast ratio increased

Fig. 10. SEM images of MVPs films fractured in liquid nitrogen (magni-fication 6000�), for degrees of neutralization of 0.5 (a) and 2 (b).

Fig. 12. Influence of polyester concentration (g of polyester/g of organicphase � 100) on Brookfield viscosity of organic phase at 20 �C.

Fig. 13. Influence of polyester concentration (g of polyester/g of organicphase � 100) on final dry film opacity and MVPs average particle size(lm). The reference concentration value corresponds to 48%.

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rapidly, reaching a maximum for the reference polyesterconcentration (48 wt.%). Vesiculation improved as the con-centration of hydrophilic acid–base ion-pairs increasedwithin the organic droplets. Above this value, a slight de-crease in contrast ratio is observed, which may be relatedto the increasing particle size becoming a determinant fac-tor. As discussed previously, dry film opacity seems to befavored by lower particle sizes. Internal particle vesicula-tion for a polyester concentration of 57 wt.% is similar tothe one obtained at the reference conditions, as shown inFig. 14.

For polyester concentrations below 40 wt.%, a distinctset of results was obtained. Despite the lower viscosity ofthe organic phase, the particle size distributions shifted to-wards larger sizes and became significantly broader aspolyester concentration decreased (Fig. 15). The dry filmsapplied for this concentration range had a heterogeneousappearance, showing agglomerated areas with non-uni-form thickness, not allowing contrast ratio measurements.For the lowest concentration tested (30 wt.%), the cureddispersion became a highly viscous mass and applicationon an opacity chart was not even possible. Fig. 16 showsa SEM image of a fractured film obtained for a polyesterconcentration of 38 wt.%. Particles are not well individual-ized and are enveloped in an unshaped coalesced polymermass. Some vesiculation is visible, but is surrounded by athick layer of compact polymer.

These results show that the stability of the organic dis-persion was compromised by reduction of polyester con-centration below a critical level. If the amount ofneutralized end groups available at the external surfaceof the organic droplets is too low, their stabilization is neg-atively affected. This confirms the importance of polyester-amine hydrophilic sites as both water-in-oil and oil-in-water stabilizers.

3.5. Effect of protective colloids concentration

The protective colloids used in MVP production werePVA and HEC, in a concentration of 2.3 and 0.43 wt.% (gof colloid/g of aqueous phase � 100), respectively, for thereference conditions. The effect of varying both concentra-tions separately was studied.

Fig. 11. Influence of polyester concentration (g of polyester/g of organicphase � 100) on MVPs average particle size (lm). Reference valuecorresponds to 48%.

Fig. 14. SEM image of MVPs produced with a polyester concentration of57% (g of polyester/g of organic phase � 100).

Concerning PVA, the tested concentrations in the con-tinuous aqueous phase were 1.6, 2.1, 2.3, 2.5, 3.0 and3.5 wt.% (corresponding to �30, �10, +10, 0, +30 and+50% in relation to reference concentration). For all cases,

Fig. 15. Influence of low polyester concentration in organic phase (g ofpolyester/g of organic phase � 100) on MVPs average particle size (lm).

Fig. 16. SEM image of MVPs film fractured in liquid nitrogen (magnifi-cation 6000�), produced with a polyester concentration of 38% (g ofpolyester/g of organic phase � 100).

Fig. 17. Influence of PVA concentration on polyester droplets sizedistribution. Reference concentration corresponds to 2.3%.

Fig. 18. Influence of PVA concentration (g of PVA/g of aqueousphase � 100) on final dry film opacity and MVPs average particle size(lm). Reference value corresponds to 2.3%.

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the MVPs dispersions produced were within the samerange of viscosities and presented similar stability (lowsyneresis and no sediment after several weeks storage).Fig. 17 shows the particle size distributions obtained. Re-sults for concentration of 2.1 and 2.5 wt.% were not pre-sented since the curves overlapped with the reference. Asexpected, as colloid concentration was increased, smallerparticles were obtained, showing PVA’s influence on drop-lets stabilization. For the highest concentration, 3.5 wt.%, asecond peak appeared, for sizes of about 20 lm, whichprobably corresponds to formation of agglomerates.

The effect of PVA concentration on final dry film opacityand average particle sizes is presented in Fig. 18. Beyond aconcentration of 2.1 wt.%, the opacity started to decreasesignificantly, even though particle sizes also tended to de-crease. This result was unexpected, since internal vesicula-tion, and hence film opacity, should not be directly affectedby the concentration of PVA in the continuous phase.

Fig. 19 shows representative SEM images of fracturedparticles corresponding to the lowest and highest PVA con-centrations tested. For the lower concentration (Fig. 19a)

the fractured particle is uniformly vesiculated. But for thehighest PVA concentration (Fig. 19b) deficient vesiculationis visible, with a reduced number of voids within particles,and large regions of dense polymer. This shows that theobserved lowering in contrast ratio with decreasing PVAconcentration was due to inefficient vesiculation. It isinteresting to note that in Fig. 19a the particle outer sur-face is seen to be porous, having sub-micrometric holesprobably due to collapse of thin vesicle walls. This fact iscommon to all previously described MVPs productions.However, in Fig. 19b the particle surfaces are distinctlynot porous, due to existence of a compact PVA film sur-rounding the cured particles. It is hypothesized thatincreasing PVA concentration in the aqueous phase origi-nates a dense layer of adsorbed colloid at the droplets sur-face, so that water diffusion into the organic phase ishindered. The vesiculation process is therefore delayedand when curing is performed, proper vesiculation hasnot yet been attained.

The HEC concentrations tested were 0.30, 0.39, 0.43,0.47 and 0.56 wt.% (corresponding to �30, �10, 0, +10and +30% in relation to reference concentration). The finaldispersion viscosities and stabilities did not change signif-icantly for the different productions.

Fig. 19. SEM images of MVPs (magnification 6000�) produced withdifferent PVA concentrations (g of PVA/g of aqueous phase � 100): (a)1.6%; (b) 3.5%.

Fig. 20. Influence of HEC concentration (g of HEC/g of aqueousphase � 100) on particle size distribution. Reference concentration cor-responds to 0.43%.

Fig. 21. Influence of HEC concentration (g of HEC/g of aqueousphase � 100) on final dry film opacity and MVPs average particle size(lm). Reference concentration corresponds to 0.43%.

Fig. 22. SEM images of MVPs produced with different HEC concentrationsin aqueous phase (g of HEC/g of aqueous phase � 100): (a) 0.30%; (b)0.56%.

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Cured particle size distributions are shown in Fig. 20.Results for 0.39 and 0.47 wt.% are not shown because thedistribution curves almost overlapped with the one for0.43 wt.%. The largest deviation obtained was for the

higher HEC concentration, 0.56 wt.%, which yielded dis-tinctively smaller particle sizes.

Fig. 21 shows that the influence of HEC concentrationon dry film opacity and average particle sizes is not as

674 Â. Dias et al. / European Polymer Journal 49 (2013) 664–674

strong as in the case of PVA. The contrast ratio does notchange significantly, except for the higher HEC concentra-tions, where it decreases slightly.

Representative SEM images for the lowest and highestconcentrations tested are shown in Fig. 22. For 0.30 wt.%HEC concentration, the particle internal morphology issimilar to the reference results. However, for the highestconcentration (0.56% wt), some regions of dense, poorlyvesiculated polymer are visible in some particles, indicat-ing this is the reason for the lower measured film opacity.As in the case of PVA, discussed above, HEC is not expectedto interfere directly with the vesiculation process. A possi-ble explanation is again that high concentration of HEC ad-sorbed at the droplet surface may hinder water diffusioninto the organic phase and consequently delay vesicleformation.

4. Conclusions

The work performed allowed for a better understandingof the process of formation of cross-linked polyester multi-vesiculated particles. As the polyester/styrene phase is dis-persed in the aqueous medium, diffusion of water into theorganic phase originates growing water-in-oil droplets. If asufficient long dispersion time is allowed, these end upcoalescing, eventually forming a homogeneous water coresurrounded by an organic shell. During the initial stages ofdispersion, organic droplets decrease in size due tomechanical shearing, even though water is being absorbedand vesicle size is apparently increasing.

The added amine (DETA) was shown to play a crucialrole in the process of formation of the water-in-oil-in-water double emulsion, affecting both the quality of vesic-ulation and particle size. The first effect is well describedby the mechanism suggested by Horie and co-workers forinverted emulsions in polyester/styrene solutions [12]:the polyester salts formed by combination of the aminewith the carboxyl end-groups are responsible for the emul-sification of water within the organic phase. On the otherhand, we suggest that the contribution of the amine tothe stabilization of the organic droplets in the aqueousmedium involves a mechanism equivalent to the one ob-served in aqueous polyurethane dispersions (PUDs).

The polyester concentration in the organic phase affectsthe amount of carboxyl end-groups available for stabiliza-tion of the double emulsion. At too low concentrations,well defined particles cannot be obtained and, in addition,vesiculation is incipient. Higher concentrations allow for-mation of homogeneously vesiculated particles, whoseaverage particle sizes tend to decrease with increasing

polyester concentration, probably due to higher viscosityof the dispersed phase.

Increasing PVA concentration in the continuous aque-ous phase leads to a decrease in particle size, as a conse-quence of more efficient steric stabilization. Surprisingly,vesiculation is negatively affected by high PVA concentra-tion, probably because this hinders water diffusion intothe organic droplets. Varying HEC concentration showssimilar consequences, but to a lower extent.

Acknowledgements

Funding for this work was provided by FEDER, throughPrograma Operacional Factores de Competitividade –COMPETE, and by national funding through FCT – Fun-dação para a Ciência e a Tecnologia, in the framework ofproject PTDC/EQU-EQU/112151/2009. Ângela Dias thanksCIN – Corporação Industrial do Norte, S.A., Resiquímica –Resinas Químicas, S.A., and FCT for PhD grant SFRH/BDE/33432/2008.

References

[1] Terblanche J. The development of vesiculated beads: faculty ofEngineering at Stellenbosch University; 2003.

[2] Engelbrecht JF, De Wet-Roos D, Smit AC, Cooray B. Vesiculatedpolymer particles. US patent: 10/529764; 2006.

[3] Ferguson LD, Hayes PC, Macas TS. Vesiculated polymer granules. USpatent: 4917765; 1990.

[4] Jonathan Banford PD. Polymer granules and compositions containingthem. US patent: 5055513; 1991.

[5] Ritchie PJA, Serelis AK. Vesiculated polyester granules. US patent:2003/0040557 A1; 2003.

[6] Kershaw RW, Lubbock FJ, Polgar L. Vesiculated polymer granules. USpatent: 3933579; 1976.

[7] Karickhoff M. Vesiculated beads. US patent: 4489174; 1984.[8] Gunning RH, Henshaw BC, Lubbock FJ. Process of making porous

polyester granules. US patent: 3879314; 1975.[9] Gillan J, Kershaw RW. Process of preparing vesiculated cross-linked

polyester resin granules. South African patent: 70/8592; 1974.[10] Geoffrey Willison JH. Bead polymerization process. US patent:

4363888; 1982.[11] Horie K, Mita I, Kambe H. Copolymerization of unsaturated polyester

with styrene in inverted emulsion. J Appl Polym Sci1967;11(1):57–71.

[12] Razumovskii SD, Medvedev SS. Kinetics of the reaction betweencumene hydroperoxide and triethylenetetramine in presence of ironsalts in aqueous solutions. Russ Chem Bull 1958;7(8):944–51.

[13] Nanda AK, Wicks Da, Madbouly Sa, Otaigbe JU. Effect of ioniccontent, solid content, degree of neutralization, and chain extensionon aqueous polyurethane dispersions prepared by prepolymermethod. J Appl Polym Sci 2005;98:2514–20.

[14] Galgoci EC, Hegedus CR, Snyder JM, Lawson DC, Lindenmuth DL.Urethane-acrylic hybrid polymers. AIR PRODUCTS; 2001.

[15] Chinwanitcharoen C, Kanoh S, Yamada T, Tada K, Hayashi S, SuganoS. Preparation and shelf-life stability of aqueous polyurethanedispersion. Macromol Symposia 2004;216:229–40.