synthesis and characterization of paraffin wax microcapsules with acrylic-based polymer shells

Synthesis and Characterization of Paraffin Wax Microcapsules with Acrylic-BasedPolymer Shells

Luz Sanchez-Silva,*,† John Tsavalas,‡ Donald Sundberg,‡ P. Sanchez,† and Juan F. Rodriguez†

Department of Chemical Engineering, UniVersity of Castilla-La Mancha, AVda. Camilo Jose Cela s/n13004 Ciudad Real, Spain, and Nanostructured Polymers Research Center, Materials Science Program,UniVersity of New Hampshire, Durham, New Hampshire 03824, United States

Microencapsulation of PRS paraffin wax was carried out by means of suspension-like homopolymerizationof methyl methacrylate (MMA) and by the copolymerization of this monomer with methyl acrylate (MA)and methacrylic acid (MAA). The influence of the type of monomers and their proportion as shell materials,the mass ratio of polyvinylpyrrolidone to monomers (PVP/monomers), and the mass ratio of PRS paraffinwax to monomers (PRS/monomers) on the properties of phase change material (PCM) microcapsules hasbeen studied. The analytical techniques used for the characterization of the particles were differential scanningcalorimetry (DSC) and modulated DSC, dynamic light scattering, environmental scanning electron microscopy(ESEM) and gel permeation chromatography (GPC). The chemical property differences between theencapsulating “shell” material, acrylic polymer in this case, compared to the “core” material (PRS paraffinwax), such as polarity and interfacial tensions, largely determine the thermodynamically favored morphologyof the microcapsules. While the equilibrium morphology was a core/shell structure, kinetic factors relating tothe competition between the fast acrylic polymerization rate and the diffusion limited rate of phase separationwere found to constrain the ability of the system to approach that equilibrium structure. For bothhomopolymerization of methyl methacrylate (PMMA) and copolymerizations of methyl methacrylate withmethyl acrylate (P(MMA-co-MA)), “pomegranate” microspheres were observed with a thin acrylic shellsurrounding an inner composite structure constituted by a paraffin matrix with a dense assortment of PMMAspherical domains. Ter-polymerizations incorporating methacrylic acid (P(MMA-co-MA-co-MAA) had aremarkable effect on the morphology and average particle size, resulting again in “pomegranate” microparticlesbut where the internal PMMA spheroids were more densely populated closer to the outer portion of thecapsule; approaching a core/shell structure. As the amount of polyvinylpyrrolidone stabilizer in the aqueousmedium was increased and the PRS/monomers proportion was decreased, the mean particle size of themicrocapsules decreased. However, higher PVP/monomer and PRS/monomer ratios led to less efficientencapsulation of the PRS paraffin wax. Near perfect encapsulation efficiency was obtained for two pairingsof these ratios via P(MMA-co-MA-co-MAA) polymerization, but only up to an equal mass ratio of PRS toacrylic polymer.

1. Introduction

The diminishing supply of nonrenewable energy sources andthe increase in energy demand make research into new energysolutions attractive. Thermal energy storage can be consideredan important advanced energy technology, and increasingattention has been paid to utilization of the technique for thermalapplications.1,2 Phase change materials (PCMs) are able to storethermal energy in small temperature intervals very efficientlydue to their high latent heat.3 Research performed in recent yearshas increased the number of available phase change materials(PCM), knowledge about their behavior, and their possibleapplications. Paraffin waxes compared to other PCMs arechemically inert, noncorrosive, long-lasting, inexpensive, eco-logically harmless, and nontoxic.2,4,5

Microcapsules of paraffin wax have been widely applied totextiles and fabrics for clothing,6-8 building insulation,9 andsolar and nuclear storage systems.10,11 Many methods have beenexplored for encapsulation of paraffin waxes, such as a coa-cervation and a spray drying method,10 in situ polymerization,12-14

emulsion polymerization,5 interfacial polymerization,15,16 andsuspension polymerization.17,18

Choosing a shell material for the microencapsulation of PCMsplays an important role in controlling the structure, permeability,and thermal stability of the microcapsules. The shell of thecapsules can be formulated with a variety of starting materialsincluding natural and synthetic polymers. Melamine-formal-dehyde resin, urea-formaldehyde resin and polyurea are mostoften selected as the microcapsule shell materials for the PCMsprotection.13,16,19 Hawlader et al.10 encapsulated paraffin withgelatin and Arabic gum by complex coacervation. Park et al.20

prepared polystyrene particles containing paraffin wax byminiemulsion polymerization. Zhang et al.21 and Jiang et al.22

also prepared PCM microcapsules with polystyrene, polymethylmethacrylate (PMMA), and phenol resin as shell materials.However, several of these works did not report the phase changeenthalpy of their microcapsules, while others reported low PCMconcentrations (<40%), and thus low phase change enthalpies(19-90 J g-1).

In our previous papers18,23 polystyrene microcapsules con-taining paraffin wax were successfully obtained using a suspen-sion-like polymerization. However, cross sections of the mi-crocapsules revealed a composite salami-like internal morphology(as opposed to a more traditional core/shell) due to the lack ofa strong driving force for phase separation of the polystyreneformed within the paraffin wax droplet. The values of polarityand interfacial tensions of polystyrene are quite similar to

* To whom correspondence should be addressed. E-mail:[email protected].

† University of Castilla-La Mancha.‡ University of New Hampshire.

Ind. Eng. Chem. Res. 2010, 49, 12204–1221112204

10.1021/ie101727b 2010 American Chemical SocietyPublished on Web 10/25/2010

paraffin wax, and thus a core/shell morphology was notthermodynamically favored during the polymerization process.Challenges regarding preparation of polystyrene-based capsulescontaining hydrophobic paraffin in a core/shell morphology hasalso been reported by other authors.24,25

On the other hand, poly(methyl methacrylate) (PMMA) hasa dry glass transition temperature (Tg) of 119 °C, modulus ofelasticity of 3300 MPa, hardness of 195 MPa, and a thermalconductivity of 0.19 W m-2 K-1.26 Thus, the use of methylmethacrylate (MMA) as the dominant constituent in the shellmaterial composition should afford increased thermal stabilityand mechanical strength of the polymeric microparticles makingthem more robust to manufacturing process requirements.Polymers based on one or more C1-C24 alkyl esters of acrylicand/or methacrylic acid as shell materials have been widelyreported in the literature27 for these very reasons. Berg et al.28

achieved the encapsulation of hydrocarbon oils by polymeri-zation of MMA. Loxley et al.29 reported a phase separationmethod to encapsulate n-hexadecane with PMMA. Yang et al.30

studied the encapsulation of tetradecane by poly(vinyl acetate),poly(styrene), poly(methyl methacrylate), and poly(ethyl meth-acrylate) by in situ polymerization.

In the present study, we build upon the capsule formationtechniques developed in our previous styrene based system,23

yet we focus here on suspension-like polymerization of a fullyacrylic polymer composition with the objective of improvingthe paraffin encapsulation efficiency. The hypothesis is that witha capsule morphology closer approximating a core/shell minimalparaffin will be left unencapsulated. Our contention is not thata perfect core/shell structure would be a more effective PCMthan a more occluded internal structure in terms of latent heat,but rather that conditions that thermodynamically favor core/shell might improve the encapsulation efficiency.

For the polymeric material, we have chosen to compare threedifferent compositions with increasing polarity. MMA has awater solubility of 15 g L-1, which is almost 40 times greaterthan that of styrene, and it is considerably more polar thanparaffin wax. Homopolymerization of MMA as an encapsulatingmaterial for the paraffin was thus chosen as the first acrylatesystem for this study. To increase the polarity gap, the nextsystem was a copolymerization of MMA with methyl acrylate(MA), whose saturated water solubility is 3.5 times greater thanthat of MMA. Finally, ter-polymerizations of MMA, MA, andmethacrylic acid (MAA) were chosen to produce an even greaterpolarity gap between the acrylic composition and the paraffinwax.

The high water solubility of these more polar monomersmeans that they will partition to that extent in the continuousaqueous phase during these batch suspension polymerizations.To avoid secondary nucleation of pure acrylic (no paraffin)particles in the water phase, we chose to use a hydrophobicoil-soluble initiator (benzoyl peroxide) to prevent radicalformation in the aqueous phase that could react with thatpartition of the acrylic monomer in that phase.

2. Experimental Section

2.1. Materials. Methacrylic acid, methyl acrylate, and methylmethacrylate (all Merck Chemical Co.) were purified frominhibitors by passing them through a column packed withaluminum oxide active base (Fisher). Benzoyl peroxide (97 wt%) was used as the initiator (Fluka Chemical Co., Ltd.). PRSparaffin wax (478 g mol-1) was used as the core material. Thisparaffin was a mixture of hydrocarbons C19-C27 produced andcommercialized by the petrochemical company Repsol YPF(Spain), with energy storage capacity of 202.7 J g-1 and witha melting temperature range of 40-45 °C. Reagent gradepolyvinylpyrrolidone (K30, Mw ) 40000 g mol-1) (FlukaChemical Co., Ltd.) was used as a stabilizer, and methanol wasused to filter unencapsulated paraffin from the samples. Thereagents were all used as received. Water was purified bydistillation and subsequent deionization using ion-exchangeresins.

2.2. Microencapsulation of Paraffin Wax. Microencapsu-lations were carried out in a 1-L glass reactor equipped with anoil thermostat bath, a reflux condenser and a nitrogen gas inlettube. A schematic diagram of this experimental setup and thesuspension-like polymerization technique was described previ-ously.18

The reaction medium involves two phases: a continuous phasecontaining water and polyvinylpyrrolidone and a discontinuousphase containing the acrylic monomers, PRS paraffin wax, andbenzoyl peroxide initiator. The base recipe and process em-ployed in this study is based on the optimum formulationobtained in our previous styrene-based work.31

The continuous phase was transferred to the glass reactor withmild agitation. The initiator was premixed with monomers andphase change material. Next, the discontinuous phase was addedinto the continuous phase and was maintained under vigorousagitation (400 rpm) and a constant temperature of 70 °C. Thepolymerization process was carried out for 5 h at temperatureunder a nitrogen atmosphere.

Once obtained, the polymerized microcapsules were repeat-edly washed with methanol and filtered to remove impuritiesand unencapsulated paraffin wax. The purified microcapsuleswere then dried at room temperature. Table 1 shows the recipeconditions used in the preparation of all the samples in thisstudy.

2.3. Environmental Scanning Electron Microscopy (ESEM)Observation. The surface and internal morphological featuresof the microcapsules after polymerization were observed byusing a XL30 (LFD) ESEM. Fracture of the microcapsules toreveal the internal microstructure was carried out by using anUltramicrotome Leica EM UC6. Owing to the size and thestiffness of the microcapsules, the ESEM technician couldperform the cutting of microcapsules without embedding thesamples in an epoxy resin matrix. Fracture of the microcapsuleswas done using the Leica Ultramicrotome diamond bladedirectly while samples were observed by optical microscope.

Table 1. Experimental Conditions Used in the Preparation of Microparticles Containing PRS Paraffin Wax

experiments monomers proportions PVP/monomers (g/g) PRS/monomers (g/g)

1 100% MMA 0.0943 1.022 90% MMA/10% MA 0.0943 1.023 89% MMA/10% MA/1% MAA 0.0943 1.024 88% MMA/10% MA/2% MAA 0.0943 1.025 89% MMA/10% MA/1% MAA 0.0471 1.026 89% MMA/10% MA/1% MAA 0.1885 1.027 89% MMA/10% MA/1% MAA 0.1885 0.338 89% MMA/10% MA/1% MAA 0.1885 1.339 89% MMA/10% MA/1% MAA 0.1885 2.00

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2.4. Number-Average and Volume-Average Diameter ofMicrocapsules. Particle size and particle size distribution (PSD)were determined on a Malvern Mastersizer Hydro 2000 SMlight scattering apparatus in a diluted dispersion of the particlesin methanol.

2.5. Differential Scanning Calorimetry (DSC) and Modu-lated DSC. The melting point and heat of fusion of the variousmaterials employed and obtained were determined using amodulated differential scanning calorimeter (DSC), TA Instru-ments model Q100. The DSC was equipped with a refrigeratedcooling system and nitrogen as the purge gas.

DSC measurements were performed by ramping the temper-ature from -30 to 80 °C for two cycles with a heating rate of10 °C min-1 to observe the melting transitions and correspond-ing enthalpy. Each sample was analyzed at least twice, and theaverage values were recorded. This procedure was performedfor both the pure paraffin wax and the paraffin/acrylic compositecapsules.

For the measurement of dry glass transition temperature (Tg),samples were subjected to an initial short preheat step to atemperature equal to the Tg of the most glassy componentfollowed by two successive heating steps ranging from -20 to130 °C. This DSC measurement was run in modulated modewhere the heating ramps used a modulation amplitude of (3°C, with 60 s period and an underlying heating rate of 3 °Cmin-1.

2.6. Gel Permeation Chromatography (GPC) Mea-surement. The molecular weight distribution (MWD) of thepolymer forming the encapsulating shell was measured by gelpermeation chromatography (GPC) using a chromatographmodel 150-GPCV LC from Waters (USA). Tetrahydrofuran(THF) was used as the elution solvent. Poly(styrene) standardsfrom Waters were used for MWD calibration.

3. Results and Discussion

The objective of this work was to design a microencapsulationsystem suitable of producing phase change materials (PCMs)based on a paraffin-rich core and an acrylic polymeric shell.We contrast this work to previous studies which employedstyrene-based polymeric shells where there was little drivingforce for strong phase separation of the polymer formed fromthe paraffin payload. Here we contend that by exploring a seriesof significantly more polar acrylic polymer compositions forthe encapsulation we can test the influence of the polymericcomposition on the morphology of capsules produced and theencapsulation efficiency of the system. As a basis for compari-son, the styrene-based microparticles of our previous work onlyshowed an encapsulation efficiency of around 40%.18 Thefollowing discussion focuses on three aspects of the workrelating to particle size control, morphology, and microcapsulestructural evaluation, and the ultimate thermal storage capacityand encapsulation efficiency.

3.1. Microcapsule Particle Size. The target particle size ofa microencapsulated material depends strongly on the end use

application. For use in textiles, the appropriate particle sizeranges between 0.5 and 100 µm in diameter.3 In a suspensionpolymerization there are several factors that influence theachievable particle size and size distribution. The dominant threefactors of these are the polarity difference between the dispersedand continuous phases and the resulting tendency for phaseseparation, the balance between the viscosity of the dispersedphase relative to the shear imposed on the system, and theamount/type of surface active stabilizer used to lower theinterfacial tension between the dispersed phase and the continu-ous phase. The stabilizer also helps protect dispersed dropletsor particles from coalescing should they come in close proximityto each other in the reaction medium.

3.1.1. Influence of the Polarity of the Acrylic Composi-tion on Particle Size. In Table 2, particle sizes and othercharacteristics of experiments 1 through 4 (Table 1) are shownwhere both the PVP/monomer (0.0943) and PRS/monomer(1.02) weight ratios were held constant. In this series, thepolarity of the acrylic phase increased with each experimentboth by copolymerizing with more polar monomers and by theuse of different loadings of acid functional monomers.

Experiment 1 showed a particle size approaching 500 µm,significantly larger than the target size range of below 100 µm.Relative to the other experiments of this study, this was themost hydrophobic acrylic composition, so it is not surprisingthat as the composition became increasingly more polar theparticle size also was reduced. Moving from experiment 1 to 2did not have as marked an effect on particle size as comparedto when the methacrylic acid entered the acrylic compositionfor all subsequent polymerizations. For this stabilizer/monomerratio, the incorporation of MAA acid had a remarkable effectby nearly halving the particle size. This is likely due to thestrong reduction in interfacial tension between the organic andwater phase that this monomer provides.32 Nonetheless, Experi-ments 3 and 4 were still found to be larger than the target sizerange for these capsules designed for textile applications.

3.1.2. Influence of Polyvinylpyrrolidone/Monomers (PVP/Monomers) Mass Ratio on Particle Size. With the polaritydifference between the monomer and aqueous phase significantlyreduced, in order to further reduce particle size, the amount ofPVP stabilizer was varied. In Table 3, characterizations ofsystems where the PVP/monomer ratio was varied are illustrated,and experiments 5 and 6 can be compared to experiment 3 atthe same paraffin/monomer ratio and acrylic composition.

In experiment 5, even with the same strongly polar acidcontaining composition as in experiment 3, the particle sizedoubled to 418 µm when the concentration of PVP stabilizerwas reduced by half. However, by doubling the PVP stabilizerto monomer concentration the resulting particle size was reducedby that same factor to produce a microcapsule in experiment 6of 147 µm, quite near the target range.

The particle size distributions (PSDs) for microparticlesobtained from experiments 3, 5, and 6 are shown in Figure 1.Wide particle size distributions were obtained for all PVP/

Table 2. Average Diameters of Microcapsules, Glass Transition Temperature, Molecular Weights of Microparticles, and EncapsulationEfficiency Obtained for Experiments 1 through 4

experiments dpv0.5 (µm)a dpn0.5 (µm)b Tg (°C) Mw (g mol-1) Mn (g mol-1)encapsulation

efficiency (%)c

1 456.1 71.7 116.6 85 427 48 388 82.32 409.9 37.7 98.4 89 715 55 544 80.53 244.3 14.1 106.4 78 214 45 220 93.54 272.9 30.2 106.9 82 635 45 243 63.7

a dpv0.5 represents 50% microcapsule particles whose mean volumetric diameter is less than this value. b dpn0.5 represents 50% microcapsule particleswhose mean numeric diameter is less than this value. c Encapsulation efficiency is a normalized thermal storage capacity defined in section 3.3.

12206 Ind. Eng. Chem. Res., Vol. 49, No. 23, 2010

monomers mass ratios studied, and bimodality was observedin experiments 3 and 5 at the lower PVP concentrations. Weare uncertain at this point as to the reason for this bimodality.However in experiment 6, with the largest PVP concentration,not only did the average diameter of microparticles decreasebut also the distribution looks to be more uniform. It is well-known that the suspension stabilizer forms a film or skin aroundthe droplet/particle surface, and this layer prevents coalescenceand agglomeration by a mechanism analogous to stericstabilization.33-35 It could be that the lower PVP concentrationexperiments appeared to have larger particle size simply due tovarying degree of flocculation. This is in agreement with theresults previously found31 for the styrene based system, wherethe microcapsules looked to be composed of multiple aggregatesof smaller primary capsules at low PVP/monomer ratios. Here,since the PVP/monomers mass ratio of 0.1885 produced thelowest average particle size, the subsequent experiments in thisstudy were all performed with that concentration.

3.1.3. Influence of Paraffin/Monomers (PRS/Monomers)Mass Ratio on Particle Size. With a strongly polar acryliccomposition (P(MMA-co-MA-co-MAA)) and a PVP/monomerratio that led to microcapsules close to the target particle sizerange, the influence of paraffin/monomer mass ratio wasexplored for its influence on particle size. Experiments 1-6(Tables 1 and 2) all employed an equal mass loading of paraffinto monomer. When the PRS/monomer ratio was reduced to 0.33

(experiment 7), the smallest particle size of the series wasobtained at 134 µm. The balance between the viscosity of thedispersed phase relative to the shear imposed on the systemstrongly influences the achievable particle size. With lessparaffin (the most viscous component) the dispersed phaseviscosity is reduced and leads to smaller droplets. In experiment8, a greater PRS/monomer ratio was used (1.33) which then ismore difficult to shear and correspondingly led to a largerparticle size (245 µm, Table 3). Finally, at the highest PRS/monomer ratio tested (2.0), the particle size could not be testeddue to excessive coagulation of the capsules formed. This willbe described in more detail shortly, yet this instability likelyrelates to incomplete coverage of the paraffin by the acrylicpolymer which is not surprising at that PRS/monomer ratio.

3.2. Microcapsule Morphology and Structural Evaluation.While the thermal storage capacity of PCM paraffin microcap-sules relates most directly to the absolute amount of paraffinencapsulated in each capsule, the morphology and structuralaspects of the microcapsules play an important role in theirrobustness to mechanical and thermal processing. From amechanistic standpoint, the final particle morphology alsoreveals important hints into the dynamic history of the evolutionof that structure during the microencapsulation.

In Figure 2, the result of the MMA-based encapsulation(experiment 1) is contrasted against an example of a previousstyrene-based encapsulation.18 In the styrene-based encapsula-

Table 3. Average Diameters of Microcapsules, Encapsulation Efficiency, and Morphological Descriptions Obtained for Experiments 5-9 UsingDifferent PRS/Monomers and PVP/Monomer Mass Ratios

experiments dpv0.5 (µm)a dpn0.5 (µm)bencapsulationefficiency (%) morphology (ESEM)

5 418.6 38.9 62.3 spherical and rough surface6 147.2 11.2 64.9 spherical and rough surface7 134.0 10.5 98.0 spherical and rough surface8 245.3 10.1 66.5 irregular9 aggregates of collapsed containers

a dpv0.5 represents 50% microcapsule particles whose mean volumetric diameter is less than this value. b dpn0.5 represents 50% microcapsule particleswhose mean numeric diameter is less than this value.

Figure 1. Particle size distribution in volume for microparticles prepared using different PVP/monomers mass ratios corresponding to experiments 5 (0.0471),3 (0.0943), and 6 (0.1885).


tion, shown in the top left corner of the image, we notice threedistinct features: a salami-like morphology of a distribution oflarge sphere sizes encased within the large capsule, a capsulediameter of approximately 800 µm, and a smooth nature to theouter shell of the capsules. It is interesting to consider that thesmall capsules also shown in that image look to be in a similarsize range to those spheres comprising the internal structure inthe large capsule; one might consider the large capsule to haveformed from agglomeration of smaller ones during the polym-erization process. When the MMA-based capsules of this studyare contrasted in the remaining images of Figure 2 it is clearthat these three morphological features differ dramatically. Asdiscussed previously, the average capsule size is dramaticallysmaller. What is more interesting is that the internal micro-structure seems to be somewhat a uniform mixture of bothparaffin and PMMA polymer (Figure 2c and correspondingmagnified inset). The bottom left magnified image howevershows a continuous shell encasing the capsule of around 10µm in thickness, while Figure 2b shows the shell surroundingthe core contents to be rather bumpy and irregular.

Elemental analyses by energy dispersive X-ray spectroscopy(EDS) were performed on the cross-sectioned ESEM imagesto determine the average composition of core and shell regions

of the microparticles. For Figure 2c, the EDS showed an averageof 90.5% of carbon and 9.5% of oxygen and 85.5% of carbonand 14.5% of oxygen, respectively, in the core and shell regions.These results are consistent with the inner structure of theparticle being formed by a mixture of both paraffin matrix andPMMA domains, with the outer shell significantly dominatedby PMMA. Since the capsules in these images were previouslywashed with methanol, we can safely conclude that the shellmust be PMMA based as otherwise they would not havesurvived that cleaning step.

While from a thermodynamic perspective the resultingmorphology of the PMMA-based encapsulation of paraffinshould be fully phase separated, Figure 2 clearly shows thiswas not the case. From a kinetic point of view, this result islikely due to the high polymerization rate of methyl methacrylatemonomer resulting in rapid polymeric chain growth to highmolecular weight before those chains have a chance to phaseseparate from the paraffin matrix and diffuse. Furthermore, themobility of the polymeric radicals is dependent on whether ornot the polymer is glassy relative to the reaction temperature(70 °C).36 In this case, since the wet polymer glass transitiontemperature (94 °C)37 is markedly higher than the reactiontemperature, the diffusion coefficient of that polymer is low due

Figure 2. ESEM micrographs of PS microparticles18 vs PMMA microparticles (experiment 1, Table 1) containing paraffin wax: (a) cross-section of PS/paraffin microparticle, (b) general view of experiment 1 particles, and (c) cross-section and higher magnifications of experiment 1 inner structure and shellof the microtomed microparticle.


to the glassy environment. This, compounded with the reactionrate leads to a case where the morphology is not determined bythe balance of interfacial forces, but instead produces nonequi-librium mixed-phase structures.38

A similar morphology was obtained for the P(MMA-co-MA)copolymerization of experiment 2. While the polarity of theacrylic phase was increased by incorporation of MA, its evenhigher propagation rate coefficient further exacerbates thebalance between the polymerization rate and phase separationrate, favoring a kinetically limited structure. The incorporationof MA reduces the wet Tg of the acrylic polymer and thedifference between that and the reaction temperature, butapparently this composition was not sufficiently different toproduce a morphology difference in the capsules obtained. Whilethe difference between the Tg and the reaction temperatureinfluences the polymer chain diffusion, the molecular weightof chains also plays a role. Interestingly, no marked differencein molecular weight or distribution (each was approximately 8× 104 g mol-1 with a distribution index of 1.7) was observedfor the different experiments carried out (Table 2).

Synthesis of the P(MMA-co-MA-co-MAA) terpolymers,experiments 3 through 9, however yielded spherical “pomegran-ate”-type microparticles, as shown in Figure 3 for experiment5 (Table 3). We describe this morphology similar to apomegranate fruit as the acrylic polymer appears to have formedseed-like spheres of about 3 µm in diameter through most ofthe core of the capsule with a thin but continuous shell aroundthat interior. The wrinkled thin shell was observed on most ofthe capsules in this study, coating the matrix of acrylic spheresembedded in the paraffin core. Remnants of the inner matrix ofparaffin are also observable in the core of the broken capsuleof Figure 3. While phase separation is evident in the presenceof the pomegranate-seed-like domains in the interior of thesecapsules, ripening and coalescence of those domains nearthe exterior of the capsule were apparently restricted for thesame diffusion limited reasons described for the previous twosystems. To produce capsules that more resemble a core/shell,either the polymer should be more plasticized to facilitatediffusion by lowering the reaction rate during synthesis viaintroduction of a slower rate-limiting comonomer or the capsulescould be annealed above the glass point of the acrylic polymerto allow for rearrangement and more complete phase separation.However, as long as the encapsulation efficiency is high, thedegree of phase separation in the capsule morphology may notbe important to the performance as a PCM.

Figure 4 shows the morphology of microparticles obtainedfor different proportions of PRS/monomers; 0.33 (experiment7), 1.02 (experiment 6), and 1.33 (experiment 8). As can beseen, when a low PRS/monomers mass ratio of 0.33 wasemployed (Figure 4a), the microcapsules presented a sphericalshape and a rough wrinkled surface. However, when the payloadof paraffin was increased to 1.33 or greater, the morphologytended to be irregular (Figure 4b,c). In some cases, brokenparticles and incomplete spheres were obtained, and higher PRS/monomer ratios also tended to produce greater amounts ofcapsule aggregation (Figure 4c). These results indicate that the

Figure 3. ESEM cross-section micrograph of a P(MMA-co-MA-co-MAA)microparticle with 1 wt % of MAA (experiment 5, Table 2).

Figure 4. ESEM micrographs of microparticles prepared at mass ratios PRS/monomers of (a) 0.33, (b) 1.02, and (c) 1.33.


structure of the microparticle strongly depends on the amountof paraffin wax used in the recipe with respect to monomers.Similar results have been reported by other authors.24,25,39-41

Obviously, the thermal storage capacity of the capsule isadvantaged by higher paraffin payloads but at the expense ofmore challenging encapsulation conditions.

3.3. Thermal Storage Capacity and Encapsulation Ef-ficiency. The efficiency of a PCM material is dependent on theencapsulated quantity and energy storage capacity per unit massduring its melting and solidifying.7 The encapsulation efficiencyof paraffin wax by acrylic polymer in the microcapsules wascalculated with the following equation based on enthalpy valuesmeasured by the DSC:

where ∆Hcapsule is the enthalpy of melting for the analyzedcomposite microcapsules (J g-1) and ∆Hparaffin is the enthalpyof melting for pure PRS paraffin wax (202.7 J g-1).18 Theparenthetic factor in the denominator normalizes the totalavailable enthalpy of melting to the percentage of paraffin in arepresentative capsule of the recipe employed. Lower than fullefficiency simply means that a portion of the paraffin in thesystem is left unencapsulated. To remove this portion ofunencapsulated paraffin from the dispersion of microcapsules,each experiment’s dispersion was filtered and washed withmethanol. The resulting cleaned capsules could then be driedand isolated for characterization.

Thermal storage capacity for a particular encapsulated systemwas determined by DSC, as shown in Figure 4. Integration ofthe melting peak gives the ∆Hcapsule in J g-1, and those valueswere used to calculate the encapsulation efficiencies shown inTables 2 and 3.

As can be seen in Table 3, when the PRS/monomers ratio isincreased the polymers/paraffin wax interfacial area increases

making it less favorable to form the capsule structure.25 Thisbehavior has also been reported by other authors on the paraffinmicroencapsulation process.10,23,42,43 At the lowest PRS/monomer ratio of 0.33 in experiment 7, almost perfect encap-sulation (98%) was achieved. The actual thermal storagecapacity of that system, however, was quite low (49.7 J g-1) asa consequence of the low paraffin payload. On the other hand,experiment 8 had the highest paraffin content that formedcharacterized capsules, but with only 66.5% encapsulationefficiency the effective thermal storage capacity was still only76.9 J g-1. However, experiment 3 showed the best balance offeatures at a PRS/monomer ratio of 1.02 with a high encapsula-tion efficiency (93.5%, Table 2) and a correspondingly highthermal storage capacity (94.8 J g-1). All of the acryliccompositions used outperformed the previous styrene-basedstudy which at the same PRS/monomer ratio as experiment 3only achieved a 40% encapsulation efficiency and a 41.7 J g-1

thermal storage capacity.

4. Concluding Remarks

Microparticles containing PRS paraffin wax encapsulated witha shell based on acrylate compositions were synthesized bymeans of suspension-like polymerization. Incomplete phaserearrangement to a core/shell morphology was observed whilephase separation of the paraffin and acrylic polymer was evidentin the internal structural morphology observed for all of theacrylic compositions attempted: PMMA, P(MMA-co-MA), andP(MMA-co-MA-MAA). A “pomegranate” morphology wasobserved for most compositions tested with a thin continuousshell encasing a dense population of acrylic spheres in a paraffinmatrix within the core. The average diameter of microparticlescontaining paraffin wax PRS decreased with increased amountof suspension agent and with the amount of paraffin wax in thesuspension medium. At higher PRS/monomer mass ratios, itwas increasingly more difficult to attain useful encapsulations.The most effective balance of encapsulation efficiency (93.5%)and paraffin payload was best achieved at a PRS/monomer

Figure 5. DSC thermograms of microparticles obtained using PVP/monomers mass ratio of 0.0471 (experiment 5), 0.0943 (experiment 1), and 0.1885(experiment 6).

paraffin encapsulation efficiency (%) )∆Hcapsule

( PRS/monomerPRS/monomer + 1)∆Hparaffin

100


weight ratio of 1.02, for a P(MMA-MA-MAA) composition ofthe shell, and at a PVP/monomer weight ratio of 0.0943. Therewas slight bimodality in the capsule size distribution, but theaverage size approached the target range for textile applicationsand the latent heat of 94.8 J g-1 was more than double thatfound in a similar styrene-based encapsulation system.

Acknowledgment

Financial support from ASINTEC S.A. and ref. PBC08-0243-1458 from Consejerıa de Ciencia y Tecnologıa (JCCM), aregratefully acknowledged.

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ReceiVed for reView August 15, 2010ReVised manuscript receiVed October 6, 2010

Accepted October 11, 2010

IE101727B


synthesis and characterization of paraffin wax microcapsules with acrylic-based polymer shells

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