Materials Science and Engineering C 29 (2009) 415–419

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Materials Science and Engineering C

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Kinetic advantages of using microwaves in the emulsion polymerization of MMA

C. Costa a, A.F. Santos b, M. Fortuny b, P.H.H. Araújo a, C. Sayer a,⁎a Departamento de Engenharia Química, Universidade Federal de Santa Catarina, Campus Universitário, CEP: 88040-900, Florianópolis, SC, Brazilb Programa de Mestrado em Engenharia de Processos, Universidade Tiradentes, Instituto de Tecnologia e Pesquisa, Av. Murilo Dantas, 300, CEP: 49032-490, Aracaju, SE, Brazil

⁎ Corresponding author.E-mail address: csayer@enq.ufsc.br (C. Sayer).

0928-4931/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.msec.2008.08.013

a b s t r a c t

a r t i c l e i n f o

Article history:

Microwave irradiation has be Received 30 April 2008Received in revised form 13 August 2008Accepted 15 August 2008Available online 23 August 2008

Keywords:Microwave irradiationEmulsion polymerizationMethyl methacrylate

en an interesting alternative for heating systems and several chemical reactions. Inpolymerization processes, microwaves can enhance reaction rates or improve specific characteristics of theformed polymer. In this work, the use of microwave irradiation in emulsion polymerization reactions has beenstudied, using a commercialmicrowave reactor,which is able to performsynthesesundercontrolled conditions oftemperature and power.Methylmethacrylate emulsionpolymerization reactionswere faster, resulting in smallerpolymer particles, in comparison to the conventional heating method (reactions in a jacketed reactor). Differenteffects were observed in the emulsion polymerization of butyl acrylate. To study the effect of high powermicrowave irradiation upon the emulsion polymerization, a pulsed irradiation strategy was developed, inwhichthe samples were repeatedly heated within short intervals of time (about 27 s) at the maximum microwavepower. A significant reduction of the total time of irradiation was observed in reactions carried out under thepulsed scheme, showing the kinetic advantages of using microwaves in emulsion polymerization processes.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

The application of microwave irradiation in polymer chemistry isan emerging research field. A lot of activity has been developed in thelast ten years regarding the use of microwave irradiation forpolymerization purposes [1–10]. This is because the microwaveirradiation offers a clean, convenient, and inexpensive method ofheating which often results in higher yields and shorter reaction times[11]. The heating of liquids using microwaves can be explained by theinteraction of matter with the electric field of the incident radiation,causing the movement of ions as well as that of induced or permanentmolecule dipoles. Molecules exhibiting a permanent dipole momenttry to align to the applied electromagnetic field resulting in rotation,friction, and collision of molecules and, thus, in heat generation (theso-called dielectric heating) [11,12]. As a result, the heating rate andefficiency of microwave heating strongly depends on the dielectricproperties and the relaxation times of the reaction mixture [14].

Besides the advantages of fast and selective heating of the materials,as well as the possible high-temperature chemistry, some authorsconsider that the microwaves can provide specific effects (not purelythermal) generally connected to the selective absorption of microwaveenergy by polar molecules [12,13]. Nevertheless, the occurrences ofthese effects have not been explained yet [7,10,15]. In spite of that and asstated before, the microwave irradiation has become an interesting toolfor a number of applications in the polymerization field.

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Concerningemulsionpolymerizationapplications, it seems thatCorreaet al. (1998) [1] were the first to report the successful use of microwaveirradiation for the synthesis of polymer latex. The system studied by theauthors consisted in the styrene emulsion polymerization, usingpotassium persulfate (K2S2O8) as initiator. The work revealed that themicrowave irradiationprovides high reaction rates (70-times faster)whencomparing with the conventional heating method. Since this pioneeringwork, several interesting studies have been published aiming at theemulsion polymerization in short times [2–10]. Most of them support theidea that the microwave irradiation increases the initiator decompositionrates and thefinal conversion of certainmonomers [2–6]. In this sense, Heet al. (2001) [2] obtained higher K2S2O8 decomposition rate at 70 °C usingthe microwave heating (10-times higher) when compared with thedecomposition rate obtained under the conventional heating method. Asimilar study was conducted by Zhu et al. (2003) [3], resulting in K2S2O8

decomposition rates 3-times higher when using the microwave heatingmethod. With regard to the acceleration of emulsion (and miniemulsion)polymerization reactions undermicrowave irradiation, increased reactionrates were found for a number of monomers, including styrene [1,3,8,9],methyl methacrylate [4–6,10] and butyl methacrylate [2].

In the work described in the current paper, the use of microwaveirradiation in emulsion polymerization processes was investigated, andcompared with conventional water bath heating. Emulsion polymeriza-tions of methyl methacrylate (MMA) and butyl acrylate (BuA) werecarried out under conventional heating in a glass reactor. The effect ofhighpowermicrowave irradiation in emulsionpolymerizationswas alsostudied. For this purpose, pulsed reactions were conducted, in whichthe samples were repeatedly heated from room temperature to 80 °Cthrough microwave radiation at 1400 W, within short intervals of time

Table 1Molecular structures of the emulsifiers used in the emulsion polymerizations

Emulsifier Molecular structure

Disponil FES32 CH3–(CH2)11–(O–CH2–CH2)4–OSO3 NaDisponil A3065 CH3–(CH2)11–(O–CH2–CH2)19–OH

Fig. 1. Power irradiation profile, for MMA pulsed reactions.

416 C. Costa et al. / Materials Science and Engineering C 29 (2009) 415–419

(about 27 s), and cooled down in an ice bath immediately afterwards.Results obtainedwith thepulsed scheme show thekinetic advantages ofusing microwaves in polymerization processes.

2. Experimental

2.1. Materials

Methyl methacrylate (MMA) and butyl acrylate (BuA) monomers,of analytical grade, were purchased from Merck and Aldrich,respectively. Potassium persulfate P.A. (K2S2O8), from Vetec, wasused as initiator. Disponil FES32, anionic emulsifier, and DisponilA3065, nonionic emulsifier, were obtained from Cognis. Table 1 showsthemolecular structures of the emulsifiers, determined by elementaryanalysis [16]. All reagents were used as received, without furtherpurification. Water was deionized by filtration through a Milliporecartridge and used as the continuous phase.

2.2. Experimental procedures

All reactions were carried out with 19% P/P of monomers, and emul-sifiers concentration above the criticalmicelar concentration (CMC). TheCMC values, for both emulsifiers, were determined from liquid surfacetensions, thatweremeasured by theWilhelmyplatemethod, employinga tensiometer (Dataphysics, DCAT 11). Experimental conditions andformulations of the polymerizations are shown in Table 2.

2.2.1. Conventional heated polymerizationsConventional heated polymerizations were conducted in a

1000 mL jacketed glass reactor, equipped with mechanical stirrer(150 rpm), nitrogen inlet, condenser and thermostatic bath. Deionizedwater, emulsifiers andmonomer, MMA or BuA, were transferred to thereactor, stirred under nitrogen atmosphere and heated up to thereaction temperature. When the reaction medium reached thistemperature, 10 mL of an aqueous K2S2O8 solution was added to thereactor. Polymerization reactions were carried out under constanttemperature. Samples were taken out at regular intervals, cooleddown by immersion into an ice bath and 4 drops of an aqueoushydroquinone solution (5 wt.%) were added to fully stop the reaction.

2.2.2. Microwave polymerizationsThemicrowave reactionswere conducted inaSynthos3000microwave

reactor, fromAnton Paar. Themicrowave sourcewas of 2.45GHz frequencymagnetronpoweredbya1400Wvariablepowergenerator,which couldbeoperated at different power levels. To prevent nonuniform heating, thecavity was designedwith a rotating rotor onwhich eight quartz vials could

Table 2Experimental conditions and formulations employed in emulsion polymerizations withmicrowave and with conventional heating

MMA reactions (wt.% relatedto total mass of the reaction)

BuA reactions (wt.% relatedto total mass of the reaction)

MMA 19.04 –

BuA – 19.08Deionized water 80.24 80.33K2S2O8 0.08 0.01Disponil FES32 0.14 0.07Disponil A3065 0.50 0.51Temperature ( °C) 80 70

be placed. In addition, samples could be stirred through amagnetic stirrer.The temperature of the sample was monitored by a gas sensor, inserted inoneof the vials. Infrared sensors, locatedonall vial bases,measured the vialexternal temperature. The microwave reactor was also equipped with apressure transmitter which provides pressure information of all vials. Themicrowave reactor could be adjusted to keep constant either sampletemperature, or the applied power, varying the other parameter.

2.2.2.1. Microwave polymerization — constant temperature. To performthe emulsion polymerization under constant temperature, the vialswere filled with aliquots (16 mL) of a previously prepared solution(consisting of water, initiator and emulsifiers), and purged withnitrogen. The MMA or BuA monomer was then added. Thereafter, thevials were closed with screw caps and submitted to microwaveirradiation, for individual reaction times. The vials were promptlyheated up to the reaction temperature and after being kept at thedesired temperature (70 or 80 °C) for a certain interval, the sampleswere taken out, quenched in an ice bath and 4 drops of hydroquinonesolutionwere added to stop the reaction. This experimental procedurewas repeated for different reaction times. In order to compare results,the experimental conditions were similar to those used in reactionswith conventional heating.

2.2.2.2. Microwave polymerization — pulsed irradiation at constantpower. The effect of high power microwave irradiation in emulsionpolymerizations was also studied. For this purpose, pulsed reactionswere conducted, in which the samples were submitted to heating andcooling cycles. Sampleswere repeatedly heated from room temperatureto 80 °C through microwave radiation at constant 1400 W, within shortintervals (about 27 s each pulse). Between the applied microwaveradiation pulses, the sampleswere cooled down in an ice bath for 4min.

Fig. 2. Power irradiation profile, for MMA reactions at constant temperature.

Fig. 3. Evolution of conversion during MMA emulsion polymerizations with microwave(constant temperature) and conventional heating.

Fig. 4. Evolution of reaction rate duringMMAemulsionpolymerizationswithmicrowave(constant temperature) and conventional heating.

417C. Costa et al. / Materials Science and Engineering C 29 (2009) 415–419

After each procedure, samples were collected, hydroquinone solutionwas added and reserved for further characterization. Experiments werecarried out applying 1 to 6 irradiation pulses, all tests in duplicate. Thisprocedure is schematically shown in Fig. 1.

In the experiments conducted with constant temperature, on theother hand, the microwave reactor was programmed to maintain thedesired temperature by adjusting the applied power, which variesfrom 1400 W to 0 W. Fig. 2 shows the power irradiation profile ofMMA polymerization for the first 5 min of reaction. The decrease inpower irradiation was already expected, once polymerization reac-tions are exothermic, and, as a consequence, the energy required tomaintain a desired temperature decreases along reaction.

2.3. Characterization

Polymerization conversions were determined by gravimetry.Average diameters of polymer particles were measured using adynamic light scattering equipment (Zetasizer Nano S, fromMalvern).

3. Results and discussion

In order to investigate the microwave effects on the emulsionpolymerization process, batch MMA and BuA emulsion polymeriza-tion reactions were carried out under both, microwave and conven-tional, heating methods. To exclude the influence of temperaturedifferences between both conditions, the temperature of the reactionmedium in the microwave reactor was controlled, by adjusting theapplied power (see Fig. 2), to keep the same temperature as in theconventional heating polymerization condition.

3.1. Comparison between conventional heated MMA polymerizations andmicrowave MMA polymerizations

The monomer (MMA) conversion profile, as a function of time, isshown in Fig. 3. For the same reaction times, higher conversions wereobtained under microwave irradiation, in comparison with conven-tional heating. Under microwave irradiation, 93% MMA conversion

Table 3Time reaction, conversion and Rp, for conventional heating and microwave (constanttemperature) MMA polymerizations

Conventional heating Microwave — constant temperature

Time(min)

Conversion Rp×103

(mol L−1s−1)Time(min)

Conversion Rp×103

(mol L−1s−1)

4 0.24 1.97 1.5 0.32 2.206 0.40 2.57 2.5 0.55 7.248 0.73 5.20 3 0.67 7.6410 0.93 3.28 4 0.89 6.9812 0.96 0.43 6 0.93 0.59

was achieved within 6 min, while for conventional reaction, 10 minwere necessary to achieve the same conversion.

These results indicate that microwave irradiation increases MMAreaction rates. A better discussion is provided through the analysis ofthe polymerization rates (Rp) of both reactions. The calculated Rpvalues are shown in Table 3 and represented in Fig. 4, as a function ofconversion. It can be observed that higher reaction rates were ob-tained when the microwave heating method was used. For conver-sions of about 70%, Rp under microwaves was about 40% larger thanthat for conventional heating.

Fig. 5 shows the average particle diameters of polymers obtainedunder microwave (constant temperature) or conventional heating forMMA polymerizations. In the microwave experiment polymerparticles were smaller compared to particles obtained with thermalheating. This can be ascribed to an increased thermal decompositionrate of the initiator due to microwave irradiation. Studies on themicrowave-assisted K2S2O8 decomposition [3,4,17] showed anincrease of 2.4 to 4.8 times in the decomposition rate constant (kd)of K2S2O8 undermicrowave irradiation, in comparison to conventionalheating. An increase in the decomposition rate of the initiator resultsin the formation of a greater number of particles, and, as aconsequence, in smaller particle sizes. Moreover, it is well establishedthat primary particles formed by micellar nucleation are the majorpolymerization locus in conventional ab initio batch emulsionpolymerizations. So, an acceleration of the polymerization rate canbe obtained when a higher number of particles is formed.

Particle number results of the MMA polymerizations agree withthis theory. In the conventional reaction 4.2×1017 particles per liter ofreaction medium were obtained, whereas in the microwave process,

Fig. 5. Evolution of average particle diameters during MMA emulsion polymerizationswith microwave (constant temperature) and conventional heating.

Fig. 6. Evolution of conversion during BuA emulsion polymerizations with microwave(constant temperature) and conventional heating.

Fig. 7. Evolution of reaction rate during BuA emulsion polymerizations with microwave(constant temperature) and conventional heating.

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the number of formed particles was 1.5×1018 per liter of reactionmedium. This increase affects directly the polymerization rate.

3.2. Comparison between conventional heated BuA polymerizations andmicrowave BuA polymerizations

BuA conversion, as a function of time, is shown in Fig. 6. It may beobserved that, contrasting with the MMA polymerizations, microwaveheating at constant temperature decreased the initial reaction rates,resulting in a slower polymerization than under conventional heating.This effect was observed only for initial reaction times. In Table 4, thatpresents conversions and Rp values calculated for BuA emulsion poly-merizations under microwave and conventional heating, it is observedthat 91% conversion is achieved within 30 min of reaction for bothheating methods, showing no differences in the total reaction time.

Fig. 7 presents the polymerization reaction rates, for microwave(constant temperature) and conventional heated polymerizations. Rpvalues for microwave irradiated reactions are only lower for lowconversions. As reaction evolves, Rp for microwave heated reactionsbecomes higher than that for conventional ones. This suggests thatmicrowave irradiation decreases the initial BuA polymerization rate, butpromotes slightly higher reaction rates after a certain time of irradiation.

Average particle diameters of polymers obtained under microwave(constant temperature) or conventional heating for BuA polymeriza-tions are shown in Fig. 8. Also in the case of the BuA polymerizations,polymer particles of the microwave experiment were smaller thanparticles obtained with thermal heating. Nevertheless, while in theMMA microwave polymerization final particle diameter was 40%smaller than that of the conventional reaction (Fig. 5), in the BuAmicrowave polymerization final particle diameter was only 8% smallerthan that of the conventional reaction (Fig. 8).

These different microwave effects, observed in MMA and BuA poly-merizations, can be ascribed to monomer properties and its reactionkinetics. The aqueous-phase solubility of BuA (6.2×10−3 mol dm−3, at50 °C) is considerably lower than the MMA aqueous-phase solubility

Table 4Time reaction, conversion and Rp, for conventional heating and microwave (constanttemperature) BuA polymerizations

Conventional heating Microwave — constant temperature

Time(min)

Conversion Rp×103

(mol L−1s−1)Time(min)

Conversion Rp×103

(mol L−1s−1)

3 0.22 1.375 0.34 1.54 5 0.10 0.787 0.47 1.329 0.56 1.07 10 0.48 1.7215 0.77 0.63 15 0.71 1.0820 0.84 0.35 20 0.85 0.6430 0.91 0.10 30 0.91 0.14

(1.5×10−1mol dm−3, at 50 °C) [18]. As a consequence, the kinetics of BuAin the aqueous phase is very different from that of MMAmonomer. In amicrowave emulsion polymerization, energy is preferentially absorbedbywater, due to its polar characteristics. So, theaqueousphase kinetics isaffected mostly by microwave irradiation. This could result in specificmicrowave effects on each monomer system. Since in MMA emulsionpolymerization aqueous phase kinetics are much more important thanin BuA emulsion polymerizations, MMA polymerizations are affected ina stronger way by microwave irradiation than BuA polymerizations.

Another possibility could be the microwave promoted monomerheating. Under microwave irradiation, monomers are mostly heated byconduction from water, instead of irradiation absorption, due to its nonpolar characteristics. As the solubility of BuA inwater is very low, this heatconduction, fromwater to monomer, is lower for BuA than forMMA. As aconsequence, the heating rate of BuA is also lower than that of MMA.Microwave experiments were started at room temperature, whileconventional ones were started already at the desired reaction tempera-tures. Therefore, this decrease in the initial reaction rate of BuA can be aconsequence of the heating time delay under microwave irradiation.

3.3. MMA microwave polymerizations at pulsed irradiation

Fig. 9 shows MMA conversion as a function of the number of pulsesand itmight be observed that it was possible to achieve 97% ofmonomerconversionwith only 6 pulses of microwave radiation at 1400W (about27 s each pulse). The total time that samples were submitted tomicrowave irradiation was less then 3 min, while for the reaction atconstant temperature (80 °C) it was necessary about 6 min undermicrowaveheating toachieve93%conversion (Fig. 3). These results agreewith those already reported [8,9] for miniemulsion polymerizations.

Fig. 8. Evolution of average particle diameters during BuA emulsion polymerizationswith microwave (constant temperature) and conventional heating.

Fig. 9. MMA conversion for pulsed reactions (from room temperature to 80 °C), underhigh power (1400 W) microwave irradiation.

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Analyzing power profiles (Figs. 1 and 2) and conversion results(Figs. 3 and 9), it can be concluded that the fast reaction rates observedwhen the pulsedmethodwas used are due to higher amount of energysupplied to the system, in comparison with constant temperaturereactions.

4. Conclusions

The use of microwave irradiation accelerated the emulsion poly-merization of methyl methacrylate. Nevertheless, this behavior was notobserved in the butyl acrylate polymerizations, which presented equalor lower reaction rates in the microwave experiments compared to theconventional ones. This fact was attributed to the different aqueous-phase solubilities and dielectric parameters of the monomers. As aconsequence, specific microwave effects on each monomer system arelikely to occur. In addition, in all microwave experiments polymerparticles were smaller compared to the particles obtained with thermalheating. Thiswasascribed to an increased thermal decomposition rate ofthe initiator (potassium persulfate) due to microwave irradiation,resulting in a greater number of formed particles.

A significant reaction rate increase was observed in the pulsedreactions. In methyl methacrylate emulsion polymerization it waspossible to achieve 97% of monomer conversion with only 6 pulses of

about 27 s at 1400W (from room temperature to 80 °C for each pulse),while for the reaction at constant temperature (80 °C) it was necessaryabout 6 min under microwave heating. The obtained results arerelated to the selective heating property of microwave irradiation, themicrowaves interaction with different substances and the acceleratedinitiator decomposition.

These results highlight several kinetic advantages of usingmicrowaves in polymerization processes. Using cycles of heating andcooling, samples can be submitted to a great energy amount withinshort intervals of time. As a result, we can dramatically reduce theirradiation time and promote rapid reactions, without achievinghigher temperatures.

Acknowledgement

The authors thank CNPq— Conselho Nacional de DesenvolvimentoCientífico e Tecnológico for supporting this work and providingscholarship.

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