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Mixing effect on emulsion polymerization in a batch reactor.

INTRODUCTION

During polymerization, mixing and heat transfer have significant influence on the colloidal stability of latex particles. In emulsion polymerization, mixing, which results in the bulk movement of the fluid, plays a vital role in maintaining the homogeneity of the reaction mass. The importance of mixing in polymerization has been the subject of a few studies [1-4]. In an emulsion polymerization system, mixing can play a significant role in the kinetics of the chemical reaction. In the beginning of the emulsion polymerization, reaction is controlled by diffusion mechanism; i.e. the monomer is supplied by diffusion to the growing submicron polymer particles. In case of very low agitation rates, larger droplets are generated and phase separation may occur which will limit diffusion mechanism [5].

In the stirred dispersion, deformation of the droplets occurs as a result of shear force in the turbulent flow field. The droplets experience viscous shear stress, pressure variation along their surface, and turbulent velocity fluctuation [5]. Break up occurs if the hydrodynamic force exceeds the stabilizing force originating from the interfacial tension and drop viscosity [6]. The deformation and break up is characterized by the Weber number, which is proportional to the ratio of the inertia forces to the surface tension force. For vinyl acetate emulsion polymerization in a CSTR, the monomer conversion and the particle size were increased with an increase in the impeller speed [7]. A vigorous agitation can result in reduced nucleation of particles. In spite of the influence of mixing on emulsion polymerization kinetics, it has not been largely debated in the open literature. However, in some reported cases, conflicting results were obtained by different researchers on the effect of stirring on the rate of polymerization reaction [6, 7]. Type of impeller has been investigated on the emulsion polymerization [8-12]. Some researchers [10] reported an optimum range of stirring speed over which the polymerization rate was not affected by the speed. Nevertheless, it was reported [12] that the number of particles decreased by using a larger impeller diameter and faster speed. This was attributed to the shear stress and its effect on the nucleation mechanism, and the aggregation of the unstable nuclei.

The increase in conversion by enhancing the mixing speed in the Taylor-Couette polymerization reactor was also reported. The mass transfer between the vortices was poor at low rotational speed [13]. Generally, emulsification of the monomers has a critical effect in the formation of particles in emulsion polymerization [1]. This is due to the fact that insufficient emulsification leads to a broad particle size distribution, which also affects the other stages of the emulsion polymerization reaction [14] and ultimately decreases the product quality. For the particles formed in batch polymerization, the maximum size of the particles is achieved at the end of the growth stage, where monomer droplets cease to exist and polymer particles become continuously richer in polymer as polymerization proceeds [15]. Basically, the factors that can reduce the colloidal stability of latex are broadly divided into physical and chemical types. The destabilization due to the physical factors occurs through an increase in the average kinetic energy and the collision frequency of the particles, whereas the destabilization due to the chemical factors happens through the reduction of the interparticle potential energy barrier. In particular, the particle aggregation by agitation occurs when particles collide with the sufficient transporting force, which is caused by the high velocity gradients in the fluid, to surmount the repulsive barriers due to the adsorbed surfactant. The prediction of the rate of the inter-particle collisions in the fluid was first reported by Von Smoluchowski (16] for the orthokinetic flocculation. This type of the flocculation mechanism is based on the generation of the velocity gradients within the wastewater to promote the particle interaction. In this case, a mild mechanical agitation promotes the rate of shear-induced aggregation. However, any further increase in the agitation speed or power does not appreciably enhance the solids distribution in the fluid. Latex particle size is an important characteristic in emulsion polymerization. Some studies [17, 18] indicated that the overall rate of the monomer conversion decreased as the particle size increased for the macro or mini emulsion. The formation of the larger particle sizes at low conversion was also reported [19] for the MMA emulsion polymerization in a batch reactor with a redox initiator system and it has been attributed to the instability of particles at low conversions and the presence of a large amount of unreacted monomer. It was claimed that the particles became increasingly swollen by the monomer or there was the coalescence possibility before the sample analysis [19]. However, in the aforementioned studies, little information has been provided regarding the mixing characterization in emulsion polymerization.

The molecular weight and the molecular weight distribution of emulsion polymer exhibit a significant influence on the mechanical and application properties of the polymer [20]. The molecular weight of polymer in the latex is related to the growth of the polymer radicals which depends on the propagation rate constant, the concentration of the monomer in the particles, and the average number of the free radicals per particle.

The effect of mixing is more pronounced as the reactor size increases because the creation of the effective mixing throughout the reactor becomes more difficult. Therefore, an appropriate criterion should be selected for the scale-up of the mixing system [16]. For the emulsion polymerization scale-up, the goal is to produce at commercial scale latexes of the same quality as those developed in the laboratory. Because of geometric considerations, the larger the volume of the reactor, the smaller is its heat transfer area/volume ratio. Therefore, larger reactors require longer process times to carry out the process under good thermal control. Although agitation may improve the heat transfer, the range of the mixing intensity is limited because a vigorous agitation may cause shear induced coagulation. In large-scale reactors, it is difficult to reproduce uniform mixing similar to that in small reactors, and this is a common source of variability in particle nucleation and hence in particle size distribution. The size distribution affects the radical distribution, which in turn influences the molecular weight distribution and polymer architecture, and consequently latex properties. Besides, most researches are performed using jacket cooling on a lab-scale reactor. However, for large reactors, jacket cooling is not always sufficient to obtain reasonably short batch times and supplemental heat removal through external heat exchangers, internal cooling coils, and baffles must be used [21].

In general, important characteristics of the latex products are particle size, molecular weight and their relative distributions, chemical composition distribution, and flow properties. The choice of the recipe, reactor configuration, and the process conditions strongly determine the quality of the latex product. The ability to control the emulsion polymerization process is essential to guarantee constant product properties [22]. The main focus of most researchers in this field for the last few decades has been the incorporation of particular reactive agents during the course of polymerization to adjust the polymer properties. Even though significant advances have been achieved in emulsion polymerization in recent decades, the basic concept of mixing which is the basis for the formation of emulsion mixture has not been fully delineated yet. For the progress in future commercial latex products, it is essential to develop the cost efficient methods to control the polymer colloid properties as well as the polymer chemistry researches. In this study, the emphasis is to elucidate important aspects of operating conditions such as reactor configuration (baffle installation) and speed that can affect the monomer conversion, the polymer molecular weight, particle size and number of particles.

EXPERIMENTAL

Reactor Setup

The schematic diagram of the experimental setup is shown in Fig. 1. The reactor was comprised of a flat bottomed cylindrical tank with a diameter of 10.16 cm and a height of 26.67 cm with a total capacity of two litres. The vessel was equipped with the following items: a 45[degrees] pitched blade impeller, which is an axial-flow impeller, with a diameter of 5.08 cm and off-bottom clearance of 1.3 cm, a U-shaped cooling coil connected to a circulator, a thermocouple jacket, and inlet and outlet pipes for sampling and gas purging. The impeller was rotated with a 1/4 hp motor.

Materials

The following reagents were used in the polymerization reaction: methyl methacrylate (MMA) with purity of 99% as the monomer, sodium dodecyl sulfate (SDS) with purity over 99% as the surfactant, deionized (DI) water, potassium persulfate as the initiator, and Hydroquinone with 99% purity as the inhibitor. All reagents were supplied by Sigma-Aldrich Canada and used as received. Emulsions were prepared according to the following recipe: 700 g water, 250 g methyl methacrylate, and 8.64 g of sodium dodecyl sulfate (SDS). The amount of KPS was 0.5455 g or 2.9 X [10.sup.-3] mol [L.sup.-1] water that is typical for an emulsion polymerization system [23, 24]. The volumetric ratio of monomer to water was 4/10.

The desired amount of SDS was dissolved in 80 g of water while stirred with a magnetic stirrer bar. Nearly 600 g of water and the SDS aqueous solution were added to the reactor. About 250 g of MMA were then poured into the reactor. The reactor temperature was set to 50 or 60[degrees]C and was sealed and purged with nitrogen gas for 20 min.

The venting valve was opened and closed several times in order to remove the air completely from the reactor. The impeller speed was varied from 20 to 350 [+ or -] 2 rpm. The potassium persulfate (i.e., initiator) was then dissolved in 20 g of water and injected into the reactor with a syringe at 45[degrees]C for the reactor set point of 50[degrees]C and at 55[degrees]C for the cases that reactor set point was 60[degrees]C. Sampling (20 mL) was started 5 min after adding the initiator by opening the sampling valve and sampling procedure continued at 10-15 min time intervals for a total reaction time of 130 min after adding the initiator.

The samples were poured into the vials and two drops of 1% hydroquinone solution was added to each sample. The vials were placed in the ice and then refrigerator. The conversion was calculated using the gravimetric method. A portion of each sample was transferred directly into a dry and clean aluminum cup, where the reaction was short-stopped by addition of hydroquinone. The aliquot was weighed, and the free liquid evaporated in a vacuum oven and the resulting product was dried in an oven at 40[degrees]C for 24 h. The average particle size and particle size distribution significantly impact on the quality and applications of the latex product. The particle size and the particle size distribution were determined using Microtrac (modelS3000/ S3500) particle analyzer. Molecular weight measurements were performed using Viscotek (Model 302-040) GPC equipped with a triple detector array and tetrahydrofuran (THF) as the mobile phase at a nominal flow rate of 1.0 ml [min.sup.-1]. The molecular weight averages were obtained using universal calibration, which was performed with PMMA standards.

Experimental Design

A multilevel factorial design of experiments were employed by varying the reactor variables such as the impeller speed (20, 100, 250, and 350 rpm), baffles (yes or no) and temperature (50 and 60[degrees]C). The reaction temperature of 60[degrees]C has been previously employed by some researchers [25, 26]. Furthermore, we selected a lower temperature, i.e., 50[degrees]C to examine the mixing effects at this condition. Generally, in higher temperature, the conversion will normally increase as already reported in literature [27], In contrast, in a lower reaction temperature the particle size distribution is more uniform. In our experiments, we intended to investigate whether improving the mixing condition can enhance the polymer characteristics at lower temperatures i.e., 50-60[degrees]C.

The complete design consisted of 16 runs. Recipe was constant for all experiments listed in Table I. Some runs were repeated three times.

RESULTS AND DISCUSSION

In this study, the impact of the stirring rate, baffles, and reaction temperature on the monomer conversion, the mean particle size, and the average molecular weight were extensively explored.

In this research, the final number of particles, [N.sub.p] was calculated using the following equation [28]:

Experiments 7, 11, 12, 14, and 16 were repeated three times.

[N.sub.p] = 6[M.sub.t][x.sub.f]/[pi][[rho].sub.p][D.sup.3.sub.P] (1)

where [M.sub.t] is the total amount of monomer and polymer present in the reactor, [x.sub.f] is the final weight ratio of the polymer in the reactor to the total amount of monomer fed into the reactor by the time t, [[rho].sub.P] is the density of polymer (1.004 g [cm.sup.-3]), and [D.sub.P] is the volume average diameter of particles.

In our study, the experiments were performed from low to high agitation rates at a fixed chemical recipe. Sampling was started 5 min after adding initiator at 5[degrees]C below the set point and a total of 13 samples were taken for each run. Figure 2a and b shows monomer conversion as a function of reaction time at different impeller speeds of 20, 100, 250, and 350 rpm, with and without baffles at the isothermal reactor temperature of (a) 50[degrees]C and (b) 60[degrees]C. According to literature [29], the induction time is the period before the reaction takes place. Therefore, in our experiments, even though the sampling was started after 5 min of adding the initiator, the conversion results, revealed that the experiments performed at lower impeller speeds (20-100 rpm) had longer induction time before the monomer conversion was raised compared to the runs at the higher agitation rates (250-350 rpm).

At the impeller speed of 20 rpm, the very low conversion indicates that pooling of monomer occurred on top of the reaction mixture. This was predictable due to the insufficient mixing at this low agitation rate. The conversion increased when the impeller speed was varied from 20 to 250 rpm. However, further increase in impeller speed (i.e., from 250 to 350 rpm) resulted in lower conversion profile. Thus, the maximum conversion was achieved at 250 rpm. Conversion enhancement with an increase in the impeller speed can be anticipated as the mechanical agitation directly influences the emulsification and nucleation stages. Emulsification affects the rate of polymerization at the beginning of the reaction. At the higher impeller speeds, the mixing of the reaction mixture is improved. This enhances the probability of the reaction between reactants and thus the rate of polymerization [4]. Besides, the initiator performance improves as the result of the improved recirculation achieved at the higher impeller speed within the reactor. For impeller speeds >250 rpm, the conversion decreased. Vigorous stirring at 350 rpm induced instability in emulsion as foaming appeared on the surface of the emulsion samples. In fact, the intensive mixing exerted an excessive shear rate on the emulsion mass. Instability in emulsion polymerization systems can occur at higher impeller speeds [11]. Figure 2a and b also shows the polymerization profiles at 50 and 60[degrees]C. The effect of temperature on the conversion has already been studied in literature [30]. Nevertheless, we intended to obtain the results at two different temperatures to study the effect of incorporation of baffles and the impeller speed at these two temperatures to better demonstrate the impact of these factors compared to the temperature effect on the polymer properties. As expected, these data indicate that the temperature has an appreciable effect on the monomer conversion. At higher temperature, the decomposition rate of the initiator increases and thus more radicals are produced, which in turn lead to the higher monomer conversion [30].

On the other hand, as it can be observed in Fig. 2, the complete conversion couldn't be achieved. According to Soh and Sundberg [31], it has often been noted that polymerizations carried out at temperatures significantly below the glass transition temperature of the pure polymer do not appear to reach full conversion. Besides, the limiting conversions can be attributed to the decreased initiator efficiency and decrease in the decomposition rate of the initiator at the isothermal reaction temperature [32]. Furthermore, in our study, the monomer was not purified as it was intended to resemble the actual industrial case. Therefore, all the mentioned reasons can be attributed to the limited conversion as observed in Fig. 2.

Furthermore, the influence of the baffles on the conversion was studied in this research. The installation of baffles decreased the conversion about 7% at 50[degrees]C and 250 rpm when the baffles were mounted as shown in Fig. 2a. The reduction in conversion at 50[degrees]C due to the use of baffles was about 2% at 20 rpm and 350 rpm.

As depicted in Fig. 2b, the use of baffles at 60[degrees]C decreased the conversion by about 9% at 250 rpm and by about 2-3% at 20 and 350 rpm. At both set point temperatures, the maximum reduction in conversion due to the use of baffles was observed at 250 rpm and the least effect was observed at 350 and 20 rpm. It means that the baffles did not have a significant effect on the conversion at very low and very high impeller speeds. In contrast, the baffles had the most impact on the conversion at the impeller speed of 250 rpm.

Generally, baffles increase the axial velocity component that promotes circulation and reduce the tangential or swirl velocity. This lower tangential velocity leads to a higher relative velocity and shear rate near the impeller. Baffling is always required for liquid-liquid dispersion, with the exception of suspension polymerization and certain highly shear-sensitive emulsion polymerizations [33]. In our study, the use of baffles, increased the shear inside the reactor and as the latex was shear sensitive, the agglomeration occurred which is more notable at the impeller speed of 250 rpm at both temperatures of 50 and 60[degrees]C which contributed to the slight reduction in conversion. In addition, according to literature [33], the baffle placement is important in determination of the surface flow of the dispersion. The location of the top edge of the baffles relative to the liquid surface can creating eddies that are helpful in facilitating drop suspension. When baffle tips are just below the surface, unrestricted eddy motion facilitates engulfment of surface materials into the bulk liquid. If baffles extend through the surface, they create local stagnation, causing slow surface engulfment and sometimes pooling. In our experimental setup, the baffle set was extended through the surface of liquid. Therefore, the slight pooling of monomer at the surface may have reduced the monomer conversion. Besides, a minimum distance of baffles from the wall enables liquid to pass between the baffle and the wall. On the other hand, in our study, due to restriction of available space inside the reactor, the baffles were attached to the wall. Therefore, it is possible that the baffle surface wetted by the dispersed phase collected the monomer droplets which resulted in lower conversion.

By comparing the number of particles in Table 2, it is evident that the number of particles was larger in the reactor without baffles runs compared to the reactor with baffles experiments, which verifies the elevated conversion. Furthermore, baffles led to agglomeration of colloidal particles as will be discussed afterward in this section.

Figure 3 shows the polymer mean nano particle size as a function of conversion at the different impeller speeds (20, 100, 250, and 350 rpm) with and without baffles at the isothermal reactor temperature of (a) 50[degrees]C and (b) 60[degrees]C. These data demonstrate that the nano particle size increased when the impeller speed was varied from 20 to 250 rpm. In other words, the emulsification and the distribution of the initiated radicals and particles were enhanced when the efficient mixing was generated with the elevation of the impeller speeds. Regardless of which particle nucleation mechanism predominates in the particle formation process, the amount of surfactant available for stabilizing particle nuclei controls the size of population of latex particles [34]. The emulsifier or surfactant molecules required to stabilize these primary particles come from those dissolved in the continuous aqueous phase and those adsorbed on the emulsified monomer droplet surfaces [20].

Generally, in emulsion polymerization, monomer can be transported to the growing latex particles by molecular diffusion from the continuous aqueous phase or by the shear induced collision between the monomer droplets and particles [20], Therefore, increase in the particle size with improvement in mixing from 20 to 250 rpm can be attributed to both molecular diffusion and shear induced collision mechanisms. In the other hand, a further increase in the impeller speed (e.g., >250 rpm) resulted in slight reduction of the nano particle diameters. As the surfactant concentration was constant in all runs at different mixing rates, the instability of emulsion at excessive stirring can only be due to collision of radicals and termination reaction inside the particles to stop the polymerization. Figure 3a and b depicts the thermal effect during the polymerization on the particle size profile. Larger nano particle sizes were obtained at 50[degrees]C while the baffles were installed. Raising the reaction temperature from 50 to 60[degrees]C (Fig. 3b) had an adverse effect on the particle sizes despite an elevation in conversion. The reduction of the nano particle size with an increase in temperature has also been reported in literature [4, 32, 34, 35], The overall rate of polymerization increases with an increase in temperature. Temperature increases the rate by increasing both propagation rate constant and number of particles. The increase in the number of particles is due to the increased rate of radical generation at higher temperatures (Table 2). Opposing this trend is the small decrease in the concentration of monomer in the particles at higher temperatures. Hence, as it can be seen from Fig. 3, the particle size was reduced at the elevated temperature. Figure 3a and b also shows the influence of the baffles on the particle size at the two temperatures, 50 and 60[degrees]C. These data indicate that by removing the baffles, the particle size became smaller. Therefore, the presence of baffles enhanced the shear flow and consequently slightly larger polymer particles were produced due to agglomeration. Another assumption is that removing the baffles resulted in the vortex flow and the growth of the particles slowed down due to the swirling and solid flow. It may be postulated that the monomer had less chance to be absorbed into the micelles or radical oligomers in the aqueous phase since the monomer transportation was limited due to vortex flow. Overall, it can be concluded that the largest particle size profile as a function of conversion was achieved at the lower temperature in a reactor equipped with the baffles.

It has been reported [36] that during the course of isothermal polymerization (without monomer addition), the molecular weight decreases mildly due to the monomer depletion. Besides, according to some researchers [37], higher initial oxygen levels in the vapor space reduced the molecular weight and led to smaller latex particles. In this research, the influence of the impeller speed, reactor temperature, and the presence of baffles on the average molecular weight as a function of conversion is plotted in Fig. 4. Samples were taken at time intervals of 25, 75, and 130 min after the injection of the initiator into the reactor. As can be discerned in Fig. 4 (both plots), the weight average molecular weight increased with a rise in the impeller rotational speed from 20 to 350 rpm for the first sample taken 25 min after the initiator injection. For the second and third samples taken from the reactor after 75 and 130 min, the increase in the weight average molecular weight was observed with an increase in the impeller speed up to 250 rpm. However, further increase in the rotational speed, eventuated in a decrease in the molecular weight. Again, the interpretation is analogous to the particle size results as discussed for Fig. 3 and shows that better recirculation of the reaction mixture at higher agitation rates can improve the initiated radical distribution throughout the reactor, which led to an enhancement of the particle growth and therefore the molecular weight. In contrast, the instability of emulsion at vigorous stirring slowed down the growth of polymer particles at higher stages of polymerization reaction with lower molecular weights. Furthermore, an increase in the reaction temperature from 50 to 60[degrees]C resulted in a decrease in the average molecular weight of the sample polymers as presented in Fig. 4. As mentioned before, at higher temperature, the rate of propagation rate and number of particles are enhanced. In the other hand, it is likely that the transfer to monomer and polymer was increased due to the higher movement of chains inside the particles, and the depletion of monomer and growing radicals was also enhanced under the thermal effect. These phenomena might contribute to the reduction of the molecular weight.

Profiles of the average molecular weight in the reactors with and without baffles are also depicted in Fig. 4. It can be observed that the installation of the baffles resulted in the higher average molecular weights. As mentioned earlier, the use of the baffles eliminated the formation of the vortex in the reactor and converted it into an axial flow. From this data trend of polymer product with higher molecular weights we can postulate that when the vortex flow is eliminated in the reactor vessel, there is a higher probability of the monomer absorption into the micelles or into insoluble radical chains in the aqueous phase. According to literature [38], the influence of baffles on the molecular weight distribution in an emulsion polymerization reactor equipped with the internal angular baffles was also investigated and the results showed that the use of baffles had only a marginal effect on the molecular weight distribution [38], Overall, the highest average molecular weight was achieved in the reactor with baffles at the reaction temperature of 50[degrees]C. Figure 5 shows plots of the polydispersity of the polymer samples obtained in this study. These data show that the polydispersity increased with conversion. This trend was due to the growth of the polymer particles, which resulted in the production of the larger particles with time. Polydispersity of a polymer increases with the conversion as a consequence of chain branching reactions, transfer to polymer, and terminal double bond polymerization, which becomes more important as the polymer concentration increases [21]. This explanation can be applied to our study as well. The only exception was observed in Fig. 5a (100 rpm, without baffles), in which the second sample had a slightly higher PD1 than the next sample (from 2.4 to 2.3), which was attributed to the sample preparation issues for the GPC analysis. A comparison between the data shown in Fig. 5a and b reveals that the use of baffles resulted in a narrower molecular weight distribution. It can be implied that the molecular weight was better controlled by the use of the baffles. An increase in the temperature did not show a significant effect on PD1 at the early stage of polymerization, but contributed to the higher polydispersity as the conversion was enhanced.

In Fig. 6, the conversion, mean particle size, and average molecular weight as a function of the impeller speed are separately plotted after 130 min for the reactors with and without baffles at 60[degrees]C. In Fig. 6a-c, the conversion, mean particle size, and average molecular weight of the final latex samples were enhanced with an increase in the impeller speed due to better recirculation and better chance of reaction and emulsification. However, further raise in the agitation rate (>250 rpm) resulted in instability of emulsion and lowered all the mentioned polymer qualities. Use of baffles in the reactor resulted in excessive shear flow and stagnation of monomer droplets on the surface of the reaction mixture. Furthermore, the wetted surface of baffles collected the monomer droplets and therefore the monomer conversion slightly decreased. Besides, installation of baffles in our experiment led to agglomeration of particles and increase in the mean particle size (Fig. 6b). The average molecular weights as shown in Fig. 6c had a slight decrease when the baffles were removed. We can assume that the swirling How in the reactor without baffles limited the monomer transport to the growing particles and thus the molecular weight decreased.

The volumetric size distribution of particles at the impeller speed of 250 rpm and the reaction temperature of 60[degrees]C in the reactors with and without baffles is plotted in Fig. 7. Three samples were taken at 25, 75, and 130 min after the initiator was injected into the reactor. The size distribution of the particles produced in both reactors with and without baffles drifted distinctly to the broader profiles as the reaction time progressed from 25 to 130 min. Besides, the effect of the baffle is clearly observed in this figure as at each specified reaction time, the distribution curve measured for the reactor without baffles was wider than that for the reactor with baffles. The particle size distribution is a consequence of the distribution of times at which different polymer particles are nucleated. An explanation for a broader distribution with an increase in the reaction time is proposed as follows. When particles are first formed at the initial stage, the distribution is normal. The free radicals formed in the aqueous phase diffuse into the primary particles and the majority of them escape into water again. There is a higher chance that a particle with a larger volume gets more radicals than a small one. Thus, a large particle will propagate at a faster rate to form a larger particle. During stage II of the polymerization reaction, more polymer primary particles are formed by micellar and homogeneous nucleation. This increases the population of the small particles while the large particles receive more radicals and propagate at a rapid rate [20], This is an explanation as why the particle size distribution is broad in a conventional emulsion polymerization. Despite the occurrence of excessive shear flow in our experiments due to use of baffles as already discussed, the vortex formation was minimized in the reactor with baffles, and therefore the possibility of having a more uniform reaction mixture increased. Thus, the size distribution of the particles yielded in the reactor with baffles was narrower than that produced in the reactor without baffles during the polymerization process.

Figure 8 shows the refractive index (RI) response peaks extracted from GPC data for 250 rpm in the reactor with baffles operated at 60[degrees]C. We showed the RI signal as a function of elution volume [39] to confirm the change of molecular weight with increase in reaction time and conversion. The different molecular species are eluted from the GPC column in order of their molecular size [40]. Therefore, the larger molecules leave the column earlier than the smaller molecules. In Fig. 8, the size exclusion chromatography traces of the polymer samples in THF exhibit the shift towards lower elution volumes with the increase in reaction time from 25 min to 75 and 130 min, with conversion of 39, 72and 79% respectively. This approves higher molecular weight due to growth of polymer particles in the reactor. Besides, the well-resolved unimodal peaks in the GPC chromatograms shows a uniform distribution in the molecular weight of polymer particles.

Finally, Fig. 9 demonstrates the scanning electron microscopy (SEM) images (Hitachi S-2150) of the dried end product latex at different impeller speeds of 100, 250, and 350 rpm. The reaction temperature was 60[degrees]C and the reactor was equipped with the baffles. These images show that the dried particle size increased when the agitation speed was varied from 100 to 250 rpm. However, the particle sizes decreased slightly at 350 rpm. Besides, the polymer particles had more uniform shape and size distributions at the higher stirring rates (especially at 250 rpm). The information obtained in this study will enable us to improve the existing emulsion polymerization processes and has the potential to contribute to the development of the novel polymerization processes.

CONCLUSIONS

Emulsion polymerization of MMA was carried out in a 2-1 reactor equipment with a pitched blade turbine. The effects of the impeller speed, baffles, and temperature on the conversion, molecular weight, and particle size were investigated. It was found that the monomer conversion, particle size, and molecular weight were increased when the impeller speed was raised from 20 to 250 rpm. However, further increase in the rotational speed (from 250 to 350 rpm) resulted in the reduction of the monomer conversion, particle size, and molecular weight of the polymer. Installation of the baffles decreased the monomer conversion particularly at 250 rpm, but the particle size and the molecular weight of the polymer produced in the reactor with baffles were slightly enhanced. Besides, the number of particles was higher for the reactor without baffles. However, installation of baffles led to the formation of the polymer with a narrower size distribution. While an increase in the reaction temperature from 50 to 60[degrees]C had an adverse effect on the polymer particle sizes and molecular weights, the conversion of the monomer was improved at the higher temperature. Also, the number of particles was higher at 60[degrees]C compared to that at 50[degrees]C. The highest values for the average particle size and molecular weight were observed at 50[degrees]C and the reactor with baffles.

An increase in the temperature did not result in the variation of PD1 at the early stage of polymerization, but contributed to the higher PDI as the conversion enhanced. SEM images demonstrated that the polymer particles had more uniform shape and size distributions at the higher stirring rates (250 and 350 rpm) and the largest particle sizes were produced at 250 rpm.

The finding of this study can be applied to improve the reactor design, to optimize the polymerization conditions, and to adjust the end product quality with the minor likeliness of adding the so-called reactive agents, which would be definitely reduces the environmental impact and the production cost. In our future work, the effects of other mixing parameters such as the impeller type and the number of impellers will be discussed in detail.

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Shideh Fathi Roudsari, Ramdhane Dhib, Farhad Ein-Mozaffari

Department of Chemical Engineering, Ryerson University, Toronto, Ontario M5B 2K3, Canada

Correspondence to: Farhad Ein-Mozaffari; e-mail: fmozaffa@ryerson.ca Contract grant sponsors: Natural Sciences and Engineering Research Council of Canada (NSERC); Ryerson University.

DOI 10.1002/pen.23963

Published online in Wiley Online Library (wileyonlinelibrary.com).

TABLE 1. Experimental design of the experiments.

      Reaction temperature
No.       ([degrees]C)       Impeller speed (rpm)   Baffles

1              50                     20              Yes
2              50                     20              No
3              50                    250              Yes
4              50                    250              No
5              50                    100              Yes
6              50                    100              No
7              50                    350              Yes
8              50                    350              No
9              60                     20              Yes
10             60                     20              No
11             60                    250              Yes
12             60                    250              No
13             60                    100              Yes
14             60                    100              No
15             60                    350              Yes
16             60                    350              No

TABLE 2. Final particle numbers at different reactor conditions.

                          Particle numbers in the mixture

Reactor condition        50[degrees]C,        50[degrees]C,
Impeller speed (rpm)        Baffled             Unbaffled

20                     8.78 x [10.sup.15]   1.09 x [10.sup.16]
100                    1.47 x [10.sup.16]    2.1 x [10.sup.16]
250                    3.15 x [10.sup.16]    3.9 x [10.sup.16]
350                     3.3 x [10.sup.16]   3.86 x [10.sup.16]

                          Particle numbers in the mixture

Reactor condition        60[degrees]C,        60[degrees]C,
Impeller speed (rpm)        Battled             Unbafiled

20                     1.6 x [10.sup.16]    2.3 x [10.sup.16]
100                    3.5 x [10.sup.16]    5.3 x [10.sup.16]
250                    4.6 x [10.sup.16]    6.2 x [10.sup.16]
350                    4.5 x [10.sup.16]    6.2 x [10.sup.16]
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Author:Roudsari, Shideh Fathi; Dhib, Ramdhane; Ein-Mozaffari, Farhad
Publication:Polymer Engineering and Science
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Date:Apr 1, 2015
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