Particle Size Characterization During Emulsion Polymerization

Chapter 18

Particle Size Characterization During Emulsion Polymerization J. Venkatesan and Cesar A. Silebi

Downloaded by UNIV LAVAL on October 28, 2015 | http://pubs.acs.org Publication Date: June 10, 1998 | doi: 10.1021/bk-1998-0693.ch018

Department of Chemical Engineering and Emulsion Polymers Institute, Lehigh University, Bethlehem, PA 18015-4735

Particle size distribution (PSD) information in an emulsion polymerization reactor is critical to understanding the complex kinetics therein and to develop control strategies to produce latexes with pre -specified PSDs. These important benefits of implementing particle size as a quality control parameter in the manufacture of latexes has resulted in extensive research in recent years. The ability to precisely monitor the PSD is critically dependent on the sensor technology employed. Accordingly, the present study demonstrates the use of Capillary Hydrodynamic Fractionation (CHDF) to monitor an emulsion polymerization process using styrene as the model system. In the first part of the paper attention is focussed on implementing a hybrid sensor by coupling the high resolution fractionation capability of CHDF with turbidimetric methods. This is achieved by passing the fractionated species through a photo-diode array detector which measures the turbidity at several wavelengths, instantaneously, thereby enabling the use of turbidimetric methods for the final particle size analysis. The distinct advantages offered by this absolute procedure to measure PSDs is found to be particularly advantageous to the monitoring scheme. The sensor is then used to monitor an emulsion polymerization process and specifically focus on two experiments. The first experiment shows the validity of the sensor as an accurate, sensitive and robust tool to track uni-modal growth patterns. The presence of monomer swollen particles does not hinder the particle size measurements. The second experiment demonstrates the sensitivity of the sensor to changes in the monomer feed rate. The results of these experiments provide a sound foundation to monitor a more complex series of polymerizations in the future.

Characterizing, monitoring and controlling particle growth in an emulsion polymerization process enable the user to gain a handle on an important parameter 266

©1998 American Chemical Society

In Particle Size Distribution III; Provder, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

267


that defines the molecular architecture of the end product; namely, the Particle Size Distribution (PSD). Important application properties of a latex such as opacity, colorability, viscosity, film forming ability etc. are intimately tied to the particle growth process and the final size distribution achieved. However, despite these far reaching benefits offered by potential solutions to this problem of particle size control, the literature is surprisingly thin on research conducted in this area. A careful examination of the literature suggests that a crucial factor that has prevented the implementation of effective control of PSD in a reactor is the lack of a sensor technology that is accurate and reproducible to make PSD information available fast enough to enable appropriate control action. Dynamic Light Scattering. Recently Kourti et al. [1,2] have demonstrated the utility of using dynamic light scattering (DLS) to measure on-line particle growth during the production of polyvinyl acetate) latex in a continuous stirred tand reactor. The data showed good reproducibility in the measured average diameters (intensity average and volume average) derived from turbidimetric measurements. This technique is found to work very well for monitoring reactions where growth of one size population is of interest. However, size distribution is assessed using the difference between the weight and intensity average diameters [3]. If these vary significantly, the distribution can be described as relatively broad; but the exact nature of the distribution viz. partial or complete bimodal, broadly distributed etc. cannot be quantitatively described. Fiber-Optic Dynamic Light Scattering (FODLS). Dynamic light scattering (DLS) measurements on concentrated samples, called Fiber Optic Dynamic Light Scattering (FODLS) [4,5] has an edge over regular dynamic light scattering techniques because it eliminates the need for dilution thereby making this technique more robust for industrial reactors. Additionally, FODLS systems are more compact and have cheaper optical systems. They operate on a similar principle to the DLS with the exception that the DLS measurements are made on the backscattered light. This technique is susceptible to erroneous results from particle-particle interactions in a concentrated dispersion and the fiber optic probe fouling due to an impermeable coating of impurities. In general, the interpretation of the results from FODLS is not straightforward and since the technique is essentially based on DLS it is susceptible to the same problems experienced by DLS. Hydrodynamic Chromatography (HDC). Hydrodynamic chromatography (HDC) and liquid exclusion chromatography (LEC) have also been reported as viable techniques to track in-situ particle growth. Studies conducted using H D C were successful in monitoring the uniform growth of particles and tracking phenomena like electrolyte induced agglomeration and secondary nucleation during a seeded polymerization of Styrene-Butadiene systems [6]. Sampling and analysis times, however, allowed the measurement of three or four samples during the course of the reaction. This often becomes essential when a certain control action is desired over relatively short interval of time. Issues such as reproducibility and sensitivity to manipulated variables remain unaddressed.


268 Application of Capillary Hydrodynamic Fractionation (CHDF) for Particle Size Distribution Analysis.


In CHDF, fractionation according to size is achieved by pumping the sample to be analyzed under laminar flow conditions through micro-bore capillary tubes resulting in the larger particles eluting the column before the smaller ones. Separations of various mixtures have been demonstrated using this principle and have been extensively discussed by Silebi et al [7,8]. Subsequent to these studies, optimization of the fractionation process resulted in higher resolution separations [9,10] which in turn facilitated shorter residence times and hence shorter overall particle size analysis times. Absolute Particle Sizing technique. Despite these improvements in the separation efficiency, one shortcoming of the technique, however, still involved the use of a calibration curve for the final calculation of the distribution. The calibration curve in general is influenced by three major parameters: (i) eluant velocity, (ii) eluant composition and (iii) sample concentration. Practically speaking, during a period of prolonged operation, changes in these parameters alters the calibration curve. Failure to account for concomitant changes in the calibration curve correctly will result in erroneous particle size assessments. The process of continuously accounting for these changes will render the procedure tedious and time consuming - two factors that cannot be compromised upon during monitoring of particle growth during polymerization. This was the motivation to attempt a particle sizing methodology that does not rely on a previously calibrated system, namely, an absolute particle sizing procedure. The evolutionary development of the CHDF therefore continued with the incorporation of the multi-wavelength detector as the light sensing device to detect the particulate species after fractionation. This facilitated turbidity measurement of the fractionated species at several wavelengths instantaneously, thereby enabling the use of turbidimetric methods for the final particle size analysis. This methodology therefore couples the advantages offered by each of the individual techniques (fractionation by CHDF and turbidimetric measurements by light scattering) while simultaneously eliminating their inherent deficiencies. For instance, while turbidimetric methods by themselves, perform well for narrow PSDs; broad distributions have been difficult to analyze. The ability of CHDF to fractionate broad samples into narrower ones before resorting to turbidimetric analysis, helps overcome this deficiency. Moreover, it does not rely on a calibrated system thereby making the information absolute in nature. These new developments were tested to examine if the sensor is independent of variations in the three manipulated parameters (alluded to in the previous paragraph) that have been shown to have a tremendous influence on the calibration curve. The results of these tests are summarized below, details on which can be found elsewhere [8,9]. The particle size calculation procedure was found to be independent of changes in the eluant velocity which becomes an important factor when considering polymerization process monitoring. Frequently, during the course of a reaction, the dynamics of evolution of particles may require a faster monitoring policy. This demand can be met by operating the CHDF at the higher velocity, thereby allowing a quicker response by the sensor. Moreover, the absolute calculation


269 procedure eliminates the need to re-calibrate the system at this higher velocity. Similarly, the procedure was found to be independent of eluant composition variations. Again in a process monitoring situation, components such as initiators, monomers, chain transfer agents and in particular buffer electrolytes can influence the ionic composition of the eluant. The absolute procedure eliminates the tedious process of having to accurately monitor the eluant ionic strength at all times during the course of the polymerization process. Finally, the independence of the absolute technique to the detrimental effects of sample concentration eliminates the need to establish an accurate and cumbersome dilution procedure to ensure that the sample concentration being injected in the column is constant at all times.


Experimental Recipe Considerations and Materials. Styrene was chosen as the model system for study. Polymerizations were performed with a seeded system where the reactor is furnished with a previously prepared latex referred to as the seed latex. The average number of particles introduced through the seed latex is determined from its average particle size. Monomer and initiator are added to this seed latex allowing the particles to grow to a larger size. Details on the exact proportions of the various components used in the recipe are included in the sections describing the individual experiments. The seeded polymerizations were conducted in semi-batch mode where the monomer was fed continuously throughout the course of the reaction. The monomer feed rates were designed to ensure monomer starved conditions throughout the course of the reaction which was accomplished by maintaining the monomer feed rate lower than the maximum rate of polymerization. Unimodal particle growth is ensured by limiting the free emulsifier concentration to a value below the critical micelle concentration while simultaneously maintaining a sufficient concentration of emulsifier to prevent coagulation. The batch times for the growth stage of the experiments are estimated from the monomer feed policies employed. The reaction is stopped at the end of this batch time and the system is shut down. Styrene was distilled twice under reduced nitrogen pressure. The distilled monomer was stored at - 2 °C and was used no more than three weeks after distillation and storage. Sodium lauryl sulfate used as the surfactant (SLS - Aldrich Chemicals) and sodium bicarbonate used as the buffer (ACS certified grade) were used as received. Potassium persulfate was recrystallized from distilled deionized water and then dried at room temperature. The water used was distilled and deionized (DDI). Reactor Configuration. Polymerizations were carried out in a 1000 ml glass flask. The schematic of this laboratory scale reactor and associated components are presented in Figure 1. The reactor is equipped with baffles and an agitator to ensure proper homogeneous mixture of the reactants in the vessel. The glass reactor is jacketed for heating which is accomplished by recirculating hot water through the jacket using an immersion circulator. An internal cooling coil is provided to maintain the temperature constant. A gear pump is used to circulate cold water through the cooling loop. The system is equipped with a nitrogen line for blanketing and oxygen removal for start-up. The same line is used as the inlet port for initiator injection. Two stainless-steel lines are used for monomer addition. The monomer feed rates can



270

Figure 1: Diagram of the reactor and associated apparatus: (1) jacketed glass reactor; (2) N supply; (3) injection port; (4) reflux condenser; (5) cold trap; (6) double impeller agitator; (7) baffles; (8) emulsifying charge inlet; (9) catalyst addition funnel; (10) catalyst addition pump; (11) jacket water heater and circulator; (12) internal cooling coil; (13) cooling water pump; (14) water buffer and overflow; (15) jacketed glass emulsifying tank; (16) impeller agitator; (17) cold water supply; (18) emulsifier addition pump. 2


271

be manipulated very accurately using FMI fluid metering pumps that are designed to deliver very precise amounts of liquid at low flow rates. Thermocouples are introduced at three different spatial locations in the reactor; two to measure the liquid phase temperature and one for the gas phase temperature. Data acquisition and signal conditioning tasks are performed through a control interface using the / / M A C communications system.


Monitoring Scheme. The monitoring scheme involves sampling at regular intervals during the course of the reaction. The amount of sample withdrawn is a negligible fraction of the reactor volume and hence does not interfere with the progress of the polymerization. The sample withdrawn is split into two parts; one part is used for the gravimetric analysis and the other is used for the particle size analysis. Gravimetric Analysis. The samples for the gravimetric analysis are quenched using a 1% solution of hydroquinone as the short-stop. The samples are then dried in an oven at 60 C following which the final weight of the samples is recorded. The overall conversion is calculated using the following formula: _ Conversion =

^polymer ^seed

+

^monomer

(1)

The instantaneous conversion at any point in time is calculated by replacing the total monomer added W in equation (1) by the amount of monomer fed up to that particular instant in time. A coded algorithm developed to calculate the overall and instantaneous conversions at each sampling point is compiled to accurately account for the fractional changes in the concentration of the components in the reactor due to intermittent sampling. m

o

n

o

m

e

r

Particle Size Analysis. Particle size analyses are performed using both the absolute particle sizing methodology discussed earlier and the conventional calibration based method. The fraction of the total sample reserved for the particle size analysis is first diluted to approximately 2% (weight) using DDI water. This dilution process in itself acts as a quenching step and freezes the reaction thereby eliminating the need for the addition of a short-stop. Addition of hydroquinone as a short-stop would interfere with the light scattering results because it is a strong U V absorber. The CHDF is operated at 4000 psi with an average eluant velocity of 2.0 cm/sec and a residence time of 185 sec. in the capillary. The ability to achieve the fractionation and obtain the particle size analysis results within 7 - 1 0 min. allows enough time for analyses to be performed twice in between any two successive samplings from the reactor which are typically 20 min. apart. In this fashion the monitoring scheme essentially mimics the ideal case of implementing an on-line monitoring and analysis system. The sections below discuss two case studies identified to demonstrate the ability of the above-mentioned sensor to monitor the growth of particles in a seeded emulsion polymerization process. The first case study demonstrates the validity of the sensor to monitor changes in the distributions and the average diameter during


272 unimodal particle growth. The second case study evaluates the response of the sensor to a change in a manipulated input variable; namely, monomer feed rate. Monitoring Unimodal Particle Growth.


The following paragraphs report experimental approaches to monitor the growth of a previously prepared polystyrene seed latex with a measured diameter of 92 nm (details of the analysis presented in the next section). This seed latex, denoted Dow LS-1030-B, was cleaned using the standard serum replacement cell which ensures the removal of surface active agents adsorbed onto the surface of the particles. This seeded polymerization was performed in the semi-batch mode at 50°C. The exact proportions of the various components used in the recipe are shown in the Table I. Table I. Recipe used for the semi-batch polymerization run SB01 at 50°C. Ingredients

SB01

Dow latex Seed: LS-1039-B (92 nm) Styrene (feed)

20 gm. 24.01 gm. 1.00 gm.

Emulsifier: (Sodium Lauryl Sulfate) Initiator: K S 0

8

1.0061 gm.

Buffer: N a H C 0

3

1.0005 gm.

2

DDI water

2

480.00 gm.

Particle Size Analysis of Seed. Figure 2 shows a comprehensive view of the basis for particle size analysis using the turbidity ratio approach for the seed latex used in this experiment. The column of graphs on the left hand side show the fractograms measured at two wavelengths (indicated on top of each graph, in nm) and the colum of graphs on the right hand side represent the intensity ratios taken with respect to the corresponding wavelengths. The solid lines in the graphs on the right hand side are the experimental values for the turbidity ratio and the dashed lines are the average of the turbidity ratio; the average taken across 80% of the fractogram. These average values for the turbidity ratio are now compared with the theoretically calculated value in order to ascertain the average particle size. The average particle size of this seed latex is calculated to be 92 nm. The figure shows four independent turbidity ratios primarily computed in the absorption range. Among the four ratios it is evident that 'recomputed at 220:254 yields the most stable value and is consequently chosen as the preferred x to perform the particle size analysis during the monitoring scheme. r

Profile of Reaction. Figure 3 shows the profiles of some of the key variables monitored during the semi-batch polymerization process. The plots are experimental values obtained using the gravimetric analysis discussed earlier. Figure 3 a shows three variables: (i) dashed line showing the cumulative increase of the monomer (fed at a constant rate of 0.16 g/min for a total feed time of 145 min.); (ii) dotted line showing the instantaneous amount of polymer formed; and (iii) the solid line showing



273

220:254

220:254

150

200

100

150

200

Elution time (sec.)

Elution time (sec.)

190:220

190:220 1.5

150 Elution time (sec.)

200

100

150

200

Elution time (sec.)

Figure 2: Demonstration of the application of the Mie theory for the particle size analysis of a monodisperse polystyrene standard (average particle size = 92 nm) at wavelengths of 190:254, 200:254, 220:254, and 190:220 nm. Left-hand column: optical density (intensity) versus elution time; right-hand column: turbidity ratio versus elution time.



274 the difference between (i) and (ii) which translates to the amount of residual monomer at any instant of time. Figure (3b) plots the amount of polymer produced during the reaction. Three distinct regimes are evident from this %solids profile: (a) regime 1 with a minimal slope extending up to 45 min. of the reaction time; (b) regime 2 corresponding to the maximum rate of polymer formation, ranging from 45 min. to about 145 min.; and (c) regime 3 which takes effect when the monomer feed is exhausted (145 min.), corresponds to the final stages of the reaction. Figure 4 shows the measured overall conversion curve, using Equation 1, for reaction SB01 (asterisks). As expected, the overall conversion curve also reflects the description of the three regimes identified above. The instantaneous conversion curve shown by the triangles in Figure 4 decreases initially and then attains a plateau value around 75%. This indicates that some fraction of the monomer exists as a finite amount of residual monomer within the growing polymer particles and another monomer fraction is lost as fugitive emissions. In other words, the monomer fed to the reactor is not completely and instantaneously. Hence although the theoretical calculation for the monomer feed rate was originally designed to ensure completely starved conditions in the reactor; in reality there always exists some finite residual monomer in the system. Particle Size Distribution Development (PSD). The evolution of the size distribution during the course of the polymerization is discussed in detail in the following paragraphs. Figures 5-8 plot the particle size distributions (by weight) during the course of the reaction described above at various sampling periods. Details on the calculation of the distributions can be found elsewhere [10]. The analysis of the evolution of PSD begins with the first stage of the reaction comprising the first 50 minutes (described earlier as regime 1) which corresponds to the lowest rate of polymerization. Figure 5 shows the weight distributions corresponding to the seed (solid line) and those for samples taken at 25 minutes (dashed) and 50 minutes (dotted); all of which are plotted against a diameter axis. As dictated by the progress of the polymerization, the distributions show a gradual increase towards larger diameters. An interesting point is evident upon a closer look at Figure 5. The shift in the distribution from the seed stage (solid curve) to the 25 min. sample (dashed curve) appears to be more drastic than the corresponding growth from the 25 min. sample (dashed curve) to the 50 min. sample (dotted curve). This suggests that the rate at which particle diameter grows or the rate of polymerization is faster in the first 25 min. of the reaction compared to the next 25 minutes. However, the overall conversion curve, Figure 4, reflects a situation quite to the contrary; i.e. the conversion in the first 25 min. is seen to be negligible and significant conversion of monomer into polymer is observed only following the first 25 min. of reaction. This apparent paradox can be explained by the fact that during the first 25 minutes of the reaction the monomer that is fed into the reactor primarily gets distributed throughout the reaction volume and is absorbed by the seed particles resulting in monomer swollen polymer particles. In the case of styrene, the density of the monomer (0.906 gm/cc) is significantly lower than the density of its polymer (1.11 gm/cc). Therefore by the conservation of mass, the volume of a given mass of a monomer droplet is larger than its equivalent polymer particle. Along the same lines, the volume of a polymer particle swollen by a certain amount of styrene monomer is larger than its equivalent fully converted polymer particle. Since, physically the first


275 SB01 polymerization - Figure 2(a)

100 Time (min.)


Solids profile - Figure 2(b)

100 Time (min.)

Figure 3: Semi-batch polymerization data for the reaction denoted SB01: (a) monomer feed rate (dashed line), polymer formation rate (dotted line), and residual monomer amount (solid line); (b) solids content profile during the reaction.

100 150 Reaction Time (min.)

250

Figure 4: Overall and instantaneous conversion profiles for the reaction denoted SB01.


276 seed - 25 - 50 min.

-a 3 Xi


'B

60

80

100

180

120

Diameter (nm)

Figure 5: Particle size weight distributions corresponding to the first regime: seed latex particles (solid line), 25 minutes reaction time (dashed line), and 50 minutes reaction time (dotted line).

90-110- 130 min.

80

100

120

140

160

180

Diameter (nm)

Figure 6: Particle size weight distributions corresponding to the intermediate regime: 90 minutes (solid line), 110 minutes (dashed line), and 130 minutes reaction time (dotted line).



277

seed-50- 110-210 min.

Diameter (nm)

Figure 8: Particle size weight distributions during the reaction denoted SB01: seed latex particles (solid line); 50 minutes (dashed line), 110 minutes (dotted line), and 210 minutes reaction time (dot-dashed line).



278 25 min. essentially represent the swelling stage followed by the actual conversion of monomer, the corresponding size distribution during the first 25 min. appears to have an exaggerated diameter followed by a more steady growth pattern in the following 25 min. of the reaction. This effect is particularly prominent in monomer-polymer systems where the density difference between a monomer and its corresponding polymer is significant, which is indeed the case for styrene. It should be noted here that the onset of significant conversion of the monomer does not necessarily imply complete disappearance of monomer swollen polymer particles. During seeded emulsion polymerization, in-situ growth of particles occurs continuously as long as the monomer feed is sustained and is the only mode of propagation of the polymerization reaction. Hence unless the monomer that is being fed is instantaneously reacted, the seed particles can always be expected to exhibit a certain degree of swelling. Figure 6 shows the development of the size distributions corresponding to regime 2 which represents the highest rate of polymerization. Sampling times of 90 min. (solid), 110 min. (dashed) and 130 min. (dotted) are chosen to represent this interval of interest. During this period the monomer swollen particles are polymerized resulting in the overall growth of the polymer particles to newer sizes. The evolution of the larger sized particles is evident from the distribution curves shown in this figure. The final stage of the polymerization is characterized in Figure 7, which plots the distributions corresponding to 150 min. (solid), 170 min. (dashed) and 195 min. (dotted). The monomer being fed to sustain particle growth is exhausted at 145 min. and therefore in this final stage of the reaction the last residual monomer in the system gets converted into polymer. The shape of the distributions in this regime, therefore, do not exhibit any dramatic changes with the exception of a gradual refinement of the distribution to its final form at 195 min. The summary of the characterization of the reaction with respect to the particle size distribution development is illustrated in the distributions in Figure 8. The four distributions plotted are representative of the various stages monitored during particle growth. Data validation using measured average diameter. The description of the development of particle growth can be further analyzed by plotting the average diameters as a function of reaction time as shown in Figure 9. The points plotted on the graph are experimentally measured values; the weight average diameter (asterisks) are expectedly larger than the number average diameters (triangles). The solid curves (A and B) are the calculated values of the average diameter; computed using stoichiometric considerations. The experimentally measured points show a monotonic increase in particle diameter over the course of the reaction. The top solid curve (A) is the stoichiometrically calculated diameter assuming that all the monomer fed is instantaneously reacted and since this is known not to be the case, the calculated diameter overpredicts the measured diameter. The solid curve (B), on the other hand, is calculated using the instantaneous conversion data and appropriately accounts for the fraction of monomer that is not instantaneously converted. This curve as expected shows better agreement with the experimental data. The calculated diameter is the volume average diameter and therefore lies between the measured number and weight average diameters. Additionally, the diameter calculated is the dry particle diameter and does not take into effect the swollen state of the particles.



279

( i

0

1

50

1

1


1

1

200

250

Figure 9: Particle growth profiles based on the number and weight average sizes: experimental data (symbols); calculated diameters (solid lines).


280


This explains the fact that the calculated diameter (B) slightly underpredicts the weight average diameter. In order to ensure that the particle growth monitored is indeed unimodal and is devoid of newly generated particle sizes, the Polydispersity Index (PDI) is calculated from the average diameter data. The PDI is the ratio of the weight to number average particle size and is indicative of the extent of polydispersity of the distributions. Values closer to 1 imply more monodisperse distributions. We found that throughout the reaction, the PDI was esentially constant with a value of 1.05. This demonstrates that the growth patterns monitored were indeed unimodal without generation of new particles and/or multimodal species. The diameters were also measured using an alternate sensor; i.e. NICOMP (a dynamic light scattering instrument). Table II shows the measured and calculated values for the average Table EL Measured and theoretical final weight average diameters. Method

Initial Diameter (nm)

Final Diameter (nm)

Theoretical Dia. (nm)

% error

CHDF

87.2

109.3

109.8

-0.46

NICOMP

90.3

112.0

113.7

-1.50

diameter. The first two columns compare the measured initial and final diameters using the CHDF and the NICOMP submicron analyzer. The third column shows stoichiometric calculations for the final theoretical diameter assuming the starting diameter respectively measured by each technique. The error between the theoretical and calculated final diameters is found to be minimal. It should be noted here that the particle size analysis using the NICOMP took 30 min. (approx.) before a stable value was obtained and hence could not be used as a valid tool to monitor the growth of the seed particles during the course of the polymerization. Effect of Monomer Feed Rate. Having established the feasibility of the technique to monitor unimodal particle growth, the next course of action is to test the sensitivity of the measurements to variations in manipulated input variables. This paragraph focuses on studying and demonstrating the ability to monitor the effect of a change in the monomer feed rate. Accordingly, the seed (SD01) is first synthesized in batch mode. The recipe followed for this stage of the reaction is shown in Table III. The final diameter of the seed latex was 76 nm. The seed latex SD01 was then apportioned equally and used in two separate semi-batch polymerizations, operated at two different monomer feed rates. The recipe used for these two polymerizations, termed SB03 and SB04, are shown in Table IV. It is evident from the table that the two recipes have nearly identical proportions of the various components. This is done with the purpose of ensuring that the only difference in the two recipes is the monomer feed rates. Table IV also shows the differences in the operating conditions between SB03 and SB04. Figure 10 shows the cumulative profile of the amount of monomer fed to the reactor at any instant of time. The feed rate policy for SB04 is


281 Table DDL Recipe used for the semi-batch polymerization run SD01 at 50°C. Ingredients

SB01

Styrene (feed) Emulsifier: (Sodium Lauryl Sulfate) Initiator: K S 0 Buffer: N a H C 0 DDI water 2

2

20.14 gm. 2.004 gm. 1.003 gm. 1.003 gm. 500.0 gm.

8

3


Table IV. Recipe used for the semi-batch polymerization run SB03 and SB04 at 50°C. Ingredients and Operating Conditions

SB03

SB04

Seed latex: SD01 (76 nm) Styrene (feed) Emulsifier:C H S 0 Na Initiator: K S 0 Buffer: N a H C 0 DDI water Feed rate Feed time (min) Reaction time (min)

200.00 gm. 21.13 gm. 0.5002 gm. 1.013 gm. 1.0095 gm. 308.03 gm. 0.20 (gm/min) 110 min. 210 min.

200.00 gm. 21.14 gm. 0.5042 gm. 1.007 gm. 1.0098 gm. 308.00 gm. 0.10 (gm/min) 195 min. 250 min.

12

2

2 5

2

4

8

3

half that of SB03. Consequently the instantaneous monomer concentration at any time in the reactor is higher for SB03 and hence the monomer concentration per polymer particle is also higher. In emulsion polymerization the rate of polymerization is given by:

k NftM] p

p

where kp is the propagation rate constant, N is the number of particles, n is the average number of radicals per particle, [M] is the monomer concentration in the polymer particle and Avogadro number. It is clear from this equation that the rate of polymerization is directly proportional to the monomer concentration in the polymer particles. Therefore, the rate of reaction for SB03 should be higher than SB04. Figure 11 plots the overall conversion history from the gravimetric analysis for the two cases. The slope of the overall conversion curve yields the rate of polymerization. Since the slope of the curve for SB03 (asterisks) is higher than SB04 (triangles), it can be concluded that the experiment is consistent with the arguments stated above. Figure 12 shows the development of the average diameter as a function of reaction time. The faster monomer feed rate is represented by asterisks and the s

p

l s m

e



282

300 Reaction time (min.)

Figure 11: SB04.

Overall conversion history for the reactions denoted SB03 and

10O 90807060 504030^

0.2 gm/min

20-

• 0.1 gm/min

10O

50


200

250

Figure 10: Monomer feed profiles (cumulative) for the reactions denoted SB03 (monomer feed rate = 0.20 g/min) and SB04 (monomer feed rate = 0.10 g/min).


283 slower feed rate by triangles. As mentioned in the previous paragraph, when the monomer feed rate is increased, at any given instant of time the concentration of the monomer in the reactor is higher than the slower monomer feed rate. Therefore the rate of polymerization will be higher and hence the seed particles will grow more rapidly at faster monomer feed rates. The growth of the polymer particles is also directly proportional to the monomer concentration in the polymer particles. When a slower monomer feed rate is employed (SB04), the average diameter monitored yields a profile (triangles in Figure 12) that is lower than that obtained for SB03. The sensor therefore accurately monitors the effect on the particle growth profile due to the change in the monomer feed rate.


100

95-

ML

E

• •

i_

CD CD E b C D D)

I

* j •

• j * ML

*

* 0.2 gm/min

A

•

80"

"'JL'[

0.1 gm/min 75-

I 50

i

I

100 150 Reaction Time (min)

200

250

Figure 12: Comparison of the average diameter growth profiles for two semibatch emulsion polymerizations (reactions denoted SB03 and SB04).

Literature Cited 1. Kourti, T. and Macgregor J. F. In Particle Size Distributions II: Assessment and Characterization; Provder, T. Ed.; ACS Symposium Series No. 472; American Chemical Society: Washington, DC, 1991; pp 34-63. 2. Nicoli, D. F., Kourti, T., Wu, J. S., Chang, Y. J., and MacGregor, J. F. In Particle Size Distributions II: Assessment and Characterization; Provder, T. Ed.; ACS Symposium Series No. 472; American Chemical Society: Washington, DC, 1991; pp 86-97. 3. Husain, A., J. Vlachopolous and A. E. Hamielec, J. of Liquid Chromatography, 1979, 2, 517 4. Thomas, J. C. In Particle Size Distributions II: Assessment and Characterization; Provder, T. Ed.; ACS Symposium Series No. 472; American Chemical Society: Washington, DC, 1991; pp 98-105.


284


5. Thomas, J. C. and Dimonie, V. Applied Optics, 1990, 36, 5332. 6. Van Gilder, R. L. and Langhorst, M. A. In Particle Size Distributions: Assessment andCharacterization;Provder, T. Ed.; ACS Symposium Series No. 464; American Chemical Society: Washington, DC, 1987; pp 272-292. 7. Silebi, C. A.; DosRamos, J. G. J. Colloid and Interface Sci., 1989, 130, 14. 8. DosRamos, J. G.; Silebi, C. A. J. Colloid and Interface Sci., 1990, 135, 165. 9. Venkatesan, J.; DosRamos, J. G.; Silebi, C. A. In Particle Size Distributions II: Assessment and Characterization; Provder, T. Ed.; ACS Symposium Series No. 472; American Chemical Society. Washington, DC, 1991; pp279-291. 10. Venkatesan, J., Particle Size Sensor Design and Application, Ph.D. Dissertation, Lehigh University, 1992.


Particle Size Characterization During Emulsion Polymerization

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