Shell Latexes in a Continuous Stirred Tank Reactor

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Synthesis of Core/Shell Latexes in a Continuous Stirred Tank Reactor Cyril Landier,†,§ Marı´a J. Barandiaran,† Xavier Drujon,‡ and Jose´ M. Asua*,† Institute of Polymer Materials “POLYMAT” and Grupo de Ingenierı´a Quı´mica, Departamento de Quı´mica Aplicada, Facultad de Ciencias Quı´micas, Universidad del Paı´s Vasco/Euskal Herriko Unibertsitatea, Apdo 1072, 20080, Donostia-San Sebastia´ n, Spain, and Atofina, Building Applications-Additives Division, CECA, Immeuble IRIS, La Defense 2, F-92062 Paris La Defense CEDEX, France

The feasibility of obtaining core/shell (poly(butyl acrylate-butadiene)/poly(methyl methacrylate)) latexes in a continuous stirred tank reactor was investigated. The effect of mean residence time, premixing of the reactants, nitrogen purge, core/shell ratio, and initiator type and concentration on the kinetics was studied. It was observed that neither the nitrogen purge nor the premixing significantly affected monomer conversion. On the other hand, initiators giving hydrophobic radicals are highly efficient at polymerizing the monomer. Furthermore, the grafting efficiency of the shell polymer on the core was analyzed, and higher grafting was found when the shell proportion increased. The morphology and the performance as impact modifiers of these core/ shell latexes were compared with those of conventional products synthesized by a semibatch process. Similar coverage level and comparable properties to the impact performance were obtained. Introduction Poly(butyl acrylate)/poly(methyl methacrylate) (pBA/ pMMA) core/shell latexes are widely used as impact modifiers of PVC. To minimize strains, the core has to be made of a highly elastomeric material, such as poly(butyl acrylate) (pBA) or poly(butyl acrylate-butadiene) (p(BA-BD)). The role of the pMMA shell is to ensure compatibility with the PVC matrix.1 Traditionally, the latexes have been obtained in a two-stage batch/semibatch emulsion polymerization process where the elastomeric seed is first prepared and then followed by the pMMA polymerization. The application properties are dependent, among others, on the coverage level and homogeneity of the shell. With increasing pMMA shell coverage, the latexes evolve from elastomeric toward more stiff and rigid particles. Batch and semibatch processes are not attractive from the economic point of view when high production rates are desired. Continuous reactors offer the economical advantage derived from high production rates and, in some cases, less capital investment and operating costs. Furthermore, uniform product quality is obtained, because batch-to-batch variations are avoided. One of the main drawbacks of the continuous stirred tank reactor (CSTR) is the broad residence time distribution, which can produce core/shell particles with different coverage levels, which in turn yields different application properties. Nevertheless, it is worth investigating this process. The main objective of this work is to investigate the feasibility of synthesizing core/shell (p(BA-BD)/pMMA) latexes useful as impact modifiers in a CSTR. The work is organized as follows. First, the influence of several operational variables on the kinetics of the MMA conversion in the CSTR seeded polymerization is pre* To whom correspondence should be addressed. Tel.: +34943-018181. Fax: +34-943-212236. E-mail: [email protected]. † Universidad del Paı´s Vasco/Euskal Herriko Unibertsitatea. ‡ Atofina, Building Applications-Additives Division, CECA. § Present address: Clariant LSM, Zone Industrielle de Laville, 47240 Bon-Encontre, France.

sented. Second, the grafting efficiency of the shell polymer is studied. Then, the effect of the type of process (continuous vs semibatch) on the morphology of the latexes is investigated. Finally, the impact performance of those core/shell impact modifier latexes made using the continuous process is compared with the impact performance of latexes produced through a conventional semibatch process. Experimental Section The continuous reactions were carried out at 80 °C in a 0.48-L glass unbaffled reactor equipped with a mechanical stirrer (six-blade turbine, 200 rpm), a sampling device, nitrogen inlet, feed inlet, and exit tubes. Before starting the process, the reactor was completely filled with the seed (core polymer) and mixed with a solution of reducing component of initiator. The process was started by feeding the reactants into the reactor through a 1/4-in. stainless steel tube located near the turbine. The reactants were fed in three streams: the first was pure MMA, the second was the seed (prepared with sodium lauryl sulfate) mixed with the solution of reducing component of initiator, and the last was an aqueous solution of oxidizing component of initiator. The flow rates of these streams were controlled by means of weight-based flow controllers. The products left the reactor through a 1/4in. stainless steel tube located at the top of the reactor. There was no headspace in the reactor, which ensured a constant volume. The temperature of the reactor was controlled by means of cascade control. The thermal fluid was pumped through the reactor jacket. An electrical resistance was placed in the circuit of the thermal fluid. Additionally, cooling was provided by injecting cool water in the circuit by means of an electrovalve. The streams containing the seed and the monomer were premixed in a 10-mL magnetically stirred flask before their introduction into the reactor. The aim was to facilitate the transport of MMA to the seed particles. In addition, this acted as a filter for the small amounts of coagulum that sometimes formed from the seed latex, because of the shear caused by the feeding pump.

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Samples were withdrawn during the run and the polymerization was short-stopped with hydroquinone. The MMA conversion was determined gravimetrically. The particle size was measured by dynamic light scattering (Coulter N4 Plus). The extent of grafting was determined as follows. The core/shell latex was coagulated with a solution of CaCl2. A 2-wt-% solution of this polymer in methyl ethyl ketone (MEK) was prepared by ultrasonification during 5 min and further dissolution for 24 h under mechanical agitation. This solution was centrifuged at 15 000 rpm during 3.5 h to separate the grafted polymer which precipitated from the free polymer which remained in solution. The solids content of the upper phase (free polymer in MEK solution) was determined and the percentage of free polymer was estimated. The free polymer was actually a mixture of nongrafted core (BA/ BD) oligomers and free pMMA. The evolution of the mean particle size (Figure 4) showed that the free polymer did not correspond to new pMMA particles. This was double checked by transmission electron microscopy for some samples. The morphological characteristic of interest in the frame of this study is the extent of the coverage of the core polymer by the pMMA shell. In this work, the coverage was estimated by determining the area covered by one molecule of emulsifier (as) by using the Maron adsorption method.2,3 The as was first determined for both core polymer (seed) and shell polymer (latex of pMMA) and later for core/shell particles of different compositions. Those latexes were obtained in seeded semibatch polymerization carried out under monomerstarved conversions. Assuming that as of a core/shell latex is a linear combination of the as of the uncovered core and the as of the shell, the coverage level of core/ shell particles can be estimated. The parameter as is experimentally determined by using the Maron adsorption method,2 by measuring surface tension with a KSV Sigma 70 tensiometer equipped with a DuNouy ring. Clean latexes, namely latexes which are totally or partially free of surfactant (sodium lauryl sulfate), were titrated with the sodium lauryl sulfate surfactant.When the amount of surfactant was high enough, [S]*, the latex particles were saturated and the micellation point reached. This point corresponded to a break in the slope of the surface tension vs log [surfactant] curve. The value of as was calculated from the following expression:

as )

Np ap NA [S]* + [S]1 - CMC

(1)

where ap is the surface area of one latex particle (cm2/ particle), Np is the particle number (particle/cm3), NA is Avogadro’s number, [S]* is the surfactant concentration added during the titration to reach the saturation point of the particles (mol/cm3), [S]1 is the surfactant concentration in the latex after the cleaning ([S]1 ) 0 for total cleaning), and CMC is the critical micellar concentration of emulsifier (mol/cm3). To improve the accuracy in the determination of as, eq 1 was rearranged as follows:

[S]* + [S]1 ) CMC +

ap Np as NA

(2)

Figure 1. Determination of as. Table 1. Characteristics of the Seeds

seeds

solids content (%)

particle diameter (nm)

residual monomer (%)

BA/Bd copolymer (90.5/9.5) A B

32.47 38.03

81.7 79.0

0.4 1.5

and samples of different particle number concentration were titrated. The value of as was calculated from the slope of ([S]* + [S]1 - CMC) vs Np. An example is presented in Figure 1. The serum replacement, which allows a relatively fast and total cleaning of the latexes, was used to clean latexes with high Tg, i.e., the pMMA latex and the core/ shell latexes with high proportion of MMA. Dialysis was used to clean latexes with low Tg (seed particles and core/shell particles with a low MMA content). To save time, some latexes were partially cleaned. In this case, it was necessary to evaluate the concentration of surfactant that remained in the latex. This concentration was obtained by subtracting the concentration of surfactant removed from the initial latex from the initial concentration of surfactant. The amount of surfactant removed from the initial latex corresponds to the amount of surfactant in the serum and was evaluated by a biphasic titration of the anionic surfactant.4 Two p(BA-BD) seeds obtained in different batches, differing in both the solids content and amount of residual monomer, were used. The characteristics of the seeds are summarized in Table 1. The B seed was diluted at the same solids content as the A seed but with different residual monomer levels, which provides slightly different core/shell weight ratios of the particles, i.e., 80.2/19.8 (with A seed) and 80.4/19.6 (with B seed). Table 2 summarizes the polymerizations carried out in the CSTR. The effect of mean residence time, premixing of the reactants, nitrogen purge, core/shell ratio, and initiator type and concentration on the monomer conversion was studied. Table 3 summarizes the formulation and procedure for the synthesis of the copolymer latexes obtained in semibatch. Technical grade methyl methacrylate was used as received. Potassium persulfate (KPS), ammonium persulfate (APS), sodium bisulfite (SBS), tert-butyl hydroperoxide (TBHP), and sodium formaldehide sulfoxilate (SFS) were also used as received. Deionized water was used throughout the work.

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Table 2. Summary of the Characteristics of the Reactions Carried Out (Solids Content 36 wt %)

run

τ (min)

C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13

19.3 29.0 29.0 29.0 29.0 29.0 29.0 29.0 29.0 29.0 29.0 29.0 29.0

a

seed

core/shell (weight ratio)

initiator (ox/red)

ox/MMA (molar ratio)

red/MMA (molar ratio)

A A A A A A A B A A A B A

80.2/19.8 80.2/19.8 80.2/19.8 80.2/19.8 80.2/19.8 80.2/19.8 80.2/19.8 80.4/19.6 80.2/19.8 80.2/19.8 80.2/19.8 69.8/30.2 69.6/30.4

KPS/SBS KPS/SBS KPS/SBS KPS/SBS KPS/SBS KPS/SBS KPS/SBS KPS/SBS APS/SBS APS/SBS TBHP/SFS KPS/SBS TBHP/SFS

4.89 10-4 4.89 10-4 1.08 10-3 1.08 10-3 1.08 10-3 1.08 10-3 1.74 10-3 3.44 10-3 5.79 10-4 1.28 10-3 3.44 10-3 1.08 10-3 3.44 10-3

4.42 10-4 4.42 10-4 9.85 10-4 9.85 10-4 9.85 10-4 9.85 10-4 1.58 10-3 3.12 10-3 4.42 10-4 9.85 10-4 2.38 10-3 9.84 10-4 2.38 10-3

premixing

nitrogen purge

T (°C)

MMA conversiona (%)

no no no no yes yes yes yes no no yes yes yes

no no no yes yes no yes no no no no no yes

80 80 80 80 80 80 80 80 80 80 80 80 80

65.6 77.9 81.9 78.6 81.6 79.8 81.1 82.2 70.6 72.1 87.1 87.5 90.9

At the steady state.

Table 3. Summary of the Seeded Polymerization Reactions Carried out in Semibatch (Seed A, Temperature 80 °C) core/shell (weight ratio)

initiator (ox/red)

ox/MMA (molar ratio)

red/MMA (molar ratio)

addition time (min)

batch reaction time (min)

33.4/66.6 65.0/35.0 73.0/27.0 79.1/20.9 88.3/11.7 93.8/6.2 97.4/2.6

KPS/SBS KPS/SBS KPS/SBS KPS/SBS KPS/SBS KPS/SBS KPS/SBS

7.30 10-4 7.30 10-4 7.30 10-4 7.30 10-4 7.30 10-4 7.30 10-4 7.30 10-4

6.66 10-4 6.66 10-4 6.66 10-4 6.66 10-4 6.66 10-4 6.66 10-4 6.66 10-4

451.7 122.9 81 60 30 15 6

60 60 60 60 60 60 54

Figure 3. Influence of the nitrogen purge on the time evolution of MMA conversion: (b) run C3, no purge; (O) run C4, nitrogen purge. Figure 2. Effect of the mean residence time on MMA conversion: (b) run C1, τ ) 19.3 min; (O) run C2, τ ) 29 min.

Kinetic Study Effect of the Mean Residence Time. Figure 2 shows the time evolution of the fractional conversion of MMA (defined as the ratio between the polymer content in the reactor and the monomer content if there was no reaction) for runs C1 and C2. These runs were carried out at two different residence times (C1 τ ) 19.3 min and C2 τ ) 29 min), using potassium persulfate/sodium bisulfite (KPS/SBS) as initiator system. It can be seen that the longer the mean residence time, the higher the conversion of MMA. Furthermore, no oscillations in the conversion were observed at τ ) 29 min, which is an indication that secondary nucleation was negligible, and the steady state was apparently reached after about 3 residence times. On the other hand, the conversion reached in runs C1 and C2 was too low. Additional increments of the monomer conversion could be obtained by increasing the residence time, but this would result in a loss of the productivity of the reactor. Therefore, a τ ) 29 min was retained for the rest of the work and

other variables were changed in an attempt to increase the MMA conversion. Influence of the Nitrogen Purge. In a continuous reactor, the inhibitor, which is continuously fed into the reactor, causes a decrease of the polymerization rate.5 In practice, the inhibitor cannot be economically removed from the feedstock. On the other hand, the effect of some inhibitors is enhanced by the presence of O2 (which is an inhibitor itself). Therefore, a reduction of the oxygen content might be beneficial to the monomer conversion. In a continuous reactor, the main sources of oxygen contamination are the feed tanks. Figure 3 compares the evolution of monomer conversion in run C3, in which no nitrogen purge was used, with that of run C4, in which the feed tanks were purged with nitrogen during the whole process. It can be seen that monomer conversion at the stationary state was almost not affected by the nitrogen purge. In this process, secondary nucleation should be avoided because this is a waste of pMMA, which results in a poor coverage of the p(BA-BD) core. Figure 4 shows that the particle diameter was equal to that calculated assuming that

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Figure 4. Influence of the nitrogen purge on the time evolution of mean particle size: (b) run C3, no purge; (O) run C4, nitrogen purge; (______) theoretical particle diameter.

Figure 5. Effect of premixing on the MMA conversion: (b) run C4, no premixing; (O) run C5, premixing.

no secondary nucleation occurred. This means that no new particles were formed. Effect of Premixing Seed and Monomer. A possible reason for a low monomer conversion is that the monomer suffered from diffusional limitations. Monomer mass transport can be enhanced by premixing the monomer and the seed. Figure 5 shows that this has no effect on the conversion of the shell monomer. Nevertheless, it has been observed that the small premixing flask acted as a filter for the little coagula formed in the seed latex stream due to the shear of the pump. Influence of the Initiator System. Figure 6 presents the effect of the initiator concentration on monomer conversion for the potassium persulfate/sodium bisulfite (KPS/SBS) redox system. It can be seen that, with the exception of the run C2 with the lowest concentration of initiator, this variable had no significant increase in the monomer conversion. A possible explanation of these results can be found by analyzing the value of the average number of radicals per particle (n˜ ). The expected dependence of the polymerization rate on the initiator concentration ranges from zero order for Smith-Ewart case 2 (n˜ ) 0.5) kinetics to a 0.5 order for Smith-Ewart6 cases 1 (n˜ < 0.5) and 3 (n˜ > 1). The values of n˜ at the steady state (kp ) 1318 L mol-1 s-1)7 were close to 0.5, except for run C2 (n˜ C2 ) 0.32, n˜ C5 ) 0.40, n˜ C7 ) 0.39, n˜ C8 ) 0.39). This suggests that the system was almost a Smith-Ewart case 2, where there is no effect of the initiator concentration on the polymerization rate. However, when analyzing these results

Figure 6. Influence of the initiator concentration on the time evolution of MMA conversion for the KPS/SBS system: (b) run C2, [KPS/SBS] ) 4.89 10-4/4.42 10-4; (O) run C5, [KPS/SBS] ) 1.08 10-3/9.58 10-4; (4) run C7, [KPS/SBS] ) 1.74 10-3/1.58 10-3; (2) run C8, [KPS/SBS] ) 3.44 10-3/3.12 10-3.

Figure 7. Influence of the initiator type (KPS/SBS vs APS/SBS) on the time evolution of MMA conversion: empty symbols (KPS/ SBS); filled symbols (APS/SBS); (b) run C9; (O) run C2; (4) run C3; (2) run C10.

one should take into account that for an 80% conversion of MMA the amount of unreacted MMA represented only 4% of the organic phase. This means that the apparent conversion was 96%, and it is hard to polymerize the last few percents of monomer. In some cases, the use of ammonium persulfate is preferred, because of its higher water solubility which allows feeding of more concentrated initiator aqueous solution, avoiding the solids content reduction. Figure 7 compares the efficiency in the polymerization of MMA of the potassium persulfate/sodium bisulfite (KPS/SBS) redox system with that of the ammonium persulfate/ sodium bisulfite (APS/SBS). It can be seen that the KPS/ SBS system was more effective. On the other hand, Figure 8 shows that the tert-butyl hydroperoxide/sodium formaldehyde (TBHP/SFS) system was more effective than the KPS/SBS in the polymerization. This could be due to the higher hydrophobicity of the radicals formed with the TBHP/SFS system, which are readily absorbed in the polymer particles.8 Effect of the Core/Shell Ratio. The residence time distribution of a CSTR may yield a significant number of particles with negligible core coverage. This effect may be counteracted in some extent by decreasing the overall core/shell ratio. Runs C12 and C13 were carried out with a core/shell weight ratio of 69.8/30.2 and using KPS/SBS and TBHP/SFS, respectively. Table 2 shows that the MMA conversion achieved in these experiments was significantly higher than those obtained in runs C5 and C11, in which the same initiator systems, but a core/ shell ratio of 80.2/19.8, were used. However, it should

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Figure 8. Influence of the initiator type (KPS/SBS vs TBHP/SFS) on the time evolution of MMA conversion: (O) TBHP/SFS (run C11), (b) KPS/SBS(run C8).

Figure 10. Effect of the temperature on the evolution of residual MMA in the storage tank: (0) 60 °C; (9) 80 °C. Table 4. Percentage of Free Polymer in Products of CSTR Reactions

Figure 9. Time evolution of residual MMA in the storage tank at room temperature (run C8).

be pointed out that the amount of unreacted monomer per total amount of polymer (including the seed) was not affected by the core/shell ratio. Thus, in the experiments initiated with KPS/SBS, the unreacted monomer was 3.6 wt % (based on the total amount of polymer) in run C5 (core/shell ) 80.2/19.6) and 3.7 wt % in run C12 (core/shell ) 69.8/30.2). On the other hand, in the experiments initiated with TBHP/SFS, the unreacted monomer was 2.5 wt % in run C11 (core/shell ) 80.2/ 19.6) and 2.7 wt % in run C13 (core/shell ) 69.6/30.4). The effect of the core/shell ratio on the core coverage is discussed below. Conversion of the Residual Monomer in the Storage Tank. The latex leaving the continuous reactors contained an excess of unreacted monomer which should be eliminated later. Some of this monomer may be converted in the storage tank reducing the problem. Therefore, a kinetic study was carried out to evaluate the time evolution of the residual MMA in the storage tank. The amount of residual monomer was determined by gas chromatography, using acetone as internal standard. The results were confirmed by gravimetry, to check that the diminution of the amount of residual monomer was not due to evaporation. Figure 9 shows the time evolution of residual MMA in the storage tank at room temperature for reaction C8. It clearly appears that the residual MMA was significantly reduced within 1 day in the storage tank at room temperature. The process may be sped up by increasing the temperature of the tank. Figure 10 shows the results of a similar kinetic study carried out by heating a sample of reaction C12 at two different temperatures, i.e., 60 and 80 °C. Less than 1 h was enough to achieve 99% conversion of the shell monomer

polymer

core/shell ratio

initiator type

% free polymer

free polymer/ shell polymer ratio

C8 C11 C13

80.4/19.6 80.4/19.6 69.6/30.4

KPS/SBS TBHP/SFS TBHP/SFS

18 16.5 19

0.90 0.82 0.63

at 80 °C. Logically, more time was necessary at 60 °C to reach the same conversion. It should be pointed out that no additional initiator was used in these postpolymerizations. Those high conversions were obtained despite the well-known effect of the glass transition temperature on the high conversion bulk polymerization of methyl methacrylate. However, the current system differs from bulk polymerization in that both a rubbery phase (the core) and an aqueous phase are present. MMA partitions between the shell and those phases, and in the core and in the aqueous phase, its polymerization is not affected by the glass effect. Consequently, conversions higher than those in bulk could be achieved. Grafting Efficiency The role of the pMMA shell is to compatibilize the core with the PVC matrix. To achieve this goal, the shell should cover the p(BA-BD) core and should remain in place during the processing of the material. This means that the shell should not be detached from the core. This can be achieved by grafting the shell on the core. As explained in the Experimental Section, the grafting efficiency was evaluated in terms of the amount of soluble polymer: the greater the amount of soluble polymer the poorer the grafting. Table 4 summarizes the results obtained with three different products. It can be seen that the initiator type had a small effect on grafting, with that for the KPS/SBS system (run C8) being slightly lower than the one for TBHP/SFS (run C11). This behavior can be related with the different hydrophobicity of the initiator systems. The more hydrophobic TBHP/SFS system produces radicals with higher hydrophobicity which will enter more readily into the interior of the particles.8 Comparing the results of C11 product with C13, it appears that a higher shell proportion yields a higher level of free polymer. Nevertheless, the ratio (% of free polymer/% of shell polymer) is higher for a 80.4/19.6 core/shell composition than for a 69.6/30.4 one, which means that more pMMA is grafted in the 69.6/30.4 polymer than in the 80.4/19.6 one.

Ind. Eng. Chem. Res., Vol. 43, No. 3, 2004 705 Table 5. Coverage Level for Core/Shell Particles Obtained in a Continuous Reactor

polymer

as (Å2/molecule)

experimental coverage level (%)

seed A seed B pMMA

56 ( 2.8 57 ( 3.4 111 ( 14

0 0 100

C3 C4 C5 C6 C8 C11 C12 C13

94 ( 2.8 94 ( 4.7 85 ( 3.4 86 ( 3.4 93 ( 2.8 89 ( 3.6 99 ( 4 100 ( 4

69 69 53 55 67 60 78 80

theoretical coverage level (%)

54 52 52 53 54 56 75 76

Table 6. Coverage Level for Core/Shell Particles of Different Compositions Obtained in a Semibatch Reactor

polymer (seed A/pMMA) 33.4/66.6 (seed A/pMMA) 65.0/35.0 (seed B/pMMA) 73.0/27.0 (seed A/pMMA) 79.1/20.9 (seed A/pMMA) 88.3/11.7 (seed A/pMMA) 93.8/6.2 (seed A/pMMA) 97.4/2.6

as (Å2/molecule)

coverage level (%)

theoretical shell thickness (nm)

114 ( 6.6

100

17.27

110 ( 3.8

98

5.97

98 ( 3.3

76

4.23

86 ( 1.9

55

3.12

78 ( 3.4

40

1.63

75 ( 2.7

35

0.82

65 ( 3.2

16

0.33

Core-Shell Morphology Table 5 summarizes the calculated as values and the coverage level for the BA/BD copolymer seeds, pMMA latex, and for the products of the continuous reactions. The coverage level of the core/shell particles was estimated by assuming that the as of the core/shell latex is a linear combination of the as of the unconvered fraction of the core and the as of the shell. Table 6 presents the as values and coverage levels of core/shell latexes of different composition obtained in seeded semibatch polymerization carried out under monomer-starved conditions. These data are easier to interpret than those in Table 5, because in a seeded semibatch system all particles have the same residence time. Table 6 shows that about 35 wt % of shell polymer was needed to completely cover the seed latex. Smaller amounts of shell polymer left a fraction of the core uncovered. Table 6 also includes the value of the shell thickness calculated assuming that all the pMMA formed a continuous shell around the core. This table shows that it is difficult to obtain a continuous shell thinner than 6 nm. Therefore, in latexes containing less than 25 wt % of pMMA, this polymer was likely forming patches on the surface of the core polymer.9 Considering the results presented in Table 6 for the semibatch polymerizations, it is not surprising that only a partial coverage of the core polymer was achieved in the runs carried out in CSTR. Table 5 shows that the coverage decreased when the monomer and the seed were premixed (compare C3 with C6, and C4 with C5). This suggests that when the monomer and the seed were not premixed, a monomer concentration profile was developed in the polymer particles due to limitations to the monomer diffusion

in the polymer particles. On the other hand, the average coverages achieved in the CSTR were similar to those obtained in the semicontinuous reactor. The decrease of the core/shell ratio to 69.8/30.2 yielded a significant increase of the average coverage (78% in run C12 vs 55% in run C6). The coverage values compared well with those obtained for the semibatch latexes. No clear effect of the hydrophobicity of the initiator system was observed (run C8 vs run C11, and run C12 vs run C13). To assess the relative importance of these factors, a mathematical model was developed. Assuming that all of the newly formed polymer (pMMA) is located on the outer shell, the residence time distribution, E(t), and the shell thickness distribution, E(s), are related by the following equation:

E(t)‚dt ) E(s)‚ds

(3)

Rearranging eq 3, one gets

E(s) ) E(t)/(ds/dt)

(4)

The shell thickness of a core/shell particle s is given by

s ) r - r0

(5)

where r is the radius of a core/shell particle and r0 is the radius of the core particle. The particle growth can be calculated by

r(t)3 ) r03 + at

(6)

with

a)

3 Pm r 4π p Fp

(7)

where rp is the polymerization rate per particle, Pm is the molecular weight of the shell monomer, and Fp is the density of the shell polymer. For a CSTR with a mean residence time of τ, rp can be calculated by the following expression:

rp )

[M]0 × xMMA τ × Np

(8)

where [M]0 is the concentration of MMA in the feed, x is the MMA conversion, and Np is the particle number. Then ds/dt can be obtained by deriving eq 6:

ds/dt ) dr/dt ) (1/3)a(r03 + at)-2/3

(9)

The residence time distribution in a CSTR is given by

E(t) ) (1/τ)exp (-t/τ)

(10)

Substituting eqs 9 and 10 in eq 4, the following equation for the distribution was obtained:

(

)

(r0 + s)3 - r03 1 exp τ a×τ E(s) ) 1 × a × (r0 + s)-2 3

(11)

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Figure 11. Effect of the core/shell ratio on the shell thickness distribution: (O) 70/30; (b) 80/20. Table 7. Rigid PVC Formulation to Evaluate the Impact Performance PVC SIIOP lead stabilizer processing aid calcium stearate lubricant PE wax antioxidant calcium carbonate titanium oxide

100 parts 4 0.5 0.8 0.3 0.2 0.1 5 4

100 parts 4 0.5 0.8 0.3 0.2 0.1 5 4

100 parts 4 0.5 0.8 0.3 0.2 0.1 5 4

8

8.5

9

impact modifier

Figure 11 shows the shell thickness distribution for 80/20 and 70/30 core/shell ratios, when τ ) 29 min and xMMA ) 1. The average shell thickness is given by

∫0∞ sE(s)ds sj ) ∞ ∫0 E(s)ds

(12)

The average shell thickness values for these distributions are sj80/20 ) 2.71 nm and sj70/30 ) 4.46 nm. The data of shell thickness (s) and coverage level (c) presented in Table 6 may be correlated by means of the following equation:

c ) 1 - exp(-0.347 × s)

(13)

Equation 13 can be used to estimate the average coverages of the core/shell latexes obtained in CSTRs. For the latexes used in this work, and considering total conversion of the monomer, these values are cj80/20 ) 61% and cj70/30 ) 79%. The theoretical coverage level for each experiment is presented in Table 5, and these values compare well with those measured experimentally. Evaluation Of The Impact Performance In Rigid PVC Formulation The hard pMMA shell of the impact modifier has several functions: first, the pMMA phase must help the

dispersion of the elastomeric nodules (core of the latex particles) in the matrix. Second, the pMMA phase must provide adhesion between the elastomeric phase and the matrix. For these two functions, compatibility of proper physical or chemical interactions of the shell with the matrix are required. The core/shell latexes made in a CSTR are characterized by a broad shell thickness distribution, or more precisely by the presence of many latex particles with some patches of pMMA partially covering the core and few latex particles with a complete pMMA shell. Whether the particles with a high core/shell ratio will be dispersed in the PVC matrix and function as impact modifier was an open question. The aim of this part of the study was to assess the performance of the core/ shell latexes obtained in a CSTR as toughening agents for PVC. A CSTR latex (C8), after a thermal treatment to convert the residual monomer, and a latex of the same composition made with a conventional semibatch process were spray dried on a NIRO pilot plant atomizer. The recovered powders were added to a rigid PVC formulation (Table 7). The formulation was malaxed on a two-roll mill at a temperature of 180 °C for 6 min. Sheets 4 mm thick were prepared in a hydraulic press heated to 190 °C. Test samples corresponding to the specifications of ISO 179-type 2 (50 mm × 6 mm × 4 mm) were cut. Ten samples were evaluated for each formulation (room temperature Charpy Impact). The results (% ductile failure) are presented in Table 8. From those results, it can be concluded that the impact performance of an impact modifier made in a CSTR is comparable to the impact performance of a conventional product. Actually, the ductile-fragile transition seems to occur at a slightly lower concentration of impact modifier in the case of C8. This means that the distribution of the shell thickness seems to have no effect on the dispersion of the elastomeric nodules and their adhesion to the PVC matrix. This may be because some additives contained in the PVC formulation may act as a compatibilizer between the core and the matrix under the high shear and high temperature at which the compounding is done. Part of the nongrafted pMMA shell could also migrate during compounding at the core/matrix interphase. Therefore, if a good dispersion of the modifier in the matrix is achieved, a low shell coverage may not be prejudicial to impact resistance of the material. Conclusions The feasibility of obtaining core/shell (poly(butyl acrylate-butadiene)/poly(methyl methacrylate)) latexes in a continuous stirred tank reactor, using the core polymer as the seed, was investigated. The effect of mean residence time, premixing of the reactants, nitrogen purge, core/shell ratio, and initiator type and concentration on the kinetics was studied. It was observed that both the conversion and stability of the

Table 8. Charpy Impact; Rigid PVC Formulation with Impact Modifier Made According to a Continuous or Semibatch Process conventional (semibatch process)

type of impact modifier level of impact modifier % ductile failures energy (kJ/m2)

C8 (continuous process)

8

8.5

9

8

8.5

9

0% 15.4 ( 0.5

20% >17.0

100%

70% 22 ( 0.4

90% >18.6

100%

Ind. Eng. Chem. Res., Vol. 43, No. 3, 2004 707

operation improved with the residence time. The residence time could not be increased beyond a certain value, because this would result in a loss of production of the reactor, therefore τ ) 29 min was retained for the work. It was observed that neither the nitrogen purge of the feed tanks nor the premixing of the monomer with the seed had a substantial influence on the shell monomer conversion. Three different redox initiators were studied, i.e., potassium persulfate/sodium bisulfite (KPS/SBS), ammonium persulfate/sodium bisulfite (APS/SBS), and tertbutyl hydroperoxide/sodium formaldehide sulfoxilate (BTHP/SFS). It was found that BTHP/SFS was the most efficient, followed by KPS/SBS, and finally APS/SBS. The higher efficiency of BTHP/SFS was probably due to the higher hydrophobicity of the radicals formed, which were readily absorbed in the polymer particles. It was found that MMA conversion decreased with the core/shell ratio, but the amount of unreacted monomer per total amount of polymer remained unchanged. On the other hand, the grafting efficiency decreased with the core/shell ratio. The average coverage level of the latex obtained in the CSTR was similar to or even slightly better than that of the latex obtained in the semicontinuous reactor. However, only a partial coverage of the core polymer was achieved in both cases. This means that the pMMA was likely forming patches on the surface of the core polymer. The coverage was improved when the shell monomer and the seed were not premixed, likely due to the diffusion limitations of the monomer into the particles. In addition, a significant increase of the coverage level was achieved with the increase of the shell/core ratio. No clear effect of the hydrophobicity of the initiator system was observed. The evaluation of the impact performance in a rigid PVC formulation of the core/shell particles made in the CSTR showed properties comparable to the impact performance of a conventional product manufactured in a semicontinuous reactor. This suggests that the shell

thickness distribution does not have any effect on the compatibility of the shell and the matrix. Acknowledgment The finantial support of Atofina is acknowledged. Literature Cited (1) Sommer, F.; Duc, T. M.; Pirri, R.; Meunier, G.; Quet, C. Surface Morphology of Poly(butyl acrylate)/Poly(methyl methacrylate) Core Shell Latex by Atomic Force Microscopy. Langmuir 1995, 11, 440. (2) Maron, S. H.; Elder, M. E.; Ulevitch, I. N. Determination of Surface Area and Particle Size of Synthetic Latex by Adsorption. I. Latexes Containing Fat-Acid Soaps. J. Colloid Sci. 1954, 89, 9. (3) Okubo, M.; Yamada, A.; Matsumoto, T. Estimation of Morphology of Composite Polymer Emulsion Particles by the Soap Titration Method. J. Polym. Sci., Part A: Polym. Chem. 1980, 16, 3219. (4) Reid, V. W.; Longman, G. F.; Heinerth, E. Determination of Anionic-Active Detergents by Two-Phase Titration. Tenside 1967, 4, 292. (5) Poehlein, G. W. Polymerization Reactors and Processes; Henderson, J. N., Bouton, T. C., Eds.; ACS Symp. Ser. 104; American Chemical Society: Washington, DC, 1979. (6) Smith, W. V.; Ewart, R. H. Kinetics of Emulsion Polymerization. J. Chem. Phys. 1948, 16, 592. (7) Beuermann, S.; Buback, M.; Davis, T. P.; Gilbert, R. G.; Hutchinson, R. A.; Olaj, O. F.; Russell, G. T.; Schweer, J.; Van Herk, A. M. Critically Evaluated Rate Coefficients for Free-Radical Polymerization. Part 2. Propagation Rate Coefficients for Methyl Methacrylate. Macromol. Chem. Phys. 1997, 198, 1545. (8) Ilundain, P.; Alvarez, D.; da Cunha, L.; Salazar, R.; Barandiaran, M. J.; Asua, J. M. Knwoledge-Based Choice of the Initiator type for Monomer Removal by Postpolymerization. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 4245. (9) Gonza´lez-Ortiz, L. J.; Asua, J. M. Development of Particle Morphology in Emulsion Polymerization. 1. Cluster Dynamics. Macromolecules 1995, 28, 3135.

Received for review February 7, 2003 Revised manuscript received July 17, 2003 Accepted July 24, 2003 IE030101G