Particle Formation in Vinyl Chloride Emulsion ... - ACS Publications

Oct 4, 2008 - Hugo M. Vale and Timothy F. L. McKenna*. LCPP-CNRS/ESCPE-Lyon, 43 Boulevard du 11 Novembre 1918, 69616 Villeurbanne Cedex, ...
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Ind. Eng. Chem. Res. 2008, 47, 8107–8118

8107

KINETICS, CATALYSIS, AND REACTION ENGINEERING Particle Formation in Vinyl Chloride Emulsion Polymerization. Experimental Study Hugo M. Vale† and Timothy F. L. McKenna*,‡ LCPP-CNRS/ESCPE-Lyon, 43 BouleVard du 11 NoVembre 1918, 69616 Villeurbanne Cedex, France

Particle formation in vinyl chloride emulsion polymerization has been investigated by means of ab initio and seeded experiments. A series of ab initio polymerizations was run in order to obtain reliable data regarding the dependence of the particle number on the emulsifier (SDS and SDBS) concentration, as well as to analyze the effect of the initiator concentration, stirring rate, and monomer-to-water ratio upon the particle number and the polymerization kinetics. Seeded polymerizations carried out at different concentrations of seed latex and surfactant made it possible to quantitatively evaluate the influence of these factors on the onset and extent of secondary particle formation. 1. Introduction 1.1. Kinetics of PVC. Much of what is known about the emulsion polymerization of vinyl chloride monomer (VCM) is due to Ugelstad and co-workers. Ugelstad et al.1 investigated the effect of initiator concentration, amount and type of emulsifier, and number of particles upon the rate of batch emulsion polymerization of VCM. The following results emerged from this study (and some corroborating previous work2,3): • The reaction order with respect to the initiator is 0.5. • The reaction order with respect to particle number (N) is very low, increasing with N between about 0.05 and 0.15. • The polymerization rate increases with conversion (autocatalytic effect). • The value of nj is below 0.5, and is usually between 0.001 and 0.1. To explain the low value of nj, Ugelstad et al.1 proposed a mechanism involving rapid desorption and reentry of radicals. Such a mechanism was considered especially favorable in the case of VCM because of the high rate of chain transfer to monomer, kfM/kp ) 1.1 × 10-3 at 50 °C.4,5 Independent evidence for the low value of nj was also obtained by experiments where the rate of radical production was changed during the run.6 1.2. Ab Initio Particle Formation. Peggion et al.2 investigated the emulsion polymerization of VCM in presence of several surfactants (SDS and aerosols DBM, AY, and MA), using potassium persulfate as the initiator. They observed that the particle number remained constant between 6 and 100% conversion, and that (to within the limits of experimental error) the concentration of initiator had no influence on the number of particles formed. The authors found an approximately linear (log-log) relationship between the particle number and the emulsifier concentration for SDS and aerosol DBM, but a * To whom correspondence should be addressed. Tel.: (613) 5336582. E-mail: [email protected]. † Current address: Synthetic Rubber Group, Michelin Technology Center, Clermont Ferand, France. ‡ Current address: Department of Chemical Engineering, Queen’s University, Kingston, ON, Canada.

marked increase at the critical micelle concentration (cmc) for aerosols AY and MA (cf. Figure 1). Ugelstad et al.1 also verified that particle number was independent of conversion (in the range covered, 10-90%) and initiator concentration. With respect to the relationship between particle number and surfactant concentration, they observed a linear dependence for SDS and a smooth curve for aerosol DBM (Figure 1). These results led Ugelstad et al.7 to affirm that the absence of an abrupt change in the N-[S] curve at the cmc is an important feature of VCM emulsion polymerization (it should be pointed out that these authors do not in fact define the value of the cmc). However, the findings of Peggion et al.,2 mentioned in the previous paragraph, raise serious doubts on the universality of such a statement. It is also worth noting that significant discrepancies are observed in the N-[SDS] data reported by these two research groups (cf. Figure 1). Goebel et al.8,9 investigated particle coagulation by following the evolution of the number of particles in the emulsion polymerization of VCM. They observed that, after going through

Figure 1. Influence of surfactant concentration on number of particles formed in ab initio VCM emulsion polymerization, for various types of surfactants.1,2 Trend lines are drawn in agreement with the original references.

10.1021/ie0715153 CCC: $40.75  2008 American Chemical Society Published on Web 10/04/2008

8108 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 Table 1. Critical Particle Concentrations To Avoid SPF According to Gatta et al.13 djseed (nm)

Nseed (particles/L)

(Ndj)seed (m/L)

229 310 457 570

2.4 × 10 2.8 × 1015 1.4 × 1015 1.2 × 1015

5.5 × 108 8.7 × 108 6.4 × 108 6.8 × 108

15

a maximum, N decreased throughout the entire reaction: more rapidly in the first instance and then more slowly. From the initial slopes of the 1/N versus time curves, the authors computed the corresponding aggregation rates, which they found to decrease with the initial concentration of emulsifier. Additionally, they observed that N remained constant when the polymerization was interrupted by addition of inhibitors or by monomer consumption. These findings led the authors to suppose the existence of a dependency between aggregation and polymerization. Indeed, in a subsequent paper,10 they considered the coagulation rate to be determined by the rate of radical exit. The results of these last authors are somewhat unexpected and are in disagreement with those of Peggion et al.2 and Ugelstad et al.,1 who used TEM to show that the number of particles remained constant after 10% conversion. Tauer et al.11 measured monomer conversions at very short reaction times (40-280 s) and observed a maximum in the polymerization rate at about 160 s. By manipulating approximate expressions for the rate of polymerization, Rp(t), the authors showed that a maximum in Rp implies the existence of a maximum in N, thereby supporting the role of particle coagulation in VCM emulsion polymerization. Finally, Vidotto et al.12 made use of dilatometry to accurately follow the kinetics of VCM emulsion polymerization. They identified an initial period where the reaction occurs at a rate independent of [S] (at least above the cmc), but whose length depends on [S]. During this stage, the surface tension is constant and has a value corresponding to the presence of micelles. The second period begins with a sudden drop of the polymerization rate, with the new reaction rate being higher for higher values of [S]. This stage lasts up to the disappearance of the monomer droplets. The authors suggested a correspondence between the initial period observed in the conversion-time plots and the period of particle formation. The decrease of the polymerization rate from the first to the second stage was attributed to (i) particle coagulation or (ii) decrease of the initiation efficiency caused by the disappearance of micelles. 1.3. Secondary Particle Formation. Gatta et al.13 were perhaps the first to study the formation of new particles in the seeded emulsion polymerization of VCM. A series of runs were conducted at 55 °C, with varying amounts of monodispersed seeds of different diameters, and with calculated amounts of surfactant (SDS) to ensure that no micelles were formed. They found that a limited amount of emulsifier was not enough to prevent secondary particle formation (SPF). In order to obtain a final monodispersed latex, the number of seed particles per unit volume of water also had to be above a certain critical value. Their results are summarized in Table 1, and show that the minimum number of particles decreases with increasing seed diameter. As we may infer from the third column of the table, this is consistent with an interpretation (not done by the authors) in terms of a minimum value for the product (Ndj)seed, whose average value is 6.8 × 108 m/L. Tauer and Petruschke14 performed similar experiments and observed that the formation of new particles in the seeded emulsion polymerization of VCM was affected by the concentration of both the surfactant (an alkylsulfonate) and the product

(Ndj)seed. At 50 °C, it was found that SPF could be prevented if (Ndj)seed > 3.7 × 109 m/L for surfactant concentrations in the range 3.7 < [S] < 22 mM. For [S] > 22 mM, SPF was again observed independently of high values of (Ndj)seed. Butucea et al.15 proposed to quantify the onset of SPF in terms of a parameter that they designated “maximum critical concentration of initiator per unit surface of particles” (MCCI). A series of seeded experiments was conducted, at 50 °C, with two seed diameters and a constant concentration of potassium persulfate, from which they determined MCCI ) 2.1 × 10-6 mol/m2. Note, however, that the observation cited earlier that [I] does not seem to influence the number of particles in ab initio polymerizations is not coherent with the observation here that [I] seems to influence SPF. Furthermore, the effect of [I] was not experimentally investigated. In sum, the works of Gatta et al.13 and Tauer and Petruschke14 seem to corroborate each other, as both relate the onset of SPF to a minimum value of the product (Ndj)seed, although very distinct values are presented for this parameter. The work of Butucea et al.,15 on the other hand, points in a totally different and less evident direction. 1.4. Current Status and Objectives. The above paragraphs show that certain facts about particle formation in VCM emulsion polymerization are fairly well established, namely: • The contribution of the homogeneous nucleation mechanism is rather significant, as attested by the high values of N obtained at emulsifier concentrations well below the cmc. • The number of particles formed does not depend on the initiation rate, in contrast to what is usual. Such behavior is believed to be due to the participation of desorbed radicals in the particle formation process (this will be discussed in more detail in a subsequent paper from this group). • Particle coagulation occurs during interval I. • SPF can take place in the absence of micelles, as expected from the significant role of homogeneous nucleation. • SPF can be prevented if the number of seed particles is sufficiently high. At the same time, it is also clear that some issues require further attention. There appears to be a lack of reliable data relating the dependence of particle number on emulsifier concentration, which is essential to test particle formation models. In addition, no experimental data or explanations are reported in the literature about the extent of SPF with VCM and its relation to the operating conditions (e.g., Nseed, djseed, [S], etc.). It is the purpose of this work to help fill this gap. To do so, a series of ab initio and seeded experiments was carried out under well-defined and well-characterized conditions, as described in the following sections. Hopefully, these data will be of value in improving our qualitative and quantitative understanding of ab initio and secondary particle formation. 2. Experimental Section 2.1. Materials. Vinyl chloride monomer, without inhibitor, was supplied by Cires S.A., with a purity of 99.99% measured by gas chromatography. Ammonium persulfate (APS) (ACS grade, Acros Organics) was used as the initiator. Sodium dodecyl sulfate (SDS) (99%, Acros Organics) and sodium dodecyl benzenesulfonate (SDBS) (88%, Acros Organics) were used as the emulsifiers. All reagents were employed as received. Deionized water was used throughout the work. 2.2. Polymerization Procedure. All polymerizations were carried out in a 1.5 L jacketed stainless steel reactor, equipped

Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 8109 a

Table 3. Recipes of Seeded Polymerizationsa

Table 2. Recipes of ab Initio Polymerizations run

surfactant

VCM (g)

[APS] (mM)

[S] (mM)

run

seed

mass of seed (g)

[S] (mM)

A1 A2 A3 A4b A5 A6 A7 A8 A9 A10 A11 A12 A13 A14 A15 A16 A17 A18 A19 A20 A21 A22 A23 A24 A25 A26 A27 A28 A29 A30 A31 A32 A33

SDS SDS SDS SDS SDS SDS SDS SDS SDS SDS SDS SDS SDS SDS SDS SDS SDS SDS SDS SDS SDS SDS SDBS SDBS SDBS SDBS SDBS SDBS SDBS SDBS SDBS SDBS SDBS

100 100 100 100 50 100 100 100 100 100 100 100 100 100 100 100 100 50 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100

1.0 1.0 1.0 1.0 1.0 1.0 2.0 2.0 1.0 1.0 1.0 1.0 2.0 1.0 1.0 1.0 1.0 1.0 2.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 2.0 1.0 1.0 1.0 1.0 2.0 1.0

0.08 0.16 0.34 0.34 0.34 0.77 0.77 0.77 1.6 3.1 3.1 3.1 3.1 4.3 4.3 6.2 6.2 6.2 6.2 8.7 12.4 12.4 0.06 0.13 0.28 0.28 0.28 0.58 1.4 2.8 2.8 2.8 5.6

B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14 B15 B16 B17 B18 B19 B20 B21

A20 A20 A20 A20 A20 A11 A11 A11 A11 A11 A11 A11 A11 A11 A11 A11 A3 A3 A3 A3 A3

1.0 2.5 5.0 10.0 20.0 2.5 2.5 5.0 5.0 5.0 5.0 10.0 10.0 10.0 10.0 20.0 5.0 5.0 7.5 10.0 10.0

0.62 0.65 0.73 0.89 1.2 0.63 2.0 0.68 2.1 3.8 6.8 0.76 2.4 4.5 7.9 5.7 0.66 3.7 0.70 0.72 4.0

a In all batches, the mass of water was 1125 g. stirring rate was 450 rpm.

b

In this run the

with a 45° pitched four-blade impeller. The temperature of the reactor contents was kept at 50.0 ( 0.5 °C using a circulating water bath. Unless otherwise stated, the stirring speed was set at 650 rpm. The initial charge, composed of surfactant and water (plus seed latex in seeded experiments) was first added to the reactor. Residual oxygen was then removed by purging with nitrogen, followed by 30 min of vacuum drawn with an oil-sealed rotary vane pump. The desired amount of vinyl chloride was then charged from a steel cylinder mounted on top of a balance, and the reactor contents were heated to the polymerization temperature. In the seeded experiments, the system was allowed to equilibrate for 1.5 h at the reaction temperature. The polymerization was started by introducing the initiator, dissolved in 25 mL of water, via nitrogen pressure. Latex samples were taken at various time intervals by means of a 10 mL stainless steel cylinder connected to a valve at the bottom of the reactor. After taking the sample, the cylinder was depressurized in a hood and the latex was collected. The seeds were prepared by washing previously synthesized latexes with a mixture of anionic and cationic ion-exchange resins (Dowex MR-3, Aldrich). The withdrawal of the ionic species was monitored by measuring the latex conductivity, and the washing step was assumed complete when the conductivity no longer decreased with successive passes over the resins. A summary of the recipes used in the polymerizations is provided in Tables 2 and 3. 2.3. Latex Characterization. Monomer conversion was determined by conventional gravimetric analysis; the samples were dried at 70 °C to avoid polymer degradation. The particle size of the latexes was measured by dynamic light scattering (Malvern Zetasizer 1000HS) and/or static light

a In all batches, SDS was used as the emulsifier, [APS] ) 1 mM, and the total mass of water was 1125 g.

scattering (Coulter LS230). In seeded polymerizations, the extent of secondary particle formation was determined by examining the final latex by TEM and counting the number of new and seed particles.16 Surface charge densities were measured by titrating a given mass of washed latex with standard sodium hydroxide solution (0.010 M) until the isoelectric point was reached. Titrations were conducted at room temperature and under nitrogen atmosphere, and the conductivity was monitored with a Radiometer CDM 83 conductivity meter equipped with a platinum cell. 3. Results and Discussion 3.1. Ab Initio Polymerizations. 3.1.1. Effect of Surfactant Type and Concentration. A series of runs (see Table 2) was performed over a wide range of concentrations of SDS and SDBS to investigate the effect of surfactant type and concentration on the number of particles formed and the polymerization kinetics. In all polymerizations, the number of particles was found to become constant after 5-20% conversion, as illustrated in Figure 2. These results corroborate the findings of Peggion et al.2 and Ugelstad et al.,1 but do not concord with those of Goebel et al.8,9 It should be pointed out that while Goebel et al. used a higher concentration of surfactant, as well as a buffer, and were therefore working at a higher ionic strength in their studies, Ugelstad et al.1 used similar conditions of ionic strength without seeing the same results as Goebel et al. The complete set of N-[S] data for the ab initio polymerizations is summarized in Figure 3. The reproducibility of the experiments is rather satisfactory, with standard deviations remaining below 12%. It is important to point out that the particle numbers presented here were computed from the average diameter measured by dynamic light scattering (djDLS ) dj6,5),17 which is not the appropriate average to calculate N from monomer conversion (the correct one is dj3,0). Consequently, one should expect the N values to be affected by a certain systematic error. This error should not be very significant though, since the polydispersity index of the latexes was low (typically around 0.05), except in the samples taken at short reaction times. The first thing we notice in Figure 3 is that the two curves have very distinct shapes. In the case of SDS we identify

8110 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008

Figure 2. Evolution of number of particles formed in ab initio polymerization as a function of monomer conversion. The length of the error bar corresponds to a deviation of (15%.

Figure 3. Evolution of number of particles formed in ab initio polymerization as a function of surfactant concentration. The length of the error bar corresponds to a deviation of (15%.

(although there is some room for subjectivity) two linear sections separated by a sharp rise at about 5 mM. This behavior seems consistent with what one could expect from a transition of nucleation mechanism (from homogeneous to predominantly micellar), as has been observed with other monomers (e.g., methyl methacrylate18). Considering the effects of temperature and APS concentration, but neglecting eventual effects of the monomer, the cmc of SDS at reaction conditions was evaluated at 8 mM (see the Appendix). This value can be estimated from measurements of the cmc in water at 50 °C, and measurements of the effect of APS at 20 °C (see Appendix A1), if we admit that the effects of temperature and APS concentration are independent. The eventual influence of VCM upon the cmc could not be evaluated due to experimental difficulties, but measurements with 1,2-dichloroethane (cf. Appendix A2) showed no effect. This value of the cmc is close to, but somewhat higher, than the emulsifier concentration at which the transition occurs (5 mM). At present, the cause of this

Figure 4. Number of particles formed as a function of SDS concentration: comparison between the values found in this work and the ones reported by Ugelstad et al.1 and Peggion et al.2 Trend lines are drawn in agreement with the original references.

discrepancy is not clear: it may well be a simple consequence of the approximations done in estimating the cmc, but it may also be due to some other phenomenon that we do not clearly understand. With SDBS, in contrast, there is no clear sign of a transition near the estimated cmc value of 1.3 mM. Instead, a smooth concave curve is found. A categorical explanation for this difference of behavior with respect to SDS is difficult to give. One possibility is that it is a consequence of the purity of the SDBS grade used (88%), and more precisely of the presence of homologues with different chain lengths and structures. As the cmc varies (decreases) with chain length, the existence of homologues would smooth the rise in particle number (not very pronounced with SDS, by the way) and result in a concave curve. It can also be seen in Figure 3 that in the region [S] < 1.3 mMswhere, in principle, no SDS or SDBS micelles exists approximately 2 times more particles are formed with SDBS than with SDS at equal molar concentration of emulsifier. In this range of concentrations, the number of particles created is determined by the balance between the rate of homogeneous nucleation and the rate of coagulation. Consequently, the fact that more particles are produced with SDBS indicates that this surfactant is more efficient in preventing the coagulation of the precursor particles. The adsorption isotherms19 of the surfactants offer a consistent explanation for this observation. Indeed, the adsorption constant of SDBS is significantly higher than that of SDS, resulting in larger equilibrium surface concentrations of the former emulsifier. Although thermodynamic equilibrium is probably not attained under polymerization conditions, it is nevertheless reasonable to expect that this tendency be respected. As mentioned above, the N-[SDS] data reported in the literature1,2 are contradictory. It is therefore interesting to see how our results compare to the values obtained by these authors. As shown in Figure 4, there are significant differences among the three curves. We note, nevertheless, that there is good correspondence between the slope of the first part of the curve of Peggion et al. and the slope of the first zone of our data (0.54 vs 0.56), as well as between the slope of Ugelstad et al.’s data and the slope of the second zone of our curve (1.3 vs 1.4).

Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 8111

Figure 5. Conversion-time curves of ab initio polymerizations at different SDS concentrations: (a) [APS] ) 1 mM; (b) [APS] ) 2.0 mM.

At least part of the deviations between our values and those reported by Peggion and Ugelstad could be attributed to systematic errors in the particle sizes measured by these two teams. Indeed, the method used in these works, TEM, is known to underestimate the diameter of PVC latexes by as much as 40%, because the particles shrink under the electron beam.20 As a matter of fact, if we correct the data of Peggion et al. for this effect (assuming 40% shrinking), their data overlap ours. The differences with respect to the data of Ugelstad et al. are harder to interpret: even correcting for TEM errors, their N values are still much higher, and the N-[S] plot evinces no sign of a change of slope. The only hypothesis we can put forward is that perhaps the properties of the emulsifier they used (Empicol-8043, an industrial grade of SDS) are significantly different from those of analytical grade SDS. This would be coherent with the proposal advanced earlier to explain the absence of transition in the N-[SDBS] plot. The influence of the emulsifier concentration on the polymerization kinetics is illustrated in Figures 5 and 6 for SDS and SDBS, respectively. In the case of SDS (Figure 5), it is seen that an increase in the surfactant concentration results in an increase of the polymerization rate (measured at a given

Figure 6. Conversion-time curves of ab initio polymerizations at different SDBS concentrations: (a) [APS] ) 1 mM; (b) [APS] ) 2.0 mM.

conversion) that parallels the increase in particle number, i.e., Rp∝Nb. In the range corresponding to the first part of the SDS curve (Figure 3) we have b ) 0.14, whereas for the second part we have b ) 0.22. These low exponents are in qualitative agreement with the results of Ugelstad et al.1 discussed above. Different behavior is observed in the case of SDBS. In the runs carried out at [APS] ) 1.0 mM, the polymerization rate tends to a constant value (about 0.09 mol · mw-3 · s-1 at 50% conversion) that does not vary with [S] (Figure 6a). Apparently, the only effect of increasing the concentration of emulsifier is to increase the initial polymerization rate. In this respect, it is worth noting the significant increment of the initial polymerization rate between the runs carried out at 2.8 and 5.6 mM SDBS. This could well be a sign of a change in the main nucleation mechanism not perceptible in the N-[S] plot. At [APS] ) 2.0 mM, the available data indicate a different behavior, similar to that found with SDS. Indeed, we find exactly the same exponent as above in this case: b ) 0.14. The results of the polymerizations performed with SDBS seem to indicate the existence of a nonnegligible interaction between this surfactant and the initiator, at least at low

8112 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 Table 4. Effect of Monomer-to-Water Ratio and Stirring Rate on Particle Formation run A3 A4 A5 A16 A18

[S] (mM) 0.34 0.34 0.34 6.2 6.2

VCM (g) 100 100 50 100 50

ω (rpm)

N (particles/L)

650 450 650 650 650

3.9 × 10 3.8 × 1016 3.9 × 1016 5.2 × 1016 5.3 × 1016 16

concentrations of the latter. If it were important to elucidate the physicochemical mechanism(s) responsible for these observations, more experiments would need to be done at initiator concentrations of 2 mM and above. In any case, the observations shown here clearly demonstrate something that is well-known to emulsion polymerization practitioners, but rarely taken into account in emulsion polymerization models: an emulsifier (anionic in our case) cannot, in general, be reduced to a simple set of physical properties (cmc, adsorption isotherm, etc.); the aqueous-phase chemistry is probably important as well. 3.1.2. Effect of Initiator Concentration. The results of Figure 3 demonstrate that, to within the limits of experimental error, doubling the initiator concentration has no observable impact on the number of particles formed. Moreover, this appears to be so, independently of the type and concentration (above and below the cmc) of surfactant. However, it should be pointed out that the effect of [I] was not studied at constant ionic strength. Therefore, the hypothesis that a positive effect of [I] on the nucleation rate is being exactly compensated by an increase of the coagulation rate cannot be categorically excluded. Such a hypothesis is likely remote, though, given the low values of [I] employed and the fact that N remains unchanged independently of the surfactant concentration. In sum, our results confirm the findings of Peggion et al.2 and Ugelstad et al.,1 and point to a negligible influence of the initiation rate on the rates of homogeneous and micellar nucleation. 3.1.3. Effects of Monomer-to-Water Ratio and Stirring Rate. The influence of the monomer-to-water ratio (M/W) on the formation of particles and the polymerization kinetics was investigated by carrying out some polymerizations with half the mass of monomer normally employed (Tables 2 and 4). As we can see from Table 4, there is no measurable effect of M/W on the particle number. Likewise, Figure 7 shows that the polym-

Figure 7. Effect of monomer-to-water ratio on the kinetics of ab initio polymerizations carried out with SDS.

Table 5. Effect of Polymerization Conditions on Concentration of Strong Acid Ionic End Groups at the Surface of the Particles strong acid groups latex

surfactant

[S] (mM)

[I] (mM)

A1 A6 A12 A17 A19 A24 A25 A27 A28 A29 A31 A32 A33

SDS SDS SDS SDS SDS SDBS SDBS SDBS SDBS SDBS SDBS SDBS SDBS

0.08 0.77 3.1 6.2 6.2 0.13 0.28 0.28 0.58 1.4 2.8 2.8 5.6

1.0 1.0 1.0 1.0 2.0 1.0 1.0 2.0 1.0 1.0 1.0 2.0 1.0

N (particles/L) 2.0 × 1016 7.4 × 1016 1.5 × 1017 4.7 × 1017 6.0 × 1017 5.7 × 1016 8.3 × 1016 7.5 × 1016 1.3 × 1017 2.7 × 1017 5.4 × 1017 4.8 × 1017 8.7 × 1017

µmol/g

mC/m2

0.7 1.1 3.4 3.2 3.0 1.8 2.1 1.5 3.6 10 9.9 9.1 6.1

2.7 3.0 6.7 4.2 3.6 4.9 5.0 3.7 7.0 16 12 12 6.4

erization rate is independent of M/W up to the end of interval II (which corresponds to a polymer concentration of about 20 g/kg in the runs conducted with 50 g of VCM). The fact that M/W has no observable effect on N and Rp is coherent with the classical view of emulsion polymerization, according to which the monomer phase is simply a reservoir of monomer molecules. As long as the initial amount of monomer is sufficient to guarantee constant values of [M]w and [M]p during the entire period of particle formation (interval I), the actual value of M/W will have no effect on N. The only influence of M/W will be upon the extent of interval II. Of course, this interpretation only holds for systems and experimental conditions leading to a well-defined period of particle formationsas is the case in our experiments (Figure 2)sand when monomer droplets are not directly involved in particle nucleation. The effect of the stirring rate was analyzed by conducting two ab initio polymerizations differing only in the agitation rate, ω (Table 4). The variation in ω was chosen so that the difference with respect to the reference value could be significant, but yet ensure that monomer transport was not rate limiting. Additionally, a low surfactant concentration was used in order to render the eventual effect of ω more visible. The experiments revealed no effect of ω upon N or Rp, thereby allowing us to draw the following conclusions about our experiments: • At least for ω e 650 rpm, the contribution of the orthokinetic (shear) aggregation mechanism to the total particle aggregation rate is negligible. This is in agreement with the experimental findings of Chern et al.,21 Kemmere et al.,22,23 and Zubitur and Asua.24,25 • The area of the monomer/water interface (which increases with M/W and ω) does not significantly affect the partitioning of the emulsifier between the aqueous phase and the latex particles. If this were not the case, increasing M/W or ω would cause a reduction in N (by decreasing the effective surfactant concentration).26,27 3.1.4. Surface Ionic End Groups. The ionic end groups present at the surface of the particles contribute (together with adsorbed surfactant, etc.) to the total surface charge density of the particles and, consequently, to their colloidal stability. It is therefore important to have an idea of the magnitude of the surface charge density due to ionic end groups (σeg) and to understand how it is affected by the polymerization conditions. To examine the importance of this quantity, the concentrations of surface acid groups of a few latexes were determined by conductometric titration with NaOH. The results are summarized in Table 5, in terms of both the concentration of acid groups

Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 8113 Table 7. Results of Seeded Polymerizations

Table 6. Characteristics of the Seed Latexes latex A3 A11 A20

djDLS (nm) 143 88 56

dj4,3a (nm) 140 90 -

PIb 0.05 0.04 0.11

σeg (mC/m2) 6.0 5.2 4.0

a

Volume-average diameter determined by static light scattering. This technique could not be used with latex A20, because it is limited to d > 40 nm. b Polydispersity index determined by dynamic light scattering.

per unit mass of polymer and the corresponding surface charge density; only strong acid groups were detected (distinct Vshaped curve). Although these results are limited, some interesting tendencies emerge: • There seems to be no direct relation between [I] and σeg. • At low particle numbers, similar values of σeg are obtained with SDS and SDBS. • At high particle numbers, higher values of σeg are obtained with SDBS. The σeg values found in the latexes stabilized with SDS are coherent with the figures reported by Neelsen et al.28 Unlike us, however, these researchers also found a significant amount of weak (carboxylic) acid groups on the surface of the particles. In our case, the only situation where weak acid groups were detected was in latexes characterized several months (in our case, 6 months) after their synthesis. These tendencies concerning σeg suggest that the generally assumed explanations for the origin of the surface ionic end groups are perhaps inadequate for this system. Indeed, if one assumes the ionic end groups to arise from initiator fragments, it is not easy to interpret the apparent absence of effect of the initiator concentration and the significant influence of the emulsifier type upon σeg. Neelsen et al.29 claim that the surface ionic groups in PVC latex particles mainly result from covalently bound fragments of the surfactant. In Figure 4 of their paper, relative to a polymerization with SDS at 50 °C, the amount of surfactantderived groups is shown to be 5-7 times higher than the amount of initiator-derived ones. A chain transfer reaction between surfactant and aqueous-phase oligo radicals was proposed to explain these observations. To the best of our knowledge, no one has attempted to verify their findings; it does seem, nevertheless, that such a theory could account for the results exposed here. If the emulsifier affects σeg, as it appears to be the case, this effect could play a role in determining the number of particles formed. In particular, this could provide a complementary explanation for the higher number of particles formed in polymerizations stabilized with SDBS. 3.2. Seeded Polymerizations. A series of seeded polymerizations (see Table 3) were conducted with the purpose of investigating the effects of seed particle number (Nseed), seed diameter (djseed), and surfactant concentration on the onset and extent of secondary particle formation. The main characteristics of the three PVC latexes used as seeds are listed in Table 6. The initial concentration of free surfactant in the aqueous phase ([S]w,0) was computed from the total surfactant concentration ([S]) by estimating the amount of surfactant adsorbed on the surface of the seed particles via the corresponding adsorption isotherm (see Vale and McKenna19); the increase of the particle size due to swelling by the monomer was taken into account. According to these calculations, in all experiments, [S]w,0 was kept below the cmc of the emulsifier (estimated at 8 mM, as discussed above). The results of the seeded polymerizations are summarized in Table 7, indicating the runs where SPF was detected and,

SPF run

djseed (nm)

Nseed (particles/L)

[S]w,0 (mM)

θ0a (%)

Y/N

Nnew/Nseedb

B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14 B15 B16 B17 B18 B19 B20 B21

56 56 56 56 56 88 88 88 88 88 88 88 88 88 88 88 143 143 143 143 143

7.0 × 1015 1.7 × 1016 3.5 × 1016 7.3 × 1016 1.5 × 1017 4.4 × 1015 4.4 × 1015 8.9 × 1015 8.9 × 1015 8.9 × 1015 8.9 × 1015 1.8 × 1016 1.8 × 1016 1.8 × 1016 1.8 × 1016 3.6 × 1016 2.1 × 1015 2.1 × 1015 15 3.1 × 10 4.2 × 1015 4.2 × 1015

0.6 0.6 0.6 0.6 0.6 0.6 1.9 0.6 1.9 3.4 6.0 0.6 1.9 3.4 6.0 3.4 0.6 3.4 0.6 0.6 3.4

10 10 10 10 10 10 30 10 30 50 80 10 30 50 80 50 10 50 10 10 50

Y N N N N Y Y Y Y Y Y ? Y Y Y Y Y Y Y N Y

1.0 ( 0.2 0 0 0 0 1.7 ( 0.3 12 ( 3 small 2.6 ( 0.6 5.0 ( 0.6 high (>10) / / / high (>10) / 10 ( 3 21 ( 6 1.2 ( 0.5 0 /

a Initial surface coverage of the seed particles. b The symbol “/” denotes cases where the ratio of new to seed particles could not be measured. The confidence intervals correspond uniquely to the statistical error (R ) 90%) associated with the determination of the ratio Nnew/ Nseed from the number of particles counted.

Figure 8. Number of new particles formed as a function of number of seed particles times swollen seed diameter. The arrows indicate the number of particles formed in ab initio polymerization (i.e., the value of Nnew corresponding to Nseed ) 0).

when experimentally possible to determine, the extent of SPF. In most cases, determining whether SPF had occurred could be easily done by analyzing the TEM images of the final latexes: two populations with quite different diameters could be identified. In some instances, however, the final latexes were polydisperse, making an objective evaluation more difficult. With respect to the extent of SPFsexpressed in terms of the ratio of new to seed particlessaccurate measurements were in general hard to obtain due to experimental constraints/difficulties: (i) limitation of the number of micrographs that could be taken, (ii) overlapping of new and seed particles, and (iii) polymer particles that often appeared surrounded by a sort of sticky substance (present in the latex) that could not be removed by ion exchange.

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Figure 9. Effect of amount of seed on conversion-time curves of some seeded polymerizations: (a) seed ) A20, [S]w,0 ) 0.6 mM; (b) seed ) A11, [S]w,0 ) 0.6 mM; (c) seed ) A11, [S]w,0 ) 1.9 mM.

Although limited, the results of Table 7 clearly show that, for a given seed, the number of new particles formed, Nnew, increases with [S]w,0 (at constant Nseed) and decreases with Nseed (at constant [S]w,0). The fact that Nnew increases with the surfactant concentration (below the cmc) is evidence that coagulation plays a role in secondary particle formation. Indeed, at constant Nseed, the rate of production of precursor particles via homogeneous nucleation is also constant. Therefore, a variation of Nnew can only be due to the effect of the emulsifier upon the rates of coagulation of the precursor particles with themselves and/or with the seed particles. The influence of Nseed upon the extent of SPF, on the other hand, can be explained in terms of two effects: (i) increasing Nseed decreases the rate of production of precursor particles because more aqueous-phase radicals preferentially enter seed particles and (ii) increasing Nseed augments the rate of coagulation between precursor and seed particles. If SPF occurred without a significant amount of particle coagulation (which, as we have seen, is not the case), the important parameter in determining the number of new particles

formed would be the product (Ndjs)seed (assuming that the entry of radicals into particles is diffusion controlled30,31). It is thus interesting to see how our data correlate with this parameter, which, we recall, has been used by certain authors to define conditions for the onset of SPF. This is what is done in Figure 8, which represents the number of new particles formed as a function of (Ndjs)seed for the various seeds and experimental conditions employed. For comparison, the plot also indicates the particle numbers obtained in the absence of seed (cf. Figure 3). As we can see, for the lowest surfactant concentration considered (0.6 mM), the results of the three seeds agree fairly well. Moreover, we can estimate a critical value of (Ndjs)seed of about (0.5-1) × 109 m/L, which is in good accord with the value of 6.8 × 108 m/L that can be deduced from the work of Gatta et al.13 For the other concentrations of emulsifier, not much can be said because of missing data. It appears, nevertheless, that if a critical value of (Ndjs)seed exists, such value is an increasing function of the surfactant concentration. This would explain the higher figures (3.7 × 109 m/L for surfactant

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concentrations 3.7 < [S] < 22 mM) reported by Tauer and Petruschke.14 The literature is scarce in quantitative studies of SPF addressing simultaneously the effects of seed concentration and emulsifier concentration, with one of the few exceptions being the work of Hansen and Ugelstad32 on the seeded emulsion polymerization of styrene (a single seed was considered). Although vinyl chloride and styrene are known to be very different systems, our results are in qualitative agreement with those of Hansen and Ugelstad. In particular, these authors point out that the effect of seed particles in reducing the number of secondary particles decreases with emulsifier concentration, a phenomenon also seen in our experiments: in Figure 8 the slope of the data at 1.9 mM is considerably smaller than the slope at 0.6 mM. In addition to the extent of SPF, the conversion profiles of all runs were also determined. The analysis of such curves seems to suggest two distinct types of behavior. In certain cases, presumably when the extent of SPF is small, the monomer conversion is for the most part determined by the seed particles, increasing (as expected) with Nseed and djseed. Examples of this behavior are given in Figure 9a,b. In other cases, presumably when the extent of SPF is large enough, the contribution of the secondary particles to the polymerization kinetics becomes sufficient to offset that of the seed particles. A nice illustration of this is presented in Figure 9c, where we see that run B7 is actually faster than run B9 despite the smaller (half) number of seed particles in B7.

type and concentration of surfactant. Moreover, at the stirring rates employed, the contribution of shear aggregation to the total particle aggregation rate can be safely neglected. The results obtained with SDS contradict the ideas of Ugelstad et al.,7 who defend that the absence of an abrupt change in the N-[S] curve at the cmc is an important feature of VCM emulsion polymerization. Further experiments with pure, well-defined SDBS would be necessary to establish the cause of the distinct behavior of this surfactant with respect to SDS. Seeded polymerizations were performed to help quantify the role of the seed and the surfactant on secondary particle formation. It is seen that the number of new particles formed is determined not only by the rate of homogeneous nucleation, but also by the rates of homo- and/or heterocoagulation. Despite some experimental difficulties (mostly related to the characterization of the latexes by TEM), this approach has the potential to provide useful data to the development of particle formation models. Additional work in this area clearly deserves to be done given the limited number of available studies, not only with vinyl chloride, but also with monomers like styrene or vinyl acetate, which are simpler to manipulate.

4. Conclusions

Appendix: Effects of APS and 1,2-Dichloroethane on the cmc’s of SDS and SDBS

In this work, we have experimentally investigated the formation of latex particles in the ab initio and seeded emulsion polymerization of vinyl chloride. Ab initio experiments carried out over a wide range of concentrations of SDS and SDBS were used to determine the N-[S] relationships for these two emulsifiers. In the case of SDS, a sharp rise in the particle number is observed somewhat below the estimated value of the cmc. With SDBS, in contrast, a smooth N-[S] curve is found without any sign of a transition near the cmc. Doubling the concentration of initiator has no impact on the number of particles formed, independently of the

Acknowledgment The authors are grateful to CIRES (Estarreja, Portugal) for supplying the monomer. H.M.V. thanks the Portuguese Science and Technology Foundation for financial support (Grant SFRH/ BD/10513/2002).

. A1. Effect of APS Concentration. It is a well-known fact that the presence of electrolytes decreases the value of the cmc. This effect is not simply a function of the ionic strength of the medium (as sometimes suggested, e.g., Meadows et al.33), but actually depends on the type of electrolyte ions.34 In a previous study from our group on the absorption of SDS and SDBS, values were given for the cmc’s of these two surfactants in pure water.19 To quantify the effect of ammonium persulfate (the only electrolyte in the reaction mixture), ad-

Figure A1. Curves of surface tension versus surfactant concentration at various APS concentrations and 20 °C: (a) SDS and (b) SDBS.

8116 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008

The surface tension curves are shown in Figure A1. In the case of SDS, a small dip before the cmc point is observed, denoting the presence of traces of dodecanol (not fully removed in the recrystallizations). The transition point can, nonetheless, be estimated with sufficient accuracy for our purposes. The cmc values obtained from the surface tension curves of both emulsifiers are plotted in Figure A2. The values found for SDS are in good agreement with those reported by Tajima36 using NaCl as electrolyte (at equal cation concentration). The effect of APS concentration on the cmc’s is well described by the following empirical relation:34 cmc 1 + a[APS] ) cmc0 1 + b[APS]

Figure A2. Effect of APS concentration on cmc’s of SDS and SDBS at 20 °C. The solid lines represent the predictions of eq A1. Table A1. Parameters of Equation A1 emulsifier

a (mM-1)

b (mM-1)

SDS SDBS

0.000 0.053

0.133 0.642

ditional tensiometric measurements were carried out to determine the cmc’s of SDS and SDBS in aqueous APS solutions. The measurements were performed at 20 °C with a Kru¨ss K12 automatic tensiometer, equipped with a Du Nou¨y ring and a Metrohm 665 Dosimat. SDS was purified by recrystallization from water (two times).35

(A1)

where cmc0 is the cmc value in water. The values of the parameters a and b are determined by fitting the experimental data and are summarized in Table A1. As we can see in Figure A2, for the concentrations of APS used in this work (1-2 mM, or 0.23-0.46 g/L), the effect on the cmc of SDS is relatively small (10-20%). In contrast, a significant decrease is registered for SDBS (40-50%). A2. Effect of 1,2-Dichloroethane. The presence of monomer molecules solubilized in the aqueous phase may also decrease the cmc value. In principle, this effect can be quantified by measuring the cmc in a solution saturated with monomer. In the case of vinyl chloride, however, the measurement is not straightforward since VCM is a vapor at normal conditions. In view of this difficulty, we decided to use a substitute molecule as similar as possible to VCM, but liquid at normal conditions. The molecule chosen was 1,2-dichloroethane (1,2DCE). This same compound was used by Saethre et al.37 as a replacement for VCM in swelling experiments and cmc measurements. The solubility of 1,2-DCE in water is 8.6 g/L at

Figure A3. Effect of 1,2-DCE on the cmc’s of SDS and SDBS in pure water and aqueous APS solution (0.25 g/L) at 20 °C.

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20 °C, a value quite close to the solubility of VCM in water at the polymerization temperature, 10 g/L.38 The effect of 1,2-DCE on the cmc of SDS and SDBS was measured by tensiometry. The aqueous solution saturated in 1,2DCE was prepared by “dissolving” an excess of 1,2-DCE in water; the emulsion was then stirred vigorously for a long period and decanted. No attempt was made to detect the presence of droplets. The surface tension curves are shown in Figure A3. As we can see, the presence of 1,2-DCE lowers the surface tension of the solutions, but has no measurable effect on the cmc values. We also conclude that there is no interaction effect between APS and 1,2-DCE. These results are in line with those of Saethre et al.,37 who found the cmc of SDS in the presence of 1,2-DCE and some electrolytes (not specified) to be 6.9 mM (i.e., very close to the value in pure water). Of course, these experiments do not permit us to draw categorical conclusions on the effect of VCM on the cmc values. We can say, nonetheless, that they point to an absence of effect. Nomenclature dj ) average particle diameter djs ) average swollen particle diameter djseed ) average seed diameter [I] ) initiator concentration [M]p ) monomer concentration in the particle phase [M]w ) monomer concentration in the aqueous phase nj ) average number of radicals per particle N ) number of particles per unit volume of aqueous phase Nnew ) number of new particles per unit volume of aqueous phase Nseed ) number of seed particles per unit volume of aqueous phase Rp ) rate of polymerization [S] ) total surfactant concentration [S]w,0 ) initial concentration of free surfactant in the aqueous phase Greek Symbols θ0 ) initial surface coverage of the seed particles σeg ) surface charge density due to ionic end groups ω ) agitation rate AbbreViations APS ) ammonium persulfate AY ) sodium diamylsulfosuccinate cmc ) critical micellar concentration DBM ) sodium dibutylsulfosuccinate MA ) sodium diahexylsulfosuccinate M/W ) monomer-to-water ratio PVC ) poly(vinyl chloride) SDBS ) sodium dodecylbenzenesulfonate SDS ) sodium dodecyl sulfate SPF ) secondary particle formation TEM ) transmission electron microscopy VCM ) vinyl chloride monomer

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ReceiVed for reView November 07, 2007 ReVised manuscript receiVed June 18, 2008 Accepted June 19, 2008 IE0715153