To implement the strategy outlined in Figure 1, it is necessary to translate the ultimate product performance in terms of polymer microstructure by means of quantitative polymer micro-structure–property relationships. Although examples of quantitative micro-structure–property relationships are available (e.g., for the effect of copolymer composition profile82 and MWD83, 84 on adhesive properties and for the effect of PSD on latex rheology85), much work is needed in this area.
Once the desired microstructure of the polymer is known, an optimal trajectory should be computed. When a mathematical model is available, the optimal trajectory can be calculated with standard optimization algorithms.86–90 The first-principles mathematical models for emulsion polymerization are often complex and neural networks91–93 have been used to reduce the complexity of the problem. In some cases, a good understanding of the process allows the simplification of the optimization problem. Thus, for the production of copolymers with a given composition (YA), it is sufficient to maintain the ratio of monomer concentrations at the value calculated from the Mayo–Lewis equation94–96
where [i]p is the concentration of monomer i in the polymer particles, K1=YA (1−YA), and ri are the reactivity ratios. To correlate the concentrations of the monomers in the polymer particles with the total amount of monomers in the reactor, the partitioning of the monomers among the different phases should be calculated. This can be done with models of different complexity. The models have been reviewed by Gugliotta et al.,97 who on the basis of using the simplest but sufficiently accurate model, recommended constant partition coefficients for monomers of low and moderate water solubility (<5/100 g of water) and the Morton model98 for highly water-soluble monomers (e.g., acrylic acid). The reactivity ratios used in eq 1 are based on the ultimate model, which has been reported99 to accurately describe the evolution of the copolymer composition. Methods to estimate the reactivity ratios from emulsion polymerization experiments with the whole range of monomer conversions have been reported.100
In addition, maximum production in emulsion polymerization reactors is limited by the heat-removal capacity of the reactor. Therefore, the optimal process is a process in which the rate of heat generation by polymerization is equal to the safe, maximum heat-removal rate of the reactor. In this context, the safe, maximum heat-removal rate means that some additional cooling power is available as a safety margin. When the heat-removal capacity of the reactor is known, the optimal trajectory is readily available and has been used in both open-loop101 and closed-loop102 control strategies. In addition, the heuristic knowledge of the plant operators can be used to build a fuzzy system to determine online the optimal trajectory.103
To follow the optimal polymerization trajectory, both open-loop and closed-loop control may be used. Early strategies were based on open-loop strategies,94–96, 101, 104 but the run-to-run irreproducibility forced the development of closed-loop control. A successful closed-loop control strategy needs robust and accurate online monitoring devices.
The development of accurate and robust online monitoring devices is the limiting factor for the implementation of control strategies in emulsion polymerization reactors. Excellent reviews on online monitoring of (emulsion) polymerizations are available.105–108 There are three main reasons for these limitations. First, latices are thermodynamically unstable multiphase systems prone to suffer coagulation. Second, emulsion polymers are complex materials with multiple characteristics (copolymer composition, chemical composition distribution, MWD, branching, gel, PSD, particle morphology, etc.), whose determination requires the use of a variety of experimental techniques. Third, some of the analysis requires long times (sometimes longer than the polymerization time), the equipments are expensive, and are not adapted for online purposes.
In practice, only a few characteristics are observable online. Here, the term observable is used in a somehow loose way and includes the characteristics that can either be directly measured online or that can be estimated in a relatively precise way from online measurements. Monomer concentration, the polymerization rate, and copolymer composition are the characteristics most often measured online. The three magnitudes are related through the material balance.
Online analysis of unreacted monomers in high solids (55 wt %) emulsion polymerization was achieved with gas chromatography (GC).109 The setup was used to monitor emulsion copolymerizations110 and terpolymerizations111 as well as the consumption of CTAs.87 However, online GC is prone to suffer mechanical problems.
The polymerization rate is best measured with either heat-flow or heat-balance reaction calorimetry.112 Heat-flow reaction calorimetry is best adapted for small lab reactors. The main limitation of this technique is that the value of the overall heat-transfer coefficient must be known. This limitation may be overcome with oscillatory heat-flow calorimetry that allows the online estimation of the overall heat-transfer coefficient.113 The usefulness of oscillatory calorimetry is limited to small reactors.114, 115 Heat-balance calorimetry is best suited for large-scale commercial reactors. Reaction calorimetry allows the online determination of the rate of heat generation by polymerization, which is proportional to the polymerization rate. Integration of the polymerization rate over time allows the estimation of monomer conversion and copolymer composition. This was first demonstrated by Urretabizkaia et at.,116 and since then both open-loop117–121 and closed-loop122, 123 estimators have been proposed.
Under starved conditions, which are commonly used in emulsion polymerization, the accuracy of reaction calorimetry for the estimation of the amount of unreacted monomer is limited. This can be illustrated by the following example. If an emulsion polymerization is conducted in such a way that the instantaneous conversion is 0.9 and applying reaction calorimetry the estimated conversion is 0.94, the error in monomer conversion is about 4%, which for most applications would be acceptable. However, the estimated fraction of unreacted monomer would be 0.06 instead of 0.1, which represents a 40% error. This may have severe consequences in process control because the polymerization rate and polymer characteristics depend on the amount of unreacted monomer in the reactor.
Spectroscopic techniques are in principle able to provide a direct measurement of the unreacted monomer. An excellent discussion of the strengths and weaknesses of the different spectroscopic techniques has been published by Hergeth.107, 124 The recent development of fiber-optic probes suitable for remotely collecting spectra via optical fibers has given rise to the possibility of making in situ measurements in remote and harsh environments125 (high temperatures, pressures, toxic environments, etc.). The spectroscopic techniques coupled with fiber optics have a high potential for online monitoring and can provide important information about the state and nature of the samples under analysis. An additional advantage is that a fiber-optic probe can be installed in an existing reactor without time-consuming and expensive modifications.
Raman spectroscopy is well suited for online monitoring in emulsion polymerization because water has a very weak Raman response, and double and triple bonds in monomers and polymers are very strong Raman scatterers. Styrene/butadiene,126, 127 styrene/n-BA,128 and Veova 9/BA129 emulsion copolymerizations have been monitored by means of Raman spectroscopy. For systems containing styrene, usually the peak associated with the ring-breathing mode of styrene at 1000 cm−1 is used to normalize the spectral intensity, and the calibration stage does not present any difficulties.125 All acrylic copolymerizations are much more complex because the bands of the main functional groups of the different acrylic monomers of the formulation overlap because of the similarity in the chemical structure. Therefore, univariate calibration methods are not appropriate, and multivariate calibration techniques such as partial least-squares regression130 are required. An online monitoring technique based on Fourier transform Raman for all acrylics high-solids-content (50 wt %) emulsion copolymerizations has been developed.131 The method was applied to a system containing n-BA and MMA. Unreacted monomer amounts, solids content, and cumulative copolymer compositions were the variables monitored.
Particle size and PSD strongly affect the emulsion polymerization process as well as the application properties of the latex. The accurate offline determination of the latex PSD is still an unsolved issue,132 and the advances in online monitoring of this variable are modest.133–136 Artificial neural networks were evaluated as soft sensors to monitor particle size online during the 55 wt % emulsion polymerization of VAc and Veova 10 carried out in a continuous loop reactor.137
The ultimate goal of the control strategies is to achieve maximum production of emulsion polymers of consistent quality under safe and environmentally friendly conditions. Because emulsion polymerization is prone to suffer run-to-run irreproducibility, only feedback control may ensure the consistency of the product quality. In addition, product quality depends on many microstructural characteristics of the latex including copolymer composition, MWD, branching, crosslinking, gel content, particle morphology, and PSD. However, no attempts to simultaneously control all of these properties have been reported. Reviews on this subject are available.138, 139
Polymer composition is the characteristic more frequently controlled. Control schemes based on GC monitoring were developed,140 and although in some cases the composition of copolymers110 and terpolymers111 of relatively high solids content (55 wt %) was controlled, online GC was prone to suffer mechanical problems. Successful strategies for copolymer118, 119, 141 and terpolymer120, 142 composition control based on reaction calorimetry have been reported. The problem of the maximum production of latices of well-defined composition was addressed by Saenz de Buruaga et al.102 The control scheme developed by these researchers was also able to avoid monomer accumulation in the reactor that may lead to potentially dangerous thermal runaways.
Emulsion linear homopolymers of well-defined MWD were obtained by means of control strategies based on online GC measurements of both unreacted monomer and CTA87 and on reaction calorimetry.143 The simultaneous online control of copolymer composition and MWD of linear copolymers based on reaction calorimetry has also been reported.144 The strategy for MWD control, which is summarized in Figure 8, was based on the fact that for linear polymers produced by free-radical polymerization, the polymer chains do not suffer modifications once they are formed. This opens the possibility of decomposing the desired final MWD in a series of instantaneous MWDs to be produced at different stages of the process. Each of the instantaneous MWDs is characterized by a single parameter, the number-average molecular weight (Xn), which depends on the monomer/CTA ratio
where kfCTA is the CTA constant. When combining the two pieces of information, the evolution of the monomer/CTA ratio required to achieve the desired MWD is calculated (Fig. 8).
The formation of nonlinear polymers involves processes such as chain transfer to polymer and propagation to terminal and pendant double bonds, which imply that the inactive chains may reenter in the polymerization modifying their molecular weight. This makes the online control of the MWD of nonlinear polymers more challenging.145, 146 Although open-loop control strategies have been developed,89 the closed-loop control of the MWD of these polymers is still a pending issue.
In emulsion polymerization the reaction rarely proceeds to completion and inevitably some amount of unreacted monomer remains in the polymer. Because of environmental regulations and market preferences, it is necessary to remove unreacted monomers and other organic compounds (VOCs) from the latex.
Both postpolymerization and devolatilization are used to reduce the residual monomer content in latices.147 Postpolymerization consists of adding, after the end of the main polymerization process, initiators to polymerize the residual monomer. This is the preferred method for monomer removal because it may be carried out in the polymerization reactor or in the storage tank, and no additional equipment is needed. Water-soluble redox initiators yielding hydrophobic radicals present advantages for monomer removal by postpolymerization, independently of the water solubility of the monomers.148 The main reason is that hydrophobic radicals can enter into the polymer particles, where most of the residual monomer is, much easier than the hydrophilic radicals, which must undergo a number of propagation steps before becoming hydrophobic to be able to enter into the polymer particles. However, some of these redox systems (e.g., those containing tert-butyl hydroperoxide) may suffer secondary reactions that would give VOCs as byproducts.149 Model-based optimal postpolymerization strategies minimizing the amounts of both the residual monomers and VOCs have been developed.150
Postpolymerization cannot be applied to the removal of nonpolymerizable VOCs. These compounds may be impurities contained in the raw materials as well as products of side reactions occurring during the polymerization and/or postpolymerization. When nonpolymerizable VOCs are present, devolatilization must be used. In the devolatilization, the latex is stripped with either steam or inert gas in vacuo conditions until acceptable low concentrations of residual monomer and VOCs are reached. The main advantage of this process is that both monomer and nonpolymerizable VOCs can be removed. However, devolatilization is highly energy-consuming and requires additional investments in equipment. In addition, under some conditions, foaming and coagulation may occur. Devolatilization experiments carried out with VAc/BA/acrylic acid latices of different particle sizes and under different agitation and sparger geometries showed that the mass transfer from the aqueous phase to the gas phase was the controlling step.151 This implies that the process variables involved in the mass transfer between the aqueous phase and the gas phase, such as agitation, geometry of the sparger or gas-flow rate would improve devolatilization.
High-Solids, Low-Viscosity Latices
The synthesis of high-solids, low-viscosity latices has raised great interest from both industry and academia.152 Possible advantages of highly concentrated emulsions, understood by a highly concentrated latex with a solids content above 60 wt %, are numerous, including the higher unitary usage of industrial installations and the faster drying rates during application. Low viscosity is required for a higher heat-removal rate and a better mixing during the polymerization process that allows improvements in safety, production capacity, and product quality. For a given solids content, the latex viscosity decreases with the broadness of the PSD. In addition, bimodal PSDs containing about 20 wt % of small particles and 80 wt % of large particles yield low-viscosity latices. The viscosity of these bimodal PSDs is further reduced by increasing the size of the large particles. This heuristic knowledge has prompted a number of polymerization strategies. Masa et al.153 and Unzué and Asua154 used semicontinuous processes in which both the initial charge and the feed were monomer miniemulsions. The continuous nucleation of the miniemulsion droplets resulted in a broad PSD that allowed obtaining a 65 wt % solids constant latex of low viscosity. Leiza et al.155 obtained a 61 wt % solids content latex by preparing by miniemulsion polymerization a latex with a rather broad PSD as initial charge and using this latex as seed in a conventional, semicontinuous emulsion polymerization. Bimodal PSDs have been prepared in semicontinuous emulsion polymerizations by generating a second crop of particles through the addition of a shot of emulsifier.156 The use of several seeds is a popular way of producing bimodal latices. Chu and Guyot157 used a large particle size seed in the initial charge, and a small one was added as a shot during the process. Schneider et al.158, 159 favored the growth of the large particles with an oil-soluble initiator.
To a great extent, the strategies for the production of high-solids-content, low-viscosity latices outlined above were developed based on heuristic knowledge through trial-and-error approaches. To implement the strategy outlined in Figure 1 to the production of high-solids, low-viscosity latices, a quantitative relationship for the effect of PSD on latex rheology should be available. Recently, do Amaral et al.85 experimentally assessed the capability of Sudduth's160 viscosity equation to account for the influence of both the PSD and the physicochemical characteristics of the dispersion. Combining the viscosity equation with a polymerization model, do Amaral et al.161 developed a knowledge-based approach, which was used to explore possible polymerization scenarios (Fig. 9) in such a way that the most promising reaction conditions were identified and experimentally checked.162, 163 In this way, high-solids-content latices with fine-tuned viscosity were obtained164 (Table 2).
Figure 9. Effect of the sizes of the small and large particles on the viscosity latex with a trimodal PSD [solids content: 70 vol %, weight proportion of particles: 80/10/10 (large/medium/small), medium particle size equal to 1.5 dsmall.
Download figure to PowerPoint
Table 2. High-Solids Content Latices with Fine-Tuned Viscosity
|Solids Content (wt %)||Viscosity (mPa · s)|
Process intensification refers to technologies that replace large, expensive, energy-intensive equipment or processes with ones that are smaller, less costly, and more efficient. The development of the continuous loop reactor165 (CLR) is an example of process intensification in emulsion polymerization. This reactor consists of a tubular loop that connects the inlet and the outlet of a recycle pump. Reactants are continuously fed into the reactor, and the product is continuously withdrawn from the reactor. Because of its large heat-transfer area/reactor volume ratio, high conversions in short residence times can be achieved. This results in a substantial reduction of the reactor volume.166 Because of the small volume and the short residence time, the CLR can be used with great flexibility and minimum losses in the manufacture of different emulsion polymers. The small volume and the absence of head space make the process intrinsically safe. Likely, the main drawback is that because of the presence of the recycle pump, formulations with high mechanical stability are required to prevent shear-induced coagulation.
Abad et al.167 compared the performance of a CLR with that of a continuous stirred tank reactor (CSTR), finding that the behavior of both reactors is almost the same at low heat-generation rates; otherwise, thermal runaway occurred in the CSTR while the temperature of the CLR was easily controlled. It has been reported168 that the startup procedure did not affect the steady-state values of the monomer conversion, number of polymer particles, and MWD, but the smoothness of the operation could be substantially improved if the reactor was initially filled with previously formed latex. Araujo et al.169 examined the effect of temperature, residence time distribution, and initiator concentration on the performance of a CLR in the redox-initiated emulsion copolymerization of VAc and Veova 10 under industrial-like conditions. For these latices, the technological goal is to achieve high conversions in short residence times (maximizing production rate) of a high-molecular-weight (maximum wet-scrub resistance of the paint formulated with this latex) and water-resistant polymer. However, it was found that to achieve high conversion at long residence times, high temperatures (that reduce the molecular weight) or high initiator concentrations (that reduce both the molecular weight and the water resistance of the polymer) should be used. Nevertheless, it has been demonstrated170 that it is possible to take advantage of the particular reactor dynamics and conduct the polymerization in such a way that a pseudosteady state is reached in which high monomer conversions at short residence times, low temperatures, and low initiator concentrations are obtained. Basically, this strategy consists of starting the process at relatively high values of the residence time, temperature, and initiator concentration and later reducing them to the desired values. If these final conditions were implemented from the beginning of the process, low conversions, and sometimes coagulation, would be obtained.