Semibatch emulsion polymerization kinetics

Types of Reactor Processes Batch Reactors Semibatch Reactors Continuous Reactors Emulsion Polymerization Kinetics Other Preparation Methods... 

A first study of reaction kinetics with ACOMP was made for the semibatch emulsion polymerization of MMA at 70 °C [50]. The use of continuous monitoring method described in the previous sections offered a robust means of determining the characteristic features of the starved and flooded monomer conditions and identifying them during the experiments. 

Wessling [9] studied the reaction kinetics of semibatch emulsion polymerization of relatively hydrophobic monomers such as styrene and derived the following expression for the rate of polymerization at pseudo-steady state (Rp) ... 

Chern [42] developed a mechanistic model based on diffusion-controlled reaction mechanisms to predict the kinetics of the semibatch emulsion polymerization of styrene. Reasonable agreement between the model predictions and experimental data available in the literature was achieved. Computer simulation results showed that the polymerization system approaches Smith-Ewart Case 2 kinetics (n = 0.5) when the concentration of monomer in the latex particles is close to the saturation value. By contrast, the polymerization system under the monomer-starved condition is characterized by the diffusion-con-trolled reaction mechanisms (n > 0.5). The author also developed a model to predict the effect of desorption of free radicals out of the latex particles on the kinetics of the semibatch emulsion polymerization of methyl acrylate [43]. The validity of the kinetic model was confirmed by the experimental data for a wide range of monomer feed rates. The desorption rate constant for methyl acrylate at 50°C was determined to be 4 x 10 cm s ... 

Sajjadi [47] developed two mechanistic models for the particle nucleation process involved in the semibatch emulsion polymerization of styrene under the monomer-starved condition. In the first model, Smith-Ewart theory was extended to take into account the particle nucleation under the monomer-starved condition. The number of latex particles per unit volume of water is proportional to the surfactant concentration, the rate of initiator decomposition, and the rate of monomer addition, respectively, to the 1.0,2/3, and -2/3 powers. The second model considers the aqueous phase polymerization kinetics and its effect on the efficiency of free radical capture by the monomer-swollen micelles. This model is capable of predicting some features of the particle nucleation process. 

It is not straightforward to successfully manufacture a particular latex product, which is generally developed in a laboratory batch or semibatch reactor, in a commercial continuous emulsion polymerization system (e.g., a continuous stirred tank reactor). This is simply because the characteristics of continuous stirred tank reactors are dramatically different from those of batch and semibatch reactors. As a consequence, the particle nucleation process and kinetics experienced in batch or semibatch emulsion polymerization systems cannot be directly applied to continuous systems consisting of stirred tank reactors. 

The batch emulsion polymerization is commonly used in the laboratory to study the reaction mechanisms, to develop new latex products and to obtain kinetic data for the process development and the reactor scale-up. Most of the commercial latex products are manufactured by semibatch or continuous reaction systems due to the very exothermic nature of the free radical polymerization and the rather limited heat transfer capacity in large-scale reactors. One major difference among the above reported polymerization processes is the residence time distribution of the growing particles within the reactor. The broadness of the residence time distribution in decreasing order is continuous>semibatch>batch. As a consequence, the broadness of the resultant particle size distribution in decreasing order is continuous>semibatch>batch, and the rate of polymerization generally follows the trend batch>semibatch>continuous. Furthermore, the versatile semibatch and continuous emulsion polymerization processes offer the operational flexibility to produce latex products with controlled polymer composition and particle morphology. This may have an important influence on the application properties of latex products [270]. 

Emulsion polymerization studies reported in the scientific literature are usually based on experiments with batch or semibatch reactor systems. Since most workers in the field are familiar with such reactors, the thrust of this discussion will be to compare continuous reactors with batch and semi-batch operations. The particular areas to be reviewed include (i) inhibitor effects, (ii) particle age distributions, (iii) particle nucleation, (iv) copolymerization, (v) particle morphology, (vi) temperature control and heat removal and (vii) polymerization kinetic models. 

Semibatch processes are usually carried out using two different feed types neat monomer feed, where only monomer is flowed into reactor, or monomer emulsion feed, in which aqueous emulsifier is added with the monomer. The systems, for which a critical flow rate has been reached, such that the rate of polymerization is controlled by the rate of monomer addition, are called starved [46]. In this case, there are no more monomer droplets and the monomer concentration in the polymer particles is lower than the saturated concentration. If the feed rate is higher than the polymerization rate, the monomer accumulates in the reactor as monomer droplets. This is usually termed a flooded system. In this case, the particles are completely saturated with monomer, and reaction kinetics resembles those of a batch emulsion polymerization. 

Several reports have been published on the in-line monitoring of vinyl acetate emulsion polymerization reactions in semibatch mode [22]. With appropriate models, this approach can provide good feedback about the polymerization reaction kinetics. Heat flow calorimetry (Hfc) is frequently used to... 

There is no doubt that the discipline of interfacial phenomena is an indispensable part of emulsion polymerization. Thus, the goal of this chapter is to offer the reader an introductory discussion on the interfacial phenomena related to the emulsion polymerization process, industrial emulsion polymerization processes (primarily the semibatch and continuous reaction systems), some important end-use properties of latex products, and some industrial apphcations. In this manner, the reader may effectively grasp the key features of emulsion polymerization mechanisms and kinetics. Some general readings in this vital interdisciphnary research area [1-6] are recommended for those who need to familiarize themselves with an introduction to the basic concepts of colloid and interface science. 

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