Semibatch emulsion polymerization of butyl acrylate. II. Effects of emulsifier distribution

The effects of emulsifier distribution ratio between the initial charge and the feed on particle formation and kinetics of butyl acrylate emulsion polymerization, using sodium lauryl sulfate as emulsifier and potassium persulfate as initiator, were investigated. The number of particles increased with initial emulsifier concentration in the reactor charge. It was shown that traditional ranking, in terms of number of particles produced, of semibatch emulsion polymerization with monomer emulsion feed is not always justified and a semibatch emulsion polymerization can produce far more particles than a conventional batch emulsion polymerization. The number of polymer particles was found to be practically independent of the emulsifier distribution ratio between the charge and the feed for a high overall emulsifier concentration, while for a low overall emulsifier concentration, the number of particles increased with initial loading of the emulsifier. The polydispersity index (PDI) of the final latexes showed a minimum with emulsifier distribution. A bimodal particle size distribution, and a latex with a large PDI, was obtained when there was no emulsifier in the charge. As the initial emulsifier charge increased, a unimodal PSD with a smaller PDI was obtained. With higher proportions of emulsifier in the initial charge, the PDI rose again due to particle nucleation at monomer‐starved conditions, and a skewed unimodal PSD was obtained. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 582–597, 2001     

Semibatch emulsion polymerization of butyl acrylate. I. Effect of monomer distribution

The effects of initial monomer charge on the particle formation and on the rate of polymerization were investigated for semibatch emulsion polymerization of butyl acrylate using sodium lauryl sulfate (SLS) as a surfactant and potassium persulfate (KPS) as an initiator. For the semibatch process with monomer (M) feed, it was found that by varying the monomer distribution ratio between the initial reactor charge and the feed it is possible to alter the contribution of monomer‐flooded and monomer‐starved nucleation mechanisms to the whole nucleation process. The number of particles increases as the initial monomer charge decreases, if the monomer concentration is below a critical value for any fixed system. The increase in number of particles is associated with a broad particle‐size distribution which might depict an emerging second peak on the particle‐size distribution curve. For low emulsifier concentration systems, a larger number of particles was obtained for a lower amount of monomer charge. Particle coagulation and emulsifier adsorption on the monomer droplets were counted as the main reasons for such behavior. For a semibatch process with monomer emulsion (ME) feed, the larger number of particles was formed at a lower initial monomer charge, similar to an M‐add semibatch process. However, the application of monomer charge to an ME‐add process was found to increase the possibility of secondary nucleation and led to the occurrence of a bimodal particle‐size distribution. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 3094–3110, 1999     

Semibatch anionic ring-opening polymerization of octamethylcyclotetrasiloxane in emulsion

The semibatch anionic ring-opening polymerization of octamethylcyclotetrasiloxane in emulsion was studied under the condition of no seed particles used in polymerization system. Concentration of emulsifier was set above critical micelle concentration. Effect of monomer feed rate on the chemical kinetics, average particle size and distribution and on the polymer molar mass and distribution was investigated. During monomer addition, polymerization rate was constant and controlled by the monomer feed rate. According to the proposed mechanism for anionic polymerization in emulsion, it was assumed that also the number of active polymerization sites and monomer concentration at the particle surface, where propagation occurs, remain constant. A continuous increase of average particle size and absence of extensive condensation reactions at high conversion suggested that the freshly added monomer diffuses toward hydrophobic polymer particles, where it is consumed in propagation reaction and/or accumulated in the particle core.     

Winsor I-like (micro)emulsion polymerization of styrene initiated by oil-soluble initiator

Batch emulsion polymerization of styrene initiated by an oil-soluble initiator and stabilized by non-ionic emulsifier (Tween 20) has been investigated. The rate of polymerization vs. conversion curve shows the two non-stationary rate intervals typical for the non-stationary-state polymerization. This behavior is a result of the continuous particle nucleation and the decrease of monomer concentration at the reaction loci with increasing conversion. The initial increase of the polymerization rate is attributed to the increase of particle number and the polymerization proceeding under the monomer-saturated condition—the Winsor I-like (micro)emulsion polymerization. The decrease of the polymerization rate is the result of the depressed transfer of monomer from the monomer reservoir to the reaction loci. Above 50 °C the monomer emulsion separates into two phases: the upper transparent monomer phase and the lower blue colored (microemulsion) phase. The polymerization mixture consists of the microdroplets (act as the reaction loci) and large degradable monomer droplets (act as the reservoir monomer and emulsifier). The continuous release of emulsifier from the monomer phase and the microdroplets induce the continuous particle nucleation up to high conversion. The initial formation of large particles results from the agglomeration of unstable growing particles and monomer droplets. The size of large (highly monomer-swollen) particles decreases with conversion and they merge with the growing particles at ca. 40-50% conversion. The coarse initial emulsion transformed during polymerization to the fine (semitransparent) polymer emulsion as a result of the continuous particle nucleation, the shrinking of highly monomer-swollen polymer particles and the depletion of monomer droplets. The low overall activation energy of polymerization is mainly ascribed to the decreased barrier for entering radicals into the latex particles with increasing temperature.     

Anionic ring-opening polymerization of octamethylcyclotetrasiloxane in emulsion above critical micelle concentration

Batch anionic ring-opening polymerization of octamethylcyclotetrasiloxane in emulsion using nonionic and cationic emulsifiers was studied. The concentration of emulsifiers was set above their critical micelle concentration. Effects of emulsifier concentration, nonionic/cationic emulsifier ratio and cationic emulsifier/initiator (KOH) ratio on the kinetics, average particle size and distribution and on the average molecular weight and distribution were investigated and discussed. At the beginning of the polymerization, empty micelles, active micelles (polymer particles) and monomer droplets co-exist in emulsion. The transport of monomer from monomer droplets toward empty micelles was confirmed by monomer droplets and empty micelles disappearance and by formation of smaller particles. The transport of monomer from monomer droplets toward polymer particles was not confirmed, since the average polymer particle size did not increase during polymerization. It was proposed, that at lower conversions, monomer diffuses from polymer particle interior to particle surface, while at higher conversions, the monomer diffuses from larger to smaller polymer particles. Emulsifier concentration, nonionic/cationic emulsifier ratio and cationic emulsifier/KOH ratio have an evident effect on the kinetics and on the average molecular weight, thus demonstrating that cationic emulsifier participates to the initiation reaction.     

Population balance modeling of particle size distribution in monomer‐starved semibatch emulsion polymerization

The evolution of particle size distribution (PSD) in the monomer‐starved semibatch emulsion polymerization of styrene with a neat monomer feed is investigated using a population balance model. The system under study ranges from conventional batch emulsion to semicontinuous (micro)emulsion polymerization depending on the rate of monomer addition. It is shown that, contrary to what is often believed, the broadness of PSD is not necessarily associated with the length of nucleation period. The PSDs at the end of nucleation are found to be independent of surfactant concentration. Simulation results indicate that at the completion of nucleation the particle size is reduced and the PSD narrows with decreasing rate of monomer addition despite nucleation time increasing. The broad distribution of particles frequently encountered in semibatch emulsion polymerizations is therefore attributed to stochastic broadening during the growth stage. The zero‐one‐two‐three model developed in this article allows perceiving that the dominant kinetic mechanism may be different for particles with different sizes. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Particle nucleation mechanism for the emulsion polymerization of styrene with a novel polyester emulsifier

The particle nucleation mechanism in emulsion polymerization of styrene with a novel polyester emulsifier, 5‐sulfoisophthalic acid dimethyl ester sodium salt‐modified tetracarboxylic acid‐terminated polyester (SMTAPE), was investigated. The consumption of SMTAPE micelles was monitored by the measurement of surface tension during the emulsion polymerization. Kinetic studies and emulsifier consumption clearly showed that a continuous nucleation mechanism without Smith–Ewart interval II was characteristic of this system. It was attributed to the high concentration of SMTAPE emulsifier in the polymerization, which led to a large surface area and a vast number of micelles around 10 nm in size that served as the major locus of particle nucleation. A broad particle size distribution was observed throughout the reaction, and the nucleation period lasted well into the reaction until the disappearance of the micelles or the disappearance of monomer droplets. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1061–1070, 2001     

Semibatch anionic ring‐opening polymerization of octamethylcyclotetrasiloxane in emulsions: effect of the amount of seed polymer particles

Semibatch anionic ring‐opening polymerization of octamethylcyclotetrasiloxane with the use of seed polymer particles in emulsions was studied. The concentration of the emulsifier was set above the critical micelle concentration. We investigated the effect of the amount of seed polymer particles on the chemical kinetics and the average particle size and distribution. During monomer starving conditions the polymerization rate strongly depended on the monomer feed rate and not on the amount of seed particles. Throughout the entire monomer feed period the average particle size increased. This increase depended on the number and the size of seed particles. In emulsions with higher particle sizes higher equilibrium conversions were obtained. In our opinion, a greater extent of backbiting reactions is responsible for lower equilibrium conversions during and at the end of the process. The seeded semibatch process seems a reasonable choice for designing emulsion products with high monomer conversion and desired particle size. © 2012 Society of Chemical Industry     

Particle size distribution control in emulsion polymerization

Effects of the operating policies—the initial initiator amount; the initial emulsifier amount; the monomer addition mode: batch or semibatch; and the monomer addition rate under “monomer‐starved conditions” for the control of particle size distribution (PSD)—were studied through a model that simulates batch and semibatch reactor operations in conventional emulsion polymerization. The population balance model incorporates both the nucleation stage and the growth stage. The full PSDs were reported, which have normally been omitted in earlier studies. It was shown through simulations that the broadness of the distributions, both initial (obtained after the end of nucleation) and final (after complete conversion of monomer), can be controlled by the initial initiator amount and the emulsifier amount. The higher initiator amounts and the lower emulsifier amounts favor narrower initial and final distributions. The shape of the initial PSDs and the trends in the average size and range were preserved with subsequent addition of monomer in the batch or in the semibatch mode, although the final PSD was always considerably narrower than that of the initial PSD. The addition of monomer in the semibatch mode gave narrower distribution compared to that of the batch mode, and also, lower monomer addition rates gave narrower distributions (larger average sizes), which was a new result. It was further shown through simulations that, under monomer‐starved conditions, the reaction rate closely matched the monomer feed rate. These conclusions are explained (1) qualitatively—the shorter the length of the nucleation stage and the larger the length of the growth stage (provided the number of particles remains the same), the narrower is the distribution; and (2) mathematically—in terms of the “self‐sharpening” effect. Experimental evidence in favor of the self‐sharpening effect was given by analyzing the experimental particle size distributions in detail. The practical significance of this work was proposed. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2884–2902, 2004     

Effect of stirring on the emulsion polymerization of styrene

The purpose of this paper is to clarify the effect of stirring on the course of emulsion polymerization of, for example, styrene. It establishes the existence of an optimum range of stirring speed and three important factors which must be considered in carrying out emulsion polymerization. (1) Stirring significantly affects the course of reaction in the presence of an imperfectly purified nitrogen atmosphere. Consequently, the number of polymer particles produced and the polymerization rate per particle will be affected. (2) At higher stirring speeds, polymer particles coagulate and coalesce. At lower stirring speeds, the reaction rate is controlled by the monomer transport rate from monomer droplets to the aqueous phase. (3) Stirring contributes to the reduction of the number of micelles because emulsifier molecules are adsorbed onto the surfaces of monomer droplets finely dispersed by the stirring. At low emulsifier concentrations near the critical micelle concentration, this effect cannot be neglected.     

Modeling and simulation of semibatch emulsion polymerization

By taking advantage of recent theoretical developments in emulsion polymerization and radical polymerization kinetics at high conversion, a model is developed for simulating seeded semibatch emulsion polymerization. The model is used to calculate the time evolution of the polymerization rate, the monomer conversion, the instantaneous degree of polymerization, the monomer concentration in particles, the average radical number per particle, and the propagation rate coefficient and termination rate coefficient over the whole course of the polymerization. Experimental observations by previous workers, including the existence of steady-state phenomenon and the broad distributions of molecular weight, are reproduced in the results of the simulation. The gel effect on emulsion polymerization and the open-loop stability of the semibatch process are discussed. © 1993 John Wiley & Sons, Inc.     

Miniemulsion and macroemulsion copolymerization of vinyl acetate with vinyl versatate

The miniemulsion and macroemulsion polymerization of vinyl acetate with vinyl versatate in batch and semibatch systems was investigated. Vinyl versatate was added either as an emulsion with the vinyl acetate, or as a neat liquid stream. In the batch runs, there is a poor dispersion of vinyl versatate during the nucleation period for the runs in which the vinyl versatate was added neat at the beginning of the polymerization. This led to smaller particles, lower polymerization rate, and different polymer composition evolution when compared with runs in which the vinyl versatate was emulsified with the vinyl acetate. In seeded semibatch runs, residual surfactant in the seed latex, along with the propensity for homogeneous nucleation in vinyl acetate emulsions, resulted in continuing nucleation during the entire semibatch interval. The polymerization rate was primarily affected by monomer feed rate rather than the feeding mode. The effect of monomer feeding mode on copolymer composition was weak when the semibatch feed rate was low, indicating some level of vinyl versatate mass transfer resistance. In all runs, only one glass transition temperature was observed, indicating effective copolymerization. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2219–2229, 2002     

Kinetics of emulsifier adsorption and nucleation of latex particles

Evidence suggesting that the ultimate latex particles formed in the emulsion polymerisation of styrene are formed by the coalescence of unstable primary particles appears incompatible with the usual assumption that latex particles are predominantly nucleated from monomer-swollen micelles in this system. However, this is not necessarily the case if the rate of emulsifier adsorption is not always sufficient to maintain a saturated monolayer of adsorbed emulsifier at the surface of growing latex particles so long as emulsifier micelles remain in the system. This appears to be the situation particularly when non-ionic emulsifiers are used. Even with ionic emulsifiers abnormally high emulsifier/monomer ratios are needed to prevent the coalescence of primary latex particles. Some very high molecular weight polymer (controlled by the transfer constant to monomer) would be expected in the initial stage of styrene emulsion polymerisation if the micellar nucleation of latex particles is dominant.     

Thiol-ene emulsion polymerization using a semi-continuous approach

The thermally initiated thiol-ene emulsion polymerization of diallyl phthalate (DAP) diene and ethylenedioxy diethanthiol (EDDT) dithiol monomers in batch and semibatch emulsion polymerization is investigated. The batch process leads to larger and broader particle sizes than when the polymerization is carried out in semibatch. The evolution of the particle size and the final particle size distribution indicate that the stability of the latexes is limited and, hence, aggregation phenomena occurred in both processes. In particular, the evolution of the particle size distribution (PSD) in the semibatch process indicates nucleation, growth, and aggregation occurring simultaneously that produced a bimodal particle size. When the diene monomer was changed to diallyl terephthalate (DATP), the semibatch polymerization yielded smaller particles and narrow distribution without any indication of aggregation. The partial substitution of the dithiol by a trithiol monomer that is substantially more water insoluble affected nucleation of the particles, yielding for both systems smaller particles. The polythioether polymers synthesized present low glass transition temperatures (~ −30/−40°C) and those containing the therephthalate yield crystalline films. The potential application of the polymers as pressure sensitive adhesives (PSAs) was preliminarily assessed.     

Kinetic investigations of acrylic–polyurethane composite latex

The effect of various reaction parameters on the rate of polymerization, R_p, and on the particle size and morphology of aqueous acrylic–polyurethane hybrid dispersions, prepared by semibatch emulsion polymerization, was investigated. The particles of polyurethane dispersion were used as seeds during the polymerization of acrylic component: methyl methacrylate (MMA), butyl acrylate (BA), and a mixture of MMA and BA with the ratio of 1:1. These emulsions were found to form structured polymer particles in aqueous media using scanning electron microscopy. The kinetics of the emulsion polymerization was studied on the basis of Wessling's model. The influence of emulsifier and initiator concentrations, including the monomer feed rates, R_m, on the rates of polymerization and on the properties of the resulting dispersions were studied. The number of particles and the particle size were also measured during the polymerization process. The final values were found to be independent of the concentration of the emulsifier, initiator and the monomer feed rate in monomer starved conditions. In the steady‐state conditions, during the seeded semibatch hybrid emulsion polymerization, the rate of polymerization and the monomer feed rate followed the Wessling relationship 1/R_p = 1/K + 1/R_m. The dispersions MMA/PU, BA/PU, and MMA/BA/PU have K values of 0.0441, 0.0419 and 0.0436 mol/min, respectively. The seeded BA/PU hybrid polymerization proceeded according to Smith‐Ewart Case I kinetics, while the MMA/PU hybrid emulsions demonstrate Case II of the Smith‐Ewart kinetic model. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2639–2649, 2002     

Continuous tube-CSTR reactor system for emulsion polymerization kinetic studies

A plug-flow tubular reactor (PFT) was used to generate a latex feed stream for a CSTR. Measurements of monomer conversion, particle number, particle size distribution and molecular weight distribution in the effluent streams of both reactors were used to study transport of free radicals and chain transfer agents in the emulsion polymerization of styrene.     

乳液聚合粒子成核和粒子大小的影响因素

介绍了乳液聚合中胶束,均相和单体液滴三种不同的粒子成核机理。讨论了引发速率和乳化剂浓度以及不同聚合方法和各种条件对乳胶粒子大小的影响。     

Synthesis and kinetics of microgel in inverse emulsion polymerization of acrylamide

The kinetic behavior is studied in inverse emulsion polymerization of acrylamide. The polymerization rate in a micelle is proportional to the monomer concentration and the effective average radical concentration. When concentrations of monomer and radical are too low to proceed the polymerization only, some finite final conversion is obtained. The micelles of various sizes have different effective average radical concentrations at the same monomer concentration. Therefore, the polymerization rate and the final conversion are different for various sized micelles. The minimum amounts of emulsifier required to get stable state during reaction is 10 and 7 wt% (based on oil phase) in toluene and n-heptane systems, respectively. Phase inversion is observed with the increase of viscosity during polymerization. The final latex coagulates obviously in the toluene system even with 10 wt% of emulsifier, however, it can be improved more in the n-heptane system with 10 wt% of emulsifier. The crosslinking agent which copolymerizes with the monomer was supposed to remain in the interface of micelles. Such interfacial copolymerization not only hardens particles but improves the stability of the system also, therefore avoiding coagulation among particles. An increase in the content of crosslinking agent leads to a more uniform size of final particles. The microgel added in the alkyd resin decreases the drying time. The greater the amount of microgel added, the shorter the drying time.     

Transitional emulsion polymerisation: Zero-one to pseudo-bulk

The transitional behaviours of emulsion polymerisation for styrene and butyl acrylate (BA) monomers from zero-one to pseudo-bulk regime were mechanistically investigated. A dynamic mathematical model, which incorporates cross-over mechanism from zero-one to pseudo-bulk kinetics was developed for emulsion polymerisation and compared with experimental data for conversion, particle size and molar mass. Particles smaller than cross-over size follow zero-one kinetics and particles greater than cross-over size, they follow pseudo-bulk kinetics. In our mechanistic approach, particles nucleated from micelles, grow until the cross-over size is attained, based on zero-one kinetics, and subsequently continue to grow based on pseudo-bulk kinetics. Key findings from our work are that the developed transitional model predictions agree reasonably with experimental data on process and product attributes such as conversion, average molecular weight, molecular weight distribution (MWD), average particle size and particle size distribution (PSD). Optimal strategies for semibatch operation was developed using reaction temperature and monomer feed rate as process variables with specified initial conditions.     

乳液聚合成核阶段的模拟与分析

建立了乳液聚合成核阶段的Monte Carlo模型，并用计算机对一个体积为10^-17m^3的微型反应器中苯乙烯的乳液聚合进行了模拟。以计算机生成随机数作为自由基被胶束和乳胶粒捕获的几率，模拟了在微型反应器中每一个自由基的生成、被胶束或乳胶粒捕获的过程以及每一个乳胶粒的生成及增长过程。通过对每一个乳胶粒在增长过程中各参数的统计计算，研究了乳液聚合成核阶段诸参数(乳胶粒数目、乳胶粒直径与粒径分布、单体转化率、聚合反应速率等)与乳化剂浓度[S]及引发剂浓度[I]的关系。结果表明，苯乙烯的乳液聚合体系中乳胶粒数目与[S]^0.5996[I]^0.4016成正比：在成核阶段乳胶粒直径分布先变宽后变窄，乳液聚合过程中乳胶粒直径分布有自动变窄的趋势；成核阶段持续时间t12与[S]^0.60[I]^0.60成正比，成核阶段结束时的单体转化率X12与[S]^1.20[I]^0.20成正比。