The 41st Annual Short Course
Advances in Emulsion Polymerization
and Latex Technology
Living Radical Polymerization and Recent Advances in Emulsion Polymerization
While significant advances have occurred in emulsion polymerization in recent decades, in both our fundamental understanding and in practice, the basic chemistry of the process has seen little change until recently. Significant for the development of future commercial products are advances being made in polymer chemistry and catalysis that allow synthesis of polymer colloids with control of the polymer microstructure, polymer colloids made using monomers not previously polymerizable in water-based systems, and in the development of polymer colloids from renewable resources including natural polymers. As this chemistry matures, new classes of polymer colloids will emerge, possibly ushering in entirely new fields of application and considerable opportunities for product innovation. For example, “living” (or “controlled”) radical polymerizations (LRP/ CRP) provide a novel and potentially inexpensive route to designing polymers with controlled microstucture (e.g. block copolymers) and narrow molecular wei ght distributions. Earlier studies focused on homogeneous bulk and solution living radical polymerizations, but our ability to conduct LRP in aqueous dispersed phase systems has now progressed to a point where commercial applications are feasible. This presentation introduces the three major living radical polymerization chemistries (nitroxide-mediated radical polymerization (NMRP), atom transfer radical polymerization (ATRP) and reversible-addition-fragmentation-transfer polymerization (RAFT)), and summarizes recent progress of these systems in bulk and emulsion-based systems. The emphasis will be on those aspects of operating in a heterogeneous environment that influence the polymerization rate, the molecular weight distribution and the livingness of the system. The presentation will also highlight recent advances in the use of other non-radical chemistries to make polymer colloids, and in progress to make “green” polymer colloids from renewable feedstocks and natural polymers.
Semi-Continuous Emulsion Polymerization and Structured Latexes
Semi-continuous (or semi-batch) polymerizations in which the monomer is added incrementally during the course of reaction are commonly used in industrial processes because they allow control of the polymerization rate, and because they can be used to control the particle morphology. Structured latexes are emulsion polymer particles in which the internal morphology an/or composition vary through the particle. Examples include core-shell particles, and particles with radial composition gradients between the particle core and surface. The discussion will describe how semi-continuous processes are run, the unique features of operating an emulsion polymerization in semi-continuous mode, and how structured latexes can be synthesized.
Scale-Up of Emulsion Polymerization Processes
Converting a laboratory scale emulsion polymerization process to a production scale process requires consideration of a range of issues including reaction kinetics, heat transfer and mixing (mass transfer). Emulsion polymerizations often pose a difficult scaleup challenge since by their nature the polymerization kinetics are coupled with both heat and mass transfer. Consequently almost any change to the process during scaleup is likely to impact product properties, whereas the primary goal of scaleup is to reproduce the latex properties obtained in bench scale experiments. Typical challenges encountered in scaleup include inability to obtain the desired latex properties, coagulum formation, and problems controlling the reactor temperature. The principles of scaling up an emulsion polymerization will be introduced, and these specific challenges will be discussed.
The Role of Surfactants in Emulsion Polymerization Processes and Kinetics
Surfactants play major roles in emulsion polymerization during the particle nucleation and growth stages, with direct impact on latex particle size, size distribution, polymerization rate, polymer molecular weight, and particle morphology. Surfactants are also essential during post-polymerization processes: stripping, storage, shipping, and formulation for several applications. The general characteristics of surfactants and their adsorption profiles on latex particles will be reviewed. The specific role of surfactants (single and mixtures of surfactants) on the kinetics of emulsion polymerization (rate of polymerization and evolution particle number as a function of polymerization time according to the various nucleation mechanisms) will be described. The influence of water-solubility of monomers, partition of the surfactant between the monomer and aqueous phases, and the use of single vs. mixtures of anionic and non-ionic surfactants on the kinetics results of emulsion polymerization will also presented and discussed. Three alternatives to conventional surfactants, including ionic monomers, block copolymers, and the recent work on reactive surfactants in emulsion polymerization and characterization results of their loci in the final copolymer latex particles as well as properties of films cast from these latexes be discussed.
Colloidal Stabilization and Destabilization Mechanisms of Latex Systems
Colloidal stability are essential both during the entire course of an emulsion polymerization process in order to eliminate coagulum formation and to avoid reactor fouling. Latex stability is also essential for many of the post-polymerization processes such as storage, transportation, steam stripping of residual monomer, as well as formulation involving additives (such as pigments, fillers or coalescing aids,) and latex applications methods which my involve subjecting the latex system to mechanical sheer.
In this lecture we will introduce the electrostatic and steric colloidal stabilization mechanisms of latex systems. We will discuss in details the key parameters responsible for repulsion and attraction between latex particles in light of the Derjaiguin - Landau - Verwey - Overbeek (DLVO) theory, for latex particles that carry negative or positive surface charges due to adsorption of surfactants and/or chemically bound ionized functional groups. We will also discuss the entropy and enthalpy contributions to steric stabilization of latex systems in light of the 2nd law of thermodynamic, due to the presence of non-ionic species at the particle surface such as non-ionic surfactants and/or anchored polymer chain molecules with affinity to the surrounding aqueous phase. The reverse of colloidal stabilization, namely destabilization or aggregation (coagulation and flocculation) of latex systems will be discussed. In this regards, the influence of chemical additives (such as electrolytes, acids, solvents on non-solvents) on stabilization/destabilization will be presented. Also the influence of physical effects (such as agitation and mechanical sheer as well as temperature changes including freeze/thaw cycles) on stabilization/destabilization will be presented. Ways to assess the critical concentrations of additives, and levels of temperature changes as well as mechanical sheer that cause destabilization of a colloidally-stable latex particles will be discussed. Experimental results will be used to illustrate some of the basic concept and the influence of chemical additives and physical effects on stabilization/destabilization mechanisms of latex systems.
Miniemulsions: Their Latex Systems via Polymerization in Submicron-Size Droplets and
Direct Emulsification of Polymer
Despite the fact that the first miniemulsion polymerization was carried out at Lehigh University in 1972, the term "miniemulsion" was first coined only in 1981. The number of publications on miniemulsions has been increasing exponentially over the past decade, including a few patents.
Miniemulsions are relatively stable oil-in-water emulsions with average droplet diameters ranging from 50 to 500 nm. These are typically prepared using a mixture of a surfactant and a low molecular weight, highly water-insoluble costabilizer (sometimes referred to as cosurfactant). In miniemulsion polymerization, the submicron monomer droplets are the main sites for particle nucleation and growth via free radical initiation using oil-soluble or water-soluble initiators. The stability behavior of miniemulsions has been explained theoretically based on the well know concepts of Ostwald ripening and thermodynamics. Miniemulsions have been exploited in making new types of polymer colloids (latexes) that were difficult and sometimes impossible to make using conventional emulsification or emulsion polymerization processes. These include preparation of artificial latexes and hybrid latexes, high solids latexes, polymerization of highly water-insoluble monomers and macromonomers, controlled polymer microstructure and morphology, encapsulation of pigments and dyes, and controlled molecular weight via living free radical polymerization. In this lecture both the theory and practice of miniemulsions will be discussed.
Branching and Grafting in Emulsion Polymerizations
Branching in polymers produced by free-radical polymerization arises from chain transfer to polymer and has important effects on polymer properties. In emulsion polymerization, intermolecular chain transfer to polymer can lead to grafting of water-soluble polymers to latex particles, facilitating control of colloidal stability and latex rheology. Such branching and grafting is used to good effect in the emulsion polymer industry to control the end-use performance of latexes and emulsion polymers. This lecture will begin with an overview of the chemistry of branching and grafting. Case studies of branching in acrylate and vinyl acetate homopolymerizations and synergistic effects in copolymerization will then be presented, together with strategies for controlling the level of branching. This will provide the basis for considering grafting of water-soluble polymers used as colloid stabilizers in emulsion polymerizations. The chemical processes which the most commonly-used water-soluble polymers may under go during emulsion polymerization will be illustrated through case studies that highlight the key principles for their control.
Understanding film formation is important for all applications where latexes are dried, which includes obvious applications such as water-borne
paints, inks and adhesives, but also those which are less obvious, such as binding of non-woven fabrics, sealants and foamed products.
This lecture will describe the fundamental principles underlying the process of film formation from latexes, including the key stages
and the molecular processes that are necessary for the formation of coherent films. Factors that influence film formation and the
quality of the films produced will be described.
Emulsion Polymerization Mechanisms and Kinetics
Reaction mechanisms and kinetics of free radical polymerization will be reviewed. The unique features of emulsion polymerization will be outlined and the influence of the colloidal size of the reaction sites discussed. Kinetic theories due to Smith and Ewart, Stockmayer, O'Toole, Roe, Fitch, Ugelstad, and Gilbert will be discussed.
Engineering of Emulsion Polymerization Reactors
The various types of reactors (batch, semi-batch and continuous), used to produce synthetic latexes will be reviewed. Pros and cons of various types of processes will be discussed and theoretical reactor models will be presented where appropriate. Reactor design and operating factors that influence product properties will also be reviewed.
Review of experimental studies illustrating the various factors that influence the rheological properties of latexes. Topics to be covered include the effects of solids concentration, particle size and distribution, electrolyte content, particle aggregation, adsorbed surfactants, non-spherical particle morphology, particle swelling, and the use of water-soluble associative and non-associative polymeric thickeners. Consideration will also be given to thickened latexes and variables affecting their rheological flow curves.
Experimental Methods for the Characterization of Latex Particle Size
The application of fractionation and non-fractionation methods for the determination of particle size distribution, the range of applicability, and advantages and disadvantages and their on-line measurement capability will be discussed. Among the methods examined are: classical and dynamic light scattering, sedimentation, disc centrifugation, electrozone sensing, sedimentation field flow fractionation, capillary hydrodynamic fractionation, and recent advances in hybrid methods of analysis. Comparisons of several of these methods will be used to illustrate problems often encountered in the particle size distribution determination of latexes.
The Kinetics of Free Radical-Initiated Polymerization
A review of the principles of free radical-initiated polymerization, including the basic reactions of initiation, propagation, termination and transfer;inhibition, molecular weight and molecular weight distribution, effect of temperature and pressure, autoacceleration and diffusion control of termination and propagation, and copolymerization including copolymerization reactivity ratios and copolymer sequence distribution.
Sensors and Control of Emulsion Polymerization Reactors
Recent developments in the area of on-line sensors, coupled with the availability of high-performance digital control systems has opened up new opportunities for the efficient operation and control of latex reactors. Available sensors for on-line analysis will be discussed. The use of such measurements in the application of advanced control techniques to batch and continuous polymerization reactors will be reviewed, with special emphasis on controlling the undesirable process dynamics associated with continuous emulsion polymerization, and optimizing controllers for batch polymerization.
A Mechanistic Study of Water Evaporation from Wet Acrylic Latex Films / Glass Transition Evolution of Plasticized Latex Films: An Important Process in the Application of Everyday Latex Paints
This talk discusses factors that control the evaporation rate of water from acrylic latexes and solvents during the film formation process. For stage 1 of the film formation process a mechanistic model is developed that shows that the instantaneous drying rate of either latex decreases linearly as the fractional-surface area of water decreases during the drying process. This mechanistic model postulates acrylic particles at the liquid-air interface that inhibit the evaporation of water. At 42 to 45% solids the initial instantaneous drying rate for the two latexes is ~26% less than that of pure water. At 75% solids a change in slope of the instantaneous drying rate as a function of time identifies stage 2 of the drying process.
The second part of this lecture focuses on the use of filming aids or solvents during the film formation process of latex paints. Important parameters such as the glass transition temperature of filming aids and polymers, the volatility of filming aids in the presence of water and polymeric particles, the distribution coefficients of filming aids, and the Fox-Flory equation are used to predict the MFT of latex particles at deformation. A new experimental method that obtains the activity coefficients of filming aids during the drying process of latex films is demonstrated. These activity coefficients are used to predict the total solvent loss during the wet evaporation stage of the film formation process. Additionally, the clear film composition is modeled for the ensuing “volatility-controlled” stage that defines the time line where solvent evaporation is not diffusion controlled. The ability of the model to predict or follow the Tg of a “drying system” is demonstrated. The model presented can assist in the selection of filming aids for water-borne latex-based formulations and can provide important criteria for optimizing particle composition and morphology.
Latexes for Industrial Applications and Methods of Reducing Residual Monomers
Latexes for Industrial Applications: More than 10 million metric tons (more than 20 billion pounds) of dry latex polymers are being consumed annually in a very large number of industrial applications, including paints and coatings (~26% of the total annual latex consumption), paper and paperboard applications (~24%), adhesives (~23%), carpet backsizing (~10%), etc. This part of the talk will review the major industrial applications and types of latexes, and then the important latex variables affecting the properties of latexes for various industrial applications will be discussed. Furthermore, industrial latexes will be grouped in terms of their Tg ranges for various applications. They are also grouped in terms of filler levels. Finally, some specific applications will be highlighted and their latex requirements and future needs will be discussed.
Methods of Reducing Residual Monomers: Historically, butadiene-containing copolymer latexes, such as gel-free SBR (styrene butadiene rubber) and
crosslinked S/B latexes, have been steam-stripped to remove their residual monomers, whereas the residual monomers of non-gel forming polymer
latexes, such as acrylic latexes, have been further polymerized (i.e., cooked down) in their post-polymerization steps by using hydrophobic initiators, such as tertiary butyl hydroperoxide, and reducing agents known as “chaser catalysts” in the industry. However, public demands and government regulations for ever lower amounts of residual monomers and VOC’s contained in latexes and latex-containing coating formulations may require the industry to consider many different approaches to meet the demands and regulations. For example, in some cases where the post-polymerization cook-down alone may not be sufficient to meet the demands, the cook-down approach must be either combined with nitrogen stripping or entirely switched to steam-stripping. This part of talk will discuss the mechanisms for both batch and continuous steam-stripping processes, the post-polymerization cook-down mechanisms, various initiator systems for the cook-down, and other considerations.