LECTURE ABSTRACTS
The 47th Annual Short Course
Advances in Emulsion Polymerization
and Latex Technology
Living Radical Polymerization and Recent Advances in Emulsion Polymers
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 and/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.
Reactor Design and Scale-up in Emulsion Polymerization
The principles of designing and safely operating emulsion polymerization reactors will be presented in the context of how the selection of reactor type, mode of operation (batch, semi-batch/semi-continuous, continuous) and specific reaction conditions influence latex product properties and reactor productivity. The first part of the presentation will emphasize the inter-action of the chemistry and kinetics of emulsion polymerization with the physical design and operation of the reactor. The second part of the presentation will examine the issues involved in converting a laboratory scale emulsion polymerization process to a production scale process, including consideration of reaction kinetics, heat transfer and mixing. 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. The principles of scaling up an emulsion polymerization will be introduced, and 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 and Their Latex Systems via Polymerization in Monomer Droplets and
Direct Emulsification of Polymer Solutions
In this lecture the early development of the miniemulsion concept will be reviewed, and current state-of-the-art including theory and practice of miniemulsions will be discussed.
The concept of miniemulsions was invented at Lehigh University in 1972. Despite the fact that the first miniemulsion polymerization was also carried out at the same time, the term "miniemulsion" was coined only in 1981. Since that time the number of publications and patents on miniemulsions has been increasing exponentially.
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 for the preparation of polymer colloids (latexes), since the surfactant concentration in the aqueous phase is below the CMC, the submicron monomer droplets are the main sites for particle nucleation (and growth) via free radical initiation using oil-soluble or water-soluble initiators. An alternative approach for making latexes based on miniemulsions is the direct emulsification of polymer solutions. The formation and shelf-life stability of miniemulsions are explained based Ostwald ripening and the 2nd law thermodynamics. Miniemulsions have been exploited in making new types of polymer colloids (latexes) that were difficult and sometimes impossible to make using conventional emulsification and/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, controlled polymer molecular weight distribution via living free radical polymerization, and encapsulation of liquids, inorganic particles, inorganic and organic pigments and dyes.
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 undergo during emulsion polymerization will be illustrated through case studies that highlight the key principles for their control.
Latex Film Formation
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
Water-Borne Soft-Soft Nanocomposites: Principles and Application Case Studies
Many applications of water-borne polymers prepared by emulsion polymerisation require use of soft polymers with glass transition temperatures below room temperature so that films can be formed. In this lecture an overview of the principles underlying the soft-soft nanocomposite concept for enhancing the performance of soft films produced from water-borne polymers prepared by methods of emulsion polymerisation will be presented and then exemplified through three commercially-relevant application case studies: (i) water-borne pressure-sensitive adhesives; (ii) polybutadiene films as models for nitrile rubber glove applications; and (iii) acrylic latexes for use as binders in surface coatings.
Film Formation and Film Properties of Acrylic Latexes for Coatings and Adhesives
Water based polymer dispersions are providing their final function and properties only in the dried state. Upon drying, the individual latex particles undergo coalescence to form a coherent polymer film. Understanding and control of the film formation process of industrial acrylic latexes is of utmost importance to achieve optimum application properties.
Post film formation crosslinking is often used to improve the mechanical properties of latex films. For this purpose, a variety of reactive crosslinker monomers and multifunctional crosslinker additives are available, which undergo chemical crosslinking during and after the film formation. The curing kinetics and the film properties of one of the most common one-component room temperature latex crosslinking systems, which is based on diacetone acrylamide and adipic acid dihydrazide, will be described in detail.
For revealing details of the film formation process like particle coalescence or polymer chain interdiffusion, specific scattering and fluorescence techniques are required. Using such methods, the influence of hairy layer and crosslinking monomers on film formation, mechanical properties and water uptake of latexes for coatings and pressure sensitive adhesives has been studied.
Correlation between Colloidal Structure and Properties of Acrylic Latexes
For tailoring properties of polymer latexes, a good understanding of their fundamental structure property relationships is essential. In this talk, we will discuss the influence of stabilization system, colloidal interactions, coalescence behaviour and chemical crosslinking on various properties of acrylic latexes. Electrosteric colloidal stabilization is commonly provided by combining anionic and non-ionic surfactants with hydrophilic functional monomers like acrylic acid and acrylamide. Alternatively, amphiphilic polymers or block copolymers can be used as protective colloids for latex stabilization. The stabilizers and the hairy layer structure determine the colloidal interactions of polymer dispersions with other coating formulation ingredients like pigments, fillers, dispersants and rheology additives. The understanding and design of these interactions is crucial for tailoring paint properties like rheology, workability, open time and hiding power. Water based polymer dispersions are providing their final function and properties in the dried state. Upon drying, the individual latex particles of the dispersion undergo coalescence to form a coherent polymer film. Understanding and control of the film formation process helps to achieve optimum properties. To improve the mechanical properties, it is possible to incorporate specific functional monomers and reactive species, which undergo chemical cross-linking during and after the latex film formation.
Free Radical Polymerization Mechanisms and Kinetics
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.
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, and Ugelstad will be discussed.
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.
Latex Rheology
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 and Particle Size Distribution
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.
