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.
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.
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
Latex particles that comprise two or more phases/components are important both academically and industrially. They are used in a diverse range of applications, for example: toughening of plastics; adhesives; architectural coatings; inks; controlled-release of actives; and diagnostics. This lecture will build upon the "Semi-Continuous Emulsion Polymerization and Structured Latexes" lecture by describing in greater detail the parameters which control the development of particle morphology when attempting to prepare, and control the morphology of, multi-phase latex particles in which there are two or more polymer phases. The importance of thermodynamic versus kinetic control will be emphasized and strategies for achieving control of morphology will be described together with their limitations. Some examples will be given to illustrate the principles. The focus will then switch to preparation of multi-component latex particles in which there are both polymeric and non-polymeric materials present. Different approaches to preparation of multi-component latex particles will be described through examples of encapsulating non-polymeric materials, templating of particle morphology, and synthesis of surface-functionalized particles.
Sustainability Topics for Polymer Latexes
In addition to application properties, the sustainability characteristics of products have become very important differentiators in the competitive market. An important measure of sustainability is the product carbon footprint (PCF). The basic concepts and key drivers of the PCF of polymer colloids will be discussed. One important driver of the PCF is the use of petroleum-based feedstocks. In this talk, we will describe routes to “green” polymer colloids from biobased and renewable feedstocks, as well as from feedstocks derived from polymer waste recycling processes. Another important sustainability topic is the input of microplastics into the environment. Recent regulatory activities on microplastics will be reviewed.
Latex-based Adhesives for Pressure Sensitive, Laminaton and Construction Applications
Water-borne adhesives based on polymer latexes have become the industry standard in many applications and have largely replaced solvent-borne adhesives. The most prominent examples are pressure-sensitive adhesives (PSAs) used for self-adhesive labels and various types of adhesive tapes. In this talk, we will highlight how the balance between tack, adhesion and cohesion depends on the molecular architecture of the acrylic latex polymers used in such materials. The role of tackifier resins will also be discussed.
Another important application is lamination adhesives. They are used, for example, to laminate different film substrates into composite films for packaging, to bond decorative foils to automotive interior parts, or to bond them to furniture surfaces made of engineered wood. For lamination adhesives, crosslinking acrylic and polyurethane dispersions are the most commonly used latex types.
Adhesives for construction applications are based on either mastic or cementitious formulations. In mastic construction adhesives, e.g. for textile floor coverings or ceramic tiles, a latex polymer with a low glass transition temperature (Tg) provides initial tack and strong cohesion after drying. Dry cementitious adhesive formulations, e.g. for ceramic tiles or insulation panels, usually contain spray-dried latex powder. After water is added and the adhesive slurry is prepared, the redispersed latex provides good workability and wet adhesion, as well as sufficient strength and flexibility after drying.
Correlation between Colloidal Structure and Properties of Acrylic Latexes
Acrylic polymer latexes are used in numerous everyday applications. Typical examples include interior and exterior architectural coatings, waterproofing membranes, pressure-sensitive and lamination adhesives, printing inks, and binders for industrial and medical nonwovens. The versatility of the semicontinuous emulsion polymerization process allows the cost-efficient manufacture of polymers with tailored application properties in large-scale reactors. A broad chemical toolbox of main and functional monomers, as well as modern property-by-process concepts, can be used for the development of advanced latexes that meet increasing application challenges. To tailor the properties of polymer latexes, a good understanding of their fundamental structure–property relationships is essential. In this talk, we will discuss the colloidal stabilization and the formation of the so-called “hairy layer structure” of hydrophilic monomers during emulsion polymerization, as well as their influence on coating application properties such as paint rheology, pigment interaction, film formation, and water resistance. The mechanism and importance of chemical crosslinking for the application properties of coatings will also be discussed.
Characterization of Latex Particle Size and Particle Size Distribution: Experimental Methods
Particle size and particle size distribution are key properties for the development and application of polymer latexes. This lecture will give an overview of the experimental methods available for characterizing latex particle size. The key features, operating principles, and theoretical background of the various fractionating and non-fractionating measurement techniques will be described. After discussing their major advantages and disadvantages, guidelines for selecting suitable methods for different colloidal materials and work requirements will be given.
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 & Ewart, Ugelstad, Gilbert and others will be discussed. Effects of phase equilibrium and mass transfer on polymer properties will be explored.
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.
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.
