Living (or controlled) radical polymerizations provide a novel and potentially inexpensive route to designing polymers with controlled microstucture (e.g. block copolymers, star polymers) and narrow molecular weight distributions. While extensive research has been conducted into homogeneous bulk and solution living radical polymerizations, investigations into aqueous dispersed phase systems (emulsion and miniemulsion polymerization) have only recently appeared. Although little progress has been realized with emulsion polymerization, considerable success has been achieved using miniemulsion polymerization with living radical systems. 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, miniemulsion and emulsion. The emphasis will be on heterogeneous systems, and more specifically on those aspects of operating in a heterogeneous environment that influence the polymerization rate, the molecular weight distribution and the livingness of the system.
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.
Surfactants play major roles during the particle nucleation and growth stages, with direct impact on latex particle size, size distribution, polymerization rate, 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 in determining the particle number according to the various nucleation mechanisms will be described. Three alternatives to conventional surfactants will be reviewed.
The basic concepts and terminology of colloid science will be introduced. The principles of electrostatic and steric stabilization mechanisms will then be reviewed. The inverse problem of coagulating and flocculating latexes will also be discussed.
Despite the fact that the first miniemulsion polymerization was carried out at Lehigh University in 1972, the word miniemulsion was 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 (referred to as cosurfactant). In miniemulsion polymerization, the submicron size monomer droplets are the main sites for particle nucleation and growth via free radical initiation using oil-soluble or water-soluble initiators. The Theory of Miniemulsions has been developed based on the well known 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 by 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.
The cohesive properties of polymer films from latexes are dependent on the film formation mechanism. Polymer film formation from latex occurs either when: (a) the molecules from the individual polymer particles interdiffuse and entangle as the particle boundaries gradually disappear, or, when (b) the molecules partially interpenetrate and cross-link, forming interparticle spot welds, or when (c) water-soluble molecules react or interact with functional groups on the particle surface. In the last two cases, the particle boundaries remain distinct. The role of interpenetration depth and the diffusion rate on cohesive strength development will be discussed, using model latex systems.
The scale-up of the mixing process in emulsion polymerization involves breaking the process down into individual but interrelated steps. The effect of mixing on the microscopic heterogeneity of the continuous phase, fluid shear rates and heat transfer allows each to be considered separately. A few of these effects will be discussed and illustrated with specific examples. The utility of bench scale experimentation with a view toward scale-up with some early experimental results, will also be discussed.
The cohesive properties of polymer films from latexes are dependent on the film formation mechanism. Polymer film formation from latex occurs either when: (a) the molecules from the individual polymer particles interdiffuse and entangle as the particle boundaries gradually disappear, or, when (b) the molecules partially interpenetrate and cross-link, forming interparticle spot welds, or when (c) water-soluble molecules react or interact with functional groups on the particle surface. In the last two cases, the particle boundaries remain distinct. The role of interpenetration depth and the diffusion rate on cohesive strength development will be discussed, using model latex systems.
High-solids latex technology is based on three basic considerations from the viewpoints of dispersion rheology: (1) The maximization of latex particle packing volume fractions, (2) The minimization of the effective volumes of latex particles, and (3) The minimization of latex medium viscosity. With these considerations at hand, the technology is concerned with the maximization of the volume solids of latexes, while meeting their respective end-use property requirements for a variety of applications. For this reason, its objective is to increase the volume solids of the existing latexes by 5 to 15% by considering a bimodal approach only for the packing efficiency, although the technology is capable of achieving 70% or higher volume solids latexes. This talk will describe the three basic considerations involved in the high-solids dispersion technology, and then discuss post-polymerization blending (i.e., blending large and small particle size preformed latexes) and in-situ emulsion polymerization methods (i.e., bimodal emulsion polymerization by either surfactant or seed addition) for the preparation of high-solids bimodal latexes. Some high-solids latex examples will be also presented.
Biobased latex binders adopted in the paper industry in 2008 were the first use of biopolymer-based microgels and nanogels for large-scale industrial applications [1], although they had been explored and used for drug delivery and other bio-medical applications for a long time [2]. Both biobased latex binders and biopolymer-based microgels and nanogels can be broadly classified as a special type of latexes whose particles are made up of water-swollen crosslinked hydrophilic polymers. Since the biobased latex binders currently used in the paper industry are water-swollen crosslinked starch nanoparticles, their wet and dry properties depend mainly on their particle size and crosslink density. The crosslink density of starch molecules forming the nanoparticles is especially important because it controls the extent of water swelling (swell ratio) [3,4], that is, as the crosslink density increases, the swell ratio of crosslinked starch nanoparticles decreases. Varying swell ratios of the water-swollen starch nanoparticles not only set them apart from conventional starches and synthetic latexes in their rheological behavior, but also differentiate themselves in paper coating performance. Their unique rheological behavior and paper coating performance will be discussed based on theoretical considerations as well as some laboratory testing, pilot coater and mill trial results.
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.
Pressure sensitive adhesives (PSAs) are viscoelastic materials which adhere to substrates on the application of slight pressure over short periods of time. They are ubiquitous in everyday life as self-adhesive tapes and labels used in a wide variety of applications (e.g., bonding, signing and marking, healthcare, automotive, electronics, furniture, security, food packaging and retailing). Over the past few decades, water-borne PSAs based on latexes prepared by emulsion polymerization have gained market share at the expense of flammable, environmentally-unfriendly solvent-borne PSAs and now comprise the largest proportion of the overall PSA market. The growth has arisen not only because they replace solvents with water, but also because the latexes have low viscosity at high solids contents which brings benefits in formulation, handling, transport and coating. This lecture will describe the different types of PSAs before focusing on the principle components used in preparation of acrylic water-borne PSAs and their roles in controlling latex properties and adhesive performance. Effects of latex particle composition/morphology, polymer properties and branching on the performance of PSA films will be discussed. A case study will be presented to demonstrate principles for control of adhesive performance through careful design of structured latex particles that determine the sub-micron and nanoscale morphology of PSA films as a consequence of the mechanism of film formation from latexes.
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.
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.
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.
This introduction to the rheology of latexes covers the type of rheological measurements that can be made and the effects of the many variables found in latexes; solids concentration, particle size and distribution, surface charges, adsorbed surfactants, particle aggregation, non-spherical particle morphology, swellable particles, and the use of water-soluble polymer thickeners.
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.