Latexes are currently under going extensive research and development as key replacement materials for many solvent-based systems. They are being used in a broad range of fields from adhesives, inks, paints, coatings, drug delivery systems, medical assay kits, gloves, paper coatings, floor polish, films, carpet backing and foam mattresses to cosmetics. Latex is also used to improve properties, for example as an impact modifier in polystyrene and to improve tensile properties in cement. Currently, more than 8 million dry metric tons of latex are produced globally each year.
Natural rubber latex is only one of the many types of latex currently in use. The term latex covers emulsion polymers, polymer dispersions and polymer colloids. Latexes are liquids (typically aqueous) in which microscopic polymer particles are dispersed. They are formed by the polymerization of monomer emulsions. Solvent-based latexes do exist, but have limited applications. However, the use of organic solvents in latexes is discouraged for environmental reasons. In fact, the desire to discontinue the use of solvent-based polymer solutions has been a major driving force for the development of water-based latexes.
The different methods of emulsion polymerization are reviewed here. Each technique has advantages and disadvantages in terms of cost, control over the polymerization process and stability of the resulting emulsion. Monomer feed systems can also be used to control reactions and polymer composition. In some techniques very uniform polymers and latex particles can be produced. Particle structure is important and can be varied or maintained uniform during the process, depending on the system.
The low viscosity of latexes allows a high rate of heat transfer during polymerization and excellent flow over substrates to be coated. Water can then be evaporated to form a polymer film.
The technology and science of emulsion polymerization are described clearly and succinctly in this review from two authors who are at the forefront of latex research and development. The text is referenced extensively to permit further reading on this subject.
Quality control is an important feature. The different tests and analytical methods used on latex polymers are discussed.
The review is accompanied by around 400 abstracts from papers and books in the Rapra Polymer Library database, to facilitate further reading on this subject. A subject index and a company index are included.
1 General Introduction
1.1 Aims and Scope
1.2 Importance of Emulsion Polymers
1.3 Advantages of Emulsion Polymers
2 Scientific Principles
2.1 Colloidal Stabilization
2.2 Diffusional Degradation
2.3 Free Radical Polymerization
2.4 Particle Nucleation Mechanisms
2.5 Emulsion Polymerization Intervals
2.6 Smith-Ewart Kinetics
3 Emulsion Polymerization Processes
3.1 Conventional Emulsion
3.4 Inverse Emulsion
3.7 Artificial and Hybrid Latexes
4 Latex Preparation
4.1 Recipe Formulation
4.1.1 Polymerization Components
4.1.2 Post-Polymerization Additives
4.2 Feed Strategies
4.2.5 Power Feed
4.3 Sensors and Process Control
4.4 Reactor Agitation
5 Latex Characterization
5.1 Colloidal Stability
5.3 Rate of Polymerization
5.4 Particle Size and Size Distribution
5.5 Particle Morphology
5.6 Molecular Weight
5.7 Copolymer Composition
5.8 Latex Rheology
5.9 Film Formation
5.10 Mechanical Properties
5.11 Particle Surface Characterization
6 Classes of Emulsion Polymers
6.1 Natural Latexes
6.1.1 Natural Rubber Latexes
6.2 Synthetic Latexes
6.2.1 Styrene-Butadiene Rubber
6.2.3 Polyvinyl Acetate
6.2.4 Polyvinyl Chloride
7 Industrial Applications
7.1 Vehicle Tires
7.2 Latex Gloves
7.3 Latex Paints
7.4 Industrial Coatings
7.5 Paper Coatings
7.6 Textiles and Nonwovens
7.7 Carpet Backing Binders
7.9 Other Commercialized Uses
8 Specialized and Potential Applications
8.1 Latexes as Scientific Tools
8.2 Nanofabrication and Optoelectronics
8.3 Biomedical Technology
Abbreviations and Acronyms
The authors of this report are on the staff at the Emulsion Polymers Institute (EPI), Lehigh University, formed in 1975. Its aim is to carry out interdisciplinary research to gain fundamental information on the preparation and properties of polymer latexes and also to discover and develop industrial applications.
Eric S. Daniels is a Principal Research Scientist and Executive Director at the Emulsion Polymers Institute. He received his Ph.D. in Polymer Science and Engineering from Lehigh University in 1987, and also has B.S. and M.S. degrees in Chemical Engineering. Research interests include the biomedical applications of latexes, the mechanism for the formation of composite latexes, particle morphology, interfacial crosslinking and film formation in emulsion polymer systems, particle technology, and the role of surfactants in emulsion polymerization.
Christopher D. Anderson is a Postdoctoral Research Associate at the Emulsion Polymers Institute. He has six years of experience in polymer colloid science and has a Ph.D. in Chemical Engineering. Previously, he worked at the ARCO Chemical Company as a pilot plant process engineer.