Yvon G. Durant

Latex Particle Morphology, Fundamental Aspects: A Review

The control of particle morphology is an essential part of producing high quality latex products for applications in coatings, adhesives, impact modifiers, and medical diagnostics, among others. A great variety of formulation and process variables are available to manipulate the particle structure and many different morphologies have been created. Techniques to characterize these morphologies are varied, but electron microscopy of both whole and sectioned particles is the most common one used. Atomic force microscopy is gaining in utility and often two or more characterization methods are simultaneously used to gain clarity of interpretation. A great deal of basic understanding of the factors controlling the morphology has been achieved by applying equilibrium thermodynamics to phase separated particles in aqueous media. Interfacial tensions at the polymer‐water interface and at the polymer–polymer interface, along with crosslinking density are found to be the dominant factors controlling the equilibrium morphology. In turn there is a significant number of formulation variables which determine the interfacial tensions and the crosslinking density. Successful models have been developed and applied to a number of different polymer systems. Much less progress has been made in understanding the development of non‐equilibrium morphologies where the possible number of particle structures is essentially infinite. Here, characterization techniques become somewhat less precise in identifying exact structure and further work is needed to advance this capability. In the dynamic reaction environment of the latex process the morphology develops within very viscous phases and is a result of competitive reaction and diffusion processes. Some progress has been made to quantitatively describe these phenomena, but more work is needed. This is even more evident when extension to carboxylic and hybrid (e.g. polyurethane/acrylic) latices is desired.

Non-equilibrium particle morphology development in seeded emulsion polymerization. 1: penetration of monomer and radicals as a function of monomer feed rate during second stage polymerization

Abstract Starve feeding of monomers is often used in an attempt to control latex particle morphology, especially when non-equilibrium structures are desired. For the case of a polar seed polymer and a non-polar second stage polymer, we have analyzed the relative probabilities of reaction and diffusion of polymer radicals and monomers as they penetrate the seed particle. The resultant penetration ratios (for polymer radicals and monomers) and fractional penetration values (depth of penetration) correlate well with a number of different non-equilibrium morphologies obtained from a wide variety of experimental reaction conditions. We conclude that the lack of polymer radical penetration is responsible for non-equilibrium core-shell structures for the glassy PMMA seed/PS system, while the styrene monomer easily penetrates the entire particle, even at very slow monomer feed rates. When the polar, low T g PMA is substituted for the PMMA seed, the polymer radicals cannot be excluded from the particle center and an inverted core-shell equilibrium structure is obtained at all monomer feed rates.

Mathematical Model for the Emulsion Polymerization Reaction Kinetics of Two Phase Latex Particles

Many of todays industrial applications of emulsion polymers require the creation of polymer particles with specifically designed structures. A fair amount of work has been done on predicting and controlling particle morphology, and it is recognized that the reaction kinetics often play an important role in determining that morphology. The present work describes a procedure to calculate the reaction rate for two‐phase latex particles by extending the classic Smith–Ewart concepts to the more complicated situation. The model involves a complete account of radical events in both polymeric phases. Eight rate parameters are necessary to completely describe the system. A particle population distribution is generated to reflect the probability of specific particles to have two distinct numbers of radicals in each phase. Simple statistics on this population allow the calculation of the average number of radicals for each polymeric phase. The model collapses to the correct answers when applied to single‐phase particles and results in the prediction of the number of radicals in each of the phases when applied to structured particles. Several examples are described for core–shell (or inverted core–shell) and hemispherical particles in which the polymer Tg, monomer concentration and radical entry rates are varied. In addition, the effects of radical transfer rates between the polymer phases are highlighted. Such effects are found to be important for some polymer systems and not so important for others. The model is readily extendible to occluded structures. Since the reaction kinetics are concluded to be significantly dependent upon the particle morphology, it is clear that morphology development and reaction kinetics are coupled processes and need to be treated simultaneously in order to produce an effective overall model for two‐phase emulsion polymers.

Thermodynamic and Kinetic Aspects for Particle Morphology Control

Composite latex particles offer a wide variety of physical properties to the end user and find application in coatings, adhesives, graphic arts, and impact resistant thermoplastics, among other areas. The physical properties are achieved by a balance of polymer composition, molecular weight, and latex particle morphology. There is a wide variety of particle morphologies produced, some of which are in their most stable configuration and some which are not. Because of its importance to the final properties of latex derived polymers, particle morphology is a subject of intense interest and a great deal of effort is being expended to learn how to control the final particle structure. The frequency of articles appealing in the scientific literature over the past 15 years attests to the heightened interest in this area.

Monitoring of Seeded Batch, Semi‐batch, and Second Stage Emulsion Polymerization by Low Resolution Raman Spectroscopy

Low resolution Raman spectroscopy (LRRS) offers a convenient way to monitor many different polymerization processes and reactor environments. LRRS can be used as an additional tool for process control to determine the conversion inside the reactor in real time without disruption to the reaction. Critical to polymerization monitoring, the efficiency of LRRS eliminates the lag between taking samples and calculating conversion. For each system, the phenyl ring of styrene or polystyrene provides an internal reference, which can be used to eliminate fluctuations in laser intensity. This paper demonstrates that the LRRS system can monitor seeded emulsion homopolymerizations in batch and semi‐batch as well as second stage emulsion polymerizations with some acceptable level of confidence.

Low cost mold development for prototype parts produced by vacuum assisted resin transfer molding (VARTM)

A low cost prototype mold, used in the vacuum assisted resin transfer molding (VARTM) process, was developed and manufactured using inexpensive materials and tooling methods. Several available mold materials were investigated from Polytetrafluoroethylene (PTFE) and aluminum to less conventional materials such as ultra high molecular weight polyethylene and oak. Through a multiple generation approach, a fully functional mold was developed capable of producing parts of accurate and reproducible dimensions. The use of conventional machine shops over fully automated machine shops was utilized for additional savings. Race tracking is a common problem in the VARTM process. A two-part room temperature vulcanizing rubber was used to seal the ends of the fibers and act as a mold gasket for vacuum. These baffles, created by sealing the fiber ends to the mold, forced the resin through the fibers and eliminated the race-tracking problem.