281396 Transport Fundamentals of Micro-Encapsulated Small Molecules Embedded in a Polymer Matrix: Model Predictions and Experiment Findings
Micro-encapsulation technology is recognized as a sizable market opportunity for a number of specialty chemical businesses supporting agricultural, adhesives/epoxy, protective coatings, food, and personal care markets. A synthetic approach that leads to the encapsulation of actives/functional molecules by establishing core-shell morphology is often speculated to addresses the needs of these market opportunities. Particles of a core-shell morphology could potentially enable the controlled delivery of a number of actives/functional molecules including bioactive small molecules (biocides, insecticides, herbicides, etc.), fragrances, drugs, dyes/pigments, coalescing agents, reactive intermediates (hardeners, accelerators and catalysts for epoxy and other 2K reactive systems) and photoactive agents.
While each application is different, common underlying physics connect the fundamental performance drivers. In the broadest sense, these diverse applications can be described as a micro-encapsulated small molecule distributed in some type of matrix continuum (air, water/solvent, or polymer composite). The nature of this work is to demonstrate a detailed Fickian diffusion model that describes the case for simultaneous diffusion of a small molecule out of an embedded encapsulated particle through a polymer film to an exterior free surface. Case studies were examined and the model was validated by simulating experimental data. Using ATR-IR and a uniquely designed microfluidic weathering cell, the diffusion coefficient and surface loss factor of the small molecule were determined for various polymer properties (composition and Tg). It will be shown that the model can be extended to extract performance information which is challenging to observe experimentally. The simulation output includes (1) the distribution of active small molecule through the film at any given time, (2) the surface concentration of small molecule at any time point and (3) the “replenishment” rate (or surface flux) of the small molecule to the surface. This type of simulation is particularly useful to understanding the migration of bioactives, fragrances, drugs and coalescing agents that might be distributed in a polymer composite film where the initial concentration of small molecule in the film is much smaller than 1%. Since many of the applications of interest require formulations where the particle size cannot be larger than 50 um, it will be shown that manipulating the diffusion coefficient of the active through choice of shell wall chemistry is critical to enabling a successful application.