Modeling of Oxygen Scavenging
Polymers and Composites
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Susana Carranza |
INTRODUCTION
Polymers are widely used as packaging of a variety of items, from food and drugs, to consumer electronics. One of the roles of the polymer is to provide protection against mechanical damage during transport and handling. Another important role is the use of polymers to prevent contamination, spoilage or aging of the internal contents.
Over time, gases can permeate through polymers, and the rate of permeation depends on the type of polymer. Polyethylene terephthalate (PET) is widely used as beverage container since it combines good mechanical properties with low permeability to oxygen. However, for products that are highly sensitive to oxygen, the transport rates of oxygen in PET may not be slow enough. One alternative is the use oxygen scavenging polymers.
Materials that are known as good scavengers usually don't have the required mechanical properties for protective packaging, so they are used dispersed in other polymers (such as PET) or as part of a multi-layer structure. Material development and experimental characterization is being done jointly by Dr. Benny Freeman's and Dr. Don Paul's research groups. The objective of this research is to model the transport phenomena occurring in such composite materials.
SHRINKING CORE MODEL
The polymer film consists of PET with evenly dispersed scavenging particles, as illustrated in Figure 1. Each particle is modeled as an unreactive core with a moving reaction front. The radius of the sphere, R, it is unchanged throughout the process. The radius of the unreactive core, a, reduces in size as the oxygen uptake progresses. At the reaction front (r = a), diffusion equals the reaction rate.
Figure 1 - Oxygen scavenging polymer particles dispersed in PET matrix
CONTINUUM APPROACH
The particles are assumed to be uniformly dispersed, small enough compared to the film thickness L, and in large enough numbers to enable to model the film as a continuum with effective properties.
The equations for concentration and unreacted radius for each sphere are used to generate the equations to describe the composite film. Solving these equations provides the time evolution of concentration and radius profiles.
References
- M.C. Ferrari, S. Carranza, R.T. Bonnecaze, K.K. Tung, B.D. Freeman, D.R. Paul, "Modeling of oxygen scavenging for improved barrier behavior: Blend films," J. Membr. Sci. 329 (2009) 183-192.