Q: From what is a flavor & aroma barrier of plain & metallized film derived?

By Dr. Eldridge M. Mount

A: In my 2018 Q1 column, I focused on scalping and tainting of packaged products by the packaging film used to wrap them. These interactions between the film and the packaged product speaks to a subject not often touched on: that is, flavor and aroma barrier or more precisely the chemical barrier of packaging films. This is because flavors and aromas as well as environmental chemicals, such as gasoline or household products which packaged goods may be exposed to, are simply chemicals.

The chemical (i.e.: flavor and aroma) barrier, or perhaps better the permeability (P), of any package is primarily controlled by the polymer used in the package. Permeability is simply the solubility (S) of a chemical times the diffusion rate (D) of that chemical in the polymer [1] or:

P = SD

Polymers are simply chemicals and therefore susceptible to chemicals of the same chemical family as the polymer. The old chemist adage that “like dissolves like” is at play here. That is because, for a molecule to permeate through a polymer film, it must first dissolve into the film surface. In practice it means that by and large polar polymers, such as polyesters and nylons, are less susceptible to diffusion of non-polar chemicals and more susceptible to permeation by polar molecules. At the same time, non-polar polymers, such as polypropylene and polyethylene, are less susceptible to diffusion of polar chemicals and more susceptible to permeation by non-polar molecules. Molecular weight of the permeant also will have an impact on solubility and permeation in a polymer. Of course, there are many intermediate situations. So, to obtain control of permeants in a film it is important to understand the chemical interactions of your polymer films with a wide range of chemicals. Know the chemical resistance of your polymer film.

Misplaced metallized assumptions

But what about metallized film? With a layer of aluminum, it should have great flavor and aroma barrier correct? That seemed a good assumption, but experimentally (it’s always best to test an idea), it just wasn’t the case. Experimentally, chemical flavor and aroma permeation of metallized films was not much better than the permeation of plain films [2] when exposed to d-limonene (citrus flavors), ethanol and diacetyl (butter flavor). For these permeants, the metallized and plain coextruded films were not as good as films coated with acrylic layers, such as 75 ABX in Table 1. Acrylic and Saran coatings have long been used to add film heat-sealing and/or flavor and aroma barrier.

So, we begin to understand that chemical barrier is a bit different than simple gas permeation in that layers other than the metallizing layer can impact the overall outcome of the flavor and aroma barrier. Let’s explore this statement a bit more. For instance (see Figure 1), in the plain metallized-film case of 70 MET, based on the general metallized-film product design:

FIGURE 1. 70 MET metallized-film product design

Impact of acrylic coatings

Table 1 shows that the chemicals d-limonene, ethanol and diacetyl permeate through the metallized films at a slightly reduced but essentially the same order of magnitude rate as the plain coextruded films (compare the base film 70-MB-HB to 70 MET-HB). However, if an acrylic-barrier coating is added to the film opposite the metal layer, as in the 60MAC example in Table 1 (see Figure 2), then the flavor and aroma barrier is significantly reduced and lower than the simply acrylic-coated film.

FIGURE 2. 60 MAC product design

However, for the MET-UHB (see Figure 3), the aluminum is deposited on an EVOH skin and here the aroma barrier is very good and similar to that of the acrylic-coated metallized film 60 MAC.

FIGURE 3. MET-UHB product design

I believe that the source of the improved chemical barrier for the 60 MAC and the MET-UHB products is due to the inherent chemical resistance of the acrylic coating and the EVOH layer and its protection of the film adjacent to the aluminum layer from the impact of the flavor chemical on the base polymer layers.

What I believe is happening in the 70 MET-HB product is that the flavor and aroma chemicals are absorbed by the film and then begins to act like a solvent and swells the film layer adjacent to the aluminum, perhaps disrupting the continuity of the aluminum layer. In the case where the acrylic and EVOH layers are present, the chemicals cannot enter and swell the layers adjacent to the aluminum, and thus the aluminum layer remains undisturbed and functions as the barrier layer it is.

TABLE 1. Comparison of film types, coatings and permeants

References

1.      InTech-Diffusion_in_polymer_solids_and_solutions.pdf (intechopen.com)

2.      Mount III, Eldridge, M., Wagner, John R., “Aroma, Oxygen and Moisture Barrier Behavior of Coated and Vacuum Coated OPP Films for Packaging,” J. Plastic Film & Sheeting, vol. 17 (July), 2001, pp 221-237.