Permeability

There are different approaches for providing permeability resistance in plastic packaging. The more extensively used is barrier plastics with nonbarrier types to meet cost-to-performance requirements. Most plastics can be considered barrier types to some degree, but as barrier properties are maximized in one area (as gases such as 02, N2, or C02), other properties as permeability and moisture resistance diminish. This is achieved through coextrusion, coinjection, corotation, and other such processes. The other chemically modifies the plastics' surfaces.

There are plastics that have different behaviors to protect and preserve products in storage and distribution. They provide different diffusion (transport) action of gases, vapors, and other low molecular weight species through the materials. Important selection in packaging (food, etc.) is based on the permeability of the materials to factors such as oxygen and water vapor. There are special applications such as packaging bananas where an ethylene gas remains in a package is used to artificially ripen the bananas. There is other industrial gas separation systems that use the selective permeability of plastics to separate their constituents.

This diffusion rate is related to the resistance, within the plastic wall, to the movement of gases and vapors. Important aspects of the diffusion process are permeability and migration of additives. Possible migrants from plastics can include the many different additives and fillers used (Chapter 1).

The conditions inside and outside packages relate to the gas and vapor pressure forces penetrating or diffusing through permeable packages. As the engineering handbooks report diffusing substance's transmission rate is expressed by mathematical equations commonly called Fick's First and Second Laws of Diffusion very popular in the industry:

where: F = flux (the rate of transfer of a diffusing substance per unit area), D = diffusion coefficient, C = concentration of diffusing substance,, t = time, and X = space coordinate measured normal to the section.

Different test methods are used to conduct permeability behaviors of plastics to measure gas, water, and other material vapor permeability. Permeability test procedures used to measure the permeability of plastic films include those identified as the absolute pressure method, the isostatic method, and the quasi-isostatic method. Basic approach mounts a plastic film sample between two cell chambers of a permeability cell (two piece closed container). One chamber holds the gas or vapor to be used as the permeanr. The permeant then diffuses through the film into a second chamber, where a detection method such as optical devices, infrared spectroscopy, a manometric, gravimetric, or coulometric method; isotopic counting; or gas-liquid chromatography provides a quantitative measurement.

The absolute pressure method (ASTM D1434, Gas Transmission Rate of Plastic Film and Sheeting) is used when no gas other than the permeant in question is present. In this test the change in pressure on the volume of the low-pressure chamber measures the permeation rate. Between the two chambers a pressure differential provides the driving force for developing permeation.

Isostatic testing equipment has been used for measuring the oxygen and carbon dioxide permeability of both plastic films and complete plastic packages. Pressure in each test chamber are held constant by keeping both chambers at atmospheric pressure. With gas permeability measurement, there must be a difference in permeant partial pressure or a concentration gradient between the two cell chambers. Gas that permeates through the film into the lower-concentration chamber is then conveyed to a gas sensor or detector by a carrier gas for quantification.

A variation of the isostatic method is the quasi-isostatic method where at least one chamber is completely closed and no exposure to atmospheric pressure. However, there is a difference in penetrant partial pressure or a concentration gradient between the two cell chambers. The concentration of permeant gas or vapor that has permeated through into the lower-concentration chamber can be quantified by a technique such as gas chromatography.

There is the chemically modifying approach. It changes the plastic's surface during or after fabrication permiting the control of the permeation behavior of such products as film, sheets, diaphragms, and containers. These techniques are becoming increasingly important. There is an endless search for better barrier materials for packaging applications. As an example in blow-molded gasoline containers/tanks, the amount of gasoline permeation through HDPE even though it is very low, is still excessive per the standard requirements, thus has required some type of barrier. Including a barrier in a multilayer construction has created a satisfactory solution. The approach to functionalized PE formed tanks on the inside wall is by a chemical reaction, mosdy by exposing the surface to sulfonation or fluorination.

Oxifluorination has the fluorine gas combined with nitrogen to which several percent of oxygen by volume have been added. Subjecting PE to fluorine and oxygen at the same time leads to functionalization of the PE making it impermeable. This method permits substantially reducing the required amount of fluorine, resulting in a cost-to-performance improvement. Barrier plastics using oxifluorination are used for foods. They provide barriers that are needed to protect them against spoilage from oxidation, moisture loss or gain, and changes or losses in favor, aroma, or color.

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