Gas Barrier
Gas barriers are technological solutions able to reduce or slow down the passage of gases and vapors. The typical function of blocking or reducing the gas migration is to protect what is stored on the other side of the barrier. The use of gas barriers is ubiquitous in packaging to protect sensitive goods, such as electronic components, medical devices, pharmaceuticals, cosmetics, food, and so on. No material can actually provide an infinite barrier to gases; the definition of sufficient barrier properties depends upon the end use requirements, e.g. in relation to the intended product shelf life.
Today there is a wide acceptance of polymer materials in the packaging industry in spite of their poor barrier properties compared to glass and metals, due to the favorable trade-off with their attractive features of flexibility, light weight, and formability. The permeation in polymers is directly related to the concentration gradient of the gas diffusing from one side of the barrier to the other. The gas molecules naturally tend to move from the high-concentration side to the low-concentration side, crossing the barrier interface. The mechanism involved depends on the intrinsic physical properties of the barrier material and of the gas, as well as on the temperature conditions. Considering a sheet of barrier material, the permeation occurs via: i) the dissolution of the gas into the upstream side of the film, ii) the diffusion through the film, and iii) the desorption from the downstream side. The rate-limiting step of this process is the diffusion through the film.
The permeability coefficient in dense polymers is defined as the molar flux of the gas (rate of permeation per unit area) normalized by the film thickness and the difference between upstream and downstream partial pressures. Thus the higher the permeability, the higher the gas transfer across the barrier. Permeability is therefore measured in SI units of mol m-1 s-1 Pa-1, or g m-1 s-1 Pa-1, although Barrer are commonly used (1 Barrer = 10-10 cm3 (STP) cm cm-2 s-1 cmHg-1). The permeability can be used to evaluate at first glance the performance of the barrier structure for packaging applications.
One method to measure permeation is to feed the gas on one side of the sample cell and to carry the permeated gas to the detector by a sweep gas. The conditions under which the measurement is made have a considerable influence on the result. Both temperature and humidity gradient across the sample must be measured, controlled, and recorded with the result. It would be meaningless to compare two results unless the measuring conditions are known.
Oxygen transmission rate (OTR) is the measurement of the amount of oxygen gas that passes through the barrier sample over a given period of time in steady state conditions. Standard test methods are available for measuring the OTR of packaging materials. Analogously, carbon dioxide transmission rate is the measurement of the amount of CO2 gas that passes through a substance over a given period of time. Moisture or water vapor transmission rate (WVTR) is the measurement of the steady state rate of passage of water vapor through a substance. The SI unit for WVTR is g m-2 day-1. Typical rates in aluminum for laminates may be as low as 0.001 g m-1 day-1.
Active barriers are systems that can selectively prevent the gas dissolution by capturing it thanks to the action of materials suitably introduced in the system. The mechanisms are mainly based on the chemical properties of the active component. The low gas transmission rate of the barrier is reinforced by the blocking function of the active material, and a breakthrough time can be defined, during which the passage of gas into the permeate is zero. The incorporation of oxygen scavengers or oxygen absorbers into packaging films is an example of how to build an active barrier.
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