Flexible packaging Barrier Properties

Flexible packaging films are used to provide barrier to gases such as the following:

• oxygen 

• nitrogen 

• carbon dioxide 

• water vapor. 

Other applications may require taste and odor resistance properties. Permeation occurs in polymer films and this mechanism allows interaction with the environment. Molecules may permeate through a package in a three-step process. First, the molecules must dissolve into the film structure. Then the molecules will diffuse through the film layers. And, finally, the molecule will desorb on the opposite side. The molecule can move either from outside the film into the package or from inside to outside. This permeation happens due to a concentration or pressure gradient and is sensitive to temperature and, for many polymers, relative humidity. The permeation rate of a material through a polymer is a function of the diffusivity at steady state times solubility in the polymer. Diffusivity is also a function of solubility especially at the low solubility levels.

Permeation rates are usually measured at steady state conditions. The adsorption of a material can significantly influence the permeation rates, and if the adsorption mechanisms are of sufficiently long duration, this may prevent steady-state conditions from being reached during the expected shelf life of the package. Thus, in these instances, the polymer location in a film structure could influence actual permeation rates during a package’s shelf life. The gas transmission rate at steady state through a given polymer is inversely proportional to the layer thickness. The permeation coefficient through a multilayer film structure may be estimated by summing the permeation resistance of each layer.