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How to improve the water vapor and gas permeability for plastic-film packaging
July 24, 2008
Roseliza asks the question: How to improve the water vapor and gas permeability for plastic-film packaging. This is a very old question with an ever-ongoing answer, and of course, the unspoken part of the question is without increasing (but even better lowering) the cost.
The first answer is to choose the very best polymer for the job. In many regards this might be PVDC which has a unique combination of moisture and oxygen barrier. However, it is very dense and therefore expensive in terms of cost/area and has a bad environmental image. For situations not requiring oxygen or aroma barrier, then polyethylene is a good moisture barrier, and in this polymer we can explore some of the ways to increase moisture barrier.
For example, in polyethylene, it is well documented that moisture barrier is increased by increasing the percent crystallinity. Therefore, a cast HDPE has a much improved moisture barrier than a cast LDPE because of the increased density (percent crystallinity). Next, improvements can be made for any given film process by controlling quenching and annealing conditions in a manner which will impact percent crystallinity and therefore barrier properties.
Once all the improvement has been wrung out of the polymer itself, there are additives which can be added which change the transport properties of the amorphous phase, such as polyterpenes or synthetic polyterpenes. These appear to dissolve, or at least concentrate, in the amorphous phase of a polymer and will improve moisture barrier (also stiffness and optical properties). This has been practiced for many years in OPP films.
Next, there is orientation, which can improve the moisture barrier by a factor of two or three times. Of course, the best ways to improve moisture barrier is to combine the base film, selected for its other properties, and add additional materials in layers to the film as by coextrusion, extrusion coating, laminating or by coating. This allows material combinations which are tailored to the application, i.e. no need to buy more barrier than is needed for the application.
The real work of film manufacture and design is to determine the right level of moisture protection required and then to generate a combination of process changes, material combinations and subsequent processing to reach the level within the economic constraints of the application.
Posted by Eldridge M. Mount on July 24, 2008 | Comments (0)