Converting Magazine

Eldridge M. Mount

Improving moisture barrier
May 19, 2008

I thought I would start a series on moisture barrier improvement as it is a key barrier property for many substrates and in particular for substrates used for packaging, and especially for food packaging. In both industrial and food packaging, the level of moisture barrier which is important will depend a great deal on the impact of moisture on the product and the length of time it is to be kept. 

There is not any difference in this regard for industrial applications than for food packaging but the impact of the product spoilage is different. In industrial packaging spoilage of the product could be a moisture induced corrosion (oxidation) of a surface as in surface rusting of an iron part or the swelling of a component which makes it difficult to operate etc. In food packaging the principle change in the product is the direct loss, or gain of moisture by the product which then changes the texture of the product. The change in texture is recognized by the consumer as “stale”. 

When things get stale, two things happen, first the texture change leads to a change in the feel of the product when first bitten into. In moist products, such as bread the product dries out and gets stiff and hard while for dry products, the moisture gain changes a stiff product, such as a chip, from a crunchy to a limp feel. As the product change from the moisture loss or gain changes the product from its anticipated characteristic it is easily recognized as “stale”. This does not necessarily mean the product is unfit for consumption, just that it is not what was desired, and for instance dry bread is also sold as a product.

Moisture gain and loss is an important thing to control because it is one of the most noticeable changes in a product and therefore the first failure of the product in inadequate moisture barrier packaging. If the moisture barrier is improved what will happen is that at some point the product failure first noticed by a consumer will change and at this point the moisture barrier is as good as it needs to be, until the new failure is corrected (flavor loss, rancidity due to oxygen etc.). This is an interesting point that the improvement in one barrier property inevitably leads to a different product “Failure”. In part it means there will always be work for improving packaging, because the demand for improvements in packaging is real (increased shelf life for longer distribution chains) and this will inevitably lead to changing failure mechanism for packaged products.

So how do we change moisture barrier of a substrate? The simplest method is in the initial material selection as each packaging material will have a different base moisture barrier level. Next we can modify the barrier of many materials by controlling the morphology (internal structure) of the material, generally by the thermal and mechanical processing of the substrate. We can also chemically modify the base material by blending in other materials which changes the substrates morphology or its sensitivity to moisture. Other methods are to create composite structures by the combination of additional materials into layered structures as in coextrusion, coating or metallization. 

In the next few postings I will expand on these various methods of substrate modifications for improving moisture barrier, and in particular I will explore the variations which can be induced by modifications to the materials and the impact of various processing conditions on substrate morphology and compare various materials together.

Posted by Eldridge M. Mount on May 19, 2008 | Comments (2)