This made me think that I have not spoken much about moistures impact on polymer substrates. First about my friend’s question, we remember that the “Bond Test” that most of us do is really just a peel test and only a portion of the measured force comes from the strength of the interface bond where a failure may be forced. Much of the measure force is made up of the bending of the substrates, any tape backing applied to the sample, any energy lost due to stretching of a component of the structure etc. So in that sense it should not be too surprising that if the mechanical properties of any component of the structure changes than the measured “Bond Strength” should change (might get higher but mostly will get lower). In this instance it is likely that the moisture is impacting the various elements of the lamination along with the mechanical properties of the adhesive.
I think that what does come as a surprise to many is that the substrate itself might be affected by moisture absorption. Especially to those working in the food packaging industry where packages are seldom subjected to liquid water immersion as opposed to those in the industrial lamination industry where it is much more common. Of course the impact of water on a substrate will depend very much on the physical as well as chemical nature of the product. For instance, paper as a substrate is very sensitive to water and has poor wet strength if immersed so that the fiber to fiber bonding is disrupted. It also becomes quite brittle if too dry. In comparison a spun bonded HDPE such as TYVEK® will retain its mechanical strength when wet in part because the fibers are fused together and also because HDPE will not absorb an appreciable amount of water. The same would generally be true for all the polyolefins such as PP, PS, LDPE, LLDPE, poly-4-methyl pentene etc. They are hydrophobic and are not generally affected by water. In contrast, the more polar polymers such as cellulose (Cellophane), polyesters (PET), Nylons, PVOH, EVOH, polyurethanes, epoxies etc will absorb an appreciable amount of water. The absorbed water will plasticize the polymer and change its mechanical, diffusion and dimensional properties. Dimensional changes in one polymer in a coextruded substrate can lead to curling if the structure absorbs moisture.
In some instances, depending on the polymer backbone chemistry and the temperature of the exposure, the absorbed moisture can chemically attack the polymer chain and cause a drop in molecular weight. This is especially common in the condensation polymers where the polymer molecular weight distribution is in equilibrium with the moisture in the polymer. Hot water is especially damaging in these polymers. Next time I will go a little further and get some examples of water on the properties of various polymers.