Layer strength can help resist curling
As the film structure begins to absorb moisture from the environment, due to the coefficient of hygroscopic expansion, the outer layer will generally begin to get longer. This will create a force causing the film to curl away from the moisture-absorbing layer. Curling can be minimized if the other polymer layers are strong enough to resist the force developing in the skin layer. Experimental work used in the development of US Patent #5,591,520 for a metallized-nylon skin layer found that curling was a function of the nylon-skin thickness, and it was necessary to maintain a thin skin layer of nylon to prevent curling.
Should a moisture-sensitive layer be chosen for its barrier properties as opposed to its surface properties, then the layer can be placed within the film structure. In this instance, the outer layers will resist the change in length of the moisture-sensitive layer and can prevent curling, but if one outer layer is not as stiff as the other, there could be curling. Changes in the outer-layer relative thickness by offsetting the barrier polymers position within the film, or by changing the outer layer’s polymer composition, can usually be made to create a stable, flat film.