Converting Magazine

Eldridge M. Mount

Dimensional stability considerations
February 25, 2008

Dimensional stability is a key substrate property and is more or less critical depending on the application. Most times, dimensional stability is simply considered as thermal shrinkage, but there are more aspects to substrate stability than thermal shrinkage. 

First, on heating, not all films shrink and get uniformly smaller. The dimensional change observed will depend on how the film was made and the temperature to which it is heated. Oftentimes we only get the dimensional stability (Dim Stab) at a single temperature which the film manufacturer uses as a production control, so we get only a little bit of information on the stability. Ideally, we would have the dim-stab data for a range of temperatures. For instance, biaxially oriented polymer films will shrink in both directions at elevated temperatures, but at temperatures slightly lower at the onset of significant shrinkage, one dimension will shrink while the other direction will actually grow in length (MD shrinkage, TD growth). Also at low temperatures below the point where polymer molecules are free to relax, they will grow in length when heated just like metals. But as the temperature is increased, a point will be reached where the polymer molecules begin to relax and if they contain residual stresses will begin to shrink. At this point, the relative balance of orientation will affect how the balance in shrinkage and growth appears. But as the temperature is increased further, a more general relaxation will occur, and the film will shrink in all directions (but thicken).

Other factors which affect the dim stab of a substrate are chemicals which are soluble in the substrate. For instance, for polymers which absorb water, such as cellulose, nylon or polyester, as the relative humidity changes the size of the film changes, growing with moisture absorbance and shrinking when dried. In contrast, for polyolefin films such as polyethylene, polypropylene and other moisture-insensitive polymers while inert to the effect of moisture will be affected by solvents such as hexane, toluene and perhaps alcohols. This can have a profound impact on packaging when products contain materials which can diffuse into the films. For instance, we once had an unusual case when packaging gummy bears in OPP films, the film was quite inert for most of the product but the purple gummy bears would cause the PP film to swell and get sticky.

Manufacturing methods and thermal treatments, post-film formation, will impact the dim stabs of the substrate. There are large differences in the behavior of cast, blown and oriented films which we will explore in more detail. These allow films of the same polymer to serve as shrink films as well as heat-stable films capable of withstanding high temperatures. These differences are widely used, and examples are easily found in the polyesters, nylons and polyolefins used for packaging and industrial applications. 

When getting a new film product or comparing and new film suppliers product to an existing film product, rather than relying on a single value of dimensional stability, a better way to compare them and determine potential performance differences would be to measure the dimensional stability as a function of temperature or on exposure to a variety of chemicals (flavors?). What temperatures and chemical? Look at your manufacturing process and see with what temperatures and chemical exposures the film will be contacted.

Posted by Eldridge M. Mount on February 25, 2008 | Comments (2)