By Dr. Eldridge M. Mount
A: Over the years, this column has described the properties of coextruded films and how they are made. Polymers for coextruded structures are chosen based on the intrinsic properties of the polymer, melting point, barrier, refractive index, etc.
So, the coextruded films are designed based on layer properties, but generally it is not enough to simply have the polymer present (the level of the desired properties are extensive [controlled by the layer thickness of the polymer]). Even for metallized films where the metallized barrier is controlled by the surface polymer, other properties such as metal-fracture resistance or %metal-lift may be controlled by the metallizing skin-layer thickness. Consequently, the uniformity of the film property depends on the uniformity of the layer thickness both across the film (layer distribution from the die) as well as along the length of the film in a roll (manufacturing output stability). This means it is important to know the absolute thickness and the thickness uniformity of each polymer layer during film production.
Take a look; use a ruler
So, how is layer thickness and layer uniformity determined? The best way is by direct observation or by measuring the layer thickness directly, essentially using a ruler. This is time-consuming and perhaps tedious but important work. Layer thickness is then oftentimes correlated to particular film-performance properties and a secondary laboratory measure such as seal strength, hot slip, barrier or production setting, such as output measured as melt-pump RPM, can be used in manufacturing and control charted.
Of course, this type of layer thickness/film property correlation must be made for each production line and combination of production equipment on which the coextruded film is manufactured. Initial layer thickness/performance data are generally developed during the “Fitness-For-Use” product-design portion of a project to set product-design parameters for coextruded-film manufacturing.
Samples and more samples
To measure film-layer thickness, a film’s full-width web sample is collected from a finished roll. The film is indexed by position, and the number of samples to be taken is determined. During the qualification of a product on a film line, the number of samples would be expected to be higher to determine the uniformity of the layer distribution. Later, for control charting purposes, the number of samples across the web can be reduced.
Samples are then measured for total thickness and prepared for observation and measurement on a microscope. Cutting of the film sample for microscope observation is the critical part of the whole process. Relatively thick or stiff samples can be placed in a cutting jig, cut, perhaps stained and observed on the microscope stage directly in the jig.