By Dr. Eldridge M. Mount
A: Over the years, this column has described the properties of coextruded films and how these films are made. Polymers for coextruded structures are chosen based on the intrinsic properties of the polymer, melting point, barrier, refractive index, etc.
The actual layer measurement can be made manually using a calibrated measuring eyepiece, or with camera software designed and calibrated to measure film layers and total thickness, or by printing a picture and knowing the sample thickness measuring the relative layer thickness with a ruler or calibers. From the physical measurements, the thickness percentage and thickness can be directly calculated.
The only difference in the approaches is the time and money involved in equipment and software. Layer thickness can then be plotted against position in the web to obtain a layer-thickness distribution. Today, with additional equipment, it also is possible to identify individual layer polymers with IR spectra taken directly through the microscope.
Microtome for very thin, soft samples
For very thin or soft samples, cutting can be done supported between two polymer sheets with a microtome. The microtome is a mechanical device designed to cut very thin samples. With the turn of a crank, a knife blade cycles up and down while the sample holder advances a fixed distance allowing sequential cuts to be made. Often times used in medicine to section a tumor sample, it also can be used to cross-section film defects or films as described here.
Modern film-thickness scanners used in film production usually use a radiation source to measure total film mass (related to thickness by density) by Bears Law attenuation. Today, however, there are IR thickness scanners which can be used to directly measure the thickness of individual polymer layers with strong IR absorbance bands, such as nylons and EVOH. Once calibrated, these can be used to measure layer distribution and thickness in real time.
Know your parameters
The importance of layer thickness and layer thickness uniformity for a product’s Fitness-For-Use and expected film performance as well as customer product acceptance are important parameters to know and control. While time-consuming to directly measure, once determined it can be correlated with film properties and used as an indirect measure of layer thickness for manufacturing quality control.
The classic example is the correlation of sealant-layer thickness with seal strength. Consequently, during film manufacture, the uniformity of seal strength across the film web is an inexpensive and quicker but effective substitute quality measurement for absolute film-layer thickness and thickness distribution.