Analysis of Plastic Laminates and Fibres

Plastic laminates are used extensively from the ordinary crisp packet to high specification products for medical and pharmaceutical use. In all these cases, laminates are used since the application demands a number of properties which no single polymer exhibits. 

Taking the example of the crisp packet, the irmermost layer must be inert to the product and provide a barrier to air, the outside layer has to be capable of being printed onto and the middle layer (or tie layer) has to be able to stick both of the other two together. 

One of the principle analytical techniques for the analysis of laminates is the infrared microscope. This instrument is capable of recording transmission infrared spectra from sample down to 10 square microns in size. The approach is usually to prepare a section of the laminate by microtoming a section and then recording a spectrum of each layer aided by the masking off of the other layers. The sectioning is often undertaken obliquely in order to maximise the apparent layer thickness. The microscope also allows the thickness of each layer to be determined. 

It is also possible to use an ATR technique to record infrared spectra of the outside layers and reserve the IR microscope for the central layers. This is possible because the ATR technique only penetrates a few microns into the surface of the layers. It is also possible by choosing the appropriate crystal (e.g., germanium. 

Using the knowledge gained by infrared spectroscopy of the generic polymer types present, the assignments can be refined by the use of DSC. Due to the fact that laminates by their nature are often constructed of completely different plastic types, the complete structure can be analysed in one operation and the melting endotherm transitions obtained used to differentiate between various grades. For example in the case of a laminate which had been shown to be comprised of a polyethylene and a nylon, it would be possible to differentiate between LLDPE, LDPE and HDPE, and nylon 6 from nylon 6,6. 

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