Many packaging materials are made of multiple layers. Characterisation of these materials is often required either to identify the layers or investigate the consequences of processing. The topographic image of a section through a film used as food packaging is shown above The layer located in the middle is a gas barrier designed to prevent the ingress of oxygen and the egress of food flavour.

Thermal analysis experiments were then performed on the different layers. The image below shows the sample after analysis. Although the ‘mounds’ created by the micro-thermal analysis are relatively large, 10-20 µm, the actual analysis is carried out on a few square µm. The probe is heated so that the sample beneath the tip melts then the tip penetrates as melting occurs. When the tip is retracted, this causes material to rise with it to form a bump. However, the contact area of the tip is sub-micron and this contact area largely dominates the onset of penetration, although as penetration proceeds a progressively larger area of contact is established. In this case, one of the layers investigated, the tie layer, was only 5 µm thick.

The results of the local thermal analysis are shown below.

The central gas barrier layer (red curves) can be seen to have a high melting point and this is poly(ethylene-co-vinyl alcohol). The bulk of the film (blue curves) is high-density polyethylene with an intermediate melting point. The tie layer (green curves) has the lowest melting point and is low-density polyethylene.