From transparent rigid films that keep sliced sausage and cheese fresh for longer, to stable yogurt cups to colorful, flexible coffee packaging, composite films have become indispensable in the packaging industry. Depending on the application, these highly developed products may need to be oxygentight, transparent or printable and possess a certain flexibility and/or stability. Such properties can only be achieved through the use of a variety of components, such as multiple polymer layers. To this end, polymers are in turn selected according to their own properties.
For the identification of individual polymers in a multi-layer film, DSC has proven itself in the packaging industry as a fast and easily accessible method.
In the following example, a commercially available composite film was investigated by means of the DSC 204 F1 Phoenix®. The sample was prepared in the Concavus® crucible and evenly pressed onto the crucible bottom by means of a slidein lid (figure 1) which was specially developed for measurements on very thin samples such as films.
Figure 2 shows the results of the DSC measurement from the 1st and 2nd heating runs. In both heating runs, multiple overlapped peaks were detected between 108°C and 121°C. This indicates the presence of different polymers; the temperature range here is typical for various low-density polyethylene types.
In the 1st heating, a peak at 176°C was additionally detected which indicates the presence of EVOH (polyethylene vinyl alcohol). EVOH is also known as a barrier plastic, and is widely used in the packaging industry due to its good impermeability to substances such as oxygen. Its melting temperature is dependent on its ethylene content; a melting temperature of 176°C corresponds to an ethylene content of between 35 mol-% and 38 mol-% . In the 2nd heating, the peak at 176°C is shifted to a lower temperature (159°C). This shift is probably due to the melting of a mixed phase formed between polyethylene and EVOH. The broad effect between 230°C and 280°C will be investigated in more detail in the following.
For this, the composite film was separated into two layers: a flexible, aluminum-colored film and a second, thinner, printed film (see figure 3). Between the two layers was an additional paper layer.
The two films on either side of the paper layer were measured separately from one another. The DSC curves are presented in figure 4.
The printed film (blue curve) – except for the peak at 254°C (figure 2) – exhibits the same effects as the composite material as a whole. In contrast, the aluminum-colored film (black curve) produces only one peak, at 255°C (1st heating) and 248°C (2nd heating), respectively. This temperature range is typical for the melting of PET.
With these results, the following can be concluded about the composition of the composite film: The thinner, printed film consists of different polyethylene types as well as EVOH; the aluminum-colored one is PET. The appearance of the PET layer in terms of color indicates an aluminum coating which may be used, for example, as a light shield in packaging . The aluminum melting peak (660.4°C) is outside the measured temperature range and was therefore not detected.
In order to be able to clearly identify the three overlapping peaks between 108°C and 121°C, the DSC curve from the 2nd heating (figure 4, dotted line) was imported in the Peak Separation software program. Peak Separation allows for the presentation of experimental data in the form of the additive overlapping of peaks. This program offers different curves types such as Pearson, Gauß, Cauchy, etc. Here, the Fraser-Suzuki curve progression along with a mixture of the Fraser- Suzuki- and asymetrical Cauchy curve progresssion was selected. By applying these profiles to the measured DSC curve, it becomes possible to mathematically separate the overlapping peaks.
Figure 5 shows the results of the Peak Separation. Four calculated peaks can be related to the experiment's DSC curve (blue dotted line). The peaks at 108°C, 118°C and 120°C are typical for different lowdensity polyethylene types (PE-LD, PE-LLD).
An additional peak at 92°C (orange curve) can be attributed to the melting of small crystallites.
The correlation coefficient between the sum of the four calculated curves and the measured curve is determined to be 0.999 and thus confirms the good fit of the calculated endothermal peaks to the measured data.
DSC measurements yield valuable information on the composition of packaging films. These complex materials consist of different layers, which can sometimes be identified with just a single DSC measurement. The packaging shown in our example consists, at a minimum, of PET, EVOH and several polyethylene types of different densities.
The melting ranges of the different polymers often lie close together. However, complete separation of the peaks and/or precise material characterization can be achieved by means of careful sample preparation and the application of the Peak Separation software.