## 3 Results and Discussion
### The Properties of the Manufactured Foil
For most of the prepared samples, the haze parameter, which is an indirect measure of foil transparency, does not exceed 2%, which proves good optical properties. It is worth mentioning that for the reference APET film, the haze parameter was the lowest (about 1%), which results mainly from the homogeneous nature of the material forming the cross-section of the film. Considering the issue of materials homogeneity, it is not surprising that the haze value is slightly higher for the film made of PET/PE laminate (\(\approx\)12%). The reduced transparency is  
Figure 4: The results of the mechanical properties measurements obtained during the tensile test. (**A**) Tensile strength, (**B**) elongation at break. (**C**) The results of haze factor measurements and appearance of foil samples. mainly influenced by the addition of PE and an adhesive layer. PE, as a polymer with a higher level of crystallinity itself, has an increased haze index; additionally, the transparency is influenced by the differences in the refraction angle for the PET core, the adhesive, and the PE layer. On the other hand, the results of measurements for materials with the addition of the M1 modifier are less expected. It is visible that each use of PMDA in the process increases the haze parameter, even up to 50%, as for the MIX-M1 sample. Such a significant change is difficult to explain, given the relatively low content of the modifier. Even for materials containing the addition of fillers or recycled resin [38; 39], the transparency did not deteriorate in this way; hence there is no clear cause for the increase in the haze factor of the manufactured foils. Due to its multifactorial nature, this issue will be analyzed in more detail in further research. Due to the fact that PET belongs to the group of semi-crystalline polymers, in many cases, its properties depend on the morphology of the crystalline phase, despite the fact that the crystallinity level for most of the foil did not exceed a few percent. The DSC analysis used for the evaluation of the crystallinity level is also helpful for a more deep understanding of the polymer phase transition differences. For the purpose of the study, the obtained results are presented in the form of DSC signal plots, while the crystallinity level is presented in a separate graph (see Figure 5). The individual figures present the comparison of the DSC plots of the 1 st heating, cooling, and 2 nd heating measurements. Additionally, Table 2 contains some of the basic thermal properties read from the DSC plots. Figure 5: The results of the DSC analysis for prepared foil samples recorded at subsequent measurement steps: (**A**) 1 st heating, (**B**) cooling, (**C**) 2 nd heating. (**D**) The results of the crystallinity calculations from 1 st and 2 nd heating stages. The appearance of 1 st heating thermograms indicates some typical thermal properties of thermoplastic polyester. For all of the tested samples, the glass transition range, visible as a slight change in the baseline level, was very close to 75 \({}^{\circ}\)C. That is a typical value for pure PET resin since many other studies indicate similar results [26; 40; 41]. Another important feature visible on the DSC curve is reflected by a clear exothermic peak around 130 \({}^{\circ}\)C. That behavior is correlated with the cold crystallization phenomenon. The appearance of the visible peaks and the area of the transition (enthalpy) suggest the dominant amorphous nature of the prepared materials. For all tested samples, the maximum temperature (T\({}_{\rm cc}\)) and the peak area (\(\Delta\)H) are very similar; hence again, it is difficult to find any differences for individual materials. The lack of significant differences in the appearance of DSC thermographs was also visible for the melting range. The enthalpy values obtained from the cold crystallization and melting were used for evaluation of the crystallinity level, which results are shown in Table 2.  
According to the calculations, the content of the crystalline phase varies from 11% to 13%.