## 3 Results and Discussion  
### In-Line Rheological Tests  
_Viscosity Difference for Standard and Reactive Extrusion_  
The results of in-line rheological measurements are presented in the form of a graph of viscosity change over time, measured during the production cycle lasting several hours (see Figure 3A). The reaction scheme of the PET chain elongation is presented in Figure 3B. As can be concluded during a cursory analysis, the values of viscosity change during the process and may fluctuate significantly. This is especially true of the two areas of the graph which characterize the viscosity jump following the addition of the M1 compound in step no. 3 and no. 6. In both cases, where virgin type PET was processed (V and VD), the addition of the M1 chain extender resulted in a sudden increase in viscosity up to 380 Pa-s. In these two cases, the feeding time of the material containing the M1 (PMDA) modifier was limited to about 20 min, preventing further increase in the viscosity of the  
Figure 2: The flow chart of the research methodology. polymer. In addition to the two listed examples, for the remaining materials, the viscosity measurements show only a slight variation in flow characteristics. Starting from stage No. 1, where the VD material was processed, the addition of the B1 modifier did not change properties much. The viscosity values recorded by the VIS viscometer changed from 150 Pa-s for the VD sample to around 200 Pa-s for VD-B1 materials. It is worth mentioning that the viscosity fluctuations during measurements for individual materials are also significant and can reach even \(\pm\)15 Pa-s. In the next step, no. 3, a sharp increase in viscosity can be observed for the VD-M1 sample. After the viscosity exceeded the value of 350 Pa-s, there was another change in the composition of the charge material to undried virgin PET (V). The change of material allowed to quickly reduce the viscosity and stabilize its value again at the level of about 200 Pa-S; this value  
Figure 3: (**A**) The results of the in-line viscosity measurements conducted during the production of the PET foil. (**B**) Possible course of the reaction in compositions containing the addition of the modifier M1 (PMDA) to PET resin. remained despite the addition of B1 in the step no. 5. Changing the composition in step no. 6 to V-M1 repeated the situation with a sharp increase in viscosity. Again, the material composition was changed after exceeding the viscosity reading of 350 Pa-s. The material processed at stage no. 7 consists of the recycled PET flakes/preforms with PMDA agent addition (MIX-M1). Again, changing the material composition caused the viscosity to drop, and it stabilized at about 180 Pa-s; however, this time, the main factor changing the polymer characteristics was the replacement of the virgin PET resin with a recycled mixture; therefore, as can be concluded, the efficiency of M1 modifier (PMDA) in such blends is lower than for unprocessed PET. A further change of the composition in step no. 8 to the MIX-B1 material does not bring any visible changes in viscosity, while for the unmodified batch extruded in the last step no. 9, a reduction in viscosity to the level of about 130 Pas is noticed. Taking into account the obtained results, it should be stated that for virgin materials, the effectiveness of modifier B1 is negligible, while the addition of M1 is characterized by the very high efficiency of the PET chain extension reaction. The presumed course of the reaction between the polymer chains of PET and the addition of M1 (PMDA) is presented in Figure 3B. For this type of reactive extrusion process; it is possible to obtain different efficiency, where a single PMDA molecule can connect from two to four PET chains with each other. The intensity of this reaction depends mainly on the residence time of the material in the extruder system, where a more extended time period increases the probability of bond formation [26; 37].