This study aims to evaluate the role of the polymer memory function in characterizing the rheological and thermal behavior of semi-safe plastic materials used in direct-contact applications between bottled water container walls and drinking water. The significance of the study stems from the fact that the degradation of rheological properties of polymeric materials, such as the decrease in dynamic viscosity and elastic modulus, may lead to structural changes that contribute to the release of potential contaminants from plastic container walls into drinking water. The study was based on thermo-rheometric tests conducted on three polymer samples: virgin low-density polyethylene LDPE-2570/1 as a reference sample, previously extruded low-density polyethylene LDPE-2570/2 as a sample carrying the memory of prior processing stresses, and polypropylene PP-2570/4 as a sample belonging to the same olefinic group. Rheometric measurements were performed at temperatures of 175°C and 212°C, with particular focus on time sweep tests to investigate the relationship between elastic modulus and thermal exposure time. The results showed a significant decrease in the values of dynamic viscosity and elastic modulus for the LDPE-2570/2 sample compared with the virgin sample. The activation energy also decreased from 77 kJ/mol to 70 kJ/mol, indicating the loss of part of the polymer branching structure and the influence of previous thermal and mechanical stresses on the molecular structure. Thermal stability tests further revealed that the LDPE-2570/2 sample followed an increasing trend with a positive slope, indicating the occurrence of crosslinking within the polymer chains, whereas the PP-2570/4 sample exhibited a decreasing trend with a negative slope due to the phenomenon of polymer chain scission. The study confirms that the polymer memory function represents an important tool for evaluating the suitability of semi-safe plastic materials for drinking-water contact applications. The results also indicate that XL-LDPE possesses better thermal and structural stability than PP under the investigated testing conditions.............. Keywords: ..............Polymer degradation, rheological properties, low-density polyethylene, polypropylene, polymer memory, thermal and mechanical stress.