This thesis aimed at achieving UV-stabilized polymers, using the incorporation of few-layer graphene (FLG), and to evaluate the dominant action mechanisms of FLG, and optimize its role, as a photo-stabilizer. The FLG performance as a photo-stabilizer and its effect on both macro and microstructural changes of High-Density Polyethylene (HDPE) composites, occurring during photo-degradation, were investigated. HDPE composites containing different FLG concentrations (0, 0.1, 0.25, and 0.5 wt%) were prepared through melt mixing using a twin-screw extruder, followed by exposure to UV-A irradiation for several durations. Rheological and mechanical properties were evaluated as a function of UV exposure time. To determine FLG's photo-stabilizing mechanisms, a specific controlled test, involving the UV exposure process of the mixture of hydrogen peroxide (H2O2) and the FLG aqueous suspensions, was conducted. Then, the amount of created free radicals was measured using an Electron Paramagnetic Resonance (EPR) test. Finally, the effect of the FLG addition on the penetration of photo-degradation within HDPE, and the correlation between photo-degradation depth at which chemi-crystallization occurs, and elongation at break, were investigated using Raman microscopy.
It was found that the addition of only 0.25 wt% FLG into HDPE showed a notable UV resistance for an exposure time of 672 hours, evident from sustained retention of rheological and mechanical properties.
Additionally, it was shown that 57% of the observed reduction in EPR signals, indicating the reduction in the amount of created free radicals, is due to UV absorption/reflection and 43% due to the free radical scavenging abilities of FLG. It is demonstrated that UV absorption/reflection and free radical scavenging mechanisms are the dominant ones among the three FLG photo-stabilizing mechanisms (UV absorption/reflection, free radical scavenging, and physical barrier to oxygen).
Finally, it was found that embrittlement occurred in neat HDPE when photo-degradation depth reached around 10 % of the sample thickness. Conversely, the samples containing 0.5 wt% FLG maintained a ductile failure behavior, despite a notable 12.5% depth of photo-degradation within the sample, and in the presence of surface cracks. The persistence of ductile behavior was attributed to the photo-stabilizing effect of FLG, as well as, to the tendency of the degraded layer to detach from the ductile core of the composite. The ability of the FLG-containing composites to maintain ductility through the detaching mechanism led to the deviation from the superposition principle observed in neat HDPE, between sample thickness, UV exposure time, and UV damage
