Environmental aging and degradation of multiwalled carbon nanotube reinforced polypropylene.

The degradation of polypropylene (PP) and PP-multiwalled carbon nanotube (PP-MWCNT) panels during environmental weathering resulted in an increased degree of crystallinity, making them brittle, and creating surface cracks. The degradation led to a breakdown of the panels and increased the potential for nanorelease. Thermal analysis revealed that the thickness of the test panels and reinforcement with MWCNTs had a significant influence on the stability of PP-MWCNT composites. Differential scanning calorimetry indicated that the MWCNTs acted as nucleation points, increasing the crystallization temperatures of PP-MWCNT, which reduced the extent of aging. Weathering decreased both the melting and crystallization temperatures of PP by as much as 20 o C. The reduction in the temperatures was inversely proportional to the thickness of the panels. The activation energy (E a ) obtained using isoconversional kinetics of the TGA analysis showed that the effective thermo-oxidative degradations of PP changed during aging. The E a for the initial stages of thermal degradation decreased from ~330 kJ/mol to ~100 kJ/mol for aged PP. During the late degradation stages, the E a values increased to ~300 kJ/mol. These results suggest that early degradation were altered because of the changes in the molecular structure of the aged P and a shift in the degradation rate-limiting steps.