Study on Aging and Recover of Poly (Lactic) Acid Composite Films with Graphene and Carbon Nanotubes Produced by Solution Blending and Extrusion

The aging, annealing, and reprocessing of the biodegradable poly (lactic) acid (PLA) based composite films incorporating graphene and carbon nanotubes were investigated in this work. Various monofiller and bifiller nanocomposite films with 6 wt.% filler content were produced by a solution-phase technique followed by extrusion. The freshly produced films were compared with the aged films after 18 months of shelf life in a room environment. The effects of aging, annealing, and melt reprocessing on the crystalline structure, the thermal stability, the hardness, and Young's modulus were analyzed by differential scanning calorimetry (DSC), TGA, and nanoindentation methods. The fresh and the aged samples were found to have semi-crystalline materials with 3%-7% crystallinity, while the crystallinity was significantly enhanced to 34%-38% by annealing at 80 degrees C and subsequent slow cooling. A good dispersion was observed in the bifiller films with filler ratios of 4.5:1.5 and 1.5:4.5 [graphene nanoplatelets (GNP) to carbon nanotubes (CNT)], which affected the crystallization processes. The reprocessing at 200 degrees C followed by fast cooling resulted in amorphous films, which significantly reduced the hardness and Young's modulus. The nanoindentation properties were dependent on the dispersion of nanofillers at the surfaces. The efficiency of annealing and reprocessing for the recovery and the reuse of aged nanocomposite films is discussed herein. The paper underlines that properties of the nanocomposites under investigation were influenced not only by the composition, the chemical nature of the added filler, and the processing condition, but also by the aging processes, which in turn depended on the type of nanopartcles added to PLA and the compositions. The paper provides valuable information for selection of material and processing conditions.