Mechanical behavior of solid and foamed polyester/expanded graphite nanocomposites

Abstract

Poly(ethylene 2,6-naphthalate)- PEN is a thermoplastic polyester characterized by a high glass transition temperature (125°C), comparable to that of polyetheretherketone (143°C), but with a significantly lower melting temperature (265°C). Its physical and chemical properties are very promising for applications in transport industry and aeronautics. Nanocomposite matrices based on PEN and expanded graphite were developed to be used as matrix for foams. Expanded graphite was melt blended with the polymer by means of extrusion process and its effects on the foaming properties were investigated through solid state foaming process. Graphite nanoparticles increased the crystallization kinetics of the polymer, inducing the formation of small crystals but lowering the total amount of crystalline phase. Transmission electron microscopy analysis showed a good dispersion of the nanofiller but some aggregates were still present, as also confirmed by graphite peak in the X-ray diffraction patterns of all nanocomposites. The elastic modulus of nanocomposites with amorphous matrix increased with respect to the neat amorphous PEN, while the modulus of crystallized nanocomposites decreased. Nanocomposite foams were successfully prepared, and an higher cell density was obtained when compared to the neat PEN. In the latter case, a strong increase in both yield and strain at break was measured. Furthermore, the elastic modulus and compressive yield stress of foamed PEN nanocomposites increased with the expanded graphite.