Preparation And Properties Of Graphite Reinforced Poly(Lactic Acid) And Thermoplastic Polyurethane Composites

Poly(lactic acid)/graphite and thermoplastic polyurethane/graphite composites have been prepared by solution mixing-coagulation-melt pressed processing techniques and solution casting method respectively using laboratory-made graphite oxide and expanded graphite as fillers. Their structures, morphology, mechanical, thermal stability, crystalline and electrical properties were evaluated in detail by Fourier Transform Infrared Spectroscopy (FT-IR), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Dynamic Mechanical Analysis (DMA), Dynamic Mechanical Thermal Analyzer (DMTS), Thermo-gravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) techniques.First, graphite oxide (GO) was prepared by the Hummers methods using natural flake graphite as raw material. And then expanded graphite (EG) was prepared by rapid thermal expansion of GO. SEM showed that under ultrasonic treatment GO can be exfoliated in water. The GO aqueous dispersion was brown in color with layered structure. The flat size was from a few hundred nanometers to a few microns and the thickness was from several nanometers to several tens of nanometers. FT-IR analysis showed that GO was heavily oxygenated, bearing hydroxyl, epoxy, carbonyl and carboxyl groups on their basal planes and the sheet edges. XRD analysis showed that the GO had good crystal structure and the interlayer distance was increased. EG are black floccules with low density. Compared with GO, oxygen-containing groups on the surface of EG were much lowered, layer spacing was decreased and the crystal structure was destroyed. The originalπ-conjugated structure of graphite was restored. It can be used as conductive filler.Polylactic acid (PLA)/graphite composites were prepared by coagulation method and the morphology and properties were studied. SEM images and XRD results showed that neat PLA exhibits relatively smooth fracture surface. PLA/GO composite exhibits relative rough fracture surface with GO layers dispersed homogeneously in the PLA matrix. A higher magnification of SEM demonstrated that the GO were completely wrapped with the PLA, and a blurred interface was formed between GO and PLA. EG particles were also dispersed homogeneously in the matrix but the fracture surface of PLA/EG composite was smooth. DSC analysis showed that PLA/GO composites exhibit cold crystallization and decreased crystallinity. The presence of dispersed EG in PLA accelerated the crystallization process and the crystallinity of the corresponding PLA composites increased. As the content of GO increased, both the initial thermal degradation temperature and the temperature of maximum degradation rate of PLA/GO composites increased substantially. The addition of EG has the same effect on thermal stability of the PLA composites as GO. DMA tests showed that the increase of tan value of PLA/GO composites was the largest, indicating good bonding force between PLA and GO and low crystallinity of the composites.Thermoplastic polyurethane (TPU)/graphite composites were produced by solution mixing method and their morphology and mechanical properties were studied. SEM images and XRD results showed that both the exfoliation structure and intercalation structure coexisted in TPU/GO composite. GO dispersed in the TPU matrix with a layered structure. EG was dispersed well in the TPU matrix but micro-dimples appeared on the fracture surface. Tensile tests showed that in the range of 0wt.% to 5wt.% of the filler content, the mechanical strength of TPU/GO composites was significantly increased with the increase of GO content while elongation at break was not significantly reduced. DMTS testing showed that the addition of GO improved storage modulus. The addition of EG can significantly enhance the storage modulus of TPU composite, but the dynamic loss data shows that GO has higher affinity to TPU than EG. A TPU composite at filler (EG) loading of 3wt.% had a volume resistivity of 105Ω?cm, six order lower than that of pristine TPU, whereas those of PLA/GO composites remained constant regardless of the GO content.