Microfibrillar reinforced composites from PET/LDPE blends: Morphology and mechanical properties

Microfibrillar reinforced composites (MFC) comprising an isotropic matrix from a lower melting polymer, i.e., low density polyethylene (LDPE), reinforced by microfibrils of a higher melting polymer, recycled from bottles, i.e., poly(ethylene terephthalate) (PET), were processed under industrially relevant conditions via injection molding in a weight (wt) ratio of PET/LDPE equal to 30/70 and 50/50, respectively. Dogbone samples with MFC structure were characterized by means of light microscopy (LM), scanning (SEM), and transmission (TEM) electron microscopy, and mechanical testing. The SEM observations of cryogenic fracture surfaces show an isotropic LDPE matrix reinforced by more or less randomly distributed PET microfibrils while LM and TEM on thin slices indicate on orientation in machine direction. By means of TEM on stained ultrathin slices, one observes the formation of transcrystalline layers of the LDPE matrix on the surface of the PET microfibrils where the crystalline lamellae are parallel to each other and are placed perpendicularly to the fibril surfaces. By this transcrystallization and the relatively high aspect ratio of the fibrils (at least 50), the impressive mechanical properties of the injection-molded samples can be explained. The elastic modulus is about 10 times higher than that of LDPE and about three times higher than LDPE reinforced with glass spheres, approaching the modulus of LDPE reinforced with 30 wt% short-glass fibers (GF). The tensile strength is at least two times higher than that of LDPE or of LDPE reinforced with glass spheres, approaching that of LDPE reinforced with 30 wt% GF.