Single polymer composites made of slowly crystallizing polymer
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Composites are widely used in an increasing number of applications in diverse fields. However, most traditional composite materials are difficult to recycle. Because of their enhanced recyclability, thermoplastic single-polymer composites (SPCs), i.e., composites with fiber and matrix made from the same thermoplastic polymer, have attracted much attention in the recent years. High-performance polymer fibers in combination with same polymer matrices would lead to a fully recyclable single polymer composite that has major ecological advantages. However, because a single polymer is involved in the composite, thermoplastic SPCs manufacturing presents a unique set of technical problems, and different approaches from those in standard composites manufacturing are frequently needed. Two specific issues in SPCs manufacturing are how to produce distinct forms of the same polymer and how to consolidate them. So far, most investigations have been reported on a single-component hot compaction method and two-component molecular methods. However, in these methods, either the processing window is too narrow or some impure materials are introduced into the system. The key issue in thermoplastic SPCs processing is how to melt-process the matrix without significantly annealing or even melting the fiber. To overcome the above drawbacks in existing SPCs processing, particularly to widen the SPCs processing temperature window and to purify the SPCs, a novel SPCs manufacturing process utilizing the characteristics of slowly crystallizing polymers was developed and investigated. Highly oriented and highly crystalline fibers made of a slowly crystallizing polymer are mixed with the amorphous form of the same polymer and then consolidated together under heat and pressure. In this dissertation research, two slowly crystallizing polymers, poly(ethylene terephthalate) (PET) and poly(lactic acid) (PLA), were used as model systems for SPCs processing.. To study the deformation and failure mechanisms of PET and PLA SPCs, the SPCs were characterized using tensile test, tearing test, impact test, SEM, optical microscopy, and other methods. The change of crystallinity and orientation of the material forms during SPCs processing were characterized by DSC and XRD. The effects of major process conditions on the performance of the SPCs were studied. It was found that the processing temperature played a profound role in affecting the fiber-matrix bonding property. The compression molded SPCs exhibited enhanced mechanical properties. For the PET SPCs with 45% by weight fiber content the tensile strength is four folds of that of non-reinforced PET. After reinforcement, the tearing strength of the PLA SPCs is almost an order higher than that of the non-reinforced PLA. The fusion bonding behavior of two crystallizable amorphous PET sheets was also studied. Several characterization methods including SEM, TEM and polarized microscopy (either on etched or on non-etched samples) were used to observe interfacial bonding morphology of the crystallizable amorphous PET sheets. For a bonded sample, a layer of transcrystals with a thickness of 1-2 Ým was found right at the interface. A secondary but much larger zone with a distinct morphology was observed outside the transcrystal layer. With increase of the heating time, the width of the whole interfacial region decreases. The interfacial morphology was found to significantly affect the interfacial bonding quality. The testing results further indicated that high bonding temperature with an appropriate holding time promotes interfacial bonding of two crystallizable amorphous PET.