Fiber reinforced polypropylene nanocomposites

The aim of this thesis is to assess the feasibility of integrating nanoparticles into glass fiber (GF) reinforced isotactic polypropylene (iPP) composites via existing thermoplastic processing routes, and to investigate whether this results in significant improvements in the mechanical properties of the final composites. A longer term aim will be to extend the approach to the preparation of hybrid composites with added non-structural functionality. However, the nanoparticles that have provided the focus for the present project, montmorillonite layered silicates (MMT) and nanocarbons, were chosen for their potential as structural reinforcing elements. A melt-spinning grade and a film grade of iPP were used to prepare iPP-based nanocomposite precursors in the form of melt-spun fibers and extrusion-calendered films respectively. Long glass fiber (LGF) and glass mat thermoplastic (GMT) composites were then compression-molded from co-woven, co-wound and intercalated semi-finished products. The processing behavior and structural performance of the resulting composites are discussed in terms of the matrix morphology and its influence on the matrix rheological and mechanical properties, and interactions between the matrix and the reinforcing fibers. The nanocomposites were prepared by either (i) combined solvent and melt-mixing or (ii) direct melt-mixing. Combined solvent and melt-mixing was more suitable for dispersing carbon nanofibers (CNF), which tended to agglomerate. With iPP/MMT, both routes gave a mixed intercalated-exfoliated morphology with an MMT interlayer spacing up to 57 % greater than in the as-received MMT. However, direct melt-mixing was considered to be better suited to industrial requirements, and also more convenient for laboratory scale preparation. Melt-compounded iPP/MMT injection moldings showed a monotonic increase in stiffness with increasing MMT content, a 40 % increase in tensile modulus being measured at 13.5 wt% MMT, for example. The tensile strength, on the other hand, reached a maximum 10 % increase over that of the pure iPP at about 3 wt% MMT, but fell off at higher MMT contents. iPP/MMT and iPP/CNF fibers were melt-spun using a laboratory-scale industrial spinning line. Processability was consistent with the melt rheology, the maximum MMT content for which fiber spinning was possible being about 5 wt%. The MMT platelets were aligned with the fiber axis over the whole range of MMT loadings and fiber draw ratios. MMT particle aspect ratios of about 150 were observed by TEM in this case, i.e. greater than in the as-compounded iPP/MMT, for which the particle aspect ratios were about 50. An aspect ratio of 150 was found to be consistent with micromechanical modeling of the observed increases in fiber stiffness with MMT content, which reached 170 % for iPP/2 wt% MMT fibers melt-spun with a draw ratio of 1 and a drive-roll velocity of 360 m/min. The tensile strength again reached a maximum at about 3 wt% MMT. The thermal stabil