4 Conclusions - POLYTECH - ETH Zürich

The properties of such laminated strands and embedded spread filaments are wellunderstood and can readily be calculated with well-known theories. 66 Unfortunately,there are considerable difficulties in manufacturing such products of high fibercontent. The theoretical maximum fiber volume fraction for random packing is82 %. 67 In practice, however, that fraction appears to be limited to about 55-65 %only. 68 As the stiffness of the matrix or encompassing foils, generally, is low incomparison with that of the high-performance fibers, the upper limit of the axialstiffness that is reached - even for composites and laminates comprising perfectlyalignedfilaments - is approximately only half of the actual value of the reinforcingelements.Recent attempts to employ no foreign matrix and produce “single-species” fiber-basedcomposites have been met with some success, notably in the case of PE 69, 70 andisotactic polypropylene (i-PP). 71-74 Typically, in the processes developed, uniaxiallyoriented fibers or tapes are hot compacted in the vicinity of their melting temperature,causing partial melting of the polymer, which leads them to fuse together - invariably,however, with a partial loss of their initial properties.In summary, since the development of high-performance polymer fibers, considerableprogress has been made in constructing advanced products thereof retaining asignificant fraction of their outstanding mechanical characteristics. But also, clearly,there is room for improvement and better exploitation of the unique intrinsicmechanical properties of assemblies of oriented extended-chain macromolecules.-23-