Poly p-(phenylene benzobisoxazole) or PBO fibres are highly anisotropic polymeric fibres used in high-performance structural applications owing to their outstanding properties, such as high strength and stiffness as well as low density. The anisotropic fibre structure, which is responsible for the excellent tensile properties, makes PBO fibres relatively weak under axial compression, and their compressive strength is one order of magnitude lower than the tensile one. Failure under axial compression is triggered at low strains by the initiation of a kink band from the surface by fibril microbuckling which propagates rapidly through the fibre cross-section to form a knuckle. Under further compressive or tensile deformation, the strain is localized in the knuckle leading to a considerable reduction of the fibre tensile strength. This mechanism obviously limits the application of PBO fibres in structural components subjected to both tensile and compressive stresses or when axial compressive loads may appear during service.
In order to alleviate this limitation, a multidisciplinary research team of IMDEA Materials Institute has developed a nitrogen plasma surface treatment (patent pending) to improve the mechanical behaviour of PBO fibres under axial compression. Specifically, the new surface treatment leads to a dramatic improvement (of the order of 40%) in the compressive strain for knuckle formation with a negligible effect on the tensile strength. This significant improvement opens a wide range of new possibilities for the use PBO fibres in advanced composite materials, ballistic protection textiles, structural cables, etc.
Figure: Development of a knucle through the PBO fibre cross-section under axial compression as observed during in situ testing inside a scanning electron microscope.
This technological innovation comes out from a R&D project (FUTURE PBO), partially funded by the Spanish Ministry of Science and Innovation, in which IMDEA Materials Institute collaborates with FUTURE FIBRES RIGGING SYSTEMS S.L.U.. This company, located in Valencia (Spain), manufactures synthetic fibre cables used by leading teams in international sport competitions in motor (Formula 1) and marine sectors (America’s Cup, Volvo’s Cup, etc). Since its start, Future Fibres has been worldwide recognized for its innovative strategy to improve performance by means of state-of-the-art materials technology.
Within the above mentioned project, a second result with potential industrial application has been submitted for patent protection. This covers a new design method of synthetic fibres cables which main application is their optimization by stress states calculation of the mechanical behaviour under external loads.