Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

1 Publication
Adhesion Issues in PBO/Epoxy Composites
Liu, J.; Gao, S. L.; Mäder, E.; Bianchi, A. D.; Herrmannsdörfer, T.; Zherlitsyn, S.; Zvyagin, S. A.; Wosnitza, J.;
This study aims to address interfacial adhesion requirements for composites with high electrical insulation properties for extreme high magnetic field and pressure environments. The magnetic field is one fundamental parameter influencing the physics of many systems. Over the recent years, research with high magnetic fields has led to a range of enhanced modern technologies. These efforts were triggered by experiments such as the integer and fractional quantum Hall effect and as a result several high magnetic field facilities were built in the US, Japan, and Europe. The current technology limits the generation of static magnetic fields to 45 T. Magnetic fields of up to about 70 T are only available for experiments with pulse duration in the ms range. The Hochfeld-Magnetlabor Dresden, as well as the Los Alamos branch of the National High Magnetic Field Laboratory, are aiming at generating fields up to 100 T. The access to this magnetic field range possesses a large number of technological issues, particularly abnormal heating and high stress, generated by the Lorentz forces from the electrical current and the magnetic field of the magnet. In a 100 T magnet these forces amount to a pressure of the order of 4 GPa and ordinary steel would burst under the stresses involved in confining the high magnetic field inside the magnet. This problem is circumvented by reinforcing all conductor layers of the coil with high modulus fibre reinforced composites. Because the pulsed field magnets are operated at high voltages and large currents, this demands high electrical insulation properties of the composite and makes the use of electrically conducting carbon fibres problematic. Today, the high ultimate tensile strength of 5 GPa, high heat resistance/service temperature and the good insulation properties make poly p-phenylene-2,6-benzobisoxazole (PBO) fibre of good choice. However, it is challenging to get adequate interfacial adhesion of PBO fibre due to the nonpolar surface nature of PBO preventing any chemical bonding the smooth PBO surface with polymer matrix.
We have characterised the interfacial adhesion strength and critical interphase energy release rate by single fibre pull-out tests. The PBO fibre surfaces were de-sized by extraction of the commercial finish, and sized with different sizings. In addition, both O2 and NH3 gas plasma treatments and plasma modification with Maleic anhydride were used. It was found that the rather low surface free energy of the as-received PBO fibres (34 mJ/m²) could be increased by appropriate surface modification, particularly the oxygen-plasma and Maleic anhydride graft. However, the adhesion strength increased marginally with increasing surface free energy due to induced limited hydrogen and covalent bonds at their interface, which is in the same level like aramid, i.e. below the adhesion strength of glass fibre / epoxy systems. A short time plasma treatment (30s) with NH3 gas results in both reduction of surface energy and interfacial adhesion strength. AFM was used to characterize the surface topography varied by different fibre surface modifications and to evaluate the fracture surfaces. The fractographs are dominated by adhesive failure, that is to say, the interfacial crack propagation occurs primarily in the fibre/matrix interface plane, suggesting a further improvement of chemical bonding/mechanical interlocking is required. Moreover, for the application in a high magnetic field also the temperature change in the coil during operation has to be taken into account. Epoxy resins with different temperature resistance have not shown significant differences in the interfacial shear strength. The micro-mechanical results agree well with those of compression shear tests performed with real composite samples.
  • Lecture (Conference)
    Polymer Fibres 2006 Conference, 12.-14.07.2006, Manchester, UK

Publ.-Id: 8727 - Permalink