Estimation of Percolation Threshold and Its Influence on the Properties of Epoxy Resin-Based Polymer Composite Materials Filled Carbon Fibers and Carbon Nanotubes

Carbon fillers are widely used in the polymer composite materials to control electrophysical properties, which makes them promising for shielding/absorbing electromagnetic radiation in various bands, protection against radio interference and antistatic coatings, etc. The study of percolation transitions in such materials is a priority for such tasks. Epoxy resin-carbon fiber (ER-CF) and epoxy resin-carbon fiber-carbon nanotubes (ER-CF-CNT) systems with different filler contents were investigated. Electrophysical studies were carried out in the frequency range of 8–12 GHz by the non-contact method, and electrical conductivity at low frequencies of 0.1, 1 and 10 kHz was measured by the two-contact method. The bending strength was tested on a 2167 P-50 tensile tester. It was found that: the electrical conductivity at low frequencies and the complex permittivity at 9 GHz of the composites change significantly (percolation transition) in the range of 0.001–0.005 volume fraction of the combined filler; the values of the real and imaginary components of the complex dielectric constant of the composites are ~ 50, which indicates a significant level of interaction between the components of the system and a uniform distribution of the conductive component in the composite; the maximum values of the relative flexural strength are observed at the content of CF-CNT proportional to the percolation threshold in the systems. The composites have a high level of strength at a low content of fillers, which, makes them attractive for use as protective coatings for absorbing or shielding from microwave EMFs.