Milled carbon fibers added S-glass epoxy composites were fabricated using milled carbon fibers as fillers. Milled carbon fibers having five different lengths namely 7, 20, 60, 200 and 400 µm were used to fabricate five different S-glass epoxy composites. Optical microscope and scanning electron microscope studies of S-glass epoxy composites indicated that milled carbon fibers having 400 µm length settled at weave openings as large clusters besides spreading on the tow surfaces at the interfilament undulations. Milled carbon fibers having 200 µm length were found to form smaller clusters at weave openings with an array of interconnected network in the resin-rich grid zones on the tow surfaces at the interfilament undulations. Milled carbon fibers having 60 µm and further lower lengths were found to get arrested at the interfilament undulations with random orientations without interconnectivity. Electromagnetic properties namely permittivity, loss tangent, and reflection loss of the fabricated composites measured in the range of 8 GHz to 18 GHz indicated that controlled agglomeration of the milled carbon fibers with interconnectivity is required for observing increase in the electromagnetic properties. This study indicated that S-glass epoxy composites added with milled carbon fibers having 400 µm length can show better conductivity due to increased interconnectivity but cannot effectively absorb incident microwave energy due to their inability to confine within the resin-rich grid zones as well as increased reflections from the large clusters that are formed at weave openings.
Comparison of different surface treatments of carbon fibers used as reinforcements in epoxy composites: Interfacial strength measurements by in-situ scanning electron microscope tensile tests