Exploring Mechanical Adhesion in Fiber Reinforced Composites with Aramid and Recycled Carbon Fibers

Polymer composites are a versatile group of advanced materials with high strength, low weight and high stiffness. To achieve these properties special care must be taken in the manufacturing process of each composite structure starting from material selection to compounding and assembly. Composites are composed of matrix material and reinforcing material. The combination of these two elements is what gives the composite its unique properties. However, if these elements are not joined together properly through interfacial adhesion the load carrying capacity of the structure is compromised. In the worst case, poor adhesion may cause catastrophic failure of the whole composite.

The objective of this thesis is to identify what effect does increased mechanical adhesion at the fiber-matrix interphase in fiber reinforced polymer composites have on the adhesion properties. In addition, the goal is to determine if this effect can be observed in composite structures of different size scales and material combinations. To achieve these goals, two case studies were investigated: development of a novel surface treatment method for aramid fibers to create increased surface topography and utilization of recycled carbon fibers with pronounced surface topography acquired through an adjoining project. Using these two fiber types, micro and macroscale composites were manufactured and the adhesion between fibers and matrix measured. In addition, the effect of the surface treatment on the fibers was tested with single filament tensile tests, electron microscopy and atomic force microscopy.

The results of this study showed that by increasing mechanical adhesion at the fiber-matrix interphase the overall interfacial adhesion is increased also. Even nanoscale structures that are firmly attached to the fiber surface, have a positive effect on the adhesive properties without affecting the fiber properties negatively. In addition, it was shown that the increased adhesion can be observed in thermoset, thermoplastic and rubber matrices in different size scales. This broadens the use- potential of aramid and recycled carbon fibers and enables a transition towards more sustainable composite manufacturing with these two advanced fiber materials.