Interfacial bonding between thermoset and thermoplastic polyurethane reinforced textile grade carbon fiber: Structure property relationships

The research work focused on examining the interfacial adhesion of unsized, epoxy, and urethane-sized textile grade carbon fiber (TCF) reinforced in different classes of polyurethane (PU) thermoplastic (TPU) and thermoset (TSU) polyurethane (PU) through the structure-property relationship. The Carbon Fiber Technology Facility (CFTF) at Oak Ridge National Laboratory (ORNL) has produced TCF to reduce the cost of commercial-grade carbon fiber. The first part of the research examined the fundamental relationships between (a) soft segment thermoplastic polyurethane (S-TPU), (b) hard segment thermoplastic polyurethane (H-TPU), (c) thermoset polyurethane (TSU) and TCF reinforcement’s molecular behavior at the interface using the surface and thermal analysis (e.g., atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA)). The results showed that S-TPU, H-TPU and TSU could produce strong interaction with urethane sizing TCF through nucleophilic addition reaction.

In the second part of the research, the surface and thermal results obtained from material characterization were used to validate the interfacial adhesion of TCF-TPU and TCF-TSU composites, respectively. The mechanical properties showed that segmented TPU is compatible with epoxy-sized TCF, whereas urethane-sized TCF is compatible with the TSU system. Furthermore, the joining of dissimilar materials is a significant step to produce cost-effective automotive and aircraft components. Hence, the fusion bonding study was conducted using epoxy-sized TCF-TPU and urethane-sized TCF-TSU composites. The lap shear strength of fusion boded TCF-TSU and TCF-TPU showed 21% improvement compared to PA66/GF and epoxy/CF bonded composites.

The third part of the research studied the effect of sizing thickness on composites' mechanical properties using the Dehomogenized approach. The results showed that increasing sizing thickness from 0.01 to 0.5 enhances the tensile strength by 1200% of TCF-TSU composites. The TCF and TSU properties obtained through reverse engineering mechanism further validated for the cross-ply laminate. The simulations results showed less than 10% variation in the tensile properties than experimental findings. The overall outcome of this work can advance state of the art in TCF polyurethane composites for a range of applications such as aerospace, automotive, sports, and industrial products.