Effect of heat treatment on mechanical properties of laminated carbon fiber reinforced polymeric composites

In recent years use of electrospun nanofibers and nanoparticles to improve the interlaminar properties have increased significantly. In most of the cases the additional interlaminar phase of nanofibers is required to go through various thermal and/or chemical processes. There has been emphasis to optimize the interlaminar nanofiber layers to achieve the optimum desired mechanical properties such as interlaminar strength. One common practice is to disperse nanofibers into the resin and then use the nanofiber enhanced resin to fabricate the laminated composites. However, proper dispersion and fiber filtering out are some of the problems that exist in fabrication using the nanofiber mixed resin approach. To alleviate this problem, an innovative approach of growing PAN (polyacrylnitrile) nano fibers directly on carbon fabric by electrospinning seems to solve the dispersion and fiber filtering problem. However, as PAN fibers require stabilization and carbonization, it is obvious that carbon fabric with PAN fiber deposition will have to undergo stabilization and carbonization process. The effect of stabilization and carbonization heat treatment on the mechanical properties of carbon fiber fabric is not yet fully understood. This paper presents the effects of heat treatment on carbon fabric used for fabricating laminated carbon fiber reinforced composite with epoxy resin. The heat treatment was performed at 280oC in air for six hours, and 1200oC for one hour in nitrogen which are similar to stabilization and carbonization of pure PAN fibers. The effects, due to heat treatment, were mainly characterized in terms of mechanical properties by performing tensile tests and shear tests. Fiber surface topography was observed by SEM to analyze physical changes. Chemical changes, corresponding to the existing groups with carbon fibers, were examined through FTIR. The results obtained are compared with a set of control laminated composite specimens, which were fabricated using heat vacuum assisted resin transfer molding (HVARTM) process and cured at 149oC. The two sets of composite were infused with resin in a single vacuum bag to ensure that both sets of specimens have identical resin infusion and cure cycle. Laminates used for making control specimens were fabricated using carbon fabric which did not undergo any heat treatment. A change in laminate thickness for heat treated carbon fabric was observed indicating a possible bulk up of the carbon fibers due to loss of sizing compounds, which also resulted into significant change in tensile properties.