Molding Process And Properties Of Carbon Fiber Reinforced Polyamaid6Composite

Thermoplastic composite materials show significant social and economic potential benefitsbecause of those recyclability, rapid processing speed, and high toughness. Hence this kind ofmaterials achieve a resurgence of interest. Nowadays, thermoplastic replacing thermosettingcomposites becomes a trend in aviation and automobile industries. In the past, most carbon fiber(CF) reinforced polyamide6(PA6) composites which were investigated contained less than30wt.%of non-continuous fibers such as short or long fibers. Mechanical properties of thesematerials were much lower than continuous fiber reinforced composites. For load bearingstructures, the composite materials should be reinforced with carbon fiber fabrics (CFF). However,this kinds of composite materials are difficult to be processed, compared with non-continuousfibers reinforced composites or thermosetting composites. Owing to high content of CF (alwaysmore than40wt.%) and high viscosity of PA6, wettability of PA6onto CF is rather low, whichmay bring pore defects into composites. Therefore, it is necessary to improve the wettability andinterface bonding strength between CF and PA6so as to obtain high mechanical performances ofcomposite materials.In this thesis, we prepared CFF/PA6composite laminates by mold pressing. First weinvestigated on the influences of processing parameters, e.g., temperature, time and pressure, onmicrostructures and mechanical properties of CFF/PA6composites. Based on the experimentalresults, we found out when processed at a temperature of250C, under a pressure of2.0~2.5MPafor30min, the composites show best mechanical performances, as a tensile strength of349.5MPa,a flexural strength of497.7MPa, a interlaminar shear strength of23.1MPa and an impact strengthof67kJ/m2.Further, modifications of wettability and interface bonding strength between CF and PA6were studied. The sizing agent coated on the surface of CF in commercial CFF will bedecomposed at high temperatures during preparation of CFF/PA6, forming defects and greatlyreducing the adhesion between CF and PA6. In this thesis, we proposed an efficient way to produce CFF/PA6composites based on modifications of CFF: first annealing CFF at400C innitrogen for4h, in order to remove the sizing agents coated on surface of CF; then immersing theannealed CFF in formic acid solution of PA6, where hydrogen bonds would be formed betweenCF and PA6and PA6could easily permeate into CFF at rather low viscosity; at last moldingmethod in hot press was conducted to process CFF/PA6laminates using modified CFF and PA6thin films. In characterization on microstructures and evaluation of mechanical properties, thecomposites containing modified CFF exhibited much enhanced mechanical performances (e.g., atensile strength of464.4MPa, a flexural strength of698.5MPa, a interlaminar shear strength of82.7MPa and an impact strength of95.3kJ/m2) and lower porosity (of less than0.81%) comparedwith the non-modified. SEM images on tensile fractural sections showed the pull-out CFs werecovered with much PA6matrix, which indicates an enhancement of interface bonding strengthbetween CF and PA6.Different processing methods and their effect of joints forming for CFF/PA6composites werealso studied. We used several ways such as gluing and thermal welding to form single lap jointsbetween two CFF/PA6composite laminates, and investigated on the influences of differentmethods and processing parameters on the strength of joints. Results showed that traditionalgluing is quite easy to handle and may form rather stronger connections; thermal welding based onelectric conduction by CFF can be used to form thermoplastic joints with design flexibility;thermal welding by hot pressing can further increase the connection strength; when formed by hotpressing under an optimum conditions of250C and2.5MPa, the shearing strength of joint is138.85MPa.