Synthesis of different types of carbon nanostructure on carbon fiber and their application as fillers in polypropylene composite

The potential application of carbon nanoparticles such as carbon nanotubes (CNT),carbon nanofibers (CNF) and graphene (G) flakes grown on carbon fiber (CF) surface as fillers in polypropylene composite is discussed. Carbon fiber surface must be modified before it can be used as fillers in composites. A one-step process using the chemical vapor deposition (CVD) method has been used to synthesize CNT, CNF and G and also G-CNF and G-CNT on the carbon fiber to modify its surfaces. In this study, CFs (Toho Tenax Co. Ltd.) was utilized as a substrate to grow carbon nanostructures and also as a filler in polypropylene pellets (PP 600G) polymer matrix. To the best of our knowledge, so far nobody has reported any work being widely carried out on synthesizing G layers on the CNF and CNT grown on CF by using a bimetallic catalyst (Ni/Cu) in a one-step CVD method in order to increase the CF surface area as well as to improve its properties. The synthesis of nanostructures on CF was accomplished using high purity acetylene (C2H2) as a carbon source, and nitrogen (Air Product,99.9995) and hydrogen as carrier gases. Two types of catalysts namely copper nitrate trihydrate (Cu(NO3)2.3H2O) and nickel nitrate hexahydrate (Ni(NO3)2.6H2O) were utilized as bimetallic catalyst in the synthesis. All the operating parameters of CVD process for growing the carbon nanostructures were optimized in order to obtain uniform and high quality carbon nanostructures. These parameters include catalyst concentration (from 50 mM to 150 mM), reaction temperature (different for each kind of carbon nanomaterial), reaction time (from 10 to 50 min) and carbon source flow rate (from 25 sccm to 100 sccm). Based on the SEM,TEM, TGA, BET surface area and Raman spectroscopy results, it was concluded that the optimum conditions are at 100mM catalyst concentration at 50sccm acetylene flow rate for 30 min at 550oC, 800 oC and 1050 oC for CNF, CNT and G growth, respectively. The changes in the surface characteristics of CFs was studied with scanning electron microscopy (SEM), transmission electron microscope (TEM), Raman spectroscopy and BET surface area analyzer. By analyzing SEM and TEM images, the morphology,structure, size and diameter of the resulting carbon nanostructures were obtained. In Raman spectra, the ID/IG ratio of the samples decreases when graphene flakes are present. When the ratio of CF/catalyst was at the maximum value, the ID/IG ratio (≈1.13) coated with CNF, CNT and G, decreased to 0.94, 0.88, and 0.47 respectively. The ID/IG ratio of CF-CNF-G (0.85) and CF-CNT-G (0.81) indicates the effect of graphene growth on the crystallinity of the substrate. Based on the results obtained from the surface modification of CF with various nanostructures, it is concluded that the carbon fiber coated with CNT-G (CF-CNT-G) with 80% yield and 46 m2/g BET surface area is the best method for surface modification of CF. Other fillers such as CF-CNF produced 24% yield and 2.31 m2/g surface area, CF-CNT with 46% yield and 5.22 m2/g surface area, CF-CNF-G with 56% yield and 21 m2/g surface area and, CF-G with 54% yield and 10.21 m2/g surface area. Polypropylene (PP) composites with different carbon-based fillers such as G on CF (GCF),CNF on CF (CNF-CF), CNT on CF (CNT-CF) and also G-CNF-CF and G-CNTCF were prepared by the melt mixed method and the effects of these nanoparticles on the mechanical and thermal behavior of the composites were analyzed. The mechanical behavior and thermal resistance of the produced composites were evaluated using the tensile test and thermal gravimetric analysis (TGA), respectively. The Raman images were then used to explain the observed mechanical behavior of the different types of fillers/PP composites. The tensile stress and young’s modulus of neat PP are 28MPa and 1400MPa. The values increased when various nanostructures were grown on the CF, to about 8.9% and 14.5% for CF/PP, 21% and 30.5% for CF-CNF/PP, 30.7% and 50% for CF-CNF-G/PP, 58.9% and 58% for CF-CNT/PP, 98.2% and 114.2% for CFCNT-G and finally 82.8% and 97% for CF-G. Based on the results, the PP composite reinforced with CF-CNT-G showed the highest improvement in tensile stress at 55.5 MPa, young’s modulus at 2998.9 MPa and enhancement in thermal stability to 130oC.

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