Carbon Fiber / Fluoropolymer Composite Materials Research

In recent years, carbon fibers (CFs) are a new kind of reinforcing material for preparating of composites. CFs not only have inherent characteristics of carbon materials, but also exist many advantages such as high electrical conductivity, large aspect ratio, low density, high strength, good corrosion resistance, heat resistant and anti-oxidation. Owning to excellent physical and chemical properties of CFs, high temperature resistant and good surface properties of fluoropolymers, CFs/Fluoropolymers composites which were synthesised by different methods have excellent merits. The studies on the preparation and properties of CFs/Fluoropolymers composites are very important.Using different CFs (marked CFs-1 and CFs-2) as reinforcements, CFs/poly(trifluoroethyl methacrylate) (PTFEMA) composites were prepared by blending and in-stiu polymerization methods, and heat and electrical performances of composites were analyzed.1. The structure and properities of CFs-1 and CFs-2 were studied. According to analytical results:both CFs-1 and CFs-2 existed the graphite (002) characteristic diffraction peaks. Carbon and oxygen were main elements, but the diameter of fiber was different. After oxidation treated by nitric acid, the peroperties of CFs (labeled CFs-11 and CFs-22) changed, such as the diameter of fiber became thicker, surface roughness increased, the oxygen content of surface increased and the carbon content of surface decreased. These results showed that after oxidation, oxygen groups of CFs surface increased and the combination of CFs and PTFEMA improved effectively.2. In scCO2, CFs-1/PTFEMA composites were prepared by in-situ polymerization. The structue of composites was characterized though 1H NMR, FT-IR, and molecular weight measurement and molecular weight distribution. Compared with the homo-polymerization of TFEMA under the same conditions, conversion of TFEMA and molecular weight of polymer decreased. Conversion of TFEMA reduced from 80% to 60%, and molecular weight of polymer decreased from 19,000 to 13,000. Reasons for these results may be due to the existence of epoxy coating of heat-treated CFs-1, which can absorbe oxygen. Therefore, as retarder of radical polymerization, oxygen consumed initiator and made monomer inadequate initiated, which lead to the decrease of monomer conversion. As chain transfer agent or chain terminating agent, epoxy coating might reduce molecular weight of polymer. Therefore, in the further investigation, carbon fibers must be modificated.3. As a reference to CFs-1/PTFEMA composites prepared by blending method, the conductivity of CFs-1/PTFEMA composites prepared by in-situ polymerization in SCCO2 medium and heat-treated CFs-1 as reinforcement was discussed. For CFs-1/PTFEMA composites, dispersion of CFs-1 synthesised by in-situ polymerization was better than that of blending method; CFs-1/PTFEMA composites which prepared by two different methods had higher conductivity; With increase of CFs-1 content, the resistivity (ρv) of composites decreased. The resistivity of composites prepared by in-situ polymerization was higher in 3 orders of magnitude than that of blending method.Taking composites of 12%(CFs-1:PTFEMA) CFs-1 prepared by in-stiu polymerization as an example, the effect of temperature on the resistivity of composites was analyzed. The results showed that with increase of testing temperature, the composites showed a PTC effect at the beginning. However, the composites appeared a NTC effect while the temperature rose to 80-85℃. The presence of NTC effect might reduce conductive properties of composites. Therefore, we should effectively control NTC effect of composites.4. As references to CFs-1/PTFEMA and CFs-11/PTFEMA composites prepared by blending method, the dielectric properties of CFs-2/PTFEMA and CFs-22/PTFEMA composites prepared by in-situ polymerization in scCO2 medium and reduced CFs-2 and CFs-22 as reinforcement were discussed. The results showed that at a room temperature and 1×103 Hz frequency, dielectric constant percolation threshold of CFs-11/PTFEMA composites was lower than that of CFs-1/PTFEMA composites, which may be part of oxidation fibers fractured in blenging method, made long fibers shorted, so the electric network could not be easily formed in the matrix. Dielectric constant percolation threshold of CFs-22/PTFEMA composites was higher than that of CFs-2/PTFEMA composites, resulting from not only the surface roughness of the oxidation fibers increased, but also the interface polarization effect in in-situ polymerization enhancd.With increase of CFs-11 content at a room temperature and frequency of 2×105 Hz, the dielectric loss of CFs-11/PTFEMA composites increased slowly at the beginning. Approaching to the percolation threshold (5%), the dielectric loss increased sharply, and the dielectric loss of composites was lower than that of CFs-1/PTFEMA composites. Due to the adhesion and compatibility of CFs-11 surface with the substrate of PTFEMA were better than that of CFs-1 surface with PTFEMA, which made the gap reduced between the filler and matrix, and the loss of composites caused by interfacial polarization reduced.With increase of frequency, the changes of dielectric constant of composites did not show a sudden increase or a sudden decrease, which was a slow process of reduction. In the frequency of (0.01-3)×105 Hz, both dielectric losses of CFs-11/PTFEMA and CFs-22/PTFEMA composites remained at less of 0.15, and the trend slightly decreased. These results showed that the composites prepared by two methods had a certain dielectric properties, and which expanded applications of carbon fibers composites.