"Ball-Stick"-like Short Carbon Fiber Composite Reinfircement Design And The Properties Of Its Epoxy Composites

Short carbon fiber (SCF) is an important reinforcement. Due to the low cost and good formability, short carbon fiber reinforced epoxy composites are widely used in various fields. However, the interfacial adhesion of SCFs and resin is weak, and because of the length of fiber, the ends of short fibers easily lead to the stress concentration and induce the cracks causing material failure. In recent years, scientists have designed different structural short fibers to solve the fiber length and the weak interfacial adhesion resulting that the improvement of properties is limited, and has made certain achievements, but the choice of the short fiber and the preparation technology of short fiber with complex structure are significant limitations. Therefore, carrying out the works combined short-fiber design, materials preparation with improving properties of short fiber reinforced polymer composites are of great significance.In this work, the targets are the interfacial design and the property improvement of short carbon fiber reinforced epoxy composites. Combining the oxidization modification introduced the reactive functional groups on the carbon fiber surface with electrodeposition technologies, we designed a new type of short carbon fiber reinforcement, which was some metal balls beaded the short carbon fiber. We investigated the effects of the interfacial microstructure on the properties of the composites, and enriched the interfacial theory of the composite.(1) The carbon fiber was preteated by air oxidation. XPS and FT-IR analysis showed that the oxidation treatment could effectively introduce the O-containing groups to the carbon fiber surface and O 1s/C 1s increased from 23.05% (as-received carbon fiber) to 42.83% (400℃treated carbon fiber). The weight loss and DSC curves showed that the oxidation reaction in air began at about 328℃and reached a peak at 515.6℃, along with the carbon fiber weight loss increasing dramatically. FESEM photographs showed that the surface roughness of SCFs after oxidation had a significant increase.(2) The adsorption experiment of 400℃treated SCFs showed that O-containing groups on the carbon fiber surface could reduce Cu ions in the copper sulfate solution and played a catalytic role on Cu electrodeposition. Electrochemical test results showed that the O-containing groups on the carbon fiber surface increased, the deposition potential needed to start the electrochemical reaction decreased.(3) Cu-SCFs composite reinforcements with different morphology were prepared by electrodeposition. It could be seen from the FESEM photographs that when the deposition voltage was lower than 1.0 V, the discontinuous Cu particles were obtained on the carbon fiber surface; deposition voltage of 1.5~2.5 V, resulting smooth uniform Cu deposits; greater than or equal to 3.5 V, resulting Cu-"ball-stick"-like short carbon fiber composite reinforcements.(4) The formation mechanism of Cu-SCF composite reinforcement with different morphology was: under the applied potential, Cu ions was first reduced to Cu in the positions with active functional groups on the SCF surface and adsorbed at the points. Then under the different instantaneous deposition potential, the initial nucleation and growth process were controlled by the deposition voltage. When the deposition potential gradually increased, the uniform and smooth Cu deposits were formed. While the initial deposition voltage was very high, the Cu grains improved the conductive. With the tip effect, Cu grains continued to grow, eventually the "ball-stick"-like SCF was formed.(5) The effects of Cu-"ball-stick"-like SCFs on the mechanical properties of epoxy composites were studied. The results showed that the mechanical properties of Cu-"ball-stick"-like SCFs/epoxy composites were greater than straight SCFs/ epoxy composites. When the fiber content was 0.1 wt%, tensile strength, tensile modulus, bending strength and bending modulus of Cu-"ball-stick"-like SCFs/epoxy composites reached the maximum, and they were 35.27 MPa, 4.93 GPa, 106.9 MPa and 2.51 GPa, respectively, than that of pure epoxy increased by 123.1%, 74.8%, 54.1% and 14.5%; than 0.5 wt% 400℃treated SCFs/epoxy composites increased by 75.9%, 39.3%, 28.3% and 1.2%; than 0.5 wt% Cu-SCFs/epoxy composites increased by 25.2%, 25.8%, 24.0% and 0.4%.(6) The strengthening mechanism of "ball-stick"-like SCFs on the mechanical properties of epoxy composites was revealed. On the one hand, the pull-out resistance of "ball-stick"-like SCFs was greater than the straight fibers; on the other hand, compared to straight fibers, "ball-stick"-like SCFs could significantly reduce the interface shear stress in the composites, and the metal balls made the stress uniformity and avoided the matrix cracking. Therefore, "ball-stick"-like SCFs enhanced the mechanical properties of composites.(7) When the deposition voltage was greater than or equal to 5.6 V, Ni-"ball-stick"-like short carbon fiber composite reinforcements were prepared, and the mechanical properties of its epoxy composites were greater than straight SCFs/epoxy composites. Tensile strength, tensile modulus, bending strength and bending modulus were 27.76 MPa, 3.88 GPa, 120.62 MPa and 3.21 GPa, respectively, than the pure epoxy resin increased by 75.6%, 37.6%, 39.4% and 45.9 %; than 400℃treated SCFs/epoxy composites increased by 38.5%, 9.6%, 16.1% and 28.9%; than Ni-SCFs/epoxy composites increased by 30.0%, 6.6%, 3.4% and 3.9%.(8) The surface resistance of Cu- and Ni-"ball-stick"-like SCFs/epoxy composites was lower than 400℃treated SCFs/epoxy composites with the same fiber content, but higher than uniform Cu- and Ni-SCFs/epoxy composites.The main innovations of this paper are as follows:(1) The interfacial structure and properties of composites were combined organically. A new "ball-stick"-like short carbon fiber was proposed and the mechanical properties of its epoxy composites were greatly improved, parts of the properties were even greater than carbon nanotubes or carbon nanofibers reinforced epoxy composites. The results fully reflected the great advantages of "ball-stick"-like composite reinforcement, which was significant to improve the mechanical properties of resin composites and reduce the cost.(2) Combined oxidation modification with electrodeposition technique, by controlling the functional groups on carbon fiber surface and the deposition voltage, a simple preparation method for the "ball-stick"-like short carbon fiber composite reinforcement was developed, which was significant to promote the use of "ball-stick"-like short fiber.As a first attempt, we provided a useful method to prepare short carbon fiber composite reinforcement with special structure. It's important for further improving the shape of short fiber, perfecting the interfacial design of the composites, and improving the properties of composites with weak interfacial adhesion.