Search

The density of T300 carbon fiber, as well as the amount of sizing and micro-structure were measured, before and after removing the sizing. The results showed that there was apparent stratification when measuring the density of the carbon fiber by the sink/float method, without removing the sizing. The upper sample layer was seriously flocculated with an excessive sizing amount of 2.17%, while the bottom-layer also flocculated. Two methods including acetone extraction and pyrolysis, were conducted to removing the sizing. It was shown that the measured amounts of sizing for the two methods were 1.20% and 1.53%, respectively. For the sample after immersion in acetone, there was still stratification in the sink/float test, with the upper sample layer flocculated because of the remaining sizing; while samples at the bottom dispersed relatively well. However, for the thermally treated samples the sizing was adequately removed, which resulted in the uniform density and dispersion during the test. This indicates that the non-uniform distribution of sizing agent on the surface of the CFs is the main reason for stratification during the density measurement. Therefore, the sizing must be fully removed in order to obtain an accurate density for the carbon fibers. Moreover, it can be seen that pyrolysis is an effective way to remove the CF sizing agent.

The aim of this work is to investigate the effect of moisture on the measured tensile strength of polyacrylonitrile (PAN) carbon fibers immersed in epoxy matrix under various moisture conditions. Tensile tests are carried out for the specimens, and a sharp decrease in measured tensile strength of the carbon fibers/epoxy system A immersed under high-moisture conditions is observed. Scanning electron microscope (SEM) is used to observe the effect of moisture on the surface structures of specimens. The results show that the decrease in the measured tensile strength is attributed to the microvoids of epoxy matrix formed in the preparation of the specimens. And the formation of microvoids in carbon fiber/epoxy matrix depends on viscosity change and curing condition of epoxy system. During the early stage of epoxy curing, both the solvent and absorbed water in the epoxy system escape from the immersed carbon fibers and form channels in the epoxy matrix. Once the viscosity of the epoxy system increased rapidly at a low curing temperature, the channels cannot be filled quickly by the surrounding resin and form microvoids finally. In order to verify these results, epoxy system B with a high curing temperature is chosen to prepare the specimens for tensile tests, and SEM images demonstrate that there are no microvoids that can be observed on the surfaces of specimens, even at such a high-moisture condition. And the measured tensile strengths remain unaffected in both low and high-moisture conditions. These results indicate that epoxy system with a high curing temperature is suitable for preparing the specimens for tensile tests regardless of moisture condition in the surrounding air.

The aim of this work is to investigate the effect of a process-induced resin bead on the measured tensile strength of polyacrylonitrile-based carbon fibers. It is found that the resin bead was formed on the surface of carbon fiber specimens if the carbon fibers impregnated with resin were positioned horizontally during the tensile specimen preparation. Mechanical testing results indicated that the measured tensile strength of carbon fibers noticeably decreased by about 10 % due to the formation of the resin bead, and the initial fracture of carbon fibers usually occurred around the resin bead. Moreover, the bending of the filaments in the resin bead had been observed by optical microscope. It is concluded that the flowing and curing of excess resin lead to the bending of loose filaments in carbon fiber tow during specimen preparation; thus, internal stress was subsequently formed in the fiber specimen, which is detrimental to the measured tensile strength. Finally, based on the microscopic observations, theoretical analysis under the maximum stress criterion was employed to predict the tensile strength of carbon fibers with a resin bead, and the theoretical results agree well with the experimental results. It is verified that the bending angle and the fraction of the bending filaments in the resin bead play important roles in deteriorating the measured tensile strength. These conclusions can help to improve the tensile testing method for accurately measuring the tensile strength of high modulus carbon fibers.

Major advancements in the evaluation of carbon fibers at home and abroad are briefly summarized in this paper. In addition, the requirements on microstructure, composition, surface elements, ash, processing and stability of carbon fibers used in the aerospace field are introduced. Future trends for the evaluation of aerospace carbon fibers are discussed. [New Carbon Materials 2014, 29(3): 161–168]