The polymer composites designed for high-performance applications are mostly based on carbon fiber reinforcement. The two most common precursors used currently for carbon fiber production are poly(acrylonitrile) and pitch (petroleum- or coal- based). As of today, the most promising alternative to these fossil originated raw materials is lignin. Previous research has mainly focused on carbon fiber production from pre-treated hardwood lignin with the addition of different softening agents. Softwood lignin has been considered difficult to melt, and thus, impossible to process by conventional melt spinning processes.
The aim of the presented work is to find the way for melt spinning of softwood kraft lignin, by using lignin-derived additives. A method for isolation of kraft lignin was recently developed, making large amounts of high purity lignin available. The thermal properties of this type of lignin make it an interesting candidate for carbon fiber production.
During this study, both unfractionated hardwood and softwood kraft lignin were used with addition of their fractionated counterparts, acting as softening agents. The spinning process of the lignin blends was optimized by adapting the processing temperature to the thermal properties of blends of different compositions. Different batches of lignin fibers were produced and characterized with scanning electron microscopy to evaluate the fiber diameter, the surface smoothness, the presence of pores and the shape of fiber cross-section.
The fiber batch containing softwood kraft lignin and 10% fractionated hardwood kraft lignin was relatively easy to melt spin, despite the small amount of added fractionated hardwood lignin. Therefore, this batch was further processed into carbon fibers by oxidative stabilisation followed by carbonization in nitrogen atmosphere. X-Ray/Energy Dispersive Spectroscopy confirmed that carbon fibers containing above 90% carbon had been obtained.
Mechanical characterization of produced lignin based carbon fibers was carried out. Single fiber tensile tests were performed to evaluate the stiffness and the strength of carbon fibers. In order to determine the properties of the lignin-based CFs, and to estimate the impact of the manufacturing parameters (such as die sizes and winding speeds), fibers of different diameters (≈30, 60 and 90 microns) were made and tested.
Carbon fibers are brittle materials and therefore the experimental results (fiber strength) were treated by use of Weibull statistical distribution. Three fiber lengths (10, 20 and 40 mm) for each diameter were tested and strength data was approximated by two-parameter Weibull equation in order to obtain parameters of the strength distribution. The experimental results and predictions based on Weibull statistics showed a good fit.
Although strength of the produced fibers is still significantly lower than that of commercially available carbon fibers, this thesis reports the first mechanical characterization of softwood kraft lignin based CFs.
The carbon fiber production process differs depending on the raw material used. Most of the studies on lignin have considered hardwood lignin as raw material. As a first step towards a process optimized for softwood kraft lignin based carbon fibers, the stabilization step in the carbon fiber process was developed further.