The selection of a retting method for the extraction of bast fibers as response to challenges in composite reinforcement

This article presents an evaluation of bast fiber properties conditioned by the selection of a retting method in terms of meeting requirements for the final application of the fibers as composite reinforcement. Two different methods of fiber extraction were used in the experiment: dew retting and osmotic degumming. The fibrous material was extracted from flax (Modran variety) and hemp (Bialobrzeskie variety). In addition, retted kenaf fibers (from China) were evaluated for the comparison of fiber properties. The properties of the retted and degummed fibers were evaluated according to relevant valid standards regarding color, linear mass, breaking tenacity, aspect ratio, microscopic images and chemical composition, which included the determination of wax and fat, cellulose, hemicellulose, lignin and pectin content and the thermogravimetric analysis coupled with Fourier transform infrared spectroscopy technique. The Attenuated Total Reflection FTIR spectroscopic technique was used for the characterization of the fibers. Also, fogging and volatile organic compound (VOC) emissions were tested in order to evaluate the suitability of the bast fiber for composite formation. The results of the study proved that osmotic degumming applied for bast fiber extraction improves significantly the fiber quality in terms of color, odor, aspect ratio and VOC emission. The aspect ratio of osmotically degummed flax fibers increased by about 46% and hemp fibers by about 22% in comparison with dew-retted fibers. VOC emission of osmotically degummed hemp decreased by about 35%, but in the case of flax fibers, increasing of VOC has been observed. For this reason, osmotically degummed fibers can be recommended as more suitable for composite reinforcement.