Study On The Morphology And Structure Of Coarse Yak Hair And Its Processing And Mechanism Of Stretching Slenderization

Yaks are mainly bred in China, and its number is over 92% in the world, thus there is rich resource of yak hair fiber. But most fibers among them are coarse and long, so they are basically used for slub materials with lower economic value. With the increase of living standard, the need for the production of wigs is increasingly strong by people. Due to the limit of the quantity of human hair resource, coarse yak hairs are widely used instead of it now. The longer the length of yak hair fiber is, the high the price of wig, thereby it is an effective way for raising fiber value to increase its length and decrease its diameter. The stretching slenderization is most efficient method at present for increasing fiber length, while the relatively research is mainly focused on wool fiber, and there is rare work to be report about the slenderization of coarse yak hair fiber. In addition, due to the tiny knowledge for coarse yak hair fiber, it will be very difficult to modify these fibers further.This paper studied amply the morphologic structure, cortical cell character, cell and substance between cells, and chemical structure of Chinese coarse yak hair fiber for the first time. Using scanning microscope electron (SEM) and optical microscopy observed the appearance morphological structure of coarse yak hair fiber and obtained the diameter distribution situation of single fiber and between fibers, at the same time it is revealed for the surface scale structure and character of cross section of fiber. Making use of ultrasonic separation achieved the cortical cell and observed its morphological and packing structure in fiber. The "length/diameter ratio" was put forward to characterize the shape of cortical cell for the first time, and the result show that the shape of coarse yak hair is thicker and shorter than that of wool fiber. Furthermore, the dimension distribution of cortical cell was also studied. The ultrathin section dyed was achieved very well using embedding method, and through the TEM the microfine structure of coarse yak hair fiber was investigated in detail. It was found that the medulla existed in most coarse yak hair fiber, and fiber is made up of one sort of cortex-cell but not the structure of ortho-and para-cortex. By FTIR techniques studied the basic absorbent apics characters of coarse yak hair and compared with wool fiber, it presented that two kind fiber have same inner molecular structure and bonding forms but the disordered structure in coarse yak hair fiber is more than wool.For the commercial development of stretching slenderization of coarse yak hair, the stretching process of the untwisted fiber sheet has been investigated, and the processing parameters which will affect the continuous production have been studied mainly. By means of strain-stress curves of coarse yak hair fiber stretching with different technological conditions and with lower initial module, longer yield region, lower breakage ratio as a evaluation criterion, the optimum process conditions were decided, i.e., the time of pretreating process is 10min, and the best stretching method is varying speed mode with 6 grade, and the stretch ratio should not exceed 60%. Using the retraction ratio of slenderized fiber in boil water as the evaluating indicator, the optimum setting time were found, i.e., first setting 5min, and second setting 3min, and drying process 20min, and baking process 3min. According to the slenderization process of coarse yak hair sheet, a whole new method of continuous stretching has been developed, and the relative machine has also been simulated, particularly adopted a new stretching method, i.e., all-in-one of pretreating and stretching.Through laser confocal Raman microscopy, the secondary structure transformation of coarse yak hair fiber during stretching process was studied in depth. It was proved that amide I, amide III and C-C backbone vibration regions all took notable change except for S-S bond. The result also showed that the conformation transformation occurred from the a-helical toβ-pleated-sheet in coarse yak hair fiber during stretching, meanwhile some a-helical macromolecule transformed would restore fromβ-pleated-sheet. This change could also be found when the fiber slenderized undergo boiling treatment. In addition, the degree of crystallinity and orientation has been analyzed quantificationally on coarse yak hair fiber after and before stretching. The results showed that the degree of crystallinity was changed obviously, while the change of degree of orientation is tiny. It follows that the slippage of macromolecule or microstructure mass exists during the stretching process. On the basis of the microstructure analysis to coarse yak hair, it was found that the mechanism should be a more complicated process for stretching slenderization of coarse yak hair, which involved "macromolecular conformation transformation" and "microstructure mass slippage", and the latter was primary. The mechanism of "macromolecule transformation and slippage" is put forward in present paper. Firstly, the crosslinks between the a-helical macromolecule are taken apart by the pretreating solution and keep partially the crosslinks inside the molecules. Secondly, the fibers are stretched by force, and the conformation transformation happened on the macromolecule first, and then the slippage of macromolecule and microstructure mass begin present, at the same time the reciprocate of the conformation of molecule arise. Finally, the stretched fibers are set quickly and restructure the crosslinks between molecules. This will keep the length of slenderized fiber and elasticity.For the sake of the convenience of treatment and use in the future, the morphology, structure and properties of slenderized coarse yak hair fiber were studied. The results showed that the scale structure has been deteriorated in the surface of coarse yak hair fiber stretched, and the density of that decreased gradually with the stretching ratio increase. Some scales turned up at the edge of cell, even the partial of scales come forth situation slippage and damage. After stretching slenderization, the diameter of fibers decrease evidently, especially that of the fiber stretched at 90% ratio could reduce approximate 30% than the original fiber. Moreover, the shape of cross-section of fibers change from near circular form to oval. The cortical cells have been elongated and slenderized, and the orientation along the fiber axis became more obvious, and the density of packing of those gets increase. The breaking strength and initial module of slenderized fibers are both higher than the original fiber, and with the increase of stretching ratio the value of those raise gradually. However, the variation of the extension at break was different. Furthermore, the bending rigidity of fibers is lower than its original fiber after stretching. Slenderized fibers have a higher friction coefficient than the original coarse yak hair fiber have, and the variation quantity of that reduce gradually with stretching ratio increase. The hydroscopic property of the fibers has been improved obviously, so that the slenderized fiber's dyeability is made better greatly and it become possible for dyeing at lower temperature.