关键词: 桑蚕丝, 强制抽丝, 力学性能, 二级结构, 彩色蚕丝

Abstract: As people pay more attention to ecological environment and health, the demand for high mechanical properties, good biocompatibility and biodegradable fiber materials is increasing in various industries. Bombyx mori silkworm silk is a well-known natural protein fiber, and is considered as an ideal biomaterial because of its excellent properties, such as outstanding toughness, safety degradability, and good compatibility with human body. Mulberry silk fibers have an interesting and elaborate structure, but its mechanical properties are inferior to spider dragline silk. Therefore, it is of great significance to prepare mulberry silk comparable to spider silk. Traditional silk processing technology, however, is difficult to enhance the performance of cocoon silk, and even damage its morphology and properties. In addition, artificial silk produced by a variety of spinning methods cannot fully replicate the properties of natural silk. Herein, we obtained two different force-reeled silks from white cocoon male silkworms and yellow cocoon male silkworms with the same reeling speed using a conical forced reeling device. The mechanical properties and internal structure of the force-reeled silks were analyzed to elucidate the effect of forced reeling on the silk fibers.
In this study, the silkworms were forcibly reeled at 4 cm/s, resulting in the force-reeled silks (WM-4 and YM-4). During forced reeling, the silk was fully stretched due to a recombination of the reeling force and the gravity of silkworm, which increased the orientation and crystallinity of force-reeled silk fibers. Meanwhile, the cocoon silk fibers (WM-0 and YM-0) selected from the same batch were set as a control group. In addition, the characteristics of fiber fineness, mechanical properties, FTIR, XRD of all silk samples were performed. The result shows that the fineness of the force-reeled silk fibers decreases with the increase of the force-reeled speed. The mechanical properties including elongation, strength initial modulus, and specific work of rupture of force-reeled silk fibers are superior to that of cocoon silk fibers. The average values of initial modulus, breaking strength, specific work at break and elongation of WM-4 are 98.65, 4.00, 0.68 cN/dtex and 23.54%, respectively, which are about 24.76%, 32.45%, 30.77% and 9.08% higher than those of WM-0. The average values of initial modulus, breaking strength, specific work at break, and elongation of YM-4 are 117.35, 4.55, 0.75 cN/dtex, and 26.54%, respectively, which are about 80.43%, 43.99%, 29.31%, and 5.82% higher than those of YM-0. It can be seen that the mechanical properties of both force-reeled silks increase compared to cocoon silks, which is due to the fact that forced reeling improves the internal structure of the force-reeled silks. This results in a uniform distribution of tensile stress and enhances the ability of the force-reeled silks to resist damage by external force. Furthermore, YM-4 has superior mechanical properties, with the mean values of initial modulus, breaking strength, specific work at break and elongation increasing by about 18.86%, 13.75%, 10.29% and 12.74%, respectively, compared with WM-4. This is because the yellow male silkworms are robust and well-adapted to forced reeling, resulting in a more uniform force-reeled silk with a more regular internal structure and better mechanical properties.
In summary, the force-reeled silks obtained from different Bombyx mori silkworms show further improved mechanical properties compared with cocoon silks, but the dispersion of the tensile curves of the force-reeled silks increases; the mechanical properties of the force-reeled silk of yellow male silkworms are better, and the dispersion of the tensile curve is less, so yellow male silkworms could be selected for the preparation of stable high-strength force-reeled silk; the resulted forced reeled silks have smaller grain size and higher crystallinity, leading to the excellent mechanical properties.

Key words: Bombyx mori silkworm silk, forced reeling, mechanical properties, secondary structure, colored silk