Abstract:

Objective The poor breaking strength and elongation at break are the main reasons that limit the wide applications of alginate fibers. Many approaches such as cross-linking, grafting and blending modifications have been developed to improve the physical and mechanical properties of alginate fibers. In this research, the reinforcing mechanisms of montmorillonite (MMT) were studied, and nano MMT was blended into sodium alginate(SA) solution to prepare modified calcium alginate(CA) fibers using wet spinning technique aiming to improve the mechanical properties of the alginate fibers.

Method Montmorillonite (MMT) suspension was blended with sodium SA aqueous solution to prepare the mixed spinning solution, before the SA/MMT fibers were prepared by wet spinning method using CaCl₂ solution as the coagulation bath. The microscopic morphologies and structural characteristics of the prepared fibers were characterized by using TEM, SEM, FT-IR, XRD and the mechanical properties and thermal stabilities of different fibers were tested by using single fiber tensile tester and thermogravimetric analyzer respectively to study the influences of different swelling time and contents of MMT on the corresponding fiber properties.

Results The layer distance of MMT was increased desirably after swelling for 120 h, and the swollen MMT was well dispersed in the spinning solution, which provided favorable conditions for SA to get into the macromolecular layers of MMT and eventually peel off the layers of MMT. The breaking strength of the CA/MMT fibers containing 0.1% MMT that swelled for 120 h was the best. This phenomenon is owing to the intercalation of SA into the uniformly dispersed MMT in the spinning solution, which is conducive to the combination of MMT and SA. Meanwhile, the result of the elongation at break was similar to that of the breaking strength of fibers, which could be explained that the formed hydrogen bonds among the hydroxyl groups on MMT and the ionized carboxyl groups on SA were conductive to overcoming stress concentration and increasing the crystallinity of SA molecules, and toughening the fibers. Compared with SA fibers,the addition of MMT made the fiber surface gully. Evidence showed that the MMT has been successfully loaded onto the CA fiber. The reason for this is that MMT in SA exists as a monolithic structure, which makes the viscosity of the spinning liquid increase and the entanglement point increase, affecting the SA macromolecular flow, and finally the spinning liquid bears a stronger tensile effect during spinning and increasing the friction during fiber formation. The hydrogen bond interaction between the hydroxyl group on the MMT and the ionized carboxyl group on the SA was confirmed, and the Si—O stretching vibration peak from MMT was also identified in the spectrum of SA/MMT fibers, indicating the successful blending of MMT and SA. The addition of MMT increased the crystallinity of CA fibers hence strengthening and toughening the fiber, and the test results corroborated with the above conclusions. The TG results indicated that the addition of MMT also improves the thermal stability of the fibers. The decomposition temperatures of the two types of fibers in the three stages were quite similar, but the carbon residue rate of CA fibers blended with MMT was higher than that of CA fiber, indicating that the addition of MMT can improve the thermal stability of the fibers.

Conclusion It is found feasible to use inorganic nanoclay for toughening the alginate fibers. Hydrogen bonding occurs between MMT and calcium alginate, increasing the intermolecular spacing. The addition of MMT did not obviously changes the chemical structure and crystalline structure of alginate fibers, making the thermal stability properties of alginate fibers improved to some extent.

Key words: montmorillonite, alginate fiber, strengthening and toughening, intercalation, sodium alginate, wet spinning