Spandex is an important functional textile raw material, which has the structural characteristics of alternating soft and hard. It is the most elastic synthetic fiber in the market at present. This paper introduces the preparation method of conventional spandex fiber.

1.Preparation of spandex by dry spinning

Dry spinning is also called dry solution spinning. The production process is divided into three parts: prepolymerization, chain extension and spinning forming. The spinning source liquid is sprayed through the spinneret under the hot air flow, the solvent volatilizes at a higher temperature, and the solute solidifies before winding. Dry spinning is the most used method in its production. About 80% of spandex is spun by this method, and the spinning process has a great impact on its resilience and strength.

In general, dry spinning has a high level of industrialization, sound production equipment and the best product performance. However, the dry spinning process is relatively complex, with many equipment, high energy consumption and high production cost. The solvents in the production process do great harm to human body and the environment, and the recovery and treatment are complex, which limits the further development of dry spinning. To improve the competitiveness of dry spinning, we must look for new green solvents and simplify the process steps.

2.Preparation of spandex by melt spinning

Melt spinning is a process of heating up to the melting point of thermoplastic polyurethane (TPU) chips and then spinning to form fibers. There are three common methods: one-step method, two-step method and end capping method. The reaction temperature is about 170~210 ℃. Because TPU is unstable at high temperature, crosslinking agent is usually added to improve the mechanical properties of the product and prevent its thermal decomposition at high temperature.

Melt spinning has the characteristics of simplified process, simple equipment, low cost, fast production process and convenient industrialization. Moreover, compared with solution spinning, the solvent-free production system is more environmentally friendly and friendly, and solves the problems of high toxicity and poor post-treatment of solution spinning. However, the process of melt spinning is not yet fully mature, the consumption rate of spinneret is fast, and the mechanical skills and heat resistance of products are still a certain gap with those of dry process products. Most of them are used in the production of medium and low-grade textiles, and the choice of high-end textiles is still based on dry process. In recent years, with the gradual improvement of melt spinning technology, some products have been comparable to dry spandex. Coupled with the characteristics of low cost, environmental protection and sustainability, melt spinning is expected to become an important direction for the future development of its products.

3.Preparation of spandex by wet spinning

Wet spinning is similar to dry spinning. In the forming step, unlike the dry spinning method, the spinning solution enters the high-temperature tunnel, is extruded from the pump to the jet head, and then sprayed into the coagulation bath (usually at a temperature lower than 90 ℃), the solute slowly condenses into silk, and finally is bundled, dried, and wound into its product. Due to the low production temperature, functional spandex can be prepared with biological macromolecules and nanoparticles.

Wet spinning mainly produces polyester elastic fibers, which can produce differentiated functional fibers at low temperature, and the chlorine resistance of the products is the best. Like dry spinning, the wet spinning process is complex, the solvent pollution in the production process is relatively large, the solvent recovery cost is high, and the post-treatment is complex. Although the production cost of wet spinning is lower than that of dry spinning, the production speed of wet spinning is slow, and the product performance is far lower than that of dry spinning. At present, it has been gradually replaced by other methods.

4.Preparation of spandex by chemical spinning

As an early spandex production method, chemical spinning uses chemical reaction to control the curing speed of spinning solution in the spinning process. The production process is long, the cost is high, and the pollutant diamine produced has great environmental pollution. Except for a few companies that produce fine denier silk, this spinning method has basically been eliminated.

With the development of spandex industry and the upgrading of daily necessities, the continuous optimization of existing spinning processes and the research and development of new efficient and environmentally friendly spinning processes are the main trends of spandex fiber in the future.