Spandex is a polyurethane elastic fiber, which is a linear macromolecule composed of at least 85% polyurethane segments. Its molecular structure is a block copolymer with alternating soft and hard segments, and it has the characteristics of high elongation and high recovery. The elongation rate is 500%~600%, and the instantaneous elastic recovery rate is above 95%. Adding a small amount can improve the elasticity and wearing comfort of the fabric. Based on the above characteristics, spandex is known as the "industrial MSG" of textiles. At present, spandex is mainly used in textiles such as core spun yarn, knitted fabrics, and woven fabrics. More and more new fabrics and functional clothing are using spandex as an efficiency enhancing fiber. In addition to being used in ordinary textile fabrics, spandex also has important applications in hygiene products and medical fields, such as elastic fabrics in cycling clothes, yoga clothes, aviation clothes, as well as pressure clothing and medical elastic bands for patients with varicose veins. The application of spandex is becoming increasingly widespread, and its usage in fabrics is also increasing. With the development of differentiated spandex, its application fields are constantly expanding.

Although spandex has many advantages, in terms of dyeing performance, ordinary spandex lacks functional groups that can bind with dyes, and the dye uptake and color fastness are far from meeting the requirements for use. When blended with other fibers, the phenomenon of "whitening" often occurs, especially when combined with nylon, cotton fibers, and polyester cations to dye black or bright red, the problem of "whitening" is particularly serious. The problem of "whitening" of spandex will seriously restrict its application in some high-end fabrics and textile fabrics with high spandex content. Therefore, improving the dyeing performance of spandex is of great significance for expanding its application scope, improving the grade and added value of spandex fabrics.

Spandex structure

Spandex is a block copolymer of urethane and urea groups, mainly composed of polyurethane. The molecular chain contains both flexible and rigid segments. Its structural formula is shown in Figure 1.

In general, the soft chain segment is in an amorphous state, and the proportion of amino ester groups in the molecular chain is relatively low. However, the glass transition temperature of spandex is below minus 40 ℃, so spandex fibers are in a high elastic state at room temperature. Overall, the following structural characteristics of spandex may affect its dyeing performance: (1) spandex molecular chains do not contain polar groups, and the content of methylene and aryl groups is relatively high, so spandex fibers are hydrophobic fibers; (2) The molecular crystallinity of spandex fiber is high; (3) In the block copolymer structure of spandex, the hard segments are tightly packed, while the soft segments are loose but have weak binding ability with dyes; (4) There is a small amount of terminal amino groups present on spandex molecules, which can provide dye sites for dye uptake. Due to the small number of terminal amino groups, the dyed color is relatively light.

Plasma treatment technology solves the problem of spandex dyeing

Plasma is an ionized gaseous substance composed of atoms and atomic clusters that have been partially electron deprived and ionized to produce positive and negative ions. It is a macroscopic electrically neutral ionized gas with a scale larger than the Debye length, and its motion is mainly dominated by electromagnetic forces, exhibiting significant collective behavior. It is an unstable gas in a highly excited state, composed of electrons, ions, free radicals, excited molecules, and neutral particles, exhibiting electrical neutrality. It is the fourth state of existence for substances other than solids, gases, and liquids.

Compared to traditional chemical wet treatment processes, plasma treatment of polymer materials has the following advantages: (1) plasma treatment can only improve the surface properties of the material without changing the properties of the substrate; (2) The effect of plasma on the surface of materials only involves a few to a few hundred nanometers, which can only improve the surface properties of materials and does not affect the properties of the matrix; (3) Plasma treatment is a physical process with minimal consumption of chemical substances. Plasma technology is a high-energy physics technology that does not involve water during the processing, which can greatly reduce environmental pollution. It is an environmentally friendly processing method that meets the requirements of carbon peaking and carbon neutrality.

The energy level of the active factor in low-temperature plasma is higher than the chemical bond energy of organic compounds, and it is in a chemically active state. Using this plasma to treat organic compounds can open their chemical bonds. It can form new chemical structures on the surface by generating free radicals. By utilizing this effect, low-temperature plasma can generally cause the following changes in materials: surface cross-linking, generation of free radicals, introduction of new functional groups, and surface etching. It must be pointed out that the use of low-temperature plasma to treat textile materials does not significantly affect the mechanical and other properties of the materials.

The types of active particles produced by ionization of different gases are different, therefore, the modification effect on the material surface is also different. Reactive gases include O2, N2, CO, CO2, and air. The chemical active species contained in the plasma generated can directly react with the material surface, thereby changing the surface chemical structure; Non reactive gases include Ar and He, and the chemical active species in the plasma generated cannot directly react with the material surface. However, high-energy particles can be used to bombard the material surface, generating a large number of highly active free radicals and changing the chemical and physical properties of the material surface. Different gases can obtain different functional groups after plasma treatment, for example, hydrogen can obtain hydroxyl groups, oxygen can obtain carboxyl groups, methane can obtain aldehyde groups, ammonia can obtain amino groups, etc. These functional groups are all active groups that can significantly enhance the surface activity of materials and play a role in activation modification.

The use of plasma treatment technology can greatly increase the number of amino groups in spandex, provide a dyeing base for acidic dyes to dye spandex, improve the dye uptake rate, increase the dyeing depth, and solve the problem of "whitening" of spandex. In addition, low-temperature plasma has a certain etching effect on spandex, which makes the surface of spandex fibers rough, increases the path of repeated absorption and emission of incident light, and improves the overall absorption intensity, resulting in a dark color effect.