The main properties of spandex fibers are: they usually have an elongation of over 500% to 700% and good elastic recovery performance. It retracts by 97% when stretched by 200% and by more than 99% when stretched by 50%.

The relative densities of various spandex fibers vary slightly, generally ranging from 1.20 to 1.25, with breaking strengths of 4.41 to 8.82cN/tex(0.5 to 1.0g/denier). Polyester-based spandex fibers have lower strength, while polyether-based spandex fibers have higher strength. The moisture absorption rate is generally 0.3% to 1.2%. The moisture absorption rate of multifilament is slightly higher than that of monofilament. It is heat-resistant and will not damage the fibers when stored for a short time at 95 to 150℃. The safe ironing temperature is below 150℃.

Fibers burn slowly and form gel-like residues after burning. It has excellent dyeing performance, can dye various colors, and has a strong affinity for dyes. It has good acid and alkali resistance, can be bleached and does not mold.

3. Spandex dyeing

Spandex can be dyed with disperse dyes and acid dyes, but the fastness of these two dyes is too poor. Moreover, experiments have shown that reactive dyes (for dyeing nylon stretch fabric) and disperse cationic dyes (for dyeing CDP stretch fabric) for nylon basically do not stain spandex. This does not mean that these two dyes are not suitable for dyeing spandex. In fact, they are not. Under the premise of using appropriate auxiliaries as media, nylon can be dyed with reactive dyes on spandex, and both the fastness and depth are relatively excellent.

There are few pure spandex fabrics on the market, so spandex dyeing is relatively unfamiliar. When pure spandex fabric is used in combination with low-elasticity fabric or non-elastic fabric, it can increase the elasticity and resilience of the fabric. Moreover, when the elastic fabric is stretched or molded, if the color of the spandex does not match that of the filament, the problem of spandex color leakage will occur, which requires dyeing the spandex.

For dyeing spandex in light colors

Dyeing can be carried out under acidic bath conditions with acid dyes or disperse dyes. Under the same dye dosage, the fastness of disperse dyes on spandex in all aspects is superior to that of acid dyes, but the fastness of different disperse dyes to spandex varies. Generally, disperse dyes can be used for dyeing when the dye dosage is less than 0.5%.

Spandex is a high-elasticity fiber. Considering that long-term dyeing at high temperatures can cause spandex to lose its elasticity, it is generally dyed below 100℃. Moreover, spandex is not resistant to alkali. Additionally, disperse dyes and acid dyes are suitable for dyeing under acidic conditions. Generally, spandex is dyed in an acidic bath with a pH of around 5, which is more appropriate.

All kinds of dyes can be used as auxiliaries to dye spandex. Such auxiliaries are called spandex colorants or spandex coloring agents in the market. They are mainly used for dyeing spandex with reactive dyes and acid dyes for nylon, and are generally of amphoteric ionic nature.

The principle of its operation is roughly as follows: Under acidic conditions, the amide bonds and other groups within spandex ionize and become positively charged. The spandex colorant reacts with them, and the fixative adheres to the spandex. Since the spandex colorant contains amino positive ions, the dye can also combine with the spandex colorant.

Spandex colorant

The general rule for the fastness of spandex after dyeing is: water washing > sweat (acid) > water immersion, and the wet rubbing fastness is much better than the dry rubbing fastness.

The reason for this is that after spandex is colored, under wet conditions, the dye or the combination of spandex colorant and dye will migrate under the long-term action of external pressure, and this is a slow process (spandex is a hydrophobic fiber, water molecules will not quickly enter the fiber interior, and the dye will not migrate quickly either). Therefore, its washing fastness is relatively good. Secondly, under sweat and acidic conditions, It is conducive to the ionization of amide bonds on spandex or amino groups on spandex colorants, which can capture migrating dyes or the combination of spandex colorants and dyes. Therefore, the acid perspiration fastness is superior to the neutral water immersion fastness. Under the effect of friction, dry grinding generates heat, causing the dye on the surface of spandex to undergo thermal migration. This situation does not occur under wet conditions.

2. After dyeing spandex and boiling it in soda ash, its fastness will decrease by about 0.5 grades. This is because under alkaline conditions, the combination of spandex colorant with fibers and dyes will be destroyed, thereby reducing the fastness. Secondly, after dyeing spandex, cationic fixation agents are used to fix the color. The improvement in fastness is greater than that of anionic fixation agents. Cationic fixation agents can react with the water-soluble groups (-SO3-) generated by the dye, seal the water-soluble groups, and reduce the water solubility of the dye. Moreover, the fixation agent forms a film on the fiber surface, which can isolate the dye, and the dry rubbing fastness is accordingly improved.

3. The light fastness of acid, disperse and nylon reactive dyes for dyeing spandex is worse than that for dyeing polyester or nylon filament. The main reason for this is that spandex has a low crystallinity and is in a highly elastic state at room temperature, making it easier for air to enter the fiber interior. Under the effect of sunlight, it undergoes color changes. Additionally, spandex itself turns yellow under sun exposure, which is also a cause of its low light fastness.

When cotton stretch fabric is dyed black, spandex may show white. Generally, a small amount of dispersed or acidic black can be added to the dyeing bath to dye spandex on it. However, attention should be paid to the fastness issue.

Secondly, regarding the issue of black spandex color exposure when dyeing polyester stretch fabric and nylon stretch fabric, a small amount of spandex colorant, about 0.3%, can be added to the dyeing bath when dyeing polyester stretch fabric. For dyeing nylon stretch fabric, spandex colorant can be directly added to the dyeing bath for dyeing. In this way, the problem of spandex color exposure can basically be solved.

A solution to mitigate the elasticity loss of spandex

For the denim garments that need chlorine bleaching, apply 1-3g/L of spandex chlorine bleaching anti-brittleness agent (1/10 of the bleaching water) in a water bath for 1-2 minutes in advance. Gradually increase the temperature to 40℃. Add the bleaching water three times: the first time, add 1/5 of the total bleaching water; the second time, add 2/5 of the total bleaching water; and the last time, add the remaining 2/5 of the bleaching water. Operate according to the actual bleaching time.

After bleaching, while draining water, water is added at the same time. The water temperature will gradually drop. The reason for this is to prevent the spandex from becoming brittle and losing its elasticity due to a sudden drop in temperature.

3. Softness: Due to the different formulations of softeners, silicone oils, etc. by various additive manufacturers, some softeners and silicone oils can damage the elasticity of spandex. This is mainly because a small amount of solvents in their components have an impact on the elasticity of spandex. Some silicone oils and softeners basically do not contain this solvent and have no effect on the elasticity of spandex. Choose the appropriate silicone oil for softening treatment.

4. Solvents: Due to the presence of silicone-containing oiling agents in the spandex weaving process, the oiling agents need to be removed during the dyeing and finishing process. Many refining agents contain solvents, which have a significant impact on the elasticity damage of spandex.

5. Physical tearing: Due to the nostalgic nature of denim, many cases require physical tearing and cutting treatments. Such tearing causes uneven force on the spandex elastic filament, leading to bubbling of the spandex. The exposed yarn is also prone to breakage, resulting in the loss of elasticity.

6. Drying temperature: Generally, the drying temperature for denim fabrics containing spandex elastic filament is below 60℃. Therefore, excessively high temperatures can cause significant damage to the spandex elasticity.