Spandex can also lose its elasticity! What are the reasons?

1. What are the different forms of spandex elastic fiber yarn?

Spandex elastic fiber is blended with other fibers to form elastic yarn. Currently, there are three forms of this yarn: spandex core-spun yarn, spandex twisted yarn, and spandex covered yarn.

2. The origin and main properties of spandex fiber

Spandex fiber is a type of elastic fiber. Elastic fibers are collectively known as Spandex internationally, while in my country, the commercial name is simply "spandex." In reality, spandex is just one broad category of elastic fibers. Polyurethane fiber (PU fiber) has two varieties: one is a copolymer of aromatic diisocyanate and hydroxyl-containing polyester segments, simply called polyester-type spandex; the other is a copolymer of aromatic diisocyanate and hydroxyl-containing polyether segments, simply called polyether-type spandex. A representative commercial name for polyester-type spandex is Vyrent, and a representative commercial name for polyether-type spandex is Lyera.

The main properties of spandex fiber are: It typically has an elongation of 500% to 700% or more and good elastic recovery. It shrinks by 97% at 200% elongation and by over 99% at 50% elongation.

The relative density of various spandex types varies little, generally between 1.20 and 1.25. Its breaking strength is 4.41 to 8.82 cN/tex (0.5 to 1.0 g/denier). Polyester-type spandex has lower strength, while polyether-type spandex has higher strength. Its moisture absorption rate is generally 0.3% to 1.2%, with multifilament having slightly higher absorption than monofilament. It is heat-resistant; short-term storage at 95–150℃ will not damage the fiber, and the safe ironing temperature is below 150℃.

The fiber burns slowly, leaving a gel-like residue after combustion. It has excellent dyeing properties, can be dyed in a variety of 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 colorfastness of these two dyes is quite poor. Furthermore, experiments have shown that reactive dyes for nylon (for dyeing nylon elastic fabric) and disperse cationic dyes (for dyeing CDP elastic fabric) have virtually no colorfastness on spandex. Does this mean that these two dyes are unsuitable for dyeing spandex? Not at all. With appropriate auxiliaries as a medium, reactive dyes for nylon can indeed dye spandex, and the colorfastness and depth are quite good.

Pure spandex fabrics are rare on the market, so spandex dyeing is relatively unfamiliar. Using pure spandex fabric in combination with low-elasticity or non-elastic fabrics can increase the fabric's elasticity and resilience. Also, when stretching or molding elastic fabrics, if the spandex color is inconsistent with the filaments, color bleeding will occur, necessitating spandex dyeing.

For dyeing spandex in light colors:

It can be dyed with acid dyes or disperse dyes under acidic bath conditions. Under the same dye dosage, disperse dyes show better fastness to spandex than acid dyes in various aspects. However, different disperse dyes have different fastness to spandex. Generally, disperse dyes can be used when the dye dosage is less than 0.5%.

Spandex is a highly elastic fiber. Considering that prolonged dyeing at high temperatures will cause spandex to lose elasticity, dyeing is generally done below 100℃. Furthermore, spandex is not alkali-resistant. Also, disperse dyes and acid dyes are suitable for dyeing under acidic conditions. Generally, dyeing spandex under acidic bath conditions with a pH of around 5 is most suitable.

Various dyes can be applied to spandex using suitable auxiliaries as a medium. These auxiliaries are commercially known as spandex coloring agents or spandex colorants. They are mainly used for dyeing spandex with reactive dyes and acid dyes, and are generally amphoteric.

Its working principle is roughly as follows: Under acidic conditions, the amide bonds and other groups within the spandex ionize, becoming positively charged. The spandex dye reacts with this positive charge, and the fixing agent adheres to the spandex. Because the spandex dye contains amino cations, the dye can also combine with the spandex dye.

Spandex Dye

1. General rules for fastness after spandex dyeing: Water washing > Perspiration (acid) > Water immersion, and wet rubbing fastness is much better than dry rubbing fastness.

The reasons for this are as follows: 1. After dyeing, spandex undergoes migration under prolonged external pressure in a wet state due to the influence of dye or spandex dyeing agent on the dye-resin bond. This migration is a slow process (spandex is a hydrophobic fiber; water molecules do not quickly penetrate the fiber, and the dye does not migrate rapidly). Therefore, its wash fastness is relatively good. 2. Under acidic perspiration conditions, the amide bonds on the spandex or the amino groups on the spandex dyeing agent ionize, which can capture the migrating dye or spandex dye-resin bond. Therefore, acidic perspiration fastness is better than neutral water immersion fastness. 3. Under friction, dry rubbing generates heat, causing thermal migration of the dye on the spandex surface. This does not occur under wet conditions.

4. Boiling spandex in soda ash after dyeing will reduce its fastness by about 0.5 grades. This is because under alkaline conditions, the bond between the spandex dyeing agent and the fiber and dye is broken, thus reducing the fastness. Secondly, using cationic fixing agents to fix the color after dyeing spandex results in a greater improvement in fastness compared to anionic fixing agents. Cationic fixing agents can react with the water-soluble groups (-SO3-) of the dye, blocking these groups and reducing the dye's water solubility. Furthermore, the fixing agent forms a film on the fiber surface, isolating the dye and thus improving dry rubbing fastness.

3. The light fastness of spandex dyed with acid, disperse, and nylon reactive dyes is worse than that of polyester or nylon filaments. This is mainly because spandex has low crystallinity and is in a highly elastic state at room temperature, allowing air to more easily penetrate the fiber. Under sunlight, this causes color change. Additionally, spandex itself yellows under sunlight, further contributing to low light fastness.

When dyeing cotton elastic fabric black, the spandex may show through. This can generally be addressed by adding a small amount of disperse or acid black to the dye bath to allow the spandex to adhere, but attention must be paid to the fastness.

Secondly, regarding the issue of spandex showing through when dyeing black polyester and nylon elastic fabrics, for polyester elastic fabrics, a small amount of spandex dyeing agent (approximately 0.3%) can be added to the dye bath. For nylon elastic fabrics, the spandex dyeing agent can be added directly to the dye bath. This should largely resolve the spandex showing through issue.

Solutions to mitigate spandex elasticity loss:

1. For denim garments requiring chlorine bleaching, pre-treat with 1-3g/L spandex chlorine bleach anti-brittleness agent (1/10 of the bleach solution) in a water bath for 1-2 minutes, gradually increasing the temperature to 40℃. Add the bleach solution in three stages: first, add 1/5 of the total bleach solution; second, add 2/5; and finally, add the remaining 2/5, following the actual bleaching time.

2. After bleaching, drain the water while simultaneously adding water. The water temperature will gradually decrease. This is to prevent a rapid drop in temperature from causing the spandex to become brittle and lose elasticity.

3. Softening: Due to differences in the formulations of softeners and silicone oils by various auxiliary manufacturers, some softeners and silicone oils can damage the elasticity of spandex. This is mainly because of the small amount of solvent in their components, which affects the elasticity of spandex. Other silicone oils and softeners contain virtually no solvent and have no effect on the elasticity of spandex. Choose a suitable silicone oil for softening treatment.

4. Solvents: Because spandex weaving involves silicone-containing oils, these oils need to be removed during the dyeing and finishing process. Many scouring agents contain solvents, which significantly damage the elasticity of spandex.

5. Physical Disruption: Due to the vintage feel of denim, many fabrics require physical distressing or cutting. This uneven stress on the spandex elastic yarns can cause bubbling, and exposed yarns are prone to breakage, leading to loss of elasticity.

6. Drying Temperature: Generally, denim fabrics containing spandex elastic yarns are dried at temperatures below 60℃. Excessively high temperatures can severely damage the elasticity of spandex.