Why Does Spandex Lose Its Elasticity? Causes and Solutions

1. What Are the Main Forms of Spandex Yarn?

2. Origin and Main Properties of Spandex

This is one type of elastic fiber. It is commonly known as Spandex in international markets and elastane in many textile applications.

Its chemical name is polyurethane fiber, often abbreviated as PU fiber. Spandex mainly includes two types:

• Polyester-based spandex, made from aromatic diisocyanate and hydroxyl-containing polyester segments.
• Polyether-based spandex, made from aromatic diisocyanate and hydroxyl-containing polyether segments.

Typical commercial names include Vyrent for polyester-based spandex and Lyera for polyether-based spandex.

Key Properties of Spandex

Spandex can usually stretch by 500% to 700% or more and offers excellent elastic recovery.

• At 200% extension, it can recover by approximately 97%.
• At 50% extension, its recovery can exceed 99%.
• Its density usually ranges from 1.20 to 1.25.
• Breaking strength generally ranges from 4.41 to 8.82 cN/tex, or approximately 0.5 to 1.0 g/denier.
• Polyester-based spandex generally has lower strength.
• Polyether-based spandex usually offers higher strength.
• Moisture regain typically ranges from 0.3% to 1.2%.
• Multifilament spandex usually absorbs slightly more moisture than monofilament spandex.
• Short-term exposure to temperatures between 95°C and 150°C generally does not damage the fiber.
• The recommended ironing temperature is below 150°C.

This burns relatively slowly and forms a gel-like residue after burning. It offers good dyeability and can be dyed in various colors because it has a strong affinity for certain dyes.

It also provides good resistance to acids and alkalis, can withstand bleaching under suitable conditions, and does not easily develop mold.

3. Spandex Dyeing

Manufacturers can dye spandex with disperse dyes or acid dyes. However, both dye types may show limited colorfastness on spandex.

Tests have shown that reactive dyes designed for nylon and disperse cationic dyes designed for CDP stretch fabrics generally do not easily stain spandex.

This does not mean these dyes cannot dye spandex. With the right auxiliaries, nylon reactive dyes can be applied to spandex and may provide good depth and colorfastness.

Pure spandex fabrics are uncommon in the market. More often, manufacturers combine spandex with low-stretch or non-stretch fabrics to improve elasticity and recovery.

During stretching or molding, the spandex may show through if its color does not match the filament yarn. In that case, manufacturers need to dye the spandex to prevent color leakage or exposed white spandex.

Dyeing Spandex in Light Colors

Manufacturers can dye light-colored spandex in an acidic bath with acid dyes or disperse dyes.

Under the same dye concentration, disperse dyes usually provide better fastness on spandex than acid dyes. However, the fastness of disperse dyes can vary significantly.

In general, disperse dyes may work well when the dye concentration remains below 0.5%.

Because spandex is highly elastic, prolonged dyeing at high temperatures may cause it to lose elasticity. Therefore, manufacturers usually dye spandex below 100°C.

Spandex also has limited alkali resistance. Since disperse and acid dyes generally work under acidic conditions, an acidic dye bath is normally more suitable for spandex.

A pH level of around 5 is often appropriate.

How Spandex Colorants Work

With suitable auxiliaries, different dyes can be applied to spandex. These auxiliaries are commonly known as spandex colorants or spandex dyeing agents.

They mainly help nylon reactive dyes, acid dyes, and similar dyes bond with spandex. Many of these agents have zwitterionic properties.

The working principle can be summarized as follows:

Under acidic conditions, amide groups and other groups in spandex become ionized and carry a positive charge. The spandex colorant reacts with these groups, while a fixing agent helps secure the colorant on the fiber.

Because the colorant contains positively charged amino groups, the dye can also combine with the colorant and remain on the spandex.

4. Colorfastness of Dyed Spandex

General Colorfastness Pattern

The general colorfastness pattern of dyed spandex is:

Washing fastness > Acid perspiration fastness > Water fastness

Wet rubbing fastness is usually much better than dry rubbing fastness.

The reason relates to the behavior of dyes and dye-colorant complexes under wet conditions. When pressure acts on the wet fiber for a long time, the dye or dye-colorant complex may gradually migrate.

However, this migration occurs slowly because spandex is hydrophobic. Water does not enter the fiber quickly, and the dye does not move rapidly through the fiber structure.

As a result, dyed spandex generally shows good washing fastness.

Under acidic perspiration conditions, the amide groups in spandex or the amino groups in the spandex colorant can ionize. These groups can capture the migrating dye or dye-colorant complex.

Therefore, acid perspiration fastness may be better than neutral water fastness.

During rubbing, dry friction generates heat. This heat can cause the dye on the spandex surface to migrate. Under wet conditions, this heat buildup is less likely to occur, so wet rubbing usually produces better results.

The Effect of Alkali Treatment

After dyeing, boiling spandex with soda ash can reduce colorfastness by approximately 0.5 grade.

Under alkaline conditions, the bonds between the spandex colorant, fiber, and dye may break down. As a result, the colorfastness decreases.

Cationic fixing agents usually improve the colorfastness of dyed spandex more effectively than anionic fixing agents.

Cationic fixing agents can react with the water-soluble groups in dyes, such as -SO₃⁻. They block these groups, reduce the water solubility of the dye, and form a film on the fiber surface.

The film helps isolate the dye from external friction, which can improve dry rubbing fastness.

Lightfastness

The lightfastness of spandex dyed with acid dyes, disperse dyes, or nylon reactive dyes is usually lower than that of dyed polyester or nylon filament yarns.

Spandex has a relatively low degree of crystallinity and remains highly elastic at room temperature. Air can enter its internal structure more easily, which may accelerate color change under sunlight.

In addition, spandex itself may yellow during long-term exposure to sunlight. This also contributes to its lower lightfastness.

5. Why Does Spandex Lose Its Elasticity?

When manufacturers dye black cotton-spandex fabric, the spandex may appear white or show through.

A small amount of disperse or acid black dye can be added to the dye bath to color the spandex. However, manufacturers must pay close attention to the resulting colorfastness.

For polyester-spandex and nylon-spandex fabrics, manufacturers can use spandex colorants to reduce color leakage.

• During polyester-spandex dyeing, adding approximately 0.3% spandex colorant may help.
• During nylon-spandex dyeing, manufacturers can add the spandex colorant directly to the dye bath.

This approach can largely solve the problem of exposed or mismatched spandex. Other colors can follow the same principle.

However, spandex may lose its elasticity during dyeing, bleaching, softening, solvent treatment, physical finishing, or high-temperature drying.

6. Ways to Reduce Spandex Elasticity Loss

1. Control Chlorine Bleaching

For denim garments that require chlorine bleaching, first treat them with 1 to 3 g/L of a spandex chlorine-bleach anti-brittleness agent.

This amount equals approximately one-tenth of the bleach liquor dosage.

Run the bath for 1 to 2 minutes, then gradually raise the temperature to 40°C. Add the bleach liquor in three stages:

• Add one-fifth of the total bleach liquor during the first stage.
• Add two-fifths during the second stage.
• Add the remaining two-fifths during the final stage.

Then continue the process according to the required bleaching time.

2. Avoid Sudden Temperature Changes

After bleaching, drain and refill the bath at the same time so that the water temperature decreases gradually.

This process helps prevent sudden temperature changes, which may cause spandex to become brittle and lose elasticity.

3. Select the Right Softener

Different suppliers use different formulas for softeners and silicone products.

Some softeners and silicone oils contain small amounts of solvents that may damage spandex elasticity. Other products contain little or none of these solvents and have much less effect on the fiber.

Therefore, manufacturers should select a silicone softener that suits the spandex and the required finishing process.

4. Control Solvent Exposure

Silicone-based oils are often used during spandex knitting and weaving. The dyeing and finishing process must remove these oils.

However, many scouring agents contain solvents, and these solvents may seriously damage spandex elasticity.

Manufacturers should therefore check the solvent content of scouring and cleaning products before use.

5. Prevent Excessive Physical Damage

Denim products often require physical distressing to create a vintage appearance. Cutting, tearing, and other damage treatments can apply uneven stress to the spandex yarn.

This uneven stress may cause bubbling or distortion. Exposed yarns may also break easily, which can lead to permanent loss of elasticity.

6. Control Drying Temperature

For denim fabrics containing spandex, the drying temperature should generally remain below 60°C.

Excessive heat can seriously damage the elastic fiber and reduce its recovery performance.

Conclusion

provides excellent stretch and recovery, but its elasticity can decline when the fiber faces excessive heat, strong alkali, solvents, unsuitable auxiliaries, chlorine bleaching, or physical damage.

To protect spandex performance, manufacturers should control dyeing temperature, maintain a suitable pH, select compatible auxiliaries, avoid sudden temperature changes, and keep drying temperatures within a safe range.

In denim and other stretch fabrics, careful control of dyeing, finishing, and garment processing is essential. Proper process management can help preserve elasticity, improve garment durability, and reduce problems such as exposed spandex, bubbling, and fabric deformation.