Modified Polyester Dyeing Process And Performance Analysis

I. What is Modified Polyester?

Modified polyester is a fiber material that chemically modifies traditional polyester by introducing a third or fourth monomer into the molecular structure of polyester (PET).

Common modification methods include:

Third monomer: Sodium isophthalate-5-sulfonate (SIPM)Fourth monomer: Polyethylene glycol (PEG)

Among them:

Adding only the third monomer (CDP) introduces sulfonic acid groups, improving dyeing performance. Adding the fourth monomer (PEG) further improves moisture absorption and dyeing conditions, but affects fiber strength. 

2.Fiber Characteristics of Modified Polyester

（1).Structural and Performance Changes

Modified polyester exhibits significant performance differences compared to ordinary polyester:

With only the addition of the third monomer, the accessibility of sulfonic acid groups is approximately 16%, making dyeing still relatively difficult.

With the addition of the fourth monomer:

Dyeing temperature can be lowered

However, fiber strength decreases significantly

Application range is somewhat limited.

(2) Performance Improvements

Modified polyester offers the following improvements compared to ordinary polyester:

Improved pilling resistance

Increased air permeability

Enhanced moisture absorption

Strengthened antistatic properties

Softer hand feel

(3) Engineering and Processing Issues

Despite significant performance improvements, some engineering challenges arise:

Increased fiber rigidity

Decreased elasticity

Reduced strength

Increased weaving and processing difficulty

Therefore, modified polyester is typically used more in blended fabric systems, such as:

CDP + Polyester

CDP + Nylon

CDP + Acrylic

3. Dyeing Characteristics

(1) Applicable Dye Systems

Modified polyester can use the following dyes:

Catonic dyes (suitable for some modified systems)

Disperse dyes (most commonly used system)

(2) Dyeing Process Characteristics

Dyeing temperature usually needs to be close to 120°C.

High-temperature conditions pose a certain risk of fiber decomposition.

Fibers are relatively sensitive to alkali.

Alkali reduction and pretreatment require strict control of process conditions.

(3) Dyeing Performance Characteristics

Advantages:

Bright colors

Can create multi-color effects

Lighter staining on elastic fibers

High wet fastness

Disadvantages:

Lower light fastness

Poor reproducibility

More pronounced ring dyeing phenomenon

Unstable fiber saturation value

Susceptible to hydrolysis (especially under strong acid/alkali conditions)

4.Main Uses of Modified Polyester (CDP)

Modified polyester is widely used in the following areas:

Sportswear, Casual wear, Lining materials, Socks, School uniforms, Carpets, etc.

5. Process (Key Steps)

（1）Pretreatment

Pretreatment is crucial for ensuring dyeing uniformity and stability.

Scouring (Removal of Spinning Oil)

• Chemicals: 1 g/L Dyamul GS new, 2 g/L Sodium Carbonate

• Conditions: 20 min at 70°C

• Purpose: Removes spinning oil, improves wetting, and prepares the fabric for uniform dyeing.

Scouring (Removal of Silicone on Spandex)

• Chemicals: 1–2 g/L Dyamul SLX

• Conditions: Slightly acidic, 20 min at 80°C

• Purpose: Removes silicone on spandex, reduces internal fabric stress, improves hand feel, and prevents wrinkles during subsequent processing.

（2）Pre-setting

Setting time: 30-45 seconds, 160-180°C. Excessive setting temperature or time will damage fibers, affecting dyeing performance and causing yellowing. Fabrics containing spandex can be set up to 190°C; the setting temperature should be kept as low as possible. 

 (3) Dyeing process

 (4) Skyzon SD 

uses water below 50°C. After adding the dye, stir thoroughly until there are no precipitates or lumps, then add it to the auxiliary tank. Do not use hot water above 50°C – it will damage the dye dispersion. Do not add glacial acetic acid to pre-dissolve the dye – it is unnecessary.

Reason: In Skyzon SD dye production, the addition of dispersants forms a precipitated cationic dye, which is an aqueous dispersion.