Generation and Removal of Oligomers
Defects caused by oligomers are a common issue in dyeing production. They seriously compromise product quality, contaminate equipment and leave stubborn stains that are difficult to clean. The relevant issues are summarized as follows:
1. Definition
Oligomers, also known as low polymers, are low-molecular substances inside polyester fibers with the same chemical structure as polyester. They are by-products generated during polyester spinning. Generally, polyester fibers contain 1% to 3% oligomers.
An oligomer is a polymer composed of a small number of repeating units, with a relative molecular mass between small molecules and macromolecules. The prefix “oligo-” derives from the Greek word oligos, which means “a few”. Most polyester oligomers are cyclic compounds formed by three ethylene terephthalate molecules.
2. Impacts
Oligomers cause color spots and stains on fabrics, as well as white powder on dyed yarns.
When the temperature exceeds 120°C, oligomers dissolve in the dye bath, then crystallize out and combine with aggregated dyes. Upon cooling, they deposit on the surfaces of equipment and fabrics, resulting in defects such as color stains and spots. For dyeing with disperse dyes, maintaining the temperature at 130°C for around 30 minutes is normally required to ensure sufficient color depth and fastness.
Corresponding solutions: For light-colored fabrics, keep the temperature at 120°C for 30 minutes. For dark-colored fabrics, pre-treatment is mandatory before dyeing. In addition, dyeing under alkaline conditions is also an effective way to tackle oligomer-related problems.
3. Comprehensive Solutions
Pre-treat grey fabrics before dyeing with 3% pure sodium hydroxide (NaOH) and 1% surfactant detergent. Conduct the treatment at 130°C for 60 minutes with a liquor ratio of 1:10 to 1:15. This pre-treatment slightly erodes polyester fibers but works exceptionally well for removing oligomers. It can eliminate the aurora effect on polyester filament fabrics and improve pilling resistance of medium and short staple polyester fibers.
Control the dyeing temperature below 120°C and adopt carrier dyeing. This method reduces oligomer formation while achieving the required color depth.
Add dispersing and protective colloid auxiliaries during dyeing. They not only ensure level dyeing, but also prevent oligomers from depositing on fabrics.
Drain the hot dye liquor out of the machine rapidly within 5 minutes after dyeing. Oligomers disperse evenly in the dye bath at 100–120°C, yet tend to accumulate and precipitate on dyed materials when the temperature drops below 100°C. Note that this method may cause creases on heavy-weight fabrics.
Perform dyeing under alkaline conditions. This effectively inhibits oligomer generation and removes residual oil agents on fabrics. Dyes specially formulated for alkaline dyeing must be used.
Conduct reductive cleaning after dyeing. Prepare the solution with 3–5 mL/L of 32.5% (38°Bé) sodium hydroxide and 3–4 g/L of sodium dithionite. Treat the fabrics at 70°C for 30 minutes, followed by cold washing, hot washing and another cold washing step, and finally neutralize with acetic acid.
4. Solutions to White Powder on Yarns
(1) High-temperature liquor draining (thorough solution)
Open the drain valve immediately after holding the temperature at 130°C. A temperature of 120°C is also acceptable, but it cannot be lower, as 120°C is the glass transition temperature of polyester.
Though the operation seems simple, safety is the top concern. Dramatic noise and intense mechanical vibration will occur during high-temperature draining. Aging equipment may crack or suffer loose screws, and cracked dyeing machines carry a risk of explosion (Extreme caution is required). Any equipment modification must be designed and implemented by the original manufacturer; never conduct unauthorized alterations.
There are two draining modes: draining to a water tank and direct outdoor draining. Backflow after drainage is a common issue, which professional dye vat manufacturers are well aware of. High-temperature draining shortens the overall dyeing cycle, but it poses challenges for factories with unstable production reproducibility.
(2) Alternative for factories unable to implement high-temperature draining
Replace conventional detergents with oligomer cleaners in the reductive cleaning process. However, this method cannot completely eliminate the problem.
Clean dye vats regularly after dyeing. For medium and dark shades, clean the vat after every 5 batches. If a large amount of white powder scale has built up on jet dyeing machines, cleaning the equipment shall be the top priority.
Post time: Jun-08-2026