Formation and Removal of Oligomers
Problems related to dye defects caused by oligomers are quite common. They severely compromise product quality, contaminate equipment and lead to difficult cleaning. Relevant details are summarized as follows:
1. Definition
Oligomers, also known as low polymers, are low-molecular substances with the same chemical structure as polyester fibers, existing inside polyester fibers. They are by-products generated during polyester spinning. Generally, polyester contains 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, meaning “a few”. Most polyester oligomers are cyclic compounds formed by three ethylene terephthalate units.
2. Impacts
Oligomers cause color spots and stains on fabrics, as well as white powder formation during yarn dyeing.
When the temperature exceeds 120°C, oligomers dissolve in the dye bath and then crystallize out, combining with aggregated dyes. Upon cooling, they deposit on equipment or fabric surfaces and result in defects such as color spots and stains. Dyeing with disperse dyes normally requires heat preservation at 130°C for around 30 minutes to ensure satisfactory dye depth and color fastness.
Corresponding solutions are as follows: For light-colored products, maintain the temperature at 120°C for 30 minutes. For dark-colored products, pre-treatment is mandatory prior to dyeing. In addition, dyeing under alkaline conditions is an effective way to control oligomers.
3. Comprehensive Solutions
-
Pre-treat grey fabrics with 3% 100% caustic soda and 1% surfactant detergent at 130°C for 60 minutes, with a liquor ratio of 1:10 to 1:15. This pre-treatment slightly etches polyester fibers but delivers excellent effects on oligomer removal. It can reduce the glare of filament fabrics and improve the pilling resistance of staple fiber fabrics.
-
Control the dyeing temperature below 120°C and apply carrier dyeing appropriately. This method inhibits oligomer formation while achieving the required dye depth.
-
Add dispersing protective colloid auxiliaries during dyeing. They deliver level dyeing performance and prevent oligomers from depositing on fabrics.
-
Drain the dye liquor rapidly from the machine immediately after high-temperature dyeing, with the draining time limited to a maximum of 5 minutes. Oligomers disperse evenly in the dye liquor at 100–120°C but tend to accumulate and deposit on dyed materials when the temperature drops below 100°C. Note that this method may cause creases on heavy-weight fabrics.
-
Conduct dyeing under alkaline conditions. This effectively suppresses oligomer generation and removes residual oil agents on fabrics. Only dyes suitable for alkaline dyeing shall be adopted.
-
Carry out reduction cleaning after dyeing. Prepare the cleaning solution with 3–5 mL/L of 32.5% (38°Bé) caustic soda and 3–4 g/L of sodium hydrosulfite. 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 for White Powder on Yarns
(1) High-temperature draining method (thorough solution)
Open the drain valve immediately after constant temperature treatment at 130°C. A temperature of 120°C is also acceptable, while a lower temperature is not recommended, as 120°C is the glass transition temperature of polyester.
Though the operation seems simple, safety is the top concern and the biggest challenge. Dramatic noise and intense mechanical vibration occur the moment high-temperature liquor is drained. Aging equipment is prone to cracking, loose screws or even rupture, which may trigger explosions of dyeing machines (extreme caution required).
Any equipment modification must be designed and implemented by the original manufacturer. Do not carry out unauthorized alterations.
There are two draining modes: draining to the water tank and direct discharge to the outside atmosphere. Watch out for the backflow phenomenon after draining, which is well known to experienced dyeing vat manufacturers.
High-temperature draining shortens the overall dyeing cycle, yet it may cause troubles for factories with poor process reproducibility.
(2) Alternative for factories unable to perform high-temperature draining
Replace conventional cleaning agents with dedicated oligomer removers during reduction cleaning. This method cannot achieve a 100% removal effect.
Clean the dye vats regularly after dyeing. For medium and dark shades, clean the vat after approximately 5 dyeing batches. If heavy white powder scale has built up on jet dyeing machines, vat cleaning shall be prioritized immediately.
Post time: May-27-2026