Generation and Removal of Oligomers
Problems of dye defects caused by oligomers occur frequently, which seriously downgrade product quality and contaminate equipment with stubborn residues difficult to clean. Key points are summarized as follows:
1. Definition
Oligomers, also named low polymers or short-chain polymers, are low-molecular-weight substances with the same chemical structure as polyester fibers existing inside polyester filaments, generated as by-products during polyester spinning. Conventional polyester contains 1%~3% oligomers by weight.
An oligomer is a polymer composed of a small number of repeating units, with molecular weight between small molecules and high polymers. The prefix oligo- derives from Greek oligos, meaning “a few”. Most polyester oligomers are cyclic compounds formed by three ethylene terephthalate units.
2. Adverse Impacts
Oligomers lead to color spots and stains on finished fabrics as well as white powdery deposits on dyed yarns.
When temperature rises above 120℃, oligomers dissolve in dye liquor before recrystallizing and combining with agglomerated dyestuffs. Upon cooling, precipitates attach to machine surfaces or fabrics to form speckles and stains. Disperse dye dyeing normally requires holding at 130℃ for around 30 minutes to secure satisfactory color depth and color fastness.
Corresponding solutions: Pale-shade fabrics can be dyed at 120℃ with 30 min holding time; deep-shade fabrics need dedicated pre-treatment prior to dyeing. Alkaline dyeing is another effective measure against oligomer-related defects.
3. Comprehensive Countermeasures
- Pre-dye pre-scouring for grey fabric: apply 3% 100% caustic soda plus 1% surfactant detergent, treat at 130℃ for 60 min with liquor ratio 1:10~1:15. This process slightly etches polyester fiber yet delivers excellent oligomer removal; it eliminates bright streak marks on filament fabrics and improves pilling performance of staple polyester fabrics.
- Restrict dyeing temperature below 120℃ and adopt carrier dyeing to curb oligomer generation while maintaining target color depth.
- Add dispersing protective colloid auxiliaries during dyeing to level dye uptake and inhibit oligomer deposition on fabrics.
- Drain hot dye liquor rapidly within maximum 5 minutes immediately after dyeing. Oligomers disperse uniformly in liquor at 100–120℃ yet readily precipitate onto substrates below 100℃; note this quick draining may cause creases on heavy-weight fabrics.
- Conduct dyeing under alkaline conditions to suppress oligomer formation and eliminate residual spinning oils; select alkali-resistant disperse dyes accordingly.
- Post-dye reduction clearing: add 3–5 mL/L 32.5% (38°Bé) caustic soda and 3–4 g/L sodium dithionite, treat at 70℃ for 30 min, followed by sequential cold wash → hot wash → cold wash and acetic acid neutralization.
4. Remedies for White Powder on Yarn
(1) High-temperature rapid draining (thorough solution)
Open drain valve instantly once temperature stabilizes at 130℃; 120℃ is also acceptable (do not go lower, as 120℃ is the glass transition temperature of polyester).
Despite straightforward operation, safety is the critical bottleneck: drastic noise and intense mechanical vibration happen instantly during high-temp draining. Aged dyeing machines risk crack propagation, loose fasteners or even equipment explosion (critical safety warning).
Modification must be designed and implemented by original equipment manufacturers; unauthorized refitting poses severe safety hazards.
Two draining options are available: draining to waste water tank or direct atmospheric venting; pay close attention to back-siphonage after full drainage (well-known among professional dye vat manufacturers).
High-speed hot draining shortens dyeing cycles yet brings unstable repeatability for poorly managed factories.
(2) Alternative for factories unable to implement high-temperature draining
Replace conventional clearing agent with dedicated oligomer cleaner during reduction clearing (removal efficiency not 100%).
Implement regular vat cleaning post-dyeing: clean dye vat after every five batches for medium and deep shades. If existing jet dyeing machines are heavily fouled with white oligomer scale, priority goes to thorough tank cleaning first.
Post time: Jun-02-2026