Generation And Prevention of Polyester Oligomers | Analysis And Solutions To PET Dyeing Problems

I. Basic Structure and Characteristics of Polyester (PET)

Polyester, scientifically known as polyethylene terephthalate (PET), has the following molecular structure:

[-OC-Ph-COOCH₂CH₂O-]ₙ

Polyester is produced by esterification or transesterification of terephthalic acid (PTA) or dimethyl terephthalate (DMT) with ethylene glycol (EG), followed by polycondensation. The resulting polymer typically has a molecular weight between 18,000 and 25,000.

The polymerization process is a typical step-growth polycondensation reaction. In industrial production, the degree of polymerization gradually increases, but inevitably, a certain amount of monomers and oligomers remain.

II. Formation Mechanism of Polyester Oligomers

The polyester polymerization process can be divided into three stages:

Initial Stage: Monomers gradually form oligomers.

Intermediate Stage: Oligomers continue to condense and form polymers.

Late Stage: Molecular weight increases, system viscosity rises, and the reaction tends to end.

Since the polymerization reaction is stepwise, a small amount of oligomers inevitably exists in the finished PET product.

Among them, the most important oligomer is:

▶ Cyclic Trimers

Cyclic trimers account for approximately 70%–80% of the total polyester oligomers and have the following characteristics:

Highly symmetrical structure

Easy to crystallize and precipitate

High melting point (approximately 310℃)

Extremely low solubility (<2 mg/L) under high-temperature dyeing conditions (130–135℃)

Therefore, it is the most important "contaminant" in the polyester dyeing process.

▶ Linear Oligomers

Linear oligomers account for approximately 25% of the composition. They possess certain polar end groups and exhibit some solubility in water, resulting in relatively minor impacts on dyeing.

III. The Impact of Polyester Oligomers on Dyeing and Processing

In high-temperature, high-pressure dyeing processes using disperse dyes, polyester oligomers can cause several problems:

Deposition on equipment surfaces, affecting circulation and heat exchange efficiency.

Precipitation on fabric surfaces, causing color spots and blemishes.

Migration to fiber surfaces, affecting hand feel and luster.

Increasing the yarn friction coefficient, reducing processing performance.

Combining with disperse dyes, forming stubborn stains or color bleeding problems.

Typical Phenomenon: White powdery or waxy deposits appear on the fabric surface.

IV. Prevention and Control Methods for Polyester Oligomers

To address the issue of polyester oligomers, control can be implemented through both process and auxiliary agent aspects:

1. Alkaline Dyeing System Control

Under certain alkaline conditions, the ester bonds in oligomers can undergo hydrolysis, generating water-soluble carboxylates, thereby reducing the tendency to precipitate.

Advantages: Reduces oligomer crystallization.

Current Status: Some alkali-resistant disperse dye systems are already used in industrial production.

2. Use of Oligomer Removal Agents

High-temperature oligomer removal agents are typically composed of anionic/nonionic surfactant blends, which can:

Improve oligomer dispersibility

Inhibit crystal precipitation

Reduce deposition on equipment and fabrics

In practical applications, it can significantly improve color spot and staining problems.

3. High-Temperature Direct Drainage Process

Using a high-temperature drainage method in the later stages of dyeing can effectively reduce the adhesion of oligomers to fibers and equipment.

4. Bio-enzyme Treatment Technology

Studies have shown that cutinase can effectively degrade cyclic trimers.

Using it in conjunction with a high-efficiency dispersant can further improve removal efficiency, representing an environmentally friendly development direction.

5. Reduce the use of carrier dyeing auxiliaries

Some dyeing carriers can promote oligomer migration or deposition; therefore, their use should be minimized or replaced as much as possible during process optimization.

V. Conclusion

Polyester oligomers, especially cyclic trimers, are one of the important factors affecting the dyeing quality of polyester fibers.

By optimizing the dyeing process, selecting appropriate auxiliaries, and adopting environmentally friendly enzyme treatment technology, their negative impact on:

Dyeing uniformity

Equipment operating efficiency

Finished product appearance quality

This improves overall production stability and product quality.