Talk about the thermal stress of polyester filament POY

In the quality control system for polyester filament, there’s a fickle factor that constantly influences product performance: thermal stress. For pre-oriented yarn (POY), thermal stress is not only a key indicator during the production process but also directly impacts the smoothness of subsequent textile processing and the quality of the final fabric. Today, we’ll explore the history of POY thermal stress, its effects, and the factors that subtly influence it.

Thermal Stress: POY’s “Intrinsic Tension”

First, we need to understand what thermal stress actually is.

Simply put, during the spinning process, POY undergoes high-temperature melting, high-speed stretching, and cooling, forcing its molecular chains into alignment. However, this alignment is unstable, creating a latent internal stress that seeks to return to its natural state. This is thermal stress. Invisible and intangible, it acts like an invisible spring, impacting POY’s subsequent performance.

So, what are its specific effects?

1. Determining Post-Processing Stability

POY yarn subsequently undergoes texturing (DTY) to achieve a more elastic yarn. The degree of thermal stress directly impacts the smoothness of the texturing process. If the thermal stress is too high, the yarn strands are prone to breakage, fuzzing, and even stiffness during texturing. If the thermal stress is too low, the yarn strands are insufficiently tensile, and the DTY stretchability after texturing is reduced, affecting the elasticity and bulk of the fabric.

2. Affecting the Dimensional Stability of the Finished Fabric

POY yarn with high thermal stress, when woven into fabric and subjected to high-temperature treatments such as dyeing and ironing, will experience internal stress release, leading to excessive shrinkage and deformation. Conversely, properly controlled thermal stress improves the fabric’s dimensional stability, making it less prone to wrinkling and deformation.

3. Affecting POY’s Storage Performance

POY yarn with excessive thermal stress may experience slow stress release during storage due to temperature fluctuations (such as high temperatures in summer), resulting in “natural shrinkage,” loosening the yarn cake, and even hindering subsequent unwinding.

What Factors Secretly “Control” Thermal Stress?

Polyester thermal stress is not static; it’s like a sensitive child, easily affected by various factors during production. To effectively control it, you must first understand these “under-the-hood drivers”:

1. Spinning Temperature: The “Starting Switch” for Thermal Stress

During spinning, the temperature of the molten polyester (spinning temperature) is crucial. If the temperature is too high, the molecular chains move more vigorously, leading to more chaotic alignment during cooling and lower thermal stress. If the temperature is too low, the molecular chains freeze before they have time to fully stretch, resulting in a strong sense of internal tension after cooling and higher thermal stress. Therefore, maintaining a stable spinning temperature is the first step in controlling thermal stress.

2.Cooling Conditions: A Critical Step in “Fixing”

During POY spinning, the melt stream exiting the spinneret needs to be rapidly cooled and solidified by cooling air. The “cooling intensity” (air speed, temperature, and humidity) has a significant impact here:

☆ Fast cooling air speeds and low temperatures cause the melt to cool more quickly, resulting in a more abrupt “freeze” of the molecular chains, leaving them with less time to relax and increasing thermal stress.

☆ Uneven cooling (such as unstable blower air speed) can also lead to significant variations in thermal stress within a batch of yarn, resulting in “batch variation.”

3. Spinning Speed: Stress Induced by “Stretching”

The spinning speed of POY is typically 2500-3500 m/min. High-speed stretching forces the molecular chains into proper orientation. Higher speeds increase the stretching force, tightening the molecular chains, and increasing thermal stress. However, too low a speed can lead to insufficient orientation, resulting in low thermal stress and reduced POY strength.

4. Oil Performance: The Balance Between “Lubrication” and “Stability”

During the spinning process, the yarn passes through an oiler. Oil not only reduces friction but also helps stabilize yarn tension. Improper oil concentration and application amount can cause yarn tension fluctuations during cooling and winding, indirectly affecting thermal stress uniformity. For example, too little oil application results in increased yarn friction, unstable tension, and easily fluctuating thermal stress.

5. Winding Tension: The Impact of the “Last Mile of Production”

After cooling, the yarn is wound into a bobbin. The tension during winding also “adds” to thermal stress. Excessive winding tension further tightens the yarn, resulting in excessively high thermal stress. Too little tension results in loose bobbin winding, leading to tension fluctuations during subsequent unwinding, which also affects thermal stress stability.

In short, all factors that can affect POY tension (man-machine-material-process) will also affect thermal stress.

Summary

Although “invisible,” thermal stress is the “bridge” that determines POY quality from production to application. Its magnitude directly impacts post-processing efficiency, fabric performance, and storage stability. Factors such as spinning temperature, cooling conditions, spinning speed, finishing agent, and winding tension collectively constitute the key variables in regulating thermal stress.

For technical managers at chemical fiber companies, optimizing these parameters to maintain thermal stress within a reasonable range (usually adjusted based on downstream texturing process requirements) and carefully controlling the thermal stress CV value are crucial for producing POY that is “easy to spin, easy to use, and easy to sell.” For downstream textile mills, understanding POY’s thermal stress characteristics also helps them better adapt the texturing process and reduce production losses.