The molecular changes of POY during the texturizing (draw-texturing, DTY) process
Today we will follow the industry chain and talk about the molecular changes of POY during the texturizing (draw-texturing, DTY) process.
The molecular-level changes of POY during texturizing (DTY) are a dynamic, multi-stage process. The core mechanism is that molecular chains, under external forces (stretching, twisting) and thermal energy, gradually reorganize from a metastable state into a more stable, ordered form that possesses permanent crimp. Below is a summary of the key molecular changes at each processing step:
Ⅰ、Starting molecular state of POY feedstock at the POY input stage: freshly spun POY is in a metastable state.
1. Structural inhomogeneity: there is a “higher at the surface, lower at the core” nonuniformity in molecular orientation — surface molecular chains have higher orientation due to faster cooling and stronger shear, while the core has lower orientation.
2. High internal stress: after the molecular chains have been forcibly stretched they adopt a “straightened–tensioned” conformation, storing a large amount of orientation stress and volumetric contraction stress.
1. Imperfect crystallization: the degree of crystallinity is very low, mostly consisting of amorphous quasi-crystals or microcrystals; chains in the amorphous regions are highly entangled, and there are few stable crosslinking points.
II.、Drafting and false-twist (core forming steps)
These occur in the first heating chamber and the downstream false-twist zone and constitute the key stage for intense molecular rearrangement.
Heating and drawing (first roll → first heating chamber → second roll)
① Thermal activation: in a heated environment above the glass transition temperature (Tg) (for polyester roughly 160–220 °C), segmental motion of the polymer chains is activated.
② Chain rearrangement under drawing
Orientation and disentanglement: under the drawing action at the second roll, the tense, entangled chains slip, extend and align in the direction of the applied force (fiber axis), increasing the proportion of straightened chains and significantly raising molecular orientation.
Stress-induced crystallization: the tensile stress supplies energy and a driving force for ordered arrangement of chains along the fiber axis, promoting the formation of microcrystals and a marked increase in crystallinity. Some literature indicates this stage is the start of POY’s transition from low to higher crystallinity.
③ Tension relaxation and internal stress release: heating renders the chains more flexible so that part of the internal stresses stored during spinning (orientation stress, volumetric stress) can be released and relaxed.
False-twist (false-twister as the core region)
① Twist and shear: the false-twister applies rotational shear to the yarn, forcing polymer chains in a thermoplastic state to undergo torsion and winding.
② Conformation fixation and crimp formation: under the twist imposed by the false-twister, the already drawn and heated chains are temporarily shaped into specific conformations (e.g., helical), forming the embryonic crimp. This part is less often detailed in literature, but from a process perspective the false-twist is the direct cause of imparting crimp morphology.
III、Heat-setting and post-treatment (occurring in the second/setting heating chamber and afterward)
The aim is to stabilize the newly formed structure.
Heat-setting (second heating chamber)
① Relaxation and permanentization of molecular structure: after untwisting from the false-twister, the twist is removed, but by moderate heating in the second heating chamber chains undergo controlled relaxation motions without mechanical constraints.
② Elimination of temporary torsional stresses: residual torsional stresses introduced during false-twisting are relieved.
③ Promotion of crystal perfection and recrystallization: energy is supplied to allow microcrystals to grow and homogenize in size or to undergo recrystallization; crystallinity continues to increase and the crystalline lattice becomes more perfect and stable.
④ Structure locking: the new crimped structure formed by drawing and twisting is permanently fixed by newly created crystallization points and intermolecular forces (e.g., hydrogen bonding, van der Waals forces), thereby giving DTY its stable crimp and elastic recovery.
⑤ Reduced thermal shrinkage and dimensional stability: by improving the crystalline regions and increasing physical crosslinking between chains, future chain slippage upon heating is limited, reducing boiling-water shrinkage and improving finished-product dimensional stability.
Oiling and winding
① Reduction of frictional damage: lubricants form a protective film on the surface, reducing chain breakage (damage) caused by shear forces when the yarn contacts metal parts such as guides and rolls, and preventing excessive degradation and generation of “fuzz” or powder (oligomers, oil, abraded fibers).
② Surface state modification: lubricant wetting and adsorption on the fiber surface can slightly alter the arrangement or relaxation state of surface chains, but does not affect the internal bulk structure.
IV、 Summary: The macroscopic manifestations of molecular changes throughout the POY-to-DTY texturizing process are mainly reflected at the molecular level in:
1. Orientation: from non-uniform to more uniform, increased and stabilized.
1. Crystallinity: from very low and disordered to moderate, uniform, and more perfected (generally increasing, though excessive processing or certain materials can cause a decrease due to damage).
2. Entanglement and internal stress: from highly entangled and high internal stress to moderately disentangled with stresses largely released.
3. Morphology: from a straightened, tensioned conformation to a stable helical/crimped conformation that is locked in by crystallization.
Ultimately, these molecular-level changes determine the macroscopic properties of DTY: tensile strength generally increases (due to higher orientation/crystallinity); elongation at break is greatly reduced (orientation/crystallinity); boiling-water shrinkage decreases significantly (due to setting and crystallization locking); and elastic recovery is markedly improved (due to stabilized crimp structure).
It is important to note that initial molecular inhomogeneities in POY (in orientation and crystallinity) can be amplified during texturizing, becoming the root cause of uneven dyeing and variability in physical properties of DTY. Therefore, supplying high-quality POY with uniform molecular structure is a fundamental prerequisite for achieving high-quality DTY processing.
