Density Increase
Physical reduction of free volume within non-crystalline polymer regions occurs when heat and pressure are applied above the glass transition temperature. This process of amorphous matrix compaction alters the density of synthetic fibers during heat setting or calendering. Laboratory tests on finished yarn often reveal this change through increases in specific gravity.
The boundary of this state is reached when the polymer chains achieve their maximum stable density for a given temperature.
Thermal Threshold
Temperature control governs the rate at which the non-crystalline domains compress. During the finishing stage, amorphous matrix compaction occurs rapidly if the fabric passes through heated rollers. High temperature increases molecular mobility to allow polymer chains to slide into closer alignment.
This change is monitored through thermal analysis.
Molecular Rearrangement
Tension applied during the processing of synthetic yarns modifies the behavior of the non-crystalline regions. When tension is high, the polymer chains align along the fiber axis instead of compressing into a dense, disordered state. Thus, amorphous matrix compaction is suppressed under high mechanical load but occurs freely during relaxed thermal treatment.
This difference determines the residual shrinkage of the fabric. It also dictates how the material behaves during subsequent dyeing operations in the mill.
Production Consequence
Reduced dye uptake represents the main industrial outcome of this dense fiber structure. Tight packing of the polymer chain network limits the rate of dye diffusion into the fiber core during bulk dyeing.
