Structural Densification
Shrinkage of the sub-microscopic void spaces between polymer chains occurs when a wet-spun fiber is dried or cooled below its glass transition temperature. This free volume collapse locks the polymer molecules into a denser, less permeable arrangement, which directly affects the physical properties of the filament. During the early stages of spinning, the swollen fiber holds a large volume of empty space, which shrinks as the liquid phase is removed.
Thermal Transition
Heat setting and drying steps must be carefully managed to prevent this structural contraction from occurring too rapidly. Once free volume collapse happens, the fiber becomes more rigid and less flexible, which can lead to brittle filaments if the cooling process is not controlled.
Fiber Aging
Longterm storage of synthetic fibers can lead to gradual, spontaneous compaction of the amorphous regions. This slow free volume collapse over weeks or months increases the density of the yarn, which might alter its mechanical behavior during subsequent knitting or fabric formation.
Dyeability Effect
Dye molecules penetrate denser polymer structures much more slowly because the microscopic pathways are restricted. By avoiding excessive free volume collapse during yarn drying, mills ensure that the fibers remain open and receptive to dye liquors, preventing uneven coloring and poor dye uptake. Lower temperatures during drying preserve some open structure, which improves subsequent exhaust dyeing efficiency.
