Size Separation
Separation of solutes in a liquid phase based on their physical dimensions prevents larger compounds from penetrating dense polymer matrices or membrane pores. The principle of molecular weight exclusion determines which chemical species can diffuse into the porous structure of a fibre or pass through a filtration membrane during textile processing. This structural barrier allows selective treatment of the outer surface of a yarn or purification of textile wastewater.
It is used to separate active dyes from spent salts.
Barrier Effect
Pores within a swollen textile fibre have a specific size distribution that varies with the polymer type and temperature. Molecules with a molecular mass above a certain threshold cannot enter these micro-pores, so they remain in the dye bath or on the outer fibre boundary. This mechanism is exploited when applying specific finishing agents that are meant to sit exclusively on the fabric surface to provide soft hand or water repellency.
Keeping larger molecules on the surface avoids altering the internal structure.
Wastewater Treatment
Membrane filtration systems use this size-based selection to treat dyehouse effluent before discharge. Ultrafiltration and nanofiltration membranes are selected with specific cut-off values that allow water and salt molecules to pass while retaining larger synthetic dyestuff molecules. This separation enables the reclamation of water for subsequent dyeing cycles, which reduces overall water usage and lowers chemical waste treatment expenses.
Threshold Selection
Selecting the correct pore size is essential for efficient filtration. An incorrect threshold leads to membrane fouling.
