Effect of Impurities on Yellowing of Viscose Filaments

The quality of pot-spun viscose filament yarn (VFY) is adversely affected by yellowness due to cake-to-cake as well as inside-to-outside variations. Yellowness arises due to inefficient after treatment (AT) of cakes, which prevents effective removal of various impurities from the filaments. To identify the primary source of yellowness, viscose filaments from different AT stages were collected and analyzed to map the extent of total sulfur and metallic impurities in each of the process steps. Inductively coupled plasma-optical emission spectroscopy (ICP-OES), color spectrophotometry, and iodometric titration were used. Free sulfur content related best with the yellowness index, indicating the predominant effect of free sulfur on viscose filament yellowness. This enabled specific optimization of the AT process to reduce yellowness in viscose filaments.