Single Bath Full Bleaching of Flax Fibers Using an Activated Sodium Chlorite/Hexamethylene Tetramine System

Abstract

A novel chemical formulation for bleaching flax fibers (machine tow) in one-step process was established. The process is based on activation of sodium chlorite by hexamethylene tetramine (HMTA) in the presence of a nonionic wetting agent. The factors affecting the bleaching such as HMTA concentration, temperature, and duration of the treatment were studied to optimize the bleaching conditions. The bleached flax fibers were assessed for critical properties; namely, whiteness index, loss in fiber weight, tensile strength, carboxyl groups, and carbonyl groups. Based on the results obtained the optimum formulation for bleaching the flax fibers is consisting of: [NaClO2] = 5 g/l, [HMTA] = 0.25 g/l and [wetting agent] = 1 g/l provided that bleaching is carried out at 90°C for 3 hrs using a material-to-liquor ratio (M/L) of 1:50. For comparison purposes, different types of flax fibers; namely, grey, card, and waste fibers were bleached under the optimum bleaching conditions of the machine tow flax fibers, and their properties after bleaching were also examined. Furthermore, tentative mechanisms for the reactions involved in bleaching using NaClO2/HMTA system were suggested. It seems that, when the optimum formulation was used, HMTA activates decomposition of NaClO2 to mainly liberate nascent oxygen rather than chlorine dioxide.