Influence of fabric structure on sodium hydroxide release from woven lyocell

Abstract

The sodium hydroxide release (alkali wash-off) from lyocell fabrics with different weave constructions (plain, twill, sateen) was studied by conductivity measurements in the wash bath. An alkali transport coefficient (K) was established to quantify alkali release, which represents the liquid-side mass transfer of alkali release after a pad-batch process into the wash bath. Three regions of NaOH-release behaviour were observed and two competitive phenomena related to swelling, increase of the substrate surface, and the substrate compactness, have been recognized. Firstly (up to 2.25 mol dm-3), the influence of fabric-alkali interactions and non-uniform access of NaOH determines alkali release, and also the absolute K value is mainly determined by the bulk densities (plain 0.4998, twill 0.4622 and sateen 0.4096 g cm-3) and porosities (plain 0.671, twill 0.696 and sateen 0.731) and thus, it increases continuously. The maximum swelling of cellulose fibers occurs between 2.25—3.00 mol dm-3 NaOH; here, maximum K is observed as well as dependency on fabric construction is still determining factor. Above 3.75 mol dm-3 NaOH, the treated substrates are highly swollen sheets and the K does not show dependency on fabric constructions, thus, the fabric construction is not relevant at this range anymore for the alkali-diffusion. Because materials consisting of the same type of fiber and with similar mass per area are currently considered to behave identically, the findings herein which contrast with the existing theory are of particular relevance in optimization of technical processing of regenerated cellulosic fabrics during alkalization.