Brightening of grey jute—Mechanism and application

摘要

Ferric iron in combination with phenolic groups of tannin and phenolic acid lignin imparts grey colour in jute. Treatment with EDTA removed almost all the Fe(Ⅲ), improving the brightness significantly but caused over-yellowing of fibres. Evidences presented suggest that the removal of metal ions bound with phenolic groups generated free phenolic—OH which on aerial oxidation caused the over-yellowing. Reducing action of dithionite minimized such oxidation and prevented over-yellowing. Besides, dithionite reduced Fe(Ⅲ) to soluble Fe(Ⅱ), thereby enhanced the brightening process. Thus, the combined action of EDTA and dithionite induced a high brightness and restored the natural yellow colour of jute. Brightening of jute under submerged condition proved uneconomical due to the expenses of post-treatment drying. Treatment at a low level of moisture eliminated the above. This was done by spraying brightening chemicals on fibres before piling. The prolonged piling time and inherent acidity of fibres favoured the brightening reactions. The resultant fibres were brightened appreciably and the yarn spun thereof was found to be superior in quality compared to control.