Over dyeing of Lyocell fabric with natural indigo and rhubarb

Abstract

In order to enrich the range of natural dye chromatography, this study used rhubarb to dye indigo dyed Lyocell fabric to obtain green Lyocell fabric. Firstly, the Lyocell fabric was dyed with indigo using a typical reduction method. The K/S and b* values of dyed fabrics were used as evaluation indicators to determine indigo concentration of 1.2 g/l before the over dyeing. The effects of rhubarb concentration, pH value, and dyeing temperature on the K/S value and color characteristic value of rhubarb dyed Lyocell fabric are discussed. The results showed that acidic solution, higher rhubarb concentration, and lower dyeing temperature were beneficial to obtain higher K/S and b* values for the rhubarb dyed Lyocell. Finally, these three factors were taken as independent variables in over dyeing, and the red-green value of dyed fabric was taken as response value for response surface analysis. The response surface analysis of three factors and three levels showed that the pH value of the dye solution had a significant impact on the a* value. The a* value of the dyed fabric decreased with the increase in rhubarb concentration and the decrease in the pH value and dyeing temperature. The optimization process suggested by the regression model statistical analysis was as follows: rhubarb concentration was 2.67 g/l, dye bath pH 3.5, and dyeing temperature 57°C. The verification result showed that a* value of the over dyed fabric was –10.75, which was close to the predicted value –10.98. The infrared results showed that the cellulose characteristic groups of the undyed and dyed Lyocell fabrics were almost same. The Lyocell fabric dyed with indigo and rhubarb showed a blue-green color, and 50+ ultraviolet protection factor. The washing color fastness of the over dyed Lyocell fabric was 2–3 and the rubbing fastness was above 3. This over dyeing method may enrich the chromatogram of Lyocell fabric dyed with natural dyes.