Preparation and characterisation of dope‐dyed regenerated cellulose films using 1‐butyl‐3‐methylimidazolium chloride: A comparison of indigo and direct dyes

Abstract

This study investigates the dissolution behaviour and dyeing performance of indigo and CI Direct Blue 1 in water, 1‐butyl‐3‐methylimidazolium chloride ([BMIM]Cl) and cellulose solutions and evaluates their effects on the properties of regenerated cellulose films (RCFs). Indigo exhibited limited solubility in [BMIM]Cl and cellulose solutions, whereas CI Direct Blue 1 dissolved uniformly in both. A comparative analysis of dope‐dyed and conventionally dyed RCF revealed distinct differences in dye distribution and color development. Microscopic observations confirmed that dope dyeing incorporated dyes into the amorphous regions of the RCF, whereas conventional dyeing resulted in surface‐localised coloration. Although FTIR did not detect dye‐specific peaks due to low dye content, UV–Vis spectroscopy distinguished spectral differences between dope‐dyed regenerated cellulose films containing indigo (RCF/i) and conventionally dyed films (Dyeing RCF/i), whereas films containing Direct Blue 1 prepared via dope dyeing (RCF/d) and conventional dyeing (Dyeing RCF/d) showed similar spectra. Thermal analysis indicated that indigo did not influence the thermal decomposition behaviour of RCF/i or Dyeing RCF/i, whereas CI Direct Blue 1 reduced the thermal stability of both RCF/d and Dyeing RCF/d. Mechanical testing revealed improved elongation at break for indigo‐dyed samples, correlating with differences in crystallinity. Lightfastness and dye elution tests demonstrated that dope‐dyed RCF achieved colorfastness comparable to or exceeding that of conventionally dyed RCF. These findings highlight the influence of dye composition and molecular state on dye incorporation, stability and performance in regenerated cellulose. They also demonstrate the potential of dope dyeing using [BMIM]Cl to produce materials with improved color stability.