Study On Dyeing Of Ramie Fiber In Supercritical Carbon Dioxide

Abstract: Ramie, originally produced in China, is also called China Grass. It has high tensile strength and excellent thermal conductivity when it is used as textile material.The conventional dyeing process requires a large amount of water asa dyeing medium causes the emission of waste-water with various toxic chemical additives to the environment. The supercritical fluid dyeing (SFD) is a waterless dyeing process, in which supercritical carbon dioxide (scCO₂) is used as the dyeing medium and the emission of waste-water is avoided.Recently, studies on dyeing of synthetic fiber in scCO₂ have got several successes, however, dyeing of natural fiber in scCO2 is still of some technical difficulties. Especially reports on dyeing of ramie fiber in scCO₂ are few.For the affinity between cellulose fiber and disperse dye is low, pretreatments before scCO₂ dyeing are required to modify the surface of fiber and improve the affinity. Several modification methods are attempted to before the dyeing: alkali-benzoyl chloride modification, alkali-acetic anhydride modification, alkali-br-isobutyl-acyl-bromine modification, ionic liquids adsorption, cation modification and sodium tetraphenylborate adsorption, urea moistening, cellulase hydrolysis and phthalic anhydride esterification. By improving the structure of ramie fiber and introduction of functional radicals, the modifications are aimed on increasing the infinity of ramie fiber, making the disperse dye enter into the channels of fiber easily and combine with fiber covalently. Through comparison of the dyeing result, alkali-benzoyl chloride modification was finally chosen to pretreat the ramie fiber before scCO₂ dyeing.The dyeing experiments of ramie fiber in scCO₂ are carried out at different temperature (70-130℃), pressure (12-20 MPa) and with different time (15-150 min). The dyeing result of the fiber was evaluated by color strength measurements (value of K/S). The effects of temperature, pressure and time have been investigated. The color fastness to washing and the color fastness to rubbing were tested.The morphological and structure transformations in fiber induced by pretreatment were determined by a Fourier transform infrared spectrometer (FT-IR), X-ray diffraction (XRD), scanning electro-microscopy (SEM), differential scanning calorimetry (DSC), and thermogravimetry analysis (TGA). The results show that the alkali modification changes the celluloseâ… into celluloseâ…¡, decreases the crystallinity and increases the amorphous region of fiber. The benzoyl chloride modification introduces benzoyl group into the fiber and decreases the crystallinity further. The scCO₂ dyeing also decreases the crystallinity of ramie fiber in some extent and changes the thermal behavior of fiber.The color strength of dyed ramie fiber increases with the increases of temperature and pressure, or with the increase of the density of CO₂.Dyeing of modified fiber with different dye concentration ranging from 0.5-5% was taken under 130℃and 20 MPa. Beyond the dye concentration of 2.5%, the rising trend of the color strength becomes gentle. As a result, dye concentration of 2.5% was chosen for further research. The dyeing result showed that the desired dyeing effect can be obtained in 90 min, under the pressure arranging from 18 to 20 MPa and at about 130℃. The color fastness to washing and the color fastness to rubbing is the same as that being dyed in the water.