Production of alizarin extracts from Rubia tinctorum and assessment of their dyeing properties

In this work several experiments were performed in 50 cm 3 shaken-tubes, thus allowing methanol to be selected as the most appropriate leaching solvent for alizarin from roots of common madder (Rubia tinctorum). Methanol at 25 degrees C was found to be able to extract not only free alizarin but also its glycosidic forms, thus resulting in an overall alizarin extraction yield of 2.9 +/- 0.1 gkg(-1) of dried material when leaching madder root particles with 100 dm(3) of methanolkg(-1). Further extraction tests using a liquid-solid ratio of 40 dm(3) kg(-1) in a 1-dm(3) stirred extractor allowed the production of a methanolic extract, which was then dried under vacuum. The solid residues were re-dissolved in ethanol so as to avoid methanol vapours exhaling from dyeing baths. Dyed standard specimens of raw cotton and wool exhibited almost the same reddish-yellow hue, even if those coloured with the ethanolic extract had a lighter colour intensity and a more pinkish shade than those dyed with Rubia root particles. Whatever the dyeing procedure used, the colour intensity or hue of cotton specimens was found to be brighter or more pinkish than the wool ones. These characteristics were also more evident for the cotton specimens dyed with the ethanolic extract. The fastness properties of dyed cotton and wool specimens were evaluated and it was found that all the dyed specimens were not or just slightly affected by manual washing at 40 degrees C, acid or basic perspiration tests, and it was also found that the resistance to fading of dyed wool specimens was generally greater than that of cotton ones. (C) 2007 Elsevier B.V. All rights reserved.