Open Access ISSN: 2165-8064 Journal of Textile Science & Engineering Research Article Volume 11:3, 2021 Rare earth salts mediated improved rubbing fastness for Indigo dye Abstract Indigo dye though rich in blue colour has poor affinity for cellulose cannot penetrate too well and thus it mostly remains at the surface of the fabric after dyeing. This phenomenon is called ring dyeing. Such ring-dyed materials have poor rubbing fastness towards dry and wet test methods. Our objective in this paper has been to use rare earth (RE) salts to overcome the rubbing fastness problem. The RE metals used in this research work is Cerous sulphate, Lanthanum chloride and Yttrium chloride. Keywords: Indigo dyeing • Rare earth salts • Cerous sulphate • Lanthanum chloride • Yttrium chloride• Post mordanting Padma S Vankar* and Archana Gangwar Bombay Textile Research Association (BTRA), Mumbai *Address for Correspondence: Padma S Vankar, Head of Department at Maharashtra Institute of Technology Email: padma.vankar@gmail.com Copyright: © 2021 Vankar PS, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Received 14 February 2021; Accepted 17 March 2021; Published 24 March 2021 Introduction Indigo is a vat dye and has the following properties: It is water-insoluble; before application it is made soluble by reduction with caustic soda and sodium hydrosulphite and oxidised after application on yarn or fabric. It has poor affinity for cellulose fibres. Its colour fastness to light and washing is moderate. The colour index of Indigo dye is C.I. Vat Blue 1 and its chemical formula is C 16 H 10 O 2 . The Indigo dye is a small molecule which can exist in trans form mostly, however cis form also is present. As indigo dye has poor affinity for cotton it is important to understand the morphology of the indigo dyed fabric. It is very well documented through microscopy studies that in indigo dyeing, the penetration of the dye into the cross-section of the cotton yarn solely depends on the pH of the bath. The best colour yield is obtained in the pH range of 10.5–11.5. Monitoring the dyebath pH carefully can be an effective means of consistent and reproducible dyeing results. Depending on dyebath pH, reduced indigo can exist in three forms: i) nonionic enolic acid leuco compound, ii) monoenolate anion which is more soluble, and iii) doubly charged bisenolate anion which is less substantive. At the pH range 10.5-11.5, maximum colour is obtained in the dye bath. This colour yield correlates closely with the fractional amounts of these three species. Cis–trans isomerization around double bonds in conjugated compounds can be triggered by heat, light, or catalysts such as the addition of protons, transition metal ions, Lewis acids and others [1]. The presence of several heteroatoms with free electron pairs, often in optimum position for chelate coordination, provided another dimension for several classes of dye molecules, allowing for the modification of absorption and redox properties via metal coordination. Chelate complexes are distinguished by a metal acyclic structure, with five- and six-membered rings as the most common configurations. Dye molecules can also offer more than one coordination site for metals within extended p systems, thereby allowing dye ligand mediated metal-metal interaction such as in mixed- valent systems [2]. The indigo molecule exists in a Tran’s configuration regarding the central CC bond (Figure. 1), however, the cis configuration is a high-energy alternative. - The acidic N-H protons may be removed stepwise and replaced by one or two metals (Figure 2). Indigo forms as depicted in Figure 2 can bridge two metals, allowing for metal-metal interaction, including mixed valency. - The metal coordination for the trans conformer of indigo derivative results in six- membered chelate rings (N, O or N, O: N coordination) while the cis conformer may lead to five- and seven-membered ring chelates. Thus the proposed structures for metal chelated products with cerous (Ce), lanthanum (La) and yttrium (Y) are shown in Figure 2. Metal chelates are readily formed by these rare earth metals [3]. The stabilization of rare earth metal chelates can be rationalized on the basis of the interplay of crystal-lattice, acid-base, charge, size and redox factors. Several researchers have attempted to improve the rub fastness of indigo dyed fabrics. The fastness to crocking improvement of Indigo and CI Sulphur Black 1 dyed cellulosic fabric, whilst retain its other desirable properties [4]. Since rubbing fastness is a major issue faced by indigo dyers. We tried to experiment with rare earth salts -Cerous sulphate, Lanthanum chloride and Yttrium chloride through post mordanting. It is known that rare earth metals H N O H N O H N O N H O Cis Trans Figure 1; Cis and Trans conformers of Indigo. N O N M M O O O O O O O O O M = La , Ce , Y N O N O M M O O O O O O O O M = La , Ce , Y Figure 2: Metal chelation sites for cis and Trans conformers.