G. A. BAIG: Studies on dyeing wool with indigo, Tekstil 61 (1-6) 65-73 (2012.) 65 Studies on dyeing wool with indigo Gulzar A. Baig University College of Textile Engineering Multan, Punjab, Pakistan e-mail: [email protected] Received May 2, 2011 UDK 677.016.4:677.31 Original scientific paper Wool fabric was dyed with indigo through an exhaust technique at various pH values. The non-ionic form of indigo showed the highest substantivity, while the ionic ones exhibited relatively poor substantivity for wool. The wool fibres were best dyed in the appropriate acidic pH range. The chemistry of the indigo dyebath and wool fibres was taken into account to explain the ef- fects of pH on colour strength. The results suggested that hydrogen bonding and ionic interactions were the main reasons for the exhaustion of indigo dye on wool fibres. The hydrogen bonding were estabilished between the hy- droxyl groups and amide groups of the dye and the fibres, respectively. The ionic interactions worked in the low pH range between the end-amino and ionized groups of the wool fibres and indigo dye, respectively. Key words: wool, indigo, dyeing, dyebath analysis, colorimetric properties 1. Introduction % of the world wool production. The with vat dyes and studying the effect other producers are China and New of different variables, mainly pH and Although indigo is now exclusively Zealand with 18 and 11% share, re- temperature. Because of the popular- applied on cotton, historical records spectively [6]. The main applications ity and use, indigo was selected for show that about 4000 years ago it was include the apparels, carpets, felt and dyeing wool. One of our clients in- wool that had first been dyed with insulation. Wool is dyed mainly with tended to produce denim look on indigo and it was about in 1500 A.D, acid dyes; however other dyes such woollen fabrics. Very little data was when cotton was dyed with it [1, 2]. as basic, reactive and vat dyes are available about the effect of pH on Stonewashed woolen fabrics have also used though their percentage been reported in the literature [3]. the dyeability of wool with vat dyes. Wool is a fibre that is obtained from share is very less [7-9]. The applica- It was not known at what pH values, animals in the Caprinae family, main- tion of different classes of colorants could we obtain ring or deep dyeing ly sheep, although hair of other mam- clearly shows the versatile chemical and what would be the optimum dye- mals e.g. goats, llamas and rabbits nature of the fibre. Being Proteinous ing temperature conditions. Polyes- may also be considered as wool [4]. in nature, it contains acidic as well as ter-wool blends are important after Wool has several qualities that distin- basic sites in addition to sulphur-sul- Polyester-cotton ones. Previous work guish it from hair or fur: it is crimped; phur bonds and ionic salt linkages. It showed that good colour yield could it has a different texture or handle; it is very susceptible to hydrolysis; be produced on polyester and nylon is elastic; and it grows in staples [5]. therefore, most of the dyeing of wool [10, 11]. It showed that PET-Wool These properties impart greater bulk is carried out in appropriate acidic blends could be dyed with the same and heat insulation properties for medium to avoid loss in mechanical dye thereby simplifying the process. which it is known. Out of 1.3 million properties. In our case it worked very well be- tons per annum of wool, 60 % goes The present work was undertaken to cause poor-light and sublimation into apparel. Australia is the leading dye wool with Indigo. The goal was fastness of indigo on polyester had producer of wool and accounts for 25 to examine the dyeability of wool helped in achieving very nice faded- 66 G. A. BAIG: Studies on dyeing wool with indigo, Tekstil 61 (1-6) 65-73 (2012.) look effects which could be produced dithionite 10 g/L, Temperature 60 °C, 2.5. SEM analysis either by light or through chemical Time 30 min and LMR 40:1 was Surface morphology of the Wool fi- means. fixed while pH was varied from acid- bers was investigated using Scanning Wool fabric dyed with indigo exhib- ic to alkaline regions. All of the sam- Electron Microscope. Samples were ited excellent shade depths as well as ples having been dyed were subjected sputter-coated with Au in Polaron good fastness properties. It could be to cold water rinsing. The dye was coating unit, model E5100 followed dyed in the acidic as well as basic pH oxidized at ambient conditions with by loading sampler holder in the Hi- regions. However, higher pH values atmospheric oxygen. All of the sam- tachi SEM, model S-3000N. Gun-to- almost destroyed the wool fabric. ples were soaped with Sandopan sample distance was 8-9 mm and Wool, being a polypeptide, contains DTC (soaping agent, Clariant) 5 g/L voltage of 5kV was applied to acce- a large many amide groups in its at 40 °C for 10 min; air-dried and lerate electron toward the samples structure. It also contains free amine then tested for colour values. Fig.1 under high vacuum. Electron beam groups at the end of its polymeric showed the complete dyeing cycle focusing, image magnification and chains, although the number of free and the scheme of the Reduction- brightness/ contrast were adjusted to amine groups is less than the carbox- Oxidation Cycle of Indigo. take the micrographs. SmatSEM sof- ylic groups so that the fibre posses a tware was used to acquire the micro- negative charge unless in the appro- 2.3. pH Measurement graphs. priate acidic pH range. These amide pH values of the fresh as well spent and amine groups provide excellent 3. Results and discussion hydrogen bonding sites for the dye dyebaths were measured at room molecules and are considered to temperature. ΔpH (difference in pH 3.1. Analysis of dyebath pH mainly contribute to the substantivity between any two adjacent pH values) for the dye molecules. were measured and presented along Fig.2 showed pH values of the che- Dyebath pH values at the very start as with the pH values. mical bath solution, in the absence of well as at the end of dyeing were indigo, at the start of simulated dyeing in the presence of wool. A detailed monitored and the results presented. 2.4. Colour measurement Effects of varying pH on the dyebath description of dyebath behaviour is composition, colour strength and on Colour Strength (K/S) by SWL given elsewhere [10]. Fig.3 showed the structure of the fibre were dis- method [13] were measured at a spe- pH at the end of simulated dyeing. cussed. This work finds potential uses cified wavelength (λ) using the fol- The behaviour of wool dyebath at the in the hosiery industry where polyes- lowing Kubelka-Munk equation: end of dyeing is quite different from K/S= [(1.0-R )2]/2.0 R ter/wool blends are dyed with dis- λ λ other fibres [10, 11]. Here pH did not perse/acid dye system. These polyes- Where, K - Coefficient of absorption change that abruptly and this was at- of dye at λ , S - Coefficient of scat- ter/wool blends can be dyed in a sin- max tributed to hydrolysis of the peptide tering at λ R - Reflectance of the gle bath with a single dye and excel- max, λ bonds in the fibre. Hydrolysis led to specimen at λ . lent shade depth can be obtained. max the appearance of new amine end Reflection of the samples was measu- groups which absorbed the acidic 2. Experimental red in the range of 400 – 700 nm at moieties. Alkaline pH had drastic ef- intervals of 20 nm. fects on the strength of the fibre. Wool 2.1. Material and machinery Mathis Labomat IR dyeing machine 80 oC was used to dye wool samples and the 30 min pH measured before and after dyeing 5 oC /min using Henna Digital pH Meter PH- 210. Reflectance and colour strength were measured on Datacolour 500 o Spectrophotometer. Wool: 100% Wool 2 C /min yarn plain woven scoured fabric. Indi- go dye (85 %), granular form, was obtained from BASF. All other chemi- Indigo dye NaOH cals were of laboratory grade. Na2S2O4 Drain Fabric 3x rinising o 2.2. Dyeing 20 C Samples were dyed with Indigo to 0.1-2 % o.w.f shade depth. Sodium Fig.1 Dyeing process profile G. A. BAIG: Studies on dyeing wool with indigo, Tekstil 61 (1-6) 65-73 (2012.) 67 fibres were almost destroyed and lost O OH H H their integrity. The micrograph sho- N + - N 2H 2e wed that surface of fibres were peeled off and the cuticles were completely N N H H dissolved away. O HO Fig.4 showed pH of the dyebath at the - [O] OH start of dyeing process in the absence [O] - O OH of wool. The derivative of this curve H H showed two distinct peaks denoted as N N “a” and “b”. Peak “a” appeared when - N OH N alkali was increased from 0 to 0.2 g/ - H - H O O L. Sodium dithionite produced acidic Scheme I Reduction-oxidation cycle of indigo. products during decomposition even at ambient conditions (25 °C). These acidic by-products lowered down the Chemical bath pH analysis-Start dyebath pH. When alkali was increa- pH-start D?pH start sed this started neutralizing the acidic b 14 20 products and so pH started increasing 18 12 as could be seen from 0.4 upto 0.8 16 10 14 g/L.