Effect of a cationising agent on the conventional reactive dyeing of cotton M Javaid Mughal, a M Naeem, a Ausaf Aleem, a, * Rehana Saeed b and Kamran Ahmed a a PCSIR Laboratories Complex, University Road, Karachi 75280, Pakistan Email: m_n_pearl@yahoo.com b Department of Chemistry, University of Karachi, University Road, Karachi 75280, Pakistan Received: 14 August 2007; Accepted: 27 November 2007 Cationisation of cotton is emerging as an effective tool that may help to solve the environmental problems associated with the dyeing of cotton with reactive dyes. The efficiency of the cationising agent CA200 has been investigated and was found to be more effective when compared with the usual method for reactive dyeing of cotton. Pretreatment of the cotton fabric with the cationising agent increases the rate of dyeing compared with the existing method of reactive dyeing. The colour yields, in terms of the Kubelka–Munk values as a function of the amount of dye fixed, showed that cationisation enhances the colour strength. It also improves the washing fastness, rubbing fastness and depth of shade. The positive environmental impact of this cationisation process is significant and the cationised cotton shows a similar fabric quality as with the normal dyeing process. Introduction The effects of pH, electrolyte concentration and temperature on the fixation of reactive dyes on cotton fibre have been studied for over 50 years [1–5]. Fibre reactive dyes are known to be the best for cotton with their wide range of applications and better fastness properties. However, reactive dyeing systems require huge amounts of electrolyte and alkali to exhaust and fix the dye, respectively. These electrolytes are neither exhausted nor destroyed and hence remain in the dyebath after dyeing and only 60–65% dye utilisation is attainable, even with the use of salt in the normal dyeing system. When alkalinity is added to the bath in order to facilitate the formation of a covalent bond between the fibre and the functional groups of the reactive dye, dye utilisation is attainable, even with the use of salt in the normal dyeing system [6]. The abundance of hydroxy ions causes significant hydrolysis of the reactive dyes. Those hydrolysed dyes are called ‘dead’ dyes as they have no affinity towards cotton and therefore remain in the dyebath; deposition of this on the fibre significantly lowers the fastness properties, and requires a large amount of washing off. A higher amount of electrolyte concentration in the effluent causes effects such as: – the impairment of the delicate biochemistry of aquatic organisms; – a destructive attack on concrete pipes if sodium sulphate is used as the electrolyte as a result of the formation of alumino–sulphato complexes, which swell and crack concrete with a considerable aluminum content; – evolution of hydrogen sulphide gas under anaerobic conditions, when sodium sulphate is used as the electrolyte; – dissolution of such sulphides and subsequent bacterial oxidation to harmful sulphuric acid [7–10]. Cationisation of cotton to improve its dyeability with reactive dyes is therefore very important [11–13]. It has been found that cationisation before dyeing can offer an effective method of improving dye fibre affinity by avoiding the need for salt in the dyebath. The aim of this study was to investigate whether the dyeing behaviour of typical commercial reactive dyes could be improved by modifying cotton fabric with the cationic agent CA200. The functionality and reactivity of the cationic agent were compared with conventional dyeing methods. Using this treatment, the dyeing is electrolyte free, thus reducing the cost of dyeing and improving the fastness properties. This study also involved the prediction of certain pollution parameters; for example, chemical oxygen demand (COD), biochemical oxygen demand (BOD) and total dissolved solids (TDS) of the reactive dyeing effluent after cationisation. Experimental Fabric A desized, scoured and bleached fabric was used for this study; 100% grey cotton fabric, twill weave with a weight of 238 g ⁄ m 2 and a density of 46 threads ⁄ cm in the warp and 20 threads ⁄ cm in weft directions. Dyes and chemicals Commercial samples of vinyl sulphone base reactive dyes were used in this study, i.e. Remazol Blue R and Remazol Red RB (Dystar, Pakistan). Cationic agent CA200, [(CH 3 O) 3 Si(CH 2 ) 3 (CH 3 ) 2 –N–C 18 H 39 ] + Cl ) , was obtained from Penta Chemicals (Pakistan). An enzymatic desizer JRL-40, wetting agent Sandozin MRN, which is a polyglycol ether derivative, and scouring agent Sandozin NI extra, which contains alkylene oxide and silicate free stabiliser contavan gel, were purchased from the local market. Sodium hydroxide, hydrogen peroxide 50% w ⁄ w, doi: 10.1111/j.1478-4408.2007.00122.x 62 ª 2008 The Authors. Journal compilation ª 2008 Society of Dyers and Colourists, Color. Technol., 124, 62–65