© Color. Technol. , 117 (2001) 193 Web ref: 20010402 Coloration Technology Society of Dyers and Colourists Photofading of reactive dyes on silk and cotton: effect of dye–fibre interactions Deepali Rastogi, a, * Kushal Sen b and Mohan Gulrajani b a Department of Textiles and Clothing, Lady Irwin College, Sikandra Road, New Delhi 110001, India Email: rastogideepali@hotmail.com b Department of Textile Technology, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India Fading characteristics of selected bifunctional and monofunctional reactive dyes have been studied on silk and cotton. Dyes were applied in the reactive and hydrolysed forms in order to study the influence of covalent bonding on the photofading of reactive dyes. The active dye–fibre interaction was found to influence the light fastness of dyes. Reactive dyes forming covalent bonds exhibited better fastness especially on cotton. Only a slight difference in light fastness of fixed and unfixed dyes was observed in the case of silk. The bifunctional reactive dyes, due to the formation of crosslinks, maintain the integrity of the fibre particularly for short periods of light exposure. Introduction On exposure to light, the possible photochemical reactions that a dye molecule may undergo are varied and complex [1–3]. The fading reaction sequence for any dye on exposure to light is shown in Scheme 1. Initially an excited state of the dye is formed which then undergoes reaction with a neighbouring molecule (X) or radical or, in some cases, a radical derived from a molecule of the substrate [3,4]. Dye + h Dye* Dye* + X Breakdown products Scheme 1 A wide variety of influences affect the stability of dyes to light, viz. temperature, humidity, physical and chemical nature of the dye, substrate, concentration of the dye, etc [1–6]. Reactive dyes deserve a special mention in this regard owing to their unique property of forming covalent bonds with the fibre. The effect, if any, of such strong dye– fibre association on the light fastness has been a subject of interest, as well as contradictions, amongst researchers. It has been suggested that the covalent bond formed between a dye and the fibre facilitates the transfer of energy from the excited state of the dye to the fibre, thereby reducing the rate of photodegradation of the dye [7–11]. For instance, Daruwalla and Rastogi found that the covalently bonded reactive dye shows a substantially higher light fastness compared to the hydrolysed unreactive dye on cotton [7]. Independent studies on a range of Procion [8,10,12] and Remazol dyes on cotton and cellophane film [13] gave similar results. On the other hand, Krichevskii and Vachobov observed that the dye–substrate bond does not affect the light fastness [14]. In fact, the light fastness of unreactive absorbed dye on cellophane film was found to be somewhat superior to that of the reacted dye. Similar observations were made by Giles et al. [15] and Shah and Srinivasan [16]. Studies on wool fibre dyed with reactive and hydrolysed Remazol and Lanasol dyes also did not show any difference in light fastness attributable to the dye–fibre bond [17,18]. Furthermore, Daruwalla and Rastogi observed that for equivalent duration of exposure, breakdown of polymer chains in the fibre was greater in the case of dyed cotton containing reacted dye than in the case of cotton containing unreacted hydrolysed dye [7]. This further substantiated the probability of the transfer of energy taking place from the excited dye to the fibre. On the contrary, Rusznak et al. [19] and Baumann [20] have reported that reactive dyes protect wool against photodegradation. In the case of silk too, the presence of reactive dyes has been found to inhibit its photochemical degradation [21,22]. The stabilising effect of the dyes depends on their structure, for instance whether the dye is monofunctional or bifunctional. The efficiency of dyes as light stabilisers has been ascribed to the deactivation of excited electronic states. Thus, it can be seen that the studies on light fading of reactive dyes show markedly contradictory results. Also, no significant work has apparently been carried out on the relative light fading of reactive and hydrolysed reactive dyes on silk. Most of the work has been carried out on wool or gelatine films and the results have been accepted as such for other protein fibres. However, it is a well-known fact that the physical nature of the substrate may have a significant influence on their dyeing and fastness properties. In the case of bifunctional reactive dyes, the presence of two reactive groups results in the formation of crosslinks in the fibre which are reported to impart a fibre protective effect [23,24]. It is, therefore, of interest to study the effect of bifunctional reactive dyes on light fastness properties