© Color. Technol. , 118 (2002) 319 Web ref: 20020608 Coloration Technology Society of Dyers and Colourists Diffusion of disperse dyes into microfibres and conventional polyester fibres K H Park, M Casetta and V Koncar* GEMTEX/ENSAIT, 9 rue de l’Ermitage, BP 30329, 59056 Roubaix, Cedex 01, France Email: keun-hoo.park@ensait.fr; mathilde.casetta@ensait.fr; vladan.koncar@ensait.fr The dyeing properties of polyester microfibres are quite different from those of conventional polyester fibres. In this paper, the sorption isotherms, the diffusion coefficients and the amount and rate of dye uptake into the fibres are compared for both conventional fibres and microfibres. Shibusawa’s approximation of Hill’s equation is used to compute the diffusion coefficient, which depends on the initial dye concentration, the time and the fibre count for a fixed temperature (130 °C). The kinetic properties are analysed only under infinite bath conditions. The sorption isotherms and diffusion coefficients as functions of time for conventional polyester fibres and microfibres are compared by considering the surface area and the diffusional boundary layer influence. Introduction A microfibre is defined as a fibre or filament of linear density of less than 1 dtex. † The reduction in the filament linear density is also accompanied by an increase in the surface area per unit volume of the filament, the specific surface increasing markedly with decreasing filament linear density [1]. Compared to conventional fibres, micro- fibres exhibit several important differences, even if there is no difference in the chemical structure or morphology. These differences apply both to the properties of the final dyeing, and the dyeing process itself [2–4]. In the context of dyeing, the increase in surface area that accompanies a decrease in filament linear density serves, firstly, to increase the extent of both dye adsorption and desorption (these two effects resulting in, respectively, an increase in the rate of dyeing and often decreased wet and light fastness) and, secondly, to reduce the visual and instrumental depth of shade obtained. Disperse dyes exhibit a faster rate and greater extent of uptake on polyester microfibres than on conventional fibres; microfibres can absorb 2–3, or even 4–5, times as much disperse dye, the magnitude of this differential dye uptake depending on dye structure as well as fibre fineness and cross-sectional shape. A difference between the amor- phous zones of polyester microfibres and conventional fibres has also been invoked to explain this behaviour [3,4]. Fully penetrated dyeings are more rapidly attained on microfibres than on coarser fibres and, as a result, shorter times are required under high temperature dyeing conditions to achieve good dye penetration. The temper- ature range over which dye adsorption occurs, as well as the time required to achieve dyebath exhaustion and fully penetrated dyeings, depends on the fineness of the fibres. † The unit ‘dtex’ expresses linear density as the mass in g/10 km of fibre As a consequence of disperse dyes exhibiting a faster rate, and a greater extent, of uptake on polyester micro- fibres than on conventional fibres, the levelling behaviour of the dyes on microfibres is often poorer. To improve levelling, microfibre dyeing usually begins at a lower temp- erature than that employed for conventional fibres. Furthermore, a slower rate of increasing the temperature up to the desired level is usually used. More dyestuff is required to achieve a given visual depth of shade on microfibres, with the greater surface area of microfibre providing a greater surface reflectance. Disperse dyes display lower fastness to light on micro- fibres than on conventional polyester fibres because of the greater fibre surface area that is exposed to light. However, dyeings on microfibres do exhibit lower wet fastness than comparable dyeings on conventional fibres, especially in heavy depths. The lower wet fastness of disperse dyes on microfibres can be attributed to the greater uptake of dye within the fibre and to the greater specific surface from which dye desorption can occur. The fastness of dyeings to sublimation on microfibres are lower than on conventional fibres, this phenomenon being due to the greater uptake of dye on microfibres. Theory Most theoretical equations describing the rate of dyeing have been derived by assuming that the overall dyeing rate is determined by the dye diffusion rate within a fibre [5– 10]. Diffusion of dyes into polyester fibres can occur under both infinite and finite dyebath conditions during the dyeing procedure. In the case of infinite dyebaths, the dye concentration in the dyebath does not change during the sorption process, whereas for finite dyebaths, the dye concentration at the fibre surface continuously decreases during the sorption process until equilibrium between the dye concentration within the fibres and in the dyebath is achieved [11]. The diffusion rate of dye molecules into polyester fibres