Polyester microfibres dyeing with disperse dyes

Microfibres of PET for production of fabrics with a lush handle are a fairly recent development. Microfibres have a fineness of less than 1.0 dtex per filament, an arbitrarily chosen value. Normal PET filaments are in the range 2–5 dtex. The introduction of microfibres has created a number of dyeing problems with disperse dyes. Firstly, microfibres require more disperse dye than regular denier fibres to achieve the same depth of shade. The concentration of dye (% owf) required to achieve a given depth of shade is usually assumed to be inversely proportional to the square root of the filament fineness:

Add captionPolyester microfibres dyeing with disperse dyes

In this equation, CM and CR are the required concentrations of dye in the polyester microfibre and regular denier fibre respectively, rM and rR the respective filament radii, and dtexM and dtexR the respective filament fineness. This approximate relationship predicts that a 0.5 dtex microfibre will require (2.5/0.5)1/2 or about 2.2 times as much dye in the fibre to give the same depth of shade as a 2.5 dtex filament. It only applies, however, when the regular and microfibres being considered are identical in all other properties. In fact, it is the dyeing rate that should be proportional to the available filament specific surface area (m2 g–1) and therefore inversely proportional to the filament radius and to the square root of the filament decitex. The value of the diffusion coefficient of the dye in the fibre does not change when the filaments are finer.

Secondly, because of the more rapid uptake of dye by microfibres, level dyeing requires greater control. The greater specific surface area of microfibres also means that dye desorption during washing is more rapid and the washing fastness is less than for fabrics made of conventional filaments. Dyed microfibre fabrics also have lower fastness to light at equal apparent depth. In addition, the closeness of microfibre packing in yarns makes wetting and yarn penetration difficult. Nevertheless, the lush handle and special surface effects that are possible for fabrics made of microfibres have ensured their success.