Dyeing modified polyesters with disperse dyes

Dyeing modified polyesters

A variety of modified polyester fibres are available that can be dyed with disperse dyes, and other types of dyes, at temperatures not exceeding 100 °C. The so- called non-carrier types can be dyed with disperse dyes at the boil, although very deep shades may require a small amount of carrier. The basic polymer is PET but contains a comonomer with a more flexible molecular chain such as suberic acid (1,8-octanedioic acid). The polymer has a more open molecular structure, a lower Tg and dye penetration is therefore easier. The new polytrimethylene terephthalate fibre (Corterra) also has a lower Tg than PET and can be dyed with disperse dyes in a bath at the boil under normal pressure.

Polyester modified to have anionic sites contains comonomers such as 5- sulpho-isophthalic acid. It is readily dyed with disperse dyes, and with cationic dyes. These types of modified polymer are also more easily hydrolysed. Therefore, during processing, the pH of solutions must not be excessive and the maximum pressure dyeing temperature should not be above 120 °C. Additions of Glauber’s salt to the dyebath protect basic dyeable polyester fibres from hydrolysis. Modified polyester fibres are also more sensitive to heat setting before dyeing, the maximum setting temperature being around 180 °C. Cationic dyes require some acetic acid in the dyebath and dyeing at pH of 4–5 at 100–120 °C is typical. The brightly coloured dyeings with cationic dyes have good fastness to washing and light. Combinations of regular and basic dyeable polyester can be dyed with mixtures of cationic and disperse dyes to produce two colour effects. The carpet industry is a major outlet for this type of fibre. The new polyester fibre poly(trimethylene terephthalate) produced from terephthalic acid and 1,3-propanediol, rather than the usual 1,2-ethanediol, is also initially intended for use in carpets.

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