Fibre structure and Dyeability

The dyeability of synthetic fibres depends, apart from the chemical nature of fibre, on the fine structure of the fibre. The dye uptake of a given fibre remarkably affected by thermal and mechanical history of fibre. The two factors drawing and heat setting greatly influence the dye uptake properties because these processes affect the crystallinity in the material.

The thermal history of the fibre is related to the heat setting or annealing. The dye uptake of PET annealed at different temperatures shows a minimum dye uptake at temperature of 200°C with aqueous dyeing method. A typical relationship between dye uptake vs heat setting temperature is shown in fig. with increase in heat setting temperature up to 180°C dye uptake decreases, between 180°C to 200°C remain some what constant and on further increase in heat setting temperature beyond 210°C the dye uptake increases sharply.

At low temperature heat setting shows low crystallinity build up due to many small crystals formations while at high temperature heat setting these large numbers of small crystals melts to form large crystals and composed of fewer large crystals together with large adjacent amorphous regions due to this the amorphous region per crystal increases. The crystal can be considered completely inaccessible to the dye molecules and dyeing takes place only in the amorphous regions. Actually the crystals are considered physical cross-links and the presence of large number of small crystals have a great cross-linking effect than fewer big ones so decrease the segmental mobility and the dye uptake. The effect of increase in the annealing temperature is made up of two opposite factors; (a) a decrease in the amorphous content and (b) an increase in accessibility due to big size crystals. The balance of these two effects gives the observed result in fig.