The Structural Causes of the Dyeing Variations of Nylon Yarns Subjected to Dry Heat

When nylon 6 and nylon 6.6 yarns are subjected to dry heat in a texturing machine, the subsequent rate of dyeing with Durazol Blue 2R at first falls and then increases with increasing temperature of heat treatment. It becomes greater than that of the unheated yarn for samples that have been preheated to temperatures near the melting point of the nylons. It has been shown that Fick's law is valid for nylon 6.6 dyed with Durazol Blue 2R in an infinite dyebath. This does not hold for nylon 6, so that, in consequence, for characterising the dyeing process for both nylons the rate of dyeing is a better parameter than the diffusion coefficient. Measurements of density, birefringence, and degree of lateral order by an X‐ray method show that these are all increased by the heat treatments. The moisture regain is, however, decreased, but little change is produced in the amine end‐group content and X‐ray orientation in the samples. An explanation of the dyeing results in relation to the physical and chemical changes is advanced. A morphological structure is envisaged of long ribbon‐like units held together by binding material that allows an increase in the relative size of voids, or the total void content, on heating the nylons above a certain temperature, this temperature depending on the particular nylon.