No abstract

A brief historical survey is given of previous work on the mechanism of adsorption of alkali by cellulose, in which three possibilities are considered: adsorption, because the hydroxyl ion has affinity for cellulose, compound formation between alkali and cellulose, and neutralisation of the alkali by the cellulose acting as a weak acid. Using the third mechanism, a theoretical treatment involving the normal concepts of dyeing thermodynamics has been set out; it successfully predicts the amount of sodium hydroxide that will be adsorbed from alkaline baths by both viscose rayon and cotton. A discussion is given of the way in which this treatment enables affinities to be calculated for adsorption of dye from aqueous alkaline dyebaths; and it is also shown how the concept of ionised cellulose hydroxyl groups, produced by neutralisation of alkali by the fibre, provides a mechanism for dyeing with reactive dyes. A brief historical survey is given of previous work on the mechanism of adsorption of alkali by cellulose, in which three possibilities are considered: adsorption, because the hydroxyl ion has affinity for cellulose, compound formation between alkali and cellulose, and neutralisation of the alkali by the cellulose acting as a weak acid. Using the third mechanism, a theoretical treatment involving the normal concepts of dyeing thermodynamics has been set out; it successfully predicts the amount of sodium hydroxide that will be adsorbed from alkaline baths by both viscose rayon and cotton. A discussion is given of the way in which this treatment enables affinities to be calculated for adsorption of dye from aqueous alkaline dyebaths; and it is also shown how the concept of ionised cellulose hydroxyl groups, produced by neutralisation of alkali by the fibre, provides a mechanism for dyeing with reactive dyes.

The colour changes produced in vat‐dyed cellulose fibres by the normal soaping aftertreatment have been examined quantitatively by spectrophotometric methods using dyed viscose film. From the increase in the orientation of the dye as shown by measurements of dichroism and from the analogous behaviour observed during the ageing of colloidal suspensions of vat dyes, it is concluded that the changes aie associated with crystallisation of the dye. The efficiency of various soaping agents has been measured and also the effect of the temperature of the treatment, and it has been shown that soap and Lissapol N at the boil are the most efficient methods of treatment of those examined. It has also been found that the effects observed on viscose film occur on normal commercial fabrics, although here the visual effect is diminished by the nature of the substrate and by the greater loss of dye which occurs in the soaping treatment owing to the greater relative surface area of fibrous materials. The light fastness of a series of vat dyes on cotton in the unsoaped and soaped states has been measured, and shows that soaping has, in general, little effect on light fastness. In those cases where changes are observed the effect may be either to increase or to decrease the light fastness according to the dye.

The various interactions that can occur in a dyebath containing nylon, an anionic dye, an anionic agent (Lissapol D) and a mildly cationic agent (Dispersol CWL) are described and discussed. A mechanism by which these agents influence the dyeing of nylon is postulated. The importance of considering the individual behaviour of anionic dyes in such a dyeing system is emphasised, since it is demonstrated that the strengths of the complexes formed between dye and cationic agent vary from dye to dye, in some instances being more stable and in others less stable than that between the anionic and the cationic agents.

The effects of random variations in all aspects of the dyeing process are considered in terms of the visual differences from target of the dyed material and the statistical chances of being within certain tolerance limits at the first attempt. This work is supported by a study of the accuracy with which dye is presented to substrate in bulk dyeing and this shows that two major causes of irreproducibility are the faulty weighing of dye and the inaccurate measurement of volume. This conclusion is based on work done with the co‐operation of four large dyehouses and there is no reason to suppose that their performance in these respects is grossly misrepresentative of the trade as a whole.

Novel experimental techniques have been used to show that in thermofix dyeing disperse dycs are transferred from cotton to polyester libres-in a blend or union fabric-solely by a vapour-phase mechanism. Other possible modes of transfer were examined, but no evidence was found to support them. The heat fastness of particulate disperse dyes on cotton was correlated with their transfer properties to polyester fibres. The various types of behaviour of disperse dyes in bulk application are explained. Functions of paddin!: assistants and, in particular, a migration inhibitor and the importance of time and temperature in thermofixation procedures arc dealt with in practical terms in Part 11 of the paper.

The more recent developments in the theoretical treatment of the dyeing of cellulosic materials with anionic dyes are described, and the usefulness of the derived equation in interpreting the effects of electrolytes and pH is illustrated.

A method is described for determining the affinities and the integral diffusion coefficients of reactive dyes in alkaline solution. This entails measuring these quantities for two inactive compounds of structure similar to that of the dye, followed by extrapolation of the values for the active dye assuming parallel behaviour. In the measurement of diffusion coefficients, an initial experiment was carried out in which the ‘cell constant’ of the film was determined from the diffusion of potassium chloride. From this constant the approximate effective dimensions, rather than the physical dimensions of the film, were calculated.