The Effect of Dye Structure on Affinity 

 for Fibers 



H. E. Schroeder and S. N. Boyd 



E. I. duPont de Nemours & Co., Wilmington, Del. 



DYEING PROCESSES HAVE BEEN FOUND to involve Straightforward physi- 

 cal chemical principles in all of the cases which have been studied 

 carefully. I plan to discuss only the chemistry of systems essentially 

 at equilibrium. Dyeing kinetics are diffusion controlled and too complicated for 

 treatment in a short period of time. 



There is a wide variation in the chemical features of specific systems since 

 the characteristics are those of a particular dye structure responding to a par- 

 ticular environment. In general, dyeing may be regarded as a competition be- 

 tween water and fiber for dye. Therefore, as the solubility of dye in water be- 

 comes less and the solubility in the fiber more, there is greater absorption of dye 

 by fiber. Conversely, relatively greater solubility of dye in water favors desorp- 

 tion. The physical-chemical question concerns the forces capable of favoring 

 the energy and the entropy changes involved and resulting in a concentration 

 of dye inside the polymer structure. 



The problem can be broken down into simple elements: first, the primary 

 processes that attract individual dye molecules into the polymer; next, the 

 more subtle forces that result in marked superiority of one dye structure to 

 another, despite their superficial equivalence. A good deal can be learned about 

 both of these by considering the isolated physical-chemical systems at equilib- 

 rium, on the basis of the particular dye structures which show affinity for or 

 react with different polymers. 



Fibers vary from essentially hydrophobic types like ethylene terephthalate 

 and nylon to hydrophilic materials like cotton. Also they may be relatively non- 

 polar or may contain a great number of ionizable groups. Since a similar varia- 

 tion is possible in dye structures, practically all the normal chemical forces can 

 be involved, ranging from those occurring in simple intermixing of non-polar 

 molecules through intermolecular association even to formation of ion pairs. 



In the simplest case, a fiber devoid of ionizable groups, like cellulose acetate 

 or polyethylene terephthalate, is dyed only by water insoluble molecules lacking 

 ionic groups — compounds such as: 1 ,4-diamino-anthraquinone or aminoazo- 

 benzene derivatives — while ionic dyes highly soluble in water will not even 

 stain these fibers. With dyes of the former type the fibers become colored rap- 

 idly when heated in the presence of an aqueous dispersion of the dye. 



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