118 



H. E. SCHROEDER AND S. N. BOYD 



and are limited in number. If sites were involved a typical Langmuir isotherm 

 would result. The linearity of the experimental isotherms suggests that the dye 

 is monomolecularly dispersed in both phases. Isotherms for two dissimilar 

 dyes dyed together (Fig. 2) show complete independence of solubility in accord 

 with simple solution theory. In the case of a limited number of chemical sites, 

 there would be competition between dyes for the sites. The dyeing mechanism 

 is best described as solution in the fiber, probably only in the non-crystalline re- 

 gions. As the curves show, ethylene terephthalate is an excellent solvent capable 

 of dissolving substantial amounts of many non-polar dyes. The heat of dyeing 

 of ethylene terephthalate calculated from isotherms at various temperatures is 

 large, —14.7 Kcal, indicating non-ideal solution and interaction between dyes 

 and liber, possibly by hydrogen bonding. A very similar picture is observed for 

 cellulose acetate (Fig. 3). 



For these fibers the equilibrium constant and the rate of dyeing are profoundly 

 influenced by structural variations in size, shape and polarity of the dyes, as 

 shown for polyethylene terephthalate in Fig. 4 and 5. These indicate that prac- 

 tical dyes must be of restricted size and shape and possess very low water solu- 



25 



20 



^«» 

 o 



"♦- 

 o 



T 



DISTRIBUTION OF DYE BETWEEN 

 CELLULOSE ACETATE AND H pO 



SATURATION^ QK^SO^hQ 



.0025 



.005 

 Cf^MG/ML 



Fig. 3 



.0075 



bility. The amount and rate of dye pick up are a function of structure of both 

 dye and fiber. They are also certainly greatly influenced by the degree of in- 

 ternal order or crystallinity of the libers. Dyeing is believed to occur in the non- 

 crystalline regions and is promoted by agents which decrease internal order 

 even temporarily, such as heat or soluble carriers like benzoic acid (Fig. 6). 



