120 



H. E. SCHROEDER AND S. N. BOYD 



'DACRON" DYEING RATE 



2I2»F. 

 -t- CARRIER 



2I2»F. 



MINUTES 



Fig. 6 



These considerations of solution mechanism appear to apply in general to 

 polymers of the relatively non-polar type. Coloration occurs by a reversible 

 process involving solution of the dye in the iiber at no specific sites. 



In the case of the hydrophilic polymers such as the various forms of cellu- 

 lose, forces of intermolecular association appear to play a dominant role, al- 

 though the dyeing mechanisms involved are not clearly defined. The signifi- 

 cance of structural coincidence probably reaches its maximum here. Cellulose 

 is a highly ordered hydrophilic molecule which absorbs dyes very rapidly from 

 dilute aqueous solutions of appropriate anions and cations. The dyeing process 

 appears to involve association, probably hydrogen bonding, between polar dye 

 molecules and polar cellulose. The water soluble direct dyes which show affinity 

 for cellulose have groups capable of associating through hydrogen bond forma- 

 tion. In a few cases in which precise measurements have been made the free 

 energy involved approximates that involved in formation of two, three or more 

 hydrogen bonds; i.e., 5-10 Kcal (Fig. 7). 



Characteristics that these direct dyes possess in general are relatively great 

 molecular length, a large ratio of mass to charge and great molecular size (sur- 

 face area). It is particularly important that the direct dye be not too soluble in 

 water if it is to be strongly affinitive, since this displaces eciuilibrium in favor 

 of the water. These are all features favoring apparent laying down of long flat 

 dye molecules on the long flat cellulose chains. Their effect on affinity is por- 

 trayed in Figs. 8, 9, 10 and 11. In Fig. 11 the methoxyl groups of the second 

 molecule force the biphenyl nuclei out of a co-planar situation and destroy 

 affinity of the dye for cellulose. 



