April 27, 2008

Reaction Rates of Reactive Dyes: A Practical Dyers's Guide to Reactive Dyeing Cotton - Part-3

The rate of reaction with cellulose may be written:
Rate = Kc (D.Cl)f (Cell O)
The rate constants of reactive dye with cellulose may not be available but the rate constant with water is known and since the two are proportional, may be used as a guide to behavior. The rate constants of some procion dyes are shown in the table below. The rate of reaction of Procion “H” at 20°C is extremely low ad difficult to measure. The values shown in the table are extrapolation from the values at 50°C and 70°C.


Bi molecular reaction

 Constant 20°C1 gram mloe/c- 50°C1 gram mloe/c- 50°C
Orange 2R13.5   
Red 8B13.2  
Yellow GR11.0  
Blue 3G3.22  
Yellow 6G2.82  
Scarlet G2.06  
Orange G1.61  
Rubine HB0.181.999.99
Yellow H3B0.0540.815.06
Yellow H3G0.0340.553.62
Blue HB0.0440.462.23
Blue HGR0.0270.341.89

It will be observed that there are significant differences between the reactivity of individual dyes in each group the most reactive being roughly 10 times more reactive than the least reactive. However the difference between hot brand and cold brand is even more marked, the latter being 50 times as active as the former.

The rate constant of a chemical reaction increases with increasing temperature by between two and three times for every 10↑8C increase in temperature. Clearly then an increase in temperature of 50° (from 20 to 70°) may be expected to increase the rate 50 times. This is seen in the above table, where rate constants pf Procion H dyes at 70°C are similar to those of Procion dyes at 20°C.

Rate of reaction can be changed by varying the concentration of cellulose ions in the fibre, by changing the pH of the external bath.

If 1 unit pH of the dye bath is increased, the concentration of cellulose ions will increase tenfold. An increase of 1.7 pH units will increase the concentration 50 fold and the rate of reaction similarly. Thus Procion H dyes at pH 12.5 should react at the same rate as the Procion M dyes at pH 10.5. This proves to the case but the yield of the combined dye is relatively low.

The reason is, if the pH exceeds 12, the exhaustion of dye bath falls rapidly. Below pH 11, the concentration of cellulose ions is small compared with that of dye, at pH 11 it is roughly equal and at pH 12 it is considerably greater than that of the dye. Because of the increasing ionization the fibre acquires a large negative charge that depresses the absorption of the dye.

Thus the degree of exhaustion at pH 12 is so low that though the reaction takes place with cellulose in cold in one hour, the efficiency is low.

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