Chemicals and auxiliary products

The rate of fixation of dyes on nylon is controlled by the pH during steaming: the
lower the pH, the more rapid is the fixation. A wide variety of acid-producing agents
has been suggested from time to time ranging from acid-producing salts (such as
ammonium sulphate, ammonium dihydrogenphosphate and ammonium tartrate) to
organic acids of varying strength (such as acetic, glycollic, formic and citric acids,
which increase in strength in this order). Not all of these acids produce constant pH
during the steam fixation stage, however, owing to their steam volatility. 


This is especially the case when acetic or formic acid is used. Table 4.7 shows the effect of
steaming a series of prints on the pH of extracts and, from this data, the variable
conditions (depending on the steaming conditions) which are likely to result from the
use of volatile acids are evident. Such variations can lead to end-to-end or side-to-side
variations in dye yield, particularly when applying the slower-fixing dyes, the fixation
of which is pH-sensitive. 

The use of volatile acids also leads to increased corrosion
problems within the steamer. Thus the potential advantages of using the nonvolatile
acids outweigh their higher cost; the most economical are usually citric acid or the byproduct acid from nylon 6.6 manufacture (a mixture of adipic acid homologues). Even
stronger acids, such as oxalic or sulphamic acids, are used occasionally.

The necessity for maintaining a constant low pH throughout the steaming process is
illustrated in Figure 4.16, which shows the effect of pH on the fixation of four different
types of dye within the pH range 3–7. Monosulphonated equalising acid dyes are much
less sensitive to pH variations than are disulphonates, but rather surprisingly, monoand di-sulphonated metal-complex and milling acid dyes also show the effect. Thus

using an acid which gives a constant low pH enables the carpet printer to select dyes
from a wider range of products than if a weaker acid or acid-producing salt were used.
(The weaker acids and acid salts are, of course, used by the conventional textile
printer, who dries the goods and afterwards steams them for 30 min or more.)
The addition of the acids necessary to attain rapid dye fixation tends to reduce the
solubility of the dyes, and a solubilising agent may be required.