Observations on Properties of Partially Formylated Cottons; and Formyl Groups as Adsorption Sites for Direct Dyes

Partially formylated cottons in the form of yarn or cloth absorbed up to two or three times as much direct dye as the original cottons, when samples of both were dyed in competition in the same bath. This higher absorption is shown to be due to the formyl groups acting as dyeing sites. The dye absorption increased with combined formic acid within the range studied, up to a D. S. of 0.35. Absorption was much higher by mercerized cotton than by unmercerized material of the same percentage combined formic acid. It is suggested that formyl groups substituted in cotton are more accessible to dyes than the hydroxyl groups of the original cotton, particularly if reacted with primary hydroxyl groups where they add to the lengths of side chains. Also, the carbonyl groups of the formates may have more attraction for the dyes than the original hydroxyl groups. The results appear to support the view that bonding of a dye to a substrate is a main factor in the direct dyeing of cellulose, as distinguished from "fixing" by aggregation alone of dye molecules within the fiber. It has been shown by indirect evidence from physical tests and special dyeing tests that the increased dye absorption is not due to any unusual swelling of the partially formylated cottons. Such tests indicate that cotton swells in formic acid about the same as in water, and that partially formylated cottons do not swell more in water than the original cottons. Incidental observations on the storage of partly formylated cottons showed losses of two-thirds of their combined formic acid in 5 years, while in other materials about 1% combined formic acid was retained after 25 years. Low-formylated cottons were unstable to boiling water in simulated direct dyebaths. This instability makes difficult, if not impossible, the establishment of a quantitative correlation between dye absorption and formyl content.