The Dyeing of Cotton with Mineral Khaki
Authors: RACE E. , ROWE F. M. , SPEAKMAN J. B. 
Published: July 2010

For the production on cotton of a mineral khaki with bactericidal and fungicidal properties sufficient to be effective under stringent conditions of service, we believe that a further component is essential in the chromium‐iron impregnating bath used in mineral khaki dyeing.

Of the cations which are toxic towards a wide range of micro‐organisms, the most effective (e. g. silver, gold, platinum, mercury) are prohibited on economic grounds and, of those commercially possible, the cupric ion is one of the most effective. During exposure to weather conditions, however, the leaching of copper from cotton material impregnated with copper compounds is severe, whereas the leaching of chromium and iron from mineral khaki‐dyed material is considerably less. This may be due, in part, to the acidity of both atmosphere and rain, particularly in industrial areas, and an attempt has been made to improve the fastness of copper on yarn by incorporating a copper salt with the chromium and iron salts used in the impregnating bath during mineral khaki dyeing. Fastness to leaching has been tested in the laboratory by washing yarns, the pigment on which contained copper alone, copper and chromium, copper and iron, and copper, chromium and iron, in running water, pH 5‐3 to 5‐5, which was thought closely to simulate rain water.† Thus, it has been found that incorporation of a chromium salt with a copper salt in the impregnating bath, resulting, after development with sodium carbonate solution at room temperature, in a mixture of copper carbonate and hydrated chromium oxide on the fibre, does not substantially improve the fastness to washing at pH 5‐3 to 5‐5 of the copper carbonate. By the incorporation of a ferric salt with the copper salt in the impregnating bath, however, thereby producing, after development with sodium carbonate solution at room temperature, a mixture of copper carbonate and hydrated ferric oxide on the fibre, the leaching of copper from yarn washed in running water, pH 5‐3 to 5‐5, is greatly diminished, whilst, by using chromium, iron and copper in the impregnating bath, followed by similar development, the fastness to leaching of the copper is slightly improved further.

The temperature of the sodium carbonate developing bath, although having little effect on the amounts of copper, chromium and iron developed on the fibre, does affect the nature of the copper compound deposited; hydrated copper oxide rather than copper carbonate tending to be formed at development temperatures above 50°C. In the various microbiological tests carried out on materials containing copper, chromium and iron, or copper and iron, the pigments were developed with sodium carbonate solution at room temperature, and the tests therefore refer to material pigmented with copper carbonate, hydrated chromium oxide and hydrated ferric oxide.

Changes in concentration of chromium salt in an impregnating bath containing copper, chromium and iron greatly affect both the chromium content and the copper content of the pigment on the fibre. Since the effectiveness in combating attack by micro‐organisms is dependent upon the presence on the material of a minimum amount of copper, too great a chromium concentration relative to the copper concentration in the impregnating bath can result in a pigment, the copper content of which is not great enough to prevent attack of the cotton by cellulose‐decomposing bacteria. Changes in concentration of ferric salt in the impregnating bath, on the other hand, do not greatly affect either the copper content or the iron content of the yarn. Consequently, careful control of the concentration of chromium salt in an impregnating bath containing copper, chromium and iron is of greater importance than careful control of the concentratration of ferric salt in the impregnating bath.

In testing the relative fungicidal and bactericidal efficiencies of pigments containing chromium and iron, copper, chromium and iron, and copper and iron, the methods used were those which had been shown previously to give the most reliable results, viz. (i) a “contact” test in which samples of processed yarns were placed on the surface of prepared horse dung in Petri dishes incubated at 30°C. and 100% R. H.; (ii) a “contact” test in which samples of processed yarn, inoculated with prepared horse dung, were incubated with free access of air at 30°C. and 100% R. H. (a) in glass jars using the glass wick technique, and (b) on Berkefeld Filter Candles; and (iii) pure culture tests in which yarns, inoculated with a culture of the cellulose‐decomposing fungus Chætomium globosum, were incubated in the presence of a nutrient medium and with free access of air (a) in glass jars using the glass wick technique, and (b) on Berkefeld Filter Candles.

All the above tests proved that copper carbonate, even when present in relatively small quantities along with iron or with chromium and iron on the fibre, provides a toxicity towards cellulose‐decomposing fungi and a wide range of non‐cellulose‐decomposing fungi. There appears to be, however, a minimum value for the copper carbonate content of the pigment on the fibre below which toxicity towards cellulose‐decomposing bacteria is not achieved. Thus, when the material contains < 0–5% approx. copper (calc. as CuO on the weight of the fibre), although, due to the absence of visible fungal growth, there are no apparent signs of deterioration, especially in the early stages, degradation of the fibre is liable to occur by bacterial action. When the copper content of the pigment on the material is > 0–5% (calc. as CuO on the weight of the fibre), attack by cellulose‐decomposing bacteria is virtually inhibited and degradation of the cotton, even after incubation for comparatively long periods, is slight. Consequently, in order to ensure maximum protection of the fibre from attack by fungi and bacteria, it is suggested that the pigment on the material should have a copper content of not less than 0–5% (calc. as CuO on the weight of the material) and, in the case of a pigment containing copper, chromium and iron, a total oxide content of not less than 2% (calc. as oxides on the weight of the material), or, in the case of a pigment containing copper and iron, a total oxide content of not less than 1–4% (calc. as oxides on the weight of the material). The concentration of chromium salt in the impregnating liquor should not be allowed to exceod the equivalence of 4–8 g. approx. Cr2O3 per 1. (i. e. 32 g. chrome alum per 1.), since too great a concentration of chromium salt in the impregnating bath results in an appreciable decrease in the copper content of the pigment on the fibre.