The Dyeing of Cotton with Mineral Khaki
Authors: RACE E. , ROWE F. M. 
Published: August 2010

Two series of exposure tests have been carried out on yarns proofed with chromium; chromium and iron; copper and chromium; copper and iron; copper, chromium and iron; and various copper compounds. Specially constructed frames have been used by means of which an accurate and comparatively rapid exposure test is possible; the variations in tensile strength of lengths from any one sample after exposure by this method are not greater than those of lengths from the unexposed sample.

Every fourth week throughout the 36 weeks of the first exposure test, attempts made to cultivate cellulose‐decomposing micro‐organisms from the exposed yarns have been unsuccessful, indicating that degradation of cotton, free from contamination by soil or other materials providing nutrition for micro‐organisms, during exposure in an industrial region, is caused solely by weathering agencies and not by micro‐organisms.

The results obtained have confirmed that deposition on cotton of trivalent chromium compounds (by impregnation of cotton with a soluble chromium salt, followed by development with sodium carbonate solution), in the absence of copper, imparts considerable resistance to photochemical degradation of cellulose. The simultaneous deposition of chromium and iron on cotton, as in the mineral khaki process, also retards photochemical tendering of the cellulose. When copper is deposited simultaneously with chromium, however, the protective value of the latter against tendering by weather is either greatly diminished or completely destroyed.

There is evidence that, under certain conditions at present not completely ascertained, the exposure of cotton proofed with copper compounds (e. g. copper carbonate, copper‐chromium‐iron) can result in accelerated photochemical tendering of the cotton. This accelerated tendering may occur when the cotton is initially in an alkaline condition, since when cotton proofed with copper compounds (e. g. cuprammonium hydroxide, copper naphthenate, copper carbonate, copper‐chromium, copper‐iron) is of pH 7 approx. before exposure, tendering during exposure is of the same order as that of unproofed material.

The leaching of chromium from chrome‐tinted or mineral khaki‐dyed yarn during exposure is not severe; thus, it is probable that a considerable proportion of the initial amount of chromium on cotton proofed by these processes will remain on the material throughout its service life. The loss of iron from mineral khaki‐dyed yarn during exposure is much less than that of chromium. There is, however, a serious loss of copper from yarn proofed with copper compounds during exposure in an industrial atmosphere in which the pH values of both atmosphere and rain are exceedingly low. Thus, during exposure for 12 winter weeks, when the pH values of atmosphere, rain and snow were as low as 3‐1, at least 94% of the original amount of copper was lost from yarns treated with cuprammonium hydroxide, copper carbonate, copper‐chromium and copper‐iron, and 88.3% of the original amount of copper was lost from copper naphthenate‐treated yarn. During this time, the losses in tensile strength of these copper‐treated yarns were 30% approx., and, therefore, it would appear that none of the copper treatments in use at the present time will withstand long exposure in the acidic conditions which prevail in an industrial region in this country. There is evidence, however, that the incorporation of iron with copper improves to some extent the fastness to leaching of the latter, although this increased fastness is not sufficient to prevent loss of copper from copper‐iron treated materials as great as that from cotton treated by any other copper process during exposure under severe acid conditions.

It appears that material which is to be subject to exposure in a temperate climate, free from contamination by soil or a nutrient medium highly susceptible to attack by micro‐organisms throughout its service life, will be degraded by photochemical action rather than by attack by micro‐organisms, and should, therefore, be proofed with trivalent chromium compounds or by the mineral khaki process. On the other hand, the resistance to microbiological attack of materials treated with trivalent chromium compounds or with trivalent chromium and iron, is very limited, and in those cases where the material for use in temperate climates is liable to come into contact with soil or similar media, proofing with trivalent chromium is not sufficient and the use of copper compounds becomes essential. Similarly, in tropical, and especially jungle zones, in which the chances of infection by micro‐organisms are great and conditions are optimum for growth, it is probable that, even for materials which are to be subject to clean exposure, proofing with trivalent chromium compounds is inadequate and the use of a toxic proofing agent such as copper becomes necessary. Further, as the serious leaching of copper from cotton during exposure in industrial regions is due to the excessively acid conditions which prevail, it is probable that leaching of copper during exposure in non‐industrial tropical zones, where the rain, although heavy, is reported to be approximately neutral, will be less serious, unless photochemical action in conjunction with rain is a prime factor under such conditions. Whether or not that is the case can be tested only by carrying out exposure tests in these regions.