The document discusses the yellowing of textiles due to atmospheric pollutants over the past few decades. It identifies over 20 ways that white or pale textiles can undergo this undesirable color change. Key factors that contribute to yellowing include exposure to air, NOx, auxiliaries, moisture, and packaging. Indigo-dyed textiles are also at risk of dye destruction from environmental influences like UV radiation, ozone, and nitrogen oxides.

3. YELLOWING OF TEXTILES DUE TO ATMOSPHERIC POLLUTANTS Over the past two three decades the problem of yellowing of textiles has become progressively more widespread. It would be wrong to regard yellowing as a phenomenon arising from a single cause. There are more than 20 different ways in which white or pale-coloured textiles can undergo this undesirable change in appearance.

5. The critical factors identified are: The interior of the pile is protected against detrimental influences by its relative inaccessibility. Due to the extreme ring dyeing of the warp yarn, the indigo dyestuff situated on the surface is highly accessible to noxious gases, such as NO x and O 3 . This phenomenon is, for instance, observed when looking at the folded edges of made-up jeans. In a pile of folded jeans, the folded edges are particularly affected since they are especially exposed to light & air.

6. ATTACKING AGENTS Ozone O 3 : Ozone is the strongest oxidizing agent after fluorine. In organic chemistry it serves to detect C-C double bonds. The general bleaching effect of ozone on indigo is well-known. Therefore the indigo dyestuff is also used as an indicator dyestuff in detection systems (e.g. Drager) which serve to measure the ozone concentration. Nitrogen oxides No x : Nitrogen oxides are also referred to as “nitrous gases”. Nitrogen oxides are mainly composed on nitrogen monoxide & nitrogen dioxide. They are known to be highly aggressive agents.

7. PROBLEMS CAUSED, AMONGST OTHERS, BY NOXIOUS GASES : Yellowing of optical brighteners due to nitrogen oxides / ozone. Destruction of natural rubber by ozone Changes of shade caused by nitrogen oxides Yellowing of polyurethane fibres due to nitrogen oxides & sulphur dioxide Destruction of indigo by ozone and nitrogen

8. COUNTER - MEASURES : Storage of the textiles in a controlled environment. Optimum ventilation of the store rooms (circulating air). No x is heavier than air and accumulates on the ground. Packing e.g. in polyethylene foil that does not contain any migrating, light-inhibiting stabilizers. Do not expose goods to sunlight. Protective finishing with AQUASORB-OZ & Powersil-NOZ

9. MECHANISM OF YELLOWING OF INDIGO-DYED TEXTILES DUE TO AIR POLLUTANTS, SUCH AS OZONE & NITROGEN OXIDES Ultraviolet radiation UV ozone O 3 nitrogen oxides No x . Short wavelength portion partly formed of the sunlight by UV & No x e.g. from the combustion of fossil fuels photochemical smog formation of aggressive radicals Radical attack against the double-bonded, conjugated indigo system

10. H H Indigo Formation of peroxide & other radicals destruction of the C-C double bond formation of mostly yellow degradation products are N COOH H H N O O Phenylglycine (Colourless) Isatine (Yellow) N N O O

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