Restoration and prevention of yellowing

Benefit from our many years of experience in the restoration of yellowed goods and our tried and tested preventative finishes to improve UV resistance! Especially after the summer vacations, we often hear from our customers: “On my very first day at the company, our warehouse and order picking manager came to meet me with a nasty surprise. He showed me fading and yellowing on various colors, shapes and sizes of our stock collection. Denim and jeans were particularly affected.”

Storage yellowing (also often referred to as light damage) can be caused by various factors. Insufficiently neutralized/washed out bleaching agents/enzymes are an old problem with denim, but non-ionic surfactants rinsed on to improve the handle can also promote yellowing. These are often more or less hydrophilic, i.e. they attract water. Nitrogen dioxide from the ambient air dissolves in the water adsorbed in the fabric, is reduced to nitrogen monoxide in the solution and at the same time oxidizes the indigo dye to isatin. This happens above 22.3 degrees, above which temperature nitrogen dioxide becomes gaseous, below which it is present as an aerosol. In gaseous form, it penetrates the surface of the material and dissolves in the water bound there. The nitrogen monoxide produced during the process escapes again in gaseous form and is oxidized again with the atmospheric oxygen to form nitrogen dioxide and the process starts all over again. As the yellowing process increases explosively at high summer temperatures, there is much to support this hypothesis. In addition, more aggressive radicals are naturally formed in our acidic air atmosphere by the sun and temperature than at normal temperatures – just as the measurable concentration of near-earth ozone increases. For their part, these radicals also have an oxidizing, yellowing effect.

We can

• Neutralize, wash out or precipitate bleach residues from ozone, enzyme, chlorine, etc.
• Either wash out non-ionic surfactants or convert them into a non-hydrophilic form (bind)
• remove the yellowing (usually isatin) with a special wash
• an active substance that docks onto the indigo’s NH groups and prevents or at least significantly delays their future tendency to oxidize – prevention.
• if necessary, remove any bleach streaks remaining after the restoration (if the indigo has already been broken down) or completely overdye the item (especially with denim)

There are also cases of yellowing outside of denim and indigo.

• Brightening and odor in sulfur stains.
  Other, very complex relationships apply here, which are difficult to control

However, phenolic yellowing should also be mentioned, a problem that has been researched for many years and for which the chemical industry has proven remedies. There are tried and tested remedies from the chemical industry. It occurs with

• Polyamide parts, especially in the underwear sector with light-colored dyes.
  The cause can be sterically hindered phenols (e.g. BHT from packaging materials), as well as chem.
  Similar to indigo, under unfavorable conditions (temperature, in this case high humidity), these substances cause yellow organic nitro compounds.