Sequestering agents or Chelating agents remove a metal ion from a solution system by forming a complex ion that does not have the chemical reactions of the ion that is removed. Sequestering agents or Chelating agents are used to eliminate water hardness and heavy metals, such as iron and copper which can affect the scouring process.
These agents bind polyvalent cations such as calcium and magnesium in water and in fibres, thus preventing the precipitation of soaps. If polyvalent ions are present, insoluble soaps may form, settle on the fabric and produce resist spots. There are four major types of sequestering agents to choose from: inorganic polyphosphates, aminocarboxylic acids, organophosphonic acids, and hydroxycarboxylic acids. The inorganic polyphosphates such as sodium tripolyphosphate and sodium hexametaphosphate are probably the best overall in that in addition to sequestering most metals they also aid in cleansing the fibres. They may, however, hydrolyze at high temperature and loose their effectiveness.
The aminocarboxylic acid types such as ethylenediaminetetraacetic acid (EDTA) are very good in that they sequester most metal ions and are very stable under alkaline conditions. They are the most used types. The organophosphonic acid types such as ethylenediaminetetra (methylene phosphonic acid) are also very effective but comparatively expensive. Oxalates and hydroxycarboxylic acids (citrates, etc.) are excellent for sequestering iron but not effective for calcium and magnesium. In order to quickly and effectively bring the chemicals to the textile material, i.e. to improve their wettability and to ensure that the fibrous impurities will be removed as far as possible, it is necessary to add surfactants with good wetting and washing/emulsifying properties. A surfactant of optimal versatility to be used for preparation, and in particular for the scouring and bleaching processes, ought to meet the following requirements.
Uses of Sequestering Agents
The three main stages in which sequestering agents are used are