What is Sequestering Agent? It is often necessary in the dyeing of all fibers to take action to restrict the impact of dissolved metal salts, by the addition of sequestering agent. Sequestering agents work by a mechanism of complex formation, often in the form of chelation. Sequestering agent or Chelating agent 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 are used to eliminate water hardness and heavy metals, such as iron and copper which can affect the scouring process.
Mechanism and Functions of Sequestering Agent: The purity of its water supply is considered an important issue in dyehouse. The presence of metal ions in water can adversely affect a number of operations by reacting with dyes or chemicals used in processing. For example, magnesium (Mg2+) or calcium (Ca2+) ions in hard water can form insoluble complexes with soaps, and if these complexes are deposited on textile substrates, they will create difficulties in subsequent dyeing processes.
The presence of transition metal ions such as iron (Fe2+) and copper (Cu2+) can catalyse the decomposition of hydrogen peroxide in bleaching processes, and if these ions are already adsorbed into the substrate, then localised accelerated attack may occur. Sequestering agents are compounds that are able to form soluble complexes with metal ions in what is termed a chelation reaction. For this reason, sequestering agents are also known as chelating agents. The soluble complex formed with the metal ions does not interfere with the process (such as bleaching or dyeing) and is washed out at the end of the operation with the exhaust liquors. Sequestering agents have structures that enable them to form closed rings with polyvalent metal ions by the sharing of a lone pair of electrons with them. In this way they ‘lock up’ the metal ions and prevent them from any further reaction.
Sequestering agent or chelating agent are negatively charged and are capable of forming strong ring structures (see below Fig) with the metal ions present in hard water and in pectins of cotton.
Fig: A typical chelate structure
The positively charged metal ions, particularly Fe3+ and Ca2+ are readily available for reaction with any negatively charged anion such as OH– or CO3-2 and insolubilise soap in the fiber which may disturb subsequent operation. This problem is much more acute when scouring is carried out in continuous process involving padding bath where liquor ratio is much lower than the batch process. Thus, the functions of the chelating agents in the soap and detergent formulations are for the prevention of –
Film and scum formation,
Precipitation of hard water,
Calcium and magnesium inhibition of foaming properties,
Clogging of liquid dispersions,
Haze turbidity in liquid solutions, and
Rancidity and oxidation that cause discoloration of formulation.
Types of Sequestering Agents: A great many chemicals exhibit sequestering capability but not all are of commercial value in textile processing. However, environmental awareness, in addition to commercial and technical exploitation, has resulted in considerable activity in this area, leading to a greatly expanded range of products in recent years, as well as some conflicting statements with regard to their environmental properties.
There are four major types of sequestering agents to choose from:
Aminopolycarboxylates and their analogues, e.g. hydroxyaminocarboxylates
Phosphates and phosphonates
Hydroxycarboxylates
Polyacrylic acids and derivatives.
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 fibers. 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.
Uses of Sequestering Agent:
The three main stages in which sequestering agents are used are
Pretreatment
Bleaching
Dyeing
References:
The Coloration of Wool and other Keratin Fibres Edited by David M. Lewis and John A. Rippon
Chemical Technology in the Pre-Treatment Processes of Textiles by S. R. Karmakar
Colorants and Auxiliaries (Second Edition) Volume 2 – Auxiliaries Edited by John Shore
Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.
