Sequestering agents - sequesterant for hard water

Excerpt: A SEQUESTERING agent is a substance that removes ions from a solution system by forming a complex that does not have chemical reactions with the ion which is removed

What are Sequestering Agents ?

A SEQUESTERING agent is a substance that removes ions from a solution system by forming a complex that does not have chemical reactions with the ion which is removed. Sequestering agents are commonly used for removing water hardness. They form molecules in which the ions are held so securely that they can no longer react. Sequestering agents are also known as chelating agents. A chemical sequestering agent surrounds another molecule or atom and holds it "in seclusion". In this process, the chemical sequestering agent hides the molecule or atom and prevents it from interfering into chemical reactions.

Role of sequestering agents in textile wet processing

Sequestering agent is used in textile wet processing for minimizing the negative effect of calcium, magnesium, ferric ions which are present in hard water. Cotton fibre/fabric also contains varying quantities of metal traces (iron, copper), which mainly comes from fertilizers and insecticides. Also, the iron content of caustic soda may exceed to 100 mg/lit. There are many routes through which the metal traces enters in the wet processing operation.

Cotton fabric is usually bleached by treatment with hydrogen peroxide under alkaline conditions in order to achieve a satisfactory level of whiteness through oxidation of coloured impurities. The trapped iron residues in the fabric acts as catalyst in this process and causes H2O2 to undergo homolytic fission to two hydroxyl radicals (OH). This metal ions causes abnormal decomposition of H2O2. The rate of decomposition may be so high that cellulose is converted into oxy-cellulose. The oxy-cellulose thus produced becomes visible in form of tiny holes commonly addressed as iron pin-holes in the industry. Fabrics comprising pinholes results into poor mechanical properties and are rejected as waste and are the cause of major concern to textile processing organizations.

Some sequestering agents like sodium tri poly-phosphate and sodium hexa meta phosphates are specially useful in scouring as they not only sequester calcium and magnesium but also cleanse the equipment and filters from any clogging. However, they hydrolyze at higher temperatures and therefore have limited use. EDTA (Ethylene di-amine tetra acetic acid) is another sequestering agent that fixes calcium and magnesium in an alkaline medium however it cannot sequester ferric ions.

Oxalic acid treatments will remove ferric ion but it is more hazardous, as the chance of fabric tendering is high if proper precautions are not taken. The presence of metals as salts of iron, copper, zinc, manganese, tin, aluminium etc., in the dye bath also are highly undesirable as it adversely affects the tone and brightness of the colour.

Need to sequester the calcium and magnesium ions

Calcium and Magnesium ions present in the hard water reacts with natural soaps generated during the alkaline scouring to form waxy substance on the textile material, which creates patchy dyeing and discoloration of the fibre. This waxy substance also deposits on the machinery surface. This is termed as Lime soap deposits.

The characteristic properties of Sequestering agents are (i) Not all sequestering agents posses the same sequestering power & (ii) Specific metal chelating efficiency. Some sequestering agents are better for masking ferric ions whereas some are used for masking calcium ions. The capacity of sequestering agent is generally denoted by the binding capacity of calcium and the binding capacity if iron.

Looking at the overall scenario of the need of sequestering agents to overcome the deleterious effects in the scouring and bleaching bath, Sarex has developed series of sequestering agents with different chelating efficiencies.

Unique features : Saraquest-W(Conc)

  • An HEDP (1-hydroxyethane 1,1-diphosphonic acid) based sequestering agent
  • High power sequestering agent in powder form specially developed for scouring and bleaching of cotton and polyester/cellulosic blends.
  • High chelating action for calcium, magnesium and iron under alkaline conditions and at high temperature
  • Prevents precipitation of insoluble hydroxides and carbonates which are formed due to alkali used in scouring and bleaching.
  • It also chelates iron impurities preventing catalytic damage and pin-hole marks during peroxide bleaching.
  • It has neutral pH so can also be used in dyeing.

Unique features : Quest-AMP

  • An ATMP Acid based (Amino trimethylene phosphonic acid) sequestering agent for scouring, bleaching and dyeing of cellulosic and its blends.
  • Good chelating action for calcium, magnesium and iron under alkaline conditions and high temperature.
  • It can also be used for water treatment.

Unique features : Ironstrong(Conc)

  • High iron chelating capacity
  • Effectively chelates Fe+++ and hence prevents it from interfering with the chemical processing of textile material.
  • Prevents pinholes and catalytic damage of cotton in peroxide bleaching
  • Applicable in bleaching and scouring process.

Unique features : Hiquest-CA(Conc)

  • A polymeric based sequestering agent with higher calcium chelation efficiency for textile wet processing.
  • Should be applied in textile treatment bath whenever a high calcium content may have negative influence in textile processing.
  • Posses good dispersing action.

Author Details

Dr. Naresh Saraf

Sarex Chemicals A division of Saraf Chemicals Ltd., Andheri (W), India