Sequestering Agents | Functions of Sequestering Agents in Dyeing | Characteristics of Sequestering Agents

Sequestering agent is a dyeing auxiliaries which is used during dyeing for removing hardness of water.Sequestering agents combine with calcium and magnesium ions and other heavy metal ions in hard water. They form molecules in which the ions are held so securely (sequestered) that they can no longer react.

Sequestering agent
The most undesirable impurities in Fibre, Common salt, Glauber salt, Caustic Soda and Soda ash are the di- and tri-valent cations, e.g., Ca++, Mg ++ Cu ++, Fe+++ etc. These ions increase hardness of the process bath and generate iron oxides in the bath. Calcium and Magnesium reacts with alkali and precipitates as a sticky substance on the textile material, which creates patchy dyeing and discoloration of the fibre. The ferric oxide with cellulose and creates small pinhole on the fibres also damages the machinery by scale formation in the nozzles and base.

To overcome these deleterious effects in the scouring and bleaching bath adequate amount of sequestrant must be used. Sequestrants prevent di-and tri-valent metal ions, e.g., Cu++, Fe +++ , Mn ++, Ca++, Mg++ etc from interfering with the chemical processing of the textile material. It prevents catalytic damage of cellulosic fibres in bleaching hath during hydrogen peroxide bleaching.

Thus, unwanted metal salts cause a lot of problems in processing. Now, with the focus on minimising costs and maximising efficiency, consistency and fastness are two important parameters that every dyer would like to achieve first time. This reduces reprocessing costs, making him competitive.

The dyer has to use a suitable sequestering agent in the process, wherever it is required. Selection of the right sequestering agent is very important. First and foremost, the sequestering agent should chelate offending metal ions under the given condition and should form a stable complex, which does not decompose over a prolonged processing period.

Now a days many other types of sequestering agents are widely used in textile processing. The three main stages in which sequestering agents are used are
  1. Pretreatment
  2. Bleaching
  3. Dyeing
There are some main type of commercial sequestering agents are:
  1. Aminocarboxylic acid base products
  2. Phosphates and Phosphonates
  3. Hydroxy carbroxylates
  4. Polyacrylates
  5. Sugar acrylates
1. Aminolycarboxylates
In aminopolycarboxylates, it is assumed that one molecule of sequestering agent complexes with one ion of metal. Depending upon the pH of the medium, i e acidic, neutral or alkaline, the preferential sequestering order or each product could change.

Some of the characteristics of some of these sequestering agents are summarised as below:

EDTA: Good sequestering agent for calcium and magnesium at alkaline pH but no sequestering agents on Fe3+ at alkaline pH. Not stable with oxidising agents. Low solubility in acidic medium.

NTA: Sequestering of Fe3+ only at acidic pH but sequestering of Cu2+ between pH 3 to 12. Low solubility in acidic medium. Not stable with oxidising agents.

DTPA: Good sequestering action Fe3+ under alkaline pH but complexes with alkaline earth salts are less stable than EDTA. slightly more resistant to oxidising agents. Low solubility in acidic medium.

2. Phosphates and Phosphonates
These sequestering agents are divided in two broad classes:
  • Inorganic polyphosphates such as sodium hexameta phosphate (SHMP), sodium polyphosphate, sodium tripolyphosphate, sodium trimeta phosphate, sodium pyrophosphates
  • Phosphonated aminopolycarboxylates such as EDTMP, DETMP, ATMP, HEDP, DTPMP
Inorganic phosphates work under specific conditions and work as sequestering agents by converting troublesome metal ions into water soluble complex by a process of ion exchange.

Phosphates of aminopolycarboxylic acids or phosphonates are derivatives of phosphorous acid and are characterised by a C-P bond, which has stronger hydrolytic stability than the P-O-P bond of polyphosphates. This type of sequestering agent has emerged as a major class of sequestering agent, since these possess more features than mere chelation. These characteristics are: Threshold effect, i e inhibition of precipitation of CaCO3, CaSo4 with sub-stiochiometric quantities of inhibitor
  1. Corrosion inhibition
  2. Resistance to hydrolysis
  3. Deflocculation i.e. stability effect on colloidal suspensions
Compared to popular amino polycarhoxylic acid based sequestering agents, these phosphonates based sequestering agents have a high chelation ratio. Apart from better chelation value or better chelation ratio, these phosphonates also have better iron chelation than EDTA and NTA.

3. Hydroxy Carboxylic Acids
Organic compounds that have several hydroxylic groups often have the property of preventing precipitation of bi and trivalent metal cationis in an alkaline medium. Some of the well known products in this category are:

Citric acid, Tartaric acid, Gluconic acid and Oxalic acid.

These are less important sequestering agents, compared to aminocarboxylic acid or phosphonates. Gluconic acid/sodium gluconate has been found to be an effective chelating agent for iron under alkaline conditions.

4. Polyacrylates
Polyacrylates are effective dispersants, with mild chelation values and protective colloid properties. The chelation values of polyacrylates have no demetallising effect on metal containing dyestuffs. They are completely non foaming.

They are very suitable as dyebath conditioners, soaping agents and washing aids. Being non surface active agents they are easily rinsable and thus reduce the quantity of water required for removing their traces from the substrates, unlike all surfactants. The typical chelation values offered by polyacrylates do not come close to the chelation values offered by amino polycarboxylates or the phosphonates. This problem has been overcome by development of sugar acrylates.

5. Sugar Acrylates
Sugar acrylates have sequestering values as high as amino polycarboxylates or the phosphonates. They are biodegradable, effective components in cellulosic fabric pretreatment during desizing, scouring, bleaching and mercerising. These products are characteristed by good chelation values from the acidic to the alkaline range and from temperatures of 45 to 115øC. They also exhibit no demetalising effect on metal-containing dyestuffs and are non-foaming. They are ideally recommended in pretreatment for desizing, scouring and bleaching and as dyebath conditioners during the cellulosic dyeing.

Factors to be taken into consideration while selecting a sequestering agent for the process :

1. Stability Constant: 
As chelation is a reversible reaction, the equilibrium is dependent on the process pH and the concentration of the metal ions and chelating agent, which react together to form a chelate. The stability of the metal complex is expressed in terms of its stability constant. If we represent chelation of metal ion, Mm+ with sequestering agent, An- as: Mm+ + An- MA(m-n) then the stability constant is Ks = MA (m-n) (Mm+) (An-)

A high value of Ks indicates high sequestering effect. For example, in the case of aminopolycarboxylates, the stability constant for same metal iron increases in the order NTA, EDTA, DTPA.

In the case of metal ions, the stability constant increases in the order.

From the above information it can be deduced that the NTA-Mg2+ complex has the least stability, whereas DTPA - Fe3+ has the highest stability. Thus, in a process, if more than one metal ion is present, the metal ion having the highest stability will be chelated preferentially. If chelating agent is present in sufficient quantity, the metal with the highest stability constant will be chelated completely, followed by the next metal ion in te order given above. Even after chelation is complete in this order, if additional metal impurity, with metal having a higher stability constant, is introduced, then this metal ion will displace low stability constant metal ions from the complex. For example, Fe3+ displaces Ca2+ from a Ca2+ chelating agent complex. Of course, the chelating agent should be capable of chelating Fe3+ under given conditions.

2. The pH of the Process: 
The pH of the system will influence the formation of the chelation complex. For example, NTA, EDTA cannot chelate Fe3+ under alkaline conditions, whereas DTPA can. HEDP can chelate Fe3+ up to pH 12, and so also gluconic acid.

3. Demetalisation:  
This property is particularly important for dyeing and printing with premetallised dyes - for example, some direct, reactive and premetallised metal complex dyes. If Ni2+, Cu2+, Cr3+, Co2+ or Fe3+ is present in premetallised dyes, these could be preferentially chelated ahead of Ca2+ and Mg2+, due to the higher stability constant of these metal ions. Therefore pretrials in the lab are required to establish the suitability of the chelating agent, and also to arrive at the optimum concentration for the given process, when premetallised dyes are to be used.

4. Other Features:  
Stability of chelate to prolonged process periods, dispersing properties, crystal-growth inhibition, effect on equipment, etc. are also to be considered when selecting a commercial sequestering agent.


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