t ext ile learner.blo gspo t .in http://textilelearner.blogspot.in/2012/02/sequestering-agents-functions-o f.html

Se q ue ste ring ag e nt

Sequestering Agents | Functions of Sequestering Agents inDyeing | Characteristics of Sequestering AgentsIntroduction: Sequestering agent is a dyeing auxiliaries which is used during dyeing f or removing hardness ofwater.Sequestering agents combine with calcium and magnesium ions and other heavy metal ions in hardwater. They f orm molecules in which the ions are held so securely (sequestered) that they can no longerreact.

The most undesirable impurit ies in Fibre, Commonsalt, Glauber salt, Caustic Soda and Soda ash arethe di- and tri-valent cations, e.g., Ca++, Mg ++Cu ++, Fe+++ etc. These ions increase hardnessof the process bath and generate iron oxides inthe bath. Calcium and Magnesium reacts with alkaliand precipitates as a sticky substance on thetextile material, which creates patchy dyeing anddiscoloration of the f ibre. The f erric oxide withcellulose and creates small pinhole on the f ibresalso damages the machinery by scale f ormation inthe nozzles and base.

To overcome these deleterious ef f ects in thescouring and bleaching bath adequate amount ofsequestrant must be used. Sequestrants preventdi-and tri-valent metal ions, e.g., Cu++, Fe +++ ,Mn ++, Ca++, Mg++ etc f rom interf ering with thechemical processing of the textile material. Itprevents catalytic damage of cellulosic f ibres inbleaching hath during hydrogen peroxide bleaching.

Thus, unwanted metal salts cause a lot of problems in processing. Now, with the f ocus on minimising costsand maximising ef f iciency, consistency and f astness are two important parameters that every dyer wouldlike to achieve f irst t ime. This reduces reprocessing costs, making him competit ive.

The dyer has to use a suitable sequestering agent in the process, wherever it is required. Selection of theright sequestering agent is very important. First and f oremost, the sequestering agent should chelateof f ending metal ions under the given condition and should f orm a stable complex, which does notdecompose over a prolonged processing period.

Now a days many other types of sequestering agents are widely used in textile processing. The three mainstages in which sequestering agents are used are

1. Pretreatment2. Bleaching3. Dyeing

There are some main type of commercial sequestering agents are:

1. Aminocarboxylic acid base products

2. Phosphates and Phosphonates3. Hydroxy carbroxylates4. Polyacrylates5. Sugar acrylates

1. Aminolycarboxylates In aminopolycarboxylates, it is assumed that one molecule of sequestering agent complexes with one ionof metal. Depending upon the pH of the medium, i e acidic, neutral or alkaline, the pref erential sequesteringorder or each product could change.

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

EDTA: Good sequestering agent f or calcium and magnesium at alkaline pH but no sequestering agents onFe3+ 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 solubilityin acidic medium. Not stable with oxidising agents.

DTPA: Good sequestering action Fe3+ under alkaline pH but complexes with alkaline earth salts are lessstable 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 pyrophosphatesPhosphonated aminopolycarboxylates such as EDTMP, DETMP, ATMP, HEDP, DTPMP

Inorganic phosphates work under specif ic conditions and work as sequestering agents by convertingtroublesome metal ions into water soluble complex by a process of ion exchange.

Phosphates of aminopolycarboxylic acids or phosphonates are derivatives of phosphorous acid and arecharacterised by a C-P bond, which has stronger hydrolytic stability than the P-O-P bond ofpolyphosphates. This type of sequestering agent has emerged as a major class of sequestering agent,since these possess more f eatures than mere chelation. These characteristics are: Threshold ef f ect, i einhibit ion of precipitation of CaCO3, CaSo4 with sub-stiochiometric quantit ies of inhibitor

1. Corrosion inhibit ion2. Resistance to hydrolysis3. Def locculation i.e. stability ef f ect on colloidal suspensions

Compared to popular amino polycarhoxylic acid based sequestering agents, these phosphonates basedsequestering agents have a high chelation ratio. Apart f rom 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 of ten have the property of preventing precipitationof bi and trivalent metal cationis in an alkaline medium. Some of the well known products in this categoryare:

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 f ound to be an ef f ective chelating agent f or iron under alkalineconditions.

4. Polyacrylates Polyacrylates are ef f ective dispersants, with mild chelation values and protective colloid properties. Thechelation values of polyacrylates have no demetallising ef f ect on metal containing dyestuf f s. They arecompletely non f oaming.

They are very suitable as dyebath conditioners, soaping agents and washing aids. Being non surf ace activeagents they are easily rinsable and thus reduce the quantity of water required f or removing their tracesf rom the substrates, unlike all surf actants. The typical chelation values of f ered by polyacrylates do notcome close to the chelation values of f ered by amino polycarboxylates or the phosphonates. This problemhas been overcome by development of sugar acrylates.

5. Sugar Acrylates Sugar acrylates have sequestering values as high as amino polycarboxylates or the phosphonates. Theyare biodegradable, ef f ective components in cellulosic f abric pretreatment during desizing, scouring,bleaching and mercerising. These products are characteristed by good chelation values f rom the acidic tothe alkaline range and f rom temperatures of 45 to 115C. They also exhibit no demetalising ef f ect onmetal-containing dyestuf f s and are non-f oaming. They are ideally recommended in pretreatment f ordesizing, 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 concentrationof the metal ions and chelating agent, which react together to f orm a chelate. The stability of the metalcomplex is expressed in terms of its stability constant. If we represent chelation of metal ion, Mm+ withsequestering 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 ef f ect. For example, in the case of aminopolycarboxylates,the stability constant f or 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 inf ormation it can be deduced that the NTA-Mg2+ complex has the least stability, whereasDTPA - Fe3+ has the highest stability. Thus, in a process, if more than one metal ion is present, the metalion having the highest stability will be chelated pref erentially. If chelating agent is present in suf f icientquantity, the metal with the highest stability constant will be chelated completely, f ollowed by the next metalion in te order given above. Even af ter chelation is complete in this order, if additional metal impurity, withmetal having a higher stability constant, is introduced, then this metal ion will displace low stability constantmetal ions f rom the complex. For example, Fe3+ displaces Ca2+ f rom a Ca2+ chelating agent complex. Ofcourse, the chelating agent should be capable of chelating Fe3+ under given conditions.

2. The pH of the Process: The pH of the system will inf luence the f ormation of the chelation complex. For example, NTA, EDTAcannot chelate Fe3+ under alkaline conditions, whereas DTPA can. HEDP can chelate Fe3+ up to pH 12, andso also gluconic acid.

3. Demetalisation: This property is particularly important f or dyeing and printing with premetallised dyes - f or example, somedirect, reactive and premetallised metal complex dyes. If Ni2+, Cu2+, Cr3+, Co2+ or Fe3+ is present inpremetallised dyes, these could be pref erentially chelated ahead of Ca2+ and Mg2+, due to the higher

stability constant of these metal ions. Theref ore pretrials in the lab are required to establish the suitabilityof the chelating agent, and also to arrive at the optimum concentration f or the given process, whenpremetallised dyes are to be used.

4. Other Features: Stability of chelate to prolonged process periods, dispersing properties, crystal-growth inhibit ion, ef f ect onequipment, etc. are also to be considered when selecting a commercial sequestering agent.

7 0 0 00 195

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