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Chemistry and Working Mechanism of Stabilizing Agents

The basic mechanism of stabilizer involves the stabilization of chemical reactions by inhibiting the reaction or slowing down the reaction. Here we will discuss the chemical nature and working mechanism of stabilizing agents.

Chemistry and Working Mechanism of Stabilizing Agents

Hydrogen Peroxide Bleach Stabilizer

Chemistry

Stabilizers must be added to the bleach solution to control the decomposition of hydrogen peroxide. Stabilizers function by providing buffering action to control the pH at the optimum level and to complex with trace metals which catalyze the degradation of the fibres. Stabilizers include sodium silicate, organic compounds and phosphates.

The addition of alcohol, glycerin or berbituric acid also stabilize hydrogen peroxide. Hydrogen peroxide also decomposes in the presence of finely divided heavy metals such as copper, iron, manganese, nickel, chromium etc. or their oxides with liberation of oxygen.

2H202 →2H20 + O2

Mechanism

Sodium silicates are the most commonly used and most effective hydrogen peroxide bleach stabilizers. They may be used as colloidal silicate (water glass), ortho silicate or metasilicate.

The mechanism by which hydrogen peroxide bleach stabilizer stabilize is not completely understood, however it is known that silicates have a natural affinity for ferrous ions and ferrous ions are naturally present in cotton. Hydrogen peroxide is a weak acid and ionizes in water to form a hydrogen ion and per hydroxyl ion which is an active bleaching species.
H2O2 + H20 → H+ + OOH-

This reaction is catalyzed by metal ions such as ferrous ions. This reaction is not desired as it is ineffective use of peroxide and causes fiber damage.
H2O2 + H20 → H2O + ½ O2

It is possible that the silicates are adsorbed onto the ferrous ions in the fiber, producing a species that catalytically enhances bleaching while reducing bleach decomposition and fiber damage. Stabilization is enhanced by the presence of magnesium ions. Magnesium serves as a pH buffer. As the concentration of OOH- rises during bleaching, magnesium hydroxide (Mg(OH)2) precipitates, reducing the OOH- concentration. Bleach solutions containing only magnesium ions have good stability but the bleaching effectiveness is not as good as when silicates are included.

Magnesium salts can be used in textile bleaching as pH buffers. It is used in hydrogen peroxide bleaching as pH buffers.

Sequestering Agents

Chemistry

The sequestering agents bind polyvalent cations such as calcium and magnesium in water and in fibers, 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, amino-carboxylic acids, organo-phosphonic 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 fibers. They may, however, hydrolyze at high temperature and lose their effectiveness.

The amino-carboxylic 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.

Mechanism

Sequestering agents work by a mechanism of complex formation, often in the form of chelation. Sequestering agents or Chelating agents remove a metal ion to from a solution system by forming a complex ion that does not have the chemical reactions of the ion that is removed.

A chelating agent contains substituents suitably located to form one or more chelate rings by electron donation to the metal ion, the resulting complex remaining soluble under the conditions of processing.

The most useful donating atoms are nitrogen, as found in amines or substituted amines, and oxygen in the form of carboxyl, phosphate or ionized hydroxyl groups.

Emulsifying Agents

Chemistry

The sodium salt of carboxymethyl cellulose (CMC) is commonly used as a stabilizer or an emulsifier. Emulsifier is used to stabilize emulsion. One type of emulsifiers, Polymeric emulsifiers, are adaptable to a variety of emulsion preparation methods. Polymeric emulsifiers are water insoluble polymers that readily anchor at the oil-water interface independent of oil type. Emulsions, being liquids, do not exhibit a static internal structure. The droplets dispersed in the liquid matrix (called the "dispersion medium") are usually assumed to be statistically distributed.

Two liquids can form different types of emulsions. As an example, oil and water can form, firstly, an oil-in-water emulsion, where the oil is the dispersed phase, and water is the dispersion medium. Secondly, they can form a water-in-oil emulsion, where water is the dispersed phase and oil is the external phase. Multiple emulsions are also possible, including a "water-in-oil-in-water" emulsion and an "oil-in-water-in-oil" emulsion. In all types of emulsions, emulsifier is responsible for stabilizing the dispersed molecules of liquid in another medium.

Mechanism

There are 3 kinds of mechanisms by which emulsifier stabilizes the emulsion:

Surface tension theory – according to this theory, emulsification takes place by reduction of interfacial tension between two phases.

Repulsion theory – the emulsifying agent creates a film over one phase that forms globules, which repel each other. This repulsive force causes them to remain suspended in the dispersion medium.

Viscosity modification – emulgents like acacia and tragacanth, which are hydrocolloids, as well as PEG (or polyethylene glycol), glycerin, and other polymers like CMC (carboxymethyl cellulose), all increase the viscosity of the medium, which helps create and maintain the suspension of globules of dispersed phase.

Muhammad Rehan Ashraf

I am a Textile Engineer, founder and editor of "Textile Trendz". Currently working in an export-oriented textile organization. I love to share my knowledge about textiles.