'Black Magic' Bleaching

Clariant's no-rinse processTextile World Asia Special Report

Dyeing, Printing & Finishing
Textile World Asia Special Report"Black Magic"BleachingOne of the most significant innovations in dyeing, printing and finishing developed this year for the textile industry was presented by Switzerland-based Clariant Ltd. Sometimes referred to as “black magic,” the no-rinse or NR-prep process is a new exhaustion bleaching process for cellulose fibers and blends. The technology draws on a patented chemical process called Singulet Bleaching, which uses a regulation medium for bleaching with hydrogen peroxide. For textile specialists, the main objective is to obtain a good pre-treatment quality within a shorter cycle and with lower water and energy consumption.The ProcessThe black magic process is based on a little known property of hydrogen peroxide bleaching — the progressive exhaustion of the alkaline agent during the oxidation process. The H2O2 reduction (exhaustion), dependent on the length of process time, is well known; however, this can’t be said about the pH, which sinks during bleaching. This is the “magic” aspect of the process.And the magic aspect corresponds to the pH decline to the neutral value in the phase of alkali peroxide bleaching.The use of SIRRIX SB, which produces a controlled excitation of the active oxygen during hydrogen peroxide bleaching, thanks to Singulet Bleaching, is also unusual in that it is an auxiliary that is black in color.Under optimum conditions, the initial pH value for a bleaching process with a classical recipe lies between 11 and 11.5. After one hour of treatment, the pH drops by one point to values of about 10 to 10.5. As the pH is a logarithmic value, there is a 10-time reduction of the alkali quantity between the beginning and the end of the bleaching operation.On the contrary, with the black magic technology, the initial pH is only 10.5 (the process requires less alkali at the start), and the final value comes down to a value close to 7.5.This means 3 logarithmic points fewer, and hence a 1,000-time lower quantity of alkali after only 30 minutes of treatment — including the heating and cooling phases of the bleaching bath. From the above, it follows that with the classical process, the end of treatment is reached with a very high pH (from 10 to 10.5), which is brought down to about 9 by the first rinsing bath. It is therefore necessary to carry out a second wash step, and also to perform neutralization by means of an acid, in order to “adjust” the dyeing pH close to 6.5 for reactive dyes.In addition, it is necessary to add the catalase to the bath prior to dyeing to eliminate the residual peroxide traces. The entire cycle is long and involves high water consumption. The process actually entails four baths in total (including dyeing), lasting about two hours (including rinsing operations) before dyeing is started.The black magic process drastically simplifies the cycle — it requires only two baths and less than one hour of process time. This includes the intermediate emptying and filling operations, as well as the elimination of peroxide residues by the enzymatic catalase — Bactosol ARL (Anti-Radicali-Liberi). All things considered, advantages are considerable yielding a 50-percent reduction in time and water, and approximately a 40-percent saving in energy (electric and thermal) consumption.

Sirrix SB: Regulator Of Singulet BleachingIn terms of efficiency, one can count on improved hydrophilicity because of the higher saponification of waxes and natural greases (hydrophobic) of the cotton. Improved hydrophilicity automatically means a better predisposition to dyeing, with the achievement of deeper, more lively and uniform shades.The degree of whiteness of the cotton bleached by the black magic process will be high, and without any significant degradation of the cellulose polymerization degree. Referring to orbital physics, the source of this new chemistry lies in the capability of causing oxygen “to dance,” knowing its “pitches” of vibration and of excitation of its active atom, a “softening” (by means of Sirrix SB) is necessary.Sirrix SB enables the ability to maintain this special “Singulet Oxygen” at its highest energy level. This allows a quicker and more intense oxidation of the cotton natural pigments, without affecting the textile fiber. The secret is in the fact that the alkaline agent (NaOH), a powerful activator of the H2O2, which drops depending on time, allows more moderate bleaching reactions compared to the normal conditions of a conventional process. In the conventional process, the alkaline agent remains in high concentration duringthe entire treatment cycle.The most appropriate definition of Sirrix SB could be a “collapser” of pH. The higher its concentration is raised, the lower the final pH shall be achieve: a 1.5 concentration in the bleaching bath has demonstrated to be the optimum value. Given that the Sirrix SB represents the heart of the system, other treatment auxiliaries are worth mentioning. The Surfactant Sandoclean JSFThe function of this soaking/detergent agent, selected for the black magic process, is to break the interface surface tension between the textile substrate and the bath. This enhances the penetration inside the fiber by exploiting its quick soaking properties, capable of overcoming the obstacles presented by the hydrophobic natural substances present in the cotton.The Sandoclean JSF facilitates the saponification of waxes and fats, obtaining an improved final hydrophilicity. The textile material circulation in jet- or overflow-type equipment is not adversely effected, in that the product does not foam, does not contain silicone, and contributes to the prevention of irreparable tears and streaking on the fabric. Moreover, such a modern auxiliary doesn’t contain organic solvents, and also is biodegradable.Bactosol ARL:A Powerful “Peroxide-Killer”This “extremephile enzyme,” according to Clariant’s original definition, is the final result of complex biological reactions, which lead to a sophisticated bio-catalyst product.This is capable of lowering the reaction energy to the lowest level of the H2O2 demolition, enhancing the rapid transformation into water compounds and molecular oxygen. Moreover, this catalase is not an ordinary product — it is active in a large temperature range (from 20 degrees Celsius to 70 degrees Celsius).It also performs in a vast spectrum of pH (from 4 to 10). This peculiarity eliminates the need for controlled cooling of the bath at the end of the cycle, and of carrying out a precise adjustment of the pH. Consequently, in the black magic process, the socalled “extremephile” properties of Bactosol ARL are strongly appreciated.The kinetics of the H2O2 demolition is quick and in less than 10 minutes all peroxide residues are eliminated, thereby avoiding all risks in dyeing with reactive dyestuffs sensitive to trace oxidizer. Nonetheless, to ensure ideal dyeing conditions and to prevent overrapid dye migration onto the fiber, it is advisable to bring the pH value close to 6.5. In order to lower the pH from the 7.5 value present at the end of the black magic process to the 6.5 level (ideal for reactive dyeing), a special acidification agent is employed. Sirrix NE:For Effective NeutralizationThe use of this new neutralization agent eliminates the need for corrosive mineral acids, such as hydrochloric, sulfuric, phosphoric acids, or the use of volatile organic acids, such as formic and acetic acids.Because the black magic process does not go through intermediate rinsing before dyeing is carried out, the (light) neutralization agent must not interfere with dyeing. Nor should it cause the demetallization of certain dyestuffs of the Ftalocianine type. Ideal for this purpose, Sirrix NE is an acid-type complex that contains neither phosphorous nor nitrogen, not even chloride ions. It has a strong neutralization power and it is perfectly biodegradable.In the black magic process, no neutralization is actually carried out; rather, a slight correction of the pH (which is lowered from 7.5 to 6.5) to ensure a subsequent safe dyeing. For this purpose, a minimum dose of product (about 0.1 gram per liter) should be added.Black MagicAround The WorldThe black magic process has been tested around the world and it has been adopted by many specialists for its operational and qualitative advantages. Black magic has been applied to the machinery (jets/overflows/ kiers) of many manufacturers, such as Brazzoli, MCS, Sclavos, Thies, Then, Scholl, Laip, Fong’s, Gaston County, Loris Bellini and others.
Winter 2004