Striping & Discharge Printing of Reactive Dyes

Striping & discharge printing of reactive dyes Page: 1 Compiled by: M. Rezaul Karim Tutul

Striping & discharge printing of reactive dyes There are two different ways to attack a dye, chemically, to remove its colour. One is by oxidation, in which electrons are removed, while other is by reduction, in which electrons are added. Here, each of discharge chemicals that are used by hand dyers to discharge dye. Please note that household bleach is a toxic chemical. All discharge agents should be used with appropriate safety precautions.

1) Oxidative discharge: a) Chlorine bleach (NaOCL, Sodium hypo-chlorite)

The most familiar discharge agent is ordinary household bleach, whose active ingredient is sodium hypochlorite. It removes or change colour of many dyes, but not all dyes can be bleached. Some dyes resist changing colour no matter what we do to them.

How to use chlorine bleach It is impossible to predict how susceptible to bleach colour in commercially-dyed fabric may be. Some dyes lose their colours almost instantly even with diluted bleach, some take longer or require more concentrated bleach solutions, & some never change colour at all.

Ordinary household bleach can be diluted to 5% hypochlorite, 60ml in one litre of water, or one cup in a gallon. Before you begin, prepare a basin (or washing machine) containing your Anti-chlor so that you are ready to use it immediately, or set out your bottle of 3% H2O2 ready to hand if that is what you are going to use. Dip fabric in bleach solution, or paint solution onto fabric, or spray it on (take adequate precautions to avoid exposure to bleach mist or damage to furnishings). Watch to see when changes colour. Once colour is lightened sufficiently, immediately plunge fabric into clean rinse water or rinse under faucet, wearing rubber gloves, squeeze it out, then immerse it in anti-chloro solution or pour 3% H2O2 over until it is thoroughly saturated. After allowing 15mins for Anti-chlor to react with residual bleach, wash it thoroughly.

Use thickened bleach. For printing or painting with bleach, thickened bleach can be used so that it does not run & creep on fabric. The most convenient method, for small projects, is to use a Clorox Bleach Pen, which contains a thickened formula of hypo-chlorite bleach. These pens are extremely convenient to use, & limit exposure to bleach fumes considerably, although they are expensive compared to liquid hypochlorite bleach. They also have a limited shelf life after opening; it is recommended to check every after six months to make sure whether bleach pen still works, before embarking on another project, as it might be needed to buy a new one.

Dishwashing machine detergent gel with bleach is said to be an ideal substance to use for discharge printing, as it is already thickened. To thicken liquid bleach, do not use ordinary dye thickeners such as alginate, as they are broken down quickly by hypochlorite. Instead, a thickener can be used that is sold specifically for this purpose. Monagum is a modified starch gum that is only thickener for discharge printing with hypochlorite bleach that stays thick for up to one day, rather than breaking down & becoming thin quite soon after mixing with bleach.

Avoid chlorine bleach on delicate or synthetic fibres


Chlorine bleach is extremely damaging to both synthetic fibres, such as nylon or polyester, & to animal fibres such as wool or silk. Never use chlorine bleach on any fibre that is not 100% cellulose, such as cotton, linen, or hemp. Chlorine bleach causes a permanent yellowing of polyester fibre.

Neutralize chlorine bleach after use! Even after chlorine bleach has been rinsed from a garment, either chlorine itself, or perhaps free radicals produced by chlorine's reaction with fabric, continue to eat away at your fibre. You should use a good bleach stopping chemical on garment after bleaching. The most economical choice is Anti-Chlor (sodium meta-bi-sulfite); other good choices are Bleach Stop (sodium thio-sulfate) or H2O2. Do not use vinegar to neutralize bleach, as reaction of acid with hypochlorite produces dangerous chlorine gas.

Take appropriate safety precautions Chlorine bleach is much more dangerous than dyes we use, & more dangerous than reductive discharge chemicals (see below). Wear gloves so that you do not get bleach on your hands; not only is it damaging to skin, but it can also be absorbed, causing a poisoning of whole body under extreme conditions. Do not work with chlorine bleach without respiratory protection. Work outside &/or use an acid gas respirator. A dust mask will not provide any protection.

b) Peroxide Hydrogen peroxide is used to whiten wool, but it does not remove colour of most dyes. Peroxides are basis for chlorine-free "colour safe" bleaches for use in the laundry.

Benzoyl peroxide is oxidative bleach often inadvertently used to remove dye. It is answer to common question of "Why are my towels getting light spots?" Many skin care regimens include benzoyl peroxide for its acne-fighting abilities, but it is also noted for its ability to remove colour of common commercially used blue dyes that can happen even for those who take great care in washing their hands after use. Interestingly for a skin care product, benzoyl peroxide is also an explosive & is used as a rocket fuel. Users of benzoyl peroxide skin care products, if bothered by ruined towels, should either use white towels only, or consider switching to another acne-fighting agent.

Fast fade for jeans: Sodium di-chloro-iso-cyanurate An excellent product for discharging indigo-dyed denim, sodium di-chloro-iso-cyanurate was until recently sold under name Rit Fast Fade for Jeans, but has apparently been discontinued. Chemical is still sold as Dylon Easy Bleach & also for use in swimming pool disinfection. Indigo can be discharged only with oxidative discharges, because reductive discharges react with vat dyes such as indigo to form water-soluble leuko form of dye.

Other oxidative discharge chemicals: There are several oxidative discharging agents used in textile industry not recommended at all for use at home. NaClO2, is closely related to NaOCl; its solutions become particularly dangerous at low pHs. (Household bleach has NaOH added to keep pH of its hypochlorite at a less hazardous high pH; low pHs are hazardous primarily as they lead to production of lethal chlorine gas.). Potassium permanganate is industrially used to discharge indigo-dyed denim, but it is extremely poisonous & also becomes dangerously explosive, if its solution is inadvertently allowed to dry up.


2) Reductive discharges Just as for chlorine bleach, described above, not all dyes can be removed or changed in colour by reductive discharges, though results are often quite different for reductive discharges than for chlorine bleach. So, following reductive bleaches may be required: ⇒ Sulfur di-oxide, SO2: It can be used as reducing, no matter which of following chemicals should

be used to reduce dye. Originally it was produced by burning yellow sulfur in presence of fibre to be bleached. But it is not good for health; better to use one of other reducing discharge agents to produce it on fabric.

⇒ Thio-urea di-oxide (Colour Index reducing agent 11), also known as amino-imino-methane-sulfonic acid or formamidine sulfinic acid, is sold under the brand names thiox, spectralite, a dye-house colour remover & chemical formula is H2NC(=NH)SO2H. It is used in indigo dyeing & other vat dyeing as well as for discharge. It is costlier than sodium hydrosulfite, but needs to use only one-fifth as much. Jacquard colour remover contains thio-urea di-oxide & soda ash.

⇒ Sodium hydrosulfite (Colour Index reducing agent 1), also known as sodium dithionite, sodium sulfoxylate, & sodium sulfoxylate, is active ingredient in Rit Colour Remover, Tintex Colour Remover , Dylon Run away for Whites, & Carbona Colour Run Remover , all of which also contain sodium carbonate (soda ash). Its chemical formula is Na2O4S2. It’s storage in large quantities is unsafe due to its flammability, but it is easy to find this product at local drugstores or sewing stores, so there is no need to keep storing bulk quantity at once. Sodium hydrosulfite can also be used on stovetop or in washing machine; latter is less effective but far more convenient for use on clothing.

⇒ Sodium hydroxyl-methane-sulfinate (Colour Index Reducing Agent 2), also known as sodium formaldehyde sulfoxylate, sodium hydroxymethanesulphonate, sodium hydroxymethanesulfonate, and formaldehyde sodium sulphoxylate, is chemical used in Formosul & Rongalit (most likely including BASF's Rongalit C & Rongalit ST, also sold as Jacquard Rongolit ST). Its chemical formula is CH3NaO3S. It can be used under acid as well as basic conditions.

⇒ Zinc formaldehyde sulfoxylate (Colour Index reducing agent 6), Zn (HOCHSO2]2, is another discharge chemical, used commercially for screen-printing t-shirts.

⇒ Calcium formaldehyde sulfoxylate (Colour Index reducing agent 12), Ca (HOCH2SO2]2, is a discharge chemical that is manufactured in paste form. It is also main ingredient in a different Rongalit product, Rongalite H, though a suitable source for it is not found.

Other reductive discharge chemicals: Both sodium sulfite (Na2SO3) & sodium bi-sulfite (NaHSO3) are listed in some documents as reductive bleaches, but it is not found a recipe for either's use for this purpose, though it's easy to find recipes for use of other reducing agents above. At our level it seems that these are only used to neutralize hypochlorite bleach. But Na2SO4 should not be used as it destroyes cellulose.

Jacquard discharge paste apparently contains Rongalit ST (as above), along with water, urea, & ammonia. This product is highly recommended by dyers who like convenience of not having to make their own paste & like its stable shelf life. To use it, paint or print design on fabric, allow to dry, & then steam-iron to activate reaction. There are 3 different kinds of discharge paste:


a) Thio-urea dioxide print paste, b) Rongalit print paste, & c) Jacquard discharge paste.

Tin (II) chloride is unlike above reductive discharges in that it does not involve SO2 at all. Also known as "tin salt", it has a formula of SnCl2:2H2O, & has been used in discharge printing on wool. It reacts to form HCl in steamer, is corrosive to equipment. It must not be used with thio-di-glycol as a dye solvent, as reaction of HCl with thio-di-glycol produces deadly mustard gas.

Heating All of reductive discharge chemicals require heat to activate reaction that breaks double bonds in dye chemicals. Some recipes call for hot tap water in washing machine; most call for higher temperatures of a discharge bath (heated in a cooking pot), a steamer, or heating with a steam iron.

An alternative way to provide moist heat required to activate discharge agents is to steam treated items in a spare microwave oven that has been placed out-of-doors during use. It is recommended to use a microwave oven that is not used for food. Do not microwave dry fabric, as it burns. Wrap damp fabric in plastic so that it does not dry out as it cooks, or place a cup of water in microwave while using. Do not use a microwave that is indoors for heating discharge chemicals, as irritating fumes produced can be bad for lungs.

Cautions Although reductive discharge chemicals are less toxic, in general, than chlorine bleach, they all produce SO2, which may be particularly dangerous for people who have asthma. All should be used only with gloves & with care to avoid overexposure to any vapours produced. Be sure to use proper ventilation &/or an acid gas respirator while working with them. Note that a dust mask provides no protection at all. MSDS for each chemical, we work with should be followed.

SO2 fumes may also have undesirable effects on fabrics that have been dyed with indigo or other vat dyes, causing blues to turn yellow or green. Do not leave indigo-dyed fabrics exposed to air in a room in which we use any reductive discharge chemical.

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Striping & Discharge Printing of Reactive Dyes