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Historically, the word oxidation referred to the combination of an element with oxygen to give an oxide, like iron combines with oxygen to give iron oxide or rust. The word reduction referred to the removal of oxygen from an oxide to yield the element.
The term oxidation was first applied to the combining of oxygen to other elements. Iron rusts to become iron oxide and carbon burns to produce carbon dioxide. Both are oxidation reactions; the element that is oxidized is losing electrons.
The term reduction was originally limited to the type of reaction in which the mineral ores like iron or copper oxide were “reduced” to the elemental form. Other elements besides oxides can be made. In reduction, there is a gain of electrons by atoms or ions.
The two processes, oxidation and reduction, always occur together. The oxidizing agent oxidizes (gives up electrons) and the agent itself gets reduced, while the reducing agent gains electrons and the agent itself gets oxidized. Sodium hypochlorite (chlorine bleach) oxidizes the stain, rendering it colorless and itself gets reduced to the chloride ion.
Bleaching action, or color removal, can be accomplished with either oxidation or reduction. Nearly any oxidizing agent would do the job. However, some would harm the fabrics, some would be unsafe, some would produce undesirable by-products and some would simply be too expensive.
Chemical bleaching agents function by solubilizing colors, thus facilitating their removal from the fiber. They also function by reacting with these substances in such a manner as to alter or destroy their sites of unsaturation, like conjugation bonds (areas in the molecule that contribute to the color), due to loosely-bound electrons which are boosted to higher energy levels by absorption of visible light. Bleaching agents do their work by removing or tying down these mobile electrons.
Stain removal is not nearly so simple a process as bleaching. A few stain removers are oxidizing or reducing agents, while others have quite different chemical natures. Nearly all stains require rather specific stain removers.
Hydrogen peroxide in cold water removes blood stains from cotton and linen fabrics. Potassium permanganate removes most stains from white fabrics (except for rayon); the permanganate stain then can be removed by treatment with oxalic acid. Sodium thiosulfate readily removes iodine stains by reducing iodine to a colorless ion.
Common oxidizing bleaches include sodium perborate, a relatively mild material that can be used on all fibers; hydrogen peroxide, a somewhat stronger chemical that is widely employed for commercial cotton bleaching and for lightening human hair; and sodium hypochlorite, the familiar chlorine bleach used in the home. Because it both disinfects and bleaches, sodium hypochlorite can be employed for heavily soiled garments, such as diapers, or for hospital laundry.
The reducing type bleaches can include sodium bisulfite, sodium hydrosulfite, sodium borohydride, titanium sulfate and oxalic acid, which is employed in rust removers. Three common reducing bleaches, which are safe on all fabrics but not on all colors, are sodium bisulfite, sodium hydrosulfite and titanium stripper. The sulfur dioxide used in bleaching wool also works by reduction.
Reducing bleaches are commonly used in dry cleaning stores, as are some oxidizing bleaches. The process in which oxidizing and reducing agents are used in textiles is called bleaching.
The purpose of bleaching is to remove or lessen discoloring stains and to remove colored contaminants from raw fibers. The term bleach is the process of producing change toward a lighter shade of an object. Basically, it means to increase the reflection of visible light at the expense of absorption.
Colors of soil
The color of organic soiling is due to the presence of chromophoric (color producing) systems; these are conjugated double bond systems.
If the conjugated bonds can be disrupted by making them shorter, or modifying their structure, the soil will become less colored or lose the color altogether. Oxidative bleaching is a destructive process, meaning the dye molecule is dismembered.
Bleaching effects can occur through mechanical, physical and/or chemical means through change or removal of dyes and soil adhering to the bleached object. The relative significance of each is determined in part by the nature of the soil present.
Mechanical/physical mechanisms (like hot water extraction/shampooing) are effective for the removal of water soluble, particulate or greasy soils. Chemical bleaching is employed for the removal of non-washable soils adhering to fibers, accomplished by oxidation or reduction of the colored stains.
The oxidizing bleaches are used to a greater extent than the reducing type. The reason is the commonly encountered stains may become colorless initially with reducing agents but may return to the colored form as a result of subsequent air oxidation. This does not rule out the use of special reductive (reducing) bleaches like sodium bisulfite, sodium hydrosulfite and sodium thiosulfate to treat specific types of discoloration encountered in residential or commercial settings.
Bleaching is dependent on a number of factors. Examples include the type of bleach, its concentration, contact time (also called residence time), temperature, the type of soil to be bleached and the nature of the fiber, whether nylon, olefin, etc.
The stains or soils to be bleached can consist of a wide range of substances. They could be dyes from fruit like blueberries, soft drinks dyes like FD&C Red No. 40, Curcuma (turmeric) dyes from curry and mustard; brown tannin stains from tea, wine, coffee; urobilin from urine, hair coloring agents and inks etc.
As a general rule, oxidizing and reducing agents should never be mixed. However, in certain situations, such as halting the effect of chlorine bleach (an oxidizer) on a fiber, at times sodium bisulfite (a reducer) is employed to neutralize the chlorine bleach. In this case, sodium bisulfite is known as an anti-chlor.
Before using oxidizing or reducing agents, where possible, the cause of stain should be identified and if the fabric of fiber will be able to stand the rigors of the chemical. The age of the stain, sensitivity of the stain, dyes, fibers and finishes should be looked into.
What is the pH within the yarn? Can the fiber be treated with a water-based reagent or should a solvent be employed?
Test the chemical in an inconspicuous are or on a spare sample. After employing the stain removal technique, the area treated should be thoroughly rinsed to remove the dissolved materials originating from the stain and excess reagent. &
Aziz Ullah, Ph.D., MBA, is president of Fabpro Manufacturing, a leading formulator of carpet and upholstery cleaning products. He is a member of the American Chemical Society, senior member of the American Association of Textile Chemists and Colorists and a member of The Textile Institute (UK). He can be reached at www.Fabpro.com.