Saturday, December 1, 2018

Chemical Finishes - Bleaching
Art Resource

Marie-Therese Wisniowski

Preamble
This is the eighty-third post in the "Art Resource" series, specifically aimed to construct an appropriate knowledge base in order to develop an artistic voice in ArtCloth.

Other posts in this series are:
Glossary of Cultural and Architectural Terms
Units Used in Dyeing and Printing of Fabrics
Occupational, Health & Safety
A Brief History of Color
The Nature of Color
Psychology of Color
Color Schemes
The Naming of Colors
The Munsell Color Classification System
Methuen Color Index and Classification System
The CIE System
Pantone - A Modern Color Classification System
Optical Properties of Fiber Materials
General Properties of Fiber Polymers and Fibers - Part I
General Properties of Fiber Polymers and Fibers - Part II
General Properties of Fiber Polymers and Fibers - Part III
General Properties of Fiber Polymers and Fibers - Part IV
General Properties of Fiber Polymers and Fibers - Part V
Protein Fibers - Wool
Protein Fibers - Speciality Hair Fibers
Protein Fibers - Silk
Protein Fibers - Wool versus Silk
Timelines of Fabrics, Dyes and Other Stuff
Cellulosic Fibers (Natural) - Cotton
Cellulosic Fibers (Natural) - Linen
Other Natural Cellulosic Fibers
General Overview of Man-Made Fibers
Man-Made Cellulosic Fibers - Viscose
Man-Made Cellulosic Fibers - Esters
Man-Made Synthetic Fibers - Nylon
Man-Made Synthetic Fibers - Polyester
Man-Made Synthetic Fibers - Acrylic and Modacrylic
Man-Made Synthetic Fibers - Olefins
Man-Made Synthetic Fibers - Elastomers
Man-Made Synthetic Fibers - Mineral Fibers
Man Made Fibers - Other Textile Fibers
Fiber Blends
From Fiber to Yarn: Overview - Part I
From Fiber to Yarn: Overview - Part II
Melt-Spun Fibers
Characteristics of Filament Yarn
Yarn Classification
Direct Spun Yarns
Textured Filament Yarns
Fabric Construction - Felt
Fabric Construction - Nonwoven fabrics
A Fashion Data Base
Fabric Construction - Leather
Fabric Construction - Films
Glossary of Colors, Dyes, Inks, Pigments and Resins
Fabric Construction – Foams and Poromeric Material
Knitting
Hosiery
Glossary of Fabrics, Fibers, Finishes, Garments and Yarns
Weaving and the Loom
Similarities and Differences in Woven Fabrics
The Three Basic Weaves - Plain Weave (Part I)
The Three Basic Weaves - Plain Weave (Part II)
The Three Basic Weaves - Twill Weave
The Three Basic Weaves - Satin Weave
Figured Weaves - Leno Weave
Figured Weaves – Piqué Weave
Figured Fabrics
Glossary of Art, Artists, Art Motifs and Art Movements
Crêpe Fabrics
Crêpe Effect Fabrics
Pile Fabrics - General
Woven Pile Fabrics
Chenille Yarn and Tufted Pile Fabrics
Knit-Pile Fabrics
Flocked Pile Fabrics and Other Pile Construction Processes
Glossary of Paper, Photography, Printing, Prints and Publication Terms
Napped Fabrics – Part I
Napped Fabrics – Part II
Double Cloth
Multicomponent Fabrics
Knit-Sew or Stitch Through Fabrics
Finishes - Overview
Finishes - Initial Fabric Cleaning
Mechanical Finishes - Part I
Mechanical Finishes - Part II
Additive Finishes
Chemical Finishes - Bleaching
Glossary of Scientific Terms
Chemical Finishes - Acid Finishes
Finishes: Mercerization
Finishes: Waterproof and Water-Repellent Fabrics
Finishes: Flame-Proofed Fabrics
Finishes to Prevent Attack by Insects and Micro-Organisms

There are currently eight data bases on this blogspot, namely, the Glossary of Cultural and Architectural Terms, Timelines of Fabrics, Dyes and Other Stuff, A Fashion Data Base, the Glossary of Colors, Dyes, Inks, Pigments and Resins, the Glossary of Fabrics, Fibers, Finishes, Garments and Yarns, Glossary of Art, Artists, Art Motifs and Art Movements, Glossary of Paper, Photography, Printing, Prints and Publication Terms and the Glossary of Scientific Terms, which has been updated to Version 3.5. All data bases will be updated from time-to-time in the future.

If you find any post on this blog site useful, you can save it or copy and paste it into your own "Word" document etc. for your future reference. For example, Safari allows you to save a post (e.g. click on "File", click on "Print" and release, click on "PDF" and then click on "Save As" and release - and a PDF should appear where you have stored it). Safari also allows you to mail a post to a friend (click on "File", and then point cursor to "Mail Contents On This Page" and release). Either way, this or other posts on this site may be a useful Art Resource for you.

The Art Resource series will be the first post in each calendar month. Remember - these Art Resource posts span information that will be useful for a home hobbyist to that required by a final year University Fine-Art student and so undoubtedly, some parts of any Art Resource post may appear far too technical for your needs (skip over those mind boggling parts) and in other parts, it may be too simplistic with respect to your level of knowledge (ditto the skip). The trade-off between these two extremes will mean that Art Resource posts will hopefully be useful in parts to most, but unfortunately may not be satisfying to all!


Introduction
Chemical finishes are some of the oldest finishes used. Natural dyes, iron rust dyeing, caustic from ashes and urine, and bleaching by sunlight are examples. Chemical finishes are usually permanent; and unlike the additive finishes, they do not add weight to the fabric.

Some of the following chemicals are now used in large quantities by the textile industry:
1. Caustic soda (alkali).
2. Sodium phosphate.
3. Soda ash.
4. Chlorine.
5. Sulfuric acid.
6. Sodium sulfate.
7. Hydrogen peroxide.

Today's post will center on bleaching and the other posts on this topic will center on acid finishes, and mercerisation.


Bleaching
Most bleaches are oxidizing agents. The actual bleaching is done by active oxygen. A few bleaches are reducing agents. These are used to strip color from dyed fabrics. Bleaches may be either acid or alkaline in nature. They are usually unstable, especially in the presence of moisture. Bleaches that are old or have been improperly stored will lose their oxidizing power.

Any bleach will cause damage, and since damage occurs more rapidly at higher temperatures and concentrations, these factors should be carefully controlled.



The same bleach is not suitable to all kinds of fibers. Because fibers vary in their chemical reaction, bleaches must be chosen with regard to fiber content. The anklets in the photograph below had been all white, but when bleached with a chlorine bleach, the wool-ribbed cuff section became discoloured, while the cotton feet remained white.

Slide 1: White anklets of cotton and wool after bleaching in chlorine bleach.

The consumer users bleaches to remove stains, and off-white "tattle-tale" grey caused by soil and soap curds from wash water, which should be removed by reconditioning treatments rather than by a bleach. Better still, wash often and use correct washing procedures from the start. Reconditioning is done by soaking the article in a solution made with a non-precipitating water softener and then washing it in hot, sudsy, softened water. This must be repeated several times. Mild bleaches may be used on fabric to remove stains without damage to the original color of the fabric. Garments that have a resin finish should not be bleached with chlorine since yellowing will result.

Fabric Bleach Art.

The finisher uses bleaches to clean and whiten grey goods. The natural fibers are an off-white color because of the impurities they contain. Since these impurities are easily removed from cotton, most cotton grey goods are bleached without damaged. The bleaching step is often omitted with wool because it has good affinity for dyes and other finishes even if not bleached.

Liquid chlorine bleaches were, for many years, the common household bleaches. They are efficient bactericidal agents, and as such can be used for sterilizing fabrics. They are cheap and efficient bleaches for cotton and rayon. The bleaching is done by the hypochlorous acid liberated during the bleaching process. This tenders cellulose fibers and the bleach must be thoroughly rinsed out or an antichlor (sodium thiosulfate) should be used. Chlorine bleaches, other than sodium chlorite, are of no value on the protein and thermoplastic fibers and if used will cause yellowing. Man-made fibers are often damaged or are unaffected by household chlorine bleaches. The weekly wash also contains many fabrics with crease-recovery and embossed finishes, which should not be bleached with chlorine.

Chlorine bleach can be used not only on white fabric, but also on colorfast colored fabrics.

Peroxide bleaches are common factory bleaches for cellulose and protein fibers and fabrics. Hydrogen peroxide is an oxidizing bleach. A 3% solution is relatively stable at room temperature and is safe to use. Peroxide will bleach best at temperatures of 180 to 200oF in an alkaline solution. These bleaching conditions make it possible to do peroxide bleaching of cellulose grey goods as the final step in the kier boil.

In the peroxide cold bleach procedure, the fabric is soaked overnight or for a period of eight hours. This procedure is often used on cotton knit goods and wool to preserve a soft hand. Peroxide is good for removing light scorch stains.

Sodium perborate is a powder bleach which becomes hydrogen peroxide when it combines with water. It is a safe bleach for home use with all kinds of fibers. Satisfactory results have also been obtained with thermoplastic fibers by use of the cold bleach process. Powder bleaches are recommended for regular use in the wash water to maintain the original whiteness of the fabric rather than as a whitener for discoloured fabrics.



Acid bleaches such as oxalic acid and potassium permanganate have limited use. Critic acid and lemon juice are also acid bleaches, which are good rust spot removers.



Reducing bleaches are good for stripping color from dyed fabrics. Sodium hydrosulfite is available in pharmacies.

Optical brighteners are also used to whiten off fabrics. They are fluorescent white compounds, not bleaches. The fluorescent white compounds are absorbed into the fiber and emit a bluish fluorescence that covers up yellowish tinges. At the mill, optical temperatures give best results when used in combination with the bleach rather than a substitute for it. These new fluorescent whites are often incorporated is soaps and detergents as "whiter than new" ingredients. They also added to the spinning solution of some man-made fibers.

The way optical whiteners work.


Reference:
[1] N. Hollen and J. Saddler, Textiles, 3rd Edition, MacMillan Company, London (1968).

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