Saturday, March 6, 2010

Chemistry of Textiles and Dyes: pH

Chemistry of Textiles and Dyes

pH of fibers and dyes

 

Ever wonder why adding lemon to iced tea changes the color?  It is due to pH.

 

What is pH?

 

pH is a logarithmic measure of the acidity of a solution. Water, which has equal amounts of OH- and H+ ions, is pH 7 or neutral pH.  An acid, which is pH 1, is 10 times more acidic than pH 2 and 100 times more acidic than pH 3.  Each unit on the pH scales differs by a factor of 10.  The scale ranges from 0 to 14.  Above 7 is basic/alkaline and below 7 is acidic.

 

What is an acid?

Acids are proton (H+) donors.  Examples of acids used in dyeing include vinegar, acetic acid, tartaric acid (cream of tartar) and lemon juice.

 

 

What is an alkali?

An alkali or base is a proton acceptor.   Examples of alkalis used in dyeing include household  ammonia, baking soda (sodium bicarbonate), washing soda/soda ash (sodium carbonate).  Lye or sodium hydroxide NaOH is not recommended for the home dyer for respiration problems and potential to damage eyes and skin.  Ammonia should be used with a fume hood.

 

Safety Precaution:  Under no circumstances should acid and bases be mixed!

 

How does pH affect fibers?

pH affects natural fibers  more than synthetics. 

 

 

A review: 

 

Cellulose fibers are fibers from plants.  They include 1) natural – cotton, linen (flax), ramie, jute, hemp, sisal, sea grass, coir and bamboo and 2) regenerated cellulose which can be made from cotton linters, wood and bamboo and includes rayon, lyocell (Tencel), acetate and triacetate. 

 

Paper is made from cellulose fibers and has the same pH requirements. 

 

Protein fibers

Silk  is made up of beta keratin protein polymer consisting of glycine, alanine, serine and tyrosine amino acid monomers.

 

Wool amino acids – the internal amino functional groups become protonated at pH 3-4 and the carboxylic groups become protonated at pH 2, which gives wool fiber a positive charge that will attract anionic (negatively charged) acid dyes.  Hence wool is acid dyed in acidic solutions.

 

The dyeing of the protein in Easter eggs is facilitated by dipping the eggs in vinegar before dyeing with acid food color dyes.

 

Acid dyes

Acid dyes contain anionic functional groups like sulphonic acid.  They are attracted to positively charged amino groups on polyamide fibers like wool, silk and nylon.

 

 

Textile dyeing

A good rule of thumb for dyeing is acid pH for protein fibers and nylon and alkaline pH for cellulose fibers.  For safety reasons the pHs of the pH scale should be avoided and dyers should wear gloves and safety goggles at all time.  The pH range for dyeing fabric can range from pH 11.5 for using vinyl sulfone Ramazol dyes on cellulose fibers like cotton down to pH 4 for Lanasol fiber reactive dyes used on polyamides like wool, silk and nylon.

 

Of the protein fibers wool is the most resistant to acids.  Silk is not damaged by organic acids like acetic acid, cream of tartar and vinegar but will be degraded by inorganic mineral acids like sulfuric, nitric and hydrochloric. 

 

In fact, wool, hair and fur are strengthened by mild acids like shampoo. Lemon juice, cream rinse and rainwater.  The acid causes the wool/hair cuticle to lie down flat, sealing in the oils (sebaceous for humans and lanolin for sheep). Mild acid treatment in cold water produces more lustrous fiber surface, by reflecting light in parallel specular direction.

 

Conversely wool and silk fibers are easily damaged by high pH cleaning products.  These same products will strength cotton and linen.

 

Organic and inorganic acids, even dilute acetic acid or vinegar weakens all cellulose fibers.  Acid hydrolysis breaks up the ether links holding the glucose monomers in the cellulose polymer, essentially converting the ether group to two alcohol groups and breaking the chain. 

 

In general synthetic fibers like polyester, acrylic and olefin are unaffected by pH changes. 

 

Solubility

Protein fibers are soluble in strong alkali at elevated temperatures.  They are not soluble in strong acids.

 

Nylon is soluble in formic acid.

 

Cellulose fabrics like cotton and rayon are not soluble in high pH liquids, but are soluble in strong acids such as concentrated sulfuric acid.

 

Cleaning Protein fibers-   Mild hair shampoos are recommended to clean wool, hair and silk, because of their neutral to mild pH.  If washing with hard alkaline water, it is advisable to add a cap of vinegar to the bath. 

 

The lanolin grease of wool and the sericin gum of silk can be effectively removed by rinsing fibers/fabrics with the same alkali detergent, which will weaken the fiber, if left on too long.  

 

 

Cleaning nylon fibers-   Cleaning products with pH> 10 will lessen the stain resistance of nylon.

 

Cleaning Cellulose fibers-   Cellulose fibers are often strengthened by alkalis.  Test your detergent solutions with universal indicator pH paper to determine the pH.

 

Dye color change with pH

Some dyes change color when the pH changes.  The color is determined by the molecular structure of the dye colorant.  Litmus paper is made with a lichen dye, which is red at pH 4.5 and blue at pH 8.5.

 

Many of these pH indicators belong to the red-purple-blue flavonoid molecule class known as anthocyanins. Dyes in this class include the dye molecules in red cabbage, blueberries, poppies and blue cornflower. The addition of a hydrogen proton to the colorant anthocyanin molecule increases at lower pH values, which changes its light absorbance spectra. Typically anthocyanin colors range from red (low pH) to purple and blue as pH increases and the hydrogen ions come off the molecule.

 

Madder root Rubia tinctorum is the source an ancient red dye used to dye Turkish carpets red. The British red coats were dyed with madder root dye.  The roots contain ruberthyrin acid, which is converted to glucose, alizarin pigment and purpurin pigment molecules by reaction with acid.  Purpurin is colorless, but changes to red when dissolved in acid.  Madder lake, an organic pigment, is produced by mixing the pigment with clay, alum and ammonia.

 

The root sawdust is soluble in sulfuric acid H2SO4, which holds a concentrated alizarin dye. It ranges from yellow at pH of 5.5 to orange to red a pH 7.

 

Carminic acid molecule, found in dyes derived from cochineal insects range from yellow at low pH to purple at higher pH.  Curcumin found in Indian curry powder ranges from yellow at pH 7 to red at pH 8.5.  Logwood  and Brazilwood, both heartwood dyes will vary in color depending on pH.

 

 

 

Discharge bleaches and pH

Some sulfur containing discharge chemical like thiourea and sodium hydrosulfite are activated by alkaline conditions to decompose to sulfur dioxide – the active molecule that decomposes the dye molecule/chromaphore.  A discharge paint can be made by combining sodium alginate, water, thiourea and washing soda (sodium carbonate Na2CO3).

Alternatively hot aqueous solutions can be used without the alginate paste.

When discharging protein fibers, always neutralize afterwards with an mild acid bath and water rinse.

 

 

Fiber Reactive dyes and pH

Fiber Reactive (Procion) dyes are aided by high pH.  The alkali removes the hydrogen atom from the alcohol group on the cellulose fiber, which then bonds to the dye molecule, forming a covalent bond.  Soda ash and lye are used to aid in dyeing.   When dyeing silk baking soda is recommended, as it is less harsh on protein fibers. 

 

Vat dyes and pH

Vat dyes like indigo are activated at high pH.  The alkali activates the reducing agent to convert indigo into the leuco or reduced green-yellow form, which is water-soluble. The higher the pH the faster the blue pigment reduced to the water-soluble dye.

 

Milder alkalis should be used when indigo dyeing wool, silk and nylon.

 

pH and wood pulp paper

Most paper made from wood pulp at not 100% cellulose, but comprise 20-30% lignins.  Over time the lignins break down and become more acidic which, in turn, causes the paper to break down.  To be acid-free and archival, paper must be delignified at the mill.  As paper acidifies with age or acid exposure, it often yellows.

 

Alum size is another cause of paper acidification.  Some alum-sized paper have a pH below 5.

 

Buffered paper is paper, which is stuffed with alkaline chemicals similar to stomach antacid with a pH around 8.  Buffers can cause some watercolor pigments to change color.  Prussian blue pigment will turn brown in alkaline-buffered paper.

 

Mercerization, named for John Mercer is an alkaline finishing process for cellulose fibers, particularly cotton, linen and rayon to strengthen them.  Fabrics are placed on a stretcher/strainer and dipped into a vat of alkali such as lye or ammonia.  The fibers are more lustrous, have increased tensile strength, are more crystalline and accept dye better.

 

 

pH and wool felt

Alkali damages wool, as it does all protein fibers and is useful in making wool felt.  Alkali opens up the wool cuticles, which helps them interlock to form felt.

 

References

 

B.B. Buchanan, W. Gruissem and R.L. Jones, Biochemistry and Molecular Biology of Plants, Wiley Publ. 2002.

 

J. Gordon Cook, Handbook of Textile Fibres: I Natural Fibres, 5th Edition, Merrow Publ. Durham, UK 1984. 

 

Hoechst Celanese Corporation, Dictionary of Fiber and Textile Technology, Charlotte NC, 1990.

 

U.U. Modibbo, B.A. Aliyu and I.I. Nkafamiya, “The effect of mercerization media on the physical properties of local plant bast fibres,”  Int. J. Phys. Sci, p. 698.

 

Numerous articles about the chemistry of natural dye pH indicators have appeared in the Journal of Chemical Education, published by the American Chemical Society.

 


 

 

3 comments:

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