Tuesday, May 5, 2009

Synthetic dyes

All dyes were natural dyes until 1856.  Englishman William Henry Perkin, 18, was trying to make quinine from coal tar.  By serendipity he made a purple goo - the first basic (cationic/aniline) dye called Mauve. His whole family jumped into the venture and they started the first synthetic dye factory in 1857.  Perkin was later knighted for his efforts.  

The basic dye has an amine functional group that becomes positively charged in water.  It is attracted to negatively charged carboxylic acid groups on protein fibers and some nylons, as well as the cyanide (nitrile) groups on acrylic fiber.

Congo red was invented in 1884. It was the first direct dye.  Direct dyes bond weakly by hydrogen bonding to functional groups containing nitrogen or oxygen on natural fibers.  It is usually applied with hot water  immersion and sodium chloride.  Typically direct dyes are used to dye cotton and the cellulose fibers which contain the alcohol functional group.   These dyes are fairly large molecules and are not the most wash fast.  Rit and other union dyes often contain direct and acid dyes of the same hue.

Fiber reactive or Procion dyes have been used since the sixties to dye cellulosic fabrics.  These have a chlorine (halide) functional group.  An alkali (sodium carbonate) removes the hydrogen proton of the alcohol groups on cellulose fibers and silk.  This hydrogen combines with the chloride of the dye to form HCl.

The dye covalently bonds to the alcohol oxygen atom on the fibers.   This dye should be heat set.

Many synthetic fibers such as polyester, nylon and acrylics were invented at Dupont and elsewhere in the forties and fifties.  Disperse dyes (also in crayola wax crayons) worked well on hydrophobic fibers such as polyester and triacetate, which both have the ester functional group.  London dispersion forces are involved in the bonding.  Disperse dyes are small molecules which sublime readily and can “torpedo” into the synthetic fiber.   Heat and pressure facilitate the dyeing and water is not necessary.

Today nylon fibers as well as protein fibers are dyed by acid (anionic) dyes. The negative sulphonate acid group on the acid dye bonds to the positive amine/amide groups on these fibers in water.

More than 8000 synthetic dyes are made today.

Type of attachment:

Ionic bonds - acid and basic dyes

Covalent bond - fiber reactive dye

Hydrogen bond - direct dye

London dispersion force - disperse dye

Physical entrapment - vat dye