DYEING is a messy business. Factories that colour natural fabrics use masses
of salts—sometimes as much by weight as the fabric itself—to
encourage dyes to stick to the material. But those salts often end up in waste
water, killing fish in rivers or ruining crop soil. Now there may be a way to
avoid the salts altogether when dyeing cotton.
Dyes and natural fibres both become negatively charged in water, so they tend
to repel each other, making it tough to get strong colours to bind to cloth
quickly. Adding salts such as sodium sulphate helps overcome this. All salts
dissociate into positive and negative ions when they dissolve. When positive
ions come close to the negatively charged dye they reduce the repulsion,
allowing the dye molecules to get close to the fibres. Once in contact with the
cloth, the dye binds strongly to it.
Every year, the US textile industry uses about 95,000 tonnes of sodium
sulphate in its dyeing processes, and it says it cannot afford to remove all the
dissolved salt from waste water. So sodium-rich effluent ends up contaminating
fresh water and can make soils too alkaline to support crops.
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“Salt is the most problematic chemical that is released when dyeing,”
says Richard Blackburn from the University of Leeds. Now Blackburn says he
has found a harmless polymer solution that can replace the salt. In his patented
process, you soak the cotton in a bath of cellulose-like polymers before dyeing
it. Since cellulose is the major constituent of plant carbohydrates, the polymer
and the fibres have a very similar physical structure. That and the polymer’s
positive charge mean it bonds readily to the fibres. This gives the cloth a
positive charge, which then attracts the negatively charged dye and forms an
even stronger chemical bond due to the action of the opposite charges. This
stronger bonding means that the dyeing and rinsing process uses up to 80 per
cent less water—and wastes less dye. Blackburn’s method will be revealed
in the journal Textile Chemist and Colorist.
But Peter Hauser from North Carolina State University says that Blackburn’s
polymer molecules are too big to penetrate the fibres, so the dye stays on the
surface rather than soaking in and is therefore more likely to fade over
time.
To avoid this, Hauser has developed a different method of giving the cloth a
positive charge, using a chemical called Quat-188, which is based on
trimethylammonium chloride (Coloration Technology, vol 5, p 282). He
claims that the colour holds faster this way.
Quat-188 is a modified form of a suspected carcinogen, but Hauser says
reacting the chemical with cotton removes its mutagenic properties, so the cloth
would be safe to wear and the chemicals would only be hazardous to dye workers
if they touched or inhaled it. Hauser thinks this won’t be a problem, but
Blackburn believes it best to avoid the chemical altogether.