Innovation: Researchers cotton on to dry printing process

SCIENTISTS at Leeds University have invented a chemical treatment that makes it possible to transfer print designs on to cotton with insoluble dyes, thus avoiding the water pollution associated with existing wet printing processes.

The man-made fabric polyester is already transfer-printed, using insoluble - or disperse - dyes. The procedure is well established and popular, because it is a dry process that does not involve washing the fabric after printing to remove excess dye.

In their untreated state, cotton and other natural-fibre textiles are not suitable for transfer-printing with disperse dyes. Attempts to do so result in poor colour yield, dull shades and patterns that run and fade.

Now research in the Department of Colour Chemistry and Dyeing at the university, supported by Gamega International, a textile design and print company in Leicester, has led to the development of a chemical that bonds to the fibres in cotton, making them receptive to disperse dyes.

The compound has been patented and University of Leeds Innovation Ltd will set up a company in the next six months to market it.

Worldwide consumption of cotton is expected to grow from the current 18 million tons a year to 24 million tons by the end of the 1990s.

In the new process, cotton fabric is treated with a 20 per cent mixture of the chemical in water. The fabric is passed through a padding machine, a kind of mangle that presses the chemical into the cotton. It is then dried and baked to fix the chemical.

''The chemical forms covalent bonds with the cotton fibres, becoming physically part of the fabric,' said Peter Broadbent, who invented it. After the cotton is washed to remove any residue, it can be transfer-printed like polyester.

'Cotton printed in this way shows the same high colour yields and wet and light fastness as polyester,' Dr Broadbent said.

The chemical has other positive effects on the fabric, making it resistant to creasing and shrinking. According to Dr Broadbent it is also non-toxic.

Transfer-printing from paper to fabric has other advantages. Designs can be more complicated, the process is faster, and it is easier to set the print up. The process does not affect the feel of the cotton.