Page 201 - Basic Principles of Textile Coloration

190 AN INTRODUCTION TO DYES AND DYEING

compatible. Dye manufacturers go to considerable lengths to select combinations
of compatible dyes.

One useful way of testing for the compatibility of dyes in mixtures is to carry
out a large number of dyeings, but after different dyeing times, a sample of the
dyed fabric is removed from a given container and replaced with an identical piece
of undyed material. Each dyeing is then continued for the same total time. The
series of samples removed from the dyebath, arranged in order of the sampling
time, will have decreasing colour depths. The series of replacement samples, in
order of their introduction time, will have increasing colour depths. For
compatible dyes, all the dyed samples will have the same hue, with colour depths
in relation to the actual amount of time spent in the dyebath. Otherwise, the two
series of dyeings show irregular differences in both hue and in depth.

10.5 CONTINUOUS DYEING

10.5.1 Introduction

Continuous dyeing is most economic for production of very large lots of a single
colour. This presents particular problems of process control to ensure constancy of
the shade over lengths of material that may exceed 50 000 m running for over 8
hours. Today, there is more demand for smaller lots in diverse colours. This often
makes continuous processing uneconomical unless rapid colour change-over is
possible. A typical continuous dyeing range may contain up to 400 m of fabric and
the customer may only want as little as 8000 m. Rapid establishment of steady-state
dyeing conditions and of shade consistency are essential when running at speeds of
up to 100 m min–1. Continuous dyeing of woven fabrics of cotton and polyester, and
their blends, is predominant. Knitted fabrics are rarely dyed continuously because
they undergo considerable elongation when under tension. Continuous dyeing of
carpets is also quite popular, particularly in the USA.

A continuous dyeing range may have as many as 4–12 pieces of equipment in
line (Figure 10.6). Some of these may be omitted, or passed over, for particular
dyeing processes. Section 12.6 gives specifics about various types of equipment. A
leader cloth will already be threaded through the various units. This is the end
cloth remaining from the last lot of material that was dyed. The start of the fabric
to be dyed is stitched to this leader and fed into the range from a large roller
mounted on a sturdy frame. The end of this piece will be stitched to the start of
the next roll and so on, without stopping production, if possible. This necessitates
use of a fabric accumulator where the remainder of a roll of fabric can be quickly