Chemistry Reference
In-Depth Information
fabric is described when entering the bleaching bath, and when leaving the
bath, respectively. Particularly with the dry-wet treatment, the surface
energy barrier between fabric and bleaching solution is high, so that the
humidification rate restricts the rate of the process. To avoid this, tensio-
active substances are added to the bleaching solution, as these substances
decrease the surface energy barrier
17-18
.This facilitates the penetration
of hydrogen peroxide (over a smaller energy barrier) in the fabric and
oxidises or reduces the chromophore groups. Adding tensio-active
substances can even double the daily production. However, many tensio-
active products have one considerable disadvantage: namely, the intense
formation of foam due to turbulent streaming profiles present in a bleach-
ing bath.
In order to control this disadvantageous effect, de-foamers and de-
aeraters are added
19
.The defoamer's task is to control the foam formation
due to the tensio-active substances, while the de-aerater removes gas
bubbles, in solution and in the fibre structure. In this way, the contact surface
between fabric and bleaching solution is kept maximal, which can only
favour the rate of the process. By adding the de-aerater, lower quantities
of tensio-active substances are needed, which helps to minimise foam
formation.
Finally, sodium persulphate is added in order to weaken the fibre. Before
the cotton is spun to a thread, this thread is reinforced with a layer of
cellulose to reduce the incidence of failures during the spinning process.
Obviously, the cellulose layer will have to be removed later, and this can be
achieved either by means of enzymes in a process preceding the bleaching,
or with sodium persulphate simultaneously with the bleaching process.
The various producers launching the above-mentioned additives onto the
market use trade names that nearly always relate to mixtures of reagents,
of which neither the identity nor the concentration is made public because
of corporate propriety. A comparative analysis has shown that the various
additives contain similar products; a minor modification to the structure
common to all the stabilisers is the EDTA structure, although the terminal
groups on the amine positions can be different
11
.
4.3
Determination methods of hydrogen peroxide
The most commonly used technique for the determination of hydrogen per-
oxide in an industrial environment is the classical manual titration of a bath
sample with potassium permanganate after acidification with sulphuric
acid. In the past, numerous attempts have been made to perform the deter-
mination by means of an instrumental technique; however, disadvantages
were always encountered, which, up to now, have hindered the break-
through of an alternative method.
