Chemistry Reference
In-Depth Information
bleached in strongly alkaline medium in which oxidative products, such as
sodium persulphate, potassium persulphate or ammonium persulphate, are
used to remove cellulose from the fabric. However, wool is bleached in an
acidic or slightly alkaline environment
21
by the use of hydrogen peroxide,
which acts only as oxidising agent for acetic acid anhydride, the reaction
product that possesses the necessary bleaching properties
22
.The hydrogen
peroxide formed in Equation 3.6 can oxidise the acetic acid anhydride to
reaction products that bleach the cellulose-free textile. The main problems
to be solved in order to allow industrial use of the method are related
to the concentration of hydrogen peroxide obtained from the oxidation
process, the optimal pH and temperature for enzyme activity and the
method used for measuring the process parameters and controlling them
at optimal values for the best quality of cellulose-free and bleached product.
Dyeing on a cellulose layer is totally different to dyeing directly on the
textile, because the structure of the textile is different from the structure of
cellulose. Depending on the type of dye used, there is either a chemical, an
electrochemical
18
, an ionic
18
, an electrostatic
19
or an adsorptive
20
interaction
with the textile structure. It is clear that this interaction would be totally
different if the dye were to be used directly on the cellulose layer. The
interaction could be weaker, or even zero, or could be based on a different
mechanism.
In this chapter, a method is described to remove simultaneously cellulose
and bleach fibre products in the same bath with an acidic medium. An
account is also given of a sensor system for measuring and controlling the
hydrogen peroxide concentration. For the first purpose, enzymes are used
that catalyse the oxidation of the cellulose-structure. In this cellulose-
removal process oxygen, dissolved in the process liquid, is consumed and
hydrogen peroxide is formed (Equation 3.7):
Alkali
HO
CH
2
HO
CH
2
O
O
5
O
2
+
+H
2
O
2
1
4
+(O)
OH
O
O
O
O
2
-H
2
O
HH
3
OH
OO
[3.7]
As can be seen from Equation 3.7, a strongly alkaline medium initiates a
break of the bonding between the oxygen and the fifth carbon atom of the
ring structure. The opening of this structure gives the cellulose the oppor-
tunity to react chemically with the wool structure and results in its fixation
to the wool structure. This is a second reason why wool is not bleached
and/or treated to remove cellulose in strongly alkaline solutions.
