2. Preparation of cellulose materials for dyeing and printing

Preparation is a set of operations to which the textile material before dyeing and printing is exposed. The size and type of bleaching depends on the type of fabric, the production required and whether the fabric is to be dyed or finished white. For cotton materials preparation consists of following stages: desizing, alkaline clearing (sliver scouring), mercerizing, bleaching. Depending on assortment of fabrics and a way of preparation the part of these stages can be excluded, some operations are combined. The basic part of natural cellulose fibers is cellulose (-C₆H₁₀O₅-)n. Besides, fibers contain impurity which influence technological processes of processing. The structure of cotton and linen fibers is lower resulted.

Table 1.2. natural impurity of cotton and flax, %

Name	cotton	flax
Cellulose	94 - 96	74 - 76
Wax	0,5-0,6	2,7
Nitrogenous	0,9-1,2	1,9-2,1
Pectinaceous	1,2	4,9-5,0
Mineral	1-1,3	1,3
Organic acids	0,8	-
lignin	-	4,8
Sugar	0,3	-
painting pigment	traces	traces
not investigated part	0,3-1,3	9,4-11,9

All these impurity inform a material rigidity, bad moistening, yellow-grey color, etc. As a result dye hardly gets into textiles and is badly fixed on a fiber. The basic indicators characterised quality of preparation: capillarness, hydrophility, moistening, whiteness.

Capillarness normally prepared cotton fabric makes about 125 mm for 30 minutes, and whiteness degree approximately 84 %.

Alkaline clearing it is considered the most important operation which purpose -to give to a material capillaries and moistening. Thus there is a maximum removal of all natural impurity of a cellulose fiber (clap) such, as: pectinaceous, nitrogenous, wax. Presence wax causes low hydrophility a severe fiber. The basic reagent destroying impurity, is NaOH. Except NaOH into a solution for alkaline clearing enter, for example, Na₂Si03 which affinity products of destruction of natural impurity of a clap and iron oxides, interfering their with return sorption on a fiber. Introduction of surface-active substances promotes reconducting wax impurity in emulsion. The weak reducer is entered into a solution also, for example, NaHSО₃, which prevents degradation of cellulose fiber air oxygen, connecting oxygen:

2NaHS0₃ + 0₂ → 2 NaHS0₄.

Bleaching it is carried out basically by oxidizers which translate a natural painting pigment in the colorless soluble connections deleted by washing. Now more than 80 % of textiles bleach hydrogen peroxide H₂O₂. Use of chlorine-containing bleaches is complicated because of environmental problems arising thus, including because of danger of formation toxic dioxines.