The processes by which we remove sizes are known as desizing. Sizing is the need for the weaving but is an obstacle for the dyeing. It can be done by many ways such as acid steep, rot steep, enzymatic etc. in all these enzymatic desizing is dominating because of its eco-friendliness and also because of its characteristics that is it acts at specific sites only at definite pH, temperature and concentration. Mainly starch is used as the ingredient in sizing.
Chemically starch is poly-α-glucopyranose in which straight chain (amylase) and branched chain (amylopectin) polymers are present. Both constituents of starch are insoluble in water, but they can be solubilised by hydrolysis of these long chain compounds to shorter ones. Thus under suitable conditions starch can be progressively hydrolyzed to the following stages. In desizing the hydrolysis reaction is carried out up to the stage of soluble dextrin only and not further to a-glucose.
Like starch polyvinyl alcohol is also common. Since it is a powerful film forming sizing agent and because of the ease with which it can be removed (it is soluble in water) it is an ideal sizing agent. The molecular weight and the degree of hydrolysis are the two primary factors, which influence its solubility in water, the solubility decreasing with increasing molecular weight. The desizing of polyvinyl-treated fabrics involves three steps- swelling, dissolving and dispersing. In contrast to starch, enzymes, normally used for desizing starches, do not hydrolyze it. The principle steps in the desizing procedure are
Wetting out with suitable wetting agent.
Steeping for affecting the swelling and softening of the polyvinyl alcohol film.
Rising thoroughly in overflowing water.
Desizing efficiency is found in two ways conventional and
In this method we first take the weight of the sized fabric, let it be W1. Then desize the fabric, dry & take the weight, let it be W2. After that the fabric is treated with 3gpl (35%) HCl at 700 C for 30 min. dry & take the weight of the fabric. Let it be W3.
Total size = W1-W3.
Residual size = W2-W3.
Desizing Efficiency = (Total size – Residual size)/Total size X 100.
Reagent: potassium iodide (10 gm. Of KI (100%) in 100 ml water, add 0.6358 gm of iodine (100%) stir and shake; iodine is completely dissolved. Fill up to 800 ml with water then complete to 1000 ml with ethanol. (Shelf life approx 6 months only).
Spot drop wise solution onto fabric.
2. Rub in gently.
3. Assess change of colour.
Note: the test must be carried on fabric cooled down to room temperature; residual alkalinity has to be neutralities prior to the test.Assessment:
No change of colour = no starch size present.
Pale blue to bluish = presence of starch size or blend
Violet =of starch size with synthetic size
Pale blue to bluish violet = refer to violet scale TEGEWA This indicates residual Starch content.